Analysis of Water Transport in Polymer Electrolyte Fuel Cells using

DISS. ETH NO. 20548
Analysis of Water Transport in
Polymer Electrolyte Fuel Cells
using Neutron Imaging
A dissertation submitted to
ETH ZURICH
for the degree of
Doctor of Sciences
presented by
Pierre Oberholzer
MSc in Mechanical Engineering
EPFL LAUSANNE
born May 8th 1983
citizen of Goldingen (SG)
accepted on the recommendation of
Prof. Dr. A Wokaun, examiner
Prof. Dr. J.F. Mesot, co-examiner
Dr. G.G. Scherer, co-examiner
Dr. P. Boillat, co-examiner
2012
Science sans conscience n’est que ruine de l’âme.
(Science without conscience is ruin of soul.)
Ferdinand Rabelais
Writer, Doctor, 1532
Abstract
The Polymer Electrolyte Fuel Cell (PEFC), using hydrogen as fuel, represents a promising alternative to internal combustion engines in cars since it can be integrated in an energy conversion
chain using renewable sources and reducing polluting emissions. Beyond the issues related to
the hydrogen distribution, the main obstacles to overcome for a worldwide commercialization are
the durability and cost of the PEFC. Understanding the major role of water on the performance
losses is fundamental to mitigate its detrimental effects, improve the performance and reduce
the cost of the system. This thesis originated from this general motivation.
By using neutron imaging as a visualization tool of liquid water combined with other in situ
characterization methods, unique insights can be gained on the loss mechanisms occurring in an
operating PEFC.
Deuterium labeling consists in replacing 1 H atoms by 2 H atoms and observing the subsequent
change of intensity obtained with neutron imaging. By applying transient changes of isotopic
composition in the gases surrounding a hydrated membrane, parameters related to the bulk and
interfacial transport can be extracted. A methodology based on a simple diffusive model fitted
on experimental data was developed. It allowed identifying a maximal exchange rate of H atoms
between the membrane and the gas phase. It also permitted to investigate the effect of various
diffusive limitations in the porous media on this exchange.
By using helox gas (79% He and 21% of O2 ), a helox pulsing method was developed as a characterization tool of the bulk diffusive losses of oxygen occurring in a PEFC operating under air.
In particular, validation experiments showed that the behavior of the cell was not changed by
the application of this method.
A new experimental set-up, called multi-cell, that enables the simultaneous imaging and testing
of six cells, was developed and used in combination with the discussed methods.
The study of the effect of the microporous layer (MPL) on performance and water distribution
was investigated by means of the proposed methods. The role of this largely used component,
still debated in the community, could be pointed out. The beneficial effects of this material
appear to be a prevention of water accumulation in the catalyst layer environment rather than
a reduction of the water saturation level in the gas diffusion layer (GDL).
v
The comparison of different flow fields was realized with the same set of characterization tools,
in order to assess the effect on the performance of the diffusive gas flow distribution of oxygen in
the GDL. While the effect of the channel-rib distribution was identified, other results suggested
the presence of important proton and mass transport losses in the electrode.
At last, cold-starts of a cell maintained at constant subfreezing temperature were visualized
by dynamic neutron imaging. The observation of the water distribution and the final voltage
drop strongly suggest the presence of water in super-cooled state in the system for most of the
experiments realized.
vi
Résumé
La Pile à Combustible à Electrolyte Polymère (PEFC en anglais), utilisant l’hydrogène comme
combustible, représente une alternative prometteuse au moteur à combustion interne des voitures.
En effet, cette technologie peut être intégrée dans une chaı̂ne de conversion d’énergie utilisant
des sources renouvelables et limitant les émissions polluantes.
Au-delà des difficultés liées
à la distribution de l’hydrogène, les principaux obstacles devant être surmontés pour une
commercialisation mondiale sont la durabilité et le coût de la PEFC. La compréhension du
rôle majeur de l’eau sur les pertes de performance est fondamentale en vue de limiter ses effets
détrimentaux, d’améliorer la performance et de réduire le coût du système. Cette thèse s’inscrit
dans cette motivation générale.
L’utlisation de l’imagerie neutronique, en tant qu’outil de visualisation de l’eau liquide, combinée
à d’autres méthodes de caractérisation in situ permet de mieux comprendre les méchanismes de
pertes effectifs dans une PEFC en opération.
Le traçage au deutérium consiste à remplacer les atomes 1 H par des atomes 2 H et observer
par imagerie neutronique le changement d’intensité qui s’ensuit. L’application de changements
transitoires de composition isotopique des gaz dans l’environnement d’une membrane hydratée
permet d’extraire des paramètres de transport volumique et interfacial. Une méthodologie basée
sur un modèle diffusif simple ajusté aux données expérimentales fut developpé. L’identification
d’un taux d’échange maximal d’atomes H entre la membrane et la phase gazeuse fut obtenue par
cette méthode. L’effet de différentes limitations diffusives dans les couches poreuses a également
pu être testé.
Utilisant du gaz helox (79% He et 21% O2 ), une méthode de pulsage d’helox a été développée
afin de caractériser les pertes diffusives volumiques de l’oxygène d’une PEFC fonctionnant sous
air. En particulier, des expériences de validation ont pu montrer que le comportement de la pile
n’était pas affecté par l’application de cette méthode.
Une nouvelle station expérimentale, nommée multi-pile, permettant le test et l’imagerie
neutronique de six piles simultanément, a été développée et utilisée en combinaison avec les
méthodes discutées précédemment.
vii
L’étude des effets de la couche microporeuse (MPL en anglais) sur la performance et la
distribution de l’eau dans la pile a été effectuée au moyen des méthodes proposées. Le rôle
de ce composant largement utilisé, encore débattu dans la communauté, a été mis en exergue.
L’effet bénéfique de ce matériau semble plutôt être de prévenir l’accumulation d’eau dans
l’environnement de la couche catalytique que de réduire le niveau de saturation d’eau dans la
couche de diffusion poreuse (GDL en anglais).
La comparaison de différents champs d’écoulement fut réalisée avec le même ensemble d’outils de
caractérisation dans le but de juger de l’effet sur la performance de la distribution du flux diffusif
d’oxygène dans la couche GDL. Tandis que l’effet de distribution canal-nervure fut identifié,
d’autres résultats suggèrent la présence d’importantes pertes par transport de masse et transport
protonique dans la couche catalytique.
Enfin, des démarrages à froid d’une pile maintenue à température constante au-dessous de 0°C
furent visualisés par imagerie neutronique dynamique. L’observation de la distribution d’eau
et de la chute de tension finale laissent fortement supposer la présence d’eau surfondue dans le
système pour la majorité des expériences réalisées.
viii
Acknowledgements
This thesis work was realized in the framework of a PhD student position in the Fuel Cell Group
of the Electrochemistry Laboratory (ECL) at the Paul Scherrer Institut (PSI) in Switzerland.
First of all, I would like to sincerely thank Prof. Alexander Wokaun and Günther G. Scherer
for having given to me the opportunity to join the team and participate to its exciting research
activities. The impressions I got on the recruiting day turned out to be founded: I really enjoyed
this position and appreciated that curiosity was one of its requirements. Their experimented and
sharp-eyed comments motivated my commitment. For his acute sense of humor and in response
to his warmhearted advice, I sincerely wish to Günther Scherer invigorating and jubilant afterwork times. His recent successor Prof. Thomas J. Schmidt is also acknowledged for his positive
feedbacks and for his involvement. For having accepted to be co-examiner of this work as director
of our institute, Prof. Joel Mesot of course receives my full gratitude.
For their strong support during the hard times that represent neutron imaging measurement
campaigns, the members of the Neutron Imaging and Activation Group (NIAG) I directly worked
with are thanked: Gabriel Frei, Stefan Hartmann, Jan Hovind, Anders Kaestner, Eberhard H.
Lehmann and Peter Vontobel. They provided the powerful infrastructure and the required knowhow for fruitful measurements.
Aside of this thesis work, an intensive contribution was supplied to our collaboration with different industrial partners. Without explicitly mentioning them all, I would like to express them
my sincere respect for their patience, serenity and professionalism during the sometimes stressful
moments shared together.
Being essentially experimental, the present study hugely relied on technical and engineering
works. I would like to thank Christian A. Marmy for his strong logistic support during the
stressful pre-campaign periods. All the contributors to the development and the construction of
the new experimental set-up are also acknowledged.
The administrative work realized in such an institute is considerable, and also at the scale of our
laboratory. For this, Isabella Kalt and Esther Schmid receive my thanks for their collaboration.
The scientific team I had the chance to work with was composed of very engaged people with
strong scientific skills. To avoid omitting anyone, the whole laboratory team is thanked. A
ix
sincere thank is given to Kaewta Jetsrisuparb for her moral support in the very last moments of
writing as well as to Yves Buchmüller for his nice company both in and out of the office.
As a member of the fuel cell neutron imaging group, I express my thanks to Raffaella Perego for
her collaboration. For his precise work and also for his friendly personality, Raphael Siegrist is
sincerely acknowledged. More recently arrived, Felix Neuschütz is thanked for his engagement
and receives my truthful wishes of a pleasant continuation in his work. Also, I wish to the newest
colleague and successor of this work, Johannes Biesdorf, exciting times on the topic, and I can
already thank him for his nice and engaged collaboration. Finally, I deeply thank my colleague
and supervisor Pierre Boillat. I would not have achieved this work without him. He taught me
a lot about science and technique and gave me all the background and tools, at all steps of the
work, for pursuing and completing it. His state of mind, readiness, humor and professionalism
will serve me as an example. The tough times we had together turned out to be positive thanks
to his remarkable personality. My full engagement at this position was due to his presence.
At last, I would like to thank my closest friends for their support and express my regrets for
having been so absent during these times. In particular and in response to an old dedication,
I wish to express my sincere friendship to Jerôme Comte, whose company was always greatly
estimated. Hoping they will understand these lines, I profoundly thank my family, brother and
sister, Elsa, Frédéric, Gilles and Anne for having visited and supported me and, of course, for
all the rest. Last but not least, I express to Isabelle my deepest thanks for having been aside
me during these very special years and for having corrected some of these words. The rest will
be kept unwritten.
x
Contents
1 Introduction
1
1.1
Motivation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1.2
Working principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
1.3
Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
1.3.1
Thermodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
1.3.1.1
Standard conditions . . . . . . . . . . . . . . . . . . . . . . . . .
4
1.3.1.2
Non-standard conditions . . . . . . . . . . . . . . . . . . . . . .
5
1.3.2
Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
1.3.3
Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
1.3.3.1
Charge transfer losses . . . . . . . . . . . . . . . . . . . . . . . .
6
1.3.3.2
Ohmic losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
1.3.3.3
Mass transport losses . . . . . . . . . . . . . . . . . . . . . . . .
9
Cell components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
1.4.1
Membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
1.4.2
Catalyst layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
1.4.3
Gas diffusion layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
1.4.4
Flow field plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
Water management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
1.5.1
Local effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
1.5.2
Global effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
1.6
Differential cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
1.7
Neutron imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
1.8
Scope of this work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
1.4
1.5
2 Diagnostic tools
2.1
25
Deuterium labeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
2.1.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
2.1.2
Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
26
xi
CONTENTS
2.1.3
Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
28
2.1.3.1
Gas mixture . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
28
2.1.3.2
Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
28
2.1.3.3
Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
2.1.3.4
Set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
31
2.1.3.5
Image processing . . . . . . . . . . . . . . . . . . . . . . . . . . .
32
Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
2.1.4.1
Bulk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
2.1.4.2
Border conditions . . . . . . . . . . . . . . . . . . . . . . . . . .
34
Analytical solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
2.1.5.1
Steady-state . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
2.1.5.2
Transient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
37
Numerical solving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
38
2.1.6.1
Discretization . . . . . . . . . . . . . . . . . . . . . . . . . . . .
38
2.1.6.2
Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
2.1.6.3
Validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
40
2.1.7
Fitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
2.1.8
Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
43
2.1.9
Effect of resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44
2.1.10 Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
46
Helox pulsing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
47
2.2.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
47
2.2.2
Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
47
2.2.3
Set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
49
2.2.4
Validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
50
2.2.4.1
Short timescale . . . . . . . . . . . . . . . . . . . . . . . . . . . .
51
2.2.4.2
Long timescale . . . . . . . . . . . . . . . . . . . . . . . . . . . .
56
Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
60
Multi-cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
62
2.3.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
62
2.3.2
General characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
64
2.3.3
Gas supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
64
2.3.4
Electrical control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
64
2.3.5
Temperature control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
65
2.3.6
Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
66
2.3.7
Compression rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
67
2.1.4
2.1.5
2.1.6
2.2
2.2.5
2.3
xii
CONTENTS
2.3.8
Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
67
2.3.9
Flow field design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
68
2.3.10 Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
70
2.3.11 Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
71
3 Results
3.1
Membrane transport properties . . . . . . . . . . . . . . . . . . . . . . . . . . . .
73
3.1.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
73
3.1.2
Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
73
3.1.3
Diffusivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
74
3.1.3.1
Gas flow dependency . . . . . . . . . . . . . . . . . . . . . . . .
75
3.1.3.2
Experimental statistics . . . . . . . . . . . . . . . . . . . . . . .
76
3.1.3.3
Relative humidity dependency . . . . . . . . . . . . . . . . . . .
80
3.1.3.4
Temperature dependency . . . . . . . . . . . . . . . . . . . . . .
81
Exchange rate with water vapor . . . . . . . . . . . . . . . . . . . . . . .
82
3.1.4.1
Gas flow dependency . . . . . . . . . . . . . . . . . . . . . . . .
83
3.1.4.2
Relative humidity dependency . . . . . . . . . . . . . . . . . . .
86
3.1.4.3
Temperature dependency . . . . . . . . . . . . . . . . . . . . . .
88
3.1.4.4
Structure dependency . . . . . . . . . . . . . . . . . . . . . . . .
90
3.1.5
Exchange rate with hydrogen . . . . . . . . . . . . . . . . . . . . . . . . .
93
3.1.6
Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
97
Effect of the microporous layer (MPL) . . . . . . . . . . . . . . . . . . . . . . . .
99
3.2.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
99
3.2.2
Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
3.2.3
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
3.1.4
3.2
3.2.4
3.3
73
3.2.3.1
RH-series - 0.5 A/cm2 . . . . . . . . . . . . . . . . . . . . . . . . 101
3.2.3.2
RH-series - 1 A/cm2 . . . . . . . . . . . . . . . . . . . . . . . . . 108
3.2.3.3
IV-curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
3.2.3.4
RH-step . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
Effect of flow field design
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
3.3.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
3.3.2
Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
3.3.3
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
3.3.3.1
RH-series - 1 A/cm2 . . . . . . . . . . . . . . . . . . . . . . . . . 125
3.3.3.2
IV-curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
3.3.3.3
Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
xiii
CONTENTS
3.3.3.4
3.3.4
3.4
RH-step . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
Cold-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
3.4.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
3.4.2
Literature overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
3.4.3
Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
3.4.4
3.4.3.1
Cell components . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
3.4.3.2
Cooling set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
3.4.3.3
Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
3.4.4.1
Reference experiment . . . . . . . . . . . . . . . . . . . . . . . . 148
3.4.4.2
Temperature dependency . . . . . . . . . . . . . . . . . . . . . . 154
3.4.4.3
Current dependency . . . . . . . . . . . . . . . . . . . . . . . . . 155
3.4.5
Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
3.4.6
Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
4 Conclusions and outlook
165
4.1
Specific aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
4.2
General aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
Bibliography
169
List of Tables
179
List of Figures
181
Oral presentations
193
xiv
1
Introduction
1.1
Motivation
Fossil fuels have served during the last century as our primary source of energy for heating,
electricity production and transportation. The intensive consumption and the finite amount
of the reserves will sooner or later lead to a situation of global depletion. The International
Energy Agency even recognized in 2008 that the worldwide rate of crude oil field production
had already peaked in 2007 [1]. Further investments will be needed to open up new production
fields and to prospect about undiscovered ones. Ineluctably, the extraction processes will be
more complex, such as exploiting resources in deep-water offshore or oil sands for instance, and
prices will increase [1].
Basically, the energy extracted from fossil fuels is obtained by a combustion reaction:
y
y
O2 → xCO2 + H2 O
(1.1)
C x Hy + x +
4
2
whose enthalpy can be directly used in a heating circuit or converted into mechanical energy by
combustion engines. Mechanical energy can be directly used like in automotives or converted
into electrical energy by a generator. Both products of the combustion reaction, namely water
and carbon dioxide, are greenhouse gases. At high concentration, carbon dioxide can be toxic on
local scale. At a global scale, the increase of the carbon dioxide concentration in the atmosphere
due to human activities appears to be a cause of the global warming observed in the last decades
[2]. If the combustion reaction is incomplete, carbon monoxide, a toxic gas for human beings, is
formed. Furthermore, due to the possible participation of nitrogen (from air) or sulfur (from fuel)
in the reaction, oxidized species like NOx and SOx can be produced, which are also toxic species.
At last, various kinds of unburned hydrocarbons can result from an incomplete combustion and
also increase the risks of diseases.
Consequently, other energy sources than fossil fuels have to be searched, both to reduce
the dependence on them and the detrimental impact on the environment and humans. Using
1
1. INTRODUCTION
renewable energies such as solar, wind, hydro, biomass or geothermal energies, can help to fulfill
these requirements. The availability of some of these sources being naturally fluctuating, like
for solar or wind, storage of energy is of prime importance in order to optimize the energy
production.
For these reasons, hydrogen is considered as a valuable candidate to be a non-carbon and
renewable fuel. In particular, it can be efficiently produced by electrolysis of water or by water
splitting with help of thermochemical cycles and can therefore integrate well in a renewable
energy conversion chain. As a fuel, hydrogen has the advantage of presenting, when pressurized,
a reasonable volumic energy density.
An efficient system to recover the chemical energy of hydrogen is to make it electrochemically
react with oxygen to produce electricity and water. This is the role of the fuel cell. The sufficient
volumic energy density of hydrogen combined to the fairly good power density of the fuel cell
together with a good efficiency are the reasons that have motivated the academic and industrial
research to invest much effort into this technology mainly in the last two decades, even if the
fuel cell principle discovery is much older and is attributed to C.F. Schoenbein (1799-1868) or
more often to Sir William Grove (1811-1896) [3]. The Polymer Electrolyte Fuel Cell (PEFC) is
one particular type of fuel cell whose main feature is a low operating temperature (< 100 °C).
It is therefore suited for application needing low temperature and rapid start-up, such as mobile
power supplies. In particular, the replacement of internal combustion engines in automotive is
the main application targeted for this technology. The main obstacles to overcome for a worldwide commercialization are durability and cost [4], such that research is still needed to help
improve the technology.
1.2
Working principle
The following explanations are based on the illustration below (Figure 1.1). The different cell
components mentioned in the following will be described in details later (section 1.4). In particular, the catalyst layer (CL) and the gas diffusion layer (GDL) will be abbreviated from here
on.
In a PEFC, hydrogen (H2 ) electrochemically reacts in the catalyst layer (CL) located on
the anode side of the polymer electrolyte membrane (PEM) by oxidizing into protons H+ and
electrons e− . On the cathode side of the membrane, oxygen (O2 ) electrochemically reacts in
the corresponding CL by accepting the electrons and the protons to reduce into water. Each
of these two half reactions implies the appearance of an electrochemical potential, which in-turn
leads to a potential difference between the two electrodes, namely the cathode and the anode. It
represents a given voltage U which is the driving force of the global reaction. Therefore, when
electrically contacting the electrodes, the voltage makes the electrons produced at the anode
2
1.3 Fundamentals
Flow field
Gas diffusion layer Catalyst layer (CL) Membrane Catalyst layer (CL)
(GDL)
(anode)
(cathode)
O2
e
-
H
+
H2
e
e
Channel
-
Rib
H 2O
-
U
Figure 1.1: PEFC: working principle and components.
attracted by the cathode, which delivers a given power to the user, whereas the protons cross
the proton-conducting membrane.
In the following sub-chapters of this introduction, fundamentals of the electrochemical reactions at thermodynamical equilibrium will be explained and the different origins of the voltage
losses will be discussed (section 1.3). The main components of a cell unit will be presented along
with specific features relevant for this work (section 1.4). The role of water in the system will be
addressed (section 1.5) and the differential cell concept will be introduced (section 1.6). Neutron
imaging will be motivated as a visualization tool of liquid water in operating cells (section 1.7).
Finally, the scope of this work will be defined (section 1.8).
1.3
1.3.1
Fundamentals
Thermodynamics
The Polymer Electrolyte Fuel Cell (PEFC) is a system in which hydrogen reacts electrochemically with oxygen to form water:
1
H2 + O2 H2 O
2
(1.2)
Precisely, this global reaction is the result of two half-reactions occurring at each electrode,
namely the hydrogen oxidation reaction (HOR) at the anode:
H2 2H+ + 2e−
(1.3)
and the oxygen reduction reaction (ORR) at the cathode:
1
O2 + 2H+ + 2e− H2 O
2
3
(1.4)
1. INTRODUCTION
Both HOR and ORR reactions are at equilibrium as long as no electron is exchanged between
the two electrodes, meaning that no electrical current is supplied by the cell. In this case, the
global reaction (Equation 1.2) is characterized by a given molar reaction enthalpy (lowercase
symbols) that can be expressed as follows:
∆r h = ∆r g + T ∆r s
(1.5)
in which ∆r g is the change of Gibbs free energy, T is the reaction temperature, and ∆r s is
the change of entropy. In a reversible reaction, the term T ∆r s is the amount of heat energy
produced by the reaction whereas the Gibbs free energy represents the maximal useful work
that this reaction can provide at constant temperature and pressure.
1.3.1.1
Standard conditions
Commonly, these values are tabled in standard conditions (p0 = 1 bar and T 0 = 298 K) and the
above equation (Equation 1.5) becomes:
0
∆r h0298 = ∆r g298
+ 298∆r s0298
(1.6)
where ∆r h0298 = -286 kJ/mol is used and supposes water in liquid state, ∆r s0298 = -0.16 kJ/(mol
0
= -237 kJ/mol.
K) and ∆r g298
The so-called thermodynamical efficiency of the fuel cell can be calculated as:
th =
0
∆r g298
= 83%
∆r h0298
(1.7)
In a Carnot cycle, which defines the maximum efficiency reachable by a heat engine, a very
high combustion temperature for the reaction (Equation 1.2) would be needed to achieve this
efficiency (3802 K) [5], which would obviously imply severe constraints for materials as opposed
to the PEFC operating at low temperatures (typically less than 100°C).
In an electrochemical conversion, the Gibbs free energy corresponds to the electrical work
done by the electrical charge z in a particular voltage, so-called reversible voltage, defined as the
maximum voltage reachable by a perfect cell in standard condition:
0
Urev,298
=−
0
∆r g298
= 1.23V
zF
(1.8)
where z = 2 is the number of electrons transferred between the two half-reactions (Equation 1.3
and Equation 1.4) and F = 96’485 C/mol is the Faraday’s constant. By extension, it is also
quite common to use the thermodynamic voltage as a translation of the reaction enthalpy:
0
Uth,298
=−
∆r h0298
= 1.48V
zF
4
(1.9)
1.3 Fundamentals
1.3.1.2
Non-standard conditions
Actually, the PEFC is operated at higher temperature than ambient temperature for thermal and
water management purposes. At constant pressure and due to the weak temperature dependence
of the entropy, it is of practical use to calculate the temperature effect on the Gibbs free energy
by [6]:
0
∆r g 0 (T ) = ∆r g298
− T ∆r s0298
(1.10)
In the present study, all experiments with operating cells are realized at 70 °C (= 343 K)
0
0
and a value of ∆r g343
= -230 kJ/mol is found, which leads to Urev,343
= 1.19 V.
Moreover, the partial pressures of the species in the present work are not equal to unity as
defined in the standard conditions. This leads to a change of Gibbs free energy and therefore to
a reversible voltage at non-standard conditions, expressed by the Nernst equation:
!
a
RT
H
O
2
0
Urev = Urev
(T ) −
ln
2F
a H2 a0.5
O2
(1.11)
where z = 2 was inserted, R = 8.314 J/(mol K) is the ideal gas constant and ai are the activities
of the different species. If liquid water is produced, a H2 O = 1 can be assumed. For the gaseous
species, the definition of the activity is:
ai = φi xi
p
p0
(1.12)
where φi is the fugacity coefficient (φ = 1 for ideal gases), xi is the molar fraction, p is the
pressure considered and p0 = 1 bar is the standard pressure. The molar fractions are therefore
x H2 = 1 in pure hydrogen and x O2 = 0.21 in air, as utilized in this work. For p = 1 bar, a value
of Urev,343 (p = 1bar) = 1.18 V is found. In this work, a typical pressure of p = 2 bar will be
applied on each side, so that Urev,343 (p = 2bar) = 1.19 V is obtained.
It can be useful to convert the activities in (Equation 1.11) into molar concentrations:
ai = φi
ci
c0
(1.13)
where ci is the molar concentration of the species i in a total molar concentration c0 at standard
pressure p0 = 1 bar. The latter can be expressed by using the ideal gas law as:
p0
RT
(1.14)
ci = xi c0
(1.15)
c0 =
and the term ci as:
The Nernst expression found before (Equation 1.11) can be rewritten as:
RT
(c0 )1.5
0
Urev = Urev −
ln
2F
c H2 c O2 ,ref 0.5
5
(1.16)
1. INTRODUCTION
In this expression, the subscript ”ref ” was added to the oxygen concentration to designate
the concentration of oxygen when no current is applied, or in other words in absence of any
concentration gradient, as it will be discussed below.
To summarize, the voltage values found in this chapter represent the maximal voltage that
could be obtained in absence of losses. In reality, kinetic losses appear as soon as an electrical
current is supplied. Therefore, the voltage value that is the closest to this theoretical reference is
measured without electrical current and is called open circuit voltage (OCV) and written UOCV .
But even in this case, the UOCV value is in practice lower than Urev , essentially due to hydrogen
crossover [7]. Values for UOCV between 0.95 and 1.1 V are typically measured [8].
1.3.2
Definitions
To compare the performances of cells operating under current I and having different active area
A, the current density is defined:
i = I/A
(1.17)
that is usually expressed in A/cm2 .
The Faraday’s law allows relating the electrical current density i to the surface molar flow
of a reactant ṅ (at stoichiometry unity):
i = ṅzF
(1.18)
where z = 2 if ṅ refers to a flow of H2 and z = 4 if O2 is considered by virtue of (Equation 1.3)
and (Equation 1.4) respectively.
1.3.3
Losses
By connecting the electrical circuit to a load, an electrical current I is delivered by the PEFC
and the HOR (Equation 1.3) and ORR (Equation 1.4) reactions are not at equilibrium. This
implies the apparition of overpotentials ηi whose effect is to reduce the real voltage U compared
to the OCV voltage UOCV :
U = UOCV −
X
ηi
(1.19)
The different loss mechanisms, detailed in the following, are summarized on the plot hereunder
(Figure 1.2).
1.3.3.1
Charge transfer losses
An electrical current circulating between the two electrodes implies the deviation of the HOR and
ORR reactions from their steady-state equilibria. The non-zero net reaction rate in these two
half-reactions in-turn results into activation overpotentials or charge transfer overpotentials ηCT
6
1.3 Fundamentals
U[V]
Uthermo
Entropy losses
Urev
H2 crossover
UOCV
ηCT: Charge transfer losses
ηΩ: Ohmic losses
ηC: Mass transport losses
ilim
2
i [A/cm ]
Figure 1.2: Representation of the different losses in the PEFC.
at each electrode, for both anodic and cathodic directions of each half-reaction. The ButlerVolmer equation expresses the relation between the electrical current and these overpotentials.
For the HOR reaction, it gives:
zF
zF
i = i+ + i− = i0,HOR · eαA,HOR ·ηCT,HOR RT − i0,HOR · e−αC,HOR ·ηCT,HOR RT
(1.20)
where i+ and i− refer to the anodic and cathodic directions of the HOR reaction, and αA,HOR
and αC,HOR are the corresponding transfer coefficients. The parameter i0,HOR is the exchange
current density and is directly proportional to the reaction rate of the HOR half-reaction at
the steady-state equilibrium. Thus, it depends on the activation energy of the reaction and
consequently on the catalyst activity. This value will be estimated in the present work by
isotopic exchanges (section 3.1).
Similarly, for the ORR reaction, we have:
zF
zF
i = i+ + i− = i0,ORR · eαA,ORR ·ηCT,ORR RT − i0,ORR · e−αC,ORR ·ηCT,ORR RT
(1.21)
Actually, the i0,HOR value is order of magnitudes bigger than the i0,ORR value, as reflected
by the values found in [9] (lowest value listed: i0,HOR = 1 mA/cm2Pt ) and in [10] (highest value
listed: i0,ORR = 8.7 nA/cm2Pt ). Therefore, the HOR overpotentials are very low compared to
the ORR overpotentials so that the HOR overpotentials are in practice neglected and only the
ORR overpotentials are taken into account, which can be expressed by simplifying the equation
(Equation 1.21) into:
zF
zF
i = i+ + i− = i0 · eαA ·ηCT RT − i0 · e−αC ·ηCT RT
(1.22)
In this expression, the exchange current density can be expressed as:
i0 = zF cO2 ,ref k0− e
0
−αC zF Urev
RT
(1.23)
where k0− is the pre-exponential term of the Arrhenius expression of the cathodic direction of
the ORR reaction and contains the expression of the activation energy. If the current density
7
1. INTRODUCTION
is sufficiently high, the first right-hand term in this above expression (Equation 1.22) can be
neglected so that it can be written:
zF
i ≈ i− = −i0 · e−αC ·ηCT RT
(1.24)
and the overpotential can be expressed as:
|ηCT | =
RT
ln
αC zF
|i|
i0
(1.25)
Usually, this expression is converted into decimal logarithms:
|ηCT | =
2.3RT
2.3RT
log(i0 ) +
log(|i|)
αC zF
αC zF
(1.26)
This is the Tafel equation and it has the form:
|ηCT | = a + b · log(|i|)
(1.27)
where the parameters a and b can be read on the previous expression (Equation 1.26). The
parameter b is often referred to as Tafel slope. In practice, it must also be mentioned that the
term a, being function of i0 and therefore of the catalyst activity, presents a dependence on
the voltage U , for instance due to Pt oxidation at high voltages. Changing the current, and
accordingly the voltage of the cell, can affect this term. The Tafel estimation of the ηCT losses
at higher currents may consequently be inappropriate.
1.3.3.2
Ohmic losses
The ohmic losses originate from the limited proton conductivity of the ionomer materials (e.g.
proton conducting material: the membrane and the ionomer of the CL), from the limited electrical conductivity of the materials (CL, GDL, flow fields), and from the electrical contact
resistances at the interfaces between these materials. The ohmic overpotential ηΩ is considered
to be proportional to the current density i:
ηΩ = r · i
(1.28)
where r is the resistivity and is expressed in Ω·cm2 . Since the ionomer hydration can strongly
vary with the current, in particular under dry conditions, the parameter r can also depend on the
current. The resistivity is typically estimated by using high frequency measurement, meaning by
measuring the AC perturbation of the voltage in response to an AC excitation current, which is
superposed to the DC current supplied by the cell. This method allows for being independent of
the losses originating from the electrochemical reactions, since the AC current tends to favor the
pathway across the double layer capacity formed at the interface between the electrical and the
8
1.3 Fundamentals
ionic conducting phases of the CL. The value measured is therefore referred to as high frequency
resistivity (HFR).
Inherently, some misestimation of the ohmic losses can result from the fact that the AC
current does not follow the same path as the DC current of the reaction, for instance due to 2D
effects in the GDL. Typically, the oxygen diffusion across the GDL induces a non-homogeneous
current distribution between the areas facing the channels and those facing the ribs of the
flow field [11, 12], as represented below (Figure 1.3). Being independent on the electrochemical reaction, the AC current will present a homogeneous pattern compared to the DC current.
Therefore, the loss estimated based on the simple multiplication of the integral current i by the
measured value r with (Equation 1.28) can introduce a given bias depending on the operating
conditions. Similar effects can appear across the ionomer of the CL. In case of a poorly conducting CL ionomer, some losses can appear on the DC current that are not captured by the
AC current, since the latter will tend to avoid the ionomer path by favoring the path across the
carbon content of the electrode. Such an effect will be observed and further discussed in this
work (section 3.3).
Electrochemical reaction
(DC current)
H
+
i1 i2 ...
Measurement
(AC current)
... iN
H
rΩ rΩ rΩ rΩ rΩ rΩ rΩ rΩ
O2
e
Channel
+
i1 i2 ...
... iN
rΩ rΩ rΩ rΩ rΩ rΩ rΩ rΩ
-
e
O2
-
Rib
ηΩ,1 ≠ ηΩ,2 ≠ ...
ηΩ,1 = ηΩ,2 = ...
... ≠ ηΩ,N≠ ηΩ,tot,real = rreal∙itot
... = ηΩ,N = ηΩ,tot,meas. = rmeas.∙itot
Figure 1.3: High frequency measurement artifact.
1.3.3.3
Mass transport losses
General equations The following effects can be expected when operating a cell under air
rather than under pure oxygen.
9
1. INTRODUCTION
If the cell is supplied by low stoichiometry gases (< 2), which is typically the case in technical applications for efficiency purposes, a reduction of the oxygen concentration along the
channel path will be caused by the reactant consumption, a process often referred to as reactant
depletion. Subsequently, the concentration c O2 ,ref will change and both the Nernst equation
(Equation 1.16) and the exchange current density (Equation 1.23) will be affected.
At a local scale, which is the scale focused in the present study, the oxygen has typically
to be supplied by a diffusive flow from the channels to the CL via the GDL, perpendicular to
the convective flow of oxygen inside the channels. A gradient of concentration will therefore
establish across the porous media, changing from the reference value cO2 ,ref in the channels
to the concentration cO2 on the active sites in the CL. In a 1D distribution, the concentration
reduction can be related to the current density by the following expression, that combines the
1D Fick’s law and the Faraday’s expression of the reactant flux (Equation 1.18):
cO2 ,ref − cO2
i = zF D
δ
(1.29)
At this point, it will be useful for some considerations to make the absolute pressure appear in
this expression. This can be realized by using the molar fraction x:
xO2 ,ref − xO2
p
i = zF D
RT
δ
(1.30)
The maximal current that can be achieved, called limit current density, is obtained when
the oxygen concentration at the electrode is zero, so that the expression above (Equation 1.29)
becomes:
ilim = zF D
cO2 ,ref
δ
(1.31)
The effect of a reduced concentration at the electrode will both change the Nernst potential at
the electrode (Equation 1.16) and the exchange current density (Equation 1.23). These effects
are the origins of the mass transport loss ηC and can be expressed as:
cO2 ,ref
RT
1
ηC = 1 +
ln
αC zF
cO2
(1.32)
By using the limit current density (Equation 1.31) and the actual current (Equation 1.29), this
expression can be rewritten as:
1
ηC = − 1 +
αC
RT
i
ln 1 −
zF
ilim
(1.33)
If the total current density i is used, then such an expression is valid only in presence of a
1D gradient of oxygen concentration, meaning with a homogeneous current distribution on the
active area. As discussed for the case of the ohmic losses, inhomogeneities might appear between
the areas facing the channels and the ones facing the ribs regions, such that a 2D gradient of
10
1.3 Fundamentals
oxygen concentration appears in the porous media, and that this expression cannot be utilized
with the global current i.
Traditionally, the estimation of the contribution of the mass transport losses on the total
loss is made indirectly by estimating the Tafel slope at low current to find the term ηCT (Equation 1.26), and by measuring r to obtain ηΩ (Equation 1.28). Then, these two contributions
are subtracted from the measured voltage U to get ηC . As discussed, the validity of the Tafel
estimation at higher current as well as the artifacts on the resistivity measurements can lead
to errors on the estimation of ηC with this method. In this work, an experimental tool will be
proposed to estimate ηC directly (section 2.2).
Diffusion mechanisms Essentially, three diffusion mechanisms can be considered in the case
of the porous media of the PEFC [13]: bulk diffusion, Knudsen diffusion and thin film diffusion.
If the medium exhibits pores of large diameter compared to the mean free path of the gas
molecules, then bulk diffusion dominates. In the case of media of porosity filled at a given
saturation level s of liquid water, the relative diffusivity is commonly expressed by:
Drel = [m (1 − s)n ]
(1.34)
where the values for the exponents m = n = 1.5 have been widely used [14, 15] but are still
discussed in the literature [16]. If the gas mixture is composed of dry air only, the resulting bulk
diffusion coefficient to be used in (Equation 1.31) can be expressed as:
Dg = Drel DO2 ,N2
(1.35)
Actually, the air present on cathode side contains a certain amount of water vapor. Therefore,
a ternary diffusivity coefficient can be calculated using the Maxwell-Stefan equation [17] to
get a more exact gas diffusivity of oxygen:
Dg = Drel DO2 ,N2 ,H2 O
(1.36)
The term ηg will be employed in this work to designate the mass transport losses attributed to
bulk diffusion limitations.
In case of a pore diameter d clearly smaller than the molecule mean free path, Knudsen
diffusion is dominant and the following diffusion coefficient must be used [17]:
DKn
8RT
= 2/3
πm
1/2
d
2
(1.37)
where m is the molar mass of the gas mixture. In this case, the gas molecules interact with
the pore walls and not with the other molecules. In a gas mixture, the diffusion limitation of
each gas will be independent of the other gases present. In particular, the mass transport loss
11
1. INTRODUCTION
of oxygen in very small pores will only depend on its partial pressure and not on the carrier gas
present (N2 or He in this work). The term ηKn will be used in this work to designate this kind
of diffusive losses.
If both bulk (Equation 1.36) and Knudsen (Equation 1.37) diffusivities are relevant, the total
diffusivity can be expressed as [18]:
m
n
Dg,Kn = [ (1 − s) ]
1
DO2 ,N2 ,H2 O
1
+
DKn
−1
(1.38)
An easy way to investigate experimentally the relative contributions of the bulk and Knudsen
diffusion is to change the absolute pressure. According to [19], the diffusion coefficient of a binary
mixture is inversely proportional to the pressure of the mixture:
Dg ∝
1
p
(1.39)
whose explicit expressions can be found in [20] or in [21]. On the contrary, the Knudsen diffusion coefficient is independent of the pressure, according to (Equation 1.37). Based on (Equation 1.30), it follows that for a given current i and a reference molar fraction x O2 ,ref , a change
of pressure will not affect x O2 in the case of bulk diffusion, while it will change this term in case
of Knudsen diffusion to the following value:
x O2 ,p2 =
(α − 1)x O2 ,ref − x O2 ,p1
p2
with α =
α
p1
(1.40)
Finally, we will designate by thin film diffusion the possible diffusion of oxygen across the
ionomer film or water film. Such a mechanism is typically expected to occur onto catalyst
particle surfaces covered by such a film. A corresponding diffusion coefficient must therefore be
used in (Equation 1.31):
D = DO2 ,f ilm
(1.41)
In this case, the term δ in (Equation 1.31) describes the film thickness. In case of a CL particle
covered by ionomer, the film thickness is in the nm domain and the following expression can be
used for Nafion [13]:
2768
[cm2 s−1 ]
DO2 ,N af ion = 3.1 · 10−3 exp −
T
(1.42)
Similarly to the Knudsen diffusion, a thin film limitation can induce a gradient of oxygen that
will only depend on the partial pressure of oxygen and will not be affected by the other species
present in the gas phase. Generally, the term ηf ilm will designate in the following the diffusive
losses occurring in a Nafion or a water film.
1.4
Cell components
The components described hereafter are illustrated on the figure presented before (Figure 1.1).
12
1.4 Cell components
1.4.1
Membrane
The polymer electrolyte membrane (PEM) constitutes the electrolyte of the PEFC. It has to
be ion conductive, electrically insulating and gas-tight. The polymer electrolyte is usually a
perfluorosulphonic acid ionomer (PFSA). It is made of a hydrophobic and chemically inert
polytetrafluoroethylene (PTFE) backbone that ensures the mechanical stability of the structure.
Chemically linked to this structure, the side chains are made of hydrophilic perfluorinated
ethers terminated by sulfonic acid groups HSO3 that guarantee a given ion exchange capacity
(IEC) and therefore a sufficient proton conductivity. The most widely used PFSA materials are
Nafion , developed by the firm DuPont (Figure 1.4), or Nafion-like materials. State-of-the-art
Nafion membranes can reach ionic conductivities of 10 Sm−1 or higher [22], with thicknesses
ranging from 25 µm (Nafion 211) to 175 µm (Nafion 117). In the present work, Nafion 117 will
be used as well as 18 µm Gore-Select membranes, in which a PFSA ionomer similar to Nafion
is employed that is reinforced by an expanded PTFE (ePTFE) micro reinforced composite [23].
CF2
*
CF2
*
CF
CF2
m
O
CF2
CF
m = 5-13
n = 1-3
CF2
O
F3C
n
SO3H·λ H2O
CF2
Figure 1.4: Nafion structural formula [24].
PFSA membranes present the characteristic of accepting a given amount of water molecules
in their bulk, strongly bound or not to the hydrophilic structure. An important characterization
value of the hydration state of the membrane is the number of water molecules pro sulfonic
group HSO3 , referred to as lambda coefficient:
λ=
[H2 O]
[HSO3 ]
(1.43)
This value is dependent on the ambient relative humidity (RH) of the surrounding gas phase and
is expressed by λ(RH) relations called sorption isotherms, experimentally estimated in [25–28]
for instance. By increasing the λ value, the membrane volume expands, a phenomenon called
swelling process.
Another important parameter that will be used in the present work is the concentration of H
atoms in the membrane, called cH . Using the density of Nafion ρN af ion and the equivalent weight
13
1. INTRODUCTION
(EW), which is the mass of Nafion pro SO−
3 molecule, we can express the molar concentration
of SO−
3 in Nafion:
c SO− =
3
ρN af ion
EW
(1.44)
It follows that the concentration of H atoms is:
cH = c SO− · (1 + 2λ)
3
(1.45)
With ρN af ion = 2 [g/cm3 ] and EW = 1100 [g/mol], the following value is found:
cH = 0.0018 · (1 + 2λ)
(1.46)
In the membrane, some water molecules are strongly attracted by the acidic group while
others molecule tend to behave like bulk water, but with specific characteristics due to their
confinement in small pores [22], as revealed by differential scanning calorimetry (DSC) studies
[29]. Like in water, the proton transport is supposed to occur via two mechanisms. By the
vehicular mechanism, the proton is permanently attached to the water molecule, meaning that
a displacement of the proton requires a motion of the water molecule. However, each water
molecule is involved in three or four hydrogen bounds [30] and therefore a proton can hop from
a molecule to the next one, mechanism referred to as proton hopping (Figure 1.5.a). It is of
interest to note that the displacement of the positive charge is not necessarily the same as
the proton. In the example given on the figure, one sees that if a given proton is labeled, for
instance by replacing 1 H by 2 H, then a transfer of the charge is possible that does not require a
displacement of the labeled proton. More generally, a possible rotation of the water molecule can
however induce a displacement of the labeled proton (Figure 1.5.b). The hopping mechanism
plus the water rotation are generally referred to as Grotthuss mechanism [30, 31].
The Nernst-Einstein equation provides a relation between the ionic conductivity σ and
the diffusion coefficient of the charge carrier Dσ [28]:
σ=
e2 nH Dσ
kB T
(1.47)
where e is the elementary charge, nH = c SO− is the density of charge carriers, kB is the Boltz3
mann’s constant and T is the absolute temperature. Usually, this formula is employed to
describe the diffusivity of the water molecule transporting the charge by a vehicular mechanism
in form a hydronium ion H3 O+ . In light of the comments given about the Grotthuss mechanism,
it appears that the diffusivity coefficient estimated by this equation rather reflects the virtual
equivalent mobility of the proton associated with the charge, since these two processes are partly
decoupled. On the contrary, the estimation of the proton diffusion in water by nuclear magnetic
resonance (NMR) or, in the case of this work, by neutron imaging of labeled species (section 2.1),
is likely to yield a lower value of diffusivity compared to the Nernst-Einstein diffusivity. This
will be explored in the result section (section 3.1).
14
1.4 Cell components
a) Proton hopping without molecule rotation
1)
2)
3)
1
H
2
H
O
b) Proton hopping with molecule rotation
1)
2)
3)
4)
Figure 1.5: Grotthuss mechanism.
1.4.2
Catalyst layer
The catalyst layer (CL) is needed to lower the energy barriers existing for the HOR and ORR
reactions, while providing a three phase boundary condition, meaning a simultaneous access of
reactants, electrons and protons for the reaction. The CL is constituted of catalyst particles,
usually Pt nanoparticles, deposited on carbon particles. In order to ensure the ionic connection
of these actives sites to the membrane, carbon supported Pt-particles are mixed with an ionomer
that can be the same as the membrane material. At last, a given porosity must be present to
permit the access of the reactants to the active sites, as well as to allow the water removal on the
cathode. For the operating cells tested in this work, the CLs are integrated to the catalyst coated
membrane (CCM) delivered by Gore and the whole part constitutes a membrane-electrodeassembly (MEA). For the operating cells tested in this work, the Pt-loading used were 0.1
mgPt /cm2 on the anode and 0.4 mgPt /cm2 on the cathode, since the HOR activation barrier is
lower than the ORR one, as mentioned earlier.
1.4.3
Gas diffusion layer
The role of the gas diffusion layer (GDL) is to ensure the electrical and thermal contact of
the MEA with the flow fields while providing an access of the gases to the CL. It is a porous
layer made of carbon fibers bound with carbon particles. To allow the access of the reactants in
presence of liquid water, this layer can be impregnated by a hydrophobic agent like PTFE. Being
15
1. INTRODUCTION
a compressible layer, it also compensates for any change of volume due to membrane swelling.
Typical values of pore size diameters are 10 to 50 µm [32].
The GDL used in the current state-of-the-art technical and experimental cells are coated
with a microporous layer (MPL), made of small carbon particles, whose diameter is 50 nm for
instance [17], and mixed with PTFE. Pore size diameters around 0.1 µm are reported in [32].
The MPL is deposited on the side of the GDL contacting the CL. The influence of such a layer
on the performance and the water content will be investigated in this work (section 3.2).
1.4.4
Flow field plates
The function of the flow field plates is to distribute the reactants to the entire active area and
remove the water produced by means of gas channels. It also serves as current collector and
must be electrically conductive. In addition, it must remove the heat produced. Therefore, it is
equipped of ribs (also referred to as lands) areas that ensure the electrical and thermal contact
with the GDL. In a fuel cell stack, where the cells are connected in series, the flow fields can
have gas channels on both sides so as to distribute the gases to the anode of one cell and to
the cathode side of the next cell. In that case, they are referred to as bipolar plates (BiP) or
separators. Flow field common materials are graphite or metal. The latter option is preferred
for manufacturability, permeability and durability to vibrations. However, due to the acidic
environment in PEFC, metallic plates have to be coated to avoid corrosion and provide good
electrical contact [33].
1.5
Water management
The role of the water in the loss mechanisms is essential in the PEFC [34, 35], as revealed by
the phenomena discussed hereafter.
1.5.1
Local effects
On one hand, since water is a reaction product, it has to be removed to provide a good access
to reactants. While the water transport in vapor phase only slightly affects the diffusivity of
the oxygen in air, the accumulation of water in liquid form occurring when the water vapor
removal is not sufficient to compensate the current production is detrimental to the diffusion of
reactants across the different porous media, as expressed by the effect of the saturation level s
on the effective diffusivity (Equation 1.34). Water can also accumulate in form of thin film at
the interfaces or on the CL particles, in which case a film diffusion will be taken into account in
(Equation 1.41).
16
1.5 Water management
On the other hand, a given hydration level has to be maintained to ensure a high proton
conductivity, both in the membrane and in the ionomer of the CL.
Additionally, it must be mentioned that other detrimental effects due to the lack of water
could affect the anode kinetics according to [36]. Beyond losses mechanisms, it is also recognized
that relative humidity can impact the various degradation mechanisms [37].
These are the basic trade-offs on which the performance optimization relies and constitute
the so-called water management.
Water spatial distribution and dynamics directly depend on the water transport mechanisms,
summarized in the following figure (Figure 1.6).
Water content
(anode)
Electro-osmotic drag
H 2O
Cathode water
(cathode)
H 2O
H
+
O2
H2
Vapor/liquid
transport
Back-diffusion
Vapor/liquid
transport
Transport of H2O molecules
Figure 1.6: PEFC: water transport.
The water produced on the cathode can either penetrate into the membrane or be evacuated
across the cathode porous media by vapor diffusion or by a capillary flow of liquid water.
Then, it is removed out of the cell by the convective gas flow in the channels, either in form of
water vapor or water droplets. When electrical current is applied, the proton, or more precisely
the hydronium ion H3 O+ , tends to drive a given number of water molecules from the anode
side towards the cathode side, phenomenon called electro-osmotic drag [25]. In addition to
this extra amount of water brought from the anode, the production of water on cathode side
tends to increase locally the hydrostatic pressure and/or the relative humidity compared to the
anode side, which creates back-permeation and/or back-diffusion bringing water from cathode
to anode side [25]. Usually, the two latter concepts are merged into the single denomination of
back-diffusion flow.
17
1. INTRODUCTION
All this mechanisms are local effects in the sense that they occur on the local scale of the
cell.
1.5.2
Global effects
Along the channels supplying reactants to the active area, and if technical stoichiometries (<
2) are used, the gas mixture changes along the channel path. On the cathode side, the oxygen
concentration reduces due to the reaction consumption and the water concentration increases
up to its saturation level. After that, water droplets can accumulate in the channels. On the
anode side, a change of hydrogen concentration can occur at a given pressure only if another gas
is present, that can be for instance nitrogen permeating from the cathode side. Moreover, due
to back-diffusion, water will tend to accumulate on the anode side as well. While a change of
oxygen concentration c O2 ,ref can affect the performance as discussed before (change of Nernst
potential and ηCT ), the effect of water accumulation in the channels is essentially to disturb
the flow distribution, which can impact the global performance and durability of the cell. Such
disturbances can occur between the different channels of a cell, or between the different cells of
stack. These are the main processes referred to as global effects of the water distribution.
1.6
Differential cell
Studying the local behavior of a full-scale cell can be made either by a segmented cell or by
emulating only a portion of the whole area by a small-size cell. To do so, same velocities as in
the full-size cell must be applied. Hence, the technical stoichiometry of the full cell translates
to a high stoichiometric flow in the small-size cell. It follows that the operating parameters
are expected to be homogeneous in the along-the-channel dimension, so that this system is well
adapted to investigate the local losses discussed earlier. A differential cell designates such a
system and the present work is dedicated to its study. The choice of the operating conditions
under which the differential cell is operated allows emulating a chosen portion of the full-size
cell.
1.7
Neutron imaging
The work realized is based on the development of the neutron imaging method applied to
fuel cells, realized at PSI. While fundamentals were given by Kramer [38], the state-of-the art
neutron imaging method and data processing used in this work were developed by Boillat [8].
Consequently, only the basic features are given hereafter.
18
1.7 Neutron imaging
All experiments involving neutron imaging were realized at the ICON (Imaging for Cold
Neutrons) beam line of the SINQ (Swiss Spallation Neutron Source) facility at the Paul Scherrer
Institut. The main characteristics of the beam line can be found in [39].
Due to the high cross-section of the H atom for neutrons, condensed phases of H2 O (liquid
phase, ice phase, membrane content) demonstrate a low transmission to neutrons. On the
contrary, the constitutive materials of the cell, in particular aluminum and graphite, present a
high transmission. In consequence, condensed phase of water present a high contrast compared
to the other species in presence. Therefore, neutron imaging is especially adapted to the in
situ visualization of the water distribution in operating cells, whose designs can be very close to
technical designs.
The attenuation of an impinging neutron beam with an intensity I0 is described by the
Beer-Lambert law:
I = I0 e−Σδ = I0 e−σnδ
(1.48)
where δ is the water thickness and Σ is the attenuation coefficient, which can be measured
experimentally and corresponds to the microscopic cross-section σ multiplied by the volumetric
atomic density n. By measuring the intensity I0 in absence of water, the referencing process
leads to the water thickness:
1
δ = − ln
Σ
I
I0
(1.49)
The real referencing process is however more detailed, since other contributions have to be
eliminated, as detailed in [8].
As developed in [8, 38], the spatial resolution of neutron imaging is typically limited by
the pixel size of the recording system, a charge-coupled device (CCD) camera in the present
case, the geometrical unsharpness provoked by the divergent beam and the inherent unsharpness
originating from the scintillator or from the optical set-up. In the last years, major improvements
in terms of spatial resolution were realized at the ICON beam line.
Based on the micro set-up developed by Lehmann et al. [40], Boillat et al. further improved
the anisotropic resolution [8, 41] as follows. Based on the figure below (Figure 1.7), it can be
seen that the geometrical unsharpness rG originating from the divergence of the beam can be
expressed by the L/D ratio according to:
rG = Ld
1
L
D
= Ld
D
L
(1.50)
where D is the neutron aperture, L is the distance from the source to the sample, and Ld is the
distance between the detector and the sample. Thus, it is possible to reduce the geometrical
unsharpness in one direction only by means of a beam limiter so as to get:
rG,x = Ld
19
e
L − Lbl
(1.51)
1. INTRODUCTION
Figure 1.7: ICON beam line (picture from [42]) and anisotropic collimation (from [8, 41]).
where e is the beam limiter aperture and Lbl is the distance between the beam limiter and the
source. This way, the reduction of the concomitant neutron flux can be mitigated.
The inherent unsharpness rI originates from the conversion process of neutrons into visible
light by the scintillator material. This was addressed by the second improvement of Boillat et
al. [8, 41] who developed a tilted detector. As represented on the figure hereafter (Figure 1.8),
it consists in positioning the detector screen in a plane tilted of a given angle Θ compared to
the standard detector plane. After re-scaling the image, the effective inherent unsharpness rIef f
of the detector is theoretically improved of a factor cos Θ [8], as expressed in (Equation 1.52).
The resolution values given in the next chapters will correspond to the full width at half
maximum (FWHM) values, meaning the width of the line spread function (LSF) signal measured
at half of its amplitude, as defined on the figure (Figure 1.8).
The combined effect of the geometrical rG and inherent resolution rI values can be expressed
20
1.7 Neutron imaging
Figure 1.8: Effect of the tilted detector on the resolution.
into the resulting resolution rR according to the following empirical formula [8]:
q
q
3
3
3
3
3
rR ≈ (rI cos Θ) + rG = (rIef f )3 + rG
(1.52)
where rI corresponds to a Gaussian blurring and rG to a box-car function [8], Θ is the tilting
angle represented on the figure (Figure 1.8) and rIef f is the effective inherent resolution of the
tilted detector. This expression assumes a negligible mean free path of neutrons inside the tilted
detector and is well appropriate for the Gadolinium scintillator used in this study [8]. It is valid
for the x direction and also for the y direction, in which case a value of Θ = 0 must be used.
An improvement of resolution is made at the expense of a loss of intensity. Since a sufficient
intensity must be kept to obtain a high signal-to-noise ratio, trade-offs have to be found according
to the desired features [8]. For some applications, a high temporal resolution is needed, which
implies a short exposure time and therefore a low intensity per image [8]. In the present work,
different imaging set-ups were used and their specific parameters will be given along with the
experiments they are used for.
21
1. INTRODUCTION
Based on the improvements reported, in-plane imaging was made possible, meaning a configuration in which the cell membrane is parallel to the neutron beam. This mode offers the
ability to distinguish the different layers of the cell, in particular the anode and cathode GDLs.
On the contrary, the through-plane mode, less demanding in spatial resolution, is preferred when
the visualization of water distribution over the entire active area is desired. In this case, the
anode side cannot be distinguished from the cathode side. In the present work, only in-plane
imaging is realized to focus on the water distribution across the layers of the differential cell.
1.8
Scope of this work
Reducing mass transport losses at the local scale allows minimizing the cell size needed for a
given power density, and therefore mitigating the costs. It is then of prime importance to deeply
understand the effect of water on the performance in order to reduce its detrimental effect. This
is the general motivation of this thesis, which pursues the predecessors’ studies addressing the
same topic [8, 38, 43].
The present work consists of two contributions: the development of diagnostic tools combined
to neutron imaging (chapter 2) and the application of these tools for the production of results
(chapter 3).
The deuterium labeling, meaning the replacement of the protium isotope of hydrogen 1 H by
the deuterium isotope 2 H, can be used to acquire fundamental knowledge about diffusion and
exchange transfer mechanisms of H atoms between the membrane and the surrounding gases. A
simple system, namely a one-sided membrane exposed to H-containing gases (H2 O and H2 ) will
serve as an experimental base to fit a diffusion model and to estimate diffusive and exchange
parameters. The model and the fitting procedure will be presented in (section 2.1) whereas the
results of this method will be discussed in (section 3.1).
The estimation of the mass transport losses can be realized by replacing the carrier gas of
oxygen in air, that is nitrogen, by another inert gas allowing a better diffusivity of oxygen, like
helium for instance. However, this replacement must be performed during short time periods
to avoid changing the working parameters of the cell, such as the saturation level of the porous
media or the resistivity for example. The helox pulsing method aims to this target and will be
presented and validated in (section 2.2).
In order to use the neutron imaging beam time more efficiently and to perform systematic
studies of differential cells made of different materials but operated under same conditions, a
set-up enabling the simultaneous imaging and electrochemical testing of six cells was developed.
This so-called multi-cell set-up will be presented in (section 2.3).
22
1.8 Scope of this work
The effect of the microporous layer (MPL) on the water distribution and the cell performance
was investigated by means of the multi-cell set-up, helox pulsing and neutron imaging. The
results obtained will be presented and discussed in (section 3.2).
Using the same combination of diagnostic tools, different flow fields were tested to assess the
effect of the diffusive pattern of oxygen in the GDL on the performance. The design of these
flow fields will be described in (subsection 2.3.9) and the results acquired will be presented and
discussed in (section 3.3).
Cold-starts of a single differential cell were realized and visualized by neutron imaging. The
dedicated set-up and the results of this study will be presented in (section 3.4).
While specific conclusions and outlook will be attached to each sub-chapter, general conclusions and outlook will be discussed in the last chapter of this work (chapter 4).
23
2
Diagnostic tools
2.1
2.1.1
Deuterium labeling
Introduction
Understanding and quantifying the water transport mechanisms across the PEFC membrane
is decisive for improving the models and the real system itself. On one hand, the membrane
represents a water reservoir that has to be maintained at a sufficiently high hydration level to
ensure a good proton conductivity. On the other hand, the membrane allows for the transport of
water from one side of the cell to the other, which is crucial to mitigate the detrimental impact
of liquid water on the oxygen diffusivity.
In an operating PEFC membrane, the migration of the proton H+ is recognized to induce
an electro-osmotic drag of water molecules from the anode to the cathode, as mentioned before
(section 1.5). This mechanism plus the electrochemical production of water tend to accumulate
water on the cathode side, which can lead to the formation of a back-diffusion flow between
the cathode and the anode (section 1.5). The observation and quantification of these fluxes in
situ is a difficult task, making the model validation studies very scarce compared to the amount
of literature available on the water management in general [34, 35]. The method presented
hereafter aims to help bridging this gap.
Contrary to the case of the protium isotope of hydrogen 1 H (the most abundant hydrogen
isotope), the deuterium isotope 2 H features a low microscopic total cross-section to neutrons.
The liquid phase of water molecules made of deuterium atoms is called heavy water 2 H2 O and
is in consequence almost transparent to neutrons compared to light water, or in usual words
normal water 1 H2 O. Visualization with neutron radiography of the replacement of light water
by heavy water (or inversely) can be interpreted as a deuterium labeling of the water transport.
In the context of a PEFC system, a basic motivation to perform deuterium labeling is
to track the pathways of the different water sources: the water electrochemically produced,
25
2. DIAGNOSTIC TOOLS
the humidification content supplied on anode side and the humidification content supplied on
cathode side. As it was revealed by previous studies conducted by Boillat et al. [8, 44, 45], the
exchange processes existing between the different H-containing species can make straightforward
interpretations intricate, in particular for the general case of a fuel cell operating in realistic
conditions. Deuterium labeling combined to nuclear magnetic resonance imaging (MRI) was used
to observe the effect of gas flow configurations on the through-plane distribution of water [46]
or to investigate the effect on the membrane hydration of the water electrochemically produced
compared to the effect of the humidified gases, as reported in [47]. Combined with neutron
imaging, deuterium labeling was used for example to estimate exchange rates on water droplets
in channels in through-plane mode [48] or to discuss the role of the MPL on the distribution of
the water flows in in-plane mode [49]. In these studies however, the exchanges occurring between
the membrane and the gas phase are not explicitly taken into account.
As an attempt to provide reliable data on which the future gain of knowledge should be
facilitated, the labeling study performed in this work was realized on a simplified system, namely
a single Nafion membrane subject to simple experimental conditions: no electrical current and
no humidity gradient. Labeling such a system allows quantifying the diffusion of H-atoms in
the membrane bulk, as well as the interfacial exchange rates of H-atoms between the membrane
and its surroundings, as already reported by Boillat et al. in [8, 44, 45]. The method proposed
for this estimation will be exposed in the present chapter, whereas the values found for these
parameters will be reported in the result chapter (section 3.1).
2.1.2
Basics
The protium isotope and the deuterium isotope will be referred to as 1 H and 2 H respectively.
The symbol H will be used to describe an arbitrary mixture of 1 H and 2 H. The local 1 H isotope
fraction is defined as:
1
F=
c1 H
c1
= H
c1 H + c2 H
cH
(2.1)
where cx is the molar concentration of the atom x. The value cH can be expressed in function
of the λ value by (Equation 1.46). The complementary 2 H isotope fraction is logically defined
as:
2
F=
c2 H
cH
(2.2)
and thus:
1
F + 2F = 1
(2.3)
Therefore, the development that follows can be made based on only one of the species. The
transport of 1 H is chosen and the 1 F fraction will be used for further considerations. It will
26
2.1 Deuterium labeling
simply be called isotope fraction in the following and will be written:
F = 1F =
c1 H
cH
(2.4)
Intrinsically, the membrane used in the PEFC contains a given amount of H atoms. In
the case of a PFSA membrane such as a Nafion membrane, the polymer structure is free of
H atoms (Figure 1.4) and these are essentially present in form of water molecules H2 O. The
protons H+ are commonly attached on water molecules to constitute hydronium ions H3 O+ or
other aqueous complexes [45]. In the case of a membrane immersed in a gaseous environment
without any macroscopic flow of water (no electro-osmotic drag or back-diffusion), the amount
of water in the membrane is determined by the λ(RH) value [25–28]. On the following figure
(Figure 2.1), two systems are considered. On the left side, a 1-sided cell is illustrated, which
consists of a membrane sample exposed to a gaseous phase on one side only, the other side being
covered by a gasket. This is the system studied in the present work. On the right side of the
figure, an usual 2-sided cell is shown, in which the gas phases can access the two sides of the
membrane. This is the system investigated by Boillat et al. [8, 44, 45] and it will be mentioned
in the following for comparison purposes.
If there is no macroscopic flow of water, the net flow of water molecules at the interface of
the membrane is zero. Even in such a condition, a dynamic equilibrium can be expected that
is characterized by a certain exchange of water molecules between the membrane and the water
vapor of the gas phase. It will be noted kW hereafter. Moreover, if H2 is present in the gas phase
1-sided cell
Membrane
CL
FH(t)
2-sided cell
Membrane
Anode CL
FHA(t)
kH
kHA
F(x,t)
FW(t)
Cathode CL
kHC
FHC(t)
kWC
FWC(t)
F(x,t)
FWA(t)
kW
kWA
x
0
x
L
0
L
Figure 2.1: System that can be studied with deuterium labeling. Left: the 1-sided cell studied in
the present work. Right: the 2-sided cell investigated by Boillat et al. [8, 44, 45]
and if there is a CL on the surface of the membrane, the dihydrogen molecule can dissociate into
protons H+ according to the HOR reaction (Equation 1.3) at a given exchange rate kH . This
27
2. DIAGNOSTIC TOOLS
exchange rate can of course be related to the exchange current density of the HOR reaction by
the Faraday’s law (Equation 1.18):
i0,HOR
zF
where z = 1 since the term kH represents a flow of H atoms.
kH =
(2.5)
Given that a given isotope fraction can be attributed to each of the two H-containing species,
called FW in the water vapor and FH in hydrogen, it can be expected that the isotope fraction
in the membrane F can be related to these terms by means of the exchange rates kW and kH .
In addition to that, a transient regime of F across the membrane thickness can be characterized
by a diffusion value of the H-atom in the membrane bulk.
The aim of the present work is to apply controlled isotope fractions in the gas phase, FW
and FH , and to measure by neutron radiography the resulting change of the isotope fraction in
the membrane F (subsection 2.1.3). The comparison of the experimental results with a simple
diffusion model (subsection 2.1.4) will allow estimating the values of kW and kH , as well as the
bulk diffusivity of H (subsection 2.1.7). The numerical values for various kind of samples and
experiments will be presented in the result chapter (section 3.1). In the whole study that follows,
no electrical current and no permeation flow were applied.
2.1.3
2.1.3.1
Experimental
Gas mixture
The different gas mixtures used in this work are summarized on the scheme below (Figure 2.2).
As only pure isotopic contents (F = 1 or 0) will be used, 8 basic combinations are obtained
(A1 to D2). In the case of the gas mixtures A and B, the absence of H2 implies a priori an
exchange rate kH equal to zero. The comparison between N2 and He as carrier gas will serve as
an indicator of a possible influence of diffusivity limitations in the gas phase on the estimation
of kW , since water vapor diffusion in N2 is lower than in He, as it will be observed in another
chapter (section 2.2).
2.1.3.2
Experiments
Two types of experiments can be realized to investigate the transport parameters: steady-state
and transient experiments.
Steady-state
A steady-state experiment, meaning when:
∂F
=0
∂t
(2.6)
can be realized by applying constant isotope fractions FH and FW in the gas fed and waiting
until the distribution of F satisfies this criterion.
28
2.1 Deuterium labeling
Water vapor Dry gas
A
N2
1
H 2O
C
B
1
H2O
1
H 2O
2
H2O
2
H 2O
FW = 1
FW = 0
(kH = 0)
FW = 1
FH = 1
A1
B1
C1
B2
H2
1
FW = 1
FW = 0
(kH = 0)
A2
2
H2
He
2
D
H2O
1
H 2O
FW = 0
FH= 1
C2
FW = 1
FH = 0
D1
2
H 2O
FW = 0
FH= 0
D2
Figure 2.2: Gas mixtures used for deuterium labeling.
On the 1-sided cell system investigated (Figure 2.1), the gas combinations without H2 , that
are A and B on (Figure 2.2), are of course useless to investigate transport processes, since the
isotope fraction in the membrane will be: F = FW . Such a condition will be used as reference
for the image processing described hereafter (subsubsection 2.1.3.5).
If mixed isotope fractions are applied on the 1-sided cell, meaning if FH 6= FW , like for the
C2 and D1 mixtures of (Figure 2.2), some conclusions can be made on the exchange rates kH and
kW , as it will be discussed below. On the contrary, nothing can be deduced about the diffusion
processes in the bulk membrane, since F is at steady-state equilibrium.
Similarly to the 1-sided cell, steady-state experiments on the 2-sided cell are useless for transport investigations if all isotope fractions are identical. Transport properties can be obtained
according to two types of experiments that yield information about both bulk transport and
interfacial exchanges (subsubsection 2.1.5.1), as reported by Boillat et al. [8, 44, 45]. The first
case is by using FA = FC but FH 6= FW , which requires applying C2 (or D1) on anode and D1
(or C2) on cathode. The second case is by using FH = FW but FA 6= FC , which requires applying C1 on anode (respectively cathode) and D2 on cathode (respectively anode). The output
results of these conditions will be discussed below (subsubsection 2.1.5.1) for comparison, but
no experiments were realized on 2-sided cells in the present work.
Transient
In the following (subsection 2.1.7), it will be argued that both bulk and interfacial
transport parameters can be obtained by analyzing transient regimes of the isotope fraction F
in the membrane. Again, only the 1-sided cell is considered in the present work.
To obtain a transient behavior of F , a change of isotope fraction must be applied in the gas
supplied. In this study, only changes from 0 to 1 (or from 1 to 0) are realized, which can be
summarized by the different gas transitions in the table hereunder (Table 2.1).
29
2. DIAGNOSTIC TOOLS
Transition
Step direction
Gases
Isotope fractions
1
2
(FW = 1) ® (FW = 0)
1
2
(FW = 1) ¬ (FW = 0)
1®0
(N2, H2O) ® (N2, H2O)
0®1
(N2, H2O) ¬ (N2, H2O)
1®0
(He, H2O) ® (He, H2O)
0®1
(He, H2O) ¬ (He, H2O)
1®0
( H2, H2O) ® ( H2, H2O)
0®1
( H2, H2O) ¬ ( H2, H2O)
TN2
1
2
1
2
(FW = 1) ® (FW = 0)
THe
(FW = 1) ¬ (FW = 0)
1
1
2
2
(FH = 1, FW = 1) ® (FH = 0, FW = 0)
1
1
2
2
(FH = 1, FW = 1) ¬ (FH = 0, FW = 0)
TH2
Table 2.1: Gas transitions performed for transient experiments.
2.1.3.3
Protocol
A gas transition (TN2 , THe or TH2 ) is chosen and automatically repeated at a given frequency
(Figure 2.3.a). By opening the camera shutter with the desired exposure time (c), neutron
radiograms are collected and for each of them, the spatial distribution of the isotope fraction
in the membrane F (x, t) can be estimated, according to the data processing described hereafter
(subsubsection 2.1.3.5). On this figure, the average isotope fraction in the membrane Favg (t) is
represented (e). Since a good temporal resolution is needed, a quite low exposure time is applied,
namely 2.7 s. As a consequent drawback of the short exposure time, a quite low signal-to-noise
ratio is obtained, which can be characterized by a reduction of the Poisson’s law standard
deviation, as described in [8, 38], and called σ hereafter. To reduce this value, a merging of the
sequences is performed, namely a reconstitution of one single period (f) based on an averaging
of all values recorded corresponding to the same moment of the different periods (e). Compared
to the raw signal (e), the merged signal (f) is expected to exhibit a higher signal-to-noise ratio
that can be expressed by [8, 38]:
1
σunmerged
σmerged = p
Nperiod
(2.7)
with Nperiod = 38 in the present case. A 30 s duration was attributed for each step at a given
isotope fraction, since it was long enough to get an almost stabilized isotope fraction.
This protocol was applied for the different types of gas transitions (TN2 , THe , TH2 ) presented
before.
30
2.1 Deuterium labeling
(a)
(b)
38 min
FH, FW [-]
30 s
30 s
1
1
0
0
t [s]
(c)
Camera
t [s]
2.7 s
(d)
Merging
(38 periods)
on
on
off
off
t [s]
t [s]
Favg [-]
(e)
(f)
1
1
0
0
t [s]
t [s]
Figure 2.3: Repetition of isotopic transitions in the gas mixture (symbolic scheme).
2.1.3.4
Set-up
Ideally, the temporal profiles of the isotope fraction in the gas phase FH (t) and FW (t) should be
step transitions:
FH (t), FW (t) =
0 if t < 0
1 if t ≥ 0
(2.8)
Firstly, this is needed for a correct comparison with the model developed hereafter, since these
values will be the border conditions. Secondly, a rapid transition is required compared to the
diffusivity time constant of the membrane.
To satisfy this requirement, the following gas management set-up was implemented (Figure 2.4). One humidifier is dedicated to light water (”Hum.
(”Hum.
2 H O”).
2
1H
2 O”)
and one to heavy water
This system prevents the apparition of unacceptable start-stop delays that
would occur if one single humidifier were used for both species. The gases flowing through these
humidifiers are independently controlled by mass flow controllers (”MFC”). This is required by
the use of separate humidifiers. Moreover, this allows for a separate calibration of the MFCs
when different species (1 H2 and 2 H2 ) are used. Placed before the humidifiers, two automatically
controlled electrical valves ensure that the desired gas is circulated through the cells (”In”) and
that the second gas is deviated to another line (”By-pass”). The gas transition is realized by
31
2. DIAGNOSTIC TOOLS
changing the position of both valves simultaneously. To maximize the external gas flow while
being able to control independently the cell flow, only a small part of the main gas flow goes
through the cells (”Out”). In the multi-cell set-up described elsewhere (section 2.3), each flow
will be independently controlled (”MFC n”). The main part of the input flow returns to the
by-pass and the pressure of the whole system is controlled at the outlet of this flow by a pressure
controller (”PC”). With this system, a high flow is used in the (”In”) line, typically 3 Nl/min,
and a small flow in circulated in the cell (”Out”), with a maximal value of 0.4 Nl/min for the
labeling investigations realized in this work.
1
2
H2
(or He or N2)
H2
(or He or N2)
MFC
MFC
1
2
Hum. H2O
In
Hum. H2O
By-pass
3m
piping
Cell n
PC
Out
MFC n
Exhaust
: heated section
MFC: mass flow controller
PC:
Exhaust
pressure controller
n:
Hum.: humidifier
1 for single-cell
1 to 6 for multi-cell
Figure 2.4: Gas management set-up for isotope labeling.
2.1.3.5
Image processing
Based on the Beer-Lambert’s law already expressed in the simple case (Equation 1.48), the
attenuated intensity I of the neutron beam due to a given water thickness δ with an isotope
fraction F can be calculated as follows [41]:
I = I0 e[−ΣLW F δ−ΣHW (1−F )δ)]
(2.9)
where ΣLW and ΣHW are the attenuation coefficients for light and heavy water respectively,
and I0 is the intensity of the impinging neutron beam. If a radiogram is taken with all H atoms
32
2.1 Deuterium labeling
present in form of 1 H atoms, that is when F = 1, the intensity measured is:
Iref,1 H = I0 e[−ΣLW δ]
(2.10)
Similarly, an image taken when all H atoms are present in form of 2 H atoms, that is with F = 0,
yields:
Iref,2 H = I0 e[−ΣHW δ]
(2.11)
Using these three expressions (Equation 3.36, Equation 2.10 and Equation 2.11), the isotope
fraction can be calculated by:
F =
ln(I) − ln(Iref,2 H )
ln(Iref,1 H ) − ln(Iref,2 H )
(2.12)
This estimation is realized pixel-wise on each radiogram of the sequence (Figure 2.3.d). The
matrix resulting from the process will be called Fexp (xi , tj ) or simply Fexp in the following.
2.1.4
Model
The purpose of the model presented hereunder is to simulate the spatio-temporal distribution of
the isotope fraction in the membrane F in response to a step transition of FH (t) and FW (t). If
the model can estimate the measured values with good agreement, then an interpretation of the
mechanisms taking place will be possible as well as a quantification of the important transport
parameters by a fitting method.
2.1.4.1
Bulk
As already mentioned, no electrical current and no RH gradient (i.e. no back-diffusion) are
applied, meaning that the total concentration of H atoms is supposed to be constant:
∂cH
∂cH
= 0 and
=0
∂x
∂t
(2.13)
On the contrary, the transient transport of the isotope 1 H is considered. In the bulk of the
membrane, Fick’s diffusion is supposed:
D1 H,M
∂ 2 c1 H
∂c1 H
=
2
∂x
∂t
(2.14)
where D1 H,M is the diffusion coefficient of 1 H in the membrane. This coefficient is assumed to
be independent on the local isotope fraction, so that one single diffusion coefficient is used for
the whole membrane bulk:
∂D1 H,M
∂F
33
=0
(2.15)
2. DIAGNOSTIC TOOLS
The possible effects of this simplification will be discussed in the result chapter (section 3.1).
Since the transport of 1 H is considered, the expression of the diffusion coefficient can be simplified
as follows:
DM = D1 H,M
(2.16)
In the usual case of a PEFC membrane with a pure isotopic content F = 1, this diffusion coefficient would of course correspond to the self-diffusion coefficient of H in the membrane. Based
on the comments made before on the water transport mechanisms occurring in the membrane
(subsection 1.4.1), it can be expected that such a coefficient contains contributions of the vehicular mechanism and the rotational part of the Grotthuss mechanism. These assumptions will
be further discussed in the result chapter (section 3.1).
Using the definition of the isotope fraction (Equation 2.4) and the simplification (Equation 2.16), the transport expression (Equation 2.14) can be rewritten as:
DM
∂(cH F )
∂ 2 (cH F )
=
2
∂x
∂t
(2.17)
which, due to (Equation 2.13), simplifies into:
DM
∂2F
∂F
=
2
∂x
∂t
(2.18)
an expression that is often referred to as heat equation.
2.1.4.2
Border conditions
In a simple biphasic system made of liquid water and vapor, the steady-state equilibrium is
reached when the vapor pressure equals the saturation pressure so that the net flow of molecules
between the two phases is zero. Even in such a condition, a permanent exchange of water
molecules occurs between the two phases that can be described by compensating flows coming
in and out. A same situation is expected for the membrane at equilibrium with the gas phase.
As mentioned earlier, the experiments were realized in absence of electrical current and RH
gradients across the membrane. This implies that a dynamic equilibrium is supposed to occur
between the membrane and the gas phase (using positive flows), so that:
ṅH,in − ṅH,out = 0
(2.19)
where the flows are defined positive: ṅH,in ≥ 0 and ṅH,out ≥ 0. Taking into account the isotope
content of these flows, this equation can be expressed as:
ṅ1 H,in + ṅ2 H,in − ṅ1 H,out − ṅ2 H,out = 0
34
(2.20)
2.1 Deuterium labeling
By introducing the exchange rate k as:
k = ṅH,in = ṅH,out
(2.21)
and by distinguishing between the exchange rate between the membrane and the water vapor,
called kW , and the exchange rate between the membrane and the dihydrogen, called kH , and
assuming:
k = kH + kW
(2.22)
it can be supposed that the input flow of 1 H is expressed by:
ṅ1 H,in = kH FH + kW FW
(2.23)
Similarly, the output flow of 1 H can be expressed using the local isotope fraction at the interface
of the membrane. If the border x = 0 is considered, it can be written:
ṅ1 H,out = F (x = 0) · ṅH,out = F (x = 0)(kH + kW )
(2.24)
Using (Equation 2.23) and (Equation 2.24), the net flow 1 H across the membrane interface is
obtained:
ṅ1 H = ṅ1 H,in − ṅ1 H,out = kH (FH − F (x = 0)) + kW (FW − F (x = 0)
(2.25)
Since this can be related to the isotope fraction by:
−DM cH
∂F
= ṅ1 H
∂x
(2.26)
the expression of the border conditions is possible. It will be described hereafter for the two
systems considered, the 1-sided and the 2-sided cell.
For these two cases however, the same initial condition will be used, that is an isotope
fraction fixed to zero at the initial time, meaning that all H atoms are present as 2 H atoms:
F (x, t = 0) = 0
1-sided cell
The flow at x = 0 can be expressed using (Equation 2.26):
∂F
− DM cH
= ṅ1 H
∂x x=0
which becomes by using (Equation 2.25):
∂F
− DM cH
= kH (FH − F (x = 0)) + kW (FW − F (x = 0))
∂x x=0
As the other border is in contact with the gasket, a zero flow condition is applied:
∂F
− DM cH
=0
∂x x=L
35
(2.27)
(2.28)
(2.29)
(2.30)
2. DIAGNOSTIC TOOLS
2-sided cell In this case, the isotope fractions for both sides have to be taken into account.
By using the subscripts A for the ”anode” side (this is formally not an anode since there is no
reduction reaction), supposed to be located at x = 0, and C for the ”cathode” side at x = L,
and assuming that:
kH = kHA = kHC
(2.31)
kW = kWA = kWC
the expression (Equation 2.29) can be generalized as:
∂F
− DM cH
= kH (FHA − F (x = 0)) + kW (FWA − F (x = 0))
∂x x=0
and at the other border as:
∂F
− DM cH
= −kH (FHC − F (x = L)) − kW (FWC − F (x = L))
∂x x=L
2.1.5
2.1.5.1
(2.32)
(2.33)
Analytical solutions
Steady-state
In the case of a steady-state system, the right-hand term of (Equation 2.18) is zero and the net
flux ṅ1 H is constant. Interesting solutions can be obtained with dedicated border conditions
[8, 45].
Firstly, by applying symmetric isotope fractions on the 2-sided cell:
FHA = FHC = FH
(2.34)
FWA = FWC = FW
the isotope fraction is constant and can be calculated using the border condition (Equation 2.29):
kH FH + kW FW
(2.35)
kH + kW
Since the neutron imaging experiment yields the value F , it is more relevant to put this equation
F = F (x = 0) = F (x = L) =
into the form:
kW
FH − F
=
(2.36)
kH
F − FW
This set of equation is also valid for the steady-state case of the 1-sided cell. That means that
only the ratio of the interfacial exchanges can be obtained by using a 1-sided cell in steady-state
mode. Results of this experiment will be presented in (subsection 3.1.5)
A second experiment that can be realized on the 2-sided cell is to apply identical isotope
fractions in hydrogen and in vapor, but with different values for the two sides of the cell:
FHA = FWA = FA
FHC = FWC = FC
36
(2.37)
2.1 Deuterium labeling
According to [45], the constant gradient of 1 H can be calculated:
∂F
(kH + kW )(FC − FA )
=
∂x
L(kH + kW ) + 2DM cH
(2.38)
where L is the thickness of the membrane, and the following expression can be obtained:
kH + kW =
2 ∂F
∂x DM cH
FC − FA −
(2.39)
∂F
∂x L
Hence, the knowledge of the value DM is required to find the sum kH + kW , that it turns allows
extracting the single values of kH and kW by the ratio found before (Equation 2.36).
2.1.5.2
Transient
Contrary to steady-state experiments on the 2-sided cell, transient regimes on the 1-sided cell
allow finding separately the parameters DM and the interfacial exchanges kH and kW . The
analytical solution presented hereafter corresponds to such a system.
By using the following border condition instead of (Equation 2.29), which is equivalent to
an infinite exchange rate k = ∞:
F (x = 0, t) = 1
(2.40)
and by keeping a zero flux condition on the other border in respect to (Equation 2.30), an
analytical solution of F in (Equation 2.18) is given by:
2
nq
4X
−π DM (2q − 1)2 t
sin [(2q − 1)] exp
Ftheo,sin (x, t) =
π
4L2
(2.41)
q=1
with nq = ∞. Since the numerical implementation is realized with a finite value of nq , instabilities of the solution occur for low x and t values. For low values of t, it can be considered however
that the zero flux condition (Equation 2.30) can be approximated by a zero flux at x = ∞:
∂F
− DM cH
=0
(2.42)
∂x x=∞
Using this new border condition and keeping the condition (Equation 2.40), the error function
can be used as analytical solution:
Ftheo,erf (x, t) = 1 − erf
x
√
4DM t
(2.43)
which corresponds to a solution for semi-infinite diffusion. This solution does not suffer from unstability issues but is not adapted for high values of t since the border condition (Equation 2.42)
does not correspond to a finite system. On the plot of these functions (Figure 2.5), it can be
observed that the instabilities featured by Ftheo,sin at t = 0.005 s have totally disappeared at t
= 0.5 s, while the wrong border condition of Ftheo,erf at x = 200 µm becomes unacceptable at t
37
2. DIAGNOSTIC TOOLS
1
1
1
t = 0.5 s
t = 10 s
0.8
0.8
0.6
0.6
0.6
F [-]
0.8
F [-]
F [-]
t = 0.005 s
0.4
0.4
0.4
0.2
0.2
0.2
0
0
0
0
50
100
x [μm]
150
200
0
50
100
x [μm]
150
Ftheo,sin
200
0
50
100
x [μm]
150
200
Ftheo,erf
Figure 2.5: Unstability effect on Ftheo,sin
(nq = 20[-] and D = 1 · 10−5 [cm2 /s]).
= 10 s. In the case of the present study, the experimental isotope fraction will be measured with
the temporal resolution of the neutron imaging set-up chosen, that will be 2.7 s (section 3.1).
Therefore, only the function Ftheo,sin will be considered hereafter, and it will be simply described
by:
Ftheo (x, t) = Ftheo,sin (x, t)
(2.44)
with Ftheo,sin (x, t) defined in (Equation 2.41).
2.1.6
Numerical solving
The analytical solution presented in the previous paragraph will not be directly used in the
model for the following reasons. Firstly, the numerical implementation of the real border condition (Equation 2.29 instead of Equation 2.40) would lead to a more complex implementation.
Secondly, a solving procedure based on analytical solutions would be restricted to the present
system and would not be easily upgradable to other transport mechanisms, such as electroosmotic drag or back-diffusion fluxes. Therefore, it will serve only as a validation tool for the
numerical solving method (subsubsection 2.1.6.3).
In the present chapter, the numerical solving of the equation (Equation 2.18) using the
real border conditions (Equation 2.29) and (Equation 2.30) is realized by a finite-differences
technique.
2.1.6.1
Discretization
The following notation is used hereafter:
Fij = F (xi , tj )
38
(2.45)
2.1 Deuterium labeling
Using this notation, the derivative terms are approached by the expressions:
j
Fi+1
− Fij
∂F (x, t)
≈
∂x
∆x
j
j
j
2
F
∂ F (x, t)
i+1 − 2Fi + Fi−1
≈
∂x2
∆x2
j+1
F
− Fij
∂F (x, t)
≈ i
∂t
∆t
(2.46)
It follows that the analytical equation (Equation 2.18) can be expressed by:
!
!
j
j
Fi+1
− 2Fij + Fi−1
Fij+1 − Fij
=
DM
∆x2
∆t
(2.47)
where
i ∈ [0, ..., N ] and j ∈ [0, ..., T ]
(2.48)
with N and T being the number of spatial and temporal points respectively used to simulate
the real domains [0, L] and [0, P ].
The border condition (Equation 2.29) can be written as:
!
F1j − F0j
j
− F0j ) + cH ∆x
−DM cH
= kH (FHj − F0j ) + kW (FW
∆x
and the border condition (Equation 2.29) becomes:
!
FNj − FNj −1
−DM
= ∆x
∆x
FNj+1 − FNj
∆t
F0j+1 − F0j
∆t
!
(2.49)
!
(2.50)
Since the border conditions are expressed on finite volume elements and not on infinitesimal
interfaces, the expressions presented above show accumulation terms (right-hand terms) that
did not appear on the analytical expressions.
The linear set of equations (Equation 2.47, Equation 2.49, Equation 2.50) can be set into a
matrix form:
F~ j+1 = AF~ j + ~bj
(2.51)
where the dimension of the system is 1 x (N + 1). This linear system is solved for each time
t until having T + 1 points. The numerical solving of this equation is detailed hereafter. The
matrix obtained is called Fnum and represents the numerical estimation of F (x, t).
2.1.6.2
Stability
Given a number T of temporal iterations to simulate a duration P , the temporal increment used
in the system (Equation 2.51) is:
∆t =
39
P
T
(2.52)
2. DIAGNOSTIC TOOLS
Similarly, the simulation of a spatial domain of length L with N points gives the spatial increment:
∆x =
L
N
(2.53)
A convergence criterion has to be established based on the spatial and temporal increments.
Taking the expression (Equation 2.47) and assuming the specific border condition:
j
j
Fn+1
= Fn−1
=0
(2.54)
the evolution of the F in the middle of the segment is given by:
2DM ∆t
j+1
j
Fn = Fn 1 −
∆x2
(2.55)
For any non zero value of Fnt , the stability criterion of this equation is given by:
2DM ∆t
62
∆x2
(2.56)
By introducing (Equation 2.52) and (Equation 2.53) in (Equation 2.56), we find the stability
relation expressed in terms of T and N :
T = αst N
2
with αst >
DM P
L2
(2.57)
Using a realistic membrane thickness of L = 200 µm, a typical experiment duration of P = 60
s and with an expected maximum value of D = 10−4 cm2 /s, we find a minimum value of αst =
15. So as to keep some flexibility, a value of αst = 100 will be used.
2.1.6.3
Validation
The choice of the absolute values for N and T relies on a trade-off between a sufficient precision
and a reasonable computing time for Fnum . To estimate the precision achieved, the following
parameter is introduced to compare Fnum with Ftheo according to the number of points N and
T:
∆Fmax (N ) = max |Fnum (xi , tj ) − Ftheo (xi , tj )|
(2.58)
where the number of iterations T was determined using the criterion T = αst N 2 (Equation 2.57)
with αst = 100 . On the figure (Figure 2.6), this parameter is plotted in function of the spatial
iteration number N . The computing time for the matrix Fnum is also reported. Based on this
figure, a value of N = 60 is chosen as it leads to a maximal error lower than 1 % and it keeps
the computing time at a reasonably low value, that is 1.4 s. Consequently, the following number
of temporal iterations is obtained:
T = 100 · 602 = 3.6 · 105
40
(2.59)
0.016
1000
0.012
100
0.008
10
0.004
1
0
0
100
200
300
400
500
Computing time [s]
ΔFmax [-]
2.1 Deuterium labeling
0.1
N [-]
Figure 2.6: Effect of the number of iteration N on the error and the computing time.
2.1.7
Fitting
Having developed a process to generate the matrix Fnum , a fitting procedure can now be applied
to estimate the transport parameters based on the experimentally measured isotope fraction
Fexp .
In the bulk equation (Equation 2.47), the unique parameter present is the diffusive coefficient DM , since the other parameters kW , kH and cH appear only in the border condition
(Equation 2.49). Hence, the fitting process presented here consists in two steps. In a first step,
a fitting of DM solely is conducted on a sub-domain of the membrane bulk and the measured
values of Fexp is used as border condition on the left border. The initial condition in the membrane is measured as well, since it can be different than F = 0 or F = 1. In a second step, the
value found for DM is used to fit the exchange parameters kW and kH on the full spatial domain.
The matrix calculated for the first fitting step b Fnum , where the superscript b stands for bulk,
can be expressed as follows:
j

bF
b j b j
b F t+1 −b F t

i+1 −2 Fi + Fi−1
i
i

D
=
M

∆t
∆x2



 b j
FiL =Fexp (xiL , tj )
b
j+1
Fnum =
b F j −b F j
bF
b j

N
N −1

N − FN
 −DM
=

∆t
∆x2


 b 0
Fi = Fexp (xi , 0)
∀ i ∈ [iL , ..., N ]
(2.60)
where the limits of the domain chosen are such that iL > 0.
f it
The fitted value of DM , called DM
, is searched on this domain:
v
u N T
uXX
b
u
(Fnum
(xi , tj , DM ) − Fexp (xi , tj ))2
u
t i=iL j=0
f it
DM
= min
DM
NT
41
(2.61)
2. DIAGNOSTIC TOOLS
This method allows being independent on the interface exchange rates kW and kH used at the
border (Equation 2.49) and therefore the estimation of DM is more reliable.
Once this value is found, the full domain is considered, i.e. i ∈ [0, ..., N ] and the original
border conditions (Equation 2.49) and (Equation 2.50) are used :

j
j
Fi+1 −2Fij +Fi−1
Fit+1 −Fit

f
it

=
∀ i ∈ [0, ..., N ]
D
 M
∆t

∆x2




F0j+1 −F0j
F1j −F0j

j
j
j
j
f it
= kH (FH − F0 ) + kW (FW − F0 ) + cH ∆x
−DM cH
∆x
∆t
Fnum =
j j j+1 j 

FN −FN

f it FN −FN −1

=
 −DM
∆t

∆x2


 0
Fi = Fexp (xi , 0)
(2.62)
As mentioned earlier (Table 2.1), the experiments were realized with identical isotope fractions
in the water vapor and in hydrogen so that: FH = FW = Fout . Furthermore, the exchange rates
can be included in one single term k = kH + kW according to (Equation 2.22). Consequently,
the system (Equation 2.62) can be simplified:

j
j
Fi+1 −2Fij +Fi−1
Fit+1 −Fit

f
it

∀ i ∈ [0, ..., N ]
=
D

M
∆t

∆x2




F1j −F0j
F0j+1 −F0j

f it
j
j
k
−DM cH
= ∆x (Fout − F0 ) + cH
∆t
∆x2
Fnum =
j j j+1 j 

FN −FN

f it FN −FN −1

−DM
=

∆t

∆x2


 0
Fi = Fexp (xi , 0)
(2.63)
Hence, the two remaining unknown parameters of this system are k and cH . Since the latter can
be estimated by dedicated experiments (section 3.1) or calculated based on a λ value known a
priori, the fitting process is applied on the parameter k, called k f it hereafter, such that:
v
u N T
uX X
u
(Fnum (xi , tj , k) − Fexp (xi , tj ))2
u
t i=0 j=0
(2.64)
k f it = min
k
NT
f it
Both DM
and k f it are searched iteratively by finding a minimum on a changing domain,
as presented on the scheme below (Figure 2.7). An interval is chosen to begin the process that
must include the minimum location (”Iteration 1”). Once the minimum is found, a new interval
is defined around the minimum location found before and with a refined mesh (”Iteration 2”).
Then, the minimum location is found, the mesh is centered and refined, and if the criterion of
the minimal mesh size pmin is reached, the process is stopped and the fitted values are given
in output (”Iteration Imax ”). For example, if the precision to be reached is 1%, then we get
10pmin = 1.01, which leads to Imax = 9 different iterations when using p = 1 for the first step.
Using the computing time of 1.4 s found before (subsubsection 2.1.6.3), a total computing time
42
2.1 Deuterium labeling
Search interval for (D or k)
Iteration 1
10
a
10
p
10
b
Iteration 2
opt
Iteration Imax
opt
D or k
pmin
10
Figure 2.7: Search of Dopt and k opt on a changing interval.
of roughly 60 s for each fitting is obtained. In the following, the parameters found in this section
will be simply written:
f it
and k = k f it
DM = DM
2.1.8
(2.65)
Sensitivity
The fitting procedure presented (subsection 2.1.7) being applied successively on the two parameters DM and k, it is relevant to investigate the sensitivity of k to DM . For that, a given
ref
ref
profile at a chosen condition Fnum (DM
, kW
) is used as base array for the fitting procedure of
k (Equation 2.64):
k f it (DM ) = min
v
u N T
uX X
ref
u
(Fnum (xi , tj , DM , k) − Fnum (xi , tj , DM
, k ref )2
u
t i=0 j=0
NT
k(DM )
(2.66)
ref
ref
In the present case, the values DM
= 1.3 · 10−5 cm2 s−1 and kW
= 4 · 10−5 [mol cm−2 s−1 ]
(values corresponding to the reference condition: RH = 80 %, T = 70 °C, 400 Nml/min, THe )
were chosen to obtain the sensitivity curve (Figure 2.8). The value used for k ref corresponds
ref
therefore to kW
. It can be observed that the sensitivity of kW to DM becomes very high for
43
12
30%
10
20%
8
10%
Error [%]
kW [mol cm-2 s-1] · 10-5
2. DIAGNOSTIC TOOLS
6
0%
4
-10%
2
-20%
0
0
0.5
1
1.5
2
2.5
2
-1
3
3.5
4
-30%
0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8
4.5
2
-5
-1
-5
DM [cm s ] · 10
DM [cm s ] · 10
ref
ref
Figure 2.8: Sensitivity of DM on kW for DM
= 1.3 · 10−5 cm2 s−1 and kW
≈ 4 · 10−5 [mol
cm−2 s−1 ] . Left: full domain of DM . Right: focus on the region of interest.
low values of DM . This numerical result reveals the strong coupling existing between these two
values in the model. On the contrary, for high values of DM there is no big influence on the
estimation of kW . Moreover, it can be remarked that an underestimation of DM leads to an
overestimation of kW . The error defined as:
Error =
ref
kW − kW
ref
kW
(2.67)
is plotted on the right-hand graph for the domain of interest of DM that will be found in the
result section (section 3.1). It can be noticed that an error of 10% on DM leads to a 5% error
on kW .
2.1.9
Effect of resolution
Since the estimation of the parameters is extracted from a fitting procedure that uses the experimental data as input, it is relevant to explore the effect of a limited spatial resolution on
the estimation of DM . It can be demonstrated [8] that the FWHM resolution rR resulting from
a geometric and inherent unsharpness of the detector, called rG and rI respectively, can be
approximated as (Equation 1.52):
rR ≈
q
3
3
rI3 + rG
(2.68)
Moreover, it has also been numerically evaluated that in case the geometrical and inherent
blurring values are similar:
rG
≈1
rI
44
(2.69)
2.1 Deuterium labeling
which is the case of the imaging set-up considered here, then the resulting line spread function
(LSF) SR can be well approximated by a Gaussian function:
2
SR (x) =
rR
r
ln2 − r4ln2
2 2
e Rx
π
(2.70)
Hence, a way to evaluate the blurring effect is to calculate, at each time tj , the convolution
product of a simulated value Fnum by the transfer function SR as follows:
blur
Fnum
(xi , tj )
∞
X
= Fnum (xi , tj ) ∗ SR (xi ) =
Fnum (xi − m, tj ) · SR (xi )
(2.71)
m=−∞
Based on this result, the fitting process exposed before is applied using the convoluted profiles
blur to obtain the new values of D blur . This operation is then repeated for different values of
Fnum
M
rR and over a given range of the original values DM (Figure 2.9).
4
rR = 100 μm
16%
rR = 50 μm
3
14%
rR = 25 μm
rR = 100 μm
12%
2.5
Error [%]
DMblur [cm2 s-1] · 10-5
3.5
18%
rR = 75 μm
2
10%
8%
6%
1.5
rR = 75 μm
4%
1
2%
0.5
rR = 50 μm
0%
rR = 25 μm
0
0
0.5
1
1.5
2
2.5
2
-1
3
3.5
4
4.5
0
0.5
1
1.5
-5
2
2.5
2
DM [cm s ] · 10
-1
3.5
3
4
4.5
-5
DM [cm s ] · 10
Figure 2.9: Blurring effect on the estimation of DM .
It can be seen that the blurring introduced has only a negligible effect for the best resolutions,
namely around 1 % for a resolution of 50 µm. In our case, the worst resolution obtained was
estimated to rR ≈ 35 µm. This leads to a blurring effect inferior to 1 %, which is considered to
be satisfying. Let us however note that for poor resolutions, like 100 µm, an increased error is
observed for low values of DM , which can be understood as a higher effect of blurring on highly
curved profile of DM (x). It can also be noticed that the blurring effect introduces an error in
the sense of an overestimation only. At last, it can be remarked that the curves presented here
demonstrate a slight level of noise, which is probably due to the use of fitting polynomials as
blur (x , t ) in (Equation 2.60).
expressions of the border condition b Fnum
iL j
45
2. DIAGNOSTIC TOOLS
2.1.10
Summary and outlook
The use of deuterium labeling as a tracer of the different water transport mechanisms in the
PEFC was motivated. Due to the important exchanges occurring between the H-containing
species that will be measured (section 3.1), a simple system was chosen as base for the study.
It consists of a membrane exposed to a gas phase on one side only. Moreover, the membrane
hydration is at equilibrium with the RH in the surrounding gases, meaning that no gradient of
humidity is expected and no net flow of water (electro-osmotic drag or back-diffusion) can occur.
In response to a change of the isotope fraction in the gases, the study of the isotope fraction in
the membrane can yield information about the diffusive transport in its bulk and the exchange
rate transfers with the gas phase. The experimental protocols and set-up were exposed, as well
as image processing basics.
Based on this system, a diffusion model was presented in which the diffusion and exchange
rate parameters are fitted on the experimental results. The numerical implementation of the
model and the fitting procedure were described. Validation aspects and resolution effects were
discussed. The method will be implemented in the result chapter (section 3.1).
As further developments, the effect on the measured experimental value as input for the
border condition could be analyzed. The effect of non-statistical noise on the experimental data
used as base for the fitting procedure should be tested as well. The successive study of spatial
sub-domains would be an option to investigate possible local changes of the diffusion coefficient
according to the local isotope fraction.
The extension of the model to more complex systems would be a next step. The modelisation
of the catalyst layer would constitute a major improvement of the model. By considering a 2sided cell, a redefinition of the border conditions would be needed. Then, back-diffusion flow
could be investigated, in particular by combining to this the water transport measurement
method recently developed by Boillat et al. [50]. Finally, the electro-osmotic drag should be
integrated.
46
2.2 Helox pulsing
2.2
Helox pulsing
2.2.1
Introduction
In usual flow field patterns (as presented in section 2.3) and when using air as reactant, the
oxygen needed by the electrochemical reaction is basically transported by convection inside the
gas channels and by diffusion across the porous media (GDL and CL) up to the reaction sites.
Hence, the gas consumption induces a depletion of oxygen along the gas channels as well as a
diffusive gradient of concentration across the porous media. Mass transport losses occur and
the cell voltage is reduced according to (Equation 1.32). While the oxygen depletion along the
channels translates into a reduction of the reference concentration cO2 ,ref , the diffusive transport
reduces the concentration at the reaction sites cO2 . Being dominant at higher current densities,
mass transport losses limit the maximum power that a fuel cell stack can supply. Hence, reducing
mass transport losses allows for a reduction of the cost of the system, for instance by using
reduced active areas, smaller number of cells or less Pt-loading.
By operating the cell in differential mode (section 1.6), the depletion effect of the gas along
the channel is suppressed and the diffusive losses can be more easily studied. Since the possible
accumulation of liquid water in some regions of the GDL further increases the diffusive losses,
due to a reduced gas diffusivity in (Equation 1.34), it is of prime importance to be able to
estimate the contribution of the presence of liquid water, especially in the GDL, on the diffusive
transport. In this work, in-plane neutron imaging of a differential cell is performed for this
purpose.
2.2.2
Basics
The ideal way to quantify mass transport losses of a cell would be to compare the voltage measured under air with the voltage measured under conditions for which mass transport losses
would be completely suppressed, while keeping other parameters identical. Based on the expression of the cell voltage (Equation 1.19), this would mean suppressing the term ηC and
maintaining ηCT and ηΩ constant. The use of helox gas, that is a gas mixture made of 79 % He
and 21 % O2 , helps to pursue this objective. The binary diffusivity of oxygen in such a mixture
is increased of the following factor (at 100°C, 1 bar [51]):
DO2 ,He ∼
= 3.56
DO2 ,N2
(2.72)
If bulk diffusion plays an important role in the total diffusive losses, the increased limit current
density (Equation 1.31) in helox should lead to ηC,helox ηC,air . On the contrary, if Knudsen or
thin film diffusion dominates, no change of ηC is expected, since these two phenomena depend
on the oxygen concentration only and not on the gaseous diffusivity (Equation 1.3.3.3). As
47
2. DIAGNOSTIC TOOLS
the partial pressure of oxygen is kept the same, it could also be expected that i0,helox = i0,air
according to (Equation 1.23) and that consequently ηCT,helox = ηCT,air . If the experiment is
performed by preventing changes of resisitivity, then the condition ηΩ,helox = ηΩ,air would be
achieved and all basic conditions would be fulfilled for a proper comparison. In a fuel cell cathode
however, the presence of water vapor in the gas mixture must be taken into account by a ternary
diffusive coefficient. In this case, the following improvement factor is reported to be (65°C, 1
bar [13]):
DO2 ,He,H2 O ∼
= 2.2
DO2 ,N2 ,H2 O
(2.73)
In the case of a 2 bar pressure, as in the present work, the vapor molar fraction will be reduced
and the improvement factor for a diffusive flow in helox is expected to be better [52] than the
value proposed here (Equation 2.73).
The simplest way to conduct an experiment is to operate the cell in steady-state under air
and then in steady-state under helox [13, 53–57] to get the difference of voltage between the
two experiments. However, this comparison requires that other parameters remain unchanged
between the two experiments, which cannot be achieved with such a protocol, as it will be
observed in this chapter.
Since the molecular diffusivity of water vapor in He is increased compared to its value in
N2 , dry-out effects can occur under helox [55–57]. In consequence, the hydration state of the
membrane and CL ionomer may change and this can increase ohmic losses.
Besides dry-out effects, another issue that emerges when successively testing a fuel cell under
air and helox is the change of the catalyst oxidation state due to the change of potential [58].
Since a lower voltage is expected under air, the coverage of Pt particles with oxygenated species
will be relatively low in that case [52]. Under helox operation, the cell voltage will be improved
and the coverage of Pt particles will tend to increase, which can induce a higher charge transfer
overpotential ηCT , making the comparison between air and helox experiments inappropriate.
While operating the cell in potentiostatic mode would relieve this issue, the change of total
current between the experiments would change the water and current distributions, which is not
desired either.
A last issue for steady-state comparisons is that the repetition of experiments can be subject
to changes of operating conditions due to errors produced by control devices and to experimental
uncertainties.
Therefore, to avoid as much as possible changing the mentioned parameters, a relevant
estimation can be achieved by operating the cell under helox during very short periods, while
operating under air most of the time. Attempts for that were made by Slade et al. [59] who
operated the cell for 3 min under helox and O2 successively, each time after 30 min operation
48
2.2 Helox pulsing
under air. This is though not sufficient for avoiding dry-out effects, as it will be observed
hereafter.
In the technique presented in the following, the cell is operated under helox during short
periods only, called pulses henceforth, and under air between the pulses. Beside avoiding disturbing operating conditions too much and depending on repeatability issues, this method also
offers a reasonable temporal resolution for tracking transients of mass transport losses, typically
after a change of relative humidity. In the following chapter, neutron imaging is used to validate
that the water distribution is globally not affected. The results provided by the method will be
presented in the result chapters (section 3.2) and (section 3.3).
The pulse method described was also extended to pulses of pure oxygen. This characterization brings additional information to the helox pulse method. The absence of bulk diffusion
transport in pure oxygen should allows evaluating the contribution of other diffusive contributions in ηC than bulk diffusion, namely Knudsen and thin film diffusive transports. Based on
this, specific interpretations of the results will be made in the result chapters (section 3.2) and
(section 3.3). In particular, it will serve to validate that the bulk diffusion losses in helox are
negligible.
2.2.3
Set-up
To perform short time gas pulses, a quick replacement of the gases in the piping is desired. The
gas flow has therefore to be maximized while a fairly low pressure drop in the cell is requested
to ensure a homogeneous distribution of the parameters along the channel path, as defined by
the differential cell mode. The gas set-up described hereafter was designed for this purpose
(Figure 2.10). It represents of course the gas supply on the cathode side.
On the inlet gas lines of the test bench, 3 calibrated MFCs (”MFC a”, ”MFC b” and ”MFC
c”) are used to control the main gas flow rates for air, He and O2 . The helox mixture is obtained
by mixing He and O2 . The relative humidity of each gas mixture is controlled by a dedicated
humidifier (”Hum. a”, ”Hum. b”). Automatically controlled electric valves are used to circulate
the desired gas in the cell (”In”) while deviating the unused gas in the by-pass line (”By-pass”).
This configuration allows performing rapid changes of gas by avoiding the huge disturbances that
would occur if the MFCs had to be started and stopped for each pulse. In a previous version
of the gas set-up presented in [52], the gas switches were realized by valves located before a
single humidifier. Due to the change of kinematic viscosity between He and N2 , an important
change of pressure drop through the humidifier happened when changing the gas, which needed
to be compensated by an extra pressure controller. Even in that configuration, pressure gas
flow fluctuations occurred. By switching the gases after the humidifiers like here, the different
pressure drops are mitigated and the fluctuations are quite low. This solution was implemented
49
2. DIAGNOSTIC TOOLS
Air
He
O2
MFC a
MFC b
MFC c
Hum. a
In
Hum. b
By-pass
Buffer
3m
piping
Cell n
PC
Out
MFC n
pressure controller
n:
Hum.: humidifier
Exhaust
: heated section
MFC: mass flow controller
PC:
Exhaust
1 for single-cell
1 to 6 for multi-cell
Figure 2.10: Gas management set-up for the helox pulsing method.
by means of an oven so as to maintain electric valves and piping at high temperature (85°C)
and avoid condensation.
A high flow rate is supplied from the test bench to the cell (”In”) through a low diameter
piping (3 mm). A value of 7.5 Nl/min is used in the case of the multi-cell set-up (section 2.3).
At the cell inlet, only a part of this main flow is derived to feed the cell (”Out”). This MFC
controls the cell flow at a typical value of 1 Nl/min. In this configuration, most part of the main
flow (”In”) returns to the test bench (”By-pass”) where the pressure is controlled (”PC”). This
solution allows to keep a low flow in the cell and to avoid big pressure drops in the cell channels.
Before the pressure controller, the flow passes through a free volume (”Buffer”), whose function
is to mix the gases and to mitigate the change of pressure drop in the pressure controller itself
when changing the gas.
2.2.4
Validation
If not specified, all the experiments presented in the next subchapters will be realized at 1
A/cm2 , with a cell temperature of 70°C, with pressures of 2 bar (anode) / 2.1 bar (cathode),
and with cell flows of 0.4 Nl/min (anode) / 1 Nl/min (cathode). Moreover, the reference flow
field used is made of 5 parallel channels, as further described in (subsection 2.3.9).
50
2.2 Helox pulsing
2.2.4.1
Short timescale
On the following graph (Figure 2.11), the voltage and high frequency resistivity (HFR) signals
recorded during the application of a helox pulse are presented. The time period during which the
air
helox
air
0.75
0.18
Uhelox
0.15
ΔUhelox-air
0.65
0.6
0.12
0.09
Uair
0.55
0.06
0.5
0.03
0.45
0
-2
0
2
0.7 1
4
Time [s]
6
Resistivity [Ω·cm2]
Voltage [V]
0.7
without filtering
8
Figure 2.11: Helox pulse and measurement locations. Condition: 1 A/cm2 , T = 70 °C, RH an. =
100%, RH ca. = 0%
electric valves (see Figure 2.10) of the testbench are switched to flow helox throughout the cell,
is represented by the grey area. The valves are switched at t = 0 s. This moment is chosen for
measuring the voltage value under air (Uair ). After approximately 0.5 s, an increase of voltage
is measured, as expected by the reduction of mass transport losses in helox. The delay between
the valve opening and the voltage increase corresponds to the travel time of the gas from the
valve location to the active area of the cell.
It can be observed that the voltage increases rapidly, within a 200 ms range, which is required to apply short-time pulses. In addition to that, the voltage transition does not show any
signal disturbance that could have been induced by pressure transients for example. Concerning
the HFR, the signal presented on the detail view exhibits strong variations during the voltage
transition. Since the change of voltage is steep, high frequency components will disturb the
voltage signal needed by the resistance measurement. It is therefore reasonable to attribute
these disturbances to a pure measurement artifact and not to a physical change in the state of
the membrane. In consequence and for sake of clarity, a filter is applied to remove this noise.
During the pulse, both voltage and resistivity measurements are stable. This important
result validates that the introduction of helox in the system does not change the resistivity.
At t = 1 s, the gas valves are switched again to flow air throughout the cell and the voltage
51
2. DIAGNOSTIC TOOLS
reaches its nominal value, again without any visible signal disturbance, except a short voltage
peak preceding the decrease. The duration of the valve opening was chosen so as to fulfill the
following trade-off. On one hand, the exposure time to helox has to be sufficiently long to ensure
that the voltage measurement is recorded when the whole active area is covered with helox.
On the other hand, the pulse duration must be reasonably low to mitigate the disturbances
introduced by helox, so that the steady-state under air can be reached quite rapidly after the
helox pulse and that the next pulse can be applied shortly afterwards. The voltage value for
the operation under helox (Uhelox ) is measured at t = 0.7 s. As seen on the figure, this value
allows for a short stabilization time after the maximal value is obtained. The difference between
the two values measured is the targeted value of the method, namely the estimator of the bulk
diffusive transport losses given by the helox pulsing method :
∆Uhelox−air = Uhelox − Uair
(2.74)
Beyond the operating condition presented (Figure 2.11), the possible disturbances introduced
by helox pulses must also be tested for other operating or structural parameters. In the summary
figure (Figure 2.12), 3 different relative humidity conditions are presented for 4 types of cell
operating simultaneously, thanks to the multi-cell set-up presented later (section 2.3). The
first cell presented (Figure 2.12.a - c) is compressed at a 45% rate. The ”reference” condition
(Figure 2.12.b) is identical as the one presented before (Figure 2.11).
Contrary to the ”reference” condition, the helox pulse recorded under the ”dry” condition
exhibits a different feature. It can be observed that during the helox pulse, the resistivity slightly
increases whereas the voltage decreases. This can be attributed to a dry-out effect induced by
the high diffusivity of water vapor in helox. Being in ”dry” condition, the water saturation level
of the GDL is probably low compared to the ”reference” condition (as it will be confirmed in
section 3.2). It follows that a higher removal rate by vapor diffusion in helox will directly affect
the membrane hydration in the ”dry” case, whereas it will essentially lower the GDL saturation
level in the ”reference” (and ”wet”) case. Consequently, the resistance increases during the helox
pulse and the voltage decreases. This has un undesirable effect on the estimation ∆Uhelox−air ,
which is now affected by an error due to a change of resistance. However, by measuring Uhelox at
the very beginning of the pulse, this detrimental effect stays limited compared to the amplitude
of the pulse. Another consequence of the dry-out is that a given time is needed after the helox
pulse to reach a steady-state value under air. It can be observed on this graph that a waiting
time of approximately 2 s is required, which is short enough compared to the waiting time chosen
between the pulses (29 s, as detailed in subsubsection 2.2.4.2).
Concerning the ”wet” condition (Figure 2.12.c), no change of resistivity is observed during
the pulse. Like the example given before, it can be argued that a high water saturation level of
52
2.2 Helox pulsing
“Reference” condition
“Wet” condition
RH an. 40%
RH ca. 0%
RH an. 100%
RH ca. 0%
RH an. 100%
RH ca. 100%
0.09
0.55
0.06
0.5
0.03
0
0.12
0.6
0.09
0.6
0.09
0.55
0.06
0.55
0.06
0.5
0.03
0.5
0.03
0.33
0.4
0.3
0.35
0.27
0.3
0.24
0.25
0.12
0.35
0.27
0.55
0.09
0.3
0.24
0.5
0.06
0.21
0.25
0.21
0.45
0.03
0.18
0.2
0.18
0.4
0
0.4
0.09
0.35
0.06
0.3
0.03
0.55
0.5
0.15
0.5
0.45
0.12
0.4
0.09
0.35
0.06
0.3
0.03
0.25
0
6
0.2
0.6
0.17
0.55
0.14
0.5
0.11
0.45
0.4
0
2
4
Time [s]
6
(i)
0.45
0.25
0.09
0.2
0.06
0.15
0.03
0
0
6
2
4
Time [s]
6
0.7
0.65
0.2
0.65
0.2
0.6
0.17
0.6
0.17
0.55
0.14
0.55
0.14
0.5
0.11
0.5
0.11
0.08
0.45
0.08
0.45
0.08
0.05
0.4
0.05
0.4
(k)
0
2
4
Time [s]
Uair
6
Voltage [V]
0.65
6
0.23
Voltage [V]
0.7
2
4
Time [s]
0
2
4
Time [s]
0.1
0
0
0.23
Voltage [V]
0.18
(h)
(f)
0
6
Resistivity [Ω·cm2]
Voltage [V]
0.12
2
4
Time [s]
Voltage [V]
0.6
(e)
Resistivity [Ω·cm2]
0.3
Voltage [V]
0.4
Resistivity [Ω·cm2]
0.15
0.45
(j)
0.18
0.65
0.15
0.7
6
0.33
0.5
2
4
Time [s]
2
4
Time [s]
0.45
0.55
0
0
0.7
0.18
0.25
0
0.45
6
0.36
6
(g)
2
4
Time [s]
Resistivity [Ω·cm2]
2
4
Time [s]
0
0
Resistivity [Ω·cm2]
Voltage [V]
0.65
0.45
0.55
Voltage [V]
0.12
0.5
(d)
0
Voltage [V]
0.65
0.45
Resistivity [Ω·cm2]
Voltage [V]
Cell “45%”
Cell “5%”
0.15
6
0.2
Cell “60%”
0.7
0.36
0.5
Cell “0D”
0.15
0.23
(l)
0.05
0
Uhelox
Resistivity [Ω·cm2]
0.6
2
4
Time [s]
0.7
Resistivity [Ω·cm2]
0.12
0.18
(c)
Resistivity [Ω·cm2]
0.65
0
0.75
Voltage [V]
0.15
0.45
0.18
(b)
Resistivity [Ω·cm2]
0.7
Voltage [V]
0.75
(a)
Resistivity [Ω·cm2]
0.18
0.75
Resistivity [Ω·cm2]
“Dry” condition
2
4
Time [s]
6
Application of helox
Figure 2.12: Helox pulse morphology for different relative humidity conditions (RH) and different
compression rates (a to i) and design (j to l), at 1 A/cm2 and T = 70 °C.
53
2. DIAGNOSTIC TOOLS
the GDL will reduce the effect of a possible dry-out of the membrane, similarly to the ”reference”
condition. The new feature appearing here is a local minimum of the voltage during the pulse.
This is probably the sign of a gas transient perturbation induced by the limited dynamics of
the mass and pressure controllers in response to the change of pressure drop that happens when
circulating helox instead of air. The reason why such a perturbation would occur in the case
of this condition can be hypothesized to be due to an increased pressure drop in the cathode
(”By-pass”) line side or in the pressure controller due to the presence of water droplets under
high relative humidity. Further optimization of the gas buffer (subsection 2.2.3) or controllers
with higher dynamics could be tested to solve this issue. However, the amplitude of the voltage
variation is in the present case 9 mV only, which yields a reasonable error of roughly 5% for the
value ∆Uhelox−air = 180 mV.
The helox pulses recorded for another cell with a low compression rate of 5 % are presented
(Figure 2.12.d-f). In this case, the increased porosity expected in the GDL should lead to
a higher diffusivity Drel in (Equation 1.34). It follows that the ”5 %” cell is more sensitive
to dry-out effects than the ”45 %” cell, as revealed by the resistivity presented for the ”dry”
condition (Figure 2.12.d). In this case, an error of 3.5 % is obtained for ∆Uhelox−air = 100 mV.
The ”reference” condition (Figure 2.12.e) also exhibits a slight change of resistivity, whereas the
”wet” condition shows a similar perturbation during the pulse as for the ”45 %” cell.
A cell with a high compression rate, that is 60 %, is presented to complete the study (Figure 2.12.g-i). In ”dry” condition, even with a very low change of resistivity, an important change
of voltage can be observed during the pulse, which can therefore not be interpreted as the consequence of a dry-out effect in the membrane. Moreover, one observes that a given recovery
time is needed after the pulse to reach the steady-state voltage under air. It can be supposed
that this change of voltage would originate from the drying effect in the CL, according to the
mechanisms that will be described in (subsubsection 3.3.3.3). An effect of Pt-covering is less
probable since the dynamics of the oxidation is expected to be much slower [58].
The last example of the figure (Figure 2.12.j-l) concerns another type of flow field, namely a
micro-interdigitated flow field denoted ”0D” (see section 2.3 for details). In such a flow field, the
gases are essentially transported in the GDL by convection and not by diffusion. This implies
that the water vapor removal is very effective compared to the reference design. In consequence,
dry-out effects are important in this case, as reflected by the changes of resistances in the ”dry”
and ”reference” conditions (Figure 2.12.j,k). However, the value Uhelox measured at t = 0.7 s
is judged to be sufficiently precise to characterize this design in the result chapter (section 3.3).
Moreover, the helox characterization is not very useful in the case of the ”0D” flow field, since
this cell type should not be subject to diffusive losses. The helox method can however serve in
54
2.2 Helox pulsing
this case to validate that the ”0D” flow field is exempt from diffusion losses, which requires a
precise adjustment of the oxygen concentration in helox.
Finally, it must be emphasized that the different cells were simultaneously operated under
helox thanks to the multi-cell set-up presented in (section 2.3). In particular, the signals of the
cells (Figure 2.12.b,e,h,k) were recorded at the very same moment. On the figure, for instance
during the pulse of the ”reference” condition, it can be seen that no important gas delay exists,
which is an important feature of the set-up for its use in transient experiments.
As mentioned, the gas pulsing method can be extended to other gases, for instance to pulses
of pure oxygen. In the following figure (Figure 2.13.a-c), the pulses recorded under oxygen
are presented for the ”45 %” cell, operated under the same conditions as before. Contrary
to the dry-out effect observed under helox, the ”dry” condition under oxygen (Figure 2.13.a)
does not show any visible change of resistivity. This is in agreement with the little change of
gas diffusivity expected between water vapor in nitrogen and water vapor in oxygen. The slight
change of voltage remaining during the oxygen pulse for the 3 conditions (Figure 2.12.a-c) might
therefore be attributed to a change of catalyst coverage due to oxygenated species, as already
discussed. It must also be pointed out that the oxygen pulses do not exhibit any disturbance
compared to the helox case. This tends to confirm that the disturbances observed under helox
can be attributed to a change of pressure drop, which is not the case in the present experiment,
since the kinematic viscosity of oxygen is close to the one of nitrogen.
“Reference” condition
“Wet” condition
RH an. 40%
RH ca. 0%
RH an. 100%
RH ca. 0%
RH an. 100%
RH ca. 100%
0.15
0.8
0.15
0.75
0.12
0.75
0.12
0.7
0.09
0.6
0.09
0.65
0.06
0.55
0.06
0.6
0.03
0.5
0.03
0.75
0.12
0.7
0.09
0.65
0.06
0.6
0.03
0
0.55
0
2
4
Time [s]
6
Voltage [V]
0.15
0
0.55
0
2
4
Time [s]
Uair
Voltage [V]
0.8
0.8
(b)
Resistivity [Ω·cm2]
0.85
0.85
(a)
Resistivity [Ω·cm2]
Voltage [V]
Cell “45%”
0.18
0.18
0.85
0.18
(c)
0.45
0
0
6
Uhelox
2
4
Time [s]
6
Application of helox
Figure 2.13: Oxygen pulse morphology for different relative humidity conditions (RH), at 1 A/cm2
and T = 70 °C.
55
Resistivity [Ω·cm2]
“Dry” condition
2. DIAGNOSTIC TOOLS
2.2.4.2
Long timescale
Following the discussion made about the effect of helox and oxygen pulsing on a local time scale,
that is within the few seconds during and after the pulse, the influence of pulses on the global
behavior of the cell, meaning in the minute time scale, must now be analyzed. So as to have a
value Uair (see Figure 2.11 for definition) representative of a cell operating under air, a sufficient
waiting time has to be completed between each helox pulse. It was fixed at 29 s in this work, as
compromise between a reasonable time for reaching steady-state operation after the pulse while
keeping sufficient time resolution for transient experiments.
To compare the influence of the helox pulses on the behavior of the cell, a series of different
relative humidity conditions is performed and repeated 3 times: one time operating under air
only, one time operating under air with helox pulses and one time operating under helox only.
The result of this set of experiments is presented here (Figure 2.14). On the left-hand part of the
figure (Figure 2.14.a,c,e,g), the operation under air only is compared with the operation under
air with helox pulses. On the right-hand part of the figure (Figure 2.14.b,d,f,h), the operation
under air with helox pulses is compared with the operation under helox only. Contrary to
the experiments presented in the previous paragraph (subsubsection 2.2.4.1), the present set of
experiments was performed on an older set-up [52]. Major differences are that the voltage Uair
is measured 1.2 s after the valve switching instead of 0.7 s. The pulse duration is 2 s in the
present case compared to 1 s in the former case. But one pulse is performed each 30 s, as before.
Instead of the multi-cell set-up, a single-cell set-up was used here with 4 parallel channels, whose
complete description is given elsewhere in this work (subsection 3.4.3). Anode cell flow was 1
Nl/min instead of 0.4 Nl/min and cathode pressure was 2 bar instead of 2.1 bar.
On the first graph (Figure 2.14.a), the voltage values are compared for the operation under
air and for the operation under air with helox pulses. In the latter case, the voltage shown is the
voltage Uair , as defined earlier (Figure 2.11). It can be observed that the signals are almost the
same for all conditions. The slight differences appearing can be explained as follows. In the case
of the cell operating under air without pulses (grey curve), the acquisition rate is quite high,
which makes the signal more noisy and the slopes steeper than the signal recorded when helox is
pulsed (black curve), for which the acquisition rate is quite low (1 value each 30 s). Concerning
the steady-state values, it can be observed that the voltage under air without pulses appears to
be slightly higher, which can be attributed to imperfect repeatability. The resistance curves of
these two experiments (Figure 2.14.c) are virtually the same, except for the presence of a peak
at t = 5 min for one of the operation with pulses, that can be attributed to a noisy value. On the
lower graphs (Figure 2.14.e,g), the water evolution profiles measured in the cathode GDL with
neutron imaging are presented, for the areas under the channels (Figure 2.14.e) and under the
ribs (Figure 2.14.g). One measurement was taken each 10 s approximately. It can be observed
56
2.2 Helox pulsing
“Dry”
“Std.”
condition condition
RH an. 40%
RH ca. 0%
100%
0%
“Wet”
condition
“Dry”
condition
100%
100%
40%
0%
RH an. 40%
RH ca. 0%
0.8
0.8
0.75
0.75
0.7
Uair
0.65
0.55
0.55
(b)
Uhelox
(d)
0.15
Resistivity [Ω·cm2]
Resistivity [Ω·cm2]
40%
0%
0.18
(c)
0.12
0.09
0.06
0.12
0.09
0.06
0.03
0.03
0
0
30
30
(e)
Ca.
Water content [% vol tot]
An.
Water content [% vol tot]
100%
100%
0.65
0.6
0.15
25
20
15
10
5
0
30
An.
(g)
Ca.
An.
Ca.
(f)
An.
Ca.
(h)
25
20
15
10
5
0
30
25
Water content [% vol tot]
Water content [% vol tot]
100%
0%
“Dry”
condition
0.7
0.6
0.18
“Wet”
condition
0.85
(a)
Voltage [V]
Voltage [V]
0.85
“Dry”
“Std.”
condition condition
20
15
10
5
0
25
20
15
10
5
0
-5
0
5
10
Time [min]
15
20
-5
Experiment
0
5
10
Time [min]
Experiment
Air with helox pulsing
Air with helox pulsing
Air only
Helox only
Figure 2.14: Comparison of experiments with or without helox pulses.
57
15
20
2. DIAGNOSTIC TOOLS
that the water saturation levels are the same, during both steady-state and transient periods,
which shows that the pulsing method does not induce any visible redistribution of water in the
long-term.
On the top graph of the right-hand part of the figure (Figure 2.14.b), the voltage Uhelox
measured during the pulse and defined before (Figure 2.11) is compared to the voltage obtained
when operating the cell continuously under helox. An important difference can be observed
between the two signals. Precisely, the voltage recorded under continuous helox operation is
systematically lower than the voltage corresponding to pulsed helox. The resistivity measurements (Figure 2.14.d) show that, except for the ”wet” condition, the operation under continuous
helox exhibits a higher resistivity than in the pulsed mode. On the corresponding water profiles
(Figure 2.14.f,h), it can be observed that the ”dry” condition leads to a same water content for
both experiments. This means that the GDL is dry in both cases and that the whole removal
of water from the electrode takes place in form of a diffusive flow of vapor, which explains the
low hydration state of the membrane measured. For the ”reference” condition, that it between
t = 0 and 5 min, the ribs are kept dry when operating under continuous helox, whereas some
liquid water appears in that region for the case of the operation with pulsed helox. Again,
the higher diffusivity of the vapor in helox can explain this feature. Since the water removal
is increased in pure helox, the membrane is less hydrated and the resistivity is higher. For
the ”wet” condition, namely at full humidity, the operation under helox leads to slightly lower
saturation levels for both channel and rib areas. This could be the sign that a diffusive flow
of vapor also exists in fully humidified conditions, perhaps due to the presence of temperature
gradients, as suggested in [52]. However, the resistivity differences shown are not sufficient to
explain the difference of voltage. An explanation of this difference could be a change of the ionic
conductivity of the electrode, as proposed in [52], that would not be measured by the resistivity
(subsubsection 1.3.3.2).
A same set of experiments was used to evaluate the effect of the oxygen pulses (Figure 2.15).
Similarly as before, the left-hand part of the figure (Figure 2.15.a,c,e,g) compares the operation
under air only with the operation under air pulsed with oxygen. The pulsing set-up employed
is the same as before. On the right-hand part of the figure, the operation under air pulsed with
oxygen (Figure 2.15.b,d,f,h) is compared to the continuous operation under oxygen.
The voltages recorded under air are almost the same for both operations with or without
pulses (Figure 2.15.a), except for the slight differences of dynamics and reproducibility already
identified before. The resistivities (Figure 2.15.c) and the water content profiles (Figure 2.15.e,g)
are identical. The oxygen pulsing method seems therefore not to disturb the operation under
air. Similarly to the helox case, we can define the following indicator:
∆U O2 −air = U O2 − Uair
58
(2.75)
2.2 Helox pulsing
“Dry”
“Std.”
condition condition
RH an. 40%
RH ca. 0%
100%
0%
“Wet”
condition
“Dry”
condition
100%
100%
40%
0%
RH an. 40%
RH ca. 0%
0.8
0.8
0.75
0.75
0.7
Uair
0.65
0.55
0.55
Uoxygen
Uoxygen
(d)
0.15
Resistivity [Ω·cm2]
Resistivity [Ω·cm2]
40%
0%
0.18
(c)
0.12
0.09
0.06
0.12
0.09
0.06
0.03
0.03
0
0
30
30
(e)
Ca.
Water content [% vol tot]
An.
Water content [% vol tot]
100%
100%
(b)
0.65
0.6
0.15
25
20
15
10
5
0
30
An.
(g)
Ca.
An.
Ca.
(f)
An.
Ca.
(h)
25
20
15
10
5
0
30
25
Water content [% vol tot]
Water content [% vol tot]
“Dry”
condition
0.7
0.6
0.18
100%
0%
“Wet”
condition
0.85
(a)
Voltage [V]
Voltage [V]
0.85
“Dry”
“Std.”
condition condition
20
15
10
5
0
25
20
15
10
5
0
-5
0
5
10
Time [min]
15
20
-5
Experiment
0
5
10
Time [min]
Experiment
Air with oxygen pulsing
Air with oxygen pulsing
Air only
Oxygen only
Figure 2.15: Comparison of experiments with or without oxygen pulses.
59
15
20
2. DIAGNOSTIC TOOLS
By comparing the voltage measured during the oxygen pulses with the one recorded under
continuous oxygen (Figure 2.15.b), it can be seen that the difference appears to be constant
for the entire experiment, except during the transients, which is due to the different acquisition
rates. The resisitivities (Figure 2.15.d) and the water evolution profiles (Figure 2.15.f,h) can be
judged to be very similar. These results are coherent with the fact that the diffusive transport of
vapor is expected to be the same for both experiments, contrary to the helox case. The constant
offset existing between the voltages can be explained by a higher coverage of the catalyst by
oxygenated species in the case of the pure oxygen, due to the relatively high voltage during
the experiment compared to pulsed oxygen. Indeed, the cell operating with oxygen pulses is
actually most of the time operating under air (28 s over 30 s period in this experiment), that is
at a relatively low voltage (Figure 2.15.a) compared to the operation under continuous oxygen
(Figure 2.15.b). The coverage of Pt by oxygenated species can therefore be expected to be
lower in the former case, which leads to a higher voltage during the oxygen pulses than for the
continuous oxygen operation.
Having validated the measurements of Uhelox and U O2 during the pulses, it will be possible
to alternate these two kinds of measurement during steady-state conditions where Uair keeps a
constant value. The following indicator can be defined:
∆U O2 −helox = U O2 − Uhelox
(2.76)
and the following calculation method will be employed:
∆U O2 −helox = ∆U O2 −air − ∆U helox−air
(2.77)
The interpretation of this indicator will be realized in the result chapters of this work (section 3.2 and section 3.3). In particular, it will be used to validate that the bulk diffusive losses
in helox are negligible.
2.2.5
Summary and outlook
The use of helox instead of air was motivated as a measurement method of the bulk diffusive
losses. The drawbacks of the continuous helox operation as a characterization tool were discussed
and the helox pulsing method was presented as a reliable alternative.
In particular, an experimental set-up and protocols were described and the morphology of
the voltage signal during the helox pulse was studied, so as to chose adequate measurement
points. Different operating conditions and cell designs were tested and the slight disturbances
affecting the voltage measurement during the helox pulses were discussed.
The operation of the cell under pulsed helox was compared to the operation under pure
air and the operation under pure helox. This was realized on a long timescale (in the minute
60
2.2 Helox pulsing
range) and for different relative humidity conditions. It was confirmed that the pulsed helox
method has a negligible impact on the operation under air. Specifically, it does not affect the
water distribution and does not increase the resistivity either, which can be major issues under
continuous helox operation.
This method will be implemented in the result chapters (section 3.2) and (section 3.3).
As further developments, the reduction of the gas disturbances might be attempted, by
testing different gas buffers or controllers for instance. The reduction of the pulse duration
would serve as motivation for this. The combined use of electrochemical impedance spectroscopy
(EIS), in particular with the acquisition of spectra during the pulse, would be a further option.
The extension of the method to other kinds of gas mixtures would also be possible, for instance
with reduced concentration of oxygen.
61
2. DIAGNOSTIC TOOLS
2.3
2.3.1
Multi-cell
Introduction
Structural parameters of cell components (e.g. morphology, materials) can have a strong impact
on the performance, the stability and the durability of a fuel cell stack. Even at the local scale,
understanding the effects of structural parameters remains very challenging for both modelers and experimentalists. The usefulness of simplified systems is obvious to gain reliable and
straightforward knowledge and for this purpose, the differential cell (section 1.6) is examined in
this work.
A systematic way of studying structural parameters is to compare different types of differential cells operating under identical conditions. An option for that is to repeat the same
test protocol by successively changing the cell investigated [8, 60, 61]. However, this method
suffers from an important time consumption as well as repeatability uncertainties of operating
conditions between the different tests. Consequently, an efficient set-up should allow operating
all cells synchronously under common parameters, like the gas composition and the current for
instance. In the context of this work, such a set-up has to be compact and optimized for neutron
imaging. A new set-up was therefore realized and is presented in the present chapter and in
[62] under the name multi-cell (Figure 2.16). So far, the existing studies dedicated to neutron
imaging of cells with different materials have used portioned active areas. For example, Preston
et al. [63] realized neutron imaging on a cell with a MPL on half the active area and without
MPL on the other half. This set-up was not equipped for local current mesurement so that the
comparison between the two areas is difficult. Another experiment from Schröder et al. [64] was
dedicated to the effect of various PTFE contents, again by portioning the active areas in two
halves. Even if this set-up allowed for the local measurement of current, the gas flow distribution
remained unknown. This can lead to important changes of behavior, especially in presence of
water droplets in the channels. The multi-cell set-up offers the advantage of controlling these
parameters.
Besides the realization of the multi-cell itself, a new scintillator with extended field of view
(2.3 x 120 mm) was constructed by the neutron imaging and activation group (NIAG) based on
the existing small-size (5 x 25 mm) tilted detector [41], so as to fit in the ”Midi” imaging set-up
of the ICON beam line [39]. The multi-cell imaging is one application amongst others that this
scintillator offers.
At last, the multi-cell features the decisive advantage of an efficient beam time occupation
in the context of an increasing demand.
62
2.3 Multi-cell
Flowfield
Spacer
Gasket
MEA+GDL
Cell heater
Printed
circuit
board
Cooling gas
Printed
circuit
board
Heating
liquid
Out
(MFCs)
In
By-pass
(PC)
Out
(MFCs)
Air
MFC: mass flow
PC: pressure controller
In
By-pass
(PC)
H2
Figure 2.16: Multi-cell set-up.
63
2. DIAGNOSTIC TOOLS
2.3.2
General characteristics
The multi-cell assembly is constituted of two external housing enclosing the different cells (Figure 2.16). So as to fit in the available field of view while keeping sufficient cell sizes, a number
of 6 cells was chosen. The cells are mounted and tightened separately prior to their insertion in
the external housings. To achieve an easy interchangeability, cells are designed as independent
ready-to-operate modules, easily pluggable in the external housings of the multi-cell. This offers
the advantage or pre-testing the cells independently before mounting them in the multi-cell.
The gas tightness between the cell and the external housings is ensured by gaskets (see subsection 2.3.8), which are compressed once the external housings are tightened together. The cells
are electrically insulated from the external housings. The alignment issues discussed hereafter
(subsection 2.3.8) led to use a thicker external block on one side. Further important features
are detailed hereafter.
2.3.3
Gas supply
Two common gases, for anode and cathode, at defined conditions (temperature, relative humidity, gas composition) are supplied to the 6 cells (called ”In” on the figure) from the testbench
via the external housings. Cell flows are derived from the common flows and are controlled by
12 mass flow controllers (MFCs) located after the cells (”Out”). This configuration implies that
all cells are supplied by gases at identical conditions. The independent control of cell flows is
an important feature compared to a fuel cell stack, where flows can be inhomogeneously distributed, especially in presence of liquid droplets in channels. The use of transient application
of gas composition, such as deuterium labeling (section 2.1) or helox pulses (section 2.2), with
reduced delays, sharp changes and small perturbations, was considered. For this reason, the
common flow is applied in excess so as to be maximized, independently of the cell flows, and
part of it returns to the testbench (”By-pass”) where it flows through a pressure controller (PC)
and a humidity sensor.
2.3.4
Electrical control
All cells are connected in series and controlled by a single electronic load. Each cell is connected
to the main printed circuit board (PCB) by means of 4 gold coated spring-probes, contacting
the cell surface at defined locations (Figure 2.17). They allow for a 4 points measurement:
2 probes are used for the current collection and 2 other probes are dedicated to the voltage
measurement. These compressible probes provide a low contact resistance when the external
housings are tightened together. The voltage signals are measured for each cell independently,
whereas the current is measured at one location for all cells. The measurement of AC voltages
64
2.3 Multi-cell
: Voltage
: Heater current
: Current
: Temperature measurement
Figure 2.17: Measurement probes locations.
in response to the application of an AC current (typically 1 to 20 kHz) allows measuring high
frequency resistance (HFR) or more generally recording electrochemical impedance spectra (EIS)
for each cell independently. The acquisition set-up enables simultaneous measurements, which
is of great interest to obtain transient responses of HFR of the cells. The cell design is such that
the electrical signals from anode and cathode sides are measured very close to each others, so
as to reduce detrimental effects of inductive loops on AC signals. Finally, relays placed on the
PCB allow disconnecting some chosen cells while keeping applying current in the other ones,
which is particularly useful during cold-start experiments (section 3.4).
2.3.5
Temperature control
The thermal management described hereafter aims to permit a precise control of the cell temperatures. In particular, experiments with identical cell temperatures must be possible.
The external housings are maintained at the desired temperature using a heating liquid
(”Heating liquid” on Figure 2.16 and Figure 2.18), namely water in normal operation or water
with antifreeze in cold-start experiments. Although this solution ensures a homogeneous temperature of the external housings, it does not guarantee a precise control of the cell temperatures.
This is due to the thermal resistances existing between the cell and the external housings, in
the gaskets and insulation materials, as well as to the possibly different heat production rates
of the cells.
To solve this issue, additional heaters are placed on each flow field of each cell (”Cell heater”
on Figure 2.16). They are composed of electrical heating resistances connected to a copper
area acting as a heat distributor (Figure 2.18). A small-size PCB is used as substrate for
these components. Moreover, it also supports a Pt1000 resistive element for the temperature
measurement of the flow field. As previously, 2 probes are used for the heating current collection
and 2 other probes are used for the voltage signal of the temperature measurement (Figure 2.17).
They contact the small-size PCB by means of gold-plated copper pads. On the other side, they
65
2. DIAGNOSTIC TOOLS
are contacted to the main PCBs located in the external housings. The heating power is regulated
in closed loop control. Each flow field temperature is controlled independently to avoid important
temperature differences in the cell.
Figure 2.18: Thermal management (case of over-humidification: cell colder than housings).
Finally, the possibility of performing experiments under full and over-humidification conditions is taken into account. In full-humidification, this requires that the housing temperature
must be higher than the cell temperature, so as to condense water in the cell itself and not
in the housing. For over-humidification, the same requirement holds. If the whole quantity of
water would condensate and stay in the housing, the condition in the cell would be fully but
not over-humidified. So as to cool down the flow fields without impacting the heating of the
external housings, a gas (”Cooling gas”) is circulated through the channel shaped by the current
collectors of the flow fields (3D detail on Figure 2.18).
The whole thermal management set-up was validated, under operation, by imposing a same
temperature for each flow field whereas keeping the external housings at a higher temperature.
2.3.6
Materials
External housings are made of aluminum to get high thermal conductivity, good mechanical
stiffness and easy machinability. A passivation layer made by anodizing is realized to ensure
chemical stability and electrical insulation. The cell flow fields are made of aluminum for the
same reason and are coated with gold to guarantee a good chemical stability and a low contact
resistance. The cell spacer is made of stainless steel to limit the deformation. An insulation
Kapton sheet is placed on the spacer to ensure the electrical insulation between the two flow
66
2.3 Multi-cell
fields. Cell gaskets are made of PTFE for the excellent chemical stability of this material.
Moreover, it presents the major advantage of having a low attenuation to neutrons due to the
absence of H atoms in its composition.
2.3.7
Compression rate
The compression rate is an important parameter that must be precisely controlled. Firstly, it
determines the electrical and thermal resistances between the gas diffusion layer (GDL) and the
adjacent constituents. Secondly, it fixes the porosity of the GDL, which in-turn influences the
distribution of oxygen and water and consequently the performance of the cell. Two options can
be considered to control the compression rate of a cell: a stress control or a strain control. In
case of a small-size cell, the stress control does not result in a precise compression because the
PTFE gasket represents a significant part of the compression force. The direct control of the
strain is therefore more appropriate. In the present design, spacers (”Spacer” in Figure 2.16) of
known thickness are mechanically contacted to the two adjacent flow fields by tightening the two
flow fields together. Various compression rates can be obtained with different spacer thicknesses.
2.3.8
Alignment
In in-plane imaging configuration, the constitutive layers of the cell (GDLs, MEA) are positioned
in a plane parallel to the beam direction. It is therefore crucial to align the cell correctly in
this plane to avoid any blurring [38]. In practice, a correct alignment can be easily achieved for
a single cell if the rotation table positioning system is sufficiently accurate, as it is the case in
the ICON beam line. For the multi-cell, the difficulty consists in correctly aligning the 6 cells
relatively to each others so as to perform simultaneous imaging. The following solutions were
realized to this purpose.
One surface of the external housing, machined with a fine flatness tolerance, is chosen as
reference and serves as contact surface for the cells (Figure 2.19). The housing in which this
surface is machined must present a high stiffness to limit its deformation. This housing is chosen
to be on cathode side. The anode side block is thinner, so as to allow a close detector-to-sample
distance in through-plane imaging mode (subsection 2.3.10).
To achieve gas tightness, o-ring gaskets are used on the reference side whereas thicker flat
gaskets are used on the other side. When tightening the external housings, the thick flat gasket deforms more than the o-rings. The o-ring is compressed in the throat and the cell is in
mechanical contact with the reference surface. Thanks to thickenings made on the cell surface
(Figure 2.19), the contact points between the flow field and the reference surface are precisely
adjusted, avoiding this way an uncertain positioning due to a non-flat surface. On the opposite
67
2. DIAGNOSTIC TOOLS
H2
Flat gasket
Neutron
beam
Rotation
axis
External
housings
MEA
Flowfields
Reference surface
O-Ring
Air
Thickenening
Contact point
Figure 2.19: Alignment set-up: adaptation to cell deformations.
H2
Flat gasket
Air
O-Ring
Figure 2.20: Alignment set-up: adaptation to various sizes of cells.
side of the cell, the important deformation of the flat gasket compensates for any possible deformation of the cell itself (Figure 2.19) or for differences of cell thicknesses (Figure 2.20), which
is a requirement for imaging cells with different compression rates. The radiograms presented
show that a correct alignment of the cells is achieved (Figure 2.21).
2.3.9
Flow field design
Three types of flow field design have been realized so far (Figure 2.22).
In usual flow fields operating under air, the air is supplied by a convective flow in the channels,
from which the O2 needed by the reaction is transported mainly by diffusion throughout the
GDL up to the MEA. In such a structure, ribs are defined by the flow field surface that is in
contact with the GDL.
If the channels and ribs widths are similar and for the dimension range presented here
(mm domain), the diffusive mass flow of O2 and according to it the current density can feature a
68
2.3 Multi-cell
Figure 2.21: Alignment verification radiograms.
strongly inhomogeneous distribution [11, 12]. In a dry structure, this originates from the different
path lengths and porosities existing between the channel and the rib regions. It results in poorly
supplied rib areas compared to channel areas. In other words, a two-dimensional pattern of gas
concentration is obtained in the GDL with such a flow field, which is consequently denominated
”2D” (Figure 2.22).
If the rib width is small and the channels are wider than the ribs, the diffusive flow pattern
is expected to tend towards a more homogeneous distribution, leading to a mono-dimensional
concentration gradient in the GDL, such as in the ”1D” design.
At last, the gas diffusion effect can be suppressed by using an interdigitated structure [65],
called ”0D” in the following. Indeed, if the gas channels are interrupted along the gas flow
path, the pressure gradient obtained forces the gas to flow through the GDL by convection, so
that the O2 supply to the CL does not (or less) rely on diffusion. Contrary to the structure
presented by Kramer et al. [66] where channels were 1 mm wide and 1 mm deep, the channels
presented here are 0.15 mm wide and 0.55 mm deep, so that the structure can be referred to as
micro-interdigitated structure. By employing channels with reduced width compared to the ribs,
the possible influence of diffusive flows in the center of the channels is diminished. Moreover,
a more homogenous compression rate of the GDL is achieved. The channels are machined in
the flow fields by die sinking. This method allows for deeper and perpendicular channels than
in the structure presented by Boillat et al. [41], for which the process of Seyfang et al. [67]
led to trapezoidal channels, with a typical depth of 0.1 mm and a width of 0.1 mm at the top
side [68]. The use of deeper channels allows reducing the pressure drop in the flow field and
in consequence mitigating any change of the parameters along the flow field path, which is the
definition of the differential cell (section 1.6).
The denominations ”2D”, ”1D” and ”0D” will be used in the following to designate these
flow field types and by extension the type of distribution expected in the porous media. This
represents of course a simplification since changes of parameters across the CL and the MEA
can still induce gradients, even in a ”0D” structure.
69
2. DIAGNOSTIC TOOLS
0D
1D
2D
(micro-interdigitated)
Gas distribution
Through-plane
Rib
Channel
In-plane
MEA
0.55
0.5
GDL
0.15 0.55
0.2 0.75
1
1
Convective flux
Diffusive flux
Dimensions in mm
Figure 2.22: Flow field designs.
2.3.10
Detector
As mentioned, an elongated tilted detector was constructed by the members of the NIAG group
that allows imaging simultaneously all active areas of the 6 cells. Thanks to the optical field
of view available, a through-plane scintillator was positioned next to the in-plane scintillator
(Figure 2.23). The multi-cell is fastened on the rotating table at such a location that a change
from in-plane to through-plane can be realized by a simple rotation without translation. The
external block that is close to the detector in through-plane mode, as mentioned before, is
thinner than the other housing so as to keep a minimal distance between the multi-cell and the
detector. As a result of this optimization, the multi-cell can be rapidly changed from in-plane
70
2.3 Multi-cell
Top view
3D view
Scintillator for
in-plane imaging
Multi-cell
Detector box
Neutron
beam
Neutron
beam
Scintillator for
through-plane imaging
Figure 2.23: Multi-cell positioning configurations (in-plane or through-plane) in front of the
corresponding scintillator.
to through-plane configuration or inversely.
2.3.11
Summary and outlook
A new set-up was presented, called multi-cell, that permits the simultaneous electrochemical
testing and neutron imaging of six differential cells. The prerequisite of this development was
the construction by the members of the NIAG team of a new tilted detector with a field of view
extended in one direction. The main improvements brought by this set-up are the followings.
Firstly, it allows for a more efficient use of the beam time. Secondly, it represents a suitable test
platform for comparing, under identical operating conditions, various designs of cells. In this
work for instance, it will be used for deuterium labeling of different samples (section 3.1), GDL
samples with or without MPL (section 3.2), and various flow field designs (section 3.3).
Future improvements should be realized to adapt this set-up to cold-start experiments, meaning by developing an anti-freeze device similar to the one presented later for the single-cell set-up
(section 3.4). The integration of a dedicated device to measure the water transport inside the
cell based on the method of Boillat et al. [50] for each cell would constitute an ambitious but
advanced characterization set-up. Since it allows for an independent control of each cell, the
investigation of temperature gradients between the two flow fields would be another idea.
The efficient comparison of cell designs opens the way for a large spectrum of applications,
such as the comparison of GDL materials, compression rates, membrane characterization, flow
field material, etc. In particular, the electrode characterization by means of ”0D” flow fields
is thought to be a specifically interesting study. More detailed comments will be given in the
result chapters of this work (section 3.2 and section 3.3).
71
3
Results
3.1
3.1.1
Membrane transport properties
Introduction
f it
In the present chapter, the estimations of the parameters DM
and k f it defined previously (sub-
section 2.1.7) are presented. For the sake of simplicity, the superscript ”fit” will be omitted in
this chapter. Moreover, based on the comments about gas mixtures made before (subsubsection 2.1.3.1), the exchange rate will be called kW for the transitions TN2 and THe (Table 2.1)
where only water vapor is involved (subsection 3.1.4), whereas the total interfacial exchange
k will be considered for the transition TH2 where hydrogen also participates to the exchange
(subsection 3.1.5).
3.1.2
Experimental
Different operating and design parameters were tested during three measurement campaigns
(Table 3.1). All experiments presented in this chapter were realized on 1-sided cells. The
samples consisted of a Nafion 117 membrane (N117), either without or with CL, in which case it
contained a Pt loading of either 0.15 or 0.5 mgPt /cm2 (Paxitech SA, France). The Pt/C (mass
of platinum per mass of carbon) ratios of the CL were chosen so as to have the same carbon
content. The flow field patterns described previously (Figure 2.22) were used and are expected
to show different effects of the diffusivity. Two types of GDL were employed. The 25 type GDL
is chosen for its higher porosity compared to the 24 type used elsewhere in this work. The first
letter after the number indicates the PTFE content of the substrate (A: 0 % wt; B: 5% wt)
whereas the second letter indicates the presence or not of a MPL (A: without MPL; C: with
MPL). While the use of the 25AA GDL aims at getting a high diffusivity, the 24BC GDL is
employed to explore the effect of diffusive limitations. Being the reference material in this work,
the 24BC GDL will be compared with other experiments. A more precise description of the
73
3. RESULTS
GDL types can be found in the manufacturer’s documentation [69]. The varying parameters
investigated are the following: the relative humidity (RH) of the gas mixture, the temperature
of the cell, the mass flow rate of gas and the pressure. In the 2009 campaign, a fixed mass flow of
45 Nml/min was applied. The upgrade from the single-cell to the multi-cell set-up (section 2.3)
implied changing the imaging set-up from the Micro to the Midi set-up with the corresponding
changes for the imaging parameters (subsection 2.3.10).
Design
parameters
Measurement
campaign
2009
2010
Operation
parameters
Experimental
set-up
Pt loading
2
[mgPt/cm ]
Flowfield
GDL
no CL
0D
25AA
0.15
0D
25AA
RH
Single-cell
0.50
0D
25AA
Gas flow
= 45 Nml/min
Micro set-up
0.50
0D
25AA
no CL
0D
25AA
0.15
0D
25AA
0.50
0D
25AA
0.50
0D
24BC + 3 x 24BA
0.50
1D
24BC
0.50
2D
24BC
(ANDOR™ camera)
Gas flow
Gas flow
2011
Pressure
RH
Multi-cell
Midi set-up
(PCO™ camera)
Table 3.1: Summary of experiments realized with deuterium labeling.
If not specified, the ”reference” condition will be: T = 70°C, RH = 80 %, p = 1.2 bar and
a flow rate of 400 Nml/min. The cells were mounted with the corresponding spacers to target
a compression of 30%.
3.1.3
Diffusivity
All experiments presented in this chapter that concern the evaluation of the diffusivity DM were
realized using the ”0D” (micro-interdigitated) flow field (subsection 2.3.9). As discussed, this
structure is expected to allow reducing as much as possible the diffusive losses. The reference
74
3.1 Membrane transport properties
sample used for the diffusivity results presented hereafter was a N117 membrane with a Ptloading of 0.5 mgP t /cm2 with a 25AA GDL, both for the single-cell set-up in 2010 and for the
multi-cell set-up in 2011.
3.1.3.1
Gas flow dependency
As a preliminary result, the effect of the gas flow rate on the estimation of DM is presented (2010
campaign, micro set-up). Being a bulk property, the diffusivity DM is not supposed to depend
on the flow rate. The present experiment can therefore be used as a validation experiment.
On the following graph (Figure 3.1) the estimation of DM is plotted for different gas flows.
For each flow, both directions of the isotopic change (0 → 1) and (1 → 0) are shown. The test
was realized once by applying the THe (left-hand graph) transition and once by applying the
TH2 transition (right-hand graph) defined earlier (subsubsection 2.1.3.2). On the upper graphs,
the measured isotope fraction F and the fitted profiles are presented for four conditions.
1
a)
0.8
F[-]
b)
Data
Fit
c)
Data
Fit
Data
Fit
x [μm]
Data
Fit
20
40
60
80
100
120
140
160
180
0.6
0.4
0.2
0
0
6
12
18
t [s]
24
30
0
6
12
18
t [s]
24
30
0
6
12
18
t [s]
24
30
0
6
12
18
t [s]
24
30
2.0
1.9
DM [cm2 s-1] · 10-5
1.8
0Y1
0Y1
1Y0
1Y0
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0
100
200
300
400
500 0
100
200
300
400
500
Gas flow [nml/min]
Gas flow [nml/min]
Figure 3.1: Gas flow dependency of the diffusivity estimation (”reference” condition). Left: THe ,
right: TH2 .
Without commenting yet the differences appearing between the two directions of the transition (0 → 1 or 1 → 0), it can be observed that the estimation of DM for the THe transition
presents a relatively high scattering of data at low flows, while the dispersion tends to be nar75
3. RESULTS
rower at higher flows. This can be explained by the limitation introduced by the gas supply at
low flows. If the flow of H atoms supplied to the membrane is low compared to the diffusive flow
inside the membrane bulk, then the change of the isotope fraction in the gas phase affects the
isotope fraction in the membrane very slowly, which results in a small change of amplitude of
F during the period elapsed (30 s) before the next step is applied. Moreover, such a limitation
leads to a very low spatial gradient of F , as it can be observed (Figure 3.1.a). When the next
step is performed, the effect of a slow change also implies that the isotope fraction F is not
equilibrated with the one of the gas phase, in which case it would equal 0 or 1 depending on
the step direction. On the contrary, the value F oscillates within a restricted range, in the
present case 0.55 to 0.6. This corresponds to a mean value of 0.575 approximately and not to
the expected value of 0.5, which can be attributed to an isotopic effect on the interfacial rates,
as it will be confirmed later (subsection 3.1.4). Since the estimation of the diffusivity depends on
the spatial gradient of F , a very low spatial gradient will lead to a lower accuracy of the fitting
procedure. This can be solved by increasing the flow. As it can be observed, higher flows imply
bigger spatial gradients of F in the membrane (Figure 3.1.b), making the fitted parameter DM
more reliable in this case. Another way to solve this issue is to use H2 as carrier gas instead of
He. If a CL is present for the dissociation of H2 into H atoms, the flow of H atoms will also be
increased in that case and the estimation of DM , even for low gas flows, will gain in reliability
(Figure 3.1.c) by converging towards a stable value. At higher flows, the evolution of F in the
membrane can be considered to be essentially limited by the diffusion and not by the incoming
flow, as it can be seen by the similar profiles obtained with THe (Figure 3.1.d) and with TH2
(Figure 3.1.b).
Based on these results, it can be argued that sufficiently high flows are necessary to estimate
DM with reliability. In the following, the reference flow used will be 400 Nml/min. It can
be noticed on the experimental summary (Table 3.1) that a quite low flow (45 Nml/min) was
used during the 2009 campaign. The reason for that was to mitigate the pressure drop (and
therefore the change of RH) in the 0.1 mm deep channels available at that time [41]. As already
detailed (subsection 2.3.9), they were replaced by the 0.55 mm deep channels in the 2010 and
2011 campaigns, which allowed using higher gas flows while keeping a moderate pressure drop.
3.1.3.2
Experimental statistics
Having determined the minimum gas flow needed to estimate DM , the repetition of an experiment at the ”reference” condition is presented to illustrate the dispersion of the estimation, this
with the two different imaging set-ups employed (Figure 3.2).
The data gathered with the micro set-up exhibit a quite low scattering (Figure 3.2, lefthand side). The remaining deviation from the mean value can be attributed, for the worst
76
3.1 Membrane transport properties
5
2010, Micro-setup
2011, Midi-setup
1.7
4.5
1.6
4
1.5
DM [cm2 s-1] · 10-5
DM [cm2 s-1] · 10-5
1.8
1.4
1.3
1.2
1.1
3.5
3
2.5
2
1.5
1
1
0.9
0.5
0.8
0
1
0Y1
2
3
4
5
Experiment number [-]
Average
1Y0
6
7
50
Average
0Y1
100
150
Experiment number [-]
Median
1Y0
200
Median
Figure 3.2: Diffusivity estimation for repeated experiments (”reference” condition: RH = 80 %,
T = 70°C, flow = 300 and 400 Nml/min).
cases, here DM,min = 1.1·10−5 cm2 /s and DM,max = 1.7·10−5 cm2 /s, to experimental error
of the measurement rather than to a real variation of the diffusivity provoked by a change of
experimental conditions. Indeed, it will be observed hereafter that a change of diffusivity of
∆DM = 0.2·10−5 cm2 /s would represent a change of relative humidity of ∆RH = 10%, which is
assumed to be higher than the error range inherent to the RH control. Therefore, some imaging
noise or effects of inappropriate border conditions may explain these extreme values. In case of
the more narrow variations, experimental variation of parameters can play a role, too.
These results, together with a check on the quality of the data fittings realized, tend to
validate the reliability of the estimations based on the 2010 set-up compared to the 2011 set-up,
for the reasons developed in the following. The numerical values of the mean value and the
deviation are given hereafter (Table 3.2).
The difference in average between the (1 → 0) and the (0 → 1) transitions tend to demonstrate a slight isotopic effect on DM . However, the median values obtained with the midi
set-up seem to contradict this. Moreover, it will be observed in other experiments that no clear
trend can be identified on DM between the two directions (subsubsection 3.1.3.3 and subsubsection 3.1.3.4). The isotopic effect seen here cannot be treated as a general behavior but must
rather be attributed to experimental error.
Nevertheless, it is actually known that various physico-chemical properties differ between
1H
2O
and 2 H2 O [8], among which the viscosity, the melting point or the surface tension. The dif-
fusivity of the proton DM being partly dependent on the vehicular transport of water molecules
(subsection 1.4.1), a difference of self-diffusion is expected between a medium consisting of 1 H2 O
and another made of 2 H2 O. Besides this, since the Grotthuss mechanism can also participate
77
3. RESULTS
in the diffusivity DM by the rotation of the molecules, an isotopic effect on the hydrogen-bond
might also affect the measured diffusivity depending on the direction of the transition. At last,
the participation in the diffusivity of hydration shells around the HSO3 groups can also induce
modifications due to a change of the hydrophilic properties of the molecules present.
Even if only the species of water 1 H2 O and 2 H2 O were present in the system, which is not
the case due to the presence of 1 H2 HO and other charged aqueous complexes (H3 O+ , H2 O+
5 or
H4 O+
9 [8]), the assumption that the value of DM would be independent of F (Equation 2.15)
should be suppressed in the model in order to take into account a local diffusivity varying with
the local isotopic fraction. On the contrary, the use of a constant value of DM , spatially and
temporally, probably results in mixing the contributions of different phenomena.
Since the values DM (1 → 0) and DM (0 → 1), assumed to be independent of F in our model,
are not expected to furnish a precise knowledge of the mechanisms occurring, and because the
isotopic effect observed is quite small in regard of the change of DM provoked by the change
of the operating conditions discussed below (temperature and relative humidity), the average
value will be used in the following and will serve, if not specified, as the indicator of the proton
diffusivity:
DM =
DM (1 → 0) + DM (0 → 1)
2
(3.1)
The data acquired with the multi-cell combined with the midi set-up are presented on the
right-hand side of the graph (Figure 3.2). Compared to the previous case, a large scattering is
observed. Moreover, the histogram of this data distribution (Figure 3.3) exhibits an asymmetric
behavior. These results are to be attributed to the poor quality of the images recorded with the
camera used in this setup (PCO camera). By comparing the radiograms provided by the two
imaging setups (Figure 3.4), it can be observed that the midi-setup image shows a higher level
of noise. In the membrane, the grey level appears strongly inhomogeneous and some clusters of
pixels with the same intensity value can be observed. Moreover, the grey level of some pixels
was sometimes blocked for a while to a given value. This provokes the presence of a nonstatistical noise that, contrary to the statistical noise, cannot be removed after cumulating and
merging the sequences (as explained on Figure 2.3). In consequence, the isotope fraction profiles
measured Fexp are disturbed and the estimation of DM is not only noisy but also affected by
some bias. The median value is therefore preferred to the average value as an indicator of the
real DM value for this set of experiments. By calculating the median values, one remarks that
the isotope effect identified earlier is now inverted, namely that the (1 → 0) transition shows
a higher value. Again, this tends to show that no clear isotopic effect on DM can be observed
with the method proposed.
78
3.1 Membrane transport properties
50
45
40
Frequenccy [-]
35
30
25
20
15
10
5
0
0
0.5
0Y1
1
1.5
2 2.5 3 3.5
2
-1
-5
DM [cm s ] · 10
1Y0
Median
4
4.5
5
Median
Figure 3.3: Distribution of the diffusivity estimation for repeated experiments (”reference” condition: RH = 80 %, T = 70°C, flow = 300 and 400 Nml/min) in the case of the midi set-up.
2011, Midi-setup
2010, Micro-setup
Figure 3.4: Comparison between the micro set-up and the midi set-up for a similar sample (N117
membrane under 1 H2O in steady-state).
Based on the results presented, the following deviation estimator can be calculated (average
absolute deviation):
Nexp
s=
1 X
m
|(DM,i − DM
)|
Nexp
(3.2)
i=1
where
m
DM
is the mean value of the data set for the micro set-up and the median value for the
midi set-up. By using this indicator to calculate the relative deviation:
σ=
s
m
DM
(3.3)
the following values are found for the transitions (0 → 1) and (1 → 0) (Table 3.2). The
deviation presented for the ”Mean” case in this table is found by using the average diffusivity
defined (Equation 3.1) in the above expressions (Equation 3.2 and Equation 3.3).
79
3. RESULTS
2010, Micro set-up
2
-1
-5
DMm [cm s ] · 10
σ [%]
2011, Midi set-up
0Y1
1.5
1.4
1Y0
1.3
1.8
Mean
1.4
1.6
0Y1
5.1%
35%
1Y0
5.7%
32%
Mean
3.2%
22%
Table 3.2: Diffusivity values for two different imaging set-ups (”reference” condition: RH = 80 %,
T = 70°C, flow = 300 and 400 Nml/min).
3.1.3.3
Relative humidity dependency
The effect on DM of the relative humidity of the gas is reported (Figure 3.5) and discussed in
the following, based on the data from the 2010 campaign realized with the micro set-up.
2.0
1.8
0Y1
1.6
1Y0
Average
DNMR
DM [cm2 s-1] · 10-5
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
20
30
40
50
60
70
RH [%]
80
90
100
Figure 3.5: Relative humidity dependency of DM (T = 70°C). Values of DNMR taken from [70]
.
Similarly to results reported in the literature with other methods [26–28, 70] for D(λ) and
because the relative humidity can be correlated to the membrane hydration λ [25, 28], the DM
coefficient can be observed to increase with the relative humidity. No clear isotopic effect can be
seen in this case. The diffusion coefficients DNMR measured by Kreuer et al. [70] with pulsed-field
gradient nuclear magnetic resonance (PFG-NMR) are reported on the same graph and appear
to be systematically lower than ours. Moreover, their data exhibit a slight exponential behavior.
Since the coefficient DNMR also refers to the self-diffusion of the proton, the parameter measured
with NMR should be inherently the same as with our method. These discrepancies cannot not
be justified either by the error range estimated before (Table 3.2). A source of discrepancy
could be the evaluation of the λ(RH) relationship used here [25] to convert the data DNMR (λ)
80
3.1 Membrane transport properties
into RH values. Moreover, the presence of different isotopic species in the membrane can also
induce an error compared to the λ(RH) relation measured for normal water. The use of the
direct quantification of λ by neutron imaging should be first validated by dedicated experiments
in the future to be judged reliable. Although the λ values is not known with precision, it must
be emphasized again that the method presented here for the estimation of DM does not require
knowing the λ value, since the value cH (Equation 1.46) does not appear in the bulk diffusion
(Equation 2.17). This is not the case for the NMR measurements where a calibration must be
realized to estimate λ [70]. Whereas possible differences of sample preparation are also possible,
inherent differences of the methods must still be considered.
The values reported here also seem slightly higher than the proton diffusion coefficient Dσ
estimated by the Nernst-Einstein equation (Equation 1.47) and reported by Ocha et al. [28]. As
already discussed (subsection 1.4.1), since the Nernst-Einstein diffusivity represents the proton
diffusion equivalent to the ionic conductivity measurement, it should fix the upper limit to the
value measured with neutrons. However, the method of Ocha et al. also relies on an estimation
of λ, which can introduce some error, for example due to swelling effects that could change the
λ(RH) behavior expected.
3.1.3.4
Temperature dependency
The effect of the temperature on DM is plotted hereunder (Figure 3.6), still based on the data
from the 2010 campaign realized with the micro set-up.
T [°C]
70
60
55
50
0.3
2.0
1.8
Average
0Y1
1.6
0.2
1.2
ln (DM [cm2 s-1] · 10-5)
1Y0
Average
DNMR
1.4
DM [cm2 s-1] · 10-5
65
1.0
0.8
0.6
0.1
0
-0.1
0.4
Linear
interpolation
Ea = 190 meV
Ea,NMR = 165 meV
0.2
0.0
40
50
60
T [°C]
70
80
-0.2
2.9
2.95
3
1000/T [K-1]
3.05
3.1
Figure 3.6: Temperature dependency of DM (RH = 80%). Values of DNMR and Ea,NMR from [70].
An increasing value of DM with the temperature is found, similarly to the NMR estimations
[28, 70], but again with higher values than for the case of DNMR . No clear isotopic effect can be
81
3. RESULTS
identified. By considering an Arrhenius law for the dependence of DM on the temperature:
0
DM = DM
e
−Ea
RT
(3.4)
the activation energy of the process Ea can be found by transforming the above equation into:
0
ln(DM ) = ln(DM
)−
Ea
RT
(3.5)
The Arrhenius plot corresponding to the logarithm expression is plotted on the right-hand graph
(Figure 3.6). Since Ea = ∆Ga and ∆Ga = ∆Ha - T ∆Sa , one can also transform this expression
into:
∆Sa ∆Ha
∆Ha
0∗
−
= ln(DM
)−
(3.6)
R
RT
RT
This means that the energy estimated represents the diffusion activation enthalpy, whereas the
0
ln(DM ) = ln(DM
)+
diffusion activation entropy is included in the pre-exponential factor. The value obtained here
is slightly higher than the one found by NMR. Following the comments of Kreuer et al. [70]
who states that the activation enthalpy denotes the strength of hydrogen bonds, then the result
obtained here would reflect stronger bonds than for the NMR, where such a numerical value was
obtained for a much lower λ value (λ ≈ 3.5).
3.1.4
Exchange rate with water vapor
As presented in the method description (section 2.1), the evaluation of the exchange rate kW
requires to know the value DM . In light of the results presented about the measurement of DM
(subsection 3.1.3), the average value of DM (Equation 3.1) is chosen for the estimation of kW
since it allows reducing the experimental error and because no clear isotopic effect was identified.
It must be emphasized that this choice will not mask or create an artificial isotopic effect on
kW , since the maximal isotopic effect on DM can be estimated to ∆DM = 14% (Figure 3.2) and
would give, due to the sensitivity results (Figure 2.8), a deviation of around ∆kW = 10%, which
is below most of the differences identified hereafter.
As discussed, the DM values estimated with the micro set-up are judged more reliable than
m = 1.6 · 10−5 cm2 /s
the ones evaluated with the midi set-up. Therefore, the median value DM
calculated (Table 3.2) was chosen for the estimations of kW realized with the midi set-up, since all
experiments were realized at the ”reference” condition, meaning at the same RH. For sensitivity
reasons again, this will not affect the trends that will be presented.
The experiments presented in the following that concern the variation of the gas flow, the
relative humidity and the temperature (subsubsection 3.1.4.1, subsubsection 3.1.4.2, subsubsection 3.1.4.3) were realized by using the ”0D” flow field, a N117 membrane with a 0.5 mgP t /cm2
and a 25AA GDL, both for the single-cell set-up in 2010 and the multi-cell set-up in 2011. On
the contrary, the study dedicated to the structural effects (subsubsection 3.1.4.4) was performed
only with the multi-cell in the 2011 campaign.
82
3.1 Membrane transport properties
3.1.4.1
Gas flow dependency
The estimation of kW is realized under different gas flows consisting in a mixture of water vapor
and He (Figure 3.7).
6
6
0Y1
4
3
2
2010, Micro-setup
1Y0
Average
4
3
2
2011, Midi-setup
1
1
0
0Y1
5
1Y0
Average
kW [mol cm-2 s-1] · 10-5
kW [mol cm-2 s-1] · 10-5
5
0
0
100
200
300
Flow[Nml/min]
0
400
100
200
300
Flow[Nml/min]
400
Figure 3.7: Gas flow dependency of kW estimation (”reference” condition, THe transitions).
It can be observed that the values estimated with the micro set-up demonstrate a continuous
trend, which is not the case for the midi set-up, which can be again attributed to misestimations
of the real parameters in the latter case due to noisy data.
The exchange rate kW exhibits a clear dependence on the gas flow, contrary to the flow
dependency of DM presented before (subsubsection 3.1.3.1), in the sense of an increased value
at higher flows, as explained now.
A situation where the exchange rate kW would be the rate limiting transport of H can be
reached only if no other limitation of H transport occurs. By varying the flow, and therefore
the flow of H atoms supplied by the water vapor flow to the membrane, this behavior can
be investigated. If the diffusion of water vapor can be neglected in He, which will be studied later, then the gas supply must be limiting at low flow rates. The present results confirm
these expectations. An artifact due to uncertainties on the evaluation DM at low flows (subsubsection 3.1.3.1) can be rejected, since the magnitude of the change in kW observed here is
higher than the small deviation on kW estimated by the sensitivity analysis (roughly ±∆ = 20%
according to Figure 2.8).
Contrary to the estimation of DM , a systematic isotopic effect seems to happen in the case
of the micro set-up between the (1 → 0) and the (0 → 1) directions. This will be further
commented in the temperature dependency experiment (subsubsection 3.1.4.3).
Based on these comments, a simple transport model can be proposed to introduce the limitation of the gas supply on the total exchange rate measured. The H flow supplied by the water
83
3. RESULTS
vapor content of the gas can be calculated as follows:
kG = ṅH = 2 · ṅ H2 O = 2
p H2 O
V̇ n
·
n
v
p − p H2 O
(3.7)
where V̇ n is the normal volumetric flow of dry gas controlled by the MFC, v n = 22.4 l/mol is the
molar volume of ideal gas in normal conditions, p H2 O = RH · psat is the partial pressure of the
water vapor and p is the absolute pressure of the gas. It can be figured out that the gas occupying
the GDL acts like a reservoir that exchanges H-atoms with the cell flow. Such a representation
allows for modeling this gas buffer in a similar way as the membrane. It is therefore coherent to
compare this flow with the interfacial exchange of the membrane as follows. The flow resistances
of the maximal interfacial transfer and of the gas supply are respectively defined as:
max
RW
=
1
and RG =
max
kW
1
kG
(3.8)
The superscript max refers of course to an ideal condition (i.e. in absence of limitation of the gas
max will be reached when R
supply) and then the value RW
W will be minimal. It can be supposed
that the total limitation measured RW can be expressed by putting these two resistances in
series:
max
RW = RW
+ RG
(3.9)
In terms of exchange rates, this expression is equivalent to:
kW =
1
kmax +
1
W
(3.10)
1
kG
The calculated resistances using the values presented before (Figure 3.7) are reported here
(Figure 3.8).
3
2010, Micro-setup
RW [mol-1 cm2 s1] · 105
2.5
2011, Midi-setup
2
RW = RG
1.5
1
0.5
0
0
0.5
1
1.5
-1
2
2
1
2.5
3
5
RG [mol cm s ] · 10
Figure 3.8: Gas flow dependency of kW expressed in terms of equivalent resistances (same data as
(Figure 3.7).
84
3.1 Membrane transport properties
It can be observed that the linear regressions fit well with the experimental data, both for
the micro and the midi set-up. In addition, the slopes of the linear regressions are identical to
1, meaning that the model proposed seems to be appropriate. In particular, since the H supply
from the gas must be the upper limit for kW , it is theoretically not possible that the experimental
slopes is inferior to 1. The present result confirms this, meaning that, if the RG values can be
considered reliable, the kW values are statistically not over-estimated. On the other hand, since
the experimental slopes are not bigger than 1, it can be deduced that no under-estimation is
made either. The consequence of these results is the validation of the coefficient cH chosen.
Indeed, since the kW is proportional to cH (Equation 2.63), an over-estimation of the latter
would have led to an over-estimation of kW and therefore to a slope lower than 1 in this graph.
A too low value for cH would of course have had the opposite effect.
max corresponds to the y-intercept. This
On this graphical representation, the value of RW
value can be interpreted as the closest value to the real interfacial exchange rate, if no other
limitations are taking place. Values of kW = 9 · 10−5 and 4.6 · 10−5 [mol cm−2 s−1 ] are obtained
for the micro and midi set-up respectively. As the general trend of the RW values is judged
max between the two setups
reliable for both set-ups due to the slope of 1, the differences of RW
can be attributed to differences in the samples (change of ionomer/gas interface area) or to other
differences of limitations (gas diffusion in the electrode, H diffusion in the electrode ionomer).
These values can be compared to the values available for pure water. The expression of kW
for pure liquid water is given by the Hertz-Knudsen equation [71]:
kW = γ p
psat
2πm H2 O kB T
(3.11)
where γ is the correction coefficient to the theoretical maximal rate (γ ranging from 1 to 10−2
[-] according to [71]), m H2 O is the molecular mass of H2 O, kB is the Boltzmann’s constant and
T the temperature. At 70°C, theoretical values of 5.5 · 10−2 to 5.5 · 10−4 [mol cm−2 s−1 ] are
found. The values estimated in the present work for Nafion are therefore clearly lower than the
values of pure water. One possible explanation is that limitations in the CL (in the gas phase
or in the ionomer) detrimentally affect the value kWmax compared to pure water. This kind of
limitations will be further discussed in (subsubsection 3.3.3.3). Another possibility is that the
ionomer material behaves differently against vapor than pure water does. For instance, it can be
hypothesized that Nafion surface may be rather hydrophobic in presence of water vapor, which
could change the interfacial properties compared to the hydrophilic surface exposed in presence
of liquid water [22]. At last, it can also be assumed that the presence of strongly bonded water
(non-freezing water) in the case of Nafion, as already observed by DSC studies [29, 72], might
slow down the exchange process compared to normal hydrogen-bonds in liquid water.
85
3. RESULTS
3.1.4.2
Relative humidity dependency
In this sub-chapter, the effect of RH on the exchange rate kW is presented (Figure 3.9).
1.6
8.0
0Y1
1.5
7.0
1Y0
Average
1.4
ΔTdew = 5°C
6.0
Ratio [-]
kW [mol cm-2 s-1] · 10-5
9.0
5.0
4.0
1.3
1.1
3.0
2.0
1
1.0
0.9
0.0
20
ΔTdew = 2°C
1.2
30
40
50
60
70
RH [%]
80
90
100
0.8
20
30
40
50
kW (0Y1)
p
1
H 2O
kW (1Y0)
p
2
H 2O
60
RH [%]
70
80
90
100
Figure 3.9: Relative humidity dependency of kW (T = 70°C). In the right-hand graph, the change
of humidity under 2 H2 O at constant dew point temperature was taken into account.
A clear dependency appears in form of increasing values of kW with RH. But the variation is
not linear as it could have been expected by the change of partial pressure in the Hertz-Knudsen
equation (Equation 3.11). Another information given by this result is the presence, as before,
of an isotopic effect on kW , namely that systematic higher values appear for kW (0 → 1) than
for kW (1 → 0).
The effect of the different saturation pressures between 1 H2 O and 2 H2 O has to be detailed.
In the present experiment, a constant dew point temperature Tdew was applied with a precision
of ∆Tdew so as to approach a constant RH value between the two step directions. This can lead
to the following error:
2H O
1H O
Tdew2 ± ∆Tdew = Tdew2
(3.12)
Based on heavy water saturation pressures from [73] and assuming a maximal error of ∆Tdew =
2°C, the following variation is estimated:
RH2 H2 O = (0.98 ± 0.1) · RH1 H2 O
(3.13)
In terms of change of partial pressure in the condition here (RH = 80%), the following variation
is obtained:
p2 H2 O = (0.91 ± 0.1) · p1 H2 O
(3.14)
The ratio found for the kW values is compared to the ratio of partial pressures (right-hand graph
of Figure 3.9). It can be observed that except for one condition at low RH, the ratio of kW is
systematically higher than the ratio of partial pressures:
p 1 H2 O
kW (0 → 1)
>
kW (1 → 0)
p 2 H2 O
86
(3.15)
3.1 Membrane transport properties
Furthermore, a deviation of ∆Tdew = 2°C is not sufficient either to explain the ratio found for
kW . A less probable error of ∆Tdew = 5°C would be needed to explain these differences.
Another effect that reinforces the discrepancy could be a slight drying out of the membrane
during the (1 → 0) transition, since the constant dew point temperature implies a lower RH
under 2 H2 O. Based on the λ(RH) relationship [25], a maximal change of ∆cH = 5% is estimated,
which should affect kW the same manner and is therefore not sufficient to explain the effect
observed.
Besides this, further comments can be made on the fundamental nature of the exchange. The
hydration equilibrium between the membrane and the saturated water vapor is reached when
the evaporation and condensation fluxes are equal, in other words when the net flux across the
interface is zero, as represented on the first scheme of the figure below (Figure 3.10.a). When
Base model
a)
b) Thermodynamic deviation
(partial pressures)
c)
Speculation
(kinetic deviation during transient)
Membrane
FW(t)
F(x,t)
FW(t)
F(x,t)
FW(t)
evap
kW, H O
F(x,t)
cond
kW, H O kW, H O
1
1
2
1
2
2
kW
kW, H O
evap
cond
kW, H O kW, H O
2
2
2
2
2
2
Figure 3.10: Interpretation of the isotopic effect measured on kW .
considering both isotopic species 1 H2 O and 2 H2 O, the interfacial exchange will be determined
by the Hertz-Knudsen equation (Equation 3.11), or a similar expression extended to Nafion,
meaning that the partial pressures of the respective species will fix the exchange rate of the
equilibria 1 H2 O(Nafion) 1 H2 O(g) or 2 H2 O(Nafion) 2 H2 O(g) . The corresponding equilibrium
will be reached when the evaporation equals the condensation for each species of the content. In
such a system, the exchange rates of each species would be proportional to their respective partial
pressure (Figure 3.10.b). In the experimental system considered here, even if the membrane is
equilibrated in humidity with the gas phase, a net flow of species is present during the transient
regime, meaning:
ṅ1 H,in 6= ṅ1 H,out
(3.16)
In our model, such a flow is described by using:
ṅ1 H,in − ṅ1 H,out = kW (FW − F (x = 0))
87
(3.17)
3. RESULTS
based on the assumption that the ratio of the evaporation/condensation flows are fixed only by
the ratio of isotopic fraction in the respective media (membrane or gas phase):
ṅ1 H,out
ṅ2 H,out
=
ṅ1 H,in
F (x = 0)
FW
and
=
1 − F (x = 0)
ṅ2 H,in
1 − FW
(3.18)
but the interfacial rates kW were supposed identical for all species (1 H2 O or 2 H2 O ) and for all
directions (evaporation or condensation). However, it has been shown in the literature [74, 75]
in the case of water that these ratio can strongly depend on the isotope fraction, which can
be attributed to shifts of frequencies associated with the molecular interactions of the surface
species. This leads in a general case to:
evap
kW,
1H
2O
evap
6= kW,
2H
2O
cond
and kW,
1H
2O
cond
6= kW,
2H
(3.19)
2O
Such a deviation would then serve as a prerequisite for the difference between the evaporation
and the condensation rates during the transient:
evap
kW,
1H
2O
cond
6= kW,
1H
(3.20)
2O
as represented on the scheme (Figure 3.10.c). This speculative argument would then support
the result obtained here, namely that kW (1 → 0) 6= kW (0 → 1). However, other experimental
misestimations must still be considered to put these results in perspective.
3.1.4.3
Temperature dependency
The effect of the temperature on kW is investigated hereafter (Figure 3.11).
T [°C]
70
60
55
50
2.5
9
8
0Y1
7
1Y0
Average
0Y1
ln(kW [mol cm-2 s-1] · 10-5)
kW [mol cm-2 s-1] · 10-5
65
6
5
4
3
2
2
1Y0
Average
1.5
1
Ea = 379 meV
Ea = 362 meV
0.5
1
0
0
40
50
60
70
80
T [°C]
2.9
2.95
3
-1
1000/T [K ]
3.05
3.1
Figure 3.11: Temperature dependency of kW (RH = 80%).
It is observed that the value of kW increases with the temperature. By plotting the evolution
of kW in function of the corresponding partial pressures over the temperature range (Figure 3.12),
88
3.1 Membrane transport properties
a linear behavior can be observed. However, no linear dependence on the partial pressure was
observed in the case of the relative humidity dependency discussed before (subsubsection 3.1.4.2),
thus the influence of other parameters on this behavior cannot be rejected.
kW [mol cm-2 s-1] · 10-5
9
8
0Y1
7
1Y0
Linear
6
5
4
3
2
1
0
50
100
150
200
pH O [mbar]
250
300
2
Figure 3.12: Partial pressure dependence of kW for different temperatures. The value p1 H2 O and
p2 H2 O are taken for the directions (1 → 0) and (0 → 1) respectively.
Again, an isotopic effect appears in terms of a higher kW in the (0 → 1) direction that cannot
be explained by partial pressure differences, nor by a large experimental error on the dew point
control (Figure 3.13) that would change these partial pressures. An Arrhenius behavior is
searched and the activation energies are reported on the right-hand side of the graph above
(Figure 3.11) for both isotopic transitions. A slightly higher value is found for the (0 → 1) case,
which would be consistent with a higher activation barrier for the evaporation of 2 H2 O.
1.5
Ratio [-]
1.3
1.1
ΔTdew = 5°C
ΔTdew = 2°C
0.9
0.7
0.5
40
45
50
55
kW (0Y1)
pHO
kW (1Y0)
p
60
65
1
2
2
70
75
T [°C]
Figure 3.13: Isotopic effect on kW .
89
H 2O
80
3. RESULTS
3.1.4.4
Structure dependency
Having characterized the main trends showed by the kW behavior based on the ”0D” flow field,
the effect of the diffusive mass transport of the water vapor in the porous media on kW can now
be investigated and will be compared to the values found previously. The basic expectation is
that the diffusive losses of H2 O across the porous media (GDL, CL) should detrimentally affect
the kW , mostly when using N2 as carrier gas.
As a first experiment, the gas flow dependency is compared for the different cell designs tested
in the 2011 campaign (Table 3.1). Similarly as for the previous results (Figure 3.8), the expression of equivalent resistances (Equation 3.9) are chosen to present these results (Figure 3.14).
3
3
0D (0.5 mgPt /cm2)
2.5
0D (0.15 mgPt /cm2)
2
RW [mol-1 cm2 s1] · 105
RW [mol-1 cm2 s1] · 105
2.5
0D (no CL)
1.5
1
RW = RG
2
1.5
1D
1
2D (Channel)
0.5
0.5
2D (Rib)
R W = RG
1D (4 x GDLs)
0
0
0
0.2
0.4
0.6
0.8
1
-1
2
1
5
RG [mol cm s ] · 10
1.2
0
1.4
0.2
0.4
0.6
0.8
1
-1
2
1
5
RG [mol cm s ] · 10
1.2
1.4
Figure 3.14: Gas flow dependency of kW expressed in terms of equivalent resistances for various
cell designs.
It can be observed on the left-hand graph that the ”0D” flow field cells show a similar
behavior. In particular, they all exhibit a slope of 1 as expected, as well as a same y-intercept
max . This result tends to demonstrate that the various Ptindicating an identical value of RW
loadings or even the presence or not of a CL have no influence on the exchange. In particular,
this could indicate that the diffusion of the water vapor inside the CL is negligible. But another
hypothesis could be that the large ionomer surface of the CL would be compensated by mass
transport limitations in the CL pores (subsubsection 3.3.3.3) so that the resulting exchange
would be similar as the membrane without CL.
On the right-hand graph, the results of the ”1D” and ”2D” flow fields are presented. It can
be noticed that the slopes are close to 1, except for the rib area of the ”2D” cell. This exception
can be attributed to the poor statistics gathered. The important difference emerging from this
set of cells is the higher values at the y-intercept, that can be interpreted as lower values of the
max . Whereas the ”1D” and ”2D (Channel)” estimations deliver the
maximal interfacial rate kW
90
3.1 Membrane transport properties
max , higher values are found for the other cases. These differences are coherent
lowest values of RW
with the diffusive losses expected from these designs. This is further detailed in the following.
A refined characterization of the diffusive behaviors can be realized using a variation of the
absolute pressure for the following reasons. Similarly to the value kW , that can be considered
as a flow of 1 H atoms under an isotope fraction gradient of 1, an equivalent diffusive flow can be
defined. The 1 H flow that can be provided by a mono-dimensional Fick’s diffusive flow of water
vapor across a porous medium can be expressed, based on (Equation 1.30), as:
dif f
dif f
kD = Ṅ H
= 2 · Ṅ H
= 2Drel D H2 O,x
2O
1 p H2 O RT
δ
(3.21)
where x refers to the gas considered (He, N2 or H2 here) and p H2 O = RH · psat is the partial
pressure of water vapor in the gas. The use of binary diffusivity instead of ternary diffusivity,
which would involve the different isotopic species of water (1 H2 O, 2 H2 O and 1 H2 HO), represents
a simplification, though it is not expected to affect the trends identified hereafter.
Besides this, it has also been mentioned that the bulk diffusion coefficient is inversely proportional to the absolute pressure (Equation 1.39), which is not the case for the Knudsen diffusivity
coefficient (Equation 1.37). Therefore, the origin of the diffusive losses can be pointed out by
changing the absolute pressure p and keeping a constant relative humidity and a constant value
of p H2 O .
By defining the diffusion resistance as:
RD =
1
kD
(3.22)
it can be assumed that the kW values measured for different absolute pressures behave like:
max
RW = RW
+ RG + RD
(3.23)
Actually, since the real values of the geometric parameters δ and Drel in (Equation 3.21)
are not known with precision, another diffusive flow indicator is preferred that is independent
of these values and only relies on the gas mixture present:
kD0 =
δ
kD
Drel
(3.24)
Based on this, the gas diffusion resistance indicator can be expressed as:
0
RD =
Drel
RD
δ
(3.25)
This leads to a new expression of (Equation 3.23):
max
RW = RW
+ RG +
91
δ
R 0
Drel D
(3.26)
3. RESULTS
and the slope of this expression should give:
dRW
δ
=
0
Drel
dRD
(3.27)
Such an estimation is of course valid for the one-dimensional cases only.
0 values calculated for the different
The RW values measured are plotted in function of the RD
pressures (Figure 3.15). This experiment was performed once with He and once with N2 as
0
carrier gas (transitions THe and T N2 ) which leads to different values of RD .
3
3
2
0D (0.5 mgPt /cm )
0D (0.15 mgPt /cm2)
0D (no CL)
2
1.5
1
TN2
THe
2
1.5
1
0.5
0.5
0
0
0
1
1D
2D (Channel)
2D (Rib)
1D (4 x GDLs)
2.5
RW [mol-1 cm2 s1] · 105
RW [mol-1 cm2 s1] · 105
2.5
2
3
-1
1
5
R’D [mol cm s ] · 10
4
5
TN2
THe
0
1
2
3
-1
1
5
R’D [mol cm s ] · 10
4
5
Figure 3.15: Absolute pressure dependency of kW expressed in terms of equivalent resistances for
various cell designs.
On the left-hand graph, it can be observed that the values obtained for RW for the ”0D”
cells are virtually constant, both for He and N2 , which validates the basic expectation that no
bulk diffusion occurs in this flow field design. Moreover, the y-intercept is the same as for the
previous experiment, which demonstrates the repeatability of the estimation.
0
On the right-hand graph, a clear dependence of the RW value on RD , or in other words on
the pressure, can be observed. The data are well fitted by the linear regression covering the
contribution of the two gases. As expected by the slopes of the curves (Equation 3.27), that
reflect the geometrical term δ/Drel , the structural diffusive losses observed are coherent with
the assumptions. On one hand, the ”1D” cell exhibits a low slope, together with the channel
region of the ”2D” cell. This tends to validate that the channel region of the ”2D” cell behaves
like a ”1D” structure. On the other hand, the rib region of the ”2D” cell as well as the cell
with stacked GDLs demonstrate a higher slope. However, the GDL thickness of the latter is not
identical to the other cases.
Based on the GDL thicknesses measured on the radiograms, namely δ = 165 µm for the
normal ”1D” cell and δ = 500 µm for the cell with stacked GDLs, the values of Drel = 0.18 [-]
and 0.22 [-] are obtained for these two cells respectively, which is comparable to the values found
by Baker et al. (from 0.23 to 0.25 [-]) with a similar compression but with a 25BC GDL [76],
92
3.1 Membrane transport properties
which is expected to be slightly more porous than the 24BC type used here [69]. Let us note
that the limit current density was estimated on a similar cell with stacked GDL and a value of
Drel = 0.28 [-] was obtained, which shows a quite good agreement with the value found here by
deuterium labeling.
max . Under reserve of
At the y-intercept, the different cells demonstrate different values of RW
reliable experimental data, this could indicate the presence of non-bulk diffusive losses between
the different samples, for example Knudsen diffusive losses or thin film diffusion in Nafion,
that would not be the same between the samples. However, it must be kept in mind that the
simplification realized by using a binary coefficient in (Equation 3.21) will be less appropriate
at lower absolute pressure. Indeed, reducing the pressure at a given relative humidity results in
reducing the partial pressure of the carrier gas (N2 or He) but not the one of vapor. Consequently,
the effect of the water auto-diffusivity in the mixture (1 H2 O, 2 H2 O and 1 H2 HO) will not be
max
negligible at low absolute pressure, and then a contribution of the bulk diffusivity on RW
cannot be excluded.
3.1.5
Exchange rate with hydrogen
Transient
By replacing the inert carrier gas by hydrogen, meaning by applying the transition
T H2 (subsubsection 2.1.3.2), the effect of hydrogen dissociation on the total exchange rate, called
k hereafter, can be studied (Figure 3.16). The exchange rate with water vapor kW found in the
previous chapter are reported for comparison. The curve fitting the kW values correspond to
the relation given previously (Equation 3.10).
Different behaviors appear from this graph. By focusing first on the data acquired with
the micro set-up in 2010 (Figure 3.16.a), one observes that the measured values for the total
exchange k appear to be systematically higher than kW . Due to the presence of a catalyst, the
dihydrogen molecule is dissociated into protons according to (Equation 1.3), which results in
an increased total exchange transfer. Furthermore, it seems that this extra exchange exhibits
an almost constant value even with different values of kW , which would tend to validate the
additive behavior assumed in the model (Equation 2.22). Based on these results, a value of
kH = k−kW = 1.2·10−5 [mol cm−2 s−1 ] is obtained. On their side, Chen et al. [9] report values for
the exchange current density (per surface of platinum) in the range of i0,HOR = 1−80 mA/cm2Pt .
Assuming an electro-catalytically active area (ECA) of 100 cm2Pt /cm2 [24] for the comparison,
and converting these values into exchange rates by (Equation 2.5), the range kH = 0.1−8.3·10−5
[mol cm−2 s−1 ] is obtained from [9]. The value found with the present method would be in this
range.
However, the additivity of the exchanges is put in perspective by the data obtained with
the midi set-up in the 2011 campaign. The values of the total exchange rate k measured on a
93
3. RESULTS
6
6
(a)
2
2010, Micro set-up (0D, 0.5 mgPt /cm )
k [mol cm-2 s-1] · 10-5
k [mol cm-2 s-1] · 10-5
5
4
3
2
k
kW (exp.)
1
100
200
300
Flow[Nml/min]
k
kW (exp.)
2
kW (fit)
0
400
100
200
300
400
Flow[Nml/min]
6
6
(c)
2011, Midi set-up (0D, no CL)
(d)
2011, Midi set-up (0D, 0.15 mgPt /cm2)
5
k [mol cm-2 s-1] · 10-5
5
k [mol cm-2 s-1] · 10-5
3
0
0
4
3
k
kW (exp.)
2
1
4
3
k
kW (exp.)
2
1
kW (fit)
kW (fit)
0
0
0
100
200
300
Flow[Nml/min]
0
400
100
200
300
400
Flow[Nml/min]
6
6
5
5
k [mol cm-2 s-1] · 10-5
k
kW (exp.)
4
kW (fit)
3
2011, Midi set-up (1D, 0.5 mgPt /cm2 ,
GDL 1x24BC + 3x24BA)
(e)
2011, Midi set-up (1D, 0.5 mgPt /cm2, GDL 24BC)
k [mol cm-2 s-1] · 10-5
4
1
0
2
(f)
k
kW (exp.)
4
kW (fit)
3
2
1
1
0
0
0
100
200
300
0
400
100
200
300
400
Flow[Nml/min]
Flow[Nml/min]
6
6
2011, Midi set-up (2D, 0.5 mgPt /cm2, GDL 24BC, Chan.)(g)
2011, Midi set-up (2D, 0.5 mgPt /cm2, GDL 24BC,Rib)
(h)
5
5
k
kW (exp.)
4
k [mol cm-2 s-1] · 10-5
k [mol cm-2 s-1] · 10-5
(b)
2
2011, Midi set-up (0D, 0.5 mgPt /cm )
5
kW (fit)
3
2
k
kW (exp.)
4
kW (fit)
3
2
1
1
0
0
0
100
200
300
400
0
Flow[Nml/min]
100
200
Flow[Nml/min]
Figure 3.16: Effect of hydrogen on the total exchange rate k.
94
300
400
3.1 Membrane transport properties
similar cell (Figure 3.16.b) show that the change induced by the presence of hydrogen is very
small, but not negligible, as it can be seen by the two data points measured at low flow (25
and 50 Nml/min). For what concerns the cells without catalyst layer (Figure 3.16.c) and with
reduced Pt-loading (Figure 3.16.d), the effect of hydrogen can be considered as negligible.
A clear effect of the hydrogen on the exchange rate appears for the cells having diffusive
transport losses (Figure 3.16.e-h). Indeed, the value of k is systematically higher than that
of kW . However, the difference k − kW is not the same between these different cells, which is
surprising since the CLs are a priori similar. Moreover, the change of water vapor diffusivity
in hydrogen compared to helox is too small to explain an improved kW in hydrogen that would
be the same for all structures. It is remarkable that the k values of these cells seem to be very
similar. In particular at the highest flow, a value of k ≈ 3 · 10−5 [mol cm−2 s−1 ] is reached for
all the 6 cells of the set-up. The supposition that the exchange rates kW and kH are additive
seems therefore inappropriate.
Instead of considering additive exchanges, a modified model of the exchanges can be proposed
(Figure 3.17). By taking into account a more realistic structure of the CL, it can be supposed
1-sided cell
Zoom
CL
CL
Membrane
FH(t)
FH(t)
kW
kH
F(x,t)
F(x,t)
FW(t)
FW(t)
k
Figure 3.17: Modified model of the exchanges kW and kH .
that the the ionomer phase of the electrode can act like a common limitation for all H atoms
supplied from the gas phase, regardless from which gas they are supplied, namely water vapor
or hydrogen. This limitation would introduce a maximal value for the total exchange k, which
would be coherent with the results presented.
Steady-state
As discussed in the methodology part (section 2.1), the steady-state regimes
can be used to determine the ratio kW /kH according to (Equation 2.36) based on the isotope
fraction F . The results of these experiments are presented here (Figure 3.18). The cell without
CL is not presented on this graph, since there is no effect of hydrogen on the exchange rate,
which leads theoretically to an infinite ratio and in practice to meaningless values.
95
1
∞
0
0.9
FW = 1, FH = 0
9
0.1
0.8
FW = 0, FH = 1
4
0.3
0.7
2.3
0.4
0.6
1.5
0.7
0.5
1
0.4
0.7
0.3
0.4
2.3
0.2
0.3
4
0.1
0.1
9
0
0D
0.15 mgPt /cm2
0D
2
0.5 mgPt /cm
1D
0.5 mgPt /cm2
GDL 24BC
2D (Channel)
0.5 mgPt /cm2
GDL 24BC
2D (Rib)
1D
0.5 mgPt /cm2 0.5 mgPt /cm2
GDL 24BC GDL 1x24BC
+ 3x24BA
kW/kH [-]
F [-]
3. RESULTS
0
(FW =1, FH = 0)
1
1.5
∞
(FW =0, FH = 1)
Figure 3.18: Ratio of exchange rates for the different cells (2011, midi set-up).
It can be observed that all cells demonstrate a strong variation of F (i.e. of kW /kH ) according
to the values chosen for the gaseous isotopic fraction of water vapor FW and hydrogen FH , which
is the consequence of isotopic effects on the exchange rates. The comparison with the isotope
effects on kW discussed earlier cannot be directly made, since an unknown isotopic effect on kH
is a priori also expected to play a role.
Consistently with the previous observations, the cell with a 0.15 mgPt /cm2 Pt-loading exhibits high kW /kH ratios, which is due to its very low kH value. On the contrary, the cell with
the same structure (0D) but with the 0.5 mgPt /cm2 Pt-loading exhibits a higher sensitivity to
hydrogen. The ratios are higher than those measured on the cells with diffusive limitations (1D,
2D), in agreement with the results of the transient regimes discussed in the previous chapter. For
these three samples, the ratio tend to decrease along with the increase of structural limitations,
similarly to the transient regimes, and the lowest ratios are obtained for the cell mounted with 4
GDLs and for the ”2D (Rib)” area. This is consistent with the expectation that in steady-state
mode, there are effectively non-zero flows ṅ1 H,in = ṅ1 H,out 6= 0 exchanging H atoms between
the membrane and the gas phase. Therefore, in the case of the water vapor, a gradient of the
isotopic fraction across the porous media is expected, similarly to the transient case, that will
lead to more or less reduced kW values depending on the structure. On the contrary, due to
the relatively high diffusivity of hydrogen in the mixture, a constant kH value can be presumed.
The values observed here are in accordance with the trends identified previously: the ”0D” cells
present the highest kW /kH ratios, the ”1D” cell and the channel area of the ”2D” exhibit lower
values and finally, the rib area of the ”2D” cell and the ”1D” cell with stacked GDLs show the
lowest values.
96
3.1 Membrane transport properties
3.1.6
Summary and outlook
The deuterium labeling method was applied, that consisted in estimating the diffusivity and
interfacial transport parameters of a membrane surrounded by gases with changing isotope
fraction, by means of a model fitted on experimental data (section 2.1).
Estimations of the proton diffusivity in the membrane were presented. It was shown that a
sufficient gas flow rate has to be supplied for a correct estimation of this parameter. The results
obtained with two set-ups were compared: the micro imaging set-up combined with the singlecell and the midi imaging set-up combined with the multi-cell. For the latter, issues were raised
concerning the poor image quality delivered by the camera and its impact on the estimation of
the diffusivity. Statistical estimations of the errors were proposed.
The proton diffusivity in the membrane was estimated for various relative humidities and
different temperatures. Whereas the expecting trends were captured (diffusivity increases with
RH and with temperature), the values appear to be higher than the ones found with NMR.
Some arguments were proposed to explain these differences but open questions remain about
other possible differences between the two methods.
As an outlook about the proton diffusivity measurement, calibration experiments on pure
water would be of great help to clarify the origin of these differences and more generally to
gain in reliability, by being independent on the contributions of the water transport mechanisms
specific to Nafion. By a temperature dependency experiment, the diffusion enthalpy should also
be compared with the one from NMR.
In the present method, the estimation of the proton diffusivity is required as a first-step in
the model for the estimation of the exchange rates. But in future perspectives, it could serve
per se as a useful in situ characterization tool of the membrane properties. Being based on
visualization, the estimation of the diffusivity with the proposed method can give new insights
compared to traditional conductivity measurements. Potentially, it can give information about
local distribution of diffusivity in the membrane. Under operation, it would also permit to investigate possible channel-rib distributions of the diffusivity. The comparison between this method
and the conductivity measurement could also yield information about the relative importance
of the Grotthuss mechanism on the charge transfer. At last and compared to NMR methods,
neutron imaging allows for using realistic fuel cell systems (materials and dimensions). Diffusivity measurements could be integrated in more general experimental protocols, for instance to
study degradation effects in the membrane.
Based on the proton diffusivity values, the exchange rate of water vapor was estimated for
various operating conditions (gas flow, RH, temperature, pressure, carrier gas) and on a series
of samples with different properties (presence or not of a CL, Pt-loading, diffusive limitations).
The gas flow dependency showed that the exchange rate tends to reach a maximal value. A
97
3. RESULTS
simple representation of the flows by equivalent resistances was used to estimate this maximal
exchange rate, which was observed to be lower than the value reported for pure water. The
comparison between the samples showed that the presence of a catalyst layer or the change of
the Pt-loading had no visible effect on this value. Isotopic effects on the exchange rates were
identified and discussed.
The diffusive limitations in the porous media could be clearly identified. In particular,
differences of diffusivity were observed between the channel and the rib region of the GDL in
a typical ”2D” flow field. As a direct output, the relative diffusivities of the ”1D” cells were
quantified.
Finally, the effect of the presence of hydrogen on the total exchange rate was investigated.
The expected increase of the total exchange rate greatly varied between the samples. While it
was moderate in absence of water diffusion limitations, it demonstrated an important effect for
the cells presenting diffusive limitations, meaning a clear increase of the total exchange rate.
These results tend to show that the total exchange rate cannot be assumed to be a simple
addition of the two exchange rates (membrane-vapor and membrane-hydrogen). Rather, the
existence of a common limitation, for instance in the ionomer of the CL, was hypothesized.
As a recommendation for future progress, the experiments presented with the multi-cell
should be repeated with another camera featuring a better image quality. If the results proposed
can be validated, then other prospects can be made. The isotopic effect could be investigated by
performing the isotopic gas changes under constant partial pressure of water. The use of water
vapor for diffusivity estimations could be considered. In particular, it was shown qualitatively
that is gives access to the 2D channel-rib distribution of the diffusive flows, which is not a value
accessible by measurements of integral limit current density. Reducing the effect of the vapor
exchange rate could allow for a larger relative contribution and also a better estimation of the
hydrogen exchange rate. Operating at high pressure and low temperature should offer adapted
conditions for this. Then, the hydrogen exchange rate could be used as a diagnostic tool to
perform investigation of the catalyst state, in the context of degradation studies for example.
If the water vapor effect can be minimized, then hydrogen could also be used for diffusivity
measurements. Unlike water vapor, it would not interact with the water content of the GDL
and would be suited for diffusivity estimations with various saturation levels in the porous media.
Based on the comments made in the methodology chapter (section 2.1), the extension of the
model to more complex systems can be considered. In particular, a possible modeling of the
catalyst layer could perhaps helps understanding the nature of the limitations proposed in this
chapter. The in situ estimation of the net fluxes of water across the membrane (back-diffusion
and electro-osmotic drag) would be of prime relevance.
98
3.2 Effect of the microporous layer (MPL)
3.2
3.2.1
Effect of the microporous layer (MPL)
Introduction
It is commonly recognized that the presence of a microporous layer (MPL) on the GDL helps to
improve the local performance of the cell [77], especially under wet conditions. The commonly
positive effects attributed to the presence of a MPL are to maintain a low saturation level inside
and/or on the CL [78–80], to favor the back-transport of the water produced towards the anode
[81, 82], to keep a low saturation level in the cathode GDL [80, 81, 83] and to improve the
CL/GDL electrical contact [80]. However, its real impact on the water distribution is still a
debated question.
In modeling studies, the MPL is traditionally considered as a homogeneous medium and
the water profiles predicted [79–81, 84, 85] are rarely validated by visualization results. When
the predictions are confronted to in-plane measurements, some disagreements appear [86], for
instance about the localization of the maximum saturation level in the layer, so that inhomogeneities across the MPL thickness have to be taken into account to fit the model on the reality
[87, 88]. When the presence of defects in the MPL [89] is implemented in the models [90], a
reduction of the capillary barrier is obtained and the water amount in the GDL is higher than
when a MPL without defect is considered.
Ex situ studies dedicated to the effect of the MPL have been mainly focused on the estimation of the capillary pressure [83, 91] as a function of the liquid saturation [92], on porosity
measurements [89], or on the morphological observation of liquid water droplets [93]. Other ex
situ optical visualization studies could demonstrate the positive effect of the MPL by reducing
the water accumulation on the CL surface, for instance by freezing and visualizing the sample
after operation [94].
Since the MPL is currently included in most of the state-of-the-art technical and experimental
cells, the majority of the results reported by in situ visualization studies, being realized with
neutron imaging [63, 88, 95, 96] or X-ray synchrotron tomography [97], address the effect of the
MPL without comparison with a case without MPL. Moreover, such studies rarely include the
cell performance. On the other hand, studies reporting the performance of different types of
samples with or without MPL do not include visualization results [77, 98].
Finally, very few in situ visualization works comparing the presence or not of a MPL were
reported in the literature so far, for example with in-plane neutron imaging [49]. The performances of the cells and the average water contents in the porous media were however not given
in this paper. The asymmetric configurations (MPL on anode only and MPL on cathode only)
were not considered in this reported study either.
99
3. RESULTS
3.2.2
Experimental
In the present work, four combinations of cells (with no MPL, with a MPL on anode, with a MPL
on cathode and with MPLs on both sides) are simultaneously visualized with neutron imaging
and electrochemically characterized by means of the multi-cell setup (section 2.3). In particular,
the helox pulsing method presented previously (section 2.2) was continuously applied.
The design and operation parameters studied are summarized on the following table (Table 3.3). All experiments were realized in in-plane configuration with an exposure time of 20 s.
Design
parameters
Measurement
campaign
2011
Operation
parameters
MEA
Flowfield
GDL
anode
GDL
cathode
Primea 5710
Gore
2D
24BA
24BA
Primea 5710
Gore
2D
24BA
Primea 5710
Gore
2D
Primea 5710
Gore
2D
Experimental
set-up
Current
(IV-curve)
Multi-cell
24BC
RH
(RH-series)
Midi set-up
24BC
24BA
RH
(RH-step)
(PCO™ camera)
24BC
24BC
(BC: GDL with MPL ; BA: GDL without MPL)
Table 3.3: Summary of experiments realized to study the effect of the MPL.
Three types of experiments were realized. The so-called RH-series experiment consists in changing the RH conditions at constant current every 16 min. The IV-curve consists in increasing
the current at constant RH every 10 min. The neutron imaging data for these two steady-state
experiments were obtained by merging the radiograms over 5 min periods. At last, the RH-step
experiment consists in changing the RH conditions at constant current with a measurement of
all parameters during the RH transients. The RH step was applied from dry to wet conditions
and inversely as follows: 10 min at RH = 40%/0%, 40 min at RH = 100%/100% and 10 min
again at RH = 40%/0%. The helox pulsing characterization described earlier (section 2.2) was
continuously applied in form of a 1 s pulse duration every 30 s. Oxygen pulses were applied
instead of helox pulses during 2 min at the end of each current or RH condition for the RHseries and IV-curve experiments. The values ∆Uhelox−air and ∆U O2 −helox reported correspond
respectively to the definitions (Equation 2.74) and (Equation 2.77) given previously.
For all experiments, the temperature was 70°C. For anode and cathode respectively, the
absolute inlet pressures were 2 and 2.1 bar and the cell flows were 0.4 and 1.0 Nml/min. The
cells were automatically stopped when the voltage was below 0.1 V.
100
3.2 Effect of the microporous layer (MPL)
The cells were mounted with the corresponding spacers to target a compression of 30%. In the
following, cells will be referred by the type of GDL they are constituted with, and by omitting the
number ”24”, so that only the letters will be used (”BC/BC”, ”BA/BC”, ”BC/BA”, ”BA/BA”)
for the corresponding combinations (MPL on both sides, MPL only on cathode, MPL only on
anode, no MPL). The notation ”xx/BC” (or ”xx/BA”) will designate the two cells having a
MPL on cathode (respectively no MPL on cathode).
In the following, the relative humidity conditions of the experiments will be designated with
the notation ”RH = x%/y%”, meaning a RH of x% on the anode side and y% on the cathode
side.
3.2.3
3.2.3.1
Results
RH-series - 0.5 A/cm2
The first experiment investigated is a series of steady-state conditions under different RHs, at a
constant current of 0.5 A/cm2 (Figure 3.19).
Performance
It appears that the voltages of the ”xx/BC” cells are systematically higher
than the ”xx/BA” cells. Moreover, the ”BA/BC” cell does not exhibit a lower voltage than
the ”BC/BC” cell, contrary to other observations [98]. By correcting the voltage U by the
ohmic losses ηΩ = ri, the IR-free voltage U + ri can be calculated. Since the values between
the ”xx/BC” and the ”xx/BA” cells are still different, it can be deduced that the differences of
resistance r are not the reason for the differences of performance.
Under the lowest RH values, it appears that the IR-free voltages present a slight change
of slope, which may be attributed to differences of catalyst state due to the preceding experiment. The slight change of slope appearing under the highest RH for the ”xx/BA” cells can
be attributed to the difficulty of controlling the RH around 100%/100%. Consequently, the
fully-humidified condition is perhaps already reached before the 100%/100% point presented
here.
Having a look on the U O2 + ir values, and remembering that the voltage U O2 is measured
during the O2 pulse, one sees that the values are almost identical for all cells, meaning that
there is no major difference of ηCT between the cells.
The ∆Uhelox−air values of the ”xx/BC” cells are very similar. On the contrary, the ”xx/BA”
cells exhibit both an increasing value of ∆Uhelox−air with RH, but not in accordance with each
other. On one hand, the ”BA/BA” cell shows, at the driest condition, an identical value as for
the ”xx/BC” cells, that increases then constantly with RH. On the other hand, the ”BC/BA”
cell presents an offset under the driest conditions and the increase of losses happens only in the
101
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.4
ΔUhelox-air [V]
U [V]
3. RESULTS
80%
0%
100%
20%
100%
60%
0
100%
100%
RH an. 40%
RH ca. 0%
80%
0%
100%
20%
100%
60%
100%
100%
100%
100%
RH an. 40%
RH ca. 0%
80%
0%
100%
20%
100%
60%
100%
100%
100%
100%
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0 RH an. 40%
RH ca. 0%
80%
0%
100%
20%
100%
60%
100%
100%
0.4
0.4
0.3
0.3
ΔUO2-helox [V]
r [Ω·cm2]
0.2
0.1
RH an. 40%
RH ca. 0%
0.2
0.1
0
80%
0%
100%
20%
100%
60%
2
UO +ri [V]
RH an. 40%
RH ca. 0%
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.2
0.1
0
U+ri [V]
0.3
RH an. 40%
RH ca. 0%
80%
0%
100%
20%
BC/BC
100%
60%
BC/BA
BA/BC
BA/BA
Figure 3.19: Performance indicators for steady-state steps of RH realized at i = 0.5 A/cm2 .
higher RH domain. Concerning the meaning of the ∆Uhelox−air value, it must be reminded that
the basic assumption is that:
∆Uhelox−air = ηgair − ηghelox
(3.28)
since the partial pressures of O2 are supposed to be identical in both mixtures. It is therefore
expected that the value of ηCT is the same between helox and air. The fact that the U O2 + ri
voltages are the same between the cells with different ∆Uhelox−air values show that there is
102
3.2 Effect of the microporous layer (MPL)
no difference of ηCT between the cells. It follows that the ∆Uhelox−air values can reliably be
attributed to differences of bulk diffusion losses between the cells.
Further explanations are needed to interpret the ∆U O2 −helox values. By considering the
different diffusive contributions in the general expression of the mass transport losses (subsubsection 1.3.3.3), it can be expected that the difference between pure oxygen and helox can be
conceptually (valid in 1D only) expressed as:
O
O
O
2
2
2
O2 ,helox
helox
helox
∆U O2 −helox = (ηrev
+ ηCT
+ ηghelox + ηKn
+ ηfhelox
ilm ) − (ηCT + ηKn + ηf ilm )
O2
In this expression, ηg
O ,rev
= 0 was considered. The term ηrev2
O
helox
= Urev2 − Urev
(3.29)
was added to
take into account the difference of reversible voltages (Equation 1.11). This value as well the
differences of kinetic losses can be grouped in one single term:
O
of f set
O2 ,helox
helox
∆U O
= (ηrev
) + (ηCT
− ηCT2 )
2 −helox
(3.30)
since this value is not expected to change with the working current and was estimated to be
45 mV in [13]. It can also be noticed that the possible losses at OCV (mixed potential due
to H2 permeation or short-circuit) would negligibly affect this term for the practical current
densities considered in this study. The expression of ∆U O2 −helox (Equation 3.29) can therefore
be reformulated as:
O
O
of f set
2
2
helox
∆U O2 −helox = ∆U O
+ ηfhelox
+ (ηghelox + ηKn
ilm − ηKn − ηf ilm )
2 −helox
(3.31)
In the present case, we observe that the values of ∆Uhelox−air for the different cells are not
correlated to differences of ∆U O2 −helox , meaning that a possible change of ηghelox is not reflected
in this value, which tends to validate that ηghelox ≈ 0 and that:
∆Uhelox−air ≈ ηgair
(3.32)
of f set
Besides this, the term ∆U O
is not expected to change with RH either. Having stated
2 −helox
that ηghelox ≈ 0, it follows from (Equation 3.31) that the change of ∆U O2 −helox would reflect a
change of the non-bulkian diffusion terms:
O
O
of f set
2
2
helox
∆U O2 −helox = ∆U O
+ (ηKn
+ ηfhelox
ilm − ηKn − ηf ilm )
2 −helox
(3.33)
In the present case, only the ”BC/BA” cell and to a lesser extend the ”BA/BA” cell seem to
exhibit a slight change of ∆U O2 −helox with RH. Moreover, the absolute value is approximately
of f set
100 mV, which is higher that the ∆U O
= 45 mV discussed, as already observed by Boillat
2 −helox
et al. [52]. This may reflect a contribution of Knudsen and/or thin film diffusion to the overall
mass transport losses.
103
3. RESULTS
Average water amount in GDL
By considering the water amount in the GDL (Figure 3.20),
it can be seen that all cells present the same behavior with exception of two cases: the ”BC/BC”
cell has a higher water amount in the rib area of the anode GDL at high RH and the ”BA/BC”
cell has a higher water amount in the rib area of the cathode GDL at high RH. Since these two
30
30
Cathode GDL - Channel area
Water content [% vol tot]
Water content [% vol tot]
Anode GDL - Channel area
25
20
15
10
5
20
15
10
5
0
0
RH an. 40%
RH ca. 0%
80%
0%
100%
20%
100%
60%
100%
100%
RH an. 40%
RH ca. 0%
80%
0%
100%
20%
100%
60%
100%
100%
100%
60%
100%
100%
30
30
Water content [% vol tot]
Anode GDL - Rib area
Water content [% vol tot]
25
25
20
15
10
5
Cathode GDL - Rib area
25
20
15
10
5
0
0
RH an. 40%
RH ca. 0%
80%
0%
BC/BC
100%
20%
100%
60%
RH an. 40%
RH ca. 0%
100%
100%
BA/BC
80%
0%
BC/BA
100%
20%
BA/BA
Figure 3.20: Water content for steady-states step of RH realized at i = 0.5 A/cm2 .
cells were observed to have a same value of ∆Uhelox−air , these singular features demonstrate
that an increase of the water under the ribs has virtually no effect, at least at higher RH,
on their mass transport losses. Moreover, according to the interpretations made by Boillat et
al. [52] on a ”BC/BC” single-cell, one observes that the important increase of water content
in the rib area of the cathode GDL between RH = 80%/0% and 100%/20% does not induce
any noticeable increase of ∆Uhelox−air in this RH domain, with exception of the ”BA/BA” cell.
These observations reflect the lesser impact on the performance of the water accumulation under
the ribs, which can be expected by the lower oxygen diffusive flow in the rib region [52]. The
probable lower current density under the ribs than under the channels [11] would reinforce this
phenomenon.
104
3.2 Effect of the microporous layer (MPL)
The case of the ”BA/BA” cell might be different. Due to its higher resistivity, it can be
expected that the differences of diffusivity between the channel and the rib regions will have a
smaller effect on the channel-rib distribution of the current. Then, it can be hypothesized that
the rib regions of the ”BA/BA” cell would show a higher current than the rib regions of the
”xx/BC” cells. Consequently, the sensitivity of the mass transport losses to water accumulation
under the ribs would be higher, as it appears here.
At last, the surprisingly high value of ∆Uhelox−air measured for the ”BC/BA” cell at low RH
can be attributed to unexpected higher structural losses, which can originate from a variation
of the sample or imperfections in the assembly resulting in an over-compressed GDL in given
areas. It can also be expected that the CL structure is maybe degraded in the sense of reduced
pore sizes that would increase mass transport losses in the CL. In any case, this cell seems to
show a singular behavior compared to what could have been expected based on the ”BA/BA”
cell. This sample may therefore hide the real effect of the anode MPL.
Without losing generality by taking into account a rib-channel effect or not, it is obvious
that the different values of ∆Uhelox−air between the cells cannot be explained by different water
amounts in the GDLs. In particular, this means that the higher sensitivity of ∆Uhelox−air to
RH observed for the ”xx/BA” cells cannot be correlated to a change of mass transport losses
that would be due to a higher saturation level in the cathode GDL. It can be deduced that
the extra amount of water responsible for the increased mass transport losses in the ”xx/BA”
cells is to be searched either in differences of in-plane water distribution profiles and/or in little
detrimental water accumulations that may not be detectable by neutron imaging.
Water spatial distribution The water spatial distribution is plotted for the fully-humidified
condition (Figure 3.21). On this graph, the position of the MEA was estimated based on the
non-referenced images of the ”BA/BA” cell. The water profiles of the other cells were then
shifted so as to align the bottom of the channels between them in accordance to the different
cells: the bottoms of the cathode channels of the ”BA/BA” and ”BC/BA” cells are aligned
each others, then the ”BC/BC” cell anode channels are aligned with the ones of the ”BC/BA”
cell, at last the ”BA/BC” anode channels are aligned with the ones of the ”BA/BA” cell. As
validation, the cathode channels of the ”BC/BC” and ”BA/BC” cells are observed to be well
aligned. The thickness of the MEA presented here is 35 µm. The grey area representing the
GDL corresponds to the thickness of the ”BC/BC” cell.
It can be observed that all cells show a maximum of water accumulation located in the
cathode GDL. In the case of the ”xx/BA” cells, the maximum is located near the MEA. On
the contrary, the ”xx/BC” cells exhibit a water peak that is located closer to the channel, in
accordance with other observations [49, 96]. The location and the amplitude of the peak look
105
3. RESULTS
GDL
MEA
GDL
30
Water content [% vol tot]
25
20
15
10
5
0
-300
-200
-100
0
100
200
300
Position [µm]
BC/BC
BA/BC
BC/BA
BA/BA
Figure 3.21: Water distribution profiles at the RH = 100%/100% step of RH steady-state experiment realized at 0.5 A/cm2 (Figure 3.19).
identical for the ”xx/BC” cells. The ”BC/BA” cell seems to exhibit a truncated maximum
compared to the ”BA/BA” cell.
Compared to the ”BA/BA” cell, it seems that the presence of a cathode MPL tends to
maintain a low saturation level in the vicinity of the MEA. This result corresponds to the
observation of Cho et al. [49], who also remarked a shift of the accumulation peak (by comparing
a cell with no MPL and a cell with MPL on both sides) and is in accordance with model
predictions made in [79, 80], too. However, our results show that the presence of a MPL on the
cathode does not change the saturation level of the GDLs, in particular that the water amount
in the cathode GDL is not reduced, even in under-saturation conditions, which is contrary to
other simulations [84, 99] or ex situ observations [83, 92]. A consequence is that the presence
of a cathode MPL does probably not induce a sufficiently high increase of hydrostatic pressure
to repel the water produced towards the anode side. This assumption is in agreement with
propositions made by Owejan et al. [91] but is contrary to other suppositions [49, 63, 84, 99].
Our results find a good consistency with the predictions of Medici et Allen [90]. According
to their simulations, the presence of defects in the MPL can significantly reduce the hydrostatic
pressure compared to an ideal MPL without defect, and the capillary pressure required to
percolate throughout the defects was approximately the same as for a GDL without MPL. They
106
3.2 Effect of the microporous layer (MPL)
also reported that if the number of defects in the MPL was increased, then the saturation level
of the GDL would approach the one of a GDL without MPL.
The case of the ”BC/BA” is more ambiguous, since the water amount is comparable to the
one of the ”xx/BC” cells on the virtual MEA surface represented on this figure. However, it
must be kept in mind that blurring effects due to the limited imaging resolution can result in
smoothing the sharp transitions of the water content. It follows that a value of water content
measured on a location where the profile presents a strong gradient cannot be interpreted as the
real absolute value at this precise location. Moreover, there is an uncertainty concerning the
membrane thickness, its flatness, and its exact positioning between the different cells. Therefore,
it seems reasonable to consider that the shift observed for the ”BC/BA” cell is rather due to
sample imperfections than to a low saturation domain in the GDL at this location, which would
be in conflict with the behavior of the ”BA/BA” cell.
Discussion
Having stated about the performance losses and the water distribution, it can be
proposed as an explanation that the absence of MPL on cathode side tends to favor the local
accumulation of water in some crucial areas resulting in a strong increase of the mass transport
losses. The covering of the CL/GDL interface or the clogging of CL pores represent the most
probable candidates for this accumulation, as suggested by Nam et al. [80] or Owejan et al.
[91]. On the contrary, the presence of a MPL tends to lower the saturation level in the vicinity
of the cathode CL and to mitigate or suppress the detrimental accumulation of water.
Regarding the higher saturation levels in the GDL observed with MPL (Figure 3.20), and
under reserve of repeatability of this result, it can be suggested that the slightly increased
capillary pressure provoked by the presence of a MPL combined with the stochastic distribution
of defects could allow overcoming the breakthrough pressure of a higher number of pores than
in the case of a simple GDL without MPL, as illustrated (Figure 3.22). In the next experiments,
higher saturation levels for the ”xx/BC” cells will be observed again.
Resisitivity
The resistivities measured in this experiment (Figure 3.19) show the positive
effect of the MPL on lowering the resistance with a consistent behavior: the cell having two MPLs
is the less resistive, the cell having no MPL is the more resistive and the cells with one MPL
have a same and intermediate resistivity. This beneficial effect was already mentioned in the
literature [100] for different types of samples (SGL 10BA compared with SGL 10BC 25% PTFE).
In our case, we can suppose that the membranes of the different cells are similarly hydrated. In
addition to that, the GDL portion of the ”xx/BC” cells is expected to have identical properties
(material, porosity, thickness) as the GDL in the ”xx/BA” cells. Furthermore, the addition of
a MPL constitutes an extra layer and consequently an increased electrical resistance compared
to the single GDL. In consequence, the observation made here can be interpreted as a positive
107
3. RESULTS
MPL-few defects
No MPL
pc,low
pc,high
MPL-many defects
pc,high >pc,inter >pc,low
Defect
CL MPL
GDL
Figure 3.22: Hypothesis for explaning the higher GDL saturation obtained with MPL.
effect of the MPL on lowering the contact resistance at the CL/MPL interface compared to the
CL/GDL interface. Differences of compression can also contribute in differences of resistivity.
3.2.3.2
RH-series - 1 A/cm2
In the following, a series of steady-state conditions under different RHs is performed at 1 A/cm2
(Figure 3.23).
The differences between the U voltages of the ”xx/BA” and the ”xx/BC” cells are clearly
confirmed and again, they cannot be entirely explained by differences of resistivity. The ”xx/BA”
cells could not be operated at higher RH than 100%/60%.
Let us concentrate on the ”xx/BC” cells first. The behavior of their ∆Uhelox−air values look
similar as for the lower current 0.5 A/cm2 , but exhibit more visible features now, namely the
presence of an optimal condition at intermediate RH (between RH = 80%/0% and 100%/20%)
in which a minimum value of ∆Uhelox−air is reached. This minimum value of ∆Uhelox−air can
be interpreted as an estimation of the structural loss, meaning the minimum of mass transport
losses achievable in absence of water. Because this value also depends on the current, it is
higher here at 1 A/cm2 than at 0.5 A/cm2 . At higher RH (from 100%/20% to 100%/100%), the
∆Uhelox−air value increases as expected due to the negative effect of water on the bulk diffusion.
At lower RH (from 40%/0% to 80%/0%), one observes that the ∆Uhelox−air is higher than at
the optimal point, with exception of the ”BA/BA” cell. As proposed by Boillat et al. [52],
this effect can be induced by the presence of a higher current under the ribs at low RH than at
intermediate RH. Indeed, the high resistivity in that case will tend to homogenize the current
distribution compared to a case where mass transport losses are dominating. Consequently, the
total mass transport losses measured can be higher in dry conditions than at intermediate RH.
108
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.4
ΔUhelox-air [V]
U [V]
3.2 Effect of the microporous layer (MPL)
0.2
0.1
RH an. 40%
RH ca. 0%
80%
0%
100%
20%
100%
60%
0
100%
100%
0.4
0.4
0.3
0.3
ΔUO2-helox [V]
r [Ω·cm2]
0.3
0.2
0.1
0
RH an. 40%
RH ca. 0%
80%
0%
100%
20%
100%
60%
0
100%
100%
100%
100%
RH an. 40%
RH ca. 0%
80%
0%
100%
20%
100%
60%
100%
100%
100%
60%
100%
100%
100%
60%
100%
100%
Cathode GDL - Channel area
Water content [% vol tot]
Water content [% vol tot]
100%
60%
30
25
20
15
10
5
RH an. 40%
RH ca. 0%
80%
0%
100%
20%
100%
60%
25
20
15
10
5
0
100%
100%
RH an. 40%
RH ca. 0%
80%
0%
100%
20%
30
30
Anode GDL - Rib area
Water content [% vol tot]
Water content [% vol tot]
100%
20%
0.2
Anode GDL - Channel area
25
20
15
10
5
0
80%
0%
0.1
30
0
RH an. 40%
RH ca. 0%
RH an. 40%
RH ca. 0%
80%
0%
BC/BC
100%
20%
100%
60%
Cathode GDL - Rib area
25
20
15
10
5
0
100%
100%
RH an. 40%
RH ca. 0%
BC/BA
BA/BC
80%
0%
100%
20%
BA/BA
Figure 3.23: Performance indicators and water content for steady-state steps of RH realized at i
= 1 A/cm2 .
109
3. RESULTS
Regarding the ”BA/BA” cell, it appears that the shape of the ∆Uhelox−air curve is similar
as the one observed at lower current, with a minimum found under the driest condition. By
comparing the values of ∆Uhelox−air with the ones of the ”xx/BC” cells, one observes that
the difference at 40%/0% is now higher than what was observed at 0.5 A/cm2 . Following the
argument already given, it can be expected that the ”BA/BA” cell, due to its higher resistivity,
has a more homogeneous rib-channel distribution of current than the ”xx/BC” cells, which
would explain the offset of ∆Uhelox−air appearing here at 40%/0%, as well as the shape of the
curve (strictly increasing, as for 0.5 A/cm2 ).
The ”BC/BA” cell shows very similar values as the ”BA/BA” cell in the high RH domain
(100%/20% to 100%/60%).
Further information is given by the ∆U O2 −helox parameter. Whereas it was almost constant
for all cells at low current, it exhibits here noticeable features in the case of the ”xx/BA” cells. In
the lower RH domain (40%/0% to 100%/0%), it shows a steep increase that is not correlated to
the evolution of ∆Uhelox−air . In the high RH domain (100%/0% to 100%/100%), the evolution
rate of ∆U O2 −helox is reduced, whereas the value of ∆Uhelox−air shows a steep increase. Based
on the supposed signification of ∆U O2 −helox given before (Equation 3.33), it can be argued that
a non-bulk diffusion limitation is occurring for the ”xx/BA” in the low RH domain, whereas
the bulk diffusion gets dominant in the high RH domain. This behavior might be consistent
with the observation made earlier. As the RH increases, the CL or CL/GDL interface of the
”xx/BA” cells would retain more water than the ”xx/BC” cells, resulting in non-bulk diffusion
losses, namely Knudsen or thin film diffusion, leading to the poor performances of these cells.
By further increasing the RH, the bulk diffusion is getting important, which translates into an
increase of ∆Uhelox−air . These results are however not directly comparable to the lower current
0.5 A/cm2 where the changes of ∆U O2 −helox were hardly perceptible.
By supposing that the changing value of ∆U O2 −helox with RH is attributed to non-bulk
diffusion losses, this term can be written as:
O −helox
∆ηnb2
O
O
2
2
helox
= (ηKn
+ ηfhelox
ilm ) − (ηKn + ηf ilm )
(3.34)
where the subscript ”nb” stands for non-bulk losses. Under air operation and if the bulk diffusion
is getting important, the non-bulk loss would exceed this value due to the non-linearity of the
mass transport losses:
O −helox
air
ηnb
> ∆ηnb2
O −helox
It also appears here that the ∆ηnb2
(3.35)
value is higher for the ”xx/BA” than for the
”xx/BC” cells, which was not that clear at lower current. But due to the non-linearity, the
O −helox
difference of ∆ηnb2
between the ”xx/BA” and the ”xx/BC” cells can be much smaller at
O −helox
lower current. However, the slight difference of ∆ηnb2
110
measured at 0.5 A/cm2 between the
3.2 Effect of the microporous layer (MPL)
”xx/BA” and the ”xx/BC” cells was maybe the pre-requisite for the high value of ∆Uhelox−air
measured at high RH.
Interestingly, it can be observed that the ”xx/BA” cells exhibit a steep change of ∆U O2 −helox
at lower RH, while showing a constant value of ∆Uhelox−air . At around RH=100%/0%, this
behavior suddenly changes and the ∆Uhelox−air value starts increasing while the ∆U O2 −helox
value stays almost constant. This result could be the sign of successive and partly decoupled
phenomena. As an interpretation, it can be proposed that the change of ∆U O2 −helox would
reflect the increasing non-bulk losses in the CL due to the accumulation of water in its pores.
Then, by the formation of a water film, for instance at the CL-GDL interface, the access to most
of the CL pore would be impeded and the loss would be dominated by bulk diffusion limitations,
as revealed by the increase of ∆Uhelox−air .
To summarize, the following hypothesis can be formulated based on the observations: the
absence of MPL on cathode tends to favor the accumulation of water, not directly visible with
neutron imaging, whose effect is a dramatic increase of the mass transport losses, among which
the Knudsen and/or film diffusion losses are thought to have a significant contribution.
3.2.3.3
IV-curves
IV-curve experiments are discussed hereafter for three different RH conditions: at RH = 100%/100%
(Figure 3.24), 100%/0% (Figure 3.25) and 40%/0% (Figure 3.26).
Regarding first the IV-curve at RH = 100%/100% (Figure 3.24), it can be observed that
the ”xx/BC” cells exhibit a better performance than the ”xx/BA” cells. Whereas the first ones
demonstrate very similar voltage values as the ones reported by Boillat et al. [52] at RH =
90%/90%, the latter show severe losses at already 0.5 A/cm2 . The ”BC/BA” cell could not be
operated at a higher current and the ”BA/BA” cell was operated up to 0.8 A/cm2 . One can
also notice that the ”BA/BA” cell shows a slightly lower OCV voltage than the other cells.
The ∆Uhelox−air values are accordingly much higher for the ”xx/BA” cells than for ”xx/BC”
cells. In particular, the differences of ∆Uhelox−air values between the cells are well correlated
with the differences of U . As expected, the lower OCV voltage of the ”BA/BA” cell is not
reflected in the value of ∆Uhelox−air .
The resistivity measurements do not show important change over the whole range of current
densities.
Based on these results, it can be assumed that all cells are increasingly affected by mass
transport losses for i ≥ 0.5 A/cm2 . Furthermore, the differences of performance between the
”xx/BA” and the ”xx/BC” cells can be attributed to mass transport losses. It can be observed
again that the average water content in the cathode GDL do not explain these differences.
111
3. RESULTS
0.4
ΔUhelox-air [V]
U [V]
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
0.2
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
1.2
1.4
1.6
1.2
1.4
1.6
0.4
ΔUO2-helox [V]
0.16
r [Ω·cm2]
0.3
0.12
0.08
0.3
0.2
0.1
0.04
0
0
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
30
30
Cathode GDL - Channel area
Water content [% vol tot]
Water content [% vol tot]
Anode GDL - Channel area
25
20
15
10
5
0
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
25
20
15
10
5
0
1.6
30
0
0.6
0.8
1
2
i [A/cm ]
Cathode GDL - Rib area
Water content [% vol tot]
Water content [% vol tot]
0.4
30
Anode GDL - Rib area
25
20
15
10
5
0
0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
25
20
15
10
5
0
0
0.2
0.4
2
BC/BC
0.6
0.8
1
2
i [A/cm ]
i [A/cm ]
BA/BC
BC/BA
BA/BA
Figure 3.24: Performance indicators for an IV-curve realized at RH = 100%/100%.
112
3.2 Effect of the microporous layer (MPL)
0.4
ΔUhelox-air [V]
U [V]
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
0
0.2
ΔUO2-helox [V]
0.12
0.08
0
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
1.2
1.4
1.6
1.2
1.4
1.6
0.3
0.2
0
0
1.6
30
30
Cathode GDL - Channel area
Water content [% vol tot]
Anode GDL - Channel area
Water content [% vol tot]
0.4
0.1
0.04
25
20
15
10
5
0
25
20
15
10
5
0
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
0
1.6
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
30
30
Cathode GDL - Rib area
Water content [% vol tot]
Anode GDL - Rib area
Water content [% vol tot]
0.2
0.4
0.16
r [Ω·cm2]
0.3
25
20
15
10
5
25
20
15
10
5
0
0
0
0.2
0.4
0.6
BC/BC
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
BA/BC
0
0.2
0.4
BC/BA
0.6 0.8
2
i [A/cm ]
1
BA/BA
Figure 3.25: Performance indicators for an IV-curve realized at RH = 100%/0%.
113
0.4
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
ΔUhelox-air [V]
U [V]
3. RESULTS
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
0
0
1.6
0.5
ΔUO2-helox [V]
r [Ω·cm2]
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
0.4
0.4
0.3
0.2
0.3
0.2
0.1
0.1
0
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
0
0
1.6
30
30
Anode GDL - Channel area
Water content [% vol tot]
Water content [% vol tot]
0.2
0.1
0
25
20
15
10
5
Cathode GDL - Channel area
25
20
15
10
5
0
0
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
0
1.6
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
1.2
1.4
1.6
30
30
Anode GDL - Rib area
Water content [% vol tot]
Water content [% vol tot]
0.3
25
20
15
10
5
0
0
0.2
0.4
0.6
BC/BC
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
BA/BC
Cathode GDL - Rib area
25
20
15
10
5
0
0
0.2
0.4
BC/BA
0.6
0.8
1
2
i [A/cm ]
BA/BA
Figure 3.26: Performance indicators for an IV-curve realized at RH = 40%/0%.
114
3.2 Effect of the microporous layer (MPL)
On the contrary to the ∆Uhelox−air values, the ∆U O2 −helox values show only a small increase
with the current. As discussed, the fact that the changes of ∆Uhelox−air are not reflected in
∆U O2 −helox validates that ∆Uhelox−air ≈ ηgair , at least for i ≤ 0.5. An interesting feature of
the ∆U O2 −helox value is that is seems to be linearly dependant on the current in the case of
the ”xx/BC” cells. As expected yet, the non-bulk diffusive losses should be more important at
higher current density. This result could be in agreement with the assumption made by Boillat
et al. [52] who attributed this to the limited ionic conductivity of the electrode. In presence
of pure O2 , it can be assumed that the current density in the electrode will be concentrated
towards the MEA. On the contrary, a limited partial pressure of O2 throughout the electrode
could result in a more homogeneous current distribution and therefore to a higher effect of ionic
losses in the CL ionomer. Moreover, the ∆U O2 −helox value for the ”xx/BC” cells seem to be the
highest under the driest RH, as it can be seen at RH = 40%/0% (Figure 3.26). In the latter case,
this effect would be reinforced by a drying of the CL ionomer. These aspects will be discussed
in the next chapter (subsubsection 3.3.3.3).
The ”xx/BA” cells seem to exhibit, at least at RH = 100%/0%, a stronger increase of
the ∆U O2 −helox values. Similarly to the RH-series experiment discussed previously (subsubsection 3.2.3.2), one remarks that a given decoupling of the ∆U O2 −helox and the ∆Uhelox−air can
occur. On one hand, it can be observed that the ∆Uhelox−air value shows a major increase at
RH = 100%/100% whithout important change of ∆U O2 −helox . On the other hand at RH =
100%/0%, the increase of ∆U O2 −helox is higher than the change of ∆Uhelox−air . This could be,
as hypothesized before, the sign of different effects of water accumulation, that would at first
occupy the small pores of the CL at intermediate humidity and then constitute a film at higher
humidity.
At last, it can be noticed that the value found for ∆U O2 −helox at the origin, around 50 mV
of f set
for the different IV-curves, matches well with the value ∆U O
= 45 mV [13] presented
2 −helox
earlier.
3.2.3.4
RH-step
A transient step of RH is applied to change from a dry (40%/0%) to a wet condition (100%/100%),
by a so-called ”dry-wet” step (Figure 3.27) and inversely (Figure 3.28) , by a ”wet-dry” step,
at a constant current of 0.5 A/cm2 . As defined, the voltage Uhelox is measured during the helox
pulses (1s pulse every 30s).
Performance
In the following, the ”dry-wet” transition is chosen as a base for the comments
(Figure 3.27), but analog comments could be made for the ”wet-dry” transition (Figure 3.28).
115
3. RESULTS
0.4
0.8
0.4
0.35
0.75
0.35
0.7
0.3
0.7
0.3
0.65
0.25
0.65
0.25
0.6
0.2
0.6
0.2
0.55
0.15
0.55
0.15
0.5
0.1
0.5
0.05
0.45
Uhelox
Uair
0.45
ΔUhelox-air
0
0.4
r [Ω·cm ]
0
30
30
Water content [% vol tot]
Channel
Rib
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
0
10
Channel
Rib
25
20
15
10
5
0
-2
-1
0
1
2
Cell “BA/BC”
4
5
6
7
8
9
10
Cell “BC/BC”
0.4
0.8
0.4
0.35
0.75
0.35
0.7
0.3
0.7
0.3
0.65
0.25
Uhelox
Uair
ΔUhelox-air
0.6
0.55
0.2
0.15
U [V]
0.8
0.75
ΔUhelox-air [V]
U [V]
3
Time [min]
Time [min]
0.55
0.1
0.5
0.45
0.05
0.45
0
r [Ω·cm ]
0.1
0
30
Water content [% vol tot]
20
15
10
5
0
-2
-1
0
1
2
3
4
5
6
7
8
9
0.05
0
0.1
0
Channel
Rib
10
0.15
0.2
30
25
0.2
0.1
0.4
2
r [Ω·cm2]
Water content [% vol tot]
0.2
Uhelox
Uair
ΔUhelox-air
0.6
0.5
0.4
0.25
0.65
ΔUhelox-air [V]
-1
0.05
0.1
0
-2
ΔUhelox-air
0.2
2
r [Ω·cm2]
0.1
25
0.1
Uhelox
Uair
0.4
0.2
Water content [% vol tot]
U [V]
0.8
0.75
ΔUhelox-air [V]
Cell “BC/BA”
ΔUhelox-air [V]
U [V]
Cell “BA/BA”
Channel
Rib
25
20
15
10
5
0
-2
-1
0
1
2
3
4
5
6
7
8
9
10
Time [min]
Time [min]
Figure 3.27: Performance indicators and water evolution for a RH transient applied at t = 0 from
40%/0% to 100%/100% with a constant current of 0.5 A/cm2 .
116
3.2 Effect of the microporous layer (MPL)
Cell “BC/BA”
Uhelox
Uair
ΔUhelox-air
0.8
0.4
0.35
0.75
0.35
0.3
0.7
0.3
0.65
0.25
0.65
0.25
0.6
0.2
0.55
0.15
0.5
0.1
0.5
0.45
0.05
0.45
r [Ω·cm ]
2
0.1
0
30
Water content [% vol tot]
20
15
10
5
0
-1
0
1
2
3
4
5
6
7
8
9
0
Channel
Rib
25
20
15
10
5
0
10
-2
-1
0
1
2
Time [min]
Cell “BA/BC”
4
5
6
7
8
9
10
Cell “BC/BC”
Uhelox
Uair
ΔUhelox-air
0.7
0.4
0.8
0.4
0.35
0.75
0.35
0.3
0.7
0.3
ΔUhelox-air [V]
0.8
0.75
0.65
0.25
0.6
0.2
0.55
0.15
0.5
0.1
0.5
0.1
0.45
0.05
0.45
0.05
0.65
0.55
0
0.4
2
r [Ω·cm ]
0.2
0.1
0.25
Uhelox
Uair
ΔUhelox-air
0.6
0.1
0
30
Channel
Rib
20
15
10
5
0
-2
-1
0
1
2
3
4
5
6
7
8
9
10
Channel
Rib
25
20
15
10
5
0
-2
Time [min]
0.15
0.2
30
25
0.2
0
0
Water content [% vol tot]
Water content [% vol tot]
r [Ω·cm2]
0.4
U [V]
U [V]
3
Time [min]
ΔUhelox-air [V]
-2
0.1
0.05
Cathode GDL
Channel
Rib
0.15
0.1
30
Cathode GDL
0.2
0.2
0
25
Uhelox
Uair
ΔUhelox-air
0.55
0.4
0.2
r [Ω·cm2]
0.6
0
0.4
Water content [% vol tot]
U [V]
U [V]
0.7
0.4
ΔUhelox-air [V]
0.8
0.75
ΔUhelox-air [V]
Cell “BA/BA”
-1
0
1
2
3
4
5
6
7
8
9
10
Time [min]
Figure 3.28: Performance indicators and water evolution for a RH transient applied at t = 0 from
100%/0% to 40%/0% with a constant current of 0.5 A/cm2 .
117
3. RESULTS
The study of the voltage U reveals different behaviors between the cells. While all voltage
evolutions present a maximum during the transient, the values reached after stabilization differ.
For the ”BA/BA” cell, the U value reaches approximately the same value after the step as
before. In the case of the ”BC/BA” cell, it is reduced after the change. The ”xx/BC” cells show
both an increase of the U value after the transition and a quite small peak.
These behaviors illustrate the complex interplay resulting from a beneficial humidification
on the membrane resistance and a detrimental increase of the mass transport limitations. The
voltage under air U contains these different contributions and is therefore difficult to be directly
interpreted.
The resistance values show a decrease in all cases. Regarding the absolute values, one remarks
again a consistent order between the cells, the ”BA/BA” cell having the higher resistance. The
changes have approximately the same amplitude.
The Uhelox value is improved for all cells, since it reflects the change of resistances and is not
affected by mass transport losses.
The ∆U helox−air values exhibit a monotone increase for all cells, but with very different
amplitudes. Whereas the change is small for the ”xx/BC” cells, it shows a bigger difference for
the ”xx/BA” cells. In particular, it is the most important for the ”BA/BC” cell.
Average water amount in GDL
The average water content measured in the cathode GDL
for both the regions under the channels and under the ribs are reported on the same figures
(Figure 3.27 and Figure 3.28). Considering the ”dry-wet” transition, it can be noticed that the
values of water amount are in good agreement with the ones found in the steady-state experiment
discussed previously (RH-series). For the ”wet-dry” transition however, the ”BC/BA” cell shows
a particular behavior in terms of a slightly higher value under the channel than under the rib,
which was not the case for the steady-state experiment. This discrepancy can be attributed to
the inherent difficulty to control properly the humidity at 100%/100%. Another reason can be
the different equilibrium times at RH = 100%/100%: 40 min in the present experiment instead
of 10 min for the RH-series. Indeed, it seems here that the water amounts are not stabilized
after 10 min.
Discussion
It can be attempted to compare the different time constants of the process dis-
cussed by means of normalized values, for both the ”dry-wet” (Figure 3.29) and ”wet-dry”
transitions (Figure 3.30).
For the water contents, the value before and after the step are
taken for the normalization. For the ∆Uhelox−air value of the ”xx/BC” cells in the ”wet-dry”
condition, the minimum value reached during the transition is taken as 0, since the subsequent
changes are attributed to dry-out effects. With this method, the different time constants of the
process occurring can be compared.
118
3.2 Effect of the microporous layer (MPL)
Cell “BC/BA”
1.2
1.2
1
1
Water content [-] / ΔUhelox-air [-]
Water content [-] / ΔUhelox-air [-]
Cell “BA/BA”
0.8
0.6
0.4
0.2
GDL under channels
GDL under ribs
ΔUhelox-air
0
0.8
0.6
0.4
0.2
GDL under channels
GDL under ribs
ΔUhelox-air
0
-0.2
1
1
r [-]
r [-]
-0.2
0
0
-2
-1
0
1
2
3
4
5
6
7
8
9
10
-2
-1
0
1
2
Time [min]
Cell “BA/BC”
4
5
6
7
8
9
10
Cell “BC/BC”
1.2
1.2
1
1
Water content [-] / ΔUhelox-air [-]
Water content [-] / ΔUhelox-air [-]
3
Time [min]
0.8
0.6
0.4
0.2
GDL under channels
GDL under ribs
ΔUhelox-air
0
0.8
0.6
0.4
0.2
GDL under channels
GDL under ribs
ΔUhelox-air
0
-0.2
1
1
r [-]
r [-]
-0.2
0
-2
0
-1
0
1
2
3
4
5
6
7
8
9
10
-2
Time [min]
-1
0
1
2
3
4
5
6
7
8
9
Time [min]
Figure 3.29: Normalized values of performance indicators and water evolution for a RH transient
applied at t = 0 from 100%/0% to 40%/0% with a constant current of 0.5 A/cm2 .
119
10
3. RESULTS
Cell “BA/BA”
Cell “BC/BA”
1.2
GDL under channels
GDL under ribs
ΔUhelox-air
1
Water content [-] / ΔUhelox-air [-]
Water content [-] / ΔUhelox-air [-]
1.2
0.8
0.6
0.4
0.2
GDL under channels
GDL under ribs
ΔUhelox-air
1
0.8
0.6
0.4
0.2
0
0
-0.2
-0.2
1
r [-]
r [-]
1
0
-2
0
-1
0
1
2
3
4
5
6
7
8
9
-2
10
-1
0
1
2
Cell “BA/BC”
4
5
6
7
8
9
10
Cell “BC/BC”
1.2
1.2
GDL under channels
GDL under ribs
ΔUhelox-air
GDL under channels
GDL under ribs
ΔUhelox-air
1
Water content [-] / ΔUhelox-air [-]
1
Water content [-] / ΔUhelox-air [-]
3
Time [min]
Time [min]
0.8
0.6
0.4
0.2
0.8
0.6
0.4
0.2
0
0
-0.2
-0.2
1
r [-]
r [-]
1
0
0
-2
-1
0
1
2
3
4
5
6
7
8
9
10
-2
Time [min]
-1
0
1
2
3
4
5
6
7
8
9
Time [min]
Figure 3.30: Normalized values of performance indicators and water evolution for a RH transient
applied at t = 0 from 100%/0% to 40%/0% with a constant current of 0.5 A/cm2 .
120
10
3.2 Effect of the microporous layer (MPL)
By looking at the ”dry-wet” transition (Figure 3.29), it can be remarked that whereas the
time constants between the channel and rib regions are similar for the ”xx/BC” cells, a slight
difference is revealed by the profiles of the ”xx/BA” cells in form of a slower humidification time
in the channel region of the GDL. The ∆Uhelox−air value changes in accordance but cannot be
clearly correlated to a precise area.
A difference also appears in the case of the ”wet-dry” transition between the ”xx/BA” and
the ”xx/BC” cells. On one hand, the water profiles under the rib and channel regions of the
”xx/BA” cells demonstrate a similar time constant. On the other hand, the water profiles under
the rib and channel regions of the ”xx/BC” cells exhibit different time constants, with a slower
drying process under the ribs than under the channels. In this case, the time constant of the
∆Uhelox−air value tends to be closer to the time constant of the water profile in the channel
region. Moreover, after having reached a minimum, the ∆Uhelox−air value increases again along
with the resistance. Contrary to the ”dry-wet” transition, a delay appears between the changes
of water amount and the change of ∆Uhelox−air . This could be the sign that some part of the cell,
crucial for the water transport, would be dried-out and that this effect would not be captured
by the measurement of the total water amount in the GDL.
The disparities observed between the ”xx/BC” and the ”xx/BA” cells might reflect differences of channel-rib distribution of the current, under the assumption of similar water removal
between the two cases. As a possible interpretation, this could indicate that the channel-rib
distribution of current would be comparatively more homogeneous under wet conditions in the
”xx/BC” cells that in the ”xx/BA” cells.
3.2.4
Summary and outlook
The effect of the presence or not of a microporous layer (MPL) was investigated by means of
four combinations of cells (with no MPL, with a MPL on anode, with a MPL on cathode and
with MPLs on both sides). The cells equipped with a MPL on cathode showed, for all conditions
tested here, a better performance than the cell having no MPL on cathode side.
The differences of performance between these two types of cells were attributed, based on the
∆Uhelox−air values, to important differences of mass transport losses. Based on the ∆U O2 −helox
values, it is thought that both bulk and non-bulk losses can play a role in the diffusive transport
losses, but their relative importance my change depending on the operating conditions.
The average saturation level in the GDL was measured to be the same for all cells. This
important result demonstrates that the differences of mass transport losses between the cells
having a MPL on cathode and those having no MPL on cathode does not originate from different
water saturation levels in the GDL. Concerning the water transport mechanisms, this shows that
the MPL does not imply a higher back-diffusion flow towards the anode side and does not allow
121
3. RESULTS
reducing the water content in the GDL on cathode side, which was even measured to be higher
in given areas for the cells having a cathode MPL.
The spatial in-plane distribution profiles revealed that the maximum water content in the
cathode porous medium was located close to the MEA in case of the cells having no cathode
MPL, and close to the channels for the cells having a cathode MPL. It was deduced that the
cells having no MPL on cathode present a higher tendency to accumulate water in critical areas,
which could be the CL pores and/or the CL/GDL interface.
At last, transient experiments gave some information about possible differences of channel-rib
current distribution between the different types of cells.
The effect of the MPL on anode side only could not be clearly pointed out. Due to its high
mass transport losses in dry state, it is supposed that the sample used in this study was affected
by other parameters that were non representative of a possible effect of the MPL on anode side.
A new set of tests using ”1D” geometry of channels would probably help further understanding the effect of the MPL, both on the performance and the water distribution. The use of
MPLs with more or less defects could also be considered. Repeatability tests would be helpful
to estimate the statistical distribution of average water content, since the filling of the GDL is
expected to be stochastic.
The characterization of a possible effect of the contact resistances on the current distribution would be interesting to be studied, for instance by a RH transient combined with a local
measurement of the current distribution. So as to keep the same structure on cathode side, the
modification of the contact resistance on the anode side could be an option to realize such a
test.
At last, a breakthrough in spatial resolution allowing distinguishing the catalyst layer would
be of course of high relevance to investigate the assumptions made here about the water accumulation in the pores.
122
3.3 Effect of flow field design
3.3
3.3.1
Effect of flow field design
Introduction
The morphology and dimensions of the gas channels can have a strong impact on both the cell
performance and the water distribution. Considerations such as the efficient removal of water
droplets, the gas distribution on the cell surface or the machining cost, play an important role at
the scale of the technical stack design. The scientific works dedicated to the effect of flow channel
geometry therefore mainly address its effect on liquid water accumulation and dynamics, or the
parameter distribution [101, 102], this by working on cells supplied with gas flows at technical
stoichiometries (< 2), as detailed in a review paper [103].
However, other effects are revealed on the local scale that can also impact the global performance of the stack. Basically, the major trade-off that has to be found to optimize the
performance relies on the positive effect given by small ribs, which allow reducing the oxygen
diffusion pathways across the GDL, and of wide ribs, which provide a low contact resistance
(thermal and electrical) and maintain a good hydration level in the membrane by limiting the
water removal.
At a more fundamental level though, the origins of this losses are not clearly attributed.
Therefore, beyond conducting optimization experiments, it is also of prime relevance to be
able to simplify the system so as to make reliable interpretations. The differential cell aims
at suppressing the along-the-channel dimension, passing from a 3D to a 2D system. The characterization of such a system is then greatly simplified, but remains too complex for clearly
isolating the different mechanisms. Namely, the channel-rib dimension of the system can have
a strong impact of the current distribution [11] or water repartition [8, 104]. As a further step,
the suppression of the channel-rib distribution, using a ”1D” instead of a ”2D” flow field (subsection 2.3.9), can be realized to point out the diffusive transport processes in the GDL. Finally,
by forcing a convective flow throughout the GDL in the ”0D” (subsection 2.3.9) flow field cell
(micro-interdigitated), the diffusion processes tend to disappear (subsubsection 3.1.4.4) and a
homogeneous distribution of the parameters should be obtained on the MEA surface. Then,
some insights about the transport mechanisms in the MEA, and in particular in the CL, can be
obtained.
If the studies conducted about ”2D” differential cells are quite rare, the systematic comparison of ”2D”, ”1D” and ”0D” differential cells has, to our knowledge, never been reported so
far.
123
3. RESULTS
3.3.2
Experimental
In the present work, the different flow field geometries (”2D”, ”1D” and ”0D”) presented earlier
(subsection 2.3.9) are investigated by means of in situ electrochemical characterization and inplane neutron imaging of several cells thanks to the multi-cell setup (section 2.3). The helox
pulsing method was used simultaneously (section 2.2).
The different types of samples, as well as the operating conditions tested, are summarized
on the table hereafter (Table 3.4). The experimental protocol was rigorously the same as in
the previous chapter (subsection 3.2.2), with exception of the cell flows. The cell flows were
0.4 Nml/min on anode and 1 Nml/min on cathode for the ”2D” cell. Due to its bigger channel
section, the ”1D” cell was supplied with higher mass flows so as to keep same gas velocities as
in the ”2D” cell: 0.6 Nml/min on anode and 1.5 Nml/min on cathode. In the ”0D” cell, the
gas flows circulate convectively through the GDL and water removal mechanisms are inherently
different than for the other cells. It is therefore not relevant to adapt the flow in that case and
the same flows as for the ”2D” cell are applied.
At last, it must be mentioned that the ”0D” and ”2D” cells were tested simultaneously,
whereas the ”1D” cell was tested in another multi-cell assembly, together with the different
cells investigated for the MPL study presented before (section 3.2). Moreover, the ”2D” cell
presented in this chapter represents another sample than the one used for the ”BC/BC” sample
in the MPL study. The repeatability of the experiment will be positively verified throughout
this chapter.
As before, the notation ”RH = x%/y%” will be utilized, meaning: RH = x% on the anode
side and RH = y% on the cathode side.
Design
parameters
Measurement
campaign
2011
Operation
parameters
Experimental
set-up
MEA
Flowfield
GDL
anode
GDL
cathode
Primea 5710
Gore
0D
24BC
24BC
Current
(IV-curve)
Multi-cell
Primea 5710
Gore
1D
24BC
24BC
RH
(RH-series)
Midi set-up
Primea 5710
Gore
2D
24BC
24BC
RH
(RH-step)
(PCO™ camera)
Table 3.4: Summary of experiments realized to study the impact of the flow field design.
124
3.3 Effect of flow field design
3.3.3
Results
3.3.3.1
RH-series - 1 A/cm2
As a first experiment, a series of steady-states at different RHs and with a constant current of
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.4
ΔUhelox-air [V]
U [V]
1 A/cm2 is realized (Figure 3.31).
80%
0%
100%
20%
100%
60%
0
100%
100%
RH an. 40%
RH ca. 0%
80%
0%
100%
20%
100%
60%
100%
100%
RH an. 40%
RH ca. 0%
80%
0%
100%
20%
100%
60%
100%
100%
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0 RH an. 40%
RH ca. 0%
80%
0%
100%
20%
100%
60%
100%
100%
0.4
0.4
0.3
0.3
ΔUO2-helox [V]
r [Ω·cm2]
0.2
0.1
RH an. 40%
RH ca. 0%
0.2
0.1
0.2
0.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0 RH an. 40%
RH ca. 0%
80%
0%
100%
20%
100%
60%
0
100%
100%
2
RH an. 40%
RH ca. 0%
UO +ri [V]
0
U+ri [V]
0.3
80%
0%
100%
20%
100%
60%
100%
100%
2D
1D
0D
Figure 3.31: Performance indicators for steady-state steps of RH realized at i = 1 A/cm2 .
125
3. RESULTS
Performance
The voltage curves show that under dry conditions, the ”0D” cell shows the
poorest performance, while the ”1D” cell voltage is a little higher and the ”2D” cell exhibits
the best performance. On the other hand, the ”2D” cell shows the lowest voltage under wet
conditions compared to the ”0D” and ”1D” cells. The ”2D” cell presents an optimal voltage at
intermediate RH (100%/20%).
The differences between the cell voltages at low RH partly disappear on the IR-free values U +
ri, meaning that the resistance differences explain part of the voltage differences. Accordingly,
the resistance values of the ”0D” and ”1D” cells seem to be more sensitive to drying than for
the ”2D” cell. However, a slight difference of the U + ri value remains for the ”0D” cell at RH
= 40%/0%. Under wet conditions, the ”2D” cell shows the lowest U + ri voltage.
The IR-free values under oxygen U O2 + ri are constant over the whole RH range for all cells.
O
This indicates that the losses contained in the U O2 + ri term, that are the kinetic losses ηCT2
O
O
2
, are independent of RH. Whereas the ”0D” and
and possible non-bulk losses ηKn2 and/or ηf ilm
the ”1D” cell have an identical value, the ”2D” cell shows a somewhat higher value, which can
O
be attributed to slightly smaller kinetic losses ηCT2 for this cell.
The ∆Uhelox−air values show different behaviors. The curve obtained for the ”2D” cell is
similar to what was observed in the MPL study (subsubsection 3.2.3.2) and to what is reported
by Boillat et al. in [52]. To summarize the results again: an optimal point with the lowest
mass transport losses is located at intermediate RH, whereas higher mass transport losses are
respectively attributed to channel-rib effect under dry conditions and to water accumulation
(essentially under the channels) in the cathode GDL under wet conditions.
The ”0D” cell tends to show the same behavior but the nature of the losses are different
in this case. Comparatively to the ”2D” and the ”1D” flow fields, the bulk diffusive losses
in the GDL are expected to be minimal (ideally inexistent) in the ”0D” cell. Moreover, it was
pointed out during the validation experiment of the helox pulsing method (subsubsection 2.2.4.1)
that important dry-out effects can disturb the measurement of Uhelox during the helox pulse,
in particular for the ”0D” structure under dry conditions. The value ∆Uhelox−air measured is
therefore judged to be less reliable, as illustrated by the helox pulses taken during this experiment
(Figure 3.32). The peak value obtained in the beginning of the pulse is rather to be attributed
to transients of O2 concentration or pressure. However, a real change of ∆Uhelox−air cannot be
excluded, but it will be discussed in the next experiment where the pulse results are considered
more reliable (subsubsection 3.3.3.2). Similarly to the ”0D” cell, the measurement of Uhelox
for the ”1D” cell in dry conditions is also subject to dry-out disturbances, as illustrated above
(Figure 3.32). The differences of ∆Uhelox−air appearing between the ”1D” and the ”0D” cells
will be discussed in the next chapter as well (subsubsection 3.3.3.2).
126
3.3 Effect of flow field design
2
U [V]
r [Ω·cm ]
0D
0.7
0.6
Uhelox
0.5
0.4
2
U [V]
0.2
0.7
0.1
0.6
0
0.5
r [Ω·cm ]
1D
Uhelox
-2
-1
0
1
2
t [s]
3
4
5
-3
0.2
0.8
0.1
0.7
0
0.6
r [Ω·cm ]
2D
0.2
0.1
Uhelox
0
0.5
0.4
-3
2
U [V]
-2
-1
0
1
2
t [s]
3
4
5
-3
-2
-1
0
1
2
t [s]
3
4
5
Figure 3.32: Helox pulses during the driest condition (40%/0%) of the RH-series realized at i =
1 A/cm2 : comparison of different flow fields.
At intermediate RH (Figure 3.31), the ∆Uhelox−air values for the ”0D” and ”1D” are still
affected by dry-out effects. One can only notice that both values are lower than for the 2D cell.
Under wet conditions (Figure 3.31), where the ∆Uhelox−air values are judged reliable, it
appears that the ”0D” and the ”1D” cells exhibit a slight increase of ∆Uhelox−air that is not
reflected in the ∆U O2 −helox value. This can be attributed to increased mass transport losses.
Since the diffusive losses in the GDL should a priori be different in the ”0D” and the ”1D” cells,
the MPL and/or the CL can be proposed as possible candidates for these limitations. In the case
of the MPL, it can be imagined that due to its low permeability to gases, the convective flow
in the ”0D” cell would essentially by-pass the MPL and circulate through the GDL. It would
follow that the gas transport in the MPL would be dominated by diffusion, as for the ”1D” cell.
This will be further detailed in the following. In comparison, the higher value of ∆Uhelox−air
for the ”2D” cell under wet condition can be attributed to bulk diffusion losses occurring in the
GDL.
Average water amount in GDL
The water contents in the anode and cathode GDLs are
plotted (Figure 3.33). Whereas the channel and rib regions of the GDL are presented for the
”2D” and ”1D” cells, one single zone referred to as channel area is given for the ”0D” cell .
The channel regions of the cathode GDL present the same value for the ”2D” and the ”1D”
cells, contrary to the drier ”0D” cell. In the latter case, this can be explained by the efficient
water removal of the convective gas flow in the GDL. The water amounts in the rib areas of
the cathode GDL are clearly different between the ”2D” and the ”1D” cell. As expected, the
value of the ”1D” cell is similar as the value measured under the channels, which validates the
”1D” behavior targeted. On the contrary, the rib region of the ”2D” shows a much higher water
content.
Consistently with the observations made in the previous chapter (subsubsection 3.2.3.1) and
in [52], it turns out that the important increase of the water amount in the rib regions of the
cathode GDL of the ”2D” cell has no strong impact on the mass transport losses measured
127
3. RESULTS
30
Anode GDL - Channel area
Water content [% vol tot]
Water content [% vol tot]
30
25
20
15
10
5
0
RH an. 40%
RH ca. 0%
80%
0%
100%
20%
100%
60%
Cathode GDL - Channel area
25
20
15
10
5
0
100%
100%
30
100%
20%
100%
60%
100%
100%
100%
60%
100%
100%
Cathode GDL - Rib area
Water content [% vol tot]
Water content [% vol tot]
80%
0%
30
Anode GDL - Rib area
25
20
15
10
5
0
RH an. 40%
RH ca. 0%
RH an. 40%
RH ca. 0%
80%
0%
100%
20%
100%
60%
25
20
15
10
5
0
100%
100%
2D
1D
RH an. 40%
RH ca. 0%
80%
0%
100%
20%
0D
Figure 3.33: Water content for steady-states step of RH realized at i = 1 A/cm2 .
with the ∆Uhelox−air value, which is illustrative of the low oxygen diffusive flow in this region
compared to the channel region. Moreover, the differences of ∆Uhelox−air between the 1D and
the 2D cells cannot, in the present case, be explained by differences of water amount in the
channel region. It can be assumed that due to the differences of channel-rib distributions of the
current [11, 12], the current in the channel region of the cathode GDL may be higher in the ”2D”
than in the ”1D” case, which would be the reason for the higher mass transport losses in the
”2D” cell. More generally, it will be observed that the ∆Uhelox−air value will be systematically
higher for the ”2D” than for the ”1D” cell for all the operating conditions presented in this
chapter.
Besides, it can also be noticed that the differences of water amount between the ”0D” and
the ”1D” cell are not reflected in the ∆Uhelox−air values. It must also be reminded at this point
that important differences of diffusive limitations were observed in the deuterium labeling study
(subsubsection 3.1.4.4) between these two designs. Then, while it is expected that the water
accumulation in the GDL has no effect on the ∆Uhelox−air value of the ”0D” cell, it is interesting
to observe that it does not imply either a higher value of ∆Uhelox−air for the ”1D” cell. This
128
3.3 Effect of flow field design
tends to reinforce the hypothesis that bulk losses occur elsewhere: in the MPL and/or in the
CL. However, by looking at the channel region of the cathode MPL (Figure 3.34) one observes
Water content [% vol tot]
30
Cathode MPL - Channel area
25
20
15
10
5
0
RH an. 40%
RH ca. 0%
80%
0%
100%
20%
2D
100%
60%
100%
100%
1D
0D
Figure 3.34: Water content in the channel regions of the MPL for steady-states step of RH realized
at i = 1 A/cm2 .
at high RH that the water content in the ”1D” cell is higher than in the ”0D” and ”2D” cells.
This indicates that there is no direct correlation between the different water contents in the
MPL region and the different ∆Uhelox−air values. Under reserve of no sample variation and of
a proper definition of the areas, the bulk diffusion losses can therefore be assumed to originate
from the CL. This will be further discussed in the next chapter.
3.3.3.2
IV-curves
In the following, IV-curve experiments are presented for different RH conditions: 100%/100%
(Figure 3.35), 100%/0% (Figure 3.36) and 40%/0% (Figure 3.37).
Performance
The voltage values obtained for the different RH conditions clearly reveal the
relative advantages of the different flow field designs suggested in the previous experiment.
Under fully humidified conditions (RH = 100%/100%), the ”0D” and ”1D” designs demonstrate a better performance compared to the ”2D” design. While resistivities are close to each
others, important differences of ∆Uhelox−air values are observed. For the ”2D” cell, the high
values can be attributed as previously to bulk mass transport losses originating from the channel
regions of the cathode GDL. On the contrary, the ∆Uhelox−air values are quite low for the ”0D”
and ”1D” cells. Moreover, one sees that the ∆Uhelox−air value is clearly higher for the ”1D”
than for the ”0D” cell. Whereas part of the loss can be attributed to bulk diffusion in the GDL
for the ”1D” cell, the loss observed for the ”0D” cell is to be searched elsewhere. Again, possible
losses in the MPL and/or the CL can be considered.
129
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.4
ΔUhelox-air [V]
U [V]
3. RESULTS
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
0
1.6
ΔUO2-helox [V]
r [Ω·cm2]
0.16
0.12
0.08
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
1.2
1.4
1.6
1.2
1.4
1.6
0.3
0.2
0.1
0.04
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
0
1.6
30
30
Anode GDL - Channel area
Water content [% vol tot]
Water content [% vol tot]
0
0.4
0.2
25
20
15
10
5
0
Cathode GDL - Channel area
25
20
15
10
5
0
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
0
30
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
30
Anode GDL - Rib area
25
Water content [% vol tot]
Water content [% vol tot]
0.2
0.1
0
0
0.3
20
15
10
5
0
Cathode GDL - Rib area
25
20
15
10
5
0
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
2D
0
1D
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
0D
Figure 3.35: Performance indicators for an IV-curve realized at RH = 100%/100%.
130
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.4
ΔUhelox-air [V]
U [V]
3.3 Effect of flow field design
0.2
0.1
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
0
1.6
0
0.2
0.4
0.6
0
0.2
0.4
0.6
ΔUO2-helox [V]
0.16
0.12
0.08
0
1.4
1.6
1
1.2
1.4
1.6
1
1.2
1.4
1.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
0.3
0.2
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
2
0.8
2
i [A/cm ]
i [A/cm ]
30
Water content [% vol tot]
30
Water content [% vol tot]
1.2
0.1
0.04
Anode GDL - Channel area
25
20
15
10
5
Cathode GDL - Channel area
25
20
15
10
5
0
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0
0.2
0.4
0.6
2
0.8
2
i [A/cm ]
i [A/cm ]
30
Water content [% vol tot]
30
Water content [% vol tot]
0.8
1
2
i [A/cm ]
0.4
0.2
r [Ω·cm2]
0.3
Anode GDL - Rib area
25
20
15
10
5
Cathode GDL - Rib area
25
20
15
10
5
0
0
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
2D
0
1D
0.2
0.4
0.6
0D
Figure 3.36: Performance indicators for an IV-curve realized at RH = 100%/0%.
131
0.4
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.3
ΔUhelox-air [V]
U [V]
3. RESULTS
0.1
0
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
0
ΔUO2-helox [V]
0.4
0.3
0.2
0.6
1
0.8
2
i [A/cm ]
1.2
1.4
1.6
0.3
0.2
0
0
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
1.2
1.4
1.6
1.2
1.4
1.6
30
Water content [% vol tot]
30
Water content [% vol tot]
0.4
0.1
0.1
Anode GDL - Channel area
25
20
15
10
5
Cathode GDL - Channel area
25
20
15
10
5
0
0
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
0
0.2
0.4
0.6
1
0.8
2
i [A/cm ]
30
Water content [% vol tot]
30
Water content [% vol tot]
0.2
0.4
0.5
r [Ω·cm2]
0.2
Anode GDL - Rib area
25
20
15
10
5
Cathode GDL - Rib area
25
20
15
10
5
0
0
0
0.2
0.4
0.6
0.8
1
2
i [A/cm ]
1.2
1.4
1.6
2D
0
1D
0.2
0.4
0.6
1
0.8
2
i [A/cm ]
0D
Figure 3.37: Performance indicators for an IV-curve realized at RH = 40%/0%.
132
3.3 Effect of flow field design
The ∆U O2 −helox values increase with the current and are similar for all cells. Due to the fact
that the values will be observed to be higher at lower RH and different between the cells, it can
be proposed that the ohmic loss in the CL ionomer is the origin of these losses. This argument
will be explained in details in the following (subsubsection 3.3.3.3). At this stage, we can only
remark that the differences of bulk diffusion, measured with ∆Uhelox−air , are not correlated with
a change of ∆U O2 −helox .
At intermediate relative humidity (RH = 100%/0%), the performances of the cells are very
similar. The ”2D” cell shows a lower resistivity than the other cells. The ”2D” cell also shows
a higher value of ∆Uhelox−air , which reveals the sensitivity of this cell to mass transport losses
even with a low water saturation level in the channel region of the cathode GDL. Similarly as
the previous condition, the values of ∆U O2 −helox are similar for all cells.
Finally, the driest condition (RH = 40%/0%) illustrates the drawback of the ”0D” and
”1D” cells, whose performances are lower than the ”2D” cell over the whole range of current
density. Important differences of resistivity appear, but it can be remarked that for the ”0D”
and the ”1D” cells, these differences are not reflected in the voltage value U . In addition, strong
discrepancies are also shown by the ∆Uhelox−air values. Furthermore, the ∆U O2 −helox values are
also different here, contrary to higher RH conditions.
As discussed later, it can be suggested that bulk diffusion limitations can have an effect on
the ”0D” cell under dry conditions, whereas being negligible for the ”1D” cell. Moreover, since
the ∆Uhelox−air value of the ”0D” cell is higher than for the ”1D” cell at RH = 40%/0%, which
was not the case at RH = 100%/0% or RH = 100%/100%, it can be assumed that this loss does
not originate in this case from water accumulation, in the MPL for instance, as proposed before.
Then, drying effects on the CL combined to diffusive limitations can be proposed, as developed
in the following.
Average water amount in GDL At RH = 40%/0%, the water contents are negligible,
except for the rib region of the GDL cathode of the ”2D” cell. This region can act like a water
reservoir preventing the membrane of severe dry-out, which can explain the lower resistance and
the high performance of this cell.
At RH = 100%/0%, the rib areas of the cathode and anode GDLs of the ”2D” cell show
a higher water content. In the channel regions, the water contents of all the different cells are
very similar.
At last, the water amounts at RH = 100%/100% show the following trends. The ”2D”
cell exhibits a slightly higher water content than the ”1D” cell. On the contrary, the values
were found to be identical in the previous experiment (RH-series, 1A/cm2 , 100%/100%). This
133
3. RESULTS
can be attributed to the difficulty to obtain a repeatable water content under fully-humidified
conditions.
3.3.3.3
Comments
In the following, a development is proposed to explain the behaviors of the ∆Uhelox−air values for
the ”0D” and ”1D” cells in dry conditions, in terms of possible drying effects in the CL ionomer.
The IV-curve at RH = 40%/0% will serve as base experiment for this. Based on the observation
of the corresponding helox pulses, it is assumed that the ∆Uhelox−air values measured for the
”0D” cell are reliable and representative of a real phenomenon. It will also be assumed that
the difference between the ∆Uhelox−air values of the ”0D” and the ”1D” cells do not originate
from a different oxygen concentration in helox than in air, which would be masked in the ”1D”
cell due to a fully Knudsen diffusion. This assumption seems reasonable since the U and Uhelox
values are identical at low current and for the different cells, as it can be seen on the graph
(Figure 3.39).
Drying-out effects in CL
Two extreme configurations can be imagined for the distribution
of current across a CL according to the simplified representation illustrated below (Figure 3.38).
If some diffusion limitations occur in the gas phase, being bulk diffusion or Knusden diffusion,
the ionomer pathway will tend to be preferred by the current than the gas pathway, leading to
relatively high ohmic losses in the CL ionomer. On the contrary, if the gas diffusion limitations
are low, the gas pathway will tend to be preferred by the current instead of the ionomer pathway.
It would follow that the ohmic losses in the CL ionomer would be reduced and the mass transport
increased compared to the previous case.
High gas diffusion limitations
=> ηΩ high
Low gas diffusion limitations
=> ηΩ low
Membrane
Ionomer
p
CL
O2
jiono
j
MPL
Figure 3.38: Symbolic illustration of the drying effect in the electrode.
134
3.3 Effect of flow field design
Based on this representation, the following real cases can be assumed. If the bulk diffusion
plays a non-negligible role, then the operation during the helox pulse will induce a better gas
diffusion than under air, and therefore a comparatively lower ohmic loss in the CL ionomer.
If Knudsen losses would dominate, then the diffusion limitations under air or helox would be
similar. In both cases however, the operation during the O2 pulse would tend to lower the ohmic
losses in the CL ionomer.
If two RH conditions are compared for the same gas, it can be expected that a better
hydration will lead not only to a lower ohmic loss, but also to a more homogeneous current
distribution along the ionomer. However, as the water accumulates in the CL pores in humidified
condition, the gas diffusion limitations would increase again. An optimal operating point in the
CL should consequently allow for a well hydrated ionomer and a free path for gases.
Relation to observation Based on the results, the following interpretations can be made.
The differences of the ∆Uhelox−air values between the cells obviously change according to the
RH conditions. At RH = 100%/100%, the behaviors are the less surprising since they reflect
the expected differences in terms of bulk diffusion limitation in the GDL: the ”0D” cell has
the lowest value due to the low (or inexistent) diffusion limitation in the GDL, the ”1D” cell
shows an intermediate value and the ”2D” exhibits the highest value. Moreover, as observed in
the RH-series experiment, the difference of ∆Uhelox−air between the ”1D” and the ”2D” cells
cannot be explained by differences of water amount in the cathode GDL, which supports the
assumption that the inhomogeneous channel-rib current distribution in the ”2D” cell plays a
dominant role on the mass transport losses.
At RH = 100%/0%, the fact that the ∆Uhelox−air value of the ”1D” cell is similar to the
value of the ”0D” cell tends to show that bulk diffusion limitations probably occur elsewhere
than in the GDL: the MPL and the CL would be the candidates. On the contrary, the higher
losses of the 2D cell would still be attributed to an inhomogeneous current.
At RH = 40%/0%, the differences of ∆Uhelox−air values between the cells show a surprising
behavior, namely that the ”0D” cell has a higher value than the ”1D” cell. Precisely, the
∆Uhelox−air value of the ”0D” cell is higher than at RH = 100%/0% and the ∆Uhelox−air of
the ”1D” cell is lower. Diffusion limitations in the MPL seem therefore inappropriate and
drying effects in the CL can be reasonably considered. Based on the explanations given before
(Figure 3.38), it can be supposed that, if bulk diffusion plays a non-negligible role in the CL,
then a difference of CL ionomer hydration could give an explanation for these differences.
Let us compare two identical CLs but with different hydration level of their ionomer contents,
working under air operation. On one hand, a dry ionomer will tend to force the current towards
the membrane side creating this way an important gradient of oxygen concentration. On the
135
3. RESULTS
other hand, a well hydrated ionomer will tend to favor the ionic current and reduce the oxygen
concentration gradient.
By changing from air to helox operation, the current distribution across the CL would change
as well, tending to by-pass the dry CL ionomer. If the CL ionomer is drier in the ”0D” than in
the ”1D” cell, then the improvement of voltage would be higher for the ”0D” than for the ”1D”
cell, as reflected by the higher ∆Uhelox−air value. Nevertheless, it is not trivial to attribute the
improvement to a lower ohmic loss in the ionomer and/or to lower mass transport losses in the
CL pores due to a better diffusivity of oxygen in helox than in air.
Resistivity
A contradiction is opposed to the proposed explanation by the values of resistivity:
the value measured for the ”0D” cell being lower than for the ”1D” cell. But consistency can
be founded on the following development.
First, there can be an inherent bias in the resistance measurement, as already discussed
(subsubsection 1.3.3.2). Basically, the AC current pathway in a resistance measurement can be
different than the effective DC current pathway under real operation. Indeed, the resistance
measurement does not rely on the three-phase boundary condition, or in other words it does
not need access to gas for being realized. Under dry conditions in particular, the CL ionomer
pathway will tend to be ignored by the resistance measurement current, since the electronic
phase of the CL will be preferred. Under real operation and due to possible limitation in the
gas phase, a given current will be forced throughout the CL ionomer so that a given ohmic loss
will occur in the CL ionomer that will not be measured by the total resistivity measurement.
This effect can be identified on the present results by considering the IR-free voltages U + ri,
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
2
i [A/cm ]
2D
1D
2
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
2
i [A/cm ]
UO +ri [V]
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Uhelox+ri [V]
U+ri [V]
Uhelox + ri and U O2 + ri of the IV-curve at RH = 40%/0% (Figure 3.39).
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
2
i [A/cm ]
0D
Figure 3.39: IR-free voltages for the IV-curve realized at RH = 40%/0%.
It can be observed that whereas a major part of the loss measured on U disappears on U + ri
in the case of the ”1D” cell, the correction for the ”0D” cell is insufficient to compensate the
136
3.3 Effect of flow field design
loss. Under helox, the Uhelox + ri value of the ”0D” cell gets closer to the value of the ”1D” cell,
but is not identical. At last, the U O2 + ri values of the ”0D” and ”1D” cells are the same. It
can therefore be hypothesized that the ohmic losses estimated by the resistivity measurement
would tend to be more representative, in the case of a dry CL ionomer, of the operation during
the O2 pulse than during the helox pulse or the air operation.
Then, an important hydration gradient between the membrane ionomer and the CL ionomer,
specially in the case of the ”0D” cell, would be possible without being directly observed on the
total resistivity value.
Interpretation
Based on these comments, it can be supposed that the high value of ∆Uhelox−air
of the ”0D” cell compared to the ”1D” cell originates from a drier CL ionomer in the case of
the ”0D” cell, whose effect is to create relatively high ohmic loss, not measured by the total
resistivity, and to induce a higher concentration gradient of oxygen across the CL.
The differences of ∆U O2 −helox between the ”0D” and the ”1D” cells would therefore reflect
the hydration state of the CL ionomer. The absence of variation of ∆Uhelox−air for the ”1D”
cell could be attributed to the absence of a oxygen concentration gradient in the CL due to the
a better hydrated ionomer.
Concerning the ”2D” cell, the quite low value of ∆U O2 −helox would be the sign of an even
better hydrated CL ionomer. The higher value found here for ∆Uhelox−air compared to the
100%/0% condition can be attributed to a redistribution of the channel-rib current in favor of
higher current under the ribs due to the higher resistance, as proposed by Boillat et al. [52].
3.3.3.4
RH-step
To conclude this chapter, a transient step of RH is investigated, namely a change from a dry
(40%/0%) to a wet condition (100%/100%) (Figure 3.40), referred to as ”dry-wet” transition
hereafter, and inversely (Figure 3.41), by a ”wet-dry” transition, at a constant current of 1
A/cm2 .
As already observed on the steady-state experiments, the ”1D” and ”2D” cells demonstrate
an increase of ∆Uhelox−air when the RH is changed from dry to wet condition, whereas the
change in negligible in case of the ”0D” cell. One also observes that the peak of performance
appearing for the ”2D” cell during the RH step is almost absent in the case of the ”1D” cell.
At last, whereas the water evolution is the same for the channel and rib regions of the cathode
GDL of the ”1D” cell, a slight difference appears for the ”2D” cell.
Same comments could be made based on the ”wet-dry” step. The only singular feature
appearing in this case is the step-like evolution of the voltages and resistivity for the ”0D” cell
after the RH step. It could reveal the successive drying regimes of different locations in the cell.
137
3. RESULTS
Cell “2D”
0.8
0.35
0.75
0.7
0.3
0.7
0.65
0.25
0.6
Uhelox
Uair
ΔUhelox-air
0.55
0.5
0.45
0.2
0.2
0.15
0.1
0.5
0.1
0.05
0.45
0.05
0.2
0.1
0
0
30
30
Cathode GDL
Water content [% vol tot]
Channel
Rib
20
15
10
5
0
-2
-1
0
1
2
3
4
5
6
7
8
9
0
0.4
r [Ω·cm2]
r [Ω·cm2]
Water content [% vol tot]
0.25
0.6
0.1
Cathode GDL
0.3
0.55
0.15
0.2
25
0.35
0.65
0
0.4
0.4
Uhelox
Uair
ΔUhelox-air
10
Channel
Rib
25
20
15
10
5
0
-2
Time [min]
ΔUhelox-air [V]
0.4
U [V]
0.8
0.75
ΔUhelox-air [V]
U [V]
Cell “1D”
-1
0
1
2
3
4
5
6
7
8
9
10
Time [min]
0.8
0.4
0.75
0.35
0.7
0.3
0.65
0.25
0.2
0.6
0.55
Uhelox
Uair
ΔUhelox-air
0.5
0.45
0.15
0.1
0.05
0
0.4
r [Ω·cm2]
ΔUhelox-air [V]
U [V]
Cell “0D”
0.2
0.1
0
30
Water content [% vol tot]
Channel
25
20
15
10
5
0
-2
-1
0
1
2
3
4
5
6
7
8
9
10
Time [min]
Figure 3.40: Performance indicators and water evolution for a RH transient applied at t = 0 from
40%/0% to 100%/100% with a constant current of 1 A/cm2 .
138
3.3 Effect of flow field design
Cell “1D”
Cell “2D”
0.75
0.3
0.7
0.65
0.25
0.4
Uhelox
Uair
ΔUhelox-air
0.35
0.3
0.65
0.25
0.6
0.2
0.55
0.15
0.6
0.2
0.55
0.15
0.5
0.1
0.5
0.1
0.45
0.05
0.45
0.05
0
r [Ω·cm2]
0.2
0.1
Water content [% vol tot]
0.2
0.1
0
0
30
30
Channel
Rib
25
20
15
10
5
0
-2
-1
0
1
2
3
4
5
6
7
8
9
0
0.4
r [Ω·cm2]
0.4
10
Channel
Rib
25
20
15
10
5
0
-2
Time [min]
ΔUhelox-air [V]
0.8
0.35
Water content [% vol tot]
U [V]
0.7
0.4
U [V]
Uhelox
Uair
ΔUhelox-air
ΔUhelox-air [V]
0.8
0.75
-1
0
1
2
3
4
5
6
7
8
9
10
Time [min]
Cell “0D”
0.4
Uhelox
Uair
ΔUhelox-air
U [V]
0.7
0.35
0.3
0.65
0.25
0.6
0.2
0.55
0.15
0.5
0.1
0.45
0.05
0
0.4
r [Ω·cm2]
ΔUhelox-air [V]
0.8
0.75
0.2
0.1
0
30
Water content [% vol tot]
Channel
25
20
15
10
5
0
-2
-1
0
1
2
3
4
5
6
7
8
9
10
Time [min]
Figure 3.41: Performance indicators and water evolution for a RH transient applied at t = 0 from
100%/0% to 40%/0% with a constant current of 1 A/cm2 .
139
3. RESULTS
Cell “2D”
1.2
1.2
1
1
Water content [-] / ΔUhelox-air [-]
Water content [-] / ΔUhelox-air [-]
Cell “1D”
0.8
0.6
0.4
0.2
GDL under channels
GDL under ribs
ΔUhelox-air
0
0.8
0.6
0.4
0.2
GDL under channels
GDL under ribs
ΔUhelox-air
0
-0.2
1
1
r [-]
r [-]
-0.2
0
0
-2
-1
0
1
2
3
4
5
6
7
8
9
-2
10
-1
0
1
2
4
5
6
7
8
9
10
Cell “2D”
Cell “1D”
1.2
1.2
1
Water content [-] / ΔUhelox-air [-]
GDL under channels
GDL under ribs
ΔUhelox-air
0.8
0.6
0.4
0.2
GDL under channels
GDL under ribs
ΔUhelox-air
1
0.8
0.6
0.4
0.2
0
-0.2
-0.2
1
1
r [-]
0
r [-]
Water content [-] / ΔUhelox-air [-]
3
Time [min]
Time [min]
0
0
-2
-1
0
1
2
3
4
5
6
7
8
9
-2
10
-1
0
1
2
3
4
5
6
7
8
9
10
Time [min]
Time [min]
Figure 3.42: Normalized values of performance indicators and water evolution for RH transients
applied at t = 0 from 40%/0% to 100%/100% (top) and 100%/0% to 40%/0% (bottom) with a
constant current of 1 A/cm2 .
140
3.3 Effect of flow field design
In particular, since the GDL appears to be quickly dried, an influence of the CL drying can be
proposed.
A normalization of the water content and the ∆Uhelox−air values is realized for both the
”dry-wet” and ”wet-dry” transitions (Figure 3.42). The ”0D” cell profiles are not reported on
this figure due to the negligible change of ∆Uhelox−air for this cell.
In the case of the ”1D” cell, the channel and rib regions of the cathode GDL show very
similar behaviors, which again validates the ”1D” behavior desired. In the case of the ”2D” cell,
it appears that different time constants can be remarked for these two regions in the case of the
”wet-dry” condition. Regarding the ∆Uhelox−air value of the ”2D” cell, it exhibits an undershoot for both the ”dry-wet” and ”wet-dry” transitions, while such an event is not observed on
the ”1D” cell.
Previously, the under-peaks shown by the ”2D” cell were attributed to an effect of current
redistribution between the channel and the rib regions. It seems therefore that this can be
removed by using a ”1D” flow field and that the supposition was justified.
3.3.4
Summary and outlook
Basically, the advantages/drawbacks of the different designs could be pointed out. The ”2D”
structure is very sensitive to water accumulation whereas the ”1D” and ”0D” flow fields show a
high sensitivity to drying.
The channel-rib effects identified in the ”2D” cell were imperceptible in the ”1D” cell, which
validates that a ”1D” behavior could be emulated. Relatively to the ”1D” cell, the reduced
mass transport losses under wet conditions as well as the efficient water removal observed in the
”0D” cell tend to demonstrate the high diffusivity desired for a ”0D” behavior. Generally, the
”0D” and ”1D” cells showed bad performance in dry conditions and good performances in wet
conditions, whereas the ”2D” cell showed the opposite trend.
Based on these validation observations, effective results could be deduced. On one hand, the
importance of the channel-rib current distribution on the bulk transport losses of the ”2D” cell
could be identified. On the other hand, the benefit provided by the rib region could be shown
as it helps retaining water and avoiding important membrane dehydration.
Comparatively to the ”2D” design, the ”1D” cell was utilized to show the contribution of
mass transport losses in absence of channel-rib effect. As a result, it turned out that the GDL
was apparently not the single area involved in mass transport losses. Instead, the comparison of
the ”1D” and ”0D” cells suggested that other regions (MPL and/or the CL pores) were probably
subject to bulk diffusion limitations.
The investigations conducted on dry conditions showed the complexity of the results obtained. The total resistivity measurement was observed to be unable to explain all the losses in
141
3. RESULTS
dry conditions. Rather, the drying of the CL ionomer was pointed out as an important source
of ohmic losses, in particular for the ”0D” design. Possible coupled effect of dry ionomer and
increased diffusion limitations in the CL pores were discussed.
Based on the present work, the following outlook can be formulated. As evidenced, the
reduction of the dimensions of the system allows for isolating the different mechanisms that
can affect the performance. Primarily, the ”0D” design can serve as a powerful tool to further
characterize the transport mechanisms inside the CL. The influence of the MPL on the electrode
could be tested on such a design. The effect of different O2 concentrations in the gas could be
investigated, at different RH. The impact of the volumetric flow on a possible CL ionomer
dehydration could be studied. More generally the use of such a structure is recommended for all
characterization studies that directly address the electrode. A comparison between the effect of
different electrode structures on possible mass transport losses would be interesting. The role
of the reversible oxide formation on the performance could also be investigated. The study of
the irreversible degradation processes, in particular their correlation with the water presence, is
another example. The use of a ”0D” design would also hugely benefit to EIS characterization.
The use of the ”1D ”design opens a wide way to the characterization of various GDL types
by avoiding the channel-rib effects. The compression rate, the PTFE content or again the
influence of the MPL could be tested. Such a one-dimensional system should permit an easier
and more straightforward validation of the model simulating the transport processes in the GDL,
in particular the water transport.
Finally, these different contributions can be compared, possibly to same experiments realized
in 2D mode, so as to extract the changes due to a 2D distribution of the parameters. The study
of the channel-rib compression of the GDL would be an example. To summarize, the simplified
flow field should help understanding better the 2D structure which is of course representative of
real cells in technical stacks.
142
3.4 Cold-start
3.4
3.4.1
Cold-start
Introduction
In the context of automotive applications, the subfreezing temperature environment is one specific condition in which the PEFC must be able to start up and operate, process referred to as
cold-start, without an external heating device that would increase the cost of the system. The
polymer electrolyte membrane (PEM) of the PEFC offers the advantage of being proton conductive also at sub-zero temperatures, provided that it is sufficiently hydrated so that a reasonable
current can be applied. The major drawback is that the water produced tends to accumulate
in form of ice in the pores of the CL and/or on the surface of the active sites. In the worst
case, this process can lead to a complete blockage of the oxygen access to the reaction sites.
Hence, the electrochemical reaction terminates and the voltage drops to zero. This event will be
designated as voltage failure henceforward and the time period elapsed from the cell start until
this moment will be referred to as working time.
In a technical stack however, the heat produced by the electrochemical reaction increases
the temperature of the components. If this temperature elevation is rapid enough, ice will melt
before the voltage failure occurs and the cold-start will be successful. While this requirement is
in practice taken into account in the fuel cell stack thermal management, the complete process
leading to the voltage failure is not well understood yet, as reflected by the numerous modeling
and experimental works dedicated to this topic (subsection 3.4.2). In particular, visualization
studies of PEFC during cold-start are scarce and among them, no one was performed so far in
situ, in in-plane and dynamic mode.
Aiming to close this gap, the present work addresses the visualization of cold-starts in a PEFC
by means of in-plane dynamic neutron imaging on a cell maintained at a constant subfreezing
temperature. The isothermal is preferred to the non-isothermal cold-start because it makes the
experiment independent of the thermal design and is therefore specifically adapted to the study
of the voltage failure.
After a literature overview (subsection 3.4.2), the experimental set-up and protocols will
be described (subsection 3.4.3), the results will be presented (subsection 3.4.4) and discussed
(subsection 3.4.5). Further details of this study are available in the published papers [105, 106].
3.4.2
Literature overview
The topic of cold-start in PEFCs has been discussed by different approaches during the last
decade.
Focusing on the membrane properties at sub-zero temperatures, various techniques could
show that different types of water exist in the membrane bulk, whose properties can strongly
143
3. RESULTS
deviate from those of normal water, in particular due to the influence of the hydrophilic sulfonate endgroups. Differential scanning calorimetry is (DSC) is one straightforward method to
investigate water properties based on the temperature dependency of freezing enthalpies. Hence,
a wide scattering of freezing peaks was obtained by cooling down a Nafion 117 membrane [72],
indicating that different states of water were simultaneously present: a given amount of water was freezing at -20°C while another part remained non-frozen down to -40°C. Moreover, a
hysteresis behavior was found on the DSC peaks between cooling down and heating, that was
interpreted as a pronounced super-cooling of water during the cooling mode [29]. Ionic conductivity measurements showed discontinuities at -13°C that could depend on the water content
[107, 108]. More recently, X-ray scattering spectra of a Nafion 117 membrane [109] quenched
down to -173°C showed that no crystalline water, so called by the authors, was present for
λ < 18, while super-cooled water was identified at λ ≈ 22 and crystalline water only appeared
in hyperswollen membranes at λ ≈ 50.
A common method to investigate possible damages provoked by freezing is to perform
freeze/thaw cycling experiments, meaning to cool down a cell to a subfreezing temperature,
to wait a given time without operating it, to heat it up again and to characterize the performance at ambient temperature. Earliest work dedicated to cold-start of PEFCs used such a
protocol, which sometimes yielded contradicting results. For instance, whereas the absence of
any degradation was reported after 385 cycles on Nafion 112 membranes [110], another study
revealed an increased pore size diameter in the CL as well as a reduced electrochemical active
area after 4 cycles already [111]. The formation of ice lenses at the CL/GDL interface was mentioned [112, 113] that could induce a higher HFR. Among the abundant literature available on
the freeze/thaw cycling, a review listing the damages caused by freezing can be found in [112].
Whereas numerous modeling works were dedicated to cold-start, only a few of them took
into account the water transport. Furthermore, water vapor diffusion was often retained as the
unique mechanism to remove the water produced from the MEA [114]. Other works assumed
a given ice fraction in the GDL but without commenting about the transport mechanisms
[115, 116]. In some cases, the experimental validation led to non-reproducibility, which was
argued by a change of initial conditions [116]. The presence of liquid water due to freezing point
depression was taken into account in [117, 118] but a minor effect was reported. The occurrence
of liquid water in super-cooled state was mentioned in [99, 119] but not taken into account in
these models.
Among experimental works, the removal of liquid water was reported and justified by a
temperature gradient in [120–122]. The distribution of the current across the CL was supposed
to play a role on the maximal ice saturation level reachable in the CL pores [123–125]. Liquid
water was observed on the CL surface with optical imaging [126]. By performing simultaneous
144
3.4 Cold-start
optical and infra-red imaging, the presence and the freezing of super-cooled water on the GDL
surface was demonstrated [127, 128].
Neutron imaging of cold-starts was performed, to our knowledge, only by Mukundan et al.,
namely in through-plane [129, 130] and in-plane [131] configurations. Similarly to our work, ice
could not be distinguished from liquid water, but the assumption of the presence of ice phase was
preferred. At -10°C, liquid water was assumed to be present due to freezing point depression,
but only in the CL [130]. A wide scattering of working times was observed at -10°C but not
interpreted. In-plane imaging revealed that ice was located in the CL and also in the GDL [131].
3.4.3
3.4.3.1
Experimental
Cell components
The cell assembly employed for the cold-starts, also described elsewhere [8, 105], consists in
a single differential cell. The flow fields (see Figure 3.43) are made of graphite (BMA5, SGL
Carbon Group, Germany) machined with 4 parallel channels (5 mm long, 1 mm wide and 0.6
mm deep). The active area is 0.5 cm2 (5 mm along the channels and 10 mm perpendicularly to
them). A catalyst coated membrane (CCM) is used (Primea 5710, Gore, USA) with carbon paper
GDLs (Sigracet GDL 24BC, SGL Carbon Group, Germany). The whole assembly is compressed
between the cell housings, made of gold-coated aluminum, so as to reach a compression rate of
approximately 20 % in the GDLs.
3.4.3.2
Cooling set-up
In order to maintain a constant subfreezing temperature, the cell housings (see Figure 3.43) are
mechanically contacted to cooling elements. These are made of aluminum blocks in which a
coolant liquid is circulated (Fluorinert FC-770, 3M). The coolant liquid is cooled by a refrigerated circulator (Julabo FP50). A minimal flow field temperature of -20 °C was applied in the
experiments. The cell temperature is measured by thermocouples placed in holes that are drilled
in the housings.
When this set-up is cooled down below the dew point temperature of the ambient air, the
water vapor present in the surroundings freezes on the external surfaces. Since neutron imaging
is sensitive to condensed phases of water, the frost present in the field of view can disturb the
estimation of the water inside the cell. So as to avoid the frost formation, heated plates are
placed on the external surfaces of the set-up. They consist of thin aluminum plates on which
electrical heating elements are glued with an adhesive band. Insulation foam is inserted between
these elements and the external housings to avoid creating a thermal bridge. Finally, dry N2 is
circulated in the free space formed by the cell flow fields and the heated elements to prevent the
frost formation in that region, which was successfully tested.
145
3. RESULTS
Heated plate
(T = 25 °C)
Insulation
Cooling
elements
Flow field
Neutron
beam
Cell housings
(T < 0°C)
Dry N2
Figure 3.43: Cold-start set-up (links: full-view, right: sectional view).
3.4.3.3
Protocol
As mentioned, the membrane has to be correctly hydrated to have sufficient proton conductivity
at subfreezing temperatures. In a technical stack, the purging time realized before the shut-down
at ambient temperature must consider this requirement by avoiding drying the membrane too
much. However, the membrane also plays the role of a reservoir to accumulate part of the water
produced before the system reaches the melting temperature. In consequence, a sufficient water
storage capacity of the membrane is needed, meaning that a trade-off must be found for a correct
humidification. In the case of this work, the hydration state of the membrane is controlled by
the relative humidity of the gases applied during the conditioning.
The following protocol was utilized. The whole process was realized at ambient pressure.
At first, 0.5 Nl/min flows of dry N2 were applied to dry the cell until the HFR reached 0.5
Ohm·cm2 . Afterwards, 1 Nl/min N2 flows at RH = 35% were circulated for 35 min (only on
cathode side due to a problem of humidifier on anode side). Then, the humidifiers were stopped
and the gas flows were reduced to 0.2 Nl/min to blow out possible water droplets in the piping
and cell channels. Subsequently, the gases were stopped and the cell was cooled down to the
desired subfreezing temperature value. After the temperature was stabilized, reference images
were taken during 10 min. Then, flows of 0.1 Nl/min of dry H2 and 0.1 Nl/min of dry air were
circulated. Current was applied in form of a 1 min increasing ramp up to the desired value and
was then kept constant until the voltage failure occurred. At this moment, the current and the
gas flows were automatically shut down. A maximal working time of 60 min was attributed
before stopping the experiment. Finally, the set-up was heated up to ambient temperature and
the protocol was repeated with other parameters. The operation parameters investigated are
listed in the table (Table 3.5). A reference condition is defined, namely with a current of 0.1
A/cm2 and a start-up temperature of -10°C. It was repeated twice with neutron imaging.
146
3.4 Cold-start
Neutron radiograms are continuously taken with a 10 s exposure time by using the tilted
imaging micro set-up described in [41] (FWHM resolutions: 20 µm horizontally and 200 µm
vertically).
The HFR measurement was realized with a Tsuruga 3566 device at a 1 kHz frequency.
Besides, another measurement campaign was realized without neutron imaging during which
approximately 400 cold-starts were realized on the same cell and by varying different operation
parameters (Table 3.5). Only one operating parameter was changed at once from the reference
experiment (0.1 A/cm2 , -10°C). The differences existing between the experiments realized with
or without neutron imaging were as follows: the absolute pressure was 1.2 bar instead of atmospheric pressure, the conditioning RH was 30% and gases were supplied to both anode and
cathode side.
Design
parameters
Measurement
campaign
MEA
Flowfield
Operation
parameters
GDLs
Current
2
(0.05, 0.1, 0.2 A/cm )
2009
Primea 5710
Gore
2D
Experimental
set-up
Single-cell
Micro set-up
24BC
Temperature
(-10, -15, -20°C)
(ANDOR™ camera)
Current
2
(0.05, 0.1, 0.2 A/cm )
Temperature
(-10, -15, -20°C)
2011
Primea 5710
Gore
2D
Conditioning RH
(10, 20, 30, 40,
50, 60, 70, 80%)
24BC
Conditioning duration
(35, 180 min)
Drying before conditioning
(5, 15, 30, 60 min)
Operation mode
(normal, dead-end)
Table 3.5: Summary of cold-start experimental conditions.
147
Single-cell
No imaging
3. RESULTS
3.4.4
Results
3.4.4.1
Reference experiment
Detailed analysis It must be emphasized that in the state of the art, neutron imaging does
not allow distinguishing ice from liquid water. Indeed, the attenuation of water is described by
the Beer-Lambert law (Equation 1.48):
I = I0 e−Σδ = I0 e−σnδ
(3.36)
where δ is the thickness of water and Σ is the attenuation coefficient, which can be further
expressed by the microscopic cross-section σ and by the volumetric atomic density n. When
liquid water freezes, its volume expands and its density n reduces. If the expansion is possible
along the beam direction, as it is the case in a partly saturated GDL, then a change of thickness
δ can be expected as well. Furthermore, since there is no major change of σ when using the
full energy spectrum of the beam, as it is the case here, then the quantity σnδ will be hardly
affected by a phase change. Changing the energy spectrum [132] would yield a different σ value
for ice or liquid water. In the following results, the water visible with neutrons can therefore
be interpreted as a condensed phase of water, being ice or liquid or a mixture of them. It will
simply be referred to as water.
The reference experiment (0.1 A/cm2 , -10°C) will be described in details since it is symptomatic of the whole set of experiments visualized. The corresponding radiograms and electrochemical data are presented in the following figure (Figure 3.44). A video of this experiment
can be found in [105].
At t = 0 min, the cell is at OCV and no water can be observed even in the hydrated
membrane. This is due to the fact that this condition is used for the reference radiogram for the
whole sequence. Therefore, the water visible on the other radiograms will correspond to a relative
amount compared to the OCV condition. The use of the image of the dry membrane at ambient
temperature would have yielded too important misalignment and dilatation disturbances.
After the current is applied, the voltage rapidly drops. Then, it slightly decreases during
the whole experiment. This slow decrease cannot be attributed to any channel clogging, as it
will be seen hereafter, nor to a change of HFR due to phase transition or contact resistance for
instance.
At t = 20 min, it can be observed that water is present in the MEA and also in the GDL.
Therefore, this result contradicts the commonly modeled representation according which water
accumulates only in the MEA region and in its close vicinity. Rather, it proves that an efficient
water removal does exist between the MEA region and other regions of the cell. In particular,
it does no seem realistic to assume a direct transport of ice phase to justify this result. The
148
3.4 Cold-start
Anode
Cathode
MEA
GDL
13.6mm
2mm
0min
40min
20min
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
Voltage [V]
2
0.9
Current density [A/cm ]
2
Resistivity [Ohm·cm ]
1
1
0
0
20
40
60
80
100
0
t [min]
100min
80min
60min
Voltage
High frequency resistivity
Current density
Figure 3.44: Sub-zero start-up at -10 °C and 0.1 A/cm2 visualized with neutron imaging.
transport of water vapor in supersaturated state due to a possible limiting desublimation rate
[133] was considered. The rate of water electrochemically produced by surface unit is defined
as:
ṅprod =
i
zF
(3.37)
The total desublimation surface flow can be calculated:
ṅdesub = kp ssur c H2 O
(3.38)
where kp is the desublimation rate from the gas kinetics theory, ssur is the sursaturation level of
the water vapor and c H2 O the water vapor concentration above ice. By using the desublimation
149
3. RESULTS
rate kp = 13.76 m/s reported in [133] at -15°C, one finds that already at a 0.3% sursaturation
level, one reaches:
ṅdesub ≈ ṅprod
(3.39)
This means that even under the very conservative assumption of a desublimation surface that
would equal the geometric surface of the active area (0.5 cm2 in this set-up), the whole amount
of water produced would instantly desublimate and the prerequisite to have an important flow
of sursaturated vapor would not hold.
A second hypothesis to explain the transport of saturated vapor would be to consider a
sufficiently high temperature gradient that would induce a saturation pressure gradient (process
sometimes referred to as heat pipe effect). Based on the expression of the diffusive mass transport
(Equation 1.30), and by using a conservatively high value of relative diffusivity Drel = 0.5 [-],
as well as a water vapor diffusivity at low temperature given by the following equation [133]:
1.94 n T
p
D H2 O,Air = 0.211
(3.40)
n
T
p
with T n = 273.15 K, pn = 1013.25 mbar, and a GDL thickness of δ = 180 µm, a saturation
pressure difference of approximately ∆p = 2 mbar is obtained, which leads to the following
temperature gradient:
∆T > 6◦ C
(3.41)
This value, that corresponds to a 1D structure, is much higher than the maximal temperature
difference ∆T ≈ 0.6◦ C found by the simulation presented later for a 2D structure.
It seems therefore justified to assume that a capillary flow of liquid water is occurring and
that the presence of liquid water must be hypothesized, as examined later (subsection 3.4.5).
At t = 40 min, some water can be observed expanding onto the channel walls, as revealed
by the detail view (Figure 3.45). The fact that the water propagation front locates towards the
~21min
+1.5min
+1.5min
Figure 3.45: Emerging water onto the channel walls.
channel is hardly compatible with the growing of an ice crystal. Instead, the accumulation of
liquid water spreading over the wall of the graphite flow field seems to be more appropriate.
It can be observed on the full sequence (Figure 3.44) that two of the four channels are clogged
by water. An exact analysis of the video sequence [105] would show that the slight decreases
150
3.4 Cold-start
of voltage visible on the curve at t = 70 min and t = 85 min are correlated with these events.
This means that the final voltage failure does not directly originate from the channel clogging.
At t = 100 min, the voltage failure eventually occurs so that the current and the gases are
automatically stopped.
On the following graph (Figure 3.46), the voltage and the water amount evolution in chosen
areas are presented. During the first 15 min, one notices a change of accumulation trend in the
Area definition
MEA
Channel
GDL
1
25
Voltage [V]
0.8
20
0.7
0.6
15
Channel
0.5
0.4
10
GDL
0.3
0.2
MEA
5
0.1
0
-10
0
10
20
30
40
50
60
70
80
Water content [% vol tot]
0.9
0
90 100 110
t [min]
1
0.9
Channel
23
22
0.7
0.6
0.5
21
0.4
0.3
20
0.2
19
0.1
GDL
9
Water content [% vol tot]
Voltage [V]
0.8
8
MEA
7
90
95
100
105
110
t [min]
Figure 3.46: Evolution of voltage and water content in chosen areas during the voltage failure.
MEA and the cathode GDL areas. Comparing the conductivity evolution during this period
(Figure 3.47), it can be observed that the conductivity value, representing the membrane hydration state, has already reached its maximum before the accumulation rate starts decreasing.
This could be the sign that a change of back-diffusion flow would be less probable, since the
151
3. RESULTS
MEA conductivity is already at its steady-state value. Rather, the progressive contribution of
a capillary flow seems more adequate.
At t = 70 min, one observes a change of trend on the water evolution profile in the top
channel. This moment corresponds to the clogging of the bottom channel.
For what concerns the MEA area, one remarks a sudden change of the accumulation rate at
around t = 100 min that is well correlated with the voltage failure. This important result tends
to confirm the following hypothesis. The sudden increase of water which is visible would be the
sign that a strong limitation of water removal is happening at that moment. Consequently, the
water electrochemically produced would not be able to completely escape the CL pores so that a
detrimental accumulation would occur in form of pore clogging and/or of coverage of the active
sites. Then the oxygen access would be impeded and the voltage would drop. Simultaneously
in the top channel area, a sudden increase of the water content is perceived, indicative that a
sudden limitation of water removal is occurring also in this region. If pure ice or pure liquid
were present during this event, there would be no reason for explaining the apparition of a new
limitation. Consequently, it can be assumed that part of the water (if not the whole) is freezing
at this precise time and that the supposed capillary flow of liquid water is suddenly stopped.
Since the water produced in the CL cannot escape, it starts to accumulate detrimentally until
blocking the reaction. Because the cathode GDL does not exhibit any change of trend compared
to the MEA, it can reasonably be argued that this detrimental accumulation affects (at least
partly) the CL pores and/or particle surfaces. The occurrence of a sudden freezing will be
8.5
14
7.5
12
6.5
10
5.5
8
6
4.5
MEA
3.5
4
2
2.5
1.5
-1
0
1
2
3
4
5
6
7
8
9
Water content [% vol tot]
Conductivity [S/cm2]
discussed afterwards (subsection 3.4.5).
0
10
t [min]
Figure 3.47: Conductivity (inverse value of HFR) and water amount evolution in the MEA area
in the beginning of the cold-start.
Let us finalize the analysis with a quantification of the mass of water produced during the
working time ∆twork :
Mprod = Ṁprod · ∆twork
152
(3.42)
3.4 Cold-start
where Ṁprod = ṅprod · m̃ H2 O is the water production rate expressed in mass units based on the
molar flow ṅprod given before (Equation 3.37), and m̃ H2 O is the molar mass of water. With
a working time ∆twork = 100 min and a water production rate of Ṁprod = 0.56 [mg cm−2
min−1 ] corresponding to the current density of 0.1 A/cm2 , a total mass of Mprod = 56 mg/cm2
is estimated to be produced during this cold-start. This value if far higher than the maximal
MEA water storage capacity estimated to be 0.85 mg/cm2 [120] or 1.6 mg/cm2 [134] for materials
similar as ours. This proves that the water storage capacity is not, at least under the present
experimental conditions, the determining parameter that predicts the working time.
Besides, the amount of water corresponding to the fast accumulation during the voltage
drop can be measured on the neutron radiograms to be ∆Mdrop,M EA = 0.07 mg/cm2 , this by
taking a voltage drop duration of 40 s. This value is lower than the CL water storage capacity
estimated to 0.35 mg/cm2 [120] and 0.65 mg/cm2 [134] for a similar CL. This comparison tends
to indicate that only a small part of the CL storage capacity would be crucial for the complete
oxygen blocking. On the contrary, the accumulation is 0.3 mg/cm2 in the top channel during
the same lapse of time for a total production of 0.4 mg/cm2 . Most of the water produced is
therefore leaving the MEA area, even during the voltage failure.
Statistics
The experiment presented was repeated (34 cold-starts) in similar condition but
without neutron imaging (subsection 3.4.3). The working times obtained are presented on the
following figure (Figure 3.48). It must be emphasized that although the working times recorded
here are all shorter than the 100 min found before, a similar evolution of the voltage and the
HFR during the voltage failure was observed, which tends to confirm that the failure mechanism
is the same in both cases.
The working times measured are widely scattered, with higher probabilities towards shorter
working times. The HFR value measured before applying the current is reported so as to
evaluate the reproducibility of the cell hydration state. The values obtained range from 1.5 to
2.6 Ω · cm2 , which was measured to correspond to a 10% variation of RH (this observation is
not presented here). The working times do not correlate with the HFR values and thus, the
scattered working time values cannot be attributed to the different HFR values. Moreover, as
indicators of a possible degradation, the maximum voltage reached during the cold-start and the
HFR measured at this moment are reported on the same figure. Since no change of these values
is identified, one can exclude any dependence of the working time to a degradation mechanism
measurable with these parameters, such as a loss of electrochemical active area or an increase of
contact resistances for instance. In conclusion, the voltage failure seems to occur like a stochastic
process. Further explanations will be given in (subsection 3.4.5).
153
3. RESULTS
8
6
4
2
0.9
4.5
0.8
4
0.7
3.5
0.6
3
0.5
2.5
0.4
2
0.3
1.5
0.2
1
0.1
0.5
0
0
10
20
30
40
50
5
(b)
0
60
0
10
20
30
40
Working time [min]
Working time [min]
Voltage during cold-start
Resistivity [Ω·cm2]
1
(a)
Voltage [V] / Resistivity [Ω·cm2]
Number of cold-starts [-]
10
0
60
50
High frequency resistivity before cold-start
High frequency resistivity during cold-start
Figure 3.48: Working time distribution obtained with 34 cold-starts repeated at the same condition: 0.1 A/cm2 and -10°C. The voltage and HFR values measured during the cold-start are taken
after the current is stabilized and when the voltage reaches its maximum value.
3.4.4.2
Temperature dependency
Cold-starts are realized at -15°C and -20°C, both with a current of 1 A/cm2 (Figure 3.49). On the
x-axis, the time is converted into the water amount produced by means of the conversion given
earlier (Equation 3.42) to facilitate further comparisons at different currents. The radiograms
presented were taken at the very end of the voltage failure. The data of the reference experiment
(-10°C, 0.1 A/cm2 ) are mentioned again for comparison.
T = -10°C, i=0.1 A/cm
2
(a)
10
1
9
0.9
0.8
8
0.7
7
0.6
6
7
6
0.5
5
0.4
4
0.3
3
0.2
2
1
4
3
0.2
2
0.1
1
0.1
0
0
0
2
2.5
3
3.5
4
4.5
0
0
5
9
0.6
0.3
1.5
10
8
0.4
1
T = -10°C, i=0.1 A/cm
(b)
2
0.7
5
0.5
2
0.8
0.5
0
T = -20°C, i=0.1 A/cm
Water content [% vol tot]
2
Voltage [V]
Voltage [V]
T = -15°C, i=0.1 A/cm
Water content [% vol tot]
1
0.9
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Water production [mg]
Water production [mg]
Figure 3.49: Evolution of voltage and water content in the MEA for different cold-starts.
One observes at -15°C (Figure 3.49.a) that even with a much shorter working time than at
-10°C (5.7 min here instead of 100 min before), a little amount of water can be observed in
the cathode GDL. The accumulation rate of water in the MEA is identical as for the reference
154
3.4 Cold-start
condition. This indicates that the effect of the reduced water vapor pressure at -15°C does not
result in a higher accumulation rate. The voltage failure occurs earlier, meaning that a lower
amount of water has been produced. However, the water amount evolution during the voltage
failure is similar as the one observed for the reference experiment and the voltage failure exhibits
the same duration (40 s). Due to the presence of water in the GDL, it can be assumed that a
capillary flow is present in this case, too. The fact that the voltage failure occurs earlier can
be expected by a reduced stability of liquid water at lower temperature, as expected by the
hypothesis given afterwards (subsection 3.4.5).
This experiment was repeated (13 cold-starts) without neutron imaging and the working
times obtained showed good repeatability between ∆Twork = 2 and 3 min, as it will be reported
in the graph below (Figure 3.53).
At -20°C, no water can be observed in the GDL and only a little amount is seen in the
MEA. This value can be measured and was estimated to 0.4 mg/cm2 , which is still lower than
the 0.9 mg/cm2 produced water calculated based on the current, under reserve of an exact
quantification. If it were confirmed, this would mean that a significant water quantity, invisible
on the radiograms, would be evacuated from the MEA region, even at this low temperature.
3.4.4.3
Current dependency
A cold-start realized at a higher current density of 0.2 A/cm2 and -10°C is reported along with
the reference condition (Figure 3.50.a). A video of this experiment can be found in [105]. A
lower current density of 0.05 A/cm2 was tested at -10°C, but since no voltage failure occurred
within 60 min, the experiment was aborted. Then, a cold-start realized at 0.05 A/cm2 and
-15°C is presented here and is compared with an experiment performed at 0.1 A/cm2 and -15°C
(Figure 3.50.b).
2
i=0.1 A/cm , T = -10°C
0.8
8
0.7
7
0.6
6
0.5
5
0.4
4
0.3
3
0.2
2
0.1
0
0
1
2
3
4
5
6
7
8
9
1
9
(b)
2
i=0.05 A/cm , T = -15°C
0.9
9
2
i=0.1 A/cm , T = -15°C
0.8
10
8
0.7
7
0.6
6
0.5
5
0.4
4
0.3
3
0.2
2
1
0.1
1
0
0
10
0
0
Water production [mg]
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Water production [mg]
Figure 3.50: Evolution of voltage and water content in the MEA for different cold-starts.
155
5
Water content [% vol tot]
(a)
Voltage [V]
0.9
Voltage [V]
10
2
i=0.2 A/cm , T = -10°C
Water content [% vol tot]
1
3. RESULTS
During the beginning of the cold-start at 0.2 A/cm2 , the accumulation rate in the MEA is
very similar as the one of the reference experiment. The reduction of the accumulation rate
occurs at a same value of Mprod = 2 mg, which indicates that a same threshold of water must
be reached before the trend changes. The working time is much shorter (14 min) than for the
reference case.
This experiment was repeated (18 cold-starts) without neutron imaging and a wide scattering
of working times (between ∆T = 3 and 50 min) was obtained (Figure 3.53), but with a lower
mean value than at 0.1 A/cm2 .
Regarding the low current density (0.05 A/cm2 ), one remarks that the voltage failure duration is much longer than for the other experiments discussed so far. The radiogram reveals
that the cathode GDL contains a little amount of water. The water accumulation rate in the
MEA is also reduced compared to the other experiments. The role of a possible water vapor
removal by undersaturated gas flows can be proposed as an explanation. The freezing of the
water content in the GDL under the ribs would concentrate the current under the channels.
Since the production rate is low, the water vapor removal would dictate the accumulation rate
in the MEA, unlike the other experiments at higher current densities. Then, the voltage failure
duration would last a longer time.
Cold-starts at 0.05 A/cm2 and -10°C were repeated (11 times), as summarized in (Figure 3.53). Whereas most of the working times obtained were below 10 min, a 60 min working
time was obtained in one case. The voltage failure durations were not repeatable under this
condition. Precisely, two trends of voltage failures were identified: long-duration drops similar
as the one presented here and short-duration drops similar as the one of the reference condition.
3.4.5
Comments
Having stated that liquid water was present in the system, different hypothesis have to be
explored to support this assumption.
As a first hypothesis, a hydrostatic pressure effect can be excluded since unrealistic pressures
would be needed to keep the water in liquid state in the subfreezing temperatures investigated
here (600 bars at -4°C, 1200 bars at -10 °C [135]).
A second hypothesis is to consider the transfer of the beam power to the sample. Based on
the flux and energy values corresponding to the beam line [41, 42], a negligible power of 10−14
W was found by assuming a total absorption of the impinging neutrons by the sample. Thus,
the consecutive temperature increase of the sample can be neglected.
A third hypothesis relies on a possible deviation of the freezing point due to surface tension
effect that would change the hydrostatic pressure of the water confined in small pores. Referred
156
3.4 Cold-start
to as Gibbs-Thompson effect, the deviated freezing temperature can be estimated as follows
[136, 137]:
Tf,pore
= Tf 1 −
4γSL
∆hf ρL d
(3.43)
where Tf = 273.15 K is the freezing temperature of water at standard pressure, γSL = 0.033
J/m2 is the solid-liquid surface tension, ∆hf = 3.35 · 105 J/kg is the fusion enthalpy (neglecting
density differences between liquid and ice), ρL = 1 · 103 kg/m3 is the water density and d is the
particle diameter. Typical pore size diameters are reported [32] to be 0.5 µm in the MPL and 50
µm in a carbon-paper type GDL. Hence, deviated freezing temperatures of Tf,pore = −0.22◦ C
and Tf,pore = −0.0022◦ C are obtained in the MPL and the GDL respectively. To reach Tf,pore =
−10◦ C, a pore diameter of d ≈ 10 nm should be considered, which can be relevant only in the
CL or in the smallest pores of the MPL.
The fourth hypothesis takes into account the presence of a sufficiently high temperature
gradient that would be induced by the heat flux produced by the reaction. A simulation is
realized to evaluate this gradient. The heat flow can be estimated by:
0
− U )i
q = (Uth,298
(3.44)
0
= 1.48 V is the thermodynamic voltage with liquid water as product (Equation 1.9),
where Uth,298
U = 0.7 V is a typical voltage measured for instance in the reference condition and i = 0.1
A/cm2 is the current density of the reference condition. A homogeneous current distribution
is assumed. A heat production of q = 0.078 W/cm2 is obtained. As first boundary condition,
this heat source is imposed at the MEA/GDL interface. On the other side, that is at the flow
field/endplate interface, a temperature of -10°C is fixed. Using the thermal parameters reported
below (Table 3.6), a 2D simulation is realized on a thermal conduction model, by using the
Ansys platform (Figure 3.51).
The bulk thermal conductivities used (Table 3.6) correspond to materials in dry state, which
can be considered as a worst case in terms of thermal conduction since the presence of liquid
water or ice in the pores can only increase the effective thermal conductivity. However, the
formation of an ice layer at the GDL/flow field interface could induce a higher thermal contact
resistance. But this should have been reflected by a higher electrical resistance, which was not
observed during the cold-start. Any unconsidered heat conduction mechanism (e.g. heat pipe
effect) would also increase the heat removal and reduce the temperature gradient.
On the result presented below (Figure 3.51), the highest temperature obtained is -9.33°C
so that the biggest temperature difference is ∆T = |−10 − (−9.33)| = 0.66 °C. Therefore, the
presence of liquid water cannot be argued by a sufficiently high temperature.
The last hypothesis retained assumes the presence of liquid water in super-cooled state, as
proposed by Ishikawa et al. [127, 128]. It is in fact known, since the discovery of Fahrenheit in
157
3. RESULTS
Figure 3.51: Simulation (Ansys ) of the 2D temperature distribution obtained during a cold-start
at -10 °C and 0.1 A/cm2 with thermal parameters listed in the below table (Table 3.6)
.
1724, that in absence of an ice nuclei, water can be kept in a meta-stable liquid state in the temperature range from 0°C down to -42°C, its stability depending on the probability of formation
and growth of ice nuclei. Freezing occurs in it like a random process that has to be described by
probability laws [140–142]. Various parameters can influence the freezing probability, such as the
droplet size [140, 143], the substrate composition [142] or the surface characteristics [141, 144],
as already proposed in the earlier review [145]. In absence of substrate (solid surface, foreign
particles) on which nucleation can initiate, the freezing probability depends only on the chance
of getting an ice-like orientation of the water molecules. The phenomenon of super-cooling was
extensively studied in the context of physics of clouds and precipitation [145].
By using a population of pure distilled water droplets of similar size deposited on a plate
maintained at constant temperature, it could be shown that the freezing probability was timedependent and extremely dependent on the subfreezing temperature [146]. After having been
initiated, the freezing process happens in a short time, ranging from milliseconds for micrometer
droplets at -30°C, as modeled in [147], up to 25 s for millimeter-size samples at -6°C, as measured
in [148]. Super-cooled water was already identified in porous ion-exchange resin with 5 nm pore
158
3.4 Cold-start
Thermal conductivity
Value
Reference
GDL in-plane
2 W/(mK)
Another material (Toray paper) is used in the reference [138], but it shows that the in-plane conductivity of paper is about one order of magnitude
higher than the through-plane conductivity.
GDL through-plane
0.31
W/(mK)
For a Sigracet GDL BA [139]
Membrane
0.16
W/(mK)
For dry Nafion [139]
Graphite
20 W/(mK)
Interface graphite/GDL
1.5 · 103
W/(m2 K)
This value corresponds to the through-plane conductivity. Since the in-plane conductivity is higher,
this represents a conservative assumption [69].
This value corresponds to a Toray paper contacted with a metallic flow field (worst case in
[139])
Table 3.6: Parameters used for the 2D simulation (Figure 3.51) of the temperature distribution
obtained during a cold-start at -10°C.
diameter even in presence of ice [149]. The freezing front was recognized to propagate through
a 130 µm thick cation exchange membrane [133], but its composition was not specified.
In our system, the requisite that no ice is present is a priori satisfied. The low RH conditioning realized at ambient temperature and without current should ensure that no liquid water
is present outside the MEA before cooling the cell. In the membrane, a water content of λ < 14
can be assumed, so that no crystalline phase should be present, neither in the bulk nor on
the surface, as predicted by the X-ray small angle scattering results in [109]. Furthermore, the
presence of super-cooled water was already identified in such a λ domain in Nafion 117 membranes by using DSC [29]. At last, the electrochemical production of water can be considered as
impurity-free, under reserve of significant amount of degradation products. For these reasons,
it seems that our system features the sine qua non conditions for the absence of ice nuclei and
that super-cooled water can be considered.
The assumption that super-cooled water is present brings the results observed to light. It
would support the presence of a capillary flow removing the water produced from the MEA
towards the GDL and channel regions. Furthermore, the rapid freezing of super-cooled water is
in accordance with what was observed in the MEA and the channel regions during the voltage
failure, namely the sudden apparition of a limitation of water removal. It has to be emphasized
that a rapid increase of the HFR value was measured during the voltage failure (Figure 3.52),
which could indicate a phase change in the membrane and/or an increase of the contact resistance
159
3. RESULTS
due to the formation of ice lenses at the interfaces [130, 134]. A validation of the resistivity
measurement was realized at a higher frequency of 20 kHz and the same increase of HFR was
observed, meaning that the effect cannot be attributed to a change of charge transfer resistance.
The unpredictable working times recorded for the different cold-starts realized at the same
operating condition (Figure 3.48) are consistent with a stochastic process of super-cooled water
freezing.
15
2
Resistivity [Ohm·cm ]
0.3
0.28
14
0.26
13
0.24
0.22
12
0.2
11
0.18
0.16
97
Water content [% vol tot]
16
0.32
98
99
100
101
102
103
104
10
105
t [min]
Figure 3.52: HFR and water amount evolution in the MEA area during the voltage failure of the
cold-start.
At last, it has been observed that water in meta-stable state can be set into a stable state by
applying an energy input to the system, for instance in form of a mechanical shock [150–152].
The application of a mechanical shock was empirically realized for 4 cold-starts: in all cases,
the shock initiated a similar voltage failure and HFR increase as in the other experiments where
super-cooled was assumed to be present. The shock was realized by raising the cell set-up by
hands at a height of 35 mm and hitting it rapidly against its base plate, as it can be observed on
the corresponding movie [105]. Based on this sequence, the speed v of the set-up at the moment
of the impact was estimated to 0.25 m/s, so that the following kinetic energy can be estimated
with a mass M of 1.5 kg:
1
E = M v 2 ≈ 47 mJ
2
(3.45)
This rough estimation is indicative of an order of magnitude but not of the exact amount of
energy effectively transmitted to the cell and to the exact location where it would concentrate.
However, this experiment qualitatively demonstrates that a voltage failure could be initiated by
a mechanical shock, which would support the hypothesis of a phase transition of super-cooled
water into ice at this moment.
As mentioned before, a set of experiments was realized without neutron imaging by performing approximately 400 cold-starts under various operation conditions, as summarized on
160
3.4 Cold-start
the figure below (Figure 3.53). Only one parameter was changed at once from the reference
condition (-10°C, 0.1 A/cm2 ). Similarly as the case presented, no change of maximal voltage or
HFR during the cold-start was observed. Moreover, the scattered HFR values measured before
the cold-start were not correlated to the working times either. Again, we suppose that possible
degradation effects or changes in membrane hydration are not the reason for explaining the
distribution of working times observed.
Reference condition
Working time [min]
60
34
18
1
34
34
50
40
2
30
11
2
20
11
13
10
0
0
0.05
0.1 0.15 0.2
0.25 -20
-15
-10
-5
0
Start-up current [A/cm ]
Start-up temperature [°C]
0
60
120
180
2
Working time [min]
60
34
34
Conditioning time [min]
34
7
50
10
40
30
12 12
28
13 14
13
20
13
17
10
10
0
0 10 20 30 40 50 60 70 80 90
Relative humidity
during conditioning [%]
One cold-start
0
10 20 30 40 50 60
Drying time before
conditioning [min]
Standard Dead-end
Start-up mode
Average value for each condition (mentioned if number of
cold-starts > 5 and if no cold-start aborted after 60 min).
The numbers of cold-starts is indicated in the graph area for each condition.
Figure 3.53: Working times recorded for 400 cold-starts realized under various operation parameters. One parameter was changed at once from the reference condition (-10°C, 0.1 A/cm2 ).
Basically, it can be seen on the figure (Figure 3.53) that none of the changed parameters seems
to imply a repeatable distribution of the working times, except perhaps the lower temperature
(-15°C). But as observed previously at this temperature, some water was present in the GDL.
This makes the assumption of super-cooled water presence still relevant in that case. It can be
161
3. RESULTS
suggested that a lower subfreezing temperature will induce a lower expected value of the working
time in the probabilistic distribution, as well as a reduced standard deviation, thus tending to
a repeatable behavior but with non-negligible probabilities for higher working times. This is in
agreement with the reduced stability of super-cooled water at lower subfreezing temperatures
observed in [146].
Among the experiments reported here, cold-starts were realized in dead-end mode, meaning
by closing the outlet valves of both hydrogen and air lines. Since no major trend is observed on
the working times, it can be deduced that the influence of water removal by gases has no visible
impact.
Finally, super-cooled water was already observed on the GDL surface in an operating fuel
cell by simultaneous optical and infra-red imaging [127, 128]. When a dry purging was applied as
conditioning, they could observe after a given time the freezing of a super-cooled water droplet
by a change of morphology and a subsequent release of heat.
3.4.6
Summary and outlook
Isothermal cold-starts of a differential PEFC were visualized with dynamic in-plane neutron
imaging and extensively repeated without imaging. Super-cooled water was identified in most
experiments realized with neutron imaging at different subfreezing temperatures and current
densities. The major observations supporting the presence of super-cooled water are as follows.
First, a condensed phase of water was visualized in the GDL and in the cathode channel regions.
Secondly, the rapid voltage failure was observed to occur simultaneously as a rapid increase of
the water accumulation rate. This indicates a sudden change of the water removal rate, for
which the fast freezing process of super-cooled water is thought to be responsible. At last,
the repetition of the cold-starts under various operating conditions yielded an unpredictable
behavior of the working times, which is in accordance with the stochastic freezing inherent to
super-cooled water.
Lowering the temperature seemed to reduce the working times but a condensed phase of
water was also present in this case in the GDL. One cold-start realized at lower current density
exhibited a longer duration of the voltage failure, which was attributed to a non-negligible effect
of water vapor removal in that case.
In conclusion, it seems that super-cooled water is, in our experimental set-up and at least
for subfreezing temperatures higher than or equal to -10°C, rather the rule than the exception.
Lower temperatures probably make the presence of super-cooled water less stable so that the
working times should be more predictable.
Regarding the real operation of technical stacks, it can be stated that the presence of supercooled water is a positive phenomenon that will lead to increased working times compared to
162
3.4 Cold-start
the direct freezing of the water produced. However, since the freezing is a probabilistic process,
it seems hardly possible to design a realistic and reliable system that would perfectly control the
stability of this meta-stable phase. The present study seems therefore not be directly exploitable
for technical applications.
Rather, the results obtained put into perspective the working times predicted by models. In
particular, the core contribution of this study is to address a warning message to the model validations that could be made based on too little experimental data, specially when no visualization
is included, that can in turn lead to wrong models of water transport.
Further investigations could take into account the size-effect of the cell on the probabilistic
trends identified here. Namely, it could be expected that with a bigger active area, the larger
amount of water produced would increase the freezing probability in the total amount of water.
Since the freezing of super-cooled water propagates in the water phase, one can assume that the
probability of a voltage failure after a given working time would tend to increase.
Other investigations could be dedicated to the influence of the λ value on the freezing probability, for instance by increasing λ by a conditioning with application of current. Materials
variation could be tested to identify any influence on the working time, for instance by changing
the hydrophilicity/hydrophobicity of the GDL or of the flow fields. The application of mechanical shocks or other external excitations must be seriously considered as a useful tool to validate
or not the presence of super-cooled water.
Finally, energy-selective imaging should be experimented for trying to distinguish liquid
water from ice in the operating cell. As a further challenge, the development of dynamic and
energy-selective neutron imaging may provide the direct observation of super-cooled water freezing.
163
4
Conclusions and outlook
The conclusions of this thesis can be formulated by specific and general comments and outlook.
In the first part (section 4.1), the essential contributions and perspectives already presented at
the end of the different chapters will be summarized. In the second part (section 4.2), a global
big picture of the work will be proposed.
4.1
Specific aspects
The present work contributed to the development of new diagnostic tools and to the exploration
of their practical applications.
The deuterium labeling method was described. Basically, it consists in replacing the 1 H
atoms by 2 H atoms in the PEFC and observe by neutron imaging the subsequent change of
intensity to draw conclusions about water transport. To gain a fundamental understanding of the
phenomena, a simple diffusive transient model was fitted on a simplified experimental system.
Precisely, this method allowed for quantifying parameters related to the bulk and interfacial
properties of a membrane in contact with water vapor and hydrogen. The full methodology
(model, numerical resolution, validation, experimental set-up and protocols) was described and
recommendations were proposed towards further validation steps. Future developments should
consist in modeling more complex systems by taking into account net flows of water across the
membrane as well as the bulk behavior of the catalyst layer.
The deuterium labeling method was applied on different samples in various operating conditions. Whereas some issues were raised regarding the image quality delivered by one of the
cameras used, the results obtained offered new insights about fundamental mechanisms. The
proton diffusivity values estimated are consistent with the expected trends, but some disagreements with NMR measurements from literature were observed. Further validation experiments
and applications were proposed. The exchange rates between the membrane and the water vapor were estimated. Like bulk water, the exchange at the membrane interface seems limited to
165
4. CONCLUSIONS AND OUTLOOK
a maximal rate that is probably lower than for pure liquid water equilibrated with the vapor
phase. Hydrogen was introduced in the system and different effects were observed according
to the diffusive limitations taking place. As a result, it seemed that a maximal exchange rate
could limit the global H-atom exchange. Moreover, this total exchange is probably not simply
the result of additive processes between the membrane-vapor and membrane-hydrogen exchange
rates, which suggests that a common limitation could affect the whole process. The ionomer
content of the catalyst layer was proposed as a possible location of limitations but this has to be
further investigated. The analysis of the exchange rates can be of practical use. In particular, it
could serve as a 2D in situ diffusimetry method or as an indicator of the catalyst activity state.
The helox pulsing method was proposed as an estimation tool of the bulk diffusive transport
losses. Different validation experiments were realized and showed the relevance of the pulsed
operation of gas since it allows mitigating the changes of the operation under air. Then, it can
serve as a characterization tool of the classical operation under air. Beside helox, the type of
gas pulsed can of course be extended to various mixtures according to the different needs. Pure
oxygen was used in this work.
An experimental set-up was constructed that allows testing and imaging six different cells
simultaneously. The main features of this set-up were described. The major advantages brought
by this set-up are a more efficient use of the beam time and a high repeatability of the operating
parameters between the different cells. The potential of this set-up was fully exploited in this
work so that a significant part of the results presented in this thesis were acquired in about four
days of measurement campaign. It can serve in the future as an useful test station for a wide
variety of studies, as long as the differential cell system will not be completely understood.
By means of the different tools presented, the impact of the microporous layer (MPL) on the
performance and the water distribution was systematically studied. Whereas the presence of a
MPL was beneficial for the performance, it was observed, contrary to many expectations, that
it does not reduce the saturation level of water in the gas diffusion layer (GDL) on cathode side.
The use of helox and oxygen pulsing methods allowed pointing out that mass transport losses
were much higher for the cells having no MPL on cathode side. A corroborating visualization
of the water distribution across the layers led to attribute these mass transport losses to water
accumulation in the catalyst layer (CL) and/or at the CL-GDL interface. Propositions were
made for further experiments. In particular, the use of simplified flow field structure should
allow suppressing undesirable 2D effects of the parameter distribution in the porous media.
The effect of different flow field designs was studied by means of similar tools. Instead
of realizing a technical approach of flow field optimization, the focus of this investigation was
to compare systems with more or less complex patterns of diffusivity pathways in the porous
medium. In consequence, new insights could be gained about the possible mass transport losses
166
4.2 General aspects
and ohmic losses, in particular in the catalyst layer. Future experiments were proposed according
to the different flow field design tested.
At last, cold-starts were visualized in isothermal mode to examine the water distribution and
its possible correlation with the voltage failure happening after a given operation duration. Based
on the direct observations of the water and on the stochastic occurrence of the voltage failure,
liquid water was hypothesized to be present in super-cooled state. Future investigations could
address the possibility of distinguishing super-cooled liquid from ice so as to further inspect the
conditions in which super-cooled water appears and the parameters that influence its stability.
4.2
General aspects
The whole study was realized on a differential cell system that represents a local portion of a
technical cell. It is then directly illustrative of the local phenomena occurring in a full-size cell
and by extension in a fuel cell stack. Based on the results obtained, it seems that the differential
cell has not yet said its last word. Indeed, it is still a quite complex system whose behavior is not
yet fully understood. Rather, it can benefit from further simplifications, as suggested by the flow
field comparison presented. The gap existing between model predictions and the experimental
data can probably be reduced by using model experiments. It will still be challenging enough for
the modelers to capture the physics and demanding enough for the experimentalists to control
the parameters.
Whereas further investigations can address the transport mechanisms in the gas diffusion
layer and the microporous layer, the results obtained by different approaches tend to point out
the core role of the catalyst layer in the system, perhaps also at high current densities. Like
matryoshka dolls interlocking each others, the catalyst layer would exhibit identical features
as the whole system, meaning significant ohmic losses in the ionomer in dry conditions and
important mass transport losses in wet conditions. Experimental in situ investigation of the
catalyst layer are very rare in the current research, in particular those addressing transport
aspects, compared to the huge effort attributed to the ex situ characterization of the catalyst
activity. The combined visualization of liquid water inside the catalyst layer would of course be
a decisive tool to identify the origin of the losses.
Research efforts towards the characterization and the understanding of the Polymer Electrolyte Fuel Cell (PEFC) at the local scale are more relevant than ever as an essential contribution to increase the global performance of the fuel cell stack. Also, future improvements realized
at this scale will directly allow reducing the cost of the system. This will in-turn make the
PEFC technology more competitive and increase its chances for a worldwide commercialization,
in particular to replace the internal combustion engines of our cars.
167
Bibliography
[1] R. G. Miller, Energ. Policy 39, 1569 (2011).
[2] S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. Averyt, M. Tignor,
and H. Miller, Contribution of Working Group I to the Fourth Assessment Report of
the Intergovernmental Panel on Climate Change, Technical report, Cambridge University
Press, 2007.
[3] B. C. Seyfang, Simplification and Investigation of Polymer Electrolyte Fuel Cells using Micro-patterned Glassy Carbon Flow Fields, PhD thesis, Swiss Federal Institute of
Technology Zürich (ETHZ), 2009.
[4] Y. Wang, K. S. Chen, J. Mishler, S. C. Cho, and X. C. Adroher, Appl. Energ.
88, 981 (2011).
[5] A. E. Lutz, R. S. Larson, and J. O. Keller, Int. J. Hydrogen Energy 27, 1103 (2002).
[6] A. A. Kulikovsky, Fuel Cell Basics, chapter 1, pp. 1–38, Analytical Modelling of Fuel
Cells, Elsevier, 2010.
[7] S. A. Vilekar and R. Datta, J. Power Sources 195, 2241 (2010).
[8] P. Boillat, Advanced characterization of polymer electrolyte fuel cells using high resolution neutron imaging, PhD thesis, Swiss Federal Institute of Technology Zürich (ETHZ),
2009.
[9] S. Chen and A. Kucernak, J. Phys. Chem. B 108, 13984 (2004).
[10] H. A. Gasteiger, W. Gu, R. Makharia, M. Mathias, and B. Sompali, Beginningof-life MEA performance - Efficiency loss contributions, volume 3 of Handbook of Fuel
Cells: Fundamentals, Technology and Applications, chapter 46, pp. 593 – 610, Wiley,
2003.
[11] I. A. Schneider, M. H. Bayer, and S. von Dahlen, J. Electrochem. Soc. 158, B343
(2011).
169
BIBLIOGRAPHY
[12] M. Reum, Sub-Millimeter Resolved Measurement of Current Density and Membrane Resistance in Polymer Electrolyte Fuel Cells (PEFC), PhD thesis, Swiss Federal Institute of
Technology Zürich (ETHZ), 2008.
[13] S. S. Kocha, Principles of MEA preparation, volume 3 of Handbook of Fuel Cells: Fundamentals, Technology and Applications, chapter 43, pp. 538 – 565, Wiley, 2003.
[14] J. H. Nam and M. Kaviany, Int. J. Heat Mass. Tran. 46, 4595 (2003).
[15] L. Matamoros and D. Brueggemann, J. Power Sources 172, 253 (2007).
[16] P. Das, X. Li, and Z.-S. Liu, Appl. Energ. 87, 2785 (2010).
[17] R. Flückiger, Transport Phenomena on the Channel-Rib Scale of Polymer Electrolyte
Fuel Cells, PhD thesis, Swiss Federal Institute of Technology Zürich (ETHZ), 2009.
[18] P. Chippar, K. O, K. Kang, and H. Ju, Int. J. Hydrogen Energy 37, 6326 (2011).
[19] S. Chapman and T. Cowling, The Mathematical Theory of Non-Uniform Gases, 1990.
[20] H. Li, Ø. Wilhelmsen, Y. Lv, W. Wang, and J. Yan, Int. J. Greenh. Gas Con. 5,
1119 (2011).
[21] http://en.wikipedia.org/.
[22] K. Jiao and X. Li, Prog. Energ. Combust. 37, 221 (2011).
[23] S. Cleghorn, J. Kolde, and W. Liu, Catalyst coated composite membranes, volume 3
of Handbook of Fuel Cells: Fundamentals, Technology and Applications, chapter 44, pp.
566 – 575, Wiley, 2003.
[24] S. von Dahlen,
Ortsaufgelöste in situ Charakterisierung von Polymerelektrolyt-
Brennstoffzellen in Kanal- und Stegregionen, PhD thesis, Swiss Federal Institute of Technology Zürich (ETHZ), 2012.
[25] T. E. Springer, T. A. Zawodzinski, and S. Gottesfeld, J. Electrochem. Soc. 138,
2334 (1991).
[26] T. Zawodzinski, T. Springer, J. Davey, R. Jestel, C. Lopez, J. Valerio, and
S. Gottesfeld, J. Electrochem. Soc. 140, 1981 (1993).
[27] T. Zawodzinski, C. Derouin, S. Radzinski, R. J. Sherman, V. Smith, T. Springer,
and S. Gottesfeld, J. Electrochem. Soc. 140, 1041 (1993).
170
BIBLIOGRAPHY
[28] S. Ochi, O. Kamishima, J. Mizusaki, and J. Kawamura, Solid State Ionics 180, 580
(2009).
[29] E. Thompson, T. Capehart, T. Fuller, and J. Jorne, J. Electrochem. Soc. 153,
A2351 (2006).
[30] N. Agmon, Chem. Phys. Lett. 244, 456 (1995).
[31] D. Marx, ChemPhysChem 7, 1848 (2006).
[32] K. Yoshizawa, K. Ikezoe, Y. Tasaki, D. Kramer, E. H. Lehmann, and G. G.
Scherer, J. Electrochem. Soc. 155, B223 (2008).
[33] H. Tawfik, Y. Hung, and D. Mahajan, J. Power Sources 163, 755 (2007).
[34] A. Biyikoğlu, Int. J. Hydrogen Energy 30, 1181 (2005).
[35] M. Ji and Z. Wei, Energies 2, 1057 (2009).
[36] K. Wiezell, P. Gode, and G. Lindbergh, J. Electrochem. Soc. 153, A749 (2006).
[37] F. A. de Bruijn, V. A. T. Dam, and G. J. M. Janssen, Fuel Cells 8, 3 (2008).
[38] D. Kramer, Mass transport aspects of polymer electrolyte fuel cells under two-phase flow
conditions, PhD thesis, Technische Universität Bergakademie Freiberg, 2007.
[39] A. Kaestner, S. Hartmann, G. Kuehne, G. Frei, C. Gruenzweig, L. Josic,
F. Schmid, and E. Lehmann, Nucl. Instr. Meth. A 659, 387 (2011).
[40] E. Lehmann, G. Frei, G. Kühne, and P. Boillat, Nucl. Instr. Meth. A 576, 389
(2007).
[41] P. Boillat, G. Frei, E. H. Lehmann, G. G. Scherer, and A. Wokaun, Electrochem.
Solid St. 13, B25 (2010).
[42] G. Kühne, G. Frei, E. Lehmann, P. Vontobel, A. Bollhalder, U. Filges, and
M. Schild, Swiss neutron news 28, 20 (2005).
[43] A. Geiger, Characterization and Development of Direct Methanol Fuel Cells, PhD thesis,
Swiss Federal Institute of Technology Zürich (ETHZ), 2002.
[44] P. Boillat, G. Scherer, A. Wokaun, G. Frei, and E. Lehmann, Electrochem.
Commun. 10, 1311 (2008).
171
BIBLIOGRAPHY
[45] P. Boillat, P. Oberholzer, B. C. Seyfang, A. Kaestner, R. Perego, G. G.
Scherer, E. H. Lehmann, and A. Wokaun, J. Phys.: Condens. Matter 23, 234108
(2011).
[46] K. W. Feindel, S. H. Bergens, and R. E. Wasylishen, J. Power Sources 173, 86
(2007).
[47] T. Kotaka, S. Tsushima, and S. Hirai, ECS Transactions 11, 445 (2007).
[48] I. Manke, C. Hartnig, N. Kardjilov, M. Messerschmidt, A. Hilger, M. Strobl,
W. Lehnert, and J. Banhart, Appl. Phys. Lett. 92 (2008).
[49] K. T. Cho, A. Turhan, and M. Mench, ECS Transactions 41, 513 (2011).
[50] P. Boillat, P. Oberholzer, F. Neuschuetz, G. G. Scherer, and A. Wokaun,
Electrochem. Commun. 22, 12 (2012).
[51] http://www.hbcpnetbase.com/.
[52] P. Boillat, P. Oberholzer, A. Kaestner, R. Siegrist, E. H. Lehmann, G. G.
Scherer, and A. Wokaun, J. Electrochem. Soc. 159, F210 (2012).
[53] Y. W. Rho, O. A. Velev, S. Srinivasan, and Y. T. Kho, J. Electrochem. Soc. 141,
2084 (1994).
[54] P. Berg, K. Promislow, J. St.-Pierre, J. Stumper, and B. Wetton, J. Electrochem. Soc. 151, A341 (2004).
[55] R. Carter, D. Baker, R. Reid, and M. Mathias, ECS Meeting Abstracts 502, 1015
(2006).
[56] R. S. Fu, X. Zhang, and U. Pasaogullari, ECS Transactions 25, 323 (2009).
[57] O. E. Herrera, D. P. Wilkinson, and W. Mérida, J. Power Sources 198, 132 (2012).
[58] F. A. Uribe and T. A. Zawodzinski, Electrochim. Acta 47, 3799 (2002).
[59] S. Slade, J. Smith, S. Campbell, T. Ralph, C. P. de Len, and F. Walsh, Electrochim. Acta 55, 6818 (2010).
[60] J. Zhang, D. Kramer, R. Shimoi, Y. Ono, E. Lehmann, A. Wokaun, K. Shinohara, and G. G. Scherer, Electrochim. Acta 51, 2715 (2006).
[61] J. J. Kowal, A. Turhan, K. Heller, J. Brenizer, and M. M. Mench, J. Electrochem. Soc. 153, A1971 (2006).
172
BIBLIOGRAPHY
[62] P. Oberholzer, P. Boillat, R. Siegrist, A. Kaestner, E. H. Lehmann, G. G.
Scherer, and A. Wokaun, Electrochem. Commun. 20, 67 (2012).
[63] J. S. Preston, U. Pasaogullari, D. S. Hussey, and D. L. Jacobson, ECS Transactions 41, 319 (2011).
[64] A. Schröder, K. Wippermann, W. Lehnert, D. Stolten, T. Sanders,
T. Baumhöfer, N. Kardjilov, A. Hilger, J. Banhart, and I. Manke, J. Power
Sources 195, 4765 (2010).
[65] A. Kazim, H. T. Liu, and P. Forges, J. Appl. Electrochem. 29, 1409 (1999).
[66] D. Kramer, J. Zhang, R. Shimoi, E. Lehmann, A. Wokaun, K. Shinohara, and
G. G. Scherer, Electrochim. Acta 50, 2603 (2005).
[67] B. C. Seyfang, R. Fardel, T. Lippert, G. G. Scherer, and A. Wokaun, Appl.
Surf. Sci. 255, 5471 (2009).
[68] B. C. Seyfang, P. Boillat, F. Simmen, S. Hartmann, G. Frei, T. Lippert, G. G.
Scherer, and A. Wokaun, Electrochim. Acta 55, 2932 (2010).
[69] http://www.sglgroup.com.
[70] K.-D. Kreuer, T. Dippel, W. W. Meyer, and J. Maier, Nafion membranes: Molecular diffusion, proton conductivity and proton conduction mechanism, in Proceedings of the
Second Symposium on Dynamic in Small Confining Systems, Boston, MA, 1992, volume
293 of Materials Research Society Symposium Proceedings, pp. 273–282, 1993.
[71] W. S. Drisdell, C. D. Cappa, J. D. Smith, R. J. Saykally, and R. C. Cohen,
Atmos. Chem. Phys. 8, 6699 (2008).
[72] M. Saito, K. Hayamizu, and T. Okada, J. Phys. Chem. B 109, 3112 (2005).
[73] P. G. Hill and R. D. C. MacMillan, Ind. Eng. Chem. Fund. 18, 412 (1979).
[74] C. D. Cappa, W. S. Drisdell, J. D. Smith, R. J. Saykally, and R. C. Cohen, J.
Phys. Chem. B 109, 24391 (2005).
[75] C. D. Cappa, J. D. Smith, W. S. Drisdell, R. J. Saykally, and R. C. Cohen, J.
Phys. Chem. C 111, 7011 (2007).
[76] D. R. Baker, C. Wieser, K. C. Neyerlin, and M. W. Murphy, ECS Transactions
3, 989 (2006).
173
BIBLIOGRAPHY
[77] H. K. Atiyeh, K. Karan, B. Peppley, A. Phoenix, E. Halliop, and J. Pharoah,
J. Power Sources 170, 111 (2007).
[78] Z. Qi and A. Kaufman, J. Power Sources 109, 38 (2002).
[79] U. Pasaogullari and C.-Y. Wang, Electrochim. Acta 49, 4359 (2004).
[80] J. H. Nam, K.-J. Lee, G.-S. Hwang, C.-J. Kim, and M. Kaviany, Int. J. Heat Mass.
Tran. 52, 2779 (2009).
[81] X. Wang and T. V. Nguyen, J. Electrochem. Soc. 157, B496 (2010).
[82] M. Baghalha, ECS Transactions 41, 521 (2011).
[83] Z. Lu, M. M. Daino, C. Rath, and S. G. Kandlikar, Int. J. Hydrogen Energy 35,
4222 (2010).
[84] A. Z. Weber and J. Newman, J. Electrochem. Soc. 152, A677 (2005).
[85] E. Nishiyama and T. Murahashi, J. Power Sources 196, 1847 (2011).
[86] Y. Wang and K. S. Chen, J. Electrochem. Soc. 157, B1878 (2010).
[87] Y. Wang and K. S. Chen, J. Electrochem. Soc. 158, B1292 (2011).
[88] J. S. Preston, R. S. Fu, U. Pasaogullari, D. S. Hussey, and D. L. Jacobson, J.
Electrochem. Soc. 158, B239 (2011).
[89] Z. Fishman and A. Bazylak, J. Electrochem. Soc. 158, B846 (2011).
[90] E. F. Medici and J. S. Allen, J. Electrochem. Soc. 157, B1505 (2010).
[91] J. P. Owejan, J. E. Owejan, W. Gu, T. A. Trabold, T. W. Tighe, and M. F.
Mathias, J. Electrochem. Soc. 157, B1456 (2010).
[92] J. T. Gostick, M. A. Ioannidis, M. W. Fowler, and M. D. Pritzker, Electrochem.
Commun. 11, 576 (2009).
[93] M. Yoneda, M. Takimoto, E. Ejiri, and S. Koshizuka, ECS Transactions 16, 787
(2008).
[94] K. Kadowaki, Y. Tabe, and T. Chikahisa, ECS Transactions 41, 431 (2011).
[95] M. A. Hickner, N. P. Siegel, K. S. Chen, D. S. Hussey, D. L. Jacobson, and
M. Arif, J. Electrochem. Soc. 155, B427 (2008).
174
BIBLIOGRAPHY
[96] A. Turhan, S. Kim, M. Hatzell, and M. M. Mench, Electrochim. Acta 55, 2734
(2010).
[97] C. Hartnig, I. Manke, R. Kuhn, S. Kleinau, J. Goebbels, and J. Banhart, J.
Power Sources 188, 468 (2009).
[98] T. Tanuma, ECS Transactions 33, 1099 (2010).
[99] Y. Wang, P. Mukherjee, J. Mishler, R. Mukundan, and R. L. Borup, Electrochim.
Acta 55, 2636 (2010).
[100] M. Ismail, T. Damjanovic, D. Ingham, M. Pourkashanian, and A. Westwood, J.
Power Sources 195, 2700 (2010).
[101] Y.-S. Chen and H. Peng, J. Power Sources 196, 1992 (2011).
[102] N. Akhtar and P. Kerkhof, Int. J. Hydrogen Energy 36, 5536 (2011).
[103] A. Aiyejina and M. K. S. Sastry, J. Fuel Cell Sci. Tech. 9, 011011 (2012).
[104] P. Boillat, D. Kramer, B. Seyfang, G. Frei, E. Lehmann, G. Scherer,
A. Wokaun, Y. Ichikawa, Y. Tasaki, and K. Shinohara, Electrochem. Commun.
10, 546 (2008).
[105] P. Oberholzer, P. Boillat, R. Siegrist, R. Perego, A. Kaestner, E. H.
Lehmann, G. G. Scherer, and A. Wokaun, J. Electrochem. Soc. 159, B235 (2012).
[106] P. Oberholzer, P. Boillat, R. Siegrist, R. Perego, A. Kaestner, E. H.
Lehmann, G. G. Scherer, and A. Wokaun, ECS Transactions 41, 363 (2011).
[107] M. Cappadonia, J. Erning, and U. Stimming, J. Electroanal. Chem. 376, 189 (1994).
[108] M. Cappadonia, J. Erning, S. S. Niaki, and U. Stimming, Solid State Ionics 77, 65
(1995).
[109] H. Mendil-Jakani, R. J. Davies, E. Dubard, A. Guillermo, and G. Gebel, J.
Membr. Sci. 369, 148 (2011).
[110] R. McDonald, C. Mittelsteadt, and E. Thompson, Fuel Cells 4, 208 (2004).
[111] E. Cho, J.-J. Ko, H. Ha, S.-A. Hong, K.-Y. Lee, T.-W. Lim, and I.-H. Oh, J.
Electrochem. Soc. 150, A1667 (2003).
[112] S. Kim and M. Mench, J. Power Sources 174, 206 (2007).
175
BIBLIOGRAPHY
[113] S. Kim, B. K. Ahn, and M. Mench, J. Power Sources 179, 140 (2008).
[114] J. Ko and H. Ju, Applied Energy 94, 364 (2012).
[115] M. Oszcipok, D. Riemann, U. Kronenwett, M. Kreideweis, and M. Zedda, J.
Power Sources 145, 407 (2005).
[116] R. Ahluwalia and X. Wang, J. Power Sources 162, 502 (2006).
[117] L. Mao, C.-Y. Wang, and Y. Tabuchi, J. Electrochem. Soc. 154, B341 (2007).
[118] K. Jiao and X. Li, Electrochim. Acta 54, 6876 (2009).
[119] H. Meng, Int. J. Hydrogen Energy 33, 5738 (2008).
[120] S. Ge and C.-Y. Wang, Electrochem. Solid St. 9, A499 (2006).
[121] S. Ge and C.-Y. Wang, Electrochim. Acta 52, 4825 (2007).
[122] K. Tajiri, Y. Tabuchi, F. Kagami, S. Takahashi, K. Yoshizawa, and C.-Y. Wang,
J. Power Sources 165, 279 (2007).
[123] K. Tajiri, Y. Tabuchi, and C.-Y. Wang, J. Electrochem. Soc. 154, B147 (2007).
[124] E. L. Thompson, J. Jorne, W. Gu, and H. A. Gasteiger, J. Electrochem. Soc. 155,
B887 (2008).
[125] E. Thompson, J. Jorne, W. Gu, and H. Gasteiger, J. Electrochem. Soc. 155, B887
(2008).
[126] K. Nishida, H. Ito, S. Tsushima, and S. Hirai, Kagaku Kogaku Ronbun. 37, 57 (2011),
(Japanese only).
[127] Y. Ishikawa, T. Morita, K. Nakata, K. Yoshida, and M. Shiozawa, J. Power
Sources 163, 708 (2007).
[128] Y. Ishikawa, H. Hamada, M. Uehara, and M. Shiozawa, J. Power Sources 179, 547
(2008).
[129] R. Mukundan, Y. Kim, T. Rockward, J. Davey, B. Pivovar, D. Hussey, D. Jacobson, M. Arif, and R. Borup, ECS Transactions 11, 543 (2007).
[130] R. Mukundan, J. Davey, R. Lujan, J. Spendelow, Y. Kim, D. Hussey, D. Jacobson, M. Arif, and R. L. Borup, ECS Transactions 16, 1939 (2008).
176
BIBLIOGRAPHY
[131] R. Mukundan, R. Lujan, J. Davey, J. Spendelow, D. Hussey, D. Jacobson,
M. Arif, and R. Borup, ECS Transactions 25, 345 (2009).
[132] L. Torres, J. Granada, and J. Blostein, Nucl. Instrum. Meth. B 251, 304 (2006).
[133] G. Lu, D. Depaolo, Q. Kang, and D. Zhang, J. Geophys. Res. 114, D07305 (2009).
[134] C. Chacko, R. Ramasamy, S. Kim, M. Khandelwal, and M. Mench, J. Electrochem.
Soc. 155, B1145 (2008).
[135] M. Akyurt, G. Zaki, and B. Habeebullah, Energ. Convers. Manage. 43, 1773 (2002).
[136] L. Makkonen, Langmuir 16, 7669 (2000).
[137] H. K. Christenson, J. Phys.: Condens. Matter 13, R95 (2001).
[138] E. Sadeghi, N. Djilali, and M. Bahrami, J. Power Sources 196, 3565 (2011).
[139] M. Khandelwal and M. Mench, J. Power Sources 161, 1106 (2006).
[140] E. Bigg, Proc. Phys. Soc. B 66, 688 (1953).
[141] A. Saito, Y. Utaka, S. Okawa, K. Matsuzawa, and A. Tamaki, Int. J. Heat Mass.
Tran. 33, 1697 (1990).
[142] S. Okawa, A. Saito, and H. Suto, Int. J. Refrig. 25, 514 (2002).
[143] S.-L. Chen and T.-S. Lee, Int. J. Heat Mass. Tran. 41, 769 (1998).
[144] S. Okawa, A. Saito, and R. Minami, Int. J. Refrig. 24, 108 (2001).
[145] B. Mason, Adv. Phys. 7, 221 (1958).
[146] B. Vonnegut, J. Colloid Sci. 3, 563 (1948).
[147] S. Tabakova and F. Feuillebois, J. Colloid Interface Sci. 272, 225 (2004).
[148] D. Bertolini, M. Cassettari, and G. Salvetti, Chem. Phys. Lett. 136, 67 (1987).
[149] L. Homshaw, J. Colloid Interface Sci. 84, 141 (1981).
[150] G. Goyer, T. Bhadra, and S. Gitlin, J. Appl. Meteorol. 4, 156 (1965).
[151] G. Edwards, L. Evans, and S. Hamann, Nature 223, 390 (1969).
[152] A. Saito, S. Okawa, A. Tojiki, H. Une, and K. Tanogashira, Int. J. Heat Mass.
Tran. 35, 2527 (1992).
177
List of Tables
2.1
Gas transitions performed for transient experiments. . . . . . . . . . . . . . . . .
30
3.1
Summary of experiments realized with deuterium labeling. . . . . . . . . . . . . .
74
3.2
Diffusivity values for two different imaging set-ups (”reference” condition: RH =
80 %, T = 70°C, flow = 300 and 400 Nml/min).
. . . . . . . . . . . . . . . . . .
80
3.3
Summary of experiments realized to study the effect of the MPL. . . . . . . . . . 100
3.4
Summary of experiments realized to study the impact of the flow field design. . . 124
3.5
Summary of cold-start experimental conditions. . . . . . . . . . . . . . . . . . . . 147
3.6
Parameters used for the 2D simulation (Figure 3.51) of the temperature distribution obtained during a cold-start at -10°C. . . . . . . . . . . . . . . . . . . . . . . 159
179
List of Figures
1.1
PEFC: working principle and components. . . . . . . . . . . . . . . . . . . . . . .
3
1.2
Representation of the different losses in the PEFC. . . . . . . . . . . . . . . . . .
7
1.3
High frequency measurement artifact. . . . . . . . . . . . . . . . . . . . . . . . .
9
1.4
Nafion structural formula [24].
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
1.5
Grotthuss mechanism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
1.6
PEFC: water transport. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17
1.7
ICON beam line (picture from [42]) and anisotropic collimation (from [8, 41]). . .
20
1.8
Effect of the tilted detector on the resolution. . . . . . . . . . . . . . . . . . . . .
21
2.1
System that can be studied with deuterium labeling. Left: the 1-sided cell studied
in the present work. Right: the 2-sided cell investigated by Boillat et al. [8, 44, 45] 27
2.2
Gas mixtures used for deuterium labeling. . . . . . . . . . . . . . . . . . . . . . .
29
2.3
Repetition of isotopic transitions in the gas mixture (symbolic scheme). . . . . .
31
2.4
Gas management set-up for isotope labeling. . . . . . . . . . . . . . . . . . . . . .
32
2.5
Unstability effect on Ftheo,sin . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
38
2.6
Effect of the number of iteration N on the error and the computing time. . . . .
41
2.7
Search of Dopt and k opt on a changing interval. . . . . . . . . . . . . . . . . . . .
43
2.8
Sensitivity of DM on kW for
ref
DM
= 1.3 ·
10−5
cm2 s−1
and
ref
kW
≈ 4·
10−5
[mol
cm−2 s−1 ] . Left: full domain of DM . Right: focus on the region of interest. . . .
44
Blurring effect on the estimation of DM . . . . . . . . . . . . . . . . . . . . . . . .
45
2.10 Gas management set-up for the helox pulsing method. . . . . . . . . . . . . . . .
50
2.9
2.11 Helox pulse and measurement locations. Condition: 1 A/cm2 , T = 70 °C, RH an.
= 100%, RH ca. = 0% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
51
2.12 Helox pulse morphology for different relative humidity conditions (RH) and different compression rates (a to i) and design (j to l), at 1 A/cm2 and T = 70
°C.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
53
2.13 Oxygen pulse morphology for different relative humidity conditions (RH), at 1
A/cm2 and T = 70 °C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
181
55
LIST OF FIGURES
2.14 Comparison of experiments with or without helox pulses. . . . . . . . . . . . . .
57
2.15 Comparison of experiments with or without oxygen pulses.
. . . . . . . . . . . .
59
2.16 Multi-cell set-up. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
63
2.17 Measurement probes locations. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
65
2.18 Thermal management (case of over-humidification: cell colder than housings). . .
66
2.19 Alignment set-up: adaptation to cell deformations. . . . . . . . . . . . . . . . . .
68
2.20 Alignment set-up: adaptation to various sizes of cells. . . . . . . . . . . . . . . .
68
2.21 Alignment verification radiograms. . . . . . . . . . . . . . . . . . . . . . . . . . .
69
2.22 Flow field designs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
70
2.23 Multi-cell positioning configurations (in-plane or through-plane) in front of the
corresponding scintillator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1
Gas flow dependency of the diffusivity estimation (”reference” condition). Left:
THe , right: TH2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2
75
Diffusivity estimation for repeated experiments (”reference” condition: RH = 80
%, T = 70°C, flow = 300 and 400 Nml/min). . . . . . . . . . . . . . . . . . . . .
3.3
71
77
Distribution of the diffusivity estimation for repeated experiments (”reference”
condition: RH = 80 %, T = 70°C, flow = 300 and 400 Nml/min) in the case of
the midi set-up. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4
79
Comparison between the micro set-up and the midi set-up for a similar sample
(N117 membrane under 1 H2O in steady-state). . . . . . . . . . . . . . . . . . . .
79
3.5
Relative humidity dependency of DM (T = 70°C). Values of DNMR taken from [70] 80
3.6
Temperature dependency of DM (RH = 80%). Values of DNMR and Ea,NMR from
[70]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
81
3.7
Gas flow dependency of kW estimation (”reference” condition, THe transitions). .
83
3.8
Gas flow dependency of kW expressed in terms of equivalent resistances (same
data as (Figure 3.7). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.9
84
Relative humidity dependency of kW (T = 70°C). In the right-hand graph, the
change of humidity under 2 H2 O at constant dew point temperature was taken
into account. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
86
3.10 Interpretation of the isotopic effect measured on kW . . . . . . . . . . . . . . . . .
87
3.11 Temperature dependency of kW (RH = 80%). . . . . . . . . . . . . . . . . . . . .
88
3.12 Partial pressure dependence of kW for different temperatures. The value p1 H2 O
and p2 H2 O are taken for the directions (1 → 0) and (0 → 1) respectively. . . . . .
3.13 Isotopic effect on kW .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
89
89
3.14 Gas flow dependency of kW expressed in terms of equivalent resistances for various
cell designs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
182
90
LIST OF FIGURES
3.15 Absolute pressure dependency of kW expressed in terms of equivalent resistances
for various cell designs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
92
3.16 Effect of hydrogen on the total exchange rate k. . . . . . . . . . . . . . . . . . . .
94
3.17 Modified model of the exchanges kW and kH . . . . . . . . . . . . . . . . . . . . .
95
3.18 Ratio of exchange rates for the different cells (2011, midi set-up). . . . . . . . . .
96
3.19 Performance indicators for steady-state steps of RH realized at i = 0.5 A/cm2 . . 102
3.20 Water content for steady-states step of RH realized at i = 0.5 A/cm2 . . . . . . . 104
3.21 Water distribution profiles at the RH = 100%/100% step of RH steady-state
experiment realized at 0.5 A/cm2 (Figure 3.19). . . . . . . . . . . . . . . . . . . . 106
3.22 Hypothesis for explaning the higher GDL saturation obtained with MPL. . . . . 108
3.23 Performance indicators and water content for steady-state steps of RH realized
at i = 1 A/cm2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
3.24 Performance indicators for an IV-curve realized at RH = 100%/100%. . . . . . . 112
3.25 Performance indicators for an IV-curve realized at RH = 100%/0%. . . . . . . . 113
3.26 Performance indicators for an IV-curve realized at RH = 40%/0%. . . . . . . . . 114
3.27 Performance indicators and water evolution for a RH transient applied at t = 0
from 40%/0% to 100%/100% with a constant current of 0.5 A/cm2 . . . . . . . . 116
3.28 Performance indicators and water evolution for a RH transient applied at t = 0
from 100%/0% to 40%/0% with a constant current of 0.5 A/cm2 . . . . . . . . . . 117
3.29 Normalized values of performance indicators and water evolution for a RH transient applied at t = 0 from 100%/0% to 40%/0% with a constant current of 0.5
A/cm2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
3.30 Normalized values of performance indicators and water evolution for a RH transient applied at t = 0 from 100%/0% to 40%/0% with a constant current of 0.5
A/cm2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
3.31 Performance indicators for steady-state steps of RH realized at i = 1 A/cm2 . . . 125
3.32 Helox pulses during the driest condition (40%/0%) of the RH-series realized at i
= 1 A/cm2 : comparison of different flow fields. . . . . . . . . . . . . . . . . . . . 127
3.33 Water content for steady-states step of RH realized at i = 1 A/cm2 . . . . . . . . 128
3.34 Water content in the channel regions of the MPL for steady-states step of RH
realized at i = 1 A/cm2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
3.35 Performance indicators for an IV-curve realized at RH = 100%/100%. . . . . . . 130
3.36 Performance indicators for an IV-curve realized at RH = 100%/0%. . . . . . . . 131
3.37 Performance indicators for an IV-curve realized at RH = 40%/0%. . . . . . . . . 132
3.38 Symbolic illustration of the drying effect in the electrode. . . . . . . . . . . . . . 134
3.39 IR-free voltages for the IV-curve realized at RH = 40%/0%. . . . . . . . . . . . . 136
183
LIST OF FIGURES
3.40 Performance indicators and water evolution for a RH transient applied at t = 0
from 40%/0% to 100%/100% with a constant current of 1 A/cm2 . . . . . . . . . 138
3.41 Performance indicators and water evolution for a RH transient applied at t = 0
from 100%/0% to 40%/0% with a constant current of 1 A/cm2 . . . . . . . . . . . 139
3.42 Normalized values of performance indicators and water evolution for RH transients applied at t = 0 from 40%/0% to 100%/100% (top) and 100%/0% to
40%/0% (bottom) with a constant current of 1 A/cm2 . . . . . . . . . . . . . . . . 140
3.43 Cold-start set-up (links: full-view, right: sectional view). . . . . . . . . . . . . . . 146
3.44 Sub-zero start-up at -10 °C and 0.1 A/cm2 visualized with neutron imaging. . . . 149
3.45 Emerging water onto the channel walls. . . . . . . . . . . . . . . . . . . . . . . . 150
3.46 Evolution of voltage and water content in chosen areas during the voltage failure. 151
3.47 Conductivity (inverse value of HFR) and water amount evolution in the MEA
area in the beginning of the cold-start.
. . . . . . . . . . . . . . . . . . . . . . . 152
3.48 Working time distribution obtained with 34 cold-starts repeated at the same
condition: 0.1 A/cm2 and -10°C. The voltage and HFR values measured during
the cold-start are taken after the current is stabilized and when the voltage reaches
its maximum value. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
3.49 Evolution of voltage and water content in the MEA for different cold-starts. . . . 154
3.50 Evolution of voltage and water content in the MEA for different cold-starts. . . . 155
3.51 Simulation (Ansys ) of the 2D temperature distribution obtained during a coldstart at -10 °C and 0.1 A/cm2 with thermal parameters listed in the below table
(Table 3.6) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
3.52 HFR and water amount evolution in the MEA area during the voltage failure of
the cold-start.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
3.53 Working times recorded for 400 cold-starts realized under various operation parameters. One parameter was changed at once from the reference condition (10°C, 0.1 A/cm2 ). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
184
Symbols, Indices and Abbreviations
Latin symbols
Symbol
Description
Unit
a
A
c
D
d
E
Ea
F
∆r g
∆r h
∆r s
I
I
i
i0
k
kp
L
M
Ṁ
m
Activity
Active area
Concentration
Diffusion coefficient
Pore size diameter
Energy
Activation energy
1 H isotope fraction
Specific Gibb’s reaction energy
Specific reaction enthalpy
Specific reaction entropy
Intensity
Current
Current density
Exchange current density
Exchange rate
Desublimation rate
Membrane thickness
Mass
Mass flow
Molar mass
[-]
[cm2 ]
[mol·cm−3 ]
[cm2 ·s−1 ]
[µm], [nm]
[mJ]
[meV]
[-]
[J· mol−1 ]
[J· mol−1 ], [J· kg−1 ]
[J· mol−1 · K−1 ]
[a.u.]
[A]
[A/cm2 ]
[mA/cm2Pt ],[nA/cm2Pt ]
[mol · cm−2 · s−1 ]
[m · s−1 ]
[µm]
[mg]
[mg · s−1 ]
[g · mol−1 ]
185
Symbol
Description
Unit
N
ṅ
P
p
q
R
r
RH
S
s
s
ssur
t
T
T
U
v
Number
Surface molar flow
Period
Pressure
Heat flow
Equivalent exchange rate resistance
Resistivity
Relative humidity
Line spread function
Saturation
Deviation estimator
Sursaturation level
Time
Number
Temperature
Voltage
Velocity
Volumetric flow
Across-the-membrane coordinate
Molar fraction of species i
Number of electrons
[-]
[mol · cm−2 · s−1 ]
[s]
[bar]
[W · cm−2 ]
[mol−1 · cm2 · s]
[Ω · cm2 ]
[%]
[-]
[-]
[a.u.]
[-]
[s],[min]
[-]
[K],[°C]
[V]
[m · s−1 ]
[l/min],[Nl/min]
[µm]
[-]
[-]
V̇
x
xi
z
Greek symbols
Symbol
Description
Unit
α
αst
αA
αC
γ
γ
δ
δ
η
Coefficient
Stability coefficient
Anodic charge transfer coefficient
Cathodic charge transfer coefficient
Correction coefficient
Surface tension
GDL thickness
Transmitted material thickness
Efficiency
Overpotential
[-]
[-]
[-]
[-]
[-]
[J · m2 ]
[µm]
[mm]
[%]
[V],[mV]
186
Symbol
Description
Unit
φ
ρ
λ
σ
σ
σ
σ
Σ
Fugacity coeeficient
Mass density
Membrane hydration
Conductivity
Estimator
Noise standard deviation
Microscopic cross section
Attenuation coefficient
[-]
[g · cm3 ]
[-]
[S · m−1 ]
[-]
[-]
[barn]
[mm−1 ]
Symbol
Description
Value
e
F
kB
R
vn
Elementary charge
Faraday constant
Boltzmann constant
Universal gas constant
Ideal gas molar volume in normal condition
1.6 · 10−19 [C]
96’485 [C · mol−1 ]
1.38 · 10−23 [m2 · kg · s−2 K−1 ]
8.3145 [J · mol−1 · K−1 ]
22.4 [l · mol−1 ] at 0°C
Constants
Indices
Symbol
Description
()blur
()cond
()dif f
()evap
()f it
()n
()0
()A
()a
()C
()C
()CT
Blurring
Condensation
Diffusive
Evaporation
Fitted
Normal condition
Standard condition
Anode
Activation
Cathode
Concentration
Charge transfer
187
Symbol
Description
()D
()desub
()dew
()exp
()f
()G
()G
()g
()H
()HW
()I
()Kn
()L
()lim
()LW
()M
()num
()nb
()Ω
()prod
()R
()r
()rel
()ref
()rev
()SL
()sat
()sur
()th
()theo
Diffusion
Desublimation
Dew point
Experimental
Fusion
Gas glow
Geometric
Gas
Hydrogen
Heavy water
Inherent
Knudsen
Liquid
Limit
Light (i.e. normal) water
Membrane
Numerical
Non-bulk
Ohmic
Production
Resulting
Reaction
Relative
Reference
Reversible
Solid-liquid
Saturation
Sursaturation
Thermodynamic
Theoretical
188
Symbol
Description
()W
()wrk
b ()
x ()
Water
Working
Bulk
Isotope with atomic mass x
Abbreviations
Abbreviation Description
BiP
CCD
CCM
CL
DSC
ECA
EIS
EW
FWHM
GDL
HOR
HFR
ICON
IEC
LSF
MEA
MPL
MRI
NIAG
NMR
OCV
ORR
PCB
Bipolar Plates
Charge Coupled Device
Catalyst Coated Membrane
Catalyst Layer
Differential Scanning Calorimetry
Electro-catalytically Active Area
Electrochemical Impedance Spectroscopy
Equivalent Weight
Full Width at Half Maximum
Gas Diffusion Layer
Hydrogen Oxidation Reaction
High Frequency Resistance (or Resistivity)
Imaging with Cold Neutrons
Ion Exchange Capacity
Line Spread Function
Membrane-Electrode-Assembly
Microporous Layer
Magnetic Resonance Imaging
Neutron Imaging and Activation Group
Nuclear Magnetic Resonance
Open Circuit Voltage
Oxygen Reduction Reaction
Printed Circuit Board
189
Abbreviation Description
PEFC
PEM
PFG
PFSA
SINQ
Polymer Electrolyte Fuel Cell
Polmyer Electrolyte Membrane
Pulsed-Field Gradient
Perfluorosulphonic Acid ionomer
Spallation neutron source at PSI
190
Publications list
Peer reviewed papers (as author or co-author)
P. Boillat, P. Oberholzer, F. Neuschütz, T.J. Schmidt, and A. Wokaun, An electrochemical
method for high accuracy measurements of water transfer in fuel cells, Electrochem. Commun.
22, 12 (2012)
P. Boillat, P. Oberholzer, A. Kaestner, R. Siegrist, E.H. Lehmann, G.G. Scherer, and A. Wokaun,
Impact of water on PEFC performance evaluated by neutron imaging combined with pulsed helox
operation, J. Electrochem. Soc. 159, F210 (2012)
M. Weiland, P. Boillat, P. Oberholzer, A. Kaestner, E.H. Lehmann, T.J. Schmidt, G.G. Scherer,
and H. Reichl, High resolution neutron imaging for pulsed and constant load operation of passive
self-breathing PEFCs, In press, Accepted Manuscript
P. Oberholzer, P. Boillat, R. Siegrist, A. Kaestner, E.H. Lehmann, G.G. Scherer, and A. Wokaun,
Simultaneous neutron imaging of six operating PEFCs: experimental set-up and study of the
MPL effect, Electrochem. Commun. 20, 67 (2012)
P. Oberholzer, P. Boillat, R. Siegrist, R.Perego, A. Kaestner, E.H. Lehmann, G.G. Scherer, and
A. Wokaun, Cold-start of a PEFC visualized with high resolution dynamic in-plane neutron
imaging, J. Electrochem. Soc. 159, B235 (2012)
P. Boillat, P. Oberholzer, B.C. Seyfang, A. Kaestner, R. Perego, G.G. Scherer, E.H. Lehmann,
and A. Wokaun, Using 2 H labeling with neutron radiography for the study of solid polymer
electrolyte water transport properties, J. Phys.: Condens. Matter 23, 234108 (2011)
I.A. Schneider, S. von Dahlen, M.H. Bayer, P. Boillat, M. Hildebrandt, E.H. Lehmann, P. Oberholzer, G.G. Scherer, and A. Wokaun, Local transients of flooding and current in channel and
land areas of a polymer electrolyte fuel cell, J. Phys. Chem. C 114, 11998 (2010)
191
Peer reviewed conference proceedings
P. Oberholzer, P. Boillat, R. Siegrist, R.Perego, A. Kaestner, E.H. Lehmann, G.G. Scherer, and
A. Wokaun, Neutron Imaging of Isothermal Sub-Zero Degree Celsius Cold-Starts of a Polymer
Electrolyte Fuel Cell (PEFC), ECS Trans. 41, 363 (2011)
P. Boillat, P. Oberholzer, R. Perego, R. Siegrist, A. Kaestner, E.H. Lehmann, G.G. Scherer, and
A. Wokaun, Application of Neutron Imaging in PEFC Research, ECS Trans. 41, 27 (2011)
E.H. Lehmann, P. Oberholzer, and P. Boillat, Neutron Imaging Methods for the Investigation
of Energy Related Materials: Fuel Cells, Batteries, Hydrogen Storage, and Nuclear Fuel, Mater.
Res. Soc. Symp. Proc. 1262, 95 (2010)
Y. Utaka, Y. Tasaki, K. Waki, D. Iwasaki, N. Kubo, K. Shinohara, P. Boillat, P. Oberholzer,
G.G. Scherer, and E.H. Lehmann, Characteristics of moisture distribution and mass transfer in
MPL by neutron radiography, Trans. JSME Series B 76, No. 772, 1995 (2010). (Article in
Japanese.)
Y. Utaka, Y. Tasaki, S. Wang, D. Iwasaki, K. Waki, N. Kubo, K. Shinohara, P. Boillat, G.
Frei, P. Oberholzer, G.G. Scherer, and E.H. Lehmann, Mass transfer characteristics in porous
media applying simultaneous measurement method of water visualization and oxygen diffusivity
by neutron radiography, Trans. JSME Series B 76, No. 771, 1964 (2010). (Article in Japanese.)
192
Oral presentations
P. Oberholzer, Instrumentation and research at PSI: Imaging of liquid water in operating polymer
electrolyte fuel cells, European Technical School on Hydrogen and Fuel Cells 2012, Heraklion,
Greece, September 24 - 28, 2012
P. Oberholzer, Ground breaking research at PSI: Neutron imaging combined with helox pulse
analysis in fuel cells, European Technical School on Hydrogen and Fuel Cells 2012, Heraklion,
Greece, September 24 - 28, 2012
P. Oberholzer, P. Boillat, R. Siegrist, A. Kaestner, E.H. Lehmann, G.G. Scherer, and A. Wokaun,
Simultaneous neutron imaging of 6 cells: set-up and results (MPL study), 9th Symposium on
Fuel Cell Modeling and Experimental Validation (MODVAL), Sursee, Switzerland, April 3 - 4,
2012
P. Oberholzer, P. Boillat, R. Siegrist, A. Kaestner, E.H. Lehmann, G.G. Scherer, and A. Wokaun,
Neutron imaging of Isothermal Sub-Zero Degree Celsius Cold-Starts in a Polymer Electrolyte
Fuel Cell (PEFC), 220th Meeting of the Electrochemical Society (ECS), Boston, USA, October
10 - 14, 2011
P. Oberholzer, P. Boillat, R. Siegrist, A. Kaestner, E.H. Lehmann, G.G. Scherer, and A. Wokaun,
Sub-zero isothermal start-up of PEFC visualized with neutron imaging, 8th Symposium on Fuel
Cell Modeling and Experimental Validation (MODVAL), Bonn, Germany, March 7 - 8, 2011
P. Oberholzer, P. Boillat, R. Siegrist, R. Perego, A. Kaestner, E.H. Lehmann, G.G. Scherer,
and A. Wokaun, Study of transport in PEFC membranes using neutron radiography with deuterium labeling, 61st Annual Meeting of the International Society of Electrochemistry (ISE),
Nice, France, September 27 - October 1, 2010
P. Oberholzer, P. Boillat, R. Siegrist, R. Perego, A. Kaestner, E.H. Lehmann, G.G. Scherer,
and A. Wokaun, Study of transport in PEFC membranes using neutron radiography with deuterium labeling, 7th Symposium on Fuel Cell Modeling and Experimental Validation (MODVAL),
Morges, Switzerland, March 23 - 24, 2010
193

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