CURRENT STATUS AND FUNDAMENTAL RESEARCH NEEDS IN THERMAL
MANAGEMENT WITHIN A PEMFC STACK
Satish G. Kandlikar a,∗, [email protected] Zijie Lu a, [email protected] Thomas A. Trabold b, [email protected]
a
Rochester Institute of Technology, Rochester, NY, USA
General Motors Corporation, Honeoye Falls, NY, USA
b
ABSTRACT
Understanding the thermal effects is critical in optimizing the performance and durability of proton exchange
membrane fuel cells (PEMFCs). A PEMFC produces a similar amount of waste heat to its electric power output
and tolerates only a small deviation in temperature from its design point. These stringent thermal requirements
present a significant heat transfer challenge. The heat balance also interplays with the water balance. Although a
considerable amount of work has been reported on the experiments and comprehensive thermal models of the
fuel cells, the fundamental heat transfer mechanisms in each individual fuel cell component are still not clearly
understood. In this work, these thermal transport issues at component level (including polymer, catalyst layers,
gas diffusion media and bipolar plates) are reviewed. The current status of PEMFC cooling technology is also
reviewed and suggestions for future research are outlined.
1 INTRODUCTION
Thermal management is a key technical challenge that must be resolved for proton exchange membrane
(PEM) fuel cell technology before it can be commercialized (Wang 2004; Faghri and Gao 2005). In this paper
the published literature on PEM fuel cell (PEMFC) thermal management is reviewed, the heat transfer issues in
individual PEMFC component are outlined, and different practical cooling systems are discussed. After
reviewing the current status in these areas specific needs for further research are identified.
BACKGROUND
In this section, the thermal constraints of current PEMFCs caused by using Nafion membranes are first
presented. The heat and water transport phenomena within a fuel cell are then outlined. It is followed by a brief
discussion on the objectives for developing accurate thermal models.
Thermal Constraints of Current PEMs
Current PEM fuel cells operate in the temperature range of 60 – 80 °C. This range is dictated by the
material properties of the PEM. The most commonly employed PEM is Nafion (a trademark of E.I. DuPont de
Nemours, Wilmington, DE), which exhibits high proton conductivity only in the hydrated state (Marechal et al.
2007). The hydration requirement of Nafion limits the maximum fuel cell operating temperature to about 80 °C.
Above this temperature, the membrane dry out occurs resulting in a decreased proton conductivity. The thermal
and mechanical degradation also occurs at elevated temperatures due to the relatively low glass transition
temperature (Tg) of the hydrated Nafion (in the range of 90 – 120 °C) (Bauer et al. 2005; Young and Mauritz
2001). On the other hand, a cell temperature below 60 °C may lead to water condensation and flooding of
electrodes, with a resultant voltage loss caused by added resistance to reactant mass transport. A low operating
temperature is also undesirable from consideration of proton conductivity and electrochemical reaction kinetics.
Water and Heat Balance
The main elements composing a PEMFC are: bipolar plates, gas diffusion and microporous layers, catalyst
layers and membrane. Figure 1 shows a schematic representation of a PEMFC and the water and heat transport
processes within it. The water transport includes contributions from electro-osmotic drag, back diffusion from
the cathode to the anode, and diffusion and convection to or from the respective air and hydrogen gas streams.
Water is produced at cathode catalyst layer by electrochemical reactions and/or in gas channels due to
condensation. An imbalance between production and removal can result in either dehydration of the membrane,
or flooding of the electrodes. Both are detrimental to fuel cell performance and durability.
A PEMFC produces an amount of waste heat similar to its electric power output, thus limiting its energy
efficiency to about 50% (Wang 2004). Heat production in the PEMFC includes entropic heat of reactions,
irreversibilities of the electrochemical reactions and ohmic resistances, as well as water condensation (Wang
2004; Shan and Choe 2005; Weber and Newman 2006). This heat is removed by the cooling system or
transferred by conduction-convection across the faces of the stack. The balance of these processes determines
the temperature distribution in the PEMFC.
The thermal and water transport in PEMFC are inherently coupled because (i) evaporation and
condensation processes are respectively accompanied with absorption and release of latent heat; (ii) water and
heat transport occur in conjunction with each other due to heat pipe effect (a temperature gradient induces phase
change and net mass transfer of water) (Shan and Choe 2005); and (iii) the saturation vapor pressure is strongly
dependent on local gas temperature. The water transport occurs at multiscale, from molecular diffusion of vapor
in the CCL to the two-phase flow in the gas distribution channels. Kandlikar (2007) presents a review of current
literature on the water transport processes and identifies current research needs in this area.
2 Thermal Modeling
M
t the compleexity and inteeractions of thhe water and heat transporrt processes itt is difficult to
Due to
experimenttally measure the temperatu
ure distributionn inside a fuell cell. Such innformation is usually
u
obtaineed
through nu
umerical simulaation. An accurrate thermal m
model is neededd to meet the foollowing objecttives:
1. Obtain
O
accuratee temperature profiles in tthe different cell componeents. Evaluate the effects of
teemperature on the individuall component ddegradation andd durability. D
Develop a propper heat removval
sttrategy.
2. Prrovide componnent-level therm
mal inputs to aassess the part--load, off-desiggn (different innlet gas pressurre,
teemperature and
d flow rates) and transient performance (fuel
(
utilizatioon, polarization
n curve, curreent
diistribution, etc.) of the stack
k while integraating with suitaable electrocheemical and watter/gas diffusioon
(ssingle-phase annd two-phase fllow with embeedded gravity and
a orientation effects) models.
3. Iddentify dryout conditions in
n the membraane and flood
ding conditionns in the cathhode side GDL,
m
microporous
layyer, catalyst laayer and gas chhannels. Predicct water transpport rates and hydration leveels
thhroughout the stack.
s
Project humidity
h
requirrements of the inlet gas stream
ms.
4. Prrovide results from simulatioons to determiine the effects of changing component
c
pro
operties, such as
poorosity of the GDL, micropo
orous layer struucture, surfacee energies of G
GDL and the channels, coolaant
flow rates, channel size and coonfiguration, ettc.
D
an oveerall performannce prediction model to alloow accurate prredictions of thhe local/averagge
5. Develop
cuurrent distributtions, maximum
m temperaturess, and water traansport and its distribution.
Properr thermal mannagement is crritical in the eefficient operaation of a fuell cell and hass been receivinng
considerabble attention, esspecially in thee last five yearrs. Presenting a detailed revieew of these moodels itself couuld
be the topiic of a separatee paper. The prresent paper iss limited to ideentifying the m
major challengees in componenntlevel and stack-level
s
therrmal issues thatt still remain unresolved
u
at bo
oth microscopiic and macrosccopic scales.
h balance (riight) of a PEM
MFC. NOT TO SCALE.
Figg. 1. Schematicc of water balannce (left) and heat
COMPON
NENT LEVEL
L THERMAL TRANSPOR
RT ISSUES
ugh there are a large numbeer of publicatioons on thermaal managementt of a PEMFC
C (e.g. Shan annd
Althou
Choe 2005
5; Weber and Newman 20066; Fuller and N
Newman; Nguuyen and White 1993; Amphhlett et al. 19995;
Zhang et al.
a 2004; Maggio et al. 1996; Djilali and Luu 2002; Faghri and Gao 2005;; Hwang 2006;; Ju at all 20055a;
Muller andd Stefanopouloo 2006; Zong et al. 2006; and
a Xue and Tang
T
2005), m
most of these offer an overaall
3 predictive capability without providing a detailed insight into the transport inside individual components. In this
section the fundamental heat transfer issues in each individual fuel cell components (membrane, catalyst layer
(CL), gas diffusion layer (GDL) and bipolar plate (BPP)) are discussed.
Proton Exchange Membrane
The PEM acts as both a protonic conductor and a separator between the anode and the cathode. The most
commonly used PEM is perfluorosulfonic acid, in particular Nafion. The proton conductivity is the most
important property of Nafion membrane in fuel cell application and is strongly dependent on its water content
and temperature (Springer et al. 1991; Zawodzinski et al. 1993). Table 1 lists Nafion’s proton conductivity
together with some other properties that are critical in PEMFC application and their variation with temperature
and water content.
Table 1. Some properties and their variation with temperature and water content for Nafion 117 membrane
Properties
Symbol
Proton
conductivity
(S/cm)
σ
Dependence on T and water content
0.5139
1.3
at 30 – 80 °C
0.326 exp 1268
.
10 exp 14
References
at 80 – 140 °C
From water vapor:
0.043
Water content
λ
0.3
17.81
10.8
39.85
16.0
36.0
14.1
at 30 °C
at 80 °C
From liquid water:
8.38
Electroosmotic
drag
nd
Glass-transition
temperature
Tg
at 25 – 130 °C
0.138
.
at 30 – 80 °C
90 – 120 °C for hydrated membranes
(Springer et
al. 1991;
Zawodzinski
et al. 1993;
Yang et al.
2004)
(Springer et
al. 1991; Ju
et al. 2005b)
(Ju et al.
2005b)
(Hinatsu et
al. 1994)
(Springer et
al. 1991)
(Bauer et al.
2005; Young
and Mauritz
2001)
Note: 1). Water content λ is defined as the number of water molecules per SO3H group; 2). nd is defined as the
number of water molecules per proton; 3). a - water vapor activity; 4) T - temperature (°C).
Dehydration. The air and hydrogen gas streams are often humidified in order to maintain proper hydration
in the PEM. In fact, many fuel cells are operated at or above saturation conditions to ensure that the membrane
remains well hydrated. In spite of this, dehydration of Nafion membrane can still occur (Tsushima et al. 2004;
Minard et al. 2006; Li and Pickup 2004; Andreaus et al. 2002; Freire and Gonzalez 2001; Buchi and Scherer
2001). Dehydration is most commonly observed on the anode side of the membrane at high current densities
(Andreaus et al. 2002; Buchi and Scherer 2001). This is caused by electro-osmotic drag, whereby water
4 molecules are pulled from anode to cathode by the flow of protons. If water back-diffusion from cathode to
anode is insufficient to compensate for the electro-osmotic drag, a reduction in the water content occurs on the
anode side of the membrane. This leads to increased proton resistance, and consequently a loss in performance.
Membrane dehydration is sensitive to the operating conditions (such as gas stream relative humidity,
temperature, and current density) and the properties of the membrane itself. Generally, thicker membranes are
more prone to the anode dehydration because back-diffusion becomes more difficult (Andreaus et al. 2002;
Freire and Gonzale 2001). Several approaches have been applied to increase the water back-diffusion and thus
overcome the local dehydration of the membrane. Voss et. al. (1995) proposed an anode water removal
technique by introducing a large pressure drop in the hydrogen stream (by increasing its flow rate) to draw water
through the membrane by back diffusion, thereby reducing the membrane dehydration significantly. This
technique significantly impacts the operating conditions of the fuel cell and needs to be carefully evaluated with
the overall system performance. Utilizing a thinner PEM can significantly increase the water back-diffusion and
thus improve water management (Andreaus et al. 2002; Freire and Gonzale 2001).
Non-uniform temperature distribution. Another important but often overlooked issue is the non-uniform
temperature distribution in the membrane. A higher temperature on the cathode side of the membrane has been
observed through both experimental measurements (Vie and Kjelstrup 2004) and modeling (Weber and
Newman 2006; Nguyen and White 1993; Muller and Stefanopoulo 2006; Zong et al. 2006). This temperature
difference, on the order of several degrees C, has considerable impact on the water content of the membrane
(Wang and Shi 2006; Yan et al. 2004). These non-uniform distributions are caused by local variations in water
content and gas availability at the reaction sites.
Non-uniform temperature distribution also occurs along the flow length. For a typical PEMFC operating
under high humidity (75 – 100% RH), a high temperature region (i.e., hot spot) is often observed at the gas inlet
resulting from the intense reaction in this area (Shimoi et al. 2004). The hot spot must be avoided for the
membrane reliability because the peak temperature of the hot spot may exceed the membrane reliability limit.
Heat transfer consideration. Heat is generated in a PEM from two sources – energy released from the
electrochemical reactions, and the ohmic heating due to proton conduction across the PEM. Part of this heat is
transferred through the PEM by heat conduction, as the contribution from water transport is minimal. However,
the relevant thermal properties (thermal conductivity and heat capacity) of Nafion membrane at various
hydration levels are rarely available in literature. Vie and Kjelstrup (2004) estimated the thermal conductivity of
a fuel cell membrane by measuring in situ temperature gradients in a fuel cell. An average thermal conductivity
of 0.2 ±0.1 W/m.K was obtained for Nafion 115 membrane. Khandelwal and Mench (2006) measured the
thermal conductivity of dry Nafion with a steady state measurement method and a value of 0.16 ±0.1 W/m.K
was obtained at 30 °C. Various values in the range of 0.1 – 0.95 W/m K have been used in fuel cell modeling
(Berning et al. 2002; Maggio et al. 1996). There is a clear need for more accurate measurements of these
properties.
New developments in the PEM include high temperature polymer materials and self-humidification
membranes. For example PBI based polymers would enable the PEMFC to operate at a temperatures above 120
°C (Labato et al. 2007), and Pt-SiO2-Nafion composite self-humidification membranes would enable the
PEMFC to operate at low relative humidity levels (Zhu et al. 2006; Watanabe 1996). This would greatly
simplify the water and heat management within the fuel cell. Also the higher operating temperature would
improve the heat transfer rate from the device to the coolant.
Cathode Catalyst Layer (CCL)
The heat generation in a fuel cell is mainly the result of electrochemical reactions. The entropic heat at
standard state is 0.104 J/mol K for the anode and -326.36 J/mol K for the cathode reactions (Lampinen and
Fomino 1997). Due to much greater heat generation and water production rates at the cathode than at the anode,
the CCL experiences a more severe water and heat management problem. For this reason, the following
discussion is focused on CCL.
5 Microoscopic nature of heat genera
ation and tran
nsport in CCL.. Figure 2 shoows a schematiic of the catalyyst
layer geom
metry and its composition. The porous catalyst layers consist of plaatinum nanopaarticles (3-5 nnm
diameter), carbon nano
opowders (10--20 nm diam
meter) and thee ionomer maaterial. Two distinctive poore
distributionns have been id
dentified in thee catalyst layerr, namely the primary
p
pores ((diameter of 6--20 nm) existinng
inside the Pt/C agglom
merates and thhe secondary ppores (diametter of 20-100 nm) existingg between theese
agglomerattes (Uchida et al. 1995; Ihhonen 2002; Sun
S et al.; 20005 and Gloaguuen and Durrand 1997). Thhe
secondary pores offer thee major pathwaay for the transsport of reactaant gases and pproduct water (Eikerling 20066).
g
in a PEMFC comees from the eleectrochemical rreactions, whicch take place on
o
Most of thhe waste heat generated
the triple-pphase (ionomerr - platinum - gaseous
g
reactannt) boundaries in the catalyst layer.
2 Schematic reepresentation of
o the catalyst llayer structure and its compoosition. NOT TO SCALE.
Fig. 2.
ocal reactions mainly
m
take pllace in the secondary pores as
a will be illusstrated in an ovverall water annd
The lo
heat transpport representattion in the next section (Fig. 3). In the CCL
L, oxygen moleecules diffuse through
t
the vooid
space in thhe larger pores and are transpo
orted to the cattalyst sites as dissolved
d
moleecules in the ionnomers (Ayad et
al. 2004). Protons
P
and eleectrons transpoort through the proton conduccting ionomers and the interconnected carboon
particles, respectively. All these chemiccal species reacct on the platinnum surface annd produce water and heat.
h
transfer inn CCL is couppled with the water transporrt and phase cchange dynam
mics, Although a
The heat
PEMFC opperates above atmospheric pressure
p
and 800 °C, water vaapor is favoredd in the CCL for
f the followinng
reasons: (aa) the nanometter scale of thhe pores in CC
CL greatly incrreases the satuuration vapor pressure;
p
(b) thhe
water evapporation is sign
nificantly enhaanced in these nano-sized
n
porres; and (c) thee local high tem
mperature in thhe
CCL furthher increases the saturationn vapor pressuure and waterr evaporation rate. The firsst point is beest
highlighted
d by an exam
mple: the equiliibrium vapor pressure, calcuulated from thhe Kelvin equuation (Kaufmaan
2002), in a micropore wiith a diameter of
o 20 nm increeases by 11% at
a 25 °C and byy 9% at 80 °C when compareed
to a flat su
urface. For thee hydrophobic pores in CCL
L (Yu et al. 20006) the saturaation vapor preessure is furthher
increased. As for the seco
ond point, Eikerling (2006) has demonstraated that at currrent density upp to 1 A/cm2, thhe
evaporationn rate in the porous
p
structuree is sufficient to convert all the product w
water from liquiid state to vapor
phase. Froom both the thhermodynamic and kinetic viiewpoints it caan be concludded that the caatalyst layers are
a
efficient to
o convert liquiid product watter into vapor state. According to Eikerlinng (2006) the fine
f
pores in thhe
catalyst lay
yers promote evaporation
e
of liquid phase aand thus allow the water to fllow out in the vapor phase annd
permit reacctants to diffusse toward the reeaction sites.
The reeaction heat inn the CCL is allso conducted through the GD
DL and finallyy removed by the coolant. Thhe
effective thermal conducctivity of the catalyst layerr is an important parameter from a therm
mal managemeent
6 perspective. A few experimental measurements exist in the literature. Khandelwal and Mench (2006) measured
the combined thermal resistance of an MEA sandwiched between two Toray TGP-H-60 carbon paper diffusion
media. By subtracting the thermal resistance of the membrane and the diffusion layers, which were measured
independently, the effective thermal conductivity of the catalyst layer was estimated to be 0.27 ± 0.05 W/m K.
This value is similar in magnitude to that of the diffusion media. At present no attempt has been made to
calculate the thermal conductivity of the catalyst layers because of the wide variation in the properties of the
component materials, including ionomers and Pt/C particles.
Over the last decade, a few new types of non-conventional catalysts have been developed, e.g. the
nanostructured thin film (NSTF) catalysts developed by 3M (Debe et al. 2006) and the non-Pt catalysts (Wang
2005) Since these new catalysts have different compositions and structures as well as different reaction
mechanisms, the water and thermal management issues may be quite different from the conventional Pt/C
catalyst. This is beyond the scope of this paper.
The Gas Diffusion Layer (GDL) and Microporous Layer (MPL)
The most commonly used gas diffusion layer (GDL) materials for PEM fuel cells are carbon cloth and
carbon paper. These materials are highly porous to allow gas transport to the catalyst layer, as well as liquid
water transport from the catalyst layer. In order to facilitate the removal of liquid water these fibrous materials
are usually further hydrophobicized with a polytetrafluoroethylene (PTFE) coating. GDLs also serve as the
main media to conduct heat from the catalyst layers to the flow-field plates. A fine microporous layer (MPL),
consisting mainly of carbon powders and PTFE particles, has been widely used by fuel cell researchers to
improve the performance of a PEMFC (Pasagullari et al. 2005; Qi and Kaufman 2002; Jordan et al. 2000; Kong
et al. 2002; Beattie et al. 1999).
GDL flooding, caused by liquid water accumulating in the pores of the GDL and consequently hindering
oxygen transport to the catalyst sites, is one of the most common issues in proper water and thermal
management. Flooding may occur when a PEMFC is operated at moderate or high current densities and/or with
fully humidified reactants. GDL flooding is usually described by “water saturation”, which is expressed as the
ratio of volume of water filled pores to the total volume of pores in a GDL. According to Nam and Kaviany
(2003), the water saturation in a GDL is dominated by the condensation/evaporation processes. Generally, the
oxygen depletion due to the electrochemical reaction results in water condensation, while the total pressure drop
along the gas channel leads to water evaporation. The phase change inside the GDL is also strongly affected by
the temperature distribution. The heat production due to both the electrochemical reactions in the CL and the
ohmic heating in the GDL increases the temperature in the GDL, thus increasing the water vapor saturation
pressure. Both modeling results (Berning and Djilali 2003) and experimental data (Vie and Kjelstrup 2004)
indicate that significant temperature gradients (∼ 5 °C) exist across the GDL.
The heat transfer within the GDL occurs by conduction and convection mechanisms. Heat is conducted
through both the solid carbon fiber matrix and the liquid water, whereas convection is mainly through the gas
phase. The thermal conductivity of the GDL is an important parameter and has been measured by Khandelwal
and Mench (2006). The thermal and electrical conductivity of GDL are likely anisotropic, which complicates the
thermal measurements and heat transfer modeling (Pharoah et al. 2006). Another complication comes from the
temperature difference between the fluids and the solid matrix in the GDL. Hwang et al. (2007) showed that the
local fluid and matrix temperatures have considerable influence on the current density distribution.
The presence of a microporous layer has been shown to improve the fuel cell performance by a number of
inestigators, although the exact mechanism is not clearly understood. Several authors (Qi and Kaufman 2002;
Beattie et al. 1999) have demonstrated that the MPL improves the humidification of the membrane at the anode
side, thus improving the water management. Jordan et al. (2000) and Kong et al. (2002) concluded that the MPL
enhances oxygen diffusion by reducing flooding in the cathode. Owejan et al. (2007) conducted experiments on
cracked and crack-free MPLs with the same GDL material and found a significant reduction in the resistance to
water flow in the cracked case during ex-situ experiments. However, there was no difference in the
electrochemical performance between the two MPLs, indicating that during the PEMFC operation, the water
flow through the MPL may not be in the liquid form. In addition they looked at the effect of MPL position on
7 the GDL. Based on theirr experiments, they further cconcluded thatt the vapor cappillary effects within the GD
DL
w in the catalysst layer has litttle
have an innsignificant efffect on mass trransport overpootential, and thhe Darcy flow
influence in
i the transportt process. As noted
n
by some investigators, introducing ann MPL also imp
proves electriccal
contact bettween GDL and catalyst layers thereby impproving the fuel cell performaance (Qi and Kaufman
K
2002)..
h
and waterr transport mechanisms in CCL, MPL an
nd GDL
Coupled heat
The lo
ocal temperatuure profiles annd water vapor partial presssures play a crucial role in the transpoort
processes in
i these regionns. As discusseed under CCL
L section abovee, the favorable state of wateer in the CCL is
vapor phasse. At low and
d moderate cuurrent densitiess, the particulaar pore structuure in the catallyst layer favoors
water in vapor
v
phase, benefiting
b
the water and heeat managemeent. At high ccurrent densitiies, when watter
productionn rate increasess beyond the evaporation
e
cappability of the porous catalyyst layer, catalyyst flooding will
w
occur. A detailed
d
review
w of the transsport mechanissms of water is given by Kandlikar
K
(20007). The role of
microporou
us layer is crittical in water management
m
ass it acts like a surface tensioon based gate that
t
prevents thhe
backflow of
o water from GDL into thee CCL, while facilitating
f
flow
w of water aw
way from CCL
L into GDL. Thhe
local tempeerature is anothher key elemennt in ascertaininng that the con
ndensation occuurs in the GDL
L.
Figuree 3 shows a prroposed transpoort model for w
water from its generation at CCL to its rem
moval in the gas
g
channels. When
W
the high
h temperature water vapor in the CCL paasses through tthe microporou
us layer and thhe
GDL, it iss cooled and partly
p
condenseed into liquid water. Conden
nsation in the MPL with finner pores is leess
desirable than
t
in the GD
DL which has a coarser poree structure. If the local satuuration conditioon results in thhe
condensatiion in the MPL
L, it may lead
d to flooding. A condensatioon front thereffore is expecteed to exist undder
optimum conditions
c
som
mewhere at the interface betw
ween the MPL and
a the GDL oor within the GDL
G
as shown in
Fig. 3, deppending on thhe local tempeerature. The fl
flow of vapor from the reacction sites and
d its subsequeent
condensatiion inside the GDL
G
also enhaances the heat transfer due to release of laatent heat durin
ng condensatioon.
The thermal gradients within
w
the GDL
L thus play a siignificant role in the water vvapor transportt toward the gas
g
channels annd its condensation within thhe GDL matrix.
p
CCL water transporrt mechanism sshowing the electrode reactioon and the transsport of producct
Fig. 3. A proposed
wateer in a catalyst layer. NOT TO
O SCALE.
Bipolar Pllates
c
the mechanical
m
bacckbone of a fuuel cell stack. T
They conduct current betweeen
The biipolar plates constitute
cells and provide
p
conduitts for reactant gases and coollant. They are commonly maade of graphitee composites duue
8 to their high corrosion resistance and low surface contact resistance. Considerable attempts are being made to
replace the BPP made from graphite composites with metals due to their high mechanical strength, better
durability to shocks and vibrations, and much supererior manufacturability and cost effectiveness (Tawfik et al.
2007). In order to prevent the corrosion of metal in the harsh acidic and humid environment inside the PEMFC,
metals are usually coated with an anti-corrosion coating. The main challenge however is that the anti-corrosion
coating or the passivating oxide layer on the metal surface usually causes an undesirable contact resistance.
The removal of waste heat is a major function of the bipolar plates. Due to the high inlet water vapor
pressure, only a small amount of heat is removed in the form of latent heat by the gas stream. Most of the heat is
carried away as the sensible heat by the coolant stream. A significant cooling duty is therefore required in a
PEMFC with a rather small temperature difference between the cooling stream and the fuel cell. In addition to a
higher operating temperature desired from the electrochemical perspective, a higher operating temperature is
favorable from a cooling perspective as well. Recent developments in the cooling systems are reviewed in a
separate section.
In addition, the temperature distribution along the gas channel is often non-uniform. Several variations
occur along the gas channel, including (i) dilution of oxygen by the addition of water vapor into gas stream, (ii)
water condensation caused by oxygen depletion from inlet to outlet, and (iii) water evaporation caused by the
pressure drop along the channel (Berning and Djilali 2003). A comprehensive heat transfer model therefore
needs to accurately track the electrochemical reaction and evaporation/condensation processes in various
regions of a fuel cell. Such issues become even more important during dynamic operation (Yan et al. 2007).
Temperature Distribution in PEMFC Components
The balance between the heat generation and heat removal determines the steady-state operating
temperature of a PEMFC. The relevant terms for each of the heat transfer component are described below:
∑
Π , where is the current,
is
1. Heat production due to electrochemical reaction:
overpotential, Π is Peltier coefficient of an electrode reaction, and h represents either the anode or
cathode. The first term on the right side represents the irreversible heat due to overpotential, and the
second one represents the reversible entropic heat. The Peltier coefficients for the hydrogen-oxidation
reaction (HOR) and the oxygen-reduction reaction (ORR) have been measured experimentally.
Averages are -13 mV for HOR (Sun et al. 2005; Gloaguen and Durand 1997) and -240 mV for ORR
(Pasaogullari et al. 2005) at 25 °C.
2.
3.
4.
, where σ represents the proton conductivity
Heat generation due to ohmic heating:
or electronic conductivity. This conductivity is generally a function of thickness due to the varying
composition.
Heat generation/consumption due to water vaporization/condensation:
∆
is
, , where ∆
the heat of vaporization and , is the rate of vaporization or condensation. The local temperature and
humidity ratio are interlinked with the release of the latent heat and the location of the condensation
front as shown in Fig. 3.
Heat transfer due to convection and conduction:
∆ ,
, respectively, where h is the
heat transfer coefficient, Aconv the convective area, ΔT the temperature difference, k the thermal
conductivity, Acond the conduction area and
is the temperature gradient in the heat flow direction.
The balance between heat generation and various heat transport mechanisms determines the temperature
distribution in a PEMFC. A schematic of the anticipated temperature profile across a PEMFC having cooling
channels in each bipolar plate is shown in Fig. 4. A peak temperature is expected within the CCL due to the
large amount of heat generation from the electrochemical reaction. This has been verified by experimental
measurements (Vie and Kjelstrup 2004) and various thermal models (Wang 2004; Weber and Newman 2006;
Hwang 2006). The thermal conductivity of each component and their thickness are listed from experimental
measurements (Khandelwal and Mench 2006). Also shown in Fig. 4 are the heat generated in each component
due to reaction and/or ohmic heating is estimated for a 50 cm2 single fuel cell operating at 1 A/cm2. A cell
9 voltage of 0.6 V is assumed in estimating the irreversible heat generation in the CCL due to the overpotential
(η). To limit the stack thickness, its weight and cost, cooling channels are usually not provided in each bipolar
plate. The temperature profile is accordingly modified. Developing accurate thermal models is therefore quite
challenging as it needs to account for all the heat transfer terms along with the water relative humidity,
concentrations of various species and phase distribution (liquid or vapor phase of water) to accurately predict
the thermal profile.
HEAT TRANSFER MODE
Conduction (+)
+
+
Convection (+)
+
+
+
+
Evap./Cond (+)
+
+
+
+
+
+
+
+
+
+
+
Land Land T
2W
HEAT
PRODUCT
Coolant
Phase Change
(channels)
Anode BPP
Thickness: 1.2mm
Thermal
conductivity 30 W/m K
Ohmic Heating
0.7 W
7.5 W
Ohmic Heating Ohmic Heating Ohmic Heating
Reaction Heat
Reaction Heat
Anode MPL+GDL
Anode CL
Membrane
200 + 50 μm
10 μm
30 μm
0.22 W/m K
32 W
Cathode CL
10 μm
0.27 W/m K 0.16 W/m K 0.27 W/m K
2W
Ohmic Heating
Phase Change
(channels)
Cathode MPL+GDL
Cathode BPP
50 + 200 μm
1.2 mm
Coolant
0.22 W/m K
Fig. 4. A representative temperature profile across a PEMFC (with embedded cooling channels in each bipolar
plate) with individual layer thermal properties and typical heat generation values. NOT TO SCALE.
CURRENT STATUS OF PEMFC COOLING SYSTEMS
Two factors are critical in designing a cooling system for PEM fuel cells: First, the nominal operating
temperature of a PEMFC is limited to about 80 °C. This means that the driving force for heat rejection is far less
than that in a typical internal combustion engine cooling system. Second, nearly the entire waste heat load must
be removed by an ancillary cooling system, unlike a combustion process where a significant fraction of the heat
is carried out of the engine with the reaction product streams, or dissipated internally. These two factors account
for the need to have relatively large radiators in automotive fuel cell systems, and providing space for the
radiators and the associated air handling ducts represents a significant design challenge (Ashley 2006).
A common fuel cell stack cooling method involves design of the bipolar plate such that there are internal
cooling channels between the anode and cathode, an example of which in shown in Fig. 5. The channels on all
plates are connected through a common header which is formed when the plates are assembled into the “stack”.
With this approach, the channel geometry is designed to accommodate the heat transfer medium of choice.
For some smaller fuel cell systems (up to 2 – 5 kW in electrical power output) it is possible to use air as the
heat transfer fluid, and thus rely on a relatively inexpensive blower for coolant delivery. The 1.2 kW Mark 1020
system offered by Ballard is an example of this approach (Fuels Cells Bulletin 2006). The coolant air can simply
be exhausted to ambient, removing the need for a radiator. For fuel cells with power output in excess of about
100 W, use of a single air stream would require extremely high cathode stoichiometric ratios, resulting in
10 unacceptably dry operating conditions (Larminie and Dicks 2003). For automotive and many stationary fuel cell
systems, the waste heat loads are large enough that a closed cooling loop, comprised of a liquid heat transfer
fluid, coolant pump, and radiator, is required. Various liquids have been used, with differences related mostly to
cooling capacity, cost, corrosion control, or adjustment of electrical conductivity. Electrical conductivity of
coolants must be as low as possible to avoid electrical current loss through the cooling subsystem. Aside from
the glycol-based coolants typical of internal combustion systems, some variations that have been patented
include:
• A low-cost fluid based on kerosenic hydrocarbons or kerosene-water mixtures (Abd Elhamid et al.
2004)
• Maintaining low fluid conductivity by incorporation of an ion exchange medium into the coolant loop
(Imaseki 2006).
• Addition of carboxylic salts to the heat transfer fluid, to maintain electrical conductivity less than 100
μS/cm (Meas and Lievens 2007).
• Improving durability of glycol-water coolants by incorporation of a filter to remove oxidation products
(Matsuzaki 2007).
• Control of the coolant loop pressure to enable phase change, thereby increasing the effective heat
transfer coefficient, and reducing the total amount of coolant required (Lee and Skala 2005).
• Reducing the size of the coolant system by use of a refrigerant fluid (Yoshii et al. 2006).
Fig. 5. Typical fuel cell bipolar plate design in which coolant channels are formed by joining anode and cathode
flow fields. In this example, the individual plates are fabricated from corrugated metal (Wilkinson and
Vanderleeden 2003).
Additional concepts have been proposed which rely on evaporative processes to facilitate fuel cell stack
cooling. In one embodiment, a dispersed liquid droplet stream is introduced into the reactant flows to provide
both the humidification of PEM and evaporative cooling (Brambilla and Mazzucchelli 2004). Other approaches
involve heat pipes or heat pumps which rely on combined evaporation and condensation processes within a
separate subsystem to extract heat generated within the stack and reject it to the ambient environment. In
another example, a wicking structure is used on the cathode side to form the flow field channels, facilitate
internal distribution of liquid water, and simultaneously provide cooling and gas humidification (Goebel 2005).
11 A noteworthy departure from the internally cooled plate design illustrated in Fig. 5 is the concept advanced
by UTC Power, in which the bipolar plates are fabricated from a porous, hydrophilic material. This allows direct
water exchange with the cathode side of the MEA (Meyers et al. 2006). To maintain water balance in the
system, some of the product water must be returned to the stack, by directing the cathode exit stream through a
condenser, which is either the primary radiator of a fuel cell vehicle, or an intermediate heat exchanger coupled
to the primary radiator (Fig. 6). The overall volume of cooling water required is reduced by relying on the heat
of vaporization. A vent above the fuel cell stack ensures that the liquid water pressure is equilibrated with the
ambient. This pressure difference and capillary force pulls water into the porous plate, maintaining even water
distribution with no moving parts (Reiser et al. 2006).
An important consideration is that in order to meet long-term performance and material durability
requirements it is essential that the rate of cooling be reasonably uniform across the fuel cell active area. Large
temperature gradients in the fuel cell materials introduce significant in-plane stresses that can lead to premature
failure. Moreover, for automotive fuel cell systems in particular, the stringent cost targets make it difficult to
introduce additional components or subsystems, even for concepts that may offer improvements in heat transfer
effectiveness and/or uniformity. Therefore, it is reasonable to expect that internally liquid-cooled bipolar plates
will be widely used in early introduction of large stationary and automotive PEM fuel cell systems.
Fig. 6. Evaporatively-cooled stack design, using porous bipolar plate technology (Meyers et al. 2006)
CONCLUDING REMARKS
Thermal management is a key design consideration for PEMFCs, from the microscopic scale of the catalyst
particles (nanometer scale) and microporous layer, the component level gas diffusion layers and bipolar plates,
to the integration of the fuel cell stack with various external subsystems. The temperature and water vapor
pressure profiles within the membrane-electrode assembly dictate the phase of water present in various regions
and its transport from the PEM to gas channels. In this sense, fuel cell thermal and water transport mechanisms
are intimately interlinked, and one cannot study the fuel cell performance without considering the heat transfer.
As fuel cells continue to develop toward commercialization, there are a number of critical research needs
related to thermal management. Some of the important needs identified in the above review are:
12 1.
2.
3.
4.
5.
6.
Accurate experimental data for the thermal conductivity and contact resistance of different components
(e.g. proton exchange membranes, catalyst layers, gas diffusion layers and bipolar plates) are needed
for thermal and water transport modeling. Particular focus is needed on understanding multidimensional effects in components which are highly anisotropic.
New materials are desired for certain individual components. For example: a new PEM that can operate
at high temperature and low relative humidity, a new catalyst material or design that provides better
mass and heat transport properties, a new GDL that further mitigates water accumulation, and a new
BPP that combines the advantages of metal and graphite.
A clear understanding of the fundamental water and heat transfer mechanisms in each component is
needed. A variety of fundamental phenomena, such as membrane dehydration, liquid water production
in CL, anode-cathode water balance, water saturation and transport in the microporous layer and GDL,
and the non-uniform distributions of temperature and current density along the gas channel, all need
further investigation.
Defining a set of easily measurable and rigorously meaningful thermal parameters for the catalyst
layer, microporous layer and GDL is critical in developing better models.
Most currently used equations for description of transport phenomena in the GDL are derived for
granular porous materials. Their extension to porous media in PEMFC (including CL and GDL) is
open to question. Better transport models are needed to describe the water transport in the GDL.
The transient heat transfer problems, such as the start-up, shut-down, and freeze-thaw cycling, need to
be accurately modeled for developing appropriate control algorithms, especially in the case of
automotive systems which encounter highly dynamic load profiles.
NOMENCLATURE
a
nd
h
ΔH
i
Q
Tg
T
σ
λ
η
Water activity (dimensionless)
Electro-osmotic drag (dimensionless)
Convection heat transfer coefficient (W/cm2 K)
Change of enthalpy (J/mol)
Current density (A/cm2)
Heat (J)
Glass transition temperature (°C)
Temperature (°C)
Electrical conductivity (S/cm)
Water uptake (dimensionless)
Overpotential (V)
REFERENCES
Abd Elhamid, M., Mikhail, Y.M., Blunk, R.H., Lisi, D.J., 2004, Inexpensive Dielectric Coolant for Fuel Cell
Stacks, U.S. Patent 6,740,440, assigned to General Motors Corporation.
Amphlett, J.C., Baumert, R.M., Mann, R.F., Peppley, B.A., Roberge, P.R., Harris, T.J., 1995, Performance
Modeling of BALLARD MARK IV Solid Polymer Electrolyte Fuel Cell I. Mathematical Model
Development, J. Electrochem. Soc., Vol. 142, pp. 9-15.
Andreaus, B., McEvoy, A.J., Scherer, G.G., 2002, Analysis of Performance Losses in Polymer Electrolyte Fuel
Cells at High Current Densities by Impedance Spectroscopy, Electrochim. Acta, Vol. 47, pp. 2223-2229.
Ashley, S., 2006, Fuel Cells Start to Look Real, Automotive Engineering International Online, SAE
International.
Ayad, A., Naimi, Y., Bouet, J., Fauvarque, J.F., 2004, Oxygen Reduction on Platinum Eelectrode Coated with
Nafion, J. Power Sources, Vol. 130, pp. 50-55.
13 “Ballard Introduces Latest Air-Cooled Stack,” Fuel Cells Bulletin, p. 8, November 2006
Bauer, F., Denneler, S., Willert-Porada, M., 2005, Influence of temperature and humidity on the mechanical
properties of Nafion® 117 polymer electrolyte membrane, J. Polym. Sci. B: Polym Phys., Vol. 43, pp. 786.
Beattie, P.D., Basura, V.I., Holdcroft, S., 1999, Temperature and Pressure Dependence of O2 reduction at
Pt/Nafion® 117 and Pt/BAM® 407 Interfaces, J. Electroanal. Chem., Vol. 468, pp. 180-192.
Berning, T., Djilali, N., 2003, A 3D, Multiphase, Multicomponent Model of theCathode and Anode of aPEM
Fuel Cell, J. Electrochem. Soc., Vol. 150, pp. A1589-A1598.
Berning, T., Lu, D.M., Djilali, N., 2002, Three-Dimensional Computational Analysis of Transport Phenomena
in PEM Fuel Cell, J. Power Sources, Vol. 106, pp. 284-294.
Brambilla, M. and Mazzucchelli, G., 2004, Fuel Cell with Cooling System Based on Direct Injection of Liquid
Water, U.S. Patent 6,835,477, assigned to Nuvera Fuel Cells Europe S.r.l.
Buchi, F.N., Scherer, G.G., 2001, Investigation of the Transversal Water Profile in Nafion Membranes in
Polymer Electrolyte Fuel Cells, J. Electrochem. Soc., Vol. 148, pp. A183-A188.
Debe, M.K., Schnoeckel, A.K., Vernstrom, G.D., Atanasoski, R., 2006, High Voltage Stability of
Nanostructured Thin Film Catalysts for PEM Fuel Cells, J. Power Sources, Vol. 161, pp. 1002-1011.
Djilali, N., Lu, D.M., 2002, Influence of Heat Transfer on Gas and Water Transport in Fuel Cells, International
J. Therm. Sci., Vol. 41, pp. 29-40.
Eikerling, M., 2006, Water Management in Cathode Catalyst Layers of PEM Fuel Cells: A Structure-Based
Model, J. Electrochem. Soc., Vol. 153, pp. E58-E70.
Faghri, A., Guo, Z., 2005b, Challenges and Opportunities of Thermal Management Issues Related to Fuel Cell
Technology and Modeling, International J. Heat and Mass Transfer, Vol. 48, pp. 3891-3920.
Freire, T.J.P., Gonzalez, E.R., 2001, Effect of Membrane Characteristics and Humidification Conditions on the
Impedance Response of Polymer Electrolyte Fuel Cells, J. Electroanal. Chem., Vol. 503, pp. 57-68.
Fuller, T.F., Newman, J., 1993, Water and Thermal Management in Solid-Polymer-Electrolyte Fuel Cells, J.
Electrochem. Soc., Vol. 140, pp. 1218-1225.
Gloaguen, F., Durand, R., 1997, Simulations of PEFC Cathodes: an Effectiveness Factor Approach, J. Appl.
Electrochem., Vol. 27, pp. 1029-1035.
Goebel, S.G., 2005, Evaporative Cooled Fuel Cell, U.S. Patent 6,960,404, assigned to General Motors
Corporation.
Hinatsu, J.T., Mizuhata, M., Takenake, H., 1994, Water Uptake of Perfluorosulfonic Acid Membranes from
Liquid Water and Water Vapor, J. Electrochem. Soc., Vol. 141, pp. 1493-1498.
Hwang, J.J., 2006, Thermal-Electrochemical Modeling of a Proton Exchange Membrane Fuel Cell, J.
Electrochem. Soc., Vol. 153, pp. A216-A224.
Hwang, J.J., 2007, A Complete Two-Phase Model of A Porous Cathode of A PEM Fuel Cell, J. Power Sources,
Vol. 164, pp. 174-181.
Ihonen, J., Jaouen, F., Lindbergh, G., Lundblad, A., Sundholm, G., 2002, Investigation of Mass-Transport
Limitations in the Solid Polymer Fuel Cell Cathode, J. Electrochem. Soc., Vol. 149, pp. A448-A454.
Imaseki, M., Ushio, T., Shimoyama Y., 2006, Cooling Method for Fuel Cell, U.S. Patent 7,070,873, assigned to
Honda Giken Kogyo Kabushiki Kaisha.
Jordan, L.R., Skukla, A.K., Behrsing, T., Avery, N.R., Muddle, B.C., Forsyth, M., 2000, Effect of Diffusionlayer Morphology on the Performance of Polymer Electrolyte Fuel Cells Operating at Atmospheric Pressure,
J. Appl. Electrochem., Vol. 30, pp. 641- 646.
Ju, H., Meng, H., Wang, C.Y., 2005a, A Single-Phase, Non-isothermal Model for PEM Fuel Cells, International
J. Heat and Mass Transfer, Vol. 48, pp. 1303-1315.
Ju, H., Wang, C.Y., Cleghorn, S., Beuscher, U., 2005b, Nonisothermal Modeling of Polymer Electrolye Fuel
Cells, J. Electrochem. Soc., Vol. 152, pp. A1645-A1653.
Kandlikar, S.G., 2007, Microscale And Macroscale Aspects Of Water Management Challenges In Pem Fuel
Cells, Accepted for publication in International Journal of Transport Phenomena.
Kaufman, M., 2002, Principles of Thermodynamics, Marcel Dekker, New York.
Khandelwal, M., Mench, M.M., 2006, Direct Measurement of Through-Plane Thermal Conductivity and
Contact Resistance in Fuel Cell Materials, J. Power Sources, Vol. 161, pp. 1106-1115.
14 Kong, C.S., Kim, D.Y., Lee, H.K., Shul, Y.G., Lee, T.H., 2002, Influence of Pore-size Distribution of Diffusion
Layer on Mass-Transport Problems of Proton Exchange Membrane Fuel Cells, J. Power Sources, Vol. 108,
pp. 185-191.
Lampinen, M., Fomino, M., 1997, Analysis of Free Energy and Entropy Changes for Half-Cell Reactions, J.
Electrochem. Soc., Vol. 140, pp. 3537-3546.
Larminie, J., and Dicks, A., 2003, Fuel Cell Systems Explained, 2nd Edition, John Wiley and Sons, West
Sussex, England.
Lee, J.H. and Skala, G.W., 2005, Cooling System for Fuel Cell Stack, U.S. Patent 6,866,955, assigned to
General Motors Corporation.
Li, G., Pickup, P.G., 2004, Measurement of Single Electrode Potentials and Impedances in Hydrogen and Direct
Methanol PEM Fuel Cells, Electrochim. Acta, Vol. 49, pp. 4119-4126.
Lobato, J., Canizares, P., Rodrigo, M.A., Linares, J.J., 2007, PBI-Based Polymer Electrolyte Membrane Fuel
Cells Temperature Effects on Cell Performance and Catalyst Stability, Electrochim. Acta, Vol. 52, pp. 3910-3920.
Maes, J.-P., and Lievens, S., 2007, Methods for Fuel Cell Coolant Systems, U.S. Patent 7,201,982, assigned to
Texaco, Inc.
Maggio, G., Recupero, V., Mantegazza, C., 1996, Modeling of Temperature Distribution in a Solid Polymer
Electrolyte Fuel Cell Stack, J. Power Sources, Vol. 62, pp. 167-174.
Marechal, M., Souquet, J.L, Guindet, J., Sanchez, J.Y., 2007, Solvation of Sulphonic Acid Groups in Nafion
Membranes from Accurate Conductivity Measurements, Electrochemistry Communications, Vol. 9, pp. 10231028.
Matsuzaki, T., 2007, Cooling System for Fuel Cell and Prevention Method for Degradation of Coolant Therefor,
U.S. Patent 7,160,468, assigned to Calsonic Kansei Corporation.
Meyers, J.P., Darling, R.M., Evans, C., Balliet, R. and Perry, M.L., 2006, Evaporatively-Cooled PEM Fuel-Cell
Stack and System, ECS Transactions, Vol. 3, pp. 1207-1214.
Minard, K.R., Viswanathan, V.V., Majors, P.D., Wang, L., Rieke, P.C., 2006, Magnetic Resonance Imaging
(MRI) of PEM Dehydration and Gas Manifold Flooding During Continuous Fuel Cell Operation, J. Power
Sources, Vol. 161, pp. 856-863.
Muller, E.A., Stefanopoulou, A.G., 2006, Analysis, Modeling, and Validation for the Thermal Dynamics of a
Polymer Electrolyte Membrane Fuel Cell System, J. Fuel Cell Science and Technology, Vol. 3, pp. 99-110.
Nam, J.H., Kaviany, M., 2003, Effective Diffusivity and Water-Saturation Distribution in Single- and TwoLayer PEMFC Diffusion Medium, International J. Heat and Mass Transfer, Vol. 46, pp. 4595-4611.
Nguyen, T., White, R., 1993, A Water and Heat Management Model for Proton Exchange Membrane Fuel
Cells, J. Electrochem. Soc., Vol. 140, pp. 2178-2186.
Owejan, J.P., Owejan, J.E., Tighe, T.W., Gu, W., Mathias, M., 2007, Investigation of Fundamental Transport
Mechanism of Product Water From Cathode Catalyst Layer in PEMFCs, Proceedings of FEDSM2007, 5th
Joint ASME/JSME Fluids Engineering Conference, July 30 – Aug. 2, 2007, San Diego, CA, USA.
Pasaogullari, U., Wang, C.Y., Chen, K.S., 2005, Two-Phase Transport in Polymer Electrolyte Fuel Cells with
Bilayer Cathode Gas Diffusion Media, J. Electrochem. Soc., Vol. 152, pp. A1574-A1582.
Pharoah, J.G., Karan, K., Sun, W., 2006, On Effective Transport Coefficients in PEM Fuel Cell Electrodes:
Anisotropy of the Porous Transport Layer, J. Power Sources, Vol. 161, pp. 214-224.
Qi, Z., Kaufman, A., 2002, Improvement of Water Management by a Microporous Sublayer for PEM Fuel
Cells, J. Power Sources, Vol. 109, pp. 38-46.
Reiser, C.A., Meyers, J.P., Johnson, D.D., Evans, C.E., Darling, R.M., 2006, Fuel Cells Evaporatively Cooled
with Water Carried in Passageways, U.S. Patent Application US2006/0141330.
Shan, Y., Choe, S.Y., 2005, A High Dynamic PEM Fuel Cell Model with Temperature Effects, J. Power
Sources, Vol. 145, pp. 30-39.
Shimoi, R., Masuda, M., Fushinobu, K., Kozawa, Y., Okazaki, K., 2004, Visualization of the Membrane
Temperature Field of a Polymer Electrolyte Fuel Cell, J. Energy Resources Technology, Vol. 126, pp. 258261.
Springer, T.E., Zawodzinski, T.A., Gottesfeld, S., 1991, Polymer Electrolyte Fuel Cell Model, J. Electrochem.
Soc., Vol. 138, pp. 2334-2342.
15 Sun, W., Peppley, B.A., Karan, K., 2005, An Improved Two-Dimensional Agglomerate Cathode Model to
Study the Influence of Catalyst Layer Structural Parameters, Electrochim. Acta, Vol. 50, pp. 3359-3374.
Tawfik, H., Hung, Y., Mahajan, D., 2007, Metal Bipolar Plates for PEM fuel cell – A Review, J. Power Sources,
Vol. 163, pp. 755-767.
Tsushima, S., Teranishi, K., Hirai, S., 2004, Magnetic Resonance Imaging of the Water Distribution Within a
Polymer Electrolyte Membrane in Fuel Cells, Electrochem. Solid-State Lett., Vol. 7, pp. A269-A272.
Uchida, M., Aoyama, Y., Eda, N., Ohta, A., 1995, Investigation of the Microstructure in the Catalyst Layer and
Effects of Both Perfluorosulfonate Ionomer and PTFE-Loaded Carbon on the Catalyst Layer of Polymer
Electrolyte Fuel Cells, J. Electrochem. Soc., Vol. 142, pp. 4143-4149.
Vie, P.J.S., Kjelstrup, S., 2004, Thermal Conductivity from Temperature Profiles in the Polymer Electrolyte
Fuel Cell, Electrochim. Acta, Vol. 49, pp. 1069-1077.
Voss, H.H., Wilkinson, D.P., Pickup, P.G., Johnson, M.C., Basura, V., 1995, Anode Water Removal: A Water
Management and Diagnostic Technique for Solid Polymer Fuel Cells, Electrochim. Acta, Vol. 40, pp. 321-328.
Wang, B., 2005, Recent Development of Non-Platinum Catalysts for Oxygen Reduction Reaction, J. Power
Sources, Vol. 152, pp. 1-15.
Wang, C.Y., 2004, Fundamental Models for Fuel Cell Engineering, Chem. Rev., Vol. 104, pp. 4727-4766.
Wang, J., Shi, M., 2006, Study on Two-Phase Countercurrent Flow and Transport Phenomenon in PEM of a
Direct Methanol Fuel Cell, Science in China E: Technological Science, Vol. 49, pp. 102-114.
Watanabe, M., Uchida, H., Seki, Y., Emori, M., Stonehart, P., 1996, Self-Humidifying Polymer Electrolyte
Membranes for Fuel Cells, J. Electrochem. Soc., Vol. 143, pp. 3847-3852.
Weber, A.Z., Newman, J., 2006, Coupled Thermal and Water Management in Polymer Electrolyte Fuel Cells, J.
Electrochem. Soc., Vol. 153, pp. A2205-A2214.
Wilkinson, D.P., and Vanderleeden, O., 2003, Serpentine flow field design, Chapter 30 in Handbook of Fuel
Cells – Fundamentals, Technology and Applications, eds. W. Vielstich et al., Volume 3: Fuel Cell
Technology and Applications, John Wiley and Sons, Ltd..
Xue, X., Tang, J., 2005, PEM Fuel Cell Dynamic Model with Phase Change Effect, J. Fuel Cell Science and
Technology, Vol. 2, pp. 274-283.
Yan, W., Chen, F., Wu, H., Soong, C., Chu, H., 2004, Analysis of Thermal and Water Management with
Temperature-Dependent Diffusion Effects in Membrane of Proton Exchange Membrane Fuel Cells, J. Power
Sources, Vol. 129, pp. 127-137.
Yan, X., Hou, M., Sun, L., Cheng, H., Hong, Y., Liang, D., Shen, Q., Ming, P., and Yi, B., 2007, The Study on
Transient Characteristic of Proton Exchange Membrane Fuel Cell Stack during Dynamic Loading, J. Power
Sources, Vol. 163, pp. 966-970.
Yang, C., Srinivasan, S., Bocarsly, A.B., Tulyani, S., Benziger, J.B., 2004, A Comparison of Physical Properties
and Fuel Cell Performance of Nafion and Zir-conium Phosphate/Nafion Composite Membranes, J. Membrane
Sci., Vol. 237, pp. 145-161.
Yoshii, K. and Takeda, Y., 2006, Cooling Apparatus for Fuel Cell Utilizing Air Conditioning System, U.S.
Patent 7,086,246, assigned to DENSO Corporation.
Young, S.K., Mauritz, K.A., 2001, Dynamic mechanical analyses of Nafion®/organically modified silicate
nanocomposites, J. Polym. Sci. B: Polym. Phys., Vol. 39, pp. 1282-1295.
Yu, H.M., Ziegler, C., Oszcipok, M., Zobel, M., Hebling, C., 2006, Hydrophilicity and Hydrophobicity Study of
Catalyst Layers in Proton Exchange Membrane Fuel Cells, Electrochim. Acta, Vol. 51, pp. 1199-1207.
Zawodzinski, T.A., Derouin, C., Radzinski, S., Sherman, R.J., Smith, V.T., Springer, T.E., Gottesfeld, S., 1993,
Water Uptake by and Transport Through Nafion 117 Membranes, J. Electrochem. Soc., Vol. 140, pp. 1041-1047.
Zhang, Y., Ouyang, M., Lu, Q., Luo, J. Li, X., 2004, A Model Predicting Performance of Proton Exchange
Membrane Fuel Cell Stack Thermal Systems, Appl. Therm. Eng., Vol. 24, pp. 501-513.
Zhu, X., Zhang, H., Zhang, Y., Liang, Y., Wang, X., Yi, B., 2006, An Ultrathin Self-Humidifying Membranes
for PEM Fuel Cell Application: Fabrication, Characterization, and Experimental Analysis, J. Phys. Chem. B,
Vol. 110, pp. 14240-14248.
Zong, Y., Zhou, B., Sobiesiak, A., 2006, Water and Thermal Management in A Single PEM Fuel Cell with Nonuniform Stack Temperature, J. Power Sources, Vol. 161, pp. 143-159.
16