Fundamental Research Needs in
Combined Water and Thermal
Management Within a
Proton Exchange Membrane
Fuel Cell Stack Under Normal
and Cold-Start Conditions
Satish G. Kandlikar1
e-mail: [email protected]
Zijie Lu
e-mail: [email protected]
Rochester Institute of Technology,
Rochester, NY 14623
Each fuel cell component of a proton exchange membrane fuel
cell (PEMFC) used in automotive application operates most effectively (from performance and durability standpoints) within
specific ranges of water content and temperature. The water and
heat transport processes are coupled and present a challenge in
providing the right balance over the entire range of operating
conditions. Another important related aspect is CO poisoning of
the electrocatalyst, which adversely affects the fuel cell performance. Freezing and cold-start present additional challenges for
automotive PEMFCs. A critical review of the recent developments
on these topics is presented in this paper. The study covers both
the microscopic and macroscopic aspects of the transport within
membrane, catalyst layers, gas diffusion layers, and gas channels,
and an overview of the current PEMFC cooling technology. After
discussing the current status, suggestions for future work on the
above topics are presented. 关DOI: 10.1115/1.3008043兴
Keywords: thermal and water management, PEM fuel cell,
cold-start, PEMFC cooling, CO poisoning, coupled heat and water transport
1
Introduction
With the world concern about environmental pollution and fossil oil depletion, alternative clean energy solutions are in urgent
demand. The hydrogen fuel cell, primarily the proton exchange
membrane fuel cell 共PEMFC兲, is a promising energy conversion
system for future automobiles and stationary applications. Though
PEMFC technology has undergone significant development over
the past decade, high performance with increased stability and
reliability as well as a low cost has yet to be achieved. This is
essential before fuel cells can be commercialized. One of the critical technical challenges of PEMFCs is water and thermal management 关1,2兴. This is mainly dictated by the current polymer electrolyte membrane whose proton conductivity determines the
performance and longevity of PEMFC. The most commonly employed PEM is Nafion 共a trademark of E.I. DuPont de Nemours,
Wilmington, DE兲, which exhibits high proton conductivity only in
the hydrated state 关3兴. The hydration requirement of Nafion limits
the maximum fuel cell operating temperature to about 80° C.
Above this temperature, membrane dry-out occurs resulting in
decreased proton conductivity. Mechanical degradation also oc1
Corresponding author.
Manuscript received August 25, 2007; final manuscript received May 6, 2008;
published online August 18, 2009. Review conducted by Shripad T. Revandar.
curs at elevated temperatures due to the relatively low glass transition temperature 共Tg兲 of the hydrated Nafion 共in the range of
90– 120° C兲 关4,5兴. On the other hand, if the product water is not
removed efficiently, it may cause water condensation and flood
the electrodes. This will result in a voltage loss from the added
resistance to reactant mass transport. Flooding often occurs when
the fuel cell operates at lower temperatures. Because the water
level in a fuel cell strongly affects not only the membrane properties but also reactant transport and electrode reaction kinetics,
maintaining an optimal water balance between the anode and
cathode is important in achieving higher levels of cell performance.
In this paper, recently published literature on water and thermal
management of PEMFCs is reviewed. This paper is organized as
follows. First the research aimed at stack level, which targets on
the determination of the optimal operating conditions of a
PEMFC, is briefly reviewed in Sec. 3. In Sec. 4, the current understanding of water and thermal transport at the component level
is introduced. The important water and thermal management issues in individual PEMFC components are outlined as well. Special attention is drawn to the composition-structure-property relation for each component. Following this, in Sec. 5 water and
thermal management involved during freezing and cold-start conditions are discussed due to their importance in automotive application. Finally, in Sec. 6, the coupled water and heat transport
mechanisms are discussed since they are closely interrelated, and
further research needs in these areas are identified.
2
Background
In a PEMFC, water is produced at the cathode catalyst layer
共CCL兲 by electrochemical reactions and/or in gas channels due to
condensation from the saturated gas streams. The product water at
the CCL must be removed efficiently to prevent electrode flooding. Figure 1共a兲 is a schematic of the water transport processes in
a typical PEMFC, which mainly comprises of bipolar plates
共BPPs兲, gas diffusion layer 共GDL兲 共including microporous layer兲,
catalyst layers, and a polymer electrolyte membrane 共PEM兲. The
water transport mechanisms are electro-osmotic drag 共EOD兲 of
water 共i.e., motion of water molecules by the flow of proton
through the membrane from anode to cathode兲, back diffusion
from the cathode 共due to the concentration gradient兲, and diffusion
and convection through the GDL to/from the gas channels. The
electro-osmotic drag together with the back diffusion affects the
water balance in a PEMFC and determines the membrane hydration. A detailed review of water transport processes within different components of a fuel cell is presented in Ref. 关6兴.
Similarly, heat generated within a PEMFC is removed, as
shown schematically in Fig. 1共b兲. Heat sources in a PEMFC include entropic heat of reactions, irreversibilities of the electrochemical reactions, and Ohmic resistances, as well as heat produced during water condensation 关1,7,8兴. This heat is removed by
the cooling system, exiting gas streams, and conductionconvection transfer across the faces of the stack. The balance between the heat generation and removal rates at different locations
determines the temperature distribution within the PEMFC.
Experimental investigations of water transport in a PEMFC are
mostly conducted on the GDL/gas channel interface and in the gas
channel due to their easy accessibility. Tuber et al. 关9兴 were the
first group to study water buildup in a cathode side gas channel by
using the optical visualization technique. Wang and co-workers at
the Penn State 关10,11兴 extended this technique to study water
transport in gas channels under automotive conditions, i.e., at high
current densities 共about 0.8 A / cm2兲 and elevated temperatures
共70– 80° C兲. These studies showed that liquid water emerged at
preferential locations on the GDL surface in the form of droplets.
The droplets are then detached from GDL surface and removed
via one or more of the following three flow patterns: mist flow,
where the water flows with the gas stream in the form of tiny
droplets; corner and annular flow along the channel wall; and slug
Journal of Fuel Cell Science and Technology
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Fig. 1 Schematic of water balance „a… and heat balance „b… of a PEMFC; not to scale
flow, which is the most common pattern at normal fuel cell operating conditions. Water distribution in the GDL, under the lands,
and in the gas channels in operating fuel cells has been investigated using neutron radiography by several groups 关12–14兴. Liquid water is prone to accumulate at the locations where the gas
flow is limited, such as under the channel land 关14兴 and at 180 deg
turns of the serpentine channels 关12,15兴. The difficulty in differentiating water distribution in the anode side from the cathode
side posed a challenge in neutron radiography due to its twodimensional nature, but has been overcome recently with special
cell design where the anode and cathode channels are shifted to
avoid overlap 关16兴. Liquid water formation and transport on the
anode side was also studied by Trabold et al. 关17兴 and Owejan et
al. 关18兴. No water droplets were found on the anode GDL surface,
which is in sharp contrast to the cathode side. Liquid water in the
anode channels results from condensation of vapor that comes
either from the cathode through membrane transport or due to an
increase in humidity ratio of the anode gas stream resulting from
hydrogen consumption.
Temperature distribution in a PEMFC has also been measured
by some researchers. Vie and Kjelstrup 关19兴 measured the temperature profile across an operating fuel cell. A temperature difference as high as 5 ° C was measured across the GDL and is
explained by a combination of its low thermal conductivity, a low
heat transfer coefficient near the catalyst layer, and a high heat
production rate in these areas. Temperature distribution along the
gas channel has been visualized by Shimoi et al. 关20兴 using the
thermography technique. One major observation was the existence
of a hot spot, the peak height of which increased with increasing
current density. Thus, thermal management becomes more challenging at higher current densities.
The operation of a fuel cell and the resulting water and heat
distributions depend on numerous transport phenomena including
charge-transport, multicomponent/multiphase flow, and heat transfer in different components. The complexity and interaction of
these processes and the difficulty in making detailed in situ measurements have prompted the development of a number of numerical models, e.g., by Wang et al. 关21兴, Weber and Newman
关22兴, Um and Wang 关23兴, and Pasaogullari et al. 关24兴, just to name
a few. These models provide comprehensive details on the distribution of reactants and allow performance prediction under vari044001-2 / Vol. 6, NOVEMBER 2009
ous conditions. The effects of temperature and humidity variations
on the condensation as well as water and heat transport are integrated in some of the modeling studies. Wang and Wang 关25兴
showed for the first time that the evaporation of water at the
catalyst layer and its condensation at the colder land region presents a new mechanism similar to the heat pipe effect. Later, Weber and Newman 关8兴 also integrated the heat pipe effect in their
coupled thermal and water management model. More complete
and comprehensive models have been created by a number of
researchers, e.g., Refs. 关26–40兴. Detailed three-dimensional models incorporating two-phase flow, species transport, heat transfer,
and electrochemical processes have been developed to study the
transient characteristics of fuel cells 关33,39,40兴. A complete description of some of the important models is presented by Wang
关1兴 and Yao et al. 关41兴.
3
Component Level Transport Processes
In this section the fundamental water and heat transfer mechanisms in each individual fuel cell component 共membrane, CL,
GDL, and BPP兲 are discussed. The composition-structureproperty relation for each component is emphasized.
3.1 PEM. 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, due to
its high proton conductivity, chemical and mechanical stability,
and commercial availability. Nafion is a polymer that consists of a
perfluorinated backbone and pendent vinyl ether side chains terminated with SO3H groups. The proton conductivity and other
properties of Nafion are determined by its nanophase separated
morphology, which has been verified by the small angle X-ray and
neutron studies 关42,43兴. Upon hydration, the water molecules are
preferentially absorbed by sulfonic groups and aggregate to form
the hydrophilic aqueous phases 共or pores兲, which have a dimension of several nanometers, while the fluorocarbon backbone
forms hydrophobic microphase. Above a certain hydration level
共percolation limit兲, the hydrophilic regions within the materials
coalesce to yield a continuous ionic/aqueous pathway 关44兴 that
facilitates proton transport. Fully hydrated Nafion exhibits high
proton conductivity due to high proton mobility and concentration. However, at lower humidity, the proton conductivity is much
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Table 1 Some properties and their variation with temperature and water content for Nafion 117
membrane; note that „1… water content ␭ 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 is the water vapor
activity, and „4… T is the temperature „°C…
Properties
Proton
conductivity
共S/cm兲
Water content
Electro-osmotic
drag
Glass-transition
temperature
Thermal
conductivity
共W/m K兲
Symbol
Dependence on T and water content
冋 共
Tg
1
1
−
303 273+ T
1.3⫻ 10−7exp共14a0.2兲 at 80– 140° C
From water vapor:
0.043+ 17.81a − 39.85a2 + 36.0a3 at 30° C
0.3+ 10.8a − 16.0a2 + 14.1a3 at 80° C
From liquid water:
8.38+ 0.138⫻ T at 25– 130° C
2.5␭
at 30– 80° C
22
90– 120° C for hydrated membranes
k
0.16–0.2
␴
␭
nd
共0.5139␭ − 0.326兲exp 1268
lower because of a concomitant drop in the proton mobility 关45兴.
The chemical structure, morphology, and the physical properties
of dry and hydrated Nafion membranes have been summarized by
Mauritz and Moore 关42兴. Table 1 lists Nafion’s proton conductivity together with some relevant properties and their variation with
temperature and water content.
There are two fundamental mechanisms of water transport
through the membrane, electro-osmosis drag and back diffusion,
defined through individual transport coefficients: the EOD coefficient, measuring the water molecules dragged by the flow of each
proton, and water diffusion coefficient. The measured EOD values
of the membrane are found to depend on whether the membrane is
equilibrated with vapor or liquid water. Generally, the EOD values
equilibrated with water vapor, with a number of about 1.0 关3兴, are
much smaller than those of the same membrane equilibrated with
liquid water, which have a value in the range of 3–6 关51兴. For an
operating PEMFC at 80° C and atmospheric pressure, the EOD of
Nafion and Gore–Select membrane 共a membrane made by Gore,
similar to Nafion兲 was measured to be about 1.1 for the inlet gas
relative humidity between 40% and 95% 关52兴. Since water is produced at the cathode side of the membrane, a water concentration
gradient exists across the membrane that results in the water diffusion from cathode to anode. The diffusion coefficient of water in
Nafion has been extensively measured by using different techniques 关53兴. The water diffusivity is found to depend on the water
content, with a maximum value occurring at a water content 共defined as the number of water molecules per SO3H group兲 of
around 3–6 关54兴. Water balance in the polymer electrolyte is determined by the combination of the electro-osmotic drag and back
diffusion. Since the electro-osmosis is proportional to the current
density, the electro-osmotic drag may become dominant at high
current densities, resulting in dry-out of the membrane and large
Ohmic loss near the anode side 关55–60兴. Dehydration is most
commonly observed on the anode side of the membrane at high
current densities 关58,60兴. To solve this problem, both fuel and air
streams are humidified to provide the desired water on the anode
side of the membrane and to reduce the evaporation of water in
the cathode side. In addition, utilizing a thinner PEM can significantly increase the water back diffusion and thus improve water
management 关58,60兴. A thinner membrane is also beneficial to
lowering proton resistance. Other approaches, such as an anode
water removal technique by introducing a large pressure drop in
the hydrogen stream 关61兴, have been proposed.
A great concern regarding the automotive PEMFC is the freezing of water in the membrane at subzero temperatures and its
influence on the fuel cell performance. Based on the differential
Journal of Fuel Cell Science and Technology
References
兲册
at 30– 80° C
关46,47兴
关46,48兴
关48兴
关49兴
关46兴
关4,5兴
关19,50兴
scanning calorimetry 共DSC兲 studies, water in the Nafion membrane can be classified into nonfreezable and freezable water 关62兴.
Nonfreezable water is situated along the pore walls, whereas
freezable water is located near the pore center. Water furthest
away from SO3 bears the closest resemblance to bulk water, so it
is expected to crystallize first upon freezing. The ice crystal will
continue to grow with further decrease in temperature until the
residual water molecules cannot reorient themselves and pack into
a crystal lattice, giving rise to the nonfreezable water. Differential
scanning calorimetry data show that up to 14 H2O molecules per
SO3H group remain unfrozen at subzero temperatures 关63兴. The
proton conductivity of Nafion at low temperature was measured,
and a fourfold decrease in proton conductivity was observed 关63兴.
The activation energies before and after freezing of the membranes were measured to be 0.15 eV and 0.4 eV, consistent with
proton transport through liquid water and strongly bound water,
respectively. The nonfreezable water is thus believed to be responsible for the low temperature conductivity. Similar results were
also obtained by Thompson et al. 关64兴, who studied the effect of
water content on the low temperature proton conductance of
Nafion.
An important but often overlooked thermal management issue
is the nonuniform temperature distribution in the membrane. A
higher temperature on the cathode side of the membrane has been
reported both through experimental measurements 关19兴 and modeling 关8,27,34,35兴. This temperature difference, measured to be
about 0.9° C 关19兴 during in situ experiments, affects the water
content of the membrane 关65,66兴. Nonuniform temperature distribution also occurs along the flow length. For a typical PEMFC
operating under high relative humidity range 共75–100% RH兲, a
high temperature region 共i.e., hot spot兲 is often observed at the gas
inlet resulting from the intense reaction in this area 关20兴. The hot
spot must be avoided for the membrane reliability because the
peak hot spot temperature may exceed the membrane reliability
limit.
It is desirable to operate a PEMFC at elevated temperatures not
only to improve water management but also to increase the reaction rate and enhance the CO-tolerance of the Pt catalysts. These
points have been reviewed by Zhang et al. 关67兴. However, high
temperature fuel cell development is dictated by the new membrane materials, which can be used at higher temperatures and at
near zero relative humidity. Much effort has been devoted to
develop such membranes with some success. For example,
polybenzimidazole 共PBI兲 based polymers would enable the
PEMFC to operate at temperatures above 120° C 关68兴, and
NOVEMBER 2009, Vol. 6 / 044001-3
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Fig. 2 Schematic representation of the catalyst layer structure
and its composition; not to scale
Pt– SiO2-Nafion composite self-humidification membranes would
enable operation at low relative humidity levels 关69,70兴. This
would greatly simplify the water and thermal management within
the fuel cell. Also the higher operating temperature would improve the heat transfer rate from the device to the coolant. The
recent developments in the high temperature membrane have been
reviewed by Zhang et al. 关67兴.
3.2 CL. The porous catalyst layers consist of platinum nanoparticles 共3 – 5 nm diameter兲, carbon nanopowder 共10– 20 nm diameter兲, and distributed ionomers, as shown in Fig. 2. Two distinctive pore size distributions have been identified, namely, the
primary pores 共diameter of 6 – 20 nm兲 existing inside the Pt/ C
agglomerates and the secondary pores 共diameter of 20– 100 nm兲
existing between these agglomerates 关71–74兴. It is found that the
secondary pores offer the major pathway for the transport of reactant gases and product water 关75兴. A key question is how these
microscopic pore level characteristics affect the transport characteristics at the macroscopic level over the entire CL.
The electrochemical kinetics and mass transport in the catalyst
layer have been modeled by using either an agglomerate model
关73,76,77兴 or a macrohomogeneous model 关78,79兴. The agglomerate model states that the catalyst layer is a porous medium composed by platinum and carbon particles mixed together with polymer electrolyte. The local reactions mainly take place on the
triple-phase 共ionomer-platinum-gaseous reactant兲 boundaries in
the secondary pores. In the cathode catalyst layer, oxygen molecules diffuse through the void space in the larger pores and are
transported to the catalyst sites as dissolved molecules in the ionomers 关80兴. Protons 共in practice, hydronium ions兲 electrodiffuse
across the ionomer, driven by the concentration gradient and the
electric field, and electrons transport through the interconnected
carbon/Pt agglomerates. The relation between the agglomerate
structure and the effective Butler–Volmer equation has been investigated by Pisani et al. 关81兴. In contrast to the agglomerate
model, the homogeneous models focus on macroscopic effects,
and the underlying structure enters the model through certain key
parameters of the reactive medium. Though these models are two
extreme cases of catalyst composition, in reality catalyst layer
representation could be in between macrohomogeneous and agglomerate models.
Water flux in the catalyst layer comprises the water sorption by
the ionomers and the water transport in the void spaces 共pores兲.
These two processes are coupled because the water in the ionomer, determined by the water sorption isotherm, is expected to be
in equilibrium with water vapor in the pores, which is produced
by evaporation. This fine balance depends on various variables,
044001-4 / Vol. 6, NOVEMBER 2009
such as ionomer water concentration, ionomer proton concentration, local temperature and pressure, local reaction rate, and oxygen and vapor concentration, as well as the local ionomer and
carbon phase potential. Diffusion, in fact, often modeled by Knudsen diffusion, is the dominant transport mechanism for gases in
the catalyst layer due to the small permeability of the void pores
关82兴.
The heat transfer in the CCL is coupled with the water transport
and phase change dynamics. Although a PEMFC operates at 80° C
and near atmospheric pressure, water vapor is favored in the CCL,
especially at lower water production rate 共i.e., at lower current
density兲, for the following reasons: 共a兲 the nanometer scale of the
pores in the CCL increases the saturation vapor pressure, 共b兲 the
water evaporation is significantly enhanced in these nanosized
pores, and 共c兲 the local high temperature in the CCL further increases the saturation vapor pressure and water evaporation rate.
The first point is best illustrated by an example: The equilibrium
vapor pressure, calculated from the Kelvin equation 关83兴, in a
micropore with a diameter of 20 nm increases by 11% at 25° C
and by 9% at 80° C when compared to a flat surface, approximately corresponding to an increase of 1.9° C in the saturation
temperature at 80° C. For the hydrophobic pores in the CCL 关84兴
the saturation vapor pressure is further increased. As for the second point, Eikerling 关75兴 demonstrated that at a current density up
to 1 A / cm2, the evaporation rate in the porous structure is sufficient to convert all product water from the liquid to the vapor
state. From both thermodynamic and kinetic viewpoints, it can be
concluded that the catalyst layers are efficient to convert liquid
product water into the vapor state. According to Eikerling 关75兴 the
fine pores in the catalyst layers promote evaporation of the liquid
phase and thus allow the water to flow out in the vapor phase and
permit reactants to diffuse toward the reaction sites. This
evaporation-condensation mechanism provides an additional pathway for heat removal as described by Wang and Wang 关25兴.
Over the past decade, a few new types of nonconventional catalysts have been developed, e.g., the nanostructured thin film
共NSTF兲 catalysts developed by 3M 关85兴 and the non-Pt catalysts
关86兴. 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 the present
paper.
3.3 Carbon Monoxide Poisoning of Catalyst Layer. The
platinum and platinum-alloy catalyst currently used in anodes are
highly susceptible to carbon monoxide 共CO兲 poisoning, resulting
from the strong binding of CO to active catalyst sites normally
available for hydrogen chemisorptions and dissociation. The industrial H2, usually obtained by reforming hydrocarbons or alcohols, generally contains traces of CO. A CO content as low as
10– 20 ppm results in a significant reduction in anode catalyst
activity and consequently a drastic decrease in performance
关87,88兴. Bhatia and Wang 关89兴 studied the transient behavior of
CO poisoning with diluted hydrogen fuel. Their results demonstrated that in a diluted hydrogen stream, even low CO concentrations of 10 ppm, which are traditionally considered safe for
PEMFC, were found to be harmful to cell performance. Qi et al.
关90兴 reported that trace amounts of CO also poison the cathode
after passing through the membrane. As a consequence, CO levels
in the H2 stream need to be kept below about 10 ppm for safe
operation.
CO competes with H2 for the active sites on the platinum surface at normal anode operating potential. Hydrogen adsorption
and dissociation require two free adjacent Pt sites, while CO can
adsorb to one or two Pt sites by linear- and bridge-bonded adsorption 关91,92兴. Igarashi et al. 关92兴 provided direct experimental evidence for Pt-site poisoning by adsorbed CO, showing a linear
decrease in kinetic currents of hydrogen oxidation on a Pt surface
with increasing CO coverage. Recent work of Papageorgopoulos
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et al. 关93兴 shows that 1% CO blocks 98% of the active sites at
25° C and renders an almost zero H2 oxidation current. Springer et
al. 关94兴 studied the surface coverage of H2 and CO by using both
Langmuir and Temkin isotherms and found that the Temkin isotherm provides a better agreement with experimental data in terms
of steady-state cell performance. Baschuk et al. 关95兴 formulated a
steady-state CO poisoning model in which the adsorption of CO
and H2 is modeled with Temkin and Langmuir models, respectively. Previous CO poisoning data were extensively reviewed by
Baschuk and Li 关96兴 and Cheng et al. 关97兴.
Considerable efforts have been made to reduce the CO poisoning effects. Generally they can be classified into the following
four categories.
A.
B.
C.
D.
Pretreatment of reformate: Pretreatment of reformate is
one of the most popular and straightforward ways to purify
H2 stream to reduce the CO concentration. Pure H2 can be
obtained by preferential or selective oxidation of CO 关98兴,
water-gas shift reaction 关99兴, or methanation 关100兴. However, the extra pretreatment stages add complexity and cost
to the fuel cell system.
Air—共or oxygen—or hydrogen peroxide-兲 bleeding: The
levels of CO can be reduced by bleeding low levels of an
oxidant such as air, oxygen, or hydrogen peroxide into the
anode fuel stream 关101,102兴. It was reported that the COtolerance of the anode catalyst can be increased up to
100 ppm at 80° C by using this technique 关103兴. However,
the air bleed can cause overheating at the anode if the air is
not controlled and mixed properly.
Use of CO-tolerant catalyst: Development of CO-tolerant
catalysts is a major research area for mitigation of CO
poisoning in PEMFC. Alloying or codepositing Pt with
one or more other metals has been widely investigated.
The PtRu alloy catalysts, which have been commercially
used in PEMFC, were shown to be the best CO-tolerant
catalysts. Other Pt-based alloy catalysts have also been
investigated, such as Pt–Co, Pt–Mo, and Pt共Ru兲 – WO3.
Cheng et al. 关97兴 comprehensively reviewed these catalysts. The CO tolerance of these alloy catalysts was generally interpreted by bifunctional and electronic mechanisms. The bifunctional mechanism suggests that the
second alloying metal promotes the dissociation of water
molecule to form Me– OHads, which then reacts with a
neighboring CO-adsorbed Pt atom to form CO2 关104,105兴.
The potential for water dissociation and adsorption on Ru
is lower than that on Pt 关104兴. The 共intrinsic兲 electronic
mechanism suggests that the alloying metal modifies the
electronic properties of platinum and decreases the stability of CO bonding more than that of H on the catalyst
surface 关106,107兴. However, even the use of the COtolerant catalyst still results in a substantial loss of cell
potential—about 200 mV loss for 100 ppm CO with
Pt/ Ru catalyst 关108兴—when compared with the use of
pure hydrogen as fuel.
High temperature operation: It is well known that adsorption of CO on the metal surface is an exothermic process,
indicating that CO adsorption is strongly favored at low
temperatures and disfavored at higher temperatures 关109兴.
For example, when operating at high temperatures, CO
tolerance was greatly improved from 10 ppm at
80° C to 1000 ppm at 130° C and 30,000 ppm at 200° C
关110兴. The key issue in high temperature operation is to
develop alternative PEMs that can be operated at temperatures higher than 100° C. Zhang et al. 关67兴 reviewed the
high temperature development and the high temperature
PEMFC operation.
3.4 GDL. The GDL plays a critical role in fuel cell operation.
The transport of reactant gases from the gas channels to the catalyst layers and transport of product water in the opposite direction
Journal of Fuel Cell Science and Technology
are both dictated by the properties of the GDL. The presence of
liquid water further complicates these processes. It is therefore
essential to know whether and where water is present in liquid
form within the GDL. The surface temperature of the GDL matrix,
and the local temperature, pressure, and composition of the gas
phase mixture determine whether water vapor will condense and
appear in the liquid form. Because of the difficulty in gaining
optical access and measuring local temperature and pressure
within the microstructure of the GDL, useful information can be
obtained through modeling efforts.
The GDL matrix consists of hydrophobic and hydrophilic fibers
that are designed to provide efficient passageways for water and
gases. The most commonly used GDL materials for PEMFCs are
carbon paper and carbon cloth. 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 共PTFE兲 coating. GDLs
also serve as the main media to conduct heat from the catalyst
layers to the flow-field plates. A fine microporous layer 共MPL兲,
consisting mainly of carbon powder and PTFE particles, has been
widely used to improve the performance of a PEMFC 关111–114兴.
The presence of an MPL has been shown to improve the fuel
cell performance. Wang et al. 关115兴 showed that the water transport in GDL is mainly governed by the capillary forces, and the
presence of two different pore sizes was responsible in the effective drainage of water from the CCL-GDL interface. Several authors 关112,113兴 have demonstrated that the MPL improves the
humidification of the membrane at the anode side. Jordan et al.
关113兴 and Kong et al. 关114兴 concluded that the MPL enhances
oxygen diffusion by reducing flooding in the cathode. Owejan et
al. 关116兴 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 studied the effect of
MPL location on the GDL 共toward the catalyst layer or the gas
channel兲 and further concluded that the capillary effects within the
GDL have an insignificant effect on mass transport overpotential,
and the Darcy flow in the catalyst layer has little influence on the
transport process. Furthermore, introducing a MPL is shown to
improve the electrical contact between GDL and catalyst layers,
thereby improving the fuel cell performance 关112兴.
The mechanism of water transport within the GDL is not yet
completely understood. The porous medium models developed in
soil mechanics provide some guidance in this regard, and many
GDL models are based on similar concepts. Udell 关117兴 developed
a model for water transport through porous wick structures in heat
pipes. Extending this approach, Nam and Kaviany 关118兴 used the
effective diffusivity and water saturation in modeling water transport. Litster et al. 关119兴 presented images of the water flow pathways, indicating that the capillary branching with a number of
dead-ends was seen to be present. This indicated that the transport
process is quite different in the fibrous GDL from the homogeneous porous models. A number of porous network models are
being developed currently in literature, e.g., Gostick et al. 关120兴,
Sinha and Wang 关121兴, and Markicevic et al. 关122兴, utilizing
square or cubic pore network and associated capillary forces. A
number of researchers are pursuing the models based on effective
porosity and effective diffusivity in 2D or 3D domain. A survey of
these models is presented by LaManna and Kandlikar 关123兴.
These recent efforts are directed toward understanding the fundamental water transport process in the fiber matrix of a GDL.
GDL flooding, caused by liquid water accumulating in the
pores and consequently hindering oxygen transport to the catalyst
sites, is one of the most common problems faced in PEMFC water
and thermal management. Flooding may occur when a PEMFC is
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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 关118兴, 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. For example, a pressure drop
of about 10 kPa has been measured in the author’s laboratory in a
250 mm long, 700⫻ 400 ␮m2 rectangular channel, operating at a
stoichiometric ratio of 10. This pressure drop corresponds to a
reduction of approximately 4.5° C in the saturation temperature at
around 80° C. 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 its temperature, thus increasing the
water vapor saturation pressure. Both modeling results
关30,32,33,124兴 and experimental data 关19兴 indicate that significant
temperature gradients 共 ⬃ 5 ° C兲 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 关50兴. The thermal and electrical conductivities
of GDL are likely anisotropic, which complicates the thermal
measurements and heat transfer modeling 关125兴. A recent study by
Pasaogullari et al. 关126兴 indicates that the temperature difference
was predicted to be 5 ° C at a cell voltage of 0.4 V with isotropic
GDL properties, while it reduced to 3 ° C by incorporating the
anisotropic properties. Another complication comes from the temperature difference between the fluids and the solid matrix in the
GDL. Hwang 关127兴 showed that the local fluid and matrix temperatures have considerable influence on the current density
distribution.
It should be recognized that the heat transfer and water transport in the GDL are intimately linked. The local saturation temperature of the gas phase depends on the water vapor content and
the pressure, while the local temperature of the GDL matrix is
determined by the heat transport processes. If the local GDL matrix temperature is below the local saturation temperature of the
gas phase, condensation will occur within the GDL 共or the microporous layer兲. Because the interface thermal resistance between the GDL and the lands is lower 共due to high contact pressure兲 than the convection resistance between the GDL and the gas
flow channels, the GDL temperature underneath the lands is expected to be lower as compared to that underneath the channels
under steady-state conditions 共local temperature of the gas stream
needs to be considered in a more detailed analysis兲. Water condensation and accumulation are thus favored under the land regions. The design of the land-GDL interface thus becomes critical
from a water transport standpoint 共from GDL into the channels兲 as
well, in addition to its role in the flow of electrons. Since the heat
and water transport are coupled processes, a detailed thermal and
gas transport model is needed to predict the location of the condensation front, especially under transient and start-up conditions.
3.5 Bipolar Plates and PEMFC Cooling Systems. The bipolar plates constitute the mechanical backbone of a fuel cell
stack. They conduct current between cells and provide conduits
for reactant gases and coolant. They are commonly made of
graphite composites due to their high corrosion resistance and low
surface contact resistance. Considerable attempts are being made
to replace the graphite composites with metals due to their high
mechanical strength, better durability to shocks and vibrations,
and superior manufacturability and cost effectiveness 关128兴. The
main challenge, however, is that the anticorrosion coating or the
passivating oxide layer on the metal surface usually causes an
044001-6 / Vol. 6, NOVEMBER 2009
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. 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 unacceptably dry operating conditions 关129兴. In practice, 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.
Two factors are critical in designing a cooling system for PEMFCs. 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 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 关130兴.
A common fuel cell stack cooling method involves designing
the bipolar plates with internal cooling channels. With this approach, the channel geometry is designed to accommodate the
heat transfer medium of choice. For some smaller fuel cell systems 共up to 2 – 5 kW in electrical power output兲 it is possible to
use air as the heat transfer fluid. The 1.2 kW Mark 1020 system
offered by Ballard is an example of this approach 关131兴. For automotive and many stationary fuel cell systems, the waste heat
loads are large enough that a liquid heat transfer fluid 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 the following:
•
•
•
•
•
•
a low-cost fluid based on kerosenic hydrocarbons or
kerosene-water mixtures 关132兴
maintaining low fluid conductivity by incorporation of an
ion exchange medium 关133兴
addition of carboxylic salts to the heat transfer fluid to maintain the electrical conductivity below 100 ␮S / cm 关134兴
improving durability of glycol-water coolants by incorporation of a filter to remove oxidation products 关135兴
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 关136兴
reducing the size of the coolant system by use of a refrigerant fluid 关137兴
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 关138兴. 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 关139兴.
A noteworthy departure from the internally cooled plate design
is the concept advanced by UTC Power, in which the bipolar
plates are fabricated from a porous, hydrophilic material. This
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Fig. 3 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 along the channel region section AA and land region
section BB. Thicknesses and temperature gradients; not to scale
allows direct water exchange with the cathode side of the membrane electrode assembly 共MEA兲 关140兴. 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.
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 pull water into
the porous plate, maintaining even water distribution with no
moving parts 关141兴.
3.6 Temperature Distribution in PEMFC Components.
The balance between the heat generation and heat removal determines the steady-state operating temperature distribution within a
PEMFC. The relevant terms for each of the heat transfer component are described below.
1. Heat production due to electrochemical reaction: Qrxn
= ⌺hih共␩h + ⌸h兲, where ih is the current, ␩h is the overpotential, ⌸h is the 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 共HOR兲 and the oxygen-reduction reaction 共ORR兲
have been measured experimentally. Averages are −13 mV
for HOR 关73,74兴 and −240 mV for ORR 关111兴 at 25° C.
2. Heat generation due to Ohmic heating: Qohm
= 兰 共i2 / ␴共y兲兲dy, where ␴ represents the proton conductivity
Journal of Fuel Cell Science and Technology
or electronic conductivity. This conductivity is generally a
function of thickness due to the varying composition.
3. Heat generation/consumption due to water vaporization/
condensation: Q = ⌬Hvaprvᐉ, where ⌬Hvap is the heat of vaporization and rvᐉ 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. It is clear that in-plane temperature variation needs to be considered for accurate prediction of water transport.
4. Heat transfer due to convection and conduction: hAconv⌬T,
kAcond共dT / dy兲 respectively, where h is the heat transfer coefficient, Aconv is the convective area, ⌬T is the temperature
difference, k is the thermal conductivity, Acond is the conduction area, and dT / dy is the temperature gradient in the heat
flow direction.
A representative schematic of the anticipated temperature profile
across a PEMFC having cooling channels in each bipolar plate is
shown in Fig. 3. This representation is based on the current understanding 关1,8,19–41,112–116兴 and is expected to undergo
changes as the further insight is gained. 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 关19兴 and various thermal models
关1,8,32兴. The thermal conductivity of each component and their
thickness are listed from experimental measurements 关50兴. Note
that the thicknesses and the temperature gradients within each
component layer are not to scale. The temperature profiles across
the channel region are represented by section AA, while the proNOVEMBER 2009, Vol. 6 / 044001-7
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file across the land regions is shown by BB. Also shown in Fig. 3
are the heat generation estimates in each component due to the
reaction and/or Ohmic heating for a 50 cm2 single fuel cell operating at 1 A / cm2. A cell voltage of 0.6 V is assumed in estimating
the irreversible heat generation in the CCL due to the overpotential 共␩兲. The temperature profiles in the GDL under the channels
and the lands are shown to be different due to differences in the
thermal resistances offered at the GDL-bipolar plate interface and
the GDL-channel interface. 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 共liquid or vapor
phase of water兲. On the anode side, no severe condensation is
foreseen within the GDL. However, the consumption of hydrogen
may lead to significant condensation and subsequent flooding in
the anode channels toward the exit.
4 Water and Thermal Management in Cold-Start
Conditions
The automotive fuel cells face an additional challenge over
some of the stationary fuel cells in providing operation under
subfreezing conditions. The development in this area is guided by
the targets set by the DOE on cold-start-up conditions 关142兴. According to these targets, a start-up time of 30 s is recommended at
−20° C from start to 50% rated power for a 80 kWe fuel cell
stack, with a total energy consumption limit of 5 MJ for start-up
and shut-down protocols and with 5000 h durability under cycling
conditions.
The research in this regard has been thus focused on developing
shut-down and start-up protocols as they affect the combined energy consumption targets and start-up times. At shut-down, prolonged purges help in removing the water, but will contribute
toward excessive energy consumption. On the other hand, certain
amount of water is needed in the membrane to allow for rapid
start-up times.
The freeze-damage under freeze-thaw cycling has been studied
in literature. For example, Yan et al. 关143兴 reported that the fuel
cell was able to start from −5 ° C if it was properly purged and
insulated. At −10° C start-up temperature, there was no damage
with a current-step of 0 – 100 mA cm−2, but the cell failed to start
with a current-step of 0 – 220 mA cm−2, resulting in extensive cell
damage, including damage to MEA, GDL and membrane, delamination of the catalyst layer, minor damage to the backing Teflon
coating, and minor damage to the binder due to ice formation.
Analysis from an ANSYS model showed that the freeze-thaw cycle
of water in a cylindrical pore resulted in local stresses in the
carbon fibers in excess of their yield strength, and some visual
damage to the binder was seen during such cycling experiments
conducted by Pelaez et al. 关144兴. Hou et al. 关145,146兴 studied the
polarization losses as a function of different amounts of residual
water. For the conditions tested, they found no change in the
Ohmic resistance, but the mass transport resistance greatly increased even at low current densities.
Ge and Wang 关147–149兴 studied the start-up characteristics as a
function of purging times and directly observed the ice formation
on the CCL by employing microholes or silver screen GDLs.
Their results indicated that a purge of less than 30 s appeared to
be insufficient and that between 90 s and 120 s was found to be
most useful in reducing water content to a desirable level prior to
cold-start. The freezing point depression in the CCL was found to
be quite small, 共1.0⫾ 0.5兲 ° C, and its effect on the cold-start was
negligible. Cho et al. 关150兴 reported that performance degraded by
an average of 2.3% without purge over 15 freeze-thaw cycles,
while negligible degradation was observed after purging with N2,
O2, 30% methanol solution, or 35% ethylene glycol solution.
The performance of fuel cell under cold conditions is also relevant in reducing the start-up times to a target of 30 s to 50%
power. The work by Hishinuma et al. 关151兴 confirms that the
044001-8 / Vol. 6, NOVEMBER 2009
performance decreases at higher current densities, higher pressures, and lower temperatures. Below a temperature of −5 ° C,
extra heating power was needed for operation. Research work on
free-breathing fuel cells becomes of interest in this regard with
specific ramp up strategies 关152兴.
A number of investigators studied the cold-start performance by
employing numerical methods. Jiang 关153兴 developed a numerical
tool to delineate interaction between ice formation and heat generation. Their model correctly predicted decreased ice formation
with rising start-up temperatures. Their model also confirms that
the pre-start-up conditions significantly influence the cold-start
characteristics. A detailed heat transport model by Mao et al. 关154兴
analyzed the heat transport within a fuel cell during cold operation
and found that the distribution of ice formation and melting depends on the multidimensional heat transport over the entire cell.
Their model correctly predicted the dynamic effects of current
density and water amounts within the cell with 20% accuracy.
Meng 关155兴 developed a transient multiphase multidimensional
model to simulate the cold-start behavior. Their results indicate
that the water vapor concentration inside the cathode gas channel
affects ice formation in the CCL. Initially, the membrane would
absorb the product water during cold-start process and become
hydrated under both constant cell voltage and constant cell current
conditions. Subsequently, the performance degrades due to ice
formation in the CCL. The ice growth was seen to be faster under
the lands and at the interface of CCL and GDL.
Khandelwal et al. 关156兴 developed a one-dimensional model to
study the cold-start ability and corresponding energy requirement
over different operating and ambient conditions. The start-up energy requirement is also a DOE target. They reported that a 20 cell
model was representative to simulate the full size stack behavior.
They recommended thermal isolation of end stacks to reduce
start-up times. Different heating strategies were considered, but
the flow of coolant above 0 ° C was found to be most effective in
reducing the start-up time.
The specific effects of gas purge were modeled by Sinha and
Wang 关157兴 to develop basic understanding of the water removal
processes during gas purge. The model considers water transport
in the GDL and predicts the drying times of the GDL as well as
the energy requirements. The model correctly predicts that a low
gas relative humidity, high gas flow rates, and high cell temperature would favor efficient water removal. Tajiri et al. 关158兴 combined the experimental and numerical techniques to explore the
effects of various system parameters, such as the GDL thickness,
different purge methods, start-up temperature, and current density
on cold-start.
5
Discussion and Research Needs
5.1 Coupled Heat and Water Transport in PEMFC
Cathode. The thermal and water transport in PEMFC are inherently coupled because 共i兲 evaporation and condensation processes
are, respectively, accompanied with absorption and release of latent heat; 共ii兲 water and heat transport occur in conjunction with
each other due to heat pipe effect 关8,25兴; and 共iii兲 the saturation
vapor pressure is strongly dependent on local gas conditions.
Wang and Wang 关25兴 showed for the first time that the evaporation
of water at the catalyst layer and its condensation at the colder
land region presents a new mechanism for heat transfer that is
similar to the heat pipe effect. The water transport occurs at multiscale, from molecular diffusion of vapor in the CCL to the twophase flow in the gas distribution channels 关6兴.
The local temperature profiles and water vapor partial pressures
play a crucial role in the transport processes. As discussed in the
CCL section earlier, the favorable state of water in the CCL is the
vapor phase under normal operating conditions. At low and moderate current densities, the particular pore structure in the catalyst
layer favors water in the vapor phase, facilitating water removal.
At high current densities, with high water production rates, catalyst flooding is seen to occur more frequently. The role of the
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Fig. 4 A proposed CCL water transport mechanism showing the electrode reaction and the
transport of product water in a catalyst layer; not to scale
microporous layer is critical in water management as it acts like a
surface tension based gate that prevents the backflow of liquid
water from the GDL into the CCL, while facilitating flow of water
vapor away from the CCL into the GDL. The local temperature is
another key element in ascertaining that the condensation occurs
in the GDL.
Figure 4 shows a proposed transport model for water from its
generation at the CCL to its removal in the gas channels based on
our current understanding 关1,8,19,32,112–116兴. When the high
temperature water vapor in the CCL passes through the microporous layer and the GDL, it is cooled and partly condensed
into liquid water. Condensation in the MPL with finer pores is less
desirable than in the GDL, which has a coarser pore structure. If
the local saturation condition results in condensation in the MPL,
it may lead to flooding. A condensation front therefore is expected
to exist under optimum conditions somewhere at the interface
between the MPL and the GDL or within the GDL, as shown in
Fig. 4, depending on the local temperature. The flow of vapor
from the reaction sites and its subsequent condensation inside the
GDL also enhances the heat transfer due to the release of latent
heat during condensation. The thermal gradients within the GDL,
in both in-plane and through-plane directions, thus play a significant role in the water vapor transport toward the gas channels and
its condensation within the GDL matrix. The flow of liquid water
from underneath the lands into the channels, and subsequent twophase flow in the channels is a topic of current research interest. It
should be pointed out that the model presented in Fig. 4 is a
conceptual model based on the available information, and further
changes are expected to be made as our understanding improves.
Also, the water transport mechanism in certain new types of catalysts, e.g., the NSTF catalyst by 3M with no ionomers, may be
quite different. Further research is warranted on this topic for
different types of catalysts.
2.
3.
4.
5.
5.2 Research Needs. As fuel cells continue to develop toward
commercialization, there are still a number of outstanding critical
research needs related to water and thermal management. Some of
the important needs identified in the above review are as follows.
6.
1. Accurate experimental data for the thermal conductivity and
contact resistance of different components 共e.g., proton ex-
7.
Journal of Fuel Cell Science and Technology
change membranes, catalyst layers, gas diffusion layers, and
bipolar plates兲 are needed in thermal and water transport
modeling. Particular focus is needed on understanding multidimensional effects in components that 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 water removal characteristics, 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 thermal
transport mechanisms in each component is needed. A variety of fundamental phenomena, such as membrane dehydration, liquid water production in the CCL, anode-cathode water balance, water saturation and transport in the
microporous layer and GDL, in-plane and through-plane
temperature variations within the GDL, and the nonuniform
distributions of temperature and current density along the
gas channels, require further investigation. Liquid water
transport and the effects of GDL structure, its morphology,
and channel wall structure and surface energy are topics that
need further investigation.
Defining a set of easily measurable and rigorously meaningful thermal and fluid transport 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 fibrous media 共including CL and
GDL兲 is open to question. Better transport models are
needed to describe the water transport in PEMFC.
The transient heat transfer problems, such as the start-up,
shut-down, and freeze-thaw cycling, are being addressed in
literature. There is a need to develop appropriate control
algorithms, especially in the case of automotive systems that
encounter highly dynamic load profiles.
The cold-start targets by DOE set specific goals for meeting
the start-up times and total energy consumption during shutNOVEMBER 2009, Vol. 6 / 044001-9
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down purge and start-up conditions. In order to meet these
targets, the first step is to understand the detrimental effects
of freeze-thaw cycles on the material and performance. A
few studies have addressed these issues, but long-term durability under different purge and start-up protocols will be
needed. It is important to determine the locations where water is retained, and then develop new GDL materials that
effectively remove water from these locations during purge
cycles. It is also imperative to develop materials that are
more resistant to accidental freeze-damage. Such damage
occurs especially where the fibers are joined by the binders.
Developing new binders or GDL manufacturing techniques
also needs to be considered.
8. Developing an overall numerical system-level model incorporating both thermal and water transport within various
components is also important as it will help in identifying
the critical parameters and their influence on the water removal process. Furthermore, such tools will be effective in
optimizing the fuel cell performance.
6
Conclusions
Combined water and thermal management is a key design consideration for PEMFCs, from the microscopic scale of the catalyst
particles 共nanometer scale兲 and microporous layer 共tens of micrometers兲, the component level gas diffusion layers and bipolar
plates 共both microscopic and macroscopic scales兲, 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. Although our understanding of these processes has greatly improved in the past decade, some critical research needs are identified to provide better
predictive models and improved fuel cell performance, under normal as well as freezing conditions.
Acknowledgment
The work was conducted under a project “Visualization of Fuel
Cell Water Transport and Performance Characterization Under
Freezing Conditions” sponsored by Department of Energy under
Grant No. DE-FG36-07GO17018. The financial support of DOE
is gratefully acknowledged. The authors would also like to thank
Dr. Thomas A. Trabold from General Motors Corporation, Fuel
Cell Research Laboratory, Honeoye Fall, NY for his valuable input in preparing this manuscript.
Nomenclature
⫽
⫽
⫽
⫽
⫽
⫽
⫽
⫽
⫽
⫽
⫽
␩ ⫽
A
a
nd
h
⌬H
i
Q
Tg
T
␴
␭
area 共m2兲
water activity 共dimensionless兲
electro-osmotic drag 共dimensionless兲
convection heat transfer coefficient 共W / m2 K兲
change of enthalpy 共J/mol兲
current density 共A / m2兲
heat 共J兲
glass transition temperature 共°C兲
temperature 共°C兲
electrical conductivity 共S/m兲
water uptake 共dimensionless兲
overpotential 共V兲
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关11兴 Zhang, F. Y., Yang, X. G., and Wang, C. Y., 2006, “Liquid Water Removal
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关12兴 Turhan, A., Heller, K., Brenizer, J. S., and Mench, M. M., 2006, “Quantification of Liquid Water Accumulation and Distribution in a Polymer Electrolyte
Fuel Cell Using Neutron Imaging,” J. Power Sources, 160, pp. 1195–1203.
关13兴 Pekula, N., Heller, K., Chung, P. A., Turhan, A., Mench, M. M., Brenizer, J.
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关14兴 Chen, Y., Peng, H., Hussey, D. S., Jacobson, D. L., Tran, D. T., Abdel-Baset,
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关15兴 Ge, S., and Wang, C. Y., 2007, “Liquid Water Formation and Transport in the
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2006, “In Situ Investigation of Water Transport in an Operating PEM Fuel Cell
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关18兴 Owejan, J. P., Trabold, T. A., Jacobson, D. L., Baker, D. R., Hussey, D. S., and
Arif, M., 2006, “In Situ Investigation of Water Transport in an Operating PEM
Fuel Cell Using Neutron Radiography: Part II—Transient Water Accumulation
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关20兴 Shimoi, R., Masuda, M., Fushinobu, K., Kozawa, Y., and Okazaki, K., 2004,
“Visualization of the Membrane Temperature Field of a Polymer Electrolyte
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关34兴 Muller, E. A., and Stefanopoulou, A. G., 2006, “Analysis, Modeling, and
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Cell System,” ASME J. Fuel Cell Sci. Technol., 3, pp. 99–110.
关35兴 Zong, Y., Zhou, B., and Sobiesiak, A., 2006, “Water and Thermal Management
in a Single PEM Fuel Cell With Non-Uniform Stack Temperature,” J. Power
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关36兴 Xue, X., and Tang, J., 2005, “PEM Fuel Cell Dynamic Model With Phase
Change Effect,” ASME J. Fuel Cell Sci. Technol., 2, pp. 274–283.
关37兴 Yu, X., Zhou, B., and Sobiesiak, A., 2005, “Water and Thermal Management
of Ballard PEM Fuel Cell Stack,” J. Power Sources, 147, pp. 184–195.
关38兴 Yan, X., Hou, M., Sun, L., Cheng, H., Hong, Y., Liang, D., Shen, Q., Ming, P.,
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NOVEMBER 2009, Vol. 6 / 044001-13
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