Water Management
EC Kumbur and MM Mench, The Pennsylvania State University, University Park, PA, USA
& 2009 Elsevier B.V. All rights reserved.
Introduction
Fuel cells hold great promise to meet the basic requirements of many future energy conversion systems. The
factors driving the promotion of fuel cell technology arise
from the converging needs for decreased pollution, economic stability, and global security. For decades, various fuel
cell systems have been developed, with some reaching
limited market penetration. For example, more than two
hundred and forty 200-kWe phosphoric acid–based fuel cell
systems developed by ONSI Corporation (now United
Technologies Corporation Fuel Cells) for stationary power
applications have been sold and are in service since 1992,
with >95% service reliability. Other types of fuel cell
systems including the alkaline electrolyte fuel cell, solid
oxide fuel cell, and molten carbonate fuel cell have been, or
continue to be, developed. However, ubiquitous integration
into the global power production profile has not yet
occurred for a variety of reasons beyond the scope of this
article. In particular, the high cost of materials or
Bipolar
plate
DM
manufacturing, low durability, and lack of hydrogen infrastructure represent major bottlenecks that must be
overcome.
Among the various fuel cell types, hydrogen-based
proton-exchange membrane fuel cells (PEMFCs) are the
most suited for many stationary power, portable power, and
automotive applications because they offer low operating
temperature (o100 1C), high efficiency (>50%), rapid
startup time, and suitable transient response characteristics.
Figure 1 shows a schematic of a typical PEMFC assembly.
In a PEMFC, the electrolyte (15–180 mm thick) is a flexible
polymer membrane. The catalyst layer (CL) electrodes
(B10–20 mm thick) consist of nanosized (B2–5 mm
diameter) catalyst particles that are generally based on the
noble metal platinum. These catalysts are typically supported on larger carbon particles (B45–90 mm diameter).
The fuel (typically hydrogen (H2)) and oxidizer (typically
air) are distributed through separate channels and flow
parallel to the electrode surface, reaching the reaction site
by diffusive and convective transport through a porous
CL
Channel
Land
H2
Air
PEM
e−
Anode
Load
e−
Cathode
Figure 1 Schematic of a typical proton-exchange membrane fuel cell (PEMFC) assembly and components. CL, catalyst layer; DM,
diffusion medium, PEM, polymer electrolyte membrane. Reproduced with permission from Mench MM (2008) Fuel Cell Engines. New
Jersey: John Wiley & Sons Inc.
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Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
carbon diffusion medium (DM), which is also referred to as
the gas diffusion layer (GDL) or porous transport layer.
This critical component functions to deliver reactant to,
and product away from, the electrodes, as well as provide
high electrical conductivity with low contact resistance and
appropriate heat transfer.
At the anode, hydrogen is split into hydrogen ions and
electrons by an electrochemical oxidation reaction. The
semipermeable polymer membrane is conductive to H þ
ions but not electrons; therefore, only H þ ions migrate to
the cathode, whereas the electrons flow through the DM
and channel structure. The electrons passing through the
external circuit reunite with H þ ions and oxygen molecules at the cathode, undergoing an electrochemical
reduction reaction. As a result, water vapor is generated at
the cathode. The fuel cell generates waste heat as a result
of ionic, electronic, kinetic, and mass transport losses.
Although convenient in terms of pollution, the fact that
water is produced and must be properly managed represents a major technical challenge. A complex relationship
exists between the water content and the performance of
a PEMFC. The water generated by reaction needs to
be both removed and retained at the same time. The
829
fluoropolymers and hydrocarbon membranes conventionally used for the electrolyte require moisture to be ionically
conductive; thus, local moisture is critical to operation.
However, the water vapor generated by the electrochemical
reaction often condenses into liquid phase. The presence of
excess liquid water in the DM, CL, or flow channel can
block available pathways for reactant flow, thereby hindering the transport of reactant to the electrochemically
active sites. Real-time performance loss and operational
instability resulting from liquid water accumulation are
generically referred to as ‘flooding’, although flooding losses
can result from local and discrete accumulations in the
electrodes, DM, flow channels, or along interfaces between
the DM and CL as illustrated in Figure 2.
Although short-term performance loss is to be avoided, in the long term, excess liquid overhead, even if it is
not responsible for immediate performance loss, can also
result in reduced long-term performance through several
mechanisms:
1) Electrolyte internal stresses : Locations within the
fuel cell can have highly inhomogeneous water
distributions and temperatures. High levels of internal
Channel
Flow
Channel flooding
Water
DM
Flow channel
Channel
Flow
Diffusion media flooding
Water
Reactant
flow
DM
DM
CL
Catalyst layer
Catalyst layer flooding
DM
Water
CL
Electrochemical
reaction sites
Figure 2 Illustration of possible locations of flooding in a proton-exchange membrane fuel cell (PEMFC) including catalyst layer (CL),
diffusion media (DM), and channel level flooding. Interfacial accumulation and reactant restriction is another mode of flooding-related
performance loss not shown.
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Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
stresses can develop inside the membrane, because the
electrolyte membrane swells significantly in the
presence of water vapor and, even more, when in
contact with liquid-phase water (a condition known as
Schroeder’s paradox). As a result, accelerated physicochemical degradation of the CL and interfaces can
result.
2) Ionic contamination : Ionic impurities from metals are
most easily transported in liquid droplets and will
readily absorb into the fuel cell electrolyte, because it
is an ionic conductor. Ionic impurity in the membrane
greatly reduces conductivity and water transport,
thereby reducing performance, even with very minute
quantities of impurities. Postmortem testing of membrane–electrode assemblies (MEAs) has demonstrated
the presence of a surprising array of impurities, such
as calcium, iron oxides, copper, magnesium, and
various other metals. As a result, most fuel cell systems are designed to avoid contact of the reactant flow
stream with any metal connectors or couplings, and
special plastics deemed compatible for fuel cell systems have been developed.
3) Frozen condition damage : Residual water in the CLs at
shutdown to a frozen state has been shown to cause
irreversible damage to the CL and DM–CL interface
under certain conditions; therefore, adequate purge at
shutdown is essential to remove residual water even if
it is not responsible for flooding performance loss.
Typical water overhead stored in the membrane, porous
media, and flow channels in an operating PEMFC is
around 5–20 mg cm"2, depending on flow field design,
operating conditions, and materials. Although the adequate removal of the product water is essential, adequate water in the membrane electrolyte is also a
necessity for achieving high performance, because
membrane proton conductivity depends on hydration of
the membrane. The paradox is that although the fuel cell
is a net producer of water, water is typically carried into
the fuel cell by humidification of the reactant gas streams
to avoid local dry-out.
Because of these conflicting requirements, the efficient
operation of PEMFC requires a delicate balance between
membrane hydration and the local avoidance of cathode
flooding to achieve high ionic transport and low mass
transport resistance. In order to identify the requirements
of proper microfluidic management and understand the
global water balance in polymer electrolyte membrane
(PEM) fuel cells, we need to first understand the modes
of local water transport in the components of PEMFC. It
must be emphasized that flooding is a localized phenomenon. Any local loss of active catalyst sites increases
the charge transfer burden on the remaining sites,
thereby reducing overall performance.
Water Transport in the Polymer
Electrolyte Membrane
To date, most modern solid electrolytes used in the
PEMFCs are perfluorinated ionomers with a fixed side
chain of sulfonic acid bonded to the inert, but chemically
stable, polymer polytetrafluoroethylene (PTFE) structure, as illustrated in Figure 3. The development of
hydrocarbon- and alkaline-based membranes is also
under way, and these structures also need some water for
sufficient conductivity.
One widely used example of this type of membrane is
Nafions, manufactured by E. I. DuPont de Nemours and
Company. In terms of chemical composition, these
membranes consist of two very different substructures:
(1) a hydrophilic and ionically conductive phase due to
the attached sulfonic acid groups and (2) a hydrophobic
and relatively inert polymer backbone, which is not
ionically conductive but provides chemical stability and
durability. When the Nafion structure is hydrated, the
hydrophilic sulfonic acid chains imbibe water and enable
the motion of H þ ions. Depending on the hydration
level, there are two modes of proton transport in the
electrolyte: (1) vehicular or diffusion mechanism (occurs
at low hydration level) and (2) Grotthuss or protonhopping mechanism (occurs at high hydration level).
These modes are illustrated in Figure 4. In the vehicular
diffusion mode, the ionically conductive SO3 " chains are
distributed as generally isolated clusters in the membrane, and proton transport mechanism from one cluster
to another is dominated by the diffusion mechanism. In
the Grotthuss mechanism, the ionically conductive SO3"
clusters share increased connectivity in a highly hydrated
environment, and the proton can be transported through
a more efficient proton-hopping mechanism.
Water Uptake
Water uptake (l) of the membrane is defined as the
number of water molecules per sulfonic acid site:
l¼
H2 O
SO3 H
½1%
The water uptake of a Nafion membrane at 30 1C in
contact with a gas-phase flow is
l ¼ 0:043 þ 17:18a " 39:85a2 þ 36:0a 3
a ¼ RH ¼
yv P
Psat ðT Þ
for 0rar1 ½2%
½3%
where a is the water activity, i.e., the relative humidity
(RH), yv is the mole fraction of vapor, P is the total
pressure of the air–vapor mixture, and Psat represents the
Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
831
H+
O−
S
O
F C
F
F C
F
O
Sulfonated
side chain
O
F C
F
PTFE
F F
F
F F
F F F
F F
F
F F
F F F
F F
F
F
C F
F O F F F
F F F
F F F
F F
C C C C C C C C
C C C C C C C C
C C C C C C C C C C C C C C C C
F F
F F
F F
F
F F
F F F
F
F O F F F
F
F F
F F F
F F F
F O F
F C
F
F
C F
F C
F
F
C F
O
F
O
F
C F
F
F
C F
S O
O−
F F
O
C F
F C
Repeating
O S
O−
H+
O
H+
Figure 3 Schematic of fluoropolymer membranes with connected sulfonated side chains. PTFE, polytetrafluoroethylene. Reproduced
with permission from Mench MM (2008) Fuel Cell Engines. New Jersey: John Wiley & Sons Inc.
saturation pressure at a given temperature (T). Although
the relationship given in eqn [2] was derived for 30 1C, it
has been applied to higher temperature conditions. For a
fully humidified condition, the relationship in eqn [2]
gives a maximum achievable water uptake value (l) of 14;
however, this value actually decreases with increasing
temperature, to a value around l ¼ 10 at 80 1C. When the
membrane is equilibrated with liquid water, the water
uptake of expanded Nafion is much higher, approaching a
value of l ¼ 22. The sharp difference in water uptake
characteristics of Nafion between water- and vaporequilibrated conditions is a result of Schroeder’s paradox.
Schroeder’s paradox is critical, because the abrupt change
in water content results in a similarly abrupt variance in
ionic conductivity, membrane swelling, and other important transport parameters. The ionic conductivity of
the membrane (si) can be defined in terms of water
uptake as
#
!
"$
1
1
"
si ðS cm Þ ¼ exp 1268
303 T
"1
( ð0:005 193l " 0:003 26Þ
½4%
where T is in kelvin. The correlation given in eqn [4] was
derived based on conductivity measurement from 20 to
90 1C and a fully humidity range, and predicts a value of
10 S cm"1 proton conductivity for a well-hydrated
PEMFC membrane. The sensitivity of ionic conductivity
in response to a change in RH is shown in Figure 5.
Although operating at high RH is beneficial, at high
temperature, the water vapor mole fraction becomes
excessively large in the gas phase, which reduces reactant
availability and performance.
Water Flux through the Polymer Electrolyte
Membrane
Water is transported across the membrane via four different modes, as illustrated in Figure 6.
Electro-osmotic drag (potential-driven flow)
Electro-osmotic drag is the flux of water resulting from a
polar attraction of the water molecules to the positively
charged protons moving from the anode to the cathode
through the electrolyte. When H þ ions migrate from the
anode to the cathode, they tend to attract and drag water
molecules along with them. The number of water molecules transported per hydrogen proton (H þ ) is called
the electro-osmotic drag coefficient (x). Considering the
direction of proton flux in the membrane, the transport of
water molecules via electro-osmotic drag always occurs
832
Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
Figure 4 Schematic of (a) Grotthuss and (b) vehicular water motion mechanisms. Reproduced with permission from Mench MM
(2008) Fuel Cell Engines. New Jersey: John Wiley & Sons Inc.
from anode to cathode, and can be described as
%
&
i
nw;e-o mol s"1 ¼ x
F
½5%
where i is the local current density, F is Faraday’s constant, and x is the electro-osmotic drag coefficient.
Although there is some discrepancy in the measured
values of the drag coefficient among different groups and
between membrane materials, in general, a value of
x ¼ 1.0 for lo14 is appropriate. It has been shown that
significantly higher values of x ¼ 2–5 result from a liquid-equilibrated membrane.
Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
14
0.12
12
Diffusivity × 10−6 (cm2 s−1)
0.14
!i (S cm−1)
0.10
0.08
0.06
0.04
0.02
0.00
0.0
0.2
0.4
0.6
Relative humidity
0.8
0.10
Figure 5 Specific conductivity (si) of Nafion as a function of
environmental humidity at 80 1C.
Anode
Cathode
PEMFC electrolyte
Temperature/
heat flux
Figure 6 Illustration of different modes of water transport inside
the fuel cell. PEMFC, proton-exchange membrane fuel cell.
Diffusion in Nafion (concentration-driven flow)
The diffusion of water through the polymer membrane
occurs as a result of a concentration gradient across the
membrane, and can be modeled as a single-component
diffusion mechanism according to Fick’s law:
diff ðmol
s"1 Þ ¼ "Dw
DCc"a
Dx
353 K
6
4
303 K
2
0
0
2
4
6
8
10
12
14
16
Membrane water content, " (H2O/SO3H)
18
Figure 7 Membrane water diffusivity as a function of water
content.
%
&
Dw ðcm2 s"1 Þ ¼ 4:17 ( 10"4 l 1 þ 161exp"l
#
$
"2436
for 0olr17
( exp
T ðKÞ
H2O vapor inlet/
humidified flow
H2O hydraulic
permeability
(liquid and gas phase)
nw;
8
%
&
Dw ðcm2 s"1 Þ ¼ 3:10 ( 10"3 l "1 þ exp0:28l
#
$
"2436
for 0olr3
( exp
T ðKÞ
H2O
generation
H+ transport
H2O vapor inlet/
humidified flow
10
the ‘Further Reading’ section) reported a corrected
Fickian diffusion coefficient for 1100 EW Nafion, which
accounts for the effect of temperature and membrane
swelling as a result of water uptake:
Electro-osmotic
Drag
H2O diffusion
833
½6%
where Dw is the diffusion coefficient (a function of local
water content of the membrane (l)) and DCc–a/Dx is the
water concentration gradient across the membrane of
thickness Dx. The water diffusion in the ionomer phase,
Dw , has been measured as a function of ionomer water
content by several groups. On the basis of the measured
water self-diffusion data, Montupally and coworkers (see
½7%
½8%
This nonlinear relationship is plotted in Figure 7 for two
different temperatures. It should be noted that the diffusion coefficient reported by different groups actually
varies over several orders of magnitude. The reason for
this is not yet resolved, although it is likely due to the
differences in the measurement approach. Some recent
studies (see Benziger and coworkers in the ‘Further
Reading’ section) suggest that the discrepancy is a result
of relatively slow membrane interfacial uptake of water.
Water concentration is usually higher at the cathode
because of water generation in the CL. The transport of
water from cathode to anode is termed back-diffusion, and
it can play an important role in maintaining a uniform
water distribution across the membrane during operation.
In an operating PEMFC, electro-osmotic drag can result
in anode dry-out, whereas the cathode tends to get flooded
as a result of the cumulative effect of electro-osmotic drag
and water generation. In that sense, back-diffusion of water
from cathode to anode serves to compensate the water loss
of the anode, and flatten the water activity profile across
the membrane, especially for thinner membranes.
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Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
Hydraulic permeability (pressure-driven flow)
Hydraulic permeation of water through the membrane
can occur as a result of a pressure difference between the
anode and the cathode, and can be described using
Darcy’s law:
k DPc"a
nw-hydraulic ðmol s Þ ¼
m Dx
"1
½9%
where k is the effective permeability of the membrane, m
is the liquid viscosity, Dx is the membrane thickness, and
DPc–a is the pressure difference between the cathode and
anode, which can be a gas-phase difference or a liquidto-liquid (capillary) pressure difference. The gas-phase
difference is typically small because the anode and
cathode pressures are usually similar, so this effect can be
ignored. In the liquid phase, however, a capillary pressure
difference between the anode and the cathode can result
in a net flux of water across the electrodes.
Thermo-osmosis (temperature-driven flow)
A fourth mode of transport in the membrane is thermoosmosis, which is a temperature-driven flow through the
membrane. This mode of transport has not commonly
been included in the analysis of normal operation, because this effect is obscured by the net diffusive and
electro-osmotic drag transfer, although it is a known
phenomenon in the polymer community. Under startup
or shutdown, the net water flux from this mode can be
significant. The thermo-osmotic diffusion coefficient has
been directly measured for a reinforced membrane in the
author’s laboratory, and is shown in Figure 8. The coefficient has been determined to be a function of
membrane type, preconditioning, and heat treatment, but
is always in the direction of cold to hot side for fuel cell
membranes.
Net Water Flux Coefficient
The net water flux through the membrane can be described as a combination of various modes of transport
explained earlier, and a net drag coefficient, ad, can be
defined to account for the cumulative effect of all
transport modes:
nw-net
a"c
¼ nw;e-o þ nw-hydraulic þ nw;diff þ nw;temp
nw-net
a"c
¼ ad
iA
F
Thermo-osmotic diffusivity, Log10 (|D|) (kg m−1Ks)
−7.2
−7.4
−7.6
−7.8
y = −997.75 × −4.7671
−8.2
0.002 6
R² = 0.9995
Reinforced membrane
0.002 8
0.003
½11%
In an ideal situation for water management, ad would be
uniformly " 0.5 (i.e., toward anode) along the electrode.
In this case, the water generated by the reaction is exactly
balanced by directing half of the generated water to the
anode side. In practice, for the thin (B15–25 mm)
membranes that are used in automotive applications, the
high back-diffusion to the anode nearly compensates for
the electro-osmotic drag overall, yielding zero or slightly
negative value of net drag. On the contrary, for thicker
membranes, which are generally preferred in stationary
power application, the overall net drag coefficient can be
slightly positive.
The assumption of uniform net drag within the fuel
cell is rarely justified. Figure 9 shows the measured net
drag coefficient distribution along the flow channel of a
PEMFC operating at either relatively dry anode or dry
cathode inlet conditions, with a thin 18-mm electrolyte
−7
−8
½10%
0.003 2
0.003 4
Inverse temperature (K−1)
Figure 8 Measured thermo-osmosis diffusion coefficient for 18-mm-thick dry reinforced membrane.
0.003 6
Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
835
2
Net drag coefficient (αd)
0
−2
Net drag for the full cell αd = − 0.116
−4
−6
−8
−10
−12
0.0
0.2
0.4
0.6
0.8
Fractional distance from the channel inlet
1.0
Figure 9 Measured local and net effective drag coefficients
along the gas flow channel of a proton-exchange membrane fuel
cell (PEMFC) operating at 0.7 V, with a dry anode inlet and
cathode inlet at 50% relative humidity (RH) at 95 1C. The net drag
is negative, indicating a net drag of water toward the dry anode.
Toward the exit, however, the local effective drag becomes
positive.
membrane. Even though the electrolyte is very thin, the
net drag coefficient is not near zero as a result of the
initial imbalance between anode and cathode humidity.
For this dry anode inlet case, the overall net drag coefficient is " 0.27, representing a net back-diffusion flow
toward the anode. However, after the initial 40% of the
fuel cell flow channel, the net flux is slightly positive,
toward the cathode, because the anode has become humidified, reducing the diffusion concentration gradient.
Water Transport in Porous Components
of Proton-exchange Membrane Fuel Cells
Water transport mechanism in the porous components of
PEMFCs, i.e., in the CL, microporous layer (MPL), and
DM, is a critical topic of interest. A description of the
porous components is summarized in this section.
Catalyst Layer
The CL is the heart of the PEMFC, producing power and
water, as well as much of the waste heat, especially at the
cathode. Catalyst layers in PEMFCs consist of a porous,
three-dimensional structure with a thickness of 5–30 mm
(see Figure 10). Owing to its vital role in facile transport
of ions, electrons, reactants, and products, the CL has a
highly porous structure (typical porosity 0.4–0.6). The CL
typically contains a considerable fraction of ionomer (up
to B30% by weight (wt)) to form the ionic pathways for
effective proton transport to or from the main PEM.
Some fraction of the pores may also contain hydrophobic
PTFE to promote local water removal and enhance the
reactant gas transport. Because of the existence of ionomer, catalyst, and PTFE coating, the CL consists of a
Figure 10 Transmission electron micrograph of a 40 wt%
platinum/chromium catalyst on carbon support. Reproduced with
permission from Thompsett D (2003) Pt alloys as oxygen
reduction catalysts. In: Vielstich W, Lamm A, and Gasteiger HA
(eds.) Handbook of Fuel Cells – Fundamentals, Technology and
Applications, vol. 3, ch. 37, pp. 467–480. New Jersey: John Wiley
& Sons Inc.
mixed pore network, having both hydrophobic and
hydrophilic surfaces and mixed ionic/electronic
conductivity.
Diffusion Media
The DM provides pathways for gas transport to the CL,
as well as enables transport of product water, electrons,
and excess heat of the reaction from the CL. The DM is
typically constructed from electrically conductive macroporous substrates with varying degrees of mixed
wettability, such as a nonwoven carbon paper, felt, or a
woven carbon cloth. All types of DM have complex internal structures with pore size ranging from a few microns to tens of microns. The porosities vary between 40
and 90%, and thicknesses between 90 and 500 mm.
Figure 11 shows the scanning electron microscopy
images of nonwoven carbon paper and a woven carbon
cloth. Cloth DMs are as flexible as any textile, whereas
paper DMs are fairly brittle because of the presence of
thermoset polymer resin, and can easily be broken under
strain. Felt DMs are in between paper and cloth DMs in
terms of flexibility. These two types of DM have different
836
Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
SE
22-Dec-04
CL01
WD 8.6 mm 20.0 kv
x100
500 µm
(a)
SE
22-Dec-04
CP01
WD 8.7 mm 20.6 kv
x100
500 µm
(b)
Figure 11 Scanning electron micrograph of nonwoven fiber carbon paper diffusion medium structure: (a) nonwoven paper and (b)
woven carbon cloth. Reproduced with permission from Mench MM (2008) Fuel Cell Engines. New Jersey: John Wiley & Sons Inc.
characteristics of spatial uniformity and degree of
anisotropy.
Because of the hydrophilic nature of these packed
carbon fibers, the carbon fiber substrates are typically
treated with a nonuniform coating of PTFE to achieve
the desired wettability for effective water removal. As a
result, the DM internal pore structure exhibits mixed
wettability characteristics, with different fractions of
hydrophobic and hydrophilic (B20–40%) and partially
hydrophobic/hydrophilic (mixed) pores, depending on
the PTFE additive amount.
Microporous Layer
A thin (5–20 mm thick), highly dense, and almost completely hydrophobic MPL with pore sizes ranging from
100 to 500 nm is commonly introduced between the DM
and CL for water management and electrical conductivity enhancement. There are two basic types of MPLs:
slurry based and polymer sheet type. The slurry-based
MPLs are generally coated directly to the catalyst side of
the DM surface, and consist of carbon particles (5–20%
of weight), polymeric binder, and PTFE (5–20% of
weight). The other type of MPL is a porous polymer
sheet bonded to the outer surface of the CL. The MPL
was originally designed to provide improved electrical
conductivity between the CL and DM, but has shown an
improved water management during PEMFC operation.
In addition, the MPL also functions to protect the CL
and membrane from carbon fiber intrusion damage from
the DM.
level of understanding is based on the application of
porous media theory from civil and petroleum engineering studies of flow of water or oil through packed soil
beds. The reader is referred to the ‘Further Reading’
section for this background material. Even though the
traditional approaches adopted from other disciplines
provide useful starting point, there are key differences
between the transport characteristics of common soils
and the fuel cell porous media that must be considered:
1. The transport length scale in fuel cell media is much
smaller. Therefore, the porous media in fuel cells (i.e.,
DM, MPL, and CL) has a very large surface area to
volume ratio, indicating the importance of interfacial
effects, which are not treated in the bulk flow theory
adopted from soil science.
2. Soil science literature has very limited treatment of
vaporization/condensation, which has a major impact
on low-temperature fuel cells.
3. Most soil science studies have been conducted with
hydrophilic media, whereas the PEMFC medium
typically has a highly heterogeneous surface energy
distribution.
4. Most soil science modeling/work is performed in
saturated domain (i.e., the pores of the soil are completely filled with fluid), whereas fuel cell porous
materials are nearly always only partially saturated.
5. The models and physics attempt to account for bulk
flow, while generally ignoring the orphan droplets
trapped in the isolated pores, which are likely to be
common in fuel cell media, as a result of condensation
and mixed wettability.
Modes of water transport in proton-exchange
membrane fuel cell porous media
To date, the science of multiphase flow through thin-film
mixed wettability porous media such as the CL and DM
is not yet well developed; therefore, much of the present
When fuel cell operations are considered, the liquid
water transport within the pores of the fuel cell porous
media can be driven by several forces:
Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
1. Capillary action : This is a result of a pressure difference
between the phases, and dominates for small pores.
2. Gravitational forces : Gravitational effects are typically
very small compared to the capillary forces because of
the small pore size, although gravity affects the flow of
liquid in the channels and manifolds.
3. Convective forces : The importance of this effect depends
on the flow field design. An interdigitated flow field is
designed to force flow through the DM, which can
assist in removing stored water.
4. Evaporation/condensation : These processes have a major
effect because of the low operating temperature of
PEMFCs.
5. Interfacial effects : The interfacial surface area and
morphology within the different layers of fuel cell porous
media have a substantial effect on the accumulation and
storage of liquid water. Of these considerations, the
interfacial transport and phase-change-related flow are
the least studied, and considerable uncertainty still exists.
Capillary Pressure-Driven Flow
The most important relationship that must be established
for an accurate prediction of the liquid–gas capillary
transport is the relationship between the capillary pressure (PC) and the liquid saturation (sl). In the cathode,
gas-phase pressure generally remains constant because of
the low viscosity; therefore,
PC ¼ Pl " Pg
-
-
where DP C EDP l
½12%
Several empirical and semiempirical expressions are
available in soil science literature. As many porous media
share similar characteristic behavior, for PEMFC porous
materials, a generic Leverett approach from soil science
has been commonly used to describe the capillary
transport behavior of the fuel cell porous media. This
semiempirical correlation relates the capillary pressure
and saturation data for clean unconsolidated sands of
various permeability and porosity by means of defining a
dimensionless capillary pressure function:
'e(1=2
PC ¼ g cos y
J ðsÞ
k
½13%
where k, e, and y are permeability, porosity of the porous
media, and a representative contact angle, respectively.
J(s) represents the Leverett J-function for scaling
drainage capillary pressure curves.
8
2
3
>
>
> 1:417ð1 " s1 Þ " 2:120ð1 " s1 Þ þ1:263ð1 " s1 Þ
>
< if yo901 ) hydrophilic
½14%
J ð s1 Þ ¼
>
1:417s1 " 2:120s12 þ 1:263sl3
>
>
>
: if y > 901 ) hydrophilic
837
Although this Leverett approach serves as a useful
starting point, the applicability of the generic Leverett
approach function to the highly anisotropic thin-film
porous materials such as in PEMFCs has been challenged. In particular, the definitions of wetting and
nonwetting phases used to determine capillary pressure
and surface tension angle are taken as a statistical average
over the entire medium, obscuring local effects, which
differ from the whole. Because the droplet/bubble sizes
are on the same order of magnitude as the DM thickness,
the complex bimodal (hydrophobic and hydrophilic) pore
size distribution, and droplet eruption from the DM
surface has been observed to be a highly localized discrete event, a volume-averaged approach such as this
may not be appropriate at all.
Several recent studies have presented modified versions of this relationship (see the ‘Further Reading’ section). One such approach derived from the direct
capillary pressure–saturation measurements of different
paper-based DMs tailored with PTFE content ranging
from 0% to 20% of weight at different operating conditions is shown as
6
PC ¼ ð293=T Þ (gðT Þ )
|fflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflffl}
Temperature effect
rffiffiffiffi
ec
2
k
|fflfflfflffl {zfflfflfflffl }
0:4C
Compression effect
)
K ðsnw Þ
|fflffl{zfflffl }
½15%
Mixed wettability
where C, ec, k, g, and T represent the compression pressure, compressed porosity, absolute permeability, surface
tension, and temperature, respectively; K(snw) represents
a modified function:
2
K ðsnw Þ ¼ ð%wtÞ ) I0:0469 " 0:00152 ) ð%wtÞ " 0:0406 ) snw
3
þ 0:143 ) snw
m þ 0:0561 ) lnsnw
for 0osnw r0:50
½16%
where %wt and snw represent PTFE weight percentage
of the DM and nonwetting liquid saturation, respectively.
A similar approach for the CL has not yet been developed. It should be noted that this relationship covers
capillary transport in the hydrophobic regions of the DM
only, whereas liquid water will tend to first accumulate in
hydrophilic regions. This residual water in hydrophilic
pores can be significant (up to 20% or more), and is
removed only by evaporative or convective forces. The
MPL is almost completely hydrophobic, and capillary
pressures are so high because of the small pore size in
these regions that the MPL in any practical situation will
have almost no saturation of water. This useful feature
provides an open pathway for gas transport across the
DM–CL interface, which is one of the main performance-enhancing features of the MPL. Modeling approaches to flow in the DM and the CL have included
Leverett-based average approaches modeling capillary
flow in the hydrophobic pores only, with hydrophilic
838
Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
regions represented by an arbitrarily chosen irreducible
saturation of around 10%. Other approaches that attempt
to realistically follow the fluid interface involve percolation-based pore level models, again borrowed from soil
and oil recovery science. A powerful new approach that
uses the lattice Boltzmann method to simulate the
multiphase flow is also under development. The reader is
referred to the ‘Further Reading’ section for more details.
To date, however, modeling approaches have not yet fully
resolved experimental observations and further work is
needed.
Phase-Change-Driven Flow
A largely ignored but important mode of transport in the
porous media is that related to phase change. Although
assumptions of thermodynamic equilibrium are useful for
modeling purposes, in reality, small local changes in
surface energy and temperature result in the potential for
significant amounts of water transport through phasechange-related mechanisms. As the DM provide the
dominating thermal resistance in the cell, in general, the
largest temperature gradient exists in this material, which
can amount to >5 1C during operation. As water is produced in the CL, it will condense at locations with
suitable surface conditions and temperatures, potentially
in the CL or in the DM. Surface impurities, roughness,
and hydrophobicity affect the nucleation site condensation rate and temperature.
Another mode of transport that is beginning to be
understood is the so-called heat-pipe effect. Because the
channels are a cooler location than the CL, condensation
of water on the channel walls can cause local desaturation
of the gas phase, drawing more water out of the liquid
phase, and thus forming a continuous transport pathway
from the liquid-saturated regions to the condensation
plane, as illustrated in Figure 12. This effect is particularly relevant during shutdown and startup, where
significant plate-to-plate variation in temperature can
exist. During cooling, even small gradients have been
shown to draw a very significant amount of water from
the porous media into the colder channel locations. This
effect should be delineated from membrane thermo-osmosis, which has been shown to occur from cold to warm
locations in sulfonated fluoropolymer membranes. Flow
is exclusively from the hot to cold locations with the
heat-pipe effect. Figure 13 shows a side-on view neutron
imaging picture of flow in a fuel cell initially with liquid
on the left side of the image, and a hydrophobic DM with
MPL. When there is no temperature gradient, there is no
flow from the liquid- to gas-phase sides. However, when a
modest temperature gradient is introduced, as shown,
liquid flow moves rather quickly to the colder location.
This effect has been quantified for a typical PEM
membrane and a 5% wet-proofed SGL 10BB DM with
MPL material set, and typically dominates the modest
thermo-osmotic flow in the opposite direction. The rate
of transport by this mode is a function of absolute temperature, temperature gradient, and material set, and has
recently been shown by the author’s laboratory to be well
fit using an Arrhenius expression to relate transport rate
to the temperature gradient and average temperature.
Interfacial Effects
Perhaps the least understood fundamental issue regarding
water transport is the role of the various interfaces. As
discussed, the normal and perpendicular land interfaces
with the DM have been shown to play a key role in water
removal from under the lands of the DM. The membrane–CL interface is a complex three-dimensional
structure that changes with water content as a result of
ionomer swelling. Most models deal with the CL only as
an infinitely thin layer, and attempts at resolving the CL
generally assume homogeneous properties with empirical
relationships. The DM–CL interface is also of particular
Water transport due to
capillary action
Evaporation
Vapor
diffusion
Condensation
Figure 12 Illustration of heat-pipe effect between diffusion media (DMs) and flow channel.
Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
relevance because there is little known and experimental
visualization tools have yet to achieve the resolution required to investigate these ultrathin regions. For an
unbonded DM, delamination as a result of freeze–thaw or
mechanical stresses has been shown to occur in extreme
circumstances (see Figure 14), which can lead to severe
performance degradation. The larger relative gap size
Hot (70 °C)
side – initially
liquid filled
Colder (60 °C)
side – initially
dry
839
created serves as a local pooling location for water because of the low relative capillary pressure, which can
partially flood the electrode. Additionally, the current
redistribution required by the lack of physical contact will
result in increased ohmic losses. Because of this, as well as
manufacturing assembly concerns, it is common to hotpress the DM onto the CL to create an intimate bond. In
this case, the interfacial behavior is even less understood.
For a bonded MEA, ionomer and catalyst are squeezed
into the gaps along the MPL–CL interface during the hot
press, creating a complex structure. During operation, the
morphology at this interface will change as a result of
swelling and dry-out, and may alter the transport at this
interface in ways not yet understood.
Modern View of Transport and Flooding in
the Porous Media
1 min:
Cold side
is starting
to fill.
4 min:
Cold side is
completely
filled.
As discussed in Introduction, performance reduction from
flooding can be a result of (1) channel, (2) DM, (3) CL, or
(4) interfacial water accumulation at discrete locations
within the fuel cell. The exact nature of the liquid water
structure in the DM and CL is not precisely known, but it
is generally believed from capillary theory that the liquid
water flows through large hydrophobic and hydrophilic
pores, and the gas phase flows through small hydrophobic
pores. However, evaporation and condensation are also
important, because a unified understanding based on capillary pressure arguments alone has been shown to be
insufficient.
Description of Channel Flooding
Figure 13 Side-view image of water transport by heat-pipe
effect taken with neutron imaging. In 4 min, the initially gas-filled
colder flow channels are completely filled by heat-pipe flow from
the liquid-filled warmer side channels.
Channel level flooding is commonly observed under low
stoichiometry or low current conditions, when the stack
temperature is relatively cool and the channel flow rates
are insufficient to clear slug accumulations. Considering
that most fuel cell stacks contain more than 100 plates
mounted in parallel hydraulically (generally in series
electrically), any slight perturbation in the pressure drop
Figure 14 Scanning electron micrograph of interfacial delamination caused by freeze–thaw cycling in a proton-exchange membrane
fuel cell (PEMFC).
840
Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
across a single cell can result in a drastic reduction of
flow from the manifold, further exacerbating the flooding
problem and eventually resulting in parasitic and potentially damaging voltage reversal in the flooded cell
from reactant starvation.
In the channels and manifolds, gravity can assist in
draining the excess liquid, depending on cell orientation.
In general, any gravity-unfavorable design features of the
flow field are to be avoided (see Figure 15), for example,
any wells that would tend to accumulate water. Stack
orientation and flow field design are generally arranged
so that the natural progression of droplets in the channels
is at worst neutral and at best assisted by gravitational
forces. In neutral or unfavorable orientations, excess liquid water removal is dominated by drag forces. As the
DM becomes saturated with liquid water, droplets form
at discrete locations along the DM surface and increase
in size, eventually turning into water slugs in the flow
channel. On the anode side, channel blockage can cause
voltage reduction and fuel starvation for the CL, leading
to the oxidation of carbon support and accelerated degradation. Surface conditions of the channel can also
Neutral zone
Favorable zone
(gravity effect)
Pooling location
Neutral zone
Figure 15 Illustration of favorable, neutral, and unfavorable
channel configurations based on gravity and local pooling.
influence the water management. Although it would seem
logical that a hydrophobic surface would be favorable for
water removal, testing has revealed that flooding is worse
and performance less stable with hydrophobic channel
walls. The increased flooding is believed to be a result of
restraining capillary force projected from the land–
channel interface into the DM that prevents regular
drainage into the channel. The increased instability is a
result of droplet accumulation that causes sporadic
pressure variation, rather than the smoothly varying
pressure change resulting from a liquid film buildup
along a hydrophilic channel wall, as shown in Figure 16.
Description of Diffusion Media Flooding
Diffusion media flooding can occur as a result of excess
water accumulation by condensation or through capillary
introduction from the CL as a result of insufficient
channel gas flow rate. Diffusion media flooding occurs
most commonly under high current conditions. Pure
capillary flow will follow the path of least resistance, first
moving to and filling connected hydrophilic pores. Thus,
the PTFE additive in the DM (typically 5% or more by
wt) is essential to avoid DM flooding. At the CL where
water is generated, heat is also generated, and the resulting flux of water can condense either within the CL
or within the MPL–DM. By condensation, water can be
introduced into the hydrophobic areas of the DM and
MPL without first filling all hydrophilic pores. Neutron
imaging has revealed that liquid accumulation under
lands is common, which tend to be much colder than
channel sections because of enhanced heat transfer.
Connected liquid in hydrophobic pores will build up a
positive capillary pressure favoring expulsion to lower
Flow channel
DM
Flow channel
DM
Hydrophilic channel walls
Hydrophobic channel walls
Figure 16 Illustration of water droplet distribution inside flow channels having different wettability characteristics. DM, diffusion
medium.
Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
pressure areas such as hydrophilic sections or the flow
channel. Because the MPL is much more hydrophobic
than any other component, water is forced toward the
channels from these locations.
Channel-to-land ratio and land–channel interfacial
properties have also been shown to be important. The
channel-to-land ratio is critical because the heat transfer
profile between the DM and the channel or the lands is
very different. Convection into the channels is significantly less efficient than direct conduction into the lands,
so locally cool regions that tend to accumulate liquid
water typically exist under the lands. As the water accumulates under the land, it is pushed by capillary forces
laterally under the channels, where it erupts into the
channel along the interface between the land and the
channel. There is some discrepancy in the literature
between the particular distribution and mode of motion
and rejection within the DM. One conceptual model
assumes that the liquid water in the DM saturates in a
tree-like structure, with large connected channels soaked
with water and smaller branches emanating from the
main structures. More recently, an alternate view that
explains the experimentally observed presence of droplets that erupt from the DM and are removed, or erupt
and recede on the DM surface, assumes that the liquid
water in the DM follows a fingering pathway, where
drainage of one finger results in the recession of other
fingers. Recent experimental observations more strongly
support the eruptive model.
Assembly compression and operating temperature
also play a major role in the DM flooding. For instance,
the DM placed in a fuel cell stack assembly is exposed
to nonuniform compression, which in turn can generate
strong local stresses that can alter the morphological
structure and consequently the multiphase transport
characteristics of the DM. It has been established that
for a given thin-film fuel cell DM, any compression is
followed by a decrease in porosity and an increase in
tortuosity, and some studies have suggested that permanent damage to stiff papers results in a local decrease
in effective hydrophobicity, which results in the experimentally observed behavior of sporadic liquid water
eruption along the channel–land interface. In addition,
the discontinuity of the surface contact area at the DM–
flow channel interface creates inhomogeneous compression distributions, yielding substantial changes in
local physical properties of the DM. The portion of the
DM in contact with the landings is subjected to higher
compression, whereas the portion under the flow
channel experiences less compression and tends to intrude into flow channels, thus resulting in increased
reactant flow field pressure drop. The axial variation
of the compression in the cell assembly yields
discrete regions that have different capillary transport
characteristics.
841
Description of Catalyst Layer Flooding
Catalyst layer flooding can occur as a result of liquid
condensation and pore filling or localized film formation,
resulting in a significant decrease in the diffusion of
reactant gases to the catalyst sites. Two common approaches to reduce the catalyst flooding are (1) to
introduce PTFE additive to the CL and (2) to use an
MPL.
Compared to the extensive efforts to understand the
role of DMs and flow channels, few fundamental studies
have been performed to analyze the flooding mechanisms
in the cathode CL of a PEMFC. In most computational
models, the CL is treated as an infinitesimally thin
interface, even though it acts as a vital component for the
conversion of liquid water to vapor, regulating the water
flux across the entire porous electrode of a PEMFC.
Some studies have shown that the location of liquid
flooding between the CL and DM can be controlled by
PTFE content. A more hydrophilic electrode compared
to the DM results in predominantly CL flooding.
Description of Interfacial Flooding
At equilibrium, a pressure balance in the gas and liquid
phases across interfaces must exist. Therefore, for a
perfect interface, we can predict the water distribution
and flooding locations depending on the hydrophobicity
of the mating surfaces and the pore size distribution.
Figure 17 shows the liquid water saturation distribution
in the porous media assuming an isothermal condition
with no phase change and hydrophobic surfaces at steady
state. In steady state, there is a flow of condensed water
from the catalyst to the channel and out of the fuel cell as
SGDL
SMPL
iA
at SS
2F
iA
2F
SW
0
CL
MPL
GDL
Channel
Figure 17 Illustration of the typical water distribution in the fuel
cell porous media under steady state (SS), after sufficient time to
allow equilibrium in the porous media between stored and flowing
liquid. The discontinuities in the saturation level are a result of the
changing pore size and hydrophobicity between the different
layers. Hydrophobic layers are shown. CL, catalyst layer; GDL,
gas diffusion layer; MPL, microporous layer. Reproduced with
permission from Mench MM (2008) Fuel Cell Engines. New
Jersey: John Wiley & Sons Inc.
842
Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
(a) Top view
(b) Side view
Figure 18 Scanning electron micrographs of proton-exchange membrane fuel cell (PEMFC) catalyst layer (CL) with relatively largescale macro cracking.
slugs. Discontinuities in the liquid saturation as a result
of pore size variations result in a saturation jump for the
same capillary pressure. At each interface, the liquidphase capillary pressure is balanced (in steady state).
Although this approach is appropriate for perfectly
mated surfaces, in reality, there is a distribution of pores
in the DM and MPLs, and the CL or MPL can have
significantly large cracks and gaps between interfaces in
the catalyst surface, as shown in Figure 18. Thus, it is
unlikely that such abrupt discontinuities in saturation, as
shown in Figure 17, really exist. Catalyst layer surface
cracking is typically present from manufacture and is a
result of the presence of volatile compounds in the
catalyst slurry or the manufacturing process. From a
multiphase flow perspective, this situation is very different from a continuous homogeneous phase, as commonly modeled, and some larger gaps and cracks may
dominate flow physics in these regions. These cracks and
gaps are orders of magnitude larger (they can be up to
B10 mm wide) than the normal pore size in the CL.
Thus, these cracks are regions of reduced capillary
pressure that promote local liquid pooling. In terms of
gas-phase transport, these macro cracks may enhance
reactant transport to catalyst regions by reducing flow
resistance. These cracks increase the effective CL porosity, enabling high reactant species flux to the catalyst
surface.
In PEMFCs, the interface between the channel, land,
and DM, and the interface between the DM and CL can
also affect transport of liquid. If there is a gap, this can
serve as a pooling location for water. Neutron imaging
has also been used to confirm the important role of the
land interface in storing the liquid water at equilibrium.
For fuel cells with otherwise identical operating conditions, the greater the number of hydrophilic land
interfaces, the lesser the liquid water content in the DM.
On the contrary, hydrophobic land surfaces have been
shown to retain water under the lands and promote
flooding by restricting drainage from the DM, as
discussed.
Overall View of Flooding
Figure 19 is an illustration of the typical water distribution in the fuel cell porous media under the lands and
channels. As the coldest location during operation is
generally under the lands, water vapor tends to condense
in these cold spots. As the saturation increases, water
pushes out laterally from under the lands and either
erupts along the channel–land interface or forms connections between the lands under the channels, resulting
in DM flooding and performance degradation. The removal of water from under the lands into the channels is
important to avoid flooding, because lateral connectivity
between the water under adjacent lands in the DM can
induce severe performance loss through reactant blockage. Thus, larger channel-to-land width ratios provide
enhanced resistance to flooding. Besides under the lands,
there are other locations in a fuel cell where liquid water
tends to accumulate. These locations have been identified
primarily using neutron imaging, which enables a direct
nonintrusive quantification of the liquid water content in
the operating fuel cell and is used by several research
institutions for this purpose.
Liquid accumulation also commonly occurs around
channel switchbacks, as shown in Figure 20. This is a
result of flow recirculation, stagnation, and pressure drop
at locations of sudden momentum reversal. Additionally,
the local flow separation near the corner accelerates the
core flow, promoting annular flow of liquid water. Interestingly, even in very dry operating conditions, accumulation of liquid water under the lands has been
observed, as shown in Figure 20, implying that effective
Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
Land
843
Land
DM
Figure 19 Illustration of the typical liquid water accumulation behavior under the lands and channels in a proton-exchange membrane
fuel cell (PEMFC). DM, diffusion medium.
change and temperature gradients. Under current, the
polarization losses will generate a majority of the heat
dissipated by the fuel cell. As most of the entropy change
and activation polarization are generated at the cathode,
this is typically the hottest location in the fuel cell, up to
5 1C or more under high current depending on the
thermal properties of the DM. At the cathode CL, vaporphase water will be transported into the electrolyte and
back to the anode or outward to the cathode flow channel.
As the temperature cools down, a saturation plane will
develop in a location preferably where liquid water
condensation occurs. The location of condensation will
have a tremendous effect on the flooding. For the mixed
wettability CL, the condensation would tend to take
place in the predominantly hydrophilic pores first,
flooding them, while the hydrophobic pores remain
mostly liquid free. If the condensation plane is beyond
the CL and in the microporous or diffusion layer, the
highly hydrophobic nature of the MPL will prevent
backflow into the CL.
Figure 20 Neutron radiograph showing liquid water
accumulation along the corners of a 1801 switchback in an
operating proton-exchange membrane fuel cell (PEMFC). Water
is often observed to preferentially accumulate at switchback
locations and along the channel walls.
conduction heat transfer occurs through the lands, and
access to the channel is blocked.
While isothermal conditions may be true at low current density, higher current densities include phase
Overall Role of Materials in Water
Management and Flooding
On the basis of the current understanding of the gas- and
liquid-phase transport in the fuel cell, a unified view of
the role of the various porous media can be constructed,
and is shown in Figure 21. Because of the lack of direct
experimental observation of minute, small MPLs and
CLs, there exist several different theories to resolve the
844
Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
DM
MPL
MPL
Membrane
H2 in
DM
Liquid
H2O out
Capillary
flow
from MPL
Restriction
of H2O
vapor out
O2 in
Cathode
Anode
CL
CL
Potential condensation
plane
Figure 21 Overall schematic showing the roles of the different porous media in a proton-exchange membrane fuel cell (PEMFC).
CL, catalyst layer; DM, diffusion medium; MPL, microporous layer.
observed influences of the material properties on fuel cell
performance.
Microporous Layer
For the same interfacial liquid pressure and hydrophobicity, the media with the smallest hydrophobic pores
(CL) will have the lowest liquid saturation. If there were
similar levels of hydrophobicity in each layer, the capillary pressure gradient will drive the liquid flow from the
CL through the MPL and DM into the flow channel.
Acknowledging the relatively smaller pores and more
hydrophobic pore structure of the MPL, the MPL can act
as a barrier for liquid flow unless the breakthrough
pressure is reached. Typically, MPLs have a breakthrough
pressure around 5–10 kPa. Some studies have shown that
the MPL can act to force liquid water toward the anode,
preventing dry-out. This can happen once the CL is
completely flooded and cannot store more water to
overcome the breakthrough pressure of the cathode MPL.
Cathode-Side Microporous Layer
An MPL on the cathode side has been experimentally
determined to enhance performance under high current
density and high humidity conditions, where CL and DM
flooding is prevalent. As the performance in this situation
is generally limited by the flooding and not the gas-phase
oxygen transport, the additional diffusion resistance of
the MPL does not significantly reduce performance.
Several experiments have demonstrated that the DM
coated with MPL is observed to have a more uniform
water distribution in the MEA. Existence of such a fine
layer is found to prevent drying out of the membrane and
reduce the flooding of the CL.
The wetting characteristics of the MPL are found to
cause a discontinuity in the liquid saturation at the
MPL–DM interface, yielding a reduction in the amount
of liquid water in the cathode by directing the flow to the
anode side. It is believed that the MPL serves as a highly
hydrophobic boundary to prevent water accumulation,
forcing water back to the anode. This back-diffusion of
water is observed to be improved by increasing MPL
thickness, rendering the MPL more hydrophobic, and
decreasing the pore size and bulk porosity of the MPL.
The backflow of water would occur primarily through
the hydrophilic pore network in the CL, because complete pore saturation in the CL would result in nearly
total performance loss, and high saturation in the
hydrophobic pores of the CL would likely overcome the
breakthrough pressure of the MPL, resulting in sporadic
liquid slug emission, a phenomenon that has been observed to occur experimentally. However, with no MPL
Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
case, small water droplets will appear to emerge from the
CL in preferential locations and move into a macroporous DM, pooling the liquid into this location.
Anode-Side Microporous Layer
The role of the MPL on the anode side is completely
different from that on the cathode side, although use of
an MPL on the anode side has also been observed to
enhance performance in low-humidity environments
where the water in the anode ionomer can be easily lost
to the dry hydrogen stream. The mass diffusivity of water
vapor into hydrogen is up to four times greater than that
into air. Thus, combined with electro-osmotic drag of
water from the anode to the cathode, local anode dry-out
is a common reason for low performance and reduced
longevity in low-humidity conditions. The use of an
anode MPL limits the moisture removal from the anode
by acting as a diffusion barrier, reducing this dry-out
effect. Under high-humidity conditions, the use of an
anode MPL does not normally have much effect on
performance. At high-humidity conditions, anode MPL
should not have a significant effect on the overall net drag
coefficient, which has been validated experimentally.
Mechanically, the MPL has also been suggested to serve
to protect the electrolyte from puncture by protruding
fibers from the macroporous layer.
Diffusion Media: Anode and Cathode Sides
In terms of optimizing gas-phase transport in the macrodiffusion layers or the MPLs, there is a key engineering
trade-off that needs to be carefully considered. Obviously,
high diffusivity of reactant to the catalyst is desired to
promote reaction and limit concentration polarization.
However, high moisture in the electrolyte is also desired to
achieve high proton conductivity. High-temperature and
low-humidity conditions appear to simplify the system
design in that regard, but can lead to anode dry-out with
accelerated degradation and poor performance. For high
reactant diffusivity, liquid saturation must be minimized,
and there must be a high hydrophobic porosity. On the
cathode side, oxygen transport is already limited by a low
initial mole fraction, high water saturation, and reduced
diffusivity coefficient, compared to hydrogen. Here, the
focus is generally on prevention of flooding. On the anode,
however, water vapor diffusivity loss into hydrogen can be
severe and there is very little concentration polarization
limitation at the anode, and hence a flow-restricting
structure is preferred. The dominating role of the DM on
the cathode side depends on the operating conditions.
Under high-current conditions, the oxygen transfer to the
electrode is limiting performance, and an open hydrophobic structure promoting gas-phase transport with good
liquid water removal is necessary. In low-humidity environments, the lack of moisture dominates, and a closed-
845
pore, less hydrophobic structure is needed to restrict vapor
loss to the flow channel.
From an overall porous media design perspective, the
various porous media should be tailored to achieve the
desired liquid- and gas-phase transport behavior. In
the examples mentioned, and in most common materials,
the properties are mostly uniform. However, the potential
for enhanced vapor and liquid flow with hydrophobicity or
pore size gradients exists and has been exploited in some
specialized materials. Overall, different MEA configurations and materials are preferred by different manufacturers, and the exact nature of transport in these regions
is not yet perfectly understood. Complicating factors that
must be considered include the tightly coupled heat
transport phenomena, nonisotropic material transport
properties, and highly nonhomogeneous current density
along the electrode. Because phase change plays a key role,
the thermal conductivity is another design parameter of
interest for designing these media. Other factors include
assembly sag into gas channels, which has been linked to
flow maldistribution. Thus, a stiff DM is generally preferred for the case of assembly and prevention of interfacial gaps.
Unresolved Issues and Diagnostic Needs
Although much progress has been made to understand,
measure, and quantify the liquid water distribution and
transport in the PEMFC, there are still many unresolved
issues and diagnostic needs. There are still some questions as to the nature of the flow in the DM between a
tree-like and fingering model, although this is beginning
to be clarified with the use of experimental diagnostics
such as neutron imaging, synchrotron radiography, and
fluorescence microscopy. Although the nature of the
multiphase flow in the DM is fairly well described now,
there is still a significant amount of fundamental research
required to link the characteristics of the DM and the
microfluidic flow in it to the performance of the fuel cell
and flooding mechanism in a meaningful way. Other
major remaining gaps in fundamental understanding involving water transport include
1. the nature and description of the flow in the CL,
2. the effect of the DM–CL interfacial structure on
transport on multiphase and electrical transport and
performance,
3. the effect of the PEM–CL interfacial structure on
transport on multiphase and ionic transport and
performance,
4. the effect of ionomer swelling in the CL on durability
and transport,
5. natural and forced liquid drainage from the porous
media, and
846
Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
6. the nature of condensation and evaporation and related transport in the fuel cell.
Most of the required understanding above is limited by
diagnostic capability. There is not yet an established
experimental tool capable of the micron-level (or lower)
resolution required to actually visualize much of the
phenomena listed. Newly developed advanced models
are being published almost everyday covering different
aspects and approaches of multiphase flow in PEMFC,
but even if fully developed and resolved, the experimental capabilities required to validate these models do
not yet exist. The truly grand challenge remaining in
microfluidic study of PEMFCs is to develop an experimental approach to visualize accurately multiphase flow
in the CL, MPL, DM, and along the interfaces. Synchrotron radiography (X-ray-based imaging) has shown
greater resolution than neutron imaging, approaching
5 mm, but has limited field of view. Neutron imaging can
have a field of view large enough to image a full-size fuel
cell plate, but is limited in resolution to around 10 mm.
Following development of a new imaging system capable
of better resolution, or further improvement of existing
approaches, more accurate models can be developed that
can be used to truly link the salient features with performance and durability, which is the ultimate goal of
these models.
P
Pl
PC
Pc – a
Pg
Psat
RH
SGDL
sl
SMPL
snw
SW
T
x
yv
pressure (Pa)
liquid pressure
capillary pressure; compression
pressure
pressure difference between the
cathode and anode
gas pressure
saturation pressure
relative humidity (dimensionless)
gas diffusion layer surface
liquid saturation
microporous layer surface
nonwetting liquid saturation
water surface
temperature (K)
distance (m)
mole fraction of water vapor
(dimensionless)
drag coefficient
surface tension (N m " 1)
thickness of the membrane
porosity (dimensionless)
compressed porosity
contact angle (degree)
water content (dimensionless)
viscosity (Pa s)
electro-osmotic drag
conductivity (S cm " 1)
specific conductivity
Nomenclature
ad
c
Dx
e
ec
h
k
l
n
r
ri
Symbols and Units
Abbreviations and Acronyms
a
A
C
Cc – a
CL
DM
GDL
MEA
MPL
PEMFC
D
Dw
F
i
J(s)
k
K(s)
K(snw)
n
nw, diff
nw;e" o
nw – hydraulic
nw – net
nw,temp
a–c
water activity
geometric surface area (cm2)
concentration (mol s " 1)
concentration from cathode to anode
(mol s " 1)
diffusivity (cm2 s " 1)
diffusivity of water (cm2 s " 1)
Faraday constant
current density (A cm " 2)
Leverett function
permeability (m2)
modified Leverett function
empirical Leverett function
molar rate (mol s " 1)
molar rate of water transport by
diffusion (mol s " 1)
molar rate of water transport by electroosmotic drag (mol s " 1)
molar rate of water transport by
hydraulic permeability (mol s " 1)
net molar rate of water transport from
anode to cathode (mol s " 1)
molar rate of water transport by
thermo-osmosis (mol s " 1)
PEM
PTFE
PTL
RH
SS
catalyst layer
diffusion medium
gas diffusion layer
membrane–electrode assembly
microporous layer
proton-exchange membrane
fuel cell
polymer electrolyte membrane
polytetrafluoroethylene
porous transport layer
relative humidity
steady state
See also: Applications – Transportation: Electric
Vehicles: Fuel Cells; Fuel Cells – Overview: Lifetime
Prediction; Fuel Cells – Proton-Exchange Membrane
Fuel Cells: Catalysts: Life-Limiting Considerations;
Dynamic Operational Conditions; Freeze Operational
Conditions; High Temperature PEMFCs; Membrane: LifeLimiting Considerations; Membranes; Membranes:
Ambient Temperature; Membranes: Design and
Characterization; Membranes: Elevated Temperature;
Fuel Cells – Proton-Exchange Membrane Fuel Cells | Water Management
Modeling; Overview Performance and Operational
Conditions; Systems.
Further Reading
Atiyeh HK, Karan K, Peppley B, Phoenix A, Halliop E, and Pharoah J
(2007) Experimental investigation of the role of a microporous layer
on the water transport and performance of a PEM fuel cell. Journal
of Power Sources 170: 111.
Bazylak A, Sinton D, Liu ZS, and Djilali N (2007) Effect of compression
on liquid water transport and microstructure of PEMFC gas diffusion
layers. Journal of Power Sources 163: 784.
Benziger J, Chia JE, Kimball E, and Kevrekidis IG (2007) Reaction
dynamics in a parallel flow channel PEM fuel cell. Journal of the
Electrochemical Society 154: B835.
Doyle M and Rajendran G (2003) Perfluorinated membranes.
In: Vielstich W, Lamm A, and Gasteiger HA (eds.) Handbook of Fuel
Cells – Fundamentals Technology and Applications, vol. 3, ch. 30,
pp. 351--395. New Jersey: John Wiley & Sons Inc.
Gostick JT, Fowler MW, Ioannidis MA, Pritzker MD, Volfkovich YM, and
Sakars A (2006) Capillary pressure and hydrophilic porosity in gas
diffusion layers for polymer electrolyte fuel cells. Journal of Power
Sources 156: 375.
He S, Kim SH, and Mench MM (2007) 1D transient model for frost
heave in polymer electrolyte fuel cells. Journal of the Electrochemical
Society 154: B1024.
Kowal JJ, Turhan A, Heller K, Brenizer JS, and Mench MM (2006)
Liquid water storage, distribution, and removal from diffusion media
in PEMFCs. Journal of the Electrochemical Society 153: A1971.
Kumbur EC, Sharp KV, and Mench MM (2006) Liquid droplet behavior
and instability in a polymer electrolyte fuel cell flow channel. Journal
of Power Sources 168: 356.
Kumbur EC, Sharp KV, and Mench MM (2007) Validated Leverett
approach for multiphase flow in PEMFC diffusion media. Journal of
the Electrochemical Society 154: B1315.
Kusoglu A, Karlsson AM, Santare MH, Cleghorn S, and Johnson WB
(2007) Mechanical behavior of fuel cell membranes under humidity
cycles and effect of swelling anisotropy on the fatigue stresses.
Journal of Power Sources 170: 345.
Lin G and Nguyen TV (2006) A two-dimensional two-phase model of a
PEM fuel cell. Journal of the Electrochemical Society 153: A372.
Litster S, Sinton D, and Djilali N (2006) Ex-situ visualization of liquid
water transport in PEM fuel cell gas diffusion layers. Journal of Power
Sources 154: 95.
847
Liu J and Eikerling M (2008) Model of cathode catalyst layers for
polymer electrolyte fuel cells: The role of porous structure and water
accumulation. Electrochimica Acta 53: 4435.
Manke I, Hartnig C, Gr|nerbel M, et al. (2007) Investigation of water
evolution and transport in fuel cells with high resolution synchrotron
X-ray radiography. Applied Physics Letters 90: 174105.
Mauritz AK and Moore RB (2004) State of understanding of Nafion.
Chemical Reviews 104: 4535.
Mench MM (2008) Fuel Cell Engines. New Jersey: John Wiley & Sons
Inc.
Montupally S, Becker AJ, and Weidner JM (2000) Diffusion of water in
Nafion 115 membranes. Journal of the Electrochemical Society 147:
3171.
Okada T (2003) Effect of ionic contaminants. In: Vielstich W, Lamm A,
and Gasteiger HA (eds.) Handbook of Fuel Cells – Fundamentals,
Technology and Applications, vol. 3, ch. 48, pp. 627--646. New
Jersey: John Wiley & Sons Inc.
Park J and Li X (2008) Multi-phase micro-scale flow simulation in the
electrodes of a PEM fuel cell by lattice Boltzmann method. Journal of
Power Sources 178: 248.
Pasaogullari U and Wang C-Y (2004) Two-phase transport and the role
of micro-porous layer in polymer electrolyte fuel cells. Electrochimica
Acta 49: 4359.
Satija R, Jacobson DL, Arif M, and Werner SA (2004) In situ neutron
imaging technique for evaluation of water management systems in
operating PEM fuel cells. Journal of Power Sources 129: 238.
Spernjak D, Prasad A, and Advani SG (2007) Experimental investigation
of liquid water formation and transport in a transparent singleserpentine PEM fuel cell. Journal of Power Sources 170: 334.
Springer TE, Zawodzinski TA, and Gottesfeld S (1991) Polymer
electrolyte fuel cell model. Journal of the Electrochemical Society
138: 2334.
Thompsett D (2003) Pt alloys as oxygen reduction catalysts.
In: Vielstich W, Lamm A, and Gasteiger HA (eds.) Handbook of Fuel
Cells – Fundamentals, Technology and Applications, vol. 3, ch. 37,
pp. 467--480. New Jersey: John Wiley & Sons Inc.
Wang C-Y (2004) Fundamental models for fuel cell engineering.
Chemical Reviews 104: 4727.
Weber AZ and Newman J (2004) Modeling transport in polymer
electrolyte fuel cells. Chemical Reviews 104: 4679.
Zaffou R, Yi JS, Kunz HR, and Fenton JM (2006) Temperature-driven
water transport through membrane electrode assembly of proton
exchange membrane fuel cells. Electrochemical and Solid State
Letters 9: A418--A422.