ORIGINAL RESEARCH PAPER
DOI: 10.1002/fuce.201000172
Water Management in A PEMFC: Water
Transport Mechanism and Material
Degradation in Gas Diffusion Layers~
S. G. Kandlikar1*, M. L. Garofalo1, and Z. Lu1
1
Department of Mechanical Engineering, Rochester Institute of Technology, 76 Lomb Memorial Drive, Rochester, NY 14623, USA
Received November 19, 2010; accepted October 09, 2011
Abstract
It has now been well recognized that both the performance
and durability of proton exchange membrane fuel cells
(PEMFCs) are closely related to the water accumulation and
transport inside its porous components, particularly in the
gas diffusion layer (GDL), and microporous layer (MPL). In
this paper, the key GDL and MPL properties that affect
water transport through them are first discussed and a
review of GDL degradation mechanisms is presented. An
intermittent water drainage mechanism across the GDL is
discussed. The capillary breakthrough pressure (CBP) and
the dynamic capillary pressure (DCP), or recurrent breakthrough dynamics, have been identified as key GDL properties that affect its water management performance and func-
1 Introduction
Polymer electrolyte membrane fuel cells (PEMFCs)
directly convert the chemical energy of the reactants into electrical energy and usually consist of a proton exchange membrane (PEM) sandwiched between two catalyst layers (CLs),
two porous gas diffusion layers (GDLs) often coated with a
microporous layer (MPL) on the CL side, and two bipolar
plates with embedded gas channels. Protons and electrons
produced by a hydrogen oxidation reaction in the anode CL
flow through the membrane and the external circuit, respectively, and participate in the oxygen reduction reaction in the
cathode CL producing water and waste heat.
–
~
Paper presented at the Second CARISMA International
Conference “Progress in MEA Materials for Medium and High
Temperature Polymer Electrolyte Fuel Cells”, La Grande Motte,
France, 19–22 September 2010.
814
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
tion as indicators of the degradation of GDL material. This
work uses a novel ex situ experiment to degrade a GDL by
exposing it to an accelerated stress test (AST) that subjects
the GDL to elevated operation conditions seen at the cathode
side of a PEMFC for an extended period of time. In turn, the
effect of the AST on the CBP and DCP is investigated. As a
result, a loss of hydrophobicity occurred on the MPL surface. This altered the CBP and DCP, thus decreasing water
management in the GDL.
Keywords: Degradation, Dynamic Breakthrough, Gas Diffusion Layer, Intermittent Drainage, PEM Fuel Cell, Water
Management, Water Transport
Despite considerable progress in the overall cell performance made in the past decade, a pivotal performance, and
durability limitation centers on the water management of the
PEMFC, namely the transport of product water (both liquid
and vapor) and the resulting drying and/or flooding in the
constituent components. In current PEMFC technologies,
water content of the membrane determines the membrane
performance and its durability. Although sufficient water
content is required to maintain high proton conductivity of
the membrane, excessive liquid water in the fuel cell can
flood, and block the pores of the CLs, GDLs, and gas channels. Both a dry membrane and flooded electrode hinder the
performance and lead to an accelerated degradation of the
fuel cell.
–
[*] Corresponding author, [email protected]
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Kandlikar et al.: Water Management in A PEMFC: Water Transport Mechanism and Material Degradation
2 GDL and MPL Intrinsic Properties and Their
Influence on Water Management and
Durability of PEMFCs
In this section, the most important material properties of
the GDL and MPL that influence the long-term water and
thermal management and the performance are reviewed. A
property matrix related to the water management performance and degradation of a GDL and MPL is established and
discussed at the end of this section.
2.1 Properties of the Gas Diffusion Layer (GDL)
The GDL is one of most important components in a
PEMFC. The primary functions of a GDL are: to supply reactant gases and remove product water from the CLs, to conduct electricity and heat between adjacent components, and
to provide mechanical support for the membrane electrode
assembly (MEA). These functions impose stringent requirements on the electrical, water transport and mechanical properties of the GDL.
The most commonly used GDLs are carbon-fiber based
®
paper (e.g., Toray TGP-H, SGL SIGRACET ). These materials
are highly porous to provide efficient passageways for water
and gases. In order to improve the water management performance of a PEMFC, these fibrous materials are usually wetproofed with polytetrafluoroethylene (PTFE) [5]. A fine MPL,
which mainly consists of carbon powder and PTFE particles,
is often coated on the GDL side near the CL to improve the
fuel cell performance [6, 7]. The most important properties to
characterize a GDL are pore structure, permeability, and
CBP. PTFE content, which determines the wetting property of
a GDL, is also an important material property of the GDL. Each
of these properties is briefly reviewed in the following sections.
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2.1.1 Pore Structure of a GDL: Porosity and Pore-size
Distribution
The porosity and pore-size distribution [8–12] are the most
important micro-structural properties of a GDL. Higher GDL
porosity improves mass transport, leading to higher limiting
currents; the adverse effect of a high porosity is a decrease in
the through-plane electronic conductivity as well as in the
mechanical properties. Liquid water accumulation inside the
GDL and the subsequent reduction of GDL effective porosity
presents a challenging issue in quantifying the effects of porosity [13]. Nam and Kaviany [14] studied the formation–distribution of condensed water in PEFC diffusion media, and
its tendency to reduce the local effective mass diffusivity and
to influence cell performance. They found that the larger porosity is better for both the reduction of water saturation and
increase in limiting current density. Kong et al. [10] investigated the influence of GDL pore-size distribution on mass
transport and found it to be more pronounced than the influence of the total porosity. They suggested that enlarging the
macropore volume in the GDL reduced the performance loss
that resulted from mass transport limitations.
The vital role of GDL porosity in determining two-phase
transport in the GDL has been widely recognized. It is generally believed that there is a decrease of water saturation
across the GDL from the GDL/CL interface to the GDL/channel interface, which is necessary for the capillary transport of
liquid water [15]. Owing to the spatially varying water content within the structure, it is important to consider the GDL
as having a non-uniformly dispersed porosity [16]. As a consequence, graded porosity of the GDL, both in the thickness
and laterally across the layer, has been reported to improve
performance by assisting water removal and access of gases
[17, 18]. Han et al. [19] controlled the porosity and pore size
by filling the carbon paper GDL with carbon/PTFE filler and
obtained superior performance over single- and dual-layer
GDLs, despite its lower porosity (67%) and smaller average
pore diameter (4.7 lm). Hiramitsu et al. [20] found that flooding originates at the interface between the GDL and the CL
and that the flooding could be mitigated by control of the
pore size in the GDL at this interface.
Fuel cell compression is another factor that affects the GDL
morphology and porosity. Bazylak et al. [21] found that compression changes the GDL microstructural morphology and
liquid water transport behavior due to the damage of fibers
and a possible loss of PTFE coating. However, the change of
GDL porosity or pore size distribution over long term fuel
cell operation is rarely observed. Lee and Merida [22] studied
the GDL durability under steady-state freezing conditions
and did not observe any change in porosity after 50 consecutive freeze–thaw cycles between –35 and 20 °C. Wu et al. [23]
investigated the degradation behavior of a GDL under a combination of elevated temperature and elevated flow rate conditions and found no obvious change in total porosity. Other
GDL degradation studies also indicated that no significant
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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ORIGINAL RESEARCH PAPER
Water transport and accumulation in the PEM and the CL,
and their effects on performance and durability in terms of
material integrity and electrical performance have been
extensively addressed in literature. For example, several comprehensive reviews have been reported on water management in the membrane and in the CL [1–4]. However, the
degradation of GDL material and its relation to fuel cell durability as it affects water management performance have not
received much attention. This paper focuses on the GDL durability issues related to water management performance.
Important GDL and MPL properties related to water management in PEMFCs are discussed along with a review of degradation mechanisms in GDLs. A water transport mechanism
through the GDL is presented along with a novel ex situ GDL
degradation study that employed an accelerated stress test
(AST) to degrade the GDL and subsequently observe the
capillary breakthrough pressure (CBP) and the dynamic
capillary pressure (DCP). It was found that a decrease in the
surface hydrophobicity of the MPL occurred due to the AST.
ORIGINAL RESEARCH PAPER
Kandlikar et al.: Water Management in A PEMFC: Water Transport Mechanism and Material Degradation
change in porosity could be detected for GDLs aged at various conditions [24–26].
2.1.2 Permeability
The permeability of the GDL is one of the major intrinsic
properties influencing reactants and water transport as well
as thermal management in PEMFCs [9]. Higher permeability
is often related to larger pores, which aid in oxygen transport
by enhancing bulk diffusion and pressure driven hydraulic
permeation through the GDL. Williams et al. [9] reported that
a GDL with higher gas permeability had the ability to avoid
water flooding under nearly saturated operating conditions.
Ahmed et al. [27, 28] found that the cell performance deteriorated due to the reduced water removal from a cathode GDL
with low permeability. Due to the anisotropic orientation of
carbon fibers as well as the addition of an MPL, the permeability of a GDL is also highly anisotropic, with much higher
in-plane permeability than through-plane permeability [9, 29,
30]. The water and thermal management in PEMFCs depend
on both the in-plane and through-plane permeability characteristics of the GDL, especially under the land region [27,28].
Attempts have been made to measure the in-plane [5, 31]
and through-plane permeabilities of GDLs with different
PTFE loadings [32], with and without an MPL [30, 32, 33] and
with varying thicknesses [33]. Moreover, Gostick et al. [30]
showed experimentally that the compression of a GDL to half
of its initial thickness causes its permeability to decrease by
an order of magnitude. Water and air relative permeabilities
in a partially saturated GDL have also been experimentally
determined [34].
The gas permeability is likely to change as the GDL is
degraded. Lee and Merida [22] observed an increase in the
in-plane and through-plane permeabilities in the GDLs subjected to freeze–thaw cycles between –35 and 20 °C and attributed the effect to the material (mainly MPL and PTFE)
loss. It was believed that aging under freezing conditions
weakened the MPL structurally such that it was prone to
material loss from air flow through the GDL, resulting in
increased permeability. A much bigger influence of the material (PTFE) loss on liquid (relative) permeability would be
expected for a degraded GDL. However, the experimental
data on this topic is rare in literature. This is one of the motivations for the current degradation study reported in the
present paper.
2.1.3 Capillary Breakthrough Pressure (CBP) and Dynamic
Capillary Pressure (DCP)
The CBP is an important transport parameter of the GDL.
It corresponds to the first appearance of a non-wetting phase
(e.g., water in hydrophobic GDL) on the outlet face of a sample. The physical meaning of the CBP corresponds to the incipient formation of a continuum of the non-wetting phase
through a pore network of arbitrarily large size. The CBP provides a direct indicator on the liquid water transport through a
GDL.
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© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
One of the critical constitutive relationships for describing
the capillary flow in a porous material is the capillary pressure versus liquid water saturation. This has been the focus
of several recent investigations [35–37]. A CBP in the range of
5–15 kPa is normally found for a hydrophobic treated GDL
[35–38]. Water flow was found to occur through less than 1%
of the void volume in the GDL; the small pores remain free of
water and permit gas to get to the CL [38]. PTFE loss is
believed to have a critical impact on the CBP as well as the DCP.
The DCP is defined as the recurrent breakthrough phenomena through the GDL. Current work is being conducted
by the author’s group and the results are outlined in a later
section.
2.1.4 PTFE Content in the GDL
To improve water management, the GDL is often treated
with hydrophobic PTFE to modify its wetting characteristics
[8, 33, 39]. Proper PTFE content helps in effectively removing
the liquid water from the GDL into the flow channels. The
PTFE treatment leads to pockets of hydrophilic and hydrophobic pores in the GDL. It is thought that the hydrophobic
regions allow a pathway for gas transport whereas the hydrophilic regions facilitate liquid transport [40]. However, excessive PTFE loadings can have adverse effects.
(i) Excessive PTFE loading causes the CBP to become too
high at the CL/GDL interface and increases water transport resistance from the CL to GDL, leading to an extensive flooding in the CL as well as an increased Ohmic
resistance.
(ii) The addition of PTFE increases the contact resistance
between the GDL and the bipolar plate and lowers the
porosity of the GDL.
The practical PTFE concentration used in the GDL is
between 5 and 40% by weight. Paganin et al. [41] reported an
optimal PTFE content of 15 wt.%. Staiti et al. [42] found that a
30 wt.% fluoroethylenepropylene (FEP) treated GDL gave the
best cell performance. An optimum value of 10 wt.% of FEP
was reported by Lim and Wang [43]. A distribution of PTFE
across a GDL has also been found to be beneficial to water
management and thus fuel cell performance [44].
The PTFE content has been found to gradually decrease
with the fuel cell operation [22], especially in the cathode
GDL. In fact, material (PTFE) loss is a major degradation
mechanism in a GDL [22, 23]. The PTFE coating is not
strongly bound to the carbon fibers and can be washed off by
the liquid water and humid gas streams, especially at higher
temperatures. Hot pressing and mechanical compression during the assembly of fuel cell stacks may also lead to both
damage and loss of PTFE coating. The most important consequence of PTFE loss has been reported to be in the change of
the GDL wettability, including surface contact angle and
internal wettability, leading to degradation in the desirable
properties from a water management perspective.
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FUEL CELLS 11, 2011, No. 6, 814–823
Kandlikar et al.: Water Management in A PEMFC: Water Transport Mechanism and Material Degradation
An MPL, which is either coated on one side of the GDL or
serves as an independent layer, is a porous layer of carbon
particles impregnated with PTFE throughout the structure.
Many works have reported that an MPL can significantly
improve the performance and durability of PEMFCs, especially at high current densities.
However, it is interesting that many researchers have different or even opposite explanations for the effect of the MPL.
Different MPL mechanisms presented in the literature for
performance improvement are summarized below:
(i) The MPL increases the back diffusion of water and
improves the humidification of the membrane at the
anode side [6, 9, 45, 46];
(ii) The MPL increases the hydraulic pressure differential
across the membrane due to strong capillary pressure in
the MPL, which enhances the water transport from the
cathode to the anode [7];
(iii) The MPL reduces the liquid water saturation in the cathode and improves oxygen diffusion [8, 10, 14, 47];
(iv) The MPL increases effective drainage of water from the
CL/GDL interface by the capillary forces due to the presence of two different pore sizes [48];
(v) The MPL enhances the formation and transport of the
water vapor in the CL and the MPL [49];
(vi) The MPL improves the electrical contact between the
GDL and the CL [6];
(vii) The MPL on the cathode neither enhances back diffusion
nor increases water removal from the cathode CL to the
GDL [50, 51];
(viii) The MPL plays a role in controlling the water configuration (or morphology) by limiting the liquid entry locations from the CL to the GDL and in reducing the water
saturation in the GDL [52–55].
The experimental results [50, 51] of the net water drag
indicate that an MPL on the cathode GDL has a negligible
effect on back diffusion and water removal from the cathode
CL, despite the fact that an improvement in fuel cell performance was observed by adding an MPL to the cathode GDL.
Dai et al. [1] proposed an internal water recycling mechanism
between the anode and cathode to account for this discrepancy. The MPL increases the efficiency of this internal water
recycling that facilitates the water content distribution in the
membrane. Further work is warranted to reveal the underlying mechanism that is responsible for the performance
improvement due to the MPL.
2.2.1 Wettability (PTFE Content)
PTFE content used in an MPL can influence PEMFC performance [6, 33, 44, 56]. The incorporation of PTFE serves two
functions:
(i) Binding the high surface area carbon particles into a
cohesive layer.
(ii) Imparting a hydrophobic character to the layer.
FUEL CELLS 11, 2011, No. 6, 814–823
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Antolini et al. [56] found that the polymer coats the pores
of sizes smaller than 1 lm (carbon inter-agglomerate pores),
while the pores larger than 1 lm are not influenced by the
presence of PTFE. Above 40 wt.% PTFE, a further supply of
polymer does not fill the pores, but increases the thickness of
the layer. Qi and Kaufman [6] recommended an optimal loading of 35% PTFE and 2.0 mg cm–2 carbon, based on their
study.
2.2.2 MPL Pore Structure
The MPL pores range from 0.1 to 0.5 lm in diameter, making them much smaller than the pores in the GDL, which
range from 10 to 30 lm in diameter. This fact indicates that
the influence of capillary effects on water balance is more
pronounced in the MPL [57–59]. The MPL causes a sharp
increase in capillary pressure and, for a given saturation, the
capillary pressure increases with an increase in the PTFE content [58]. Lin and Nguyen [33] concluded that the hydrophobic pores are used for gas transport while the hydrophilic
pores are used for liquid water transport within the MPL.
Park et al. [60] suggested that the capillary driving force
might be the dominant water transport mechanism in the
MPL. In addition, Gostick et al. [40] found that the pore-size
distribution for the hydrophilic pores was similar in shape to
the overall distribution for standard substrate materials. Their
work indicates that proper hydrophobic and hydrophilic pore
distributions are very important for liquid water removal and
the transport of reactant gases.
Different types of carbon particles [61–63] and carbon
loading [6] in the MPL have been investigated. The MPL
thickness was also investigated by Qi and Kaufman [6] and
Pasaogullari and Wang [47]. The latter concluded that an
MPL thickness of approximately 50 lm would offset the positive effects.
2.3 Summary of Desirable GDL Properties
Table 1 gives a summary of the various GDL and MPL
properties, how they are characterized, their optimal values,
and how degradation has an effect on them. Optimal values
are taken from [64] while the characterization and property
degradation sections are proposed from knowledge of the
author’s group.
3 AWater Transport Mechanism through the GDL
Understanding the fundamental mechanisms of two-phase
water transport in the GDL is a key step for the effective management of liquid water. The extreme structural and chemical
heterogeneity of GDLs substantially complicates the studies
of liquid water transport. In this section, the main water
transport mechanisms through a GDL with and without an
MPL are presented.
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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ORIGINAL RESEARCH PAPER
2.2 Properties of the Microporous Layer (MPL)
ORIGINAL RESEARCH PAPER
Kandlikar et al.: Water Management in A PEMFC: Water Transport Mechanism and Material Degradation
Table 1 Summary of carbon paper GDL intrinsic properties related to water management, degradation, and cell performance.
Properties
Characterizations
Optimal value
GDL properties
Porosity and poresize distribution
Pore structure
70–90% [8–12]
In-plane (IP) and throughplane (TP) gas and liquid
transport
IP = 10–12–10–11 m2 [5, 31]
Permeability
TP = 10–14–10–12 m2 [32]
Both depend on PTFE
content and compression
5–15 kPa [35–38] for most
commercial GDLs
Breakthrough capillary pressure
Pore structure and liquid
water transport
PTFE coating
Wetting properties and
liquid transport
5–30 wt.%
Binding agent; pore
structure and wetting
properties
30–40 wt.% [6, 56]
Pore structure, capillary
pressure
0.1–0.5 lm
MPL properties
PTFE loading
MPL pore size
3.1 Liquid Water Transport Mechanisms
In the previous work by the authors’ group, the CBP and
DCP across GDLs with and without an MPL were studied in
an ex situ setup that simulates a real fuel cell configuration
and operating conditions [52]. The major findings will be
reproduced here, while the detailed experimental procedure
can be found in Ref. [52].
A wavy channel array measuring 30 mm long was used in
the test section. The water breakthrough locations were
recorded with a charge-coupled device camera using a
hydrophilic wicking medium (Porex X-4588), which was
placed on top of the GDL. The capillary pressure of water in
the GDL was directly determined by a differential pressure
transducer (Honeywell FDW2AR), which measures the liquid
pressure referenced to the atmospheric pressure. Liquid
water was delivered at a very low rate of 10 lL min–1, which
corresponded to an equivalent water production rate at a current density of about 1.2 A cm–2. The GDLs studied in the
experiment consisted of a Baseline-A with a 10 lm thick
MPL, a Baseline-B with no MPL, an SGL 25 BC (SGL Carbon
Group, Wiesbaden, Germany) with a 30–50 lm thick MPL,
and an SGL 25 BA with no MPL. All of these GDLs are carbon
paper based and treated with PTFE to increase their hydrophobicity.
In the first experiment, the water emergence and CBP
through the GDLs without MPLs were studied. The most outstanding result was the dynamic characteristics of water
breakthrough in the GDL, or DCP, and the dynamic breakthrough locations. The former is apparently seen from the
fluctuation of the capillary pressure after the breakthrough,
which is commonly observed for all GDL samples. The DCP
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© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
observed reveals a breakdown of the
water pathways caused by the water
Property degradation
drainage on the GDL surface. The
second distinct characteristic is the
Rarely observed at normal
dynamic water breakthrough locaand freezing condition
tions. This means that the break[22–26].
through location changed with time
Likely degrade due to
freeze-thaw cycling [22].
and was closely related to the DCP.
The multiple breakthrough locations
are shown in Figure 1 along with the
DCP [52].
Believed to be significantly
In the second experiment, the
decreased due to PTFE loss
water
breakthrough in the GDLs with
but experimental data is
an MPL was investigated. In these
limited.
PTFE loss is a major GDL
tests, the MPL was placed facing the
degradation mechanism
water inlet, simulating the configura[22, 23].
tion during fuel cell operation. Recurrent breakthroughs were observed,
PTFE loss due to carbon
corrosion is a main
similar to the cases of GDLs without
degradation mechanism
an MPL, and indicate the dynamic
for MPL [22].
feature of the breakthrough process.
Not specifically studied.
However, no shifting of water breakthrough locations was observed in
either the Baseline or SGL 25BC
GDLs. This is in sharp contrast to the GDL samples without
an MPL in which the changing of breakthrough locations was
always observed. This difference must originate from the
MPL, suggesting that the MPL plays a role in stabilizing the
preferential water pathways through the GDL. Table 2 outlines the results found from the two experiments [52].
It was found that water flows through defects in the MPLs
including the cracks and discontinuities in the MPL. These
defects provide the preferential water transport pathways
through the MPL and decrease the CBP substantially. From
Table 2, much lower water saturations were observed for
GDLs with an MPL than GDLs without an MPL. This result
indicates that the MPL greatly reduces the water saturation in
Fig. 1 Water breakthrough behavior through an initially dry Baseline-A
GDL (without MPL). The numbers in the figure indicate the peak pressures.
BT denotes “breakthrough”.
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GDL
Structure
Thicknessa) (lm) PTFEb) (wt.%)
Baseline-A
Baseline-B
SGL 25BA
SGL 25BC
No MPL
w/MPL
No MPL
w/MPL
200
208
183
225
a)
b)
±
±
±
±
3
3
3
3
7.0
7.0
5.0
5.0
Porosity (%)
Vpore (lL)
Pb (kPa)
Sw,b (%)
87
80
88
80
29
28
27
30
7.4 ± 1.1
12.7 ± 1.4
1.7 ± 0.5
6.7 ± 1.2
4.7–12.2
2.4 ± 0.2
2.6–7.1
0.8 ± 0.2
New break
locations
Yes
No
Yes
No
Rc (lm)
19.5
11.3
80.9
21.5
Thickness was measured with a micrometer.
PTFE content was taken as the manufacturer value.
a GDL. A similar result has been reported in literature and
was explained by the limitation of water access to the GDL by
the MPL [54, 55].
Based on these observations, a new water transport mechanism was proposed by the author’s group to account for the
water breakthrough in GDLs [52]. This mechanism is based
on Haines jumps, a description of the discontinuous drainage
displacement employed in geological disciplines [65, 66]. In
slow drainage processes, the interfaces between fluids remain
unmoved until the pressure in the displacing fluid increases
to a value exceeding the capillary pressure at the largest
restriction. At this point, the invading fluid suddenly moves
into the adjacent pores, accompanied by a negative capillary
pressure drop as a result of the readjustment of the interfaces
between the fluids and porous medium. In the case of water
breakthrough in a GDL, the bursting droplet grows fast as it
carries away water from the adjacent GDL pores. However,
the supply of water is often not sufficient for the droplet to fill
the larger pore (i.e., gas channel). This “choke off” effect leads
to empty pores in the GDL, which break down the continuous
water paths. These emptied pores are refilled afterwards as
water is constantly injected and the bursting process occurs
again, leading to the recurrent breakthrough behavior. As the
“choke offs” break down the original water paths, water
spontaneously readjusts its interfaces inside the GDL pores.
This water/air interface relaxation process may lead to a new
preferential pathway in the GDL and result in a new breakthrough location.
4 GDL Degradation Study
The study of degradation mechanisms of the GDL is an
area that has only been touched on lightly in literature. In
order to compete with current automotive technology, an
automotive fuel cell has to sustain at least 5,000 h without
degrading over 3–5% and even longer for stationary applications [64]. Fuel cells that are carefully designed can last over
40,000 h under ideal conditions but the degradation phenomenon that ultimately leads to their performance loss is the
GDL’s loss of hydrophobicity [67]. An ex situ study of the
effect of GDL degradation on the CBP and DCP was recently
carried out by the author’s group as a continuation of the
work presented in Section 3. A GDL was subjected to an AST
that included exposing the GDL to conditions seen at the
cathode side of the fuel cell. These conditions included:
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(i) Constant accelerated current density
(ii) Constant accelerated liquid water flow rate
(iii) Typical fuel cell compression
(iv) Typical fuel cell operating temperature.
The CBP was measured periodically during the AST and
the DCP was also observed. Post-degradation analysis was
carried out via surface contact angle measurement.
4.1 GDL Specifications and AST Conditions
The GDL tested was a carbon fibrous paper SGL 25 BC
sample. The manufacturer states that the GDL is about
235 lm thick and has a 5 wt.% PTFE loading on the substrate
along with an MPL on one side. Table 3 summarizes the AST
conditions that the GDL was subjected to. An elevated current density of 2 A cm–2 with a corresponding theoretical
cathode water production rate of 36.4 lL min–1 were chosen
to accelerate the degradation of the GDL and MPL. These values are approximately twice that of what a GDL would see
under normal fuel cell operating conditions. Both the temperature and the compression of the GDL were chosen to be typical values so as not to conflict with the degradation due to the
current and water flow.
4.2 AST Experimental Setup
The AST was designed to incorporate subjecting the GDL
to specified temperatures, liquid water flow rates, current
densities, and compressions. Also it needed to be capable of
measuring the CBP along with the applied compressions as
well as the current. Pressure data was logged over time during the breakthrough tests in order to observe the DCP.
Figure 2 shows a detailed schematic of the GDL degradation
experimental setup.
In the AST setup, the GDL substrate is sandwiched
between two gold plated copper current collectors (CCs) and
is surrounded by a 7 mil thick PTFE gasket. The GDL is oriented in the system such that the MPL side faces the water
delivery inlet and thus the water must travel through the
Table 3 Summary of AST conditions.
Condition
Quantity
Unit
Current density
Water flow rate
Compression
Temperature
2
36.4
1.4
80
A cm–2
lL min–1
MPa
°C
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Table 2 GDL properties, water breakthrough pressures (Pb), water saturation at breakthrough (Sw,b), equivalent capillary radius corresponding to breakthrough (Rc), and information about the emergence of new break sites in different GDLs.
Kandlikar et al.: Water Management in A PEMFC: Water Transport Mechanism and Material Degradation
ORIGINAL RESEARCH PAPER
4.4 DCP Results
Fig. 2 Schematic of GDL degradation experimental setup.
MPL and then the GDL substrate just as it does at the cathode
side of a PEMFC. To prevent corrosion during the AST, each
CC was gold plated at a 1.27 lm thickness. Each CC also features a 25 mm × 25 mm array of 13.1 mm wide through
channels and 12.1 mm wide lands for passing water and current through the GDL, respectively. Four aluminum blocks
sandwich the two CCs and both supply water to and heat the
GDL. The water heating loop is driven by a recirculating bath
(VWR Industries 1196D). The GDL water supply is driven by
a syringe pump (Kent Scientific Co. GENIE). Data acquisition
equipment included a load cell (Omega Engineering LC3041K) and a differential pressure transducer (Honeywell
FDW2AR) for compression and CBP measurement, respectively. Finally a screw driven clamping device compresses
the entire system at the arrowed locations specified in
Figure 2.
The plot of the initial DCP is as
follows in Figure 3 and portrays a
transport mechanism in which
water pressure builds up to close
to 1.5 kPa and spikes instantaneously and drops off slowly back
to approximately the same value
in a burst-like fashion. It is
believed that the channels fill up
with water and periodically burst
and empty through the GDL. This
water buildup is suspected
because of the hydrophobicity of
the undisturbed MPL. The plot
of the DCP after subjecting the
GDL sample to the AST for
500 hours is shown in Figure 4.
From Figure 4, it can be determined that the DCP changed due
to the AST in multiple respects as
follows:
(i) The pressure builds up to a higher value than before of
approximately 4 kPa before the first pressure spike
occurs and eventually falls back to around the same
value after spiking.
(ii) The pressure spike peak values are lower at below 7 kPa
and the pressure does not fall off as quickly after spiking
as it did before but does so in a rather linear fashion
instead.
(iii) The period of the breakthrough pressure spikes was
much shorter than before.
The change in period is suspected to be due to the loss of
hydrophobicity of the MPL and, in turn, the increased overall
ease of saturation of the GDL. The lower peak pressures
Initial Dynamic Capillary Pressure
10
4.3 AST Procedure
820
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
9
8
7
Pressure / kPa
The AST consisted of initially taking a fresh GDL and compressing and sealing it in the test section. The syringe pump
was filled with de-ionized water produced by a water purification system (Millipore Direct-Q 3) and was 18.2 MX cm at
25 °C. The water was then delivered at 70 lL min–1 to measure the initial CBP and observe the dynamics through the
fresh GDL. Subsequently, the AST conditions were set and
the system was run until the degradation time reached 500 h.
At that point, the GDL was purged with purified air at less
than 2 ppm of water content for 1 hour at 1 SLPM and then
left to dry for 1 week while still in the test section. The DCP
was, in turn, observed again at 70 lL min–1 and the test section was disassembled to remove the GDL for further investigation.
6
5
4
3
2
1
0
0
100
200
300
400
500
600
700
800
900
1000
Time / s
Fig. 3 Plot of DCP through fresh GDL sample.
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FUEL CELLS 11, 2011, No. 6, 814–823
Kandlikar et al.: Water Management in A PEMFC: Water Transport Mechanism and Material Degradation
Specimen
Static
Advancing
Receding
9
Fresh
Degraded
147°
140°
145°
142°
144°
137°
8
Table 5 Summary of MPL surface contact angle time study.
Pressure / kPa
7
6
5
Specimen
0 min
15 min
30 min
45 min
Fresh
Degraded
144°
142°
141°
134°
143°
123°
144°
104°
4
3
2
1
0
0
100
200
300
400
500
600
700
800
900
1000
Time / s
Fig. 4 Plot of DCP through degraded GDL sample.
coupled with the shorter linear fall off time can also be
coupled with the GDL becoming more saturated.
4.5 GDL Hydrophobicity Investigation
The effect of the AST on the GDL’s hydrophobicity was
investigated through measuring the surface contact angle on
a fresh GDL and on the degraded GDL. The measurement
system used was an AST Products, Inc. VCA Optima. The
degraded GDL as well as a separate fresh GDL cut from the
same sheet were both investigated and the surface contact
angle on the MPL side of both GDLs was measured. Table 4
outlines the averaged results for static, advancing, and receding contact angles that were taken at three different locations
on each GDL sample. It can be drawn from the table that,
with respect to the degraded GDL, a decrease in all three
angles occurred and, in turn, a decrease in hydrophobicity of
the MPL surface occurred.
The areas where water was passed through the GDL during the AST were found to be much less hydrophobic than
the areas where current was passed. When dispensing the
droplets onto the narrow strips of MPL surface where current
was applied during the AST, the droplet would move to the
areas of the MPL where water was passed. This was consistent throughout the contact angle measurement process and,
in turn, forced all the contact angles measured on the
degraded MPL surface to be at the areas where the water was
passed through.
A time study was also conducted to further investigate the
hydrophobicity change of the degraded MPL surface. Identical sized water droplets were dispensed onto the both the
fresh and degraded MPL surfaces and the static contact angle
was measured every 15 min over the course of 45 min.
Images of the water droplets taken for both MPL surfaces
during the time study are pictured in Figure 5. Table 5 lists
the static contact angles measured over time for both specimens.
It can be seen that the degraded sample had a much smaller contact angle after 45 min and showed a steady decrease in
contact angle over time while the fresh MPL surface maintained approximately the same contact angle throughout the
study.
It should also be noted that after dispensing a water droplet on the degraded MPL surface, it was impossible to withdraw it completely back into the syringe. On the contrary, the
droplet on the fresh MPL surface was able to be entirely withdrawn back into the syringe after the time study was completed. This along with the marked
decrease in contact angle ensures that the
degraded MPL surface did indeed
become less hydrophobic due to the AST
it was subjected to.
4.6 Degradation Study Summary
Fig. 5 Images of water droplet contact angle time study on both fresh and degraded MPL surfaces
of GDL samples.
FUEL CELLS 11, 2011, No. 6, 814–823
www.fuelcells.wiley-vch.de
The AST performed in this study
exposed the GDL sample to conditions it
would see on the cathode side of the fuel
cell throughout its life with the exception
of passing humidified reactant gases
through it. This experiment took a
method of investigating the CBP and
DCP through the GDL earlier conducted
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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ORIGINAL RESEARCH PAPER
Table 4 Summary of MPL surface contact angle study.
Final Dynamic Capillary Pressure
10
ORIGINAL RESEARCH PAPER
Kandlikar et al.: Water Management in A PEMFC: Water Transport Mechanism and Material Degradation
by the author’s group and applied it in a novel ex situ setup
to further investigate the phenomenon as well as how it
affects GDL degradation. It is proposed that the MPL area
where water was passed during the AST became much less
hydrophobic. This caused the DCP of the GDL to change and
the water to be absorbed more easily into the GDL and thus
made it more susceptible to flooding. The contact angle measurement time study showed that the surface area of the MPL
became much less hydrophobic while the areas where current
was passed did not. Thus it can be concluded that the water
flow degraded the hydrophobicity of the MPL surface more
than the current flow. Further work is being continued by the
author’s group to investigate other GDLs without MPLs or
PTFE loading in the substrate using the same procedure.
5 Conclusion
The GDL was successfully degraded as a result of the AST
and in turn both the CBP and DCP changed. The CBP
dropped by approximately 1.6 kPa and the spikes fell to a
pressure approximately 2.75 kPa lower than before. Also the
period of the spikes became much shorter and the spikes
became much steadier. A surface contact angle study
revealed a decrease in the hydrophobicity of the MPL surface
especially when the droplet was left on the surface over time.
The contact angle decreased about 40° over the course of
45 min on the degraded MPL surface while the contact angle
remained the same on the fresh MPL surface. It also showed
that the area of the MPL surface where water was passed
showed a larger decrease in hydrophobicity than the area
where the current was applied. From this study, it was concluded that the changes observed show that the degradation
mechanism caused by the AST is the loss of hydrophobicity
of the MPL. A more detailed and extended test plan is recommended under higher current loading and water flow rate
conditions to provide useful information for developing more
durable GDL and MPL materials.
The GDL is a complex component of PEMFCs and it has
been shown that its degradation can affect the CBP and DCP
which in turn affects water management in the cell. Degradation studies that pertain to the CBP are warranted as well as
visuals of how the morphology of the MPL changes with degradation time. These types of experiments can broaden the
spectrum of knowledge of the GDL and MPL and in turn
allow for the design of more robust GDLs.
Acknowledgements
The authors would like to acknowledge the US Department of Energy for the financial support under award No.
DE-EE0000470.
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© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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