Journal of New Materials for Materials for Electrochemical Systems 4, 227-231 (2001)
c J. New. Mat. Electrochem. Systems
Membrane Durability in PEM Fuel Cells
Wen Liu ∗, Kathy Ruth, and Greg Rusch
Gore Fuel Cell Technologies
W. L. Gore & Associates, Inc., 201 Airport Road, Elkton, MD, 21922-1488
( Received June 4, 2001 ; receved in revised form September 21, 2001 )
Abstract: The PRIMEA MEA Series 5600 is a durable, high power density Membrane Electrode Assembly (MEA) targeted directly at the commercial stationary polymer electrolyte fuel cell markets. In order to meet the demand of long lifetime in this market, an extensive research and
development effort was carried out at W. L. Gore & Associates, Inc. (Gore) to understand and improve MEA durability, and membrane is one of the
vital components.
Accelerated fuel cell life tests were performed to understand factors that may control the durability of membranes. It was observed that the
most significant effect influencing membrane durability in the accelerated fuel cell life tests was mechanical strength, i.e., reinforced vs. nonreinforced membranes. GORE-SELECT membrane exhibited lifetime an order of magnitude longer than a non-reinforced membrane of comparable
thickness. Furthermore, a 25-micron GORE-SELECT membrane outlasted commercial membranes three times its thickness, while providing
higher power density by a considerable margin. Membrane failure characteristics, exhibited by an increase of H2 crossover rate, also showed
a significant difference: non-reinforced commercial membranes exhibited immediate sudden failure, while GORE-SELECT membranes showed
gradual increases in gas crossover until pinhole failures. It was also concluded, from fluoride release analysis of product water, that membrane
failure in the testing hardware was highly localized.
Two groups of GORE-SELECT membranes were identified to have significantly longer lifetime and one of these technologies has been selected
as the new membrane in the PRIMEA MEA Series 5600 product. We conclude that e-PTFE reinforcement in GORE-SELECT membranes
plays a very important role in membrane durability. Combined with Gore’s electrode technology, PRIMEA MEAs are able to provide a unique
combination of power density and lifetime.
Key words : PEM fuel cells, membranes, and durability.
1. INTRODUCTION
It was identified from our internal fuel cell tests, that improper
selections of cell assembly parameters such as MEA compression and gasket design could give rise to premature membrane
failure. In addition, many parameters including fuel cell operation temperature, pressure, and relative humidity of reactant
gas may have a significant effect on membrane life. However,
within a given set of operation conditions, we observed that certain intrinsic membrane characteristics could impact membrane
durability dramatically. In order to provide the market with a
MEA having the best combination of power density and durability, an extensive study was performed to understand important
membrane attributes that affect lifetime.
To commercialize PEM (Polymeric Electrolyte Membrane) fuel
cell technology, one of the most critical criteria is to achieve
long lifetime of the overall system. This is especially important for the residential stationary power market, which requires
durable performance for many years of operation. Such market
demands pose a great challenge in MEA (Membrane Electrode
Assembly) technology development, and membrane is a vital
component.
∗ Fax:
+1 506 7633; email: wliu@wlgore.com
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W. Liu et al./ J. New Mat. Electrochem. Systems 4, 227-231 (2001)
Membrane type
Standard GSM A
GSM B
FSM
Nafion 101
Nafion 1035
Table 1: Identification of various membranes
Thickness
Mechanical
Process
MEA
(µm)
reinforcement
25
ePTFE
—PRIMEA Series 5510
35
ePTFE
—PRIMEA Series 5620
25
None
Solution Casting
—25
None
Extrusion
—88
None
Extrusion
—-
Intrinsic membrane characteristics were categorized into two
groups: chemical and mechanical. Developmental membranes
having drastically different properties were fabricated and evaluated in accelerated fuel cell life tests. In addition, several types
of commercial NAFION membranes were also evaluated for
comparison. This paper will present some of the test results as
well as conclusions of intrinsic membrane properties that impact MEA life.
2.
2.1
EXPERIMENTAL PROCEDURE
Membrane sample type
Many types of membranes were evaluated. All the ionomers
used in this study were perfluorinated sulfonic acid (PFSA)
ionomers. Membranes were fabricated using different processes
including solution casting, extrusion, and Gore’s own proprietary technique, and therefore, possessed different properties.
Identification of various membrane samples is presented in Table 1. It is worthwhile to mention that NAFION membrane
112 was initially included in the tests. However, the MEA with
this membrane was incapable of providing sufficient fuel cell
performance under the stringent accelerated test conditions;
therefore, the results will not be discussed here.
2.2
Cell construction
All MEAs used Gore electrodes with a loading of 0.4 mg/cm2
Pt on both the anode and cathode sides. CARBELT M gas diffusion media CL was used. All tests were performed with 25cm2
cells with triple-channel serpentine flow field.
2.3
Accelerated fuel cell life test protocol
The conditions for the accelerated life test were Tcell = 90o C,
D.P.a/c = 83/83o C, FlowH2/air = 1.2/2.0 stoich, Pressurea/c = 5/15 psig, and i=800 mA/cm2 . To determine the membrane integrity, every week electrochemical H2 crossover rate
(CRXH2 ) measurements were performed at Tcell = 60o C, Ta/c
= 60/60o C, FlowH2/air = 50/50 cc/min, Pressurea/c = 0/0 psig.
If CRXH2 >10 mA/cm2 , a physical pinhole test was performed.
The test consisted of applying a 2 psi differential cathode over
pressure while capping the anode inlet and leading the anode
Figure 1: Lifetime of various membranes in accelerated fuel cell
conditions
outlet to a water reservoir. Membrane failure was declared and
test was terminated if the bubble count through the membrane
exceeded 10 bubbles/min. Accelerated life testing was resumed
if the membrane did not fail.
2.4
Standard residential fuel cell test protocol
In order to correlate lifetime of membranes in accelerated fuel
cell life tests and less stringent residential fuel cell operation
conditions, several standard fuel cell tests were performed using PRIMEA MEAs Series 5510, which incorporate standard
GORE-SELECT membrane A. The test conditions were
Tcell = 70o C, D.P.a/c = 70/70o C, FlowH2 /air = 1.2/2.0 stoich,
Pressurea/c = 0/0 psig, and i=800 mA/cm2 . H2 crossover rate
measurement was performed periodically to monitor the membrane’s integrity.
2.5
Fluoride release rate analysis
In the past, a correlation between ionomer degradation, measured by fluoride release rate, and membrane life was suggested
[1]. In this study, the amount of fluoride released into the product water was monitored to evaluate this correlation. Product
Membrane Durability in PEM Fuel Cells . / J. New Mat. Electrochem. Systems 4, 227-231 (2001)
Figure 2: H2 crossover rate as a function of lifetime
water of fuel cell reactions was collected at exhausts throughout the tests using plastic containers. The collected water was
then concentrated about 20 fold (for example, 2000ml to 100ml)
in PTFE beakers heated on hot plates. Fluoride concentration
in the concentrated water was determined using a F− -specific
electrode. Fluoride release rate (g of F− /cm2 hr) was then calculated, and a total percentage loss of ionomer throughout the
test was calculated.
3.
3.1
Results and Discussions
Lifetime of various membranes
Lifetime of various membranes in the accelerated fuel cell tests
(conditions listed above) is presented in Figure 1. Two important observations stand out from these results: First, test reproducibility is sufficient to determine differences between membrane types. Second, mechanical reinforcement had a large
effect on membrane life. GORE-SELECT membranes with
ePTFE reinforcement offered much longer lifetime compared
with non-reinforced films. For example, GORE-SELECT membrane A provided more than 5 times longer life than its direct,
non-reinforced comparison, FSM; further more, it offered more
than twice as long life as NAFION membrane 101, and even
outlasted a membrane which was over three times its thickness,
NAFION membrane 1035. It seems that the unique mechanical properties of GORE-SELECT membranes allow them to
achieve high performance with long life when compared with
non-reinforced films.
Within the GORE-SELECT membrane group, change of certain mechanical properties, which may result from the interaction between reinforcement and membrane thickness, seems to
play a very important role in membrane durability. Developmental GORE-SELECT membrane B, being thicker and having different mechanical properties than standard
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Figure 3: Comparison of lifetime of GSM A under accelerated
and standard fuel cell life test conditions.
GORE-SELECT membrane A, proved to have significantly
longer life in the accelerated fuel cell conditions.
3.2
H2 crossover rate as a function of lifetime
Figure 2 presents H2 crossover rate as a function of lifetime.
The last data point of each line, indicated by symbol “*”, represents the point of membrane failure.
It can be seen clearly that the trend of H2 crossover rate with
time is different between ePTFE reinforced GORE-SELECT
and non-reinforced membranes. In the GORE-SELECT membrane group, H2 crossover rate gradually increased until failure,
while non-reinforced membranes appeared to display a sudden,
drastic jump, which is characteristic of a catastrophic failure.
Such a distinctive difference may provide insights of membrane
failure mechanisms in fuel cells. One can image that a local
imperfection, that formed during MEA fabrication, or damage
caused during fuel cell testing, such as a crack, could act as a
stress concentrating point. Further mechanical stresses as a result of fuel cell operation and changes in the membrane environment could cause crack enlargement. Mechanical reinforcement
with the three-dimensional nodes and fibril structure of ePTFE
could provide an effective blunting mechanism that might slow
down crack enlargement or propagation. Non-reinforced membranes, on the other hand, may be vulnerable to crack growth
and propagation through membrane thickness and severe tearing along x-y plane, leading to catastrophic failures.
Figure 3 compares H2 crossover rate as a function of lifetime
for GORE-SELECT membrane A under accelerated and standard fuel cell life tests. We observed an approximately 10x relationship for membrane life under the two different conditions.
In addition, the trend of H2 crossover rate increase over time
for standard residential fuel cell tests also appeared to be grad-
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W. Liu et al./ J. New Mat. Electrochem. Systems 4, 227-231 (2001)
ual, indicating that membrane failure mechanism was the same
under the two different conditions. Therefore, we accept the accelerated fuel cell life test protocol as a valid testing condition
to evaluate membrane durability for residential fuel cell applications.
In summary, it appeared that crack propagation might be the
leading cause for catastrophic failures of non-reinforced membranes. Employing ePTFE as reinforcement seems to effectively slow down this mechanism.
3.3
Fluoride release rate in life spans of various membranes
Figure 4 plots F− release rate of different membranes during the
life tests. Each data point is an average F− release rate value for
the time period between the current and the previous data points.
Figure 4: Fluoride release rate of various membranes
Figure 6: Fuel cell performance of GSM B compared with standard GSM A under two different conditions: a. standard residential; b. accelerated.
membrane, F− release rate was observed to vary significantly in
some tests, starting high then dropping low in some cases, such
as for one of the GORE-SELECT membrane B samples, or
continuously increasing as in one case of NAFION membrane
101. In addition, reproducibility was very poor within all three
sets of replicates.
Figure 5: Total loss of ionomer during accelerated membrane
fuel cell tests
It appeared that overall, F− release rate of various membranes
did not follow any particular trend, nor did it always replicate
within the same membrane. In addition, within the life span of a
Total percentage loss of ionomer throughout the test for various membranes is shown in Figure 5. Similar to F− release rate
during fuel cell tests, total percentage loss of ionomer at the end
of membrane life span did not seem to correlate with lifetime,
nor did it show satisfactory reproducibility between replicates.
For example, total loss of ionomer for all of the membrane samples at the end of their lifetimes varied between 1.7% to 28%.
One of the NAFION membranes 101 managed to lose 28% of
ionomer within approximately 300 hours of life while the other
one lost only a total of 7% ionomer within a slightly longer
Membrane Durability in PEM Fuel Cells . / J. New Mat. Electrochem. Systems 4, 227-231 (2001)
lifetime of 450 hours. Such evidence may suggest random, localized rather than uniform degradation.
The measurement method for F− release rate was carefully
tested with standard fluoride solutions. Accuracy and reproducibility of the measurement was proven to be within 5%.
Therefore, it is believed that the measured F− release rate value
reflects actual signals. Yet, such erratic behavior is not understood. It is conceivable that F− release rate of membranes in
the accelerated fuel cell tests might have been controlled by
factors such as local chemical or thermal degradation resulting
from non-uniform current distribution and/or random electrical
shorting. More careful experimental design is needed to correlate F− release rate with controlled parameters. Until then, it is
suggested that one can not predict membrane life based on F−
release.
3.4
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NAFION membrane 1035, on the other hand, not only is less
durable than GORE-SELECT membrane A despite its greater
thickness, but also offers poorer fuel cell performance.
4.
CONCLUSION
From this study, it was concluded that:
1. Under accelerated membrane fuel cell test conditions,
membrane life is dominantly controlled by mechanical reinforcement. Composite membranes, such as ePTFE reinforced GORE-SELECT membrane, can provide more
durable fuel cell performance because of the mechanical
properties provided by reinforcement.
2. Membrane failure mechanism appears to be localized and
mechanical in nature. Defect propagation could be the
controlling mechanisms that lead to membrane failure.
In GORE-SELECT membranes, ePTFE reinforcement
may provide effective barriers to defect propagation.
Performance of GORE-SELECT membranes showed
improved life
Comparisons of fuel cell performance of standard GORESELECT membrane A vs. GORE-SELECT membrane B,
which showed improved life, and NAFION membrane 1035
are presented in Figures 6 and 7, respectively.
3. Because of highly localized nature of the failure, degree
of ionomer degradation monitored by fluoride release rate
does not correlate with membrane life.
5.
ACKNOWLEDGMENT
The authors wish to thank technical support and discussion provided by team members of Gore Fuel Cell Technologies: Tim
Sherman, Dr. A. Singh, Dr. J. Rudolph, Thong Le, and Dr. C.
Martin.
6.
LIST OF SYMBOLS
GSM GORE-SELECT Membrane
Tcell fuel cell operating temperature (o C)
D.P.a/c dew Point temperature of anode and cathode inlet gas (o C)
Figure 7: Fuel cell performance of NAFION 1035 compared
with standard GSM A under accelerated conditions
The developmental GORE-SELECT membrane B gave the
same high power density output as standard GORE-SELECT
membrane A under two different test conditions despite the 40%
increase in membrane thickness. We conclude that this membrane can be used in PRIMEA MEAs to provide durable and
high power performance in residential power market. Therefore
GORE-SELECT membrane B has been selected as the choice
of membrane in the PRIMEA Series 5600 MEA targeted for
the commercial residential and stationary PEM fuel cell markets.
FlowH2 /air anode and cathode gas stoichiometry or minimum flow
rate (cc/min)
Pressurea/c anode and cathode operating pressure (psig)
CRXH2 H2 crossover rate (mA/cm2 )
i current density of fuel cell (mA/cm2 )
CARBEL, GORE-SELECT, and PRIMEA are trademarks of W.
L. Gore & Associates, Inc.
NAFION is a registered trademark of E. I. DePont de Nemours
& Company
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REFERENCES
[1] R. Baldwin, M. Pham, A. Leonide, J. McElroy, and T.
Nalette, Journal of Power Sources, 29, 399 (1990).