Journal of The Electrochemical Society, 155 共6兲 B532-B537 共2008兲
B532
0013-4651/2008/155共6兲/B532/6/$23.00 © The Electrochemical Society
Membranes and MEAs Based on Sulfonated Poly(ether ketone
ketone) and Heteropolyacids for Polymer Electrolyte Fuel
Cells
Vijay Ramani,a,*,c,z Steven Swier,b,d M. T. Shaw,a,b R. A. Weiss,a,b H. R. Kunz,a,*
and J. M. Fentona,**,e
a
Department of Chemical Engineering and bInstitute of Materials Science, University of Connecticut,
Storrs, Connecticut 06269-3136, USA
Organic sulfonated poly共ether ketone ketone兲 共SPEKK兲 membranes with different ion-exchange capacities 共IECs兲, and composite
membranes prepared by the addition of 20 wt % phosphotungstic acid 共PTA兲 to SPEKK were used to prepare membrane electrode
assemblies 共MEAs兲. The proton conductivity of the membranes increased with increasing IEC of the SPEKK, and with the
addition of PTA. The proton conductivity attained at 80°C and 75% relative humidity was 20 ⫾ 2 mS/cm. The feasibility of
using SPEKK in the cathode layer of the MEAs was investigated. The electrochemically active surface areas 共ECAs兲 of the
SPEKK-based cathodes were lower than that of the Nafion-based cathode and decreased further as the operating relative humidity
was lowered. These observations were reflected in the single-cell polarization data, which indicated that the MEAs with the
SPEKK-based electrodes were outperformed by their Nafion-based counterparts. Furthermore, a mismatch in SPEKK IEC between the membrane and cathode resulted in immiscibility at the interface. While the additive stability in the composite membrane
was very good, the long-term stability of the membranes was poor when compared to perfluorosulfonic acid membranes such as
Nafion, with failure occurring by scission along the gasket edges of the MEA after limited operation.
© 2008 The Electrochemical Society. 关DOI: 10.1149/1.2898171兴 All rights reserved.
Manuscript submitted October 23, 2007; revised manuscript received February 13, 2008. Available electronically April 8, 2008.
Sulfonated hydrocarbons offer an alternative to perfluorosulfonic
acid membranes such as Nafion for polymer electrolyte fuel cell
共PEFC兲 and direct methanol fuel cell 共DMFC兲 applications. Advances in the area of hydrocarbon-based membranes for fuel cell
applications have been summarized in several recent reviews.1-8
Composite membranes based on Nafion and heteropolyacids 共HPAs兲
have been previously studied9-11 in an attempt to reduce the dependence of membrane conductivity on water content. Stabilization of
the HPA additive within the ionomeric matrix, designed to prevent
HPA leaching in aqueous environments, has recently been demonstrated in Nafion-based systems.12 Studies have also been performed
on composite membranes prepared using hydrocarbon-based matrices that contain HPAs.13-15 These studies attest to the utility of HPA
additives in membranes designed for medium to high relative humidity 共RH兲 PEFC operation.
Sulfonated poly共ether ketone ketone兲 共SPEKK兲16 is a protonconducting material with good film-forming properties. However,
the conductivity of SPEKK is strongly dependent on the water uptake of the membrane and, concomitantly, on the ion exchange capacity 共which in turn is a function of the degree of sulfonation兲. The
operating range and composition of SPEKK membranes are therefore limited to fully saturated conditions and high ion exchange
capacities 共IECs兲, respectively. Perfluorosulfonated polymer electrolyte membranes 共PEMs兲 such as Nafion 共IEC = 0.9 mequiv/g兲 intrinsically have a higher acid ionization constant 共lower pKa兲 when
compared to SPEKK, which implies that a higher IEC is needed for
SPEKK to obtain the same conductivity under a given set of operating conditions. However, the mechanical properties of the SPEKK
films deteriorate with increasing IEC. This results in low operational
lifetimes for membrane electrode assemblies 共MEAs兲 prepared using high IEC SPEKK. Hence, a trade-off exists between proton conductivity and operational lifetime in the case of SPEKK membranes.
One approach to get around this trade-off is to prepare composite
membranes with a low IEC organic matrix supplemented with inorganic additives to enhance proton conductivity. An added benefit of
this approach is that the inorganic particles will also serve to impede
the crossover of methanol from anode to cathode. To this end,
SPEKK/HPA composite membranes using low IEC SPEKK were
prepared and studied, with the results obtained discussed in this
paper.
While significant attention has been devoted to membrane development, many of the hydrocarbon-based membranes detailed in the
references above have been evaluated ex situ to determine proton
conductivity. MEAs have been prepared using these alternate
membranes.17-20 However, until recently the electrode layers in
these MEAs have been either devoid of proton-conducting material
or employed perfluorosulfonic acid ionomers such as Nafion as the
proton-conducting component and binder. While Nafion is an admirable proton-conducting binder, concerns exist about the integrity of
the membrane electrode interface due to the mismatch that exists
between the hydrocarbon-based membranes and the fluorocarbonbased Nafion. Commercialization of hydrocarbon-based membranes
could well be contingent on the concomitant development of compatible proton-conducting materials to be used in the electrode layers. Recognizing this, Lakshmanan et al.21 and Easton et al.22 investigated the influence of the addition of sulfonated poly共ether ether
ketone兲 共SPEEK兲 as a binder in PEM gas diffusion electrodes. Jung
et al.23 have investigated the effect of SPEEK as an electrode binder
in DMFCs.
In this study, MEAs were prepared for the first time using
SPEKK binders with varying IECs in the cathode. Unlike prior studies involving SPEEK, the MEAs were prepared by applying the
catalyst layer directly to the membrane 共as opposed to applying the
catalyst layer to the gas diffusion layer, followed by hot pressing
onto the membrane兲. This technique improves the utilization of electrocatalyst in the MEA. The MEAs were characterized by linear
sweep voltammetry 共LSV兲, cyclic voltammetry 共C-V兲, and polarization tests to determine the feasibility of using SPEKK as the protonconducting material in the cathode layer to promote interfacial stability.
Experimental
* Electrochemical Society Active Member.
** Electrochemical Society Fellow.
c
Present address: Illinois Institute of Technology, Chicago, Illinois, USA.
d
Present address: Dow Corning, Midland, Michigan 48640, USA.
e
Present address: Florida Solar Energy Center, University of Central Florida,
Orlando, Florida 32922, USA.
z
E-mail: [email protected]
Materials.— SPEKK.— PEKK with a terephthaloyl 共T兲 to isophthaloyl 共I兲 ratio of 6/4 共OXPEKK SPb, Tg = 154°C兲 was obtained
from Oxford Performance Materials, Enfield, CT. Sulfonation of
PEKK was performed in a 5% 共w/v兲 mixture of 53/47 共v/v兲 concentrated sulfuric acid and fuming sulfuric acid as described in Ref. 16.
Journal of The Electrochemical Society, 155 共6兲 B532-B537 共2008兲
The sulfonation level, expressed as an ion-exchange capacity, defined as the concentration of sulfonate groups in milliequivalents per
gram, was determined by titration.
SPEKK/HPA and SPEKK-modified HPA membranes.— SPEKKs
with IEC between 1.2 and 2.5 mequiv/g were dissolved in dimethyl
acetamide 共DMAc兲 to produce a 5 wt % solution. Appropriate quantities of phosphotungstic acid 共PTA兲 and stabilized PTA were added
to this precursor solution, followed by stirring at room temperature
for 1 h. Stabilized PTA 共insoluble in water兲 was prepared as described previously.12 The desired composite membrane was obtained
by casting the solution onto a flat glass plate followed by evaporation at 60°C for 15 h to yield membranes that could be readily
peeled from the plate surface. In all cases, the additive loading in the
composite membranes was 20% by weight.
Membranes prepared with IEC 2.4 SPEKK had poor film properties and could not be directly used to prepare MEAs. Reinforced
membranes were prepared by impregnating a porous poly共tetrafluoroethylene兲 共PTFE兲 matrix with a SPEKK 2.4 IEC solution in
methanol, followed by drying.
MEA preparation and assembly.— The catalysts used were 46.5
wt % Pt/C on the cathode and 30.1% Pt–23.4% Ru/C on the anode.
Both anode and cathode catalysts were purchased from Tanaka
Kikinzoku Kogyo, Japan. MEAs were prepared by spraying catalyst
ink containing 25 wt % Nafion, SPEKK 共1.4 IEC兲, or SPEKK 共2.1
IEC兲, and with no ionomer binder present onto one side of the
membrane to produce the cathode, followed by spraying catalyst ink
containing 25 wt % 共Nafion on the opposite side兲 to produce the
anode. The noble metal loadings on the cathode were between 0.35
and 0.4 mg/cm2 in each MEA. The inks prepared using SPEKK
also contained small quantities of DMAc that was used to dissolve
the methanol insoluble SPEKK. As a consequence, the cathode layer
was applied in 8–10 steps, with a small amount of catalyst dispersion sprayed onto the membrane surface during each step. This ensured that the residual DMAc did not excessively swell and/or dissolve the membrane during MEA fabrication.
Subsequent to anode and cathode catalyst application, the MEAs
were placed between 2 thin PTFE 共i.e., Teflon兲 sheets, which were
then placed between two rubber sheets. The entire assembly was
introduced between two stainless steel plates and hot-pressed at a
temperature of 120°C and a pressure of 207 kPa 共30 psig兲. The
MEAs were assembled in a 5 cm2 hardware with single serpentine
flow fields 共Electrochem Inc., model FC05–01SP兲. Commercial gasdiffusion layers 共SGL Carbon, model 10BB兲 were used. To seal the
MEA, we used 250 ␮m thick PTFE gaskets on each side of the
MEA. The thickness of the gasket was carefully chosen to ensure
that a pinch 共defined as 关tMEA + 2ⴱtGDL − 2ⴱtgasket兴, where t stands
for thickness and GDL means gas diffusion layer兲 of 300 ␮m was
in place. This pinch was chosen after detailed experimentation to
identify the pinch that yielded the lowest contact resistance while
maintaining the integrity of the MEA during testing. Finally, the
hardware was closed by applying a uniform torque of 3.5 N m to
each of eight bolts.
Techniques.— Transmission electron microscopy.— SPEKK/HPA
and SPEKK/stabilized HPA membranes and an SPEKK/SPEKK
blend 共different IECs兲 were embedded in an epoxy resin 共EponAraldite embedding mixture兲 followed by ultramicrotomy with a
diamond knife to obtain thin sections, which were placed on copper
grids and studied using a transmission electron microscope 共TEM,
Philips 420, 80 kV兲. The cross-sectional morphologies of the
samples were thus investigated.
Fourier transform infrared (FTIR) spectroscopy.— The IR spectra
of SPEKK and SPEKK/HPA membranes were obtained directly in
transmittance mode before and after washing samples in hot water
and methanol. The spectra were analyzed to determine if the HPAs
were stable in these media.
Voltammetry.— LSV experiments were performed at room temperature 共⬃25°C兲 to evaluate and monitor fuel crossover and to check
for the presence of electronic short-circuits. C-V experiments were
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carried out at room temperature and at 80°C and different relative
humidities to determine the electrochemically active surface area
共ECA兲 of the different cathode compositions studied. LSV and C-V
experiments were performed using an electrochemical interface 共Solartron Analytical, model 1286兲. Briefly, hydrogen was passed
through the fuel cell anode 共counter/reference electrode for this experiment兲 and nitrogen was passed through the fuel cell cathode
共working electrode for this experiment兲. For both experiments, the
working electrode potential was varied between 0 and 0.8 V vs
reference, with a reverse sweep included for the C-V. The sweep rate
for the LSV experiments was 4 mV/s and for the C-V experiments
was 30 mV/s. Further details of the experimental procedures have
been reported in a previous publication.11
Resistance and conductivity estimation.— The resistance of the
membrane was obtained using the current-interrupt technique built
into the fuel cell testing system. The conductivity was estimated
from the measured value of resistance using recorded values of the
active area 共5 cm2兲 and thickness of the membrane. The resistance
measurements were made with the cell temperature at 80°C, with
anode and cathode reactant gas temperatures at 73°C, corresponding
to an inlet RH of 75%. The measurements were automatically recorded when polarization curves were obtained, as described below.
MEA performance–polarization curves.— The performance of
the cell was evaluated by obtaining polarization curves at 80°C
共both anode and cathode gases saturated at 73°C for 75% RH operation兲. Pure hydrogen was used at the anode and air or oxygen was
used at the cathode. All data were obtained at an outlet pressure of 1
atm. Briefly, the reactant gases were introduced and the fuel cell was
placed under load at a constant voltage of 0.55 V until a constant
value for current and resistance were obtained. At this point, a current scan experiment was performed with data at each current density being collected for 5 min to ensure that the recorded voltage
was stable. The experiment was stopped when the cell voltage
dropped below 0.15 V. To perform the tests, we used a model 890 B
electronic load-box from Scribner Associates alongside a flow loop
共with humidifiers and mass flow controllers兲 built in-house. The load
box was preprogrammed with a current interruption routine above a
current of 1 A. The voltage gain during the short current interruption
period was recorded and used to estimate the resistance of the cell.
The resistance obtained using this technique is attributed to
membrane + contact resistances. Contact resistances were minimized by adjusting the pinch and compression of the fuel cell hardware. Further details about the test system and test conditions used
are provided in a previous publication.11
Results and Discussion
TEM.— TEM images of SPEKK, SPEKK 20% PTA, and
SPEKK 20% stabilized PTA are shown in Fig. 1a-d, respectively兲.
The IEC of the SPEKK in each case was 1.4 mequiv/g. The composite membrane prepared by adding unmodified PTA 共which is
completely soluble in the precursor solution兲 to SPEKK results in a
fairly uniform morphology, suggesting that the PTA was well dispersed within the polymer and that the particle size of the PTA
formed was very small. The stabilized PTA 共which is insoluble in
the precursor solution兲 yielded larger, sedimented particles. While
the sedimentation is not desirable 共a homogeneous dispersion of
additive is sought兲, the stability of the modified additive in methanol
solutions as discussed in the next section is highly encouraging.
Additionally, a higher resolution image of the stabilized PTA clusters 共Fig. 1d兲 indicated that the clusters were comprised of aggregates of 15–20 nm particles of stabilized PTA. This suggests that by
proper optimization of the membrane preparation process, a better
dispersion of the stabilized additive throughout the membrane may
be achieved.
TEM images of pure SPEKK 共1.4 IEC兲 and a SPEKK 共1.4 IEC兲/
SPEKK 共2.1 IEC兲 blend are shown in Fig. 2a and b, respectively.
These images constitute micrographs of the cross sections of
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Journal of The Electrochemical Society, 155 共6兲 B532-B537 共2008兲
Figure 1. TEM images of 共a兲 SPEKK, 共b兲 SPEKK/PTA, and 共c, d兲 SPEKK/
stabilized PTA membranes. The SPEKK IEC was 1.4 mequiv/g.
DMAc-cast membranes. The morphology of pure SPEKK with IEC
of 1.4 mequiv/g 共Fig. 2a兲 was, as expected, homogeneous. However,
the blend formed with dissimilar SPEKK IECs 共Fig. 2b兲 revealed
significant immiscibility. The consequences of such immiscibility
with respect to fuel cell performance is further discussed in forthcoming sections.
FTIR–additive stability.— FTIR spectra of SPEKK, SPEKK/
PTA, and SPEKK/stabilized PTA membranes after boiling in water
for 3 h are shown in Fig. 3. The SPEKK IEC was 1.4 mequiv/g. The
presence of PTA was confirmed by the feature at 980 cm−1 corresponding to the W = Ot 共terminal oxygen兲 bond in the PTA 共see
dashed line in Fig. 3兲. A feature was also seen at 890 cm−1, corresponding to the W–Oc–W 共tungsten–corner shared oxygen兲 bond in
PTA. These features were absent in the pure SPEKK membrane.
Figure 3 shows that both the PTA and the modified PTA additives
were stable within the membrane matrix upon washing in boiling
water. In contrast, the FTIR spectra taken after treatment in methanol at 50–60°C for 3 h 共Fig. 4兲 indicates that the unstabilized PTA
leached out upon treatment with methanol 共absence of the absorbance at 980 cm−1, the spectrum resembles pure SPEKK兲, while the
stabilized PTA was still present within the membrane matrix. This
qualitative study suggests that while both varieties of composite
membranes are suitable for PEFC application, composite membranes prepared using stabilized PTA additives are better suited for
DMFC applications.
Figure 3. FTIR spectra of 共a兲 SPEKK, 共b兲 SPEKK/PTA, and 共c兲 SPEKK/
stabilized PTA after treatment in boiling water for 3 h. The SPEKK IEC was
1.4 mequiv/g.
ode layer兲. Both membranes possessed uniformly low hydrogen
crossover limiting current densities of 0.2 mA/cm2 in the region
between 0.3 and 0.4 V on the voltammogram. Experiments on membranes with different IECs suggested that the IEC and the PTA did
not affect the crossover current. Internal shorting in the MEAs was
not significant, as evidenced by the negligible slope in the limiting
current region of Fig. 5.
C–V.— C–V images obtained at room temperature on electrodes
containing Nafion, 1.4 IEC SPEKK, and 2.1 IEC SPEKK are presented in Fig. 6. The ECA of each electrode was determined from
the area of the oxidation or reduction peaks between 0.05 and 0.35
V on the cyclic voltammograms. The ECA of SPEKK-based electrodes was less than half of that of Nafion-based electrodes for similar noble metal loadings. Increasing the IEC of SPEKK from 1.4 to
2.1 mequiv/g improved the activity of the electrode, as was reflected
by a larger peak area. This was attributed to the higher concentration
of proton conductive sulfonated groups and hence, better proton
conductivity in the electrode.22
Figure 7 presents C–Vs of MEAs containing SPEKK 1.4 IEC
共80°C, 100 and 75% RH兲. The observed electrochemical activity
decreased in the SPEKK-based MEA as the RH was reduced from
100 to 75%, with the oxidation/reduction peaks nearly disappearing
LSV.— Figure 5 shows representative LSV images obtained at
room temperature using SPEKK 共IEC of 1.2 mequiv/g兲 and SPEKK/
PTA composite membranes 共Nafion used as the ionomer in the cath-
Figure 2. TEM images of 共a兲 SPEKK 1.4 IEC and 共b兲 SPEKK 1.4 IEC/
SPEKK 2.1 IEC blend.
Figure 4. FTIR spectra of 共a兲 SPEKK, 共b兲 SPEKK/PTA, and 共c兲 SPEKK/
stabilized PTA after treatment in methanol at 60°C for 3 h. The SPEKK IEC
was 1.4 mequiv/g.
Journal of The Electrochemical Society, 155 共6兲 B532-B537 共2008兲
B535
Figure 5. Room temperature LSV images of 共a兲 SPEKK and 共b兲 SPEKK/20
wt % PTA composite membranes. The IEC of SPEKK used was 1.2
mequiv/g. The sweep rate used was 4 mV/s.
Figure 7. C–V images of MEAs with cathodes containing 共a兲 1.4 IEC
SPEKK 共80°C and 100% RH兲 and 共b兲 1.4 IEC SPEKK 共80°C and 75% RH兲.
The sweep rate used was 30 mV/s.
at the latter condition. The double-layer capacitance 共the plateau
region between 0.4 and 0.6 V兲 was significantly lower at the lower
RH, which indicated a reduced interaction between SPEKK and the
carbon catalyst support in the cathode. The voltammograms shown
in this paper are representative of results obtained using several sets
of MEAs.
and SPEKK/PTA-based MEAs. Hence, we believe our comparison
between the two conductivities is valid. The composite membranes
possessed higher conductivities than pure SPEKK over the entire
range of IECs, demonstrating that the PTA additive has a beneficial
effect. The improvement obtained was attributed to the higher conductivity and larger water uptake of the PTA additive. Data for
PTFE-reinforced membranes with a SPEKK IEC of 2.4 mequiv/g
are also shown for comparison. The lack of significant improvement
in conductivity as the IEC was raised from 1.7 to 2.4 mequiv/g was
attributed to the fact that these membranes contained up to 30%
nonconductive PTFE as reinforcement.
Resistance and conductivity.— Figure 8 shows the conductivities of SPEKK and SPEKK/PTA membranes with SPEKK IECs
ranging from 1.2 to 1.7 mequiv/g obtained at 80°C and 75% RH.
The membrane conductivity was determined from resistance values
obtained in situ using the current-interrupt technique. One must recognize that the current-interrupt technique will be convoluted by the
presence of contact resistances. Hence, only an effective resistance
and “apparent conductivity” can be obtained. While our pinch has
been optimized to minimize the influence of contact resistance between hardware components and the MEA, the contact resistances
between the membrane and the electrode is difficult to eliminate,
especially because dissimilar proton conductors are used in the
membrane and the electrode 共Nafion is used in the electrode and
SPEKK in the membrane兲.22 However, we also note that the techniques used to prepare the MEA were identical in both cases and the
same contact resistances will manifest themselves for both SPEKK-
Figure 6. Room-temperature C–V images of MEAs containing 共a兲 Nafion,
共b兲 1.4 IEC SPEKK, and 共c兲 2.1 IEC SPEKK in the cathode. The sweep rate
used was 30 mV/s.
Single-cell performance.— Figure 9 shows polarization data and
area specific resistances 共ASRs兲 obtained on oxygen and air at 80°C
and 75% RH using pure SPEKK, SPEKK/PTA, and SPEKK/
stabilized PTA membranes 共SPEKK IEC = 1.4 mequiv/g in all
cases; all MEAs contained 25 wt % Nafion in the anode and cathode兲. The calculated electrode noble metal loading was 0.4 mg/cm2
for the pure SPEKK and SPEKK/stabilized PTA MEAs and
0.35 mg/cm2 for the SPEKK/PTA MEA. The performance on a
membrane resistance-free basis reflected the trend in Pt loading. The
SPEKK/PTA and SPEKK/stabilized PTA both had similar ASRs
when normalized to a 25 ␮m membrane thickness. These ASRs
were lower when compared to pure SPEKK. In conjunction with
Figure 8. Conductivities of SPEKK and SPEKK/20 wt % PTA at 80°C and
75% RH as a function of SPEKK IEC.
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Journal of The Electrochemical Society, 155 共6兲 B532-B537 共2008兲
Figure 9. Performance 共closed symbols; left axis兲 and ASR 共open symbols;
right axis兲 data at 80°C and 75% RH using SPEKK 关共쎲兲 H2 /O2兴, SPEKK/
PTA 关共쒀兲 H2 /O2, 共쑽兲 H2 /air兴, and SPEKK/stabilized PTA 关共䊐兲 H2 /O2兴
membranes 共SPEKK IEC = 1.4 mequiv/g兲. The ASRs were normalized to a
membrane thickness of 25 ␮m.
superior stability in methanol, this result illustrated the advantage of
additive stabilization and highlighted the utility of the SPEKK/
stabilized PTA membrane.
Figure 10 shows the performance at 80°C and 75% RH of MEAs
containing 25 wt % SPEKK 共1.4 IEC兲, 25 wt % SPEKK 共2.1 IEC兲,
and no ionomer, respectively, in the cathode layer. The latter MEA
was prepared using a cathode catalyst ink without a binder. The
platinum loading in these MEAs was similar to the Nafion-bonded
electrodes discussed earlier. The performances of the MEAs with
both types of SPEKK in the cathode layers 共Fig. 10兲 were inferior to
MEAs containing a Nafion-bonded cathode 共Fig. 9兲 and a nonbonded cathode 共Fig. 10兲. The slight deterioration in performance
seen when the IEC of SPEKK in the cathode was increased is in
contrast to the enhancement seen in ECA of the electrode under
these conditions 共Fig. 6兲. This apparent contradiction was attributed
to enhanced contact resistances at the membrane cathode interface
arising from the interfacial immiscibility of SPEKK 1.4 IEC and
SPEKK 2.1 IEC, evidence for which is shown by the TEM micrograph in Fig. 2b. In general, the lower performance of MEAs with
SPEKK in the electrode may be attributed to the lower ECAs obtained as well as the lower oxygen permeability in these materials
when compared to Nafion.
Figure 10. Performance data at 80°C and 75% RH on MEAs containing 共a兲
SPEKK 共1.4 IEC兲, 共b兲 SPEKK 共2.1 IEC兲, and 共c兲 no ionomer in the cathode
layer using air as an oxidant. The membrane used was SPEKK 1.4 IEC.
Figure 11. Hydrogen crossover data on SPEKK-based MEAs 共a兲 before
operation, 共b兲 after operation at 75% RH with no cycling, and 共c兲 after
cycling between 75 and 50% RH during operation. The experiments were
conducted at room temperature and humidity; the sweep rate used was 4
mV/s.
The structural stability of the membrane during MEA testing
appeared to be independent of the presence or absence of the PTA
additive. Figure 11 shows LSV images of a SPEKK 1.4 IEC/
stabilized PTA membrane before and after operation at 75% RH and
80°C for 1 day. The LSV images were obtained at room temperature
and humidity at a scan rate of 4 mV/s, and an estimate of the hydrogen crossover flux was obtained from the crossover current obtained from the experiment. The low crossover and absence of internal shorting were indicative of membrane stability over this time
period. The enhanced peak seen in the voltammogram after operation was attributed to the wet-up of the MEA and concomitant enhancement in ECA of the cathode. LSV data for an MEA that was
cycled between 50 and 75% RH during operation is also shown in
Fig. 11. Clearly, there was a large increase in crossover current and
an induced electronic short 共evidenced by the slope in the diffusion
limiting current region of the voltammogram兲 after operation. This
observation was attributed to the swelling–deswelling of the membrane during RH cycling. All RH-cycled MEAs failed by scission
along the gasket edge. The observation was reproducible on multiple
MEAs subjected to identical RH cycling experiments. In contrast,
Nafion membranes had a crossover current of 1–2 mA/cm2 initially
and even after RH cycling, maintained this value.
In any event, the membranes were not stable for more than 2–3
days of operation 共8–10 h of actual operation each day, with the cell
being shut down at the end of each day and restarted the next morning兲. This problem was attributed to temperature and RH cycling
during startup and shutdown, as the most common cause of failure
was once again scission along the edges of the gasket. Membranes
Journal of The Electrochemical Society, 155 共6兲 B532-B537 共2008兲
B537
reinforced with PTFE did not demonstrate such scission even after
exposure to deliberate and repeated RH cycling. This observation
may be explained by considering that the PTFE functioned as a
protective matrix that prevented rapid swelling and shrinkage, and
thereby preserved the integrity of the membrane. Efforts to prepare
PTFE reinforced membranes with lower IEC SPEKK 共lower methanol crossover兲 are ongoing.
cathodes yielded much higher performance than those containing
SPEKK. The increase in ECA with increased cathode SPEKK IEC
共for a membrane SPEKK IEC of 1.4 mequiv/g兲 was not reflected in
MEA performance as it was more than offset by immiscibility at the
interface between the membrane and the electrode. Evidence for this
interfacial mismatch was obtained using transmission electron microscopy.
Conclusions
ACKNOWLEDGMENT
Proton conductivity increased with SPEKK IEC for pure SPEKK
and for SPEKK/PTA composite membranes. Relative to pure
SPEKK, the composite membranes demonstrated superior conductivities across the entire range of IECs studied. MEAs evaluated at
80°C and 75% RH confirmed this observation, with both PTA-based
and stabilized-PTA-based membranes offering advantages over pure
SPEKK in terms of conductivity. No detrimental effects of stabilization of PTA on proton conductivity were observed, despite the
formation of clusters of larger particles in this case. On the contrary,
membranes containing a stabilized additive offered superior stability
in a methanol environment, which bodes well for their application in
DMFCs. The membranes possessed uniformly low H2 crossover
currents and had no internal shorting during the initial stages of
operation. Cycling the MEA between 75 and 50% RH resulted in
MEA failure after only a few hours of operation. The mode of failure was observed to be scission along the gasket edge. This failure
mode was attributed to swelling–deswelling of the membrane during
RH cycling. In most MEAs, failure usually occurred after a few
days of operation 共even without deliberate RH cycling兲 by scission
along gasket edges induced by swelling–deswelling during start-up
and shut-down. The operational lifetime could be enhanced by impregnating a strong porous matrix 共such as porous PTFE兲 with high
IEC SPEKK to yield a reinforced membrane. MEAs prepared with
the reinforced membranes did not fail even after deliberate RH cycling over several days.
The feasibility of using SPEKK in the electrode layers to promote interfacial stability was qualitatively investigated. Although an
increase in ECA was seen when the IEC of SPEKK in the cathode
was increased from 1.4 to 2.1 mequiv/g, the activity of the SPEKKbased cathodes was lower than that of the Nafion-based cathode.
The ECA of the SPEKK-based cathodes decreased further as the
operating RH was lowered. MEAs prepared using Nafion-containing
This research was supported in part by a grant from the U.S.
Department of Energy 共grant no. 69797-001-03 3D兲.
Illinois Institute of Technology assisted in meeting the publication costs
of this article.
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