Journal of The Electrochemical Society, 154 共2兲 B271-B278 共2007兲
B271
0013-4651/2006/154共2兲/B271/8/$20.00 © The Electrochemical Society
Improving PEMFC Performance Using Low Equivalent Weight
PFSA Ionomers and Pt-Co/C Catalyst in the Cathode
Hui Xu,a,z H. Russell Kunz,a,* Leonard J. Bonville,b and James M. Fentonc,*
a
Department of Chemical Engineering, and bCenter for Environmental Science and Engineering,
University of Connecticut, Storrs, Connecticut, USA
c
Florida Solar Energy Center, University of Central Florida, Cocoa, Florida, USA
The effects of lower equivalent weight 共EW兲 perfluorosulfonic acid 共PFSA兲 ionomers and Pt-Co/C catalyst on the cathode
performance of proton exchange membrane fuel cells 共PEMFCs兲 were investigated at two atmospheric pressure operating conditions: low temperature/high relative humidity 共RH兲, 80°C/100% RH, and high temperature/low RH, 120°C/35% RH. Cell voltage
at a current density of 400 mA/cm2 was used for the performance comparison. The optimized content in the electrode changed
with the ionomer EW, from 32% for 1100 EW Nafion, 28% for 920 EW Nafion to 25% for a developmental PFSA 800 EW
ionomer. Compared to 1100 EW Nafion, 800 EW ionomer significantly improved the cell performance by 39 mV at 120°C/35%
RH; however, at 80°C/100% RH, its effect was not apparent. The introduction of Pt-Co/C catalyst into the cathode increased the
cell performance by 43 mV at 80°C/100% RH, which was much higher than a performance improvement at 120°C/35% RH.
Compared to electrodes made of Pt/C and Nafion 1100 EW, the combination of 800 EW Ionomer and Pt-Co/C catalyst resulted
in a 55 mV cell voltage increase at 80°C/100% RH and a 48 mV cell voltage increase at 120°C/35% RH.
© 2006 The Electrochemical Society. 关DOI: 10.1149/1.2401059兴 All rights reserved.
Manuscript submitted May 30, 2006; revised manuscript received September 14, 2006.
Available electronically December 29, 2006.
In proton exchange membrane fuel cells 共PEMFCs兲, an effective
cathode must provide transport for all involved species for effective
oxygen reduction. The three required reactants are protons from the
membrane to the catalyst layer, electrons from the current collector
to the catalyst layer, and reactant oxygen and product water to and
from the catalyst layer and the gas channels.1 Optimization of
PEMFC electrodes was previously described;2 however, this optimization was targeted for low-temperature PEMFC operation.
PEMFCs operated at elevated temperatures 共⬎100°C兲 have a number of advantages.3,4 However, operation of PEMFCs at high temperature and ambient pressure results in a low RH. The requirements
of the cathode structure for two different operation conditions, low
temperature/high RH and high temperature/low RH, may be different.
Our previous study showed that the catalytic activity for the oxygen reduction increased with an RH increase until 60-70%.5,6 Above
60–70% RH, the effect of the RH on the catalytic activity was not
apparent. The variation of catalytic activity with RH is due to two
changes: proton activity and oxide coverage on the platinum
surface.7,8 Higher proton activity accelerates the oxygen reduction
reaction; however, increased oxide coverage suppresses oxygen reduction because the reactive sites on the platinum surface are
blocked by oxygenated species.9 To increase the proton activity at
high temperature and low RH, low equivalent weight perfluorosulfonic acid 共PFSA兲 ionomers 共800 EW and 920 EW兲 were introduced
into the electrodes in this study. It was hoped that the increased
proton activity could improve the oxygen reduction rate. Some platinum alloys have shown higher activity for oxygen reduction than
platinum because the platinum coverage with oxidized species is
inhibited by the introduced second metal.10,11 Carbon-supported
Pt-Co was used as the cathode catalyst to evaluate its effect on the
cell performance at the two operating conditions, low temperature/
high RH and high temperature/low RH. The optimized content of
each ionomer for both Pt/C and Pt-Co/C was obtained. The effect of
catalyst loading on the cell performance was also investigated.
Experimental
Membrane and electrode assembly.— The membrane electrode
assemblies 共MEAs兲 were prepared by applying both the anode and
the cathode catalyst layers directly onto a composite membrane,
Nafion and phosphotungstic acid impregnated into porous Teflon
* Electrochemical Society Active Member.
z
E-mail: [email protected]
共NTPA兲,12 with a thickness of 30 ␮m. The base electrode catalyst
was 46.6 wt% Pt/C 共Tanaka Kikinzoku Kogyo, Tokyo, Japan兲. The
catalyst was mixed with different ionomer solutions by stirring and
ultrasonic dispersion before being applied onto the membrane on
both sides. The platinum loading for both anode and cathode was
0.4 mg/cm2. The catalyst-coated membrane was sandwiched between two SGL gas diffusion layers 共SGL Carbon Group, Wiesbaden, Germany兲 to obtain three 25 cm2 MEA for single-cell polarization measurements. The gasket pinch was 300 ␮m each on each
side of the MEA, for a total pinch of 600 ␮m. A single serpentine
25 cm2 active area graphite flow field was used 共ElectroChem, Inc.,
Woburn, MA兲.
Cyclic voltammetry.— Cyclic voltammetry 共CV兲 was performed
using a PAR 273A Potentiostat 共Princeton Applied Research, Oak
Ridge, TN兲 with hydrogen at the anode and nitrogen at the cathode.
The RH of anode and cathode gases was controlled by saturators in
a MEDUSA test station 共Teledyne Energy Systems, Inc., Los Angeles, CA兲. Before the measurement, the MEA was equilibrated at the
experimental condition for 1 h. The CV was scanned from 0.05 to
0.8 V with a scan rate of 20 mV/s. Five CV cycles were performed
and the last cycle was recorded for analysis.
Electrochemical impedance spectroscopy.— Electrochemical
impedance spectroscopy was conducted using an Electrochemical
Interface SI 1287 共Solartron Analytical, Houson, TX兲 combined
with a frequency response analyzer SI 1260 共Solartron Analytical,
Houson, TX兲 and ZPlot 2.6b software 共Scribner Associates, Inc.,
Southern Pines, NC兲. Impedance spectroscopy was performed with
hydrogen at the anode and oxygen at the cathode. The voltage was
0.9 V with an amplitude of 30 mV. The frequency was from 104 to
0.1 Hz. Before the impedance test began, the cell was conditioned at
200 mA/cm2 for 1 h.
Polarization curves.— A 890C load box 共Scribner Associates,
Inc., Southern Pines, NC兲 was used to measure cell voltage and
resistance. Five minutes were spent at each current density with the
cell voltage collected every 20 s. Cell internal resistance, R, was
measured and recorded at current densities higher than 100 mA/cm2
using the current interrupt technique with the load box and the Fuel
Cell V3.2 software 共Scribner Associates, Inc., Southern Pines, NC兲.
IR-corrected cell voltages are equal to the cell voltages plus IR. The
utilization of hydrogen in the anode was 33% for current densities
above 400 mA/cm2. Below this current density, a minimum flow
rate of 0.2 L/min was used. The utilization of air or oxygen in the
cathode was 25% for current densities above 120 mA/cm2. Below
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Journal of The Electrochemical Society, 154 共2兲 B271-B278 共2007兲
this current density, a minimum flow rate of 0.2 L/min was used.
The total pressure on both sides was maintained at 1 atm.
Effect of low EW ionomers.— In addition to the 5% Nafion 1100
EW dispersion solution 共Solution Technologies, Mendenhall, PA兲,
two lower EW ionomers were used in this study, Nafion 920 EW
共DuPont, Wilmington, DE兲 and developmental PFSA ionomer 800
EW 共confidential supplier兲. These three solutions would be called
1100 EW ionomer, 920 EW ionomer, and 800 EW ionomer in short,
respectively. For 1100 EW ionomer, 15, 25, 32, 35, and 40 wt %
共weight percent兲 contents were introduced to the cathodes, respectively. For 920 EW ionomer, the investigated content was 15, 25, 28,
32, and 35%, and for 800 EW ionomer, it was 10, 15, 25, 28, and
32%. The effects of the ionomer EW and ionomer content were
investigated at two conditions, low cell temperature and high RH,
80°C/100% RH and high cell temperature and low RH, 120°C/35%
RH.
Effect of Pt-Co/C.— To study the effect of Pt-Co/C cathode
catalyst on the cell performance, 46.6 wt % Pt/C 共Tanaka Kikinzoku
Kogyo, Tokyo, Japan兲 was consistently used as anode catalyst. At
the cathode, 53.2 wt % Pt-Co 共atomic ratio 3:1兲/C 共Tanaka Kikinzoku Kogyo, Tokyo, Japan兲 was used. The platinum loading for both
cathode and anode was 0.2 mg/cm2. The optimized ionomer loading
with the Pt-Co/C–based cathode was also investigated.
Results and Discussion
Effect of low EW ionomers.— First, the H2/air performance of
1100 EW ionomer-based cell was measured, shown in Fig. 1; Fig. 1a
for 80°C/100% RH and Fig. 1b for 120°C/35% RH. The insert
shows the low current density portion 共0–100 mA/cm2兲 of Fig. 1a
or b. At 80°C/100% RH, the cell voltage increased with the increase
of the ionomer content from 15 to 32%. This was expected because
the introduction of ionomer to the electrodes improves the proton
transport through the entire catalyst layer. In a previous study, the
ionic conductivity of the catalyst layer was found to be proportional
to the volume fraction of Nafion in the catalyst layer.13,14 In the
present study, a cell voltage of 0.701 V was obtained at
400 mA/cm2 when the ionomer content was 32%. However, when
the ionomer content was increased from 32 to 35%, the cell performance slightly decreased. The cell voltage decreased significantly
when the ionomer content was further increased from 35 to 40%.
The cell voltage at 400 mA/cm2 dropped to 0.61 V at 40% ionomer.
The large performance loss was due to increased transport resistance
which was observed at high current densities as the cell voltage bent
very steeply with increasing current density. At high content, ionomer blocks the catalyst layer as an electronic insulator; this increases
the electronic resistance in the catalyst layer. More seriously, the
excess of ionomer in the electrode blocks gas diffusion in the reaction sites and resulted in high transport resistance. An ionomer film
model proposed by Gasteiger and Mathias may better explain this
phenomenon.15 In this model, it is assumed that there is a thin film
of Nafion ionomer around each platinum particle. The oxygen must
penetrate through this film to reach the platinum surface to be reduced. Because the reduction rate is influenced by the local oxygen
concentration on the platinum surface, the rate is determined by
oxygen permeability through the film and the film thickness. The
ionomer content in the catalyst must then be controlled precisely to
achieve the best cell performance with a good balance of catalytic
activity, proton conductivity, and gas permeability. This balance is
more important in the case of high temperature and low RH, as
shown in Fig. 1b. For the same ionomer content, the performance at
120°C/35% RH is much less than that at 80°C/100% RH. This is
due to significant membrane and electrode resistance losses at the
low RH condition 120°C/35% RH. The polarization curve at
120°C/35% RH almost displays the same tendency as that at
80°C/100% RH. The cell performance improved with an increase in
ionomer content 15 to 32%; the cell performance went down after
the ionomer content was further increased. The highest cell voltage
Figure 1. Effect of 1100 EW ionomer content on the cell performance with
Pt/C-based electrodes at H2/air with 1 atm total pressure: 共a兲 80°C/100%
RH; 共b兲 120°C/35% RH. Platinum loading was 0.4 mg/cm2. The insert
shows low current density portion.
0.57 V at 400 mA/cm2 was obtained at 32% ionomer content. The
cell voltage of the electrode with relatively low ionomer contents,
15% and 25%, were 0.41 and 0.52 V at 400 mA/cm2, respectively.
At the very high ionomer content of 40%, the cell performance was
0.32 V, even lower than that at 15% and 25%. Oxygen permeability
is affected by the RH; Broka et al.16 showed that the oxygen
permeability was 9.0 ⫻ 10−12 mol/共cm s atm兲 for a fully saturated
Nafion membrane. However, it decreased to 3.0 ⫻ 10−12 mol/
共cm s atm兲 under dry membrane condition. Therefore, low RH in a
combination with a thick ionomer film around the platinum results
in serious oxygen transport problems thus significantly decreasing
the cell performance.
Figure 2 shows the cell performance of the electrodes with 920
EW ionomer. Figure 2a is for 80°C/100% RH and Fig. 2b is for
120°C/35% RH. At 80°C/100% RH, the cell performance went up
when the 920 EW ionomer content was increased from 15 to 28%,
then decreased when the ionomer content was further increased. The
maximum cell voltage was 0.715 V at 400 mA/cm2 with a 28%
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Journal of The Electrochemical Society, 154 共2兲 B271-B278 共2007兲
B273
Figure 2. Effect of 920 EW ionomer content on the cell performance with
Pt/C-based electrodes at H2/air with 1 atm total pressure: 共a兲 80°C/100 RH;
共b兲 120°C/35% RH. Platinum loading was 0.4 mg/cm2.
Figure 3. Effect of 800 EW ionomer content on the cell performance with
Pt/C-based electrodes at H2/air with 1 atm total pressure: 共a兲 80°C/100%
RH; 共b兲 120°C/35% RH. Platinum loading was 0.4 mg/cm2.
content of 920 EW ionomer. At 120°C/35% RH, the cell voltage
also peaked at 28% content, which was 0.59 V at 400 mA/cm2.
Figure 3 shows the cell performance of the electrodes with 800 EW
Ionomer. Figure 3a is for 80°C/100% RH and Fig. 3b is for
120°C/35% RH. At 80°C/100% RH, the cell performance increased
when the 800 EW ionomer content was increased from 10 to 25%,
then decreased when the 800 EW ionomer content was further increased. The maximum cell voltage was 0.721 V at 400 mA/cm2
with a 25% content of 800 EW ionomer. At 120°C/35% RH, the cell
voltage also peaked at a 25% ionomer content, which was 0.604 V
at 400 mA/cm2.
To better compare the performance of the cells made with various EW ionomers, the cell voltages vs the ionomer content at two
current densities, 10 and 400 mA/cm2, were plotted, as shown in
Fig. 4 and 5. Figure 4 is for 80°C/100% RH. The cell voltage at
10 mA/cm2 was plotted, as shown Fig. 4a, because at low current
density the cell voltage is mostly kinetically controlled so that the
catalytic activity for the ORR could be compared. This figure shows
that the effect of different EW ionomers on the cell performance is
different at various contents. At 32% content 共the optimized content
for 1100 EW ionomer兲, the cell voltage increases from 0.877 to
0.891 V 共14 mV gain兲 when the EW is reduced from 1100 to 800;
however, at 25% content, there is a gain of 31 mV 共0.87 to 0.901 V兲
with the same EW lowering. At low current density, the performance
is basically controlled by the cathode activation, which is related to
the oxygen reduction kinetics. The oxygen reduction rate is influenced by the proton activity in the cathode. The same content of
lower EW ionomer provides more protons. Figure 4b shows the
performance at 400 mA/cm2. The performance changes little with
the lower EW at 32% content at which the cell voltage increases
from 0.701 to 0.715 V. At a 25% ionomer content, however, the
improvement is about 34 mV, from 0.689 to 0.723 V. More importantly, it was observed that the maximum performance was achieved
at different contents for different EW ionomers. These optimum
contents are 32% for 1100 EW ionomer, 28% for 920 EW ionomer,
and 25% for 800 EW ionomer. This observation has a significant
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Figure 4. Effect of ionomer EW and content on the cell performance with
Pt/C-based electrodes at H2/air with 1 atm total pressure at 80°C/100% RH:
共a兲 cell voltage at 10 mA/cm2; 共b兲 cell voltage at 400 mA/cm2. Platinum
loading was 0.4 mg/cm2.
Figure 5. Effect of ionomer EW and content on the cell performance with
Pt/C-based electrodes at H2/air with 1 atm total pressure at 120°C/35% RH:
共a兲 cell voltage at 10 mA/cm2; 共b兲 cell voltage at 400 mA/cm2. Platinum
loading was 0.4 mg/cm2.
meaning in that with the small percentage of lower EW ionomer in
the electrodes, the required amount of protons is satisfied and the
ionomer film around the platinum particles becomes much thinner.
This will lower the oxygen transport resistance in the electrodes. At
low RH where the oxygen permeability is low, the advantage of the
lower EW ionomers in the electrodes is more significant.
Figure 5 shows the comparison at the 120°C/35% RH condition.
Figure 5a is the performance comparison at 10 mA/cm2. At 32%
ionomer content, the cell voltage changes from 0.871 to 0.892 V, a
21 mV gain with an EW lowering from 1100 to 800; at a 25%
ionomer content, this gain is 51 mV. At both ionomer contents, 32%
and 25%, the voltage gain with lower EW ionomer is higher than
that at 80°C/100% RH, which is 14 and 31 mV, respectively. Figure
5b is the performance comparison at 400 mA/cm2. At 32% content,
the cell voltage changes from 0.565 to 0.598 V 共32 mV voltage
gain兲 when the EW was lowered from 1100 to 800; however, at a
25% content, the cell voltage increases from 0.545 to 0.604 V, a
59 mV gain. At both contents, 32% and 25%, the voltage gain with
lower EW ionomer at 400 mA/cm2 is bigger than that at
10 mA/cm2. This is because, at low current density, the lower EW
ionomer increases the proton activity and which improves the kinetics of the oxygen reduction. At high current density, the lower EW
ionomer also decreases the protonic resistance of the electrodes. As
shown in a previous study, this protonic resistance may become
significant at high temperature and low RH.17,18 The maximum performance for 1100 EW ionomer is 0.565 V at 400 mA/cm2 at a 32%
content; the best performance with 800 EW ionomer was 0.604 V at
400 mA/cm2 at a 25% content. The difference is ⬃39 mV. This
significant performance improvement is partially due to the decreased oxygen diffusion resistance across the electrodes considering that 32% 1100 EW ionomer and 25% Ionomer 800 EW may
provide a comparable amount of protons.
One advantage of the lower EW ionomer is that at the same
content more protons are provided. This can also be seen from CV
results as shown in Fig. 6. The electrodes used here had 28% content
of each ionomer. Figure 6a shows CVs at 80°C/100% RH and Fig.
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Journal of The Electrochemical Society, 154 共2兲 B271-B278 共2007兲
B275
Figure 7. Effect of Ionomer EW and content 共28%兲 on electrochemical
impedance spectroscopy with Pt/C-based electrodes at H2/air with 1 atm
total pressure at 120°C/35% RH. Anode: H2, cathode:O2, impedance was
performed at 0.9 V with an amplitude of 30 mV and the frequency changes
from 104 to 0.1 Hz. Platinum loading was 0.4 mg/cm2.
Figure 6. Effect of Ionomer EW and content 共28%兲 on the CV with
Pt/C-based electrodes at H2/air with 1 atm total pressure: 共a兲 80°C/100%
RH; 共b兲 120°C/35% RH. Anode: H2, cathode:N2, scan rate: 20 mV/dec.
Platinum loading was 0.4 mg/cm2.
which had a 28 wt % ionomer content in both anode and cathode.
The cell was operated at 120°C/35% RH. The anode reactant is
hydrogen and the cathode reactant is oxygen. The impedance was
performed at 0.9 V with an amplitude of 30 mV. The frequency
changes from 104 to 0.1 Hz. The difference first occurs at the highfrequency intercept, which is 0.48 ⍀-cm2 for 1100 EW ionomer,
0.42 ⍀-cm2 for 920 EW ionomer, and 0.37 ⍀-cm2 for 800 EW
ionomer. The high-frequency intercept is the cell resistance. Provided that the membrane resistance is the same for the three cells,
the difference is due to electrode resistance, primarily the protonic
resistance of the electrode. It is apparent that the introduction of
lower EW ionomer has reduced the protonic resistance of the electrodes at low RH. The second difference is the length of the semicircle that represents the charge transfer resistance. The lower the
charge transfer resistance, the higher the oxygen reduction rate. The
semi-circle is not standard due to the electrode resistances, which
shows a 45° slope at the high-frequency intercept. The extraction of
the exact charge transfer resistance needs a suitable transmission
model. Nevertheless, the much smaller diameter of the semi-circle
with the 800 EW ionomer indicates that the cathode electrode has a
lower charge transfer resistance thus a higher oxygen reduction rate.
6b for 120°C/35% RH. In Fig. 6a, the three CV curves almost
overlap, therefore the hydrogen adsorption and desorption 共0.1 to
0.35 V兲 are almost identical to each other. This is because at the
high RH condition 80°C/100% RH, it is relatively easy to obtain
free protons in the electrodes for proton transfer, so the lower EW
ionomer in the electrodes is not significant. However, for the low
RH case 120°C/35% RH, as shown in Fig. 6b, the situation is completely different. The electrodes with 800 EW and 920 EW ionomers
have much higher hydrogen adsorption and desorption areas than
those with Nafion 1100 EW. The calculated ECA based on hydrogen
adsorption area 共from 0.1 to 0.4 V兲 is 35 m2 /g for 1100 EW ionomer, 46 m2 /g for 920 EW ionomer, and 55 m2 /g for ionomer 800
EW ionomer. Therefore, the effect of lower EW ionomer is more
significant at low RH. This is consistent with the performance results as previously discussed. With the 32% content, the 800 EW
ionomer improved the performance only 20 mV at the 80°C/35%
RH condition, compared to 39 mV at the 120°C/35% RH condition.
Figure 7 shows the electrochemical impedance spectroscopy of
three cells with different EW ionomers in the electrodes, each of
Effect of Pt-Co/C.— Recent studies have shown that Pt-Co/C
catalyst has a much higher catalytic activity than Pt/C due to a
reduced coverage of oxidized species and a changed electronic
structure. However, most of the studies in PEM fuel cells were carried out at low temperature and high RH.11,19 In this study, the effect
of the Pt-Co/C catalyst on the PEMFC performance at both high RH
and low RH was investigated.
Figure 8 shows the effect of Pt-Co/C on the cell performance.
The metal loading used here is 0.2 mg/cm2 with 32% Nafion 1100
EW in the electrodes. Figure 8a shows the effect of the IR-corrected
voltage for H2 /O2. There are four polarization curves, two for the
Pt/C catalyst and two for the Pt-Co/C catalyst. At 80°C/100% RH,
the introduction of Pt-Co/C produced a significant improvement in
the cell performance. The IR-corrected cell voltage with Pt-Co/C is
0.923 V at 10 mA/cm2 and 0.797 V at 400 A/cm2; the cell voltage
with Pt/C catalyst is 0.873 V at 10 mA/cm2 and 0.745 V at
400 mA/cm2. There is a 50 mV cell voltage gain when the
Pt-Co/C catalyst is used as the cathode catalyst. The use of
Pt-Co/C in PEM fuel cells to improve cell performance has been
studied by several researchers.11,19,20 One theory is that the introduction of Co inhibits the formation of the –OH group on the platinum
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Journal of The Electrochemical Society, 154 共2兲 B271-B278 共2007兲
Figure 8. Effect of Pt-Co/C on the cell performance with 32% 1100 EW
ionomer in the electrodes with 1 atm total pressure: 共a兲 H2 /O2; 共b兲 H2/air.
Platinum loading was 0.2 mg/cm2.
surface and thus improves the kinetics of the ORR. X-ray absorption
near-edge data 共XANES兲 shows that the onset of the –OH formation
and adsorption typically occurs at 0.8 V on Pt/C, but shifts to a
higher potential on Pt-Co/C. This leads to a lowering of the overpotential by 60 mV. Another explanation is that Pt alloying with
transition elements increases the Pt d-band vacancy and decreases
the Pt-Pt bond distance, which eventually improves the oxygen reduction kinetics on the platinum surface.19
Figure 8a also shows the performance improvement with the
Pt-Co/C at 120°C/35% RH. The cell voltage with the Pt-Co/C catalyst is 0.851 V at 10 mA/cm2 and 0.692 V at 400 mA/cm2 and the
voltage with the Pt/C catalyst is 0.833 V at 10 mA/cm2 and
0.674 V at 400 mA/cm2. There is only an 18 mV again with the
Pt-Co/C catalyst at low RH. It is also noted that all four-polarization
curves in Fig. 8a are nearly straight lines over two decades of current density 共10–1000 mA/cm2兲, which demonstrates good oxygen
transport property of the electrodes. Tafel slopes calculated from 10
to 1000 mA/cm2 are 65 and 98 mV/dec at 80°C/100% RH and
120°C/35% RH, respectively. There is no effect of Pt-Co/C on the
Tafel slope. Figure 8b shows the effect of the Pt-Co/C on the
hydrogen/air performance. At 80°C/100% RH, the cell voltage at
400 mA/cm2 for the Pt-Co/C catalyst is 0.711 V, which is 45 mV
higher than that of the Pt/C catalyst-based cell with 0.2 mg/cm2
platinum loading and the same 32% content of 1100 EW ionomer.
At 120°C/35% RH, however, this improvement is only 18 mV at
400 mA/cm2. The difference in the voltage gain with the Pt-Co/C
catalyst can be explained by the RH effect on –OH formation on the
platinum surface in our previous study, which was performed using
cyclic voltammetry.5 At low RH, the current density produced by
water oxidation 共Pt + H2O → Pt-OH + H+ + e−兲 is much lower
than that at high RH; the potential at which water oxidation is of a
maximum is much higher with the low RH. Therefore, operation of
PEM fuel cells at low RH delays the –OH formation and adsorption
on the platinum surface. In this case, the advantage of the Pt-Co/C
catalyst may be reduced. It is also possible that the Nafion content
with respect to Pt-Co/C in the electrodes is not optimized. A 32%
content of 1100 EW Nafion was used in the electrodes for this
Pt-Co/C catalyst study. At this content, Pt/C catalyst has the highest
cell performance, but this may not be the case for the Pt-Co/C
catalyst. Therefore, it is worthwhile to optimize the Nafion content
in Pt-Co/C-based electrodes.
Figure 9 shows the effect of 1100 EW ionomer content with
respect to the Pt-Co/C catalyst on the H2/air performance. Figure 9a
is for 80°C/100% RH. It shows that good performance occurs in a
region with an ionomer content from 28 to 32%. The best performance was obtained with a 28% content of 1100 EW ionomer,
0.738 V at 400 mA/cm2. The optimized ionomer content of the
Pt-Co/C-based electrodes is different from that of Pt/C-based electrodes. The latter had an optimized 1100 EW ionomer content of
32%, at which the cell voltage was 0.656 V at 400 mA/cm2. For an
optimized content of 1100 EW ionomer, the Pt-Co/C improves the
cell performance 82 mV in comparison with the Pt/C. When the
content of 1100 EW ionomer is above 32% or below 24%, the cell
performance declines very steeply. Figure 9b shows the dependence
of the performance on the ionomer content at 120°C/35% RH. The
electrodes with 28% 1100 EW Nafion had the best performance,
0.536 V at 400 mA/cm2. However, the optimized Pt/C-based electrode 共at 32% content of Nafion 1100 EW兲 has a cell voltage of
0.51 V at 400 mA/cm2. The improvement with Pt-Co/C was only
25 mV. The lower Nafion content required for the Pt-Co/C may be
due to the different chemical composition and property of this catalyst. The metal percentage of the Pt/C catalyst is 46.6%; however,
the Pt-Co/C catalyst has a 53.1% metal percentage. The introduction
of Co metal in the catalyst may also change the surface structure of
the platinum. All these changes may have affected the interface between the platinum crystallite and ionomer and their interactions. At
120°C/35% RH, the cell performance does not change as much as
that at 80°C/100% RH when the ionomer content is above 28% or
below 24%. This shows that the dependence of cell performance on
Nafion content is not only related to the catalyst in the electrodes,
but also affected by the cell operating condition 共high or low RH兲.
The combined effect of lower EW ionomer with the Pt-Co/C
catalyst in the cathode was also investigated. In this study, the metal
loading was still 0.2 mg/cm2 with 28% ionomer content. Figure 10a
is for 80°C/100% RH. The performance can be seen to increase
slightly with the lowering of the EW. At 400 mA/cm2, the cell voltage was 0.755 V for the 800 EW ionomer-based cell, in contrast to
the cell voltage of 0.739 V for ionomer 1100 EW-based cell. Figure
10b is for 120°C/35% RH. The effect of the lower EW ionomer on
the performance at low RH is greater. At 400 mA/cm2, the cell
performance was 0.573 V for the 800 EW ionomer-based cell,
which is 38 mV higher than that of 1100 EW ionomer-based cell. As
shown in our previous studies, the optimized content of Pt/C-based
electrodes changes with the ionomer EW. For the Pt-Co/C catalystbased electrode, the optimized content for the 1100 EW ionomer is
28%. This percentage may change with the EW of the ionomers. So
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Journal of The Electrochemical Society, 154 共2兲 B271-B278 共2007兲
Figure 9. Effect of 1100 EW ionomer content on the cell performance with
Pt-Co/C-based electrodes at H2/air with 1 atm total pressure: 共a兲 80°C/100%
RH; 共b兲 120°C/35% RH. Platinum loading was 0.2 mg/cm2.
it is worthwhile to investigate the optimized content of 920 EW
Nafion and 800 EW ionomer for Pt-Co/C-based electrodes in the
future.
Conclusions
The effects of lower EW ionomers and Pt-Co/C catalyst in the
cathodes electrodes were investigated at two PEM fuel cell conditions: low cell temperature and high RH, 80°C/100% RH, and high
cell temperature and low RH, 120°C/35% RH. The lower EW ionomer improves the cell performance more significantly at
120°C/35% RH than at 80°C/100% RH due to the low proton activity and high electrode resistance at 120°C/35% RH. The optimized content in the electrodes changed with the ionomer EW. The
optimization was 32% for 1100 EW ionomer, 28% for 920 EW
ionomer and 25% for 800 EW ionomer. The introduction of Pt
-Co/C catalyst into the cathode increased the cell performance more
at 80°C/100% RH than at 120°C/35% RH. This is because at
120°C/35% RH, the water oxidation and –OH group adsorption on
the platinum surface is much less than those at 80°C/100% RH. In
B277
Figure 10. Effect of ionomer EW on the cell performance with
Pt-Co/C-based electrodes at H2/air with 1 atm total pressure: 共a兲 80°C/100%
RH; 共b兲 120°C/35% RH. Platinum loading was 0.2 mg/cm2.
this case, the effect of Pt-Co/C on inhibiting the –OH group is not
apparent. Compared to electrode made of Pt/C and 1100 EW ionomer, the combined 800 EW ionomer and Pt-Co/C catalyst resulted
in a 55 mV cell voltage increase at 80°C/100% RH and a 48 mV
cell voltage increase at 120°C/35% RH.
Acknowledgment
Financial support for some of this work was provided by UTC
Fuel Cells through DOE agreement DE-FC04C-02-A1-67608 for
the Development of High-Temperature Membranes and Improved
Cathode Structures.
The University of Central Florida assisted in meeting the publication
costs of this article.
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