Electrochemical and Solid-State Letters, 6 共12兲 A272-A274 共2003兲
A272
0013-4651/2003/6共12兲/A272/3/$7.00 © The Electrochemical Society, Inc.
Effects of Hydrogen Sulfide on the Performance of a PEMFC
R. Mohtadi,a,* W.-k. Lee,a,** S. Cowan,a J. W. Van Zee,a,**,z
and Mahesh Murthyb,**
a
Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208, USA
W. L. Gore & Associates, Incorporated, Elkton, Maryland 21921, USA
b
An exploratory study of H2 S poisoning of membrane electrode assemblies 共MEAs兲 in proton exchange membrane fuel cells
共PEMFCs兲 consisting of Pt and Pt-Ru alloy electrodes is presented. Steady-state polarization curves are reported for each electrode
after exposure to 50 ppm H2 S at 70°C. Significant findings include 共i兲 partial recovery of the MEA after 3.8 h of exposure to H2 S;
(ii) the degree of the recovery is influenced by the electrochemical oxidation of two surface species observed during cyclic
voltammetry experiments; (iii) in contrast to CO poisoning, Ru has no effect on increasing MEA tolerance toward H2 S poisoning;
and (i v ) increasing the Pt loading by 60% appears to quadruple the partially recovered current density at 0.6 V 共i.e., 0.125 A/cm2
for Pt-Ru alloy and 0.575 A/cm2 for Pt electrodes兲 after exposure to neat H2 for 24 h.
© 2003 The Electrochemical Society. 关DOI: 10.1149/1.1621831兴 All rights reserved.
Manuscript submitted May 29, 2003; revised manuscript received July 16, 2003. Available electronically October 7, 2003.
In a ‘‘hydrogen challenged’’ economy, the fuel for proton exchange membrane fuel cells 共PEMFCs兲 is produced mainly from
reformed hydrocarbons. Thus the anode may be exposed to undesirable by-products, such as carbon monoxide, ammonia, and hydrogen
sulfide. While it is well known that H2 S severely poisons Pt, the
extent of H2 S poisoning of PEMFCs has received only minimal
attention in literature. Also, H2 S poisoning of Pt-Ru alloy catalysts
has not received any attention. Here we present some preliminary
data that may help focus work on sulfur tolerant catalysts for
PEMFCs.
Ex situ testing of H2 S poisoning of Pt has been studied in both
the aqueous,1-4 and the gaseous phases.5-8 The hydrogen chemisorption capacity on Pt decreases during H2 S poisoning. Studies showed
evidence of the presence of polysulfides or neutral sulfur when a Pt
electrode was sulfided with H2 S in aqueous medium.2,8 The effect of
gaseous phase H2 S on Pt supported on alumina catalysts was
reported5 to show that coverage of the Pt surface atoms by sulfur is
a result of H2 S dissociation. Therefore, the poisoning effect of H2 S
on Pt appears to be a result of the blockage of the Pt active sites by
a sulfur adsorbed species.
One in situ study9 showed that exposure of a membrane electrode assembly 共MEA兲 with Pt catalyst to concentrations of H2 S up
to 8 ppm considerably degraded cell performance, and that no recovery of the cell performance was observed after exposure to neat
H2 . These results contradict our observations with high performance
MEAs as discussed below. We are interested in testing the recovery
of performance after exposing the MEA to neat H2 and after performing cyclic voltammetry 共CV兲 on the poisoned MEA. We are
also interested in differences between Pt and Pt-Ru alloy catalysts
and thus we compare the cell performance for two readily available
MEAs. This paper reports the results obtained for MEAs poisoned
with 50 ppm H2 S in H2 as the first step in an ongoing project
studying the effects of H2 S poisoning on PEMFCs. We select this
relatively high concentration 共i.e., 50 times higher than the 1 ppm
suggested in a DOE report10兲 because we are interested in significant
signals that allow us to develop accelerated durability tests 共ADTs兲.
Experimental
All experiments were performed using test stations manufactured
by Fuel Cell Technology, Inc. 共Albuquerque, NM兲. The gases used
were high purity neat H2 共99.997%兲, high purity neat H2 premixed
with 50 ppm H2 S, bottled industrial grade air, and high purity N2 .
The flow rates were set manually according to the measured current
* Electrochemical Society Student Member.
** Electrochemical Society Active Member.
z
E-mail: [email protected]
for a fixed stoichiometry of 1.2 for the anode and 2.0 for the cathode. A triple path serpentine flow field was used. Other experimental
details can be found in Ref. 11. The anode and the cathode flows
were cocurrent. The temperature of the cell was fixed at 70°C for all
experiments, and the pressures were 101 kPa on each side. Each cell
was held at 0.60 V for 60 h before the polarization data were obtained. The humidification temperatures were 85/75°C.c The estimated dew points were 80/70°C, respectively, from the humidity
calibration data.12 No indication of MEA flooding due to excess
water was observed at these humidification temperatures during initial experiments in neat hydrogen.
Two types of MEAs were used in this study. The Gore PRIMEA
MEA series 5510 共25 ␮m nominal membrane thickness兲 has a catalyst loading of 0.40 mg/cm2 Pt on both the anode and cathode sides,
and our cell used an active area of 20.0 cm2. The other MEA, the
PRIMEA MEA series 5621 共35 ␮m nominal membrane thickness兲
has 0.45 mg Pt-Ru/cm2 catalyst on the anode, 0.60 mg Pt/cm2 on the
cathode, and our cell used gaskets, so this MEA has an active area of
23.0 cm2. We used these loadings because we wanted to compare the
effects of Pt loading, while maintaining approximately the same
degree of dispersion of the catalyst on a mass basis. That is, the 0.45
mg/cm2 Pt-Ru alloy and the 0.40 mg/cm2 Pt dispersions may be
more similar than 0.40 mg/cm2 and 0.20 mg/cm2 Pt loadings. This
means that the microscopic structures of the two electrodes used in
this study are probably similar due to their similar catalyst loadings.
The gas-diffusion layers used were CARBEL CL GDM
(16 mils ⫽ 0.406 ⫻ 10⫺3 m兲 and silicone coated glass fiber gaskets
with a thickness of 10 mils (0.254 ⫻ 10⫺3 m兲 were used on the
anode and the cathode sides.11 A thin gasket (1.2 mils ⫽ 0.305
⫻ 10⫺4 m兲 referred to as subgasket, was placed between the MEA,
the GDM, and the gasket. Eight lubricated bolts were threaded into
tapped holes on one of the end plates, and the cell was compressed
with a torque of 50 in.-lbf /bolt.
The current-voltage polarization curves were measured in neat
H2 before the MEA was exposed to 50 ppm H2 S in H2 for 3.8 h.
During the 3.8 h of exposure to H2 S, the cell was held at a constant
voltage of 0.69 V for the Pt and 0.67 V for the Pt-Ru alloy anodes
because these voltages resulted in 12 A in neat H2 prior to exposure.
The flow rate was 101/420 standard cm3 based on 0°C and 101 kPa
corresponding to a stoichiometry of 1.2/2.0 at 12 A in neat H2 . The
measurement of the polarization curves required approximately 2 h
of additional exposure of H2 S and they were recorded after the 3.8 h
of exposure and prior to reintroducing neat H2 . The reintroduction
c
Our convention for specifying operating conditions is to list the anode conditions
first followed by the cathode conditions. Thus, the humidification temperatures
were 85°C for the anode and 75°C for the cathode.
Electrochemical and Solid-State Letters, 6 共12兲 A272-A274 共2003兲
Figure 1. Effect of 50 ppm H2 S/H2 on cell performance. Open symbols and
dashed lines correspond to Pt-Ru alloy 共i.e., PRIMEA MEA series 5621兲 and
closed symbols and solid lines correspond to Pt only catalyst 共i.e., PRIMEA
MEA series 5510兲. History of exposure to H2 S is indicated by symbols: 共䊊
and 䊉兲 Initial MEA in neat H2 ; 共〫 and ⽧兲 after 3.8 h of 50 ppm H2 S/H2 ;
共䊐 and 䊏兲 after exposure of poisoned MEA to 24 h of neat H2 ; 共䉮 and 䉲兲
after CV.
subjected the anode side to neat H2 for 24 h while the cell performance was monitored at the 0.69 and 0.67 cell voltages. After 24 h
of exposure to neat H2 , a polarization curve in neat H2 was recorded. Next, CV was performed according to the procedure described below and then a second polarization curve in neat H2 was
measured. The polarization data reported below were reproducible
to within ⫾10 mV and the results of the CVs did not depend on
whether they were measured directly after exposure to the 3.8 h of
H2 S or if the H2 S poisoned electrode was first exposed to 24 h of
neat H2 . The data should be considered as representative of beginning of life 共BOL兲 and they were with multiple MEAs.
The CV was measured while flowing N2 on the anode 共used as
the working electrode兲 and neat H2 on the cathode.13 The cathode
was then used as the counter/reference electrode. An EG&G Instruments model 263A potentiostat/galvanostat was used for performing
CV measurements. The potential range was between 0.05 and 1.4 V
for the Pt anode. Due to concerns about the oxidation of Ru, the CV
scan range was limited between 0.05 and 0.90 V for the Pt-Ru anode. The scanning rate employed for both MEAs was 5 mV/s and
four voltammograms were measured in each case.
Results and Discussion
During the 3.8 h exposure, the current decreased in an approximately exponential fashion for both catalysts. For the Pt MEA 共held
at 0.69 V兲 the current decreased 1.3 A in the first hour and another
3.6 A in the second hour. The final value of 0.49 A was observed
after 3.7 h of exposure for the Pt MEA. For the Pt-Ru alloy 共held at
0.67 V兲 the current decreased 2.1 A in the first hour and 5.2 A in the
second hour. The final value of 0.07 A was observed after 3.2 h of
exposure for the Pt-Ru alloy. The faster poisoning in the Pt-Ru alloy
electrode may be due to the lower number of Pt sites as discussed
below. Future work is sought to characterize these decreases for
other concentrations. Thus, H2 S is shown to severely degrade the
performance of both MEAs and Fig. 1 indicates the degree of poisoning in terms of the polarization performance. These effects are
consistent with the literature on H2 S poisoning of Pt electrodes1,2,4,6
and Pt supported catalysts.5,8 These poisoning effects could be assigned to strong H2 S adsorption on Pt causing the Pt sites to be
inaccessible to hydrogen according to the following mechanism proposed by Mathieu and Primet5
H2 S-Pt → Pt-S ⫹ H2
关1兴
A273
Figure 2. Comparison of anode overpotentials 共calculated by difference兲 for
PRIMEA MEA series 5621 and MEA series 5510 exposed to 50 ppm
H2 S/H2 . Open symbols and dashed lines correspond to Pt-Ru alloy 共i.e.,
PRIMEA MEA series 5621兲, and closed symbols and solid lines correspond
to Pt only catalyst 共i.e. PRIMEA MEA series 5510兲. History of exposure to
H2 S is indicated by symbols: 共〫 and ⽧兲 after 3.8 h of 50 ppm H2 S/H2 ; 共䊐
and 䊏兲 after exposure of poisoned MEA to 24 h of neat H2 ; 共䉮 and 䉲兲 after
CV.
Pt-H ⫹ H2 S → Pt-S ⫹ 3/2H2
关2兴
This decrease in current may also occur because H2 S dissociation
produces a sulfur block that prevents hydrogen adsorption on Pt.
During the H2 S adsorption, Najdeker and Bishop2 suggested that
Pt-S 共platinum sulfide兲 could be formed electrochemically according
to Reaction 3. They also suggested that Pt-S2 共Pt disulfide兲 could be
formed according to Reaction 4
Pt ⫹ H2 S ↔ Pt-S ⫹ 2H⫹ ⫹ 2e⫺
⫹
Pt-S ⫹ H2 S ↔ Pt-S2 ⫹ 2H ⫹ 2e
Eⴰ ⫽ 0.30 V
⫺
ⴰ
E ⫽ 0.01 V
关3兴
关4兴
The CV data discussed below appears consistent with multiple adsorption sites rather than the formation of Pt-S2 .
Figure 1 shows the difference between Pt and the Pt-Ru alloy in
the partial recovery of the cell performance after exposure of the
MEAs to neat H2 and after CV measurements. The MEA recovery in
neat H2 can be explained by the reaction of H2 with sulfur according
to the reverse of Reaction 1 or 2 described above. These reactions
form H2 S that is flushed away with the continuously fed stream of
neat H2 . A comparison of the two MEAs and hence the two catalysts is aided by a graph of anode overpotential such as that shown
in Fig. 2. That is, it is difficult to compare the poisoning effects
since the baseline performances with neat hydrogen for the Pt and
Pt-Ru anodes are different as described above. These overpotentials
are calculated from the difference between the cell potential with
neat hydrogen and the cell potential of interest at the same current
density. Therefore, to call this an overpotential, it is assumed that the
hydrogen overpotential with neat hydrogen is negligible, and that
the ohmic contribution to the cell voltage and the cathodic overpotential depend only on the current density. The recovery in neat H2
for the Pt-Ru anode 共series 5621兲 is lower than the corresponding
recovery for the Pt anode 共series 5510兲 as seen from the higher
overpotential values. The presence of Ru appears to provide no additional tolerance for the MEA toward H2 S and this is in contrast to
its major role with CO as an impurity.14 The Pt loading on the anode
of the series 5510 is approximately 60% higher and this increase
seems to play a major role in providing higher recovery in neat H2 .
In Fig. 2, the recovery is larger after performing the CV measurements, and higher recovery is obtained for the Pt anode compared to
the Pt-Ru anode, probably as a result of the scan limits 共see Fig. 3兲.
Continued exposure to H2 S beyond the 3.8 h yields higher anode
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Electrochemical and Solid-State Letters, 6 共12兲 A272-A274 共2003兲
the H2 S poisoned Pt at 80°C in aqueous medium. This means that
the species formed during H2 S poisoning appear to be mainly Pt-S.
During the potential sweep, the adsorbed sulfur is oxidized to SO3
4
or SO⫺
4 according to the following reactions proposed by Loučka in
the aqueous phase
Pt-S ⫹ 3H2 O ↔ SO3 ⫹ 6H⫹ ⫹ 6e⫺ ⫹ Pt
关5兴
⫹
⫺
Pt-S ⫹ 4H2 O ↔ SO2⫺
4 ⫹ 8H ⫹ 6e ⫹ Pt
关6兴
Thus in addition to the formation of sulfides 共i.e., Reactions 3兲,
sulfate ions can also be produced as reported by Wang et al.6,7
Conclusions
For high performance MEAs such as the PRIMEA MEA series
5510 and 5621, a concentration of 50 ppm hydrogen sulfide is sufficient to study performance losses due to catalyst poisoning in
shorter time durations. This concentration and exposure time may be
used for ADTs of new anodes. The presence of Ru does not appear
to provide higher tolerance toward H2 S poisoning in contrast to the
role it plays during CO poisoning. After H2 S poisoning, the lower
recovery in neat H2 of the Pt-Ru anode compared to the Pt anode
appears to be due to the lower Pt loading. A trifunctional electrode
may be useful if the MEA is exposed to a fuel containing both CO
and H2 S, so that higher tolerance than a Pt-Ru electrode toward both
contaminants is provided. Partial recovery of the cell performance is
observed after exposure to neat H2 and after CV, and this indicates
that the adsorption of H2 S is reversible even at the relatively high
concentrations of H2 S. Higher recovery is obtained after CV measurements compared to recovery in neat H2 indicating that electrochemical cleaning of the surface by oxidation, or using an electrochemical filter15 may allow for a recovery of poisoned electrodes.
Acknowledgments
Figure 3. Comparison of CVs after exposure to 50 ppm H2 S/H2 , 共a兲 for Pt
anode and 共b兲 for Pt-Ru alloy anode. CV scan cycles from first to fourth
cycle are denoted by numbers from 1 to 4.
overpotentials than those shown in Fig. 2; thus continued exposure
to H2 S at the 50 ppm level appears to result in continued poisoning.
These data for larger exposure times are not shown or discussed here
because the 3.8 h exposure accomplishes our goal of establishing a
baseline suitable for ADTs.
The CV in Fig. 3a shows the presence of two distinct oxidation
peaks at 0.89 V 共oxidation peak I兲 and 1.09 V 共oxidation peak II兲
共note the Pt reduction peak at 0.75 V兲, whereas these peaks are
absent for the Pt-Ru anode in Fig. 3b, probably due to the maximum
applied voltage of 0.90 V. The current increase observed for Pt-Ru
anode during the CV scan at high potentials is responsible for the
partial oxidation of the species that is noticeable at 0.89 V for Pt
anode. This means that voltages higher than 0.90 V are required to
oxidize the adsorbed sulfur yielding higher cell performance recovery at this temperature and pressure. Due to catalyst characteristic
differences between Pt and Pt-Ru alloy anodes, the Pt reduction
peak observed in the Pt anode CV scan is absent for the Pt-Ru alloy
CV scan. The peaks that are observed for the CV scan on the Pt
anode give evidence of the presence of two forms of chemisorbed
sulfur that are strongly and weakly bound to the Pt forming Pt sulfides. These two peaks have been observed by Contractor and Hira1
at 0.97 and 1.10 V during potentiodynamic scan measurements on
The authors gratefully acknowledge that W. L. Gore & Associates, Inc., supported this work. One author, S.C., was supported
during a summer NSF-REU program 共NSF grant no. DMR-973227兲
while she was an undergraduate at Central Florida University.
PRIMEA, GORE, and CARBEL are trademarks of W. L. Gore &
Associates, Inc.
University of South Carolina assisted in meeting the publication costs of
this article.
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