Journal of The Electrochemical Society, 156 共7兲 B848-B855 共2009兲
B848
0013-4651/2009/156共7兲/B848/8/$25.00 © The Electrochemical Society
Oxygen Reduction Reaction Kinetics of SO2-Contaminated
Pt3Co and Pt/Vulcan Carbon Electrocatalysts
Yannick Garsany,*,z Olga A. Baturina,* and Karen E. Swider-Lyons*
U.S. Naval Research Laboratory, Washington, DC 20375, USA
Sulfur dioxide, SO2, is a common impurity in air that is known to deactivate electrocatalysts for the oxygen reduction reaction
共ORR兲 at proton exchange membrane fuel cell cathodes. The SO2 poisoning of a Vulcan-carbon-supported platinum cobalt alloy
共Pt3Co/VC兲 is compared to that of a standard platinum 共Pt/VC兲 electrocatalyst using cyclic voltammetry 共CV兲 and rotating
ring-disk electrode 共RRDE兲 methodology at controlled concentrations of S共IV兲 in an oxygen-free solution. The CV and RRDE
measurements show that for electrodes with the same Pt loading, the Pt3Co/VC is two times more active than the Pt/VC. Upon
exposure to S共IV兲 solutions, the Pt3Co/VC nanoparticle electrocatalysts are more poisoned than the Pt/VC ones, and their initial
sulfur coverage is higher. The poisoning of both catalysts is accompanied by an increase in the amount of H2O2 production, as
adsorbed sulfur species inhibit the four-electron ORR. The Pt3Co/VC electrocatalyst loses 80% activity in a 0.0001 M S共IV兲
compared to a 30% loss by the Pt/VC electrocatalysts. The adsorbed sulfur species are more easily removed from the Pt3Co/VC
than the Pt/VC by potential cycling, implying a weaker bonding between Sx species and Pt3Co/VC. We conclude that Pt3Co is
more susceptible to poisoning by SO2 than Pt at a given Pt loading, but its activity is more easily recovered.
© 2009 The Electrochemical Society. 关DOI: 10.1149/1.3126383兴 All rights reserved.
Manuscript submitted February 6, 2009; revised manuscript received April 6, 2009. Published May 20, 2009.
For a successful operation of proton exchange membrane fuel
cells 共PEMFCs兲, the cathode 共air兲 catalyst must maintain a high
activity for the oxygen reduction reaction 共ORR兲. The standard
carbon-supported platinum catalysts lose performance due to trace
air contaminants including sulfur dioxide 共SO2兲, nitrogen dioxide
共NO2兲, and organic contaminants.1-5 Poisoning studies have been
performed on fuel cell membrane electrode assemblies with Pt/
Vulcan carbon 共Pt/VC兲 catalysts to determine their sensitivity to
these impurities and to show methods for the recovery of the catalyst performance.2-5
We have previously reported using the solution-based methods
cyclic voltammetry 共CV兲 and thin-film rotating ring-disk electrode
共RRDE兲 methodology to study how SO2 poisoning affects the kinetics of Pt/VC electrocatalysts.6,7 All studies were carried out in a
liquid electrolyte, with dissolved S共IV兲 in a deaerated electrolyte as
the contaminant. For Pt/VC, a significant loss in Pt mass activity
共33%兲 occurred when the Pt was poisoned by 0.0001 M S共IV兲 and
1.2% of the Pt surface had adsorbed sulfur-containing species. Sulfur surface coverage of 14% from poisoning in 0.001 M S共IV兲
caused a 95% loss of Pt mass activity. When the Pt/VC was exposed
to 0.01 M S共IV兲, ⱖ37% of the Pt surface was covered by sulfur
species, the reaction pathway for the ORR on the Pt/VC catalyst
changed from a four-electron to a two-electron process reaction.
Our goal here is to extend our S共IV兲-poisoning studies to a high
activity nanoparticulate platinum cobalt alloy supported on Vulcan
carbon, Pt3Co/VC. Pt alloyed with first-row transition metals such
as Co, Ni, Cr, Fe, and Mn can exhibit 2–10 times the electrocatalytic
activity of pure Pt for the ORR.8-19 The transition metals in the Pt
alloys are soluble under the operating conditions of PEMFC cathodes 共i.e., low pH and high potential兲, so they are leached out from
the surface leading to the formation of a more stable “Pt skin,”
which is formed over the Pt-depleted second layer.16-19 Catalytic
improvement for the Pt skin is caused by electronic modification of
the Pt atoms on the top of the transition-metal-enriched layer underneath. The subsurface transition metals shift the Pt 5d band center
down from the Fermi level and cause a decrease in the adsorption
strength of oxygenated species on the Pt surface, leading to faster
electroreduction kinetics of the reaction intermediates.11,19-21 The
high activity alloys, Pt3Co and Pt3Ni, inhibit OH− adsorption from
water activation and ORR intermediates on Pt sites, which leaves
more Pt sites to participate in O2 reduction.11,16-19 The improvement
in catalytic activity has been hypothesized to be caused by electronic
* Electrochemical Society Active Member.
z
E-mail: [email protected]
modification; however it is by no means the generally accepted
mechanism. Min et al. suggested that the increase in the electrocatalytic activity of the Pt alloys shows a strong correlation with a decrease in the interatomic distance between Pt atoms and therefore
with smaller Pt–Pt bond distances, resulting in more favorable sites
for dissociative adsorption of O2.15 Mukerjee et al.22 attributed the
increase in ORR activity to a decrease in the Pt–Pt bond and Pt–Pt
coordination number. Paffet et al.9 and Beard and Ross23 attributed
the ORR improvement to surface roughening of the Pt alloys, leading to higher activity via higher surface area.
We extend our previously developed methods for Pt/VC to
Pt3Co/VC to compare the relative susceptibility of these nanoparticle alloys to sulfur poisoning. First, the effect of the S共IV兲 concentration in the initial solution is compared for the ORR on Pt3Co/VC
and Pt/VC using RRDE. The electrocatalysts are poisoned in deaerated S共IV兲 solutions of varying concentrations at 0.65 V vs a reversible hydrogen electrode 共RHE兲 and then brought to 0.05 V to reduce
any Sx species to S0.6,7 At 0.05 V, sulfur is in a zero-valent state
共S0兲, but that sulfur adsorbed on Pt is easily electro-oxidized at high
24-35
potentials to sulfate 共SO2−
4 兲, which desorbs from the Pt surface.
RRDE is used to isolate the ORR kinetics 共in an oxygenated electrolyte兲 as the results are well understood mathematically and can be
easily corrected for diffusion limitations of the oxygen gas in solution at high potentials.36-39 The use of the additional ring, which
surrounds the central disk with the electrocatalyst, allows a quantitative detection of hydrogen peroxide 共H2O2兲.40-42 Hydrogen peroxide production is an independent measure of whether the catalyst is
reducing O2 gas by the four-electron conversion to water 共Eq. 1兲, or
by a two-electron process to hydrogen peroxide 共Eq. 2兲
O2 + 4e− + 4H+ → 2H2O
关1兴
O2 + 2e− + 2H+ → H2O2
关2兴
CV is used to determine the initial sulfur coverage, ␪S,i, of the
Pt3Co/VC and Pt/VC exposed to S共IV兲 solutions by correlation to
the charge consumed for the irreversible six-electron oxidation of S0
to water-soluble sulfate. The sulfur oxidation reaction, given in Eq.
3, occurs when the electrodes are cycled up to about 1.50 V
+
−
S0 + 4H2O → SO2−
4 + 8H + 6e
关3兴
For Pt/VC, ␪S,i is determined from the ratio of adsorbed S, as determined from Eq. 3, to the total Pt surface area as determined from the
integration of the area of its hydrogen adsorption region. Because
the hydrogen adsorption region of the Pt3Co/VC surface area is
likely affected by its modified electronic structure, the absolute Pt
surface area is difficult to determine from the usual integration of
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Journal of The Electrochemical Society, 156 共7兲 B848-B855 共2009兲
B849
the charge from hydrogen adsorption, and approximations must be
used to compare its sulfur coverage to that of Pt/VC. Further, the
state of the Pt3Co/VC is poorly understood. As Co dissolution occurs during voltammetric testing in acid electrolyte,19,43 it is very
difficult to know the actual surface and bulk composition of the
alloy catalyst.
We compare the ORR activity of Pt3Co/VC and Pt/VC electrodes
exposed to the same S共IV兲 solution, and then the poisoned electrodes are cycled as a measure of how easily the sulfur is removed
from the electrocatalyst surfaces, from which we infer information
about their relative strength of S adsorption. From these results, we
can better understand how these electrocatalysts might comparatively perform best in the midst of sulfur impurities.
Experimental
We used a precommercial catalyst comprising 30% Pt:Co alloy
共3:1 a/o兲 supported on Vulcan carbon XC72 共E-TEK兲 and a commercially available catalyst comprising 19.7% Pt supported also on
Vulcan carbon XC72 共E-TEK兲. The particle size of the Pt3Co nanoparticles was 4.3 nm, as determined by X-ray diffraction 共XRD兲, and
that of the Pt nanoparticles was 4.2 nm, as determined by both XRD
and high resolution transmission electron microscopy.
The procedures for electrochemical analysis have been reported
for our prior studies of S共IV兲 poisoning of Pt/VC but are repeated
here for clarity. Thin-film electrodes were prepared for voltammetric
analysis using a method adapted from Ref. 44. Inks were prepared
by ultrasonically dispersing 10 mg of the as-received Pt3Co/VC or
Pt/VC powders in 5 mL of a stock solution composed of 20 mL of
isopropanol 共Mallinckrodt, ACS grade兲, 79.6 mL of 18 M⍀ cm
water 共Barnstead Nanopure兲, and 0.4 mL of 5 wt % Nafion ionomer
共Ion Power, Liquion 1100兲 for 30 min in an ultrasonic bath
共Branson-2510兲. Glassy carbon disk electrodes 共0.283 and
0.196 cm2, Pine Instruments兲 were used as a substrate for the supported catalyst and polished for 5 min to a mirror finish using a
0.05 ␮m alumina-particle suspension 共Buehler兲 on a moistened polishing cloth. The electrodes were then rinsed well and washed in
18 M⍀ cm water in an ultrasonic bath for 5 min. An aliquot of the
suspension was loaded onto the glassy carbon disk and dried at
room temperature in air to yield a smooth film that covered the
entire surface of the electrode and a Pt loading of 20 ␮gPt cm−2
共geometric兲.
All electrochemical measurements were conducted using a threeelectrode potentiostatic circuit in a glass cell at room temperature
共⬃25°C兲. The electrodes were attached to a Pine AFMSRX ringdisk rotator coupled with an Autolab bipotentiostat. A gold mesh
was used as a counter electrode. All potentials were measured with
respect to a sealed RHE45,46 using a bridge containing the same
concentration of electrolyte used in the experiment 共0.1 M HClO4兲.
The 0.1 M HClO4 electrolyte was prepared from double distilled
HClO4 共GFS Chemicals, Inc.兲 and 18 M⍀ cm water. Electrolytes
were deoxygenated prior to use with bubbling Argon 共Ar兲 and kept
under a stream of this gas during measurement. Before studies in
other electrolytes, the potential of the Pt3Co/VC and Pt/VC electrodes was cycled between 0.05 and 1.30 V in 0.1 M HClO4 until a
reproducible voltammogram of the expected form was recorded.47
The potential of the Pt ring electrode was held at 1.20 V where the
oxidation of hydrogen peroxide was under pure diffusion control.
The collection efficiency 共N兲 of the ring electrode was determined to
be 0.25 from the ratio of the oxidation current of potassium ferricyanide at the Pt ring vs its reduction current at the glassy carbon
disk.
A S共IV兲 stock solution was prepared by the decomposition of the
appropriate amount of anhydrous Na2SO3 共Fisher Scientific, ACS
grade兲 in 1 M HClO4. Na2SO3 reached equilibrium between SO2−
3
and SO2 in the acid. The concentration of the electroactive species is
reported based on the total amount of S共IV兲 initially dissolved. The
stock solution was then diluted to between 0.0001 and 0.01 M S共IV兲
by injection of a certain volume of the S共IV兲 stock solution into 50
Figure 1. 共Color online兲 Cyclic voltammograms for 30% Pt3Co/VC and
19.7% Pt/VC electrocatalysts in Ar-purged 0.1 M HClO4 Ar at 25°C at a
scan rate 共␷兲 of 50 mV s−1 between 0.05 and 1.30 V vs RHE. The Pt
loading is 20 ␮gPt cm−2 in both electrodes.
mL of deoxygenated 0.10 M HClO4 and homogenized by stirring for
5 min. Throughout the remainder of the paper, the poisoning electrolyte solution is designated by its S共IV兲 molarity, and the 0.1 M
HClO4 solution is implicit.
The Pt3Co/VC and Pt/VC electrodes were poisoned with S共IV兲
by holding them at an adsorption potential 共Eads兲 of 0.65 V for 2 min
while submerged in the deaerated S共IV兲 solution. Then, the electrode was withdrawn covered with a droplet of this solution, rinsed
with 18 M⍀ cm water, and introduced at an initial potential 共Ei兲 of
0.65 V into a clean 0.1 M HClO4 electrolyte in an auxiliary electrochemical cell. For the determination of the sulfur coverage, the poisoned, catalyst-coated electrodes were cycled at a scan rate 共␷兲 of
50 mV s−1 from 0.05 to 1.50 V in Ar-purged 0.1 M HClO4 at room
temperature 共⬃25°C兲. For the ORR experiments, the electrodes
were rotated at 1600 rpm and cycled from 1.03 to 0.0 V and back to
1.03 V at a scan rate of 10 mV s−1 in 200 mL of O2-saturated 0.1
M HClO4 at room temperature 共our prior RRDE measurements on
Pt/VC were carried out at 20 mV s−1, but they were measured here
at 10 mV s−1 to decrease contributions from capacitive current兲.
The gas atmosphere in the cell was controlled by bubbling O2 at a
constant flow of 100 mL min−1 through the electrolyte.
The electrocatalytic activity of the clean and poisoned catalysts
is best compared by their mass activities using the mass transportcorrection for thin-film rotating disk electrodes 共RDEs兲44 in Eq. 4
Ik = 共Id ⫻ I兲/共Id − I兲
关4兴
In this equation, I is the experimentally obtained current, Id is the
measured diffusion-limited current, and Ik is the mass-transport free
kinetic current. For this relative comparison, I is taken from the
value of the curve at 0.90 V and Id is taken from that at 0.30 V. The
mass activities are estimated via the calculation of Ik and the normalization to the Pt loading of the disk electrode. The applicability
of this analysis to our electrodes was supported by the Levich analysis of clean Pt3Co/VC and Pt/VC electrodes at 400, 625, 900, 1225,
1600, and 2500 rpm, which produced a linear plot passing through
the origin, indicative of a mass-transfer-controlled reaction in the
potential range of 0.05 ⱕ E ⱕ 0.70 V.
Results and Discussion
Figure 1 shows the typical CV response for the Pt3Co/VC and
Pt/VC catalysts in Ar-saturated 0.1 M HClO4 electrolyte. Both
Pt3Co/VC and Pt/VC exhibit the typical features of Pt with the
hydrogen adsorption–desorption potential region at 0.05 ⬍ E
⬍ 0.35 V and the Pt–OH adsorption/reduction potential region at
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B850
Journal of The Electrochemical Society, 156 共7兲 B848-B855 共2009兲
Figure 2. 共Color online兲 共A兲 Voltammetry
to 1.50 V of an unpoisoned Pt3Co/VC
electrode compared to 1–5 sequential
cycles of a Pt3Co/VC electrode poisoned
in 0.001 M S共IV兲. 共B兲 Voltammetry to
1.00 V for a Pt3Co/VC electrode poisoned
in 0.001 M S共IV兲. ␷ = 50 mV s−1; 25°C.
0.70 ⬎ E ⬎ 1.30 V. The hydrogen adsorption–desorption peaks
are poorly defined on Pt3Co/VC compared to those of Pt/VC and
also have about one-half the area, despite having similar particle
sizes 共⬃4 nm兲 and Pt loadings. The smaller area of the H desorption peak for Pt3Co/VC and related catalysts was thought to be due
to the occupation of some of the Pt surface atoms by transitionmetal atoms,48 but a decrease in area of about one-third may also be
attributed to the change in the hydrogen adsorption properties for Pt
alloy nanoparticles due to differences in their electronic states.19
Figure 1 also indicates that the onset of –OH adsorption shifts to
more positive potentials for the Pt3Co/VC alloy. For Pt/VC, –OH
adsorption in the anodic going scan starts at around 0.70 V, whereas
on Pt3Co/VC the onset of –OH adsorption is observed near 0.78 V.
This positive voltage shift for –OH adsorption on Pt3Co/VC has
been attributed to an alloy-induced decrease in the bond strength of
the chemisorption of OH on the Pt at potentials above 0.80 V, which
is also the source of its higher ORR activity.16-18
Figure 2A shows a sequence of five voltammetric cycles from a
Pt3Co/VC electrode that has been poisoned with Sx at 0.65 V in a
0.001 M S共IV兲 solution. The polarization program 共0.05 → 1.50
→ 0.05 V兲 starts at the low potential of 0.05 V to ensure that all the
adsorbed Sx species are converted to S0. The curve for the unpoisoned Pt3Co/VC in the Ar-purged 0.1 M HClO4 is shown as a reference in the same figure. The results are similar to what we have
reported for Pt/VC 6,7 and for Pt polycrystalline electrodes.28-35
The adsorbed S0 species on the Pt3Co/VC electrode modifies the
CV curve characteristic of the Pt3Co nanoparticles. During the first
positive sweep from 0.05 to 1.50 V 共CV1兲, S0 is oxidized to Sx
species, causing the broad anodic peak at 1.35 V. During the first
negative sweep from 1.50 to 0.05 V, reduction currents appear at
0.70 and 0.10 V. The peak at 0.70 V is probably due to the reduction
of the oxygen species which adsorb on the Pt at E ⬎ 0.70 V. This
reduction peak becomes more pronounced in subsequent cycles as
the Sx species is oxidized from the Pt surface, leaving more sites for
O adsorption. The second reduction peak 共0.10 V兲 is observed in the
hydrogen region due to H electrochemical adsorption, and it is also
less pronounced in the presence of adsorbed Sx. An additional reduction current is visible between 0.60 and 0.05 V, particularly in
the first CV 共CV1兲 and is attributed to the conversion of remaining
adsorbed Sx species to S0.29 As cycling progresses, the peak near
1.35 V decreases slowly and shifts toward more negative potentials
共1.35 to 1.00 V兲, implying that it is more dominated by the adsorption of oxygen on the Pt rather than Sx oxidation. The area of the
hydrogen adsorption/desorption and adsorbed oxygen reduction
peaks increase with cycling, also indicating that the Sx species have
been removed from the Pt3Co/VC surface. The fifth voltammogram
共CV5兲 is essentially identical to that of an unpoisoned Pt3Co/VC
surface, suggesting that all adsorbed Sx species are removed from
the Pt3Co/VC surface. The electrodes are cycled up to 15 times.
Figure 2B shows a sequence of 10 voltammetric cycles from a
Pt3Co/VC that has been poisoned at 0.65 V in a deaerated 0.001 M
S共IV兲 solution but is cycled only to 1.00 V 共rather than 1.50 V兲.
Again, the presence of the adsorbed S0 species modifies the cyclic
voltammetric curve characteristic of the Pt-metal nanoparticles.
However, no S0 oxidation peak is visible on the first anodic sweep
from 0.05 to 1.00 V in the deaerated electrolyte. Only very small
voltammetric changes are seen over the 10 consecutive scans 共for
clarity of the figure only CV1 and CV5 are shown兲: an extremely
small decrease in the oxidation current at the highest potentials
共E ⱖ 0.80 V兲 and an extremely small decrease in the reduction
current at the lowest potentials 共0.25 V ⱕ E兲. After 10 consecutive
scans up to 1.00 V, a steady voltammogram is obtained. No hydrogen adsorption/desorption peaks 共0.05 ⬍ E ⬍ 0.40 V兲 and oxide
oxidation–reduction 共E ⬎ 0.70 V兲 on the Pt surface are observed,
indicating that the S0 species are not oxidatively removed from the
Pt3Co/VC surface upon cycling up to 1.00 V in an inert atmosphere.
The RRDE results are shown in Fig. 3 for a clean Pt3Co/VC
electrode and a series of S共IV兲-poisoned electrodes poisoned with
S共IV兲 concentrations of 0.01, 0.0025, 0.001, 0.0001, and 0.0005 M.
The ORR results from the disk electrodes are shown in Fig. 3a, the
H2O2 oxidation at the ring electrode is shown in Fig. 3b, and the
calculated fraction of H2O2 produced is shown in in Fig. 3c. The
ORR measurement for the clean Pt3Co/VC has a well-defined
−2
diffusion-limiting current density 共Jlim兲 of −6 mA cmgeometric
at
0.10–0.80 V followed by a region under mixed kinetic-diffusion
control at 0.82 ⬍ E ⬍ 1.00 V. Its ring currents 共Ir兲 in both potential regions are a small fraction of the Jlim. The fraction of H2O2
produced on the Pt3Co/VC is negligible at E ⬎ 0.20 V, indicating
that the reduction in oxygen proceeds almost entirely by a 4e− pathway to water, in agreement with previous work.17,18,48 The activity
−1
of the Pt3Co/VC is about 0.30 A mgPt
, matching the state-of-the−1
art results,49,50 and is about two times higher than the 0.16 A mgPt
that we measure for the Pt/VC.
As the Pt3Co/VC is poisoned with 0.0001–0.01 M S共IV兲, the
onset potential for the ORR shifts to a lower potential, as summarized in Table I. The lower onset potential for the ORR indicates a
higher catalytic overpotential, or deviation from its 1.23 V theoretical value. The Pt3Co/VC poisoned with 0.01 M S共IV兲 exhibits
about 0.46 V more overpotential at J = −1.5 mA cm−2 relative to
the clean Pt3Co/VC. The ORR of the Pt3Co/VC poisoned with 0.01
M S共IV兲 starts at ⬃0.81 V 共compared to 1.01 V for the clean
Pt3Co/VC兲 and is probably under kinetic-diffusion control in the
wide potential window 0.30 ⬍ E ⬍ 0.81 V. The mass activity of
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Journal of The Electrochemical Society, 156 共7兲 B848-B855 共2009兲
B851
Figure 3. 共Color online兲 First anodic scan
from RDE voltammetry of clean
Pt3Co/VC and Pt3Co/VC electrodes poisoned in various S共IV兲 solutions. 共a兲 Disk
current for ORR. 共b兲 Ring currents for
H2O2 oxidation. 共c兲 Fraction of H2O2
formed. Polarization program: 1.03 → 0
→ 1.03 V, in O2-saturated 0.1 M HClO4,
1600 rpm, 25°C, and ␷ = 10 mV s−1.
longer reduced by a 4e− process on this S共IV兲-poisoned electrode.
Other adsorbed species, such as Br− anions,51 Cl− anions,52 or organic adsorbates,53 also inhibit the complete 4e− reduction of the O2
and thus increase the H2O2 formation.
The ORR results reported in Table I for Pt3Co/VC and Pt/VC
show that Pt3Co is much more susceptible to poisoning than the
Pt/VC electrodes when they have the same Pt loading and are exposed to the same concentration of S共IV兲. For instance, Pt3Co/VC
electrodes poisoned with 0.0001 M S共IV兲 have only 20% of their
mass activity for the ORR, while the Pt/VC electrodes are 69%
active. This result may be due to one or several effects. Our
Pt3Co/VC electrodes have a lower Pt electrochemical surface area
共ECSA兲 for the same Pt loading than the Pt/VC electrodes, so the
electrochemically available Pt in the Pt3Co/VC is effectively exposed to a higher S共IV兲 concentration. The Pt in Pt3Co also binds
the S共IV兲-poisoned electrodes decreases with increasing poisoning,
and it is less than 1% of its initial value when poisoned with S共IV兲
concentrations of 0.005 M and greater. These results are also tabulated in Table I. The results for Pt/VC, as reported in our earlier
work, are also given in Table I for comparison.
The decrease in the ORR activity occurs in parallel with an increase in the amount of H2O2 oxidized on the ring electrode, pointing to a change in the ORR pathway. Below 0.30 V, the Pt3Co/VC
poisoned with 0.01 M S共IV兲 produces a diffusion-limiting current of
⬃ − 2.7 mA cm−2 which corresponds to 1.8e− per O2 molecule or
significant peroxide formation according to Eq. 2. As discussed in
our earlier work, the presence of adsorbed Sx species disrupts the
adsorption of O2 on Pt bridge sites on the Pt surface necessary for
the ORR, leaving the opportunity only for H2O2 formation. As expected, a H2O2 oxidation current is detected on the ring electrode,
and H2O2 formation is close to 90%, confirming that oxygen is no
Table I. Initial sulfur „␪S,i… coverage resulting from varying pretreatment solutions, both measured from CVs at a scan rate of 50 mV s−1. The
error for the ␪S,i for the Pt3CoÕVC does not take into account the uncertainty of the Pt surface area.
Pt3Co/VC
Pt/VC
Overpotential/V
关S共IV兲兴
−1
at 0.9 V % mass activity 共J = −1.5 mA cm−2兲
共M兲 A mgPt
0
0.0001
0.0005
0.001
0.0025
0.01
3.0
6.2
2.5
3.7
2.1
2.1
⫻
⫻
⫻
⫻
⫻
⫻
10−1
10−2
10−3
10−4
10−3
10−3
100
20
0.82
0.12
0.70
0.81
0.93
0.88
0.73
0.65
0.55
0.47
Overpotential/V
−1
A mgPt
at 0.9 V % mass activity 共J = −1.5 mA cm−2兲
␪S,i
0.23
0.45
0.54
0.83
0.96
0
⫾
⫾
⫾
⫾
⫾
0.07
0.04
0.06
0.09
0.26
1.6 ⫻ 10−1
1.1 ⫻ 10−1
—
8.0 ⫻ 10−2
—
1.4 ⫻ 10−4
100
69
—
5
—
0.11
0.92
0.9
—
0.81
—
0.69
␪S,i
0
0.012 ⫾ 0.08
NA
0.14 ⫾ 0.06
NA
0.47 ⫾ 0.14
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Journal of The Electrochemical Society, 156 共7兲 B848-B855 共2009兲
B852
Figure 4. 共Color online兲 Polarization
curves for the ORR in 0.1 M HClO4 solution for 共A兲 clean Pt3Co/VC and
Pt3Co/VC electrode covered with Sx species 共␪S,i = 0.45兲, 共B兲 clean Pt/VC and
Pt/VC electrode covered with Sx species
共␪S,i = 0.47兲 as a function of successive
scan, 共C兲 ring currents for H2O2 oxidation
共Er = 1.20 V兲 for clean Pt3Co/VC and
Pt3Co/VC electrodes covered with Sx species 共␪S,i = 0.45兲, 共D兲 for clean Pt/VC and
Pt/VC electrodes covered with Sx species
共␪S,i = 0.47兲 as a function of successive
scans. Ei = 0.65 V, 1600 rpm, 25°C, ␷
= 10 mV s−1, and polarization program:
1.03 → 0.0 → 1.03 V. Pt loading is identical for both electrodes and is equal to
20 ␮gPt cm−2.
hydroxyl groups more weakly than Pt/VC 共as discussed for Fig. 1兲,
which might leave less adsorbed oxygen species and more Pt sites
available for Sx adsorption in the Pt3Co/VC vs the Pt/VC.
While the data reported above show that Pt3Co/VC poisons more
readily than Pt/VC catalysts under the same S共IV兲 exposure, another
comparison of electrocatalyst activity is done by comparing the
catalysts on the basis of their sulfur coverages. We use our prior
method to calculate the initial sulfur coverage, ␪S,i, by determining
the total charge for complete oxidation of adsorbed sulfur 共Eq. 1兲,
兺QSox, divided by the charge for hydrogen desorption 共or hydrogen
oxidation兲, QHox, for the sulfur-free electrode,32 as written in Eq. 5
␪S,i =
兺Q
Sox /6QHox
关5兴
The use of this approximation is less accurate when used for
Pt3Co/VC because of the uncertainty in how the charge of the H
desorption region correlates to the Pt surface area for alloy nanoparticles. As noted in the introduction, the area under the curve for H
oxidation 共electrochemical desorption兲 can be easily correlated with
Pt surface area for pure Pt on a one-to-one basis of Pt to Hads, but
the H adsorption properties are different for the Pt alloys and for
nanoparticles. The surface composition of the Pt3Co/VC may also
change with Sx adsorption and potential cycling.
For this paper, we make the most conservative assumption and
use QHox for both materials and thus interpolate the electrochemical
Pt surface area the same way for the Pt3Co/VC alloy and Pt/VC,
taking into account no approximation for the difference in their hydrogen adsorption/desorption properties. Pt3Co/VC then has about
half the Pt surface area of Pt/VC based on the CVs in Fig. 1, despite
having the same particle size 共4 nm兲. The actual Pt electrochemical
surface area of the Pt3Co/VC is probably higher, and so the data
given below for the initial surface coverages of the Pt3Co/VC are
probably skewed high. But, even a two times correction in ␪S,i for
the Pt3Co/VC does not affect the findings below.
As observed in our Pt/VC studies, ␪S,i increases with increasing
S共IV兲 concentration in the poisoning electrolyte. ␪S,i is listed in
Table I for 5 S共IV兲 concentration solutions measured from the average of 6 replicates from 2 different electrodes 共12 measurements
total兲. The results obtained for the Pt/VC electrocatalyst from Ref. 7
are also included for comparison at selected concentrations. For both
catalysts, the ␪S,i coverage increases with S共IV兲 concentration. The
initial sulfur coverage is significantly higher on the Pt3Co/VC than
on the Pt/VC electrocatalyst for Sx adsorption from the same concentration S共IV兲 solution 关23 vs 1.2% when poisoned in the 0.0001
M S共IV兲 solution and 54 vs 14% when poisoned in the 0.001 M
S共IV兲兴. As hypothesized above, the difference in the increased susceptibility of the Pt to S共IV兲 poisoning in the Pt3Co/VC vs the
Pt/VC may be due to either its lower Pt ECSA 共and exposure to an
effectively higher S共IV兲 concentration兲 or its lower coverage with
adsorbed oxygen species.
The experiments above give little indication of the relative effect
of the Sx adsorbed on the electocatalyst surfaces. For a better comparison of the poisoned catalysts, we compare the ORR activity and
cycling behavior of Pt3Co/VC and Pt/VC electrodes in two ways:
first, with similar ␪S,i values, Fig. 4, poisoned in solutions with a 20
times difference in S共IV兲 concentration and, second, by electrodes
poisoned in the same concentration of S共IV兲 solution, Fig. 5. The
cycling behavior can be used as an indication of how easy it is to
electro-oxidize sulfur from the surface of the electrocatalysts.
For the comparison of the electrodes with similar ␪S,i values in
Fig. 4, the starting electrodes are Pt3Co/VC poisoned in a 0.0005 M
S共IV兲 solution and Pt/VC poisoned in a 0.01 M S共IV兲, with approximate ␪S,i values of ⬃45 and 47%, respectively 共see Table I兲. The
error within the estimation of the ␪S,i value for Pt3Co/VC is discussed above. The ORR on the poisoned Pt3Co/VC with ␪S,i
= 0.45 has an onset potential for the ORR of 0.73 V at
−1.5 mA cm−2 or 0.20 V lower than that of the clean electrode
共Fig. 4A and 3兲. The poisoning results in a decrease in the magnitude of diffusion-limiting current density from ⬃ − 6 to
⬃ − 4.1 mA cm−2, corresponding to 2.7e− per O2 molecule and
the formation of peroxide. Peroxide oxidation is also measured on
the ring electrode for CV1 共Fig. 4C兲. On successive scans, the onset
potential for the ORR shifts toward more positive potentials, and the
ORR potential at J = −1.5 mA cm−2 increases from 0.73 V for
CV1 to 0.84 V for CV10, coming within 0.09 V of the potential
measured at the clean electrode. The value of the limiting diffusion
current also increases with successive scans, recovering by CV10 to
−5.6 mA cm−2. In parallel with the decreasing overpotential and
increasing limiting current, the H2O2 oxidation current on the ring
decreases considerably from ⬃63% for CV1 to ⬃15% for CV10.
All these data suggest that the ORR efficiency of the sulfur-poisoned
electrocatalyst improves with cycling.
The ORR results are shown in Fig. 4B for Pt/VC for an unpoi-
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Journal of The Electrochemical Society, 156 共7兲 B848-B855 共2009兲
B853
Figure 5. 共Color online兲 Comparison of
the ORR activities of Pt3Co/VC and
Pt/VC electrodes poisoned in S共IV兲 solutions for cycles 1, 5, and 10 between 0.05
and 1.03 V. Pt mass activities are reported
from the kinetic current as measured from
the RDE measurement of the ORR at 0.90
V. 共A兲 Pt3Co/VC and Pt/VC percent Pt
mass activity when poisoned in 0.0001 M
S共IV兲; 共B兲 Pt3Co/VC and Pt/VC Pt mass
activity when poisoned in 0.0001 M
S共IV兲; 共C兲 Pt3Co/VC and Pt/VC percent
Pt mass activity when poisoned in 0.001
M S共IV兲; 共D兲 Pt3Co/VC and Pt/VC mass
activity when poisoned in 0.001 M S共IV兲.
soned electrode and one with a ␪S,i = 0.47. The results show that for
the same 共approximate兲 sulfur coverage, Pt/VC is much more poisoned than Pt3Co/VC, and its performance does not recover as fully
with cycling. The ORR potential for the poisoned electrode at a
current density of −1.5 mA cm−2 is 0.55 vs 0.92 V for the clean
electrode, a change of 0.37 V, which is almost double the 0.20 V
decrease in the onset potential measured for the Pt3Co/VC. The
diffusion-limiting current density of the poisoned Pt/VC drops to
−2.7 mA cm−2, or 1.8e− per O2 molecule, rather than 2.7e− per O2
molecule measured for the Pt3Co/VC. Within 10 cycles, the Pt/VC
current density and limiting current does not recover as quickly as
on the alloy, with the limiting diffusion current only reaching
−3.7 mA cm−2 for CV10, rather than −5.6 mA cm−2 for
Pt3Co/VC. The response obtained at the ring electrode is similar
共see Fig. 4D兲. The percentage of H2O2 produced is higher for all
cycles on the Pt/VC, changing from ⬃93% for CV1 to ⬃75 and
⬃64% for CV5 and CV10, respectively, far higher than the 15%
H2O2 production measured for the Pt3Co/VC at cycle 10. The comparison of the cycling behavior of Pt3Co/VC and Pt/C suggests that
sulfur species are easier to remove from Pt3Co/VC than Pt/VC probably due to a weaker interaction between Pt3Co/VC and adsorbed Sx
species.
The cycling behavior of the Pt3Co/VC and Pt/VC electrodes poisoned in the same S共IV兲 concentration also gives information about
the relative adsorption strength of Sx on the two electrocatalysts.
Figure 5 shows the results from Pt3Co/VC and Pt/VC electrodes
poisoned in 0.0001 and 0.001 M S共IV兲 solutions. In CV1 of the
Pt3Co/VC exposed to 0.0001 M S共IV兲, 80% of its initial activity is
lost; only 30% of the Pt/VC activity is lost 共Fig. 5A兲. In terms of the
absolute mass activity, Pt3Co/VC poisoned in 0.0001 M S共IV兲 has a
−1
and is only 45% less active than
mass activity of 0.06 A mgPt
−1
兲 due to its initially higher mass activity 共Fig.
Pt/VC 共0.11 A mgPt
5B兲. The same trend is observed for Pt3Co/VC and Pt/VC poisoned
in 0.001 M S共IV兲. Pt3Co/VC loses more than 99% of its Pt mass
activity, while the Pt/VC loses 95% 共Fig. 5C兲.
Figure 5 also shows the result of how the mass activities of
Pt3Co/VC and Pt/VC recover when the sulfur is electro-oxidized
from the surface by cycling between 0.05 and 1.03 V according to
Eq. 3. For electrodes poisoned in both 0.0001 and 0.001 M S共IV兲,
the Pt mass activity for the Pt3Co/VC recovers more quickly than
for the Pt/VC. In 0.0001 M S共IV兲, Pt3Co/VC recovers from 20 to
35% of its Pt mass activity, while Pt/VC recovers only from 68 to
75% during the 10 cycles 共Fig. 5A兲. Because of the higher initial
mass activity of Pt3Co/VC vs Pt/VC, its activity is close to that of
−1
Pt/VC by the 10th cycle 共0.10 vs 0.12 A mgPt
兲 共Fig. 5C兲. For the
electrodes poisoned in 0.001 M S共IV兲, the Pt3Co/VC is more active
−1
than Pt/VC by the 10th cycle 共0.012 vs 0.020 A mgPt
兲 共Fig. 5D兲.
We interpret the faster rate of recovery during cycling of
Pt3Co/VC vs Pt/VC electrocatalyst to the weaker adsorption
strength of the S to the alloy, making the S adsorbed to the
Pt3Co/VC more easily oxidized and desorbed as a sulfate. The
weaker bond strength of S to the alloy is consistent with the theoretical predictions of Pillay and Johannes.54 They performed density
functional theory 共DFT兲 calculations to understand the poisoning
effect of S on the electronic and chemical properties of Pt共111兲 and
Pt3Ni共111兲 surfaces and showed, using an explicit calculation of the
bonding strengths of coadsorbates, that a single S adatom poisons
both neighboring OH and H adsorption sites, with diminishing but
finite effects even further away. The bond strength of 1/12th of a
monolayer of S occupying surface sites on the Pt skin of a
Pt3Ni共111兲 surface was 2.71–4.62 eV, while it was 2.98–5.26 eV on
a pure Pt共111兲 surface, indicating a weaker bond strength. This observation is consistent with the results in the Pillay and Johannes
paper and with prior calculations and measurements showing that
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B854
Journal of The Electrochemical Society, 156 共7兲 B848-B855 共2009兲
Figure 6. 共Color online兲 Two sets of ORR
polarization curves for Pt3Co/VC electrodes poisoned in 0.001 M S共IV兲 solution
共i.e., ␪S,i = 0.54兲 as a function of successive scans 共anodic scan shown 0.0
→ 1.03 V兲: Set 1 共black and green or
gray circles兲 was recorded immediately
after the poisoned electrode was cycled up
to 1.00 V in argon-saturated 0.1 M HClO4
electrolyte; Set 2 was recorded immediately after the poisoning step and not subjected to cycling up to 1.00 V in Arsaturated 0.1 M HClO4 electrolyte 共black
and green or gray triangles兲. Ei = 0.65 V,
␷ = 10 mV s−1, 1600 rpm, 25°C, and polarization
program:
1.03 → 0.0
→ 1.03 V.
OH− adsorbs more weakly to alloy surfaces and leads to their higher
activity.55 A future publication of ours will address how a similar
DFT analysis can be used to understand the poisoning effect of S on
the electronic and chemical properties of Pt3Co共111兲 vs Pt共111兲 surfaces.
Does the weaker bond strength of the adsorbed Sx species mean
that it can be removed from the Pt3Co/VC surface by cycling up to
1.00 V 共under an inert atmosphere兲 vs the usual 1.50 V? Figure 6
shows two sets of polarization curves recorded at 1600 rpm for the
ORR of a Pt3Co/VC electrode poisoned in 0.001 M S共IV兲 solution
共i.e., ␪S,i = 0.54兲 as a function of successive scan 共anodic scan
shown 0.0 → 1.03 V兲. This first set of polarization curves 共black
and green or gray circles in Fig. 6兲 was recorded immediately after
the poisoned Pt3Co/VC electrode was cycled up to 1.00 V in the
argon-saturated 0.1 M HClO4 electrolyte 共results shown in Fig. 2B兲.
The other set of polarization curves was obtained for a Pt3Co/VC
electrode poisoned in the same 0.001 M S共IV兲 solution but not subjected to cycling up to 1.00 V in Ar-saturated 0.1 M HClO4 electrolyte prior to the ORR, and is shown on the same figure for comparison 共black and green or gray triangles in Fig. 6兲. The poisoned
electrodes were cycled 10 times from 1.03 → 0.0 → 1.03 V, but
for clarify of Fig. 6 only CV1 and CV10 are shown. Figure 6 clearly
indicates that the adsorbed Sx species is still present on the
Pt3Co/VC surface after cycling the poisoned electrode up to 1.00 V
in argon-saturated 0.1 M HClO4 electrolyte. Again, the onset potential for the ORR is shifted toward lower potentials, and the oxygen
reduction current decreases to a Jlim of ⬃ − 4.91 mA cm2 共for
CV1兲. As cycling continues, the onset potential for the ORR shifts
toward higher potentials, and the limiting current density also increase 共Jlim = −5.41 mA cm2 for CV10兲.
The mass activities of the Pt3Co/VC calculated at 0.90 V via Ik
from data given in Fig. 6 and normalization to Pt loading are shown
in Table II. The data obtained for a poisoned Pt3Co/VC electrode in
the same 0.001 M S共IV兲 solution but not subjected to Sx oxidation
up to 1.00 V are summarized in Table III for comparison. It can
clearly be seen that the poisoning effect of adsorbed Sx species is
diminished on the Pt3Co/VC electrode subjected to Sx oxidation up
to 1.00 V before ORR. For CV1, the onset potential for the ORR is
shifted to 0.80 V at J = −1.5 mA cm2, compared to 0.65 V for the
other electrode, and there is a 98% loss of mass activity compared to
99.9% loss of mass activity for the Pt3Co/VC electrode not subjected to Sx oxidation. As cycling continues, the recovery of the
mass activity is more consequent for the Pt3Co/VC electrode subjected to Sx oxidation. After 10 scans, 87% loss in mass activity is
observed for the electrode cycled up to 1.00 V prior to ORR compared to 93% for the electrode not subjected to Sx oxidation. These
observations imply that some oxidation of the adsorbed Sx species
might occur at 1.00 V, changing the initial Sx surface coverage 共i.e.,
␪S,i兲.
Conclusion
The ORR on Pt3Co/VC electrodes is more poisoned by S共IV兲
than that on Pt/VC electrodes with the same Pt loading. The ORR
performance loss for both electrocatalysts is accompanied by the
formation of hydrogen peroxide. The Pt3Co/VC electrodes appar-
Table II. Pt3CoÕVC mass activities at 0.90 V calculated from data
given in Fig. 6. This data set was recorded immediately after the
poisoned Pt3CoÕVC electrode was cycled up to 1.00 V in argonsaturated 0.1 M HClO4 electrolyte (results shown in Fig. 6).
CVs
CV1
CV2
CV5
CV9
CV10
Mass activities
−1
兲
共A mgPt
% mass
activity
Overpotential/V
共J = 1.5 mA cm−2兲
0.006
0.009
0.020
0.036
0.040
2
3
7
12
13
0.80
0.81
0.85
0.87
0.88
Table III. Pt3CoÕVC mass activities at 0.90 V as a function of
successive scans. Electrode poisoned in 0.001 M S„IV… + 0.1 M
HClO4 solution, ␪S,i = 0.54.
CVs
CV1
CV5
CV10
Mass activities
−1
兲
共A mgPt
% mass
activity
Overpotential/V
共J = 1.5 mA cm−2兲
0.0004
0.006
0.02
0.1
2
7
0.65
0.78
0.82
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Journal of The Electrochemical Society, 156 共7兲 B848-B855 共2009兲
ently have a higher sulfur coverage than the Pt/VC ones due to their
lower Pt electrochemical surface area or lower coverage with surface oxygen species. For electrodes with similar sulfur coverage,
Pt3Co/VC is more active than Pt/VC. The adsorbed sulfur is partially removed by potential cycling to 1.5 V under an inert atmosphere, during which adsorbed Sx species are oxidized to sulfate.
This potential cycling is more effective for the relative recovery of
the Pt3Co/VC electrodes vs the Pt/VC electrodes, suggesting low
adsorption strength for the Sx species on the alloy. The Pt/VC electrodes have higher overall activity even with potential cycling due to
their initially lower sulfur coverage. Cycling to 1.0 V, or a voltage
more relevant to fuel cell operating conditions, is not very effective
at oxidizing the Sx adsorbed to the Pt3Co/VC surface. We conclude
that Pt/VC is probably a better choice for fuel cells that are exposed
to high amounts of sulfur in the air inlet, but there are opportunities
with Pt3Co/VC if potential cycling is possible.
Acknowledgments
We are grateful to the Office of Naval Research for the support
of this research. This research was performed while Y.G. held a
National Research Council Post-Doctoral Fellowship Award at the
U.S. Naval Research Laboratory.
US Naval Research Laboratory assisted in meeting the publication costs
of this article.
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