Simulated air bleed oxidation of adsorbed CO on carbon supported Pt. Part 2. electrochemical measurements of hydrogen peroxide formation during O2 reduction in a double-disk electrode dual thin-layer flow cell

Article abstract of DOI:10.1021/jp0499171

The reaction of preadsorbed CO with O₂ and the simultaneous O₂ reduction reaction (ORR) on a carbon supported Pt catalyst (20%Pt/Vulcan) in an O₂-saturated electrolyte at potentials of 0.06 V and, for comparison, at 0.26 V_RHE were investigated by electrochemical measurements in a dual thin-layer flow cell under continuous flow conditions. Following a previous differential electrochemical mass spectrometry study (J. Electroanal. Chem, 2003, 554-555, 427), which demonstrated very slow CO₂ formation, in combination with significant 0: consumption, even at the most cathodic potential (0.06 V_RHE), we here show that the O₂ consumption on the CO_ad-saturated catalyst is due to almost exclusive H₂O₂ formation. Both the CO oxidation rate and the selectivity for H₂O₂ formation decrease with lower CO_ad coverage, reaching slightly above 25% at the limiting coverage of 80% of the saturation value (0.6 monolayers). The experimental observations are explained by a mechanism, where H ₂O₂ formation and, with a much lower probability, reaction with a neighboring CO_ad occur upon O₂ adsorption in monomer vacancies. For O₂ adsorption on larger vacancy islands dissociative adsorption and subsequent H₂O formation prevails, reaction with CO_ad at the perimeter of the vacancy island is practically inhibited, Consequences for the air bleed operation of polymer electrolyte fuel cells are discussed.

Full text of DOI:10.1021/jp0499171

J. Phys. Chem. B 2004, 108, 7893-7901
7893
Simulated “Air Bleed” Oxidation of Adsorbed CO on Carbon Supported Pt. Part 2.
Electrochemical Measurements of Hydrogen Peroxide Formation during O2 Reduction in a
Double-Disk Electrode Dual Thin-Layer Flow Cell
Z. Jusys and R. J. Behm*
Department of Surface Chemistry and Catalysis, UniVersity Ulm, D-89069 Ulm, Germany
ReceiVed: January 7, 2004; In Final Form: April 1, 2004
The reaction of preadsorbed CO with O2 and the simultaneous O2 reduction reaction (ORR) on a carbon
supported Pt catalyst (20%Pt/Vulcan) in an O2-saturated electrolyte at potentials of 0.06 V and, for comparison,
at 0.26 VRHE were investigated by electrochemical measurements in a dual thin-layer flow cell under continuous
flow conditions. Following a previous differential electrochemical mass spectrometry study (J. Electroanal.
Chem. 2003, 554-555, 427), which demonstrated very slow CO2 formation, in combination with significant
O2 consumption, even at the most cathodic potential (0.06 VRHE), we here show that the O2 consumption on
the COad-saturated catalyst is due to almost exclusive H2O2 formation. Both the CO oxidation rate and the
selectivity for H2O2 formation decrease with lower COad coverage, reaching slightly above 25% at the limiting
coverage of 80% of the saturation value (0.6 monolayers). The experimental observations are explained by
a mechanism, where H2O2 formation and, with a much lower probability, reaction with a neighboring COad
occur upon O2 adsorption in monomer vacancies. For O2 adsorption on larger vacancy islands dissociative
adsorption and subsequent H2O formation prevails, reaction with COad at the perimeter of the vacancy island
is practically inhibited. Consequences for the air bleed operation of polymer electrolyte fuel cells are discussed.
1. Introduction
chemical mass spectrometry (DEMS). In that study, we found
a significant depletion of O2 in the electrolyte in the potential
region of hydrogen underpotential deposition (H-upd) on the
COad-blocked surface, which was tentatively attributed to
hydrogen peroxide formation. Continuing that study, we now
directly investigated and determined the hydrogen peroxide
yields on COad-blocked Pt/Vulcan catalyst electrodes in O2-
saturated solution by using a dual disk-electrode dual thin-layer
flow cell (DDE- DTLFC). The design of the setup is described
in more detail and characterized in ref 7. The results of these
measurements, which show an extremely high current efficiency
of up to nearly 100% for hydrogen peroxide formation on a
CO-blocked Pt/Vulcan catalyst in the H-upd region, are reported
and discussed in the present paper.
The “air bleed” reaction,1 the reaction of adsorbed CO on
the anode catalyst with small amounts of O2 added to the feed
gas, was proposed as a simple and inexpensive method to reduce
the poisoning of the anode catalysts (carbon supported Pt or Pt
alloy catalysts) by adsorbed CO in low-temperature polymer
electrolyte membrane fuel cells (PEFCs), which are operated
by CO-contaminated H2-rich gases produced by reforming of
hydrocarbons and methanol (ref 2 and references in ref 3).
Inspite of its technical importance, however, this reaction has
rarely been studied, and details of the reaction mechanism and
kinetics of this process are largely unknown (see also ref 3). In
this context, it is important to note that under PEFC anode
operation conditions, at potentials around 0.05-0.1 VRHE, both
oxygen reduction and hydrogen peroxide formation can occur
at the fuel cell anode, when O2 is present in the anode feed,
either from direct O2 admission via the feed gas (air bleed) and/
or due to possible O2 crossover through the ionomer membrane
from the cathode side.4 Formation of hydrogen peroxide at the
membrane electrode assembly (MEA) is highly undesirable,
since it will cause increased degradation of both the gas diffusion
layer and the ionomer membrane under fuel cell operating
conditions.5,6
The formation of hydrogen peroxide during the oxygen
reduction reaction (ORR) on Pt surfaces and catalysts at fuel
cell relevant potentials in the range of 0.05-0.1 VRHE is well
documented and has been widely studied using the rotating ring-
disk electrode (RRDE) technique (for polycrystalline Pt see,
e.g., refs 8-10, for single-crystal Pt(hkl) electrodes see refs 11-
13, and for supported Pt catalysts see refs 14-20). O2 reduction
to water (four-electron process), which is the dominant process
on Pt electrodes/catalysts at potentials >0.3VRHE (see above
references), is known to be hindered by adsorbed species,21,22
such as adsorbed (bi)sulfate, chloride or bromide anions,18,23,24
adsorbed selenium or sulfur,25 or adsorbed hydrogen, upon
H-upd.11 Similar effects have been reported for Pt surfaces
modified by oxide formation (OH adsorption)26 or by Cu-upd
deposition.27,28 On the other hand, rotating ring disk electrode
(RRDE) measurements showed a significant increase of hydro-
gen peroxide production on the adsorbate blocked surfaces
compared to the clean Pt surfaces.11,18,21,23-28 This change in
reaction pathway, from a four-electron pathway to a two-electron
We have therefore started to systematically investigate the
reactions accompanying the air bleed operation in more detail.
In a preceding paper,3 we reported results of an on-line mass
spectrometric study of a simulated air bleed reaction on a Pt/
Vulcan catalyst, monitoring on-line both CO2 formation and
O2 consumption on the COad-covered catalyst as a function of
the electrode potential and reaction time by differential electro-
* To whom correspondence should be addressed. E-mail: juergen.behm@
chemie.uni-ulm.de.
10.1021/jp0499171 CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/12/2004

7894 J. Phys. Chem. B, Vol. 108, No. 23, 2004
Jusys and Behm
pathway, was attributed to a blocking of surface sites on the Pt
surface required for the dissociative adsorption of O2.
are quoted, however, vs that of the reversible hydrogen electrode
(RHE)). The electrolyte flow (flow rate ca. 4 µL/s) was driven
by the hydrostatic pressure of the electrolyte in the supply bottle,
which was continuously purged by Ar or saturated with O2.
Under these flow conditions electroactive species formed at the
generator electrode were detected at the potential-controlled
collector electrode with a delay of ca. 3 s and a collection
efficiency of 76% (for details of the calibration see ref 7).
The supporting electrolyte (0.5 M sulfuric acid) was prepared
using Millipore Milli Q water and ultrapure sulfuric acid (Merck,
suprapur) and purged by high purity Ar (MTI Gase, N 6.0) or
saturated with O2 (MTI Gase, N 5.7) in the electrolyte supply
bottle. For the chronoamperometric and CO stripping experi-
ments, CO was preadsorbed at a constant electrode potential
for 10 min, after introducing 2 mL of 0.5 M H2SO4 solution
saturated with CO (Messer-Griesheim, N 4.7) by an all-glass
syringe through a separate port. Then the cell was rinsed by
introducing Ar-saturated solution through the same port, using
another syringe. Finally, the O2-saturated electrolyte was
admitted to the cell by opening an open-close valve at the
electrolyte supply bottle. COad stripping experiments were
performed either immediately after having started the flow of
O2-saturated electrolyte or at the end of a chronoamperometric
measurement in the same electrolyte. In both cases, CO stripping
was performed in O2-saturated electrolyte. The CO stripping
measurements were followed by a second complete potential
cycle, detecting the O2 reduction behavior on a CO-free
electrode. By comparison with previous data (see ref 7 and
references therein), the latter measurements can also be used to
check the cleanness of the electrode. All measurements were
conducted at room temperature.
In the present situation, where we are interested in hydrogen
peroxide formation on a COad-covered surface in O2-containing
electrolyte at potentials typical for PEFC anodes (0.05-0.1
VRHE), the commonly used RRDE approach is hardly applicable
because of experimental problems, due to the relatively long
time required for electrolyte exchange (or saturation by O2) and
the immediate onset of COad oxidation upon contact with the
oxygen-containing electrolyte. Hence, in an RRDE setup, CO
oxidation would start already before saturation of the electro-
lyte.21 This is different in a flow cell, and our DDE-DTLFC
design allows a fast change of electrolyte, from a CO-saturated
electrolyte to an O2-containing electrolyte, under well controlled
conditions (for details see ref 7).
After a brief description of the experimental setup and details,
we will present and discuss the results of potentiodynamic and
transient potentiostatic measurements on the ORR kinetics and
on the CO adlayer oxidation in O2-saturated solution COad-
covered Pt/Vulcan electrodes. For comparison, we include also
measurements on COad-free Pt/Vulcan electrodes and on Pt-
free carbon Vulcan electrodes. The resulting data will demon-
strate that under fuel cell relevant conditions on a fully or largely
COad-covered Pt/Vulcan electrode the majority of the consumed
O2 will be reacted into H2O2 rather than being used for CO
oxidation.
2. Experimental Section
The experimental setup for the dual disk electrode thin-layer
flow cell measurements was described and characterized in detail
in a previous publication.7 In short, it is based on the dual thin-
layer flow cell design described in ref 29, which had been used
for DEMS measurements on supported catalyst electrodes in
our group (see ref 3 and references therein) and which was
modified for the present measurements. The cell itself cell
consists of a circular Kel-F plate (5 mm thick) with capillaries
for the electrolyte inlet and outlet, whose openings are located
at the center of the Kel-F plate, on the opposite sides, and two
electrodes which are pressed via two sealing gaskets (Kalrez,
ca. 50 µm thick) against the Kel-F plate. The working electrode
in the first compartment (generator) is a mirror-polished glassy
carbon disk 9 mm in diameter covered with a thin Pt/Vulcan or
Vulcan carbon film (6 mm diameter), and the collector electrode
is a mirror-polished polycrystalline Pt disk (15 mm in diameter,
exposed area 1.33 cm2).
3. Results
3.1. Oxygen Reduction on Pt/Vulcan Catalyst Electrodes
- Potentiodynamic Measurements. 3.1.1. Oxygen Reduction
on COad-Saturated Pt/Vulcan Catalysts. The oxidation of the
CO adlayer and the O2 reduction behavior in the O2-saturated
electrolyte on COad-precovered Pt/Vulcan electrodes were first
investigated in potentiodynamic DDE-DTLFC experiments,
starting with a Pt/Vulcan catalyst saturated with preadsorbed
COad at 0.06 V. The combined oxygen reduction and CO
oxidation current on the Pt/Vulcan generator electrode and the
hydrogen peroxide oxidation current on the polycrystalline Pt
collector electrode at 1.16 V were detected simultaneously. The
resulting cyclic voltammetry DDE-DTLFC data for the ORR
are shown in Figure 1. The solid black line in Figure 1a (total
Faradaic current IF(total)) represents the first positive-going scan,
recorded immediately after starting the flow of O2-saturated 0.5
M H2SO4 solution through the DDE-DTLFC cell. The gray solid
line in Figure 1a describes a subsequent CV of the Pt/Vulcan
electrode in O2-saturated solution, corresponding to the ORR
in O2-saturated base electrolyte on the unmodified Pt/Vulcan
catalyst electrode. This CV is basically identical with previous
DDE-DTLFC data published in ref 7, underlining both the
cleanness of the electrode and the reproducibility of the
measurements.
The thin-film electrode was prepared as described previ-
ously,30 by pipetting an aliquot of an aqueous suspension of
the Pt/Vulcan catalyst (E-TEK, 20 w.% Pt) onto the glassy
carbon disk (Sigradur G, from Hochtemperatur Werkstoffe) and
then a defined amount of Nafion solution on top of the dried
catalyst layer. The latter results in a very thin (ca. 0.1 µm
thickness) Nafion film. The geometric area of the accessible
part of the electrode is 0.28 cm2, the Pt loading 28 µg cm-2
.
The average particle size and the particle size distribution of
the Pt/Vulcan catalyst had been characterized recently by X-ray
diffraction (XRD) and transmission electron microscopy (TEM),
yielding an average particle size of 3.7 nm for the conditioned
catalyst and a dispersion of 26%.31 Prior to the calibration and
kinetic measurements, both electrodes were characterized by
cyclic voltammetry (CV) in 0.5 M H2SO4 solution.7
Two Pt wires connected through separate ports at the inlet
and outlet of the cell served as counter electrodes. The reference
electrode, a saturated calomel electrode (SCE), was connected
trough a Teflon capillary at the outlet of the cell. (All potentials
The measured Faradaic current (IF(total)) response to the
electrode potential scan shows a complicated behavior: Im-
mediately after starting the electrolyte flow (still at 0.06 V),
the current on the COad-covered Pt/Vulcan electrode increases
to ca. -0.15 mA, which is about 1/3 of the mass transport limited
ORR current in the subsequent scan. When the potential is
increased, the current first diminishes to ca. -0.8 mA at about
0.15 V and then increases again, approaching nearly the mass

Simulated “Air Bleed” Oxidation
J. Phys. Chem. B, Vol. 108, No. 23, 2004 7895
line). At 0.06 V, the hydrogen peroxide formation rate on the
COad-covered Pt/Vulcan surface is more than 1 order of
magnitude higher than on the CO-free catalyst surface as
measured in the second scan (Figure 1b, solid gray line). For
better visualization, the latter is included also on a 10-fold
magnified scale (Figure 1b, dotted gray line). The difference is
even more pronounced, by a factor of 20, in the double-layer
region, between 0.3 and 0.6 V. The ratio between the collector
current (corrected by the time constant of the setup) and the
generator current is about constant at ca. 0.76 in the potential
range 0.06-0.15 V (Figure 1c, solid black line). Depending on
the potential, it is 1-2 orders of magnitude higher than on the
CO-free Pt/Vulcan catalyst in the potential range up to 0.6 V
(Figure 1c, gray line). Taking into account the collection
efficiency of our DDE-DTLFC setup of 76%,7 the current
efficiency for hydrogen peroxide formation reaches about 90%
on the COad-saturated Pt/Vulcan surface at potentials negative
of 0.15 V (Figure 1c, dashed line). Similarly high current
efficiencies for hydrogen peroxide production during the ORR
of up to 100% have been reported in several RRDE studies on
adsorbate-covered Pt electrodes, e.g., on selenium-modified
polycrystalline Pt in the range of Se-adlayer stability,25 on
adsorbed bromide-covered Pt(111) in the double-layer region
(0.2-0.4 V),23 and on Had-covered Pt(111) in the H-UPD
region,11 i.e., on Pt surfaces which were blocked by adsorbed
species under reaction conditions. In a recent RRDE measure-
ment of the ORR on a COad-blocked Pt/Vulcan catalyst,21 the
initial H2O2 yield was much lower, about 10%, but increased
to a similar value of about 80%. This discrepancy is tentatively
attributed to the undefined, low O2 concentration in the
electrolyte at the beginning of that experiment.
Figure 1. Simultaneously recorded cyclic voltammogram on a COad-
blocked Pt/Vulcan catalyst (Pt loading 28 µg/cm2, CO preadsorption
at 0.06V) in the first compartment of a dual thin-layer flow cell in
O2-saturated 0.5 M H2SO4 solution (a), simultaneously measured H2O2
oxidation current measured on a polycrystalline Pt electrode (E ) 1.16
V) in the second compartment (b), and hydrogen peroxide yield (c).
Electrolyte flow rate ca. 4 µL/s, potential scan rate 10 mV/s, room
temperature. Black solid lines correspond to the first positive-going
scan. Solid gray lines correspond to subsequent cyclic voltammogram
in a supporting electrolyte. Dotted gray line in (b): similar to the solid
gray line, but enhanced sensitivity (×10). Dashed black line (a): partial
current for oxygen reduction to hydrogen peroxide calculated according
to eq 1. Dotted black line (a): partial current for oxygen reduction to
water calculated as the difference between the net current and partial
hydrogen peroxide formation current.
The two partial currents IH O and IH O, which were calculated
2
2
2
from the time constant-corrected collector current Icoll via the
relations
IH O ) Icoll/N
(1)
2
2
where N represents the collection efficiency (N ) 0.76) and
IH O ) Igen - IH O ) Igen - Icoll/N (2)
2
2 2
transport limited oxygen reduction current in the potential region
between 0.5 and 0.7 V (ca. -0.4 mA). This is followed by the
characteristic COad electrooxidation peak centered at ca. 0.77
V, and finally, the current signal traces the behavior of a Pt/
Vulcan catalyst in an O2-saturated solution at potentials positive
of 0.82 V (Figure 1a, solid gray line). The complex character
of the current response results from several contributions,
namely oxygen reduction (both to hydrogen peroxide and water),
adsorbed CO oxidation (both electrochemical and chemical
oxidation), which may contribute directly (electrochemical
oxidation) and indirectly via a COad-coverage dependent ORR
rate, and the voltammetric features of the Pt/Vulcan electrode
itself.
Information on the amount of CO oxidation and on the
evolution of the COad coverage during the CV comes from
previous DEMS measurements of the same reaction and on the
same catalyst,3,32 which had shown that the increase in Faradaic
current during the positive-going scan goes along with the
oxidation of ∼0.05 ML of COad in the potential range 0.06-
0.4 V, and additional ∼0.15 ML (0.2 ML in total) at the onset
of the main CO stripping peak.
are plotted as a dashed black line (IH O ) and dotted black line
2 2
(IH O), respectively, in Figure 1a as a function of the electrode
2
potential, up to the onset of adsorbed CO electrooxidation. (At
higher potentials, this method can no longer be applied due to
increasing contributions of COad stripping as a third electro-
chemical process). The fraction of O2 being reduced to hydrogen
peroxide, XH O , can be calculated from the partial currents via
2
2
XH O ) 2IH O /(2IH O + IH O
)
(3)
2
2
2
2
2
2
2
At potentials negative of 0.15 V, the Faradaic current corre-
sponds nearly quantitatively to the partial current for O2
reduction to hydrogen peroxide, at essentially zero current for
oxygen reduction to water, indicating practically 100% current
efficiency for the former reaction pathway under these condi-
tions (Figure 1c, dashed black line). Going to more positive
potentials, the partial current for hydrogen peroxide formation
reaches a plateau at ca. -0.05 mA in the potential region 0.2
to 0.4 V, and then drops to zero at potentials between 0.4 and
0.6 V (Figure 1a, dashed black line). At the same time, the
partial current for oxygen reduction to water increases about
linearly with potential, starting from ca. 0.15 to ca. 0.5 V (Figure
1a, dotted black line). These results suggest a strong potential
dependence of the ORR on the COad-covered Pt surface. A
The formation of hydrogen peroxide during reaction of the
CO presaturated Pt/Vulcan electrode in the O2 saturated solution
was monitored via the collector current (Figure 1b, solid black

7896 J. Phys. Chem. B, Vol. 108, No. 23, 2004
Jusys and Behm
Figure 3. Simultaneously recorded cyclic voltammogram on a COad-
blocked Pt/Vulcan catalyst (Ead ) 0.26 V) (a), H2O2 oxidation current
(b), and a hydrogen peroxide yield (c) in O2-saturated 0.5 M H2SO4
solution, after exposure for 9 min to O2-saturated 0.5 M H2SO4 solution
at 0.06 V. (Experimental conditions and setup as well as assignment
of the lines as in Figure 1.)
Figure 2. Simultaneously recorded cyclic voltammogram on a COad-
blocked Pt/Vulcan catalyst (Ead ) 0.26 V) (a), H2O2 oxidation cur-
rent (b), and a hydrogen peroxide yield (c) in O2-saturated 0.5 M H2-
SO4 solution, starting with a negative going scan at 0,0.26 V.
(Experimental conditions and setup as well as assignment of the lines
as in Figure 1.)
peroxide formation increases during the negative-going potential
scan up to values which are about 1 order of magnitude higher
than those obtained on a COad-free catalyst surface. In the
subsequent positive-going scan, it decreases again, but remains
significantly higher than in the negative-going scan and is about
20 times higher than on the COad-free electrode over the entire
potential region up to ca. 0.7 V. The ratio of the (time constant
corrected) collector current to generator current (Figure 2c,
solid black line) on the CO-blocked electrode surface reaches
a value of ca. 0.7 at the negative potential limit, which
corresponds to a hydrogen peroxide yield of ca. 95% (Figure
2c, dashed black line). Finally, the partial currents for oxygen
reduction to hydrogen peroxide (Figure 2a, dashed black line)
and water (Figure 2a, dotted black line), calculated following
the procedure described above, show a clear potential depen-
dence for hydrogen peroxide formation in the H-upd region on
the COad-blocked catalyst. This confirms the conclusions drawn
from DEMS experiments in refs 3 and 32, where a loss of COad
was ruled out as the origin for the decay of the H2O2 yield during
the positive-going scan, and instead potential effects were
favored.
possible decrease of the COad coverage as a result of a chemical
reaction of the CO adlayer with O2 during the positive-going
potential scan cannot be ruled out, however, as origin for the
decreasing H2O2 yield (see below). Finally, for comparison, we
also include the hydrogen peroxide yield on the COad-free Pt/
electrode determined in the second potential cycle (Figure 1c,
gray line). On the scale in this graph, the hydrogen peroxide
yield is hardly detectable. The maximum hydrogen peroxide
yield obtained at 0.06 V is about 4%, which agrees very well
with previous RRDE data.7,17
To clarify the question of whether the decrease in hydrogen
peroxide yield with increasing potential is due to a potential
effect or results from a decrease of COad coverage, we performed
an analogous ORR cyclic voltammetry experiment, where the
Pt/Vulcan electrode was saturated with CO at 0.26 V and where
the potential scan was started in the negative-going potential
scan, immediately after having started the flow of O2-saturated
electrolyte (Figure 2). The measured (negative) Faradaic current
(Figure 2a, solid black line) first increases to ca. -0.2 mA in
the negative-going scan, followed by a further increases to ca.
-0.4 mA at about 0.5 V during the positive-going scan. It then
transforms into the peak for COad electrooxidation, which is
centered at ca. 0.77 V. At more positive potentials the signal
traces the current response on the COad-free Pt/Vulcan electrode,
which was determined in the subsequent scan (Figure 2a, gray
solid line). The simultaneous measurement of the collector
current (Figure 2b, solid black line) shows hydrogen peroxide
formation on the COad-blocked Pt/Vulcan surface already at 0.26
V, at a rate that is about two times higher than that reached on
the COad-free Pt/Vulcan catalyst in the second cycle at the
negative potential limit (Figure 2b, gray line). Hence, hydrogen
3.1.2. Oxygen Reduction on Partly COad-CoVered Pt/Vulcan
Electrodes after Reaction with O2. To test for possible effects
of COad losses on the interaction with O2-containing electrolyte,
we recorded CVs after having exposed the COad-saturated Pt/
Vulcan electrode to an O2-saturated solution for 9 min at two
different potentials, 0.06 and 0.26 V. The resulting CVs are
plotted in Figures 3 and 4, respectively. The corresponding
current transients recorded during these 9 min reaction time are
reproduced in Figures 5 and 6 and will be discussed later in
section 3.2. Again the second CV is included as well, reflecting

Simulated “Air Bleed” Oxidation
J. Phys. Chem. B, Vol. 108, No. 23, 2004 7897
the Faradaic current increase because of the decreasing H2O2
yield with decreasing COad coverage (see below), and, more
directly, by the immediate CO2 formation right above 0.06 V
observed in similar potentiodynamic DEMS experiments on the
same catalyst.32
Simultaneous measurements of the hydrogen peroxide oxida-
tion current on the collector (Figure 3b, solid black line) show
a ca. 5-fold increase compared to the value measured on the
COad-free Pt/Vulcan electrode (Figure 3b, solid gray line).
Compared to the immediate positive-going scan on the COad-
saturated Pt/Vulcan electrode (Figure 1b, solid black line),
however, the initial hydrogen peroxide oxidation current is
1
reduced to about /5. The hydrogen peroxide oxidation current
corresponds to a maximum yield for hydrogen peroxide forma-
tion of ca. 18% during the ORR on the partly (ca. 20%) oxidized
CO-adlayer (Figure 3c, dashed line), compared to about 95%
on the fully COad-saturated Pt/Vulcan electrode (Figure 1c,
dashed line). Correspondingly, the time constant corrected partial
current for oxygen reduction to hydrogen peroxide after 9 min
exposure to O2-saturated solution (Figure 3a, dashed line) is
1
only about /5 of that in the immediate positive-going scan
(Figure 1a, dashed line), whereas the partial current for oxygen
reduction to water on the partly COad-covered electrode is close
to the mass transport limited current (Figure 3a, dotted line),
compared to negligible currents on a Pt/Vulcan electrode
blocked by a full COad monolayer (Figure 1a, dotted line).
Hence, the ORR pathways are highly sensitive to relatively small
changes in the COad coverage, changing from nearly quantitative
hydrogen peroxide formation on a Pt/Vulcan electrode blocked
by a saturated CO adlayer, to about 18% hydrogen peroxide
Figure 4. Simultaneously recorded cyclic voltammogram on a COad-
blocked Pt/Vulcan catalyst (Ead ) 0.26 V) (a), H2O2 oxidation current
(b), and a hydrogen peroxide yield (c) in O2-saturated 0.5 M H2SO4
solution, after exposure for 9 min to O2-saturated 0.5 M H2SO4 solution
at 0.26 V. (Experimental conditions and setup as well as assignment
of the lines as in Figure 1.)
1
formation when about /5 of the CO-adlayer is removed by
reaction with oxygen.
Analogous cyclic voltammetry experiments were performed
on the COad-covered Pt/Vulcan surface after COad-saturation
and subsequent reaction (9 min reaction time) with O2-saturated
electrolyte at 0.26 V (Figure 4). Similarly to the results shown
in Figure 3a, the ORR current reaches almost the mass transport
limiting current within 9 min reaction time (the corresponding
transients are shown in Figure 6). Scanning the potential first
to more negative potentials, to 0.06 V, and then returning to a
positive-going scan, we find that the H-upd features are still
largely suppressed. Also, the pronounced CO stripping peak
centered at ca. 0.76 V in the positive-going scan suggests that
much of the adsorbed CO is still present on the catalyst surface.
The quantitative evaluation of these data, following the proce-
dure described above, shows that also in this case ca. 80% of
the CO adlayer is still adsorbed on the Pt particles after 9 min
exposure to the O2-saturated sulfuric acid solution at 0.26 V,
similar to the change in COad coverage during reaction at 0.06
V (Figure 3). On the other hand, the amount of hydrogen
peroxide produced in the H-upd region at Ead ) 0.26 V (Figure
4a-c) is only half of that generated after CO adsorption and
reaction at 0.06 V (Figure 3a-c). A more detailed interpretation
of these results will be given in the Discussion
the O2 reduction behavior on a COad-free Pt/Vulcan electrode
(Figure 3, solid gray line).
Comparison of the CV in Figure 1a, where the positive-going
scan was started immediately after admitting the O2-saturated
electrolyte flow to the COad-saturated Pt/Vulcan electrode, and
that in Figure 3a, where the positive-going scan was started
after the COad-saturated Pt/Vulcan electrode was exposed for 9
min to a continuous flow of the O2-saturated electrolyte at 0.06
V, shows that during these 9 min waiting time at 0.06 V the
total ORR current has increased from about -0.15 to -0.4 mA,
which is close to the mass transport limited current of -0.5
mA. (The latter current value is reached during the positive-
going scan at potentials above the H-upd region, at >0.3 V
(Figure 3a, solid black line).) The increase in ORR current
indicates a partial loss of the blocking CO-adlayer due to
chemical oxidation by reaction with oxygen during the waiting
time. At more positive potentials, the characteristic COad
stripping peak appears in the positive-going scan (Figure 3a,
black solid line). By subtracting the ORR current in the second
positive-going scan (Figure 3a, gray solid line) and integrating
the COad stripping current in the potential range from 0.45 to
0.8 V, the charge under this peak was determined to be 1.84
mC. This is about 80% of the charge resulting from electrooxi-
dation of a saturated CO adlayer at the same Pt/Vulcan loading
(about 2.3 mC, equivalent to 0.75 monolayer (ML)3,33), indica-
tive of an absolute coverage of about 0.6 ML at the end of the
chemical oxidation experiment. Hence, the COad coverage has
decreased during the waiting time due to chemical oxidation in
the O2-saturated electrolyte at 0.06 V. Most of the COad
oxidation actually occurs during the first 1-2 min, which is
indicated both by the initially faster increase of the Faradaic
current, note that the decrease in COad coverage is steeper than
3.2. Oxygen Reduction on Partly COad-Covered Pt/Vulcan
Catalyst Electrodes - Transient Chronoamperometric Mea-
surements. The effect of the CO adlayer on the mechanism
and kinetics of the ORR is much more directly evaluated in
potentiostatic transient measurements, where the kinetic effects,
the change in the overall ORR kinetics and in the contributions
of the different reaction pathways with varying CO adlayer
coverage, do not interfere with potential-dependent features such
as H-upd or the onset of COad electrooxidation in the “prewave”
region.33 Therefore, the current transients on both generator and

7898 J. Phys. Chem. B, Vol. 108, No. 23, 2004
Jusys and Behm
Figure 5. Simultaneously recorded current transients after starting O2-
saturated electrolyte flow over a COad-blocked Pt/Vulcan catalyst (black
thick lines), COad-free Pt/Vulcan catalyst (gray lines) (Pt loading 28
µg/cm2) or Vulcan carbon (black thin lines) at 0.06 V. (a) Faradaic
current transients in the first compartment and (b) H2O2 oxidation
currents on polycrystalline Pt (E ) 1.16 V) in the second compartment
of the dual thin-layer flow cell; (c) corresponding hydrogen peroxide
yields. (Electrolyte flow rate ca. 4 µL/s, room temperature, assignment
of the lines as in Figure 1.)
Figure 6. Simultaneously recorded current transients after starting O2-
saturated electrolyte flow over a COad-blocked Pt/Vulcan catalyst (black
thick lines), COad-free Pt/Vulcan catalyst (gray lines) (Pt loading 28
µg/cm2) or Vulcan carbon (black thin lines) at 0.26 V. (a) Faradaic
current transients in the first compartment and (b) H2O2 oxidation
currents on polycrystalline Pt (E ) 1.16 V) in the second compartment
of the dual thin-layer flow cell; (c) corresponding hydrogen peroxide
yields. (Electrolyte flow rate ca. 4 µL/s, room temperature, assignment
of the lines as in Figure 1.)
collector were recorded continuously during the combined ORR
and chemical CO oxidation at 0.06 (Figure 5) and 0.26 V (Figure
6) on the COad-covered Pt/Vulcan catalyst surface, respectively,
for 8.5 min. Subsequently, the residual COad was removed by
electrochemical oxidation (CO stripping) to detect the COad
coverage (see Figures 3 and 4). For comparison, similar
transients were recorded on a COad-free Pt/Vulcan electrode
(gray lines) and on a Vulcan support electrode (thin black lines)
in O2-saturated sulfuric acid solution (Figures 5 and 6). For all
three electrodes, our previous DEMS measurements had shown
a measurable consumption of O2 during the reaction time3).
For reaction at 0.06 V (Figure 5, zero time corresponds to
the onset of O2-saturated electrolyte flow) on the Vulcan
electrode and on the COad-free Pt/Vulcan electrode, the O2
reduction current almost instantaneously reaches its final value,
which is equivalent to the mass transport limited current for
the latter one and about 40% of that for the former one, (Figure
5a, solid gray line and solid thin black line). In contrast, on the
COad-blocked Pt/Vulcan electrode, the ORR current first exhibits
a sharp spike and then returns to a value slightly lower than
that on the Vulcan electrode. It then increases steadily with time,
reaching about 80% of the ORR current on the CO-free Pt/
Vulcan electrode after 8.5 min. The transient ORR behavior on
the CO-blocked Pt/Vulcan electrode points to a significant effect
of the slowly decreasing CO adlayer coverage on the ORR
characteristics.
The proposition of a pronounced coverage adlayer effect on
the formation of hydrogen peroxide is supported by the
simultaneous measurements of hydrogen peroxide formation at
the second electrode (Figure 5b, solid black line): The hydrogen
peroxide oxidation current on the Pt collector shows, after an
initial current spike, a continuous change on the COad-blocked
Pt/Vulcan electrode, first passing through a maximum value of
ca. 0.1 mA ca. 50 s after starting the electrolyte flow and then
dropping to ca. 0.06 mA after 8.5 min. The sharp collector
current increase when starting the electrolyte flow is attributed
to an experimental artifact, to oxidation of CO residues in the
solution rather which were not completely removed during the
brief flush of the flow cell with CO-free electrolyte, rather than
to oxidation of hydrogen peroxide formed at the CO blocked
Pt/Vulcan generator. According to the subsequent CO stripping
data (Figure 3a, solid black line), the COad coverage decreased
to about 80% of its initial value during that time, reaching a
final value of about 0.6 ML. In contrast, on both the COad-free
Pt/Vulcan electrode (Figure 5b, solid gray line) and on the
Vulcan electrode (Figure 5b, thin black line), the hydrogen
peroxide oxidation current and hence the hydrogen peroxide
formation rate assume almost instantaneously their final,
constant value, which on the latter is about 3-fold higher than
on the COad-free Pt/Vulcan electrode, at only 40% of the overall
ORR current. These currents correspond to constant hydrogen
peroxide yields of ca. 6% on the CO-free Pt/Vulcan electrode

Simulated “Air Bleed” Oxidation
J. Phys. Chem. B, Vol. 108, No. 23, 2004 7899
(Figure 5c, gray dashed line) and about 42% on the Vulcan
electrode (Figure 5c, thin black dashed line). The latter results
agree very well with literature data.17,34 The corresponding
partial currents for O2 reduction to hydrogen peroxide (dashed
lines) and water (dotted lines) on carbon Vulcan (thin black
lines), Pt/Vulcan (thick gray lines), and CO-blocked Pt/Vulcan
(thick black lines), calculated according to eqs 1 and 2, are
plotted in Figure 5a. On the fully COad-covered Pt/Vulcan
electrode, the hydrogen peroxide yield is initially constant for
about 20 s, reaching ca. 90%, and then drops steadily to a value
of ca. 30% after 8.5 min (Figure 5c, thick dashed line). As
mentioned above, this loss in hydrogen peroxide yield goes
along with a decay in the COad coverage, from saturation (0.75
ML) to 80% of the saturation value (0.6 ML). As had been
discussed before, similarly high hydrogen peroxide yields (up
to 100%) where found during the ORR in RRDE measurements
on several adsorbate covered Pt electrode surfaces (see section
3.1.1).
On the COad-free Pt/Vulcan electrode (Figure 6a, thick solid
gray line) and on the Pt-free Vulcan electrode (Figure 6a, thin
solid black line) the initial and constant ORR currents at 0.26
V correspond to the mass transport limited value for the ORR
and to the initial ORR current on the COad-blocked Pt/Vulcan
electrode, respectively. The latter result indicates that the initial
ORR activity on the CO-blocked Pt/Vulcan electrode is
determined by the ORR on the carbon support. Note that for
reaction at 0.06 V (Figure 5a) the initial ORR activity on the
COad-blocked Pt/Vulcan electrode was slightly lower than that
on the Pt-free Vulcan electrode.
The collector current measured for the ORR on the COad-
saturated Pt/Vulcan electrode shows a qualitatively similar
transient behavior at 0.26 V as at 0.06 V. It immediately
increases after the flow of O2-saturated electrolyte was started,
then decreases to a minimum after 10-15s, increases again and
passes through the maximum after ca. 40s, and then slowly
decays to finally reach a value of about 0.08 mA (Figure 6b,
thick black line). On a quantitative scale, however, the hydrogen
peroxide formation current is significantly higher at 0.06 V than
at 0.26 V, by about a factor of 4, despite of the similar losses
in COad coverage in both cases (see the Discussion section).
For comparison, the ORR current on the COad-free Pt/Vulcan
electrode (Figure 6a, thick solid gray line) reaches its mass
transport limited value within a few seconds after having started
the flow of O2-saturated electrolyte, similarly to observations
at 0.06 V (Figure 5a, thick solid gray line). The hydrogen
peroxide production at 0.26 V, however, reaches less than 20%
of that at 0.06 V (Figures 6b and 5b, solid gray lines) and,
correspondingly, also lower hydrogen peroxide yields (Figures
6c and 5c, dashed gray lines). Finally both the total ORR current
and the hydrogen peroxide formation current on the Vulcan
electrode (Figures 6a,b, thin black lines) are only about 10%
of that at 0.06 V (Figure 5a,b thin black line), resulting in a
similar hydrogen peroxide yield of about 40% (Figure 6c, thin
dashed line). Compared to the reaction on the COad-free Pt/
Vulcan electrode at a similar reaction potential of 0.26 V the
total ORR current reaches only about 10% (Figure 6a, thin solid
black and solid gray lines) at similar hydrogen peroxide
formation currents (Figure 6b, thin black and thick gray lines),
in agreement with previous findings that carbon is a poor ORR
catalyst compared to Pt.10,34
Analogous current transients for the ORR recorded at a
constant electrode potential of 0.26 V are shown in Figure 6
(CO stripping of the CO adlayer remaining after these experi-
ments is shown in Figure 4). In their general characteristics,
these data closely resemble those obtained during reaction at
0.06 V (see above). On the COad-blocked Pt/Vulcan surface,
the total ORR current (Figure 6a, thick solid black line) also
first increases almost instantaneously after having started the
flow of O2-saturated electrolyte. The value reached here,
however, is only about -0.02 mA, which is only slightly more
than 10% of that obtained at 0.06 V. It then decays steadily to
the mass transport limited value of about 0.5 mA. Furthermore,
despite the rather similar final values of the ORR current, the
increase in ORR current is much faster at the higher potential.
At 0.26 V, the ORR current reaches the same value in about 1
min as after 8.5 min at 0.06 V and attains the ORR value
measured on the COad-free Pt/Vulcan electrode at 0.26 V in ca.
4 min (Figures 6a and 5a, thick solid black and gray lines).
Nevertheless, the amount of COad remaining after the reaction,
evaluated from the stripping peak after 9 min of CO-adlayer
exposure to O2-saturated solution at 0.26 V (Figure 4a, thick
solid black line), is about the same as that after reaction at 0.06
V (Figure 3a, thick solid black line), i.e., about 80% of the
saturated CO adlayer coverage, equivalent to about 0.6 ML.
These results explain in a simple way the different observa-
tions for the coverage of the remaining CO adlayer and hence
for the rate for chemical COad oxidation by chemical reaction
with oxygen in our previous DEMS experiments,3 where we
found a much stronger decay of the CO adlayer coverage and
hence faster COad oxidation kinetics for reaction at 0.26 V than
at 0.06 V, and in the present experiments, where the losses of
the CO adlayer were similar at both potentials. The rates are
indeed different for shorter reaction times, being significantly
higher at 0.26 V than at 0.06 V. However, the reactions becomes
very slow at longer reaction times and levels off the mass
transport limited value, at a COad coverage of about 0.6 ML.
Apparently, at a critical COad coverage of around 80% of the
saturation coverage, there are enough free Pt sites available to
reach the transport limited ORR rate, leaving very little O2 for
the chemical reaction with adsorbed CO (see the Discussion
section). In the DEMS experiments, which were performed using
a 2% O2-in-Ar gas mixture, it takes much longer to reach this
critical COad coverage than in the present DDE-DTLFC experi-
ments, where from experimental reasons we had to work with
pure O2. Hence, we had not waited long enough in the DEMS
experiments in ref 3 to reach this CO critical coverage.
4 Discussion and Conclusions
The data presented above in combination with the DEMS
data in ref 3 lead to the following conclusions on the kinetics
and mechanism of the competing chemical COad oxidation and
the ORR on COad-covered Pt/Vulcan catalysts.
1. At a fuel cell relevant reaction potential of 0.06 V, the
chemical oxidation of COad is rather slow. At a mass transport
limited O2 impingement of at least 10 ML per second, which is
calculated from the mass transport limited ORR rate assuming
a reaction probability of 1 on the COad-free Pt/Vulcan catalyst
under these conditions, it takes about 500 s to remove 20% of
the adsorbed CO. For a COad coverage independent reaction
rate, we can calculate a reaction probability for chemical CO
oxidation of at most 10-5 (5000 ML O2 per 0.15 ML COad) per
impinging O2 molecule from the above numbers.
2. The reaction rate for chemical CO oxidation depends
strongly on the COad coverage and on the potential, which is
evidenced both by the Faradaic current, using the ORR rate as
a measure for the COad coverage, and by the CO2 formation
rate.3,32 Under present reaction conditions, the limiting COad
coverage is about the same for both potentials investigated,

7900 J. Phys. Chem. B, Vol. 108, No. 23, 2004
Jusys and Behm
around 0.6 ML. At the same time, the O2 consumption has
reached the mass transport limited value. We therefore explain
the observation of a limiting coverage by the combination of
two effects, (i) a significant decrease in the probability for
chemical CO oxidation (see point 4) and (ii) an increase in the
O2 consumption for the ORR, which leaves increasingly less
oxygen for the competing CO oxidation.
3. Also the activity and selectivity of the ORR depend
strongly on the COad coverage. At both reaction potentials the
initial H2O2 yield, on a fully COad-blocked Pt/Vulcan catalyst,
is between 80% and 90%, though at much lower rates at the
higher potential. With increasing reaction time, and therefore
decreasing COad coverage, the H2O2 yield decreases steadily,
reaching about 25% at the limiting COad coverage of 0.6 ML,
whereas the (negative) Faradaic current increases to the O2 mass
transport limited value.
4. The formation of a pronounced maximum in the rate for
H2O2 formation after about 45-60 s under present reaction
conditions, both at 0.06 and 0.26 V reaction potential, in
combination with a steady increase in faradaic current and
decrease in H2O2 yield with decreasing COad coverage, points
to a complex relation between the total ORR rate, described by
the Faradaic current, and the H2O2 yield. Whereas in the first
60 s the increase in reactivity prevails, resulting in an increasing
partial current for H2O2 formation, the decrease in H2O2 yield
overcompensates this effect in the later stages. The maximum
in H2O2 formation appears long before the mass transport limited
O2 consumption is reached, so that transport effects cannot be
made responsible.
A mechanistic explanation of this phenomenon can be given
when we include the observation of a similar maximum, after
comparable reaction times, in the CO2 formation rate.3 The
correlation between CO oxidation and H2O2 formation leads to
the proposal that O2 adsorption in small vacancies, most likely
mono-vacancies, can result either in H2O2 formation or in
reaction with a neighboring COad (CO2 formation) plus forma-
tion of H2O. The probability for COad oxidation, however, is
much lower than that for H2O2 formation, pointing to a
considerable reaction barrier for the latter process. Though we
cannot rule out the reaction with two neighboring COad
molecules, it appears unlikely considering the very low prob-
ability for COad oxidation, which even for a saturated CO adlayer
is much less likely than reduction to H2O. On the larger
vacancies the ORR prevails, resulting in H2O formation, while
reaction of the reaction intermediate with COad, at the perimeter
of the vacancies, is highly improbable.
measurements imply that the oxidation of adsorbed CO by O2
at fuel cell relevant anode potentials is a very slow process.
Only 10-5-10-4 of the consumed O2 molecules are used for
CO oxidation;3 the vast majority is used in the ORR, with a
very high yield for H2O2 formation, between 80 and 90% at
saturation and still more than 25% at a COad coverage of 80%
of the saturation coverage. On the other hand, the loss of ca.
20% of the CO adlayer is sufficient to reach the diffusion-limited
ORR current under present experimental conditions. Likewise,
for the hydrogen oxidation reaction (HOR) on a CO blocked
Pt/Vulcan electrode the loss of ca. 5% of the CO adlayer was
found to be sufficient to reach the mass transport limited HOR
rate under similar experimental conditions.33
On the basis of the effective CO sticking probability at a
given CO adlayer coverage,33 we can calculate the required
minimum amount of O2 in the feed to maintain a constant
steady-state coverage of adsorbed CO. For 80% of the saturation
coverage, the effective sticking probability of CO is around
0.3.33 With a reaction probability of 3 × 10-5 for chemical CO
oxidation and a CO contamination level of 50 ppm, this would
require 15% O2 in the feed gas to maintain this coverage. For
95% of the COad saturation coverage, the required O2 content
would be somewhat lower because of the lower CO sticking
coefficient (<0.1) and the higher reaction probability at the
higher COad coverage. Taking the H2O2 yield, however, the
reduction in the amount of O2 required would at least partly be
offset by the much higher H2O2 yield at the higher COad
coverage, which is estimated to be between 60 and 80%
compared about 25% at 80% of the saturation coverage. Hence,
in both cases, about 3-4% of the feed would be turned into
H2O2 under these conditions, which is not tolerable because of
the possible degradation of the carbon support and the mem-
brane.5
These numbers may vary significantly in the presence of H2,
at higher temperatures, and under the mass flow conditions
present in an operating fuel cell. Nevertheless, it is instructive
to visualize in a well-defined model setup the impact of the
very low probability for chemical CO oxidation, by reaction
with O2, on a supported Pt catalyst. In a simplified picture, the
“price” for increasing the CO tolerance of a fuel cell anode using
the air-bleed method very likely is an enhanced long-term
degradation of the MEA, i.e., of the catalyst layer, carbon
support, and membrane due to the reasons discussed above.
Acknowledgment. We gratefully acknowledge financial
support by the state of Baden-Wu¨rttemberg, within the “Zuku-
nftsoffensive Junge Generation”, by the Federal Ministry of
Education and Research (Grant 01SF0053) and by the Deutsche
Forschungsgemeinschaft (Be 1201/8-4).
Reaction between a molecularly adsorbed peroxo species and
COad was in fact found in recent density functional calculations
for CO oxidation on Pt(111) at high coverages as the lowest
barrier reaction path.35
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