Electrochemical and Solid-State Letters, 7 共12兲 A474-A476 共2004兲
A474
1099-0062/2004/7共12兲/A474/3/$7.00 © The Electrochemical Society, Inc.
Effect of Agglomeration of PtÕC Catalyst on Hydrogen Peroxide
Formation
Minoru Inaba,*,z Hirohisa Yamada, Junko Tokunaga, and Akimasa Tasaka*
Department of Molecular Science and Technology, Faculty of Engineering, Doshisha University, 1-3
Tadara-Miyakodani, Kyotanabe, Kyoto 610-0321, Japan
Various amounts of 20 wt % Pt/C catalysts (56.7-5.7 ␮gcarbon cm⫺2 ) were loaded on glassy carbon 共GC兲 disk electrode, and the
effect of agglomeration on hydrogen peroxide formation in oxygen reduction was investigated by the rotating ring-disk electrode
technique. The formation of H2 O2 was enhanced with a decrease in agglomeration of Pt/C. Even in the operating potential range
of polymer electrolyte fuel cell cathodes 共0.6-0.8 V兲, 10% hydrogen peroxide was formed at 5.7 ␮gcarbon cm⫺2 Pt/C loaded on GC.
These results revealed that series two-electron reduction pathway, which is negligible on clean bulk Pt surface, does exist on Pt
particles supported on carbon.
© 2004 The Electrochemical Society. 关DOI: 10.1149/1.1814595兴 All rights reserved.
Manuscript submitted May 20, 2004; revised manuscript received June 21, 2004. Available electronically October 25, 2004.
Durability is one of the most important issues in commercialization of polymer electrolyte fuel cells 共PEFCs兲. A long life over
90,000 h (⫽10 yr) is required for PEFC stacks in the small-scale
cogeneration systems; however, sufficient durability to meet this demand has not been established. Various mechanisms are being considered for the deterioration of the cell stacks during a long
operation. Hydrogen peroxide formation, which is formed electrochemically or chemically during operation, is one of the potential
deteriorating factors of PEFCs.1 Hydrogen peroxide is a strongly
oxidative compound, and may deteriorate the components of the
membrane-electrode assemblies 共MEAs兲 in PEFCs. For example, it
is widely known that H2 O2 deteriorates perfluorinated sulfonic acid
membranes in the presence of a small amount of Fe2⫹ ion.1
In the MEAs, fine platinum particles 共several nm in diameter兲
supported on carbon black 共Pt/C兲 are used as the catalyst of the
cathode in PEFCs. Oxygen reduction reaction 共ORR兲 on clean bulk
Pt surface has been thoroughly studied for decades,2-5 and it is
widely believed that oxygen is reduced predominantly to water
through direct four-electron reduction. ORR on Pt/C catalysts has
been also investigated using the rotating ring-disk electrode 共RRDE兲
technique,6,7 and it has been reported so far that hydrogen peroxide
formation is negligible 共less than 1%兲 in the operating potential
range of PEFC cathodes 共0.6-0.8 V兲. We have also studied ORR on
Pt/C catalysts using the RRDE technique, concentrating on H2 O2
formation as a deteriorating factor of the MEAs.8 We recently found
that a large amount of H2 O2 is formed on Pt/C catalysts when they
are highly dispersed on glassy carbon 共GC兲, which may be a key to
understand the mechanism H2 O2 formation at the MEAs in PEFCs.
Here we preliminary report the effect of agglomeration of Pt/C on
H2 O2 formation.
Experimental
A commercially available catalyst of 20 wt % Pt supported on
Vulcan XC-72 共E-TEK兲 was used throughout in the present study.
The nominal diameter of the Pt particles is 2.5 nm.9 The RRDE
共Nikko Keisoku兲 consisted of a GC disk and a platinum ring sealed
in a polytetrafluoroethylene holder. The geometric surface area of
the disk electrode was 0.28 cm2 共6 mm diam兲, and the collection
efficiency N determined using a solution of Fe(CN) 3⫺ was
0.36 ⫾ 0.02 (N th ⫽ 0.38). 10 The RRDE was polished to a mirror
finish with a 0.05 ␮m alumina suspension 共Baikalox兲 and then
cleaned ultrasonically in highly pure water. The Pt/C catalyst was
ultrasonically dispersed in ethanol 共Nakalai Tesque兲 at a concentration of 1 g dm⫺3 . An aliquot of the suspension was carefully
dropped on the GC disk electrode with a microsyringe to adjust Pt
* Electrochemical Society Active Member.
z
E-mail: [email protected]
loadings at 5.67, 11.3, 28.3, and 56.7 ␮gcarbon-cm⫺2 , and dried overnight at 60°C in an electric oven. The dispersed states of the Pt/C
catalysts on the GC disk were observed with an optical microscope
共Olympus, DP10兲. Nafion coating to hold the catalyst layer on GC11
was not used in the present study. Loss of Pt/C during electrochemical measurements was not observed from cyclic voltammograms
共CVs兲 before and after the measurements. Perchloric acid 共70%,
Tama Chemical Co., Inc., analytical grade兲 was diluted with highly
pure water from a water purification system 共Yamato, Autopure
WD500兲 to prepare 1.0 M HClO4 . Argon and O2 gases were of
ultrahigh purity 共Japan Fine Products兲. Hydrogen gas was obtained
from a water electrolysis system 共Parker, H2-500兲.
A glass cell that was filled with 1.0 M HClO4 was used for
electrochemical measurements.12,13 The counter and reference electrodes were a Pt plate and a reversible hydrogen electrode 共RHE兲,
respectively. Electrochemical measurements were carried out using
a dual-potentiostat 共ALS, model 700A兲 and a motor speed controller
共Nikko Keisoku, RDE-1兲. The disk and ring potentials were scanned
repeatedly between 0.0 and 1.4 V at 50 mV s⫺1 under argon atmosphere to remove residual impurities and finally CVs were recorded.
Current-potential relations for ORR were measured in O2 -saturated
solutions at various rotation rates. The potential of the GC disk was
swept at 2 mV s⫺1 in the negative-going direction from 1.0 to 0.05
V to obtain current-potential curves. The ring potential was kept at
1.4 V, at which all H2 O2 molecules that reached the ring were oxidized to O2 , to detect H2 O2 formed at the disk electrode All measurements were carried out at 25°C.
Results and Discussion
Figure 1 shows optical micrographs of GC 20 wt % Pt/C catalysts dispersed on GC with different amounts. The catalysts loaded
with 56.7 ␮gcarbon cm⫺2 were highly agglomerated, and formed a
thick layer on GC 共Fig. 2a and e兲. The agglomeration decreased with
a decrease in the loading amount of Pt/C, and bare part of GC
surface increased. The catalysts loaded with 5.7 ␮gcarbon cm⫺2
seemed highly dispersed at a low magnification 共x 10, Fig. 2d兲;
however, agglomerates of about 10 ␮m were still observed at a high
magnification 共x 500, Fig. 2h兲.
CVs at GC loaded with various amounts of Pt/C in 1.0 M HClO4
under argon atmosphere are compared in Fig. 2, in which the ordinate is shown in current per mass of Pt. Note that contribution from
double-layer charging of GC surface became significant when Pt/C
loading decreased in Fig. 2. The currents per mass of Pt for characteristic peaks of Pt 共i.e., hydrogen adsorption/desorption and oxide
formation/reduction兲 did not change noticeably after the charging
current was subtracted, which indicates that the mass activity of Pt
was not changed by a change in Pt/C loading and almost all of the
catalyst particles were electrically connected to the GC disk electrode. At 5.7 and 11.3 ␮gcarbon cm⫺2, additional broad peaks origi-
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Electrochemical and Solid-State Letters, 7 共12兲 A474-A476 共2004兲
A475
Figure 1. Optical micrographs of 20 wt % Pt/C dispersed on GC. Pt/C loadings: 共a, e兲 56.7, 共b, f兲 28.3, 共c, g兲 11.3, and 共d, h兲 5.7 ␮g cm⫺2 .
nating from oxygen species 共quinone/hydroquinone type兲 on the carbon support were seen at around 0.6 V.
Figure 3 shows current-potential relations for O2 reduction at
Pt/C with different loadings in O2 -saturated 1.0 M HClO4 . Though
bare GC disk electrode did not have appreciable catalytic activity for
oxygen reduction, Pt/C catalyst loading greatly improved the catalytic activity. The onset of cathodic O2 reduction current (I D) shifted
positively with an increase in Pt/C loading on GC. This is due to an
increase in the total amount of platinum, though mass activity of
Pt/C was not changed by a change in Pt/C loading on GC. For 5.7
and 11.3 ␮gcarbon cm⫺2 Pt/C, the limiting currents for O2 reduction
were lower than those for 28.3 and 56.7 ␮gcarbon cm⫺2 Pt/C. The
thickness of diffusion layer of RRDE depends on the rotation speed,
and is calculated to be about 20 ␮m at 900 rpm in aqueous
solution.10 As is clear in Fig. 1, the distance between adjacent Pt/C
agglomerates expands with decreasing Pt/C loading. Therefore, the
diffusion length of oxygen parallel to the GC surface cannot be
Figure 2. CVs at 20 wt % Pt/C loaded with different amounts on GC in 1 M
HClO4 under argon atmosphere.
ignored and the true surface area of the disk electrode decreases,
which leads to the observed decrease in limiting current for O2
reduction. On the other hand, the ring current (I R), which is oxidation current of H2 O2 formed at the disk electrode, was detected at
potentials more negative than 0.8 V, and increased with lowering the
disk potential. Because the amount of H2 O2 formed on GC surface
was negligibe 共curve e兲, the hydrogen peroxide detected at the ring
electrode was formed at the Pt/C catalysts loaded on GC. I R increased with a decrease in Pt/C loading on GC, which indicates that
the ratio of H2 O2 formation increases with a decrease in agglomeration of Pt/C. For each electrode, a significant increase in I R was
observed in the Hupd region (⬍0.20 V), which is generally attributed
to the blocking effect of the adsorbed hydrogen atoms on the dissociative adsorption of oxygen molecules.5 Here H2 O2 formation increased with a decrease in agglomeration of Pt/C.
Figure 3. Hydrodynamic voltammograms for oxygen reduction in 1 M
HClO4 . Disk electrodes: 20 wt % Pt/C loaded with different amounts on GC,
Ring electrode: Pt. Rotating rate: 900 rpm.
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Electrochemical and Solid-State Letters, 7 共12兲 A474-A476 共2004兲
A476
reduction. As mentioned above, in most of the studies on ORR on
Pt/C catalysts, it has been reported that hydrogen peroxide formation
in the normal operating potential range of PEFC cathodes 共0.6-0.8
V兲 is negligible 共less than 1%兲.6,7 In these studies, a large amount of
Pt/C was loaded on GC and hence Pt/C catalysts should have been
highly agglomerated. Note that about 10% of oxygen was transformed to H2 O2 on 5.7 ␮gcarbon cm⫺2 Pt/C at 0.6 V in the present
study. As shown in Fig. 1, even 5.7 ␮gcarbon cm⫺2 Pt/C was not free
from agglomeration, and it is therefore reasonable to think that more
H2 O2 would be produced on a single Pt/C particle. The mechanism
for the appearance of the two-electron reduction pathway on Pt/C is
not clear at present, but it may originate from (i) particle size effects
of platinum or (ii) interactions between Pt particles and the substrate 共carbon兲. Further investigation on the mechanism for H2 O2
formation is needed to minimize H2 O2 formation on Pt/C and to
improve the durability of MEAs of PEFCs.
Conclusions
Figure 4. The ratio of H2 O2 formation upon oxygen reduction at 20 wt %
Pt/C loaded with different amounts on GC in 1 M HClO4 .
The ratio for H2 O2 formed from one oxygen molecule (X H2 O2 ) at
the disk electrode can be calculated using5
X H2 O2
2I R
N
⫽
IR
ID ⫹
N
关1兴
The variations of X H2 O2 with disk potential are shown in Fig. 4.
X H2 O2 increased with decreasing the loading amount of Pt/C catalysts, and even in the normal operating potential range of PEFC
cathodes 共0.6-0.8 V兲, GC disks with the lowest Pt/C loading
(5.7 ␮gcarbon cm⫺2 ) produced hydrogen peroxide as high as 10%.
X H2 O2 increased with lowering potential, and increased significantly
in the Hupd region. Surprisingly, 65% of oxygen molecules were
transformed to H2 O2 at 5.7 ␮gcarbon cm⫺2 Pt/C loaded on GC.
Cathodic O2 reduction is a multielectron reaction associated with
the formation of many intermediates.2,14 Oxygen can be electrochemically reduced either directly to water without the intermediate
formation of hydrogen peroxide 共direct four-electron reduction兲, or
to hydrogen peroxide 共series two-electron reduction兲. The adsorbed
hydrogen peroxide (H2 O2 ) ads can be further reduced to water 共series
four-electron path兲, oxidized back to O2 by catalytic decomposition
at the platinum surface 共disproportionation兲, or desorbed and diffused into the bulk of the solution. It has been believed for decades
that oxygen is reduced to water predominantly via direct fourelectron reduction at clean bulk Pt surface except in the Hupd
region.2-5 Nevertheless, the results obtained in the present study revealed that two-electron reduction pathway leading to H2 O2 formation does exist on Pt particles supported on carbon. On hydrogen
peroxide formation, (H2 O2 ) ads is desorbed from Pt surface, and diffuses away to the bulk of the solution. When Pt/C catalysts are
highly agglomerated, the diffusing H2 O2 molecules easily meet
nearby Pt/C particles, where they are transformed to water by further two-electron reduction 共series four-electron reduction兲 or catalytic decomposition. Both processes lead to apparent four-electron
The effect of agglomeration of 20 wt % Pt/C on ORR was investigated using the RRDE technique. H2 O2 formation was greatly
enhanced with a decrease in agglomeration of Pt/C. In the Hupd
region (⬍0.2 V), 65% of oxygen molecules were transformed to
H2 O2 at 5.7 ␮gcarbon cm⫺2 Pt/C loaded on GC. Even in the normal
potential range of PEFC cathodes 共0.6-0.8 V兲, 10% hydrogen peroxide was formed at maximum. These results revealed that series
two-electron reduction (H2 O2 formation兲, which is negligible on
clean bulk Pt surface, does exist on Pt particles supported on carbon.
However, in agglomerated state, water is predominantly formed
through apparent four-electron reduction, which results from the series four-electron reduction or catalytic decomposition of H2 O2 at
nearby Pt/C catalysts in the agglomerates. The facts obtained in the
present study would be a key to clarify the mechanism for H2 O2
formation in the MEAs and the resulting deterioration of the MEAs
of PEFCs.
Acknowledgment
This work was supported by Research and Development of Polymer Electrolyte Fuel Cells from New Energy and Industrial Technology Development Organization 共NEDO兲, Japan.
Doshisha University assisted in meeting the publication costs of this
article.
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