Platinum-free catalyst for efficient reduction of oxygen at air cathode

Article abstract of DOI:10.1246/cl.2006.1166

Electrocatalytic activity and selectivity for oxygen reduction to water on platinum-free catalysts prepared from transition metal hexacyanometallate precursors have been examined using a rotating ring-disk electrode technique. The results obtained indicated that a catalyst containing copper and iron promotes so-called four-electron reduction reaction of oxygen at a potential close to or higher than that of a Pt electrode, so that it could be a possible candidate substituting for Pt-based catalysts of air cathodes. Copyright

Full text of DOI:10.1246/cl.2006.1166

1166
Chemistry Letters Vol.35, No.10 (2006)
Platinum-free Catalyst for Efficient Reduction of Oxygen at Air Cathode
Keijiro Sawaiꢀ and Daisaku Uda
Department of Applied Chemistry, Graduate School of Engineering, Osaka City University (OCU),
3-3-138 Sugimoto, Sumiyoshi, Osaka 558-8585
(Received August 21, 2006; CL-060951; E-mail: sawai@a-chem.eng.osaka-cu.ac.jp)
Electrocatalytic activity and selectivity for oxygen reduc-
tion to water on platinum-free catalysts prepared from transition
metal hexacyanometallate precursors have been examined using
a rotating ring-disk electrode technique. The results obtained in-
dicated that a catalyst containing copper and iron promotes so-
called four-electron reduction reaction of oxygen at a potential
close to or higher than that of a Pt electrode, so that it could
be a possible candidate substituting for Pt-based catalysts of
air cathodes.
into a vigorously stirred suspension of the pretreated carbon
black in 0.02 M K3M00(CN)6 (M00 ¼ Fe and Co) solution. The
molar ratio of M0 to M00 in the mixed solution was 1.2. The
loaded amount of the Prussian blue analogs was ca. 5 ꢂ 10ꢁ4
mol on 0.1 g of carbon support. The obtained precursor was
heated up to 800 ꢃC at a rate of ca. 200 deg/h under a nitrogen
flow, followed by keeping at 800 ꢃC for ca. 10 min. For brevity,
the obtained sample was denoted as HT-M0[M00]PB/C.
Polarization measurements for oxygen reduction were
carried out in a glass cell with a rotating ring-disk electrode. A
platinum ring–glassy carbon (GC) disk electrode assembly (disk
diameter 5 mm, ring inner diameter 5.5 mm, ring outer diameter
8 mm) was used as a working electrode. The collection efficien-
cy of the ring-disk electrode was determined to be 0.48 by using
Fe(CN)64ꢁ/Fe(CN)63ꢁ redox couple. The GC disk electrode was
coated with a catalyst layer by applying catalyst dispersion in a
diluted Nafionꢀ solution. The amount of catalyst loaded on GC
disk was ca. 0.25 mg (ca. 0.1 mg as transition-metal weight). The
thickness of Nafion film was estimated to be 0.12 mm by assum-
ing a density of 2 g cmꢁ3. Oxygen reduction reaction was also
examined using a Pt ring–bright Pt disk electrode for compari-
son. The electrolyte used was a 1 M potassium phosphate buffer
solution (pH 7.6). An Ag/AgCl/satd. KCl was used as a refer-
ence electrode, while the electrode potential was represented
against a hydrogen electrode (RHE). All electrochemical meas-
urements were carried out at room temperature.
Electrochemical reduction of oxygen has been of great
importance in various applications, such as air cathodes of fuel
cells and metal/air batteries to generate electrical energy, and
air cathodes of electrolytic processes to reduce power consump-
tion. For oxygen reduction at low and moderate temperatures,
Pt or Pt alloy catalysts have been most widely used, because
Pt-based catalysts are the most efficient catalyst for oxygen
reduction by transferring totally four electrons,1 so-called four-
electron reduction. However, inexpensive catalysts with em-
ploying abundant nonnoble elements are desired. Many efforts
have been made to explore Pt-free catalysts, such as transition-
metal oxides,2 transition metal macrocycles,3 and alternative cat-
alysts,4 for both high activity and selectivity for four-electron re-
duction of oxygen to yield water as product.
Recently, we have reported an easy and inexpensive method
to prepare Pt-free catalysts from the nitrogen-containing inor-
ganic precursors (transition metal hexacyanometallates, Prussian
blue analogs, PBs) dispersed on a carbon support.5 An advantage
of using such inorganic precursors is that a series of compound
consisting of various combinations of transition metal ions locat-
ed at nitrogen-coordinated and carbon-coordinated sites can be
easily prepared from popular transition-metal salts to examine
the effect of combination of transition elements together with
the effect of coordination field in the precursor upon the catalytic
activity for oxygen reduction. By taking such an advantage, it
was found that the combination of cobalt and iron leads to the
prominent synergistic effect of cobalt and iron on the catalytic
activity for oxygen reduction.5 However, there still seemed to
be a problem concerning generation of an intermediate, hydro-
gen peroxide, during oxygen reduction. Hydrogen peroxide is
a strong oxidizing species, so that the peroxide generation during
oxygen reduction is a crucial factor on deterioration of air
cathode. Here, we report a novel Pt-free catalyst showing
both high activity and selectivity for four-electron reduction of
oxygen.
Figure 1 shows rotating ring-disk electrode voltammograms
for oxygen reduction on the catalyst-coated GC disk electrode in
0.015
(a)
0.010
(b)
0.005
Ring
0.0
(c)
-0.1
Disk
-0.2
(a)
-0.3
(b)
-0.4
-0.5
0.0
0.2
0.4
0.6
0.8
1.0
E / V vs. RHE
Figure 1. Rotating ring-disk electrode voltammograms for oxy-
gen reduction on (a) HT-Co[Fe]PB/C, (b) HT-Cu[Fe]PB/C, and
(c) bright Pt in 1 M potassium phosphate buffer solution (pH 7.6)
saturated with oxygen. The collection efficiency of the ring-disk
electrode is 0.48. The rotation rate of the electrode was 1500
rpm. The ring electrode potential was set at 1.35 V vs. RHE.
The catalysts were prepared as described previously.5 The
carbon black powder (Vulcan XC-72R, Cabot Corp.) was pre-
treated in 9 M (1 M ¼ 1 mol dmꢁ3) HNO3 at room temperature
for ca. 12 h. The precursors were prepared by slowly adding
0.024 M aqueous M0SO4 (M0 ¼ Mn, Fe, Co, Ni, and Cu) solution
Copyright ꢁ 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.10 (2006)
1167
0.015
(b)
(c)
(a)
(b)
(e)
(f)
0.010
4.0
3.5
3.0
(a)
(d)
0.005
Ring
Disk
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
(c)
(d)
(a)
(b)
(c)
(d)
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
E / V vs. RHE
E / V vs. RHE
Figure 3. Average number (ne) of electrons transferred per O2
during oxygen reduction on the disk: (a) HT-Cu[Fe]PB/C, (b)
HT-Cu[Co]PB/C, (c) HT-Fe[Fe]PB/C, (d) HT-Co[Fe]PB/C,
(e) HT-Co[Co]PB/C, and (f) bright Pt.
Figure 2. Rotating ring-disk electrode voltammograms for oxy-
gen reduction on (a) HT-Fe[Fe]PB/C, (b) HT-Co[Co]PB/C, (c)
HT-Cu[Co]PB/C, and (d) HT-Cu[Fe]PB/C. Other conditions
were the same as in Figure 1.
a neutral buffer solution saturated with oxygen. The rest poten-
tial of the catalyst-coated GC disk electrode was somewhere be-
tween 0.85 and 0.9 V vs. RHE. Oxygen reduction current on HT-
Co[Fe]PB/C, for which the synergistic effect of cobalt and iron
upon oxygen reduction activity was observed,5 increases at po-
tentials below 0.85 V and draws a limiting current plateau in a
potential region lower than 0.5 V. The onset potential of oxygen
reduction reaction on HT-Co[Fe]PB/C was almost the same as
that obtained by floating electrode measurements. The ring cur-
rent for peroxide oxidation increases with decreasing disk poten-
tial with drawing a maximum at ca. 0.6 V. While the disk cur-
rents for HT-Co[Fe]PB/C was slightly smaller than that for Pt,
the ring current for HT-Co[Fe]PB/C was quite larger than that
for Pt, indicating that peroxide generation on HT-Co[Fe]PB/C
is rather large at usual oxygen reduction potentials. To explore
a Pt-free catalyst promoting four-electron reduction of oxygen,
a series of Pt-free catalysts were prepared and examined. There-
by, it was found that HT-Cu[Fe]PB/C shows a prominent oxy-
gen reduction activity with hardly any generation of peroxide
during oxygen reduction. As can be seen in Figure 1, oxygen
reduction reaction on HT-Cu[Fe]PB/C sets in higher potential
region than others, while the ring current is comparably small
as that for a bright Pt electrode.
Figure 2 shows an effect of combination of transition ele-
ments in the catalyst upon the peroxide generation and catalytic
activity for oxygen reduction. Although active site structure and
reaction mechanism for oxygen reduction reaction on the present
Pt-free catalysts are not clear yet, peroxide generation is highly
dependent on the combination of the transition elements incor-
porated in the catalysts. The ring current for HT-Fe[Fe]PB/C
is smaller than that for HT-Co[Co]PB/C, while the polarization
curves for oxygen reduction on both catalysts look alike. Substi-
tution of copper for a half cobalt in HT-Co[Co]PB/C, i.e., HT-
Cu[Co]PB/C, leads to slightly improved catalytic activity with
lower generation of peroxide in a potential region lower than
0.7 V. When copper and iron are incorporated together in the
catalyst, i.e., HT-Cu[Fe]PB/C, peroxide generation is quite de-
pressed and oxygen reduction activity is remarkably enhanced,
as can be seen in Figure 2. To examine a contribution of so-
called two-electron reduction generating hydrogen peroxide,
the average number (ne) of electrons transferred during oxygen
reduction was calculated from the ring and disk current data in
Figures 1 and 2 with taking the collection efficiency (0.48) into
account.6 The results are shown in Figure 3. The ne value for HT-
Cu[Fe]PB/C was calculated to be ca. 3.95 in the whole potential
region, whose value was almost the same as that for the Pt
electrode under these conditions.
As were briefly described above, a highly active Pt-free cat-
alyst showing the enhanced activity and selectivity for four-elec-
tron reduction of oxygen is prepared from popular transition
metal salts. Such a cost-effective catalyst could be a possible
candidate substituting for Pt-based catalysts of air cathodes. De-
tailed investigation together with optimization of preparation
conditions is under way in our laboratory.
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Published on the web (Advance View) September 16, 2006; doi:10.1246/cl.2006.1166