Cobalt(III)-d-metal triethanolamine complexes as catalysts of electrochemical reduction of oxygen

Article abstract of DOI:10.1023/A:1013071018716

Carbon-supported oxide catalysts of oxygen electroreduction were prepared from Co(III)-M(II) ethanolamine complexes. The electrochemical properties of these catalysts were studied. The apparent activation energies of the reduction were measured. 2001 MAIK

Full text of DOI:10.1023/A:1013071018716

Russian Journal of Applied Chemistry, Vol. 74, No. 7, 2001, pp. 1147 1150. Translated from Zhurnal Prikladnoi Khimii, Vol. 74, No. 7,
2001, pp. 1116 1119.
Original Russian Text Copyright
2001 by Kublanovskii, Pirskii.
CATALYSIS
Cobalt(III) d-Metal Triethanolamine Complexes as Catalysts
of Electrochemical Reduction of Oxygen
V. S. Kublanovskii and Yu. K. Pirskii
Vernadsky Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine,
Kiev, Ukraine
Received September 12, 2000; in final form, April 2001
Abstract Carbon-supported oxide catalysts of oxygen electroreduction were prepared from Co(III) M(II)
ethanolamine complexes. The electrochemical properties of these catalysts were studied. The apparent activa-
tion energies of the reduction were measured.
Development of new electrocatalysts free from
noble metals and electrodes on their base for chemical
current sources is an urgent problem [1]. One of the
most promising catalysts for oxygen electroreduction
in metal air chemical current sources and electro-
problem. The electrocatalytic activity of oxides is
known [1, 7] to depend mainly on the nature of sur-
face cations and structural defects. The aim of this
work was to prepare catalysts of electrochemical oxy-
gen reduction by pyrolysis of cobalt(III) d-metal(II)
triethanolamine complexes on carbon support to form
metal oxides with the spinel structure.
chemical generators are d-metal N complexes [2 5].
4
Catalysts with required properties can be prepared by
variation of the ligands and the central metal. Thermal
treatment of carbon materials modified with organo-
EXPERIMENTAL
metallic N complexes enhances their catalytic activ-
4
ity in electrochemical reduction of oxygen and in-
creases the discharge voltage and capacity of voltaic
cells. The structure of catalytically active centers
depends on the ligand nature and on the temperature
and atmosphere of pyrolysis. For example, the active
centers of the catalysts prepared by thermolysis of
cobalt(II) and iron(II) polyacrylonitrile complexes
at 800 900 C are anchored to the carbon support via
nitrogen atoms [2]. When a carbon material modified
with products of condensation of ethylenediamine and
formaldehyde with d-metals are used as the precursors,
the most active catalysts are formed at 500 600 C
[3]. Effective catalysts of oxygen electrochemical
reduction were prepared by heat treatment of poly-
meric cobalt(III) complexes with p-phenylenediamine
and 2,6-diaminopyridine at 923 1123 C in a vacuum,
with the catalytic activity increasing with the tem-
perature of the heat treatment. Tarasevich and Ra-
dyushkina [5] showed that effective catalysts based on
cobalt(II) and iron(II) porphyrin and phthalocyanine
complexes can be prepared by calcination at 800
900 C. Among the best catalysts are 3d-metal oxides
with the spinel structure [6]. However, the electrical
conductivity of these oxides is low, and preparation of
catalysts with high surface conductivity by application
of these oxides to carbon support is rather complex
Catalysts of electrochemical reduction of oxygen
were prepared by treatment of AG-3 activated carbon
with methanolic or aqueous-methanolic solutions of
[Co(HTEA)H TEA] M(OOCCH ) (where H TEA
2
2
3 2
2+
3
2+
2+
is triethanolamine and M is Mn , Ni , Cu , or
2+
Zn ).
The initial complexes were prepared by addition of
a 0.2 M methanolic or aqueous methanolic solution
2+
2+
2+
(10 ml) of appropriate d-metal (Mn , Ni , Cu , or
2+
Zn ) acetates to 0.004 M methanolic solution (5 ml)
of cobalt aminoalcoholate [Co(HTEA)H TEA] pre-
2
2
pared by the procedure in [8]. Methanol was used as
the solvent of nickel acetate and zinc acetate and
water methanol mixture (1 : 1), as the solvent for
manganese acetate and copper acetate. Manganese(II)
acetate was prepared by dissolving metallic manga-
nese in glacial acetic acid, followed by recrystalliza-
tion from water. The other acetates were of analytical-
ly pure grade. AG-3 activated carbon (1 g) with par-
ticles smaller than 50 m and the BET specific sur-
2
1
face area of 850 m g was added to a solution of
appropriate metal acetate. The mixture was allowed to
stand for 12 h. Then 80% of the solvent was distilled
off. The impregnated carbon was filtered off, washed
with a small amount of isopropanol and then with
1070-4272/01/7407-1147$25.00 2001 MAIK Nauka/Interperiodica

1148
KUBLANOVSKII, PIRSKII
ether, and dried at 120 C to constant weight. The
weight gain of the activated carbon after sorption of
[Co(HTEA)H TEA] Mn(OOCCH ) , [Co(HTEA)H
and after bubbling of oxygen through the electrolyte
was measured by an OP-210/3 microanalyzer [12].
2
2
3 2
2
The working floating gas-diffusion electrode was
prepared by the following procedure. A nickel wire
TEA] Cu(OOCCH ) ,
[Co(HTEA)H TEA] Zn
2
3 2
2 2
(OOCCH ) , and [Co(HTEA)H TEA] Ni(OOCCH )
2
3 2
2
2
3 2
was pressed at a pressure of 50 70 kg cm in acetyl-
was 51, 55, 20, and 64%, respectively.
ene black (400 mg) hydrophobized with 30% poly-
tetrafluoroethylene. A thin layer of finely divided
catalyst (<20 m) was applied to the surface of the
resulting single-layer porous electrode (d = 10.15 mm,
The currents passed through the catalysts were
adjusted depending on the weight of the complex
applied to the carbon.
3
density 0.95 g cm , thickness 1 mm). The com-
The triethanolamine complexes have the following
structure:
2
posite was pressed at 50 60 kg cm . The amount of
applied catalysts was determined by weighing the
electrode before and after pressing of the catalyst
powder. To determine the state of the catalyst surface
before and after the electrochemical reduction, polari-
zation curves were recorded in an argon atmosphere
1
with a sweeping rate of 2 mV s . The potentiostatic
,
polarization curves were constructed from the current
densities measured at constant potentials adjusted with
a 10 mV increment by a PI-50-1.1 potentiostat. The
current was registered with an M-104 milliammeter
with extrapolation to t
.
Since the thermal treatment was performed in an
inert atmosphere, we suggest that centers active in
the oxygen reduction are metal atoms or their oxides
dispersed in AG-3 activated carbon. As seen from the
kinetic parameters of oxygen electroreduction on the
tested catalysts (see table), the active catalysts are
formed after calcination of the cobalt triethanolamine
complexes on the carbon support at 300 500 C.
where curve segments denote the C H fragments.
2
4
As seen from this figure, the double-charged metal
located in the equatorial plane is bound via oxygen
bridges to the triple-charged cobalt ions. There are
reasons to believe that this fragment in the form of
spinel or substoichiometric mixed oxides can be
retained on the carbon surface after pyrolysis of the
complexes at definite temperatures. Thermal degrada-
tion of the complexes of this type in a carbon matrix
is known [5] to start from elimination of the terminal
groups. The products are incorporated into the struc-
ture of the carbon matrix.
The influence of the calcination temperature on
the properties of catalysts based on Co(III) M(II) tri-
ethanolamine complexes is seen from the results
presented in the table. The maximal exchange currents
are observed for the catalysts prepared at 400 C,
which is probably due to formation of active centers.
With increasing calcination temperature to 800 C,
the currents decrease owing to degradation of these
centers to form inactive species. The slopes of the
stationary polarization curves lie in the following
ranges (mV): b = 44 64 and b = 100 127. The
A weighed portion of carbon-supported metal com-
plexes (200 mg) was placed in a quartz tube furnace.
The sample was gradually heated in an argon counter-
flow to the required temperature, kept at this tempera-
ture for 1 h, and cooled to room temperature. The heat
treatment was performed at temperatures from 200 to
800 C with a 100 C increment. The catalytic activity
of the resulting samples in electrochemical reduction
of oxygen was studied.
1
2
dependence of oxygen electroreduction at a constant
potential E = 0.125 V (vs. silver chloride electrode)
on the calcination temperature is shown in Fig. 1.
The maximal current was observed with the catalysts
calcined at 400 C. This is the best temperature for
preparing active catalysts from cobalt(III) d-metal tri-
ethanolamine complexes. The activity of the tested
catalysts decreases in the following order: Co Mn >
Co Zn > Co Cu > Co Ni.
The electroreduction of oxygen was performed at
room temperature in 1 M KOH on a floating gas-
diffusion electrode [11] in an electrochemical cell
with separated cathodic and anodic compartments.
The electrochemical measurements were performed
in a PI-50-1.1 potentiostat using an Ag/AgCl refer-
ence electrode. The partial pressure of oxygen before
The most active catalysts were prepared from
Co Mn triethanolamine complexes. We suggested that
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 7 2001

COBALT(III) d-METAL TRIETHANOLAMINE COMPLEXES AS CATALYSTS
1149
Kinetic parameters of oxygen electroreduction in 1 M KOH at 20 C on catalysts prepared by pyrolysis of the
Co(III) Mn(II) triethanolamine complexes
Catalyst
E / log j, V
j0 103,
mA cm
1
T,
C
Est, V
Wapp, kJ mol
2
b1
b2
AG-3
Co Mn(AG-3)
0.007
0.053
0.098
0.007
0.003
0.100
0.084
0.092
0.073
0.097
0.065
0.102
0.090
0.044
0.105
0.072
0.100
0.098
0.061
0.111
11.35
4.78
5.68
5.31
12.32
4.54
2.59
1.97
2.84
1.60
5.38
3.81
4.69
10.0
3.09
3.21
1.60
1.98
2.74
0.99
0.058
0.061
0.046
0.049
0.064
0.048
0.052
0.056
0.048
0.060
0.060
0.057
0.055
0.047
0.054
0.047
0.047
0.044
0.046
0.046
0.116
0.117
0.124
0.119
0.126
0.123
0.115
0.109
0.103
0.118
0.116
0.127
0.126
0.120
0.128
0.106
0.105
0.100
0.105
0.104
60.1
56.7
56.0
55.6
54.0
59.1
60.2
59.5
58.8
60.5
60.6
60.4
59.5
59.0
60.4
60.8
60.4
60.2
59.1
60.3
200
300
400
800
Co Ni(AG-3)
Co Zn(AG-3)
300
400
800
200
300
400
800
Co Cu(AG-3)
200
300
400
800
* j is the exchange current density; b and b are the slopes of stationary polarization curves.
0
1
2
the Co Ni catalysts would be the next in the sequence
of the catalytic activity. However, this was not the
case. The X-ray diffraction study showed the absence
of the crystalline phase. Since the complexes contain
oxygen and metal atoms, we suggest that in this case
double metal oxides with spinel or nonstoichiometric
oxide structure are formed. The catalytically active
centers of this catalyst are formed on structural defects
[1, 7] containing excess oxygen. The defects promote
formation of cationic vacancies and increase the elec-
trical conductivity. As a result, the electrochemical
properties of the catalysts are improved.
These data agree with the results of [13] where
spinel and nonstoichiometric oxides prepared also at
300 400 C were used as catalysts.
The ponetiostatic polarization curves of oxygen
electroreduction on the catalyst prepared from Co Mn
triethanolamine complexes supported on AG-3 ac-
tivated carbon, AG-3 activated carbon without sup-
ported metal, and hydrophobized carbon black sub-
strate are shown in Fig. 2. As seen from Fig. 2, the
activity of the catalysts is higher than that of AG-3
carbon. The stationary polarization curve of the cata-
lysts is shifted by 50 mV to the positive potentials.
We also calculated the apparent activation energy of
oxygen electroreduction on the catalysts prepared
from Co(III) M(II) triethanolamine complexes sup-
ported by AG-3 activated carbon. For this purpose the
potentiostatic polarization curves of Co(III) M(II)
electrocatalysts were measured in an oxygen at-
T, C
Fig. 1. Rate of oxygen electroreduction in 1 M KOH at
20 C on catalysts prepared by pyrolysis of Co(III) M(II)
triethanolamine complexes on AG-3 activated carbon
as a function of the pyrolysis temperature T. Potential
E = 0.125 V vs. silver chloride electrode. ( j) Current
density. Catalyst: (1) Co Mn, (2) Co Ni, (3) Co Cu, and
(4) Co Zn.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 7 2001

1150
KUBLANOVSKII, PIRSKII
spinel and nonstoichiometric oxide structure of the
catalysts was suggested.
(2) The most active catalyst was prepared by cal-
cination of Co(III) Mn(II) triethanolamine complex
on AG-3 carbon support at 400 C in an inert atmos-
phere.
ACKNOWLEDGMENTS
This work was financially supported by the State
Foundation for Basic Research of Ukraine.
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lnj0
Wapp
=
2.303R
,
1
(T )
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is the apparent activation energy of oxy-
app
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0
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1
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RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 7 2001