Journal of Electroanalytical Chemistry 464 (1999) 149 – 157
An old workhorse of oxide investigations: new features of Co
3
O
4
Mariangela Longhi, Leonardo Formaro *
Department of Physical Chemistry and Electrochemistry, Uniersity of Milan, Via Golgi 19, 20133 Milan, Italy
Received 27 April 1998; received in revised form 13 December 1998
Abstract
The electrochemical behaviour of Co
3
O
4
from exhaustive cobalt nitrate decomposition (T =260 – 850°C) is investigated on
graphite-supported electrodes, mainly relying on quasi-reversible results. Two well defined 1 -e
-
redox systems are observed
related to Co
2 +
/Co
3 +
and Co
3 +
/Co
4 +
reactions of non-equivalent oxide surface sites. Oxide stoichiometry is directly estimated
by voltammetry in ‘wet’ conditions and depends on preparation temperature as in ex situ solid state data. Adsorption of surface
intermediates involved in charge transfer is examined. Langmuir and Temkin-type adsorption isotherms are obeyed for the
Co
2 +
/Co
3 +
and Co
3 +
/Co
4 +
redox systems, respectively. Unusually negative (attractive) lateral interaction parameters are
calculated for the latter system. Separation of oxide and graphite currents is achieved to permit real electrode surface area
determination. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Co
3
O
4
; Voltammetry; Surface stoichiometry
1. Introduction
In the search for catalytic materials to improve the
efficiency of large-scale electrolytic processes cobalt
based oxides and unsubstituted Co
3
O
4
continue to at-
tract considerable interest [1–21]. This is mainly due to
their good performance in, e.g. Cl
2
and O
2
evolution or
O
2
reduction. Besides composition, preparation is most
important in controlling the overall behaviour of cata-
lytic coatings and films. Several techniques, such as salt
decomposition on metals (Ti, Ni, Pt) [1–9], spray pyrol-
ysis [10–14], powder immobilisation [15–19] and
plasma sputtering [2,13,20,21], were developed in time
and actually represent significant steps towards better-
behaved electrodes. Interest in mainly new material
properties, however, also resulted in incomplete or in-
adequate coverage of the basic interfacial behaviour of
materials investigated. The situation perhaps emerges
more readily for Co
3
O
4
whose behaviour was investi-
gated several times on electrodes from one or another
of the above preparations. The voltammetric oxide
‘fingerprint’ between O
2
and H
2
evolution in alkali
shows a characteristic system of coupled anodic and
cathodic maxima near O
2
evolution, with shape, aspect
ratio and symmetry across the potential axis, which
depend on details of the electrode preparation used. A
striking example of preparation dependent behaviour
concerns plasma sputtered thin-film electrodes. The
above current maxima are missing [20] in otherwise
‘featureless [20]’ I /E profiles recorded just after sputter-
ing deposition, and develop only weakly after prelimi-
nary cathodic treatments. In [2] the maxima are instead
already present from the beginning with, moreover,
very favourable kinetic peak potential/sweep rate fea-
tures. Stronger variability in I /E results occurs ap-
proaching open circuit potentials [11 – 14,19], as for
seemingly unrelated electrode materials. Crystallo-
graphic orientation, cell size, bulk and surface defectiv-
ity are indicated among reasons for a given behaviour
[2,4,8,20,21]. These are the same influencing factors
relevant to heterogeneous catalysis. As such, the oxide
appears a suitable subject in further investigations
aimed at establishing a common set of properties from
* Corresponding author. Tel.: +02-26603229; fax: +02-70638129.
E-mail address: formaro@icil64.cilea.it (L. Formaro)
0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII:S0022-0728(99)00012-1