Int. J. Hydrogen Energy, Vol. 16, No. 1, pp. 1-7, 1991. Printed in Great Britain. 0360-3199/91 $3.00 + 0.00 PergamonPressplc. © 1990International Association for HydrogenEnergy. MECHANISTIC STUDIES OF THE OXYGEN REACTIONS ON NiCo204 SPINEL AND THE HYDROGEN EVOLUTION REACTION ON AMORPHOUS Ni-Co SULPHIDE A. C. D. ANGELO,* E. R. GONZALEZt and L. A. AVACA Instituto de Fisica e Quimica de Sao Carlos, USP C.P. 369, 13560 Sao Carlos, SP, Brazil (Received for publication 13 June 1990) A~tract--The mechanism of the oxygen evolution and reduction reactions on teflon-bonded NiCo204 spinel has been examined several times, mainly because of the potential application of this material in water electrolysersand fuel cells. However, some controversy still remains. This work presents the results of electrochemical studies of the oxygen evolution and reduction on NiCo2 04 spinel in alkaline solutions and a proposal for the mechanisms involved. Kinetic parameters were obtained by steady state polarization experiments, complemented with cyclic voltammetry to examine the surface of the catalyst. The role of the oxidation state of the surface on the mechanism of the oxygen evolution reaction is examined. Results of the study of the hydrogen evolution reaction on amorphous Ni-Co sulphide in alkaline solutions are also presented. This material showed an improved performance in comparison with other catalysts and a mechanism for the reaction is proposed on the basis of the experimental parameters. INTRODUCTION Oxygen electrode reactions have an important role in electrochemical technology because they constitute basic processes in systems like water electrolysers, fuel cells, metal-air batteries, sensors, etc. Both the reduction of oxygen and the oxidation of oxygen present high over- potentials which is the reason for the reduced efficiences in those electrochemical devices. Because of this, there is a marked interest in the development of appropriate electrocatalysts and in understanding their participation in the mechanistic pathways of the reactions. The mixed oxides of transition metals are receiving increased attention as promising electrocatalysts for the oxygen reactions. In general, when the oxygen evolution takes place on a metallic electrode, the surface is already covered with an oxide layer. If the electrode is an oxide, then it is covered with a higher oxide. So, good electro- catalysts have a rest potential of either the couple metal/oxide or the oxide/higher oxide which is near the equilibrium potential of the oxygen evolution reaction. From this point of view the mixed oxides of transition metals are candidates to be good electrocatalysts for the oxygen electrode reactions [1-9]. Of these, the mixed oxides with a spinel cubic crystalline structure seem to be particularly suitable because the couple oxide/higher oxide presents an adequate rest potential and the reticu- lar parameters favour the adsorption of oxygen [3, 7]. *Present Address: Departamento de Quimica, UNESP-- Bauru, C.P. 473, 17100, Bauru, SP, Brazil. tAuthor to whom correspondence should be adressed. Additionally, these materials are good catalysts for the chemical decomposition of hydrogen peroxide which is an undesirable intermediate in the reduction of oxygen [3]. The oxyspinel NiCo:O4 presents high stability and high activity with respect to the oxygen evolution reac- tion (OER) and the oxygen reduction reaction (ORR) in alkaline media [4-7, 9]. In spite of many papers devoted to this material, few mechanistic s~udies have been presented [8, 10, 11] and it is apparent that controversial points still exist about some aspects of the reaction. Thus, we decided to carry out a study of the OER and ORR on NiCo204 spinel in alkaline media to get a deeper understanding of the electrochemical behaviour of this material. A related compound, nickel cobalt sulphide [6], was also prepared and found to be a good electrocatalyst for the hydrogen evolution reaction (HER) in alkaline media. EXPERIMENTAL The NiC0204 spinel was prepared by freeze-drying [12] from 10% solutions of Ni(NO3)2 (Riedel p.a.) and C0(NO3)2 (Riedel p.a.) mixed in the stoichiometric proportions followed by thermal decomposition at 320°C. The spinel was characterized by X-ray diffrac- tion. The nickel-cobalt sulphide was prepared by co- precipitation from the same mixed solutions with the addition of sodium sulphide (Merck, p.a.). Characteriz- ation by X-ray diffraction showed that this material is essentially amorphous. The teflon-bonded electrodes were prepared with an active mass constituted of the