ABSTRACT
References ..................................................................................................................................... 289
Different ferrous metal oxides or defined characteristic structures, like spinel and bronzes, have been investigated as substrates for electrocatalytic processes [1-3], since they offer large activities and a rather good selectivity. Spinels are usually prepared by thermal procedures starting from either a homogenized mixture of solid metal oxides, or by the evaporation of a solution containing the nitrates of the corresponding metals followed by the thermal decomposition of the salt residue, or by the co-precipitation of the metal hydroxides and the subsequent thermal decomposition of the solid [1,4]. The preparation procedures can be carried out on different substrates such as titanium, iron, and carbon, with the objective of developing highly dispersed materials [5,6]. The oxidation of the substrate during the course of the thermal treatment modifies the composition of the spinel-type metal oxide [5]. Depending on the preparative method, the electrocatalytic properties of the spinel oxides exhibit different features upon electrode reactions, such as hydrogen and oxygen evolution, chloride evolution, etc. The results obtained on the iron-group metal hydroxide electrodes show that their electrochemical behavior changes substantially on potential cycling as should be expected when the changes in the oxide layer composition and structure occur [7-10]. Furthermore, those changes are considerably dependent upon the entire history of the electrode, but under certain circumstances, the metal hydroxide electrodes attain a very reproducible and stabilized voltammetric response, which could be attributed to the formation of a definite oxide layer structure. Relatively thick hydrous cobalt oxide overlayers have been grown on cobalt electrodes that are subjected to a triangular potential cycling at low potential sweep rates in an alkaline solution [11].
