Formation of hydrous oxide films on cobalt under potential cycling conditions

Abstract

Subjecting a cobalt electrode in base to repetitive triangular potential sweeps over the range 0–1.2 V(RHE) resulted in the formation of a thick hydrous oxide film. The electrochemical behaviour of the system is complicated due to metal dissolution over the first few cycles (this reaction was less important with thicker films due to the passivating nature of the latter) and and the variety of oxidation states (0, II, III and IV) accesible to this element within the range 0–1.5 V(RHE). As with rhodium, the rate of oxide growth on cycling and the general reactivity of the film decreased with decreasing hydroxide ion concentration. A notable feature of the cobalt system is that enhanced oxide growth was observed using a lower limit at which Co(II)—rather than the base metal—is thermodynamically stable. However, since Co(OH) 2 is slightly soluble, the reduced form of the hydrous oxide is non-protective and further oxidation of the metallic substrate can occur during the subsequent anodic sweep. A considerable proportion of the cations present in the surface layer did not participate in the redox reactions, especially at high sweep rates. Slight enhancement of oxygen gas evolution rates was observed with a hydrous film present on a cobalt anode in base. An advantage of cobalt (relative to RuO 2) is that it retains its low Tafel slope behaviour for oxygen gas evolution in base to quite high current densities.