The oxygen electrode. Part 7.—Influence of some electrical and electrolyte variables on the charge capacity of iridium in the anodic region

Abstract

The charge storage properties of an iridium electrode activated by potential cycling are attributed to stoichiometry changes in an oxide film. The rate of growth of this film in acid solution decreased with increasing frequency and was highly dependent on the upper and lower potential limits of the activating cycle. These results are explained in terms of nucleation of corrosion centres at low potentials and subsequent irreversible oxide formation in pitted regions at more anodic potentials. The unusual shift of the main peak of the cyclic voltammogram on changing from acid to base is assumed to be due to a pH-induced alteration in the nature of the anodic film. Although activation in base was negligible, the charge storage process in the case of an electrode activated in acid was effective in the alkaline medium. Inability to activate is explained in terms of oxide dissolution in solutions of high pH. Similarly, decreasing activation efficiency with increasing sulphate concentration is probably due to the formation of soluble sulphate complexes.