Electrocatalysis in the anodic evolution of oxygen and chlorine

Requisites for electrode materials to be suitable for technological applications are outlined and discussed. Oxides with metallic or quasi-metallic conductivity meet these requirements best. Most of these electrodes are prepared by thermal procedures. It is shown that the temperature of preparation affects the catalytic activity through the surface area and the chemical composition (non-stoichiometry). The co-variation of these parameters is best followed in situ by voltammetric curves and point of zero charge measurements. Examples are given for pure RuO2, IrO2, Co3O4 and IrO2 + RuO2 mixtures. Kinetic and mechanistic details are discussed for O2 evolution on RuO2, IrO2 and Co3O4 and for Cl2 evolution on RuO2 and Co3O4. Finally, the electrocatalytic properties of different oxides are correlated with the energy change involved in the lower → higher valency state transition. Experimental data for both O2 and Cl2 evolution can thus be organized into a “volcano” curve enabling predictive interpolations to be made.