The kinetics of electrode reactions and the electrode material

The effect of the nature of the electrode on the kinetics of electrode reactions is considered. This effect may be primary in the sense that the electrode exerts a direct influence on the state of the reactants or may itself be a reactant. On the other hand, secondary effects occur when the nature of the electrode affects the state of non-reactants which then influence the rate or course of the reaction. Two limiting types of reaction may be distinguished, 1) Simple electron transfer reactions in which the reactants form only the weakest of bonds with the electrode surface, 2) Reactions in which the reactants or some intermediate form strong bonds with the electrode surface. The theory of type 1) reactions and the very limited experimental evidence suggest that when the electrode is a metal, it has little primary influence on the kinetics, although the secondary effects may be quite large. When the electrode is a semiconductor, however, the kinetics are markedly affected by the nature of the electrode. The primary effect of the electrode material on reactions of type 2) is important for all types of electrodes. At metal electrodes it is expressed through the strength of the adsorption bond. This effect is discussed in terms of a simple model. The features which are probably of general applicability are indicated and are compared with the available experimental evidence. Information about the structure of solid electrode surfaces is thus largely to be obtained from the analysis of the kinetics of electrode reactions in terms of the adsorption of reactants or intermediates. The use of steady state kinetic measurements is shown to have the advantages of simplicity but the disadvantage of insensitivity. A more powerful technique in principle is the study of transients.