Chloride ion adsorption effects in the recombination-controlled kinetics of anodic chlorine evolution at Pt electrodes

Abstract

The role of chloride ion adsorption in the anodic Cl₂-evolution reaction at Pt electrodes in aqueous chloride solutions is examined for a range of Cl^– ion concentrations. A new method of testing the applicability of a recombination-controlled mechanism is given. It enables both the rate constant of the rate-controlling recombination step and the quasi-equilibrium constant of the prior Cl^– discharge/Cl^˙ electrosorption step to be evaluated quantitatively as a function of Cl^– concentration.

It is shown that the Cl₂-evolution process at Pt is recombination-controlled over a wide-range of Cl^– ion concentrations. Despite the approach of the anodic current–potential relation towards limiting-current behaviour, consistent with recombination-controlled kinetics, the limiting current densities are dependent on Cl^– ion concentration. It is suggested that this is due to (a) effects of specifically adsorbed Cl^– ion on the Cl^˙ recombination rate constant and (b) changes of state of the Pt anode surface due to competitive effects of Cl^– ion adsorption on the extent of surface oxidation of the Pt surface on which the catalytic recombination of discharged Cl^˙ takes place in aqueous medium. These two factors complicate the interpretation of reaction orders in [Cl^–] for the Cl₂ evolution reaction since both specific adsorption effects and the competitive effects in surface oxidation have to be taken into account in relating rates to Cl^– ion concentration.