Evaluation of Cl? adsorption in anodic Cl2 evolution at Pt by means of impedance and potential-relaxation experiments. Influence of the state of surface oxidation of the Pt

Abstract

The behaviour of the adsorbed Cl˙ intermediate in anodic Cl2 evolution at Pt electrodes has been investigated by means of impedance spectroscopy. Because, in aqueous solution, surface oxide formation is competitive with Cl– ion and Cl˙ adsorption, experiments have been conducted (a) at Pt anodes pre-oxidized under controlled conditions and (b) at electrodes ‘freshly reduced’ before each polarization measurement or immediately prior to each a.c. frequency scan.At the freshly reduced electrodes, the complex-plane plots exhibit two semi-circles while at the pre-oxidized ones, only single semi-circles are observed. This difference indicates appreciable coverage and resulting adsorption pseudocapacitance of adsorbed Cl˙ at the ‘reduced’ surfaces while at the pre-oxidized surfaces, the presence of the oxide film greatly diminishes the pseudocapacitance, corresponding to much lower Cl˙ coverages. However, under both conditions of electrochemical surface pretreatment, the mechanism of anodic Cl2 formation appears to involve a rate-controlling Cl˙ recombination process leading to observable non-diffusion-controlled limiting currents. The impedance measurements are complemented by potential-relaxation studies which give corresponding major differences in the adsorption behaviour of Cl˙ at pre-oxidized and ‘reduced’ Pt electrodes.The presence of the oxide film modifies sensitively the Pt anode surface for chemisorption of the kinetically involved Cl˙ intermediate in anodic Cl2 evolution and, on that account, also inhibits electrocatalysis in that reaction.