Surface electrochemistry and kinetics of anodic bromine formation at platinum

Abstract

Studies are reported on the kinetics of anodic formation of Br2 at Pt in relation to chemisorption of the Br– ion, and the role of the adsorbed Br˙ intermediate in the reaction steps. The results are compared with those for the analogous anodic Cl2 evolution reaction. Cyclic voltammetry, recording of potential-relaxation transients and impedance spectroscopy methods are used to elucidate the question of chemisorption of Br– and Br˙ species in competition with surface oxide formation at Pt, which otherwise occurs in Br–-free aqueous solutions, and to evaluate the kinetic features of the reaction.Measurements have been made of the kinetics of Br2 formation at ‘freshly reduced’ Pt electrodes, at which virtually no anodic oxide film exists, and at ‘pre-oxidized’ ones where a well defined oxide film has been grown prior to the anodic Br2 formation experiments.The kinetics are shown to be dominated by the step of Br˙ recombination under all conditions, as similarly found for the Cl2 evolution reaction. However, the role of Br3– ion in the reaction in relatively concentrated Br– solutions has to be taken into account as well as chemisorption of Br– as a partially discharged reactant anion.