SURFACE ELECTROCHEMISTRY AND KINETICS OF ANODIC BROMINE FORMATION AT PLATINUM

Studies are reported on the kinetics of anodic formation of Br-2 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 Cl-2 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 Br-2 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 Br-2 formation experiments. The kinetics are shown to be dominated by the step of Br-. recombination under all conditions, as similarly found for the Cl-2 evolution reaction. However, the role of Br-3(-) 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.