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.