Voltammetry, nanogravimetry and double-layer capacitance studies on chemisorption of Cl − and Br −, competitive with potential-dependent electrosorption of O species at Pt electrodes

Abstract

The change of double-layer capacitance ( C dl) behaviour of polycrystalline Pt electrodes upon controlled formation of thin oxide films is evaluated. The capacitance behaviour of the oxide film interface with aqueous H 2SO 4 and HClO 4 is quite different from that at the unoxidized Pt surface, while the presence of Cl − ion lowers C dl of the latter interface. This indicates that extrapolation of C dl data for the so-called ‘double-layer region’ into potential ranges where an oxide film exists, would be a quite unreliable basis for double-layer corrections to recorded transient data. The presence of Cl − or Br − ions causes appreciable lowering of C dl until onset of surface oxidation. By means of sensitive EQCN nanogravimetry measurements, the competitive effects of Cl − or Br − adsorption on oxide film formation at Pt can be quantitatively evaluated. Interestingly, adsorption of Cl − leads to net negative mass changes while Br − causes net positive ones over the oxide region. These results are interpreted in terms of specific mass changes due to diminution of O coverage concomitant with increasing halide coverage as a function of potential and Cl − or Br − concentration.