BEHAVIOR OF THE ABSORBED CL INTERMEDIATE IN ANODIC CL-2 EVOLUTION AT THIN-FILM RUO2 SURFACES

Studies of the adsorption of the CI' intermediate in anodic CI, evolution at a series of four thin-film RuO, electrodes formed on a Ti substrate have been made by means of potential-relaxation measurements, following interruption of previous steady-state currents, coupled with determination of the current vs. overpotential kinetic relationships. Experiments at rotated RuO, electrodes show only a small effect, indicating that neither diffusion-controlled supply of CI- nor effects of CI, supersaturation in the boundary region at the electrode materially effect the kinetic behaviour. This behaviour is manifested as continuously curved Tafel relations, which approach limiting currents at overvoltages of 0.2-0.3 V. Therefore, on these thin-film RuO, materials, kinetic control of CI, evolution seems to be by the CI' recombination pathway; this is supported by linearity of Conway-Novak test plots for the recombination mechanism. Analysis of the potential-relaxation transients enables the pseudocapacitance, C,, for adsorbed CI to be determined. C, shows ascent to large values below 50-100 mV of overpotential, depending on temperature. Some problems arise, however, in reconciling this adsorption behaviour with expectations associated with recombination control. The near-surface composition profiles of the RuO, films have been determined by means of Rutherford back- scattering and the average compositions, deeper into the films, by energy-dispersive X-ray emission analyses.