Behaviour of the adsorbed Cl˙ intermediate in anodic Cl2 evolution at thin-film RuO2 surfaces

Abstract

Studies of the adsorption of the Cl˙ intermediate in anodic Cl₂ evolution at a series of four thin-film RuO₂ 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 RuO₂ electrodes show only a small effect, indicating that neither diffusion-controlled supply of Cl^– nor effects of Cl₂ 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 RuO₂ materials, kinetic control of Cl₂ evolution seems to be by the Cl˙ 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_ϕ, for adsorbed Cl˙ to be determined. C_ϕ 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 RuO₂ 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.