ELECTROCATALYSIS OF H-2 EVOLUTION ON RUO2 PLUS IRO2 MIXED-OXIDE ELECTRODES

H-2 evolution from acid solutions has been studied on IrO2 + RuO2 electrodes over the whole oxide composition range. Kinetics has been investigated by means of cyclic voltammetry, quasistationary current-potential curves and reaction order determination. Results have shown that the Tafel slope for as-prepared electrodes is close to 40 mV and independent of oxide composition. The Tafel slope changes on electrodes aged by H-2 evolution: it varies from about 60 mV for pure RuO2 to about 20 mV for pure IrO2. The anomalously low Tafel slope for IrO2-rich electrodes, and the apparent reaction order varying from 1 to ca. 2.5 have been interpreted on the basis of a single mechanism with different rate determining steps in which the back-reaction may have an appreciable effect due to oversaturation of the solution adlayer in the presence of quasi-equilibrium of the H-2 reaction in the pores of the oxide layer.