Hydrogen Evolution on Oxide Electrodes

Abstract

H2 evolution on RuO2, IrO2 and CO3O4 prepared by thermal decomposition of suitable precursors at temperatures between 300 and 500°C has been studied in alkaline solution (1 mol dm-3 NaOH) by means of quasi stationary potentiostatic curves and reaction order determination. The electrodes have been characterized by voltammetric curves before and after each set of experiments to monitor their surface state. The variation of the voltammetric charge with use, and hysteresis phenomena in recording polarization curves are explained in terms of wetting of the oxide surface promoted by hydrogen discharge. The same kinetic parameters are observed for RuO2 and IrO2 . A Tafel slope close to 40 mV and a fractional reaction order of -0.5 with respect to OH- are explained with a mechanism involving a step of reduction of the active sites, a pH dependence of the potential at the reacting plane and a pH dependent surface concentration of OH-. The mechanism is different for Co3O4, which exhibits an appreciably higher overpotential. In the case of RuO2 and IrO2 electrodes there is no evidence that oxide surfaces are reduced down to metal.