Observation of two stationary states of low and high H2O2-reduction currents at a Pt electrode, arising from the occurrence of a positive feedback mechanism including solution-stirring by gas evolution

Abstract

Experiments have revealed that H₂O₂ reduction at Pt electrodes in acidic solution have two stationary states of low and high current densities (j), called LC- and HC-mechanism states, respectively. The LC-mechanism state, to which all the reported results belong, shows a potential-independent low j, determined by the rate of the dissociative adsorption of H₂O₂ (the first step of the H₂O₂ reduction), over a wide potential region. On the other hand, the HC-mechanism state shows a potential-dependent (high) j, determined by the rate of the following electrochemical reduction of resultant adsorbed OH, throughout the potential region measured. The LC-mechanism state tends to appear for rough-surface Pt and in relatively low H₂O₂ concentrations (⩽0.5 M), whereas the HC-mechanism state tends to appear for flat-surface Pt and in high H₂O₂ concentrations (0.9 M). Under intermediate conditions, both states appear, depending on the experimental parameters. It is concluded that the HC-mechanism state results from double positive feedback mechanisms, i.e., increased adsorbed OH due to the increase in surface H₂O₂ concentration by effective solution stirring through catalytic oxygen-gas evolution from the increased adsorbed OH, and increased adsorbed OH due to the increased dissociative adsorption of H₂O₂ as an autocatalytic effect of the increased adsorbed OH.