Reversible hydrogen evolution and oxidation mediated by molecular ion

The paper revisits the mechanism of reversible hydrogen evolution and oxidation reactions (HER and HOR) based on detailed thermodynamic analysis and survey of recent literature data. An assumption about the participation of adsorbed hydrogen atoms as intermediates in these processes, which was made about half a century ago and cannot account for a number of recent experimental observations, is critically analyzed. We propose that adsorbed molecular ion (H+2 )ad acts as intermediate in HER and HOR reactions described by the overall reversible process as follows: 2H3O+ + 2e (H+2)ad H2'' + 2H2O. New insights into the surface electrochemical processes taking place on Pt electrode at E ERHE (RHE { reference hydrogen electrode) as well as electrocatalysis of HER at E < ERHE, i.e. under overvoltage conditions, are obtained. The presented concept explains quantitatively the HER and HOR phenomena observed experimentally.