GENERALIZED ANALYTICAL EXPRESSIONS FOR TAFEL SLOPE, REACTION ORDER AND AC IMPEDANCE FOR THE HYDROGEN EVOLUTION REACTION (HER) - MECHANISM OF HER ON PLATINUM IN ALKALINE MEDIA

Following the generally accepted mechanism of the HER involving the in itial proton discharge step to form the adsorbed hydrogen intermediate , which is desorbed either chemically or electrochemically, generalize d expressions for the Tafel slope, reaction order and the a.c. impedan ce for the hydrogen evolution reaction are derived using the steady-st ate approach, taking into account the forward and backward rates of th e three constituent paths and the lateral interactions between the che misorbed intermediates. Limiting relationships for the Tafel slope and the reaction order, previously published, are deduced from these gene ral equations as special cases. These relationships, used to decipher the mechanistic aspects by examining the kinetic data for the HER on p latinum in alkaline media, showed that the experimental observations c an be consistently rationalized by the discharge-electrochemical desor ption mechanism, the rate of the discharge step being retarded on inac tive platinum compared to the same on active platinum.