The hydrogen evolution reaction under mixed kinetic control

Abstract

A theoretical description of steady-state current–potential curves for the hydrogen evolution reaction has been given in which the rates of all forward and reverse steps have been taken into account. In addition to the limiting ranges of quasi-equilibrium and coupled reactions, the treatment includes the intermediate interval in which the Tafel relationship is not linear, and the reaction order with respect to H⁺ ions can have fractional values which vary with the potential. The derivation of volcano curves according to Parsons and Gerischer has been extended to the case of mixed control by the kinetics of the first and second steps. By combining the assumptions underlying the theory of volcano curves with the derivation of polarisation curves, it is possible to deduce the dependence of these polarisation curves on the nature of the metal. It appears that Tafel slopes are determined primarily by the potential range in which hydrogen evolution is studied, but are in addition dependent on the free energy of adsorption of hydrogen; these predictions are supported by experimental data. The theory further confirms that the customary volcano curve only indicates the catalytic activity at the standard potential, but that at more negative potential the highest rate of hydrogen evolution is found on a metal with a low positive free energy of adsorption of hydrogen.