The polarisation resistance, exchange current density and stoichiometric number for the hydrogen evolution reaction: Theoretical aspects

A relationship between the polarisation resistance and the equilibrium reaction rate of the Volmer, Heyrovsky and Tafel steps has been developed for the hydrogen evolution reaction. Furthermore, the exchange current density for the simultaneous occurrence of both Volmer-Heyrovsky and Volmer-Tafel routes has been analysed on the basis of an extension of the Temkin development for a single reaction route. In this context, a relationship between the true exchange current density and that obtained by extrapolation of the Tafel lines has been obtained, as well as their relation to the mean stoichiometric number. The application of these expressions to some selected experimental results is also analysed. It is concluded that the determination of the dependence of the current density on overpotential should be carried out in a potential range as wide as possible, in order to evaluate the polarisation resistance at low overpotentials and to determine whether a Tafel region with a slope equal to RT/(1 - alpha)F is present at high overpotentials.