Kinetic criteria for the mechanism of the hydrogen evolution reaction

Additional criteria for distinguishing between the Volmer—Heyrovský and the Volmer—Tafel reaction mechanisms for the hydrogen evolution reaction (HER) have been derived from a theoretical treatment of polarization curves under conditions of continuous surface renewal. From data published by Tomashov and Vershinina for the HER on Pd, Ni, Fe, Sn and Pb, estimates of hydrogen coverages and rate constants have been made. Approximate values of standard exchange current densities could thus be predicted for the two reaction mechanisms; these have been compared with experimental data. In order to reconcile the various kinetic criteria with each other, it has been proposed that the Volmer—Heyrovský reaction mechanisms is operative on the majority of metals. Alternative interpretations within the Volmer—Heyrovský mechanism have been presented for Tafel slopes in the region of −30 mV dec−1 and for the relative positions of volcano curves, which might otherwise have been considered indicative of the Volmer—Tafel reaction mechanism.