Despite its major importance in electrocatalysis and mechanistic aspects of electrode processes, the experimental determination of the adsorption behaviour of kinetically involved intermediates, e.g. H in H₂ evolution, in electrode reactions proceeding at appreciable Faradaic currents has hitherto remained little developed. A new method of analysis of the kinetics of the decay of overpotential on open-circuit, following interruption of a polarizing current, has been applied to the study of the adsorption behaviour of the ‘overpotential-deposited’(o.p.d.) H generated as an intermediate in the cathodic evolution of H₂ at appreciable currents. Results are described for Ni and electrodeposited Ni–Mo–Cd composite cathode materials.

The method enables, for the first time, the H coverage to be reliably determined as a function of potential, at appreciable overpotentials, and the corresponding adsorption pseudocapacitance behaviour to be directly and accurately evaluated for the o.p.d. species in a continuous Faradaic reaction. The results show that H coverage at the surfaces of these metals attains limiting values at overpotentials of –0.15 to –0.25 V, so that the Faradaic process of H₂ evolution proceeds, at these metals, on a surface incompletely but appreciably covered by H atoms. By application of a steady-state analysis to a discharge and electrochemical-desorption mechanism this behaviour is explained theoretically and the ratios of rate constants required to account for the observed H-adsorption behaviour are evaluated. The Tafel slopes for the H₂-evolution reaction at the metals studied are discussed in terms of the observed H-adsorption behaviour and steps in the reaction mechanism.