Hysteresis effects in the hydrogen evolution reaction at lead in aqueous perchloric acid

In hydrogen overpotential measurements on electropolished Pb electrodes in charcoal-cleaned aqueous HClO4, new hysteresis effects, not due to anion adsorption processes, have been observed. Overpotentials changed both reversibly and irreversibly, a substantial increase of electrode activity for hydronium ion discharge occurring in the course of time. Relations developed between overpotentials and the total amount of cathodic charging of the electrode lead to an explanation of the observed effects in terms of continuous accumulation of H atoms at the interface and in the bulk metal. Observed changes in the Tafel slope are consistent with enhancement of the discharge rate as a result of specific adsorption of hydronium ions on H atom-occupied sites on the electrode.