Hydrogen evolution on lead cathodes.

The work reported in this thesis is an examination of the hydrogen evolution reaction on lead cathodes in aqueous acid solutions. It involved the investigation of the electrochemical behaviour of the electrode in aqueous perchloric acid which had been rigorously purified by adsorptive cleaning on activated carbon. The effects of trace quantities of platinum and surface contaminants were examined and discussed. The nature of electrode surfaces was studied under an electron microscope and by Auger spectroscopy. Measurements under the purified conditions indicated that two essentially different current density - overvoltage relationships (Tafel plots) found by previous workers were both arms of a single overall hysteresis loop. The results, together with the behaviour of the electrode on entry to the solution, impedance measurements, potential step transients and rapid sweep Tafel plots, were used to support a proposed mechanism for the hydrogen evolution reaction. The proposed mechanism involves two parallel reaction paths, via two forms of adsorbed hydrogen atoms and absorption of hydrogen atoms within the lattice structure of the electrode. The rate-determining step for the reaction is the slowest step of the fastest overall path. The results of the investigation are considered sufficient to seriously question the traditionally accepted theory of a rate-determining initial slow discharge step for the hydrogen evolution reaction on lead electrodes.