Gold electrocrystallization on carbon and highly oriented pyrolytic graphite from concentrated solutions of LiCl

The early stages of the electrolytic deposition of gold on carbon electrodes from concentrated LiCl electrolytes have been investigated by voltammetry, chronoamperometry, and microscopy. The analysis of current-time transients according to existing theories indicates that this process occurs by multiple three-dimensional nucleation, followed by diffusion-controlled growth of nuclei. Diffusion coefficients calculated on the basis of nucleation theory were found to be higher than those from rotating disk experiments (RDE), suggesting that gold species are adsorbed on the electrode surface. Gold nucleation from 6 M LiCl cannot be classified as either instantaneous or progressive nucleation. The kinetic parameters for nucleation, aN(0) (nucleation rate) and N-0 (number density of active sites on the substrate surface), were estimated by using a general Poisson nucleation law. Both quantities were found to vary with potential and with concentration of gold. The potential dependence of the nucleation rate, aN(0), was interpreted according to classical and atomistic theory For low gold concentrations, where adsorption of AuCl or AuCl3 might occur prior to nucleation, the number of atoms in the critical nucleus (N-c) was less than unity over the entire potential range analyzed. For high gold concentrations the number of atoms forming the critical nucleus depends on overpotential. In all solutions studied nucleation takes place on a limited number of active sites.