First stages of Ni deposition onto vitreous carbon from sulfate solutions

Electrochemical and morphological aspects of the first stages of Ni deposition were studied in neutral sulfate solutions by means of voltammetry, potentiostatic steps and AFM techniques. Both of the two deposition mechanisms identified in this medium were analysed. At low overpotentials, the nucleation of crystallites follows a mechanism involving the adsorption of NiOH+, which is suppressed at low pH values and in the presence of complexing anions. Due to the local alkalisation by hydrogen evolution, the initial kinetic-controlled growth on H covered Ni-crystallites is inhibited by a preferential adsorption of Ni(OH)2. A model based on an inhibited three-dimensional growth was proposed. At more cathodic potential, nucleation is controlled by the adsorption of non-complexed Ni(I) and crystal growth occurs in the presence of a porous hydroxide layer limiting the cation transport.