Experimental and modeling study of nickel electrodeposition including H+ and water reduction and homogeneous reactions

A comprehensive physicochemical model is presented for nickel electrodeposition onto nickel and copper rotating disk electrodes in sulfate media buffered with boric acid at various NiS O₄ concentrations. The model accounts for three contributions comprising the total current: nickel deposition, H+ reduction, and water reduction. Also considered are homogeneous reactions and transport of dissolved species to and from the electrode by diffusion and convection. The model is statistically fit to experimental linear sweep voltammetry data to obtain the various kinetic parameters. It is not possible to satisfactorily fit the curves obtained on a nickel substrate for the different NiS O₄ concentrations with a single set of parameters. Analysis of the experimental data and fitted model shows that the cathodic reactions reach completion in the following order as the potential becomes more negative during the scans: H+ reduction, nickel deposition, and water reduction. The extent of water reduction during scans does not vary with the metal ion concentration in solutions containing 50 mol m^-3 NiS O₄ or more, but increases when the NiS O₄ concentration is reduced to 10 mol m^-3. The model confirms that B (OH)3 hydrolysis mitigates the increase of the surface pH due to H+ and water reduction.