Transient and steady-state model of cobalt deposition in borate-sulfate solutions

A model for cobalt deposition on a cobalt rotating disk in borate-sulfate solutions is fit to experimental impedance spectroscopy (EIS) data obtained at different CoSO₄ concentrations and electrode potentials and steady state polarization curves obtained in the same solutions. The model also accounts for the evolution of hydrogen (HER), adsorption-desorption of boric acid and the transport of dissolved species to and from the electrode. The kinetic parameters are estimated by minimization of the sum-of-squares error between the measurements and model predictions using a genetic algorithm. The model could not be satisfactorily fit to all of the EIS spectra in this study with a unique set of kinetic parameters. A single set of parameters could fit the spectra well over the range of potentials at a given CoSO₄ concentration, but the parameters obtained vary with the CoSO₄ concentration. Conversely, the steady-state current-potential curves obtained at the same CoSO₄ concentrations are very well fit with a single set of parameters. Model analysis indicates that the Co(II) reduction rate is controlled by the first step involving the formation of the adsorbed intermediate Co(I)_ads. The situation is less clear for the HER where both the first and second steps appear to influence its rate.