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.