INFLUENCE OF TEMPERATURE ON THE THERMODYNAMICS AND KINETICS OF COBALT ELECTROCHEMICAL NUCLEATION AND GROWTH

From potentiodynamic and potentiostatic experiments, the cobalt electrochemical nucleation and growth process, onto the glassy carbon electrode surfaces, from an aqueous solution containing 10⁻² M CoCl₂ and 1 M NH₄Cl (pH = 4.66) at different temperatures (15–60 °C) is reported. It was found that while the equilibrium potential moves to more negative values as the temperature of the system was increased, the contrary was observed for the nucleation overpotential, η_nucleation, required for the onset of cobalt nucleation onto the surfaces of vitreous carbon. From Tafel plots recorded at the different temperatures considered, both: the transfer coefficient, α, and the exchange current density, j₀, associated with the Co(II)/Co(0) system were assessed, and from the Arrhenius plot (ln j₀ vs. T⁻¹) the activation energy for Co(II) reduction of (27.9 ± 0.3) kJmol⁻¹ was estimated. From analysis of the potentiostatic current density transients according with the formalism proposed by Palomar-Pardavé et al. (Electrochim. Acta 50 (2005) 4736-4745), the total current density was de-convoluted into individual contributions, due to formation of multiple mass-transfer controlled 3D Co nuclei and that associated to the proton reduction occurring simultaneously on the growing surface of the Co nuclei. The latter contribution was practically null at 10 °C, however, it drastically increased as the temperature of the system did, provoking that the cathodic efficiency for Co deposition diminished. Furthermore, it was found that both the number density of active sites, N₀, and the nucleation rate, A, for Co nucleation depend exponentially with η_nucleation, regardless of temperature, except at 60 °C where N₀ diminished linearly with η_nucleation. From variations of the Co(II) ions diffusion coefficient with temperature, a value of (8.5 ± 0.2) kJmol⁻¹ was obtained for the activation energy for bulk diffusion. SEM analysis showed that the cobalt nuclei are bigger as the temperature increases; however, the coverage of the electrode surfaces becomes lower.