Electrochemical nucleation of cobalt onto glassy carbon electrode from ammonium chloride solutions

In the usual aqueous electrolytic baths, cobalt electrodeposition is accompanied by the hydrogen evolution reaction (HER), and therefore, in such circumstances, a quantitative evaluation of the kinetic parameters of the cobalt electrocrystallization process, is almost impossible. Here, we show a possibility to separate these two processes. The existence of a potential range where the cobalt deposition on glassy carbon electrode (GCE) occurs independently of HER, from an aqueous solution of 10^-2 M CoCl₂, in 1 M NH₄Cl (pH = 4.66), was found, and the nucleation kinetic parameters, using the potentiostatic method and existing theoretical formalism, were determined. It was concluded that the mechanism of the cobalt deposition in 1 M NH₄Cl occurs by multiple, progressive three-dimensional nucleation with growth controlled by mass transfer. Regarding the potential dependence of the steady-state nucleation rate, and atomistic theory of the electrolytic phase formation, it was shown that an active site plays the major role in critical nucleus formation over the entire potential range.