Based on cyclic voltammetry (CV) and chronoamperometry (CA) experiments, the effects of deposition potential and temperature on Co(II) reduction and electrocrystallization in ethylene glycol (EG) are reported. CV observations show that Co(II) reduction reaction is irreversible, and both the equilibrium potential and nucleation overpotential, ηnucleation, move to more positive values as the temperature is improved. The diffusion coefficient of Co(II) is 2.24 × 10-6 cm2 s-1 at 323 K and the activation energy for bulk diffusion (Ebd) of Co(II) is 22.03 ± 0.8 kJ mol-1. CA observations show that the Co electrocrystallization mechanism is a 3D nucleation and growth of a new phase under diffusion control. Its mechanism is an instantaneous regime at 323 K at various applied potentials, however, the mechanism changes from a progressive nucleation to an instantaneous nucleation at -1.10 V as the temperature increases. The cobalt nuclei are spherical microstructure, and these innumerable spherical crystal particles can align compactly to form many wirelike crystals bundles. XRD observation confirms the characteristic peak of crystalline Co with a preferred orientation direction and the average size of Co grains is 19 nm. It is also found that the Co coating thickness is affect by the potential and temperature.

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Nanocrystalline Co–Ni catalysts with a high electrochemical active surface area (ECSA) for HER are prepared from ethylene glycol system.

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