Insights into Electronucleation and Electrodeposition of Nickel from a Non-aqueous Solvent Based on NiCl2·6H2O Dissolved in Ethylene Glycol

This work deals with nickel electronucleation and growth processes onto a glassy carbon electrode from NiCl2·6H2O dissolved in ethylene glycol (EG) solutions with and without 250 mM NaCl as a supporting electrolyte. The physicochemical properties of EG solutions, namely, viscosity and conductivity, were determined for different Ni(II) concentrations. From cyclic voltammetry, it was found that in the absence of the supporting electrolyte, the cathodic efficiency of Ni electrodeposition is about 88%; however, in the presence of the supporting electrolyte, the cathodic efficiency was reduced to 26% due to water (added along the supporting electrolyte) reduction on the growing surfaces of Ni nuclei. This side reaction produced both H2(g) and OH- ions. Part of the former was occluded in Ni, and the latter reacted with Ni(II) ions in EG forming passivation products such as Ni(OH)2(s). Moreover, it was shown that metallic Ni did not catalyze the EG reduction in this system. From chronoamperometry, it was shown that in the absence of the supporting electrolyte, the amount of Ni electrodeposits, for the same overpotential and time, was higher than in the presence of the supporting electrolyte. The j-t plots recorded in the latter system, for different Ni(II) concentrations, were analyzed using a model which involves a contribution due to multiple 3D nucleation and diffusion-controlled growth and another related to the simultaneous reduction of water on the Ni nuclei growing surfaces. This model allows not only the quantification of the Ni nucleation kinetic parameters but also the effective deconvolution of the individual contributions to the total current; thus, from the integration of the j-t plots of these contributions, it was demonstrated that the charge amount of each process depends on the Ni(II) concentration. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, micro-Raman spectroscopy, and X-ray photoelectron spectroscopy revealed the presence of pure Ni nanoparticles electrodeposited on the electrode surface. Moreover, X-ray measurements verified the formation of a high-crystallinity face-centered cubic structure with preferred orientation growth on the ⟨111⟩ direction, which were also corroborated by the magnetic measurement performed in a physical property measurement system.