Surface sensitive nickel electrodeposition in deep eutectic solvent

Abstract

The first steps of nickel electrodeposition in a deep eutectic solvent (DES) are analysed in detail. Several substrates from glassy carbon to Pt(111) were investigated pointing out the surface sensitivity of the nucleation and growth mechanism. For that, cyclic voltammetry and chronoamperometry, in combination with scanning electron microscopy (SEM), were employed. X-ray diffraction (XRD) and atomic force microscopy (AFM) were used to more deeply analyse the Ni deposition on Pt substrates. In a 0.1 M NiCl2 + DES solution (at 70ºC), the nickel deposition on glassy carbon takes place within the potential limits of the electrode in the blank solution. Although, the electrochemical window of Pt|DES is considerably shorter than on glassy carbon|DES, it was still sufficient for the nickel deposition. On Pt electrode, the negative potential limit was enlarged while the nickel deposit growed, likely because of the lower catalytic activity of the nickel towards the reduction of the DES. At lower overpotentials, different hydrogenated Ni structures were favoured, most likely because of the DES co-reduction on the Pt substrate. Nanometric metallic nickel grains of rounded shape were obtained in any substrate, as evidenced by the FE-SEM. Passivation phenomena, related to the formation of Ni oxide and Ni hydroxylated species, were observed at high applied overpotentials. At low deposited charge, on Pt(111) the AFM measurements showed the formation of rounded nanometric particles of Ni, which rearranged and formed small triangular arrays at sufficiently low applied overpotential (Scheme 1). This particle pattern was induced by the orientation and related to surface sensitivity of the nickel deposition in DES. The present work provides deep insights into the Ni electrodeposition mechanism in the selected deep eutectic solvent.