Electrodeposition of Single γ-phase Zn–Ni Alloy from Deep Eutectic Solvents using Metal Oxides as Precursors

The electrodeposition behavior of Zn–Ni alloy on a Cu electrode has been studied in the choline chloride-urea-ethylene glycol (ChCl-urea-EG) based deep eutectic solvent (DES). The nickel and zinc sources are introduced into the solvent by the dissolution of nickel oxide (NiO) and zinc oxide (ZnO), respectively. Cyclic voltammetry indicates that the electroreduction of Ni(II) or Zn(II) species in the DES is a diffusion-controlled process and the diffusion coefficient of Zn(II) species is an order of magnitude larger than that of Ni(II) species. Under the examined condition, the anomalous codeposition of Zn–Ni alloy is observed, which is different from normal codeposition of Zn–Ni alloy in similar DESs using metal chlorides as precursor. The deposition potential is found to play an important role in controlling the composition and surface morphology of the resultant Ni-Zn alloys. The phase structure of the deposits depends on the Ni content, and changes from a mixture of η-phase and γ -phase to single γ -phase with (411) plane orientation. A mechanism for this electrodeposition process has been proposed on the basis of the formation of complex ions [NiO·urea·Cl] − , [ZnO∙urea·Cl] − and [ChCl·O·urea] 2− in ChCl-urea-EG-ZnO-NiO electrolyte during the electrodeposition, emphasizing that the metal oxides were dissolve in DES by forming metal complex ions, the cathodic reduction of metal complex ions generates complex ions [ChCl·O·urea] 2− with alloys (Ni x Zn y ), and [ChCl·O·urea] 2− is oxidized to release O 2 at anode.