The effect of the electrode material on the electrodeposition of zinc from deep eutectic solvents

The voltammetric behaviour of the ZnCl2 containing deep eutectic solvent choline chloride/ethylene glycol 1:2 was investigated on glassy carbon, stainless steel, Au, Pt, Cu, and Zn electrodes. While cyclic voltammetry on glassy carbon and stainless steel showed a cathodic peak for zinc electrodeposition only in the anodic reverse sweep, a cathodic peak was found also in the cathodic forward sweep on Au, Pt, Cu, and Zn. This behaviour is in agreement with the proposed mechanism of zinc deposition from an intermediate species Z, whose formation depends on the cathodic reduction potential of the solvent. The voltammetric reduction of the electrolyte involves hydrogen evolution and as a result the formation of Z and its reduction to zinc depend on the hydrogen overpotential for each electrode material. On Au, Pt, and Cu also the anodic stripping was different from that on glassy carbon and steel due to the formation of surface zinc alloys with the three former metals. The morphology of the zinc layers on Cu has been characterised by scanning electron microscopy and focussed ion beam. X-ray diffraction confirmed the presence of crystalline zinc and a Cu4Zn phase. Spectroelectrochemistry by means of polarization modulation reflection-absorption spectroscopy (PM-IRRAS) on a glassy carbon electrode in the ZnCl2 containing deep eutectic solvent showed characteristic potential dependent changes. The variation of band intensities at different applied potentials correlate with the voltammetry and suggest the formation of a compact blocking layer on the electrode surface, which inhibits the electrodeposition of zinc at sufficiently negative potentials.