Existence forms and electrodeposition behavior of various zinc compounds in choline chloride − Urea deep-eutectic solvent

The influence of existing form of different zinc compounds on the electrochemical behavior of zinc was investigated in choline chloride-urea (ChCl-urea) deep eutectic solvent (DES). Raman and ESI-MS analyses indicate that complexes [ZnCl 4 ] − , [Zn(H 2 O)Cl 3 ] − and [ZnO⋅urea⋅Cl] − are presented in the ChCl-urea DES containing ZnCl 4 , ZnSO 4 ⋅7H 2 O and ZnO, respectively. The zinc compounds were found to vary reduction overpotential of Zn(II) ions in the order ZnO > ZnSO 4 ⋅7H 2 O > ZnCl 4 , while their reduction and diffusion rate in a sequence ZnSO 4 ⋅7H 2 O > ZnCl 4  > ZnO. This may be due to the formation of various Zn(II) complexes and oligomers. The chronoamperometric transient analysis shows that Zn electrodeposition in ChCl-urea DES with ZnCl 2 or ZnO as a precursor follows instantaneous nucleation mode, while ZnSO 4 ⋅7H 2 O is changed to a more progressive one. Besides, the zinc compounds can alter the rate of nucleation, influencing the morphology of zinc deposits. The ChCl-urea DES containing ZnSO 4 ⋅7H 2 O or ZnO is conducive to electrodeposition of a smooth and dense zinc coating. The XRD analysis of the deposits displays that part of the deposited zinc is able to alloy with copper substrate to form the Zn-Cu surface alloy, and the amount of the alloy depends on zinc compound. These findings promise a cheap and simple route for production of Zn coating directly form Zn secondary resources.