Electrochemical and spectroscopic study of Zn(ii) coordination and Zn electrodeposition in three ionic liquids with the trifluoromethylsulfonate anion, different imidazolium ions and their mixtures with water

In this paper we report on the use of three ionic liquids, 1-methylimidazolium trifluoromethylsulfonate ([MIm]TfO), 1-ethyl-3-methylimidazolium trifluoromethylsulfonate ([EMIm]TfO) and 1-ethyl-2,3-dimethylimidazolium trifluoromethylsulfonate ([EMMIm]TfO) containing zinc trifluoromethylsulfonate as electrolytes for zinc electrodeposition. By varying the cations from [MIm]+via [EMIm]+ to [EMMIm]+, the vibrational band in the Far-IR spectra below 200 cm−1, characterizing the cation–anion interaction, is shifted to lower wavenumbers, which suggests that the interaction between cations and anions is arranged in order of [MIm]TfO > [EMIm]TfO > [EMMIm]TfO. The coordination of Zn2+ ions in these electrolytes was investigated by Raman spectroscopy. The Raman spectra show obvious differences in terms of the solvation of Zn2+ ions in the dried electrolytes. The average number of TfO− anions bound to each Zn2+ ion is lower in [MIm]TfO than in [EMIm]TfO and in [EMMIm]TfO, respectively. In ionic liquid–water mixtures, aqueous zinc species were formed in all cases. The differences in zinc species present in the electrolytes should have an influence on their electrochemical behavior and on the morphology of the deposits. In dried ionic liquids, the cyclic voltammograms reveal that the potentials for the deposition of zinc were shifted to more negative values by varying the cations, while in ionic liquid–water mixtures, the deposition of zinc occurs at almost the same potential. The SEM and XRD results show that the surface morphology, crystal shape and size as well as crystallographic orientation of the deposits are markedly affected by varying the cations of the ionic liquids.