Lewis acidity dependency of the electrochemical window of zinc chloride-1-ethyl-3-methylimidazolium chloride ionic liquids

Negative ion fast atom bombardment mass spectra (FAB-MS) recorded for ZnCl2-1-ethyl-3-methylimidazolium chloride (ZnCl2-EMIC) ionic liquids with various compositions indicate that various Lewis acidic chlorozincate clusters (ZnCl3-, Zn2Cl5- and Zn3Cl7-) are present in ZnCl2-EMIC ionic liquids depending on the percentage of ZnCl2 used in preparing the ionic liquids; higher ZnCl2 percentage favors the larger clusters. Cyclic voltammetry reveals that the potential limits for a basic 1:3 ZnCl2-EMIC melt correspond to the cathodic reduction of EMI+ and anodic oxidation of Cl-, giving an electrochemical window of approximately 3.0 V which is the same as that observed for basic AlCl3-EMIC ionic liquids. For acidic ionic liquids that have a ZnCl2/EMIC molar ratio higher than 0.5:1, the negative potential limit is due to the deposition of metallic zinc, and the positive potential limit is due to the oxidation of the chlorozincate complexes. All the acidic ionic liquids exhibit an electrochemical window of approximately 2 V, although the potential limits shifted in the positive direction with increasing ZnCl2 mole ratio. Underpotential deposition of zinc was observed on Pt and Ni electrodes in the acidic ionic liquids. At proper temperatures and potentials, crystalline zinc electrodeposits were obtained from the acidic ionic liquids. (C) 2002 Elsevier Science Ltd. All rights reserved.