Anion Effects on the Solid/Ionic Liquid Interface and the Electrodeposition of Zinc

Abstract

The effect of anions on the solid/ionic liquid (IL) interface and the electrodeposition of zinc have been investigated. The employed ILs are composed of 1-ethyl-3-methylimidazolium ([EMIm]+), bis(trifluoromethylsulfonyl)imide (TFSI–), trifluoromethylsulfonate (TfO–), methylsulfonate (OMs–) and acetate (OAc–). These anions show an increasing cation–anion interaction strength in the order TFSI– < TfO– < OMs– < OAc–, as probed by far-infrared spectroscopy below 200 cm–1. It was shown by in situ AFM that the anion has a profound impact on the interfacial properties. Multilayered structures were observed at the electrode/IL interface for [EMIm]TFSI and [EMIm]TfO, respectively, while only a few layers with rather a low push-through force were found at the interface for [EMIm]OMs and [EMIm]OAc, respectively. The coordination of Zn(II) ions in these ILs by varying zinc salts was investigated by Raman spectroscopy. The differences in metal species and interfacial layers have a strong influence on the electrochemical process and on the quality of the deposits. Dense zinc deposits with nanowire-like and hexagonal plate-like structures were obtained from ILs with TFSI– and TfO– anions, respectively. Thin layers of zinc with porous and spongy structures were obtained in ILs with OMs– anion containing 0.2 mol/L zinc salts, while homogeneous and smooth deposits with a fine-grained structure were obtained with 1 mol/L zinc salts. However, no deposits were found in Zn(OAc)2/[EMIm]OAc under the same conditions. These results indicated that the anions of ILs strongly affected the solid/IL interface, the speciation of Zn(II) ions in ILs, and the morphology of zinc deposits.