Influence of Water on the Electrified Ionic Liquid/Solid Interface: A Direct Observation of the Transition from a Multilayered Structure to a Double-Layer Structure

Abstract

Ionic liquids are potential designer electrolytes for energy storage devices such as batteries and capacitors wherein by changing the cation and anion of the ionic liquid (IL) the solid/liquid interface can be tuned, thereby influencing the charge and mass transfer processes. In this paper, we show the influence of water on the electrified ionic liquid 1-ethyl-3-methylimidazolium trifluoromethylsulfonate ([Emim]TfO)/Au(111) interface using in situ atomic force microscopy (AFM) and spectroscopy. A clear “water in IL” to “IL in water” transition could be observed in the range of 20–30 vol % of water using vibrational spectroscopy. Above 30 vol % of water the cation–anion interaction in the ionic liquid drastically reduced, which was ascertained by both spectroscopy and interfacial studies using in situ AFM. In situ AFM results further revealed that the structure of the innermost (Stern) layer depends both on the applied electrode potential and the amount of added water. A transition from a multilayered structure to a classical double-layered structure occurred at −1.0 V on changing the water concentration from 30 to 50 vol %. Furthermore, the morphology of the electrodeposited Zn could be altered with addition of water to the electrolyte which has some potential for Zn-based batteries.