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.

Atomic Force Spectroscopy on Ionic Liquids

“…In additionally wetted ionic liquids, layering was also present, but the jumps between the innermost layer and surface were larger, suggesting that water forms an adsorption layer on the mica surface, altering the interface structure [105]. The effect of higher amounts of water on the interface structure was examined by Cui et al [114] using 1-ethyl-3-methylimidazolium trifluoromethylsulfonate ([EMIm][TfO]) in contact with Au (111). They found that the number of layers detected by the force-separation curves decreased with increasing water content.…”

“…This compression is weakened at an intermediate concentration, indicating a mixture of separated molecules and micelles, while at a high concentration, the compression was found to be the highest at 4.4 nN/Å, reflecting the dense micellar layering. The effect of higher amounts of water on the interface structure was examined by Cui et al [114] using 1-ethyl-3-methylimidazolium trifluoromethylsulfonate ([EMIm][TfO]) in contact with Au (111). They found that the number of layers detected by the force-separation curves decreased with increasing water content.…”

“…For the pure ionic liquid, five layers were seen, whereas after the addition of 50 vol% of water, only one layer was resolved under cathodic polarization. Assisted by complementary experiments using vibrational spectroscopy, Cui et al [114] concluded that a "water in IL" to "IL in water" transition takes place at 20-30 vol%. As a result, the water molecules weaken the interaction between the ions and the multilayer structure at the interface is diluted to a classical Helmholtz-like double layer.…”

The study of ionic liquid behavior at solid-liquid interfaces

“…[176][177] A recent paper by Endres et al using in situ atomic force microscopy and vibrational spectroscopy demonstrates that the structure of the innermost (Stern) layer in 1-ethyl-3methylimidazolium trifluoromethylsulfonate + water on Au(111) depend on both the applied potential and quantity of water. 178 Their work reveals a transition from a multilayer structure to a classical double layer structure at -1.0 V vs Pt Quasi-reference electrode (QRE) upon changing the water concentration from 30 to 50 vol. %.…”

“…%. 178 Water will certainly be active within the electrochemical double layer (EDL), affecting RED/OX processes, altering electrochemical windows, and possibly degrading the electrode surface. 179 Water sorption from the atmosphere has been studied in aprotic imidazolium-based ILs by gravimetric analysis, 164 with near-infrared spectroscopy, 162 by volumetric analysis from a levitated ionic liquid droplet, 174 and in tandem with electrochemistry.…”