Study of M(hkl)| Ionic Liquid Interfaces in Well-Defined Surroundings
Authors: Jorda-Faus Pepe , Herrero Enrique , Arán-Ais Rosa 
Published: July 2022

Ionic liquids have very interesting properties for their use in electrochemistry: they have a wide potential window of stability, high electrical conductivity and can replace water in processes in which water is not an appropriate solvent. Because of that, the number of studies has increased exponentially in recent years. When used in electrocatalytic applications, the structure of the interphase alters the electrocatalytic response of the electrode. For this reason, the characterization of the interphase, and especially the nature of the interaction of the ionic liquid with the metal surface becomes an essential element in the study of these reactions. In most cases, the interaction between the ions and the metal surface is specific, that is, a chemical interaction between the ionic liquid and the surface is established. Then, the interaction will depend on the nature of the ionic liquid and the electronic properties of the surface. This implies that this interaction is structure-sensitive. Practical surfaces are composed of several types of sites, which have different geometries and different interactions with the ionic liquid so that the observed responses are difficult to analyze. The best way to simplify the problem is the use of single crystal electrodes, which have a well-defined surface atomic structure, which allows establishing correlations between surface structure and observed behavior.

One of the main disadvantages of using ionic liquids in electrochemistry is their high viscosity, which hinders the diffusion of the electroactive species towards the electrode surface and thus, lowering the reaction rates. This has triggered the study of low viscosity ionic liquids, which are usually those based on the imidazole cation ([Im+]) and/or the bis(trifluoromethyl)sulfonyl imide anion ([NTf2-]). It should be noted that the ionic liquid-electrode interphase is more complex than one in an aqueous medium. Ionic liquids have a higher electrostatic charge and interaction between the ions, which complicates the application of simple double-layer models. It is also important to highlight that, being organic compounds, they are prone to have impurities related to their synthesis processes, which can affect their physicochemical properties. In electrochemical systems in which ionic liquids are used, these liquids play the role of both solvent and electrolyte, which complicates the study of interactions, since the concentration of the ions is very high. To simplify the problem and to have a system with which to compare, ionic liquids can be dissolved in water. In this case, the concentration of the ions is reduced, facilitating their study. In addition, models are available to study and analyze their behavior. Thus, it is possible to analyze the effect of the concentration of the ionic liquid on its behavior and to extrapolate it to concentrations in which water is absent. In addition, due to the high hygroscopicity of ionic liquids, it is very difficult to completely eliminate water in practical applications. Furthermore, the presence of ionic liquids in aqueous solutions has been shown to catalyze some electrochemical reactions where small amounts of these have been shown to significantly increase the reaction rate. Thus, the aim of this work is to characterize the electrochemical behavior in water of the different ions that form the ionic liquids based in [Im]+ cation and ([NTf2]-) anion separately on platinum single crystal electrodes. To determine the role of each ion, different salts composed with at least one of the ions of interest ([Im]+ or ([NTf2]-)) will be used. The interactions of the ions and the electrode will be characterized using electrochemical and spectroelectrochemical techniques.