Application of Electrochemical Impedance Spectroscopy (EIS) and X-Ray Photoelectron Spectroscopy (XPS) to the Characterization of RTILs for Electrochemical Applications

Abstract

Ionic liquids (ILs) are low temperature molten salts, that is, a salt in the liquid state. ILs used to present a very low vapour pressure and this property makes of the ILs key materials for the development of a wide variety of emerging technologies. The stability of ILs at high temperatures (several hundred degrees), low combustibility, and even the relatively high viscosity of some of them compared to conventional solvents, are characteristic of interest for some applications. Due to the large diversity of ILs components, they may present wide structural variations which can be used to design the IL with more adequate properties for a particular application. These applications might include new types of lubricants and fluids for thermal engines, electrodeposition, energy and CO2 capture devices, biomimetics, double layer capacitors,.....in fact, the scientific and technological importance of the ILs spans nowadays to a wide range of applications [1-5]. Among the energy devices, polymer-electrolyte membranes for fuel cell application are under development as a way to reduce global warming and energy cost and ILs incorporation in the structure of Nafion, a typical membrane for fuel cell use, is under study [6-8]. Since transport properties of porous and dense membranes can be modified with the addition of substances which could favour/reject the pass of some of the particles or ions in a mixture, the incorporation of a particular IL in the structure of a membrane may increase its selectivity and/or specificity. Chemical characterization and determination of electrical parameters for different ILs as well as the changes associated to water incorporation, a subject of interest for different electrochemical applications, is considered in this work. Moreover, due to the importance that membrane separation technology has nowadays, modification of membranes with different structures by incorporation of RTILs or IL-cations and their effect on mass and charge transport is also presented. Ionic liquids, membranes and membranes/IL-modified samples were chemically characterized by X-ray photoelectron spectroscopy (XPS). This technique allows the