## 1 Introduction  
Nafion membranes play a relevant role in many applied areas with interest in science and technology. In spite of the old discovery of Nafion more than 50 years ago, this family of polymers has numerous commercial applications, mainly in electrochemical processes as proton-exchange membranes [1], where the high proton conduction polymer is used as electrolyte in membrane fuel cells (PEMFCs). However, there are many drawbacks concerned with their relatively poor mechanical and thermal stability at operation temperatures that need to be overcome. Basically, Nafion consists of a backbone of polytetrafluoroethylene (PTFE) with side-chains of perfluorovinylether (PFAE) with a sulfonic group end that is constituted as a cation exchange site. This communicates to the material a dual response when it is in contact with liquid of different polarity. Thus, Nafion exhibits a high hydrophobicity due to the presence of PTFE backbone and, at the same time, a high hydrophilicity due to the pendant sulfonic groups. In order to improve the thermal stability of Nafion when working at temperatures of about 100 \({}^{\circ}\)C, the incorporation of ionic liquids (ILs) is one of the best alternatives due to their stability and low vapour pressure. However, there are many ILs in the market and the choice has to consider different factors such as cost, availability and environmental friendliness. Other considerations concern the degree of incorporation of the IL into the Nafion membrane, where the nature of the anion can also have a relevant role [2]. The incorporation of ILs in the structure of Nafion is under study [3; 4; 5]. Ionic liquids (ILs) are low temperature molten salts, that is, a salt in the liquid state. ILs are known to present a very low vapour pressure and this property makes them a key material 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 characteristics 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 CO\({}_{2}\) capture devices, biomimetics, and double layer capacitors. In fact, the scientific and technological importance of the ILs spans nowadays to a wide range of applications [6; 7; 8; 9; 10; 11; 12]. 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; 7; 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 thiswork. Moreover, due to the importance that membrane separation technology has nowadays, modification of membranes by incorporation of room temperature ionic liquids 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).