Abstract
Surface and bulk changes in a Nafion membrane as a result of IL-cation doping (1-butyl-3-methylimidazolium tetrafluoroborate or BMIM+BF4 and phenyltrimethylammonium chloride or TMPA+Cl−) were studied by X-ray photoelectron spectroscopy (XPS), contact angle, differential scanning calorimetry (DSC) and impedance spectroscopy (IS) measurements performed with dry samples after 24 h in contact with the IL-cations BMIM+ and TMPA+. IL-cations were selected due to their similar molecular weight and molar volume but different shape, which could facilitate/obstruct the cation incorporation in the Nafion membrane structure by proton/cation exchange mechanism. The surface coverage of the Nafion membrane by the IL-cations was confirmed by XPS analysis and contact angle, while the results obtained by the other two techniques (DSC and IS) seem to indicate differences in thermal and electrical behaviour depending on the doping-cation, being less resistive the Nafion/BMIM+ membrane. For that reason, determination of the ion transport number was obtained for this membrane by measuring the membrane or concentration potential with the samples in contact with HCl solutions at different concentrations. The comparison of these results with those obtained for the original Nafion membrane provides information on the effect of IL-cation BMIM+ on the transport of H+ across wet Nafion/BMIM+ doped membranes.
Highlights
Nafion membranes play a relevant role in many applied areas with interest in science and technology
The hydrophobic/hydrophilic character of the membrane surfaces was determined from contact angles measurements, and they were performed with a Teclis (Lyon, France) T2010 instrument equipped with a video system
Results show the surface coverage of Nafion112 as a result of both BMIM+ and TMPA+ doping, which reduced the hydrophobic character of both modified membranes and give rise to thermal stable membranes when compared with the original Nafion112 membrane
Summary
Nafion membranes play a relevant role in many applied areas with interest in science and technology. 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. In order to improve the thermal stability of Nafion when working at temperatures of about 100 °C, the incorporation of ionic liquids (ILs) is one of the best alternatives due to their stability and low vapour pressure. 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. 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 presented. A comparison of a characteristic electrochemical parameter (H+ transport number) obtained from membrane potentials measured with HCl-aqueous solutions through wet Nafion/BMIM+ doped and original Nafion membranes is presented
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