Broadband Electric Spectroscopy of triethylammonium-methanesulfonate and triethylammonium-perfluorobutanesulfonate Ionic Liquids

In recent years Proton Exchange Membrane Fuel Cells (PEMFCs) have drawn considerable interest from both the scientific community and the industry due to: a) a high energy conversion efficiency; b) a low environmental impact; and c) the possibility of applications ranging from portable electronics to the automotive sector. The heart of a PEMFC is its proton exchange membrane (PEM). The most widely used PEMs consist of perfluorinated polymer electrolytes such as Nafion. These systems have drawbacks such as their low proton conductivity at temperatures higher than 90°C and at low relative humidity which severely limits their large-scale commerciol use. In order to overcome these limitations, PEMs composed of perfluorinated ionomers doped with proton-conducting ionic liquids (PCILs) were recently proposed due to their high conductivity under anhydrous conditions, large electrochemical stability window and good thermal stability. Here, we report the investigation of triethylammonium-methanesulfonate (MST) and triethylammonium-perfluorobutanesulfonate (PFBuT) PCILs by techniques such as Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC) and Broadband Electric Spectroscopy (BES). These measurements allow us to investigate the relationship between the thermal behaviour and the proton conduction mechanism of this type of ionic liquid. The TGA/DSC results indicated that: a)ILs are thermally stable up to 200°C; b) MST shows a glass transition at -17°C and two endothermic transitions at 20 and 38°C; and c) PFBuT shows a glass transition at -20°C and two endothermic transitions at 17 and 61°C. The BES results reveal that at 120°C, MST and PFBuT showed conductivity values of 1.4x10-2 and 7.8x10-3 S·cm-1, respectively. These results make the ILs promising materials to be used in the preparation of membrane for PEMFCs operating above 100°C.