A Study on Proton Conductivity of Composite Membranes with Various Ionic Liquids for High-Temperature Anhydrous Fuel Cells

In this study composite membranes based on ionic liquids (ILs) containing 1-ethyl-3-methylimidazolium (EtMeIm(+)), 1-butyl-3-methylimidazolium (BuMeIm(+)) cations, and tetrafluoroborate (BF4-), triflate (CF3SO3-, Tf-) anions and sulfonated polymers such as sulfonated random copolymers, sulfonated poly(aryl ether ketone) (SPAEK, SPAEK-6F), and recasted Nafion have been prepared and characterized for high-temperature anhydrous polymer electrolyte fuel cells. The ionic conductivity of composite membranes and activation energy for proton conduction has been used to investigate the behavior of ionic conduction in composite membranes at elevated temperatures along with the pairing relation between the polymer matrix and IL. The ionic conductivity of composite membranes increases with ascending temperature and also depends upon the intrinsic conductivity of the ILs. Composite membranes consisting of SPAEK-6F and EtMeImBF(4) show the highest ionic conductivity, approximately 0.023 S cm(-1) at 180 degrees C. The trend of ionic conductivity of the composite membranes is consistent with that of the ILs. Composite membranes with a higher amount of IL and more degree of sulfonation of polymer matrix showed higher ionic conductivity. It is proposed that the IL in the composite membranes at elevated temperatures behaves as water does in the ionomeric membranes at temperatures below 100 degrees C. The pairing relation between the polymer and ionic liquid arose from the fact that the ionic conductivity of composite membranes depends upon the polymer and ionic liquid used. It has been observed that in addition to the cations and anions of conjugate ionic liquids, the conjugate polymer matrix also contributes to ionic conduction in composite membranes. Moreover, the conjugate polymer matrix also plays an important role in the immobilization of ILs in the composite membranes.