Small-Angle X-ray Scattering Study of Water Free Fuel Cell Membranes Containing Ionic Liquids

The effect of polymeric membranes containing ionic liquids in replacement of water on the morphology and the formation of ionic aggregates has been studied using mainly small-angle X-ray scattering (SAXS) techniques. The polymeric membranes were based on recast Nafion and sulfonated poly(aryl ether ketone) (SPAEK-6F) containing ionic liquids, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF 4 ) and 1-ethyl-3-methylimidazolium hexafluorophosphate (EMIPF 6 ). The small-angle scattering maximum (ionomer peak) has been observed in the recast Nafion membranes containing the ionic liquids (Nafion/EMIBF 4, Nafion/EMIPF 6 ), whereas it was absent in the membranes based on SPAEK-6F (SPAEK-6F/EMIBF 4, SPAEK-6F/EMIPF 6 ). This has been explained to be due to the different level of ion-clustering ability of the hydrophilic parts (i.e., sulfonic acid groups) in the Nafion and SPAEK-6F polymers. It is also confirmed in that the mean Bragg distance between the ionic clusters, which is reflected in the position of the ionomer peak, is larger in the membranes containing EMIBF 4 than the EMIPF 6 . In addition, the ionic conductivity of the membranes follows the same trend and is higher for the membranes containing EMIBF 4 . However, the matrix peak is present at low scattering angles in all the membranes based on recast Nafion and SPAEK-6F due to the crystalline content of the membranes. The membranes containing ionic liquids have ionic conductivity ∼0.01 S cm −1 above 100 °C under ambient humidity, good mechanical strength, and thermal stability up to temperatures above 300 °C and are hence suitable for potential use in polymer electrolyte fuel cells (PEFCs) at medium temperatures (100−200 °C) in the absence of external humidification.