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.
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Sekhon et al. (2009) studied this question.
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