Modification of Nafion membranes by impregnation with ionic liquids

Nafion 117 membranes were impregnated with various imidazolium (1-hexyl-3-methyl-imidazolium/HMI, 1-butyl-3-methyl-imidazolium/BMI) and pyrrolidinium (1-butyl-1-methyl-pyrrolidinium/BMPyr) based ionic liquids bearing hydrophobic (tris(pentafluoroethyl)trifluorophosphate/FAP, bis(trifluoromethylsulfonyl)imide/BTSI, hexafluorophosphate/PF6) and more hydrophilic (tetrafluoroborate/BF4) anions. The modified membranes were characterized in terms of uptake behavior, washing out of the ionic liquids by water, swelling in humidified environment, thermal stability, mechanical properties, ion exchange capacity and ion conductivity. Upon this treatment, the ionic liquids' cations partially replace the protons of the sulfonic acid groups in Nafion. While the ionic liquids act as potent plasticizers in the polymer matrix, thermal stability of these systems remains unchanged and swelling by water of the dried ionomer membranes is reduced. Ion conductivities at 120 degrees C under dry conditions have been found to be up to 100 times higher than for dry Nafion 117. In particular, modification of Nafion by ionic liquids bearing the bulky hydrophobic FAP anion seems promising.