Proton exchange membranes (PEMs) are a key component of a proton exchange membrane fuel cell. Sulfonated polyimides (SPIs) were doped by protic ionic liquid (PIL) to prepare composite PEMs with substantially improved conductivity. SPIs were synthesized from diamine, 2,2-bis[4-(4-amino-phenoxy)phenyl]propane (BAPP), sulfonated diamine, 4,4'-diamino diphenyl ether-2,2'-disulfonic acid (ODADS) and aromatic anhydride. BAPP improved the mechanical and thermal properties of SPIs, while ODADS enhanced conductivity. A PIL, 1-vinylimidazolium trifluoromethane-sulfonate ([VIm][OTf]), was utilized. [VIm][OTf] offered better conductivity, which can be attributed to its vinyl chemical structure attached to an imidazolium ring that contributed to ionomer-PIL interactions. We prepared sulfonated polyimide/ionic liquid (SPI/IL) composite PEMs using 50 wt% [VIm][OTf] with a conductivity of 7.17 mS/cm at 100 °C, and in an anhydrous condition, 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride (DSDA) was used in the synthesis of SPIs, leading to several hundred-times improvement in conductivity compared to pristine SPIs.

A novel sulfonated diamine was synthesized from 1,4-bis(4-aminophenoxy) benzene [pBAB]. Sulfonated polyimides (SPIs) were synthesized from sulfonated pBAB, 1,4-bis(4-aminophenoxy-2-sulfonic acid) benzenesulfonic acid [pBABTS], various diamines and aromatic dianhydrides. Composite proton exchange membranes (PEMs) made of novel SPIs and a protic ionic liquid (PIL) 1-vinyl-3-H-imidazolium trifluoromethanesulfonate [VIm][OTf] showed substantially increased conductivity. We prepared an SPI/PIL composite PEM using pBABTS, 4,4′-(9-fluorenylidene) dianiline (9FDA) as diamine, 3,3′,4,4′-diphenylsulfone tetracarboxylic dianhydride (DSDA) as dianhydride and 40 wt % [VIm][OTf] with a high conductivity of 16 mS/cm at 120 °C and anhydrous condition. pBABTS offered better conductivity, since the chemical structure had more sulfonated groups that provide increased conductivity. The new composite membrane could be a promising anhydrous or low-humidity PEM for intermediate or high-temperature fuel cells. OPEN ACCESSPolymers 2015, 7 1047

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