Preparation and evaluation of a proton exchange membrane based on oxidation and water stable sulfonated polyimides

A series of novel oxidation and water stable sulfonated polyimides (SPIs) were synthesized from 4,4′-binaphthyl-1,1′,8,8′-tetracarboxylic dianhydride (BTDA), and wholly aromatic diamine 2,2′-bis(3-sulfobenzoyl) benzidine (2,2′-BSBB) for proton exchange membrane fuel cells. These polyimides could be cast into flexible and tough membranes from m-cresol solutions. The copolymer membranes exhibited excellent oxidative stability and mechanical properties due to their fully aromatic structure extending through the backbone and pendant groups. Moreover, all BTDA-based SPI membranes exhibited much better water stability than those based on the conventional 1,4,5,8-naphthalenecarboxylic dianhydride. The improved water stability of BTDA-based polyimides was attributed to its unique binaphthalimide structure. The SPI membranes with ion exchange capacity (IEC) of 1.36–1.90 mequiv g−1 had proton conductivity in the range of 0.41 × 10−1 to 1.12 × 10−1 S cm−1 at 20 °C. The membrane with IEC value of 1.90 mequiv g−1 displayed reasonably higher proton conductivity than Nafion® 117 (0.9 × 10−1 S cm−1) under the same test condition and the high conductivity of 0.184 S cm−1 was obtained at 80 °C. Microscopic analyses revealed that well-dispersed hydrophilic domains contribute to better proton conducting properties. These results showed that the synthesized materials might have the potential to be applied as the proton exchange membranes for PEMFCs.