Synthesis, proton conductivity and chemical stability of novel sulfonated copolyimides-containing benzimidazole groups for fuel cell applications

A new diamine monomer, 2,2-bis(4-aminophenyl)-5,5-bibenzimidazole (BAPBI), has been simply synthesized via a one-step reaction procedure of 4-aminobenzoic acid and 3,3-diaminobenzidine in polyphosphoric acid (PPA) at 190 degrees C. A series of sulfonated copolyimides (SPIs)-containing benzimidazole groups have been prepared by random copolymerization of 1,4,5,8-naphthalenetetracarboxylic dianhydride, BAPBI, 4,4-bis(4-aminophenoxy)biphenyl-3,3-disulfonic acid and nonsulfonated diamines in m-cresol at 180 degrees C. Transparent and tough SPI membranes are obtained by solution cast method. The SPIs show high decomposition temperatures (approximate to 300 degrees C) of sulfonic acid groups, indicating good thermal stability. Covalent cross-linking was successfully achieved by treating the SPI membranes in PPA at 170 degrees C for 10h. The covalently cross-linked SPI membranes show much improved radical oxidative stability due to the synergic action of the covalent cross-linking and the incorporation of benzimidazole groups into the polymer structure. The covalent cross-linking is also favorable for improving the hydrolytic stability of the SPI membranes. However, the proton conductivity decreased to some extent because the sulfonic acid groups had been partially consumed during the process of covalent cross-linking.