Synthesis and properties of sulfonated poly[bis(benzimidazobenzisoquinolinones)] as hydrolytically and thermooxidatively stable proton conducting ionomers

Novel sulfonated poly [bis(benzimidazobenzisoquinolinones)] as hydrolytically and thermooxidatively stable electrolyte for high -temperature fuel cell applications are reported. A series of sulfonated polymers (SPBIBI-x, x refers to molar percentage of sulfonated dianhydride monomer) were synthesized from 6,6'-disulfonic-4,4'-binaphthyl-1,1',8,8'-tetracarboxylic dianhydride (SBTDA), 4,4-binaphthyl-1,1,8,8-tetracarboxylic dianhydride (BTDA), and 3,3'-diaminobenzidine. The chemical structures of those polymers as well as model compounds synthesized from SBTDA and o-phenylenediamine were confirmed by nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR). The resulting polymers generally showed good solubility in in-cresol and DMSO. Flexible and tough membranes with high mechanical strength were prepared. They show very high thermal, thermooxidative, and hydrolytic stabilities and reduced swelling in water at enhanced temperature compared to other sulfonated aromatic polymers such as sulfonated polyimides. The proton conductivity of SPBIBI was slightly lower than that of the perfluorinated ionomer (Nafion 117) below 100 degrees C but comparable at higher temperature and 100% RH. The highest conductivity of 1.9 x 10(-1) S cm(-1) was obtained for SPBIBI-100 at 140 degrees C.