High temperature proton exchange membranes based on poly(arylene ether)s with benzimidazole side groups for fuel cells

A new benzimidazole containing monomer has been synthesized for the preparation of poly(arylene ether sulfone) (PAES) and poly(arylene ether benzimidazole) (PAEB) with benzimidazole side groups by nucleophilic substitution polymerization. PAES and PAEB had inherent viscosities of 0.56 and 0.93 dL g^-1, respectively, measured in N,N-dimethylacetamide (DMAc) at a concentration of 0.5 g dL^-1. The structures of the benzimidazole containing monomer, PAES and PAEB were characterized by FTIR, ¹H NMR, and elemental analyses. These polymers showed excellent solubility in common organic solvents, such as DMAc, dimethyl sulfoxide (DMSO), and N-methyl-pyrrolidinone (NMP) at room temperature. Due to the strong intermolecular hydrogen bonding from the amide and imidazole groups in the side chains, PAES and PAEB had unusually high T_gs at 374 and 381 °C, respectively. The 5% weight loss temperatures of PAES and PAEB were around 472 and 522 °C in air, respectively. The phosphoric acid doping levels of PAES and PAEB membranes were 5.6 and 15.3. The proton conductivity of phosphoric acid doped membranes increased with increasing temperatures and reached to a range of 10^-3 to 10^-2 S cm^-1 at 160 °C.