Poly(imide benzimidazole)s for high temperature polymer electrolyte membrane fuel cells

• A novel polymer of poly(imide benzimidazole)s (PIBIs) is synthesized.
• The PIBI is chemically stable in polyphosphoric acid at up to 200 °C.
• The PIBI is capable of acid doping to achieve a single phase solid membrane.
• High proton conductivity is achieved of 0.26 S cm−1 at 170 °C and 0% RH.
• Uses of membranes are demonstrated in H2/air fuel cells at 180 °C.

A series of poly(imide benzimidazole) random copolymers (PIBIs) was synthesized by condensation polymerization of biphenyl-4,4'-diyldi(oxo)-4,4'-bis(1,8-naphthalenedicarboxylic anhydride) (BPNDA), 2-(4-aminophenyl)-5-aminobenzimidazole (APABI) and 4,4'diaminodiphenyl ether (ODA) in m-cresol in the presence of benzoic acid and isoquinoline at 180 °C for 20 h. The resulting PIBIs showed excellent thermo-oxidative as well as radical-oxidative resistance and, depending on the composition of the random copolymers, the PIBI membranes could be readily doped in polyphosphoric acid (PPA) or in 85 wt% orthophosphoric acid under pressure at 180 °C to give acid uptakes as high as 780 wt% and anhydrous proton conductivity of up to 0.26 S cm−1 at elevated temperatures. The PIBI membrane with a 1:1 molar ratio of APABI:ODA (PIBI-1/1) and with an acid uptake of 300 wt% showed an elastic modulus of 0.1 GPa at 160 °C, which is an order of magnitude higher than that of the common polybenzimidazole membranes with similar acid contents. A preliminary H2/air fuel cell test at 180 °C showed a peak power density of 350 mW cm−2 of the fuel cell equipped with the phosphoric acid doped PIBI-1/1 membrane with a 300 wt% acid uptake, demonstrating the technical feasibility of the novel electrolyte materials.