Synthesis and properties of poly(aryl sulfone benzimidazole) and its copolymers for high temperature membrane electrolytes for fuel cells

Abstract

Poly(aryl sulfone benzimidazole) (SO₂PBI) and its copolymers with poly[2,2′-p-(phenylene)-5,5′-bibenzimidazole] (pPBI), termed as Co-SO₂PBI, were synthesized with varied feeding ratios of 4,4′-sulfonyldibenzoic acid (SDBA) to terephthalic acid (TPA). Incorporation of the stiff para-phenylene and flexible aryl sulfone linkages in the macromolecular structures resulted in high molecular weight copolymers with good solubility. The chemical stability towards radical oxidation was improved for SO₂PBI and its copolymer membranes due to the electron-withdrawing sulfone functional groups. Upon acid doping, the membrane swelling was reduced and the mechanical strength was improved, as compared with their meta structured analogues. At an acid doping level of 11 mol H₃PO₄ per average molar repeat unit, the Co-20%SO₂PBI membrane exhibited a tensile strength of 16 MPa at room temperature and an H₂-air fuel cell peak power density of 346 mW cm⁻² at 180 °C at ambient pressure. Durability tests with the membrane under a constant current density of 300 mA cm⁻² at 160 °C showed a degradation rate of 6.4 μV h⁻¹ during a period of 2400 h, which was significantly lower than that for meta PBI membranes with a similar acid doping level.