Functionalized 4-phenyl phthalazinone-based polybenzimidazoles for high-temperature PEMFC

A series of novel polybenzimidazoles containing 4-phenyl phthalazinone moieties and functional hydroxyl groups (PPBIOH) were synthesized from 4-(4-(4-(4-carboxyphenoxy)phenyl)-1-oxophthalazin-2(1H)-yl)benzoic acid (CPPBC), 2,5-dyhydroxyterephthalic acid (TPA-OH) and 3,3′-diaminobenzidine (DAB) in polyphosphoric acid (PPA). The resultant polymers showed good solubility in 98% H2SO4 and NMP. The inherent viscosities of the polymers in 98% H2SO4 at 25 °C were in the range of 0.33–2.10 dL/g, increasing obviously with decreasing hydroxyl content in the main chain. The corresponding acid doped membranes were prepared directly from the polymerization solution by the PPA process. The chemical structures of PPBIOH polymers and membranes were characterized by FT-IR. These membranes showed reasonable doping levels (10.1–12.2 mol H3PO4), high proton conductivities (0.10–0.21 S cm−1 above 120 °C), good mechanical properties (stress of 1.7–2.9 MPa and strain of 28.1–68.0%) and high oxidative stability (breaking time in the range of 52–155 h). The structures of PPBIOH membranes were analyzed by the Wide-angle X-ray diffraction (WAXD) and polarized optical microscopy (POM). The WAXD and POM results indicate the acid doped PPBIOH membranes exhibit semi-crystalline character, from which the mechanism of proton conduction can be further investigated. The combination performance makes these PPBIOH membranes promising candidates for potential applications in high-temperature proton exchange membrane fuel cells (PEMFCs).