Acid doped polybenzimidazoles containing 4-phenyl phthalazinone moieties for high-temperature PEMFC

A novel series of polybenzimidazoles containing 4-phenyl phthalazinone moieties and p-phenylene linkages (p-PPBI) were synthesized from 4-(4-(4-(4-carboxyphenoxy)phenyl)-1-oxophthalazin-2(1H)-yl)benzoic acid (CPPBC), terephthalic acid (TPA) and 3,3′-diaminobenzidine (DAB) in polyphosphoric acid (PPA). The chemical structures of p-PPBI polymers were characterized by FT–IR, 1H NMR, 13C NMR and WAXD. The resultant polymers have inherent viscosities from 1.78 to 3.43 dL/g in 96% sulfuric acid at 30 °C and show good solubility in aprotic polar solvents, such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc) and dimethyl sulfoxide (DMSO). These p-PPBI polymers exhibit high glass transition temperatures (Tg) in the range of 399–426 °C and excellent thermal stability with the 5% (Td5%) and 10% (Td10%) weight loss temperatures in the range of 516 to 594 °C and 560 to 672 °C, respectively. These amorphous polymers were cast into membranes and further investigated as proton exchange membranes. Membranes with high acid doping level (∼15.2 mol H3PO4) and high proton conductivity (∼0.13 S/cm) were obtained after doping with phosphoric acid. These obtained membranes present good oxidative stability and mechanical property even with high doping levels. The p-PPBI polymers are promising candidates for high-temperature proton exchange membranes.