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

Abstract 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(1 H )-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, 1 H NMR, 13 C 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 ( T g ) in the range of 399–426 °C and excellent thermal stability with the 5% ( T d5% ) and 10% ( T d10% ) 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 H 3 PO 4 ) 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.