Synthesis And Characterization Of Poly(2,2’-m(4-hydroxyl Pyridine)-5,5’-bibenzimidazole) For Novel Proton Exchange Membranes

Proton exchange membrane fuel cell (PEMFC) is a new kind of primary cell. It has broad application prospects in many fields such as transportation. Proton exchange membrane (PEM) is the one of the core components to the fuel cell. The properties of the PEM directly determine the performance of the fuel cell. For instance, the low operational temperature of the PEM will limit the improvement on the performance of the fuel cell. Therefore, it has great significance to develop the high temperature proton exchange membrane (HT-PEM) that can work above100℃. The phosphoric acid doped polybenzimidazole membrane seems so far the most successful system in this field. It has excellent thermal stability, good chemical stability, and fine mechanical properties.This paper proposed a way to synthesize poly(2,2’-(m-4-hydroxyl pyridine)-5,5’-bibenzimidazole)(OH-PPBI) and its copolymer, i.e. OH-PPBI/PBI, with poly(2,2’-(m-phenylene)-5,5’-bibenzimidazole)(PBI) in PPA at200℃for3h using microwave as an assistant. The percentage of hydroxypyridine could be controlled via changing the molar ratios of dicarboxylic acids. The introduced pyridine in PBI backbones is expected to increase the acid doping level (ADL) and thus high proton conductivity of the membranes. The solubility could be enhanced by preparation of the copolymer. Meanwhile, the introduced hydroxyl can react with other function groups for further modification.The obtained polymer exhibited excellent thermal stability until500℃in nitrogen atmosphere according to the data of thermogravimetric analysis. The chemical stability was good according to the Fenton test. The solubility of the copolymer OH-PPBI/xPBI is higher than that of the OH-PPBI in organic solvents such as N,N-dimethylacetamide (DMAc), dimethylsulfoxide (DMSO) and N-methyl-2-pyrrolidone (NMP). For the OH-PPBI/PBI membrane with an acid doping level of4.3, the tensile stress at break was55.3MPa, the maximum elongation was107%, respectively, at room temperature. A proton conductivity of this membrane was0.099S/cm at150℃without humidification.