Studies On Membrane Preparation And Performance Of Poly [2, 2'-(m-phenylene)-5, 5'-bibenzimidazole] As The High Temperature Proton Exchange Membrane Material

The proton exchange membrane fuel cell has advantages of high efficiency, no pollution, no electrolyte leakage and so on. The proton exchange membrane is one key component of the proton exchange membrane fuel cell. Recently, study of the high temperature (120-200℃) proton exchange membrane materials has obtained progress, having good application prospect. Among the various membrane materials, poly [2,2'-(m-phenylene)-5,5'-bibenzimidazole] (PBI) has attracted attention of the researchers for its excellent thermal and chemical stabilities. Based on the fact that the proton conductivity of the pure PBI is as low as about 10-7 S/cm at room temperature, PBI can be modified by doping with acid and alkali, sulfonation, and blending with other polymer electrolytes and inorganic materials. In the family of polybenzimidazoles, another polymer-poly (2,5-benzimidazole) (ABPBI) has easier chemical structure. And, ABPBI membrane can exhibit high conductivity after being doped with acids.In this study, the PBI and the ABPBI were synthesized under microwave irradiation, and the time period for PBI synthesis was shortened within 3 hours. Then the obtained PBI polymers were used to prepare the PBI membranes by solution casting method. The membranes were characterized by FT-IR, conductivity and mechanical strength. Generally, high polymerization degree of PBI polymer is good for the mechanical strength of the PBI membrane, while the solubility of the PBI decreases dramatically with the increase of polymerization degree. Taking this problem into account, the thermoreversible gel of PBI in phosphoric acid was introduced to prepare phosphoric acid doped PBI membranes directly, shortening experiment period and resolving the difficult solubility of PBI in organic solvents. By utilizing the thermoreversible gel of PBI, the phosphoric acid doped PBI membrane was directly prepared and the acid doping level was higher than 20 (phosphoric acid doping level is defined as moles number of phosphoric acid in each mole of PBI repeat unit). The PBI membranes were characterized by FT-IR, conductivity, mechanical strength, XRD speatra and so on. The tensile strength of the as-prepared PBI membrane with acid doping level of 3.0 was 13.05 MPa at room temperature, and the conductivity of the PBI membranes with acid doping level of 21.7 was 0.22 S/cm under dry atmosphere at 200℃. Furthermore, the PBI/ABPBI composite membrane was prepared by utilizing the thermoreversible gelation property of PBI in phosphoric acid. In case of no effects on the mechanical strength, the obtained composite membrane had higher conductivity than the PBI membrane with the same phosphoric acid doping level. In addition, the PBI membrane was modified by p-xylylene dichloride to process cross-link. At the low phosphoric acid doping level (<5), the cross-linked PBI membranesenhance exhibited higher mechanical strength and lower swelling ratio, comparing to the uncross-linked PBI membranes.