Novel Proton Exchange Membranes Based On Polybenzimidazole For Fuel Cell Applications

Proton exchange membrane fuel cell (PEMFC for short) has attracted the interest of scientists worldwide as a kind of high efficient environmentally friendly energy conversion system. One major barrier for the commercialization of PEMFCs is the lack of cheap and reliable membranes for their high-temperature operation. H3PO4-doped polybenzimidazole (PBI) membranes have been regarded as one of the potential materials for high temperature proton exchange membranes due to their unique advantages. However, there are still some problems need to be solved for the realistic application of PBI membranes.By changing the structure, feeding ratio and polymerization method of the monomers, series of novel PBI copolymers were prepared. The corresponding polymer membranes were fabricated and the corresponding fuel cell properties were studied. The performance of the membranes were improved. Main researches are listed as follows.(1) A series of copolymers were synthesized by copolymerization of two kinds of self-poly condensation monomers:3,4-diaminobenzoic acid and 4-(3,4-diamino-phenoxy)-benzoic acid (DPBA). The copolymers exhibit better process ability than original ABPBI. Their corresponding membranes show good mechanical properties even at high H3PO4-doping level. The optimal polymer therein, PO-AB-50 (50 molar percentages of DPBA in total monomers), exhibits considerable proton conductivity (53.47mS/cm at 160?) and tensile strength (1.9MPa) after doped with phosphoric acid.(2) A kind of bulky monomer, dimethyl-4,4'-quinoxaline-2,3-diyldibenzoate(DMQB), was designed and synthesized to improve the acid retention ability of the PA-doped membranes. The bulky monomer was then used for the copolymerization with pyPBI. The copolymers show good processability and high acid doping level. Their acid retention ability was also improved to some extents. The optimal polymer therein, bpPBI-30 (30 molar percentages of DMQB), exhibits considerable proton conductivity (51.89mS/cm at 160?) and tensile strength (5.6MPa) after doped with phosphoric acid. After immersed in deionized water for 160 min, PO-AB-100 membrane shows the best acid retain ability with almost 70% acid among all the membranes.(3) In order to further improve the anti-oxidative capacity of polymer, two kinds of diamine-terminated PBI oligomers were synthesized from 3-methoxycarbonylmethylsulfanylpropionic acid methyl ester (MTMA) and dipicolinic acid (PDA), respectively. A series of block copolymers were afterwards synthesized with these two kinds of PBI oligomers and DMQB. The copolymer membranes show improved properties in many aspects. The proton conductivity of sPBI-50 reached up to 61.18mS/cm at 160?. Under enhanced Fenton test after 30 h, sPBI-50 membrane shows the best oxidation stability among all the membranes with only less than 10% weight loss.