Novel Proton Exchange Membranes Based On Polybenzimidazole For Fuel Cell Applications

Polybenzimidazole has become one of the most promising materials for high temperature proton exchange membranes due to its unique advantages,which has received more and more attention.However,some of the shortcomings of commercialized polybenzimidazole limited its further development in the fuel cell application,which included the poor solubility resulting in a difficulty for forming membrane,chemical oxidation stability,and acid-retention ability,and so on.In this paper,the research work would be carried out to solve these problems by changing the structure of polymerized monomer and introducing the method of block copolymerization.Three serial copolymers were prepared by microwave polycondensation technology.The corresponding membranes have been prepared and some properties for fuel cell have been studied.The main results are as follows.(1)Preparation and performance of non-planar PBI copolymers and membrane materials containing bipyridine units have been studied.The BPY-PBI-x serial copolymers were prepared by copolymerization of three kinds of monomers including BPY,2,6-pyridinedicarboxylic acid,and 3,3',4,4'-Tetraaminobiphenyl under microwave-assisted conditions.By introducing bipyridine units with large size into the PBI backbone,the poor-process problem and low conductivity of PBI have been improved.The anti-oxidative stability of BPY-PBI-x serial membranes increased because the pyridine unit can quench radicals by the formation of pyridine-N-oxide.In the aspect of acid doping,the series of films have a high acid doping level,which makes the proton conductivity of BPY-PBI-100%membrane could reach 78.6 mS·cm-1 at 160?.(2)The PBI block copolymers and membrane materials containing thio-ether units and BPY monomer have been studied.The bpy-block oligomer was prepared by BPY monomer design and the S-block oligomer containing thio-ether unit was prepared.The serial block copolymers with strong oxidation resistance were prepared under microwave-assisted conditions.When the proportion of the prepared thio-ether units was 40%,the corresponding conductivity of s-PBI-40%membrane was up to 60.5 mS·cm-1 at 160?.The oxidation resistance of the membrane had also been significantly improved.In the high concentration of hydrogen peroxide,the loss of S-PBI-40%membrane was only about 5%at 80?,which exhibits better anti-oxidative stability.(3)The PBI copolymers and membrane materials containing piperazine structural unit have been studied.We had designed and synthesized PZDT monomers containing piperazine structural units.The series of membrane materials with strong acid-retention capacity had been prepared by copolymerization of three kinds of monomers including PZDT,2,6-pyridinedicarboxylic acid,and 3,3',4,4'-tetraaminobiphenyl under microwave-assisted conditions.The acid-doping level will increase with the increase content of PZDT monomer,and the proton conductivity of PZDT-PBI-40%membrane could reach 46.8 mS·cm-1 at 160?.We found that PZDT-PBI-40%could maintain nearly 85%phosphoric acid by testing,which also demonstrated that it was very effective to reduce the loss of phosphoric acid by increase the alkaline strength of alkaline sites.