Preparation And Characterization Of Novel Polybenzimidazole Membranes For High Temperature Fuel Cells

Proton exchange membrane （PEM） is the core components of protonexchange membrane fuel cells （PEMFC）, which plays the critical role ofseparating the fuel between anode and cathode, transferring the proton andinsulating electron. Recently, perfluorosulfonic acid membranes, such asNafion membrane used extensively, have high cost and low protonconductivity under high temperature due to membrane degradation anddehydration. It hinders commercial process of fuel cells. Polybenzimidazole（PBI） membrane doping phosphoric acid has excellent thermal and chemicalstability, good mechanical properties and high proton conductivity under hightemperature and low humidity. The price of PBI is about ten percent of Nafionmembrane, which has the potential to be used in the field of high temperaturefuel cells.In this thesis,4,4’-diaminodiphenyl ether as raw materials synthesized3,3’,4,4’-diaminodiphenylether （TADPE） via five-step reaction. Two series ofunreported PBI polymers were prepared by polymerization between TADPE and2,5-pyridinedicarboxylic acid （2,5-PDA） or2,6-pyridinedicarboxylic acid（2,6-PDA）, and trimesic acid （TMA） was used as crosslinking reagent. Themembranes from the above PBI polymers prepared and researched thecorresponding properties of fuel cell. The main contents in this research aregiven as follows.1.4,4’-diaminodiphenyl ether as raw materials by acetylation （aminoprotection）, nitrification, acid hydrolysis, reduction, and neutralizationreaction synthesized3,3’,4,4’-diaminodiphenylether （TADPE）. The SnCl₂-HClreduction system was chosen and optimized, and n （nitro）: n （SnCl₂）=1:12,80C,6h. The total yield of TADPE was up to75.1%, and the molecularstructure was confirmed by furier transform infrared spectrum and1H-NMRspectrum.2. The PBI polymers were prepared by solution polycondensationbetween the monomers TADPE,2,5-pyridinedicarboxylic acid （2,5-PDA） or2,6-pyridinedicarboxylic acid （2,6-PDA）, and trimesic acid （TMA） was usedas crosslinking ragent to improve the corresponding properties of resin. Thepolymerization conditions were optimized for the reaction mixture in PPAsolvent under nitrogen atmosphere kept at room temperature30min,50C2h,100C3h,130C4h,160C5h,190C10h. Under the optimized condition,two series PBI including novel2,5-pPBI-TMA-X （X=0,2.5,5.0,7.5,10） and2,6-pPBI-TMA-X （X=0,2.5,5.0,7.5,10） polymers were prepared. Allpolymers were characterized by furier transform infrared spectrum, inherent viscosity, solubility and thermogravimetric analysis.3. The membranes from the above-mentioned polymers were prepared bysolution-casting method, and the membranes from2,6-pPBI-TMA-X （X=0,2.5）polymers were obtained. All membranes were charactierized by swelling ratio,mechanical properties, oxidation resistance, X-ray diffraction and scanningelectron microscope. The corresponding performance of fuel cell wasmeasured. All membranes were doped by immersing in phosphoric acidsolutions with different concentrations for at least7days at room temperature.The results indicated that the membranes of2,6-pPBI-TMA-X （X=0,2.5） havehigh proton conductivity, and the proton conductivity of2,6-pPBI-TMA-2.5（cross-linked membrane） membrane is up to0.053S·cm-1in80%phosphoricacid under170C. Totally, the prepared membranes have a potentialapplication for high temperature fuel cells.