The Preparation And Properties Of Novel Cross-linked Polybenzimidazole Membranes For Fuel Cell Applications

Fuel cells have attracted much attention due to their high energy conversion efficiency.Sulfonated hydrocarbon membranes are potential alternatives of Nafion~?due to their merits such as low cost,wide resources of raw materials,excellent thermal stability and good mechanical properties.In comparison with many other sulfonated hydrocarbon polymer membranes,sulfonated polybenzimidazoles(S-PBIs)possess much better chemical stability,which is favorable for improving long-term durability of fuel cells.However,because of the acid-base interaction between sulfonic acid groups and benzimidazole groups,S-PBIs generally exhibit rather low proton conductivity.This is the major drawback associated with the S-PBI membranes,which hinders their practical applications in fuel cells.In order to solve this problem,in this thesis a tetramine monomer,4,4'-(3,4-diaminophenoxy)biphenyl(BDAPB)was synthesized,and the relevant polybenzimidazole copolymers were prepared by copolymerization of BDAPB,4,4'-dicarboxyldiphenyl ether(DCDPE)and 3,3'-diaminobenzidine(DAB).Because the BDAPB moiety possesses many sulfonation sites(theoretical value:4),the resulting sulfonated polybenzimidazole membranes exhibited exceptionally high degrees of sulfonation and greatly enhanced proton conductivities.In chapter three,excess DCDPE was allowed to react with DAB in Eaton's reagent at 140oC to yield carboxyl-end-capped polybenzimidazole oligomers with varied degrees of polymerization.Without isolation the resulting polybenzimidazole oligomers were further polymerized with BDAPB and DCDPE to give a series of sequenced polybenzimidazole copolymers(PBI-x-Pms,here,x and m refer to the molar fraction of BDAPB in the tetraamine monomers and the average block length of the PBI oligomers,respectively).Post-sulfonation of the sequenced copolymers in fuming sulfuric acid resulted in the highly sulfonated polybenzimidazole copolymers(S-PBI-x-Pms).A series of covalently cross-linked sulfonated polybenzimidazole copolymer membranes with varied ion exchange capacities(IEC)were prepared by solution cast method using bisphenol A diglycidyl ether(BADGE)as the cross-linker(the membranes are denoted as S-PBI-x-Pm-C15,here,C15 means that the molar ratio of BADGE to benzimidazole groups is 15%).The resulting cross-linked membranes displayed very high IEC(3.36-3.84 meq/g),high tensile strength(53-58MPa)and high proton conductivities(0.087-0.134 S/cm,60oC,in water).The S-PBI-40-P10-C15 membrane showed even higher conductivity in water than that of Nafion 212 in the whole test temperature range.However,the membrane swelling ratios were fairly large(18-50%,80oC,in water)despite the fact that they became completely insoluble in water due to the covalent cross-linking.More work is needed to further reduce the swelling ratio of the membranes.In chapter 4,BDAPB,DAB and DCDPE were randomly copolymerized in Eaton's reagent at 140oC to give a series of polybenzimidazole random copolymers(PBI-x,x refers to the molar fraction of BDAPB in the tetraamine monomers).Post-sulfonation of the PBI-x yielded a series of highly sulfonated polybenzimidazole copolymers(S-PBI-x).The covalently cross-linked sulfonated polybenzimidazole random copolymer membranes prepared by using BADGE as the cross-linker(the membranes are denoted as S-PBI-x-Cy,here,y refers to the molar ratio of epoxy groups to benzimidazole groups)exhibited significantly higher tensile strength(62-80 MPa)than the foregoing sequenced copolymer membranes.The covalently cross-linked sulfonated polybenzimidazole random copolymer membranes exhibited exceptionally high IECs(3.16-4.06 meq/g)due to many potential sulfonation sites of the BDAPB moiety.As the content of BDAPB moiety increased,the proton conductivities of the membranes increased remarkably;when the molar fraction of BDAPB reached 30%,the conductivities in water were even higher than that of Nafion 212.The covalent cross-linking treatment led to reduction in membrane swelling ratio and the higher cross-linking density the lower swelling ratio.The fuel cell performances of the covalently cross-linked sulfonated polybenzimidazole random copolymer membranes under different operation conditions were investigated in detail and compared with those of Nafion212.The single cell assembled with S-PBI-30-C15 showed a maximum output power density of 1.11 W/cm~2 at 90oC and 70%relative humidity,which is significantly higher than that of Nafion 212(0.95 W/cm~2)under the same conditions indicating excellent performance of the former.