Preparation And Properties Of Crosslinked Branched PBI High Temperature Proton Exchange Membranes

Polybenzimidazole（PBI）has attracted wide attention because of its excellent mechanical properties and thermal stability.Phosphoric acid（PA）doped PBI membranes are of great interest to researchers because of their excellent proton conductivity at high temperatures and low humidity.However,PBI/PA membranes also have some defects which limit their application:（i）At high PA doping levels,the conductivity of PBI/PA membranes is good,but the mechanical properties and dimensional stability are generally poor;（ii）When working under medium or high relative humidity（RH）conditions,PA in the membrane is easy to lose,resulting in performance attenuation;（iii）PBI prepared by melt polymerization method is not easy to form membrane;（iv）PBI membrane is easy to undergo oxidative degradation.Therefore,this dissertation focuses on these problems and studies on the preparation of PBI typed high-temperature proton exchange membrane（HT-PEM）with excellent performance:（1）Given the problems of PA loss,poor mechanical properties,and dimensional stability of PBI/PA membrane,three types of new solid proton conductors with organic-inorganic composite,organic polymer,or organic structure were designed and synthesized based on the compatibility between proton conductor and PBI,which have good compatibility with PBI.Water-insoluble organic-inorganic hybrid proton conductors（Fe ATMP）were synthesized from metal ions(Fe²⁺)and amino trimethylene phosphonic acid（ATMP）.By adjusting the molar ratio of Fe²⁺to ATMP,water-insoluble high-temperature proton conductors with excellent yield and ion exchange capacity were obtained.Sulfonated hyperbranched polybenzimidazole（SHBPBI）with an organopolymeric structure was prepared by controlling the reaction conditions.The proton conductor not only has the organic macromolecular framework of PBI,but also has free sulfonic groups,which makes hydroxyl groups fully exposed and is conducive to proton conduction.An organic proton conductor 3,3’-（1,4-phenyl bis（azazadienyl））bis（propane-1-sulfonic acid）（PBSA）was synthesized.Although the proton conductor is soluble in water,it contains crosslinkable NH bond,which can be fixed in the membrane by crosslinking agent without loss.These three kinds of proton conductors have organic or organic-inorganic hybrid structures and have good compatibility with PBI.The structures of Fe ATMP,SHBPBI,and PBSA were verified by ¹H NMR and FT-IR.TG-DSC shows that Fe ATMP,SHBPBI,and PBSA have good thermal stability up to 200°C.The proton conductivity of Fe ATMP,SHBPBI,and PBSA at 180°C and 100%,50%and 0%RH are0.124,0.079 and 0.038 S cm^-1,respectively.The proton conductivity of SHBPBI can reach0.0283 S cm^-1 at 180°C and 0%RH.The proton conductivity of PBSA can reach 0.0230 S cm^-1 at 180°C and 0%RH.（2）In order to solve the problem that PBI prepared by melt polymerization is not easy to form membrane,1,3,5-trioxy-2,4,6-triazine-2,4,6-trioxybenzoic acid（TPCA）was synthesized firstly,and then 3,3’-diaminobenzidine（DAB）and isophthalic acid（IPA）were synthesized by melt polymerization,Branched PBI（br PBI）with different branching degrees were synthesized from IPA and TPCA at different TPCA molar dosages.The branching degrees of br PBI were determined by ¹H NMR and its molecular weight was determined by viscosity method.The br PBI has the characteristics of large free volume and good segment flexibility.The branched structure limits the movement of molecular chain,which is similar to the effect of crosslinking.It can improve the problem that PBI is not easy to form membrane.br PBI-KH230/Fe ATMP composite membranes were prepared by using monofunctionalγ-chloropropyl triethoxysilane（KH230）and br PBI3 with different content of Fe ATMP.The cross-linked membrane composed of br PBI3 and KH230 can form POSS（Si-O-Si）structure during acidification,which can improve the water absorption of the membrane.The structure of the composite membrane was characterized by FT-IR,and the morphology was characterized by SEM.The TG-DSC data showed that the composite membrane had good thermal stability below 200°C.HRTEM was used to study the morphology of clusters formed by proton conducting groups in the membrane.The mechanical properties,dimensional stability,oxidative stability,proton conductivity,and gas permeability property of br PBI3-KH230/Fe ATMP composite membrane were tested.The proton conductivity of br PBI3-KH230（5%）/Fe ATMP（50%）at 180°C and100%,50%,0%RH reached 0.136,0.073,and 0.041 S cm^-1,respectively.（3）The molecular weight of PBI prepared by melt polymerization is low,so the high molecular weight o PBI with ether bond structure was synthesized by solution polymerization using DAB and 4,4’-diphenyl ether dicarboxylic acid（OBA）as raw materials.o PBI-TAIC/SHBPBI composite membranes were prepared by crosslinking o PBI with triallyl isocyanurate（TAIC）and doping SHBPBI.TAIC has three crosslinking groups,which can significantly improve the performance of the membrane at low crosslinking degree（5%）.The structure,morphology,and thermal stability of o PBI-TAIC/SHBPBI composite membrane were characterized.The mechanical properties,dimensional stability,oxidative stability,proton conductivity,and gas permeation property of o PBI-TAIC/SHBPBI composite membranes were tested.The proton conductivity of o PBI-TAIC（5%）/SHBPBI（50%）reached 0.147,0.074 and 0.034 S cm^-1 at 180°C and100%,50%,0%RH,respectively.（4）brPBI has large free volume and good flexibility,so bro PBI was synthesized by solution polymerization from DAB,OBA and TPCA.The bro PBI has branched structure and ether bond,good flexibility,large free volume and high molecular weight.The macromolecular crosslinking agent Br ABPBI was prepared from self-made ABPBI.bro PBI,PBSA and Br ABPBI were crosslinked to form bro PBI-Br ABPBI/PBSA monolayer.The structure of bro PBI-Br ABPBI/PBSA composite membrane was characterized by FT-IR,and the morphology of bro PBI-Br ABPBI/PBSA composite membrane was characterized by SEM.TG-DSC showed that the composite membrane had good thermal stability in the working range of HT-PEMFCs.The mechanical properties of bro PBI-Br ABPBI/PBSA composite membrane were significantly improved.The mechanical properties,dimensional stability,oxidative stability,proton conductivity,and gas permeation of bro PBI-Br ABPBI/PBSA composite membranes were tested.The proton conductivity of bro PBI-Br ABPBI（20%）/PBSA（50%）reached 0.140,0.0701 and 0.031 S cm^-1 at 180°C and 100%,50%,0%RH,respectively.