Preparation And Performance Study Of Branched Polybenzimidazole/Polymeric Ionic Liquid As High Temperature Proton Exchange Membrane

High temperature proton exchange membranes（HT-PEMs）are important components of high temperature proton exchange membrane fuel cells（HT-PEMFCs）,which play roles in proton transport in isolating fuel and proton transport in the system,so in order to improve the performance of HT-PEMFCs,a host of researchers have studied PEMs.Among them,polybenzimidazole（PBI）has basic amphoteric nitrogen heterocyclic structure and it can both act as a donor and acceptor of protons,and it usually has excellent mechanical properties,thermal stability and oxidation stability,however,the proton conductivity of PBI is low,so the current method to improve the proton conductivity of PBI is to doping with phosphoric acid（PA）,the presence of PA can improve the proton conductivity of the system through the grotthuss mechanism.However,PA has plasticizing effect,with the increase of PA doping level,the mechanical properties of the membrane are greatly reduced,and during the operation of the fuel cell,excess PA will leach out,which will affect the long-term durability of HT-PEMs.Therefore,how to balance the long-term properties such as mechanical properties of PA-PBI and proton conductivity is a major problem for researchers.In this paper,the OPBI was first synthesized,and then a rigid triazine structure with a large free volume was introduced into the polymer,then a series branched OPBI membranes with different branching ratio was synthesized.The membrane has unique large-volume three-dimensional dendritic structure,the free cavity created by these structures is conducive to accommodating the absorbed PA,so more PA can be doped while maintaining sufficient mechanical strength.Among them,the BOPBI-6%membrane obtains splendid comprehensive performance,when the PA doping level（ADL）is 9.55,the proton conductivity of BOPBI-6%is 99.2 m S cm^-1 at 180℃,which is 1.8 times higher than OPBI(54.8 m S cm^-1),and it performs well in long-term oxidation stability test,after 144 h,it still has a mass retention rate of 90%.In order to further improve the performance of the membrane,functionalized polymeric ionic liquid（PIL）was continuously introduced into the system,when the membrane was soaked into an alkaline solution,the formation of ion-pairs（H⁺-H₂PO₄^-）can be used as both a proton donor and receptor,providing a binding site for proton transmission,which further improves the performance of the composite membrane.Among them,BOPBI-PIL-30%membrane has sufficient mechanical properties（5.50±0.8 MPa）,and the proton conductivity(146.9 m S cm^-1)at 180℃is also excellent.During this period,the introduction of PIL will reduce the long-term properties of the membrane（such as oxidation stability）to a certain extent,and after 144 hours of testing,the BOPBI-PIL-X membrane has more than60%residual weight,so in the following experiments,on the basis of the introduction of large-volume PIL,KH560 crosslinker was further introduced into the system,and the crosslinking network structure further resisted the attack of oxidative free radicals.The modified membrane has excellent proton conductivity(155.8 m S cm^-1 at 180°C),mechanical properties（4.68±0.5 MPa）,excellent long-term oxidation stability,and a mass retention rate of more than 80%after up to 144 hours of testing.