Synthesis And Properties Of Polybenzimidazole Based On Hyper-branched Cross-linker As Polymer Electrolyte Membrane

Polymer electrolyte membrane fuel cell has a good prospect in power stations,transportation application and mobile portable equipment,which can be divided into proton exchange membrane fuel cell?including low-temperature proton exchange membrane fuel cell and high-temperature proton exchange membrane fuel cell?and alkaline polymer electrolyte fuel cell.Polybenzimidazole is a kind of heterocyclic polymer membrane with benzimidazole as structure unit,which has excellent thermal stability and chemical resistance.After being doped with phosphoric acid or base,polybenzimidazole membrane can be used as high-temperature proton exchange membrane or alkaline polymer electrolyte membrane.However,the hydrogen bond between the molecular chains of doped?by acids or bases?polybenzimidazole is destroyed and the tensile stress is reduced.In the operation of fuel cells,the phosphoric acid in the membrane is"taken away"by the water produced by the redox reaction,which causes doped polybenzimidazole still cannot meet the requirement of long-term operation of fuel cell.Cross-linking can effectively improve the tensile tress and acid retention of polybenzimidazole,but it reduces the electrochemical properties of the polymer electrolyte membrane,leading to lower power density.Firstly,cross-linkers may dilute the effective functional groups in the polymer,which may hinder the proton conduction.Secondly,cross-linked structure may restrict the movement of polymer chains,which makes the polymer too tight to accommodate more acid or base.Regarding the issues above,a new cross-linker with hyper-branched structure was synthesized in this thesis,which contained multiples of basic sites.The introduction of basic sites in cross-linker can enhance ion conduction.The purpose was to improve ion conductivity and tensile stress of polybenzimidazole membrane materials.The research contents and results are as follows:?1?The synthesis of brominated hyper-branched polybenzyl was based on Friedel-Crafts reaction and then NBS bromination reaction.Cross-linked polybenzimidazole membranes containing quaternary ammonium groups as high temperature proton exchange membrane were obtained by quaternization reaction.The results showed that cross-linked polybenzimidazole membranes had excellent tensile stress,oxidative stability and phosphoric retention.All the tensile stress values of doped cross-linked membranes were more than 20 MPa.These cross-linked membranes showed only a little weight loss and no crack after immersing in Fenton's reagent for 200 hours.The introduction of quaternary ammonium groups enhanced proton conduction of membranes.Under hydrogen/air atmosphere,the fuel cell based on the QOPBI-15 showed a power density of 260 mW cm^-2 at 160°C.?2?In order to the improve proton conductivity,cross-linked polybenzimidazole containing imidazolium salts,which are base group with a conjugated ring structure,have been fabricated.The results showed that proton conductivity of these membranes obviously enhanced.Also,these membranes exhibited great tensile stress,oxidative stability and acid retention ability.At 160°C,the fuel cell based on ImOPBI-15 membrane showed a power density of 610 mW cm^-22 under hydrogen/oxygen atmosphere.?3?Due to the excellent tensile stress and good conductivity,cross-linked polybenzimidazole membranes with quaternary ammonium groups were used as alkaline polymer electrolyte membrane.This chapter focused on the research of fuel cell performance.The results showed that ionic conductivity of these cross-linked membranes was greater than0.1 S cm^-1 and tensile stress was higher than 50 MPa.The fuel cell based on QOPBI-15 showed a power density of 75.6 mW cm^-2.