Preparation And Properties Of High Temperature Proton Exchange Membranes Based On 1,2-benzimidazole Based Polymers

Polybenzimidazole（PBI）has attracted much attention because of its excellent temperature resistance and mechanical properties.When phosphoric acid（PA）is doped into PBI,the prepared PA/PBI membrane shows good proton conductivity under high temperature（100～200°C）and low humidity conditions,which has aroused great interest among researchers.However,PA/PBI membranes have some shortcomings that limit their application:when the PA doping degree is high,although the proton conductivity of the membrane is improved,its mechanical properties and resistance to dry and wet deformation are reduced;In the presence of liquid water,the PA in the membrane is easily lost,which leads to a rapid decrease in proton conductivity.This paper mainly focuses on these problems,and designs and prepares 1,2-benzimidazolyl-based high-temperature proton exchange membrane（HT-PEM）.The main research contents of this paper include:（1）Aiming at the problem of PA loss of PA/PBI membrane,two proton conductors are designed and synthesized,namely a new organic-inorganic composite solid proton conductor MHPA and an acidic polymer proton conductor SPEEK.In order to optimize the ion exchange capacity（IEC）and yield of MHPA,the metal ion types and the feeding ratio of HPA to metal salts were screened,and the optimal preparation process conditions for water-insoluble solid proton conductors were obtained:proton conductor Zr HPA（1:2）.MHPA has an organic-inorganic composite structure and good compatibility with PBI,which solves the problem of poor compatibility between inorganic proton conductor materials and PBI.Zr HPA and SPEEK have good thermal stability within 200°C and high proton conductivity,which is suitable for doping into PBI composite membranes as high-temperature proton conductors.Due to the high content of phosphonic acid group,Zr HPA（1:2）exhibits high IEC,which can reach 1.36 meq.g^-1,and can form a consecutive hydrogen bond network structure,which reduces the dependence of its proton conductivity on RH,and endows good proton conduction performance even at low RH.（2）PBI membranes are prone to oxidative degradation,mainly because that the end NH₂ groups and NH on the imidazole rings are easy to be attacked by free radicals.In order to solve the problem of oxidative degradation of PBI membranes,a novel 1,2-imidazolyl polymer PBESK without terminal amino group and N-H bonds was prepared by block copolymerization,which fundamentally solves the problem of oxidative degradation of composite membranes caused by PBI terminal amino groups and N-H bonds on imidazole rings.The synthesized PBESK was reacted with 3-bromopropene to synthesize APBESK,and doped with Zr HPA as a proton conductor,the APBESK/Zr HPA composite membrane was prepared by thermal crosslinking after membrane formation.APBESK with different degrees of crosslinking was obtained,and APBESK2 had the best mechanical properties.APBESK/Zr HPA composite membrane has uniform microscopic morphology and ideal thermal stability.SEM characterized the topography of APBESK/Zr HPA,and TG-DSC test data showed that the composite membrane had good thermal stability at below 200°C.The mechanical properties,wet and dry deformability,oxidation resistance,proton conductivity,methanol resistance and gas penetration rate of APBESK/Zr HPA composite membrane were tested.At 180°C,the proton conductivity of APBESK2/Zr HPA（50%）at 100%,50%,and 0RH reached 0.165,0.087,and 0.05 S cm^-1,respectively.（3）SPEEK is used as the proton conductor for PBESK membrane to prepare acid-base blended membranes.PBESK/SPEEK composite membranes were prepared by doping SPEEK with different contents,and PBESK/SPEEK（50%）had the best mechanical properties.The structure,morphology and thermal stability of PBESK/SPEEK composite membranes were characterized,and the mechanical properties,wet and dry deformability,oxidation resistance,proton conductivity,methanol resistance,and gas penetration rate of the composite membranes were tested.The proton conductivity of the PBESK/SPEEK（60%）composite membrane reached 0.147,0.053 and 0.0058 S cm^-1 at 100%,50%and 0%RH,respectively.