Preparation Of Anion Exchange Membrane Materials With Steric Quaternary Ammoniums Via "Click Chemistry" And The Study Of Structure-property Relationship

As an important part of anion exchange membrane fuel cell,anion exchange membrane plays an important role in the performance of anion exchange membrane fuel cell.At present,most anion exchange membranes are still characterized by low conductivity,poor stability and high processing cost.Based on this reason,from the perspective of molecular structure design,we synthesized a series of side-chain anion exchange membrane materials through "click chemistry",and further studied the influence of steric quaternary ammonium and isomeric quaternary ammonium on the performance of anion exchange membrane materials and anion exchange membrane fuel cells.The main contents and conclusions are as follows:Steric hindrance engineering is of great significance to the preparation of new stable organic cations and durable polymeric anion exchange membrane materials.In this work,we presented a study of the structure-property relationship in quaternized poly(2,6-dimethyl-1,4-phenylene oxide)containing bulky n-propyl,isopropyl,or cyclohexyl-functionalized quaternary ammonium moieties.PPO-iP membranes possessing isopropyl-based QA cations showed higher water uptake than n-propyland cyclohexyl-containing analogues(PPO-nP and PPO-CH),in which obvious microphase-separated morphology was found.Minor loss(<2%)in conductivity was observed for PPO-CH membranes after the treatment in 2 M NaOH at 60℃ for 216 h,highlighting the excellent alkaline stability,while significant degradation of PPO-iP and PPO-nP was confirmed by NMR characterization of the aged samples.In addition,the prepared PPO-based AEMs having sterically crowded QA cations were compared in alkaline H2/O2 fuel cells,demonstrating that the highly conductive PPO-iP membrane exhibited a peak power density of 182 mW/cm2 under optimized testing conditions.Benzyltrimethylammonium(BTMA)is most frequently-used organic cations in anion exchange membrane(AEM)materials.However,BTMA-based AEMs always suffer from low ionic conductivity and insufficient alkaline stability.Here,we present a systematic investigation of a series of side-chain-type poly(2,6-dimethyl-1,4phenylene oxide)(PPO)AEMs with constitutional isomerism in BTMA cations.Three isomeric BTMA cations,e.g.meta-BTMA,ortho-BTMA,and para-BTMA,were tethered onto PPO backbones via a flexible spacer using CuAAC reaction,producing side-chain-type AEMs,namely m-QPPO,o-QPPO,and p-QPPO membranes,respectively.As expected,side-chain-type PPO AEMs displayed higher hydroxide conductivity as compared to a control PPO-QA membrane where BTMA cations were directly linked on PPO backbones,due to the microphase-separated morphologies as confirmed by small-angle X-ray scattering(SAXS)results.Although these isomeric quaternized PPO copolymers had identical chemical composition and polymer architectures,they did not share similar properties.Specifically,among three side-chain-type AEMs,the highest hydroxide conductivity of 42.8 mS/cm was observed for m-QPPO membrane having meta-BTMA cations with an ion exchange capacity of 1.93 meq./g at 20℃,as a result of its high water uptake.In addition to high conductivity,m-QPPO membrane showed superior alkaline stability with respect to o-QPPO and p-QPPO membranes.After 200 h of aging in 1 M NaOH at 60℃,85%of the hydroxide conductivity was retained for m-QPPO AEMs,while more than 30%conductivity loss was observed for o-QPPO and p-QPPO membranes.Furthermore,the AEM fuel cells using these PPO AEMs with isomeric BTMA cations were investigated,and the cell with highly conductive and durable m-QPPO membranes exhibited the best performance with a peak power density of 333 mW/cm2 at a current density of 700 mA/cm2 at 60℃,comparable to other AEMFCs with PPO based AEMs.