Preparation And Properties Of Nitrogen-containing Heterocyclic Polyaryl Anion Exchange Membranes

As a clean energy source,hydrogen energy is a crucial strategic option to facilitate the achievement of carbon peak and carbon neutrality targets.As a representative hydrogen energy development and utilization technology,fuel cells are widely used in industrial and mining enterprises,ships,aircraft and other fields.They are an important solution for realizing our country’s clean and low-carbon energy transformation.Anion exchange membrane fuel cell（AEMFC）is considered as a new generation of fuel cell technology that is the most promising alternative to proton exchange membrane fuel cell（PEMFC）due to its faster oxygen reduction kinetics and allowing the use of non-noble metal catalysts.Anion exchange membrane（AEM）is a vital ingredient of AEMFC,and its performance directly determines the performance level of AEMFC.After years of development,the performance of AEM has been greatly improved,but they remain encounter numerous problems such as insufficient ion conductivity and durability.In view of the current bottleneck problems of AEM,with the starting point of comprehensively improving the comprehensive performance of AEM,new ether-free polyaryl polymer backbones with super alkali resistance were designed.On this basis,the molecular design is carried out from the all-round and mutli-angle comprehensive application of comb-shaped grafting,cross-linking,side chain grafting and cationic string grafting.At the same time,taking into account the structural design criteria of the anion exchange ionomer（AEI）,we hope to develop a new feasible preparation method for AEM with excellent comprehensive performance and achieve an overall improvement in AEMFC performance,thereby promoting the progress of AEMFC industrialization.The specific research contents of this article are summarized as follows:（1）Double cation grafted comb-shaped polycarbazole anion exchange membranesFrom the perspective of regulating the microphase separation morphology in membrane,a series of novel dual-cation grafted comb-shaped polycarbazole（n-DTMA-PHCZBDO）anion exchange membranes were prepared using synergetic strategies of multi-cations strings and flexible hydrophobic alkyl chain comb-grafting.The thermodynamic incompatibility between the flexible comb pendant chains on the backbones and the cations provides a strong driving force for the self-aggregation of ions.The side-chain-type double cations not only improved the hydrophilicity of the alkyl side chain but also ensured sufficient free space for the movement of ions,which synergistically promoted the formation of microphase separation morphology in the membrane,effectively improving the ion conductivity of AEMs.The ionic conductivity of6-DTMA-PHCZBDO AEM is as high as 169.16 m S/cm at 80°C.The ether-free polymer backbone combined with the appropriate level of water uptake brought by double cations and the cation remotely-grafted on the backbone further reduce the probability of cations being attacked by OH^-.After 6-DTMA-PHCZBDO AEM was soaked in 1 M Na OH at 80°C for 2000 h,its ionic conductivity loss ratio was as low as 12.43%,and its mechanical strength loss ratio was as low as 9.83%.（2）“Star”rigid cross-linked polycarbazole anion exchange membranesBased on the previous work,a rigid“hyperbranched”type 1,3,5-tris（4-aminophenyl）benzene crosslinker was introduced to construct a new star-shaped cross-linking network（PBHCZ-x%TAPB）in AEM.Rigid crosslinker can enlarge the interchain distance of polymers,reduce the accumulation effect of polymer chains,give AEM a large free volume,and reduce the resistance of ion transport,thus providing a new direction to solve the“trade-off”problem between ion conductivity and mechanical properties and dimensional stability in traditional hydrophobic crosslinking structures.PBHCZ-10%TAPB AEM has the highest OH^-conductivity at 80°C,which is 146.03 m S/cm.The swelling ratio of PBHCZ-10%TAPB AEM was effectively controlled within the ideal range thanks to the restraining effect of rigid cross-linked network.The rigid branched crosslinkers with large volume increase the flexibility of AEMs and weaken the attack of OH^-,providing further guarantee for the stability of AEMs in alkaline environment.After PBHCZ-20%TAPB AEM was soaked in 2 M Na OH at 80°C for 1500 h,its OH^-conductivity only lost 7.6%.（3）Poly（terphenyl piperidine isatin）anion exchange membrane containing dipolar crosslinkersSeveral hydrophilic crosslinking AEMs（PTPBHIN-O_x）were synthesized by selecting poly（p-terphenyl piperidine isatin）polymer with better performance as backbone and using 1,2-bis（2-aminoethoxy）ethane containing dipole ethylene oxide group as hydrophilic crosslinker,and the effect of dipole crosslinker on the performance of AEMs was studied.The cation-dipole interaction between the cations and dipolar crosslinkers in PTPBHIN-O_x AEMs enhances the self-assembly ability of the cationic groups,and PTPBHIN-O₁₉AEM exhibits a high OH^-conductivity of 151.69 m S/cm at 80°C.The flexible cross-linked network strengthens the interaction between polymer chains and ensures the dimensional stability of the membrane.The in-situ cross-linking effect in PTPBHIN-O_x AEMs is supplemented by the dynamic water-rich hydrogen bonding networks,which further enhance the chemical alkali resistance of AEMs.The OH^-conductivity of PTPBHIN-O₁₉ AEM decreased only 12.28%in 3 M Na OH at 80℃for 1600 h.（4）Polyaryl isatin anion exchange polyelectrolytes with flexible monomersConsidering the structural design principles of AEM and AEI,a series of side-chain-type poly（terphenylene-co-dibenzyl isatin）copolymers（QAPTDBHI-n）were synthesized and the effect of flexible monomers in the backbone on the performance of anion exchange polyelectrolytes was analyzed.The introduction of flexible bibenzyl monomers endows the copolymers with a spatially foldable structure,which is conducive to the aggregation of ion clusters and strengthens the interchain interactions.Compared with the AEMs without flexible monomers,QAPTDBHI-n exhibited higher OH^-conductivity and superior mechanical stability.The QAPTDBHI-20 polymer with optimal mechanical and dimensional stability was selected as AEM.Membrane electrode assembly were prepared based on different AEIs to explore the impact of AEIs on AEMFC performance.Higher WU,OH^-conductivity,and lower phenyl concentration of QAPTDBHI-40 AEI effectively avoided the cathode drying and reducing the phenyl adsorption.Both H₂-O₂ and H₂-Air（CO₂ free）AEMFC based on it showed the highest single cell peak power density,which are 1.14 W/cm² and 0.86 W/cm²,respectively.（5）Alkyl imidazole cation-supported poly（terphenyl-co-fluorenyl isatin）anion exchange membranesBased on the previous research concept and from the perspective of selecting polymers with low phenyl adsorption energy on catalyst,a novel ether-free poly（terphenyl-co-fluorenyl isatin）polymer backbone was developed.Alkylimidazole with a uniqueπ-conjugated delocalized structure was selected as the ion transport groups to develop a series of imidazole cation-supported comb-shaped poly（terphenyl-co-fluorenyl isatin）polyelectrolyte PTFI-IZ-n.The additional hydrophobic structure on the imidazole cation provides a driving force for the self-assembly of ion clusters.PTFI-IZ-8 AEM exhibits the most well-developed phase separation morphology and exhibits a high OH^-conductivity of 140.61 m S/cm at 80°C.The high-strength rigid conjugated backbone gives it ideal mechanical properties,with a tensile strength of up to74.74 MPa.The long comb-shaped alkyl chain at the end of the cation in PTFI-IZ-8 AEM effectively protects the stability of the cation.After 1200 h in 2 M Na OH at 80°C,its ionic conductivity only lost 10.32%and its tensile strength lost 11.05%.The non-rotatable 9,9-dimethylfluorene monomer effectively reduces the phenyl adsorption of the ionomer.The peak power density of H₂-O₂AEMFC based on PTFI-IZ-8 can reach 1.03 W/cm² at 80℃.