Structural Design And Performance Tuning Of Quaternary Ammonium-based Polyarylanion Exchange Membranes

Anion exchange membrane fuel cells（AEMFCs）are considered to be the most potential power generation technology to replace proton membrane fuel cells（PEMFCs）in the future due to their low oxygen reduction overpotential,allowing the use of non-precious metal catalysts,simple water management,which is expected to alleviate the high cost of PEMFCs under the current technology saturation and break through the technological limitations.As the“heart”component of AEMFCs,the performance of anion exchange membranes（AEMs）is critical to the overall cell.However,the current development of AEMs is not comprehensive,and key issues such as alkali resistance,dimensional stability and mechanical strength need to be resolved urgently.To solve the above bottlenecks,a new highly stable polyaryl backbone and alkali-resistant quaternary ammonium cationic groups were prepared based on the molecular structure design,and the matching mechanism between them was optimized by combining the strategies of side chains,ordered branching,hydrogen bond cross-linking,covalent cross-linking,and ion-dipole to improve the intrinsic alkali resistance of AEMs and realize the regulation of the microphase separation morphology in the membrane.Moreover,a simple and inexpensive universal synthesis strategies were used for the preparation of high-performance AEMs.The main work of this thesis is as follows:（1）Side-chain cation grafted poly（biphenyl piperidine）anion exchange membranesTo construct continuous cationic hydrophilic domain,ether free backbone poly（biphenyl piperidine）（PBP）was prepared by superacid-catalyzed Friedel-Crafts polycondensation reaction.Subsequently,alkali stable cyclic piperidine cations were introduced to the PBP backbone through flexible hydrophobic alkyl chains with different lengths to develop the side chain-type AEMs（PBP-n-Pip）,and the effect of flexible spacer chain length on the membrane properties was investigated.Attributed to the core structural advantage of remote grafting,the flexible hydrophobic chain segments improved the freedom of QAs and gave them spatial protection effects,which led to the improvement of both ionic conductivity and chemical stability.Among them,PBP-6-Pip has an ionic conductivity of 117.1 m S cm^-1at 80℃and only 15.96%degradation after 1500 h of alkali resistance test.（2）Cation-dipole comb shaped anion exchange membranesBased on the nature of high-speed ion transport channel construction,cation-dipole comb AEMs（m-PTP-O-x%）were developed by grafting different ratios of hydrophilic alkoxy side chains onto part of the hydrophobic poly（m-triphenyl-piperidine）backbone（m-PTP）,while the remaining sites were given pure carbon side chains without dipoles,and the influence of the ratio of alkoxy side chains on membrane performance was investigated.Several characterization results consistently confirmed that the introduction of cation-dipole driving force promoted the aggregation of ion-hydrophilic regions,induced the construction of continuous ion-transporting water channels,and increased the ion conductivity by expanding the microphase separation size with the help of the polarity difference of the hydrophilic and hydrophobic side chains when a small amount of hydrophobic alkyl chains was held in the membrane.Among them,m-PTP-O-67%has the highest ion conductivity of 101.55 m S cm^-1,which is nearly 1.2 times higher compared to m-PTP-O-100%with all grafted hydrophilic dipole chains.The alkali resistance test showed that the retention of cationic groups was increased by 56.26%when 100%of alkoxy side chains were grafted in the membrane compared to m-PTP-O-0%without ion-dipole interaction,confirming that the electron donating effect of alkoxy reduces the sensitivity of quaternary ammonium cations to OH^-and improves the alkali resistance of the membrane.（3）Hydrogen bond cross-linked“windmill”branched poly（p-terphenyl piperidine）anion exchange membranesTo further optimize the comprehensive performance of AEMs,aromatic branched reagent 1,3,5-triphenylbenzene was used to build a“windmill”branched structure in the membrane.Then,2-（2-chloroethoxy）ethanol containing hydrophilic alkoxy and terminal hydroxyl group was introduced onto the branched backbone,and the hydrogen bond cross-linked"windmill"branched AEMs TPTP-Pip-OH-x%was prepared by combining the side chain piperidine cations with high degree of freedom and strong alkali resistance.The“windmill”branched structure provides more free volume within the membrane,which could break through the accumulation and entanglement of molecular chains,facilitating the aggregation and connectivity of hydrophilic regions.Its inherent steric hindrance effect could weaken the attack probability of hydroxides.The C-O-C and terminal hydroxyl groups in the hydrophilic side chain can cooperate to build a hydrogen bonding network for the"Hopping"transport of hydrated OH^-,which can break the transport inertia of the hydrophobic backbone while enriching the ion conduction pathway.Moreover,the terminal hydroxyl groups allow AEMs to obtain dynamic cross-linking effect without significant adjustment of molecular structure,optimizing dimensional stability and mechanical strength.Attributed to the molecular structure,the ionic conductivity of TPTP-Pip-OH-20%was 143.2 m S cm^-1and the tensile strength was up to 59.12 MPa,and the ionic conductivity remained 87.40%of its initial value after 1500 h of alkali resistance test（80℃2M Na OH aq.）,showing excellent overall performance.Then,the MEA was prepared for single fuel cell testing,and the peak power density reached 405 m W cm^-2.The synthesis strategy proposed in this work has the advantages of simplicity and universality,which is beneficial for the micromorphology and electrochemical properties of AEMs.（4）Doublecross-linkedorderedbranched poly（m-triphenyl-piperidine）anion exchange membranesBased on the core structural advantages of the previous work,a novel branched reagent triphenylmethane was introduced.Compared with1,3,5-triphenylbenzene,the aromaticity of triphenylmethane monomer was slightly reduced,so it could hold excellent solubility while improving the branching degree.Subsequently,a simple in-situ cross-linked strategy was used to introduce the multi-cationic cross-linked structure,and in combination with the strong hydrophobic segment 1,1,1-trifluoro-3-iodopropane to enhance the polar driving force for the successful preparation of double cross-linked poly（m-terphenyl piperidine）anion exchange membrane TPM-m PTP-x%.The double cross-linking effect of covalent and hydrogen bonds enhanced the directional aggregation ability of the hydrophilic region in the membrane,improved ion transport ability and alkali resistance,and alleviated the“Trade-off”effect faced by AEMs due to excessive IEC and swelling ratio.TPM-m PTP-30%exhibits ionic conductivity of 126.14 m S cm^-1at80°C.After 1500 h of alkali resistance testing（80°C 2 M Na OH aq.）,the ion conductivity remained at 89.58%of its initial value.Besides,the single H₂/O₂fuel cell based on TPM-m PTP-30%exhibits a peak power density of 333 m W cm^-2at 80°C.