咪唑鎓盐聚芳醚类梳型共聚物的拓扑结构设计及性能调控

Alkaline fuel cells based on anion exchange membranes have received significant interest in recent years, because non-noble metals or inexpensive metal oxides can be used as catalysts to greatly reduce the cost of the device. Recently, polymers with pendant imidazolium cationic groups other than quaternary alkylammonium have been developed due to their relatively high chemical stability, which is largely attributed to the presence of the π conjugated structure. However,degradations of imidazolium cations were also observed under vigorous conditions due to the nucleophilic attack of hydroxide ions to the imidaozole rings. In this project, a variety of C2 and N3-substituted imidazolium cations will be synthesized to find the effective strategy for stabilizing imidazoliums. The influence of substituents on alkaline stability of imidazolium cations and imidazolium-based alkaline polymer electrolytes will be systematically studied, including the competing deprotonations, electronic resonance and sterically hinderance. The project also intends to prepare three types of comb-shaped poly(aryl ether) copolymers by using the RAFT polymerization technique. First,the addition of long alkyl side-chains in the comb-shaped poly(aryl ether) ionomers is expected to enhance the microscopic phase separation between hydrophilic and hydrophobic domains in the membrane, such that the hydroxide ion will locally aggregate and the OH- hopping conduction can become more effective. Second, the long alkyl side chain could signficantly mitigate water swelling of the membrane and reduce the nucleophilic attack of hydroxide at the imidazolium moieties resulting in good alkaline stability of the comb-shaped poly(aryl ether) ionomers. By regulating their topological structure, new anion exchange membranes having micro-phase seperated structure, high ionic conductivity and alkaline stability can be exploited. The findings will provided a general strategy for the rational design of alkaline polymer electrolytes.

基于阴离子交换膜的电化学器件有望摆脱贵金属催化剂Pt的使用，可大幅降低成本，具有重要的应用前景。非季铵化的咪唑鎓盐型阴离子交换膜发展较晚，耐碱性优劣存在很大争议，需要更多实验事实的积累和研究方法学的建立。本项目拟通过对咪唑环C2位和N3位取代基的合理设计，从去质子化效应、共振效应和位阻效应等三个层次对咪唑鎓盐的稳定和降解机理进行系统地研究，回应目前的争论，并筛选出稳定性高的咪唑鎓盐结构。针对阴离子交换膜存在的离子传导率偏低的问题，本项目拟结合RAFT聚合接枝技术，制备三种拓扑结构的咪唑鎓盐聚芳醚类梳型共聚物。通过对聚合物拓扑结构的调控，构筑连通的离子传输通道，提高离子传导率；通过空间延展的憎水性烷基侧链的位阻效应，增强耐碱稳定性；通过对聚合物结构-微相分离形貌-膜性能关系的全面研究，阐明影响碱性聚合物电解质电导率和稳定性的关键结构因素，为设计更加优异、稳定的阴离子交换膜提供借鉴思路。

基于阴离子交换膜的碱性燃料电池有望摆脱贵金属催化剂Pt 的使用，可大幅降低成本，具有重要的应用前景。本项目针对目前文献中季铵及咪唑鎓阳离子基团的耐碱性优劣存在很大争议入手，设计合成了苄基季铵及C2 和N3 位取代的苄基咪唑鎓盐小分子模型化合物，并利用NMR 等技术手段研究了苄基位季铵及咪唑鎓盐等阳离子基团的稳定与降解机理。设计合成了不同链长烷基取代的咪唑单体，与溴甲基化聚芳醚反应后，合成了不同烷基链长的咪唑鎓盐聚芳醚类梳型聚合物。以溴甲基化的聚芳醚酮为大分子引发剂，引发4-乙烯基苄基氯化铵接枝聚合，制备了不同链段长度的季铵化梳型共聚物。以含双硫酯的聚芳醚类聚合物为大分子链转移，与苯乙烯单体进行RAFT 聚合反应，制备聚芳醚类梳型共聚物。设计合成了一端为烷基溴，另一端为三甲胺季铵盐的小分子化合物1-溴-6-（三甲基铵）基溴化物（BTAH）。利用季铵化反应，获得了侧链含有三个季铵基团的聚芳醚酮梳型共聚物，实现了引入柔性长侧链和侧链多官能化的目标。通过上述对聚芳醚类梳形共聚物的结构设计与调控，实现了对阴离子聚合物电解质膜的亲水/疏水相分离结构的有效调控，达到了构筑高效的离子传输通道的目的，提高了膜的氢氧根离子传导率。同时，空间延展的憎水性长烷基侧链可将离子基团四周保护起来，有效降低OH-对阳离子基团的进攻，增强了膜的耐碱稳定性。通过该项目，我们对作为基质阴离子交换膜膜材料的聚芳醚类聚合物的设计制备、性能表征等方面积累了丰富经验，初步完善了改进阴离子耐碱稳定性的策略和建立了阴离子交换膜的结构与性能之间的关系。在该项国家自然科学基金的资助下，发表标注国家自然科学基金资助的SCI检索论文21篇，已授权国家发明专利2项。受邀在《科学通报》“燃料电池阴离子交换聚合物膜”专题发表研究论文1篇。在中国化学会第十八次电化学大会做了题目为“碱性燃料电池用聚芳醚类阴离子交换膜的分子设计及性能调控”的口头学术报告，并获得口头报告奖。