High-Performance Anion Exchange Membranes with Para-Type Cations on Electron-Withdrawing C═O Links Free Backbone
Fan Zhang, Tiantian Li, Wanting Chen, Xiaoming Yan, Xuemei Wu, Xiaobin Jiang, Yang Zhang, Xiaozhou Wang, Gaohong He
Abstract
As critical triggers, both the electron-withdrawing links and neighboring cationic groups induce and accelerate the aryl ether cleavage in the widely used aryl ether containing anion exchange membranes (AEMs). Herein, a novel alkaline stable polyelectrolyte is proposed by converting the electron-withdrawing C═O links in the widely used poly(aryl ether ketone) backbone into the electron-donating C–NH2 links, and then grafting faraway para-type cations by Leuckart-Menshutkin reaction without using noble metal catalysts. Contributions of the elimination of electron-withdrawing C═O links and graft of para-type cations to the excellent alkaline stability of the aryl ether containing polyelectrolytes are demonstrated through density functional theory, as the increasing electron cloud density on the ether-connected carbon atom elevates the activation energy barrier to OH– initiated aryl ether cleavage, and the para-type cation is more stable than the conventional ortho-type cation due to the sterical protection and low electrophilicity to benzylic carbon. By converting C═O to C–NH2 groups and then tuning the quaternization degree, the remaining C–NH2 groups in the polymer backbone could form hydrogen bonding networks to reinforce the membranes. The novel quaternized poly(aryl ether amine) membrane exhibits high conductivity (92.2 mS cm–1 at 80 °C), excellent mechanical properties (48.2 MPa, 51.3%), and suppressed swelling ratio (12.8% at 80 °C). Excellent alkaline stability is also confirmed experimentally with no polymer backbone degradation even in harsh conditions (4 M KOH, 80 °C, 400 h), which is at the top level among the state-of-the-art side chain and aryl ether-free types of AEMs.