Influence of Typical Cation Structures of the Functionalized Poly(phenylene ether)s as Anion Exchange Membranes in Vanadium Redox Flow Batteries

The exploitation of anion exchange membranes (AEMs) based on tailored ionomers has become an investigating hotspot of the membranes for vanadium redox flow batteries (VRFB), but it is challenging to guide the design of AEMs for VRFB based on the reported findings about combinations of specific backbones and ions with functionality tuning. Therefore, a series of typical cationic groups of saturated monocationic and dicationic quaternary ammoniums and imidazolium with good oxidation resistance have chosen to modify poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) to give a tailor-made series of cognate ionomers PPO-x (x denotes a certain cation, including benzylbutyldimethylammonium (DMBA), linear dication of N1-benzyl-N2-butyl-N1,N1,N2,N2-tetramethylethane-1,2-diaminium (TMEDA-Bu) and caged dication of 1-benzyl-4-butyl-1,4-diazabicyclo[2,2,2]-octan-1,4-diium (DABCO-Bu), and N-methylimidazolium (IM) of the conjugated five-membered heterocycle) at the close functionality of about 0.35. The results reveal that the conjugated structure of N-methylimidazolium ions affords PPO-IM better VRFB performance than those of the other cognates and the commercial perfluorosulfonic acid membrane because of its better conductivity, vanadium permeation resistance, and good oxidation resistance. The butyl modification of saturated monoquaternary ammonium further improves oxidation resistance and self-discharge duration in its application of VRFB. These results enlighten that the coupling strategy of short side-chain-modified imidazolium groups and oxidation-stable polymer backbones is promising to make progress in VRFB technology.