Influence of Multiscale Tuning in Membrane Structure of Poly(arylene ether) Anion Exchange Membranes on Their Performance in Vanadium Redox Flow Batteries

Poly(arylene ether)-based anion exchange membranes (AEMs) exhibit superior vanadium ion resistance and lower cost than perfluorosulfonic acid (PFSA) membranes in the field of vanadium redox flow batteries (VRFBs), but they are challenging in good balance of oxidation stability, conductivity, and vanadium blocking. In this work, we have tailored five kinds of AEM-based commercial poly(arylene ether)s to reveal the effects of multiscale tuning in membrane structure on balancing capacities in fundamental membrane properties and their VRFB performance. The considering three scales lie in the backbones with or without side arylmethyl, cationic groups with methyl or octadecyl, and interfacial interactions between the pore-filling matrix of micro-nanoporous polyethylene (PE) ultrathin film and the cognate ionomers with various side chains of the cation groups. The results declare that interfacial interactions resulting from the van der Waals forces between weak polar alkyls and the PE interfaces positively reduce vanadium ion permeability owing to their control of the excessive swelling of the filled ionomers, but they are not strong enough to stand the operation of over a hundred circles due to the failure in resolving the oxidation of side arylmethyl groups. Instead, the combination of conjugated imidazolium (IM) and oxidatively inert backbone of side arylmethyl-free poly(arylene ether sulfone) (BPA-PES) endows the derivate AEM of BPA-PES-IM with good oxidative stability, conductivity, and lower vanadium ion permeability, affording longer self-discharge time, higher Coulombic and energy efficiencies than those of commercial NEPEM, and a long-time stable 375-circle operation (about 212 h) of VRFB. These results confirm that combining arylmethyl-free poly(arylene ether sulfone) with imidazolium is a promising strategy to balance the cost and the application performance in VRFBs.