Synergistic Tuning of Hydrophobic Tails of Linear Dication and Ionomer Functionalities for Effective Performance Enhancement of Polyethersulfone-Based Vanadium Redox Flow Batteries

Ion exchange membranes with excellent ion selectivity and chemical stability are essential to offer their vanadium redox flow batteries (VRFBs) with high working performances. Here, based on the highly conductive tetramethylethylenediaminium (TMEDA), a linear aliphatic dication, formerly developed by our group, four ionomers with various functionalities and hydrophobic alkyl tails have been tailor-made with the side aromatic methyl-free oxidation-stable backbones of chlorinated bisphenol A poly(ether sulfone) (Cl-BPA-PES) with tailored functionalities. They and their anion exchange membranes (AEMs) have been uniformly named QPAES-T/Cn-x (n is the number of carbon atoms in the tails, and x is the specific functionality). The synergistic tuning effects of varied ionomer functionalities and hydrophobic alkyl tails on the membrane basic properties and VRFB performances are explored. The results confirm that QPAES-T/C6–1 ionomer with the functionality of 1.25 and the TMEDA appending hydrophobic hexyl tails has the best synergistic effect, which endows the cognominal membrane with the best key basic properties, including good conductivity, low vanadium ion permeability, and good chemical stability. Naturally, the VRFBs assembled with QPAES-T/C6–1 show better comprehensive performance than those with the commercial perfluorosulfonic acid membrane NEPEM at 100 mA cm–2, giving a Coulombic efficiency of 98.07%, voltage efficiency of 84.52%, and an energy efficiency of 82.89%. Moreover, QPAES-T/C6–1 maintains a stable Coulombic efficiency higher than 98.0% during the 965 charge/discharge cycles with a total running time of more than 600 h. These facts confirm that grafting linear aliphatic dications with suitable hydrophobic alkyl tails to cheap and appropriately halogenated poly(ether sulfone) mother polymers, with absent side aromatic methyl groups, is an effective strategy to create nonfluoro AEMs with simultaneously improved ionic conductivity, vanadium ion permeation resistance, and chemical stability, promising for VRFB. The good electrochemical characteristics and chemical stability of QPAES-T/C6–1 make it competitive in VRFB applications.