Enhanced Vanadium Redox Flow Battery Performance with New Amphoteric Ion Exchange Membranes

Vanadium redox flow batteries (VRFBs) depend on the separator membrane for their efficiency and cycle life. Herein, two amphoteric ion exchange membranes are synthesized, based on sulfonic acid group-grafted poly(p-terphenyl piperidinium), for VRFBs. Using ether-free poly(p-terphenyl piperidine) (PTP) as the polymer matrix, and sodium 2-bromoethanesulphonate (ES) and 1,4-butane sultone (BS) as grafting agents, We achieve quaternization of PTP through an environmentally friendly process without alkaline catalysts. PTP-ES and PTP-BS membranes exhibit low area resistance, high H⁺ permeability, and significantly reduced vanadium ion permeability, leading to exceptional ion selectivity, which is 3.06 × 10⁶ S min cm^-3 and 4.34 × 10⁶ S min cm^-3, respectively, three orders of magnitude higher than that of Nafion115 (0.27 × 10⁴ S min cm^-3). The VRFB with PTP-BS achieves a self-discharge duration of 190 h, compared to 86 h for Nafion 115. Additionally, under current densities of 40-160 mA cm^-2, PTP-BS shows coulombic efficiencies of 98.1-99.1% and energy efficiencies of 92.0-82.1%, outperforming Nafion 115. The VRFB with PTP-BS also demonstrates excellent cycle stability and discharge capacity retention over 300 cycles at 100 mA cm^-2. Therefore, the amphoteric PTP-BS membrane shows remarkable performance, offering significant potential for VRFB applications.