Sulfonate side chains grafted poly(terphenyl piperidine) membranes with improved chemical stability for vanadium redox flow battery applications

Ether-free ion exchange membranes with high ion selectivity and excellent oxidative stability are highly desirable for vanadium redox flow batteries (VRFBs), yet achieving a balanced combination of fast proton transport and suppressed vanadium ion crossover remains a major challenge. Herein, an ether-free, N -unsubstituted poly(terphenyl piperidine) (PTP) was synthesized via superacid-catalyzed polymerization and employed as the polymer backbone for membrane fabrication. Subsequently, sodium 2-bromoethanesulphonate (ES), 1,3-propane sultone (PS), and 1,4-butane sultone (BS) are grafted onto the active N H site in piperidine ring to introduce sulfonic acid side chains with different alkyl lengths and grafting degrees under mild conditions without additional catalysts. Unlike conventional N -methyl substituted poly(terphenyl N -methyl-4-piperidine)-based membranes, the grafted PTPs don't contain quaternary ammonium structures in polymer main chains, which improves oxidative stability in strongly acidic VO 2 + environments. The effects of side-chain alkyl length and sulfonation degree on membrane properties were systematically investigated. Among all samples, the PTP-55%PS membrane exhibited the best overall performance, achieving a low area resistance of 0.52 Ω cm 2 , low vanadium ion permeability of 2.0 × 10 −7 cm 2 min −1 and high ion selectivity of 4.0 × 10 4 S min cm −3 , which was approximately 13 times higher in ion selectivity than Nafion 115. When applied in VRFBs, the PTP-55%PS membrane delivered a maximum energy efficiency of 82.7% at 100 mA cm −2 and maintained coulombic efficiencies above 97% with stable electrochemical performance over 200 charge-discharge cycles. This work demonstrates that the regulation of sulfonic acid side-chain length and grafting degree of PTP provides an effective strategy for constructing high-performance membranes for VRFB applications.