A novel sulfonated polyimide membranes inserting flexible decane chain and rigid triptycene-based crosslinked network for vanadium redox flow batteries

To resolve the trade-off between proton conductivity and vanadium ion permeability of sulfonated polyimide membranes, cross-linked structures bearing sulfonic acid groups were introduced in this work, and a novel cross-linked sulfonated polyimide membrane (CSPI-DH) with abundant sulfonic acid groups and functionalized cross-linked networks was fabricated. The incorporation of benzidine disulfonic acid cross-linking structure introduces additional sulfonic acid groups and creates more proton transport pathways, thus elevating the proton conductivity of the membrane. Meanwhile, the cross-linked framework effectively restrains vanadium ion penetration and markedly improves ion selectivity. Furthermore, cross-linking tightly connects polymer chains and greatly enhances membrane stability. The proton conductivity of CSPI-DH-5 reaches 0.110 S cm −1 , comparable to that of Nafion 212 (0.112 S cm −1 ). Its ion selectivity is up to 1.57 × 10 5 S min cm −3 , 3.7 times that of Nafion 212 (4.26 × 10 4 S min cm −3 ). At a high current density of 200 mA cm −2 , the single cell assembled with CSPI-DH-5 delivers an energy efficiency of 80.91 %. After 100 charge-discharge cycles at 200 mA cm −2 , CSPI-DH-5 retains 62.49 % of its initial discharge capacity. In addition, the Coulombic efficiency and energy efficiency remain relatively stable during 500 charge-discharge cycles. These results demonstrate that CSPI-DH-5 provides a favorable balance between proton transport, vanadium ion resistance, and electrochemical performance, although further optimization of its long-term durability is still required for practical VRFB applications.