Cross-linked amphoteric membrane: Sulphonated poly(ether ether ketone) grafted with 2,4,6-tris(dimethylaminomethyl)phenol using functionalized side chain spacers for vanadium redox flow battery

Herein, we report a novel synthesis of sulphonated poly(ether ether ketone) (SPK) grafted with 2,4,6-tris(dimethylaminomethyl)phenol (TAP) for preparation of amphoteric ion-exchange membrane (AIEM). Cross-linking has been achieved by using di-bromo alkane with different number of carbon chains (C6 & C10), and different AIEMs are assessed for vanadium redox flow battery (VRFB). These AIEMs are architected to contain -SO₃H groups maintained conductivity, while -NH₃⁺ groups for reduction in vanadium permeability with improve charge-discharge time via acid-base interactions. Structural and surface morphology of AIEMs have been elucidated by different spectral techniques (FTIR, ¹H NMR, SEM, and AFM). Cross-linking and acid-base interactions effectively reduced water uptake, swelling ratio, VO⁺² permeability and improved ion-exchange capacity, conductivity, along with membrane stabilities. Due to good synergy between conductivity and VO⁺² permeability, cross-linked AIEM with six number of carbon chains (CQSPK-6) exhibited higher Coulombic (CE: 98.4% vs 91.2%) and energy efficiency (81.4% vs~76%) than Nafion 117 membrane at 60 mA cm^-2 in single cell VRFB. Further, CQSPK-6 AIEM maintained stable performance during in-situ cycle test (100 cycles) at 60 mA cm^-2. Hence, AIEM based on cross-linked SPK grafted with high molality of functional groups via side chain spacer is a promising candidate for an efficient VRFB system.