Partial Fluorinated Zwitterionic PVDF-<i>co</i>-HFP Copolymer Membrane Doped with Acid-Functionalized Graphene Oxide for Vanadium Redox Flow Batteries

For energy storage, vanadium redox flow batteries (VRFBs) are very efficient, while stability and durability of membrane separator control battery performance. We synthesized sulfonated propyltrimethoxysilane graphene oxide (SPGO) as a nanofiller. Furthermore, the zwitterionic monomer [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammoniumhydroxide (DMAPS) was chemically grafted with partially fluorinated PVDF-co-HFP copolymer by free radical reaction with a 42.17% degree of grafting. Different zwitterionic membranes (P-DMAPS/SPGO) were prepared by incorporating SPGO as a nanofiller, and observed that the membrane with 3.0 wt % SPGO nanofiller (P-DMAPS/SPGO-3.0) exhibits superior VRFB performance. Incorporation of SPGO nanofillers significantly improved membrane stability due to the appropriate arrangement of functional groups responsible for hydrogen bonding. The suitably optimized P-DMAPS/SPGO-3.0 membrane exhibited excellent VRFB performance in a single cell with 98.23% Coulombic efficiency (CE) and 84.21% energy efficiency (EE) without any deterioration in the performance up to 350 cycles, which were significantly high in comparison with commercial Nafion-117 membrane (CE: 91.02%, EE: 70.84%). This was attributed to good ionic conductivity (9.89 × 10–2 S cm–1) and remarkably low vanadium ion permeability (5.33 × 10–7 cm2 min–1) P-DMAPS/SPGO-3.0 zwitterionic membrane. Polarization curve of the P-DMAPS/SPGO-3.0 membrane also revealed a 442.8 mW/cm2 peak power density at 540 mA/cm2. In addition to VRFB, the synthesized ultrarobust and high-performance zwitterionic membranes may also find potential applications in diversified electrochemical applications.