Proton Conducting Organic-Inorganic Composite Membranes for All-Vanadium Redox Flow Battery

The quest for a cost-effective, chemically-inert, robust and proton conducting membrane for flow batteries is at its paramount. Perfluorinated membranes suffer severe electrolyte diffusion, whereas conductivity and dimensional stability in engineered thermoplastics depend on the degree of functionalization. Herein, we report surface-modified thermally crosslinked polyvinyl alcohol-silica (PVA-SiO₂) membranes for the vanadium redox flow battery (VRFB). Hygroscopic, proton-storing metal oxides such as SiO₂, ZrO₂ and SnO₂ were coated on the membranes via the acid-catalyzed sol-gel strategy. The membranes of PVA-SiO₂-Si, PVA-SiO₂-Zr and PVA-SiO₂-Sn demonstrated excellent oxidative stability in 2 M H₂SO₄ containing 1.5 M VO₂⁺ ions. The metal oxide layer had good influence on conductivity and zeta potential values. The observed trend for conductivity and zeta potential values was PVA-SiO₂-Sn > PVA-SiO₂-Si > PVA-SiO₂-Zr. In VRFB, the membranes showcased higher Coulombic efficiency than Nafion-117 and stable energy efficiencies over 200 cycles at the 100 mA cm^-2 current density. The order of average capacity decay per cycle was PVA-SiO₂-Zr < PVA-SiO₂-Sn < PVA-SiO₂-Si < Nafion-117. PVA-SiO₂-Sn had the highest power density of 260 mW cm^-2, while the self-discharge for PVA-SiO₂-Zr was ~3 times higher than Nafion-117. VRFB performance reflects the potential of the facile surface modification technique to design advanced membranes for energy device applications.