Amphiprotic Side-Chain Functionalization Constructing Highly Proton/Vanadium-Selective Transport Channels for High-Performance Membranes in Vanadium Redox Flow Batteries

A novel amphiprotic side-chain-functionalized membrane was for the first time designed for vanadium redox flow battery (VFB). Different from frequently used blending amphiprotic membranes, the one proposed here is allowed to possess high anion-exchange capacity (IEC_a) without sacrificing the cation-exchange capacity (IEC_c) because both IEC_a and IEC_c increased with the grafting degree of side chains. Having a high IEC_a, the membrane prepared here exhibits an ultralow vanadium permeability (<10^-8 cm² s^-1), which leads to very high Coulombic efficiencies (97-98% at 40-200 mA cm^-2) of VFB and good cell self-discharge durability. Moreover, the high IEC_c contributes to a decent ionic conductivity (area resistance: 0.5 Ω cm^-2), which ensures a high-voltage efficiency of the cell. On the basis of these good properties, the VFB single cell with this membrane achieves a high energy efficiency (e.g., 77.4% at 200 mA cm^-2) that is higher than those of Nafion 212 and other reported amphiprotic membranes. These results indicate that the approach proposed here is an ideal option to prepare amphiprotic membranes for VFBs with high efficiency and good durability.