High Ion Conductive and Selective Membrane Achieved through Dual Ion Conducting Mechanisms

Conventional ion-selective membranes, that is ion-exchange and porous membranes, are unable to perform high conductivity and selectivity simultaneously due to the contradictions between their ion selecting and conducting mechanisms. In this work, a bifunctional ion-selective layer is developed via the combination of nanoporous boron nitride (PBN) and ion exchange groups from Nafion to achieve high ion conductivity through dual ion conducting mechanisms as well as high ion selectivity. A template-free method is adopted to synthesize flake-like PBN, which is further enmeshed with Nafion resin to form the bifunctional layer coated onto a porous polyetherimide membrane. The double-layer membrane exhibits excellent ion selectivity (1.49 × 10⁸ mS cm^-3  min), which is 22 times greater than that of the pristine porous polyetherimide membrane, with outstanding ion conductivity (64 mS cm^-1 ). In a vanadium flow battery, the double-layer membrane achieves a high Coulombic efficiency of 97% and outstanding energy efficiency of 91% at 40 mA cm^-2 with a stable cycling performance for over 700 cycles at 100 mA cm^-2 . PBN with ion exchange groups may therefore offer a potential solution to the limitation between ion selectivity and conductivity in ion-selective membranes.