Design High Performance Double Layer Ion Selective Membrane with Porous Boron Nitride and Polyetherimide

Flow batteries is one of the most attractive large-scale energy storage technologies, while the high cost and ion crossover issues of the commercial ion selective membranes limit scale-up of flow batteries. Herein, we designed a double-layer membrane with a unique bifunctional porous boron nitride (PBN) layer coated on a low cost porous polyetherimide (PEI) layer. PBN, synthesized through a one-step template free method from a precursor with boric acid and excess urea, has high nanoporosity and great stability. Through sonicating in isopropanol, PBN obtained hydroxyl and amino groups during the decomposition of the amorphous part, which increased the hydrophilicity and crystallinity simultaneously. PBN was further mixed with Nafion to constitute the ion selective layer, integrating ion selectivity with proton conductivity. The nanoporous PBN structure decorated by sulfonic acid groups from Nafion formed high-speed proton transfer channels. On the other hand, the nano-sized and tortuous channels from PBN effectively prevented the crossover of other ions and increased the ion selectivity. The ultra-thin (5 µm) PBN ion selective layer was coated on the high ion conductive and low-cost porous PEI layer prepared by nonsolvent induced phase separation method to obtain free-standing membranes with both high mechanical strength and ion conductivity. The PBN-PEI double layer membrane achieved superior coulombic efficiency of 97.02%, voltage efficiency of 93.79%, and energy efficiency of 91.18% at 40 mA cm^-2 in vanadium redox flow battery compared to the Nafion 115 membrane (coulombic efficiency of 95.01%, voltage efficiency of 92.32%, and energy efficiency of 87.71%). Meanwhile, the PBN-PEI displayed long cycling stability with 700 cycles at 100 mA cm^-2. This work presents a novel approach to designing ion selective membrane by imparting ion-exchange groups into the PBN inorganic porous structure to break the tradeoff between ion selectivity and ion conductivity and provides a thread for cost reduction by double-layer design.