Near-frictionless ion transport from rigid nanoconfinement by triazine framework membranes

Abstract Many separation, purification processes and electrochemical technologies such as water electrolysers1,2, fuel cells3,4, redox flow battery5,6 and ion-capture electrodialysis7 hinge on the development of low-resistance and high-selectivity ion-transport membranes (SIMs). The transport of ions through SIMs depends on the overall energy barrier which are imposed by the collective interplay of pore architecture and pore-analytes interaction8,9. However, it remains challenging to design efficient, scalable and low-cost SIMs with low-energy-barrier ion channels. Here we pursue a strategy to approach the diffusion limit of ions in water for large-area free-standing synthetic membranes using covalently bonded polymer frameworks with rigidity-confined ion channels. The near-frictionless ion flow is synergistically fulfilled by robust micropore confinement and multi-interaction between ion and membrane, which afford, for instance, a Na+ diffusion coefficient of is 1.18×10-9 square meter per second, close to the value in pure water at infinite dilution and an area-specific membrane resistance as low as 0.17 Ohm square centimeter. We demonstrate the great efficacy of membranes in fast-charging aqueous organic redox flow batteries that deliver both high energy efficiency and high capacity utilization at extremely high current densities (up to 500 milliampere per square centimeter), while avoiding crossover-induced capacity decay. This membrane design concept may be broadly applicable to membranes for a wide range of electrochemical devices and for precise molecular separation.