Near-frictionless ion transport within triazine framework membranes

Abstract

The authors develop a strategy that allows the diffusion limit of ions in water to be approached for large-area, free-standing, synthetic membranes using covalently bonded polymer frameworks with rigidity-confined ion channels. The enhancement of separation processes and electrochemical technologies such as water electrolysers^ 1 , 2 , fuel cells^ 3 , 4 , redox flow batteries^ 5 , 6 and ion-capture electrodialysis^ 7 depends on the development of low-resistance and high-selectivity ion-transport membranes. The transport of ions through these membranes depends on the overall energy barriers imposed by the collective interplay of pore architecture and pore–analyte interaction^ 8 , 9 . However, it remains challenging to design efficient, scaleable and low-cost selective ion-transport membranes that provide ion channels for low-energy-barrier transport. Here we pursue a strategy that allows the diffusion limit of ions in water to be approached 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 1.18 × 10^−9 m^2 s^–1, close to the value in pure water at infinite dilution, and an area-specific membrane resistance as low as 0.17 Ω cm^2. We demonstrate highly efficient membranes in rapidly charging aqueous organic redox flow batteries that deliver both high energy efficiency and high-capacity utilization at extremely high current densities (up to 500 mA cm^–2), and also that avoid 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.