Heterogeneous Covalent Organic Framework Membranes Mediated by Polycations for Efficient Ions Separation

Abstract Precise ions sieving at angstrom‐scale is gaining tremendous attention thanks to its significant impact at the water‐energy nexus. Herein, a novel polycation‐modulated interfacial polymerization (IP) strategy is developed to prepare a heterogeneously charged covalent organic frameworks (COFs) membrane. Cationic poly(diallyldimethylammonium chloride) (PDDA) regulates the growth and assembly of anionic COFs nanosheets, which thus provides a negative, smooth top surface and positive, rough bottom surface, indicating the presence of heterogeneously charged angstrom‐scale channels through the membrane. Experiments and simulations are conducted to understand the facilitated ions transport behavior relative to specific interactions raised by heterogeneously charged channels and angstrom‐scale steric hinderance as well, rendering the membrane with robust mono‐/divalent cations sieving capabilities. The selectivity (61.6) of Li + to Mg 2+ in mixed saline under the continuous cross‐flow filtration mode is superior to most of the reported nanofiltration membranes. This polycation‐mediated interfacial polymerization strategy offers a compelling opportunity to develop versatile heterogeneously charged COF membranes for exquisite ion sieving.