Hydrogen bond-mediated assembly of homo-charged COF nanosheets and polyelectrolytes towards robust Li+/Mg2+ separation membrane

Developing membranes with ordered channels and high positive charge density is crucial for Li⁺/Mg²⁺ separation. Ionic covalent organic framework (COF) membranes are promising candidates, yet they face challenges like pore size mismatch with ions and the liable structural defects. Herein, we proposed a hydrogen bond-mediated strategy to assemble membranes from homo-charged COF nanosheets and polyelectrolytes. Compared with the quaternary amines in poly (diallyl dimethyl ammonium chloride), the abundant primary and secondary amines in polyethyleneimine facilitate multiple hydrogen bonding interactions with COF nanosheets. These interactions effectively overcome the electrostatic repulsion between positive charges, endowing membrane with structural robustness. Furthermore, the intercalation of polyelectrolytes eliminates the structural defects, reduces the membrane pore size, and enhances the Donnan effect. The optimized COF membrane exhibited a pure water flux of 10.2 L m⁻² h⁻¹ bar⁻¹, separation factor of up to 30 at high Mg²⁺/Li⁺ mass ratio of 100, and excellent stability under various operating conditions. Strikingly, our strategy facilitates the fabrication of membranes in large area (>450 cm²) while maintaining consistent separation performance, showcasing substantial potential of scalable manufacturing.