Surface-Engineered Asymmetric Covalent Organic Framework Membranes with Polymers for Improved Ion Transport

Highly oriented two-dimensional (2D) covalent organic frameworks (COFs) possess aligned channels that facilitate efficient ion transport. This structural advantage makes COFs promising membrane materials for reverse electrodialysis (RED) systems. However, to develop ion-exchange membranes suitable for commercialization, it is essential to devise additional strategies that overcome the inherent trade-off between ion conductivity and selectivity. A promising strategy is to create asymmetric properties that enhance ion permeability by suppressing ion collisions while simultaneously improving ion selectivity by restricting reverse co-ion transport. To this end, we introduce polymers onto the COF surfaces to create an asymmetric structure with respect to charge density and hydrophilicity. We demonstrate that even minimal surface modification with polymer chains can induce meaningful asymmetry in membranes. The anchored polyelectrolytes characterized by strong negative charges and high hydrophilicity generate effective coordination sites for counterions, thereby enhancing ion transport and reducing the internal resistance of the membrane. In particular, the effect of surface modification becomes more pronounced with longer polymer chains, which possess more functional groups and exhibit a higher affinity for counterions, as revealed by molecular dynamics simulations. Consequently, the polymer-integrated COF membrane with relatively long polymer chains achieves more than a 15-fold performance enhancement, which is well maintained more than 50 days. This study highlights how simple and subtle tuning of surface properties can effectively modulate ion transport behavior and enhance membrane performance for osmotic power generation.