Quantitative Skeletal-Charge Engineering of Anion-Selective COF Membrane for Ultrahigh Osmotic Power Output

Osmotic energy contained in water bodies can generate abundant renewable electricity through reverse electrodialysis (RED) that relies on ion permselective membranes. Anion-permselective membrane RED offers a stable and sustainable energy output potential by maintaining a consistent driving force across the membrane, providing advantages in sustainability and versatility over cation-selective membranes. But significant challenges persist in developing anion-selective membranes that feature high selectivity and low impedance. Herein, this study presents the development of an anion-permselective osmotic power generation system using a free-standing chloride-selective covalent organic framework (COF) membrane. Inspired by biological chloride channels, the membrane is engineered with smooth, straight, and highly charged nanochannels for rapid chloride-anion transport. Its inner structure is stoichiometrically controlled to atomically distribute positive charges on the COF intraplane rings without introducing heterometal atoms or branch groups, enabling selective and efficient single-directional movement of anions. The RED device with this ionic-COF membrane achieves a remarkable output power density of 239.6 W m^-2, outperforming commercial benchmarks by 2 orders of magnitude, with low intermediate resistance under demanding gradients. Theoretical simulations corroborate that anion transport within ionic-COF membranes is governed by electrostatic interactions with the charged skeletons, thereby enhancing the anion selectivity and permeability. The findings highlight the potential of ionic-COF membranes for high-efficiency osmotic energy capture, demonstrating a substantial step toward sustainable and stable energy output from salinity gradients.