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

Abstract

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.