Highly Ion-Selective Covalent Organic Framework/Cellulose Nanofiber Nanofluidic Membranes for Efficient Conversion of Osmotic Energy

Covalent organic frameworks (COFs) with tunable nanochannels and high charge density have shown great potential for constructing ion-selective membranes. However, COF membranes are limited by their mechanical brittleness and complex fabrication processes in large-scale applications. To address these limitations, we introduced highly ion-selective membranes composed of COFs and cellulose nanofibers (CNFs) for osmotic energy conversion. The COF/CNF nanofluidic membranes (CCNMs) were obtained by vacuum-assisted filtration of COF/CNF suspensions, where ionic COFs providing ion-selective channels were combined with high-strength CNFs through strong hydrogen bonding interactions. Consequently, the CCNMs achieved high ion selectivity (t₊ = 0.97) and power density (5.25 W m⁻²) under a 50-fold NaCl gradient, meeting the commercial benchmark for osmotic energy conversion (5 W m⁻²). Furthermore, the CCNMs exhibited stable open-circuit voltage (VOC), short-circuit current (ISC), and power density during continuous testing for 120 days, suggesting their excellent long-term stability. When used for harvesting osmotic energy between natural river water and seawater, as well as between natural river water and high-salinity lake water, power density values reached 5.24 and 12.47 W m⁻², respectively, highlighting the significant potential of CCNMs as robust and high-performance nanofluidic generators for osmotic power.