Asymmetric Pore Engineering in Covalent Organic Framework Membrane for Effective Osmotic Energy Conversion

Reverse electrodialysis (RED) generates electrical energy from salinity gradients via selective ion transport across membranes. Covalent organic frameworks (COFs) are attractive membrane materials due to their well-defined porous architecture and robust covalent linkages. Particularly, imine-based COF membranes exhibit high ion selectivity and excellent structural stability from intramolecular hydrogen bonds and aligned channels; however, these hydrogen-bond networks create hydrophobic nanochannels that disturb the interaction with water molecules and limit ionic conductivity. Here, we introduce an imine-based COF membrane with asymmetric channel structures to improve the RED performance by modulating ion transport along the diffusion path. The asymmetric channels are formulated by treating one side of the membrane with an alkaline solution. This process partially hydrolyzes imine bonds, regenerates original functional groups, disrupts hydrogen bonding, and imparts hydrophilicity to the treated region while retaining the crystalline framework on the untreated side. The resulting gradient in pore chemistry enhances ionic conductivity without fully compromising selectivity. The optimized membrane achieves an output power density of 6.07 W/m² under a 50-fold salinity gradient─14.8 times higher than the pristine COF. These results demonstrate that localized chemical modifications of nanochannel environments can effectively modulate ion transport, providing a versatile strategy for designing asymmetric membranes with a tunable balance between conductivity and selectivity for advanced energy conversion and environmental applications.