Synergistically Cross-Linked, Antiswelling Cellulose Nanofibril Hydrogel Membranes Enabled by Metal Coordination for Efficient Osmotic Energy Conversion

Osmotic energy harvesting via reverse electrodialysis presents a promising “blue energy” solution. Charged hydrogel membranes, with their three-dimensional interconnected ion channels, are emerging as efficient osmotic energy conversion materials. However, their practical application is severely hampered by the inevitable swelling-induced loss of mechanical stability and ion selectivity. In this study, a “multilevel cross-linking” strategy is employed to fabricate a highly water-resistant, charged hydrogel ion-selective membrane. A hydrogen-bonded, covalent primary cross-linked polymer network is first constructed, followed by ion-enhanced cross-linking. This process yields a hydrogel with remarkable integrated performance, combining high mechanical strength with exceptional antiswelling properties─a tensile strength of 15.1 MPa (a 39-fold increase) and a swelling ratio of only 30% (a 13-fold reduction). Under a 50-fold KCl concentration gradient using a salt-bridge electrode, the membrane achieved an osmotic energy output power density of 1.08 W·m–2. No significant output decay was observed over a 30-day test period. In a practical demonstration, 12 series-connected osmotic energy harvesting units based on this hydrogel membrane generated a maximum voltage of 1.6 V, successfully powering electronic devices such as an LED and a calculator. This work presents a viable approach for fabricating water-stable, ion-selective hydrogel membranes with strong potential for stable renewable osmotic energy harvesting.