Three-dimensional hydrogel membranes for boosting osmotic energy conversion: Spatial confinement and charge regulation induced by zirconium ion crosslinking

Ion-exchange membranes have been widely used to harvest osmotic energy in the past decades. However, conventional ion-exchange membranes suffer from low output power and poor conversion efficiency due to their limited pores and high membrane resistance. Herein, a sodium alginate (SA)/3-sulfopropyl acrylate potassium salt (SPAK) hydrogel membrane which has good cationic selectivity and can effectively harvest osmotic energy is designed, yielding a maximum power density of 16.44 W/m² under a 50-fold NaCl concentration gradient and 36.85 W/m² with ion selectivity of 0.73 at 500-fold. Furthermore, by introducing Zr⁴⁺, post-crosslinking reaction was employed to prepare tougher hydrogel membranes at room temperature for breaking a trade-off between selectivity and permeability, boosting a maximum power density up to 25.07 W/m² under a 50-fold NaCl concentration gradient and 121.66 W/m² with a high cation selectivity of 0.87 at 500-fold. Importantly, the resultant SA/SPAK/Zr⁴⁺ membrane reveals excellent osmotic energy harvesting property with the largest thickness of 500 μm, exceeding other reported porous nanofluidic membranes. Theoretical calculations correlate the enhanced power density of SA/SPAK/Zr⁴⁺ membranes with the enriched Cl^- and smaller pore size after the introduction of Zr⁴⁺. This work paves an avenue to design and develop the 3D hydrogel membranes for high-performance osmotic energy generators.