Acid-Induced Protonation Engineering of Hydrogels for High-Performance Salinity Gradient Energy Harvesting

Direct conversion of Gibbs free energy from salinity gradients into electrical power through ion-exchange membranes holds great promise for mitigating the energy crisis. However, traditional ion-exchange membranes generally suffer from high internal resistance, poor ion selectivity, and low ion permeability, resulting in suboptimal power density and limiting their practical applications. Herein, we develop a series of ion-selective membranes with a three-dimensional (3D) interconnected network for salinity gradient energy (SGE) harvesting by incorporating acidic anions with different electronegativity into a poly(vinyl alcohol) (PVA)-chitosan (CS)-acrylamide (AM) hydrogel. Compared with the weak acidic compound (acrylic acid, AA), the stronger acidic anions (methanesulfonic acid, MSA, and phosphoric acid, H 3 PO 4 ) can effectively improve ion selectivity and permeability, thereby enhancing the osmotic energy conversion efficiency. The results show that the MSA-modified (NS) and H 3 PO 4 -modified (NP) hydrogels achieved maximum power outputs of 29.15 and 16.96 W m –2, respectively, at a 50-fold concentration gradient (0.5 M/0.01 M NaCl), significantly exceeding the commercial benchmark of 5 W m –2 . The acid-modified hydrogels also maintained good structural integrity and stable output during long-term operation. This composite hydrogel fabrication strategy offers a viable, cost-effective, and scalable approach to boost SGE conversion in ion-selective membranes, paving the way for their application in sustainable energy harvesting.