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, H3PO4) can effectively improve ion selectivity and permeability, thereby enhancing the osmotic energy conversion efficiency. The results show that the MSA-modified (NS) and H3PO4-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.