Highly Stretchable Hydrogel Membranes with Metal‐Ion Coordination for Scalable Stable Osmotic Energy Conversion

Abstract Hydrogels show promise in osmotic energy harvesting, yet most studies focus on small membrane areas (≤0.1 mm 2 ), limiting their practical application. Larger areas are facing challenges like salinity gradient dilution, extended ion transport paths, reduced ion selectivity, and ion concentration polarization, diminishing energy conversion efficiency. Herein, a highly stretchable cation‐selective membrane using polyacrylamide‐acrylic acid‐vinyl imidazole‐metal ions (PAM‐AA‐VI‐Metal) hydrogel via combined physical and chemical cross‐linking is developed. Hydrogen bonding stabilizes physical cross‐linking, while metal‐ion coordination and olefins enhance chemical cross‐linking, improving membrane robustness and osmotic energy conversion. These membranes exhibited excellent tensile strength, maintained structural integrity, and swelled less than 1% over a week in saline solutions. The synthesized hydrogel demonstrated a high output power density of 36.35 W m −2 at 50‐fold under a testing radius of 0.1 mm ( ≈ 0.03 mm 2 ). The testing area of our membrane can be scaled up to 177 mm 2 , and the power density can be stabilized at 0.1 W m −2 (5/0.5 m NaCl), surpassing the value of 0.003 W m −2 obtained at a traditional seawater/river water gradient (0.5/0.01 m NaCl). This work indicates that hypersaline conditions mitigate scalable issues, stabilizing ion flow, power output, and offering a pathway toward scalable osmotic energy conversion of hydrogels.