Robust Polymer Hydrogels Improve Electric‐Fish‐Inspired Batteries

ABSTRACT Electric‐fish‐inspired hydrogel batteries based on ion‐concentration gradients offer an attractive route to soft power sources; however, the poor mechanical properties of existing hydrogels limit device assembly and performance. Here, we report poly(ethylene glycol) methyl ether acrylate hydrogels that enable ion‐gradient batteries composed of thin, mechanically resilient layers. Using a photopolymerization process with LiCl to form high‐ and low‐salinity compartments, or with charged monomers to create ion‐selective membranes, we produce defect‐free layers with thicknesses as low as 117 µm. All components exhibit excellent tensile properties (elongation >300%), enabling facile handling and assembly. Reducing internal resistance through sequential curing and bilayer formation, and minimizing layer thickness, improves battery performance. Single‐cell devices exhibit open‐circuit voltages up to 211 mV and power densities up to 10.3 W·m −2 . Gravimetric and electrical measurements reveal pronounced self‐discharge under open‐circuit conditions, a general phenomenon in ion‐gradient hydrogel batteries driven by coupled ion and water transport. The batteries can be recharged repeatedly under fixed‐current conditions, but the discharge kinetics of recharged batteries differ from pristine devices, suggesting distinct underlying processes. Overall, this work establishes a robust and scalable hydrogel platform for next‐generation soft batteries and provides insights into mitigating self‐discharge and enabling rechargeability.