Electric‐Fish‐Inspired Thin Hydrogel Electrocytes Achieve High Power Density and Environmental Robustness

Electric-fish-inspired hydrogel-based power sources offer a promising platform for powering soft, wearable, and implantable electronics due to their compliance, biocompatibility, and biodegradability. They typically consist of high- and low-salinity gel layers separated by anion- and cation-selective gel compartments, generating an electric potential that emulates the diffusion-based energy mechanisms of electrocytes in electric fish. However, their development has been hindered by high internal resistance, limited power density, and poor environmental stability. Here, a scalable layer-by-layer spin-coating strategy is introduced to fabricate hydrogel electrocytes with precise thickness control, yielding 106.1 µm-thick units comparable to biological electrocytes. This thin architecture significantly reduces resistance and enables high instantaneous power density (44.0 kW m^-3) with low area-normalized resistance (2.0 × 10^-3 Ω m².). By tailoring the hydrogel composition with a glycerol-carboxylated chitosan mixture, long-term hydration (>98.7% after 120 h at 60% RH) and antifreezing performance down to -80 °C are achieved without encapsulation. Furthermore, varying layer thickness provides tunable energy density, while integration of PEDOT:PSS hydrogel electrodes preserves material compliance and yields robust, ready-to-use power systems. These advances overcome critical barriers in hydrogel-based energy storage, establishing a versatile, scalable pathway toward stable, bioinspired power sources for next-generation wearable, implantable, and autonomous devices.