Versatile Ultrahigh‐Output Bilayer Hydrogel for Electricity Generation and Passive Cooling.

The gradient concentration in nature has garnered significant attention as a promising source for energy harvesting. Researchers have explored various methods to harness electricity from gradient‐concentration‐induced flows, including evaporation‐driven nanogenerators and humidity‐gradient‐based power generators. However, their low current and power density are the main obstacles toward practical applications. Herein, a Bilayer Hydrogel Electricity Generator (BHEG) is presented to enable efficient energy harvesting through the synergy between ion gradient concentration and galvanic effects. A BHEG unit employing identical materials for both electrodes demonstrates an open‐circuit voltage of 0.7 V and a short‐circuit current of 4.34 mA—surpassing the currently reported average by over 73 times—and achieves a maximum power density of 72.2 mW m−2. Moreover, another BHEG unit using Zn─C electrode materials exhibit an open‐circuit voltage of 1.86 V and a short‐circuit current of 92 mA. Furthermore, the versatility of the BHEG extends beyond power generation, effectively providing passive thermal management for electronics, resulting in a maximum temperature reduction of ≈20 °C. Consequently, the study contributes insights into the design and fabrication of efficient hydrogel‐based power generators, providing a promising avenue for leveraging natural‐flow‐induced energy for various applications. [ABSTRACT FROM AUTHOR]