Passive Cooling Enabled Enduring Electricity Generation with a Single Water Droplet under Solar Exposure

Evaporative electricity generation (EEG) is a fast-emerging technology for sustainable energy supply by converting low-grade thermal energy from the surrounding environment. Unfortunately, all current EEG nanogenerators quickly cease functioning once they are disengaged from bulk water bodies, e.g., rivers and lakes, greatly limiting their wider geographical applicability. Realizing enduring electricity generation with only a single water droplet is greatly desired for their practical deployment. However, under practical outdoor scenarios, the inevitable solar exposure parches the device to inhibit the electricity generation ability due to the solar-thermal effect. Herein, we report a strategy that integrates the passive cooling capability and electricity harvesting ability into a bifunctional ceramic. The ceramic possesses a unidirectional and hierarchical porous structure that not only regulates water transport but also enhances sunlight scattering and thermal radiative emissivity. The self-cooling effect keeps the device constantly at a subambient temperature even under strong solar exposure with a peak temperature reduction of 14.2 °C. As a result, the ceramic-based nanogenerator can produce a voltage of 2.1 V for a long duration of 36 min with only 100 μL of water under outdoor solar intensity of 780 W m^-2. This work demonstrates the passive cooling-enabled enduring electricity generation device with only a single droplet, addressing the critical challenge in the practical viability of EEG technology.