High‐Performance Flexible Cotton Fiber‐Based Water Evaporation‐Induced Generator: Sustainable Energy Harvesting Enabling Self‐Powered Ammonia Gas Sensing

In recent years, water evaporation‐induced generators (EEGs) have attracted significant attention for powering distributed sensor nodes. However, the complex fabrication of EEG devices remains a major bottleneck for industrialization. This study innovatively develops a high‐performance cotton‐fiber‐based EEG (CFEEG) using discarded shoelaces. The natural fibrous structure enables efficient capillary water absorption, yielding highly efficient and stable electrical output without additional processing, which demonstrates significant application advantages. Furthermore, coating the CFEEG surface with MXene enhances its output current. Experiments show that the CFEEG achieves an output power density of up to 220 nW cm − 2 at 25% relative humidity, setting a record for flexible plant‐based EEGs. Additionally, leveraging the room‐temperature gas‐sensing properties of MXene, the first self‐powered ammonia (NH 3 ) detection system based on an MXene‐coated CFEEG is reported. This system enables self‐powered NH 3 detection within the 1–100 ppm concentration range by directly monitoring changes in the CFEEG's output voltage without power management, achieving a response time of <20 s. This work establishes a novel “power generation–sensing” integrated paradigm, providing new insights for next‐generation self‐powered systems integrating power supply and sensing.