Sustainable and portable kapok-based hydroelectric generators with high power density for wearable applications

Natural fibers offer great opportunities for broad sustainable applications due to their unique microstructures and environmental abundance. In this study, we present the Raw-Kapok Cell Generator (RCG), a device that generates electricity through water evaporation based on the raw kapok fibers without the delignification treatment. Benefiting from its hydrophilic inner wall and hydrophobic outer shell, each kapok fiber can act as an individual power unit. By assembling these fibers in an aligned manner, the RCG can achieve an continuous output power density as high as 4.5 μW/cm 2 after optimizing the physical dimensions, fiber alignment, and packing density. It is also revealed that the continuous electrical output is driven by the contact electrification at water-fiber interface and water evaporation-induced charge migration, which in turn is strongly dependent on fiber microstructures and water transport. This work demonstrates the potential of kapok fibers for eco-friendly and reliable energy generation, representing a green solution of portable power sources for wearable devices. • Natural kapok fiber efficiently generates power through water evaporation. • Each kapok fiber is an independent power cell due to its special structure. • RCG units achieve a peak power output of 4.5 μW/cm 2 . • Only 3 drops of water sustain a stable output for 11 h.