A high voltage direct current droplet-based electricity generator inspired by thunderbolts

Relatively low output of alternating current (AC) pulsed electricity generated by impinging a water droplet on a hydrophobic surface is difficult for practical utilization. Though high peak power density has been achieved by using a delicate transistor-like architecture and further been simplified to single electrode, the output is still AC pulsed. Methods have been developed to achieve direct current (DC) output, but either an integrated mechanical switch or an air breakdown electrode is necessary. Furthermore, additional rectification, conversion and storage components are prerequisites for further application. Here we demonstrate a DC droplet-based electricity generator to eliminate the unavoidable energy loss by integrating the rectification, energy conversion and storage components into the same device, through a simple yet delicate structure. We show that the water droplet can not only act as a source of charge generation in the process of contact electrification and electrostatic induction, but also act as a charge shuttle by adjusting its gravity-driven movement to sequentially deliver negative charges to the top charge reservoirs and positive charges to the bottom charge reservoirs, outputting DC electricity. Then the water droplet returns to its initial neutral state. A high output voltage over 1600 V has been achieved without any power management circuit. 400 LEDs can be lightened up by a single droplet with volume of 80 μL, showing a great potential for real-life application on water energy harvesting.