Researchers from City University of Hong Kong developed a new type of droplet-based electricity generator, which is powerful enough to generate a voltage of over 140V, enough to light up 100 small LED bulbs, with a single drop of water.

A conventional droplet energy generator (DEG) based on the triboelectric effect can generate electricity induced by contact electrification and electrostatic induction when a droplet hits a surface. However, the amount of charge generated on the surface is limited by the interfacial effect, and as a result, the energy conversion efficiency is quite low.

The new DEG has a field-efficient transistor (FET)-like structure that allows for high energy-conversion efficiency, a thousand fold of other FET-like structures. The innovation lies in two crucial factors. Firstly, continuous droplets impinging on PTFE, an electret material with quasi-permanent electric charge, will cause saturation of the surface charge. This realisation helped the researchers overcome the bottleneck of the low-charge density encountered in previous work.

Secondly, the FET device consists of an aluminium electrode and an indium tin oxide electrode (ITO) with a film of PTFE on it. The latter is responsible for the charge generation, storage and induction. When a falling water droplet hits and spreads on the PTFE/ITO surface, it naturally “bridges” the aluminium electrode and the PTFE/ITO electrode, translating the original system into a closed-loop electric circuit.

With this special design, a high density of surface charge can be accumulated on the PTFE through continuous droplet impinging. Meanwhile, when the spreading water connects the two electrodes, all the stored charges on the PTFE can be fully released for the generation of electric current. As a result, both the instantaneous power density and energy conversion efficiency are much higher. A single drop of 100 microlitres, one-millionth of a litre, of water released from a height of 15 cm (5.9 inch) can generate a voltage of over 140V, enough power to light up 100 small LED lights.

The researchers hope this study will contribute to solve the global problem of renewable energy storage.