Light up 100 LEDs with a drop of water

Print edition : February 28, 2020

Schematic diagrams showing (a) a droplet-based electricity generator (DEG) and (b) four parallel DEG devices fabricated on a glass substrate. Photo: City University of Hong Kong / Nature

A TEAM led by scientists from the City University of Hong Kong (CityU) has developed a droplet-based electricity generator (DEG), featured with a field-effect transistor (FET)-like structure that allows for high energy-conversion efficiency and instantaneous power density increased by thousands of times compared with its counterparts without an FET-like structure. This would help to advance scientific research of water energy generation and tackle the energy crisis, said the release from CityU. The results of this collaborative research by scientists from CityU, University of Nebraska-Lincoln and the Beijing Institute of Nanoenergy and Nanosystems were published in a recent issue of “Nature”.

While hydropower is not new, low-frequency kinetic energy contained in waves, tides and even raindrops are not efficiently converted into electrical energy owing to limitations in current technology. For example, a conventional DEG based on the triboelectric effect can generate electricity induced by contact electrification and electrostatic induction when a water drop hits a surface. However, the amount of charges generated on the surface is limited by the interfacial effect, and as a result, the energy conversion efficiency is quite low. In order to improve the conversion efficiency, the research team spent two years developing the new DEG. Its instantaneous power density can reach up to 50.1 W/m2, a thousand times higher than other similar devices without the use of FET-like design. And the energy conversion efficiency is markedly higher.

Wang Zuankai from CityU pointed out that there were two crucial factors for the invention. First, the team found that droplets continuously impinging on the surface of polytetrafluoroethylene (PTFE), an electret material with a quasi-permanent electric charge, provided a new route for the accumulation and storage of high-density surface charges. They found that the surface charges generated will accumulate and gradually reach saturation. This new discovery helped overcome the bottleneck of low-charge density encountered in previous work. Another key feature of their design is a unique set of structures similar to a FET, the basic building block of modern electronic devices. The device consists of an aluminium electrode and an indium tin oxide (ITO) electrode with a film of PTFE deposited on it. The PTFE/ITO electrode is responsible for the charge generation, storage and induction. When a falling water droplet hits and spreads on the PTFE/ITO surface, it “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 charges 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. “Our research shows that a drop of 100 microlitres of water released from a height of 15 cm can generate a voltage of over 140 V. And the power generated can light up 100 small LED light bulbs,” said Wang.

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