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Triboelectric Nanogenerators

April 9, 2018

Winter in the Northeastern United States, where Tikalon is based, is responsible for more discomfort than cold weather. The low humidity permits the discharge of huge electrical sparks from fingers to switchplates. Rubbing of the dissimilar materials of socks, slippers, or shoes on carpeting generates electrical charges by the triboelectric effect, and these charges flow to ground through the grounded switchplate. I wrote about the triboelectric effect in two earlier articles (Triboelectric Generators, July 25, 2012, and Triboelectric Generators, February 8, 2016).

The triboelectric effect was known in ancient Greece at least as early as 600 BC when Thales of Miletus (c. 600 BC) observed that certain combinations of materials were better at spark production than others. In particular, the combination of rabbit fur and amber was especially good. The word, electron, comes from the Greek word for amber, ηλεκτρον. Materials are better at spark production when they're widely separated in the triboelectric series, shown in the diagram below.

The triboelectric series.

The triboelectric series. Items high on this list have a tendency to donate electrons to items lower on the list, and the separation of materials on this list indicates the intensity of the triboelectric effect. (Drawn using Inkscape from Wikipedia data.)


While the word triboelectric comes from the Greek verb for rubbing, tribo, (τριβω), rubbing is not required. It's just necessary for the dissimilar materials to contact each other, then separate. Chemical bonds formed between the materials on contact will then separate charges when they're pulled apart, although rubbing helps by increasing the contact area. Engineers, of course, endeavor to make use of physical effects, so the British engineer, James Wimshurst (1832-1903), built an electrical generator called the Wimshurst machine that produced electricity by rubbing two disks together to generate charges collected by metal patches and directed to electrodes.

An improved version of this rubbing concept was devised by Princeton University physics professor, Robert Van de Graaff(1901-1967), to generate the high voltages needed to accelerate particles in nuclear physics experiments. His eponymous Van de Graaff generator is very simple, as the figure shows.

Diagram of a Van De Graaf generator



Diagram of a Van de Graaff generator

In this device, a pulley acts as one member of a triboelectric couple with an insulating belt.

Charge is transferred through small gaps between the belt and the brushes.

(Modified Wikimedia Commons image).)


Two years ago, I used the triboelectric effect to implement a switch.[1] This switch used the rubbing of Teflon® (PTFE) against Kapton (polyimide) to generate sufficient voltage to toggle a CMOS 4013 set-reset flip-flop. The power needed to trigger this digital circuit is small, essentially five volts into a 3.3-megohm load resistance. Appealing to the power formula, P = V2/R, shows that the required power is of the order of ten microwatts. This underscores one problem with triboelectric generators - Their output power is very small.

A triboelectric switch

My triboelectric switch, as published in Electronic Design Magazine.[1] Voltage is generated by pushing in either of the directions shown, which causes sliding of the Kapton against the Teflon® (PTFE). The generated charges are collected by copper electrodes. (Created using Inkscape)


There is much electronic circuitry, however, that requires very little operating power. In some cases, a small amount of power, generated over a long period of time and stored, can be used to transmit wireless data. In 2012, a research team at the Georgia Institute of Technology (Atlanta, GA) created a triboelectric generator from a sheet of polyester, an electron donor, rubbing against a sheet of polydimethylsiloxane, an electron acceptor.[2-3]

Triboelectric
efficiency was enhanced in the Georgia Tech device by micropatterning the surfaces (see photograph). Continually rubbing and separating the micropatterned sheets generated a small alternating current of about 0.10 microamps per square centimeter at nearly 20 volts, giving a peak power of 2 microwatts per square centimeter.[2-3]

Micrograph of Georgia Tech triboelectric material

Micrograph of pyramidal patterns created in a polymer sheet for the Georgia Tech triboelectric generator.

(Georgia Tech image by Zhong Lin Wang).[2]


Polydimethylsiloxane has also been used to produce a triboelectric nanogenerator by a research team with members from the Chinese Academy of Sciences (Beijing, China) and the State University of New York (Buffalo, New York).[4-5] This generator, based on a crumpled gold film, gave a maximum voltage of 124.6 V, and a maximum current of 10.13 μA.[4] The maximum power density achieved for this device, 0.22 mW/cm2, is two orders of magnitude greater than that of the Georgia Tech device.[4] This research is reported in a recent issue of Nano Energy.[4]

The crumpled gold film makes the nanogenerator bendable, a feature that's desireable for self-powered wearable electronics.[4] Reliability testing showed no obvious degradation in performance after a half-year's storage in an ambient environment.[4] Says Qiaoqiang Gan, associate professor of electrical engineering at Buffalo's School of Engineering and Applied Sciences and lead author of the paper,
"No one likes being tethered to a power outlet or lugging around a portable charger. The human body is an abundant source of energy. We thought: 'Why not harness it to produce our own power?'"[5]

The device structure is simple, consisting of polydimethylsiloxane sandwiched between two thin layers of gold.[5] One of the gold layers is initially stretched, so it crumples upon release, creating a nanostructured surface. An applied force, as from a finger bending, causes rubbing friction between the gold layers and the polydimethylsiloxane.[5] The triboelectricity produced by the rubbing produces charges at the gold layers. A 1.5 centimeter long by 1 centimeter wide device was able to flash a string of 48 red LEDs.[5]

Bendable triboelectric nanogenerator

Bendable triboelectric nanogenerator.

Image - State University of New York at Buffalo/Nano Energy


Several things can make such triboelectric generators more useful. One of these is a way to store generated power, and the research team is working on a compatible battery.[5] Another research team from the Chinese Academy of Sciences is working on high voltage diodes with high rectification ratio and high breakdown voltage to aid in energy management of such generators.[6-7] The triboelectric nanogenerator research was supported by the US National Science Foundation and various Chinese agencies.[5]

References:

  1. Dev Gualtieri, "Simple, Novel Switch Exploits Triboelectric Effect," Electronic Design, June 3, 2016.
  2. John Toon, "Plastic Power: Triboelectric Generator Produces Electricity by Harnessing Frictional Forces Between Transparent Polymer Surfaces," Georgia Tech Press Release, July 9, 2012.
  3. Feng-Ru Fan, Long Lin, Guang Zhu, Wenzhuo Wu, Rui Zhang, and Zhong Lin Wang, "Transparent Triboelectric Nanogenerators and Self-Powered Pressure Sensors Based on Micropatterned Plastic Films," Nano Letters, vol. 12, no. 6 (June 13, 2012), pp. 3109-3114.
  4. Huamin Chen, Lin Bai, Tong Li, Chen Zhao, Jiushuang Zhang, Nan Zhang, Guofeng Song, Qiaoqiang Ganc, and Yun Xua, "Wearable and robust triboelectric nanogenerator based on crumpled gold films," Nano Energy, vol. 46 (April, 2018), pp. 73-80, https://doi.org/10.1016/j.nanoen.2018.01.032.
  5. Your gadget's next power supply? Your body, State University of New York, Buffalo, Press Release, February 9, 2018.
  6. Yonghui Zhang, Zengxia Mei, Tao Wang, Wenxing Huo, Shujuan Cui, Huili Liang, and Xiaolong Du, "Flexible transparent high-voltage diodes for energy management in wearable electronics," Nano Energy, vol. 40 (October, 2017), pp. 289-299, https://doi.org/10.1016/j.nanoen.2017.08.025.
  7. Yonghui Zhang, Zengxia Mei, Tao Wang, Wenxing Huo, Shujuan Cui, Huili Liang, and Xiaolong Du, "Flexible transparent high-voltage diodes for energy management in wearable electronics," arXiv September 20, 2017.

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