Designing Self-Powered Triboelectric Nanogenerators Based on Triboelectric Effect

Many of us would have felt the sting while touching a doorknob after walking across a carpet or witnessed the way a balloon sticks to a fuzzy surface after being rubbed against another material.

Georgia Tech professor Zhong Lin Wang poses with an array of 1,000 LED lights that can be illuminated by power produced by the force of a shoe striking a triboelectric generator placed on the floor. (Image credit: Rob Felt, Georgia Tech)

Although the effects of static electricity have had researchers and casual observers spellbound for millennia, specific particulars on the way the electricity is produced and stored on surfaces have always been mysterious.

At present, scientists have unearthed greater details on the way specific materials store a charge even after the separation of two surfaces. This information can help enhance devices that tap such energy to be their source of power.

We’ve known that energy generated in contact electrification is readily retained by the material as electrostatic charges for hours at room temperature. Our research showed that there’s a potential barrier at the surface that prevents the charges generated from flowing back to the solid where they were from or escaping from the surface after the contacting.

Zhong Lin Wang, Regents’ Professor in the School of Materials Science and Engineering at the Georgia Institute of Technology

In a study published in the Advanced Materials journal in March, the scientists discovered that electron transfer is the principal process for contact electrification between two inorganic solids and elucidates on specific properties of static electricity observed earlier.

There has been some debate around contact electrification—namely, whether the charge transfer occurs through electrons or ions and why the charges retain on the surface without a quick dissipation,” stated Wang.

Wang and his colleagues first reported the study on triboelectric nanogenerators eight years ago, which involves using materials that produce an electric charge while in motion and could be developed to tap energy from a broad range of sources, for example, ocean currents, wind, or sound vibrations.

Previously we just used trial and error to maximize this effect,” stated Wang. “But with this new information, we can design materials that have better performance for power conversion.”

The team devised a technique by using a nanoscale triboelectric nanogenerator, including layers of titanium and aluminum oxide, or titanium and silicone dioxide, to enable quantification of the amount of charge that gets assimilated on surfaces at the moments of friction.

The technique could carry out real-time tracking of the assimilated charges and works well over a broad array of temperatures, even at higher temperatures. The data collected from the research showed that the properties of the triboelectric effect—that is, the way electrons traveled across barriers—were in agreement with the electron thermionic emission theory.

The researchers developed triboelectric nanogenerators with the ability to withstand investigations at higher temperatures and discovered that temperature had a crucial role in the triboelectric effect.

We never realized it was a temperature dependent phenomenon,” stated Wang. “But we found that when the temperature reaches about 300 Celsius, the triboelectric transfer almost disappears.”

The team investigated the potential of surfaces to retain a charge at temperatures of nearly 80 °C to 300 °C. Based on the data derived from the tests, the team advanced a mechanism for elucidating the physical process behind the triboelectrification effect.

As the temperature rises, the energy fluctuations of electrons become larger and larger,” the team wrote. “Thus, it is easier for electrons to hop out of the potential well, and they either go back to the material where they came from or emit into air.”

The Hightower Chair Foundation, the National Key R & D Project from the Minister of Science and Technology of China, the National Natural Science Foundation of China, and the Six Talent Peaks Project in Jiangsu Province, China, supported the study. The authors take sole responsibility for any conclusions or recommendations, which do not necessarily represent the official views of the sponsoring organizations.

CITATION: Cheng Xu, Yunlong Zi, Aurelia Chi Wang, Haiyang Zou, Yejing Dai, Xu He, Peihong Wang, Yi-Cheng Wang, Peizhong Feng, Dawei Li, and Zhong Lin Wang, “On the Electron-Transfer Mechanism in the Contact-Electrification Effect,” (Advanced Materials, March 2018). http://dx.doi.org/10.1002/adma.201706790

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