TENGs work thanks to two effects, the triboelectric effect and electrostatic induction, to generate small amounts of electrical power from mechanical motion. The triboelectric effect occurs when certain materials become electrically charged after they come into moving contact with a surface made from a different material. The electricity generated by TENGs could replace or supplement batteries for a broad range of potential applications and the technology, which has come along in leaps and bounds over the last few years, is now good enough to power nanoelectronics devices.

“The structure of a TENG is in itself quite simple and can be thought of as a variable capacitor connected to a 'virtual' voltage source and load,” explains team member Aihua Zhang in Beijing. “Work done to change the capacitance is converted to electric current in the circuit driven by the voltage source.”

Three types of TENG

TENG devices can be classified into three types: those that use conductor-conductor, conductor-dielectric and dielectric-dielectric contacting interfaces. Previous research has mainly focused on conductor-dielectric and dielectric-dielectric TENGs in which the triboelectric charge produced by the devices can be distinguished from the electrostatic charge. Indeed, the first prototype of these nanogenerators can be traced back to an old device called the “electrophorus” from the 18th century.

“In our work, we studied the conductor-conductor contacting interface in which it is impossible to separate the triboelectric charge and induced charge,” says Zhang. “The prototype of this type of triboelectric nanogenerator may be traced back to the Kelvin probe, first put forward by Lord Kelvin in 1861 to measure the contact potential difference between different materials. (Kelvin’s method was subsequently developed into a powerful nanoscale characterisation instrument, called Kelvin Probe Microscopy in 1991).”

A simple model of contact electrification

In a conductor-conductor TENG, the voltage source is provided by the electrochemical potential difference between the two contact parts. "This arrangement might overcome a major problem in conductor-dielectric or dielectric-dielectric TENGs - that is, under normal circumstances, a load as large as several hundred megaohms is needed to reach optimal power outputs," explains Zhang.

The researchers, led by Yan Zhang, say they have now analyzed the working behaviour of the three types of TENG and developed a single mathematical formalism that describes it. They did this by including a simple model of contact electrification and looked at how the average power output from the devices depends on their load resistance, motion speed and form.

“Based on how the output power depends on charge and displacement, we have proposed three different ways for obtaining higher output power and better load resistance matching for a conductor-conductor TENG,” says Aihua Zhang. “These are by: changing the nanogenerator’s motion form (from "uniform" to "cosine"); switching the device’s circuit on and off during certain stages of motion; and flash charging by short-circuiting the device.” These three strategies improve output power and reduce optimal load resistance and will be helpful for future experimental TENG designs, he tells nanotechweb.org.

The work is detailed in Nanotechnology 26 42 and features in Nanotechnology Select.