Triboelectric nanogenerators (TENGs) work using the triboelectric effect and electrostatic induction. They generate small amounts of electrical power from mechanical motion or external vibrations. 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 greatly improved over the last few years, is now good enough to power nanoelectronics devices.

However, there is a problem in that compared to traditional electromagnetic generators (EMGs), TENGs produce only a small amount of current and a relatively high voltage. EMGs, for their part, produce a rather low voltage compared to their relatively high current output, which is better for real-world applications.

Hybrid device combines TENGs and EMGs

To overcome the shortcoming of both types of generator, a team led by Zhong Lin Wang of the Georgia Institute of Technology and the Beijing Institue of Nanoenergy and Nanosystems and Weiqing Yang of Southwest Jiaotong University decided to make a hybrid device that combines the two. The new system contains four TENGs and two EMGs that work together and at the same time in a double-deck sandwiched structure.

The TENG in the device is made from aluminium–kapton–aluminium films sandwiched by aluminium back-coated polytetrafluroethylene (PTFE) thin films. These two materials make up the triboelectric layers. The EMG is made up of two copper coils inlaid into a top and bottom substrate with a magnet anchored into a middle movable acrylic plate. The aluminium foils and PTFE are pasted onto the copper coils.

Current is generated by triboelectric and Faraday effects

To improve the performance of the TENG, Wang, Yang and colleagues created PTFE nanowire arrays by an inductively coupled plasma process to increase the surface roughness and also the effective contact area between the triboelectric layers.

“By shaking the device, the middle substrate moves up and down,” explains Yang. “This changes the contact surface between the aluminium and PTFE area and the magnetic crossing coils. Current is thus generated by the triboelectric effect and the Faraday effect.”

Output power density of 167.22 W/m3

By then connecting the device to transformers and a rectifier, the researchers measured an output power density of 167.22 W/m3 when it was shaken. They demonstrated this by using the device to instantaneously power up 1000 commercial light-emitting diodes (LEDs). Basically, the alternating current (AC) generated by the nanogenerators is transformed into direct current by the rectifier and stored in a capacitor in the system. AC can power applications such as LED lighting but is not ideal for mobile devices so it must be converted into DC.

A safety helmet containing the device does not weigh much more than a normal safety helmet and can convert vibrational energy into electricity. The helmet can also wirelessly send signals – to a mobile phone, for example. “Here, transmitted signals underground are capable of being received by the people above ground, which could be especially useful for mine-workers for positioning and communication,” write the researchers in their paper published in ACS Nano DOI: 10.1021/acsnano.6b03760.

At the moment, the self-powered helmet generates electricity when shaken. Ideally, the team says it would like to improve the circuitry in its device and add supercapacitors for sustainable energy storage. “In this new configuration, the helmet could be powered by vibrations produced as the wearer goes about his normal, everyday activities – walking, running, jumping,” Yang tells nanotechweb.org.