Bend your finger, generate electricity

Dr. Neil Canter, Contributing Editor | TLT Tech Beat May 2018

Researchers find a way to more efficiently convert friction into mechanical motion.

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© Can Stock Photo / GeorgeJmclittle

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KEY CONCEPTS
The movement toward alternative energy sources has focused on such areas as batteries and fuel cells. Attention also has been paid to developing smaller, energy-producing sources such as the human body to generate electricity. 

In a previous TLT article, a technique known as reverse electrowetting was developed that might enable individuals to use energy from walking to power mobile electronic devices (1). The mechanical energy produced from walking caused a liquid droplet to move when interacting with a dielectric-film-coated electrode leading to the generation of electricity. In one experiment, 0.4 milliwatts was produced.

Another approach to convert mechanical motion into electricity is friction. Qiaoqiang Gan, associate professor of electric engineering in the School of Engineering and Applied Sciences at the University of Buffalo in Buffalo, N.Y., says, “A process known as triboelectric generation works by collecting static electricity through rubbing two surfaces together. This effect is similar to what happens to an individual in a low-humidity environment such as during the winter when picking up a sweater and then touching a metallic object. Often an individual will receive a shock as the electricity discharges from the body.”

Initial work conducted by Dr. Yun Xu of the Institute of Semiconductors, the Chinese Academy of Sciences in Beijing, China, and Gan was directed toward developing a triboelectric nanogenerator that could power small electric devices such as cell phones. The researchers first decided to work with graphene as one of the layers in the device. 

Xu says, “Graphene is a widely studied material that has provided beneficial results in many applications. We felt that using it in the triboelectric nanogenerator would lead to a significant production of electricity over a long period of time. While the graphene-based device showed good initial results, operational problems occurred due to oxidation that limited the lifetime of the triboelectric generator.”

The researcher worked with a different material to produce a device that displayed superior electrical generation capability over a long period of time. 

Crumpled gold
Xu, Gan and their fellow researchers developed a triboelectric nanogenerator based on two gold layers that functioned as electrodes sandwiching a polydimethylsiloxane triboelectric layer. Gan says, “We decided to work with gold due to our previous work in using this material in nanophotonics and plasmonics sensing. Gold works well in this application because it does not easily oxidize and is much more durable than graphene.”

One of the important aspects of the device is that one of the electrodes consisted of a crumpled gold layer. Xu says, “We found that the microstructure of the crumpled gold layer produced a larger surface area than pristine gold, leading to greater power generation when stressed by a mechanical force.”

The crumpled gold layer also acts as a second friction layer.

The device was prepared by depositing a gold layer with a thickness of 30 nanometers onto a 500-micron-thick polydimethylsiloxane layer using thermal evaporation. A stamping process then transferred this film onto a biaxially prestrained tape. As the tape was relaxed, the gold layer shrank with the tape and became crumpled. The degree of crumpling was controlled by adjusting the prestrains of the tape.

The device generated electricity when an external force brought the crumpled gold and polydimethylsiloxane layers together. The friction produced when the two layers contact each other led to the flow of electrons between the gold layers. Xu says, “The more friction, the greater the amount of power produced.”

Triboelectric nanogenerator performance was measured in two ways. Xu says, “We evaluated the output current and power density.”

The device achieved a maximum voltage of 124.6 volts and a maximum current of 10.13 microamps that corresponded to a maximum power density of 0.22 milliwatts per centimeter. This was sufficient power to light 48 red LEDs simultaneously. 

A significant increase in power was seen when crumpled gold was used compared to pristine gold. The device displayed no drop-in performance after being stored for six months at ambient conditions, an indication of good durability. 

Gan says, “The microscopic mechanism of the device was examined through the use of scanning electron microscopy. Crumpling of the gold layer clearly led to more complex and disordered features that produced more triboelectric charges.”

The researchers demonstrated the performance benefits by attaching the device to a finger joint as shown in Figure 1. In moving the finger, the researchers were able to produce a voltage that was dependent upon the rate of bending. 


Figure 1. Bending of the finger enabled an attached device to generate electricity through the process of triboelectric generation. (Figure courtesy of the University of Buffalo.)

Xu believes the technology can be commercialized. She says, “We are now working on developing a portable battery that can store energy produced by the triboelectric nanogenerator to power larger electronic devices.” 

The researchers also are looking to improve the triboelectric nanogenerator because long-term durability is a concern. Xu says, “We have to induce continuous friction, which might reduce the durability of the materials used in our device. We are working to identify better wear-resistant materials than the ones we are currently using.”

Additional information can be found in a recent article (2) or by contacting Xu at xuyun@semi.ac.cn and Gan at qqgan@buffalo.edu. 

REFERENCES
1. Canter, N. (2011), “Powering your cell phone as you walk,” TLT, 67 (11), pp. 6-7.
2. Chen, H., Bai, L., Li, T., Zhao, C., Zhang, J., Zhang, N., Song, G., Gan, Q. and Xu, Y. (2018), “Wearable and robust triboelectric nanogenerator based on crumpled gold films,” Nano Energy, 46, pp. 73-80.
Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat can be submitted to him at neilcanter@comcast.net.