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Flexible generators convert movement into energy.

Flexible generators convert movement into energy.

An electron microscope image shows a cross section of a laser-induced graphene and polyimide compound created at Rice University for use as a triboelectric nanogenerator. Devices can convert movement into energy that can then be stored for later use. Credit: Tour Group / Rice University.

Portable devices that gather energy from movement are not a new idea, but a material created at Rice University can make them more practical.

The laboratory of Rice of chemist James Tour has adapted the grafeno induced by laser (LIG) in small devices free of metal that generate electricity. Like rubbing a balloon in the hair, putting the LIG compounds in contact with other surfaces produces static electricity that can be used to power devices.

Therefore, thanks to the triboelectric effect, through which materials accumulate through contact. When they come together and then separate, charges accumulate on the surface that can be channeled into energy generation.

In experiments, the researchers connected a folded strip of LIG to a chain of light-emitting diodes and discovered that touching the strip produced enough energy to cause them to flash. A larger piece of LIG embedded within a flip-flop allows the user to generate energy with each step, since the repeated contact of the graphene compound with the skin produces a current to charge a small capacitor.

"This could be a way to recharge small devices simply by using excess energy from heel hits during the march, or the movements of the arms against the torso," Tour said.

Flexible generators convert movement into energy.

The Rice University postdoctoral researcher, Michael Stanford, holds a flip-flop with a triboelectric nanogenerator, based on laser-induced graphene, attached to the heel. Walking with the flip-flop generates electricity with repeated contact between the generator and the user's skin. Stanford connected the device to store energy in a condenser. Credit: Jeff Fitlow / Rice University

The project is detailed in the American Chemical Society magazine. ACS Nano.

LIG is a graphene foam produced when chemicals are heated on the surface of a polymer or other material with a laser, leaving only interconnected scales of two-dimensional carbon. The laboratory developed LIG with common polyimide, but extended the technique to plants, food, treated paper and wood.

The lab converted polyimide, cork and other materials into LIG electrodes to see how well they produced energy and resisted wear. They obtained the best results from the materials at the opposite ends of the triboelectric series, which quantifies their capacity to generate static charge by contact electrification.

In the folding configuration, the LIG of the tribo-negative polyimide was sprayed with a protective polyurethane coating, which also served as a tribo-positive material. When the electrodes were joined, the electrons were transferred to the polyurethane of the polyurethane. Subsequent contact and separation led charges that could be stored through an external circuit to rebalance the accumulated static charge. The collapsible LIG generated approximately 1 kilovolt and remained stable after 5,000 cycles of flexion.

The lab video shows that repeatedly hitting a folded triboelectric generator produced enough power to power a series of connected light-emitting diodes. The test showed how generators based on laser-induced graphene could be used to power sensors and electronic devices with human movement. Credit: the travel group

The best configuration, with electrodes of polyimide-LIG compound and aluminum, produced voltages higher than 3.5 kilovolts with a maximum power of more than 8 milliwatts.

"The integrated nanogenerator within a flip-flop was able to store 0.22 millijoules of electrical energy in a condenser after a 1-kilometer walk," said Rice postdoctoral researcher Stan Stanford, lead author of the paper. "This energy storage rate is enough to power sensors and electronic devices with human movement."

Laser-induced graphene hardens with help

More information:
Michael G. Stanford and others, laser-induced triboelectric graphene nanogenerators, ACS Nano (2019). DOI: 10.1021 / acsnano.9b02596

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Flexible generators convert movement into energy (2019, May 31)
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