Flexible generators transform the movement into energy

Portable devices that recover the energy of movement are not a new idea, but a material created at Rice University could make them more practical.

Chemist James Tour's laboratory, specializing in rice processing, has adapted laser-induced graphene (LIG) using small, metal-free devices that generate electricity. Just like rubbing a balloon on the hair, contacting LIG composites with other surfaces produces static electricity that can be used to power devices.

For that, thank the triboelectric effect, thanks to which materials accumulate by contact. When they are assembled and then separated, surface charges can be channeled towards the production of electricity.

During experiments, the researchers connected a folded strip of LIG to a string of light – emitting diodes and discovered that tapping the tape produced enough energy to flash them. A larger piece of LIG embedded in a rocker allows the wearer to generate energy at each step because the repeated contact of the graphene composite with the skin produces a current to charge a small capacitor.

"This could be a way to reload small devices simply by using the excess energy of the heel strikes when walking or swinging the arms against the torso," Tour said.

The project is detailed in the American Chemical Society 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 two-dimensional carbon flakes. The laboratory first made LIG on common polyimide, but extended the technique to include plants, feeds, treated paper and wood.

The laboratory has transformed polyimide, cork and other materials into LIG electrodes to determine their ability to produce energy and resist wear. They obtained the best results with the materials located at the opposite ends of the triboelectric series, which makes it possible to quantify their capacity to generate a static charge by electrification of contact.

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

The best configuration, with polyimide-LIG and aluminum composite electrodes, produced voltages greater than 3.5 kilovolts with a peak power greater than 8 milliwatts.

"The nanogenerator built into a flip-flop could store 0.22 millijoules of electrical energy on a capacitor after a 1-kilometer walk," said Postdoctoral Researcher Rice, Michael Stanford, lead author of the paper. "This energy storage rate is enough to power portable sensors and electronics with human movement."