Devices that convert the energy from footsteps, arm movements, and breathing into electricity could charge smart watches and wearables. But they are complicated and costly to make. Now a team has made flexible energy-scavenging devices by simply blasting organic materials with a laser to create graphene films (ACS Nano, 2019. DOI: 10.1021/acsnano.9b02596). This should give hardy, affordable power generators that can be sewn onto clothes or embedded in shoe soles.
Triboelectric nanogenerators (TENGs) that convert motion into electric power have been studied for several years. They rely on the charge transfer between two different materials that are pressed or rubbed together. The surfaces build up oppositely charged ions, which generates an electric current when a wire connects the two surfaces.
A typical TENG consists of an electron-giving conductor—usually a metal--and an electron-grabbing dielectric polymer placed close to each other. An electrode placed on the polymer extracts current. Researchers used metal or carbon nanomaterials such as graphene as electrodes. But fabricating the electrodes requires costly, involved techniques like vacuum deposition or chemical vapor deposition.
Rice University chemist James Tour and his colleagues instead used a one-step method they have developed for making porous graphene foams on carbonaceous materials. They zap things like cloth, paper and food multiple times with infrared laser pulses. This rearranges the carbon atoms in the materials to form a foamy black layer made of nanoscale sheets of graphene. This thicker graphene is 100 times as conductive as its single-layer cousin, Tour says.
To make TENGs, the researchers lased the dielectric polymer, polyimide, rearranging its surface carbon atoms to make a graphene electrode. They then stacked the polyimide on an aluminum film that served as the triboelectric conductor. Pressing and releasing the device produced 8.5 mW, enough to power 60 light-emitting diodes. “Laser-induced graphene makes it really easy to create TENGs,” Tour says. “You don’t need a clean room.” Graphene-covered polyimide film can be generated in large sheets on a typical “roll-to-roll” manufacturing line. “Just put the polyimide in contact with rigid or flexible substrate and you’re in business,” Tour says.
Swapping the aluminum for a polyurethane layer gives a highly flexible metal-free TENG that produces 2.75 mW of power even when bent 5000 times. The researchers also made a wearable TENG where skin acts as the triboelectric electron-donor. By transferring laser-induced graphene made on polyimide to a polydimethylsiloxane sheet they made flexible, stretchable TENG that produces power when rubbed against skin. As a demonstration, they attached the PDMS device on the heel pad of a flip-flip that generates energy when a wearer walks with it on. The device produced enough energy to run a low-power sensor but stopped working after about 2 km because it got dirty.
Graphene’s mechanical flexibility and easy bonding on organic surfaces makes it an ideal electrode for making robust TENGs, says Zhong Lin Wang, a materials scientist and engineer at Georgia Tech, whose group invented TENGs in 2012. Along with making electrodes simpler to produce, laser-induced graphene allows researchers to “make designed patterns at a high resolution” using the laser, which could lead to designer TENG structures on any material, Wang says.