Triboelectric generators, has no little ring of sci-fi about it, but throw the phrase laser-induced graphene into the equation and this sounds like full-on futurism! Nevertheless, a team at Rice University in Texas has built a flexible device that can generate electricity from movement and could be used to power wearable, self-powered sensors and devices. [Stanford, M.G., et al., ACS Nano (2019) DOI: 10.1021/acsnano.9b02596]

Rice chemist James Tour has adapted laser-induced graphene (LIG) into small, metal-free devices that can generate electricity when they come into contact with another surface and then are pulled apart through the triboelectric effect, which builds up a static charge. The team has tested a range of materials as the substrate against which their LIG might work, including common polyimide, plant materials, foodstuffs, treated paper, and wood. Polyurethane-coated polyimide worked well and is likely to be a composite most amenable to a range of applications, but the other substances tested also work well and point to novel applications. The team reports that the folding LIG could generate about 1 kilovolt, and continues working even after 5000 bending cycles. The optimal configuration reported has electrodes made from the polyimide-LIG composite and aluminum and generates more than 3.5 kilovolts with a peak power of more than 8 milliwatts.

The team has tapped into this effect to power a string of light-emitting diodes. Just tapping the strip was sufficient to cause the LEDs to flash. By incorporating the LIG into a shoe and adding circuitry and a capacitor, walking could be used to charge the capacitor as the walker's feet repeatedly come into contact and separate again from the underfoot LIG. "This could be a way to recharge small devices just by using the excess energy of heel strikes during walking, or swinging arm movements against the torso," Tour explains. Team member Michael Stanford adds that their nanogenerator on the part of a flip-flop shoe that comes into contact with the heel could store 0.22 millijoules of electrical energy on a capacitor after a 1-kilometer walk. "This rate of energy storage is enough to power wearable sensors and electronics with human movement," says Stanford.

Opening their paper in ACS Nano, the team tells us that "triboelectric nanogenerators show exceptional promise for converting wasted mechanical energy into electrical energy". Indeed.