Dont you hate it when your phone battery dies? In the future, you might be able to recharge your gadget by breathing on it, thanks to a technique that harvests electricity from moisture in the air. Water droplets might seem an unlikely source of energy, but then can hold a form of static electricity called hygroelectricity, which is thought to be the source of electric charge that causes lightning. This charge can be transferred from droplets to other tiny particles, such as dust. Researchers have previously shown that this energy can be harvested by structures made from graphene, the two-dimensional form of carbon. As tiny holes in these structures absorbs water molecules, they become ionised, generating an electric current. The structures in this previous work were bulky and rigid, making them impractical for use in small-scale electronics like a smartphone. To fix this, a team at Tsinghua University and the Beijing Institute of Technology in China have made generators out of thin films of graphene oxide that can be rolled, stretched and folded into a range of structures, including tubes, pyramids and balls. It may be used in wearable devices in the future, says Huhu Cheng at Tsinghua University. Graphene is formed from a single-layer of carbon atoms, making it difficult to manufacture, but graphene oxide, which is made from a sheet of carbon atoms interlaced with oxygen-containing molecules, is easier and cheaper to produce. It can be laid as a film onto other materials and is used in flexible electronics and solar cells. To make their flexible hygroelectric generators, the researchers used a laser to etch patterns on graphene oxide laid on the surface of flexible films. They were able to partly restore the structure and properties of pure graphene by removing most of the oxygen. The films could then be rolled and folded into a variety of shapes without losing their electricity harvesting ability. Spraying a mist of water droplets onto the surface of a generator or even breathing through one rolled into a tube produced voltages up to 1.5V, which is comparable to household AA or AAA batteries. Ron Mertons, a graphene industry analysis in Herzliya, Israel, thinks this technology could be great for powering low-energy sensors or tiny computers that you want to leave in the environment without an energy supply. It is likely, though, that it could take years for such devices to reach the market, if ever, he says.
New Scientist, 23 November 2018 ; http://www.newscientist.com/