Friction May Someday Charge Your Cell Phone

A nanogenerator made from inexpensive materials harvests mechanical energy and produces enough power to charge personal electronics.

The phenomenon that causes a painful shock when you touch metal after dragging your shoes on the carpet could someday be harnessed to charge personal electronics.

Researchers at Georgia Tech have created a device that takes advantage of static electricity to convert movement—like a phone bouncing around in your pocket—into enough power to charge a cell phone battery. It is the first demonstration that these kinds of materials have enough oomph to power personal electronics.

Excess energy produced when you walk, fidget, or even breathe can, in theory, be scavenged to power medical implants and other electronics. However, taking advantage of the energy in these small motions is challenging.

Zhong Lin Wang, a professor of materials science at Georgia Tech, has been working on the problem for several years, mostly focusing on piezoelectric materials that generate an electrical voltage under mechanical stress. Wang and others have amplified the piezoelectric effect by making materials structured at the nanoscale. So far, though, piezoelectric nanogenerators have not had very impressive power output.

Now Wang’s group has demonstrated that a different approach may be more promising: static electricity and friction. This is the effect at work when you run a plastic comb through your hair on a dry day, and it stands on end. The Georgia Tech researchers demonstrated that this static charge phenomenon, called the triboelectric effect, can be harnessed to produce power using a type of plastic, polyethylene terephthalate, and a metal. When thin films of these materials come into contact with one another, they become charged. And when the two films are flexed, a current flows between them, which can be harnessed to charge a battery. When the two surfaces are patterned with nanoscale structures, their surface area is much greater, and so is the friction between the materials—and the power they can produce.

The Georgia Tech nanogenerator can convert 10 to 15 percent of the energy in mechanical motions into electricity, and thinner materials should be able to convert as much as 40 percent, Wang says. A fingernail-sized square of the triboelectric nanomaterial can produce eight milliwatts when flexed, enough power to run a pacemaker. A patch that’s five by five centimeters can light up 600 LEDs at once, or charge a lithium-ion battery that can then power a commercial cell phone. Wang’s group described these results online in the journal Nano Letters.....

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