Sustainable Energy

How Friction May Someday Charge Your Cell Phone

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

Extending the battery life of portable gadgets would let users go longer between charges.

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 (see “Harnessing Hamster Power with a Nanogenerator”). 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.

“The choice of materials is wide, and fabricating the device is easy,” says Wang. Any of about 50 common plastics, metals, and other materials can be paired to make this type of device.

“I’m impressed with the power density here,” says Shashank Priya, director of the Center for Energy Harvesting Materials and Systems at Virginia Tech. Other smart materials haven’t produced enough power for practical applications, he says.

Whether the new nanogenerator will work outside the lab remains to be seen. “They need to demonstrate that this can generate power from mechanical vibrations in real life,” says Jiangyu Li, professor of mechanical engineering at the University of Washington in Seattle. To work in the real world, an energy scavenger will have to be able to pick up on vibrational frequencies that provide the most energy. A nanogenerator that can only pick up on low-energy mechanical vibrations would take way too long to charge a cell phone, Priya notes. Wang says he is in talks with companies about developing the energy scavenger for particular applications, and envisions it being worn on an armband.

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