Communications of the ACM
Harnessing Everyday Motion to Power Mobile Devices
Nanostructures increased the surface area of polymer surfaces hence the triboelectric effect produced when they are rubbed together.
Credit: American Chemical Society
Imagine powering your cellphone by simply walking around your office or rubbing it with the palm of your hand. Rather than plugging it into the wall, the user become the power source. Researchers presented these commercial possibilities and a unique vision for green energy at the 247th National Meeting & Exposition of the American Chemical Society (ACS), which is being held in Dallas this week.
Zhong Lin Wang and his team, including graduate student Long Lin who presented the work, have set out to transform the way we look at mechanical energy. Conventional energy sources have so far relied on century-old science that requires scattered, costly power plants and a grid to distribute electricity far and wide.
"Today, coal, natural gas, and nuclear power plants all use turbine-engine driven, electromagnetic-induction generators," Wang says. "For a hundred years, this has been the only way to convert mechanical energy into electricity."
But a couple of years ago, Wang's team at the Georgia Institute of Technology was working on a miniature generator based on an energy phenomenon called the piezoelectric effect, which is electricity resulting from pressure. To their surprise, it produced more power than expected. They investigated what caused the spike and discovered that two polymer surfaces in the device had rubbed together, producing what's called a triboelectric effect — essentially what most of us know as static electricity.
Building on that fortuitous discovery, Wang then developed the first triboelectric nanogenerator, or "TENG." He paired two sheets of different materials together — one donates electrons, and the other accepts them. When the sheets touch, electrons flow from one to the other. When the sheets are separated, a voltage develops between them.
Since his lab's first publication on TENG in 2012, the team has since boosted the power output density by a factor of 100,000, with the output power density reaching 300 Watts per square meter. Now with one stomp of his foot, Wang can light up a sheet with a thousand LED bulbs.
His group has incorporated TENG into shoe insoles, whistles, foot pedals, floor mats, backpacks, and ocean buoys for a variety of potential applications. These gadgets harness the power of everyday motion from the minute (think vibrations, rubbing, stepping) to the global and endless (waves). These movements produce mechanical energy that has been around us all along, but scientists didn't know how to convert it directly to usable power in a sustainable way until now.
The key to the huge leap in output and future improvements is the chemistry.
"The amount of charge transferred depends on surface properties," Wang says. "Making patterns of nanomaterials on the polymer films' surfaces increases the contact area between the sheets and can make a 1,000-fold difference in the power generated."
With those improvements, Wang says his group is now working on commercializing products to recharge cellphones and other mobile devices using TENG. Down the road, he envisions these nanogenerators can make a far bigger impact on a much larger scale. Researchers could use the technology to tap into the endless energy of ocean waves, rain drops, and the wind all around us — empowering tiny generators rather than towering turbines — to help feed the world's ever-growing energy demand, he says.
Wang acknowledges funding from the U.S. National Science Foundation, the U.S. Department of Energy, the National Institute for Materials in Japan, Samsung, and the Chinese Academy of Sciences.
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