Researchers at the Georgia Institute of Technology have taken advantage of the triboelectric effect, which sees an electric charge generated through friction between two different materials, to develop a generator that could supplement power produced by piezoelectric nanogenerators previously developed at Georgia Tech. The triboelectric generator could be used to produce electricity from activities such as walking and even has the potential to create touchscreens that generate their own power.

“The fact that an electric charge can be produced through this [triboelectric] principle is well known,” said Zhong Lin Wang, a Regents professor in the School of Materials Science & Engineering at the Georgia Institute of Technology. “What we have introduced is a gap separation technique that produces a voltage drop, which leads to a current flow, allowing the charge to be used. This generator can convert random mechanical energy from our environment into electric energy.”

The team’s triboelectric generator generates a charge when a sheet of polyester, which tends to donate electrons, rubs against a sheet of polydimethylsiloxane (PDMS), which accepts electrons. Immediately after the two polymer surfaces rub together, they are mechanically separated, creating an air gap that isolates the charge on the PDMS surface and forms a separation of the positive and negative charges (known as a diploe moment).

Connecting an electrical load between the two surfaces will result in the flow of a small electric current to equalize the charge potential. Therefore, by continuously rubbing the surfaces together and then quickly separating them, the generator can produce a small alternating current. An external deformation is used to press the surfaces together and slide them to create the rubbing motion.

“For this to work, you have to use to two different kinds of materials to create the different electrodes,” Wang explained. “If you rub together surfaces made from the same material, you don’t get the charge differential.”

The researchers say the technique could be used to create a very sensitive self-powered active pressure sensor for potential use with organic electronic or opto-electronic systems. Since the sensors can detect pressure as low as around 13 millipascals, they would be sensitive enough to produce a small current that can be detected to indicate contact from something as small as a feather or water droplet touching the surface of the triboelectric generator.

Additionally, because the devices can be made around 75 percent transparent, there is the potential for the technology to be used in touch screens to replace existing sensors. “Transparent generators can be fabricated on virtually any surface,” said Wang. “This technique could be used to create very sensitive transparent sensors that would not require power from a device’s battery.”

Although rubbing smooth surfaces together will generate a charge, Wang and his team have managed to increase the current by using micro-patterned surfaces. After testing line, cube and pyramid surface patterning, they found that surfaces patterned with pyramids generated the most electrical current: as much as 18 volts at about 0.13 microamps per square centimeter.

This enhanced generating capacity of the pyramid-patterned surface was due to the air voids created between the patterns improving the capacitance change and facilitating charge separation.

The team fabricated the triboelectric generators by first creating a mold from a silicon wafer, onto which the friction-enhancing patterns were formed in recess using traditional photolithography and either a wet or dry etching process. The molds were then treated with a chemical to prevent the PDMS from sticking.

The liquid PDMS elastomer and cross-linker were then mixed and spin-coated onto the mold, and peeled off as a thin film after thermal curing. The resulting PDMS film, complete with surface patterning, was then fixed onto an electrode surface made of indium tin oxide (ITO) coated with polyethylene terephthalate (PET) by a thin PDMS bonding layer. The entire structure was then covered with another ITO-coated PET film to form a sandwich structure.

“The entire preparation process is simple and low cost, making it possible to be scaled up for large scale production and practical applications,” Wang said.

Wang added that the generators are robust, continuing to generate a current even after days of use and after more than 100,000 cycles of operation. The team’s next step is to create systems that include a way to store the current generated.

“Friction is everywhere, so this principle could be used in a lot of applications,” Wang added. “We are combining our earlier nanogenerator and this new triboelectric generator for complementary purposes. The triboelectric generator won’t replace the zinc oxide nanogenerator, but it has its own unique advantages that will allow us to use them in parallel.”

The research was funded by the National Science Foundation, the Department of Energy and the U.S. Air Force. Details of the triboelectric generator are reported in the June issue of the journal Nano Letters.

Source: Georgia Tech

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