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Researchers have discovered yet another way to harvest small amounts of
electricity from motion in the world around us—this time by capturing the
electrical charge produced when two different kinds of plastic materials rub
against one another. Based on flexible polymer materials, this “triboelectric”
generator could provide alternating current (AC) from activities such as
walking.
The triboelectric generator could supplement power produced by
nanogenerators that use the piezoelectric effect to create current from the
flexing of zinc oxide nanowires. And because these triboelectric generators can
be made nearly transparent, they could offer a new way to produce active
sensors that might replace technology now used for touch-sensitive device
displays.
“The fact that an electric charge can be produced through this principle is
well known,” said Zhong Lin Wang, a Regents professor in the School of Materials Science
and 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 research was funded by the National Science Foundation, the Department
of Energy, and the U.S. Air Force. Details were reported in Nano Letters. In addition to Wang,
authors of the paper included Feng-Ru Fan, Long Lin, Guang Zhu, Wenzhuo Wu, and
Rui Zhang from Georgia Tech. Fan is also affiliated with the State Key
Laboratory of Physical Chemistry of Solid Surfaces at Xiamen
University in China.
The triboelectric generator operates when a sheet of polyester rubs against
a sheet made of polydimethysiloxane (PDMS). The polyester tends to donate
electrons, while the PDMS accepts electrons. Immediately after the polymer
surfaces rub together, they are mechanically separated, creating an air gap
that isolates the charge on the PDMS surface and forms a dipole moment.
If an electrical load is then connected between the two surfaces, a small
current will flow to equalize the charge potential. By continuously rubbing the
surfaces together and then quickly separating them, the generator can provide a
small alternating current. An external deformation is used to press the
surfaces together and slide them to create the rubbing motion.
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“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 technique could also be used to create a very sensitive self-powered
active pressure sensor for potential use with organic electronic or optoelectronic
systems. The force from a feather or water droplet touching the surface of the
triboelectric generator produces a small current that can be detected to
indicate the contact. The sensors can detect pressure as low as about 13
millipascals.
Because the devices can be made approximately 75% transparent, they could
potentially 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.”
While smooth surfaces rubbing together do generate charge, Wang and his research
team have increased the current production by using micropatterned surfaces.
They studied three different types of surface patterning—lines, cubes, and
pyramids—and found that placing pyramid shapes on one of the rubbing surfaces
generated the most electrical current: as much as 18 V at about 0.13 microamps
per square centimeter.
Wang said the patterning enhanced the generating capacity by boosting the
amount of charge formed, improving capacitance change due to the air voids
created between the patterns, and by facilitating charge separation.
To fabricate the triboelectric generators, the researchers began by creating
a mold from a silicon wafer on which the friction-enhancing patterns are formed
using traditional photolithography and either a dry or wet etching process. The
molds, in which the features of the patterns are formed in recess, were then
treated with a chemical to prevent the PDMS from sticking.
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The liquid PDMS elastomer and crosslinker were then mixed and spin-coated
onto the mold, and after thermal curing, peeled off as a thin film. The PDMS
film with 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.
The generators are robust, continuing to produce current even after days of
use—and more than 100,000 cycles of operation, Wang said. The next step in the
research will be to create systems that include storage mechanisms for 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.”
Source: Georgia Institute of Technology
