Researchers at the Georgia Institute of Technology have discovered yet another way to harvest small amounts of electricity from motion and friction in the world around us by capturing the electrical charge produced when two different kinds of plastic materials rub against one another.
It’s not the common ‘static’ electricity we’re all familiar with; the new generator is based on flexible polymer materials. The new “triboelectric” generator could provide alternating current (AC).
Because these triboelectric generators can be made nearly transparent, they could for example offer a new way to produce active sensors that might replace technology now used for touch-sensitive device displays.
Zhong Lin Wang, a Regents Professor in the School of Materials Science & Engineering at the Georgia Institute of Technology said, “The fact that an electric charge can be produced through this principle is well known. 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 paper published in the June issue of the journal Nano Letters. Additional authors of the paper include Georgia Tech’s 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 simplified technical explanation is a 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 or simply a charged state.
So when 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.
Wang explains, “For this to work, you have to use to two different kinds of materials to create the different electrodes. If you rub together surfaces made from the same material, you don’t get the charge differential.”
While smooth surfaces rubbing together do generate a charge, Wang and his research team have increased the current production by using micro-patterned 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 volts 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 team 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.
The liquid PDMS elastomer and cross-linker 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.
Wang said, “The entire preparation process is simple and low cost, making it possible to be scaled up for large scale production and practical applications.”
Here’s the opening surprise – 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.
The technique could also be used to create a very sensitive self-powered active pressure sensor for potential use with organic electronic or opto-electronic systems. The force from a feather or water droplet touching the surface of a triboelectric generator produces a small current that can be detected to indicate the contact. These sensors can detect pressure as low as about 13 millipascals.
Because the devices can be made approximately 75 percent transparent, they could potentially be used in touch screens to replace existing sensors. “Transparent generators can be fabricated on virtually any surface,” Wang points out. “This technique could be used to create very sensitive transparent sensors that would not require power from a device’s battery.”
Wang extends the research with, “Friction is everywhere, and so this principle could be used in a lot of applications.”
Wang’s Georgia Tech group has already built a zinc oxide nanogenerator that use the piezoelectric effect to create current from the flexing of zinc oxide nanowires. The new triboelectric generator could be used as a complement. Wang said, “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.”
It does seem small – until you do the math. 18 volts at about 0.13 microamps per square centimeter works out to over 6.45 cm2 or .84 microamps. Still, its 84 millionths of an amp per square inch.
It’s a grand start. The materials and processes are dirt cheap and the motion out there is incredible. And now we know what a “triboelectric generator” is.