Researchers at Georgia Institute of Technology have discovered a way to produce electricity from random mechanical motion using triboelectric nanogenerators. These are devices fabricated from flexible polymeric materials with a high degree of transparency. Rubbing the surface of these devices can generate alternating current from friction.
The research was published in the June issue of Nano Letters. The proponents of the study were Zhong Lin Wang, Feng-Ru Fan, Long Lin, Guang Zhu, Wenzhuo Wu and Rui Zhang. Fan is also connected with Xiamen University of China.
Triboelectric nanogenerator operates when a sheet of polyester rubs against a sheet made from polydimethylsiloxane (PDMS). The polyester donates electrons while the PDMS accepts electrons. When the two surfaces are mechanically separated, a voltage drop develops between them. Connecting an electrical circuit between them causes a current flow to equalize the charge differential.
Continuously rubbing and quickly separating the two surfaces can allow the generator to provide a small alternating current. An external deformation can be used for this purpose.
Wang said that it is essential to use two surfaces made of different materials to create different electrodes (anode and cathode). The same materials will not result to a charge differential.
In order to maximize current output, micropatterns were introduced into the surfaces of the polymeric materials. The researchers tested surface patterns consisting of lines, cubes and pyramids.
Pyramid shapes on one of the rubbing surfaces generated the most current at around 0.13 microampere per square centimeter with a potential of 18 volts.
Wang explained that patterns enhanced current generating capacity by boosting the charge formed, improving the capacitance charge due to the air voids between the patterns, and facilitating the separation of charges.
The first step in the fabrication process of the nanogenerator was the creation of the mold using a silicon wafer. Friction-enhancing patterns were introduced into the silicon wafer using traditional photolithography and either dry or wet etching process. Then, the silicon mold was treated with a chemical to prevent sticking of the PDMS layer.
Next, a mixture of PDMS elastomer and crosslinker were spin-coated on the mold, followed by thermal curing. The resulting polymer was then peeled-off as a thin film from the mold.
The polymer with patterns was fixed onto an electrode surface made of indium tin oxide (ITO) coated with polyethylene teraphthalate (PET) by a thin PDMS bonding layer. The entire assembly was again covered with ITO-covered PET to form a sandwich structure.
According to Wang, the fabrication process is simple, inexpensive and easily amenable for large-scale productions. Furthermore, the fabricated generator is robust, usable for days on end, and capable of more than 100,000 cycles of operation.
Friction is everywhere; hence, the practical applications of triboelectric nanogenerators are many and diverse. It can be applied to create very sensitive self-powered active pressure sensor for potential use with organic or opto-electronic systems. It can sense force from a feather or a water droplet touching its surface and can generate current that can be detected to indicate contact. It can sense pressure as low as 13 millipascals.
The generator can be made 75 percent transparent, which is ideal for potential use in touch screens to replace existing sensors that would not need power from a device’s battery. These transparent generators can be fabricated in virtually any surface.
The researchers reported that the next phase of the study is to create a system with storage mechanisms for the generated current.