Energy is one of the most critical resources that dictates the quality of our life. Over the past two decades, vast applications and distributions of mobile electronics have reached every corner of our lives. Energy collection is the order of the day; many solutions sometimes use toxic materials, while other forms lend themselves to structural electronics. The triboelectric effect is defined as a small amount of static electricity generated by the contact and movement of surfaces, used to generate energy that is collected and stored to power sensors and electronics at very low power.
The triboelectric effect is a phenomenon that consists in the transfer of electric charges between two bodies of a different material (of which at least one is an insulator) when they are rubbed together, or even when contacted and removed. Thus a tension is generated starting from the movement.
The intensity of the charge depends on many factors, from the type of materials to the width of the contact surface, the intensity of the rubbing, and more. Triboelectricity is the name of a phenomenon known to all (and since ancient times): a classic example is the ability of a glass or plastic rod to attract bits of paper after being rubbed with a particular cloth; another is the effect that acrylic clothing makes on our hair.
The formation of electrostatic charge does not necessarily require rubbing. In reality, the transfer of electrons from one material to another is also manifested by simple contact. An example is unrolling of the adhesive tape (of cellulose). Here there is no rubbing: the strip of adhesive tape is in contact with the layer of adhesive when the tape is unrolled the cellulose strip is removed from the glue and in the detachment, electrons are transferred from the glue to the tape. The glue is positively charged, the underlying tape negatively. The electrical potentials that are generated are in the order of tens of kV. Triboelectric effects can also occur between solids and liquids or gases.
In published literature there is the “triboelectric series”, that is the list of the materials most likely to yield or to acquire electrons, and among the first ones there are glass, nylon, wool, leather, and human hair, while among the latter there are Teflon, PVC, polyethylene, polyester and polyurethane.
A triboelectric energy “harvester” is a device that uses the principle of electrification contact between two materials to capture electricity transferred. In order to continually capture the energy, there must be a constant separation between the corresponding materials. In recent years, progress has been made in the development of triboelectric energy collection systems, called triboelectric nanogenerators (TENG). These systems require a minimum of necessary components: at least two layers of triboelectric material, physical separation between them, and electrodes for collecting electricity, as well as a conditioning circuit to maximize collection efficiency. Different materials can be used in the structure of a TENG.
The first step in the energy management strategy is to maximize the transfer of current from the TENG to the back-end circuit. The TENG can be schematized as in Figure 1 with a rectifier and a serial switch for synchronizing and releasing the maximum energy.
Circuit application of triboelectric energy harvesting (Image courtesy of Reference 1)
A triboelectric nanogenerator was first demonstrated in the group of Prof. Zhong Lin Wang at the Georgia Institute of Technology in 2012. The triboelectric nanogenerator can be applied to collect all available, but wasted, mechanical energy in our daily life as human movement, walking, vibration, mechanical priming, rotating tire, wind, running water and more.
The primary cell conceived in this way measures 4.5 x 1.2 cm and is formed by a thickness of 125 μm of Kapton and one of 220 μm of PET both sliding and therefore 'rubbed' and, moreover, interposed with two electrodes constituted from two malleable sheets of Au/Pd-Au. The rubbing between the Kapton and the PET is collected by an electrical circuit that can obtain an electrical power density of about 10.4 mW/cm3 with a potential difference at the terminals of 3.3V and an emitter current of about 0.6 μA (Figure 2).
Schematic view of typical Vertical nanowire Integrated Nanogenerator with different contacts(Image courtesy of Reference 1)
What are your experiences with Triboelectric Energy Harvesting?
1 Universal power management strategy for triboelectric nanogenerator, Fengben Xia, Yaokun Panga, Wei Lia, Tao Jianga, Limin Zhanga, Tong Guoa, Guoxu Liua, Chi Zhanga, Zhong Lin Wanga, Nano Energy, 2017