Droplet-based Triboelectric Nanogenerators

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The rising demand for energy due to the development of science and technology is inevitable for finding new sustainable power technologies that make life easier and better for people without any harm to the earth's atmosphere and environment. Unfortunately, fossil fuels are the main source of energy that people use right now, but they are also the main cause of so many troubles that our world is facing such as the greenhouse effect for rising world temperature, rising sea levels, other carbon gas emission is making the environment polluted and increasing the toxicity in land and water that making human life more unhealthy and miserable all around the world. As a result, it is very important to create technology that can use energy from natural, sustainable resources instead of fossil fuels to get clean energy. A hopeful device that has gotten a lot of interest is droplet nanogenerators, which use different types of water energy in the form of droplets for effective energy conversion. This technology is becoming more popular because it is easy to make these devices, doesn't cost much, and provides a longer time duration of power supply, which is enough to run small electronics for a long time without much effort. It is possible to make droplet nanogenerators more efficient by using new designs and a wider range of basic materials with favorable characteristics. This means that they can be used in more versatile situations from application aspects.

Making our existing cities into smart cities is becoming more common, and they make life easier for people who live there by using many different technologies that require a lot of energy without any interruptions. However, greener energy resources can make the city healthier and long-lasting for future generations. Fossil fuels are the main energy source that meets the world's energy needs for almost all kinds of energy conversion. However, these fuels are also the leading cause of environmental issues which must be addressed for environmental protection. In order to solve ecological problems, experts are working hard to get energy from green sources like the sun, the seas, the wind, water, and others. The triboelectric nanogenerator (TENG), which was discovered by Professor Zhong Lin Wang and his research team in 2012, has opened up a new way to convert low-frequency mechanical energy into electrical energy. There are also new ways to improve the characteristics of fabricating materials to fabricate these TENG devices, like their pyroelectricity, piezoelectricity, thermoelectricity, and more. Thus the power outputs from these devices can be extended significantly for real-life applications.

Water is a green energy source because it is natural and easy to obtain. Water can give us energy in the form of tide waves, moving water on the surface, and raindrops or other water droplets. Researchers are interested in rain energy drops because it is plentiful and lasts a long time. When it comes to the different ways to get energy from water, droplet nanogenerators get a lot of attention because low cost device fabrications and simple operation. Furthermore, these devices can be made in different shapes, which makes them useful for many purposes, such as self-powered sensors and energy harvesters. Though the outputs are significantly low for these TENGs they can be used in the distributed sensor networks for future IoT applications. Further research can also extend the output power by new structures and materials with multiple device integration.

The nanogenerators for droplet energy harvesting are getting more attention because of recent growth in nanotechnology and energy gathering; liquid droplet-based nanogenerators are now being used more. This is considered as a new type of device that uses the special qualities of liquid drops to power small electronics in an environmentally friendly way. Droplet nanogenerators are different from other types of energy harvesters in a number of important ways. One of the best things about them is the materials used in their production process are very cheap. These materials are not only cheap but also good for the earth because they are not harmful to the ecosystem during production. Droplet nanogenerators have a relatively simple design structure compared to other nanogenerators. This makes them easy to change depending on the purpose. Notably, water drops, which are easy to find worldwide, can be used as a plentiful and long-lasting energy source. If you leave out dry deserts, this is especially useful in most places, which makes the study field very hopeful for further growth. It becomes even clearer that droplet nanogenerators could be very important for finding long-term energy solutions if the right materials are carefully chosen and the devices' designs are better.

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There are a few main ways that water droplet nanogenerators work. These include contact electrification or triboelectrification between droplets and dielectric layers, droplets sliding on semiconductors, droplets interacting with carbon nanostructured surfaces, droplets moving on dielectric materials, and more. Triboelectrification is the main way droplet nanogenerators work, and it has been written extensively in scholarly literature. The triboelectric effect, triboelectrification, or contact electrification, is the electrical event in which charge is transferred due to electron transfer from one material to another after their contact on the surface.

The working system of droplet nanogenerators is based on the contact of water droplets and a dielectric material surface, combining the triboelectric and electrostatic induction effects. The droplet nanogenerator's general process is shown using the simplest design possible in Figures a, b, and c. In this case, the electrode is covered with a PTFE layer and linked to the ground through an external load to complete the circuit. The PTFE film (dielectric material A) and the water droplet (material B) have different electron clouds (an imaginary area where the free electron exists) that don't touch before they come into contact with each other (Fig. a, first picture). This is because the outer shell electrons are loosely coupled. When two materials touch, the droplet comes in contact with the PTFE surface. Their electron clouds meet, which causes electrons to be transferred, which is called contact electrification, which is shown in the second picture. The electron movement creates charges on the dielectric disc surface, which causes the positive current to flow toward the electrode under the exterior of the PTFE film. Most of the electrons being transmitted will be held back if the temperature stays low because of the energy barrier (E2) in the water droplet (third picture). Accordingly, contact electricity with both the positively charged PTFE and negatively charged water droplets takes place. Electrons can either return to the atom they came from or be let go into the air, as shown in the fourth picture. The electron clouds touch and meet when there is a strong mechanical force that will generate the flow of electrons to generate electricity.

The illustration of the simple working process is presented in figure b and c. When water drops rub against air, they can be positively charged before hitting PTFE film (Fig. b (i)). When a positively charged droplet hits the PTFE surface, it creates a positive potential difference (Fig. b (ii)). This makes electrons move from the ground to the electrode until the balance is reached, which creates an instantaneous positive current (Fig. b (iii)). How much charge is passed to the electrode depends on how densely charged the droplet's surface is. This changes the output of the water-TENG. A negative electric potential difference is made when the droplet moves away. This makes electrons run from the electrode to the ground (Fig. b (iv)), which creates a negative current right away. A steady flow of water droplets can produce an AC output. The most electricity is produced when each droplet is completely taken from the PTFE surface before the next one falls. In the second case, a water droplet that is not charged hits the thin film of PTFE. This causes triboelectricity, which charges both the water droplet and the surface of the PTFE (Fig. c (ii)). When the PTFE surface is ionized, a negatively charged PTFE and a positively charged electrical double layer (EDL) form on the face of the water droplet (Fig. c (iii)). A negative electric potential difference is made when the water droplet moves away from the PTFE film. This makes electrons move from the electrode to the ground (Fig. c (iv)) until the balance is restored (Fig. c (v)), which creates an instantaneous negative current. The PTFE film's negative charges pull in counter-ions from the next water droplet, making another positively charged EDL and making a positive electric potential difference. This makes electrons flow from the ground to the electrode (Fig. c (vi)) until a new balance is reached (Fig. c (vii)), which causes an instantaneous positive current. As the water droplet moves away from the PTFE surface, a negative potential difference forms again. This makes the electrons move from the electrode to the ground (Fig. c (viii)).

If you feel the mechanisms are difficult, then you can read the full articles from the references. I think the article has let you know something new about science. In the next post, I will discuss the application area of these droplet-based nanogenerators, which will attract you more. The article is getting longer, so I am ending this today. I will catch you at the next one. Until then, take care and enjoy your life to the fullest.
Have a great weekend!

References:

1. Fan, Feng-Ru, Zhong-Qun Tian, and Zhong Lin Wang. "Flexible triboelectric generator." Nano energy 1.2 (2012): 328-334. (3)
2. Cheng, Tinghai, Jiajia Shao, and Zhong Lin Wang. "Triboelectric nanogenerators." Nature Reviews Methods Primers 3.1 (2023): 39. (35)
3. Xu, Cheng, et al. "On the electron‐transfer mechanism in the contact‐electrification effect." Advanced materials 30.15 (2018): 1706790. (37)
4. Lin, Zong-Hong, et al. "Harvesting water drop energy by a sequential contact-electrification and electrostatic-induction process." Adv. Mater 26.27 (2014): 4690-4696. (38)