Waste materials from plants and animals — think flower petals, fruit peels, shed fur, or snake skin — have the potential for a second life in electronics. Scientists have figured out how to reuse these materials to generate electricity with the help of a nifty little device known as a triboelectric nanogenerator (TENG for short).

Invented in 2012, TENGs convert random, often wasted background energy into electricity without requiring fuel or batteries. These devices capture energy from the environment — such as ocean waves, wind, raindrops, or vibrations — and even from a person’s normal movements, like walking or even sleeping, enabling them to be self-powered. Their simple and economical design harnesses friction to generate electricity.

They do this through a phenomenon known as the triboelectric effect, which occurs when two materials that have different tendencies to accept and donate electrons come into contact and then separate.

The voltage generated when this process is repeated over and over is sufficient to power small electronic devices, where the larger the charge difference between the two materials, the higher the voltage output.

The future of energy could be triboelectric

Petroleum-based polymers containing fluorine, such as polytetrafluoroethylene, are normally used as electron-accepting “tribonegative” materials due to their high negative charge, but these synthetic plastics are not biodegradable. This is also true for the metal films and metal oxides typically used as electron-donating “tribopositive” materials.

“In biowaste-based triboelectric nanogenerators, one or both sides of the triboelectric layers are made from processed or unprocessed biowaste materials, which need to be optimized for their ability to induce charge generation when in contact with other surfaces,” explained Tae-Jun Ha, a professor in the Department of Electronic Materials Engineering at Kwangwoon University, South Korea.

The carbon-based materials that make up plant and animal wastes –– like cellulose, chitosan, collagen, gelatin, and keratin –– are abundant in electron-donating chemical groups known as carboxyl, amino, amide, and hydroxyl groups, allowing them to be used in place of metals as tribopositive materials.

Although most biowaste materials donate rather than accept electrons, weakly tribopositive materials can be used in combination with strongly tribopositive materials to prepare completely natural triboelectric nanogenerators, Ha said.

These nitrogen- and oxygen-containing groups can also form hydrogen bonds, weak bonds that stabilize a material when many are present. For example, the protein keratin contains chains of amino acids that form multiple hydrogen bonds, strengthening human hair, fingernails, feathers, and animal fur.

Hydrogen bonds also improve the mechanical properties of triboelectric nanogenerators, Ha said.

The flip side is that extensively hydrogen-bonded biowaste materials are often sensitive to humidity, reducing their energy-conversion efficiency and making them impractical for some applications.

Practical considerations

Although some degree of processing is needed for certain applications, like those that require water resistance, unprocessed biowaste like agricultural residues have been shown to be effective tribopositive materials.

Even a dry leaf, when paired with polytetrafluoroethylene as the tribonegative material, can generate enough voltage to power light-emitting diodes. 

Despite their advantages, triboelectric nanogenerators that run on biowaste have yet to be commercialized.

According to Tan, the main issues preventing this are their low output voltages, poor durability, and short service life — a tradeoff of their biodegradability. But certain applications are promising, especially smart healthcare, which requires materials that are compatible with the human body. 

For example, scientists developed a chitosan-based smart sensing patch that patients could potentially use to nonverbally communicate simple needs to doctors or nurses. When worn on the finger, they would simply tap the patch in a certain voltage sequence corresponding to a letter of the alphabet. In this way, words could be spelled out.

Triboelectric nanogenerators based on waste materials could also be used as real-time environmental sensors that detect pollution, humidity, temperature, and pH levels.   

In one study, researchers built a unique humidity and gas sensor from the skins of onions, scallions, and leeks. Unlike polymers, which are uniformly positively or negatively charged, plant skins can have different charges on their outer and inner surfaces, giving them both electron-donating and electron-accepting capabilities. In this case, only one material is needed for the triboelectric nanogenerator to function.

Although these applications are interesting and the raw materials themselves are cheap, further manufacturing processes are required, which could amplify the cost of the devices. But Ha believes that the biowaste-derived devices will one day be able to compete with the current state-of-the-art TENGs.

In the beginning, solar cells were not cost-competitive for generating electricity, he pointed out. In the 1980s, the average cost of solar cells was around $30 USD per watt but has been driven down to only $0.50 per watt thanks to advancement of the technology over the past few decades.

“I believe energy-harvesting technologies based on biowaste-based TENGs can be cost-effective to manufacture, as researchers all over the world are working very hard,” Ha stated.    

This work includes improving device stability, durability, and energy-conversion efficiency by exploring different combinations of biowaste materials, Ha said.

It may be a while before triboelectric nanogenerators that run solely on biowaste become mainstream, but incremental steps can be made in the meantime. For example, their energy-conversion efficiency could be improved by incorporating some non-biodegradable components into the devices as a compromise.

Reference: Abhisikta Bhaduri and Tae-Jun Ha, Biowaste-Derived Triboelectric Nanogenerators for Emerging Bioelectronics. Advanced Science (2024). DOI: 10.1002/advs.202405666

Feature image credit: Del Barrett on Unsplash