About ten years ago, researchers at the Georgia Institute of Technology in Atlanta demonstrated a way to harvest vibrational energy. Their device was simple: essentially two sheets of material placed in contact and then folded. If the materials are carefully selected, this process transfers the charge from one sheet to another, generating a voltage between them, a phenomenon known as the triboelectric effect. Bending the material in the other direction reverses the polarity.
The resulting device, a triboelectric nanogenerator, is now the subject of intense study. The hope is that it can power a new generation of internet-connected devices by harvesting energy from nearly any vibration or mechanical movement.
But there is an almost omnipresent problem in the electronics industry: waste. Triboelectric nanogenerators consist of sheets of different types of plastics, for example polytetrafluoroethylene or (PTFE), fluorinated ethylene propylene and polyethylene terephthalate (PET).
These are common, relatively inexpensive, and easy to make. But ultimately they are derived from petroleum, stable and durable with various known environmental impacts. This is an unsightly environmental legacy that would be best avoided. But how?
We now have an answer thanks to the work of Jianfeng Ping of Zhejiang University in China and colleagues who have found a way to make triboelectric nanogenerators from plant proteins that are biodegradable. The result is an energy harvesting device that can be disposed of like any other form of organic waste.
The team began by creating biofilms using proteins that are natural by-products of processing crops such as rice, wheat, peanuts and soybeans. They combined these proteins with a polylactic acid film to create a layer that could test triboelectric effects when placed in contact with a film of PDA, another biodegradable polymer.
Rice protein turns out to produce the most potent tension effect. The team believes this is a result of the protein’s chemical structure, specifically the way the amide groups bind to water molecules.
It is this hydrogen bond that friction appears to affect, creating a tension. “We can conclude that the degree of coupling with water molecules is an important factor in triboelectric positivity,” Jianfeng and colleagues say.
So what can you do with a biodegradable triboelectric nanogenerator? Researchers say one application is in encouraging plant growth. They indicate that various studies have shown that plants grown in electric fields tend to be larger and grow faster, possibly because the field encourages the flow of highly polar water molecules through plant tissue.
This technique has never been widely used due to the electronic infrastructure required to make it work. But biodegradable sheets capable of producing the required tension could change that.
As a proof of concept, Jianfeng and colleagues grew celery bok choy, or bok choi, through perforated sheets of biodegradable nanogenerators that generated a field up to 180 V. They compared the growth rates and sizes of plants grown under similar conditions. but without the voltages. “Our results suggest that the use of bio-[triboelectric nanogenerators] as mulch films can promote bok choi growth, and plants closer to the electric field produced by the mulch film show more noticeable growth effects, ‘the team says.
Next, they observed the biodegradation of the sheets, stating that the rice protein film completely degraded in 127 days.
This is interesting work that paves the way for further research into the agricultural potential of biodegradable nanogenerators. Clearly there are many questions to investigate, such as the overall yield of this process, how it affects unwanted plant growth and herbicide use, the overall cost and so on.
But the idea that electronic nano generators can be made using food processing waste products is certainly important and worth investigating in more detail.
Ref: Biodegradable triboelectric nanogenerator enabled for vegetable proteins for sustainable agriculture: arxiv.org/abs/2110.01891