Some 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 flexed. If the materials are carefully selected, this process transfers charge from one sheet to the other, generating a voltage between them, a phenomenon known as the triboelectric effect. Flexing the material in the other direction reverses the polarity.
The resulting device--a triboelectric nanogenerator-- is today the subject of intense study. The hope is that it can power a new generation of internet-connected devices by harvesting energy from almost any mechanical vibrations or movement.
But there is a problem, one that is almost ubiquitous across the electronics industry: waste. Triboelectric nanogenerators are made from sheets of different kinds of plastics—for example, polytetrafluoroethylene or (PTFE), fluorinated ethylene propylene and polyethylene terephthalate (PET).
Questions of Waste
These are common, relatively cheap and easy to make. But they are ultimately derived from oil, stable and long-lasting with various known environmental impacts. That’s an unsavory environmental legacy that would be better to avoid. But how?
Now we have an answer thanks to the work of Jianfeng Ping at Zhejiang University in China and colleagues who have found a way to make triboelectric nanogenerators from plant-based 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 from the processing of crops such as rice, wheat, peanuts and soyabeans. They combined these proteins with polylactic acid film to create a layer they could test for triboelectric effects, when placed in contact with a film of PDA, another biodegradable polymer.
It turns out that rice protein produces the most powerful voltage effect. The team thinks this is the result of the chemical structure of the protein, in particular the way amide groups bond with water molecules.
It is this hydrogen bonding that friction seems to influence, creating a voltage. “We can conclude that the degree of coupling with water molecules is an important factor in triboelectric positivity,” say Jianfeng and colleagues.
So what can you do with a biodegradable triboelectric nanogenerator? The researchers say one application is in encouraging plant growth. They point at that various studies have shown that plants grown in electric fields tend to be bigger and grow faster, probably because the field encourages the flow of highly polar water molecules through the plant tissue.
This technique has never been widely used because of the electronic infrastructure required to make it work. But biodegradable sheets that can produce the required voltage might change that.
As a proof-of-concept, Jianfeng and co grew Chinese celery cabbage, or bok choi, through perforated sheets of biodegradable nanogenerators that generated a field of up to 180V. They compared growth rates and size of plants grown under similar conditions but without the voltages. “Our results suggest that using bio-[triboelectric nanogenerators] as mulch films can promote the growth of bok choi, and plants closer to the electric field produced by the mulch film exhibit more obvious growth effects,” say the team.
Afterwards, they observed the biodegradation of the sheets, saying the rice protein film completely degraded in 127 days.
That’s interesting work that paves the way for more research on the agricultural potential of biodegradable nanogenerators. Clearly there are many questions to investigate, such as the overall yield from this process, how it affects the growth of unwanted plants and the use of herbicides, the overall cost and so on.
But the notion that electronic nanogenerators can be made using waste products from food processing is surely important and worth investigating in more detail.
Ref: Plant-Protein-Enabled Biodegradable Triboelectric Nanogenerator for Sustainable Agriculture : arxiv.org/abs/2110.01891
