But now a Chinese research team has figured out a way to convert fallen leaves into devices that store energy.
The scientists used a multi-step, yet simple, process to convert tree leaves into a porous carbon material that can be used to produce high-tech electronics. Their research was recently reported in the Journal of Renewable and Sustainable Energy.
Many of the roads throughout northern China are lined with trees locally known as “phoenix trees.” And despite the government’s disapproval, residents still burn the fallen leaves. This, of course, only compounds the country’s air pollution. For instance, in Beijing alone, about two million tons of leaves and other plant waste are burned every year.
The scientists at the Qilu University of Technology are using the fallen leaves of the phoenix tree to make “organic capacitors.” These can then be used to store energy (like batteries). All while potentially reducing some of that air pollution.
The team’s process involves first grinding the dried leaves into a powder. Then they heat the powder to 220°C (428° F) for 12 hours. The resulting powder is composed of tiny carbon microspheres. These microspheres are then treated with a solution of potassium hydroxide and gradually heated up to 800° C (1472°F).
This chemical treatment corrodes the surface of the carbon microspheres, making them extremely porous. The final product is a black carbon powder with a very high surface area. This is due to the presence of many tiny pores that have been chemically etched on the surface of the microspheres.
And the high surface area gives the final product its extraordinary electrical properties.
The team ran a series of standard electrochemical tests on the porous material to determine its potential for use in high-tech electronics. They discovered that the substance could make an excellent capacitor.
Turning biomass into capacitors isn’t new. (It’s been done with wood and coal.) But the leaf-based devices turned out to be “supercapacitors.” They were much better at storing charge than similar capacitors made from coal.
The Chinese research team has also been researching other types of waste biomass that can be used in energy storage technology. So far, they’ve successfully converted potato waste, corn straw, pine wood and rice straw into carbon electrode materials.
But the supercapacitive properties of the substance made from the phoenix tree leaves are higher than those derived from other biowaste materials.
The process of making organic capacitors does release a small amount of carbon dioxide. But it’s not nearly as much as would be emitted if you let the same quantity of material burn or decay. That’s according to researchers at Penn State University. According to them, “Any type of use of any waste material is a good thing.”
Some researchers are concerned that the natural variability in leaves could make the idea of converting them difficult to commercialize.
Apparently, leaves are less consistent in their character than say, wood or coal. So supercapacitors made from them may also be variable in character.
Will this new discovery ever make a dent in China’s severe air pollution problem? That remains to be seen.