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Northern China has a smoky problem caused by autumn leaves, but now there could be a fix: simply turn them into devices that store energy.
Many roads in this part of China are lined with trees of the genus Paulownia, sometimes called phoenix trees. Despite the government’s disapproval, many locals burn the fallen leaves, worsening the country’s notorious air pollution. In Beijing alone, about 2 million tonnes of leaves and other plant waste are burned every year.
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Now, Hongfang Ma at the Qilu University of Technology in Jinan and her colleagues have figured out how to turn phoenix tree leaves into organic capacitors. These could be used like batteries to store energy, potentially avoiding some of that air pollution into the bargain.
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The process of making organic capacitors does release a little carbon dioxide, but not nearly as much as would be emitted if you let the same quantity of material burn or decay, says Caroline Burgess Clifford at Penn State University. “Any type of use of any waste material is a good thing.”
The researchers cleaned and dried the leaves before grinding them into a fine powder. They then dispersed the powder in water and heated the mixture at 220°C for 12 hours before filtering out any ash or contaminants. This process leaves behind a brown powder of carbon microspheres.
Opening up pores
Adding the microspheres to a potassium hydroxide solution and heating it again at up to 800°C corroded their surfaces, leaving behind a black powder covered in minuscule pores. The pores give the microspheres a high surface area, which allows capacitors made of the material to store more charge.
Turning biomass into capacitors isn’t new, says Fred Cannon at Penn State University. “Others have done the same sort of thing, only with wood or coal rather than leaves,” he says.
In tests, however, the leaf-based devices turned out to be supercapacitors, much better at storing charge than similar capacitors made from coal.
Despite this feat, Cannon is concerned that the natural variability in leaves could make the idea hard to commercialise.
“Leaves are less consistent in their character, and so supercapacitors made from them would be variable in their character, I would anticipate,” he says.
Journal reference: Journal of Renewable and Sustainable Energy, DOI: 10.1063/1.4997019
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