Scientists from the University of Maryland have used an anode made of carbonised oak leaf to boost performance of a sodium battery.
Using the ‘leaf’ anode, scientists found a battery could hold 90% of its charge after 200 cycles and kept its charge efficiency at around 75%. The results were unveiled in a paper published in the journal ACS Applied Materials Interfaces.
Researchers used pyrolysis to burn off everything but the leaf’s carbon structure, then soaked it in hydrogen chloride for six hours before pumping it full of sodium.
The paper explains how the leaf’s upper layers of carbon became sheets of nanostructured carbon which absorbed the sodium-ions.
Hongbian Li, from the National Centre for Nanoscience and Technology in Beijing, China, and visiting student Fei Shen were the main authors of the paper. Both have visited the University of Maryland’s department of materials science and engineering.
Shen said: “The natural shape of a leaf already matches a battery’s needs: a low surface area, which decreases defects; a lot of small structures packed closely together, which maximizes space; and internal structures of the right size and shape to be used with sodium electrolyte.”
On paper, sodium is a good competitor to lithium-ion because it can hold more charge.
However, a stumbling block to comercialisation comes in finding a cheap anode material that allows sodium-ions to pass through it. Once achieved, this will allow better charge-and-discharge cycle rates, opening up use in different applications including large-scale energy storage.
Liangbing Hu, an assistant professor of materials science and engineering and a researcher with the University of Maryland Energy Research Center (UMERC), said: “A leaf is designed by nature to store energy for later use, and using leaves in this way could make large-scale storage environmentally friendly.”
Other studies have shown that melon skin, banana peels and peat moss can be used in this way, but a leaf needs less preparation.
The work was supported by the Department of Energy’s Energy Frontier Research Center program, as part of Nanostructures for Electrical Energy Storage.
The next step will involve the scientists investigating different types of leaves to find the best thickness, structure and flexibility for electrical energy storage.