The "green" supercapacitor, say the researchers, is flexible, lightweight, and cost-effective, and in the near future could charge devices - even electric cars - within a few minutes.
"Integrating biomaterials into energy storage devices has been tricky because it is difficult to control their resulting electrical properties, which then gravely affects the devices’ life cycle and performance," said Prof Hong Liang, in the Department of Mechanical Engineering. "Also, the process of making biomaterials generally includes chemical treatments that are hazardous. We have designed an environmentally friendly energy storage device that has superior electrical performance and can be manufactured easily, safely and at much lower cost."
Supercapacitors store charge on metal plates or electrodes. However, unlike basic capacitors, they can be made in different sizes, shapes, and designs, depending on the intended application. Furthermore, supercapacitor electrodes can also be built with different materials.
For their work, the researchers were attracted to manganese dioxide nanoparticles for designing one of the two supercapacitor electrodes.
"Manganese dioxide is cheaper, available in abundance and is safer compared to other transition metal oxides, like ruthenium or zinc oxide, that are popularly used for making electrodes,” says Liang. “But a major drawback of manganese dioxide is that it suffers from lower electrical conductivity."
Past research, say the researchers, has shown that lignin - a natural polymer that glues wood fibers together - used with metal oxides enhances the electrochemical properties of electrodes. However, there have been few studies looking into combining manganese dioxide and lignin to leverage both of their useful properties.
To create their electrode, the researchers treated purified lignin with a commonly available disinfectant called potassium permanganate. They then applied high heat and pressure to initiate an oxidation reaction that results in the breaking down of potassium permanganate and the deposition of manganese dioxide on lignin.
Next, they coated the lignin and manganese dioxide mixture on an aluminum plate to form the "green" electrode. Finally, the