Researchers have created a flexible, lightweight, cost-effective plant-based energy storage device that in the near future could charge devices — even electric cars — within a few minutes.
Energy storage devices are generally in the form of either batteries or supercapacitors. Although both types of devices can deliver electrical currents when required, they have some fundamental differences. While batteries can store large amounts of charge per unit volume, supercapacitors are much more efficient at generating a large quantity of electric current within a short duration. This burst of electricity helps supercapacitors quickly charge devices, unlike batteries that can take much longer.
Supercapacitors have an internal architecture that is more in line with basic capacitors. Both these devices store charge on metal plates or electrodes; however, unlike basic capacitors, supercapacitors can be made in different sizes, shapes, and designs, depending on the intended application. Furthermore, supercapacitor electrodes can also be built with different materials.
The researchers used manganese dioxide nanoparticles for designing one of the two supercapacitor electrodes. The material is available in abundance and is safer compared to other transition metal oxides, like ruthenium or zinc oxide, that are popularly used for making electrodes; however, manganese dioxide suffers from lower electrical conductivity.
Past research has shown that lignin — a natural polymer that glues wood fibers together — used with metal oxides enhances the electrochemical properties of electrodes. There have been few studies looking into combining manganese dioxide and lignin to leverage both of their useful properties.
To create the 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, they assembled the supercapacitor by sandwiching a gel electrolyte between the lignin-manganese dioxide-aluminum electrode and another electrode made of aluminum and activated charcoal.
Upon testing the green electrode, they found that the supercapacitor had very stable electrochemical properties. In particular, the specific capacitance (the ability of the device to store an electrical charge) changed little, even after thousands of cycles of charging and discharging. Also, for an optimal lignin-manganese dioxide ratio, the specific capacitance was observed to be up to 900 times more than what has been reported for other supercapacitors.