Supercapacitors made from autumn leaves

With spring finally having arrived Down Under, China is now heading into autumn, resulting in an abundance of leaves falling from its roadside phoenix trees in the process. Now, researchers led by Professor Hongfang Ma of Qilu University of Technology have discovered a way to convert this leaf litter into a porous carbon material that can be used to produce high-tech electronics.

The investigators used a multistep process to convert tree leaves into a form that could be incorporated into electrodes as active materials, with the results published in the Journal of Renewable and Sustainable Energy. The dried leaves were first ground into a powder, then heated to 220°C for 12 hours. This produced a powder composed of tiny carbon microspheres. These microspheres were then treated with a solution of potassium hydroxide and heated by increasing the temperature in a series of jumps from 450 to 800°C.

The chemical treatment corrodes the surface of the carbon microspheres, making them extremely porous. The final product, a black carbon powder, has a very high surface area due to the presence of many tiny pores that have been chemically etched on the surface of the microspheres. The high surface area gives the final product its extraordinary electrical properties.

Scanning electron microscopy (SEM) image of porous carbon microspheres. Image credit: Hongfang Ma, Qilu University of Technology.

The investigators ran a series of standard electrochemical tests on the porous microspheres to quantify their potential for use in electronic devices. The current-voltage curves for these materials indicate that the substance could make an excellent capacitor. Further tests show that the materials are in fact supercapacitors, with specific capacitances of 367 farads per gram — more than three times higher than values seen in some graphene supercapacitors.

Supercapacitors can typically store 10 to 100 times as much energy as an ordinary capacitor, and can accept and deliver charges much faster than a typical rechargeable battery. For these reasons, supercapacitive materials hold great promise for a wide variety of energy storage needs, particularly in computer technology and hybrid or electric vehicles.

In addition to tree leaves, the research team and others have successfully converted potato waste, corn straw, pine wood, rice straw and other agricultural wastes into carbon electrode materials. However, the supercapacitive properties of the porous carbon microspheres made from phoenix tree leaves are higher than those reported for carbon powders derived from other biowaste materials. The fine scale porous structure seems to be key to this property, since it facilitates contact between electrolyte ions and the surface of the carbon spheres, as well as enhancing ion transfer and diffusion on the carbon surface.

Professor Ma and her colleagues hope to improve even further on the electrochemical properties of porous carbon materials by optimising the preparation process and allowing for doping or modification of the raw materials.

Top image: Phoenix tree (Paulownia imperialis) leaves. Image credit: US National Park Service.