A scanning electron microscopy image of porous carbon microspheres made from phoenix tree leaves. Image: Hongfang Ma, Qilu University of Technology.
A scanning electron microscopy image of porous carbon microspheres made from phoenix tree leaves. Image: Hongfang Ma, Qilu University of Technology.

The roadsides in Northern China are peppered with deciduous phoenix trees, producing an abundance of fallen leaves in autumn. These leaves are generally burned in the colder season, exacerbating the country's air pollution problem. A team of researchers in Shandong, China, has now discovered a new use for this organic waste matter, by converting it into a porous carbon material that can be used to produce high-tech electronics. The advance is reported in a paper in the Journal of Renewable and Sustainable Energy.

The investigators used a multistep, yet simple, process to convert tree leaves into a carbon material that could be incorporated into electrodes as active materials. The dried leaves were first ground into a powder, then heated to 220°C for 12 hours to produce a powder composed of tiny carbon microspheres. The researchers then treated these microspheres with a solution of potassium hydroxide and heated them by increasing the temperature in a series of jumps from 450°C 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 the 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.

The researchers 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 indicated that the substance could make an excellent capacitor. Further tests showed that the materials are, in fact, supercapacitors with specific capacitances of 367 Farads/gram, over three times higher than values seen in some graphene supercapacitors.

A capacitor is a widely used electrical component that stores energy by holding a charge on two conductors, separated from each other by an insulator. Supercapacitors can typically store 10–100 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.

The researchers, led by Hongfang Ma at Qilu University of Technology, have been looking for ways to convert waste biomass into porous carbon materials that can be used in energy storage technology. In addition to tree leaves, the team, together with others, has successfully converted potato waste, corn straw, pine wood, rice straw and other agricultural wastes into carbon electrode materials.

The supercapacitive properties of the porous carbon microspheres made from phoenix tree leaves are higher than those reported for carbon powders derived from these other biowaste materials. The fine-scale porous structure seems to be key to this property: 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. Ma and her colleagues hope to improve even further on these electrochemical properties by optimizing the preparation process and allowing for doping or modification of the raw materials.

This story is adapted from material from the American Institute of Physics, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.