A team from two universities in Shandong, China, has developed a new technique that converts the organic waste from the leaves of the local deciduous phoenix trees into a porous carbon material that could find uses in high-tech electronic devices. The leaves, which are abundant in the area in autumn, are usually burned, resulting in a worsening of the country’s air pollution problem.

With such porous biomass carbon being seen as a new functional material with uses as an absorbent as well as an electrode material, this study, which was published in the Journal of Renewable and Sustainable Energy [Ma et al. IRESR (2017) DOI: 10.1063/1.4997019], was based on a straightforward multi-step process that converts the leaves into microspheres that can be incorporated into electrodes as active materials. By first drying the leaves and grinding them into a powder, and then heating them to 220oC for 12 hours, the team produced a powder made up of tiny carbon microspheres. These were then treated with a solution of potassium hydroxide and heated by increasing the temperature in a series of stages from 450oC to 800oC.

The chemical treatment corrodes the surface of the microspheres so that they become very porous, leaving a black carbon powder with an extremely high surface area because of the many pores that have been chemically etched onto their surface. It is this large surface area that provides the useful electrical properties.

The team carried out a range of electrochemical tests on the microspheres to quantify if they could be used in electronic devices, with the current–voltage curves showing that they could make an excellent capacitor, and other tests showing they can act as supercapacitors, with specific capacitances over three times greater than that seen in some graphene supercapacitors. As capacitors store energy by holding a charge on two conductors that are separated by an insulator, and can accept and deliver charges much quicker than a standard rechargeable battery, such supercapacitive materials have great potential for a range of energy storage applications in computer technology and hybrid/electric vehicles.

The supercapacitive properties are more than those reported for carbon powders derived from other biowaste materials, perhaps due to the fine-scale porous structure enabling contact between electrolyte ions and the surface of the carbon spheres, in addition to enhancing ion transfer and diffusion on the carbon surface. The team are now looking to improve the electrochemical properties of the materials by optimizing the preparation process, and achieving doping or modification of the raw materials.