Northern China’s roadsides are lined with leaves from Phoenix trees, which turn into a problem when autumn rolls around. The abundance of leaves is usually burned when it starts to get cold, which just adds to China’s air pollution problem. Researchers in Shandong, China have discovered a new method to convert the organic waste matter into a porous carbon material that can be used to produce high-tech electronics.
Phoenix tree (Paulownia imperialis) leaves (US National Park Service, Public Domain)
The researchers used a multi-step process to convert tree leaves into a form that could be incorporated into electrodes as active materials. The dried leaves are first ground into a powder and then heated to 220 degrees Celsius for 12 hours. This produces a powder composed of tiny carbon microspheres. These microspheres were 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 and makes them porous. The final product is a black carbon powder with a high surface area due to the presence of the tiny pores that were chemically etched on the surface of the microspheres. The high surface area gives the final product electrical properties.
The investigators then ran a series of standard electrochemical tests on the microspheres to quantify their potential for use in electronic devices. The current-voltage curves for these materials indicate the substance could make a great capacitor. Further tests show that the materials are supercapacitors with capacitances of 367 farads/gram.
A capacitor is a widely used electrical component that stores energy by holding a charge on two conductors that are separated by an insulator. Supercapacitors typically store 10-100 times as much energy as an ordinary capacitor. It accepts and delivers charges faster than a typical rechargeable battery. Supercapacitive materials have great promise for a wide variety of energy storage needs like computer technology and hybrid or electric vehicles.
The research has been focused on looking for ways to convert waste biomass into porous carbon materials that can be used in energy storage technology. Along with the tree leaves, the team has successfully converted potato waste, corn straw, pine wood, rice straw and other agricultural wastes into carbon electrode materials. The team hopes to improve even further on electrochemical properties of porous carbon materials by optimizing the preparation process and allowing for doping or modification of the raw materials.
The super capacitive properties of the porous carbon microspheres from the phoenix tree leaves are higher than the carbon powders derived from other bio-waste materials. The final scale porous structure is key to the property because it facilitates contact between electrolyte ions and the surface of the carbon spheres and enhancing ion transfer and diffusion on the carbon surface. The team hopes to improve even further on the electrochemical properties by optimizing the process and allowing for doping or modification of the raw materials.
A study on this research was published in Journal of Renewable and Sustainable Energy