The growing demand for the production and storage of energy with minimal CO2 emissions represents a continuous research challenge. Research on increasing the amount of energy that can be stored in packages such as lithium-ion batteries and capacitors is particularly active, and achieving increased performance while at the same time reducing CO2 emissions is an attractive goal. In a collaboration between the Max-Planck Institute for Colloids and Interfaces in Germany, the University of Science and Technology in Beijing, China, and the Chinese Academy of Sciences in Taiyuan, Li-Zhen Fan, Magdalena Titirici and colleagues have now demonstrated that high-performance supercapacitors can be constructed using nitrogen-containing carbon materials derived from biomass rather than coal1.

One of the most common types of supercapacitors is the electrical double layer capacitor, which comprises two electrodes on either side of two compartments (double layer) of an electrolyte. Activated carbon is typically used for the capacitor layers because its high porosity — and therefore high surface area — favors good charge accumulation at the interface with the electrolyte. However, activated carbon is typically obtained from special petroleum coke, which is produced from coal and requires complex and expensive processing techniques.

Fig. 1: Scanning electron microscopy image of the activated carbon containing nitrogen.

Fan, Titirici and their colleagues have now explored the use of carbon materials derived from renewable sources. Based on the knowledge that the presence of dopants such as oxygen or nitrogen atoms within the carbon structure increases the storage capacity of porous activated carbon — an effect known as pseudocapacitance — the researchers focused on carbon-based materials that naturally contain nitrogen atoms, such as those obtained by hydrothermal carbonization of biomass derivatives like glucosamine. Increasing the amount of potassium hydroxide used to activate the carbon materials was found to increase the surface area through the formation of micropores while retaining high nitrogen content. The highest performance for the biomass-based capacitors was obtained using activated carbon with an optimum balance between nitrogen content and surface area (Fig. 1).

“This methodology represents a simple procedure to increase the capacitance in double-layer capacitors through a combination of relatively high surface area and the pseudocapacitance effect due to the presence of nitrogen,” says Fan. The team is now attempting to increase the capacity further by introducing more nitrogen atoms, or other elements such as boron.