Beating graphene to push supercapacitors closer to batteries

Scientific Method / Science & Exploration

Adding nitrogen to carbon materials boosts capacitance above that of graphene.

by Shalini Saxena - Dec 30, 2015 11:44am CST

Oak Ridge National Lab

Most people think of batteries when they consider energy storage, but capacitors are an alternative in some use cases. Capacitors are used in almost all electronic devices, often to supply temporary power when batteries are being changed to prevent loss of information. In addition to everyday devices, they are also used in more obscure technologies, including certain types of weapons.

Understanding the supercapacitor

Unlike batteries, capacitors use static electricity to store energy. In their simplest form, they contain two conducting metallic plates with an insulating material (dielectric) placed in between. A typical capacitor charges instantly but usually cannot hold a great deal of charge.
Supercapacitors can at least partly overcome this shortcoming. They differ from the typical capacitor in that their "plates" provide significantly larger surface area and are much closer together. The surface area is increased by coating the metal plates with a porous substance. Instead of having a dielectric material between them, the plates of a supercapacitor are soaked in an electrolyte and separated by an extremely thin insulator.
Carbon supercapacitors offer high electrical power, low weight, and fast charge-discharge cycles. But it's difficult to get carbon to provide a high enough surface area to bring the energy density up to where it could compete directly with batteries.
Though some carbon materials have been made to exhibit a high supercapacitance in theory, they are not able to translate those gains into real-world applications. For example, graphene supercapacitors exhibit a theoretical capacitance of 550 F/g but only reach 300 F/g when used in real-life applications.

Improving one atom at a time

Recently, scientists have focused on altering the surface of carbon-based supercapacitors to increase their potential to store charge. In this case, the supercapacitor system under investigation is composed of carbon plates that contain nano-sized pores (mesoporous carbon) with a polymer insulator. They have altered the surface of the mesoporous carbon plates by the addition of nitrogen. Doping in nitrogen has allowed for reactions between the nitrogen and carbon. These "redox" reactions result in the movement of electrons from one species to another.  
In this study, the scientists demonstrated the ability to produce an electrochemically active substance from layered carbon (similar to graphene) by nitrogen doping.
In order to make the material, they used a sacrificial porous silica template containing self-assembled tubes. This material was then covered with a thin layer of carbon. The silica was then etched away, leaving a self-supported ordered superstructure, with a thickness of only a few atomic layers of carbon.
Since this process was rather involved, they also demonstrated a simplified, template-free method to produce a similar carbon structure with the same overall performance.
These materials were imaged and found to have nanometer-sized tubes that are evenly spaced throughout the material. The tubes themselves were found to be composed of graphene-like sheets with fewer than five total layers. After nitrogen doping, these structural features still remained, but the surface area increased dramatically, as did the total pore volume. The higher surface area should allow it to store more charges.

Performance doping with nitrogen

The scientists tested the capacitance of the nitrogen-doped structure using an aqueous electrolyte. The system was found to have a capacitance of 855 F/g—quite a big step above the graphene-based materials. They also found that the system can be charged and discharged very quickly.
The unusually high capacitance exhibited by this system can be attributed to robust redox reactions. In the course of these reactions, the nitrogen reacts with the carbon on the surface of the plates, forming a variety of compounds. As a result, the layered carbon material is transformed into an electrochemically active substance while maintaining its electrical conductivity. Nitrogen doping also altered other physical properties that are conducive to supercapacitive performance, including resistance, hydrophobicity, and electrostatic charge.
After nitrogen doping, these carbon-based systems can store 41 watt-hours per kilogram. Though that's still not enough to compete with batteries in their energy-storage capabilities, this approach demonstrates that significant improvements in supercapacitor energy storage are still possible.
Science, 2015. DOI: 10.1126/science.aab3798 (About DOIs).