Researchers at The University of Texas at Austin’s Cockrell School of Engineering have synthesized a new carbon with a continuous three-dimensional network of highly curved, atom-thick walls that form primarily 0.6–5 nm width pores. Two-electrode supercapacitor cells constructed with this material yielded high values of gravimetric capacitance and energy density with organic and ionic liquid electrolytes.
Supercapacitors made from the material have an energy density that is approaching the energy density of lead-acid batteries, while retaining the high power density that is characteristic of supercapacitors. The processes used to make this carbon are readily scalable to industrial levels, the group, led by materials science and mechanical engineering Professor Rodney S. Ruoff, reports in a paper published in Science.
Supercapacitors are similar to batteries in that both store electric charge. Batteries do so through chemical reactions between metallic electrodes and a liquid electrolyte. Because these chemicals take time to react, energy is stored and released relatively slowly. But batteries can store a lot of energy and release it over a fairly long time.
Supercapacitors, on the other hand, store charge in the form of ions on the surface of the electrodes, similar to static electricity, rather than relying on chemical reactions. Charging the electrodes causes ions in the electrolyte to separate, or polarize, as well—so charge gets stored at the interface between the electrodes and the electrolyte. Pores in the electrode increase the surface area over which the electrolyte can flow and interact, thereby increasing the amount of energy that can be stored.
Because most supercapacitors can’t hold nearly as much charge as batteries, their use has been limited to applications where smaller amounts of energy are needed quickly, or where long life cycle is essential, such as in mobile electronic devices.
The new material, synthesized by using chemical activation of microwave exfoliated graphite oxide (MEGO), has a BET surface area of up to 3100 m2 per gram, a high electrical conductivity, and a low O and H content.
Ruoff’s research team of about 40 people collaborated with faculty and students from The University of Texas at Dallas, scientific staff at Brookhaven National Laboratory in New York and staff members at QuantaChrome Instruments in Florida.
Ruoff had formed a hypothesis that the activated MEGO (a-MEGO) material consisted of a continuous three-dimensional porous network with single-atom-thick walls, with a significant fraction being “negative curvature carbon,” similar to inside-out buckyballs. He turned to Erich Stach at Brookhaven for help with further structural characterization to verify or refute this hypothesis.
Stach and Brookhaven colleague Dong Su conducted a wide range of studies at the Lab’s Center for Functional Nanomaterials (CFN), the National Synchrotron Light Source (NSLS), and at the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory, all three facilities supported by the DOE Office of Science. Their observations confirmed Ruoff’s hypothesis.
The University of Texas at Austin’s Office of Technology Commercialization has filed a patent with the US Patent Office on behalf of the inventors.
Yanwu Zhu, Shanthi Murali, Meryl D. Stoller, K. J. Ganesh, Weiwei Cai, Paulo J. Ferreira, Adam Pirkle, Robert M. Wallace, Katie A. Cychosz, Matthias Thommes, Dong Su, Eric A. Stach, and Rodney S. Ruoff (2011) Carbon-Based Supercapacitors Produced by Activation of Graphene. Science doi: 10.1126/science.1200770