Researchers Develop Graphene-Based Ultracapacitors; Potential for Doubling Storage Capacity of Current Materials
|Low and high (inset) magnification SEM images of CMG particle electrode surface. Click to enlarge. Credit: ACS|
Researchers at The University of Texas at Austin have developed a new class of carbon material—chemically modified graphene (CMG)—that is made from a 1-atom thick sheet of carbon. CMG materials can be functionalized as needed, and the researchers demonstrated their performance in an ultracapacitor cell. A paper on their work was published online 13 September in the journal Nano Letters.
The surface area of a single graphene sheet is 2,630 m2/g, substantially higher than values derived from BET surface area measurements of activated carbons used in current electrochemical double layer capacitors. The group measured specific capacitances of 135 and 99 F/g in aqueous and organic electrolytes, respectively, in devices using the CMG material.
Through such a device, electrical charge can be rapidly stored on the graphene sheets, and released from them as well for the delivery of electrical current and, thus, electrical power. There are reasons to think that the ability to store electrical charge can be about double that of current commercially used materials. We are working to see if that prediction will be borne out in the laboratory.—Professor Rod Ruoff
EDLC ultracapacitor units consist of two porous carbon electrodes isolated from electrical contact by a porous separators. The separator and the electrodes are impregnated with an electrolyte, which allows ionic current to flow between the electrodes while preventing electronic current from discharging the cell. Current collectors of metal foil or carbon impregnated polymers are used to conduct electrical current from each electrode.
The CMG system of individual sheets does not depend on the distribution of pores in a solid support to give it its large surface area, rather every chemically modified graphene sheet can “move” physically to adjust to the different types of electrolytes (their sizes, their spatial distribution). Thus, access to the very high surface area of CMG materials by the electrolyte can be maintained while preserving the overall high electrical conductivity for such a network.—Stoller et al. (2008)
Unlike conventional commercial electrodes which use additives such as carbon black to enable rapid electrical charge transfer from the cell, the high electrical conductivity of the graphene materials eliminates the need for conductive fillers and allows increased electrode thickness. Increasing the electrode thickness and eliminating additives leads to an improved electrode material to collector/separator ratio, which in turn further increases the energy density of the packaged ultracapacitor.
The morphology of the CMG material used in the initial study results in only a portion of the graphene sheets (those at the surface of the particles) being exposed to electrolyte. Nevertheless, the measured specific capacitances showed that the graphene material works well with current commercial electrolytes, has good electrical conductivity, and has very promising charge storage capability.
Chemically modified graphenes with good electrical conductivity and very large (and in principle completely accessible) surface areas, are extremely promising candidates for EDLC ultracapacitors. Our results applying these materials to ultracapacitors show they are compatible with commonly used electrolyte systems. In addition, these CMG materials are based on abundantly available and cost-effective graphite. Ultracapacitors based on these materials could have the cost and performance that would dramatically accelerate their adoption in a wide range of energy storage applications.—Stoller et al. (2008)
Funding and support was provided by the Texas Nanotechnology Research Superiority Initiative, The University of Texas at Austin and a Korea Research Foundation Grant for fellowship support for Dr. Park.
Meryl D. Stoller, Sungjin Park, Yanwu Zhu, Jinho An, and Rodney S. Ruoff (2008) Graphene-Based Ultracapacitors. ASAP Nano Lett., doi: 10.1021/nl802558y