By Timo A. Lähde, Helsinki Institute of Physics / Department of Applied Physics
The isolation of graphene, a single atom thick layer of graphite, in 20041 has propelled the study of two-dimensional electron systems to the forefront of theoretical as well as technological progress. This development has recently culminated in the award of the 2010 Nobel Prize to Andre Geim and Konstantin Novoselov of the University of Manchester, UK, for “groundbreaking experiments regarding the two-dimensional material graphene.” While the unusual electronic properties of graphene are exciting in their own right,2 graphene is also having a major impact in the industrial sector, with applications being developed from ultra-sensitive nanomechanical resonators to transparent electrodes. Recently, graphene has become a hot topic in “macroscopic” applications as well, notably in the design and manufacture of ultracapacitors, which have recently become highly valued as power storage systems in electrical light-rail vehicles, diesel-electric battery drive systems, and forklift trucks.
Unlike traditional capacitors, ultracapacitors make use of the “electrical double layer” effect, whereby charge separation is achieved on a length scale of a few nanometers at the interface between the capacitor plates and an electrolyte.3 The “plates” in commercially available ultracapacitors are formed of nanoporous, sponge-like substances, such as activated charcoal. These maximize the surface area exposed to the electrolyte, thereby achieving capacitances thousands of times larger than parallel-plate or electrolyte capacitors. To be specific, the energy densities of current ultracapacitors reach ~10 Wh/kg, whereas lithium-ion batteries feature ~160 Wh/kg. In comparison, a typical gasoline-driven automobile operates at ~ 2400 Wh/kg. While ultracapacitors feature very high charge and discharge rates, little degradation over hundreds of thousands of recharge cycles and low toxicity of component materials, the amount of energy stored per unit weight is considerably lower than that of an electrochemical battery. As a result, finding improved materials that would rectify this situation is a big priority.
Recently, the group led by Rodney Ruoff of the University of Texas at Austin has demonstrated prototypical ultracapacitors with energy densities up to ~100 Wh/kg4 using a novel material, derived from exfoliated graphite oxide and dubbed “chemically modified graphene” (CMG). This research is driven by the high conductivity and extraordinarily high specific surface area of graphene, which reaches ~2630 m2/g, about the area of a football field in 1/500th of a pound of carbon. Graphene thus enables the storage of an exceptional level of charge by allowing a superior number of positive and negative ions to form.
The possibilities opened by graphene go beyond improvements in the energy density of ultracapacitors, as the team of John Miller, president of JME, an electrochemical capacitor company based in Shaker Heights, Ohio, and Ron Outlaw, of the College of William & Mary, Williamsburg, VA, have recently shown.5 By using electrodes made from vertically oriented graphene nanosheets, the team of Miller was able to improve drastically on the RC time constant of extant ultracapacitors, opening up new possibilities for the miniaturization of AC filtering and rectifier circuits. Other ultracapacitors fail in this respect, as the porous electrodes act like resistors in filter circuits. The graphene nanosheets resemble ~600 nm tall “potato chips” standing on edge in rows. The novel ultracapacitors charge and recharge in ~200 microseconds, compared to ~1 second for nanopore-based designs.
These exciting developments indicate that graphene-based ultracapacitors are poised to take a further step in the “graphene revolution,” which has been ongoing since the groundbreaking work of Geim and Novoselov in 2004. However, graphene is also facing stiff competition from a host of other revolutionary materials, such as carbon nanotubes, aerogels, and conductive polymers. The adventure of carbon-based, environmentally friendly energy storage is likely just beginning.
- K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306, 666 (2004).
- A.H. Castro Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, A.K. Geim, Rev. Mod. Phys. 81, 109 (2009).
- J.R. Miller, P. Simon, The Electrochemical Society Interface 17, 31 (2008).
- M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Nano Lett. 8, 3498 (2008).
- J.R. Miller, R.A. Outlaw, B.C. Holloway, Science 329, 1637 (2010).