Carbon Nano-Onion Supercaps

August 26, 2010

A capacitor is a convenient device for storing electrical charge to produce electrical power. In this age of increasing device complexity, the capacitor is resplendent in its simplicity, since it's just two conductors separated by a dielectric material. A few simple equations tell you everything you need to know about energy storage in a parallel plate capacitor.

 C = κεoA/d C is capacitance, κ is the dielectric constant, Q = C V εo is the permittivity of free space, I = ∂Q/∂t A is the area of the capacitor plates, W = (1/2) C V2 d is the plate separation, Q is the charge, V = Vo e(-t/RC) V is the voltage, Vo is the initial voltage, Q = C Vo e(-t/RC) W is the stored energy, t is time, I = (Vo/R) e(-t/RC) and R is the load resistance.

The real trick here is to keep all your units straight. I tend to use a value of 8.854 x 10-12 F/m for εo, the permittivity of free space, since my brain is tuned for picofarads (10-12 F). Farads and volts will give the stored energy in joules. A watt is a joule per second. Since capacitors lose voltage as they discharge, this leads to some problems in extracting energy from them. It can be seen from these equations that large plate area, small plate separation, high dielectric constant and high voltage leads to high capacitance and high energy storage. What doesn't appear in any equation is the material requirement that the breakdown voltage cannot be exceeded as you reduce the separation of the capacitor plates.

Electrolytic capacitors have been with us for more than a century. They address the problem of generating high capacitance in a small volume by using an extremely thin anodization layer on a conductor (usually aluminum or tantalum foil) as the dielectric layer. Since an electrolyte is used as a conductor, high frequency operation is not possible, and this relates also to the charging and discharging times. One way to increase capacitance is to use electrodes of porous materials that have high surface area. A recent advance in electrolytic capacitor technology has been the electric double-layer capacitor, also called a supercapacitor. The dielectric layer is actually an interface between conductive electrolyte layers. At low voltages there is no current flow through the interface, so the assembly acts like a capacitor with a very thin dielectric. From the capacitor equations, you can see that this very thin dielectric, combined with the use of very high area carbon electrodes made from activated charcoal, gives capacitors of many farads in a few cubic inches, albeit at low operating voltages (about 3 - 5 volts).

A research team at Drexel University, led by Yury Gogotsi, Director of the Drexel Nanotechnology Institute, has improved on the supercapacitor concept by using nanotechnology. Their nano-synthesis was a little extreme, and it might not be practical for large scale production. As described in the supplementary information to their Nature Nanotechnology paper[6], explosives were used to form diamonds about 5 nanometers in size. Subsequent baking of these diamonds at 1800oC transforms them into carbon "onions" that have about 6-10 shells. Using these nano-onions, deposited electrophoreticly, as the capacitor electrode rather than porous carbon gave capacitors that have a thousand-fold faster charging and discharging rate, albeit at a lower quantity of stored charge. The quoted discharge rate is 200 volts per second, which is likely derived from something like two volts in ten milliseconds. The Drexel researchers believe that the speed-up is from a faster ionic diffusion rate at the electrode surface. These nano-onion assemblies have capacitances that are four orders of magnitude higher, and energies per volume that are an order of magnitude higher than electrolytic capacitors. The surface area reduction of the electrodes was about a factor of four; namely, 520 square meters per gram for the nano-onions as compared to 2,000 square meters per gram for a conventional porous carbon electrode. However, if you need millisecond response from your energy source, this may be a solution.

Drs. Vadym Mochalin (left) and Yury Gogotsi (right) in front of a vacuum furnace used for the synthesis of onion-like carbon. Photo supplied by Drexel University.

Science is increasingly becoming a team effort and an international effort. The Drexel team collaborated with scientists in Toulouse, France, at several institutions (CNRS, the Université de Toulouse and the Université Paul Sabatier de Toulouse) A grant from the Partner University Fund of the French-American Cultural Exchange allowed two of the Toulouse researchers to spend some time at the Drexel laboratories. Additional funding was from the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

As to why I've published a photograph of two guys in a laboratory, rather than some graph of capacitor properties, it's refreshing to see a real laboratory environment, and not a television version of a laboratory. If you pan over a few feet, you're bound to find all the old, but still useful, pieces of equipment that were moved out of camera range for the photo opportunity. I must confess, however, that the CBS television show, "The Big Bang Theory," has some very believable laboratories.

References:

Linked Keywords: capacitor; dielectric; dielectric constant; permittivity of free space; farad; joule; breakdown voltage; Electrolytic capacitors; aluminum; tantalum; electric double-layer capacitor; activated charcoal; Drexel University; Drexel Nanotechnology Institute; nanotechnology; Nature Nanotechnology; diamond; electrophoretic deposition; Toulouse, France; CNRS; Université de Toulouse; Université Paul Sabatier de Toulouse; Partner University Fund of the French-American Cultural Exchange; U.S. Department of Energy; The Big Bang Theory.