We tend to focus on charge storage in terms of the batteries that power our electronic devices and, increasingly, our cars. But charge storage devices now make an appearance on scales ranging from tiny implanted medical devices to on-grid electrical storage. No single technology performs well across that range of applications, so scientists have continued to experiment with different kinds of batteries and capacitors. A paper that was released by Nature Nanotechnology describes a new kind of capacitor, built from onion-like shells of graphene, that may give us another option for specific needs: discharge rates of up to 200V per second, "three orders of magnitude higher than conventional supercapacitors."

Capacitor technology isn't suitable for all use cases since, at the moment, it simply can't store as much power per unit volume as a battery can. They do, however, have some very useful properties: capacitors retain their performance through an indefinite number of discharge cycles, and energy stored in a capacitor can be mobilized much more quickly than that in a battery. The new work takes this last feature, already a strength, and improves upon it.

As anyone who didn't sleep through high school physics will recall, at its most basic level, a capacitor involves two plates separated by an insulator. Load one plate with a charge, and like charges will be repelled from the other, effectively storing the charges. There are a number of ways to achieve this in practice that are far more sophisticated than two plates, but the device here is a variant of the electrochemical capacitor.

Electrochemical capacitors rely on a combination of charge-carrying ions and an electrode that doesn't chemically interact with them. The charges in the ions act as the equivalent of one plate, while the electrode acts as the other. As a result, the greater the surface area of the electrode, the greater the charge that can be stored. Electrodes made from activated or porous carbon are some of the highest rated supercapacitors available. Although this provides a high storage capacity, there is a tradeoff: it takes some time to shuffle ions into and out of the mesh of carbon, which slows down the rate at which charging and discharging can occur.

The paper's authors decided that a more ordered, less porous, but otherwise similar material might enable charges to meet up with and depart the carbon a bit more efficiently. For this, they turned to what's apparently termed "onion-like carbon" (OLC), a collection of concentric spheres formed of single-atom sheets of carbon. Think of a set of Russian dolls crossed with a buckyball, and you're on the right track. The stuff isn't easy to make—the authors describe the most efficient process as "the annealing of detonation nanodiamond powders"—but it's apparently quite easy to work with once it forms.

To create a capacitor out of the OLC, all that was required was an exposed electrode; the OLC could be electrodeposited on it, and would adhere without any binding agent. Once this was submerged in an appropriate ionic solution (1M tetraethylammonium tetrafluoroborate in propylene carbonate, for the curious), the capacitor was ready to go. For comparison, the authors prepared a similar device using activated carbon on a similar electrode.

As expected, the activated carbon outperformed the OLC when it came to total capacitance, but only by a factor of about seven (9.0 Farads/square centimeter vs. 1.3 for the OLC). And, when it came to moving charge in and out of the device, the OLC came out well ahead. Before the rate of charge or discharge cleared 20V/s, the performance of the activated carbon device dropped below the OLC one; its performance remained relatively stable out to 200V/s. Using Watts (which incorporates an element of time), the authors calculate that the power density of the device is over 100 times that of the activated carbon version.

Compared to a thin film lithium battery, its energy per volume is an order of magnitude lower, but its power is over 10,000 times higher. In the paper's final graph, which compares these two measures, the authors show that the device is the only one with this sort of performance, which means that OLC could find a home for applications that require large bursts of power, long lifetimes, and a decent storage capacity.

They also suggest that there's significant room for improvement. Smaller, more uniform onions, more tightly packed electrodes, and a better electrolyte could all boost performance.

Nature Nanotechnology, 2010. DOI: 10.1038/NNANO.2010.162 (About DOIs).