Graphene Ultracapacitors

Graphene is very much the material of the moment.

But graphene actually dates back to 1961. Hanns-Peter Boehm and coauthors Clauss, Fischer, and Hofmann isolated and identified single graphene sheets by transmission electron microscopy (TEM) and X-ray diffraction in 1961 and authored the IUPAC (International Union of Pure and Applied Chemistry) report formally defining the term graphene in 1994. He must have been surprised to learn of its discovery in 2004.

Graphene is a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, and is a basic building block for graphitic materials of all other dimensionalities. It can be wrapped up into 0D fullerenes, rolled into 1D nanotubes or stacked into 3D graphite.

“Electrons in graphene, obeying a linear dispersion relation, behave like massless relativistic particles. This results in the observation of a number of very peculiar electronic properties – from an anomalous quantum Hall effect to the absence of localization – in this, the first two-dimensional material. It also provides a bridge between condensed matter physics and quantum electrodynamics, and opens new perspectives for carbon-based electronics.” (M.I. Katsnelson)

Properties of graphene are still being discovered and are leading to new studies of relativity and a wave of potential applications in physics, electronics, chemistry and biology (transistors, gas molecule detection, nano-ribbons, nano-tubes, bio-devices and transparent electrodes).


The IEEE reports that the ultracapacitor—the battery’s quicker cousin—just got faster and may one day help make portable electronics a lot smaller and lighter.  John Miller, president of the electrochemical capacitor company JME, in Shaker Heights, Ohio, and his team reported the new ultracapacitor design this week in Science.

Ultracapacitors don’t store quite as much charge as batteries but can charge and discharge in seconds rather than the minutes batteries take. Using nanometer-scale fins of graphene, the researchers built an ultracapacitor that can charge in less than a millisecond. This agility, its designers say, means that the devices could replace the ubiquitous bulky capacitors that smooth out the ripples in power supplies to free up precious space in gadgets and computers.

ultracapacitor cell:

One team member, Ron Outlaw, a material scientist at the College of William and Mary, in Williamsburg, Va., came up with an electrode consisting of up to 4 sheets of graphene —a one-atom-thick form of carbon with unusual electronic properties. The graphene was formed so that it stuck out vertically from a 10-nanometer-thick graphite base layer.

Miller’s team, which also included Brian Holloway, a program manager at the Defense Advanced Research Projects Agency (DARPA), tested its graphene ultracapacitor in a filtering circuit, part of an AC rectifier. Many rectifiers leave a slight AC echo behind, called a “voltage ripple,” and it’s the capacitor’s job to smooth it out. In order to do that, the capacitor needs to respond well at double the AC frequency—120 hertz in the United States. Most commercial ultracapacitors fail at this filtering role at around 0.01 Hz, but when Miller’s team tested its ultracapacitor in such a 120-Hz filtering circuit, it did the job. That means the smaller ultracapacitors could replace the big electrolytic capacitors that do the filtering now. Miller estimates that a commercial version, operating at 2.5 volts, could be less that one-sixth the size of any other 120-Hz filtering technology.

But even if graphene proves to be more promising than carbon nanotubes, silicon isn’t going away anytime soon.

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