Graphene for the Green Grid

Ultracapacitors that store more could help the grid run smoothly.

Integrating irregular sources of renewable energy, such as wind and solar, with the electrical grid, while keeping power output steady, is going to be a big challenge. Energy-storage devices called ultracapacitors could help by storing sudden surges of power. But much will depend on developing a new generation of ultracapacitors with enough storage capacity to meet the likely demand.

Graphene power: Graphene Energy hopes that graphene electrodes such as this one will increase the energy-storage capacity and power output of ultracapacitors. This image, which shows the edge of a graphene electrode, was made with a scanning-electron microscope. Credit: Meryl Stoller

Graphene Energy, a startup based in Austin, TX, hopes that ultracapacitors with electrodes made of graphene--sheets of carbon just an atom thick--will be the solution. The storage capacity of an ultracapacitor is limited only by the surface area of its electrodes, and graphene offers a way to greatly increase the area available.

Ultracapacitors store energy electrostatically, instead of chemically, as in batteries. During charging, electrons come to the surface of one electrode, and electron "holes" form on the surface of the other. This draws positive ions in an electrolyte to the first electrode and negative ions to the second. By contrast, the chemical reactions used to charge batteries limit the speed with which they can be charged and eventually cause the electrode materials to break down. Ultracapacitors can be charged and discharged very rapidly, in seconds rather than minutes, and can be recharged millions of times before wearing out.

However, ultracapacitors currently on the market can't match batteries for energy density, so they're mostly used in hybrid systems alongside batteries or for niche applications. Because these devices can handle a rapid influx of large amounts of energy, they're often used to recover energy--for example, when a city bus brakes or a gantry crane lowers its cargo. Ultracapacitors employed in this way have reduced by 40 percent the energy needed by some cranes used in Japanese ports. A few power tools, including an electric drill, take advantage of the rapid recharging ability of ultracapacitors.

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Graphene Energy hopes to open up new ultracapacitor applications by developing devices with far higher power output. These ultracapacitors could perhaps be used to regulate surges in the electrical grid or to power hybrid transportation vehicles. The company has $500,000 in seed funding to commercialize graphene ultracapacitors developed by Rodney Ruoff, a professor of mechanical engineering at the University of Texas at Austin. Ruoff is a cofounder of Graphene Energy and also serves as the company's technology advisor.

Existing ultracapacitors use electrodes made from activated carbon--a porous, charcoal-like material that has a very high surface area. Activated carbon stores charge in tunnel-like pores, and it takes about one second for it to travel in and out. This is very fast compared with the fastest batteries, but activated carbon has a limited power output.