Thursday, February 05, 2009
Ultracapacitors that store more could help the grid run smoothly.
By Katherine Bourzac
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
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 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 breaks 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.
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 and chair 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.
To make the graphene for its electrodes, Ruoff's team starts by putting graphite oxide in a water solution. This causes the material to flake into atom-thin sheets of graphene oxide. Next, the oxygen atoms are removed, leaving the graphene behind. So far, Ruoff's lab has made graphene ultracapacitors that match the performance of those made using activated carbon. With further refinements, he says, they should outperform activated carbon, although the steps that his company is taking to achieve this remain secret.
Based on a description of the graphene ultracapacitors published last September in the journal Nano Letters, John Miller of JME, a research and consulting firm that specializes in electrochemical capacitors, says that it should indeed be possible to improve their performance. The graphene electrode described in this paper is "wadded into a ball like a crumpled piece of paper," says Miller. "You don't have full access to the surface."
If Graphene Energy can grow the electrodes in vertical arrays, like a row of perfectly flat sheets of paper standing on edge, Miller says that the power output could be increased dramatically. In this arrangement, every single carbon atom would be exposed and able to store energy, with virtually no waiting time for the charge to travel down the tunnels found in activated carbon.
However, in addition to improving the performance of its ultracapacitors, Graphene Energy must also develop a method for making them at larger scales--a common challenge across all graphene research.
Dileep Agnihotri, CEO of Graphene Energy, says that the company hopes to test its first prototype product incorporating graphene electrodes by the end of this year.
Another group of researchers hopes to make better ultracapacitor electrodes using carbon nanotubes--rolled-up tubes of graphene that have many of the same properties. "I think both approaches can work in principle," says Joel Schindall, a professor of electrical engineering and computer science at MIT who is working on the nanotube electrodes. "The key will be getting the growth process right, then working on ways to manufacture it in a cost-effective manner."