Graphene for the Green Grid

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."

http://www.technologyreview.com/business/22062/?nlid=1752&a=f

New Graphene-Based Material Clarifies Graphite Oxide Chemistry

September 25, 2008

AUSTIN, Texas — A new "graphene-based" material that helps solve the structure of graphite oxide and could lead to other potential discoveries of the one-atom thick substance called graphene, which has applications in nanoelectronics, energy storage and production, and transportation such as airplanes and cars, has been created by researchers at The University of Texas at Austin.

To get an idea of the nanomaterial graphene, imagine a lightweight material having the strongest chemical bond in nature and, thus, exceptional mechanical properties. In addition it conducts heat better than any other material and has charge carriers moving through it at a significant fraction of the speed of light. Just an atom thick, graphene consists of a "chickenwire" (or honeycomb) bonding arrangement of carbon atoms—also known as a single layer of graphite.

Mechanical Engineering Professor Rod Ruoff and his co-authors have, for the first time, prepared carbon-13 labeled graphite. They did this by first making graphite that had every "normal" carbon atom having the isotope carbon-12, which is magnetically inactive, replaced with carbon-13, which is magnetically active. They then converted that to carbon-13 labeled graphite oxide and used solid-state nuclear magnetic resonance to discern the detailed chemical structure of graphite oxide.

The work by Ruoff's team will appear in the Sept. 26 issue of the journal Science.

"As a result of our work published in Science, it will now be possible for scientists and engineers to create different types of graphene (by using carbon-13 labeled graphene as the starting material and doing further chemistry to it) and to study such graphene-based materials with solid-state nuclear magnetic resonance to obtain their detailed chemical structure," Ruoff says. "This includes situations such as where the graphene is mixed with a polymer and chemically bonded at critical locations to make remarkable polymer matrix composites; or embedded in glass or ceramic materials; or used in nanoelectronic components; or mixed with an electrolyte to provide superior supercapacitor or battery performance. If we don't know the chemistry in detail, we won't be able to optimize properties."

Graphene-based materials are a focus area of research at the university because they are expected to have applications for ultra-strong yet lightweight materials that could be used in automobiles and airplanes to improve fuel efficiency, the blades of wind turbines for improved generation of electrical power, as critical components in nanoelectronics that could have blazing speeds but very low power consumption, for electrical energy storage in batteries and supercapacitors to enable renewable energy production at a large scale and in transparent conductive films that will be used in solar cells and image display technology. In almost every application, sensitive chemical interactions with surrounding materials will play a central role in understanding and optimizing performance.

Ruoff and his team proved they had made such an isotopically labeled material from measurements by co-author Frank Stadermann of Washington University in St Louis. Stadermann used a special mass spectrometer typically used for measuring the isotope abundances of various elements that are in micrometeorites that have landed on Earth. Then, 100 percent carbon-13 labeled graphite was converted to 100 percent carbon-13 labeled graphite oxide, also a layered material but with some oxygen atoms attached to the graphene by chemical bonds.

Co-authors Yoshitaka Ishii and Medhat Shaibat of the University of Illinois-Chicago then used solid state nuclear magnetic resonance to help reveal the detailed chemical bonding network in graphite oxide. Ruoff says even though graphite oxide was first synthesized more than150 years ago the distribution of oxygen atoms has been debated even quite recently.

"The ability to control the isotopic labeling between carbon-12 and carbon-13 will lead to many other sorts of studies," says Ruoff, who holds the Cockrell Family Regents Chair in Engineering #7.

He collaborates on other graphene projects with other university scientists and engineers such as Allan MacDonald (Departments of Physics and Astronomy), Sanjay Banerjee, Emanuel Tutuc and Bhagawan Sahu (Department of Electrical and Computer Engineering) and Gyeong Hwang (Department of Chemical Engineering), and some of these collaborations include industrial partners such as Texas Instruments, IBM and others.

Co-authors on the Science article include: Weiwei Cai, Richard Piner, Sungjin Park, Dongxing Yang, Aruna Velamakanni, Meryl Stoller and Jinho An (all of the Ruoff research group at The University of Texas at Austin); Sung Jin An, formerly of Pohang University of Science and Technology (POSTECH-Korea) and a visiting graduate student in the Ruoff group during the study; Dongmin Chen (Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences); Stadermann; and Ishii and Shaibat of the University of Illinois-Chicago.

A high-resolution photo of Ruoff is available. Learn more about Ruoff's work.

For more information, contact: Daniel Vargas, Cockrell School of Engineering, 512-471-7541; Rodney Ruoff, Department of Mechanical Engineering, Cockrell School of Engineering, 512-471-4691.

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