By Marlene Cimons, National Science Foundation
When talking about the differences between a battery and an ultracapacitor, scientist Rodney Ruoff says it’s like comparing marathon runners to sprinters.
“Batteries are relatively slow,” he says. “They can store energy, but take a while to charge up, and then they distribute energy slowly, over time. Ultracapacitors can be charged very quickly, within seconds, and discharge very quickly, but, right now, they can’t store very much electrical energy. They can take it in and get it out very quickly, but they don’t store very much.”
Ruff, a professor in the mechanical engineering department at the University of Texas at Austin, is studying the basic science in the development of graphene, a new material that holds great promise for use in electronics and other fields. Graphene, the focus of considerable research in recent years, is a one-atom thick layer of carbon molecules with potential as a fast, efficient substance from which to make computer chips, sensors--and ultracapacitors.
“They’re remarkably efficient,” Ruoff says of graphene-based ultracapacitors. “They are much more robust than batteries. You can put that electrical energy in and take it out thousands of times, whereas batteries might give you only a couple of hundred cycles.”
Ruoff and his colleague, Christopher Bielawski, a professor in the university’s department of chemistry and biochemistry, and Junhong Chen, associate professor of mechanical engineering at the University of Wisconsin, Milwaukee, are studying graphene applications in ultracapacitors and sensors, among other things, under a series of three-year grants from the National Science Foundation totaling roughly $862,359, as part of the American Recovery and Reinvestment Act of 2009.
Chen has been studying graphene as a material in making sensors, tools that could sniff out chemicals in industry, warfare and possibly even those used in terrorist attacks, such as sarin or ricin. Graphene-based sensors can operate at room temperature, in contrast to current commercial sensors of metal oxides, which require higher temperatures, making them more energy efficient. And they are highly sensitive.
“Graphene has great potential to advance chemical sensing technologies in terms of sensitivity, energy consumption, portability and flexibility and so on,” Chen says. “Graphene can detect gas adsorption [the binding of molecules or particles to a surface] events down to a single molecule level. Graphene also has excellent sensing performance for detecting chemical warfare agents and explosives.”
In graphene-based ultracapacitors, the devices store energy electrostatically, unlike batteries, which store it chemically. The development of stable and less expensive ultracapacitors could be a key step in using wind or solar-generated power, particularly if researchers can find ways to enable capacitors to store energy longer, which is not yet possible.
“At this point, the field is still developing,” Ruoff says. “For ultracapacitors to be able to make a strong contribution to the electric grid, they need to store more electrical energy than they do now. It’s of great interest to see whether we can push that energy storage up, but we can’t do that yet. When I talk to experts in the commercial arena, and we lay out our arguments about their potential, they don’t laugh and say ‘it’s absurd.’ They say: ‘it might happen.’ I’m hopeful. We have to just do the science and find out.”
Even with their current storage capacity, the graphene devices could provide quick energy when needed in certain situations - and in an environmentally friendly way. They can be used, for example, to absorb the heat used in braking an automobile or train, stored for a short time, and then used for the electrical needs of the vehicle, Ruoff says. “It’s definitely less polluting in terms of being able to extract energy that otherwise would just be lost,” he says. “If you generate heat and don’t use it to generate more power, it’s just lost to the environment.”