[tt] graphene in ultracaps

[Eugen Leitl]

http://www.sciencedaily.com/releases/2008/09/080916143910.htm

Breakthrough In Energy Storage: New Carbon Material Shows Promise Of Storing
Large Quantities Of Renewable Electrical Energy

ScienceDaily (Sep. 17, 2008) — Engineers and scientists at The University of
Texas at Austin have achieved a breakthrough in the use of a one-atom thick
structure called "graphene" as a new carbon-based material for storing
electrical charge in ultracapacitor devices, perhaps paving the way for the
massive installation of renewable energies such as wind and solar power.

The researchers believe their breakthrough shows promise that graphene (a
form of carbon) could eventually double the capacity of existing
ultracapacitors, which are manufactured using an entirely different form of
carbon.

"Through such a device, electrical charge can be rapidly stored on the
graphene sheets, and released from them as well for the delivery of
electrical current and, thus, electrical power," says Rod Ruoff, a mechanical
engineering professor and a physical chemist. "There are reasons to think
that the ability to store electrical charge can be about double that of
current commercially used materials. We are working to see if that prediction
will be borne out in the laboratory."

Two main methods exist to store electrical energy: in rechargeable batteries
and in ultracapacitors which are becoming increasingly commercialized but are
not yet as popularly known. An ultracapacitor can be used in a wide range of
energy capture and storage applications and are used either by themselves as
the primary power source or in combination with batteries or fuel cells. Some
advantages of ultracapacitors over more traditional energy storage devices
(such as batteries) include: higher power capability, longer life, a wider
thermal operating range, lighter, more flexible packaging and lower
maintenance, Ruoff says.

Ruoff and his team prepared chemically modified graphene material and, using
several types of common electrolytes, have constructed and electrically
tested graphene-based ultracapacitor cells. The amount of electrical charge
stored per weight (called "specific capacitance") of the graphene material
has already rivaled the values available in existing ultracapacitors, and
modeling suggests the possibility of doubling the capacity.

"Our interest derives from the exceptional properties of these atom-thick and
electrically conductive graphene sheets, because in principle all of the
surface of this new carbon material can be in contact with the electrolyte,"
says Ruoff, who holds the Cockrell Family Regents Chair in Engineering #7.
"Graphene's surface area of 2630 m2/gram (almost the area of a football field
in about 1/500th of a pound of material) means that a greater number of
positive or negative ions in the electrolyte can form a layer on the graphene
sheets resulting in exceptional levels of stored charge."

The U.S. Department of Energy has said that an improved method for storage of
electrical energy is one of the main challenges preventing the substantial
installation of renewable energies such as wind and solar power. Storage is
vital for times when the wind doesn't blow or the sun doesn't shine. During
those times, the stored electrical energy can be delivered through the
electrical grid as needed.

Ruoff's team includes graduate student Meryl Stoller and postdoctoral fellows
Sungjin Park, Yanwu Zhu and Jinho An, all from the Mechanical Engineering
Department and the Texas Materials Institute at the university. Their
findings will be published in the Oct. 8 edition of Nano Letters. The article
was posted on the journal's Web site this week.

This technology, Stoller says, has the promise of significantly improving the
efficiency and performance of electric and hybrid cars, buses, trains and
trams. Even everyday devices such as office copiers and cell phones benefit
from the improved power delivery and long lifetimes of ultracapacitors.

Ruoff says significant implementation of wind farms for generation of
electricity is occurring throughout the world and the United States, with
Texas and California first and second in the generation of wind power.

According to the American Wind Energy Association, in 2007 wind power
installation grew 45 percent in this country. Ruoff says if the energy
production from wind turbine technology grew at 45 percent annually for the
next 20 years, the total energy production (from wind alone) would almost
equal the entire energy production of the world from all sources in 2007.

"While it is unlikely that such explosive installation and use of wind can
continue at this growth rate for 20 years, one can see the possibilities, and
also ponder the issues of scale," he says. "Electrical energy storage becomes
a critical component when very large quantities of renewable electrical
energy are being generated."

Funding and support was provided by the Texas Nanotechnology Research
Superiority Initiative, The University of Texas at Austin and a Korea
Research Foundation Grant for fellowship support for Dr. Park.  Adapted from
materials provided by University of Texas at Austin.