03/20/2008
CONTACT: Jade Boyd
PHONE: 713-348-6778
E-MAIL: jadeboyd@rice.edu
Tiny buckyballs squeeze hydrogen like giant Jupiter
Carbon cages can hold super-dense volumes of nearly metallic hydrogen
Hydrogen could be a clean, abundant energy source, but it's difficult
to store in bulk. In new research, materials scientists at Rice
University have made the surprising discovery that tiny carbon capsules
called buckyballs are so strong they can hold volumes of hydrogen
nearly as dense as those at the center of Jupiter.
The research appears on the March 2008 cover of the American Chemical Society's journal Nano Letters.
"Based
on our calculations, it appears that some buckyballs are capable of
holding volumes of hydrogen so dense as to be almost metallic," said
lead researcher Boris Yakobson, professor of mechanical engineering and
materials science at Rice. "It appears they can hold about 8 percent of
their weight in hydrogen at room temperature, which is considerably
better than the federal target of 6 percent."
The
Department of Energy has devoted more than $1 billion to developing
technologies for hydrogen-powered automobiles, including technologies
to cost-effectively store hydrogen for use in cars. Hydrogen is the
lightest element in the universe, and it is very difficult to store in
bulk. For hydrogen cars to be competitive with gasoline-powered cars,
they need a comparable range and a reasonably compact fuel system. It's
estimated that a hydrogen-powered car with a suitable range will
require a storage system with densities greater than those found in
pure, liquid hydrogen.
Yakobson said scientists have long
argued the merits of storing hydrogen in tiny, molecular containers
like buckyballs, and experiments have shown that it's possible to store
small volumes of hydrogen inside buckyballs. The new research by
Yakobson and former postdoctoral researchers Olga Pupysheva and Amir
Farajian offers the first method of precisely calculating how much
hydrogen a buckyball can hold before breaking.
Buckyballs,
which were discovered at Rice more than 20 years ago, are part of a
family of carbon molecules called fullerenes. The family includes
carbon nanotubes, the typical 60-atom buckyball and larger buckyballs
composed of 2,000 or more atoms.
"Bonds between carbon
atoms are among the strongest chemical bonds in nature," Yakobson said.
"These bonds are what make diamond the hardest known substance, and our
research showed that it takes an enormous amount of internal pressure
to deform and break the carbon-carbon bonds in a fullerene."
Using
a computer model, Yakobson's research team has tracked the strength of
each atomic bond in a buckyball and simulated what happened to the
bonds as more hydrogen atoms were packed inside. Yakobson said the
model promises to be particularly useful because it is scalable, that
is it can calculate exactly how much hydrogen a buckyball of any given
size can hold, and it can also tell scientists how overstuffed
buckyballs burst open and release their cargo.
If a
feasible way to produce hydrogen-filled buckyballs is developed,
Yakobson said, it might be possible to store them as a powder.
"They
will likely assemble into weak molecular crystals or form a thin
powder," he said. "They might find use in their whole form or be
punctured under certain conditions to release pure hydrogen for fuel
cells or other types of engines."
The research was supported by the Office of Naval Research and the Department of Energy.