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."
Posted 25th March 2008
