Hydrogen fuel, because its only byproduct is steam, should be the ultimate
in green alternatives to fossil fuels, but it hasn't delivered on its
promise yet because of one enormous stumbling block, storage. Now a team of
chemical engineers at the University
of Massachusetts Amherst has developed a computational model that shows
that carbon nanotubes may offer a surprising solution. Results are presented
in the current online issue of the journal, Applied Physics Letters.
“If this works as we expect, it's perhaps no longer science fiction
to hope for a briefcase-sized hydrogen battery to run a bus or car,” says
UMass Amherst chemical engineering professor Dimitrios Maroudas. “Hydrogen
storage has been a huge problem in the energy field for the past 10 years because
no one has been able to demonstrate a truly viable storage medium. We've
shown that it's possible to achieve hydrogen storage capacity up to 8
percent by weight using carbon nanotubes. This is an outstanding level, higher
by 1 percent than the 2010 United States Department of Energy target for on-board
hydrogen storage systems,” Maroudas adds. “The method we propose
may lead to breaking the bottleneck.”
The UMass Amherst computational model strongly lends itself to verification
in laboratory experiments, say Maroudas and colleagues, and it provides ample
testable hypotheses for future experimental research. “People had been
losing faith, but I think our predictions show that hydrogen should be back
on the table and in a most promising way. We come up with modeling predictions
for technologically relevant problems every day, but this cute model is special,”
he notes.
Specifically, Maroudas, his graduate student Andre Muniz and their collaborator
M. Meyyappan, chief scientist for exploration technology at the Center for Nanotechnology
at NASA Ames Research Center, Moffett Field, Calif., show that proper arrangement
of carbon nanotubes can overcome hydrogen transport limitations in nanotube
bundles. It should also prevent ineffective and nonuniform hydrogenation, which
is caused by nanotube swelling due to chemisorption of hydrogen atoms on the
nanotube walls.
If one were to think of carbon nanotube bundles as something like a toothbrush,
one strategy that Maroudas and colleagues recommend for holding hydrogen atoms
most efficiently is that the brush arrangement should not be too dense. If it
is, when the tubules swell they'll block efficient passage and diffusion
of the hydrogen, Maroudas explains. In addition to an optimal bundle density,
further improvement can be achieved by optimizing the individual nanotube configurations
to limit their swelling upon hydrogenation.
Following this approach should result in one hydrogen atom being able to chemisorb
onto — form a chemical bond with — each carbon atom of the nanotubes,
leading to 100 percent (atomically) storage capacity, he adds. This chemisorbed
hydrogen, bound to the surface, can then be easily released by applying heat.
Maroudas says, “We propose recipes that will be very easy for others
to try, by which carbon nanotubes can be arranged to accomplish practically
100 percent storage atomically, which is nearly 8 percent by weight. You can't
get any greener than hydrogen as fuel, and if the experiments we envision lead
to new technology that's economically viable, that's as good as
it gets.” This work was supported by a National Science Foundation grant
and a Fulbright/CAPES scholarship to Muniz.
Posted October 26th, 2009
