UCLA Engineering Solution to Chemical
Mystery could yield More Efficient Hydrogen Cars
Environmentally friendly vehicles that use hydrogen gas can
dramatically reduce greenhouse emissions and lessen the country's
dependence on fossil fuels. While several hydrogen-fueled vehicles
are currently on the market, there is still much room for improvement
in the way they store and utilize hydrogen gas.
Now researchers at the UCLA Henry Samueli School of Engineering
and Applied Science, using molecular dynamics simulations, have
solved a decade-old mystery, and their findings could eventually
lead to commercially practical designs of storage materials
for use in hydrogen vehicles. Their research, currently available
on the Web
site of Proceedings of the National Academy of Sciences,
will be published in the journal's print edition March 4.
With current technologies, hydrogen gas storage tanks have to
be as large as or larger than the trunk of a car to carry enough
fuel for a vehicle to travel only 100 to 200 miles. While liquid
hydrogen is denser than gas and takes up less space, it is expensive,
difficult to produce and reduces the environmental benefits
of hydrogen vehicles. Widespread commercial acceptance of hydrogen
vehicles has therefore hinged on finding materials that can
store hydrogen gas at high volumetric and gravimetric densities
in reasonably sized, lightweight fuel tanks.
The search for solutions has generally involved the use of metal
hydrides — metal alloys that absorb and store hydrogen
within their structure and release the hydrogen when subjected
to heat.
In 1997, scientists discovered that adding a small amount of
titanium to sodium alanate, a well-known metal hydride used
in onboard hydrogen gas storage, not only lowered the temperature
of the hydrogen released, making the reaction more efficient,
but it also allowed for easier refueling and storage of high-density
hydrogen at reasonable pressures and temperatures. In fact,
the weight-percent of stored hydrogen was instantly doubled
in comparison with other inexpensive materials.
"Nobody really understood what the titanium did,"
said the UCLA study's lead author, Vidvuds Ozolins, an associate
professor of materials science and engineering and a member
of UCLA's California NanoSystems Institute. "The chemical
processes and the mechanisms were really a mystery."
Using computers and the power of basic physics, chemistry and
quantum mechanics, Ozolins' group decided to take a step back
and examine sodium alanate in its pure form, without added titanium.
The group analyzed the atomic processes occurring in the material
and what happens to the chemical bond between the hydrogen and
the material at the temperatures of hydrogen release. The computation
gave the researchers information that would have been very difficult
to obtain experimentally.
Their findings suggest that the
reaction mechanism essential for the extraction of hydrogen
from sodium alanate involves the diffusion of aluminum ions
within the bulk of the hydride. By comparing the calculated
activation energies to the experimentally determined values,
Ozolins' group found that aluminum diffusion is the key rate-limiting
process in materials catalyzed with titanium. Thus, titanium
facilitates processes in the material that are essential for
turning on this mechanism and extracting hydrogen at lower temperatures.
Schematics of metal vacancy mediated hydrogen
release from sodium alanate. The bulk diffusion rate of metal
ions determines the overall rate of hydrogen release. Na, Al
and H atoms are represented as indigo, amber and white spheres,
respectively. The released hydrogen gas (H2) is color coded
red.
"This method and this knowledge can now be used to analyze
other materials that would make for better storage systems than
sodium alanate," said Hakan Gunaydin, a UCLA graduate student
in Ozolins' lab and one of the study's authors. "We are
still on the fundamental end of the study. But if we can figure
this out computationally, the people with the technology in
engineering can figure out the rest."
"Sodium alanate in itself is a prototypical complex hydride
with a reasonable storage density and very good kinetics,"
Ozolins said. "Hydrogen goes in and comes out quickly,
but it wouldn't be practical for a car, simply because it doesn't
contain enough hydrogen. So that's why we are so interested
in understanding how the hydrogen comes out, what happens exactly
and how we can take this to other materials."
What Ozolins' group — along with UCLA chemistry and biochemistry
professor Kendall Houk, also a member of the California NanoSystems
Institute — hopes to do now is to apply the methods and
lessons learned to those materials that would make for a commercially
practical hydrogen gas storage system. They hope their findings
will one day facilitate the design and creation of an affordable
and environmentally friendly hydrogen vehicle.
The study was funded by the U.S. Department of Energy and the
National Science Foundation.
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