Solution To Chemical Mystery Could Yield More
Efficient Hydrogen Cars
3/4/2008
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 material 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.
"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.
SOURCE: University of California |