UCLA Research Holds Promise for Hydrogen Fuel
Researchers at
the UCLA Henry Samueli School of Engineering and Applied Science, with
the use of molecular dynamics simulations, have solved a decade old
mystery that could one day lead to commercially practical designs of
storage materials for use in hydrogen gas-fueled vehicles. The study
appears on the Proceedings of the National Academy of Sciences (PNAS)
Web site.
In 1997, it was discovered that adding a small amount of titanium
to a well-known metal hydride, sodium alanate, not only lowers the
temperature of hydrogen release from the material but also allows for
easy 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. The chemical
processes and the mechanisms were really a mystery," said Vidvuds
Ozolins, associate professor of material science and engineering, a
member of the California NanoSystems Institute, and lead author of the
study.
With computers and the power of basic physics, chemistry and
quantum mechanics, Ozolins’ group analyzed the 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.
The computation suggested a reaction mechanism that is essential
for the extraction of hydrogen from the material that involves
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. 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," said Hakan Gunaydin, a UCLA
graduate student in Ozolins’ lab and another one of the study's authors.