Washington, Feb 28 : Researchers, with the use of molecular dynamics simulations, have solved a decade old mystery that could one day lead to environmentally friendly and commercially practical designs of storage materials for use in hydrogen gas fueled vehicles.
The study was done by researchers from the UCLA (University of California - Los Angeles) Henry Samueli School of Engineering and Applied Science.
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 an 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 and lead author of the new study.
With computers and the power of basic physics, chemistry and quantum mechanics, Ozolins' group decided to take a step back and analyze 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 which 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," said Ozolins.
According to Ozolins, sodium alanate in itself is a prototypical complex hydride with a reasonable storage density and very good kinetics.
What the researchers now hope to do 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.