By Wileen Wong Kromhout
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.
“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.
Main Image: 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.