A major challenge with using hydrogen as an environmentally-friendly source of energy is finding a way to store large quantities of it. A practical hydrogen storage system must be safe, reversible, efficient, as cheap as possible, and based on plentiful materials.
Craig Jensen (University of Hawaii, Honolulu) and coworkers have addressed this challenge. They have reversibly stored hydrogen in aluminum metal.
Why focus efforts on aluminum?
There are myriad hydrogen storage devices reported in the science technical literature, some more successful than others. A recurring theme in most of this research is the investigation of exotic materials; "new and different" is very fashionable.
However, a more practical approach is to focus on materials that are cheap and readily available. Aluminum is one of these materials.
Aluminum is easily converted into lithium aluminum hydride, which has a theoretical hydrogen storage capacity of roughly 8 weight percent. This meets the United States Department of Energy's year 2010 goal of a material with 6 weight percent hydrogen storage capacity.
Challenges of using aluminum.
Unfortunately, the uptake of hydrogen by aluminum requires a lot of energy. It is therefore widely thought that aluminum cannot be easily recharged with hydrogen.
Jensen and coworkers note, however, that this recharging issue has been partially addressed by other scientists through the use of a solvent. The energetically favorable solvation of lithium aluminum hydride (the reaction product of hydrogen, lithium hydride, and aluminum) is thought to contribute to favorable recharging energetics.
However, the chemical reaction still requires a large amount of heat. These scientists, in contrast, have demonstrated a more efficient method of coaxing aluminum to store hydrogen.
The improved process.
The scientists mixed a small amount of titanium chloride with a much larger amount of lithium aluminum chloride. When heated to 120°C in a vacuum, hydrogen was released as a gas.
Lithium hydride and aluminum powder were left behind. Thus, hydrogen was released from the aluminum.
In an inert atmosphere, lithium hydride and aluminum powder were dissolved in methyl ether, a low-boiling solvent. Hydrogen gas, at approximately 100 atmospheres of pressure, was subsequently added to the reaction mixture.
This mixture was stirred at room temperature for 1 day. Afterwards, the pressure was carefully reduced, and the solvent was evaporated, the hydrogen uptake being complete.
Thus, hydrogen was stored in the aluminum. These scientists have therefore stored hydrogen, in an almost fully (nearly 90%) reversible manner.
Future prospects.
The scientists note that repetitive hydrogen storage/release cycles become less efficient over time. Less hydrogen is released after numerous cycles.
However, the reversibility, efficiency, cost, and ease of fabrication of this development are clear. With further development, aluminum may be set to outperform many other far more exotic hydrogen storage materials.
for more information:
Liu, X.; McGrady, G. S.; Langmi, H. W.; Jensen, C. M.
Facile cycling of Ti-doped LiAlH4 for high performance
hydrogen storage.
J. Am. Chem. Soc. 2009, 131, 5032-5033.