The use of hydrogen as an energy storage material continues to attract great interest. Since its combustion "waste" product is water, hydrogen is environmentally-friendly as long as it is produced from renewable resources, such as water.
A challenge is to find a material that is capable of reversibly storing and releasing hydrogen. Utilizing gaseous or liquid hydrogen is not a realistic possibility, for safety and other reasons.
Many materials have been proposed for hydrogen storage. Some of them are simple and realistic for use in the near term, such as aluminum, while others are far more complex and would require many years of future development, such as carbon nanotubes.
Dieter Lentz (Freie Universität Berlin) and coworkers have shown how to dramatically increase the hydrogen storage capacity of hydrazine borane. However, they have not yet developed a protocol for regenerating the material after hydrogen release.
Hydrazine borane.
Hydrazne borane is readily prepared from the chemical reaction of dihydrazine sulfate with sodium borohydride. In addtion to storing hydrogen in the product (hydrazine borane), this chemical reaction also produces hydrogen gas.
The scientists found that decomposing hydrazine borane by applying gentle heat released hydrogen gas, to a greater and initially faster extent when more heat was applied. At temperatures greater than 100°C, most of the hydrogen that will be released is released within a few minutes.
There is further release over the course of hours. In the optimal case of heating to 140°C, 6.5 weight percent of hydrogen was eventually released after 16 hours.
This meets the year 2010 hydrogen storage goal of the United States Department of Energy, which is a material with 6 weight percent hydrogen storage capacity. Is it possible to improve this further, in terms of storage extent and release speed?
Hydrazine borane mixed with lithium hydride.
The scientists were inspired by previous research which suggested to them that the efficacy of hydrazine borane may be improved by mixing it in with lithium hydride. In the optimal case of heating to 150°C, approximately 10 weight percent of hydrogen is released within a few minutes, with up to 12% after a few hours.
At optimal temperatures, more hydrogen is initially released with this protocol, almost double that observed without lithium hydride. However, minutes are still required for this release.
Future challenges.
The scientists readily note that they have not yet figured out a way to regenerate hydrazine borane after hydrogen is released. Reversible hydrogen storage under controlled conditions is essential if this material is to be of practical use.
These and other scientists will hopefully attempt to overcome this reversibility limitation, and to solve the challenge of fully identifying the molecule into which hydrazine borane becomes after hydrogen release. This development shows a great deal of promise, and hydrazine borane should be fully investigated as a possible hydrogen storage material.
for more information:
Hügle, T., Kühnel, M. F., & Lentz, D. (2009). Hydrazine Borane: A Promising Hydrogen Storage Material Journal of the American Chemical Society, 131 (21), 7444-7446 DOI: 10.1021/ja9013437