Alternative sources of energy beyond conventional petroleum are desperately needed, and take your pick of reasons; both environmental and security concerns can bring a wide range of people onboard. Hydrogen is an attractive option, in that the "waste" product of hydrogen combustion is water.
Zillions of research papers have been published on this topic. Comparatively few of them offer any real hope of practical implementation.
To use hydrogen as a fuel, it must be stored safely, which isn't as easy as it may sound. The safest method of storing hydrogen is probably within a molecule, by chemically bonding hydrogen atoms within the molecule in a reversible manner (hydrogen storage and release).
One of the promising ideas is the use of hydrazine borane as a hydrogen storage material, due to its high hydrogen storage capacity. Ten weight percent of hydrazine borane was released as molecular hydrogen within a few minutes of heating at 150°C.
Unfortunately, a method of easily regenerating hydrazine borane after molecular hydrogen has been released has not been reported. The regeneration problem is also a limitation of the application of hydrous hydrazine as a hydrogen storage material.
Efficient hydrogen release and regeneration is needed.
One needs to be able to reversibly store hydrogen if a material is to be of practical use for energy purposes. Imagine if your car could run on gasoline, but couldn't be easily re-filled when the tank was empty; that wouldn't be useful.
Shin-Yuan Liu (University of Oregon, United States) and coworkers have worked towards addressing this challenge. Their use of an ammonia borane derivative is not yet ready for commercial rollout.
However, they have crystal structures which clearly identify chemical intermediates in the regeneration pathway. This should facilitate experimental optimization for real-world applications.
The promise and challenges of ammonia borane.
Almost twenty weight percent of ammonia borane is comprised of hydrogen atoms. Ammonia borane therefore has a high hydrogen content, theoretically enabling it to be a hydrogen storage material.
As noted previously, the major limitation is that it's difficult to regenerate ammonia borane after the hydrogen has been released. This is partly due to the wide variety of products that can result from the dehydrogenation of ammonia borane, depending on the reaction conditions.
Additionally, it's not necessarily easy to even determine what the products are. If you don't know what's happening, you can't rationally optimize the reaction conditions to yield the desired product.
Ammonia borane derivatives.
Liu and coworkers have devised a new hydrogen regeneration protocol based on cyclic derivatives of ammonia borane. The intermediates in the system are readily identified.
Their hydrogen storage material can lock away over 7 weight percent hydrogen. This exceeds the United States Department of Energy goal of 6 weight percent hydrogen storage capacity by the year 2010.
Furthermore, the spent fuel is a low melting point liquid, and the regenerated fuel is also a liquid (above 100°C) that doesn't decompose upon repeated freezing and melting. These features are attractive for practical applications.
The scientists have focused their efforts on hydrogen regeneration, often the more difficult aspect of release/regeneration studies. This research will facilitate future studies on the hydrogen release properties of their hydrogen storage material.
Regenerating the hydrogen storage material from spent fuel.
The basic idea is that Liu and coworkers started with a molecule (N-t-butyl-1,2-dihydro-1,2-azaborane) that is able to bond to six more hydrogen atoms (one at each of the four carbon atoms, and one each at the two nitrogen and boron atoms), under the proper experimental conditions.
After a certain amount of trial and error, they found that they could achieve this in 71% yield through incubation in 45 pounds per square inch hydrogen gas at 80°C for four hours (which are relatively mild reaction conditions), followed by treatment with potassium hydride and hydrochloric acid. The hydrogen storage material was thus regenerated from spent fuel.
Crystal structures of intermediates in the reaction indicate what reaction happens when. A computational analysis further suggests that, from a thermodynamic viewpoint, hydrogen can be either stored or released readily, depending on the experimental conditions.
This in turn suggests that hydrogen release is also feasible with the scientists' system, in addition to the more challenging problem of hydrogen regeneration on which they have initially foucsed their efforts.
Further development.
Liu and coworkers point out that the use of potassium hydride needs to be avoided, due to the danger of starting an out-of-control reaction. Surely this limitation is one that can be overcome, with enough experimental tinkering.
Similar hydrogen regeneration results are expected with analogous ammonia borane derivatives that have a higher weight percent hydrogen content. This means that it should be possible to enhance the hydrogen storage capacity of their initial model test system even further.
The highlight of this research is that these scientists have regenerated hydrogen fuel after it has been spent. Most research on the use of hydrogen as fuel neglects this critical aspect of a practical hydrogen storage and release system, probably because it's usually more challenging to regenerate the fuel than to use the fuel.
Furthermore, when certain experimental difficulties are resolved, the scientists' system will be practical and realistic for large-scale deployment. These results will likely be used in real-world applications for the benefit of society, rather than the lab-toy results so common in this scientific field that every scientist knows will never be of practical use.
for more information:
Campbell, P. G., Zakharov, L. N., Grant, D. J., Dixon, D. A., & Liu, S.-Y. (2010). Hydrogen Storage by Boron-Nitrogen Heterocycles: A Simple Route for Spent Fuel Regeneration Journal of the American Chemical Society, 132 (10), 3289-3291 DOI: 10.1021/ja9106622