Hydrogen is an attractive energy storage material; the only chemical byproduct of breaking it down for energy is water. However, a large obstacle remains for its realistic use: how can hydrogen be stored safely?
Hydrogen gas can spontaneously detonate in air. Additionally, hydrogen fires are almost invisible, even though they are very hot (which can easily lead to accidental and severe burns).
At first glance, it would seem that hydrogen is too dangerous a material to use for energy storage. However, its environmental advantages have driven many scientists to try to solve the safety challenges associated with hydrogen.
Qiang Xu (AIST, Japan) and coworkers have made an important breakthrough on this issue. They have achieved complete chemical conversion of hydrous hydrazine, a relatively safe material, to nitrogen and hydrogen at room temperature.
Why hydrous hydrazine?
Hydrazine has a hydrogen content of over 12%. This exceeds the 6% hydrogen storage capacity desired by the United States Department of Energy.
Unfortunately, the most efficient methods of chemically converting hydrazine into hydrogen are explosive. Hydrazine by itself is therefore not a solution to the hydrogen storage safety issue.
Hydrazine borane is another potential hydrogen storage material, with approximately 10% hydrogen release after a few minutes of heating at 150°C. However, hydrazine borane cannot be readily regenerated after hydrogen release.
Hydrous hydrazine has a hydrogen content of nearly 8%. This still exceeds the hydrogen storage goal of the Department of Energy.
Furthermore, hydrous hydrazine is much safer to handle than plain hydrazine, and it is easily recharged with hydrogen. These are important advantages for a realistic hydrogen storage material.
Unfortunately, to date the maximum observed percent chemical conversion of hydrous hydrazine to hydrogen is less than 50% at room temperature. Xu and coworkers have achieved 100% chemical conversion at room temperature with the aid of a metal nanoparticle catalyst.
Releasing hydrogen.
The scientists found that the amount of hydrogen release strongly depends on the ratio of rhodium metal to nickel metal in the nanoparticles. The optimum ratio is 4 parts rhodium to 1 part nickel.
Complete chemical conversion was observed after 160 minutes. Although at first glance this may seem slow, as long as the entire stock of hydrogen is not needed immediately, a release time of less than 3 hours shouldn't be a problem.
The scientists are still faced with the puzzle of why nickel enables the catalyst to function. Although nickel does not directly participate in the chemical reaction, it must be there (and in the proper amount) for the nanoparticle catalyst to work.
Incorporating a random metal into the nanoparticles doesn't work either. Incorporating iron, cobalt, or copper metal into the rhodium metal catalyst gives sub-optimal hydrogen production.
The scientists suspect that the nickel in the nanoparticles alters the chemical interaction between the rhodium metal and the nitrogen and hydrogen atoms in hydrous hydrazine. However, specific chemical details have yet to be elucidated.
Implications.
Xu and coworkers have developed a safe chemical method to store hydrogen, and release it at room temperature. However, as the scientists note, the nanoparticles aren't cheap (rhodium is a rare metal).
This means that a practical catalyst for this hydrogen storage and release system still needs to be developed for realistic, large-scale applications. If such a catalyst can be found, a hydrogen-based economy may be arriving in the near future.
for more information:
Singh SK, & Xu Q (2009). Complete conversion of hydrous hydrazine to hydrogen at room temperature for chemical hydrogen storage. Journal of the American Chemical Society, 131 (50), 18032-3 PMID: 19928987