The Interocitor

Not Hydrogen -- Ammonia!

One of the real problems with the supposed transition to a Hydrogen Economy -- much beloved by technologists like myself -- is the issue of hydrogen storage. It's quite hard and we have no good way to store acceptable amounts of hydrogen in vehicles, nor do we have a good line on how to do it in the future. A good discussion of this can be found in last year's Physics Today article The Hydrogen Economy.

At least up to now. A recent letter to Physics Today, by Sandia National Laboratory scientist Peter J. Feibelman, offers a surprisingly easy path to hydrogen storage: store it as ammonia:

In their article "The Hydrogen Economy", George Crabtree, Mildred Dresselhaus, and Michelle Buchanan say that "basic research must provide breakthroughs . . . to make a hydrogen-based energy system . . . vibrant and competitive." This statement overlooks the near-term feasibility of an ammonia-mediated hydrogen-based system....

Because ammonia forms hydrogen bonds, unlike H2 or methane, it liquefies at about 8 atmospheres and room temperature, or ambient pressure and –33 °C. Indeed, because of this favorably situated phase transition, anhydrous ammonia was used as a household refrigerant for much of the 20th century.

Pipelines are in place to distribute anhydrous ammonia. To fertilize their fields, farmers routinely pull tank trucks up to ammonia "filling stations." An ammonia-fueled automobile with an internal-combustion engine was reported in the 1970s. Commercial catalytic cells are available to break ammonia into nitrogen and hydrogen and thus produce feedstock for a hydrogen fuel cell. Solid-electrolyte ammonia fuel cells have been demonstrated....

Unlike CH4 and CO2, ammonia is not a greenhouse gas. In the atmosphere, it quickly forms hydrogen bonds to water vapor and returns to the ground in alkaline rain. However, NH3 is toxic, chills its surroundings rapidly on vaporizing, and releases heat on contact with water. Engineering a safe fuel tank for an ammonia-fueled vehicle would be a key priority.

Ammonia is an excellent material for hydrogen storage. As Crabtree and coauthors report in their figure 4, the volume density of hydrogen in liquid NH3 is more than 40% greater than in liquid H2, and the comparison becomes much more favorable when one considers the weight of the required fuel tank and peripherals....

The original authors respond (scroll down in the link above) by observing:

The use of ammonia in a hydrogen economy has been discussed since at least the 1970s; Ali T-Raissi summarizes its history and its possibilities. The subject remains vibrant today; new mechanisms for the release of hydrogen from ammonia over catalysts at acceptable temperatures are continuing topics of research. A major challenge is toxicity, as Feibelman points out, but all hydrogen storage proposals come with safety issues.

Ammonia can be used effectively in other hydrogen storage media as well, notably in combination with its borane analog, BH3. NH3BH3 releases more than 12% of its mass as H2 in decomposing to NHBH at low temperature and ambient pressure. Its release rate and decomposition chemistry can be significantly improved by nanoscale structuring in porous hosts. This example shows how the richness of hydrogen chemistry and the influence of nano-patterning lead to new horizons in hydrogen storage.

The original article and these responses are must reading for anyone who intends to participate in the debate on where to go after gasoline. RTWT.

Posted by Kevin Murphy at June 2, 2005 12:29 PM