Technical University of Denmark researchers have come up with a new way of storing hydrogen for fuel cell vehicles, and it seems quite clever. Instead of storing pure hydrogen, they store ammonia (NH3), and instead of storing it as a pressurized liquid, they store it in the crystal lattice of a solid (in this case, orinary sea salt.) This is similar to earlier methods of storing hydrogen in metal hydrides (quick explanation here, benefits here). However, the most promising implementation of metal hydride storage so far was by Energy Conversion Devices a few years back, and has yet to be commercialized because its storage capacity decays over time. Can the Danes do better with ammonia?
The TU Denmark press release says of its salt-tablet storage, "when hydrogen is needed, ammonia is released through a catalyst that decomposes it back to free hydrogen. When the tablet is empty, you merely give it a 'shot' of ammonia and it is ready for use again." This needs to be more clearly stated, because it looks from the picture like "releasing" the ammonia means heating it to a few hundred degrees C, and unless the catalyst that pulls of the hydrogen also bonds the nitrogens to each other as N2, the reaction will cause nitrous oxide emissions, which are a more potent greenhouse gas than CO2.
Back in march, Physorg reported on advances the US's Pacific Northwest National Laboratory had made using a nanotech formulation of ammonia borane to store hydrogen; it released hydrogen 100 times faster than before, and at a temperature of just 80 C, which may be better than the Danish implementation; however, their system was not easy to recharge with more ammonia borane.
If these forays into storing hydrogen as ammonia work, does this mean that "the hydrogen economy" should mean "the ammonia economy"? In a letter to Physics Today, Peter J. Feibelman from Sandia National Labs makes a strong case for the hydrogen economy being run by ammonia, pointing out that ammonia "liquefies at about 8 atmospheres and room temperature, or ambient pressure and 33 C" as opposed to the enormous pressures and/or cryogenic temps required for liquid hydrogen; and even when they're both liquid, "the volume density of hydrogen in liquid NH3 is more than 40% greater than in liquid H2". He also points out that ammonia infrastructure is a solved problem: "To fertilize their fields, farmers routinely pull tank trucks up to ammonia 'filling stations'... 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... Thousands of miles of NH3 pipeline in the US stand as evidence that reliable infrastructure for NH3 transport and storage has been engineered." However, as Feibelman freely admits, "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."
