In their article “The Hydrogen Economy” (December 2004, page 39), 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. 1 A research breakthrough might reduce the cost of ammonia production, by emulating its biosynthesis, 2 for example. But we have known how to make NH3 economically for almost a century. Nowadays, between 1% and 2% of the world’s energy is devoted to synthesizing ammonia from air and hydrocarbons, notably natural gas, via the Haber–Bosch process. 3  

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. 4 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. 5  

Because Bosch synthesis is performed in large industrial plants, the carbon dioxide byproduct can be captured and sequestered relatively easily—for example, by pumping it back into the wells that supplied the natural-gas feedstock. Any means of producing hydrogen based on a renewable energy source could substitute for the Haber–Bosch process, and thereby allow for “renewable” ammonia production.

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. Unlike H2 gas, ammonia explodes in air only over a narrow range of concentrations. Shipping ammonia from production site to point-of-use does not require a great deal of cooling or high pressure. Thousands of miles of NH3 pipeline in the US stand as evidence that reliable infrastructure for NH3 transport and storage has been engineered. In sum, liquid NH3 is not just an excellent hydrogen-storage material but also an ideal medium for moving hydrogenic energy from place to place.

Given these advantages, it is hard to avoid the conclusion that relatively modest investments in the science and engineering of NH3 synthesis and fuel cells, and in safer transport, storage, and delivery of NH3, are the best hope for making the hydrogen economy a reality in our lifetimes (and by the way, I am 62).

2.
See, for example,
D. V.
Yandulov
,
R. R.
Schrock
,
Science
301
,
76
(
2003
) .
3.
International Fertilizer Industry Association, http://www.fertilizer.org/ifa/statistics/indicators/ind_reserves.asp.
4.
J. W.
Hodgson
,
Mech. Eng.
7
,
22
(
1974
).
5.
A.
McFarlan
,
L.
Pelletier
,
N.
Maffei
,
J. Electrochem. Soc.
151
,
A930
(
2004
); also see http://chem.ch.huji.ac.il/~eugeniik/history/kordesch.html.