The Haber−Bosch process for the industrial synthesis of ammonia has, over the past century, led to a global revolution in agriculture to the extent that almost half the crops grown across the world today depend on ammonia-based fertilizers. The reverse reaction may have a similarly transformative potential, where the decomposition of ammonia into nitrogen and hydrogen enables the provision of hydrogen for a low-carbon energy economy. Indeed, high-density, affordable, and efficient hydrogen storage is one of the key steps in the realization of a hydrogen-based energy sector.

Hydrogen (H₂) is an attractive chemical fuel, with very high gravimetric energy content (120 MJ/kg) and an emissions profile free from carbon dioxide. Despite these advantages, its use has been hindered by the lack of an effective and efficient method for its storage. Current high-pressure (∼700 bar) storage is expensive and energy-intensive, and imposes practical restrictions on tank dimensions. In response to this challenge, there has been a significant research effort over the past 15 years that has focused on new chemical hydrogen storage materials, particularly solid-state materials which offer impressive volumetric and gravimetric hydrogen densities. Arguably, this focus may have diminished the consideration of reversibility, cost, and practicality of use of these materials. To date, very few candidates show potential beyond that of the seminal work on titanium-doped sodium alanate.

Ammonia (NH₃) has a high gravimetric (17.8 wt% H₂) and volumetric (121 kg H₂ m⁻³ in the liquid form) H₂ density and is produced on an industrial scale. Furthermore, it has an existing extensive distribution network and is easily stored by liquefaction at moderate pressure (ca. 10 bar at room temperature). While both perceived and real safety risks due to the toxicity of NH₃ have detracted from its appeal, its adoption as a vector for H₂ has not yet been realized largely because of the absence of an efficient, low-cost method for cracking NH₃ to H₂ and N₂.

—David et al.

Sodium amide is used extensively as a reagent in a variety of synthesis processes, but its modest hydrogen capacity and high decomposition enthalpy suggest that it would be an unattractive hydrogen storage candidate. However, the team noted, it was observed more than a century ago that NaNH₂ decomposes to its constituent elements: i.e., sodium, nitrogen and hydrogen.

The bulk production of NaNH₂ is by reaction of sodium metal (Na) with gaseous (or liquid) NH₃. Run concurrently, the RAL team proposed, these two reactions should effect the chemical decomposition of NH₃ by cycling between sodium amide and sodium metal, thusly:

The RAL researchers performed their NH₃ decomposition experiments in simple cylindrical flow reactors.

Currently, ruthenium shows the highest catalytic activity for low-temperature cracking of ammonia; the RAL results showed that the performance of as-found NaNH₂ for continuous stoichiometric NH₃ decomposition is as effective as a supported ruthenium catalyst—or more so. The material costs, however, are very significantly less, the team observed.

We believe this combination of high efficiency and low cost of the Na/NaNH₂ system indicates the potential of this new class of NH₃ decomposition catalyst, and invites reconsideration of the potential of NH₃ decomposition as a viable delivery method of H₂, for applications ranging from small-scale distributed power to massive grid-balancing and on-board use for transportation.

—David et al.

Low-temperature fuel cells—either alkaline fuel cells or proton exchange membrane-based fuel cells—could utilize the hydrogen produced. Given that a 1 kW fuel cell requires a H₂ supply of ∼13.5 L/min, the RAL team calculated that linear scale-up of the highest production rate obtained in their work to this power output would require a reactor volume of 0.62 L, using 14.5 g of NaNH₂.

They suggested, however, that the direct combustion of NH₃ might be the most attractive short-term option. Although ammonia alone is difficult to ignite, a 2.5 wt% hydrogen-in-ammonia mixture is sufficient to enable ammonia combustion. Using the current best conversion rates obtained in their experiments, they calculated that this mix could be provided, at sufficient flow rates, using a total reactor volume of 3.2 L, containing 75 g of NaNH₂.

(The calculations are detailed in the paper’s Supporting Information.)

These calculations indicate that ammonia-based transportation is achievable. While advances in the containment and turnover frequency of the amide are necessary steps toward this goal, we anticipate that significant improvements in these areas will be realized through the optimization of the reactor design and materials properties of NaNH₂.

—David et al.

Resources

• William I. F. David, Joshua W. Makepeace, Samantha K. Callear, Hazel M. A. Hunter, James D. Taylor, Thomas J. Wood, and Martin O. Jones (2014) “Hydrogen Production from Ammonia Using Sodium Amide,” Journal of the American Chemical Society doi: 10.1021/ja5042836