(55d) Ammonia and Metal Ammines for High Density Hydrogen Storage

The challenge of efficient storage and transportation of hydrogen for use in fuel cells still needs to be solved. In the following it is described how ammonia has potential to become one of the important indirect hydrogen carriers. Aside from hydrogen, ammonia provides the only carbon-free chemical energy carrier solution for the transportation sector, giving no CO₂ emission at the end user. Ammonia is not a green house gas (GHG), and in light of the enormous global climate challenges with respect to GHG outlined, e.g. in the Kyoto protocol, combined with a high hydrogen density, NH₃ could be an interesting alternative to hydrogen. Ammonia is also interesting because the infrastructure is well known technology from the fertilizer industry. The synthesis of ammonia in the Haber-Bosch process is one of the largest chemical processes with an yearly production of 120 million tons, but this is only enough to support a minor fraction of the hydrogen that will be needed in a future hydrogen economy. Ammonia is primarily produced from natural gas, but hydrogen for the synthesis could be supplied from wind or solar power via electrolysis or any other green source of hydrogen. Thus synthesis of ammonia can be positive regarding CO₂ segregation. Bulk ammonia is transported and stored as a liquid under pressure or cooled to -33ºC. For use in the transportation sector ammonia can be stored as a high density solid in a range of metal ammine salts. The metal ammines lower the ammonia vapor pressure making them safe to handling and use. In figure 1, mass and volumetric hydrogen densities for storage of 10 kg hydrogen reversibly by 8 different methods is shown¹. It can be seen that the metal ammines has higher volumetric hydrogen densities than liquid ammonia at room temperature. Desorption of ammonia from metal ammines is easy and only limited by heat transportation². The metal ammines can be compressed into a solid media and still maintain a high desorption rate. This is due to the formation of nanopores during desorption of ammonia³. The ammonia released from metal ammines, can be used directly in a solid oxide fuel cell or a direct ammonia fuel cell. The ammonia can also be decomposed to nitrogen and hydrogen over a catalyst and used in low temperature fuel cells like PEMFC or PAFC. To optimize the ammonia decomposition a range of different ruthenium based catalysts has been tested and compared to gain further knowledge on the decomposition reaction. The ruthenium catalysts supports include nanostructured titanates with incorporated alkaline promoters, high surface area graphite and MgAl₂O₄-spinel. The results have been used in selection of the optimal catalyst for development of an ammonia cracker. The cracker will work by burning ammonia to supply heat for decomposition and desorption of ammonia. To avoid ammonia poisoning of the fuel cell there has also been looked at employing an ammonia scrubber. Figure 1: Mass and volume densities of 10 kg hydrogen stored reversibly by 8 different methods. Based on the best obtained reversible densities reported in the literature without considering the space or weight of the container¹. References: ¹: A. Klerke, C. H. Christensen, J. K. Nørskov and T. Vegge, J. Mater. Chem. 18 (2008) 2304. ²: R. Z. Sørensen, J. S. Hummelshøj, A. Klerke, J. B. Reeves, T. Vegge, J. K. Nørskov and C. H. Christensen, J. Am. Chem. Soc. 130 (2008) 8660. ³: J. S. Hummelshøj, R. Z. Sørensen, M. Y. Kustova, T. Johannesen, J. K. Nørskov, C. H. Christensen, J. Am. Chem. Soc., 2006, 128, 16-17.