Indirect hydrogen storage in metal ammines

HUMMELSH0J, T. J0HANNESSEN, J. K. N0RSK0V and C. H. CHRISTENSEN, Technical University of Denmark, Denmark 

This chapter is focused on the potential of indirect hydrogen storage by use of ammonia stored in metal ammines. Section 19.2 describes ammonia as a potential hydrogen storage medium, ammonia production and infrastructure, safety concerns and the energy costs involved in indirect storage. Section 

3 discusses the storage of ammonia in solid form using MgCl2 as the model carrier material. The improved safety of ammonia stored in metal ammines, methods for preparation and powder compaction, low materials cost and easy scale-up are covered. This section is followed by a description of the thermodynamic properties of different metal ammines, e.g. van t Hoff plots, desorption properties, reversibility and reloading, and selection of specific ammines by weighting parameters such as safety and desorption temperature. 

Section 19.5 deals with the development of novel metal ammine systems and focuses on the design of superior metal ammines by closely integrating experimental and calculational work. From a detailed understanding of structure and stability, porosity and particle size, desorption and diffusion, and alloy formation, it is possible to engineer these materials on the nano-, micro-, and macro-scales. 

The last section (19.6) is focused on the commercial potential and perspectives of using metal ammines in connection with, for example, polymer electrolyte membrane (PEM) and solid oxide fuel cells (SOFCs) as well as selective catalytic reduction (SCR)-DeNO c (NO c removal) in the transport sector, and it includes comments on the global availability and low cost of the carrier salts. This section also provides the authors perspectives on future trends and challenges in metal ammine research, along with links to the interested reader for further information on key articles, companies and websites. 

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AM Indirect, Reversible Hydrogen Storage in Metal Ammine Salts Recent Progress and Prospects ... 

The important properties of the metal ammines when used as materials for indirect hydrogen storage can be understood and explained at the atomic scale. In this section, an atomic scale model of the sample system Mg(NH3)6Cl2 based on density functional theory (DFT) calculations shows how atomic structures, energies and dynamics can be connected to macro-scale properties such as shape, uptake and decomposition rates, porosity and reaction enthalpies. The model is based on experimentally observed structures (Olovsson, 1965 Partin and O Keeffe, 1991 Leineweber et al, 1999, 2000 Hummelshpj et al, 2006) and it is expected to be applicable to other metal ammines. A detailed understanding of the systems at the atomic scale is a prerequisite for the design of new and better materials. 

Sprensen R Z, Hummelshpj J S, Klerke A, Reves J B, Vegge T, Nprskov J K, Christensen C H (2008), Indirect, reversible high-density hydrogen storage in compact metal ammine salts , J. Am. Chem. Soc. 130(28), 8660-8668. 

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