Metal ammine complexes for hydrogen storage

The hopes of using hydrogen as an energy carrier are severely dampened by the fact that there is still no safe, high-density method available for storing hydrogen. We investigate the possibility of using metal ammine complexes as a solid form of hydrogen storage. Using Mg(NH 3 ) 6 Cl 2 as the example, we show that it can store 9.1% hydrogen by weight in the form of ammonia. The storage is completely reversible, and by combining it with an ammonia decomposition catalyst, hydrogen can be delivered at temperatures below 620 K. Storing hydrogen in a safe, high-density, condensed phase is a notoriously difficult problem. 1 Storage in the form of metal hydrides has been studied for decades, 2 and most recently attention has been focused on the so-called complex hydrides based on alanates 3 or borates. properties, but most of them also suffer from problems relating to the density of hydrogen being too low, the kinetics of hydrogen release being too slow, or the regeneration of the hydride being too difficult. 5 In the present paper we explore a new way of storing hydrogen in the form of metal ammine complexes. They decompose thermally by evolving ammonia at a temperature which can be varied by changing the composition of the complex. By combining such complexes with an ammonia decomposition catalyst one obtains a very versatile hydrogen source. We consider Mg(NH 3 ) 6 Cl 2 Metal ammine complexes of the form M(NH 3 ) n X m , where M is a metal cation like Mg, Ca, Cr, Ni, and Zn, and X is an anion like Cl or SO 4 , have been know for more than a century. 9 Mg(NH 3 ) 6 Cl 2 was prepared by leading 1 bar of ammonia (Hede Nielsen, .99.9%) over anhydrous MgCl 2 (Merck, .98%) at 300 K. The purity and phase composition was verified by X-ray powder diffraction. pellet to obtain a high volumetric density. The saturated salt was then put in a small cell, where the rate of ammonia desorption from the sample could be measured quantitatively. This was done by absorbing the ammonia immediately at the outlet of the cell in a miniaturized scrubber using a small flow of distilled water. The ammonia content was then determined using an on-line conductivity cell