Formation and Stability of Ternary Imides in the Li−Mg−N−H Hydrogen Storage System

The thermodynamic nature of the mixtures consisting of Mg(NH 2 ) 2 and LiH determines the reaction sequence and pathways between the components. That is, the reaction resulting in the formation of Li 2 Mg(NH) 2 and H 2, which is useful for on-board hydrogen storage, first takes place in the mixtures in spite of various stoichiometries. By varying the molar ratio of Mg(NH 2 ) 2 to LiH in the proximity of 1:2, a maximal amount of H 2 release and low level of ammonia generation were observed for the mixture of Mg(NH 2 ) 2 and LiH at a 1:2 molar ratio. When LiH was in deficit (<2), severe ammonia evolution occurred concomitantly during dehydrogenation. On the other hand, excess in LiH (higher than 2) lowered the hydrogen storage efficiency. In the molar ratio range from 1:1.5 to 1:2.7 of Mg(NH 2 ) 2 to LiH, both dynamic and quasi-equilibrium dehydrogenation resulted in the formation of the ternary imide Li 2 Mg(NH) 2, indicating the ternary imide is a thermodynamically favored dehydrogenation product in this hydrogen storage system and may explain the cyclic stability of this system.