Destabilization of light element hydrides with high hydrogen capacities: metal imides/nitrides

High hydrogen capacity materials are highly desirable for hydrogen storage for on-board applications. Some light elements form hydrides with high hydrogen capacities, such as LiH (12.7 wt{\%}) and MgH$_{2}$ (7.6 wt{\%}). These hydrides, however, are very stable, releasing hydrogen only at very high temperature, above 600$^{o}$C and 350$^{o}$C, respectively, with poor kinetics. These hydrogen storage features are unsatisfactory for on-board application. Chen et al [1] reported the hydrogen storage properties of lithium nitride/imide. According to their results lithium imide can absorb hydrogen at 1 bar at 285$^{o}$C reversibly with hydrogen capacity of 6.5wt{\%}. Lithium nitride, on the other hand, can absorb 5wt{\%} more hydrogen, however, it is much more stable compared with lithium imide. \begin{center} Li$_{3}$N + 2H$_{2}$ (Li$_{2}$NH + LiH + H$_{2}$ ( LiNH$_{2}$ + LiH \end{center} This indicates that it is an effective method to destabilize lithium hydride by converting hydride to nitrogen-containing material, such as lithium imide/nitride. Here we report a new approach to further de-stabilize lithium imide by partial substitution of lithium by magnesium in this system. This Mg-substituted material releases hydrogen of significant higher pressure at much lower temperature than those for lithium imide, with minimal capacity reduction [2]. One of the examples is the mixture of (LiNH$_{2}$-MgH$_{2})$, which can release hydrogen of approximately 30 bar at 200$^{o}$C reversibly, with hydrogen capacity of 5 wt{\%}. This material has the potential to deliver hydrogen of 3 bar at 100$^{o}$C. It may be further dehydrogenated to nitride with total hydrogen capacity of approximately 9wt{\%}. The destabilization mechanism for this system will be discussed since this may provide clue in the future searching for high capacity hydrogen storage materials [1] P. Chen, Z. Xiong, J. Luo, J. Lin, L Tan, Nature Vol. 420 (2002) 302-304. [2] W. Luo, J. Alloys and Compounds, 381 (2004) 284-287.