Studies of covalent amides for hydrogen storage systems: Structures and bonding of the MAl(NH2)4 phases with M = Li, Na and K

Mixtures of metallic amides and LiH are studied as hydrogen storage materials. We show that the amides decomposition temperature decreases as the polarizing effect of the metallic cation increases, hence our interest for amides with strong polarizing cations such as Al3+. Studying the MAl(NH2)4 phases with M = Li, Na and K, we tried to rationalize such metal-decomposition temperature dependence and found from IR spectroscopy investigations supported by DFT calculations that smaller is the alkali cation M+, more the Al(NH2)4 and M(NH2)4 tetrahedra are interconnected in the crystal structures and higher is the decomposition temperature. Regarding applications, the possibility of using the LiAl(NH2)4–LiH mixture as a reversible hydrogen storage material is discussed. If such mixture is able to release up to 6.2 mass% of hydrogen at 130 °C, it is shown that the LiAl(NH2)4 decomposition leads to the formation of amorphous LiAl(NH)2 imide, which is unfortunately metastable and exothermically transformed into LiNH2 + AlN. This last reaction is highly problematic for hydrogen storage applications as it is fully irreversible and AlN does not react with hydrogen under moderate temperature and pressure conditions.