Interpretation of dehydrogenation ability of high-density hydrogen storage materials by density functional theory

Abstract

A first-principles plane-wave pseudopotential method based on the density functional theory was used to investigate the dehydrogenation properties and its influence mechanics on several high-density hydrogen storage materials (MgH2, LiBH4,LiNH2 and NaAlH4) and their alloys. The results show that MgH2, LiBH4, LiNH2 and NaAlH4 high-density hydrogen storage materials are relatively stable and have high dehydrogenation temperature. Alloying can reduce their stability, but the stability of a system is not a key factor to the dehydrogenation properties of high-density hydrogen storage materials. The width of band gap of hydrogen storage materials can characterize the bond strength basically, the wider the energy gap is, the harder the bond breaks, and the higher the dehydrogenation temperature is. The bonding peak of the valence band top of LiNH2 is attributed mainly to the Li—N bonding, the N—H bond constitutes the low peak, which makes the dehydrogenation temperature of LiNH2 high, though LiNH2 has a narrow band gap in respect to LiBH4 and NaAlH4, which makes the ammonia release in the dehydrogenation process. Alloying makes the band gap narrow, and the Fermi level goes into the conduction band, which improves the dehydrogenation properties. It was found from the charge population analysis that B—H bond in LiBH4 is the strongest, H—N bond in LiNH2 is the weakest, so LiNH2 is relatively easy to release hydrogen. After alloying, the bond strength of X—H is weakened in every hydrogen storage material, and the N—H bond strength in LiMgNH2 is the lowest. Therefore, it is perspective to develop LiNH2 as hydrogen storage from the lowering of dehydrogenation temperature.