Ab initio study of the structures and hydrogen storage capacity of (H2)nCH4 compound

The hydrogen-rich compound (H-2)(n)CH4 (for n = 1, 2, 3, 4) or for short (H-2)(n)M is one of the most promising hydrogen storage materials. The (H-2)(4)M molecule is the best hydrogen-rich compound among the (H-2)(n)M structures and it can reach the hydrogen storage capacity of 50.2 wt.%. However, the (H-2)(n)M always requires a certain pressure to remain stable. In this work, we first investigated the binding energy of the different structures in (H-2)(n)M and energy barrier of H-2 rotation under different pressures at ambient temperature, applying ab initio methods based on van der Waals density functional (vdW-DF). It was found that at 0 GPa, the (H-2)(n)M is not stable, while at 5.8 GPa, the stability of (H-2)(n)M strongly depends on its structure. We further investigate the Raman spectra of (H-2)(n)M structures at 5.8 GPa and found the results were consistent with experiments. Excitingly, we found that boron nitride nanotubes (BNNTs) and graphite and hexagonal boron nitride (h-BN) can be used to store (H-2)(4)M, which give insights into hydrogen storage practical applications.