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

Abstract

The hydrogen-rich compound ( H 2)n CH 4 (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.