A comparative investigation of metal (Li, Ca and Sc)-decorated 6,6,12-graphyne monolayers and 6,6,12-graphyne nanotubes for hydrogen storage

Based on first-principle calculations, this study comparatively investigated the structural stability of 6,6,12-graphyne monolayers (GMs) and 6,6,12-graphyne nanotubes (GNs) decorated with light metal atoms (Li, Ca, Sc) for hydrogen storage. It is found that the binding strengths and diffusion energy barriers of metal atoms on GNs are much higher than those of metal atoms on GMs due to the curvature effects of nanotubes. Such strong binding would prevent the agglomeration of metal atoms on GNs, which suggests that metal-decorated GNs will be more suitable for hydrogen storage. Studying the adsorption of molecular hydrogen on metal-decorated GNs revealed that the hydrogen gravimetric density of Li-decorated GNs is only 2.5 wt% because of the high adsorption energy of H-2 (-0.060 to -0.099 eV). However, the gravimetric densities can be as high as 5.6 wt% and 5.4 wt% for Ca- and Sc-decorated GNs, respectively, due to the strong orbital hybridization interaction between metal (Ca and Sc) and hydrogen. Our results suggest that the metal-GN complex is as a highly promising material for hydrogen storage.