Calcium decorated two dimensional carbon allotropes for hydrogen storage: A first-principles study

Graphene has attracted tremendous interest as a potential hydrogen storage medium due to its extraordinary properties granted by its two-dimensional nature. In this study, we investigated the hydrogen storage ability of five 2D carbon allotropes: C₄₀, C₄₁, C₆₃, C₆₄ and C₆₅ with calcium decoration by adopting the first-principles method based on density functional theory. The probable adsorption sites for a Ca atom and different Ca coverages were examined, and the hydrogen molecules were added one by one to investigate the maximum hydrogen gravimetric density. Our results show that Ca atom prefers to stay on the hollow site above the largest polygons instead of small polygons. Up to 6 hydrogen molecules can be adsorbed on one single Ca atom with the average adsorption energy in the reasonable range. The hydrogen gravimetric density for these carbon allotropes with Ca decoration are 3.53–4.48 wt.% at low Ca coverage and 7.5–8.57 wt.% at high Ca coverage.