High Capacity Hydrogen Storage in Ca Decorated Graphyne: A First-Principles Study

Ca decorated carbon allotropes have a potential for high density hydrogen storage, except that the Ca-graphene and Ca-fullerenes binding is not strong enough to prevent the formation of a Ca cluster. Using first-principles calculations, we show that Ca can bind strongly to s-p and s-p(2) bonded graphyne without the formation of a Ca cluster. This enhanced binding energy is due to the additional in-plane pi states which do not exist in the s-p(2) bonded graphene and fullerenes. The H-2 binding to the Ca-graphyne system is similar to the Ca fullerenes system with a maximum of six H-2 molecules per Ca atom and a 0.2 eV per H-2 binding energy which is optimal as hydrogen storage materials. With two Ca atoms per unit cell, this leads to 9.6 wt % hydrogen storage capacity in theory.