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 π 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.