Thermodynamically Stable Calcium-Decorated Graphyne as a Hydrogen Storage Medium

Recently, carbon nanomaterials decorated with Ca atoms as attractors of H-2 molecules have been suggested as room temperature hydrogen storage media because of their large surface area and low weight. However, clustering of Ca atoms was found to significantly reduce the hydrogen storage capacity of Ca-carbon nanostructures. Through the first-principles density functional calculations, we explored new carbon allotropes known as graphynes for hydrogen storage. Using equilibrium statistical thermodynamics, we found that individual Ca-decorated graphynes are thermodynamically stable, unlike previously studied Ca-carbon complexes. Up to five H-2 molecules can be adsorbed on a Ca atom, each with a binding energy of similar to 0.2 eV/H-2 required for H-2 filling and delivery under an achievable operation condition of temperature and pressure. We also show that Ca-decorated graphynes can serve as promising hydrogen-storage materials with a capacity of similar to 7 wt %.