Transition Metal Decorated Porphyrin-like Porous Fullerene: Promising Materials for Molecular Hydrogen Adsorption

Transition metal decorated carbon materials like fullerenes and nanotubes have been studied extensively for hydrogen adsorption applications. However, the weaker metal binding energy makes these materials unsuitable for making a stable hydrogen storage material. Here, using ab initio based density functional theory (DFT) calculations, we have studied the hydrogen adsorption in transition metal doped porphyrin-like porous fullerene, C24N24. This porous fullerene is generated through truncated doping of 24 carbon atoms by 24 nitrogen atoms, and the resulting fullerene contains six N4 cavities with a cavity diameter of 3.708 Å. These N4 cavities are found to bind with transition metal atoms (Sc, Ti, and V) very strongly, and the binding energies are found to be considerably larger than (nearly double) the corresponding metal cohesive energies. These transition metal sites are found to adsorb molecular hydrogen through well-known Kubas-type interactions. The calculated adsorption energies of molecular hydrogen around the different metal sites are found to be in the range of −9.0 to −3.0 kcal/mol, which is considered to be the enthalpy range required for ambient condition hydrogen storage. In the case of C24N24Sc6, the maximum hydrogen adsorption capacity is found to be ∼5.1 wt % with a total of 24 H2 molecules adsorbed around the metal doped fullerene.