DFT Investigations of Kubas-Type Hydrogen Storage in C₁₉X Heterofullerenes: d-Orbital Preservation Under Substitutional Transition Metal Doping

Abstract

Hydrogen is a promising clean energy carrier, but safe and efficient solid-state storage remains a major challenge. Here, dispersion-corrected density functional theory (PBE-D3BJ/def2-TZVPD) was employed to investigate C₁₉X heterofullerenes (X = Si, Ge, Ti, V, Ni, and Zr) as hydrogen storage materials and elucidate their binding mechanisms. Transition-metal doping substantially enhances H₂ binding, with energies ranging from −0.289 eV (Zr) to −0.450 eV (Ni), within the desirable reversible-storage range, whereas Si and Ge provide negligible enhancement. ESP, FMO, NCI, and NBO analyses confirm Kubas-type interactions involving σ-donation and π-backdonation, with Ni exhibiting the strongest backdonation. NBO-derived natural charges (Ti: 0.758, V: 0.469, Ni: 0.315, Zr: 1.264) demonstrate preservation of substantial metal d-electron character and reveal a correlation between partial charge and binding strength. All transition-metal systems exhibit a three-H₂ saturation limit, achieving gravimetric capacities of 1.86–2.14 wt% with adsorption energies remaining within the reversible-storage window. Thermodynamic and kinetic analyses indicate favorable adsorption and rapid H₂ release, while formation energies show that substitution is endothermic, highlighting the need for non-equilibrium synthesis strategies. Overall, the results demonstrate that single-metal substitution provides robust Kubas-active sites but imposes a capacity ceiling below the DOE target, emphasizing increased active-site density through multiple dopants or extended doped-carbon frameworks as a pathway toward higher hydrogen storage capacity.