Scandium Substituted Adamantane, Si Adamantane, Ge Adamantane Cages and Their Derivatives for Hydrogen Storage: A DFT Analysis

Adamantane and its derivatives are designed and analyzed for their hydrogen storage properties after scandium substitution using density functional theory approach. A total of nine structures are considered viz. adamantane (C₁₀H₁₆) denoted by C_adm, Si adamantane (Si₁₀H₁₆-Si_adm) Ge adamantane (Ge₁₀H₁₆-Ge_adm), germanium substituted adamantane (C₄Ge₆ (CG1) and Ge₄C₆(CG2)), silicon substituted adamantane (C₄Si₆ (CSi1) and Si₄C₆ (CSi2)), Si and Ge substituted adamantane (Si₄Ge₆ (SiG1) and Ge₄Si₆ (SiG2)). To enhance hydrogen uptake capacity, four hydrogen atoms in these structures are replaced with four Sc atoms. The negative Sc substitution energies for all the structures suggest that the substitution of Sc atoms is an endothermic process. The number of H₂ molecules adsorbed on these structures is either 24 or 28 with H₂ uptake capacity in a range of 6.08–13.4 wt% meeting the U.S. Department of Energy's 2025 target of 5.5 wt.%. The inorganic structures exhibit H₂ adsorption characteristics that fall between physisorption and chemisorption whereas the structures containing carbon atoms demonstrate a physisorption nature for H₂ molecules. The hydrogen molecules strongly bind with SiG1(Sc)₄ which is supported by its higher H₂ desorption energy and H₂ desorption temperature. In conclusion, the derivatives of adamantane show better hydrogen storage performance than adamantane.