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 H-2 molecules adsorbed on these structures is either 24 or 28 with H-2 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 H-2 adsorption characteristics that fall between physisorption and chemisorption whereas the structures containing carbon atoms demonstrate a physisorption nature for H-2 molecules. The hydrogen molecules strongly bind with SiG1(Sc)(4) which is supported by its higher H-2 desorption energy and H-2 desorption temperature. In conclusion, the derivatives of adamantane show better hydrogen storage performance than adamantane.