Sc/Ti functionalized defective C₂₀ as promising materials for hydrogen storage: a theoretical study

In present work, we theoretically designed and addressed the H₂ uptake properties of TM₂C₁₆ (TM = Sc, Ti) cages originating from C₂₀ to overcome TM clustering. The Sc₂C₁₆/Ti₂C₁₆ cluster can adsorb a maximum of 12/10 H₂ molecules with a gravimetric H₂ uptake capacity of 7.85/6.50 wt% and a volumetric density of 97.39/83.10 g H₂·L^{− 1}, surpassing the target set by the US-DOE for onboard hydrogen storage in light-duty vehicles. For Sc₂C₁₆(H₂)₁₂, 4 H₂ molecules are decomposed into H atoms and bonded with the parent body, forming Sc₂C₁₆H₄(H₂)₁₀. The average binding energy of Sc₂C₁₆H₄(H₂)_2n and Ti₂C₁₆(H₂)_2n (n = 1–5) fall within the ideal range for reversible hydrogen storage. The results of Gibbs free energy correction suggest that all H₂ molecules adsorbed in molecular form in Sc₂C₁₆H₄(H₂)_2n and Ti₂C₁₆(H₂)_2n (n = 1–5) can be efficiently adsorbed and desorbed under atmospheric pressure. The reversible characteristics of the adsorption/desorption cycle were verified through ADMP-MD simulations. The thermodynamic analysis indicates that the majority of the adsorbed hydrogen molecules in Sc₂C₁₆H₄(H₂)₁₀/Ti₂C₁₆(H₂)₁₀ can be desorbed under practical temperature and pressure conditions, with a practical capacity of 5.31/5.60 wt%, being close to the target specified by the US-DOE.