Sc/Y/Ti functionalized N-substituted defective C₂₄ as promising materials for hydrogen storage

Incorporating transition metal atoms and creating porphorin-like N₄ cavity are effective strategies for improving the hydrogen storage capability of carbon materials. In this view, we designed and addressed the hydrogen uptake properties of TM₄C₈N₈ (TM = Sc, Y, Ti) cages originating from C₂₄ to overcome TM clustering. The stabilities of three clusters were confirmed by molecular dynamic simulation. Sc₄C₈N₈/Y₄C₈N₈/Ti₄C₈N₈ can bind up to 20/20/36H₂ molecules with a higher gravimetric density of 9.35/6.67/15.27 wt%, compared to the goal of 5.5 wt% proposed by the US-DOE. The interaction between H₂ molecules and the hosts can be identified as van der Waals attractions. The average binding energies of Sc₄C₈N₈(H₂)₂₀/Y₄C₈N₈(H₂)₂₀/Ti₄C₈N₈(H₂)₃₆ at M06(D3) and PBE(D3) functional are 0.159/0.173/0.128 eV/H₂ and 0.139/0.141/0.102 eV/H₂, respectively, meeting the desirable range of reversible hydrogen storage. Furthermore, an investigation was conducted on the impact of pressure and temperature on the hydrogen storage capabilities of TM₄C₈N₈ (TM = Sc, Y, Ti). The desorption temperature exhibits a positive correlation with increasing pressure. The realization of hydrogen release at near mild conditions and the reversibility of hydrogen adsorption/desorption cycles were demonstrated using the van't Hoff equation and ADMP-MD simulations, respectively. Compared with the monomer, the hydrogen storage capacities of (TM₄C₈N₈)₂ (TM = Sc, Y, Ti) dimers are slightly reduced.