DFT Study on the H₂ Storage Properties of Sc-Decorated Covalent Organic Frameworks Based on Adamantane Units

We follow the idea of using metal-decorated polyhydroxy adamantane structures as H₂ adsorption sites and develop it further by integrating such structures into porous framework structures. We use density functional theory calculations to investigate the hydrogen adsorption in five Sc-decorated, porous, covalent organic frameworks. Each of them consists of polyhydroxy adamantane units connected by molecular linkers. A polyhydroxy adamantane unit acting as an anchor site for four Sc atoms, in return, can bind up to four H₂ molecules per Sc site. At full coverage, the average H₂ adsorption energy is between −0.17 and −0.19 eV/H₂. We use a simplified thermodynamic model to estimate the gravimetric and volumetric hydrogen uptake as a function of temperature and H₂ pressure. The most promising framework found here is a structure with benzene units as linkers, which is predicted to achieve 3.04 wt % or 27.28 g/L H₂ uptake at 233 K and 100 bar H₂ pressure. We also show that the relatively weak framework–H₂ interaction leads to the circumstance that at typical operating conditions, the hydrogen uptake deviates significantly from the full coverage, illustrating the necessity to account for the temperature and pressure dependency of the H₂ uptake.