Abstract:

The effect of inserting coordinatively unsaturated metal sites (CUS) into porous aromatic frameworks (PAFs) on their hydrogen storage capacity was investigated systematically by density functional theory and grand canonical Monte Carlo simulations. The results indicate that the maximum excess gravimetric uptake of hydrogen possible with PAF-302MgO₂_PBE100 is 7.97% (w) at 77 K. The total uptakes of hydrogen by PAF-302 and PAF-303 functionalized with 100% magnesium alkoxide at 77 K and 10 MPa were determined to be 9.9% (w) (65.9 g∙L^-1) and 15.0% (w) (50.5 g∙L^-1), respectively. These uptake values are 80% (64.8%) and 173% (26.3%), respectively, more than the gravimetric and volumetric targets set by the Department of Energy (DOE) of USA. They also exceed the targets set by NU-1101, presenting the highest measured performance of 9.9% (w) (46.6 g∙L^-1) under the same conditions. Even at 243 K and 10 MPa, the total gravimetric and volumetric uptakes of hydrogen in the former are up to 5.13% (w) and 34.19 g∙L^-1, which are about 93.3% and 85.5% of the targets set by DOE, respectively. By analyzing the isosteric heat of adsorption (Q_st), radial distribution function, and mass center probability density, it is found that increasing the length of the organic linkers of PAFs incorporated with CUS will result in decreasing volumetric surface areas in spite of the increase in void fractions, which is the root of trade-offs between the total gravimetric and volumetric H₂ uptake in porous materials. Additionally, CUS incorporation improves the affinity of PAF materials to H₂ molecules, resulting in an enhancement of the volumetric hydrogen storage capacity.

Key words: Porous aromatic framework, CUS, Hydrogen storage, Molecular adsorption, GCMC simulation