Tuning the hydrogen adsorption properties of Zn–based metal–organic frameworks: Combined DFT and GCMC simulations

Abstract

This study aims to guide the future design of metal-organic frameworks (MOFs) for hydrogen storage. At 77 K, azulenedicarboxylate (ADC)-substituted MOF-5 increases the hydrogen adsorption rate and hydrogen uptake capacity when the pressure exceeds 12 bar. At 297 K, ADC substitution significantly increases hydrogen adsorption when the pressure exceeds 5 bar. The grand-canonical Monte Carlo (GCMC) detailed analyses indicate that with increased pressure, hydrogen molecules in MOFs tend to be located at the corner region, organic linkers, and the center of cores. Meanwhile, high pressure adsorption shows that the large space due to ADC substitution leads to reduced steric effect around metal sites. Analysis of density of states illustrates that the introduced organic linkers may contribute sufficient electrons and enhance electron flow. These electrons can induce an electric field-effect, which increases the polarisation of hydrogen atoms and molecules surrounding linkers. Amongst the studied MOFs, substitution with Ni exerts no obvious effect on hydrogen storage.