Theoretical investigation of mixed-metal metal-organic frameworks as H2 adsorbents: insights from GCMC and DFT simulations

Molecular hydrogen (H-2) is a renewable energy carrier, however, its practical applications are limited due to the challenges of developing safe and efficient H-2 storage devices. Metal Organic Frameworks (MOFs) containing at least two different metal ions in their structures are called as mixed-metal MOFs (MM-MOFs) and they could adsorb H-2 in higher amounts compared to structures containing single metal nodes. We theoretically examined the H-2 storage capacities of 26 MM-MOFs having various physical and chemical properties applying Grand Canonical Monte Carlo (GCMC) and Density Functional Theory (DFT) simulations. H-2 adsorption isotherms were calculated using a five-site anisotropic H-2 model. QIXSOG, YOMVIG, OSOYUR, Cu-Mg-BTC, Fe-Mg-BTC, and Cr-Mg-BTC were selected as top-performing MM-MOFs maximising H-2 adsorption gravimetrically and volumetrically at near-ambient conditions (233 K and 100 bar), approaching the DOE targets. YOMVIG has the largest H-2 adsorption enthalpy, calculated as -9.93kJ/mol at 233 K and 100 bar. DFT simulations have been conducted to analyse preferable H-2 adsorption sites as well as identify guest-host interactions. Electron density difference analysis showed that adsorbed H-2 molecules in the OSOYUR, Cr-Mg-BTC, Cu-Mg-BTC, and Fe-Mg-BTC are polarised. Our study challenges existing literature by identifying promising MM-MOFs as potential next-generation hydrogen storage adsorbents at near-ambient conditions.