Understanding hydrogen adsorption performance of lithium-doped MIL-101(Cr) by molecular simulations: Effects of lithium distribution

Abstract

Metal-organic frameworks (MOFs) have been recognized as one of the most compelling physical adsorption hydrogen storage materials owing to their ultrahigh surface area and excellent hydrogen adsorption performance. In order to further improve their hydrogen adsorption performance, lithium doping is an effective approach to increase the number of hydrogen adsorption sites as well as enhance the interaction strength towards hydrogen molecules according to grand canonical Monte Carlo(GCMC) simulations. However, in previous simulation studies, lithium ions were commonly assumed to be randomly distributed in MOF frameworks. In fact, the lithium-doped MOFs were prepared by immersing MOFs in a lithium salt solution and then drying them under high temperatures, in which the distribution of Li+ in MOF frameworks is elusive. In this work, the lithium-doped MIL-101 models (i.e., Immersion model) with varying lithium contents were constructed according to experimental operation and their hydrogen adsorption performance from GCMC simulations was also investigated in comparison with the equivalent models with randomly distributed lithium ions (i.e., Random model). It is found that in contrast to the uniform distribution of lithium ions in Random model, the accumulation of lithium ions was inspected in Immersion models especially at high loadings, leading to the reduced pore size. On the contrary, the hydrogen adsorption capacities of Immersion models are significantly improved owing to the enhanced interaction strength with hydrogen molecules resulting from the reduced pore size and the strengthened charged-induced dipole interaction.