Design requirements for metal-organic frameworks as hydrogen storage materials

Storing an acceptable density of hydrogen in porous materials by physisorption at room temperature and reasonable pressures is a challenging problem. Metal-organic frameworks (MOFs) are a new class of nanoporous materials that have shown early promise for meeting this goal. They have extremely large specific surface areas, but the heats of adsorption to date are too low to provide significant storage at room temperature. In this work, molecular simulations are used to provide guidelines for the design of MOFs for hydrogen storage. To learn how much the heat of adsorption must be increased to meet current targets, we artificially increase the hydrogen/MOF Lennard-Jones attraction. The correlation of the amount of hydrogen adsorbed with the heat of adsorption, the surface area, and the free volume is revisited. We also review the distinction between excess and absolute adsorption and show that comparing the density of hydrogen within the free volume of materials provides useful insight. The simulation results yield a graph showing the required heats of adsorption as a function of the free volume to meet gravimetric and volumetric storage targets at room temperature and 120 bar.