Design Requirements for Metal-Organic Frameworks as Hydrogen Storage Materials

Abstract

Storing an acceptable density of hydrogen in porous materials by physisorption at room temperature and\nreasonable pressures is a challenging problem. Metal-organic frameworks (MOFs) are a new class of nanoporous\nmaterials that have shown early promise for meeting this goal. They have extremely large specific surface\nareas, but the heats of adsorption to date are too low to provide significant storage at room temperature. In\nthis work, molecular simulations are used to provide guidelines for the design of MOFs for hydrogen storage.\nTo learn how much the heat of adsorption must be increased to meet current targets, we artificially increase\nthe hydrogen/MOF Lennard-Jones attraction. The correlation of the amount of hydrogen adsorbed with the\nheat of adsorption, the surface area, and the free volume is revisited. We also review the distinction between\nexcess and absolute adsorption and show that comparing the density of hydrogen within the free volume of\nmaterials provides useful insight. The simulation results yield a graph showing the required heats of adsorption\nas a function of the free volume to meet gravimetric and volumetric storage targets at room temperature and\n120 bar.