Hydrogen storage enhancement via transition metal decoration on metal organic frameworks: A first-principles study

We investigate the hydrogen storage capacity of the light transition metal (TM)-decorated metal organic frameworks (MOFs) by performing ab initio density functional theory calculations. We find that among all the light TM elements, divalent Ti and Fe are suitable for decorating MOFs to enhance the hydrogen uptake, considering the H₂ binding energy on the TM atom and the reversibly usable number of H₂ molecules attached to the metal site. In general, the magnetization of metal atoms undergoes a high-spin to low-spin state transition when H₂ molecules are adsorbed, which helps to stabilize the system energetically. By analyzing the projected density of states on each TM atom, it is shown that the d-level shift induced by the ligand field of the adsorbed H₂ molecules contributes substantially to the H ₂ binding strength. We also study the stability of selected TM-decorated nanostructures against the attack of foreign molecules by examining the energetics of those contaminating molecules around the metal sites.