5-fold increase of hydrogen uptake in MOF74 through linker decorations*

We present \emph{ab initio} results for linker decorations in Mg-MOF74---i.e.\ attaching various metals $\mathcal{M}$ = Li, Na, K, Sc, Cr, Mn, Fe, Ni, Cu, Zn, Rb, Pd, Ag, and Pt near the ring of the linker---creating new strong adsorption sites and thus maximizing small molecule uptake.\footnote{C. Arter, S. Zuluaga, D. Harrison, E. Welchman, and T. Thonhauser, Phys. Rev. B {\bf 94}, 144105 (2016).} We find that in most cases these decorations influence the overall form and structure of Mg-MOF74 only marginally. After the initial screening we chose metals that bind favorably to the linker and further investigate adsorption of H$_{2}$, CO$_{2}$, and H$_{2}$O for $\mathcal{M}$ = Li, Na, K, and Sc. For the case of H$_2$ we show that up to 24 additional guest molecules can be adsorbed in the MOF unit cell, with binding energies comparable to the original open-metal sites at the six corners of the channel. This leads to a 5-fold increase of the molecule uptake in Mg-MOF74, with tremendous impact on many applications in general and hydrogen storage in particular---where the gravimetric hydrogen density increases from $1.63$~mass\% to $7.28$~mass\% and the volumetric density from 15.10 g H$_{2}$ L$^{-1}$ to 75.50~g~H$_2$~L$^{-1}$.