Metal doped tetrahedral silsesquioxane cages for hydrogen storage

Abstract

The hydrogen adsorption properties of Tetrahedral Silsesquioxane Si4O6H4 (TS) cages doped with Be, Li, Sc and Ti atoms are studied in different spin state using density functional theory with M062X functional. Initially-one of the four hydrogen atoms in TS cage was substituted by metal atoms. Doublet and singlet are the lowest energy state for TS-Be and TS-Li respectively with respective H2 uptake capacity of 1.79 and 3.56 wt%. Triplet and quartet are the lowest energy state for TS-Sc and TS-Ti respectively. TS-Sc (singlet) and TS-Ti (doublet) cage interacts with seven and six hydrogen molecules respectively with respective gravimetric H2 capacity of 5.25 and 4.51 wt%. TS-Sc in triplet state could adsorb six H2 molecules whereas TS-Ti in quartet state could adsorb five H2 molecules with respective H2 uptake capacity of 4.5 wt% and 3.7 wt%. Then all the four hydrogen atoms in a TS cage are substituted by four Be, Li or Sc atoms (lowest energy state is Singlet for all the three) with respective hydrogen uptake capacity of 6.19, 12.69 and 12.01 wt%. When all the four hydrogens in TS cage are substituted by four Ti atoms, clustering of Ti atoms occurs and the cage structure gets distorted and therefore not considered for H2 adsorption. H2 adsorption on one Be(doublet), Sc(triplet) and Ti(quartet) atom doped TS cage is thermodynamically favourable below 210, 160 and 320 K respectively at 1 atm. pressure. At room temperature, it is thermodynamically favourable at higher pressure also. Four Sc and Be atom doped TS cages are found to be thermodynamically suitable for hydrogen storage below 210 K at 1 atm pressure. It is also suitable at room temperature but at higher pressure. One as well as four Li atom doped TS cages are not suitable for hydrogen storage for the entire temperature and pressure range considered. Molecular dynamics simulations performed using atom centered density matrix propagation (ADMP) show that the loosely bonded H2 molecules fly away during the simulations.