Computational investigation of hydrogen adsorption in silicon-lithium binary clusters#

Abstract

Theoretical studies on hydrogen adsorption properties of silicon-lithium binary clusters are carried out. We have considered three different clusters viz., Si5Li\(_{5}^{-}\), Si5Li6 and Si5Li\(_{7}^{+}\) and for each cluster, the geometries of different possible isomers are optimized. In all the minimum energy isomers of the three clusters considered, two of the lithium atoms are found to be situated in the axial positions and the remaining lithium atoms are in the equatorial position in the Si5 plane. The lithium atoms which are in Si5 plane are bonded to the Si-Si edge through a bridged bond instead of a corner in the Si5 ring. From the calculated atomic charges, it is found that there is a charge transfer from lithium to silicon leaving a partial positive charge on the Li atoms and the axial lithium atoms are more charged as compared to the remaining lithium atoms. In the case of Si5Li6 and Si5Li\(_{7}^{+}\), the Li sites can trap a total of 14 and 17 H2 molecules, respectively, with each bridge bonded Li site adsorbing three H2 molecules and each axial Li adsorbing one H2 molecule which corresponds to a gravimetric density of 13.33 wt% and 15.25 wt%, respectively. Silicon–lithium binary clusters have been investigated for their hydrogen adsorption properties through ab initio based density functional theory calculations. All the lithium sites in these clusters are found to be cationic in nature and can trap molecular hydrogen with a gravimetric density range of 13–15 wt%.