  This article presents the reversible hydrogen storage capacities of
Li-decorated Si6C14 and Si8C12 using Density Functional Theory (DFT). The
chemical stabilities of the designed Si6C14Li6 and Si8C12Li4 nanocages are
investigated using HOMO-LUMO gaps and various global reactivity descriptors
such as chemical hardness and electrophilicity index. Our study reveals that
each Li atom decorated over the designed Si6C14 and Si8C12 nanocages can hold
up to 5H2 molecules with adsorption energy lying in the optimum range of
0.14-0.085 eV, thereby yielding an overall gravimetric density of 13.8% and
9.2% respectively. The interaction between adsorbed H2 molecules and the Li
metal sites is found to occur via non-covalent and closed shell type of
interaction. The H2 molecules are adsorbed in a quasi-molecular fashion with
elongated bond length. The molecular dynamics study reveals that most of the H2
molecules get desorbed from the designed host nanocages at 300K without causing
any significant structural changes, which confirms their reversibility. When
the adsorption and desorption conditions are set at 100K/60bar and 240K/1bar
respectively, the practical storage gravimetric densities of Si6C14Li6 and
Si8C12Li4 cages are estimated to be 13.73 wt% and 9.08 wt%, which are
relatively high in comparison to the US-DOE target of 5.5 wt% by the year 2020.
Hence, the computationally designed Si6C14Li6 and Si8C12Li4 nanocages can be
regarded as prospective systems for hydrogen storage applications.
