High capacity reversible hydrogen storage in Si substituted and Li decorated C <sub>20</sub> fullerene: Acumen from density functional theory simulations
Ankita Jaiswal, Brahmananda Chakraborty, Sridhar Sahu
Abstract
This article addresses the reversible hydrogen storage capacities of lithium (Li) decorated and silicon (Si) substituted C 20 fullerene using density functional theory. The stabilities of the newly designed Si 2 C 18 Li 6 and Si 4 C 16 Li 6 cages have been verified from their chemical hardnesses and HOMO−LUMO gaps. The interaction between H 2 molecules and the Li centers occurs via a Niu-Rao-Jena type interaction. Topological analysis reveals that the nature of the bonding between H 2 and the host clusters as a weak non-covalent type. The H 2 molecules in Si2C18Li6-nH2 and Si4C16Li6-nH2 are found to be adsorbed in quasi-molecular fashion with the average adsorption energies in the range of 0.119 eV - 0.139 eV and 0.131 eV - 0.140 eV, respectively. The molecular dynamics simulation confirms the thermal stability and structural integrity of Li decorated Si 2 C 18 and Si 4 C 16 cages at a relatively high temperature of 400 K. It has been observed that between 300 K and 400 K, most of the hydrogen molecules get desorbed from the host cages, and the structures of the clusters remain almost intact after the complete desorption, which confirmed their reversibility. The practical storage capacities of Si 2 C 18 Li 6 and Si 4 C 16 Li 6 cages at temperature and pressure ranges of 40 K - 140 K, 40 K - 120 K, and 1 - 60 bar are found to be 16.09 % and 14.77 wt%, which are fairly high as compared to the target of the United States Department of Energy (5.5 wt% by 2020). Moreover, we have found that at the temperature and pressure of 200 K and 40 bar, the gravimetric density of both the cages are approximately 5.5 wt%. Hence, the newly designed Si 2 C 18 Li 6 and Si 4 C 16 Li 6 cages can be considered as promising materials for hydrogen storage systems.