Geometry, Electronic Properties, and Hydrogen Adsorption Properties of Li3N-Based Nanostructures

Recent studies of hydrogen storage have focused on lithium metal atoms as dopants in a variety of substrates as Li is the lightest metallic element in the periodic table. In this work, we have explored the role of Li3N nanostructures in hydrogen storage as they possess Li atoms with varying degrees of coordination. We have performed detailed calculations of geometries, electronic structures, and hydrogen adsorption properties of free (Li3N) n (n = 1−7) clusters and those supported on BN nanoribbons by using density functional theory and generalized gradient approximation for the exchange and correlation potential. We found general motifs of (Li3N) n clusters where N sites form polygons for n ≤ 4 and polyhedrons n ≥ 5. The binding energies per formula unit increase with size, whereas the HOMO−LUMO gaps decrease. The HOMO is mainly contributed by Li and the LUMO by N. The bonding between Li and N has both ionic and covalent character. Lithium sites with low coordination are found to have a stronger adsorption energy for hydrogen molecules, which varies in the range of 0.08−0.11 eV/H2. When deposited on a BN nanoribbon, the Li3N molecules show stronger adsorption of hydrogen due to the changes in charge distribution. This suggests that Li3N molecules or small clusters can be introduced in porous substrates for enhancing hydrogen storage.