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

Abstract

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.

Lithium-doped triazine-based graphitic C3N4 sheet for hydrogen storage at ambient temperature

“…To understand the adsorption mechanism, we calculated the charge distribution in Li doped g-C 3 N 4 using the natural bond orbital (NBO) analysis, which has been found to be more accurate than standard Mulliken analysis or alternative theoretical approaches [31]. Li site is found to carry a charge of 0.774 |e|, suggesting that the bonding between Li and the g-C 3 N 4 is essentially ionic, and the positively charged Li ions would produce local electric field which would polarize H 2 molecules and enhance the adsorption [32][33][34][35][36][37][38].…”

Chemical engineering of adamantane by lithium functionalization: A first-principles density functional theory study

“…They have found that the average optimal adsorption enthalpy should be in the range of 0.1-0.2 eV/H 2 . 21,22 It is seen that our designed materials satisfy the criteria for the gravimetric density of hydrogen in storage media by the DOE as well as above the predicted optimal energy window for hydrogen adsorption. Therefore diamondoid-Li/Li + complexes might be good candidates for hydrogen storage materials.…”

“…In all our calculations, it was observed that due to the very high stability of ADM, 31 the binding energy of hydrogen molecules on such compounds was very small, i.e., on the order of ∼−0.001 eV, which is out of the energy range from −0.1 to −0.2 eV desirable for reversible H 2 adsorption/desorption near room temperature for hydrogen storage applications. 21,22 Hence, pristine diamondoids are not considered to be good candidates for hydrogen storage applications. However, experimentally it has been shown that by treating the surfaces of diamondoids with chemical solutions, 32-36 the hydrogen atoms of diamondoids can be selectively replaced by other compounds or elements, resulting in significant changes in their electronic structures.…”

Low dimensional nanostructures of fast ion conducting lithium nitride

“…Unlike these composites, however, the nanofibres do not suffer from the gravimetric capacity penalty imposed by an inactive component. Computational studies have demonstrated that pseudo-molecular (Li 3 N) n ( n = 1–7) clusters would bind H 2 via coordinatively unsaturated Li atoms with an adsorption energy approximately an order of magnitude smaller than that of the bulk material 34 . By extension, earlier DFT calculations show that the most favourable adsorption sites for both H 2 and dissociated H atoms are to N positions on the (001) surface of Li 3 N 35 .…”

“…Unlike these composites, however, the nanofibres do not suffer from the gravimetric capacity penalty imposed by an inactive component. Computational studies have demonstrated that pseudo-molecular (Li 3 N) n (n = 1-7) clusters would bind H 2 via coordinatively unsaturated Li atoms with an adsorption energy approximately an order of magnitude smaller than that of the bulk material 34 Calculations on nanostructured α-Li 3 N electronic properties. Electronic properties were computed on α-Li 3 N using DFT to compare the behaviour of a monolayer of Li 3 N with respect to the bulk material and to develop an understanding of the evolution of electronic structure with the number of layers as compared to graphene and other van der Waals nanostructures (vdWNs).…”