Abstract
The solid-state structures of LiH, NaH, KH, LiAlH4, NaAlH4 and Li3AlH6 have been explored in details as potential hydrogen-storage materials using computational electron density methods; the full-potential linearized augmented plane wave (FPLAPW) method plus local orbital (FPLAPW+lo) embodied in the WIEN2k package code. Topological analysis of their DFT-computed electron densities in tandem with Bader’s Atoms in Molecules (AIM) theory reveals a plethora of stabilizing interactions some of which are really strong. With the exception of NaH and KH, which do not contain the hydride-hydride bonding, the rest of the metal hydrides; LiH, LiAlH4, NaAlH4 and Li3AlH6 show an increasing number of hydride-hydride interactions that contribute to the stabilization of their three-dimensional (3-D) solid-state structures. Even though these hydride-hydride interactions are weaker compared to the M-H counterparts, their multiplicity greatly contributes to the stability of these metal hydrides. Results from their electron density studies reveal that the number of hydride-hydride interactions in these binary and complex metal hydrides increase with the complexity of the solid-state structures. LiAlH4 is more stable compared to NaAlH4, Li3AlH6, and LiH. NaH and KH were seen to be the least stable solid-state structures. It is suggested that the presence of these hydride-hydride interactions play a significant role in the mediation or understanding of the reaction mechanism leading to the release of hydrogen from these solid-state systems.
Highlights
Much experimental research has been done and is ongoing to select potential solid-state systems for hydrogen storage, but the mechanism behind the release of hydrogen in these systems is still largely unclear
In order to be suitable for vehicular applications, solid-state hydrogen storage materials are required to meet the requirements or targets proposed by the US Department of Energy (DOE), viz; a hydrogen storage capacity of at least 5.5 wt. %, low desorption temperature, low cost, and low toxicity [1,2,3,4,5]
To expand on the immense continuous experimental research already conducted on these and similar metal hydrides to improve and modify their properties for hydrogen storage using periodic density functional theory (DFT) calculations in conjunction with a topological analysis of the electron density derived from the Bader's Quantum Theory of Atoms in Molecules (QTAIM) approach [8, 9], we investigate the structures and properties of LiH, NaH, KH, NaAlH4, LiAlH4 and Li3AlH6
Summary
Much experimental research has been done and is ongoing to select potential solid-state systems (materials) for hydrogen storage, but the mechanism behind the release of hydrogen in these systems is still largely unclear. The temperatures and pressures required to release hydrogen from these systems are generally either too high or too low for on-board vehicular applications. Binary and complex metal hydrides of period 2 and 3 metals are potential candidates for hydrogen storage and other mobile applications. The chemical simplicity of these binary and complex solid-state systems as well as their high hydrogen contents and low desorption temperatures have prompted enormous research geared at improving and enhancing their thermodynamic, kinetic, and materials properties for use as hydrogen storage materials and other mobile applications [6, 7]
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