Dehydrogenation of Alkali Metal Aluminum Hydrides MAlH 4 (M = Li, Na, K, and Cs): Insight from First-Principles Calculations.

Complex aluminum hydrides with high hydrogen capacity are among the most promising solid-state hydrogen storage materials. The present study determines the thermal stability, hydrogen dissociation energy, and electronic structures of alkali metal aluminum hydrides, MAlH₄ (M = Li, Na, K, and Cs), using first-principles density functional theory calculations in an attempt to gain insight into the dehydrogenation mechanism of these hydrides. The results show that the hydrogen dissociation energy (E_d-H₂) of MAlH₄ (M = Li, Na, K, and Cs) correlates with the Pauling electronegativity of cation M (χ_P); that is, the E_d-H₂ (average value) decreases, i.e., 1.211 eV (LiAlH₄) < 1.281 eV (NaAlH₄) < 1.291 eV (KAlH₄) < 1.361 eV (CsAlH₄), with the increasing χ_P value, i.e., 0.98 (Li) > 0.93 (Na) > 0.82 (K) > 0.79 (Cs). The main reason for this finding is that alkali alanate MAlH₄ at higher cation electronegativity is thermally less stable and held by weaker Al-H covalent and H-H ionic interactions. Our work contributes to the design of alkali metal aluminum hydrides with a favorable dehydrogenation, which is useful for on-board hydrogen storage. [ABSTRACT FROM AUTHOR]