| Summary: | 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, MAlH<sub>4</sub> (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<sub>d</sub>-H<sub>2</sub>) of MAlH<sub>4</sub> (M = Li, Na, K, and Cs) correlates with the Pauling electronegativity of cation M (χ<sub>P</sub>); that is, the E<sub>d</sub>-H<sub>2</sub> (average value) decreases, i.e., 1.211 eV (LiAlH<sub>4</sub>) < 1.281 eV (NaAlH<sub>4</sub>) < 1.291 eV (KAlH<sub>4</sub>) < 1.361 eV (CsAlH<sub>4</sub>), with the increasing χ<sub>P</sub> 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 MAlH<sub>4</sub> 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.
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