Dehydrogenation of Alkali Metal Aluminum Hydrides MAlH4 (M = Li, Na, K, and Cs): Insight from First-Principles Calculations
Rui Zhou, Xiaohua Mo, Yong Huang, Chunyan Hu, Xiaoli Zuo, Yu Ma, Qi Wei, Weiqing Jiang
Abstract
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 (Ed-H2) of MAlH4 (M = Li, Na, K, and Cs) correlates with the Pauling electronegativity of cation M (χP); that is, the Ed-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 χP 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.