Alkali metal silanides alpha-MSiH3: A family of complex hydrides for solid-state hydrogen storage

摘要

The alkali metal silanides alpha-MSiH3 appear to be a promising family of complex hydrides for solid-state hydrogen storage. Herein the structural, energetic and electronic properties of alpha-MSiH3 silanides (M = Li, Na, K, Rb, Cs) and MSi Zintl phases are systematically investigated for the first time by using first -principles calculations method based on density functional theory. The structural parameters of alpha-MSiH3 and MSi including lattice constants and atomic positions are determined through geometry optimization. The obtained results are close to the experimental data analysed from X-ray and neutron powder diffraction. The calculations of formation enthalpy show that alpha-KSiH3, alpha-RbSiH3 and alpha-CsSiH3 silanides are easier to be synthetized relative to alpha-LiSiH3 and alpha-NaSiH3, which interprets well the lower thermostabilities of experimental alpha-LiSiH3 and alpha-NaSiH3. Nevertheless, LiSi, KSi and CsSi phases are easier to be formed relative to NaSi and RbSi. The calculations of hydrogen desorption enthalpy reveal that the dehydrogenation abilities of alpha-MSiH3 silanides along the decomposition path of alpha-MSiH3 -> MSi + H-2 are gradually enhanced in the order of alpha-CsSiH3, alpha-RbSiH3, alpha-KSiH3, alpha-NaSiH3 and alpha-LiSiH3, which may be originated from their decreasing thermostabilities. From a comprehensive point of view including hydrogen storage capacity, thermostability and dehydrogenation ability, alpha-KSiH3 (similar to 4.29 wt%) is identified as the most promising alkali metal silanide for reversible hydrogen storage. Analysis of electronic structures indicates that a significant charge transfer leads to positively charged M ions and negatively charged SiH3 complex, which constitutes the ionic bonding between them. The bonding within SiH3 complex not only involves the covalent hybridization between Si (3s) (3p) and H (1s) orbitals, but also exhibits some ionic bond characteristics due to the partial charge transfer from Si to H. The covalent bonding interactions between H and Si atoms within SiH3 mainly dominate the thermostabilities and dehydrogenation properties of alpha-MSiH3 silanides. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.