Enthalpy–Entropy Compensation Effect in Hydrogen Storage Materials: Striking Example of Alkali Silanides MSiH3 (M = K, Rb, Cs)

Abstract

Safe storage of hydrogen is a key issue for development of the hydrogen economy. Solid-state hydrogen storage through reversible formation of hydrides is one of the most promising methods. Herein reports for the very first time a combined experimental and theoretical study of the hydrogen absorption–desorption of MSi Zintl phases (M = K, Rb, and Cs). Upon direct hydrogenation of the MSi phases at 100 °C, the α-MSiH3 silanides (4.3, 2.6, and 1.85 wt % H2 for M = K, Rb, and Cs, respectively) are formed and the corresponding alloys are reobtained upon desorption. Besides the full resolution of the crystal structures of the α-MSiH3 silanides and their low-temperature β-phases, by neutron powder diffraction, we show an enthalpy–entropy compensation effect for the MSi/α-MSiH3 equilibrium: a simultaneous linear increase in the enthalpy (more stable silanide) and entropy change (more localized H atoms in the crystal structure) for the larger cation (Cs+). Through this phenomenon, a desorption temperature of ca. 410 K is obtained at 0.1 MPa hydrogen equilibrium pressure for all three systems. Indeed, this study shows for the first time that the alkali silanides form a new family of complex hydrides that can be considered as promising materials for reversible hydrogen storage near ambient conditions in the solid state.