Article pubs.acs.org/JPCC
Enthalpy−Entropy Compensation Effect in Hydrogen Storage Materials: Striking Example of Alkali Silanides MSiH3 (M = K, Rb, Cs) Wan Si Tang,† Jean-Noel̈ Chotard,† Pascal Raybaud,‡ and Raphael̈ Janot*,† †
Laboratoire de Réactivité et Chimie des Solides (LRCS), UMR 7314 CNRS, Université de Picardie Jules Verne, 33 rue Saint-Leu, 80039 Amiens Cedex, France ‡ IFP Energies nouvelles, Rond-point de l’échangeur de Solaize BP 3, 69360 Solaize, France S Supporting Information *
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.
1. INTRODUCTION Solid-state silicon−hydrogen bonds are very rarely reported in the literature. Evidence of short bonds found between silicon and hydrogen are detected in the well-known hydrogenated amorphous silicon (a-Si:H), which are typically used in thin films, solar cells, and electronics.1 Passivation of the highdefect-containing surface of amorphous silicon with hydrogen reduces the number of dangling bonds, allowing the material to undergo p/n doping. From infrared (IR) spectroscopy, the −Si−H bonds have usual vibration wavenumbers of 2000 and 630 cm−1, corresponding to the stretching and bending modes, respectively.1 For potential applications involving hydrogen as an energy carrier, elemental silicon is usually mixed together with different hydrides as a reactive additive to destabilize the system. For example, when silicon is added to LiH2,3 or MgH2,2,4 the exothermic formation of the more stable alloys (LixSi and Mg2Si, respectively) reduces the enthalpy of desorption of the binary hydrides, which in turns lowers the temperature of desorption at the same pressure of 0.1 MPa H2. However, no silicon-based complex hydrides are found for these reactions. Instead, silicon always remains in an elemental form upon absorption and as an alloy upon desorption. No actual silicon− hydrogen bonds are formed in the whole process. In crystalline phases, the distance between silicon and hydrogen (deuterium) atoms within the unit cells are usually found to be very large (more than 3 Å), i.e., silicon and © 2014 American Chemical Society
hydrogen are not directly bonded to each other. Also, pertaining to their use as hydrogen storage materials, these compounds have, unfortunately, very poor theoretical hydrogen contents. Some examples taken from the past 10 years include the following. Eu5Si3H2: dSi−H = 3.95 Å, 0.24 wt % H2.5 Li4Si2H0.9: dSi−H = 3.59 Å, 1.10 wt % H2.6 CaAlSiD: dSi‑D = 3.22 Å, 1.05 wt % H2.7 TbNiSiD1.78: dSi‑D = 2.37 Å, 1.44 wt % H2.8 Thus far one of the more promising results in the literature is CaSiD1.19, formed by direct hydrogenation of the CaSi Zintl phase:9 it has one of the shortest Si−D bonds of 1.82 Å and a slightly higher hydrogen content of 1.73 wt %.10,11 However, when looking further back in the literature, a notable class of compounds is the alkali silanide [MSiH3] family. The first alkali silicide, KSiH3, was synthesized in 1961 by Ring et al. through a wet chemistry method: excess potassium was mixed with silane gas (SiH4) in 1,2-dimethoxyethane (monoglyme) at 195 K for 1 month.12 Roomtemperature powder X-ray diffraction (PXRD) analysis on the colorless crystals after solvent removal showed that KSiH3 crystallizes in the cubic NaCl-type Fm-3m space group with a cell parameter value of a = 7.15(2) Å.12,13 The hydrogen atomic positions were not solved due to few reflections and probable rotation of the anionic group.13 Subsequently, in 1968, Received: November 18, 2013 Revised: January 20, 2014 Published: January 21, 2014 3409
dx.doi.org/10.1021/jp411314w | J. Phys. Chem. C 2014, 118, 3409−3419
The Journal of Physical Chemistry C
Article
c monoclinic unit cell33 and MSi (M = K, Rb, Cs) in P-43n cubic ones.34 Other than starting with KSi to form KSiH3 through a solid−gas reaction,17,18 NaSi and LiSi have also been studied for their hydrogenation properties. NaSi disproportionates to NaH and the Na8Si46 clathrate phase,18 and LiSi forms LiH and Si under hydrogen pressure.35 Unfortunately, their corresponding alkali silanides are not formed, and presently, the stabilization of the [SiH3]− anion with small cations in the solid state is still a great challenge. However, since RbSiH3 and CsSiH3 are already isolated solid-state phases in the literature,14,15 it would be interesting to synthesize them through direct hydrogenation and subsequently determine the hydrogen positions within their unit cells to fully resolve the crystal structures. Therefore, in order to fully explore the whole alkali monosilicides series, herein we describe a comparative study combining experimental work and theoretical approaches on the hydrogenation properties of the MSi (M = K, Rb, Cs) Zintl phases, elaborating on the structural characterization of the different α-/β-MSiH3 silanide phases formed upon hydrogenation (deuteration) as well as the thermodynamics of the MSi/α-MSiH3 equilibrium.
Amberger et al. used a similar wet chemistry method to successfully synthesize the potassium to cesium analogues by mixing the alkali metal with SiH4 in monoglyme/diglyme at 293 K (reaction times KSiH3 = 24 days, RbSiH3 = 13 days, and CsSiH3 = 7 days).14 Weiss et al. then performed PXRD experiments in 1970 on these compounds at room temperature: all three silanides crystallize in the same Fm-3m space group with cell parameter values of a = 7.23(1), 7.52(1), and 7.86(1) Å for KSiH3, RbSiH3, and CsSiH3, respectively, but the hydrogen atomic positions still remain unsolved.15 Solid-state proton nuclear magnetic resonance (1H NMR) performed on KSiH3 showed that at low temperatures (
