Hydrogen Storage In A Novel Destabilized Hydride System, Ca₂SiH_x:  Effects of Amorphization

Abstract

Crystalline Ca 2 Si was synthesized via the 873 K evacuation of a 2CaH 2 + Si ball-milled mixture. The resulting Ca 2 Si readily absorbs hydrogen below 0.1 MPa within the temperature range 473-523 K, leading to the formation of crystalline Ca-Si-based hydrides as well as the reformation of crystalline CaH 2 . In contrast, hydrogen absorption at pressures greater than 0.5-0.7 MPa leads to an unusual amorphous metal-hydride phase, stable in a range up to 523 K as evidenced by neutron powder diffraction, neutron vibrational spectroscopy, and absorption isotherm measurements. The formation of such a phase is discussed in relation to other known amorphous intermetallic hydrides and the role of "chemical frustration." The hydrogenation process from Ca 2 Si to Ca 2 SiH x is completely reversible but requires high desorption temperatures.

Structure and hydrogenation properties of the ternary alloys Ca2−xMgxSi (0≤x≤1)

“…This interesting morphological behavior will be discussed in detail elsewhere in a separate publication. The formation of this amorphous structure is consistent with the observed disappearance of virtually all NPD reflections associated with the Ca 2 Si phase upon hydriding [6].…”

“…Recent studies of the preparation and storage properties of CaSi and Ca 2 Si alloys by neutron methods and other techniques have revealed the structure and bonding of hydride phases created under different synthesis conditions (pressure, temperature, stoichiometry, etc.) [3][4][5][6]. The more Ca-rich Ca 2 Si alloy has the potential for higher hydrogen uptake (i.e., 3.59 wt.% max.…”

Hydrogenous Zintl Phase Ba3Si4Hx (x = 1–2): Transforming Si4 “Butterfly” Anions into Tetrahedral Moieties

“…11 For Ca 5 Si 3 and Ca 2 Si, H incorporation leads to the formation of amorphous hydrogenous phases. 12,13 Amorphization and/or H-induced oxidative decompositions are frequently observed upon hydrogenation of Zintl phases. 3,14 The compositional range of hydrogenous Zintl phases is not yet explored.…”

Predictions of H isotope separation using crystalline and amorphous metal membranes: A computational approach

“…In the amorphous Fe 3 B we examined, the presence of extremely favorable binding sites for interstitial H and its isotopes strongly limited the selectivity of the material as a membrane. The existence of a broad range of interstitial site binding energies is typical in amorphous metals (Kirchheim, 1988;Kirchheim et al, 1988;Wu et al, 2007), so poor isotopic selectivity is likely to be common in many materials of this type. Another conclusion from our results is that the isotopic permeability selectivity of the crystalline materials is only weakly dependent on temperature over a wide range of temperatures.…”