Hydrogen Encapsulation in a Silicon Clathrate Type I Structure:  Na_5.5(H₂)_2.15Si₄₆:  Synthesis and Characterization

Abstract

A hydrogen-encapsulated inorganic clathrate, which is stable at ambient temperature and pressure, has been prepared in high yield. Na5.5(H2)2.15Si46 is a sodium-deficient, hydrogen-encapsulated, type I silicon clathrate. It was prepared by the reaction between NaSi and NH4Br under dynamic vacuum at 300 degrees C. The Rietveld refinement of the powder X-ray diffraction data is consistent with the clathrate type I structure. The type I clathrate structure has two types of cages where the guest species, in this case Na and H2, can reside: a large cage composed of 24 Si, in which the guest resides in the 6d crystallographic position, and a smaller one composed of 20 Si, in which the guest occupies the 2a position. Solid-state 23Na, 1H, and 29Si MAS NMR confirmed the presence of both sodium and hydrogen in the clathrate cages. 23Na NMR shows that sodium completely fills the small cage and is deficient in the larger cage. The 1H NMR spectrum shows a pattern consistent with mobile hydrogen in the large cage. 29Si NMR spectrum is consistent with phase pure type I clathrate framework. Elemental analysis is consistent with the stoichiometry Na5.5(H2.15)2Si46. The sodium occupancy was also examined using spherical aberration (Cs) corrected scanning transmission electron microscopy (STEM). The high-angle annular dark-field (HAADF) STEM experimental and simulated images indicated that the Na occupancy of the large cage, 6d sites, is less than 2/3, consistent with the NMR and elemental analysis.

Accelerating the Understanding and Development of Hydrogen Storage Materials: A Review of the Five-Year Efforts of the Three DOE Hydrogen Storage Materials Centers of Excellence

“…From this plot, it can be seen that a number of materials fall close to the gravimetric and temperature ranges considered necessary for vehicular applications. Others, such as silicon nanoparticles [58][59][60][61] release hydrogen at too high a temperature, and do not release hydrogen with adequate gravimetric capacity. When the CHSCoE concluded its research, chemical regeneration of spent fuel from ammonia borane had been demonstrated in the laboratory with two major variants with over ten regeneration schemes having been partially or completely demonstrated, thereby showing potential to overcome regeneration as a barrier to the technological implementation of chemical hydrogen storage; however, more R&D is need to reduce cost and increase efficiencies.…”

Semiconducting Clathrates Meet Gas Hydrates: Xe24[Sn136]

“…[51] The synthesis was carried out by reacting NaSi and NH 4 Br under dynamic vacuum at 300 °C. Similarly to Na 5.5 (H 2 ) 2.15 Si 46 , the noble gas atoms could also coexist in a clathrate structure with other types of guest atoms such as alkali metals.…”

Down Select Report of Chemical Hydrogen Storage Materials, Catalysts, and Spent Fuel Regeneration Processes

“…There are two seminal papers from this work just published in peer-recognized journals at the end of 2007. The first is on Ca(NH 2 BH 3 ) 2 4 . The second regards LiNH 2 BH 3 and NaNH 2 BH 3 materials 5 .…”