Ca7Ge-type hydride Mg6VNaxHy (0≤x≤1): High pressure synthesis, synchrotron X-ray analysis and hydrogen storage properties

• Published in: Journal of power sources Vol. 210; pp. 158 - 162
• Main Authors: Takeichi, N., Yan, J., Yang, X., Shida, K., Tanaka, H., Kiyobayashi, T., Kuriyama, N., Sakai, T.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 15.07.2012; Elsevier
• Subjects: Alternative fuels. Production and utilization; Applied sciences; Energy; Exact sciences and technology; Fuels; High pressure; Hydrogen; Hydrogen storage; Rietveld analysis; Synchrotron X-ray; Hydrogen storage; Rietveld analysis; Synchrotron X-ray; High pressure; Hydrogen; Enthalpy; Instrumentation; Hydrides; X ray diffraction; Powder; Phase estimation; Synchrotrons; Magnesium; Synchrotron radiation
• ISSN: 0378-7753; 1873-2755
• DOI: 10.1016/j.jpowsour.2012.03.034

► Mg6VNaxHy hydride was synthesized under high pressure H2 (8GPa). ► Synchrotron X-ray diffraction and Rietveld crystallographic structure analysis. ► Mg6VNaxHy hydride has the Ca7Ge-type cubic structure. ► Reversible reaction with H2 was observed. ► Comparison with related hydride Mg7TiNaxHy. A powder mixture of MgH2:VH2:NaH=6:1:n is treated under 8GPa at 873K using an eight-anvil apparatus in order to investigate the influence of NaH addition to Mg6VHy, a Ca7Ge-type FCC hydride. Synchrotron radiation X-ray diffraction (SR-XRD) of the obtained sample reveals that Na occupies the vacant 4b site in Mg6VHy to form Mg6VNaxHy (0≤x≤1) as the main product while retaining its Ca7Ge-type structure. The Rietveld analysis of the SR-XRD data suggests that the bond lengths between hydrogen and magnesium remain constant through the Na addition. All the samples reversibly desorb and absorb hydrogen at 620–630 and 590–600K, respectively, under 0.5MPa (H2). These temperatures are, respectively, about 70 and 120K lower than those of MgH2. The hydrogen capacity of the main product phase, Mg6VNaxHy, is estimated to be 5–6mass% from the pressure-composition isotherms (PCIs) by taking its content rate in the specimen into account. The reaction enthalpies calculated from the van’t Hoff relation of the PCIs do not significantly differ from that of MgH2. The bond lengths and energies between hydrogen and magnesium are not affected by the perturbation by the NaH addition in the lattice in Mg6VNaxHy.