Structural Changes of Hydrogen Storage Alloys during Hydrogen Absorption and Desorption

Abstract

  Interstitial metal hydrides for hydrogen storage were studied by in situ powder X-ray and neutron diffractions. This paper reports structural change and distribution of hydrogen atoms in La₂Ni₇- and La₅Ni₁₉-based hydrides with stacking structures consisting of La₂Ni₄ cells and LaNi₅ cells. La₂Ni₇ formed two hydrides, La₂Ni₇H_7.1 and La₂Ni₇H_10.8. During formation of La₂Ni₇H_10.8, La₂Ni₄ and LaNi₅ cells expanded by 69.8% and 12.2%, respectively, indicating that most of absorbed hydrogen was located in La₂Ni₄ cells. In contrast, La₅Ni₁₉-based La₄MgNi₁₉ formed only one hydride, La₄MgNi₁₉H_∼24. Hydrogen was distributed almost evenly in LaMgNi₄ and LaNi₅ cells (0.8-1.0 H/M) in La₄MgNi₁₉D_∼24 from Rietveld refinement of in situ neutron diffraction data. This result indicated that Mg substitution for La changes preference of hydrogen occupation in the La₂Ni₄ and LaNi₅ cells, which leads to difference in hydrogenation properties.