Structural Properties of (Ti, Zr)(Mn, Cr)₂M_0.1 (M = None, Fe, Co, Ni, and Cu) Hydrogen Storage Alloys: Composition Distribution and Occupied Site of Doped Element

Abstract

CO₂ tolerance of hydrogen storage alloys of AB₂-type (C14 Laves phase) Ti_0.515Zr_0.485Mn_1.2Cr_0.8M_0.1 (AB₂-M, M = none, Fe, Co, Ni, and Cu) depends on dopant M. Since our goal is to clarify this mechanism, we determined the elemental analysis using X-ray absorption spectroscopy (XAS), scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (XRD), and neutron powder diffraction (NPD) with Rietveld refinement in this study. As a result of XAS analysis, a strong evidence of all doped elements occupying B site in AB₂ was obtained. SEM-EDX showed inhomogeneous composition with vacancy in B site and linear correlation of Ti/Zr and Mn/Cr ratio. The peak width in XRD patterns of AB₂-M depends on the magnitude of homogeneity, therefore the Rietveld analysis using NPD patterns could not be well refined. Thus, homogeneity is not important but element of B site would be important for CO₂ tolerance as well as AB₅ type alloys.

Additive metal M occupies B site in AB₂-type hydrogen storage alloys. Therefore, CO₂ tolerance depends on B element.