Effect of Pd/Mn Substitution on Characteristics of Amorphous Zr-Ni-Ti-V Alloy Hydride Electrodes
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Effect of Pd/Mn Substitution on Characteristics of Amorphous Zr-Ni-Ti-V Alloy Hydride Electrodes Hiroshi Miyamura, Yoshihisa Fujita and Balachandran Jeyadevan The University of Shiga Prefecture, 2500 Hassaka-cho, Hikone 522-8533, JAPAN ABSTRACT Hydrogenation properties of some amorphous Zr-Ni-Ti-V based alloys were investigated. Pressure-composition(P-C) isotherms and hydrogen storage capacities at room temperatures were measured and effects of elemental substitution of the components with Pd or Mn were studied. The alloy electrodes were prepared by using amorphous (Zr-Ni-Ti-V)-(Pd,Mn) alloys prepared by the melt spinning method. The amorphous alloys in the electrode kept their amorphous structures during cycles of charge and discharge. The electrochemical hydrogen storage capacities were strongly affected by the substitution amounts of Pd or Mn. Even a small amount of substitution, changed the equilibrium dissociation pressures of the alloy. In the present study, the rechargeable capacity was optimized up to H/M=0.5 for the alloy electrode with the composition of (Zr45Ni30Ti25)-3at%Pd. The slope in the P-C isotherm suggested that the maximum H/M of the alloy would exceed 1.0 at higher hydrogen pressure than 1.0 MPa, however, the wide distribution of hydrogen site energy in the amorphous hydride resulted in extremely large slope in P-C isotherms, and consequently restricted the rechargeable capacities of the electrodes.
INTRODUCTION Since metal hydrides were first applied to battery electrode (“metal hydride electrode” or “MH electrode”) [1,2], considerable efforts have been made to improve their properties such as capacities, kinetics, and cycle lives. Since the report about the excellent properties of LaNi5 based MH electrode by Willems[3], most of the LaNi5 based alloy for electrode materials have been developed. Because hydrogen atoms occupy the interstitial sites in the alloy crystal, their storage capacities are limited by the arrangement of the interstitial sites. Recently, their storage capacities were further improved by using a superstructure alloy mateials[4,5]. Some amorphous alloys form hydrides. Especially, reactions with hydrogen of Cu-Ti, ZrNi, and Ti-Ni amorphous alloys have been well investigated[6-8]. Change in the microstructures by hydrogenation was mainly examined by investigating the radial distribution function (RDF) using X-ray diffraction (XRD) and neutron diffraction. Maeland[7] reported that an amorphous Cu-Ti alloy prepared by rapid-quenching kept its amorphous structure during hydrogenation. Aoki et al.[8] reported on thermodynamic data for hydrogen absorption of amorphous ZrNi alloys. They reported that the hydrogen storage capacity increase with increasing Zr content. They also studied hydrogenation of amorphous Hf-Ni and Ti-Ni alloys[9], and reported similar properties for Zr-Ni alloy hydrides. Because amorphous alloys do not have specific hydrogen sites, hydrogen site energies vary in a wide range according to a Gaussian distribution. Although hydrogen concentration in amorphous al
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