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Tetsuya Okuyama Graduate School, Shibaura Institute of Technology, Toyosu, Koto-ku, Tokyo 135-8548, Japan

Janusz S. Szmyd Faculty of Energy and Fuels, AGH–University of Science and Technology, Krakow 30-059, Poland (Received 19 January 2010; accepted 31 March 2010)

Mechanical alloying of Ti45Zr38–xNi17þx and Ti45–xZr38Ni17þx (0  x  8) elemental powders produced an amorphous phase, but subsequent annealing converted the amorphous phase into an icosahedral quasicrystal phase, along with a Ti2Ni-type phase. The discharge capacities, measured in a three-electrode cell at room temperature for both the amorphous and quasicrystal electrodes, increased with increasing Ni substitution for Zr or Ti. The highest discharge capacities, which were about 60 mAh/g for the amorphous electrode and 100 mAh/g for the quasicrystal electrode, were obtained from (Ti45Zr30Ni25) after substitution of Ni for Zr. For the Ti45Zr30Ni25 composition, the discharge performance of the quasicrystal electrode was stable over charge/discharge cycling, but that of the amorphous electrode gradually decreased with cycling. The structure of the quasicrystal phase in the electrodes was stable, even after 15 charge/discharge cycles, but the amorphous phase converted to a (Ti, Zr)H2 f.c.c. hydride.

I. INTRODUCTION

LaNi5 and MmNi5 intermetallic compounds, and their modified forms after the substitution of other metals for La (Mm) or Ni, or after the addition of a third or fourth element, have been studied by numerous researchers. This is because LaNi5 is currently used as an anodic material for nickel/metal hydride (Ni/MH) secondary batteries, which do not contain the toxic cadmium present in alkaline batteries. The H/M (number of hydrogen atoms per metal atom) ratio of LaNi5 is 1.0 (the hydride form of LaNi5 is LaNi5H6), which corresponds to a gravimetric hydrogen capacity of 1.4 wt%. Furthermore, the theoretical charge capacity of LaNi5 estimated from its chemical composition is 348 mAh/g. The actual discharge capacity was about 330 mAh/g, which is almost identical to the theoretical capacity, indicating that most of the hydrogen absorbed in the compound can easily be removed electrochemically. Ti-Zr-Ni icosahedral (i) quasicrystal phases, which have a new type of translational long-range order and display noncrystallographic rotational symmetry, are believed to possess a large number of tetrahedral interstitial sites in the cluster,1 which makes the i-phase alloys a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0202 J. Mater. Res., Vol. 25, No. 8, Aug 2010

attractive candidate materials for hydrogen storage. One of the authors previously reported that Ti45Zr38Ni17 quasicrystal powders could be synthesized by mechanical alloying (MA) and subsequent annealing, and that the hydrogen capacity of the i-phase powder reached about 2.8 wt%,2 which is superior to that of LaNi5. We have recently reported electrochemical hydrogenation/ dehydrogenation properties of Ti45Zr38Ni17 amorphous and i-phase e