Study of hydrogen storage and electrochemical properties of AB₂-type Ti_0.15Zr_0.85La_0.03Ni_1.2Mn_0.7V_0.12Fe_0.12 alloy

A C15 AB₂ Laves-type Ti_0.15Zr_0.85La_0.03Ni_1.2Mn_0.7V_0.12Fe_0.12 alloy was prepared by arc melting and annealing. Phase-structural composition, microstructure, hydrogen absorption-desorption properties, thermodynamic and electrochemical performances were characterized by X-ray diffraction, scanning electron microscopy, hydrogen absorption-desorption measurements and electrochemical characterization and were related to the use of the alloys as metal hydride battery anodes. The alloy contains a C15 FCC intermetallic compound as the main phase and a LaNi secondary phase as the minor constituent (∼1 wt%). During the electrochemical tests, the anode electrodes quickly, after just a few activation cycles, reached a maximum discharge capacity. This was related to the catalytic effect of the La-rich secondary phase which acted as a catalyst of hydrogen absorption-desorption. Annealing resulted in increase of the maximum discharge capacity from 345 mAh/g for the as cast alloy to 370 mAh/g. Furthermore, the annealed alloy showed a better high rate dischargeability and a higher cyclic stability. After 100 cycles with 100% DOD at discharge current density of 1C, the discharge capacity of the annealed alloy was very high, at a level of 90% of the initial capacity. The rates of hydrogen diffusion have been characterized by Potentiostatic Intermittent Titration Technique and Electrochemical Impedance Spectroscopy. With increasing an extent of transformation into the hydride, the H diffusion rate in the bulk of the alloy particles decreased. The maximum value of D_H measured by PITT for the annealed alloy was observed for the nearly fully discharged electrode, (SOC 2%).