The correlation of C14/C15 phase abundance and electrochemical properties in the AB2 alloys

The C14/C15 phase abundance is an important parameter in the design of a suitable AB(2)-based metal hydride electrode for use in nickel metal hydride batteries. In order to separate the contribution due to chemical composition from that of phase abundance, three multi-phase and multi-element AB(2) alloys with different C14/C15 phase abundances before and after annealing were studied by scanning electron microscope, transmission electron microscope, X-ray diffraction, pressure-concentration isotherm, half-cell, and full-cell measurements. After annealing at 800 degrees C for 14 h, the C14 phase abundances increased from 71 to 94% for the first alloy (Ti12Zr21.5V10Ni40.2Co1.5Cr8.5Mn5.6Al0.4Sn0.3), remained unchanged at about 50% for the second alloy (Ti12Zr21.5V10Ni40.2Co5.0Cr5.5Mn5.1Al0.4Sn0.3), and decreased from 32 to 7% for the third alloy (Ti12Zr21.5V10Ni40.2Co8.0Cr3.5Mn4.1Al0.4Sn0.3). The effect of reduction in the amount of non-Laves ZrxNiy secondary phases by annealing was established with the second alloy while the contribution of additional major phase (either C14 or C15) by annealing can be distilled by comparing various properties between samples before and after annealing. After the comparison, C15 structure was found to have a higher hydrogen storage capacity and higher reversibility in gas phase hydrogen storage, and better high-rate dischargeability, hydrogen bulk diffusion, specific power, and low temperature performance with a shortcoming of an inferior cycle life in electrochemistry. Results from gas phase hydrogen storage measurement also agree with those from electrochemical testing. Besides, the non-Laves ZrxNiy secondary phases are found to play importance roles in the reversibility of hydrogen storage in the gas phase and battery performance in activation, rate capability, charge retention, and cycle life. (C) 2010 Elsevier B.V. All rights reserved.