Studies of Ti1.5Zr5.5V0.5(MxNi1−x)9.5 (M = Cr, Mn, Fe, Co, Cu, Al): Part 2. Hydrogen storage and electrochemical properties

In Part 2 of this two part series of papers, the gas phase and electrochemical properties of the quinary alloy Ti1.5Zr5.5V0.5(MxNi1−x)9.5 with M = Cr, Mn, Fe, Co, Cu, or Al and x = 0.1 or 0.2 are studied and compared to the structural properties reported in Part 1. The electrochemical properties of the alloys are strongly related to the non-Laves phases. For example, the Zr7Ni10 phase provides a more catalytic surface and improves the high rate discharge capability of the alloy at the expense of discharge capacity due to the higher Ni to Zr ratio; the Zr9Ni11 phase, on the contrary, improves the discharge capacity but hinders the high rate discharge capability. In the Laves phases, C14 provides a stable hydride with lower equilibrium pressure, higher gas phase storage capacity and lower electrochemical rate capability. The C15 phase provides a less stable hydride with higher equilibrium pressure, lower gas phase storage capacity and higher electrochemical storage capacity. The main effect of each modifying element can be summarized as follows: Mn is the most beneficial substitution element found in this study. Mn increases both the gas phase and electrochemical capacities with good reversibility; it also improves the activation, hydrogen diffusion, surface reaction, and high rate discharge capability. Co increases the degree of disorder in the alloy, reduces both gas phase and electrochemical capacities, destabilizes the hydride, improves activation, and reduces surface reactivity. Fe decreases the gas phase capacity but improves the electrochemical discharge capacity. Fe facilities activation, but its surface reaction is the least active among the substituents used in this study. Al, Cr, and Cu are less desirable because they reduce: (1) the electrochemical and gas phase capacities, (2) the reversibility, and (3) the rate capability. They also slow activation.