Effects of Zn-addition to C14 metal hydride alloys and comparisons to Si, Fe, Cu, Y, and Mo-additives

A series of Ti₁₂Zr_21.5V₁₀Cr_7.5Mn_8.1Co_8.0Ni_32.2−xSn_0.3Al_0.4Zn_x, x = 0, 1, 2, and 3 alloys made by an arc melting process was studied. As the Zn-content increases, the TiNi phase abundance increases, which reduces the Ni-content in the main C14 phase causing an isotropic expansion of the unit cell. Both the equilibrium pressure and heat of hydride formation are reduced via a synergetic effect between the main phase and secondary phases, which causes small increases in both gaseous phase maximum H-storage capacity and full discharge capacity measured electrochemically. The C15 phase abundance shows a peak value for improvement in activation and high-rate performance in the alloy containing 1 at% Zn and a passivated surface is formed to hinder the activation and high-rate performance. Despite of the passivated surface at room temperature, the −40 °C charge-transfer resistance of a 3 at% Zn-addition was much improved by increase of both surface area and surface catalytic ability. In a sealed cell, the addition of 1 at% Zn in the AB₂ metal hydride alloy improves the −10 °C capacity and cycle stability at the expense of power reduction. The effects of Zn-substitution were also compared with those obtained from Si, Fe, Cu, Y, and Mo. - Highlights: • Partial replacement of Ni by Zn in AB2 metal hydride alloy was studied. • With increase in Zn, TiNi phase increases, unit cell expands, and ΔH decreases. • C15 phase is crucial for the high-rate performance and activation behavior. • 1 at% of Zn can improve the storage capacity, activation, and cycle stability. • High Zn-content (>1 at%) passivates the surface and impedes high-rate capability.