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

• 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.

A series of Ti12Zr21.5V10Cr7.5Mn8.1Co8.0Ni32.2−xSn0.3Al0.4Znx, 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.

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