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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