Effect of annealing on the discharge characteristics of a Zr-V-Mn-Ni hydrogen storage alloy

Abstract

In order to improve the discharge performance of the ZrV0.7Mn0.5Ni1.2 alloy electrode, the alloy was annealed at 1000 degrees C for 12 h. It is shown that the total hydrogen storage capacity is unchanged,but the plateau pressure is increased in the lower hydrogen concentration range of the pressure composition isotherm curve after annealing. The annealed alloy has a higher rate capability and a higher discharge capacity than the as-cast one. In order to analyze the above phenomena, the reaction surface area and surface reaction kinetics were examined by Brunauer, Emmett, and Teller method (BET) and electrical impedance spectroscopy (EIS) analyses, respectively. The measurement of the fully activated electrode surface area by BET analysis shows that there is little difference between as-cast and annealed alloy. The higher rate capability is due to the decrease of charge-transfer resistance for the hydrogenation reaction in the KOH electrolyte after annealing. It is well. known that the Ni component plays a role in improving the electrocatalytic activity of the alloy electrode. The microstructures of the as-cast and annealed alloys were investigated by scanning electron microscope and energy-dispersive spectroscopy analyses. The second phase (Zr-Ni phase) is widely distributed in as-cast alloy, but this second phase disappears after annealing, resulting in the homogenizing of the alloy and an increased Ni content in the matrix. The annealed alloy has a higher exchange current density than any single phase of as-cast alloy. Consequently, it is concluded that the improved rate capability of the annealed alloy is due to increased Ni content in the matrix resulting from the disappearance of the secondary phase.