Studies of copper as a modifier in C14-predominant AB2 metal hydride alloys

Abstract

The structure, gaseous storage, and electrochemical properties of Cu-modified C14-predominant AB2 metal hydride alloys with compositions of Ti12Zr21.5V10Cr7.5Mn8.1Co8.0−xNi32.2CuxSn0.3Al0.4, x=0 to 5 were studied. As the Cu-content in the alloy increases, the lattice constant a increases (except at the highest Cu-content); both the lattice constants a, c, and the volume of the C14 unit cell increase; a/c ratio decreases; and the secondary phase shifts from a combination of C15 and TiNi to Zr7Ni10. In the gaseous phase reaction (analyzed by the pressure–concentration–temperature isotherm), the plateau pressure decreases; both the slope factor and hysteresis increase; and both the maximum and reversible hydrogen capacities decrease with the addition of Cu. Calculations of the heat of formation and the change in entropy indicate incompleteness of an ordered metal hydride structure. While the secondary phase plays a more important role in the gaseous reaction, the main phase determines the electrochemical properties. With the addition of Cu, both the high-rate and full discharge capacities increase; however, the ratio between the two capacities decreases due to lower bulk diffusibility of hydrogen, and activation becomes easier due to a higher pulverization rate. A good tradeoff between surface area and charge-transfer resistance, 1at.% Cu-addition is recommended for the low-temperature application of C14-predominant AB2 alloys.