Electrochemical and metallurgical characterization of ZrCr_1-xNiMo_x AB₂ metal hydride alloys

The effects of partial replacement of chromium by molybdenum was studied on the structure and electrochemical kinetic properties of ZrCr_1-xNiMo_x(x = 0.0, 0.3 and 0.6) metal hydride alloys. The arc-melting prepared alloys were metallurgically characterized by X-ray diffraction and energy dispersive spectroscopy microanalysis, which showed AB₂ (with hexagonal C14 structure) and Zr_xNi_y (Zr₇Ni₁₀, Zr₉Ni₁₁) phases. After a partial substitution of chromium by molybdenum, secondary phases monotonically increase with the C14 unit cell volume indicating that most of molybdenum atoms locate in the B-site. The alloys were electrochemically characterized using charge/discharge cycling, electrochemical impedance spectroscopy and rate capability experiments that allowed the determination of hydriding reaction kinetic parameters. The presence of molybdenum produces a positive effect for hydrogen diffusion in the alloy lattice, and ZrCr_0.7NiMo_0.3 alloy depicts the better kinetics associated with a fast activation, lower charge transfer resistance and the best high rate discharge behavior. This fact would be related to a lower diffusion time constant and a bigger value of the product between exchange density current and surface active area. There is a trade-off in the amounts of secondary phase and Laves phases in order to improve the kinetic performance. - Highlights: • Metallurgical characterization evidences the presence of Zr_xNi_y and C14 phases. • The partial replacement of Cr by Mo promotes the segregation of Zr_xNi_y phase. • The incorporation of molybdenum improves the kinetics for the hydriding process. • Mo produces a decrease in the diffusion time constant