Effects Of B And Mo On Electrochemical And Kinetic Properties Of AB₅-Type Hydrogen Storage Alloys

In this paper, the hydrogen storage alloys MmNi_3.70-xMn_0.35Co_0.60Al_0.25B_x (x = 0.00, 0.10, 0.15, 0.20, 0.25) and La_0.35Ce_0.65Ni_3.54Co_0.80-xMn_0.35Al_0.32Mo_x(x = 0.00, 0.10, 0.15, 0.20, 0.25, 0.30, 0.40) were prepared by inductive melting method and arc-melting method, respectively. The phase structure of the prepared hydrogen storage alloys was characterized by XRD. The electrochemical properties of alloy electrodes were measured by galvanostatic charge-discharge methods and the kinetic performances were tested using the methods of linear polarization, anodic polarization, potentialstatic step discharge and cyclic voltammogram.XRD analyses indicate that the alloy (x = 0) has a single LaNi5 phase with CaCu₅-type structure and B-adding alloys (x > 0) consist of the main LaNi5 phase and a small amount of second phase with CeCo4B-type structure, and the amount of the secondary phase increases with the increase of B content in the MmNi_3.70-xMn_0.35Co_0.60Al_0.25B_x(x = 0.00, 0.10, 0.15, 0.20, 0.25) storage hydrogen alloys system; For La_0.35Ce_0.65Ni_3.54Co_0.80-xMn_0.35Al_0.32Mo_x (x = 0.00, 0.10, 0.15, 0.20, 0.25, 0.30, 0.40) storage hydrogen alloys, after partial substitution of Mo for Co, the alloys remain a single LaNi5 phase with a CaCu₅-type structure。In the MmNi_3.70-xMn_0.35Co_0.60Al_0.25B_x (x = 0.00, 0.10, 0.15, 0.20, 0.25) storage hydrogen alloys, the activation properties of alloy electrodes are improved because of the formation of the second phase; with the increase in B content, the HRD first increases and then decreases, and the alloy with x = 0.20 has the highest HRD value; As x increases from 0.00 to 0.25, the exchange current density I0 first increases from 284.17 mA/g (x = 0.00) to 327.07 mA/g (x = 0.20), and then decreases to 293.04 mA/g (x = 0.25), and the hydrogen diffusion coefficient D value first increases from 0.93×10^-10 cm²/s (x = 0.00) to 1.58×10^-10 cm²/s (x = 0.20), and then decreases to 1.12×10^-10 cm²/s (x = 0.25). The alloy with x = 0.20 has better electrochemical kinetic properties.For the La_0.35Ce_0.65Ni_3.54Co_0.80-xMn_0.35Al_0.32Mo_x (x = 0.00, 0.10, 0.15, 0.20, 0.25, 0.30, 0.40) storage hydrogen alloys, the substitution of Mo for Co makes the activation of alloy electrodes easier and is also beneficial for the HRD of alloy electrodes (the optimum content is x = 0.25). The HRD at 1200 mA/g discharge current density increases from 38.3% (x = 0.00) to 59.9% (x = 0.25), and then decreases to 40.2% (x = 0.40). As the Mo content increases, the exchange current density I0 of alloy electrodes gradually increases, the hydrogen diffusion coefficient D increases firstly and then decreases, reaching the maximum at x = 0.25. The alloy with x = 0.25 exhibits better electrochemical and kinetic properties.