Electrochemical studies on La_0.7Mg_0.3Ni _3.4-xCo_0.6Mn_x metal hydride electrode alloys

The effect of Mn substitution for Ni on the structural and electrochemical properties of the La_0.7Mg_0.3Ni_3.4-xCo _0.6Mn_x (x=0.0, 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys has been studied systematically. The results by X-ray powder diffraction show that the alloys are all consisted of the (La, Mg)Ni₃ phase and the LaNi₅ phase, and the lattice parameters and cell volumes of the component phases all increase with increasing x. P-C isotherms curves reveal that the hydrogen storage capacity increases to a maximum and then decreases with increasing x, but the equilibrium hydrogen pressure decreases continuously with increasing x. The electrochemical studies show that the maximum discharge capacity increases first when x increases from 0.0 to 0.4 and then decreases when x reaches 0.5. The high-rate dischargeability of the alloy electrodes are all rather high increasing from 72.9 (x=0.0) to 83.8% (x=0.3) and then decreasing to 78.9% (x=0.5) at the high discharge current density I _d=1000mAg^-1. Meanwhile, the results of the electrochemical impedance spectroscopy, linear polarization, Tafel polarization, and hydrogen diffusion coefficient measurements all indicate that the exchange current density I₀, the limiting current density I_L and the hydrogen diffusion coefficient D of the alloy electrodes also all increase first and then decrease with increasing Mn content in alloys, which signifies that the electrochemical kinetics property of the La_0.7Mg _0.3Ni_3.4-xCo_0.6Mn_x (x=0.0, 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys has a most optimum value of Mn content. This phenomena is mainly attributed to the good electrocatalytic activity of Ni element at the surface and the Mn substitution for Ni leads to the increase of Ni content at the surface of alloy electrodes when the amount of Mn substitution is appropriate.