Influence of Mn content on the structural and electrochemical properties of the La_0.7Mg_0.3Ni_4.25-xCo_0.75Mn _x hydrogen storage alloys

The effects of Mn content on the structure and the main electrochemical properties of La_0.7Mg_0.3Ni_4.25-x Co_0.75Mn_x (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys were investigated systematically. X-ray powder diffraction (XRD) analysis shows that all the alloys mainly consist of a (La,Mg)Ni₃ phase and a LaNi₅ phase. The lattice parameters and the unit cell volumes of the two phases increase with increasing x. P-C-T curves reveal that the plateau pressure decreases progressively and the hydrogen storage capacity increases first and then decreases with an increase in Mn content. Electrochemical studies showed that the maximum discharge capacity and the high rate dischargeability (HRD) of the alloy electrodes increased with increasing x. Electrochemical impedance spectroscopy (EIS), linear polarization, anodic polarization and potential-step measurements, showed that the exchange current density I₀, the limiting current density I_L and the hydrogen diffusion coefficient D all first increase and then decrease with increasing x from 0 to 0.5. Considering the global effect of Mn substitution for Ni on the overall performance of the La_0.7Mg_0.3Ni_4.25-xCo_0.75 Mn_x alloy electrodes, the optimum composition was found to be x=0.4.