Structural and Electrochemical Properties of the La_0.7Mg _0.3Ni_2.975-xCo_0.525Mn_x Hydrogen Storage Electrode Alloys

The effect of partial substitution of Mn for Ni on the structural and electrochemical properties of the La_0.7Mg_0.3Ni _2.975-xCo_0.525Mn_x (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys has been investigated systematically. The results of X-ray powder diffraction and Rietveld analyses showed that all alloys consisted of the (La, Mg)Ni₃ phase and the LaNi₅ phase, and the content of the (La, Mg)Ni₃ phase first remained unchanged (~77 wt%) and then decreased, but the content of the LaNi₅ phase increased progressively with increasing x. Meanwhile, the lattice parameters and cell volumes of the (La, Mg)Ni₃ phase and the LaNi₅ phase all increased with increasing Mn content. The pressure composition isotherms showed that the hydrogen storage capacity first remained almost unchanged and then decreased with increasing x from 0.0 to 0.5, and the equilibrium pressure decreased from 0.51 atm to 0.06 atm. The electrochemical measurements indicated that the maximum discharge capacity first remains unchanged (~400 mAh/g) with increasing x from 0.0 to 0.2 and then decreased when x increased further. Moreover, the high rate discharge-ability, the exchange current density I₀, the limiting current density I _L, and the hydrogen diffusion coefficient D of the alloy electrodes all increased first and then decreased with increasing x, which indicates that the kinetics of hydriding/dehydriding of the La_0.7Mg _0.3Ni_2.975-xCo_0.525Mn_x (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys increased first up to x = 0.1 and then decreased with further increasing x.