Phase structure and electrochemical properties of La 0.67Mg 0.33Ni 3.0 − xCo x ( x = 0.0, 0.25, 0.5, 0.75) hydrogen storage alloys

Abstract

La 0.67Mg 0.33Ni 3.0 − x Co x ( x = 0.0, 0.25, 0.50, 0.75) hydrogen storage alloys were prepared by induction melting. Influences of partial substitution of Co for Ni on phase structure and electrochemical properties of La 0.67Mg 0.33Ni 3.0 were investigated by means of X-ray diffraction (XRD), electron probe X-ray microanalysis (EPMA) and electrochemical measurements. XRD patterns and back scattered electron images show that the alloys were composed of the (La,Mg)Ni 3 phase with the PuNi 3-type structure and the (La,Mg) 2Ni 7 phase with the Ce 2Ni 7-type structure. The lattice parameters a, c and the unit-cell volumes v vary with the increase of Co content x. The electrochemical measurements show that partial Co substitution for Ni had no influence on the initial activation rate of the alloys. The maximum electrochemical discharge capacity increases firstly then decreases, the high-rate dischargeabilities (HRDs) of La 0.67Mg 0.33Ni 3.0 − x Co x alloy electrodes increase with the increase of Co content. Moreover, the cycle stabilities of La 0.67Mg 0.33Ni 3.0 − x Co x is not improved by small quantity replacement Ni by Co except for x = 0.75.