Phase structure and electrochemical properties of La0.67Mg0.33Ni3.0 − xCox (x = 0.0, 0.25, 0.5, 0.75) hydrogen storage alloys

La0.67Mg0.33Ni3.0 − xCox (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 La0.67Mg0.33Ni3.0 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)Ni3 phase with the PuNi3-type structure and the (La,Mg)2Ni7 phase with the Ce2Ni7-type structure. The lattice parameters a, c   and the unit-cell volumes vv 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 La0.67Mg0.33Ni3.0 − xCox alloy electrodes increase with the increase of Co content. Moreover, the cycle stabilities of La0.67Mg0.33Ni3.0 − xCox is not improved by small quantity replacement Ni by Co except for x = 0.75.