A study on the microstructures and electrochemical properties of La0.7Mg0.3Ni2.45-xCrxCo0.75Mn0.1Al0.2(x=0.00–0.20) hydrogen storage electrode alloys

In this work, the microstructures and electrochemical properties of the La0.7Mg0.3Ni2.45-xCrxCo0.75Mn0.1Al0.2La0.7Mg0.3Ni2.45-xCrxCo0.75Mn0.1Al0.2 (x=0.00x=0.00–0.20) hydrogen storage alloys were investigated systematically for the purpose of improving the cycling stability of this type alloy. X-ray powder diffraction (XRD) analysis showed that all of the alloys mainly consisted of an (La,Mg)Ni3Ni3 phase with PuNi3PuNi3-type structure and an LaNi5LaNi5 phase with CaCu5CaCu5-type structure. The abundance of the (La,Mg)Ni3Ni3 phase in the alloys decreased from 69.6% to 42.9%, accordingly that of the LaNi5LaNi5 phase increased with increasing xx value. With the increase of xx, the lattice parameters and the unit cell volumes of the two phases increased gradually due to the larger atom radius of Cr (1.85A˚) than that of the Ni (1.62A˚). Electrochemical studies showed that the cycling stability (C100/CmaxC100/Cmax) of the alloy electrodes firstly increased from 66.2% (x=0.00)(x=0.00) to 70.6% (x=0.10)(x=0.10), and then decreased to 62.8% (x=0.20)(x=0.20) with the increase of xx value. However, the maximum discharge capacity (Cmax)(Cmax) of these alloy electrodes decreased from 369.7 (x=0.00)(x=0.00) to 311.5 mA h/g (x=0.20)(x=0.20), and the high rate dischargeability (HRD) also showed a decreasing tendency with increasing Cr content. Further, the electrochemical impedance spectra, the linear polarization, the anodic polarization and the potential-step measurements revealed that the decrease of the HRD of this type alloy electrodes can be ascribed to the decrease of both the charge-transfer rate on the surface of the alloy electrodes and diffusion rate of the H atom in the bulk of the alloys with increasing xx.