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-xCrxCo0.75Mn0.1Al0.2 (x=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)Ni3 phase with PuNi3-type structure and an LaNi5 phase with CaCu5-type structure. The abundance of the (La,Mg)Ni3 phase in the alloys decreased from 69.6% to 42.9%, accordingly that of the LaNi5 phase increased with increasing x value. With the increase of x, 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/Cmax) of the alloy electrodes firstly increased from 66.2% (x=0.00) to 70.6% (x=0.10), and then decreased to 62.8% (x=0.20) with the increase of x value. However, the maximum discharge capacity (Cmax) of these alloy electrodes decreased from 369.7 (x=0.00) to 311.5 mA h/g (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 x.