A study on the microstructures and electrochemical properties of hydrogen storage electrode alloys

In this work, the microstructures and electrochemical properties of the La 0.7 Mg 0.3 Ni 2.45 - x Cr x Co 0.75 Mn 0.1 Al 0.2 ( 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) Ni 3 phase with PuNi 3 -type structure and an LaNi 5 phase with CaCu 5 -type structure. The abundance of the (La,Mg) Ni 3 phase in the alloys decreased from 69.6% to 42.9%, accordingly that of the LaNi 5 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 ( 1.85 A ˚ ) than that of the Ni ( 1.62 A ˚ ). Electrochemical studies showed that the cycling stability ( C 100 / C max ) 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 ( C max ) 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 .