Effect of La partial substitution for Zr on the Structural and electrochemical properties of Ti0.17Zr0.08-xLaxV0.35Cr0.1Ni0.3 (x = 0–0.04) electrode alloys

In order to elucidate the effects of metallic La addition on the performance of Ti–V-based hydrogen storage alloys as negative electrodes for nickel/metal-hydrides batteries, Ti0.17Zr0.08-xLaxV0.35Cr0.1Ni0.3 (x = 0, 0.01, 0.02, 0.03, 0.04) alloys were prepared and their structural and electrochemical properties were systematically investigated. X-ray powder diffraction (XRD) results showed that these alloys were mainly consisted of C14 Laves phase with a hexagonal structure, V-based solid solution phase with BCC structure and C15 Laves phase with a cubic structure. The electrochemical measurements indicated that the maximum discharge capacities of the alloy electrodes decreased from 337.3 mAh/g (x = 0) to 262.5 mAh/g (x = 0.04) and that the substitution of Zr with metallic La in the alloys had no obvious effect on the capacity retention rate (C100/Cmax, C200/Cmax). The high-rate dischargeability (HRD) of the alloy electrodes at the discharge current density of 800 mA/g first increased from 69.01% (x = 0) to 71.13% (x = 0.01) and then decreased to 65.35% (x = 0.04). In brief, the HRD was improved with an optimum La content in the alloy (x = 0.01). The electrochemical hydrogen kinetics of the alloy electrodes was further studied by means of electrochemical impedance spectroscopy, linear polarization, anodic polarization and potential-step measurements. The charge-transfer reaction resistance Rct decreased for x = 0.01 with respect to x = 0 and then increased with the increase of x, while exchange current density I0, limiting current density IL and hydrogen diffusion coefficient D were all increased for x = 0.01 with respect to x = 0 and then decreased with the increase of x. The optimal content of La in Ti0.17Zr0.08-xLaxV0.35Cr0.1Ni0.3 alloys for negative electrodes in alkaline rechargeable secondary batteries is x = 0.01 in this study.