Electrochemical hydrogen storage properties of La0.7Mg0.3Ni3.5–Ti0.17Zr0.08V0.35Cr0.1Ni0.3La0.7Mg0.3Ni3.5–Ti0.17Zr0.08V0.35Cr0.1Ni0.3 composites

For further improving the electrochemical properties of nonstoichiometric AB3AB3-type La0.7Mg0.3Ni3.5La0.7Mg0.3Ni3.5 alloy as negative electrode of Ni-MH battery, its related composites La0.7Mg0.3Ni3.5La0.7Mg0.3Ni3.5–xx wt% Ti0.17Zr0.08V0.35Cr0.1Ni0.3Ti0.17Zr0.08V0.35Cr0.1Ni0.3 (x=5x=5, 10, 25, 40) are prepared by ball milling method. It is found that all these composites are mainly composed of (LaMg)Ni3Ni3 and LaNi5LaNi5 phases according to XRD patterns. When x⩽25x⩽25, the V-based solid solution BCC phase and C14 Laves phase in Ti0.17Zr0.08V0.35Cr0.1Ni0.3Ti0.17Zr0.08V0.35Cr0.1Ni0.3 alloy are not detectable. As xx further increases to 40 wt%, however, these two phases appear in XRD pattern. Electrochemical studies show that the maximum discharge capacity of these electrodes decreases slightly from 348.5 mAh/g (x=5)(x=5) to 335.2 mAh/g (x=40)(x=40), while the cyclic capacity retention rate of composite electrodes is significantly improved from 29.7% (x=5x=5) to 63.0% (x=40)(x=40) after 70 charge–discharge cycles. This improvement of cyclic life can be ascribed to the enhanced anti-corrosion performance of composite electrode in alkaline solution. Moreover, the electrochemical kinetics properties, including the charge-transfer resistance RctRct, the polarization resistance RpRp, the exchange current density I0I0, the limiting current density ILIL and the hydrogen diffusion coefficient DD, of the composite electrodes were retarded first with xx increasing from 5 to 25 and then improved when xx reaches to 40.