Effect of Al Content on Structure and Electrochemical Characteristics of Re0.8Mg0.2(Ni0.85-xCo0.15Alx)3.5 Hydrogen Storage Alloys

Abstract:

Hydrogen storage alloy La0.6Pr0.2Mg0.2(Ni0.85-xCo0.15Alx)3.5(x=0, 0.01, 0.02, 0.03, and 0.04) were prepared by induced melting method followed by annealing treatment at 900 ℃. The effects of partial substitution Al for Ni on the structure, electrochemical properties of the alloys were investigated systematically. X-ray diffraction (XRD) results indicate that the alloys were composed of La2Ni7 and LaNi5 phase with the Ce2Ni7-type and CaCu5-type structure. The value of c/a increases with the increase of Al content. The electrochemical analysis shows that the increase of Al leads to some decrease in both the discharge capacity and the HRD (high-rate dischargeability), but leads to a significant improvement in cycling stability of alloys. The discharge capacity decreases from 394.6 mAh/g (x=0.01) to 380.6 mAh/g (x=0.04), and the HRD decreases from 61% (x=0.01) to 35% (x=0.04) as the electrodes were discharged at 1200 mA/g. However, the rate of attenuation decreases from –0.32 mAh/(g·cycle) (x=0.01) to –0.20 mAh/(g·cycle) (x=0.04). This research reveals higher discharge capacity was remained, and cycling stability was improved remarkably when x≤0.02

Key words:hydrogen storage alloy; La2Ni7 phase; LaNi5 phase; PCT; electrochemical properties