Effect of annealing time on the structure and electrochemical properties of La_(0.72)Nd_(0.08)Mg_(0.2)Ni_(3.4)Al_(0.1) hydrogen storage alloys

摘要

The effect of annealing time on the structure and electrochemical properties of La_(0.72)Nd_(0.08)Mg_(0.2)Ni_(3.4)Al_(0.1) hydrogen storage alloys was investigated. The alloys were annealed at 1173 K for different time, and the results of Rietveld refinement for the XRD data showed that the alloys were composed of Gd_2Co_7-type, Ce_2Ni_7-type, Pr_5Co_(19)-type, PuNi_3-type, LaNi_5-type and MgCu_4Sn-type phases. Lengthening annealing time obviously promoted the formation of Gd_2Co_7-type phase which possessed high hydrogen storage capacity, and the largest content of Gd_2Co_7-type phase was obtained when the alloys were annealed for 20 h. The maximum discharge capacity increased from 365.0 (as-cast) to 389.6 mAh/g (annealed for 20 h), which was ascribed to Gd_2Co_7-type phase abundance increasing from 35% (as-cast) to a maximum (81.71%, annealed for 20 h). Also, the discharge capacity retention over 180 cycles at charge/discharge current density of 300 mA/g was remarkably heightened from 56.6% (as-cast) to 80.5% (annealed for 20 h). In addition, annealing treatment at 1173 K for 6 h can enhance high rate discharge ability.