Effect of annealing time on the structure and electrochemical properties of La0.72Nd0.08Mg0.2Ni3.4Al0.1 hydrogen storage alloys

The effect of annealing time on the structure and electrochemical properties of La0.72Nd0.08Mg0.2Ni3.4Al0.1 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 Gd2Co7-type, Ce2Ni7-type, Pr5Co19-type, PuNi3-type, LaNi5-type and MgCu4Sn-type phases. Lengthening annealing time obviously promoted the formation of Gd2Co7-type phase which possessed high hydrogen storage capacity, and the largest content of Gd2Co7-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 Gd2Co7-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.

► Lengthening annealing time increases discharge capacity and cyclic stability.
► Lengthening annealing time promotes the formation of Gd2Co7-type phase.
► The maximum discharge capacity reaches 389.6 mAh/g.
► The cyclic stability remains 80.5% over 180 cycles.