Synthesis of La_2Ni_7 hydrogen storage alloy by the electro-deoxidation technique

摘要

La_2Ni_7 alloy was synthesized in the molten CaCl_2 electrolyte by the electro-deoxidation method at 850 ℃ and the electrochemical hydrogen storage characteristics of the synthesized alloy was observed. Sintering at 1200 ℃ for 3 h was determined as the optimum condition to have the electrode pellet with enough porosity and mechanical strength. The hydroscopic La_2O_3 disappeared and the non-hydroscopic La_2NiO_4 formed during the sintering. The X-ray diffraction peaks indicated that the sinter product La_2NiO_4 and NiO reduced to LaNi_5 and Ni, respectively, within 4 h electro-deoxidation process. The reduction kinetics of LaOCl was relatively slow. This sluggish reduction caused formation of the target La_2Ni_7 phase only after 6 h electro-deoxidation. The final La_2Ni_7 alloy structure with small amount of retained LaNi_5 was obtained after 10 h electro-deoxidation and this structure did not change up to 25 h. It was observed that the synthesized alloy had maximum discharge capacity of 207 mA h g~(-1) and the alloy kept more than 90% of its discharge capacity within 20 charge/discharge cycles. According to the gathered EIS data La_2Ni_7 alloy was estimated to have no considerable surface barrier layer after discharging to hinder the atomic hydrogen diffusion on its surface.