Effect of substituting Y with Mg on the microstructure and electrochemical performance of LaY2Ni9 hydrogen storage alloy

Abstract

LaY2Ni9 showed a much lower discharge capacity than theoretical electrochemical capacity due to the hydrogen induced amorphization (HIA). To improve its electrochemical performance, Mg was added into LaY2Ni9 alloy and the effects of Mg on microstructure and electrochemical performance of LaY2Ni9 alloy were investigated. LaY2-xMgxNi9 (x = 0, 0.25, 0.50, 0.75, and 1.00) hydrogen storage alloys were prepared by vacuum induction melting followed by annealing at 1173 K for 12 h. The X-ray diffraction (XRD) analysis and Rietveld refinement showed that Mg addition has significant effects on the phase component. A small amount of Mg addition (x ≤ 0.50) can lead to a transformation between C15 and C14 phases. Further increase of Mg content (x = 0.75) is responsible for the formation of Gd2Co7-type phase. Electrochemical measurements indicated that the maximum discharge capacity and high rate discharge ability (HRD) of the alloys can benefit from a suitable amount of Mg addition. The maximum discharge capacity of LaY2-xMgxNi9 increases from 267.3 mA h g−1 for LaY2Ni9 to 366.4 mA h g−1 for LaY1.5Mg0.5Ni9, which is due to the suppression effect of Mg on HIA of LaY2Ni9 phase. HRD1500 (70.82%) is the best when x = 0.75, which contributes to the existence of Gd2Co7-type phase by accelerating the hydrogen diffusion rate.