Effect of the partial substitution of samarium for lanthanum on the structure and electrochemical properties of La15Fe77B8-type hydrogen storage alloys

Abstract

La15Fe77B8 hydrogen storage alloys were prepared using a vacuum induction-quenching furnace. The results of X-ray diffraction (XRD) and scanning electron microscopy (SEM) suggested that La15−xSmxFe2Ni76Mn5B2 (x=0, 2, 4, 6) alloys had multiphase structure including the main LaNi5 phase, La3Ni13B2 phase and (Fe, Ni) phase. With the increasing substitution of Sm for La, the main phase structure of alloys did not change, while the unit cell volumes decreased, the cycle stability was improved and the maximum discharge capacity decreased, but the low temperature maximum discharge capacity of the same substitution alloy was gradually approaching the maximum discharge capacity at room temperature, which showed that La15Fe77B8 hydrogen storage alloys of the partial substitution of Sm for La had better low-temperature dischargeability (LTD). For the same substitution alloys, self-discharge characteristics and cycle stability at low temperature were better than that at room temperature. Furthermore, the high-rate dischargeability (HRD) and the exchange current density I0 first increased and then decreased with the increasing of Sm content, whereas the hydrogen diffusion coefficient D in alloy bulk decreased gradually, which indicated that appropriate substitution of Sm for La improved the electrochemical kinetics properties of the alloys. The HRD was mainly dominated by the charge-transfer rate on the alloy surface.