The influence of boron content on the structural and electrochemical properties of the La15Fe77B8-type hydrogen storage alloy

Abstract

La15Fe10−xNi70Mn5Bx (x = 0, 1, 1.5, 2, 3) hydrogen storage alloys are prepared using a vacuum induction-quenching furnace. A structure analysis shows that the boron-free alloy is composed of a La0.99Mn0.32Ni4.71 phase and an (Fe, Ni) phase and that the alloys containing boron consist of the La0.99Mn0.32Ni4.71, (Fe, Ni) and La3Ni13B2 phases. As the x increase, the parameter a of the La0.99Mn0.32Ni4.71 and La3Ni13B2 phases increases, while the c decreases. The relative abundance of the La3Ni13B2 phase and the gather degree of the (Fe, Ni) phase increase with increasing x. Electrochemical testing shows that the boron content has an unapparent effect on the alloy electrodes' activation property. The maximum discharge capacity of the alloy electrodes is stable (approximately 310 mAh g−1) when x increases from 0 to 2. The cycling capacity retention rate at the 50th cycle monotonically increases from 56.6% (x = 0) to 76.7% (x = 3). The high-rate dischargeability of the alloy electrodes first increases with increasing x from 0 to 1.5 and then decreases until x increases to 3. The HRD is controlled by the hydrogen diffusion rate in the bulk of the alloy (x = 0–1.5) and the charge-transfer reaction of hydrogen on the alloy electrode surface (x = 1.5–3).