The influence of boron content on the structural and electrochemical properties of the La_(15)Fe_(77)B_8-type hydrogen storage alloy

摘要

La_(15)Fe_(10-x)Ni_(70)Mn_5B_ x(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 La_(0.99)Mn_(0.32)Ni_(4.7) phase and an (Fe, Ni) phase and that the alloys containing boron consist of the La_(0.99)Mn_(0.32)Ni_(4.7), (Fe, Ni) and La_3Ni_(13)B_2 phases. As the x increase, the parameter a of the La_(0.99)Mn_(0.32)Ni_(4.7) and La_3Ni_(13)B_2 phases increases, while the c decreases. The relative abundance of the La_3Ni_(13)B_2 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)) whenx 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 increasingx 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).