Study On Re-Mg-Ni Based A₂B₇Type Hydrogen Storage Alloys Modified By Element Substitution And Heat Treatment

Re-Mg-Ni-base hydrogen storage alloys were prepared by vacuum induction meltingand heat treatment technology. Inductive coupled plasma (ICP), X-Ray diffraction (XRD),Scanning electron microscope (SEM)-Energy dispersive spectrometer (EDS), Hydrogenabsorption/desorption performance test were utilized to characterize their phase structure andhydrogen storage properties. The effect of Sm substitution for Nd and heat treatment time ofRE-Mg-Ni-based hydrogen storage alloy on microstructure and the electrochemicalhydrogen storage performance were investigated.XRD analysis of La0.5Nd0.35-xSmxMg0.15Ni3.5(x=0~0.3) alloys shows that the phasecomposition of the alloys remains unchanged after partial substitution of Sm for Nd,all thealloys have multiphase structure including the major phases A2B7phase and A5B19phaseand residual phase AB5phase. The abundance of the A2B7phase and A5B19phase firstincrease and then decrease with the increasing of x value from0to0.30, however the AB5phase abundance shows the reverse trend. Moreover, it can be found that a-axis parameter,c-axis and unit cell volume of the major phases and the residual phase gradually decreasewith the increasing Sm content as a whole. The activation property of the alloy electrodesis improved by substituting Nd with Sm. The maximum discharge capacity of alloyelectrodes first increases from334.6mAh g-1(x=0) to346.5mAh g-1(x=0.20) and thendecreases to331.1mAh g-1(x=0.30). HRD2000first increases from41.4%(x=0) to63.8%(x=0.20), and then decreases to52.1%(x=0.30), which is determined by the exchangecurrent density and hydrogen diffusion coefficient. The cycling capacity retention rate atthe200th cycle is monotonically increases from65.6%(x=0) to69.2%(x=0.30), whichshould be ascribed to the improvement of corrosion resistance of alloy electrodes in thecharging/discharging cycle.XRD analysis of La0.5Nd0.15Sm0.20Mg0.15Ni3.5alloys show that all the alloys havemultiphase structure including the major phases which is A2B7phase and A5B19phase, and the residual phase which is AB5phase. Pr5Co19type phase disappears after annealingtreatment prolonged to9h. Moreover, the abundance of A2B7phase decrease with theincreasing of t value which is from6to12, but the A5B19phase and the AB5phaseabundance shows the reverse trend. The maximum discharge capacity of alloy electrodesincreases from326.6mAh g-1(t=6h) to368.4mAh g-1(t=12h) with the increasing of tvalue but the HRD2000decreases from63.8%(t=6h) to55.5%(t=12h), which isdetermined by the hydrogen diffusion coefficient.The cycling capacity retention rate at the200th cycle monotonically increases from67.0%(t=6h) to70.6%(t=12h), which shouldbe ascribed to the improvement of corrosion resistance of alloy electrodes in thecharging/discharging cycle.