Hydrogen Storage Properties And Electrochemical Properties Of Non-stoichiometric Tirple Alloy

The hydriding/dehydriding and charging/discharging performance of as-castnon-stoichiometric LaNi_3.8-xAl_x（0≤x≤0.5） alloy and triple La-Mg-Ni alloy were studied in thiswork. X-ray diffraction （XRD）, scanning electron microscopy （SEM） and energy dispersivespectroscopy （EDS） were used to characterize crystal the structure of hydrogen storage alloys.Hydrogen storage properties and electrochemical properties were characterized by P-C-T testsand electrochemical tests.The main results are shown as following:1. LaNi_3.8-xAl_x（0≤x≤0.5） alloy was prepared by arc melting under argon atomosphere.XRD result showed that the alloy consisted of Pr₅Co₁₉-type, Ce₅Co₁₉-type, LaNi₅-type andCe₂Ni₇-type phase.With increasing Al, the phase abundance of each phase varied, whichaffected the hysteresis and the hydriding/dehydriding plateau pressure ofLaNi_3.8-xAl_x（0≤x≤0.5） alloy. With increasing Al, the discharge capacity ofLaNi_3.8-xAl_x（0≤x≤0.5） alloy firstly increased, and then slightly decreased. The good cyclicstability was obtained at x=0.3. The polarization resistance and the electrochemicalimpedance slightly decreased with increasing x, whereas the exchange current density slowlyincreased. Correspondingly, the HRD increased.2. La-Mg-Ni alloy were prepared by induction melted method under argon atomosphere.The influence of elemental composition on the crystal structure, hydrogen storage andelectrochemical properties for Mg-containing alloys was investigated. As La/Mg ratiodecreases, the slight change of Ni content was detected. The hydrogen storage capacityreached¹.6wt%for La/Mg≥3:1and decreased to⁰.71wt%for La/Mg=1. Two hydrogenabsorption processes were observed because of the existence of the multiphases forLa/Mg≥3:1. With decreasing La/Mg ratio, the equilibrium pressure rose due to the shrinkageof the lattice parameter. The electrochemical result showed the discharge capacity of alloy（La/Mg≥3:1） was higher than alloy （La/Mg≤2）, but the cyclic stability was poor for La/Mg≥3:1due to the partial amorphisation. With the decline of La/Mg ratio, the polarizationresistance increased from121.5m to225.8m. Correspondingly, the exchange currentdensity reduced from207.6mA/g to111.8mA/g. The results and electrochemical impedance spectroscopy （EIS） results were consistent.3. The （La0.83Mg0.17）Ni3.9alloy were prepared by induction melted method, then it washeated for several days.It is demonstrated that Ni and La are almost evenly dispersed, but Mgis nonuniform distribution in these alloys by EDX. Rietveld analysis results illustrated that thealloys consisted of LaNi₅, Pr₅Co₁₉, Ce₂Ni₇and PuNi3-type phase. The mass fraction of eachphase in these alloys varied after annealed. The hydriding capacity of annealing alloy washigher than as-cast alloy. Estimating from pressure-composition isotheral curves, it wasconsidered that the hydriding is governed by two processes. The electrochemical resultshowed the alloy annealed3d showed the better electrochemical cyclic stability, after100cycles, the discharge capacity retention rate was up to83.7%. The impedance of the annealedalloy was significantly reduced, decreased from0.41to0.24. The trend of the polarizationresistance of the alloy first decreased and then increased, the exchange current density hadalso undergone a corresponding change. The results indicated that annealing treatment canimprove the electrochemical properties of the alloy.