Study On The A₂B₇Type Rare Earth Magnesium Based Hydrogen Storage Alloys Without Pr And Nd

Based on the researches of La-Mg-Ni based hydrogen storage alloys, hydrogen storage alloys were selected in this work. The effects of Sm and Y substitution for La on phase structure and electrochemical and gas/solid hydrogen storage properties of the as-cast alloy and the influence of the heat treatment were studied by XRD, SEM, PCT and electrochemical measurements.PCT tests showed that cell volume and hydrogen storage capacity reduced and hydrogen storage platform pressure increased after the substitution of Sm and Y for La,there are two hydrogen storage platforms. For the as-cast alloys, the low platform area is mainly corresponding to PuNi3phase, Ce2Ni7phase and Ce5Co19phase. The high plateau area is mostly corresponding to CaCu5phase.The substition of Sm or Y and their interaction can reduce the abundance of CaCu5phase. The optimizing alloy composition is La0.4Gd0.2Smo.1Y0.15Mg0.15Ni3.35Al0.15, La0.4Gd0.2Sm0.15Y0.1Mg0.15Ni3.35Al0.15,La0.4Gd0.2-Sm0.2Y0.1Mg0.15Ni3.35Al0.15and La0.45Gd0.2Sm0.1Y0.1Mg0.15Ni3.35Al0.15.On the basis of the results above, the La0.35Gd0.2Sm0.2Y0.1Mg0.15Ni3.35Al0.15alloy was selected for heat treatment. The results showed that the CaCus phase decreased, Ce2Ni7phase and Ce5Co19phase increased gradually with the increase of heat treatment temperature. When the heat treatment temperature was lower than1223K, the main phase of the alloy is PuNi3type. When heat treatment temperature was1223K, the main phase transformed from PuNi3type to Ce2Ni7type. After heat treatment, the hydrogen storage capacity increased, hydrogen storage platform pressure decreased and the cyclic stability enhanced. Through the heat treatment at1273K for8h, the discharge capacity is346mAhg-1and the discharge capacity retention rate S90reached to93.01%.The effects of annealing time on the phase structure and properties of La0.35Gd0.2Sm0.2Y0.1Mg0.15Ni3.35Al0.15alloy under1273K were investigated. The SEM results showed that the alloy approximated to the single phase structure after heat treatment. Extending the time of heat treatment had small effects on the phase structure. The cycle stability enhanced, but hydrogen storage performance, discharge properties and were almost the same. Considering the comprehensive performance of the alloy,the suitable heat treatment condition is1273k X8h. Furthermore, The effects of heat treatment on the La0.45Gdo.2Sm0.1Y0.1Mg0.15-Ni3.35Al0.15、La0.4Gd0.2Sm0.15Y0.1Mg0.15Ni3.35Al0.15and La0.4Gd0.2Sm0.1Y0.15Mg0.15Ni3.35-Alo.15alloys were investigated systematically. The results showed that the alloy phase transformed from PuNistype to Ce2Ni7type at1223K. When the temperature was up to1273K, the PuNi3phase in the alloy further decreased and the abundance of Ce2Ni7phase became highest. When the heat treatment temperature was1293K, The Pr5Co19phase appeared, CaCu5phase increased and Ce2Ni7phase reduced. PCT test results showed that the annealed alloy had the largest amount of hydrogen storage, the lowest platform slope and the platform pressure when the temperature was1273K. According to the results above, the suitable condition of heat treatment is1273k×8h.After a comprehensive comparison of the hydrogen storage properties and electrochemical properties of four kinds of alloy, we found that the hydrogen storage capacity and discharge capacity of all the annealed alloys were close to each other. The La0.45Gd0.2Sm0.2Y0.1Mg0.15Ni3.35Al0.15alloy had the lowest pressure platform and the La0.45Gd0.2Sm0.1Y0.15Mg0.15Ni3.35Al0.15alloy had minimum hydrogen storage platform slope, whose capacity retention rates after100charge-discharge cycles were92.88%and93.96%.