Study On Properties Of Y-Fe Based Rare Earth Hydrogen Storage Alloys

In this paper, taking YFe₂ and YFe₃ alloy as the research object, the structure and hydrogen absorption capacity were studied with the substitution of A, B side elements of the alloys which were tested by XRD, PCT, SEM/EPMA etc. The purpose was to develop low cost, high performance non nickel rare earth based new materials for hydrogen storage.Firstly, the structure and hydrogen storage properties of YFe₂ alloy were investigated. The as cast alloy had a multiphase structure, in addition to the main phase of YFe₂, and there were still a certain amount of YFe₃ and oxide of Y. The first principle calculation showed that the free energy of YFe₂ and YFe₃ was very close, Non-uniform alloy composition and local energy fluctuation were very easy to lead to the transformation of the two phases. Heat treatment improved the content of YFe₂ phase and the initial hydrogen absorption capacity of the alloy reached 1.998 wt.%, but YFe₂ phase had a disproportionation and caused the hydrogen absorption capacity decay after several times hydrogen absorption and desorption.Through the substitution of Ce for Y, the kinetics performance and cycling stability of hydrogen absorption and desorption of Y_1-xCexFe₂ alloys（x=0,0.15,0.25） were improved. Meanwhile, the YFe₃ phase content and the stable hydrogen absorption capacity of the alloys increased with the increase of Ce substitution. The alloy had the highest hydrogen absorption capacity and the best cycle stability when x=0.25. In addition, the alloy had great hydrogen absorption kinetics and the lowest capacity decrease after seven hydrogen absorption and desorption cycles. But when x=0.5, the content of YFe₃ phase and the cycling stability of the alloy decreased.Moreover, the structure and hydrogen storage properties of YFe₃ alloy were investigated. The as cast alloy also had a multiphase structure, in addition to the main phase of YFe₃, and there was a amount of Y6Fe₂₃ phase. After adding excessive Y（2 wt.%） and annealing for 72 hours under 1100 degrees Celsius, the content of YFe₃ phase of the alloy increased to 75.8 wt.% and the content of YFe₂ phase decreased. What’s more, the cycling stability of the alloy was improved and the stable hydrogen absorption capacity reached to 1.433 wt.%. But the hydrogen absorption plateau of the alloy was very low. In order to improve the hydrogen absorption plateau characteristics, B-side elements were substituted for Fe of YFe₃ alloy. The results showed that the hydrogen absorption capacity of the YFe₃-xMx（M=Mn, Al） alloys decreased with the increase of substitution and the hydrogen absorption plateau was more inclined.The La, Ce and other elements were used to substitute the Y to explore the possibility of improving the characteristics of the platform. It was found that La could not replace the Y element in YFe₃ alloy, and La was existed in the form of oxide. With the increase of La, the hydrogen absorption capacity and the dynamic performance of the alloys decreased gradually. Moreover, the hydrogen absorption plateau of the alloy was more inclined. The moderate substitution of Ce for Y in Y_1-xCexFe₃ alloys（x=0, 0.15, 0.25 and 0.5） could effectively improve the hydrogen absorption plateau pressure of the alloys. When the substitution were x =0.15 and 0.25 for Ce, the alloy got a well platform characteristics. However, with the increase of Ce substitution in the alloy, the content of YFe₂ phase increased gradually. What’s more, the hydrogen absorption and desorption cycle stability decreased. Meanwhile, the substitution of Mg for Y element（x≦0.15） did not improve the hydrogen absorption plateau characteristics of Y_0.85-xMgxCe_0.15Fe₃ alloys.Compared all the studied alloys, we found that the YFe₃ phase was more stable than that of YFe₂ phase. And the YFe₃ alloy had a well hydrogen storage capacity. Moderate substitution of Ce for Y in Y_1-xCexFe₃ alloys could effectively improve the hydrogen absorption plateau pressure of the alloys. Hence, the substitution of Ce for Y should be less than 0.25.