Influence Of Element Substitution On Phase Structure And Hydrogen Storage Properties Of Non-stoichiometric Yttrium-Nickel-based Alloys

The actual hydrogen storage capacity of rare earth AB2 type hydrogen storage alloy is lower than the theoretical hydrogen storage capacity.The alloy is prone to hydrogen-induced amorphization and hydrogen-induced disproportionation during the hydrogen absorption and desorption process,and there is a large capacity attenuation.In addition,due to the large atomic radius of rare earth elements,different numbers of vacancies are formed on A-side to relieve the stress.The rare earth-based AB2 alloys form A1-xB2 phases to reduce the stress and stabilize structure.When using Y with a smaller atomic radius(1.801 A)as the A-side element and Ni as the B-side element,the alloy forms Y0.95Ni2 phase.In this paper,based on YNi2.1(Y0.95Ni2)alloy,the effects of stoichiometric ratio and substitution of Mg and Co elements on the phase structure and hydrogen storage properties of the alloy were systematically studied.The phase structure and hydrogen storage properties of YNiy(y=1.9,2.1,2.3,2.5)alloys with different stoichiometric ratios were investigated.With the increase of y from 1.9 to 2.5,YNi phase disappears,Y0.95Ni2 phase abundance decreases,YNi3 phase appears,and the abundance increases gradually.When x=2.5,the abundance of YNi3 phase reaches 92.85 wt%.The hydrogen storage performance shows that the increase of stoichiometric ratio can significantly improve the cyclic stability and stable hydrogen absorption capacity of the alloy.The capacity of YNi2.5 alloy does not decay after cycling.The maximum stable hydrogen absorption capacity of the alloy is 1.72 wt%.From the structural changes after hydrogen absorption and desorption,the capacity attenuation of the alloy is mainly due to the hydrogen induced disproportionation of Y0.95Ni2 phase.The YNi3 phase structure does not change after hydrogen absorption and desorption.The YNi3 phase can stabilize the structure of the Y0.95Ni2 phase and suppress the capacity attenuation.For Y1-xMgxNi2.1(x=0,0.1,0.2,0.3,0.4,0.5)alloys,with the increase of Mg substitution,the abundance of(Y,Mg)Ni2 phase increase,and the abundance of(Y,Mg)Ni3 phase increase first and then decrease.The content of Mg in the(Y,Mg)Ni2 phase is higher than that in the(Y,Mg)Ni3 phase.With the increase of Mg substitution,the structural stability of the alloy is gradually improved,and the capacity attenuation rate gradually decreases.When x ?0.1,the capacity attenuation is mainly due to the hydrogen induced disproportionation of Y0.95Ni2 phase.When x?0.2,the capacity attenuation comes from the(Y,Mg)Ni2 phase with less Mg content.When x=0.5,the capacity does not decay.With the increase of Mg substitution,the Mg content in(Y,Mg)Ni2 phase increases,and the increment of Mg content in(Y,Mg)Ni3 phase is small.In the PCT curves,the equilibrium pressure of the low platform corresponding to(Y,Mg)Ni2 increases gradually.The equilibrium pressure of the high platform corresponding to(Y,Mg)Ni3 does not change significantly.For Y0.7Mg0.3Ni2.1-xCox(x=0,1.05,2.1)alloys,with the increase of Co substitution amount,the content of Mg in(Y,Mg)(Ni,Co)3 phase decreases,and the content in(Y,Mg)(Ni,Co)2 phase increases.When the substitution amount of Co increases from 0 to 2.1,the capacity attenuation rate in the first four cycles of the alloy decreases from 2.84%to 0%,and the stable hydrogen absorption increased from 1.37 wt%to 1.79 wt%.For Y1-xMgxNiy(x=0,0.05,0.1,0.2,0.3,0.4,0.5;y=1.9,2.1,2.3,2.5)alloys with different stoichiometric ratios,with the increase of Mg substitution,Y0.95Ni2 phase in the alloys disappears.When the Mg substitution is less than or equal to 0.2,the alloys preferentially generate(Y,Mg)Ni3 phase,and then the phase abundance gradually decreases with the increase of Mg substitution.For Y-Mg-Ni2.5 alloys,the abundance of(Y,Mg)Ni3 phase decreases with the increase of Mg substitution.For Y-Mg-Ni2.1-2.5 alloys,when x>0.4,the(Y,Mg)Ni3.5 phase appears.All alloys contained(Y,Mg)Ni2 phase,and the phase gradually increases with the increase of Mg substitution.Mg preferentially enters the phase with low stoichiometric ratio,and the minimum Mg content required for the formation of(Y,Mg)Ni2 phase is 0.20.With the increase of Mg content,Mg content in(Y,Mg)Ni2 phase saturates or no longer increases at 0.50,and Mg content in high stoichiometric phase increases.With the increase of Mg content in(Y,Mg)Ni2 phase,the structural stability of the phase gradually increases.When the alloy only contains(Y,Mg)Ni2 phase,the higher Mg content required for the phase without hydrogen-induced disproportionation is 0.50.When the alloy contained(Y,Mg)Ni3 phase and the higher the abundance of the phase,the less Mg content is needed to stabilize(Y,Mg)Ni2 phase.(Y,Mg)Ni3 phase can inhibit the hydrogen induced disproportionation and hydrogen induced amorphization of(Y,Mg)Ni2 phase.With the increase of Mg content in(Y,Mg)Ni2 phase,the equilibrium pressure corresponding to the low plateau of the(Y,Mg)Ni2 phase is significantly increased.The equilibrium pressure corresponding to the high plateau of(Y,Mg)Ni3 phase has little change due to the small change of Mg content.The reversibility of the PCT curve is improved with the increase of the Mg content in the(Y,Mg)Ni2 phase and the(Y,Mg)Ni3 phase abundance.