Alloying Substitution Tuning The Phase Structure And Hydrogen Storage Characteristics Of YFe₂ Hydrogen Storage Alloy

Among many types of hydrogen storage alloys,YFe₂rare-earth hydrogen storage alloy with AB₂-type Laves phase structure is characterized by its high theoretical hydrogen storage capacity,abundant resources and low cost.However,hydrogen-induced amorphization and phase disproportionation would occur during de-/hydrogenation process in YFe₂ compound,which severely damages the reversibility.In this paper,a series of YFe₂ based compounds containing Sc are designed by the strategies of alloying and non-stoichiometric.All compounds were prepared by arc melting and high-temperature annealing;the phase structure and composition were characterized by XRD and SEM;hydrogen absorption kinetics and PCI plot were measured to evaluate hydrogen storage performance.The main results are as follows:Based on YFe₂ compound,Sc with a smaller atomic radius for Y is used to partially replace Y,and Y_1-xSc_xFe₂?x=0.1,0.2,0.3,0.4,0.5?compounds with a single C15-type Laves phase were prepared.By alloying with Sc,the fully reversible de-/hydrogenation process has been achieved for the Y_1-xSc_xFe₂ compounds,which could be attributed to the decrease in the R_A/R_Bratio and in the degree of expansion and contraction of interatomic distances in C15 phase as well as the increase in bulk modulus of compounds.In addition,the hydrogen absorption capacity is effectively improved,and Y_0.5Sc_0.5Fe₂compound has the highest hydrogen storage capacity,which is about 1.94 wt.%.The dehydrogenation thermodynamic properties have also been significantly improved.The hydrogen desorption capacity,dehydriding plateau pressure and dehydriding reaction enthalpy of Y_0.5Sc_0.5Fe₂compound is about 1.50 wt.%,0.192 atm,and60.31 k J/mol,respectively.Furthermore,the binding energy of Y-Sc-Fe hydrides was calculated by first principle method and the improvement of dehydrogenation properties can be attributed to the reduced stability of the hydrides,caused by the reduction of the volume of tetrahedral interstices.Based on the Y_0.5Sc_0.5Fe₂ compound,non-stoichiometric Y_0.5Sc_0.5Fe_2-x?x=0.1,0.3,0.5?compounds were further designed to investigate the influence of B-site deficiency on the hydrogen storage properties of Y-Sc-Fe compounds.Y_0.5Sc_0.5Fe_1.9 compound,with decreasing a small amount of Fe,consists of a C15 type Laves single phase,and Y_0.5Sc_0.5Fe_2-x?x=0.3,0.5?compounds,with decreasing more amount of Fe,consists of a C15-type Laves phase???and the Y???.All the C15 phases can maintain the structural stability after de-/hydrogenation process.In addition,the?-phase content of Y_0.5Sc_0.5Fe_2-x?x=0.3,0.5?compounds would change after first de-/hydrogenation process.By reducing Fe content,the hydrogen storage properties of Y_0.5Sc_0.5Fe₂ compound have been significantly improved.As the degree of Fe deficiency in Y_0.5Sc_0.5Fe₂ compound increases,hydrogen absorption capacity is increased and dehydriding plateau pressure is elevated.For Y_0.5Sc_0.5Fe_1.5 compound is about 2.08 wt.%and 0.984 atm?200??,respectively.In order to study the influence of A-side substitution on the phase structure and hydrogen storage properties of Y-Sc-Fe compounds,based on the Y_0.5Sc_0.5Fe₂ compound,Y_0.5Sc_0.5Zr_0.5-xFe₂?x=0.1,0.25?and Y_0.5-xTi_xSc_0.5Fe₂?x=0.1,0.2?compounds were prepared.Both Y-Sc-Zr-Fe and Y-Ti-Sc-Fe compounds are a C15-type Laves single phase,and maintain structural stability during de-/hydrogenation process.The addition of Zr and Ti has an adverse effect on the hydrogen storage capacity,and the effect of adding Ti is greater.Compared with Y_0.5Sc_0.5Fe₂compound,the dehydrogenation thermodynamic properties of Y-Sc-Zr-Fe and Y-Sc-Zr-Fe compounds have been improved,and the effect of adding Ti is better.The dehydriding plateau pressure of Y_0.3Ti_0.2Sc_0.5Fe₂ compound is about 5.99 atm at 200?and can achieve a completely reversible de-/hydrogenation process at 110?.Though Zr substituted by Ti,Y_0.7Zr_0.3-xTi_xFe₂?x=0.03,0.06?compounds were prepared and all the main phases are C15 phase.The addition of a small amount of Ti increases the hydrogen absorption capacity and elevate dehydrogenation plateau pressure.However,by adding excessive Ti,a segregated phase that can not absorb hydrogen is formed,resulting in a deterioration in hydrogen storage performance.