Effect Of Element Substitution On Hydrogen Storage Performance And Long-term Service Behavior Of Rare Earth-based AB₅ Alloys

Due to the excessive consumption of non-renewable fossil energy,the energy crisis and environmental pollution problems have become increasingly severe.Hydrogen energy can be used as an emerging sustainable clean energy to replace traditional fossil energy due to its advantages such as pollution-free,high energy efficiency,and abundant reserves.AB5-type hydrogen storage alloys have the advantages of easy activation,good kinetics performance,moderate plateau pressure,and are widely used in many fields such as fuel cells,hydrogen storage tanks,and hydrogen compressors.However,the AB5-type hydrogen storage alloys are easy to pulverize and break during repeated hydrogen absorption/desorption,and their cycle stability is poor.In addition,the AB5 alloys are prone to plateau splitting,which severely restricts the hydrogen utilization efficiency.Element substitution is considered to be an effective method to improve the overall hydrogen storage performance of AB5 alloys.In this paper,in order to improve the cycle stability of the AB5-type hydrogen storage alloys and increase the hydrogen storage capacity,Co and Mn are used to partially replace the Ni on the B side of the LaNi5 alloy to prepare LaNi5-xCox(x=0,0.25,0.5,0.75,1)and LaNi4.75Mn0.25 alloys.In order to suppress the plateau splitting of AB5 alloys,improve hydrogen utilization efficiency,and increase cycle life,Y and Ce are used to partially replace La on the A side of LaNi4Co alloy to prepare La1-yYyNi4Co(y=0,0.1,0.2)and La1-zCezNi4Co(z=0,0.4,0.5)alloys.The composition and microstructure of the alloys are studied by ICP,SEM,EDS,XRD and other characterization methods.In order to fully study the effects of the addition of Co,Mn,Y,and Ce elements on the intrinsic hydrogen storage performance and long-term service behavior of AB5 alloys,this paper conducts 1000 cycles hydrogen absorption/desorption performance test on the prepared alloys.The results are as follows:With the increase of Co content,the cell volume of LaNi5-xCox series alloys increases,the anisotropy parameter and plateau pressure decrease,the hydrogen absorption rate increases,and the hydrogen storage capacity and cycle stability first increase and then decrease.Compared with the original LaNi5 alloy,the maximum hydrogen storage capacity of the LaNi4.5Co0.5 alloy is increased from 1.536 wt.%(LaNi5)to 1.556 wt.%(LaNi4.5Co0.5).Due to the partial mixing of ? and ? phases when the LaNi4.5Co0.5 alloy absorbs/desorbs hydrogen,its cycle stability is also increased from 82.5%(LaNi5)to 86.7%(LaNi4.5Co0.5).By adding Mn with a larger atomic radius,the lattice parameters and anisotropy parameters of LaNi4.75Mn0.25 alloy are higher than those of LaNi5 and LaNi4.5Co0.5 alloys.Due to the larger cell volume,the alloy has excellent kinetics performance.With cycling,the anisotropy parameter increase,and the microstrains,grain destructions,and particle powdering are relieved.Compared with LaNi5 alloy,due to the smaller microstrains with cycling,the cycle stability is improved from 82.5%(LaNi5)to 90.3%(LaNi4.75Mn0.25).Regarding the La1-yYyNi4Co series alloys,as the Y content increases,the cell volume decreases,the anisotropy parameter increases,the plateau pressure increases,and the hydrogen storage capacity decreases.The addition of Y to the AB5 alloys helps to relieve the plateau splitting,extend the length of the first plateau,and improve the cycle stability.The effective utilization rate of hydrogen of the alloys increase from 57.0%(LaNi4Co)to 77.4%(La0.8Y0.2Ni4Co),and the cycle stability increased from 80.5%(LaNi4Co)to 88.1%(La0.8Y0.2Ni4Co).For the La1-zCezNi4Co series alloys,with the increase of Ce content,the cell volume decreases,the anisotropy parameter increases,the plateau pressure increases,but the hydrogen storage capacity first decreases and then increases.Compared with Y,Ce suppresses the AB5 alloy plateau splitting and improves the cycle stability more significantly.Among them,the second plateau of the La0.5Ce0.5Ni4Co alloy has completely disappeared,the effective utilization rate of hydrogen is as high as 95.5%,and the cycle stability is increased to 96%.In addition,the lower price of Ce reduces the production cost of the alloys.