| As environmental problems become increasingly serious and fossil fuels become depleted,the search for alternative clean energy sources is urgent.Hydrogen,as a renewable and clean energy source with high calorific value,high efficiency and zero pollution,is considered to be one of the perfect alternatives to fossil fuels.As a typical representative of hydrogen storage alloys,rare earth based AB5 type alloys have been widely studied for their advantages of moderate platform pressure,easy activation and fast hydrogen absorption/desorption.Despite these advantages,the long-term cycling performance of AB5 alloys still needs to be improved for wider applications;Among the many hydrogen storage materials,the body-centred cubic(BCC)solid solution alloys had been widely investigated for their high hydrogen storage density.However,due to the low desorption pressure of its monohydride phase,the overall hydrogen storage performance of BCC alloys,especially the reversible hydrogen storage performance,is relatively poor under ambient conditions.Therefore,improving its hydrogen desorption reversibility would significantly facilitate the practical application of this alloy system.The elemental modifications are considered to be an effective way to improve the hydrogen storage properties of hydrogen storage alloys.In this paper,the LaNi5-xSnx(x=0,0.25,0.5,0.75,1)alloys were prepared by substituting large atomic radius Sn elements and the V35Ti30Cr25Fe10,V35Ti30Cr25Mn10,V30Ti30Cr25Fe10Nb5 and V35Ti30Cr25Fe5Mn5 and V0.3Ti0.3Cr0.25Mn0.1Nb0.05 high entropy alloys.The structure of the alloys was studied by means of structural characterization such as XRD,SEM and EXAFS.The effect of element substitution on the structure and properties of the superlattice alloys was analyzed in conjunction with the hydrogen storage properties of the alloys.The results of the study are as follows:It is found that the segregation effect caused by Sn addition leads to the multi-CaCu5 phase structure with different cell parameters.The interaction between different CaCu5 phases plays a buffer role in the hydrogen absorption/esorption,which stabilizes the crystal structure and improves the cycle life.The capacity retention after 1000 cycles increases from 83.2%(x=0)to 95.8%(x=0.75).Moreover,the addition of Sn significantly reduces the hysteresis of the alloys from 0.212(x=0)to 0.023(x=0.5)at 383 K,owing to the reduction of the microstrain during hydrogen absorption/desorption.It is found that among the four alloys,the V35Ti30Cr25Mn10 alloy shows the highest hydrogen absorption capacity while the V35Ti30Cr25Fe5Mn5 alloy exhibits the highest reversible capacity.The cause of the loss of desorption capacity is mainly due to the high stability of the hydrides.The higher room-temperature desorption capacity of the V35Ti30Cr25Fe5Mn5 alloy is due to higher hydrogen desorption pressure.After pumping at 400 oC,the hydrides can return to the original BCC structure with only a small expansion in the cell volume.It is found that for the V0.3Ti0.3Cr0.25Mn0.1Nb0.05 high entropy alloy,the first hydrogenation is possible at room temperature without incubation time and reaches a maximum hydrogen storage capacity of 3.45 wt%.The pressure composition isotherm(P-CI)at 298 K shows a reversible hydrogen desorption capacity of 1.78 wt%and a desorption plateau pressure of 80 kPa.The capacity loss is mainly due to the stable hydride with the desorption enthalpy of 31.1 kJ/mol and entropy of 101.4 J/K/mol.The hydrogen absorption capacity decreases with cycling due to incomplete desorption at room temperature.The hydrogen absorption kinetics increases with cycling and the rate-limiting step is diffusioncontrolled for hydrogen absorption. |
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