Structure-activity Relationship Of Zr-based Laves Phase Hydrogen Storage Alloys And Poisoning Resistance Against Gaseous Impurities

Zr-based Laves phase alloys with high storage capacity and security,relatively excellent cyclic stability and ultra-low plateau pressure,have been considered as suitable getter materials to be used in the field of storage and separation of hydrogen and its isotopes.However,the hydrogen cannot be released easily from these high capacity hydrides due to the limitations of thermodynamics,kinetics or both of them.The shortage in hydrogen absorption kinetics,the serious hysteresis between absorption and desorption and the high cost of vanadium and high pyrophoricity of ZrV2 also make it difficult to meet the requirements of applications.In this dissertation,To improve the hydrogen storage properties and overcome these obstacles,Z-V based Laves phase alloys were prepared by alloying element substitution?Al?Ni??high energy ball milling?HEBM?and the addition of multi-walled carbon nanotube?MWCNTs?.The phase compositions,microstructures,lattice parameter,surface states,hydrogen absorption/desorption kinetics and thermodynamics of Zr-based alloys have been investigated.In addtion,the poisoning effect of gaseous impurity O2 on the hydrogen absorption properties of Zr-based Laves phase alloys was also investigated.The main research contents and conclusions are as follows:The as-cast Zr0.9Ti0.1?V1-xAlx?2 alloys preserve multiphase structures including C15 Laves main phase with cubic structure,V-based solid solution phase and?-Zr phase.The minority phases of as-cast alloys eliminate during the annealing treatment.The P-t curves indicate that the maximum hydrogen absorption capacity decreases with increasing Al substitution,which is believed mainly due to the existence of ZrAl2.In addition,the fully activated Zr0.9Ti0.1?V1-xAlx?2 alloys can absorb maximum capacity within 25 s under⁵0 kPa hydrogen pressure and perform rapid hydrogenation kinetics.Meanwhile,the absorption and desorption curves are almost coincidence in the?-phase region.No significant hysteresis between absorption and desorption can be observed for each alloy at 823 K.The Zr(V_1-xNix)₂?x=0.02,0.05,0.1,0.15,0.25?intermetallic compounds were prepared by the arc-melt method and annealed 1273 K for 168 h.The solidification segregation of as-cast Zr?V1-xNix?2 alloys almost disappear under the homogenizing treatment at 1273 K for 168 h.With increasing Ni content in the alloys,the hydrogen absorption capacities decrease and equilibrium pressure increases.Meanwhile,the standard enthalpy of formation?H and standard entropy of formation?S for the alloys'hydrides have been obtained from Van't Hoff's equation in the??+??two-phase region of Zr?V1-xNix?2 alloys.It is found that the Zr?V1-xNix?2alloys with higher Ni content preserve higher absolute values of?H and more stable metal hydride roughly.After 95 hydrogen absorption/desorption cycles,the capacity of Zr?V0.05Ni0.95?2 alloy was 1.508 H.A^-1.Zr?V0.05Ni0.95?2 still preserves 94.1%of the first hydrogen storage capacity at 823K and shows excellent cyclic stability.The homogeneously annealed Zr?V0.95Ni0.05?2 alloys have been mechanically milled with and without multi-walled carbon nanotube?MWCNTs?by high energy ball milling?HEBM?under the protection of Ar atmosphere.The extending of the milling time contributes to the formation of amorphous Zr-based alloy.The sharp diffraction peaks in crystalline Zr?V0.95Ni0.05?2 alloy become broader continuously and the intensities of Bragg peaks gradually reduce with the extension of milling time,which indicates the crystallite size reduction and lattice strains accumulation.The crystalline Zr?V0.95Ni0.05?2 almost transforms to amorphous after 60 min of HEBM.Meanwhile,the unit cell volume of ZrV2 phase increases obviously with the increase of milling time.After HEBM,the hydrogen storage capacities of Zr?V0.95Ni0.05?2 alloy with and without MWCNTs both decrease due to the amorphization,which decreases the number of tetrahedral interstices for hydrogen storage.In addition,the optimum milling time for production of the smallest sized Zr?V0.95Ni0.05?2 particle is found to be 5 min and the powders prepared under this condition show faster hydrogen absorption kinetics than those milled for longer time.The samples ball milled with MWCNTs preserve faster hydrogen absorption kinetics compared to powders without MWCNTs.The results show that the as-milled Zr?V0.95Ni0.05?2 alloys with different concentration MWCNTs preserve a ZrV2 phase with C15 type structure and the lattice parameter of ZrV2 phase increases with increase of the MWCNTs concentration.During the hydrogenation process,it takes 35,10,14,10 seconds to reach the 90%of their maximum hydrogen storage capacity at 773K for the MWCNTs concentration is 1,2,5 and 10 wt.%,respectively.It is believed the addition of MWCNTs increase the hydrogenation kinetics in the way of the remarkably increased diffusion rate of H-atoms originating from MWCNTs with hollow structure.In addition,the activation property of Zr?V0.95Ni0.05?2 alloy is not affected significantly by the addition of MWCNTs.The slope of plateau decreases and the width of plateau increases with increasing MWCNTs concentration,which accounts for the increase of the reversible hydrogen storage capacity.In addition,the equilibrium pressure of Zr?V0.95Ni0.05?2 increases with the increase of MWCNTs concentration.The absolute value of enthalpy and entropy decrease as the MWCNTs concentration increase from 1 to 10 wt.%,indicating that the addition of MWCNTs dramatically decreases the stability of Zr?V0.95Ni0.05?2.When H2 containing 1 vol%O2 is employed as the hydrogen source,the Zr?V0.95Ni0.05?2 alloy can also be activated,whereas,more activation time and higher temperature are needed due to the continually forming passivation layer caused by the interaction between oxygen and alloy surface.The hydrogen storage capacity of the alloy is found to degrade evidently with increasing of the O2 concentration and the alloy almost loses the entire hydrogen absorption capacity(0.07 H.A^-1)as O2concentration reaches 2 vol%.In the process of hydrogenation in the gaseous impurities,the passivation layers formed on alloy surface containing oxides and hydrocarbons inevitably block the dissociation of hydrogen molecules and hydrogen atoms diffusion to the inside of the alloy.During the cycling stability tests in O2gaseous impurity,the maximum hydrogen capacity in gaseous mixtures decreases with various degree comparing with that in pure hydrogen.In addition,the hydrogen desorption capacity decreases gradually in each absorption with the increase of absorption/desorption cycles and the alloy can hardly absorb hydrogen at the third cycle as the temperature below 423 K.With increasing temperature,the hydrogen storage capacity in the first cycle even shows a little rise at 573 K comparing with that at 308 K,indicating a strong temperature dependence of the hydrogen storage degradation for experimental alloys.The hydrogenation performance degradation of Zr?V0.95Ni0.05?2 introduced by oxygen gaseous impurities can be regenerated with the value is 4.23 H.A^-1 by the out-gassing method in vacuum at 823 K for 1 h,slightly lower than that hydrogenated in pure hydrogen(4.34 H.A^-1).The hydrogenation storage performance of Zr?V0.95Ni0.05?2 after pre-exposed in O2 has been investigated.It is found that introduced pure oxygen can be rapidly absorbed by the fully activated Zr?V0.95Ni0.05?2 alloy within 40 s.All of the four poisoned samples contain dominant ZrV2 Laves phase and minor Zr3V3O oxides,with the further increase of oxygen pressure in specimen chamber,Bragg peaks of Zr2V,V-rich BCC solid solution phase and ZrO2 phase including baddeleyite with monoclinic structure??-ZrO₂?and small amount of tazheranite with cubic structure??-ZrO2?are also observed in addition to the dominant ZrV2 and minor Zr3V3O phases.The unit cell volume of cubic C15-ZrV2 is found to be increased by 0.86 vol.%with the increase of O2 pressure from 1.0 to 4.0 kPa,which indicates more O2 has been dissolved into the lattice of ZrV2.Meanwhile,all of the oxygen poisoned Zr?V0.95Ni0.05?2 alloys can still absorb hydrogen and the capacity decreases from 3.39to 2.22 H.A^-1 with the increase of O2 pressure from 1.0 to 4.0 kPa,respectively,comparing with the maximum hydrogen storage capacity(4.34 H.A^-1)of a fully activated fresh Zr?V0.95Ni0.05?2 alloy.The existence of oxygen is found to decrease both capacity and the kinetics.The metallic Ni precipitates after pre-exposure process and distributes on the alloy surface can provide an active surface for the dissociation of hydrogen molecule.