Study Of Preparation,Microstructure And Hydrogen Storage Properties Of TiFe-based Composites

TiFe based hydrogen storage alloys are considered to be the most promising materials for hydrogen storage due to their high storage capacity,abundant storage capacity,suitable room temperature decomposition pressure and low price.However,the harsh activation conditions（450℃、6.5 MPa H₂）of TiFe based alloys limit their practical application.The addition of rare earth elements to alloying can significantly improve the activation properties and hydrogen storage characteristics of TiFe based alloys.Therefore,the as-cast Ti_1.1-xZr_0.1M_xFe_0.6Ni_0.3Mn_0.2（M=La,Y;x=0-0.08）poly alloys were synthesized by vacuum induction melting in this paper.The effects of rare earth elements La and Y and their additions on the microstructure and hydrogen storage properties of TiFe based hydrogen storage alloys were investigated in detail.Then,the selected Ti_1.08Zr_0.1Y_0.02Fe_0.6Ni_0.3Mn_0.2alloy with excellent performance was subjected to short-time high-energy ball milling with different transition metal catalysts to maximize its activation performance and hydrogen storage performance.The effects of the modification methods such as alloying with rare earth elements addition,mechanical ball milling and transition metal（Ni,V,Pd）catalysis on the microstructure and gaseous hydrogen storage performance of TiFe based hydrogen storage alloy were systematically investigated using test methods such as XRD,SEM,TEM,and Sivert’s constant volume device（P-C-T）etc.The results of quantum mechanical derivation were used to compare the electronic density of states of Ni,V,and Pd（Pd>Ni>V）,and the mechanism of the improved hydrogen storage performance was also investigated in detail by combining the microstructure,hydrogen absorption/release kinetics and thermodynamic evolution of the alloy.The as-cast Ti_1.1-xZr_0.1La_xFe_0.6Ni_0.3Mn_0.2（x=0-0.08）alloy contains mainly Ni-rich TiFe main phase and Zr Mn₂second phase which is segregated on the TiFe phase.The addition of La alloy also produces La-rich phase which promotes grain refinement of the alloy.After hydrogen absorption,TiFe H,TiFe H₂and Zr Mn₂H_3.46phases were present in the alloy without La adding,while La H₂phase appeared in the alloy containing La.The addition of La significantly improves the activation performance of the alloy,which can be fully activated in only one pass at 150℃and 3 MPa hydrogen pressure.the presence of La H₂phase leads to pulverization and grain refinement of the alloy,which is the main reason for the enhanced activation performance.In addition,chemical reaction La+H₂→La H₃,La H₃→La H₂+H,can rapidly supplying H atoms to the TiFe matrix phase,resulting in the"hydrogen pumping effect",which significantly enhances the hydrogen absorption of the alloy.The La_0.06alloy exhibits the fastest hydrogen absorption saturation rate and the largest hydrogen desorption capacity of 96.04%and 1.182wt.%,respectively.The absolute values of the enthalpy and entropy changes during hydrogen absorption and desorption increased significantly with the increase of La addition in the alloy,which was caused by the high stability of the rare earth hydride La H₂.The phase composition of the as-cast Ti_1.1-xZr_0.1Y_xFe_0.6Ni_0.3Mn_0.2（x=0-0.08）alloy consisting of Ni-rich TiFe main phase,a small amount of Zr Mn₂phase biased on the TiFe phase in the alloy without Y adding.and Y-rich phase.After hydrogen absorption,there are TiFe H phase and Zr Mn₂H₃phase in the alloy without the addition of Y.The addition of Y alloy also produces Y-rich phase which promotes grain refinement of the alloy.The YH₂phase appears in the alloy with Y substitution,and the Y-added alloy can be fully activated at 150°C and 3 MPa hydrogen pressure in only 1 time.The alloy chalking and grain refinement caused by the presence s of YH₂phase are the main reasons for the enhanced activation performance.In addition,chemical reaction Y+H₂→YH₃,YH₃→YH₂+H,can rapidly supplying H atoms to TiFe matrix phase,resulting in the"hydrogen pumping effect",and add Y alloy still exhibits a fast hydrogen absorption rate even at-10°C.The addition of Y promotes the multi-phase nucleation of the alloy,which increases the phase boundary and provides more channels for the diffusion of hydrogen atoms and enhances the hydrogen absorption an desorption kinetics of the alloy.And the Y_0.02alloy exhibited the fastest hydrogen absorption saturation rate and the largest hydrogen release capacity of 92%and 0.837 wt.%,respectively.With the increase of Y addition,the absolute values of the enthalpy and entropy changes during hydrogen absorption and desorption increase significantly,which is caused by the high stability of rare earth hydride YH₂.The effect of ball milling time on the hydrogen storage properties of Ti_1.08Zr_0.1Y_0.02Fe_0.6Ni_0.3Mn_0.2+2 wt.%Ni alloy was investigated.The phase structure of the ball milling alloy includes TiFe main phase,a small amount of Zr Mn₂phase deviated on TiFe phase and Ni phase.and the diffraction peak gradually broadens with the extension of ball milling time,and the particle size of the composite alloy decreases and gradually transforms into nanocrystalline and amorphous.The ball milling alloy enhances the activation and hydrogen absorption kinetic properties of the alloy.The amorphization of the alloy leads to the decrease of the hydrogen absorption and discharge capacity.The absolute values of enthalpy and entropy changes during the hydrogen absorption and desorption process show a tendency to decrease and then increase significantly with the increase of ball milling time,which are caused by the grain refinement of the short-time ball milling alloy and the agglomeration of the long-time ball milling alloy powder,respectively.The composite alloy was prepared by adding transition metals V,Ni and Pd to Ti_1.08Zr_0.1Y_0.02Fe_0.6Ni_0.3Mn_0.2alloy through mechanical ball milling,and the effects of transition metals V,Ni and Pd on the hydrogen storage properties of Ti_1.08Zr_0.1Y_0.02Fe_0.6Ni_0.3Mn_0.2alloy were investigated in detail.It was shown that the composite Ni,V and Pd ball milling treatment resulted in the grain size reduction and grain refinement of the alloy.The composite Ni and Pd ball milled alloys showed better activation performance,and the saturation rate of hydrogen absorption was Pd>Ni>V.The hydrogen desorption rates of composite V,Ni and Pd ball milled alloys were 97.24%,98.81%and 94.45%at 90°C,respectively,and the hydrogen desorption rates of composite V and Ni ball milled alloys were faster,and the absolute values of enthalpy and entropy changes during hydrogen absorption and desorption were smaller for composite V alloys,indicating that V is more capable of reducing the stability of TiFe based alloy hydrides.The composite V ball milling alloy was prepared by mechanically ball milling Ti_1.08Zr_0.1Y_0.02Fe_0.6Ni_0.3Mn_0.2alloy by adding different contents of V.The effect of V addition on the hydrogen storage properties of Ti_1.08Zr_0.1Y_0.02Fe_0.6Ni_0.3Mn_0.2alloy was investigated.It was shown that the composite ball mill alloy with V addition of 7 wt.%exhibited good activation properties and hydrogen absorption and desorption kinetics.The composite ball mill alloy with7 wt.%V addition exhibited the fastest hydrogen absorption rate of 98.1%at 90°C and 100 s and the maximum hydrogen desorption capacity of 0.72 wt.%at 90°C and 1000 s.With the increase of V addition,the absolute values of enthalpy and entropy change of hydrogen absorption and desorption of the composite V ball mill alloy showed a gradual decrease,and the absolute value of enthalpy change of hydrogen absorption and desorption of the composite V ball mill alloy reached the minimum value when the V addition was 7 wt.%.In summary,it can be seen that multiple alloying,particle size reduction,grain refinement and transition metal cladding catalysis can not only improve the activation performance and hydrogen absorption and desorption kinetic performance of the alloy,but also reduce the stability of metal hydrides and the enthalpy change of hydrogen desorption of metal hydrides,thus improving the thermodynamic properties of the alloy.