| Magnesium-based hydrogen storage alloys are widely concerned and studied with their high storage capacity,abundant resources,light weight,low cost and so on.They are considered as one of the most promising candidate materials for large-scale hydrogen storage.However,the application of Mg-based hydrogen storage alloys in commercial applications is limited to a large extent due to the excessive stability of hydrogen ab/desorption thermodynamics and the slow kinetics.The composition,internal microstructure and surface catalysis of the Mg-based hydrogen storage alloy can be adjusted to further improve the hydrogen ab/desorption heat/kinetics of the alloy.The eutectic point Mg76.87Ni12.78Y10.35 alloy in the Mg-Ni-Y ternary system is taken as the research object,and the Mg-Ni-Y composite hydrogen storage materials are prepared by adjusting the internal microstructure of the alloy through melt-spun and surface catalysis with the help of low dimensional nanometer carbon materials supported catalyst in this paper.The research focuses on the effects of alloying,melt-spun process and ball milling catalyst on the phase composition,microstructure,activation behavior and hydrogen storage properties of the alloy.Combined with the microstructure evolution of nanocrystalline/amorphous alloy and the catalytic effect of Ni loaded carbon nanotubes,the hydrogen ab/desorption mechanism of Mg76.87Ni12.78Y10.35 alloy is revealed.The main research contents and conclusions are as follows:(1)In this work,the Mg-Ni-Y alloy with high hydrogen storage capacity and amorphous formation ability was selected.The as-cast alloy was melted in a resistance furnace by covering agent protection in this work.The as-cast alloy was composed of Mg2Ni,Mg,Mg15NiY and MgNi4Y phases.Stacking fault and 14H type LPSO structure were observed in Mg15NiY phase.After hydrogenation,the as-cast alloy consisted of Mg2NiH4,Mg2NiH0.3,MgH2,YH2,YH3 and MgNi4Y phases.Due to hydrogenation-induced decomposition,the LPSO structure was decomposed into Mg2NiH0.3 Mg2NiH4,MgH2.YH2 and YH3 during hydrogenation.Based on the activation curves of Mg76.87Ni12.78Y10.35 alloy and the kinetic curves at 573 K,the activation properties and hydrogen absorption kinetics of Mg76.87Ni12.78Y10.35 alloy were investigated.During the activation process,the first hydrogen uptake 3 wt.%within 1 min,and 3.69 wt.%within 20 min.When the activation was completed,under the condition of 573 K,and 3 MPa,the hydrogen absorption capacity up to 4.45 wt.%in the 60 min.The improvement of hydrogen absorption kinetics of the alloy came from two aspects:one was the fine ternary eutectic structure of the as-cast Mg76.87Ni12.78Y10.35 alloy with Mg2Ni,Mg and LPSO structure.The refined eutectic structure had a large number of grain boundaries,which can provide more diffusion paths and reduce the diffusion distance to promote hydrogen diffusion.The second was that the LPSO structure will decompose in situ to form nano-catalysts during the hydrogen absorption process,and these nano-catalytic phases were dispersed in the alloy matrix to improve the hydrogen absorption kinetics.(2)The high-density boundary in amorphous/nanocrystalline structure can promote the hydrogenation reaction and the rapid diffusion of hydrogen molecules.In this paper,the ternary eutectic Mg76.87Ni12.78Y10.35 ribbons amorphous and nanocrystalline coexisting structures were prepared by melt-spinning method.The ribbons showed obvious amorphous/nanocrystalline characteristics,and the grains were obviously refined.After hydrogenation,the ribbons were composed of MgH2,Mg2NiH4,Mg2NiH0.3,YH2 and YH3.By means of hydrogen absorption kinetics Pt curve,hydrogen ab/desorption thermodynamics PCT curve and DSC curve,it was found that the hydrogen absorption capacity of the ribbons at 523 K exceeded 80%of the total hydrogen absorption capacity within 1 min,and reached hydrogen absorption saturation within 10 min,and the hydrogen absorption capacity was about 3.96 wt.%.The improvement of hydrogen absorption kinetics can be attributed to the introduction of amorphous/nanocrystalline structures by melt-spun,in which there were a large number of high-energy disordered sites that can act as active reaction centers for hydrogen absorption.In addition,the Van't Hoff formula was used to calculate the formation enthalpy and entropy of the ribbons.The calculated values of enthalpy and entropy of formation of Mg2Ni hydride were-53.25 kJ/mol and 107.74 J/(K·mol),respectively.A large number of nanocrystalline/amorphous structures with a size less than 5 nm were formed during melt-spun process,which reduced the enthalpy of hydride formation.(3)The loading of nano-Ni catalyst on the surface of carbon nanotubes can improve the catalytic efficiency,and Ni@MWCNTs can reduce the agglomeration of nanomaterials.In this paper,Ni@MWCNTs was doped into Mg76.87Ni12.78Y10.35 melt-spun ribbons by high-energy ball milling.The balling samples were labeled as BM30h and BM30h+Ni@MWCNTs,respectively.The hydrogenated samples of both samples were composed of MgH2,Mg2NiH4,Mg2NiH0.3,YH2,YH3 and MgO.The dehydrogenation sample is composed of Mg,Mg2Ni,YH2 and MgO.The introduction of Ni@MWCNTs presents two states:one was loaded on the particle surface,and the other was partly embedded inside the particle and partly exposed outside the particle.The hydrogen absorption kinetics of the two samples were studied based on the Pt curve.The hydrogen absorption equilibrium of BM30h was reached within 5 min at 623 K,and the hydrogen absorption was about 3.86 wt.%.The hydrogen absorption capacity of BM30h+Ni@MWCNTs was 3.97 wt.%,and it had not reached saturation and still had an upward trend.The optimization of hydrogen absorption kinetics was that the presence of Ni can accelerate the dissociation of hydrogen atoms,and the MWCNTs embedded in the alloy can prevent the agglomeration of nanoparticles and increase the diffusion gap of hydrogen atoms.Both PCT curves showed obvious platform characteristics with small lag coefficients.The formation enthalpies of BM30h samples at low and high platform pressures were-72.96 kJ/mol and-51.02 kJ/mol,respectively.The formation enthalpies of BM30h+Ni@MWCNTs samples at low and high platform pressures were-60.59 kJ/mol and-41.22 kJ/mol,respectively.The decrease of formation enthalpy indicated that the addition of catalyst can effectively reduce the stability of hydride and improve the hydrogen desorption performance of the alloy. |
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