Effects Of Ball Milling And Different Types Of Additives On The Structure And Hydrogen Storage Properties Of La₅Mg_95-xNi_x?x= 5,10,15? Alloys

Mg-based alloys have become one of the most attractive hydrogen storage materials due to their abundant raw material reserves,low cost and large hydrogen storage capacity.However,the kinetic performance of hydrogen absorption and desorption reactions of such materials is poor,which greatly affects their hydrogen storage performance and application.To improve the performance of magnesium-based hydrogen storage materials,La5Mg95-xNix(x = 5,10,15)alloys were selected as the research object.The alloy was prepared by vacuum induction melting method,and the structure and hydrogen storage properties of the alloys was systematically studied.On this basis,the effects of nanocrystallization of ball milling and different types of additives on the microstructure and hydrogen storage properties of La5Mg85Ni10 alloy were studied.The mechanism of hydrogen storage performance was also studied from kinetics and thermodynamics performance.The as-cast La5Mg95-xNix(x= 5,10,15)alloys were all based on La2Mg17 as the main phase,with Mg2Ni,La2Ni3,and Mg comprising the second phase,whereas LaMg2 was only found in x = 5 alloy.The amount of Mg2Ni increased as the amount of Mg decreased with increasing Ni content.The alloys after hydriding were all composed of MgH2,Mg2NiH4,and the rare earth hydride LaH3,whereas those after dehydriding were all composed of Mg2Ni,Mg,and the rare earth hydride LaH3.We can infer that the reactions that occurred during hydrogen absorption and desorption were Mg+ H2+(?)MgH2 and Mg2Ni+ H2(?)Mg2NiH4,respectively.The increased Ni content in this range remarkably improved the hydrogen storage kinetic performance of the alloys but reduced their hydrogen absorption/desorption capacity.The La5Mg80Ni15 alloy showed the lowest Ea value of 57.7 kJ/mol and the highest hydrogen absorption/desorption rate.In addition,the thermodynamic properties initially improved and then degraded with increasing Ni content.The La5Mg85Ni10 alloy proved to have the best thermodynamic properties with AH at-72.1 kJ/mol and?S at-123.2 J/mol/K.As the milling time is extended,the diffraction peak of La2Mg17 phase is gradually broadened but the diffraction peak of the Mg2Ni phase remained stable,the Ea-value of hydrogen evolution decreases firstly and then increases.With the increase of ball milling time,the ?H and AS of as-milled alloy decrease firstly and then increase.The composite La5Mg85Ni10 alloy milled for 10 h exhibits the best thermodynamics and kinetics performances,the lowest Ea of 47.6 kJ/mol,absorb 5.4 wt.%within 5 min and desorb 5.2 wt.%within 3 min at 360 ? and the lowest ?H and AS of-72.1 kJ/mol and-123.2 J/mol/K.It can be seen that nanostructures have a positive effect on improving the hydrogen storage kinetics and thermodynamic properties of hydrogen storage materials.The effects of the type and amount of transition metal catalyst on the microstructure and hydrogen storage performance of La5Mg85Ni10 + x wt.%M(x = 1,3,5,7;M = TiF3,NbF5,Cr2O3)alloys milled for 10 h have been investigated.The results show that the hydrogen storage capacity of the alloy decreases as the catalyst increases.On the one hand,the catalytic effects of different amount of catalyst TiF3 were studied.TiF3 exists in form of MgF2 and TiH2 phases and Ea decreases firstly and then increases as the amount of TiF3 increases.When 5 wt.%TiF3 is added,the hydrogen desorption activation energy shows the lowest Ea= 45.2 kJ/mol.On the other hand,the catalytic effects of TiF3,Cr2O3 and NbF5 are compared in detail.It was found that TiF3 has better catalytic effect than Cr2O3 and NbFs due to TiF3 nanoparticles can refine the grains better,provide hydrogen diffusion channels and reduce the nucleation driving force of the alloys.