Effects Of Ball Milling And Graphene Compounding On The Structure And Properties Of Nd-containing Mg Based Hydrogen Storage Alloy

The Mg-based hydrogen storage alloys are considered as one of the most promising hydrogen storage alloys,offering the combination of large amount of hydrogen storage,low density,abundant resources,low cost,clean and environmental protection.Therefore,it has been widely concerned by the public.However their difficult hydrogen-release conditions,the higher temperature towards hydrogen absorption/desorption,bad kinetics properties in hydrogen absorbing process and alloys are subject to oxidation and corrosion are the blocks in the road for Mg-based hydrogen stroge alloys to become a strong competitor in this field.Researchers often use changes in stoichiometric ratio and composition,addition of catalysts and changes in alloy preparation methods to improve the overall performance of hydrogen storage alloys.Although the adoption of the above methods has achieved certain results,it is still far from practical application,and the influence mechanism is not completely clear.In this paper,the effects of the above three conditions on the gas-solid hydrogen storage performance of magnesium-based alloys were studied by adding rare earth,ball milling and graphene composite methods.The effects of the above three conditions on the gas-solid hydrogen storage performance of magnesium alloy were studied.As well as the correlation with the hydrogen absorption and desorption properties of the alloy,the hydrogen release activation energy of the alloy and the enthalpy change and entropy change value during the hydrogen absorption and desorption process were calculated under different conditions.The effects of different amounts of rare earth element Nd addition on the phase composition,microstructure and hydrogen storage performance of Mg₂₄Ni₁₀Cu₂ alloy were studied.It was found that the amount of Nd addition did not change the phase composition of the alloy.The as-cast alloys of the four Nd additions mainly included Mg₂Ni phase,NdMgNi₄phase and a small amount of Mg₃Nd phase.The microstructure of as cast alloy is smooth and flat,and has three kinds of contrast structure.The white structure with sharp edge is inlaid in the gray matrix structure,while a small amount of dark gray structure is scattered.The phase composition of the alloy after hydrogen absorption becomes Mg₂NiH₄,Nd₃H₂ phase and a small amount of MgH₂ phase.The composition phases after hydrogen release are mainly Mg₂Ni and Nd₃H₂ phases,while the NdMgNi₄ phase remains unchanged before and after hydrogen absorption.At the same time,it was found that the surface of the alloy sample became rough and there were a lot of cracks after hydrogen absorption and desorption cycles.It was found that the addition of Nd reduced the hydrogen activation energy of the alloy from 70.82 kJ/mol of Mg_23.5Nd_0.5Ni₁₀Cu₂ alloy to 60.26 kJ/mol of Mg_22.5Nd_1.5Ni₁₀Cu₂ alloy.The addition of Nd also improves the thermodynamic properties of hydrogen absorption and desorption of the alloy.Among them,the hydrogen absorption（35）H and（35）S of Mg₂₃Nd_1.0Ni₁₀Cu₂ alloy are-48.61 kJ/mol and-80.03 J/K/mol H₂,while the hydrogen desorption（35）H and（35）S of Mg_23.5Nd_0.5Ni₁₀Cu₂ alloy are 59.61 kJ/mol and 90.18 J/K/mol H₂.The effect of ball milling time on the microstructure and gas-solid hydrogen storage properties of the alloy was studied.It was found that a small amount of Mg₃Nd phase disappeared in the as-cast alloy.The main phase Mg₂Ni and NdMgNi₄ remain unchanged.The particle size of the alloy was obviously reduced by ball grinding,but with the extension of ball grinding time,the particle size was reduced and agglomeration phenomenon appeared.In the study of hydrogen absorption and desorption of alloys,it is found that the phase composition of the alloy is the same as that of the as-cast alloy after hydrogen absorption and desorption.The ball-milled structure does not change significantly after hydrogen absorption and desorption.There are cracks on the surface.The results of performance tests show that the activation and hydrogen release kinetic properties of the alloy are obviously changed by ball grinding.The Ea of Mg₂₃Nd_1.0Ni₁₀Cu₂ cast alloy was 60.57 kJ/mol,which was reduced to 55.71 kJ/mol after ball grinding for 20h.At the same time,it was found that the thermodynamic properties of absorption and emission of hydrogen were slightly improved by ball grinding preparation.The hydrogen release（35）H and（35）S of Mg₂₃Nd_1.0Ni₁₀Cu₂ alloy ball mill for 10h were 80.66 kJ/mol and127.01 J/K/mol H₂,while the hydrogen release（35）H and（35）S after ball mill for 40h were 72.12kJ/mol and 118.45 J/K/mol H₂.The effect of composite graphene on the structure and hydrogen storage properties of the alloy was studied.It was found that graphene can aggravate the degree of amorphization of the alloy,but does not change the phase composition of the alloy.The composite graphene has little effect on the activated hydrogen absorption rate,and has no improvement on the activated hydrogen release rate.However,the addition of graphene can significantly improve the low-temperature hydrogen absorption and absorption performance of the alloy.At a low temperature of 150℃,the addition of 1wt.%and 10wt.%graphene ball milled alloy has good hydrogen absorption capacity.When the temperature exceeds 250℃,graphene has little effect on the rate of hydrogen absorption and desorption.Trace graphene does not improve the hydrogen release kinetics,but it can reduce the pressure of the hydrogen release platform to a certain extent.Graphene can also improve the thermodynamic properties of the alloy.The hydrogen evolution（35）H and（35）S added with 1wt.%graphene are 79.13 kJ/mol and 122.98 J/K/mol H₂,while the hydrogen evolution（35）H and（35）S added with 10wt.%graphene become 75.81 kJ/mol and 119.37J/K/mol H₂.