Effects Of Rare Earth Alloying And Ball Milling On Hydrogen Storage Behavior Of Mg-Ni Based Alloys

Magnesium based hydrogen storage alloys have received considerable attentions due to the high hydrogen storage ability,abundant resources and low cost.However,the higher temperature towards hydrogen absorption/desorption and sluggish hydrogen kinetics deprive the practical application of these alloys.Improvements by chemical modification,catalysts or preparing by different routes have been extensively studied and positive effects have been achieved by a certain extent.However,problems towards the hydrogen storage thermodynamic and kinetic properties still need further development,and understanding of the exact mechanism of the above mentioned processes is still confused.In the present dissertation,Mg₂₄Ni₁₀Cu₂ has been adopted as the master alloys,and influences of substitution of rare earth elements,ball milling and compounding with Ni on the microstructure and hydrogen storage properties have been systematically studied.Evolution of thermodynamics and kinemics under various hydrogen absorption/desorption processes was quantitatively investigated and exact the hydrogenation/dehydrogenation mechanism associated with these modification methods was discussed.Effects of adding of Y,Sm and Nd on the microstructure and hydrogen storage performances have been investigated.Adding RE promotes the microstructural refinement and formation of REMgNi₄?RE=Y,Nd,Sm?.Moreover,it is found that adding of Y promotes solid solution of Cu substituting of Ni in the Mg₂Ni phase.RE substitution improves the electrochemical cycling stability,gaseous activation performances,and the hydrogen absorption/desorption kinetics and thermodynamics.The results show that Y substitution is more benefit to modification of the hydrogen performances.The dehydrogenation activation energies change from 77.4 kJ/mol H₂ to 72.0 kJ/mol H₂ for the master alloy and Y substituted alloy respectively.The enthalpies of hydrogenation and dehydrogenation change from-59.2 and 67.1 kJ/mol H₂ to-53.6 and 63.7 kJ/mol H₂respectively.And the entropies change from-113.7 and 123.1 J/K/mol H₂ to-104.3 and117.4 J/K/mol H₂.It is illustrated that hydrogen absorption and desorption processes follow the three-dimensional diffusion and boundary reaction-controlled mechanism respectively,which has not been altered by RE addition.Subsequently,effects of Y substitution contents on the microstructure and hydrogen storage performances have been investigated by both the first principle calculation and experimental analysis.Adding Y promotes dendritic microstructure changing to eutectic morphology which mainly contains Mg₂Ni and YMgNi₄.Abundance of YMgNi₄ increases with the Y contents.And solid solution of Cu substituting of Ni in the Mg₂Ni phase increases with elevation of Y substitution,where Cu/Ni reaches to 0.36 in the alloy with highest Y content.Y promotes solid solution of Cu in the Mg₂Ni phase,which decreases the thermal stability of Mg₂NiH₄ confirmed by the first principle calculation.During the hydrogenation process,Mg₂Ni and Mg start absorb hydrogen at almost the same time.While,Mg₂NiH₄ begins to release hydrogen more easily during the dehydrogenation process.Y substitutions improves the hydrogen absorption/desorption kinetics and thermodynamics,for which the activation energy of Mg₂₂Y₂Ni₁₀Cu₂ is of 63.7 kJ/mol H₂,the enthalpies of hydrogenation and dehydrogenation are of-50.5 kJ/mol and 61.1 kJ/mol H₂,and the entropies are of-100.7 kJ/mol and 115.4 kJ/mol H₂ respectively.In addition,Y atoms can occupy the sites of Mg atoms in the YMgNi₄ phase and decrease the thermal stability of YMgNi₄,which has been demonstrated using the first principle calculation.Effects of ball milling time on the microstructure and gaseous and electrochemical hydrogen storage performances have been studied.The results show that grain size can be significantly decreased by ball milling.And transformation from crystalline to amorphous occurred after ball milling.Ball milling dramatically elevates the electrochemical discharge capacity where the alloy made by 40 hours reaches 231%of that of the as-cast alloy.Ball milling also improves the electrochemical kinetic property and the alloy adopted by 20hours exhibits the most excellent kinetic property.Ball milling improves the hydrogenation and dehydrogenation kinetics but decrease the reversible gaseous hydrogen storage content.The alloy milled by 20 hours has the best hydrogenation kinetic property that the activation energy is of 62.4 kJ/mol H₂ which is 13.3%less than that of the as-cast alloy.The alloy milled by 40 hours has the best hydrogenation thermodynamic property where the hydrogen absorption enthalpy and entropy are 49.1 kJ/mol and-94.6 kJ/mol H₂,and that for dehydrogenation are 61.3 kJ/mol and 110.6 kJ/mol H₂ respectively.It IS also found that the hydrogen absorption and desorption mechanisms have not been changed by ball milling.Effects of ball milling compounding with Ni on the microstructure and hydrogen storage performances have been studied.Ball milling with Ni significantly improves formation of amorphous and nanocrystalline.Adding with Ni is benefit to the electrochemical discharge capacity,cycling stability and electrochemical kinetic property.The alloy with 100 wt.%Ni presents the highest discharge capacity of 729.3 mAh/g.And that with 150 wt.%Ni exhibits excellent cycling stability and electrochemical kinetic property.Adding of Ni cannot improve the gaseous thermodynamic and kinetic hydrogen storage properties.While,the hydrogenation and dehydrogenation mechanisms can be significantly altered by adding with Ni.Both the hydrogenation and dehydrogenation processes of the alloy with 50 wt.%Ni are random nucleation and subsequent grow-controlled mechanism.And that with 100 wt.%Ni are diffusion for the hydrogenation process and random nucleation and subsequent grow-controlled mechanisms for the dehydrogenation process.