Preparation Of Magnesium-based Hydrogen Storage Materials By Mechanical Alloying And Research On Their Mechanis

Hydrogen energy has the characteristics of high energy density,abundant resources,and cleanliness,and is considered to become an ideal choice to solve the problems of consumption of traditional fossil fuel resources and environmental pollution.However,energy storage is one of the biggest challenges to achieving large-scale hydrogen applications.Currently,solid-state hydrides offer a compact,safe and economical way to store hydrogen and are one of the most promising ways to meet the requirements of in-vehicle applications.MgH₂ has great application prospects in the field of solid-state hydrogen storage due to its high hydrogen storage capacity（～7.6wt.%）,abundant resources,low cost,environmental friendliness and high safety.However,the transition from MgH₂ to commercial solid-state hydrogen storage still faces many challenges,such as high hydrogen absorption and desorption temperatures,and slow kinetics.To solve these problems,copper-aluminum intermetallic compounds,transition metal Ni and high-entropy alloys as additives were introduced into Mg by mechanical ball-milling,and explores the influence of mechanical ball-milling and additives on the hydrogen absorption and desorption performance of Mg-based composites and the synergistic catalytic mechanism in the hydrogenation-dehydrogenation process,and then seeks an effective method to improve the hydrogen storage performance of Mg-based materials.The main points of this paper are summarized below:（1）The effects of mechanical milling time on the phase,micromorphology and structure and hydrogen storage performance of pure Mg were investigated.It was found that the mechanical milling destroyed the dense oxide film on the surface of Mg particles,and promoted the contact between hydrogen molecules and the surface of Mg particles.In addition,mechanical milling introduces many lattice defects to the Mg matrix,and the emergence of nanocrystals produces a large number of grain boundaries,which provides an effective channel for the diffusion of hydrogen atoms.The hydrogen storage test results show that the first hydrogen absorption and desorption times of pure Mg after 240 min mechanical ball-milling are shortened from 3068 min and 47.6 min untreated to 330 min and 5 min,respectively,and the dehydrogenation activation energy decrease from 154.97 k J·mol^-1 to 122.79 k J·mol^-1,and the hydrogen storage capacity is about 6.40 wt.%.（2）The intermetallic compound of Cu₉Al₄ prepared by mechanical alloying method as additive was introduced into Mg by mechanical ball-milling,and the effects of milling time and Cu₉Al₄ addition on the microstructure and hydrogen storage properties of the material were systematically studied,and the effect of Cu₉Al₄compound in the process of hydrogen absorption and desorption was explored.The results show that the Mg -Cu₉Al₄ hydrogen storage material prepared at the ball-milling time of 240 min and the addition amount of 20 wt.%has a small hungry particle size and uniform particle size distribution,the hydrogen storage performance of the material is the best,the reversible hydrogen storage capacity is about 5.30 wt.%,the dehydrogenation activation energy is 96.84 k J·mol^-1,the first hydrogen absorption and desorption times are 150 min and 3 min,and the initial hydrogen desorption temperature is 68 K lower than that of pure Mg （after mechanical ball-milling）.The mechanism analysis shows that during the heating process,the metastable phase Cu₉Al₄and Mg matrix in the material react in situ to generate(Cu_1.3Al_0.7)Mg ,and in the subsequent hydrogen absorption and desorption cycles,the(Cu_1.3Al_0.7)Mg phase exist stably,providing more nucleation sites for the hydrogen absorption and desorption reactions.It is also proved by density functional theory（DFT）calculation that Cu₉Al₄will alloy with Mg ,and the ternary cluster formed has higher activity,promotes the adsorption of hydrogen,and accelerates the migration of hydrogen during the process of hydrogen absorption and desorption.（3）To further reduce the hydrogen absorption and desorption temperature and improve the kinetic properties,the transition metal Ni was doped in Mg -Cu₉Al₄composites,and the effects of doping amount on the phase,micromorphology,particle size and hydrogen storage performance of the materials were studied in detail.The results show that Ni doping makes the material to generate Mg ₂Ni,Al_0.96Ni_1.04 and Mg ₂Cu in addition to(Cu_1.3Al_0.7)Mg during the heating process,when the Ni content reaches 15 wt.%,an additional Mg ₃Al Ni₂ phase is generated in the material.However,(Cu_1.3Al_0.7)Mg ,Al_0.96Ni_1.04 and Mg ₂Cu do not participate in the reaction in the subsequent hydrogen absorption and desorption process,which will significantly reduce the reversible hydrogen storage capacity of the material,while Mg ₂Ni and Mg ₂Cu play the role of"overflow"and"hydrogen pump"in the process of hydrogen absorption and desorption,which can improve the kinetic properties of the material.When the addition amount of Ni is 7 wt.%,the dehydrogenation activation energy is77.05 k J·mol^-1,and the initial hydrogen absorption and desorption temperatures are reduced by 26 K and 70 K compared with Mg -Cu₉Al₄ composite,respectively.（4）Based on the Mg-Ti-V-Zr-Nb system,a high-entropy alloy（HEA）with BCC structure and hydrogen storage capacity was designed and prepared by using valence electron concentration（VEC）,atomic radius difference（δ）,mixed entropy(△S_mix),mean enthalpy of hydrogen solution at infinite dilution（（?））,mean standard enthalpy of the concentrated hydrides（（?））and a new dimensionless parameter（?）,and introduced into the Mg matrix through mechanical ball-milling as an additive,the effects of HEA addition on the phase,micromorphology and hydrogen storage performance of the material were systematically studied.HEA containing a variety of transition metals can accelerate the dissociation of hydrogen molecules and weaken Mg-H bonds.The results show that with the increase of HEA content,the dehydrogenation activation energy and hydrogen absorption and desorption temperatures of the composite gradually decrease.After adding 15 wt.%HEA,the reversible hydrogen storage capacity of the material is about 5.50 wt.%,the initial hydrogen absorption and desorption temperatures are 418 K and 526 K,respectively,the dehydrogenation activation energy is 97.51 k J·mol^-1,and the first hydrogen absorption and desorption times are 68 min and 4.4 min.In summary,magnesium-based hydrogen storage materials were prepared by mechanical alloying,and the effects of mechanical milling and additives on the phase,microstructure and hydrogen storage performance of the materials were systematically studied,which provided a reference for the preparation of high-performance magnesium-based hydrogen storage materials.