Preparation Of Magnesium-based Solid-state Hydrogen Storage Materials Under Moderate Conditions

As the carrier of the future sustainable energy system,hydrogen has broad development prospects,safe and efficient hydrogen storage technology is the key to the development of hydrogen energy.Solid-state hydrogen storage technology is one of the effective ways to solve the problem of efficient and safe storage and transportation of hydrogen energy.In this paper,two types of solid hydrogen storage systems are studied.The first class is the amino metal-metal hydride system.The amino metal-metal hydride composite hydrogen storage system has a high theoretical reversible hydrogen storage capacity,using Mg（NH₂）₂ instead of Li NH₂ in the Li-Mg-N-H hydrogen storage system has better hydrogen storage performance than other amino metals-Metal hydride systems.However,there are still some shortcomings such as high hydrogen desorption temperature and slow kinetics in practical application.In this study,three borohydrides Mg（BH₂）₂,Li BH₄ and Na BH₄ were doped into the pure Li-Mg-N-H system respectively,and the effect of doping on the hydrogen desorption kinetics of the system was investigated.All of the three metal borohydrides can improve the dehydrogenation kinetics of the system,and the dehydrogenation reaction activation energy decreases in the order of Mg（BH₂）₂>Li BH₄>Na BH₄.However,further research into the mechanism of the kinetic improvement found that the three borohydrides had different mechanisms in action.There is the weak interaction between Na BH₄ and the system components,and it basically remains unchanged during the dehydrogenation process.Li BH₄ reacts with the intermediates in the dehydrogenation process of the system to form multiple boron hydrogen-amino complexes to reduce the dehydrogenation activation energy,and finally release the amino part to obtain Li BH₄ again,which is a typical catalytic effect.However,after doping with Mg（BH₂）₂,the borohydride compound and the original metal hydride components in the system undergo a metathesis reaction during the ball milling process,and the hydrogen release process also participates in the intermediate reaction,shortening the reaction steps of the hydrogen release process,to speed up the reaction.Another category involves magnesium-based solid hydrogen storage materials.Magnesium hydride（Mg H₂）is considered to be one of the most promising solid-state hydrogen storage candidates due to its high hydrogen storage density（7.6%,110 g/L）,and has also received extensive attention in chemical heat storage.However,due to the presence of oxides on the surface of metal magnesium（Mg）,the traditional preparation of Mg H₂ from Mg can only be achieved under high temperature and high pressure（200 atm and 350°C）.In recent years,my country’s import of various hydrogen storage materials including Mg H₂has been restricted,which not only affects research work,but also poses hidden dangers to future applications.In this paper,by reviewing the research progress of Mg-based hydrogen storage materials at home and abroad,the research plan for preparing Mg H₂ by mechanical ball milling surface modification and adding catalysts is determined.Due to the ductility of metal powder,the cold welding effect of direct mechanical ball milling will lead to powder agglomeration and difficult sample processing.In this study,the effect of different ball milling conditions,solvents and catalytic additives on the hydrogenation reaction of commercial Mg after treatment was studied by using ball milling in solvent medium.The results show that the hydrogenation reaction of Mg after ball milling can be carried out under milder conditions under the following three test conditions,and the conversion rate can reach more than 95%.Among them,magnesium powder mixed with anthracene-Ni Cl₂ and ball milled has the fastest hydrogen absorption kinetics.XRD analysis found that the magnesium crystal planes moved after ball milling,the intensity of Mg（101）crystal plane decreased and the diffraction intensity of Mg（002）crystal plane increased.Density functional theory（DFT）calculations show that Mg（002）is exothermic from the initial state（IS）to the final state（FS）,that is,the thermodynamics is a spontaneous reaction,which is conducive to the formation of Mg H₂,while the thermodynamics of the Mg（101）crystal plane unfavorable.X-ray photoelectron spectroscopy（XPS）analysis shows that the corresponding transition metals can still be detected on the surface of the Mg sample after ball milling,in which the Ti is still dominated by the Ti⁴⁺valence state,and the Cr signal is weak,which is roughly the same as when it was added.The Cr³⁺valence remains unchanged,while Ni is a Ni⁰ valence single substance.These transition metals generally have a catalytic effect on the hydrogenation reaction,and it is inferred that it may be another factor for the improvement of the hydrogenation reaction.The results of differential scanning calorimetry（DSC）of magnesium powder hydrogenation reaction products show that the hydrogenation product Mg H₂ of Mg treated by ball milling is significantly better than ordinary commercial Mg H₂.The activation energy of dehydrogenation reaction determined by Kissinger method is lower than that of commercial Mg H₂.This indicates that the introduction of catalysts through the ball milling stage can affect the hydrogen storage properties of the hydrogenation products.In other words,the hydrogen absorption and desorption performance of magnesium hydride can be regulated by optimizing the synthesis process,which is beneficial to subsequent practical applications.