Study On Property Regulation And Mechanism Of Magnesium-based Borohydride Hydrogen Storage Materials

Hydrogen energy is a clean energy with great development potential.The key to the realization of hydrogen energy practicality is to develop safe,efficient and economic solid-state hydrogen storage technology.Mg（BH₄）₂ has been widely concerned because of its high theoretical hydrogen storage density（14.9 wt%）.However,its practical application is limited due to the defects of over stable thermodynamic performance and poor dehydrogenation kinetics.In this paper,the thermodynamic and kinetic properties of Mg（BH₄）₂-based hydrogen storage material for ab/dehydrogenation have been controlled by means of additive doping,high capacity derivatives confining and multiphase compounding,and the corresponding mechanism of modification has been revealed.Firstly,the typical accordion like Ti₃C₂ MXene material was successfully prepared by high concentration HF solution corrosion method.It was introduced into Mg（BH₄）₂hydrogen storage system by ball milling and wet chemical method.The effect of Ti₃C₂MXene as additive on Mg（BH₄）₂ was studied systematically.The results show that the initial dehydrogenation temperature of the modified system is 82℃,and 7.5 wt%hydrogen can be released at 260℃,which are superior to that of Mg（BH₄）₂ under the same conditions,and the activation energy of dehydrogenation is significantly reduced.The-F terminal groups on the surface of Ti₃C₂ MXene participate in the dehydrogenation reaction,and MgF₂ is formed to promote the dehydrogenation at low temperature.The effect of Ti₃C₂MXene as a confined material on Mg（BH₄）₂·6NH₃ was further studied.The results show that the initial dehydrogenation temperature of the system is reduced to 50℃,and the interlayer spacing of Ti₃C₂ MXene can be effectively controlled during the dehydrogenation process.The appearance of Mg₃B₂N₄ in the dehydrogenation product improved the purity of hydrogen.Ti₃C₂ MXene has a space limiting effect on hydrogen storage materials,and the nanosize effect can improve the hydrogen release performance.Secondly,the mixture of NaBH₄ and Mg（BH₄）₂ was milled to obtain Mg（BH₄）₂-NaBH₄composite system.The results show that the mixture of the two components is physical mixing and eutectic melting occurs in the heating and dehydrogenation stage.The initial dehydrogenation temperature of Mg（BH₄）₂-NaBH₄ composite decreased by 10℃compared with pure Mg（BH₄）₂ and 272℃compared with pure NaBH₄ respectively.The thermodynamic performance is significantly improved,and the dynamic performance needs to be further improved.Finally,NdF₃ was introduced into Mg（BH₄）₂-NaBH₄ composite system as an additive.The results show that the initial dehydrogenation temperature of the modified system decreases to 174℃.The rate and capacity of dehydrogenation were improved at 300℃.And it has partial reversibility.NdF₃,as the destabilizing agent of the reactant,participates in the dehydrogenation reaction and generates NaMgF₃ to promote the reaction.During ball milling,the homogeneous dispersion of NdF₃ and the sudden drop of NaBH₄ particle size significantly improved the kinetics of dehydrogenation.