金属有机骨架材料在镁基储氢材料中的应用

Application of Metal-Organic Frameworks on Magnesium-Based Hydrogen Storage Materials

能量密度高、热值大、资源丰富、无污染、可储存、可再生、可燃烧和可发电的氢能,被誉为21世纪解决能源危机和缓解温室效应的“终极能源”。MgH_(2)因其较高的理论储氢容量,有望成为未来车载氢能源载体而备受关注,但其过高的热力学稳定性、缓慢的吸放氢动力学等缺点限制了其工程应用。比表面积高、结构性质可调以及金属离子可高效利用的金属有机骨架(MOFs)材料,在镁基材料储氢性能的改善方面展现出良好的应用前景。概述了MOFs材料对镁基材料储氢性能的催化掺杂改性、纳米限域催化改性,以及MOFs材料的常见制备方法,并对该领域的研究前景进行展望。

In recent years,rapid economic development has driven the growing energy demand,leading to environmental pollution problems and energy shortage caused by the widespread use of fossil fuels.Hydrogen energy,with high energy density,large calorific value,abundant resources,zero pollutions,storable,renewable and electric and combustible,has been termed as the"ultimate energy"to solve the energy crisis and alleviate the greenhouse effect in the 21st century.However,the promotion and practical application of hydrogen energy have been limited by its storage and transportation.Nowadays,there are three major hydrogen storage technologies,compressed gaseous storage,cryogenic liquid storage and solid-state materials storage.Among these,the solid-state hydrogen storage technology has gained popularity because of the unique features of good safety,convenient transportation,long service life,high volume storage density,low storage pressure and high purity of hydrogen release,making it perfect for hydrogen fuel cell vehicles.With a mass hydrogen storage density of 7.69%and volume hydrogen storage density of 106 kg·m^(-3),MgH_(2)had been identified as one of the most promising on-board hydrogen energy carriers.However,before it could be used for industrial purposes,the high thermodynamic stability and sluggish reaction kinetics needed to be modified,which were caused by its large formation enthalpies(ΔH=76 kJ·mol^(-1)H_(2))and high reaction activation energies(ΔE=160 kJ·mol^(-1)).Metal-organic frameworks(MOFs)with large surface areas,adjustable structural properties,and efficient utilization of metal ions,had shown promising application prospects in improving the hydrogen storage performance of MgH_(2)by catalysis and nanoconfinement with in-situ catalysis.MOFs with different structures including isoreticular metal-organic framework(IRMOF),Zeolitic Imidazolate Framework(ZIF),Materials of Institute Lavoisier(MIL)and University of Oslo(UIO),could be synthesized by various methods,such as hydrothermal,solvothermal,conventional solution,electrochemistry,microwave,ultrasonication and so on.The continuous research and development of synthetic methods and structural systems had made MOF materials more diversified,which could satisfy complicated and changeable application scenarios.Catalyst doping was an important method to improve the reaction kinetics of hydrogen release and uptake,which could accelerate the dissociation and reconstruction of hydrogen on MgH_(2)/Mg surface,and thereby reducing the activation energy and enhancing the rates of hydrogen uptake and release.The high specific surface area and superior metal ion utilization of MOFs could significantly increase the catalytically active specific surface area and active site density,which regarded the perfect catalysts for MgH_(2)/Mg system.Furthermore,the regular and dense pore channels inside MOFs would provide additional hydrogen diffusion channels.However,grain growth and particle agglomeration would happen with a long time’s exposure to the high-temperature cycles of hydrogen release and uptake,which would definitely lead to the degradation of hydrogen storage performance,especially the hydrogen storage capacity and reaction kinetics.Therefore,the key to breaking through this process was to develop more efficient MOFs catalysts,which could not only slow down or inhibit grain growth and particle agglomeration,but also decrease the hydrogen release temperature,and thereby modifying the cycling stability of hydrogen storage.By organically combining nanoengineering and catalysis,the novel process called“nanoconfinement with in-situ catalysis”had been proposed lately,which employed nanoporous MOFs catalysts as the scaffold to confine MgH_(2)/Mg particles,provided a promising strategy for the exploitation of advanced hydrogen storage materials,and it might be the ultimate solution to achieve the practical application of magnesium-based hydrogen storage materials for the following reasons:(1)Highly active MgH_(2)/Mg nanoparticles confined in the independent nanopore,were able to maintain the nanostructures during cycles,which would not only destabilize the thermodynamics but also shorten the hydrogen diffusion path;(2)The metal ions of MOFs had the capability of catalyzing hydrogen desorption and absorption as required;(3)The nanoporous scaffold was responsible for the uniform distribution of MgH_(2)/Mg particles,and the prevention of agglomeration and aggregation.It was believed that reducing the particle size of MgH_(2)/Mg to nanometers(<5 nm)could not only improve the kinetic performance but also reduce the thermodynamic stability.Along this direction combining nanoconfinement and catalysis might lead to the simultaneous improvement of kinetics and thermodynamics.However,several methods involved in repeating the impregnation process,extending the impregnation time,and increasing the porosity of the MOFs materials,had been proposed to increase the hydrogen storage capacity of Mg-based@MOFs composites to meet the requirement for future industrial application.The current application of hydrogen energy in transportation was mainly focused on vehicle fuel cells,which had potential to apply to other fields such as distributed energy storage and natural gas blending with more research efforts.Therefore,safe and efficient magnesiumbased solid-state hydrogen storage materials with mild working conditions were still the primary and challenging problem to overcome.This article outlined the recent advances in modified properties of magnesium-based solid-state hydrogen storage materials through catalysis modification and nanoconfinement with in-situ catalysis by MOFs,as well as the preparation methods of MOFs.On this basis,the perspective of study on MOFs modified magnesium based solid-state hydrogen storage materials was proposed.