Modification On Hydrogen Storage Properties Of Magnesium Nanocrystals And Its Mechanism

Under the global energy crisis,hydrogen energy has attracted much attention because of its advantages such as high energy density,non-toxic and environmental protection.Hydrogen storage is one of the bottlenecks in the development of the hydrogen industry,but the solid-state hydrogen storage technology,especially the solid-state hydrogen storage technology based on magnesium hydride（Mg H₂）,has a high mass and volume storage density,and is expected to promote the practical application of hydrogen energy.At present,in the face of the obstacles of high thermodynamic stability and slow reaction kinetics of Mg H₂,alloying,nanocrystalline and catalytic doping methods have achieved remarkable results in improving the dehydrogenation and hydrogenation performance of Mg H₂.In view of this,on the basis of reviewing the research progress of magnesium based hydrogen storage materials at home and abroad,the relevant factors affecting the hydrogen storage performance of magnesium nanocrystals were first explored,and then the hydrogen storage performance of magnesium based hydrides was improved by means of nanocrystalline and catalysis.Firstly,a series of carbon coated magnesium nanocrystals were prepared by a simple wet chemical ball milling method.Four kinds of magnesium nanocrystals with different carbon thickness were obtained at 100°C,150°C,200°C and 250°C by adjusting the heat treatment temperature.With the increase of heat treatment temperature,the hydrogen absorption kinetics of magnesium nanocrystals decreased,but the hydrogen storage capacity and dehydrogenation kinetics increased.The properties of magnesium nanocrystals obtained at 150°C are the best after comparing the hydrogen storage capacity and the rate of hydrogen absorption and discharge.Then,the two catalysts Co₃O₄and Co Cl₂were incorporated into magnesium nanocrystals for control variable experiments,and it was found that the timing of catalyst addition had a great influence on the hydrogen storage performance of the composite system.The hydrogen absorption rate of Mg-150+5 wt%Co₃O₄-A was the highest when the catalyst and magnesium nanocrystals were wet chemical ball milling together.When the heat treatment temperature was changed,the hydrogen storage capacity of the composite material increased with the increase of the heat treatment temperature,and the hydrogen storage capacity of the composite material at 250°C reached5.6 wt%.In addition,under the above optimal conditions,the hydrogen absorption and emission kinetics and hydrogen storage capacity of Co Cl₂-doped magnesium nanocrystals are superior to those of Co₃O₄-doped magnesium nanocrystals.In order to improve the hydrogen absorption and dehydrogenation kinetics of magnesium nanocrystals while maintaining high hydrogen storage capacity,the Nb₂O₅@MOF catalyst was successfully prepared by hydrothermal synthesis method,and Mg-Nb composites were constructed,so as to better understand and improve the hydrogen storage properties of magnesium nanocrystals.The results showed that the dehydrogenation of pure magnesium samples after 2 h,4 h,6 h and 8 h were 0.49 wt%,4.21 wt%,6.75 wt%and 7.30 wt%,respectively.Nb₂O₅@MOF The doped Mg-Nb-A（2 h）,Mg-Nb-B（4 h）,Mg-Nb-C（6 h）and Mg-Nb-D（8 h）released 3.24 wt%,5.78 wt%,6.84 wt%and 7.02 wt%hydrogen,respectively.The average grain sizes of Mg-A,Mg-B,Mg-C and Mg-D are18.95,14.43,13.02 and 12.15 nm,respectively,calculated by XRD pattern and Scherrer formula.The grain sizes of Mg-Nb-A,Mg-Nb-B,Mg-Nb-C and Mg-Nb-D are 19.13,15.89,13.22,12.38 nm.The results of constant temperature experiments showed that the apparent activation energy（E_a）of Mg H₂hydrogenation decreased from 72.3±2.9 k J/mol to 43.1±5.2k J/mol with the addition of 5 wt%Nb₂O₅@MOF,and the hydrogen absorption kinetics of Mg H₂was improved greatly with the addition of catalyst.According to the Chou model analysis,the addition of catalyst changes the controlled rate of penetration（slow hydrogen uptake and release rate）into the controlled rate of diffusion（fast hydrogen uptake and release rate）,which explains the superior kinetic properties of Nb₂O₅@MOF catalyst.Finally,the cyclic performance of Mg-Nb-D was tested,and the capacity retention rate was83.3%after 10 cycles.