Preparation Of Fe And Co-based Transition Metal Additive And Their Effects On Hydrogen Storage Properties Of MgH₂

MgH₂ has the advantages of high theoretical hydrogen storage capacity?mass hydrogen storage density of 7.6 wt.%?,reversible hydrogen absorption and desorption performance,and is considered as a hydrogen storage material with great application potential.However,the Mg-H bond in MgH₂ has a high bond energy,which results in its high hydrogen release temperature and poor kinetic properties,making it difficult to be practically applied.In order to improve the hydrogen storage performance of MgH₂,the transition metal compounds Fe₇S₈ and Fe_0.96S were prepared by hydrothermal method,and the carbon nanotube-loaded transition metal cobalt nanoparticles Co/CNTs were prepared by using ZIF-67 as the precursor template.The effects of three transition metal-based additives on the hydrogen storage performance and hydrogen storage mechanism of MgH₂were studied.The results show that the initial temperature of hydrogen release of the composite system of MgH₂-Fe₇S₈ and MgH₂-Fe_0.96S prepared by hydrothermal combustion of Fe₇S₈,Fe_0.96S and magnesium powder by hydrogenation combustion and ball milling method is420K and 550K,which is better than that of pure ball milling The MgH₂ decreased by290K and 160K respectively.Under the condition of 573K,hydrogen release,MgH2-Fe0.96S system can release 2.8wt.%H2 within 20 minutes.The hydrogen absorption test shows that the hydrogen release product of the composite material can complete the hydrogenation reaction within 700 s at 523K,and after 10 cycles of the MgH₂-Fe_0.96S system,the hydrogen absorption capacity remains basically unchanged.XRD patterns show that during ball milling and the first hydrogen evolution,the iron-based transition metal compounds Fe₇S₈ and Fe_0.96S will undergo oxidation-reduction reactions with MgH₂to produce metallic Fe elemental and MgS.In the subsequent hydrogen absorption and desorption process,the existence forms of Fe element and MgS did not change.The presence of elemental Fe and MgS is conducive to the breaking of Mg-H bonds,thereby promoting the separation of MgH2 from hydrogen.Using ZIF-67 as a precursor to prepare carbon nanotube-loaded transition metal cobalt Co/CNTs,using hydrogen combustion and ball milling method to prepare MgH₂-Co/CNTs composite hydrogen storage materials,studied its hydrogen absorption and desorption behavior and mechanism.Studies have shown that during the ball milling process,MgH₂ reacts with Co/CNTs to generate nanocrystalline Co₃MgC_0.5.5 in situ,which keeps the structure stable during the cycle and plays a catalytic role in absorbing and releasing hydrogen in the composite system.Among them,the MgH₂-Co/CNTs composite hydrogen storage system starts to absorb hydrogen from 423 K,which is 282K lower than that of pure MgH₂.This sample emits hydrogen at 573K and can emit 1.5 wt.%H₂ within30 minutes.The hydrogen evolution product can absorb 4.0 wt.%H₂ at 423K.The XRD pattern shows that the product Co₃MgC_0.5.5 is produced during the ball milling process,which reduces the kinetic energy barrier of the hydrogen absorption and desorption reaction of MgH₂,and the carbon nanotubes in Co/CNTs can play a good role in dispersion in the composite system.And limit the growth of Co particles nucleation,the combined effect of the two reduces the temperature of hydrogen absorption and release of MgH₂,and improves the rate of hydrogen absorption and release.