Study On The Hydrogen Absorption And Desorption Performance Of Molybdenum Based Two-Dimensional Catalysts Modified Magnesium Hydride And Its Mechanism

As a safe,clean,efficient and sustainable new energy,hydrogen energy has become one of the main forces of the low-carbon energy system and has a huge market potential in the fields of energy,transportation and industrial production.In China’s"14th Five-Year Plan",hydrogen energy is listed as one of the"frontier"fields and one of the six key industries to be promoted.The use process of hydrogen energy mainly includes hydrogen production,storage and transportation,application,etc.The key to hydrogen energy application is safe and efficient hydrogen energy storage and transportation technology.Magnesium hydride（MgH₂）,a coordination hydride formed by the reaction of alkali metal and hydrogen,is regarded as one of the representatives of the new generation of complex hydrogen storage materials because of its high hydrogen storage capacity（110 g/L,7.6 wt%）,low cost,rich reserves and good reversibility.However,the high thermodynamic stability and slow kinetic performance of MgH₂have been restricting its development in the field of hydrogen storage.Therefore,based on the research of MgH₂ at home and abroad,this thesis systematically studied the catalytic modification of MgH₂by designing different molybdenum（Mo）based two-dimensional layered catalysts,and deeply analyzed its internal mechanism.Firstly,this article designed and synthesized FeCoNiCrMo high entropy alloy nanosheets,and doped 6 wt%samples with MgH₂ after mechanical ball milling and wet chemical ball milling,respectively.The results showed that the samples after mechanical ball milling were completely dehydrogenated at 415℃,while the samples after wet chemical ball milling were completely dehydrogenated at 380℃.Therefore,wet chemical ball milling can significantly improve the performance of the catalyst.Secondly,in order to further investigate the effect of wet ground FeCoNiCrMo on the hydrogen storage performance of MgH₂,composite systems with different doping ratios were constructed.MgH₂ doped with 9 wt%FeCoNiCrMo began dehydrogenation at 200℃and released up to 5.89 wt%hydrogen at 325℃for 60 minutes.Completely dehydrogenated composite materials can absorb 3.23 wt%of hydrogen gas within50 minutes at temperatures as low as 100℃.The calculated activation energies for dehydrogenation and hydrogenation decreased by 44.21%and 55.22%respectively compared to MgH₂.In addition,mechanism studies have shown that the hydrogen capacity of the composite material maintained 96.2%of its initial size after 20 cycles,with only a loss of0.28 wt%,demonstrating significant cycling stability.The mechanism study shows that,finally,the two-dimensional Mo₂TiC₂ MXene is synthesized by etching Mo₂TiAlC₂,and then doped into MgH₂ to test its hydrogen storage performance.The initial hydrogen desorption temperature of Mo₂TiC₂ doped MgH₂ decreased significantly from 330℃（original MgH₂）to 187℃.Isothermal dehydrogenation analysis showed that MgH₂+9 wt%Mo₂TiC₂ composite rapidly discharged 6.4 wt%hydrogen in 4min at 30℃.For hydrogenation reaction,dehydrogenated MgH₂+9wt%Mo₂TiC₂ absorbed6.5wt%hydrogen in 5 minutes at 175℃.The activation energy of desorption reaction and absorption reaction of MgH₂ doped with Mo₂TiC₂ is 135.6±1.9 k J/mol and 46.1±0.2 k J/mol respectively.After 20 cycles,the hydrogen capacity of MgH₂+9 wt%Mo₂TiC₂ composite lost1.0 wt%.The mechanism study shows that the presence of Mo in Mo₂TiC₂ enhances the thermal stability of MXene,reduces the content of active Ti in the cycle process,and results in a worse catalytic effect than Ti₃C₂.