Study On Modulation And Optimization Of Hydrogen Storage Kinetics And Thermodynamic Properties In Magnesium Hydride By Iron-based Nano Additives

In order to achieve sustainable energy development,the use of clean and renewable energy has become the main theme of development.Hydrogen energy,as a kind of secondary energy with abundant sources,green and low carbon and wide application,can absorb renewable energy on a large scale,accelerate the decarbonisation of energy applications and is the clean energy with the most development potential.However,there are many technical bottlenecks to the large-scale application of hydrogen energy and the introduction of hydrogen energy into households,among which the storage and transportation technology of hydrogen energy is the key.Among various hydrogen storage technologies,solid-state hydrogen storage has attracted much attention because of its high efficiency and safety.Magnesium hydride（MgH₂）is considered as one of the most promising solid-state hydrogen storage materials due to its high storage capacity,low price,abundant resources and good environmental compatibility.However,the poor kinetic performance and high thermodynamic stability of this material limit its practical application in the field of hydrogen storage.Therefore,in this thesis,MgH₂hydrogen storage materials are doped with various iron（Fe）-based additives to study their effects on the reversible hydrogen storage performance of MgH₂and the mechanism of their effects,in order to achieve significant improvement in the kinetics and thermodynamics of hydrogen absorption and desorption and cyclic stability of Mg-based composite systems.Firstly,graphene loaded FeOOH nanodots（FeOOH NDs@G）were prepared by water bath method and doped into MgH₂by mechanical ball milling method.The hydrogen storage performance test results showed that the MgH₂doped with 10wt%FeOOH NDs@G started to release hydrogen at 229.8℃,which was 106.8℃lower than the initial hydrogen release temperature of MgH₂.The MgH₂-10wt%FeOOH NDs@G composite after complete hydrogen release could absorb 6.0wt%hydrogen again at 200℃and 3.0 MPa hydrogen pressure.By incorporating 10wt%FeOOH NDs@G into MgH₂,the dehydrogenation and rehydrogenation activation energies of MgH₂were reduced from156.05 and 82.80 k J/mol to125.03 and 58.20 k J/mol,respectively.In addition,the composite system maintained 98.5%of its initial hydrogen storage capacity after 20 cycles,showing good cycling stability.Through the mechanism analysis,the catalytic effect of FeOOH NDs@G on MgH₂can be attributed to the synergistic effect between the graphene nanosheets and the in situ formed Fe.On the one hand,graphene with large specific surface area acts as a carrier to achieve uniform dispersion of FeOOH nanodots,and graphene also has the effect of inhibiting Mg lattice expansion.On the other hand,the in situ formed Fe on the Mg/MgH₂surface becomes an effective catalytic active site,which accelerates the hydrogen diffusion during the cycling process,thus the MgH₂-10wt%FeOOH NDs@G composite exhibits excellent hydrogen storage performance.Further,three different sizes of Fe nanoparticles were synthesized by regulating the content of buffer LiCl,and the hydrogen storage performance of the Fe nanoparticle-doped MgH₂and the degradation mechanism were systematically investigated by combining microstructure analysis and hydrogen storage performance testing.Compared with MgH₂,the hydrogen absorption and desorption temperature and activation energy of MgH₂-Fe composites were significantly lower.The onset temperature of MgH₂doped with 5wt%Fe was reduced to 182.3℃.At 300℃,the MgH₂-5wt%Fe composite released 6.7wt%H₂in 15min.At 325℃,the hydrogen release reached 7.0wt%in 15 min,which was better than undoped MgH₂under the same conditions.Maintaining good cycling stability is a key challenge for the practical application of MgH₂.In this experiment,the cycling process was simulated with a long-time high temperature incubation for practical applications,and a significant deterioration in hydrogen storage capacity was found.Mechanistic investigation suggests that the grain growth phenomenon occurring in MgH₂and Fe nanocatalysts directly contributes to the capacity decline.High entropy alloys have unique high entropy effect and hysteresis diffusion effect,while their own solid solution structure can maintain their stability under high temperature conditions,and exhibit excellent catalytic activity in the field of catalysis.Therefore,in this thesis,FeCoNiCrTi high entropy alloy（HEA）nanosheets were designed and synthesized,and introduced into MgH₂to improve the hydrogen storage performance.The experimental results showed that HEA exhibited excellent catalytic activity on MgH₂,and the initial hydrogen desorption temperature of the constructed MgH₂-HEA composite was reduced from 330.0℃to 198.5℃,with a significant reduction of 51%in dehydrogenation activation energy compared with MgH₂without additives.In addition,the time required for the MgH₂-HEA composite to absorb 5.0wt%of H₂at 225°C after hydrogen release is one-twentieth of that required for MgH₂.Meanwhile,HEA enhanced the cycling stability of MgH₂,and the retention of hydrogen storage capacity after 20 cycles was 94.4%.Microstructural analysis showed that the synergistic"hydrogen pumping"effects of Mg₂Ni/Mg₂NiH₄and Mg₂Co/Mg₂Co H₅and Fe,Cr and Ti catalytic sites improved the hydrogen desorption and absorption performance of MgH₂-HEA composites.