Mechanism Studies On Electrochemical Properties Of Transition Metal Compounds

Transition metal compounds are rich in resources,green and environmental protection,and have received widespread attention in energy storage and conversion devices.Research shows that Fe/Ni transition metal-organic frameworks（MOFs）have the characteristics of porous structure,large specific surface area and abundant active sites,and exhibit good performance in catalytic systems.The conductivity and stability of MOFs are difficult to meet the needs of energy storage electrode materials,but the calcined oxides retain high specific surface area and porous characteristics,and can also exhibit excellent electrochemical performance in energy storage.Transition metal compounds also show excellent electrochemical performance in novel Li-CO₂ energy storage devices,especially Mo-based metal compounds have low charging potential,high energy efficiency and unique reaction mechanism.Therefore,from the perspective of the structural design of transition metal compounds and their electrochemical reaction mechanism,this thesis constructs transition metal compound composites with various morphological and structural characteristics to improve the electrical conductivity,specific surface area and structural stability of transition metal compounds.The correlation law between structure and electrochemical performance is summarized and condensed,and the electrochemical reaction mechanism is further discussed in depth.These studies can provide theoretical and experimental guidance for the promotion and screening of transition metal compounds in energy storage and conversion devices.The research conclusions of this paper are mainly divided into four parts:（1）In this chapter,Fe Ni-MOF and carbon nanotube composites（F1N2@CNT-10）with high specific surface area,high conductivity,and abundant exposed active sites were prepared by modulating the morphology,which showed good reactivity and stability in both OER and HER.And further study the effect of alternating magnetic field on the OER and HER reactivity of F1N2@CNT-10 catalyst:in OER,the alternating magnetic field can promote the OER activity at low current density,but the alternating magnetic field can inhibit the OER activity at high current density;In HER,alternating magnetic fields can promote HER reactivity at both low and high current densities.（2）In this chapter,based on the organic precursors of Ni/Fe MOFs prepared in the previous chapter,carbon-modified transition metal oxide composites（Ni O@C,Fe₃O₄@C and Ni Fe₂O₄@C）with different micro-nano structures were prepared by carbonization,and then microstructure characterization and electrochemical tests were performed.The results show that Ni Fe₂O₄@C double transition metal oxides have suitable specific surface area,smaller particle radius,loose particle packing and bet ter electrochemical performance.（3）Based on the different electrochemical properties of Ni Fe₂O₄,Fe₃O₄ and Ni O in the previous chapter,this chapter adopts the first-principles theoretical calculation method to study the crystal structure and electronic structure of transition metal oxides,and reveal the influence mechanism of the bimetallic synergistic effect.The results show that the electronic conductivity of Ni Fe ₂O₄ is better,the surface provides more adsorption sites for Li ions,the adsorption energy for Li ions is larger and the migration energy barrier is smaller,and the Fe Ni alloy has a smaller work function,which makes Ni Fe₂O₄ exhibit high performance.Excellent electrochemical performance.Finally,combined with ex-situ X-ray diffraction characterization and ex-situ AC impedance characterization methods,the reversible conversion reaction mechanism of Ni Fe ₂O₄ as anode material for lithium-ion batteries was analyzed..（4）Based on the fact that Mo-based transition metal compounds show different electrochemical reaction mechanisms from other transition me tal compounds in Li-CO₂battery energy storage devices,this chapter selects Mo ₂C as the research object,adopts the first-principles theoretical calculation method,and systematically studies the transition metal Electrochemical reaction mechanism of meta l compound Mo₂C as cathode catalyst for Li-CO₂ batteries.The calculation results show that the final discharge product on the surface of Mo₂C cathode catalyst in Li-CO₂ batteries is Li₂C₂O₄,which is thermodynamically and kinetically favorable.In addition,under the influence of the Mo₂C catalyst surface,the disproportionation reaction of Li ₂C₂O₄decomposition to Li₂CO₃ is suppressed.These theoretical studies can provide a new way to find suitable cathode catalysts for Li-CO₂ batteries.