Regulation Of Composites And Properties Of Ni/Fe-based Spinel Oxides

The construction of multiphase composite structure is of great significance to the development of inorganic solid chemistry.In composite structures,fine tuning can bring huge performance improvements.Non-noble metal composite catalysts are widely used in electrocatalysis because of their abundant reserves and adjustable structure.Based on the inherent characteristics of composite solid,multi-scale strategies including electronic structure adjustment,microstructure engineering,phase structure and recombination are adopted to realize interface coupling,and then affect the overall properties of solid materials.The optimization of microstructure and interfacial coupling can also accelerate proton diffusion,improve electronic conductivity and interfacial charge transfer.However,there are still many theoretical gaps in the structural regulation and preparation methods of complex composite structures.Although a wide range of multiphase bonds based on composite structures have active interfaces,a small number of interfaces are not enough to change the overall intrinsic activity of the composite solid.Therefore,it is a promising method to construct composite structures in multiple ways and expand the interface effect to affect the whole composite structure.Due to the presence of different catalytic sites in the spinel structure,its electronic structure can be adjusted by doping or defect engineering.Phase and composition modulation can change the position and oxidation state of octahedron,thus optimizing the surface adsorption energy and improving the reaction kinetics.In consideration of the advantages of rich and adjustable cations and variable structure of spinel oxide,this paper takes nickel-iron spinel oxide as the research model and constructs multi-interface composite based on it by different methods.The effects of different compounding methods on the overall structure of spinel were compared.The practical synthesis methods and the theoretical results are expected to be extended to other composite systems besides spinel oxide.This provides new opportunities for synthesizing composite solids with controllable electronic states and crystal structures.The specific research contents are as follows:1.NiS/NiFe₂O₄ composites were synthesized by applying high-energy plasma in S doped NiFe₂O₄ system through in situ phase transformation.In the defective S-NiFe₂O₄,the plasma destroys the long-range order of NiFe₂O₄ and generates the short-range order NiS with intrinsic defects.The unique composite structure produced by in situ phase transformation is beneficial to improve electrochemical catalytic performance.For oxygen evolution reaction(OER),NiS/NiFe₂O₄ composite catalyst has a low overpotential of 230 m V at the current density of 10 m A·cm^-2.NiS/NiFe₂O₄ exhibits a half-wave potential of 0.81V for oxygen reduction(ORR).Based on the excellent bi-functionality of the composite material,a liquid zinc-air battery was assembled using catalyst supported carbon paper as a positive electrode.Experimental results show that the power density reaches 148.5 m W·cm^-2.The NiS/NiFe₂O₄ composite interface induced by the in-situ phase transition was discovered by spherical differential electron microscopy,and the possibility of this structure was verified by density functional theory calculation,and the formation of this active structure optimized the adsorption energy of H₂O and accelerated the reaction process.In-situ phase reconstruction is important for synergistic lowering of energy barriers,accelerating reaction kinetics and catalyzing oxygen release.2.To further illustrate the unique advantages of in-situ reconstituted composite structures,we compared the two composite structures based on NiFe₂O₄ with electrocatalytic activity.Firstly,an effective method of NiSe₂/NiFe₂O₄ composites with strong interaction was developed by constructing competitive environment.Among them,the stable composite structure is realized by Niatom sharing.Two phase composite structure(W-NiSe₂/NiFe₂O₄)was synthesized by contrast.Compared with the composite grown at the same time,the NiSe₂obtained is uniformly dispersed over NiFe₂O₄,resulting in a smaller interfacial energy of the active component.M(?)ssbauer spectroscopy revealed the in-situ growth process of stable and highly dispersed NiSe₂by sharing Niatoms.The X-ray absorption fine structure confirmed that the strongly interacting structure excites more charge transfer than the extraneous heterogeneous structure.Theoretical calculation further verifies that the charge transfer of NiSe₂/NiFe₂O₄ significantly optimizes the overall electron state and further reduces the dissociation energy barrier of water in the reaction.The OER performance of the composite is greatly improved with the change of the composite strength.Compared with the original NiFe₂O₄ and heterogeneous structure composite sample,it shows lower overpotential and higher current density.The superiority of"bottom-up"composite material construction is verified.3.Inspired by the ideas summarized in the first two chapters,we further construct strongly interacting polyphase composites on the basis of weak crystallization system.We have designed a weak crystalline phase of NiFe₂O₄ with abundant pore structure and induced a solidly dispersed dissolution phase by S atom.Porous structure facilitates S adsorption and exposes more reaction sites.Then,room temperature plasma was used to excite and adsorb S on the surface of the precursor.With the continuous irradiation of plasma,the precursor underwent obvious phase transformation,and obvious NiFe alloys and bimetallic hydroxyl oxides appeared.Compare the aggregation state of NiFe alloy in the sample without S.S-anchored samples tend to form dispersed composite phase and produce more bimetallic hydroxyl oxide content.Combining with density functional theory,we show that the presence of S can significantly reduce the binding energy of composite structures,which proves the anchoring effect of S.At the same time,the strong interaction of multibody phase dispersion can optimize the whole electronic state.Amorphous NiFe₂O₄ increases the charge transfer efficiency.Therefore,the resulting S-anchored NiFe/NiFe₂O₄ has excellent ORR and OER bifunctional catalytic activity.The zinc-air battery(ZABs)assembled from it also has stable discharge/charge voltage,high energy efficiency and long life.