Electronic State Modulation Of Amorphous Transition Oxide Composite Structures And Their Electrocatalytic Oxygen Evolution Performance

Composite structures are applied in the development of electrocatalytic materials due to their unique advantages such as synergistic effects,strain effects,and strong electron interactions.The composite structure construction can drive electron migration at the interface,optimize electron density and surface electron potential distribution,and thus improve the oxygen evolution kinetics of composite materials.Transition metal-based composites have attracted much attention in the field of electrocatalysis due to their low price,rich elemental composition,and easily tunable microstructure.At present,the transition metal-based composite structures are mainly crystalline composite structures,but the stable structure and limited active site of crystalline composite hinder its catalytic activity.In contrast,amorphous structures are characterized by long-range disorder,isotropy,and rich orbital energy levels,the flexible and tunable electronic structure provides an ideal platform for studying the Structure-activity relationship of amorphous composite structures.Based on this,a series of Ni/Fe/Co-based amorphous transition metal oxides composite catalysts were constructed from the aspects of synthesis and electronic state regulation in this paper.The structure-activity relationship between its electronic structure and electrocatalytic activity is deeply explored,and the optimization mechanism of the electronic state of the composite structure is understood from the atomic level,which provides a new perspective for establishing the relationship between its composition-structure-performance,as well as a novel synthetic strategy for the design and development of catalysts with high-performance composite structures catalysts.This paper mainly focuses on p-d orbital coupling,e_g orbital occupancy,charge transfer and spin state modulation of amorphous transition metal oxide composite structures.The main research contents are as follows:1.Synthesis of Ni Fe（OH）_x/Ni Fe₂O₄ composite structures and their electrocatalytic oxygen evolution performanceAn ultrasonic-assisted Na BH₄ reduction method was applied to the in-situ construction of amorphous Ni Fe（OH）_x/crystalline Ni Fe₂O₄ structures,and the composites showed enhanced electrocatalytic water oxidation performance.The ultrasonic-assisted reduction as an efficient innovative method,which is able to combine the cavitation effect of ultrasonic with the reduction effect of active hydrogen,imposes a local high temperature,high pressure,and strong reduction environment on the Ni Fe₂O₄ spinel oxide surface,resulting in a kinetic-driven ultrafast reaction with the reactant Ni Fe₂O₄ particles and the migration of Ni and Fe cations from Ni Fe₂O₄ to the crystal surface through a hydrogenation substitution mechanism,achieving in situ growth of amorphous Ni Fe（OH）_x on Ni Fe₂O₄ surface,thus forming a Ni Fe（OH）_x/Ni Fe₂O₄ composite structure.The composite exhibits relatively excellent electrochemical oxygen evolution performance with an overpotential of 276 m V at a current of 10 m A cm^-2 and a Tafel slope of 68 m V dec^-1.The study demonstrates a novel approach to the preparation of amorphous materials,which provides new opportunities for the development of highly efficient electrocatalytic materials.2.Modulation of orbital hybridization in Ni Fe（OH）_x/Ni Fe₂O₄ composite structures and their oxygen evolution performanceIn the previous chapter,the ultrasonic-assisted reduction method was used to achieve a controlled regulation of the chemical composition of the amorphous Ni Fe（OH）_x/Ni Fe₂O₄ catalyst.Based on this,this chapter explores the relationship between the chemical composition and electronic structure of Ni Fe（OH）_x/Ni Fe₂O₄composites in more depth by changing the concentration of Na BH₄.Structural characterization demonstrates that a moderate concentration of reducing agent（4M）leads to the strongest Fe 3d-O 2p hybridization in the composite,effectively inducing charge density redistribution at the active site.In addition,density functional theory（DFT）calculations reveal that the d-band center of the active site in optimized composite shifts down and away from the center of the Fermi energy level due to the strong p-d orbital hybridization,which optimizes the oxygen adsorption and desorption process of the intermediate.Ultimately,the overpotential of the optimized composite material is 150 m V lower than that of the pristine material.This study opens up new ideas for the design of efficient electrocatalysts through rational manipulation of orbital hybridization.3.The e_g electronic occupancy modulation and oxygen evolution performance of Ni Fe₂O₄-based spinel via bridging amorphous Mo S_xThe e_g orbital filling of octahedral nickel cations(Ni _Oh)and octahedral iron cations(Fe _Oh)in Ni Fe₂O₄-based spinel is controllably regulated by introducing an external radical of amorphous Mo S_x as an electron acceptor through an ultrasonic-anchored pyrolysis strategy.XAS and X-ray photoelectron spectroscopy（XPS）were used to confirm that the electron occupied in e_gorbit of M_Oh emigrates with the amount of Mo-S hanging on the apical of octahedral sites,and results in a salutary transition from high to medium e_g occupancy state.In addition,benefiting from the abundant unsaturated S atoms in amorphous Mo S_x,the Fe _Oh and Ni _Oh at the surface furthest activates and the composite consequently shows a superior water oxidation performance.Density functional theory（DFT）also reveals that the e_g fillings of Ni and Fe decrease to 1.4 and1.2 after Mo S_x modification,which can effectively reduce the free energy of the OOH*intermediates in the OER process.This work opens up a pathway to further release the electrocatalytic activity of octahedral sites by bridging an external phase with electron capture capability,which is important for the development of catalysts with better performance for water oxidation or other reactions.4.The charge transfer and Co spin state regulation of amorphous Co S_y/Co O_xcomposite structure and their oxygen evolution performanceA simple and rapid one-step wet chemical method is proposed to achieve the large-scale synthesis of amorphous Co S_y/Co O_x nanostructures in a short period of time.According to the differences in ion diffusion rates,S^2-with a slower diffusion rate combines with Co²⁺to form Co S_y nanoparticles on the surface of Co O_x nanosheets,ultimately active amorphous Co S_y/Co O_x composite structures with tunable electronic structures were constructed.Compared with the annealed crystalline composite structures,the amorphous composites with abundant defect exhibits a lower overpotential,a smaller Tafel slope,and long-time stability.The high catalytic activity of composite materials results from the strong charge transfer between Co S_y and Co O_x,which facilitates Co²⁺to lose electrons and generate active Co³⁺.This change in the valence state of cobalt cations induces a spin state transition,accelerating the electrocatalytic reaction kinetics and lowering the energy barrier of the oxygen evolution reaction process.This study highlights that electron spin state regulation provides a valuable guide to the exploration of efficient and low-cost catalysts.In this paper,a series of amorphous transition metal oxide composite structures were synthesized via the ultrasonic reduction method,ultrasonic anchored pyrolysis method,and one-step co-precipitation methods.Taking advantage of the unique structure of the amorphous composites,the relationship between their electronic structure and highly efficient performance is explored in depth,in order to provide new ideas for the construction of efficient oxygen evolution composites in the future.