Controllable Construction And Selective Regulation Of Oxygen Reduction Reaction Mechanism Based On Cu_2-xX/CNTs Nanocatalysts

Oxygen reduction reaction(ORR)plays a pivotal role in electrochemical energy conversion devices.The complete generation of H₂O by 4e^-ORR pathway is a vital cathode reaction in fuel cells and metal-air batteries,which is important for the efficient operation of these electrochemical devices,whereas the production of H₂O₂by electrocatalytic 2e^-ORR can serve as an effective strategy to replace the traditional anthraquinone process,and this synthetic method has the advantages of good safety,low cost,simple opration and mild conditions.However,whether it is the4e^-ORR patahway to generate H₂O or the 2e^-ORR pathway to generate H₂O₂,the sluggish kinetics,high overpotential for oxygen reduction potential and occurrence of competitive side reaction are the bottlenecks restricting the development.On the other hand,nobel metal Pt and Pd based catalysts have been respectively considered as efficient catalysts in both 4e^-ORR and 2e^-ORR,but their limited resources and prohibitive cost extremely hinder large-scale applications.Hence,the development of low cost,high active and selective electrocatalysts is of crucial significance for advancing the ORR reaction.To solve the above problems,this thesis focuses on low-cost,resourceful and eco-friendly transition metal oxides/sulfides.The ORR reaction pathway of the catalysts was designed and regulated around the microstructure through control strategies such as morphology regulation,atomic-chemical environment regulation,composition regulation and defect regulation,revealing the relationship between catalyst structure and catalytic performance.Combined with density functional theory(DFT)calculation,the ORR reaction mechanism of catalysts was investigated.It provides experimental and theoretical reference for the design of ORR catalysts with low cost,high activity and selectivity.The main research results of this thesis are as follows:Three novel Au-Cu₂O nanoparticles with controllable morphology(spheres,cubes and petals)were designed and synthesized by different methods,and then were dispersed onto carbon nanotubes(CNTs)matrix with high-surface area and strong conductivity as nanocomposite catalysts(Au-Cu₂O/CNTs).The presence of Au O^-active species involves the charge transfer and electron rearrangement between Au and lattice oxygen in Cu₂O,and the Au-Cu₂O petal/CNTs distributes the most abundant Au O^-species on its surface due to unique morphology.Electrochemical tests showed that the ORR activity of the catalysts has obvious morphology-dependent,and enhancement in ORR activity and stability of Au-Cu₂O petal/CNTs is related to the most abundant Au O^-actives species on the surface.DFT indicates that Au O^-species as surface active sites can improve the catalytic performance of ORR by promoting 4e^-reaction mechanism,and the optimal compromise of the adsorption and bond lengths of O₂ on the surface of Au-Cu₂O are beneficial to activate and accelerate the kinetics of O-O band splitting.This study deepens the insight into how the morphology of nanocomposite catalysts affects the catalytic activity,which provides an implication for the design of ORR catalysts with high activity and stability.Cu₂X/CNTs(X:S,Se,SSe)nanohollow composite catalysts were synthesized by anion exchange method that can regulate the chemical environment around adjacent Cu atoms by different atoms using Cu₂O as sacrificial template,revealing the relationship between the Cu₂X structure and properties of 2e^-ORR to H₂O₂.In the reaction process,the O atoms in the catalyst are gradually replaced by other atoms,which will change the chemical environment and surface charge around the Cu atom,and the Cu⁺in Cu₂S crystal will participate in electron transport and charge transfer.In this series of electrocatalysts,the Cu₂S/CNTs exhibits excellent electrocatalytic activity,selectivity and superior stability.The Cu₂S/CNTs catalyst has the largest electrochemical active surface area due to the unique hollow structure,which enhances the diffusion of electrolyte inos and facilitates charge transfer.The adsorption model of the catalysts indicates that the O₂ molecule is adsorbed on the Cu atom sites on the surface of Cu₂S in an"end-site"configuration,which facilitates the formation of OOH*intermediates and ultimately promotes the reaction to H₂O₂.The M-Cu_2-xS/CNTs nanohollow composite catalysts were constructed by doping Cu₂S with trace noble metals(Au,Ag,Pd,Pt)using cation exchange method,which effectively improves the catalytic activity and H₂O₂ selection of Cu₂S/CNTs.The doping of trace noble metals causes the lattice expansion of Cu₂S,resulting in a certain lattice distortion and a small amount of Cu defects in Cu₂S to some extent.There is charge transfer between noble metals(especially Au)and Cu_2-xS,which improves the electronic structure of Cu_2-xS,and the existence of Au^?+active species promotes the adsorption of O₂ molecules on the surface of Au-Cu_2-xS/CNTs catalyst and enhances the overall electrocatalytic activity of the catalysts.In alkaline medium,the Au-Cu_2-xS/CNTs improves the catalytic activity and H₂O₂ selectivity compared with Cu_2-xS/CNTs,which indicates that after doping Au,the activity and selectivity of catatlyst are improved due to the charge transfer between Au and Cu_2-xS.At the same time,the catalyst also displays excellent stability due to the synergistic effect between Au and Cu_2-xS.Au@Cu_2-xS/CNTs nanocore-shell catalysts with Cu defects were constructed by adjusting the introduction of S^2-using etching method,and the intrinsic relationship between defects and activity was investigated.Electrochemical tests show that the Cu-defective Au@Cu_2-xS/CNTs catalysts exhibit higher catalytic ORR activity and H₂O₂ specific productivity than Au@Cu₂O/CNTs,indicating that the introduction of Cu defects effectively improves the two-electron transfer of ORR,especially,the Au@Cu_1.75S/CNTs sample with the abundant Cu defects exhibits high H₂O₂selectivity(?94%)and good stability.DFT calculations reveal that the introduction of Cu defects can significantly lower the reaction energy barrier of the determinant intermediate OOH*and prevent the O-O band splitting,thus improving the catalytic performance of 2e^-ORR to H₂O₂.