Insight On 3d Electronic Structure Regulation And Oxygen Evolution Mechanism Of Cobalt-iron Based Layered Double Hydroxides

As a sustainable renewable and clean energy source,hydrogen energy can satisfy the growing global energy demand will be realized only through its efficient,low-cost,and environmentally friendly production.Electrolyzed water with the production of hydrogen is a very promising method.The hydrogen evolution reaction for electrocatalyzing water spitting is accompanied with oxygen evolution reaction(OER).However,the OER reaction process is a four-electron-proton reaction process,resulting in kinetically sluggish kinetics.Therefore,the OER reaction severely hinders the hydrogen evolution reaction process.The efficient and stable OER electrocatalysts can be used to accelerate the reaction and reduce the overpotential for the OER.Although currently the precious metal catalysts(i.e.,Ru O₂ and Ir O₂)have shown very excellent OER catalytic performance,the high cost of precious metals has hindered their widespread application.Therefore,designing efficient,stable,and inexpensive transition metal-based OER catalysts has become the key to solving the hydrogen energy problem.The purpose of this thesis is to study the catalytic performance of OER electrocatalysts and the OER reaction mechanisms based on Co₂Fe LDH structural compounds.Cu,Mn,Cr and V-doped Co₂Fe LDH were synthesized to study high-performance OER catalysts,and I studied high-efficiency OER catalysts and the mechanism to improve the catalytic performance of OER catalysts by density functional theory.The main conclusions are as follows:(1)In this thesis,bimetallic Co₂Fe LDH,Cu-doped Co₂Fe LDH,Mn-doped Co₂Fe LDH,Cr-doped Co₂Fe LDH,and V-doped Co₂Fe LDH were synthesized by low-temperature wet-chemical method,and the samples were grown on carbon papers by wet-chemical method in this dissertation.It was found that the Cu element did not completely enter the Co₂Fe LDH,and the Cu-doped Co₂Fe LDH grown on carbon fiber paper were nanoparticles;Mn,Cr,and V were completely doped into the Co₂Fe LDH,and the in-situ doped Co₂Fe LDHs grown on carbon fiber paper are hexagonal nano-sheet structures that grow verticality to the carbon fiber paper.The nanosheets of electrocatalyst was varied after and before doping with by XRD and IR characterization.(2)The thesis systematically tests the OER catalytic performance of Co Fe-based LDH electrocatalysts.After the concentration of Cu-doped Co₂Fe LDH gradually increasing,it is found that the OER catalytic activity gradually decreases.However,after the optimal doped content with Mn,Cr,and V elements in the Co₂Fe LDH,the OER catalytic activity of Co₂Fe LDH has been significantly improved.Especially,it has been found that the electrocatalysst of V-doped Co₂Fe LDH shows the best OER catalytic performance.According to the obtained OER catalytic performance,I have obtained an interesting rule,that is,the d orbital electrons of the doping element is closely related to the OER catalytic performance.When the d electrons of the doping element Cu is 9,the d orbital is the electrons is more than Co and Fe,and the OER catalytic performance of Cu-doped Co₂Fe LDH is reduced.When the d electrons doped with elements such as Mn,Cr and V is less than Co and Fe,the catalytic activity of Mn,Cr,and V-doped Co₂Fe LDH for OER was increased.The d electrons of doping with V is the smallest,and the performance of the electrocatalyst is the best.(3)In the thesis,I studied the projected density of states(DOS)of bimetallic Co₂Fe LDH via density functional theory(DFT).After doping with Mn,Cr,and V in the Co₂Fe LDH,it is found that the electronic structures of Co,Fe,and O elements has varied.Among them,the changed electronic structure of Fe element is particularly significant,especially the projected density of states(PDOS)of Fe 3d orbital.After doping with metal element,the d-band center of Fe element has shifted significantly relative to that of Fe in the Co₂Fe LDH,indicating that the d-band center of the doped metal element Fe site is favored for the Fe site adsorption process of intermediates.As the d electrons of Mn,Cr,and V elements gradually decreases,I also found that the d band center of Fe element gradually increases.The number of d electrons in V element is the least,and the d-band center of Fe in Co₂Fe LDH doped with V element is the lowest.(4)The OER reaction process of Cr-doped Co₂Fe LDH was studied by density functional theory.The relationship between the d-band center and the OER catalytic activity was systematically studied.According to the Gibbs free energy analysis of the OER reaction process,Cr element is not the OER catalytic active site.However,the introduction of Cr element effectively optimizes the d-band center of Fe element,and then the improved OER performance of Co₂Fe LDH,and further explained the relationship between the OER catalytic activity and the center of the d-band,that is,the appropriately increased d-band center of Fe accelerates the OER reaction.The work in the thesis could provide some references to develop cheap and simple synthesis scheme for the synthesis of materials with poor thermal stability.At the same time,it also provides a new way of the growth of materials.The method is also simple and cheap.It also provides some method reference methods for the phase characterization and electrochemical performance research of OER catalysts,and also has certain reference for the methods to improve the OER performance of OER catalysts.The design and construction of OER catalysts provide a certain idea and theoretical basis.