Controllable Synthesis Of Spinel Eletrocatalyst And Their Defect Modification And Electrocatalytic Performance

At present,the energy crisis and environmental pollution are becoming more and more serious,so it is urgent to develop a kind of efficient and clean energy conversion and storage device.Metal air battery is divided into two half reactions,oxygen evolution reaction（OER）and oxygen reduction reaction（ORR）,which are limited by the slow kinetics and high energy barrier caused by multi-electron transfer.Therefore,additional catalysts are needed to catalyze the reaction.At present,noble metal base and corresponding oxide catalysts are the best electrocatalytic materials for OER and ORR.Considering the cost of catalyst and other practical problems,the development of relatively cheap non-noble metal catalyst has become an urgent problem to be solved.In this paper,cobalt-based spinel material was used as the research object.However,due to the lack of active site regulation strategies,further improvement of the activity of spinel oxide is still limited.Therefore,in this paper,the electrocatalytic activity and stability of spinel based materials are improved through morphology regulation,defect construction,metal and non-metal heteroatom doping and geometric structure adjustment.In addition,the mechanism of catalytic reaction is studied in depth,so as to provide theoretical guidance and material basis for the study of oxygen evolution reaction and oxygen reduction reaction of spinel catalyst.（1）Metal ion substitution and anion exchange are two effective strategies for regulating the electronic and geometric structure of spinel.Herein,Co Fe₂O₄-based hollow nanospheres with outstanding oxygen evolution reaction activity are prepared by Cr³⁺substitution and S^2-exchange.X-ray absorption spectra and theoretical calculations reveal that Cr³⁺can be precisely doped into octahedral（Oh）Fe sites and simultaneously induce Co vacancy,which can activate adjacent tetrahedral（Td）Fe³⁺.Furthermore,S^2-exchange results in structure distortion of Td-Fe due to compressive strain effect.The change in the local geometry of Td-Fe causes the*OOH intermediate to deviate from the y-axis plane,thus enhancing the adsorption of the*OOH.The Co vacancy and S^2-exchange can adjust the geometric and electronic structure of Td-Fe,thus activating the inert Td-Fe and improving the electrochemical performance.（2）In the spinel structure,there is a large gap between the tetrahedral and octahedral centers,so there is enough space to accommodate different metal cations.However,the study of structure and mechanism of spinel ferrite becomes more complicated due to the phenomenon of structure inversion.Based on this,this part introduces a series of metal ions into the tetrahedral A and octahedral B sites of Co Fe₂O₄ respectively according to the differences of crystal field stability energy and octahedral preference energy of different metals,and studies the influence of metal doping at different sites on OER activity.Finally,the optimum doping position of spinel ferrite metal and the optimum active site of catalyst were determined.The electrochemical results show that the metal substitution at octahedral sites is more favorable than that at tetrahedral sites,and the introduction of Cr³⁺improves the electrocatalytic performance of Co Fe₂O₄ most obviously.（3）Herein,an effective approach is proposed to fabricate high-performance electrocatalysts based on Co Fe alloy and Co C_X nanoparticles sandwiched in nitrogen-doped carbon nanotubes.The preparation of Co Fe-Co Cx@NCNT is achieved by the calcination of Co Fe₂O₄ spinel and dicyandiamide under reducing atmosphere.The Co Fe-Co C_X@NCNT catalyst exhibits remarkable oxygen reduction reaction（ORR）performance with the onset and half-wave potential of 1.01 V and 0.89 V,respectively,exceeding the commercial Pt/C catalyst.The super electrocatalytic performance is attributed to the multiple heterointerface and strong coupling effect between Co Fe alloy,Co C_X,and NCNT,which can regulate conductivity and electron structure of the catalyst.（4）The defection-rich Zn S nanoparticles were uniformly and tightly anchored on the surface of Ni Co₂S₄ nanosheet to form Ni Co₂S₄/Zn S heterojunction.Transmission electron microscopy（TEM）and positron annihilation spectroscopy（positron annihilation spectroscopy）showed that there were abundant Zn vacancy defects in Ni Co₂S₄/Zn S heterojunction.The anchoring of Zn S nanoparticles inhibited the structural expansion and collapse of Ni Co₂S₄ nanosheets during long-term electrochemical reaction and enhanced the stability of the materials.Theoretical analysis showed that the synergistic effect of Zn S and Ni Co₂S₄,the introduction of Zn vacancy defects and the electrochemical reconstruction of metal sulfide under OER voltage promoted the OER and ORR performance of the catalyst.（5）Single-atom catalysts（SACs）have aroused extensive attention due to their ultrahigh activity and selectivity.Here,a facile ionic liquid（IL）modification strategy is creatively proposed to obtain N and P dual-coordinated Fe single atoms with N unsaturated coordination(denoted as Fe _SAC-N₂P/C)on pre-designed Fe _SAC-N₄/C sites.The using a hydrophobic IL can alter the binding affinity of O₂,maintain a higher O₂concentration at the catalyst interface,and protect Fe single atom sites from surface oxidation and methanol toxicity.This method achieves the function of regulating the coordination and hydrophobic/hydrophilic microenvironment of the SAC simultaneously at room temperature.The optimized IL modified catalyst shows an ultrahigh oxygen reduction reaction（ORR）activity with a half-wave potential of 0.92V and a superior stability with 40000 cycles.This study offers an effective approach for accurately controlling the coordination electronic structure and interface environment of SACs and optimizing ORR performance at room temperature.