Controllability Modulation Of Electron Pathway During Oxygen Reduction Reaction Of Cobalt-based Materials

As an important electrocatalytic process,oxygen reduction reaction（ORR）can convert chemical energy into electric energy,so as to achieve energy conversion.However,the kinetics of ORR is slow,so electrocatalysts with high activity and stability are needed to promote ORR.As a common non-noble metal catalyst,Co-based material has attracted wide attention for its high activity and adjustability.Meanwhile,the ORR is divided into four-electron pathway and two-electron pathway,which show a competitive relationship.The geometric structure and electronic structure of the catalyst are the main factors that affect the electron pathway of electrocatalyst for ORR.Therefore,the construction of cobalt-based catalysts with different geometric or electronic structures is the key to the preparation of materials with controllable electronic pathway.This thesis mainly realizes the synthesis of cobalt-based electrocatalysts with high activity and high selectivity of ORR through different strategies,successfully realizes the controllable modulation of electron pathway in the oxygen reduction process of cobalt-based electrocatalysts.The main contents of this thesis are as follows:（1）CoN nanoparticles anchored on ultra-thin nitrogen-doped graphene electrocatalyst（CoN/UNG）were synthesized by the combination strategy.The coordination of 1,10-phenanthroline with metal Copromotes the dispersion of metal Co,which can effectively control the size of CoN nanoparticles.Polyethylene imine can intercalate into layers of graphene and block the stacking of graphene to achieve ultra-thin nanosheet structures.The small-sized CoN nanoparticles distributed on ultra-thin nitrogen-doped graphene provide a large number of ORR active sites.Consequently,the CoN/UNG has a half-wave potential of 0.87 V vs.RHE with excellent electrochemical ORR performance and high stability.The power density of CoN/UNG-based zinc-air battery is 149.3 mW cm^-2,the specific capacity is 793.2 mAh g^-1 and the cycle stability is 350 h.（2）CoAl₂O₄/CoO electrocatalyst was synthesized by the activation strategy of Lewis acid site Al.The introduction of Lewis acid site Al can improve the selectivity of cobalt oxide for two-electron ORR.Density functional theory（DFT）calculations reveal that the Lewis acid site Al can regulate the electronic structure of metal Coactive site,thereby decreasing the state density of Co3d orbital and reducing the Gibbs free energy barrier of 2-electron ORR.Thus,the ORR on the CoAl₂O₄/CoO catalyst can be carried out through two-electron pathway.Meanwhile,both metal Coand Al can be used as the metal active sites of oxygen reduction reaction.Consequently,the MgAl2O4/Co3O4 electrocatalyst has excellent electrocatalytic performance.The H₂O₂selectivity of CoAl₂O₄/CoO catalyst can reach 85%at 1600 rpm and 94%at 400 rpm at the voltage range of 0.2～0.6 V with excellent stability.（3）On the basis of regulating the ORR electron pathway of cobalt metal oxide by Lewis acid site Al,a heterojunction electrocatalyst（MgAl₂O₄/Co₃O₄）with two oxide of MgAl₂O₄ and Co₃O₄ was constructed by adding metal Mg.It is beneficial to enhance the ORR activity of catalyst.DFT calculations reveal that the heterostructure of MgAl₂O₄/Co₃O₄ facilitates electron transfer,promotes the adsorption of O₂ and the desorption of intermediates during ORR,which is conducive to improve the activity of two-electron pathway.Consequently,the MgAl₂O₄/Co₃O₄ electrocatalyst has excellent electrocatalytic performance.The H₂O₂ selectivity of the MgAl₂O₄/Co₃O₄electrocatalyst can reach 95%at 1600 rpm and 100%at 400 rpm at the voltage range of 0.2～0.6 V with excellent stability.