First-Principles Study Of Oxygen Reduction Reaction On Metal-Embedded Nitrogen-Doped Graphene

To address the severe challenges of ever-growing energy crisis and environmental pollution,enormous efforts have been devoted to exploring green,efficient,and renewable electrochemical energy conversion and storage devices.Proton exchange membrane fuel cells（PEMFCs）can directly convert the chemical energy into the electric energy,and have many advantages such as simple structure,high energy density and energy conversion efficiency,environmental friendliness,and wide applications,thereby attracting much attention in recent years.However,the oxygen reduction reaction（ORR）at the cathode in PEMFCs is kinetically sluggish and highly dependent on Pt-based catalysts to accelerate the reaction.The natural scarcity and prohibitive cost of Pt significantly impede the widespread application of PEMFCs.Therefore,there is an urgent demand to develop the substitutes for the Pt-based catalysts.Metal-embedded nitrogen-doped graphene single/double-atom catalysts not on ly can maintain the good physicochemical properties of graphene,but also h ave significant merits of fully atomic utilization,lowly cost and excellent catalytic activity,thereby becoming one of the promising substitutes for the Pt-based catalysts.However,its electrocatalytic performance still cannot meet the actual needs in applications,especially in acidic medium.In this dissertation,the first-principle calculations based on density functional theory were performed to study the metal-mbedded nitrogen-doped graphene single/double-atom ORR catalysts,including the geometric and electronic properties,the surface active sites,the reaction mechanism,the origin of catalytic activity,and the effect of ligands on catalytic activity.It is expected that this thesis can provide valuable theoretical guidance for design and synthesis of highly efficient and cost-effective ORR catalysts.The main research contents and results are presented as follows.（1）The adsorption and dissociation of O₂ molecule on a series of 3d,4d,and 5d transition metal atom embedded N-doped graphene（MN₄-G）single-atom catalysts were systematically studied.The results show that Ag,W,Re,and Os atoms cannot stably bond with N₄-G,NiN₄-G,CuN₄-G,PdN₄-G,PtN₄-G,AuN₄-G only can physically adsorb O₂ molecule,and other MN₄-G structures can chemically adsorb O₂ molecule.Moreover,the side-on adsoprtion conguration is more conducive to the activation and dissociation of O₂ molecule compared to the end-on conguration.MnN₄G,RuN₄-G,CrN₄-G,TcN₄-G,and MoN₄-G were indentified to have good adorption and dissociation activity for O₂ molecule,and MnN₄-G was indentified to be the most active single-atom catalyst.（2）The surface active sites and ORR pathways of singe Mn embedded N-doped graphene（MnN₄-G）were comprehensively investigated.The results show that the top of C and N（T2 and T5）are the major H+ adsorption sites,and the initial state（IS）structure of H+adsorbed on T5 is more beneficial for hydrogenation.The maximum energy barrier for the three optimal O₂ hydrogenation pathways was only 0.13 eV.（3）The structural stability and ORR catalytic properties of metal embedded Ndoped graphene（M₁M₂N₆,M1,M2=Fe,Co,Ni,Cu,and Zn）double-atom catalysts with or without*OH ligand modification were systematically studied.The results show that all configurations except ZnZnN₆ present good structural stability.NiNiN₆,NiCuN₆,CuCuN₆,and CoNiN₆（OH）were found to have excellent catalytic activity superior to Pt catalysts.The O₂ adsorption energy,the electronegativity of the metal atoms,and the adsorption free energy of*OH were closely related to the overpotential,hence can be served as the descriptors to evaluate the ORR catalytic activity.（4）The geometric and electronic structures and ORR catalytic activities of Fe and Co atoms embedded N-doped graphene（FeCoN₆）isomers before and after being modified by ligands of*O,*OH and*O₂ were systematically investigated.The results show that*O,*OH,and*O₂ mainly obtain charges from Fe and Co atoms and make the d-orbitals of Fe and Co atoms shift toward the Fermi level,thus regulating the adsorption strength for reaction intermediates to boost the catalytic activity of FeCoN₆.Especially,the overpotentials of FeCoN₆-I（OH）and FeCoN₆-I（O₂）are as low as～0.23 V,markedly lower than FeCoN₆ and Pt catalysts.