First-principles Study On Oxygen Reduction Of Graphene Doped With Zinc And Cobalt Diatoms

At present,human beings are increasingly demanding energy,and traditional fossil energy cannot meet the future energy needs of human society.Fuel cells can directly convert chemical energy into electrical energy with the advantages of high conversion efficiency,cleanliness and high specific energy,which have a wide application in actual life.Although Platinum-based catalysts are regarded as one of the best catalysts for cathodic oxygen reduction reaction?ORR?of fuel cells,but the high price and slow kinetic effect seriously restrict its commercial application.Therefore,developing one kind of novel non-precious metal catalysts with low-cost,high catalytic activity has a significant scientific meaning.Carbon-based materials have attracted much attention due to their high catalytic activity,good electrical conductivity,and high anti-toxicity.Theoretical studies on the oxygen reduction activity and mechanism of carbon-based catalysts can not only provide a theoretical basis for improving the catalytic activity of new carbon-based catalysts,but also promote the development and utilization of catalysts.In this thesis,the electronic structure and oxygen reduction activity of Zn-N,Co-N,Zn-Co-N,Zn-N-S,Co-N-S,Zn-Co-N-S co-doped graphene have been systematically studied by using the first-principles calculation method based on density functional theory.The main contents and results are as follows:1 The theoretical study on oxygen reduction mechanism of Zn-Co-N co-doped graphene:The Zn-Co-N co-doped graphene model was established by using Materials Studio software.The electronic structure and oxygen reduction mechanism of Zn-N,Co-N,Zn-Co-N co-doped graphene were calculated by density functional theory?DFT?.Firstly,the formation energy of Zn-Co-N co-doped graphene with different structures were calculated to determine the stable configuration of the doped structures.Then,the Bader charge were calculated for determining the catalytic active sites,and the conductivity of the doped structures was compared through the density of states?DOS?calculation.Finally,the process of oxygen reduction with stable structures were analyzed by using Gibbs free energy as criterion.It has been found that Zn-Co-N co-doped graphene not only has good electrical conductivity,but also promotes the activation of oxygen molecules and lengthens the O-O bond.The strong adsorption of Zn and Co atoms on OH makes it difficult to be reduced.At this time,the OH was firmly bonded to the Zn-Co-N co-doped structure in a ligand form,and a new structure of ZnCoN₆?OH?catalyst was constructed,which exhibits more excellent oxygen reduction catalytic activity.2.The theoretical study on oxygen reduction mechanism of Zn-Co-N-S co-doped graphene:The effects of sulfur on the electronic structure and oxygen reduction activity of ZnN₄,CoN₄ and ZnCoN₆₁ were discussed.Firstly,the model structures of S doping into ZnN₄,CoN₄ and ZnCoN₆₁ were established,and the stable configurations were determined by the formation energy.Next,the oxygen reduction activity of the stable structure was carried out,and the calculation results showed that:S can't increase the catalytic activity of oxygen reduction after doping to ZnN₄ and CoN₄ in the near position.And ZnCoNS₄ formed by doping the same site to ZnCoN₆₁ at the same site which shows excellent oxygen reduction activity.This can be mainly attributed to the obvious wrinkling of the base structure of ZnCoNS₄,which is favor for exposing more active sites than the planar structure when undoped with S.At the same time,the introduction of S leads to the rearrangement of the electronic structure of the surface of ZnCoNS₄,which weakens the bonding between Zn and Co atoms and hydroxyl groups,so that ZnCoNS₄ can effectively activate oxygen molecules and promote the reduction of OH intermediates.