Study On Electrocatalytic Oxygen/Nitrogen Reduction Reaction Mechanism Of Bimetallic Atom Doped Graphene

The energy crisis and environmental problems brought about by the extensive use of fossil energy have begun to restrict human development and threaten human survival.Therefore,mankind must develop new energy and change the way of energy utilization to deal with the difficulties caused by energy shortage and environmental degradation.As an efficient energy exchange device,fuel cell can supply clean power.Traditional ammonia synthesis method consumes a lot of energy and emits a lot of greenhouse gases.In contrast,electrochemical ammonia synthesis improves energy efficiency and reduces greenhouse gas emissions,which provides a new way to solve the current severe energy and environmental problems.Ammonia is a widely used chemical raw material and energy storage material.The electrochemical synthesis of ammonia provides a clean conversion and storage method for electrical energy.Energy conversion and energy storage by electrochemical reaction is an effective way to solve energy and environmental problems.However,whether these electrochemical reactions can occur efficiently depends on electrocatalysts with high activity,good selectivity and stability.Based on density functional theory（DFT）,15 kinds of bimetal doped graphene,which are composed of five transition metals（Fe,Co,Ni,Cu and Zn）,are systematically studied.The effects of different metal combinations on the catalytic activities of oxygen reduction（ORR）and nitrogen reduction（NRR）are investigated.The main conclusions are as follows:（1）Formation energy and binding energy calculations confirmed that the diatomic catalyst has strong stability,and molecular dynamics simulations also confirmed this.The charge distribution and partial density of states（PDOS）confirm the stability of the catalysts and metal atoms are the possible active sites.And the calculation of the energy band structure confirmed that the incorporation of the metal atom is usually beneficial to improve the conductivity.（2）Theoretical study on oxygen reduction reaction:the adsorption configuration and adsorption energy of ORR intermediates were calculated by DFT,and the free energy change data based on the four basic steps of ORR were obtained.Electronic structure calculations show that the pairing between different metal atoms changes the d-band center,which in turn modulates the catalytic performance of ORR.Some metal dimers（Fe-Fe,Fe-Zn,Co-Zn and Zn-Zn）exhibit excessive adsorption capacity,resulting in OH groups remaining on the substrate as ligands to form new catalysts,significantly improving the catalytic activity.The linear relationship of the free energy of adsorption of oxygen-containing species is calculated and the classic volcano diagram is derived.Among them,Co Zn OH@NC shows the best catalytic performance.（3）Theoretical study of nitrogen reduction reaction:Partial density of states analysis confirmed that the unoccupied and occupied d orbitals of metal atoms are the key to effective N₂ activation.Among the 15 metal dimers,Fe-Fe and Co-Co can selectively adsorb and activate N₂.Their limiting potentials are-0.44 and-0.45 V,which are superior to most catalysts and also superior to similar single-atom catalysts.They can well inhibit the hydrogen evolution reaction（HER）.Moreover,the NH₃ desorption free energies of Fe-Fe and Co-Co are 0.54 and 0.57 e V respectively,which ensure the good cycle durability of the catalysts.