Impact Of Active-Site Density On Oxygen Reduction Reactions Using Monodispersed Fe-N-C Catalysts

Single-atom catalysts(SACs)have attracted intensive attention due to merits of high catalytic activity and maximized atomic utilization.When the particle size is reduced to the atomic level,the highly exposed surface often holds high surface free energy.Single-based atoms could be easily diffused to form aggregates,so the loading of metal atoms in the experiment is very low and the distance between adjacent active sites is relatively far(>>lnm).Normally,theoretical studies treat the active sites as isolated centers assuming that they operate independently from each another.However,since low density SACs are ultimately difficult to meet the needs of practical applications,extensive studies have been focused on developing highly dense atomically dispersed catalysts.With the development of advanced fabrication and characterization techniques,the density of active sites increases rapidly and the distance between each other is getting closer and closer.In many cases,the neighboring interactions between adjacent SACs active sites cannot be ignored,as demonstrated by many recent works.Therefore,it is very important to investigate the interactions between adjacent sites and their influence on the catalytic reaction,so as to optimize the activity of single-atom based catalysts and strengthen the mechanistic understanding of heterogeneous catalysts by manipulating such neighboring effect.Proton exchange membrane fuel cells(PEMFCs),the devices that convert chemical energy into electrical energy through the reaction of hydrogen and oxygen to generate water,are of key importance for developing a sustainable economy.However,the slow reaction rate of oxygen reduction reaction(ORR)at the cathode has severely hindered the development of PEMFCs.Recent Research has found that monodispersed transition metal(TM)anchored in two-dimensional nitrogenated carbon materials(often referred to as TM-N-C)have emerged as promising alternatives to precious-metal-based ORR catalysts due to the excellent ORR activity,environmental friendliness,long life and low cost.In this work,we have performed density functional theory(DFT)study on the interaction between Fe-N-C SAC active sites with varying distribution densities and its influence on ORR performance.Based on the graphene supported FeN3 SAC models,our DFT simulations have revealed that the thermodynamic limit potential(UL)of FeN3@Gr is strongly dependent on the inter-site distance and the coverage of the reactants.In comparison,the neighboring effect on ORR is appreciably weaker in the FeN4@Gr systems,except in the case of extremely highly dense FeN4 distribution(Fe-Fe distance is 4.1 A).Further,under certain Fe-Fe distance conditions,the SAC-SAC interactions would induce efficient charge transferring between the reactive species and the graphene support,which promotes the physisorbed-O2-initiated two-electron(2e')pathway toward a better ORR performance.Our investigations not only provide new insights into fundamental mechanisms of SAC systems,but also help to guide the experimental preparation of high-density SACs.