Investigation On The Designing And Synthetic Technology Of Single-atom M-N-C Catalysts For ORR

Metal fuel cells（metal-air batteries）are one kind of environmental-friendly power sources with high energy densities.The oxygen reduction reaction（ORR）catalysts in their cathodes exert significant influences on the performance of metal fuel cells.In this work,transition metal-nitrogen-carbon（M-N-C）single-atom active sites with outstanding ORR catalytic activities were designed and screened based on Mn and Fe through DFT computations.The N/C coordination effects on the d bands of the metal atoms,the relations between the d bands and the corresponding ORR performance were investigated.Based on the optimized Fe-N₈ active site,column-like nanowire catalysts with highly efficient ORR catalysis and durability were synthesized on graphene membranes through the organic molecular tailoring-self-assembling-rebuilding vapor deposition（TSRVD）method.SEM,TEM,XRD,FTIR,Raman spectra,XPS and TG characterizations were performed to study the morphologies and structures of the nanowire catalysts.LSV,CV,Tafel,EIS,etc.electrochemical measurements were used to investigate their corresponding electrocatalytic activities and durabilties.A series of models based on Mn/Fe-N-C multi-coordination sites embedded in carbon six-member rings were established.Density functional theory（DFT）calculations were performed to investigate their corresponding electronic structures and ORR catalytic activities.The computational results reveal that a volcano relationship was found between the ORR activities and the*OH adsorption free energies.Particularly,five-coordination（Cyan）Mn-N₄/D is calculated positioned near the apex of the volcano relationship,exhibiting an theoretical equilibrium potential of0.90V,which is even higher than that of Pt（111）（0.89V）.In addition,the eight-coordination Fe-N₈/D site exhibits a similar equilibrium potential（0.88V）compared with Pt（111）.Electronic structure calculations reveal that the d-band structures of metal centres could be tuned by their surrounding coordinates,it is more efficient for the 3-dimensional sites compared with the 2-dimensional sites on modificating the d bands of transition metals,which will further determine their ORR activities.Through Chemical vapor deposition（CVD）technique,high-quality single-layer graphene membranes were synthesized on electrodeposited copper foils which were used as the growth substrates.The results reveal that the lattice defects and layer number of graphene can be modified by elaborately varying the synthesis parameters:the increase in the electroplating current density,thickness of copper foils and the decrease in the CH₄:H₂ flowing ratios lead to fewer defects and layer number of graphene.Prolonging the CVD deposition time leads to larger layer number while exert little effects on the defects.Increasing the thermal treating time of copper foils will significantly decrease the layer number of graphene while exert subtle influence on the corresponding defects.Focusing on the theoretical Fe-N₈ catalytic structure,Fe Pc and monolayer graphene were used as the precursor and growth substrate,respectively.3-dimensional nanowire-structural ORR catalysts were synthesized through TSRVD process at450℃.The results reveal that Fe Pc molecules undergo molecular tailoring,self-assembling,rebuilding process and finally grow into 3-dimensional structures with possible Fe-N₈ active sites.Moreover,the 30min-deposited nanowire catalyst possesses the same onset potential with Pt/C（0.93V vs.RHE）,while a higher ORR current density of 3.9 m A·cm^-2 at 0.85 V vs.RHE(Pt/C,3.05 m A·cm^-2).The new catalyst also exihibit excellent durability in 0.1M KOH aqueous solution,which exhibits only 6%decrease in the ORR current density at 0.85 V vs.RHE after 10000s discharging process compared with Pt/C（almost 20%）.Electrochemical analyses indicate that the ORR process of the synthesized catalyst is definitely a high-efficiency 4-electron transfer pathway.