N Doped Carbon Supported Single-atom Catalyst:Preparation And Electrocatalytic Performance

Fuel cells（FCs）are clean and efficient energy storage and conversion devices because they can convert chemical energy into electric energy efficiently.However,the cathodic oxygen reduction reaction（ORR）in fuel cells requires catalyst to reduce the energy barrier and accelerate the reaction due to its slow reaction kinetics.Despite its excellent catalytic activity,the low abundance,high price,and insufficient stability of commercial Pt/C catalyst limit the large-scale application of fuel cells.Therefore,the development of cheap catalysts with high activity and stability has become the key to overcome the above difficulties.Carbon based single atom catalysts（C-SACs）are considered to be one of the most promising catalysts for large-scale application of fuel cells due to their excellent properties such high activity,conductivity and selectivity.High temperature pyrolysis is one of the most common methods.However,the excessively high temperature accelerates the aggregation of metal atoms,which leads results in the unstable existence of metal single atoms.At the same time,heteroatomic organic precursors decompose readily at high temperature,which will reduce the yield of catalyst.Molten salt means the inorganic salt system that exist in molten form under certain temperature.The strong polar environment provided by molten salt can strongly inhibit the aggregation of metal atoms.At the same time,the molten salt can well dissolve the organic molecular precursors,inhibit the thermal decomposition.Based on the above discussion,the molten salt assisted pyrolysis method was applied to the preparation of C-SACs to in this paper in order to obtain C-SACs with excellent catalytic activity and stability.We prepared N,Fe doped carbon supported catalyst materials Fe-NC-x in one step method by using Zn Cl₂ and KCl as molten salt system and 6 kinds of heterocyclic organic molecules as precursors respectively.And compared with Fe-NP-x,which was prepared without adding molten salt in order to explore the feasibility of molten salt assisted pyrolysis method in the preparation of single atom catalyst.TGA results show that the residual mass of 6-Aminoquinoxaline increase from 0 to 60%,indicating that the addition of KCl/Zn Cl₂ inhibit the thermal decomposition of small molecules effectively.XRD results show that KCl/Zn Cl₂system can inhibit the aggregation of Fe atoms,which makes it possible for the metal to disperse as the form of single atoms.KCl/Zn Cl₂ salt system brings the higher specific surface area for Fe-NC-x(the maximum is 2092.9 m² g^-1).The synthesized Fe-NC-x catalysts exhibit good catalytic activity when applied to the catalytic process of ORR.We successfully prepared Fe single atom catalyst（Fe SA-NC）by pyrolyzing hypoxanthine in Zn Cl₂/KCl systems.The content of Fe is 0.57 wt%according to ICP test.The existence of Fe single atom in the form of O₂-Fe-N₄is verified by AC-HAADF-STEM and XAFS analysis.The surface area of Fe SA-NC reaches 2072.5 m² g^-1,which proves the pore-forming action of Zn Cl₂ in the molten salt system.Fe-SA-NC has excellent ORR catalytic activity,the half-wave potential reached 0.85 V and the limiting current density reached 5.79 m A cm^-2,both are higher than that of 20 wt%Pt/C.Fe SA-NC also has good electrochemical stability and methanol resistance.Through characterization analysis,it is concluded that the excellent ORR activity of Fe SA-NC not only comes from the introduction of Fe single atom,but also comes from the large surface and multi-pore carbon carrier,which can fully expose the active sites and accelerate the mass transfer process rapidly.