Multi-dimensional Fe₃C@FeN₄C Catalysts: Preparation And Their Oxygen Reduction Properties

Fuel cell and metal-air cell industries are in the ascendant.However,the inherent retardation kinetics of（ORR）in oxygen reduction reaction on the cathode restricts the process of its commercial application.At present,the most effective ORR catalysts are platinum（Pt）based catalysts.Unfortunately,their low cost performance,poor stability and poor methanol resistance limit their large-scale application..Therefore,it is of more practical significance to develop non-precious metal catalysts with high activity and high stability.Based on ionic liquids（ILs）has the advantages of low volatility,high conductivity,excellent thermal stability and adjustable structure of anions and cations.In this paper,ionic liquids with different structures were selected as precursors of carbon sources,and metal Fe sources were introduced by electrostatic self-assembly.Fe₃C/FeNC-16 catalysts with two-dimensional"anisotropic"layered structure and three-dimensional cross-linked tunnel structure Fe₃C/FeNC catalysts were successfully prepared.The main research contents are as follows:（1）The long straight-chain cetylpyridinium bromide ionic liquid was selected as the main body of the functionalization and potassium ferricyanide was self-assembled to form the precursor of the catalyst,and then the Fe₃C/FeNC-16 catalyst with two-dimensional"anisotropic"layered structure was prepared by carbonization and etching.Fe₃C/FeNC-16catalyst has abundant graphene edge sites and hierarchical pore structure with coexistence of micropores and mesopores.The activity and stability were tested in 0.1 M KOH alkaline solution.The half-wave potential of Fe₃C/FeNC-16 catalyst can reach 0.88 V,which is 60 m V higher than that of commercial 20%Pt/C catalyst.At the same time,the long-cycle voltammetry test of 2000 cycles shows that the catalyst also has good stability.（2）The ionic liquid（IL）with short branched chain structure was prepared by using 1,2-dibromoethane and 1-vinyl imidazole.The[Fe（CN）₆]^3-structure was anchored by electrostatic interaction,and then the Fe₃C/FeNC catalyst with three-dimensional cross-linked tunnel structure was synthesized by carbonization and etching.The surface of the catalyst has a single atomic structure of FeN₄C and a Fe₃C structure covered by the surrounding carbon layer,which makes the catalyst have excellent electrochemical activity and stability.The half-wave potential of alkaline solution in 0.1 M KOH is 0.85 V;the long-cycle voltammetry test of 2000 cycles shows that the catalyst has good stability.At the same time,it also has good activity in 0.1 M HCl O₄ acid solution,and the half-wave potential can reach 0.70 V.（3）On the basis of experiments,the model of FeN₄C-coated Fe₃C structure is established.Through density functional calculation,it is found that Fe₃C structure can regulate the electronic structure of surface FeN₄C,such as the increase of charge and spin number.The change of these electronic structure is beneficial to the improvement of oxygen reduction activity.The Fe₃C structure can stabilize the surface FeN₄C structure,and the calculated bonding energy is-22.10 e V,so the Fe₃C/FeNC catalyst has high activity and high stability,which is consistent with the experimental results.