ZIFs Derived Iron-and Nitrogen-codoped Carbon Electrocatalyst For Oxygen Reduction Reaction

Fuel cell,as a type of energy conversion devices that directly converts chemical energy into electrical energy,which has large application prospects in the fields of electric vehicles and portable power supplies due to its high efficiency and cleanness.However,fuel cell cathode adopts the commercial noble-metal platinum(Pt)-based catalyst to catalyze oxygen reduction reaction(ORR),The high cost and limited reserve of Pt-based catalyst still hinders the large-scale application of fuel cells.Therefore,it is of great significance to develop low-cost and high-performance non-precious metal catalysts.Metal organic frameworks(MOFs),which are composed of periodically distributed metal atoms and organic linkers and have permanent porosity and large specific surface area,have emerged as a new platform for the synthesis of transition metals and nitrogen co-doped carbon(TM-N-C)non-precious metal electrocatalyst.However,structural collapse may be occurred during high temperature pyrolysis of MOFs precursor,resulting in the demage of pore structure with severe decrease in specific surface area,and the derease of effective active sites that part of active sites were embedded in the carbon matrix.These limit the further improvement of ORR catalytic activity for TM-N-C.Herein,we designed and prepared two new types of MOFs precursors by means of the coordination effect between metals and ligands,consisting of bimetallic zeolitic imidazolate frameworks(ZIFs)with core-shell structure and a well-defined metal composition gradient,respectivley.By pyrolyzing these ZIFs precursors,a series of high-performance iron and nitrogen co-doped carbon electrocatalysts with aboudant pore structure and high utilization rate of active sites were obtained.Furthermore,the effect of composition and structure of ZIFs on ORR performance were also clarified.Firstly,the core-shell structured ZnO@Fe-ZIF synthesized by using ZnO as core that was prepared by one-step oxidation of ZIF-8,followed by the epitaxial growth of 2-Methylimidazole(MeIm)and Fe to obtain the Fe-ZIF shell.Then,the iron-and nitrogen-co-doped carbon electrocatalyst(Fe-N-C)was prepared through one-step pyrolysis.Results showed that the volatilization of a large number of Zn in ZnO core at high temperatre can create abundant pores.The resultant Fe-N-C possesses a specific surface area(SSA)of 180.67 m2·g-1 with a high percentage of mesoporous SSA(73.2%),which is benefit for the mass transport of ORR reactants and product.Meanwhile,the ZnO core can promote the uniform pyrolysis of the Fe-ZIF shell that favors to achieve homogeneous dispersion of Fe-N4 active sites.Electrochemical tests showed that,the optimized Fe-N-C electrocatalyst exhibits the highest ORR catalytic activity among all these samples as the molar ratio of Fe/MeIm is 1/30,with the half-wave potential(E1/2)of 0.893 V(50 mV higher than that of the commercial Pt/C)in 0.1 M KOH electrolyte and E1/2 of 0.74 V in 0.1 M HClO4 electrolyte.Also,the electrochemical durability of Fe-N-C is better than that of Pt/C.In order to further improve the ORR activity especially in acidic electrolyte by optimizing the pore structure and increasing the utilization of active sites of Fe-N-C electrocatalyts,we have firstly designed and synthesized a new class of bimetallic zeolitic imidazolite framework(Fe,Zn-ZIF)materials with a cylindrical morphology and a well-defined Fe/Zn composition gradient through one-step hydrothermal reaction of Fe salt,Zn salt and benzimidazole.The Fe/Zn gradient in the Fe,Zn-ZIF cylinders—with a Zn-rich core and a Fe-rich shell—results from a balance of the kinetics and thermodynamics of Fe-and Zn-benzimidazole coordination reactions.Pyrolysis of Fe,Zn-ZIF results in evaporation of Zn from the core of the cylinders generating an inter-connected hierarchical microporous(0-2 nm)and mesoporous(2-10 nm)structure,and the concomitant formation of highly exposed and accessible Fe-N4 sites on the surface of the resulting carbonized material.Results show that when the hydrothermal time is 24 h,the Zn/Fe molar ratio is 20,and the pyrolysis temperature is 1000?,the as-obtained iron-and nitrogen-codoped carbon material outperforms a commercial Pt/C in alkaline electrolyte(with a E1/251 mV higher)and possesses comparable activity to that of Pt/C in acidic electrolytes(with a E1/2?50 mV lower).Furthermore,the electroccatalyst is also superior to Pt/C in both acid and alkaline electrolyte in electrochemical durability and resistance to methanol poisoning.