Controllable Construction And Efficient Utilization Strategy Of Catalytic Active Sites In Fe-N-C Catalyst

Proton exchange membrane fuel cells（PEMFCs）are green and efficient energy conversion device.The performance of PEMFCs largely depends on the oxygen reduction reaction（ORR）kinetics in the cathode.So far,platinum group metal（PGM）catalyst is the most commonly used cathode catalyst in PEMFCs.However,the high cost,scarcity and poor durability of such catalysts have seriously hindered their wide application.Iron and nitrogen-doped carbon（Fe-N-C）materials have good activity,low cost and structural adjustability at the atomic level and are considered to be the most promising alternative for Pt/C catalyst.However,typical Fe-N-C catalysts suffer from low density of Fe-N₄ active sites.In addition,most of the reported Fe-N-C catalysts lack optimized porous features to ensure full utilization of Fe-N₄ active sites.To solve the problems above,the strategy of strengthening the interaction of N precursors and Fe precursors is proposed for the construction of Fe-N-C catalysts with high density of Fe-N₄ sites.Furthermore,approach for the introduction of desired porous feature enabling efficient utilization of Fe-N₄ sites in Fe-N-C catalysts is developed.The contents of the thesis are summarized below.（1）Guest ionic liquid molecule（IL,[EMIM][NTf₂]）with high N content and excellent thermal stability is encapsulated within the pores of Fe-ZIF-8 host.Favorable interactions between Fe in Fe-ZIF-8 and N in IL promoted targeted generated of Fe-N₄ active sites during the carbonization synthesis.The loading amount of IL and Fe content in Fe-ZIF-8 is systematically optimized.The optimal catalyst developed in this work(Fe_0.05-N-C/10)has a high active site density of 1.88×10¹⁹site g^-1 and excellent electrochemical performance comparable to commercial Pt/C catalyst in both acid and alkaline condition.The PEMFC assembled with Fe_0.05-N-C/10 could deliver a power density of 300 m W cm^-2 uner H₂-O₂ condition（1.5 bar）.（2）Taking advantage of the protection effect of the NaCl salt template,a two-step process for carbonization synthesis of Fe-N-C catalyst with high Fe utilization（Fe to Fe-N₄ yield）is developed.In the first carbonization step,molten NaCl enters into the pores of Fe-ZIF-8 and coats onto the surface of Fe-ZIF-8 skeleton,which effectively reduces the loss of N and undesired generation/aggregation of Fe compounds,thereby improving the Fe utilization.By optimizing the ratio of Fe-ZIF-8to NaCl,high performance Fe-ZIF-0.5 catalyst with active site density of 4.9×10¹⁹site s^-1 is obtained.The Fe utilization in Fe-ZIF-0.5 reaches up to～50%.Structural analysis shows that the catalyst has high specific surface area and abundant pore structure,which provide efficient pores for oxygen and proton transport.Secondly,the abundant microporous structure can accommodate more active sites.Besides,such porous features improve the accessibility of catalyst active sites,which increases the utilization of active sites of the catalyst.Electrochemical tests show that the Fe-ZIF-0.5 catalyst exhibits electrocatalytic ORR activity comparable to that of commercial Pt/C catalysts.