Preparation Of ZIF-8-derived Iron/cobalt-based Nitrogen-doped Carbon Catalyst And Oxygen Reduction Performance

As an efficient and environmentally-friendly new energy technology,proton exchange membrane fuel cells（PEMFCs）are widely used in transportation and portable mobile power sources.The key to limiting the efficiency of PEMFCs is the oxygen reduction reaction（ORR）process.The widely used Pt-based precious metal catalysts have the disadvantages of high cost,easy poisoning and poor stability.Therefore,the development of high-performance and high-stability catalysts has always been a key issue in the commercial application of PEMFCs technology.In order to solve the above problems,this thesis deeply explored ZIF-8-derived iron/cobalt-based nitrogen-doped carbon catalysts,and prepared Fe_x@Co-N-C catalysts and Co_x-Fe-N-C catalysts.So as to improve the activity of Co-N-C catalysts,the second transition metal Fe is introduced during the preparation of Co-N-C catalyst.We firstly prepare Zn Co-MOF,which can be obtained by reaction of 2-methylimidazole,cobalt nitrate and zinc nitrate,the molecular size of ferrocene and the cavity size of Zn Co-MOF are more suitable,so ferrocene（Fc）is selected as the iron source,dissolved in the reaction solution,the Fc can be encapsulated in the Zn Co-MOF cavity during the growth of Zn Co-MOF,thereby obtaining a new structure of the Fc@Zn Co-MOF precursor.After heating,Fe-doped Co-N-C catalyst（Fe@Co-N-C）was prepared.The unique nano-restriction effect of Zn Co-MOF can make Fc highly dispersed in the Fc@Zn Co-MOF precursor.After high temperature calcination,Fe atoms can form monodisperse Fe-NC active centers instead of agglomerating into Fe particles.In addition,the volatilization of Zn atoms at high temperature has the effect of creating pores and defects,so that the Fc@Zn Co-MOF precursor can obtain a porous carbon material after heat treatment.This material has the characteristics of high specific surface area,the porous carbon skeleton with a high specific surface area can expose more Fe/Co reaction active sites and improve the mass transfer of the intermediates in the ORR process,thereby improving the reduction activity of catalyst.Fe-doped Fe@Co-N-C catalyst has excellent ORR activity.The half-wave potential of catalyzing ORR in 0.1 M HCl O₄ solution reaches 0.791 V,which is 12 m V higher than that of undoped Co-N-C catalyst.So as to improve the stability of Fe-N-C catalysts,the second transition metal Co was introduced during the preparation of Fe-N-C catalyst.We firstly prepare Fc@ZIF-8,which can be obtained by reaction of 2-methylimidazole,zinc nitrate and errocene dissolving in the reaction solution,thereby obtaining Fc@ZIF-8 precursor,Co-doped Fe-N-C catalyst can be obtained by introducing cobalt nitrate during the preparation process.After the introduction of Co,Co2.5-Fe-N-C showed ORR activity comparable to Fe-N-C,but had better stability.After 20,000 cycles,the half-wave potential and current density only decreased by 19 m V and 21%.