Preparation And ORRerformance Investigation Of Double Transition Metals Co-doped Carbon-based Catalysts Derived From MOF

The proton exchange membrane fuel cells（PEMFCs）have been widely studied for its advantages of environmental friendliness and low temperature operation.Currently,in order to solve the problem of slow kinetics of its cathodic oxygen reduction reaction（ORR）,Pt-based catalysts are widely used in PEMFCs,but high cost and low reserves of Pt-based cathodic oxygen reduction reaction（ORR）catalysts seriously hinder the large-scale commercial development of the PEMFCs,so it is of great practical significance to study and develop non-Pt catalysts with high activity,low cost and long life.However,the design and synthesis of non-noble metal ORR catalysts with high activity and stability is still a great challenge.In this paper,we mainly from the point of view of improving the ORR activity and stability of M-N-C catalysts in acidic media,ZIF-8（Z-8）as carbon-nitrogen precursor,using its high specific surface area and high porosity,a series of highly dispersed bimetallic nitrogen-doped carbon-based catalysts（M₁/M₂-NC,m=Fe,Mn,Co）derived from Z-8.We systematically investigated the effects of metal doping content,heat treatment temperature,heat treatment time and nitrogen source on the ORR properties of the synthesized catalysts,combined with the physical properties characterization of X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,specific surface area and pore size（BET）,X-photoelectron spectroscopy（XPS）,Raman spectrum（Raman）to study the relationship between morphology,structure,composition and properties,the following are the main contents of the study:（1）Using Z-8 as carbon-nitrogen precursor,Fe and Mn salts were adsorbed under hydrothermal conditions,we obtained a dual-metal catalyst Fe/Mn-NC-50 derived from Fe and Mn co-doped Z-8 after high temperature heat treatment.When m _Fe/m _Mn=10/1,the obtained Fe/Mn-NC-50 catalyst is a hollow rhombic dodecahedron structure,with the best electrochemical performance,the half-wave potential was 0.814 V（vs.RHE,0.1 M HCl O₄）,only 21 m V less than commercial Pt/C.After 5000 potential cycles the half-wave potential losses only 17 m V,but Pt/C lost 67 m V.After chrono-amperometry test of 40000 s the relative current density remained at 76.15%,Pt/C reduced to 34.4%.RRDE shows,Fe/Mn-NC-50catalyst yields low hydrogen peroxide,close 4 e^-reaction paths.Although only a small amount of Mn added,it effectively enhanced the oxygen reduction activity of the Fe-NC catalyst.（2）With high specific surface area and porosity of Z-8 as the host,adopting the strategy of host-guest,Co and Fe(n(_Co+Fe)/n _Zn=1/6)were introduced into the channel of Z-8 under hydrothermal conditions.After high temperature heat treatment,bimetallic doped Co/Fe-NC catalysts with different Co and Fe ratios were obtained.From the composition point of view,investigated the Co/Fe ratio,on the effect of catalyst morphology,structure and electrochemical properties in acidic medium.The results show that when the amount of Fe is reduced,n._Co/n _Fe=9/1,Co_0.9/Fe_0.1-NC catalyst has complete rhomboid dodecahedron shape,and has the best electrochemical activity.The half-wave potential of Co_0.9/Fe_0.1-NC catalyst can reach 0.814 V（vs.RHE,0.1 M HCl O₄）.After 5000 cycles of CV test,the half-wave potential of Co_0.9/Fe_0.1-NC catalyst decreases only 21 m V.The relative current of after 40000s test is 88.56%.The RRDE test results showed that,compared with the single metal nitrogen doped carbon catalyst,the catalytic process was close to the 4e^-reaction path by using the synergistic effect of Co/Fe,which greatly reduced the hydrogen peroxide production rate and greatly improved the catalytic activity and stability of the catalyst.（3）Based on the Fenton effect of removing Fe element,the Fe-free catalyst Co/Mn-NC was prepared by chemical doping and physical adsorption.Co-NC catalysts with different Co content were prepared by introducing Co in situ into Z-8 by chemical doping method.The morphology,structure and activity of the catalysts were studied.When Co/Zn=1/4,the oxygen reduction activity of the catalyst was the highest,and the half-wave potential reached0.73 V（vs.RHE,0.1 M HCl O₄）.On this basis,Mn metal was introduced by physical adsorption method to increase the active site density of the catalyst.When the adsorption amount of Mn was 2.5 mg,the oxygen reduction activity of the catalyst was the best(E_1/2=0.78 V).