Preparation Of Transition Metal Nitrogen-carbon Catalysts By In-situ Polymerization Of Pyrrole Using Template Method And Study Of Their ORR Performanc

With the overuse of fossil fuels,global environmental problems are becoming more serious.Hydrogen fuel cells are considered the most potential new energy devices to replace traditional energy due to their advantages,such as low operating temperature,fast startup,convenient operation,and greenness.However,the slow kinetics of the cathode oxygen reduction reaction（ORR）reduces the conversion efficiency of fuel cells and restricts the development of hydrogen fuel cells.Currently,Pt-based catalysts are used in commercial hydrogen fuel cells due to their excellent ORR catalytic performance.However,the scarcity and high price of Pt resources seriously hinder the large-scale commercial application of hydrogen fuel cells.（1）Using MnO₂nanorods and Fe₃O₄microspheres as templates,Mn-N-C nanorod catalysts and Fe-N-C nanomicrosphere catalysts were successfully prepared.Through the ORR electrocatalytic performance test and compared with the Pt/C catalyst,the results show that the half-wave potential E_1/2of the prepared Mn-N-C catalyst is 0.811 V（vs.RHE）,which is close to the half-wave potential of the commercial Pt/C catalyst（0.845V）.In addition,the RRDE test results show that the Mn-N-C catalyst possesses higher four-electron selectivity.（2）Using solid bimetallic oxide MnFe₂O₄as template,MnFe-N-C and Fe Cx/MnFe-N-C catalysts were prepared by controlling the polymerization time.Electrochemical test results show that the avarage electron transfer numbers of bimetallic doped catalysts are about 3.80,indicating that the doping of Mn element can improve the four-electron selectivity of the catalyst,and the presence Fe C_xis beneficial to the ORR catalytic performance of the catalyst.（3）The hollow bimetallic oxide H-MnFe₂O₄microspheres were used as templates to prepare catalysts with hollow spherical structures,and the H-Fe C_x/MnFe-N-C and H-MnFe-N-C catalysts with different compositions were obtained.The H-Fe C_x/MnFe-N-C catalyst with the best performance has a half-wave potential of 0.87 V（vs.RHE）and a limiting current density of 4.95 m A·cm^-2,which are better than commercial Pt/C catalysts.By testing the LSV curves at different rotational speeds and calculating by the K-L equation,the average electron transfer number for the ORR reaction of the H-Fe C_x/MnFe-N-C catalyst is 3.99.By comparing the Tafel slopes of different samples,the result shows that the construction of the hollow sphere structure can significantly accelerate the kinetics of the ORR reaction.The results of this work show that the introduction of Mn element can significantly improve the 4-electron selectivity of the ORR reaction.In addition,this method also provides a new idea for the preparation of transition metal-based catalysts in the future.