Preparation And Performance Of Fe/Co-Based Nitrogen Doped Carbon Catalysts For Oxygen Reduction Reaction

Oxygen reduction reaction(ORR)is the core reaction of energy conversion devices,such as proton exchange membrane fuel cells and Zn-air batteries.However,its sluggish kinetics seriously hinders the large-scale commercialization of energy conversion devices.Thus,there is an urgent demand for efficient and stable electrocatalysts to accelerate the ORR process.At present,platinum and other precious metal-based catalysts exhibit high activity,but are limited by their high cost and severe scarcity.In the long run,developing high-efficiency and cost-effective non-precious metal-based catalysts is the key to solve the above problems.However,at present,the non-precious metal-based catalysts still face the problems of insufficient activity and poor stability.In this thesis,the Co-,Fe-,and bimetallic FeCo-based nitrogen doped carbon catalysts with high performance and high stability were prepared by using dual-nitrogen-source,dual-template and one-step impregnation-pyrolysis strategies,respectively,and the structure-activity relationships and active centers were studied and analyzed.The Co/Co-N-C catalyst was synthesized by dual-nitrogen-source strategy(i.e.1,10-phenanthroline(Phen)and dicyandiamide(DCDA)as nitrogen sources)through a high-temperature pyrolysis route.The influence of nitrogen source on the morphology,composition and structure of the catalyst was studied.It was found that DCDA was beneficial for enlarging the specific surface area of the catalyst.Compared with the introduction of a single nitrogen source,the introduction of dual nitrogen sources could significantly increase the contents of the doped N and contribute to the formation of more Co-N_x moieties.In addition,the ORR activity and stability of the catalysts were further investigated.The half-wave potential(E_1/2)of Co/Co-N-C catalysts under alkaline condition was much higher than that of individual-nitrogen-source derived Co-Phen-C and Co-DCDA-C catalysts,indicating that the dual-nitrogen-source strategy could significantly enhance the ORR activity.The XRD and XAFS results verified the co-existence of both the Co-N_x moieties and metallic Co particles in the Co/Co-N-C catalyst.Furthermore,the experiments and theoretical calculations were performed to study and analyze the active sites of the catalyst.The obtained results displayed that both Co-N_x site and metallic Co particles contributed to the ORR activity.In addition,the Zn-air battery assembled with Co/Co-N-C catalyst showed outstanding discharge performance with a peak power density of 122.5 m W cm^-2.The Fe-N-C catalyst was synthesized by one-step pyrolysis and post-acid treatment using an improved dual-nitrogen-source strategy(i.e.Phen and urea as nitrogen sources).The influence of nitrogen source on the morphology,composition and structure of the catalyst was investigated.It was found that urea was conducive to enhancing the specific surface area of the catalyst.In addition,the introduction of dual nitrogen sources could significantly increase the content of doped N and Fe-N_x moieties.Moreover,the electrochemical properties of the catalysts were studied.The E_1/2 of Fe-N-C catalyst was 0.883 V under alkaline condition,which was higher than that of individual-nitrogen-source derived Fe-Urea-C and Fe-Phen-C catalysts,indicating that the dual nitrogen sources could significantly boost the ORR activity.The EXAFS results revealed that the Fe-N_xsite was the main active structure for ORR.Furthermore,the mechanism of the dual-nitrogen-source strategy on the formation of Fe-N_x active sites was reasonably speculated and analyzed from the microstructure.It was found that the dual nitrogen sources synergically promoted the controllable formation of dense Fe-N_x active sites,thereby enhancing the ORR activity.In addition,the Zn-air battery assembled with Fe-N-C catalyst exhibited outstanding performance in terms of a peak power density of 135.3 m W cm^-2.The hierarchical porous Fe-N-HPC catalyst were designed and fabricated using a dual-template strategy(i.e.MgO and ZnCl₂as templates).The influence of templates on the morphology,pore structure and composition of the catalysts was investigated.The results revealed that the MgO template played a leading role in the formation of catalyst morphology.In addition,the two templates exhibited different pore-forming fuctions.The ZnCl₂ template contributed to the formation of micropores,and the MgO template led to the formation of urtra-macropores.Moreover,the dual templates could significantly increase the density of Fe-N_x sites.At the same time,the constructed hierarchical porous structure was beneficial for the exposure of Fe-N_x active sites.Benefitting from the densely accessible Fe-N_xsites,the Fe-N-HPC catalyst exhibited significantly enhanced ORR activity in both alkaline and acid media with E_1/2 of 0.910 V and 0.800 V,respectively.Besides,both liquid and all-solid-state Zn-air batteries assembled with Fe-N-HPC catalyst delivered outstanding discharge performance.Importantly,the hierarchical porous structure constructed by dual templates greatly improved the mass transfer in both liquid and all-solid-state Zn-air batteries.The bimetallic FeCo-N-C catalyst with dual metal centers was prepared via a one-step impregnation-pyrolysis route.The similarities and differences of individual metal catalysts and dual-metal catalysts in morphology,structure and composition were compared.In addition,the ORR performance of FeCo-N-C,Co-N-C,and Fe-N-C catalysts in acidic and alkaline media was further studied.The FeCo-N-C bimetallic catalyst exhibited enhanced ORR activity and good stability.The XAFS results revealed that the configuration of the bimetallic center is Fe-N₄and Co-N₄.Furthermore,the origin of high activity of the bimetallic center was investigated.The theoretical calculation results showed that ORR intermediates have moderate adsorption strength on the Fe-N₄&Co-N₄ site,indicating that compared to the individual metal center,the Fe-N₄&Co-N₄ dual-metal center can effectively regulate the adsorption of ORR intermediates,thus exhibiting high ORR activity.The free energy diagrams showed that the bimetallic Fe-N₄&Co-N₄ site exhibited a higher limiting potential in both acidic and alkaline media,indicating that the bimetallic Fe-N₄&Co-N₄ site possessed higher ORR activity than the single metal Fe-N₄,and Co-N₄ sites.Both liquid and all-solid-state Zn-air batteries assembled with FeCo-N-C catalyst showed outstanding discharge performance.Remarkably,the peak power density of the liquid Zn-air battery reached up to 196.3m W cm^-2.