Design And Modification Investigation Of High-Performance Fe/Co/Ni-Based Oxygen Catalysts

In order to solve the energy crisis and environmental concerns,it’s necessary to search for cleaner and more efficient energy conversion devices,including fuel cells,metal-air batteries,and water electrolysis devices.However,the half-reactions such as ORR or OER in these devices involve multi-step reactions,which demand high overpotentials and require catalysts to accelerate the reactions.Nowadays commercial ORR catalysts are restricted to Pt/C,and OER catalysts are mainly Ir O₂ and Ru O₂,which limit the large-scale application of these devices due to the high cost and low reserves.Therefore,the design of clean,efficient and low-cost alternative catalysts is significant for solving environmental problems and the greenhouse effect.In this paper,we choose iron,cobalt,and nickel-based catalysts due to the simple preparation process,abundant reserves and low cost,including porous iron and nitrogen co-doped porous carbon,perovskite oxide and layered double hydroxide.And we aim to improve the ORR or OER catalytic performance by adjusting the morphology,electronic structure or accelerating interfacial reaction and study the corresponding mechanism.It mainly includes the following three parts:（1）A novel single-atom-involved electrochemical catalyst（Fe₃O₄@Fe NC）was synthesized with PAN as the precursor,ZnCl₂and 200 nm-SiO₂ as the template and Fe Cl₃ as the Fe source by carbonization and acid washing.The half-wave potential is0.890 V and the Tafel slope is 58.8 m V dec^-1.It is demonstrated that the porous structure not only affects the mass transfer and the exposure of active sites,but also influences the nucleation of Fe₃O₄ nanoparticles,the degree of graphitization of the carbon support,the chemical environment of the elements,and the ORR reaction pathway.Density functional theory shows that the loading of Fe₃O₄ nanoparticles on carbon favors O₂adsorption,therefore increases the concentration of reactants and promotes overall activity.（2）We report a novel and simple method to improve the ORR performance and stability of Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃（BSCF）through the engineering of the surface electronic structure with ionic liquid（IL）.The optimized catalyst,BSCF-IL possesses2.7 times intrinsic activity that of BSCF.The half-wave potential is positively shifted by 40 m V and the stability is also improved significantly,which is caused by the strong electronic interaction between BSCF and IL.The electronic interaction triggers the surface amorphization and selective elemental preservation,which is beneficial for the exposure of active sites.Synchrotron-based X-ray absorption spectroscopy and density functional theory calculations reveals the charge transfer from BSCF to IL and the oxidation of surface Co and Fe,leading to the optimized e_g filling,lifted O-2p band center and improved metal 3d-O 2p hybridization,contributing to better ORR performance.（3）The optimized LDH-IL-Br catalyst was synthesized with the addition of[BMIm][NTf₂]via simple hydrothermal process and surface modification with[BMIm]Br.LDH-IL-Br possesses current density up to 369 m A cm^-2at 1.6 V,which is2.4 times that of LDH-IL and 5.9 times that of LDH,indicating that the two-step ionic liquid modification promotes the OER activity significantly and accelerates the escape of O₂ bubble under high potential.XANES and XPS characterization shows IL modification triggers the oxidation of surface Ni and Fe and accelerate the reaction kinetics,promoting OER activity and stability.