Preparation Of Iron-based Compounds/Carbon Nanocomposite Catalysts For Electrochemical Oxygen Reaction Studies

Zn-air batteries,which convert chemical energy into electrical energy through electrochemical process,are a clean,efficient and considered the most promising energy conversion device.However,the oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）at the air cathode suffer from slow kinetics,which limit the further development of Zn-air batteries.So far,noble metal（Pt,Ru,Ir）and its alloy catalysts have been regarded as the most efficient oxygen reaction catalysts,but they still suffer from low reserves,high cost,and poor stability.These problems also limit the development of Zn-air batteries,so it is urgent to explore non-precious metal catalysts that can replace noble metal catalysts.Although the introduction of transition metals,especially iron-based metals into carbon materials,can significantly improve the activity and stability of the catalyst,the research on its catalytic mechanism is not enough.This work combines iron-based metals and carbon materials to synthesize oxygen reaction catalysts,and uses theoretical calculations combined with a series of characterization methods to deeply analyze the source of catalyst activity,which provides a new research idea for the design of iron-based catalysts.The specific research contents are as follows:A composite of metallic Fe and Fe₃C confined in N-doped carbon nanotubes（Fe-Fe₃C-NCT）via a pyrolysis strategy,which can be used as ORR electrocatalysts.DFT calculations show that the interaction between Fe and Fe₃C can enhance the electron density of the central metallic iron species,resulting in stronger O₂ adsorption and faster ORR kinetics.In addition,the oxygen intermediates are more easily dissociated at the Fe-N_xsites formed by the coordination of metallic Fe and N.Electrochemical tests show that Fe-Fe₃C-NCT has a high onset potential（0.99 V）,a large limiting current density(8.00 m A cm^–2),and excellent stability（negative shift of half-wave potential by 4 m V after 10,000 cycles）,better than Pt/C.It was assembled into a primary Zn-air battery,which exhibited a power density of 195 m W cm^–2 and power density of 840 m Ah g^–1 at 10 m A cm^–2,which was much better than that of Pt/C(123m W cm^–2,647.7 m Ah g^–1).A catalyst with CoNi alloy nanoparticles embedded in bamboo-like N-doped carbon nanotubes（CoNi-NCT）was obtained by pyrolysis of a mixture of CoNi Prussian blue analogs and melamine.Density functional theory calculations indicate that oxygen molecules are more easily adsorbed on the Ni sites of these catalysts,while Co site can reduce the energy barrier of the rate-determining step,which is conducive to the rapid occurrence of ORR.In addition,the combination of CoNi alloy and N-doped carbon favors electron transfer and promotes electrocatalytic activity,and CoNi-NCT shifts the half-wave potential positively by 70 and 80m V compared with single-metal catalysts.CoNi-NCT and NiFeP/CC assembled into a Zn-air battery,the power density(167 m W cm^–2)and stability（17%decay after 300 hours of working）were better than those of noble metal materials(127 m W cm^–2,18%decay after 60 hours of working).FeNi Prussian blue analogs formed FeNi alloys during pyrolysis,which subsequently catalyzed melamine to form N-doped carbon nanotubes（FeNi-CNT）embedded in FeNi alloys.Theoretical calculations indicate that the interaction between metals in FeNi alloys leads to a positive shift of the center of the d-band,which helps to optimize the adsorption energy of the active sites for oxygen-containing intermediates.The site promotes the rapid progress of OER,which is assembled into a Zn-air battery with no degradation in battery performance after 900cycles.The lamellar arrays of iron-nickel bimetallic MOFs（MIL-53（FeNi）/NF）were synthesized by solvothermal formation of NiFe hydrotalcite as a template on nickel foam.XPS and ICP showed that during the formation of MOF,Ni ions released in situ from nickel foam in acidic solution together with Ni and Fe on hydrotalcite act as metal centers,which are organically linked to terephthalic acid through strong coordination bonds.Molecular connection,the formed MIL-53（FeNi）/NF has high valence Ni,and the higher valence Ni atoms have stronger electron accepting ability,which is beneficial to accelerate the charge transfer process between OH–and MIL-53（FeNi）,optimized the activity of OER.At 200 m A cm^–2,the potential of MIL-53（FeNi）/NF is only 1.48 V,and the performance is not degraded after 100 h of continuous operation.Combining MIL-53（FeNi）/NF with ORR catalyst（Fe-Fe₃C-NCT）as the cathode of Zn-air battery can work continuously for 500 h without performance degradation.