Controlled Synthesis And Electrocatalytic Performance Of Carbon Nanotube-based Bimetallic Monoatomic Catalysts

Hydrogen evolution reaction（HER）and oxygen reduction reaction（ORR）,which are key electrochemical processes in water electrolysis cell and fuel cell as clean,efficient and sustainable energy conversion devices,must rely on efficient and stable electrocatalysts.Catalysts dominated by Pt group metals are still the most efficient HER and ORR catalysts.However,the high cost and scarcity of precious metals hinder their application.Therefore,it is urgent to develop low-cost and highly active HER and ORR catalysts.At present,metal monatomic catalysts show excellent electrocatalytic performance with unique electronic structure and maximum atomic utilization,and greatly reduce the cost,which is expected to replace the high cost of noble metal catalysts.At the same time,bimetallic monatomic catalyst can further enhance the electrocatalytic performance through the synergistic action between metals.However,the preparation process of most of the reported metal monatomic catalysts is complicated,the conditions are harsh,and toxic gases will be released.Therefore,it is urgent to develop new synthesis strategies that are simple,quick,mild,and non-toxic.In addition,the most developed monatomic catalyst are in the form of powder,the existence of complex electrode preparation process,catalyst is easy to fall off from the electrode surface,and active site cannot be fully exposed,the poor electronic conductivity,therefore in urgent need of development of three dimensional self supporting metal monatomic catalyst to solve existing problems powder catalyst.In order to solve the above problems,a novel cyclic voltammetry oxidation-reduction electrodeposition method was developed in this paper,and a novel three-dimensional self-supported carbon nanotube-based bimetallic monatomic catalyst was constructed for efficient electrocatalysis of HER and ORR.The main research contents and results are as follows:1.The nitrogen-doped carbon nanotubes（Co@Co SA-NCNTs/CC）embedded with Co nanoparticles were obtained by using the basic cobalt nitrate nanosheets（Co LNH）grown in situ on the surface of carbon cloth by low-temperature solvothermal method as metal source and catalyst,and dicyandiamide as carbon and nitrogen sources during high-temperature pyrolysis under the protection of nitrogen.A novel three-dimensional self-supported nitrogen-doped carbon nanotube-based Co and Ir monatomic catalyst（Co@Co-Ir SAs-NCNTs/CC）was constructed by anchoring Co and Ir monatomic atoms by cyclic voltammetry electrochemical oxidation-reduction deposition strategy.The phase,morphology,structure,composition,surface defects and electronic states of the catalysts were systematically analyzed by MEANS of XRD,SEM,TEM,STEM,Raman spectroscopy and XPS,and the electrocatalytic performance of the catalysts in acidic and alkaline media was evaluated.The results show that,due to the three-dimensional open porous structure formed by interlacing between carbon nanotubes,excellent electron transport performance,abundant surface defect active sites,high density monatomic active centers,electron coupling between Fe and Ir atoms and synergistic interaction between components,Co@Co-Ir SAs-NCNTs/CC-4000 catalyst showed excellent HER activity and stability.2.On the basis of the first part of the study,Co was extended to other transition metals（M,M=Fe,Co,Ni）,and a three-dimensional self-supported M,Ir bimetallic monatomic electrocatalyst M@M-Ir SAs-NCNTs/CC was constructed.The electrocatalytic performance of the three dimensional self-supported M,Ir bimetallic monatomic electrocatalyst was systematically studied using ORR reaction as a model reaction.The effects of the types of transition metals and the number of cycles of cyclic voltammetry electrochemical deposition on the performance of M@M-Ir SAs-NCNTs/CC electrocatalytic ORR were investigated.Fe@Fe-Ir SAs-NCNTs/CC-4000,which was deposited by 4000 cycles of electrochemical oxidation-reduction treatment,showed the best ORR performance,its half-wave potential and limiting current density were comparable to Pt/C catalyst,and its methanol poisoning resistance and stability were better than Pt/C catalyst.The crystal phase,morphology,structure,composition,surface defects,electronic structure and charge transfer properties of M@M-Ir SAs-NCNTs/CC catalysts,especially Fe@Fe-Ir SAs-NCNTs/CC electrocatalysts,were systematically studied by various characterization methods.Combined with the characterization and test results,it can be seen that the excellent ORR performance of Fe@Fe-Ir SAs-NCNTs/CC-4000 is mainly attributed to the optimization of the electron configuration of the active center by the electron coupling between Fe and Ir.The reduced reaction energy barrier and more carbon defects anchor a large number of metal single atoms and increase the exposure of non-metallic active centers and metal active centers.3.A sulfur-nitrogen co-doped carbon nanotube-based Fe,Ir bimetallic mono-atom catalyst（Fe3C@Fe-Ir SAs-NSCNTs/CC）was constructed by introducing a second non-metallic heteroatom S into the 3d self-supported nitrogen-doped carbon nanotube and replacing Fe nanoparticles with Fe3C.The crystal phase,morphology,structure,surface defects and electron transfer state were also studied.The effect of cyclic voltammetry on the ORR performance of Fe₃C@Fe-Ir SAs-NSCNTs/CC catalyst was investigated.The results showed that the ORR performance of Fe₃C@Fe-Ir SAs-NSCNTs/CC-4000catalyst,Its half-wave potential and limiting current density are 0.935 V vs.RHE and4.24 m A cm^-2,respectively.Better than Pt/C catalyst(0.875 V vs.RHE and 4.06 m A cm^-2)and Fe@Fe-Ir SAs-NCNTs/CC-4000 catalyst(0.884 V vs.RHE and 4.20 m A cm^-2).Combined with the characterization results and ORR performance test results,S doping further increases the number of non-metallic defect active sites in the tube wall,and anchors more Fe and Ir single atom active sites,and synergies between Fe and Ir single atom,N and S heteroatoms and Fe3C nanoparticles.Furthermore,the electronic structure and geometric structure of the active site were optimized to further improve the catalytic performance of ORR.