Preparation And Electrocatalytic Properties Of Transition Metal (Co, Ni) Doped Carbon Nanowire

With the continuous consumption of non-renewable energy and the increasingly serious environmental pollution caused by fossil energy,it is of great significance to develop energy storage and conversion technologies that can efficiently use renewable and clean energy.Electrocatalytic oxygen evolution reaction（OER）and oxygen reduction reaction（ORR）provide an efficient,economical and clean means of energy production and utilization through the process of oxygen evolution and reduction.These two oxygen catalytic reactions are the key reactions of emerging energy devices such as water decomposition,rechargeable metal air cells and fuel cells.However,due to the influence of multi electron/proton transfer process,the reaction kinetics of OER and ORR is slow,which restricts the energy conversion efficiency.The development of high efficiency oxygen electrocatalysts,especially stable bifunctional non noble metal electrocatalysts,still faces great challenges.Metal organic frameworks have the characteristics of large specific surface area,high porosity,diverse structures and easy functionalization.Meanwhile,the metal source and carbon source in the frameworks have intertwined porous network structures.Taking them as self sacrificial templates,the metal doped carbon nanostructures with various morphologies can be obtained by pyrolysis derivatization,and a bifunctional non noble metal electrocatalyst can be constructed.In this paper,bimetallic doped carbon nanomaterials with one-dimensional nanofibers and two-dimensional layered structures were prepared by controlled carbonization derivatization using Co-Ni bimetallic organic framework as precursor.The samples of the micro morphology and elements were characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM/HRTEM）,electron spectroscopy（EDS）,X-ray photoelectron spectroscopy（XPS）,Raman spectroscopy and nitrogen adsorption.In addition,the OER and ORR performance of the samples were studied by electrochemical test.The specific research contents are as follows:（1）Synthesis of CoNi-BDC nanosheetsUsing layered solution diffusion method,2-D layered CoNi-BDC metal organic framework precursors were prepared with transition metals Co²⁺and Ni²⁺as metal coordination ions,terephthalic acid（BDC）as organic ligands and N,N-dimethylformamide and acetonitrile as organic solvents.Their synthesis route,reaction mechanism and crystal structure were studied and characterized.（2）Synthesis and electrocatalytic properties of CoNi-N-CNFs nanofibersThe CoNi-BDC precursor was mixed with dicyandiamide（DCD）and polyacrylonitrile（PAN）for electrospinning to produce CoNi-BDC based nanofibers（CoNi-N-NFs）.Samples with different Co/Ni content were obtained by changing the ratio of CoNi-BDC precursor to PAN,which were pyrolysis at 800℃in an argon environment for two hours to obtain nitrogen-doped carbon nanofibers embedded with CoNi nanoalloy particles（CoNi-N-CNFs）.The elemental composition,crystal structure,micromorphology and electrocatalytic properties of CoNi-N-CNFs nanofibers were characterized.It was found that the synthetic CoNi-N-CNFs nanofibers have a unique one-dimensional/two-dimensional（1-D/2-D）composite structure,and show excellent OER/ORR bifunctional catalytic performance.In 1 M KOH electrolyte,CoNi-N-CNFs-60 can provide 229 m V of low OER overpotential(current density of 10 m A cm^-2)and high ORR half wave potential（0.823 V）.The zinc air battery assembled with it as air electrode has high energy density and stability.The high performance of nanofiber electrocatalyst is due to its uniformly dispersed bimetallic CoNi-N active center,hierarchical porous carbon structure and their synergistic effect.The combination of high conductivity 1-D carbon carrier and flake CoNi nanoalloy fully exposes the high active sites on the surface of carbon fiber,ensuring effective charge transfer and stability.The hierarchical porous structure of nitrogen doped carbon fiber promotes the permeability of ions and the rapid transport of oxygen,accelerating OER and ORR kinetics.（3）Synthesis of magnetic CoNi@C nanosheets and enhancement of electrocatalytic performance by magnetic fieldUsing dicyanodiamine（DCD）as nitrogen source,CoNi-BDC precursor was carbonized at high temperature to obtain CoNi@C nanosheets.The elemental composition,morphology,magnetic properties and electrocatalytic properties of CoNi@C were characterized.It is found that CoNi@C is a two-dimensional thin layer structure,which the CoNi nanoalloy particles are evenly embedded in the nitrogen doped carbon thin layer.The samples exhibit ferromagnetism at room temperature and have high saturation magnetization（more than 50 emu/g）.The experimental results show that the external magnetic field significantly improves the catalytic activity of the catalyst for oxygen evolution under alkaline conditions.Under the action of external constant magnetic field（～360 m T）,the overpotential and Tafel slope of the catalyst are reduced in varying degrees.The analysis shows that this magnetic enhancement effect is due to the spin polarization and orientation of transition metals,which promote electron transfer and oxygen adsorption.Our research shows that it is a feasible strategy to improve the oxygen evolution activity of magnetic electrocatalyst by electromagnetic induction.