Synthesis Of Iron And Cobalt Based Carbide / Nitride Compounds Anchored By Nitrogen Rich Carbon Materials And Their Applications In Zinc-air Batteries

The research and development of green,environment-friendly and efficient electrochemical conversion energy storage devices has become the main means to solve the current energy crisis,environmental pollution and other problems.Among many energy storage devices,zinc-air batteries（ZABs）have attracted extensive attention because of their high energy density,low cost and high safety.However,the slow four electrons dynamic process on the cathode surface（oxygen reduction reduction/oxygen evolution reaction,ORR/OER）greatly inhibits the overall efficiency of electrochemical devices and seriously hinders the commercial development of ZABs.Although noble metal based catalysts can effectively catalyze ORR and OER,their practical application is limited by the defects of high price and poor stability.Among many catalysts that can replace precious metals,transition metals have an electron cloud structure close to precious metals,which is conducive to the contact between the catalyst and ORR and OER reactants and the removal of reaction products.They have electrocatalytic activity comparable to precious metals.However,due to the small specific surface area brought by its traditional preparation methods,the catalytic activity of transition metal catalysts（such as transition metal carbides and transition metal nitrides）is still far from the theoretical value.In this paper,4,6-diamino-1,3,5-triazine-2-imino sodium salt monomer（Melamine-Na）was constructed by the addition reaction of cyano and amino groups;Further,based on the hydrogen bond between it and cyanuric acid（CA）,N-rich supramolecular carriers（CT）with different morphologies were obtained by adjusting the experimental conditions such as reaction solvent;On this basis,transition metal supramolecular complexes were constructed by using the domain limiting anchoring effect of a large number of N atoms in supramolecular support on transition metals.Iron and cobalt based carbon/nitrogen compound nanoclusters were obtained after heat treatment,and showed good ORR and OER properties.The specific research is as follows:（1）Based on the sensitivity of supramolecular structure to solvent polarity,water（-H₂O）,dimethyl sulfoxide（-DMSO）,ethanol（-Et OH）and their mixed solvents were selected as reaction solutions to obtain supramolecular precursors with rod（CT-1,H₂O/DMSO system）,block（CT-2,H₂O/Et OH system）and cone（CT-3,H₂O/H₂O system）morphology,the abundant N atoms in its structure provide a good coordination environment for the domain limiting anchoring of transition metals.On this basis,the transition metal iron（Fe）,cobalt（Co）,nickel（Ni）and silver（Ag）sources were introduced into the CT supramolecular carrier.Based on the coordination bond theory,the transition metal←N coordination bond replaced the hydrogen bond（N...H）in the supramolecular,and the transition metal supramolecular complex was constructed.Take CT-1 as an example:in Fe-CT-1 system,the introduction of Fe source makes CT-1 change from rod structure to tubular structure,which is mainly due to the formation of Fe←N coordination bond,which changes the original hydrogen bond structure in the supramolecular structure,and the phase transition leads to the change of macro morphology;By further increasing the feed ratio of Fe（from 1 wt%to 20 wt%）,it can be found that with the increase of transition metal content,the phase transition of supramoleculars is more complete,and Fe-CT-1 will show a more uniform tubular structure;Similarly,the introduction of transition metals such as cobalt（Co）,nickel（Ni）and silver（Ag）has also brought about the transformation of supramolecular bonding mode.However,due to the difference of their coordination ability,they show rod（Co-CT）,sheet（Ni-CT）and nanowire（Ag-CT）structures respectively,which realizes the limited anchoring of transition metals in supramolecular system,it provides a prerequisite for the preparation of small-size transition metal carbon/nitrides.（2）The uniform tubular Fe₁-CT-1 precursor was selected as the research object,and the iron nitride nanoclusters/nitrogen doped carbon catalyst（Fe₃N/NC）was prepared by high temperature pyrolysis.Spherical aberration electron microscopy showed that the size of Fe₃N was much smaller than most of the reported Fe₃N sizes at 3 nm;At the same time,XRD and XPS further show that there is no Fe particle in the composites obtained by high-temperature pyrolysis,which proves that the supramolecular confinement anchoring method can effectively avoid the agglomeration of metal atoms.In alkaline electrolyte,the half wave potential of Fe₃N/NC catalyst is 0.80 V vs.RHE,the limiting current density is-4.9 m A cm^-2,and the tafel slope is only 96.10 m V dec^-1,indicating that it has good ORR catalytic activity.Meanwhile,in the stability test of 30000 seconds,Fe₃N/NC catalyst showed better stability than commercial platinum carbon（20 wt%）.In addition,Fe₃N/NC catalyst was further prepared into slurry and coated on the composite substrate with a load of 2 mg cm^-2 as the cathode of aqueous ZABs.The results showed that the open circuit voltage of ZABs was 1.43 V,the peak power density was 85.06 m W cm^-2,and the specific capacity was 691.59 m A h g^-1 under the condition of 10m A cm^-2.（3）In order to further study the difference between bimetallic supramolecular complexes and monometallic supramolecular complexes as precursors,iron and cobalt bimetallic supramolecular complexes were selected as precursors,and cobalt carbide/iron nitride/nitrogen doped carbon heterostructures（Co C_x/Fe₃N/NC）were obtained by high temperature pyrolysis.Raman results show that adjusting the feed ratio of Co/Fe can effectively improve the defect degree of Co C_x/Fe₃N/NC composite catalyst and provide more active centers for OER.In 1 M potassium hydroxide（KOH）electrolyte,the overpotential of Co C_x/Fe₃N/NC catalyst at 10 m A cm^-2 is about 320 m V vs.RHE,which is significantly better than that of ruthenium dioxide（Ru O₂overpotential 350 m V vs.RHE）,and the tafel slope is only 77.68 m V dec^-1.At the same time,the optimal electrochemical active surface area of Co C_x/Fe₃N/NC catalyst was determined by cyclic voltammetry.In addition,Co C_x/Fe₃N/NC catalyst also showed excellent stability in the stability test of tens of thousands of seconds.