Investigations On Electrocatalytic Mechanism Of Heteroatom Doping And Application In Zinc-air Batteries

Battery is one of the essential components in the development of new energy technology.Among many different types,the zinc-air battery has the advantages of safety,reliability,high energy density,simple structure,low price,green and environmental protection,which will satisfy the demand required by the emerging technology.The electric energy in the electrochemical reaction of zinc-air battery cannot be 100%converted into the chemical energy since a large percentage is lost due to the overpotential from the cathode.Therefore,it is critical to study catalysts with excellent catalytic effect for electrochemical oxygen reduction reaction(ORR)and oxygen evolution reaction(OER).So far,researches on this aspect mainly focus on noble metals and non-precious metallic M-N-C materials.Due to the high price and resource scarcity of the former,the latter may be applied to zinc-air batteries.At the same time,the single atom catalysts have the highest atomic utilization rate of 100%.However,it is still quite challenging whether the material preparation based on the M-N-C structure can meet the requirements of large-scale application,and can produce materials having excellent catalytic activity and stability.In this dissertation,several points addressing these aspects have been done:(1)The preparation of effective catalysts by one-pot synthesis method is demonstrated to be effective.Template-induced pyrolysis method was used to solve the heterogeneity of Fe-N-C carrier doping and the precision of local atomic structure regulation via precise coordination and regulation of pyrolysis process.First,the P-O doped Fe-N-C carbon nanosheets were prepared,and the formation of P-O bond under the condition of high temperature without reducing gas has been studied.Second,the Fe-N-C carbon nanotubes were produced by B-N regulation,and the Fe-N bond formed by the migration of pyridine N and pyrrole N to the high energy field by 0.3 e V has been investigated.The Fe _SA/B,N-CNT and Fe _SA/N-CNT were measured by X-ray absorption near-edge structure(XANES)spectroscopy.Analysis of the related data reveals that the valence state of Fe has increased after doping B.Third,the porous bimetallic Fe Cu N₆nanomaterials were prepared.The aberration corrected transmission electron microscope(ACTEM)diagram directly shows the formation of bimetallic sites.The x-ray absorption near-edge spectroscopy(XANES)results imply that the atomic valence of Fe in Fe Cu-N-HC is close to 0 and that of Cu is close to+1.Based on the curve fitting of extended x-ray absorption fine-structure(EXAFS)parameters of Fe Cu-N-HC,the complete atomic structure information of Fe Cu N₆ active sites were obtained.The wavelet transforms further confirmed that Fe-Fe and Cu-Cu bonds do not exist in the material.By precisely adjusting the local atomic structure,the required materials were successfully synthesized.Their morphology,structure,atomic valence,element content and specific surface area were carefully analyzed.(2)By tuning the type and number of the central metal atoms,the coordination atoms and the doped hetero-atoms,the bifunctional catalysts of P-O doped Fe-N-C carbon nanosheets were shown to have better eletrocatalysis activity.The experimental results reveal that the P-O doped atom dispersion catalyst has excellent bifunctional performance,i.e.,?E is 0.74 V and the electron transfer number is greater than 3.97,which is better than most of the reported non-noble metal catalysts.Therefore,this type of catalyst demonstrates superior power density and stable cycling performance in the zinc-air battery.(3)With help of the density functional theory(DFT)calculation,the adsorption energy of O₂ molecule was analyzed.Specifically,its difficulty at the single atom catalyst active center was deduced.The degree of electron transfer between the active center of single atom and O₂ molecule was deduced by calculating the change of electron cloud density before and after the adsorption of O₂ molecule.The molecular dynamics and transition state parameters of O₂ molecule on the surface of the single atom catalyst active center were calculated to reveal the migration path and activation energy of O₂ molecule in the monatomic active center.(4)In order to solve the sensitivity problem about the constrained atomic systems responding to the external control parameters,charge extrapolation method was employed in the ORR DFT calculation to make proton-electron migration under constant potential.The corrected DFT calculations considering charge denotation show that the double metal atoms on the coordination of porous nano-materials Fe Cu N₆ system of O-O bond is extended,and is helpful for subsequent dissociation,bimetal site cooperation with providing double active site adsorption,single active site dissociation,and break of the O adsorption-energy scaling relation between active sites.This clearly improves the efficiency of the oxygen reduction reaction,and hence the performance of zinc-air battery.(5)The d-band theory is used in the research of transition metal catalysis.The design of B doped with Fe-N-C material,the d band center closer to the Fermi energy,and the smaller O adsorption suggest that the dissociation reaction can be easier.Therfore,the d band center can be used as a judgment of adsorption intermediate state dissociation with the electronic descriptors,At the same time,B-doped Fe-N-C material has very excellent ORR performance,and has a half-wave potential(E_1/2)of 0.933 V.