Electronic Structure Design And Electrochemical Properties Of Co,Fe-based Electrocatalysts

Zinc-air batteries have become one of the most promising energy conversion and storage devices due to their broad application prospects and reliable safety.In the rechargeable zinc-air batteries（ZABs）,oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）are two key electrochemical reactions in the air cathode.But their slow kinetic processes hider the large-scale application of ZAB.Precious metal catalysts（Pt/Ru）are expensive and scarce,either single Pt/C or Ru O₂ cannot accelerate both ORR and OER simultaneously,which cannot meet thereaction of a bifunctional catalysis.There is a great significance to developing non-precious metal catalysts.However,the traditional transition metal compounds have the problems of single activity and poor electrical conductivity.In this work,on the one hand,the defect structure and metal coordination was adjusted from the atomic level to improve the spin structure of metal compounds and optimize the intrinsic electrochemical activity of materials.On the other hand,the morphology of the substrate material is designed to establish the structure-activity relationship between the morphology structure and the electronic defects.Based on this,a highly active bifunctional catalyst is designed for liquid/solid zinc air battery.The main works are as follows:（1）A series of novel ZIF materials was successfully designed and synthesized by room temperature solution method.Controlling the Co/Zn ratio in the precursor and optimizing the heat treatment conditions can synthesize different 3D nitrogen-doped porous carbon materials（NPC）,including single-atom-based catalysts with different loadings,metal nanoparticle-based catalysts,and metal-free heteroatom-doped porous carbon.When the mass addition ratio of Co/Zn in the precursor was 1:1,cobalt-atom nitrogen-doped porous carbon materials（Co SAs@NPCs）can be obtained.The ORRcurrent density of Co SAs@NPC(6.19 m A cm^-2)is better than that of Co@NPC(5.25 m A cm^-2)and NPC(3.55 m A cm^-2).The Co atomic-loaded catalyst can maximize the metal utilization rate and effectively increase the number of catalytic active center（Co-N_x）.The higher the metal atom load,the much the ORR activity.（2）The Zn-ZIF-based NPC was selected as a microreactor to adsorb and anchor metal ions(Co²⁺,Fe³⁺),nitridescatalysts were obtained through further two-step"carbonization-amination"heat treatment.The formation of the intermediate cubic phase was designed to obtain the nanoscale hexagonal phase（Co,Fe）₃N particles,and a layer of 10-15 nm onion carbon was precipitated around it.This is mainly because the intermediate cubic phase has a small carbon dissolution,and the mismatch of iron nitride and carbon in the cubic phase is smaller（5%）,which contributes to the formation of nano nitride.Furthermore,(Co_xFe_1-x)₃N@NPC materials were prepared by adsorbing different amounts of Co²⁺.The incorporation of Co moves up the d-band center of Fe,which can adjustits electron spin state.The upward d-band centerwill cause the upward movement of the anti-bonding orbital,further the decrease of the electron filling in the anti-bonding orbital will form a stronger adsorption.Therefore,adjusting the metal ion adsorption ratio of Co/Fe to 1:1 can optimize the electronic structure of(Co_0.17Fe_0.83)₃N@NPC with ideal spin(t_2g⁵e_g¹).It exhibits good cycling stability as cathode catalysts for both rechargeable solid-state and liquid zinc-air batteries.The assembled solid-state battery displayed a peak power density of 66.7m W cm^-2,which is more higher than that of Pt/C+Ir O₂(39.8 m W cm^-2).（3）Co SAs-NGST was obtained in the presence of dicyandiamide through the heat treatment of Co Zn-ZIFprecursor.The transformation of morphology and structure is attributed to the existence of dicyandiamide（DCDA）,in which ZIF precursor can be destroyed the tri-s-triazine molecules from DCDA at higher temperature.Then the bamboo-like carbon tubes are catalyzed and grown by metal cobalt,the redundant carbon nitride moleculesform the single-atom-supported graphite sheet.The Co SAs-NGST exhibits excellent bifunctional catalytic activity.The pathways of Co N₄-sheets and Co N₄-tubes for ORR and OER processes were calculated by DFT.Although the rate-determining steps of the two structures are different,the synergistic effect is key toenhance the excellent catalytic activity of Co SAs-NGST.（4）Based on the analysis of the reaction paths of Co N₄-sheets and Co N₄-tubes,it is found that the overpotential of the nitrogen-doped graphitic nanotubes（NGT）issmaller.In this section,DCDA was selected as the carbon precursor material,and metal salt as the metal source to prepare Fe Co Ni Cu Zn@NGTby mechanical mixing and further carbonization.Encapsulated by the derived carbon tubes,the high-entropy alloy nanoparticles were well crystallized and uniformly dispersed.By adjusting the temperature,components and types of carbon precursors,the synthesis conditions of stable high-entropy nanoparticles can be concluded:high temperature,high entropy（the proportion of elements is relatively uniform）,and carbon with good crystallinity.The bifunctional catalytic activity of Fe Co Ni Cu Zn@NGT is higher than that of Ir O₂,especially at high currents.Since the high-entropy alloys exhibit good ferromagnetism,the catalytic performance of Fe Co Ni Cu Zn@NGT for ORRat an external magnetic field（350 m T）was investigated,in which the half-wave potential and the current densitywere increased by 10 m V and 0.98 m A cm^-2.High-entropy metal particles can be magnetized into nanomagnets with high spin polarization,which can effectively promote the adsorption and electron transfer processes of oxygen intermediates and significantly improve the catalytic efficiency.