Tuning The Electronic Structure Of Carbon-based Nanomaterials And Their Applications In Electrocatalysis

As one of the important clean and sustainable energy,electrochemical energy has attracted more and more attention in view of the world energy shortage and environmental pollution.So far,a series of electrochemical energy conversion devices and technologies have been developed,including water splitting,proton-exchange membrane fuel cells(PEMFC)and metal-air batteries.Electrocatalysts with high activity and low cost are of great importance for reducing the equipment costs and improving the energy conversion efficiency of the above-mentioned devices.Although progress has been made in the development of electrocatalysts and the regulation of their intrinsic catalytic activity over the past few years,noble metal-based catalysts(Pt,Ir,Pd,etc.)are still considered as the most efficient catalysts for a series of electrocatalytic reactions including hydrogen evolution reaction(HER),oxygen evolution reaction(OER),oxygen reduction reaction(ORR)and hydrogen oxidation reaction(HOR).However,their commercial applications in the field of electrocatalysis are greatly hindered by the high-cost and scarcity.Due to their unique structure and electronic properties,such as high electrical conductivity,good chemical stability,and easy process for doping and modification,carbon-based materials(such as graphene,etc.)have emerged as a class of promising catalysts for replacing metal catalysts.Though pure graphene exhibits poor electrocatalytic activity resulted from their improper adsorption of many intermediates of electrocatalytic reactions,their intrinsic activities can be altered through doping and surface modification.It has been found that the electronic structure of the carbon atoms adjacent to the heteroatom in carbon materials can be tuned by doping of various non-metallic heteroatoms(such as nitrogen doping)due to the electronegativity difference between heteroatoms and carbon atoms,thus,improving the intrinsic electrocatalytic activity of the carbon-based materials.Besides,researchers also found that the methods of introducing single metal atoms into the graphene lattice can also significantly enhance the intrinsic catalytic activity of the graphene/carbon-based materials.Therefore,carbon-based materials with non-metallic heteroatoms doping and the single metal atom catalysts incorporated into carbon-based materials such as graphene have become promising electrocatalyst.Owning to their high surface areas,unique pore structures,rich variety of metal ions and organic ligands,grafting of functional groups on organic ligands and so on,metal-organic frameworks(MOFs)(or metal-organic complexes)possess fascinating structural and functional characteristics.They can be served as templates and precursors to fabricate multifunctional carbon-based materials.Specifically,the carbon atoms and other non-metallic heteroatoms in the organic ligands of metal-organic frameworks(or metal-organic complex)can be used as a source of carbon and other heteroatoms to form heteroatom-doped carbon materials in situ during the pyrolysis process in a certain atmosphere.Meanwhile,the metal ions in it will form corresponding metals and their compounds,alloys,or a small amount of them will be embedded in the lattice of the carbon substrate,which can be further used to prepare metal-carbon composite materials,carbon materials encapsulated metal or alloy(M@C)and single-atom catalysts(SACs)through subsequent processes.Therefore,through rational design of the structure of metal-organic frameworks(or metal-organic complexes),we can construct the doping and coordination structures that we need in the carbon materials to a certain extent and further study the relationships between its structure and electrocatalytic activities.In the research works of the subject,we design and synthesize a series of metal-organic frameworks(or metal-organic complexes)as precursors or templates.Through thermolysis,acid leaching,solvothermal reaction and other follow-up treatment processes,we have prepared novel heteroatom-doped carbon-based materials and carbon-supported single-atom catalysts.Their HER and ORR catalytic activity as well as corresponding active sites and catalytic mechanism are further investigated by a combination of electrochemical tests,advanced characterization technologies and theoretical calculations.The main contents of this dissertation are as follows:1.Compared with pure graphene,non-metallic heteroatoms doped carbon-based materials(such as heteroatoms doped graphene)can tailor the electronic structure of the carbon atoms adjacent to the heteroatoms,thereby improving the electrocatalytic HER activity of the corresponding carbon-based materials.Nitrogen doping is one of the important non-metallic heteroatom doping modes.However,although the carbon materials with conventional pyridinic,pyrrolic and graphitic nitrogen doping exhibit improved catalytic activities towards HER compared to their undoped counterparts,the performance of the reported nitrogen-doped carbon materials are still at a relatively low level,which hinder their applications in the field of hydrogen evolution.Herein,we prepared graphene analogous particles with a novel nitrogen-doped structure(structure of dual graphitic-N doping in a six-membered C-ring)for hydrogen evolution reaction.The optimal HER-catalytic activity of the synthesized carbon material is superior to most documented carbon-based catalysts(the overpotential of catalyst is only 57 mV at the current density of 10 mA/cm2 under acidic electrolyte).Also,it exhibits high activity under alkaline conditions.Density functional theory calculations reveal that the carbon atoms bonded to two graphitic N atoms in the novel dual graphitic-N doping structure shows a ?GH*value of 0.01 eV,which is very close to zero,making it an excellent active site towards HER.The bonding structure is beneficial to enhance the C-H bonding,thereby boosting the catalytic activity.2.Transition metal based single-atom catalysts are a kind of promising electrocatalysts that can be used to replace platinum group metal(PGM)-based catalysts for oxygen reduction reactions.Previous studies have shown that transition metals Fe or Co based single-atom catalysts(Fe-N-C or Co-N-C)possess high ORR catalytic activity.Compared with other transition metals with high ORR catalytic activity such as Fe and Co,manganese(Mn)is generally regarded as less active element for oxygen reduction reactions.Herein,we predicted that the configurations of Mn-N4 moiety embedded in the graphene framework possess high ORR intrinsic catalytic activity and successfully achieved such configurations in the carbon substrate through one-step pyrolysis of a nitrogen-rich MOFs precursor(ZIF-8),which adsorb manganese ions in a water-methanol mixed solvent system.The prepared Mn single-atom catalyst(Mn-SA)exhibits excellent ORR catalytic activity(E1/2=0.87V)and zinc-air battery performance under alkaline conditions,which is superior to commercial Pt/C catalysts to a certain extent.The fascinating performance of the Mn single-atom catalyst is ascribed to the monodispersed Mn-N4 moiety in the carbon framework confirmed by the aberration-corrected HAADF-STEM technique and the analysis and fitting of the XAFS data.Theoretical calculation results demonstrate that the d-orbital electronic state of the manganese single atom in the carbon material is tuned to a reasonable state through the coordination of the nearest four nitrogen atoms,which promotes the adsorption process between the catalyst and the ORR intermediates,enabling fast ORR kinetics.3.Although many transition metal-based single atom catalysts exhibit high ORR intrinsic catalytic activity,their practical applications are impeded because most of them suffer from unsatisfactory stability during ORR test in the electrolyte,especially under acidic media,which is one of the bottleneck in the PEMFC.It has been proven that a number of metal centers in transition metal-based single-atom catalysts(transition metal centers with intermediate valence states or transition metal centers with unsaturated coordination structures)can catalyze the decomposition of hydrogen peroxide(a byproduct of the two-electron ORR)to a certain extent.As a result,active oxygen-containing hydroxyl and hydroperoxyl radicals are generated that can attack the carbon substrate of the catalysts,leading to the durability problem of the catalyst system.Different from transition metals,the s-block main group metal elements generally exhibit a single valence,which is expected to effectively alleviate Fenton reaction and further improve the durability of the electrocatalysts during the ORR process.Herein,using amino acid metal complexes as precursors,we first prepare a catalyst of s-block metal Ca-based single atom coordinated with N and O and realize its applications for oxygen reduction reactions under acidic and alkaline conditions.The as-prepared s-block metal Ca-based single-atom catalyst shows good catalytic stability under acid and alkaline conditions while maintaining high ORR catalytic activity.Combined with the aberration-corrected HAADF-STEM characterization,analysis and fitting of the XAFS data and density functional theory calculations,it shows that the p-orbital electronic state of s-block metal Ca can be tuned through the N and O atoms co-coordination.Especially for the model of Ca-N3O1,the highest peak of the projected density of states(PDOS)of the Ca active center is close to the Fermi level,and the p bands of Ca active center have a wide overlap with the p-orbital of the adatom in adsorbed species when they interact with each other during the ORR process,which enhances the adsorption of the ORR oxygen-containing intermediate,promoting the ORR process.