Defect engineering of carbon-based electrocatalysts for the CO2 reduction reaction: A review

LU Yan-kun,CHENG Bai-xue,ZHAN Hao-yu,ZHOU Peng

（State Key Laboratory of Bio-fibers and Eco-textiles,College of Materials Science and Engineering,Collaborative Innovation Center of Shandong Marine Biobased Fibers and Ecological Textiles,Institute of Marine Biobased Materials,Qingdao University,Qingdao 266071, China）

Abstract：Electrocatalytic carbon dioxide (CO2) reduction is an important way to achieve carbon neutrality by converting CO2 into high-value-added chemicals using electric energy.Carbon-based materials are widely used in various electrochemical reactions,including electrocatalytic CO2 reduction,due to their low cost and high activity.In recent years,defect engineering has attracted wide attention by constructing asymmetric defect centers in the materials,which can optimize the physicochemical properties of the material and improve its electrocatalytic activity.This review summarizes the types,methods of formation and defect characterization techniques of defective carbon-based materials.The advantages of defect engineering and the advantages and disadvantages of various defect formation methods and characterization techniques are also evaluated.Finally,the challenges of using defective carbon-based materials in electrocatalytic CO2 reduction are investigated and opportunities for their use are discussed.It is believed that this review will provide suggestions and guidance for developing defective carbon-based materials for CO2 reduction.

Key words: Defect engineering；Carbon-based materials；Electrocatalysis；CO2 reduction

1 Introduction

With the rapid development of the economy and industry,the excessive consumption of fossil fuels such as coal and petroleum has caused a shortage of resources and also resulted in the emission of large amounts of greenhouse gas carbon dioxide (CO2),which has destroyed the carbon cycle in nature and induced a series of environmental problems[1–3].In order to reduce the concentration of CO2in the atmosphere,there are two main strategies: (1) CO2collection and storage;(2) CO2conversion and utilization.However,CO2storage faces the problem of high energy consumption and easy leakage,on the other hand,the conversion of captured CO2into energy-rich carbon fuels and chemicals is a very efficient way[4–8].While the C＝O in linear CO2molecules is very stable,and the energy barrier of its conversion into the target product is high,it is difficult to achieve the reduction and conversion of CO2molecule[9–12].

At present,a variety of techniques have been developed for the reduction and conversion of CO2,including biological catalysis,photocatalysis,thermal catalysis,and electrocatalysis[13–16].Among these,the technique of electrochemical reduction of CO2plays a key role in the future sustainable energy use and development.The electrochemical reduction of CO2has the following advantages: (1) CO2can be directly converted into high-value chemicals and liquid fuels,such as carbon monoxide,formic acid and ethanol under relatively mild reaction conditions.(2) This method can be combined with renewable energy,such as using electric energy generated by renewable sources including solar,wind and tidal as driving force.(3) The reaction process can be controlled using applied potential and electrocatalyst,so the energy consumption in the whole reaction process can be minimized,and no CO2is produced during the reaction process.(4) The electrochemical reaction system has a compact structure with modular device,this can be applied to large-scale industrial applications.Based on the above analysis,electrochemical CO2reduction reaction (ECRR) is one of the most promising CO2conversion technologies and has become a hot research topic in the field of energy storage[17–24].

Among many other factors,the selection and design of catalysts play a crucial role in improving the activity and efficiency of electrocatalytic CO2reduction.As one of the most abundant elements in nature,carbon plays a leading role in the ecological environment and human economic and social development.In the recent decades,significant progress has been made in the synthesis and application of carbon-based materials with various nanostructures[25–27].Due to their unique characteristics,carbon materials can be used as both catalyst carrier and catalyst,thus various types of carbon materials have been widely used in energy conversion and storage as well as in catalysis[28–30].Compared to metal-based catalysts and molecular catalysts,carbon-based catalysts (such as MOFs-derived carbon,graphene,carbon nanotubes,carbon fibers,doped carbon materials,and porous carbon materials,etc.) have great advantages,mainly reflected in the following points: (1) Low price;(2) High stability under strongly acidic or alkaline conditions;(3)Strong electrical conductivity,which can realize the rapid transfer of electrons;(4) Economical and environmentally friendly,conducive to large-scale production;(5) Strong operability,active substances can be easily introduced through certain strategies to regulate the structure of carbon materials[31–35].Although carbon-based materials exhibit many advantages,the weak intrinsic catalysis properties still restrict their further development.Therefore,adopting appropriate strategies to optimize the geometric/electronic structure of the catalysts and supports is essential to improve the performance of ECRR.

Defect engineering can precisely regulate the surface composition of the carbon materials from the atomic scale and adjust the local microenvironment of the catalysts,which is an important strategy to improve the catalytic performance of the materials[36–39].Crystals are derived from the periodic repeated arrangement of atoms in three dimensions.The ideal state of this periodic arrangement can only exist at zero temperature in thermodynamics.As soon as this temperature is exceeded,there are always defects in the crystal that deviate from the perfectly ordered ideal state.Therefore,defects in the actual crystal and various defect types (Scheme 1) and different defect concentrations have a very important impact on the physical and chemical properties of the carbon itself[40–42].By deliberately introducing defects into specific areas of the material in a certain way,the optical and electrical properties,and band structure of the material can be significantly affected,which has been confirmed by numerous reports[43–45].Therefore,because of the problems faced in electrochemical CO2reduction,it is an effective strategy to adopt defect engineering for regulating the carbon-based materials and changing the electronic structure of the catalysts to improve the CO2transformation performance,which is a topic of concern for more and more researchers.

Scheme 1.Schematic illustration for the types of the defective in carbon materials

In this review,we first provide a systematic summary of the advantages of defect engineering and the various types of defective carbon-based materials.Then,the methods of constructing defective carbonbased materials and the characterization methods of defects are described in detail.Finally,further development in the field of defect engineering has also prospected.In general,we anticipate that such a fundamental and comprehensive overview will provide scientific guidance for the applications and development of defective carbon-based materials in ECRR.

2 Effect of defect engineering of carbon-based materials for ECRR

ECRR is a complex process involving multiple reaction steps and the transfer of protons and electrons.Therefore,the local environment of the active site,the adsorption and activation of the reactive species,and the transport of charge are the key factors affecting the catalytic performance,which are closely related to the electronic structure of the electrocatalyst.Defect engineering can optimize the ECRR performance of carbon-based materials from these aspects by regulating their intrinsic properties,which will be discussed in detail in this section.

2.1 Optimize the adsorption behavior of reactive species

The first step in a catalytic reaction is the adsorption process.Therefore,the adsorption behavior of reactants and intermediates has an important effect on the catalytic performance.The introduction of defects in carbon-based materials can effectively improve the surface chemical state of the catalysts,and then regulate the adsorption behavior of reactive species.For example,the introduction of intrinsic vacancy-type carbon defects in metal organic framework (MOF)-derived carbon materials (Fig.1a) can result in the breaking of structural symmetry of the carbon materials.The results of electrostatic potential and theoretical calculation (Fig.1b,c) showed that the unsaturated coordination structure can effectively promote the adsorption of electrophilic CO2.The high surface energy of the defect site distorts the linear structure of the adsorbed reactive molecule,thus accelerating CO2activation and promoting the production of the inter- mediate COOH in the rate-determining step[46].Zhu et al.also showed that the electron interaction between pyridine N and Au atoms in the defective carbon optimized the adsorption energy of the intermediate COOH,thus improving the ECRR performance[47].

Fig.1 (a) The synthesis process of the K-defect-carbon;(b,c) The adsorption free energy change and the values of UL(CO2RR)?UL(HER) on V0,V1,V10,and V12 sites[46].Copyright 2022,Wiley-VCH.(d) The theoretical computational model molecule of the F-doped defect carbon;(e,f) The free energy change of different catalysts for ECRR and the related schematic of ECRR pathway[48].Copyright 2018,Wiley-VCH.(g) The N-doped,P-doped,and N,P-co-doped carbon configurations.(h) Difference in limiting potentials for ECRR and HER over a simulated N,P-co-doped carbon configuration[49].Copyright 2020,Wiley-VCH

In addition,hydrogen evolution reaction (HER)will occur in the process of CO2reduction,which seriously affects the Faraday efficiency (FE) of ECRR.Therefore,improving the utilization of electrons and intermediate active hydrogen by inhibiting the competitive HER process is also an important way to improve the ECRR performance of the materials.Wang et al.obtained carbon materials with defect-rich structures through interlayer F element doping.Compared with the initial porous carbon material,the introduction of F dopants can cause the positive charge density and asymmetric spin of neighboring defective carbon atoms (Fig.1d,e),hence resulting in the lowest hydrogen adsorption free energy at the local defect site.The enhanced H-bonding ability leads to the difficulty of desorption of active hydrogen intermediate in the ECRR process (Fig.1f),thus inhibiting the occurrence of competitive HER and improving the Faraday efficiency of ECRR[48].Moreover,other heteroatom-doped (N,P,etc.) defective carbon materials also showed reduced HER properties compared with unmodified ones (Fig.1g,h),further confirming that defective carbon materials can improve ECRR performance by inhibiting HER process[49].According to the published results,the creation of defect structures in carbon-based materials can effectively optimize the adsorption behavior of reactants and intermediates,and thus improve the catalytic performance.

2.2 Provide anchor sites

Because of the break in the symmetry structure,the defect sites have an unsaturated coordination structure and high surface energy.Therefore,the defect sites of the material are particularly suitable for further loading and modification of foreign species,which fills the unsaturated sites and reduces the energy of the system.For example,by using defect Ndoped graphene as a carbon carrier (Fig.2a),nitrogenconfined atomic Fe moieties (Fe―N―C sites) can be formed through the coordination of nitrogen sites and Fe,which can effectively anchor single-atom Fe species,promote the highly dispersed distribution of single atoms (Fig.2b-e),and greatly improve the ECRR performance[50].Wang et al.developed an ECRR catalyst anchored by single atom Ni in an Ndoped graphene shell with CO Faraday efficiency(FE) exceeding 90% at a current density of 60 mA mg?1.Theoretical calculations showed that the single atomic Ni site had a unique electronic structure thanks to the anchoring effect of defective graphene,which can promote the conversion of CO2to CO more than the bulk Ni (111)[51].Then they further investigated 4 different configurations of single Ni-anchored N-doped graphene nanosheets.According to the density functional theory (DFT) calculations,the CO desorption energy barrier of block Ni (111) was the highest.In contrast,Ni―N coordination structure with double vacancy exhibited the lowest CO desorption energy in the ECRR process,which was conducive to promoting the production of CO[18].In addition,reports on the improvement of ECRR performance by other metal single atom-defective carbon carriers (Co―N―C,Mn―N―C,Cu―N―C,Zn―N―C,etc.) further confirmed the anchoring effect of defective carbon carriers (Fig.2f-j)[52–55].

Fig.2 (a) The synthesis process diagram;(b) electron energy loss spectrum;(c) scanning transmission electron microscopy and energy dispersive mapping spectra;(d,e) low and high-angle toroidal dark-field scanning transmission electron microscopy of the Fe-N-C-750 material[50].Copyright 2018,Wiley-VCH.(f) The reaction mechanism diagram;(g,h) The low and high-angle toroidal dark-field scanning transmission electron microscopy (the inset exhibits the related energy dispersive mapping spectrum of Zn);(i,j) X-ray absorption near edge structure and extended X-ray absorption fine structure spectra of the Zn-microporous N-doped carbon catalyst[55].Copyright 2020,American Chemical Society.(k) The linear sweep curve;(l) electrochemical impedance spectra;m) schematic of the catalytic enhancement mechanism of various catalysts including defective carbon[59].Copyright 2020,American Chemical Society

2.3 Improve electrical conductivity

Indeed,CO2reduction on the surface of carbonbased materials involves proton and electron transfer,so the conductivity of the material significantly affects the charge transfer during the reaction.Defect regulation of carbon-based materials can destroy the original periodic symmetry structure and improve the intrinsic physicochemical properties.The appropriate defect structure can effectively regulate the band structure of the carbon-based materials and improve the electrical conductivity.Sharma et al.synthesized N-doped carbon nanotubes (NCNTs) with different surface structures and N content by regulating different precursors and carbonization temperatures.The results showed that the catalytic activity of the NCNTs depends on the type and density of N defects.The presence of graphite N defect and pyridine N defect can significantly enhance the conductivity of the material,reduce the overpotential,and improve the CO selectivity[56].Similarly,defect-rich carbon-based materials such as F-doped carbon nanocages and Sdoped carbon nanosheets also exhibit better electrical conductivity[57].As for metal-carbon materials,compared with non-defective carbon-metal materials,the presence of carbon defects (such as vacancy carbon or doped carbon,etc.) in the composite is more conducive for improving the electronic structure,thus promoting the charge transfer in the reaction process and enchancing the catalytic performance (Fig.2km)[58–60].

3 Types of carbon defects in ECRR

Because of the diversity of preparation methods and material structures,there are different defect types in carbon-based materials including vacancy,edge,topological,doping defects,etc.The diversity of defect structures have different effects on the electronic structure of the materials,and improve the catalytic performance in a variety of aspects.In this section,we will summarize in detail the several common defect types of carbon-based materials in the ECRR process.

3.1 Vacancy defect

Vacancy defect is a kind of intrinsic carbon defect,which usually refers to the defect formed by missing one or several carbon atoms.The researchers developed a K+-assisted synthesis strategy to synthesize a vacancy carbon material based on bio-MOF-1 precursors.During the pyrolysis,K+in MOF accelerated the removal of N-dopants and carbon atoms from the carbon matrix,which promoted the etching process and created a large number of vacancy defects in the carbon matrix.Benefiting from the enhanced CO2adsorption capacity and increased COOH formation rate,K-defect-C-1100 with a large number of V12defects showed excellent ECRR activity with up to 99%FECOat ?0.45 V,which was far superior to other compared samples[46].Zhang et al.found that the ECRR performance of the defective carbon catalysts (Fig.3ad) is positively correlated with the amount of intrinsic porous carbon defects contained in these catalysts.C K-edge near-edge X-ray absorption fine structure spectra and density functional theory calculations(Fig.3e-h) revealed that sp2vacancy defects was the key to the excellent ECRR activity of the porous carbon catalyst[61].

Fig.3 (a-c) The linear sweep curves.(d) Faraday efficiency.(e,f) C K-edge X-ray absorption near edge structure spectra and related expanded view;(g) theoretical computational model molecule;(h) free energy change of the different N-doped carbon materials[61].Copyright 2019,Wiley-VCH.(i) The Faraday efficiency;(j) current density of the NRMC-800,NRMC-900 and NRMC-1000 materials at different applied potentials;(k) The N element content;(l) ID/IG with FECO for NRMC-800,NRMC-900 and NRMC-1000 at the applied potential of ?0.7 V;(m) The Faraday efficiency;(n) current density of the NRMC-900.NRMC-900-2 and NRMC-900-3 materials at different applied potentials;(o) The EPR spectra;(p) Double-integrated intensity of defects in EPR spectra for NMC and NRMC catalysts[62].Copyright 2018,American Chemical Society

3.2 Edge defect

The perfect crystalline materials without boundaries do not exist.The bonding mode of the edge region is different from that of the base region,so the edge sites often exhibit unique physicochemical properties and electrocatalytic performance.The charge density of the edge carbon sites is higher and the carbon atoms in the plane are close to neutral and less active.The defect sites exposed in the edge structure of the carbon frame are mostly zigzag and arm-chair edge defects.The zigzag edge defect is filled with a large number of unpaired π bonds,which can effectively accelerate the electron transfer to the reactive molecule and decrease the formation energy of key intermediate species.The unpaired electrons of the two carbon atoms adjacent to the arm-chair edge site often modulate the interaction between the catalyst and the reactant by forming covalent bonds[39].Amai et al.removed the N element by heat treatment of N-doped mesoporous carbon to obtain the defect-rich carbon material for electrocatalytic CO2reduction.The formation of edge defects in the carbon material has been confirmed by characterization techniques including high-resolution transmission electron microscopy(HRTEM),Raman spectroscopy,electron paramagnetic resonance spectroscopy (EPR) and X-ray photoelectron spectroscopy (XPS).The results of controlled experiments and theoretical calculations(Fig.3i-l) showed that the defect site was the main catalytic site in the reaction.Benefiting from the promotion effect of defect sites,the optimal catalyst exhibited metal-like activity with the CO Faraday efficiency of 80% at a low overpotential of 490 mV(Fig.3m-p)[62].

3.3 Topological defect

Topological defects are one of the intrinsic defects of carbon materials.According to the second law of thermodynamics,intrinsic defects of the carbon structures are unavoidable and exist in several different forms.Even graphene with a very high degree of graphitization has intrinsic defects.Moreover,the existence of these intrinsic carbon defects will affect the charge distribution of the carbon framework structure in different degrees,expose more active sites,and improve the catalytic activity[63].Among the intrinsic defects of carbon-based materials,the most common one is the six-membered ring lattice disorder observed by HRTEM.This phenomenon is usually caused by the increase,loss,or transfer of one or several carbon atoms in the six-membered ring structure of the carbon frame,resulting in the deformation of the carbon frame and the formation of topological defect structures such as five-membered rings,seven-membered rings,eight-membered rings,or five-membered ringsseven-membered rings[39].

Chen et al.used the MOF template carbonization method to completely remove pyridine-N and pyrrole-N from the material and then prepared carbon nanosheets with rich topological defect structures.DFT calculation (Fig.4a) showed that the formation of asymmetric structure in the material caused the local electron rearrangement,and the five-membered ring carbon defect site was the main active center of ECRR(Fig.4b).Notably,the ECRR performance of the material was positively correlated with the content of the generated topological defects (Fig.4c,d),which can be controlled by regulating the nitriding heat treatment temperature and the content of pyridine-N and pyrrole-N in the MOF precursor[64].This group then further adopted a double N-elimination approach to anchor Fe2C nanoclusters at the topological defects.Because of the weak CO binding energy and the accelerated CO desorption process,this material showed excellent performance in electrocatalysis CO2reduction (the Faraday efficiency of CO generation was 97.1% and the current density was 8.53 mA cm?2at?0.7 V)[65].In addition,Zhang et al.synthesized Fe-N4material with abundant 585 topological defect structures.This catalyst can achieve 90% CO selectivity and 33 mA cm?2partial current density in 0.1 mol L?1KHCO3solution[66].

Fig.4 (a) The nitrogen dopants model system of reactive molecular dynamics (RMD) simulation,i.e.,pyridinic-N,pyrrolic-N,and graphitic-N.(b) The structural evolution of the active site in ECRR process.(c) The free energy diagram for ECRR at N-doped sites,penta-hole,and 585-1 sites.(d) The partial charge distribution at defect sites[64].Copyright 2020,Wiley-VCH.(e) The N-doped structure model of pyridinic-N,pyrrolic-N and graphitic-N.[71] Copyright 2017,Wiley-VCH.(f) The structural model of N-doped carbon and the correspondence between Tafel value and N content[74].Copyright 2016,Wiley-VCH.(g) The structural model of N-doped carbon nanotube[75].Copyright 2015,American Chemical Society.(h) The structural model of pyrrolic-N,graphitic-N,S-doped carbon nanosheet[88].Copyright 2018,Wiley-VCH

3.4 Doping defect

The introduction of heterogeneous elements is a promising method to modulate the electronic structure of the materials and thus improve the catalytic performance.The introduction of non-metallic elements with different electronegative properties such as B,N and F into carbon-based materials to form doping defects can destroy the original sp2hybrid carbonbased network,resulting in the redistribution of charge density,thereby improving the electrocatalytic performance of defective carbon-based materials.In 2013,Kumar and co-workers used N-doped nanofiber for the electrocatalytic reduction of CO2to synthesize CO.The experimental results indicated that the heteroatom-doped carbon nanofiber catalyst exhibited a lower overpotential (?170 mV) and more than an order of magnitude higher current density compared with bulk Ag catalyst,which was attributed to the structural advantages of the carbon nanofiber and the high positive charge induced by N doping[67].Subsequently,the effects of different heteroatom species(Fig.4e) including pyridine N,pyrrole N,graphite N,etc.,have been studied[5,68–71].Through electrochemical experiments and theoretical calculations,Hu et al.proved that the intrinsic catalytic property of the pyridine N site was higher than those of the pyrrole N site,and the metallic cobalt loaded on the defective carbon material could further reduce the energy barrier of the rate determination step at the pyridine N site,thereby improving its catalytic activity[72].Wen’s group found that pyridine N and graphite N are the active centers of ECRR.Due to the dual promotion of a high concentration of the active N site and the porous structure,the optimized defective carbon material exhibited high ECRR activity and selectivity by inhibiting HER[73].

Moreover,a large number of N-doped carbon materials with various structures and different types have been developed which showed excellent ECRR activity.Examples include but are not limited to Ndoped nanocarbon (Fig.4f)[74],N-doped carbon nanotube (Fig.4g)[56,75],N-doped carbon nanotube loaded with Cu/Ni single atom[76–78],N-doped carbon nanotube loaded with NiNxcluster[79],N-doped porous carbon loaded with Cu/Co/Ni single atom[80–81],sp-N doped graphdiyne[82],N-doped carbon nanorod/nanosheet[83],and N-doped ultra nanocrystalline diamond[84].In addition to heteroatom N,non-metallic elements such as B,F,P and S were also introduced into carbon materials to form doped defect carbon,which exhibited excellent electrocatalysis performance.Such as F-doped carbon[57],N,P co-doped carbon aerogel[49],N,B co-doped graphdiyne[85],N,B codoped graphitic carbon loaded with Co nanoparticle[86],N,F co-doped carbon nanosheet[87],and N,S co-doped porous carbon (Fig.4h)[88–89].

Electrocatalytic CO2reduction is a complex reaction,which involves many different products and reaction paths.At present,the research on carbon defects in electrocatalytic CO2reduction mainly focuses on analyzing the role of different defect types and clarifying the activity order of different defect structures.Even though a lot of work has been carried out on defective carbon-based materials in electrocatalysis CO2reduction,the roles and activity sequences of various defective structures in different reaction paths are still unclear and controversial,which needs to be further explored.Due to the lack of in-depth understanding of the effect of carbon defects in CO2reduction,the synergistic effect of different defect structures in carbonbased materials is rarely mentioned and discussed in detail,which deserves further in-depth investigation.Therefore,the complex-defect systems still need more detailed and deeper investigation to find their real origin of high activity.

4 Synthesis method of defective carbon-based materials

Defect engineering is considered to be an effective method to improve the electronic structure and physicochemical properties of the materials.To date,researchers have developed a variety of methods for the synthesis of defective carbon-based catalysts,such as the pyrolysis method,chemical vapor deposition,ball-milling,chemical etching,and plasma etching methods.Therefore,it is very important to summarize the synthesis methods of defective catalysts for further study of the internal formation mechanism of defect structure.Several commonly used methods for the synthesis of defective carbon-based catalysts are described in detail below.

4.1 Pyrolysis method

The pyrolysis method is a universal synthesis technique,which is often used in the preparation of carbon materials.High temperatures can promote the breaking of chemical bonds,causing atoms to escape from the lattice,which facilitates the formation of defects.In addition,exogenous species also easily enter the substrate lattice to form new chemical bonds under high-temperature conditions,thus forming defect structures.Yu’s group prepared an N-doped defective carbon nanofiber by high-temperature pyrolysis using biomass chitin as a precursor.Due to the high N content in chitin,a large number of heteroatoms N can be incorporated into the carbon substance to form the defect structures,resulting in excellent electrochemical performance of the material[90].Peng and co-workers fabricated N-doped defect carbon material from natural wood by high-temperature carbonization (Fig.5a),which further indicated that biomass can be converted into defect-rich carbon materials through pyrolysis[91].Telfer and Wei et al.used different MOFs as templates and precursors to prepare metal-supported porous carbon materials by high-temperature carbonization.Due to the presence of a large number of N elements in the MOF precursor and the evaporation of Zn element at high temperature,the resulting carbon materials were rich in defect structures[92–93].Suslick and co-workers synthesized porous carbon materials using alkali propionate as raw material by spray pyrolysis.Benefiting from the limiting effect of atomized droplets containing alkali propionate on the high-energy precursors,the obtained materials are rich in a large number of defect structures[94].In this way,researchers have developed a variety of defective carbon materials such as N-doped carbon nanotubes(Fig.5b),and porous carbon spheres[95–97].On the other hand,high-temperature nitrogen removal method is also a kind of pyrolysis method.Li et al.prepared a variety of fullerenes and graphene carbon materials with pentagon-heptagon-rich carbon (PHC),edge-rich carbon (EC),pentagon-octagon-rich carbon (POC),and pentagon-rich carbon (PC) defects by nitrogen removal method (Fig.5c),which exhibited improved electrocatalysis performance[98].And the researchers synthesized a variety of defective carbon materials by nitrogen removal strategy,which further demonstrated the advantages of this method in constructing defective structures[61,64,99–101].

Fig.5 (a) The schematic diagram of the process of converting wood into defective carbon material by pyrolysis method[91].Copyright 2019,Wiley-VCH.(b)The schematic diagram of the preparation process of the N-doped carbon nanotube by pyrolysis[97].Copyright 2019,Wiley-VCH.(c) The schematic diagram of the preparation process of the defective carbon materials by nitrogen removal pyrolysis method[98].Copyright 2021,American Chemical Society.(d) The model structure of the P-modified carbon material prepared by chemical vapor deposition[103].Copyright 2018,Royal Society of Chemistry

4.2 Chemical vapor deposition

Chemical vapor deposition (CVD) is the process of forming solid deposits by the reaction of gaseous or vapor substances on the gas phase or gas-solid interface.The CVD process is divided into three important stages.(1) The reaction gas diffuses to the substrate surface.(2) The reaction gas is adsorbed on the substrate surface.(3) The chemical reaction occurs on the surface of the substrate to form a solid deposit and the resulting gas phase,and byproducts are detached from the surface of the substrate.This is also one of the common methods for the synthesis of carbonbased materials.Mokaya et al.prepared N-doped ordered porous carbon material by a simple CVD method using mesoporous SiO2as a solid template and styrene or acetonitrile as the carbon source.The carbon materials with different N contents can be obtained by adjusting the deposition temperature[102].Li and co-workers prepared P-modified onion-like carbon (OLC) material by depositing exogenous species P on the OLC substrate with triphenylphosphine(TPP) as a phosphorus source (Fig.5d).Compared with materials prepared by other methods,this P-OLC material exhibited better electrocatalytic CO2reduction performance including higher Faraday efficiency and better durability due to the abundance of P―C bonds[103].Einaga and co-workers prepared B-doped diamond material by microwave-assisted CVD method using B(OCH3) as a boron source.Due to the special property of sp3bond in B-doped diamond,this material can effectively catalyze the conversion of CO2to formaldehyde in a seawater environment[104].

4.3 Ball-milling method

Ball milling method is a kind of grinding method that mainly uses the ball as the medium and realizes the crushing of materials through impact,extrusion and friction.In the process of ball milling,the grinding ball endowed with kinetic energy moves at a high speed in the sealed container and collides with the material.Upon impact,the material will be broken and split into smaller fragments,thus achieving the fine grinding of the sample.Pereira and co-workers fabricated N-doped carbon nanotubes by a simple ball-milling method,which showed excellent oxidation activity.Moreover,this method can incorporate a large amount of N-containing groups such as pyridine like N (N-6),pyrrole like N (N-5) and quaternary nitrogen (N-q) on the surface of carbon nanotubes in the presence or absence of solvents,which is a promising way to synthesize defective samples[105].Song and coworkers prepared expanded graphene (EG) from natural graphite by the methods of Hummers oxidation and rapid thermal reduction.Subsequently,the defective graphene was prepared by ball milling under argon atmosphere (Fig.6a).This approach can be used to introduce self-doped defect structures onto graphene sheets,thereby significantly improving the packing density and capacitive performance of the materials[106].In addition,Baek et al.prepared a variety of defect-functionalized graphene materials by dry ball milling in the presence of CO2,N2,SO3,I2,etc.,and these exhibited excellent electrocatalytic performance[107–109].However,the application of defective carbon-based materials prepared by this method in electrocatalytic CO2reduction is less,which needs to be further investigated.

Fig.6 (a) The schematic diagram of structural evolution of the defective carbon materials prepared by ball milling method[106].Copyright 2019,Wiley-VCH.(b) The schematic diagram of the preparation of the defective graphene by chemical etching[110].Copyright 2019,American Chemical Society.(c) The schematic diagram of the preparation of the defective graphene by plasma etching method[115].Copyright 2016,Royal Society of Chemistry.(d) The schematic diagram of the self-supporting defective carbon material prepared by plasma treatment[116].Copyright 2017,Wiley-VCH

4.4 Chemical etching

Chemical etching is a common micro-nano fabrication technique.The basic principle involved in this technique is to remove specific substances from the surface of the material through a chemical reaction when the material is in contact with the etching agent.Different defect types and defect concentrations can be obtained by regulating the type of etching agent as well as reaction temperature and time,which is an effective defect regulation strategy and has attracted much attention.Eigler and co-workers synthesized defect-rich graphene by etching some of the carbon atoms (Fig.6b) in the material through the strongly oxidizing radicals produced by the decomposition of H2O2.Electrochemical tests indicated that this material exhibited high H2O2yield of 224.8 mmol g?1h?1and high H2O2selectivity of >82% in 0.1 mol L?1KOH solution[110].Duan et al.reported a general strategy for the preparation of a series of single-atom catalytic materials by this method,in which defectrich carbon materials were obtained by H2O2etching and applied to capture single atoms[111].In addition,Ruoff et al.developed a technique for preparing porous graphene materials using KOH as an activator.The two-dimensional layer graphene can be reconstructed into three-dimensional porous defect materials by KOH activation treatment,which has a specific surface area of up to 3 100 m2/g,a nanoscale pore size distribution,and excellent conductivity[112].Then this group developed a variety of defective carbon materials by this method,expanding greatly application of the developed strategy[113–114].The materials synthesized by this method are rarely used in electrocatalytic CO2reduction and deserve more attention in the future.

4.5 Plasma etching method

In recent years,plasma treatment technology has been gradually considered by researchers and applied to the preparation of defective catalysts due to the characteristics of simplicity and high efficiency.The basic principle is to use the high energy of plasma to promote the escape of atoms or molecules from the surface of the material,resulting in defective structures,thereby changing the physical and chemical properties of the material surface.Wang and co-workers performed Ar plasma etching treatment (Fig.6c)on graphene by regulating the reaction temperature and applying power.Due to the bombardment of highenergy plasma,carbon atoms escaped from the surface of the material,resulting in a large number of holes.Since the intrinsic performance of the edge sites is higher than that of the basal sites,the existence of a large number of edge defects is conducive in increasing the active sites and improving the catalytic performance.In addition,this strategy is also applicable to carbon nanotubes and graphite,demonstrating the universal nature of the method[115].Subsequently,Wang et al.used the same Ar plasma treatment method to etch the carbon cloth (Fig.6d) and prepared an oxygen-functionalized graphene-carbon cloth electrode material with rich edge defects.Compared with pristine carbon cloth,the plasma-treated material exhibited a larger specific surface area,more active sites and better intrinsic conductivity,resulting in better catalytic performance[116].In addition,other research groups have prepared N-doped carbon,modified multiwalled carbon nanotubes,superhydrophilic graphdiyne,and other defective carbon-based materials by plasma treatment technology,and applied them to the field of energy and environment,which further enriches the application of plasma technology in carbon materials[117–119].As an efficient defect preparation method for carbon-based materials,it is worth developing this method for the preparation of catalysts in the field of electrocatalytic CO2reduction in the future.The relevant summary table which contains the defect category,synthesis strategy,electrocatalytic ability and defect effect is given in Table 1.

Table 1 Summary of defect category,defect effect,synthesis strategy and performance of reported electrocatalysts

A summary of the advantages and disadvantages of different synthesis methods for defective carbonbased materials is given below:

Pyrolysis method: The pyrolysis method holds the advantages of simple operation,high yield and good stability of the defective carbon structure.However,the disadvantages include uneven particle size of the carbon-based materials,high reaction temperature,long time and high energy consumption,which make this method unable to meet the purpose of energy saving and emission reduction,hence restricting the development and application of green production of the defective carbon-based materials.

CVD: CVD method has the good characteristics of fast deposition rate,controllable deposition temperature and gas flow,as well as high thermal stability and mechanical strength of the defective carbon-based materials.Notably the yield is low,the required reaction temperature is high,and the reaction source or residual gas is flammable,explosive,toxic and corrosive,requiring additional collection.

Ball milling method: The ball milling method includes wet ball milling and dry ball milling,which is usually carried out under normal temperature and pressure conditions.Ball milling method has the advantage of optional operation modes,such as intermittent operation and continuous operation during the synthesis process for defective carbon-based materials.However,this method has low preparation efficiency,high energy consumption,long time consumption and noise pollution.Due to continuous wear in the process of use,the life of ball mill is short and the maintenance cost is high.In addition,the preparation of the defective carbon-based materials is easy to adhere,resulting in a low yield.The easy loss of the ball in the ball milling process may also lead to contamination of the carbon-based materials.

Chemical etching: The advantages include mild reaction conditions and a high reaction rate during the synthesis process for defective carbon-based materials.However,the disadvantages of this method are also obvious,including uneven etching of the defect structure,safety and pollution risks.In addition,the chemical etching reagent after use cannot be directly discharged,and additional treatment is required.

Plasma etching method: Plasma etching methodis highly efficient,simple to operate,short in time,and low in temperature,which meets the needs of green production of the defective carbon-based materials and is very suitable for laboratory and large-scale preparation.However,it is difficult to achieve uniform etching and obtain uniformly distributed defect carbon structures,which are current challenges.

5 Characterizations of defective carbon-based materials

Advanced characterization methods are the key to understanding the types and effects of defects.The early characterization methods could only indicate the existence of a large number of defect sites in the carbon skeleton through the pores and structural distortion of the material,but could not identify the specific defect types or further clarify the catalytic reaction mechanism.With the development of various advanced characterization techniques,it is convenient for researchers to understand the structure and physicochemical properties of the materials,reveal the intrinsic catalytic mechanism,and guide the design of efficient carbon-based electrocatalysts.However,each characterization technique has its advantages and limitations,so it is necessary to fully understand the specific types of defects and the effects of defects by different characterization methods.

5.1 X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) is an advanced analytical technique,which can accurately measure the inner electron binding energy and chemical shift of atoms,and then provide information on molecular structure,valence state,constituent content,chemical bond and so on.The presence of defects in the material may cause a shift in the bond energy or a new peak in the XPS spectrum.For example,Mu and co-workers constructed abundant pentagonal carbon defects in fullerene molecules byin-situetching.The XPS results showed that C-sp3bond appeared in the carbon material,which indicated that three-dimensional topological deformation carbon structure and intrinsic defects existed in the treated fullerene material (Fig.7a-d),and this is further supported by the conclusion of the existence of defects observed in scanning transmission electron microscopy (STEM)[120].Elimelech et al.obtained defect-rich carbon materials by finely manipulating the atomic/lattice arrangement in the electrocatalyst by a simple and highly controllable thermal tuning strategy.XPS results showed that the ratio of C-sp3and C-sp2in CN-900 carbon material was highest,which proved that there were differences in lattice disorder and defect content in different samples (Fig.7e-p)[121].

Fig.7 (a,b) STEM images of the defective carbon material.(c) Raman spectra and (d) C 1s XPS spectrum of the defective carbon material[120].Copyright 2019,Wiley-VCH.(e) The comparison of elemental content ratio of different defective carbon materials.(f-l) C 1s XPS spectra of different defective carbon materials.(m) The comparison of AC-sp3/AC-sp2 ratio of different defective carbon materials.(n-p) HRTEM images of different defective carbon materials[121].Copyright 2023,Springer Nature Publishing Group.(q,r) TEM and STEM images of the defective graphene.(s) Raman spectrum;and (t) C1s XPS spectrum of the defective graphene[122].Copyright 2017,Wiley-VCH

5.2 Raman spectroscopy

Raman spectrum analysis is an analytical method based on scattering spectra to obtain information on molecular vibration and rotation and is applied to the study of molecular structure.In Raman spectra,theD-band represents the defect and disordered structural properties of the carbon materials,and theGband represents the graphitization properties of the sp2network.The disorder degree in the carbon materials is usually defined by the intensity ratio of theDpeak to theGpeak.For example,Yao et al.fabricated defective graphene carbon materials by a heteroatom N removal strategy.Compared to graphene and N-doped graphene,N-removed graphene exhibited the highestID/IGvalue (Fig.7g-j),which meant that the lattice integrity in this material was the lowest and the defect content was the highest[122].Subsequently,Wu and coworkers investigated the effect of different pyrolysis temperatures on the defect structure and catalytic performance of the carbon materials.Raman results showed that with the rise in heating temperature,theD-peak ratio increased,which meant that the content of highly disordered defect carbon increased.At the same time,the higher the temperature,the narrower theG-peak,indicating that the ideal carbon plane structure was continuously decreasing.Therefore,in the process of catalyst synthesis,higher heating temperatures will produce more disordered carbon structures,which seem to be more conducive to catalytic reactions[123].

5.3 High resolution transmission electron microscopy

The high-resolution microscopic technology can directly observe the existence of defects.HRTEM and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) with aberration correction function can magnify samples to subnanometer or even atomic scale,and then visually analyze the defect structure of the materials.Yao’s group synthesized defective graphene carbon materials by adjusting the N doping amount with high-temperature pyrolysis.The researchers found that the presence of pentagonal defect sites in graphene could be observed by HAADF-HRTEM.Combined with microscopic electrochemical test and work function analysis,it is proved that the defect site was the main catalytic site,and the performance of the defect site was even better than that of the pyridine N site[100].The Yao and co-workers prepared porous carbon materials with extremely high defect density by an interfacial self-corrosion strategy.HRTEM images and the associated Fourier transform images showed that the main region and the edge part of the porous carbon material were significantly different.The main region of the carbon skeleton was dominated by amorphous carbon,and there were some adjacent pentagon carbon defects on the edge of carbon,which proved the feasibility of high-resolution transmission electron microscopy to observe the defect structures[124].Wang et al.etched graphene by low-temperature Ar plasma treatment technology to prepare defect-rich porous carbon materials.As observed in the HRTEM images,many holes appeared on the graphene surface after plasma treatment compared to the smooth surface of the untreated sample,further confirming the presence of defect sites[115].

5.4 Scanning tunneling microscopy

As a scanning probe microscopy tool,scanning tunneling microscopy (STM) can observe and locate individual atoms in the materials.STM can investigate the arrangement of single atoms on the surface of a substance and the physicochemical properties related to the electronic behavior of the surface,so it is widely used in surface science,material science,life science and other fields.He and co-workers used STM to realize the detection and regulation of the inter-valley scattering of single carbon atom vacancy defects in graphene (Fig.8a,b) at the atomic scale[125].In addition,this research group carried out a series of studies on graphene single-atom defects using STM,which greatly expanded the application of STM in defective carbon materials studies[126–127].

Fig.8 (a,b) STM and related fast fourier transform images of the defective graphene material[125].Copyright 2022,American Physical Society.(c) EXAFS spectrum and (d) EPR spectrum of the defective carbon material[99].Copyright 2023,American Chemical Society.(e) C K-edge XANES spectrum of the defective carbon material[128].Copyright 2018,American Chemical Society.(f) The EPR spectrum of ECM-800[129].Copyright 2022,Wiley-VCH.(g) PAS spectrum of the defective PBA-60.(h,i) The schematic diagram of positron capture[130].Copyright 2019,Springer Nature Publishing Group

5.5 X-ray absorption spectroscopy

To analyze the electronic structure information of the materials at atomic scale,X-ray absorption spectroscopy (XAS) is a very effective tool specifically for characterizing defects.Among them,X-ray absorption near edge structure (XANES) can provide the details for the oxidation state and unoccupied electron state of related atoms.Extended X-ray absorption fine structure (EXAFS) can give related information about the coordination environment around the adsorbed atom and the chemical bond length,disorder,etc.Lu and co-workers designed and fabricated defect-rich Ndoped carbon materials by nitriding treatment of fullerene precursors.The absorption peaks located at 286.5,287.5-290 and 293.8 eV can be observed from the XANES spectra of the sample (Fig.8c,d).These peaks correspond to the transition from the 1s core level to the π*(C=C),π*(C=N/C=O/C―O―C/C―N―C),and σ*(C―C) orbitals,respectively.Compared with conventional N-doped graphene,this material exhibited lower π*resonance strength,indicating that there were more unsaturated defect structures in the material,which was conducive to the adsorption activation of reactive molecules[99].Yu et al.studied the electronic structure of the prepared carbon material by the C-K edge XANES spectrum (Fig.8e).The NKCNPs-900 material exhibited a weak absorption peak around 284.1 eV,which means that there was an asymmetric defect structure in the material[128].

5.6 Electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) is a magnetic resonance technique generated by the magnetic moment of unpaired electrons.It can be used to detect the unpaired electrons contained in atoms or molecules of a substance qualitatively and quantitatively and to explore the structural properties of their surroundings.The magnetic moment of electron spin in a constant magnetic field in the sample will have a resonance transition between magnetic energy levels under the action of the radio-frequency electromagnetic field.When an electromagnetic wave of frequencyνis added in the direction perpendicular to the external magnetic fieldB,the free electrons of the material have an energy ofhν.When the relation betweenνandBmeetshν=gμB,a magnetic energy level transition occurs,corresponding to the absorption peak on EPR.Thegvalue is determined by the chemical environment of the unpaired electrons,and different compounds have differentgvalues.According to the position and relative strength of thegvalue,the existence state of the unsaturated electron pair in the material can be effectively analyzed.For example,Amai and co-workers obtained a series of defective carbon materials for electrocatalytic CO2reduction by heat treatment of N-doped mesoporous carbon at different temperatures.It was found that the EPR spectroscopy showed a strong signal atg=2.004,which was caused by the local suspended bonds in the carbon material,and further confirmed the existence of local edge defects,supported by the results of Raman and XPS studies[62].Wang et al.used EPR spectroscopy to study the unpaired electron property of the synthesized carbon materials.Among them,ECM-800 material exhibited higher vibration peak strength atg=2.003,indicating the presence of unsaturated edge defect structure in the material (Fig.8f),which was consistent with the results of HRTEM,XPS and Raman studies[129].

5.7 Positron annihilation spectroscopy

Positron annihilation spectroscopy (PAS) is a nuclear physics technique in which all the mass of a positron is converted into electromagnetic radiation by the collision of a positron with an electron in the surrounding medium.This technique is mainly used to obtain the microstructure information of the materials,especially the internal defect structure.Negatively charged defects such as vacancies in the crystal structure can easily capture positrons by coulomb forces.The lifetime of the positron and the relative strength of the PAS reflect the defect type and the defect concentration in the crystal.This characterization technique does not cause any damage to the material,so it plays an important role in the research of microscopic defects.Yao et al.used PAS to study the defect types and defect concentrations in graphene.The PAS signal intensities obtained in D-HOPG and N-HOPG were similar,which meant that the concentration of intrinsic carbon lattice defects was close to that of vacancy defects.Therefore,the increase in defect concentration in D-HOPG was due to the generation of edge carbon defects rather than vacancy defects[100].Gao and co-workers prepared defective Prussian blue materials (PBA) using N2plasma treatment technique.The lifetime components (Fig.8g-i) detected in PBA-60 were in good agreement with the calculated cyanogen defect (VCN) results,which indicated that positrons annihilation trap at the defect site[130].

As we all know,the complexity of defect structure makes it difficult to accurately characterize the defects of materials.Therefore,it is an effective way to identify the specific defect types in materials by combining various characterization techniques.In addition,limited by the imperfection of characterization strategies,it is imperative to develop new and effective characterization techniques to further directly observe or indirectly verify the defect structure of materials,including defect types and defect concentrations.

6 Conclusions and prospects

In summary,defect engineering is an effective strategy to accurately regulate the geometric and electronic structure of carbon-based materials and thus enhance the activity of electrocatalytic CO2reduction.According to many reports on defective carbon-based materials in recent years,the advantages of defect engineering,types of defective carbon-based materials in ECRR,preparation methods of defective carbonbased materials,and characterization techniques of defects are introduced in detail in this review paper.Despite the exciting achievements,there are still many difficulties and problems that need to be solved,which limit the further development of defective carbon-based materials in electrocatalytic CO2reduction.The following five aspects deserve in-depth consideration by researchers:

(1) Control synthesis strategy of defects.At present,many carbon-based materials have complicated defect regulation steps and high energy consumption,which makes it difficult to achieve green and energy-saving preparation of the such materials.In addition,the controllable introduction of single or multiple defect types and regulation of defect content in the synthesis process still remain challenging.How to modulate the defect type and concentration through energy-saving and simple strategies to achieve accurate regulation of defects is particularly worthy of indepth exploration.

Defect regulation of carbon-based materials can improve the electron distribution near the Fermi level,enhance the intrinsic conductivity of the material,and promote the transfer of electrons within the material.Moreover,due to the high surface energy of defect sites,defective carbon materials have strong electron/hole/ion capture ability,which can effectively reduce the energy barrier of charge transfer and ion diffusion at the interface,and accelerate the rate of charge transfer and ion transfer at the electrode-electrolyte interface.The role of defects is reflected in many aspects,including as active sites,adsorption,charge transfer,ion transport,etc.Therefore,the application of defect engineering requires a comprehensive consideration of many aspects.The existence of defect structures in the material is undoubtedly favorable for the adsorption of reaction molecules and the transfer of electrons and ions.However excessive defect structures may lead to increased adsorption ability of reaction molecules,thus blocking reaction sites and reducing reaction efficiency.Too many or too few defect sites in the material may not be favorable to the maximization of catalytic performance,hence moderate number of defect sites may be most promising to the reaction.At present,a large number of works have demonstrated that defect regulation can improve the electrocatalytic CO2reduction performance of materials.However,in-depth research on defect concentration is still lacking.In particular,the exploration and detailed research on the comprehensive influence of defect concentration regulation on the intrinsic properties,adsorption behavior,electron transfer,ion transport ability and catalytic performance of the material is still lacking and needs further investigation.

(2) Scaling up synthesis.The large-scale synthesis of materials needs to be considered from the perspective of economy,yield and energy consumption.As the most common method for the synthesis of defective carbon materials,the pyrolysis method can achieve the preparation of kg-grade materials.However,the high temperature and long reaction time result in high energy consumption.CVD method also has the same disadvantage,thus it is difficult to achieve the purpose of energy conservation and emission reduction.Chemical etching and ball milling may be more suitable options due to mild synthesis conditions.However,the used chemical etching reagents cannot be directly discharged,and the process is long and time-consuming.Whereas the low yield of ball milling method also restricts its further development.In contrast,the plasma etching method has the advantages of low synthesis temperature,short reaction time,and low energy consumption,which is promising for large-scale synthesis.Thus,in terms of economy,energy consumption and yield,we believe that the plasma etching method is more suitable for large-scale industrial production in the future.In addition,developing new synthesis methods with high efficiency,low energy consumption,and high yield is also the future development direction for defective carbon-based materials.

(3) Potential industrial application.The reaction device of electrocatalytic CO2reduction consists of a catalyst,exchange film,gas diffusion layer,bipolar plate and plastic frame as well as other components.Therefore,the development of the industrial route of electrocatalytic CO2reduction requires the participation of many aspects.Most of the current research is based on the basic research of laboratory scale,and many factors in practical application are not taken into account.Industrial applications require larger reactors,higher reaction currents,longer operation times,and lower cell voltage as well as additional product separation and purification equipment.Therefore,to meet the needs of industrialization,the further development of efficient catalysts,gas diffusion layers,exchange membranes,and bipolar plates is an important direction for the development of electrocatalytic CO2reduction in the future.

(4) In-situ characterization techniques of defects.An electrocatalytic reaction is a dynamic process in which the catalyst may undergo structural evolution during the reaction.The transformation mode and degree of the defect structure in the catalytic process will affect the geometric/electronic structure of the carbon-based materials,and then affect the activity of the electrocatalytic CO2reduction.In addition,ECRR is a multi-step catalytic process,in which the adsorption behavior of various intermediates on the catalyst surface and the pathways of bond making and bond breaking will significantly affect the selectivity of catalytic products.The current understanding of the carbon-based defective structure and the transition of intermediate species during catalysis is not enough to effectively feedback on the design of catalysts.Therefore,the development of novel and effectivein-situcharacterization techniques to capture the dynamic evolution of defect structures and identify the adsorption,bond-making,and bond-breaking behaviors of intermediate species is helpful to further understand the catalytic mechanism of defects.

(5) Density functional theory (DFT) calculation of defective structures.DFT calculation is a computational method based on the principles of quantum mechanics,which can be used to study the structure,energy,and electronic properties of atoms and molecules.In the study of electrocatalysis,DFT calculation can be used to predict the structure and surface adsorption properties of the catalysts,as well as the energy barrier and reaction mechanism of the catalytic reactions,which helps to understand the basic process of the catalytic reactions at the atomic scale and can help in optimizing the performance of the catalysts.At present,the DFT calculation mainly predicts the physicochemical properties of the catalysts and the energy change in the reaction process by building a simple structure model,which cannot accurately reflect the actual structure of the catalysts.In addition,the actual electrocatalytic reaction is generally carried out in solution,which is affected by solution pH,solvent,solute molecules,as well as temperature and pressure.Therefore,the development of effective DFT calculation methods,which can reflect the real structure of the catalyst and the actual catalytic environment,helps deepen the understanding of the ECRR mechanism of the defective carbon-based materials.

Acknowledgements

This work was financially supported by the Natural Science Foundation of Shandong Province(ZR2023QB235),Open Fund of Hubei Key Laboratory of Processing and Application of Catalytic Materials (202306404) and the Taishan Scholar Program of Shandong Province.