MOF-derived nanocarbon materials for electrochemical catalysis and their advanced characterization

CHEN Xi ,LI Ming-xuan ,Yan Jin-lun ,Zhang Long-li,2,

（1.College of Chemistry and Chemical Engineering,China University of Petroleum (East China),Qingdao 266580, China;2.State Key Laboratory of Heavy Oil Processing,China University of Petroleum (East China),Qingdao 266580, China）

Abstract：Because of the demand for clean and sustainable energy sources,nanocarbons,modified carbons and their composite materials derived from metal-organic frameworks (MOFs) are emerging as distinct catalysts for electrocatalytic energy conversion.These materials not only inherit the advantages of MOFs,like customizable dopants and structural diversity,but also effectively prevent the aggregation of nanoparticles of metals and metal oxides during pyrolysis.Consequently,they increase the electrocatalytic efficiency,improve electrical conductivity,and may play a pivotal role in green energy technologies such as fuel cells and metal-air batteries.This review first explores the carbonization mechanism of the MOF-derived carbon-based materials,and then considers 3 key aspects: intrinsic carbon defects,metal and non-metal atom doping,and the synthesis strategies for these materials.We also provide a comprehensive introduction to advanced characterization techniques to better understand the basic electrochemical catalysis processes,including mapping techniques for detecting localized active sites on electrocatalyst surfaces at the micro-to nano-scale and in-situ spectroscopy.Finally,we offer insights into future research concerning their use as electrocatalysts.Our primary objective is to provide a clearer perspective on the current status of MOF-derived carbon-based electrocatalysts and encourage the development of more efficient materials.

Key words: MOFs；Nanocarbon materials；Electrochemical catalysis；Advanced characterizations

1 Introduction

Metal-organic frameworks (MOFs) are crystalline materials composed of self-assembled metal ions or clusters with organic ligands[1–2].In recent years,MOFs have found widespread applications in various fields such as gas adsorption and separation,catalysis,chemical sensing,energy storage and conversion due to their periodic crystal structure,structural flexibility,tunable pore topology,high surface area,and tailorable properties[3–9].Notably,Zheng et al.have discovered that the in-depth exploration,precise design,and efficient synthesis of MOFs can now be achieved through the collaboration of GPT-4 chemists and human researchers,enhancing the feasibility and efficiency of research activities,thus accelerating the progress in MOF materials[10].Furthermore,BASF,as a groundbreaking development,has announced its position as the first global producer of MOFs on a scale of several hundred tons per year.These MOFs,particularly zinc-triazole-oxalate-based MOF (CALF-20,developed from University of Calgary),have been designed for carbon dioxide storage and can also adsorb greenhouse gas methane,making industrial-level carbon capture possible.This significant achievement,as published in the journal Science,signifies the true industrialization of MOFs and the successful enhancement of economic benefits[6].

However,the presence of weak coordination between metal nodes and organic ligands in the majority of MOFs leads to issues such as low catalytic activity and challenges in catalyst recovery under demanding reaction conditions.These conditions include organic/water solvents,acidic or alkaline environments,and high temperatures,which have limited the practical application of MOFs in the field of electrocatalysis.On another note,high conductivity is another critical requirement for MOFs when used as electrocatalysts.Nonetheless,due to the presence of typical organic linkers surrounding redox-active sites,MOFs often exhibit poor conductivity,rendering them insulating materials.Additionally,the electronic interactions between metal nodes and organic linkers further affect the conductivity of MOFs[11].Carbon-based catalysts exhibit distinctive traits,displaying superior catalytic activity in electrochemical reduction reaction,chemical stability,cost-effectiveness and environmental compatibility in research applications.From economic and environmental perspectives,carbonbased nanomaterials offer certain advantages due to their abundant resources and eco-friendly nature compared to metal-based catalytic materials.These attributes have garnered significant attention for potential applications across various domains for a multitude of carbon-based materials.The carbon allotrope family primarily constitutes of a vast,periodically arranged sp2lattice,forming an extensive π-conjugated system that offers enhanced thermal and electrical conductivity.Through the efforts of numerous scholars,modifications to the lattice structure of carbon-based materials have been accomplished utilizing both covalent and non-covalent methods,altering their intrinsic properties and tailoring desired material characteristics.Particularly in oxygen reduction reaction (ORR)and oxygen evolution reaction (OER),typical electron-demanding reactions,the direct utilization of the inert π electrons within carbon-based materials poses significant challenges.Currently,the most direct strategy involves manipulating the electronic structure of carbon-based material catalysts through the introduction of heteroatoms.Hence,the preparation of nanoscale carbon-based derivatives from MOFs as precursor materials offers an effective means to achieve these targets.Over the past decade,remarkable progress has been achieved in both the synthesis strategies and electrocatalytic activities of carbonbased electrocatalysts derived from MOFs[12–16].

MOFs are renowned for their ordered structures,uniform compositions,and relatively high carbon content[16–17].They are considered appropriate sacrificial templates and metal precursors for the synthesis of carbon materials.In comparison to conventional porous materials,these materials derived from MOFs often exhibit a range of unique advantages.MOFs can be transformed into carbon-based porous materials that exhibit greater stability than their precursor MOFs,retaining characteristics such as a high surface area,structural diversity and abundant porosity.Thus,MOF-derived materials not only offer the advantages of porous carbon but also enhance the stability of the parent MOFs.Additionally,they exhibit greater resilience and recyclability under demanding reaction conditions.For ORR or OER,the metal ions/clusters within MOFs represent potentially well-defined metal active centers.However,their poor conductivity severely hinders their ORR catalytic activity.To address this issue,the thermal decomposition of MOFs to prepare carbon-based catalysts has proven to be an effective solution.In the context of electrochemical water splitting,MOFs are among the best support systems for enhancing the cathodic hydrogen evolution reaction (HER)[12].MOF-derived carbon compounds not only exhibit good conductivity and stability but also successfully maintain porosity while inheriting the substantial surface area of the MOF precursors[18].The structure and size of these materials can also be modified through carefully planned synthesis for optimal use in energy applications.Thus,continuous improvement in the preparation strategies for MOF-derived materials holds significant implications for their application in various catalytic fields,contributing to the future of industrial development.

At present,several excellent reviews have provided detailed discussions regarding the major influence of composition,structure,or morphology on MOF-derived carbon-based materials in the field of electrocatalysis[12,16].However,a systematic summary and discussion of advanced characterization methods specific to these materials are currently lacking.Therefore,we hope that this review will assist researchers in gaining a quicker understanding of the latest developments in MOF-derived carbon electrocatalysts,designing and synthesizing higher-performance electrocatalysts,and gaining a deeper understanding of advanced characterization methods for MOF-derived carbon-based materials.In this review,we first discuss the carbonization mechanisms of MOF-derived carbon-based materials.Subsequently,we focus on promotion strategies from 3 aspects (intrinsic carbon defects,metal and non-metal heteroatom doping) and systematically introduce various advanced characterization methods (in-situ mapping and in-situ spectroscopy).Finally,we provide our conclusions and prospects for future research on MOF-derived carbon-based materials as electrocatalysts.This review aims to provide a clearer understanding of the current status of MOF-derived carbonbased electrocatalysts,offering insights into the exploration of more efficient electrocatalytic materials.

2 Carbonization strategies

MOFs exhibit a wide range of compositions and structural tunability,rendering them ideal precursors for crafting porous carbon nanomaterials[17–19].To create these materials,MOFs are subjected to carbonization,involving the pyrolysis of self-ligands,adsorbed organic solvents,or guest molecules[20–21].This process involves inter-/intramolecular dehydrogenation,deoxygenation,polymerization,arylation and functional group carbonization,ultimately yielding nano carbon materials[22–23].Common preparation methods include pyrolysis or solution permeation within controlled atmospheres like Ar,N2,H2or air[24].Through the rational design of MOFs precursors and meticulous control of the synthesis process (e.g.gas environment,pyrolysis temperature,duration,heating rate and precursor addition),a diverse array of materials derived from MOFs can be prepared[25–26].These materials often retain certain characteristics inherited from the MOFs precursors,such as pore sizes,morphology,compositions and properties[27–28].This broad spectrum of MOFs-derived nanocarbon materials encompasses carbon quantum dots[20,29],porous carbons[28,30],metal nanoparticles[31],metal compounds[32],and their various nanoscale composites[3,7,33].MOFs-derived nanocarbon materials offer key advantages: (1) Diversity and tunability: They leverage the varied metal ions and organic ligands found in MOFs[3,34].(2) Preservation of porous and ordered structures: The ordered,porous MOF structure effectively prevents metal nanoparticle formation during pyrolysis and the creation of metal oxides[19,26].(3)Simple preparation: MOFs are easily prepared under mild conditions,exemplified by ZIF-67,ZIF-8 and HKUST-1,which can be synthesized at room temperature and ambient pressure[14,35].The utilization of nanocarbon materials derived from MOFs in electrocatalysis presents several advantages: (1) Enhanced active sites[35–36]: Careful control of nanocarbon material morphology and porosity increases the number of exposed active sites,thereby enhancing electrocatalytic efficiency.(2) Improved metal dispersion[37–38]: The coordination environment between metal ions and ligands in MOFs ensures well-dispersed metals in carbon nanomaterials boosting utilization efficiency.(3) Heteroatom introduction[39–40]:The incorporation of heteroatoms makes doped-carbon materials more receptive to well-defined heteroatom functional groups,customized polarity,and inherent redox-active sites.The synthesis strategies outlined in this paper for MOFs-derived carbon nano materials can be categorized into 3 key aspects.

2.1 Direct pyrolysis

This is the simplest method to prepare porous carbon materials by direct pyrolysis of MOFs precursors[14,40].The activated carbon produced by this method has an ordered pore structure compared to commercial activated carbon.For the preparation of metalfree carbon,the MOFs precursors are generally subjected to high-temperature carbonization under an inert atmosphere (e.g.,Ar,N2),which leads to the decomposition of the organic skeleton,and the metal species can subsequently be removed by in-situ evaporation or acid etching,and the removal of the metal species increases the specific surface area and pore volume of the material[7,14].For example,Zhang et al[14].synthesized nitrogen-doped graphitic porous carbons(NGPCs) by using a zeolite-type nano-metallic organic skeleton (ZIF-8) as a self-sacrificing template,which was directly subjected to a high-temperature carbonization process under an inert N2atmosphere,followed by further removal of metallic zinc particles by acid etching,and consequently the ligand imidazole used in the MOF acted as both a carbon source and nitrogen source in the carbonization process(Fig.1a,b).As shown by the transmission electron microscope (TEM) images,the NGPCs retained the nanopolyhedral morphology of the parent ZIF-8(Fig.1c-e),and had abundant nitrogen,high specific surface area and hierarchical porosity with good electrical conductivity network,which has great potential as metal-free electrocatalysts for the ORR in fuel cells.Li et al[30].used ZIF-67 as a precursor to control the formation of high graphitized carbon shell-enclosed mesoporous material (CN) from in situ-generated Co nanoparticles through acid etching by adjusting the calcination temperature,which exhibited excellent catalytic performance in the aerobic oxidation of cyclohexane and toluene as well as in the oxidative coupling reaction of amine and imine.

Fig.1 (a-b) Schematic illustration of the ZIF-8 derived highly graphitized NGPCs.(c) The corresponding rhombic dodecahedron-like structural models of ZIF-8 and NGPCs.(d-e) TEM images of typical ZIF-8 and NGPC polyhedron nanoparticle,respectively[14].Reproduced with permission.Copyright @ 2014 Royal Society of Chemistry.(f) Schematic illustration of the FA/MOF-5 composites derived highly porous carbon and its corresponding SEM image[17].Reproduced with permission Copyright ? 2008 American Chemical Society.(g) Schematic of the self-assembly process of the ZIF-8 and chitosan aerogel for the formation of ZCCA composite(up) and their corresponding SEM images (bottom)[41].Reproduced with permission.Copyright @ 2020 Elsevier Ltd.

2.2 Co-pyrolysis

Due to the low carbon content in some MOFs or the anisotropic contraction during pyrolysis,the derived carbon pores may swell or show the phenomenon of cavity collapse,and the direct pyrolysis of MOFs forms a limited number of active centers for catalysts[7].In order to maintain the stability of the pore structure,the co-pyrolysis of MOFs as templating agents with different carbon sources can be used to prepare carbon materials with high specific surface area,homogeneous particle sizes and good morphology.The incorporation of guest species into MOFs followed by pyrolysis as an effective strategy is of great interest the preparation of MOF-derived porous materials[11,28].The most commonly used externally added carbon source in the carbonization process of MOFs is furfuryl alcohol (FA),and the polymerization of FA in MOFs with the loss of weight in the pyrolysis process can lead to the derivation of porous structures (Fig.1f).Liu et al[17].subjected FA/MOF-5 composites to low-temperature (<150 °C) carbonization in an inert atmosphere of Ar,allowing FA to be immobilized in the MOF-5 pores.This was followed by high temperature carbonization (500-1000 °C) and acid treatment to form nanoporous carbon (NPC) with a specific surface area of up to 3 040 m2g?1.Wang et al[41].first synthesized ZIF-8/chitosan composites and then carbonized them to obtain derived carbon materials with large specific surface area and unique threedimensional layered pore structure (Fig.1g).Additionally,MOFs composed of hetero-element (N,S)containing organic ligands (such as methyl imidazole)or guest molecules (such as NH3,SO2) can generate N,S-doped carbon nanomaterials during the pyrolysis process,thereby enhancing the electrical conductivity and electrochemical activity of carbon-based catalysts.

2.3 Pyrolysis of MOFs composite materials

Since the electrocatalytic process requires a catalyst with good electrical conductivity,by assembling MOFs on different carriers (e.g.,graphene oxide(GO),nickel foam,carbon cloth,etc.) and then pyrolyzing them,materials with high electrical conductivity can be prepared,thereby improving the charge transfer between the electrolyte and the active component.Another interesting point is that metal ions within MOFs can create nano-sized metallic and metal oxide particles during the carbonization process.The regular structure of MOFs effectively prevents particle aggregation,making them excellent precursors for preparing carbon-loaded nanoparticles.Ding et al[19].reported the preparation of Co nanoparticles with nitrogen-doped carbon hybridization material Co@CN by high-temperature carbonization using ZIF-67/GO composites as precursors,and then advanced layered graphene oxide (HAGO/Co@CN) arrays were obtained by freeze-casting method,and highly interpenetrating oxidized graphene network in HAGO/Co@CN electrodes accelerated electron transport (Fig.2a-c).The highly interpenetrating graphene oxide network in the HAGO/Co@CN electrode accelerates electron transport,and the HAGO/Co@CN anode has an ultra-high capacity retention of >98% after 500 cycles at 1 C,with a commercial-grade reversible capacity of 2.3 mAh cm?2.An SEM image of the cross-section of the HAGO/Co@CN material is shown in Fig.2d,which shows a hierarchical structure of horizontally aligned GO/Co@CN nanosheets with gaps between neighboring GO/Co@CN layers of about 20 μm.The selected area electron diffraction(SAED) (Fig.2e) coincide well with the lattice planes of Co (111),(200),(220) and graphitic carbon (002).It can be expected that the HAGO/Co@CN electrodes will expose additional active sites and thus improve the electrochemical performance.In addition,different structures and properties of carbon nanomaterial derivatives can be obtained by regulating the pyrolysis temperature and atmosphere.Zhang et al[34].found that similar ZIF-67/GO composites can produce the three-dimensional porous hybrid material Co@N-HCCs@NG by using the low-temperature evaporation-pyrolysis strategy (Fig.2f).The TEM images (Fig.2g-j) illustrated that the Co@N-HCCs@NG hybrids are composed of the outer shell and inner openings of the closed capsule-like nanostructures.High resolution TEM (Fig.2i-j) as well as SAED mapping again demonstrated the unique hollow nanocapsule structure and hierarchical porous structure in the Co@N-HCCs@NG,which facilitates the rapid transport of reactant molecules,electrolyte ions,and products during the ORR/OER process.

Fig.2 (a) Synthetic process of the GO/ZIF-67 composites and GO/Co@CN nanosheets.(b) Randomly aligned GO/Co@CN nanosheets with electrode.(c) Horizontally aligned GO/Co@CN nanosheets with electrode.(d) The cross-sectional SEM image and (e) SAED pattern of the HAGO/Co@CN electrodes[19].Reproduced with permission.Copyright@ 2022 The Royal Society of Chemistry.(f) Schematic illustration of the preparation of the Co@N–HCCs@NG derived from GO/ZIF-67 composites.(g-h) TEM images of Co@N–HCCs@NG.(i) HRTEM image of Co@N–HCCs@NG and(j) SAED pattern of the HAGO/Co@CN electrode[34].Reproduced with permission.Copyright @ 2021 Elsevier Ltd.

3 Promotion strategies

3.1 Intrinsic carbon defects

In principle,the introduction of defects can disrupt the electron symmetry within aromatic rings.This disruption allows for the adjustment of the charge and spin density of carbon atoms,providing non-uniform composition and catalytic active centers[42–48].

Intrinsic defects in carbon materials primarily include zigzag edges and topological defects (C5,five-C ring;C7,seven-C ring;C5+7,C5 ring adjacent to C7 ring,Fig.3a,b)[36,47].The electrocatalytic activity of these intrinsic defects has been initially validated through theoretical calculations (Fig.3c-e)[47,48].For instance,regardless of the presence of oxygen atoms,zigzag edges exhibit electrocatalytic activity when compared to chair-type structures.Ball-milled carbon materials,characterized by more exposed edge structures,exhibit enhanced catalytic properties[15,46].Both zigzag-shaped edge defects and pentagon defects have lower energy barriers (Fig.3c)[48].In both the ORR and OER,edge or topological defects are more active than N-doped sites[47–48].This phenomenon arises because adjacent carbon rings create spatial curvature,induce different electron densities,and form permanent weak dipole moments,thereby resulting in moderate adsorption and excellent activity.Further theoretical calculations have indicated that C5+7 defects serve as the optimal active sites for the dual-function electrocatalysis of ORR and OER (Fig.3d-e)[47].Therefore,tuning the topological defect structure can achieve the optimal binding energy corresponding to the volcano plot peak (Fig.3d).For example,Yao et al.synthesized porous carbon containing only carbon and oxygen using Zn-MOF (IRMOF-8) precursors[35].This material exhibited excellent ORR performance,molecular selectivity and long-term durability.Such performance is mainly attributed to the removal of Zn from sp2carbon at high temperatures,which disrupts its integrity and creates defects/disorder (Fig.3f).However,it should be noted that intrinsic defects are generally present within carbon materials,as their density is typically too low to attain high electrocatalytic performance on a macroscopic scale.

Fig.3 (a) High-angle annular dark-field imaging of various carbon structural defects[36].Reproduced with permission.Copyright @ 2016 Wiley-VCH GmbH.(b) Scheme of the carbon structural defects[47].Reproduced with permission.Copyright @ 2016 Wiley-VCH GmbH.(c) DFT calculation of free energy for ORR activities at different defects[48].Reproduced with permission.Copyright ? 2015 American Chemical Society.(d) Volcano plot of both ORR and OER for the adsorption energy of OH* at different carbon defects,(e) free energy diagrams of ORR substeps on C5+7 active site[47].Reproduced with permission.Copyright @ 2016 Wiley-VCH GmbH.(f) Cyclic voltammetry and ORR performance of carbon structural defects [35].Reproduced with permission.Copyright @ 2016 Royal Society of Chemistry

3.2 Non-metal heteroatom doping

Both theoretical and experimental research has validated that introducing heteroatoms appropriately can disrupt the uniformity of π-conjugation,modulate the electronic structure of adjacent carbon atoms,thereby reducing the energy barrier for adsorption activation,or significantly enhancing electrocatalytic activity.

Nitrogen doping,which includes pyridinic N,pyrrolic N,graphitic N (also known as quaternary-N),and N-oxides of pyridinic N,is considered an effective approach for improving catalytic activity.However,the specific N-based group that serves as the electrocatalytic center remains uncertain.Fundamental questions concerning the structure-activity relationship between nitrogen functional groups and catalytic activity have yet to be thoroughly investigated[49–52].Achieving precise synthesis of targeted products and elucidating specific active sites within complex nitrogen species pose significant challenges.Guo et al[53].employed highly oriented pyrolytic graphite (HOPG) with well-defined π-conjugation as a template to fabricate materials.Their findings demonstrate a positive correlation between the concentration of pyridinic N and the electrocatalytic activity of the material,along with the generation of Lewis basic sites (Fig.4a,b).In practice,it is the carbon atoms adjacent to pyridinic N that play a substantial role,as confirmed by the formation of pyridonic N in ex-situ X-ray photoelectron spectroscopy (XPS) analysis due to reactions with OH species (Fig.4c,d)[53].

Fig.4 (a) N 1s XPS spectra of N-modified HOPG.(b) ORR results for corresponding N-modified HOPG in (a).(c) N 1s XPS of the N-modified HOPG sample catalyst before and after ORR.(d) Catalysis mechanism of the active site[53].Reproduced with permission.Copyright @ 2016 American Association for the Advancement of Science.(e) Schematic of the formation of N-doped hollow carbon from ZIF-8.(f) Current density measured at 0.75 V (vs.RHE).(g) ORR curves and (h) discharge curve and corresponding power density of N-doped hollow carbon[21].Reproduced with permission.Copyright @ 2020 Elsevier Ltd.(i) Schematic of the formation of N,P co-doped carbon from ZnO@ZIF-8.Linear sweep voltammetry curves of (j) ORR and (k) both ORR and OER at 1 600 r min?1 in the whole in O2-saturated 0.1 mol L?1 KOH solution[59].Reproduced with permission.Copyright @ 2018 Elsevier Ltd.

Direct pyrolysis of MOFs containing N organic ligands is a direct strategy for synthesizing N-doped carbon.However,most of the N in the ligands evaporates before entering the carbon framework,leading to poor utilization and availability of active N.Salt-assisted pyrolysis can effectively prevent the loss of N from organic ligands while significantly enhancing carbonization.Yan et al.synthesized N-doped hollow carbon by the direct calcination of ZIF-8 templated with NaCl[21].They adjusted the graphitization degree and key pyridinic N catalytic centers by varying the calcination temperature (Fig.4e,f).The prepared Ndoped carbon yielded an ORR half-wave potential(E1/2) of 0.86 V,a peak power density of 272 mW cm?2,a specific capacity of 740 mAh g?1,and an operational period of 160 h when used as the cathode in a Znair battery (Fig.4g,h).The morphology of the electrocatalyst is crucial,as it determines the number of accessible catalytic active sites exposed to the electrolyte.Therefore,when pyrolyzing MOFs to obtain carbon materials,it is often essential to control the sample morphology by optimizing the temperature[54],using templates[55]and electrospinning[56],and other measures to maintain an open structure.

While introducing heteroatoms is considered an effective method to enhance carbon electrocatalytic activity,density functional theory (DFT) calculations have shown that the charge transfer induced by a single heteroatom is slightly weak to significantly boost catalytic activity on neighboring carbons.Instead,when multiple heteroatoms combine to form dual/multi-heteroatom dopants,they can finely tune synergistic effects to generate higher electrocatalytic activity[57].Common approaches include modifying ligands or introducing other foreign elements during the pyrolysis process.For example,carbon materials co-doped with N―S or N―P exhibit significantly improved electrocatalytic properties compared to materials doped with a single heteroatom[57–58].This is because the cooperative action of the energy constraint field generated by co-doping optimizes the electron density of adjacent carbon atoms,thereby enhancing the selective adsorption of oxygen on the catalyst surface and the desorption of intermediates (Fig.4i)[57–59].As the result,N,P co-doped carbon dominated by ZIF-8 exhibits a higher positiveE1/2(0.83 V vs.RHE)and limiting current (5.35 mA cm?2) for ORR and a lowerEj=10(1.73 V vs.RHE) for OER,outperforming commercial Pt/C materials (Fig.4j,k)[59].It also demonstrated a peak power density of 74 mW cm?2and an energy density of 896 Wh kg?1when severed as the cathode in a Zn-air battery[59].

Overall,the graphitization degree,porous structure,pyridinic N in the carbon lattice,and edge heteroatoms of heteroatom-doped/co-doped carbon materials determine their electrocatalytic performance.While heteroatom-doped MOF-derived carbon-based materials can increase the density of active sites and enhance ORR electrocatalytic activity compared to bare carbon materials,the effect of solely doping heteroatoms is not very pronounced.Therefore,there is an urgent need to develop MOF-derived carbon-based materials that can load more active sites and exhibit higher catalytic activity.

3.3 Metal heteroatom-doping

Transition metal (TM)-N-C catalysts have emerged as the most promising platinum-free catalysts[60].The coordinated interaction between active TMs and N can significantly reduce the adsorption free energy of oxygen,thereby promoting electron transfer from carbon to oxygen[61–63].Research results have demonstrated that TM-Nxmoieties are highly effective active centers within the catalyst.Specifically,the interaction between oxygen molecules and theDband orbitals of 3d transition metal ions leads to continuous charge transfer from the metal to the π*orbitals of oxygen molecules,activating the oxygen molecules[64–65].

Various metal-based nanoparticles (NPs),including metals,alloys and compounds,can be obtained through a simple pyrolysis process,embedding them into carbon-based materials yielding high electrochemical catalysis activity.However,the aggregation of metal nanoparticles during high-temperature pyrolysis remains a significant challenge in the synthesis of these materials[66–67].To overcome this issue,an effective approach is to anchor metal NPs on the surface of carbon-based materials or confine them within nanocavities.MOFs have a distinct advantage in this regard[18,68].Compared to other synthesis methods,using MOFs as precursors to synthesize metal-doped carbon electrocatalysts enables the construction of an open,hierarchical porous structure through the control of the thermal decomposition process of organic ligands and the utilization of templates.This enhances the formation of C-sp2structure of the supporting carbon by promoting the ordered and highly dispersed distribution of metal sites within the precursor,consequently improving the overall material’s electron transfer capability.Furthermore,the combination of different metal ligands in MOFs,variations in precursor compositions,and the introduction of flexible guest molecules provide a high degree of tunability for the catalyst’s electronic structure.For example,utilizing enzyme-assisted preparation,a unique Co-ZIF material with a special morphology was synthesized as a precursor.A simple one-step carbonization process was employed to obtain N-doped carbonbased electrocatalysts loaded with Co (Fig.5a)[63].This material exhibited superior catalytic activity in both ORR and OER,characterized by rich and exposed active sites,as well as reduced mass transport pathways,resulting in a low potential gap of 0.866 V(Fig.5b).When employed as an electrode in Zn-air batteries,this material demonstrated exceptional longterm cycling stability (>200 h) and an excellent power density of 115.4 mW cm?2(Fig.5c).

Fig.5 (a) Schematic of the formation of Co nanoparticle grafted on N-doped carbon.(b) The corresponding linear sweep voltammetry curves for both ORR and OER at 1 600 r min?1.(c) Discharge curve and corresponding power density[63].Reproduced with permission.Copyright @ 2022 Elsevier Ltd.(d) Schematic of the formation of Fe single-atom catalysts.(e) The corresponding linear sweep voltammetry and (f) NH3 yield[76].Reproduced with permission.Copyright @ 2022 Elsevier Ltd.

The diverse selectivity between MOF metal ions and ligands allows MOFs to be used in the synthesis of bimetallic or multimetal-based carbon catalysts.Furthermore,by controlling the ligands,changes can be made to the metal coordination environment.Compared to single-metal-based catalysts,bimetallic or multimetal carbon materials induce electron structure rearrangements,prevent the aggregation of metal particles,and increase the density of active sites,thereby reducing electrocatalytic overpotential and enhancing overall catalytic performance and durability[69–71].For example,Ni-Fe nanoalloys are formed within N-doped porous carbon decorated with carbon nanotubes by the high-temperature carbonization of Ni-Fe bimetallic zeolitic imidazolate framework[72].The prepared material exhibits superior electrocatalytic performance compared to single-metalbased carbon catalysts,demonstrating dual functionality in both the ORR and OER.It achieves an ORR half-wave potential of 0.85 V and an OER overpotential of 330 mV at the current density of 10 mA cm?2,placing it on par with commercial Pt/C and RuO2/C catalysts.Assembled into a Zn-Air battery,this material demonstrates a low potential gap of 0.8 V and exceptional long-term cycling stability,exceeding 300 h.

“Single-atom catalysts” refer to catalysts in which active metal atoms are dispersed on a support in the form of single atomic centers.This represents the smallest spatial scale for catalytic reaction centers,maximizing the atomic utilization efficiency of metals.It is currently a research hotspot.In traditional synthesis process,to prevent the aggregation of metal atoms,it is usually necessary to reduce the concentration of metal atoms,resulting in a low density of catalytic active center sites and an uneven spatial distribution.The coordination structure advantage of MOFs allows them to control the spatial distance between metal sites[73–75].For example,by introducing bimetallic ion centers and selectively removing one metal in subsequent processes,metal single-atom electrocatalysts distributed in a porous carbon matrix can be obtained.For example,Zn and Fe ions,along with 2,2’-bipyrimidine and oxalic acid ligands,were initially used to synthesize MOFs.In the subsequent direct carbonization process,Zn ions are removed,resulting in the synthesis of Fe single-atom catalysts with N,O co-coordinated structures (FeN2O4,Fig.5d)[76].In a neutral electrolyte,the electroreduction of NH3using this catalyst achieved a high Faradaic efficiency of up to 92%,with an ammonia yield of 46 mg h?1mg?1(Fig.5e,f)[76].This outstanding performance can be attributed to the ability of Fe2O2in the catalyst to modulate theD-band center of Fe atoms,reducing the adsorption energy for intermedi- ate products.The high conductivity of the FeN2O4catalytic core,along with its high selectivity for NH3and suppression of NO2? transformation,promotes the conversion of*NOH to N*.

3.4 Applications

Tables 1-3 summarize the recent electrochemical catalysis of MOF-derived carbon materials,including ORR,OER and hydrogen evolution reaction (HER).

The equilibrium potential for the ORR is 1.23 V(vs.RHE).In contrast to two-electron reactions may potentially have detrimental effects on the catalytic materials in the battery due to the formation of H2O2,in contrast,the 4-electron reaction pathway is deemed the optimal choice[27].A critical aspect within the reaction system pertains to the interaction (adsorption energy) between the electrochemical active sites and oxygen intermediates.The fine-tuning of the catalyst’s electronic structure is of paramount importance,encompassing strategies such as heteroatom doping and the design of single-atom catalysts,as exemplified in Table 1[21,34,36,47,59,63,77–83].Furthermore,the controlled morphology of carbon-based catalysts and the uniform distribution of active sites represent common approaches to enhance ORR performance.

Table 1 Summary of the recent MOF-derived carbon-based electrochemical catalysts for ORR

The OER process represents the antithetical process to the ORR,comprising a complex 4-electron pathway with an identical equilibrium potential.Nevertheless,the design principles for OER catalysts diverge from those of ORR.Optimal choices involve the utilization of metal/metal compounds in conjunction with carbon composites with excellent stability,as succinctly outlined in Table 2[34,36,59,63,77–78,82–87].This encompasses ensuring a heightened level of graphitization or introducing nitrogen element into carbon materials to augment the catalyst’s electronic conductivity.Concurrently,the refinement of catalyst porosity through morphology design contributes to enhancing ion diffusion processes and optimizing oxygen transport to achieve a better OER property.

Table 2 Summary of the recent MOF-derived carbon-based electrochemical catalysts for OER

The HER features an equilibrium potential of 0 V(vs.RHE),constituting a 2-electron process with hydrogen as the intermediate species (H*).The critical aspect of catalysts in this context is the adsorption energy between the active site and H*,with optimal HER performance achieved when active sites exhibit an adsorption free energy of 0 eV.In comparison,the kinetics of H+to H*conversion in alkaline solutions are lower than those in acidic environments.Pt stands as the widely acknowledged and most efficient HER catalyst,and subsequent strategies involve employing MOFs) as template materials to prepare Mo-based materials,metal nitrides,sulfides,among others,as summarized in Table 3[36,81–82,84–85,88–92].Additionally,careful consideration of material structure adjust-ments and the uniform distribution of effectively exposed active sites becomes imperative in further electrocatalyst design.

Table 3 Summary of the recent MOF-derived carbon-based electrochemical catalysts for HER

4 Advanced characterizations

The typical approach typically involves using theories such as electronegativity or calculations of electron spin density to explain the electrocatalytic mechanism[42,93].However,this may not provide a clear explanation of the underlying physical principles.The development of in-situ electrochemical techniques has provided a powerful avenue for elucidating the structure-activity relationships and clarifying the electrocatalytic mechanism[94].

4.1 In-situ mapping

The scanning electrochemical cell microscopy(SECCM) technique enables high-resolution electrochemical measurements and scanning imaging by precisely controlling the three-dimensional movement and positioning of nanoscale capillary probes.When the nanoscale capillary probe approaches the substrate electrode,an electrochemical nanodroplet cell is formed by the interaction between the probe’s electrolyte nanodroplet and the electrode.This allows to measure the electrochemical activity of materials at the nanoscale[95–98].Measurements can be conducted using either a constant-distance or jump-scanning approach,ensuring complete separation of the probe from the surface after each measurement to prevent overlap.Consequently,SECCM is applicable for studying complex systems and surface features at the micro to nanoscale,including specific surface chemistry involving step sites,terraces,or regions.Zou et al.utilized SECCM to verify that the electrocatalytic activity of graphite edge structures is significantly superior to the basal plane,exhibiting a higher current density response at edge-rich sites (Fig.6a-c)[15].Mechanical exfoliation of HOPG via tapping can produce typical regions where ca.15 μm diameter droplets ensure that the electrochemical response originates from the targeted microstructure (Fig.6b).Recently,SECCM has been developed at the nanoscale,with precise control of tip aperture down to 200 nm,providing high-resolution images of nanoscale proton transport and revealing the impact of local wrinkles on electrochemical processes (Fig.6d-e)[99].Additionally,the combination of SECCM techniques with other methodologies enriches the study of electrochemical processes.For instance,the integration of SECCM with highly surface-sensitive interference reflection microscopy enables tracking the quantity,spatial distribution,and morphology of electrode materials with exceptional spatiotemporal resolution[100].

Distinguishing from SECCM,scanning electrochemical microscopy (SECM) is a scanning probe technique based on changes in the sample’s surface Faradaic current,thereby providing surface imaging information[101–103].Using a high-precision piezoelectric positioning system,the Pt probe was positioned approximately 10 μm above the N-containing carbon sphere catalyst spot,collecting current feedback in the oxidation-reduction competition mode (Fig.6f)[104].During the ORR test,a noticeable oxidation current was observed only when the voltage dropped below 1 V (Fig.6f),indicating the production of H2O2.However,once the oxygen reduction current began to increase during the negative potential scan,a sharp increase in the oxidation current at the tip was observed,indicating H2O2production.Imaging clearly reveals the distribution of catalysts at the micro and nanoscale (Fig.6f).The combination of multiple techniques has becoming a new trend.Introducing molecular characterization methods such as Raman or surface-enhanced Raman on the basis of visualizing catalyst active sites allows for real-time decoupling of the contributions of different molecular structures to the oxidation-reduction response at the same site[105–107].By co-locating SECM probes and Raman laser beams,full spectra with sub-10 μm resolution can be obtained,and graphene’s SECM imaging(Fig.6g) is highly consistent with its Raman spectrum’sGpeak value (Fig.6h)[107].The combined use of atomic force microscopy with nanoscale spatial resolution and scanning electrochemical imaging allows for the acquisition of ultra-high-resolution (approximately 50 nm) catalytic current maps[108–109].When using HOPG as a template material (Fig.6i,j),the current imaging results after co-modification with Fe-N provide detailed information (Fig.6k,l) about the local oxygen reduction process[108].The results indicate that Fe-coordinated ‘N’ sites,in conjunction with structural defects in HOPG,are the primary active sites.These specific sites exhibit higher oxygen reduction currents compared to other areas.This combined characterization enables a more profound understanding of catalytic sites on various composite materials,thereby paving the way for improved catalyst design.

4.2 In situ spectroscopy

In-situ operando Raman spectroscopy has been widely employed to provide fingerprint information regarding molecular structures and reaction intermediates[110–112].The Raman scattering signal from water molecules is relatively weak,thus hardly interfering with the Raman scattering signal of the sample.This includes techniques such as surface-enhanced Raman spectroscopy,shell-isolated nanoparticle-enhanced Raman spectroscopy,and tip-enhanced Raman spectroscopy,can further enhance the surface sensitivity and selectivity of the system,enabling the detection of trace interface intermediates.In addition to the 2 prominentD(ca.1 350 cm?1) andG(ca.1 600 cm?1)bands,which can be deconvoluted into different individual components related to the sp2/sp3structure,Raman spectroscopy can also be employed to characterize the interaction between catalysts and adsorbates.For instance,in the in situ Raman study of honeycomb carbon nanofibers with abundant oxygenated functional groups (Fig.7a,b),the shifted and broadenedD1peak (disordered sp3-hybridized carbon defects;A1g) indicates a strong interaction with intermediates (O or OOH*)[113].This stands in sharp contrast to the unchangedGpeak (graphite;E2g) and the absence of theD1peak in the carbon nanofiber reference sample,providing evidence that oxygenated edge defects[43]and intrinsic carbon defects[114]are crucial active sites for electrocatalysis.

Fig.7 In-situ Raman spectra of (a) honeycomb carbon nanofibers with abundant oxygenated functional groups and (b) solid carbon nanofibers[113].Reproduced with permission.Copyright @ 2021 Wiley-VCH GmbH.(c) Spin trapping results of the superoxide radicals using the DMPO,EPR spectra of (d)graphene oxide,(e) graphene oxide washed by base and acid solutions to remove Mn2+ impurities and oxidized debris and (f) quenched graphene oxide with phenyl carboxylic acid groups[117].Reproduced with permission.Copyright @ 2012 Macmillan Publisher Ltd.(g) In situ EPR spectra of the N-doped graphene in KOH electrolyte[124].Reproduced with permission.Copyright @ 2020 Elsevier Ltd.(h) Scheme of the in situ FTIR setup,(i) in situ FTIR spectra of high-entropy single-atom activated carbon catalysts,(j) summarised peak density at 1 347 cm?1,(k) in situ FTIR spectra at 1 082 cm?1[133].Reproduced with permission.Copyright @ 2023 Macmillan Publisher Ltd.

Electron paramagnetic resonance (EPR) spectroscopy is a highly sensitive and rapidly responsive technique based on unpaired electrons[115].Free radical trapping techniques,such as 5,5-Dimethyl-1-pyrroline N-oxide (DMPO),are commonly used for qualitative and quantitative analysis of free radical intermediates in electrocatalysis,such as hydroxyl radicals (OH*),superoxide radicals () and carb on dioxide radicals (Fig.7c)[116–117].EPR spectra of carbonbased materials are sensitive to their carbon electronic structures,including intrinsic defects and edge oxygen functional groups[118].Detailed EPR spectra can distinguish various contributions,especially catalytic active sites.Taking graphene oxide as an example,which was purified to remove oxidation debris and Mn2+impurities,it exhibits an EPR signal from dangling bond spins with a linewidth of approximately 1 G and delocalized π-electrons at the graphene edges with a broad linewidth of approximately 10 G(Fig.7d,e)[117].Only oxidized graphene with a broad signal shows catalytic activity,further confirmed by quenching experiments through diazonium coupling reactions in the absence of the broad signal (Fig.7f).This indicates that localized spins originating from non-bonding π electron states created at the edges,analogous to non-Kekulé molecules with open-shell unpaired electrons,serve as catalytic active centers[117].Moreover,the chemical stability of defect structures in a polar water electrolyte can also be studied using EPR[119],due to the fragmentation of the carbonaceous framework of graphene edges.Lastly,in situ EPR can investigate changes in the carbon electronic structure during the catalytic process[120–121].Although current in situ EPR characterization focuses on changes in the solution phase,such as charge transfers in organic small molecules,it can reveal significant alterations in the EPR signal of N-doped graphene under negative potential in a 6 mol L?1KOH electrolyte (Fig.7g)[122–124].This change reflects the transition from mobile electrons originating from the extended aromatic structure in the initial state to localized electrons due to edge functional groups and nonbonding π electrons.

Near-ambient pressure X-ray photoelectron spectroscopy (NAXPS),which can operate at pressures up to a few millibars,offers the capability to probe the electronic structure of working catalysts,the properties of adsorbed reactant molecules,and the changes in the properties of products formed at pressures close to the relevant conditions,helping establish a stronger structure-function correlation[125–128].This technique has been successfully applied in electrocatalysis involving noble metals,transition metal oxides and more.However,its application in carbon-based catalyst research is not as widespread,despite its undeniable potential.The C 1s XPS spectra,based on in situ analysis,can elucidate electronic structure shifts of carbon-based functional groups under the influence of voltage.More importantly,different forms of oxygen can be identified through O 1s spectra,allowing for in-depth analysis of the electrocatalytic mechanistic processes[128].It is possible to identify all forms of dissociated and non-dissociated chemisorbed water,including single OH species (OH―OH,O―H2O),gasphase components and liquid water[128].This can provide insights into the evolution of oxygen during catalytic processes.Similarly,oxygen species can also be detected through X-ray absorption fine structure(XAFS) at the O K-edge[129].However,it is undeniably challenging to resolve in-situ spectra due to the complex carbon-oxygen bond combinations inherent in carbon-based materials.

Fourier transform infrared spectroscopy (FTIR)is a characterization technique that uses the characteristic absorption of infrared light by molecular vibrations and rotational transitions to analyze molecular structural information.In situ electrochemical infrared spectroscopy contributes to the correct identification of adsorbate species,the bonding structures on the electrode surface,the interactions of adsorbates,and the quantitative analysis of surface adsorbate coverage and related kinetic information,among other factors[130].It should be noted that infrared signals can be easily interfered with by water molecules,as IR spectroscopy is an absorption spectroscopy.The strong absorption of ―OH groups from water molecules in the infrared region can mask the―OH signals on the catalyst’s surface[131–132].Although ―OH is also an intermediate species in the acidic OER process,in an aqueous environment,oxide surfaces will spontaneously generate ―OH,making the observation of ―OH irrelevant for exploring the mechanism of acidic OER.The provided figure illustrates a typical in-situ setup design (Fig.7h),where one side of the catalyst sample interfaces with the electrolyte,while the other side contacts an attenuated total reflection (ATR) crystal (e.g.,ZnSe,diamond).A very thin electrolyte membrane between the sample and the ATR crystal serves as the reaction site and minimizes interference from the electrolyte components on the infrared signal.Therefore,combining theoretical calculations can allow for the determination of the vibrational frequencies of different adsorbed oxygen species.Additionally,the trend of characteristic peaks in the in situ spectra with changes in potential can reveal reaction mechanisms (Fig.7i).For example,in the OER of high-entropy single-atom activated carbon,the signal at 1 347 cm?1representing the O―O―H bending of*OOH increases with an increasing potential (Fig.7j)[133].Moreover,the Tafel slope calculated from log(IR intensity) vs.Potential matches the experimental results closely,confirming that*O+OH?=*OOH+e–is the rate-determining step.Conversely,in the ORR process,the trend of the signal at 1 082 cm?1ascribed to the bending mode of*OH,as observed in the in-situ FTIR spectrum(Fig.7k),initially increases and then decreases,which is in agreement with theoretical calculations.

Overall,the application of operational characterization techniques has greatly benefited the understanding of the basic electrochemical catalysis processes.Conventional macroscopic electrochemical characterization methods provide average information about different interfacial sites,making it challenging to discern essential electrochemical information related to specific structural sites such as defects,grain boundaries and edge sites.In situ imaging techniques offer an excellent approach for detecting localized active sites on electrocatalyst surfaces.High-resolution in-situ imaging techniques enable the study of electrocatalytic reaction processes at the nanoscale,compensating for the limitations of macroscopic characterization methods.These techniques provide opportunities to explore structure-activity relationships and elucidate electrocatalytic reaction mechanisms.Moreover,in-situ electrochemical spectroscopy monitors surface oxidation states and local atomic structural changes at the molecular level during electrocatalytic processes,including the generation of intermediate products.This further clarifies the active electrocatalytic sites,forming the foundation for the development of high-performance catalytic materials.

5 Conclusion and prospects

MOFs exhibit prominent advantages,including high controllability,multifunctionality and a large surface area.When used as precursors for carbon materials,they offer precise control through the adjustment of metals,organic ligands and reaction conditions.This precision allows for the fine-tuning of chemical properties,electronic structures,and pore structures of carbon materials.In this mini-review,we explored intrinsic structural characteristics of high-performance carbon-based catalysts.Specifically,we outlined a series of in-situ characterization techniques that have recently emerged and summarized the insights gained from the results of these in-situ tests.However,it is essential to acknowledge that our understanding of carbon-based electrochemical catalysis remains incomplete.Therefore,we emphasize that future research on the MOF derived carbon materials should be directed towards strengthening several key areas.

Carbonization mechanism and the preparation of high-quality carbon-based electrocatalysts: When compared to other carbon sources,such as biomass,MOFs exhibit a simpler and more standardized structure.However,there is a lack of clarity regarding the mechanisms involved in element migration,the rearrangement of carbon bonds,and the role played by metal ions during the pyrolysis process.Consequently,it is imperative to elucidate these processes from multiple perspectives,encompassing aspects like free radicals,functional groups and electronic structures.This comprehensive understanding is essential for deciphering the evolving patterns based on organic ligands and for delineating the requisite conditions for the formation of pivotal catalytic active sites,including crucial catalytic carbon intrinsic defects and the coordination environments of metal ions.This,in turn,will facilitate the development of directed preparation techniques for functionalized MOF-based carbon materials.

Developing a multi-technology-coupled: In-situ characterization platform to elucidate the catalytic mechanisms and structure-performance relationships of carbon-based materials is of paramount importance.It is imperative to construct an integrated in-situ characterization platform that combines multiple technologies,such as atomic force microscope (AFM),Raman spectroscopy,and SECM/SECCM.This platform will not only enhance the resolution of mapping techniques but also enable the determination of catalytic key sites at the micro-nano level.Additionally,when combined with molecular characterization methods,it will facilitate the identification of the critical structural characteristics of active sites.Moreover,it is essential to focus on the changes in critical structures under different potential conditions.By integrating theoretical calculations into the research,we can systematically clarify the structure-performance relationship of MOF-based carbon materials.The development of these platforms has the potential to become a potent tool in the field of electrocatalysis research and is expected to further drive the development of high-performance carbon-based electrocatalytic materials.

Acknowledgements

The authors acknowledge the financial support from the Startup Support Grant from China University of Petroleum (East China) (23CX06009A).