碳基无金属纳米材料用于电催化合成小分子化学品.pdf

Cite this:NewCarbonMaterials,2024,39(1):42-63DOI:10.1016/S1872-5805(24)60836-XCarbon-based metal-free nanomaterials for the electrosynthesis ofsmall-molecule chemicals:A reviewSHILei1,2,LIYan-zhe1,YINHua-jie2,*,ZHAOShen-long1,*（1.CAS Key Laboratory of Nanosystem and Hierarchical Fabrication,National Center for Nanoscience and Technology,Beijing 100190,China;2.CAS Key Laboratory of Materials Physics,Institute of Solid State Physics,Hefei Institutes of Physical Science,Chinese Academy of Sciences,Hefei 230031,China）Abstract：Electrocatalysisisakeycomponentofmanycleanenergytechnologiesthathasthepotentialtostorerenewableelectri-cityinchemicalform.Currently,noblemetal-basedcatalystsaremostwidelyusedforimprovingtheconversionefficiencyofreact-antsduringtheelectrocatalyticprocess.However,drawbackssuchashighcostandpoorstabilityseriouslyhindertheirlarge-scaleuseinthisprocessandinsustainableenergydevices.Carbon-basedmetal-freecatalysts(CMFCs)havereceivedgrowingattentionduetotheirenormouspotentialforimprovingthecatalyticperformance.Thisreviewgivesaconcisecomprehensiveoverviewofre-centdevelopmentsinCMFCsforelectrosynthesis.First,thefundamentalcatalyticmechanismsanddesignstrategiesofCMFCsarepresentedanddiscussed.Then,abriefoverviewofvariouselectrosynthesisprocesses,includingthesynthesisofhydrogenperoxide,ammonia,chlorine,aswellasvariouscarbon-andnitrogen-basedcompoundsisgiven.Finally,currentchallengesandprospectsforCMFCsarehighlighted.Key words:Electrosynthesis；Electrocatalysis；Carbon-basednanomaterials；Metal-freeelectrocatalysts；Small-moleculechemicals1IntroductionChemicalmanufacturingheavilyreliesonfossilfuelsforitsenergyneeds,whichconstitutesasignific-antportionoftheworldsenergydemand1.Giventheescalatingenergycrisisandenvironmentalconcerns,thereisanurgentneedtodevelopclean,low-costandefficientrenewableenergytechnologiestoreplacethetraditionalchemicalmanufacturingprocesses.Electro-synthesisemergesasapromisinggreenstrategy,util-izingcleanelectricitytodriveelectrochemicalreac-tionsforchemicalsynthesis.Unlikeconventionalin-dustrialsynthesismethodswithhighenergyconsump-tion,electrosynthesistechnologieseffectivelyreducetheenergybarriersofelectrochemicalreactions,thusenabling the synthesis of valuable chemicals undermilderconditions.Asaresult,thedirectelectrochem-icaltransformationofabundantrawingredients,suchasH2O,CO2,O2andN2,intohigh-value-addedchem-icalsandfuelshasattractedincreasingattention2.Inthesesystems,electrocatalystsplayapivotalroleinincreasingreactionefficiencyandregulatingproductselectivity35.This inherent capability makes cata-lystsindispensable in various electrochemical reac-tions,includingtheoxygenreduction/hydrogenoxida-tionreaction(ORR/HOR)infuelcells69,hydrogenandoxygenevolutionreaction(HER/OER)inphoto-/electro-watersplitting1014,carbondioxidereductionreaction(CO2RR)intheartificialcarboncycle1516,nitrogenreductionreaction(NRR)inartificialnitro-gen fixation1719,and other electrosynthesis pro-cessesforgeneratinghigh-value-addedchemicals2022.Currently,noblemetal-basedcatalystsarecommonlyemployedtoenhancetheconversionefficiencyofre-actantstoproductsduringtheelectrocatalyticprocess.However,thedrawbackssuchashighcostandpoorstabilityseriouslyhindertheirlarge-scaleapplicationsinelectrosynthesisandsustainableenergydevices2325.Therefore,itishighlydesirabletodevel-opcost-effectiveelectrocatalystsforovercomingtheseReceived date：2023-10-29；Revised date：2023-12-17Corresponding author：YINHua-jie,Professor.E-mail:;ZHAOShen-long,Professor.E-mail:Author introduction：SHILei,Postdoctoral.E-mail:;LIYan-zhe,Researchassistant.E-mail:Homepage:http:/ advancing the progress of electro-chemicaltechnologies.Carbon-basedmetal-freecatalysts(CMFCs)ex-hibituniquephysicalandchemicalproperties,includ-ingcontrollabledimensions,largesurfacearea,excep-tionalconductivity,substantialporosity,andexcellentchemical stability.Their tunable structures,rangingfrom 0D to 3D,offer an ideal platform for precisedesignandeffectiveintegrationofdynamicreactioninterfaces.Additionally,their compositions can bepreciselycontrolledthroughelementdopingorchem-icalfunctionalization,resultinginimprovedcatalyticactivity and enabling exploration of their structure-activity relationships at the atomic/molecular level.ThestrongcovalentbondsinCMFCsgrantexception-alchemicalstabilitywhichensureslong-termcatalyt-icperformance.Furthermore,variouseffectivemeth-odologies,suchasballmilling,chemicalvapordepos-itionand chemical modification,have been estab-lished for creating CMFCs,providing an optimalfoundationforthedesignofhigh-performanceelec-trocatalysts.These features enable CMFCs to be ahighly promising alternative to noble or transitionmetal-basednanomaterials26.The development of highly efficient CMFCsholdsgreatpromiseformakingasignificantbreak-throughin electrocatalysis.In 2009,Dai and col-leaguesintroducedanewbranchofmetal-freecata-lysisbydevelopingnitrogen-dopedvertically-alignedcarbonnanotubes(N-dopedVA-CNTs)asahigh-per-formance CMFC for ORR27.The enhanced ORRactivityisattributedtochargetransferinducedbyN-doping,alteringtheadsorptionmodeofoxygenmo-lecules and facilitating the ORR process(Fig.1a).Subsequently,this principle of modifying intrinsiccatalyticpropertieshasbeenwidelyappliedindesign-ingefficientCMFCstoenhancethecatalyticperform-ance in electrocatalysis and sustainable energydevices2832.For instance,the carbon-based metal-freenanomaterialshavedemonstratedpromisingper-HCNOHSiBPCSeSIBrNCIOF1.51.00.50.51.0Electronegativity()Charge redistributionCharge-spin couplingSiBPC SI*Br*N ClOFSpin redistribution(vs.C)0Before ORRNN(a)(b)(c)(d)OHpyridinic-N398.5 eVAfter ORRpyridonic-N400.2 eVFig.1(a)CalculatedchargedensitydistributionofN-dopedCNTsandthecorrespondingadsorptionmodesofoxygenmolecule27.Reproducedwithpermis-sionfromAAAS.(b)ReactionprocessbetweenpyridinicNandOHspecies.(c)ProposedmechanismforORRonnitrogen-dopedcarbonmaterials59.Repro-ducedwithpermissionfromAAAS.(d)Heteroatom-dopingmechanisminCMFCs60.ReproducedwithpermissionfromWiley-VCH第1期SHILeietal:Carbon-basedmetal-freenanomaterialsfortheelectrosynthesisofsmall-molecule43formance in ORR26,3335,HER3637,OER3839,CO2RR40,NRR41,bi-functional4249andmulti-func-tional catalysis5052.Moreover,many CMFCs havebeenproventobestableandeffectivemultifunctionalelectrodesinapplicationssuchashydrogenperoxidephoto-electrochemical production5354,Zn-air batter-ies5556,andwatersplitting9,50.ThesebreakthroughsinCMFCsholdgreatpromiseforthedevelopmentofaffordableanddurablecatalystsforvariouskeyreac-tionsinvolvedinenergyconversionandelectrosyn-thesistechnologies31,57.ConsideringthecomprehensivereviewsonCM-FCsforHER,four-electronORRandOER26,3237,thisreviewshiftsthefocustowardtheapplicationofCM-FCsforadvancedchemicalelectrosynthesis,suchashydrogen peroxide,multi-carbon fuels,ammonia,urea,andothersmall-moleculechemicals.BasedonthebroadinterestinCMFCsforelectrocatalysis,wefirstdiscussthemechanismunderstanding,anddesignstrategiesrelatedtoCMFCs.Subsequently,itdelvesintotheapplicationofCMFCsinvariouselectrosyn-thesisreactions,involvingtwo-electrontransferORRand water oxidation reaction(WOR)for hydrogenperoxide(H2O2)production,multi-electron transferCO2RRandNRRforcarbon-andN-basedchemicals,and other electrosynthesis of emerging small-mo-leculechemicalssuchaschlorineandurea.Finally,weproposetheemergentchallengesandfuturedevel-opments of CMFCs.This review aims to providereaderswithdeeperinsightintotheintelligentdesignofCMFCswithhighactivity,exceptionalselectivityandlong-termstability.2MechanismunderstandingTomodulatetheelectrocatalyticactivityofCM-FCs,avarietyofapproacheshavebeendeveloped,in-cluding heteroatom doping,intermolecular chargetransfer and defect generation32.These strategieshavedemonstratedeffectivenessinalteringcharge/spindistributionwithincarbon-basednanoma-terials.Specifically,catalyticcentersbearingpositivechargeand/orhigherspindensityplayapivotalroleinmodifying the chemical adsorption of reactants andintermediates.Thefine-tunedelectronconfigurationscan enhance the compatibility between active sitesandadsorbedreactants,promotingtheadsorptionofreactionintermediatesandfacilitatingelectrontrans-fer.Therefore,understanding these mechanisms iscrucialfortheprecisedesignofhighlyefficientCM-FCs.Inthissection,wedelveintotheoriginsofcata-lytic performance through experiments and densityfunctional theory(DFT)calculations,focusing ontwo-electronORRandWORforhydrogenperoxideproduction,CO2RRandNRRforcarbon-andnitro-gen-basedchemicalsproduction.2.1 Two-electron oxygen reduction/water oxida-tion for H2O2Thecatalyticmechanismofmetal-freecatalysishasbeenproposedbasedonthediscoveryofN-dopedVA-CNTscatalyst.However,there is some contro-versyregardingthepotentialroleofmetalimpuritiesinORR.Toaddressthis,positivelychargedpolyelec-trolytes,exemplifiedbypoly(diallyldimethylammoni-umchloride)(PDDA),havebeenemployedtoverifythecharge-transfermechanismforORR58.Asares-ult,theORRperformanceofPDDA-modifiedCNTissignificantly enhanced,whereas the activity of theelectron-donated polyethyleneimine functionalizedCNTelectrodeisinferiortothatofbareCNTelec-trode.ThisfindingrevealedthattheORRactivityinCMFCsisattributedtodoping-inducedchargeredis-tributionratherthanthepresenceofmetalimpurities.Subsequently,the active site is conclusively con-firmedusingasuiteofmodelN-dopingcarboncata-lysts,including pyridinic N-/graphitic N-/edges-/clean-highly oriented pyrolytic graphite(HOPG)59.X-rayphotoelectronspectroscopy(XPS)studiesofthepyridinic N-doped HOPG demonstrated that carbonatomsadjacenttopyridinicNcanreactwithOHspe-cies,leadingtoatransformationofthepyridinicNtopyridonicN(Fig.1b).Thisindicatedthatthecarbonatoms adjacent to pyridinic N were the real activesites.Experiments combined with DFT highlightedthat pyridinic N-doping could significantly promotethechemisorptionofO2attheadjacentcarbonatomandthesubsequentprotonationprocess(Fig.1c).44新型炭材料（中英文）第39卷ConsideringtheORRreactionpathway,anoxy-genmoleculeaccepts2electronsandcombineswith2protons to form a reaction intermediate(OOH*),whichcanbedirectlyreducedtoformH2O2(Fig.1c).Therefore,the two-electron ORR pathway has beenconsideredapromisingapproachforgreenH2O2pro-duction54.TheCMFCshavedemonstratedgreatpo-tentialintheelectrosynthesisofH2O2duetotheirhightunabilityandchemicalstability.Theactivityandse-lectivityofH2O2productionarestronglycorrelatedtoheteroatomic species,oxygen groups,and structuraldefects54.Heteroatom-dopedCMFCsgenerallycausearedistributionofchargedensityand/orspindensitydue to differences in electronegativity/spin densitybetweencarbonatomsandheteroatoms,therebyregu-lating the adsorption of reactants and intermediates(Fig.1d)60.Moreover,thedifferenceinheteroatomspeciesleadstoatunableactivityandselectivityto-wardORR.Forexample,amongvariousNspeciesinCMFCs,thepyrrolicNspecieshaveshownhighper-formancetowardtwo-electronORRduetothesuit-ableadsorptionstrengthwith*OOHspecies61.ThepyrrolicNconfigurationhastheclosestadsorptionen-ergycomparedwiththeidealvalue,thusbeingcon-sidered a suitable species for electrochemical H2O2synthesis.In addition,two-electron ORR perform-anceisstronglylinearlycorrelatedtooxygencontent,asdemonstratedbyCuiandco-workers62.DFTcal-culationsdemonstratethatexcellentactivityandse-lectivityoriginatedfromtheCOOHandCOCfunctionalgroupsinCMFCs.Furthermore,thesyner-gisticeffectbetweenheteroatomanddefecthasalsobeenproveneffectivefortwo-electronORR.Mostre-cently,cooperationbetweenpentagondefectandN-doping can effectively regulate the geometric andelectronicpropertiesofthecarbonstructure,resultingin a high affinity for O2 and suitable strength of*OOHspecies63.Similarly,anO-modifieddefectiveCMFCexhibitshighactivityforH2O2electrosynthes-is.Comprehensive experiments and calculations re-vealthatthecombinationofdefectsandO-groupsisthekeytothetwo-electronpathwayofORR64.InadditiontoORR,WORinvolvingatwo-/four-electrontransferprocessisalsoacomplexyetcrucialreactionforbothwaterelectrolysisandH2O2electro-synthesis.Theoretical calculations suggest that N-doped CMFCs can significantly reduce the reactionenergyoftherate-limitingstep(fromO*toOOH*),demonstrating the optimal catalytic activity forWOR65.Forinstance,thepyridineN-oxidesitesin-duceenoughpositivechargeseparationthrough-delocalization,facilitatingtheinitialhydroxylionad-sorptionandenhancingtheWORprocess.Moreover,thechargeredistributioncausedbypyridinicNalsocanoptimizethethermodynamicenergybarrierofthe*OOHintermediate.Asaresult,theas-preparedN-dopedCMFC exhibits the excellent WOR perform-ance,surpassingthecommercialRuO266.Accordingto the charge redistribution principle,O-/S-dopedCMFCswerealsosuccessfullysynthesizedtoreducetheoverpotentialorenergybarrierofWOR6769.The electrosynthesis of H2O2 through WOR isgainingincreasingattentionbecausewateristhesolerawreactantintheelectrochemicalprocess.However,owingtothesignificantlylowertheoreticalpotentialofoxygenevolutioncomparedtohydrogenperoxideformation(1.23Vvs.1.76V),theintroductionofhet-eroatoms,defects,andfunctionalizationoftengener-atesahighactivityforthefour-electronOER.Inaddi-tion,the high oxidation potential of WOR to H2O2alsoleadstotheinevitableself-oxidationofCMFCsthemselvesduringthereaction,resultinginstructuralchanges and unclear identification of active sites.Therefore,theCMFCsusedinH2O2electrosynthesismustpossess highly intrinsic stability,such as nan-odiamond,graphyne,andHOPG-basedmaterials7071.Consequently,thetwo-electronWORforH2O2pro-ductionbyCMFCsposesasignificantchallenge.Todate,thereareveryfewreportsonCMFCsfortwo-electron WOR to H2O2.Recent findings show thatboron-doped diamond(BDD)nanomaterials exhibitimpressive activity for H2O2 production,with per-formance highly correlated to the doping level ofboronatoms70.Thisdiscoverypavesthewayfornewresearchinthedesignofsp3-structuredcarbonaceousmaterials for H2O2 electrosynthesis.Furthermore,第1期SHILeietal:Carbon-basedmetal-freenanomaterialsfortheelectrosynthesisofsmall-molecule45acetylene-basedCMFCshavealsoshowngreatpoten-tialforphoto/electrocatalytictwo-electronWORforH2O2 electrosynthesis.The acetylene species couldsignificantlypromotechargeseparationsandelectron-ic modulation,thereby facilitating the formation of*OOHintermediateforH2O2production71.2.2 Electrocatalytic CO2RR for chemicalsTheelectrocatalyticreductionofCO2toproducevaluablechemicalsandfuels,includingCO,HCOOH,CH4,C2H4,andCH3COOH,involvescomplexmulti-electron transfer pathways40.Carbon-based metal-freenanomaterialscandisruptthescalingrelationshipandmodulatetheadsorption/desorptionofintermedi-ates,demonstratingcomparableCO2RRperformancetotraditionalmetal-basedcatalysts.CO2isathermo-dynamicallystablemoleculewithinherentlylowelec-tron affinity.The dissociation energy of the CObond surpasses that of various other carbon-basedchemical bonds.Consequently,the electrochemicalreductionofCO2demandsaconsiderableinputofen-ergy.The resulting products in CO2RRalways de-pendonthecatalystsandexperimentalparameters.Ingeneral,electrocatalyticCO2RRinvolves3steps:ini-tial CO2 adsorption onto the carbon-based electrodesurface,charge