炭材料在电解水制氢耦合有机氧化方面的研究进展.pdf

Cite this:NewCarbonMaterials,2024,39(1):64-77DOI:10.1016/S1872-5805(24)60829-2A review of carbon-based catalysts and catalyst supports forsimultaneous organic electro-oxidation and hydrogenevolution reactionsWANGZhi-dong1,XIATian1,LIZhen-hua1,2,*,SHAOMing-fei1,2,*（1.Beijing University of Chemical Technology,Beijing 100029,China;2.Quzhou Institute for Innovation in Resource Chemical Engineering,Quzhou 324000,China）Abstract：Producingorganicelectro-oxidationandhydrogenevolutionreactions(HER)simultaneouslyinanelectrolyticcellisanappealingmethodforgeneratingvaluablechemicalsattheanodewhilealsoproducingH2atthecathode.Withinthisframework,thetaskofdesigningenergy-savingelectrocatalystswithhighselectivityandstabilityisaconsiderablechallenge.Carbon-basedcata-lysts,alongwiththeirsupports,haveemergedaspromisingcandidatesduetotheirdiversesources,largespecificsurfacearea,highporosityandmultidimensionalcharacteristics.Thisreviewsummarizesprogressfrom2012to2022,intheuseofcarbon-basedcata-lystsandtheirsupportsfororganicelectrooxidationandHER.Itdelvesintoouter-sphereelectrooxidationmechanismsinvolvingmo-lecule-mediatedoxidationandoxidativeradicalcouplingreactions,aswellasinner-sphereelectrooxidationmechanisms,encom-passingbothacidicandalkalineelectrolytes.Thereviewalsoexploresprospectiveresearchdirectionswithinthisdomain,addressingvariousaspectssuchasthedesignofelectrocatalyticmaterials,thestudyoftherelationshipbetweenthestructureandpropertiesofelectrocatalysts,aswellasexaminingtheirpotentialindustrialapplications.Key words:Carbon-basedmaterials；Hydrogen；Electrochemicalwatersplitting；Organicoxidation；Electrocatalysis1IntroductionHydrogen(H2)hasdiversesources,possessesahigh calorific value(1.4108 J kg1),and generateszero emissions.Consequently,it is regarded as the“ultimateenergy”tomitigatethecurrentenvironment-alproblemsandenergycrises14.Theelectrochemic-alconversionofwatertoH2drivenbycleanenergy(e.g.,solar,wind)isoneofthemostpromisinggreenpathwaysfordecarbonization56.However,thispro-cess exhibits high overpotential between the anodeandcathode(1.50Vat10mAcm2)710.Traditionalelectrochemical water splitting involves hydrogenevolutionreaction(HER)andoxygenevolutionreac-tions(OER),asshownbelow(Fig.1a):Inanacidicorneutralmedium:OER:2H2O4e 4H+O2(1)HER:4H+4e 2H2(2)Inanalkalinemedium:OER:4OH+4e 2H2O+O2(3)HER:4H2O+4e 2H2+4OH(4)The HER involves a two-electron transfer,whereas the OER involves a four-electron transfer.ThemainbottleneckofH2productionviawatersplit-tingisthesluggishOER,wherethegeneratedO2isnotvaluableandthismightleadtothemixingofH2andO21113.TheadditionoftheoxygenatomsofH2Otoor-ganicmoleculesinsteadoftheirevolutionasO2isamoreatom-economicalapproachtoupgradingthean-ode for H2 production,which is thermodynamicallymore favorable than OER1419(Fig.1b,c).Electro-chemicalalcohol oxidation reactions(AORs)in-volving methanol,ethanol,glycerol,glucose,5-hy-droxymethylfurfuralandarylalcoholsareusedinli-quidfuelcellsorforgeneratinghigh-valuefinechem-icals(e.g.,acids,ketonesoraldehydes)alongwithH2Received date：2023-08-23；Revised date：2023-11-23Corresponding author：LIZhen-hua,AssociateProfessor.E-mail:LZH;SHAOMing-fei,Professor.E-mail:Author introduction：WANGZhi-dong,Mastercandidate.E-mail:Homepage:http:/ stability of the corresponding elec-trocatalysts.Forinstance,Pt-andPd-basedcatalystsexhibit excellent catalytic activity in both organicelectrooxidationandHER,buthavelimitedavailabil-ityandareexpensiveinnature23.Inaddition,theirstabilityisunsatisfactory24.Inthiscontext,carbon-based materials are promising catalysts for organicelectrooxidationandHERduetotheirdiversesources,largespecificsurfacearea,highporosity,andmultidi-mensionalstructure2528.Carbonsaretypicallyutilizedasconductivesup-portstoloadanduniformlydispersecatalyticallyact-ivespecies,especiallypreciousmetals.Carbonsup-portsreducethedosageoftheactivespeciesandsta-bilize them29.Moreover,heteroatom(N,P and S)-dopedcarbonscanbedirectlyusedascatalystsforor-ganicelectrooxidationandHER3036.Thisreview summarizes the recent advance-mentsincarbon-basedcatalystsandconductivesup-portsemployedinthecontextoforganicelectrooxida-tioncoupledwithHER.Section2providesanover-viewofrecentfindingsconcerningtheutilizationofcarbonasbothelectrocatalystandsupportmaterialinthe electrooxidation of different substances such asmethanol,ethanol,glycerol,benzyl alcohol,5-hy-droxymethylfurfuralandvariousotherchemicals.Thissection also deals with the outer and inner spheremechanismsof electrooxidation.Section 3 summar-izestherecentreportsontheuseofcarbonasanelec-trocatalystandasupportmaterialinHER.Finally,thereviewprovidesaprospectonpotentialresearchdir-ectionstocommercializecarbon-basedmaterialsandsupports for bi-functional organic electrooxidationandHERactivity.2Organicelectrooxidationusingcar-bon-basedmaterials 2.1 Direct use of carbon catalystsTaube37 and Bard38 proposedthat an electro-chemical organic oxidation reaction involves outerandinnerspherereactions.Intheinnerspherereac-tion,thereactantsarechemicallyadsorbedontotheelectrodesurface(Fig.2a)andinvolveproton-coupledelectrontransfer(PCET).Thispathwayrequiresthemediationofanactiveoxygenspecies.Forinstance,ethanoloxidationongoldsurfacesrequiresthedepro-tonation of some ethanol molecules to form ethoxyanions.Inanalkalineenvironment,negativelypolar-izedreactivespecies,includingOHions,ethoxyan-ions,andneutralethanolmoleculeswithterminaloxy-genatoms,aresequentiallyrepelledfromthenegat-ivelycharged electrode.Following this,a consider-ablenumber of ethanol molecules undergo electro-chemicaladsorptionontothegoldelectrodesurface,resultingintheformationofAu(OH)adsspecies.ThesespeciesreactwiththeOHions,chemisorbedontothegoldsurface,thusoxidizingethanol.AhighernumberofOHionsontheelectrodesurfacegeneratesagreat-er number of active sites,thereby accelerating theelectrocatalyticreaction(Fig.2b)39.Incontrast,outerspherereactionsoccurneartheelectrodesurfacewithoutstronginteractions.Inthispathway,theelectronsnecessaryforthereactionpassthrough the solvent layer,facilitating the reaction.Outer sphere electrooxidation has fewer restrictionsforthechoiceofelectrodematerials.Thus,cheapercarbon-basedmaterialshavebeenpreferredforthesereactions40,such as CH bond oxidation41 andCOH bond oxidation42.However,carbon-based(a)(c)HEREOROERPotentialCurrent(b)H2H2OH2OH2CathodeHER catalystCathodeHER catalystAnodeOER catalystAnodeEOR catalystAdd-valuedproductsOrganicmoleculeO2Fig.1(a)DiagrammaticrepresentationofelectrolyticH2production.(b)SchemeforelectrolyticH2productioncoupledwithorganicelectrooxidationand(c)ComparisonoftheHER,ethanolelectrooxidationreaction(EOR)andOER第1期WANGZhi-dongetal:Areviewofcarbon-basedcatalystsandcatalystsupportsforsimultaneous65anodes only function as conductors,aiding in thechargecollectionandtransmission.Itisessentialtonotethattheydonotdirectlyengagesinthecorres-pondingchemicalreactions.Instead,theirmainfunc-tion is to provide a conductive surface for efficientelectron transfer.Notably,solvents and electrolytesdeterminethereactionpathwayselectivity,yield,andotherfactors.Outersphereelectrooxidationcanoccurinamo-lecule-mediated oxidation reaction or an oxidativeradicalcouplingreaction.Formolecule-mediatedox-idation,solubleredoxmediator,suchas2,2,6,6-tetra-methylpiperidine-1-oxyl(TEMPO),are needed todrivetheelectrooxidationprocess.Chaetal.43usedTEMPOasthemediatortoachievetheelectrooxida-tion of biomass-derived 5-hydroxymethylfurfural(HMF)toproduce2,5-furandicarboxylicacid(FDCA)with100%Faradaicefficiency(FE).FDCAisavalu-ablefeedstocktoproducerenewablepolymerssuchaspolyethylene2,5-furandicarboxylate(PEF),whichisapromising alternative to polyethylene terephthalate(PET)4446.Initially,thereactionwasconductedwithagoldanode,buttheadditionofTEMPOandHMFsubstantiallydecreasedtheinitialpotentialofthereac-tion(1.09Vvs.RHE).Asthisreactionisanoutersphere reaction,carbon-based materials with largersurface areas can theoretically drive the reaction(Fig.3a).Acarbon-basedanodeexhibitedanearlierreactiononsetpotentialthanthatofthegoldanode.However,theHMFconversionrate(100%)andthe(a)(b)CH3CH3CH3H3CH3CH3CCOOCOOHOHOHHCOCOOH(2)Gold film WEAu(OH)adsSurface gold oxideOOH(1)(3)(4)(5)eeH+eH+eH2OOCHHOHHHHOHOHCHOuter-sphere electrooxidationInner-sphere electrooxidationSubstratePCETROSROSOxidized productsAbsorbateProductRadicalsCarbon-basedmaterialseeSubstrateOxidized products=Soluble redox mediator(TEMPO etc.)=Reduced state=Oxidized stateElectrodeElectrodeFig.2(a)Schemefortheouterandinnerspherereactionsand(b)reactionmechanismfortheoxidationofethanoltotheacetateiononthesurfaceofagoldfilmworkingelectrode(WE)inanalkalinesolution.UsedwithauthorizationfromRef39.Copyrightby2019AmericanChemicalSociety(a)(b)ProcessingLignocellulose(0.7 eq)2(1 eq)1ORadical chain reactionfaradaic efficiency 4700%Yield 94%Yield 91%O OOOO3HydrogenHydrodeoxygenation4-ethylnonaneCarbon-basedmaterial anodeMetalcathodeeevH+H2HMFFDCATEMPO+TEMPOFig.3(a)SchemeforTEMPO-mediatedHMFelectrooxidation.(b)Form-ationof4-ethylnonane,avaluableliquidfuel,fromtheelectrooxidationof2-methylfuran(2-MF).ReproducedwithpermissionfromRef48.Copyrightfrom2019AmericanChemicalSociety66新型炭材料（中英文）第39卷FDCA yield(98.8%)from the carbon-based anodewerecomparabletothoseofthegoldanode(99.8%FDCAyield).Hence,noblemetal-basedanodescanbe replaced by inexpensive carbon-based anodes insuchreactions.Oxidativeradicalcouplinghasproventobeanefficientmethod,toincreasethecarbonnumberofor-ganicchemicals through outer sphere electrooxida-tion,withoutanyneedformediators.Inthisprocess,the organic reactant first involves a single-electrontransfertobeoxidizedtoradicalcations.Thuscoup-ling of radical cations can afford multi-carbonproducts47.Forinstance,Chenetal.48reportedtheuseofgraphiteelectrodetooxidize2-methylfuran(2-MF).Then,3-(5-methylfuran-2-yl)hexane-2,5-dionewasproducedviathereactionbetween3-hexene-2,5-dione and the initially formed radical cation.Moreover,4-ethylnonane,avaluableliquidfuel,wasobtained in 91%yieldvia further hydrodeoxygena-tion(Fig.3b).2.2 Use of carbon as a support materialTheprocessofinnersphereelectrooxidationne-cessitatestheutilizationofanelectrocatalystcapableofpreciselyregulatingkeyreactionelements.Theseencompass the reaction rate,selectivity and productyields.However,pure carbon materials(such asamorphous carbon,graphene and carbon nanotubes)usuallyshowapoorcatalyticactivity.Hence,carbonmaterialshavebeenusedasconductivesupportsinin-nersphereelectrooxidationreactions.Theprototypic-alelectrocatalystcontainingcarbon-basedsupportisthePt/Ccatalyst,whichiswidelyusedinvariousor-ganicreactions.Overrecentyears,therehasbeenex-tensiveresearchontheelectrocatalystsgrowninsituon self-supported carbon substrates,such as carbonpaper.Thisapproachpromotestheexposureofreact-ivesitesandfacilitatesmasstransfer,therebyenhan-cingthereactionrate.Thissectionhighlightsthere-centadvancementsintheinnersphereelectrooxida-tionreactionsofmethanol,ethanol,glycerol,benzylalcohol,HMFandvariousotherchemicals(Table1)conductedoncarbon-basedsupports.Methanolelectrooxidationreaction(MOR):Thisreactionshowcasestheversatilityofmethanolasanidealreactantforalcoholelectrooxidation,duetoitswidespreadavailability,cost-effectivenessandinher-entreactivity.Itisreactiveandwater-solubleowingtoits hydroxyl groups and high hydrogen content(12.6%,massfraction),whichenhancesitssuitabilityforelectrochemicalprocesses4950.TheMORmech-anismvarieswiththeacidityoralkalinityofthesolu-tion.Inanacidicenvironment,MORoccursviathefollowingpathway:CH3OH(aq)+H2O CO2+6H+6e(5)Conversely,inanalkalineenvironment,MORisdiverse,involvingtheformateandCO2pathways,asshownbelow:CH3OH+6OH CO2+5H2O+6e(6)CH3OH+5OH HCOO+4H2O+4e(7)AstheonsetpotentialofMORlagsbehindthatofOERandCO2,whichisproducedinanacidicen-vironment,researchers have primarily focused onMORinanalkalineenvironment5152.Fuetal.53syn-thesizedPt-Co3O4oncarbonpaper(CP)asabifunc-tional catalyst for MOR and HER(Table 1).Pt-Co3O4/CP demonstrated the need for a potential ofTable 1 Summary of reported electrocatalysts containing carbon substrates for organic oxidation reactionsSampleReactantElectrolyte(vs.RHE)/VforJ=10mAcm2FE/%Stability/hRef.Pt-Co3O4/CPMethanol1.0MNaOH+3.5%NaCl+2Mmethanol0.56802053Co3O4/CPEthanol2MKOH+2Methanol1.459858Ni-Mo-N/CFCGlycerol1MKOH+0.1Mglycerol1.361001064MnO2/CPGlycerol0.005MH2SO4+0.2Mglycerol1.366085065NCCuCo2Nx/CFBenzylalcohol1MKOH+15mMbenzylalcohol1.55956066Ni3NCHMF1.0MKOH+10mMHMF1.5510072Note:MmolL1,CPcarbonpapers,NCN-dopedcarbon,CFcarbonfibric第1期WANGZhi-dongetal:Areviewofcarbon-basedcatalystsandcatalystsupportsforsimultaneous670.56 V(vs.RHE)to reach a current density of10mAcm2in1.0molL1NaOH+3.5%NaClcon-taining2molL1methanol.ThepotentialforMORis102mVlessthanthatofOER,suggestingthatMORisthermodynamicallymoreadvantageousrelativetoOER.Moreover,formateisthemainproductofMORwithFaradaicEfficiency(FE)80%,andthecurrentdensityexhibitedlittlechangeafter20hat1.0V(vs.RHE).ThiswasattributedtothereductionofthePtionsintoPtclustersbytheelectronsaroundtheoxy-gen vacancy of Co3O4.The uniformly dispersed Ptclusters on Co3O4 facilitateintimate interfacial con-tact,whichisbeneficialforlong-termelectrocatalysis.In addition,researchers interest in the use ofnon-preciousmetalcatalystsforMORhasenlarged.Thisinterestisdrovebyconcernsaboutthelimitedavailabilityandconsiderablecostassociatedwithpre-cious metal resources.Jin et al.54 synthesized aNiMn-layereddouble hydroxide(LDH)electrocata-lystforMOR,whichcouldreachacurrentdensityof10mAcm2at1.33V(vs.RHE)in1molL1KOHcontaining 3 mol L1 MeOH.This potential is lessthan that required for OER.Furthermore,formateemergesastheprimaryproductofMORwithanFEexceeding95%.Therefore,advocatingfortheuseofnon-preciousmetalsinMORiswarranted.Ethanolelectrooxidationreaction(EOR):Ethan-olisanon-hazardous,liquidalcoholatroomtemper-aturethatiseasilyavailableinnature.EORalsooc-cursviatwodifferentpathwaysinacidicandalkalineenvironments55.EOR under acidic conditions pro-duces CO2,whereas EOR under alkaline conditionsgenerates acetate anions,this could further affordhigh-valuechemicalssuchasaceticacidorethylacet-ate51.Preciousmetal-basedcatalystsarethepreferredanodicEORcatalysts.Vizzaetal.56usedcarbon-sup-portedRhandPtasanodicEORandcathodicHERcatalysts,thisproduceacetateandH2,respectively.Ahighcurrentdensity(500mAcm2)wasachievedat0.7V(vs.RHE)in2molL1KOHwith2molL1ethanol.Lucas-Consuegraetal.57usedPtRh/Castheanodic EOR catalyst to produce acetaldehyde in1molL1KOHwith6molL1ethanol,whereacur-rentdensityof250mAcm2wasachievedat1.1V(vs.RHE).Zhengetal.58synthesizedcarbonpaper-supported Co3O4 nanosheets as an EOR catalyst,which demonstrated high FE(98%)for acetate andreachedacurrentdensityof10mAcm2at1.445V(vs.RHE),which is less when compared to OER(1.50V).Thereducedenergyrequirementwasattrib-utedtotheabundantpresenceofCo3+onthe(111)surfaceactivesites5960.Notably,transitionmetal-basedcatalystsrequirehighreactionpotentialscomparedtothepreciousmet-al-basedcatalysts,whichsuggestsahighenergyre-quirement.Therefore,furtherresearchshouldfocusonenhancingtheselectivityoftheoxida