氮掺杂空心碳纳米球嵌入氮掺杂石墨烯负载钯纳米粒子作为甲酸氧化的高效电催化剂.pdf

Cite this:NewCarbonMaterials,2024,39(2):321-333DOI:10.1016/S1872-5805(24)60844-9N-doped hollow carbon nanospheres embedded in N-doped grapheneloaded with palladium nanoparticles as an efficient electrocatalyst forformic acid oxidationFANGYue1,YANGFu-kai1,QUWei-li1,2,*,DENGChao1,WANGZhen-bo3,4（1.College of Chemistry and Chemical Engineering,Harbin Normal University,Harbin 150025,China;2.Key Laboratory of Photochemical Biomaterials and Energy Storage Materials,Harbin Normal University,Harbin 150025,China;3.MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage,School of Chemistry and Chemical Engineering,State KeyLab of Urban Water Resource and Environment,Harbin Institute of Technology,Harbin 150001,China;4.Shenzhen Key Laboratory of Special Functional Materials,Shenzhen Engineering Laboratory for Advance Technology of Ceramics,Guangdong ResearchCenter for Interfacial Engineering of Functional Materials,College of Materials Science and Engineering,Shenzhen University,Shenzhen 518060,China）Abstract：Efficientelectrocatalystswithalowcost,highactivityandgooddurabilityplayacrucialroleintheuseofdirectform-icacidfuelcells.PdnanoparticlessupportedonN-dopedhollowcarbonnanospheres(NHCNs)embeddedinanassemblyofN-dopedgraphene(NG)withathree-dimensional(3D)porousstructurebyasimpleandeconomicalmethodwereinvestigatedasdirectform-icacidfuelcellcatalysts.Becauseoftheuniqueporousconfigurationofinterconnectedlayersdopedwithnitrogenatoms,thePd/NHCNNGcatalystwithPdnanoparticleshasalargecatalyticactivesurfacearea,superiorelectrocatalyticactivity,ahighsteady-state current density,and a strong resistance to CO poisoning,far surpassing those of conventional Pd/C,Pd/NG,andPd/NHCNcatalystsforformicacidelectrooxidation.WhentheHCN/GOmassratiowas11,thePd/NHCNNGcatalysthadanoutstandingperformanceinthecatalyticoxidationofformicacid,withanactivity4.21timesthatofPd/C.Thisworkindicatesawaytoproducesuperiorcarbon-basedsupportmaterialsforelectrocatalysts,whichwillbebeneficialforthedevelopmentoffuelcells.Key words:Formicacidelectrooxidation；N-dopedhollowcarbonnanosphere；N-dopedgraphene；Supportmaterial；Three-dimen-sionalinterconnectedlayeredporousconfiguration1IntroductionWiththecontinuousdevelopmentofurbanmod-ernization,thedemandforenergyisincreasing.Theexcessivedependenceonfossilfuelshascausedmanynegativeimpactsonthehumanenvironment.There-fore,the development of clean energy and energystoragedeviceshasbecomeanewdirectionforcon-temporaryscientificresearchers12.Fuelcellsarere-cognizedasanenvironmentallyfriendlynewenergyutilization technology.Due to their versatility andflexibility34,directformicacidfuelcells(DFAFCs)anddirect methanol fuel cells(DMFCs)have re-ceived extensive research interest5.Compared tomethanol,formic acid has the advantages of beingnon-toxic,easier to handle,higher energy density,lowerpermeabilitythroughtheprotonexchangemem-brane,andmorepotentialforinclusioninregulargas-olineinfrastructure611.Commonlyusedformicacidfuel cell catalysts are palladium-based catalysts andplatinum-based catalysts12.The palladium-basedcatalystsarehighlyactiveandlowcostbutslightlylessstable13.Therefore,inordertorealizethelarge-scaleapplicationofDFAFCs,itisnecessarytodevel-oppalladium-basedcatalystswithhighstabilityandhighactivity6.Usually,theperformanceofpalladi-um-basedcatalystscanbeimprovedbypreparingal-loycatalystswithdifferentmorphologies,addingmet-alcompound promoters,doping non-metallic ele-ments,andimprovingthedispersionofpalladiumonthesupport1416.Asupportmaterialhasacrucialef-fectontheperformanceofacatalyst17.Excellentsup-Received date：2023-11-27；Revised date：2024-02-02Corresponding author：QUWei-li,Ph.D,Professor.E-mail：Author introduction：FANGYue.E-mail:Homepage:http:/ specific surface area and strongmetal-supportinteraction.Atpresent,carbonmateri-alsaremostwidelyusedtodisperseactivespeciesinfuelcellcatalysts.Moreover,differentcarbonmateri-als have different structures and properties1820.Amongthem,graphene,whichhasatwo-dimensionalstructure,is composed of a single layer of carbonatomsconnectedbysp2hybridization.Ithasanultra-high electron mobility and specific surface area,whichmakesitsuitableasasupportforfuelcellcata-lysts21.However,thetwo-dimensional(2D)structuretendstostackundervanderWaalsforces,leadingtoinaccesibilitytoreactionsitesandpoordispersionofmetalparticles.Ontheotherhand,comparedtoidealgraphene,thedeteriorationoftheelectricalconductiv-ityof graphene prepared from GO significantly re-duces the electrocatalytic performance of graphene-supportedpreciousmetalcatalysts.Therefore,itisde-sirabletoaddothercarbon-basedmaterialstomain-taintheconductivityofthecarrierandsimutaneouslyreducethestackingof2Dgraphene22.Asaresult,weconsider the introduction of spacers into graphenesheetstoprepareathree-dimensional(3D)intercon-nectedlayered porous configuration,which can im-provethedispersionofpreciousmetalsandtheacces-ibilityofactivesitestoreactants2325.Duetothepresenceofbothhydrophobicandhy-drophilicgroupsinGO,othercarbonmaterialscanbeefficientlydispersedonitssurface,whichisbenefi-cialforconstructinga3Dstructure.Amongnumer-ous carbon materials,hollow carbon nanospheres(HCNs)arecommonlyusedaselectrodematerialsbe-causeoftheiruniquehollowstructure,whichcanactas an ion reservoir,improve electron conduction,providelargesurfaceareaandshortendiffusionpathstofacilitatemasstransport26.ThusHCNscanbein-troducedintographeneasspacers.Usually,HCNsaresynthesizedbythetemplateagentmethod,whichisusedtocontrolthemorphologyofHCNs,butitspre-parationtimeislong,thestepsaretedious,andtheor-ganictemplate agent is not easily removed com-pletely,which may block the active sites ofpalladium2728.Therefore,weuseasimplenon-tem-platemethodtoprepareHCNs,whichgreatlysavesthepreparationtimeandachievesthegoalofmakingHCNsastwo-dimensionalgraphenespacers.Inaddition,dopingnitrogenatomscanoptimizethe electrical conductivity of graphene and hollowcarbonspheres,whichinturncanchangetheelectron-icstructure of the catalyst and improve the elec-trocatalyticactivity29.Thedopednitrogenatomscanalso effectively anchor the Pd NPs and change thenucleationprocessofthecatalyst,resultinginasmallparticlesizeoftheloadedPdNPs30.Also,nitrogendopingalsofacilitatestheremovaloftoxicintermedi-atesandprolongsthecatalystlifetime31.Wechosetoachieveeffectivedopingofnitrogenbyin-situdopingwith a mild nitrogen-containing precursor urea32.Herein,wecompositeHCNswithGOandcarryoutnitrogendopingtreatmenttoconstructauniquepor-ous NHCNNG support material.NHCNNGloadedwithPdNPs(denotedasPd/NHCNNG)arefabricatedasacatalystforformicacideletrooxidation.Theelectrochemicalperformanceoftheas-preparedsamples are investigated.Through electrochemicaltesting,itisfoundthatPd/NHCNNGcatalystexhib-itssignificantly improved electrocatalytic perform-anceduetotheuniqueporousconfigurationandnitro-gendoping.2Experimental 2.1 ChemicalsFlake graphite(50 m,99.9%)was purchasedfrom Forsman Corporation.Spherical carbon black(VulcanXC-72)waspurchasedfromCabotCorpora-tion.Palladium()chloride(PdCl2,99.9%)waspur-chased from J&K Chemica.Sodium borohydride(NaBH4,98.0%)was bought from Tianjin AoranFineChemicalResearchInstitute.Urea(CH4N2O,99.0%)wasboughtfromTianjinWindshipChemicalReagentTechnologyLtd.5%(massfraction)NafionwassuppliedbySigma-Aldrich.2.2 Sample synthesis2.2.1PreparationofNHCNNGGOwassynthesizedfromflakegraphitebythe322新型炭材料（中英文）第39卷modifiedHummersmethod33.TheHCNswerepre-paredaccordingtothefollowingmethods34.TogetHCNs,1gofsphericalcarbonblackwasputintoa100mLreactionkettle,15mLofconcentratednitricacidwasaddedandmixedthoroughly.Themixturewasputintoanautoclave,hydrothermallytreatedat150Cfor20h,washed,anddriedundervacuumfor4h.40mgofGOand40mgofHCNswereultrason-icallydispersedseparatelyin40mLultrapurewater.Then,thedispersantsweremixeduntiluniform.2gureanitrogenprecursorwasaddedintothemixture,stirredmechanicallyfor15min,putintothereactionkettle and treated at 160 C for 4 h.The resultingproductwaswashedwithwateranddriesundervacu-umfor4htoobtainNHCNNG-1:1.NHCNNG-1:2andNHCNNG-2:1supportmaterialswerepre-paredbythesamemethodbyadjustingthemassra-tiosofHCNstoGOto1:2and2:1,respectively.2.2.2SynthesisofPd/NHCNNG30mgNHCNNGand30mLultra-purewaterwereultrasonicallymixedfor1h,andthenewlypre-pared14.1mL5mmolL1PdCl2solutionwasaddedtotheabovemixtureunderagitation.ThepHvaluewas adjusted to between 9 and 10 using NaOH(1molL1).Then,22.8mgNaBH4wasslowlyaddedandstirredfor2h,washedwithultrapurewateranddriedundervacuumfor4htoobtainPd/NHCNNG.2.3 CharacterizationThemorphology of the Pd/NHCNNG cata-lystswasinvestigatedusingaHitachiS-4800scan-ning electron microscope(SEM)and a JEOL-JEM2100F transmission electron microscope(TEM).X-raydiffraction(XRD,RigakuD/maxTTR-)andX-rayphotoelectron spectroscopy(XPS,Thermo Sci-entific K-Alpha+)were used to analyze the crystalstructure and surface compositions of the catalysts.Raman spectra of different materials were collectedusingaHORIBAScientificLabRAMHREvolutionRaman microscope.Nitrogen adsorption-desorptiontest(BET,Quantachrome NOVA-3000)was carriedoutat77K.2.4 Electrochemical measurementsAll electrochemical testing of catalysts in thiswork was done in a three-electrode system on aCHI650Eelectrochemicalworkstation(thewholesys-temwaskeptina25Cwaterbath).Thethree-elec-trodesystemusedforelectrochemicaltestingconsistsofaglassycarbonelectrodecoatedwithacatalystastheworkingelectrode,aHg/Hg2SO4electrodeasthereference electrode,and a Pt plate electrode as theauxiliary electrode.The preparation method of theworkingelectrodeisasfollows.5.0mgofthepre-paredcatalystwasmixedwith2.5mLofethanolandultrasonicallydispersedfor20min.10uLofthewell-dispersedsuspensionwasthentransferredtoaglassycarbonelectrodewithadiameterof4mm,andthen5uLof5%(massfraction)Nafionwasdrippedontothesuspension-coatedelectrode,andtheelectrodewasplacedintheairtoallowittodrynaturally.CyclicvoltammetrytestswereaccomplishedinaN2-saturatedelectrolytecontaining0.5molL1H2SO4withorwithout0.5molL1HCOOH,withascanrateof50mVs1andascanrangeof0.630.32V.Elec-trochemicalimpedancetestswererealizedatatestpo-tential of 0.45 V and a test frequency of between0.01and100000Hz.Amperometrici-tcurvesweretestedatatestpotentialof0.4Vandatesttimeof3600s.The electrochemically active surface area(EC-SA)was evaluated based on the reduction peak ofPdOwithapotentialintervalof0.05to0.4VintheCVcurveobtainedin0.5molL1H2SO4solution.ThefollowingformulaisusedtocalculateECSA.ECSA=Q420mPdWhereQisthechargeadsorbedbyoxygenduringtheunderpotentialdeposition,Q=S/v,Sistheintegrationarea,visthesweepspeed(0.05Vs1)duringthetest,420Ccm2isthechargedensitycorrespondingtooxygenadsorptiononasinglelayerofPd,andmPdistheloadingmassofPdontheglassycarbonworkingelectrode(g).3ResultsanddiscussionInthisstudy,weestablishedasyntheticmethodasshowninFig.1.TheNHCNNGsupportmaterial第2期FANGYueetal:N-dopedhollowcarbonnanospheresembeddedinN-dopedgraphene323witha3Dinterconnectedporoushierarchicalconfig-uration was synthesized by embedding N-dopedHCNsintoN-dopedgraphenewithabottom-upsyn-thesismethod.Afterwards,PdNPsweredepositedontheobtainedsupport.Thesynthesismethodconsistsof2mainsteps:(1)self-assemblyofNHCNNGsup-portbyasolvothermalmethodusingureaasthenitro-gen source and(2)loading of Pd NPs ontoNHCNNGsupport by sodium borohydride reduc-tionmethod.Fig.2a-cshowstheSEMimagesofdif-ferentsamples,andHCNsofuniformsizearetightlystackedtogetherinFig.2a.Fig.2bshowsthefoldedstructureofreducedGO,wherethelayersofgrapheneareclusteredwitheachothertomakethelayerspa-cinglimitedduetovanderWaalsforces,showingathick edge structure,and some of the granular Pdnanocrystals are dispersed on the graphene surface.AfterNHCNsareembeddedinNG(Fig.2c),NHCNsarearbitrarilydistributedbetweenNG.Thethicknessofthegraphenelayeredgesdecreasesanda3Dinter-connectedlayeredporousmorphologyappears,whichisduetotheintroductionofNHCNsthathinderthestackingofNGlayers.ItindicatesthatHCNscanactasnanoscale spacers between graphene layers27.Fig.2dshowstheTEMimageofHCNs.Asshowninthefigure,thetreatedsphericalcarbonblackshowsahollowstructure,withashelllayerthicknessofabout20nmandacentralcavityofabout50nm.Owingtotheinhomogeneityofthecrystallizationofsphericalcarbonblack,the reaction activity of graphite do-mainsintheprimaryparticlesisdifferentfromthatofamorphousdomains.Theamorphousdomainsintheprimaryparticleshavehigherreactionactivityandarepreferentially oxidized by concentrated nitric acid,resultingintheformationofthehollowstructure34.TheTEMimagesofPd/NHCNareshowninFig.2eand2i,whichrevealsthatthesphericalcarbonblackappearstohaveanobvioushollowstructureafterox-idation,thus proving the successful synthesis ofHCNs34.Inaddition,PdNPscanbeuniformlysup-portedonHCNs,butasmallamountofaggregatesarestillseen.Pd/NG(Fig.2f,j)showsthelamellarstruc-tureofGO,thePdNPsisslightlylessagglomeratedcomparedwiththatontheHCNs,andthesizeofPdNPsis smaller.As seen in the TEM images ofPd/NHCNNG-1:1(Fig.2g,k),theHCNsaredis-tributedinthelamellargraphene,indicatingthattheHCNsaresuccessfullyinsertedintothelamellarstruc-tureofgraphene,anditisseenthatthedistributionofPdNPsinPd/NHCNNG-1:1ismoreuniformandthe agglomeration is further reduced.The averageparticle sizes of the measured Pd/NHCN,Pd/NG,Pd/NHCNNG-1:1are6.19,5.89and3.34nm,re-spectively.IntheHRTEMimageofPd/NHCNNG-1:1(Fig.2h),theclearlatticestripeof0.23nm,cor-responding to the(111)lattice plane of the face-centered cubic Pd lattice,further demonstrates thesuccessfulloadingofPdNPs.Inaddition,EDSdatawereprovidedtodeterm-ine the elemental compositions of Pd/NHCNNGcatalystswithdifferentproportionsandtheresultsareshowninFig.2l-n.EDSdatashowsthatthePdload-GraphiteOxidationExfoliationGraphene oxideHNO3NaBH4PdCl2Urea150 C,20 h160 CSpherical carbonNitrogen doped hollow carbonCarbonPalladium nanoparticlesNitrogenHollow carbon nanospheresPd/NHCNNGFig.1SchematicillustrationofthesynthesisofPd/NHCNNG324新型炭材料（中英文）第39卷inginallsamplesisaround20%(massfration),whichissimilartothetheoreticalvalue,andtheNcontentincreaseswiththeincreaseofGOintherawmaterial,whichindicatesthatNatomsareselectivelydopedonGO,enabling the high deposition efficiency on thesupport35.While excess N doping may reduce thegraphitizationdegreeoftheNHCNNGsupport,thusaffectingelectronicconductivity36.The crystal structures of Pd/NHCN,Pd/NG,Pd/NHCNNG-2:1,Pd/NHCNNG-1:1 and Pd/NHCNNG-1:2wereanalyzedbyXRD(Fig.3a).ItcanbeobservedthatthepeakofGOat10.5isthe(002)characteristicdiffractionpeakoflayeredGO.InPd/NGtheGOcharacteristicpeakdisappearsandtheC(002)broadpeakat25appears,indicatingthatGOisreducedtographene,resultinginthecharacteristicpeak of graphitic carbon material37.The Pd(111),(200),(220)and(311)characteristicpeaksappearinPd/NHCN,Pd/NG,Pd/NHCNNG-2:1,Pd/NHCNNG-1:1andPd/NHCNNG-1:2,indicatingthatPdisintheformofaface-centeredcubicstructure.Accord-ingtoSchellersformula,theparticlesizeofPdinthePd/NHCNNG-1:1catalystis4.05nm,whichisthesmallest among several samples,followed byPd/NHCNNG-1:2,Pd/NHCNNG-2:1,Pd/NG,and Pd/NHCN,whose particle sizes are 4.58,5.96,6.12and6.46nm,respectively.Fig.3bdepictstheRa-manspectraofGOandPd/NH