ResearchArticlewww.acsami.orgCoNi2S4NanosheetArraysSupportedonNickelFoamswithUltrahighApplications
CapacitanceforAqueousAsymmetricSupercapacitorWeiHu,RuqiChen,WeiXie,LilanZou,NiQin,andDinghuaBao*
StateKeyLaboratoryofOptoelectronicMaterialsandTechnologies,SchoolofPhysicsandEngineering,SunYat-SenUniversity,Guangzhou510275,China
*
SSupportingInformationABSTRACT:WereportthatCoNi2S4nanosheetarraysexhibitultrahighspeci?ccapacitanceof2906Fg?1andarealcapacitanceof6.39Fcm?2atacurrentdensityof5mAcm?2,aswellasgoodratecapabilityandcyclingstability,andsuperiorelectrochemical1performanceswithanenergydensityof33.9Whkg?atapowerdensityof409Wkg?1havebeenachievedinanassembledaqueousasymmetricsupercapacitor.TheCoNiby2Sa4nanosheetarrayswereinsitugrownonnickelfoamsfaciletwo-stephydrothermalmethod.TheformationmechanismoftheCoNi2S4nanosheetarrayswasbasedonananion-exchangereactioninvolvingthepseudoKirkendalle?ect.ThetwoaqueousasymmetricsupercapacitorsinseriesusingtheCoNi2S4nanosheetarraysasthepositiveelectrodescanpowerfour3-mm-diameterred-light-emittingdiodes.TheoutstandingsupercapacitiveperformanceofCoNi2S4nanosheetarrayscanbeattributedtoravine-likenanosheetarchitectureswithgoodmechanicalandelectricalcontact,lowcrystallinityandgoodwettabilitywithoutanannealingprocess,richredoxreactions,aswellashighconductivityandtransportrateforbothelectrolyteionsandelectrons.OurresultsdemonstratethatCoNi2S4nanosheetarraysarepromisingelectrodematerialsforsupercapacitorapplications.KEYWORDS:CoNi2S4,nanosheetarrays,anion-exchangereaction,supercapacitor,energystorage1.INTRODUCTIONTransition-metaloxides,hydroxides,andconductingpoly-Recently,urgentandincreasingdemandsforclean,e?cient,mersaredemonstratedinsupercapacitorapplicationsbasedonandsustainableenergyhavegreatlystimulatedsubstantialtheirpseudocapacitiveproperties.7?11Morerecently,metalresearchonalternativeenergystorageandconversion.Amongsul?deshaveattractedextensiveattentionowingtotheirthee?ectiveandpracticaltechnologiesforenergystorageandexcellentpotentialapplicationsinoptics,catalysis,sensing,solarconversion,supercapacitors,alsocalledelectrochemicalcapaci-energy,andbatteries.12Furthermore,metalsul?desarealsotors,havebeenconsideredasoneofthemostpromisingespeciallynotablecandidatesforpseudocapacitorsbecauseofenergy-storagedevicesandwildlyinvestigatedforapplicationstheir13?low17electronegativityandhighelectrochemicalactiv-inportableelectronicdevicesandelectricvehiclesbecauseofity.Severalmetalsul?descanbeeasilysynthesizedbyatheiradvantagessuchasfastcharge?dischargeprocess,highfacileande?ectivehydrothermalanion-exchangereactionpowerdensity,andlongcyclelife.1?4Generally,electro-method.Puetal.preparedCo9S8nanotubearrayssupportedchemicalcapacitorshavebeendividedintotwotypesintermsonnickelfoamforahigh-performancesupercapacitor.13Xiaetofthecharge-storagemechanism.Oneisanelectrochemicalal.synthesizedNiS,Co9S8,CoS,andCoS/NiSnanostructuresdouble-layercapacitor(EDLC),whichisdominatedbyforelectrochemicalenergystorage.14Besides,ithasbeenelectrostaticchargedi?usionandaccumulationattheinterfaceproventhatNi?Cosul?deshavemuchloweropticalband-gapofthecarbonelectrodeandelectrolyte.Thecarbon-basedenergyandhigherconductivitycomparedtoNi?Cooxides.18Theelectrodematerials(e.g.,activecarbon,carbonnanotubes,andgraphene)?substitutionofoxygenwithsulfurcouldcreatea?6exhibitexcellentconductivityandchemicalthanexiblethatstructureofoxygen.because19Intheaddition,electronegativityternaryofNisulfur?Coissullowermore?desstability.4However,thecarbon-basedelectrodessu?erfrompossessricherredoxreactionandhigherelectronicconductivity
relativelylowspeci?ccapacitance.6,7Theotherone,incontrast,isapseudocapacitor,whichismainlydominatedbyreversiblefastsurfaceFaradaicredoxreactionsandcanprovidemuchReceived:August11,2014Accepted:October16,2014higherspeci?ccapacitancethanEDLC.3,7,8Published:October16,2014
?2014AmericanChemicalSociety
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ACSAppliedMaterials&InterfacesResearchArticlecomparedwithbinarymetalsul?desincludingNiS,CoS,and2.2.AsymmetricSupercapacitorFabrication.ToconstructanCo9S8.15,20VariousmorphologiesandstructuresofNi?Coasymmetricsul?des,suchasusedsupercapacitor,theNi?18NiCo2S4nanotubes,15?17NiCoelectrode.astheCosul?denanosheetarrayswere21Ni?Cosul?denanowires,20NiCo2S4urchin-likenanostructures,carbon,Thepositiveelectrodeandactivecarbonasthenegative23shape-controlledNiCo2S4nano-networks,andNi2S4,22CoNi2S4mushroom-likearrays,ratioxCo3?xS4hollownanoprisms,24havebeenobtainedtoinvestigatethesupercapacitiveperformance.For°instance,Pengetal.havesynthesizedNiCoseparatorCforof8:1:1,acetylenenegativeblack,andelectrodepoly(vinylidenewasprepared?uoride)bymixingwithaactivemass12h.whichThewasasymmetricthenpressedsupercapacitoronnickelfoamwasanddriedat80graphenewithhighcapacitanceandlongcycle2Slife.4nanosheets25onChenetal.was(NKK,MPF20AC-100),anda2Mseparatedbya2.3.usedKOHaqueoussolutionMaterialastheelectrolyte.
Characterization.ThestructuresandmorphologieshaveinsitufabricatedNiCooftoachieveultrahigharealcapacitance.2S4nanotube15arraysonnickelfoamdiHigh-performancesupercapacitorelectrodesshoulddisplayλ?ractionthesesamples(XRD;EMPYREAN,werecharacterizedPANalyticalbygrazing-incidentwithX-raycharacteristicssuchashighutilizatione?ciencyandhighmass250),=0.15406loadingofelectroactivematerialsaswellashightransportrateTA?eld-emissionnm),X-rayCuKαradiation,scanningphotoelectronelectronspectroscopymicroscopy(XPS;ESCALABforbothelectrolyteionsandelectrons.Thus,itisofsigni?cance(TEM;400F),andhigh-resolutiontransmissionelectron(FESEM;microscopyQUAN-topreparehighmassloadingactivematerialsdirectlythroughtroscopyJEOLmechanicalandelectricalcontactwiththecurrentcollectorforments(EDS)JEM-2010HR)detector.withanenergy-dispersiveX-rayspec-usingimprovedsupercapacitorperformance.Ithasbeenreported2.4.awereElectrochemicalsurfaceperformedareaanalyzertoinvestigateNitrogenMeasurements.(Micromeriticstheadsorption/desorptionsurfacecharacteristicsmeasure-at77KTheASAPnominal2020).
areaofthethatNi27CoNi2S4nanoparticles,26CoNi2Swithadditivebinder,andCoNi4/graphenenanocompo-sitesimmersed?CooxideandNi?Cosul?arrays23onnickelfoamassupercapacitorelectrodes.2S4mushroom-likeHowever,electrochemicalintheelectrolyteiscontrolleddenanosheetataroundarrays1on×nickel1cm2.foamthereisnoreportonCoNichemicalforsupercapacitor2S4nanosheetarraysdirectlydepositedonnickelfoamsapplications.Itiselectrolyteworkstationtests(Chenhua,wereconductedShanghai)withinanaaqueousCHI660E2.0Melectro-TheKOHexpectedthatCoNicounter2S4nanosheetarraysgrowndirectlyonnickelfoamscanleadtosuperiorpseudocapacitiveperform-referenceelectrodewithathree-electrodeelectrode.
andastandardcellcalomelwhereplatinumelectrodefoil(SCE)servesasasthetheance.Inthiswork,usingafaciletwo-stephydrothermal3.RESULTSANDDISCUSSION
method,wehavesuccessfullyinsitusynthesizedCoNiarraysonnickelfoams.Theas-fabricatedCoNi2Snanosheet4Figure1illustratesthefabricationprocessoftheNi?Cooxidenanosheetarraysmanifestultrahighspeci?candareal2S4andNi?Cosul?denanosheetarraysonnickelfoamthrougha
capacitancevaluesaswellasgoodcyclingstability.Theassembledaqueousasymmetricsupercapacitorcellexhibitshighenergydensityandpowerdensity.OurresultssuggestthattheCoNi2S4nanosheetarrayscanactashigh-performanceelectrodematerialsforsupercapacitorapplications.
2.EXPERIMENTALSECTION
2.1.MaterialsPreparation.AllofthereagentswereanalyticalgradedepositionandimmersedinofwereaNiusedwithoutfurtherpuri?cation.Beforethe3?MCoHClprecursorsolutiononfornickel10minfoam,totheremovenickelthefoampossiblewasFigure1.SchematicillustrationofthefabricationprocessforNi?Cosurfaceprecursor,oxide,andsul?denanosheetarrays.
wateroxidelayerandcleanedsequentiallyinethanolanddeionized15Typically,for15min,respectively.
mMhexamethylenetetramine2.5mMNi(NO3)2·6Hwere2O,dissolved5mMCo(NOin75mL3)2of·6Hdeionized2O,andwatertwo-stepmethod.First,theNi?Coprecursornanosheetwastransferredbymagneticstirringfor30min.Then,directlygrownonnickelfoambythehydrothermalprocess.pieceThentheNi?CooxideandNi?Cosul?denanosheetarraysofwereconvertedfromtheNi?Coprecursornanosheetwastheofuniformlyautoclavenickelintofoama100coveredat95wasmLTe?on-linedstainlesssteelthesolutionautoclave.wasA°Cimmersedwithforpoly(tetra8h,andinthe?thesolutionuoroethylene)topsidefollowedofthebyheatingtapenickeltopreventfoamsubsequentlybythecalcinationandhydrothermalanion-theexchangereactionprocesses,respectively.Theion-exchangetemperature,solutioncontamination.Afterreactionincludingcation-andanion-exchangereactionsisanethanole?ectiveandlow-costThen,anddeionizedtheNi?Coprecursoronthenickelautoclavefoamcooledwaswashedtoroomwith30methodforchemicaltransformationofnanomaterials.28?NanostructuredCdS,CdS-CuhydrothermaltheNienvironment?Coprecursorwaterseveralwithsodiumontimesnickelanddriedat60°Cfor5h.sul?de.foamInbrief,was25treatedmMNainaSmaterialshavebeensynthesizedbyacation-exchange2S,andCdS-Ag9H2S·precursor2Owasreaction.231,32TheformationmechanismofNi?Cosul?desinstainlessourworkisbasedonananion-exchangereactionbetweenS2?Thesteelanddissolvedautoclave.solutionin75weremLofdeionizedwater,andthentheNi?CoThetransferredautoclavewasintoheateda100tomL120Te°C?on-linedinthesodiumsul?desolutionandOH?andCO32?intheNi?washedNi?withCoethanolsul?denanosheetanddeionizedarrayswater,onnickelanddriedfoamatwere60°Cobtained,for8h.for5h.Coprecursornanosheetathydrothermalenvironment.WealsoForcontrast,theNi?Cooxidenanosheetdemonstratedbeen2?thattheanion-exchangereactioncantakeplacebetweenSandO2?,andthustheNi?CooxidenanosheetairarrayscouldbeconvertedintoNi?Cosul?denanosheetarrays.minambientsynthesizedbyannealingtheNi?Coarraysprecursoronnickelnanosheetfoaminhavean?1FigureS1intheSupportingInformation(SI)showsthesul?de.atmosphereat350°Cfor2hwithaheatingrateof1°CnanosheetTheaveragearraysmassonloadingsnickelfoamofNiwere?Coapproximatelyprecursor,oxide,1.4,and1.2,FESEMimagesofNi?Cosul?denanosheetarraysbyanion-and2.2mgcm?2,respectively.
exchangereactionfromNi?Cooxidenanosheetarrays.
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ACSAppliedMaterials&InterfacesResearchArticleFigure2showsthetypicalFESEMimagesoftheNi?Coravine-likefeatureisrougherthanthatoftheNi?Cooxideprecursor(Figure2a,b),oxide(Figure2c,d,g),andsul?de
nanosheetarrays.Inaddition,thethicknessofNi?Cosul?denanosheetarraysincreasesslightly.ThroughBrunauer?Emmett?Teller(BET)surfaceareaanalysis(FigureS3intheSI),itcanbeseen2thattheBETsurfaceareaofbothNi?Cooxide(106.7mg?1)andNi?Cosul?de(22.9m2g?1)nanosheetarraysishigherthanthatofNi?Coprecursor(18.2m2g?1)nanosheetarrays.ThehugeincreaseoftheBETsurfaceareaofNi?CooxidecouldbecausedbytheformationofquantitiesofmesoporesafterannealingNi?Coprecursor.TheincreasedBETsurfaceareaofNi?Cosul?decouldbeascribedtothegrowthofaravine-likesurfaceandafewlargeporesafterthehydrothermalanion-exchangereaction.
Figure3ashowstheXRDpatternsoftheNi?CooxideandNi?Cosul?denanosheetarraysonnickelfoams.Thetwo
Figure3.(a)XRDpatternsofNi?Cooxideandsul?denanosheetarraysFigure2pforonNi?nickelCosulfoam.?deXPSnanosheetspectraarrays.
of(b)Co2p,(c)Ni2p,and(d)Sb)Ni?2.CoTypicalprecursorFESEMnanosheetimagesatarrays,di?erent(cmagniand?d)cationsNi?Coof(aoxideandnanosheetontypicalpeaks,markedwithasterisks,originatefromnickelfoam.nanosheetnickelarrays,arraysfoamand(eandonnickel(gf)andNi?foamh)CowithNisul??denanosheetarrayssupportedhighCooxidemagni?andcation,Ni?respectively.
Cosul?deAlthoughthepeaksofNi?Cooxidenanosheetarraysarerelativelyweakandbroad,theXRDpeaksat36°and64°,correspondingtothe(311)and(440)planes,respectively,and(Figure2e,f,h)nanosheetarrayssupportedonnickelfoam.theselected-areaelectrondi?ractionpattern(SAED)ofNi?CoObviously,thenanosheetarraysuniformlycovernickelfoam,oxidenanosheetarrays(FigureS4intheSI),indicatingtheandtheyinterconnectwitheachothertoformawall-like(311),(400),and(620)planes,con?rmtheformationofthestructure.AsshowninFigure2c,d,theinterconnectedwall-likeNiCofeatureofNi?Cooxidenanosheetarrays,convertedfromNi?2O4spinelphase.ThisresultisconsistentwiththepreviousreportonNiCoCoprecursornanosheetarraysafterthermaltransformation,2O4nanosheetarrayssynthesizedbyasimilarfabricationmethod.33Thedi?ractionpeaksofNi?Coretainedperfectly.TheNi?Cooxidenanosheetarrayshaveasul?denanosheetarraysarealsoweak,indicatingthelowthicknessofapproximately45nm,whichisfavorableforfullcrystallinity.Fourdi?ractionpeaksat31.4°,38.2°,50.1°,andutilizationoftheactivematerials.Meanwhile,theNi?Co55.1°,correspondingtothe(311),(400),(511),and(440)sul?denanosheetarraysalsoretainedthewall-likenanosheetdi?ractionplanes,respectively,canbeindexedtothecubicmorphologywellafterthehydrothermalanion-exchangephaseofCoNireactionprocess.FigureS2intheSIdisplaystheelemental2S4(JCPDS24-0334)orNiCoandchemical2S4(JCPDS43-1477).TheelementalcompositionstateofthemappingimagesofNi?Cosul?denanosheetarraysonnickelNi?Cosul?denanosheetarrayshavebeeninvestigatedbyXPSfoam,whichindicatethatthenickel,cobalt,andsulfurelementsmeasurements,andthecorrespondingresultsarepresentedindisperseduniformlyandcontinuously.Fromthehigh-Figure3b?d.TheCo2pandNi2phigh-resolutionspectracanmagni?cationFESEMimagesinFigure2g,h,thereisalittlebe?ttedwithtwospin?orbitdoubletsandtwoshakeupdi?erenceinthemorphologiesbetweentheNi?Cooxideandsatellites(Sat.)byusingaGaussian?ttingmethod.Asshowninsul?denanosheetarrays.ThesurfaceofNi?Cosul?dewitha
Figure3b,theintensityoftheCo2ppeaksisrelativelyweak.
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ACSAppliedMaterials&InterfacesResearchArticleThebindingenergiesataround777.62and794.13eVofthespondingSAEDimageshowninFigure4dimpliestheCo2ppeaksareassignedtoCo3+andthebindingenergiesatpolycrystallinenatureofNi?Cosul?denanosheetarrays,and783.25and798.42eVtoCo2+.Likewise,intheNi2pspectrathedi?ractionringscanbereadilyindexedtothe(311),(400),showninFigure2+3c,thepeaksat855.88and873.50eVand(440)planesoftheCoNiindicated3+Niandthepeaksat857.40and875.18eVdenotedcorrespondstotheaforementioned2S4orNiCoXRDresult.
2S4phase,whichNi.IntheS2pspectra(Figure3d),thebindingenergyInaddition,EDSanalysiswasconductedtoexaminethecenteredat168.83eVcanbe?ttedbyonemainpeakandonecompositionofNi?Cooxideandsul?denanosheetarrays.Theshakeupsatellitepeak.Thepeakcenteredatabout163.22eVelementscopperandcarbon(fromtheTEMcoppergrid),agreeswiththebinding18,27,34energiesofmetal?sulfurbonding(Ni?chromium(fromthespecimenholder),nickel,cobalt,sulfur,SandCo?Sbonding).Therefore,thesurfaceoftheNi?andoxygenweredetected,andtheelementratioofCo/Ni/SisCo3+sul?denanosheetarraysiscomposedofCo2+,Co3+,Ni2+,1:1.85:3.5inNi?Cosul?denanosheetarrays,asshowninNi,andS2?.FigureS5intheSIshowstheXPSspectraoftheFigure4e.OnthebasisofanalysisoftheXRD,XPS,andTEMNi?Cooxidenanosheetarrays.Theresultsareinaccordanceresults,itcanbeidenti?edthatNi?Cosul?denanosheetarrayswiththereportedchemicalstatesofNiCoarecomposedofCoNidetailedmorphologyandstructure2O4.33Thecharacteristicsof2S4nanosheetarrays.TEMimagesandEDSspectraofNi?CooxidenanosheetarraysareshowninNi?Cosul?denanosheetarraysarefurtherinvestigatedbyFigureS4intheSI,indicatingthatNi?CooxidenanosheetTEMcharacterization.AsshowninFigure4a,b,thesurfaceofarraysexhibitmesoporousstructureandtheelementratiooftheNi?Cosul?denanosheetarraysisdenselywrinkledandtheCo/Ni/isapproximately1.15:1.Ni?Cooxidenanosheetarraysplanarsizeisabout0.8μm,whichareingoodagreementwitharenonstoichiometricNiCotheresultsofFESEMimages.Thelatticefringesshownin2O4,whichalsoimpliesthattheNi:Coelementratioisabout1.15:1inNi?CoprecursorFigure4ccanbeindexedtothe(111)crystalplaneofthecubicnanosheetarrays.Thevariationofthecomponentsandratioofphase.TheinsetofFigure4cshowsthecorrespondingfastNi/CoinNi?Cosul?decouldbeattributedtothepseudoFouriertransform(FFT)pattern.Simultaneously,thecorre-
Kirkendalle?ect.Atthebeginningofhydrothermalanion-exchangereactionprocess,thereactionsbetweenthesulfurionsandNi?CoprecursorresultedintheformationofaravinesurfacelayerofNi?Cosul?de.Then,thereactionscontinuedbythedi?usionofsulfurionsthroughtheinterface.Yinetal.haveproposedthatthedirectconversionoftheprecursorcoretothemetalsul?deshellisthereforeobstructedbythesurfacelayer,andsomepreviousworkdemonstratedthatvoidmetalsul?denanostructuressuchasnanotubesandhollownanoplatescouldbesynthesizedbecauseoftheunequaldi?usionofthecobaltandsulfurions.13,17,29,30,34,35However,inourstudy,noobviousvoidnanostructuresbutafewlargeporesandachangedNi/Coelementratioofnanosheetarraysformedafteranion-exchangereaction.Thiscouldbeattributedtotherelativelysu?cientanion-exchangereactionbetweentheNi?CoprecursorandsulfurionsbecauseoftheuniquenanosheetarchitectureandthethinthicknessoftheNi?Coprecursornanosheet.ThevariationoftheNi/Coelementratio,whichchangedfromabout1:1.15oftheNi?Coprecursortoabout1.85:1ofNi?Cosul?de,couldbebecausetheoutwarddi?usionofthecobaltionsismuchfasterthantheinwarddi?usionofthesulfurions.13,17,29,30,34Inaddition,theremightbealittlereactionbetweennickelfoamandsulfurions.
Todemonstratethepotentialapplicationsinsupercapacitors,weinvestigatedtheelectrochemicalperformanceofNi?Coprecursor,oxide,andsul?denanosheetarraysonnickelfoam.Figure5ashowsthetypicalcyclicvoltammetry(CV)curvesofCoNipotential2S4nanosheetarrayssupportedonnickelfoamwiththewindowfrom?0.1to0.6V(vsSCE)atvarioussweepratesrangingfrom3to20mVs?1.Clearly,theshapeoftheCVcurvesshowsapairofredoxpeaksduringthecathodicandanodicsweeps,whichisapparentlydistinctfromtheEDLCscharacterizedbynearlyrectangularCVcurves.ThisindicatesthatthecapacityofCoNireactions.2SWith4nanosheetarraysresultsfromtheFaradaicredoxanincreaseofthesweeprates,theanodicpeakcurrentdensityincreasesandthecathodicpeakFigure4.(a?c)TEMimagesofNi?Cosul?denanosheetarraysatcurrentdensitydecreases,suggestingarelativelylowresistancedioftheelectrodeandfastredoxreactionsattheinterfaceofthepattern?erentelectrodeandelectrolyte.36Ni?Coofmagnisula?deNi??cations,nanosheetCosul?derespectively.array.nanosheetThearray.(d)Correspondinginset(e)SAEDofpartEDScspectrashowsoftheaFigure5bshowsthegalvanostaticcharge?discharge(GCD)correspondingFFTpattern.
curvesofCoNi2S4nanosheetarraysonnickelfoamwitha
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