PCCPFig.5(a)to(c)areCVcurvesofsupercapacitorsbasedonthe3DG(DeviceA)andGQD–3DGcompositeswith5h(DeviceB)and10h(DeviceC)depositiontime,respectively.Scanratesare0.2Vsà1,0.4Vsà1,0.6Vsà1,0.8Vsà1and1Vsà1,respectively,andthearrowsindicatethedirectionofincreasingscanrates.(d)to(f)aregalvanostaticcharge–dischargecurvesatdifferentdischargecurrentsforDevicesA,BandC,respectively.
promisingmaterialforsupercapacitors,andasplottedinFig.5a,DeviceAshowedtypicalcapacitiveCVcurvesclosetotheidealelectrochemicaldoublelayerbehavior.UponthedepositionofGQDsonthesurfaceof3DG,theCVcurvesofDevicesBandCremainedundistortedinbothanodicandcathodicdirectionsevenathighscanrates(Fig.5bandc),implyingthatelectrodepositingGQDsdidnothaveanyadverseeffectontheEDLattheelectrode/electrolyteinterface.Mean-while,thecapacitivecurrentofDevicesBandCnotablyincreasedatallscanratesaftertheanchorageofGQDsonthe3DG,sug-gestingthatthedepositionoftheGQDswellimprovedtheelectrochemicalcapacitanceofthecompositeelectrode.Com-paredtothe3DG,theGQD–3DGcompositeshadalargerspecificsurfaceareatorendermoresurfaceactivesitesandaccessibleedgesfortheionadsorption–desorption,therefore,improvingtheEDLcapacitivepropertiesoftheelectrode.31ItisinterestingtonotethattheareasurroundedbytheCVcurvesforDevicesAtoCincreasesinthefollowingorderofAoBoC,implyingthatDeviceCwouldhavethehighestspecificcapacitanceasthespecificcapacitanceofthesupercapacitorwasreportedtobedirectlyrelatedtotheareasurroundedbytheCVcurvesmeasuredatthesamescanrate.38AsshowninFig.5dtof,thespecificcapacitancesoftheDevicesAtoCwerefurthermeasuredbyagalvanostaticcharge–dischargeexperimentbetween0and+0.8Vatdifferentdis-chargecurrents.NoobviousIRdropwasobservedatthestartofalldischargecurves,indicatingthatallthedeviceshadasmallinternalseriesresistanceandefficientextractionofstored
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Table1Directcomparisonofthespecificcapacitance(Cm)ofsuper-capacitorsbasedondi?erent3DGmaterials
No.ElectrodematerialsCm(Fgà1)Ref.1Ni(OH)2/grapheneandporousgraphene218.41023Dgrapheneaerogel
128403MnO2depositedongrapheneoxidecomposites21684N-dopedgraphiticcarbonnanocages2484153DGraphenehydrogelfilms
186426Silvernanoparticlesdecoratedgraphenefoam110437Mn3O4/reducedgrapheneoxidehydrogel
148448CompositeofMn2O3with3Dgraphene/CNTs280459N-dopedgraphene–CNTnetworks18046103DMnO112/graphenehydrogel
24247Compositeofgrapheneandactivatedcarbon116.514
12
GQD–3DG-10hcomposite
268
Current
energy,aswasalsoconfirmedbythelinearandsymmetricalcharge–dischargecurves.Themass-specificcapacitanceCmiscalculatedfromtheequation:Cm=2I/m(dV/dt),39whereIisthedischargecurrent,dV/dtistheslopeofthedischargecurveandmisthemassofoneelectrode,respectively.Thecalculatedspecificcapacitancesare136Fgà1,192Fgà1and268Fgà1atadischargecurrentdensityof1.25Agà1(I=125mA)forDevicesA,BandC,respectively.InagreementwiththeCVcurvesinFig.5atoc,thespecificcapacitancesofDeviceBandCarelargerthanDeviceA,suggestingtheeffectivenessofdepositingGQDsinimprovingtheperformanceof3DGsupercapacitors.AnoptimizedGQDdepositiontimeof10hrendersthebestdevice,DeviceC,withahighcapacitanceof268Fgà1,whichisamongthebestvaluesreportedforsupercapacitorsbasedon3Dgrapheneorgraphene–metaloxidecomposites(Table1).ToruleoutanypossibilitythattheGQDdepositionprocesscouldcontributetotheperformanceimprovementinthesuper-capacitor,controlsamplesof3DGwereimmersedinaGQD-freesolutionandsubjectedtoa+2Vbiasfor10hbeforetheywereeventuallyassembledinthesamesymmetricaltwo-electrodeconfigurationtogiveDeviceD.Fig.S1(ESI?)showsthegalvano-staticcharge–dischargecurvesofDeviceDatacurrentdensityof1.25Agà1,2.5Agà1and5Agà1,respectively.Thecalculatedcapacitancefromthecharge–dischargecurveis131Fgà1atadischargecurrentdensityof1.25Agà1,matchingthatoftheDeviceA(3DG)andconfirmingthattheobservednotableimprovementinthesupercapacitorperformanceisrelatedtothedepositionofGQDsratherthantheapplicationofthebiasintheelectrodepositionprocess.
Electrochemicalimpedancespectroscopy(EIS)wasper-formedtofurtherevaluatetheelectrochemicalperformanceofDevicesAandC.ForanidealEDLC,thelowfrequencyregionofitsNyquistplotisastraightlineperpendiculartotherealaxisoftheimpedance.Fig.6acomparestheEISspectraofbothdevices,wherebothcurvesareclosetotheimaginaryaxisatlowfrequenciesandshowcharacteristicsapproachingtheidealEDLC.However,theEISspectrumofDeviceC(redcurve)wasmoreverticaltotherealaxisthanthatofDeviceA,revealingamoreidealcapacitivebehaviorofDeviceCthanDeviceA.48,49ThelackofanobviousRCsemicircleathighfrequenciesinbothdevicesindicatesthefastchargetransferacrossthe
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Published on 24 July 2014. Downloaded on 12/09/2014 08:19:05. PaperFig.6(a)NyquistplotsofDevicesA(black)andC(red)measuredunderopen-circuitconditions.Theinsetshowstheenlargedareawithimpe-dancevaluesbetween0and50O.(b)BodeplotsofDevicesAandC.
electrode/electrolyteinterfaceinboth3DGandGQD–3DGcompositematerialsthatleadstotheexcellentconditionsoftheassembledsupercapacitors.
Meanwhile,thehigh-frequencyinterceptwiththerealaxisistheequivalentseriesresistance(Rs),representingthesumoftheelectrolytesolutionresistance,theintrinsicresistanceoftheactivematerialandthecontactresistanceattheelectrode/electrolyteinterface.50Fromthemagnifiedhigh-frequencyregionintheinsetofFig.6a,itisclearlyobservedthattheDeviceChasalowerRsvaluethanthatofDeviceA,showingthatthedepositionofGQDshelpedtodecreasetheundesiredRsandcontributetotheimprovedperformanceoftheGQD–3DGsupercapacitor.51ThedependenceofthephaseangleonthefrequencyofthesupercapacitorsisplottedinFig.6b.IntheBodeplot,anidealEDLCwouldhaveaphaseanglecloseto901atlowfrequencies.Herein,thephaseanglesofDeviceAandDeviceCare83.81and87.31at0.01Hz,respectively.Moreimportantly,thephaseangleofDeviceCishigherthan801atanyfrequencylowerthan1Hz,implyingthatcomparedtoDeviceA,DeviceCapproachesanidealEDLCandthedeposi-tionofGQDseffectivelyimprovedthecapacitivepropertiesofthe3DGsupercapacitor.36ItisreportedthatthecapacitivebehaviorofEDLCscouldbeclassifiedintotwodifferentcate-goriesinviewoftheporesizeoftheelectrodematerial,wheremesoporouscarbonsofporeslargerthan2nmaregovernedbythetraditionaldoublelayermodelandcarbonmaterialsofmicroporescommensuratewith1nmorlessarewelldescribedbyanelectricdouble-cylindercapacitor(EDCC)modelasso-ciatedwiththecurvatureeffectoftheelectrodesurface.4,52SincethesizeoftheGQDsisonly2–5nm,itisprobablethatsubnanometerporeswerecreatedbetweendepositedGQDsandthe3DGscaffoldorevenbetweenneighboringGQDs,aligningpartiallyorcompletelydesolvatedelectrolyteionsin
Fig.7(a)ThecapacitanceofDeviceCunderthecyclingtestsfor5000charge–dischargecyclesand(b)theprocessoftencharge–dischargecycles.
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PCCP
thesecylindricalporesandformingthe‘‘electricwireincylinder’’.4ThisiswellsupportedbythenotablyincreasedpresenceofB1nmandsubnanometerporeswiththedeposi-tionofGQDsshowninFig.4b,andconsequently,thesuper-capacitorbasedontheGQD–3DGelectrodescanbebetterdescribedasacombinationofEDLCandEDCC,aphenomenonalsoobservedbyHahmetal.inasupercapacitorconstructedonacarbonnanotube–nanocuphybridstructure.53Thecyclingstabilityofthesupercapacitorisanimportantparametertoevaluateitspotentialforpracticalapplications.Fig.7summarizesthetemporalevolutionofthespecificcapa-citanceofDeviceCoverconsecutivecharge–dischargecyclesatacurrentdensityof5Agà1.Thedeviceretainedmorethan90%ofitsinitialcapacitanceafter5000charge–dischargecyclesandnoobviousdegradationinthecapacitancecouldbeobserveddur-ingthecyclingexperiment,indicatingthatourGQD–3DGsuper-capacitorhasagoodlong-termelectrochemicalstability.
Conclusions
Insummary,wehavesuccessfullydemonstratedanelectro-chemicaldepositionmethodtoassembletheGQDsonthe3DGelectrodesandtestedtheirperformanceassupercapacitors.TheelectrochemicalassemblyoftheGQDsonthe3DGpro-ceededsmoothlyandledtotheformationofauniformfilmonthesurfaceofthe3DG.SupercapacitorsfabricatedfromGQD–3DGcompositeelectrodeswith10hGQDdepositionexhibitedahighcapacitanceof268Fgà1,representingamorethan90%improvementoverthatofbare3DGelectrodes(136Fgà1).ConsideringtheconvenienceoftheelectrodepositionofGQDs,thecurrentmethodcouldalsobeusedinotherwell-definedelectrodematerials,suchascarbonnanotubes,carbonaerogelsandsoon,tofurtherboosttheperformanceofthesupercapacitors.
Acknowledgements
WethankthefinancialsupportfromtheNSFC(21103010,21373027,21325415,21174019and51161120361).
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