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Therefore,theiondiffusion(slowerthanelectrontransport)andaccumulationarekeyfactorsaffectingthecapacitiveperformanceofsupercapacitors.Atareactiontimeof10min,MnO2/IL–RGOnanocompositeswereassembledintothehier-archicalstructuresthroughtheelectrochemicalassembly.ThestructuraleffectofMnO2/IL–RGOnanocompositesonthefastiondiffusionandlargesurfaceareawascon?rmedbyimpedanceandBETanalyses.Inparticular,hierarchicalMnO2/IL–RGOnanocompositesshowedsmallerinternalresistanceandcharge-transferresistanceaswellashigherspeci?ccapacitancethanthoseofphysicallymixedMnO2/IL–RGOnanocompositesdespitetheidenticalcontentsofMnO2(Fig.S8?).Accordingly,the3Dinterconnectedstructuresprovidedashortdiffusionpathwayforthefastiontransportandaneasyaccessibilitytotheporesurface.Furthermore,thelayerofMnO2ofnano-compositescontributedtotheadditionalcapacitancebymeansoftheredoxchemistry.DespitethelowerloadingofMnO2atadepositiontimeof10min,theMnO2/IL–RGOnanocompositesobtainedamaximumcapacitanceduetothecontributionofthehierarchicalstructuretothefastiondiffusion,easyaccessibility,andlargesurfacearea.
RatecapabilityandcyclestabilityofMnO2/IL–RGOnano-compositesobtainedatadepositiontimeof10min,whichiscriticalforpracticalpowerapplications,weredemonstratedherebyfullytakingadvantageofhierarchicalstructures.Galvano-staticcharge–dischargetestswereperformedbyvaryingcurrentdensitiesandpureMnO2?lmsandIL–RGOnanocompositeswereusedascontrolsamplesasshowninFig.6.ThepreparationprocedureofpureMnO2?lmbyelectrodepositionisprovidedintheESI.?IL–RGOandMnO2/IL–RGOnanocompositesrevealedidealcapacitivebehaviorsintermsofsmallinternalresistanceandsymmetriccharge–dischargeresponse.Incontrast,thepureMnO2?lmhasalargeinternalresistance(IRdrop)andasymmetricresponse.Thisresultwasattributedtotheirrevers-ibleredoxreactionsandunfavorableionandelectrontransferasaconsequenceoftheintrinsicallypoorelectricalconductivityofMnO2.Thespeci?ccapacitanceofMnO2/IL–RGOnano-compositesatacurrentdensityof200mAgà1wasmeasuredtobe266Fgà1,higherthanthoseofMnO2?lm(159Fgà1)andIL–RGOnanocomposite(53.4Fgà1).TheeffectofthehierarchicalstructureinMnO2/IL–RGOnanocompositesonthefavourablecapacitativebehaviorwassupportedbydemonstratingtheratecapability(Fig.6b).TheMnO2/IL–RGOnanocompositesstillretained61%ofspeci?ccapacitance(from266to164Fgà1),varyingthecurrentdensityfrom200mAgà1to30Agà1.ThehighratecapabilityofMnO2/IL–RGOnanocompositeswasattributedtotheinterconnectedco-assemblyofMnO2andIL–RGOintothehierarchicalstructure.Owingtothehierarchicalporousstructureandhighsurfacearea,theMnO2providedhighenergystoragecapacity,whilethehighlyconductiveIL–RGOsheetfacilitatedtheelectrontransportandiondiffusionintotheredoxsitesofnanocomposites.
Thelong-termcyclestabilityoftheMnO2?lm,IL–RGO,andMnO2/IL–RGOnanocomposites(10minofdepositiontime)wasinvestigatedbyrepeatingtheCVtestat10mVsà1for2000cycles,asshowninFig.7.TheIL–RGOnanocompositesshowedagoodcyclestability(95%retentionafter2000cycles)becauseoftheintrinsicstabilityofthedouble-layercapacitiveIL–RGOnano-composites,buttheoverallspeci?ccapacitancewaslimitedto
5398|Nanoscale,2012,4,5394–5400
Published on 21 June 2012. Downloaded by ZHENG ZHOU UNIVERSITY on 06/06/2014 13:17:38. Fig.6(a)Galvanostaticcharge–dischargebehavioursofMnO2?lm,IL–RGO,andMnO2/IL–RGOnanocompositesataconstantcurrentdensityof200mAgà1in1MNa2SO4solution(insetisthemagni?edimagesofMnO2?lmandMnO2/IL–RGOnanocomposites).(b)Speci?ccapacitancesofMnO2?lm,IL–RGOandMnO2/IL–RGOnano-compositesintherangeofcurrentdensityfrom200mAgà1to30Agà1.
61Fgà1.Incontrast,thepureMnO2?lmdisplayedpoorcyclestability(35.7%retentionof167Fgà1atinitialcycling)after2000cycles.AlthoughMnO2/IL–RGOnanocompositesobtainedthepseudocapacitanceinananalogousmannertotheenergystoragemechanismofMnO2?lm,thecapacitancelosswasonly8%of245Fgà1oftheinitialcapacitance.Thissigni?cantincreaseinthecycleperformanceofMnO2/IL–RGOnanocompositesinthe2000cycleswasattributedtotheeffectofthehierarchicalstructureratherthanthatofthesimpleblendingofpseudoca-pacitativeMnO2anddouble-layercapacitativeIL–RGO(simplymixedMnO2/IL–RGO,72%retentionafter2000cycles).Afterthestabilitytest,nostructuralcollapseofMnO2/IL–RGOnanocompositeswasobserved,becauseoftheef?cientstressreleaseofhierarchicalstructureswithhighsurfaceareaaswellasthestrongmechanicalstrengthofRGOs(Fig.7b).Thecontri-butionoftheMnO2tothespeci?ccapacitanceofMnO2/IL–RGOnanocompositescanbecalculatedbythefollowingequation:45CeMnO2T?
QeMnO2=IL--RGOTàQeIL--RGOT
DV?meMnO2T(3)
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Published on 21 June 2012. Downloaded by ZHENG ZHOU UNIVERSITY on 06/06/2014 13:17:38. electrochemicalconditionsandcontrolledatadepositiontimeof10minfortheconstructionofhierarchicalstructures.AsshowninNyquistplots,MnO2/IL–RGOnanocompositespromotediondiffusionintotheporesandelectrontransportduetothehier-archicalcomplexstructure.Accordingly,MnO2/IL–RGOnanocompositesshowedhigherspeci?ccapacitance(266Fgà1)andratecapability(61%retentionat30Agà1)comparedto159Fgà1and<30%retentionofpureMnO2?lm.Inaddition,MnO2/IL–RGOnanocompositesrevealedalongcyclelife(92%retentionafter2000cycles),whichwashigherthan35.7%retentionofpureMnO2?lm.NostructuraldeformationofMnO2/IL–RGOnanocompositeswasobserved,becauseoftheef?cientstressreleaseofhierarchicalstructures.Considering72%retentionofmixedMnO2/IL–RGOafter2000cycles,further-more,thecyclestabilityofMnO2/IL–RGOnanocompositeswasthoughttobeduetotheeffectofthehierarchicalstructureratherthanthatofthesimpleblendingofpseudocapacitativeMnO2andconductiveIL–RGO.Therefore,theelectrochemicalco-assemblyinahierarchicallyheterostructuredmannerdescribedhereinprovidesasimpleandversatilemethodtopreparetheadvancedelectrodesforhighperformanceenergystoragedevices.
Acknowledgements
Weacknowledgethe?nancialsupportbytheNationalResearchFoundationofKoreaGrantfundedbytheKoreanGovernment(MEST,NRF-2010-C1AAA001-0029018)andBasicScienceResearchProgramthroughtheNationalResearchFoundationofKorea(NRF)fundedbytheMinistryofEducation,ScienceandTechnology(2011-0007677).
Fig.7(a)Speci?ccapacitanceofMnO2?lm,mixedMnO2/IL-RGOcomposite,IL-RGOnanocomposite,andMnO2/IL-RGOnano-compositeduring2000cyclingtests.(b)Speci?ccapacitanceandcoulombicef?ciencyofMnO2onMnO2/IL-RGOnanocompositeduring1000cyclingtests.(InsetistheSEMimageofMnO2/IL–RGOnano-compositesafter2000cyclingtests.)
Notesandreferences
1Y.Min,M.Akbulut,K.Kristiansen,Y.GolanandJ.Israelachvili,Nat.Mater.,2008,7,527;B.E.Conway,ElectrochemicalSupercapacitors:Scienti?cFundamentalsandTechnologicalApplications,KluwerAcademic/Plenum,NewYork,1999.2M.WinterandR.J.Brodd,Chem.Rev.,2004,104,4245.3E.Frackowiak,Phys.Chem.Chem.Phys.,2007,9,1774.
4V.L.Pusharaj,M.M.Shaijumon,A.Kumar,S.Murugesan,L.Ci,R.Vajtai,R.J.Linhardt,O.NalamasuandP.M.Ajayan,Proc.Natl.Acad.Sci.U.S.A.,2007,104,13574.
5B.FangandL.Binder,J.Phys.Chem.B,2006,110,7877.6E.FrackowiakandF.B??eguin,Carbon,2001,39,937.
7M.D.Stoller,S.Park,Y.Zhu,J.AnandR.S.Ruoff,NanoLett.,2008,8,3498.
8T.Brezesinski,J.Wang,S.H.TolbertandB.Dunn,Nat.Mater.,2010,9,146.
9W.Sugimoto,H.Iwata,Y.Yasunaga,Y.MurakamiandY.Takasu,Angew.Chem.,Int.Ed.,2003,42,4092.
10R.LiuandS.B.Lee,J.Am.Chem.Soc.,2008,130,2942.
11C.Liu,Z.Yu,D.Neff,A.ZhamuandB.Z.Jang,NanoLett.,2010,10,4863.
12D.-W.Wang,F.Li,M.Liu,G.Q.LuandH.-M.Cheng,Angew.Chem.,Int.Ed.,2008,47,373.
13P.SimonandY.Gogotsi,Nat.Mater.,2008,7,845.
14C.-C.Hu,K.-H.Chang,M.-C.LinandY.-T.Wu,NanoLett.,2006,6,2690.
15C.Meng,C.Liu,L.Chen,C.HuandS.Fan,NanoLett.,2010,10,4025.
16R.K.Das,B.Liu,J.R.ReynoldsandA.G.Rinzler,NanoLett.,2009,9,677.
17H.Wang,H.S.Casalongue,Y.LiangandH.Dai,J.Am.Chem.Soc.,2010,132,7472.
whereC(Fgà1)isthespeci?ccapacitance,Qisthevoltammetriccharge,DVisthewidthofpotentialwindow,andmisthemassofMnO2depositedonIL–RGOnanocomposites.Remarkably,C(MnO2)onIL–RGOnanocompositeswasrevealedtobeashighas511Fgà1,whichissigni?cantlyhigherthanthoseofpureMnO2?lm(167Fgà1)andotherpreviousreports.51–53Inaddi-tion,MnO2depositedonIL–RGOnanocompositesshowedonly7.8êpacitancefadingofspeci?ccapacitancesafter1000cycles(Fig.7b).ThehierarchicalcomplexstructureofMnO2/IL–RGOcompositespromotesthefastiondiffusion,easyaccessibilitytoredoxsites,andhighsurfacearea,givingrisetohighspeci?ccapacitance,goodratecapability,andlong-termstability.
Conclusions
Inthisresearch,thehierarchicalheterostructureofMnO2/IL–RGOnanocompositeswasreadilyconstructedthroughelectro-chemicalassembly.MnO2/IL–RGOnanocompositesexhibited?ower-likenanostructures,whichwereinterconnectedwiththinleaf-likenano?akesconsistingofconformallyMnO2-depositedIL–RGO?akes.Inparticular,themorphologyofMnO2/IL–RGOnanocompositeswasmanipulatedbychangingthe
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Nanoscale,2012,4,5394–5400|5399
View Article Online
Published on 21 June 2012. Downloaded by ZHENG ZHOU UNIVERSITY on 06/06/2014 13:17:38. 18J.-S.Ye,H.F.Cui,X.Liu,T.M.Lim,W.-D.ZhangandF.-S.Sheu,Small,2005,1,560.
19S.L.Chou,J.Z.Wang,S.Y.Chew,H.K.LiuandS.X.Dou,Electrochem.Commun.,2008,10,1724.
20C.Z.Yuan,B.Gao,L.F.Shen,S.D.Yang,L.Hao,X.J.Lu,F.Zhang,L.J.ZhangandX.G.Zhang,Nanoscale,2011,3,529.21W.Wei,X.Cui,W.ChenandD.G.Ivey,Chem.Soc.Rev.,2011,40,1697.
22R.Liu,J.DuayandS.B.Lee,Chem.Commun.,2011,47,1384.23L.L.Zhang,R.ZhouandX.S.Zhao,J.Mater.Chem.,2010,20,5983.
24L.L.ZhangandX.S.Zhao,Chem.Soc.Rev.,2009,38,2520.
25W.Wei,X.Cui,W.ChenandD.G.Ivey,J.Phys.Chem.C,2008,112,15075.
26A.J.RobertsandR.C.T.Slade,Electrochim.Acta,2010,55,7460.27S.Chen,J.ZhuandX.Wang,ACSNano,2010,4,6212.
28Z.-S.Wu,W.Ren,D.-W.Wang,F.Li,B.LiuandH.-M.Cheng,ACSNano,2010,4,5835.
29S.Chen,J.Zhu,X.Wu,Q.HanandX.Wang,ACSNano,2010,4,2822.
30Z.Li,J.Wang,X.Liu,S.Liu,J.OuandS.Yang,J.Mater.Chem.,2011,21,3397.
31J.-K.Chang,M.-T.Lee,W.-T.Tsai,M.-J.DengandI.-W.Sun,Chem.Mater.,2009,21,2688.
32J.P.Hill,S.Alam,K.Ariga,C.E.AnsonandA.K.Powell,Chem.Commun.,2008,383.
33Y.Hou,Y.Cheng,T.HobsonandJ.Liu,NanoLett.,2010,10,2727.34G.-X.Wang,B.-L.Zhang,Z.-L.YuandM.-Z.Qu,SolidStateIonics,2005,176,1169.
35S.C.Pang,M.A.AndersonandT.W.Chapmanb,J.Electrochem.Soc.,2000,147,444.
36J.Yan,X.J.Fan,T.Wei,J.Cheng,B.Shao,K.Wang,L.P.SongandM.L.Zhang,J.PowerSources,2009,194,1202.37H.Zhang,G.Cao,Z.Wang,Y.Yang,Z.ShiandZ.Gu,NanoLett.,2008,8,2664.
38S.Chou,F.ChengandJ.Chen,J.PowerSources,2006,162,727.39G.-R.Li,Z.-P.Feng,Y.-N.Ou,D.Wu,R.FuandY.-X.Tong,Langmuir,2010,26,2209.
40W.S.HummersandR.E.Offeman,J.Am.Chem.Soc.,1958,80,1339.
41B.G.Choi,H.Park,T.J.Park,M.H.Yang,J.S.Kim,S.-Y.Jang,N.S.Heo,S.Y.Lee,J.KongandW.H.Hong,ACSNano,2010,4,2910.
42M.H.Yang,B.G.Choi,H.Park,W.H.Hong,S.Y.LeeandT.J.Park,Electroanalysis,2010,22,1223.
43T.Y.Kim,H.W.Lee,M.Stoller,D.R.Dreyer,C.W.Bielawski,R.S.RuoffandK.S.Suh,ACSNano,2011,5,436.
44S.W.Lee,J.Kim,S.Chen,P.T.HammondandY.S.Horn,ACSNano,2010,4,3889.
45H.S.Park,B.G.Choi,S.H.Yang,W.H.Shin,J.K.Kang,D.JungandW.H.Hong,Small,2009,5,1754.
46B.G.ChoiandH.S.Park,ChemSusChem,2012,5,709.
47P.L.Taberna,P.SimonandJ.F.Fauvarque,J.Electrochem.Soc.,2003,150,A292.
??n,M.T.Mart??48F.Pico,C.Pecharroman,A.Anso?nezandJ.M.Rojo,
J.Electrochem.Soc.,2007,154,A579.
49S.Y.Liew,W.ThielemansandD.A.Walsh,J.Phys.Chem.C,2010,114,17926.
50M.Hughes,G.Z.Chen,M.S.P.Shaffer,D.J.FrayandA.H.Windle,Chem.Mater.,2002,14,1610.
51P.Ragupathy,D.H.Park,G.Campet,H.N.Vasn,S.-J.Hwang,J.-H.ChoyandN.Munichandraiah,J.Phys.Chem.C,2009,113,6303.52A.Zolfaghari,F.Ataherian,M.GhaemiandA.Gholami,Electrochim.Acta,2007,52,2806.
53M.Nakayama,T.KanayaandR.Inoue,Electrochem.Commun.,2007,9,1154.
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