226Y.Wangetal./JournalofPowerSources327(2016)221e228
Fig.5.(a)CVcurvesofe-MXene,LDHandtheM30/LDHatascanrateof5mVsà1in6MKOH.(b)CVcurvesofM30/LDHatdifferentscanrates.(c)ThedischargecurvesofM30/LDHatdifferentcurrentdensitiesbasedonthewholecompositemass.Speci?ccapacitanceofLDH,M20/LDH,M30/LDHandM50/LDHatdifferentcurrentdensities(d)basedonthecompositemass(e)basedonLDH.(f)NyquistplotsofM30/LDHandLDH.
TheM30/LDHelectrodeindicates51%ofthecapacitance(333Fgà1)retentionat10Agà1,superiortoM20/LDH(46%)andM50/LDH(43%)electrodes.Thespeci?ccapacitancesofallsamplesbasedonLDHatdifferentcurrentdensitiesarealsoshowninFig.5e.Atacurrentdensityof1Agà1,theinitialspeci?ccapacitanceofM30/LDHisashighas1061Fgà1.Withincreasingcurrentdensityto10Agà1,thecapacitanceretentionofM20/LDH,M30/LDH,M50/LDHandLDHare46.8%,52.4%,43.2%and19.2%,respectively.TheM30/LDHshowsmuchbetterelectrochemicalperformancethanLDHandothertwocomposites.Furthermore,thespeci?ccapaci-tanceandcapacitanceretentionofM30/LDHcompositearemuchhigherthanthoseofLDH-basedcompositescontainingotherconductivesubstrates,suchascarbonnanotubesandgraphene(TableS2)[45e48].Thehighspeci?ccapacitanceandthegoodratecapabilityoftheM30/LDH,ononehand,couldbeattributedtotheexposureofmoreLDHsurfaceactivesitesduringtheelectrodereactionprocess.Ontheotherhand,thee-MXenesubstratepro-videsaneffectivelyconductivenetworkforelectrontransport.Fore-MXene,LDH,M20/LDH,M30/LDHandM50/LDHcomposites,theelectricalconductivitiesare2.65?105,0.32,1.73?104,2.15?104and2.58?104Smà1,respectively.TheresultsindicatethatMXenecoulddramaticallyimprovetheelectricalconductivityofMXene/LDHcomposites.TofurtherunderstandtheeffectoftheMXeneontheelectro-chemicalbehavioroftheMXene/LDH,electrochemicalimpedancespectroscopy(EIS)testswereinvestigated.Fig.5fshowstheNyquistplotsofLDHandM30/LDHelectrodes,whichconsistofanarcinthehighfrequencyregionfollowedbylinearshapeinthelowfrequencyregion.Thecurveonthejunctureofaxisathighfre-quencyactsasaninternalresistanceofactivespecies,ionicresis-tanceofelectrolyteandthecontactresistancewithintheelectrode.TheinternalresistanceofM30/LDHcompositeislowerthanthoseofLDHandothercomposites(Fig.S6f).Theverticallineinthelowfrequencyregionrepresentsidealcapacitivebehavior.FromtheexpendedviewintheinsetofFig.5f,M30/LDHhasmoreverticallinethanothersamplesinlowfrequencyregion,whichprobablyresultsfromthattheanchoredLDHonthee-MXenecaninhibitstheaggregationofe-MXenesheets.Meanwhile,thee-MXenesheetsbridgingtheLDHnanoplatesformaconductivenetwork,whichfacilitatesrapidelectrontransferbetweentheelectrolyteandactivematerial.Thecyclestabilityisvitaltotheelectrochemicalcapacitors.ThecyclelifetestofM30/LDHcompositeisinvestigatedbyGCDtech-niqueatacurrentdensityof4Agà1asshowninFig.6.Interest-ingly,thegraduallyincreasingofthespeci?ccapacitanceofM30/LDHat?rst50cyclesisattributedtotheactivationoftheelectrodeY.Wangetal./JournalofPowerSources327(2016)221e228227
Fig.6.CyclicperformanceoftheM30/LDHatacurrentdensity4Agà1(theinsetshowstheSEMimageofM30/LDHelectrodeaftera4000-cycletestat4Agà1).
[49].After4000cycletests,thecapacitanceretentionis70%comparingwithmaximumcapacitance.Theexcellentcyclestabil-ityoftheM30/LDHisduetothestabilityofthestructure.Itcanbecerti?edfromtheSEMimageofM30/LDHelectrodeafter4000cycletestsat4Agà1intheinsetofFig.6.Theoriginal3DporousstructureofM30/LDHcanbemaintained,whichalleviatesthevolumechangeofLDHduringthecharge/dischargeprocess.ThehighcapacitanceoftheM30/LDHupto1061Fgà1withexcellentelectrochemicalstabilityandrateperformanceduetothesynergiceffectbetweene-MXeneandLDHcanbeexplainedasfollows.Firstly,theas-preparedcompositethroughthedehydrationcondensationbetweenthehydroxylgroupsonthesurfaceofe-MXeneand[Ni(OH)x](xà2)àkeepsthetightconnectionbetweene-MXeneandLDHsheets,atthesametimedecreasestheoverlappingofLDHande-MXenesheetstoforma3Dporousstructure.Such3DuniquestructureofMXene/LDHcanofferhighlyef?cientpathwaystowardselectronsandions.Thee-MXenesheetsofferaconductivenetworkbybridgingtheLDHnanoplates,andthechannelsformedbetweenthee-MXenesubstratesandLDHnanoplatesfacilitatetheionictransportation[50,51].Then,itcanalsoprovidemoreactivesitesformakingfulluseofLDHpseudocapacitanceandallowbettercontactoftheelectrodematerialwiththeelectrolyte,whichofferslargerzoneforiondiffusionandelectrontransportduringthecharge/dischargeprocess[52].Finally,thishydridstructurewithgoodelectronicandionicconductioncouldalsoimprovethechargeàdischargeef?ciencyandrelaxthetensionfromthevolumechangeinducedbyphasetransformationofNieOH,thusmakingsurethegoodreversibilityuponcycling[53].4.ConclusionsInsummary,byusinge-MXenesheetsasaconductivesubstrate,the3DporousMXene/LDHnanocompositeassupercapacitorelectrodematerialhasbeensuccessfullypreparedbyaliquidphasedepositionmethod.TheLDHplateletshomogeneouslyanchoredonthesurfaceofthee-MXenesheetsallowtoexcellentFaradaicuti-lizationoftheelectro-activesurfaceandfacileelectrolytepene-tration,alsoalleviatethevolumechangeduringthecharge/dischargeprocess.Meanwhile,e-MXenesubstrateformsaconductivenetworkacceleratingelectrontransport.Therefore,theoptimizedM30/LDHexhibitsexcellentelectrochemicalperfor-mancewithhighspeci?ccapacitancesof1061Fgà1and556Fgà1atcurrentdensitiesof1Agà1and10Agà1respectively,andacapacitanceretentionof70ˉter4000cycletestsatacur-rentdensityof4Agà1.Theseresultssuggestitshighpromisingprospectiveforsupercapacitors.AcknowledgementsThisworkwassupportedbytheNationalBasicResearchPro-gramofChina(973Program)(No.2014CB239701),NationalNaturalScienceFoundationofChina(No.51372116),NaturalScienceFoundationofJiangsuProvince(BK20151468,BK2011030),theFundamentalResearchFundsfortheCentralUniversitiesofNUAA(NJ20160104),PriorityAcademicProgramDevelopmentofJiangsuHigherEducationInstitutions(PAPD).AppendixA.SupplementarydataSupplementarydatarelatedtothisarticlecanbefoundathttp://dx.doi.org/10.1016/j.jpowsour.2016.07.062.References[1]S.Chu,A.Majumdar,Nature488(2012)294e303.[2]A.S.Arico,P.Bruce,B.Scrosati,J.M.Tarascon,W.VanSchalkwijk,Nat.Mater.4(2005)366e377.[3]P.Simon,Y.Gogotsi,Nat.Mater.7(2008)845e854.[4]D.Dubal,O.Ayyad,V.Ruiz,P.G??omez-Romero,Chem.Soc.Rev.44(2015)1777e1790.[5]R.K