文献-MAX相刻蚀 MXene-LDH (1)

JournalofPowerSources327(2016)221e228

ContentslistsavailableatScienceDirectJournalofPowerSourcesjournalhomepage:www.elsevier.com/locate/jpowsourThree-dimensionalporousMXene/layereddoublehydroxidecompositeforhighperformancesupercapacitorsYaWang1,HuiDou*,1,JieWang,BingDing,YunlingXu,ZhiChang,XiaodongHaoJiangsuKeyLaboratoryofMaterialandTechnologyforEnergyConversion,CollegeofMaterialsScienceandEngineering,NanjingUniversityofAeronauticsandAstronautics,Nanjing,210016,PRChinahighlights??MXene/LDHwaspreparedbyliquidphasedepositionmethod.??LDHplateletshomogeneouslygrownone-MXenesubstrateformsa3Dporousstructure.??3Dporousstructurefacilitatesactivesitesexposureandelectrolytepenetration.??MXene/LDHshowsexcellentelectro-chemicalpropertiesforsupercapacitors.graphicalabstractarticleinfoArticlehistory:Received21April2016Receivedinrevisedform1July2016Accepted16July2016abstractInthiswork,anexfoliatedMXene(e-MXene)nanosheets/nickel-aluminumlayereddoublehydroxide(MXene/LDH)compositeassupercapacitorelectrodematerialisfabricatedbyinsitugrowthofLDHone-MXenesubstrate.TheLDHplateletshomogeneouslygrownonthesurfaceofthee-MXenesheetsconstructathree-dimensional(3D)porousstructure,whichnotonlyleadstohighactivesitesexposureofLDHandfacileliquidelectrolytepenetration,butalsoalleviatesthevolumechangeofLDHduringthecharge/dischargeprocess.Meanwhile,thee-MXenesubstrateformsaconductivenetworktofacilitatetheelectrontransportofactivematerial.TheoptimizedMXene/LDHcompositeexhibitsahighspeci?ccapacitanceof1061Fgà1atacurrentdensityof1Agà1,excellentcapacitanceretentionof70ˉter4000cycletestsatacurrentdensityof4Agà1andagoodratecapabilitywith556Fgà1retentionat10Agà1.?2016ElsevierB.V.Allrightsreserved.Keywords:LayereddoublehydroxideMXene3DporousstructureSupercapacitors1.IntroductionTheimpendingglobalenergycrisishaspromptedintenseresearchintothedevelopmentofvarioustypesofsustainableen-ergyconversionandstoragesystems[1,2].Recently,*Correspondingauthor.E-mailaddress:dh_msc@nuaa.edu.cn(H.Dou).1Theseauthorscontributedequallytothiswork.http://dx.doi.org/10.1016/j.jpowsour.2016.07.0620378-7753/?2016ElsevierB.V.Allrightsreserved.electrochemicalcapacitors,alsocalledsupercapacitors(SCs),haveattractedconsiderableattentionowingtotheirhighpowerdensityandexcellentcyclicstabilitycomparedwithrechargeablebatteries[3,4].Accordingtoenergystoragemechanisms,SCscanbedividedintoelectrochemicaldouble-layercapacitors(EDLCs)whichstorethechargethroughrapidionadsorption/desorptionprocessattheelectrode/electrolyteinterface,andpseudocapacitorsthatstoreenergybyreversiblefaradicreactionsproceededontheelectrodesurface[5e9].Duetothepurelyphysicalprocess,EDLCsexhibitrapidcharging/dischargingratebutrelativelowenergydensity222Y.Wangetal./JournalofPowerSources327(2016)221e228

[10e12].Incontrast,thepseudocapacitorswithmuchhigherca-pacityemergeasalternativedevices[13,14].Themostlyusedpseudocapacitiveelectrodematerialsaretransitionmetaloxides/hydroxides/sul?des,suchasNiO[15],MnO2[16],Co3O4[17],NiCo2O4[18,19],Ni(OH)2[20]andNiCo2S4[21],andconductivepolymers[22,23],suchaspolyanilineandpolypyrrole.Amongthem,layeredmetallicdoublehydroxides(LDHs),areconsideredaspotentialSCselectrodematerialsbecauseoftheirhighredoxac-tivity,lowcostandenvironmentallyfriendlynature[24e26].TofurtherimprovetheelectrochemicalperformanceofLDHs,considerableeffortsweredevotedtoengineerthemorphologyandstructureofLDHs[27e29].Forexample,NiAl-LDHmicrosphereswithtunableinteriorarchitecturewerefabricatedbyinsitugrowthmethod.ThehollowNiAl-LDHmicrospheresexhibitedahighspe-ci?cpseudocapacitanceof735Fgà1[30].However,theelectro-chemicalperformanceofLDHsisstilllimitedbythepoorconductivityandcyclicstability.Toaddresstheseissues,substantialresearcheffortshavebeenfocusedonmakingcompositematerialsofLDHsandconductiveadditives[31,32].Forexample,delaminatedNiAl-LDHswereincorporatedbetweengraphenenanosheetstoformalayeredhybridstructurewhichshowedexcellentelectro-chemicalperformancewithhighspeci?ccapacitanceandgoodcyclicstabilityaselectrodematerial[33].ButitisindispensabletodelaminatebothNiAl-LDHandgraphenenanosheetspriortopre-paringthecomposite.Andthen,ourgroupfabricatedacompositeofNiAl-LDH/graphene(NiAl-LDH/GNS)byliquidphasedepositionmethod,theNiAl-LDHhomogeneouslygrewonthesurfaceofGNSandexhibitedasigni?cantenhancementofelectrochemicalper-formance.However,GNShastobereducedfromGOinafurtherstepandGNSwaseasilyaggregatedduetoitsintrinsicproperty[34].MXenes,anovelgroupoftwo-dimensionaltransitionmetalcarbidesorcarbonitrides,includingTi3C2,Ti2C,Nb2C,V2CandTi3CN,haveattractedwideattentionowingtoultrahighelec-tricalconductivity,highspeci?ccapacitanceandchemicalsta-bility[35e38].Yuryetal.reportedthattheconductivityoftherolledMXene?lmcanreach150,000Smà1.Whenusedaselectrodematerialforsupercapacitors,theMXeneachievedahighspeci?ccapacitancewith245Fgà1at2mVsà1.Inaddition,after10,000charge/dischargecyclesatacurrentdensityof10Agà1,thecapacitanceretentionofMXeneelectrodestillmaintainedalmost100%[39].Therefore,MXenecanactasafavorableconductivesubstratetocombinewithpseudocapaci-tanceelectrodematerialsfortheenhancingperformanceconcern.Recently,Lingetal.preparedthe?exiblefreestandingPVA/MXene?lm,andtheconductivityofthe?lmwassigni?-cantlyimprovedfrom0.04Smà1to22,433Smà1withtheMXenecontentincreasingfrom40%to90%.Bene?tingfromtheenhancedconductivity,thePVA/MXene?lmexhibitedultrahighvolumetriccapacitance[40].Herein,weproposealiquidphasedeposition(LPD)methodtofabricatethree-dimensional(3D)porousnanocompositeofnickel-aluminumlayereddoublehydroxideplatelets(LDH)ontheexfoliatedMXene(e-MXene)sheets(MXene/LDH)aselec-trodematerialforsupercapacitors.Optimumratioofthecom-positecanbeachievedthroughtuningthemassratioofLDHande-MXene.TheLDHplateletshomogeneouslygrownonthesurfaceofthee-MXeneconstructaporous3DnetworkwhichprovidesfastpathwayforiontransportandlargeactiveareaforredoxreactionofLDH.Meanwhile,e-MXenesubstrateformsaconductivenetwork,whichaccelerateselectrontransportandimprovestheelectricalconductivityofMXene/LDHcomposite.Bene?tingfromtheseadvantages,theMXene/LDHcompositeexhibitsahighspeci?ccapacitance,excellentratecapabilityandstability.2.Experimental2.1.Materialssynthesis2.1.1.PreparationofmultilayeredMXene(Ti3C2)1gofMAX(Ti3AlC2)wasaddedinto10mLHFsolution(40wt%)andmagneticallystirredfor18hatroomtemperature.Multilay-eredMXene(m-MXene)wasobtainedaftercentrifuging,washingwithdeionizedwaterandabsolutealcoholrepeatedly,anddriedinavacuumovenatroomtemperature.2.1.2.Exfoliationofm-MXeneTypically,0.9gofm-MXenepowderwasmagneticallystirredin15mLdimethylsulfoxide(DMSO)for18hatroomtemperature.Afterdilutingwithdeionizedwater,DMSO-intercalatedMXenewasseparatedbycentrifugationat3500rpmfor5min.Theobtainedpowderwasdispersedindeionizedwaterwithultrasonicationfor6h.Aftercentrifugedat3500rpmfor1handdriedinavacuumovenatroomtemperature,e-MXenesheetswereobtained.2.1.3.PreparationoftheNi-containingparentsolution11mmolofNi(NO3)2$6H2Owasdissolvedin22mLofdeionizedwaterwithpowerfulstirring,andthesolutionwasslowlyregulatedtopH7.5withammoniawater.Theobtainedprecipitatewasdriedatroomtemperatureafterrepeatedlywashingwithwaterandabsolutealcohol.Thenthedriedpowderwaspouredinto40mLofNH4F(0.66molLà1),andthesolutionwasvigorouslystirredfor48hatroomtemperature.TheNi-containingparentsolutionwasachievedby?ltration.2.1.4.SynthesisoftheMXene/LDHcomposites30mgofe-MXenewasdispersedinNi-containingparentso-lutionwithultrasonicationfor1hinaplasticbeaker.Then10mmolofH3BO3and0.25mmolofAl(NO3)3$9H2Oweredissolvedin20and40mLdeionizedwater,respectively.TheH3BO3solutionandAl(NO3)3solutionwerepouredsuccessivelyintotheabovee-MXenesuspensionandthemixturewasshakenultrasonicallyun-derN2atmospheretoformahomogeneoussolution,whichwasthensetat50??Cfor48h.Subsequently,theproductwascentri-fugedandwashedseveraltimeswithdeionizedwateranddriedinavacuumovenat30??Cfor24h,whichwasdesignatedasM30/LDH.Withthesameprocedure,20mgor50mgofMXenewasaddedintothesystemtogetM20/LDHandM50/LDH,respectively.Thecontentofe-MXeneinthecompositeiscalculatedbyweighingthemassofMXene/LDH.TheweightpercentageofMXeneinM20/LDH,M30/LDHandM50/LDHcompositesareapproximately35%,38%and51%,respectively.2.1.5.SynthesisofLDHForcomparison,pureLDHwaspreparedviaacoprecipitationprocess[34].3mmolNi(NO3)2$6H2Oand1mmolAl(NO3)3$9H2Oweredissolvedin100mLwater.Theobtainedsolutionwasmixedwith50mLof0.08molLà1NaOHsolutionunderstirringandkeptat60??Cfor5h.Theprecipitatewaswashedseveraltimeswithdeionizedwaterandabsolutealcohol,thendriedinavacuumovenat60??CtogettheLDH.2.2.MaterialcharacterizationThecrystalstructuresweremeasuredthroughX-raydiffraction(XRD)byaBrukerD8AdvancedX-raydiffractometerwithCuKaradiation(0.15406nm).TheFouriertransforminfraredspectros-copy(FT-IR)spectraweretestedonaNicolet750Fouriertransforminfraredspectrometer.TheN2adsorptionàdesorptionisothermswereconductedbyaMicromeriticsBK122T-Banalyzer.Thespeci?cY.Wangetal./JournalofPowerSources327(2016)221e228223

surfaceareawascalculatedaccordingtheBrunaueràEmmettà-Teller(BET)method.TheporesizedistributionwereobtainedfromBarretàJoyneràHalenda(BJH)desorptionbranchoftheisotherm.X-rayphotoelectronspectroscopy(XPS)wasconductedonaPerkin-ElmerPHI550spectrometerusingAlKaastheX-raysource.Themorphologieswerecharacterizedbyscanningelectronmi-croscope(SEM,HitachiS4800),transmissionelectronmicroscopyandhigh-resolutiontransmissionelectronmicroscopy(TEM,HRTEM,JEOLJEM-2010).Theelectricalconductivitytestwascar-riedoutonaST-2722semiconductorpowderresistivityapparatus(SuzhouJingleElectronictechnologyCo.Ltd.,China).2.3.ElectrochemicalmeasurementsAlltheelectrochemicalperformanceswerecarriedoutin6MKOHelectrolyteusingthree-electrodesystematroomtemperature.Theworkingelectrodeswerepreparedbypressingtheas-preparedcomposites,carbonblackandapolytetra?uoroethylene(PTFE)binderintheweightratioof85:10:5ontofoamedNigrids(1cmà2)andthendriedat50??Cinthevacuumovenforseveralhours.Theelectrodematerialmassloadingis5mgcmà2.Aplatinumfoilwasusedascounterelectrodeandasaturatedcalomelelectrode(SCE)asreferenceelectrode,respectively.Thecyclicvoltammetry(CV),galvanostaticcharge/discharge(GCD)andelectrochemicalimped-ancespectroscopy(EIS)measurementswerecarriedoutwithaCHI660Celectrochemicalworkingstation.CVtestswereperformedinthepotentialrangefrom0to0.6V(vs.SCE)atdifferentscanrates.GCDcurvesweremeasuredbetween0and0.55V(vs.SCE)atdifferentcurrentdensities.3.ResultsanddiscussionThesyntheticprocessoftheMXene/LDHcompositeisillustratedinScheme1.Firstly,MAX(Ti3AlC2)powderswereaddedintoHFsolutiontoobtainthem-MXeneplates.Then,them-MXeneplatesweredelaminatedbyDMSOtoproducee-MXenesheets.Afterthee-MXenesheetsweredispersedinthemixedsolutioncontainingNiparentsolution([NiFx](xà2)à),Al3tandH3BO3,the[NiFx](xà2)àweregraduallyhydrolyzedto[Ni(OH)x](xà2)à,whichproceededdehy-drationcondensationreactionswiththehydroxylgroupsonthesurfaceofe-MXeneandlinkedtoe-MXene.Meanwhile,Al3tScheme1.IllustrationofthefabricationrouteoftheMXene/LDHcomposite.

insertedintothecrystallatticesandthenLDH?akeswereformedandanchoredonthee-MXenesheetstoyieldthe3DporousMXene/LDHnanocomposite.ThetightconnectionbetweenLDHande-MXeneinMXene/LDHconstructsahighlyef?cientconduc-tivenetworkandretainsthestructurestabilityduringelectro-chemicalprocess.AndtheporousstructureimprovestheelectrolytepenetrationandprovidesmoreactivesitesformakingfulluseofLDHpseudocapacitance.X-raydiffraction(XRD)andFouriertransforminfraredspec-troscopy(FT-IR)werecarriedouttocharacterizethecompositionofthecomposites.TheXRDpatternsofe-MXene,LDHandM30/LDHareexhibitedinFig.1a.Thediffractionpeakof(002)at2q?7.7??fore-MXenesheetsislowerthanthatform-MXeneplatesat2q?8.8??,whichillustratestheincreaseofinterlayerspacingandsuccessfulexfoliationofm-MXene(Fig.S1a)[41].TheXRDpeaksofLDHat2q?11.3??,21.5??,35.8??and61.8??correspondtothe(003),(006),(012)and(110)crystalplanes(JCPDS15-0087)ofbrucite-likecrystal.M30/LDHshowsalmostallcharacteristicpeaksbothofe-MXeneandLDH.Moreinterestingly,(002)diffractionpeakforM30/LDHshiftstoloweranglethanthatfore-MXene,suggestingdepositionofLDHone-MXenesheetsinhibitstheaggregationofe-MXene.Additionally,thecompositesM20/LDHandM50/LDHwithdifferente-MXenecontentpresentidenticalXRDpatterntoM30/LDH(Fig.S1b).AsshowninFig.1b,theFT-IRspectrumofLDHexhibitsabsorptionbandsat3448,1637and1384cmà1,whichcorrespondtotheOeHstretchingvibrationofwatermoleculesintheinterlayer,hydrogen-bondedOHgroupsandNeOstretchingvibrationfromNOà3,respectively.Someotherabsorptionpeaksbelow800cmà1areattributedtothemetalàoxygenstretchingorbendingmodesinthebrucite-likecrystallatticeofLDH[42].ThesecharacteristicIRabsorptionsofLDHcanbeclearlyidenti?edintheFT-IRspectrumofM30/LDH,alsoaccountingfortheexistenceofLDHinthecomposite.M20/LDHandM50/LDHshowalmostsimilarFT-IRspectratoM30/LDH(Fig.S2).ThesurfacecharacteristicofM30/LDHwasstudiedusingX-rayphotoelectronspectroscopy(XPS)asshowninFig.2.Fig.2ashowsNi2p,Al2p,O1s,C1sandTi2pcorelevelsfromsurveyofM30/LDHcomposite.IntheNi2pXPSspectrum(Fig.2b),thepeaksat857and875eVareassignedtothe2p3/2and2p1/2levelsofNi2t,respectively,suggestingtheexistenceofLDHinthecomposite[43].TheO1sspectrumshowsapeakat531.2eV,whichisattributedtoCeTiàOx(Fig.2c).ThehighresolutionTi2pspectrumcanbedeconvolutedintosixpeaks(Fig.2d),correspondingtoCeTiàFx(460.2eV),TieO(458.6eV),TiO2-xFx(459.3eV)andTiatoms(455.0eV,455.8eVand457.2eV)[44].TheXPSresultsfurtherdemonstratetheexistenceofthenegatively-chargedgroupsandtheformationofLDHonthee-MXene.Themorphologyofas-obtainedsampleswasinvestigatedbyscanningelectronmicroscope(SEM)andtransmissionelectronmicroscope(TEM).TheSEMimagesofthem-MXeneandM30/LDHcompositeareshowninFig.3.ItisdemonstratedinFig.3athattheAllayerswereselectivelyetchedinHFsolutionformingm-MXeneplates.Them-MXeneplateswereef?cientlyexfoliatedwithDMSOtoforme-MXenesheets(Fig.S3a).AfterreactedwithNiparentsolutionandAl3t,thesurfaceofe-MXeneiscoveredwithfrizzyLDHplatelets.However,theabundantLDHplateletsonthee-MXeneaggregatetogetherforM20/LDHbecausetheamountofe-MXenesubstrateisnotenoughforLDHplateletstodeposit(Fig.S3b).Withincreasingamountofe-MXene,LDHplateletsdistributehomogeneouslyone-MXenesurface,forminga3DporousandopenstructureforM30/LDH(Fig.3b).Fig.3crevealsthatLDHplateletsaregrownontwosidesofe-MXenesheets,ef?cientlypreventingtheaggregationofindividuale-MXenesheets.Nevertheless,withthecontinuedincreaseofe-MXenecontent,thesurfaceofe-MXenecannotbecoveredcompletelywith224Y.Wangetal./JournalofPowerSources327(2016)221e228

Fig.1.(a)XRDpatternsofe-MXene,LDHandM30/LDHcomposite.(b)FT-IRspectraofLDHandM30/LDHcomposite.

Fig.2.(a)XPSspectrasurvey(bed)thecore-levelNi2p,O1sandTi2pofM30/LDH.

LDHplateletsforM50/LDH(Fig.S3c),whichleadstorestackingofe-MXenesheets.Forcomparison,themorphologyofLDHdemon-strateslargeparticleswithaggregated?akes(Fig.S3d).Thestruc-tureofM30/LDHisfurtherevidencedbyTEMimage.TheLDHplateletsgrownonthee-MXeneexhibitgauze-likemorphologyandconnectedwitheachother(Fig.3d),whichcouldprovideef?cientiontransportpathwayandlargeactivesurfaceareafortheelectrode.TotrackthegrowthprocessofMXene/LDHcomposite,theSEMimagesofM30/LDHafterdifferentreactiontime(4,8,12,24,36and48h)areshowninFig.S4.ItcanbeclearlyseenfewLDHplatesgrowonthesurfaceofe-MXeneafter4h.Withtheincreaseofreactiontime,LDHplatesgraduallydepositonthesubstrate.Finally,LDHplateletsdistributedhomogeneouslyone-MXenesurfaceformsa3Dporousandopenstructureafter48h.Thefor-mationofsuchuniquestructurecouldbemainlyattributedtointeractionbetween[Ni(OH)x](xà2)àandhydroxylgroupsexistingonthesurfaceofe-MXene.ThetexturalpropertiesoftheM30/LDHandLDHwererevealedbyN2-sorptionmeasurements.TheN2adsorption/desorptioniso-thermsofM30/LDH,LDH(Fig.4)ande-MXene(Fig.S5)areoftypeIVwithaclearhysteresisloop,indicatingmesoporouscharacter-istics.TheirtexturalparametersarelistedinTableS1.Thespeci?csurfaceareaandporevolumeofM30/LDHare72.34m2gà1and0.34cm3gà1,respectively,whicharemuchhigherthanthoseofLDH(18.10m2gà1and0.027cm3gà1)ande-MXene(6.3m2gà1and0.021cm3gà1).AsseenintheinsetofFig.4,theBJH(Barrett-Joy-ner-Halenda)poresizedistribution(PSD)ofLDHdistributesat2e9nm.ThePSDcurveofM30/LDHshowssimilarshapebutaparticularlyhighvolumeat~3nm.Thespeci?c3DporousstructureofM30/LDHcouldenhancetheexposureofactivesitesandaccel-eratetheiontransport.Inordertoinvestigatetheelectrochemicalpropertiesoftheobtainedcompositeasanactivesupercapacitorelectrodematerial,theMXene/LDHelectrodeswereinvestigatedwithathree-Y.Wangetal./JournalofPowerSources327(2016)221e228225

Fig.3.SEMimagesof(a)m-MXeneplates,(b)M30/LDH,(c)cross-sectionSEMimageofM30/LDH,(d)TEMimageofM30/LDH.

capacitance.Withincreasingscanratefrom2mVsà1to50mVsà1,theweaklydeviatedredoxcurrentpeakssuggestgoodpseudoca-pacitivebehavior(Fig.5b).Inaddition,M30/LDHexhibitshigherredoxpeakcurrentsandlargerintegraldomainoftheCVcurvescomparedwithe-MXene,M20/LDHandM50/LDH(Fig.S6aand6b),suggestingmoreeffectiveutilizationoftheelectroactivespecies.TheelectrochemicalperformanceofM30/LDHisfurtherstudiedwithgalvanostaticcharge-discharge(GCD)measurement.Thespeci?ccapacitance(Cs,Fgà1)oftheelectrodeiscalculatedac-cordingtothefollowingequation:Cs?I$Dt=m$Dv(2)Fig.4.N2adsorption-desorptionisothermsandporesizedistributions(inset)ofM30/LDHandLDH.

electrodesystemin6MKOHaqueouselectrolyte.Fig.5ashowsthecyclicvoltammetry(CV)curvesofe-MXene,LDHandM30/LDHatascanrateof5mVsà1.TheCVcurvesofLDHandM30/LDHexhibitapairofredoxpeaks,whichcorrespondstothetypicalpseudoca-pacitivebehaviorofNi2t/Ni3tinalkalineelectrolyte.ThepossibleFaradicredoxreactionisbasedonthefollowingequation:NieOHT2tOHà/NiOOHtH2Oteà(1)ComparedwithLDHande-MXene,theM30/LDHdisplaysenhancedredoxpeakcurrents,indicatingamuchhigherwhereI,Dt,m,Dvaretheconstantcurrent(A),dischargetime(s),theactivematerialmass(g),thetotalpotentialwindow(V),respectively.Thenonlineardischargecurves(Fig.5c)ofM30/LDHelectrodeshowtypicalpseudocapacitivebehavior,whichagreeswellwithCVresults.Fromthedischargecurves,M30/LDHcompositeshowsahighspeci?ccapacitanceof655Fgà1basedonwholecompositemassat1Agà1.Thespeci?ccapacitanceofe-MXeneisonly46Fgà1atacurrentdensityof0.5Agà1and27Fgà1at10Agà1(Fig.S6c).Comparedwithe-MXene,LDHandothertwoMXene/LDHcomposites,M30/LDHelectrodepresentsmuchlongerdischargetimeattheidenticalcurrentdensity,indicatingthebestchargestorageperformance(Fig.S6cee).Fig.5dcomparesthecalculatedspeci?ccapacitancesofLDH,M20/LDH,M30/LDHandM50/LDHelectrodesatdifferentcurrentdensitiesbasedonthewholecompositemass.M30/LDHelectrodepresentsthehighestspeci?ccapacitanceatallcurrentdensities.DuetothelowmassofLDH,thespeci?ccapacitancesofM50/LDHelectrodeareevenlowerthanthoseofLDHelectrodeatlowercurrentdensities.TheLDHelectrodeshowsthepoorestratecapa-bilitywithonly19.2%retentionatahighcurrentdensityof10Agà1.

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

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