EnergyStorageMaterials36(2021)318–325
ContentslistsavailableatScienceDirect
Energy
Storage
Materials
journalhomepage:www.elsevier.com/locate/ensm
Tunable
capacitance
in
all-inkjet-printed
nanosheet
heterostructures
Yang
Wang
a,
Mohammad
Mehrali
a,
Yi-Zhou
Zhang
b,
Melvin
A.
Timmerman
a,
Bernard
A.
Boukamp
a,
Peng-Yu
Xu
a,
Johan
E.
ten
Elshof
a,∗a University of Twente, MESA + Institute for Nanotechnology, P. O. Box 217 7500AE Enschede, the Netherlands
b School of Chemistry and Materials Science, Nanjing University of Information Science and Technology, Nanjing 210044, China
a
b
s
t
r
a
c
t
Heterostructuresconstructedfromtwo-dimensional(2D)buildingblockshaveshownpromiseforfield-effecttransistors,memorydevices,photosensorsandother electronicapplications.2Dnanosheetcrystalsaretypicallyconstructedintomultilayerheterostructuresusinglayer-by-layermethods,whichcannotbeusedto fabricatelarge-scaleandthickheterostructures,duetothetime-consumingnatureandlowefficiencyoftheprocess.Analternativeapproachtodepositdifferent2D materialsinthecontrollablefashionisbyinkjetprinting.Hereweshowthefabricationofsupercapacitorsbasedon2DheterostructuresbyinkjetprintingTi3C2Tx
MXenenanosheetsaselectrodes,followedbyinkjetprintinggrapheneoxidenanosheetsassolid-stateelectrolyte.Thefreewatermoleculestrappedbetweengraphene oxidesheetsfacilitateprotonmovementthroughthelayeredsolidelectrolyte.Theas-madeheterostructuresshowhigharealcapacitance,goodcyclingstabilityand higharealenergyandpowerdensitiescomparablewithexistingprintedsupercapacitors.Moreover,thespecificcapacitancecanbeincreasedfurtherbyadditionof liquidelectrolytes.
1. Introduction
Two-dimensionalheterostructureswithverticalstacking configura-tionsareusefulforavastrangeofapplicationsduetotheirexciting prop-erties,suchassuperconductivity,magnetismandoptoelectronic prop-erties[1,2].However,2Dheterostructuresforenergystorage applica-tionhashithertoremainedunexplored.Wearguethatverticalstacking different2D materialsintoheterostructurescreatenewopportunities forenergystoragebycombiningtheadvantagesofindividualmaterials whileovercomingthelimitations[3].
Mechanicalexfoliationofthree-dimensionallayeredcompounds fol-lowedby drytransferof each 2Dnanosheetontoa substrateisstill themaintechniqueforvertical2Dheterostructurefabrication[4].The advantage of thistechnique is thatatomicallythin high-quality het-erostructurescanberealizedonasmallscale.However,thistechnique cannotbeappliedtofabricate2Dheterostructuresonalargescale. So-lutionprocessingmethodssuchasspraycoatingandvacuumfiltration havebeenattempted,butthesemethodsofferpoorcontrolover inter-faceandsurfaceroughness,resultingin poordeviceperformance[3]. Inkjetprinting,asimple,low-costandversatiletechnique,[5-9]provides analternativeroutetothefabricationoflarge-scalevertical heterostruc-tureswithcontrolledthickness,interface,roughnessandwell-designed configuration[10,11].Recently,variousheterostructure-baseddevices basedonprinted2Dmaterialssuchasfield-effecttransistors,[12] ca-pacitors,[13]photosensorsandmemorydevices[10]havebeen demon-strated.However,printedheterostructuresforenergystorage remain unexplored.Furthermore,realizingwell-controlledandsharpinterfaces stillpresents asignificantchallengeforprintedheterostructures.Full
∗Correspondingauthor.
E-mail address: j.e.tenelshof@utwente.nl(J.E.tenElshof).
controlovertheheterostructureinterfaceiskeytoachievinghigh per-formance,whichincludesavoidingredispersionofnanosheetsfromthe interfaceupondepositionofasubsequentlayer.Thepreparationof non-toxic, stableandprintable2Dinksisanothercriticalissueforinkjet printing.
2Dtransitionmetalcarbidesornitrides(MXenes)withgeneral for-mulaMn +1Xn Tx (n=1,2,3,4),whereMisanearlytransitionmetal,
Xiscarbonand/ornitrogenandTx aresurfaceterminalgroupslike-F, -O,or-OH,havebeenattractingtremendousattentionrecentlydueto theiroutstandingchemicalandphysicalproperties[14,15].MXeneswith atomicthicknessandhighelectricalconductivityhavebeenwidely stud-iedforapplicationinhydrogelsensors,[16]solarcells[17]and superca-pacitors(SCs)[18,19].MXeneshavebeencombinedwithother2D ma-terialsintomulti-materialstructureswithtunablepropertiesand func-tionalities,showingpromiseforenergystorageapplications[3]. Com-parewithother2Dmaterials,theoutstandingpropertiesofMXeneslike hydrophilicity,highelectricconductivityandexcellentdispersion qual-itymakethemsuitableforinkjetprinting[20].Ontheotherhand, hy-dratedgrapheneoxide(GO)nanosheetsareelectricallyinsulatingbut exhibithighionicconductivitywhichvariesbetween5×10-6Scm-1
and4×10-3Scm-1,suggestingtheirpotentialassolid-stateelectrolyte
andseparator.Asdemonstratedbyseveralgroups,GOnanosheetsshow high ionic conductivitywhile being electronically insulating[21-23]. Thesourceofionsforionicconductivityaretheprotonsthatarisefrom thehydrolysisofthefunctionalgroups(carboxyl,sulphonicacidand/or hydroxyl)presentonhydratedGO[21].Theprotonstransportviathe hydrogen-bonding networkor movingfreely in thehydronium form withinGOfilm[21].
https://doi.org/10.1016/j.ensm.2021.01.009
Received28September2020;Receivedinrevisedform9January2021;Accepted11January2021 Availableonline12January2021
Fig.1.Schematicillustrationofall-inkjet-printing-basedheterostructureSSC(top)andMSC(bottom)supercapacitors.Water-basedadditive-freeMXeneinkwas firstinkjetprintedintothinfilmsandinterdigitatedconfigurationsaselectrodes,followedbyinkjetprintingawater-basedGOinkontopofinterdigitatedMXene electrodestoformanall-solid-stateMSC(bottom).AsecondMXeneelectrodewasinkjetprintedontopofthesolid-stateGOelectrolytetocompletethefabrication processofanall-solid-stateSSC(above).
Owingtothewiderangeofphysicalpropertiespresentin2D materi-als,[24]wedemonstrateherethatacombinationof2Dmaterialscanbe usedtorealizeanall-solid-statesupercapacitor,withoutanyliquidorgel electrolytepresentinthesystem.Inthiswork,weusedawater-based additive-freeMXeneinktoinkjet printelectrodes andcurrent collec-torsonpolyimidesubstrates,andawater-basedGOinktoinkjet-print thesolid-stateelectrolyte.Bothsandwichedsupercapacitors(SSCs)and micro-supercapacitordevices(MSCs)wereprintedonflexiblepolyimide substrates(Fig.1).TheSSCsachievedspecificarealcapacitances(CA)
upto9.8mFcm-2atacurrentdensityof40μAcm-2.Theadditionof
aqueouselectrolytesledtoanenhancementofCA,duetotheimproved
ionicconductivityoftheelectrolyteresultingfromthepresenceof ad-ditionalionsandaliquidphase.
2. ExperimentalSection
2.1. PreparationofMXeneink
Titaniumcarbide(Ti3C2Tx )MXenewassynthesizedfollowingamild etchingmethodasoutlinedelsewhere[25].Typically,theetchant solu-tionwaspreparedbydissolving3.2goflithiumfluoride(LiF, Sigm-Aldrich,−300meshpowder,98.5%)in40mLof9MHCl(Sigm-Aldrich, 37%solutioninwater).Subsequently,2gofsievedTi3AlC2powder(400 mesh)wasslowlyaddedtotheetchantsolutionoverthecourseof10 minandthereactiontemperaturewaskeptat35°C.Afterreactionfor 24h,theresultantwaswashedwithdeionizedwaterrepeatedlyand de-laminatedmanuallybyhandshakingagitationtoobtainTi3C2Tx MXene suspension.Thepreparedsolutionwasstoredinanitrogen-sealedvial andusedastheMXeneink.
2.2. Preparationofgrapheneoxide(GO)nanosheetsink
Graphite oxidewas synthesized from naturalgraphite (Nord-Min 802,ChemicalSchmitsSolutions)byamodifiedHummersmethod[26]. Graphite(2g)wasaddedto50mLconcentratedsulfuricacid(Fluka) ina1000mLflaskunderstirringinanicebathfor2h.Then7g potas-siumpermanganate(Merck)wasslowlyaddedtothesuspensionunder vigorousstirringtokeepthetemperatureofthemixtureunder10°C.
Themixturewastransferredtoa35°Coilbathunderstirringfor20h, yieldingathickpaste.Afterthemixturehadcooleddowntoroom tem-perature,100mLDIwaterwasslowlyaddedwithvigorousstirringfor 2hwhilekeepingtheflaskinanicebath.Anadditional500mLDI wa-terwasadded,followedbyadditionof15mLH2O2(30wt%,Aldrich)
untilnofurtherbubblescameout.Themixturewaswashedby1:10 HCl(37%,Acrosorganics)solution(250mL)toremovemetalions,and subsequentlywithDIwatertoapHaround6.Theresultingsolidwas freezedried.ThefreezedriedgraphiteoxidepowderwasdispersedinDI waterbyultrasonicationfor2htogetaGOsuspension.Toincreasethe GOconcentration,theGOsuspensionwascentrifugedat15000gfor1 h.Thecollectedsedimentwasre-dispersedinprintingsolvent contain-ing0.06wt%TritonX-100(Sigma-Aldrich)and1:10propyleneglycol (Sigma-Aldrich):waterbymass.
2.3. Inkjetprinting
AllpatternsanddeviceswereinkjetprintedbyaDimatixDMP-2800 inkjetprinter(FujifilmDimatix),whichwasequippedwitha10pL car-tridge(DMC-11610).ToperformAFMmeasurementsonsingledroplets, theMXeneandGOinkswereinkjetprintedonSi/SiO2withadrop
spac-ingof80μmat30°C.Fortheelectricalconductivitymeasurements,the MXeneinkwasprintedat30°ConSi/SiO2asathinfilmwithsize5mm
by5mmandvaryingnumbersoflayersatadropspacingof20μm.The substrates includingpolyimide andSi/SiO2 werecleanedbyethanol, acetone,isopropanolandwaterfollowedbyO2plasmatreatmentfor5
minbeforeprinting.
Tofabricateall-inkjet-printed solid-statesandwiched supercapaci-tors,theMXeneinkwasfirstprintedat30°Casbottomelectrodeon polyimidesubstratewithadropspacingof20μmfollowedbydrying at50°Cfor1h.ThenGOinkwasprintedontopofMXeneelectrode withadropspacingof20μmat30°Cfollowedbydryingat50°Cfor 1h.Finally,theMXeneinkwasprintedat30°ConGOelectrolyteas topelectrodewithadropspacingof20μm.Itisworthnotingthatthe GOelectrolyteareaislargerthantheMXeneelectrodetoprevent short-circuiting.
Tomakeall-inkjet-printedsolid-statemicro-supercapacitors,the MX-eneinkwasprintedwithaninterdigitatedconfigurationonpolyimide
Y. Wang, M. Mehrali, Y.-Z. Zhang et al. Energy Storage Materials 36 (2021) 318–325 substrateasinterdigitatedelectrodesat30°Cwithdifferentlayersata
dropspacingof20μmusing2nozzles,followedbydryingat50°Cfor1 h.Then,theGOinkwasprintedat30°ContopoftheMXeneelectrodes atadropspacingof20μm.
2.4. Electrochemicalcharacterization
Allelectrochemicalcharacterizationswereconductedonan Auto-labworkstation(PGSTAT128N).Bothmicro-supercapacitorsand sand-wichedsupercapacitorswerecharacterizedinatwo-electrode configu-ration.Electrochemicalimpedancespectroscopywasperformedby ap-plyinganACvoltageof10mVamplitudeinthefrequencyrangefrom 0.01Hzto10kHz.
2.5. Materialscharacterization
X-raydiffraction(XRD)analysiswasdonewithaPANalyticalX’Pert ProwithCuK𝛼 radiation(𝜆=0.15405nm).AtomicForceMicroscopy (AFM) (Veeco Dimension Icon) was conducted in standard tapping mode.TheAFMdatawereanalyzedbyGwyddion(version2.47) soft-ware.X-rayphotoelectronspectroscopy(XPS)wasconductedusingan OmicronNanotechnologyGmbH(OxfordInstruments)surfaceanalysis systemwithaphotonenergyof1486.7eV(AlK𝛼 X-raysource)witha scanningstepsizeof0.1eV.Thepassenergywassetto20eV.The spec-trawerecorrectedusingthebindingenergyofC1softhecarbonresidual onnanosheetsasareference.Ramanspectroscopywasperformedona BrukerSenterraRamanspectrometerusinga532nmlaserunder ambi-entconditions.Highresolutionscanningelectronmicroscopy(HRSEM; ZeissMERLIN)wasperformedtoacquireinformationofprintedMXene andGOfilms.Thesurfacetensionoftheinkswasmeasuredbyacontact anglesystemOCA(DataPhysicsCorporation).Theviscosityoftheinks wasdeterminedbyanAutomatedMicroviscometerAMVn(AntonPaar GmbH).
TheelectricalconductivityofprintedMXenefilmsonSi/SiO2 was
measuredinaVanderPauwgeometrybyPhysicalProperties Measure-mentSystem(PPMS)at300K.Copperwireswerebondedonfour cor-nersofprintedMXenefilmsbysilverpaste.Rswascalculatedfromthe
followingEq.(1):
𝑅s=𝜋𝑅∕ln2 (1)
Thespecificarealcapacitance(CA)ofdeviceswascalculatedfrom
theGCDcurvesbyusingEq.(2):
𝐶A=
[
𝐼∕(d𝑉∕d𝑡)]∕𝐴device (2)
whereIisthedischargecurrent,dV/dtistheslopeofdischargecurve, andAdevice referstothetotalgeometricalsurfaceareaof thedevices
includingtheelectrodesandthegapbetweentheelectrodes. Thearealenergydensities(EA,μWhcm-2)andpowerdensities(P
A,
μWcm-2)werecalculatedfromEqs.(3)and(4)
𝐸A=𝐶A𝑉2∕(2× 3.6) (3)
𝑃A=3600×𝐸A∕Δ𝑡 (4)
WhereΔtreferstodischargetime. 3. Resultanddiscussion
Additive-freewater-basedMXeneandGOinksweresuccessfully pre-paredasshowninFig.2a.DuetoitshighGOconcentration,theGOink hasadarkbrowncolor. ThethicknessofMXeneandGOnanosheets weredeterminedbyatomicforcemicroscopy(AFM)tobearound1.5 nmand1nm,respectively,indicatingaunilamellarstructureforboth types(SupplementaryFig.S1a,b).ThelateralsizesofMXeneandGO nanosheetsestimatedfromAFMimageswereabout0.76μmfor MX-enenanosheets and0.78μmforGOnanosheets (SupplementaryFig.
S1c)[27].Toevaluatetheinkprintability,theinverseOhnesorge num-berZ,whichisdefinedas𝑍=(𝛼𝜌𝛾)1∕2∕𝜂,wasemployed.Here,𝛼 isthe
nozzlediameter,𝜌 isthedensity,𝛾 isthesurfacetension,and𝜂 isthe viscosityofthefluid.ThesurfacetensionandviscosityofMXeneink were80.3mNm-1and1.4mPas,respectively.Thenozzlediameterof
21.5μmandthevalueofsurfacetension,viscosityresultinaZvalue of about30 forMXeneink. ThesurfacetensionandviscosityofGO inkswere129.4mNm-1and20.2mPas,respectively,leadingtoaZ
valueofabout3.TheZvaluethatiscommonlyusedtoevaluatethe inkprintabilityisbasedonatheoreticalmodelwithseveralsimplifying assumptions[28].Theinkisexpectedtobeprintableif1<Z<10[29]. However,inreality therehavebeenmanyexamplesof inksthatcan be consistentlyejected withnocloggingissuesforalongtimewhile havingaZvaluewelloutsidethatrange.Forinstance,awater/Triton X-100inkwaseasilyejectedandstableduringprintingwithZvalueof 20[10].SothefactthatourinkcanbeprintedwhilehavingaZvalue outside therange1-10showsthat thecommonlyusedtheory appar-entlyhasitslimitations.This hasalreadybeenrecognizedbyseveral otherauthors[30,31]includingourselves[32]. TheZvalueof MXene inkwasalsolargerthantherecommendedvaluerange(1<Z<10) forinkjetprinting.Stroboscopicimagesofinkdropletformation ver-sustimeillustratedthequalityoftheinks(SupplementaryFig.S2).No satellitedropletswereobservedforMXeneandGOinks.Therefore, MX-eneinkissuitableforinkjetprintingwithoutsatellites,irrespectiveof theactualZvalue.Toachieveahighqualityinkjetprintingprocess,the preparationofprintableandstableinksisveryimportant.Water-based MXeneinkwithoutanyadditivesshowedhighlystableprinting behav-iorduringjetting,whichmaybeattributedtothepresenceoffunctional groupslike–O,-OHand-FonthesurfaceofMXenesheets (Supplemen-taryFig.S2a)thathelpdispersioninwater.ToprepareaprintableGO ink,TritonX-100wasaddedtothewater-basedgrapheneoxideinkin ordertooptimizetheinksurfacetension[10].AsshowninFig.S2b,no satellitedropletsweregeneratedduringjetting,indicatingaprintable andstableGOink.Bothinksshowedgoodwettingonsiliconsubstrates, asconfirmedbyAFMmappings,andthecross-sectionalprofileofthe AFMimagesfurtherconfirmedtheuniformdepositionofbothMXene andGOinks(SupplementaryFig.S3)[27].ThewrinklesinprintedGO dropletswerecausedbyinteractionsbetweenadjacentGOsheets[33]. Fig.2bshowstwoexamplesofprintedpatternsobtainedbyMXeneink onflexiblepolyimidesubstrate,demonstratingflexibilityinpattern de-signandlargeareacoatingwithmultipleprintingpasses.Thescanning electronmicroscopy(SEM)imagesofprintedMXene(Fig.2c)andGO (Fig.2d) nanosheetfilms onSi/SiO2 substrates showuniformity and
continuityoverlargesurfaceareas.Itisworthnotingthatthesheetsin bothprintedfilmsshowedahighdegreeofhorizontalorientationanda layer-by-layerstructure,whichwillfacilitatethetransportofelectrolyte ionsinin-planestructureddevicessuchasMSCs.AsshowninFig.2e, theXRDpatternofaprintedMXenefilmshowsstrongorderinginthec
directionwitha(002)peakat6.8°,thusconfirmingthehorizontal ori-entationofnanosheetsinprintedfilms[27].Thesmalleranglethanin dryMXenefilms,wherethesamepeakisat8.9° (SupplementaryFig. S4a),indicateswiderspacingbetweenthelayersintheprintedfilmand intercalationofspatiallyconfinedH2Omolecules[27,34].TheXRD
pat-ternofaprintedGOfilmshowsapeakat2𝜃 =9.6°,whichcorresponds withadspacingof0.92nm,suggestingthatelectrolyteiontransportis predominantinhorizontalratherthaninverticaldirection.Thesheet resistanceRsofprintedMXenefilmscouldbetunedbythenumberof printedlayers.AsshowninFig.2f,theRsofMXenefilmsonSi/SiO2
sub-stratesdecreasedrapidlyfrom116.7Ωsq-1(printedlayers<N>=1)to
around5.9Ωsq-1(<N>=40)withaninkconcentrationofaround4.5
mgml-1[27].
Thesolid-statesymmetricalSSCsconsistofaprintedsolid-stateGO electrolytesandwichedbetweentwoprintedMXeneelectrodes(Fig.1, SupplementaryFig.S6a,c).Aclearboundarybetweenelectrolyteand electrodescanbeidentified,demonstratingwell-definedspatial separa-tionbetweenthephases(SupplementaryFig.S6b).Toavoidremixing
Fig.2. CharacterizationofMXeneandGOnanosheets.(a)Opticalimageofwater-basedMXeneandGOinks.(b)Inkjetprinted“MESA+INSTITUTEFOR NAN-OTECHNOLOGY” logoandword“MXene” usingMXeneinkonpolyimidesubstratewithmultipleprintinglayers.Cross-sectionalSEMimageof(c)printedMXene and(d)GOfilmsonSi/SiO2substrates.(e)XRDpatternofprintedMXeneandGOfilmsonsiliconsubstrates.(f)Sheetresistance R sasfunctionofthenumberof
printedMXenelayers(5by5cm2)onSi/SiO
2.TheinsetshowsaschematicrepresentationofthesheetresistivityasmeasuredbytheVanderPauwmethod.Theink
concentrationwasaround4.5mgmL-1.
ofnanosheetsattheprintedinterfaces,printednanosheetlayerswere solidifiedbyheatingat50°Cfor1hbeforeprintinganothermaterialon top.Cross-sectionalSEManalysisindicatedanintimateandstable con-tactbetweentheMXeneelectrodesandtheGOelectrolyte(Fig.3a, Sup-plementaryFig.S6d-f).ThegoodadhesionattheGO/MXeneinterface canbeexplainedbystrongattractiveinteractionsbetweenpolar oxygen-containingfunctionalgroupspresentonthesurfaceofGO,e.g.O,OHor COOH,andthehighlypolarsurfaceofTi3C2Tx(T=O,OH,F;terminal
groups).Achievingwellcontrolledandsharpinterfacesisasignificant challengeforprintedheterostructures.Theinterfaceplaysanimportant roleindeviceperformance.Addingbinderssuchasxanthanguminto inkshasbeendemonstratedtocontrolthestructureoftheinkjetprinted heterostructureinterface[10].However,theperformancecouldalsobe affectedbythepresenceof(organic)binders.Annealingprinted het-erostructuresathightemperaturescanleadtoremovalofthebinder, butitlimitsthechoiceofsubstratestothermallystableones.Here,we successfullyinkjetprintedverticalheterostructureswithoutanysignof re-dispersionattheinterfacebydryingtheprintedpatternsbefore print-ingthenextlayerwithdifferentnanosheets.Dryingwasperformedat 50°C,whichisarelativelylowtemperatureandisapplicabletomost substratesincludingpaperandpolymersubstrates.Thetop-viewSEM imageofaMXeneelectrodeshowscontinuousfeaturesofprinted elec-trodeswithoutcracksorpinholes,illustratingthehighqualityofprinted films(Fig.3b).Duetothehighresolutionbroughtbyinkjetprinting,all printed30LSSCsexhibitedalowrootmeansquareroughness(RMS)of ≈130±25nmatadevicethicknessofaround4μm(Fig.3c).Element mappingsfromenergy-dispersivex-rayspectroscopy(EDS)further con-firmthestableandsharpinterfacebetweenMXeneelectrodesandGO electrolyte(SupplementaryFig.S7).Toinvestigatetheelectrochemical performancesofall-inkjet-printedSSCs,MXeneelectrodeswith
thick-nessesof10and30printedlayers(fromhereonreferredtoas10LSSC and30LSSC,respectively)werefabricated.AsshowninFig.3d,30LSSC showsahigherspecificcapacitancethan10LSSCfromcyclic voltamme-try(CV)atscanrateof10mVs-1[27].Thequasi-rectangularCVcurves
showthepseudo-capacitivebehaviorofthedevices.Morespecifically, 30LSSCstillexhibitsaquasi-rectangularshapeevenathighscanrate (SupplementaryFig.S8a,c).Galvanostaticcharge/discharge(GCD)data ofbothdevicesatcurrentdensitiesrangingfrom40-200μAcm-2
(Sup-plementaryFig.S8b,d)areshowninFig.3e[27].The30LSSCexhibits
CAashighas9.8mFcm-2atcurrentdensityof40μAcm-2,while10L
SSCexhibitsCAof3mFcm-2atsamecurrentdensity,indicatingthat
CAisroughlyproportionalwiththenumberofprintedMXenelayers.
Thus,theentireelectrodescontributetothechargestorageprocess,and protontransportisnothinderedbyincreasingelectrodethickness.
Bothtypesofdeviceswerecharacterizedwithoutliquidelectrolyte. The mobile electrolyte which are protons ions needed for charg-ing/dischargingthesedevicesarethereforethoughttoarisefromthe hydrolysisof functionaloxygen-bearinggroupsonthesolidGO elec-trolyte[21].FreewatermoleculespresentbetweenGOsheetsmay fa-cilitateprotontransportviatheGrotthussmechanism[35]orby diffu-sionofhydroniumionswithintheinterlayerspaces[21].Thepathof proton movementpathfromone layertothesurrounding layers oc-cursthroughnanoporesinmultilayerGO.ThesingleanddoubleGO wallssupporttheprotonmobilityandhydrogenbondreformationin GOfilms[23].IntimatecontactbetweentheGOelectrolyteandMXene electrodeswillfacilitateprotontransferbetweendifferentSSC compo-nents.ProtonmovementinsidetheMXeneelectrodesprobablyproceeds viaconfinedwatermoleculesthataretrappedbetweenMXenesheets.
Electrochemical impedance spectroscopy (EIS)was conducted on bothdevicesinthefrequencyrangefrom10mHzto10kHz.The
ex-Y. Wang, M. Mehrali, ex-Y.-Z. Zhang et al. Energy Storage Materials 36 (2021) 318–325
Fig.3. Electrochemicalperformanceofall-inkjet-printedSSCs.(a)Cross-sectionalSEMimageofinkjet-printedSSC.Thedashedlineroughlyindicatestheboundary betweenMXene(above)andGO(below)phasesasaguidetotheeye.(b)TopviewSEMoftopMXeneelectrode.(c)AFMtopographyof30×30μm2scanareain
device.(d)CVofas-made10LSSCand30LSSCat10mVs-1.(e)C
Aofas-made10LSSCand30LSSCatdifferentcurrentdensities.(f)Nyquistplotsofas-made10L
SSCand30LSSC.Theinsetshowstheequivalentcircuittowhichtheexperimentaldatawerefitted.(g)CVdiagramof1,2and4supercapacitorsconnectedinseries andinparallel.(h)C Aof30LSSCwithdifferentelectrolytes(DIwater,5MLiCland0.5MNa2SO4inwater,respectively)ontopofthedevices.(i)Ragoneplotof
all-inkjet-printedSSCwithotherdifferentsystems.
perimentaldatawerefittedtotheequivalentcircuitshownintheinset ofFig.3f[27].Itconsistsofaconstantphaseelement(CPE)Q2that
rep-resentsthesurfacecapacitanceofthedevice,inparallelwithaCPEQ3 thatrepresentstheslowerdiffusion-controlledvolumecapacitance.The electrolyteisrepresentedbytheR2(Q1)sub-circuit,andtheelectrode
resistancebyR1.Sample10LSSChasalargerR1than30LSSC,showing
thatthickerelectrodesexhibitalowerresistance(TableS1;5.9kΩ ver-sus2.1kΩ).Theimpedanceinthelowfrequencyrangesuggestsmixed
surfaceabsorptionanddiffusionalcontrolofthedevices,whicharethe doublelayercapacitance(Q2)andchargetransferdiffusionimpedance
Q3.The30LSSCdevicehasasmallerchargetransferresistance(R3)in thelowerfrequencyrangethan10LSSC(1.9kΩversus12.9kΩ).From the30LSSCEISdatainFig.3f,theionicconductivityofGOelectrolyte wascalculatedtobeatleast0.38μScm-1.
Todemonstratethepotentialforpracticalapplicationsathigh volt-ages, theas-made30LSSCswereconnectedinseriesandinparallel
Fig.4. Electrochemicalperformanceofall-inkjet-printedMSCs.(a,b)Cross-sectionalSEMimagesofall-inkjet-printedMSC.ThedashedlineroughlydividesMXene electrode(below)andGOelectrolyte(above).(c)AFMtopographyoftopGOelectrolytein(b).(d)CVof30LMSCwithandwithoutaddedwaterontopofthedevice at20mVs-1.(e)CVofprinted10L/20L/30LMSCsat10mVs-1.(f)GCDofprinted10L/20L/30LMSCsat15μAcm-2.(g)C
Aofprinted10L/20L/30LMSCswith
differentcurrentdensities.(h)Nyquistplotsofprinted10L/20L/30LMSCs.Theinsetisthefittedequivalentcircuitforthelowfrequencyrangeoftheimpedance response.(i)C Aof30LMSCwithexcessH2Oand0.5MH2SO4atcurrentdensityof50μAcm-2.
configurations.AsshowninFig.3g,thevoltagewindow reached1.2 Vand 2.4Vwith twoandfour devicesconnectedin series, respec-tively[27]. Thecurrent wasincreasedbyafactorof~ 2and4with twoandfourdevicesconnectedinparallel,respectively.The electro-chemicalperformanceofSSCscouldbeenhancedbyadditionofliquid electrolytes.AsshowninFig.3h,excessdeionized(DI)waterresultedin ahigherCAof12.1mFcm-2thanintheas-madedevice,probablydue
toenhancediontransportinliquidmedia.Aqueouselectrolytessuchas 5MLiCland0.5MNa2SO4 introduceadditionalelectrolyteionsthat
enhanceionictransportatthesametime,increasingtheCAfurtherto
13.6-14.1mFcm-2(SupplementaryFig.S9)[27].Theas-made30LSSC
exhibited anarealenergydensity(EA)of 0.49μWhcm-2atapower
density(PA)of12.55μWcm-2,whileEAincreasedto0.71μWhcm-2
atPAof12.48μWcm-2byaddingadropof5MLiClelectrolyteinto
theas-madedevice(Fig.3i). Apparently,thepresenceofliquid elec-trolyte hasa positiveeffect ontheenergydensity, howeverthe pre-cisemechanismremainsunclear.TheEAofas-made30LSSCishigher
thantherecentlyreportedextrusion-printedMXeneMSCwithH2SO4
-poly(vinyl alcohol,PVA) gelelectrolyte (EA of0.32μWhcm-2 atP A
Y. Wang, M. Mehrali, Y.-Z. Zhang et al. Energy Storage Materials 36 (2021) 318–325 printedMSCwithgrapheneelectrodesandprintedpolyelectrolyte,[38]
printedgrapheneMSC,[39]andMXene/single-walledcarbonnanotube supercapacitor[40]. The30LSSCexhibitsgoodcyclingstabilitywith acapacitanceretentionof~100%after10000cycles(Supplementary Fig.S10a).Moreover,theall-printedSSCshowshighmechanical stabil-itywithabendingradiusofabout1cm,asshowninSupplementaryFig. S10b[27].
MSCswerefabricatedbyprintingMXenenanosheetswith interdig-itated structure as electrodes on polyimide substrate, followed by a printedlayerofGOnanosheetsontopof/overtheMXeneelectrodesto serveassolid-stateelectrolyte(Fig.1,bottom).MXeneelectrodeswith 10,20and30printedlayers(referstoas10LMSC,20LMSCand30L MSC,respectively)wereprinted(SupplementaryFig.S11).Theprinted MXeneelectrodesshowsharpfeaturesattheedges,indicatingthe sta-bilityoftheMXeneinkandthehighreliabilityoftheinkjetprinting pro-cess,andthesharpedgeswerestillretainedafterprintingGOnanosheets ontop.EDSelementalmappingconfirmsthepresenceofawell-defined andstabletopinterfacebetweenMXeneelectrodeandGOelectrolyte (SupplementaryFig.S12).Theall-inkjet-printed30LMSCsexhibiteda lowRMS of≈157 ±20nmat adevicethicknessof around2.3μm (Fig.4c).However,thecontactatthecross-sectionbetweenMXene elec-trodeandGOelectrolyteispoor(Fig.4a,b),whichislikelythecauseof lowcurrentresponseintheCVmeasurement(Fig.4d).Mostlikely,only GOsheetsneartheheterostructureinterfacecontributetothe capaci-tance,becausetheprotonsaregeneratedviahydrolysisoffunctional groupsonGO.
Electrochemicalmeasurementswereperformedonas-made all-solid-stateMSCsandonMSCstowhichexcessaqueouselectrolytehadbeen added.Theelectrochemical performanceof 30LMSC improved con-siderably upon addition of water, due to enhancedproton mobility (Fig.4d)[27].TheCVcurvesofMSCswithvaryingelectrodethicknesses demonstratethatthickerelectrodeswithmoreactivesurfacesitesshow higherCA(Fig.4e,SupplementaryFig.S13a-c).GCDfurtherconfirms
that30LMSCexhibitsahighercapacitancethantheothertwodevices (Fig.4f,SupplementaryFig.S13d-f).TheCAof30LMSCreachedto3.1
mFcm-2,while10and20layerdevicesreached1.2and1.9mFcm-2
atacurrentdensityof20μAcm-2,respectively(Fig.4g).EISsuggests
thatthecharge transferresistance(R1) ofthe30layersthickMXene electrodedeviceislowerthantheothertwodevices(4.1kΩ,11.9kΩ and12.0kΩfor10LSSC, 20LSSCand30LSSC,respectively;Fig.4h, TableS2).Similar totheSSC, theequivalentcircuitsin thelow fre-quencyrangesuggestsmixedsurfaceabsorptionanddiffusionalcontrol, i.e.thedoublelayercapacitance(Q1)andthechargetransferdiffusion
impedance(Q2).Additionofa0.5MH2SO4 electrolytesolutiononto 30LMSCresultedinaahighercapacitancethanindeviceswithexcess water(Fig.4i,SupplementaryFig.S14)[27].TheH2SO4electrolyte
pro-videsadditionalprotonsthatenhancetheionicconductivity,leadingto lowerseriesresistances.
ComparedwithSSCs,as-madeMSCsshowmuchlowerareal capac-itance,whichisprobablyduetothepoorcontactatthecross-section betweenMXeneelectrodeandGOelectrolyte.Itis notedthatdevice structureoptimizationcouldfurtherimprovetheelectrochemical per-formanceoftheseMSCs.Furthermore,thenormalizedarealcapacitance inbothSSCandMSCdevicesareindependentofelectrodesthickness withinexperimentalerror,indicatingthattheentireelectrodes partici-pateinthechargestorageprocess(SupplementaryFig.S15).
4. Conclusion
Inconclusion,wedemonstratedall-inkjet-printedsolid-state super-capacitorsbasedon2DMXene/GO/MXene.Duetothehighionic con-ductivityofGOfilms,theprintedSSCwithoutliquidelectrolyteshowed anEAof0.49𝜇Whcm-2ataPAof12.55𝜇Wcm-2.TheCAcanbe
in-creasedfurtherbyaddingliquidelectrolytes.Furtheroptimizationof materials,printedelectrodesthicknessandelectrodesconfigurationwill
enhancedeviceperformancefurther.Printedsupercapacitorsshowhigh potentialforuseinsmallpowersourceunitsforflexibleelectronics. DeclarationofCompetingInterest
Theauthorsdeclarethattheyhavenoknowncompetingfinancial interestsorpersonalrelationshipsthatcouldhaveappearedtoinfluence theworkreportedinthispaper.
CRediTauthorshipcontributionstatement
YangWang:Conceptualization,Methodology,Investigation, Vali-dation,Visualization,Writing-originaldraft.MohammadMehrali: In-vestigation,Methodology.Yi-ZhouZhang:Investigation,Formal anal-ysis.MelvinA.Timmerman:Investigation,Methodology.BernardA. Boukamp:Formalanalysis.Peng-YuXu:Investigation.JohanE.ten Elshof:Conceptualization,Writing-review&editing,Supervision. Acknowledgements
Y.W. thanksDr.B.Chenfor thehelpwithelectricalconductivity measurementsandDr.Y.Liuforthehelpwithsurfacetension measure-ments.M.SmithersisacknowledgedforperformingtheHR-SEMand EDSelementalmapping.Y.W.acknowledgesthefinancialsupportofthe ChinaScholarshipsCouncilprogram(CSC,No.201608340058).Y.Z.Z. acknowledgesthefinancialsupportfromtheNationalNaturalScience FoundationofChina(21805136)andtheNaturalScienceFoundationof JiangsuProvince(BK20170999).
Supplementarymaterials
Supplementarymaterialassociatedwiththisarticlecanbefound,in theonlineversion,atdoi:10.1016/j.ensm.2021.01.009.
References
[1] K.S. Novoselov , A. Mishchenko , A. Carvalho , A.H. Castro Neto , 2D materials and van der Waals heterostructures, Science 353 (2016) aac9439 .
[2] A.K. Geim , I.V. Grigorieva , Van der Waals heterostructures, Nature 499 (2013) 419 . [3] E. Pomerantseva , Y. Gogotsi , Two-dimensional heterostructures for energy storage,
Nat. Energy 2 (2017) 17089 .
[4] Y. Liu , N.O. Weiss , X.D. Duan , H.C. Cheng , Y. Huang , X.F. Duan , Van der Waals heterostructures and devices, Nat. Rev. Mater. 1 (2016) 16042 .
[5] X. Lin , J. Wang , X. Gao , S. Wang , Q. Sun , J. Luo , C. Zhao , Y. Zhao , X. Yang , C. Wang , R. Li , X. Sun , 3D Printing of Free-Standing “O 2 Breathable ” Air Electrodes for High-
-Capacity and Long-Life Na–O 2 Batteries, Chem. Mater. 32 (2020) 3018 .
[6] X. Gao , X. Yang , S. Wang , Q. Sun , C. Zhao , X. Li , J. Liang , M. Zheng , Y. Zhao , J. Wang , M. Li , R. Li , T.-K. Sham , X. Sun , A 3D-printed ultra-high Se loading cathode for high energy density quasi-solid-state Li–Se batteries, J. Mater. Chem. A 8 (2020) 278 . [7] X. Gao , X. Yang , Q. Sun , J. Luo , J. Liang , W. Li , J. Wang , S. Wang , M. Li , R. Li ,
T.-K. Sham , X. Sun , Converting a thick electrode into vertically aligned “Thin elec- trodes ” by 3D-Printing for designing thickness independent Li-S cathode, Energy Storage Mater. 24 (2020) 682 .
[8] X.J. Gao , Q. Sun , X.F. Yang , J.N. Liang , A. Koo , W.H. Li , J.W. Liang , J.W. Wang , R.Y. Li , F.B. Holness , A.D. Price , S.L. Yang , T.K. Sham , X.L. Sun , Toward a remarkable Li-S battery via 3D printing, Nano Energy 56 (2019) 595 .
[9] X. Gao , X. Yang , K. Adair , X. Li , J. Liang , Q. Sun , Y. Zhao , R. Li , T.K. Sham , X. Sun , 3D Vertically Aligned Li Metal Anodes with Ultrahigh Cycling Currents and Capacities of 10 mA cm −2 /20 mAh cm −2 Realized by Selective Nucleation within Microchannel
Walls, Adv. Energy Mater. 10 (2020) 1903753 .
[10] D. McManus , S. Vranic , F. Withers , V. Sanchez-Romaguera , M. Macucci , H. Yang , R. Sorrentino , K. Parvez , S.K. Son , G. Iannaccone , K. Kostarelos , G. Fiori , C. Casir- aghi , Water-based and biocompatible 2D crystal inks for all-inkjet-printed het- erostructures, Nat. Nanotechnol. 12 (2017) 343 .
[11] P.H. Yang , H.J. Fan , Inkjet and Extrusion Printing for Electrochemical Energy Stor- age: A Minireview, Adv. Mater. Technol. 5 (2020) 2000217 .
[12] A.G. Kelly , T. Hallam , C. Backes , A. Harvey , A.S. Esmaeily , I. Godwin , J. Coelho , V. Nicolosi , J. Lauth , A. Kulkarni , S. Kinge , L.D. Siebbeles , G.S. Duesberg , J.N. Cole- man , All-printed thin-film transistors from networks of liquid-exfoliated nanosheets, Science 356 (2017) 69 .
[13] R. Worsley , L. Pimpolari , D. McManus , N. Ge , R. Ionescu , J.A. Wittkopf , A. Alieva , G. Basso , M. Macucci , G. Iannaccone , K.S. Novoselov , H. Holder , G. Fiori , C. Casir- aghi , All-2D Material Inkjet-Printed Capacitors: Toward Fully Printed Integrated Cir- cuits, ACS Nano 13 (2019) 54 .
[14] B. Anasori , M.R. Lukatskaya , Y. Gogotsi , 2D metal carbides and nitrides (MXenes) for energy storage, Nat. Rev. Mater. 2 (2017) 16098 .
[15] M. Naguib , V.N. Mochalin , M.W. Barsoum , Y. Gogotsi , 25th anniversary article: MX- enes: a new family of two-dimensional materials, Adv. Mater. 26 (2014) 992 . [16] Y.Z. Zhang , K.H. Lee , D.H. Anjum , R. Sougrat , Q. Jiang , H. Kim , H.N. Alshareef , MX-
enes stretch hydrogel sensor performance to new limits, Sci. Adv. 4 (2018) eaat0098 . [17] A. Agresti , A. Pazniak , S. Pescetelli , A. Di Vito , D. Rossi , A. Pecchia , M. Auf der Maur , A. Liedl , R. Larciprete , D.V. Kuznetsov , D. Saranin , A. Di Carlo , Titanium-carbide MXenes for work function and interface engineering in perovskite solar cells, Nat. Mater. 18 (2019) 1228 .
[18] M. Ghidiu , M.R. Lukatskaya , M.Q. Zhao , Y. Gogotsi , M.W. Barsoum , Conductive two-dimensional titanium carbide ’clay’ with high volumetric capacitance, Nature 516 (2014) 78 .
[19] Y. Xia , T.S. Mathis , M.Q. Zhao , B. Anasori , A. Dang , Z. Zhou , H. Cho , Y. Gogotsi , S. Yang , Thickness-independent capacitance of vertically aligned liquid-crystalline MXenes, Nature 557 (2018) 409 .
[20] Y.Z. Zhang , Y. Wang , Q. Jiang , J.K. El-Demellawi , H. Kim , H.N. Alshareef , MX- ene Printing and Patterned Coating for Device Applications, Adv. Mater. 32 (2020) e1908486 .
[21] W. Gao , N. Singh , L. Song , Z. Liu , A.L. Reddy , L. Ci , R. Vajtai , Q. Zhang , B. Wei , P.M. Ajayan , Direct laser writing of micro-supercapacitors on hydrated graphite ox- ide films, Nat. Nanotechnol. 6 (2011) 496 .
[22] M.R. Karim , K. Hatakeyama , T. Matsui , H. Takehira , T. Taniguchi , M. Koinuma , Y. Matsumoto , T. Akutagawa , T. Nakamura , S. Noro , T. Yamada , H. Kitagawa , S. Hayami , Graphene oxide nanosheet with high proton conductivity, J. Am. Chem. Soc. 135 (2013) 8097 .
[23] K. Hatakeyama , M.R. Karim , C. Ogata , H. Tateishi , A. Funatsu , T. Taniguchi , M. Koinuma , S. Hayami , Y. Matsumoto , Proton conductivities of graphene oxide nanosheets: single, multilayer, and modified nanosheets, Angew. Chem. Int. Ed. 53 (2014) 6997 .
[24] H. Zhang , Ultrathin Two-Dimensional Nanomaterials, ACS Nano 9 (2015) 9451 . [25] J.D. Cain , A. Azizi , K. Maleski , B. Anasori , E.C. Glazer , P.Y. Kim , Y. Gogotsi ,
B.A. Helms , T.P. Russell , A. Zettl , Sculpting Liquids with Two-Dimensional Mate- rials: The Assembly of Ti 3 C 2 T x MXene Sheets at Liquid-Liquid Interfaces, ACS Nano
13 (2019) 12385 .
[26] W.S. Hummers , R.E. Offeman , Preparation of Graphitic Oxide, J. Am. Chem. Soc. 80 (1958) 1339 .
[27] J.E. ten Elshof, Y. Wang, Tunable capacitance in all-inkjet-printed nanosheet het- erostructures, Mendeley Data V1 (2020), doi: 10.17632/prmcmwdsgs.1 .
[28] N. Reis , B. Derby , Ink jet deposition of ceramic suspensions: Modeling and exper- iments of droplet formation, MRS Online Proceeding Library Archive 625 (2000) 117–122 .
[29] M. Gao , L.H. Li , Y.L. Song , Inkjet printing wearable electronic devices, J. Mater. Chem. C 5 (2017) 2971 .
[30] D.J. Finn , M. Lotya , G. Cunningham , R.J. Smith , D. McCloskey , J.F. Donegan , J.N. Coleman , Inkjet deposition of liquid-exfoliated graphene and MoS 2 nanosheets
for printed device applications, J. Mater. Chem. C 2 (2014) 925 .
[31] F. Torrisi , T. Hasan , W. Wu , Z. Sun , A. Lombardo , T.S. Kulmala , G.-W. Hsieh , S. Jung , F. Bonaccorso , P.J. Paul , D. Chu , A.C. Ferrari , Inkjet-printed graphene electronics, ACS Nano 6 (2012) 2992 .
[32] Y. Wang , Y.-Z. Zhang , D. Dubbink , J.E. ten Elshof , Inkjet printing of 𝛿-MnO 2
nanosheets for flexible solid-state micro-supercapacitor, Nano Energy 49 (2018) 481 . [33] X. Shen , X.Y. Lin , N. Yousefi, J.J. Jia , J.K. Kim , Wrinkling in graphene sheets and
graphene oxide papers, Carbon 66 (2014) 84 .
[34] X.P. Mu , D.S. Wang , F. Du , G. Chen , C.Z. Wang , Y.J. Wei , Y. Gogotsi , Y. Gao , Y. Dall’Agnese , Revealing the Pseudo-Intercalation Charge Storage Mechanism of MXenes in Acidic Electrolyte, Adv. Funct. Mater. 29 (2019) 1902953 .
[35] N. Agmon , The Grotthuss mechanism, Chem. Phys. Lett. 244 (1995) 456 . [36] C.J. Zhang , L. McKeon , M.P. Kremer , S.H. Park , O. Ronan , A. Seral-Ascaso , S. Bar-
wich , C.O. Coileain , N. McEvoy , H.C. Nerl , B. Anasori , J.N. Coleman , Y. Gogotsi , V. Nicolosi , Additive-free MXene inks and direct printing of micro-supercapacitors, Nat. Commun. 10 (2019) 1795 .
[37] S. Sollami Delekta , K.H. Adolfsson , N. Benyahia Erdal , M. Hakkarainen , M. Ostling , J. Li , Fully inkjet printed ultrathin microsupercapacitors based on graphene elec- trodes and a nano-graphene oxide electrolyte, Nanoscale 11 (2019) 10172 . [38] J. Li , S. Sollami Delekta , P. Zhang , S. Yang , M.R. Lohe , X. Zhuang , X. Feng ,
M. Ostling , Scalable Fabrication and Integration of Graphene Microsupercapacitors through Full Inkjet Printing, ACS Nano 11 (2017) 8249 .
[39] S. Sollami Delekta , A.D. Smith , J. Li , M. Ostling , Inkjet printed highly transparent and flexible graphene micro-supercapacitors, Nanoscale 9 (2017) 6998 . [40] C.J. Zhang , B. Anasori , A. Seral-Ascaso , S.H. Park , N. McEvoy , A. Shmeliov ,
G.S. Duesberg , J.N. Coleman , Y. Gogotsi , V. Nicolosi , Transparent, Flexible, and Conductive 2D Titanium Carbide (MXene) Films with High Volumetric Capacitance, Adv. Mater. 29 (2017) 1702678 .
