On the supercapacitive behaviour of anodic porous WO3-based negative electrodes

On

the

Supercapacitive

Behaviour

of

Anodic

Porous

WO

3

-Based

Negative

Electrodes

Kush

K.

Upadhyay

*

,

Marco

Altomare

*

,

Sonia

Eugénio

,

Patrik

Schmuki

,

Teresa

M.

Silva

,

Maria

Fàtima

Montemor

a_{CQE,InstitutoSuperiorTécnico,UniversidadedeLisboa,Av.RoviscoPais,1049-001Lisboa,Portugal} b

DepartmentofMaterialsScience,InstituteforSurfaceScienceandCorrosionWW4-LKO,UniversityofErlangen-Nuremberg,Martensstraße7,D-91058 Erlangen,Germany

c

ChemistryDepartment,FacultyofSciences,KingAbdulazizUniversity,80203Jeddah,SaudiArabia

d

DepartmentofMechanicalEngineering,GI-MOSM,InstitutoSuperiordeEngenhariadeLisboa-ISEL,1950-062Lisboa,Portugal

ARTICLE INFO

Articlehistory:

Received22December2016

Receivedinrevisedform13February2017 Accepted22February2017

Availableonline24February2017

Keywords: electrochemicalanodization WO3nanostructure phosphoricacid supercapacitor energystorage ABSTRACT

Herein we illustrate the functionality as pseudocapacitive material of tungsten trioxide (WO3)

nanochannellayersfabricatedbyelectrochemicalanodizationofWmetalinpurehotortho-phosphoric acid (o-H3PO4). These layers are characterized bya defined nanochannel morphology and show

remarkable pseudocapacitive behaviour in the negative potential (
0.8–0.5V)in neutral aqueous electrolyte(1MNa2SO4).Themaximumvolumetriccapacitanceof397Fcm
3isobtainedat2Acm
3.The

WO3nanochannellayersdisplayfullcapacitanceretention(upto114%)after3500charge-discharge

cyclesperformedat10Acm
3.Therelativelyhighcapacitanceandretentionabilityareattributedtothe highsurfaceareaprovidedbytheregularanddefinednanochannelmorphology.Kineticanalysisofthe electrochemicalresultsforthebestperformingWO3structures,i.e.,grownby2h-longanodization,

reveals the occurrence of pseudocapacitanceand diffusional controlledprocesses. Electrochemical impedance spectroscopy measurements show for the same structures a relatively low electrical resistance,whichistheplausiblecauseforthesuperiorelectrochemicalbehaviourcomparedtotheother structures.

©2017ElsevierLtd.Allrightsreserved.

1.Introduction

Energy, environment and health are regarded as biggest

challengesinthemodernworld.Owingtothesteeplyincreased

demand of energy per capita and consequent environmental

pollution, conversion, storage and efficient management of

renewableenergy havebecomeworld-wide subjectsof intense

scientificresearch.Considerableeffortshavebeenmadetodevelop efficient,green,andrenewableenergy-relatedtechnologies,based ontheuseofe.g.supercapacitors,lithium-ionbatteries,solarcells,

fuel cells, and photocatalytic and photothermal conversion

processes[1–5].

Inthecontextofenergystorage,supercapacitors(alsoknownas

electrochemical capacitors or ultracapacitors) are currently

attracting wide attention. Remarkable characteristics, such as theirhighpowerdensity,fastcharge/dischargeratesandlongcycle lifetime,bridgethegapbetweenhigh-specificenergybatteriesand high-specificpowerconventionalcapacitors[6–9].

Presently,themostimportantdevelopmentsinthisfieldentail theuseofnovelactiveelectrodematerialstofurtherenhancethe overallenergydensityofthesupercapacitors
thegoalistofulfil theenergyrequirementatvariousscales,rangingfromsimpleand low-consumptionelectronicdevicestoelectricdrivenvehicles.

Inordertoaccomplishthedesiredenergydensity,aprimary

strategy is to increase the working potential window of the

supercapacitor.Thisresultcanbeachievedbyusingorganic-or ionicliquid-basedelectrolytes,whichenablepotentialwindowsof upto3V[10].However,organicspresentactivityonlytowardsa smallgroupofelectrodematerials,namelycarbon-based materi-als,while ionic liquidsare still expensiveto beenvisaged asa

*Correspondingauthor.

E-mailaddresses:[email protected](K.K. Upadhyay),

[email protected](M.Altomare).

1

Contributedequallytothiswork.

http://dx.doi.org/10.1016/j.electacta.2017.02.131

0013-4686/©2017ElsevierLtd.Allrightsreserved.

ContentslistsavailableatScienceDirect

Electrochimica

Acta

practicalsolution.Additionally,theseelectrolyteshavesomeother drawbacks,suchasa decreasedionicconductivity,hightoxicity,

flammability, and safety issues which severely restricts their

applicability[10].

Inthisframe,apromisingapproachistoassembleasymmetric systems,usingaqueouselectrolytes,inwhichdifferentelectrode

materials (having different active potential windows) can be

combined,resultingintheadvantageouswideningoftheworking potentialwindowofthedevice[11].

Mostoftheresearchperformedtodateonthedevelopmentof electrodematerialsforasymmetricsupercapacitorshasfocusedon electrodematerialsworkinginpositivepotential region.On the otherhand,theexplorationofredox-responsivenegativeelectrode materials is still relatively new, sothat generally the negative electrodesdonotfulfilthedemandsofhighenergydensity[10]. In thequest for more efficient negativeelectrode materials, high-surfaceareacarbonbasedmaterialsarebeingexplored[12].

Although they deliver excellent power density, carbon based

materialsdo notfulfilrequirementsin termsofenergy density, whichisaseriouslimitation.

Theuseofpseudocapacitivematerials,suchastransitionmetal oxidesandconductingpolymers[12–15],insteadofcarbonbased materials,leadstoenhanced energydensity withoutsignificant lossofpowerdensity.Thisenhancementresultsfromtheadditive effectoffastsuperficialredoxreactionsandelectricdoublelayer formationattheelectrode/electrolyteinterface.Transitionmetal oxidessuchas Fe2O3,MoO3, Mn3O4,and WO3 [12,16–23] show

promising pseudocapacitive behaviour as well as reasonable

capacityretentioninasymmetricsystemsinthenegativepotential region.

Amongthem,tungstenoxide(WOx,x3)iscurrentlyreceiving

considerableattention,notonlyinlightofitschemicalstabilityand goodelectrical conductivitybut alsoduetoitswidelytuneable composition[24–26].Particularly, WO3 materialsshowefficient

chargestorage/deliverydynamicowingtothereversiblevalence

change, i.e., change of oxidation state between W6+ _{and W}5+

centres [24–27]. Due to this reversible change, WOx derived

materialsare alsowidely used as electrode for electrochromic devices[24–27]).

Themorphologyofthematerialalsoplaysanimportantrole, i.e., a large surface areaof theelectrode is key factor towards

improved specific capacitance [28,29]. In this regard, one

dimensional (1D) nanostructured materials (such as nanorods,

nanotubes,nanochannelsetc.)areadvantageousbecausetheyoffer largespecificsurfacearea,1Dchargetransportpathway,andcan

also withstand volume changes (expansion) allowing strain

accommodation during intercalation/deintercalation processes

[30–32].

Amongdifferentnanostructuringtechniques, electrochemical

anodization has gained largeattention in the last decades for fabricatingnanopore,nanochannelandnanotubelayersofvarious

metal oxides [33–37]. This simple approach is based on the

anodizationofametalpieceinasuitableelectrolyte–theresultis

that under adequate (i.e., self-organizing) electrochemical

conditions highly-orderedvertically-aligned arraysof 1D metal

oxide nanostructures can be grown on the (conductive) metal

substrate.

Keyadvantageofthisapproachisthatthemorphologyofthe nanostructurescanbefinelyadjustedbysimple electrochemical

parameters (e.g., applied voltage, electrolyte composition,

anodizationtime, etc.)[38,39].Moreover,these1D oxide layers arealreadywellback-contactedandarehencereadytobeusede.g. forelectrochemicalapplications.Onthecontrary,powdersofWO3

nanostructures fabricated by various approaches other than

electrochemical anodization can be explored in view of their

supercapacitive behaviour only in the form of composites, e.g.

combinedwithconductivematerialssuchaspolyaniline[40,41], whichactsasbinderandconductivesupport.

Inthis work,wereportonthefabricationof1DanodicWO3

nanochannel layers and their use (without addition of binder,

conductivepolymersetc.)asnegativeelectrodesforasymmetric

supercapacitor. The WO3 nanochannel layers are formed by

self-organizing electrochemicalanodization of Wmetal foils in pure hot o-H3PO4 electrolyte [42]. The electrochemical

perfor-manceoftheWO3electrodesisexploredinrelationtothelayer

thickness. The electrochemicalresponse of theWO3 electrodes

shows a remarkable volumetric capacitance of 397Fcm
3,

measured at 2Acm
3 in the
_0.8–0.5V potential range, along withfullcapacitanceretentionafter3500charge-dischargecycles performedat10Acm
3.

2.ExperimentalSection

2.1.Materialpreparation

Self-organizedvertically-alignedWO3nanochannellayerswere

fabricated by electrochemical anodization of W metal in pure

moltenortho-phosphoricacid[42].

W foils (0.125mm thick, 99.95% purity, Advent Research

MaterialsLTD, Oxford,UK)werecut into1.5cm1.5cmpieces, cleaned by ultra-sonication in acetone, ethanol and de-ionized water(10mineach)andfinallydriedinaN2stream.

WO3nanochannellayersweregrownbyanodizationusinga

two-electrodeelectrochemicalcellwheretheWandPtfoilswere

used as working and counter electrode, respectively. Prior to

anodization,onesideoftheWfoilwascoatedwithKaptontape

(DuPont). The two electrodes were immersed into the molten

electrolyteinverticalconfigurationandplacedatadistanceoftwo cmfromeachother.TheuncoatedsideoftheWfoilfacedthePt counterelectrode,andwasimmersedintothehotelectrolytein ordertoformananodicsurfaceof2cm2_{(1.5cm1.3cm).}

The electrolyte was composed of (nominally) pure molten

ortho-phosphoricacid(o-H3PO4,99%,Sigma-Aldrich),andwas

constantlystirredduringtheanodizationexperimentsandkeptat

the desired temperature (100_{C) by thermostatic control}

provided by a heating-stirring plate that was equipped witha

Teflon-linedthermocoupleimmersedintotheanodizingmedium.

The experiments were performed under potentiostatic

conditions applying a constant potential of 5V (no sweeping)

providedbyaVolcraftVLP2403Propowersupply.Theresulting current(density)wasrecordedbyusingaKeithley210061_/

2Digit

multimeterinterfacedwithalaptop.Theanodizationexperiments lasted1,2and4h.

Theas-formedanodicoxidelayersaretypicallyamorphousand wereconvertedintocrystalline(monoclinic)WO3byannealingin

airat450Cfor1h.Thethermaltreatmentwascarriedoutusinga

rapid thermal annealer (RTA, Jipelec JetFirst100) with

heating/coolingratesetat30Cmin
1. 2.2.Physicochemicalcharacterization

AHitachifieldemissionscanningelectronmicroscope(FE-SEM S4800,Hitachi)wasusedformorphologicalcharacterizationofthe samples.Thethicknessandthemorphologyofthenanostructured filmswereassessedbySEManalysisofcross-sectionalcutsofthe anodiclayers.

X-raydiffraction(XRD)patternswerecollectedusinganX’pert PhilipsPMDdiffractometerwithaPanalyticalX’celeratordetector, usinggraphite-monochromizedCuK

a

l

Thechemicalcompositionoftheanodiclayerswasdetermined

by X-ray photoelectronspectroscopy (XPS, PHI 5600, US). XPS

apassenergyof23.5eV.TheXPSspectrawerecorrectedinrelation totheC1ssignalat284.8eV.

2.3.Electrochemicalcharacterization

TheanodicWO3nanochannellayersgrownontungstenmetal

substrates were characterized as working electrodes by

cyclic voltammetry (CV) and galvanostatic charge-discharge

measurementsina1MNa2SO4aqueoussolution(afreshlyprepared

electrolytewasusedforeveryexperiment).

All measurements were done in a three-electrode setup in

whichaplatinumfoilandasaturatedcalomelelectrode(SCE)were usedascounterandreferenceelectrode,respectively.AVoltalab PGZ100potentiostatwasusedforallthemeasurements.TheCV curveswereobtainedatdifferentscanrates(10–100mVs
1)ina potentialwindowof
1–0.5V.Thegalvanostaticcharge-discharge

Fig. 1.(a–c)Top-view(left)andcross-sectional(right)SEMimagesofWO3nanochannellayersformedbyself-organizingelectrochemicalanodizationofWfoilsinpurehot

o-H3PO4(100C
5V)fordifferenttimes:(a)1h,(b)2hand(c)4h.(d)ThicknessoftheWO3layersasafunctionoftheanodizationtime.(e)Typicalcurrentvs.time(i-t)profile

profiles were obtained by varying the current density (2–10Acm
3)inthepotentialrangeof
0.8to0.5V.

Electrochemical impedance spectroscopy (EIS) experiments

were performed by using a Gamry FAS2 Femtostat, in the

frequencyrangefrom0.01Hzto105_{Hzatopencircuitpotential}

using10mVsinusoidalperturbation. 3.ResultsandDiscussion

Fig.1a–cshowSEMimagesoftypicalanodicWO3nanochannel

layersgrowninmolteno-H3PO4.Theuseofthiselectrolyteisakey

factorforthegrowthoforderedWO3nanochannelsasitprovides

anidealequilibriumbetweenfield-assistedpassivationofWmetal andoxidedissolution[38,43–45],thusestablishingtheadequate

electrochemical conditions for the growth of defined

nano-structures.Whilerapidoxidedissolution thatlimitsthegrowth oftheanodicstructureisreportedtotakeplaceinmostcommon electrolytes[46–48],(nominally) pure hoto-H3PO4 providesan

environmentwithlimitedwatercontentandwithphosphateions thatprotect anodictungstenoxide layer fromrapiddissolution [42].

ToformWO3nanochannelelectrodesof variousthicknesses,

anodizationexperimentswerecarriedoutfor1,2and4h
this resultedinchannelswithlengthintherangeof200–800nm.For

energy storage, the electrode material mass loading and its

conductivitytypicallydeterminethedeviceperformance[49,50]. TheelectrochemicalperformanceoftheWO3layerswillbethus

discussedinrelationtotheirthickness.

Thetop-viewSEMimages(Fig.1a–c,left)showthatthetop surfaceofthelayersishomogeneous,andthatthelayerspresent openpores, withinner diameterof ca.10nm regardless of the anodizationtime.

Thecross-sectionalSEMimages(Fig.1a–c,right)obtainedfrom cross-sectionalcutsoftheWO3layers(supportedonWfoil)show

clearlythattheanodicWO3haveananochannelstructure,with

aligned vertically-oriented channels that extend from the top

surface of the anodic film to the oxide-metal interface. The

channelsareopenatthetopsurfaceandremainopen(withinner diameterofca.10nm)acrosstheentirechannellength,regardless oftheanodicoxidethickness.

Aprolongedanodizationtime,i.e.,from1to2and4h,resulted ina clearincreaseof theoxidefilm thickness(Fig.1d)and the

nanochannellengthwasfoundtobeofca.200,400and800nm, respectively–itisacceptabletoassumethatthesurfaceareaofthe nanochannellayersincreaseswithincreasingtheirthickness.

Fig.1eillustratesatypicalcurrentvs.time(i-t)curveforaWO3

nanochannellayergrownfor2h.Whentheanodicpotential(5V)is

applied, the recorded current steeply increases and values of

currentdensityarereachedthatareof6–7mAcm
2.Thecurrent density J is calculated as J=i/A, where the recorded current i13mAandtheelectrodenominalareaA2cm2.Aftertheinitial peak, thecurrent decreaseswithinafew minutes(5min) and levelsofftoasteady-statevalueof0.25mAcm
2.

Theresultingi-tbehaviourresemblesotheranodicallygrown

self-organized1Doxide structures(amostcommonexampleis

TiO2 nanotube arrays) [45,51]. Here in the first stage of the

anodization theWmetal substratedevelopsan initialcompact

oxide_filmatthemetal/electrolyteinterface.Afterthisshortstage, anequilibriumisestablishedbetweenthemetaloxidegrowthand thefield-assistedelectrochemicaldissolutionoftheformedoxide. Asaresult,anodicoxidescangrowintheformofvertically-aligned nanochannels[43,44].

The WO3 porous films were characterized in view of their

crystallographicfeaturesandchemical compositionbyXRDand

XPS.DataarecompiledinFigs.2and3,respectively.

Fig. 2a shows the XRD patternsof as-formedand annealed

anodic WO3 layers. In line with previous reports [42], the

as-formed oxide film is amorphous and only reflectionsof the

Wmetalsubstratecanbeseen,peakingat2

u

The structures were converted into crystalline WO3 by

annealing in air at 450C (1h). The XRD patternsof annealed anodiclayersshowcharacteristicre_flectionspeakingat2

u

23.7 and 24.3 corresponding to the (002), (020) and (200)

crystallographicplanesofmonoclinicWO3,respectively[53].Fora

WO3 specimen composed of randomly oriented monoclinic

crystallites, the relativeintensity of the(002), (020) and (200) reflectionsis1:1:1,thatis,thepeakstypicallyshowvirtuallythe samerelativeintensity.ItcanbenoticedfromtheXRDpatternsin Fig.2bthatwithincreasingtheanodizationtime (1,2and 4h), thereisachangeintherelativeintensityofthepeaksassociatedto the(002),(020)and(200)crystallographicplanes.Thereasonfor thismaybeapreferentialgrowthofthemonoclinicWO3crystals

Fig.2.XRDdata:(a)as-formedandannealed(450_{C, 1h,air)WO3layersformedonWmetalfoilsbyanodizingfordifferenttimes.ThemainreflectionofmonoclinicWO3and}

Wmetalareindexed.TheplotalsoshowsreferencesoftypicalreflectionofcubicWmetalandmonoclinicWO3;(b)plotoftheXRDpatternsinthe22–252urangeshowing

alongthe(002)and(020)planes
inotherwords,thelongerthe anodizationtime,themoreintensethe(002)and(020)reflections comparedtothe(200)peak.

The XPSsurvey(Fig.3a) showstheas-formedand annealed

anodicnanochannellayersarecomposedofonlyWandO,with

small amounts of phosphorus (uptake from the electrolyte,

discussedbelow)andadventitiouscarbon.

Thehigh-resolutionXPSspectraintheW4fandO1sregionsare showninFig.3bandc,respectively.Fortheannealedstructures, theW4f5/2and7/2signalspeakat38.2and36.1eV,respectively, with

D

Table1 summarizesthechemicalcompositionofamorphous

and crystalline layers determined from the XPS data. One can

notice that in any case Pis present at the surface of theWO3

structures.Additionally,theWtoOratiois1:3.2,whichisslightly higherthanexpectedforstoichiometricWO3(i.e.,1:3),andtheP2p

XPSsignal(HRspectrainFig.3d)peaksat134.3eV.Thusonecan morelikelyconcludethatPO43
speciesareadsorbedatthesurface

oftheWO3nanochannels.ThiswouldbeinlinewiththehigherW

toOratio,andalsowiththefactthattheW4fandO1ssignalsare slightlyshiftedtowardshigherB.E.comparedtodatareportedin theliterature[54,55].

TheHR-XPSspectrainFig.3bandcshowalsothattheW4fand O1ssignalsofthecrystallinelayersareshiftedtowardsslightly

lower B.E. (e.g., by 0.3–0.6eV) compared to the as-formed

structures
thisisinlinewithpreviousreports[56],andismore probablyascribedtodesorptionofspecies(e.g.,C-andP-species
seeTable1)fromthesurfaceoftheWO3nanochannels.

The electrochemicalbehaviourofall anodizedWO3 samples

wasinvestigatedbycyclicvoltammetryandgalvanostatic

charge-discharge measurements (performed in 1M Na2SO4 solution).

Fig.4ashowsacomparisonofthecyclicvoltammetry(CV)curves ofvariousWO3layers(anodizationtimeof1,2and4h,annealingat

450C)takenatascanrate of10mVs
1.Themaximumcurrent responsewas recordedforlayersanodized for2h(Fig.4a).The resultsofthegalvanostaticcharge-dischargemeasurementstaken atcurrentdensityof2Acm
3(Fig.4b)areinagreementwiththis finding,indicatingabetterredoxcapacitivebehaviourofthe2h anodizedsampleincomparisontothecounterpartsanodizedfor1

or 4h. Moreover, the plateau observed for all samples in the

charge/discharge profiles(Fig. 4b) indicatesthat charge is also storedviaredoxprocesses,whichshouldbeoccurringthroughout theactivenanochannels.Theseresultsareinaccordancewiththe voltammetricwavesobservedintheCVplots.

ThehighercapacityobtainedfortheWO3structuresgrownfor

2hincontrasttothosegrownfor1hismorelikelyascribedtothe largersurfaceareaoftheformer.Thesuboptimumperformanceof 1hanodizedsamplescomparedtothickeroxidefilmscouldbealso

Fig.3.XPSdataofas-formedandannealed(450_C,1h,air)WO

3nanochannellayersgrownby2h-longanodizationexperiments:(a)survey;(b-d)highresolutionspectra

measuredinthe(b)W4f,(c)O1sand(d)P2pregions.

Table1

W,O,PandCcompositiondeterminedbyXPSanalysisofas-formedandannealed (450_{C, 1h, air) WO3 nanochannel layers grown by 2 h-long anodization}

experiments.

Sample Atomicconcentration(at%) W:Oratio W4f O1s P2p C1s

2h
as-formed 22.8 72.0 3.8 1.4 1:3.2 2h
450_{C 22.9 73.1 2.8 1.2 1:3.2}

attributed to their poor mechanical properties, i.e., short WO3

nanochannelswerefoundtobelooselyattachedtotheWmetal

substrateand partly detached duringelectrochemical

measure-ments. On the other hand, the 4h anodized nanochannels are

significantlythickerthanthosegrownfor2h(800vs.400nm), andthisstructuremayleadtolimitedelectrolytepermeationand

ion diffusion deep through thewhole WO3 layer, which could

explain theirdecreased capacitance compared to the channels

grownfor2h.

Furtherelectrochemicalinvestigationwasperformedonthe2h anodizedsample,asshowninFig.4candd.TheCVcurves(Fig.4c) displayclearoxidationpeaksat
0.06Vand
0.3Vandabroad

Fig.4.(a)CVcomparisonplotsofWO3(anodizationtime=1h,2h,and4h)at10mVs
1;(b)Galvanostaticchargedischargecomparisonplotat2Acm
3;(c–d)CVand

galvanostaticchargedischargeplotof2hranodizedsampleatdifferentscanratesandcurrentdensities,respectively(e)specificcapacitancevs.currentdensitiesforallthe samples(f)capacitanceretentionupto3500cyclesat10Acm
3(inset:chargedischargeplotsovertime).Alltheexperimentswereperformedwithannealedsample(450_C,

reductionpeakat
0.45V.Thisredoxresponsecanbeassociated withtheintercalation/de-intercalationofaquantity(x)ofpositive ionstobalanceanequalquantityofelectrons(e
)accordingtothe followingreaction[6,57]:

WO3þxHþþyNaþþðxþyÞe
$HxNayWO3;0ðxþyÞ1 ð1Þ

The (x+y) canvary from0 to1. The peak current response

increaseswithincreasingthescanrate,suggestingthattheWO3

nanochanneledarchitecture supportedfast charge transfer and

cationdiffusion.Also,ashiftintheoxidationandreductionpeaks canbenoticed when increasingthescan rate, andthis shiftis associatedtopolarizationeffectsoftheelectrodesathigherscan

rates.Moreover,symmetricoxidationand reductionpeakswere

retainedevenathighscanrates,thisprovingthereliabilityofthe materialwhenoperatedatfastrates.

The specificvolumetriccapacitance (Cs)ofalltheelectrodes

wascalculatedfromthegalvanostaticdischargecurves(Fig.4b) datausingequation:

Cs¼I

D

D

where,Csisthevolumetricspecificcapacitance(Fcm
3),Iisthe

currentdensity (Acm
3, consideringa volume ofthe electrode materialcalculatedbymultiplyingtheheightofthenanochannels obtainedfrom SEM analysisto the nominalelectrode area, i.e. 2cm2_),

_D

_D

(s). The specific capacitance value obtained for 2h anodized

sample is 397Fcm
3at 2Acm
3, which is significantly higher

comparedto262Fcm
3and136Fcm
3obtainedfor4hand1h anodizedsamples,respectively.

The specific capacitance values at different current density

ranging from 2–10Acm
3 are depicted in Fig. 4e. The best

performingsample(anodizedfor2h)showedatacurrentdensity of 10Acm
3aspecificvolumetriccapacitance ofca.110Fcm
3,

that is28% of that measured at 2Acm
3. This volumetric

capacitance,measuredatarelatively highcurrentdensity(10A cm
3),iscomparabletodatareportedintheliterature(seee.g.refs [17,19]).

Ingeneral,the2hanodizedmaterialshowedremarkableredox capacitiveresponse(inneutral1MNa2SO4aqueoussolutions)in

comparison to WO3 electrodes previously reported in the

literature,primarilyinthenegativepotentialrange.Forexample, Pengetal.reportedWO3nanorodsactiveinthenegativepotential

range (-0.7–0.2V) which deliver specific capacitance values of 385Fg
1 at 1Ag
1 in 1M H2SO4 [19]. Shinde and co-workers

reportedonWO3 thin filmssynthesizedbySilartechniqueand

obtainedspecificcapacitanceof266Fg
1at10mVs
1measuredin potentialwindow
0.7–0.4Vin1MNa2SO4solution[17].Gaoetal.

reportedWO3nanowiresgrownoncarbon clothhaving specific

capacitance of 521Fg
1 at 1Ag
1 (-0.6–0.3V) in 2M H2SO4

solution [23]. Other works reported specific capacitances of

290Fg
1at 25mVs
1(-0.6–0.2V)in0.5MH2SO4 solution[16],

andof639.8Fg
1at10mVs
1(-0.6–0V)in1MH2SO4electrolyte

[21].

Foramoredirectcomparison,ourresults(thatweremeasured inaneutralelectrolyteandwithoutusinganyadditive)canbealso

Fig.5. Kineticanalysisofthe2hanodizedfilm;(a)thepowerlawdependenceofcurrentivs.thesweeprateshowsgoodlinearity.Thetwocurvescorrespondto
0.2V, b=0.59and0.2V,b=0.93,respectively(b)b-valuesasafunctionofpotentialforanodiccurrentforvaryingsweeprate;relationshipof(c)chargeQvs.Ѵ
1/2_and(d)Q
1_vs.Ѵ1/2

comparedtovolumetriccapacitancedatareportedintheliterature (i.e.,intermsofFcm
3).Valuesintherangeof40–340Fcm
3are reportedforvariouscompositeWO3electrodes[58–61],whilea

volumetriccapacitanceof640Fcm
3isreportedbyYoonetal. for m-WO3-x electrodes used in acidic electrolytes [62]. Worth

noting,resultsreportedforWO3electrodesaremostlymeasuredin

acidicmedia.Undertheseexperimentalconditionstungstenoxide mayalsobehaveaselectrocatalystforhydrogengeneration–asa result,thematerialworkingpotentialwindowshouldbenarrowed [63,64],whichwouldconsequentlyleadtoalimitedperformance oftheelectrodeintermsoflowerenergydensity.

Onthecontrary,theWO3nanochannellayersareexploredin

thisworkinaneutralelectrolyte,i.e.aqueous1MNa2SO4,andthis

widensthematerialworkingpotentialwindowfrom
0.8to0.5V.

The widening of the working potential window results in the

enhancement of the overall energy density when using these

electrodesinanasymmetricconfiguration.

Furthermore,theimprovedcapacitivebehaviourcanalsobea consequenceofarelativelyhighintrinsicelectronicconductivityof thematerial,providedbytheinterconnectedstructure.

In order toevaluate thestability of thematerial, long term

charge-discharge experiments were carried out. Galvanostatic

charge-discharge measurements were performed at 10Acm
3.

Theresultsin Fig.4f showremarkable capacitanceretainupto 3500charge-dischargecycles.Moreprecisely,aslightincreasein

the capacitance was observed that could be ascribed to an

increasedaccessibility/diffusionofionsdeepintothenanochannel structureafterseveralcycles,asaconsequenceofalongersoaking time(similarresultscanbefoundintheliterature[59,61,65]).The capacitance was observed toincrease upto 114% of the initial

capacitance in the first 3000 cycles. Afterwards, the measured

capacitancestabilizedandbecameconstant(upto3500cycles),as shownfromthecharge-dischargeplotintheinsetofFig.4f.More generally,the2hanodizedstructuresresultedmechanicallystable

(showed goodadhesion tothe substrate) and theirremarkable

capacitanceretaincanbeascribedtothe1Dnanochannelstructure thatcanaccommodatestrainandsustainrepeatedvolumechanges [30–32,66].

Thechargestoragemechanismwasalsostudiedfor thebest

performingsample(2hanodizedstructure).Itisknownthatthe

overall capacitance of a material can bealso ascribed toother

phenomena such as surface double layer formation or surface

faradaicadsorption,andtobulkfaradaic intercalation/de-interca-lationreactions.Theoccurrenceofthesedifferentmechanismsof

energy storage can be identified (and differentiated from the

others)byanalysingtheCVdataatvarioussweepratesaccording toequation(3),andcanbeillustratedbyplottinglog(i)vs.log(Ѵ) asafunctionofpotential[67,68]:

i=aѴb

(3) Accordingtoequation(3),themeasuredcurrentiobeystoa powerlawfunctionofthesweeprateѴ.Bothaandbarevariable

parameters. When b is close to 1, the current response is

predominantlycapacitiveinnature.Ontheotherhand,thecurrent flowatanygivenpotentialisexpectedtovarywiththesquareroot ofthescanrateforthediffusioncontrolledprocessandinthiscase bequalsto0.5.

Inordertodeterminethebvalues,logiwasplottedversuslogѴ

for different potentials. According to equation (3) this

representationgivesrisetoastraightlinewithaslopeequalsto b.Asanexample,plotsoflogiversuslogѴatappliedpotentialsof
0.2Vand0.2VareillustratedinFig.5a,showinganearlylinear

behaviour and presenting distinct b-values of 0.59 and 0.93

respectively. InFig.5b, theb-values areshownasa functionof

different applied potentials. In the positive potential range

(0.1–0.4V) theb-values approach 1 indicating that the current responsearisespredominantlyeitherfromredoxphenomenaatthe nanochannelsurfaceorfromdoublelayerformation.However,in thenegativepotentialregion,b-valuesareintherangeof0.55to 0.7 indicating the occurrenceof diffusioncontrolled processes, whichcanbeassociatedwithdiffusionofcationstowardsthebulk ofthematerial[68].

Afterobservingthepresenceofdiffusioncontrolledprocessin

the charge storagebehaviourof WO3 nanochanneled films,we

further studied and separated the contributions of surface

capacitance and diffusion-controlled insertion from the total

charge storage, using an approach reported elsewhere [28,69]. Inthismodel,therelationshipbetweencharge(Q[coulomb])and thescanrateofthecyclicvoltammogramisdescribedbytheEqs. (4)and(5)[69]:

Fig.6.(a)Nyquistplot(inset:magnifiedview)and(b-c)Bodeplotobtainedfromelectrochemicalimpedancespectroscopy(EIS)measurementsofcrystallineanodicWO3

Q=QѴ=1+constant(Ѵ
1/2) (4)

Q
1=Q
1Ѵ=0+constant(Ѵ1/2) (5)

whereQѴ=1isthesurfaceadsorptioncharge,correspondingtothe

voltammetricchargeatѴ=1andQѴ=0isthetotalcharge,whichis

thevalueatѴ=0.Therefore,byplottingQversusѴ
1/2_andQ
1

versusѴ1/2_{thesurfaceadsorbeddoublelayercapacitanceandthe}

totalcapacitance can bedeterminedfrom they-axis intercepts (Fig.5cand d). Aftercalculations,theresultsrevealed thatthe

contribution from the double layer capacitance and surface

adsorptionisabout27%andtheremainingcapacitance(73%) arisesfromthediffusion-controlledredoxprocesses.

Tofurtherstudytheelectrochemicalbehaviour, electrochemi-calimpedancespectroscopy(EIS)experimentswereperformedfor

all the anodized and annealed samples at the open circuit

potential.

Fig.6ashowstheNyquistplotfortheWO3filmsanodizedfor1,

2and4h.Fromtheinterceptoftheplotwiththerealaxisinthe highfrequencyregion(insetinFig.6a)itispossibletoassessthe equivalentseriesresistance(sumofcontactresistance,electrolyte resistanceandmaterialresistance).Itcanbeclearlyobservedthat

the electrode anodised for 2h presents the lowest equivalent

resistance, which can be associated with its highly-ordered

morphology contributing to a beneficial intrinsic electronic

conductivity.

Ontheotherhand,thelowfrequencyregionischaracterized bythepresenceofstraightlineswhichpresentlowerslopesinthe caseoftheoxidefilmspreparedby1and4h-longanodization.

Thisfact can beassociated with the development of Warburg

behaviourassociatedwiththeoccurrenceofdiffusion-controlled

processes. This is particularly in good agreement with the

morphologicalfeatures offilms anodizedfor 4h,thestructure

of which may lead to limited electrolyte permeation and ion

diffusionintothethickWO3structure.Thiscanalsobenoticedin

theBodeplot(Fig.6c),wherethe2hanodizedsamplerevealsat

medium-lowfrequencyahigherphaseangleapproaching
90_,

whichindicates amore pronouncedcapacitiveresponse.These

results,whichhighlighttheenhancedcapacitivebehaviourofthe

4h anodized layers, are in good agreement with the cyclic

voltammetryandchargedischargecomparisoncurvesshownin

Fig.4aandb.

4.Conclusion

The pseudocapacitive behaviour of anodically grown WO3

nanochannellayerswasexploredandtheelectrochemicalresults provedthatthesestructuresarepromisingmaterialsfor fabricat-inganodesforasymmetricsupercapacitors.Layersgrownby2

h-long anodization displayed superior volumetric capacitance

comparedtolayers anodizedeither for shorter orlongertimes (397Fcm
3 at 2Acm
3 in a relatively wide potential range of 1.3V).Thematerialexhibitedfullspecificcapacitanceretainover 3500charge-discharge cycles at 10Acm
3. The WO3 structures

wereinvestigatedwithafocusonmechanisticaspectsrelatedto

charge storage dynamic. The b-values obtained at different

potentialsallowedtodistinguishthecontributiontotheoverall capacitanceofthenanochannelarchitectureascribedtosurfaceor

bulk effects. Further analysis revealed that 27% of the total

capacitance is from surface adsorption/desorption process,

whereas the remaining73% results froma diffusion-controlled

process. The promising capacitance and capacitance retention

resultscanbeassociatedtothenanochannelarchitecture,which offerslargespecificsurfacearea,shortchargetransportpathways,

lowintrinsic resistance,and long-lastingmechanical robustness allowingstrainaccommodationduring intercalation/deintercala-tionprocesses.

Acknowledgements

Prof.M.F.Montemor, Prof.T.M.Silva,Dr.S. Eugénioand K.K. UpadhyaywouldliketothankFundaçãoparaaCiênciaeTecnologia (FCT)forthefundingunderthecontractUID/QUI/00100/2013.K.K.

Upadhyay would also like to acknowledge Erasmus Mundus

NAMASTE(20130674)forfunding.Prof.Dr.P.SchmukiandDr.M. AltomarewouldliketoacknowledgetheERC,theDFG,andtheDFG “Engineering of Advanced Materials” cluster of excellence for

financial support. Xuemei Zhou and JeongEun Yoo (WW4-LKO,

UniversityofErlangen-Nuremberg)areacknowledgedforhelping

withtheXRD analysis. Dr. A.Mazare (WW4-LKO, Universityof

Erlangen-Nuremberg) is acknowledged for helping with XPS

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