Advanced analytical techniques to characterize materials for electrochemical capacitors

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Advanced analytical techniques to characterize materials

for electrochemical capacitors

Zifeng Lin, Pierre-Louis Taberna, Patrice Simon

To cite this version:

Zifeng Lin, Pierre-Louis Taberna, Patrice Simon. Advanced analytical techniques to characterize

materials for electrochemical capacitors. Current Opinion in Electrochemistry, Elsevier, 2018, 9,

pp.18-25. �10.1016/j.coelec.2018.03.004�. �hal-02020693�

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This is an author’s version published in:

http://oatao.univ-toulouse.fr/21769

To cite this version:

Lin, Zifeng

and Taberna, Pierre-Louis

and Simon, Patrice

Advanced

analytical techniques to characterize materials for electrochemical capacitors.

(2018) Current Opinion in Electrochemistry, 9. 18-25. ISSN 2451-9103

Advanced analytical techniques to characterize

materials for electrochemical capacitors

Zifeng

Lin

_,

_Pierre-Louis

_Taberna

_and

_Patrice

_Simon

Thisreviewcoversrecentdevelopmentsinadvancedanalytical techniquestocharacterizematerialsforelectrochemical capacitors.Fordoublelayercapacitors,examplesoftheuseof insituX-rayphotoelectronspectroscopy(XPS),pulsed electrochemicalmassspectrometry(PEMS)technique, temperature-programmeddesorptioncoupledwithmass spectroscopy(TPD-MS)technique,insituNMRspectroscopy, andinsitudilatometrymeasurementarepresented,for studyingcarbon/electrolyteinterfacewithafocusonto electrolyteionsconfinementinnanoporesandchangesduring ageing.Forthepseudocapacitivesystem,insituX-ray (neutron)diffractionorscattering,insitudilatometrytechnique, cavitymicro-electrode,insituRamanspectroscopy,TPD-MS technique,andelectrochemicalquartzcrystalmicrobalance (EQCM)techniquehavebeenemployedforstudyingmaterials structure,electrochemicalkinetic,interfaceinteraction,and ionsadsorption/desorption.Theseadvancedanalytical techniquesprobeinsightintochargestoragemechanisms,and guidingthefastdevelopmentofsupercapacitors.

Addresses

1 _{Université PaulSabatier,LaboratoireCIRIMATUMRCNRS,Toulouse} 5085,France

2 _{RéseausurleStockageElectrochimiquedel’Energie(RS2E),FR} CNRSn°3459,France

3_{InstitutUniversitairedeFrance,1ruedesEcoles,Paris75003,France} ∗_{Correspondingauthor: Simon,Patrice (simon@chimie.ups-tlse.fr)}

https://doi.org/10.1016/j.coelec.2018.03.004

Introduction

Duringthepast15years,majorscientificadvanceshave

been made in the field of Electrochemical Capacitors

(ECs)which ledtoa2-foldincrease oftheenergy

den-sity of carbon-based (EDLCs) or to the development

ofhigh-ratepseudocapacitivematerials.Theseadvances

havebeenmainlyachievedthankstotheuseofanalytical techniques,usedincombinationwithinsitu

electrochem-icalconventionalmethodsand/orwithmodeling.Inthis

review,someadvancedtechniques,as wellastheirroles

instudyingchargestoragemechanismsareintroduced.

Electrochemical

double

layer

capacitors

InElectricalDoubleLayerCapacitors(EDLCs),

capaci-tivestorageisachievedthroughionadsorptionofan

elec-trolyteonto high surface areaporous carbon electrodes

[1].Then,mostoftheelectrochemicalprocessinEDLCs

occursatthecarbon/electrolyteinterface.Theanalytical

techniquesusedinEDLCsmainlyfocusonthisinterface.

OneoftheimportantparametersofECsistheoperating

voltagewindow,which drivestheenergydensityof the

system.In situ_{X-rayPhotoelectronSpectroscopy(XPS)}

techniquehasbeenusedtostudythestabilityofthe car-bon/electrolyteinterfaceduringelectrochemical polariza-tionofCDCcarbonsinionicliquidelectrolyte[2,3].By

tracking the changein theC1s and N1s energy levels,

Lustandco-workers identifiedthereactionmechanism

responsiblefortheelectrolytedegradationathigh poten-tial(>_{3.6V),which involves theoxidative dimerization}

ofthe imidazoliumcation viaN–Nbond formation [2].

Anothertechniquerecentlydevelopedformonitoringgas

evolutionduringtheoperationof supercapacitors isthe

pulsedelectrochemicalmassspectrometry(PEMS)

tech-nique[4•_{],aspresentedinFigure1.PEMStechnique}

al-lowsforfastquantitativemeasurementoflowgas produc-tionduringsupercapacitorcyclingorageing.Batisseand

Raymundo[4•

]evidencedandquantifiedtheformation

ofCO,CO2 andH2 atthepositiveandnegativeelectrode

duringsupercapacitorcellsageingatconstantvoltagesin

aqueouselectrolytes.Furthermore,they could correlate

gas production to the change of each electrode

poten-tialversusreferenceduringageing.Ageingmechanisms

inporouscarbonelectrodesstronglydependonthe

pres-enceofsurfacefunctionalgroupsonthecarbonsurface,as wellasthecarbonstructure(presenceofdefects)and tex-ture(surfaceareaandpore sizedistribution).Herealso,

manyadvances have been achieved withinthe past 10

years.

A key technique for analyzing the carbon surface is

the temperature-programmed desorption coupled with

mass spectroscopy (TPD-MS) technique [4•_–7].

TPD-MStechniqueallowsformeasuringthesurfacegroup

Figure1

Observationofgasevolutionatelectrode/electrolyteinterfacebyamodifiedpulsedelectrochemicalmassspectrometry(PEMS)method.Adapted withpermissionfromref.[4•_].Copyright©_{2017AmericanChemicalSociety.}

whichaccountsforthepresenceofcarbonactivesites: de-fectssuchasdislocations,stackingfaultsoratom vacan-ciesmainlylocatedintheedgeplanes[7].Usingcarbon

onionswithcontrolleddefectandsurfacegroupcontents

asmodelmaterials,adirectcorrelationwasreported

be-tweenthenumberofdefectsonthecarbonsurface

mea-suredbyTPD-MSandthecapacitanceinbothaqueous

andnon-aqueouselectrolytes.Surprisingly,the contribu-tionofthesurfacedefectstothecapacitancewasfoundto behigherthanthatofthefunctionalsurfacegroups,even

inaqueouselectrolytes.Theseresultsshowthatthe

sur-facedefectscontentalsoaffectsthecarboncapacitance.

Besidesthecarbonstructureand surfacegroupcontent,

theelectrochemicalperformancesof porous carbonsare

alsocontrolledbythecarbontexture:specificsurfacearea, porevolume,poresize,poresizedistribution.Backto15 yearsago,thecarbon-specificsurfaceareaSSAwasmainly calculatedfromN2 gassorption isothermsat77Kusing

theBrunauer–Emmett–Teller(BET)equation.The

evi-denceofthecapacitanceincreaseincarbonporeslessthan 1nm[8–10]highlightedtheneedforrefiningthe

charac-terizationmethodstofinelymeasuretheporousvolume

and pore size in the ultra-microporous range (<_1nm).

Following recommendations of the IUPAC, the BET

equationisnotsuitableforthemeasurementofspecific

surfaceareaofmicroporouscarbons[11].Instead,for mi-croporosityassessment,CO2 sorptionat273Kshouldbe

preferredto alleviatekineticsrestrictionsobserved

dur-ing measurements at low temperature with (77K with

N2 ) [11]. In the same way, calculation of the SSA

us-ingquenchedsoliddensityfunctionaltheory(QS-DFT)

avoids the fundamental limitations of the BET theory

[12].Finally,theporousvolumeaccessibletoionsshould

beconsidered,thatistheporousvolumeofporeslarger

thanthesizeofthedesolvatedion[13].Basedonthe

pre-viousrecommendations,Jäckeletal._{[13]proposetouse}

CO2 gasformeasuringporousvolumebelow0.9nmand

N2 gasforpores>0.9nm.Thechangeofthecapacitance

normalizedto QS-DFTSSAversusaccessiblepore size

showsacapacitanceincreaseinporeslessthan1nmsize forvariousporouscarbonsinnon-aqueouselectrolytes(in

acetonitrile- or propylene carbonate-based electrolyte).

Initiallyreportedin2006usingaseriesofporouscarbons with controlledpore size [10],the originof the

capaci-tanceincreaseincarbonnanoporeshasbeenextensively

studiedsincethattimemainlybyusingneworadvanced

Figure2

InsituNMRspectroscopyexperimentscarriedoutatdifferentchargestatesallowquantificationofthenumberofchargesstoringspecies.Adapted withpermissionfromref.[15].Copyright©_{2013AmericanChemicalSociety.}

workwas directedtoward theunderstanding of theion

transport and adsorption in confined carbon nanopores

(<_{1nm),i.e.wherethereisnoroomforthebuildingofa}

diffuselayer.Insitu_{NMRspectroscopyexperiments}

dur-ingelectrochemicalpolarizationofporouscarbonsinNaF

aqueous electrolyte have shown that ions could access

subnanometerporeswithpartialaniondehydrationunder

polarization[14].Usingdedicatedelectrochemicalcell,as

shownin Figure2,insitu_{NMRexperimentsduring}

po-larizationalsoevidenceddifferentchargestorage

mech-anisms depending on the electrode polarity [15–19••_].

Counter ionadsorption wasfound atthenegative

elec-trode(X=1)andionexchangeatthepositive(X=0)[18],

confirmingresultsobtainedusingelectrochemicalquartz

crystalmicrobalancetechniqueunderagravimetricmode

[20].Inaddition,theeffectiveionicdiffusioncoefficients

inside thecarbon nanoporesweredecreasedbytwo

or-dersofmagnitudecomparedwithbulkelectrolyte[19••_];

thiswasexplainedbytheincreaseoftheionpopulation

inpores.Asaresult,thechargingmechanism(counterion adsorptionversusionexchange)affectstheiontransport

kineticsinconfinednanopores.

Conventional techniques have been also developed to

studyiontransferinporouscarbons.Interestingly,insitu

dilatometry measurement duringelectrochemical

polar-izationshowsalsoanasymmetricbehaviorwithrespectto theelectrodepolarization,theelectrodethicknesschange

beingmoreimportant duringnegativepolarization[21].

Recently,Presseretal. _{improvedthetechniqueby}

cou-pling insitu_{dilatometry togetherwith X-ray absorption}

spectroscopy(XAS)[22••_{][21].First,theyconfirmedthe}

asymmetricswellingof porouscarbon electrodesduring

electrochemicalpolarizationinaqueouselectrolytes.Also, thankstotheuseofporouscarbonwithhierarchical

micro-porous/mesoporousstructure,mostofthevolumechange

couldbeassignedtothepresenceofporeslessthan1nm

size.Theoriginoftheasymmetrywasattributedto the

increaseoftheC–Cbondsduetoelectroninjectioninto

thecarbonduringnegativepolarization.Thisechoesthe

increaseintheionpopulationreportedbyForseand

co-workers by in situ _{NMR spectroscopy [15], leading to}

thedecreaseofthe ionicdiffusioncoefficientin carbon nanopores.

Finally,thedevelopmentofinsituX-rayorneutron

scat-tering techniques has been particularly successful for

studyingion adsorption in carbon nanopores[23••

–25]. Prehaletal._{[24]usedCsClaqueouselectrolytetostudy}

theionadsorptioninnanoporouscarbonsunder

polariza-tion.BycouplingSAXSandMonteCarlomodeling,they

evidencedtheionpartial desolvationwhen confinedin

carbonnanopores.Theextentofdesolvationand

confine-mentwasfoundtoincreasewiththeappliedpotential,in

agreementwithpreviousstudies[26],whichgiveshints

toexplainthecapacitanceincreaseincarbonnanopores.

Also,Futamura et al. _{recently showed the existence of}

co-ion pairs when theionic liquid electrolyte was

con-finedinto carbonpores of0.7nmsize,thatis whenthe

ionsizeiscloseto theporesize[25].Suchanimproved

co-ionpairingwastheconsequenceofthepartial

break-ingoftheelectrostaticCoulombicrepulsioninteractions

betweenco-ionsthankstothecreationofimagecharges

inthecarbon.Theycouldconfirmthecreationofa“super

ionic” statesuchaspredictedbyKornyshevandKondrat

Figure3

Schematicofthecavitymicro-electrode.Adaptedwithpermissionfromref.[40].

Asonecansee,themoreweadvanceinthe

understand-ingoftheionconfinementeffectinnanopores,themore

thingsgetcomplex.Thereisstillalottounderstand in thisfieldandalltheseanalyticaltoolswillbeofgreathelp tokeeponmovinginthisdirection.

Pseudocapacitive

and

high

charge–discharge

rate

materials

Pseudocapacitive materials store the charge through

fast,non-diffusion-limitedredoxreactions.Also,different

fromamorphousporouscarbonmaterialsusedinEDLCs,

most of the pseudocapacitivematerials show organized

crystallinestructure.So,mostof theconventional

tech-niques based on X-ray diffraction or scattering used to

characterize batteryelectrode materials have been

em-ployed with pseudocapacitive materials. We will then

just briefly mention some examples. In situ _{XRD has}

beenextensivelyusedtostudytheswelling/contraction

ofpseudocapacitivematerialsduringion intercalation/de-intercalation,suchasinMnO2 ,NiOx ,orMXene[28–30].

Withoutsurprise,theelectrodematerialvolumechanges

aredrivenbyelectrostatic repulsionbetweenthelayers

or the steric effects coming from ion

intercalation/de-intercalation to balance the charge [29]. In

monocrys-tallineNb2 O5 operatinginnon-aqueouselectrolytes,the

intercalation/de-intercalation process volume change is

drivenbystericeffect(swellingduringintercalation, con-traction during de de-intercalation) [31]. X-ray

absorp-tionwasusedtoevidencethechangeoftheTioxidation

state duringcharge/discharge of Ti3 C2 Tx MXene

elec-trode[32,33],aswellasinothermaterials[34,35].

Differently, micro-electrodes or cavity micro-electrodes

tools (Figure 3) are well-suited for the electrochemical characterizationof high-ratepseudocapacitivematerials.

Thankstothesmall amountof materialstested,alarge

rangeof potentialscan rates(v) canbeexplored—from

fewmVs−1 uptofewVs−1 —whichwasextremelyuseful forstudyingtheelectrochemicalkineticsof pseudocapac-itivereactions[36–38].Thedeconvolutionofthecurrent intonon-diffusionlimitedsurfaceprocess(changingwith v) anddiffusion-limited(changingwithv1/2 )hasmadeit

possibleto extract thepseudocapacitivecontributionto

thetotalcurrentateachpotential,whichhelpsin optimiz-ingthestructurestodesignhigh-ratematerials[36–39].

Another original initiative comes from Hu et al._{, who}

usedRamanspectroscopytocharacterizethecharge

stor-age during polarization of Ti3 C2 Tx MXene electrodes

inSO4 2 − ionscontaining aqueouselectrolyte of various

cations[41••_{].MXenesare2-Dimensionalmaterials}

pio-neeredbyBarsoumandco-workers[42],whichcontains

O- andF-surfaceterminations.Thosegroupscomefrom

thesynthesis process, thatis etching of MAX phasein

thefluorine-containingacidicsolutions[33].Itwasfound

that hydronium ions in sulfuric acid could bond with

theoxygen-containingterminationsoftheTi3 C2 Tx

MX-enenegativeelectrodeuponreduction(oxidation)while

imidazolium-basedionicliquid electrolyte [47].MXene

electrodeswellingwas measured duringnegative

polar-ization,suggesting the preferential insertion of cations. Underpositivepolarization,theelectrodecontractseven

further compared to the un-polarized sample,

suggest-inganionexchangemechanism.Similarresultswere

ob-tainedfrominsitu_{XRDmeasurements[28]anda}

molec-ulardynamicssimulation study [48],which confirm the

differenceinthechargestoragemechanismwiththe elec-trodepolarity.Also,themodestcapacitancesuggestsa ma-jorcontributionfromthedoublelayer.

Newanalyticaltoolsbasedontheelectrochemicalquartz

crystalmicrobalance(EQCM)techniquehavealsobeen

developedtostudyenergystoragematerialsthatoffer

in-terestingopportunitiesintheECarea.Differentlyfrom

thegravimetricEQCM,theEQCMwithdissipation

mon-itoring(EQCM-D)operatedwithmultipleovertones of

theresonance frequency,thus probing awide range of

penetrationdepthδ_{n [49,50].Thestructuralparametersof}

theelectrodescanbeobtainedbymeasuringthechange

in the resonance frequency 1_{F and thechange in the}

full-widthathalf-heightoftheresonancepeak1_Wover

a wide range of overtone numbers n, and fitting the

processisaccompaniedbyachangeintheoxidationstate ofTifromTi(+III)downtoTi(+II).Theredoxreaction onTiexplainstheextremelyhigh capacitanceTi3 C2 Tx

MXene canreach in acidicelectrolytes [33,43]. In

con-trast,inneutralelectrolytes,onlydoublelayeradsorption occurswithoutchargetransferonTiatoms[41••_].

Broad-eningtheuseofinsitu_{Ramanspectroscopytechniqueto}

othermaterialscouldbringnewinsightsinthe

pseudoca-pacitivechargestoragemechanism.

The discoveryof 2DMXenematerials hasboostedthe

research in pseudocapacitivematerials. One of the key

features of MXenes is the presence of surface oxygen

and fluorineterminationsontheirsurface.The

TPD-MS technique has been successfully used to measure

the change of the amount and the nature of these

groupsduringhydrazineintercalationintoTi3 C2 Tx

MX-ene material [44]. The capacitance of MXene in the

non-aqueouselectrolyteiswellbehindthatinthe

aque-ousacidicelectrolytes[45]andthechargestorage

mech-anism of MXenes in non-aqueous electrolytes is still

unclear [28,46]. In situ _{dilatometry technique,which is}

well-suitedfor 2Dmaterials,hasbeenusedto measure

theswelling/expansionof Ti3 C2 Tx MXene electrodein

Figure4

Gravimetricandnon-gravimetricapplicationsofEQCM-Dforthecharacterizationofenergy-storageelectrodes.(Bottompanel)Acousticwavesfor fundamentalfrequencyandits3rdovertone.Adaptedwithpermissionfromref.[49].Copyright©_{2018AmericanChemicalSociety.}

hydrodynamic equations.Structural parameters include electrodefilmdensityorthickness,permeabilitylength, particlesradiusandcoveragedensity(Figure4)[49].This

technique is an efficient tool for tracking in one shot

thegeometricalchangeintheelectrodes(contractionor

swelling,morphologicalchanges)aswellasthechangein reactionmechanisms(formationofpassivelayersor elec-trolytedecomposition)[51••_].

Another technique derived from EQCM is called

AC-electrogravimetry or AC-EQCM [52]. The AC-EQCM

techniqueconsistsofachievinggravimetricEQCM

mea-surements at a steady state (constant potential for

in-stance) and over-impose a sinusoidal perturbation to

the bias signal such as achieved in electrochemical

impedance spectroscopy. Differently from gravimetric

EQCM,AC-EQCMallowsthedeconvolutionofaglobal

gravimetricECQMresponseintoindividualcations,

an-ions,andsolventmoleculescontributionsbyplottingthe dQ/dE(Q:charge,E:potential)ordm/dE(m:mass,E: po-tential)transferfunctions;thisisonekeyadvantageofthis techniquewhichcantracktheelectrochemicalactivityof onetypeofanion(cation)inamixtureofanions(cations)

[53].Somepapers havejust been publisheddescribing

theuseof AC-EQCM to studypseudocapacitive

mate-rials[54•_{].Thepossibilityfordifferentiatingtheion}

con-tributions presentgreatinterestfor studyingthecharge storagereactionmechanismsinvariouselectrolytes.

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