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Electrochemical double layer capacitors: What is next
beyond the corner?
Zifeng Lin, Pierre-Louis Taberna, Patrice Simon
To cite this version:
Zifeng Lin, Pierre-Louis Taberna, Patrice Simon. Electrochemical double layer capacitors: What is
next beyond the corner?. Current Opinion in Electrochemistry, Elsevier, 2017, 6 (1), pp.115-119.
�10.1016/j.coelec.2017.10.013�. �hal-02020588�
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This is an author’s version published in:
http://oatao.univ-toulouse.fr/21768
To cite this version:
Lin, Zifeng
and Taberna, Pierre-Louis
and Simon, Patrice
Electrochemical double layer capacitors: What is next beyond the corner?
(2017) Current Opinion in Electrochemistry, 6 (1). 115-119. ISSN 2451-9103
Official URL:
https://doi.org/10.1016/j.coelec.2017.10.013
Review
Article
Electrochemical
double
layer
capacitors:
What
is
next
beyond
the
corner?
Zifeng
Lin
1,2,
Pierre-Louis
Taberna
1,2and
Patrice
Simon
1,2,3,∗Thisreviewsummarizessomerecentdevelopmentsachieved inthefundamentalunderstandingofionconfinementin microporouscarbonsupercapacitorelectrodes.Combinedwith computationalsimulations,theseadvancedtechniques providednewinsightsintothechargestoragemechanism, providingguidelinesfordesigningimprovedporouscarbon structureswithhigh-energydensity.Also,innovative
electrolyteshaverecentlybeenproposedbyintroducingsome redox-activemoietiesintoelectrolyteanions/cations,called biredoxelectrolytes,thatcombinesredoxcontributionto doublecapacitance.Thisapproachopensupnew
opportunitiestodevelophigh-energysupercapacitorsanda newfieldofbiredoxelectrolytes.
Addresses
1Université PaulSabatier,CIRIMATUMRCNRS5085,118routede Narbonne,31062ToulouseCedex,France
2RéseausurleStockageElectrochimiquedel’Energie(RS2E),FR CNRS3459,France
3InstitutUniversitairedeFrance,1rueDescartes,75231ParisCedex 05,France
∗Correspondingauthor:
Simon,Patrice (simon@chimie.ups-tlse.fr)
https://doi.org/10.1016/j.coelec.2017.10.013
Introduction
Inthepasttwodecades,electrochemicalcapacitors,also knownassupercapacitors,havereceivedspecialattention sincetheyareoneofthemostpromisingelectrochemical energystoragedevicesforhighpowerdeliveryorenergy harvesting applications [1]. Originally developed for power electronics applications, they are facing
increas-ing demand mainly driven by automotive and power
electronics applications. Those include regenerative braking,voltagestabilizationboostorstart-stopsystems. NumerouscitiesinEuropeandaroundtheworldare de-velopingtramwaysorbuseswithon-boardsupercapacitor
modulesforbrakingenergyrecoveryand short-distance electric drive (to passcrossings). Besides,they also can sometimesreplacebatteriesinelectric buseswherethe limitedautonomy is balanced by thefast charging that canbeachievedduringthepassengerexchange[2]. Today, most (not to say all) commercialized superca-pacitors use porous carbons as active materials. These devices, called electrochemical double layer capacitors (EDLCs),storethechargeattheelectrolyte/carbon inter-facethroughreversible adsorptionof ionsfroman elec-trolyte onto the carbon surface, by charging the elec-trochemicaldoublelayercapacitance,which canbe de-scribedinacrudeapproachby(1):
C=εrε0
A
d orC/A= εrε0
d , (1)
whereCisthedoublelayercapacitance(inF),εr isthe
electrolytedielectricconstant,ε0 thedielectricconstant
ofthevacuum(inFm−1),dthechargeseparationdistance
(inm),andAtheelectrodesurfacearea(inm2).
Increasingthecapacitanceof porouscarbonsisoneway to improve the energy density of supercapacitors. Be-yondthisbasicconsideration,thereareimportant scien-tificchallengestotackledealingwiththeunderstanding oftheionsfluxesandadsorptionmechanismsinsidethe porouscarbonstructure.
Ion
confinement
in
carbon
nanopores
FollowingpioneerworkfromAurbach[3],thediscovery in2006of adifferent,moreefficientstoragemechanism in nanosized pores (less than one nm) led to a drastic change of view:not only the surface area but also the poresize was importantto increase the chargecapacity
[4],alsoshowingamaximumincapacitancewhentheion sizewassimilartothecarbonmeanporesize[5].Those resultshavealsopointedouttheimportanceofthe mea-surementsofthespecificsurfaceareaandpore size dis-tributionofmicroporouscarbonsfromgassorption tech-nique[6].As a result, the International Union of Pure andAppliedChemistry hasrecently confirmedthatthe Brunauer–Emmett–Teller(BET)analysiswasnot recom-mendedformicroporouscarbons,asitover-estimatesthe surfaceof large micropores(>0.7nm)[6].Similarly,the use of N2 gas for gas sorption measurement has to be
discardedbecauseoftheexistence ofaquadripole mo-ment,whichdoesnotgiveaccurateporosityfor
ultrami-Figure1
A:specificcapacitance(inFcm−2)ofporouscarbonsnormalizedtotheaccessiblesurfaceareaforeachioninbothsolvents(acetonitrileANand propylenecarbonatePC).Theregularpatternaverageof0.095Fm−2adaptedfromRef.[7].B:changeofthespecificcapacitancenormalizedtothe DFT-specificsurfaceareaversustheporesizeofcarbide-derivedcarbons(adaptedfromRef.[4]).ThetrendissimilartothatofFigure1A.
cropores<0.7nm.Thedeterminationofthespecific sur-faceareaformicroporouscarbonsshouldthenbedone us-ingArgasat87Kanddensityfunctionaltheory(DFT)or
non-linearDFT(NLDFT)mathematicalmodelsshould
be used to calculate the pore size distribution and the meanpore sizediameter.Also,thesurfacearea accessi-bletotheionsoftheelectrolyte(definedbyconsidering theporesizelargerthantheneationsize[7])shouldbe consideredto calculatethe specificcapacity (in Fm−2) of the porous carbons. The specific surface area and poressizedistributionsofaseriesofporouscarbonshave beenmeasuredfollowingtheaboverecommendation[7].
Figure1Ashowstheplotofthespecificcapacitance ver-sus the carbon pore size measured in 1M (C2H5)4N+,
BF4−inacetonitrileorpropylenecarbonateelectrolytes.
The specific capacitance was obtained by dividing the gravimetric capacitance by the DFT accessible surface area measured using Ar gas that is the total surface of thepores larger thanthe neation size.Theplot shows anincreaseof thecapacitanceinthesmallpore size re-gion(lessthan1nm),suchasoriginallyproposedin2006 (Figure1B).Forlargerporesizes(mesopores),the capaci-tanceconvergestowardanaveragevaluebelow0.1Fm−2,
which aligns well with the “regular pattern” value re-portedbyCentenoetal.[8]andwiththecalculatedvalue
limitingthedouble-layercapacitanceattheplanarcarbon interface(orlargerthanthefew-nmpores[9]).
However, the “real” structure of porous carbons being difficultto obtainfromgassorption measurementsonly, advanced experimental techniques have been recently proposedas toolstohelp inunderstanding theion con-finementandenvironmentincarbonnanopores.Ina re-cent paper, Prehal et al. [10•] studied theconfinement
anddesolvationinnanometersizedcarbonporesof1M CsClinwaterelectrolyte,bycombininginsitusmall
an-gle X-ray scattering experiments (SAXS) together with MonteCarlosimulations.Bymodellingbackthe experi-mentalSAXSdata,theyfoundthattheaveragenumberof stripped-offwatermoleculesincreasedfordecreasing av-erageporesize.Interestingly,thepartialdesolvationalso occurredincarbonlargermesopores,althoughlimitedto about1% water molecules removedfrom the solvation shell.Thesedatathus showthat iondesolvation seems tobeauniversalphenomenonforvirtuallyallnanoporous carbons,becauseoftheporesizedispersionandpresence ofultranarrow(subnanometre)pores[10•].
Other insitutechniques have beenrecently developed thepastyearssuchaselectrochemicalquartzcrystal
mi-crobalance (EQCM).EQCM in gravimetric mode, that
trackstheweightchangeofanelectrodeduring electro-chemicalpolarization,hasbeensuccessfullyusedto ex-perimentallymeasuretheextentofdesolvationincarbon nanoporesinnon-aqueouselectrolytes[11•
–13].EQCM resultsshowed thatEMI+
cationslosehalfof their sol-vationshellinacetonitrile-basedelectrolytewhile enter-ing 1nm pores in carbon electrodes [12]. Additionally, twodifferentiontransfermechanismscouldbeidentified: counterion(cation)adsorptionatnegativeelectrodeand ionexchangemechanismatpositiveelectrode.Recently, Levietal.proposedtheuseoftheEQCM-Dtechnique
(electrochemicalQCM with dissipation monitoring), to performamultiharmonicfrequencyanalysistoassessthe viscoelasticandhydrodynamicpropertiesinporousactive films[14].FittingtheEQCM-Ddatawithsuitable hydro-dynamicmodelsallowstracking thechangeof the geo-metricparametersofporouselectrodesincontactwithan electrolyte.Suchtechniqueoffersinteresting opportuni-tiestoidentifytheimpactof severalparameters(nature oftheelectrolytes,ions,binder...)onthestructurechange oftheelectrodes.
Alternatively,notonlythegravimetricordissipationmode ofEQCMoffersopportunitiestostudyingtheionfluxes in supercapacitor or energy storage electrodes. Perrot’s grouphasdevelopedtheso-calledAC-electrogravimetry technique [15].It consistsin runningEQCM measure-ment in the gravimetric mode at a steady state (for instance constant potential) and, simultaneously, over-imposingasinusoidalperturbationtothesteady-stateand
recordsthe masschange1m or thecharge change1Q
vs. thepotential change1E [15].These plotsgive ac-cess to the type of ions as well as the number of sol-ventmoleculesinvolvedinthechargestorageprocess de-pending on the potential range,as wellas the ion dy-namicsinsidetheelectrodedependingonthefrequency rangeexplored[16•].Thistechniqueseemspromisingfor
trackingtheionfluxesandcouldpotentiallypushfurther our knowledgeof ion transfer/adsorptionin porous car-bonelectrodes,thushelpingtodesigningthebestporous carbonelectrodesforthenextgenerationofhigh-energy densitysupercapacitorelectrodes.
Electrolytes
for
EDLCs
The energy density of supercapacitors changing with thevoltagesquare(1/2.C.V²),thereisagreatinterestin designing new electrolytes with improvedvoltage win-dowstability,togobeyond3V.Today,conventional elec-trolytes for supercapacitors contain asaltdissolvedin a solvent. Acetonitrile (CH3CN, AN) is the solvent that
leadstothehighestconductivitywhenusedin combina-tionwithanammoniumcationmixedwithafluorinated anion(suchas(C2H5)4N+,BF4−).Althoughhighcell
volt-ageof3Vcanbeobtained[17],AN-basedelectrolytes suf-ferfromlowflashpoint(5°C)andhighvolatility. Alter-natively,propylene carbonate-based electrolytes canbe usedbutinthatcase,thepowercapabilityofthedeviceis greatlyaffected(dividedbythree)aswellasthelow tem-peratureapplications.Thereisthenanimportantneedfor alternativehighvoltageelectrolytes[18].
However,itiswellknownthatnotonlythe electrochem-icalvoltagewindowbutalsotheconductivity,the viscos-ity,theboiling/meltingpoints,thedielectricconstantand saltchemistrydeeplyaffectstheelectrochemical proper-ties of the electrolyte [19].As aresult, itis really chal-lengingtofindasolvent/saltmixturethatmatchesallthe criteriaatthesametime.Also,researchworkdeveloped in the Li-ionbatteries fieldhighlighted that the stabil-ityoftheactive materialsatthepositiveelectrodeisan issue when operating at high potential (>4.5V vs. Li, thatis approximately >3.5Vcell voltagefor a symmet-riccarbon/carbondevice)[20,21].However,Balducciand co-workers[22••]haveproposedaninteresting
combina-toryapproachtoselectnewsolvent/saltcouplesandsuch efforts should bedefinitelypursued.Also,ionicliquids, whicharesolventfreeelectrolyte,areofparticular inter-est toincrease thecellvoltageupto 3.5Vbutthehigh viscosity and limited conductivityat room temperature
limitstheirinterest.Theuseofeutecticionicliquid mix-turescanexpandthetemperaturerange,butthecarbon structuremustbespecificallydesignedtoensureeasyion accessibilitytothecarbonsurface[12,23].
Anotheralternativeapproachhasbeenrecentlyproposed, which consists in using redox-active electrolyte [24•].
Theideaistomodifyionicliquidselectrolytesby graft-ingaredox-activegroupsontotheanionsand/orcations, intheaim ofplayingwith boththedoublelayer charg-ingprocessthroughbasicionadsorptionand,atthesame time,achievinganelectrontransferthrougharedox reac-tionoftheredoxgroupattheoperatingpotentialofthe carbonelectrode.
Thisinnovativeconcepthasbeenreportedinrecent pa-persfromtwo groups in France [24•
] and Canada [25]. Fontaineandco-workers[24•],for instance,useda
per-fluorosulfonateanion(PFS−
)andamethylimidazolium cation (MIm+
) ionic liquid, where anion and cation arefunctionalizedwithanthraquinone(AQ)and 2,2,6,6-tetramethylpiperidinyl-1-oxyl(TEMPO) moieties, lead-ing to the (AQ–PFS−
) (MIm+
–TEMPO) biredox
elec-trolyte.These biredox were used as saltand dissolved at 0.5M in BMIm+
, TFSI−
neat ionic liquid. The re-sultingsolution(ionicliquidplusthebiredoxsalts)was used as electrolyte in supercapacitor cells in combina-tionwithactivatedporouscarbonelectrodes.Asexpected, theseionicliquidswereabletoincreasethechargestorage viabothelectrostatic—inthedoublelayer—andredox re-actionmechanisms.Theobtainedcapacitancewastwice largerinthebiredoxelectrolytecomparedtobiredox-free solution(Figure2a).
Figure 2b shows theschematic representation of a gal-vanostaticchargedischargecycle,withtherespective po-tential stability domains of the reduced and oxidized forms of the biredox species. The figure shows that a Faradiccontributionisaddedtothedoublelayer contri-butionduringchargebyoxidationoftheTEMPO moi-etiesbeyond1Vvs.ref.Inthesametime,thereductionof theAQgroupsontheanionatthenegativeelectrode be-low−0.5Vvs.Ref.reductionreactionaddstothedouble layercontributiontoincreasethetotalcapacitance.This exampleshows thatthecareful selection of thegrafted moietieshasthepotentialtogreatlyimprovethe capaci-tiveperformanceofporouscarbonelectrodes.Moreover, theworkingvoltageofthecellwaskeptashighas2.7V, whichistypicallythevoltageAN-basedelectrolytescan achieve.
Capacitancesupto 370 F g−1 weremeasured,
depend-ingonbiredoxionicliquidconcentrationsand tempera-tures,stableforatleast2000cycleswithlimited—ifany— degradation.Anotherstrikingresultwastheevidenceof theretentionof theredoxspeciesintotheporous elec-trode,as canbe seen from both the low self-discharge
Figure2
A:cyclicvoltammetryat5mVs−1with0.5MbiredoxILinBMImTFSI(solidline)andpureBMImTFSI(dashedline),respectively.B:schematic representationofthegalvanostaticvoltageresponseoftheelectrostaticandFaradaicprocesseswiththebiredoxILandthestabilityregionsforits oxidized/reducedspecies.Thedashedlinesillustratethebehaviorifonlyelectrostaticprocessesweretoprevail.AdaptedfromRef.[24•].
andleakagecurrentsmeasured.Althoughsurprising,this stillneedstobeunderstood,butspecificelectrostatic in-teractionsbetweenionsandcarbonpores(bycreationof imagechargesforinstance)couldbepossible[26].This newelectrolyte conceptopensupnewopportunities to develophigh-energysupercapacitorsandawidenewfield inredoxmaterials.
In this review, we summarized 1) recent development of fundamental understanding of porous carbon super-capacitors by using advanced experimental techniques and 2) electrolyte design strategy toward high-energy
performance. Although the capacitance increase in
sub-nanometer pores (less than 1nm)in porous carbon
electrode has been evidenced for a decade, the ion
confinement effect in nanopores is still notcompletely understood.Theoreticallyandexperimentallystudiesare neededtofurtherunveilthechargestoragemechanism.
In that aim, advanced experimental techniques such
as in situ NMR, in situ small angle X-Ray or neutrons
scatteringexperiments(SAXS,SANS),insituEQCM(in gravimetric or dissipation modes) techniquescombined withcomputermodelinghavebeendevelopedtoprobe deeplyintotheionconfinementinnanopores,thus guid-ing the designof better porous carbon electrodes with high-energy density. These efforts should be pursued duringthenextyearsand othertoolssuchas electronic microscopy(insitutransmissionelectronicmicroscopy)or
scanningelectrochemicalmicroscopy (SECM) couldbe of greathelp.Besidesthestrategyofelectrode material design,developingadvancedelectrolyteswithimproved voltagewindowisaswellimportanttoenhancetheenergy density.Althoughinterestingprogresseshavebeenmade usingacombinatoryapproach,thefindingofhighvoltage electrolyte remains challenging. Alternatively, a recent approachhasbeendevelopedbyusingactiveelectrolyte ionsfunctionalizedtogetherwithredoxmoieties,offering realopportunitiestoincreasethecapacitanceandvoltage
window by introducing redox reaction in double layer capacitors. The abounding research works highlights the real interest for developing supercapacitor devices
with improved performance for complementing—and
sometimes replacing—conventional batteries in appli-cations where high power, high cyclability and high charge/dischargeratesareneeded.
References
and
recommended
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•
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