wikiSupercapacitors2.docx
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wikiSupercapacitors2.docx
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wikiSupercapacitors2
Supercapacitor
MaxwellTechnologies"MC"and"BC"seriessupercapacitors(upto3000faradcapacitance)
Supercapacitors(SC),[1]compriseafamilyofelectrochemicalcapacitors.Supercapacitors,sometimescalledultracapacitorsorelectricdouble-layercapacitor(EDLC),don'thaveaconventionalsoliddielectric.Thecapacitancevalueofanelectrochemicalcapacitorisdeterminedbytwostorageprinciples,bothofwhichcontributetothetotalcapacitanceofthecapacitor:
[2][3][4]
∙Double-layercapacitance–electrostaticstorageoftheelectricalenergyachievedbyseparationofchargeinaHelmholtzdoublelayerattheinterfacebetweenthesurfaceofaconductiveelectrodeandanelectrolyte.Thedistanceofthestaticseparationofchargeinadouble-layerisontheorderofafewAngstroms(0.3–0.8 nm)whichisextremelysmall.[5]
∙Pseudocapacitance–Electrochemicalstorageoftheelectricalenergywithelectrontransfer,achievedbyredoxreactionswithspecificallyadsorbedionsfromtheelectrolyte,intercalationofatomsinthelayerlatticeorelectrosorption,underpotentialdepositionofhydrogenormetaladatomsinsurfacelatticesiteswhichresultinareversiblefaradaiccharge-transfer.[5]
Theratioofthestorageresultingfromeachprinciplecanvarygreatly,dependingonelectrodedesignandelectrolytecomposition.Pseudocapacitancecanincreasethecapacitancevaluebyasmuchasanorderofmagnitudeoverthatofthedouble-layerbyitself.[1]
Supercapacitorsaredividedintothreefamilies,basedonthedesignoftheelectrodes:
∙Double-layercapacitors–withcarbonelectrodesorderivateswithmuchhigherstaticdouble-layercapacitancethanthefaradaicpseudocapacitance
∙Pseudocapacitors–withelectrodesoutofmetaloxidesorconductingpolymerswithahighamountoffaradaicpseudocapacitance
∙Hybridcapacitors–capacitorswithspecialandasymmetricelectrodesthatexhibitbothsignificantdouble-layercapacitanceandpseudocapacitance,suchaslithium-ioncapacitors
Hierarchicalclassificationofsupercapacitorsandrelatedtypes
Supercapacitorsbridgethegapbetweenconventionalcapacitorsandrechargeablebatteries.Theyhavethehighestavailablecapacitancevaluesperunitvolumeandthegreatestenergydensityofallcapacitors.Theysupportupto12,000Farads/1.2Volt,[6]withcapacitancevaluesupto10,000timesthatofelectrolyticcapacitors.[1]Whileexistingsupercapacitorshaveenergydensitiesthatareapproximately10%ofaconventionalbattery,theirpowerdensityisgenerally10to100timesgreater.Powerdensityisdefinedastheproductofenergydensity,multipliedbythespeedatwhichtheenergyisdeliveredtotheload.Thegreaterpowerdensityresultsinmuchshortercharge/dischargecyclesthanabatteryiscapable,andagreatertolerancefornumerouscharge/dischargecycles.
Withinelectrochemicalcapacitors,theelectrolyteistheconductiveconnectionbetweenthetwoelectrodes,distinguishingthemfromelectrolyticcapacitors,inwhichtheelectrolyteonlyformsthecathode,thesecondelectrode.
Supercapacitorsarepolarizedandmustoperatewithcorrectpolarity.Polarityiscontrolledbydesignwithasymmetricelectrodes,or,forsymmetricelectrodes,byapotentialappliedduringthemanufacturingprocess.
Supercapacitorssupportabroadspectrumofapplicationsforpowerandenergyrequirements,including:
∙Lowsupplycurrentduringlongertimesformemorybackupin(SRAMs)
∙Powerelectronicsthatrequireveryshort,highcurrent,asintheKERSsysteminFormula1cars
∙Recoveryofbrakingenergyforvehicles
Exceptionalforelectroniccomponentslikecapacitorsarethemanifolddifferenttradeorseriesnamesusedforsupercapacitorslike:
APowerCap,BestCap,BoostCap,CAP-XX,DLCAP,EneCapTen,EVerCAP,DynaCap,Faradcap,GreenCap,Goldcap,HY-CAP,Kaptoncapacitor,Supercapacitor,SuperCap,PASCapacitor,PowerStor,PseudoCap,Ultracapacitormakingitdifficultforuserstoclassifythesecapacitors.
Contents
∙1History
o1.1Developmentofthedoublelayerandpseudocapacitancemodel
o1.2Developmentofelectrochemicalcapacitors
∙2Storageprinciples
o2.1Conventionalelectrostaticvselectrochemicalenergystorage
o2.2Electrostaticdouble-layercapacitance
o2.3Electrochemicalpseudocapacitance
∙3Typesofsupercapacitors
∙4Construction
∙5Materials
o5.1Electrodes
o5.2ElectrodesforEDLCs
o5.3Electrodesforpseudocapacitors
o5.4Electrodesforhybridcapacitors
o5.5Electrolytes
o5.6Separators
o5.7Collectorsandhousing
∙6Electricalparameters
o6.1Capacitance
o6.2Operatingvoltage
o6.3Internalresistance
o6.4Currentload
o6.5Cyclestability
o6.6Energydensityandpowerdensity
o6.7Lifetime
o6.8Self-discharge
o6.9Polarity
∙7Comparisationoftechnicalparameters
o7.1Comparisationofsupercapacitorparameters
o7.2Parametriccomparisonoftechnologies
∙8Standards
∙9Applications
o9.1Generalapplications
o9.2Heavyandpublictransport
∙10Newdevelopments
∙11Market
∙12Seealso
∙13Literature
∙14References
∙15Externallinks
History
Developmentofthedoublelayerandpseudocapacitancemodel
Helmholtz
Whenametal(oranelectronicconductor)isbroughtincontactwithasolidorliquidionic-conductor(electrolyte),acommonboundary(interface)amongthetwodifferentphasesoriginates.Helmholtz[7]wasthefirsttorealizethatchargedelectrodesimmersedinelectrolyticsolutionsrepelthecoionsofthechargewhileattractingcounterionstotheirsurfaces.Withthetwolayersofoppositepolarityformedattheinterfacebetweenelectrodeandelectrolytein1853heshowedthatanelectricaldoublelayer(DL)thatisessentiallyamoleculeardielectricachievedelectrostaticchargestorage.[8]Belowtheelectrolyte'sdecompositionvoltagethestoredchargeislinearlydependentonthevoltageapplied.
ThisearlyHelmholtzmodelpredictedaconstantdifferentialcapacitanceindependentfromthechargedensitydependingonthedielectricconstantofthesolventandthethicknessofthedouble-layer.[5][9][10]Butthismodel,whileagoodfoundationforthedescriptionoftheinterface,doesnotconsiderimportantfactorsincludingdiffusion/mixingofionsinsolution,thepossibilityofadsorptionontothesurfaceandtheinteractionbetweensolventdipolemomentsandtheelectrode.
SimplifiedillustrationofthepotentialdevelopmentintheareaandinthefurthercourseofaHelmholtzdoublelayer.
Gouy\Chapman
LouisGeorgesGouyin1910andDavidLeonardChapmanin1913bothobservedthatcapacitancewasnotaconstantandthatitdependedontheappliedpotentialandtheionicconcentration.The“Gouy-Chapmanmodel”madesignificantimprovementsbyintroducingadiffusemodeloftheDL.InthismodelthechargedistributionofionsasafunctionofdistancefromthemetalsurfaceallowsMaxwell–Boltzmannstatisticstobeapplied.Thustheelectricpotentialdecreasesexponentiallyawayfromthesurfaceofthefluidbulk.[5][11]
Stern
Gouy-ChapmanmodelfailsforhighlychargedDLs.InordertoresolvethisproblemOttoSternin1924suggestedthecombinationoftheHelmholtzandGouy-Chapmanmodels.InStern'smodel,someoftheionsadheretotheelectrodeassuggestedbyHelmholtz,givinganinternalSternlayerandsomeformaGouy-Chapmandiffuselayer.[12]
TheSternlayeraccountedforions'finitesizeandconsequentlyionshaveaclosestapproachtotheelectrodeontheorderoftheionicradius.TheSternmodeltoohadlimitations,effectivelymodelingionsaspointcharges,assumingallsignificantinteractionsinthediffuselayerareCoulombic,assumingdielectricpermittivitytobeconstantthroughoutthedoublelayer,andthatfluidviscosityisconstantabovetheslippingplane.[13]
Grahame
Thus,D.C.GrahamemodifiedSternin1947.[14]HeproposedthatsomeionicorunchargedspeciescanpenetratetheSternlayer,althoughtheclosestapproachtotheelectrodeisnormallyoccupiedbysolventmolecules.Thiscouldoccurifionslosttheirsolvationshellwhentheionapproachedtheelectrode.Ionsindirectcontactwiththeelectrodewerecalled“specificallyadsorbedions”.Thismodelproposedtheexistenceofthreeregions.TheinnerHelmholtzplane(IHP)planepassingthroughthecentresofthespecificallyadsorbedions.TheouterHelmholtzplane(OHP)passesthroughthecentresofsolvatedionsattheirdistanceofclosestapproachtotheelectrode.FinallythediffuselayeristheregionbeyondtheOHP.
Schematicrepresentationofadoublelayeronanelectrode(BMD)model.1.InnerHelmholtzplane,(IHP),2.OuterHelmholtzplane(OHP),3.Diffuselayer,4.Solvatedions(cations)5.Specificallyadsorbedions(redoxion,whichcontributestothepseudocapacitance),6.Moleculesoftheelectrolytesolvent
Bockris/Devanthan/Müller
In1963J.O'M.Bockris,M.A.VDevanthan,andK.AlexMüller[15]proposedamodel(BDMmodel)ofthedouble-layerthatincludedtheactionofthesolventintheinterface.Theysuggestedthattheattachedmoleculesofthesolvent,suchaswater,wouldhaveafixedalignmenttotheelectrodesurface.Thisfirstlayerofsolventmoleculesdisplayastrongorientationtotheelectricfielddependingonthecharge.Thisorientationhasgreatinfluenceonthepermittivityofthesolventwhichvarieswiththefieldstrength.TheinnerHelmholtzplane(IHP)passesthroughthecentersofthesemolecules.Specificallyadsorbed,partiallysolvatedionsappearinthislayer.ThesolvatedionsoftheelectrolyteareoutsidetheIHP.Throughthecentersoftheseionspassasecondplane,theouterHelmholtzplane(OHP).TheregionbeyondtheOHPiscalledthediffuselayer.TheBDMmodelnowismo
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