Combined Effect of Nitrogen- and Oxygen-Containing Functional Groups of Microporous Activated Carbon

Combined Effect of Nitrogen- and Oxygen-Containing Functional Groups of Microporous Activated Carbon

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EffectofNitrogen-andOxygen-Containing

GroupsofMicroporousActivatedCarbononElectrochemicalPerformanceinSupercapacitors

DenisaHulicova-Jurcakova,MykolaSeredych,GaoQingLu,andTeresastandingwhethermicroporesonly,[1–5]acombinationofmicroporesandmeso-pores,[6–9]ormesoporesonly[10–13]areopti-malforthemosteffectiveutilizationofthesurfaceareainadouble-layerformation.Chmiolaandco-workers[14–16]presentedadifferentviewofporeaccessibilitytoions.

Theirresultsrevealedthattheporeswithawidthclosetothesizeofelectrosorbedions,i.e.,lessthan1nmfornon-aqueouselectro-lytes,arethemostactiveinadouble-layerformation.Thistheorywassupportedbyotherresearchgroups.[7,17]Inaddition,

Huangetal.recentlyrevealedauniversal

theoreticalmodeltoexplainthechargestoragemechanismapplicabletodifferent

porosity,carbonorigin,poresizedistribu-tionsandelectrolytes.[18,19]Thismodeltakestheporecurvatureintoaccount,incontra-dictionwiththetraditionalparallel-plate

capacitor,anditappearsthatinthemicroporeregimesolvated/desolvatedionslineupalongtheporeaxistoformanelectricwire-in-cylindercapacitor,whileinthemesoporeregimetheyapproachtheporewalltoformanelectricdouble-cylindercapacitor.Theelec-tricdouble-layercapacitormodelbecomesapplicableinthemacroporeregionwherethe

porecurvatureisnolongersigni cant.Itshouldbenotedthatthismodelwassuccessfullyappliedtocarbonswithunimodalpores

1.Introduction

anddominantelectrosorptionofionsanditmaynotbeapplicableincarbonswithpseudocapacitiveenergystoragemechanism.

Carbonelectrodematerialsforelectricdouble-layercapacitorsor

Theseincludecarbonswithnitrogen-andoxygen-containing

so-calledsupercapacitorshavebeenstudiedextensivelyinrecent

surfacegroups,whichhavebeeninvestigatedextensivelyinthelast

years.Oneofthemaininterestshasbeenfocusedonunder-decade.[20–26]Surfacegroupsintroducetheacid-basepropertiestocarbon[27–29]andgiverisetoFaradaicpseudocapacitivereac-[*]Prof.T.J.Bandosz,Dr.M.Seredychtions.[30]Pseudocapacitanceinnitrogen-enrichedcarbonshasTheCityCollegeofNewYork,DepartmentofChemistrybeenproposed,butnotcon rmed,astheoxidation/reductionof160ConventAve,NewYork,NY10031(USA)

negativelychargednitrogengroupslocatedattheperipheryofE-mail:tbandosz@ccny.cuny.edu

graphene-likelayers.[22]Inthecaseofoxygenfunctionalities,theDr.D.Hulicova-Jurcakova,Prof.G.Q.Lu

mostdocumentedcaseisthereversibleoxidation/reductionofTheUniversityofQueensland

ARCCentreofExcellenceforFunctionalNanomaterialshydroquinone/quinonegroups.[21,28,31]AustralianInstituteforBioengineeringandNanotechnologySinceoxygenfunctionalitiesarealwayspresentonthecarbonandSchoolofEngineering

surfaceasaresultofactivationprocessesorasaresiduefromtheCornerCollegeandCooperRoads,StLucia,4072QLD(Australia)

carbonsource,[32]thepseudocapacitivecontributiontothetotalcapacitanceofcarbonshouldbetakenintoaccount.Therefore,inDOI:10.1002/adfm.200801236438

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ordertodevelopahigh-performancecarbonelectrodeforsupercapacitors,itiscrucialtounderstandthecombinedeffectofvarioustypesofcarbonsurfacefunctionalitiesandtheporestructureonitscapacitiveperformance.Tothebestofourknowledge,thisimportantissuehasnotbeensystematicallyinvestigatedandnofundamentalunderstandingofpseudocapa-citancefromeitherthenitrogen-oroxygen-functionalgroupshasbeenreportedtodate.

Previouslywehaveinvestigatedaseriesofmicro/mesoporousactivatedcarbonenrichedwithnitrogen.[33]Verystrongdepen-denceofcapacitanceonthechemistryofsurfacegroupsaswellasontheporosityofcarbonswasobserved.Goodcorrelationswerefoundbetweenthenumberofbasicgroupsandthegravimetriccapacitance,andbetweenthenormalizedcapacitanceinmicro-poresandthedistributionofquaternaryandpyridinic-N-oxidenitrogenspeciesonthesurfaceofthemicropores,particularlyatheavyoperationregimes.Theconclusionwasthatthepseudo-capacitiveinteractionstakeplaceonnegativelychargedpyrrolic-Nandpyridinic-N,whilethepositivechargeonquaternary-Nandpyridinic-N-oxidehelpedinelectrontransferthroughthecarbon.Themicroporeswerethemosteffectiveinadouble-layerformationwhilemesoporesappearedtohavenopositiveeffectontheelectrochemicalperformance.

Inthispaperwestudythesynergisticeffectofnitrogen-andoxygen-containingsurfacefunctionalgroupsofsolelymicropor-ousactivatedcarbononitsperformanceina1MH2SO4supercapacitor.Detailedanalyzesofporestructureallowtheinvestigationofmosttheeffectiveporesforadouble-layerformationaswellasporesinwhichthepseudocapacitiveinteractionstakeplace.Accordingly,theresultsarediscussedintermsofnitrogentocarbonratio,nitrogenandoxygencontent,speci cchemicalarrangement,strengthofthesurfacefunctionalgroups,theirbasicityandthelocationofnitrogen-andoxygen-containinggroupsonthecarbonsurface.Theeffectofpore-sizedistributionandporestructureisanalyzedonthebasisofnitrogenandcarbondioxidesorption.

caseofureamodi cationtheintroductionofoxygengroupsincreasedtheamountofnitrogenincorporatedintothecarbonmatrix.Infact,thisisinteresting,since,basedonthecontentofoxygencalculatedfromelementalanalysisasadifferenceto100%,oxidationdidnotaffecttheoverallcompositionofthiscarbon.OnehastobeawarethatthetreatmentwithHNO3certainlydecreasedtheashcontent(2.4%fortheinitialcarboncomparedto1.1%afteroxidizationandthusthecontentofoxygencanbeconsideredasunchanged.Aftermodi cationandheattreatment,thecontentofoxygeninthepreoxidizedandureamodi edsample,S-UO,issmallerthanthatinthemelaminetreatedsample,S-MO.Thismightberelatedtotheinvolvementofoxygengroupsinthechemicalbondswithurea,[40]whichledtothermallyunstablespecies.Ontheotherhand,asigni cantamountofoxygen-containinggroupsstillseemtobestableinthemelamineandureatreatedsamplesonwhichpreoxidationwasnotperformed.Eventhoughtheoxidationdidnotvisiblyaffectthetotalcontentofoxygen,andotherstudieshaveshownthatS208isoxidationresistant,[41]itmusthavechangedthenatureoftheoxygengroupssincetheeffectofoxidationonincorporationofnitrogenintheurea-treatedsamplesisclearlyseeninthedifferencesinNcontent.

OwingtothefactthatX-rayphotoelectronspectroscopy(XPS)determinesthesurfacecompositionincontrarytobulkcompositionrevealedbyelementalanalysis,therelativecontentsofsurfacecarbon,oxygenandnitrogenobtainedfromXPSdifferedfromthosefromelementalanalyzes(Table1).Moresurfacenitrogenwasdetectedinpreoxidizedandmelamine-treatedS-MOthaninitscounterpartS-Mwithoutpreoxidation.Ontheotherhand,thedifferenceinthecontentofsurfacenitrogenofurea-treatedserieswasverysmall.MoresurfaceoxygenwasmeasuredinS-UthaninS-UO,whichisinagreementwiththeresultsofelementalanalysis.

Toseeiftheappliedtreatmentschangedthenatureofsurfacechemistryofthecarbons,protonuptakecurveswerecalculated

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2.ResultsandDiscussion

S208coconut-shell-basedcarbonisconsideredstableathightemperaturesbut,despitethis,treatmentswithmelamineorurearesultedinanincorporationofasigni cantamountofnitrogen.Table1showsthattotalnitrogencontentreachedmorethan4%inmelamine-treatedsamples.Althoughthecontentofnitrogeninthemelamineseriesdidnotdependonpreoxidation,inthe

Table1.Carbon,hydrogen,oxygen,andnitrogencontents[%]inthestudiedsamplesobtainedfromelementalanalyzesandX-rayphoto-electronspectroscopy(XPS)(hydrogenisnotincluded).

SampleSS-OS-US-UOS-MS-MO

C79.081.984.991.285.188.2

H0.70.940.70.30.90.6

N0.120.802.93.74.04.1

O20.1816.3611.54.8107.1

N/C0.0020.0090.0340.0410.0470.047

CXPS94.3491.3593.2894.6093.2293.21

OXPS5.668.654.683.244.013.54

NXPS002.042.162.773.25

Figure1.Protonbindingcurvesofallinvestigatedcarbonsobtainedfrompotentiometrictitrationmeasurements.

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fromthetitrationcurvesandarecollectedinFigure1.Itisclearthattheoxidationincreasedacidityofthecarbonsurface.ForpreoxidizedS-Oonly,protonreleaseisseen,incontrasttotheprotonuptakeoftheinitialsampleS.Treatmentwithnitrogen-containingcom-poundsincreasedsurfacebasicity,especiallyforS-UandS-M.ThelowerbasicityinthecaseofpreoxidizedsamplesS-UOandS-MOmustberelatedtothelesseramountofoxygenandnitrogenfunctionalgroups,asalsorevealedbyelementalanalysis(Table1).

Thequantitativedifferencesbetweensam-plesarereportedinTable2asnumbersofacidicandbasicgroupsevaluatedfromtheBoehmtitrationmethod.TheaveragepHisalsolisted.Inagreementwiththepotentio-metrictitrationdata,oxidationincreasedthenumberofacidicgroupsbyalmostfourtimesandthebasicgroupstotallydisappeared.Aftertreatmentwithnitrogen-containingspecies,thebasicgroupswerepresentinpredominantquantities,especiallyinS-UandS-M.Fortheformersample,thenumberofacidicgroupswasverysmall.BothS-UOandS-MOhavesimilarchemistriesfromthepointofviewofacid/basesurfaceproperties.

Furtherdifferentiationbetweenthetypesofallsurfacegroupsnotrelatedtoacid/basecharacterwasobtainedusingXPSanalysis.AsindicatedbyPelsetal.,[42]aftertreatmentat9508Cthequaternary/pyridiniumandthepyridinicnitrogen,withtheformerbeingthemainnitrogenfunctionality,areexpectedtobepresentincarbonmatrices.

XPSresultsforthenitrogenandoxygen

functionalitiesforS,S-O,S-UO,andS-MOarepresentedinFigure2andtheirchemicalformulaswithincarbonmatricesareschematicallyshowninFigure3.Thetreatmentappliedresultedinchangesinthenumberandthekindofsurfacespecies.Foroxygen,thebindingenergiesaround531eV,532eV,and535eVrepresentC––Oquinonetypegroups(O-I),C–OHphenolgroupsand/orC–O–Cethergroups(O-II),andchemisorbedoxygen(COOHcarboxylicgroups)and/orwater(O-III),respectively.[42,43]Thenitrogenfunctionalitiesarerepresentedbypeaksat398eV,400eV,401eV,and403eV.Theseareassignedtopyridinic(N-6),pyrrolic/pyridone(N-5),quaternary(N-Q)nitrogen,and

pyridine-

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Table2.ResultsoftheBoehmtitration(numbersofsurfacegroupsinmmolgÀ1)andsurfacepHvalues.

SampleSS-OS-US-UOS-MS-MO

pH10.153.4310.209.159.989.30

Basicgroups

0.464–1.1810.8361.0370.858

Acidicgroups

0.3281.1340.0220.2840.1100.274

Allgroups0.7921.1341.2031.1201.1471.132

Figure3.Schematicmodelofnitrogen-andoxygen-containingsurfacefunctionalgroupsoncarbon.

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Table3.Relativesurfaceconcentrationsofnitrogenandoxygenspeciesobtainedby ttingtheN1sandO1scorelevelXPSspectra.

SampleSS-OS-US-UOS-MS-MO

N-5––27.019.922.224.5

N-6––48.253.544.746.1

N-Q––20.516.426.122.3

N-X––4.3010.107.047.09

O-I37.042.044.637.851.630.8

O-II59.554.847.954.151.657.9

O-III3.53.27.58.16.111.5

N-oxide(N-X),respectively.[42–45]Thecontributionofeachspeciesobtainedby ttingtheN1sandO1scorelevelspectraforallsamplesstudiedarelistedinTable3asrelativesurfaceconcentrations.ThecontentsofsurfacenitrogeninSandS-Owerebelowthedetectionlimits.

AsseenfromFigure2andTable3,oxidationresultedinanincreaseinthesurfaceaciditymanifestedbyanincreaseinthenumberofC––Ospecies.ThesecorrespondtoC––Oofthecarboxylicgroupsresponsibleforanincreaseinsurfaceacidity,asdemonstratedbywettitrationdata.Aftertreatmentwithureaormelamine,thenitrogen-containinggroupsappearedonthesurfaceandtheirrelativequantitiesdiffered.Itappearsthatureatreatmentfavoredtheformationofpyridinicnitrogen,whereasmelaminehadapositiveeffectontheformationofquaternarynitrogen.Thiscanberelatedtodifferentchemistriesofbothprecursorsandformationoflargepolymersasaresultofmelaminedecompositionbyheattreatment.[46]Whilepreoxida-tionincreasedtherelativeamountsofpyrrole/pyridoneandpyridinicnitrogeninthemelaminederivedsamples,inurea-treatedsamplesasigni cantincreasewasnoticedforpyridinicgroupsandpyridineN-oxideincomparisonwiththenon-preoxidizedsamples.Theamountofquaternarynitrogensigni cantlydecreasedforbothmelamine-andurea-treatedpreoxidizedsamples.

Kelemenetal.suggestedthatthereisacorrelationbetweentheoxygencontentincarbonsandquaternarynitrogen.[47]Thelattercanconsistofpyridinicnitrogenassociatedwithoxygenfromhydroxylorcarbonylgroupslocatedinclosevicinity,whichresultsinapositivechargeonthenitrogen.[42]Inourcase,thecontentofpyridinicnitrogenafteroxidation,especiallyintheurea-treatedsampleS-UO,increasedabout10%.Moreover,melaminetreatmentalsofavorsthiskindofsurfacechemistry.ItisinterestingtonotethatthecontentofN-5nitrogendecreasedbyalmost30%afterpreoxidationandureatreatment,whereasinthemelamine-treatedsampleasmallincreasewasnoticed.This

mightbelinkedtothedifferencesinthechemistriesoftheinteractionsofprecursorswithoxygen-containingsurfacefunctionalgroupsandinthemechanismsoftheirpyrolysis.[46,48]Onecanhypothesizethatless ve-member-ringsnitrogengroupsinS-UOwereduetospeci cinteractionsofsmallmoleculesofureawithoxygengroupsviahydrogenbonding,limitingtheurea–ureainteractionsowingtothespatialdistance.Theseinteractionscouldbealsoresponsibleforhighcontentsofpyridine-N-oxidesandpyridinicnitrogeninthesecarbons.Ifthismechanismindeedexists,thenitrogencontainingcentersshouldbedispersedmoreonthesurfaceofurea-modi edcarbonsthanthoseresultingfrominteractionwithmelamine.Thepatchesofnitrogenfunctionalitiesareexpectedtoexistinmelamine-treatedsamplesowingtothepossibilityofmelaminepolymerization.[46]

Thesurfacechemistry,althoughimportant,canaffecttheperformanceofamaterialinachemistrysensitive/in uencedprocessonlyifthatchemistryisaccessibleforthespeciestakingpartintheprocess.Thisaccessibilityisusuallylinkedtotheporosityofthematerialanditssurfacearea.TheporousstructureofourmaterialswasevaluatedusingbothN2andCO2adsorptionmeasurements.ThecalculatedsurfaceareasandporevolumesaresummarizedinTable4.ThedataderivedfromN2isothermsshowedthatoxidationdecreasedthesurfaceareaandvolumeofthemicroporesby10%.Thisislikelyduetothedestructionofporewallsandblockingofsomeporesbyfunctionalgroups.Fortheas-receivedsamples,adecreaseinstructuralparameterswasalsonoticedafterimpregnationandthermaltreatment;about10%forS-Uand20%forS-M.Thediscrepanciesarerelatedtotheabove-mentioneddifferencesinthemechanismsofnitrogen-precursorcarbonization.Itseemsthatthebulkypolymersformedfrommelamine[46]blockedthemicroporestoagreaterextentthanthoseformedfromurea.Thevariationsbetweentheinitialandthetreatedsamplesarelessobviousforsamplesobtainedfromtheoxidizedcarbon.ForboththemelamineS-MOandureaS-UOtreatedsamples,theporosityparametersincreasedslightlywiththemostsigni canteffectonthevolumeofverysmallporeswithsizes

.Thiscanbeexplainedonthebasisofdecompositionoflessthan5A

oxygen-containingsurfacefunctionalgroupsinlargerporesuponheattreatment.[49,50]InthecaseoftheoxidizedsampleS-O,nitrogencouldnotentertheporesblockedbyfunctionalgroups.Itisinterestingthatwithoutpreoxidationtheparametersoftheporousstructureofthemelaminemodi edsampleS-Mwerethesmallest.Thiscanbeexplainedbythenonspeci cadsorptionofthemelaminelayeronthesurfaceoftheinitialsample,whichcouldleadtolargerpolymersandthustonitrogen-containingcarbondepositsblockingthesmallpores.Ontheotherhand,ontheoxidizedsample,melaminecouldbeadsorbedspeci callyvia

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Table4.StructuralparameterscalculatedfromnitrogenadsorptionisothermsandCO2adsorptionisotherms(lastthreecolumns).

SBET[m2gS1]898804808844732829

V<5A

[cm3gS1]

V<10A

[cm3gS1]

SampleSS-OS-US-UOS-MS-MO

Vmic[cm3gS1]0.4540.4100.4060.4230.3720.413

Vmeso[cm3gS1]0.0290.0130.0260.0270.0140.024

Vt

[cm3gS1]0.4830.4230.4320.4500.3860.437

DDA ][A15.715.915.815.615.615.6

VCO2[cm3gS1]0.2500.2940.2410.2760.2520.290

SCO2[m2gS1]655770631722660760

L ][A5.86.35.46.25.26.2

0.01800.00240.00460.02620.00730.03040.2770.2510.2480.2580.2390.246

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hydrogenbondingandthusthepresenceofoxygengroupscouldhinderitsfavorablelayerformationonthesurface.ThiswouldexplainthehigherdispersionofnitrogenfunctionalitiesinthepreoxidizedsampleS-MO.

Thevolumesofverysmallpores(ultramicropores)calculatedfromCO2adsorption(VCO2)followedthetrendinthevolumeof

)whenmicroporescalculatedfromnitrogenadsorption(V<10A

thehighestvolumesofultramicroporeswerefoundinthepreoxidizedS-MOandS-UO.

Thepore-sizedistributionspresentedinFigure4showsimilaritiesintheporestructureofallcarbons,whichwerepredominantlymicroporous.Treatmentwithureaormelamineshiftedthedistributiontosmallerporeswithavisibledecreasein

.Thisismostlikelythevolumeofporeswithsizesofabout10A

theresultofincorporationofnitrogen-containingfunctionalgroups.Ontheotherhand,inthecaseofthesampleswithout

preoxidation,thevolumesinporesofsizesbetween10–30A

decreased,especiallyforS-M,which,ashypothesizedabove,istheresultofdepositionofcarbonaceousspeciesfromcarboniza-tionofmelaminepolymersonthesurfaceleadingtoadecreaseinthesizesofporesavailabletothenitrogenmolecules.

Thespeci cgravimetriccapacitancesin1MH2SO4perelectrodecalculatedfromthegalvanostaticdischargecurvesasafunctionofappliedcurrentloadsareshowninFigure5.Thecapacitanceretentionratiosbetween1AgÀ1and0.05AgÀ1currentloadsarealsoincluded.ThehighestcapacitancevalueswereobtainedfromS-O,whichwasanunexpectedresulttakingintoaccounttheveryacidicsurfaceandlowpHofthissample.Inourpreviouswork,wefoundthatawood-basedcarbon,preoxidizedunderthesameconditions,showedpoorcapacitivebehaviorandaverylargeelectricalresistance.[33]Itisbelievedthatthisdiscrepancyisduetodifferentlevelofaromatizationofwood-basedcarbonandcoconut-basedcarbon,withtheformerhavinglowerlevelofaromatizationthatcausedthemoreextensiveoxidizationofwood-basedcarbons.[41]XPSrevealedthepresence

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Figure5.Speci cgravimetriccapacitance(inFgÀ1ofoneelectrode)asafunctionofappliedcurrentload.Capacitanceretentionswerecalculatedbydividingthecapacitancevaluesat1AgÀ1byvaluesat0.05AgÀ1.

Figure4.Poresizedistributioncalculatedfromnitrogenadsorptioniso-thermsusingthedensityfunctionaltheory(DFT)

method.

ofnitricoxidessuchasnitro-typecomplexesNOÀ2andnitrates[33]À

NO3inthewood-basedcarbon,blockingthecarbonsurfaceinachargeprocessandthusresultinginapoorcapacitiveperformance.Thesurfaceoxygencontentwasashighas15%.Ontheotherhand,coconut-basedS-Owasenriched,withonly8.65%ofsurfaceoxygen,approximately3%morecomparedtostartingS.XPSrevealednonitrotypecomplexesinS-O.Incomparisonwiththeurea-andmelamine-treatedsamplesinvestigatedhere,theporosityparametersofS-OsuchasSBETandtotalporevolumesfromnitrogenadsorptionweresimilar(Table4).However,owingtothehighsurfaceoxygencontent(Table1)andthenatureoftreatmentsapplied,pseudocapacitancewasexpectedtobethemostpronouncedinS-O.Thisissuewillbediscussedlaterinthetextindetail.

Regardingthecapacitanceretentionsat1AgÀ1,thenon-preoxidizedS-MandS-UretainedmorecapacitancethantheirpreoxidizedcounterpartsS-MOandS-UO,particularlytheS-M(89.3%retention)comparedwith87.1%forS-MO.GoodcapacitanceretentionofS-McanbeexplainedonthebasisoftheXPSanalysis.TheN1speakanalysisofS-MrevealedthehighestcontributionofquaternarynitrogenN-Qamongthetreatedsamples(26.1%)andrelativelyhigh(7.04%)amountsofpyridinic-N-oxideN-X(Table3).Thesenitrogenentitiesarepositivelycharged[42]andasweunderstoodandpreviouslyreported[33]thispositivechargehelpinanelectrontransferthroughthecarbon,improvingthecapacitiveperformanceathighcurrentloads.Theresultsobtainedhereareconsistentwiththismechanism,asthepre-oxidizedandurea/melaminetreatedcarbonscontainedsmalleramountsofN-Qthanthenon-oxidizedcarbonsand,consequently,maintainedlesscapacitanceat1AgÀ1.S-MwiththehighesttotalcontributionofN-QandN-Xprovidedthebestcapacitanceretention.

Thecombinedeffectoftheporousstructureandthesurfacefunctionalitiesonthecapacitiveperformancearediscussedinthefollowingparagraphs.Firstly,tocarryouttheanalyses,thespeci ccapacitancespersurfaceareacalculatedfromCO2adsorption(CsaCO2inFmÀ2obtainedbydividingthespeci cgravimetriccapacitance(inFgÀ1)bySCO2(inm2gÀ1))wereplottedagainstthe

)calculatedfromCO2adsorptionaverageporediameters(LinA

(Fig.6).Theseplotsallowedustoinvestigatetheeffectofultramicroporesonthedouble-layerformationat

different

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normalizedcapacitanceinultramicroporeswas

(S-Msample).Thisobtainedataporesizeof5.2A

isaninterestingobservationconsistentwiththe ndingsofChmiolaandcoworkers[14–16]regard-ingthemostef cientionadsorptioninporesmatchingthesizeofsolvated(hydrated)ions.Theelectrolyteusedinourstudyis1MH2SO4andhencethecorrespondingadsorbedionswere

À

bivalentsulphateionSO2andhydroniumion4

þ

H3O.Eventhoughthenegativelychargedionsareadsorbedinabarestateinmostcases,thesulphateanionundergoeshydrationandthemostlikelynumberofhydrateswasestimatedat12.16

À[51]

moleculesofwaterperoneSO2Thecorre-4.

2À

spondingsizesofSO4(H2O)12andH3Oþwere

and4.2A ,respectively.[51,52]reportedtobe5.33A

perfectlymatchestheThus,theporesizeof5.2A

sizeofnegativelychargedsulphateionsandisnottoobigto‘‘waste’’thespaceduringaccommoda-tionofhydroniumions.Lotaetal.recently

À

reportedontheunchangedmobilityofSO24ions

,[53]inconstrainedporeswithadiameterof6A

whichisinagreementwiththisresult.Basedontheseobservations,itcanbeconcludedthatthemostef cientdouble-layerformationinultrami- ).croporesisinS-M(meanporediameterof5.2A

Inaddition,aggravationofcorrelationswithincreaseincurrentloadsupportsourpreviousconclusion[33]aboutthedominantroleofadouble-layercapacitanceatlowercurrentloadsandmaincontributionofpseudocapacitanceathigherloads.Inordertounderstandthecombinedeffectofheteroatomsonthecapacitiveperformance,het-eroatomindiceswerecalculatedasdescribedinourpreviouswork.[33]Owingtothehighercontentofsurfaceoxygencomparedtonitrogen,nitrogen(I-N)aswellasoxygenindices(I-O)werecalculatedbymultiplyingthepercentcontributionofeachnitrogen(oxygen)speciesfromXPSdeconvolutionanalyseswiththetotalcontentofsurfacenitrogen

(CsaCOinFmÀ2)asa(oxygen)anddividedbydifferentialsurfaceareaFigure6.Speci ccapacitancepersurfaceareaofporeslessthan10A2

)atthecurrentloadsof0.05AgÀ1,0.1AgÀ1,0.5AgÀ1DS.DSrepresentsadifferencebetweentheSfunctionofaverageporesize(LinABET

and1AgÀ1.SurfaceareasandmeanporediameterswerecalculatedfromCO2adsorption.andtheSCOanditcorrespondstothesurfacearea

2

Thecorrelationsonthelefthandsideincludeallsampleswhereastherighthandsidedofporesbiggerthan10A ,inwhichthesurfacegraphsincludeallbutS-Osample.

functionalitiesareexpectedtobelocalized.OurargumentforintroducingtheDSisbasedonthefactthatverysimilarporevolumeswereobtained )andCO2(VCO)adsorptions.ThereforetheSCOfromN2(V<10Acurrentloads,assumingthattheelectrochemicallyactivesurface22

inwhichfunctionalitieswerenotlocatedintheseporesduetostericalrepresentedthesurfaceareaofporessmallerthan10A

factors.ThegraphsonthelefthandsideinFigure6includealltheelectrosorption(double-layerformation)ofionstookplaceassamples.WeobservedthateventhoughthedecreasingtrendofexplainedaboveandproveninFigure6.CsaCO2withanincreaseintheporediameterwasclear,theFigure7representstherelationshipbetweentheCDS[inFmÀ2]correlationswerepoor.Itisnoticeablethatonepointliesoutofandthesumofnitrogenandoxygenindices.TheCDSlinearly

increasedwiththeincreaseofNandOindicescon rmingthelinearityinallplotsandthispointcorrespondstotheS-Osample.

WhenS-Owasomitted,thecorrelationsgreatlyimprove,uptopseudocapacitanceduetobothnitrogenandoxygenfunctional-0.97,asshowninthegraphsontherighthandsideinFigure6.ities.Excellentcorrelationssupportourtheoryofthepresenceof

electrochemicallyactivefunctionalgroupsintheporesbiggerTheexplanationcanbefoundintheoxygencontentofS-Oand

.Theseresultsfurthercon rmthatinthemicroporousconsequentsigni cantpseudocapacitivecontributiontothethan10A

overallcapacitance.Thecorrelationsclearlyshowthatthehighestcarbonstheeffectsofnitrogen-andoxygen-containinggroups

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Asanextstepofouranalyses,theindicesofthepositivelychargedN-QandN-XandtheireffectsonCDSwereinvestigated.ThecorrelationsshowninFigure10areingoodagreementwithourpreviouswork,[33]con- rmingtheirbene cialeffectonthecapaci-tance,particularlyathighercurrentloads.Thecorrelationcoef cientsincreasedwiththeincreasedcurrentloads,furthercorroboratingthatthepositivechargeonN-QandN-Xhelpsinelectrontransportthroughthecarbon,particularlyathighcurrentloads.Therefore,thegoodcapacitanceretentionofS-Mdis-cussedabovecanbeattributedtolargenumbersoftheseindices.Fastelectrontransferisacrucialrequirementforasuper-capacitorasitshoulddeliverthesameenergyatanyoperationalload.Realsupercapacitors (CDSinFmÀ2)asafunctionofthesumFigure7.Speci ccapacitanceinporesbiggerthan10A

constructedfromporouscarbonsoftensuffer

ofnitrogenandoxygenindicesatdifferentcurrent

loads.

fromacapacitancedropcausedbyincreaseintheohmicresistanceofcarbon.Controllable

enrichmentofcarbonswithN-QandN-Xentitiesmayprovideacannotbeseparatedowingtotherelativelysmallsurfaceinlarger

solutiontothisproblem.poreswheretheyco-exist.

Cyclicvoltammetry,recordedina3-electrodecellwiththeInordertoanalyzetheelectrochemicalactivityofindividual

samecarbonmaterialasboththeworkingandthecounternitrogen/oxygenfunctionalities,theCDSwasanalyzedasa

electrodeandAg/AgClasthereferenceelectrode,wasusedinfunctionofthecombinationofpyrrolicI-N5nitrogen,pyridinic

aninvestigationofFaradaicinteractionsontheoxygen/nitrogenI-N6nitrogenandI-OIquinoneoxygen(Fig.8).Thesefunction-groups.CorrespondingcyclicvoltammogramsofS,S-O,alitieswereselectedbasedonthewell-documentedcaseof

[28,31]

andS-MarepresentedinFigure11.Itisclearthatthepseudocapacitanceonquinoneoxygengroupsaswellas

pseudocapacitivepeaksat0.4Vre ectingtoreversiblereduc-proposedFaradaicreactionsonnitrogenlocatedattheedgesof

[33,22–24]

tion/oxidationofquinonetohydroquinonewerethemostgraphenelayers,i.e.,pyridinicandpyrrolicgroups.Itis

pronouncedinS-O.Thisresultisingoodagreementwiththeclearthatallthreefunctionalitiescontributedtothepseudo-surfacechemistryofthissample,asdiscussedabove,andcapacitivebehaviorsinceCDSlinearlyincreasedwithanincrease

supportsthedistinctpropertiesofS-OshowninFigure6.Theinthesumofindices.Ontheotherhand,whentheI-OIIindices

redoxhumpsatthesamepotentialcanbeobservedinthecycliccorrespondingtophenolorethergroupswereincluded(Fig.9),

voltammogramoftheinitialScarbon.ThisalsocoincideswithcorrelationdegreebetweentheCDSandsumofindicesdecreased.

theXPSstudy,whichrevealedasigni cantamountofsurfaceThisresultprovesthatphenolandethergroupsarenot

oxygen(5.66%).Incontrast,thecyclicvoltammogramofelectrochemicallyactiveinanacidicelectrolyte,aspreviously

[31,54,55]

melamine-modi edS-Mwasabsentofredoxpeaksinspiteofsuggestedbyseveralresearchgroups.

unquestionablepseudocapacitivecontributionfromnitrogengroups.Similarresultswereobservedinlow-surface-areacarbonswithlargespeci ccapacitanceduetohighnitrogencontent[25,57]andinpreviouslyreportedwood-basedcarbonstreatedatthesameconditionsasappliedhere.[33]Thisveri putationalchemistrymayprovideanswerstothisphenomenon.

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3.Conclusions

(CDSinFmÀ2)asafunctionofthesumFigure8.Speci ccapacitanceinporesbiggerthan10A

ofI-N5,I-N6,andI-OI.

Theresultsofournovelapproachleadingtounderstandingthecombinedeffectofpseu-docapacitancefromnitrogenandoxygensur-facefunctionalgroupsonthecapacitance

of

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microporouscarbonsinacidicelectrolytehavebeenpresented.Coconut-shell-basedcarbonS208wastreatedbyureaormelamineinordertointroducenitrogensurfacefunctionalgroups,anditwasfoundthatthesurfacechemistryoftheresultingcarbonswasaffectedbythetypeofnitrogenprecursorandthespeci cgroupspresentonthesurfacebeforethenitrogen-incorporationtreatment.Theelectrochemicalperformancesoftheureaandmelaminemodi edcarbonsin1MH2SO4wereanalyzed,takingintoconsidera-tiontheporousstructure,surfacechemistry,andlocationofsurfacefunctionalgroups.We

beingtheobservedporesbetween5and6A

mosteffectiveinadouble-layerformation,whichcorrelateswellwiththesizeofhydrated

(CDSinFmÀ2)asafunctionofthesumions.Pyridinicnitrogen,pyrrolicnitrogenandFigure9.Speci ccapacitanceinporesbiggerthan10A

quinoneoxygen,presumablylocatedinporesofI-N5,I-N6,I-OI,and

I-OII.

biggerthan10A,werecon rmedtohavethemostpronouncedin uenceonthecapacitanceduetotheirpseudocapacitivecontributions.Quaternaryandpyridinic-N-oxidesshowedenhancingeffectsonthecapacitanceduetotheirpositivechargeandthusanimprovedelectrontransfer,particularlyathighercurrentloadswhenthedouble-layercapacitanceislesspronouncedthanthepseudocapacitance.

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4.Experimental

Materials:Coconut-shell-basedactivatedcarbon,S208(Calgoncarbon)wasusedinthisstudy.Themodi cationprocedurewithnitrogen-containingureaandmelaminewasthesameasdescribedpre-viously[33].Beforethemodi cation,subsamplesofcarbonswereoxidizedwith50%HNO3for4handthenwashedwithwatertoremoveexcessacidandwater-solubleproductsofoxidation.Tointroducethe

(CDSinFmÀ2)asafunctionofthesumnitrogengroups,theinitialcarbonsoroxidizedFigure10.Speci ccapacitanceinporesbiggerthan10A

carbons(30g)weretreatedwithureaormelamine

ofI-NQand

I-NX.suspension(20gofureaormelaminein100mLof

ethanol)andstirredatroomtemperaturefor5h.The

mixturewasthenboiledtoevaporatealcoholandthecarbonsamplesweredriedat1208C.Thesamplesimpregnatedwithureaormelaminewereheatedinnitrogenat108CminÀ1to9508C,andmaintainedatthistemperaturefor0.5h.Aftermodi cations,thesampleswerewashedwithboilingwatertoremoveanyexcessureaormelaminedecompositionproducts.TheS208sampleisreferredtoasS.Themodi edcarbonshavetheletterUorMaddedtotheirnames,representingureaormelamine,respectively.ThepreoxidizedsamplesarereferredtowiththeletterO.Thus,forexample,theS-MOisS208preoxidized,treatedwithmelamineandheatedat9508Ccarbon.

BoehmTitration:0.5gofcarbonsamplewasplacedin25mLof0.05Nsolutionofsodiumhydroxideandhydrochloricacid.Thevialsweresealedandshakenfor24h,andthen5mLofeach ltratewaspipettedandtheexcessofbaseoracidwastitratedwithHClorNaOH.Thenumbersofallacidicsites(ofvarioustypes)werecalculatedundertheassumptionthatNaOHneutralizesthecarboxyl,phenolic,andlactonicgroups[34].Thenumberofsurfacebasicsiteswascalculatedfromtheamountof

Figure11.CyclicvoltammogramsofS,S-O,andS-Mrecordedinathree-hydrochloricacidthatreactedwiththecarbon.Sincenitrogen-containing

À1

electrodecellatthescanrateof5mVs.groupscanhaveapKasimilartothosecontainingoxygen,thebasesof

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differentstrengthtodistinguishlactonic,carboxylicorphenolicgroupswerenotused.

pHoftheCarbonSurface:Carbonpowder(0.4g)wasplacedin20mLofdistilledwaterandequilibratedduringthenight.ThenthepHofthesuspensionwasmeasuredatambienttemperature.

PotentiometricTitration:PotentiometrictitrationmeasurementswereperformedwithaDMSTitrino716automatictitrator(Metrohm).TheinstrumentwassetatthemodewhentheequilibriumpHwascollected.Subsamplesofthematerialsstudied(0.100gin50mL0.01MNaNO3)wereplacedinacontainerkeptat258Candequilibratedovernightwiththeelectrolytesolution.Toeliminatethein uenceofatmosphericCO2,thesuspensionwascontinuouslysaturatedwithN2.Thesuspensionwasstirredthroughoutthemeasurements.VolumetricstandardNaOH(0.1M)wasusedasthetitrant.TheexperimentswerecarriedoutinthepHrangeof3–10.

Thesurfacepropertieswereevaluatedusingpotentiometrictitrationexperiments[35,36].ItwasassumedthatthepopulationofsitescanbedescribedbyacontinuouspKadistribution,f(pKa).Theexperimentaldatacanbetransformedintoaprotonbindingisotherm,Q,representingthetotalamountofprotonatedsites.ThepositivevaluesofQrepresentaprotonuptakeandthusabasicsurfaceandnegativeprotonrelease,whichrepresentsanacidicsurface.

EvaluationofPorosity:SorptionofnitrogenatitsboilingpointwascarriedoutusingASAP2010(Micromeritics).Beforetheexperiments,sampleswereoutgassedat1208Ctoaconstantvacuum(10À4kPa).Thesurfaceareas(BETmethod),totalporevolumes,Vt,(fromthelastpointofisothermatrelativepressureequalto0.99),volumesofmicropores,Vmic,

,V<5A ,volumesofporessmallerthanvolumesofporessmallerthan5A

,mesoporevolumes,Vmesalongwithporesizedistributions10A,V<10A

werecalculatedfromtheisotherms.Thelastfourquantitieswerecalculatedusingdensityfunctionaltheory(DFT)[37,38].TheaveragesizeofmicroporesfromadsorptionofnitrogenwascalculatedusingtheDubinin–Astakhovapproach(DDA)[39].

Moreover,toensurethattheprobegasenteredthesmallporeswithoutanykineticlimitation,theCO2adsorptionisothermsweremeasuredat08Candusedtocalculatethevolumeofmicropores(VDR)usingtheDubinin–Radushkevichapproach[39]andthesurfaceinmicropores(SCO2).TheDubinin–Astakhov(DA)approachwasusedtocalculatetheaveragesizeofpores,L[39].LandDDAforthespeci ccarbonswereexpectedtodifferduetothefactthatCO2isabletoadsorbonlyinultramicroporesattheexperimentalconditions,whichresultsinsmallercalculatedvaluesofLthanthatofDDA.

CHON:Thebulkcontentofcarbon,hydrogen,andnitrogenwasmeasuredinthecommercialSchwarzkopflaboratory,NewYork,NY.Theanalysiswasbasedonthermalconductivitydetectionformeasuringcarbon,hydrogen,andnitrogen,aftercombustionandreduction.Acetanilidewasusedasastandard.Oxygencontentwascalculatedasadifferencebetween100%andthesumofCþHþN,assumingthatashisnotpresent.Thisassumptionresultsintheapproximatedvaluesoftheoxygencontent.

XPS:TheXPSmeasurementswereperformedonESCALAB220i-XL(VGScienti c,UK)usingmonochromatedAlKaexcitationsource.Thesurveyandhigh-resolutionspectrawerecollectedwith100eVand20eVpassenergy,respectively.ThequantitativeanalysiswasperformedwithCASAXPSsoftwareafterShirleybackgroundsubtraction.Thebestpeak tswereobtainedusingmixed30%Gaussian-LorentzianlineshapesatthesameFWHMforall ttedpeaks.

ElectrochemicalMeasurements:Thecapacitiveperformanceofallcarbonsampleswasinvestigatedin1MH2SO4usingasandwich-typetwo-electrodetestingcell.Theworkingelectrodewaspreparedbymixingthecarbonwithpolyvinylidine uoride(PVDF)andcommercialMitsubishicarbonblack(8:1:1)inN-methyl2-pyrrolidone(NMP)toformahomogeneousslurry.Theslurrywascoatedonatitaniumfoilwithatotalsurfaceareaof1cm2ofactivematerial.Theelectrodesweredriedat1508Cfor1handthenweighted.Thetotalmasswasbetween4and8mgandtwoelectrodeswithidenticalorverycloseweightswereselectedforthemeasurements.Glassy berpaperwasusedtoseparatetheelectrodes.The

electrolytewasaddedtothecellundervacuumtoreduceaircontaminationandimprovewettabilityoftheelectrodes.

TheelectrochemicalinvestigationswerecarriedoutusingSolartron1480Multistatandthecapacitiveperformanceswereevaluatedbythemeansofgalvanostaticcharge-dischargecycleswithcurrentloadsbetween0.05AgÀ1and1AgÀ1atapotentialwindowof1V.Cyclicvoltammetryatascanrateof5mVsÀ1wasalsorecordedinathree-electrodecelllayoutusingthesamecellandAg/AgClasthereferenceelectrode.

Thespeci cgravimetriccapacitancesCg(inFgÀ1)persingleelectrodewerecalculatedfromthedischargecurves.Speci ccapacitancespersurfaceareaCsa(inFmÀ2)wereobtainedbydividingtheCgwiththeSBETsurfaceareasorthesurfaceareasmeasuredbyCO2adsorption.

FULLPAPER

Acknowledgements

D.H.-J.acknowledgestheAustralianResearchCouncilCentreofExcellenceforFunctionalNanomaterialsfor nancialsupport.

Received:August21,2008Revised:October7,2008

Publishedonline:December18,2008

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