Combined Effect of Nitrogen- and Oxygen-Containing Functiona(2)

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

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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.

Adv.Funct.Mater.2009,19,438–447ß2009WILEY-VCHVerlagGmbH&Co.KGaA,

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