Combined Effect of Nitrogen- and Oxygen-Containing Functiona(7)

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

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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.

FULLPAPER

3.Conclusions

(CDSinFmÀ2)asafunctionofthesumFigure8.Speci ccapacitanceinporesbiggerthan10A

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

Theresultsofournovelapproachleadingtounderstandingthecombinedeffectofpseu-docapacitancefromnitrogenandoxygensur-facefunctionalgroupsonthecapacitance

of

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