Combined Effect of Nitrogen- and Oxygen-Containing Functiona(9)

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

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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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