Combined Effect of Nitrogen- and Oxygen-Containing Functiona(6)

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

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

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