Combined Effect of Nitrogen- and Oxygen-Containing Functiona(8)

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

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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

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

Weinheim

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