Combined Effect of Nitrogen- and Oxygen-Containing Functiona(5)

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

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hydrogenbondingandthusthepresenceofoxygengroupscouldhinderitsfavorablelayerformationonthesurface.ThiswouldexplainthehigherdispersionofnitrogenfunctionalitiesinthepreoxidizedsampleS-MO.

Thevolumesofverysmallpores(ultramicropores)calculatedfromCO2adsorption(VCO2)followedthetrendinthevolumeof

)whenmicroporescalculatedfromnitrogenadsorption(V<10A

thehighestvolumesofultramicroporeswerefoundinthepreoxidizedS-MOandS-UO.

Thepore-sizedistributionspresentedinFigure4showsimilaritiesintheporestructureofallcarbons,whichwerepredominantlymicroporous.Treatmentwithureaormelamineshiftedthedistributiontosmallerporeswithavisibledecreasein

.Thisismostlikelythevolumeofporeswithsizesofabout10A

theresultofincorporationofnitrogen-containingfunctionalgroups.Ontheotherhand,inthecaseofthesampleswithout

preoxidation,thevolumesinporesofsizesbetween10–30A

decreased,especiallyforS-M,which,ashypothesizedabove,istheresultofdepositionofcarbonaceousspeciesfromcarboniza-tionofmelaminepolymersonthesurfaceleadingtoadecreaseinthesizesofporesavailabletothenitrogenmolecules.

Thespeci cgravimetriccapacitancesin1MH2SO4perelectrodecalculatedfromthegalvanostaticdischargecurvesasafunctionofappliedcurrentloadsareshowninFigure5.Thecapacitanceretentionratiosbetween1AgÀ1and0.05AgÀ1currentloadsarealsoincluded.ThehighestcapacitancevalueswereobtainedfromS-O,whichwasanunexpectedresulttakingintoaccounttheveryacidicsurfaceandlowpHofthissample.Inourpreviouswork,wefoundthatawood-basedcarbon,preoxidizedunderthesameconditions,showedpoorcapacitivebehaviorandaverylargeelectricalresistance.[33]Itisbelievedthatthisdiscrepancyisduetodifferentlevelofaromatizationofwood-basedcarbonandcoconut-basedcarbon,withtheformerhavinglowerlevelofaromatizationthatcausedthemoreextensiveoxidizationofwood-basedcarbons.[41]XPSrevealedthepresence

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Figure5.Speci cgravimetriccapacitance(inFgÀ1ofoneelectrode)asafunctionofappliedcurrentload.Capacitanceretentionswerecalculatedbydividingthecapacitancevaluesat1AgÀ1byvaluesat0.05AgÀ1.

Figure4.Poresizedistributioncalculatedfromnitrogenadsorptioniso-thermsusingthedensityfunctionaltheory(DFT)

method.

ofnitricoxidessuchasnitro-typecomplexesNOÀ2andnitrates[33]À

NO3inthewood-basedcarbon,blockingthecarbonsurfaceinachargeprocessandthusresultinginapoorcapacitiveperformance.Thesurfaceoxygencontentwasashighas15%.Ontheotherhand,coconut-basedS-Owasenriched,withonly8.65%ofsurfaceoxygen,approximately3%morecomparedtostartingS.XPSrevealednonitrotypecomplexesinS-O.Incomparisonwiththeurea-andmelamine-treatedsamplesinvestigatedhere,theporosityparametersofS-OsuchasSBETandtotalporevolumesfromnitrogenadsorptionweresimilar(Table4).However,owingtothehighsurfaceoxygencontent(Table1)andthenatureoftreatmentsapplied,pseudocapacitancewasexpectedtobethemostpronouncedinS-O.Thisissuewillbediscussedlaterinthetextindetail.

Regardingthecapacitanceretentionsat1AgÀ1,thenon-preoxidizedS-MandS-UretainedmorecapacitancethantheirpreoxidizedcounterpartsS-MOandS-UO,particularlytheS-M(89.3%retention)comparedwith87.1%forS-MO.GoodcapacitanceretentionofS-McanbeexplainedonthebasisoftheXPSanalysis.TheN1speakanalysisofS-MrevealedthehighestcontributionofquaternarynitrogenN-Qamongthetreatedsamples(26.1%)andrelativelyhigh(7.04%)amountsofpyridinic-N-oxideN-X(Table3).Thesenitrogenentitiesarepositivelycharged[42]andasweunderstoodandpreviouslyreported[33]thispositivechargehelpinanelectrontransferthroughthecarbon,improvingthecapacitiveperformanceathighcurrentloads.Theresultsobtainedhereareconsistentwiththismechanism,asthepre-oxidizedandurea/melaminetreatedcarbonscontainedsmalleramountsofN-Qthanthenon-oxidizedcarbonsand,consequently,maintainedlesscapacitanceat1AgÀ1.S-MwiththehighesttotalcontributionofN-QandN-Xprovidedthebestcapacitanceretention.

Thecombinedeffectoftheporousstructureandthesurfacefunctionalitiesonthecapacitiveperformancearediscussedinthefollowingparagraphs.Firstly,tocarryouttheanalyses,thespeci ccapacitancespersurfaceareacalculatedfromCO2adsorption(CsaCO2inFmÀ2obtainedbydividingthespeci cgravimetriccapacitance(inFgÀ1)bySCO2(inm2gÀ1))wereplottedagainstthe

)calculatedfromCO2adsorptionaverageporediameters(LinA

(Fig.6).Theseplotsallowedustoinvestigatetheeffectofultramicroporesonthedouble-layerformationat

different

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