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Title:TuningElectronicPropertiesofPrussianBlueAnaloguesforEfficientWaterOxidationElectrocatalysis:ExperimentalandComputationalStudiesAuthors:ElifPnarAlsa,Eminelker,SatyaVijayaKumarNune,YavuzDede,andFerdiKaradasThismanuscripthasbeenacceptedafterpeerreviewandappearsasanAcceptedArticleonlinepriortoediting,proofing,andformalpublicationofthefinalVersionofRecord(VoR).
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Tobecitedas:Chem.
Eur.
J.
10.
1002/chem.
201704933LinktoVoR:http://dx.
doi.
org/10.
1002/chem.
201704933FULLPAPERTuningElectronicPropertiesofPrussianBlueAnaloguesforEfficientWaterOxidationElectrocatalysis:ExperimentalandComputationalStudiesElifPnarAlsa,[a]Eminelker,[d]SatyaVijayaKumarNune,[a]YavuzDede,[c]*FerdiKaradas[a],[b]*Abstract:WhileseveralPrussianblueanalogues(PBAs)havebeeninvestigatedaswateroxidationcatalysts,thefieldlacksacomprehensivestudythatfocusesonthedesignoftheidealPBAforefficientwateroxidationcatalysis.
Herein,aseriesofPBAswithdifferentcyanideprecursorswereinvestigatedtostudytheeffectofhexacyanometalgrouptotheirelectrocatalyticwateroxidationactivities.
Cyclicvoltammetric,chronoamperometric,andchronopotentiometricmeasurementsrevealthecloserelationbetweentheelectrondensityofelectroactivecobaltsitesandelectrocatalyticactivity,whichisalsoconfirmedbyInfraredandXPSstudies.
pHdependentcyclicvoltammetryandcomputationalstudieswerealsoperformedtogaininsightaboutthecatalyticmechanismandelectronicstructureofcyanide-basedsystemstoidentifypossibleintermediatesandtoassigntherate-determiningstepofthetargetprocess.
IntroductionIncreaseinenergydemandhasforwardedscientificcommunity,particularlyinthelasttwodecades,tofindalternativeenergysourcesthatwillreplacelimitedfossil-basedfuels.
[1]SincesolarenergythatutilizestheproductionofH2fromwaterhasbeenoneofthemostpromisingcandidatesamongsustainablesourcesofenergy,muchefforthasrecentlybeendevotedtoinvestigateefficientmethodstosplitwater.
[2–7]Sincewatersplittingprocessismostlylimitedbythehighoverpotentialofoxygenevolutionreaction(OER),manystudieshavebeenperformedtointroducenovelcatalyststhatoperateatlowoverpotentials.
[8]Manyinorganicsystemsincludingmetaloxides,[9–12]perovskites,[13–15]amorphousmaterials,[16]noble-metalbasedmaterials,[17,18]andmetalorganicframeworks(MOFs)[19,20]havebeeninvestigatedasWOCs.
Ofthese,cobaltoxidesstandforwardduetotheirhighcatalyticactivities.
[21,22]Despitetheirhighcatalyticactivities,cobaltoxideshavemainlytwodisadvantages;[23,24]i)lowstabilityandhightendencytodecomposeinacidicmedium,ii)difficultyincorrelationofthecatalyticactivitieswithstructureduetotheiramorphousnature.
Non-oxidematerialshavealsodrawnattentionasWOCsduetotheirfavorablecharacteristicssuchaseaseofpreparation,stabilityatawiderangeofpH,androbustnessduringcatalyticprocesses.
[25]Patzkeetal.
reportedacarbodiimide-basedmaterialthatcouldbeusedasaWOC,whichisstableinacidicandneutralmedia.
[26]Asimilarclassofmaterials,metaldicyanamides,hasalsoshowntobepromisingcandidatesforwateroxidationelectrocatalysis.
[27]Cobalthexacyanoferrates,membersofPrussianblueanalogue(PBA)family,arealsoexceptionalcandidatesforelectrocatalyticwateroxidationduetotheirhighcatalyticactivities,robustness,andstabilityatneutralpH.
[28–30]AfurtherstudybyPatzkeetal.
showedthatPBAscanalsobeusedforlightdrivenwateroxidationprocessinthepresenceof[Ru(bpy)3]2+asachromophore.
[31]Despitetheirhighturnoverfrequencies(TOFs),oneofthemaindrawbacksofcyanide-basedsystemsistheirlowconcentrationofelectroactivecobaltsites.
TheirlowconcentrationisattributedtotherelativelylargerdistancesbetweenCo(II)sites(~10)comparedtooxide-basedsystems(~3).
[28]Thisproblemhasrecentlybeenovercomebyourgroupwiththeuseofanovelpentacyanoferrate-boundpolymerasaprecursorforCo-FePBAs,whichresultedinadramaticdecreaseinthecrystallinitiesofPBAs,andthus,asignificantincreaseinthesurfaceconcentration.
[32]aln-asarsal.
approachedthesameproblembyusinganewsyntheticmethodforthepreparationofthinfilmsofPBAs,whichinvolveschemicaletchingofcobaltoxideswithahexacyanoferratesolutiontoformaninsituPBAfilm.
ThisnovelmethodledtoanimpressiveimprovementonthestabilityoftheelectrodeandelectrocatalyticperformanceinawiderangeofpH.
Itexhibitsamuchloweroverpotential(510mV)toobtainacurrentdensityof1mAcm2.
[33]Inaddition,Fukuzumietal.
investigatedthephotocatalyticwateroxidationperformancesofaseriesofCo-PtPBAsinthepresenceofwell-definedRu(bpy)32+/S2O82-couple.
Thesystematicstudyperformedwith[Co(CN)6]3and[Pt(CN)6]4groupsindifferentstoichiometricratiosclearlyshowedthatnumberofactivesitesishighlydependentonthenumberofdefects.
[34,35]FukuzumiandcoworkersalsostudiedtheeffectofcountercationtothecatalyticactivityandquantumefficiencydisplayedbyCo-CoPBAsinphotocatalyticwateroxidationprocessshowingthataquantumefficiencyof200%canbeachievedwithCo-CoPBAsincorporatingcalciumionsascountercations.
[36]PreviousstudiesmentionedaboveclearlyshowthatslightmodificationsinthestructureofPBAscouldleadtoasignificantincreaseintheircatalyticactivities.
Althoughpreviousstudiestookadvantageofrichandwell-establishedcyanidechemistrynostudyhasbeenperformedtoinvestigatetheeffectof[a]E.
P.
Alsa,Dr.
S.
V.
K.
Nune,Prof.
F.
KaradasDepartmentofChemistry,BilkentUniversity,06800Ankara(Turkey)E-mail:karadas@fen.
bilkent.
edu.
tr[b]Prof.
Dr.
F.
KaradasUNAM-InstituteofMaterialsScienceandNanotechnologyBilkentUniversity,06800Ankara,(Turkey)[c]Prof.
Y.
Dede,FacultyofScience,DepartmentofChemistry,GaziUniversity06500,Ankara(Turkey)dede@gazi.
edu.
tr[d]Prof.
E.
lkerDepartmentofChemistry,FacultyofArts&Sciences,RecepTayyipErdoganUniversity,53100,Rize(Turkey)SupportinginformationforthisarticleisgivenviaalinkattheendofthedocumentFULLPAPERhexacyanometalunittotheelectronicpropertiesandcatalyticperformanceofelectroactivecobaltsites.
Herein,electrocatalyticmeasurementsonaseriesofcobalthexacyanometalates(CHCMs)incorporatingdifferentM(CN)6units(M=CoIII,CrIII,FeII/III,and)togetherwithcharacterizationstudieswereperformedtoinvestigatetheeffectofthetypeandoxidationstateofthemetalinM(CN)6unittothecatalyticactivityofPBAs.
TheeffectofhexacyanometalgrouptotheelectronicpropertiesofelectroactivecobaltsitewasfurtherexaminedwithelectronicstructurecalculationsemployingDensityFunctionalTheory(DFT).
[37,38]ResultsandDiscussionElectrochemistryAlltheelectrochemicalexperimentswereconductedwithaPBAmodifiedfluorine-dopedtinoxide(FTO)electrode.
CyclicVoltammograms(CVs)ofCo[M(CN)6](M:CoIII,CrIII,andFeII/III)weretakeninaphosphatebufferwith1MKNO3astheelectrolyteina0.
2–1.
7Vvs.
NHEpotentialrange(Figure1).
[CoII-CoIII]exhibitsaquasi-reversibleredoxcouplewithanoxidationpeakat1.
210Vandareductiononeat1.
031Vvs.
NHEthatcanbeassignedtoCo2+/Co3+redoxcouple.
AsimilarredoxcouplewasobservedalsoforotherPBAs.
Anotherpeak,atamorepositivepotential,isobservedataround1.
415Vvs.
NHEfor[CoII-CoIII],whichcanbeassignedtoCo3+/Co4+redoxprocess.
[39]Tafelplotsofeachcatalystwereobtainedbyperformingchronoamperometrymeasurementsatdifferentappliedpotentialstofurtherinvestigatetheirelectrocatalyticperformances.
Alineartrendwasobtainedbetweenthelogarithmofthesteadystatecurrentdensitiesandinanoverpotentialrangeof283–483mVwithTafelslopesin90–130mVdec1range(Figure2).
Tafelslopesobtainedwith[CoII-FeII]and[CoII-FeIII]areslightlyhigherthanthosereportedprislalan-asarsal.
[28,33]ThedifferenceismainlyattributedtodifferentpreparationmethodssincePBAmodifiedelectrodespreparedviaaninsitumethodexhibitlowerTafelslopes(~90mVdec-1)comparedtothosepreparedwithdropcasting.
[26,32]Figure1.
CyclicVoltammogramsofPBderivatives([CoII-CoIII]black,[CoII-CrIII]red,[CoII-FeIII]blue,and[CoII-FeII]greenlines)in50mMKPielectrolyteatpH7with50mvsec-1sweeprate.
Thegraylineindicatestheelectrochemicalresponseofblankelectrode.
SimilarityintheTafelslopesindicatessimilarOERmechanisms.
Accordingtochronoamperometricmeasurementsonsetoverpotentialsof283,303,323,and343mVsareobtained,respectively,for[CoII-CoIII],[CoII-CrIII],[CoII-FeIII],and[CoII-FeII],whichareinlinewithcyclicvoltammetricstudies(FigureS1).
SurfacecoverageoftheelectroactiveCo2+speciesonFTOelectrodei.
e.
,surfaceconcentration,wasdeterminedbyperformingCVsatdifferentscanrates(25–225mVsec1range)recordedinthe0.
8–1.
6Vrange.
Surfaceconcentrationofderivativeswereobtainedinthe2–5nmolcm2range,whichisingoodagreementwiththepreviouslyreportedstudies(FigureS2).
[28,32]Figure2.
TafelplotsforPBderivativesfrom1.
1to1.
4VvsNHErecordedina50mMKPibufferatPh7.
0.
Surfaceconcentrationwasusedtoassessturnoverfrequencies(TOFs)ofPBAs.
TOFsatanoverpotentialof400mVwereobtainedas5.
0*102s1,3.
0*103s1,4.
4*103s1,and5.
0*103s1for[CoII-CoIII],[CoII-FeII],[CoII-FeIII],and[CoII-CrIII](FigureS3).
AcomparisonofTOFs,thus,showsthatCo(II)sitesavailablein[CoII-CoIII]exhibitthehighestcatalyticactivity.
Chronopotentiometry(CP)hasbeenperformedtomonitortheoverpotentialrequiredtoobtainacurrentdensityof1mAcm2duringa2hexperiment.
Theoverpotentialfor[CoII-CoIII],decreasesslightlyatfirstandthenmaintainsaconstantoverpotentialwhileitincreasesgraduallyuntilstabilizationforotherPBAs.
Theobservedoverpotentialsfor1mAcm2areslightlyhigherthantheonesextractedfromTafelslopesduetotheformationofO2bubblesontheelectrodesurfaceduringthemeasurement.
CPstudiesshowthat[CoII-CoIII]exhibitsthelwsrpnialandη1mAisdeterminedtobe531,578,661,and692mVsfor[CoII-CoIII],[CoII-CrIII],[CoII-FeIII],and[CoII-FeII],respectively(Figure3).
FULLPAPERTable1.
SummaryofelectrochemicalpropertiesforPBAsFigure3.
ChronopotentiometrymeasurementofPBderivativesat1mAcm-2ina50mMKPibufferatpH7.
0Longtermchronoamperometricstudiesatanappliedpotentialof1.
4VvsNHEwereperformedtoinvestigatethestabilityofthePBAmodifiedelectrodes.
Thecurrentdensityforeachcatalystdecreasesuntilitreachesaconstantvalueaspreviouslyreportedrrpandaln-asarsal.
[28,32]Thesametrendcanbeobtainedforfourrepeatingcyclesandtheclosesimilarityofcyclicvoltammetricprofilesobtainedaftereachcycleindicatesthatcatalystsretaintheirstructureevenduringlongtermcatalyticprocesses(FigureS4).
Aninterestinganomalyisobservedonlyfor[CoII-CrIII]whereadecreaseincurrentdensityisobtainedasusualfollowedbyanabruptincreaseafteraround10hours.
AcomparisonoftheCVsobtainedbeforeandaftera24helectrolysisexperimentindicatesasignificantincreaseintheonsetoverpotentialandcatalyticcurrentdensity,whichcouldbeattributedtothedecompositionof[CoII-CrIII]toamorecatalyticallyactivespecies.
Ourcharacterizationstudies,whichwillbediscussedinthefollowingsectionalsosuggestthatthedecompositionoccursonlyduringlong-termelectrolysisstudies(longerthan10h).
Furthermore,asimilarelectrolysisstudyequippedwithanO2probehasbeenperformedwith[CoII-CoIII]toinvestigatetheoriginofcurrentdensityandFaradaicefficiency.
TheperfectmatchbetweenthetheoreticalyieldobtainedfromchronocoulometrymeasurementandtheexperimentaloneobtainedfromO2probeindicatesthattheonlyoriginofcurrentdensityiscatalyticwateroxidationtoO2evolutionprocessandtherearenocompetingredoxreactions(FigureS5).
CharacterizationStudiesAllsamplesareisostructuralwithPrussianBluecrystalstructureadoptingface-centeredcubicstructure(fcc)withFm3mspacegroupasconfirmedbypowderXRDstudies.
Thecharacteristic2ha(2θ)paksfrPrssianBlhansrdfrallofthematerials(FigureS6)andlatticeparameterwasdeterminedtobearound10foreachderivative(TableS1).
XRDanalysisingracingincidencemodewasalsoperformedonthecatalystsdepositedonFTObefore(pristine)andafter(post-catalytic)theelectrocatalyticstudiestoinvestigatethestructuralstabilityofcatalystduringelectrocatalysis.
NoadditionalpeakswereobservedintheXRDofpost-catalyticsamplesandthepeakscorrespondingtoPrussianBluetypestructureremainconfirmingthestabilityofcatalysts(Figure4).
TheatomicratioofmetalsineachcompoundwasextractedbyEDXanalysis(TableS2).
Thefollowingmolecularformulaewereobtainedbasedonstoichiometricratioofmetals:K0.
76Co2.
62[Co(CN)6]2,K0.
82Co2.
59[CrIII(CN)6]2,K0.
62Co2.
69[FeIII(CN)6]2,andK1.
40Co3.
30[FeII(CN)6]2for[CoII-CoIII],[CoII-CrIII],[CoII-FeIII],and[CoII-FeII],respectively.
Eachcompoundhassimilarpotassiumcontentinthe0.
6-0.
8range,whichresultsinanaverageof~4.
5CNgroupsperCo(II)sites.
TheremainingcoordinationsphereofCo(II)sitesareoccupiedbywatermolecules,whichplayactiveroleinwateroxidation(FigureS7).
CompoundCo2+/3+(mV)V(CN)(cm1)TOF(η=400mV)SurfaceConcentration(nmolcm2)η1mAfromTafelPlot(mV)TafelSlope(mVdec-1)η1mAfromCP(mV)ηonset(CV)[CoII-CoIII]1.
01021765.
0*1024.
1153199565283[CoII-CrIII]1.
08421735.
0*1033.
9057896598303[CoII-FeIII]1.
08421204.
4*1035.
48661127717323[CoII-FeII]0.
99520723.
0*1032.
006921211079343FULLPAPERFigure4.
GI-XRDpatternsofPBderivativesforpristine(blacklines)andpostcatalytic(redlines).
ThepeaksthatbelongtoFTOelectrodearemarkedwithtriangle()andthepeaksbelongtoPrussianBluearemarkedwithasterisk(*).
InfraredstudiesshowthatPBAsexhibitthecharacteristicbandsthatareobservedforPrussianbluetypesystems;a)asharpbandataround1610cm1andabroadoneat3200–3500cm1,whichrepresentH-OHbendingandOHstretch,respectively,b)asharppeakataround490–590cm1duetoM-Cstretch,andc)asharpstretchataround2120–2180cm1thatisattributedtoCNstretch(TableS3).
PBAsexhibithigherCNstretchingfrequenciescomparedtotheirhexacyanometalprecursors,whichconfirmthebindingofnitrogenatomsofcyanidetoCo(II)sites[28,40](FigureS8).
TheInfraredanalysiswasalsoperformedonthepost-catalyticsamples.
TheclosesimilaritybetweentheinfraredspectraofcyanidestretchesofpristineandpostcatalyticsamplessuggeststhatcatalystsM-CN-CoIItypecoordinationmodeispreservedduringelectrolysis(FigureS9).
Aslightshifttohigherfrequenciesobservedinpost-catalytic[CoII-FeII],whichwasobservedalsoinpreviousstudies,canbeattributedtothepartialoxidationofironionsfrom+2to+3duringelectrocatalysis.
XPSstudiesalsoconfirmtheremarkablestabilityofPBelectrocatalysts.
InordertoinvestigatetheoxidationstateofelectroactiveCoIIsitesinpristineandpostcatalyticelectrodesCo2psignalwasexaminedinthebindingenergyregionbetween810-775eV.
InpreviousstudiesthebindingenergyofCo2p3/2andCo2p1/2signalsforCoIIsaltshavebeenreportedas782.
28and798.
38eVs,respectively.
Forthepristinesamples,Co2p3/2andCo2p1/2signalswereobservedinthesamerange.
ThesimilaritybetweenthebindingenergiesofCo2psignalsobtainedforpristinePBAsandpreviouslyreportedCoIIsaltssuggeststhattheoxidationstateofelectroactiveCoatomsis+2(Figure5).
NosignificantchangesintheCo2p3/2andCo2p1/2signalswereobservedinthepostcatalyticsamplesindicatingthestabilityoftheCoIIsites.
Figure5.
XPSofCo2pregionforpristine(blacklines)andpostcatalytic(redlines)ofPBderivativesInadditiontoCo2p,O1ssignalswerealsoexaminedforbothpristineandpostcatalyticsamples(FigureS10).
TheO1ssignalwhosebindingenergyishigherthan530eVindicatestheabsenceofanycobaltoxidespeciesbeforeandafterelectrochemicalexperimentsevenfor[CoII-CrIII].
TheobservedvaluesaredisplayedinTableS4.
AmildnoticeablebroadeningintheO1ssignalinthepostcatalyticsamples,indicatingapartialandreversibleoxidationofelectroactiveCoIIsites.
MechanismforCatalyticWaterOxidationTheCNstretchcouldbeconsideredasthefingerprintforcyanide-basedcoordinationcompounds.
Thecomparisonoftheshiftinthecyanidestretchcanbeusednotonlytoconfirmthebridgingcyanidegroupbutalsotoevaluatetheoxidationstates,andthus,electrondensitiesofmetalions.
ConsideringthattheFULLPAPERcyanidestretchshiftstohigherfrequenciesastheoxidationstateofthemetalincreasesadirectcorrelationcanbeestablishedwiththeshiftofthecyanidestretchandtheelectrondeficiencyofCoIIcenters.
ThecomparisonofcyanidestretchesimpliesthattheelectrondensitiesofCoIIsitesinPrussianblueanaloguescanbeorderedas:[CoII-CoIII]~[CoII-CrIII][CoII-CrIII]>[CoII-FeIII]>[CoII-FeII],whichpointsoutthatCoIIsitesin[CoII-CoIII]analoguehavethelowestelectrondensitiesamongall.
Theevaluationofelectrondensitiescangiveinsightabouttheratedeterminingstep(r.
d.
s.
)inwateroxidationcatalysis.
Twostepshavengenerallybeenreportedtocompetewitheachotherasther.
d.
s.
inwateroxidationprocess;i)CoIII-OH/CoIV-O(oxo)orCoIII-OH/CoIII-O(oxyl)oxidationstepandii)thenucleophilicattackofwatertotheelectrophilicoxygenatomofoxo/oxylspeciesthatresultsinO–Obondformation.
TheincreaseintheelectrondensityofCoIIsitefacilitatestheformerstepwhileitdecreasestheelectrophilicnatureofoxo-intermediateand,thus,impedesthelatter.
TheaforementioneddiscussiononthecomparisonofelectrondensitiesofCoIIsitesinPBAsandtheirelectrocatalyticperformancesclearlyshowthat[CoII-CoIII]standsoutasthemostefficientcatalystamongPBAswhileithasCoIIsiteswiththelowestelectrondensity.
ThiscorrelationpointsoutthatthenucleophilicattackofwatertoFigure6.
FTIRspectraofPBderivativesthatshowscyanidestretches.
oxo/oxylintermediateisther.
d.
s.
ofwateroxidationprocessforPBAs.
Herein,itshouldbenotedthattheelectronicpropertiesofthecatalystswilldifferwhenapotentialisapplied.
Catalyticallyactivecobaltionswillbeintheirhigheroxidationstatesparticularlywhentheappliedpotentialisabove1VvsNHE.
Nevertheless,thedifferenceintheelectrondensityofcobaltionsshouldbepreservedgiventhatstructuralintegrityofcyanideframeworkispreservedandthatmetalioninM(CN)6buildingblockisnotoxidized.
Whilethisassumptioncanbevalidwithhexacyanometalgroupsthatcontainmetalionsintheir3+oxidationstates,Fe2+ionin[Fe(CN)6]4-groupisexpectedtobeoxidizedwhenapotentialabove1Visapplied.
[41]TheoxidationofallFe2+ionsis,however,akineticallydemandingprocesssinceitrequiresmorepotassiumionstobetransportedfromtheframeworktotheelectrolyteduetochargeneutralityand,moreimportantly,therearenotenoughpotassiumionstoproduceafully-oxidized[CoIII-FeIII]system.
Therefore,thecatalyticallyactivespeciesin[CoII-FeII]containsamixtureofFeionswithoxidationstatesof2+and3+.
ThedifferenceinthecurvaturesofthebandsassignedtoFe2+/3+andCo2+/3+redoxprocessesfor[CoII-FeII]and[CoII-FeIII]alsoindicatesdifferentkineticsforthesetwoanalogues(FigureS11).
Thelowersurfaceconcentrationandturnoverfrequencyobtainedfor[CoII-FeII]couldthenbeattributedtothedifferenceinthekineticsoftheirelectrontransferandtheirelectronicproperties.
AfurtheranalysisofthemechanismwasmadebasedonthePourbaixdiagram(Figure7),whichwasobtainedbyperformingCVsfor[CoII-CoIII]atdifferentpHs(FigureS12).
ThediagramshowsthatCo2+/Co3+redoxprocessispHdependentinthepH4–10rangewithaslopeof64mVlog[H+]-1,whichreferstoa1H+–1eprocess.
Figure7.
PourbaixDiagramof[CoII-CoIII]inKPibufferatpHsfrom2to13.
CyclicVoltammogramsthatarerecordedatthesepHsareshowninFigure(S10).
Interestingly,thehalf-potentialforthesecondredoxstepispreservedregardlessofpH(97.
0%),cobaltchloridehexahydrateCoCl2.
6H2O(Sigma-Aldrich,98.
0%),PotassiumhexacyanochromateK3[Cr(CN)6](Aldrich,99.
99%),PotassiumhexacyanoferrateK3[Fe(CN)6](Sigma-Aldrich,>97.
0%),potassiumhexacyanoferratetrihydrate,K3[Fe(CN)6].
3H2O(Sigma-Aldrich,98.
5-102%).
AllthesolutionswerepreparedwithMilliporeMilli-Qdinizdwarwiharsisiif18.
2m.
m.
ExperimentalKaCob[M(CN)6]·xH2O(M=FeII,FeIII,CoIII,andCrIII)abbreviatedthroughoutas[CoII-FeII],[CoII-FeIII],[CoII-CoIII],and[CoII-CrIII].
Inthecaseof[CoII-CoIII]anaqueoussolutionofCoCl2.
6H2O(0.
15M,20mL)wasaddeddropwisetoanaqueoussolutionofK3[Co(CN)6](0.
10M,20mL)atroomtemperature.
Themixturewaskeptunderstirringfor1hourandthenallowedtowaitovernightforprecipitation.
Thesolutionwasfilteredbyvacuumsuctionandwashedwithcopiousamountsofwatertoobtainthepinkpowder.
Thepowderwasdriedfurtherindesiccator.
Thesameprocedurewasappliedfor[CoII-FeII](darkblue),[CoII-FeIII](darkbrown),and[CoII-CrIII](paleyellow).
PreparationPBAmodifiedFTOElectrodesFTOelectrodeswereprocuredfromSigma-Aldrich(with~80%ransmian,2mmwihasrfarsisanf7.
sq-1,1x2cm).
Electrodeswerewashedbysonicationfor10minutesinbasicsoapysolution,deionizedwaterandisopropanolrespectively.
Thentheywereannealedat400oCfor30minutes.
CatalystmodifiedelectrodeswereFULLPAPERpreparedbydropcastingmethod.
Amixtureof5mgofPBAcatalyst,500μLDF,500μLwarand100μLNafinslinwrmixdandsonicatedfor30minutes.
Aftermakingastalsspnsin,50μLfiwastakenanddroppedontobycovering1cm2oftheFTOelectrode.
Electrodeswerethendriedatroomtemperaturefor10minutesfollowedby80oCfor10minutesinanoven.
Thentheywereleftindesiccatoruntilfurtheruseforelectrochemicalexperimentsandcharacterization.
ElectrochemicalMeasurementsGamryInstrumentsInterface1000Potentiostat/Galvanostatwasusedforperformingelectrochemicalmeasurements.
AconventionalthreeelectrodecellwasusedwithAg/AgCl(3.
5MKCl)asreferenceelectrode,FTOastheworkingelectrode,andPtwireascounterelectrode.
YSI5100dissolvedoxygensensingelectrodeinstrumentequippedwithadissolvedoxygenfieldprobewasusedtodeterminetheoxygenevolution.
KPibuffersolutionwaspreparedbyusingKH2PO4andK2HPO4andpHofthesolutionwasadjustedbyaddingH3PO4orKOH.
Bulkwaterelectrolysiswasperformedwithatwocompartmentcellwithseparationofaglassfrit.
TheelectrolysisandsteadystatechronoamperometryexperimentswereperformedinKPibuffersolutioncontaining1MKNO3asspprinlrl.
lrTldS220SnCmpapH/InpHmeterwasusedtodeterminethepHsofbuffersolutions.
AlloftheelectrochemicalexperimentswereperformedatroomtemperatureandunderN2atmosphere.
PhysicalMeasurementsXRDparnswrmasrdsinaPananalialX'PrPrMultipurposeX-RayDiffractometer(MPD)withCuKαX-RayRadiation(λ=1.
5418).
I-XRDpatternswererecordedbyusingaPanalyticalX'Pr3RDarialRsarhDifframr(RD)wihCKαX-rayradiation(l=1.
5418a)atanincident(w)angleof0.
58.
FTIRspectraweretakenbyusingaBrukerAlphaPlatinum-ATRSpectrometerwithwavenumberrangebetween4000-400cm1.
FEI-Quanta200FEGESEMwasusedforimagingandEDAXanalysis,at5kVbeamvoltageforimagingand30kVforEDAX.
XPSanalysiswasperformedusingThermoScientificK-AlphaX-RayPhotoelectronSpectrometersystemwithaAlKαmicrofocusedmonochromatorsourceoperatingat400mmspotsizeandhγ=14.
86.
6eVaccompaniedbyafloodgun,200eVforsurveyscanand30eVforindividualscans.
InordertoplotandanalyzetheresultsOriginPro8.
5wasused.
AcknowledgementsTheauthorsthanktheScienceandTechnologyCouncilofTurkey,TUBITAK(ProjectNo:215Z249)forthefinancialsupport.
E.
U.
thanksTUBITAKforsupport(ProjectNo:1929B011500059).
Y.
D.
thanksM.
N.
ParlarFoundation,BAGEPandTBA-EBPfrninsiarawards.
TUBITAKTRGRIDinfrastructureisgratefullyacknowledgedforHPCresources.
WealsothankProf.
Buraklgütforhishelpfuldiscussionsonelectrochemistry.
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FULLPAPEREntryfortheTableofContents(Pleasechooseonelayout)FULLPAPERTheeffectofelectroinactivehexacyanometalgrouptoelectroactiveCo(II)siteinPrussianBlueanalogueswasprobedbyexperimentalandcomputationalstudies.
E.
P.
Alsa,E.
lker,S.
V.
K.
Nune,Y.
Dede,*F.
Karadas*PageNo.
–PageNo.
Title