Fe+TsOH催化剂应用碱性介质催化氧还原的电催化活性

[Article]

www.whxb.pku.edu.cn物理化学学报(WuliHuaxueXuebao)

ActaPhys.-Chim.Sin.2014,30(2),318-324February

Received:October14,2013;Revised:December11,2013;PublishedonWeb:December12,2013.∗Correspondingauthor.Email:pangs@tsinghua.edu.cn;Tel:+86-755-26957243.TheprojectwassupportedbytheNationalNaturalScienceFoundationofChina(91223202),InternationalScience&TechnologyCooperationProgramofChina(2011DFA73410),TsinghuaUniversityInitiativeScientificResearchProgram,China(20101081907),andNationalKeyBasicResearchProgramofChina(973)(2011CB013102).国家自然科学基金(91223202),国家国际科技合作专项项目(2011DFA73410),清华大学自主科研计划(20101081907)及国家重点基础研究发展规划项目(973)(2011CB013102)资助©EditorialofficeofActaPhysico-Chimica

Sinicadoi:10.3866/PKU.WHXB201312121

Fe-N/C-TsOH催化剂应用碱性介质催化氧还原的电催化活性

徐莉1,2潘国顺1,2,*梁晓璐1,2罗桂海1,2邹春莉1,2罗海梅1,2

(1清华大学摩擦学国家重点实验室,北京100084;2深圳清华大学研究院深圳市微纳制造重点实验室,广东深圳518057)

摘要:通过溶剂分散热处理方法制备了一种吡咯和对甲苯磺酸(TsOH)共同修饰的碳载非贵金属复合催化剂

(Fe-N/C-TsOH),并采用扫描电子显微镜(SEM)、X射线衍射(XRD)和X射线光电子能谱(XPS)对催化剂的形貌

和组成成分进行表征.借助循环伏安法(CV)和旋转圆盘技术研究了TsOH对催化剂在0.1mol∙L-1KOH介质中

催化氧还原性能的影响.结果表明:TsOH的存在对催化剂催化氧还原反应(ORR)的活性影响很大.以其制备的

气体扩散电极在碱性电解质溶液中催化氧还原过程时转移的电子数为3.899,远比不含TsOH修饰的催化剂催

化氧还原的电子数(3.098)高.此外,研究发现600°C热处理过的Fe-N/C-TsOH催化剂表现出最佳的氧还原催

化性能.相比未经热处理过的Fe-N/C-TsOH催化剂,起峰电位和-1.5mA∙cm-2电流密度对应的电压分别向正

方向移动30和170mV.XPS研究结果表明吡咯氮是催化剂主要活性中心,提供氧还原活性位,而TsOH加入

形成的C―Sn―C和―SOn―有利于催化剂催化氧还原活性的提高,从而使该催化剂对氧还原表现出很好的电

催化性能和选择性.

关键词:非贵金属催化剂;双杂化;热处理;碱性燃料电池;氧还原反应

中图分类号:O646

ElectrocatalyticActivityofFe-N/C-TsOHCatalystfortheOxygen

ReductionReactioninAlkalineMedia

XULi1,2PANGuo-Shun1,2,*LIANGXiao-Lu1,2LUOGui-Hai1,2

ZOUChun-Li1,2LUOHai-Mei1,2

(1TheStateKeyLaboratoryofTribology,TsinghuaUniversity,Beijing100084,P.R.China;2ShenzhenKeyLaboratoryofMicro/

nanoManufacturing,ResearchInstituteofTsinghuaUniversityinShenzhen,Shenzhen518057,GuangdongProvince,P.R.China)

Abstract:Pyrolyzedcarbonsupportedferrumpolypyrrole(Fe-N/C)catalystsweresynthesizedwithand

withoutthedopantp-toluenesulfonicacid(TsOH)throughasolvent-grindingmethodfollowedbyheat-

treatmentatthedesiretemperature.Boththecatalystswerecharacterizedusingelectrochemical

techniques,suchascyclicvoltammetry(CV),aswellastherotatingdiskelectrode(RDE)technique.Itwas

foundthatthecatalystsdopedwithTsOHshowedsignificantlybetteroxygenreductionreaction(ORR)

activitythantheundopedcatalysts.TheaverageelectrontransfernumbersforthecatalyzedORRwere

3.899and3.098fortheTsOH-dopedandundopedcatalysts,respectively.Thermaltreatmentwasfoundto

beanecessarystepforcatalystactivityimprovement.Thecatalystpyrolyzedat600°Cshowedthebest

ORRactivity:theonsetpotentialandthepotentialatthecurrentdensityof-1.5mA∙cm-2fortheTsOH-

dopedcatalystwere30and170mVmorepositivethanthosefortheun-pyrolyzedTsOH-dopedcatalyst,

respectively.ToclarifytheeffectsofTsOHdopingandpyrolyzation,scanningelectronmicroscopy(SEM),

X-raydiffraction(XRD),andX-rayphotoelectronspectroscopy(XPS)wereusedtoanalyzethemorphology,318XULietal.:ElectrocatalyticActivityofFe-N/C-TsOHCatalystfortheOxygenReductionReactioninAlkalineMediaNo.2

structure,andcompositionofthecatalysts.TheXPSresultssuggestthatthepyrrolic-Ngroupsarethe

mostactivesitesandsulfurspeciesarestructurallyboundtocarbonintheformofC―Sn―Cand

oxidized―SOn―bonds,whichisanadditionalbeneficialfactorfortheORR.

KeyWords:Non-noblemetalcatalyst;Dual-dopant;Heat-treatment;Alkalinefuelcell;

Oxygenreductionreaction

1Introduction

Inresponsetoincreasingawarenessofenvironmentalpollu-

tionandlimitingenergysource,greatefforthasbeenmade

worldwidetogeneratepowerinmoreefficientandenviron-

mentalfriendlyways.Fuelcellshaveattractedsignificantat-

tentionasapromisingtechnologyduetotheiradvantagessuch

ashighenergydensity,highpowerdensity,andhighenergy

conversionefficiency,aswellastheirloworzeroemission.1

However,therearetwofundamentalcatalyticchallengesre-

mainedatourcurrenttechnology,includingprohibitivecost

andinadequatedurability,hamperedthecommercializationof

fuelcells.Withrespecttothehighcost,Pt-basedcatalysts

whichhavebeenregardedasthemostelectrocatalystforoxy-

genreductionreaction(ORR)contributesover50%ofthetotal

costofafuelcellstack.2Onesolutiontoovercomethispredica-

mentistoreducethePtcontentbyafactoroftenbyreplacing

thePt-basedcatalystswithnon-preciousmetalcatalystsatthe

oxygen-reductioncathode.Fe-andCo-basedelectrocatalysts

(M-Nx/C)forthisreasonhavebeendevelopedformorethan50

years,buttheywereinsufficientlyactiveforthehighefficien-

cyandpowerdensityneededforapplicationssuchasstation-

ary,portable,andautomobilepowersupplies.3,4However,sev-

eralbreakthroughsoccurredhaveimprovedtheactivityanddu-

rabilityofthosekindsofcatalysts,whichcannowbetreatedas

potentialcompetitorstoPt-basedcatalysts.

Inaddition,ithasbeenrecognizedthatthenitrogensources

inthecatalystprecursorsduringtheM-Nx/Csynthesisplaya

crucialroleinamelioratingORRactivityaswellasstability.

Andthisisthereasonwhyseveraldifferenttypesofnitrogen-

containingmacrocycles,inorganicsalts,andorganometallic

compoundshavebeenemployedastheprecursorstoformmetal-

nitrogencomplexes.5-11

Polypyrrole(PPy),aconductingpolymerwithmanypyrrole-

typeNatoms,alteringsurface,andeasypreparationanddop-

ing,hasbeenwidelyusedforthesynthesisofM-Nx/Csinceit

wasfirstinvestigatedbyYuasaetal.12in2005.Yuasaetal.

foundthatCo-PPy/CORRactivityenhancedafterbeingpyro-

lyzedforthecobaltsiteformingfourcoordinatebondswith

thenitrogenofthePPyrings(Co-N4).BashyamandZelenary3

depositedPPyoncarbonblacktoformacatalystsupport(PPy/

C)viaanoxidativepolymerizationprocessusinghydrogenper-

oxide.Afterimpregnatingcobaltions,acarbon-supportedco-

baltcatalyst(Co-PPy/C)wasgenerated.Researchshowedthat

sodiumborohydrideasreductiveagentcouldimprovethecata-

lystactivityandstabilityandCo-N2maybeactivesite.Furthermore,thermal-treatmenthasbeenrecognizedasacru-

cialroleandsometimesnecessarysteptofurtherimprovethe

activityandstability.Althoughtheheat-treatmenteffectoncat-

alystshasbeenwelldocumented,themechanismsofthecata-

lystreactionduringtheheat-treatmentprocessandtheresult-

ingimprovementinactivityarecomplicatedandnotfullyun-

derstood.Reviewingmanypapers,thermalactivationhassig-

nificantimpactonthemetalparticlesizeandsizedistribution,

particlesurfacemorphology,andmetaldispersiononthesup-

portforsuchcatalyst.Otherbenefitsofheattreatmentaretore-

moveanyundesirableimpuritiesandallowauniformdisper-

sionandstabledistributionofthetransitionmetalonthesup-

port,and,therefore,toimprovetheelectrocatalyticactivity.13

Inaddition,whenthecatalystsarepyrolyzedatadesiredhigh

temperatureinaflowinginertatmosphere(nitrogenorargon),

M-Nprecursorispartiallyorcompletelydecomposed,result-

inginacatalystwithmuchbettercatalyticactivityandstabili-

tythanauntreatedcatalyst.14

Tofurtherimprovetheactivity,dual-dopedcarbonswith

twodifferentheteroatomsbecomeonepromisingoptionfor

ORRbytakingadvantageofdifferentheteroatomsinconjugat-

edcarbonbackbonethatcancreatenewnon-electron-neutral

sites.Inparticular,thesulfur(orsulfogroup),whichhasa

closeelectronegativitytocarbon,hasbeenemployedasadual-

dopantinthepreparationofM-Nxcatalysts.Itwasthoughtthat

thesulfurgroupdopedtotheM-Nx/Cmightbehelpfulforen-

trappingMionsinanenvironmentrichinpyrrole-typenitro-

genorpyridine-typenitrogen.Forinstance,N/Sco-dopedVul-

canXC-72RhasbeendemonstratedtoshowsuperiorORRper-

formancewithexcellentactivity9anddurability.14

Basedontheaboveconception,p-toluenesulfonicacid(TsOH)

andpolypyrrolewereusedasdual-dopant(SandN)tosynthe-

sizeanovelnon-preciousmetalcatalystbasedoncarbon-sup-

portedferrum,polypyrrole,andTsOHcomplex(Fe-N/C-

TsOH).Inparticular,TsOH,anorganiccompound,isknownas

tosylgroupandnotonlywasusedastheSprecursor,butalso

usedasan“organic-soluble”acidcatalysttopromotetheoxi-

dationofpyrrole.Andacomparativestudywascarriedout

throughthesystematicanalysesofFe-Ncatalystsprepared

withoutandwithTsOHdopantbycyclicvoltammeters(CVs),

rotatingdiskelectrodetechnique(RDE)inoxygen-saturatedal-

kalinesolutions,andscanningelectronmicroscope(SEM),X-

raydiffraction(XRD),andX-rayphotoelectronspectroscopy

(XPS)areusedtocharacterizethesecatalystsintermsoftheir

structuresandcompositions.319