Nanostructured platinum-free electrocatalysts in alkaline direct alcohol fuel cells PDF
Preview Nanostructured platinum-free electrocatalysts in alkaline direct alcohol fuel cells
RSC Advances REVIEW View Article Online View Journal | View Issue Nanostructured platinum-free electrocatalysts in ce. alkaline direct alcohol fuel cells: catalyst design, n 27. Lice Citethis:RSCAdv.,2016,6,89523 principles and applications 10:18:orted 017 Unp Kenneth Ikechukwu Ozoemena*abc 5/23.0 1/0on Thealkalinedirectalcoholfuelcell(ADAFC)isanenvironmentallyfriendlyelectrochemicalenergysourcethatcan n 1uti driveaplethoraofconsumerandportableelectronics.ResearchinADAFCshascontinuedtoattractmajorattention ob ded Attri duetotheirseveraladvantagesoverconventionalproton-exchangemembranefuelcells(PEMFC);theseinclude wnloamons thhigeheemrveorgluemnceetroicfeanneiorng-yedxecnhsaintiegseomfeamlcobhraonlsesco(AmEMpa)r,eedastyohhaynddrloinggeno,feliqnuhiadnaclecdohreoalcfutieolnsckoinmetpicasreodftaolchoyhdorlosgaennd, Doom oxygenreductionreactioninalkalinemedia.Furtherdevelopmentsinthisfieldaredependentonimprovingthe 16. e C performanceofnanostructuredelectrocatalystsandAEMs.Thisreviewisanoverviewofsomenotableadvances 0v mber 2Creati mtheadientienllirgeecnetndteyseigarns.aInmdpsoyrntathnetlsyi,sitopfrnoovnid-epsreacnioeuxscemlleentatlinnasingohsttrinutcotutrheedfeulnedcatrmoceanttaallysptrsinfcoirpltehsethcaatthaolldoewafnodr 8 Septeunder a aanopadreticreualacrtiocnatsaolyfstADfoArFCthse.TAhDisArFeCv?ie”,w“Wishaantaattreemthpettounfidnedrlyainnsgwperrsinctoiplqeusesthtiaotnsmsuuscthinafso“rWmhymsyhocuhlodicIeusine d on 0ensed RAcecceepivteedd95tthhJSuenpete2m0b16er2016 dcaetsaiglynsitnsgwsiuthchthaecaaptaplyrostp?r”i,aatendph“Wyshicaotcshynetmheicsaislpmroetpheordtie(ss)oforrchatigahly-sptesrufpoprmoratnscseh?o”uTldhebekncoonwslieddegreedptroovpidreepdarine ec Publishcle is li DwOwIw:.1r0sc.1.o0r3g9/a/cd6varan1c5e0s57h tvhairsioruesvioetwhecrafunelbceelalpspysliteedmns,oetleocntrlyocthoemADicAaFlCses,nsbourts,aalnsodmtoetsael–veariarlboatttheerriese)l.ectrocatalytic systems (such as s Article. This arti 1. Introduction theglobalquestforacleanandsustainableenvironment.These s e developments include the United Nations' Sustainable Devel- c c A The last ve years, more than any other in the history of opment Goal (SDG), the recent landmark agreement by 195 n pe mankind,havewitnessedthemostimportantdevelopmentsin countries to protect the environment and avoid the global O temperature rising by the ‘psychological’ 2 (cid:1)C at the Paris aEnergyMaterials,MaterialsScienceandManufacturing,CouncilforScienticand cMolecularSciencesInstitute,SchoolofChemistry,UniversityoftheWitwatersrand, IndustrialResearch(CSIR),Pretoria0001,SouthAfrica.E-mail:[email protected]. Johannesburg2050,SouthAfrica za;Tel:+27128413664 bDepartmentofChemistry,UniversityofPretoria,Pretoria0002,SouthAfrica Kenneth Ozoemena is Chief programmes spanning several areas of materials science and ResearchScientistattheCouncil engineering,fromrenewableenergytechnologies(suchasfuelcells, for Scientic and Industrial lithium-ion batteries, and supercapacitors) to smart electro- Research(CSIR),whichisSouth chemicalsensors(forhealth,environmental,andindustrialappli- Africa's premier science council. cations).ProfOzoemenahasauthored/co-authoredover140peer- HeholdsaBSc(Hons)degreein reviewed articles, 3 edited books, 10 book chapters and 7 patent Industrial Chemistry from Abia applications. He is an elected Fellow of the African Academy of State University (Nigeria), Science (FAAS), Fellow of the Royal Society of Chemistry (FRSC), double MSc degrees (Chemistry and currently holds extraordinary/visiting professorships at the and Pharmaceutical Chemistry) University of Pretoria and the University of the Witwatersrand, from the University of Lagos SouthAfrica.Heservesontheeditorial Boardsofseveral leading (Nigeria), and a PhD degree journals,includingElectrochemistryCommunicationsandCurrent (Chemistry) from Rhodes OpinioninElectrochemistry. University,SouthAfrica(2003).Heleadsinterdisciplinaryresearch. Thisjournalis©TheRoyalSocietyofChemistry2016 RSCAdv.,2016,6,89523–89550 | 89523 View Article Online RSCAdvances Review meeting of the United Nations' twenty-rst session of the protonelectrolytemembranemustbeabletolteroffmethanol Conference of the Parties and the eleventh session of the butallowprotonstopassthroughtothecathode. ConferenceoftheParties,servingasthemeetingoftheParties ADAFCs have numerous advantages over DAFCs due to the totheKyotoProtocol(COP21/CMP11),andtheObamaClimate facile electrochemistry in alkaline media. Alkaline environ- ActionPlan,tonameafew.Countriesandbusinessesthathad mentsprovideuniquecharacteristicsfortheefficientoperation ignoredtheimpactofglobalwarmingareretracingtheirsteps of alkaline-based fuel cells (hydrogen or alcohol fuels) with and supporting efforts for a sustainable environment. These respecttobothcathodeandanodekinetics:5–7(i)fasterkinetics developmentspromisetostimulatenewresearchdirectionsin oftheORRandalcoholoxidationreactions(AOR)allowtheuse e. low-carbon investments and innovations in renewable energy oflow-cost,non-noble-metalelectrocatalysts;(ii)facileAORsat c n technologies(suchasfuelcells)inthecomingdecades. low anodic overpotential; (iii) reduced alcohol cross-over; (iv) 27. Lice A fuel cell (FC) may simply be described as an electro- enhanced water management, since the electro-osmotic drag 1/05/2017 10:18:on 3.0 Unported ceb(AhlaeDescemAtdrFiiccCaafsull)edoelnepveecirrecgaelytlse.t,hiDnawitrahelccikaltaenlaindlaceliokremhacloeitlnldyeifauc;oedblniorvceteehcrlttldsciahr(leDcecomAthlFyioCcolasxl)ifeduanirezeelergaacycleciltdolos-- oewsinfidlslteuh,rreaeinvmeogloiedlvcoietnnrogwgdetahevtieetmryp;aofaotsenrsmridibaei(lldvsiit)ybartoeeiftdnhwugeactseaeudrnb-orjiedosceoktdetodihnfrgttooh;u(egvc)hosrrperteohdcsuetiacotcenoad,rthrtiihoosdnukesss n 1uti hols, generating electricity in the process. Both DAFCs and adsorbing onto the MEA, which might hamper the electro- ded oAttrib AcoDuAnFtCerspahrtasv,eprsoetvoenr-aelxcahdavnagnetamgeems borvaenre ftuheelircehllysd(rPoEgMenF-Cfesd) cinataaclyidtiiccpmroecdeisas,.bAulstot,hPistipsoeisaosinlyinpgoeisffoencetdisbvyeCryOwdeuarkinignAaOlkRas- nloaons andalkalinefuelcells(AFCs).Oneofthemainchallengeswith line environments, which means that it is possible to employ wm om hydrogen is that it is not a primary fuel; it is obtained from low-costelectrocatalystscomparedtoplatinum.8,9 Do 16. e C many other sources, such as coal-to-gas processes, electro- Recent advances in the preparation of chemically stable mber 20Creativ cnhaetumraiclalgaws.a1teAr-lsspol,ittlianrgge-osrcaleelecptrroodlyusicst,ionandandrefostromraingge ooff awliknadlionwesanoiofn-eoxpcphoarntguenimtyemfborrantehse(AEdMevse)lhopavmeebnetgunoftohoipgehn- 8 Septeunder a hatyeddrowgeitnhatrheefruasueghotfwhityhdrcohgaelnlenngeecse.ssTihtaetecdhatlhleengseesaracshsofcoir- p(AeErfMor-mAFaCnsc)ehAaFveCsseavnerdalAaDdAvFaCnsta.gAeEsMov-bearsceodnvaelknatliionnealfuAeFlCcselolsr d on 0ensed pmoatteunrteiaflulieqlucidellf,ueitlsalmtearjnoartitveecsh.nAilcthaloupgrohbtlheemAFoCf pisrothgeremssoivset AmDoAbFilCesc:a(tii)otnhserienisthneoscyasrtbemon;a(tiie)pthreecriepiistantioone,lescintrcoelyttheewreeaepreinngo; ec s Article. Publish This article is li gdcaisaieerrnn2bres0eori–mtna3iteaa0esti%dindosnuewtveooeittnfbohtetharhsteeeoftloeshvlrleeoemdcwt.oerAdrpoateltyhntethyeodecfdirarocuncogeuoreirdnttooe.sv,CiIotonhOlntea2aogcdcfeodotnnihotcteireaocnmnaatt,rrinabClotoaOiwtnoi2noscncuouapfrfnprrCoeoObnmre2tt (aprienoiliota)Tedtanhielvtceeioraliehylsloyohlclsadcoivrmneo(sispfubsloemoienveeneeddrc;oisotaahnnrteesddritdhu(rveece)relscadcott;aerh(trdiheovo)sr‘hidesoiveonni,tecniwewsesswar,seta’ebdtroeuurrtcmgetthedhane.neeyafragaasetrtemedpeaeanictttehtheaiesrt s ce forthePtcatalysts. which the subject is moving) or limited in information on c A n Alcohol-basedfuelcellsattractalotofattentionaspossible relevantsub-topics. For example, Bidault et al.10dwelt onlyon pe alternative power sources for portable and consumer elec- gas diffusion cathodes, while Antolini and Gonzalez11 focused O tronics. Low-molecular-weight alcohols (methanol, ethanol, on ADAFCs, but with limited information. As a contrast, this ethyleneglycol,andglycerol)havebeenconsideredaspotential review is aimed at closing the knowledge gap by providing alternatives to hydrogen because of their several advantages a more comprehensive insight into recent developments in over hydrogen; they are liquids at ambient temperature and AORs and ORRs in alkaline media, and AEM-based DAFCs pressure, thereby easy to handle, store, distribute, or trans- (AEM-DAFCs, Fig. 1) using Pt-free nanostructured electro- port.2–4 Moreover, the gravimetric energy densities of these catalysts are presented. Importantly, some basic principles alcohols(5–8kWhkg(cid:3)1)areclosetothatofgasoline(12kWh underlying the design and development of nanostructured kg(cid:3)1)andtheycanbeobtainedfromrenewablesourcessuchas electrocatalystsarediscussed. biomass. The direct methanol fuel cell (DMFC) has been well describedintheliteratureasthemostpromisingDAFCsystem 1.1. Basicprinciplesunderlyingmetalnanoparticle- forportableapplicationduetoitsmanyadvantages,suchasthe enhancedelectrocatalyticactivity ease of transport, storage and distribution of methanol. However, the key challenges that still conspire against the Metalnanoparticlescarryoutelectrocatalysismoreeasilythan widespreadcommercialisationofDMFCsincludetheexorbitant large single-crystal catalysts. In his book, Roduner12 elegantly cost (due to precious catalyst and membrane-electrode describedthegeneralprinciplesofcatalysiswithnanoparticles. assembly (MEA) parts) and low power density or low cell In a nutshell, there are several reasons why nanoparticles performancecomparedtotheproton-exchangemembranefuel (clusters of atoms), oen with irregular shapes, can perform cells (PEMFC) (due to poor kinetics of the anodic methanol betterchemistrythansingle-crystalsurfaces.Therstreasonis oxidation reaction, poor proton conductivity, and methanol theirregularityoftheparticlesurfaces,whichprovideasuitable crossover through the polymer electrolyte membrane). To environment for the occurrence of defects, steps, and kinks, resolve the proton conductivity and methanol crossover, the whichpermitthecreationofspecialbondingsituationsthatare favourableforthebreakingandmakingofbonds.Thesecond 89524 | RSCAdv.,2016,6,89523–89550 Thisjournalis©TheRoyalSocietyofChemistry2016 View Article Online Review RSCAdvances e. c n 27. Lice 10:18:orted 017 Unp 5/23.0 1/0on n 1uti ded oAttrib Fig.1 Schematicofanion-exchangemembrane-baseddirectalcoholfuelcell(AEM-DAFC). wnloamons reason is that the nanoparticles permit facile surface restruc- withaviewtosynthesizingnanocatalystswithshapesthatgive om Do turing, unlike single crystals. It should be noted that surface a preferential surface structure. For example, Hao and co- 16. e C restructuring constantly occurs to adapt to the adsorbate and workers15reportedthatcubicPdnanocrystals(7nm)exhibited 0v mber 2Creati tohfussmthaellcchleumstiesrtsryatrheatfrtaekerestpolamceo.vAet,osminsccelotsheetyohthaevesufrefwaceer malkuaclhinebemtteerdiaele(icntrotcearmtaslysoisf otvoewraprodtenmtieatlhaannodl loaxrgideatciuornrenint ea 8 Septunder nsienigglhebcoruyrsstaltso. obstruct their movements compared to large rseizsep.oTnhsee)rceosumltpwaraesdattotrtihbeuitresdpthoetrhicealcucobuicnntearnpoacrrtsysotfalas,siwmhiilcahr d on 0ensed undTehreaenlecotrvoedrpeorteeancttiiaoln,,Ide(amnepderaessthpeercusrurrefnatcedeanrseiatyodfratwhne cthoentsapihnemricoarlenoafntohceryhsitgahlsly.Sciamtaillyatrilcya,lNlyaarcatyiavnea{n10a0n}dfaEcle-Stsaytehda1n6 ec Publishcle is li electrode),isgivenbythefollowingequation:13 p(4r.o8vnedmi)nsh2o0w04edthmautcPhtfnaasnteorpraeratcictiloesnwkiinthetaicstectoramhpeadrreadl stohathpee ccess Article. This arti wtrhocearetajlIyi(ssAttshcumerf(cid:3)sa2pc)ee¼ciajrce(aAc)u,crmsre(cid:3)ins2t)thsdee(cnmsspi2teygci(cid:3)(a1c)mwspue(rgrfeacscmep(cid:3)ea2r)reraeaolfelte(h1ce)- trwcheoeaerlcrletcidsovporenoctunehrmdaianentngottmhe‘nodsseeatohrfo-astftphthlehoeewtrei-sctirnpaahldh’eeenrxdiacrcnaarolylcsnprtaayasnlrttopaiccllslraeyns(se4tsaa.9lrsge.nemnmIteu)rhcabahlselycmbageuoievsrneee A n electrocatalyst(realcatalystsurfaceareapermass),andwisthe poor catalytic properties, while high-index planes with a high e p amountofloadedelectrocatalyst(massperprojectedelectrode O concentrationofatomicsteps,ledges,andkinksgenerallygive surface area). Eqn (1) can be interpreted in several ways. For highelectrocatalyticactivityandstability.17–20High-indexplanes example,itsuggeststhatwhenexpensivepreciousnoblemetals are characterized by having at least one Miller index greater (e.g., Pt, Ru, or Pd) are deployed as catalysts, it makes more than 1. Today, one of the biggest challenges that confronts economicsensetosimplyincreasejandswithoutincreasingw. researchers is the ability to tune synthetic strategies that will Thevalueofjcanbeincreasedbytheco-operationofdifferent produce nanocatalysts with only, or dominated by, high-index metal catalysts (multi-metallic catalysts), by addition of ada- facets.Thischallengeshouldperhapsnotbetotallysurprising toms,orbyanalloyingprocess.Thevalueofscanbeincreased ifoneunderstandsthatthehighsurfaceenergyofahigh-index if metal catalysts are dispersed as nanoparticles on high- plane allows it to respond much faster to the particle growth surface-areasupportplatforms. rateduringsynthesisthanalow-indexplane,thusleadingtoits (i) Effectofparticle shapeand surfacestructure.Thecrit- rapid disappearance as the synthesis proceeds. According to ical determining factor for the electrocatalytic properties of Wang,21 the surface energy of different crystal planes of face- acatalystisthesurfacestructure(i.e.,particularstyleoffaceting centered cubic (fcc) metals decreased in the order g{hkl} > g or the prevailing crystallographic directions and planes in the {110} > g{100} > g{111}. The implication of this trend is that catalytic material, also known as the Miller indices) and not duringshape-controlledsynthesisofnanoparticlesviaconven- necessarily the shape of the nanocatalyst.14 However, some tional chemical methods, the growth rate along the normal shapes are preferred as they tend to give rise to the preferred directionofalow-indexfacetwithlowsurfaceenergyismuch atomic arrangement for enhanced electrocatalytic activity. slowerthaninthedirectionperpendiculartoahigh-indexfacet. Thus, what some researchers believe to be shape-dependent Inotherwords,rapidgrowthinthedirectionperpendicularto electrocatalytic activity is simply shapes of catalysts that give high-index facets leads to rapid loss of high-index facets in the right surface structure (Miller indices) for enhanced elec- favour of low-index facets, yielding NCs with cubic shapes, trocatalysis.Itisadvisable,therefore,thateverystudyattempts cuboctahedrons, tetrahedrons, and octahedrons.17 The toexplorethecorrelationbetweenshapeandsurfacestructure, Thisjournalis©TheRoyalSocietyofChemistry2016 RSCAdv.,2016,6,89523–89550 | 89525 View Article Online RSCAdvances Review inherent nature of nanocatalysts with high-index facets a nanoparticle is a clear indication of an incompletely devel- (extremelysmall-sizednanoparticles,withfeaturesonlyvisible oped band-structure, meaning that the electronic properties with high-resolution TEM) makes them suitable to positively will be different from those of the bulk metal; and (ii) a low impactonjandsineqn(1). coordination number (CN): the smaller the size of the nano- The use of unit stereographic triangles (Fig. 2) to illustrate catalyst,thesmallerthenumberofatomswithlowCNsinthe the co-ordinates of different crystal planes has been well- surface, hence the electrocatalytic properties are improved. As documented in the literature, and is not intended to be already indicated, atoms with low CNs are generally more repeated here. However, a short summary here may be appro- reactivethanthosewithhighCNs.Itshouldbepointedoutthat e. priatetoremindusoftheMillerindices.AsshowninFig.2,the electrocatalysisisasurfacereaction,whichsimplymeansthatit c n threelow-indexorbasalplanes(i.e.,(111),(100),and(110))are isonlytheatomsatthesurfaceofthecatalystparticlethatcan 27. Lice at the three vertices, while the high-index planes (i.e., (331), access the reaction intermediates, while those that are buried 1/05/2017 10:18:on 3.0 Unported ((pa30tl11oa11nm))e,,ss,(a3cnao1ndm0d)(,p1tr(1h7i0se5e)1)(c)1a,l1one0std)ec.li)pynlapasrianedceekloectcodhameatenptdrdriiaashnetisggthlhseelt.yesTpcihodaoeetrloi(dn1mi1ens1sa,)t(ewia.dehn.isd,lus((tr01f0at01hc0)ee), ooiunbbnsssdieederrervvsaetthdtaienocdnpuastbrhoteif-csroloecemtpcaahhneetnndhooreamtilrpeaPnritanti,pvcSeiaphsrtataiitgocelaeeitn.tioaFtnhli.re2s2rotmlefya,acdttthiheoeesnoCpmcNrooemscuemonsfsioq.tnThuloeey n 1uti high-index planes comprise kink atoms. Surface atoms are {111}and{100}edgesandverticesare9,8,7,and6,respectively. ded oAttrib enaastiiloyndniffuemrebnetrisat(eCdNsfr)o,mwhoicnheaarneotthheernubmy tbheerssoo-fctahlleeidrncoeaorredsit- Stheecoenddgleys,,avsetrhtiecepsa,ratincdlessuizrfeadceecarreeasinesc,rethaesefdra,cmtieoannsionfgatthoamtsthaet nloaons atoms; the smaller the CN, the better the electrocatalytic loadingofcatalystsonelectrodesforfuelcellapplicationscan wm om performanceofthenanoparticles.TheCNsdecreaseasfollows: beremarkablyreducedbynano-sizingofthecatalystparticles. Do 16. e C 6forplanesinzone{001}andinsidethetriangle,7forplanesin Finally,asthesizeofthecatalystbecomessmalleranddisper- mber 20Creativ zpolannees.1{70S1i1m},il{a1r1ly0,}f,oarntdhe(1in1t0r)i,n8sicfotrria(1n0g0l)e,tahnadtc9oofordrinthaete(s1t1h1e) saiosunbisstainntciraelarseeddu(cit.ieo.,ninincrtehaesehdigshulryfacoceoradtionmatse)d, oantoemosbs({e1r1ve1s} d on 08 Septeensed under a oc(hMrceytdNsarthCaaellsd)nsr(uoaFnrnifg,oa.cac2reny)d,sitntarhhdlesoexmvbeobarutinicncdeddseotdhddeeebcsaycshhribeabdapesreoathlnoeffaacrcmoeeoetcrstod,avliein.renae.tadentshboeoycfr{yc1psu0toba0lely}s-,, maannedd(tia{ivli1el)i0rct0ia}Ec)teffocsemo.cmstpsoiagfrnelidattctoiacnteht-sleytrlaoaffiwne-.cctoTsohtrhedeinleaaltettecicdtreoantpoiacmrsastmrauetctttehurere(ea,d)agneodsf Publishecle is lic {h1e1x1o}c,taanhded{r1a1b0}o,urnedspeedcbtiyve4l8y.{Ihnksl}i,dwehthileetirniatnhgelesiadreelinfoeusnodftthhee himenpcoerttahnet craotlaelyitnictrheeacetilveicttyr.o23d–2e6 rCeoamctpiornes;saivsetshteraisnizeplaoyfstahne Article. This arti t(TriHanHg)l,eatrraeptehzeophoedlyrhae,daranldMtNriCsso,cwtahhiecdhraarebtohuentdeetrdahbeyxah{hekd0r}a, msiveetalsltircaninanionpcarretaisceles,dethcruesasceasta(llayrtigce sauctrifvaictye airnecar)e,acsoems pwrietsh- s {hkk},and{hhl}facets,respectively.17 increasedcompressive strain.It hasbeenwell recognised that ces (ii) Effect of particle size. Eqn (1) recognises the need for the amount of strain exerted on the metallic nanoparticles is c A n nano-sizedparticles(largesurfacearea)forenhancedelectrode stronglydependentonthelocationoftheatoms,anddecreases e p reactions.Therearetwophenomenathatexplaintheenhanced asedges/vertices>{111}>{100}.27Infact,thestrainsinthelow O electrocatalysisofnanostructuredcatalysts:(i)anincompletely coordinated atoms are twice as large as those observed in the developed band structure: a small number of atoms in highly coordinated atoms of {111} and {100}. One of the Fig.2 (a)Unitstereographictriangleoffccsingle-crystalandmodelsofsurfaceatomicarrangement,(b)unitstereographictriangleofpolyhedral nanocrystalsboundedbydifferentcrystalplanes.Figureadaptedfromref.17withpermission. 89526 | RSCAdv.,2016,6,89523–89550 Thisjournalis©TheRoyalSocietyofChemistry2016 View Article Online Review RSCAdvances noticeable effects of alloying or core–shell catalysts (with the exempliesthesegregationenergies(eV)ofselectedbinaryalloy precious metal as the ‘shell’ and the transition metal as the nanocatalysts that are relevant for AFC systems. Note that ‘core’) is increased compressive strain. For example, Strasser a positive energy value, DE , means that the core–shell (X)Y et al.28 studied the impact of the Cu content and preparation structureprefersXinthecoreandYintheshell,andthelarger temperatureonthelatticestrainofthePtshellofaPt–Cu @Pt the value, the greater the tendency. As a contrast, a negative x core–shell catalyst and found that the average lattice parame- DE describestheoppositepreference(i.e.,Xintheshelland (X)Y ters of the Pt shell (a ) were smaller than those of the pure Y in the core). For example, for an Ni-core and Pd-shell shell bulk Pt, suggesting that the Pt shells are under compressive conguration, the positive SE (+0.46 eV) indicates that the e. strain. Also, higher Cu contents or an increased preparation presentcongurationispreferred,whereasanegativeSE((cid:3)1.09 nc temperature resulted in a decreased a and hence a larger eV) for a Pd-core and Ni-shell conguration means that a Pd- 27. Lice magnitude of compressive strain. Alsosh,eliln recent studies by shellandNi-coreispreferred. n 11/05/2017 10:18:ution 3.0 Unported Osacaholnzlreodoelyel–t.,mhsthTheeehenclyiloasormaebpnosh(dFedereencvCloeo:od-mtw)hleoraeenrtstkloiauecntlrettsiscsc2t5aei,rn2na6miaoneinassremsiadinlnauydtcucFbehceedeCbdPoueid@ntnw–dFPteeheder@esndtPoPitdsohdtdebacynsofcthrrheoeee–mlilcnFo(ePttrChhedo–ee) etworlcooecrmnTudhpdesryn,eentemtswhdieteietytahc)clailonadirceealsaebrtlagoeignemorar,ereWynlYitaeaisgvlmlneowaeyucirts–ocheSmhgeirsip3etm4rzgefaaorsltarslemeidsvrieuutoalsastttto(hrelmoaedwiisntceuh.rrerfTaaarhvdcueuielir.sea,Itgtneheinfoaedttolheantecnoer- ded oAttrib sshmealll;letrhweihllibgheetrhethleatttricaenspiatiroanmmeteertsalacnodnhteenntceinththeehicgohreer, tthhee kthneonwsabthlee WtoSprareddiiicotr, tdheesieglnectaronnddmenaskietietsheofdmeesitraelsd, onnaenois- nloaons compressivestrainbeinginducedintheshell. catalysts.Forexample,Table2comparestheWigner–Seitzradii wm om (iv) Effect of alloying: surface-segregation phenomenon. andelectrondensitiesofsomemetallicelementsthatareuseful Do 16. e C Fromtheequationoftheelectrodereaction(eqn(1)),thejvalue for making alloy catalysts for AFC systems. Table 2 clearly mber 20Creativ cmaenabnesiomfpardoavteodmbsyostrraatlelogyiicncgo.-1o3pAelrloaytiionngoisfdkinffoewrennttomeentahlasn,bcey isnudrfiaccaete-ssegthreagtatPedi,nfNori,eFxea,morpFlee,–Choasalaloysstr.onger tendency to d on 08 Septeensed under a tTtthhhheeeecmceaaltateaancllyntyrtseoitcrcsaa.i2ntc9a–t3iwl1vyhitTtiiychcahiasnctdtahir/veoriatraynstogtoaemfbmasilleilatnoyrtyeosofafirtshrtahsentebrgoaaentsdoegmlapytrsedthicesiepoieunsnusdrmufeaenecnttecaoelosndf. apanodshTiitchgiooihs-n-wpiiosenrrikfanoenrramsFg,e2ra5Cen,2eo6cm,3ea5l,e3ls6nouwytb,hwaeni1trdhe0PFtnhdemewocarcesFucefepoCniueotnd@dtnFhtdeoe@i‘onscPghcdeublply’cypaOttozhasoleiyets‘imsothneefnoilnlar’ ec Publishcle is li bpyarttihceleatboilitthyeofsutrhfeacaetolamyesrto(i.em.,ig‘srautrefafcreomsegtrheegactoioren’o).fTthhee anlaknaolianleloydciraetcatlysatl,c3o7hAoglisfuperledciecltledsytsotesmursf.acAel-sseog,reingataenonAgPPdd. s Article. This arti ptahnhedeonrpoyemarnfeodnrmoenxapnoecfreimsuoerfnfantceteowsbeeelgeorceftgcraoritctiaioctnaallyhismatssp.boSeroteamnnecsehooifnwtthnheebfdyaecbstoiogtrhns Iwsttariusscabtuecolriemevabeniddntaehtniaostnetmhoefbtlewenoehfffaaenccctteosd.rsAO;lmlRoRoyidnaigctcwivaaittsiyopnornoovftehthdeetAoeglaePlcdtteraroltnlhoiecy ces mentionedintheliteraturethatdeterminesurfacesegregation electronicstructuresofthetwometals(byshisinthebinding c n A includethesegregationenergy,cohesiveenergy,surfaceenergy, energy inXPS).The ensemble effect simply describes the suit- e p atomicradii,andelectronegativity.32,33Forexample,Wangand ablearrangementoftheAgnexttothePdsurfaceatoms. O Johnson33 reported that the tendency for atoms in metal nanoparticlestopreferthe‘core’or‘shell’positionofthealloys can be generally described by two independent factors; (i) Table2 RepresentativeWigner–Seitzradiiofsomerelevantmetallic cohesiveenergy(relatedtovapourpressure)and(ii)atomicsize elements for making alloy nanocatalysts for application in Pt-free (quantiedbytheWigner–Seitz(WS)radius),andtheinterplay alkalinefuelcellsystems between them. These two independent factors were found to determine the trends for surface segregation preference for atoms in nanoparticles and semi-innite surfaces. Table 1 Table1 Typicalcalculatedsegregationenergies(eV)forbinaryalloy nanoparticles Shell Core Ag Pd Ni Ir Co Fe Ag 0 (cid:3)0.82 (cid:3)2.29 (cid:3)3.54 (cid:3)2.15 (cid:3)5.20 Pd 0.70 0 (cid:3)1.09 (cid:3)1.71 (cid:3)1.29 (cid:3)3.26 Ni 0.67 0.46 0 (cid:3)0.67 (cid:3)0.20 (cid:3)2.02 Ir 1.51 1.34 0.33 0 (cid:3)0.04 (cid:3)1.97 Co 0.36 0.75 0.15 0.04 0 (cid:3)2.24 Fe 0.60 0.74 (cid:3)0.10 0.00 0.07 0 Thisjournalis©TheRoyalSocietyofChemistry2016 RSCAdv.,2016,6,89523–89550 | 89527 View Article Online RSCAdvances Review (v) Effect of catalyst support. A fuel cell is expected to (CNTs), carbon microspheres, carbon nanobers, and graphe- operate for thousands of hours or cycles during its life time. nes.Theoreticalandexperimentalstudieshaveshownhowthe Catalyst-support materials remain one of the most critical physicochemicalpropertiesofthecarbonscanimpactontheir componentsofthe fuelcellthatwill allowforsuchlong oper- ability to strongly adsorb catalysts and improve fuel cell ation.Indeed,itiscommonknowledgethatefficientinteraction performances.Twosuchkeyfeaturesofthecarbonsareshapes between the electrocatalyst and its supporting material is andstructures.Forexample,Cuongetal.44usedDFTtounravel responsibleforsomeoftheimportantparametersthat govern theinterplaybetweendifferentcarbonsupports(i.e.,graphene the efficient performance of fuel cells: particle size, catalyst sheet, a metallic single-wall carbon nanotube (SWCNT), and e. dispersion, and stability. From the equation of the electrode a series of semiconducting SWCNTs) and Pt nano-clusters, in c n reaction(eqn(1))wenotedthattheperformanceofeachofthe determiningthestabilityandelectronicpropertiesofPtnano- 27. Lice twoelectrodereactionsofthefuelcellisintrinsicallylinkedtos clusters(Fig.3).ItwasestablishedthatthePtclusterswerebest 1/05/2017 10:18:on 3.0 Unported i((tni2h.–ee.5d,vintaahlmmueeesitnpoeefrdc)si.i,aAmtchnesetuiecdrrafe)ataaocllenyssaautrspheampigoouhrfs-tstthufboerefraedlctehiesc-eptarreeoerlcaseaecstdtaurlopaycspsaton)tr.aatTlny(o1ost0pin–as5crhtr0oiecnualesmldes sgo(2trna.a2bpS1iWhlieezCVne)Ned.Tsbsuyp(4ap.do0sroetsrVp,)twbioietnihnogtnhmecuaacrdbhsoohnripgnhtaieonrnottheunabneerstg,hyraaottfhofePrtgrtcahlpuahnsteeonrnes n 1uti possess the following critical properties:38,39 (i) satisfactory The enhanced stability of the Pt cluster on SWCNTs was ded oAttrib ereledcutcriecathlceopnodsuscibtilveitdy,e(aici)tisvtartoinongcoaftathlyestc-sautaplpysotrstainntderaalcltoiwonfotor acussrvoactiuartee-dindwuicthedthepygraemomideatlriyzatoifontheanSdWCNmTissa;li“gdnumeentot thoef nloaons efficientchargetransport,(iii)largesurfacearea,(iv)asuitable p-orbitalsofcarbonatomsinSWCNT,carbonatomsontheoutside wm Doom porous structure to permit good reactant and product ux, (v) wall of the SWCNT have more sp3 nature than those on a at 16. e C good water-handling capability to avoid ooding, (vi) good graphenesheet”.44Thisndingcouldwellexplainthegenerally mber 20Creativ roepseirsatatinncgeetnovicroornrmoseionnts,toanadllo(vwii)feoarsheiogfhcasttaablyislittyreicnovfeureyl.-cell htoigcha-tpaelyrsfotsrmadisnogrbaebdiliotny gorfaCphNiTte-scuaprpboorntse.dCcoantsaildysetrsincgotmhpatarthede 8 Septeunder a intoCattwaloysct-astuepgporoiretss:fo(ir)fcuaerlbcoenll-sbamseady bseupcpoonrvtesniaenndtly(idi)ivnidoend- dm-settaatle–scaorbfotnhebomnedtianlgataormisessafnrodmp-tshteatehsyborfidaidzajaticoenntbecatwrbeoenn d on 0ensed ccaartabloynst-ssuuppppoorrtts.mCaatrebrioanlsmfaotrerifaulesl-caerell theleecmtroocsattaimlysptos.rtaBnyt aptroommso,ttehitshme eoavnesrltahpatbtehtewheeignhpcuarnvadtudrewoafvtehefuCnNcTtisoinssa,btlheutos Publishecle is lic madoorppthinoglogdieisffeorfencatrbtoynpseshaovfe bseyennthoebsitsainmede,thinocdlsu,dindgiffceoreren–t incArenaostihnegrthkeeyafdesaotruprteioonfeancearrgbyoonfmsuertfaalceonthCaNtTins.uences its Article. This arti simheplolsr,taspnhceeroefs,cahroblolonwasspahcearetasl,ynsta-nsuoptupboerst,manatderoinaliosnte-lmikse.frTohme agbroiluitpys.toThienttewroacmt awiinthfumncettiaolnaclaittaielyssatsreisoxsyugrefnacaendfunnicttrioogneanl s its unique advantages, such as easy availability, low-cost, high (Fig. 4). The commonly observed oxygen functionalities are ces stabilityinbothacidicandbasicmedia,andabilitytobeburnt carboxyl (–COOH), hydroxyl (–OH), and carbonyl (–C]O) c n A offeasily,thusallowingforeasyrecoveryofthepreciousmetal groups. Although a number of dopants are being reported on pe catalystsifneeded.Despitetheseimportantadvantages,carbons a constant basis, nitrogen-doping has emerged as an efficient O areplaguedbythefollowingshortcomings,whichhavelimited strategy for tailoring the properties of carbons and tuning the their performance as catalyst-supports: (i) severe corrosion/ physicochemical properties of carbon materials for a plethora oxidation under normal operating conditions, leading to poor of applications.45 For the nitrogen-doped carbons (Fig. 4), the durabilityoftheelectrocatalystsastheyareelectricallyisolated threemostcommonlyobservedarethepyridinic,pyrrolic,and or separatedfromthe support,and subsequent aggregation of graphitic nitrogen groups. The main goal of surface- small particles of these catalyst particles (Ostwald ripening), functionalization of carbon materials is to improve their whichcanleadtotheformationofaninhomogeneousstructure hydrophilicity (e.g., treatment with acids and bases) or hydro- over time.38,40,41 Indeed, the main reason for the loss of phobicity (e.g., treatment with organic compounds such as electrochemically-activesurfacearea(ECSA)oftheelectrodeand benzene), with a view to enhancing their dispersibility in theconsequentlossofelectrocatalyticactivityisrelatedto(i)the solventsandpermittinganchorageofthemetalcatalysts.CNTs abilityof the small-sizedparticles to aggregate into large-sized areattractivesupportsforelectrocatalystsduetotheirexcellent particlesviadiffusion,accompaniedbycoalescenceortheOst- mechanicalstrength,high-surfacearea,highconductivityand, waldripeningmechanism;(ii)thepresenceofalargeamountof of course, their inherent geometry, which allows for the high micropores (<1 nm), which can impede fuel supply to the bindingenergywithmetalcatalysts,asdescribedabove. surface,andpresentalowaccessiblesurfaceareaforthedepo- Thesurface-functional groups providethebinding sitesfor sition of metal particles; (iii) low polarity and high hydropho- thegrowthofthemetalcatalystions.Theconventionalmethod bicity,whichreducesthepermeabilityofgasesandliquids;and for surface-functionalisation of CNTs, and indeed any carbon (iv)poorstabilityattemperatureshigherthan373Kandlackof materials, involves treatment with acids or bases, a simple protonconductivity.42,43 chemical treatment that permits the introduction of surface Several types of carbons have been investigated as catalyst- groups,suchascarboxyl,hydroxyl,phenolic,sulfonyl,orsulte supports and these include carbon blacks, carbon nanotubes groups on the carbon surfaces. About a decade ago, Kim and Park46 investigated the effects of the chemical treatment of 89528 | RSCAdv.,2016,6,89523–89550 Thisjournalis©TheRoyalSocietyofChemistry2016 View Article Online Review RSCAdvances e. c n 27. Lice 10:18:orted 017 Unp 5/23.0 1/0on n 1uti ob ded Attri nloaons wm om Do 16. e C 0v mber 2Creati ea 8 Septunder Fig.3 ThegeometricstructuresofPt13clustersadsorbedoncarbonsupports:(a)metallic(5,5)SWNT,(b)semiconducting(10,0)SWNT,and(c) d on 0ensed S(8W(cid:4)N8T)sgarnadphgernapehseunrfeacseh.e(edt).dFeigsucrreibaedsathpeteddefproenmderenfc.e44ofwthitehcpuerrvmatiussrieoon.fthecarbonsupportsontheadsorptionenergyofPt13clustersonthe ec Publishcle is li carbonsupportsontheelectrochemistryofplatinumcatalysts. treating CBs with 0.2 M benzene), base-treated carbon blacks Article. This arti Tblhaeckfsol(lCoBwsi)n,gnecuatrrbaol-ntresautpedpocratrsbownerbelasctkusdi(eNdC;Bvsirogbintaicnaerdbobny (tBreCaBtesd, ocbatrabionnedbblaycktrsea(AtiCnBgsC,Bosbtwaiitnhed0.2byMtrKeaOtiHn)g, aCnBdsawciitdh- s 0.2MH3PO4).Theauthorsshowedthatthesizeandthecatalyst- s ce loadingwerestronglydependentonthesurfacecharacteristics c A n of the carbon blacks: BCB-supported Pt yielded the smallest e p particlesize(2.65nm)andthehighestloading(97%)compared O to the other carbons investigated. The electroactivity of the catalyst was enhanced with BCBs and NCBs, but deteriorated withACBs. Anotherinterestingmethodofimprovingthesurfaceproper- tiesofnanocarbonmaterials,includingCNTs,issulfonation,47–49 which was rst introduced by Hara et al. in 2004.50 In a recent review by Kang et al.,47 the authors discussed a number of synthesis methods for sulfonated nanocarbons, including their applications.Toimprovethesulfonationprocess,Ozoemenaand co-workers51 rst functionalised pristine CNTs to increase the concentrationoftheoxo-groups(mainlythe–COOH)byathree- step acid-treatment process: (i) reuxing in 2.6 M HNO for 24 3 h;(ii)sonicatinginanH SO /HNO mixture(3:1ratio)for24h; 2 4 3 and(iii)stirringinanH SO /H O mixture(4:1ratio)at70(cid:1)C. 2 4 2 2 The acid-functionalised CNTs (MWCNT-COOH) were then sulfonated using a mixture of H SO and acetic anhydride at 2 4 70(cid:1)Cfor2h,usingtheprocedurereportedbySunetal.48,49The catalystsweresupportedonthesulfonated-CNTs(MWCNT-SO H) 3 usingrapidmicrowaveirradiation,asshowninFig.5. Sun et al.48,49 showed that a Pd catalyst supported on Fig. 4 Structure depicting nitrogen functionalities in nitrogen-doped carbonmaterials,themostcommonlyobservedgroupsbeingthepyr- sulfonated MWCNT (Pd/MWCNT-SO3H) had an enhanced idinic,pyrrolicandgraphitic.Figureadaptedfromref.45withpermission. Thisjournalis©TheRoyalSocietyofChemistry2016 RSCAdv.,2016,6,89523–89550 | 89529 View Article Online RSCAdvances Review e. c n 27. Lice 10:18:orted 017 Unp 5/23.0 1/0on n 1uti ob ded Attri nloaons wm om Do 16. e C 0v mber 2Creati ea 8 Septunder d on 0ensed ec Publishcle is li s Article. This arti Fig. 5 Schematic of the sulfonation process for carbon nanotubes using microwave-assisted synthesis. Figure modified from ref. 51 with es permission. c c A n e p O catalytic activity for methanol oxidation48 and the ethylene (FeCo@Fe@Pd, #10 nm) by fast microwave irradiation glycol oxidation reaction.49 The MWCNT-SO H allowed for (Fig.6).Theauthorsclearlyprovedthatsurfacefunctionalities 3 uniformdispersionofsmaller-sizedPdnanoparticles((cid:5)4.5nm) (mainly –COOH and –SO H) on the MWCNT support played 3 comparedtoMWCNTsupportsthatwerenotsulphonated.The a crucial role in the physico-chemistry of the FeCo@Fe@Pd uniform dispersion was associated with the specic electro- catalyst toward the oxidation of polyhydric alcohols, ethylene static interactions between negatively charged MWCNTs and glycol,andglycerol.TheFeCo@Fe@Pd/MWCNT-COOH(Fig.6A positivelychargedPd(Pd2+).Ramulioetal.51adoptedsimilar and B and 7) gave smaller-sized particles (ca. 7.4 nm), more functionalizationofMWCNTsforthesynthesisofbimetallicPd- uniform dispersion or loading of the catalyst on the support, basedcatalystsusingamicrowave-assistedmethod,ratherthan ahigherelectrochemicallyactivesurfacearea(ECSA,ca.75m2 theconventionalNaBH reductionmethodusedbySunetal.48,49 g(cid:3)1)andanenhancedelectrocatalyticactivitycomparedtothe 4 The authors showed that the catalytic properties of the FeCo@Fe@Pd/MWCNT-SO H (Fig. 6C and D), with ca. 11 nm 3 PdSncatalystfollowedthistrend:PdSn/MWCNT-SO H>PdSn/ particle size and an ECSA of ca. 42 m2 g(cid:3)1. This nding is in 3 MWCNT-COOH>PdSn/C. good agreement withthe report byKim and Park,46as already To understand the effect of sulfonation on MWCNT discussedabove,wherecarbonblacktreatedwithacidshowed supports, Ozoemena and co-workers recently investigated the apoorcatalystloadingcomparedtothebase-treatedorneutral catalyticpropertiesofanFeCo@Fe@Pdnanocatalystsupported carbon blacks. It is possible that the MWCNT-COOH support on MWCNT-COOH and MWCNT-SO H. The core–core–shell (aweakacid)mighthaveledtocompletereductionofthePd2+, 3 Pd-basednanocatalysts(FeCo@Fe@Pd)wereobtainedbywhat thusallowingamoreuniformdispersionandhigherloadingof has been termed “microwave-induced top-down nanostructuring catalyststhantheMWCNT-SO Hsupport(strongacid).Simply 3 and decoration (MITNAD)”,25 a process whereby a large-sized stated, the high catalytic performance of FeCo@Fe@Pd/ precursor core–shell alloy (FeCo@Fe, $0.5 mm) was reduced MWCNT-COOH may be related to the improved electronic to a nano-sized structure and decorated with a Pd-shell properties of the catalyst, coupled with the high affinity of its 89530 | RSCAdv.,2016,6,89523–89550 Thisjournalis©TheRoyalSocietyofChemistry2016 View Article Online Review RSCAdvances e. c n 27. Lice 10:18:orted 017 Unp 5/23.0 1/0on n 1uti ob ded Attri nloaons wm om Do 16. e C 0v mber 2Creati FFiegC.6o@FFeE/SMEMWCimNaTg-eCsOdOesHcr(ib$in0g.5thmemi)m,(pBo)rFteanCcoe@oFfet@hePd“m/MicWroCwNaTv-eC-OinOduHce(#d1to0pn-mdo),w(Cn)nFaenCoost@ruFcet/uMriWngCaNnTd-dSOecoHra($tio0n.5”(mMmIT),NaAnDd)(pDro)FceeCsso:@(A-) 8 Septeunder a Fe@Pd/MWCNT-SO3H(#10nm).Figureadaptedfromref.25withpermission. 3 d on 0ensed –COOHsurfacewiththecatalyst.Itshouldbenoted,however, thebindingofthemetalcatalyst.Forexample,Fujigayaandco- ec Publishcle is li tahnadttthhiissmnaydibnegrieslaitnedcotnotsreavdeircatilofnacttoorpsr,eivnicoluusdinngdtihnegsn,4a8t,4u9r,5e1 wthoartkercsonetlaeignantnliytrowgreanppeadtomCNsTs(i.we.i,thpyproidlyimnee-rdiocpmedateproiallys- Article. This arti othfethneapturirsetionfethCeNaTlslouysse(de.(gi..e,.t,htheeararmanoguenmtoenf–tCoOftOhHeSanvaoilnabPlde)),, dbeanzozliem53i]d,azsoullef5o2n)icandacipdoly(i[.2e,.2,9-s(2u,l6fo-pnyartieddinep)-o5l,y5s9u-lbfiobneenziamnid- ss and the synthesis method used (fast microwave-assisted acid- sulfonated polyimide54) and phosphonic acid groups (i.e., e cc functionalizationvs.alengthyhydrothermalreaction). poly(vinyl phosphonic acid)-doped polybenzimidazole55). The A n Despite the advantages of chemical oxidation of carbon catalystsdisplayedexcellentstabilitywithadurability10times e p O supports to introduce surface-functional groups, it should be higherthanthatofcommercialcarbonblack/Pt. noted that the oxidised sites of the carbon supports may To mitigate the problems of carbon-based supports in fuel accelerate the degradation process of the support material, cells, especially the issue of corrosion, several non-carbon thereby inducing signicant aggregation of the catalyst nano- supports for fuel cells are being developed without sacricing particles.Oneofthestrategiestoavoidtheoccurrenceofthese the desirable properties of carbon, such as high surface area oxidation problems is to wrap the carbon support with poly- and high electrical conductivity. The non-carbon materials mericmaterialsthatcontainappropriatefunctionalgroupsfor include the following: (i) conducting polymers, such as poly- pyrrole (PPY) and polyaniline (PANI); (ii) metal oxides and hydroxides, such as ZrO , CeO , TiO , MnO , and Al O ; (iii) 2 2 2 2 2 3 transitionmetalcarbides,suchastungstenmonocarbide(WC), and(iv)transitionmetalnitrides,includingbinarynitrides(e.g., CrN,TiN)andternarynitride(e.g.,Ti Nb N)complexes.For 0.5 0.5 moreinformationonthesenon-carbonsupports,thereaderis referredtotherecentbookchapterbyEjikemeetal.56 (vi) Effect of synthesis method. All the parameters in eqn (1) (j, s, and w) can be inuenced by the synthesis protocol adoptedbytheresearcher.Ithascontinuallybeenshowninthe literaturethatcatalystswiththesameelementalcompositions showdifferentelectrocatalyticproperties.Animportantreason Fig.7 (A)High-angleannulardark-field(HAADF)-and(B)bright-field forthisdiscrepancyisthatthepropertiesofseveralmetalalloy (BF)-STEM images simultaneously acquired for FeCo@Fe@Pd/ catalysts are highly dependent on the synthesis method and MWCNT-COOH. Figure adapted and modified from ref. 25 with post-treatment protocols. This should perhaps not be totally permission. Thisjournalis©TheRoyalSocietyofChemistry2016 RSCAdv.,2016,6,89523–89550 | 89531 View Article Online RSCAdvances Review surprising if one considers that different synthetic techniques and provides homogeneous reaction conditions for obtaining or post-treatments are capable of tuning the physicochemical high-performance catalysts. The hydrothermal method is propertiesofthealloys,suchasthedegreeofalloying,particle another frequently employed technique due to its simplicity, sizeandthesurfacestructure(i.e.,Millerindices).Forexample, andeaseofshape-controlofthemetalcatalyst. Wang et al.57 proved that PdCu synthesised by a colloidal Surfactants,suchasPVPandCTAB,havetheabilitytotune 3 method exhibited high ORR activity in an acid medium the physico-chemistry of the catalysts and improve their cata- comparedtothesamePdCu catalystpreparedbyathermode- lytic performance. For example, Xia and coworkers60 demon- 3 composition process; the former method yielded a more strated the synthesis of Pt–Pd alloy nanocrystals with well- e. uniformalloy and smaller-sized particlesthan thelatter. Also, dened shapes and twinned structures by simply heating an nc otherworkers,suchasRaghuveeretal.,58showedthataternary aqueous solution of Na PdCl , K PtCl , and PVP at 80 (cid:1)C for 27. Lice PdCoAu catalyst obtained from the micro-emulsion method 18h.Itwasfoundthatth2islen4gth2yors4low-reductionsynthesis 1/05/2017 10:18:on 3.0 Unported atgthhlalievoreyaNimanagh5B9iHgashn4hdoOrewRsdemRudcaatltcilhoteiarnv-tistripyzooecusdottem-tp,rpaebaraetrtciemcadlueetssno.et,tSohisfmauttciholhaebrtalahysi,ingiLenhidceurrferadaonsemdignrugeMsetianhnogef- ptrtioarotonercee(otosafwsitnhiwnetightthwetoiapntahnwreetediahcklsyetdsrrrueoadcxttuyusclrmtteae.anrTlmtlhwsieniazPaselVsrgPefr-osomprueopadnoilsaofitbnetlhgdeepsfPloeoVrrwiPoth)rdeewdoafufoscrtatmiimboalnee- n 1uti annealing temperature of a Pd70Co30/C catalyst from 350 to beforenucleationcouldstart.Theslowprocessallowsforeasy ded oAttrib 5ca0t0al(cid:1)yCs,t,intchreeraesbeystdheecrpeaarstiincgleistiszecaantadlydtiecgraecetioviftaylltooywinagrdosftthhee creodaulecsicnegncteheofsusmrfaaclle-tpoa-rvtoiclulemseinrtaotiola,rgtehruspalretaicdliensg, tthoerethbey nloaons ORR.Thenextsectionisasuccinctdescriptionofthesynthesis formation of twinned structures. When the process was wm om methodsforvariousnanoelectrocatalystsforAEM-ADAFCsand repeated with ethylene glycol, which is a relatively high-rate Do 16. e C theireffectsonthesurfacestructure. reductant,asingle-crystalstructureofPt–Pdnanocrystalswith 8 September 20under a Creativ 1A.E2M. -ASDynAtFhCessisofnanoelectrocatalystsforapplicationin aeanqtucrLuleoenoseuceasdettesedyax,nlco.tl6huc1tseaisohvibesetldayirinnabvleyodslhvhiaAinpgugeh–-Pwisndiamdsenuoxalbtnatfaoanicceneroeytussdts.a{l5rs4e1d(}Nu,cCtbsioy)n(oFnoigef-.pAo8ut) d on 0ensed Aacstiaviltrieeasdyofexnpalnaionsetrducitnurtehdeeplerecctreodciantgalyssetcstiosntr,onthgelycdaetapleyntidc amnadniPpdulaptriencgurtshoersg.rTowhethnkainnoectricysst,ablsywcoenretromllaindeg pthoessriebllaetibvey ec Publishcle is li uwpitohntthheeiprreefleemrreedntsaulrcfoamcepsotsriuticotunr,esi(zeexpoorsseudrfsaucrefaacreeap,lashnaeps)e, apmreocuurnstosrso.fItreidsuincttaenret,stiansgcotrobiocbsaecridve(tAhAa)t,AtoA pthlaeysmaetcarliti(cMa)l s Article. This arti arceanatdacltyitoshtnesa(eir.xeet.e,unstthedeo,fawlichnoitlheeroaflocrotixaoindcaatwthioiotnhdircseuraepcatpicootrniot)sn,.P(Fid.oe-r.b,atashneedoamnxyoegdteaincl rPAodul@epiPrnedctuhcreosromer–ossrhapelhtlslolNwoCgityshowCfiTtthhAeBaNninCostc.htTaehhaeebdcsroea-nlrecsdehuaocpfteiAon(Acaop.fr4oA8duuhacneaddt ces reduction reaction), non-platinum group metal (non-PGM) 90(cid:1)C).Onthecontrary,however,theco-reductionofthemetal c A n catalysts or Pt-free catalysts are used. The catalysts (anodic precursorsaltsinthepresenceofAAatan[AA]/[M]ratioof0.75 e p and cathodic) are synthesized using similar methods. The key (where [M] is the metal precursor concentration) (2 h at room O synthesismethodscanbesummarisedaschemicalreduction, temperature),yieldedAu–PdalloyNCswith{111}-facetedocta- galvanic replacement, seed-mediated growth, and electro- hedral morphology. When the [AA]/[M] ratio was 1, rhombic chemicaldeposition.Eachofthesemethodscanbeusedalone orcombinedwithanothermethodtoprepareacatalyst. 1.2.1 Chemical reduction method. Chemical reduction (for monometallic catalysts) or co-chemical reduction (for bimetallic or ternary catalysts) is a facile strategy for the synthesis of nanoelectrocatalysts. In this method, various reducing agents, including strong reductants, such as sodium borohydride and hydrazine, and mild reductants, such as alcohols (e.g., ethylene glycol, glycerol, methanol, 1,4-butane- diol,andpoly(vinylpyrrolidone)(PVP)),ascorbicacid,andcitric acid,havebeenusedtoreduceorco-reducetonanocrystalsof correspondingmetals.Thecatalystsareusuallydispersedinsitu onto the supporting materials. Vulcan XC-72 carbon black remainsthestate-of-the-artcarbonsupportforvariouscatalysts, while nanocarbon materials (e.g., carbon nanotubes and gra- phenes) and non-carbon materials (e.g., conducting polymer Fig.8 SchematicofthecorrelationbetweenthemorphologyofAu– Pd NCs and the NC growth kinetics; (a) core–shell octahedron, (b) composite matrices, and tungsten carbide) have also been re- octahedron, (c) rhombic dodecahedron (RD), (d) hexoctahedron ported. Microwave-assisted synthesis has also emerged as (HOH),and(e)flower-likestructure.Corresponding3Dlatticemodels afacilestrategyforthesynthesisofvariouscatalysts.Microwave for the surface facets of NCs are shown below the scheme. irradiation,unliketraditionalchemicalmethods,issimple,fast Figureadaptedfromref.61withpermission. 89532 | RSCAdv.,2016,6,89523–89550 Thisjournalis©TheRoyalSocietyofChemistry2016
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