Nanostructured platinum-free electrocatalysts in alkaline direct alcohol fuel cells

28 Pages·2016·3.18 MB·English

by  Kenneth Ikechukwu Ozoemena

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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
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s Article. This arti 1. Introduction theglobalquestforacleanandsustainableenvironment.These
s
e developments include the United Nations' Sustainable Devel-
c
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A The last ve years, more than any other in the history of opment Goal (SDG), the recent landmark agreement by 195
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pe mankind,havewitnessedthemostimportantdevelopmentsin countries to protect the environment and avoid the global
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temperature rising by the ‘psychological’ 2 (cid:1)C at the Paris
aEnergyMaterials,MaterialsScienceandManufacturing,CouncilforScienticand cMolecularSciencesInstitute,SchoolofChemistry,UniversityoftheWitwatersrand,
IndustrialResearch(CSIR),Pretoria0001,SouthAfrica.E-mail:[email protected]. Johannesburg2050,SouthAfrica
za;Tel:[phone]
bDepartmentofChemistry,UniversityofPretoria,Pretoria0002,SouthAfrica
Kenneth Ozoemena is Chief programmes spanning several areas of materials science and
ResearchScientistattheCouncil engineering,fromrenewableenergytechnologies(suchasfuelcells,
for Scientic 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
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meeting of the United Nations' twenty-rst session of the protonelectrolytemembranemustbeabletolteroffmethanol
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
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n 1uti hols, generating electricity in the process. Both DAFCs and adsorbing onto the MEA, which might hamper the electro-
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ce forthePtcatalysts. which the subject is moving) or limited in information on
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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
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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), oen with irregular shapes, can perform
cells (PEMFC) (due to poor kinetics of the anodic methanol betterchemistrythansingle-crystalsurfaces.Therstreasonis
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
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Review RSCAdvances
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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
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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
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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
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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
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noticeable effects of alloying or core–shell catalysts (with the exempliesthesegregationenergies(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 conguration, the positive SE (+0.46 eV) indicates that the
e. strain. Also, higher Cu contents or an increased preparation presentcongurationispreferred,whereasanegativeSE((cid:3)1.09
nc temperature resulted in a decreased a and hence a larger eV) for a Pd-core and Ni-shell conguration means that a Pd-
27. Lice magnitude of compressive strain. Alsosh,eliln recent studies by shellandNi-coreispreferred.
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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/veoriatraynstogtoaemfbmasilleilatnoyrtyeosofafirtshrtahsentebrgoaaentsdoegmlapytrsedthicesiepoieunsnusdrmufeaenecnttecaoelosndf. apanodshTiitchgiooihs-n-wpiiosenrrikfanoenrramsFg,e2ra5Cen,2eo6cm,3ea5l,e3ls6nouwytb,hwaeni1trdhe0PFtnhdemewocarcesFucefepoCniueotnd@dtnFhtdeoe@i‘onscPghcdeublply’cypaOttozhasoleiyets‘imsothneefnoilnlar’
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s Article. This arti ptahnhedeonrpoyemarnfeodnrmoenxapnoecfreimsuoerfnfantceteowsbeeelgeorceftgcraoritctiaioctnaallyhismatssp.boSeroteamnnecsehooifnwtthnheebfdyaecbstoiogtrhns Iwsttariusscabtuecolriemevabeniddntaehtniaostnetmhoefbtlewenoehfffaaenccctteosd.rsAO;lmlRoRoyidnaigctcwivaaittsiyopnornoovftehthdeetAoeglaePlcdtteraroltnlhoiecy
ces mentionedintheliteraturethatdeterminesurfacesegregation electronicstructuresofthetwometals(byshisinthebinding
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
(quantiedbytheWigner–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-innite 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
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(v) Effect of catalyst support. A fuel cell is expected to (CNTs), carbon microspheres, carbon nanobers, 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,aAmtchnesetuiecdrrafe)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”.44Thisndingcouldwellexplainthegenerally
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,orsulte
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
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e.
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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) reuxing 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
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e.
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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 specic 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+).Ramulioetal.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, $0.5 mm) was reduced MWCNT-COOH may be related to the improved electronic
to a nano-sized structure and decorated with a Pd-shell properties of the catalyst, coupled with the high affinity of its
89530 | RSCAdv.,2016,6,89523–89550 Thisjournalis©TheRoyalSocietyofChemistry2016
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e.
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n
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10:18:orted
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mber 2Creati FFiegC.6o@FFeE/SMEMWCimNaTg-eCsOdOesHcr(ib$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 tahnadttthhiissmnaydibnegrieslaitnedcotnotsreavdeircatilofnacttoorpsr,eivnicoluusdinngdtihnegsn,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 signicant aggregation of the catalyst nano- supports for fuel cells are being developed without sacricing
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 inuenced 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.
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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, dened 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
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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
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8 September 20under a Creativ 1A.E2M. -ASDynAtFhCessisofnanoelectrocatalystsforapplicationin aeanqtucrLuleoenoseuceasdettesedyax,nlco.tl6huc1tseaisohvibesetldayirinnabvleyodslhvhiaAinpgugeh–-Pwisndiamdsenuoxalbtnatfaoanicceneroeytussdts.a{l5rs4e1d(}Nu,cCtbsioy)n(oFnoigef-.pAo8ut)
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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