Non-Precious Electrocatalysts for Oxygen Reduction Reaction in Alkaline Media

22 Pages·2016·1.65 MB·English

catalysts
Review
Non-Precious Electrocatalysts for Oxygen Reduction
Reaction in Alkaline Media: Latest Achievements on
Novel Carbon Materials
AngelikiBrouzgou1,ShuqinSong2,*,Zhen-XingLiang3,*andPanagiotisTsiakaras1,4,*
1 LaboratoryofAlternativeEnergyConversionSystems,DepartmentofMechanicalEngineering,
UniversityofThessaly,PedionAreos,Volos38334,Greece;[email protected]
2 KeyLaboratoryofLow-CarbonChemistryandEnergyConservationofGuangdongProvince,
SchoolofMaterialsScienceandEngineering,SunYat-senUniversity,Guangzhou510275,China
3 KeyLaboratoryonFuelCellTechnologyofGuangdongProvince,
SchoolofChemistryandChemicalEngineering,SouthChina,UniversityofTechnology,
Guangzhou510641,China
4 LaboratoryofElectrochemicalDevicesBasedonSolidOxideProtonElectrolytes,
InstituteofHighTemperatureElectrochemistry,RussianAcademyofSciences,
Yekaterinburg620137,Russia
* Correspondence:[email protected](S.S.);[email protected](Z.-X.L.);[email protected](P.T.);
Tel.:[phone](S.S.);[phone](Z.-X.L.);[phone](P.T.)
AcademicEditors:VincenzoBaglioandDavidSebastián
Received:1August2016;Accepted:26September2016;Published:19October2016
Abstract: Lowtemperaturefuelcells(LTFCs)areconsideredascleanenergyconversionsystems
and expected to help address our society energy and environmental problems. Up-to-date,
oxygen reduction reaction (ORR) is one of the main hindering factors for the commercialization
of LTFCs, because of its slow kinetics and high overpotential, causing major voltage loss and
short-term stability. To provide enhanced activity and minimize loss, precious metal catalysts
(containingexpensiveandscarcelyavailableplatinum)areusedinabundanceascathodematerials.
Moreover, research is devoted to reduce the cost associated with Pt based cathode catalysts,
by identifying and developing Pt-free alternatives. However, so far none of them has provided
acceptable performance and durability with respect to Pt electrocatalysts. By adopting new
preparation strategies and by enhancing and exploiting synergetic and multifunctional effects,
some elements such as transition metals supported on highly porous carbons have exhibited
reasonable electrocatalytic activity. This review mainly focuses on the very recent progress of
novelcarbonbasedmaterialsforORR,including: (i)developmentofthree-dimensionalstructures;
(ii)synthesisofnovelhybrid(metaloxide-nitrogen-carbon)electrocatalysts;(iii)useofalternative
raw precursors characterized from three-dimensional structure; and (iv) the co-doping methods
adoptionfornovelmetal-nitrogen-doped-carbonelectrocatalysts. Amongtheexaminedmaterials,
reduced graphene oxide-based hybrid electrocatalysts exhibit both excellent activity and long
termstability.
Keywords: oxygen reduction reaction; low temperature fuel cells; non-precious carbon-based
electrocatalysts;hybridelectrocatalysts;macro-mesoporousstructure;co-doping
1. Introduction
Thedesignandthedevelopmentofhighlyefficient,stableandlowcostoxygenreductionreaction
(ORR)electrocatalystsisoneofthemaindeterminingstepsforthefabricationofmorecommercially
viableelectrochemicaldevices(fuelcells,electrolyzers,metal-airbatteries,etc.).
Catalysts2016,6,159;doi:10.3390/catal6100159 www.mdpi.com/journal/catalysts
Catalysts2016,6,159 2of22
Duringthelasttwodecades,ajointefforthasbeenmadebytheinternationalresearchcommunity
towards: (i)thedevelopmentoflow-loadingpreciousmetalelectrocatalysts;(ii)thedevelopmentof
non-Ptelectrocatalysts;(iii)thedevelopmentofnewsupportingmaterials;and(iv)theinvestigationof
novelpreparationmethods[1].
Amongthevariousdevelopednon-Ptelectrocatalysts,Ir-andPd-basedoneshaveattractedmuch
moreattention,astheyexhibitedcomparableactivitywithPt-basedelectrocatalysts,thoughinthe
presenceofmethanolorethanolduetoalcoholcrossover,theyexhibitedmuchlowerstabilityand
lessactivitythanPt-basedones[1]. Moreover,differentsupportssuchastungstencarbides,carbon
nanotubes,andnitrogen-modifiedcarbonwereinvestigatedandadopted,butwithouttheexpected
results[2].
During the last decade, along with the development of alkaline membranes, their promising
performance,inmajorcasesbetterthanthatoftheacidicmembranes,significantlyboostedfurther
investigationonthenon-Ptelectrocatalysts[3]. Amongthenon-Ptelectrocatalysts,heteroatom-doped
carbonmaterialsarecurrentlyconsideredasthemostpotentialPtsubstitutesfortheoxygenreduction
reaction(ORR)[4]. Variousheteroatoms,suchasN[5–8],B[7,8],S[9–11],Fe[12,13],andP[7,14–16],
withdifferentelectronegativity,havebeendopedfornanotubes[17],graphene[18],graphite[19],etc.
Thedoped-carbonsshowexcellentabilitytoadsorbandbondOOH•,significantlyenhancing
theformationofH O . ThisfacileORRactivityhasbeenattributedtothefactthatcarbonatomsnext
2 2
toheteroatomsarethecatalyticallyactivesitesandtothemodificationofcarbonelectronegativity.
For instance, in N-doped carbon, a charge separation occurs due to the different electronegativity
values,inwhichthepositivepoleisfavorableforoxygenadsorption,enhancingtheORR.Interms
ofalcoholcrossover,thecarbon-basedcatalystshowsanextremelyhighselectivitytotheORRand
inactivitytoalcoholoxidationreaction. Thereasonmostlyliesinthehighactivationenergyforthe
chemicaldissociativeadsorptionofthehydrocarboncompoundsonsuchcatalysts.
Furthermore,ithasbeenobservedthatco-dopingwithtwoheteroatomscreatesahighernumber
ofcatalyticallyactivesitesthansingle-dopedcounterparts. Thus,manyoftheORRinvestigationsin
thelasttwoyearsweredevotedtoN-dopedcarboncatalystswithanotherelement[8]. Forexample,
Fe-N doped carbon systems have shown excellent ORR performance in both alkaline and acid
electrolytes,duetothemetal-nitrogensynergeticeffect[13]. Thesignificantcontributionofnitrogen
when being doped in carbon has been reported since 2009. Niwa et al. [20] investigated different
electronicstructuresofnitrogen(pyridinic,graphitic,pyrrolic,oxidic,proposingthatgraphitic-like
nitrogenisthemainreasonfortheenhancedORRactivityofN-dopedcarbonelectrocatalysts.
Themesoscopicmorphology,thehighsurfacearea,theporesizeandtheorderdegreereflectthe
availableactivesitesofacarbonmaterial,andconsequentlyaffectitselectrocatalyticactivity. Mostof
thesynthesisstrategiesofdopedcarbonmaterialwithhighsurfaceareaincludemulti-stepchemical
processes,withsomeofthemtobeofhighcost. Consequently,sometimesduetothecomplexityof
thesynthesisprocedureofhetero-atomdopedcarbonmaterialsitisdifficulttodesignatethenature
ofcatalyticallyactivesitesforORR,whichisofhighimportanceforunderstandingthewholeORR
process[21].
The key issue to obtain optimal ORR activities by using doped-carbons lies in: (i) uniform
dopingofheteroatoms(ii)uniquenanostructurewithhighsurfacearea;and(iii)suitablemesopore
distribution.Inmostofthepreparationmethods,carbonsupportiseitherpost-treatedwithheteroatom
substances [7,8,15], or obtained through direct co-pyrolysis of the heteroatom-containing organic
compoundswithcarbonunderinertgasatmosphere[22,23].
However, a sufficiently doped and finely controlled structure is rather difficult to be
simultaneouslyachieved. Therefore,auniformdistributionofdopantsintoawell-designedcarbon
matrixwithsignificantlyenhancedORRperformancestillremainsachallenginggoal. Asecondtarget
tobeachievedconcernsthechoiceoftheappropriateprecursorincombinationwiththepyrolysis
conditions, to obtain large surface area, high nitrogen content and, thus, high performance to
beattained.
Catalysts2016,6,159 3of22
Thedeliberateddefectsoncarbonstructuresleadtothedestructionofitsconductivity,whichis
the most essential characteristic in electrocatalytic processes [4]. For this reason, in the last years,
researchers have turned their attention to carbons with graphitic structure, such as graphene;
theconductivityofwhichisnotinfluencedbyintroducingheteroatoms.
Thecurrentreviewfocusesondifferentstrategiesveryrecentlydevelopedbydifferentresearch
groupsinordertoenhancetheactivityofnon-preciousmetal,namelycarbonbasedelectrocatalysts
towards ORR. Mainly, four strategies have been distinguished: (i) the fabrication of 3D structure
including interconnected networks by modifying carbon structure; (ii) the development of novel
hybridmaterials;(iii)theadoptionofalternativeprecursorforamorehomogeneousdistributionof
theheteroatomsintocarbonmatrix;and(iv)theco-dopingtechniqueoftwoheteroatoms,mainlyinto
highlyorderedmesoporousmaterials.
2. 2Dand3DDoped-CarbonElectrocatalysts
Two-dimensional (2D) and three-dimensional (3D) porous structures with controllable
compositionscanprovidehighactivityandstabilityforORRduetotheirinterconnectedopen-pore
structure,whichcanfacilitateelectronandmasstransfer. Inaddition,theofferedlargeactiveareaof
mesoporestructurethatcanprovidemoreactivesites,incombinationwithmicroporousstructure
thatreducestheelectrolytediffusiondistancestotheinteriorsurfacesandservesasanelectrolyte
bufferingreservoir,significantlyenhancestheORRactivity[24,25]. Theadditionofnitrogenatthis
advantageouscarbonstructurefurtherfacilitatesORR.
Recently,ithasbeenreportedthathierarchicallyporousmaterialsoffergreateractivesurfacearea
viatheirinterconnectedmacro-porosity[26]. Liuetal.[27], fortheN-dopedhierarchicallyporous
carbonpyrolysedat1000◦C,reportedakinetic-limitingcurrentdensity(J )of20.0mA·cm−2at0.853V
k
(vs. RHE),ahalfwave-potential(E )of0.773V(vs. RHE),anexcellentmethanoltolerance(0.5M)
1/2
and8.1%ofcurrentlossafter3.5operationalhoursat−0.5VvsAg/AgCl. TheobservedhighORR
activitywasattributedtotheexistenceof3D-hierarchicallyinterconnectedporousframeworkswith
highsurfacearea,largeporevolumeandhighconductivity.
N-dopedhierarchicallymacro/mesoporouscarbonpyrolysedattheoptimaltemperature(850◦C)
was also examined by Tao et al. [28]. By the aid of rotating disk electrode (RDE) measurements,
theyrecordedalimitingcurrentdensitycloseto4.4mA·cm−2(1600rpm),andahalf-wavepotential
of 0.753 V (vs. RHE); in the presence of 3.0 M methanol in 0.1 M KOH the activity was almost
unchanged. Chronoamperometrictestat−0.55Vvs. Ag/AgClrevealsthatafter12.5hdurabilitytest,
theelectrocatalystlostonly5%ofitsactivity,whileadramaticdegradationof15%wasobservedfor
Pt/Cafter2.5h.
The3Ddual-dopedhighlyorderedmacro-mesoporousstructureisobservedtoprovidemore
activeORRelectrocatalyststhanthesingle-dopedone. Namely,3DCoandN-dopedhighlyordered
macro-mesoporouscarbonelectrocatalystwiththeoptimumcontentofcobaltdisplayedhighactivity
towards ORR due to the presence of a large number of active sites. According to linear sweep
voltammetrycurves(LSV),alimitingcurrentdensityof5.8mA·cm−2 inoxygensaturatedalkaline
environment and a half-wave potential (E ) of 0.83 V (vs. RHE), was observed [29]. The kinetic
1/2
current density at 0.70 V vs. RHE was calculated close to 23.2 mA·cm−2, while in the presence of
1.0Mofmethanol,itsORRactivityremainedsatisfactorilystable,indicatingitshightolerancetothe
methanol. Additionally, after 25 h of continuous operation at 0.5 V vs. RHE, there was negligible
currentloss,incomparisonwithPt(20wt%)/Cthatlost~50%ofitsinitialactivitywhenexamined
underthesameexperimentalconditions.[29].
Transition metal oxides, especially the doped-ones, have shown very good ORR activity [30].
TheroleofNi-dopinginMnO nanoneedlesfortheORRinalkalinemediawasrecentlyinvestigated
2
byHaoetal.[31]. TheincrementofMn(III),whichservesaselectrochemicalactivesite,accordingto
theauthors,isthemainreasonforitshigheractivity,incomparisontotheothersamples. TheORR
activity is enhanced when transition metals are adopted in 3D hierarchical porous spinel hollow
Catalysts2016,6,159 4of22
nanospheres,especiallythosewithspinelstructure[32,33]. Hollowstructurealsocontributestothe
incrementofthenumberofactivesites. Moreprecisely,CoFeO 3Dhollownanospheresbenefitedthe
2
formationofalargethree-phaseboundary(solid–liquid–gas),whichisnecessaryforthetransferof
reactantsandproducts. Theyieldedlimitingcurrentdensity(5.3mA·cm−2)decreasedby34%after
12-hdurabilitytest;whereasthePt/Celectrocatalystdecreasedby54%underthesameexperimental
conditions[34]. Theenhancedstabilitywasattributedtothecombinationofitshollowmorphology
withthehierarchicallyporousstructure. However,accordingtoLSVcurves,itshalf-wavepotential
was 0.58 V vs. RHE, negatively shifted by ~0.30 V compared with Pt. In general, even without
aspecial3Dstructure,transitionmetalspineloxidenanoparticles(NPs)haveshownreallygoodORR
performance[35–37]. 3Dhollow-structuredNiCo O /CNPswithinterconnectedporesasbifunctional
2 4
electrocatalysts,whicharetransformedfromsolidNiCo alloyNPs,havealsobeeninvestigatedfor
2
ORR[35]. FromtheRDEmeasurements,alimitingcurrentdensityof5.7mA·cm−2andahalf-wave
potential of 0.68 V vs. RHE were recorded (Table 1). In addition, NiCo O /C showed a superior
2 4
long-termstabilityoverPt/Celectrocatalyst. Ataconstantpotentialof0.6Vvs. RHE,acurrentdecay
ofonly9%after10hoccurred,whereasagradualdecreaseof52%wasrecordedforPt/Cunderthe
sameconditions.
Table1.Non-precious2Dand3D-structureddoped-carbonORR(oxygenreductionreaction)electrocatalysts.
LimitingCurrent(Jd;d=disc),
Catalyst PreparationMethodandExperimentalConditions KineticCurrentDensity(Jk), Reference
HalfWavePotential(E1/2))
Jd=4.7mA·cm−2
Nm-eHsoCpSor(hoiuesrasrpchheicreasll)y-900 N20amnoVc·ass−ti1n,g16m00etrhpomd,0.1MKOH,oxygensaturated AJkg=/A20g.0ClmsAat·.cKmC−l2(@0.805.3VVvvs.s.RHE) [27]
E1/2=0.748Vvs.RHE
3D-HPC-N Htemierpalractheifcoalllloywmeadcbroy/amseimsopploerNou-dsospiliicnagapsraocheadrudre Jd=4.4mA·cm−2
(N-doped-3D-hierarchically (InactiveinMeOHpresence) [28]
and0.1MKOH,oxygensaturated
porouscarbonmaterials)-850 5mV·s−1,1600rpm(0.1MKOH+3.0MMeOH) E1/2=0.753Vvs.RHE
Dual-templatingsynthesisapproachinaone-pot
3DCo-N-OMMC-0.6 controllableprocedurebytheuseofsilicacolloidal Jd=5.8mA·cm−2
crystal(opal)asamacroporousmoldandtriblock (InactiveinMeOHpresence),
(mCaoc(rNoO-m3)e2s·o6pHo2rOou)s(ocradrebroend) caonpdo0ly.1mMerKPOluHro,noixcyFg1e2n7saastuarmateedso;p5omroVu·ss−te1m, plate EJk1/=22=3.02.8m3AV·cvms.−[email protected], [29]
1600rpm(0.1MKOH+1.0MMeOH)
pn3Doanrohouisepsr-haCreocrhFesiec2aOl4hollow Hoxyydgreonthseartmuraaltemde,t1h0omdVan·sd−01.,11M600KrOpHm, EJd1/=25=.30.m58AV·cmvs−.2RHE [34]
3DhollowNiCo2O4/C TtohrxaryongusefgnohrsmtahtaeutirKoanitrekfdre,on5mdmasloVlle·isdff−eN1c,tiC1a6on02d0a0rlp.l1omyMnKanOoHpa,rticles EJd1/=25=.70.m68AV·cmvs−.2RHE [35]
3CDo3NOa4n-dooshpeeedt-graphene M0.1icMrowKOavHe,aorxgyogne-pnlasasmtuarastyendt,h5emsisVa·sp−p1r,o1a6c0h0arnpdm EJJkd1/==235=4.7m0.m8A3Ac2m·cVm−v2−[email protected] [38]
InsitunucleationanddirectedgrowthofMOFs
(2LDD-CHo:Alaly-LerDeHd@doZuIFb-l6e7-800 arraysonthesurfaceofLDHsnanoplatelets Jd=5.2mA·cm−2
followedbyasubsequentpyrolysisprocessand (InactiveinMeOHpresence) [39]
hydroxides,ZIF:zeolitic
imidazolateframework) 100.1mMVK·sO−1H,,1o5x0y0grepnmsa(0tu.1raMteKdOH+2.0MMeOH) E1/2=0.675Vvs.RHE
PPynanotubesweresynthesizedbythechemical
3D-NCNT-900(N-doped ooxfiFdeaCtilv3easpoalnyomxeidriaznatt,ioanndofpp-tyorlruoelen,einsutlhfeonpirceasecindce (JIdn=ac5ti.2veminA·McmeO−H2 presence) [40]
carbonnanotubes) (TsOH)asadopantand0.1MKOH,
oxygensaturated,10mV·s−1,1600rpm E1/2=0.707Vvs.RHE
(0.1MKOH+5.0MMeOH)
Anitrogen-containingmolecule,terephthalonitrile,
(3tDhe-Nrm-daolipzeedd-tTrTiaFz-i7n0e0-based atrsimtheeribzaastiiconbuiniltdoinag2Dbloccokvaalnedntthtrrioauzginhe-fibrassted Jd=4.0mA·cm−2 [41]
framework) frameworkand0.1MKOH,oxygensaturated; E1/2=0.767Vvs.RHE
10mV·s−1,1600rpm
Among all carbon materials, graphene is the most promising for accommodating various
nanoparticlestoachievehighelectrontransportrate,electrolytecontactareaandstructuralstability,
all of which lead to markedly improved ORR performance. Replacing carbon with graphene in
Catalysts2016,6,159 5of22
3DstructurescombinedwithM-N-doped-carbonfurtherenhancestheirelectrocatalyticactivityand
stability(M-N-doped-3Dgraphene).TheeffectivedispersionofCo O nanosheetsamong3Dgraphene
3 4
sheets was made via argon-plasma synthesis. The Co O nanosheets formed strong bonds with
3 4
graphenesheetsenhancingitselectricalconductivityandelectrochemicalactivity. Thekineticanalysis
resultedakineticcurrentdensityof34mA·cm−2 at−0.75Vvs. RHEandahalf-wavepotentialof
0.832Vvs. RHE.Additionally,ahighstabilitywasobtainedasafter5.5operationalhoursonly2%
ofitsinitiallimitingcurrentdensitywaslost,whileinthecaseofPt(20wt%)/C,ca21%losswas
detected[38].
Thekineticparameters(theavailableones)oftheelectrocatalystsreportedinthecurrentreview,
alongwiththerespectivepreparationmethodsaresummarizedinTable1. Itisobservedthatmostof
theelectrocatalystshavealsobeeninvestigatedfortheORRinpresenceofmethanol,asitscrossover
tocathodehasbeenconsideredasoneofthemajorreasonsforthedecayofORRactivity[3]. Itis
remarkablethateachofthemshowsexcellenttolerancetomethanol,consideringthatjustfewyears
agothemaindisadvantageofthenon-preciouselectrocatalystswasrelatedwiththeirinstabilitydue
toalcoholcrossover[1,3].
It should be pointed out that the data given in literature are often problematic or misleading.
Forexample,thelimitingcurrentdensityfromRDEmeasurementvariesconsiderablyunderthesame
conditions. AccordingtoLevichequation,thesteady-statelimitingcurrentdensityforthe4e− ORRis
ca6.0mA·cm−2inadilutedelectrolyteaqueoussolution. ThedeviationobservedinTable1should
possiblybeoriginatedfromseveralreasons. Anegativedeviationiscausedbyatleast,butnotlimited,
tothefollowingreasons: (i)thepoorqualityofthefabricatedelectrodewithanincompletecoverage
oftheelectrocatalystonthesubstrate;(ii)amixedreactionpathwaywithboth2e− and4e− charge
transfer processes; (iii) poor electrocatalytic activity, and not mass transfer controlled steady state.
Apositivedeviation,theothersideofthecoin,canalsobefoundinliterature(seeTable2),whichis
understandableasfollows. Theloadingofthenon-preciousmetalcatalystisgenerallyprettyhigh,
andthereby,thethicknessofthe“thin-film”electrodecannotbeneglectedanymore.Assuch,theradial
masstransfershouldbeconsideredforsuch“column-like”electrodes. Therefore,thecomparisonis
sometimestrickyandshouldbecautiousinpractice. Inouropinion,thenormalizedkineticcurrent
density,otherthanthemasstransfer-controlledcurrent,shouldbeusedtobenchmarktheactivityin
future. Nevertheless,thedataaresummarizedinthisreview(Tables1–3),whicharephenomenally
usedforreference.
On2016,porousN-dopedcarbonnanotubes(NCNTs)withhighsurfacearea(>1000m2·g−1)are
fabricatedbyKOHactivationandpyrolysisofpolypyrrolenanotubes. TheyexhibitedexcellentORR
electrocatalytic(thoseannealedat900◦C)performanceandyieldedamorepositiveonsetpotential,
higher current density, and long-term operation stability in alkaline media, when compared with
acommerciallyavailable20wt%Pt/Ccatalyst;thisbehaviourismostlyduetothesynergeticeffect
betweenthedominantpyridinic/graphitic-Nspeciesandtheporoustubestructures. Thesemetal-free
porousnitrogen-dopedcarbonnanomaterialscouldbeconsideredaspotentialalternativestoPt/C
catalysts,forelectrochemicalenergyconversionandstorage[40].
2Dmaterials,asthe3Dones,havealsoattractedincreasedinterest,withgraphiticcarbonnitride
(g-C N )asaderivativegraphenetobeidentifiedalsoasagoodORRelectrocatalyst. Moreprecisely,
3 4
thedevelopmentofporousorganicnetworks,whichstemsfromthepolymerizationofrigidorganic
molecules, offers the opportunity of 3D controllable structured electrocatalysts preparation [42].
Recently, Hao et al. [41] exploited the 2D structure of triazine-based framework (metal organic)
and convert it to a 3D porous electrocatalyst. Its stability at 0.6 V vs. RHE was excellent as kept
stableby100%for11hofoperation,aswellasitstoleranceinpresenceof3.0Mmethanol. Moreover,
itscatalyticperformanceisconsideredhighcomparedwiththeresultsreviewedinthepresentwork,
asitshalf-wavepotentialreached0.767Vvs.RHE.
Zeoliticimidazolateframework(ZIF)constitutesanotherprecursorusedoftenforthepreparation
of 2D/3D controllable structures More precisely, this is a zeolite-type nanoscale metal-organic
Catalysts 2016, 6, 159  6 of 22 
Catalysts2016,6,159 6of22
Zeolitic  imidazolate  framework  (ZIF)  constitutes  another  precursor  used  often  for  the 
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ddeeppiicctteedd.. AAllll tthhee eexxaammiinneedd eelleeccttrrooccaattaallyyssttss aarree cchhaarraacctteerriizzeedd bbyy rreellaattiivveellyy ggoooodd ssttaabbiilliittyy;; aafftteerr 1100 hh,, 
tthheeyy lloossee lleessss tthhaann 88%% ooff tthheeiirr iinniittiiaall ccuurrrreenntt ddeennssiittyy.. LLoonnggeerr-‐tteerrmm ssttaabbiilliittyy ffoorr 2255 hh iiss oobbsseerrvveedd ffoorr 
33DD-‐ssttrruuccttuurreedd CCoo-‐NN-‐ddooppeedd hhiigghhllyy oorrddeerreedd mmaaccrroo-‐mmeessooppoorroouuss ccaarrbboonn eelleeccttrrooccaattaallyysstt [[2299]].. TToo tthhee bbeesstt 
ooff oouurr kknnoowwlleeddggee,, tthhisisi sist htheeh higighhesetstv avlauleueo folfo lnogn-gte‐rtemrmst asbtailbitiyli,tyw, hwichhicish miso mstolystalytt raibtturitbeudtteodt htoe t3hDe 
a3sDw aesl wlaesllt oast htoe tmhea cmroacarnod amndes mopeosoropuosrosutrsu scttruurcetsu.res. 
100
%)
nt ( 95
e
urr
c
d 90 1
e 2
z
ali 3
m
4
Nor 85 5
6
7
8
80
0 5 10 15 20 25
Time (h)  
Figure 1. Comparison of stability of 3D‐structured non‐precious carbon based electrocatalysts: (1) 
Figure 1. Comparison of stability of 3D-structured non-precious carbon based electrocatalysts:
NHCs  (N‐doped  hierarchically  carbon)  (at  0.5  V  vs  RHE)  [27];  (2)  3D‐HPC‐N  (N‐doped‐3D‐
(1) NHCs (N-doped hierarchically carbon) (at 0.5 V vs. RHE) [27]; (2) 3D-HPC-N (N-doped-3D-
hierarchically porous carbon materials)‐850 (at 0.55 V vs RHE) [28]; (3) 3D Co‐N‐OMMC‐0.6 
hierarchically porous carbon materials)-850 (at 0.55 V vs. RHE) [28]; (3) 3D Co-N-OMMC-0.6
((CCoo((NNOO3))2∙6·6HH2OO) )(o(ordrdeereredd mmaaccrroo‐m-meessooppoororouuss ccaarrbboonn) )((aatt 00..66 VV vvss. RRHHEE)) [[2299]];; ((44)) 33DD hhiieerraarrcchhiiccaall 
3 2 2
ppoorroouuss-‐CCooFFee22OO44 hhoollloloww nnaannoosspphheerreess (a(at t00.5.566 VV vvss .RHREH) E[)34[]3;4 ](;5) (5M)nMxCnox3CxOo34 xO(x4 =( x0.=4) 0[.346)];[ 3(66]); 
(N6)anNoasnhoeseht eCetoC3Oo4‐Odo-pdeodp‐egdra-gprhaepnhee‐3nDe- 3[D38[]3; 8(]7;)( 72)D2D‐C-CoAoAl‐Ll-DLDHH@@ZIZFI‐F6-76‐78-0800 0(L(LDDHH: :llaayyeerreedd ddoouubbllee 
3 4
hhyyddrrooxxiiddeess,, ZZIFIF: z:ezoeliotliict iicmiidmaizdoalzaotela ftreamfreawmoerwk [o3r9k]; [a3n9d]; (8a) n3dD‐(N8)‐d3oDpe-Nd‐-TdToFp e(tdh-eTrTmFali(ztheedr tmriaalziizneed‐
tbraiaszedin fer-abmaseewdofrrkam)‐7e0w0o (rakt )0-7.60 0V( vast 0R.6HVE)v [s4.1R].H  E)[41].
TThhee hhaallff-‐wwaavvee ppootteennttiiaall ((EE1/2) )isis ccoonnvveennttioionnaalllyly uusseedd ffoorr tthhee ccoommppaarriissoonn ooff tthhee OORRRR aaccttiivviittyy 
1/2
between different electrocatalysts [36]. To evaluate the catalytic activities of the electrocatalysts listed 
between different electrocatalysts [36]. To evaluate the catalytic activities of the electrocatalysts
in Table 1, the half‐wave potentials are depicted in Figure 2. As can be seen, 3D‐structured Co‐N‐
listed in Table 1, the half-wave potentials are depicted in Figure 2. As can be seen, 3D-structured
Cdoop-Ned-d hoipgehdlyh oigrhdleyreodr dmearecdro‐mmaecsroop-omreosuosp coarrobuosn cealrebcotrnocealteacltyrsotc a[t2a9l]y satn[d2 93]Da nndan3oDsheneatn Cosoh3Oee4t‐
doped‐graphene [38] exhibit the same half‐wave potential values (~0.83 V vs. RHE). The specific half‐
Co O -doped-graphene[38]exhibitthesamehalf-wavepotentialvalues(~0.83Vvs.RHE).Thespecific
3 4
wave value is almost the same as that of Pt (ca 0.8 V vs. RHE). However, they differ in their kinetic 
half-wave value is almost the same as that of Pt (ca 0.8 V vs. RHE). However, they differ in their
kcuinreretinct cvuarlrueenst: vthaelu 3eDs: ntahneo3shDeentasn Coosh3Oee4‐tdsoCpoedO‐gr-adpohpeende- g[r3a8p] hexenhieb[it3e8d] eax khiinbeittiecd cuarkreinnet tdicencusirtrye notf 
3 4
d34e nmsitAy∙comf−324  amt A0.·7c5m −V2  vast. 0.R7H5EV, vws.hiRleH  tEh,ew  3hDil‐esttrhuect3uDre-dst rCucot‐uNr‐eddoCpeod-N  h-digohplyed  ohrdigehrleyd omrdaecrreod‐
mmaecsorop-omroesuosp coarrobuosnc oafr b2o3.n2 omfA23∙c.2mm−2.A  ·cm−2.
Catalysts2016,6,159 7of22
Catalysts 2016, 6, 159  7 of 22 
0.9
0.8
0.7
E)0.6
H
R
s
v
V 0.5
al (
nti
e
ot0.4
p
e
v
a
w
alf-0.3
H
0.2
0.1
Nanosheet Co03.O0-4graphene CoN--NHm-eCOsSo M(phoMireoCruasr cshpihcealrleNys -)-d9o0p0ed-TTF-700 HPC-N-850 N- C NT-h9o0ll0 ow NiCo22OD/-Z4ICCFo-6Al7-3-L8D 0CDh0oi HeFr@ear2Oc h4ihcoalll opow rnoausn-ospheres  
Figure2.Comparisonofhalf-wavepotential(Vvs.RHE).(Referencesfromlefttorightcolumn:[38],
Figure 2. Comparison of half‐wave potential (V vs. RHE). (References from left to right column: [38], 
[29],[27],[41],[28],[40],[35],[39],[34]).
[29], [27], [41], [28], [40], [35], [39], [34]). 
3. Hy3.b Hriydbr(Mid e(MtaeltOalx Oidxeid-Ne‐Nitritorgoegnen-C‐Caarrbboonn)) EElleeccttrrooccaatatlaylsytsst s
DespDiteestphitee ftahcet ftahcatt t,hinat,a icnid aiccidmice mdiead,itah, ethtrea tnrasnitsiiotnionm meteatlaol xoixdideesse exxhhibibiitt vveerryy llooww OORRRR aaccttiivviittyy  and
and insufficient stability, in alkaline media, they exhibit high activity, mostly due to the oxygen‐
insufficientstability,inalkalinemedia,theyexhibithighactivity,mostlyduetotheoxygen-containing
containing groups [45]. The key for their enhanced activity is the amplification of their conductivity 
groups[45]. Thekeyfortheirenhancedactivityistheamplificationoftheirconductivitybyintegrating
by integrating their conductive matrix, forming the as‐called hybrid materials; metal oxide‐N‐C [46]. 
theirconductivematrix,formingtheas-calledhybridmaterials;metaloxide-N-C[46].
In Table 2, along with their respective kinetic parameters, the most active ORR hybrid 
In Table 2, along with their respective kinetic parameters, the most active ORR hybrid
electrocatalysts are listed. Most of them seem to present higher ORR activity than the 2D or 3D 
electrocatalysts are listed. Most of them seem to present higher ORR activity than the 2D or 3D
electrocatalysts (Table 1). Characteristic is that all authors report higher limiting current and more 
electrpoocsaittiavley ostnsse(tT aanbdle h1a)l.f‐wCahvaer apcotteernitsitailc viasluthesa tthaalnl aPut/tCh oelresctrreopcaotratlyhsitg (hTearbllei m2)i. tAindgdictuiornraelnlyt, amnodstm  ore
positiovfe thoonssee tthaantd whearlef -ewxaamveinpeodt efonrt imalevthaaluneosl ttohlaenraPntc/e Cweelreec tforoucnadt atloytsatll(yT aubnlaeff2e)c.teAdd bdyi tmioentahlalyn,oml’so stof
thoseptrheastenwceer. eexaminedformethanoltolerancewerefoundtotallyunaffectedbymethanol’spresence.
TabTlaebl2e. 2N. Nono-np‐prereccioiouuss hhyybbrriidd OORRRR eelelcetcrtorcoactaatlyaslytss. ts.
Preparation Method and   Limiting Current (Jd ; d=disc),  
CCaattaalylystst  PreparationEMxpeethriomdeanntdaEl xCpoenridmietinotanlsC onditions KHLiKniHamielnatifeitl icfWtni WcCgaCauCvvureuerr rrePPreneoonntttt eteDDn(nJteetdiinna;alsdsli(i tE=t(yyE1d/( 12J/(i2)kJ)s))kc) ),),,   RReeffeerreennccee
Introduction of metal precursor (cobalt nitrate) to 
CgssgChrohaero0pale.l00phsl.30l-e3@hs8@n‐0eC8eC0no0coeO0a O rc-bNa‐oNr-nbd‐oodpnoe pded  (Is00nMs(u.01.utc1r. rcM1oKMor dMosOKuesK HceOaKOt naiH,Oo Hdnon,Hdxuo+oy xr u+fg0eyr .ame0g5en.feae5Mo n ts flaMaolslMotlapuw ltMoeurreOwearedcatHOeuetbdedr)Hysd ,bo ,p)1yr 10y 0( prcmomoylbVryVoaz∙s·lliystn−1−nzg, 1ii1nta,6rn1ga06d t0ae0 n)0rpdtromp 0m.,1 ,  Ε(J(EJdIId1n 1n/=2/a=a  2=4cc4 =t.t01i.i1vv.0 8me.me81 i1AinA VnV·∙M cc MvmvmessOe−..− O2H2RR H HHpE rEpe sreensceen)ce)   [[4477] ]
CddCeVceVDocDmosmy psnoyptsnhoinetshsgiinesssgooi lfssid BooClBfi dNBC C-Bsha NCnee d‐st shaNneb-eydcot tsNnh tbea‐rycimno tianhnltgelayrpimnreiancluglyr s ors J(JdId n==a 6c6t.0i.0v memiAnA·∙MccmmeO−−2H2  presence)
BBCCNN-2‐.25.a5t .a%t. -%10‐010000  aptnreocrumrasloprrse sastu nreoarmnda0l .p1rMesKsuOrHe ,aonxdyg 0e.n1 sMatu rated, (EIn1/a2c=tiv0.e7 0in7 VMvesO.RHH pEresence)   [[4488] ]
1K0mOVH·,s −o1x,y1g6e00n rspamtu,r(0a.t1eMd, K10O HmV+∙2s.−01,M 16M00eO rHpm) ,  ΕJk1/2= =2 06..67207m VA ·vcms. −R2HE  
CoAl-LDHs(layered G(r0o.1w MCo KAOl-LHD +H s2.o0n Mth eMsueOrfaHce) ofGOin-situvia Jk = 26.62 mA∙cm−2 
doublehydroxide)/rGO coprecipitationandsubsequentlyhydrothermal Jd=4.8mA·cm−2 [49]
(r educedgraphene treatmentand0.1MKOH,oxygensaturated, E1/2=0.853Vvs.RHE
oxide) 10mV·s−1,1600rpm
Catalysts2016,6,159 8of22
Table2.Cont.
LimitingCurrent(Jd;d=disc),
Catalyst PreparationMethodandExperimentalConditions KineticCurrentDensity(Jk), Reference
HalfWavePotential(E1/2))
(ChoyIbI-rAid-r-aGm-Omonium) Smyanttehreiaslisscaotnrosiosmtintgemofpceorbaatultr-ebaosfeadrcohregtaynpoicmaelthayllbicrid Jd=5.4mA·cm−2
hgryadprhoexnidee)-reduced ccoooxmoyrgpdelinenxasetaestdu([rtCaotoeN(da-,cda1oc0p)2me]d,Vag·csra−ac1p=,h1ea6nc0ee0tsyralpanmcdet0o.1naMteK)OH, EJk1/=28=.90m.81AV·cmvs−.2RaHtE0.8Vvs.RHE [50]
Co/N-HCOs OctahedralCo(II)complexwith
(Co/N-co-doped 2,6-bis(benzimidazol-2-yl)pyridine(BBP)asthe Jd=4.9mA·cm−2 [51]
hollowed-outcarbon precursorand0.1MKOH,oxygensaturated, E1/2=0.81Vvs.RHE
octahedrons) 10mV·s−1,1600rpm
CNreo-dR(uOGcHeOd)2(gN-nra-adpnohopepenldaetoe/xide) Hoxyydgreonthseartmuraaltemde,t5homdVa·ns−d10,.116M00KrpOmH, EJd1/=24=.70.m66AV·cmvs−.2RHE [52]
N/Co-doped Pyrolysisofgrapheneoxide-supportedcobalt-based Jd=7.8mA·cm−2
zeoliticimidazolate-frameworkand0.1MKOH, (Inactiveevenafterof48h)
pPoClPy(hpeodrroouns)/caNrRboGnO o5xmygVe·ns−s1a,t1u6r0a0terdp,m(0.1MKOH+3.0MMeOH) EJk1/=21=1.06.9m3AV·cvms.−R2HaEt0.7Vvs.RHE [53]
(CcooCbaNlt@caCrobOonxi(t1r8id)/eN/G H0.1igMhtKemOpHe,roaxtuyrgeenamsamtuornaitaedn,it1r0idmatVio·ns−m1,e1th60o0drapnmd, (JIdn=ac5ti.9veminA·McmeO−H2 presence) [54]
nitrogendoped
graphene) (0.1MKOH+1.0MMeOH) E1/20.763Vvs.RHE
Icnarsbitoungnraonwotthuboefsm,feotalllo–woregdanbiycpfryarmoleywsiosraknsd(Z0I.F1-8M)on Jd=6.0mA·cm−2
C-CZ-4(N-CNTs)-1000 KOH,oxygensaturated,5mV·s−1,1600rpm(0.1M (InactiveinMeOHpresence) [55]
KOH+1.0MMeOH) E1/2=0.887Vvs.RHE
Bnaanmobpoaor-tliickleehCyNbTri/dFse-830C0 0Am.1nenlMaemaKliinOnegH,aa,nomdxiyxFgteue(nNresOoa3ft)uP3rEaaGtte8-dP0,0P1G◦0C-mPiEVnG·Ns−P21lau,nr1od6n0i0crPp1m23, EJd1/=24=.00.m86A1·cVmv−s2.RHE [56]
(Cnoit3rOog4/enN-Gdoped Hfoylldowroethdebrymaaltwreoa-csttieopnhoefaGtOtre,aMtmR,enantdanCdo0C.1l2MKOH, Jd=4.6mA·cm−2 [57]
graphene) oxygensaturated,10mV·s−1,1600rpm E1/2=0.74Vvs.RHE
(CnoitNroPgse@nN-dGoped Tmheetramlsaalltcsoanndden0s.1atMionKoOfHbi,oomxyagssenansdatcuorrarteesdp,onding Jd=7.2mA·cm−2 [58]
graphene) 10mV·s−1,1600rpm(0.1MKOH+1.0MMeOH) E1/2=1.01Vvs.RHE
C(PANI)/Mn2O3 S0.u1rMfacKeOprHo,teocxtyedgecnalscaintuartaiotendp,r1o0cmesVse·ss−a1n,d1600rpm EJd1/=25=.601.7m84AV·cmvs−.2RHE [59]
Ingeneral,Metal-N-dopedcarbon(especiallygraphene)-basedelectrocatalystsarepreferredas
conductivematrixmaterials. Theyaremorepositivethantheothers’hybridmaterials,andhalf-wave
potential as well as onset potential (Figure 3) make them more plausible candidate for cathodes.
Namely,N/Co-dopedgraphene-typeelectrocatalyst(CoNPs@NG)[58]performedhalf-wavepotential
(E )of1.01Vvs. RHE;about0.19VmorepositivethanPt/C’s,thevalueofwhichisusuallyaround
1/2
0.8Vvs. RHE.ItshighactivitywasattributedtothegrowthofN-Conanocrystalthroughthegraphene
planeandnotatitsedges,permittingtheformationofahigheramountofactivesitesavailableto
thereactants. Itsspecialstructurealsoofferedverygoodstabilityperformanceasitisindicatedfrom
Figure 4. After ca 10 h of operation, only about 8% of current decay was observed. Remarkably,
thesamecatalystalsopresentedgoodlong-termstabilityinacidicenvironment,preserving91%ofits
initialactivityafter8.5hofoperation.
Anovelhybridelectrocatalystwithreducednitrogen-dopedgraphene/cobaltembeddedporous
carbon polyhedron was developed by Hou et al. [53]. A four-electron pathway was activated,
performingE =0.93Vvs. RHE.Exceptforitsexcellentactivity,thiscatalystexhibitedalsoagood
1/2
durability,asafter5.5hoperation,itsinitialactivitydecreasedby14.5%incomparisonwithPt/C,
whoseinitialactivitywasreducedmorethan30%forthesameoperationperiod. Itsexcellentactivity
was mainly attributed to the following factors: (i) the doping effect of N and Co; (ii) the porous
structure;and(iii)thegoodcontactbetweenN/Co-dopedcarbonpolyhedronandnitrogen-doped
reducedgrapheneoxide.
Catalysts 2016, 6, 159  9 of 22 
1.1
1.0
E) 0.9
H
R 0.8
s
v
V 0.7
Catalysts2016,6,159 ential ( 0.6 9of22
Catalysts 2016, 6, 159  ot 0.5 9 of 22 
p
e
v 0.4
wa 1.1
alf- 10..03
H
0.2
E) 0.9
H 0.1
R 0.8
otential (V vs N/0000CCoo....-N5670dPospe@C-d NCPZG C-4PB/(aNNm-RbCoGNoO-Tlsi)k -e 1C00N0T/FeC3Co-8AlC0-o0LG0.D0C3@HSs-C/8ro0GO0-O N-dopeCdo-/N-HCCOosI I-A-rC(G-POCA oNIC)/NM@nC2oOO3 (x18)/N G CBoCCo3NO(-/O42.NH5 )Ga-t2n%a-n1o0p0la0te/N-R G O  
p
e
Figure 3. Comparivson0 .o4f half‐wave potentials. (References from left to right bar: [58], [53], [55], [56], 
a
w
[49], [47], [50], [51]alf-, [509.]3, [54], [57], [48], [52]).  
H
0.2
A novel hybrid electrocatalyst with reduced nitrogen‐doped graphene/cobalt embedded porous 
0.1
carbon polyhedron was developed by Hou et al. [53]. A four‐electron pathway was activated, 
wdpweuharrsofa osbmreim laiinitinynitl,gi yaa  Elsa a 1ta/c2tf rt=tiievb 0riu t.95yte3. 5wd  V haNt/ 0sC oCvooo.- N 0rdsPoptsep.he@C -edd RNCePrZGu C-H a4PB/(acfNNtm-oEReibCoGolNdo.O-Tll nsio) Ek -me ,1w xC00iNo0ctTi/seFnre epC3gCio- n8Att lC0- o0hfiLfG0.taD0oCa3@iHScs-aC/nr8rot0 lGO0 o-i O3 Nta-r0sdsco p%:teeC doi-/x(v Nf-icioH)etC CryOoltsI  Ilh- teAdh-ernC(Gee -PtdO cCA  soNarICoa)/eNcpmM@atnCii2osOvneO3e (x ig1do8t)/ yp Neb,G e fCBytorfCChoe3aN O(-/1O42cit.NHs5i 4t)Gao- t 2.nc%ao5n-na1o%f0 p0lt pa0atNe /eliNy-rn RisaG oOctn d oed .mx  IhCtpsioa be;rix ti(esciodei)ln  lae tlwhnseoti  t ahpac  otgPirvoto/iouCtdys,    
strucFFtuiiggruuerr; eea 33n..d CC oo(immii)pp aathrriiesso ognno oooffd hh aaclloff‐-nwwtaaavcvtee  bppeootttweennettieiaanlls s.N. ((R/RCeefofee‐rrdeenoncpceeess dffr rocomamr blleoeffntt  ttopo orriligyghhhtte bdbaarrro: :n[[55 8a8]n],, d[[55 3n3]]i,, t[r[55o55g]],e, [n[55‐66d]],o, ped 
redu[c[44e99d]], , g[[44r7a7]]p,, h[[55e00n]],,e [[ 55o11x]]i,,d [[55e99. ]],, [[5544]],, [[5577]],, [[4488]],, [[5522]]))..  
A novel hybrid ele1c0t0rocatalyst with reduced nitrogen‐doped graphene/cobalt embedded porous 
carbon polyhedron was developed by Hou et al. [53]. A four‐electron pathway was activated, 
95
performing E1/2 = 0.93 V vs. RHE. Except for its excellent activity, this catalyst exhibited also a good 
90
durability, as after 5.5 h operation, its initial activity decreased by 14.5% in comparison with Pt/C, 
whose initial activity%) wa8s5 reduced more than 30% for the same operation period. Its excellent activity 
was mainly attributent (d t8o0 the following factors: (i) the doping effect of N and Co; (ii) the porous 
e
structure; and (iii) thcurre g7o5od contact between N/Co‐doped carbon polyhed 1ron and nitrogen‐doped 
reduced graphene oxzed ide7.0  2
ali 3
Norm10605 45
9650 6
7
9505
8
%) 8550
ent ( 80 0 5 10 Time (h) 15 20 25 
curr 75 1
FsFchaiiggrebuullorrseen‐8 40s4.h0.  Le(laolLtsn o-−g8n0‐0gt.40e-alized  rtV(mear tv m7s−s0t aS0sbC.t4ialEbiVt)iy l[iv 4tcs7yu.]r;c Sv(uC2er)sETv )oewfs[ 4oh7o‐yd]f;bim(hr2iyde)bn Tmrswiidoaotne-madrliia ammtlesea:r nti(eas1rli)sio a:Cnlsao( l0b1.0m)a3s@CaeCtdoeo 0ro.Oi0na3‐l @NtseCb‐rdnoa 23oasOper-ydeN dso-y dngsorttaepemprenhd oaenfrg yeBr a,sc pyCahsr atbeenonmdne  
Nof (BB,(2C ata%n)dN(N5 a(tmB%(2)G65a‐t1%00)N0 (°5C)a [t4%8)]G; (-31)0 0N0/C◦oC‐)do[4p8e]d; P(3C)PN//N/CRoG-Odo (paet d−0P.2C5P V /4 /vNs RAGg/OAg(aCtl)− [503.2];5 (4V) 
vCso.CANg@/CAogOCxl()18[5)/3NorN];G(64 0()cCoboaCltN c@aCrbooOnxit(r1i8d)e//NniGtro(gcoenb adltocpaerdb ognriatprihdeen/en)i t[r5o4g]e; n( 55d) oCpoe3dOg4/rNapGh(nenitero)g[5e4n]‐;
6
(d5o)pCeod3 Og4ra/pNhGen(en)i tr(oagt e−n0-.d3o6p Ved vgsr aSpChEen) e[)5(7a]t; −(60). 3C6oVNvPss.@SNCGE) [(5n7it]r;o(6g)enC‐odNoPp se7@d NgGra(pnhiternoeg)e n[5-d8]o;p (e7d) 
55
Cgr(aPpAhNenI)e/M) [n528O];3 ((7h)yCbr(iPdA) N(aIt) /0.M8 nV2 Ovs3 R(hHyEb)r i[d59)](; aatn0d. 8(8V) Cvos.O/RMHnEO)2/[R59G];Oa (nrde d8(u8c)eCdo gOra/pMhnenOe2 /oRxiGdeO) 
((raet d0u.6c6e dV gvrsa pRhHeEn)e [o6x05i]0d. e)(at0.66Vvs.RHE)[60].
0 5 10 15 20 25
Time (h)  
N-containingmetalorganicframeworks,likeinthecaseofthree-dimensionalcategory,candeliver
reallyFiggouored 4O. RLoRnegl‐etecrtrmo csatatabliylittiyc caucrtviveist yo.f Ahynbirnidi nm-saitteurigarlso: w(1n) Cmoe0t.0a3@l-CorogOa‐nNic‐dforapmede wgroaprkhe(nMe OcaFr)b/ocna rbon
nanosthueblels‐(8C0N0 (Ta)t c−o0.m4 pVo vssi tSeC,iEn) s[t4e7a]d; (2o)fTawsou‐drfimaceengsiroonwaln moanteer,iwalsa sbapsreedp oanre tderannardy tseysstteedmb oyf BG,e Ce atnadl. [55],
usingNZ (BIF(2-8 aat%s)pNr(e5c uatr%so)Gr‐a1n00d0 b°yC)p [y4r8o];l y(3z)i nNg/Ctoh‐edocpateadl yPsCtPa/t/N10R0G0O◦ C(a.t T−0h.e25a sV- pvrse Apga/rAedgCell)e [c5t3r]o; c(a4)t alyst
reachCeodCaNh@aClof-Owx(a1v8e)/NpoGt e(ncotibaallto fca0r.b8o8n7itVridves/.nRitrHogEenan ddopaeldim girtainpghecnuer) r[e5n4t];o (f56) .C0om3OA4/·NcmG(−n2i,trroegmena‐ining
stabldeoepveedn ginratphheenper)e s(eant c−e0.o3f6 1VM vCs HSC3OE)H [.57]; (6) CoNPs@NG (nitrogen‐doped graphene) [58]; (7) 
C(PANI)/Mn2O3 (hybrid) (at 0.8 V vs RHE) [59]; and (8) CoO/MnO2/RGO (reduced graphene oxide) 
(at 0.66 V vs RHE) [60].
Catalysts2016,6,159 10of22
Yang et al. [56] used, in their turn, melamine as natural source of nitrogen, synthesizing
bamboo-likecarbonnanotubeFe CNPshybridelectrocatalyst,whichalsoexhibitedE =0.861V
3 1/2
vs. RHEaswellasexcellenttolerancetowardsmethanol. TheCo/N-dopedgraphenehybridcatalyst,
preparedbyusingureaasnaturalnitrogensource, incombinationwithsucroseandcobaltnitrate
performedahalf-wavepotentialof0.81Vvs. RHE[47]. Moreover,theas-preparedelectrocatalystafter
10hoperationat0.4Vlostonly8%ofitsinitialactivity.
UreawithboricacidandpolyethyleneglycolwereusedforthepreparationofB-N-co-doped
graphite layers [48]. The obtained ORR kinetic current density was ca 27.0 mA·cm−2, while its
half-wavepotentialwasverysatisfactory;ca0.707Vvs. RHE.Itsactivityremainedunaffectedinthe
presenceofmethanol(2.0M)anditsstabilitywasverygood,unchangedfor8.5h,at0.853Vvs. RHE,
while Pt (20 wt %)/C, examined under the same experimental conditions, decreased by 20%.
Inhybrid-BCNstheextraelectronsofnitrogenandtheemptyorbitalofboronareconnectedintothe
carbonmatrix,changingtheelectrondensitydistributionofcarbon[60–62].
Moreover, sandwich-like hybrid porous Co O nanospheres and N-doped graphene using
3 4
melamine as precursor were prepared by Zhang et al. [57]. By using melamine instead of urea
or ammonia as nitrogen source, nitrogen NPs helped being attached not only at the edges of the
graphene, but also in its inert areas; cobalt in each turn is intruded in the inert part of graphene
providingmuchmoreactivearea. However,Co O /NG(nitrogen-dopedgraphene)exhibitedgood
3 4
andclosetoPt’shalf-wavepotential,ca0.74Vvs. RHE,indicatingthatitpossesseshighORRactivity.
Meanwhile,itsstabilitywasgoodsinceafter5.5hdurabilitytestat0.913Vvs. RHElostonly6%ofits
activity,whilePt/Clost30%[57].
Additionally, Chao et al. [51] tried to make more environmentally friendly the
preparation processes of hybrid materials by using inexpensive Co(CH COO) ·4H O and
3 2 2
2,6-bis(benzimidazol-2-yl)pyridine(BBP)asrawmaterials. Asdescribedabove,mostofthefabrication
methods of hybrid materials include multiple and relatively expensive processes. The result was
ahollowed-outoctahedralCo/N-co-dopedcarbonelectrocatalystthatalsoperformedahalf-wave
potentialcloseto0.81Vvs. RHE,andalimitingcurrentdensityof4.9mA·cm−2. Itsactivityremained
almostconstantevenafter8000cyclesfrom0to1.2V,showingadditionallyremarkablemethanol
tolerance[51].
AmongtheinvestigatedN-containingprecursors,polyaniline(PANI)iscommonlyusedforthe
preparationofN-dopedcarbonelectrocatalysts. ThecombinationofPANI’saromaticringsandthe
N-containing groups result in highly efficient ORR hybrid materials [63]. Recently, Cao et al. [59]
reportedhalf-wavepotentialof0.784Vvs. RHEand23hlong-termstability,atmesoporoushybrid
shellofcarbonizedpolyanilineMn O (C /Mn O )electrocatalyst.Theirhighactivityandexcellent
2 3 PANI 2 3
stabilitywasbelievedtoberelatedwith:(i)thehighspecificsurfacearea;(ii)thesurfaceoxidationstate
ofMn;and(iii)thecompositioncodependentbehavior(Mn O andC arestronglydependenton
2 3 PANI
eachother). HybridmaterialshavealsobeendevelopedinordertoimprovethedensityofN-doped
carbonelectrocatalysts. Thelowdensity,observedinmanyN-dopedcarbons,leadstoathickelectrode
layerwithreducedmasstransferability[63,64]. Byincreasingthedensityofthecarbonstructurethe
volumetricORRactivityincreasestoo.
Recently,N-dopedreducedgrapheneoxidematerialshaveattractedmuchinterestduetotheir
ability to increase electrocatalytic activity of the simple N-doped graphene electrocatalyst [65–67].
InN-dopedgraphene,ithasbeenfoundthatoxygengroupsareresponsiblefortheformationofthe
C-Nbonds,increasingtheconductivityofcarbons. ThereductionofN-dopedgrapheneincreasesthe
oxygengroupsandconsequentlytheelectronic-conductivity;reducedgrapheneoxideexhibitn-type
electrondopingbehavior.
The introduction of metal oxides into N-doped graphene sheets, in order to form the hybrid
material, can be processed through the following two-ways: (i) on the surface of the sheets;
or(ii)through the plane of the graphene nanosheets. The second approach results in a better