DTIC ADA578769: Vertically Aligned BCN Nanotubes as Efficient Metal-Free Electrocatalysts for the Oxygen Reduction Reaction: A Synergetic Effect by Co-Doping with Boron and Nitrogen

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DOI:10.1002/anie.201105204
Oxygen Reduction
Vertically Aligned BCN Nanotubes as Efficient Metal-Free
Electrocatalysts for the Oxygen Reduction Reaction: A Synergetic
Effect by Co-Doping with Boron and Nitrogen**
Shuangyin Wang, Eswaramoorthi Iyyamperumal, Ajit Roy, Yuhua Xue, Dingshan Yu, and
Liming Dai*
The oxygen reduction reaction (ORR) is an important poisoning and crossover effect and having better long-term
process in many fields, including energy conversion (fuel operational stability than commercially available Pt-based
cells,metal–airbatteries),[1]corrosion,[2]andbiosensing.[3]For electrodes.[11–17] The enhanced catalytic activity of these
fuel cells, the cathodic oxygen reduction is a major factor metal-free nitrogen-doped carbon nanomaterials toward
limiting their performance.[4] The ORR can proceed either ORRcouldbeattributedtotheelectron-acceptingabilityof
through a four-electron process to directly combine oxygen nitrogen species, which creates net positive charges on
withelectronsandprotonsintowaterastheendproduct,ora adjacent carbon atoms to facilitate oxygen adsorption for
less efficient two-step, two-electron pathway involving the ORR with low overpotential. The well-defined high surface
formation of hydroperoxide ions as intermediate.[4] Oxygen area and intertube spacing for improved electrokinetics, as
reductionalsooccurs,albeittooslowlytobeofanypractical well as the good electrical and mechanical properties
significance, in the absence of an ORR catalyst on the associatedwithverticallyalignedN-dopedcarbonnanotubes
cathode.Platinumnanoparticleshavelongbeenregardedas (VA-NCNTs) provide additional benefits to the metal-free
thebestcatalystfortheORRandarestillcommonlyusedin nanotubeORRelectrodeinachievingrecordelectrocatalytic
fuel cells due to their relatively low overpotential and high performance.[11]Morerecently,Yangetal.[17]reportedboron-
current density with respect to other commercial catalysts.[5] doped carbon nanotubes (BCNTs) as ORR electrocatalysts
However, the ORR kinetics on the Pt-based electrode is with improved activities relative to undoped CNTs. On the
sluggish,[5]andthePtelectrocatalyststillsuffersfrommultiple basis of experimental analyses and theoretical calculations,
drawbacks, such as susceptibility to fuel crossover from the they concluded that the B atoms in the BCNT lattice are
anode,deactivationbyCO,andpoorstabilityunderelectro- positivelychargedandactastheactivesitesforORR.
chemicalconditions.[6]Inaddition,thehighcostofPtandits Incontrasttoall-carbonnanotubes[18,19]carbonnanotubes
limitednaturalreservesarethemajorbarrierstomass-market containingbothBandNatoms(BCNnanotubes),[20–23]either
fuelcellsforcommercialapplications.[5] in an aligned or nonaligned form, are bandgap-tunable by
Recently,considerableeffortshavebeenmadetodevelop means of their chemical composition.[22] Unlike CNTs, the
advancedelectrocatalystsforreducingorreplacingPt-based bandgap of BCN nanotubes is independent of the diameter
electrodes in fuel cells.[7–10] In particular, certain nitrogen- andchirality.[24–26]Thisuniquestructure–propertyrelationship
dopedcarbonnanomaterials(e.g.,carbonnanotubes(CNTs), makes BCN nanotubes attractive candidates for potential
graphene, porous carbon) were demonstrated to act as usesinmanyareaswhereCNTshavebeenexploited.[20,27]In
effective metal-free ORR electrocatalysts free from CO particular, the superb thermal stability and chemically
tunable bandgap of BCN nanotubes provide tremendous
opportunitiestotunenanotubeelectronicpropertiesfortheir
[*] Dr.S.Wang,[+]Dr.E.Iyyamperumal,[+]Dr.Y.Xue,Dr.D.Yu,
useasanefficientmetal-freeORRelectrode,evenatelevated
Prof.L.Dai
DepartmentofMacromolecularScienceandEngineering temperatures.
CaseWesternReserveUniversity Here we report, for the first time, metal-free ORR
10900EuclidAvenue,Cleveland,Ohio44106(USA) catalysts based on vertically aligned BCN (VA-BCN) nano-
E-mail:[email protected] tubes containing both B and N atoms and exploit possible
Dr.Y.Xue synergetic effects of co-doping with B and N on the ORR
Onleavefrom:SchoolofOphthalmology&Optometry activities by comparison with vertically aligned N-doped
WenzhouMedicalCollege,Zhejiang325027(China)
carbon nanotubes (VA-NCNTs) and vertically aligned B-
Dr.A.Roy
dopedcarbonnanotubes(VA-BCNTs).
ThermalScienceandMaterialsBranch,Materials&Manufacturing
While synthesis of VA-CNTs have been widely
Directorate,AirForceResearchLaboratory
reported,[18]thereismuchlessdiscussionintheliteratureon
Dayton,OH45433(USA)
[+] Theseauthorscontributedequally. the synthesis of VA-BCN nanotubes, most probably due to
technical difficulties.[20–22] In most of the previous studies,
[**] ThisworkwassupportedfinanciallybyAFOSR(FA9550-10-1-0546)
ternarycompounds(e.g.,ferrocene,melamine,boronoxide)
andMURIunderLowDensityMaterialsProgram(Dr.Joycelyn
Harrison–programManager). were used as precursors for nanotube synthesis by metal-
SupportinginformationforthisarticleisavailableontheWWW catalyzed (e.g., Ni) thermal chemical vapor deposition
underhttp://dx.doi.org/10.1002/anie.201105204. (CVD), with and without plasma enhancement.[20–22] In the
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Vertically Aligned BCN Nanotubes as Efficient Metal-Free
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Electrocatalysts for the Oxygen Reduction Reaction: A Synergetic Effect
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currentwork,VA-BCNnanotubeswerepreparedbypyrolysis BCN nanotubes. Like many B-/N-doped nanotubes,[23] the
of melamine diborate, a single-compound source of carbon, presenceofanO1speakintheVA-BCNnanotubesampleis
boron, and nitrogen for BCN nanotube growth. Use of a possibly due to the incorporation of physicochemically
single precursor compound simplifies the nanotube growth adsorbedoxygen,[30,31]whichsuggestsanadditionaladvantage
process. Melamine diborate was synthesized by treating as ORR electrode.[32] The absence of any metal peak in the
melamine with boric acid (see Supporting Information). XPS spectra shown in FigureS3 (Supporting Information)
Detailed growth conditions for the VA-BCN nanotubes can indicates that the metal catalyst particles formed at the
be found in the Supporting Information, while the VA- bottom of each of the nanotubes during the “base-growth”
NCNTsandVA-BCNTswerepreparedaccordingtoreported processwerecompletelyremoveduponmechanicalremoval
procedures.[28,29] of the nanotube sample from the growth substrate[33] and/or
subsequent HCl washing (see Supporting Information).
Among the chemically bonded C, B, and N atoms, the C
contentofVA-BCN(85.5%)dominatesoverB(4.2%)andN
(10.3%),andthehighCcontentintheVA-BCN nanotubes
ensures high conductivity, compared to other BCN materi-
als[34]forelectrochemicalapplications,whilethepresenceofB
and N could significantly enhance the ORR activity (see
below). FiguresS4–S6 of the Supporting Information show
high-resolutionXPS,FTIR,andRamanspectra,fromwhich
more detailed chemical information can be obtained, as
schematically shown in FigureS7 (Supporting Information).
All four electrode materials (i.e., VA-CNT, VA-NCNT,
Figure1. SEM(a)andTEM(b)imagesofVA-BCNnanotubes. VA-BCNT, and VA-BCN nanotubes) showed a substantial
reduction process in the presence of oxygen, whereas no
obvious response was observed under nitrogen (Figure2).
Figure1a shows a typical SEM image (Hitachi S4800-F) The onset and peak potentials of ORR on the VA-BCNT,
of the VA-BCN nanotubes. The as-synthesized VA-BCN VA-NCNT, and VA-BCN nanotube electrodes are more
nanotubes are perpendicularly aligned on the SiO/Si sub- positive,andthecurrentdensitiesmuchhigher,thanthoseon
2
strateandalmostfreefrompyrolyticimpurities(e.g.,carbon theVA-CNTelectrode,[11]thatis,heteroatomdopingwithN
particles and other carbonaceous materials) with a fairly and/orBeffectivelyimprovedtheORRactivity.Comparison
uniform length of about 15mm. The length of the aligned between VA-BCNTand VA-NCNTelectrodes shows that N
nanotubescanbevariedoverawiderange(uptoseveraltens dopingismoreefficientthanBdopingforORRintermsof
of micrometers) in a controllable fashion by changing the theonset/peakpotentialandcurrentdensity,thoughtheVA-
experimentalconditions(e.g.,pyrolysistime,flowrate).The NCNT (4.1% N) prepared by pyrolysis of ferrocene under
well-aligned relatively short VA-BCN nanotubes shown in ammonia(seeSupportingInformation)usedinthisstudydid
Figure1acouldfacilitatethediffusionofelectrolyteionsand not show the record high ORR activity reported for its
oxygenduringtheORRprocess.Alignedmorphol-
ogies of VA-BCNT and VA-NCNT were also
observed (FigureS1, Supporting Information).
High-resolution TEM (HRTEM, Zeiss, 300kV)
images of an individual BCN nanotube show a
hollowtubeofVA-BCNwithanouterdiameterof
about 40nm and a wall thickness of about 2–3nm.
The bamboo-like structure shown in Figure1b is a
characteristic feature of multiwalled BCN nano-
tubes.[20,22]
Electron energy loss spectroscopy (EELS) was
usedtocharacterizetheK-edgeabsorptionforB,C,
and N. Typical EELS spectra from an individual
BCNnanotube(FigureS2,SupportingInformation)
show three distinct absorption features at 188, 284,
and403eV,whichcorrespondtotheexpectedB,C,
andNK-edges,respectively,[22]andindicatesuccess-
ful incorporation of B, C, and N in the VA-BCN
nanotubes. These results are further confirmed by
X-rayphotoelectronspectroscopic(XPS,VGMicro-
tech ESCA 2000) measurements. As expected, the
Figure2. Cyclicvoltammetrycurvesofa)VA-CNT,b)VA-BCNT,c)VA-NCNT,and
XPSsurveyspectra(FigureS3,SupportingInforma- d)VA-BCNelectrodesinnitrogen-andoxygen-saturated0.1mKOHaqueous
tion) show B 1s, C 1s, and N 1s peaks for the VA- electrolytesolutions.Thescanratewas50mVs(cid:2)1.
2 www.angewandte.org (cid:2)2011Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim Angew.Chem.Int.Ed.2011,50,1–6
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counterpartfrompyrolysisofiron(II)phthalocyanine[11]due muchmorepositivethanthatofVA-BCNT((cid:2)0.5V)andVA-
to its relatively low nitrogen content (see Supporting Infor- NCNT((cid:2)0.3V),asshowninFigure3a.Thediffusioncurrent
mation). Of the four electrodes investigated here, the VA- densityfromtheVA-BCNelectrodeisalsomuchhigherthan
BCNnanotubeelectrodeismostactiveintermsoftheonset those from the VA-BCNTand VA-NCNTelectrodes. Thus,
and peak potentials as well as the current density, that is, a the VA-BCN nanotube electrocatalysts showed the highest
synergetic effect resulted from co-doping of the carbon activitytowardsORRamongallnanotubeelectrodesstudied
nanotubes with N and B. This is because not only both the in the present work. This again indicates a synergetic effect
isolated N and B atoms shown in FigureS7 (Supporting arising from co-doping of CNTs with both B and N atoms.
Information)canactasactivesitesforORRthroughcharge Furthermore, Figure3a also shows an even more positive
transfer with neighboring C atoms,[11,17] but also interaction half-wave potential in LSV curves and higher diffusion
between adjacent N and B atoms could facilitate charge current density for the VA-BCN nanotubes with respect to
transferwithneighboringCatoms,andhencefurtherenhance thePt/Celectrode.
the ORR performance of the VA-BCN nanotube electrode. Figure3bshowsTafelplotsforORRonvariouselectro-
To gain further insight into the ORR on VA-BCN, VA- des derived from Figure3a. The polarization curves in
BCNT, VA-NCNT, and VA-CNT electrodes, rotating disk Figure3a were corrected for diffusion effects by using
electrode (RDE) voltammetry (linear-sweep voltammetry, Equation(1)[35]
LSV)wasperformedinO-saturated0.1mKOHsolutionata
2
scanrateof10mVs(cid:2)1(Figure3).Forcomparison,RDEtests j ¼ jjD ð1Þ
k j (cid:2)j
D
wherej isthekineticcurrentdensity,j thelimitingcurrent
k D
density, and j the measured current density. As seen in
Figure3b,theTafelplotsclearlyshowtheactivitydifferences
for the nanotube electrodes. At (cid:2)0.3V, for instance, the
kineticcurrentdensityofORRontheVA-BCNnanotubesis
around10.13mAcm(cid:2)2,whichissignificantlyhigherthanthat
ofVA-CNT(0.10mAcm(cid:2)2),VA-BCNT(0.28mAcm(cid:2)2),and
VA-NCNT (4.24mAcm(cid:2)2) electrodes, and even slightly
higherthanthatofPt/C(8.34mAcm(cid:2)2).
Figure3. a)Linear-sweepvoltammetrycurvesofvariouselectrodesin
To further quantitatively characterize these nanotube
oxygen-saturated0.1mKOHelectrolyteatascanrateof10mVs(cid:2)1and
electrodes,weusedEquation(2)tocalculatethetransferred
arotationrateof1000rpm.b)TafelplotsderivedfromFigure3ain
electron number n per oxygen molecule for each of the
thelow-currentregion.
electrodes.[11,36]
4j
werealsoperformedonacommercialPt/Celectrode(C2-20, n¼ D ð2Þ
20% platinum on Vulcan XC-72R; E-TEK). As shown in jDþjNR
Figure3a,thetypicaltwo-steppathwaywasobservedforthe
VA-CNTelectrodeataround(cid:2)0.28and(cid:2)0.65V,indicatinga In this equation, j is the faradic disk current, j the faradic
D R
successive two-electron reaction pathway, instead of the ring current (Figure4a–e), and N the collection efficiency
direct four-electron pathway seen for the commercial Pt/C (0.3)oftheringelectrode.[11]Figure4fshowsthedependence
electrode. VA-BCNT shows a similar LSV profile to VA- of n on the disk potential. The n value increases as the
CNT, but with a more positive onset potential and higher potentialmovestowardsmorenegativevaluesbeforereach-
currentdensity,andhenceanoverallbetterORRactivity.Itis ingthelimitingvalueofn(cid:3)3.7.AsshowninFigure4f,ORR
believedthatborondopingcouldfacilitatechemisorptionof ontheVA-CNTelectrodeproceededviaa2epathwayinthe
oxygen, which thus led to a relatively high catalytic activity low-overpotential region with generation of hydrogperoxide
towardORRwithrespecttotheVA-CNTelectrode.[27,31]On ions. Subsequent oxidation of hydrogen peroxide also fol-
theVA-NCNTandVA-BCNelectrodes,theLSVcurvesshow lowed a 2e pathway, leading to overall nearly 4e reaction in
a single-step wide platform, indicating a four-electron ORR the high-overpotential region. Similar ORR kinetics was
process. Interestingly, the half-wave potential (i.e., the observedfortheVA-BCNTelectrode.OntheVA-NCNTand
potentialatwhichthecurrentishalfofthelimitingcurrent) VA-BCN electrodes, however, ORR followed a direct 4e
forORRontheVA-CNTelectrodein0.1mKOHsolutionof pathway over the entire potential range by directly forming
about(cid:2)0.7V(Figure3a)ismuchmorenegativethanthoseof OH(cid:2)ionsasfinalproduct,asisthecasewiththecommercial
theotherelectrodes.TheORRcurrentdensityfromtheVA- Pt/Celectrode.Thus,theVA-BCNnanotubeelectrodeshows
CNTelectrodeoverthepotentialrangecoveredisalsomuch highdiffusioncurrentdensity,highpositivehalf-wavepoten-
lower than those from other electrodes tested in this study. tial, high electron transfer number ((cid:3)4), and high kinetic
These results clearly indicate that B/N doping could signifi- current density, which already outperforms the commercial
cantly improve the electrocatalytic activity of the CNT Pt/CelectrocatalystsforORRinalkalineelectrolyte.
electrodes toward ORR. On the other hand, the half-wave In view of the potential of VA-BCNs as effective ORR
potential of the VA-BCN nanotube electrode ((cid:2)0.25V) is catalyststoreplacethecommerciallyavailablePt/Celectrode,
Angew.Chem.Int.Ed.2011,50,1–6 (cid:2)2011Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim www.angewandte.org 3
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Figure5. ChronoamperometricresponseforORRatVA-BCNandPt/C
electrodesa)onadditionof3mmethanolafterabout200sandb)on
introductionCOafterabout300sat(cid:2)0.3V.c)Durabilityevaluationof
Pt/CandVA-BCNnanotubeelectrodesfor50000sat(cid:2)0.2Vanda
rotationrateof1000rpm.
Insummary,wehave,forthefirsttime,preparedvertically
alignedBCNnanotubesbypyrolysisofmelaminediborate,a
single-compound source of carbon, boron, and nitrogen for
BCN nanotube growth. Due to a synergetic effect arising
from co-doping of CNTs with boron and nitrogen, the
resultant VA-BCN nanotube electrode has higher electro-
Figure4. RRDEtesting(LSVcurves)ofORRona)VA-CNT,b)VA-BCNT,
c)VA-NCNT,d)VA-BCN,ande)Pt/Celectrodesinoxygen-saturated0.1m catalytic activity for ORR in alkaline medium than its
KOHelectrolyteatascanrateof10mVs(cid:2)1andarotationrateof1000rpm. counterparts doped with boron or nitrogen alone (i.e., VA-
f)Plotofelectron-transfernumbernagainstelectrodepotential. BCNTor VA-NCNT). The observed superior ORR perfor-
mancewithgoodtolerancetomethanolandcarbonmonoxide
andexcellentdurabilityfortheVA-BCNnanotubeelectrode
comparedtoacommercialPt/Celectrodeopensupavenues
we further tested the electrochemical stability, possible for the development of novel, efficient, metal-free ORR
methanolcrossover,andCOpoisoning.Thecurrent-time(i– catalystsbyco-doping.
t)chronoamperometricresponses[12]forORRattheVA-BCN
and Pt/C electrodes (Figure5) show a sharp decrease in Received:July25,2011
currentonadditionof3.0m methanolforthePt/Celectrode Revised:August23,2011
Publishedonline:&&&&,&&&&
(Figure5a).Incontrast,theamperometricresponsefromthe
.
VA-BCN electrode remained almost uncharged even after
Keywords: doping·electrochemistry·fuelcells·nanotubes·
theadditionofmethanol.Therefore,theVA-BCNelectrode
oxygenreduction
has a higher selectivity toward ORR and better methanol
tolerance than the commercial Pt/C electrocatalyst. To
examine CO poisoning, 10vol% CO/O was introduced
2
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Communications
Communications
Oxygen Reduction Asingle-compoundsourceofB,C,andN
wasusedforthegrowthofvertically
S.Wang,E.Iyyamperumal,A.Roy,Y.Xue, alignedBCN(VA-BCN)nanotubes(NTs).
D.Yu,L.Dai* &&&&—&&&& Owingtoasynergeticeffectofco-doping
ofCnanotubes(CNTs)withNandB,the
VerticallyAlignedBCNNanotubesas VA-BCNNTsshowsignificantlyimproved
EfficientMetal-FreeElectrocatalystsfor electrocatalyticactivity(e.g.,current
theOxygenReductionReaction:A density)fortheoxygenreductionreaction
SynergeticEffectbyCo-Dopingwith comparedtoundopedVA-CNTs,CNTs
BoronandNitrogen dopedwithonlyBorN(VA-BCNT,VA-
NCNT),andacommercialPt/Celectro-
catalyst(seepicture).
6 www.angewandte.org (cid:2)2011Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim Angew.Chem.Int.Ed.2011,50,1–6
(cid:2)(cid:2)These are not the final page numbers!