ChineseJournalofCatalysis34(2013)1105–1111
催化学报2013年第34卷第6期|www.chxb.cn a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m j o u r n a l h o m e p a g e : w w w . e l s e v i e r. c o m / l o c a t e / c h n j c Article Effectofmethanolconcentrationonoxygenreductionreaction activityofPt/Ccatalysts
TANGShuihuaa,b,c,*,LINWenfengb,PaulA.CHRISTENSENb,GeirMartinHAARBERGc
StateKeyLabofOilandGasReservoirGeology&Exploitation,SouthwestPetroleumUniversity,Chengdu610500,Sichuan,China
SchoolofChemicalEngineeringandAdvancedMaterials,NewcastleUniversity,NE17RUNewcastleuponTyne,UnitedKingdom
cDepartmentofMaterialsScienceandEngineering,NorwegianUniversityofScienceandTechnology,Trondheim7491,Norway a
b
A R T I C L E I N F O
Articlehistory:
Received6November2012
Accepted14November2012
Published20June2013
Keywords:
Platinum
Carbon
Methanolcrossover
Oxygenreduction
Fuelcell
A B S T R A C T
Theoxygenreductionreaction(ORR)activityofPt/Ccatalystswasinvestigatedinelectrolytesof0.5
mol/LH2SO4containingvaryingconcentrationsofmethanolinahalf‐cell.ItwasfoundthattheORR
activitywasimprovednotablyinanelectrolyteof0.5mol/LH2SO4containing0.1mol/LCH3OHas
comparedwiththatin0.5mol/LH2SO4,0.5mol/LH2SO4containing0.5mol/LCH3OH,or0.5mol/L
H2SO4containing1.0mol/LCH3OHelectrolytes.ThesametendencyforimprovedORRactivitywas
also apparent after commercial Nafion® NRE‐212 membrane was hot‐pressed onto the catalyst
layers. The linear sweep voltammetry results indicate that the ORR activities of the Pt/C catalyst
werealmostidenticalinthe0.5mol/LH2SO4+0.1mol/LCH3OHsolutionbeforeandaftercoated
with the Nafion® membrane. Electrochemical impedance spectroscopy results demonstrated that
theresistanceoftheNafion®membraneissmallerintheelectrolyteof0.5mol/LH2SO4+0.1mol/L
CH3OH than in other electrolytes with oxygen gas feed. This exceptional property of the Nafion®
membraneisworthinvestigatingandcanbeappliedinfuelcellstackstoimprovethesystemper‐
formance.
©2013,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences.
PublishedbyElsevierB.V.Allrightsreserved.
1. Introduction
The direct methanol fuel cell (DMFC) is one of the most
promising candidates for portable power sources. One of the
currentchallengesforthisindustryismethanolcrossoverthat
leadstothepoisoningofcathodecatalystsandenvironmental
pollution[13].Therefore,thedevelopmentofamethanolre‐
sistant membrane and methanol‐tolerant catalysts is of great
importanceindevelopingDMFCs.
Extensivestudieshavebeencarriedouttoimprovetheac‐
tivities of catalysts [4,5], develop methanol resistant mem‐
branes [69], and methanol‐tolerant catalysts [1013]. Pd‐Pt
nanodendrites were synthesized by reducing K2PtCl4 with
L‐ascorbic acid in the presence of uniform Pd nanocrystal
seeds. On the basis of equivalent Pt mass for the ORR, these
weretwoandahalftimesmoreactiveinanaqueoussolution
than state‐of‐the‐art Pt/C catalysts. They were also five times
moreactivethanfirst‐generationPt‐blackcatalystsbecauseof
thecontrolledmorphologyofPtnanostructures[4,5].Renetal.
[6]preparedorganicsilicawiththiolgroup/Nafion®composite
membranesbyusingacastingmethodthatusedvariousaddi‐
tives including HS(CH2)3CH3Si(OCH3)2(SH–), tetraethyl ortho‐
silicate(TEOS),andHS(CH2)3CH3Si(OCH3)2‐TEOS(HS‐TEOS)in
Nafion® solutions. The 125 μm SH‐TEOS/ Nafion® composite
membranesshowedanapproximately50%decreaseofmeth‐
anolcrossovercomparedwiththatofcommercialNafion®117
membrane.However,theprotonconductivityofthemembrane
wasslightlylowered.Core‐shellPtM/C(M=Fe,Co,Ni,Pd,Cu,
etc)catalystsareregardedasgoodmethanol‐tolerantcatalysts
becauseoftheirhigherplatinumsurfaceareaanduniquealloy
*Correspondingauthor.Tel/Fax:+86‐28‐83032879;E‐mail:[email protected] DOI:10.1016/S1872‐2067(11)60480‐4|http://www.sciencedirect.com/science/journal/18722067|Chin.J.Catal.,Vol.34,No.6,June2013 1106
TANGShuihuaetal./ChineseJournalofCatalysis34(2013)1105–1111
structure[1013]. The influence of methanol on catalyst activity and catalyst
layershavebeeninvestigatedbyACimpedance[1418].Duet
al. [16] reported that the half‐cell was ideal for the Electro‐
chemicalImpedanceSpectroscopystudiesofDMFCelectrodes
asthesystemnotonlyfacilitatesanaccuratepotentialcontrol
butalsoreflectstheactualmasstransportprocessthatoccurs
inpracticalDMFCs.Thesteady‐stateresultsconfirmedthatthe
presenceofmethanolatthecathodecouldleadtoasignificant
poisoning effect on the ORR, especially when the DMFC oper‐
atesathighermethanolconcentrationsanddischargesatlower
potentials. Affoune et al. [17] measured the conductivity and
surface morphology of Nafion® 117 membranes in water and
alcohol environments, where the conductivity was 0.1013,
0.0206, 0.0311, 0.0667, and 0.0912 S/cm for water, pure
methanol, and methanol‐H2O solutions with CH3OH/H2O vol‐
ume ratios of 75%25%, 50%50%, and 25%75%, respec‐
tively. The proton conductivity decreased with increasing
methanol content [17]. Aricò et al. [18] used a gas diffusion
electrodethatmountedintoaTeflonholdercontainingaplati‐
numringcurrentcollectorandhavinganoxygengasfeed(1×
105 Pa, 200 cm3/min) as the working electrode. The oxygen
reductionbehaviorsofthe electrodescoated anduncoated by
Nafion® film were characterized in solutions of 2.5 mol/L
H2SO4and2.5mol/LH2SO4containing0.5mol/LCH3OHelec‐
trolyte. The oxygen reduction process was more favorable at
low current densities with the Nafion® coated electrode. The
opencircuitvoltagewasapproximately200mVhigherforthis
electrode than the analogous uncoated electrode. At current
densitieshigherthan100mA/cm2,theactivityoftheprepared
electrodesurpassesthatoftheNafion®coatedelectrode.These
phenomena have been interpreted on the basis of the large
oxygensolubilityoftheNafion®electrolyte‐electrodeinterface
thatcausesadecreaseofover‐potentialatlowcurrentdensity.
Ohmicdropandmasstransfercontrolsignificantlyreducethe
activity of Nafion®‐coated electrodes at high currents. At po‐
tentialslowerthanthosecorrespondingtoPt‐oxidereduction,
methanolisoxidizedeveninthecathodicpolarization[18]. Inthispaper,carbondiffusionlayerswerecoatedwithPt/C
catalysts (Johnson Matthey Company) of different Pt loadings
andinvestigatedina0.5mol/LH2SO4solutioncontainingvar‐
yingconcentrationsofmethanol.CommercialmembraneNafi‐
on® NRE‐212 was then hot‐pressed onto the surface of the
catalyst layers and the half‐membrane electrode assembly
(half‐MEA)wasinvestigatedunderidenticalconditions.Several
notable results were obtained when Pt/C catalysts and the
half‐MEAsweretestedin0.5mol/LH2SO4solutionscontaining
differentconcentrationsofmethanol.
2. Experimental
2.1. Preparationofgasdiffusionlayer
Toraycarbonpaper(TGP‐H‐030,Toray,Japan)wastreated
bya6wt%PTFEemulsionuntilthedryPTFEloadingreached
ca. 0.8 mg/cm2.The slurryof carbonblackVulcanXC72Rand
PTFE (w/o = 1:1) was then brushed onto both sides of the
PTFE‐treated carbon paper until the carbon loading was ap‐
proximately 1.5 g/cm2. The gas diffusion layer was prepared
after being treated at 340 °C for half an hour under nitrogen
flow.
2.2. Fabricationofworkingelectrodes
Pt/C (20 wt% HiSPECTM 3000, Johnson Matthey) and Pt/C
(50wt%HiSPECTM8000,JohnsonMatthey)catalystinkswere
prepared with a mass ratio of 10:90 or 15:85 for dry Nafi‐
on®/catalyst, respectively. Cutting the gas diffusion layer into
discs of Ø = 13 (s = 1.3273 cm2), the catalyst ink was then
droppedontoonesideofthediscrepeatedly.Afterthecatalyst
Pt Wire
O2
Diffusion Layer
Nafion Membrane
Catalyst Layer
O2 Inlet
Pt Ring
Teflon
Teflon Screw Lid with a
hole of Ø = 11.7
Electrode
(a)
(b)
Fig.1. Schematicstructureofelectrode(a)andconfigurationofworkingelectrode(b). TANGShuihuaetal./ChineseJournalofCatalysis34(2013)1105–1111
ink dried, the electrodes were treated in an oven at 80 °C for
half an hour. Two parallel catalyst electrodes were fabricated
andtestedforeachcatalyst.
2.3. Electrochemicalmeasurements
TheORRactivityof20wt%Pt/Ccatalystswastestedinthe
electrolytesof0.5mol/LH2SO4containingdifferentconcentra‐
tionsofmethanol.Afterthat,theelectrodewastakenoutofthe
electrolyteandrinsedwithhighpurityofwater,thendriedin
an80°Coven.SubsequentlyNafion®perfluorinatedresinsolu‐
tion(5wt%)wasdroppedontothetopofthecatalystlayerto
form a thin film, then two layers of pre‐treated commercial
Nafion® membrane NRE‐212 (Aldrich, Item# 676470) were
hot‐pressedontothecatalystlayeronebyoneat130°C,which
is denoted ashalf‐MEA. Theabovementioned testswere per‐
formedagain.Atthesametime,anewuntestedelectrodewas
hot‐pressed with Nafion® NRE‐212 membrane and subse‐
quentlyinvestigated. For 50 wt% Pt/C (HiSPECTM 8000, John‐
sonMatthey)catalysts,bothelectrodeswerehot‐pressedwith
twolayersofNafion®NRE‐212membranebeforetesting. Theconfigurationofthehalf‐MEAisshowninFig.1(a).The
electrode or half‐MEA was mounted onto a Teflon skeleton
with the catalyst layer or Nafion® membrane exposed to the
electrolyte. Oxygenwasintroduced fromthe backofthecata‐
lyst layer as shown in Fig. 1 (b), which is similar to that de‐
scribedinreference[18].Allmeasurementswerecarriedoutin
athree‐electrodecell.APtslicewasusedasthecounterelec‐
trode, and Ag/AgCl(s) electrode was used as the reference
electrode. The potentials mentioned below were recorded
against Ag/AgCl(s) unless noted. Oxygen was introduced for
morethan10minbeforelinearsweepvoltammetry(LSV)tests
or electrochemical impedance spectroscopy (EIS) measure‐
ments.After changingthe electrolyte,oxygen was stopped for
half an hour to ensure the replaced electrolyte efficiently gets
access to the catalyst layer. The electrodes were not rinsed if
they were placed in more concentrated electrolyte; however,
they were rinsed with high purity de‐ionized water when
placedinadilutedelectrolyte.
3. Resultsanddiscussion
Thecompositionsoftheelectrodesorhalf‐MEAsof20wt%
Pt/Cand50wt%Pt/CcatalystsarelistedinTable1.
3.1. InfluenceofmethanolconcentrationonORRactivity
The ORR activities of 20 wt% Pt/C catalysts in different
Table1 Theparametersofelectrodesorhalf‐MEAs.
Item
20wt%Pt/C (JohnsonMatthey)
50wt%Pt/C (JohnsonMatthey)
Catalyst
mass
(mg)
10.4wt%
15.0wt%
Nafion®
5wt%
Ptloading
loading
Nafion®
2
(mgPt/cm )
solution
(mg/cm2)
dryNafion®
0.47
24.5
0.62
N/A
dryNafion®
4.13
25.5
4.74
26.8
Current density (mA/(mgPt.cm2))
1107
50
0
-20.2
-51.0
-47.2
-51.5
-91.2
-50
-100
-113.4
-150
-200
0.5 mol/L H2SO4
0.5 mol/L H2SO4 + 0.1 mol/L CH3OH
0.5 mol/L H2SO4 + 0.5 mol/L CH3OH
-250
-300
0.2
0.4
0.6
0.8
Potential (V vs Ag/AgCl (s))
1.0
Fig.2. LSVcurvesof20wt%Pt/C(JohnsonMatthey)indifferentelec‐
trolyte at room temperature. Potential scan range of 1.10.2 V vs
Ag/AgCl(s),scanrateof5mV/s,andoxygenflowrateof50ml/min. electrolytes are shown in Fig. 2. In the potential range from
1.1–0.7 V, the current density is noted to be influenced by
methanolconcentration.Thecurveat~0.64Vcanbeassigned
asthecombinationofmethanolandoxygen,wherethemetha‐
nol creates an oxidation current and the oxygen causes a re‐
ductioncurrent.Inthe0.5mol/LH2SO4electrolyte,thereisno
oxidationcurrentofmethanol,andtheonsetpotentialofoxy‐
genreductionisapproximately0.62V.In0.5mol/LH2SO4+0.1
mol/LCH3OHelectrolyte,theoxidationcurrentofmethanolis
almostoffsetbythereductioncurrentoftheoxygen.However,
if the electrolyte contains more concentrated methanol, the
oxidation current of methanol is higher than the reduction
currentofoxygen.
At a potential of 0.5 V, the ORR activity is 47.2
mA/(mgPt.cm2) in 0.5 mol/L H2SO4. It is 51.5 mA/(mgPt.cm2)
in 0.5 H2SO4 + 0.1 mol/L CH3OH and 20.2 mA/(mgPt.cm2) in
0.5H2SO4+0.5mol/LCH3OH.Atapotentialof0.4V,theORR
activity was enhanced from 91.2 mA/(mgPt.cm2) to 113.4
mA/(mgPt.cm2)whenreplacingthe0.5mol/LH2SO4solutionby
0.5mol/LH2SO4+0.1mol/LCH3OH.However,theactivityde‐
creased to 51.0 mA/(mgPt.cm2) when replaced by 0.5 mol/L
H2SO4+0.5mol/LCH3OH.Theeffectofmethanolconcentration
ontheORRactivityishighlynoticeable.Previously,ithadonly
beenconsideredthatmethanolcausespoisoningofthecathode
Pt/C catalyst, so the study of methanol‐tolerant catalysts was
determinedasoneofthemostimportantaspectsinthedevel‐
opment of DMFCs [1013]. However, from the results above,
the ORRactivityof Pt/C catalysts hasbeen showntoimprove
significantlyin a0.5mol/LH2SO4+0.1mol/LCH3OHelectro‐
lyte.IftheORRactivityofPt/Ccatalystsina0.5mol/LH2SO4+
0.1mol/LCH3OHsolutioncanbeusedinDMFCstacksbyspe‐
cializeddesign,systemperformancecanbeimproved.
Togainmorepracticaldata,thehalf‐MEAwithtwolayersof
pre‐treated Nafion® NRE‐212 membrane hot‐pressed on the
catalyst layer were investigated. The LSV curves of the
half‐MEAof20wt%Pt/Ccatalystin0.5mol/LH2SO4,0.5mol/L
H2SO4+0.1CH3OH,and0.5mol/LH2SO4+0.5CH3OHsolutions
areshowninFig.3.AstheNafion®membranemayhavecov‐
ered some active sites of the catalyst, the ORR activity of the
1108
TANGShuihuaetal./ChineseJournalofCatalysis34(2013)1105–1111
Current density (mA/(mgPtcm2))
50
0
-32.9
-50
-73.8 -45.3
-100
-112.0
-150
-200
0.5 mol/L H2SO4
0.5 mol/L H2SO4 + 0.1 mol/L CH3OH
0.5 mol/L H2SO4 + 0.5 mol/L CH3OH
-250
0.2
0.4
0.6
0.8
Potential (V vs Ag/AgCl (s))
1.0
Fig.3.LSVcurvesof20wt%Pt/C(JohnsonMatthey)aftercoatedwith
twolayersofcommercialNafion®membraneNRE‐212.Potentialscan
rangeof1.10.2VvsAg/AgCl(s),scanrateof5mV/sandoxygenflow
rateof25ml/minatroomtemperature.
Current density (mA/(mgPtcm2))
Current density (mA/(mgPt.cm2))
catalyst in 0.5 mol/L H2SO4 solution decreased from 91.2 to
75.8mA/(mgPt.cm2).Inthe0.5mol/LH2SO4+0.5CH3OHso‐
lution, the Nafion® membrane can prevent the Pt/C catalyst
from being exposed to methanol directly, thus improving the
ORR activity. The behavior is similar to that recorded in the
absence of methanol at potentials lower than 0.5 V. As in the
0.5mol/LH2SO4+0.1mol/LCH3OHsolution,thecatalystactiv‐
ity was barely affected by the Nafion® membrane, because it
was almost identical to that of the uncoated electrode, as
showninFig.4.ItappearsthattheNafion®membranedidnot
covertheactivesitesandthemethanoldidnotleadtopoison‐
ingofthe catalystinthe0.5mol/LH2SO4 + 0.1mol/L CH3OH
electrolyte. The ORR activity was significantly enhanced com‐
paredwiththatinthe0.5mol/LH2SO4solution.Thiscouldbe
due to more open micro‐channels for proton transfer in the
Nafion® membrane other than the swelling in the 0.5 mol/L
H2SO4+0.1mol/LCH3OHelectrolyte,whichcouldsubsequent‐
ly lead to smaller resistance of the Nafion® membrane. This
tendencymayalsobebecauseofthepromotioneffectofmeth‐
anolintheoxygenreductionreactioninthe0.5mol/LH2SO4+
0.1 mol/L CH3OH electrolyte, which could lead to reduced
chargetransferresistance.
50
0
-50
-100
-150
-200
(1)
(2)
-250
Usuallyinasinglecellorstack,1.0mol/Lorhigherconcen‐
trationsofCH3OHsolutionareintroducedasfuel.Atthepoten‐
tialof 0.5V,theORRactivity ofthehalf‐MEAof20 wt%Pt/C
catalystcanincreasefrom22.5to54.0mA/(mgPt.cm2)after
the electrode is transferred from the 0.5 mol/L H2SO4 + 1.0
mol/LCH3OHtothe0.5mol/LH2SO4+0.1mol/LCH3OHelec‐
trolyte.Atthepotentialof0.4V,theORRactivitycanincrease
from 71.5 to 126.4 mA/(mgPt.cm2). Furthermore, the ORR
activityofthePt/Ccatalystinthe0.5mol/LH2SO4+0.1mol/L
CH3OH solutioncan berecoveredafterbeingtestedinthe 0.5
mol/LH2SO4+1.0mol/LCH3OHsolution,andevenenhanced
slightlyafterseveralcycles.TheLSVcurvesareshowninFig.5.
Tofurthersupportpracticalstackapplication,50wt%Pt/C
catalystswithacatalystloadingofca.4.0mgPt/cm2wereinves‐
tigated. The ORR activities of half‐MEA of 50 wt% Pt/C cata‐
lystsina0.5mol/LH2SO4solutioncontainingdifferentmetha‐
nol concentrations are shown in Fig. 6. A similar trend was
obtained for 50 wt% Pt/C catalysts. The ORR activities of the
catalysts were almost identical in 0.5 mol/L H2SO4, 0.5 mol/L
H2SO4 + 0.5 mol/L CH3OH, and 0.5 mol/L H2SO4 + 1.0 mol/L
CH3OHsolutionsatpotentialsoflowerthan0.5V,butwasen‐
hanced in 0.5 mol/L H2SO4 + 0.1 mol/L CH3OH. The most no‐
ticeable differences in electrolytes containing varying concen‐
trations of methanol were observed at potentials higher than
0.6V.Moreseriousmethanolcrossoverwasobservedinhigher
concentrations of methanol solutions that produced a larger
methanol oxidization current. However, at potentials lower
than0.5V,theydemonstratedsimilaractivities,exceptforthat
inthe0.5mol/LH2SO4+0.1mol/LCH3OHsolution.
FromFigs.3and5,themethanolcrossoveraffectstheLSV
curvesatpotentialshigherthanca.0.5V,buthaslittleinfluence
atlowerpotentials.Thisimpliesthattheoxygenreductionre‐
action is dominant at lower potentials, with methanol only
contributingaveryslightoxidationcurrentornoneatall.The
onset potentials shifted cathodically in 0.5 mol/L H2SO4 + 0.5
mol/LCH3OHor0.5mol/LH2SO4+1.0mol/LCH3OHsolutions
as the active sites of the Pt/C catalysts were occupied by
CO‐likeintermediatesfromthemethanoloxidationreactionat
50
0
-22.5
-54.0
-50
-71.5
-100
-126.4
-150
-200
0.5 mol/L H2SO4 + 0.1 mol/L CH3OH-1
0.5 mol/L H2SO4 + 1.0 mol/L CH3OH-2
0.5 mol/L H2SO4 + 0.1 mol/L CH3OH-3
0.5 mol/L H2SO4 + 1.0 mol/L CH3OH-4
-250
-300
0.2
-300
0.2
0.4
0.6
0.8
Potential (V vs Ag/AgCl (s))
1.0
Fig. 4. Comparison of LSV curves of 20 wt% Pt/C (Johnson Matthey)
before(1)andaftercoatedwithNafion®membraneNRE‐212(2).
0.4
0.6
0.8
Potential (V vs Ag/AgCl (s))
1.0
Fig.5.LSVcurvesof20wt%Pt/C(JohnsonMatthey)coatedwithNafi‐
on® membraneNRE‐212 were testedindifferentelectrolytesatroom
temperature. Numbers 1, 2, 3, and 4 were used to denote the testing
samplesequence.
TANGShuihuaetal./ChineseJournalofCatalysis34(2013)1105–1111
40
Specific activity (mA/cm2)
20
0
-20
-40
-60
-80
0.5 mol/L H2SO4
0.5 mol/L H2SO4 + 0.1 mol/L CH3OH
0.5 mol/L H2SO4 + 0.5 mol/L CH3OH
0.5 mol/L H2SO4 + 1.0 mol/L CH3OH
-100
-120
-140
0.2
0.4
0.6
0.8
1.0
Potential (V vs Ag/AgCl (s))
Fig. 6. Comparison of linear sweep voltammetry of 50 wt% Pt/C
(Johnson Matthey)coated withNafion®membrane NRE‐212in differ‐
entelectrolytesatroomtemperature.
higher potentials, leading to the poisoning of Pt/C catalysts.
However,itisnotclearwhysmallamountsofmethanolinthe
electrolyte do not lead to poisoning of Pt/C catalysts, rather
thanimprovingtheORRactivity.
Duringpreparationofthecatalystink,thecatalystparticles
were partly covered by a Nafion® thin film, so the catalysts
utilizationisusuallylessthan20%.Themethanolsolutioncan
open or enlarge the proton transfer micro‐channels of the
Nafion® membrane, which would lead to methanol crossover
andmembraneswelling.However,moreopenmicro‐channels
can lead to higher proton transfer capacity, and more oxygen
cangetaccesstoactivesitesofPt/Ccatalysts.Moreactivesites
of the Pt/C catalyst can be exposed to oxygen and/or proton
thusthethree‐phaseinterfacescanbemoreeasilymaintained,
which is defined as a promotional effect of methanol. If the
methanol concentration is too high, the methanol crossover
andmembraneswellingwillbemoreserious,thusthenegative
effects of methanol will dominate. However, if the methanol
concentration is within acceptable limits, methanol can max‐
imize the capacity of proton transfer, and more oxygen can
accessmoreactivesites,thuspromotingthepositiveeffectsof
2.0
methanol, such asinthe0.5mol/LH2SO4+ 0.1mol/L CH3OH
solution.Affouneetal.[17]measuredtheprotonconductivity
of Nafion® 117 membrane in a CH3OH/H2O electrolyte with
volume ratios of 75%25%, 50%50%, and 25%75%. They
drew the conclusion that proton conductivity decreased with
increasedmethanolcontent.However,themostdiluteelectro‐
lyteusedintheirpaperwasof25%methanol‐75%H2O,witha
molarconcentrationofca.6.0mol/L,whichistooconcentrated
as a direct feed fuel for DMFCs. Thus their conclusion is not
applicableinthisstudy.
3.2. InfluenceofmethanolconcentrationonACimpedance
Electrochemical impedance spectroscopy can be a helpful
technique to probe reaction processes [1418]. For identical
system configurations, the difference in impedance spectro‐
scopes can reflect the differences in the EC or half‐MEA.
Half‐MEAsof 20wt%Pt/C and50wt%Pt/C catalystsin a0.5
mol/L H2SO4 solution containing different concentrations of
methanolweremeasuredatdifferentpotentialsinafrequency
rangefrom100kHz–10mHz.
NyquistplotsaredemonstratedinFig.7forhalf‐MEAsof20
wt%Pt/Cand50wt%Pt/Ccatalystsin0.5mol/LH2SO4elec‐
trolytes containing different contents of methanol. Electrolyte
resistancewas3.81Ωforthehalf‐MEAof20wt%Pt/Ccatalyst
in0.5mol/LH2SO4,2.49Ωforthesolutionof0.5mol/LH2SO4+
0.1 mol/L CH3OH, and 3.88 Ω for the 0.5 mol/L H2SO4 + 0.5
mol/L CH3OH electrolyte. The resistances were slightly larger
in0.5mol/LH2SO4+0.5mol/LCH3OHthanin0.5mol/LH2SO4,
butweremuchsmallerin0.5mol/LH2SO4+0.1mol/LCH3OH.
Thesametendencywasobservedforthehalf‐MEAof50wt%
Pt/Ccatalysts.Theresistanceswere3.80Ωforthehalf‐MEAof
50 wt% Pt/C catalyst in 0.5 mol/L H2SO4, 2.63 Ω for the 0.5
mol/LH2SO4+0.1mol/LCH3OHsolution,3.96Ωfor0.5mol/L
H2SO4+0.5mol/LCH3OH,and4.12Ωin0.5mol/LH2SO4+1.0
mol/L CH3OH. The major difference between the two
half‐MEAsliesinthechargetransferresistance.Thehalf‐MEA
of50wt%Pt/Ccatalystwithhighplatinumloading(morethan
4mgPt/cm2)demonstratedasmallerchargetransferresistance,
0.6
(a)
1.5
(b)
0.3
1.0
0.0
0.5
-Zi/
-Zi/
1109
0.0
-0.3
-0.6
-0.5
0.5 mol/L H2SO4
0.5 mol/L H2SO4 + 0.1 mol/L CH3OH
0.5 mol/L H2SO4 + 0.5 mol/L CH3OH
-1.0
0.5 mol/L H2SO4
0.5 mol/L H2SO4 + 0.1 mol/L CH3OH
0.5 mol/L H2SO4 + 0.5 mol/L CH3OH
0.5 mol/L H2SO4 + 1.0 mol/L CH3OH
-0.9
-1.2
-1.5
2
3
4
5
Zr/
6
7
8
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
Zr/
Fig.7.Nyquistplotsof20wt%Pt/C(a)and50wt%Pt/C(b)coatedwithNafion®membraneNRE‐212atpotentialof500mVwithpotentialstepof
10mVandACsinewaveamplitudeof10mVbetween100kHzand10mHz,in0.5mol/LH2SO4containingvaryingconcentrationsofCH3OHatroom
temperature.
1110
TANGShuihuaetal./ChineseJournalofCatalysis34(2013)1105–1111
Acknowledgments
thatiswhyhighPtloadingisneededforDMFCs.Asmorecon‐
centrated methanol in the electrolyte was applied, smaller
chargetransfer resistanceswereobserved. Thisindicatesthat
insomecases,methanolcanfacilitatethechargetransferpro‐
cess in the oxygen reduction reaction. This is consistent with
thereportsbyAricòetal.[18].
TheEISresultswerefoundtocorrelatewiththeORRactivi‐
ty results. As mentioned previously, more micro‐channels of
the Nafion® membrane were enlarged because of the oxygen
gaspresentinthe0.5mol/LH2SO4+0.1mol/LCH3OHsolution,
thusleading to the smaller resistances andimprovedORRac‐
tivity.ThisspecialpropertyofNafion®membranescanbeused
indesigningastackthatwillimprovetheperformanceoffuel
cells. Pt/C catalysts (20 wt%) from E‐TEK company, in‐house
Pt/C and PtCo/C catalysts were also measured and the same
trends were obtained. The improved ORR activity of the Pt/C
catalysts in 0.5 mol/L H2SO4 + 0.1 mol/L CH3OH solution has
beennotedinallthetestingdata.Furtherresearchwillbecar‐
riedouttoinvestigatethetheorybehindtheseresults.
IwillbegratefultoProf. ReidarTunold,Prof.SveinSunde,
Prof. Frøde Saland for their helpful discussions and sugges‐
tions. References
[1] TangSH,SunGQ,QiJ,SunSG,GuoJS,XinQ,HaarbergGM.ChinJ
[2]
[3]
[4]
[5]
[6]
[7]
4. Conclusions
[8]
Both catalysts of 20 wt% Pt/C and 50 wt% Pt/C (Johnson
Matthey)showedmuchbetteroxygenreductionreactionactiv‐
itiesinelectrolytesin0.5mol/LH2SO4solutionscontaining0.1
mol/LCH3OH.Thiswasduetoreducedelectrolyteresistances.
The charge transfer resistances can also be reduced in more
concentratedmethanolsolutionsbyincreasingthePtloadingof
theelectrode.HighPtloadingattheelectrodecanbeadopted
for practical use and 0.1 mol/L CH3OH can be fed to a direct
methanolfuelcell.Thiscanincreasetheefficiencyoftheoverall
systemandreduceenvironmentalpollutionforportablepower
devices.
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GraphicalAbstract
Chin.J.Catal.,2013,34:1105–1111 doi:10.1016/S1872‐2067(11)60480‐4
EffectofmethanolconcentrationonoxygenreductionreactionactivityofPt/Ccatalysts
TANGShuihua*,LINWenfeng,PaulA.CHRISTENSEN,GeirMartinHAARBERG
SouthwestPetroleumUniversity,China;NewcastleUniversity,UnitedKingdom;NorwegianUniversityofScienceandTechnology,Norway
2.0
1.5
0
-32.9
-50
1.0
-73.8
-45.3
-Zi/
Current density (mA/(mgPtcm2))
50
-100
-112.0
0.5
-150
0.0
-200
-0.5
-250
-1.0
Pt/C-Nafion-0.5mol/L H2SO4
Pt/C-Nafion-0.5mol/L H2SO4 + 0.1mol/L CH3OH
Pt/C-Nafion-0.5mol/L H2SO4 + 0.5mol/L CH3OH
-1.5
0.2
0.4
0.6
0.8
Potential (V vs Ag/AgCl (s))
1.0
2
3
4
5
Zr/
6
7
8
TheORRactivityof20wt%Pt/CcatalystwasenhancednotablyduetoasmallerresistanceofNafionmembraneintheelectrolyteof0.5
mol/LH2SO4containing0.1mol/Lmethanolwithoxygengasfeed.
TANGShuihuaetal./ChineseJournalofCatalysis34(2013)1105–1111
[16] DuCY,ZhaoTS,YangWW.ElectrochimActa,2007,52:5266
[17] AffouneAM,YamadaA,UmedaM.JPowerSources,2005,148:9
1111
[18] AricoAS,AntonucciV,AlderucciV,ModicaE,GiordanoN.JAppl
Electrochem,1993,23:1107
甲醇浓度对Pt/C催化剂氧还原活性的影响
唐水花a,b,c,*, 林文锋b, Paul A. CHRISTENSENb, Geir Martin HAARBERGc
a
西南石油大学油气藏地质及开发工程国家重点实验室, 四川成都 610500
b
纽卡斯尔大学化工和先进材料学院, 纽卡斯尔 NE1 7RU, 英国
c
挪威科技大学材料科学与工程系, 特隆赫姆 7491, 挪威
摘要: 采用半池考察了Pt/C催化剂在含不同浓度甲醇的0.5mol/L硫酸中的氧还原活性(ORR).研究发现,当甲醇浓度为0.1mol/L
时,Pt/C催化剂的ORR活性最高,在催化层上热压商品NafionNRE-212膜后也出现同样趋势.线性扫描伏安曲线显示,压膜前后的
Pt/C催化剂的ORR活性在含0.1mol/L甲醇的0.5mol/L硫酸中几乎没有变化.电化学阻抗谱结果表明,在该溶液中,Nafion膜的电
阻比在其它电解液中低,这可能是导致Pt/C催化剂ORR活性提高的主要原因.有必要关注Nafion膜的这一异常性质并通过特殊设
计后用于电池堆,以提高燃料电池性能.
关键词: 铂; 炭; 甲醇渗透; 氧还原; 燃料电池
收稿日期: 2012-11-06. 接受日期: 2012-11-14. 出版日期: 2013-06-20.
*通讯联系人. 电话/传真: (028)83032879; 电子信箱: [email protected]
本文的英文电子版由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).