DTIC ADA541935: Heat-Treated Metal Phthalocyanine Complex as an Oxygen Reduction Catalyst for Non-Aqueous Electrolyte Li/air Batteries

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by  Defense Technical Information Center

ElectrochimicaActa56 (2011) 4544–4548
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Heat-treated metal phthalocyanine complex as an oxygen reduction catalyst for
non-aqueous electrolyte Li/air batteries
ShengS.Zhang∗,XiaomingRen,JeffreyRead
U.S.ArmyResearchLaboratory,RDRL-SED-C,2800PowderMillRoad,Adelphi,MD20783-1197,USA
a r t i c l e i n f o a b s t r a c t
Articlehistory: Inthisworkwestudyheat-treatedFeCu-phthalocyanine(FeCuPc)complexesasthecatalystforoxygen
Received9December2010 reductioninnon-aqueouselectrolyteLi/aircellsbysupportingthecatalystonahighsurfaceareaKet-
Receivedinrevisedform14February2011 jenblackEC-600JDcarbonblack.ItisshownthattheresultantFeCu/Ccatalystnotonlyacceleratesthe
AAcvcaeilpatbelde 1on9lFineeb r2u6a rFyeb2r0u1a1ry 2011 ttwioon-aetlieocntroofnLir2eOd2uactsio“2nLoi2fOo2x→yge2nLia2sO“+OO22+”2.ILni+L+i/2aeir→ceLllis2,Ot2h”e,bcuattaalylssotcreadtaulcyezsespothlaercizhaetmioincaolnddisipscrhoaprogre-
whilesimultaneouslyreducingthefractionofLi2O2inthefinaldischargedproducts.Ina0.2molkg−1
Keywords:
Oxygenreduction LLii/SaOir3CcFe3lls7:w3i(twhtF.)eCpuro/Cpyslheonwecaatrbleoansatte0.2(PVC)h/tirgihs(e2r,2d,i2s-cthriaflrugeorvooeltthaygle)patho0s.2phmaAtecm(T−F2P)thealencttrhoolysetew,tihthe
Electrocatalysis
Metalphthalocyaninecomplex pristinecarbon.Bymeasuringthecharge-transferresistance(Rct)ofLi/aircellsattemperaturesranging
Airelectrode between−30◦Cand30◦C,wedeterminetheapparentactivationenergyofthedischargeofLi/aircells
Li/airbattery anddiscusstheeffectofFeCu/CcatalystontheoxygenreductioninLi/aircells.
Published by Elsevier Ltd.
1. Introduction the rate-determining step to limit the discharge performance of
Li/aircellswhenthedischargecurrentrateisincreased.Therefore,
Li/air batteries are more complicated than the traditional moreeffectivecatalystsareneededtoincreasethepowercapabil-
metal/airbatteriessincetheirdischargeproducts(Li O andLi O) ityofLi/aircells.TransitionmetalN -macrocyclecomplexeshave
2 2 2 4
are insoluble in non-aqueous electrolytes. During discharge, the longbeenknowntobehighlyactiveforthecatalyticreductionof
Li O and Li O deposit and accumulate on the surfaces of a car- oxygen. However, these compounds are either soluble or chem-
2 2 2
bonaircathode,whicheventuallyblockstheaccessofoxygento icallyinstableinaqueousacidicoralkalineelectrolytesolutions.
catalytic sites and halts the operation of the battery. Therefore, For these reasons, the technique of heat-treatment with carbon
theamountthatthecarbonaircathodecanaccommodateforthe has been used to enhance catalytic activity and increase chemi-
dischargeproductsdeterminesthespecificcapacityofLi/aircells calstabilityagainsttheaqueouselectrolyteenvironments[9–11].
[1–3].Toachievehighspecificcapacity,aliquid/solidtwo-phase Theheat-treatedtransitionmetalN -macrocyclecomplexeshave
4
reactionzonebetweentheliquidelectrolyteandsolidcatalyticsites beenconsideredasanexcellentcatalystfortheoxygenreduction
isrequiredfortheLi/aircellstoachievethemaximumreactionarea inalkalinemetal/aircellsandfuelcells[9–15].Keepingthisknowl-
andthelargestaccommodationcapacity[3].However,suchcells edgeinmind,inthisworkwestudytheeffectofoxygenreduction
areintrinsicallylowpowersincediffusionofthedissolvedoxygen catalystonthedischargeperformanceofLi/aircellsbyselecting
inliquidelectrolyteisslowandthedischargeproductscannotbe aheat-treatedironandcopperphthalocyanine(FeCuPc)complex
removedfromthecatalyticsites.FortheselowpowerLi/aircells, asthecatalystandloadingitontoahighsurfaceareaKetjenblack
thepristinecarbonwithoutanyothercatalystshasbeendirectly EC-600JDcarbonblack.
usedasthecatalystfortheoxygenreduction[2–8].
Catalytic reduction of oxygen in Li/air battery takes place
2. Experimental
through such three steps as: (1) dissolution of oxygen from gas
phaseintoliquidelectrolyte,(2)diffusionofthedissolvedoxygen
Ketjenblack EC-600JD carbon black (AkzoNobel) was used as
intocatalyticsitesonthecarbonsurface,and(3)catalyticreduction
thecontrolandsupportingcarbonforcatalyst.FeCu/Ccatalystwas
ofoxygenintoLi O orLi O.Ofthem,thelaststepmightbecome
2 2 2 receivedfromActaS.p.A,Italy,whichwaspreparedbyfirstabsorb-
ingaFeCuPccomplexsolutionontoKetjenblackEC-600JDcarbon
blackandthenheatingthemixtureattemperaturesbetween800◦C
∗ and900◦Cinanargonatmosphere.ThefinalFeCu/Ccatalystcon-
Correspondingauthor.Tel.:[phone],fax:[phone].
E-mailaddress:[email protected](S.S.Zhang). tained1.5wt.%Feand1.7wt.%Cu,respectively,whichcorresponds
0013-4686/$–seefrontmatter.Published by Elsevier Ltd.
doi:10.1016/j.electacta.2011.02.072
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1. REPORT DATE 3. DATES COVERED
26 FEB 2011 2. REPORT TYPE 00-00-2011 to 00-00-2011
4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Heat-Treated Metal Phthalocyanine Complex As An Oxygen Reduction
5b. GRANT NUMBER
Catalyst For Non-Aqueous Electrolyte Li/air Batteries
5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S) 5d. PROJECT NUMBER
5e. TASK NUMBER
5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION
U.S. Army Research Laboratory,RDRL-SED-C, 2800 Powder Mill REPORT NUMBER
Road,Adelphi,MD,20783
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S)
11. SPONSOR/MONITOR’S REPORT
NUMBER(S)
12. DISTRIBUTION/AVAILABILITY STATEMENT
Approved for public release; distribution unlimited
13. SUPPLEMENTARY NOTES
Electrochimica Acta 56 Feb 2011,pages 4544-4548
14. ABSTRACT
In this work we study heat-treated FeCu-phthalocyanine (FeCuPc) complexes as the catalyst for oxygen
reduction in non-aqueous electrolyte Li/air cells by supporting the catalyst on a high surface area
Ketjenblack EC-600JD carbon black. It is shown that the resultant FeCu/C catalyst not only accelerates
the two-electron reduction of oxygen as ?O2 + 2Li+ +2e→Li2O2?, but also catalyzes the chemical
disproportionation of Li2O2 as ?2Li2O2→2Li2O+O2?. In Li/air cells, the catalyst reduces
polarization on discharge while simultaneously reducing the fraction of Li2O2 in the final discharged
products. In a 0.2 mol kg−1 LiSO3CF3 7:3 (wt.) propylene carbonate (PC)/tris(2,2,2-trifluoroethyl)
phosphate (TFP) electrolyte, the Li/air cells with FeCu/C show at least 0.2V higher discharge voltage at
0.2mAcm−2 than those with pristine carbon. By measuring the charge-transfer resistance (Rct) of
Li/air cells at temperatures ranging between −30 ◦C and 30 ◦C, we determine the
apparent activation energy of the discharge of Li/air cells and discuss the effect of FeCu/C catalyst on the
oxygen reduction in Li/air cells.
15. SUBJECT TERMS
16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF
ABSTRACT OF PAGES RESPONSIBLE PERSON
a. REPORT b. ABSTRACT c. THIS PAGE Same as 6
unclassified unclassified unclassified Report (SAR)
Standard Form 298 (Rev. 8-98)
Prescribed by ANSI Std Z39-18
S.S.Zhangetal./ElectrochimicaActa56 (2011) 4544–4548 4545
toa1:1molarratioofFetoCu.InthisworkwereferredtotheFeCu/C Table1
catalyst as a composite of the Ketjenblack carbon and FeCuPc Physicalpropertiesofcarbonmaterials,aircathodes,andLi/aircells.
pyrolyticproducts.Usingthecarbonmaterialsabove,airelectrodes KetjenblackEC-600JD CuFe/C
with a composition of 90wt.% carbon and 10wt.% polytetrafluo- BETsurface(m2g−1) 1413 751
roethylene(PTFE)werepreparedbymixingcalculatedamountsof Porevolume(cm3g−1) 2.06 1.23
carbonandPTFEemulsion(Teflon®,solidcontent=61.5%,DuPont Electrodeporevolumea(cm3g−1) 5.46 3.64
Co.)androllingthemixedpastetoformafree-standingcathode
sheet.Theresultantaircathodesheet,equalingtoatotal2.88wt.% Li/aircell Controlcell CuFe/Ccell
ofmetal(FeandCu)content,waspunchedintosmalldiskswithan CellOCV(V) 2.9166 3.331
areaof0.97cm2anddriedat100◦Cundervacuumforatleast8h. Cellimpedance((cid:2)) 270 226
A0.2molkg−1(m)LiSO3CF37:3byweightpropylenecarbonate a CalculatedbythesolventabsorptionmethodasdescribedinRef.[3].
(PC)/tris(2,2,2-trifluoroethyl)phosphate(TFP)electrolytewaspre-
paredinanargon-filledglove-boxbymixingcalculatedamountsof
LiSO CF (96%,Aldrich),PC(Ferro)andTFP,inwhichtheTFPwas
3 3
synthesizedin-houseasdescribedinRef.[16].Thiselectrolytewas
selectedasitissuperiortoothercarbonate-basedelectrolytesin
boththedissolutionkineticsandthesolubilityofoxygen[17,18].
Inadry-roomhavingadewpointofbelow−90◦C,Li/aircellswith
anairwindowof0.97cm2wereassembledbystackingaLifoil,a
Celgard®3500membrane,acarbonaircathode,aNimeshasthe
currentcollector,andanairwindowinsequenceintoacoincellcap.
Toactivatethecell,200(cid:2)Lofliquidelectrolytewasaddedthrough
theair-window,followedbyapplyingavacuumfor20stoensure
completewetting.Extraliquidelectrolytewasremovedbygently
swipingafilterpaperonthetopoftheNimesh.Theelectrolyte-
activatedcellwasclampedonacellholdertotestasaLi/aircell.The
sameprocedureswerefollowedtoassemblethree-electrodecells
exceptforusingtwopiecesofseparatorsandsandwichingasmall
pieceofLifoilasthereferenceelectrodebetweenthesetwosepa-
rators.Detaileddescriptionsonthecellconfigurationandassembly
procedurecanbefoundinRef.[3].
An Arbin BT-2000 cycler was used for the discharge testing Fig.1. Current–timeresponseofLi/aircellswhena−0.8Vofconstantpotentialwas
appliedtotheOCVofthecell.(1)Controlcellwithpristinecarbon,and(2)Li/aircell
with a 1.5V cutoff voltage. Specific capacity of the cells was
withFeCu/C.
calculatedwithreferencetotheweightofcarbonintheaircath-
ode.ASolartronSI1287ElectrochemicalInterfaceandaSI1260
basedonthefollowingreactions[21]:
Impedance/Gain-PhaseAnalyzerwereusedforcyclicvoltammetry,
polarization,andimpedancemeasurements.Impedancespectraof 2Li + O2→ Li2O2 OCV = 2.959V (1)
thecellsweremeasuredatopen-circuitvoltage(OCV)infrequen-
4Li + O → 2Li O OCV = 2.913V (2)
ciesrangingfrom0.01Hzto−106Hzbyapplyinganacperturbation 2 2
of0.10mVinamplitude.SincetheimpedancesofLi/aircellsare This phenomenon can be explained by the chemical struc-
highly dependent of the depth-of-discharge (DOD) [3], the cells tureofFeCu/Ccatalyst.Manypublicationshavereportedthatthe
weredischargedtothesameDODforeachcomparisonoftheelec- final chemical components of the heat-treated transition metal
trochemicalproperties.Theconditionsforothertestsaredescribed N -macracyclecomplexesdependonthetemperatureofheating-
4
either in discussion or in related figure captions. Before each treatment [11–15]. The most acceptable conclusion is that the
test, the cells rested for 2h to reach equilibrium of oxygen con- complex forms MN molecular moieties (where M represents a
4
centrations between the air cathode (catalytic sites) and the air transition metal) at temperatures of 500–700◦C, it further loses
environment.Unlessnotedotherwise,thetestswerecarriedoutin nitrogentoformMN /MOmixturesattemperaturesof700–800◦C
2
thedryroomandatroomtemperature(22◦C). bycombiningoxygenatomsfromthefunctionalgroupsofthecar-
bon edges, and finally converts all MN into MO at even higher
2
temperatures.SincethepresentFeCu/Cwasmadeat800–900◦C,
3. Resultsanddiscussion weconsiderthattheFeandCuinthecatalystarepresentmainly
intheformofmetaloxides.
3.1. AircathodestructureandLi/aircell’sOCV Basedonthefactsdescribedabove,webelievethattheunusu-
allyhighOCVoftheFeCu/CbasedLi/aircellsisduetothemetal
KetjenblackEC-600JDisahighlybranchedcarbonblackconsist- oxides,ratherthantheO fromair.Tosupportourhypothesis,we
2
ingof30–100nmlongaggregates[19].Afterloadingwithcatalyst, conductedapolarizationexperimentontwoLi/aircellshavingthe
thesurfaceareaandporevolumeofthecarbonaresignificantly samedepth-of-discharge(DOD).TheOCVandoverallimpedanceof
reduced as shown in Table 1. This is because many of pores in thesetwocellsarelistedinTable1,andthepolarizationcurvesof
Ketjenblack carbon were filled with the thermal decomposition thesetwocellswitha−0.8VconstantpotentialappliedtotheOCV
products of FeCuPc, including metal oxides and pyrolytic carbon arecomparedinFig.1.ItisshowninTable1thattheFeCu/Ccellhas
[20].Therefore,theFeCu/Caircathodehasmuchlowerporevolume lowerimpedancethanthecontrolcell(i.e.,226vs.270(cid:2)).Oncon-
thanthepristinecarbonaircathode. trary,theFeCu/CcellhaslowercurrentintheI–tresponsecurve
ItisinterestingtonotethatallnewlyassembledLi/aircellswith as indicated in Fig. 1, which disagrees with the ohmic rule. This
FeCu/C have constantly about 0.3V higher OCV than those with conflictionsuggeststhattheinitialOCVoftheFeCu/Ccellcorre-
thepristinecarbon.TheFeCu/CcellshaveanOCVrangingbetween spondsneithertoreaction(1)nortoreaction(2),insteaditreflects
3.35Vand3.50V,whichareevenhigherthanthetheoreticalOCV theOCVofthemetaloxidesthemselves.TheinitialOCVranging
4546 S.S.Zhangetal./ElectrochimicaActa56 (2011) 4544–4548
Fig.2. CyclicvoltammogramofthefirstcycleofnewlyassembledLi/aircells.(1)
Controlcellwithpristinecarbon,and(2)Li/aircellwithFeCu/C.Experimentcondi-
tpiootnesn:ttihalestceasntnwinagspraetrefoorfm0.e1dmoVnsa−t1h.rTehee-eploetcetnrotidaelscceallnbneintwgesteanrt2e.d2Vfroamndc4e.l3l’sVOaCtVa Fig.4. DynamicpolarizationofLi/aircellsat0.01mAs−1.(1)Controlcellwithpris-
tinecarbon,and(2)Li/aircellwithFeCu/C.
asindicatedbythesoliddotsinthefigure,firstdownwardtolowpotentialandthen
upwardtohighpotential.
3.3. DischargecharacteristicofLi/aircells
Fig.3showstheeffectofFeCu/Ccatalystonthedischargeperfor-
between3.35Vand3.50VoftheFeCu/Ccellsisinagreementwith
manceofLi/aircells.Inconsecutivedischarge(Fig.3a),theFeCu/C
thoseobservedfromtheLi/CuOandLi/Fe O cells[22,23].
2 3 cell shows about 0.2V higher voltage than the control cell. This
observation agrees with the cyclic voltammetric results as dis-
cussed in Fig. 2, and it is attributed to the reduced polarization
3.2. CyclicvoltammogramofLi/aircells by FeCu/C catalyst. Another significant difference in the voltage
curvesbetweentwocellsisthatthecatalyzedcellexhibitsasec-
Fig.2comparescyclicvoltammogramsofthefirstcycleoftwo ondvoltageplateaustartingat∼1.7V.Inearlywork[20],wehave
Li/aircells.Itisshownthatthereductionofoxygenstartsat2.94V concludedthatthissecondvoltageplateauisduetothecatalyst-
vs.Li/Li+intheFeCu/Ccell,whileat2.80Vinthecontrolcell.This inducedsolventdecomposition.Ifsubtractingthecapacityofthe
oxygenreductioncorrespondstoreaction(1)andthe0.14Vhigher secondvoltageplateau,wecanfindthatthespecificcapacityofthe
shiftinthereductionpotentialisattributedtothecatalyticeffect FeCu/Ccellisalittlelowerthanthecontrolcell.Thisresultagrees
ofFeCu/C.Furthermore,theFeCu/Ccellshowsmuchhigherreduc- withthefactthattheFeCu/Chaslessporevolumethanthepristine
tioncurrentsthanthecontrolcell.ThisfactrevealsthattheFeCu/C carbonasindicatedinTable1.
catalystreducespolarizationintheLi/aircell.Itappearsthatthe Similarresultsareobservedfromintermittentdischargeexcept
FeCu/C also catalyzes the oxidation of Li2O2, i.e., the reversal of that the specific capacities are significantly higher than those
reaction (1), as indicated by a broad oxidization current peak at obtainedfromtheconsecutivedischarge(compareFig.3aandb).
∼3.8Vvs.Li/Li+whenthepotentialisscannedbackfrom2.2V.In Thereasonforthisphenomenonisunderstandablesinceininter-
the control cell, the similar oxidation peak seems not to appear mittent discharge the rest between two discharges allows more
untilthepotentialisincreasedto4.1V.Inaddition,thecontrolcell oxygen to diffuse into the catalytic sites. On the other hand, the
showsapairofsomehowreversiblecurrentpeaksbetween3.3V catalytic effect of FeCu/C on the oxygen reduction in Li/air cells
and3.9V.Thesecurrentsmaybeassociatedwiththeoxygenradi- alsocanbeverifiedbythedynamicpolarizationexperiment(see
calintermediatesformedduringtheoxidationofLi2O2,whichare Fig.4).Inwholetestedcurrentrange,theFeCu/Ccelloutperforms
consideredtoberesponsibleforthedecompositionofcarbonate thecontrolcellandtheimprovementseemstoincreasewiththe
solvents[24,25]. current.
Fig.3. DischargecharacteristicofLi/aircellsina(a)consecutivedischargeand(b)intermittentdischarge.(1)Controlcellwithpristinecarbon,and(2)Li/aircellwithFeCu/C.
Experimentconditions:bothdischargesuseda0.2mAcm−2currentdensity.Inintermittentdischarge,thecellrestedfor2hafterevery50mAhg−1discharge.
S.S.Zhangetal./ElectrochimicaActa56 (2011) 4544–4548 4547
Fig.5. OCVrecoveryofLi/aircellsatdifferentdepth-of-discharge.(a)At50mAhg−1,(b)at300mAhg−1,(c)at600mAhg−1,and(d)at900mAhg−1.(1)Controlcellwith
pristinecarbon,and(2)Li/aircellwithFeCu/C.
3.4. RoleofFeCu/CcatalystinOCVrecovery FeCu/Cpowder,respectively,wasadded.Asaresult,theFeCu/C
powdercausedviolentdecompositionofH O accompaniedby
2 2
Fig. 5a–d compares OCV recovery of two Li/air cells at differ- numerousoxygenbubbles,whilethepristinecarboncausedno
ent DOD. A general trend is that the OCV recovery slows down change.
with an increase in the DOD. This is because the insoluble and
non-conductivedischargeproducts(Li2O2andLi2O)depositonthe TheabovetwoexperimentssuggestthattheFeCu/Calsocat-
carbonsurface,whichformsapassivationlayertosuppressthedif- alyzethechemicaldisproportionationofLi O as:
2 2
fusionofoxygenintothecatalyticsites.ForacellwithhighDOD,
2Li O → 2Li O + O (3)
longer time is required for the diffusion of oxygen into the cat- 2 2 2 2
alyticsitessincethepassivationlayergrowswithDODandsince
The faster OCV recovery of FeCu/C cell in Fig. 5a–d can be
theOCVcannotgetstableuntiltheequilibriumofoxygenconcen-
well explained by reaction (3) as follows: when the discharge is
trationsbetweentheaircathode(catalyticsites)andtheelectrolyte
halted,theLi O incontactwiththeFeCu/Ccatalyticsitesunder-
2 2
isachieved.Fig.5b–dindicatesthata2hrestisnotsufficientfor
goes reaction (3) to release O . Since the reaction occurs right
2
thefullOCVrecoveryalthoughthesecellseventuallycanrecover
on the Li O /catalyst interface, the released O can be immedi-
toalmostsameOCV(∼3.0V),asrepresentedbyreaction(1)(not 2 2 2
atelyutilizedtoestablishtheequilibriumofoxygenconcentrations
showninFig.5).
betweenthecatalyticsitesandtheelectrolyte,whichresultsina
AnotherinterestingphenomenonisthattheFeCu/Ccellshows
fasterOCVrecovery.Ontheotherhand,thefasterOCVrecoveryof
fasterOCVrecoverythanthecontrolcell.Tounderstandthis,we
theFeCu/CcellsuggeststhattheFeCu/Ccatalystmightnotaffect
conductedthefollowingtwoexperiments:
thefurtherreductionofLi O asshownbyreactionbelow:
2 2
Li O +2Li++2e → 2Li O (4)
2 2 2
1. Discharge two cells with pristine carbon and FeCu/C, respec-
tively, to the same capacity, and then dip the discharged air ThediscussionaboveindicatesthattheFeCu/Cnotonlyaccel-
cathodesinto1mLdeionizedwater.Inthisprocess,theLi O eratesthetwo-electrontransferreaction(1),butalsocatalyzesthe
2 2
formed in discharging was reacted with water to form H O non-redoxreaction(3).Sincethelatterisasolid/solidtwo-phase
2 2
and LiOH. Upon the end of reaction, the same amount of reactionthatonlycanoccurintheinterfacebetweenthecatalyst
KMnO4/H2SO4 solution was added into the clear solution to sitesandLi2O2phase,thecatalyticactivityofFeCu/Conreaction(3)
roughlydeterminetheamountofH O .Asaresult,wefound isnotaseffectiveasonreaction(1).Duetothecatalyticeffecton
2 2
thatthecolorofKMnO4 disappearedimmediatelyinthesolu- reaction(3)andthedifficultyinoxidizingelectrochemicallyLi2O
tionwithpristinecarbon,whilethatinthesolutionwithFeCu/C toLi2O2andoxygen,theFeCu/Ccatalystmaynotbeagoodchoice
decoloredslowlyandthecolorcouldnotdisappearcompletely forthecatalystofrechargeableLi/airbatteries.
evenintheend.Theseresultsindicatethatinthesamecapac-
itiestheFeCu/CcellproduceslessLi O ascomparedwiththe 3.5. ImpedanceanalysisonLi/aircells
2 2
pristinecarboncell.
2. DissolvesmallamountofcommercialLi O powderintodeion- In previous work [3], we have shown that the impedance of
2 2
izedwater.WhentheLi O wascompletelydissolvedtoform a Li/air cell is changed greatly with the cell’s DOD. Therefore, in
2 2
H O , the clear solution was split into two parts, to which the present work two identical Li/air cells were assembled and
2 2
smallamountofpristineKetjenblackEC-600JDcarbonblackand discharged to the same DOD, followed by measuring impedance
4548 S.S.Zhangetal./ElectrochimicaActa56 (2011) 4544–4548
and41.4kJmol−1fortheFeCu/Ccell.Attemperaturesbelow10◦C,
theapparentactivationenergyis86.1kJmol−1forthecontrolcell
and 69.1kJmol−1 for the FeCu/C cell. These results indicate that
theFeCu/Ccatalystcaneffectivelyreducetheapparentactivation
energyofLi/aircells.
4. Conclusions
Based on the results of this work, the following conclusions
canbemade:(1)theunusuallyhighOCVofthenewlyassembled
Li/aircellswithFeCu/CcatalystcorrespondstotheOCVofcatalyst
itself,ratherthantotheoxygenreduction,(2)theFeCu/Ccatalyst
notonlyacceleratesthetwo-electronoxygenreduction,butalso
catalyzes the non-redox Li O disproportionation. In Li/air cells,
2 2
theformerreducescell’spolarizationandthelatterincreasesOCV
recoveryrate,(3)sincethecatalyticdisproportionationofLi O is
2 2
Fig.6. ImpedancespectraofLi/aircells,inwhichtwocellsareidenticalinthesize
asolid/solidtwo-phasereactionoccurringintheinterfacebetween
andweightoftheaircathodeandinthecell’sDOD(100mAhg−1).(1)Controlcell
thecatalyticsitesandLi O particles,thecatalyticeffectofFeCu/C
withpristinecarbon,and(2)Li/aircellwithFeCu/C. 2 2
onitisnotaseffectiveasonthetwo-electronoxygenreduction,
and (4) the FeCu/C catalyst can effectively reduce the apparent
activationenergyforthedischargeofLi/aircells.
Acknowledgements
TheauthorswouldliketothankDr.K.XuforhissupplyofTFP
solventandDr.D.TranforhisassistanceinBETmeasurement.
References
[1] K.M.Abraham,Z.Jiang,J.Electrochem.Soc.143(1996)1.
[2] J.Read,J.Electrochem.Soc.149(2002)A1190.
[3] S.S.Zhang,D.Foster,J.Read,J.PowerSources195(2010)1235.
[4] S.D.Beattie,D.M.Manolescu,S.L.Blair,J.Electrochem.Soc.156(2009)A44.
[5] X.H.Yang,P.He,Y.Y.Xia,Electrochem.Commun.11(2009)1127.
[6] J.Xiao,D.Wang,W.Xu,D.Wang,R.E.Williford,J.Liu,J.G.Zhang,J.Electrochem.
Soc.157(2010)A487.
[7] C.Tran,X.Q.Yang,D.Qu,J.PowerSources195(2010)2057.
[8] C.K.Park,S.B.Park,S.Y.Lee,H.Lee,H.Jang,W.I.Cho,Bull.KoreanChem.Soc.31
(2010)3221.
[9] E.Yeager,Electrochim.Acta29(1984)1527.
[10] B.Wang,J.PowerSources152(2005)1.
Fig.7. ArrheniusplotoftheexchangecurrentofLi/aircells.(1)Controlcellwith [11] V.Neburchilov,H.Wang,J.J.Martin,W.Qu,J.PowerSources195(2010)1271.
[12] D.A.Scherson,S.L.Gupta,C.Fierro,E.Yeager,M.Kordesch,J.Eldridge,R.Hoff-
pristinecarbon,and(2)Li/aircellwithFeCu/C.
man,Electrochim.Acta28(1983)1205.
[13] K.Sawai,N.Suzuki,J.Electrochem.Soc.151(2004)A682.
toexaminetheeffectofFeCu/Ccatalystontheoxygenreduction. [14] A.L.Bouwkamp-Wijnoltz,W.Visscher,J.A.R.vanVeen,E.Boellaard,A.M.van
Fig.6showsimpedancespectraoftwocellsmeasuredataDODof derKraan,S.C.Tang,J.Phys.Chem.B106(2002)12993.
100mAhg−1.Itisshownthattheimpedancespectracontaintwo [15] TR.eSs.a1ra0k(o2n0s0r9i,)S5.8S9u.thirakun,S.Charojrochkul,T.Vilaithong,J.Ceram.Process.
overlappedsemicircles,inwhichthesemicircleathighfrequencies [16] K.Xu,M.S.Ding,S.S.Zhang,J.L.Allen,T.R.Jow,J.Electrochem.Soc.150(2003)
relatestothedischargedproductlayerandthatatlowfrequencies A161.
[17] S.S.Zhang,J.Read,J.PowerSources196(2011)2867.
correspondstothechargetransferofcellreaction.Therefore,an
[18] S.S.Zhang,K.Xu,J.Read,J.PowerSources196(2011)3906.
equivalentcircuitasshownintheinsetcanbeusedtofittheelec- [19] AkzoNobel,ProductDataSheetonKetjenblackEC-600JDcarbonblack.
tricalelementsoftheimpedancespectra.Sincethediameterofthe [20] X. Ren, S.S. Zhang, D. Tran, J. Read, J. Mater. Chem. (2011),
doi:10.1039/C0JM04170J.
semicircleatlowfrequenciescorrespondstothecharge-transfer
[21] CalculatedfromthestandardGibbsfreeenergies,ChemistryWebBook,NIST
resistance(Rct)ofthecellreaction[26,27],theexchangecurrent StandardReferenceDatabase,http://webbook.nist.gov/chemistry/.
(io)o(cid:2)fthe(cid:3)Li/aircellscanbecalculatedusingequationbelow[28]: [[2223]] PS..PKoadnhzaakjeic,kTy.,InBa.Sdcar,oTs.aMti,aJt.sPuomwuerraS,oNu.rScoesno1y6a(m19a8a,5A)3.0Y9a.mada,M.Takano,R.
RT 1 Kannoa,J.PowerSources146(2005)323.
io= nF R (5) [24] F.Mizuno,S.Nakanishi,Y.Lotani,S.Yokoishi,H.Iba,Electrochemistry78(2010)
ct 403.
whereRisthegasconstant,FistheFaradayconstant,andTisthe [25] K.Takechi,E.Sudo,T.Inaba,F.Mizuno,H.Nishikoori,T.Shiga,218thECSMeet-
ing,2010(Abs#586).
temperature.Arrheniusplotsoftheexchangecurrentsfortwocells
[26] S.S.Zhang,K.Xu,T.R.Jow,Electrochim.Acta49(2004)1057.
areshowninFig.7,fromwhichtheapparentactivationenergyof [27] S.S.Zhang,K.Xu,T.R.Jow,Electrochem.Solid-StateLett.5(2002)A92.
theLi/aircellsisestimatedasfollow:attemperaturesabove10◦C, [28] A.J.Bard,L.R.Faulkner,ElectrochemicalMethods,Wiley,NewYork,1980,p.
theapparentactivationenergyis50.2kJmol−1forthecontrolcell 213.