JOM • December 200526

Carbon NanotubesResearch Summary

Electrochemicaldouble-layercapaci-tors,orsupercapacitors,havetremen-dous potential as high-power energysources for use in low-weight hybridsystemsforspaceexploration.Electrodesbasedonsingle-wallcarbonnanotubes(SWCNTs)offerexceptionalpowerandenergy performance due to the highsurface area, high conductivity, andtheabilitytofunctionalizetheSWCNTstooptimizecapacitorproperties.1Thispaper will report on the preparationof electrochemical capacitors incor-

Carbon-Nanotube-Based Electrochemical Double-Layer Capacitor Technologies for Spaceflight Applications

S. Arepalli, H. Fireman, C. Huffman, P. Moloney, P. Nikolaev, L. Yowell, C.D. Higgins, K. Kim, P.A. Kohl, S.P. Turano, and W.J. Ready

0.1 0.5 1 5Specific Energy (Wh/kg)

Spec

ific

Pow

er (W

/kg)

10 50 100 500 1,0000.01

107

106

105

104

103

100

10

10.05

CNT-Based

Batteries

“Traditional”

ElectrochemicalCapacitors

Capacitors

FuelCells

�

poratingSWCNTelectrodesand theirperformance compared with existingcommercial technology. Preliminaryresultsindicatethatsubstantialincreasesin power and energy density are pos-sible. The effects of nanotube growthand processing methods on electro-chemicalcapacitorperformanceisalsopresented.ThecompatibilityofdifferentSWCNTsandelectrolyteswasstudiedbyvaryingthetypeofelectrolyteionsthataccumulate on the high-surface-areaelectrodes.

INTRoDuCTIoN

Energystoragedevicesareclassifiedaccordingtoenergyandpowerdensity.Powerdensityisrelatedtothestrengthofagivencurrentandvoltagecombi-nation(wattage),whileenergydensityis related to the duration of time thatwattagecanbeapplied.Electrochemi-caldouble-layercapacitors,commonlycalledsupercapacitorsorultracapacitors,areintermediatesystemsthatbridgethepower/energy gap between traditionaldielectric capacitors (highpower) andbatteries(highenergy). Batteries are currently the mostcommonformofelectricalenergystor-age.Theyaretypicallyabletostorehigherenergydensitythansupercapacitors,buttheydeliverlesspowercomparedtotra-ditionaldielectriccapacitors.However,duetotheirshortcyclelifeandlowpowerdensities(i.e.,10,000). However, the smallenergy density (i.e.,

2005 December • JOM 27

BF4–H3C

CH3

N+Nuo

iu

ExpERImENTAL pRoCEDuRES

Figure B. A schematic diagram (left) and laboratory-scale superca-pacitor (right).

Figure A. A schematic diagram and chemical name for room-tem-perature ionic liquid (RTILs) used in laboratory-scale supercapacitor manufacture. The number to the left of the structure indicates the nomenclature used to designate each RTILthroughout this effort.

Laboratory-Scale SWCNT-Based Supercapacitors

Specially treated aluminum foil sheets were sectioned intosquaresmeasuring5cm×5cmtoserveasthecurrentcollectorforthelaboratory-scalesupercapacitorsamples.A5cmsquareofCelgard™wassectionedtoserveastheseparatormaterial.Theactiveelectrodematerialforthesupercapacitorelectrodeconsistsofawell-mixedpastemadefromcarboxymethylcellulose(CMC),water, methanol, and single-wall carbon nanotubes (SWCNTs).To gauge the potential for performance differences betweenmanufacturers, two separate SWCNT suppliers were used:HiPco™ SWCNTs from Carbon Nanotechnologies, Inc. andElicarb™SWCNTsfromSwanChemicalCompany. Six different electrolytes were used in the project: benzyldi-methylpropylammoniumaluminumtetrachlorate,benzyldimethylammoniumimide,dimethylethylammoniumimide,ethylmethylammoniumbisulfate,1-butyl-3-methylimidazoliumtetraflourob-orate, and tetraethylammonium tetrafluoroborate in acetonitrile.Thefirstfiveoftheseelectrolytes(FigureA)aretermed“room-temperatureionicliquids”andareanewfamilyofchemicalcom-poundsusefulforelectrochemistryapplications.30,31

The active electrode material (i.e., the SWCNT-containingpaste)isappliedinauniformlayeracrossthetwoaluminumfoilcurrentcollectorsandthepaste is thensaturatedwithaspecific

electrolyte.TheCelgardseparatorissandwichedbetweenthetwoactiveelectrodesand theentireassembly issealedusingkaptontape(FigureB). Once manufactured, the supercapacitors were subjected to aseries of electrical tests and compared to a 10 F commerciallyavailablesupercapacitor (MaxwellTechnologiesModel#PC10).Since the academic-scale kapton tape sealing method does notproducenorretainthedesiredinternalpressurethatatraditionalcommercial-scale supercapacitor manufacturing method couldgenerate,ahydraulicpresswasusedduringthelaboratoryelectricaltesting.Thepressurebetweenthetwoplatesofthispress(withthesupercapacitorinbetween)ismaintainedat6.9MPathroughoutthetestingregimen.Theprimarydatareportedforthelaboratoryscalesamplesareresultsfromaconstantvoltagetest.Duringthisanalysis,avoltageof1.5Vismanuallyappliedviaalligatorclipsattached across the “tabs” of the supercapacitor (Figure B) forbetweenfourandfivesecondsandtheninstantaneouslyreducedtozeroandheldatzeroforbetweenfourandfivesecondswhilethecurrentismeasured.Thiscycleisthenrepeated.

Electrical Testing and Materials Characterization at Johnson Space Center

Three experimental supercapacitors with the highest reportedperformance during initial testing at Georgia Tech ResearchInstitute,inadditiontoaMaxwellPC1010Fsupercapacitor,weresubmittedfor further testingatNASAJohnsonSpaceCenter.Astandard constant current charge/discharge cycle was chosen tocharacterize capacitance and equivalent series resistance of thecapacitors. A tensile tester with a 4,536 kg load cell was usedtoapplytherecommended6.9MPaofpressure.Forthesakeofcompleteness,loadsfrom0kgto1,361kgwerealsoapplied.Thesupercapacitor leads were connected to a direct-current powersupplywithvoltagelimitedto1.5Vandcurrentsetat20mAor50mA(200mAinthecaseoftheMaxwellPC10).Thecapacitorwaschargedforafixedtime,8sto12s,whilevoltagewasmeasured.The charge current was removed and the capacitor was thendischargedbeforerepeatingthetest.Voltagewasrecordedforthechargeportionofthecycleonly.

Electrochemicaldouble-layersuper-capacitors have properties ranging inbetween these two common energystorage devices. Supercapacitors offerhighpowerdensity,highenergydensity,andlongcyclelife.5Inaddition,conven-tionalcapacitorsarelimitedbydielectricbreakdown.Dielectricmaterialsthatarenecessaryfortraditionalcapacitorsarenotneededforsupercapacitors.However,supercapacitorstypicallycontainorganic

electrolytesthatmaylimittheiruseinsome applications. As energy storagedevices,supercapacitorscouldbeappliedtomanyemergingtechnologiessuchaselectric vehicles, satellite propulsion,andpulsepowerapplications.6

Supercapacitorsincorporatingcarbonnanotubes(CNTs)canpotentiallystorehigher energy density than traditionalcapacitorswithanequivalentamountofdeliveredpower(Figure1).Thisproperty

makesthemsuitableforcertainapplica-tions (like drills and in-situ resourceutilizationsystems)envisionedforuseinhumanspaceflighttothemoon,Mars,andbeyondaspartofNASA’s“VisionforSpaceExploration.” Threemainclassesofsupercapacitorsare described in the literature: metaloxide,7,8electronicallyconductingpoly-mer,9,10andcarbon-basedsupercapaci-tors.11,12Recently,hybridsupercapacitors

JOM • December 200528

havebeendevelopedwhereanactivatedcarbon electrode is associated with afaradaicelectrode.13,14 Carbon-based supercapacitors havebeen largely investigated because oftheirlowcost,highcyclelife,andhighcapacitance (measured in Farads [F]).Large-size(i.e.,>5,000F)devicesarecommerciallyavailablefromcompaniessuch as Maxwell, Epcos, and Pana-sonic.15,16ThisworkfocusesontheuseofSWCNTstoenhancetheperformanceofcarbon-basedsupercapacitors.

Carbon Nanotubes

Carbonnanotubes,firstreportedandcharacterized by Ijima17 and Endo,18consist of high-aspect-ratio cylindersof carbon that are often capped withhemispherical buckminster fullerenes(i.e.,C

60;buckyballs).Theseintriguing

structures have sparked much excite-mentinrecentyearsandalargeamountofresearchhasbeendedicatedtotheirunderstanding.Carbonnanotubesmaybesub-classifiedasbeingSWCNTs,(some-timesabbreviatedSWNT)orconcentricmulti-walled nanotubes (MWCNTsor MWNTs). Due to their impressivematerial properties, CNTs are beingdevelopedforuseinapplicationssuchas superconductors,19 hydrogen stor-age,20fieldemission,21 logiccircuits,22andnumerousotheremergingareas.

Supercapacitors

BothMWCNTsandSWCNTshavebeenresearchedforsupercapacitorappli-cations.Inthiswork,SWCNTsareused.TounderstandthebenefitsofSWCNT-based supercapacitors, it is useful tobegin with a first-principles approachtoenergystorage.Thecapacitance,C,of a material is given by Equation 1,whereA

Eisthegeometricsurfacearea

oftheelectrode,εoisthepermittivityof

freespace,εristherelativepermittivity

of thedielectricmaterial,andd is the

Equations

(1)

(2)

(3)

CA

do r E=

⋅ ⋅ε ε

CI t

V=

⋅∆ 1

RV

IESR=

∆ 2

distance between the two oppositelybiasedelectrodes.(Note:Allequationsaregiveninthetableonthispage.) Supercapacitorsconsistoftwoelec-trodesimmersedinorimpregnatedwithanelectrolytesolutionwithasemi-per-meablemembraneservingasasepara-tor23 that prevents electrical contactbetweenthetwoelectrodesbutallowsfor ionic diffusion. When an electricpotential is applied to the electrodes,apotentialdifference iscreatedat theelectrode-electrolyteinterface.Thiselec-trostatic interfaceconsistsofadoublelayerbetweenionsintheelectrolyteandtheelectronicchargesontheelectrode24(Figure2).Theinterplanedistance,d,in

system.AsisevidentfromEquation1,inordertoachievehighcapacitance,amaterialwithhighsurfacearea(A

E)and

smallHelmolzdistance(Dd)shouldbe

chosen.25 Surface conditions areextremely important for capacitance,withporosityplayingalargerole. The most prevalent materials beinginvestigatedforuseincommercial-scalesupercapacitors are activated carbons.This work will investigate SWCNT-based active electrode materials, butunderstandingthelimitationofactivatedcarbonelectrodesisessentialtorealizingthefullpotentialapplicabilityandcom-mercial feasibility of SWCNT-basedsupercapacitors. Activated carbons arehigh-surface-area, high-porosity carbons made ofsmall hexagonal rings organized intographene sheets. These sheets can beproducedbyvariousprocessingmethodsthatresultinvaryingporesizedistribu-tionsandorientations.Activatedcarbonslacklong-rangeorderandcanthereforebeviewedasamixtureofmicrodomainsoforderedgraphenesheets.Thespecificdouble-layercapacitancecanbeseenasthe sumofeachmicrodomaincapaci-tance. Qu25hasshownthatthesemicrodo-mainscanbeconsideredasafewgra-phene sheets stacked in parallel withthickness L

c. The sheets are linked

togetherthroughoutthelateraldirectionand separated by a distance L

a. The

specific capacitance of the activatedcarbonwasshowntobeproportionaltotheaspectratioofthegraphenesheets(i.e., L

c/L

a). A key to achieving large

capacitanceisincreasingLcwhileatthe

same time reducing La. Single-wall

carbonnanotubes,withtheirextremelyhighaspect ratio,are thereforesuperbcandidates for use in supercapacitorapplications. Poresizesareclassifiedaccordingtothe International Union of Pure andApplied Chemistry. Micropores aredefinedashavingaradiuslessthan20Å.

Table I. Mean Capacitance, ESR, and Specific Capacitance

Mean Capacitance Specific Capacitance ESR*Type (F) (F/g) (Ω)

HiPco+Acetonitrile(unloaded) 0.2 0.1 7.8Elicarb+Acetonitrile(unloaded) 0.6 0.4 8.4Elicarb+Acetonitrile(loaded) 0.9 0.6 1.4PC10 10.0 1.7 0.3

*ESR=equivalentseriesresistance

Due to their impressive material properties, CNTs are being developed for use in applications such as superconductors,

hydrogen storage,

field emission, logic circuits, and numerous other emerging areas.

Equation1isnowreducedtotheHelmolzdouble-layerdistance,d

D,definedashalf

the diameter of the adsorbed solvatedionsattheelectrode/electrolyteinterface.In supercapacitors, energy storage isduetotheseparationofelectronicandionicchargesattheinterfacebetweenahigh-surface-area active electrode andtheelectrolytesolution.

Active Materials

Theactivematerialontheelectrodeisanessentialpartofthesupercapacitor

2005 December • JOM 29

Mesoporeshavearadiusbetween20Åand50Å,andmacroporesareanyporeslargerthan50Å.Activatedcarboncon-tainsawidedistributionofporesizes.Typical Brunauer, Emmett, and Teller(BET)surfaceareasforactivatedcarbonare1,000–3,000m2/g.Unfortunately,asubstantialfractionofthissurfacearearesides in micropores or unpercolatedpores which are inaccessible to ionmigrationandthereforeunabletosupportan electrical double layer.26 Ions arecapableofmigrationtosomeofthelargerpores, although this results in anincreasedresistance in theelectrolyte.Typically,suchanincreaseinresistanceresultsindecreasedcapacitance. Carbonnanotubesofferanadvantageoveractivecarbonintermsofporosity.AlthoughtheBETsurfaceareaofCNT-basedactivematerialsissometimesnotashighasinactivatedcarbon,NiureportsthatthesurfaceareaismoreaccessibleinCNTs.26Theporesarepercolatedandtheirsizedistributionliesmostlywithintheextremelybeneficialmesoporerange.Inaddition, thevolumeofdeleteriousmicroporesinsamplesofCNTsisneg-ligible. Carbon nanotubes also offerchemicalandmechanicalstability.Theyhaveadditionalmaterialandelectricalproperties that make them attractivecandidates as an active material in asupercapacitorelectrode.27–29Forexam-ple,theballisticelectricalconductivityofanarm-chairSWCNTisidyllic. Single-wall carbon nanotubes wereinvestigatedinthisstudyforincreasingthepowerperformancesofcarbon-basedsupercapacitors. This article presentsperformancecharacterizationsoflabora-tory-scale supercapacitors assembledfromtreatedaluminumcurrentcollectorscoupled with a SWCNT-containingactive electrode teamed with variouselectrolytes. Commercially availablesupercapacitorsarealsoinvestigatedandcompared to the research results.Experimentalproceduresarepresentedinthesidebar.

RESuLTS & DISCuSSIoN

Laboratory-Scale SWCNT-Based Supercapacitors

The constant voltage plot for thelaboratory-scalesupercapacitorsreportsthecurrentpulsegeneratedupon“firing”asupercapacitorthathasbeencharged

witha1.5Vpotentialforapproximately5s.Forthisevaluation,superiorperfor-manceisbasedonagreatermagnitudeofthecurrentpulse.Theslightdifferenceinthetimeapplicationofthepulsesonthehorizontalaxisisduetothemanualfiring of the supercapacitor by theoperatorevery4sto5sandshouldnotbe interpreted as a performancemetric. The performance of SWCNT-basedsupercapacitors in comparison to thecommercial benchmark reveals theimportance of electrolyte selection(Figure3).Thedifferenceinperformancebetweenthevariousroom-temperatureionicliquid(RTIL)electrolytesisprimar-

formance compared to the HiPcoSWCNTs.Yet,Figure5 andFigure6show that when RTILs #2 and #5 areused, an opposite response occurswhereby the HiPco SWCNTs offersuperiorperformance.Thesedifferencesinperformancearebelievedtobeattrib-utedtoamatingeffect,wherebyapar-ticularelectrolyteiondiameterisbettersuitedforthegivenporosityandmaterialpropertiesofeithertheHiPcoorElicarbSWCNTs. Unfortunately, the laboratory-scalesamplesexhibitedverypoorreliability.Thisisattributedtotheprimitivenatureofthekaptontapesealingmethod.Thecrudenessofthistechniqueallowedfortheelectrolytetosuffersevere(Figure3andFigure4versusFigure7)degrada-tionwithinafewdozenhoursofmanu-facture. The commercial benchmarksamples from Maxwell have yet todegradeovertheprogramperiod.Thisis,ofcourse,tobeexpectedwithacom-merciallyavailableproductwithahighlevel of quality control such as thePC10. Yet,ifthekaptonsealonthelabora-tory-scalesampleswasopenedandtheactive material re-saturated with thedesignatedelectrolyte,andthensubse-quently re-sealed, the supercapacitorperformed in a manner similar to itsoriginal performance (Figure 8). Thisshowsthatifcommercial-gradepackag-ing and sealing technologies areemployed,theSWCNT-basedsuperca-pacitors and electrolytes would havesufficient longevity that theycouldbeusedinmission-criticalapplications.

Electrical Testing and Materials Characterization

The constant current plot for thelaboratory-scalesupercapacitorsreportsavoltagechangeasthesupercapacitorischargedatconstantcurrent. The three experimental capacitorssubmittedforelectricalscreeningwerethe HiPco SWCNT with acetonitrileelectrolyte, the Elicarb SWCNT withacetonitrile,andtheHiPcoSWCNTwithRTIL#2.Followinganextendedagingperiodofseveralweeks,itwasexpectedthattheelectrolytewouldhaveevapo-ratedordeterioratedbythetimetestingoccurred.Avialofacetonitrilewassup-plied for refilling the supercapacitors.NoadditionalsupplyoftheRTILswas

In this study, SWCNTs were investigated for increasing power performances of carbon-based supercapacitors.

ily attributed to the differences in ionsizeandviscositybetweentheelectro-lytesandthereforethediffusivityoftheionsthatarethechargecarriersinthesupercapacitorisimpacted. Room-temperature ionic liquids #1and#4behavedquitepoorly.AsshowninFigure3,theywerevirtuallyindistin-guishable from the horizontal axis.However,severalelectrolytesperformedon par or superior to the commercialbenchmark. Room-temperature ionicliquid#2performednearly identicallytothecommercialbenchmarkinpulseheight. Note, however, that the com-merciallyavailableproductdischargesmorerapidlythanRTIL#2.Thesuperiorperformance of the SWCNT-basedsupercapacitorscomparedto thecom-mercialbenchmarkbecomesevenmorestrikingwhencomparingtheresponseoftheacetonitrileandtetraethylammo-nium tetrafluoroborate electrolyte(Figure4). In addition, it can be seen that theElicarbSWCNT(ifFigure4isextrapo-lated beyond the 1.05A equipmentlimitation)seemstooffersuperiorper-

JOM • December 200530

0.7

0.5

0.3

0.1

−0.1

−0.3

−0.5

−0.7Time (s)

Cur

rent

(A)

Time (s)

Cur

rent

(A)

1.5

1

0.5

0

–0.5

–1

–1.5

MaximumCurrent

is Limitedto 1.05A

Time (s)

0.25

0.2

0.15

0.1

0.05

0

−0.05

−0.1

−0.15

−0.2

Cur

rent

(A)

Time (s)

0.8

0.6

0.4

0.2

0

−0.2

−0.4

−0.6

−0.8

−1

Cur

rent

(A)

Time (s)

0.25

0.2

0.15

0.1

0.05

0

−0.05

−0.1

−0.15

−0.2

Cur

rent

(A)

availableatthetimeoftheNASA-John-sonSpaceCentertesting,sothesuper-capacitor with RTIL #2 was testedwithout refill. This supercapacitorbehavedasaresistor,soitsdataisnotincludedhere.Room-temperatureionicliquid #5 had not been tested by theNASA-JohnsonSpaceCentergroupatthetimeofthiswriting. The performance of SWCNT-basedsupercapacitors in comparison to thecommercialbenchmarkisseeninFigure9.TheMaxwellPC10capacitoracceptedchargefor60s,buttheHiPco+aceto-nitrilesupercapacitorbegantosaturateafterabout10s.Thechargetimewasreduced accordingly, and a plot of alldataonthisreducedtimescaleisshowninFigure10. An ideal charging curve resemblesthatoftheMaxwellPC10data—linearduringchargingandleveloncethecur-rent is shut off, though the constantvoltageisoffsetbyaninstantaneousdropduetoequivalentseriesresistance(ESR).Itcanbeseenthatmosttestsinvolvingtheexperimentalsupercapacitorsshowednonlinearcharging,whicharerelatedtohighESRandleakagecurrent,orinsomecasesduetothesupercapacitorsaturat-ingwithcharge.Voltagealsodroppedoffwhencurrentwasturnedoff,ratherthanholdingconstant. CapacitanceCcanbedeterminedfromconstant-currentchargingasshown inEquation2,whereIistheconstantcharg-ingcurrent, t is the timecharged,andΔV

1istheincreaseinvoltageduringthe

linear portion of the charging period.Therefore,capacitanceisrelatedtotheslopeofthelinearportionofthecharg-ingcurve.TheESRcanbedeterminedbyEquation3,whereIistheconstantchargingcurrentandΔV

2isthemagni-

tudeoftheinstantaneousdropinvoltageafter the current is shut off. Meancapacitance,ESR,andspecificcapaci-tancedataarepresentedinTableIfortheacetonitrile-basedsupercapacitors.

CoNCLuSIoNS

Thisworkhasdemonstratedthatbyalteringtheelectrolytecompositionandactive electrode material attributes,carbon-nanotube-based supercapacitorperformancecanbesignificantlyaltered.Improvedpackagingandsealingtech-niquesareexpectedtomarkedlyreduceESRandleakagecurrentintheexperi-

Figure 3. Electrical testing results—constant voltage (RTIL comparison—all with HiPco).

Figure 4. Electrical testing results—constant voltage (Elicarb vs. HiPco).

Figure 5. Electrical test-ing results—constant voltage (Elicarb vs. HiPco).

Figure 6. Electrical testing results—constant voltage (Elicarb vs. HiPco).

Figure 7. Electrical testing results—constant voltage (aging).

2005 December • JOM 31

�

Time (s)

1.5

1

0.5

0

−1.5

−1

−1.5

Cur

rent

(A)

MaximumCurrent

is Limitedto 1.05A

mentalsupercapacitors.TheapplicationofexternalpressuremaydecreaseESR.However, high pressure might alsodegradethenanotubeelectrodes,hinder-ing interaction between the electrodeandelectrolyte,ordestroytheseparator,causingelectricalshorting.

Time (s)

Volta

ge (V

)

00.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

5 10 15 20 25

0 20

1.80

1.60

1.40

1.20

1.00

0.80

0.60

0.40

0.20

0.0040 60

Time (s)

Volta

ge (V

)

80 100 120

Figure 10. Electrical testing results—constant current (Elicarb vs. HiPco), concentrating on first 24 seconds. Slope of curve is indicative of capacitance while the step height is indicative of ESR.

Figure 9. Electrical testing results—constant current (Elicarb vs. HiPco).

Figure 8. Electrical testing results—constant voltage (rejuvenation).

References

1. Reynolds et al., NASA Tech Briefs (June 2005), p. 70.2. T. Christen and M.W. Carlen, J. Power Sources, 91 (2000), p. 210.3.http://data.energizer.com/batteryinfo/other_avail-able/pdfs/typical_characteristics.pdf. 4. C. Emmenegger et al., to be published in J. Power Sources.5. M. Endo et al., Carbon Sci. 1 (2001), p. 117.6. B.E. Conway, Electrochemical Supercapacitors—Scientific Fundamentals and Technological Applica-tions (New York: Kluwer Academic/Plenum, 1999).7. Q.L. Fang et al., J. Electrochem. Soc.,Soc., 148 (2001), p. A833.8. J. Jiang and A. Kucernak, Electrochim. Acta, 47 (2002), p. 2381.9. M. Mastragostino, C. Arbizzani, and F. Soavi, Solid State Ionics, 148 (2002), p. 493.10. W.-C. Chen, T.-C. Wen, and H. Teng, Electrochim. Acta, 48 (2003), p. 641.11. D. Lozano-Castello et al. Carbon, 41 (2003), p. 1765.12. P.J. Gamby et al., J. Power Sources, 101 (2001), p. 109.13. A. Laforgue et al., J. Electrochem. Soc., 150 (2003), p. A645.14. A. Du Pasquier et al., J. Power Sources, 115 (2003), p. 171.15. A. Chu and P. Braatz, J. Power Sources, 112 (2002).16. R. Kötz and M. Carlen, Electrochim. Acta, 45 (2000), p. 2483.17. S. Ijima, Nature, 354 (1991), p. 56.18. A. Oberlin, M. Endo, and T. Koyama, J. Cryst. Growth, 32 (1976), pp. 335–349.19. M. Kociak et al., Phys. Rev. Lett., 86 (2001), p. 2416.20. A.C. Dillon et al., Nature, 386 (1997), p. 377.21. A.G. Rinzler et al., Science, 269 (1995), p. 1550.22. P.G. Collins, M.S. Arnold, and P. Avouris, Science, 292 (2001), p. 706.23. C. Emmenegger, to be published in J. Power Sources.24. From: http://pl.legoff.free.fr/us/usscap.htm.25. D. Qu, J. Power Sources, 109 (2002), p. 403.26. C. Niu et al., Appl. Phys. LettPhys. Lett., 70 (1997), p. 1480.27. E. Frackowiak et al., Fuel Process. Technol.,Technol., 77-78 (2002), p. 213.28. Ch. Emmeneger et al., Appl. Surf. Sci.,Surf. Sci., 162-163 (2000), p. 452.29. A.K. Chatterjee et al., Electrochim. Acta,Acta, 48 (2003), p. 3439.30. K. Kim et al., “Electrochemical Investigation of Qua-ternary Ammonium/Aluminum Chloride Ionic Liquids,” J. Electrochemical Society, 151 (2004), A1168-72.31. K. Kim, C. Lang, and P.A. Kohl, “Properties of Ben-zyl-substituted Quaternary Ammonium Ionic Liquids,” J. Electrochem. Soc., in press.

S.Arepalli,seniorstaffscientist,H.Fireman,C.Huffman, research scientist, P. Moloney,nanomaterialsapplicationslead,P.Nikolaev,andL.Yowell,materialsresearchengineer,arewithNASA-JohnsonSpaceCenterinHouston,Texas.K.KimandP.A.KohlarewithGeorgiaInstituteof Technology. C.D. Higgins, undergraduateresearchassistant,S.P.Turano,researchengineer,andW.J.Ready,seniorresearchengineer,arewithGeorgia Tech Research Institute in Atlanta,Georgia.

For more information, contact W.J. Ready, Georgia Tech Research Institute, 925 Dalney St., Atlanta, GA 30332; (404) 385-4497; fax (404) 894-0580; e-mail jud.ready@gtri.gatech.edu.

ACkNowLEDgEmENTS

ThisworkwasfundedinpartbytheU.S.ArmySpaceandMissileDefenseCommand (Contract #DASG60-03-1-0 0 0 4 ) , a n d NA S A ( C o n t ra c t#NNJ05HA25G).