DTIC ADA606389: Redox Deposition of Nanoscale Metal Oxides on Carbon for Next-Generation Electrochemical Capacitors

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by  Defense Technical Information Center

Redox Deposition of Nanoscale Metal Oxides on
Carbon for Next-Generation Electrochemical
Capacitors
MEGANB.SASSIN,CHRISTOPHERN.CHERVIN,
DEBRAR.ROLISON,ANDJEFFREYW.LONG*
U.S.NavalResearchLaboratory,SurfaceChemistryBranch(Code6170),
Washington,D.C.20375,UnitedStates
RECEIVEDONOCTOBER26,2011
CONSPECTUS
T ransitionmetaloxidesthatmixelectronicandionicconductivityare
essentialactivecomponentsofmanyelectrochemicalcharge-storage
devices,rangingfromprimaryalkalinecellstomoreadvancedrecharge-
able Li-ion batteries.In thesedevices,charge storageoccursvia cation-
insertion/deinsertion mechanisms in conjunction with the reduction/
oxidationofmetalsitesintheoxide.Batteriesthatincorporatesuchmetal
oxidesaretypicallydesignedforhighspecificenergy,butnotnecessarily
for high specific power. Electrochemical capacitors (ECs), which are
typically composed of symmetric high-surface-area carbon electrodes
that store charge via double-layer capacitance, deliver their energy in
timescalesofseconds,butatmuchlowerspecificenergythanbatteries.
The fast, reversible faradaic reactions (typically described as “pseudo-
capacitance”)ofparticularnanoscalemetaloxides(e.g.,rutheniumandmanganeseoxides)provideastrategyforbridging
the power/energy performance gap between batteries and conventional ECs. These processes enhance charge-storage
capacitytoboostspecificenergy,whilemaintainingthefew-secondtimescaleofthecharge(cid:1)dischargeresponseofcarbon-
basedECs.
InthisAccount,wedescribethreeexamplesofredox-baseddepositionofEC-relevantmetaloxides(MnO ,FeOx,andRuO )
2 2
and discuss their potential deployment in next-generation ECs that use aqueous electrolytes. To extract the maximum
pseudocapacitance functionality of metal oxides, one must carefully consider how they are synthesized and subsequently
integratedintopracticalelectrodestructures.Expressingthemetaloxideinananoscaleformoftenenhanceselectrochemical
utilization(maximizingspecificcapacitance)andfacilitateshigh-rateoperationforbothchargeanddischarge.The“wiring”ofthe
metaloxide,intermsofbothelectronandiontransport,whenfabricatedintoapracticalelectrodearchitecture,isalsoacritical
designparameterforachievingcharacteristicECcharge(cid:1)dischargetimescales.Forexample,conductivecarbonmustoftenbe
combinedwiththepoorlyconductivemetaloxidestoprovidelong-rangeelectronpathwaysthroughtheelectrode.However,the
ad hoc mixing of discrete carbon and oxide powders into composite electrodes may not support optimal utilization or rate
performance. As an alternative, nanoscale metal oxides of interest for ECs can be synthesized directly on the surfaces of
nanostructuredcarbons,withthecarbonsurfaceactingasasacrificialreductantwhenexposedtoasolution-phase,oxidizing
precursorofthedesiredmetaloxide(e.g.,MnO (cid:1)forMnO ).Theseredoxdepositionmethodscanbeappliedtoadvancedcarbon
4 2
nanoarchitectures with well-designed pore structures. These architectures promote effective electrolyte infiltration and ion
transporttothenanoscalemetaloxidedomainswithintheelectrodearchitecture,whichfurtherenhanceshigh-rateoperation.
Introduction give.Electrochemicalcapacitors(ECs,alsodenotedas“ultra-
Whenaburstofenergymustbedeliveredonthetimescale capacitors” and “supercapacitors”) fill a temporal perfor-
ofseconds(10(cid:1)1(cid:1)102s),batteriesmissthepowerdelivery mancevoidthatiscriticalforrecapturingwasteenergy(e.g.,
windowwhileelectrostaticcapacitorsexpendtheirenergy from braking in an electric or gas/electric hybrid vehicle)
in microseconds to milliseconds and have nothing left to or providing energy bursts for portable communication
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10.1021/ar2002717 ThisarticlenotsubjecttoU.S.Copyright.Published2012bytheAmericanChemicalSociety
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1. REPORT DATE 3. DATES COVERED
2011 2. REPORT TYPE 00-00-2011 to 00-00-2011
4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Redox Deposition of Nanoscale Metal Oxides on Carbon for
5b. GRANT NUMBER
Next-Generation Electrochemical Capacitors
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. Naval Research Laboratory,Surface Chemistry Branch (Code REPORT NUMBER
6170),,Washington,DC,20375
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S)
11. SPONSOR/MONITOR’S REPORT
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13. SUPPLEMENTARY NOTES
ACCOUNTS OF CHEMICAL RESEARCH,Vol. 46, No. 5, 2013, 1062-1074
14. ABSTRACT

15. SUBJECT TERMS
16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF
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Prescribed by ANSI Std Z39-18
RedoxDepositionofNanoscaleMetalOxides Sassinetal.
TABLE1. OxidationPotentialsofSelectMetal-OxidePrecursors
oxidant oxidationpotential,acidic oxidationpotential,alkaline
MnO (cid:1) 1.67 0.56
FeO 42(cid:1) 2.20 0.72
RuO42(cid:1) (cid:1)0.22
RuO4(cid:1) 0.59
4
RuO 1.0
4
the pseudocapacitive metal oxide is more intimately asso-
ciated with nanostructured carbon substrates. The electro-
chemical utilization of such rewired metal oxides can be
significantlyenhancedwhenexpressedasnanoscalefilms,
as shown for ultrathin MnO (tens to hundreds of nm),
2
withspecificcapacitancesapproachingthetheoreticalvalue
of 1370 F g(cid:1)1 for storing one electron per Mn center,14,15
as compared to conventional powder-composite elec-
trodes of MnO , where the capacitance rarely exceeds
2
200Fg(cid:1)1.16Thepropertiesofthin-filmmetaloxidescan
betranslatedtomorepractical electrodeanddevicesize
FIGURE1. Schematicofchargestorageviaelectrochemicaldouble-
regimes by incorporating the metal oxide as a nano-
layercapacitanceandpseudocapacitance.
scale coating on conductive carbon supports. Many pro-
devices.1,2Thefew-secondstimescaleoverwhichenergyis tocols, including electrodeposition,17 vapor deposi-
released from ECs serves as one performance criterion; tion,18 impregnation/decomposition,19 and sol(cid:1)gel
another is the magnitude of the energy released during chemistry20 have been used to synthesize such metal
the desired pulse of power. Many present and foreseen oxide(cid:1)carbon nanomaterials, but not all methods yield
applicationsrequiremoreenergytobedeliveredinseconds conformal coatings on nanostructured surfaces, and
than can be achieved with EDLCs (electric double-layer optimized contact between metal oxide and carbon is
capacitors), which store charge in the electric double-layer notnecessarilyachieved.
(Figure 1) and are characterized by relatively low specific Redoxdepositionofmetaloxides,asachievedbyreact-
energy(3(cid:1)8versus>100Whkg(cid:1)1forLi-ionbatteries),even ing the corresponding strong-oxidant precursor (MnO (cid:1),
4
usinghigh-surface-areacarbonelectrodes.3,4 FeO 2(cid:1), RuO ; Table 1) at the surface of carbon (acting as
4 4
To increase the specific energy released by ECs, re- reductant), offers an attractive alternative in terms of scal-
searchers a;repsteuurndoincgaptaocitaanncea,dodritiaondaolubclheairngsee-rsttioornagoefmechabainliitsym,cost,andmanufacturability.21Althoughaseemingly
anelectronandcation(eitherprotonsorsupportingelectro- simple deposition protocol (exposing the desired carbon
lyte cations, such as Na substratetoasolutionoftheoxidant),itisoftennecessary
þ þ
or K ) into a mixed electron/ion to tune reactionconditions in orderto promote maximum
conductive material (Figure 1).5(cid:1)7 Transition metal oxides, metal-oxideloadingwhilealsomaintainingconformalityof
suchasRuO andMnO ,areprimecandidatesasthepseu- thedeposit.Wefindthatwhenconditionsthatensureself-
2 2
docapacitive phase, particularly when used in aqueous- limiting deposition are attained, we preserve the complex
electrolyteECs.8(cid:1)12Themaindrawbacktotheuseofmost porestructuresofmesoporous(2(cid:1)50nmpores)andmacro-
metal oxides is that their limited electronic conductivity porous(>50nmpores)carbonsubstrates(Figure2),thereby
restricts power delivery on the few-seconds time scale. enablingfacileelectrolyteinfiltrationandhigh-rateelectro-
Thetraditionalworkaroundistomixtheactiveoxidewith chemical performance.22(cid:1)24 In the following sections, we
carbon powder into a composite electrode, not unlike describe specific examples of redox deposition of MnO ,
2
theapproachusedtoformelectrodesforbatteries,butthe FeOx, and RuO on a variety of nanostructured carbon
2
ad-hocnatureofthepowder-compositeelectrodestructure substrates and the implications of composition and archi-
potentiallyprecludesahigh-rateresponse. tecture when the resulting metal oxide(cid:1)carbon electrode
Amoredesign-richstrategyistomovetowardadvanced structuresareusedascharge-storingarchitecturesforaqu-
compositesandarchitectures,13particularlythoseinwhich eousECs.
Vol.46,No.5’2013’1062–1074’ACCOUNTSOFCHEMICALRESEARCH’1063
RedoxDepositionofNanoscaleMetalOxides Sassinetal.
FIGURE2. Schematicofredoxdepositionunder“poorlycontrolled”(left)versus“self-limiting”(right)conditions.
Redox Deposition Approaches to Nanoscale
Coatings of Metal Oxides
(cid:1)
Manganese Oxides. Permanganate (MnO ) is a versa-
4
tile, readily available, and water-soluble oxidizing agent
commonly used in organic synthesis,25 wastewater treat-
ment,26andcanserveasaprecursortoMnO uponreaction
2
with an appropriate reducing agent (e.g., Mn2þ, sugars,
alcohols,unsaturatedorganicacids).27,28Whilepermanga-
nate has long been used to oxidize carbon surfaces to
enhanceelectrochemicalperformance,Chenandco-work-
erswerethefirsttorecognizethesimplicityandscalabilityof FIGURE3. ElectrodepotentialandpHversustimeforreactionof
the permanganate(cid:1)carbon redox reaction for generating KMnO4withacetyleneblack.Reprintedwithpermissionfromref29.
Copyright2006TheElectrochemicalSociety.
MnOx coatings that store charge.21 The initial study per-
formedonplanargraphitehadlimitedutilityforreal-world Later studies showed that CO is generated as a bypro-
2
ECsbecauseofthesmallquantityofchargestored,butasthe duct, confirming a direct reaction between permanga-
authors predicted, redox deposition can be extended to nateandAB.30
more EC-relevant high-surface-area and 3D carbon sub-
strates,includingcarbonblacks,29,30templatedmesoporous 4MnO4(cid:1)þ3Cþ2H2O f 4MnO2þ3CO2þ4OH(cid:1) (2)
carbon,31(cid:1)34 nanotubes,35(cid:1)40 graphene,41,42 and nano-
Amorecomplexreactionschemeisproposedformulti-
foams.22,23
walledcarbonnanotubes(CNTs)withoxidationoccurring
Maandco-workerspositedthatthereactionofperman-
primarily at edge-plane sites or other defects; MnO
ganatewithacetyleneblack(AB)beginsasprotonsadsorbat 2
depositioninitiatesatthesedefectsandthenpropagates
the carbon surface from the aqueous permanganate solu-
tion(pH∼5),29,35evidencedbyasharpincreaseinsolution to other surface sites, coupled to electron transport
pH and a decrease in solution potential (Figure 3). The throughthenanotubes(Figure4).35
interfacial localization of protons from solution and elec- Permanganatereductionatcarbonisinfluencedbysuch
trons from the carbon substrate favors the reduction of factorsastemperature,pH,concentration,andthefunction-
permanganatetoamixed-valentmanganese(III/IV)oxide, ality of the carbon surface. Acidic solutions are used to
designatedasMnO ,accordingtotheequation accelerate the reaction or to modify carbon surfaces that
2
are more resistant to oxidation (e.g., the basal-plane sur-
MnO4(cid:1)þ4Hþþ3e(cid:1) f MnO2þ2H2O (1) facespredominantinCNTs).FasterMnO -depositionrates
2
1064’ACCOUNTSOFCHEMICALRESEARCH’1062–1074’2013’Vol.46,No.5
RedoxDepositionofNanoscaleMetalOxides Sassinetal.
Acidic conditions not only enhance the permanganate(cid:1)
carbon reaction rate, but also the competing autocatalytic
decomposition of permanganate (via oxidation of H O).43
2
Fischer and co-workers demonstrated the importance of
avoiding acidic deposition conditions when modifying
prefabricated, macroscale forms of an ultraporous carbon
substrate(e.g.,170μmthickcarbonnanofoampapers),22,23
asopposedtodispersionsofcarbonpowders.Underacidic
conditions, micrometers-thick MnO coatings form at the
2
exteriorofcarbonnanofoampaper(Figure6a,b),occluding
the pore structure and frustrating MnO deposition within
2
the interior void volume. These thick crusts of poorly con-
ductive MnO also impose an undesirable charge-transfer
2
resistance to the resulting electrode structure (Figure 7a),
markedlylimitingECperformance.Underneutral-pHcondi-
tions, autocatalytic permanganate decomposition is sup-
FIGURE4. SchematicofredoxdepositionofMnO onCNTsatinitial
2
andlaterstages.Reprintedwithpermissionfromref35.Copyright2007 pressed and the permanganate(cid:1)carbon reaction self-
WileyInterScience. limits,resultinginnanoscale(∼10nmthick)MnO coatings
2
that conform to the carbon walls throughout the nano-
foam paper (Figure 6c,d), enhance electrode capacitance,
andmaintainthefrequencyresponseofthenativecarbon
nanofoam(Figure7b).
The thickness, conformality, distribution, and weight
loading of the MnO coating depend not only on the
2
depositionconditionsbutalsoonthenatureofthenanos-
tructured carbon substrate; the interplay of these design
features ultimately determines the electrochemical perfor-
manceoftheresultingMnO -modifiedcarbon. Dongetal.
2
showedthatreactionatshorttimesbetweendiluteperman-
ganate and templated mesoporous carbons (ordered 3(cid:1)
5 nm pores) yields nanoscopic MnO embedded into the
2
carbon walls.31 When cycled in aqueous 2 M KCl, the
FIGURE5. Scanningelectronmicroscopyimagesof(a)pristineCNTs,
andMnO2-coatedCNTspreparedfrom(b)pH7,(c)pH2.5,and(d)pH1 resulting MnO2-modified carbons demonstrated enhanced
KMnO4solution.Reprintedwithpermissionfromref36.Copyright2007 total electrode capacitance (173 versus 105 F g(cid:1)1 for the
Elsevier. nativecarbon)andhighMnO utilization(628Fg (cid:1)1)at
2 (MnO)
2
at CNTs occur at pH 1 compared to permanganate solu- a relatively low weight loading (13 wt % MnO ); with this
2
tionsatpH2.5and7;conformal,nanoscaleMnO coating protocol,diminishingreturnsinutilizationwereobservedas
2
are achieved at all pHs, but surface morphologies were the weight loading increased to a maximum of 26 wt %
rougher for deposition at pH 1 (Figure 5).36 The sp2 MnO .32Pengetal.usedahierarchicalporestructurecom-
2
character of pristine CNT and graphene surfaces makes pr(cid:1)isi8ngnmdi)stainncdt,mbauctroinptoerrceosn(7n0ec(cid:1)te1d0,0nnemtw)otroksimopfromveesoopnotrhees(6
them difficult to oxidize, and thus additional measures, limited MnO
suchasconventionalheating(70(cid:1)140(cid:1)C)37ormicrowave uptake of templated carbons that are strictly
2
irradiation,41 may be required to drive the permangana- mesoporous.34 Oxide loadings as high as 83 wt % were
te(cid:1)carbon reaction. Alternatively, CNTs and graphene achieved,butattheexpenseofcloggingthemesoporechan-
may be first preoxidized with strong acid to make nels,whichinturn,inhibitedelectrolyteinfiltration,andresulted
them more amenable to a subsequent permanganate(cid:1) inlowertotalspecificcapacitance(166 Fg(cid:1)1with80%with80
carbon reaction step to generate the desired MnO %activeMnO (cid:1)carbonmaterial)thanobservedatmuchlower
2 2
coating.35,38(cid:1)40,42 MnO loadingswiththemesoporouscarbonsubstrates.
2
Vol.46,No.5’2013’1062–1074’ACCOUNTSOFCHEMICALRESEARCH’1065
RedoxDepositionofNanoscaleMetalOxides Sassinetal.
FIGURE6. Micrographsof(a,b)acid-depositedand(c,d)neutral-depositedMnO (cid:1)carbonnanofoams.Adaptedwithpermissionfromref22.
2
Copyright2007AmericanChemicalSociety.
Yanetal.achievedsimilarlyhighMnO weightloadings followingcomplications:(i)activeMnO sitesmaybecome
2 2
(78 wt %) on graphene, but with better electrochemical obscured when mixed with binder and other additives;
utilization,asevidencedbyaspecificcapacitanceof310Fg(cid:1)1 (ii) long-range electron-conducting pathways (the carbon
(normalized to MnO (cid:1)graphene). The enhanced capa- component) may be disrupted in the ad hoc mixture of
2
citanceongraphenesubstratesmayarisefromtheirhigher powderedcomponents;and(iii)poorlyinterconnectedvoid
electronicconductivity,whichalsosupportsperformanceat volume may frustrate electrolyte infiltration. Amore desir-
high charge(cid:1)discharge rates.41 The effect of the carbon able design would begin with a macroscale, preformed
substratewasalsoexploredbyChuetal.,whodemonstrated carbon scaffold onto which nanoscale coatings of MnO
2
higherspecificcapacitance(208Fg(cid:1)1)andlowerelectrode are deposited (Figure 8, bottom). The proposed electrode
resistance (19 Ω) when MnO was deposited onto CNTs designwouldbedevice-ready,providecontroloverpore(cid:1)
2
versusanABsubstratewithsimilarMnO loading(63wt% solidarchitectureviathechoiceofcarbonsubstrate,main-
2
versus69wt%fortheCNT),butwhichprovidedonly158Fg(cid:1)1 tain uninterrupted electron-conducting pathways through-
andhigherresistance(28Ω).37 outthemacroscalecarbonscaffold,andestablishadesired
TheearliestexamplesofMnO -basedelectrodesforECs frequencyresponse.
2
were fabricated by mixing MnO powders with carbon Inonesuchexample,CNTsarefirstappliedtothefibers
2
powders and polymer binder to form a composite elec- comprising a carbon fiber paper (CFP) to create a free-
trode,16which,althoughsimpleandscalable,hindershigh- standing architecture with micrometer-sized voids; subse-
rateperformanceduetolimitedelectroniccontactbetween quent reaction with permanganate deposits nanoscale
conductive carbon and micrometer-sized particles of the MnO attheCNTs.39Thefree-standingMnO (cid:1)CNT(cid:1)CFPelec-
2 2
poorly conductive MnO (Figure 8, top). The permangana- trodeexhibited322Fg (cid:1)1in0.65MK SO at1mVs(cid:1)1
2 (MnO) 2 4
2
te(cid:1)carbonreactionpromotesintimatecontactbetweenthe thatdecreasedto219Fg (cid:1)1at5mVs(cid:1)1.Thevoltam-
(MnO)
2
MnO andcarboncomponentsthatimproveselectrochemi- mograms were rectangular in shape and did not exhibit
2
calperformance,butthefullbenefitsofsuchMnO (cid:1)carbon significant resistance even at 200 mV s(cid:1)1, which is not
2
materialsarenotrealizedwhenprocessedintoconventional surprising considering the large open void network of the
powder-compositeelectrodes(Figure8,middle),duetothe CFPandtherelativelylowloadingofMnO (0.15mgcm(cid:1)2
2
1066’ACCOUNTSOFCHEMICALRESEARCH’1062–1074’2013’Vol.46,No.5
RedoxDepositionofNanoscaleMetalOxides Sassinetal.
FIGURE7. (a)Nyquistand(b)Bodeplotsofbarecarbon(blue),acid-
deposited(red),andneutraldepositedMnO (cid:1)carbon(black)at0.2V
2
versusSCEin1MNa SO .Adaptedwithpermissionfromref22.
2 4
Copyright2007AmericanChemicalSociety.
geometricarea).Althoughtheratecapabilitiesofthiscom-
posite are impressive, the low MnO loading and high FIGURE8. Schematicofelectrolyte-infiltratedelectrodestructuresincorpor-
2
atingelectronpaths,active,andinactivematerials,usingcomposites,versusa
fractionofvoidvolume(inthelargespacesbetweencarbon
nanoarchitectureincorporatingelectronpathswiredtotheactivematerial.
fibers)limitsthemass-andvolume-normalizedcapacitance
thatcanbeachieved.39 MnO -normalized capacitance of 350 F g(cid:1)1. While the
2
The high fraction of void volume in CFP can be more increaseinspecificcapacitanceispromising,evengreater
optimally used by filling with a sol(cid:1)gel-derived polymer enhancements in volumetric and geometric capacitance
precursor(resorcinol(cid:1)formaldehyde,RF)followedbycarbo- (factorof10)areachievedbecausetheconformal,nano-
nization to yield a device-ready carbon nanofoam paper scale MnO coating does not increase the electrode
2
that exhibits high specific surface area (200(cid:1)600 m2 g(cid:1)1), volumeorfootprint.
electronic conductivity (10(cid:1)200 S cm(cid:1)1), a through-con- The MnO weight loadings and corresponding electro-
2
nectednetworkofsize-tunablepores(10nmtoafewμm), chemicalperformancemetricscanbefurtherenhancedby
and scalability in both thickness (70(cid:1)300 μm) and geo- fine-tuning the pore structure and macroscale thickness of
metric footprint (∼100 cm2).44,45 The resulting pyrolytic thecarbonnanofoamelectrodestructure.45Byselectingan
carbonreadilyreactswithpermanganateatroomtempera- RFformulationthatyieldshigherspecificsurfacearea,one
ture to produce nanoscale conformal MnO coatings can increase the MnO loading and improve mass- and
2 2
throughout the thickness of the nanofoam.22,23 These volume-normalized capacitance (up to 200 F cm(cid:1)3). The
first examples, using a commercially available carbon geometric capacitance (electrode footprint normalized)
nanofoam paper, demonstrated a MnO weight uptake for MnO -modified nanofoams scales from 2.5 F cm(cid:1)2 for
2 2
of 37 wt % and a concomitant enhancement in total a70μmthicknanofoamto5.0and7.5Fcm(cid:1)2for120and
electrode capacitance from 53 to 140 F g(cid:1)1, with a 230μmthicknanofoams,respectively(Figure9),confirming
Vol.46,No.5’2013’1062–1074’ACCOUNTSOFCHEMICALRESEARCH’1067
RedoxDepositionofNanoscaleMetalOxides Sassinetal.
(20(cid:1)50 nm) and/or small macropores (50(cid:1)100 nm). This
trendreflectsthefactthatmorefacileinfusionoftheviscous
ferrate solution into the architecture occurs as pore size
increases.
When FeOx(cid:1)carbon nanofoams are electrochemically
cycledinmildaqueouselectrolytes,theyexhibitenhanced
charge storage via faradaic pseudocapacitance that is pro-
portionaltotheFeOxloading(Table2andFigure11).24The
FeOx-normalized capacitances of such electrode architec-
tures (>300 F g(FeOx)(cid:1)1) exceed those previously reported
(typically <120 F g(FeOx)(cid:1)1) for both thin-films and powder-
composites,49,50 demonstrating greater electrochemical
FIGURE9. Geometriccapacitanceversuspotentialof1-ply(red),2-ply utilization when theFeOx isintimately associated with a
(blue),and3-ply(green)MnO (cid:1)carbonnanofoamelectrodesin2.5M
Li SO at5mVs(cid:1)1.Adaptedw2ithpermissionfromref45.Copyright well-designed carbon substrate (Figure 10c,d). The FeOx
2 4
2011RoyalSocietyofChemistry. pseudocapacitance arises from reversibly toggling the
average Fe oxidation state between 3.0 and 2.7, as ver-
thatMnO notonlydepositsthroughoutthefullthicknessof ified by X-ray absorption spectroscopy analysis of the
2
thepaperbutiselectrochemicallyaccessible.45 FeOx(cid:1)carbonnanofoamunderelectrochemicalcontrol.24
IronOxide.Ferrate(FeO 2(cid:1))isacloserelativeofperman- TheelectrochemicalstabilityoftheFeOxcoatingishighly
4
ganateandhasbeenusedforsimilarapplications,suchas dependentonelectrolytepH;whencycledinunbuffered
wastewatertreatment.46Maoetal.demonstratedthatreac- aqueous Li SO (pH ∼ 3.5) the pseudocapacitance fades
2 4
tion with ferrate introduces oxygen functionalities on the into the double-layer capacitance background over the
surfacesofgraphite,whichenhancesreversiblecapacityand first 100 cycles, whereas stable cycling (tested to 1000
cyclingstabilitywhenusedasanegativeelectrodeforLi-ion cycles)isobservedwhenaborate-basedbufferisaddedto
batteries,althoughtheresultingFeOxdepositwasremoved theelectrolytetobringthepHto∼8.5.24
priortouse.47Lessfrequentlystudiedthanothertransition Ruthenium Oxide. Ruthenium dioxide (RuO ) is the
2
metaloxides,FeOxcanalsoserveasanactivematerialfor performancechampionamongthepseudocapacitivemetal
electrochemical chargestorage,forexampleasanegative oxides used for ECs. The specific capacitance of RuO is
2
electrodematerialforaqueousasymmetricECs.48Wewere structure-dependent and maximizes in hydrous forms
the first to report that reaction of ferrate with carbon sub- (RuO 3xH O) that possess a nanocomposite structure of
2 2
strates (e.g., carbon nanofoam papers) generates electro- electronically conductive rutile networks interpenetrated
chemicallyactiveFeOxcoatings.24 withinadisordered,proton-insertinghydrousphase.51Un-
Although ferrate(cid:1)carbon and permanganate(cid:1)carbon like MnO and FeOx, RuO is an excellent electronic con-
2 2
reactions are similar in mechanism, ferrate-based deposi- ductor and even disordered RuO 3xH O has sufficient
2 2
tions are more challenging because ferrate solutions are electronicconductivity(∼1Scm(cid:1)1)tosupporthighpseudo-
onlystableinanarrowalkaline-pHrange(∼7(cid:1)10MKOH), capacitance(upto∼720Fg(cid:1)1forRuO 30.5H O)withoutthe
2 2
havelimitedshelflife(hourstodays),andtheirhighviscosity needforconductivecarbon.8Forpracticalapplications,the
impedes infiltration into the void volume of the carbon highcostofRuO canbemitigatedbydispersingtheoxide
2
substrate. With appropriate deposition conditions (e.g., onhigh-surface-areacarbons(viasol(cid:1)gel,impregnation,
using 25 mM K FeO in 9 M KOH), self-limiting, nanoscale CVD, or electrodeposition methods), a strategy that can
2 4
FeOxcoatingsaregeneratedattheinnerandoutersurfaces enhance electrochemical utilization.18,52(cid:1)54 Redox deposi-
of carbon nanofoam papers while retaining the through- tion is another viable approach to generate RuO coat-
2
connectedporestructureofthenativenanofoam(Figures10 ings on carbon, as first demonstrated by Pickup and co-
and11).TheweightloadingofFeOxisprimarilycontrolled workers, who used perruthenate (RuO (cid:1)) and ruthe-
4
by the pore structure of the native carbon nanofoam nate(RuO 2(cid:1))aqueousprecursorstomodifycarbonfiber
4
(Table2andFigure11),wheresmallermesopores(<20nm) mats and CNTs.55,56 When cycled in 1 M H SO , the
2 4
limit FeOx incorporation despite higher specific sur- RuO (cid:1)CNTcompositesyieldamaximumelectrode-mass
2
face area compared to nanofoams with larger mesopores normalized specific capacitance of 231 F g(cid:1)1 at 30 wt %
1068’ACCOUNTSOFCHEMICALRESEARCH’1062–1074’2013’Vol.46,No.5
RedoxDepositionofNanoscaleMetalOxides Sassinetal.
FIGURE10. (a)Photograph,(b)scanningelectronmicrograph,and(c,d)transmissionelectronmicrographsofanFeOx(cid:1)carbonnanofoam;
(d)adaptedfromref24.Copyright2010AmericanChemicalSociety.
FIGURE11. (Left)Micrographsofnativecarbonnanofoams(toprow)andthecorrespondingFeOx(cid:1)carbonnanofoams(middlerow)withFe
elementalmaps(bottomrow).(Right)SpecificcapacitanceversuspotentialforFeOxcoatingson50/500(black),50/1500(blue),and40/1500(red)
carbonnanofoamsin2.5MLi SO at5mVs(cid:1)1.
2 4
TABLE2. SummaryofPhysicochemicalandElectrochemicalPropertiesofNativeandFeOx-CoatedCarbonNanofoams
BETsurface meanpore FeOxwtloading specificcapacitance geometriccapacitance volumetric
carbonnanofoama area(m2g(cid:1)1)b size(nm)b (%)c (Fg1(cid:1))d (Fcm(cid:1)2)d capacitance(Fcm(cid:1)3)d
50/500 533 9 25 54 0.28 40
50/1500 486 20 38 75 0.71 71
40/1500 332 50 38 77 0.61 61
aNanofoam designations taken from the polymer nanofoam precursor formulation: e.g., “50/500” denotes a carbon nanofoam derived from 50 wt % total
resorcinol(cid:1)formaldehyde concentration and a resorcinol-to-catalyst (Na CO ) molar ratio of 500:1. bDerived from N -sorption porosimetry for native carbon
nanofoam(priortoFeOxdeposition).cDeterminedgravimetrically.dDerive2dfro3mcyclicvoltammetricmeasurementsat52mVs(cid:1)1in2.5MLi SO .
2 4
RuO , with up to 704 F g (cid:1)1 but only at lower oxide synthesized by oxidation of lower-valent ruthenium com-
2 (RuO )
2
loadings(25wt%). pounds. Ruthenium oxide deposition onto carbon sub-
Ruthenium tetroxide (RuO ) is a related RuO precursor strates can be performed from either aqueous or non-
4 2
that is better known as a staining agent for organic and aqueous RuO solutions, but we find that the best results
4
biological samples in electron microscopy.57 This precur- areachievedwhenusingnonaqueoussolvents.Inthelatter
sor is commercially available as a dilute aqueous solution case, aqueous RuO is first extracted into a hydrocarbon
4
(0.1wt%)witharelativelylongshelflifeandcanbereadily solventthatiscooledtodryice(cid:1)acetonebathtemperatures
Vol.46,No.5’2013’1062–1074’ACCOUNTSOFCHEMICALRESEARCH’1069
RedoxDepositionofNanoscaleMetalOxides Sassinetal.
FIGURE12. Scanningelectronmicrographsofa50/1500nanofoam:(a)beforemodification,(b)afterRuO depositionoutofRuO (cid:1)petroleumether
2 4
with(c)correspondingRuelementalmap;(d)RuO crustthatformedatsurfaceofcarbonnanofoampaperoutofaqueousRuO .
2 4
((cid:1)78 (cid:1)C)58(cid:1)60 prior to introducing the desired carbon sub- and140Fg(cid:1)1for29wt%and16wt%RuO (cid:1)nanofoam
2
strate;RuO formsspontaneouslyatthecarbonsurfacesas composites,respectively.Theoxide-normalizedcapacitances
2
thesolutionwarmstoroomtemperature(Figure12a(cid:1)c). (860 and 940 F g (cid:1)1; Figure 13b) exceed that previ-
(RuO )
2
The weight loading of RuO depends not only on the ouslyreportedforunsupportedRuO 30.5H O(720Fg(cid:1)1).8
2 2 2
RuO concentration and deposition time, but also on the Althoughderivedfromanominallyanhydroussynthesis,
4
pore structure of the nanofoam; for example, a predomi- these RuO coatings exhibit pseudocapacitance on par
2
nantlymesoporousnanofoampaper(designated50/1500) withthebesthydrousrutheniumoxide,aswaspreviously
withspecificsurfaceareaof∼450m2g(cid:1)1supports29wt% noted when using the same deposition protocol to coat
RuO , while a macroporous nanofoam paper (designated silica fiber-paper substrates.59,60 The amplification of
2
40/1500) of lower surface area (350 m2 g(cid:1)1) incorporates electrode capacitance is lower when RuO is expressed
2
16 wt % RuO . Alternatively, RuO deposition can be per- as a >1 μm thick crust on the nanofoam paper: capac-
2 2
formed directly from aqueous RuO , but the deposition is itance decreases to 400 F g (cid:1)1 (corresponding to
4 (RuO )
2
lesswell-controlled.AlthoughsimilarRuO weightloadings 120Fg(cid:1)1totalelectrode),indicatingpoorerelectrochemi-
2
are achieved (30 wt %), the oxide is localized as a micro- calutilization(Figure13b).
meter-thick crust at the outer boundary of the nanofoam
(cid:1)
paper (Figure 12d). The more uniform RuO deposition Applying Metal Oxide Carbon Electrode
2
Architectures to High-Performance Electro-
observedusinganonaqueoussolutionisassistedbyinfiltra-
chemical Capacitors
tion into the pore structure of the hydrophobic carbon
nanofoam. The metal oxide(cid:1)carbon electrode materials described
The pseudocapacitance of the RuO coating provides a above express pseudocapacitance-enhanced charge-
2
>4-foldincreaseinspecificcapacitancerelativetothenative storage functionality in nanostructured forms that, when
nanofoam whencycledinanacidicelectrolyte(Figure13). used in asymmetric EC devices, yield attractive combina-
The enhancement in capacitance tracks the RuO loading, tionsofenergyandpowerinconjunctionwiththecostand
2
with total electrode-mass-normalized capacitances of 250 safetyadvantagesofaqueouselectrolytes.9,12Asymmetric
1070’ACCOUNTSOFCHEMICALRESEARCH’1062–1074’2013’Vol.46,No.5