3D-Addressable Redox: Modifying Porous Carbon Electrodes with Ferrocenated 2 nm Gold Nanoparticles

3D-Addressable Redox: Modifying Porous Carbon Electrodes withFerrocenated 2 nm Gold NanoparticlesKwok-Fan Chow,† Rajesh Sardar,†,∥ Megan B. Sassin,§ Jean Marie Wallace,⊥ Stephen W. Feldberg,‡

Debra R. Rolison,§ Jeffrey W. Long,§ and Royce W. Murray*,†

†Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599, United States‡Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States§Code 6170 Surface Chemistry Branch, U.S. Naval Research Laboratory, Washington, D.C. 20375, United States⊥Nova Research, Inc., Alexandria, Virginia 22308, United States

*S Supporting Information

ABSTRACT: Nanostructured, high-surface-area carbon elec-trodes have large electrochemical double-layer capacitancescompared to smooth-surfaced electrodes because of theirenhanced internal surface areas, e.g., several hundred m2g−1.In the present work, we demonstrate that the electricalcapacitance of carbon “nanofoams”, both in commerciallyavailable forms and as prepared by the authors, can besignificantly enhanced by the insertion into their pores of smallAu nanoparticles (∼2 nm diameter core) to whose surfaces arebonded ferrocenyl-hexane thiolate ligands (SC6Fc) (>40 pernanoparticle). The enhanced capacitive behavior of the modified nanoporous carbon (in CH3CN or CH2Cl2 with 1.0 or 2.0 MBu4NPF6 as the supporting electrolyte) is clearly seen in their cyclic voltammetric responses and is attributed to a combination ofthe ferrocene redox-capacity and the double-layer capacity of the intercalated nanoparticles. Footprint-normalized, volume-normalized, and gravimetric-normalized integral capacitances of 0.28 F cm−2, 39 F cm−3, and 66 F g−1 are realized over a 1 Vpotential range. We suggest this approach as a conceptual pathway to improve the science of electrochemically based energystorage systems (e.g., “supercapacitors”).

■ INTRODUCTIONIncorporating redox-active functionality into nanostructuredcarbon electrodes is being pursued with the goal of enhancingcharge-storage or electrocatalytic activity.1−4 Among the manyavailable forms of porous, nanostructured carbons, aerogels5 and“nanofoams”6 are particularly attractive substrates because of theirthrough-connected, size-tunable porous structures and the abilityto synthesize these “all nano” objects in device-ready form factors.Carbon aerogels and nanofoams have been modified with electro-active polymers and adsorbed redox couples,7,8 charge-storingtransition metal oxides,9−13 and electrocatalytic metal nano-particles (e.g., Pt, Pd),14,15 tactics that enhance electrochemicalperformance in such applications as electrochemical capacitors,batteries, and fuel cells.2 This report describes a further approachto enhanced electrochemical capacity by the modification ofcarbon nanofoam interior surfaces with metal nanoparticlesbearing multiple redox moieties per nanoparticle. In the resultantstructure, the carbon nanofoam serves as a porous three-dimen-sional (3D) current collector for adsorbed ferrocenyl-coated Aunanoparticles, in a 3-dimensional flow field of electrolyte,providing enhanced charge-storage capacity to the nanofoam viaa combination of the multielectron nanoparticle Fc1+/0 redoxreactions and the collective double-layer capacitances of thenanoparticle surfaces.

The nanoparticles used in this study have 2 nm (dia.) Aucores that are coated with ferrocenyl-hexanethiolate (SC6Fc)redox ligands with an average composition of Au225(SC6Fc)43.Each nanoparticle is thus capable of faradaically releasing ca. 43electrons over the potential range of the Fc+/0 redox reaction.16

The ferrocenated Au nanoparticles are strongly adsorbed to Ptand Au electrode surfaces, to the extent that stable ferrocenevoltammetry of nanoparticle monolayers can be observed evenin nanoparticle-free electrolyte.17 When adsorbed within thecarbon nanofoam, it appears that the nanoparticles serve to alsoenhance the overall nanofoam capacitance even at potentialsremote from the ferrocene wave itself, an effect akin to a“roughening” of the internal nanoporous surfaces.The nanoparticle-modified nanofoam electrode architectures

described here represent a “modular” design approach, offeringthe advantages of allowing separate design and evaluation of theultraporous carbon and of the redox nanoparticle ingredients.Cyclic voltammetry in CH2Cl2 or CH3CN containing either1.0 or 2.0 M Bu4NPF6 electrolyte shows that the ferrocene sitesare electrically addressable through the nanofoam structure.

Received: December 28, 2011Revised: March 27, 2012Published: April 12, 2012

Article

pubs.acs.org/JPCC

© 2012 American Chemical Society 9283 dx.doi.org/10.1021/jp212537q | J. Phys. Chem. C 2012, 116, 9283−9289

pubs.acs.org/JPCC

The voltammetry is compared to simulations. Imaging withtransmission electron (TEM) and scanning electron (SEM)

microscopies shows that the Au225(SC6Fc)43 nanoparticles aredistributed throughout the full volume of the nanofoamstructure, albeit with some heterogeneity. Elemental analysiswith inductively coupled plasma-mass spectrometry (ICP-MS)indicates that the Fe content of the loaded nanofoam rangesfrom 0.5 to 0.7 wt %.

■ EXPERIMENTAL SECTION

Electrodes and Procedures. The carbon nanofoamelectrodes used in the initial phase of this study were preparedby pyrolysis of ca. 0.5 × 0.5 cm squares of polymer nanofoampaper (Marketech International) under Ar in a 1000 °C tubefurnace, as outlined in the Supporting Information. Electricalcontact to these nanofoam electrodes was made by either aninsulated wire attached to the corner of the square electrode(Type A electrodes) or by a Ni foil sealed onto one side of thesquare, called Type B electrodes. The voltammetry (shownlater) in Figures 2 and 3 is based on these nanofoam materials.Later in the study, the electrode materials were purchased asprepyrolyzed carbon nanofoams (MarkeTech, Port Townsend,WA, grade #2). These were mounted by sealing to a Ni foilon one side and except for a circular portion of the nano-foam electrode exposed to the solution on the other side,

Figure 1. Cyclic voltammetry (100 mV s−1 in CH3CN/2 M Bu4NPF6electrolyte) of Au225Fc43 adsorbed on a polished 3 mm (dia.) glassycarbon electrode surface, by 2 h immersion in THF/0.1 M Bu4NPF6containing 0.05 mM nanoparticles. The charges under from the anodicand cathodic peaks are 5.17 and 5.08 μC, respectively, correspondingto a ferrocene surface coverage of 1.7 × 10−11 mol cm−2.

Figure 2. Panel A. Experimental cyclic voltammetric responses for a 0.133 cm2 (area exposed on each side) Type A carbon nanofoam electrodecontaining Au225(SC6Fc)43 nanoparticles, in 1 M Bu4NPF6/CH2Cl2 at T = 298 K at potential scan rates of 0.002, 0.005, 0.008, 0.010, and 0.012 V s−1

(lowest to highest curves). Panel B. Black curve: experimental CV at 0.002 V s−1. Red curve: CV simulated (see Supporting Information) usingΓtotal = 7.0 × 10−7 mol cm−2, Ru = 175 Ω, γ = 1.8, n = 1, E0 = 0.406 V, Cdl = 0.20 F cm−2, b = 7.0 × 10−4 AV−1 cm−2, b0 =1.0 × 10−4 A cm−2. Panel C.Black curve: experimental CV at 0.005 V s−1. Red curve: simulated CV parameters as in Panel B except Cdl = 0.16 F cm−2, b = 0.001 AV−1 cm−2, b0 =5.0 × 10−5 A cm−2. Panel D. Black curve: experimental CV at 0.012 V s−1. Red curve: simulated CV parameters as in Panel B except Cdl = 0.16 Fcm−2, b = 0.001 AV−1 cm−2, b0 = 5.0 × 10−5 A cm−2.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp212537q | J. Phys. Chem. C 2012, 116, 9283−92899284

encapsulating everything in epoxy (Type C electrodes). Thevoltammetry (shown later) in Figure 4 is based on these nano-foam materials. Carbon-fiber paper supported carbon nano-foams were also prepared as previously described;6 details arefound in the Supporting Information. Their voltammetry, uponincorporation of the ferrocenated Au nanoparticles, is shownlater in Figure 8.The geometrical areas of the nanofoam electrodes exposed to

the solution were Type A, 0.266 cm2 (0.133 cm2 exposed oneach side to the solution), Type B, 0.133 cm2 (one side onlyexposed), and Type C, 0.38 cm2 (0.19 cm2 exposed on eachside). The thickness, mass, and volume of the Type A, B, and Celectrodes were 0.017 cm/1.7 × 10−3 g/4.5 × 10−3 cm3; 0.017cm/1.7 × 10−3 g/4.5 × 10−3 cm3; and 0.017 cm/1.6 × 10−3 g/2.7 × 10−3 cm3, respectively. Further details are provided in theSupporting Information and Figure S-1.The thiolated alkylferrocene, Fc(CH2)6SH, and Au nano-

particles capped with it as ligands were synthesized as pre-viously described16 (see also Supporting Information). Theaverage nanoparticle formula is Au225(SC6Fc)43 as determinedby a combination of transmission electron microscopy (TEM),coulometry, and quantized double-layer charging characteristics.16

Type A nanofoam electrodes were loaded with Au225(SC6Fc)43

nanoparticles by soaking (24 h) in 0.05 mM CH2Cl2 solutions,followed by washing with CH2Cl2. Types B and C nanofoamelectrodes were loaded similarly except the 0.05 mM nanoparticlesolutions were in 0.1 M Bu4NPF6/THF, and the soaking timein an evacuated glass desiccator to encourage pore fillingwas2 h. The electrodes were thoroughly washed with 0.1 M Bu4NPF6/THF and then soaked in nanoparticle-free 0.1 M Bu4NPF6/CH3CN for 1 h to remove the more resistive THF solventcomponent.Cyclic voltammetry of nanoparticle-loaded Type A electro-

des was performed in 1 M Bu4NPF6/CH2Cl2 electrolytesolutions. Voltammetry of Type B and C nanofoam electrodeswas done in 1 and 2 M Bu4NPF6/CH3CN, respectively.The reference and counter electrodes of the three-electrodecell were Ag/AgCl/3 M KCl (aq) and Pt, respectively.Measurements were performed using a CH Instruments(Austin, TX) model 760C electrochemical analyzer and aPine Instruments (Durham, NC) WaveNow potentiostat.Potential scans were done over a range of −0.1 to +1.0 V vsAg/AgCl, in which the background currents appear to bemainly nonfaradaic. See Supporting Information for othermeasurement details.

Figure 3. Panel A. Experimental cyclic voltammetric responses for a 0.133 cm2 type B carbon nanofoam electrode containing Au225(SC6Fc)43nanoparticles, in 1 M Bu4NPF6/CH3CN at T = 298 K at potential scan rates of 0.002, 0.004, 0.006, 0.008, 0.010, and 0.012 V s−1 (lowest to highestcurves). Panel B. Black curve: experimental CV at 0.002 V s−1. Red curve: CV simulated (see Supporting Information) based on Γtotal = 1.12 × 10−6

mol cm−2, Ru = 49 Ω, γ = 1.6, n = 1, E0 = 0.419 V, Cdl = 0.20 F cm−2, b = 5.0 × 10−5 AV−1 cm−2, b0 = 2.5 × 10−4 A cm−2. Panel C. Black curve:experimental CV at 0.006 V s−1. Red curve: simulated CV parameters as in Panel B except Cdl = 0.20 F cm−2, b = 5.0 × 10−5AV−1 cm−2, b0 = 2.5 ×10−4 A cm−2. Panel D. Black curve: experimental CV at 0.015 V s−1. Red curve: simulated CV parameters as in Panel B except Cdl = 0.20 F cm−2,b = 5.0 × 10−5 AV−1 cm−2, b0 = 2.5 × 10−4 A cm−2.



■ RESULTS AND DISCUSSION

In previous reports,17 we have described the formation, from non-aqueous media, of highly persistent adsorbed monolayers andmultilayers of ferrocene-labeled, small Au nanoparticles on Au andPt electrodes. The degree of adsorption, monitored from thecoulometric charge under the multiferrocene redox wave, alsodepends on the electrolyte anion and is enhanced by electro-generated cationic sites in the Au nanoparticle capping layer. On Auelectrodes, the adsorption is also enhanced by the presence of self-assembled monolayers terminated by anionic groups such assulfonate or carboxylate. We have proposed17 that, in these systems,adsorption is based on forming multiple ion-pair bridges betweencationic sites at the surface of the nanoparticle and electrolyte anionsadsorbed at the electrode, as illustrated in Figure S-2 (SupportingInformation). The mechanism proposes that the tenaciousadsorption is an entropic consequence of multiple interactionsbetween nanoparticles and between nanoparticles and the electrode.To probe the interaction of ferrocenated Au nanoparticles

within a 3D porous carbon nanofoam, we first established thatnanoparticle adsorption also occurs on glassy carbon electrodes,again from nonaqueous medium, as illustrated by the Figure 1cyclic voltammogram of adsorbed Au225(SC6Fc)43 nano-particles. Glassy carbon is a reasonable planar analog of the3D carbon nanofoam substrate, and it is thus expected that thenanoparticles also readily adsorb onto interior surfaces of thehighly porous carbon nanofoams. Using nanoparticles withaverage composition Au225(SC6Fc)43, carbon nanofoam elec-trodes were soaked in nanoparticle solutions (see ExperimentalSection for details) and subsequently analyzed by cyclic voltam-metry as shown in Figures 2−4 (Panels A). The voltammetryshows a large Fc+/0 wave riding atop the substantial double-layer capacitance background of the carbon nanofoam itself.

The nanoparticle/nanofoam voltammetry is fairly stablerepetitive potential cycling for 6 h (equivalent to ca. 3 h in theferrocenium state) produced <5% decline in the total integratedcharge. The integrated charges under the ferrocene waves areequal for the anodic and cathodic peaks and are alsoindependent of the potential scan rate over a range of 2−12 mV s−1 (Table 1). The latter observation indicates that

transport of electronic charge through the conductive carbonnanofoam is facile and also that the ingress/egress of thecharge-compensating counterions through the pore network ofthe nanofoam is adequately fast, at least on the time scale of thisstudy in which a 1 V window is traversed in 83−500 s.Panels A in Figures 2−4 are the experimental voltammo-

grams obtained using the Type A−C nanofoam electrodemountings. All voltammograms show prominent separationsbetween the ferrocene oxidation and reduction current peaks,which increase with increasing potential scan rate. This effectcould be due to one or more of the following conditions: (i)uncompensated resistance in the contacting bulk electrolyte

Figure 4. Panel A. Experimental cyclic voltammetric responses for a 0.19 cm2 (area exposed on each side) Type C carbon nanofoam electrodecontaining Au225(SC6Fc)43 nanoparticles in 2 M Bu4NPF6/CH3CN at T = 298 K at potential scan rates of 0.002, 0.004, 0.006, 0.008, 0.010, and0.012 V s−1 (lowest to highest curves). Panel B. Black curve: experimental CV at 0.002 V s−1. Red curve: CV simulated (see Supporting Information)based on Γtotal = 1.0 × 10−6 mol cm−2, Ru = 20 Ω, γ = 1.4, n = 1, E0 = 0.392 V, Cdl = 0.55 F cm−2, b = 3.0 × 10−3 AV−1 cm−2, b0 = −2.0 × 10−4 Acm−2. Panel C. Black curve: experimental CV at 0.006 V s−1. Red curve: simulated CV parameters as in Panel B except Cdl = 0.35 F cm−2, b = 5.0 ×10−3 AV−1 cm−2, b0 = −2.0 × 10−4 A cm−2. Panel D. Black curve: experimental CV at 0.012 V s−1. Red curve: simulated CV parameters as in Panel Bexcept Cdl = 0.38 F cm−2, b = 7.0 × 10−3 AV−1 cm−2, b0 = 5.0 × 10−4 A cm−2.

Table 1. Coulometric Charges under Ferrocene CyclicVoltammetry Peak, As a Function of Potential Scan Rate(Type C Electrode) in 2 M Bu4NPF6/CH3CN

potential scan rate,mV/s

anodic peak charge,mC

cathodic peak charge,mC

QA/QC

12 18.9 19.2 1.0110 19.0 19.4 0.978 19.8 19.6 0.986 19.8 19.7 0.994 19.3 19.4 0.982 19.9 19.4 0.98



solution; (ii) contact resistance of the connection to thenanofoam; (iii) charge-transfer resistance in the ferrocene redoxreactions of the ferrocenated Au nanoparticles; and/or (iv)electronic resistance of the carbon nanofoam structure. Thenanofoam material has very low resistance, whether derivedfrom the commercial source (<1 Ω cm according to themanufacturer) or in-house (0.05 Ω cm),6 so the lastexplanation was discarded. Simulations were conducted of thevoltammetry at several potential scan rates (Panels B−D ofFigures 2−4). The simulations assume that all electron-transferprocesses are fast and that all of the uncompensated resistanceoccurs in the electrolyte solution exterior to the nanofoam or atthe electrode contacts. (See the discussion of simulations in theSupporting Information.) These assumptions yielded goodmatches of simulated responses to the experimental responses.We conclude that the peak-potential separations are strictlyohmic phenomena, i.e., (i) and (ii). This conclusion isconsistent with the known fast kinetics of the Fc+/0 coupleand with the order of uncompensated resistances (175, 49, and20 Ω), which is qualitatively expected18 for the particularsolvents and electrolyte concentrations in Figures 2−4 (1 Melectrolyte in CH2Cl2, 1 M electrolyte in CH3CN, and 2 Melectrolyte in CH3CN, respectively).Simulating the cyclic voltammetry of Figures 2A−4A was

complicated by the nonideal waveshapes of the Fc+/0 peaks.Ideally, electrochemically reversible one-electron electrodereactions of immobilized multisite redox species (e.g., redoxpolymers),19 when the multiple redox sites do not interact withone another, have the same peak shapes as those of surface-attached monomer redox species, where the full width at half-maximum (Efwhm) value is 90.6 mV. The ferrocene voltammo-grams in Figures 2−4 for nanofoams loaded with Au225-(SC6Fc)43 nanoparticles show much smaller Efwhm values (asdoes the voltammogram in Figure 1) than characteristic ofnoninteracting redox centers. The narrow FWHMs persist evenat a very slow potential scan rate. This effect is also seen foradsorbed nanoparticle voltammetry on flat electrode surfaces,as in Figure 1 and earlier work.17 Simulating this wave-narrowingeffect was a necessary complication in treating the voltammetryof Figures 2−4.Many factors can introduce peak narrowing (or broadening)

nonidealities in the voltammetry of surface-confined species.We use here a recently described straightforward modelingapproach.20 Discussion of the simulation scheme andcalculation details is found in the Supporting Information.With no accounting for intra-nanofoam resistance or effects ofelectrode kinetics, the resulting simulations successfully matchthe experimental responses as seen by the experimental−simulation overlays in Figures 2−4, Panels B−D. We concludethat the peak-potential separations are strictly uncompensatedresistance phenomena occurring in the external electrolytesolution. The fitted uncompensated resistance values (20−175 Ω, see Figures 2−4 legends) are typical of nonaqueouselectrolytes. The currents produced by the redox-modifiednanofoams are substantial, accentuating the large voltammetricpeak-potential splittings.A central point of interest for these modified nanofoams is

the overall charge-storing capacity of the nanoparticle/nano-foam electrode combination, data for which are presented inTable 2. Electrode mounting as Type A and B (seeExperimental Section) yields essentially identical normalizedmetrics over a 1 V window, whether the voltammetry ismeasured in methylene chloride or acetonitrile electrolyte, with

gravimetric capacitances on the order of 50 F g−1 relative to thenative carbon nanofoam at 30 F g−1. The sandwiched contactof Type C yields higher capacitance for the volume- and

Table 2. Summary of Electrochemical Capacitances fromCharge−Potential Integrations of Voltammograms LikeThose in Figures 2−4, Expressed for a 1 V PotentialExcursion

electrolyte/solvent(contact geometry)

geometric cap.(F cm−2)

volumetric cap.(F cm−3)

gravimetriccap.a (F g−1)

1 M Bu4PF6/CH2Cl2(Type A)

0.32 19 50

2 M Bu4PF6/CH3CN(Type B)

0.35 21 51

2 M Bu4PF6/CH3CN(Type C)

0.28 39 66

aBased on mass of carbon nanofoam. The value of 30 F g−1 forunmodified nanofoam cited by the manufacturer was experimentallyconfirmed.

Figure 5. Example of cyclic voltammetric comparison of “blank”carbon nanofoam (Type C electrode) with nanoparticle-loadednanofoam; note the substantial increase in double layer capacitanceat potentials more positive and negative than the ferrocene redox wave.2 M Bu4NPF6/CH3CN solution.

Figure 6. Scanning electron micrograph and energy-dispersive X-rayresults (boxed areas) for cross-section of Au225(SC6Fc)43-modifiedcarbon nanofoam (Type C electrode). The apparent Au/Fe weightratios in the EDX spectra for spectra 1−4, respectively, are 4.7, 4.7, 4.3,and 4.9. While the analytical results are uniform across the sample, theweight ratios display a 3.7-fold low bias. Scale bar is 100 μm.



mass-normalized numbers, which possibly reflects more of theelectrode structure being accessed during the voltammetricexperiment than that defined by the 6.7 mm diameter hole inthe Ni current collector.The voltammetry also exhibits a phenomenon whereby the

double-layer capacitance of the nanofoam carbon at potentialsremote from the ferrocene wave is nearly twice that of thenative nanofoam (see Figure 5). The origin of this effect is notyet adequately investigated, but one possible interpretation isthat it represents the additional contribution of double-layercapacitance from the nanoparticles summed over all of theadsorbed nanoparticles. In effect, the internal nanofoamsurfaces are “roughened” by the adsorbed, conductive nano-particles, resulting in greater electrochemical surface area. If thisexplanation is correct, then substantial gains in charge storagecould also be obtained by the use of conducting nanoparticlesmodified with ionically charged, but not necessarily redox-active, surface ligands.

The ICP-MS analysis for Fe content of a Au225(SC6Fc)43-modified nanofoam sample gave 0.53% Fe by weight. Theseresults are consistent with the ferrocene content of theAu225(SC6Fc)43-modified nanofoam as deduced from voltam-metry; the Table 1 electrochemical charge data for the fer-rocene reaction predict an Fe content of 0.69%. The ICP-MSanalysis for Au content of Au225(SC6Fc)43-modified nanofoamfailed, giving a 1.2 wt % result for Au which based on the Fecontent is lower than expected for Au by 8-fold. The energy-dispersive X-ray (EDX) analysis of the nanofoam revealed asubstantial Au content, and none for Fe; i.e., the acid digestionprocedure removed the Fe, but not the Au, from theAu225(SC6Fc)43-modified nanofoam sample. (As a check, theICP-MS analysis produced a satisfactory (within ca. 20%) Auweight % result for a Au nanoparticle of known composition,Au144(SCH2CH2)60.)The Au225(SC6Fc)43-modified nanofoam was cross-sectioned

by slicing and characterized by SEM and EDX (Figure 6). Themicrograph reveals the high-quality fill factor of the nanofoam

Figure 7. Microscopy of the pyrolyzed nanofoam carbon used in Type A and B electrodes. Panels A and B. Scanning electron micrographs ofnanofoam carbon before and after, respectively, loading with nanoparticles. Scale bars are 500 nm. Panel C. Transmission electron micrograph ofnanoparticle-loaded nanofoam carbon; scale bar 20 nm.

Figure 8. Upper Panels. Microscopic SEM images (scale bar 200 nm for both) of nanofoam materials prepared according to ref 5, at NRL andloaded with the ferrocenylated nanoparticles. Lower Panel. Cyclic voltammetry of nanoparticle-loaded nanofoam electrode (mounted as Type Belectrode) in 2 M Bu4NPF6/CH3CN at a 20 × 10−5 mV s−1 potential scan rate.



within the carbon fiber matrix with no voids sized at tens ofmicrometers, as previously reported for these materials.6,10

The EDX analyses, covering the areas boxed in red (Figure 6),reveal minimal compositional heterogeneity in the Au/Feweight ratio but a bias from the ratio expected given the knownthe Au225(SC6Fc)43 composition (details in figure legend).Figure 7 shows a higher-resolution SEM study of the exteriorsurfaces of a (locally pyrolyzed) nanofoam sample before(Panel A) and after (Panel B) incorporation of theAu225(SC6Fc)43 nanoparticles into the electrode. These imagesshow the submicrometer-scale porosity of the material and thatno obvious morphological changes occur upon modifying withnanoparticles. Panel C of Figure 7 shows yet higher-resolutionmicroscopy (transmission electron microscopy) of a nanofoamcontaining Au225(SC6Fc)43, in which numerous small spotsthe Au nanoparticlesare readily visible, including a numberof larger particles which are apparently multilayers of nano-particles or their aggregates. This micrograph also reveals thelocal heterogeneity of the nanoparticle-loaded carbon nano-foam electrodes.A comparison to the behavior of the commercial nanofoam

electrodes was also made to carbon nanofoams prepared by theauthors’ published procedure.6 This material has a tunablepore-diameter range of ∼40−100 nm and was contacted withnanoparticle solution as a Type B electrode as in theExperimental Section for the other nanofoams. The EDXanalysis of this modified nanofoam gave an apparent Au/Featom ratio of 3.0. Integration of the current−potential curve(Figure 8, lower left) gave a geometrical capacitance of 0.26F cm−2, which is similar to the data in Table 2 for the cornercontacts (Type A and B). Overall, this brief comparisonindicates that, with regard to incorporation of redox-coatednanoparticles, the locally made carbon nanofoam has generalproperties and potential that are similar to the commercialnanofoam materials.

■ CONCLUSIONS

We report here a proof-of-concept scheme for buildingfunctional complexity into nanoporous carbon electrodestructures. Incorporation of redox-coated nanoparticles into ahighly porous carbon nanofoam electrode material improveselectrochemical charge storage relative to the native nanofoam,from the Fc+/0 redox reaction and by an enhancement ofporous carbon capacitance at potentials remote from the fer-rocene redox wave (an effect attributed to the collective double-layer capacitance of the adsorbed nanoparticles). We obtain fullutilization of the ferrocene redox species around the adsorbedAu nanoparticles. Apparent rate-limiting effects, as evidencedby experimental peak potential splitting in cyclic voltammetry,arise from uncompensated solution resistance as verified bydigital simulation of the current−potential data.

■ ASSOCIATED CONTENT

*S Supporting InformationExperimental details and equipment, information on carbonnanofoam electrodes and CV simulations, and connections tonanofoam electrodes. This material is available free of chargevia the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONPresent Address∥Dept. Chem. Chem. Biol., Indiana Univ./Purdue Univ.,Indianapolis, IN 46202.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis research was supported in part by the Office of NavalResearch and the National Science Foundation. We acknowl-edge MS-ICP and EDX measurements performed by theAnalytical and Nanofabrication Laboratory of the UNCInstitute for Advanced Materials.

■ REFERENCES(1) Long, J. W.; Rolison, D. R. Acc. Chem. Res. 2007, 40, 854−862.(2) Rolison, D. R.; Long, J. W.; Lytle, J. C.; Fischer, A. E.; Rhodes, C. P.;Bourg, M. E.; Lubers, A. M. Chem. Soc. Rev. 2009, 38, 226−252.(3) Biener, J.; Stadermann, M.; Suss, M.; Worsley, M. A.; Biener, M. M.;Rose, K. A.; Baumann, T. F. Energy Environ. Sci. 2011, 4, 656−667.(4) Zhai, Y. P.; Dou, Y. Q.; Zhao, D. Y.; Fulvio, P. F.; Mayes, R. T.;Dai, S. Adv. Mater. 2011, 23, 4828−4850.(5) Pekala, R. W.; Farmer, J. C.; Alviso, C. T.; Tran, T. D.; Mayer, S. T.;Miller, J. M.; Dunn, B. J. Non-Cryst. Solids 1998, 225, 74−80.(6) Lytle, J. C.; Wallace, J. M.; Sassin, M. B.; Barrow, A. J.; Long,J. W.; Dysart, J. L.; Renninger, C. H.; Saunders, M. P.; Rolison, D. R.Energy Environ. Sci. 2011, 4, 1913−1925.(7) (a) Talbi, H.; Just, P. E.; Dao, L. H. J. Appl. Electrochem. 2003, 33,465−473. (b) Roldan, S.; Blanco, C.; Granda, M.; Menendez, R.;Santamaria, R. Angew. Chem., Int. Ed. 2011, 50, 1699−1701.(8) Long, J. W.; Dening, B. M.; McEvoy, T. M.; Rolison, D. R. J. Non-Cryst. Solids 2004, 350, 97−106.(9) Miller, J. M.; Dunn, B. Langmuir 1999, 15, 799−806.(10) Fischer, A. E.; Pettigrew, K. A.; Rolison, D. R.; Long, J. W. NanoLett. 2007, 7, 281−286.(11) Fischer, A. E.; Saunders, M. P.; Pettigrew, K. A.; Rolison, D. R.;Long, J. W. J. Electrochem. Soc. 2008, 155, A246−A252.(12) Long, J. W.; Sassin, M. B.; Fischer, A. E.; Rolison, D. R.;Mansour, A. N.; Johnson, V. S.; Stallworth, P. E.; Greenbaum, S. G.J. Phys. Chem. C 2009, 113, 17595−17598.(13) Sassin, M. B.; Mansour, A. N.; Pettigrew, K. A.; Rolison, D. R.;Long, J. W. ACS Nano 2010, 4, 4505−4514.(14) Baker, W. S.; Long, J. W.; Stroud, R. M.; Rolison, D. R. J. Non-Cryst. Solids 2004, 350, 80−87.(15) Marie, J.; Berthon-Fabry, S.; Achard, P.; Chatenet, M.;Pradourat, A.; Chainet, E. J. Non-Cryst. Solids 2004, 350, 88−96.(16) Wolfe, R. L.; Balasubramanian, R.; Tracy, J. B.; Murray, R. W.Langmuir 2007, 23, 2247−2254.(17) (a) Stiles, R. L.; Balasubramanian, R.; Feldberg, S. W.; Murray,R. W. J. Am. Chem. Soc. 2008, 130, 1856−1865. (b) Sardar, R.; Beasley,C. A.; Murray, R. W. Anal. Chem. 2009, 81, 6960−6965. (c) Beasley,C. A.; Sardar, R.; Barnes, N. M.; Murray, R. W. J. Phys. Chem. C 2010,114, 18384−18389.(18) Goldfarb, D. L.; Longinotti, M. P.; Corti, H. R. J. Solution Chem.2001, 30, 307−322.(19) Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F. C. J. Am. Chem.Soc. 1978, 100, 4248−4253.(20) Feldberg, S. W. Anal. Chem. 2011, 83, 5851−5856.



http://pubs.acs.org

Documents

3D-Addressable Redox: Modifying Porous Carbon Electrodes with Ferrocenated 2 nm Gold Nanoparticles