Article pubs.acs.org/JPCC
3D-Addressable Redox: Modifying Porous Carbon Electrodes with Ferrocenated 2 nm Gold Nanoparticles Kwok-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 electrodes have large electrochemical double-layer capacitances compared to smooth-surfaced electrodes because of their enhanced internal surface areas, e.g., several hundred m2g−1. In the present work, we demonstrate that the electrical capacitance of carbon “nanofoams”, both in commercially available forms and as prepared by the authors, can be significantly enhanced by the insertion into their pores of small Au nanoparticles (∼2 nm diameter core) to whose surfaces are bonded ferrocenyl-hexane thiolate ligands (SC6Fc) (>40 per nanoparticle). The enhanced capacitive behavior of the modified nanoporous carbon (in CH3CN or CH2Cl2 with 1.0 or 2.0 M Bu4NPF6 as the supporting electrolyte) is clearly seen in their cyclic voltammetric responses and is attributed to a combination of the ferrocene redox-capacity and the double-layer capacity of the intercalated nanoparticles. Footprint-normalized, volumenormalized, 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 V potential range. We suggest this approach as a conceptual pathway to improve the science of electrochemically based energy storage systems (e.g., “supercapacitors”).
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INTRODUCTION Incorporating redox-active functionality into nanostructured carbon electrodes is being pursued with the goal of enhancing charge-storage or electrocatalytic activity.1−4 Among the many available forms of porous, nanostructured carbons, aerogels5 and “nanofoams”6 are particularly attractive substrates because of their through-connected, size-tunable porous structures and the ability to synthesize these “all nano” objects in device-ready form factors. Carbon aerogels and nanofoams have been modified with electroactive polymers and adsorbed redox couples,7,8 charge-storing transition metal oxides,9−13 and electrocatalytic metal nanoparticles (e.g., Pt, Pd),14,15 tactics that enhance electrochemical performance in such applications as electrochemical capacitors, batteries, and fuel cells.2 This report describes a further approach to enhanced electrochemical capacity by the modification of carbon nanofoam interior surfaces with metal nanoparticles bearing multiple redox moieties per nanoparticle. In the resultant structure, the carbon nanofoam serves as a porous three-dimensional (3D) current collector for adsorbed ferrocenyl-coated Au nanoparticles, in a 3-dimensional flow field of electrolyte, providing enhanced charge-storage capacity to the nanofoam via a combination of the multielectron nanoparticle Fc1+/0 redox reactions and the collective double-layer capacitances of the nanoparticle surfaces. © 2012 American Chemical Society
The nanoparticles used in this study have 2 nm (dia.) Au cores 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. 43 electrons over the potential range of the Fc+/0 redox reaction.16 The ferrocenated Au nanoparticles are strongly adsorbed to Pt and Au electrode surfaces, to the extent that stable ferrocene voltammetry of nanoparticle monolayers can be observed even in nanoparticle-free electrolyte.17 When adsorbed within the carbon nanofoam, it appears that the nanoparticles serve to also enhance the overall nanofoam capacitance even at potentials remote 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, offering the advantages of allowing separate design and evaluation of the ultraporous carbon and of the redox nanoparticle ingredients. Cyclic voltammetry in CH2Cl2 or CH3CN containing either 1.0 or 2.0 M Bu4NPF6 electrolyte shows that the ferrocene sites are electrically addressable through the nanofoam structure. Received: December 28, 2011 Revised: March 27, 2012 Published: April 12, 2012 9283
dx.doi.org/10.1021/jp212537q | J. Phys. Chem. C 2012, 116, 9283−9289
The Journal of Physical Chemistry C
Article
microscopies shows that the Au225(SC6Fc)43 nanoparticles are distributed throughout the full volume of the nanofoam structure, albeit with some heterogeneity. Elemental analysis with inductively coupled plasma-mass spectrometry (ICP-MS) indicates that the Fe content of the loaded nanofoam ranges from 0.5 to 0.7 wt %.
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EXPERIMENTAL SECTION Electrodes and Procedures. The carbon nanofoam electrodes used in the initial phase of this study were prepared by pyrolysis of ca. 0.5 × 0.5 cm squares of polymer nanofoam paper (Marketech International) under Ar in a 1000 °C tube furnace, as outlined in the Supporting Information. Electrical contact to these nanofoam electrodes was made by either an insulated wire attached to the corner of the square electrode (Type A electrodes) or by a Ni foil sealed onto one side of the square, called Type B electrodes. The voltammetry (shown later) in Figures 2 and 3 is based on these nanofoam materials. Later in the study, the electrode materials were purchased as prepyrolyzed carbon nanofoams (MarkeTech, Port Townsend, WA, grade #2). These were mounted by sealing to a Ni foil on one side and except for a circular portion of the nanofoam electrode exposed to the solution on the other side,
Figure 1. Cyclic voltammetry (100 mV s−1 in CH3CN/2 M Bu4NPF6 electrolyte) of Au225Fc43 adsorbed on a polished 3 mm (dia.) glassy carbon electrode surface, by 2 h immersion in THF/0.1 M Bu4NPF6 containing 0.05 mM nanoparticles. The charges under from the anodic and cathodic peaks are 5.17 and 5.08 μC, respectively, corresponding to a ferrocene surface coverage of 1.7 × 10−11 mol cm−2.
The voltammetry is compared to simulations. Imaging with transmission electron (TEM) and scanning electron (SEM)
Figure 2. Panel A. Experimental cyclic voltammetric responses for a 0.133 cm2 (area exposed on each side) Type A carbon nanofoam electrode containing 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) using Γtotal = 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 F cm−2, b = 0.001 AV−1 cm−2, b0 = 5.0 × 10−5 A cm−2. 9284
dx.doi.org/10.1021/jp212537q | J. Phys. Chem. C 2012, 116, 9283−9289
The Journal of Physical Chemistry C
Article
Figure 3. Panel A. Experimental cyclic voltammetric responses for a 0.133 cm2 type B carbon nanofoam electrode containing Au225(SC6Fc)43 nanoparticles, 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 highest curves). Panel B. Black curve: experimental CV at 0.002 V s−1. Red curve: CV simulated (see Supporting Information) based on Γtotal = 1.12 × 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.
encapsulating everything in epoxy (Type C electrodes). The voltammetry (shown later) in Figure 4 is based on these nanofoam materials. Carbon-fiber paper supported carbon nanofoams were also prepared as previously described;6 details are found in the Supporting Information. Their voltammetry, upon incorporation of the ferrocenated Au nanoparticles, is shown later in Figure 8. The geometrical areas of the nanofoam electrodes exposed to the solution were Type A, 0.266 cm2 (0.133 cm2 exposed on each side to the solution), Type B, 0.133 cm2 (one side only exposed), and Type C, 0.38 cm2 (0.19 cm2 exposed on each side). The thickness, mass, and volume of the Type A, B, and C electrodes were 0.017 cm/1.7 × 10−3 g/4.5 × 10−3 cm3; 0.017 cm/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 the Supporting Information and Figure S-1. The thiolated alkylferrocene, Fc(CH2)6SH, and Au nanoparticles capped with it as ligands were synthesized as previously described16 (see also Supporting Information). The average nanoparticle formula is Au225(SC6Fc)43 as determined by 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 nanofoam electrodes were loaded similarly except the 0.05 mM nanoparticle solutions were in 0.1 M Bu4NPF6/THF, and the soaking time in an evacuated glass desiccator to encourage pore fillingwas 2 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 solvent component. Cyclic voltammetry of nanoparticle-loaded Type A electrodes was performed in 1 M Bu4NPF6/CH2Cl2 electrolyte solutions. Voltammetry of Type B and C nanofoam electrodes was done in 1 and 2 M Bu4NPF6/CH3CN, respectively. The reference and counter electrodes of the three-electrode cell were Ag/AgCl/3 M KCl (aq) and Pt, respectively. Measurements were performed using a CH Instruments (Austin, TX) model 760C electrochemical analyzer and a Pine Instruments (Durham, NC) WaveNow potentiostat. Potential scans were done over a range of −0.1 to +1.0 V vs Ag/AgCl, in which the background currents appear to be mainly nonfaradaic. See Supporting Information for other measurement details. 9285
dx.doi.org/10.1021/jp212537q | J. Phys. Chem. C 2012, 116, 9283−9289
The Journal of Physical Chemistry C
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Figure 4. Panel A. Experimental cyclic voltammetric responses for a 0.19 cm2 (area exposed on each side) Type C carbon nanofoam electrode containing 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, 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) 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 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 = 0.35 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 B except Cdl = 0.38 F cm−2, b = 7.0 × 10−3 AV−1 cm−2, b0 = 5.0 × 10−4 A cm−2.
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RESULTS AND DISCUSSION In previous reports,17 we have described the formation, from nonaqueous media, of highly persistent adsorbed monolayers and multilayers of ferrocene-labeled, small Au nanoparticles on Au and Pt electrodes. The degree of adsorption, monitored from the coulometric charge under the multiferrocene redox wave, also depends on the electrolyte anion and is enhanced by electrogenerated cationic sites in the Au nanoparticle capping layer. On Au electrodes, the adsorption is also enhanced by the presence of selfassembled monolayers terminated by anionic groups such as sulfonate or carboxylate. We have proposed17 that, in these systems, adsorption is based on forming multiple ion-pair bridges between cationic sites at the surface of the nanoparticle and electrolyte anions adsorbed at the electrode, as illustrated in Figure S-2 (Supporting Information). The mechanism proposes that the tenacious adsorption is an entropic consequence of multiple interactions between nanoparticles and between nanoparticles and the electrode. To probe the interaction of ferrocenated Au nanoparticles within a 3D porous carbon nanofoam, we first established that nanoparticle adsorption also occurs on glassy carbon electrodes, again from nonaqueous medium, as illustrated by the Figure 1 cyclic voltammogram of adsorbed Au225(SC6Fc)43 nanoparticles. Glassy carbon is a reasonable planar analog of the 3D carbon nanofoam substrate, and it is thus expected that the nanoparticles also readily adsorb onto interior surfaces of the highly porous carbon nanofoams. Using nanoparticles with average composition Au225(SC6Fc)43, carbon nanofoam electrodes were soaked in nanoparticle solutions (see Experimental Section for details) and subsequently analyzed by cyclic voltammetry as shown in Figures 2−4 (Panels A). The voltammetry shows a large Fc+/0 wave riding atop the substantial doublelayer capacitance background of the carbon nanofoam itself.
The nanoparticle/nanofoam voltammetry is fairly stable repetitive potential cycling for 6 h (equivalent to ca. 3 h in the ferrocenium state) produced
