A Versatile Material for a Symmetrical Electric Energy Storage Device

Article pubs.acs.org/JPCC

A Versatile Material for a Symmetrical Electric Energy Storage Device: A Composite of the Polymer of the Ferrocene Adduct of C60 and Single-Wall Carbon Nanotubes Exhibiting Redox Conductivity at Both Positive and Negative Potentials Piotr Pieta,*,† Ievgen Obraztsov,† Janusz W. Sobczak,† Olga Chernyayeva,† Sushanta K. Das,‡ Francis D’Souza,*,‡ and Wlodzimierz Kutner*,†,§ †

Department of Physical Chemistry of Supramolecular Complexes, Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland ‡ Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, Texas 76203-5017, United States § Faculty of Mathematics and Natural Sciences, School of Science, Cardinal Stefan Wyszynski University in Warsaw, Woycickiego 1/3, 01-938 Warsaw, Poland S Supporting Information *

ABSTRACT: A (carbon nanotube)−(fullerene−ferrocene dyad polymer) composite, pyr-SWCNTs/(C60Fc-Pd), was devised and tested as an active material of a symmetrical device for electric energy storage. The composite was redox conducting at both positive and negative potentials due to the Fc/Fc+ and C60−/C60 electrode process of the ferrocene and fullerene moiety of the dyad, respectively. The composite was prepared, first, by electrophoretic deposition of a film of noncovalently modified with 1pyrenebutyric acid stacked single-wall carbon nanotubes (pyr-SWCNTs). Then, this film was coated, by potentiodynamic electropolymerization, with a film of the palladium-doped polymer of 2′-ferrocenylfulleropyrrolidine (C60Fc-Pd). The AFM imaging showed that the C60Fc-Pd film was uniformly built of 40−150 nm diameter globules of C60Fc-Pd while the film formed a tangle of pyr-SWCNTs bundles wrapped with clusters of ∼45 nm diameter globules of C60Fc-Pd. The XPS measurements identified interactions between the Pd and C60 moieties in C60Fc-Pd. The electrochemical and viscoelastic properties of the pyrSWCNTs/(C60Fc-Pd) film in a blank acetonitrile solution of tetra-n-butylammonium perchlorate favorably compared to those of the C60Fc-Pd film as disclosed by simultaneous cyclic voltammetry (CV) and piezoelectric microgravimetry (PM) measurements. Specific capacitance (Cs) of the pyr-SWCNTs/(C60Fc-Pd) film was higher than that of the C60Fc-Pd film and for positive charging its maximum value was Cs,max ≅ 300 F g−1. The EIS measurements revealed capacitive and mixed capacitive and pseudocapacitive behavior of the composite over the narrow and wide range of the applied potentials, respectively. Alternate galvanostatic +0.72 mA cm−2 charging and −0.72 mA cm−2 discharging and then opposite polarity charging of a device constructed of two identical composite film electrodes was reversible only for low (+0.6 and −0.6 V) voltage of charging and discharging. After multiple charging and discharging (1000 cycles), the capacity increased by ∼5%, but it decreased by ∼18% for the high (+1.2 and −1.2 V) voltage. Importantly, the average power of the device was ∼7.3 times higher for the latter.

1. INTRODUCTION Composites of carbon nanotubes (CNTs), incorporated in a conducting polymer, have intensively been studied for the past few years as prospective electroactive materials for electrodes of different electronic devices like those for energy storage including batteries and supercapacitors.1 These composites synergetically combine properties of CNTs and those of the polymers, resulting in materials of high electric conductivity and largely developed surface area.1c In these composites, the © 2013 American Chemical Society

charge storage mechanism of redox pseudocapacity of a conducting polymer is combined with the electrostatic attraction and outstanding mechanic properties of CNTs.1b Moreover, the mesoporous structure of the CNT network immobilized in the polymer matrix increases accessibility of the Received: October 31, 2011 Revised: December 21, 2012 Published: January 2, 2013 1995

dx.doi.org/10.1021/jp210450y | J. Phys. Chem. C 2013, 117, 1995−2007

The Journal of Physical Chemistry C

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electrochemically active surface to counterions enhancing that way charging and discharging of the composite and increasing its capacity.2 For instance, an electrode modified with the vertically aligned multiwall CNTs, fabricated by a “transfer method”, revealed very low ohmic resistance and fast ion diffusion to the CNT surface, making this electrode promising for subsequent composite preparation.3 Several studies on application of the CNT/(conducting polymer) composites for electrochemical capacitors have already been reported.4 Either single-wall (SWCNTs)5 or multiwall (MWCNTs)6 carbon nanotubes and various πelectron conducting (ECPs) or redox conducting (RCPs) polymers were used to prepare these composites. Toward that, ECPs, such as polyaniline (PANI),7 polypyrrole (PPy),6 and fullerene-based RCPs, such as the palladium bridged (C60-Pd),8 oxygen-bridged (C60-O),9 or mixed polymers of C60-Pd and polybithiophene (PBT),10 have been extensively studied because of their relatively high both electric conductivity and pseudocapacitance. The above composites can be prepared by chemical or electrochemical polymerization of a particular monomer in the presence of CNTs, i.e., by mixing of a polymer with CNTs or electrochemical deposition of a polymer onto an electrode previously coated with CNTs.11 There are many methods of immobilization of CNTs onto an electrode surface, e.g., dropcoating,12 electrostatic spray deposition,13 filter deposition,14 inject printing,15 Langmuir−Blodgett (LB) deposition,16 layerby-layer (LBL) assembling,17 direct growth,18 or electrophoretic deposition.19 Depending on the nature of a polymer and CNTs used to make a composite, a composite preparation procedure adopted and the electrolyte applied for electrochemical treatment, the resulting composite exhibits different useful voltage limits and, in consequence, different capacity properties.20 For instance, optimum voltage limit of operation of a symmetrical capacitor of CNTs/PPy and CNTs/PANI is 0.6 and 0.5 V, respectively.20b To increase operation voltage of a supercapacitor, a hybrid systems consisting of two different electrode materials were proposed. Importantly, the extension of the supercapacitor voltage significantly increases its energy and power. An asymmetric supercapacitor was built with the CNTs/PPy and CNTs/PANI film as an active material of the negative and positive electrode, respectively.21 Properties of asymmetric supercapacitors made of composites of different conducting polymers as negative electrodes and an α-MnO2 composite as the positive electrode have already been studied.22 Fullerene-based RCPs were used to prepare composites that were electroactive in the negative potential range of −0.4 to −1.2 V. For instance, thin films of SWCNTs and C60-Pd were codeposited by potentiodynamic electropolymerization onto an electrode surface.8,23 Moreover, the C60-Pd film was deposited in a similar way on the drop-cast MWCNTs film coated electrode surface.24 Another composite of SWCNTs and mixed polymers of C60-Pd and polybithiophene (PBT) was proposed as an active material for the electrode of an electrochemical capacitor.10 This composite was electroactive in both the negative (−1.50 to −0.20 V) and positive (0.40 to 1.0 V) potential range. In another study, a polymer of oxygen-bridged C60 (C60-O) was deposited by electropolymerization involving potentiodynamic reduction of C60, in the presence of O2, on an electrophoretically deposited layer of pyr-SWCNTs to yield a pyr-SWCNTs/(C60-O) film (pyr stands for 1-pyrenebutyric acid, Scheme 1a).9

Scheme 1. (a) Structural Formula of 1-Pyrenebutyric Acid (pyr), (b) Simplified Representation of a Noncovalently Surface Modified with 1-Pyrenebutyric Acid Single-Wall Carbon Nanotube (pyr-SWCNT), and (c) Structural Formula of 2′-Ferrocenylfulleropyrrolidine (C60Fc)

The present study aims at devising and characterizing a composite material, which comprises RCP conducting in both the negative and positive potential range and, therefore, is alternately p- and n-doped with cations and anions, respectively, and by virtue of that it exhibits both high conductivity and specific capacitance in these ranges.25 For that, first, a pyr-SWCNTs (Scheme 1b) film was electrophoretically deposited onto an Au electrode, as outlined in our previous study.10 Then, this film was coated with the C60Fc-Pd film deposited by potentiodynamic electropolymerization of 2′ferrocenylfulleropyrrolidine, C60Fc (Scheme 1c), aided by the palladium acetate.26 Presumably, the C60Fc-Pd polymeric network is formed via covalent bonding between the Pd complex and the fullerene moieties.27 That is, the C60 and Pd form a polymeric backbone with ferrocene centers covalently linked to it. Using this pyr-SWCNTs/(C60Fc-Pd) composite as an active material of each of the two electrodes, we constructed a symmetric energy storage device, capable of reversing polarity of these electrodes. For this reversing, positive electrode charging was switched to negative and back. This polarity reversing appeared possible because of the newly employed C60Fc-Pd, electroactive in both the positive and negative potential range.

2. EXPERIMENTAL SECTION 2.1. Chemicals. C60 (99.5% purity) was from the M.E.R. Corp. (Tucson, AZ) or SES Research (Houston, TX). 2′Ferrocenylfulleropyrrolidine was synthesized and purified according to the literature procedure.28,29 The HiPCO singlewall carbon nanotubes were from Carbon Nanotechnology, Inc. (Houston, TX). 1-Pyrenebutyric acid (97% purity) and palladium(II) acetate, PdII(ac)2 (98% purity), were from Aldrich. The toluene (anhydrous, 99.8%), 1-methyl-2-pyrrolidone, NMP, from Aldrich, and acetonitrile (puriss, absolute) solvents were used as received from Fluka. Tetra-nbutylammonium perchlorate, (TBA)ClO4, used as the supporting electrolyte was from Fluka and was also used without purification. 2.2. Apparatus and Procedures. The apparatus setup for electrophoretic deposition of CNTs was described elsewhere.10 In this setup, a spirally flat coiled 0.5 mm diameter Pt wire and an Au film electrode served as the auxiliary (negative) and working (positive) electrode, respectively. This Au film electrode, 5 mm in diameter, was evaporated on top of a Ti underlayer coated 10 MHz plano−plano quartz resonator with 1996



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a matt surface finish (14 mm diameter) from the Institute of Tele- and Radio Communication (Warsaw, Poland). This electrode was used for simultaneous potentiodynamic or CV and piezoelectric microgravimetry (PM) measurements. A dc voltage was applied to the electrodes by using an IZS-5/71 stabilized power supply of INCO (Warsaw, Poland). For monitoring the time-resolved mass changes of the film being deposited, simultaneously with the electrophoretic deposition of the noncovalently surface-coated with pyr-SWCNTs films, the PM experiments were performed. For that, the EQCM 5710 electrochemical quartz crystal microbalance of the Institute of Physical Chemistry (Warsaw, Poland) under control of the EQCM 5710-S2 software of the same manufacturer were used. The frequency shift of a quartz resonator was recalculated into the mass of the deposited film by using the Sauerbrey equation:30 Δf = −

A 160 W power IS-3R ultrasonic bath of InterSonic (Olsztyn, Poland) was used for dissolution of C60Fc and fine dispersion of pyr-SWCNTs in dedicated electrolyte solutions. Simultaneous CV and PM experiments were performed by using an EP-21 potentiostat of Elpan (Lubawa, Poland) connected to the EQCM 5710 electrochemical quartz crystal microbalance operating under the EQCM 5710-S2 software control. This microbalance allowed for simultaneous measurement of changes of current, resonant frequency, and dynamic resistance of an Au/quartz crystal resonator as a function of the potential cycled or time. An AUTOLAB computerized electrochemistry system of Eco Chemie (Utrecht, The Netherlands) was used for the CV and EIS measurements with the film-coated electrodes. This system was equipped with the expansion cards of the PGSTAT 301N potentiostat and the FRA2 frequency response analyzer and controlled by the GPES 4.9 software of Eco Chemie. A conventional three-neck one-compartment V-shaped glass electrochemical cell was used for the CV experiments in the three-electrode system. A 4 mm diameter Au disk, coiled Pt wire, and Ag|AgCl served as the working, auxiliary, and reference electrode, respectively. The EIS measurements were performed for the frequency range of 10 kHz−0.1 Hz at various constant electrode potentials applied using a 10 mV sine-wave amplitude. The composite-film-modified electrode was kept at the selected potential for ∼120 s before performing each EIS measurement. Surfaces of the electrodes were imaged with atomic force microscopy (AFM) using a Multimode NS 3D instrument of the Veeco Metrology Group (Santa Barbara, CA). For this imaging, the films were deposited on the 7 mm wide strips of the ITO glass electrodes. The XPS spectra were recorded with an Escalab-210 spectrometer of VG Scientific (East Grinstead, U.K.) using Al Kα (hν = 1486.6 eV) X-ray radiation. The pressure in the spectrometer chamber was ∼5 × 10−9 mbar. High-resolution scans were recorded with the 20 eV analyzer pass energy at a 0.1 eV increment for the C 1s, Pd 3d, and Fe 2p detail spectrum. The spectra were analyzed by the AVANTAGE data system software of Thermo VG Scientific (East Grinstead, U.K.) using a Gauss-to-Lorentz constant ratio of 0.3. The background spectrum was accounted for by using the Shirley method.34 All experiments were performed at ambient temperature, (20 ± 1) °C. 2.3. Preparation of Films of the Modified Single-Wall Carbon Nanotubes, Polymer, and Composite. The composite film of pyr-SWCNTs and C60Fc-Pd was prepared in two steps. First, the pyr-SWCNTs film was electrophoretically deposited on the electrode. Then, the resulted pyrSWCNTs modified electrode was coated with the C60Fc-Pd film by potentiodynamic electropolymerization in the presence of PdII(ac)2. 2.3.1. Electrophoretic Deposition of the pyr-SWCNTs Film. The instrumental setup for bulk electrophoretic deposition of the pyr-SWCNTs film was described elsewhere.10 In brief, the horizontally held and facing up the Au disk or ITO, or Au/ quartz electrodes, with the spirally flat coiled Pt wire auxiliary electrode positioned parallel and above the working electrode, was positively polarized with the 24 V dc in the 0.4 mg mL−1 fine suspension of pyr-SWCNTs in NMP. Distance between the two electrodes was kept constant at ∼6 mm throughout all the experiments.

2f0 2 Δm A(μQ ρQ )0.5

(1)

where μQ = 2.947 × 1011 g cm−1 s−2 is the shear modulus of quartz, ρQ = 2.648 g cm−3 is the quartz density, A (cm2) is the acoustically active area of the quartz resonator, and f 0 (Hz) is the fundamental resonant frequency. The mass deposited on the 10 MHz resonator was calculated according to eq 2: Δm = −8.67 × 10−10Δf

(2)

The frequency change due to changes of viscosity and density of a liquid of the dynamic viscosity, ηL, and density, ρL, respectively, is31 Δf = −f

⎛ ηρ L L ⎜ Aπμ ρ ⎝ Q Q

3/2 ⎜

⎞1/2 ⎟ ⎟ ⎠

(3)

Qualitatively, considerations involving quartz resonator in contact with viscous liquid can be extended to viscous film coats.32 Therefore, dynamic resistance of the quartz resonator, R,33 a variable related to viscosity of the medium, and Δf tot were simultaneously measured herein to determine contribution of the viscosity change to the total frequency change, according to eq 4: R=

A (2πf0 ηLρL )0.5 k2

(4)

k2 = 7.74 × 10−3 A2 s2 cm−2 is the electromechanical coupling factor for the quartz resonator. From eqs 3 and 4 the frequency change due to the viscosity change (opposite to dynamic resistance) is estimated according to eq 5. Δfvis = −

k 2Rf0 πA(2μQ ρQ )0.5

(5)

The pyr-SWCNTs film was electrophoretically deposited onto the Au disk electrodes for electrochemical impedance spectroscopy (EIS) measurements and onto the indium-doped tin oxide (ITO) glass electrodes (14 Ω cm−2) of Image Optics Components, Ltd. (Basildon, U.K.) for the AFM imaging as well as for the XPS measurements. 1997



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Figure 1. Multicyclic curves of the potential dependence of (1 and 1′) current, (2 and 2′) the resonant frequency change, and (3 and 3′) the dynamic resistance change for deposition of the C60Fc-Pd film on (a) the Au/quartz electrode and (b) the Au/quartz/pyr-SWCNTs electrode in the 0.3 mM C60Fc, 4.56 mM Pd(ac)2, and 0.1 M (TBA)ClO4 solution of toluene:acetonitrile (4:1, v:v). The potential sweep rate was 100 mV s−1.

of PdII(ac)2 to Pd0(ac)2,35,36 which forms an intermediate complex with C60Fc. This complex may both initiate growth of the (-C60Fc-Pd-)n polymer chain or decompose to palladium nanoclusters depending on the C60Fc-to-PdII(ac)2 concentration ratio.36 Formation of palladium nanoclusters has already been studied with respect to the C60-Pd film.36,37 The increase of the palladium complex concentration in solution for electropolymerization increases the efficiency of formation of the palladium nanoclusters. The C60Fc-Pd film deposited from the solution of a relatively high (Pd(ac)2)3 concentration is electrochemically inactive in the negative potential range.27 In the present study, therefore, the C60Fc-to-PdII(ac)2 concentration ratio was adjusted experimentally to result in the polymer film electrochemically active in both the positive and negative potential range. Moreover, the C60-Pd film electrodeposition requires imposition of the potential limit sufficiently negative to electroreduce the PdII(ac)2 complex to the zerovalent metal intermediate. The C60Fc-Pd film was deposited onto both the bare (Figure 1a) and precoated with the pyr-SWCNTs film electrode (Figure 1b) in order to compare properties of the polymer film without and with nanotubes, respectively. The pyr-SWCNTs film mass and thickness was 30 μg and 500 nm, respectively. For measurement of the film thickness, some part of the film was carefully removed, i.e., scratched by using a small piece of Teflon, from the electrode surface under optical microscope. That way, only one part of the electrode remained coated with the film. With AFM, the height of the resulting step was measured in several surface points along the step edge (sufficiently far from its partially detached front) for determination of the average value of this height. The presence of the pyr-SWCNTs film increased the electrode capacity to ∼50 F g−1 due to the ∼230% increase of the electrode area.10 During deposition of the C60Fc-Pd film

In these experiments, the Au/quartz resonator was mounted in a quartz crystal holder of EQCM 5710, and the frequency change of the resonator was simultaneously measured by PM during deposition of the pyr-SWCNT film. After ∼6 min deposition, the frequency change became negligibly small. Therefore, the total time of each deposition was set at 6 min. When the electrophoresis was terminated, the entire surface of the Au/quartz resonator became coated with a black film of the ∼35 nm diameter bundles of pyr-SWCNTs. Then, the electrode coated with this film was rinsed with the abundant acetonitrile solvent and dried in an Ar stream. 2.3.2. Preparation of the pyr-SWCNTs/(C60Fc-Pd) Film. The C60Fc-Pd film was grown by reductive electropolymerization from the 0.30 mM C60Fc, 4.56 mM PdII(ac)2, and 0.1 M (TBA)ClO4 solution of the toluene-to-acetonitrile ratio of 4:1 (v:v) under conditions of potentiodynamic multicycling of the potential between 0 and −0.90 V at the sweep rate of 100 mV s−1 under an argon atmosphere. The polymer film was deposited on the bare electrode and, separately, on that initially coated electrophoretically with the pyr-SWCNTs film in order to prepare the polymer and composite film, respectively. After deposition of the film, the resulting modified electrode was rinsed with acetonitrile prior to use. Samples prepared for the XPS measurements were transferred between the electrochemical cell and the XPS chamber under an argon atmosphere.

3. RESULTS AND DISCUSSION 3.1. Preparation as well as Electrochemical and Viscoelastic Properties of the Composite Films. The potentiodynamic deposition of the C60Fc-Pd film is likely controlled by the reductive electropolymerization mechanism similar to that governing deposition of the C60-Pd film.27,35 That is, the polymer deposition is initiated by electroreduction 1998



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Figure 2. Multicyclic curves of the potential dependence of (1 and 1′) current, (2 and 2′) the resonant frequency change, and (3 and 3′) the dynamic resistance change for the (a) Au/quartz/(C60Fc-Pd) electrode and (b) Au/quartz/pyr-SWCNTs/(C60Fc-Pd) electrode in the 0.1 M (TBA)ClO4−acetonitrile solution. Curve 1″ in panel b shows cyclic voltammogram for 0.34 mM C60Fc in the 0.1 M (TBA)ClO4 solution of toluene:acetonitrile (4:1, v:v) on 1 mm diameter Pt disk electrode. The potential sweep rate was 100 mV s−1.

response is not that of pure liquid viscosity−density. Nearly zero slopes of the 1 and 2 lines indicate deposition of slightly inelastic polymer films, which dissipate energy to some extent. However, the ΔR changes are very small and can be considered negligible in the mass calculations. The total frequency decrease for deposition of the C60Fc-Pd film on the bare electrode (curve 2 in Figure 1a), measured after 10 potential cycles, was ∼5.8 kHz while nearly the same frequency decrease, ∼5.6 kHz, required as many as 12 potential cycles (curve 2′ in Figure 1b) for deposition of this film on the (pyr-SWCNTs)-coated electrode. The total frequency decrease was accompanied by the dynamic resistance increase of ∼48.2 and ∼38.1 Ω for deposition of the former (curve 3 in Figure 1a) and the latter film (curve 3′ in Figure 1b), respectively. Qualitatively, these dynamic resistance changes correspond to the frequency change of ∼4.8 and ∼3.8 Hz, respectively, as determined by using eq 5.8,30 Apparently, the total frequency changes for both films were by 3 orders of magnitude higher than the changes which corresponded to the viscosity changes, indicating that the rigidity of the films remained practically unchanged during the deposition. Hence, the mass of the deposited films, calculated by using eq 1 thus justified, was ∼5.0 and ∼4.9 μg, respectively.8 The quartz resonator, with its Au electrode coated either by the polymer film without or with nanotubes, was transferred to a blank 0.1 M (TBA)ClO4−acetonitrile solution, and curves of the potential dependence of the change of current, resonant frequency, and dynamic resistance were simultaneously recorded (Figure 2). Each film revealed two potential ranges of electroactivity. That is, one range covered the potentials more negative than ∼−0.40 V and the other those more positive than ∼0.20 V (curves 1 and 1′ in Figures 2a and 2b, respectively). This electroactivity is due to the charge exchange at the electrode by

on the Au/quartz resonator uncoated and coated with the pyrSWCNTs film, the electrochemical and viscoelastic properties of these films were monitored. Toward that, current, the resonant frequency change, and the dynamic resistance change as a function of linearly cycled potential were measured and presented as curves 1, 2, and 3 in Figure 1a and curves 1, 2′, and 3′ in Figure 1b, respectively. The polymer film deposition was manifested by the decrease of the resonant frequency with the increase of the number of potential cycles (curves 2 and 2′ in Figures 1a and 1b, respectively). For mutual comparison, both films were deposited to reach nearly the same value of the total frequency decrease. Adoption of this deposition procedure allowed us to assume that the same amount of an electroactive material was deposited on each electrode. Additional mass determination from the charge transferred during the electropolymerization was in accord with the PM mass determination. Generally, the frequency decrease is caused by changes of the mass of the elastic film attached to the quartz surface. In this case the linear Sauerbrey equation can be used to quantify the amount of the deposited mass.8−10,38 To determine whether the deposited film is elastic or inelastic, the dynamic resistance was additionally measured simultaneously as a variable solely related to the energy dissipation of the quartz crystal resonator into its surroundings.32 Deposition of an elastic polymer film results in a pronounced decrease in the resonant frequency and a concomitant very small change in the dynamic resistance.39 Figure S1 of Supporting Information presents the ΔR vs Δf plot in which solid curves show experimental results of the C60Fc-Pd film deposition on the Au/quartz (curve 1 in Figure S1) and Au/quartz/pyr-SWCNTs electrode (curve 2 in Figure S1). The B−C horizontal dashed line represents deposition of a purely elastic mass while the A−D line corresponds to a pure liquid viscosity−density response.39 Our experimental curves 1 and 2 are far from the A−D line, suggesting that the EQCM 1999



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Figure 2b), indicating the increase of the mass of the films. This increase is most likely associated with the TBA+ ingress to the film for compensation of the negative charge generated on the C60 moiety. This frequency decrease is accompanied by the decrease of the dynamic resistance (curve 3 in Figure 2a and curve 3′ in Figure 2b). However, this resistance is decreased only for the potential range of −0.45 to −1.00 V, corresponding to the first electroreduction of the C60 moiety. When the potential reaches the value of the second (C60−/C602−) electroreduction of C60, i.e., that of ∼−1.00 V (curves 1 and 1′ in Figure 2a and 2b, respectively), the resonant frequency still decreases (curve 2 in Figure 2a and curve 2′ in Figure 2b) but the dynamic resistance increases (curve 3 in Figure 2a and curve 3′ in Figure 2b). Presumably, an initial ingress of TBA+ to the film, i.e., that corresponding to the first electroreduction, makes both films more rigid. However, further counterion ingress, i.e., that accompanying the second electroreduction, increases viscoelasticity of these films. This behavior differs from that of the pyr-SWCNTs/(C60-Pd)-PBT film.10 Namely, there were two decreasing and two raising steps in the Δf vs E and ΔR vs E curve, respectively, for this latter composite in the −0.40 to −1.20 V range.10 For both the C60Fc-Pd and pyr-SWCNTs/(C60Fc-Pd) film, there is an anodic and cathodic CV peak in the positive, i.e., 0.20−0.60 V, potential range (curves 1 and 1′ in Figure 2a and 2b, respectively). The former corresponds to electro-oxidation of the Fc moiety and the latter to subsequent electroreduction of the Fc+ moiety. Both anodic and cathodic currents for the C60Fc-Pd film (curve 1 in Figure 2a) are lower than currents for the pyr-SWCNTs/(C60Fc-Pd) film (curve 1′ in Figure 2b). The anodic peaks for both films are accompanied by a decrease of the resonant frequency (curves 2 and 2′ in Figure 2a and 2b, respectively) presumably due to the ClO4− ingress to both films for compensation of the positive charge generated on the Fc moiety. During this positive potential excursion, dynamic resistance is virtually constant (curves 3 and 3′ in Figures 2a and 2b, respectively), indicating the absence of viscoelastic changes in the films. The extent of apparent ion transfer across the film−solution interface was estimated by examining the mass-on-charge dependence for the redox processes occurring in films.41 Based on the curves of the current and frequency change vs potential shown in Figure 2, curves of the mass change (calculated using eq 1) vs charge for the C60/C60−, C60−/C602−, C602−/C60−, C60−/C60, Fc/Fc+, and Fc+/Fc processes are plotted in Figure 3. From the slopes of tangents at potentials at which these reactions occur (Figure 3), values of the apparent electrochemical equivalents of counterions entering the film were estimated (Table 1).41 These values suggest that ingress of counterions to the film and, subsequently, their release from the film are not fully balanced for both C60Fc-Pd and pyrSWCNTs/C60Fc-Pd. That is, these equivalents for the C60/ C60−, C60−/C602−, and Fc/Fc+ couples are higher than those for the corresponding C602−/C60−, C60−/C60, and Fc+/Fc couples. This unbalance indicates some irreversibility of the mass transfer accompanying the electroreductions and electrooxidations. An additional plot of the mass change as a function of the charge passed is shown, for the first and the last CV cycle, in Figure S2. This behavior is consistent with that of the frequency change vs potential (curves 2 and 2′ in Figures 2a and 2b, respectively). That is, the value of the initial frequency is higher than that of the final for each CV cycle. It indicates that the film mass increases after each CV cycle. The values of

the C60 and Fc moiety of the polymer, respectively. That is, the C60 moiety is subjected to two reversible redox processes in the potential range of −0.40 to −1.20 V while the Fc moiety experiences one redox process in the range of 0.20−0.60 V, as for clarity illustrated with a CV curve for C60Fc in solution (curve 1″ in Figure 2b). Accordingly, the polymer becomes either n- or p-doped with counterions, i.e., cations or anions, at the negative and positive potential range, respectively. However, there was one broad peak pair for the films for the negative potential range of −0.40 to −1.20 V (curves 1 and 1′ in Figures 2a and 2b, respectively) instead of the well-defined two peak pairs for the corresponding redox couples in solution (curve 1″ in Figure 2b). This behavior is governed by the polymer structure. That is, the C60 moieties and Pd atoms form a C60Fc-Pd polymer backbone with ferrocene moieties covalently linked to it.27 This linkage causes overlapping of energy bands corresponding to different redox states of each C60 moiety within the polymer network.40 Moreover, electroreduction and electrooxidation of this moiety are accompanied by cation shuttling between the film and the solution contributing to formation of an electric double layer around static charges within the polymer matrix. Hence, a large capacity current is overlapping the faradaic C600/C60− and C60−/C602− currents. These two phenomena are most likely responsible for the lack of well developed two CV peak pairs for the polymer in the negative potential range. Moreover, electrode processes of the Fc moiety also cause ion shuttling across the electrolyte−polymer interface. However, the Fc moieties do not form the polymer backbone, but they are covalently linked to it. Therefore, electronic states of Fc do not overlap. Hence, the corresponding CV peaks appear sharp and symmetric, as expected for surface-confined reversible redox species. For the electrode solely coated with the electrophoretically deposited pyr-SWCNTs film, the shape of the CV curve is pseudorectangular in the potential range of −0.40 to −1.20 V,10 indicating the capacitive nature of the current recorded. In the central potential range, i.e., that between −0.40 and 0.20 V, the C60Fc-Pd film is electroinactive manifested by a relatively low CV capacity current (curve 1 in Figure 2a). However, this current is quite pronounced for the pyrSWCNTs/(C60Fc-Pd) film in this potential range arising from charging and discharging of the much larger surface area film (curve 1′ in Figure 2b).10 In the negative potential range, i.e., that of −0.40 to −1.20 V, both cathodic and anodic currents for the pyr-SWCNTs/ (C60Fc-Pd) film (curve 1′ in Figure 2b) are higher than currents for the C60Fc-Pd film (curve 1 in Figure 2a). These higher currents for the composite film presumably result from its higher area and, hence, also higher electrode capacity. Moreover, there are no alterations in the resonant frequency (curves 2 and 2′ in Figures 2a and 2b, respectively) during cathodic potential sweeping in this negative range until the potential reaches −0.45 V. However, there is a decrease in both the resonant frequency and dynamic resistance for the undoped C60Fc-Pd film (curves 2 and 3, respectively, in Figure 2a) and composite film (curves 2′ and 3′, respectively, in Figure 2b) for potentials more negative than this potential value. For that, the first electroreduction of C60 is, most likely, responsible. That is, when the potential during its cathodic excursion reaches the value of this first electroreduction, i.e., that of ∼−0.45 V (curve 1 in Figure 2a and curve 1′ in Figure 2b), the resonant frequency decreases (curve 2 in Figure 2a and curve 2′ in 2000



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higher sweep rates. Moreover, the difference between the charge generated during electroreduction of the C60 moiety or electro-oxidation of the Fc moiety and subsequent respective electro-oxidation of the C60− moiety or electroreduction of the Fc+ moiety does not exceed 8%. Hence, charging and discharging are not fully balanced. This apparent irreversibility may cause slow gradual film deactivation with the number of CV cycles. Apparently, the relative contribution of the faradaic and charging process to the total charge accumulated was ∼74% and ∼26%, respectively, as estimated by integrating the CV curve in the potential range of 0.60 to −1.20 V for the pyrSWCNTs/(C60Fc-Pd) film (curve 1′ in Figure 2b). For estimation of the faradaic contribution, capacity currents determined in the central −0.40 to 0.20 V potential range of the C60Fc electroinactivity were extrapolated to the −0.40 to −1.20 V negative and 0.20 to 0.60 V positive range of the C60 and Fc moiety electroactivity, respectively, and subtracted from the total current. Equation 6 was used to calculate the specific capacitance, Cs, of the electrode coated by the C60Fc-Pd or pyr-SWCNTs/ (C60Fc-Pd) film.

Figure 3. Dependence of the mass change on charge for (a) electroreduction and subsequent electrooxidation of the Fc moiety and (b) electrooxidation and subsequent electroreduction of the C60 moiety of C60Fc-Pd (black curves) and of pyr-SWCNTs/C60Fc-Pd (red curves). Data were recalculated from the CV and the frequency change vs potential curves shown in Figure 1. Dash tangents serve for determination of electrochemical equivalents of species crossing the film−solution interface.

Cs =

i vm

(6)

In this equation, i is the pseudocapacity current, m is the mass of the material deposited on the electrode surface, and v is the potential sweep rate. This capacitance strongly depends on the applied potential (Figure 5), as expected. That is, Cs is higher for the potential ranges of electroactivity of the C60 and Fc moieties and lower for the central range of redox inactivity of these moieties. Importantly, specific capacitance of the pyr-SWCNTs/(C60FcPd) film coated electrode is higher than that of the C60Fc-Pd film coated electrode. The highest determined Cs value, characteristic for the positive and negative potential range, is 300 and 120 F g−1 for the pyr-SWCNTs/(C60Fc-Pd) film compared to 260 and 100 F g−1 for the C60Fc-Pd film. Notably, specific capacitance of the presently studied composite film is higher for both the positive and negative potential range than that formerly determined for the composite film containing pyrSWCNTs and mixed C60-Pd and PBT polymers, equal to 200 and 100 F g−1, respectively.10 Moreover, for the negative potential range the previously determined Cs value for the composite film of C60-Pd and pyr-SWCNTs, prepared by insitu electropolymerization of C60-Pd in the suspension of pyrSWCNTs was lower and equal to 90 F g−1.8 3.2. Topography of the C60Fc-Pd, pyr-SWCNTs, and the pyr-SWCNTs/(C60Fc-Pd) Film. The AFM images of surfaces of films of C60Fc-Pd, pyr-SWCNTs, and pyrSWCNTs/(C60Fc-Pd), deposited on the ITO electrodes, are shown in Figures 6a, 6b, and 6c, respectively. The C60Fc-Pd film was deposited during 10 and 12 potential cycles onto the

these equivalents show that the effective molecular weight of the ions entering or leaving the film is much higher than those calculated for these ions. For instance, for the C60/C60− couple the apparent electrochemical equivalent is equal to 366.5 and 395.4 g mol−1 for the C60Fc-Pd and pyr-SWCNTs/C60Fc-Pd, respectively. The molecular weight of TBA+ and ClO4− is MW(TBA+) = 242.45 and MW(ClO4−) = 99.45 g mol−1, respectively. Hence, the film electroreduction is presumably accompanied by ingress of some solvent and/or neutral electrolyte. The charge corresponding to two reversible redox processes in the negative potential range of −0.40 to −1.20 V (curve 1″ in Figure 2b) and one reversible redox process in the positive potential range of 0.20 to 0.60 V (curve 1″ in Figure 2b) was determined for different potential sweep rates for the pyrSWCNTs/(C60Fc-Pd) film (Figure 4). This charge only slightly changed with the potential sweep rate for higher sweep rates. However, the lower the rate the higher was the charge. This result is different from that expected for a diffusion-controlled process where neither charge nor capacity is potential sweep rate dependent. Therefore, the observed here behavior may indicate restricted diffusion of ions through the film. That is, more time is available for counterions to enter and diffuse through the film compensating the charge generated, the lower the potential sweep rate is. However, counterion diffusion through the film is unable to keep up pace of the potential change, and therefore, the charge reaches its limiting value for

Table 1. Apparent Electrochemical Equivalents (g mol−1) of Species Entering and Leaving the C60Fc-Pd and pyr-SWCNTs/ (C60Fc-Pd) Films in the Course of Electrode Processes of the C60 and Fc Moieties, As Calculated from the Slopes of the Mass vs Charge Curves Shown in Figure 3 apparent electrochemical equivalents of electrode process (g mol−1) film deposited on the Au electrode

C60/C60

C60Fc-Pd pyr-SWCNTs/(C60Fc-Pd)

366.5 964.5

−

C60−/C60

C60−/C602−

C602−/C60−

Fc/Fc+

Fc+/Fc

−347.2 −897.0

395.4 752.3

−376.1 −733.0

588.3 501.5

−559.4 −482.2

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Figure 4. Charge dependence on the potential sweep rate for the pyr-SWCNTs/C60Fc-Pd film for (a) C60/C60− electroreduction (curve 1) and subsequent C60−/C60 electrooxidation (curve 2′) as well as (b) Fc/Fc+ electrooxidation (curve 1′) and subsequent Fc+/Fc electroreduction (curve 2′).

The pyr-SWCNTs/(C60Fc-Pd) film (Figure 6c) is built of the pyr-SWCNTs bundles coated with the ∼45 nm diameter globules of C60Fc-Pd. This film is almost twice as thick as the C60Fc-Pd film deposited on the bare electrode and equal to ∼500 nm. Interestingly, the C60Fc-Pd globules of the film on the bare electrode are almost twice as large as the globules deposited on the nanotubes. This size difference may arise from much larger area of the electrode coated with the composite than that with the pristine polymer. Presumably, the first monolayer of the C60Fc-Pd film is deposited on the available surface of the electrode. Then, further electropolymerization leads to deposition of the polymer globules merging to form objects of larger surface. 3.3. Composition of the C60Fc-Pd Film and the pyrSWCNTs/(C60Fc-Pd) Film. The XPS measurements provided both qualitative and quantitative information on chemical composition of the C60Fc-Pd and pyr-SWCNTs/(C60Fc-Pd) (Figure 7) film. For both these films, bands of binding energy, EB, for the Pd, Fe, and C atoms are distinguished in their survey spectra (not shown). In the EB range of the Pd 3d electrons, three different forms of palladium can be distinguished in the detail spectra for both films (Figures 7a and 7a′). That is, there are three pairs of bands with the EB values of the 3d5/2 state at 334.6, 336.4, and 338.1 eV and a spin−orbit splitting of the 3d5/2 and 3d3/2 states of 5.3 eV with the relative intensity ratio of 0.4:14:1, respectively, for the C60Fc-Pd film (Figure 7a). The bands at 334.6 and 338.1 eV can be assigned to Pd0 and Pd2+,

Figure 5. Specific capacitance dependence on potential of for the film of (1) C60Fc-Pd and (2) pyr-SWCNTs/(C60Fc-Pd) in the 0.1 M (TBA)ClO4−acetonitrile solution.

bare (Figure 6a) and nanotube film coated electrode (Figure 6c), respectively. Apparently, the topography of the resulted coats is distinctly different. The C60Fc-Pd film (Figure 6a) is relatively uniform. It is built of microscopically distinguishable 40−150 nm diameter globules sticking together to form a ∼280 nm thick film that coats the entire surface of the ITO electrode. The ∼460 nm thick uniform pyr-SWCNTs electrophoretic film (Figure 6b) consists of a tangle of ∼35 nm diameter densely packet pyr-SWCNTs bundles.

Figure 6. Atomic force microscopy (AFM) images (3 × 3 μm2 area) of the deposited on the ITO electrode film of (a) C60Fc-Pd, (b) pyr-SWCNTs, and (c) pyr-SWCNTs/(C60Fc-Pd). 2002



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Figure 7. X-ray photoelectron spectroscopy (XPS) spectra for the film of (panels a, b, and c) C60Fc-Pd and (panels a′, b′, and c′) pyr-SWCNTs/ (C60Fc-Pd) deposited onto an ITO electrode. The binding energy region is shown for electrons of (panels a and a′) Pd 3d3/2 and Pd 3d5/2, (panels b and b′) Fe 2p and Fe 2p3/2 as well as (panels c and c′) C 1s.

In the EB range of the C 1s electrons, there are five and four different forms of carbon for the C60Fc-Pd (Figure 7c) and pyrSWCNTs/(C60Fc-Pd) film (Figure 7c′), respectively. For the former film, there are bands at the EB values of 284.7, 286.2, 287.5, 289.2, and 291.0 eV with the relative intensity ratio of 37.2:3.3:1.5:1.2:1, respectively (Figure 7c). The most intense band at 284.7 eV is assigned to carbon atoms of the fullerene cage.45 Other four small bands presumably originate from carbon atoms of (i) the tetra-n-butylammonium cations (286.2 eV) entrapped in the polymer matrix during electropolymerization, (ii) carboxy groups of the acetate ligands (287.5 eV), and (iii) from a shakeup process involving energy of the π → π* transition in the C60 moiety (289.2 and 291.0 eV).37 For the pyr-SWCNTs/(C60Fc-Pd) film, these bands are at EB of 284.7, 286.3, 288.1, and 290.1 eV with the relative intensity ratio of 32.8:4.8:1.6:1, respectively (Figure 7c′). As above, the most intense band at 284.7 eV is ascribed to carbon atoms of SWCNT and fullerene.45 The origin of the other three small bands is most likely the same as that for the C60Fc-Pd film. 3.4. Impedimetric Properties of the pyr-SWCNTs/ (C60Fc-Pd) Film. The complex-plane impedance (Nyquist) plots, i.e., the real vs imaginary component of impedance (Z′ vs

respectively.42 The most intense band at 336.4 eV would then correspond to a palladium moiety of the oxidation state intermediate between 0 and +2, indicating some interactions between the Pd and C60 moieties42,43 in the C60Fc-Pd polymer matrix. For the pyr-SWCNTs/(C60Fc-Pd) film, the EB values of the palladium 3d5/2 state (Figure 7a′) are equal to 335.0, 336.5, and 338.0 eV with the relative intensity ratio of 0.2:8:1, respectively. The energy difference between the 3d5/2 and 3d3/2 state of the spin−orbit splitting was 5.3 eV. Similarly to the bands for the C60Fc-Pd film (Figure 7a), the bands at 335.0 and 338.0 eV for pyr-SWCNTs/(C60Fc-Pd) can be ascribed to Pd0 and Pd2+, respectively, while the most intense one at 336.5 eV to the palladium moiety, bound in the C60Fc-Pd chain, with an oxidation state intermediate between 0 and +2. The Fe 2p detailed spectra for the C60Fc-Pd and pyrSWCNTs/(C60Fc-Pd) film are shown in Figures 7b and 7b′, respectively. One pair of bands with EB of the 2p3/2 state at 708.0 and 707.9 eV and a spin−orbit splitting of the 2p1/2 states of 12.8 eV for the polymer and composite film, respectively, corresponds to the covalently bound iron atom in the ferrocene moiety.44 2003



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Figure 8. Complex-plane impedance plots for the pyr-SWCNTs/(C60Fc-Pd) film-coated 4 mm diameter Au disk electrode in the 0.1 M (TBA)ClO4−acetonitrile solution, for constant potential in the range of (a) 0 to −1.2 V and (b) 0 to 0.60 V. The insets show the Z″ vs Z′ curves only for high-frequency ranges. (c) Dependence of pseudocapacitance (curve 1) and current (curve 2) on potential for the Au electrode coated with the pyr-SWCNTs/(C60Fc-Pd) film.

the Z″ vs Z″ curves for potentials where the composite film is conducting surpass those for potentials where the film is insulating. This behavior can be associated with the change of the film morphology46 caused by the ingress of counterions to the film accompanying the electrode processes. Low-frequency pseudocapacitance (redox capacitance), CL, is different for different potentials, for the electrode coated with the polytetracyanoquinodimethene film.46 From the slope of our Z″ vs ω−1 curves, the CL values were determined herein (curve 1 in Figure 8c). As expected, potentials of the highest values of CL correspond to potentials of electroreduction of the C60 moiety or electro-oxidation of the Fc moiety (curve 2 in Figure 8c). 3.5. Galvanostatic Charging and Discharging of a Pair of Identical pyr-SWCNTs/(C60Fc-Pd) Film Electrodes. Two 4 mm diameter Au disk electrodes, each coated with the identical pyr-SWCNTs/C60Fc-Pd film, were immersed, ∼15 mm apart, in a blank deaerated 0.1 M (TBA)ClO4−acetonitrile solution (see Figure S3) to yield a laboratory model of a simple symmetrical charge storage device. The electrodes were galvanostatically charged and then discharged by switching constant current density from +0.72 to −0.72 mA cm−2. The corresponding voltage increased with time to the predefined limit and then decreased, respectively (Figure 9).

Z″) plots, for the electrode coated with the pyr-SWCNTs/ (C60Fc-Pd) film immersed in the 0.1 M (TBA)ClO4− acetonitrile solution, recorded at different frequencies and for different constant potentials, are shown in Figure 8. Both for the potential range of 0 to −1.20 V (Figure 8a) and that of 0 to 0.60 V (Figure 8b) the Z″ vs Z′ curves show a capacitor-like behavior with a small diffusion limitation (insets in Figures 8a and 8b). The finite slope of the Z″ vs Z′ curves is characteristic of a porous film present on the electrode surface.46 Moreover, there is a very short or even no Warburg region for the rough surface. The observed herein lack or poorly developed semicircles in the high-frequency region for potentials of electrochemical reactions indicates that the charge transfer associated with the electroreduction of the C60 moiety or electro-oxidation of the Fc moiety within the composite film is very fast compared to the mass transfer.47 In consequence, the charge transfer resistance is so low that the accuracy of its determination is insufficiently low. Moreover, a shift of the Z″ vs Z′ curves to lower values of the real part of impedance was more pronounced the more negative potential was applied (insets in Figures 8a and 8b), suggesting a decrease of the internal resistance of the composite film. Further, the slope of the Z″ vs Z′ curves in the low-frequency range is different for different potentials. That is, slopes of the low-frequency parts of 2004



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For the low voltage limit of +0.6 and −0.6 V, characteristic of the sole capacitive behavior, the capacity was nearly perfectly stable with respect to multiple (1000 cycles) charging− discharging and then opposite polarity charging and discharging indicating even a slight (5%) increase (curve 1 in Figure S6). For the high voltage limit of +1.2 and −1.2 V allowing for the mixed capacitive and faradaic behavior, however, the capacity, after some activation leading to initial ∼15% capacity increase, decreased by ∼18% after 1000 cycles (curve 2 in Figure S6), indicating that the processes is not fully reversible. We observed a similar behavior for the electrode coated with the pyrSWCNTs/C60O film.9 The present results are consistent with the behavior of both the resonant frequency (curve 2′ in Figure 2) and dynamic resistance changes (curve 3″ in Figure 2). That is, frequency decreased while resistance increased in subsequent CV cycles, indicating that the total mass of the film increased. Moreover, the irreversible changes of dynamic resistance may also suggest some changes in the composite structure due to multicyclic alternate positive and negative charging of the film. 3.6. Power of the Two Identical pyr-SWCNTs/(C60FcPd) Film Electrode Device. The average power of the device was determined for different discharge currents (Figure S7). First, the electrodes were charged with the current density of 40 μA cm−2 to reach either low (0.70 V) or high (1.40 V) voltage. Then, they were shorted using the 5, 10, 20, 30, or 40 kΩ resistor, and the voltage decay with the discharge time was recorded. From this decay, the average power was determined using eq 7.

Figure 9. Voltage vs time curves of charging at +0.72 mA cm−2 and discharging at −0.72 mA cm−2 for the pyr-SWCNTs/(C60Fc-Pd) film coated 4 mm Au disk electrodes in a two identical electrode system, in the 0.1 M (TBA)ClO4−acetonitrile solution for the voltage limit of (1) 0.15, (2) 0.35, (3) 0.45, (4) 0.65, (5) 0.75, (6) 2.5, and (7) 2.7 V.

The shape of the voltage−time transients of charging and discharging was governed by the electrochemical processes proceeding. That is, the voltage linearly increased with time (slope a of curve 6 in Figure 9) for a purely capacitive electrode behavior. However, the absolute value of the slope of this dependence was lower for the voltage limits encompassing the range of mixed capacitive and faradaic behavior (slope b of curve 6 in Figure 9). The voltage−time transients were linear only for the narrow voltage limit of ≤0.6 V (curves 1−4 in Figure 9), i.e., in the absence of faradaic processes. Deviations from linearity of these transients for higher voltages (curves 5− 7 in Figure 9) are ascribed to faradaic processes of the fullerene (C60−/C60) and ferrocene (Fc/Fc+) moieties of the C60Fc-Pd polymer, respectively. Application of excessive voltage, i.e., that encompassing the second (C602−/C60−) and third (C603−/ C602−) process (curve 1″ in Figure 2b), was avoided because of the film instability with respect to products of these processes.35 Advantageously, polarity of the pyr-SWCNTs/(C60Fc-Pd) film coated electrodes can safely be made opposite by charging them with the current flowing in the opposite direction (Figure S4). This feature was exploited for determination of the capacitance stability with respect to repetitive alternate galvanostatic +0.72 and −0.72 mA cm−2 charging and discharging and then opposite polarity charging and discharging, respectively, by recording voltage−time transients (Figure S5). This experiment allowed determining capacitance from the current and time required to discharge the electrodes (Figure S6). In one experiment, the voltage limit was set low, i.e., either at +0.6 or −0.6 V, allowing for the double layer charging and discharging. Apparently, voltage−time transients were linear and repeatability was nearly perfect (curves 1 and 1′ in Figure S5). In another experiment, this voltage limit was set high extending to +1.2 and −1.2 V. The positive polarity charging allowed then for the Fc/Fc+ and C60/C60− electrode process to proceed on WE1 and WE2, respectively, while that opposite for the C60/C60− and Fc/Fc+ process on WE1 and WE2, respectively. Under these conditions, the capacity repeatability was poorer (curves 2 and 2′ in Figure S5), presumably, due to some irreversible transformation of the composite.

P=

1 R

∫t

1

t2

U 2(t ) dt

(7)

In this equation, U is the voltage decaying from time t1 to t2 due to discharging of the device with the resistor of the resistance R. Details of this determination are given in Figure S8. The average power of the device charged to the high voltage was ∼7.3 times higher than that charged to the low voltage (Figure S7). The former charging allows for occurring the faradaic Fc/Fc+ and C60−/C60 processes of the pyr-SWCNTs/ (C60Fc-Pd) film on each electrode, thus significantly increasing the amount of the charge stored. According to the PM results, this charging caused substantial swelling of the film, making it more permeable to counterions. In consequence, the ion transport through the film was faster making the power of the device higher. However, charging of the composite to the higher voltage decreases its lifetime. The average power is slightly lower the higher is the discharge current (lower resistance) for both the low and high charging voltage (curves 1 and 2, respectively, in Figure S7), most likely resulting from a finite discharge rate due to the electrochemical processes occurring.

4. CONCLUSIONS A simple two-step procedure of the pyr-SWCNTs/(C60Fc-Pd) film preparation was developed. It consisted of (i) electrophoretic deposition of a film of the pyr-SWCNTs bundles on a gold electrode followed by (ii) one-electron potentiodynamic electroreduction of C60Fc in the presence of PdII(ac)2, which resulted in wrapping these bundles with ∼45 nm diameter globules of C60Fc-Pd. Interestingly, this polymer deposited under the same potentiodynamic conditions on the bare electrode formed globules, which were in diameter twice of those deposited on the nanotube bundles. The pyr-SWCNTs/ 2005



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(C60Fc-Pd) film was electroactive both at positive and negative potentials due to the electrode processes of its ferrocene (Fc/ Fc+) and fullerene (C60−/C60) moiety, respectively. For the negative potential range, the highest value of specific capacitance of the pyr-SWCNTs/(C60Fc-Pd) film (120 F g−1) was higher than that of the C60Fc-Pd film (100 F g−1). For the positive potential range, similarly, the highest Cs value for the pyr-SWCNTs/(C60Fc-Pd) film (∼300 F g−1) was also higher than that for the C60Fc-Pd film (260 F g−1). Alternate galvanostatic +0.72 and −0.72 mA cm−2 charging− discharging and then opposite polarity charging−discharging of the device composed of two identical pyr-SWCNTs/C60Fc-Pd film coated electrodes allowed for determination of the capacitance stability with respect to the number of current switching. That is, multicyclic voltage−time curves recorded for 1000 such cycles were linear and the capacity was nearly perfectly stable and even slightly (5%) increased for the low voltage limit, i.e., that characteristic of the absence of any faradaic processes. For the high voltage limit encompassing the faradaic C60/C60− and Fc/Fc+ process, voltage−time transients were nonlinear and the capacity decreased by 18%. Because of the faradaic processes occurring, the average power of the device was ∼7.3 times higher for the latter. In summary, the diverse SWCNTs/RCP composite plays a dual role in the symmetric two-electrode energy storage device fabricated and tested herein. This role depends upon the voltage limits of the current applied. That is, the composite is a dielectric material for an electric double-layer supercapacitor and a reversible redox material for a faradaic pseudocapacitor in the narrow (≤0.6 V) and broad (∼1.2 V) voltage limit, respectively.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S8. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Fax +48 22 632 52 76, Tel +48 22 343 3055 (P.P.); e-mail [email protected], Fax +1 940-565-4318, Tel +1 940-369-8832 (F.D.); e-mail wkutner@ ichf.edu.pl, Fax +48 22 632 52 76, Tel +48 22 343 32 17 (W.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was financially supported by the European Regional Development Fund (ERDF, POIG.01.01.02-00-008/ 08 2007−2013 to W.K.), the Foundation for Polish Science (MPD/2009/1/styp15) within the International PhD Projects Programme, cofinanced from European Regional Development Fund within Innovative Economy Operational Programme “Grants for Innovations” to W.K., Polish National Science Centre (NCN, Grant No. 2011/01/N/ST5/05615 to I.O.), European Union 7.FP under grant REGPOT-CT-2011285949-NOBLESSE to P.P., and the National Science Foundation (Grant CHE-1110942 to F.D.). 2006



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