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Formation and electrochemical properties of composites of the C60–Pd polymer and multi-wall carbon nanotubes
Electrochimica Acta 54 (2009) 5621–5628
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Formation and electrochemical properties of composites of the C60 –Pd polymer and multi-wall carbon nanotubes c ˙ Emilia Grodzka a , Piotr Pieta b , Piotr Dłuzewski , Włodzimierz Kutner b,d,∗∗ , Krzysztof Winkler a,∗ a
Institute of Chemistry, University of Białystok, Hurtowa 1, 15-399 Białystok, Poland Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Institute of Physics, Polish Academy of Sciences, Aleja Lotników 32/46, 02-668 Warsaw, Poland d Faculty of Mathematics and Natural Sciences, School of Science, Cardinal Stefan Wyszy´ nski University in Warsaw, Dewajtis 5, 01-815 Warsaw, Poland b c
a r t i c l e
i n f o
Article history: Received 26 January 2009 Received in revised form 11 April 2009 Accepted 25 April 2009 Available online 3 May 2009 Keywords: Multi-wall carbon nanotubes Fullerene polymers Conducting composites Electropolymerization Capacitance
a b s t r a c t Preparation and electrochemical properties of a novel type of the composite made of multi-wall carbon nanotubes (MWCNTs) and two-component polymer of palladium and C60 (C60 –Pd) were investigated using cyclic voltammetry, electrochemical impedance spectroscopy, and piezoelectric microgravimetry. A composite film was prepared by electrochemical deposition of C60 –Pd on the layer of MWCNTs immobilized on the electrode surface. The polymer forms islands of shells on the carbon multi-wall core. This composite is electrochemically active in the negative potential range due to the electroreduction of the fullerene moiety. In this potential range, specific pseudo-capacitance of the film of the MWCNT/C60 –Pd composite is 425 F g−1 in the acetonitrile solution of tetra(n-butyl)ammonium perchlorate. The presence of MWCNTs makes the composite conductive also at potentials less negative than potentials of the C60 electroreduction. The double-layer specific capacitance of this film is close to 15 F g−1 . © 2009 Elsevier Ltd. All rights reserved.
1. Introduction The unique properties of carbon nanotubes (CNTs) make them very attractive for applications in many fields of science and technology. That is, they can be utilized in nanoelectronics [1,2], optoelectronics [3–5], for energy storage [6–8], and to build electrochemical actuators [9,10]. CNTs have been also examined as a new hydrogen storage materials for hydrogen/air fuel cells [11–13]. The CNTs properties, such as high specific surface area, relatively high electrical conductivity, and pronounced chemical and mechanical stability, make them promising as active materials for electrochemical capacitors [14–17]. In these capacitors, ions of an electrolyte are adsorbed on charged electrodes producing a Helmholtz layer. Capacitance properties of CNTs depend upon the number of their graphene walls (single-, double-, or multiwall), the way of their surface functionalization, purity, nature of the electrode material, and composition of the electrolyte solution. Specific capacitance, Cs , of unmodified CNTs is relatively low and ranges from 2.5 to 40 F g−1 for single-wall carbon nanotubes (SWCNTs) [18–21] and from ca. 3 to 70 F g−1 for multi-wall carbon nanotubes (MWCNTs) [19–21]. The capacitance of the SWCNTs
∗ Corresponding author. Tel.: +48 85 745 7802; fax: +48 85 747 0113. ∗∗ Corresponding author. Tel.: +48 22 343 32 17; fax: +4822 343 33 33. E-mail addresses: [email protected] (W. Kutner), [email protected] (K. Winkler). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.04.066
sheets is close to 30 F g−1 [18]. Li and co-workers [19] obtained Cs of 17 F g−1 for similar materials. They reported on almost three times lower specific capacitance for MWCNTs [19]. An opposite effect was observed by Pumera [20]. The author found much higher capacitance (∼70 F g−1 ) for the double-wall carbon nanotubes (DWCNTs) film than that of ca. 30 F g−1 for SWCNTs [18]. Higher specific capacitance of the MWCNT films in comparison to that of the SWCNT films was also reported by Frackowiak et al. [21]. Special treatment of nanotubes results in the increase of Cs of the carbon nanomaterial. For example, heat treatment of CNTs increases their specific capacitance to 180 F g−1 [22]. MWCNTs oxidized by nitric acid exhibit the specific capacitance values of 130 F g−1 [23] and even 145 F g−1 [24]. However, the capacitance properties of these functionalized materials are not stable due to the faradaic reactions of surface groups [23]. Recently, coating the CNTs surface by conducting polymers to produce nanoscale composites has received particular attention due to the numerous envisioned applications. The most common procedure for preparation of these materials, used in case of conducting polymers soluble in organic solvents, involves coating the electrode surface with a drop of solution containing a polymer and suspended nanotubes, and letting the solvent evaporate [25–37]. With this procedure the poly(3hexylthiophene)/MWCNT [25], poly(3-octylthio-phene)/SWCNT [26,27], poly(1,4-phenylenevinylene)/MWCNT [28], and copoly(phenylene-vinylene)/SWCNT [29–34] composites were prepared. In the case of insoluble in common organic solvents
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polymers, such as polypyrrole and polyaniline, composites of the polymers and CNTs are prepared by chemical [35–40] or electrochemical [35,41–49] polymerization of a solution containing suspended nanotubes and dissolved monomers. However, the amount of CNTs in the resulting composite is relatively low. Another preparation route of the CNT/polymer composites involves electropolymerization of the monomer at the electrode surface coated with a CNTs film. In this case, the composite obtained contains much higher amount of CNTs. Moreover, derivatized with charged addends CNTs can be also incorporated into electrochemically formed polymeric matrix as ionic dopants [50]. Composite materials that couple conducting polymers and CNTs reveal a synergistic effect with respect to properties of individual components [46]. That is, the presence of CNTs can improve mechanical properties of the polymer. Moreover, it can change electrical conductivity of the polymer, particularly in the potential range of high resistance of the polymer, and its capacitance. Capacitance properties of the CNT/polymer composites have intensively been studied [24,35,51]. Capacitance values of the electrodes coated with composites of different nanotubular materials and conducting polymers is significantly higher than that of the pristine nanotubes. For instance, specific capacitance of the MWCNT/polyacrylonitrile composite produced by pyrolysis has been determined as 100 F g−1 [51]. Furthermore, Cs is in the range of 120–160 F g−1 for composites of MWCNTs and polypyrrole, which were prepared under either electrochemical or chemical conditions [34]. Recently, amazingly high value of specific capacitance, i.e., 500 and even 650 F g−1 , was reported for the MWCNT/polypyrrole and MWCNT/polyaniline composite, respectively [35]. So far, studies of the CNT/polymer composites were limited to conducting polymers revealing p-doped properties. These composites are electroactive at positive potentials. Recently, we have focused on the development of novel electroactive polymers based on fullerenes [52–56]. They are electrochemically formed from fullerene epoxides, C60 O and C70 O [52,53]. Moreover, films of these polymers can be grown at the electrode surface during electroreduction of a solution containing the fullerene and a transition metal complex [54–56]. In these materials, metal atoms or metal complexes are coordinated directly to the fullerene in an 2 fashion, as shown below
These films are electrochemically active at negative potentials revealing n-doped behavior. Moreover, the capacitance performance of the C60 –Pd polymer is very promising [57]. In the present paper, we report on the formation and electrochemical properties of a composite film containing MWCNTs and C60 –Pd, prepared by electrochemical polymerization. The effect of the composition of synthesized material on its electrochemical properties was investigated herein. Moreover, the capacitance properties of the composite film were studied and compared to those of the pristine MWCNT film and the C60 –Pd film. Special attention was also paid to study of the mechanical properties of the films.
2. Experimental Multi-wall carbon nanotubes, (140 ± 30) nm in diameter and (7 ± 2) m in length, (>90% purity), were procured from the M.E.R. Corp. (Tucson, AZ, USA). They were produced by chemical vapor deposition. Palladium(II) acetate, Pd(ac)2 , from Aldrich Chemical Co. and C60 from the M.E.R. Corp. (Tucson, Arizona, USA) were used as received. The supporting electrolyte, tetra(n-butyl)ammonium perchlorate, (TBA)ClO4 , was used as received from Fluka. Tetra(methyl)ammonium chloride, (TMA)Cl, tetra(ethyl)ammonium chloride, (TEA)Cl, tetra(nbutyl)ammonium chloride, (TBA)Cl, were used as received from Aldrich. Acetonitrile (99.9%) and toluene (anhydrous, 99.8%) received from Aldrich were used without additional purification. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) experiments were performed on an AUTOLAB (Utrecht, The Netherlands) computerized electrochemistry system equipped with the PGSTAT 12 potentiostat and FRA response analyzer expansion cards with a three-electrode cell. The AUTOLAB system was controlled with the GPES 4.9 software of the same manufacturer. A 1.5-mm diameter gold disk of Bioanalytical Systems, Inc. (West Lafayette, IN, USA) was used as the working electrode. Prior to the experiments, the electrode was polished with a fine Carborundum paper, and then with a 0.5-m alumina slurry. Subsequently, the electrode was sonicated in water to remove traces of alumina from the gold surface, rinsed with water, and dried. For imaging by scanning electron microscopy, SEM, the studied films were electrochemically deposited on an Au foil of Goodfellow Metals, Ltd. (Cambridge, UK). Before use, this foil was annealed in a Bunsen flame. A silver wire immersed in an acetonitrile solution of 0.01 M AgClO4 and 0.09 M (TBA)ClO4 that was separated from the substrate electrode by a ceramic frit of Bioanalytical Systems Inc. served as the reference electrode. This reference electrode, the ferrocene/ferrocinium formal redox potential in the 0.1 M (TBA)ClO4 acetonitrile solution was −19 mV. The counter electrode was a platinum tab with an area of ca. 0.5 cm2 . Simultaneous CV and piezoelectric microgravimetry (PM) experiments were performed by using the EP-21 potentiostat of Elpan (Lubawa, Poland), connected to the EQCM 5710 electrochemical quartz crystal microbalance of the Institute of Physical Chemistry (Warsaw, Poland), which was controlled with the EQCM 5710-S2 software. This microbalance allowed for simultaneous measurements of changes of current, resonant frequency, and dynamic resistance of a 10-MHz, At-cut, plano-plano quartz crystal resonator (Institute of Tele- and Radiocommunication, Warsaw, Poland) during potential cycling. Projected area of this electrode and two contacting radial strips was 0.24 cm2 . The sensitivity of the mass measurement, calculated from the Saurbrey equation, was 4.2 ng Hz−1 cm−2 . The studied films were imaged by secondary electron scanning electron microscopy (SEM) with the use of the S-3000N scanning electron microscope of Hitachi (Tokyo, Japan). For determination of elemental composition of crystals, energy dispersive X-ray fluorescence (EDX) measurements were performed using EDAX software (Mahwah, NJ, USA). Transmission electron microscopy (TEM) investigations were carried out on the JOEL2000EX microscope. Films of MWCNTs were deposited on the electrode surface by drop coating. That is, a 40-l drop of sonically dispersed in dichloromethane MWCNTs was dispensed onto the electrode surface. After solvent evaporation, the electrode remained coated with a relatively porous film of MWCNTs. The C60 –Pd film was prepared by electroreduction of an acetonitrile:toluene (1:4, v:v) solution that contained the fullerene and Pd(ac)2 as well as the supporting electrolyte, 0.10 M (TBA)ClO4 ,
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Fig. 1. Multi-scan cyclic voltammogram for (1) the C60 –Pd film deposited on the 1.5mm diameter Au disk electrode in 0.10 M (TBA)ClO4 , in acetonitrile. The potential sweep rate was 100 mV s−1 . The C60 –Pd films were grown under CV conditions in 0.27 mM C60 and 3.56 mM Pd(ac)2 , 0.10 M (TBA)ClO4 , in acetonitrile:toluene (1:4, v:v), and (2) for the MWCNTs film deposited on the 1.5-mm diameter Au disk electrode in a blank acetonitrile solution of 0.10 M (TBA)ClO4 . The potential sweep rate was 100 mV s−1 . The mass of MWCNTs deposited was 4.25 g. Direction of the cathodic current, ic , flow is indicated with the vertical arrow.
under CV conditions. The potential sweep rate was 100 mV s−1 . Mass of the deposited C60 –Pd polymer film was determined using PM at EQCM [57,58]. 3. Results and discussion
Fig. 2. (a) Multi-scan cyclic voltammograms for 0.27 mM C60 , 3.56 mM Pd(ac)2 , and 0.1 M (TBA)ClO4 , in acetonitrile:toluene (1:4, v:v) recorded at the 1.5-mm diameter Au disk electrode coated with the 4.25 g MWCNTs film. The potential sweep rate was 100 mV s−1 . (b) Cyclic voltammograms for the MWCNT/C60 –Pd composite film containing 4.25 g MWCNTs and 0.40 g C60 –Pd (1), and 0.90 g C60 –Pd (2) in 0.10 M (TBA)ClO4 , in acetonitrile. The potential sweep rate was 100 mV s−1 . The condition of C60 –Pd deposition the same as in Fig. 1.
3.1. Voltammetric study of composites of C60 –Pd and MWCNTs Fig. 1 shows a CV behavior of the gold electrode coated with the film of C60 –Pd (Curve 1) and MWCNTs (Curve 2). The C60 –Pd film was electrochemically active at negative potentials due to electroreduction of the C60 moiety. The voltammograms of MWCNTs film exhibit typical capacitance behavior. Apparently, the capacitance current depends on the amount of nanotubes deposited on the electrode surface. Specific capacitance, Cs , was calculated using the following equation:
Cs =
ic dt
Em
(1)
where ic is the capacitance current, E is the potential range of integration, and m is the mass of the material deposited on the electrode surface. The specific capacitance of the MWCNTs film was determined as 15.5 F g−1 . This value is in agreement with that of the MWCNT films reported in literature [19–21]. The C60 –Pd polymer was deposited on a porous MWCNTs layer by electrochemical polymerization under CV conditions. Fig. 2a shows a multi-scan CV curve for an acetonitrile:toluene (1:4, v:v) solution of Pd(ac)2 , C60 , and (TBA)ClO4 recorded at the gold electrode that was coated with the MWCNTs film. At negative potentials, the electroreduction results in formation of the C60 –Pd polymer deposit on the nanotubes. Due to the presence of the MWCNTs film, this electropolymerization is less reversible than that at the bare gold electrode [57]. The SEM image of the pristine MWCNTs film and
that of the film of MWCNTs bearing the C60 –Pd polymer is shown in Fig. 3a and b, respectively. Clearly, the nanotubes got shrouded with a non-uniform polymer coat. The energy dispersive spectrometry measurements were conducted to examine the composition of the MWCNTs and MWCNT/C60 –Pd films (Fig. 3c). In both cases, a strong carbon signal at 0.39 keV dominates the spectrum. Additionally, weak peaks at 1.85 and 2.84 keV corresponding to palladium are observed for the composite containing MWCNTs and C60 –Pd (Curve 2 in Fig. 3c). The SEM imaging with EDX elemental mapping of the MWCNT/C60 –Pd film sample exhibits uniform distribution of palladium in the film. It also indicates that this film is free of palladium nanocrystals and that palladium atoms are involved in bonding of the fullerene cages in the polymer network. The composite film of MWCNTs and C60 –Pd as well as the MWCNT film alone were examined with TEM (Fig. 4). The TEM image shown in Fig. 4b clearly indicate that in the case of MWCNT/C60 –Pd composite, surface of the nanotube is coated with C60 –Pd. The amount of the polymer on the nanotube depends on the time of polymer deposition. The gold electrode, coated with the film of the MWCNT/C60 –Pd composite, was transferred to a blank acetonitrile solution of the supporting electrolyte and CV curves were recorded. These curves for two different amounts of the polymer deposited on the nanotubes are shown in Fig. 2b. Voltammograms for the MWCNT/C60 –Pd composite are stable with respect to multi-scan CV
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Fig. 4. TEM images of (a) MWCNT and (b) MWCNT coated with the C60 –Pd.
Fig. 3. SEM images of (a) the film of pristine MWCNTs and (b) the film of the composite of MWCNTs and the C60 –Pd polymer. (c) EDX spectra of (1) MWCNTs and (2) MWCNT/C60 –Pd composite deposited on gold foil.
cycling between 0 and −1.20 V. For potentials more positive than ca. −0.50 V, the composite exhibits behavior typical for a double-layer capacitor. Voltammograms recorded for this potential range, for the electrode coated only with MWCNTs, show almost pseudorectangular cathodic and anodic CV profiles that are nearly mirror imaged with respect to the potential axis (Curve 1 in Fig. 5a). In this potential range, the capacitance current is almost unaffected by the presence of the C60 –Pd polymer. However, the resistance of the C60 –Pd polymer is high at potentials less negative than that of electroreduction of the fullerene moiety. Therefore, the current increase after reversing direction of the potential sweep is much slower for the MWCNT/C60 –Pd composite film than that for the pristine MWCNT film. A negative deviation from the pseudorectangular profile is observed for the composite film (Curves 2 and 3 in Fig. 5a) in contrast to that for the MWCNTs film (Curve 1 in Fig. 5a). The extent of this deviation, which is related to the resistance of the film, is larger the higher is the amount of the deposited C60 –Pd polymer.
At potentials more negative than −0.50 V, the fullerene moiety is electroreduced. At these potentials, the film of the MWCNT/C60 –Pd composite behaves like a typical pseudcapacitor. Moreover, multiscan CV curve for this film is stable. Potential can be cycled between −0.80 and −1.50 V without noticeable change in the shape of the voltammogram. Cyclic voltammograms for the potential range corresponding to the C60 moiety electroreduction, for different potential sweep rates, are shown in Fig. 5b. For sweep rates lower than ca. 50 mV s−1 , the voltammograms show almost pseudorectangular cathodic and anodic profiles, a characteristic behavior for an ideal capacitor. However, for higher sweep rates the CV curve deviates from the ideal rectangular shape. This effect is related to the relatively high resistance of the film of the MWCNT/C60 –Pd composite and, therefore, slower current response to the potential sweep. For comparison, films of the pristine C60 –Pd polymer are much more conductive and, therefore, the sweep rate range of the ideal capacitance behavior is much wider in this case [57]. The current at negative potentials includes capacitive and faradaic contributions. The magnitude of both components is predicted to linearly depend upon the potential sweep rate. Therefore, the linear dependence of the total current on the potential sweep rate is expected. This dependence was obtained, indeed (Fig. 5c). Specific capacitance of the MWCNT/C60 –Pd film can be calculated using the following equation: Cs =
ipc Vm
(2)
where ipc is the pseudo-capacitance current and V is the potential sweep rate. From the slope of the i–V dependence, for sweep rates lower than 100 mV s−1 , the specific capacitance of 75 F g−1 was determined for the MWCNT/C60 –Pd composite in the potential range of the fullerene electroreduction. This pseudo-capacitance current mainly results from electroreduction of the C60 –Pd polymer. Specific capacitance as high as 425 F g−1 was calculated based on the mass of the C60 –Pd polymer deposited on the MWCNT film. This value is almost twice as high as that of the C60 –Pd film deposited on the bare surface of the gold electrode. Presumably, this higher capacitance of the C60 –Pd polymer in the MWCNT/C60 –Pd composite is due to the open porous structure of the composite. The effect of composition of the MWCNT/C60 –Pd composite on its capacitive behavior was also investigated. Fig. 6a shows CV behavior of the MWCNT/C60 –Pd films for different ratios of the mass of MWCNTs and the C60 –Pd polymer (mMWCNT /mC60 –Pd ). In order to prepare these composites, different amounts of MWCNTs were deposited on the Au electrode surface by drop coating.
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Fig. 5. (a) Cyclic voltammograms for the film of (1) MWCNT as well as (2 and 3) the MWCNT/C60 –Pd films in 0.10 M (TBA)ClO4 , in acetonitrile, in the potential range 0 to −0.60 V. The potential sweep rate was 100 mV s−1 . The C60 –Pd films were grown under CV conditions for (2) 10 and (3) 20 CV cycles under conditions described in Fig 1. (b) Cyclic voltammograms of the MWCNT/C60 –Pd film containing 4.25 g MWCNTs and 0.90 g C60 –Pd in 0.10 M (TBA)ClO4 , in acetonitrile. The potential sweep rate was (1) 10, (2) 20, (3) 50, and (4) 100 mV s−1 . (c) Dependence of the current at–0.95 V on the potential sweep rate for the MWCNT/C60 Pd film containing 4.25 g MWCNTs and 0.90 g C60 –Pd.
Next, the C60 –Pd polymer was grown under CV conditions for 10 cycles in the potential range −0.20 to −1.05 V. The capacitance current is lower at potentials less negative than −0.60 V and the pseudo-capacitance current is pronounced in the potential range characteristic of C60 electroreduction. The specific capacitance val-
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Fig. 6. (a) Cyclic voltammograms for the MWCNT/C60 –Pd films in 0.10 M (TBA)ClO4 , in acetonitrile. Film composition was (1) 4.25 g MWCNTs and 0.90 g C60 –Pd, (2) 2.12 g MWCNTs and 0.75 g C60 –Pd, and (3) 1.05 g MWCNTs and 0.66 g C60 –Pd. The potential sweep rate was 100 mV s−1 . The condition of C60 –Pd deposition the same as in Fig. 1. (b) Dependence of the current at −0.95 V on the potential sweep rate for the MWCNT/C60 –Pd film. Film composition was (1) 4.25 g MWCNTs and 0.90 g C60 –Pd, (2) 2.12 g MWCNTs and 0.75 g C60 –Pd, as well as (3) 1.05 g MWCNTs and 0.66 g C60 –Pd.
ues, calculated for all three compositions of the MWCNT/C60 –Pd composite from slopes of the i–V straight lines obtained (Fig. 6b), are summarized in Table 1. Apparently, the larger the porosity of the composite, due to the higher content of MWCNTs, the higher is the value of Cs for the higher mMWCNT /mC60 –Pd ratio. 3.2. Electrochemical impedance spectroscopy investigations of the composite of C60 –Pd and MWCNTs Electrochemical impedance spectroscopy has also been employed to study redox properties of the MWCNT/C60 –Pd com-
Table 1 Specific capacitance for the MWCNT/C60 –Pd composites in 0.10 M (TBA)ClO4 , in acetonitrile, for different potentials. mMWCNT /mC60 –Pd in the composite
4.72 2.82 1.59 a
Specific capacitance (F g−1 ) At −0.40 V
At −1.10 V
Based on the MWCNT/C60 –Pd mass
Based on the MWCNT/C60 –Pd mass
Based on the C60 –Pd mass
12.5 11.3 12.0
75 73 91
425 285 236 200a
Specific capacitance for the C60 –Pd film from ref. [59].
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resistance related to the polymer electro-oxidation or electroreduction, ZW is the Warburg impedance related to the counter-ion transport accompanying the electro-oxidation or electroreduction of the polymer, and CL is the capacitance of the internal interface between the electrode surface and the solution inside the polymer micropores. The Warburg impedance is expressed by the following equation: ZW =
Fig. 7. (a) Complex-plane impedance plots for the MWCNT/C60 –Pd films (4.25 g MWCNTs and 0.90 g C60 –Pd) in 0.10 M (TBA)ClO4 , in acetonitrile, at (1) −0.30, (2) −0.825, (3) −0.90, (4) −1.00, and (5) −1.10 V. (b) Nyquist plots for the MWCNT/C60 –Pd films in 0.10 M (TBA)ClO4 , in acetonitrile, at −0.825 and −0.30 V (inset). Film composition was (1) 4.25 g MWCNTs and 0.90 g C60 –Pd, (2) 2.12 g MWCNTs and 0.75 g C60 –Pd, and (3) 1.05 g MWCNTs and 0.66 g C60 –Pd. Frequency was in the range of 10 kHz to 100 mHz. Solid curves represent simulated data. The condition of C60 –Pd deposition the same as in Fig. 1.
posite. Fig. 7 presents the imaginary part of impedance, Z , as a function of the real part of impedance, Z (Nyquist plots), for the electrode coated with the composite. The effect of potential (Fig. 7a) and composition of the MWCNT/C60 –Pd composite (Fig. 7b) on the impedance behavior has been investigated. The polymer/electrolyte interface can be represented by the equivalent circuit (Scheme 1) similar to that describing behavior of the C60 –Pd film [57]. In this circuit, R1 is the resistance of the electrolyte solution, Cdl is the double-layer capacitance of the external polymer/electrolyte interface, Rct is the charge-transfer
ω
(3)
where is resistivity of the film related to the ion transport, and ω is angular frequency. Fig. 7a shows EIS responses for the composite containing large amount of MWCNTs, namely 4.25 g MWCNTs and 0.9 g C60 –Pd. For potentials more positive than the potential of the C60 –Pd electroreduction, i.e., E > −0.50 V, the Z –Z response is dominated by the conductivity of MWCNTs of the composite as Rct related to the C60 –Pd electroreduction is very large. At potentials more negative than ca. −0.70 V, the C60 –Pd polymer is conductive. Therefore, at high frequencies, the impedance responses exhibit a semicircle behavior related to the C60 –Pd electroreduction. The depressed shape of this semicircle can be attributed to the perturbation in distribution of the charge transfer resistance of this electroreduction due to the porous surface of the composite. In the low frequency range, the impedance depends on the capacitance of the polymer. The imaginary vs. real part of impedance rapidly increases with the frequency decrease. The effect of non-uniform film thickness manifests itself by a positive deviation from linearity of the low frequency impedance. Shapes of experimental impedance plots for different potentials are consistent with the curves simulated for the selected equivalent circuit (Scheme 1). Solid curves represent simulated Z –Z responses. Some deviation of the simulated curves from the experimental data points at low frequencies is due to the non-uniform thickness of the composite and its porosity. Values of the equivalent circuit parameters, used to generate the simulated plots, are collected in Table 2. The Rct values corresponding to electroreduction of the fullerene moiety are lower the more negative is the potential applied. However, the Rct values determined for the MWCNT/C60 –Pd composite are significantly higher than those for the genuine C60 –Pd polymer reported earlier [57]. The CL capacitance is higher for the potential range of the film electroreduction. At negative potentials, CL reaches its maximum value corresponding to the specific capacitance of 320 F g−1 (calculated with respect to the C60 –Pd mass). This value is lower than that determined from the CV measurements (Table 1). However, CV currents at negative potentials include both the capacitance and faradaic contribution, which makes the total specific pseudo-capacitance higher.
Table 2 Electrochemical impedance parameters determined at different constant potentials applied to the 1.5-mm diameter Au electrode coated with the MWCNT/C60 –Pd composite film containing 4.25 g MWCNTs and 0.9 g C60 –Pd (mMWCNT /mC60 –Pd = 4.72) in 0.10 M (TBA)ClO4 . E (V)
R ()
Cdl (F)
Rct ()
CL (F)
( s−1/2 )
−0.30 −0.75 −0.825
342 265 270 253a 300b 261 265 270
0.52 0.70 0.73 0.41a 0.52b 0.88 1.24 1.76
– 3600 2290 90a – 1530 550 222
37 242 262 230a 132b 275 270 310
– 1.72 × 10−3 1.10 × 10−3 5.51 × 10−4 a 8.51 × 10−4 b 1.00 × 10−3 7.60 × 10−4 9.75 × 10−4
−0.90 −1.00 −1.10 Scheme 1. Equivalent electrical circuit representing behavior of the MWCNT/C60 –Pd film electrode in an electrolyte solution.
2 1/2
a b
2.12 g MWCNTs and 0.75 g C60 –Pd (mMWCNT /mC60 –Pd = 2.82). 1.05 g MWCNTs and 0.66 g C60 –Pd (mMWCNT /mC60 –Pd = 1.59).
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The impedance responses for composites of different mMWCNT –to–mC60 –Pd ratios are compared in Fig. 7b. In this experiment, mass of the C60 –Pd polymer in the composite was kept almost the same and the mass of MWCNTs was changed fourfold. Therefore, the observed effects are related to the change of the amount of MWCNTs in the composite. At negative potentials, the conductive C60 –Pd component participates in the charge transfer. The Rct value for the composite is higher the higher is the amount of MWCNTs in it. Apparently, the presence of MWCNTs hinders electroreduction of C60 –Pd. However, an opposite effect is seen at potentials less negative than the potential of the C60 –Pd electroreduction (Inset in Fig. 7b). The resistance of the film is lower the higher is the mMWCNT /mC60 –Pd ratio. A Z –Z response of the films containing small amounts of MWCNTs are dominated by the high Rct value of the oxidized form of the C60 –Pd polymer. This high Rct value for the composite is represented by a large semicircle (Inset in Fig. 7b). Since MWCNTs are responsible for charge transfer in the potential range less negative than potentials of C60 –Pd
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reduction, the semicircle region shrinks with the increase of the MWCNTs amount in the composite. 3.3. Simultaneous piezoelectric microgravimetry and cyclic voltammetry behavior of the C60 –Pd film and the MWCNT/C60 –Pd composite film Fig. 8 shows simultaneously recorded curves of the potential dependence of current, resonant frequency change, and dynamic resistance change of the quartz resonators, with their Au electrodes coated either by the C60 –Pd film or the MWCNT/C60 –Pd film in blank 0.1 M (TBA)ClO4 acetonitrile solutions. Those measurements provided information not only on electrochemical behavior of the electroactive film, deposited on the working electrode during its electroreduction and electrooxydation, but also enabled to follow viscosity changes of the film caused by ingress or egress of counterions. CV measurements revealed that each film is electrochemically active at potentials more negative than ca. −0.70 V. However, the presence of MWCNTs in the composite film results in higher currents, which are due to higher electrode area and, hence, specific capacitance (Curves 1 and 2 in Fig. 8). When the scanned potential during the negative excursion reaches the value of −0.70 V (Curves 1 and 2 in Fig. 8), the resonant frequency decreases for the polymer (Curve 3 in Fig. 8) while it increases for the composite (Curve 4 in Fig. 8). Interestingly, the behavior of the dynamic resistance is similar for both films (Curves 5 and 6 in Fig. 8). However, the resistance increase for the polymer film is ca. half that for the composite. For the polymer film, the frequency decreases due to the increase of the film mass. Most likely, this increase is associated with the TBA+ ingress into the film for − compensation of the negative charge of C60 generated electroreductively. During this ingress, the film resistance increases as a result of the visco-elasticity gain. For the composite film, the frequency increases during the negative potential scan, although the negative − charge of C60 is generated. This is presumably because the enter+ ing TBA counter-ion significantly changes the film viscosity. The contribution of this change predominates in the overall frequency change, which can be affected both by the change of the mass of the resonator and the change of viscosity of the film [58]. Despite these differences in performance, both films are stable under multiscan CV conditions in the potential range 0 to −1.20 V. That is, both frequency changes (Curves 3 and 4 in Fig. 8) and dynamic resistance changes (Curves 5 and 6 in Fig. 8) remain almost the same in consecutive potential cycles. 4. Conclusions
Fig. 8. Simultaneously recorded curves of (1 and 2) multi-scan cyclic voltammetry, (3 and 4) resonant frequency change vs. potential, and (5 and 6) dynamic resistance change vs. potential for (1, 3, and 5) the C60 –Pd film and (2, 4, and 6) the MWCNT/(C60 –Pd) film in 0.1 M (TBA)ClO4 , in acetonitrile. Potential sweep rate was 0.1 V s−1 . The condition of C60 –Pd deposition the same as in Fig. 1.
Most of composites of CNTs and conducting polymers studied so far involved p-doped polymers. Herein, we investigated a composite of MWCNTs and the C60 –Pd polymer, which exhibits ndoped properties. The MWCNT/C60 –Pd composite was deposited by electro-polymerization of C60 –Pd on a porous layer of MWCNTs. Due to relatively high conductivity of the nanotubes, the accessible potential window for the composite is wide. At negative potentials, the composite is electrochemically active (due to electroreduction of the C60 moiety) behaving as a typical redox capacitor. Pseudocapacitance of the composite is mainly related to electroreduction of the C60 –Pd component. However, the incorporation of the MWCNTs results in the capacitance increase due to the increase of the composite porosity. At potentials less negative than those of the C60 moiety electroreduction, CV behavior of MWCNT/C60 –Pd resembles that of a typical double-layer capacitor. The capacitance properties of the composite in this potential range are mainly governed by the amount of MWCNTs present.
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Electrochemical properties of MWCNT/C60 –Pd depend upon the composition of the composite. In the potential range of the C60 –Pd electroreduction, specific pseudo–capacitance is higher the higher is the nanotube content. For large amounts of the nanotubes, specific capacitance determined with respect to the mass of C60 –Pd reaches value as high as 425 F g−1 . Evidently, the MWCNT/C60 –Pd composite is a much better material for charge storage than the pristine C60 –Pd polymer. Moreover, the charge transfer resistance of the composite film increases with the increase of the mMWCNT /mC60 –Pd ratio. This resistance is relatively high for composites containing large amounts of MWCNTs. In this case, current relatively slowly responds to the potential sweep. Changes in visco-elastic properties, manifested by simultaneous changes in the resonant frequency and dynamic resistance during consecutive CV potential cycles, of the C60 –Pd differ from those of the MWCNT/C60 –Pd. That is, during the negative potential scan, changes of viscosity due to the TBA+ ingress into the composite film for compensation of the negative charge generated are twice as high as those for the polymer film. Despite these differences, however, both films are stable under multi-scan CV conditions in the potential range 0 to −1.20 V. Acknowledgments The authors acknowledge the Polish Ministry of Science and Higher Education (project No. N 204 374 733 to KW) and the European Regional Development Fund (ERDF, POIG.01.01.02-00-008/08 2007-2013 to WK) for a financial support. References [1] P.G. Collins, A. Zettl, H. Bando, A. Thess, R.E. Smalley, Science 278 (1998) 100. [2] Q.H. Wang, A.A. Setlur, J.M. Lauerhaas, J.Y. Dai, E.W. Seelig, R.P.H. Chang, Appl. Phys. Lett. 72 (1998) 2912. [3] S.A. Curran, P.M. Ajayan, W.J. Blau, D.L. Carroll, J.N. Coleman, A.B. Dalton, A.P. Davey, A. Drury, B. Mc Carthy, S. Maier, A. Strevens, Adv. Mater. 10 (1998) 1091. [4] W.B. Choi, D.S. Chung, J.H. Kang, H.Y. Kim, Y.W. Jin, I.T. Han, Y.H. Lee, J.E. Jung, N.S. Lee, G.S. Park, J.M. Kim, Appl. Phys. Lett. 75 (1999) 3129. [5] H. Ago, K. Petritsch, M.S.P. Shaffer, A.H. Windle, R.H. Friend, Adv. Mater. 11 (1999) 1281. [6] L. Diederich, E. Barborini, P. Piseri, A. Podesta, P. Milani, Appl. Phys. Lett. 75 (1999) 2662. [7] K.H. An, W.S. Kim, Y.S. Park, J.M. Moon, D.J. Bae, S.C. Lim, Y.S. Lee, Y.H. Lee, Adv. Funct. Mater. 11 (2001) 387. [8] C. Liu, A.J. Bard, F. Wudl, I. Weitz, J.R. Heath, Electrochem. Solid State Lett. 2 (1999) 5. [9] R.H. Baughman, C. Cui, A.A. Zakhidov, Z. Iqbal, J.N. Barisci, G.M. Spinks, G.G. Wallace, A. Mazzoldi, D. De Rossi, A. Rinzler, O. Jaschinski, S. Roth, M. Kertesz, Science 284 (1999) 1340. [10] G.M. Spinks, G.G. Wallace, R.H. Baughman, L. Dai, in: Y. Bar-Cohen (Ed.), Electroactive Polymer Actuators as Artificial Muscles, SPIE, Washington, DC, 2001, p. 223. [11] X. Qin, X.P. Gao, H. Liu, H.T. Yuan, W.L. Gong, D.Y. Song, Electrochem. Solid State Lett. 3 (2000) 532. [12] I. Lombardi, M. bestetti, C. Mazzocchia, P.L. Cavallotti, U. Ducati, Electrochem. Solid State Lett. 7 (2004) A115. [13] C. Nutzenadel, A. Zuttel, D. Chartouni, L. Schlapbach, Electrochem. Solid State Lett. 2 (1999) 30. [14] E. Frackowiak, F. Beguin, Carbon 40 (2002) 1775. [15] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845. [16] E. Frackowiak, Phys. Chem. Chem. Phys. 9 (2007) 1774. [17] D.N. Futaba, K. Hota, T. Yamada, T. Hiraoka, Y. Hayamizu, Y. Kakudate, O. Tanaike, H. Hatori, M. Yumura, S. Ijima, Nat. Mater. 5 (2006) 987.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Formation and electrochemical properties of composites of the C60 –Pd polymer and multi-wall carbon nanotubes c ˙ Emilia Grodzka a , Piotr Pieta b , Piotr Dłuzewski , Włodzimierz Kutner b,d,∗∗ , Krzysztof Winkler a,∗ a
Institute of Chemistry, University of Białystok, Hurtowa 1, 15-399 Białystok, Poland Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Institute of Physics, Polish Academy of Sciences, Aleja Lotników 32/46, 02-668 Warsaw, Poland d Faculty of Mathematics and Natural Sciences, School of Science, Cardinal Stefan Wyszy´ nski University in Warsaw, Dewajtis 5, 01-815 Warsaw, Poland b c
a r t i c l e
i n f o
Article history: Received 26 January 2009 Received in revised form 11 April 2009 Accepted 25 April 2009 Available online 3 May 2009 Keywords: Multi-wall carbon nanotubes Fullerene polymers Conducting composites Electropolymerization Capacitance
a b s t r a c t Preparation and electrochemical properties of a novel type of the composite made of multi-wall carbon nanotubes (MWCNTs) and two-component polymer of palladium and C60 (C60 –Pd) were investigated using cyclic voltammetry, electrochemical impedance spectroscopy, and piezoelectric microgravimetry. A composite film was prepared by electrochemical deposition of C60 –Pd on the layer of MWCNTs immobilized on the electrode surface. The polymer forms islands of shells on the carbon multi-wall core. This composite is electrochemically active in the negative potential range due to the electroreduction of the fullerene moiety. In this potential range, specific pseudo-capacitance of the film of the MWCNT/C60 –Pd composite is 425 F g−1 in the acetonitrile solution of tetra(n-butyl)ammonium perchlorate. The presence of MWCNTs makes the composite conductive also at potentials less negative than potentials of the C60 electroreduction. The double-layer specific capacitance of this film is close to 15 F g−1 . © 2009 Elsevier Ltd. All rights reserved.
1. Introduction The unique properties of carbon nanotubes (CNTs) make them very attractive for applications in many fields of science and technology. That is, they can be utilized in nanoelectronics [1,2], optoelectronics [3–5], for energy storage [6–8], and to build electrochemical actuators [9,10]. CNTs have been also examined as a new hydrogen storage materials for hydrogen/air fuel cells [11–13]. The CNTs properties, such as high specific surface area, relatively high electrical conductivity, and pronounced chemical and mechanical stability, make them promising as active materials for electrochemical capacitors [14–17]. In these capacitors, ions of an electrolyte are adsorbed on charged electrodes producing a Helmholtz layer. Capacitance properties of CNTs depend upon the number of their graphene walls (single-, double-, or multiwall), the way of their surface functionalization, purity, nature of the electrode material, and composition of the electrolyte solution. Specific capacitance, Cs , of unmodified CNTs is relatively low and ranges from 2.5 to 40 F g−1 for single-wall carbon nanotubes (SWCNTs) [18–21] and from ca. 3 to 70 F g−1 for multi-wall carbon nanotubes (MWCNTs) [19–21]. The capacitance of the SWCNTs
∗ Corresponding author. Tel.: +48 85 745 7802; fax: +48 85 747 0113. ∗∗ Corresponding author. Tel.: +48 22 343 32 17; fax: +4822 343 33 33. E-mail addresses: [email protected] (W. Kutner), [email protected] (K. Winkler). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.04.066
sheets is close to 30 F g−1 [18]. Li and co-workers [19] obtained Cs of 17 F g−1 for similar materials. They reported on almost three times lower specific capacitance for MWCNTs [19]. An opposite effect was observed by Pumera [20]. The author found much higher capacitance (∼70 F g−1 ) for the double-wall carbon nanotubes (DWCNTs) film than that of ca. 30 F g−1 for SWCNTs [18]. Higher specific capacitance of the MWCNT films in comparison to that of the SWCNT films was also reported by Frackowiak et al. [21]. Special treatment of nanotubes results in the increase of Cs of the carbon nanomaterial. For example, heat treatment of CNTs increases their specific capacitance to 180 F g−1 [22]. MWCNTs oxidized by nitric acid exhibit the specific capacitance values of 130 F g−1 [23] and even 145 F g−1 [24]. However, the capacitance properties of these functionalized materials are not stable due to the faradaic reactions of surface groups [23]. Recently, coating the CNTs surface by conducting polymers to produce nanoscale composites has received particular attention due to the numerous envisioned applications. The most common procedure for preparation of these materials, used in case of conducting polymers soluble in organic solvents, involves coating the electrode surface with a drop of solution containing a polymer and suspended nanotubes, and letting the solvent evaporate [25–37]. With this procedure the poly(3hexylthiophene)/MWCNT [25], poly(3-octylthio-phene)/SWCNT [26,27], poly(1,4-phenylenevinylene)/MWCNT [28], and copoly(phenylene-vinylene)/SWCNT [29–34] composites were prepared. In the case of insoluble in common organic solvents
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polymers, such as polypyrrole and polyaniline, composites of the polymers and CNTs are prepared by chemical [35–40] or electrochemical [35,41–49] polymerization of a solution containing suspended nanotubes and dissolved monomers. However, the amount of CNTs in the resulting composite is relatively low. Another preparation route of the CNT/polymer composites involves electropolymerization of the monomer at the electrode surface coated with a CNTs film. In this case, the composite obtained contains much higher amount of CNTs. Moreover, derivatized with charged addends CNTs can be also incorporated into electrochemically formed polymeric matrix as ionic dopants [50]. Composite materials that couple conducting polymers and CNTs reveal a synergistic effect with respect to properties of individual components [46]. That is, the presence of CNTs can improve mechanical properties of the polymer. Moreover, it can change electrical conductivity of the polymer, particularly in the potential range of high resistance of the polymer, and its capacitance. Capacitance properties of the CNT/polymer composites have intensively been studied [24,35,51]. Capacitance values of the electrodes coated with composites of different nanotubular materials and conducting polymers is significantly higher than that of the pristine nanotubes. For instance, specific capacitance of the MWCNT/polyacrylonitrile composite produced by pyrolysis has been determined as 100 F g−1 [51]. Furthermore, Cs is in the range of 120–160 F g−1 for composites of MWCNTs and polypyrrole, which were prepared under either electrochemical or chemical conditions [34]. Recently, amazingly high value of specific capacitance, i.e., 500 and even 650 F g−1 , was reported for the MWCNT/polypyrrole and MWCNT/polyaniline composite, respectively [35]. So far, studies of the CNT/polymer composites were limited to conducting polymers revealing p-doped properties. These composites are electroactive at positive potentials. Recently, we have focused on the development of novel electroactive polymers based on fullerenes [52–56]. They are electrochemically formed from fullerene epoxides, C60 O and C70 O [52,53]. Moreover, films of these polymers can be grown at the electrode surface during electroreduction of a solution containing the fullerene and a transition metal complex [54–56]. In these materials, metal atoms or metal complexes are coordinated directly to the fullerene in an 2 fashion, as shown below
These films are electrochemically active at negative potentials revealing n-doped behavior. Moreover, the capacitance performance of the C60 –Pd polymer is very promising [57]. In the present paper, we report on the formation and electrochemical properties of a composite film containing MWCNTs and C60 –Pd, prepared by electrochemical polymerization. The effect of the composition of synthesized material on its electrochemical properties was investigated herein. Moreover, the capacitance properties of the composite film were studied and compared to those of the pristine MWCNT film and the C60 –Pd film. Special attention was also paid to study of the mechanical properties of the films.
2. Experimental Multi-wall carbon nanotubes, (140 ± 30) nm in diameter and (7 ± 2) m in length, (>90% purity), were procured from the M.E.R. Corp. (Tucson, AZ, USA). They were produced by chemical vapor deposition. Palladium(II) acetate, Pd(ac)2 , from Aldrich Chemical Co. and C60 from the M.E.R. Corp. (Tucson, Arizona, USA) were used as received. The supporting electrolyte, tetra(n-butyl)ammonium perchlorate, (TBA)ClO4 , was used as received from Fluka. Tetra(methyl)ammonium chloride, (TMA)Cl, tetra(ethyl)ammonium chloride, (TEA)Cl, tetra(nbutyl)ammonium chloride, (TBA)Cl, were used as received from Aldrich. Acetonitrile (99.9%) and toluene (anhydrous, 99.8%) received from Aldrich were used without additional purification. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) experiments were performed on an AUTOLAB (Utrecht, The Netherlands) computerized electrochemistry system equipped with the PGSTAT 12 potentiostat and FRA response analyzer expansion cards with a three-electrode cell. The AUTOLAB system was controlled with the GPES 4.9 software of the same manufacturer. A 1.5-mm diameter gold disk of Bioanalytical Systems, Inc. (West Lafayette, IN, USA) was used as the working electrode. Prior to the experiments, the electrode was polished with a fine Carborundum paper, and then with a 0.5-m alumina slurry. Subsequently, the electrode was sonicated in water to remove traces of alumina from the gold surface, rinsed with water, and dried. For imaging by scanning electron microscopy, SEM, the studied films were electrochemically deposited on an Au foil of Goodfellow Metals, Ltd. (Cambridge, UK). Before use, this foil was annealed in a Bunsen flame. A silver wire immersed in an acetonitrile solution of 0.01 M AgClO4 and 0.09 M (TBA)ClO4 that was separated from the substrate electrode by a ceramic frit of Bioanalytical Systems Inc. served as the reference electrode. This reference electrode, the ferrocene/ferrocinium formal redox potential in the 0.1 M (TBA)ClO4 acetonitrile solution was −19 mV. The counter electrode was a platinum tab with an area of ca. 0.5 cm2 . Simultaneous CV and piezoelectric microgravimetry (PM) experiments were performed by using the EP-21 potentiostat of Elpan (Lubawa, Poland), connected to the EQCM 5710 electrochemical quartz crystal microbalance of the Institute of Physical Chemistry (Warsaw, Poland), which was controlled with the EQCM 5710-S2 software. This microbalance allowed for simultaneous measurements of changes of current, resonant frequency, and dynamic resistance of a 10-MHz, At-cut, plano-plano quartz crystal resonator (Institute of Tele- and Radiocommunication, Warsaw, Poland) during potential cycling. Projected area of this electrode and two contacting radial strips was 0.24 cm2 . The sensitivity of the mass measurement, calculated from the Saurbrey equation, was 4.2 ng Hz−1 cm−2 . The studied films were imaged by secondary electron scanning electron microscopy (SEM) with the use of the S-3000N scanning electron microscope of Hitachi (Tokyo, Japan). For determination of elemental composition of crystals, energy dispersive X-ray fluorescence (EDX) measurements were performed using EDAX software (Mahwah, NJ, USA). Transmission electron microscopy (TEM) investigations were carried out on the JOEL2000EX microscope. Films of MWCNTs were deposited on the electrode surface by drop coating. That is, a 40-l drop of sonically dispersed in dichloromethane MWCNTs was dispensed onto the electrode surface. After solvent evaporation, the electrode remained coated with a relatively porous film of MWCNTs. The C60 –Pd film was prepared by electroreduction of an acetonitrile:toluene (1:4, v:v) solution that contained the fullerene and Pd(ac)2 as well as the supporting electrolyte, 0.10 M (TBA)ClO4 ,
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Fig. 1. Multi-scan cyclic voltammogram for (1) the C60 –Pd film deposited on the 1.5mm diameter Au disk electrode in 0.10 M (TBA)ClO4 , in acetonitrile. The potential sweep rate was 100 mV s−1 . The C60 –Pd films were grown under CV conditions in 0.27 mM C60 and 3.56 mM Pd(ac)2 , 0.10 M (TBA)ClO4 , in acetonitrile:toluene (1:4, v:v), and (2) for the MWCNTs film deposited on the 1.5-mm diameter Au disk electrode in a blank acetonitrile solution of 0.10 M (TBA)ClO4 . The potential sweep rate was 100 mV s−1 . The mass of MWCNTs deposited was 4.25 g. Direction of the cathodic current, ic , flow is indicated with the vertical arrow.
under CV conditions. The potential sweep rate was 100 mV s−1 . Mass of the deposited C60 –Pd polymer film was determined using PM at EQCM [57,58]. 3. Results and discussion
Fig. 2. (a) Multi-scan cyclic voltammograms for 0.27 mM C60 , 3.56 mM Pd(ac)2 , and 0.1 M (TBA)ClO4 , in acetonitrile:toluene (1:4, v:v) recorded at the 1.5-mm diameter Au disk electrode coated with the 4.25 g MWCNTs film. The potential sweep rate was 100 mV s−1 . (b) Cyclic voltammograms for the MWCNT/C60 –Pd composite film containing 4.25 g MWCNTs and 0.40 g C60 –Pd (1), and 0.90 g C60 –Pd (2) in 0.10 M (TBA)ClO4 , in acetonitrile. The potential sweep rate was 100 mV s−1 . The condition of C60 –Pd deposition the same as in Fig. 1.
3.1. Voltammetric study of composites of C60 –Pd and MWCNTs Fig. 1 shows a CV behavior of the gold electrode coated with the film of C60 –Pd (Curve 1) and MWCNTs (Curve 2). The C60 –Pd film was electrochemically active at negative potentials due to electroreduction of the C60 moiety. The voltammograms of MWCNTs film exhibit typical capacitance behavior. Apparently, the capacitance current depends on the amount of nanotubes deposited on the electrode surface. Specific capacitance, Cs , was calculated using the following equation:
Cs =
ic dt
Em
(1)
where ic is the capacitance current, E is the potential range of integration, and m is the mass of the material deposited on the electrode surface. The specific capacitance of the MWCNTs film was determined as 15.5 F g−1 . This value is in agreement with that of the MWCNT films reported in literature [19–21]. The C60 –Pd polymer was deposited on a porous MWCNTs layer by electrochemical polymerization under CV conditions. Fig. 2a shows a multi-scan CV curve for an acetonitrile:toluene (1:4, v:v) solution of Pd(ac)2 , C60 , and (TBA)ClO4 recorded at the gold electrode that was coated with the MWCNTs film. At negative potentials, the electroreduction results in formation of the C60 –Pd polymer deposit on the nanotubes. Due to the presence of the MWCNTs film, this electropolymerization is less reversible than that at the bare gold electrode [57]. The SEM image of the pristine MWCNTs film and
that of the film of MWCNTs bearing the C60 –Pd polymer is shown in Fig. 3a and b, respectively. Clearly, the nanotubes got shrouded with a non-uniform polymer coat. The energy dispersive spectrometry measurements were conducted to examine the composition of the MWCNTs and MWCNT/C60 –Pd films (Fig. 3c). In both cases, a strong carbon signal at 0.39 keV dominates the spectrum. Additionally, weak peaks at 1.85 and 2.84 keV corresponding to palladium are observed for the composite containing MWCNTs and C60 –Pd (Curve 2 in Fig. 3c). The SEM imaging with EDX elemental mapping of the MWCNT/C60 –Pd film sample exhibits uniform distribution of palladium in the film. It also indicates that this film is free of palladium nanocrystals and that palladium atoms are involved in bonding of the fullerene cages in the polymer network. The composite film of MWCNTs and C60 –Pd as well as the MWCNT film alone were examined with TEM (Fig. 4). The TEM image shown in Fig. 4b clearly indicate that in the case of MWCNT/C60 –Pd composite, surface of the nanotube is coated with C60 –Pd. The amount of the polymer on the nanotube depends on the time of polymer deposition. The gold electrode, coated with the film of the MWCNT/C60 –Pd composite, was transferred to a blank acetonitrile solution of the supporting electrolyte and CV curves were recorded. These curves for two different amounts of the polymer deposited on the nanotubes are shown in Fig. 2b. Voltammograms for the MWCNT/C60 –Pd composite are stable with respect to multi-scan CV
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Fig. 4. TEM images of (a) MWCNT and (b) MWCNT coated with the C60 –Pd.
Fig. 3. SEM images of (a) the film of pristine MWCNTs and (b) the film of the composite of MWCNTs and the C60 –Pd polymer. (c) EDX spectra of (1) MWCNTs and (2) MWCNT/C60 –Pd composite deposited on gold foil.
cycling between 0 and −1.20 V. For potentials more positive than ca. −0.50 V, the composite exhibits behavior typical for a double-layer capacitor. Voltammograms recorded for this potential range, for the electrode coated only with MWCNTs, show almost pseudorectangular cathodic and anodic CV profiles that are nearly mirror imaged with respect to the potential axis (Curve 1 in Fig. 5a). In this potential range, the capacitance current is almost unaffected by the presence of the C60 –Pd polymer. However, the resistance of the C60 –Pd polymer is high at potentials less negative than that of electroreduction of the fullerene moiety. Therefore, the current increase after reversing direction of the potential sweep is much slower for the MWCNT/C60 –Pd composite film than that for the pristine MWCNT film. A negative deviation from the pseudorectangular profile is observed for the composite film (Curves 2 and 3 in Fig. 5a) in contrast to that for the MWCNTs film (Curve 1 in Fig. 5a). The extent of this deviation, which is related to the resistance of the film, is larger the higher is the amount of the deposited C60 –Pd polymer.
At potentials more negative than −0.50 V, the fullerene moiety is electroreduced. At these potentials, the film of the MWCNT/C60 –Pd composite behaves like a typical pseudcapacitor. Moreover, multiscan CV curve for this film is stable. Potential can be cycled between −0.80 and −1.50 V without noticeable change in the shape of the voltammogram. Cyclic voltammograms for the potential range corresponding to the C60 moiety electroreduction, for different potential sweep rates, are shown in Fig. 5b. For sweep rates lower than ca. 50 mV s−1 , the voltammograms show almost pseudorectangular cathodic and anodic profiles, a characteristic behavior for an ideal capacitor. However, for higher sweep rates the CV curve deviates from the ideal rectangular shape. This effect is related to the relatively high resistance of the film of the MWCNT/C60 –Pd composite and, therefore, slower current response to the potential sweep. For comparison, films of the pristine C60 –Pd polymer are much more conductive and, therefore, the sweep rate range of the ideal capacitance behavior is much wider in this case [57]. The current at negative potentials includes capacitive and faradaic contributions. The magnitude of both components is predicted to linearly depend upon the potential sweep rate. Therefore, the linear dependence of the total current on the potential sweep rate is expected. This dependence was obtained, indeed (Fig. 5c). Specific capacitance of the MWCNT/C60 –Pd film can be calculated using the following equation: Cs =
ipc Vm
(2)
where ipc is the pseudo-capacitance current and V is the potential sweep rate. From the slope of the i–V dependence, for sweep rates lower than 100 mV s−1 , the specific capacitance of 75 F g−1 was determined for the MWCNT/C60 –Pd composite in the potential range of the fullerene electroreduction. This pseudo-capacitance current mainly results from electroreduction of the C60 –Pd polymer. Specific capacitance as high as 425 F g−1 was calculated based on the mass of the C60 –Pd polymer deposited on the MWCNT film. This value is almost twice as high as that of the C60 –Pd film deposited on the bare surface of the gold electrode. Presumably, this higher capacitance of the C60 –Pd polymer in the MWCNT/C60 –Pd composite is due to the open porous structure of the composite. The effect of composition of the MWCNT/C60 –Pd composite on its capacitive behavior was also investigated. Fig. 6a shows CV behavior of the MWCNT/C60 –Pd films for different ratios of the mass of MWCNTs and the C60 –Pd polymer (mMWCNT /mC60 –Pd ). In order to prepare these composites, different amounts of MWCNTs were deposited on the Au electrode surface by drop coating.
E. Grodzka et al. / Electrochimica Acta 54 (2009) 5621–5628
Fig. 5. (a) Cyclic voltammograms for the film of (1) MWCNT as well as (2 and 3) the MWCNT/C60 –Pd films in 0.10 M (TBA)ClO4 , in acetonitrile, in the potential range 0 to −0.60 V. The potential sweep rate was 100 mV s−1 . The C60 –Pd films were grown under CV conditions for (2) 10 and (3) 20 CV cycles under conditions described in Fig 1. (b) Cyclic voltammograms of the MWCNT/C60 –Pd film containing 4.25 g MWCNTs and 0.90 g C60 –Pd in 0.10 M (TBA)ClO4 , in acetonitrile. The potential sweep rate was (1) 10, (2) 20, (3) 50, and (4) 100 mV s−1 . (c) Dependence of the current at–0.95 V on the potential sweep rate for the MWCNT/C60 Pd film containing 4.25 g MWCNTs and 0.90 g C60 –Pd.
Next, the C60 –Pd polymer was grown under CV conditions for 10 cycles in the potential range −0.20 to −1.05 V. The capacitance current is lower at potentials less negative than −0.60 V and the pseudo-capacitance current is pronounced in the potential range characteristic of C60 electroreduction. The specific capacitance val-
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Fig. 6. (a) Cyclic voltammograms for the MWCNT/C60 –Pd films in 0.10 M (TBA)ClO4 , in acetonitrile. Film composition was (1) 4.25 g MWCNTs and 0.90 g C60 –Pd, (2) 2.12 g MWCNTs and 0.75 g C60 –Pd, and (3) 1.05 g MWCNTs and 0.66 g C60 –Pd. The potential sweep rate was 100 mV s−1 . The condition of C60 –Pd deposition the same as in Fig. 1. (b) Dependence of the current at −0.95 V on the potential sweep rate for the MWCNT/C60 –Pd film. Film composition was (1) 4.25 g MWCNTs and 0.90 g C60 –Pd, (2) 2.12 g MWCNTs and 0.75 g C60 –Pd, as well as (3) 1.05 g MWCNTs and 0.66 g C60 –Pd.
ues, calculated for all three compositions of the MWCNT/C60 –Pd composite from slopes of the i–V straight lines obtained (Fig. 6b), are summarized in Table 1. Apparently, the larger the porosity of the composite, due to the higher content of MWCNTs, the higher is the value of Cs for the higher mMWCNT /mC60 –Pd ratio. 3.2. Electrochemical impedance spectroscopy investigations of the composite of C60 –Pd and MWCNTs Electrochemical impedance spectroscopy has also been employed to study redox properties of the MWCNT/C60 –Pd com-
Table 1 Specific capacitance for the MWCNT/C60 –Pd composites in 0.10 M (TBA)ClO4 , in acetonitrile, for different potentials. mMWCNT /mC60 –Pd in the composite
4.72 2.82 1.59 a
Specific capacitance (F g−1 ) At −0.40 V
At −1.10 V
Based on the MWCNT/C60 –Pd mass
Based on the MWCNT/C60 –Pd mass
Based on the C60 –Pd mass
12.5 11.3 12.0
75 73 91
425 285 236 200a
Specific capacitance for the C60 –Pd film from ref. [59].
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resistance related to the polymer electro-oxidation or electroreduction, ZW is the Warburg impedance related to the counter-ion transport accompanying the electro-oxidation or electroreduction of the polymer, and CL is the capacitance of the internal interface between the electrode surface and the solution inside the polymer micropores. The Warburg impedance is expressed by the following equation: ZW =
Fig. 7. (a) Complex-plane impedance plots for the MWCNT/C60 –Pd films (4.25 g MWCNTs and 0.90 g C60 –Pd) in 0.10 M (TBA)ClO4 , in acetonitrile, at (1) −0.30, (2) −0.825, (3) −0.90, (4) −1.00, and (5) −1.10 V. (b) Nyquist plots for the MWCNT/C60 –Pd films in 0.10 M (TBA)ClO4 , in acetonitrile, at −0.825 and −0.30 V (inset). Film composition was (1) 4.25 g MWCNTs and 0.90 g C60 –Pd, (2) 2.12 g MWCNTs and 0.75 g C60 –Pd, and (3) 1.05 g MWCNTs and 0.66 g C60 –Pd. Frequency was in the range of 10 kHz to 100 mHz. Solid curves represent simulated data. The condition of C60 –Pd deposition the same as in Fig. 1.
posite. Fig. 7 presents the imaginary part of impedance, Z , as a function of the real part of impedance, Z (Nyquist plots), for the electrode coated with the composite. The effect of potential (Fig. 7a) and composition of the MWCNT/C60 –Pd composite (Fig. 7b) on the impedance behavior has been investigated. The polymer/electrolyte interface can be represented by the equivalent circuit (Scheme 1) similar to that describing behavior of the C60 –Pd film [57]. In this circuit, R1 is the resistance of the electrolyte solution, Cdl is the double-layer capacitance of the external polymer/electrolyte interface, Rct is the charge-transfer
ω
(3)
where is resistivity of the film related to the ion transport, and ω is angular frequency. Fig. 7a shows EIS responses for the composite containing large amount of MWCNTs, namely 4.25 g MWCNTs and 0.9 g C60 –Pd. For potentials more positive than the potential of the C60 –Pd electroreduction, i.e., E > −0.50 V, the Z –Z response is dominated by the conductivity of MWCNTs of the composite as Rct related to the C60 –Pd electroreduction is very large. At potentials more negative than ca. −0.70 V, the C60 –Pd polymer is conductive. Therefore, at high frequencies, the impedance responses exhibit a semicircle behavior related to the C60 –Pd electroreduction. The depressed shape of this semicircle can be attributed to the perturbation in distribution of the charge transfer resistance of this electroreduction due to the porous surface of the composite. In the low frequency range, the impedance depends on the capacitance of the polymer. The imaginary vs. real part of impedance rapidly increases with the frequency decrease. The effect of non-uniform film thickness manifests itself by a positive deviation from linearity of the low frequency impedance. Shapes of experimental impedance plots for different potentials are consistent with the curves simulated for the selected equivalent circuit (Scheme 1). Solid curves represent simulated Z –Z responses. Some deviation of the simulated curves from the experimental data points at low frequencies is due to the non-uniform thickness of the composite and its porosity. Values of the equivalent circuit parameters, used to generate the simulated plots, are collected in Table 2. The Rct values corresponding to electroreduction of the fullerene moiety are lower the more negative is the potential applied. However, the Rct values determined for the MWCNT/C60 –Pd composite are significantly higher than those for the genuine C60 –Pd polymer reported earlier [57]. The CL capacitance is higher for the potential range of the film electroreduction. At negative potentials, CL reaches its maximum value corresponding to the specific capacitance of 320 F g−1 (calculated with respect to the C60 –Pd mass). This value is lower than that determined from the CV measurements (Table 1). However, CV currents at negative potentials include both the capacitance and faradaic contribution, which makes the total specific pseudo-capacitance higher.
Table 2 Electrochemical impedance parameters determined at different constant potentials applied to the 1.5-mm diameter Au electrode coated with the MWCNT/C60 –Pd composite film containing 4.25 g MWCNTs and 0.9 g C60 –Pd (mMWCNT /mC60 –Pd = 4.72) in 0.10 M (TBA)ClO4 . E (V)
R ()
Cdl (F)
Rct ()
CL (F)
( s−1/2 )
−0.30 −0.75 −0.825
342 265 270 253a 300b 261 265 270
0.52 0.70 0.73 0.41a 0.52b 0.88 1.24 1.76
– 3600 2290 90a – 1530 550 222
37 242 262 230a 132b 275 270 310
– 1.72 × 10−3 1.10 × 10−3 5.51 × 10−4 a 8.51 × 10−4 b 1.00 × 10−3 7.60 × 10−4 9.75 × 10−4
−0.90 −1.00 −1.10 Scheme 1. Equivalent electrical circuit representing behavior of the MWCNT/C60 –Pd film electrode in an electrolyte solution.
2 1/2
a b
2.12 g MWCNTs and 0.75 g C60 –Pd (mMWCNT /mC60 –Pd = 2.82). 1.05 g MWCNTs and 0.66 g C60 –Pd (mMWCNT /mC60 –Pd = 1.59).
E. Grodzka et al. / Electrochimica Acta 54 (2009) 5621–5628
The impedance responses for composites of different mMWCNT –to–mC60 –Pd ratios are compared in Fig. 7b. In this experiment, mass of the C60 –Pd polymer in the composite was kept almost the same and the mass of MWCNTs was changed fourfold. Therefore, the observed effects are related to the change of the amount of MWCNTs in the composite. At negative potentials, the conductive C60 –Pd component participates in the charge transfer. The Rct value for the composite is higher the higher is the amount of MWCNTs in it. Apparently, the presence of MWCNTs hinders electroreduction of C60 –Pd. However, an opposite effect is seen at potentials less negative than the potential of the C60 –Pd electroreduction (Inset in Fig. 7b). The resistance of the film is lower the higher is the mMWCNT /mC60 –Pd ratio. A Z –Z response of the films containing small amounts of MWCNTs are dominated by the high Rct value of the oxidized form of the C60 –Pd polymer. This high Rct value for the composite is represented by a large semicircle (Inset in Fig. 7b). Since MWCNTs are responsible for charge transfer in the potential range less negative than potentials of C60 –Pd
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reduction, the semicircle region shrinks with the increase of the MWCNTs amount in the composite. 3.3. Simultaneous piezoelectric microgravimetry and cyclic voltammetry behavior of the C60 –Pd film and the MWCNT/C60 –Pd composite film Fig. 8 shows simultaneously recorded curves of the potential dependence of current, resonant frequency change, and dynamic resistance change of the quartz resonators, with their Au electrodes coated either by the C60 –Pd film or the MWCNT/C60 –Pd film in blank 0.1 M (TBA)ClO4 acetonitrile solutions. Those measurements provided information not only on electrochemical behavior of the electroactive film, deposited on the working electrode during its electroreduction and electrooxydation, but also enabled to follow viscosity changes of the film caused by ingress or egress of counterions. CV measurements revealed that each film is electrochemically active at potentials more negative than ca. −0.70 V. However, the presence of MWCNTs in the composite film results in higher currents, which are due to higher electrode area and, hence, specific capacitance (Curves 1 and 2 in Fig. 8). When the scanned potential during the negative excursion reaches the value of −0.70 V (Curves 1 and 2 in Fig. 8), the resonant frequency decreases for the polymer (Curve 3 in Fig. 8) while it increases for the composite (Curve 4 in Fig. 8). Interestingly, the behavior of the dynamic resistance is similar for both films (Curves 5 and 6 in Fig. 8). However, the resistance increase for the polymer film is ca. half that for the composite. For the polymer film, the frequency decreases due to the increase of the film mass. Most likely, this increase is associated with the TBA+ ingress into the film for − compensation of the negative charge of C60 generated electroreductively. During this ingress, the film resistance increases as a result of the visco-elasticity gain. For the composite film, the frequency increases during the negative potential scan, although the negative − charge of C60 is generated. This is presumably because the enter+ ing TBA counter-ion significantly changes the film viscosity. The contribution of this change predominates in the overall frequency change, which can be affected both by the change of the mass of the resonator and the change of viscosity of the film [58]. Despite these differences in performance, both films are stable under multiscan CV conditions in the potential range 0 to −1.20 V. That is, both frequency changes (Curves 3 and 4 in Fig. 8) and dynamic resistance changes (Curves 5 and 6 in Fig. 8) remain almost the same in consecutive potential cycles. 4. Conclusions
Fig. 8. Simultaneously recorded curves of (1 and 2) multi-scan cyclic voltammetry, (3 and 4) resonant frequency change vs. potential, and (5 and 6) dynamic resistance change vs. potential for (1, 3, and 5) the C60 –Pd film and (2, 4, and 6) the MWCNT/(C60 –Pd) film in 0.1 M (TBA)ClO4 , in acetonitrile. Potential sweep rate was 0.1 V s−1 . The condition of C60 –Pd deposition the same as in Fig. 1.
Most of composites of CNTs and conducting polymers studied so far involved p-doped polymers. Herein, we investigated a composite of MWCNTs and the C60 –Pd polymer, which exhibits ndoped properties. The MWCNT/C60 –Pd composite was deposited by electro-polymerization of C60 –Pd on a porous layer of MWCNTs. Due to relatively high conductivity of the nanotubes, the accessible potential window for the composite is wide. At negative potentials, the composite is electrochemically active (due to electroreduction of the C60 moiety) behaving as a typical redox capacitor. Pseudocapacitance of the composite is mainly related to electroreduction of the C60 –Pd component. However, the incorporation of the MWCNTs results in the capacitance increase due to the increase of the composite porosity. At potentials less negative than those of the C60 moiety electroreduction, CV behavior of MWCNT/C60 –Pd resembles that of a typical double-layer capacitor. The capacitance properties of the composite in this potential range are mainly governed by the amount of MWCNTs present.
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Electrochemical properties of MWCNT/C60 –Pd depend upon the composition of the composite. In the potential range of the C60 –Pd electroreduction, specific pseudo–capacitance is higher the higher is the nanotube content. For large amounts of the nanotubes, specific capacitance determined with respect to the mass of C60 –Pd reaches value as high as 425 F g−1 . Evidently, the MWCNT/C60 –Pd composite is a much better material for charge storage than the pristine C60 –Pd polymer. Moreover, the charge transfer resistance of the composite film increases with the increase of the mMWCNT /mC60 –Pd ratio. This resistance is relatively high for composites containing large amounts of MWCNTs. In this case, current relatively slowly responds to the potential sweep. Changes in visco-elastic properties, manifested by simultaneous changes in the resonant frequency and dynamic resistance during consecutive CV potential cycles, of the C60 –Pd differ from those of the MWCNT/C60 –Pd. That is, during the negative potential scan, changes of viscosity due to the TBA+ ingress into the composite film for compensation of the negative charge generated are twice as high as those for the polymer film. Despite these differences, however, both films are stable under multi-scan CV conditions in the potential range 0 to −1.20 V. Acknowledgments The authors acknowledge the Polish Ministry of Science and Higher Education (project No. N 204 374 733 to KW) and the European Regional Development Fund (ERDF, POIG.01.01.02-00-008/08 2007-2013 to WK) for a financial support. References [1] P.G. Collins, A. Zettl, H. Bando, A. Thess, R.E. Smalley, Science 278 (1998) 100. [2] Q.H. Wang, A.A. Setlur, J.M. Lauerhaas, J.Y. Dai, E.W. Seelig, R.P.H. Chang, Appl. Phys. Lett. 72 (1998) 2912. [3] S.A. Curran, P.M. Ajayan, W.J. Blau, D.L. Carroll, J.N. Coleman, A.B. Dalton, A.P. Davey, A. Drury, B. Mc Carthy, S. Maier, A. Strevens, Adv. Mater. 10 (1998) 1091. [4] W.B. Choi, D.S. Chung, J.H. Kang, H.Y. Kim, Y.W. Jin, I.T. Han, Y.H. Lee, J.E. Jung, N.S. Lee, G.S. Park, J.M. Kim, Appl. Phys. Lett. 75 (1999) 3129. [5] H. Ago, K. Petritsch, M.S.P. Shaffer, A.H. Windle, R.H. Friend, Adv. Mater. 11 (1999) 1281. [6] L. Diederich, E. Barborini, P. Piseri, A. Podesta, P. Milani, Appl. Phys. Lett. 75 (1999) 2662. [7] K.H. An, W.S. Kim, Y.S. Park, J.M. Moon, D.J. Bae, S.C. Lim, Y.S. Lee, Y.H. Lee, Adv. Funct. Mater. 11 (2001) 387. [8] C. Liu, A.J. Bard, F. Wudl, I. Weitz, J.R. Heath, Electrochem. Solid State Lett. 2 (1999) 5. [9] R.H. Baughman, C. Cui, A.A. Zakhidov, Z. Iqbal, J.N. Barisci, G.M. Spinks, G.G. Wallace, A. Mazzoldi, D. De Rossi, A. Rinzler, O. Jaschinski, S. Roth, M. Kertesz, Science 284 (1999) 1340. [10] G.M. Spinks, G.G. Wallace, R.H. Baughman, L. Dai, in: Y. Bar-Cohen (Ed.), Electroactive Polymer Actuators as Artificial Muscles, SPIE, Washington, DC, 2001, p. 223. [11] X. Qin, X.P. Gao, H. Liu, H.T. Yuan, W.L. Gong, D.Y. Song, Electrochem. Solid State Lett. 3 (2000) 532. [12] I. Lombardi, M. bestetti, C. Mazzocchia, P.L. Cavallotti, U. Ducati, Electrochem. Solid State Lett. 7 (2004) A115. [13] C. Nutzenadel, A. Zuttel, D. Chartouni, L. Schlapbach, Electrochem. Solid State Lett. 2 (1999) 30. [14] E. Frackowiak, F. Beguin, Carbon 40 (2002) 1775. [15] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845. [16] E. Frackowiak, Phys. Chem. Chem. Phys. 9 (2007) 1774. [17] D.N. Futaba, K. Hota, T. Yamada, T. Hiraoka, Y. Hayamizu, Y. Kakudate, O. Tanaike, H. Hatori, M. Yumura, S. Ijima, Nat. Mater. 5 (2006) 987.
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