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Cyclic voltammetry and impedance studies of electrodeposited polypyrrole nanoparticles doped with 2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt
Thin Solid Films 518 (2010) 4100–4105
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
Cyclic voltammetry and impedance studies of electrodeposited polypyrrole nanoparticles doped with 2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt Sh. M. Ebrahim a,⁎, M.M. Abd-El Latif b, A.M. Gad a, M.M. Soliman a a b
Department of Materials Science, Institute of Graduate Studies and Research, Alexandria University, Postal Code 21526, Alexandria, Egypt Mubarak City for Scientific Research & Technology Applications, Institute of Advanced Technology and New Materials, Borg El-Arab City, Alexandria, Egypt
a r t i c l e
i n f o
Article history: Received 2 December 2008 Received in revised form 23 October 2009 Accepted 27 October 2009 Available online 10 November 2009 Keywords: Conducting polymers Polypyrrole Electrochemical impedance Cyclic voltammetry
a b s t r a c t Electrochemical synthesis of polypyrrole (PPY), doped with 2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt (AMPSNa), was carried out using chronoamperometric technique. Cyclic voltammetry measurements showed that the electroactivity of PPY films, doped with AMPSNa, increases with the film thickness. Scanning electron microscopy photographs revealed that the PPY particles are in the nano-scale range and that their size depends on the potential at which the PPY has formed. Electrochemical impedance spectroscopy (EIS), in the potential range of + 1.0 and − 1.0 V, revealed in the PPY film charge transfer domination with a semicircle at high frequencies, and anion diffusion dominance at low frequencies. EIS also showed that the charge transfer resistance of PPY film at − 1.0 V is lower than what is expected and that on increasing the thickness of the PPY films, the overall impedance decreases. The proposed equivalent circuit model, based on the double layer capacity and the Warburg impedance, was replaced by two constant-phase elements to fit the experimental work of this study. The values of the fractional exponent of the first constant phase element at approximately 0.5 indicate that the processes have a diffusion-limited nature. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of the conducting organic polymers and the pioneering work of Diaz et al. on the electrodeposition of polypyrrole (PPY), the physical and chemical properties of polypyrrole and other conducting polymers have been the subject of many experimental and theoretical investigations [1–5]. One of the characteristic properties of polypyrrole is its amenability to be reversibly switched between the conducting (oxidized) and the nonconducting (neutral) states. During such switching, many physical properties change dramatically and this forms the basis for using conjugated polymers as active materials in displays, sensors, and electromagnetic shielding [3–5]. Polypyrrole synthesis is carried out in two major ways: chemical and electrochemical polymerizations [6,7]. Electrochemically synthesized PPY films have some attractive features, such as good conductivity and very high adherence to the substrates. However, their electrochemical properties strongly depend on the redox state of the polymer. For electrochemical deposition of polypyrrole from aqueous or nonaqueous media, several techniques are available: constant
⁎ Corresponding author. 163 Horrya Avenue, El-Shatby, Alexandria, Egypt. Tel.: + 20 124879137; fax: + 20 34285792. E-mail address: [email protected] (S.M. Ebrahim). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.10.167
potential electrodeposition, galvanostatic deposition, cyclic voltammetry and pulse voltammetry [7]. The factors that control an electropolymerization process are electrolyte anions, electrolyte solvents, pH of the aqueous solutions, polymerization temperature and current/potential values [8]. Most − − researchers use anions as dopants, e.g., ClO− 4 , Cl , NO3 , toluenesulfonate, and dodecyl benzenesulfonate. When PPY was doped with large anionic detergents, it showed a much higher chemical stability in aqueous systems than when it was doped with smaller anions [9–14]. In this work, an inexpensive and hydrophilic 2-acrylamido-2-methyl1-propanesulfonic acid sodium salt (AMPSNa) was chosen as the dopant for PPY because of its chemical structure, which contains a sulfonic acid group. AMPSNa is used in different applications such as humidity sensor and as lithium ion-conducting polymer electrolyte in a lithium battery [15]. The effects of oxidation level and thickness of PPY/AMPSNa films on the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) plots were also investigated. 2. Experimental work Pyrrole monomer (Aldrich) was distilled before use and AMPSNa (Merck) was used without further purification. The electrochemical experiments were performed in a one-compartment cell with three electrodes connected to Gamry G750 potentiostat/galvanostat with pilot integration controlled by PHE200 and EIS300 software. The working
S.M. Ebrahim et al. / Thin Solid Films 518 (2010) 4100–4105
electrode, namely the platinum rectangular sheets (1 × 2 cm2), was mechanically polished with abrasive paper (1200 grade) and rinsed in acetone before each electrochemical experiment. A platinum plate was used as an auxiliary electrode. All potentials were measured versus saturated calomel electrode (SCE) as a reference electrode. PPY films were prepared from aqueous solutions of pyrrole and AMPSNa, following CV or potentiostatic techniques. Impedance measurements were performed mainly in the frequency range of 300,000 Hz to 1 mHz, taking ten points per decade. The amplitude of the sinusoidal voltage signal was 10 mV. Direct current (dc) potentials from −1 to +1 V vs. SCE were applied to the PPY electrodes. The PPY electrodes were polarized at the appropriate potential for 5 min before the impedance measurement to ensure that the electrodes reached the desired state. Thickness of the PPY films was controlled by monitoring the charge in Coulomb (C) passed during electropolymerization [1]. Fourier transformer infrared (FTIR) spectrum of PPY film was obtained using Perkin Elmer spectrophotometer (Spectrum Bx). Scanning electron microscope (SEM) micrographs of PPY films, coated with a thin layer of gold, were obtained by JEOL JSM 6360LA with an acceleration voltage of 30 kV. 3. Results and discussion 3.1. Preparation of AMPSNa-doped PPY films To identify the optimum experimental conditions for preparing PPY film, a preliminary electrochemical study was performed. First is determined the potential range at which polymerization of electroactive PPY film commences. Fig. 1 shows the cyclic voltammetry (5th, 15th, 25th, 40th and 50th cycles) of electropolymerization of 0.45 M pyrrole and 0.5 M AMPSNa on Pt electrode in potential scans between −1.0 and 0.7 V versus SCE with 25 mV/s scan rate. The first scan shows that the minimum oxidation potential of pyrrole is + 0.6 V; so, it follows that polymerization of PPY will occur at a potential higher than 0.6 V. The current of the oxidative wave increases with the number of cycles indicating the build-up of conducting PPY film [16,17]. PPY films were obtained on Pt electrode in the potentiostatic mode from 0.45 M pyrrole and 0.5 M AMPSNa as shown in Fig. 2. The imposed potential values (Eimp) were chosen at such intervals (from 0.6 to 1.1 V vs. SCE) as to frame the known oxidation potential of the pyrrole. It is observed that the PPY films obtained on Pt electrode at Eimp ≥ 0.6 V are homogeneous and adherent and that their thickness depends on the value of the applied potential and the polarization time. The chronoamperograms show a sharp initial current maximum, which relates to the charge of the electrical double layer or capacitive contribution at the polymer/solution interface [18,19]. After a minimum at low potentials, the current increases again. This minimum is related to the penetration of counterions into PPY chains. The duration of this transition stage decreases with the increase of potentials as shown in Fig. 2. When oxidation of pyrrole starts close to the electrode, counterions have more difficulty in penetrating the
Fig. 1. CV of 0.45 M pyrrole and 0.5 M AMPSNa aqueous solution with 10 mV/s scan rate.
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Fig. 2. Chronoamperometry of 0.45 M pyrrole and 0.5 M AMPSNa aqueous solution at different potentials.
film. As a result, the progress of oxidation becomes much slower, and so is the increase in current. This may also be due to the growth of either independent nuclei or simultaneous increase in the number of nuclei, or due to the formation of conducting PPY films. This indicates that a thin film of PPY will have less electroactivity than does a thick film. Chronoamperometric curves at higher than 0.8 V potential are characterized by the disappearance of transition region, because the high potential may breakdown the electrical double layer. From preliminary experiments using different concentrations of pyrrole and AMPSNa at 0.9 V, it is found that the optimum condition for preparation of PPY is 0.45 M for pyrrole, and 0.5 M for AMPSNa. 3.2. CV studies Fig. 3 shows the CVs of PPY films in 1 M AMPSNa at different scan rates. The AMPSNa doped-PPY film was prepared from an aqueous solution of 0.45 M pyrrole and 0.5 M AMPSNA by passing 0.795 C at 0.9 V. The oxidation peaks were observed during the positive sweep at about 0.0 V, accompanied by the appearance of reduction post-peaks during the negative sweep at about −0.8 V. In case of using small size ions as the dopant, the oxidation peaks are linked to the outward movement of cations in the PPY, and reduction peaks to that of anions. With large dopant, the cation movement becomes more dominant than anion movement, because the larger dopant anions are more strongly bound to the polymer and thus are less prone to be replaced by smaller ions. Vidanapathirana et al. [20] confirmed this using quartz crystal microbalance measurement. In this work, it is found that with the largest dopant ions, only cations move in and out of the polymer during cycling [10]. Peak current increases linearly with increasing scan rate, indicating the absence of kinetic or transport
Fig. 3. CVs of PPY film, prepared from an aqueous solution 0.45 M pyrrole and 0.5 M AMPSNa, by passing 0.795 C (1.7 μm) at 0.9 V in 1 M AMPSNa solution with different scan rates.
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limitation [21]. The reduction peak currents are higher than those of the oxidation current indicating that electrolyte ions move in the PPY film during reduction process and enhance the doping level. Generally, the reversibility of electrochemical reaction is determined by the redox peak potentials on the scan rate. The peak potentials are not dependent on the scan rate in the reversible process, but are dependent in the irreversible process. The shift of reduction peak to the negative potential and of the oxidation peak to positive potential with increasing scan rate indicates that this redox process is irreversible [22,23]. The dependence of the electroactivity of PPY film on thickness is important, because the doping levels depend on the charge consumed during polymerization process [24–27]. The AMPSNa-doped PPY film was prepared from 0.45 M pyrrole and 0.5 AMPSNA at 0.9 V by passing different amounts of charge. The thickness of the PPY films was estimated based on the charge consumed during electropolymerization, assuming that 240 mC results in 1.0 μm [1]. Redox activity of the films increases with increase in thickness as shown in Fig. 4. This is possibly because PPY film thickness is directly proportional to the charge consumed during electropolymerization [24]. The dependence of redox properties and doping level of the polymer films on the thickness was observed for titanocene- and benzensulfonatedoped polypyrrole films. It was found that doping level decreases with increase in thickness [25,26]. However, using Raman spectroscopic analysis, Chen et al. show that the doping level increases during the process of film growth and this is consistent with the findings of this study [27]. In addition, the anodic and cathodic peak potentials of the PPY film shifted to more positive and negative values, respectively, with increase in polymer thickness. This is because the amount of dopant increases with film thickness, which hinders further permeation of cations [28]. The voltammetry curves of AMPSNa-doped PPY are in good agreement with the published data on PPY doped with sodium dodecyl sulfate and benzenesulfonate [11,25]. However, the reduction peak currents are broader. 3.3. FTIR spectroscopy FTIR spectroscopy was used to characterize the AMPSNa-doped polypyrrole, prepared from 0.45 M pyrrole and 0.5 M AMPSNa solution by passing 0.795 C (1.7 μm) at 0.9 V as shown in Fig. 5. The peaks at 1317 cm− 1 are attributed to C–H bending deformation and those at 1135 cm− 1 to N–H bending deformation vibrations. The peak near 1178 cm− 1 reflects the doping state of polypyrrole owing to the formation of bipolaron charge carrier [29]. Characteristic absorption peaks of pyrrole ring at 1561 and 1486 cm− 1 are assigned to the C=C and C–C stretching vibration [30], and the band near 2927 cm− 1 to C–H stretching vibrations in AMPSNa [31]. The spectrum of PPY yields an intense band at 1083 cm− 1 emanating from the dopant. The N–H wagging band between 815 and 666 cm− 1 is also clearly seen [32].
Fig. 4. CVs of PPY films of varying thickness prepared from an aqueous solution 0.45 M pyrrole and 0.5 M AMPSNa at 0.9 V in 1 M AMPSNa solution with 50 mV/s scan rate.
Fig. 5. FTIR spectrum of AMPSNa-doped PPY prepared from an aqueous solution 0.46 M pyrrole and 0.5 M AMPSNa by passing 0.795 C (1.7 μm) at 0.9 V.
3.4. SEM morphology Fig. 6 shows SEM micrographs of the PPY films electrochemically prepared at different potentials in the oxidized state (as deposited). The electronic properties of polypyrrole films are linked to their morphology; a smoother and denser film will generally be more conductive as it is likely to be less porous and more ordered, thereby facilitating charge transport through the film [33]. At low magnifications, PPY films show a cauliflower-like nodular surface morphology; Fig. 4 shows microspherical grains of approximately 5 μm diameter [34]. The high magnification of the images shows that the cauliflowerlike and microspherical grains consist of small grains filled with very small, nano-scale range particles. The grain size decreased with increasing potential at which PPY formed. The particle size of PPY films, prepared at 0.9 V, is about 50 nm and that of films prepared at 1.1 V is 15 nm. Hence, the particle size decreases by increasing the voltage applied to prepare PPY films.
Fig. 6. SEM with different magnifications for the PPY films prepared from an aqueous solution 0.45 M pyrrole and 0.5 M AMPSNa by passing 0.795 C at different potentials.
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Fig. 7. Nyquist diagrams of PPY–AMPSNa films prepared from 0.45 M pyrrole and 0.5 M AMPSNa aqueous solution by passing 0.795 C (1.7 μm) at 0.9 V in 1 M AMPSNa solution at different potentials.
3.5. EIS studies EIS appears to be an excellent technique for investigating bulk and interfacial electrical properties of a solid or liquid material connected to or being part of an appropriate electrochemical transducer [35]. Typical Nyquist diagrams, obtained by applying different dc potentials on PPY films in 1 M AMPSNa aqueous solution, are shown in Fig. 7. The PPY film was prepared by passing 0.795 C (1.7 μm) at 0.9 V in an aqueous solution consisting of 0.5 M pyrrole and 0.45 M AMPSNa. The shape of the impedance diagrams changed significantly with negative shift in dc potentials thus indicating that the electrochemical properties (reduced/oxidized states) of PPY vary as a function of the applied potential [36,37]. The impedance curves of all the electrodes show a distorted semi-circular shape in the high-frequency region owing to the porosity of PPY films and a vertically linear spike in the low-frequency region. The high-frequency intercept of the semi-circle with the real axis yields the Ohmic resistance, while the diameter provides the charge-transfer resistance (Rcp) of the PPY/electrolyte interface. The real axis high frequency intercepts are independent of potential and coincide with the uncompensated resistance of the bulk electrolyte solution (Rs) [36,37]. The representative data can be grouped into two categories. In the first category (at −0.5, 0.0 and + 0.5 V), the impedance plots present a simple Warburg type behavior [38]. In the second category (at +1.0 and −1.0 V), at high frequencies, charge transfer dominates with a semicircle, while at low frequencies, diffusion of the cation in the PPY film dominates. It is interesting to note that the impedance values of PPY films at −1.0 V and −0.5 V are lower than the expected values.
This observation does not agree with published data of Garcia et al. [38], which may be due to the outward movement of the cations from the PPY. The fit of EIS data (Fig. 7, solid line) with the proposed equivalent circuit (Fig. 8) indicates that this circuit model can adequately represent the electrochemical processes of PPY at this potential. In this model, the double layer capacity and the Warburg impedance for semi-infinite linear diffusion were replaced by two constantphase elements CPE1 and CPE2. These correspond, respectively, to the capacitance at an inhomogeneous electrode surface and to the diffusion of ions in the electrode. Thus, the double-layer capacitance and Warburg impedance of the Randles circuit were replaced by two constant-phase elements. In general, the CPE may appear from
Fig. 8. Equivalent circuit of PPY–AMPSNa films.
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Table 1 Best-fitting values of the equivalent circuit elements in Fig. 8 from the simulation of the impedance data for PPY film at different potentials.
Table 2 Best-fitting values from the simulation of the impedance data for different thicknesses of PPY films at + 1.0 V.
Fitting parameters Potential (V) − 1.0 − 0.5 0.0 + 0.5 + 1.0
Rs (Ω) 60 60 60 60 50
Rcp (Ω) 210 1.1 17,440 5765 1800
Fitting parameters α1 0.7 0.592 0.775 0.445 0.75
α2
R7 (Ω) −3
0.01×10 1.94×10− 3 1.94×10− 3 5.6×10− 3 1.0×10− 3
400 3000 3110 3748 3700
four parameters: (i) distribution of the relaxation times as a result of inhomogeneities existing at the electrode/electrolyte interface, (ii) porosity, (iii) the nature of the electrode, and (iv) the dynamic disorder associated with diffusion [39]. In the AMPSNa-doped PPY, owing to the diffusion of the cation ions from electrolyte through the films, the large size AMPSNa anions are immobilized and the redox process sets in. Hence, incorporation of the two CPE elements in the equivalent circuit corresponds to the porosity of the electrode and thus the semi-infinite diffusion of the cations becomes necessary. The impedance of the constant phase element is defined as ZCPE = A− 1(jω)− α, where pffiffiffiffiffiffiffiffiα is a fractional exponent having values between 0 and 1; j = −1; ω = 2πf is the angular frequency and A is a frequency-independent parameter. When α = 0, the CPE is an ideal resistor; when α = 1, it is an ideal capacitor (A = C), and when α = 0.5, it represents homogeneous semi-infinite diffusion [39]. The best fitting values of all these parameters for the EIS data of PPY film are listed in Table 1. The lower values of charge transfer resistance (Rcp) at −1.0 V and −0.5 V may be due movements of dopant from PPY films, which may set in the redox process. In addition, the Rcp values obtained at −1.0 V are smaller than those obtained at +1.0 V. This is because the reduction peak currents are higher than those of oxidation currents as previously mentioned under CV studies. The values of α1 at approximately 0.5 indicate that the processes have a diffusion-limited nature and the low values of α2 confirm that CPE2 is ideal resistor. Fig. 9 shows the impedance spectra for PPY films of variable thickness at + 1.0 V in 1 M AMPSNa solution. As the thickness of the
PPY thickness (μm) 1.7 9.8 15.5 26.9
Rs (Ω) 45 60 45 45
Rcp (Ω) 1800 1000 500 480
α1 0.75 0.78 0.72 0.70
α2
R7 (Ω) −6
10×10 10×10− 6 10×10− 6 10×10− 6
3700 1750 700 630
PPY film increases, the overall impedance decreases (Table 2); this is consistent with the results obtained from CVs. This may be due to the increase in PPY film thickness, which results in higher surface area owing to the enhanced number of PPY nanoparticles [36,39]. 4. Conclusion Electrochemical polymerization of 2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt (AMPSNa)-doped polypyrrole (PPY) was carried out using chronoamperometric technique. CV measurements showed that the electroactivity of PPY films, doped with AMPSNa, increases with increase in the film thickness. SEM photographs revealed that the particles of PPY form in the nano-scale range and their size depends on the potential at which PPY forms. The CVs curves of AMPSNa-doped PPY are in good agreement with the published data. However, the reduction peak currents are broader. EIS revealed that in the potential range between +1.0 and −1.0 V, at high frequencies, charge transfer dominates with a semicircle and at low frequencies diffusion of the anion in the PPY film dominates. EIS also showed that on increasing the thickness of the PPY films, the overall impedance decreases. The equivalent circuit model, based on the double layer capacity and the Warburg impedance, was replaced by two constant-phase elements to fit the experimental work of this study. Acknowledgment The authors acknowledge the financial support provided by Alexandria University under Research Enhancement Program (ALEX REP).
Fig. 9. Nyquist diagrams of PPY–AMPSNa films of different thicknesses prepared from an aqueous solution 0.45 M pyrrole and 0.5 M AMPSNa at 0.9 V in 1 M AMPSNa solution.
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Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
Cyclic voltammetry and impedance studies of electrodeposited polypyrrole nanoparticles doped with 2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt Sh. M. Ebrahim a,⁎, M.M. Abd-El Latif b, A.M. Gad a, M.M. Soliman a a b
Department of Materials Science, Institute of Graduate Studies and Research, Alexandria University, Postal Code 21526, Alexandria, Egypt Mubarak City for Scientific Research & Technology Applications, Institute of Advanced Technology and New Materials, Borg El-Arab City, Alexandria, Egypt
a r t i c l e
i n f o
Article history: Received 2 December 2008 Received in revised form 23 October 2009 Accepted 27 October 2009 Available online 10 November 2009 Keywords: Conducting polymers Polypyrrole Electrochemical impedance Cyclic voltammetry
a b s t r a c t Electrochemical synthesis of polypyrrole (PPY), doped with 2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt (AMPSNa), was carried out using chronoamperometric technique. Cyclic voltammetry measurements showed that the electroactivity of PPY films, doped with AMPSNa, increases with the film thickness. Scanning electron microscopy photographs revealed that the PPY particles are in the nano-scale range and that their size depends on the potential at which the PPY has formed. Electrochemical impedance spectroscopy (EIS), in the potential range of + 1.0 and − 1.0 V, revealed in the PPY film charge transfer domination with a semicircle at high frequencies, and anion diffusion dominance at low frequencies. EIS also showed that the charge transfer resistance of PPY film at − 1.0 V is lower than what is expected and that on increasing the thickness of the PPY films, the overall impedance decreases. The proposed equivalent circuit model, based on the double layer capacity and the Warburg impedance, was replaced by two constant-phase elements to fit the experimental work of this study. The values of the fractional exponent of the first constant phase element at approximately 0.5 indicate that the processes have a diffusion-limited nature. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of the conducting organic polymers and the pioneering work of Diaz et al. on the electrodeposition of polypyrrole (PPY), the physical and chemical properties of polypyrrole and other conducting polymers have been the subject of many experimental and theoretical investigations [1–5]. One of the characteristic properties of polypyrrole is its amenability to be reversibly switched between the conducting (oxidized) and the nonconducting (neutral) states. During such switching, many physical properties change dramatically and this forms the basis for using conjugated polymers as active materials in displays, sensors, and electromagnetic shielding [3–5]. Polypyrrole synthesis is carried out in two major ways: chemical and electrochemical polymerizations [6,7]. Electrochemically synthesized PPY films have some attractive features, such as good conductivity and very high adherence to the substrates. However, their electrochemical properties strongly depend on the redox state of the polymer. For electrochemical deposition of polypyrrole from aqueous or nonaqueous media, several techniques are available: constant
⁎ Corresponding author. 163 Horrya Avenue, El-Shatby, Alexandria, Egypt. Tel.: + 20 124879137; fax: + 20 34285792. E-mail address: [email protected] (S.M. Ebrahim). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.10.167
potential electrodeposition, galvanostatic deposition, cyclic voltammetry and pulse voltammetry [7]. The factors that control an electropolymerization process are electrolyte anions, electrolyte solvents, pH of the aqueous solutions, polymerization temperature and current/potential values [8]. Most − − researchers use anions as dopants, e.g., ClO− 4 , Cl , NO3 , toluenesulfonate, and dodecyl benzenesulfonate. When PPY was doped with large anionic detergents, it showed a much higher chemical stability in aqueous systems than when it was doped with smaller anions [9–14]. In this work, an inexpensive and hydrophilic 2-acrylamido-2-methyl1-propanesulfonic acid sodium salt (AMPSNa) was chosen as the dopant for PPY because of its chemical structure, which contains a sulfonic acid group. AMPSNa is used in different applications such as humidity sensor and as lithium ion-conducting polymer electrolyte in a lithium battery [15]. The effects of oxidation level and thickness of PPY/AMPSNa films on the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) plots were also investigated. 2. Experimental work Pyrrole monomer (Aldrich) was distilled before use and AMPSNa (Merck) was used without further purification. The electrochemical experiments were performed in a one-compartment cell with three electrodes connected to Gamry G750 potentiostat/galvanostat with pilot integration controlled by PHE200 and EIS300 software. The working
S.M. Ebrahim et al. / Thin Solid Films 518 (2010) 4100–4105
electrode, namely the platinum rectangular sheets (1 × 2 cm2), was mechanically polished with abrasive paper (1200 grade) and rinsed in acetone before each electrochemical experiment. A platinum plate was used as an auxiliary electrode. All potentials were measured versus saturated calomel electrode (SCE) as a reference electrode. PPY films were prepared from aqueous solutions of pyrrole and AMPSNa, following CV or potentiostatic techniques. Impedance measurements were performed mainly in the frequency range of 300,000 Hz to 1 mHz, taking ten points per decade. The amplitude of the sinusoidal voltage signal was 10 mV. Direct current (dc) potentials from −1 to +1 V vs. SCE were applied to the PPY electrodes. The PPY electrodes were polarized at the appropriate potential for 5 min before the impedance measurement to ensure that the electrodes reached the desired state. Thickness of the PPY films was controlled by monitoring the charge in Coulomb (C) passed during electropolymerization [1]. Fourier transformer infrared (FTIR) spectrum of PPY film was obtained using Perkin Elmer spectrophotometer (Spectrum Bx). Scanning electron microscope (SEM) micrographs of PPY films, coated with a thin layer of gold, were obtained by JEOL JSM 6360LA with an acceleration voltage of 30 kV. 3. Results and discussion 3.1. Preparation of AMPSNa-doped PPY films To identify the optimum experimental conditions for preparing PPY film, a preliminary electrochemical study was performed. First is determined the potential range at which polymerization of electroactive PPY film commences. Fig. 1 shows the cyclic voltammetry (5th, 15th, 25th, 40th and 50th cycles) of electropolymerization of 0.45 M pyrrole and 0.5 M AMPSNa on Pt electrode in potential scans between −1.0 and 0.7 V versus SCE with 25 mV/s scan rate. The first scan shows that the minimum oxidation potential of pyrrole is + 0.6 V; so, it follows that polymerization of PPY will occur at a potential higher than 0.6 V. The current of the oxidative wave increases with the number of cycles indicating the build-up of conducting PPY film [16,17]. PPY films were obtained on Pt electrode in the potentiostatic mode from 0.45 M pyrrole and 0.5 M AMPSNa as shown in Fig. 2. The imposed potential values (Eimp) were chosen at such intervals (from 0.6 to 1.1 V vs. SCE) as to frame the known oxidation potential of the pyrrole. It is observed that the PPY films obtained on Pt electrode at Eimp ≥ 0.6 V are homogeneous and adherent and that their thickness depends on the value of the applied potential and the polarization time. The chronoamperograms show a sharp initial current maximum, which relates to the charge of the electrical double layer or capacitive contribution at the polymer/solution interface [18,19]. After a minimum at low potentials, the current increases again. This minimum is related to the penetration of counterions into PPY chains. The duration of this transition stage decreases with the increase of potentials as shown in Fig. 2. When oxidation of pyrrole starts close to the electrode, counterions have more difficulty in penetrating the
Fig. 1. CV of 0.45 M pyrrole and 0.5 M AMPSNa aqueous solution with 10 mV/s scan rate.
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Fig. 2. Chronoamperometry of 0.45 M pyrrole and 0.5 M AMPSNa aqueous solution at different potentials.
film. As a result, the progress of oxidation becomes much slower, and so is the increase in current. This may also be due to the growth of either independent nuclei or simultaneous increase in the number of nuclei, or due to the formation of conducting PPY films. This indicates that a thin film of PPY will have less electroactivity than does a thick film. Chronoamperometric curves at higher than 0.8 V potential are characterized by the disappearance of transition region, because the high potential may breakdown the electrical double layer. From preliminary experiments using different concentrations of pyrrole and AMPSNa at 0.9 V, it is found that the optimum condition for preparation of PPY is 0.45 M for pyrrole, and 0.5 M for AMPSNa. 3.2. CV studies Fig. 3 shows the CVs of PPY films in 1 M AMPSNa at different scan rates. The AMPSNa doped-PPY film was prepared from an aqueous solution of 0.45 M pyrrole and 0.5 M AMPSNA by passing 0.795 C at 0.9 V. The oxidation peaks were observed during the positive sweep at about 0.0 V, accompanied by the appearance of reduction post-peaks during the negative sweep at about −0.8 V. In case of using small size ions as the dopant, the oxidation peaks are linked to the outward movement of cations in the PPY, and reduction peaks to that of anions. With large dopant, the cation movement becomes more dominant than anion movement, because the larger dopant anions are more strongly bound to the polymer and thus are less prone to be replaced by smaller ions. Vidanapathirana et al. [20] confirmed this using quartz crystal microbalance measurement. In this work, it is found that with the largest dopant ions, only cations move in and out of the polymer during cycling [10]. Peak current increases linearly with increasing scan rate, indicating the absence of kinetic or transport
Fig. 3. CVs of PPY film, prepared from an aqueous solution 0.45 M pyrrole and 0.5 M AMPSNa, by passing 0.795 C (1.7 μm) at 0.9 V in 1 M AMPSNa solution with different scan rates.
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limitation [21]. The reduction peak currents are higher than those of the oxidation current indicating that electrolyte ions move in the PPY film during reduction process and enhance the doping level. Generally, the reversibility of electrochemical reaction is determined by the redox peak potentials on the scan rate. The peak potentials are not dependent on the scan rate in the reversible process, but are dependent in the irreversible process. The shift of reduction peak to the negative potential and of the oxidation peak to positive potential with increasing scan rate indicates that this redox process is irreversible [22,23]. The dependence of the electroactivity of PPY film on thickness is important, because the doping levels depend on the charge consumed during polymerization process [24–27]. The AMPSNa-doped PPY film was prepared from 0.45 M pyrrole and 0.5 AMPSNA at 0.9 V by passing different amounts of charge. The thickness of the PPY films was estimated based on the charge consumed during electropolymerization, assuming that 240 mC results in 1.0 μm [1]. Redox activity of the films increases with increase in thickness as shown in Fig. 4. This is possibly because PPY film thickness is directly proportional to the charge consumed during electropolymerization [24]. The dependence of redox properties and doping level of the polymer films on the thickness was observed for titanocene- and benzensulfonatedoped polypyrrole films. It was found that doping level decreases with increase in thickness [25,26]. However, using Raman spectroscopic analysis, Chen et al. show that the doping level increases during the process of film growth and this is consistent with the findings of this study [27]. In addition, the anodic and cathodic peak potentials of the PPY film shifted to more positive and negative values, respectively, with increase in polymer thickness. This is because the amount of dopant increases with film thickness, which hinders further permeation of cations [28]. The voltammetry curves of AMPSNa-doped PPY are in good agreement with the published data on PPY doped with sodium dodecyl sulfate and benzenesulfonate [11,25]. However, the reduction peak currents are broader. 3.3. FTIR spectroscopy FTIR spectroscopy was used to characterize the AMPSNa-doped polypyrrole, prepared from 0.45 M pyrrole and 0.5 M AMPSNa solution by passing 0.795 C (1.7 μm) at 0.9 V as shown in Fig. 5. The peaks at 1317 cm− 1 are attributed to C–H bending deformation and those at 1135 cm− 1 to N–H bending deformation vibrations. The peak near 1178 cm− 1 reflects the doping state of polypyrrole owing to the formation of bipolaron charge carrier [29]. Characteristic absorption peaks of pyrrole ring at 1561 and 1486 cm− 1 are assigned to the C=C and C–C stretching vibration [30], and the band near 2927 cm− 1 to C–H stretching vibrations in AMPSNa [31]. The spectrum of PPY yields an intense band at 1083 cm− 1 emanating from the dopant. The N–H wagging band between 815 and 666 cm− 1 is also clearly seen [32].
Fig. 4. CVs of PPY films of varying thickness prepared from an aqueous solution 0.45 M pyrrole and 0.5 M AMPSNa at 0.9 V in 1 M AMPSNa solution with 50 mV/s scan rate.
Fig. 5. FTIR spectrum of AMPSNa-doped PPY prepared from an aqueous solution 0.46 M pyrrole and 0.5 M AMPSNa by passing 0.795 C (1.7 μm) at 0.9 V.
3.4. SEM morphology Fig. 6 shows SEM micrographs of the PPY films electrochemically prepared at different potentials in the oxidized state (as deposited). The electronic properties of polypyrrole films are linked to their morphology; a smoother and denser film will generally be more conductive as it is likely to be less porous and more ordered, thereby facilitating charge transport through the film [33]. At low magnifications, PPY films show a cauliflower-like nodular surface morphology; Fig. 4 shows microspherical grains of approximately 5 μm diameter [34]. The high magnification of the images shows that the cauliflowerlike and microspherical grains consist of small grains filled with very small, nano-scale range particles. The grain size decreased with increasing potential at which PPY formed. The particle size of PPY films, prepared at 0.9 V, is about 50 nm and that of films prepared at 1.1 V is 15 nm. Hence, the particle size decreases by increasing the voltage applied to prepare PPY films.
Fig. 6. SEM with different magnifications for the PPY films prepared from an aqueous solution 0.45 M pyrrole and 0.5 M AMPSNa by passing 0.795 C at different potentials.
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Fig. 7. Nyquist diagrams of PPY–AMPSNa films prepared from 0.45 M pyrrole and 0.5 M AMPSNa aqueous solution by passing 0.795 C (1.7 μm) at 0.9 V in 1 M AMPSNa solution at different potentials.
3.5. EIS studies EIS appears to be an excellent technique for investigating bulk and interfacial electrical properties of a solid or liquid material connected to or being part of an appropriate electrochemical transducer [35]. Typical Nyquist diagrams, obtained by applying different dc potentials on PPY films in 1 M AMPSNa aqueous solution, are shown in Fig. 7. The PPY film was prepared by passing 0.795 C (1.7 μm) at 0.9 V in an aqueous solution consisting of 0.5 M pyrrole and 0.45 M AMPSNa. The shape of the impedance diagrams changed significantly with negative shift in dc potentials thus indicating that the electrochemical properties (reduced/oxidized states) of PPY vary as a function of the applied potential [36,37]. The impedance curves of all the electrodes show a distorted semi-circular shape in the high-frequency region owing to the porosity of PPY films and a vertically linear spike in the low-frequency region. The high-frequency intercept of the semi-circle with the real axis yields the Ohmic resistance, while the diameter provides the charge-transfer resistance (Rcp) of the PPY/electrolyte interface. The real axis high frequency intercepts are independent of potential and coincide with the uncompensated resistance of the bulk electrolyte solution (Rs) [36,37]. The representative data can be grouped into two categories. In the first category (at −0.5, 0.0 and + 0.5 V), the impedance plots present a simple Warburg type behavior [38]. In the second category (at +1.0 and −1.0 V), at high frequencies, charge transfer dominates with a semicircle, while at low frequencies, diffusion of the cation in the PPY film dominates. It is interesting to note that the impedance values of PPY films at −1.0 V and −0.5 V are lower than the expected values.
This observation does not agree with published data of Garcia et al. [38], which may be due to the outward movement of the cations from the PPY. The fit of EIS data (Fig. 7, solid line) with the proposed equivalent circuit (Fig. 8) indicates that this circuit model can adequately represent the electrochemical processes of PPY at this potential. In this model, the double layer capacity and the Warburg impedance for semi-infinite linear diffusion were replaced by two constantphase elements CPE1 and CPE2. These correspond, respectively, to the capacitance at an inhomogeneous electrode surface and to the diffusion of ions in the electrode. Thus, the double-layer capacitance and Warburg impedance of the Randles circuit were replaced by two constant-phase elements. In general, the CPE may appear from
Fig. 8. Equivalent circuit of PPY–AMPSNa films.
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Table 1 Best-fitting values of the equivalent circuit elements in Fig. 8 from the simulation of the impedance data for PPY film at different potentials.
Table 2 Best-fitting values from the simulation of the impedance data for different thicknesses of PPY films at + 1.0 V.
Fitting parameters Potential (V) − 1.0 − 0.5 0.0 + 0.5 + 1.0
Rs (Ω) 60 60 60 60 50
Rcp (Ω) 210 1.1 17,440 5765 1800
Fitting parameters α1 0.7 0.592 0.775 0.445 0.75
α2
R7 (Ω) −3
0.01×10 1.94×10− 3 1.94×10− 3 5.6×10− 3 1.0×10− 3
400 3000 3110 3748 3700
four parameters: (i) distribution of the relaxation times as a result of inhomogeneities existing at the electrode/electrolyte interface, (ii) porosity, (iii) the nature of the electrode, and (iv) the dynamic disorder associated with diffusion [39]. In the AMPSNa-doped PPY, owing to the diffusion of the cation ions from electrolyte through the films, the large size AMPSNa anions are immobilized and the redox process sets in. Hence, incorporation of the two CPE elements in the equivalent circuit corresponds to the porosity of the electrode and thus the semi-infinite diffusion of the cations becomes necessary. The impedance of the constant phase element is defined as ZCPE = A− 1(jω)− α, where pffiffiffiffiffiffiffiffiα is a fractional exponent having values between 0 and 1; j = −1; ω = 2πf is the angular frequency and A is a frequency-independent parameter. When α = 0, the CPE is an ideal resistor; when α = 1, it is an ideal capacitor (A = C), and when α = 0.5, it represents homogeneous semi-infinite diffusion [39]. The best fitting values of all these parameters for the EIS data of PPY film are listed in Table 1. The lower values of charge transfer resistance (Rcp) at −1.0 V and −0.5 V may be due movements of dopant from PPY films, which may set in the redox process. In addition, the Rcp values obtained at −1.0 V are smaller than those obtained at +1.0 V. This is because the reduction peak currents are higher than those of oxidation currents as previously mentioned under CV studies. The values of α1 at approximately 0.5 indicate that the processes have a diffusion-limited nature and the low values of α2 confirm that CPE2 is ideal resistor. Fig. 9 shows the impedance spectra for PPY films of variable thickness at + 1.0 V in 1 M AMPSNa solution. As the thickness of the
PPY thickness (μm) 1.7 9.8 15.5 26.9
Rs (Ω) 45 60 45 45
Rcp (Ω) 1800 1000 500 480
α1 0.75 0.78 0.72 0.70
α2
R7 (Ω) −6
10×10 10×10− 6 10×10− 6 10×10− 6
3700 1750 700 630
PPY film increases, the overall impedance decreases (Table 2); this is consistent with the results obtained from CVs. This may be due to the increase in PPY film thickness, which results in higher surface area owing to the enhanced number of PPY nanoparticles [36,39]. 4. Conclusion Electrochemical polymerization of 2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt (AMPSNa)-doped polypyrrole (PPY) was carried out using chronoamperometric technique. CV measurements showed that the electroactivity of PPY films, doped with AMPSNa, increases with increase in the film thickness. SEM photographs revealed that the particles of PPY form in the nano-scale range and their size depends on the potential at which PPY forms. The CVs curves of AMPSNa-doped PPY are in good agreement with the published data. However, the reduction peak currents are broader. EIS revealed that in the potential range between +1.0 and −1.0 V, at high frequencies, charge transfer dominates with a semicircle and at low frequencies diffusion of the anion in the PPY film dominates. EIS also showed that on increasing the thickness of the PPY films, the overall impedance decreases. The equivalent circuit model, based on the double layer capacity and the Warburg impedance, was replaced by two constant-phase elements to fit the experimental work of this study. Acknowledgment The authors acknowledge the financial support provided by Alexandria University under Research Enhancement Program (ALEX REP).
Fig. 9. Nyquist diagrams of PPY–AMPSNa films of different thicknesses prepared from an aqueous solution 0.45 M pyrrole and 0.5 M AMPSNa at 0.9 V in 1 M AMPSNa solution.
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