R. Schrebler, P. Cury, H. Gómez, R. Córdova and L.M. Gassa*

Instituto de Química, Universidad Católica de Valparaíso, Av. Brasil 2950, Valparaíso, Chile.
*Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA),
Suc. 4. C.C. 16, 1900 Plata, Argentina.

(Recibido: Marzo 6, 2002 - Aceptado: Octubre 1, 2002)

ABSTRACT

INTRODUCTION

Conducting polymers can be switched repeatedly between their conducting and insulating states by means of electrochemical oxidation and reduction. In view of the reversibility of this process and relatively large amount of electrical charge involved it has been suggested that such polymers could be used in batteries and as electrolytic capacitors [1]. For these reasons, there is a need to improve the mechanical properties and the formation process, but the electrical conductivity and electrochemical properties must be retained. It is well known [1] that the electrochemical activity of the conducting polymers is accompanied by the insertion and ejection of anions to electrolyte solution according to the following process:

(1)

where A^- is a dopant anion to compensate positive charges generated during the oxidation process and x is the doping level. However, the insertion of cations into the film, during the cathodic process, to compensate the slow ejection of anions, has been also described as

(2)

(3)

The formation of copolymer and composite is one of the most useful tools in polymer science in that the physical and mechanical properties of a polymer can be controlled and enhanced. Considering that the processes associated with the electrochemical behavior of the conducting polymers should involved both anions and cations, the choice of a copolymer, should not disturb the electrical properties associated with the above described processes. For this reason, copolymers that present ionic conducting characteristic should be selected. In recent years, the formation of composite has been employed for the study of conducting polymer such as polyacetylene [ 2] and polypyrrole [ 3-5] . Polypyrrole/polyethylene oxide (Ppy-PEO) composites have been studied extensively [ 6-9] and polyaniline/polyethylene oxide mixture system was also analyzed for use as an electrode material in all solid-state batteries [ 10-12] . However, polyethylenglicol, which presents similar ionic conducting characteristics, has been not extensively analyzed.

The aim of this paper was to study the electrochemical activities of polypyrrole/polyethylenglicol (Ppy-PEG) composite films, using cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) as a function of the amount of PEG, added during the pyrrole electropolymerization process.

EXPERIMENTAL

The electrochemical experiments were performed in a three-compartment glass cell. The working electrode was a Pt wire of geometric area 0.2 cm². The reference electrode was a saturated calomel electrode (SCE) properly shielded to avoid chloride ion diffusion and a large area Pt foil served as a counter electrode.

Polypyrrole films with different thickness were prepared by potential cycling at 0.05 V s^-1, between -0.50 V and 0.70 V in solutions containing 0.4 M pyrrole + 0.4 M LiClO₄ + x g/l PEG (PM: 3000) in ACN:H₂0 (1:1) (0 £ x £ 10 g/l). Then, the electroformed films were rinsed with four-fold distilled water. Cyclic voltammetry at 0.05 Vs^-1 between -0.9 V and 0.3 V, and electrochemical impedance spectroscopy at operational potential -0.5 V £ E £ 0.2 V covering the frequency range 100 mHz £ f £ 100 kHz, were carried out in 0.4 M LiClO₄ in ACN:H₂0. All potentials in the text are referred to the SCE (0.242 V vs. NHE). Detailed description of data processing by non-linear fit routines and parametric identification procedures has been given elsewhere [14].

Scanning electron microscopy (SEM) analysis was carried out ex situ, after the programmed electrosynthesis step. In this case, the electrodes were washed with ACN:H₂O, dried under argon atmosphere during 48 h. and then the Ppy film were detached from the Pt substrate. The morphology of the composite material was observed as a function of the PEG concentration and the film thickness.

RESULTS AND DISCUSSION

A typical cyclic voltammogram of polypyrrole film in a monomer free solution is shown in the insert of figure 1. The oxidation of the film is associated to a current peak at ca. -0.1 V and a corresponding reduction current peak ca. -0.2 V. It should be noted that the cathodic waved shows a broader peak than the anodic ones, which has been attributed to a lack of homogeneity of the Ppy films in the potential region of interest [15].

Fig 1. j/E response of the Pt½ Ppy-PEG½ 0.4 M LiClO4 in ACN:H2O (1:1) interface. The Ppy-PEG films were formed after 50 cycles growth in 0.4 M pyrrole + 0.4 M LiClO4 in ACN:H2O (1:1) at different PEG concentration (x g/l). Scan rate 0.05 V s-1. x = 0 (¾); x = 0.5 (- - - -); x = 1(·····) and x= 10 (-·-·-).

When the polypyrrole is formed in the presence of PEG, the voltammetric response associated with the doping/undoping processes depends on PEG concentration (Fig. 1). A large increase in reversibility is observed and the charge transfer kinetics for a PEG content of 1 g/l. This behavior suggests that PEG is present in the electrodic material and the ionic motion in the conducting polymer is enhanced. When PEG content was 10 g/l, the increase of the charge was negligible in comparison with those obtained in 1 g/l and the reversibility is lost. The same effect is observed when the Ppy-PEG film thickness increases (Fig. 2).

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Fig. 2. j/E response of the Pt½ Ppy-PEG½ 0.4 M LiClO4 in ACN:H2O (1:1) interface. The Ppy-PEG films were obtained in 0.4 M pyrrole + 0.4 M LiClO4 + 1 g/l PEG in ACN:H2O (1:1). (¾) 50 cycles growth and (·····) 100 cycles growth. Scan rate 0.05 V s-1.

Figure 3 shows the typical impedance diagrams of a composite film obtained after 50 cycles in 1g/l PEG at different potential. The shape of the Nyquits diagram at E = -0.50 V (Fig. 3a) exhibits at high frequencies a distorted capacitive semicircle followed by a second non-well defined contribution at lower frequencies. As the applied potential is shifted to more positive values the second contribution becomes enlarged (Fig. 3b-c). At E = 0.20 V (Fig. 3d), where the composite oxidized state is completely attained, the second contribution deviates from diffusion behavior towards the imaginary axis according to a pure capacitance dynamic response. It is important to note that both real and imaginary part of the impedance of the polymer in the reduced state were found to be higher than those of the oxidized state. As the electrode polarization is returned to the initial negative potential, in order to attain the initial reduced state, the impedance values an the original frequency response can be recovered (Fig. 3e-f). In the case of Ppy film obtained in PEG free solution (Fig. 4a), can be observed that the values of the impedance in the reduced state were approximately ten times higher than those of the oxidized state. This fact can be associated with the irreversibility of the doping-undoping process due to the lack of homogeneity of the films. On the other hand, sometimes, for thicker Ppy films at both oxidized and reduced state, a Warburg impedance contribution Z_W is observed in the experimental impedance diagrams which is related to a diffusion process of anions through the film. However, this diffusion contribution was not observed in the films of Ppy+PEG response.

Fig. 3. Impedance spectra corresponding to the Pt½ Ppy-PEG½ 0.4 M LiClO4 in ACN:H2O (1:1) interface. The Ppy-PEG films were obtained in 0.4 M pyrrole + 0.4 M LiClO4 + 1 g/l PEG in ACN:H2O (1:1), after 50 cycles growth at different operational potentials.

An increase of the reversibility with the PEG addition is reached, even in low concentration (Fig. 4b). The fast kinetics of the redox process was also demonstrated by voltammetric results (see Fig. 1b). For thick films prepared after 100 cycles, Nyquist diagrams (not shown here) exhibit two well-defined capacitive contribution at high and intermediates frequencies, whereas at the lower frequencies the time constant is not well defined. Similar response is obtained as PEG concentration increase (Fig. 4c-d).

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Fig. 4. Nyquist plots of the Pt½ Ppy-PEG½ 0.4 M LiClO4 in ACN:H2O (1:1) interface. The Ppy-PEG films were formed after 100 cycles growth in 0.4 M pyrrole + 0.4 M LiClO4 in ACN:H2O (1:1) at different PEG concentration (x g/l). (a)´ = 0; (b) x = 0.5; (c) x = 1 and (d) x= 10 g/l. (�ð �) E = -0.1 V and (o) E = 0.2 V.

The higher charge and the presence of a complex cathodic peak in the voltammetric profiles, as well as a new capacitive constant at intermediate frequencies in the impedance responses, when the PEG concentration increases, can be attributed to that during the redox processes, both anions and cations of the electrolyte move in Ppy-PEG film. In the case of cations participation, the electric neutrality of the film could be maintained by insertion of electrolyte cations instead of the total elimination of anions from the film, mainly in the last state of the reduction process, such as was suggested by A.F. Diaz et al. [16] and J. Kaufman et al. [17] for Ppy films in acetonitrile solutions. The last authors, also suggested that electrolyte cations were incorporated in Ppy films in the course of electrochemical reduction and that the degree of the incorporation was dependent on the kind of solvent used. On the other hand, the electrochemical behavior of Ppy-PEG electrode has been similar to those found with Ppy-Nafion electrode [18]. In this case, the good reversibility attained was explained by the large mobility of Na⁺ in nafion, which allowed the fast charge neutralization in the conducting polymer during de redox processes.

The experimental impedance data can be well described by the following transfer function

where

and w = 2p f. The high frequency limit R_W corresponds to the ohmic resistance of the electrolyte, whereas [ CPE] = [ C_dl(jw )a ] ^-1 involves the double layer capacitance, C_dl, and the parameter a , that takes account of the interface roughness.

For Ppy films at both oxidized and reduced state:

and for Ppy-PEG films:

where R_ct is associated with the charge transfer resistance of the film corresponding to the doping/undoping anions processes, R and C are related to the second time constant, which probably, could be associated with the insertion of cations into the film and C_L takes into account the so-called "finite length effects" [ 19] .

The good agreement between experimental results and simulated data, the latter according to transfer function analysis by using non-linear least-square fit routines, is illustrated in Figs. 5a and 5b for 0 V and 0.3 V, respectively.

Fig. 5. Bode plots of the Pt½ Ppy-PEG½ 0.4 M LiClO4 in ACN:H2O (1:1) interface. The electrode system was formed under the same conditions indicated in Fig. 4c (1 g/l PEG). (· ) experimental curve and (¾) fitted curve using the transfer function (4). (a) 0.0 V and (b) 0.3 V.

From the optimum fit procedure according to the transfer function given in Eqs. (4)-(7), C_dl = 6 ± 1 m F cm^-2, with a = 0.5 ± 0.05 can be calculated for Ppy + x PEG (0 £ x £ 1 g/l) films.

The obtained capacitance values are lower than those related to the C_dl. Therefore, it is possible to consider that the calculated capacitance values correspond to the addition of double-layer and polymer layer capacitances, i.e., C^-1 = C_dl^-1 + C_pol^-1. Accordingly, values of the polymer capacitance of 8 ± 1 m F cm^-2 can be calculated. On the other hand, the values of a can be related to an open structure of composite films. Such structures have been reported by other authors [ 14, 20-21] in the case of Pani and Ppy films.

In the case of Ppy + x PEG (x > 1 g/l) films, C_dl = 40 ± 10 m F cm^-2, with a = 0.8 ± 0.05 can be calculated. These values of a , which are higher than those above given, permit to assume that the porous structure of composite films with high PEG concentration is partially blocked by PEG molecules.

It is interesting to know that at lower PEG concentration the main process of charge transfer would be associated with the anionic exchange, according to Eq. 1, although PEG is a cationic conductor. This is due to the fact that polymer maintains the open structure as indicate the low a values and the insertion to cations into the film is a secondary way to charge transfer process. However, when the PEG concentration increases, a large amount of PEG is incorporated into the Ppy and a more compact structure is attained, which hinders the ejection of anions and the process associated with the Eq. 2 becomes favored.

The values obtained for the low-frequency capacitance, C_L, were in the range of 40 to100 mF cm² for Ppy, and change to 380 - 480 mF cm² for 1g/l and 150 - 210 for 10g/l. Similar C_L values have been reported previously [ 22] for polymer films. Furthermore, C_L can be correlated with the film thickness, d_p. Provided that there is no dielectric relaxation in the frequency range used, C_L is given by

where e denotes the dielectric constant of the film and e _o is the permitivity in vacuum, e_o = 8.55 10^-14 F cm^-1. It has to be noted that for Ppy and Ppy-PEG films used in this work (5-10 m m) the relation between C_L and d_p could not be applied, unless unreasonably high e values are envisaged. Very similar behaviors have been found for Ppy [ 23] and Pani [ 24] films. It was concluded that, on account of the high polymer porosity, ion diffusion within the polymer requires a much shorter path than the geometrical thickness.

Optical measurements.

The electrochemical techniques show the effect of the PEG on the electrochemical behavior of Ppy, but optical techniques, such as scanning electron microscopy SEM, provided interesting information about the Ppy-PEG film morphology. The SEM pictures Fig. 6, correspond to films obtained at different PEG concentrations. The Fig. 6a corresponds to a Ppy film without PEG which shows that the surface film exposed to the solution has a granular structure with high porosity. Otherwise, the side that was in contact with the substrate metal is smoother and compact (Fig 6b). When the Ppy film is produced in presence of 1 g/l PEG (Fig. 6c), the micrography exhibits PEG crystallites inserted into Ppy film, which present a more elongated and flat shape. These crystallites can reach the film side which was in contact with the metal. These facts could be indicated that the PEG molecules and the Ppy chains are interacting from the first state of the film polymerization. This can be attributed to that both polymeric molecules present hydrophilic and hydrophobic regions, which allows that PEG remain entrapped in Ppy and the composite formation is attained. The opened structure showed in both cases, 0 and 1 g/l PEG concentration, are in good agreement with the low a values above reported.

Figure 6. SEM images of Ppy-PEG films obtained at different PEG concentration. ]]> (a , b) x = 0 g/l PEG; (c , d) x = 1 g/l PEG and (e, f) x = 10.g/l PEG (a, c, e) correspond to view of the film face which were exposed toward the electrolyte and (b, d, f) correspond to the film faces that were in contact with the Pt surface.

When the composite is prepared in 10 g/l PEG (Fig. 6e), the SEM shows a high amount of PEG crystals both into the film and adsorbed onto the surface, which was in contact with the solution. It is interesting to note that an important amount of crystals also arrived at the substrate/film interface (Fig. 6f). The high coverage of the composite surface by PEG could hinder, both the insertion and ejection of anions, and enhancing the cation move, which could explain the complex cathodic peak in voltammetry technique and the second capacitive time constant in the impedance response.

Taking into account the morphology of the different films and the electrochemical results above described, it is possible to ensure that PEG facilitates the mobility of cations into the film, due to the large amount of oxygen in its molecule and when it is present the reaction that described this process can be discussed through as following

Although, H. Yoneyama et al. [2] proposed that the neutral Ppy can be reduced and the cations which arrive from the solution to compensate the negative charge of the polymeric film, in this case there is not experimental evidence that reduction of the composite films continues after that oxidized (Ppy)^x+ species are reduced, but it is an hypothesis non completely discard.

CONCLUSIONS

The electrochemical and optical techniques provide an interesting information about the Ppy-PEG interactions and film morphology. The incorporation of polyethylenglycol in polypyrrole films enhances the mobility of ions because it favors the insertion and ejection of ions from electrolytic solutions and therefore the electronic charge transfer of the polymer is increased. When PEG concentration is low, an open structure of the film is obtained and consequently the insertion and ejection of ions to electrolyte is accelerated. While at high PEG concentration, the polymer conduction decrease since only cations insertion and ejection processes are taking place.

ACKNOWLEDGMENTS

This research project was financially supported by FONDECYT (grant Nº 8000022) and DI-UCV of Chile, and by Consejo Nacional de Investigaciones Científicas y Técnicas, the Comisión de Investigaciones Científicas de la Provincia de Buenos Aires.

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