ELSEVIER
Reactive & Functional Polymers 35 (1997) 145-151
REACTIVE & FUNCTIONAL POLYMERS
Some aspects of the electrochemical synthesis of polypyrrole .pa:~
S. Stankovlc ' , R. Stankovi6 a, M. Risti6 a, O. Pavlovi6 b, M. Vojnovi6 a a Faculty of Technology and Metallurgy, University ofBeograd, POB 494, 11001 Beograd, Yugoslavia I, Faculty of Technology, University of Novi Sad, 21000 Novi Sad, Yugoslavia Received 24 July 1996; revised version received 15 May 1997; accepted 21 May 1997
Abstract
Polypyrrole (PPy) films were synthesized under galvanostatic conditions on Pt anodes in a solution of N-methylpyridinium perchlorate in propylene carbonate (PC). The influence of current density, temperature and the amount of water in the reaction mixture on the conductivity and morphology of the prepared PPy films was studied. It was shown that the electrochemical activity of thick PPy films (20 Ixm) in a solution of 1.0 M LiC104 in PC was satisfactory. Keywords: Polypyrrole films; Electrochemical polymerization; N-Methylpyridinium perchlorate electrolyte; Electrochemical activity
1. Introduction Conducting polymers such as polypyrrole (PPy) have attracted much attention as candidate for a wide range of applications including batteries [1-3], sensors [4] and electrocatalysis [5]. Three methods are generally used for PPy preparation: electrochemical polymerization [6], chemical polymerization in solution [7], and chemical vapor deposition [8]. With a simple electropolymerization method, PPy can easily be formed as a strongly adherent to various substrates smooth film possessing good mechanical properties and environmental stability. PPy also exhibits a high electrical conductivity and good electrochemical activity with the usual dopant ions [9]. The electrochemical polymerization is of particular interest in that the structure, thick* Corresponding author.
ness, conductivity and electrochemical properties of a resulting PPy film can be controlled by filmgrowing rates (formation potentials and current densities), formation charges, temperature, solvents and supporting electrolytes [10,11]. However, it seems that the optimal synthesis conditions, in terms of conductivity, electrochemical activity and mechanical properties have not yet been realized. In this study N-methylpyridinium perchlorate (NMPP) was used as the supporting electrolyte and effects of formation parameters (current density, the amount of water in the reaction mixture, temperature, and concentration of the supporting electrolyte) on the conductivity, morphology and electrochemical activity of the PPy were investigated. 2. Experimental The electrochemical synthesis of PPy films was performed in a three-compartment cell. The
1381-5148/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 1 3 8 1 - 5 1 4 8 ( 9 7 ) 0 0 0 9 0 - 4
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S. Stankovig et aL /Reactive & Functional Polymers 35 (1997) 145-151
working electrode was a Pt foil (1.5 cm 2) which was cleaned and dried before each experiment. The counter electrode was a Pt wire separated from the working electrode by a No. 2 glass sinter. The reference electrode was a Ag/AgC1 electrode separated from the working electrode by a Luggin capillary. PPy films were deposited galvanostatically by the anodic oxidation of pyrrole (Py) in a de-aerated (purged with purified nitrogen) and slowly agitated reaction mixture. Py (Merck, zur Synthese) was freshly distilled under reduced pressure of 100 mbar and stored under nitrogen in the dark at low temperature. NMPP was prepared as follows. Methyl iodide was reacted with pyridine at 78°C in ethanol. A yellow precipitate of N-methylpyridinium iodide was filtered, washed with ethanol and dried under vacuum. An aqueous solution of AgC104 (Aldrich, p.a.) was then reacted with N-methylpyridinium iodide to give an aqueous solution of NMPP which was easily separated from the precipitate of AgI. NMPP was recovered from a concentrated aqueous solution by the addition of diethylether. The electrical conductivity of PPy films was determined on a freshly prepared free standing samples (20 txm) by a four-probe method at a constant current of 0.5 mA. Scanning electron microscopy was employed to characterize the surface morphology of PPy films. Observations were carried out without applying gold plating. The electrochemical activity of PPy electrodes was studied by recording cyclic voltammograms. For these tests, the PPy electrodes (1 cm 2) were dried under vacuum at 50-60°C for at least 48 h and then assembled in a cylindrical Teflon cell having Li as both the counter and reference electrode. A solution of 1.0 M LiCIO4 in PC was used as the electrolyte. The electrolyte was always purged with purified Ar prior to the cyclic voltammetry experiments. The electrochemical tests were carried out in the dry-box in an atmosphere of purified argon. Particular care was taken in purification of PC and LiC104. PC
(Fluka, purum 99%) was purified by a fractional vacuum distillation and dried over activated molecular sieves 5A. LiC104 (Fluka, purum, p.a.) was dried under vacuum at 140-160°C for at least 48 h. Cyclic voltammetry measurements were carfled out using a potentiostat (PAR-M173), an universal programmer (PAR-M175) and a X-Y recorder (HP-M7046B). The cyclic voltammetric analysis was performed with the ohmic drop correction. The ohmic resistance of the solution between the PPy electrode and the Luggin capillary was determined by means of impedance measurements. 3. Results and discussion For the electropolymerization of Py in organic media (as well as in many aqueous electrolytes), the electrochemically induced cationradical coupling mechanism is commonly accepted [12,13]. Accepting this mechanism, it becomes evident that the polymerization reaction will be sensitive to the presence of a nucleophile and H + scavenger in the reaction mixture [12,14]. Under the described conditions of polymerization, the reaction on the cathode is the reduction of water producing O H - ions. These ions can diffuse trough the sinter glass into the working electrode compartment and, being strong nucleophiles, they can react with the PPy decreasing its conductivity [15]. In addition to the interaction with the PPy, O H - ions also interact with the cations of the supporting electrolyte. It seems reasonable to suppose that the interaction between the O H - ions and the cations of the electrolyte depends on the cation size and structure. It might be expected that the activity of the O H - ions will be decreased by the presence in the reaction mixture of large N-methylpyridinium cations instead of small ones (such as Li + when LiC104 is used as the supporting electrolyte). The effects of current density and amount of water in the reaction mixture on the conductivity of PPy are presented in Figs. 1 and 2 respectively.
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Fig. 1. The effect of current density on the conductivity of PPy films prepared at 0°C in 0.1 M NMPP and 0.5 M Py in PC with different amount of water in the reaction mixture. I
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decrease of the PPy conductivity at higher current densities has been explained by side reactions due to overvoltage [10]. The decrease of the PPy conductivity at low current densities could be explained by possible densification effect due to prolonged polymerization [19]. Namely, Penner et al. have assumed that, when a PPy film is formed at less positive potentials, the portion of the film in the region near the electrode surface becomes denser as polymerization proceeds. The change in morphological composition would influence conductivity by changing the total contact area between conducting centers inside the film itself [20]. In an attempt to optimize the PPy synthesis, the effect of the initial NMPP concentration and temperature on conductivity was examined. The results are presented in Fig. 3. The influence of the temperature on the PPy conductivity (Fig. 3b) is not quite in agreement with reported experimental results [10,16,21], that decreasing the temperature of PPy synthesis increases its conductivity. The decrease of
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Fig. 2. The effect of water amount in a reaction mixture (0.1 M NMPP+0.5 M Py+PC) on the conductivity of PPy prepared at 0°C with a current density of 10 m A cm -2.
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As reported by others [10,16], a rather strong dependence of the PPy conductivity both on the applied current density and amount of water is found. The overall rate of polymerization is a function of current density which determines the rate of the monomer oxidation. The presence of small amounts of water in organic aprotic solvents exerts a favorable action on the synthesis and properties of PPy [10,16,17]. A mechanism for action of water has been proposed [18]. The
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Fig. 3. The effect of initial NMPP concentration (a) and temperature (b) on the conductivity of PPy films prepared in 0.5 M Py in PC with 6 vol% of H 2 0 and a current density of 10 m A cm -2.
148
S. Stankovi( et al. /Reactive & Functional Polymers 35 (1997) 145-151
Fig. 4. SEM micrographs of a PPy film synthesized in 0.1 M NMPP and 0.5 M Py in PC with 6 vol% of water at 0°C and at a current density of 6 m A cm -2.
the PPy conductivity at low temperatures (<0°C) could be explained by the fact that NMPP is more stable at low temperatures and, therefore, the concentration of counterions in the reaction mixture is smaller. Contrary to our expectation, the highest observed conductivities of the PPy synthesized with NMPP were quite close to the conductivities of PPy obtained with LiC104 under similar conditions [10,12,16]. Thus, no effect of cation nature and size on PPy conductivity was found. However, when NMPP was used as the supporting electrolyte the highest conductivities of the PPy were obtained at higher current densities and amount of water than in the case of LiC104 [16]. It has been proved that morphologies of PPy films are greatly influenced by the film growing rates, as well as by formation charges, solvents, cosolvents and supporting electrolytes [1,2,9,11, 12,16,17]. For PPy films prepared in the presence of LiC104 dissolved in PC with 1-6 vol% of water (cosolvent), a so-called 'cauliflower' texture was formed [9,16,22,23]. However, in the presence of NMPP globules resembling hollow rasp-
berries were formed (Fig. 4). Such morphology is of interest as it represents a developed film surface, i.e. favorable property for battery application.
3.1. The electrochemical activity of polypyrrole The repetitive cyclic voltammograms of a PPy electrode obtained immediately after immersion in the 1.0 LiC104-PC solution are shown in Fig. 5. The PPy electrode was formed by electrodeposition from a 0.1 M NMPP solution in PC with 6 vol% of water at a current density of 10 mA cm -2. Although it has been shown [22] that the sharp voltammetric peaks, found with thin and ultrathin PPy layers, flatten for layers thicker than a few Ixm and disappear finally for layers thicker than some tens of Ixm, the anodic and cathodic peaks for a 20-1xm thick PPy electrode (prepared under given conditions) are quite well defined. As it has been also reported by other authors [24,25], the amount of stored and released charge is enhanced during the early cycles. The number
S. Stankovid et al./Reactive & Functional Polymers 35 (1997) 145-151
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Fig. 5. Typical repetitive voltammograms of a dry PPy electrode after immersion in a 1.0 M L i C 1 0 4 - P C solution at 25°C. The cycle numbers are shown directly in the figure. Li counter and Li reference electrode. Scan rate: 20 m Vs -1 .
of cycles requiring for the activation process are dependent on the film thickness [26]. The 20-1xm thick PPy film electrodes synthesized under the above given conditions, required about 20 cycles to become fully electrochemically active. When the steady state conditions were achieved, the anodic and cathodic waves remained well defined and the integrated charge under the reduction wave was similar to that under the oxidation wave. The electrochemical oxidation and reduction of PPy in a LiC104- PC solution, occurring during the cyclic process, can be presented by the following general scheme: doping
[PY]x + x y A -
,
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where x is the degree of polymerization (10 < x < 1000) [12], y is the charge of a polymer unit (or the doping level), and A - is the dopant ion (C104). The oxidation-reduction process of PPy is complicated as is indicated by the overall appear-
149
ance of the voltammograms presented in Fig. 5: the cyclic voltammetric peaks are broad, asymmetric and the separation between the peak potentials (AEp) is large and scan-rate dependent. It has been suggested [26-30] that the electrochemical charging, i.e. cyclic voltammetric behavior of a redox polymer film, might be influenced by an ohmic resistance of the film uncompensated by the use of three electrode cells, which may cause loss of potential control, particularly in potential sweep experiments. The kinetics of an electrochemical anion doping-undoping process and its cyclic voltammetric behavior are strongly associated with the film morphology obtained during electropolymerization [1,2,9,16,22,23,31,32]. By investigating ion and electron transport in tetrathiafulvalene polymer coated electrodes, Kaufman et al. [26] have proved that the metallic surface is completely covered by the electroactive film without pore or channel defects. In other words, a macroscopic permanent physical pore structure is not necessary for the electrochemical activity of a polymer film. However, results obtained by studying the electrochemical behavior of soluble redox couples at a PPy electrode (deposited on a solid substrate) lead to the conclusion that the PPy is microporous, i.e., it is a biphasic system composed of solvent-swollen polymer and solvent-filled pores [ 1,19,23,31-34]. It is clear that the Faradaic reaction according to Eq. 1 must be dependent on two simultaneous processes: the transfer of electrons either from or to the conductive coating and the transport of counterions into or out the coating [26,35-38]. When the electron process is rate controlling, the cyclic voltammetric response will be such that the peak current is proportional to the potential scan rate [9,35,37,39]. If the diffusion of counterions is rate controlling, the peak current will be proportional to the square root of the potential scan rate [9,35]. However, if the current flow through the microporous conducting polymer film is controlled by the ohmic resistance of the layer-pore system, both the peak current and the peak potential should be proportional to the
S. Stankovid et al. / Reactive & FunctionalPolymers 35 (1997) 145-151
150
square root of the potential scan rate [40]:
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(3)
where z (= x y ) is the number of electrons exchanged per one macromolecule, F is the Faraday constant (C mol-1), p is the PPy density (g cm-3), K is the ionic conductivity in the micropores (S cm-1), M is the molar mass of PPy (g mol-1), Ao is the total surface of the film (cm2), 0p is the part of the surface of the film through which transport of counterions takes place at I = Ip, d is the film thickness (cm), Ro is the resistance of the electrolyte ([2) and v is the potential scan rate (V s-l). The proportionalities Ip vs. v 1/2 and Ep vs. v 1/2 are verified in Fig. 6. The obtained experimental results clearly show that in the studied range of the potential scan rate, i
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PPy prepared in nonaqueous media with NMPP as the supporting electrolyte have the electronic conductivities quite close to the conductivities of PPy obtained with LiC104 under similar conditions. Contrary to the 'cauliflower' texture observed for PPy obtained with LiC104, globules resembling hollow raspberries were formed. The rate of the doping-undoping reaction is controlled by the transport rate of counterions (C104) through the microporous structure of the polymer film. References
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4. Conclusions
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the doping-undoping reaction in a 20-txm thick PPy film is controlled by the transport rate of counterions through the polymer structure. This conclusion is consistent with results of other authors that the peak current increases linearly with increasing the potential scan rate only for very thin PPy films [20,22,31,35,41,42]. However, thicker samples show a linear behavior for Ip vs. v 1/2, especially at higher scan rates [20,22,31,41-43].
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