Electropolymerization of Polypyrrole and PolyanilineâPolypyrrole from

9044

J. Phys. Chem. B 1999, 103, 9044-9054

Electropolymerization of Polypyrrole and Polyaniline-Polypyrrole from Organic Acidic Medium Florence Fusalba and Daniel Be´ langer* De´ partement de Chimie UniVersite´ du Que´ bec a` Montre´ al, Case Postale 8888, succursale Centre-Ville, Montre´ al, Que´ bec, Canada H3C 3P8 ReceiVed: May 24, 1999

Polypyrrole (PPy) grown from organic media and polymeric material produced by coelectropolymerization of pyrrole and aniline in organic acidic media were investigated by cyclic voltammetry, electrochemical impedance spectroscopy (EIS), differential scanning calorimetry (DSC), infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The cyclic voltammogram of the polypyrrole grown from non aqueous media containing an organic acid (CF3COOH) is not significantly affected up to a 0.5 M CF3COOH concentration in the deposition solution. At higher CF3COOH concentration a shift of the redox waves to more positive potential is observed but the voltammetric charge of the cyclic voltammogram of the polymer and the stabilization potential during the galvanostatic growth of the polymer are barely affected. The coelectropolymerization of pyrrole and aniline was carried out in acetonitrile in the presence of an organic acid (CF3COOH) and an appropriate supporting electrolyte (tetraethylammonium tetrafluoroborate or tetramethylammonium trifluoromethanesulfonate). The resulting polymer, named thereafter PANIPY, is electroactive over a potential range of ca. 1.5 V. The thermal properties of the films were studied by DSC, and the thermograms suggest the presence of a mixture of polypyrrole, polyaniline, and a random polyaniline-polypyrrole copolymer. This is also confirmed by the FTIR spectra of the polymer grown from a mixture of aniline and pyrrole which are not the sum of the spectra of each individual polymers. SEM photomicrographs of the PANIPY polymer deposited onto carbon paper electrode show a smooth and even surface. Low-frequency capacitance (CLF) as high as 100 mF/cm2 (60 F/g) is evaluated from the EIS data. Galvanostatic charge-discharge cycling between a cell voltage of 0 and 1.5 V shows energy and power densities of 5 Wh/kg and 1.5 kW/kg of composite polymer, respectively, for a discharge time of about 13 s.

Introduction Electronically conducting polymers have been the subject of numerous investigations in the past two decades.1-3 These materials have properties that make them suitable for several applications including light-emitting diodes, sensors, batteries, and electrochemical supercapacitors. For the majority of these studies, a single polymer was prepared by chemical or electrochemical oxidation of the corresponding monomer. This asgrown oxidized polymer is not pure and can be actually considered as a composite since counterions are included in the polymer matrix as dopant. On the other hand, the formation of copolymer displaying electronic conductivity has been much less investigated.4-13 The main motivation for preparing copolymer composites lies in the possibility that these materials will display better properties and also to overcome the limitation of the rareness of new conjugated π-bond-containing monomers. The preparation of copolymers from a pair of monomers will lead to an increase of the number of conductive polymers obtained from the same set of monomers.13 Several polypyrrole copolymers have been prepared and representative examples will be presented below.4,6-16 Copolymers of pyrrole and thiophene (or bithiophene) have been prepared from the corresponding monomers.4,6-11 Garnier et al. have prepared poly(bithiophene)-polypyrrole composite and * Author to whom all correspondence should be addressed. E-mail: [email protected].

copolymers to protect a CdSe photoanode from photocorrosion.4 The copolymer is made of randomly distributed monomer subunits and is characterized by a unique cyclic voltammogram which differs from those of polypyrrole and polythiophene. Yoneyama et al. have electrochemically copolymerized pyrrole and thiophene in acetonitrile from a high thiophene and a low pyrrole concentration solution.6 They have used IR spectroscopy to demonstrate that the resulting polymers are copolymers instead of mixtures of polypyrrole and polythiophene. Conducting copolymers made by electropolymerization of mixtures of 2,2′-bithiophene (BT) and pyrrole (Py)8 were characterized by cyclic voltammetry and UV/visible spectroscopy and consist of sequential Py and BT units and random segments of Py and BT. The distribution of these segments within the copolymer chain depends on the potential of the working electrode. The copolymer films were electronically conducting, but the conductivity was reduced as BT was incorporated into a homopolymer of Py. Sanchez De Pinto et al. have prepared PTh, PPy and PTh/PPy copolymer for active electrode material in lithium batteries and have reported enhanced energy capacity for the copolymer with respect to both homopolymers.9 Cha has investigated the competitive polymerization reaction of thiophene and pyrrole and has found that the electropolymerization rate of thiophene is decreased upon addition of pyrrole in the growth solution due to the high overpotential for polythiophene growth.10 In that study, it was also found that the rate of PPy polymerization was enhanced in the mixture compared to the

10.1021/jp9916790 CCC: $18.00 © 1999 American Chemical Society Published on Web 09/29/1999

Electropolymerization of Ppy and PANI-Ppy deposition of PPy alone due to the electrophoretic effect of the pyrrole radical cation. An innovative approach to a PPy/PTh composite proposed by Zotti at al. consists of growing electrochemically a PPy film in the presence of a sulfonated thiophene derivative.11 The later acts initially as dopant for PPy and is subsequently polymerized by cycling to the appropriate potential. The CV of the resulting polymer shows the redox waves of PPy and that of the intercalated polythiophene derivative. On the other hand, copolymers produced from pyrrole and aniline are scant because individual polymers are usually grown from two very different electrolytes. Typically, PPy is grown from nonaqueous media14 whereas polyaniline (PANI) is prepared from acidic electrolyte.15 Talu et al.13 have recently demonstrated that copolymer composites of pyrrole and aniline can be prepared despite the requirement of different polymerization conditions. Homopolymerization and copolymerization reactions were carried out in acetonitrile, using LiClO4 as supporting electrolyte and in either a perchloric acid or a sulfuric acid medium. The so-called copolymerization was carried out by growing a first layer of a polymer and on the second step the other polymer was deposited but the coelectropolymerization of the two monomers from the same electrodeposition solution was not attempted. They found that the change of media (organic vs aqueous) and changing the order of coating affected the structure and properties of polymers. In this study, we report the electropolymerization of polypyrrole and the coelectropolymerization of PANIPY from two low-cost monomers, aniline and pyrrole, from organic acidic media. First, the electropolymerization of PPy was optimized and these optimum conditions were used as a starting point for the coelectropolymerization of PANIPY. The later polymer was characterized by a variety of electrochemical and spectroscopic techniques. The morphology and the thermal properties of the polymer were evaluated by scanning electron microscopy and differential scanning calorimetry, respectively. Finally, preliminary galvanostatic charge/discharge cycling was performed on electrochemical capacitors assembled from aniline-pyrrole copolymer-coated carbon paper electrodes. Experimental Section Chemicals. Acetonitrile (ACS) from Aldrich was distilled before use. On the other hand, HPLC grade acetonitrile was used as received. Electrodeposition of Polypyrrole. The polymer films were grown from a solution containing pyrrole (0.1-1 M) and either 1 M tetraethylammonium tetrafluoroborate (Et4NBF4) or 0.5-1 M tetramethylammonium trifluoromethanesulfonate (Me4NCF3SO3) in acetonitrile. Trifluoroacetic acid (between 0.1 and 2 M) was also added to this solution. The electropolymerization was performed galvanostatically at an appropriate current density (0.05-0.5 mA/cm2). The films were then washed in acetonitrile in order to remove soluble species from the film and cycled in a monomer-free acetonitrile solution containing either 1 M Et4NBF4 or 0.5-1 M Me4NCF3SO3. Electrodeposition of Polyaniline. Polyaniline was grown from a solution containing 0.5 M aniline, 1 M Et4NBF4, and 2 M CF3COOH in acetonitrile. The electropolymerization was performed galvanostatically at a current density of 0.5 mA/cm2. The films were then washed in acetonitrile and cycled in a monomer-free acetonitrile solution containing 1 M Et4NBF4 or 1 M Me4NCF3SO3. Electrodeposition of the PANIPY Polymer. The PANIPY polymer film electrode was prepared from 0.5 M aniline, 0.1 or 1 M pyrrole, 2 M CF3COOH, and 1 M Me4NCF3SO3 in

J. Phys. Chem. B, Vol. 103, No. 42, 1999 9045 acetonitrile. In some cases, the Me4NCF3SO3 salt used as supporting electrolyte was replaced by Et4NBF4. The electropolymerization was performed galvanostatically at a current density of 0.5 mA/cm2. The film was then washed in acetonitrile and cycled in a monomer-free solution of 1 M Me4NCF3SO3 in acetonitrile. Procedure and Equipment. Sample Characterization. A differential scanning calorimeter (DSC, Perkin-Elmer Model DSC-7) was used to analyze the polymers in the temperature range 50-500 °C with a heating rate of 20 °C/min. The instrument was previously calibrated in the same analysis conditions (scan speed and temperature) with indium and zinc whose melting points are 156.61 and 419.59 °C, respectively.16 The data were collected with Pyris software for Windows (Version 2.01, 1996, Perkin-Elmer Corporation). Polyaniline, polypyrrole, and PANIPY-coated carbon paper electrodes were used for the analysis. The FTIR spectra were recorded with a Michelson Series FTIR Spectrometer (Bomem, Hartmann et Braun, MB Series, Version 1.51). Typically, 16 scans were recorded between 4000 and 400 cm-1 (resolution: 1 cm-1). FTIR data were collected with Bomem Grams/386 software for Windows (Version 3.01B, level II, 1991-1994). The samples were prepared by mixing the polymer with KBr powder and pressing the mixture into a pellet. The polymer was initially deposited onto carbon paper electrodes and then removed with a scalpel. Therefore, some carbon fibers of the carbon electrode are removed with the polymer. The morphology of the deposited films was observed using scanning electron microscopy, SEM, on a Hitachi model S-5300 microscope. The relative estimates of the atomic content of the films were obtained by using a Kevex Quantum 3600-0388 energy-dispersive (EDS) X-ray analyzer. X-ray photoelectron spectroscopy, XPS, measurements were performed with a VG Escalab 220i-XL system equipped with an hemispherical analyzer and an Al anode (KR X-rays at 1486.6 eV) used at 10 kV and approximately 15 mA. The data were obtained at room temperature and typically the operating pressure in the analysis chamber was below 1 × 10-9 Torr. XPS spectra of the PANIPY-coated platinum electrode were obtained after cycling in a 1 M Me4NCF3SO3/acetonitrile and a thorough rinse with acetonitrile. Survey scans in the range 0-1000 eV were recorded at a pass energy of 100 eV with a step size of 1 eV. Core level spectra were obtained for C 1s, S 2p, F 1s, B 1s, N 1s, and O 1s with a pass energy of 20 eV and a step size of 50 meV. Typically, five detailed scans were recorded. Curve fitting of the XPS data was carried out with the Origin software (version 5.0) from VG Instruments. A semiquantitative evaluation of the relative atomic surface concentration was obtained by considering the appropriate sensitivity factors; C 1s (1.0), S 2p1/2 (0.567), S 2p3/2 (1.11), F 1s (4.43), B 1s (0.486), N 1s (1.8), and O 1s (2.93). The binding energies were corrected for surface charging by referencing to the designated hydrocarbon C 1s binding energy as 284.5 eV. Electrochemical Measurements. All the measurements were performed in a glovebox under a dry and N2-saturated atmosphere, in a closed three- or two-electrode cell. Cyclic voltammetric studies were performed using a potentiostat model 1287 Solartron Electrochemical Interface interfaced with a PC and with the DC Corrware Software (Version 1.4, Scribner Associates). The working electrodes were carbon paper (geometric area: 1-2 cm2) having a specific surface area of 2 m2/g. A platinum wire was affixed to the carbon paper with silver epoxy (Dynaloy, Inc.) and dried at 60 °C. The resulting electrode was completed by covering the dried silver epoxy with an insulating

9046 J. Phys. Chem. B, Vol. 103, No. 42, 1999

Figure 1. Cyclic voltammogram of polypyrrole in 1 M Et4NBF4/ acetonitrile at a scan rate of 100 mV/s. Growth conditions: current density ) 0.5 mA/cm2, deposited charge ) 100 mC/cm2, A ) 1 cm2. Deposition solution: 0.1 M pyrrole/1 M Et4NBF4/acetonitrile with (- - -) and without (s) 0.25 M CF3COOH.

epoxy (The Dexter Corporation). Prior to any measurements, the carbon paper electrodes were washed in methanol in an ultrasonic bath for 1-2 min and dried at 50 °C. All potentials are quoted against an Ag/Ag+ (10 mM AgNO3 in 0.1 M tetrabutylammonium perchlorate/acetonitrile) reference electrode and a platinum flag served as a counter-electrode. During the electrochemical measurements, a total geometric area of 1 cm2 of the two carbon paper faces were exposed to the electrolytic solution. This total area has been used in reporting our results. For example, low-frequency capacitance values are given in F/cm2. For values given versus weight (i.e., F/g and Wh/kg), calculations were obtained for 2 C of the as-grown conducting polymer electropolymerized on a 1 cm2 carbon paper electrode (mpolymer ) 0.0033 g). For the film grown with a charge of 1 C/cm2, we assumed a constant polymer growth and the polymer weight was taken as 2 times smaller (0.0017 g). Electrochemical impedance measurements were performed with a model 1255 Solartron Frequency Response Analyzer coupled to a model 1287 Solartron Electrochemical Interface. Data were collected and analyzed using a PC and the Zplot Software (Version 1.4, Scribner Associates). Measurements were made in the frequency range 65 kHz-0.05 Hz at various electrode potentials using a 10 mV sine-wave amplitude for all experiments. The polymer electrode was polarized at the appropriate potential for about 50 s before the impedance measurements in order to ensure that the polymer film electrode has reached equilibrium. All the computed results are given versus the geometric area of the carbon paper and impedance data are reported for a deposition charge of 1 C/cm2. Results and Discussion Polypyrrole Growth from Organic Acid Media. The coelectropolymerization of pyrrole and aniline requires compatibility of the growth solution. Polyaniline has been recently electropolymerized from non aqueous media containing LiClO4 or Et4NBF4 as supporting electrolyte and CF3COOH as proton source to ensure high conductivity of the polymer.17-19 Thus, electropolymerization of polypyrrole, PPy, from a monomer solution containing the same electrolyte (e.g., Et4NBF4) and organic acid was attempted. Cyclic voltammograms, CVs, for PPy in the presence (- - -) and absence (s) of CF3COOH in the deposition solution are depicted in Figure 1. Both CVs show

Fusalba and Be´langer a fairly broad anodic wave centered at about - 0.35 V associated to the oxidation of the polymer and electrolyte counteranions ingress or cations expulsion.20,21 The cathodic wave which peaks at -0.4 V corresponds to the reduction of the polymer with concomitant expulsion of anions or incorporation of cations. The voltammetric charge, QCV, obtained from the integration of the anodic wave of the CV is equal to 11 mC/cm2 and a doping level, x (x ) 2QCV/(Qdep - QCV)) where Qdep is the charge consumed during polymer growth) of 0.25 can be computed. No significant difference of the CVs is observed for the film grown in the presence of CF3COOH, albeit a second set of anodic and cathodic peaks are seen as shoulders at 0 and -0.1 V, respectively. Previous investigators have shown that the CV of PPy film can also exhibit a set of two anodic waves, but no definitive explanation was given for their presence.14 On the other hand, Levi et al.21 have observed two different redox systems for polypyrrole in the presence of ClO4- and B(C6H5)4- anions which were attributed to free and bound anions. Clearly, the data of Figure 1 do not show the same behavior and the effect, if there is any, is less dramatic. Moreover, the addition of 0.25 M CF3COOH to the growth solution does not influence the voltammetric charge (11 mC/ cm2) of the polymer. The similar voltammetric charges might indicate that CF3COO- anions, which can act as dopant with BF4- anions, are quickly expelled during potential cycling. Presumably, the CF3COO- anions are not irreversibly trapped into the polymer matrix and therefore they can be exchanged by BF4- anions from the electrolyte during cycling. Nonetheless, another factor must be considered with respect to the organic acid. The pKa of CF3COOH in water is =0.2322 but it is significantly lower (6.4) in an organic solvent such as propylene carbonate.23 The pKa of CF3COOH in acetonitrile is not known but the pKa of CF3SO3H, which is one of the strongest organic acid in water, in acetonitrile is 2.6.24 Consequently, it seems obvious that CF3COOH will be partially dissociated in such solvent. Hence, the actual CF3COO- concentration will be much smaller than the initial concentration of the acid and since a high concentration (1 M) of BF4- is used it is believed that the CF3COO- concentration in the polymer will be lower than that of BF4- anions. This will be confirmed below by the X-ray photoelectron spectroscopy data for polymer grown in the presence of CF3SO3- and CF3COOH. An increase of the CF3COOH concentration in the growth solution has no significant effect on the voltammetric charge (which is maintained at about 25 mC/cm2 for a deposited charge of 200 mC/cm2) and the stabilization potential reached during the electropolymerization despite the fact that a significant shift of both the anodic and the cathodic peak potentials to more positive potential is found (Table 1). Such a shift has been recently observed for polypyrrole grown at various current densities and has been related to the degree of conjugation of the polymer.14 Low current densities yielded ordered polymer and peak potentials more negative than a less-ordered polypyrrole grown at higher current densities. A same behavior seems unlikely in our case since the stabilization potential remained at about 0.44 ( 0.04 V when the CF3COOH concentration is varied from 0 to 2 M. The role of protons from the dissociated acid in the growth of the polymer can be questionned but no definitive explanation can be given for the moment. On the other hand, the effect of current density, is clearly seen in Table 1 when the CF3COOH was kept constant. In this case, the voltammetric charge increased upon increasing the current density, reached an optimum value for 0.5 mA/cm2 and decreased for higher current density.25 This is accompanied by

Electropolymerization of Ppy and PANI-Ppy

J. Phys. Chem. B, Vol. 103, No. 42, 1999 9047

TABLE 1: Galvanostatic Electrodeposition of Polypyrrole in Acidic Condition (CF3COOH) for Various Parameters; Supporting Electrolyte: 1 M Et4NBF4/ACN pyrrole (M)

CF3COOH (M)

Ia (mA/cm2)

Qdepb (mC/cm2)

QCVc (mC/cm2)

Ea (V)d (V vs Ag/Ag+)

Ec (V)e (V vs Ag/Ag+)

Estab (V)f (V vs Ag/Ag+)

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.05 0.5 1 0.1 0.1 0.1 0.1 0.1

0 0.25 0.1 0.25 0.5 1 1.5 2 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.05 0.1 1 0.5 0.5

100 100 200 200 200 200 200 200 200 200 200 200 200 200 1000 1000

11 11 23 32 25 24 23 21 26 26 26 5 10 20 101* 116*

-0.34 -0.30 -0.19 -0.34 -0.27 -0.21 -0.14 -0.01 -0.29 -0.27 -0.29 -0.01 -0.07 -0.20 -0.05 -0.4

-0.44 -0.38 -0.31 -0.60 -0.42 -0.36 -0.29 -0.26 -0.53 -0.54 -0.53 -0.11 -0.20 -0.40 -0.21 -0.65

0.40 0.41 0.44 0.43 0.45 0.46 0.46 0.47 0.38 0.40 0.38 0.34 0.36 0.44 0.39 0.40

a Current density used for electropolymerization. b Charge of deposition during electropolymerization. c QCV ) Qoxidation, integrated value from polypyrrole cyclic voltammogram in 1 M Et4NBF4/ACN at a scan rate of 100 mV/s (*, scan rate ) 10 mV/s). d Ea ) CV anodic peak potential.e Ec ) CV cathodic peak potential. f Estab ) stabilization potential during electropolymerization at the corresponding current density.

Figure 2. Cyclic voltammogram of polypyrrole (left) and polyaniline (right) in 1 M Et4NBF4/acetonitrile at a scan rate of 10 mV/s. Growth conditions: polypyrrole electropolymerized galvanostatically at a current density of 0.5 mA/cm2 for a deposited charge of 1 C/cm2, A ) 1 cm2 from a 0.1 M pyrrole/0.25 M CF3COOH/1 M Et4NBF4/ acetonitrile solution. Polyaniline electropolymerized galvanostatically at a current density of 0.5 mA/cm2 for a deposited charge of 1 C/cm2, A ) 1 cm2 from a 0.5 M aniline/2 M CF3COOH/1 M Et4NBF4/ acetonitrile solution.

a shift of the CV to more negative potential values and an increase of the stabilization potential during polymer growth. From the data of Table 1, the optimum growth conditions were found to be 0.1 M pyrrole, 0.25 M CF3COOH at a current density of 0.5 mA/cm2. A fairly low acid concentration ensures a high voltammetric charge and would meet the requirement of electrolyte/solvent compatibility for coelectropolymerization from a pyrrole/aniline solution. The same conditions were used to grow a thicker film (deposited charge 1 C/cm2) and Figure 2 shows the CV of PPy together with that of PANI grown from 0.5 M aniline, 2 M CF3COOH, 1 M Et4NBF4/acetonitrile for the same charge. Table 1 also reports relevant CV parameters for the PPy film electrode. The overlap of the CVs of Figure 2 is not adequate for an electrochemical supercapacitor using PPy as the negative electrode and PANI as the positive electrode. The influence of supporting electrolyte ions used during the electrochemical

polymerization and cycling of polypyrrole on the electrochemical behavior has been clearly demonstrated.26-28 In an attempt to extend the CV response of polypyrrole to the most negative potential values as possible, PPy was electropolymerized in the presence of Me4NCF3SO3 instead of Et4NBF4. Rudge et al.29 have reported a shift of the CV to more negative potential when PPy is prepared and cycled in Me4NCF3SO3 (anodic and cathodic peak potential of -0.55 and -0.65 V, respectively). As expected, the CV shown in Figure 3A for PPy electropolymerized with CF3SO3- anions in organic media (0.25 M CF3COOH) is shifted in the cathodic direction in comparison to that of Figure 2. The extension of the cyclic voltammetry response of PPy to more negative potential would lead to an increase of the cell voltage of a supercapacitor using PPy and PANI as active electrode material.20 This is exemplified in Figure 3A which illustrates a representative CV of PANI together with PPy in the same electrolyte. Finally, it is hoped that the cathodic shift of the PPy oxidation potential for a polymer prepared and cycled in the presence of Me4NCF3SO3 will be maintained for the polymer resulting from the coelectropolymerization of pyrrole and aniline. PANIPY Polymer. Since polypyrrole can be electropolymerized in acidic media and polyaniline can be grown from a similar medium, the coelectropolymerization of aniline and pyrrole becomes possible. The optimum conditions for polypyrrole (0.1 M pyrrole + 0.25 M CF3COOH) and polyaniline (0.5 M aniline + 2 M CF3COOH) growth17,18 are slightly different and in the initial coelectropolymerization experiments, the optimum concentrations of both monomers were maintained but a high concentration of organic acid (2 M) was used to ensure satisfactorily PANI growth. A current density of 0.5 mA/ cm2 was selected for polymer deposition and the electrode potential stabilized at 0.40 V. This value is in the range that was found for PPy (Table 1) and PANI deposition.19,30 The CV of the resulting polymer (not shown) suggests that polyaniline deposition is favored in these conditions presumably because the pyrrole/aniline ratio is too low. On the other hand, an increase of the pyrrole concentration to 1 M yields a more appropriate CV (Figure 3B). Thus, a representative CV for PANIPY polymer grown from 1 M pyrrole, 0.5 M aniline, 2 M CF3COOH, and 1 M Me4NCF3SO3/acetonitrile is characterized by an electroactivity window range of 1.5 V and seems to be a sum of PPy and PANI CVs. The CV peaks of the individual


Fusalba and Be´langer

Figure 4. Low-frequency capacitance of the PANIPY polymer (b) in 1 M Me4NCF3SO3/acetonitrile, for a 3-electrode cell (A ) 1 cm2), deposited charge ) 1 C/cm2. Selected values at -0.3 and 0.5 V are also given for PANI (ο) and PPy (∆). Growth conditions: see Figure 3 caption.

Figure 3. (A) Cyclic voltammogram of polypyrrole (left) and polyaniline (right) in 1 M Me4NCF3SO3/acetonitrile at a scan rate of 10 mV/s. Growth conditions: polypyrrole electropolymerized galvanostatically at a current density of 0.5 mA/cm2 for a deposited charge of 1 C/cm2, A ) 1 cm2 from a 0.1 M pyrrole/0.25 M CF3COOH/1 M Me4NCF3SO3/acetonitrile solution. Polyaniline electropolymerized galvanostatically at a current density of 0.5 mA/cm2 for a deposited charge of 1 C/cm2, A ) 1 cm2 from a 0.5 M aniline/2 M CF3COOH/1 M Me4NCF3SO3/acetonitrile solution. (B) Cyclic voltammogram of PANIPY polymer in 1 M Me4NCF3SO3/acetonitrile. Growth conditions: electropolymerized galvanostatically at a current density of 0.5 mA/ cm2 for a deposited charge of 1 C/cm2, A ) 1 cm2 from a 0.5 M aniline/1 M pyrrole/2 M CF3COOH/1 M Me4NCF3SO3/acetonitrile solution.

polymers are discernible as shoulders at -0.45, -0.15, and 0.2 V on the broad anodic wave of the copolymer (Figure 3B). On these grounds, it seems that the resulting polymer is a mixture of PPy and PANI. A unique CV is expected for a true copolymer such as the one reported earlier for PPy/PTh copolymers electrochemically grown from the corresponding monomers.4,6 This is in agreement with the CV response of a thienylpyrrole polymer, synthesized from an oligomer containing thiophene and pyrrole units, which lies between those of PPy and PTh.31 However, we feel that it is difficult to rule out completely that a copolymer is not being formed in our case because the redox waves are very broad. The PANIPY polymer will be characterized by other techniques to get a more definitive answer on this point. The voltammetric charge for the PANIPY polymer in 1 M Me4NCF3SO3/acetonitrile is 146 mC/cm2 for a film grown with a 1 C/cm2 charge. As expected the voltammetric charge is larger

than that of the individual PPy (see Table 1 for PPy in 1 M Me4NCF3SO3). However, it is similar to that of PANI19 but with an expanded range of electroactivity (1.5 V instead of 1.2 V for PANI). On the other hand, the comparison of the lowfrequency capacitance should allow more reliable comparison of the charge storage properties of the polymers (vide infra). The cathodic and anodic peak currents are a linear function of the scan rate indicating that the redox process is not diffusionlimited.32 This is interesting since it means that thicker films can be prepared in order to increase energy density of capacitors based on the polymeric material. The short-term stability of a PANIPY electrode upon voltammetric cycling over more than 100 cycles is good. However long-term cycling has to be evaluated. Since the individual polypyrrole and polyaniline are stable over more than 2000 (not shown) and 10000 cycles,19 respectively, in Me4NCF3SO3/ acetonitrile, a good long-term stability can be expected for the PANIPY polymer. Electrochemical Impedance Spectroscopy. The EIS was used to evaluate the low-frequency capacitance (CLF) of the polymer electrode. This parameter gives some insight about the usefulness of a conducting polymer for some specific applications such as active electrode material in energy storage device. The complex plane impedance plots (not shown) of PPy, PANI, and PANIPY film electrode were recorded at different potentials in 1 M CF3SO3Me4N/acetonitrile. The plots are characterized by a nearly vertical line (slope is lower than 90°) at low frequencies. The capacitance of the polymer electrode is calculated from the slope of a plot of the imaginary component of the impedance, at low frequency, as a function of the reciprocal of the frequency.33 The shape of a plot of the lowfrequency capacitance (CLF) as a function of potential (Figure 4) is similar to that of the corresponding cyclic voltammogramm (Figure 3B) with a maximum at about -0.25 V. Significant capacitance values are observed over a potential range of 1.2 V and a maximun value of 100 mF/cm2 is found at -0.2 V. The abrupt decrease of capacitance at about -0.6 V is associated with the loss of conductivity of the PANIPY polymer. On the other hand, the decrease is less significant at the positive limit and a CLF value of 30 mF/cm2 is observed at 0.6 V. The polymer


Figure 5. DSC thermograms of (- - -) polyaniline, (-‚-) polypyrrole, and (s) PANIPY polymer with a heating rate of 20 °C/min. Growth conditions: polypyrrole electropolymerized galvanostatically at a current density of 0.5 mA/cm2 on carbon paper electrode from a 1 M pyrrole/2 M CF3COOH/1 M Et4NBF4/acetonitrile solution. Polyaniline electropolymerized galvanostatically at a current density of 0.5 mA/ cm2 on carbon paper electrode from a 0.5 M aniline/2 M CF3COOH/1 M Et4NBF4/acetonitrile solution. PANIPY electropolymerized galvanostatically at a current density of 0.5 mA/cm2 on carbon paper electrode from a 0.5 M aniline/1 M pyrrole/2 M CF3COOH/1 M Et4NBF4/ acetonitrile solution.

still shows some electroactivity (Figure 3B) in this potential region in contrast to the negative limit where the current of the CV decay to very small values. Low-frequency capacitances were also evaluated for both PPy and PANI grown in the same conditions. The CLF for PANI at -0,3 and 0.5 V are 145 and 85 mF/cm2, respectively (ο on Figure 4). In comparison, the CLF values for PPy at -0.3 and 0.5 V are 40 and 110 mF/cm2, respectively (∆ on Figure 4). Thus, the CLF values of the PANIPY polymer are smaller than those found for the individual polymers. This is probably related to the linkage of pyrrole and aniline units in a random polyaniline/polypyrrole copolymer which translate in less ordered and oriented polymer chains. That a random polyaniline/polypyrrole copolymer is generated in our experimental conditions will be confirmed by DSC and FTIR data. Differential Scanning CalorimetrysDSC. DSC thermograms of polyaniline, polypyrrole, and PANIPY polymer deposited on carbon paper electrodes are shown in Figure 5. Polymers were grown from acetonitrile solutions containing each 1 M Et4NBF4, 2 M CF3COOH, and either 0.5 M aniline, 1 M pyrrole or 0.5 M aniline + 1 M pyrrole at 0.5 mA/cm2. The freshly deposited polymers were analyzed after being rinsed in acetonitrile in order to minimize contamination by the supporting electrolyte salt. The DSC thermogram of polyaniline exhibits four endothermic processes at 63, 210, 300, and 390 °C. The residual solvent, acetonitrile (Teb ) 81 °C) and organic acid, CF3COOH (Teb ) 72 °C) evaporate and give rise to the first endotherm at 63 °C. The second endotherm at 210 °C can be attributed to morphological changes within the polymer matrix or polymer cross-linking.34 The small endotherm at 300 °C is related to the loss of dopant bound to the polymer chain.35 The fourth endotherm occurs around 390 °C and is due to the degradation of the skeletal polyaniline chain structure.35,36 A glass transition temperature (Tg) for PANI films in the emeraldine base form was determined to be in the range 105-220 °C.36 On the other hand, in some cases35 a Tg was not observed for PANI films presumably because it was masked by overlap-

J. Phys. Chem. B, Vol. 103, No. 42, 1999 9049 ping transition in the polymer. In the case of our PANI film, a Tg is not unequivocally observed. Normally, a Tg appears as a sudden change in slope of the DSC curve whereas the transition of PANI around 200 °C extends over 100 °C. On the other hand, this might reflect a distribution of PANI chains lengths. The thermogram of polypyrrole also indicates a loss of acetonitrile and CF3COOH from the polymer matrix to which is related the endotherm at ca. 63 °C. Three exothermic processes with their peaks at 100, 200, and 230 °C are followed by two endotherms at ca. 315 °C and around 420 °C. The interpretation of this thermogram is not straighforward since exothermic phenomena arise before endothermic ones if a kind of reorganization or recrystallization occurs during the polymer heating.37 Similarly to PANI, the endotherms at 315 and 420 °C are attributed to loss of dopant and degradation of the polymer, respectively. The thermogram of the PANIPY polymer shows the characteristic peaks of the individual polymers except the exotherms of PPy between 175 and 250 °C. In addition, another endotherm appears at 210 °C together with an exotherm at 170 °C. No definitive explanation can be presented for the exotherm, but the endotherm can be tentatively attributed to cross-linking of the polymer chains.36 Moreover, an additional phase might be present in the PANIPY polymer. In conclusion, the DSC data seems to indicate that coelectropolymerization of pyrrole and aniline leads to a mixture of PPy, PANI, and PANI/PPy copolymer. Infrared Spectroscopy. Figure 6 shows the FTIR spectra of (a) polypyrrole, (b) polyaniline, and (c) the polymer resulting from the coelectropolymerization of a mixture of aniline and pyrrole. The polymers were grown on carbon paper electrodes from a solution containing the appropriate monomer, 2 M CF3COOH, and 1 M Et4NBF4 in acetonitrile and removed by scratching with a scalpel. Thus, the polymers investigated by FTIR contain a small amount of carbon fibers (vide infra). The FTIR KBr disk transmission spectrum of polypyrrole exhibits the N-H and C-H absorption from the pyrrole ring at 3500 and 3186 cm-1, respectively.38 The characteristic pyrrole ring stretch absorption bands are observed between 1000 and 1600 cm-1.6,39-42 Some important features of polypyrrole in the 1000-1100 cm-1 region cannot be clearly identified due to an overlap with dopant anions bands (vide infra). The spectrum of polyaniline shows the characteristic quinoid (1600 cm-1) and benzenoid (1490 cm-1) phenyl ring C-C stretch, the N-H stretch (3450 cm-1), the semiquinoid -Nring mode of oxidized polyaniline (1378 cm-1), and the parasubstituted aromatic C-H out-of-plane bending (840 cm-1).43-45 Other prominent features for polypyrrole and polyaniline spectra were observed between 1000 and 1100 cm-1, and can be assigned to the BF4- dopant anions.42,46,47 The absorption bands at 2800-2900 (aliphatic C-H stretch) and 1637 cm-1 (CdO stretch) are related to the carbon paper onto which the polymers were grown since they were also observed for a KBr pellet containing only a small amount of carbon fibers.48 The presence of undissociated and dissociated CF3COOH give rise to a carbonyl band at 1785 and 1700 cm-1, respectively.43,49 The former is rather weak but is clearly noticeable on the spectra. This is in agreement with a previous study where the presence of CF3COOH was barely detected in polyaniline grown in similar conditions.50 The presence of the organic acid was also confirmed by XPS (vide infra). Despite the fact that absorption bands attributed to BF4-, carbon fibers, CF3COOH, and CF3COO- were also observed for the PANIPY polymer, its FTIR spectrum (Figure 6) is not a superposition of those of polypyrrole and polyaniline. First,


Figure 6. (A) FTIR spectra of polyaniline, polypyrrole, and PANIPY. (B) FTIR spectra of polyaniline and PANIPY between 2000 and 400 cm-1. Growth conditions: see Figure 5 caption.

some bands of the spectrum of the individual polymers are absent and new bands are detected at 925 and 1026 cm-1. A band located at 931 cm-1 was observed for a polypyrrolepolythiophene copolymer.6 This band which can be attributed to C-H out-of-plane deformation of the pyrrole units51 and the one at 1026 cm-1 (C-H in-plane deformation) are barely detected on the broad absorption wave centered at 1050 cm-1 of polypyrrole (Figure 6).51 Second, the band associated with the C-H out-of-plane bending of 1,4-disubstituted benzene at 840 cm-1 is broader for the PANIPY polymer. Third, the aromatic C-N stretching band of polyaniline at 1250 cm-1 is not observed for the PANIPY polymer, instead it may be shifted to a higher wavenumber (1300 cm-1). These observations suggest that not only pyrrole-pyrrole or aniline-aniline linkages are formed but also some aniline-pyrrole linkages, and therefore the polymer resulting from the coelectropolymerization

Fusalba and Be´langer from solution containing both aniline and pyrrole is a mixture of polypyrrole, polyaniline, and a random polyaniline-polypyrrole copolymer. This is in agreement with previous FTIR data obtained on polymers generated by electropolymerization from pyrrole-thiophene6 and pyrrole-azulene52 mixtures. Scanning Electron Microscopy. SEM photomicrographs of films of polymers grown on carbon paper and that of a bare carbon paper electrode are shown in Figure 7. The photomicrographs were obtained at fairly low magnification in order to evaluate to what extent the individual carbon fibers and the surface of the carbon paper electrodes were coated with the various polymers. The photomicrographs clearly revealed that polymerization occurred on most of the individual fibers and not only at the surface of the carbon paper electrode. This growth morphology favors a faster ionic transport within the polymer layers which could translate into faster charge/discharge and thus high power density for a capacitor based on such polymeric materials.53,54 This is due to the fact that coating most of the fibers with polymer will yield a thinner polymer layer and will be beneficial for charge transport. Finally, a photomicrograph of polyaniline (Figure 7b) at the same magnification shows a fairly smooth and even surface similar to that of the PANIPY polymer coating (Figure 7d), whereas that for polypyrrole (Figure 7c) is characterized by a very different morphology. Indeed, a granular structure is observed as was previously reported for PPy.55 The relative estimate of the atomic content of the polymer films can be determined by EDS56 and used to evaluate their doping level. Unfortunately, unrealistic doping levels in the order of 0.7-0.9 were computed in this study. This is mainly due to the relatively low sensitivity for the elements of interest (N and F) in comparison to the carbon signal and also that excess salt (Et4NBF4) might be entrapped in the carbon paper/polymer electrode. It will be shown below that X-ray photoelectron spectroscopy provides more meaningful results. X-ray Photoelectron SpectroscopysXPS. XPS has been widely used to characterize the electronic properties of polymeric materials.57-60 Figure 8 shows the XPS survey spectrum for the PANIPY polymer which exhibits the characteristic C 1s (285 eV) and N 1s (400 eV) peaks associated to the polymer backbone. The peaks at 685 eV (F1s), 530 eV (O 1s), 165 eV (S 2p), and 230 eV (S 2s) originate from CF3SO3- anions from Me4NCF3SO3 which is used as the supporting electrolyte for the polymer electrosynthesis. The presence of CF3COOH or CF3COO- in the polymer also contributes to the F 1s and C 1s signals. The C 1s core-level spectrum is shown in Figure 9. The C 1s envelope is deconvoluted into five components (Table 2). The main peak at 284.5 eV is attributed to the contribution of C-C, CdC, and C-H.58,59 The component at ca. 285.8 eV corresponds to C-N, C-S, and C-O contributions.58,59 The third and fourth peaks centered at 288.6 and 289.2 eV are indicative of the presence of the carbonyl and -CF3 from the organic acid (CF3COOH), respectively.49,61,62 The intensity of these two peaks should be equal but the contribution of the carbonyl is significantly higher. The discrepancy may be due to the presence of satellite peaks at this binding energy (=288.5 eV). Finally, the peak at the highest binding energy (291.7 eV) is assigned to the -CF3 unit of the CF3SO3- anions.63,64 XPS spectra of NaCF3SO3 and poly-(ethylene oxide) (PEO) sodium triflate have been recently reported and confirm the attribution of the fifth peak. This confirms that both CF3SO3- and CF3COOH are incorporated into the PANIPY film electrode during polymer growth. The data also suggest that the concentration



Figure 7. Scanning electron microscopy of (a) carbon paper, (b) polypyrrole, (c) polyaniline, and (d) PANIPY polymer. Growth conditions: polypyrrole electropolymerized galvanostatically at a current density of 0.5 mA/cm2 for a deposited charge of 1 C/cm2, A ) 1 cm2 from a 0.1 M pyrrole/0.25 M CF3COOH/1 M Et4NBF4/acetonitrile solution. Polyaniline electropolymerized galvanostatically at a current density of 0.5 mA/cm2 for a deposited charge of 1 C/cm2, A ) 1 cm2 from a 0.5 M aniline/2 M CF3COOH/1 M Et4NBF4/acetonitrile solution. PANIPY electropolymerized galvanostatically at a current density of 0.5 mA/cm2 for a deposited charge of 1 C/cm2, A ) 1 cm2 from a 0.5 M aniline/1 M pyrrole/2 M CF3COOH/1 M Et4NBF4/acetonitrile solution.

Figure 8. XPS survey spectrum of the PANIPY polymer on a platinum foil electrode prepared galvanostatically at a current density of 0.5 mA/ cm2 from a 0.5 M aniline/1 M pyrrole/2 M CF3COOH/1 M Me4NCF3SO3/acetonitrile. Deposited charge ) 1 C/cm2, A ) 1 cm2.

of CF3SO3- in the polymer is higher than that of CF3COOH/ CF3COO-. This will be confirmed below with the F 1s core level data. The S 2p spectrum (Figure 9, Table 3) is characterized by a doublet at binding energies of 167.8 and 168.9 eV with the full width at half-maximum (fwhm) of ca. 1.1 eV arising from S

2p spin-orbit coupling (S 2p3/2 and S 2p1/2), with a ratio of 1.9, close to expected (2) value.59,65,66 This doublet is characteristic of the sulfonated group of sulfonated PANI,65 but it is somewhat shifted by about 1 eV to lower binding energy in comparison to the binding energy reported for NaCF3SO3.63,64 This shift toward lower binding energy is consistent with the fact that the CF3SO3- interact with delocalized positive charge on the polymer backbone instead of Me4N+. The other two peak components at 169.8 and 170.9 eV with fwhm of ca. 1.5 eV suggest that some of the sulfur atoms exists in a more positive environment and result from charge extraction from some of the sulfonic units dopant by the oxidized polymer units.66 The F 1s spectrum of the PANIPY polymer consists of two peaks with binding energies of 686 and 688.1 eV (Figure 9, Table 4). The latter is attributed to the CF3SO3-,63 and is the more intense as it represents 88% of the total fluorine. Thus, the F 1s component at lower binding energy can be assigned to the -CF3 unit of CF3COOH. Sailor et al. have shown that electrolysis of Si in trifluoroacetate solution under illumination or in the dark yielded a surface with a F 1s XPS spectrum displaying a signal from CF3 at 687 eV.49 Moreover, the F 1s spectrum of a PANI film grown in the presence of Et4NBF4 and CF3COOH shows a distinct peak at 687.6 eV which is related to the -CF3 group of the acid.19 Despite the difference in the binding energies it is clear that two fluorine-based species are present in the polymer electrode.


Fusalba and Be´langer

Figure 9. XPS C 1s, F 1s, N 1s, and S 2p spectra of PANIPY polymer. Growth conditions: see Figure 8 caption.

TABLE 2: C 1s Deconvolution Parameters of PANIPY Polymer C-H, C-N, C-F CdC, C-O, C-F C-C C-S O-CdO (CF3COO-) (CF3SO3-)

C 1s

peak center (eV) 284.5 286.3 % 68 15

288.6 9

290.2 1

291.7 7

TABLE 3: S 2p Deconvolution Parameters of PANIPY Polymer S 2p

S 2p3/2

S 2p1/2

S 2p3/2σ+

S 2p1/2σ+

peak center (eV) %

167.8 43

168.9 45

169.8 6

170.9 6

TABLE 4: F 1s Deconvolution Parameters of PANIPY Polymer F 1s

CF3COO-

CF3SO3-

peak center (eV) %

686 12

688.1 88

The XPS N 1s core level spectrum (Figure 9) has been deconvoluted into four peaks at 397.9, 399.6, 400.6, and 402.1 eV (Table 5). It is generally accepted that the imine (-N)) nitrogens would give rise to a peak component at ca. 398 eV (397.7 eV for PPy and 398.2 eV for PANI), whereas the corresponding peak for the amine (-NH-) nitrogens would be at ca. 399.5 eV (399.7 eV for PPy and 399.4 eV for PANI66).

TABLE 5: N 1s Deconvolution Parameters of PANIPY Polymer N 1s

-Nd

-NH-

-N+-

-N+-

peak center (eV) %

397.9 4

399.6 75

400.9 9

402.1 12

If we assume that the polymer film consists of a mixture of PPy, PANI, and a PANI/PPy copolymer it is clear that these components cannot be resolved by deconvolution. The third and fourth peaks at 400.6 and 402.1 eV are assigned to a positively charged nitrogen of the pyrrole and aniline units which are charge compensated by CF3SO3- anions. The fraction of positively charged nitrogen for the N 1s envelop corresponds to the doping level and a value of 0.21 is obtained from the XPS data. This compares well with the XPS S/N ratio (0.20) calculated from the peak areas of the S and N components using the appropriate sensitivity factors. On the other hand, the F/N ratio (0.12) is somewhat lower than the two others, presumably due to the loss of dopant either during the rinsing of the electrode with the solvent or its degradation in either the vacuum chamber of the spectrometer or the electrochemical cell. The later is more likely because the F/S ratio is lower than that expected for CF3SO3-, suggesting a degradation of the anions. Galvanostatic Charge/Discharge Cycling. Constant-current charge-discharges at various current densities (from 0.5 to 5 mA/cm2) were performed with a maximum cell voltage of 1 V,


Figure 10. Charge-discharge curve of PANIPY-based capacitor in 1 M CF3SO3Me4N/acetonitrile at a constant current density of 3 mA/ cm2 (A ) 1 cm2). Deposited charge ) 1 C/cm2. Growth conditions: see Figure 3 caption.

on capacitors assembled from two PANIPY-polymer-coated carbon paper electrodes having the same loading in 1 M CF3SO3Me4N/ACN and representative cycling curves (with a current density of 3 mA/cm2) are shown in Figure 10. Before cycling, the potential of each two electrodes is usually set at ca. - 0.3 V vs Ag/Ag+. The cell voltage increased during the charging cycle until it reached the specified value of 1 V. A charge of 57 mC is involved in this charging cycle. The voltage was held for 100 s at the end of the charge cycle and at this stage the potentials of the negative and positive electrodes were - 0.74 and + 0.26 V, respectively. Upon discharging, the cell voltage decreased very rapidly from 1 to about 0.7 V and then decreased monotonously until the fully discharged state is reached. The initial rapid decrease is related to the uncompensated resistance (ohmic drop). By considering the initial potential drop and the discharge current, an uncompensated resistance of about 100 Ω can be evaluated in this case. The discharge charge is smaller (48 mC) than that of the charge cycle and thus a charge/ discharge efficiency of 84% can be computed. A charge/ discharge efficiency lower than 100% is mainly attributed to the uncompensated resistance, although charge transfer and concentration polarization may contribute as well to the inefficiency of the process. The ohmic drop is more important for the fully charged capacitor because, in this state, the potential of the negative electrode is very negative and in a region of low electronic conductivity. Charge/discharge cycling was also performed for a cell voltage of 1.5 V (not shown) and as expected the ohmic drop effect is more significant in this case. The same observation is made when thicker polymer films are used, confirming the contribution of the polymer to the total uncompensated resistance. Galvanostatic charge/discharge were performed at various current densities between 0.5 and 5 mA/cm2, and a Ragone plot (Figure 11) summarizes the performance of the PANIPY-based capacitor. Due to the low cell voltage (e.g., 1 V) the energy and power densities are lower than those reported for polythiophene derivative-based capacitors53,54,67-70 which can develop a cell voltage ranging between 2 and 3 V. When the cell voltage of the PANIPY capacitor is increased to 1.5 V, both the energy and power densities increased. In this case, energy and power densities of 5 Wh/kg and 1.5 kW/kg of composite polymer (Qdep ) 2 C/cm2, m ) 0.0033 g), respectively, are evaluated for a discharge time of about 13 s. These preliminary values are in the range reported for type I and type II capacitors but it can be expected that optimization of polymer growth and


Figure 11. Ragone plot for PANIPY-based capacitor in 1 M CF3SO3Me4N/acetonitrile for a cell voltage of 1 V. Deposited charge ) 1 C/cm2 (A ) 1 cm2). Growth conditions: see Figure 3 caption. The current density in mA/cm2 is given in parentheses.

charge/discharge cycling conditions will yield significantly higher energy and power densities. Conclusion An electrochemical and physicochemical characterization of polypyrrole and PANIPY film electrodes prepared in acidic media has been performed. Electroactive polypyrrole films were obtained by electrodeposition in acetonitrile in the presence of CF3COOH with either Et4NBF4 or Me4NCF3SO3 as supporting electrolyte. When PPy is electropolymerized in the presence of CF3SO3- anions, a whole CV curve shift in the cathodic direction is observed as compared to BF4- anions as it was already reported by Rudge and al.29 This shift is used to extend the overall electroactivity range of the polymer grown from the coelectropolymerization of pyrrole and aniline. Low-frequency capacitance of about 100 mF/cm2 (60 F/g) is computed for the PANIPY polymer. DSC thermograms and FTIR spectra indicate a mixture of polypyrrole, polyaniline, and a random polyaniline-polypyrrole copolymer for this new material. XPS analysis suggests a doping level of about 0.21. The morphology of the random PANIPY polymer is very smooth. Galvanostatic charge-discharge cycling indicates a specific power of 1.5 kW/ kg with a specific energy of 5 Wh/kg for a discharge time of about 13 s (active material: 0.0033 g). These values compare well with those reported for type I electrochemical supercapacitors based on polyaniline as electrode material which show specific power between 700 and 1000 W/kg of active material with a lifetime of about 100 000 cycles.71,72 However, the stability of the PANIPY electrode upon charge/discharge cycling experiments needs to be investigated and optimized. Acknowledgment. This research was funded by the Conseil de la Recherche en Science et en Ge´nie (C.R.S.N.G.) du Canada through research and strategic grants (to D.B.) and an equipment grant for an XPS spectrometer (to D.B. and nine others). The financial contribution of UQAM is also acknowledged. We also thank R. Mineau from the UQAM (De´partement des Sciences de la Terre) for the SEM measurements. The technical assistance of G. Veilleux of INRS-EÄ nergie et Mate´riaux for the XPS measurements is also acknowledged. References and Notes (1) Handbook of Organic ConductiVe Molecules and Polymers, ConductiVe Polymers: Transport, Photophysics and Applications; Nalwa, H. S., Ed.; John Wiley & Sons, New York, 1997; Vol. 4.

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