polyaniline system

Journal of Electroanalytical Chemistry, 364 (1994) 127-133

127

JEC 02956

Electrochemical study of bilayer conducting polymers: polypyrrole/polyaniline system Zhiqiang Gao

l, Johan Bobacka and Ari Ivaska

Laboratory of Analytical Chemistry, A’bo Akademi Uniuersity, SF-20500 Turku (Finland)

(Received 27 January 1993; in revised form 17 May 1993)

Abstract Cyclicvoltammetric and electrochemical impedance spectroscopic studies of polypyrrole (PPyl-polyaniline (PANI) bilayer conducting polymers were carried out in aqueous medium. The bilayers were prepared by electrochemical polymerization of the corresponding monomers at a platinum electrode surface in 1 M HCl solution. Cyclic voltammograms show additive properties of the bilayer conducting polymers compared with the individual polymers. Electrochemical impedance spectra revealed that the overall responses were mainly dominated by the inner layers. It was found that the charge transfer resistance of PAN1 decreased markedly when PPy was the outer layer, which is indicative of mediated oxidation. In contrast, when PANI was used as the outer layer, the impedance spectra were similar to those of single PPy films. The electrochemical impedance responses were also characterized with respect to different polarization potentials and thicknesses of the bilayers.

Introduction

Since the discovery of electrochemical polymerization of conducting polymers [l-3], there has been great interest in the electrochemical characteristics of thin conducting polymer films on the electrode surface with respect to both their chemistry and physics, and potential technological applications in electrochromic displays [4], batteries [5], sensors [6], electrocatalysis [7] and corrosion protection [8]. To date, considerable effort has been made to elucidate the electrochemistry as well as the influence of factors such as solvent and solution composition on the electrochemical polymerization and transformation of individual conducting polymers [9-111. However, little attention has been paid to bilayer conducting polymer films. The concept of bilayers of polymer films was first introduced by Murray’s group [12]. They studied redox polymer bilayer films and identified two primary criteria for a successful bilayer film [12]. Their initial bilayer studies demonstrated that charge transfer to the outer layer is

l

Present address: Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel. To whom correspondence should be addressed.

0022-0728/94/$7.00 SSDI 0022-0728(93)02956-I

mediated by the inner layer, and occurs at the redox potential of the inner layer. More recently, a few papers concerning bilayer conducting polymer films have been published [13-161. A rectifying heterojunction consisting of polythiophene and polypyrrole bilayer films was prepared by Kaneto et al. 1131,and the potential application in electronic devices has also been demonstrated in their and others’ work [13,14]. Hillman and Mallen [15] investigated polybithiophenepolypyrrole bilayer films. The preparation of polyaniline (PANI)/polypyrrole (PPy) bilayer films was also mentioned by Kumar and Contractor [16]. However, to our knowledge, no systematic study of the PANI/PPy bilayer conducting polymers has been reported. In this paper, we present the results of an electrochemical study of the polyaniline/polypyrrole bilayer conducting polymer system, using cyclic voltammetric and electrochemical impedance methods. 2. Experimental 2.1. Chemicals and instrumentation

Pyrrole obtained from Merck was purified by double distillation and stored in the dark at low temperature. Aniline (pa) from Merck was used as received. All other chemicals were of certified analytical grade, and 0 1994 - Elsevier Sequoia. All rights reserved

128

Z. Gaoet al. / Bilayerconducting polymers

were mainly obtained from Merck and Fluka. The solutions were prepared from distilled deionized water. Voltammetric experiments were performed with a PAR 174A polarographic analyzer (PARC, USA) or a Metrohm Polarecord E506 polarograph (Metrohm, Switzerland). Electrochemical impedance spectroscopy of the bilayers was conducted with an HP-9816 computer-controlled S-5720B frequency response analyzer and a model 2000 potentiostat/galvanostat with a GPIB interface (NF Circuit Design Block Co., Ltd., Japan) over a frequency range from 10 mHz to 100 kHz. The electrochemical impedance was measured at five discrete frequencies per decade. The typical amplitude of the sinusoidal voltage signal used was 5 mV. All electrochemical measurements were conducted using a conventional three-electrode system with a triangular arrangement and electrodes ca. 1 cm apart from each other. The working electrode was a polymer film coated platinum electrode, with a platinum foil auxiliary electrode and a saturated calomel reference electrode WE). AI1 potentials given in this work refer to the SCE. 2.2. Preparation of bilayer conducting polymer films A platinum disk electrode (area 0.07 cm’, Metrohm) was used for the electrodeposition of bilayer films. The electrode surface was polished with 0.3 pm alumina, and then rinsed with water and 1: 1 nitric acid. Thereafter it was polarized in 0.5 M H,SO, by potential scanning between -0.3 and 1.6 V for 10 min. After rinsing with water, the electrode was used for experiments. Electrodeposition of bilayer films onto the electrode surface was carried out galvanostatically at 1.0 mA cm-* using a 1.0 M HCI solution containing 0.5 M aniline and 0.1 M pyrrole respectively. The polyaniline film prepared in this manner was adherent to the platinum substrate and homogeneous in its thickness [4]. This was vital for the preparation of bilayer films which were more durable during potential cycling than films prepared by potential cycling or by potentiostatic oxidation. The thickness of polyaniline films deposited by passing 90 mC cmp2 charge was determined, in a dry state, to be 0.1 pm [4]. The film thickness of PPy was controlled by measuring the charge passed during the electropolymerization. Film thicknesses of 0.1 pm were deposited by the passage of 24 mC cmp2 of charge [ 171.

for 10 min, and the impedance spectrum in the range of 10 mHz to 100 kHz was recorded. The potential was then varied to a more positive value, the system maintained at the new potential for 10 min, and another impedance measurement was taken. This procedure was repeated at a number of potentials up to 0.60 V. 3. Results and discussion 3.1. Electrochemical polymer films

Potentials of the working electrode as a function of the thickness of the inner layer during galvanostatic polymerization of pyrrole and aniline on either PAN1 or PPy covered Pt electrodes respectively, are shown in Fig. 1. It was found that the polymer deposition potential from an initial maximum polymerization potential Eini, gradually decreased with time and stabilized to a constant value Ed_,. As can be seen from Fig. 1, the galvanostatic electropolymerization of the outer layer proceeds at potentials lower than those of the corresponding polymer on the bare platinum electrode. The differences in Edep between polymer covered and bare platinum electrodes were more pronounced when polymerizing pyrrole than aniline. For the electrodeposiPAN1 Film Thickness/pm 0.0

0

2.3. Electrochemical layer films

impedance spectroscopy of the bi-

The electrochemical impedance measurements were performed using the following procedure. The bilayer film was equilibrated in 1 M HCl solution at -0.2 V

preparation of conducting bilayer

0.5

1.5

0.8

3.0

PPy Film Thickness/pm

Fig. 1. The effect of inner layer film thickness on Eini (0) and I&, polymerization of pyrrole on PAN1 (0) of the outer layer. layer (O-O.7 pm), 0.1 M pyrrole’+ 1.0 M HCl, current density 1.0 mA cm-*; ______ , polymerization of aniline on PPy layer (O-2.5 pm), 0.5 M aniline+ 1.0 M HCI, current density 1.0 mA cme2.

Z. Gao et al. / Bilayer conducting polymers

tion of PPy onto PAN1 layers, Eini and Edep decreased significantly as the thickness of the PAN1 layer increased. For a 0.7 pm PANI layer, very little difference in Eini and Edep was observed. These results indicate that electropolymerization of the outer layer was mediated by the inner layer leading to the bilayer structure. The underlying polymer layer obviously catalyzed nucleation of the outer layer because of the much lower values. In contrast, owing to the nucleEini and Edep ation overpotentials on the bare platinum surface, electrodeposition of both polymers onto that surface starts at higher potentials and the polymer growth potentials are also higher than those obtained on the polymer layers. 3.2. Cyclic voltammetry of bilayer conducting polymers Figure 2 shows cyclic voltammograms of single layers and bilayers in 1.0 M HCl solution. The potential scan rate was 30 mV s-l. As expected, the PAN1 film exhibited one redox couple between -0.2 to 0.6 V, which corresponds to the transition leucoemeraldine e emeraldine. One redox couple was also observed for PPy film in the potential range -0.4 to 0.6 V. From the shape of the voltammogram for a bilayer film, it is clear that bilayer films exhibit essentially the sum of the individual PPy and PAN1 film responses. No obvious differences were observed by changing the combinations of the bilayer systems. This can be interpreted on the basis of the energy levels of the polymers. As pointed out by Hillman and Mallen [15], the conducting polymers give very broad responses. This broadening can result in overlap of the energy levels of the inner and outer layers, so the typical current-voltage characteristics of the bilayers [12,15] are not observed in these cases. Upon repeated potential cycling, it was

4.6 Fig. 2. Cyclic

HCl solution, (0.7/2.5 pm);

0.6

0.0 E/V(vs.

SCE)

voltammograms of conducting polymer films in 1 M Potential scan rate 30 mv s-l: PfwI/PPy - - - - - -, PAN1 (0.7 pm); .-.-.-‘, PPy 6.5 pm).

129

found that both types of bilayer films were stable in 1 M HCl solution between -0.2 and 0.6 V. If the anodic limit was exceeded, the films lost their voltammetric activity quickly although they continued to adhere to the electrode surface. This is obviously due to the oxidation of PPy and the degradation of PANI. Moreover, in 1 M HCl solution, if the potentials were scanned more negative than -0.2 V, new redox peaks appeared at around -0.3 V for both bilayer films, On repetitive scans the peak currents increased. After a few minutes the bilayer film flaked off. With decreasing acidity of the solution, these peaks disappeared gradually. These redox peaks are obviously due to the reduction of H+ and oxidation of H, at the platinum electrode surface, because the same peaks were also observed on bare platinum in the same electrolyte. As shown in Fig. 2, the redox system in the bilayer was quasi-reversible. The peak to peak potential separation in the bilayer system was about 70 mV less than that of the single PAN1 layer, suggesting the charge transfer resistance was lowered by depositing the outer layer onto the conducting polymers. This was confirmed by the electrochemical impedance measurements (see below). The values of the anodic and cathodic peak potentials were independent of the potential scan rate v between 2 and 100 mV s-l. The anodic and cathodic peak currents respectively, varied linearly with v in the range 2-100 mV s-‘. 3.3. Electrochemical

impedance spectroscopy of the bi-

layer films

Typical complex plane results of electrochemical impedance measurements on Pt/PANI, Pt/PPy, Pt/ PPy/PANI and Pt/PANI/PPy bilayer systems are presented in Fig. 3. The electrochemical impedance responses were obtained at different electrode potentials varying from -0.2 to 0.6 V. It is widely accepted that the electrochemical interface can be described adequately by an equivalent electrical circuit. The impedance diagrams in Fig. 3 give direct information about the different components of the equivalent electrical circuit. The semicircle at high frequencies is characteristic of a parallel resistor-capacitor network. The charge transfer resistance in the electrochemical redox process R,, and the capacitance due to the electrolyte/polymer film interface C,, can then be derived. This capacitance appeared to be independent of the frequency and film thickness, but it depended strongly on the nature of the outer layer and the redox state of the polymer. The charge transfer resistance was also frequency independent but varied with the applied potential at the electrode. C,, was determined using the following equation [18]: wmax= l/&G, (1)

130

Z. Gao et al. / Bilayer conducting polymers 3

0.025 -. c 3 . R I

2 A- 0.025

I

I 0

0.3 - 0.4 v

0.3 - 0 v

(b)

. .

. \

.

0.63 l

0.2

0.2 -

/

0.4

. 0.1 -

3

\

.' 0 .

Oi" . . I 0.1

0.1 -. 0

Fig. 3. Electrochemical impedance spectra of conducting polymer films (frequencies (0.7/2.5 pm); (b) l PPy single film (2.5 pm), A PPy/PANI (2.5/0.7 pm).

where w,, is the frequency at the top of the semicircle. The high frequency intercept of the semicircle with the real axis gives the value for the sum of the solution and the film resistances, i.e. the ohmic drop in the system. The low frequency part of the diagram characterized by an almost vertical line indicates the existence of a very high capacitance C, due to the charge saturation within the thin films [19]. This is directly related to the charge capacity of the polymer. It was

I+

circuits

for bilayer

PANI single film (0.7 pm),

zi, = l/WC,

films: (a) PANI/PPy

bilayer

A PANI/PPy

(2)

This high capacitance is responsible for the large current plateau observed in the cyclic voltammogram (see Fig. 2). From the above discussion, an equivalent elec-

cd+l

lbl

Ia1 Fig. 4. Equivalent

l

found that C, became frequency independent at very low frequencies. The values of C, were estimated from the imaginary part of the impedance spectrum using the following equation [18]:

r+

cd+-l

are in Hz). (a)

film and (b) PPy/PANI

bilayer

film.

Z. Gao et al. / Bilayer conducting polymers

trical circuit can be suggested for the Pt/PANI/PPy bilayer system, as shown in Fig. 4(a). As seen from Fig. 3(a), the impedance spectra of the Pt/PANI and Pt/PANI/PPy systems were rather similar at -0.2 and at 0.4 V, i.e. at potentials where both polymer layers were either reduced or oxidized. A large difference between these systems was observed at potentials where the polymers were in an intermediate state, for example at 0 V. At that potential the diameter of the high frequency semicircle of the Pt/PANI system was much larger than that of the Pt/PANI/PPy bilayer system. At 0 V, PPy is almost in its fully oxidized (conducting) state. PANI, however, only begins to become oxidized at this potential. As discussed above, this semicircle is due to the charge transfer resistance and the double layer capacitance. By introducing a PPy layer onto a PAN1 layer, a significant decrease in charge transfer resistance was obtained, which suggests that the PPy layer mediates oxidation of the underlying PAN1 layer. The charge transfer resistance R,, and other parameters obtained from analysis of the impedance spectra for different films at 0.0 V are listed in Table 1. The exchange-current density j, across the film, calculated from the following equation [20], j, = RT/nFAR,,

(3)

is also given in Table 1. It is seen in Table 1 that the charge transfer resistance decreases with increasing film thickness. Since R,, is inversely proportional to the surface area A, in eqn. (31, a lower value of R,, is expected for the thicker film, owing to the systematic increase in effective surface area [21]. C,, is almost constant for all films, because the double layer capacitance is only associated with the phase boundary of the electrode. It is understandable that the bilayer film behaves as a good electronic conductor at intermediate oxidation levels (0.2-0.6 V) because both PPy and PAN1 are in their conducting (oxidized) states. Increasing the electrode potential further results in the excessive oxidation of both polymers. In the reduced state, at less than or equal to -0.2 V, higher resistances are expected. The dependence of R,, on applied potential substantiates this expectation (Fig. 5). The dependence

TABLE 1. Electrochemical films at 0 V

impedance

data

for PANI/PPy

bilayer

Film (inner/outer)

Thickness/ pm

R,, / k0

j,/ mAcm_’

cd,/

cL/

PF

mF cm+*

PANI/PPy PANI/PPy PANI/PPy PANI/PPy

0.70/0.00 0.70/2.50 0.35/1.25 0.07/0.25

2.4 0.3 1.2 1.8

0.15 1.22 0.31 0.20

28 22 22 21

28 50 25 13

2.4

. .

0

.

l

l

.

$

r

1.0

--?

l

0 .

.

0

0

0

0

a.4 -0.3

0.2 E/V(v..

Fig. 5. The dependence potential for a PANI/PPy

0.7 SCE)

of R,, (0) and C, (*) on the bilayer film (0.7/2.5 pm).

applied

of C, on the applied potential for PANI/PPy bilayer films is also shown in Fig. 5. It appears that the capacitance C, began at a very low value in the reduced state, increased rapidly with the applied potential, reached a maximum when the potential was near the voltammetric current maximum, and decreased slightly when the electrode potential became more positive. It is clear that the high capacitance was a characteristic of the oxidized state of the bilayer film, and was related to the total charge in the film [22]. Thus in the oxidized state, C, became nearly independent of the applied potential, and the values could be considered as characteristic of the oxidized state of conducting polymers as suggested by Feldberg [23]. This has also been observed from electrochemical impedance spectra on single layer conducting polymers [21]. The value of C, increased with increasing the film thickness (see Table 1). The electrochemical impedance spectra of the Pt/ PPy and Pt/PPy/PANI systems were rather different from those of the Pt/PANI and Pt/PANI/PPy systems. Both the single layer and the bilayer showed similar impedance responses throughout the range of potentials studied. At high frequencies, there was no semicircle, but a straight line with a slope of about 45”, which is characteristic of Warburg or diffusion impedance. The nearly vertical line was also observed for the previous bilayer film. A reason for the disappearance of the charge transfer resistance might be that the PPy layer was already oxidized at the applied potential and the electrode process for this system was

Z. Gao et al. / Bilayer conducting polymers

132

mainly controlled by the diffusion of the doping ions. This is true for the PPy film in this solution (see Fig. 3(b)). On this basis, another equivalent circuit for this system can be proposed (as shown in Fig. 4(b)), where 2, is the Warburg impedance arising from the diffusion process. In order to obtain the kinetic parameters of the bilayer films the impedance spectra were analyzed in detail. In the Warburg region, where the impedance phase angle is 45”, the magnitude of the impedance is given by the following equation [24]: IZI =zc,‘(D+“*

(4)

where C,, D and 1 are the low frequency capacitance, diffusion coefficient and the polymer film thickness respectively. Values of C,, estimated using eqn. (21, are listed in Table 2. The value of D is decreased slightly by increasing the bilayer film thickness, particu-

TABLE 2. Electrochemical films at 0 V Film (inner/outer)

Thickness/ pm

impedance data for PPy/PANI RL/

R

lo9 D/ cm2 s-1 a

CL/

mFcm-*

bilayer

lo9 D/ cm* S-lb

‘

0 0 0

0

N

7

k ”

L

E

.

.

45

\

o

0”

.

t

. 0 0

0

.

.

l

0 .

I

0 4.3

-0.1

E/V(vs. Fig. 6. The dependence of

C, (0)

I

I 0.1

0.3

0.5

SCE)

and diffusion coefficient (0) on the applied potential for a

PPy/PANI

bilayer film (1.25/0.35 Frn).

Z. Gao et al. / Bilayer conducting polymers

The term dE/dc is a thermodynamic parameter which shows how the concentration of diffusing species varies with potential, where E is the electrode potential and c is the concentration of oxidized polymer sites. Other symbols have their common meanings. The resistance R, in the finite diffusion region was also frequency independent and is described by R, = ll(dE/dc)

[/3nFAD

(6) R, values were obtained directly from the impedance spectra [25] and are listed in Table 2. Eliminating dE/dc from eqns. (5) and (6) yields D = 12/3R,C,

(7) which can be used to evaluate the diffusion coefficient from the finite region. The results are also listed in Table 2. The D values obtained from the finite diffusion regime were consistently larger than those obtained from the Warburg impedance region (Table 2). A similar discrepancy was noted by Rubinstein’s group in studying redox polymers [261 and by Penner and Martin [25] in determining the diffusion coefficient of PPy. It may be due to the existence of two uncoupled diffusion pathways within the polymer films [26]. It might also be attributed to the non-uniform thickness of the film. The thickness measured is probably the maximum thickness. Because D is proportional to Z2, this overestimated film thickness could make D surprisingly large. The diffusion coefficients listed in Table 2 also indicate that with the presence of an outer PAN1 layer, no additional difficulty was caused in the transportation of doping ions to the inner PPy layer. However, for the calculation of diffusion coefficients, the contribution of PAN1 to the film thickness was taken into account. When comparing the Pt/PANI/PPy and Pt/PPy/ PAN1 bilayer systems, it is clear that the inner layers of the bilayer systems determine the electrochemical impedance responses. By changing the order of the layers, rather different electrochemical impedance spectra can be observed. The general impedance responses of the bilayers were similar to those of the single polymer films which were used as inner layers. It can also be concluded that the bilayer systems studied are real bilayers. This is in spite of the fact that the cyclic voltammograms of the bilayers were practically identical. In summary, from the above experimental results, it can be seen that the bilayer systems in this study were

133

real bilayers, because by changing the order of the bilayers, rather different electrochemical impedance spectra were obtained. Otherwise if a mixture of the two conducting polymers were formed on the electrode surface, the same impedance spectrum should be observed for both PANI/PPy and PPy/PANI combinations. Comparing the impedance spectra of single layers and the bilayer systems, it is clear that the electrode process for the bilayer films is mainly controlled by the inner layer. References 1 A.F. Diaz, K.K. Kanazawa and G.P. Gardini, J. Chem. Sot., Chem. Commun., (1979) 635. 2 A.F. Diaz and J.A. Logan, J. Electroanal. Chem., 111 (1980) 111. 3 R.J. Waltman, J. Bargon and A.F. Diaz, J. Phys. Chem., 87 (1983) 1459. 4 T. Kobayashi, H. Yoneyama and H. Tamura, J. Electroanal. Chem., 177 (1984) 281. 5 T. Osaka, K. Naoi, S. Ogano and S. Nakamura, J. Electrochem. Sot., 134 (1987) 2096. 6 A. Ivaska, Electroanalysis, 3 (1991) 247. 7 R.A. Saraceno, J.G. Pack and A.G. Ewing, J. Electroanal. Chem., 197 (1986) 265. 8 R. Noufi, D. Tenth and L.F. Warren, J. Electrochem. Sot., 127 (1980) 2310. 9 L. Alcacer (Ed.), Conducting Polymers, Special Applications, Reidel, Dordrecht, 1987. 10 T.A. Skotheim (Ed.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1986, p. 81. 11 A.F. Diaz, J.F. Rubinson and H.B. Mark, Jr., Advances in Polymer Sciences 84, Springer, Berlin, 1988, p. 113. 12 H.D. Abruna, P. Denisevich. M. Umana, T.J. Meyer and R.W. Murray, J. Am. Chem. Sot., 103 (1981) 1. 13 K. Kaneto, S. Takeda and K. Yoshino, Jpn. J. Appl. Phys., 24 (1985) L553. 14 M. Aizawa, H. Shinohara, T. Yameda and H. Shirakawa, Synth. Met., 18 (1987) 711. 15 A.R. Hillman and E.F. Mallen, J. Electroanal. Chem., 281 (1990) 109. 16 T.N.S. Kumar and A.Q. Contractor, Trans. SAEST, 21 (1986) 93. 17 R.A. Bull, F.F. Fan and A.J. Bard, J. Electrochem. Sot., 129 (1982) 1009. 18 P. Fiordiponti and G. Pistoia, Electrochim. Acta, 34 (1989) 215. 19 R.D. Armstrong, J. Electroanal. Chem., 198 (1986) 177. 20 A.J. Bard and L.R. Faulkner, Electrochemical Methods, Wiley, New York, 1980, p. 105. 21 M.M. Musiani, Electrochim. Acta, 35 (1990) 1665. 22 J. Tanguy, N. Mermilliod and M. Hoclet, J. Electrochem. Sot., 134 (1987) 795. 23 S.W. Feldberg, J. Am. Chem. Sot., 106 (1984) 4671. 24 T.B. Hunter, P.S. Tyler, W.H. Smyrl and H.S. White, J. Electrochem. Sot., 134 (1987) 2198. 25 R.M. Penner and C.R. Martin, J. Phys. Chem., 93 (1989) 984. 26 I. Rubinstein, J. Rishpon and S. Gottesfeld, J. Electrochem. Sot., 133 (1986) 729.