Spectroelectrochemical studies of proton exchange processes in the electrochemical reactions of polyaniline using pH indicators

127

Chem, 284 (1990) 127-140 Elsevier Se+oia S.A., Iausamie - Printed in The Netherlands

J. Electroanal.

Spectroekctrochemical studies of proton exchange processes in the electrochemical reactions of polyaniline using pH indicators Mieczyslaw Lapkowski Institute of Inorganic Chemistry and Technology, Silesian Technical University, 44-100 Gliwice (Poland)

Eugene M. Genies * Electrochimie Moktdaire, Laboratoires de Chimie, Dkpartement Centre d’Etudes Nuclkaires, 85X, 38041 Grenoble (France)

de Recherche Fondnmentale,

(Received 22 August 1989; in revised form 23 November 1989)

ABSTRACT The thin layer transmission spectroelectrochemical method using pH indicators such as methylviolet has been applied to study the mechanism of proton exchange accompanying the electrode reactions of polyaniline, in aqueous and non-aqueous media. The results obtained for aqueous solutions seem to suggest that after polyaniline oxidation to the first polaron state, a small insertion of protons from the solution occurs. Further oxidation of polyaniline to the bipolaron state does not induce any change in the solution acidity. In the potential range between the first and the second oxidation step no deprotonation of polyaniline occurs. This process starts simultaneously with the second oxidation peak. In non-aqueous media, no proton exchange is observed during the first oxidation step, whereas in the potential range corresponding to the second oxidation step the acidity of the electrolyte solution increases. Protons are strongly bound within the polyaniline matrix. Chronoabsorptometric measurements show that the observed processes are rapid and reversible. These last features may be utilized in the construction of new electrochromic devices.

INTRODUCTION

Polyaniline has been known for many years [1,2], but only in the last decade has a significant progress in the study of its electrochemical properties been achieved. This revival of interest in polyaniline [3-91 is associated mainly with the facility of its preparation, its high stability and high potentials for technological applications [9-111.

l

Member of Grenoble Joseph Fourier University, Grenoble, France.

0022-0728/90/%03.50 0 1990 - Elsevier Sequoia S.A.

128

The elucidation of the polymerization and redox reaction mechanisms is therefore vital for a better understanding of this extremely interesting polymer [12]. Polyaniline is the only conducting polymer whose properties are not only dependent on its oxidation state but also on its protonation state, which in turn is strongly associated with the acidity of the medium in contact with the polyaniline electrode. The polymer is electroactive in solutions of pH < 4 and its conductivity can be varied over 10 order of magnitude depending on the protonation level [13]. The mechanism of aniline polymerization seems to be well established [14,15]. In the cyclic voltammetry of polyaniline, two well defined systems of redox reactions appear [9]. Several redox mechanisms have also been proposed for polyaniline and its derivatives [6,16-231. According to the majority of authors the proposed redox reactions are accompanied by proton exchange processes. If we do not take into account the exchange of protons, a typical reaction scheme can be depicted as follows: First redox transfer (from polyleucoemeraldine). (B represents a benzene ring and Q a quinoid ring) -B-NH-B-NH-B-NH-B-NHtl

0)

-B-&H=Q=&H-B-NH-B-NH(Polyemeraldine

in acid form)

Second redox transfer (from poiyemeraldine). -B-r;H=Q=1;H-B-NH-B-NH--

TI -eIL

(3)

-B-~H=Q=~H-B-NH-B-.~H-

(4 -B-~H=Q=~H-B-~H-B-~H(Pemigraniline

in acid form)

In the above reaction sequences, which take place in polyaniline, we have not shown the exchange of protons. In fact, polyaniline is protonated reversibly and reprotonated depending on the polarization of the electrode and on the redox state of the polyaniline. However, no experimental evidence for the proton exchange in

129

the course of the redox reaction has been published to date. This work is therefore focussed on the “in situ” observation of proton transfer (local pH of the solution in contact with polyaniline film) induced by a change in the redox state of polyaniline; the evidence is obtained from the colour change of a pH indicator such as methylviolet. Similar studies have already been carried out for ruthenium dioxide [24] and polypyrrole [25] deposited on optically transparent electrodes. EXPERIMENTAL

Aniline (analytical grade, Fluka) was distilled prior to use and kept over activated alumina in purified argon. Acetonitrile (spectral grade, BDH), methylviolet (Fluka), sulphuric acid (analytical grade, Analar), lithium perchlorate (analytical grade, Fluka) and sodium sulphate (analytical grade, Analar) were used as received. A polyaniline film was deposited from 0.1 M aniline solution in sulphuric acid (pH = 0) by potential scanning between - 0.2 and + 0.7 V. The thickness of the film was monitored by coulometry and by observation of the amplitude of the oxidation peak in the registered voltammogram. The details of the electrochemical and spectroelectrochemical apparatus used in the present study can be found elsewhere [6]. The central part of the special, thin layer spectroelectrochemical cell is depicted schematically in Fig. 1. The counter

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,L,

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.:::

Me

Fig. 1. Cross electrode; (2) trode solution electrode; (6)

1.

.:‘z

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:

-,

:

.I

section of a thin layer spectroelectrochemical cell. (1) Optically transparent working electrcchemically deposited polyaniline film; (3) optically transparent element; (4) eleccontaining the indicator; (5) electrolyte solution outlet - ohmic contact of the reference solution injection inlet; (7) cell body.

130

electrode is of course outside of the thin layer in a special compartment and the products of the reactions at the counter electrode cannot diffuse into the active cell during the time of the experiment to modify the pH. In such a configuration the electrochemical solution can be removed from or injected to the cell, which is permanently fixed at the same position in the spectrometer, using a syringe. A typical experimental procedure applied in this research can be described by the following steps: (a) In a standard, thick layer, spectroelectrochemical cell a polyaniline film of a given thickness was deposited (typically 5-10 mC/cm2). (b) The standard cell was replaced by a thin layer cell (the glass window was replaced by the element denoted as 3 in Fig. 1. (c) The cell was filled with 1 M sodium sulphate solution in sulphuric acid (pH = 1.4). The polyaniline film was kept in the reduced state. (d) A series of spectroelectrochemical measurements was performed as a function of the potential. The potential of the working electrode was varied in a programmed way. (e) The spectra were recorded on a floppy disk of the OMA III spectrometer. (f) The solution was then removed from the cell with a syringe and replaced by the same solution but containing 0.02 M of methylviolet (pH indicator) and the pH adjusted to 1.4. (g) The spectra of polyaniline together with the indicator were then recorded in the course of the same potential changes as in (d). (h) From the spectra recorded, the spectra of polyaniline stored on the disk were subtracted.

(i) The spectra of the solution layer obtained in such a manner were then plotted graphically. A similar procedure was applied to the spectroelectrochemical studies carried out for a fixed wavelength and to the studies in non-aqueous solution (0.2 M lithium perchlorate solution in acetonitrile; procedure as in (c) of the previous paragraph with additionally 0.02 M methyl-violet and as in (f) of the previous paragraph). The polyaniline film was washed in aqueous, sulphuric acid solution, then with distilled water and finally protonated in a solution consisting of sulphuric acid (pH = 1.4) and 1 M sodium sulphate. For the studies in acetonitrile, polyaniline was also washed with distilled water, then protonated for 1 h in perchloric acid solution (pH = 1.4) containing 1 M lithium perchlorate and finally washed with acetonitrile. RESULTS AND DISCUSSION

The indicator selected for this research had to fulfill three requirements: had to change its colour in the pH range of polyaniline electroactivity (PH < 4); (ii) in addition it had to be soluble in water as well as in acetonitrile; (iii) it had to be electroinactive in the potential range of polyaniline oxidation. (i) it

131

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400

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0

200

ii

a

100

Wavelength/nm

Fig. 2. Pseudo-three-dimensional diagram of absorbance changes in sulphuric acid solution containing 0.02 M of methylviolet and 1 M sodium sulphate (pH =1.4). The curves were registered during voltammetric potential scans of the working electrode covered with an 8 pm thick polyaniline film. Scan rate 2 mV/s. The spectra were obtained by subtracting the corresponding spectra of polyaniline recorded in the absence of methylviolet.

Methylviolet (tetra-penta- and hexamethyl-u-rosaniline) was the best of several readily available indicators fulfilling these requirements since it changes colour in the pH range 0X-3.5 [26]. The visible spectra of methylviolet as a function of pH are close to those in Fig. 2 which are in accord with the well known spectra in the literature [26]. Aqueous solution

In Fig. 2, a pseudo-three dimensional diagram of the spectral changes observed in an aqueous solution of methylviolet is presented. The potential of the electrode covered with an 8 pm thick polyaniline film was scanned in the range from -0.5 to +0.9 V. At E= -0.5 V polyaniline is in the totally reduced, so-called polyleucoemeraldine state which can also be in several states of protonation, e.g.: -B-NH-B-NH-B-NH-B-NH-H+

II

+H’

(5)

-B-I-I&H-B-NH-B-I&H-B-NH(Of course every other redox state of polyaniline can also display proton transfer reactions.) The electrolyte solution (stdphuric acid + sodium sulphate) with a uniform concentration of protons in the bulk (pH = 1.4) is violet-yellow (A,, = 600 MI,

132

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250 -

-500 mV

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, , 680 660 640

,

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,

620

600

580

560

540

520

500

480

460

440

420

ifavelength

/ nm

Fig. 3. Absorption spectra of 0.02 M methylviolet dissolved in the solution as described in Fig. 2 aad for several potentials of the polyaniline electrode.

= 450 MI) because, for this pH value, methylviolet is in the maximum A tr;ition state. The change of its colour is not sharp and takes place in the pH interval from 0.15 to 3.2 [26]. The spectral features of the solution in contact with a polyaniline electrode are strongly dependent on its potential. It may therefore be stated that the local pH varies during the potential change due to the variations in the protonation and redox states of polyaniline [27]. When the potential of the working electrode is raised to 0 V the absorption at X = 450 nm virtually disappears. A further increase of the potential results in a decrease of the absorption due to the X = 600 nm peak together with renewed formation of the h = 450 nm absorption which is observed at potentials E > 0.65 V. It may be recalled that the evolution of the spectra in the figures is due essentially to methylviolet because the polyaniline spectra have been substracted

1191. In Fig. 3, electrolyte solution spectra recorded for selected potential values are presented. Spectral changes of the pH indicator can be observed clearly. In order to elucidate the mechanism of the polyaniline redox reactions, chronovoltabsorptometric measurements were carried out for the characteristic wavelength of h = 454 nm, which is associated with the acidic form of the indicator @H < 0.15). In Fig. 4 the results obtained for A = 454 nm are presented. The absorbance of methylviolet obtained for pH = 1.4 is taken here as the reference and as the starting absorbance point. From Fig. 4 it is clear that there exist two potential ranges for which significant changes associated with the indicator occur. They correspond strictly to the two

133

electrode processes recorded for polyaniline film (the voltammetric as the dotted line).

curve is shown

First electrode process The absorbance recorded for X = 454 nm is strongly dependent on the applied potential It decreases drastically for potentials E = -0.3 to E = -0.25 V, then slowly rises till the potential of 0 V is reached and then decreases again. The low value of the absorbance remains essentially the same till E = 0.4 V. In fact, the results are a little more complex, because we observe the spectra of methylviolet molecules which are in the PANi film and of indicator molecules which are in the thin layer solution. Sometimes it was observed that the one becomes more acid (probably in the film) as the other becomes more basic (probably in the solution). It must take time before the pH is perfectly in equilibrium in the film and in the solution. The following explanation can be proposed for the results in Fig. 4; in totally reduced polyaniline (polyleucoemeraldine), at pH = 0, only l/12 of the amine groups (-NH-) are protonated [23]. Therefore for pH = 1.4 the protonation level should be much lower. In the initial stages of the first electrode process, polyaniline is being oxidized to the first polaron (shoulder preceding the first peak in the cyclic voltammogram). At this moment only 1 in lo-12 nitrogen atoms is being oxidized, the rest remaining in the reduced state (-NH-). The oxidation changes the pK of polyaniline, which becomes more basic, significantly. Thus the protonation level of the -NH- groups increases. The local pH of the solution in direct contact with the polyaniline electrode must therefore increase, since protons are transferred from the solution to the polymer according to reaction (5) with in addition some polaron states according to reaction (1). The rather insignificant increase of the acidity occurring during further oxidation can be rationalized by the diffusion of protons from the bulk of the electrolyte. It should be noted that the oxidation from the polaron state to the bipolaron state (reaction 2) [21-231 does not change the protonation level. Most probably non-protonated -NH- groups, after being oxidized (reaction l), recombine to bipolarons as follows: -B-NH-B-NH-B-NH-B-NH-

TI

(6)

-B-NH=Q=NH-B-NH-B-NH-

Second electrode process During the second electrode process a new clear absorption band develops at A = 454 nm. This band is due to the absorption of the acidic form of methylviolet; it therefore reflects a drop of the local pH of the electrolyte solution to values < 0.15.

134

Polyaniline deprotonation starts at potentials E = + 0.35 to E = 0.40 V, corresponding to the additional (third) oxidation peak which is responsible for the cross-linking of linear polymer or for the formation of phenazine structures [15,28]. A clearer increase of the electrolyte solution acidity occurs immediately prior to the second oxidation potential at E = 0.5 V. The second electrode process is therefore accompanied by polyaniline deprotonation: -B-kH=Q=&H-B-NH-B-NHI/

-H+,

-e(7)

-B-N=Q&j-B-&-B-NH-

+H+,

-e-

Ii

(8)

-_B-N=Q=N-_B-&H=Q&H-

Till now it was postulated that polyanihne deprotonation occurs in the potential range E = 0 to E = +0.5V, in between the two electrode processes and independently of the two electrochemical reactions (reactions l-4). The observed increase of the acidity after the second oxidation (E > +0.75V) can be explained by the hydrolysis of quinone-type groups, leading to quinones and protons as described by Kobayashi et al. [3]. This reaction is proton catalyzed [29] and the local increase of acidity promotes this hydrolytic process. The existence of the basic form of methylviolet can be explained by local pH changes. The increase in acidity occurs only next to the electrode surface. In the remaining part of the solution the acidity does not change and the basic form of

454

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curves for the absorbance of the Fig. 4. Voltammetric curve ( . . . * . .) and chronovoltabsorptometric acidic form of 0.02 M methylviolet (solid line, h = 454 mn) recorded in the solution described in Fig. 2.

135

methylviolet can be observed. Of course the diffusion of mobile protons from the bulk of the solution may cause additional uniformity of the concentration. Non-aqueous solutions

Acetonitrile was chosen as the electrolyte solvent since methylviolet is readily soluble in it and the electrochemical properties of polyaniline are very well established in acetonitrilebased electrolytes [6]. In Fig. 5, a pseudo-three-dimensional diagram of the spectral changes of the electrolyte, consisting of acetonitrile, lithium perchlorate (0.2 M) and methylviolet (0.02 M) is presented. The spectra were recorded during potential scanning of the electrode covered with a ca. 8 pm thick polyaniline film. The scan range was from E = -0.5to 1.3 V and back to -0.5 V. Spectral changes corresponding to the changes in the acidity of the electrolyte are also seen clearly in this case. In Fig. 6, selected spectra of the solution recorded for some characteristic electrode potentials are presented. For the reduced form of polyaniline (polyleucoemeraldine) the solution is violet (clear maximum at h = 590 nm and shoulder at h = 540 nm, spectrum a). The colour remains the same until the potential of the electrode reaches E = +0.9V. At this point the absorbance at X = 590 mn decreases and a new maximum peak at A = 440 run is formed (spectrum b). This peak is seen more clearly when the polyaniline is totally oxidized (spectrum c). The process is essentially reversible since the changes monitored in the reduction cycle correspond to the changes observed in the oxidation cycle. It must be noted,

Wavelength/

“ill

Fig. 5. Pseudo-three-dimensional diagram of absorbance changes in acetonittile solution containing 0.02 M of methylviolet and 0.2 M lithium perchlorate. The curves were recorded during voltammetric potential scans of the working elfxtrode covered with an 8 pm thick polyaniline film. Scan rate 5 mV/s. The spectra were obtained by subtracting the corresponding spectra of polyaniline recorded in the absence of methylviolet.

136

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,

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660

660

640

620

600

660

560

540

520

500

460

460

440

Wavelength

/

420

nm

Fig. 6. Absorption spectrum of 0.02 methylviolet dissolved in the solution described in Fig. 5. Polyaniline electrode potential, E/V: (a) -0.5, (b) 0.9, (c) 1.3 and (d) 0 in the reverse run.

however, that a presently unknown intermediate between the acidic and basic forms of methylviolet is created. The maximum of its absorption is shifted to h = 620 mn (spectrum d). This phenomenon, not observed in aqueous solutions, can be seen more clearly when the polyaniline electrode’s potential is changed from E = + 0.4 V to E= -0.2 V.

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...... ...._._._,,_ :Fig. 7. Voltammetric curve ( . . . . . .) and chronovoltabsorptometric curves for the acidic (- - -) and forms recorded for 0.02 M methylviolet in the solution described in Fig. 5. The basic ( -) absorption changes for the acidic form are increased threefold with respect to the applied scale.

137

Additionally, in non-aqueous medium the transition from the basic form to the acidic one and vice versa is very sharp and may be caused by small changes in proton concentration in the solution. The method is therefore very sensitive and enables small quantities of protons absorbed or desorbed by the polymer to be detected. In Fig. 7, voltabsorptometric curves for the acid (h = 442 nm) and basic (X = 592 nm) forms of methylviolet, recorded in the potential range E = - 0.5 to + 1.3 V, are presented. Significant differences, as compared to the analogous curves registered for aqueous solution (Fig. 4), are observed. The first electrode process does not induce any changes in the local acidity of the reaction medium, and only for potentials exceeding the potential of the second redox process can the changes in the acidity of the electrolyte be registered. From the measurements described above it is clear that proton exchange between polyaniline and the electrolyte is, in this case, associated with a much higher oxidation level of the polymer. This means that protons are bound more strongly to the polymer and released less readily into the acetonitrile solution [30,31]. The deprotonation is not simultaneous with the oxidation, but follows it according to the scheme: -B-&H=Q=&H-B-NH-B-NH-

(9) -B-~H=Q=~H-B-AH-B-NH-

II

-I-I+

-B-N=Q&j-B-&-B-NH-

II

-e-

-B-N=Q&j-B-&H=Q&H-

IL -H+

-_B-_N=Q=N-_B-&H=Q=&H-

Chronoabsorptometric

measurements

Spectroelectrochemical measurements carried out in this study show clearly that protons released during the redox reactions of polyaniline may cause colour changes of the acid-base indicator present in the electrolyte solution. This phenomenon can constitute the basis for the construction of an electrochromic device [32]. It should be remembered here that polyaniline has already been considered as a candidate for

1O-3 A AU

10

20

30

40

50

60

70

80

90

100

110

120

TIME/S Fig. 8. Chronoabsorptometric curve registered for h = 442 run, of a sulphuric acid solution containing 0.02 M methylviolet and 1 M sodium sulphate (pH = 1.4) during double step potential changes from -0.5 V to +0.9 V with a frequency of 0.1 Hz.

such a device [3,33-351 since it changes its colour in accordance with its different redox states [6]. The addition of coloured indicators to the electrolyte may improve the contrast of the electrochromic device. In addition, proper selection of the indicator may lead to the construction of a device with the desired colour changes. In order to estimate the utilitarian character of the observed phenomena, preliminary chronovoltabsorptometric studies were carried out in aqueous solutions. In Fig. 8, a chronoabsorptometric curve for a solution consisting of methylviolet, sodium sulphate (1 M) and sulphuric acid (pH = 1.4) is shown. The potential was stepped from E = - 0.5 V to + 0.9 V with a frequency of 0.1 Hz. From the shape of the chronovoltabsorptometric curves several conclusions can be drawn: (a) the colour changes are reversible; (b) the rate of colour change is very high compared to other homogeneous solutions; (c) the first cycle is distinctly different from the following ones; (d) for a concentration of methyviolet of 0.02 M the observed contrast is rather low. The high rate of colour change can be explained by the significant mobility of protons (high value of the diffusion coefficient) compared to more bulky groups. The contrast can be increased by the use of more concentrated indicator solutions or, alternatively, by the application of other indicators with a higher absorption coefficient [36].

139

The difference in the optical response between the first and consecutive cycles requires some explanation. This phenomenon has already been observed in electrochemical [37] and spectroelectrochemical [33] curves of polyaniline. It is believed that this difference is due to a rather slow stabilization of the polymer structure in the stationary state. The structure destabilized during the first electrochemical run undergoes different changes for the consecutive potential steps. It is clear that the diffusion of protons to the solution for the first potential step is much slower than in the consecutive runs. Protons are bound more strongly in polyaniline which was equilibrated for an extended period of time (protonation lasting ca. 1 h). CONCLUSIONS

Spectroelectrochemical measurements carried out in this study have enabled us to elucidate the mechanism of redox processes occurring during polyaniline cycling and to verify the models presented in the literature [6,16-231. The proton exchange occurring during the two oxidation processes has been determined in both aqueous and non-aqueous solutions. The research presented has also induced us to verify the possibility of the construction of a new type of electrochemichromic device based on reversible colour changes of a selected acidbase indicator, which in turn are induced by proton exchange between the polymer and the electrolyte. If in aqueous medium we have not obtained a sufficiently large contrast (Figs. 3 and 8) for a display or electrochromic window, in organic medium the contrast (Fig. 6) is certainly large enough to show promise for R&D studies on the corresponding devices. ACKNOWLEDGEMENTS

We thank A. Pron for critical reviews of the manuscript. REFERENCES 1 2 3 4

5 6 7 8

J. Fritzche, J. P&t. Chem., 20 (1840) 453; 28 (1843) 198. R. Nietski, Ber. Bunsenges. Phys. Chem., 11 (1987) 1093. T. Kobayashi, H. Yoneyama and H. Tamura, J. EIectroanaI. Chem., 161 (1984) 419; 177 (1984) 281. A.G. MacDiarmid, J.C. Chiang, M. HaIpem, W.S. Huang, S.L. Mu, N.L.D. Somasiri, W. Wu and S.I. Yaniger, Mol. Cryst. Liq. Cryst., 121(1985) 173; A.G. MacDiarmid, N.L.D. Somasiri, W.R. Salaneck, I. Lundstrom, B. Liedberg, M.A. Hasan, R. Erlandsson and P. Konrasson in H. Kuzmany, M. Mehring and S. Roth (Eds.), Electronic Properties of Conjugated Polymers, Springer Series in Solid-State Sciences, Vol. 63, Springer-Verlag, Heidelberg, 1985, p. 218. E.M. Genies, C. Tsintavis and A.A. Syed, Mol. Cryst. Liq. Cryst., 121 (1985) 181; E.M. Genies and C. Tsintavis, J. EIectroanaI. Chem., 195 (1985) 109. E.M. Genies and M. Lapkowski, J. ElectroanaI. Chem., 220 (1987) 67. A. Kitani, J. Yano and K. Sasaki, Chem. Lett., (1984) 1565. G. MengoIi, M.T. Munari, P. Bianco and M.M. Muaiani, J. Appl. Polym. Sci., 26 (1981) 4247; M. Musiani, G. Mengoh and F. Furlaneto, ibid., 29 (1984) 4433; G. MengoIi, M.M. Musiani, B. PeIIi and E. Vecchi, ibid., 28 (1983) 1125.

140 9 E.M. Genies, M. Lapkowski and C. Tsintavis, New J. Chem., 12 (1988) 181. 10 EM. Genies, M. Lapkowski, C. Santier and E. Vie& Synth. Met., 18 (1987) 631. 11 A. Kitani, M. Kaya and K. Kitani, Denki Kagaku, 52 (1984) 847; 53 (1985) 592; J. Ehxtrochem. Sot., 133 (1986) 1069. 12 A.G. MacDiarmid, J.-C. Chiang, M. HaIpem, W.S. Huang, J.R. Krawczyk, R.J. Mammone, S.L. Mu, N.L.D. Somasiri and W. Wu, Polym. Prepr., 25 (1984) 248; A.G. MacDiarmid, S.-L. Mu, N.D.L. Somasiri and W. Wu, Mol. Cryst. Liq. Cryst., 121 (1985) 187. 13 A.J. Epstein, J.M. Ginder, F. Zuo, RW. Bigelow, H.-S. Woo, D.B. Tanner, A.F. Richter, W.-S. Huang and A.G. MacDiarmid, Synth. Met., 18 (1987) 303; J.M. Ginder, A.F. Richter, A.G. MacDiarmid and A.J. Epstein, Solid State Ccammm., 63 (1987) 97. 14 EM. Genies and M. Lapkowski, J. EIectroanal. Chem., 236 (1987) 189. 15 E.M. Genies, M. Lapkowski and J.F. Penneau, J. Electroanal. Chem., 249 (1988) 97. 16 D.M. Mohilner, RN. Adams and W.J. Argersinger, J. Am. Chem. Sot., 84 (1962) 3618. 17 A. Kitani, I. Izumi, J. Yano, Y. Hiromoto and K. Sasaki, BuIl. Chem. Sot. Jpn., 57 (1984) 2254. 18 W.-S. Huang, B.D. Humphrey and A.G. MacDiarmid, J. Chem. See., Faraday Trans. 1, 82 (1986) 2385; A.G. MacDiarmid, J.C. Chiang, A.F. Richter and A.J. Epstein, Synth. Met., 18 (1987) 285. 19 E.M. Genies and M. Lapkowski, J. Electroanal Chem., 236 (1987) 199. 20 E.M. Genies and M. Lapkowski, 192nd ACS Meeting, Anaheim, CA, U.S.A., 7-12 September 1986, Abstract COLL 094. 21 E.M. Genies and M. Lapkowski, J. EIectroanaI. Chem., 236 (1987) 199. 22 E.M. Genies and M. Lapkowski, Synth. Met., 21 (1987) 117. 23 E.M. Genies and M. Lapkowski, Synth. Met., 24 (1988) 61. 24 G.-W. Jang, E.W. Tsai and K. Rajeshwar, J. Electrochem. Sot., 134 (1987) 2377. 25 E.W. Tsai, G.-W. Jang and K. Rajeshwar, J. Chem. Sot., Chem. Commun., (1987) 1776. 26 W. Kemula and A. Axt-Zak, Ron. Chem., 43 (1969) 403 and references therein. 27 M. Nechtschein, F. Genoud, C. Menardo, K. Mizoguchi, J.P. Travers and B. ViIleret, International Conference on Science and Technology of Synthetic Metals, Santa Fe, NM, U.S.A., 26 June-2 July 1988, Abstract BII 1, p. 64; Synth. Met., 29 (1989) E211. 28 E.M. Genies, M. Lapkowski and J.F. Permeau, International Conference on Science and Technology of Synthetic Metals, Santa Fe, NM, U.S.A., 26 June-2 July 1988, Abstract P25, p. 274; J. Electroanal. Chem., 249 (1988) 97. 29 M. Lapkowski, Synth. Met., in press. 30 E.M. Genies, P. Hany and C. Santier, International Conference on Science and Technology of Synthetic Metals, Santa Fe, NM, U.S.A., 26 June-2 July 1988, Abstract Al 3, p. 321; Syntb. Met., 28 (1989) C647. 31 M. Mizumoto, M. Namba, S. Nishimura, H. Miyadera, M. Koseki and Y. Kobayashi, International Conference on Science and Technology of Synthetic Metals, Santa Fe, NM, U.S.A., 26 June-2 July 1988, Abstract Al 2, p. 320; Synth. Met., 28 (1989) C639. 32 J. Przyfrski and A. Gadon, Wiad. Chem., 35 (1981) 685. 33 E.M. Genies, M. Lapkowski, C. Santier and E. Vieil, Synth. Met., 18 (1987) 631. 34 A. Kitani, J. Yano and K. Sasaki, J. Electroanal. Chem., 209 (1986) 227. 35 A. Watanabe, K. Mori, Y. Iwasaki, Y. Nakamura and S. Niizuma, Macromolectdes, 20 (1987) 1793. 36 M. Lapkowski and J.W. Strojek, Wiad. Chem., 40 (1986) 283; 37 A.F. Diaz and J.A. Logan, J. Ehxtroanal. Chem., 111 (1980) 111.