The influence of counter-ions on nucleation and growth of electrochemically synthesized polyaniline film

Ekcf~~hw~iro Copyright

Pergamon

PII: SOOl3-4686(96)00362-3

.4mr. Vol. 42. No. 9. pp. 13X9- 1402, 1997 c’ 1997 Pubhshed by Elsevier Science Ltd Printed in Great Britain. All rights reserved 00134686’97 $17 00 + 0.00

The influence of counter-ions on nucleation and growth of electrochemically synthesized polyaniline film Zoran

MandiC,” Ljerka

DuiC*” and Franjo

KovaCiEekb

“Laboratory hFaculty

of Electrochemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, P.O. Box 177, 10.000 Zagreb, Croatia of Mechanical Engineering and Naval Architecture, University of Zagreb, 10.000 Zagreb, Croatia

(Received

14 June 1996; in revised form 2 August

1996)

Abstract-The initial stages of polyaniline (PANI) film growth are investigated by a potentiostatic technique. The obtained I-t transients exhibit current maxima which are analyzed for the nucleation type. The evidence is given on two different stages of the nucleation process. It is established that the first stage of growth is a formation of a compact PANI layer which is followed by an additional nucleation process. For that nucleation process it is found that at lower concentrations of aniline monomer 3D progressive nucleation takes place, and with the increasing monomer concentration in perchloric acid solutions 3D instantaneous nucleation prevails. At later stages ID growth takes place resulting in chain branching. The possibility to observe current maxima, indicating the particular nucleation process. depends on monomer concentration, on the potential of synthesis, and on the counter-ion present, ic> on the rate of PAN1 polymerization. It is shown that clusters resulting from the nucleation process form different shapes depending on the counter-ion present. ,:c 1997 Published by Elsevier Science Ltd. All rights reserved. Key itor&;

Polyaniline,

nucleation,

potentiostatic

synthesis.

INTRODUCTION Polyaniline (PANI) is a material the physical and chemical characteristics of which greatly depend upon the method of the preparation and on the experimental parameters of the synthesis. It is known that PAN1 morphology, conductivity and other properties depend on the counter-ion incorporated on the [l-61, and the degree of crystallinity preparation procedure [7, 81. Zotti et al. [I], who investigated the mechanism and the growth of polyaniline by cyclic voltammetry, found that polyaniline growth depends greatly on the type and the concentration of the supporting electrolyte anion. DuiC er al. [2, 31 also established the importance of the anion species on PAN1 film growth, studied by cyclic voltammetry. It has been proposed that the electrochemically synthetized PAN1 consists of two types of layers

*Author to whom correspondence

should be addressed.

[9, IO]. The first one is formed at the early stage of growth and it is of a compact structure, while the second layer is of a less dense structure. The investigations of the early stages of the electrodeposition of PANI have been reported in several papers [IO-131. It is reported that the electrodeposition of PAN1 includes a nucleation process similar to the nucleation process at metal deposition, and that the nucleation process taking place at the early stage of PAN1 growth obeys 2D progressive nucleation [IO], and 3DPN mechanism in H?SOd and 2DPN and/or 3DIN in HCl04 supporting electrolyte [ 131. The aim of this work is to elucidate the early stages in the process of the electrochemical deposition of PAN1 on a Pt electrode, especially the nucleation process taking place at the most commonly applied potentials of polyaniline synthesis (E > 800 mV). Since detailed physical properties and the rates of the electrochemical processes are likely to depend on the film morphology, it is of interest to gain an insight in

1389

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Z. MandiC et al.

the early stages of PAN1 film, the growth of which might govern the developing morphology of the film. EXPERIMENTAL The electrochemical polymerization of aniline has been carried out by the potential step method, using a standard three-electrode cell, a Pt working (A = 0.5 cm2) and counter-electrode (A = 1.5 cm*), and see as reference electrode. The experiments were carried out by the potential step technique using a Wenking LB 75L potentiostat, and the resulting transients were recorded and analysed on a PC 486. Different concentrations of aniline were employed (0.05,0.1,0.15 and 0.2 M) in 1 M solutions of the following acids: HSOI, HNOJ and HCl04. The experiments were carried out at constant temperature (t = 25 + O.lC). The experiments were performed by applying starting potential, E,,,,,, of 0 V vs see; the potential of synthesis, E,yn,hr was varied in the region of 800-l 100 mV vs see. Before each potential step, the electrode was kept at Eslart for the period of 2 min. For the scanning electron microscope (SEM) analysis, the polyaniline synthesis was carried out up to the predetermined times corresponding to the characteristic parts of current-time transients. Since SEM analysis was carried out es-situ, after the programmed synthesis step, the electrodes were washed in acetone and then vacuum dried for 48 h. RESULTS

AND DISCUSSION

Figure 1 illustrates current-time transients obtained at different potentials for 0.05 M aniline in a 1 M solution of HS04. Three characteristic regions can be recognized, depending on time of the current-time transient and on the applied potential step. The first region (region I) includes a current jump followed by the exponential current decrease to the renewed current increase. The second region (region II) includes the renewed current increase up to the current plateau. The third region (region III) covers the current at the plateau. These general characteristics apply to current-time transients obtained for all the three supporting electrolytes. However, the appearance, i.e. the timing, the position and the discernibility of the current maximum (curve c in Fig. 1) depends on the counter-ion supplied by the supporting electrolyte, and on the applied potential step. Thus, I-t transients taken for aniline solutions at the same potentials (Esynth = 840 mV) but for two different supporting electrolytes show that current-time plots depart before reaching the minimum value. In the case of HS04 electrolyte (Fig. 2(a)), there is a shoulder emerging on the part of the exponential current increase, whereas in the case of HCl04 (Fig. 2(b)) there is a current shoulder on the decreasing part of the transient, followed by an indication of an

additional current maximum after which there is no immediate exponential current increase. Figure 3 illustrates current-time transients taken for three different electrolytes at the potential E synth= 1100 mV. There are evident current maxima which are well developed in the case of HNO3, less developed, but clear, in the case of HlS04, and displaying only a shoulder in the case of the HC104 supporting electrolyte. The values of current following the maxima continue in the order H2S04 > HN03 > HCl04. Due to the specificity of the appearance of current maxima, depending on the counter-ion involved, the analysis of the maxima has been carried out for the solutions of HCl04 supporting electrolyte at lower potentials of PANI synthesis, and for the solutions of HN03 solutions at higher potentials of PANI synthesis. Current

maximum

in HCl04

supporting electrolyte

Figure 4 illustrates current-time transients taken for 0.05 M aniline/l M HC104 solution for the time interval corresponding to the region I, ie until the renewed current increase is reached. The transients show a pronounced maximum and/or a shoulder in the region of the general current decrease. At the lower potentials of synthesis (curves a and b) both the current maximum and the current shoulder are discernible. With the increase of the potential of synthesis, the value of current maximum, f”,, increases and the time of the maximum appearance, t,, decreases (curves sac in Fig. 4). With the increase of Esynthr the current maximum is shifted to shorter times and it becomes less discernible (curve d). It is known [14-171 that current maximum indicates the formation of a new phase at the metal/solution boundary, and therefore the appearance of current maximum within the region of the exponential current decrease indicates that the aniline oxidation reaction is followed by the formation of a polyaniline layer on the electrode. This is supported by the SEM micrographs taken for the naked Pt-electrode (Fig. 5(a)), and for the Pt-electrode after 100 s (region I) at EEynlh= 800 mV (Fig. 5(b)). While the SEM micrograph of the naked Pt-electrode shows grooves on the electrode surface, no noticeable grooves are shown on the other one. This difference in the appearance can be explained by the deposition of a compact PANI layer covering the electrode surface. Assuming that the number of electrons exchanged in the aniline oxidation reaction is two per molecule [18], the thickness of PAN1 layer, reached within the time covered by the current maxima in region I, is estimated to be d = 150-200 nm which is in a very good agreement with the results obtained under galvanostatic conditions [9], and we think it should be attributed to the formation of the uniform polyaniline film. Figure 6 illustrates the dependence of current-time transients on aniline-monomer concentration in

Electrodeposition

of poiyaniline

1391

film

I

I

I

100

200

300

0 0

400

t/s Fig. 1. Current-time 1100 mV.

transients obtained for 0.05 M aniline/l M HrS04 at different potentials: (a) 800. (b) 1000, and (c)

2.0

1.5

4 3 1.0

0.5

-~

0.c

0

20

b

40

60

80

100

t/s Fig. 2.

i-f

transients for 0.05 M aniline in (a) H~SCh. and (b) HCIOI supporting electrolyte. &nlh = 840 mV.

Z. Mandit etal.

1392

0 0

10

20

30

40

50

60

70

80

t/s Fig. 3. I-r transients for 0.05 M aniline in (a) 1M HzS04, (b) I M HNO,, and (c) 1 M HC104,

&nth

=

1100

mV.

0.8

0.6

0

30

60

9s"

Fig. 4. /-I transients for 0.05 M aniline/l M HCIOI for different

120

&ynth:

150

180

(a) 820, (b) 840, (c) 860, and (d) 880 mV.

Electrodeposition

I393

of poiyaniiine film

10 pm I I Fig. 5. SEM micrographs: (a) naked Pt-electrode, (b) the same electrode after 100 s at Eqsnth= 800 mV, in 0.05 M anilineU M HCIOI.

2.0

1.6

1.2

0.8

0.4

0.0

u

10

30

20

40

50

t/S Fig. 6. I-r transients at l&h = 840 mV for different concentrations 0.15 M.

of aniline in f M HCIO?: (a) 0.05 M. (b) 0. I M. (c)

Z. MandiC et al.

1394

HCl04 supporting electrolyte. It is evident that with the increase of aniline concentration the current maximum increases, and shifts to shorter times. The current shoulder, preceding the current maximum, gets incorporated into exponentially decreasing current and at these potentials and monomer concentrations cannot be analyzed. However, the maximum can be analyzed for the nucleation mechanism by comparing the experimental data with the theoretical plots given for different types of nucleation processes. Theoretical plots for progressive and for instantaneous 3D nucleation process under diffusion control are given by equations (1) and (2) [l4], respectively: (i/i,,,)l

= 1.9542(

t/t”,)-‘{ 1 - &’ 1564(“nl’1)?, (1)

Figure 7 compares the non-dimensional plots, obtained from the experimental data of the second current maximum (Fig. 6), with the theoretical plots for progressive and instantaneous 3D nucleation process under the diffusion control. A good agreement of experimental data with the theoretical plot for instantaneous nucleation process is obtained in the case of higher aniline concentration, whereas at lower concentration the plot calculated from the experimental data departs towards the plot for progressive 3D nucleation. Taking into account results obtained by C6rdova et a/. [13] on the nucleation process in perchloric solutions, one may conclude that there are two separate nucleation stages of PAN1 growth. Since the first nucleation process takes place at the naked Pt-electrode, and the second one on the already PAN1 modified surface, it can be assumed that the nucleation mechanisms might be under the control of different processes. PANI film growth is known to be very slow in perchloric solutions. and the process investigated at the potentials of the very beginning of the aniline oxidation might be under kinetic control of a CE mechanism as assumed by C6rdova et al. [l3]. However, at higher potentials of PAN1 film growth, the second current maximum indicates that a new nucleation process is taking place after the first one has been completed. It implies that there are also new circumstances under which that new nucleation process takes place. First of all there is the already formed PAN1 layer, and at the potentials of investigation, it is pernigraniline form (PG). which through the mechanism PG/EM contributes to the further growth of PANI. At the same time, at higher potentials the mechanism through oligomers [ 10, 131 contributes to the higher density of oligomers in the reaction volume, making the diffusion of aniline molecules more difficult and therefore the controlling factor in PANI film growth.

Current maximum in HN03 supporting electrolyte In the case of nitric acid solutions, only at higher potential steps was a well developed current maximum registered, again preceded by a current shoulder. Figure 8 illustrates I-t transients obtained for 0.05 M aniline/l M HN03 in the range of the potential of PAN1 synthesis, Esynth= 100&l 100 mV. As the potential of synthesis increases, the current maximum, I,, shifts towards higher values and the time, t,, to lower values. At lower potential values (curves a and b in Fig. 8) a shoulder appears before the maximum is formed, indicating that there is a nucleation process preceding the one shown by the well developed current maximum. After the current maximum there is an exponential decrease of the current reaching the same value in all current-time transients. The best developed maxima in Fig. 8 (curves d and e) are analyzed for the nucleation mechanism according to the equations (I) and (2) describing instantaneous and progressive 3D nucleation processes, respectively. Figure 9 compares the plots obtained from the experimental data with the theoretical plots for instantaneous and progressive 3D nucleation. There is a fairly good agreement of the experimental data with the theoretical plot describing the progressive 3D nucleation process, except for the values after the current maximum, when experimental data drop below the theoretical values. This might be explained by the increased resistance of the newly formed polymer phase, as the measurements were carried out at the potentials of low PAN1 conductance [19]. In the case of 3D nucleation under the diffusion control further analyses through ii,t., product can be carried out according to the following equations [ 141:

ii,t,, = O.1629O{zFc[l

- e(-“lR’)])‘,

(3)

ii,t,, = 0.2598D{=Fc[l

- e(-‘” R’)])2,

(4)

given for the instantaneous (3) and for the progressive (4) processes. At high overvoltages the term e(-‘fi’R” can be ignored in both equations, which leads to the constant values of the ii,tn, product, irrespective of the overvoltage applied. The values amount to I 10.8 mA? crnm4 s for the instantaneous and 176.6 mA? crne4 s for the progressive nucleation process. In order to compare the product values for different overvoltages, the time fn, has to be corrected by the induction time to necessary for the nucleation process to start at the corresponding potential. The value of to results from the intersection obtained by the extrapolation of the decreasing current, preceding the maximum, and of the increasing part of the current maximum. From the results given in Table I, it is evident that the product values are in a fairly good agreement with the theoretical value for the progressive 3D nucleation process under the diffusion control.

Electrodeposition

1395

of polyaniline film

0.6

0

3

1 %lax

2

Fig, 7. Non-dimensional plot of current maximum shown in Fig. 6 compared with theoretical curves for (a) instantaneous and (b) progressive, 3D nucleation under diffusion control.

3.5

I

I

I

3.0

2.5

*

2.0

E 1.5

1.0

0.5

I

0.0

_^

V

I

,

. .

blJ

4u

_^

^

a0

t/S Fig. 8. I-r transients obtained in 0.05 M aniline/l M HNO? at different ,!&th: (a) 1000, (b) 1050, (c) 1060, (d) 1075, and (e) I100 mV.

1396

Z. MandiC et al.

1.5

--I

J

I

I

I

CA

A

1075mV

0

1lOOmV

0.0I -0

1

2

4

3

%lax Fig. 9. A non-dimensional plot of the current maxima, shown in Fig. 8, compared instantaneous, and (b) progressive nucleation under the diffusion control.

The same current-time transients (d and e in Fig. 8) are plotted as log(f)-log(t) plot. According to the relation given for 3D nucleation under the diffusion control [ 141: 2zFANorr(2Dc)’ 1(t) = 3p”

with the theoretical

curves

for (a)

It is necessary to solve the system of two non-linear equations with two unknowns, x and a: ln

(6)

9

‘IV’ ‘t3 (5)

from which it follows that the increasing part of current maximum in log-log coordinates results in a straight line with the slope of 312, which is indeed obtained (Fig. IO). The experimental plots for the transients at lower potentials differ from the theoretical values due to the overlap of the first current shoulder with the current maximum (c:f. curve a and b in Fig. S), so therefore to could not be accurately determined. In the case of 3D nucleation under the diffusion control it is possible to calculate the number of active sites in the nucleation process, NO, as well as the rate of the nucleation process at the active site, AN [20].

In[l + 2.u(l - e-‘“)I

The parameter a in equation (6) is defined as a = d’D’ ?c/rr’ z and for PAN1 deposition shown in Fig. 8, it amounts to 0.015 A s’ ? cme2. The unknowns s and t( are dimensionless quantities, being defined as: .Y= N,,nkDt,,,

I,,,

6)

1075 1100

I I.8 9.1

(8)

and

&!sF!E AN

where

’

(9)

D is the diffusion coefficient of aniline, and k quantity, defined as k = (8ncM/

is a dimensionless

Table I. Time. tnl, current density, b,, and corresponding (d) and (e) transients given in Fig. 8

E WV)

- .Y+ r(l - e-’ “) = 0. (7)

in,

i,:I(f,I,- ro) values for the

to (s)

(mA cm-‘)

G(/,,>- to) (mA’ cmm4 s)

5.4 5.0

4.94 6.61

156.2 179.1

Electrodeposition

of polyaniline

film

1397

E

1.2

Cl

1lOOmV

0

1075mV

0.6

0.4

0.2

0.0 1.0

1.5

2.5

2.0 log(t)

Fig. IO. Straight

lines obtained

for the increasing

part of the maximum

p)’ ‘, which for transients shown in Fig. 8 amounts to 0.34. Instantaneous and progressive nucleation are two extremes of heterogeneous nucleation processes at the electrode surface. Instantaneous nucleation is a process of nuclei creation at a high rate on a small number of active sites, so a + 0 and x = 1.2564. Progressive nucleation is a process of nuclei creation at a low rate but on a large number of active sites, and CI+ x and .X= 2.1618~’ ?. The values of x and x. the number of active sites, No, and nucleation rates, AN. for the transients shown in Fig. 8, are given in Table 2. It is evident from Table 2 that, with the increase of potential, both the nucleation rate AN and the number of active sites NO increase. The parameters x and x increase, too, and x -+ 2.2cr”*, indicating that the nucleation process is approaching progressive 3D nucleation under the diffusion control. Bade rt ul. [IO], who investigated PAN1 electrodeposition on Pt-microelectrode by the potentiostatic

shown

in Fig. 8.

method, found that the current maximum, which they registered, fits the criteria for 2D progressive nucleation. However, the measurements were taken at lower potentials (~a 740 mV vs see), at lower aniline concentration (0.01 M) and using a Pt-microelectrode, therefore it is very likely that the analyzed maximum corresponds to the first stage of the process which leads to the formation of the compact PAN1 layer. The same may be assumed for the results obtained by C6rdova et ul. [13], since their analysis is carried out for the transients obtained at quite low potentials but also at considerably higher concentrations of monomer. We assume that the shoulder appearing before the maximum in transients, presented in Figs 2, 4 and 8, corresponds to that first stage, but because of its poor development it was not possible to analyze it. Figure II shows SEM micrographs obtained for PAN1 synthetized for the time corresponding to the onset of the exponential increase of current, which differs for each supporting electrolyte (cf. Fig. 15).

Table 2. The values of .Yand a parameters, the number of active sites, NO, and the rates of nucleation, AN. for the transients shown in Fig. 8 QmV

.Y

1055 1060 1075 1100

1.5 2 5 I3

a

2.2a’ *

0.28 0.44 6.3 31.6

1.16

1.46 5.52 12.4

No (cm-‘)

I.1 2.8 8.9 3.13

x x x x

I04 104 IO4 105

AN (s-‘1

0.3 0.49 0.61 0.7

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Z. MandiC et al.

Electrodeposition

of polyaniline film

I

1399

I

I

100

150

I/mA 15

0 0

50

t,

200

t/S Fig. 12. A comparison of f-1 transients for 0.2 M aniline/l stirring applied at time shown, (b) stirred solution.

M H604,

.&,I, = 800 mV: (a) quiet solution.

and solution

with

20

15

5

/

a/

0

h

”

LUU

I””

JUU

400

t/s Fig. 13. Current transients for H2S04 supporting and (d) 0.2 M, .&,h = 800 mV.

electrolyte

for different

concentrations

of aniline: (a) 0.05, (b) 0. I, (c) 0.15,

1400

Z. Mandic

et al.

10

8

6 6

0

50

100

150

200

250

300

t/S Fig. 14. Current

transients

for 0.1 M aniline,‘] M HzS04

at different

The micrographs show the electrode surface covered with numerous clusters for all three electrolytes. The clusters resulting from the sulphate solution are hemispherical in appearance (Fig. I I (a)), needlelike in the case of perchlorate solution (Fig. 1 l(b)), and islands of cubical clusters in the case of nitric solutions (Fig. 1l(c)). Region II-the

exponential increase of current

The region of the exponential increase of current represents the advanced growth of PANI layer, which appears to be governed by a mechanism different from the mechanism in region I. Namely, while PAN1 film growth at early stages (region I) follows 2D [IO] and 3D nucleation process, as shown above, the exponential increase of current is attributed to the one-dimensional growth of polymer chains [IO]. This suggests a mechanism of a direct monomer-unit incorporation into the already existing PAN1 chain [2l, 221, and the current increase represents the increase of active sites at which the incorporation of aniline-monomer is possible. This assumption is supported by the experiment carried out in a quiet and in a stirred solution.* Namely, as shown in Fig. 12, if PANI synthesis is carried out in a quiet solution, as well as in the solution when the stirring is applied but only at the time of the exponential current increase (region II), *The stirring was performed by the use of a magnetic and w is estimated to be 300 revlmin.

stirrer

&,,,h: (a) 800. (b) 820, (c) 840, and (d) 860 mV.

the resulting Z-t transient is identical to the one obtained for the quiet solution (curve a). However, if the stirring of the solution is being carried out all the time, the increase of current is considerably postponed (curve b). Since the stirring intensifies mass transport to, as well as from, the electrode, one may conclude that the growth begins by the precipitation of oligomers onto the electrode surface [IO-131. This is, however, inhibited in the stirred solution. It is important to point out that once the initial PAN1 layer is formed (region I) in a quiet solution, the mechanism of PAN1 growth is switched to the direct incorporation of anilinemonomer into the already existing pernigraniline form of PAN1 [2l, 221 and stirring has no effect on further growth. It remains to determine the critical time and/or the critical PAN1 coverage on the electrode at which stirring has no effect of further growth. Consequently, it is fair to assume that the exponential current increase indicates the branching of PANI chains. If this is the case, the slope of a tangent at each point of the increasing part of the transient, dI/dt, represents the rate of chain branching. Transients obtained for different concentrations of monomer (Fig. 13) result in the exponential part increasing as the monomer concentration increases, showing that the rate of chain branching depends on the monomer concentration. Transients shown for different &ynrh (Fig. 14) exhibit practically the same slope of the current increase,

Electrodeposition

of polyaniline

1401

film

r

b /

a 0.0

0

200

400

600

800

1000

t/s Fig. 15. Transients HNO?, (c) H;S04.

obtained for 0.1 M aniline all at .&I, = 800 mV.

in 1 M solutions

which means that within the potential of pernigraniline existence. where the mechanism of the direct incorporation of aniline-monomer prevails [2l, 221, the rate of PANI growth is independent of the potential (&,h > 800 mV). There is also a strong influence of the anion species on the rate of PAN1 film growth, which is illustrated in Fig. 15. All the transients are obtained for the same concentration of aniline and at .!&,h = 800 mV. The highest rate is obtained in the case of HzS04 supporting electrolyte, and the lowest in the case of HC104. This is in accordance with the previously reported influence of counter-ions on PAN1 synthesis and on PAN1 characteristics [l-6] obtained under cyclovoltammetric conditions. CONCLUSIONS The potentiostatic studies show that there are several stages of the growth, and different mechanisms in PAN1 electrochemical synthesis. The possibility of observing a particular stage depends on the counter-ion supplied by the supporting electrolyte, on aniline-monomer concentration, and on the potential of synthesis. All these factors are known to govern the rate of PAN1 polymerization; therefore, the observability of a particular stage actually depends on the rate of PAN1 synthesis. In this study, it has been shown that even region I includes two stages: one of them, shown as

of the following

supporting

electrolytes:

(a) HCIOI. (b)

the shoulder, is most probably the formation of a compact layer on the top of which a process governed by 3D nucleation under diffusion control is initiated, resulting in different shapes of clusters, depending on the counter-ion present. As shown in the case of the perchloric acid solutions, depending on monomer concentration, the nucleation process changes from progressive at lower concentrations of monomer to instantaneous nucleation at higher concentrations. The third stage, characterized by the exponential current increase (region II), represents the advanced stage of PANI growth. It results in a loosely bound structure which has been interpreted as ID growth of polymer chains with continuous branching with the direct incorporation of aniline-monomer into the emeraldine form of PANI.

ACKNOWLEDGEMENTS The financial support from the Ministry of Science of the Republic of Croatia under the grant l-07-037 is gratefully acknowledged.

NOMENCLATURE P

9 A

Density Overvoltage Area

Z. MandiC et al.

1402 N

F I I 111 i I”, M

NO R T t to t“> z

Nucleation

rate per active site Concentration Diffusion coefficient Thickness of PAN1 layer Starting potential Potential of synthesis Faraday’s constant Current Current maximum Current density Current density of maximum Molecular weight Number of active sites cm2 Universal gas constant Absolute temperature Time Induction time Time corresponding to current Number of electrons

maxlmum

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5. E. M. Genies, J. F. Penneau and E. Vieil, J. ELrtronnd. Chem. 283, 205 (1990). 6. G. Zotti. S. Cattarin and N. Comisso, J. Electroonal. Chem. 235, 259 (1987). I. Y. B. Moon, Y. Cao. P. Smith and A. J. Heeger. Polym. Commun. 30, 196 (1989). 8. J. P. Pouget, M. E. Jozefowitz, A. J. Epstein and A. G. MacDiarmid, Macromolecules 24, 779 (1991). 9. J. Rishpon, A. Redondo, C. Derouin and S. Gottesfeld, J. Elec~ronnnl. Chem. 294, 73 (1990). 10. K. Bade, W. Tsakova and J. W. Schultze, Elecwochim. Actn 31, 2255 (1992). Il. A. Thyssen, A. Borgerding and J. W. Schultze, Makromol. Chem.. Macromol. Symp. 8, 143 (1987). 12. A. Thyssen, A. Hochfeld, R. Kessel, A. Meyer and J. W. Schultze, Synch. Met. 29, E357 (1989). 13. R. Cbrdova, M. A. del Valle, A. Arratia, H. G6mez and R. Schrebler, J. Eleclroanal. Chem. 311, 75 (1994). 14. B. R. Scharifker and G. J. Hills, Eleclrochim. Acta 28, 879 (1983). 15. M. Fleischmann and H. R. Thirsk, Elecwochim. Acra 2, 22 (1960). 16. R. D. Armstrong and J. A. Harrison, J. Elecrrochem. Sot. 116, 328 (1969). 17. R. D. Armstrong, M. Fleischmann and J. A. Harrison, J. Eledronnal. Chem. 11, 208 (1966). 18. J. Bacon and R. N. Adams, J. Am. Chem. Sot. 90,6596 (1968). 19. E. W. Paul, A. J. Ricco and M. S. Wrighton, J. Phy.7. Chem. 89, 1441 (1985). 20. B. R. Scharifker and J. Mostany, J. Elecrroanal. Chem. 177, 13 (1984).

21. Lj. DuiC, MandiC and S. Kovac, Elecrrochim. Acra 40, 1681 (1995). 22. E. M. Genies, J. F. Penneau, M. Lapkowski and A. Boyle, J. Electroanal. Chem. 269, 63 (1989).