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Polythiophene electrogeneration on a rotating disk electrode
219
J. Electroanal. Chem., 310 (1991) 219-237 Elsevier Sequoia S.A., Lausanne
JEC 01509
Polythiophene electrogeneration on a rotating disk electrode The influence of water on polymerization and polymer properties T.F. Otero and J. Rodriguez Facultad de Quimicas, Dpto. de Ciencia y Tecnologia de Polimeros, Apartado 20080 San Sebastian (Spain) (Received
22 October
1990; in revised form 5 February
1072,
1991)
Abstract Polythiophene was electrogenerated from acetonitrile solutions using a platinum rotating disk electrode by consecutive potential sweeps, and potential steps. When the rotation rate increases in a 0.1 M thiophene solution, thinner and more passive films were obtained. An increase of the monomeric concentration from 0.1 M to 0.25 M promotes thick polymer layers at high rotation rates again. A competitive electrodic discharge between monomer and water was confirmed for water contents between 0.04% and 1.24% (by weight). The competitive water discharge is responsible for the polymer passivation, as was confirmed by polymer generation at constant potential in different water contents and by vohammetry of active polythiophene electrodes in the background solution with different water contents. At low monomer concentrations the water flow arriving at the electrode increases at increasing rotation rates developing thinner and more passive layers. The strong acidification close to the electrode promoted by the current flow seems to be the origin of a simultaneous chemical polymerization on blocked electrodes. The extensive polymerization at high rotation rates indicates that polymerization takes place through species grafted to the electrode surface. A model of interfacial reactions according with experimental results is proposed.
INTRODUCTION
Many of the conducting polymer applications are related to the physical properties of such materials [l-lo]. These properties can be transformed either after polymer production, with an associated expensive process, or during the polymer production, simplifying the process and depressing costs. The large scope for technological applications have promoted a rapid increase of studies related to the physical properties of these new materials at the expense of a restriction in the 0@22-0728/91/$03.50
0 1991 - Elsevier Sequoia
S.A.
220
conditions of synthesis. On the other hand, the great ease of electrochemistry in synthesizing conducting polymers [11,12], promotes a strong relation between people working in electrochemistry and solid state physicists. The simplicity of the electrochemical processes may be derived from the useful mechanism proposed by Genies, Bidan and Diaz [13,14]. This useful simplification was not developed further by workers specialized in electrochemistry or in organic electrochemistry. This is a problem for the electrochemical route to synthesizing conducting polymers, in relation to the faster increase of understanding of chemical synthesis or synthesis from precursors. In spite of the problems inherent to electrochemistry (synthesis only on conducting surfaces) a more detailed knowledge of the interfacial reaction would allow an “a priori” control of the physical properties of the polymer films. This fact should give us a great advantage in comparison with other methods of synthesis. This is the reason why, in our laboratory, we try to deepen our knowledge of interfacial reactions before and during the electrochemical generation of polymer films. This implies control of the electrochemical parameters during polymerization, the quantification of the electrochemical behaviour of electrogenerated polymer films in the background solution and the following of the polymerization kinetics by microgravimetry to obtain empirical kinetic parameters. These methods allow.us to propose a model of interfacial reactions capable of explaining contradictory results between the electrogeneration of different polymers [15]. We present here evidence of the electrode reaction complexity and how this complexity allows us to control the growth and conductivity of the polymer films obtained. In this paper we present a new approach to the influence of water on the electrogeneration and physical properties of polythiophene. Different authors tackled this problem from several different angles. Downard and Pletcher mention the strongly adverse effect of water on electrodeposition (water leads to nonconducting and passivating films [16]) and water has a significant influence on the initial stages of the formation of polypyrrole films [17]. The unexpected results obtained by Tanaka et al. [18] when the polythiophene was electrogenerated using a rotating ring-disk electrode by consecutive voltammograms and a lower electrical charge was spent on the second voltammogram (than on the first), was attributed to “different polymerization mechanisms for an electrolytic solution with a dilute concentration of monomer”. Moreover they point toward the influence of the metal oxide formation on the subsequent presence or not of electropolymerization processes on different metals. The possible influence of the water discharge on the loss of polypyrrole conductivity by nucleophilic attack has been mentioned previously [19,20]. On the other hand, Beck et al. found, when the polypyrrole was electrogenerated in acetonitrile with water, a reaction order for water of 0.4-0.8, quite different from that obtained for pyrrole: 0.2 [21] or 0.1 [22]. In.any case, this group studied the overoxidation problem, already mentioned by different authors, Dlaz et al. [23], Barendrecht et al. [24], etc. and related them to a nucleophilic attack of the OH- on the oxidized polymer. Spectroscopic evidence for this was given [25].
221 EXPERIMENTAL
A Metrohm 628-10 platinum rotating disk electrode (3 mm diameter) was employed as the working electrode. Between experiments the electrode was polished using a STRUERS DAP-7 machine and STRUERS AP alumina. Then it was rinsed using double distilled water, immersed in methanol and in an ultrasonic bath. It was rinsed with acetonitrile and placed in the working solution. The electrochemical equipment has been described previously [26]. Once generated, the working electrode was rinsed with acetonitrile and transferred into the background solution, where it was submitted to voltammetry and potential steps to verify its strate. The monomer and LiClO, were supplied by Merck AG. The electrolyte was dried at 60 o C for several hours before use. The monomer and acetonitrile were distilled under vacuum. The water content of the distilled solvent was determined by the Karl-Fischer method and was lower than 0.01% in weight. RESULTS
AND DISCUSSION
Figures 1 and 2 show the consecutive voltammogram obtained when the working electrode was submitted to consecutive potential sweeps, stationary or at 2000 r.p.m., respectively. Voltammograms were obtained between -500 mV (SCE) and 1700 mV at 50 mV s-l. We used a 0.1 M thiophene and 0.1 M LiClO, solution in acetonitrile at ambient temperature. The strong oxidation present at potentials more positive than 1300 mV, which promotes polymer generation on the electrode, can be observed on the voltammograms. The polymer is reduced and oxidized between 300 and 1300 mV. The modification of the current related to these maxima gives us an idea of the amount of polymer present on the electrode. A progressive increase of the polymer film is observed on the consecutive voltammograms when stationary electrode is used. At the same time an increase in the overpotential of the polymer oxidation maximum was observed. The potential shift of this maximum informs us about the increase of the electrical resistance in the polymer film and the overpotential needed to overcome that resistance [11,27]. We propose as the electrochemical process responsible for the current flow: Polymer
+ ClO;
+ (Polymer)
+ ClO;
+ e-
and the number of electrons flowing through the polymer, at constant potential, as controlled by Ohm’s law: E = IR. In our figures j represents the current density I. Taking two consecutive voltammograms and the potential related to the maximum of the former (E), the current density on the second (j,) is lower than that measured on the former (j,). That means (j,R, = j, R, according to Ohm’s law) R, > R,. This fact can be related to the progressive thickening and overoxidation of the polymer film. The overpotential shift, on the reduction maximum, is small due to the higher conductivity of the oxidized polymer films. All the cathodic branches
222
0.6000
I
1290
-90.0 E
/mV
Fig. 1. Consecutive voltammograms obtained in a 0.1 M thiophene and 0.1 M LiClO, acetonitrile solution, using stationary platinum disk electrode. Scan rate: 50 mV/s. The evolution of the consecutive voltammograms is indicated by the arrows.
of the voltammograms were initiated at 1300 mV where the polymer was oxidized. The increase of current related to the oxidation/polymerization processes shifts toward lower overpotentials on the consecutive voltammograms, due to the increase of the interfacial area when the polymer film grows, as well as to the catalytic properties of the polymer on the electrochemical oxidations related to the polymer generation. These facts, well-described in the literature, change drastically when the rotation rate increases. Figure 2 shows a very slow polymeric growth (slow rise of the polymer oxidation maximum) when the electrode was submitted to the same triangular sweeps of potential, in the same solution, as in Fig. 1. On the other hand, the thin reduced film has a much lower conductivity as shown by the fast shift of the polymer oxidation maxima towards more positive overpotentials on the consecutive voltammograms. This lower conductivity is present, too, with the oxidized polymer as can be deduced from the starting of the oxidation/polymerization current, appearing at increasing overpotentials on the consecutive voltammograms. In this way, at 1700 mV, the end of the potential excursion, the current flowing through the electrode diminishes on consecutive voltammograms. The diminution of the overpotential for this process on the stationary electrode is described below.
223 0.6000
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': \ ._ -0.0037
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4 -550
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630.0
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1750
/ mV
Fig. 2. Consecutive voltammograms obtained by cyclic voltammetry (- 500 to 1700 mV) at 50 mV/s in a 0.1 M thiophene and 0.1 M LiClO, acetonitrile solution, at a platinum rotating disk electrode at 2000 rpm.
If the polymerization occurs in the reaction layer, close to the electrode, followed by deposition of the polymer on the surface, [14,28-301 increasing current densities would be obtained on the oxidation polymerization region at increasing rotation rates due to the presence of a thinner diffusion layer. The progressive diminution found in this case points to a polymerization process occurring via grafted species. Similar results were obtained when polypyrrole was electrogenerated on a rotating disk electrode from acetonitrile [31] or for polythiophene film generation on gold 1321. A first conclusion can be made: the processes occurring on the electrode change drastically when the electrode rotates. These unknown changes modify the electrochemical behaviour of the polymer film obtained and, consequently, the electrical properties. Experimental results obtained on rotating electrodes at higher thiophene concentrations (0.25 M) are shown in Figs. 3 and 4. The polymer grows somewhat more slowly at 2000 rpm than at 500 rpm, but the differences are much smaller than those observed between stationary and rotating electrodes at lower monomer concentrations, and closer to those obtained when polythiophene was generated on a stationary electrode (Fig. 1) in 0.1 M thiophene solutions. These experimental results point toward the presence of competitive reactions on the electrode: the
224 0.7000
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0.1000
0.0000
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-.
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-550.0
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1750
mV
Fig. 3. Consecutive voltammograms of the platinum electrode scanned between - 500 and 1700 mV, at 50 mV/s in a 0.25 M thiophene and 0.1 M LiClO, acetonitrile solution, at 500 rpm. The evolution of the consecutive voltammograms is indicated by the arrows.
of the monomer concentration facilitates monomer discharge and polymeriat the expense of a competitive discharge which promotes the formation of non-conducting polymers or polymer passivation. Another experimental method can be used to clarify these possibilities. Polymer generation was studied, using both monomer concentrations, by potential steps. The platinum electrode was submitted to a potential step between -500 mV and 1700 mV. The current response was followed, in the form of chronoamperograms. Figure 5 shows the experimental results obtained at 25°C in 0.1 M thiophene and 0.1 M LiClO, acetonitrile solutions when the electrode was submitted to different rotation rates. A stationary electrode develops a polymeric film of increasing surface area which promotes an increase of the current flowing through the electrode with time. The presence of the minimum is related to the nucleation process of the new phase. At increasing rotation rates the chronoamperograms present lower current densities from the beginning of the potential step. After 30 s of polarization, at 500 rpm, the polymer film behaves more and more like a passivating film and the current begins to drop towards zero. The time elapsed before this passivating effect diminishes when the rotation rate increases. In this way the non-conducting nature of the film generated at increasing rotation rates and low monomer concentrations is clarified increase zation
225
0.6500
0.5562
0.4625
0.3697
0.2750
E
/mV
Fig. 4. Voltamperometric r&ponse of a platinum rotating disk electrode, at 2ooO rpm, in a 0.25 M thiophene and 0.1 M LiClO, acetonitrile solution between - 500 and 1700 mV, at 50 mV/s.
by hindering the current flow. These facts are in contradiction with the idea of a polymerization taking place in the reaction layer. If that were true, the rotation rate would promote increasing currents flowing through the electrode at increasing rotation rates due to the diminution of the diffusion layer thickness; the corresponding increase of the monomer flowing to the electrode and the oligomers flowing toward the solution. This hypothesis is confirmed by the control voltammograms obtained with the films placed in the background solution, between - 500 mV and 1400 mV at 50 mV s-i. The maxima related to the polymer oxidation show increasing overpotentials for the films obtained at increasing rotation rates (Fig. 6). These overpotentials stay constant on the consecutive voltammograms for every film, showing a characteristic of the polymer, related to the intrinsic structure formed during polymerization. The passivation of the film during the polymerization process did not occur at higher concentrations of the monomer. Figure 7 depicts the chronoamperograms obtained in 0.4 M thiophene solutions. The current decreases a little at increasing rotation rates; nevertheless, no passivating effect (drop of the current toward zero) was observed after 60 s of polarization. The effect observed on the control voltammograms is greater (Fig. 8), but in a new sense: the polymer layer was
226
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Fig . 5. Chronoamperometric curves of potential steps for the growth of a polythiophene film on a platinum rotating disk electrode, from 0.1 M thiophene and 0.1 M LiClO, acetonitrile solutions, at various rates of rotation: (a) ( -)Orpm;(b)(------)5OOrpm;(c)(~~~~~~)2OOOrpm;(d)(~-~-~) 3000 rpm.
2.400
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vs
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Fig. 6. Superposition of the control voltammograms obtained from the films generated in Fig. 5, in the film generated at 0 background solution, between -500 mV and 1400 mV at 50 mV/s: (a) ( -) t-pm; (b) (- - - - - -) at 500 rpm; (c) ( ~~~~~~)at2OOOrpm;(d)(~-~-~)at3OOOrpm.
227
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/
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Fig. 7. Chronoamperometric responses of potential steps for 0.4 M thiophene and 0.1 M LiClO, acetonitrile solutions, on a platinum rotating disk electrode, at various rotation rates: (a) (10 rpm; (b) f---) 500 rpm; (c)f.-.*.-) 2000 ‘pm; (d) (+----) 3000 rpm.
4.000
3.000
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0.000
c
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-2.000
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-0.400
I -0.000
I
I 0.400 E/V
vs
o.eoo
I 1.200
Jo0
SCE
Fig. 8. Voitammograms of films generated in Fig. 7, between -100 and 1400 mV, at 50 mV/s, background solution: (a) (fgeneratedatOrpm;(b)(.-.--)at500rpm;(c)(.~..~*)at2000rpm; (d) (- - -) at 3000 rpm.
in the
228
thinner at increasing rotation rates, without a significant change in the overpotential on the maxima. This fact supports the hypothesis of a partial polymerization in the reaction layer, promoting a progressive loss of oligomers when the rotation rate increases. This hypothesis needs to be confirmed by other experimental methods, microgravimetric determination of the polymer film or chemical analysis of the solutions. A first approximation could be obtained from the electrical charge stored in the polymer during oxidation/reduction in the background electrolyte. For a non-passivated polymer film the electrical charge consumed during polymer oxidation or reduction is proportional to the amount of polymer present on the electrode [33,34]. We determined the electrical charge consumed during polymerization at different rotation rates by integration of the corresponding chronoamperograms. The polymer films obtained were checked in the background electrolyte by voltammetry (Fig. 8). The integration of the anodic and cathodic areas of the voltammograms allow us to obtain the electrical charge stored in the polymer during the oxidation process (Q,,), or obtained from the polymer during the reduction process (Qred). In this way we can take the electrical charge stored under the anodic area of the voltammograms as a measure of the polymer present. Table 1 shows the variation of the polymerization, oxidation and reduction charges, at increasing rotation rates and different thiophene concentrations. Lower polymerization charges were observed at increasing rotation rates regardless of the monomer concentration. For 0.1 M thiophene concentration, the polymerization charge spent at 3000 rpm
TABLE
1
Variation of the polymerization charge, stored charge and storage efficiency for several rotation rates and different thiophene concentrations
w/vm
Q,, /mC
Q,, /mC
‘Q, /mC
1O*QorQw,
102Q,., /Pm,,
0.1
35.59 29.10 17.25 14.86
2.83 1.23 0.68 0.58
-2.11 -0.83 -0.36 - 0.30
7.96 4.22 3.95 3.93
5.92 2.83 2.08 2.00
0.175 0.175 0.175 0.175
41.82 43.11 37,41 33.34
3,56 3.06 2.05 1.70
-3.16 - 2.51 - 1.54 -1.19
8,50 7.09 5.48 5.11
7.55 5.83 4.12 3.58
0.25 0.25 0.25 0.25
47.16 50.57 42.96 41.30
4.12 3.77 2.46 2.31
- 3.89 -3.12 - 2.04 -1.66
8.74 7.45 5.73 5.59
8.25 6.16 4.75 4.02
0.4 0.4 0.4 0.4
55.25 56.25 47.62 45.82
4.96 4.38 3.23 2.57
-
8.97 7.78 6.78 5.61
8.43 6.51 5.32 4.07
c/M 0.1 0.1 0.1
4.66 3.66 2.53 1.86
229
was 41% of the charge consumed when the electrode was stationary. At higher concentrations of monomer the diminution of the electrical charge consumed during polymerization at increasing rotation rates was lower. The continuous presence of this diminution, regardless of the monomer concentration, is contrary to the model of polymerization taking place exclusively in the reaction layer followed by the precipitation of the oligomers on the electrode. This model would lead to the progressive increase of the polymerization current with an increase of the rotation rate: the reaction layer would be progressively eliminated with progressive loss of the oligomers toward the solution, and a progressive increase of the monomer arriving at the electrode by elimination of the diffusion layer would promote an increase of the current. The progressive diminution of the stored charge when polymer films are generated at different rotation rates was checked by voltammetry in the background electrolyte. This can be due to a lower amount of polymer formed at increasing rotation rates, a lower effectiveness of the polymer to store charges or both. The increase of the overvoltage for the polymer oxidation when a film is generated from 0.1 M thiophene at increasing rotation rates was checked by voltammetry. This seems to support the hypothesis of a lower effectiveness in the charge storage. The control voltammograms from the films obtained from a 0.175 M thiophene concentration points to a diminution of the amount of polymer formed at increasing rotation rates. This fact can be due to the presence of partial polymerization in the reaction layer by which precipitation on the electrode is hindered if the electrode rotates, or to the modification of the polymerization rate by change of the chemical conditions around the electrode. The difference between the anodic and cathodic charge on the control voltammograms, known as overoxidation in the literature, seems to be related to the presence of an irreversible reaction at potentials greater than 1100 mV. From the ratio QOJQP,,, we have a measure of the current efficiency to generate polymer. We can observe in Table 1 the fast increase of the current efficiency when the monomer concentration increases at constant rotation rate, or when the rotation rate decreases. Both facts point again to the presence of competitive reactions on the electrode. The fraction of current lost in non-polymeric reactions increases when the monomer concentration drops or the rotation rate rises. The faster polymeric passivation when the rotation rate rises could be related to the presence of competitive reactions promoting nucleophilic attacks on the growing polymer favoured by rotation as was described in the literature [12,25]. This means the presence of an electroactive impurity in the solution, present in a low concentration, whose transport towards the surface increases at increasing rotation rates. The active species generated by electrodic discharge of this compound would react with the polymer promoting a loss of conductivity. The most common compound capable of producing this effect is water, because of its great reactivity in producing oxygen on a platinum electrode submitted to such a high potential. To confirm the feasibility of this hypothesis, solutions with increasing water contents were studied. The electrode was passivated at shorter times (Fig. 9) using a
230
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Fig. 9. Chronoamperometric curves of potential steps for the growth of a polythiophene film on a stationary platinum disk electrode, from 0.1 M thiophene and 0.1 M LiClO, acetonitrile solutions, at different water contents: (a) ()0.04%;(b)(------)0.14%;(~)(~-~-~)0.44R;;(d)(~~~~~~)1.04~.
stationary electrode, at increasing water contents in 0.1 M thiophene and 0.1 M LiClO, acetonitrile solutions. The feasibility for water discharge at the beginning of the polarization and during polymerization was studied by voltammetry using platinum or active polythiophene electrodes. As expected, the presence of an increasing discharge of the water around the potential of polarization on a clean platinum electrode was proved by cyclic voltammetry in the background electrolyte with increasing water contents (Fig. 10). Increasing current densities at 1700 mV, for increasing water contents in the solution are observed. The water discharge begins towards 1200 mV. When active polythiophene electrodes (generated on a stationary electrode in a 0.2 M thiophene solution) were used in the water solutions, Fig. 11 was obtained, showing increasing current densities, related to the water discharge, at 1600 mV (a new electrode was used for every voltammogram). The overpotential for water discharge on polythiophene decreases very rapidly with increasing water content. The correlation between water discharge and polymer passivation is clear on the consecutive voltammograms obtained using an active polythiophene electrode. When the water content of the background solution was 1.04% the consecutive voltamperometric results depicted in Fig. 12 were obtained. A rapid positive shift of the polymer oxidation potential (maximum present at 800 mV on the first voltammogram) is observed on the positive branches of the consecutive voltammograms, related to the increase of the electrical resistence. The cathodic region does not show any practical reduction of the polymer on the second voltammogram. This fact points toward a
231
“E
0.500
-
0.000
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-0.500
-1.000 -1.500
-2.00
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I -0,500
I 0.000
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E/V Fig. 10. Superposition electrode, at different
I 1.500
I 1.000 vi
I 2.000
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3.000
SCE
of voltarmnograms obtained from a 0.1 M LiClO, water contents: (.-.--) 0.04%; (-. . . . .) 1.2%; (-
solution, at 50 mV/s, on a Pt - -) 2.3%; () 4.55%.
loss of the polymeric activity related to the electrodic reaction observed at potentials greater than 1000 mV (the water discharge on the first voltammogram, or at 850 mV if a deconvolution is used). This irreversible reaction has been above with the overoxidation of the polymer, and has been clearly obtained clearly at different potentials on polypyrrole studied by consecutive potential steps in the background solution [30]. The most important modifications of the consecutive voltammograms are present at potentials more positive than 1100 mV, where the water discharge begins on the polymer. On the second voltammogram a significant decrease of the current densities related to this process is observed and a passivating maximum is present near 1550 mV. This fact suggests a total polymer passivation (related to the polymer oxidation/reduction) and significant passivation related to the water discharge, as can be observed on the third voltammogram. All this points to the presence of a competitive water discharge from the beginning of the polymerization process, activated by the presence of polymer on the platinum electrode. The water content of the solutions and the competitive water discharge during the polymerization process allow us to explain the experimental results. At low monomer concentrations and increasing rotation rates the water transport toward the electrode surface increases by convection and the progressive diminution of the diffusion layer. The subsequent discharge during polymerization promotes the polymer passivation. In this way consecutive voltammograms obtained at high rotation rates show (Fig. 2) increasingly passivated polymer films, measured by the
232
2.400
I
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1.600
4 1.200 \ l-l "0 -
0.800
0.400
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I -0.400
I 0.000
I
I
0.400
O.BQQ E/V
vs
I
I
1.200
1.600
*:oo
SCE
Fig. 11. Control voltammograms obtained from polythiophene films (obtained 0.1 LiClO, acetonitrile solution), in the background solution, with different 100% (by weight); (- - - ---) l.lS%; (-.-a-.)0.45%; (a----)0.04%.
in a 0.25 M thiophene water contents: (-
and )
polymer oxidation/reduction maxima (E < 1200 mV vs. SCE) and to the monomer oxidation/polymerization current (E > 1200 mV). In these regions of potential the current flowing through the electrode decreases steadily in the consecutive voltammograms due to the progressive polymer passivation when the electrode rotates. The clearest evidence comes from the chronoamperograms in Fig. 5. The progressive increase of the water discharge when the rotation rate rises promotes a faster passivation of the generated polymer film. The control voltammograms in the background solution of the films confirm, by the positive shift of the polymer oxidation potential, the more passivated nature of the polymer films obtained (Fig. 6). The influence of the discharge of the contaminating water would explain the experimental results found by Tanaka [IS] and attributed there to “different polymerization mechanisms”. The water discharge is a competitive reaction in relation to the monomer discharge. This fact is evident in Fig. 3 and 4 where a rise in the monomer concentration promoted similar voltamperometric responses related to polymer generation on a stationary and a rotating electrode. A greater flow of monomer, favoured by convection, hinders the corresponding water discharge. Meanwhile, when the concentration of monomer is lower (Figs. 1 and 2), the water discharge becomes competitive when the electrode rotates, due to the destruction of the thick diffusion layer for water molecules present on a stationary electrode. On the other
233
1.000
0.800
0.600 Q \ -Jo
0.400
0.200
0.000
-0.2!!8,~oo -0.400 I
I
I
I
0.000
0.400
0.800 E/V
vs
I 1.200
I 1.600
2.000
SCE
Fig. 12. Superposition of first three control voltammograms obtained from a polythiophene film (generated in a 0.25 M thiophene and 0.1 M LiClO, acetonitrile solution), in a 1.04% (by weight) of water content and 0.1 M LiClO, acetonitrile solution.
hand, we can conclude that the polymerization processes take place on adsorbed active species, at least in a high fraction. If the polymerization process occurred in the reaction layer close to the electrode surface the rotation rate increase would promote a faster destruction of this layer and a corresponding diminution of polymer produced on the electrode, but the current flowing through the electrode would increase with an increase of the amount of monomer arriving at the electrode surface. Our experimental results disagree with the last deduction. The progressive increase of the current efficiency (taken as the ratio between the stored charge in the polymer during the positive sweep of the control voltammogram in the background solution and the electrical charge consumed during the anodic polymerization) ( QoX/QPO,, Table l), when the monomer concentration increases, at constant rotation rate, gives us an idea of the progressive decrease of the competitive water discharge. The current efficiency diminution when the rotation rate increases could be due to: (a) The presence of two simultaneous polymerization processes, the first grafted to the electrode, the second in the reaction layer. (b) The change of the chemical environment when the electrode rotates; i.e., the removal of the reaction layer. All the results obtained by means of the rotating electrode at increasing rotation
234
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Fig. 13. A schematic representation of the electrogeneration of a polymer at a rotating disc electrode. The mechanism is elucidated in the text.
235
rates point to the presence of a polymerization process through active species grafted on the electrode. This fact is compatible with the presence of a second polymerization process taking place in the reaction layer, followed by deposition on the electrode. The increase of the rotation rate would promote progressive destruction of the reaction layer with an elimination of oligomers and polymers toward the solution. A progressive depletion of the polymer production would be observed at increasing rotation rates if this process takes place. The lack of a corresponding increase of the current density, and the presence of a depletion when the rotation rate increases are contrary to this hypothesis. On the other hand, all the polymerization steps and the water discharge are related to protonic release, which promotes a strong change of pH close to the electrode. Parallel experiments in our laboratory show great possibilities for the pyrrole and thiophene chemical polymerization in HClO, media, which could promote a parallel chemical polymerization in the reaction layer close to the electrode. The electrode rotation destroys this reaction layer progressively, leaving only the electrogenerated polymerization process grafted to the electrode. Thus a slower polymer production results when the electrode is rotating and the subsequent depletion on the current densities at high concentrations of monomer. Both possibilities must be studied more carefully in more detail, since they are of great scientific and technological interest. All the possibilities analysed above to explain the experimental results are collected in model I (Fig. 13): (i) polymerization on grafted species (la) (ii) oligomerization in the reaction layer (lb) (iii) chemical polymerization in the reaction layer due to the pH modification (2) (iv) competitive water discharge (3a) (v) polymer passivation (3b) This model involves a greater complexity to explain polymerization processes than the single mechanisms accepted up to now. However, better agreement with experimental results exists. It is possible to use the model to predict different electrical properties for polymer films before generation according to the water content and the electrical potential of polarization. We have indicated other questions not collected in this partial model: a recombination of radical cations can explain the polymerization process but needs two electrons (lOO%, current efficiency and no parallel reactions) to incorporate a molecule of monomer into the polymer. All our experimental results were lower than this. Taking into account the simultaneous presence of the polymer oxidation, during generation, values greater than 2.25 were expected. In our laboratory we have attempted to clarify those disagreements (much greater with other monomers [35]) and have proposed a more complete model. CONCLUSIONS
The polythiophene electrogeneration takes place through competitive reactions on the electrode. Water discharge is one of the most important of those competitive
236
reactions due to the marked influence by amounts of water. This influence is related to the electrode potential of polymerization, which is much more positive than that of water discharge. Oxygen production, due to the water discharge during polymerization, promotes polymer passivation and the consequent modification of its physical properties. The polymerization process takes place through active species grafted to the electrode surface. The acidification of the solution near the electrode should be responsible for a chemical polymerization in the reaction layer. This process disappears at high rotation rates. The influence of the water discharge in competition with the monomer discharge leads to a new model to explain the electrogeneration of conducting polymers, more complex and more useful for the “a priori” control of the electrical properties of the films. The low number of electrons consumed per monomeric unit incorporated to the polymer leads to a discussion about the polymerization mechanism in electroinitiated processes. ACKNOWLEDGEMENTS
Financial support of this work by Diutacion Foral de Guipticoa (BRITE-EURAM BRE-0148) is gratefully acknowledged.
and E.E.C.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
F. Gamier, G. Tourillon, J.Y. Barraund and H. Dexpert, J. Mater. Sci., 20 (1985) 2678. L.J. Buckley, D.K. Roylance and G.E. Wnek, J. Polym. Sci., Polym. Phys., 25 (1987) 217. A.F. Diaz and B. Hall, IBM J. Res. Develop., 27 (1983) 342. T. Ikeshoji, J. Electroanal. Chem., 201 (1986) 409. T.F. Otero and E. de Larreta Azelain, J. Chim. Phys., 86 (1989) 131. K. Kaneto, G. Ishii and K. Yoshino, Jpn. J. Appl. Phys., 24 (1985) L320. G. Tourillon and F. Gamier, J. Phys. Chem., 87 (1983) 2289. H.S. Nalwa, L.R. Dalton, W.F. Schmidt and J.G. Rabe, Polym. Commun., 26 (1985) 240. A. Watanabe, M. Tanaka and J. Tanaka, Bull. Chem. Sot. Jpn., 54 (1981) 2278. J.H. Kaufman, K.K. Kanazawa and G.B. Street, J. Chem. Phys., 79 (1983) 4774. T.F. Otero and E. de Larreta-Azelain, Polymer, 29 (1988) 1522. T.F. Otero, R. Tejada and AS. Elola, Polymer, 28 (1987) 651. A.F. Diaz, in T.A. Skotheim (Ed.), Handbook of Conducting Polymers, Vol. 1, Marcel Dekker, p. 81. E.M. Genies, G. Bidan and A.F. Dlaz, J. Electroanal. Chem., 149 (1983) 101. T.F. Otero, J. Rodriguez, E. Angulo and C. Santamaria, Synth. Met., 43 (1991) 2831. A.J. Downward and D. Pletcher, J. Electroanal. Chem., 206 (1986) 147. A.J. Downward and D. Pletcher, J. Electroanal. Chem., 206 (1986) 139. K. Tanaka, T. Sbichiri, S. Wang and T. Yamabe, Synth. Met., 24 (1988) 203. S. Dong and J. Ding, Synth. Met., 20 (1987) 119. T.F. Otero, R. Tejada and A.S. Elola, Polymer, 28 (1987) 651. F. Beck and M. Oberst, Synth. Met., 28 (1989) 643.
1986,
231 22 F. Beck and M. Oberst, J. ElectroanaI. Chem., 285 (1990) 177. 23 A.F. Diaz, J.I. Castillo, J.O. Logan and W.Y. Lee, J. Electroanal Chem., 129 (1981) 115. 24 F.T.A. Vork, M.T. Ubbink, L.J.J. Jansen and E. Barendrecht, Reel. Trav. Chim. Pays-Bas, 104 (1985) 25 26 27 28 29 30 31 32 33 34 35
215. F. Beck, P. Braun and M. Oberst, Ber. Bunsenges. Phys. Chem., 91 (1987) 967. T.F. Otero, J. Rodriguez and E. de Larreta-Azelain, Polymer, 31 (1990) 220. T.F. Otero and E. de Larreta-Azelain, J. Electroanal Chem., 244 (1988) 34. F. Chao, M. Costa, P. Lang, E. Lhdritier, F. Gamier and G. Tourillon, Ann. Phys., 11 (1986) 21. A.F. Diaz and J.C. Lacroix, New J. Chem., 12 (1988) 171. C.K. Baker and J.R. Reynolds, J. Electroanal Chem., 251 (1988) 307. T.F. Otero, C. Santamaria, J. Rodriguez and E. Angulo, Proceedings of the Plenary Lectures, IX Iberoam. Electrochem. Meeting. Ed. Univ. La Laguna, Tenerife, 1990. A.R. HiIIman and E.F. MaIlen, J. EIectroanaI. Chem., 220 (1987) 351. T.F. Otero and J. Rodriguez, unpublished results, 1990. T.F. Otero, C. Santamaria, E. Angulo and J. Rodriguez, Synth. Met., 43 (1991) 2947. T.F. Otero, C. Santamaria, E. Angulo and J. Rodriguez, to be published.
J. Electroanal. Chem., 310 (1991) 219-237 Elsevier Sequoia S.A., Lausanne
JEC 01509
Polythiophene electrogeneration on a rotating disk electrode The influence of water on polymerization and polymer properties T.F. Otero and J. Rodriguez Facultad de Quimicas, Dpto. de Ciencia y Tecnologia de Polimeros, Apartado 20080 San Sebastian (Spain) (Received
22 October
1990; in revised form 5 February
1072,
1991)
Abstract Polythiophene was electrogenerated from acetonitrile solutions using a platinum rotating disk electrode by consecutive potential sweeps, and potential steps. When the rotation rate increases in a 0.1 M thiophene solution, thinner and more passive films were obtained. An increase of the monomeric concentration from 0.1 M to 0.25 M promotes thick polymer layers at high rotation rates again. A competitive electrodic discharge between monomer and water was confirmed for water contents between 0.04% and 1.24% (by weight). The competitive water discharge is responsible for the polymer passivation, as was confirmed by polymer generation at constant potential in different water contents and by vohammetry of active polythiophene electrodes in the background solution with different water contents. At low monomer concentrations the water flow arriving at the electrode increases at increasing rotation rates developing thinner and more passive layers. The strong acidification close to the electrode promoted by the current flow seems to be the origin of a simultaneous chemical polymerization on blocked electrodes. The extensive polymerization at high rotation rates indicates that polymerization takes place through species grafted to the electrode surface. A model of interfacial reactions according with experimental results is proposed.
INTRODUCTION
Many of the conducting polymer applications are related to the physical properties of such materials [l-lo]. These properties can be transformed either after polymer production, with an associated expensive process, or during the polymer production, simplifying the process and depressing costs. The large scope for technological applications have promoted a rapid increase of studies related to the physical properties of these new materials at the expense of a restriction in the 0@22-0728/91/$03.50
0 1991 - Elsevier Sequoia
S.A.
220
conditions of synthesis. On the other hand, the great ease of electrochemistry in synthesizing conducting polymers [11,12], promotes a strong relation between people working in electrochemistry and solid state physicists. The simplicity of the electrochemical processes may be derived from the useful mechanism proposed by Genies, Bidan and Diaz [13,14]. This useful simplification was not developed further by workers specialized in electrochemistry or in organic electrochemistry. This is a problem for the electrochemical route to synthesizing conducting polymers, in relation to the faster increase of understanding of chemical synthesis or synthesis from precursors. In spite of the problems inherent to electrochemistry (synthesis only on conducting surfaces) a more detailed knowledge of the interfacial reaction would allow an “a priori” control of the physical properties of the polymer films. This fact should give us a great advantage in comparison with other methods of synthesis. This is the reason why, in our laboratory, we try to deepen our knowledge of interfacial reactions before and during the electrochemical generation of polymer films. This implies control of the electrochemical parameters during polymerization, the quantification of the electrochemical behaviour of electrogenerated polymer films in the background solution and the following of the polymerization kinetics by microgravimetry to obtain empirical kinetic parameters. These methods allow.us to propose a model of interfacial reactions capable of explaining contradictory results between the electrogeneration of different polymers [15]. We present here evidence of the electrode reaction complexity and how this complexity allows us to control the growth and conductivity of the polymer films obtained. In this paper we present a new approach to the influence of water on the electrogeneration and physical properties of polythiophene. Different authors tackled this problem from several different angles. Downard and Pletcher mention the strongly adverse effect of water on electrodeposition (water leads to nonconducting and passivating films [16]) and water has a significant influence on the initial stages of the formation of polypyrrole films [17]. The unexpected results obtained by Tanaka et al. [18] when the polythiophene was electrogenerated using a rotating ring-disk electrode by consecutive voltammograms and a lower electrical charge was spent on the second voltammogram (than on the first), was attributed to “different polymerization mechanisms for an electrolytic solution with a dilute concentration of monomer”. Moreover they point toward the influence of the metal oxide formation on the subsequent presence or not of electropolymerization processes on different metals. The possible influence of the water discharge on the loss of polypyrrole conductivity by nucleophilic attack has been mentioned previously [19,20]. On the other hand, Beck et al. found, when the polypyrrole was electrogenerated in acetonitrile with water, a reaction order for water of 0.4-0.8, quite different from that obtained for pyrrole: 0.2 [21] or 0.1 [22]. In.any case, this group studied the overoxidation problem, already mentioned by different authors, Dlaz et al. [23], Barendrecht et al. [24], etc. and related them to a nucleophilic attack of the OH- on the oxidized polymer. Spectroscopic evidence for this was given [25].
221 EXPERIMENTAL
A Metrohm 628-10 platinum rotating disk electrode (3 mm diameter) was employed as the working electrode. Between experiments the electrode was polished using a STRUERS DAP-7 machine and STRUERS AP alumina. Then it was rinsed using double distilled water, immersed in methanol and in an ultrasonic bath. It was rinsed with acetonitrile and placed in the working solution. The electrochemical equipment has been described previously [26]. Once generated, the working electrode was rinsed with acetonitrile and transferred into the background solution, where it was submitted to voltammetry and potential steps to verify its strate. The monomer and LiClO, were supplied by Merck AG. The electrolyte was dried at 60 o C for several hours before use. The monomer and acetonitrile were distilled under vacuum. The water content of the distilled solvent was determined by the Karl-Fischer method and was lower than 0.01% in weight. RESULTS
AND DISCUSSION
Figures 1 and 2 show the consecutive voltammogram obtained when the working electrode was submitted to consecutive potential sweeps, stationary or at 2000 r.p.m., respectively. Voltammograms were obtained between -500 mV (SCE) and 1700 mV at 50 mV s-l. We used a 0.1 M thiophene and 0.1 M LiClO, solution in acetonitrile at ambient temperature. The strong oxidation present at potentials more positive than 1300 mV, which promotes polymer generation on the electrode, can be observed on the voltammograms. The polymer is reduced and oxidized between 300 and 1300 mV. The modification of the current related to these maxima gives us an idea of the amount of polymer present on the electrode. A progressive increase of the polymer film is observed on the consecutive voltammograms when stationary electrode is used. At the same time an increase in the overpotential of the polymer oxidation maximum was observed. The potential shift of this maximum informs us about the increase of the electrical resistance in the polymer film and the overpotential needed to overcome that resistance [11,27]. We propose as the electrochemical process responsible for the current flow: Polymer
+ ClO;
+ (Polymer)
+ ClO;
+ e-
and the number of electrons flowing through the polymer, at constant potential, as controlled by Ohm’s law: E = IR. In our figures j represents the current density I. Taking two consecutive voltammograms and the potential related to the maximum of the former (E), the current density on the second (j,) is lower than that measured on the former (j,). That means (j,R, = j, R, according to Ohm’s law) R, > R,. This fact can be related to the progressive thickening and overoxidation of the polymer film. The overpotential shift, on the reduction maximum, is small due to the higher conductivity of the oxidized polymer films. All the cathodic branches
222
0.6000
I
1290
-90.0 E
/mV
Fig. 1. Consecutive voltammograms obtained in a 0.1 M thiophene and 0.1 M LiClO, acetonitrile solution, using stationary platinum disk electrode. Scan rate: 50 mV/s. The evolution of the consecutive voltammograms is indicated by the arrows.
of the voltammograms were initiated at 1300 mV where the polymer was oxidized. The increase of current related to the oxidation/polymerization processes shifts toward lower overpotentials on the consecutive voltammograms, due to the increase of the interfacial area when the polymer film grows, as well as to the catalytic properties of the polymer on the electrochemical oxidations related to the polymer generation. These facts, well-described in the literature, change drastically when the rotation rate increases. Figure 2 shows a very slow polymeric growth (slow rise of the polymer oxidation maximum) when the electrode was submitted to the same triangular sweeps of potential, in the same solution, as in Fig. 1. On the other hand, the thin reduced film has a much lower conductivity as shown by the fast shift of the polymer oxidation maxima towards more positive overpotentials on the consecutive voltammograms. This lower conductivity is present, too, with the oxidized polymer as can be deduced from the starting of the oxidation/polymerization current, appearing at increasing overpotentials on the consecutive voltammograms. In this way, at 1700 mV, the end of the potential excursion, the current flowing through the electrode diminishes on consecutive voltammograms. The diminution of the overpotential for this process on the stationary electrode is described below.
223 0.6000
0.5138
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4 -550
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630.0
1290
1750
/ mV
Fig. 2. Consecutive voltammograms obtained by cyclic voltammetry (- 500 to 1700 mV) at 50 mV/s in a 0.1 M thiophene and 0.1 M LiClO, acetonitrile solution, at a platinum rotating disk electrode at 2000 rpm.
If the polymerization occurs in the reaction layer, close to the electrode, followed by deposition of the polymer on the surface, [14,28-301 increasing current densities would be obtained on the oxidation polymerization region at increasing rotation rates due to the presence of a thinner diffusion layer. The progressive diminution found in this case points to a polymerization process occurring via grafted species. Similar results were obtained when polypyrrole was electrogenerated on a rotating disk electrode from acetonitrile [31] or for polythiophene film generation on gold 1321. A first conclusion can be made: the processes occurring on the electrode change drastically when the electrode rotates. These unknown changes modify the electrochemical behaviour of the polymer film obtained and, consequently, the electrical properties. Experimental results obtained on rotating electrodes at higher thiophene concentrations (0.25 M) are shown in Figs. 3 and 4. The polymer grows somewhat more slowly at 2000 rpm than at 500 rpm, but the differences are much smaller than those observed between stationary and rotating electrodes at lower monomer concentrations, and closer to those obtained when polythiophene was generated on a stationary electrode (Fig. 1) in 0.1 M thiophene solutions. These experimental results point toward the presence of competitive reactions on the electrode: the
224 0.7000
0.6000
0.5000
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0.1000
0.0000
0.1000
-.
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830.0
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1750
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Fig. 3. Consecutive voltammograms of the platinum electrode scanned between - 500 and 1700 mV, at 50 mV/s in a 0.25 M thiophene and 0.1 M LiClO, acetonitrile solution, at 500 rpm. The evolution of the consecutive voltammograms is indicated by the arrows.
of the monomer concentration facilitates monomer discharge and polymeriat the expense of a competitive discharge which promotes the formation of non-conducting polymers or polymer passivation. Another experimental method can be used to clarify these possibilities. Polymer generation was studied, using both monomer concentrations, by potential steps. The platinum electrode was submitted to a potential step between -500 mV and 1700 mV. The current response was followed, in the form of chronoamperograms. Figure 5 shows the experimental results obtained at 25°C in 0.1 M thiophene and 0.1 M LiClO, acetonitrile solutions when the electrode was submitted to different rotation rates. A stationary electrode develops a polymeric film of increasing surface area which promotes an increase of the current flowing through the electrode with time. The presence of the minimum is related to the nucleation process of the new phase. At increasing rotation rates the chronoamperograms present lower current densities from the beginning of the potential step. After 30 s of polarization, at 500 rpm, the polymer film behaves more and more like a passivating film and the current begins to drop towards zero. The time elapsed before this passivating effect diminishes when the rotation rate increases. In this way the non-conducting nature of the film generated at increasing rotation rates and low monomer concentrations is clarified increase zation
225
0.6500
0.5562
0.4625
0.3697
0.2750
E
/mV
Fig. 4. Voltamperometric r&ponse of a platinum rotating disk electrode, at 2ooO rpm, in a 0.25 M thiophene and 0.1 M LiClO, acetonitrile solution between - 500 and 1700 mV, at 50 mV/s.
by hindering the current flow. These facts are in contradiction with the idea of a polymerization taking place in the reaction layer. If that were true, the rotation rate would promote increasing currents flowing through the electrode at increasing rotation rates due to the diminution of the diffusion layer thickness; the corresponding increase of the monomer flowing to the electrode and the oligomers flowing toward the solution. This hypothesis is confirmed by the control voltammograms obtained with the films placed in the background solution, between - 500 mV and 1400 mV at 50 mV s-i. The maxima related to the polymer oxidation show increasing overpotentials for the films obtained at increasing rotation rates (Fig. 6). These overpotentials stay constant on the consecutive voltammograms for every film, showing a characteristic of the polymer, related to the intrinsic structure formed during polymerization. The passivation of the film during the polymerization process did not occur at higher concentrations of the monomer. Figure 7 depicts the chronoamperograms obtained in 0.4 M thiophene solutions. The current decreases a little at increasing rotation rates; nevertheless, no passivating effect (drop of the current toward zero) was observed after 60 s of polarization. The effect observed on the control voltammograms is greater (Fig. 8), but in a new sense: the polymer layer was
226
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2.400
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227
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1
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1
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Fig. 7. Chronoamperometric responses of potential steps for 0.4 M thiophene and 0.1 M LiClO, acetonitrile solutions, on a platinum rotating disk electrode, at various rotation rates: (a) (10 rpm; (b) f---) 500 rpm; (c)f.-.*.-) 2000 ‘pm; (d) (+----) 3000 rpm.
4.000
3.000
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0.000
c
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-0.400
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vs
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Jo0
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Fig. 8. Voitammograms of films generated in Fig. 7, between -100 and 1400 mV, at 50 mV/s, background solution: (a) (fgeneratedatOrpm;(b)(.-.--)at500rpm;(c)(.~..~*)at2000rpm; (d) (- - -) at 3000 rpm.
in the
228
thinner at increasing rotation rates, without a significant change in the overpotential on the maxima. This fact supports the hypothesis of a partial polymerization in the reaction layer, promoting a progressive loss of oligomers when the rotation rate increases. This hypothesis needs to be confirmed by other experimental methods, microgravimetric determination of the polymer film or chemical analysis of the solutions. A first approximation could be obtained from the electrical charge stored in the polymer during oxidation/reduction in the background electrolyte. For a non-passivated polymer film the electrical charge consumed during polymer oxidation or reduction is proportional to the amount of polymer present on the electrode [33,34]. We determined the electrical charge consumed during polymerization at different rotation rates by integration of the corresponding chronoamperograms. The polymer films obtained were checked in the background electrolyte by voltammetry (Fig. 8). The integration of the anodic and cathodic areas of the voltammograms allow us to obtain the electrical charge stored in the polymer during the oxidation process (Q,,), or obtained from the polymer during the reduction process (Qred). In this way we can take the electrical charge stored under the anodic area of the voltammograms as a measure of the polymer present. Table 1 shows the variation of the polymerization, oxidation and reduction charges, at increasing rotation rates and different thiophene concentrations. Lower polymerization charges were observed at increasing rotation rates regardless of the monomer concentration. For 0.1 M thiophene concentration, the polymerization charge spent at 3000 rpm
TABLE
1
Variation of the polymerization charge, stored charge and storage efficiency for several rotation rates and different thiophene concentrations
w/vm
Q,, /mC
Q,, /mC
‘Q, /mC
1O*QorQw,
102Q,., /Pm,,
0.1
35.59 29.10 17.25 14.86
2.83 1.23 0.68 0.58
-2.11 -0.83 -0.36 - 0.30
7.96 4.22 3.95 3.93
5.92 2.83 2.08 2.00
0.175 0.175 0.175 0.175
41.82 43.11 37,41 33.34
3,56 3.06 2.05 1.70
-3.16 - 2.51 - 1.54 -1.19
8,50 7.09 5.48 5.11
7.55 5.83 4.12 3.58
0.25 0.25 0.25 0.25
47.16 50.57 42.96 41.30
4.12 3.77 2.46 2.31
- 3.89 -3.12 - 2.04 -1.66
8.74 7.45 5.73 5.59
8.25 6.16 4.75 4.02
0.4 0.4 0.4 0.4
55.25 56.25 47.62 45.82
4.96 4.38 3.23 2.57
-
8.97 7.78 6.78 5.61
8.43 6.51 5.32 4.07
c/M 0.1 0.1 0.1
4.66 3.66 2.53 1.86
229
was 41% of the charge consumed when the electrode was stationary. At higher concentrations of monomer the diminution of the electrical charge consumed during polymerization at increasing rotation rates was lower. The continuous presence of this diminution, regardless of the monomer concentration, is contrary to the model of polymerization taking place exclusively in the reaction layer followed by the precipitation of the oligomers on the electrode. This model would lead to the progressive increase of the polymerization current with an increase of the rotation rate: the reaction layer would be progressively eliminated with progressive loss of the oligomers toward the solution, and a progressive increase of the monomer arriving at the electrode by elimination of the diffusion layer would promote an increase of the current. The progressive diminution of the stored charge when polymer films are generated at different rotation rates was checked by voltammetry in the background electrolyte. This can be due to a lower amount of polymer formed at increasing rotation rates, a lower effectiveness of the polymer to store charges or both. The increase of the overvoltage for the polymer oxidation when a film is generated from 0.1 M thiophene at increasing rotation rates was checked by voltammetry. This seems to support the hypothesis of a lower effectiveness in the charge storage. The control voltammograms from the films obtained from a 0.175 M thiophene concentration points to a diminution of the amount of polymer formed at increasing rotation rates. This fact can be due to the presence of partial polymerization in the reaction layer by which precipitation on the electrode is hindered if the electrode rotates, or to the modification of the polymerization rate by change of the chemical conditions around the electrode. The difference between the anodic and cathodic charge on the control voltammograms, known as overoxidation in the literature, seems to be related to the presence of an irreversible reaction at potentials greater than 1100 mV. From the ratio QOJQP,,, we have a measure of the current efficiency to generate polymer. We can observe in Table 1 the fast increase of the current efficiency when the monomer concentration increases at constant rotation rate, or when the rotation rate decreases. Both facts point again to the presence of competitive reactions on the electrode. The fraction of current lost in non-polymeric reactions increases when the monomer concentration drops or the rotation rate rises. The faster polymeric passivation when the rotation rate rises could be related to the presence of competitive reactions promoting nucleophilic attacks on the growing polymer favoured by rotation as was described in the literature [12,25]. This means the presence of an electroactive impurity in the solution, present in a low concentration, whose transport towards the surface increases at increasing rotation rates. The active species generated by electrodic discharge of this compound would react with the polymer promoting a loss of conductivity. The most common compound capable of producing this effect is water, because of its great reactivity in producing oxygen on a platinum electrode submitted to such a high potential. To confirm the feasibility of this hypothesis, solutions with increasing water contents were studied. The electrode was passivated at shorter times (Fig. 9) using a
230
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60
76
Fig. 9. Chronoamperometric curves of potential steps for the growth of a polythiophene film on a stationary platinum disk electrode, from 0.1 M thiophene and 0.1 M LiClO, acetonitrile solutions, at different water contents: (a) ()0.04%;(b)(------)0.14%;(~)(~-~-~)0.44R;;(d)(~~~~~~)1.04~.
stationary electrode, at increasing water contents in 0.1 M thiophene and 0.1 M LiClO, acetonitrile solutions. The feasibility for water discharge at the beginning of the polarization and during polymerization was studied by voltammetry using platinum or active polythiophene electrodes. As expected, the presence of an increasing discharge of the water around the potential of polarization on a clean platinum electrode was proved by cyclic voltammetry in the background electrolyte with increasing water contents (Fig. 10). Increasing current densities at 1700 mV, for increasing water contents in the solution are observed. The water discharge begins towards 1200 mV. When active polythiophene electrodes (generated on a stationary electrode in a 0.2 M thiophene solution) were used in the water solutions, Fig. 11 was obtained, showing increasing current densities, related to the water discharge, at 1600 mV (a new electrode was used for every voltammogram). The overpotential for water discharge on polythiophene decreases very rapidly with increasing water content. The correlation between water discharge and polymer passivation is clear on the consecutive voltammograms obtained using an active polythiophene electrode. When the water content of the background solution was 1.04% the consecutive voltamperometric results depicted in Fig. 12 were obtained. A rapid positive shift of the polymer oxidation potential (maximum present at 800 mV on the first voltammogram) is observed on the positive branches of the consecutive voltammograms, related to the increase of the electrical resistence. The cathodic region does not show any practical reduction of the polymer on the second voltammogram. This fact points toward a
231
“E
0.500
-
0.000
\
-0.500
-1.000 -1.500
-2.00
3’ . 000
I -0,500
I 0.000
I 0.500
E/V Fig. 10. Superposition electrode, at different
I 1.500
I 1.000 vi
I 2.000
I 2.500
3.000
SCE
of voltarmnograms obtained from a 0.1 M LiClO, water contents: (.-.--) 0.04%; (-. . . . .) 1.2%; (-
solution, at 50 mV/s, on a Pt - -) 2.3%; () 4.55%.
loss of the polymeric activity related to the electrodic reaction observed at potentials greater than 1000 mV (the water discharge on the first voltammogram, or at 850 mV if a deconvolution is used). This irreversible reaction has been above with the overoxidation of the polymer, and has been clearly obtained clearly at different potentials on polypyrrole studied by consecutive potential steps in the background solution [30]. The most important modifications of the consecutive voltammograms are present at potentials more positive than 1100 mV, where the water discharge begins on the polymer. On the second voltammogram a significant decrease of the current densities related to this process is observed and a passivating maximum is present near 1550 mV. This fact suggests a total polymer passivation (related to the polymer oxidation/reduction) and significant passivation related to the water discharge, as can be observed on the third voltammogram. All this points to the presence of a competitive water discharge from the beginning of the polymerization process, activated by the presence of polymer on the platinum electrode. The water content of the solutions and the competitive water discharge during the polymerization process allow us to explain the experimental results. At low monomer concentrations and increasing rotation rates the water transport toward the electrode surface increases by convection and the progressive diminution of the diffusion layer. The subsequent discharge during polymerization promotes the polymer passivation. In this way consecutive voltammograms obtained at high rotation rates show (Fig. 2) increasingly passivated polymer films, measured by the
232
2.400
I
I
I
I
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I
2.000
1.600
4 1.200 \ l-l "0 -
0.800
0.400
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I -0.400
I 0.000
I
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0.400
O.BQQ E/V
vs
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1.200
1.600
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Fig. 11. Control voltammograms obtained from polythiophene films (obtained 0.1 LiClO, acetonitrile solution), in the background solution, with different 100% (by weight); (- - - ---) l.lS%; (-.-a-.)0.45%; (a----)0.04%.
in a 0.25 M thiophene water contents: (-
and )
polymer oxidation/reduction maxima (E < 1200 mV vs. SCE) and to the monomer oxidation/polymerization current (E > 1200 mV). In these regions of potential the current flowing through the electrode decreases steadily in the consecutive voltammograms due to the progressive polymer passivation when the electrode rotates. The clearest evidence comes from the chronoamperograms in Fig. 5. The progressive increase of the water discharge when the rotation rate rises promotes a faster passivation of the generated polymer film. The control voltammograms in the background solution of the films confirm, by the positive shift of the polymer oxidation potential, the more passivated nature of the polymer films obtained (Fig. 6). The influence of the discharge of the contaminating water would explain the experimental results found by Tanaka [IS] and attributed there to “different polymerization mechanisms”. The water discharge is a competitive reaction in relation to the monomer discharge. This fact is evident in Fig. 3 and 4 where a rise in the monomer concentration promoted similar voltamperometric responses related to polymer generation on a stationary and a rotating electrode. A greater flow of monomer, favoured by convection, hinders the corresponding water discharge. Meanwhile, when the concentration of monomer is lower (Figs. 1 and 2), the water discharge becomes competitive when the electrode rotates, due to the destruction of the thick diffusion layer for water molecules present on a stationary electrode. On the other
233
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0.800
0.600 Q \ -Jo
0.400
0.200
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I
I
I
0.000
0.400
0.800 E/V
vs
I 1.200
I 1.600
2.000
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Fig. 12. Superposition of first three control voltammograms obtained from a polythiophene film (generated in a 0.25 M thiophene and 0.1 M LiClO, acetonitrile solution), in a 1.04% (by weight) of water content and 0.1 M LiClO, acetonitrile solution.
hand, we can conclude that the polymerization processes take place on adsorbed active species, at least in a high fraction. If the polymerization process occurred in the reaction layer close to the electrode surface the rotation rate increase would promote a faster destruction of this layer and a corresponding diminution of polymer produced on the electrode, but the current flowing through the electrode would increase with an increase of the amount of monomer arriving at the electrode surface. Our experimental results disagree with the last deduction. The progressive increase of the current efficiency (taken as the ratio between the stored charge in the polymer during the positive sweep of the control voltammogram in the background solution and the electrical charge consumed during the anodic polymerization) ( QoX/QPO,, Table l), when the monomer concentration increases, at constant rotation rate, gives us an idea of the progressive decrease of the competitive water discharge. The current efficiency diminution when the rotation rate increases could be due to: (a) The presence of two simultaneous polymerization processes, the first grafted to the electrode, the second in the reaction layer. (b) The change of the chemical environment when the electrode rotates; i.e., the removal of the reaction layer. All the results obtained by means of the rotating electrode at increasing rotation
234
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Fig. 13. A schematic representation of the electrogeneration of a polymer at a rotating disc electrode. The mechanism is elucidated in the text.
235
rates point to the presence of a polymerization process through active species grafted on the electrode. This fact is compatible with the presence of a second polymerization process taking place in the reaction layer, followed by deposition on the electrode. The increase of the rotation rate would promote progressive destruction of the reaction layer with an elimination of oligomers and polymers toward the solution. A progressive depletion of the polymer production would be observed at increasing rotation rates if this process takes place. The lack of a corresponding increase of the current density, and the presence of a depletion when the rotation rate increases are contrary to this hypothesis. On the other hand, all the polymerization steps and the water discharge are related to protonic release, which promotes a strong change of pH close to the electrode. Parallel experiments in our laboratory show great possibilities for the pyrrole and thiophene chemical polymerization in HClO, media, which could promote a parallel chemical polymerization in the reaction layer close to the electrode. The electrode rotation destroys this reaction layer progressively, leaving only the electrogenerated polymerization process grafted to the electrode. Thus a slower polymer production results when the electrode is rotating and the subsequent depletion on the current densities at high concentrations of monomer. Both possibilities must be studied more carefully in more detail, since they are of great scientific and technological interest. All the possibilities analysed above to explain the experimental results are collected in model I (Fig. 13): (i) polymerization on grafted species (la) (ii) oligomerization in the reaction layer (lb) (iii) chemical polymerization in the reaction layer due to the pH modification (2) (iv) competitive water discharge (3a) (v) polymer passivation (3b) This model involves a greater complexity to explain polymerization processes than the single mechanisms accepted up to now. However, better agreement with experimental results exists. It is possible to use the model to predict different electrical properties for polymer films before generation according to the water content and the electrical potential of polarization. We have indicated other questions not collected in this partial model: a recombination of radical cations can explain the polymerization process but needs two electrons (lOO%, current efficiency and no parallel reactions) to incorporate a molecule of monomer into the polymer. All our experimental results were lower than this. Taking into account the simultaneous presence of the polymer oxidation, during generation, values greater than 2.25 were expected. In our laboratory we have attempted to clarify those disagreements (much greater with other monomers [35]) and have proposed a more complete model. CONCLUSIONS
The polythiophene electrogeneration takes place through competitive reactions on the electrode. Water discharge is one of the most important of those competitive
236
reactions due to the marked influence by amounts of water. This influence is related to the electrode potential of polymerization, which is much more positive than that of water discharge. Oxygen production, due to the water discharge during polymerization, promotes polymer passivation and the consequent modification of its physical properties. The polymerization process takes place through active species grafted to the electrode surface. The acidification of the solution near the electrode should be responsible for a chemical polymerization in the reaction layer. This process disappears at high rotation rates. The influence of the water discharge in competition with the monomer discharge leads to a new model to explain the electrogeneration of conducting polymers, more complex and more useful for the “a priori” control of the electrical properties of the films. The low number of electrons consumed per monomeric unit incorporated to the polymer leads to a discussion about the polymerization mechanism in electroinitiated processes. ACKNOWLEDGEMENTS
Financial support of this work by Diutacion Foral de Guipticoa (BRITE-EURAM BRE-0148) is gratefully acknowledged.
and E.E.C.
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