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Charge transfer in poly(o-toluidine) gold modified electrodes. An EIS study of the reduced state
Journal of Electroanalytical Chemistry 455 (1998) 49 – 55
Charge transfer in poly(o-toluidine) gold modified electrodes. An EIS study of the reduced state M.J. Rodrı´guez Presa a, H.L. Bandey a,b, R.I. Tucceri a, M.I. Florit a, D. Posadas a,*, A.R. Hillman b a
Instituto de In6estigaciones Fisicoquı´micas Teo´ricas y Aplicadas (INIFTA), Sucursal 4, Casilla de Correo 16, 1900 La Plata, Argentina b Department of Chemistry, Uni6ersity of Leicester, Leicester LE1 7RH, UK Received 25 September 1997; received in revised form 7 April 1998
Abstract The impedance response of poly(o-toluidine) (POT) electrochemically grown films was studied in the potential range, 0.0 BEB0.35 V, where the polymer is in the insulating state. The electrolytes employed were aqueous solutions of H2SO4 and HClO4 with different concentrations. The predominant impedance parameters in this potential range, the charge transfer resistance, RCT, in parallel with the corresponding double layer capacitance, Cdl, were considered in terms of the applied potential, the polymer film thickness and the nature and concentration of the electrolyte. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Charge transfer; Gold modified electrodes; Poly(o-toluidine)
1. Introduction The impedance response of conducting polymers derived from arylamines generally shows, before and during the insulator – conductor transition, a large depressed semicircle in the complex plane representation. This has been interpreted through a charge transfer resistance RCT in parallel with the corresponding double layer capacitance, Cdl. The nature of this semicircle is controversial. Some workers state that the semicircle is due to charge transfer at the metal polymer interface [1,2] and others at the polymer solution interface [3 – 5]. Data about the dependence of this charge transfer on the electrolyte composition and on the polymer film thickness are scarce. Voltammetric work, about the influence of these variables on the electrochemical response of these polymers, suffers the disadvantage of not being truly steady state and, therefore, the I/E * Corresponding author. [email protected]
Fax:
+ 54
21
254642;
e-mail:
response shows the resultant of several superimposed contributions. On the other hand, impedance measurements have the advantage that steady state can be effectively reached. In some cases RCT and Cdl have been found to be thickness dependent [3,6]. Certainly, Cdl is dependent upon thickness for thiophene derived polymers [7–9]. Recently it has been shown from the temperature dependence of the impedance of poly(o-toluidine) (POT) in 3.7 M H2SO4 that the charge transport mechanism for each of the different oxidation states of the polymer is always determined by ion movements, both in the insulating and the conducting state [10]. In a recent paper, Armstrong et al. [11] have summarized the mechanisms by which the high frequency semicircles show up in the impedance response of conducting polymers. There can be three semicircles. Besides those due to charge transfer at the metal polymer interface or at the polymer solution interface, there might be one due to the bulk resistance of the polymer phase, Rb, in parallel with its geometric capacity, Cg. Rb is expected to increase and Cg to decrease with the film
0022-0728/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 2 2 - 0 7 2 8 ( 9 8 ) 0 0 1 5 5 - 7
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M.J. Rodrı´guez Presa et al. / Journal of Electroanalytical Chemistry 455 (1998) 49–55
thickness. The frequency range at which the high frequency bulk semicircle appears is independent of the polymer thickness. The two charge transfer semicircles are expected at lower frequencies than those at which the high frequency bulk semicircle appears. These semicircles should be thickness independent. The objective of this work is to study the dependence of the semicircle, in the complex impedance plane, upon the film thickness, the nature and concentration of the external electrolyte phase, and the applied potential, with the aim of resolving the nature of the processes involved in the potential region that correspond to the insulator state of the polymer film.
2. Experimental The experimental setup for voltammetric and impedance measurements was a conventional threeelectrode electrochemical cell. POT films of different thickness were deposited upon Au wires 2 cm long and 0.5 mm diameter, as described elsewhere [12,13]. The reference electrode was a Pd (H2) electrode [13]. Potentials, E, in the text are referred to the reversible hydrogen electrode (RHE). The counter-electrode was a hollow Pt cylinder of 1 cm radius and 2.0 cm height, as reported elsewhere [14]. The films were studied by electrochemical impedance spectroscopy (EIS), at room temperature (295 K), in 0.3, 1 and 3.7 M HClO4 and 3.7 M H2SO4. The oxidation charge, Q, was determined by integrating the anodic current, between 0.0 and 1.05 V, in the voltammogram recorded in electrolyte solution without monomer, at 0.1 V s − 1. Impedance experiments were carried out at different potential values, in the region 0.0BE B 0.35 V versus RHE, i.e. before the onset of pseudocapacitive current. They were measured with a PAR 309 System, over a frequency range from 0.01 Hz to 100 kHz, with an ac amplitude of 5 mV. With the aim of obtaining a measure of the film thickness, many workers have correlated, for these type of polymers, the charge obtained by the integration of the voltammetric current within a suitable potential interval [13,15–17], with thickness values measured by ellipsometry [18–20]. Although this procedure is reasonable to within an order of magnitude, it can be risky to generalize these type of conversions since, on the one hand, the ellipsometric thickness is usually obtained under the assumption of an homogeneous film, that is averaging the optical film properties across its thickness. On the other hand, it is well known that the electrochemical response depends upon the electrolyte in which the synthesis has been carried out and also that different supporting electrolytes give different electrochemical responses [21,22]. Therefore the direct scal-
ing of the voltammetric charge may not be related to the actual thickness. For this reason, here we will quote only the voltammetric charge which should be related, although perhaps not directly, to the film thickness. Most of the impedance data were analyzed by Boukamp’s fitting program [23].
3. Results The Nyquist diagrams of POT [13] are similar to those reported previously for poly(aniline) (PANI) [3– 5,10,15,16] and other conducting polymers [6–9,11,24– 26]. The complex plane impedance representation shows a depressed semicircle at high frequencies, characterized by a charge transfer resistance, RCT in parallel with the corresponding double layer capacitance, Cdl. As the frequency decreases, the semicircle is followed by a Warburg region, characterized by a Warburg coeffiecient, s, and then, by an increase of Z%% at low frequencies. From this latter frequency region, the low frequency impedance parameters, CLF and RLF, can be obtained. In general, both Cdl and CLF manifest themselves as constant phase elements. For concentrated acid electrolytes (1 and 3.7 M HClO4 and 3.7 M H2SO4), one or other of the impedance response features predominate, according to the potential region under consideration. In the potential range between 0.0BE B0.35 V, where the polymer is in the insulating state, the semicircle predominates with no Warburg region and no increase of Z%%. (Fig. 1). After the beginning of the oxidation process (E\ 0.35 V) (Fig. 2) the semicircle almost disappears and a Warburg region, followed by an increase of Z%%, becomes apparent. This division in the impedance re-
Fig. 1. Typical Nyquist plot of POT in the reduced state: 1 M HClO4; 107 mC cm − 2; E =0.25 V versus RHE.
M.J. Rodrı´guez Presa et al. / Journal of Electroanalytical Chemistry 455 (1998) 49–55
51
Fig. 2. Complex plane impedance plots for POT in 1 M HClO4, E =0.6 V versus RHE corresponding to a thickness of (a) 107 and (b) 13.64 mC cm − 2.
sponse becomes less definite as the concentration of the acid decreases: the semicircle is observed over the whole potential range (0.0BE B 0.65 V) and the capacitive rise, albeit small, is also observed at negative potentials. The latter feature is also observed in the voltammetric response where it is evident that the current, in the potential range preceeding the voltammetric peak, is larger in 0.3 than in 1 and 3.7 M HClO4. The reason for this particular behavior is that in 0.3 M HClO4 the polymer is not completely reduced at the more negative potentials. This is evident because the polymer remains black at these potentials. Other features of the semicircle are as follows: 1. At potentials between 0.2 and 0.4 V it can be seen that the semicircle is composed of two semicircles, a smaller one at high frequencies and the other at lower frequencies. Probably the smaller one is due to a contribution of a charge transfer at the metal polymer interface or to the resistence of the bulk polymer [11] (see Section 4). Since it is very difficult to deconvolute the two semicircles with the required accuracy, we chose to fit only one. 2. The radius of the semicircle depends upon the electrolyte concentration (Fig. 3): RSC decreases as the electrolyte concentration decreases. 3. Both the value and the potential dependence of RSC depend upon the nature of the electrolyte media (Fig. 4). There is a clear difference between HClO4 and H2SO4 acids. 4. The radius of the semicircle is independent of the thickness (Fig. 5 a,b). 5. In the HClO4 electrolytes there is a maximum in the RSC versus E dependence. The potential of the maximum shifts in the negative direction as the electrolyte concentration decreases. In 0.3 M HClO4
there is no maximum and RSC increases as the potential decreases (Fig. 3). This feature is also a consequence of the polymer not being completely reduced at the more negative potentials. As the potential is increased, the polymer oxidation sets in and a fall-off in RSC occurs. This fall-off is abrupt for H2SO4 and less pronounced for HClO4. The potential dependence of the parallel capacitance Cdl, for different film thicknesses in 1 and 0.3 M HClO4, is shown in Fig. 6(a) and (b), respectively. In the potential range between 0.0BEB 0.2 V, Cdl is about 2× 10 − 5 F cm − 2 but it increases as the polymer becomes oxidized (E\0.2 V) (Fig. 6). It is interesting
Fig. 3. Semilogarithmic plot of RSC versus E for different electrolytes. Film thickness corresponding to 11 mC cm − 2. (") 3.7, () 1 and () 0.3 M HClO4.
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metal polymer or at the polymer solution interfaces. However, no clear linear portion can be seen in the semilogarithmic plot of RSC versus E (Figs. 3–5). These plots depend strongly upon the nature and concentration of the external electrolyte. The RSC values obtained in the more concentrated solution of HClO4 are higher than those measured in the more dilute solutions. Besides, the potential dependence of RSC is very different for both HClO4 and H2SO4 electrolytes. These experimental findings, together with other considerations discussed below, point to the possibility that the charge transfer process occurs at the polymer solution interface, in the potential range 0.0B EB 0.35 V. The potential dependence of Cdl may give a further indication about at which interface the charge transfer reaction occurs. The values of Cdl in the range 0.0B Fig. 4. Semilogarithmic plot of RSC versus E for different electrolytes. Film thickness corresponding to 11 mC cm − 2. ( ) 3.7 M H2SO4 and (") 3.7 M HClO4.
to note that for E \0.2 V, Cdl reaches values of about 3× 10 − 3 F cm − 2 depending upon the thickness. The values of Cdl are higher for smaller thicknesses (Fig. 6). Actually, as pointed out before, the parallel capacitance manifests itself as a constant phase element [27].
4. Discussion According to Armstrong et al. [11], there are two possibilities to explain the semicircle: either it is due to the polymer bulk resistance or it is due to a charge transfer at one of the two interfaces. Let us analyze the first possibility. If the semicircle were due to the polymer resistance we would expect a linear dependence of Rb with thickness according to: Rb = d/Akp
(1)
and an associated parallel capacitance: Cg = orooA/d
(2)
where d is the polymer thickness, A the area, kp the polymer conductivity and or the dielectric constant. Since we have changed d over more than two orders of magnitude and the changes in Rb were very small, we decided to rule out the possibility of the polymer resistance being responsible for the semicircle at high frequencies. The possibility of a compensating dependence of kp on thickness is unlikely. If the increase of conductivity in this region were a field dependent ionic motion, the log RSC versus E plots should be strongly dependent on thickness, which they are not. The fact that RSC is independent of d clearly points to some kind of charge transfer control either at the
Fig. 5. Semilogarithmic plot of RSC versus E for different film thickness. (a) 1 M HClO4, () 2.0, () 2.54, () 11.0 and (") 107 mC cm − 2. (b) 0.3 M HClO4, () 7.65 and () 50.6 mC cm − 2.
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53
were due to the contribution of the metal polymer interface the exponent a, affecting jv, should be close to unity [32], whereas, for the polymer solution interface, owing to the porous nature of the polymer, a should be smaller. Parallel capacitances that increase with potential have been reported for poly(thiophene) derived polymers [7–9]. Sunde et al. [8] found very high capacitance values at high potentials in the non-conductive region. Opposite to the other two groups, they found the capacitance values did not conform to the Mott–Schottky equation. In order to explain their high capacitance values these authors proposed a diffuse double layer in the polymer side of the polymer solution interface. In the potential range between 0.0 and 0.25 V, POT show neither faradaic nor pseudo-capacitive currents [13,14]. Therefore, in the reduced state, the polymer can be thought of as an exchange membrane in which the fixed sites are constituted by the LH + species and participate, together with protons coming from the solution, in equilibrium (1) of Scheme 1. Equilibrium (1) is presumed to favour strongly the formation of LH + [33]. Both protons and anions A − , also participate in a Donnan equilibrium:
Fig. 6. Potential dependence of the parallel capacitance, Cdl. (a) 1 M HClO4, () 2.54, () 11 and () 107 mC cm − 2. (b) 0.3 M HClO4, () 7.65 and ( ) 50.6 mC cm − 2.
EB 0.2 V correspond to an interfacial double layer which could be associated either with the metal polymer or with the polymer solution interface. However, if it were associated with the metal polymer interface it would be difficult to explain its increase for E \ 0.25 V. Moreover, it is well known that as the polymer oxidizes, bulk and surface charges develop according to routes 1 and 2 in Scheme 1 [28], yielding an increase of Cdl for E\ 0.25 V. Due to the fibrilar type of morphology of this kind of polymer [29–31], the real area is very high and this fact, together with the development of a surface charge mentioned above, would explain why the capacitance at the polymer solution interface increases so much. This would also explain why a constant phase element behavior with a B 1 is observed for this capacity. If it
H + (polymer)= H + (solution)
(3)
A − (solution)= A − (polymer)
(4)
At a given applied potential, the equilibrium concentrations of L, LH + , H + and A − will depend upon the values of the equilibrium constant of equilibrium (1), the standard Gibbs energies of transfer of Eqs. (3) and (4), Dp, wG 0t = zFDp, wf 0, where the subscripts p and w stand for polymer and water, respectively; and the concentrations in solution. The sum of the concentrations of L and LH + constitute what we will call the reduced polymer. The amount of this reduced polymer depends upon the previous history of the polymer: how long it has been kept at the negative limit, the polymer thickness, and the nature and concentration of the electrolyte in the external
Scheme 1. L and E stand for the base forms and LH + and EH + for the salt forms of leucoemeraldine and emeraldine, respectively (for the chemical formula of L, LH + , E and EH + see ref. [27]).
M.J. Rodrı´guez Presa et al. / Journal of Electroanalytical Chemistry 455 (1998) 49–55
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phase. As has been pointed out previously, the reduction of the polymer is a slow process which involves proton inclusion and/or anion exclusion [28] (Scheme 1). The amount of reduced polymer increases as the concentration of the acid is increased [34]. When a small sinusoidal perturbation is applied to the metal polymer solution system the equilibria described above are perturbed and ionic charge transfer at the polymer solution interface seems to be the rate determining step. It should be emphasized that, as remarked by Girault and Schiffrin [35], the currents involved in these processes are extremely small. The present case is similar to ion transfer at the interface between two inmiscible electrolyte solutions (ITIES) [35,36]. However, an important difference with our experimental system is that we are dealing with a solid ‘oil type’ phase (the polymer) and most probably there would be a finite Galvani potential difference at the polymer solution interface, Dp, sf= fp −fs. In this case the Butler – Volmer equation for ion transfer would apply, so the ionic charge transfer resistances can be written as: 0 R−1 CT, H = nFk 0, H{ac, H fcH, s exp[ −ac, H f(fp −fp, H)] 0 + aa, H fcH, p exp[aa, H f(fp −fp, H )]}
(5)
and R−1 CT, A = nFk 0, A{a c, A fc A, p exp[ − a c, A f(f p − f p, 0A)] 0 + aa, A fcA, s exp[aa, A f(fp −fp, A )]}
(6)
where k0 is the standard rate constant, c the concentrations, a the transfer coefficients and f = F/RT. The subindexes c, a, p, s, H and A stand for cathodic, anodic, polymer, solution, protons and anions, respectively. The Galvani potential at the solution side, fs, has been arbitrarily taken as the reference level. f 0p is the standard potential at the polymer solution interface of the corresponding ion and it is related to its standard Gibbs energy of transfer. Some comments should be made at this point. In general, fp −f 0p is an unknown quantity. The experimentally accessible applied potential, E, should be proportional to fp −f 0p, but it includes other contributions such as the Galvani potential difference at the metal polymer interface and the potential drop across the polymer thickness. Probably, some of these contributions depend upon the applied potential and this leads to curved lines in the Tafel plots (Figs. 3 – 5). From Eqs. (5) and (6), the total charge transfer, RCT, in the absence of mass transfer contributions, results as: −1 −1 1 R− CT =R CT, H +R CT, A
(7)
When there is more reduced polymer, that is at the higher acid concentration, there would be less protons within the polymer and therefore, according to Eq. (5),
the charge transfer resistance is higher as observed experimentally. The maximum in the log RSC versus E plots, corresponds to a potential value at which the partial hydrogen transfer current is equal and opposite in sign to the partial anion transfer current. The position of the maximum is a function of the electrolyte concentration. An increase of protons in solution would lead to a shift of the hydrogen partial current towards more positive potentials. This fact is also observed experimentally (Fig. 3). For the more concentrated HClO4 solutions (1 and 3.7 M), where there is a maximum in RSC, there should be proton expulsion and anion injection for potentials more positive than the maximum. The reverse, proton injection and anion expulsion, should occur at potentials more negative than that of the maximum. This is not unreasonable since it is well known that at 0.0 V the first cycle effect occurs as a consequence of a hindrance of protons entering the film [28]. In the case of 0.3 M HClO4 solutions, the film is not completely reduced and there is proton ejection and anion ingress in the whole potential range 0.0B EB 0.3 V. These should be the reasons for observing higher charge transfer resistances at negative potentials in this solution. The potential dependence of RSC in H2SO4 solution can be explained qualitatively on the basis that the free energy of transfer of HSO4− is much higher than that of ClO4− [36,37], as happens for the water nitrobenzene interface. Indirect evidence shows that in this system, HSO4− is not transferred at all during the oxidation of N,N-dimethyl-1-naphthylamine [38]. It is known that at high pH IH \ \ IA [28,39], therefore it is clear from Eq. (4) that a decrease of cH, s will increase the partial anodic current, protons leaving the polymer phase, and thus a decrease of the charge transfer resistance will occur as experimentally observed.
Acknowledgements This work was financially supported by the Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas, the Comisio´n de Investigaciones Cientı´ficas de la Provincia de Buenos Aires, the Universidad Nacional de La Plata and the Fundacio´n Antorchas-British Council Cooperation Agreement.
References [1] S.H. Glarum, J.H. Marshall, J. Electrochem. Soc. 134 (1987) 142. [2] S. Cattarin, G. Mengoli, M.M. Musiani, B. Schreck, J. Electroanal. Chem. 246 (1988) 87. [3] J. Tanguy, N. Mermilliod, M. Hoclet, J. Electrochem. Soc. 134 (1987) 795.
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[22] G. Zotti, S. Cattarin, N. Comisso, J. Electroanal. Chem. 239 (1988) 387. [23] B.A. Boukamp, Equivalent Circuit Users Manual, University of Twente, 1988 – 1989. [24] J. Tanguy, N. Mermilliod, M. Hoclet, J. Electrochem. Soc. 134 (1987) 795. [25] M.M. Musiani, Electrochim. Acta 35 (1990) 1665. [26] P. Ferloni, M. Mastragostino, L. Meneghello, Electrochim. Acta 41 (1996) 27. [27] M. Sluyters-Rebach, Pure Appl. Chem. 66 (9) (1994) 1831. [28] M. Kalaji, L. Nyholm, L.M. Peter, J. Electroanal. Chem. 313 (1991) 271. [29] P. Herrasti, R. Dı´az, P. Oco´n, New J. Chem. 17 (1993) 279. [30] P. Oco´n, P. Herrasti, J.M. Vara, L Vazquez, R.C. Salvarezza, A.J. Arvı´a, J. Chem. Phys. 98 (1994) 2418. [31] K. Kanamura, Y. Kawai, S. Yonzawa, Z. Takehara, J. Electrochem. Soc. 142 (1995) 2894. [32] U. Rammel, G. Reinhard, Electrochim. Acta 35 (1990) 1045. [33] E.M. Genies, E. Vieil, Synth. Met. 20 (1987) 97. [34] M.J. Rodrı´guez Presa, R. Tucceri, M.I. Florit, D. Posadas, unpublished results. [35] H.H. Girault, D.J. Schiffrin, in: A.J. Bard (Ed.), Electroanalytical Chemistry, vol. 15, Marcel Dekker, New York, 1989, Ch. 1. [36] H.H. Girault, in: B.E. Conway, J. O’M. Bockris (Eds.), Modern Aspects of Electrochemistry, vol. 25, Plenum Press, New York, 1993, Ch. 1. [37] T. Osakai, T. Kakutani, Y. Nishiwaki, M. Senda, Anal. Sci. Jap. 3 (1987) 499. [38] N. Vettorazzi, H. Ferna´ndez, J.J. Silver, L. Sereno, Electrochim. Acta 35 (1990) 1081. [39] G. Inzelt, G. Horanyi, Electrochim. Acta 35 (1990) 27.
Charge transfer in poly(o-toluidine) gold modified electrodes. An EIS study of the reduced state M.J. Rodrı´guez Presa a, H.L. Bandey a,b, R.I. Tucceri a, M.I. Florit a, D. Posadas a,*, A.R. Hillman b a
Instituto de In6estigaciones Fisicoquı´micas Teo´ricas y Aplicadas (INIFTA), Sucursal 4, Casilla de Correo 16, 1900 La Plata, Argentina b Department of Chemistry, Uni6ersity of Leicester, Leicester LE1 7RH, UK Received 25 September 1997; received in revised form 7 April 1998
Abstract The impedance response of poly(o-toluidine) (POT) electrochemically grown films was studied in the potential range, 0.0 BEB0.35 V, where the polymer is in the insulating state. The electrolytes employed were aqueous solutions of H2SO4 and HClO4 with different concentrations. The predominant impedance parameters in this potential range, the charge transfer resistance, RCT, in parallel with the corresponding double layer capacitance, Cdl, were considered in terms of the applied potential, the polymer film thickness and the nature and concentration of the electrolyte. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Charge transfer; Gold modified electrodes; Poly(o-toluidine)
1. Introduction The impedance response of conducting polymers derived from arylamines generally shows, before and during the insulator – conductor transition, a large depressed semicircle in the complex plane representation. This has been interpreted through a charge transfer resistance RCT in parallel with the corresponding double layer capacitance, Cdl. The nature of this semicircle is controversial. Some workers state that the semicircle is due to charge transfer at the metal polymer interface [1,2] and others at the polymer solution interface [3 – 5]. Data about the dependence of this charge transfer on the electrolyte composition and on the polymer film thickness are scarce. Voltammetric work, about the influence of these variables on the electrochemical response of these polymers, suffers the disadvantage of not being truly steady state and, therefore, the I/E * Corresponding author. [email protected]
Fax:
+ 54
21
254642;
e-mail:
response shows the resultant of several superimposed contributions. On the other hand, impedance measurements have the advantage that steady state can be effectively reached. In some cases RCT and Cdl have been found to be thickness dependent [3,6]. Certainly, Cdl is dependent upon thickness for thiophene derived polymers [7–9]. Recently it has been shown from the temperature dependence of the impedance of poly(o-toluidine) (POT) in 3.7 M H2SO4 that the charge transport mechanism for each of the different oxidation states of the polymer is always determined by ion movements, both in the insulating and the conducting state [10]. In a recent paper, Armstrong et al. [11] have summarized the mechanisms by which the high frequency semicircles show up in the impedance response of conducting polymers. There can be three semicircles. Besides those due to charge transfer at the metal polymer interface or at the polymer solution interface, there might be one due to the bulk resistance of the polymer phase, Rb, in parallel with its geometric capacity, Cg. Rb is expected to increase and Cg to decrease with the film
0022-0728/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 2 2 - 0 7 2 8 ( 9 8 ) 0 0 1 5 5 - 7
50
M.J. Rodrı´guez Presa et al. / Journal of Electroanalytical Chemistry 455 (1998) 49–55
thickness. The frequency range at which the high frequency bulk semicircle appears is independent of the polymer thickness. The two charge transfer semicircles are expected at lower frequencies than those at which the high frequency bulk semicircle appears. These semicircles should be thickness independent. The objective of this work is to study the dependence of the semicircle, in the complex impedance plane, upon the film thickness, the nature and concentration of the external electrolyte phase, and the applied potential, with the aim of resolving the nature of the processes involved in the potential region that correspond to the insulator state of the polymer film.
2. Experimental The experimental setup for voltammetric and impedance measurements was a conventional threeelectrode electrochemical cell. POT films of different thickness were deposited upon Au wires 2 cm long and 0.5 mm diameter, as described elsewhere [12,13]. The reference electrode was a Pd (H2) electrode [13]. Potentials, E, in the text are referred to the reversible hydrogen electrode (RHE). The counter-electrode was a hollow Pt cylinder of 1 cm radius and 2.0 cm height, as reported elsewhere [14]. The films were studied by electrochemical impedance spectroscopy (EIS), at room temperature (295 K), in 0.3, 1 and 3.7 M HClO4 and 3.7 M H2SO4. The oxidation charge, Q, was determined by integrating the anodic current, between 0.0 and 1.05 V, in the voltammogram recorded in electrolyte solution without monomer, at 0.1 V s − 1. Impedance experiments were carried out at different potential values, in the region 0.0BE B 0.35 V versus RHE, i.e. before the onset of pseudocapacitive current. They were measured with a PAR 309 System, over a frequency range from 0.01 Hz to 100 kHz, with an ac amplitude of 5 mV. With the aim of obtaining a measure of the film thickness, many workers have correlated, for these type of polymers, the charge obtained by the integration of the voltammetric current within a suitable potential interval [13,15–17], with thickness values measured by ellipsometry [18–20]. Although this procedure is reasonable to within an order of magnitude, it can be risky to generalize these type of conversions since, on the one hand, the ellipsometric thickness is usually obtained under the assumption of an homogeneous film, that is averaging the optical film properties across its thickness. On the other hand, it is well known that the electrochemical response depends upon the electrolyte in which the synthesis has been carried out and also that different supporting electrolytes give different electrochemical responses [21,22]. Therefore the direct scal-
ing of the voltammetric charge may not be related to the actual thickness. For this reason, here we will quote only the voltammetric charge which should be related, although perhaps not directly, to the film thickness. Most of the impedance data were analyzed by Boukamp’s fitting program [23].
3. Results The Nyquist diagrams of POT [13] are similar to those reported previously for poly(aniline) (PANI) [3– 5,10,15,16] and other conducting polymers [6–9,11,24– 26]. The complex plane impedance representation shows a depressed semicircle at high frequencies, characterized by a charge transfer resistance, RCT in parallel with the corresponding double layer capacitance, Cdl. As the frequency decreases, the semicircle is followed by a Warburg region, characterized by a Warburg coeffiecient, s, and then, by an increase of Z%% at low frequencies. From this latter frequency region, the low frequency impedance parameters, CLF and RLF, can be obtained. In general, both Cdl and CLF manifest themselves as constant phase elements. For concentrated acid electrolytes (1 and 3.7 M HClO4 and 3.7 M H2SO4), one or other of the impedance response features predominate, according to the potential region under consideration. In the potential range between 0.0BE B0.35 V, where the polymer is in the insulating state, the semicircle predominates with no Warburg region and no increase of Z%%. (Fig. 1). After the beginning of the oxidation process (E\ 0.35 V) (Fig. 2) the semicircle almost disappears and a Warburg region, followed by an increase of Z%%, becomes apparent. This division in the impedance re-
Fig. 1. Typical Nyquist plot of POT in the reduced state: 1 M HClO4; 107 mC cm − 2; E =0.25 V versus RHE.
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51
Fig. 2. Complex plane impedance plots for POT in 1 M HClO4, E =0.6 V versus RHE corresponding to a thickness of (a) 107 and (b) 13.64 mC cm − 2.
sponse becomes less definite as the concentration of the acid decreases: the semicircle is observed over the whole potential range (0.0BE B 0.65 V) and the capacitive rise, albeit small, is also observed at negative potentials. The latter feature is also observed in the voltammetric response where it is evident that the current, in the potential range preceeding the voltammetric peak, is larger in 0.3 than in 1 and 3.7 M HClO4. The reason for this particular behavior is that in 0.3 M HClO4 the polymer is not completely reduced at the more negative potentials. This is evident because the polymer remains black at these potentials. Other features of the semicircle are as follows: 1. At potentials between 0.2 and 0.4 V it can be seen that the semicircle is composed of two semicircles, a smaller one at high frequencies and the other at lower frequencies. Probably the smaller one is due to a contribution of a charge transfer at the metal polymer interface or to the resistence of the bulk polymer [11] (see Section 4). Since it is very difficult to deconvolute the two semicircles with the required accuracy, we chose to fit only one. 2. The radius of the semicircle depends upon the electrolyte concentration (Fig. 3): RSC decreases as the electrolyte concentration decreases. 3. Both the value and the potential dependence of RSC depend upon the nature of the electrolyte media (Fig. 4). There is a clear difference between HClO4 and H2SO4 acids. 4. The radius of the semicircle is independent of the thickness (Fig. 5 a,b). 5. In the HClO4 electrolytes there is a maximum in the RSC versus E dependence. The potential of the maximum shifts in the negative direction as the electrolyte concentration decreases. In 0.3 M HClO4
there is no maximum and RSC increases as the potential decreases (Fig. 3). This feature is also a consequence of the polymer not being completely reduced at the more negative potentials. As the potential is increased, the polymer oxidation sets in and a fall-off in RSC occurs. This fall-off is abrupt for H2SO4 and less pronounced for HClO4. The potential dependence of the parallel capacitance Cdl, for different film thicknesses in 1 and 0.3 M HClO4, is shown in Fig. 6(a) and (b), respectively. In the potential range between 0.0BEB 0.2 V, Cdl is about 2× 10 − 5 F cm − 2 but it increases as the polymer becomes oxidized (E\0.2 V) (Fig. 6). It is interesting
Fig. 3. Semilogarithmic plot of RSC versus E for different electrolytes. Film thickness corresponding to 11 mC cm − 2. (") 3.7, () 1 and () 0.3 M HClO4.
52
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metal polymer or at the polymer solution interfaces. However, no clear linear portion can be seen in the semilogarithmic plot of RSC versus E (Figs. 3–5). These plots depend strongly upon the nature and concentration of the external electrolyte. The RSC values obtained in the more concentrated solution of HClO4 are higher than those measured in the more dilute solutions. Besides, the potential dependence of RSC is very different for both HClO4 and H2SO4 electrolytes. These experimental findings, together with other considerations discussed below, point to the possibility that the charge transfer process occurs at the polymer solution interface, in the potential range 0.0B EB 0.35 V. The potential dependence of Cdl may give a further indication about at which interface the charge transfer reaction occurs. The values of Cdl in the range 0.0B Fig. 4. Semilogarithmic plot of RSC versus E for different electrolytes. Film thickness corresponding to 11 mC cm − 2. ( ) 3.7 M H2SO4 and (") 3.7 M HClO4.
to note that for E \0.2 V, Cdl reaches values of about 3× 10 − 3 F cm − 2 depending upon the thickness. The values of Cdl are higher for smaller thicknesses (Fig. 6). Actually, as pointed out before, the parallel capacitance manifests itself as a constant phase element [27].
4. Discussion According to Armstrong et al. [11], there are two possibilities to explain the semicircle: either it is due to the polymer bulk resistance or it is due to a charge transfer at one of the two interfaces. Let us analyze the first possibility. If the semicircle were due to the polymer resistance we would expect a linear dependence of Rb with thickness according to: Rb = d/Akp
(1)
and an associated parallel capacitance: Cg = orooA/d
(2)
where d is the polymer thickness, A the area, kp the polymer conductivity and or the dielectric constant. Since we have changed d over more than two orders of magnitude and the changes in Rb were very small, we decided to rule out the possibility of the polymer resistance being responsible for the semicircle at high frequencies. The possibility of a compensating dependence of kp on thickness is unlikely. If the increase of conductivity in this region were a field dependent ionic motion, the log RSC versus E plots should be strongly dependent on thickness, which they are not. The fact that RSC is independent of d clearly points to some kind of charge transfer control either at the
Fig. 5. Semilogarithmic plot of RSC versus E for different film thickness. (a) 1 M HClO4, () 2.0, () 2.54, () 11.0 and (") 107 mC cm − 2. (b) 0.3 M HClO4, () 7.65 and () 50.6 mC cm − 2.
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53
were due to the contribution of the metal polymer interface the exponent a, affecting jv, should be close to unity [32], whereas, for the polymer solution interface, owing to the porous nature of the polymer, a should be smaller. Parallel capacitances that increase with potential have been reported for poly(thiophene) derived polymers [7–9]. Sunde et al. [8] found very high capacitance values at high potentials in the non-conductive region. Opposite to the other two groups, they found the capacitance values did not conform to the Mott–Schottky equation. In order to explain their high capacitance values these authors proposed a diffuse double layer in the polymer side of the polymer solution interface. In the potential range between 0.0 and 0.25 V, POT show neither faradaic nor pseudo-capacitive currents [13,14]. Therefore, in the reduced state, the polymer can be thought of as an exchange membrane in which the fixed sites are constituted by the LH + species and participate, together with protons coming from the solution, in equilibrium (1) of Scheme 1. Equilibrium (1) is presumed to favour strongly the formation of LH + [33]. Both protons and anions A − , also participate in a Donnan equilibrium:
Fig. 6. Potential dependence of the parallel capacitance, Cdl. (a) 1 M HClO4, () 2.54, () 11 and () 107 mC cm − 2. (b) 0.3 M HClO4, () 7.65 and ( ) 50.6 mC cm − 2.
EB 0.2 V correspond to an interfacial double layer which could be associated either with the metal polymer or with the polymer solution interface. However, if it were associated with the metal polymer interface it would be difficult to explain its increase for E \ 0.25 V. Moreover, it is well known that as the polymer oxidizes, bulk and surface charges develop according to routes 1 and 2 in Scheme 1 [28], yielding an increase of Cdl for E\ 0.25 V. Due to the fibrilar type of morphology of this kind of polymer [29–31], the real area is very high and this fact, together with the development of a surface charge mentioned above, would explain why the capacitance at the polymer solution interface increases so much. This would also explain why a constant phase element behavior with a B 1 is observed for this capacity. If it
H + (polymer)= H + (solution)
(3)
A − (solution)= A − (polymer)
(4)
At a given applied potential, the equilibrium concentrations of L, LH + , H + and A − will depend upon the values of the equilibrium constant of equilibrium (1), the standard Gibbs energies of transfer of Eqs. (3) and (4), Dp, wG 0t = zFDp, wf 0, where the subscripts p and w stand for polymer and water, respectively; and the concentrations in solution. The sum of the concentrations of L and LH + constitute what we will call the reduced polymer. The amount of this reduced polymer depends upon the previous history of the polymer: how long it has been kept at the negative limit, the polymer thickness, and the nature and concentration of the electrolyte in the external
Scheme 1. L and E stand for the base forms and LH + and EH + for the salt forms of leucoemeraldine and emeraldine, respectively (for the chemical formula of L, LH + , E and EH + see ref. [27]).
M.J. Rodrı´guez Presa et al. / Journal of Electroanalytical Chemistry 455 (1998) 49–55
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phase. As has been pointed out previously, the reduction of the polymer is a slow process which involves proton inclusion and/or anion exclusion [28] (Scheme 1). The amount of reduced polymer increases as the concentration of the acid is increased [34]. When a small sinusoidal perturbation is applied to the metal polymer solution system the equilibria described above are perturbed and ionic charge transfer at the polymer solution interface seems to be the rate determining step. It should be emphasized that, as remarked by Girault and Schiffrin [35], the currents involved in these processes are extremely small. The present case is similar to ion transfer at the interface between two inmiscible electrolyte solutions (ITIES) [35,36]. However, an important difference with our experimental system is that we are dealing with a solid ‘oil type’ phase (the polymer) and most probably there would be a finite Galvani potential difference at the polymer solution interface, Dp, sf= fp −fs. In this case the Butler – Volmer equation for ion transfer would apply, so the ionic charge transfer resistances can be written as: 0 R−1 CT, H = nFk 0, H{ac, H fcH, s exp[ −ac, H f(fp −fp, H)] 0 + aa, H fcH, p exp[aa, H f(fp −fp, H )]}
(5)
and R−1 CT, A = nFk 0, A{a c, A fc A, p exp[ − a c, A f(f p − f p, 0A)] 0 + aa, A fcA, s exp[aa, A f(fp −fp, A )]}
(6)
where k0 is the standard rate constant, c the concentrations, a the transfer coefficients and f = F/RT. The subindexes c, a, p, s, H and A stand for cathodic, anodic, polymer, solution, protons and anions, respectively. The Galvani potential at the solution side, fs, has been arbitrarily taken as the reference level. f 0p is the standard potential at the polymer solution interface of the corresponding ion and it is related to its standard Gibbs energy of transfer. Some comments should be made at this point. In general, fp −f 0p is an unknown quantity. The experimentally accessible applied potential, E, should be proportional to fp −f 0p, but it includes other contributions such as the Galvani potential difference at the metal polymer interface and the potential drop across the polymer thickness. Probably, some of these contributions depend upon the applied potential and this leads to curved lines in the Tafel plots (Figs. 3 – 5). From Eqs. (5) and (6), the total charge transfer, RCT, in the absence of mass transfer contributions, results as: −1 −1 1 R− CT =R CT, H +R CT, A
(7)
When there is more reduced polymer, that is at the higher acid concentration, there would be less protons within the polymer and therefore, according to Eq. (5),
the charge transfer resistance is higher as observed experimentally. The maximum in the log RSC versus E plots, corresponds to a potential value at which the partial hydrogen transfer current is equal and opposite in sign to the partial anion transfer current. The position of the maximum is a function of the electrolyte concentration. An increase of protons in solution would lead to a shift of the hydrogen partial current towards more positive potentials. This fact is also observed experimentally (Fig. 3). For the more concentrated HClO4 solutions (1 and 3.7 M), where there is a maximum in RSC, there should be proton expulsion and anion injection for potentials more positive than the maximum. The reverse, proton injection and anion expulsion, should occur at potentials more negative than that of the maximum. This is not unreasonable since it is well known that at 0.0 V the first cycle effect occurs as a consequence of a hindrance of protons entering the film [28]. In the case of 0.3 M HClO4 solutions, the film is not completely reduced and there is proton ejection and anion ingress in the whole potential range 0.0B EB 0.3 V. These should be the reasons for observing higher charge transfer resistances at negative potentials in this solution. The potential dependence of RSC in H2SO4 solution can be explained qualitatively on the basis that the free energy of transfer of HSO4− is much higher than that of ClO4− [36,37], as happens for the water nitrobenzene interface. Indirect evidence shows that in this system, HSO4− is not transferred at all during the oxidation of N,N-dimethyl-1-naphthylamine [38]. It is known that at high pH IH \ \ IA [28,39], therefore it is clear from Eq. (4) that a decrease of cH, s will increase the partial anodic current, protons leaving the polymer phase, and thus a decrease of the charge transfer resistance will occur as experimentally observed.
Acknowledgements This work was financially supported by the Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas, the Comisio´n de Investigaciones Cientı´ficas de la Provincia de Buenos Aires, the Universidad Nacional de La Plata and the Fundacio´n Antorchas-British Council Cooperation Agreement.
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