J. Phys. Chem. 1993,97, 4187-4191
4187
Frequency-Resolved Faradaic Processes in Polypyrrole Films Observed by Electromodulation Techniques: Electrochemical Impedance and Color Impedance Spectroscopies T. Amemiya, K. Hashimoto, and A. Fujishima' Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Bunkyo- ku, Tokyo 1 1 3, Japan Received: January 12, I993
Dynamics of faradaic processes in polypyrrole (PPy) films (thickness 0.26 pm) at different applied dc potentials have been investigated in the frequency domain by employing electrochemical impedance and color impedance (electromodulated optical transmittance) spectroscopies. The faradaic processes in the PPy films have been resolved into a fast and a slow process over the potential range in which the PPy films are in the highly conducting or intermediately oxidized state. These faradaic processes were modeled with an equivalent circuit taking into account two kinds of ionic trapping sites which evolve with dc potentials. Counterion transport through the film is treated in terms of diffusion and migration control. The diffusion coefficients obtained from the two mechanisms suggest that the diffusion control model prevails in the highly conducting PPy films encountered in our experimental conditions.
Introduction Organic conducting polymers such as polypyrrole, polythiophene, and polyaniline can be deposited onto electrodes by electrochemical polymerization. The electrodes coated with the films are very attractive materials for both fundamental research and applications because of their unique electrical, optical and electrochemical properties. These characteristic properties can be applied to microelectronic devices,! electrochromic devices,Z and batterie~.~ In many of these applications, charge transport processes in the films play an important role. Two methods are commonly used to measure the kinetics in an electrochemical system. One is a step transient technique, and the other is a small amplitude ac modulation technique. The step technique analyzes the time response of a system displaced from steady state. This technique is applicable for conducting polymers if the amplitude of the potential or current step is small enough so as not to significantly change the electrical properties of the film^.^.^ The ac modulation technique analyzes the frequency response of a system to small amplitude modulation about steady state. Two advantages of the ac modulation (electromodulation) technique6 for conducting polymers are (i) slow kinetics in the polymers can be observed if measurements are made over a wide frequency range and (ii) the electrochemical properties of the polymers can be fixed by controlling the dc potential, and the system is only perturbed from equilibrium by a small amplitude sinusoidal voltage. The second point is of particular significance with conducting polymers, because both the electrical properties and chemical structures of the polymers are changed by large amplitude voltage perturbation. With the ac modulation technique, it is advantageous to use complex capacitance plots, defined as C = (l/Z)(l/iu) where 2 is the impedance and w is the modulation frequency, to see a very slow process more clearly.6bx7 These low-frequency responses are difficult to identify from a decay curve obtained by the step technique. Ac modulated transmittance ( T = AT/&) spectroscopy has recently been carried out on polyaniline films8 This technique may be considered an extensionof the electrochemicalimpedance9 and is referred to as color impedance spectroscopy (CIS) here. When this technique is applied to electrochromic thin films, only faradaic processes are monitored, because the modulated transmittance is expected to follow the modulation of faradaic charge in the film^.^^^ 0022-3654/93/2097-4187%04.00/0
In this paper, dynamics of faradaic processes in thin (0.26 pm) polypyrrole films as a function of applied dc potentials are investigated by employing two ac electromodulation techniques (electrochemicalimpedanceand color impedancespectroscopies). A comparison of the modulated charge (capacitance, C = AQ/ AE) with the modulated transmittance ( T = ATIAE) clarifies whether the overalldectrical response at each potential is coupled to a faradaic process in the films. Over the potential range in which the coincidence between the capacitance plots and the modulated transmittance plots is maintained, the data are simulated on the basis of an equivalent circuit taking into account a fast and a slow charge transport path in the PPy films. The charge transport through the film is considered in terms of either a diffusionbc or a migration5 model. The diffusion cofficients obtained from the two models are compared to one another. Finally, a model to explain the mechanism of the fast and the slow processesis proposed. This model accounts for the dynamics of the faradaic process in PPy films as a function of dc potentials.
Experimental Section Chemicals. Pyrrole (Tokyo Kasei Co.) and KCl (Tokyo Kasei Co.) were used as received. Saturated KCl aqueous solutions made with distilled water were used for electrolytes. Electrochemical Cells and Polypyrrole Film Deposition. A three-electrode cell having two optical windows (area, 0.78 cm2) was used for all electrochemical measurements. An indiumdoped tin oxide (ITO) coated transparent glass was fitted to one of the optical windows of the cell and was used as a working electrode. A saturated calomel electrode (SCE) and a platinum plate (area, 4.5 cm2) were used as a reference and a counter electrode, respectively. Polypyrroleand counteranion composite (PPy/Cl-) films were grown at a constant current of 1.3 mA/cm* for 60 s on the I T 0 glass from a saturated KC1 aqueous solution containing 0.1 M (M = mol/dm3) pyrrole. The potential of the working electrode was ca. 0.63 V during the polymerization. After the polymerization, the cell and the films were washed with distilled water, and then fresh electrolytes were introduced into the cell. The thickness of the films was 0.26 f 0.03 pm measured with a talystep apparatus (Taylar-Hobson). The relation between the thickness of the films and the charge used for the polymerization (3.38 pm cm2/C) in this study is similar to those (4.16 pm cmZ/C or 2.64 pm cm2/C) reported elsewhere,43l0taking into account the differences in experimental conditions. 0 1993 American Chemical Society
4188 The Journal of Physical Chemistry, Vol. 97, No. 16, 1993 Llght-Tight Box
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Figure 1. Block diagram of the experimental setup for the electromodulation spectroscopies: N. D., neutral density filter; L.l-L.4,lens; P.D., photodiode.
ElectrochemistryandElectromodulationSpectroscopies. Cyclic voltammetry for the PPy/Cl- films was performed between 0.2 and-0.75 Vat a scan rateof 50mV/s usinga potentiostat (Model 2020, Toho Technical Research) and a function generator (Model HB-Ill, Hokuto Denko, Ltd.). Small amplitude electromodulation measurements were carried out as follows: First, the potential of the working electrode coated with the PPy/Cl- film was held at a constant value for 15 min with the potentiostat, and then conventional ac impedance measurements of a sinusoidal electromodulationof 5 mVrmsin the frequency range from 1 kHz to 1 mHz were made by using a frequency response analyzer (FRA) (Model S5720B N F Circuit, Ltd.). Acquisition of the impedance data over the entire frequency range at a fixed potential took approximately 2 h. Under the same conditions as the impedance measurements, transmittance of the PPy films was measured at 700 nm. This monochromatic light (700 nm) from a tungsten lamp through a monochromator is absorbed by oxidized pyrroles2c or bipolarons.’ I The transmitted light intensity was detected with a PIN photodiode, and the photodiode output was fed via a wideband current amplifier (Model LI-76, N F Circuit, Ltd.) to the FRA. After obtaining a set of the impedance and the modulated transmittancedata, the dc potentialof the working electrode was changed to another value, and the next set of the impedance and the transmittancedata were acquired by the same procedure described above. A personal computer (PC-98, NEC) was employed for the experimental control and data acquisition and analysis in both the impedance and the transmittance data. The experimental setup for the electromodulation spectroscopies is shown in Figure 1. Data are represented in the complex planes in the form of Z (=AE/AI) and C (=AQ/AE= l / i w Z ) for the electrical response, and T (=AT/AE) for the optical response. Similar electromodulation techniques as described above have been applied for polyaniline films8 or tungsten trioxide (WO3) fiims.9a
Cyclic Voltammetry. Figure 2 shows the first two cyclic voltammograms of a PPy/Cl- film in a saturated KCl aqueous solution. Multicyclevoltammogramswere qualitativelyidentical to the second voltammogram up to 10 cycles investigated here. The large cathodic current was seen only on the first cycle. Small differences between the first voltammogram and the subsequent ones are probably due to permeability of the oxidized PPy/Clfilms in KCl aqueous solutions.I2 Charge compensation in a PPy/CI- film in aqueous KCl has been shown to be dominated by C1- transport.’*a Cyclic voltammetry was performed for two cycles before electromodulation measurements to ensure electrochemical reproducibility of the films in the electrolytes. The dc potentials selected for the electromodulation measurements were 0.2, -0.15, -0.3, and -0.5 V as shown in Figure 2.
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0.3
Figure 2. Cyclic voltammograms of a PPy/Cl- film,polymerized ( Q = 77 mC/cm2) on an ITOcoated glass, in a saturated KCI aqueous solution. Electrode area was 0.78 cm2, and scan speed was 50 mV/s. The dashed line (- - -1 and the full line (-1 indicate the first cycle and second cycle, respictiveiy.
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Figure 3. Complex plane plots of capacitance (C plots, AQ/AE) and modulated transmittance (T plots, ATIAE) at 700 nm for the PPy/CIfilm in a saturated KCI aqueous solution. These data were obtained after cyclic voltammetry for twocycles (Figure 2). Insets show the impedance plots corresponding to the Cplots. Experimental plots (0)and simulated curves (W) obtained from an equivalent circuit as shown in Figure 4 are presented together. Applied dc potentials vs S C E are indicated. The fitting parameters used for the simulation are summarized in Table I.
Electromodulation Spectroscopies. Figure 3 shows complex plane plots of capacitance (C) and modulated transmittance (T) for the PPy/Cl- film at different dc potentials in the saturated KCl aqueous solution. Complex impedance (Z)plots for the film are also shown in the insets. Two noticeable features in Figure 3 are as follows:13
Faradaic Processes in Polypyrrole Films
The Journal of Physical Chemistry, Vol. 97, No. 16, 1993 4189 or the single-pore model ist3
cd
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R, =
I
1 Figure4. Equivalent circuit used toobtain thesimulated curves in Figure 3. cd and R,,are the double layer capacitance and the charge transfer resistance at the ITO/film interface. RQ is the ohmic resistance of a solution plus an I T 0 glass. 21and 22 represent the fast and the slow faradaic processes in the PPy/Cl- film, respectively.
(i) The Cand the Tplots are nearly identical at 0.2,-0.15, and -0.3 V, however, the two plots at -0.5 V are different from one another particularly at low frequencies. Thecoincidence between the C and the T plots at 0.2, -0.1 5 and -0.3 V indicates that the chargingdischarging process in the system is coupled to the faradaic process in the PPy/Cl- films. The discrepancy between the Cand the Tplots a t -0.5 V shows that another current flowed in addition to the faradaic current for the PPy/Cl-films. Because the Cplots represent the modulated charge in the electrochemical system, while the T plots follow the modulated faradaic charge in the PPy/Cl- films. Electrolysis of water or reduction of dissolved oxygeni4 probably occurred at -0.5 V under the experimental conditions. These results suggest that the dynamic behavior of the faradaic process in the films should be represented by the T plots rather than the C plots. In particular, detailed analysis of the C and T plots for a PPy film in the presence of a redox species in electrolytes will be reported in part 2 of this series.I5 (ii) Both the C and the T plots a t 0.2 V comprise single semicircles and those at -0.15 V single deformed semicircles. However, the two plots a t -0.3 V are resolved into two modes: a fast and a slow mode. These two faradaic processes were also observed for another type of PPy film (PPy/PSS-, PSS = polystyrenesulfonate) in aqueous solutions.16 The fast and slow charging-discharging processes are so far reported for polyaniline films6a*6b and ruthenium oxide filmsS7It is noteworthy that the slow processes in these films appear when the films are in a intermediately oxidized state and disappear when they are in a highly conducting or highly reduced ~ t a t e . ~ ~ . ~ , ~ , ~ ~ Equivalent Circuit. Figure 4 shows an equivalent circuit employed to fit the experimental data. This circuit is a modified Randles’s equivalent circuit.I7 The parallel combination of two faradaic impedances ( Z l and Z2) in the circuit is so far reported e l ~ e w h e r e The . ~ ~physical ~ ~ ~ ~meaning ~ of both Z1 and 2 2 is varied whether the charge transport in the PPy films is treated by diffusion6cJOor m i g r a t i ~ n . ~However, .~] the mathematical forms of both Z I and 2 2 by diffusion in a thin film2*are identical with those by migration in a single-pore electrode.23 This electrode is represented by a finite length transmission line24with capacitive termination. The general expression ( Z ) of Z I and Zz given by the diffusion model is7319J2 Z = 2, coth [(io12/D) / (iw12/D) (1) where w is an angular frequency, I is the diffusion length, D is the diffusion coefficient of counter ion^,*^^^^ and ZOis given by zO = ( I 2 / D, / cT (2) where CT is the total distributed charge capacity of the films, measured at low frequency such that w
