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Impedance analysis of the transport of counter ions at polypyrrole-Nafion composite electrodes
ANALYTICA CHIMICA
ACTA ELSEVIER
Analytica Chimica Acta 300 (1995) 15-23
Impedance analysis of the transport of counter ions at polypyrroleNafion composite electrodes Chur-Min Chang, Hsuan-Jung Huang * Department of Chemishy, National Sun Yat-sen University, Kaohsiung 80424, Taiwan
Received 20 June 1994; revised manuscript received 1 August 1994
Abstract The ion transport accompanying the redox reactions of polypyrrole at a polypyrrole-Nafion composite electrode was investigated with systems containing specifically selected electrolytes. From the cyclic voltammetric and impedance data obtained, transport of cations was found to be responsible for the charge transfer process and parameters such as the charge transfer resistance, Rti, the low frequency polymer resistance, Re, the limiting capacitance, C, and the diffusion coefficient, D, for the related cations were estimated. The electrochemical behaviour (i.e., electronic insulation and electronic conductance) of polypyrrole-Nafion composite electrodes was found to be the same as that of the polypyrrole electrodes, except that they appeared in regions of more negative potentials. Keywords: Cyclic voltammetry; Impedance; Polyoyrrole-Nafion composite electrodes
1. Introduction Understanding the mechanism of redox reactions occurring at conductive polymer electrodes and extending the practical applications of these electrodes is of great importance. Therefore, study of the charge transport in conductive polymers has become an attractive
and popular research topic in electrochemistry and electrochemical analysis [l-16]. Besides the studies on pure conductive polymer electrodes, the incorporation of conductive polymers like polypyrrole (PPy) and polyaniline into the electroinactive polymer networks of Nafion, poly(viny1 chloride), poly(viny1 alcohol) or polystyrene has been developed for improving the mechanical properties or for modifying the electrochemical characteristics of the conductive polymer electrodes and thus extending their applications [9* Corresponding author. 0003-2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved .SSD10003-2670( 94)00379-3
191. The combination of polypyrrole with Nafion is a typical example showing such characteristics. For the PPy-coated electrodes, the redox reactions are found to be accompanied with the doping-undoping mechanism of anions [l-5]. On PPy-Naiion composite electrodes the immobilized perfluorinated group of Nafion (which is a cation exchanger) serves as a charge compensator during the anodic polymerization of polypyrrole. Cations and solvent molecules, opposite to anions in plain polypyrrole electrodes, are the diffusing species accompanying the redox processes of the Nation composited polypyrrole [ 13-16,201. Although the characteristics of electrical conductivity and charge transfer in the PPy-Nafion composite film electrodes have been investigated, no systematic studies concerning the charge transport of counter ions on the electrochemical behaviour of the PPy-Nafion composite electrodes were reported. In this article, systems with various spe-
16
C.-M. Chang, H.-J. Huang I Analytica Chimica Acta 300 (1995) 15-23
cifically selected electrolytes were adopted to identify the species responsible for the charge transport process accompanying the redox reactions of polypyrrole at the PPy-Nalion composite electrodes. Because cyclic voltammetry (CV) can provide valuable information on the reversibility of the reaction and criteria for discriminating the charge transfer process and the impedance method (a powerful technique for probing the charge and mass transport mechanism), the impedance method based on the conductive polymer film model was employed in combination with CV to elucidate the charge transport of counter ions during the redox reaction of polypyrrole at PPy-Nafion composite electrodes.
2.3. Electrochemical measurements Cyclic voltammetry was performed with a PARC 175 universal programmer associated with a PARC 173 potentiostat/galvanostat. Coulometic measurements were achieved with a PARC 179 coulometer. The ac impedance measurements were made with the PARC M378 electrochemical impedance system (including a PARC 276 interface and a 5280 lock-in amplifier). An ac voltage of 5 mV in amplitude (peak-to-peak) with a frequency range of 50 mHz to 0.1 MHz was superimposed on the dc potential and applied to the studied electrodes. The obtained current signal at the frequency range of 50 mHz to 10 Hz was analyzed with the FFT technique.
2. Experimental 3. Results and discussion 2.1. Chemicals and solutions N&on (5 wt.% dissolved in a mixed solvent of lower aliphatic alcohols and water) was purchased from Aldrich. Sodium chloride, sodium butylsulfonate, ammonium chloride, methylammonium chloride, dimethylammonium chloride, trimethylammonium chloride and tetrabutylammonium bromide used for the preparation of solutions were purchased from Merck. All the chemicals used were of analytical reagent grade or better. 2.2. Preparation of PpY-Najion composite electrodes 30 ~1 of 1 wt.% Nafion solution (diluted with methanol-water, 9:l) was applied to the platinum electrode (with an area of 1.2 cm’) and air dried to allow the solvent to evaporate. The thickness of the Nafion film prepared was estimated to be 0.8 pm. The PPy-Nafion composite electrodes were prepared by the further electrodeposition of pyrrole on the Nafion-coated Pt electrode. The anodic polymerization was performed in a deaerated aqueous solution containing 0.1 M pyrrole and 0.2 M NaCl in a conventional three-electrode chemical cell (with a Pt wire as the counter electrode and the Ag/ AgCl in saturated KC1 solution as the reference electrode) with the application of a constant current of 0.1 mA cm-‘. The same electrochemical cell was used for the following electrochemical measurements.
The polymer films obtained by the electrodeposition of polypyrrole on the Nalion-coated Pt electrodes were brown-black in colour. The PPy film formed on the Nafion-coated electrode was smoother than that formed on the bare platinum electrode. This agrees with the literature report that PPy homogenizes well with N&on to produce smoother composite layers [ 13-151. The amount and the thickness of PPy film deposited on the Nafion-coated electrodes can be calculated from the charge consumed during electrodeposition, a PPy film of 0.5 pm in thickness is supposed to be obtained from a consumed charge of 90 mC cm-’ assuming that the amount of soluble oligomer formed is negligible [ 161. To study the counter ion effect on the redox reactions of PPy, cyclic voltammetry of PPy-N&on composite electrodes in solutions of 0.2 M NaCl, 0.2 M CH,( CH&SO,Na, 0.2 M ( CH3)$JHCl and 0.2 M ( CH,CH,CH,CH,) ,NBr were run respectively with a sweep rate of 10 mV/s in the potential range of - 1.20 to 0.20 V. Fig. la shows the voltammograms obtained. From Fig. la, rather sharp and large cathodic peaks and broad anodic peaks are found for curves ( 1) and (2). The separation of peak potentials for curves ( 1) and (2) are about 40 and 60 mV, respectively. This implies that the reversibility of the redox reactions of PPy in the studied solution is very good. For curve (3)) both the anodic and cathodic peaks become smaller and broader and the separation of peak potential also becomes larger (about 500 mV) compared with those
C.-M. Chang, H.-J. Huang / Analytica Chimica Acra 300 (199.5) 15-23
500
0
-500
-1000
E / mV ( vs. Ag/AgCl
-1500
)
500
0
-500
-1000
E / mV ( vs. Ag/AgCl
-1500
)
Fig. 1. Cyclic voltammograms for PPy-Nafion composite electrodes in 0.2 M solutions of (a) NaCl (curve 1) , CH,CH,CH$HZS03Na (curve 2), (CH&NHCl (curve 3) and ( CH3CHZCH2CH2)JJ3r (curve 4) and (b) NH&l, CHJWJl, (CH&NH$l, (CH,)JWICl (curves 1,2,3 and 4, respectively). Sweep rate is 10 mV/s.
of curves ( 1) and (2). For curve (4) neither the anodic nor the cathodic peak is found. Comparing the CVs in Fig. la with the electrolytes in the solutions, it is concluded that the redox reaction of PPy on the PPyNafion composite electrode is related to the characteristics of the cations in solution. The relatively large redox peaks and very good reversibility found for curves ( 1) and (2) can be attributed to the presence of highly mobilized hydrated Na+ ions in solution. For curve (3) (with rather large and bulky trimethylammonium ions, responsible for the charge transport process), the rate of the redox reactions of PPy decreases and this results therefore in small redox currents and deterioration of reversibility. For curve (4) the tetrabutylammonium ions are so large and bulky that they are not able to diffuse into the PPy-N&on film to accomplish the charge transport process and therefore no redox currents are found. These observations are consistent with the report which states that when the PPy film is grown with large polymeric anions the cation and solvent molecules will be inserted and removed from the film to compensate the variation of charge in PPy film [ 201. To further examine the cation effect on the PPyNafion composite electrodes, CVs for solutions containing the same anion but different ammonium ions were run. Fig. lb shows the CVs obtained from 0.2 M solutions of NH&l, CH,NH,Cl, (CH,),NH,Cl and
( CH3)JHCl. The CVs obtained show essentially the same features as those in Fig. la. The current of the redox peaks decreases and a better separation of peak potential is obtained because the radius of the ammonium ions in solutions increased. This further confirms that the redox reactions occurring at the PPy-Nafion film are accompanied and dominated by the transport of cations and the rate of the redox reactions is related to the size of the cations involved. Similar to the behaviour found for the PPy film the broader anodic peaks found for curves 1 and 2 in Fig. la and b should be attributed to the effect of expulsion of cations from the PPy-Nafion composite film during the oxidation process of PPy [ 51. This expulsion effect is prominent for smaller ions. Fig. 2a and b shows the impedance spectra obtained from PPy-Nafion composite electrodes in the same solutions used in Fig. 1. The direct current (dc) potential applied is kept at values corresponding to the cathodic half peak potentials. The applied dc potentials are listed in Table 1. The complex impedance plots obtained show the same characteristics as those for the thin redox and electronically conductive polymer films [ 21-231. These plots can be interpreted using the modified Randles circuit as shown in Fig. 3 [23], where Rn is the resistance of the electrolyte between the working and the reference electrode, R, is the charge transfer resistance of the redox reaction, C,, is the double layer
C.-M. Chang, H.-J. Huang I Analytica Chimica Acta 300 (1995) 15-23
18 140
‘O”r-1
120
(b)
60
100 80 60
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-
40 20 0 -0
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0
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600
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15000
0
Zr /ohm
cm
-2
10000
5000
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Zr /ohm
cm
-2
Fig. 2. Complex impedance plots (50 mHz-5.0 kHz) obtained for PPy-Nafion composite electrodes from the same solutions as those used in Fig. la and b, respectively.
capacitance of the composite polymer/electrolyte interface, Z, is the Warburg impedance and Ce is the limiting capacitance in the non-Faradaic regimes. In the high frequency region of the plots, i.e., in the kHz range, the impedance response is associated with the electrode/electrolyte interface process. The related relaxation effect is displayed in the diagram as a semicircle. The value of R, may then be obtained from the intercept with the real axis. In the low-frequency region, i.e., in the Hz range, the impedance becomes
controlled by the diffusion of the counterions into the polymer electrode. The response assumes a linear behaviour with a frequency independent phase angle of ~14 and is represented by the Warburg impedance element, Z, in the equivalent circuit. When the frequency is reduced to very low values, e.g., in the mHz range, and if the thickness of the polymer film is sufficiently small, the diffusion process will be progressively limited in favour of a charge accumulation into the polymer film, Accordingly, the ac impedance
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C.-M. Chang, H.-J. Huang / Analytica Chimica Acta 300 (1995) IS-23
approaches a purely capacitive response with a phase angle of n/2. The associated capacitance C, is the socalled limiting capacitance. In most of the plots shown in Fig. 2, the regions characteristic for the charge transfer, the diffusion of counterions and the restriction of diffusion behaviour by the film thickness are found. From Fig. 2a, it can be seen that the impedance behaviour of the PPy-Naflon composite electrodes changes with the variation of size of the cations in solutions. The diameter of the semicircle which characterizes the charge transfer resistance Rti of the redox reactions of PPy increases with the increment of the radius of the cations in solutions. Although the semicircular arcs in Fig. 2 were a little deviated from that of the impedance plot of an idealized charge transfer process, the intercepts on the real axis were still estimated and used for the determination of R,,. To evaluate these two intercept values, a least squares fit procedure was applied by fitting the impedance data with a circular function derived from the equivalent circuit in Fig. 3. The obtained R,, values are listed in Table 1. According to Table 1 the R, values obtained from curve (4) are about three orders of magnitude larger than those obtained from curves (1) and (2). Due to the existence of such large R,, values no redox current can be found for the PPy-Nafion composite electrode in the solution of tetrabutylammonium bromide. From Fig. 2b, the diameter of the semicircle increases from curve ( 1) to (4) as the radius of the ammonium ions in solutions increases. Accompanied by the increment of charge transfer resistance in solutions, the peak current decreases and the reversibility of the reaction becomes worse compared to what has been found from the CVs in Fig. 1.
‘L
%
Fig. 3. Equivalent circuit used for the analysis of the impedance data. See text for explanation of variables.
From the impedance spectra, parameters related to the mass transfer of the studied electrolytes in the polymer film can be determined. From literature [ 21-231, in the diffusion controlled region, the impedance phase angle approaches n/4, and the magnitude of the impedance is given by ]Z] =(C&‘*el(D,w)l’*
(1)
where o is the angular frequency, e is the film thickness. C, and D,, are defined above. Values of C, are related to Zi and o by the following equation. C;‘=d(
-Zi)/d(o-‘)
(2)
This equation applies to data in the very low frequency region where O-=Xt2/Dc, and the phase angle approaches n/2. The reciprocal of C, can be obtained from the slope of the linear plot of -Zr vs. w-l. Fig. 4 shows the example of such plots from data shown in Fig. 2a. From the data in the diffusion control region (frequency ranged from 1 Hz to 200 Hz), plots of the total impedance Z versus w- “* were made (Fig. 5 ) . From the slopes of these linear plots, in addition to the redox capacititrices C, obtained (from IQ. 2) and the known film thickness t?, diffusion coefficient D, of the studied
Table 1 Impedance parameters obtained from PPy-Nafion composite electrodes in various electrolytic solutions Polymer coated electrode
Electrolyte (0.2 M)
PPy-Nafion
NaCl CH3CH2CHZCHZS03Na (CH,CH,CH,CH,),NHr NH&l CH,NH,Cl (CH&‘W~ (CH,),NHCl
Applied dc potential (V vs. Ag/AgCl)
& (Rlcm2)
Re (n/cm’)
c, x lo3 (Faraday)
D, x lOI”
- 0.35 - 0.32 - 0.60 -0.35 -0.47 - 0.50 - 0.50
10 14 13,000 10 69 259 480
21 20 1900 22 65 138 190
31.3 29.8 0.8 35.8 22.3 13.6 10.2
29.5 27.6 2.3 22.8 17.4 8.7 6.7
(cm’/s)
20
C.-M. Chang, H.-J. Huang I Analytica Chimica Acta 300 (1995) 15-23
fusion coefficient of Na+ ions in Nation film was reported to be 9 X 10M7cm2 s-r by Yeager and Steck [ 241 while the diffusion coefficient of Cp,FeTMA+ , Ru(bpy)g’ and Os(bpy):+ were reported by Fan and Bard [ 131 to be 1.7~ lo-“, 4.0X lo-” and 0.7X 10-l’ cm2 s-l, respectively. The Dd values of the studied cations obtained in this experiment are about one order of magnitude larger than those of the organometallic species but about two orders of magnitude smaller than those of Na+ in a Nafion film. Judged from the recent study of Vining and Meyer [ 251, incorporated species in the Nafion tilm are distributed between the different environments according to their charges and atomic/molecular structures. Hydrophilic ions remain in the aqueous regions and are
80
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i T
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10000 (4) .
5000
/
l
0
I
I
I
I
I
I
0.0 0 5 1.0 1.5 2.0 2.5 3.0 3.5 o-1/set
Fig. 4. Plots of imaginary part of impedance Z+versus CZJ-’(50 mHz to 0.45 Hz or 2.35 Hz). Solutions used are the same as those in Fig. la.
counter ions in the PPy-Nafion films can be calculated. The low frequency polymer resistance, Re was estimated by extrapolating the 45” straight line and the vertical line of the very low frequency limiting behaviour data to the 2, axis [ 221. The obtained parameters were summarized in Table 1. From Table 1, it can be seen that values of R,, and Re increase and values of C, and D, decrease with the increment of the radius of the cation in solution. The variation of impedance behaviour found for different ammonium ions shows this trend explicitly. For the PPy-Nafion electrodes, though the charge transfer phenomenon has been studied, no diffusion coefficient data of the cations studied in this experiment were reported. However, data of different cations incorporated into the Nafion film are available in the literature. The dif-
'; 5 E 2 \ N
800 600 400 200 / , 15000
14000 13000 12000
u.1
I
I
I
I
0.2
0.3
0.4
0.5
-l/2 w
/set
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Fig. 5. Plots of impedance magnitude 2 versus C1’* (1.65 Hz to 5.65 Hz or 0.55 Hz to 4.85 Hz) for data obtained from the same solutions as those in Fig. la.
1Analytica Chimica Acta 300 (1995) 15-23
C.-M. Chang, H.-J. Hung
0
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ifyyy!_I ;il;: 0
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;_
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-_rli
500
100015002000 -2
Zr
/ohm
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4000
Zr /ohm
8000
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12000 -2
Fig. 6. Complex impedance plot (50 mHz--5.0 kHz) obtained for PPy-Nafion composite electrodes from 0.2 M solution of NaCl with various applied dc potentials.
able to diffuse through the polymer via the ion clusters and interconnecting channels at a rate which is not much below that for aqueous solutions. More hydrophobic ions accumulate mainly in the interfacial domains where the diffusion coefficients are low. In the system studied, where the polypyrrole was intercalated with the Nafion film, the region of ion clusters and interconnecting channels available for the diffusion of cations decreased. The diffusion coefficient for the studied ions should be smaller than those obtained from a pure Nafion film. On the other hand, the radii and the hydrophobicity of the studied ions are smaller than those of the organometallic species mentioned above, the diffusion coefficients of the studied ions should thus be larger than those of the organometallic species. DC, values listed in Table 1 agree will with the conclusion of Vining and Meyer [ 25 1.
The above results show the impedance behaviour of the PPy-Nation composite lilm for electronically conductive states of the PPy film as the dc potential was held at the half peak potential. It is interesting to explore further the impedance behaviour of the PPy-Nation composite electrode for cases not related to electronic conductance. Impedance spectra of the PPy-N&on composite electrodes with a dc potential held at various values were therefore recorded in a solution of 0.2 M NaCl. The impedance plots obtained from 0.00 to - 0.70 V in 0.2 M NaCl solution are shown in Fig. 6. A rather large charge transfer resistance,&, found for electrodes with an applied potential from 0.00 to - 0.20 V implies that no reduction or a very slow reduction reaction occurs at these potentials. The magnitude of R,, decreases sharply as the applied potential becomes
C.-M. Chattg, H.-J. Huang /Analytica Chimica Acta 300 (1995) 15-23
22
Table 2 Impedance parameters obtained from PPy-Nafion composite electrodes in 0.2 M NaCl solution with various applied dc potentials Polymer coated electrode
Electrolyte (0.2 M)
Applied dc potential (V vs. Ag/AgCl)
RCI ( Wcm2)
RP (R/cm’)
c, x 10’ (Faraday)
D, x 1O’O (cm’/s)
PPy-Nafion
NaCl
-0.3 - 0.4 - 0.45 - 0.5
8 12 24 200
63 33 43 250
31 30 28 18
17.4 156.4 182.6 25.7
more negative than - 0.30 V. It reaches the minimum at - 0.40 V. After that the charge transfer resistance of the reduction reaction increases slowly as the applied potential becomes more negative. The rate of reduction becomes very slow when the applied potential is more negative than - 0.70 V. The phenomena of diffusion and restriction of diffusion behaviour can be clearly found in the potential range of - 0.30 to - 0.60 V. This impedance behaviour at different applied potentials agrees very well with the characteristics of the cyclic voltammogram of curve (1) shown in Fig. la. The related impedance parameters R,, Re, C, and Dct estimated from the impedance plots at various applied potentials are summarized in Table 2. The same variation of impedance parameters with applied potential is found in the studied potential range. A maximum D,, value was found at - 0.45 V and indicates that the reduction current will be the largest at that potential. A cathodic peak potential of -0.45 V was found from curve (1) in Fig. la and confirms this prediction. From literature the electrochemical behaviour of PPy can be divided into three regimes according to the potential applied on the electrodes [ 5,26-301. The first regime exists of a low potential region where the polymer behaves like an electronic insulator. In this regime, charge is injected into the bulk via a diffusion process [ 29,301. The second regime occurs at a higher potential region where the polymer behaves like an electronic conductor. In this regime, charge transport occurs uniformly throughout the bulk of the polymer phase via a capacitive-like mechanism [5,26]. The third regime exists at the intermediate potential region. From the impedance behaviour studied above, the PPyNafion composite electrode shows the behaviour characterizing a state of very low electronic conductivity when the applied dc potential is more negative than - 0.30 V. The state of the electronic conductor can be
found when the applied potential is larger than - 0.20 V. The change of the electrochemical behaviour of the PPy-Nafion composition electrodes upon variation of the applied potential is consistent with that reported for PPy electrodes in the literature except that the same three states have shifted to a region of more negative potential.
4. Conclusions From the CVs and impedance spectra obtained and the impedance parameters estimated, it can be concluded that transport of cations is responsible for the charge transfer process accompanying the redox reactions occurring at the PPy-Nafion composite electrodes. The efficiency of the ion transport in the PPyNafion films decreases as the radius of the cation increases. The electrochemical behaviour characterized the regimes of electronic insulator and electronic conductor, and the states of intermediate regime for the PPy film are also found in the PPy-Nafion composite electrode but more negative potentials have to be applied to exhibit this behaviour. By referring to the fact that transport of anions dominates the charge transport process in the PPy films, it is believed that there should be some sort of composite film in which the transport of cations and anions can occur simultaneously and which can contribute to the charge transfer processes in the composite film. In this experiment, the weight ratio of PPy to Nafion in the composite film was estimated to be 1:7.3. It seems that the weight ratio of Nafion to PPy is excessively large. By increasing the weight ratio of PPy or decreasing the weight ratio of Nafion in the composite film, an environment favourable for the movement of both the anions and cations may be created and thus facilitate the simultaneous
C.-M. Chang, H.-J. Huang IAnalytica Chimica Acta 300 (1995) 15-23
transport of cations and anions in the PPy-Nafion composite film. The relationship between the charge transfer efficiency and the ratio of PPy to Nafion in the PPyNafion composite film and the possible applications of the specially designed composite electrode may deserve further investigation.
[lo]
[ 111 [ 121
[ 131 [ 141 [15]
Acknowledgements
[ 161
The authors wish to express their gratitude to the National Science Council of Taiwan for the financial support of this work.
References [ l] T.A. Skotheim (Ed.), Handbookof Conducting 1 and 2, Marcel Dekker, New York, 1986.
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[2] A.F. Diaz, J.I. Castillo, J.A. Logan and W.-Y. Electroanal. Chem., 129 (1981) 115. [3] E.M. Genies, G. Bidan and A.F. Diaz, J. Electroanal. 149 (1983) 101.
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[4] C.D. Paulse and P.G. Pickup, J. Phys. Chem., 92 (1988) 7002. [S] D.J. Walton, C.E. Hall and A. Chyla, Analyst, 117 (1992) 1305. [6] P. Burgmayer and R.W. Murray, J. Phys. Chem., 88 (1984) 2515. [ 71 T. Ikeda, CR. Ledner and R.W. Murray, J. Electroanal. Chem., 138 (1982) 343. [8] H. Mao and P.G. Pickup, J. Am. Chem. Sot., 112 (1990) 1776. [9] A.F. Diaz and K.K. Kanazawa, J. Chem. Sot., Chem. Commun., (1979) 635.
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T.T. Wang, S. Tasaka, R.S. Hutton and P.Y. Lu, J. Chem. Sot. Chem. Commun., (1985) 1343. M.A. De Paoli, R.J. Wahman, A.F. Diaz and J. Bargon, J. Chem. Sot. Chem. Commun., (1984) 1015. 0. Niwa and T. Tamamura, J. Chem. Sot. Chem. Commun., ( 1984) 817. F.-R.F. Fan and A.J. Bard, J. Electrochem. Sot., 133 (1986) 301. G. Nagasubramanian, S. Di Stefano and J. Moacanin, J. Phys. Chem., 90 (1986) 4447. R.M. Penner and C.R. Martin, J. Electrochem. Sot., 133 (1986) 310. T. Hirai, S. Kuwabata and H. Yoneyama, J. Electrochem. Sot., 135 (1988) 1132. Y. Ikariysama and W.R. Heineman, Anal. Chem., 58 (1986) 1803. T.Y. Sung and H.J. Huang, Anal. Chim. Acta, 246 (1991) 275. B. Scrosati, Prog. Solid State Chem., 18 (1988) 1. K. Naoi, M. Lien and W.H. Smyrl, J. Electrochem. Sot., 138 (1991) 440. C. Ho, I.D. Raistrick and R.A. Huggins, J. Electrochem. Sot., 127 (1980) 343. T.B. Hunter, P.S. Tyler, W.H. Smyrl and H.S. White, J. Electrochem. Sot., 134 (1987) 2198. S. Panero, P. Prosperi, S. Passerini, B. Scrosati and D. Perlmutter, J. Electrochem. Sot., 136 (1989) 3729. H.L. Yeager and A. Steck, J. Electrochem. Sot., 128 (1981) 1880. W.J. Vining and T.J. Meyer, J. Electroanal. Chem., 237 (1987) 191. C.K. Chiang, M.A. Druy, S.C. Gau, A.J. Louis, A.G. MacDiarmid, Y.W. Park and H. Shirakawa, J. Am. Chem. Sot., 100 (1978) 1013. Z. Cai and C.R. Martin, J. Electroanal. Chem., 300 (1991) 35. M. Mao and P.G. Pickup, J. Phys. Chem., 93 (1989) 6480. R.M. Penner, L.S. Van Dyke and C.R. Martin, J. Phys. Chem., 92 (1988) 5274. R.M. Penner, and CR. Martin, J. Phys. Chem., 93 ( 1989) 984.
ACTA ELSEVIER
Analytica Chimica Acta 300 (1995) 15-23
Impedance analysis of the transport of counter ions at polypyrroleNafion composite electrodes Chur-Min Chang, Hsuan-Jung Huang * Department of Chemishy, National Sun Yat-sen University, Kaohsiung 80424, Taiwan
Received 20 June 1994; revised manuscript received 1 August 1994
Abstract The ion transport accompanying the redox reactions of polypyrrole at a polypyrrole-Nafion composite electrode was investigated with systems containing specifically selected electrolytes. From the cyclic voltammetric and impedance data obtained, transport of cations was found to be responsible for the charge transfer process and parameters such as the charge transfer resistance, Rti, the low frequency polymer resistance, Re, the limiting capacitance, C, and the diffusion coefficient, D, for the related cations were estimated. The electrochemical behaviour (i.e., electronic insulation and electronic conductance) of polypyrrole-Nafion composite electrodes was found to be the same as that of the polypyrrole electrodes, except that they appeared in regions of more negative potentials. Keywords: Cyclic voltammetry; Impedance; Polyoyrrole-Nafion composite electrodes
1. Introduction Understanding the mechanism of redox reactions occurring at conductive polymer electrodes and extending the practical applications of these electrodes is of great importance. Therefore, study of the charge transport in conductive polymers has become an attractive
and popular research topic in electrochemistry and electrochemical analysis [l-16]. Besides the studies on pure conductive polymer electrodes, the incorporation of conductive polymers like polypyrrole (PPy) and polyaniline into the electroinactive polymer networks of Nafion, poly(viny1 chloride), poly(viny1 alcohol) or polystyrene has been developed for improving the mechanical properties or for modifying the electrochemical characteristics of the conductive polymer electrodes and thus extending their applications [9* Corresponding author. 0003-2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved .SSD10003-2670( 94)00379-3
191. The combination of polypyrrole with Nafion is a typical example showing such characteristics. For the PPy-coated electrodes, the redox reactions are found to be accompanied with the doping-undoping mechanism of anions [l-5]. On PPy-Naiion composite electrodes the immobilized perfluorinated group of Nafion (which is a cation exchanger) serves as a charge compensator during the anodic polymerization of polypyrrole. Cations and solvent molecules, opposite to anions in plain polypyrrole electrodes, are the diffusing species accompanying the redox processes of the Nation composited polypyrrole [ 13-16,201. Although the characteristics of electrical conductivity and charge transfer in the PPy-Nafion composite film electrodes have been investigated, no systematic studies concerning the charge transport of counter ions on the electrochemical behaviour of the PPy-Nafion composite electrodes were reported. In this article, systems with various spe-
16
C.-M. Chang, H.-J. Huang I Analytica Chimica Acta 300 (1995) 15-23
cifically selected electrolytes were adopted to identify the species responsible for the charge transport process accompanying the redox reactions of polypyrrole at the PPy-Nalion composite electrodes. Because cyclic voltammetry (CV) can provide valuable information on the reversibility of the reaction and criteria for discriminating the charge transfer process and the impedance method (a powerful technique for probing the charge and mass transport mechanism), the impedance method based on the conductive polymer film model was employed in combination with CV to elucidate the charge transport of counter ions during the redox reaction of polypyrrole at PPy-Nafion composite electrodes.
2.3. Electrochemical measurements Cyclic voltammetry was performed with a PARC 175 universal programmer associated with a PARC 173 potentiostat/galvanostat. Coulometic measurements were achieved with a PARC 179 coulometer. The ac impedance measurements were made with the PARC M378 electrochemical impedance system (including a PARC 276 interface and a 5280 lock-in amplifier). An ac voltage of 5 mV in amplitude (peak-to-peak) with a frequency range of 50 mHz to 0.1 MHz was superimposed on the dc potential and applied to the studied electrodes. The obtained current signal at the frequency range of 50 mHz to 10 Hz was analyzed with the FFT technique.
2. Experimental 3. Results and discussion 2.1. Chemicals and solutions N&on (5 wt.% dissolved in a mixed solvent of lower aliphatic alcohols and water) was purchased from Aldrich. Sodium chloride, sodium butylsulfonate, ammonium chloride, methylammonium chloride, dimethylammonium chloride, trimethylammonium chloride and tetrabutylammonium bromide used for the preparation of solutions were purchased from Merck. All the chemicals used were of analytical reagent grade or better. 2.2. Preparation of PpY-Najion composite electrodes 30 ~1 of 1 wt.% Nafion solution (diluted with methanol-water, 9:l) was applied to the platinum electrode (with an area of 1.2 cm’) and air dried to allow the solvent to evaporate. The thickness of the Nafion film prepared was estimated to be 0.8 pm. The PPy-Nafion composite electrodes were prepared by the further electrodeposition of pyrrole on the Nafion-coated Pt electrode. The anodic polymerization was performed in a deaerated aqueous solution containing 0.1 M pyrrole and 0.2 M NaCl in a conventional three-electrode chemical cell (with a Pt wire as the counter electrode and the Ag/ AgCl in saturated KC1 solution as the reference electrode) with the application of a constant current of 0.1 mA cm-‘. The same electrochemical cell was used for the following electrochemical measurements.
The polymer films obtained by the electrodeposition of polypyrrole on the Nalion-coated Pt electrodes were brown-black in colour. The PPy film formed on the Nafion-coated electrode was smoother than that formed on the bare platinum electrode. This agrees with the literature report that PPy homogenizes well with N&on to produce smoother composite layers [ 13-151. The amount and the thickness of PPy film deposited on the Nafion-coated electrodes can be calculated from the charge consumed during electrodeposition, a PPy film of 0.5 pm in thickness is supposed to be obtained from a consumed charge of 90 mC cm-’ assuming that the amount of soluble oligomer formed is negligible [ 161. To study the counter ion effect on the redox reactions of PPy, cyclic voltammetry of PPy-N&on composite electrodes in solutions of 0.2 M NaCl, 0.2 M CH,( CH&SO,Na, 0.2 M ( CH3)$JHCl and 0.2 M ( CH,CH,CH,CH,) ,NBr were run respectively with a sweep rate of 10 mV/s in the potential range of - 1.20 to 0.20 V. Fig. la shows the voltammograms obtained. From Fig. la, rather sharp and large cathodic peaks and broad anodic peaks are found for curves ( 1) and (2). The separation of peak potentials for curves ( 1) and (2) are about 40 and 60 mV, respectively. This implies that the reversibility of the redox reactions of PPy in the studied solution is very good. For curve (3)) both the anodic and cathodic peaks become smaller and broader and the separation of peak potential also becomes larger (about 500 mV) compared with those
C.-M. Chang, H.-J. Huang / Analytica Chimica Acra 300 (199.5) 15-23
500
0
-500
-1000
E / mV ( vs. Ag/AgCl
-1500
)
500
0
-500
-1000
E / mV ( vs. Ag/AgCl
-1500
)
Fig. 1. Cyclic voltammograms for PPy-Nafion composite electrodes in 0.2 M solutions of (a) NaCl (curve 1) , CH,CH,CH$HZS03Na (curve 2), (CH&NHCl (curve 3) and ( CH3CHZCH2CH2)JJ3r (curve 4) and (b) NH&l, CHJWJl, (CH&NH$l, (CH,)JWICl (curves 1,2,3 and 4, respectively). Sweep rate is 10 mV/s.
of curves ( 1) and (2). For curve (4) neither the anodic nor the cathodic peak is found. Comparing the CVs in Fig. la with the electrolytes in the solutions, it is concluded that the redox reaction of PPy on the PPyNafion composite electrode is related to the characteristics of the cations in solution. The relatively large redox peaks and very good reversibility found for curves ( 1) and (2) can be attributed to the presence of highly mobilized hydrated Na+ ions in solution. For curve (3) (with rather large and bulky trimethylammonium ions, responsible for the charge transport process), the rate of the redox reactions of PPy decreases and this results therefore in small redox currents and deterioration of reversibility. For curve (4) the tetrabutylammonium ions are so large and bulky that they are not able to diffuse into the PPy-N&on film to accomplish the charge transport process and therefore no redox currents are found. These observations are consistent with the report which states that when the PPy film is grown with large polymeric anions the cation and solvent molecules will be inserted and removed from the film to compensate the variation of charge in PPy film [ 201. To further examine the cation effect on the PPyNafion composite electrodes, CVs for solutions containing the same anion but different ammonium ions were run. Fig. lb shows the CVs obtained from 0.2 M solutions of NH&l, CH,NH,Cl, (CH,),NH,Cl and
( CH3)JHCl. The CVs obtained show essentially the same features as those in Fig. la. The current of the redox peaks decreases and a better separation of peak potential is obtained because the radius of the ammonium ions in solutions increased. This further confirms that the redox reactions occurring at the PPy-Nafion film are accompanied and dominated by the transport of cations and the rate of the redox reactions is related to the size of the cations involved. Similar to the behaviour found for the PPy film the broader anodic peaks found for curves 1 and 2 in Fig. la and b should be attributed to the effect of expulsion of cations from the PPy-Nafion composite film during the oxidation process of PPy [ 51. This expulsion effect is prominent for smaller ions. Fig. 2a and b shows the impedance spectra obtained from PPy-Nafion composite electrodes in the same solutions used in Fig. 1. The direct current (dc) potential applied is kept at values corresponding to the cathodic half peak potentials. The applied dc potentials are listed in Table 1. The complex impedance plots obtained show the same characteristics as those for the thin redox and electronically conductive polymer films [ 21-231. These plots can be interpreted using the modified Randles circuit as shown in Fig. 3 [23], where Rn is the resistance of the electrolyte between the working and the reference electrode, R, is the charge transfer resistance of the redox reaction, C,, is the double layer
C.-M. Chang, H.-J. Huang I Analytica Chimica Acta 300 (1995) 15-23
18 140
‘O”r-1
120
(b)
60
100 80 60
80
-
40 20 0 -0
20
40
60
80
100
120
140
40
80
120
300
600
900
160
200
1200
1500
600
0
300
600
900
1200
15000
0
Zr /ohm
cm
-2
10000
5000
0
Zr /ohm
cm
-2
Fig. 2. Complex impedance plots (50 mHz-5.0 kHz) obtained for PPy-Nafion composite electrodes from the same solutions as those used in Fig. la and b, respectively.
capacitance of the composite polymer/electrolyte interface, Z, is the Warburg impedance and Ce is the limiting capacitance in the non-Faradaic regimes. In the high frequency region of the plots, i.e., in the kHz range, the impedance response is associated with the electrode/electrolyte interface process. The related relaxation effect is displayed in the diagram as a semicircle. The value of R, may then be obtained from the intercept with the real axis. In the low-frequency region, i.e., in the Hz range, the impedance becomes
controlled by the diffusion of the counterions into the polymer electrode. The response assumes a linear behaviour with a frequency independent phase angle of ~14 and is represented by the Warburg impedance element, Z, in the equivalent circuit. When the frequency is reduced to very low values, e.g., in the mHz range, and if the thickness of the polymer film is sufficiently small, the diffusion process will be progressively limited in favour of a charge accumulation into the polymer film, Accordingly, the ac impedance
19
C.-M. Chang, H.-J. Huang / Analytica Chimica Acta 300 (1995) IS-23
approaches a purely capacitive response with a phase angle of n/2. The associated capacitance C, is the socalled limiting capacitance. In most of the plots shown in Fig. 2, the regions characteristic for the charge transfer, the diffusion of counterions and the restriction of diffusion behaviour by the film thickness are found. From Fig. 2a, it can be seen that the impedance behaviour of the PPy-Naflon composite electrodes changes with the variation of size of the cations in solutions. The diameter of the semicircle which characterizes the charge transfer resistance Rti of the redox reactions of PPy increases with the increment of the radius of the cations in solutions. Although the semicircular arcs in Fig. 2 were a little deviated from that of the impedance plot of an idealized charge transfer process, the intercepts on the real axis were still estimated and used for the determination of R,,. To evaluate these two intercept values, a least squares fit procedure was applied by fitting the impedance data with a circular function derived from the equivalent circuit in Fig. 3. The obtained R,, values are listed in Table 1. According to Table 1 the R, values obtained from curve (4) are about three orders of magnitude larger than those obtained from curves (1) and (2). Due to the existence of such large R,, values no redox current can be found for the PPy-Nafion composite electrode in the solution of tetrabutylammonium bromide. From Fig. 2b, the diameter of the semicircle increases from curve ( 1) to (4) as the radius of the ammonium ions in solutions increases. Accompanied by the increment of charge transfer resistance in solutions, the peak current decreases and the reversibility of the reaction becomes worse compared to what has been found from the CVs in Fig. 1.
‘L
%
Fig. 3. Equivalent circuit used for the analysis of the impedance data. See text for explanation of variables.
From the impedance spectra, parameters related to the mass transfer of the studied electrolytes in the polymer film can be determined. From literature [ 21-231, in the diffusion controlled region, the impedance phase angle approaches n/4, and the magnitude of the impedance is given by ]Z] =(C&‘*el(D,w)l’*
(1)
where o is the angular frequency, e is the film thickness. C, and D,, are defined above. Values of C, are related to Zi and o by the following equation. C;‘=d(
-Zi)/d(o-‘)
(2)
This equation applies to data in the very low frequency region where O-=Xt2/Dc, and the phase angle approaches n/2. The reciprocal of C, can be obtained from the slope of the linear plot of -Zr vs. w-l. Fig. 4 shows the example of such plots from data shown in Fig. 2a. From the data in the diffusion control region (frequency ranged from 1 Hz to 200 Hz), plots of the total impedance Z versus w- “* were made (Fig. 5 ) . From the slopes of these linear plots, in addition to the redox capacititrices C, obtained (from IQ. 2) and the known film thickness t?, diffusion coefficient D, of the studied
Table 1 Impedance parameters obtained from PPy-Nafion composite electrodes in various electrolytic solutions Polymer coated electrode
Electrolyte (0.2 M)
PPy-Nafion
NaCl CH3CH2CHZCHZS03Na (CH,CH,CH,CH,),NHr NH&l CH,NH,Cl (CH&‘W~ (CH,),NHCl
Applied dc potential (V vs. Ag/AgCl)
& (Rlcm2)
Re (n/cm’)
c, x lo3 (Faraday)
D, x lOI”
- 0.35 - 0.32 - 0.60 -0.35 -0.47 - 0.50 - 0.50
10 14 13,000 10 69 259 480
21 20 1900 22 65 138 190
31.3 29.8 0.8 35.8 22.3 13.6 10.2
29.5 27.6 2.3 22.8 17.4 8.7 6.7
(cm’/s)
20
C.-M. Chang, H.-J. Huang I Analytica Chimica Acta 300 (1995) 15-23
fusion coefficient of Na+ ions in Nation film was reported to be 9 X 10M7cm2 s-r by Yeager and Steck [ 241 while the diffusion coefficient of Cp,FeTMA+ , Ru(bpy)g’ and Os(bpy):+ were reported by Fan and Bard [ 131 to be 1.7~ lo-“, 4.0X lo-” and 0.7X 10-l’ cm2 s-l, respectively. The Dd values of the studied cations obtained in this experiment are about one order of magnitude larger than those of the organometallic species but about two orders of magnitude smaller than those of Na+ in a Nafion film. Judged from the recent study of Vining and Meyer [ 251, incorporated species in the Nafion tilm are distributed between the different environments according to their charges and atomic/molecular structures. Hydrophilic ions remain in the aqueous regions and are
80
": E
900
i T
600
@i50
300
10000 (4) .
5000
/
l
0
I
I
I
I
I
I
0.0 0 5 1.0 1.5 2.0 2.5 3.0 3.5 o-1/set
Fig. 4. Plots of imaginary part of impedance Z+versus CZJ-’(50 mHz to 0.45 Hz or 2.35 Hz). Solutions used are the same as those in Fig. la.
counter ions in the PPy-Nafion films can be calculated. The low frequency polymer resistance, Re was estimated by extrapolating the 45” straight line and the vertical line of the very low frequency limiting behaviour data to the 2, axis [ 221. The obtained parameters were summarized in Table 1. From Table 1, it can be seen that values of R,, and Re increase and values of C, and D, decrease with the increment of the radius of the cation in solution. The variation of impedance behaviour found for different ammonium ions shows this trend explicitly. For the PPy-Nafion electrodes, though the charge transfer phenomenon has been studied, no diffusion coefficient data of the cations studied in this experiment were reported. However, data of different cations incorporated into the Nafion film are available in the literature. The dif-
'; 5 E 2 \ N
800 600 400 200 / , 15000
14000 13000 12000
u.1
I
I
I
I
0.2
0.3
0.4
0.5
-l/2 w
/set
0.6
l/2
Fig. 5. Plots of impedance magnitude 2 versus C1’* (1.65 Hz to 5.65 Hz or 0.55 Hz to 4.85 Hz) for data obtained from the same solutions as those in Fig. la.
1Analytica Chimica Acta 300 (1995) 15-23
C.-M. Chang, H.-J. Hung
0
1000
2000
0
3000
200
400
600
0
20 40 60 80100120
ifyyy!_I ;il;: 0
20 40 60 80100120
;_
0
20 40 60 80100120
:I-:'
0
0
100
200
300
400
-_rli
500
100015002000 -2
Zr
/ohm
cm
0
4000
Zr /ohm
8000
cm
12000 -2
Fig. 6. Complex impedance plot (50 mHz--5.0 kHz) obtained for PPy-Nafion composite electrodes from 0.2 M solution of NaCl with various applied dc potentials.
able to diffuse through the polymer via the ion clusters and interconnecting channels at a rate which is not much below that for aqueous solutions. More hydrophobic ions accumulate mainly in the interfacial domains where the diffusion coefficients are low. In the system studied, where the polypyrrole was intercalated with the Nafion film, the region of ion clusters and interconnecting channels available for the diffusion of cations decreased. The diffusion coefficient for the studied ions should be smaller than those obtained from a pure Nafion film. On the other hand, the radii and the hydrophobicity of the studied ions are smaller than those of the organometallic species mentioned above, the diffusion coefficients of the studied ions should thus be larger than those of the organometallic species. DC, values listed in Table 1 agree will with the conclusion of Vining and Meyer [ 25 1.
The above results show the impedance behaviour of the PPy-Nation composite lilm for electronically conductive states of the PPy film as the dc potential was held at the half peak potential. It is interesting to explore further the impedance behaviour of the PPy-Nation composite electrode for cases not related to electronic conductance. Impedance spectra of the PPy-N&on composite electrodes with a dc potential held at various values were therefore recorded in a solution of 0.2 M NaCl. The impedance plots obtained from 0.00 to - 0.70 V in 0.2 M NaCl solution are shown in Fig. 6. A rather large charge transfer resistance,&, found for electrodes with an applied potential from 0.00 to - 0.20 V implies that no reduction or a very slow reduction reaction occurs at these potentials. The magnitude of R,, decreases sharply as the applied potential becomes
C.-M. Chattg, H.-J. Huang /Analytica Chimica Acta 300 (1995) 15-23
22
Table 2 Impedance parameters obtained from PPy-Nafion composite electrodes in 0.2 M NaCl solution with various applied dc potentials Polymer coated electrode
Electrolyte (0.2 M)
Applied dc potential (V vs. Ag/AgCl)
RCI ( Wcm2)
RP (R/cm’)
c, x 10’ (Faraday)
D, x 1O’O (cm’/s)
PPy-Nafion
NaCl
-0.3 - 0.4 - 0.45 - 0.5
8 12 24 200
63 33 43 250
31 30 28 18
17.4 156.4 182.6 25.7
more negative than - 0.30 V. It reaches the minimum at - 0.40 V. After that the charge transfer resistance of the reduction reaction increases slowly as the applied potential becomes more negative. The rate of reduction becomes very slow when the applied potential is more negative than - 0.70 V. The phenomena of diffusion and restriction of diffusion behaviour can be clearly found in the potential range of - 0.30 to - 0.60 V. This impedance behaviour at different applied potentials agrees very well with the characteristics of the cyclic voltammogram of curve (1) shown in Fig. la. The related impedance parameters R,, Re, C, and Dct estimated from the impedance plots at various applied potentials are summarized in Table 2. The same variation of impedance parameters with applied potential is found in the studied potential range. A maximum D,, value was found at - 0.45 V and indicates that the reduction current will be the largest at that potential. A cathodic peak potential of -0.45 V was found from curve (1) in Fig. la and confirms this prediction. From literature the electrochemical behaviour of PPy can be divided into three regimes according to the potential applied on the electrodes [ 5,26-301. The first regime exists of a low potential region where the polymer behaves like an electronic insulator. In this regime, charge is injected into the bulk via a diffusion process [ 29,301. The second regime occurs at a higher potential region where the polymer behaves like an electronic conductor. In this regime, charge transport occurs uniformly throughout the bulk of the polymer phase via a capacitive-like mechanism [5,26]. The third regime exists at the intermediate potential region. From the impedance behaviour studied above, the PPyNafion composite electrode shows the behaviour characterizing a state of very low electronic conductivity when the applied dc potential is more negative than - 0.30 V. The state of the electronic conductor can be
found when the applied potential is larger than - 0.20 V. The change of the electrochemical behaviour of the PPy-Nafion composition electrodes upon variation of the applied potential is consistent with that reported for PPy electrodes in the literature except that the same three states have shifted to a region of more negative potential.
4. Conclusions From the CVs and impedance spectra obtained and the impedance parameters estimated, it can be concluded that transport of cations is responsible for the charge transfer process accompanying the redox reactions occurring at the PPy-Nafion composite electrodes. The efficiency of the ion transport in the PPyNafion films decreases as the radius of the cation increases. The electrochemical behaviour characterized the regimes of electronic insulator and electronic conductor, and the states of intermediate regime for the PPy film are also found in the PPy-Nafion composite electrode but more negative potentials have to be applied to exhibit this behaviour. By referring to the fact that transport of anions dominates the charge transport process in the PPy films, it is believed that there should be some sort of composite film in which the transport of cations and anions can occur simultaneously and which can contribute to the charge transfer processes in the composite film. In this experiment, the weight ratio of PPy to Nafion in the composite film was estimated to be 1:7.3. It seems that the weight ratio of Nafion to PPy is excessively large. By increasing the weight ratio of PPy or decreasing the weight ratio of Nafion in the composite film, an environment favourable for the movement of both the anions and cations may be created and thus facilitate the simultaneous
C.-M. Chang, H.-J. Huang IAnalytica Chimica Acta 300 (1995) 15-23
transport of cations and anions in the PPy-Nafion composite film. The relationship between the charge transfer efficiency and the ratio of PPy to Nafion in the PPyNafion composite film and the possible applications of the specially designed composite electrode may deserve further investigation.
[lo]
[ 111 [ 121
[ 131 [ 141 [15]
Acknowledgements
[ 161
The authors wish to express their gratitude to the National Science Council of Taiwan for the financial support of this work.
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