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Electrochemical impedance spectroscopic investigation of CO2 reduction on polyaniline in methanol
Electrochimica Acta 48 (2003) 1595 /1601 www.elsevier.com/locate/electacta
Electrochemical impedance spectroscopic investigation of CO2 reduction on polyaniline in methanol Fatih Ko¨leli a,*, Thorsten Ro¨pke b, Carl H. Hamann b a
b
Department of Chemistry, Faculty of Arts and Sciences, Mersin University, 33342 Mersin, Turkey Angewandte Physikalische Chemie, Universita¨t Oldenburg, Postfach 2503, 26111 Oldenburg, Germany Received 20 November 2002; received in revised form 17 January 2003
Abstract The ac response of polyaniline thin films on platinum electrodes was measured at different dc potentials during the CO2 reduction in methanol/LiClO4 electrolyte with a small amount of 0.5 M H2SO4. The complex capacitance curves were simulated and the data obtained were used to calculate kinetic parameters, based on the assumption that the thermodynamic potential E0 is in the region of /0.2 / /0.1 V versus saturated calomel electrode (SCE). With E0 / /0.2 V versus SCE and b /0.6, a j0 value of ca. 10 4 A cm 2 was found for the electroreduction of CO2 on the polyaniline electrode. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Polyaniline; CO2 reduction; Had formation; HCOOH and CH3COOH formation; Electrochemical impedance spectroscopy (EIS)
1. Introduction Conducting polymers, generally in form of thin films, deposited electrochemically on metal electrodes were investigated intensively from the point of theoretical and practical views in the last two decades [1]. The reason for the interest, on the one hand, is the understanding of their specific properties, on the other hand, the application aspect of these materials for, e.g. fabrication of electronic devices, such as diodes, transistors, sensors [2 /4], and electrocatalysts [5]. Cyclic voltammetry and related techniques are the favourite tools for investigation of electroactive polymers, most probably because the same method may, when thin films are obtained by electropolymerisation on metal electrodes, provide both, electrosynthesis and characterization. Whatever an electrochemical system or method was used for investigation, the electrochemical impedance spectroscopy (EIS) generally has advantages among these methods, involving large disturbances: the studied system only is negligible far from the steady-
* Corresponding author. Tel.: /90-324-361-0045; fax: /90-324361-0046. E-mail address: [email protected] (F. Ko¨leli).
state. These advantages are important especially for electrochemical reactions, when conducting polymer films are involved, which might show perturbations in a large potential window because of their inhomogeneous structure. Furthermore, kinetic data can be determined more easily. Among others, polyaniline (PAn) is the conducting polymer film studied most intensively by EIS [6 /15] to complement and elucidate current /voltage and kinetic data. But most of these works are considering the polymer film at the anodic side of the potential scale. However, a polyaniline film modified with platinum microparticles as electrode materials was investigated by Grzeszcuk [16] via EIS at negative potentials to achieve data of various charge transfer and transport processes involved in the electrochemical response of the polymer electrode. The electroreduction of CO2 is a remarkable process with respect to two primary reasons. Firstly, CO2 is the ultimate by-product of all processes involving oxidation of carbon compounds and its increasing presence in the atmosphere. Secondly, in view of the vastness of its supply, CO2 represents a possible potential source for Cfeedstocks for the manufacture of chemicals. For the reduction of CO2, most of the metal electrodes were used [17,18] in different electrolytes and various electro-
0013-4686/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0013-4686(03)00076-8
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chemical cells without any convenient result in respect to a technical application. All metal electrodes need highly negative potentials even lower than /1.7 V versus saturated calomel electrode (SCE) for the reduction of CO2 in both, protic and aprotic electrolytes. In the present study, we focussed our interest to investigate a polyaniline film at negative potentials in the MeOH/LiClO4 solution saturated with CO2 in presence of a small amount of H2O/H2SO4 via EIS, in order to achieve the kinetic data and to find out the optimum electrolysis potential of the electroreduction of CO2 in MeOH/LiClO4.
2. Experimental For the synthesis of polyaniline, stock solution of 0.1 M freshly distilled aniline (Fluka) in 0.5 M H2SO4 was prepared. The film used for the investigation was synthesized between potentials of /0.2 and /0.8 V versus SCE on a Pt-plate (surface area of 1 cm2) with a thickness of ca. 0.6 mm potentiostatically (BANK POS 73) in a three electrode H-cell. The counter electrode was a Pt-plate (6 cm2) and as reference electrode a SCE was used throughout the measurements. The scan rate of CV was kept at 50 mV s 1 for both growth and characterization of the polymer film. In order to remove the surplus ions in the polymer film, the electrode was washed carefully with MeOH and transferred into the three electrode H-cell (cell volume of 100 cm3, cathode compartment 50 cm3) used for the impedance measurements (Solartron FRA 1255, Solartron 1286 ECI). The EIS measurements were carried out at 20 8C at different potentials in a frequency range from 20 kHz to 0.1 Hz in MeOH/0.25 M LiClO4. As proton source, 1.4 ml of 0.5 M H2SO4 was added into the cathode compartment to determine the influence of the acid concentration to the electroreduction of CO2 on a polyaniline electrode. Prior to the EIS measurements, the electrolyte was saturated with CO2. Some of the electrical circuit models proposed earlier [11,13,15,16] were used to simulate the impedance spectra (software ZView 2.2, delivered by Scribner Associates Inc., USA). For the calculation of the kinetic data, the fitting results of the Nyquist curves were used.
3. Results and discussion In a previous study [5], the experimental results of the CO2 reduction on a polyaniline film under ambient condition and under high pressure in MeOH/LiClO4/ H /H2O solution were presented. As reported in [5], polyaniline is an excellent material for the electroreduction of CO2. Formaldehyde, formic acid and acetic acid were obtained as reduction products with high current
efficiencies. The preparative electrolysis was carried out at a potential of /0.4 V versus SCE in MeOH/LiClO4/ H /H2O electrolyte. 1.4 ml (0.01 mol l 1) of 0.5 M H2SO4 (optimum concentration under electrolysis conditions) is necessary as proton donator for the reduction of CO2 in this electrolyte system. Also, the film thickness is an important parameter for the current density and optimum film thickness was found to be ca. 0.6 mm. If the film thickness during the electrolysis is larger than 0.6 mm, a crumbling of the film is observed due to H2 evolution and if the film is thinner, the current density decreases drastically, while active electrode surface is getting smaller. Various mechanisms were proposed for CO2 reduction in the literature [17,19,20]. In all these works, + establishing a radical anionic species (COO ) was formulated as the initial step. Furthermore, Vassiliev et al. [17] have shown that the generation of these species is the rate-determining step in aprotic media. It is also well known [21 /24] that the reduction potential for the + CO2 =CO2 system lies at ca. /1.9 V versus NHE on metal electrodes in both protic and aprotic media. No reaction occurs at /0.4 V versus SCE, when a bare platinum is used as the electrode material under these conditions. The value of /0.4 V versus SCE mentioned as electrolysis potential is much more positive than the electrolysis potentials necessary at metal electrodes [18]. This leads to the conclusion, that on PAn electrode the + formation of a CO2 radical anion may not be the initial step, due to the energetical reasons. Thus, the following new reaction mechanism was proposed in Ref. [5]. H e 0 Had
(1)
CO2 Had 0 HCOOad
(2)
HCOOad Had 0 HCOOH
(3 )
HCOOad CH3 OH 0 CH3 COOH OHad
(4 )
OHad Had 0 H2 O
(5)
*Competitive reactions. The experimental results in Ref. [5] indicate that for the reduction, the adsorbtion of CO2 on polyaniline via hydrogen bonds is essential, and that the reaction starts with the formation of Had atoms on the supporting metal electrode. When Had is generated, it adds to CO2 molecules, adsorbed on the polymer film, and the species HCOOad is formed, which can recombine with further Had to give formic acid with faradaic efficiencies of ca. 13% under electrolysis conditions [5]. However, when the species HCOOad is formed, it seems that the attack of a solvent molecule (CH3OH) is easier and, as a result, acetic acid is generated with faradaic efficiencies of ca. 56% [5]. Compared to the long time electrolysis, the formation of formaldehyde can be neglected during the relatively short EIS measurements. The formation of
F. Ko¨leli et al. / Electrochimica Acta 48 (2003) 1595 /1601
formaldehyde can be explained as the consequence of the reduction of formic acid [5]. Now, we have carried out EIS measurements, systematically, in order to examine of the general cathodic impedance characteristics of a bulk PAn film, in presence of H/H2O, and in presence of H /H2O/CO2 in the electrolyte on the one hand and to achieve kinetic data on the other, respectively. The Nyquist curves of the PAn electrode recorded at /0.3 V versus SCE are presented in Fig. 1. In Fig. 1, it can be seen that all three impedance curves, A, B, and C, show a semi-circle in the highfrequency region. The shape of the curves changes significantly by addition of H /H2O (curve B) and H /H2O/CO2 (inset diagram: curve C) in the solution. In the basic electrolyte in the absence of H/H2O and CO2, a semi-circle was achieved in the frequency range from 20 kHz to 917 Hz (Fig. 1, curve A). For the characterization, the equivalent circuit in Fig. 2a was used in analogy to [13]. The Re value describes the electrolyte resistance and has a value of ca. 5 V. R1 may be the Pt/polymer depletion layer resistance. Due to the high conductivity of the film at positive potentials, e.g. /0.2 V versus SCE, the R1 has a small value. However, in the direction to negative potentials, the R1 values increase due to the decreasing conductivity of the polymer film (Table 1). These are expected results for the general behaviour of the conducting polymer films at negative potentials. Without extra protons in the electrolyte, the polymer
film is electrochemically inactive in the cathodic region. However, if the electrolyte is sufficiently acidic, then the polymer film in contact to the platinum surface may serve as a conducting material for electron transport. As can be seen in Table 1, R1, the constant phase element CPE1, infinite length Warburg impedance ZILW, and n1 values are presented, where n1 is the correlation coefficient for the constant phase element CPE1. n2 has a fixed value of 0.5, which corresponds to the definition of a ZILW. The CPE1 with C1 defines the Pt/polymer depletion layer capacity. This capacitance is a function of the applied potential, because the width of the depletion region does also vary with the applied potential. If the applied potential is increased, the depletion region extends and the capacity decreases (Table 1 /0.30 / /0.45 V vs. SCE). In Table 1, the n1 values decrease by more negative potentials. This can be explained through the diffusion capacity of the depletion layer; the diffusion character becomes dominant while the capacitive character of the film decreases. The impedance of a constant phase element ZCPE is defined by two values, C and n. With Eq. (6) the impedance ZCPE can be calculated. ZCPE
1 n
C(iv)
pffiffiffiffiffiffi (i 1):
(6)
If n /1, then the equation is identical to that of a capacitor. C is determined as real capacitance. Usually, in a model, a CPE is used instead of a capacitor to describe the non-homogeneities in the system, e.g. a rough or porous surface can cause a double-layer capacity appearing as a constant phase element with a n value smaller than 1 [25]. The CPE1 in the circuit 2a has a capacity of ca. 4.2 mF cm 2 at /0.2 V versus SCE. This value grows up to ca. 4.7 mF cm 2 at /0.3 V versus SCE and then decreases in the negative potential direction gradually due to the decreasing of the conductivity of the film. A CPE with a n value of 0.5 can also be used to produce an infinite length Warburg impedance, ZILW. A Warburg element describes the diffusion of the charge carrier in a material. Lower frequencies in an impedance plot correspond to diffusion deep into the material. Generally, if a material is thick enough and the lowest applied frequency is not fully penetrated the inner phase of the material, the Warburg impedance can be interpreted as infinite. In that case, the ILW impedance can be explained through Eq. (7) [25]. ZILW
Fig. 1. Complex impedance plots of a polyaniline electrode at /0.3 V vs. SCE in MeOH/LiClO4 solution. From 20 kHz to 0.1 Hz: (A) PAn in MeOH/LiClO4; (B) PAn in MeOH/LiClO4/1.4 ml 0.5 M H2SO4; (C) PAn in CO2 saturated MeOH/LiClO4/1.4 ml H /H2O. The solid lines represent the best fitting results according to the equivalent circuits. Inset diagram: all three curves are zoomed.
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RILW tanh(iTILW v)n (iTILW v)n
(7)
Typically, the ILW impedance leads to a 458 slope in the Nyquist diagram (Fig. 1A). The ZILW includes the transport phenomena in the solution in the presence of trace water (the autoprotolysis is mainly performed by
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Fig. 2. Equivalent circuits for the polyaniline film (a), PAn film with H /H2O in the electrolyte (b), and PAn with addition of H /H2O and CO2 saturated electrolyte (c). Re, electrolyte resistance; R1, resistance of the Pt/polymer depletion layer; R2, charge transfer resistance; ZILW, infinite length Warburg impedance; ZFLW, finite length Warburg impedance; CPE1, constant phase element of the depletion layer capacitance (contact capacitance); CPE2, constant phase element of the double layer capacitance.
proton transfer from the trace of water to the organic solvent). The depletion layer resistance increases rapidly at more negative potentials and this growth cannot be influenced by the little concentration of H ions in the solution. However, the values of TILW and RILW in Table 1 are quite independent on the applied potential. With addition of 1.4 ml of H /H2O in the basic electrolyte, the shape of the Nyquist curve (Fig. 1B) changes significantly due to the H2 evolution, and this change is characterized by using the electrical circuit in Fig. 2b. In that case, a second RCPE circuit with the charge transfer resistance R2 and CPE2 was added to the elements in the equivalent circuit in Fig. 2a instead of the ZFLW (the diffusion of H in the solution is neglected), considering the forming of Had atoms. Those results, achieved by our EIS measurements, correspond to the findings in Ref. [16], as well. The Nyquist diagram of polyaniline in presence of 1.4 ml H /H2O in MeOH/LiClO4 saturated with CO2 is presented in Fig. 3. For comparison, the Bode plot is given as well. An additional semi-circle (II) at frequencies between 1174 and 5 Hz is observable only when the electrolyte is saturated with CO2 (Fig. 3). This may correspond to the charge transfer resistance of the CO2 reduction. A twostep reaction process is the reason for the occurrence of
Fig. 3. Nyquist plot of CO2 reduction on a polyaniline electrode at / 0.3 V vs. SCE in the frequency range of 20 kHz /0.1 Hz in CO2 saturated MeOH/LiClO4/H /H2O; inset diagram: Bode plot. Solid lines indicate the simulated data with the fitted parameters in the frequency range of 60 kHz /0.1 Hz.
Table 1 The best-fit values of the equivalent circuit in Fig. 2a for a polyaniline electrode at different potentials without H /H2O and CO2 Potential (V vs. SCE)
/0.20 /0.25 /0.30 /0.35 /0.40 /0.45
R1 (V cm2)
82.71 149.60 285.10 655.00 1014.00 1322.00
(8.33) (2.74) (2.07) (1.46) (1.48) (1.77)
Infinite length Warburg impedance (ZILW, n2 /0.5) a
CPE1 C1 (mF cm 2)
n1
4.24 4.46 4.68 4.14 4.29 3.60
0.84 0.82 0.80 0.79 0.77 0.77
(26.22) (8.21) (6.43) (4.64) (4.55) (6.11)
(2.49) (0.90) (0.74) (0.59) (0.67) (0.84)
TILW (s)
RILW (V cm2)
5.95 13.01 11.68 9.25 8.11 6.86
9276 (8.63) 14 875 (10.14) 13 293 (10.47) 10 691 (8.59) 7677 (12.28) 6085 (14.89)
(20.28) (20.34) (21.41) (18.04) (26.20) (32.18)
Re /5 V cm2; n1, correlation coefficient; TILW, time constant of the infinite length Warburg impedance; RILW, resistance of the infinite length Warburg impedance. a Fixed parameter; ( ) deviation in percent.
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semi-circle (II), i.e. an intermediate state is involved. The first semi-circle (I) at high frequencies may describe the metal/polymer depletion layer. The third semi-circle (III) may represent a finite length Warburg impedance ZFLW, with the time constant TFLW and real part RFLW, describing the transport phenomena of CO2 into the polymer film and the transport of reduction products out of the film. If the material (polymer film) is thin, the low frequencies can also penetrate into the inner bulk phase completely, and create a finite length Warburg element. In that case, the impedance ZFLW can be calculated by using the following equation [25]: ZFLW
RFLW coth(iTFLW v)n : (iTFLW v)n
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reactions. The depletion layer resistance is about two decades lower than the values in Table 2. In other words, a further activity of the polymer film is given through the adsorption of CO2. In this case, the formation of molecular hydrogen is not preferred, and the formation of HCOOad is easier (Eq. (2)). Thus, the reaction obeys an EC mechanism where CO2 reacts chemically with adsorbed hydrogen. In Table 3, the time constant TFLW becomes greater at potentials from / 0.275 to /0.450 V versus SCE, which indicates diffusion of CO2 at these potentials deeper into the film pffiffiffiffiffiffiffiffiffiffiffi ( TFLW L): The current densities increase while the R2 values decrease. However, a limitation of the current densities with values of ca. /0.9 mA cm 2 occurs at more negative potentials to ca. /0.4 V versus SCE. This is also verified with a stationary I /V diagram, recorded during the impedance measurements. As can be seen in Fig. 4, the optimum electrolysis potential is found to be /0.4 V versus SCE. In Nyquist plots, the real part of the Warburg impedances RFLW becomes gradually larger in both directions, to more negative and to more positive potential values than /0.3 V versus SCE. The Nyquist plots of the CO2 reduction obtained at different potentials are presented in Fig. 5. With respect to the CO2 reduction, no EIS measurements were carried out at potentials more positive than /0.2 V versus SCE due to the fact that at such high anodic potentials, the conductance mechanism of the polymer changes, and the reduction of CO2 does not occur. The determination of the kinetic data, such as the exchange current density j0, is only possible by the prior knowledge of the thermodynamic potential for the CO2 electroreduction on polyaniline in methanol. However, according to the formulated mechanism in [5], an electron transfer directly to CO2 molecule cannot occur due to energetic reasons. Only the formation of Had atoms as initial step leads to the reaction products, so that we can assume for our following calculations the equilibrium potential of Had formation.
(8)
Eqs. (7) and (8) can only be used with fixed n values (n /0.5). TFLW and TILW must be equal to L2/D , where L is the effective diffusion thickness, and D is the effective diffusion coefficient. The equivalent circuits (Fig. 2b and c) with the elements Re, R1, R2, CPE1, CPE2 and ZFLW are describing the behaviours of the polymer film at negative potentials in the solution coupled with electrochemical electron transfer steps very well (Tables 2 and 3). With the addition of extra protons into the solution, the conductivity of the polymer film may be increased in the depletion layer and the resistance becomes smaller. This result corresponds to the decrease of the R1 values in Table 2, compared to the values in Table 1. In Table 2, one can find high charge transfer resistance values for R2. The formation of molecular hydrogen (Had/Had 0/H2) seems to be hindered. Consequently, the reaction rate of hydrogen evolution is low. Measurements on blank platinum in the same system show that the reaction rate of the hydrogen formation is ca. two decades larger. As can be seen in Table 3, the values for R1 and R2 are decreased significantly by the introduction of CO2 in the electrolyte which may indicate a coupling between the
Table 2 The fitted data of the EIS measurements on a polyaniline electrode in the presence of 1.4 ml H /H2O in MeOH/LiClO4 at different potential values obtained by using equivalent circuit in Fig. 2b Potential (V vs. SCE)
/0.20 /0.25 /0.30 /0.35 /0.40 /0.45
R1 (V cm2)
78.48 90.53 102.80 115.20 138.50 183.30
(1.66) (1.42) (2.02) (2.56) (3.45) (4.13)
R2 (V cm2)
CPE1 C1 (mF cm 2)
n1
5.66 5.53 5.49 4.32 4.08 4.60
0.76 0.76 0.76 0.78 0.78 0.77
(11.65) (9.29) (12.24) (14.03) (15.54) (13.51)
Re /5 V cm2; n1, n2, correlation coefficients; ( ) deviation in percent.
(1.36) (1.09) (1.44) (1.61) (1.78) (1.56)
61 619 (12.73) 39 580 (8.27) 12 033 (2.10) 11 662 (2.66) 10 952 (2.82) 8767 (2.11)
CPE2 C2 (mF cm 2)
n2
100.50 79.18 64.58 61.07 57.73 52.24
0.76 0.78 0.79 0.76 0.72 0.68
(1.43) (1.40) (1.90) (2.27) (2.76) (2.97)
(0.43) (0.40) (0.52) (0.65) (0.81) (0.90)
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Table 3 The fitted data of the EIS measurements on a polyaniline electrode in the presence of 1.4 ml H /H2O in MeOH/LiClO4 saturated with CO2 at different potential values obtained by using equivalent circuit in Fig. 2c Potential (V vs. SCE) R1 (V cm2)
CPE1
R2 (V cm2)
C1 (mF cm 2) n1 /0.200 /0.225 /0.250 /0.275 /0.300 /0.325 /0.350 /0.375 /0.400 /0.425 /0.450
12.36 9.18 7.00 6.58 6.49 6.98 7.66 7.85 7.74 7.94 7.84
CPE2
Finite length Warburg impedance (ZFLW, n3 /0.5) a
C2 (mF cm 2)
n2
TFLW (s)
(2.15) 512.73 (9.57) 0.50 (1.70) 936.00 (5.40) 314.26 (6.10) 1.07 (1.37) 11.57 (228.87) (3.09) 116.73 (27.85) 0.62 (3.82) 179.60 (4.72) 598.47 (3.72) 0.98 (1.23) 2.48 (12.36) (1.23) 14.26 (21.27) 0.80 (2.34) 48.07 (3.24) 715.84 (3.99) 1.00 (1.19) 1.08 (3.66) (0.99) 9.91 (19.40) 0.83 (2.06) 22.99 (2.44) 830.42 (4.99) 0.97 (1.20) 0.99 (2.31) (1.12) 16.39 (20.02) 0.78 (2.23) 11.98 (2.54) 982.41 (7.48) 0.94 (1.64) 0.99 (1.71) (1.37) 24.77 (21.53) 0.75 (2.50) 7.36 (3.37) 1051.00 (12.12) 0.93 (2.53) 1.11 (1.59) (1.85) 45.36 (23.66) 0.69 (2.92) 4.18 (5.40) 1010.70 (22.27) 0.94 (4.40) 1.28 (1.55) (1.56) 43.68 (2.43) 0.69 a ( /) 2.67 (9.01) 1874.40 (43.03) 0.78 (8.85) 1.48 (1.64) (3.07) 39.27 (3.62) 0.69 a ( /) 2.32 (12.91) 555.65 (61.11) 0.80 (11.28) 1.72 (1.78) (3.14) 36.74 (3.98) 0.69 a ( /) 2.67 (13.03) 176.25 (62.39) 0.89 (9.77) 1.83 (2.14) (3.45) 33.98 (5.90) 0.69 a ( /) 1.78 (14.05) 40.68 (29.64) 1.05 (1.70) 2.33 (3.50)
RFLW (V cm2) 387.30 130.70 91.53 75.28 68.84 77.00 91.59 113.70 146.50 193.90 278.50
(116.56) (6.53) (1.95) (1.14) (0.84) (0.80) (0.80) (0.89) (0.97) (1.02) (1.92)
Re /5 V cm2; n1, n2, correlation coefficients; TFLW, time constant of the finite length Warburg impedance; RFLW, resistance of the finite length Warburg impedance. a Fixed parameter; ( ) deviation in percent.
Fig. 4. Stationary I /V plot of the CO2 reduction on a polyaniline electrode in a MeOH/LiClO4 electrolyte system with 1.4 ml H /H2O. Data have been obtained at the impedance measurements.
The EIS measurements with respect to the CO2 reduction cannot been carried out in the region positive to /0.2 V versus SCE, due to the changing conductance behaviour of the polymer film. However, one can assume that the equilibrium potential of the reaction should be in the range of /0.2 / /0.1 V versus SCE. An equilibrium potential corresponds to the linear segment of a Butler /Volmer curve i.e. that of a maximum in the charge transfer resistance R2. The R2 values increase drastically in the direction of a maximum value positive to /0.25 V versus SCE. With the
Fig. 5. Complex impedance plots on a polyaniline electrode during the reduction of CO2 at potential values varying from /0.2 to /0.45 V vs. SCE in 0.025 V steps in CO2 saturated MeOH/LiClO4 with 1.4 ml H / H2O. Frequency range from 20 kHz to 0.1 Hz. Simulated data points are indicated by solid lines.
assumption of an equilibrium potential, the overpotential of the charge transfer process can be calculated through the R2 values from Table 3. According to Eq. (9), ln R2 versus overpotential h , a Tafel plot can be obtained [26]. Detailed information about Eq. (9) can be found in Ref. [27]. ln R2 ln
RT bzF h: bzFj0 RT
(9)
z is the number of transferred electrons. The h values
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Acknowledgements F. Ko¨leli thanks the Department of Chemistry of the Universita¨t Oldenburg, for the opportunity provided to carry out the EIS measurements.
References
Fig. 6. Tafel plots to the CO2 reduction on a polyaniline electrode in the presence of 1.4 ml H /H2O in MeOH/LiClO4 saturated with CO2. E0, assumed equilibrium potential; R2, coefficient of determination.
are IR -corrected (about 9 mVmax), including both, the part of potential-dropping on the electrolyte resistance Re and the R1 from Table 3. The Tafel plots of the CO2 reduction on the polyaniline electrode are presented in Fig. 6 by using /0.2 and /0.1 V versus SCE as the equilibrium potentials. From Eq. (9), we could determine for the transfer coefficient b a value of 0.6 by using z / 1, T / 293 K, and exchange current densities j0 between 10 4 and 10 5 A cm 2.
4. Conclusion The CO2 reduction on polyaniline electrode was studied via EIS to obtain kinetic data. The thermodynamic equilibrium potential was assumed to be between /0.2 and /0.1 V versus SCE. The transfer coefficient b , has a value of 0.6 and for the exchange current density j0, a value in the range of 10 4 /10 5 A cm 2 was found in our investigation. As optimum electrolysis potential, a value between /0.35 and /0.40 V versus SCE was determined.
[1] H. Shirakawa, E.J. Louis, A.G. MacDiarmid, C. Chiang, A.J. Heeger, J. Chem. Soc., Chem. Commun. (1977) 578. [2] E.W. Paul, A.J. Ricco, M.S. Wrighton, J. Phys. Chem. 89 (1985) 1441. [3] A. Tu¨ru¨t, F. Ko¨leli, J. Appl. Phys. 81 (1992) 48. [4] F. Ko¨leli, M. Saglam, A. Tu¨ru¨t, Turk. J. Chem. 18 (1994) 22. [5] R. Aydin, F. Ko¨leli, J. Electroanal. Chem. 535 (2002) 107. [6] M.M. Musiani, Electrochim. Acta 35 (1990) 1665. [7] K. Roßberg, G. Paasch, L. Dunsch, S. Ludwig, J. Electroanal. Chem. 443 (1998) 49. [8] M. Pasquali, G. Pistoia, R. Rosati, Synth. Met. 58 (1993) 1. [9] S.H. Glarum, J.H. Marshall, J. Electrochem. Soc. 134 (1987) 142. [10] W.C. Chen, T.C. Wen, A. Gopalan, Synth. Met. 128 (2002) 179. [11] I. Rubinstein, E. Sabatani, J. Electrochem. Soc. 134 (1987) 3078. [12] P. Fiordiponti, G. Pistoia, Electrochim. Acta 34 (1989) 215. [13] C.C. Hu, C.H. Chu, J. Electroanal. Chem. 503 (2001) 105. [14] B.J. Johnson, S.M. Park, J. Electrochem. Soc. 143 (1996) 1269. [15] W.C. Chen, T.C. Wen, C.C. Hu, A. Gopalan, Electrochim. Acta 47 (2002) 1305. [16] M. Grzeszczuk, Electrochim. Acta 39 (1994) 1809. [17] Yu.B. Vassiliev, V.S. Bagotzky, O.A. Khazova, N.A. Mayorova, J. Electroanal. Chem. 189 (1985) 295. [18] F. Ko¨leli, T. Atilan, N. Palamut, Fresenius Environ. Bull. 11 (2002) 278. [19] Y. Hori, A. Murata, R. Takahashi, J. Chem. Soc., Faraday Trans. I 85 (1989) 2309. [20] K. Hara, N. Sonoyama, S. Takata, Bull. Chem. Soc. Jpn. 70 (1997) 745. [21] W.H. Koppenol, J.D. Rush, J. Phys. Chem. 91 (1987) 4429. [22] H.A. Schwarz, R.W. Dodson, J. Phys. Chem. 93 (1989) 409. [23] J. Butler, A. Henglein, Radiat. Phys. Chem. 15 (1980) 603. [24] M. Breitenkamp, A. Henglein, J. Lilie, Ber. Bunsenges Phys. Chem. 80 (1976) 973. [25] J.R. MacDonald, W.B. Johnson, Impedance Spectroscopy, Emphasizing Solid Materials and Systems, Wiley-VCH, New York, 1987. [26] C.H. Hamann, A. Hamnett, W. Vielstich, Electrochemistry, Wiley-VCH, Weinheim, 1998. [27] T. Ro¨pke, Ph.D. Thesis, Universita¨t Oldenburg, 2002.
Electrochemical impedance spectroscopic investigation of CO2 reduction on polyaniline in methanol Fatih Ko¨leli a,*, Thorsten Ro¨pke b, Carl H. Hamann b a
b
Department of Chemistry, Faculty of Arts and Sciences, Mersin University, 33342 Mersin, Turkey Angewandte Physikalische Chemie, Universita¨t Oldenburg, Postfach 2503, 26111 Oldenburg, Germany Received 20 November 2002; received in revised form 17 January 2003
Abstract The ac response of polyaniline thin films on platinum electrodes was measured at different dc potentials during the CO2 reduction in methanol/LiClO4 electrolyte with a small amount of 0.5 M H2SO4. The complex capacitance curves were simulated and the data obtained were used to calculate kinetic parameters, based on the assumption that the thermodynamic potential E0 is in the region of /0.2 / /0.1 V versus saturated calomel electrode (SCE). With E0 / /0.2 V versus SCE and b /0.6, a j0 value of ca. 10 4 A cm 2 was found for the electroreduction of CO2 on the polyaniline electrode. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Polyaniline; CO2 reduction; Had formation; HCOOH and CH3COOH formation; Electrochemical impedance spectroscopy (EIS)
1. Introduction Conducting polymers, generally in form of thin films, deposited electrochemically on metal electrodes were investigated intensively from the point of theoretical and practical views in the last two decades [1]. The reason for the interest, on the one hand, is the understanding of their specific properties, on the other hand, the application aspect of these materials for, e.g. fabrication of electronic devices, such as diodes, transistors, sensors [2 /4], and electrocatalysts [5]. Cyclic voltammetry and related techniques are the favourite tools for investigation of electroactive polymers, most probably because the same method may, when thin films are obtained by electropolymerisation on metal electrodes, provide both, electrosynthesis and characterization. Whatever an electrochemical system or method was used for investigation, the electrochemical impedance spectroscopy (EIS) generally has advantages among these methods, involving large disturbances: the studied system only is negligible far from the steady-
* Corresponding author. Tel.: /90-324-361-0045; fax: /90-324361-0046. E-mail address: [email protected] (F. Ko¨leli).
state. These advantages are important especially for electrochemical reactions, when conducting polymer films are involved, which might show perturbations in a large potential window because of their inhomogeneous structure. Furthermore, kinetic data can be determined more easily. Among others, polyaniline (PAn) is the conducting polymer film studied most intensively by EIS [6 /15] to complement and elucidate current /voltage and kinetic data. But most of these works are considering the polymer film at the anodic side of the potential scale. However, a polyaniline film modified with platinum microparticles as electrode materials was investigated by Grzeszcuk [16] via EIS at negative potentials to achieve data of various charge transfer and transport processes involved in the electrochemical response of the polymer electrode. The electroreduction of CO2 is a remarkable process with respect to two primary reasons. Firstly, CO2 is the ultimate by-product of all processes involving oxidation of carbon compounds and its increasing presence in the atmosphere. Secondly, in view of the vastness of its supply, CO2 represents a possible potential source for Cfeedstocks for the manufacture of chemicals. For the reduction of CO2, most of the metal electrodes were used [17,18] in different electrolytes and various electro-
0013-4686/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0013-4686(03)00076-8
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chemical cells without any convenient result in respect to a technical application. All metal electrodes need highly negative potentials even lower than /1.7 V versus saturated calomel electrode (SCE) for the reduction of CO2 in both, protic and aprotic electrolytes. In the present study, we focussed our interest to investigate a polyaniline film at negative potentials in the MeOH/LiClO4 solution saturated with CO2 in presence of a small amount of H2O/H2SO4 via EIS, in order to achieve the kinetic data and to find out the optimum electrolysis potential of the electroreduction of CO2 in MeOH/LiClO4.
2. Experimental For the synthesis of polyaniline, stock solution of 0.1 M freshly distilled aniline (Fluka) in 0.5 M H2SO4 was prepared. The film used for the investigation was synthesized between potentials of /0.2 and /0.8 V versus SCE on a Pt-plate (surface area of 1 cm2) with a thickness of ca. 0.6 mm potentiostatically (BANK POS 73) in a three electrode H-cell. The counter electrode was a Pt-plate (6 cm2) and as reference electrode a SCE was used throughout the measurements. The scan rate of CV was kept at 50 mV s 1 for both growth and characterization of the polymer film. In order to remove the surplus ions in the polymer film, the electrode was washed carefully with MeOH and transferred into the three electrode H-cell (cell volume of 100 cm3, cathode compartment 50 cm3) used for the impedance measurements (Solartron FRA 1255, Solartron 1286 ECI). The EIS measurements were carried out at 20 8C at different potentials in a frequency range from 20 kHz to 0.1 Hz in MeOH/0.25 M LiClO4. As proton source, 1.4 ml of 0.5 M H2SO4 was added into the cathode compartment to determine the influence of the acid concentration to the electroreduction of CO2 on a polyaniline electrode. Prior to the EIS measurements, the electrolyte was saturated with CO2. Some of the electrical circuit models proposed earlier [11,13,15,16] were used to simulate the impedance spectra (software ZView 2.2, delivered by Scribner Associates Inc., USA). For the calculation of the kinetic data, the fitting results of the Nyquist curves were used.
3. Results and discussion In a previous study [5], the experimental results of the CO2 reduction on a polyaniline film under ambient condition and under high pressure in MeOH/LiClO4/ H /H2O solution were presented. As reported in [5], polyaniline is an excellent material for the electroreduction of CO2. Formaldehyde, formic acid and acetic acid were obtained as reduction products with high current
efficiencies. The preparative electrolysis was carried out at a potential of /0.4 V versus SCE in MeOH/LiClO4/ H /H2O electrolyte. 1.4 ml (0.01 mol l 1) of 0.5 M H2SO4 (optimum concentration under electrolysis conditions) is necessary as proton donator for the reduction of CO2 in this electrolyte system. Also, the film thickness is an important parameter for the current density and optimum film thickness was found to be ca. 0.6 mm. If the film thickness during the electrolysis is larger than 0.6 mm, a crumbling of the film is observed due to H2 evolution and if the film is thinner, the current density decreases drastically, while active electrode surface is getting smaller. Various mechanisms were proposed for CO2 reduction in the literature [17,19,20]. In all these works, + establishing a radical anionic species (COO ) was formulated as the initial step. Furthermore, Vassiliev et al. [17] have shown that the generation of these species is the rate-determining step in aprotic media. It is also well known [21 /24] that the reduction potential for the + CO2 =CO2 system lies at ca. /1.9 V versus NHE on metal electrodes in both protic and aprotic media. No reaction occurs at /0.4 V versus SCE, when a bare platinum is used as the electrode material under these conditions. The value of /0.4 V versus SCE mentioned as electrolysis potential is much more positive than the electrolysis potentials necessary at metal electrodes [18]. This leads to the conclusion, that on PAn electrode the + formation of a CO2 radical anion may not be the initial step, due to the energetical reasons. Thus, the following new reaction mechanism was proposed in Ref. [5]. H e 0 Had
(1)
CO2 Had 0 HCOOad
(2)
HCOOad Had 0 HCOOH
(3 )
HCOOad CH3 OH 0 CH3 COOH OHad
(4 )
OHad Had 0 H2 O
(5)
*Competitive reactions. The experimental results in Ref. [5] indicate that for the reduction, the adsorbtion of CO2 on polyaniline via hydrogen bonds is essential, and that the reaction starts with the formation of Had atoms on the supporting metal electrode. When Had is generated, it adds to CO2 molecules, adsorbed on the polymer film, and the species HCOOad is formed, which can recombine with further Had to give formic acid with faradaic efficiencies of ca. 13% under electrolysis conditions [5]. However, when the species HCOOad is formed, it seems that the attack of a solvent molecule (CH3OH) is easier and, as a result, acetic acid is generated with faradaic efficiencies of ca. 56% [5]. Compared to the long time electrolysis, the formation of formaldehyde can be neglected during the relatively short EIS measurements. The formation of
F. Ko¨leli et al. / Electrochimica Acta 48 (2003) 1595 /1601
formaldehyde can be explained as the consequence of the reduction of formic acid [5]. Now, we have carried out EIS measurements, systematically, in order to examine of the general cathodic impedance characteristics of a bulk PAn film, in presence of H/H2O, and in presence of H /H2O/CO2 in the electrolyte on the one hand and to achieve kinetic data on the other, respectively. The Nyquist curves of the PAn electrode recorded at /0.3 V versus SCE are presented in Fig. 1. In Fig. 1, it can be seen that all three impedance curves, A, B, and C, show a semi-circle in the highfrequency region. The shape of the curves changes significantly by addition of H /H2O (curve B) and H /H2O/CO2 (inset diagram: curve C) in the solution. In the basic electrolyte in the absence of H/H2O and CO2, a semi-circle was achieved in the frequency range from 20 kHz to 917 Hz (Fig. 1, curve A). For the characterization, the equivalent circuit in Fig. 2a was used in analogy to [13]. The Re value describes the electrolyte resistance and has a value of ca. 5 V. R1 may be the Pt/polymer depletion layer resistance. Due to the high conductivity of the film at positive potentials, e.g. /0.2 V versus SCE, the R1 has a small value. However, in the direction to negative potentials, the R1 values increase due to the decreasing conductivity of the polymer film (Table 1). These are expected results for the general behaviour of the conducting polymer films at negative potentials. Without extra protons in the electrolyte, the polymer
film is electrochemically inactive in the cathodic region. However, if the electrolyte is sufficiently acidic, then the polymer film in contact to the platinum surface may serve as a conducting material for electron transport. As can be seen in Table 1, R1, the constant phase element CPE1, infinite length Warburg impedance ZILW, and n1 values are presented, where n1 is the correlation coefficient for the constant phase element CPE1. n2 has a fixed value of 0.5, which corresponds to the definition of a ZILW. The CPE1 with C1 defines the Pt/polymer depletion layer capacity. This capacitance is a function of the applied potential, because the width of the depletion region does also vary with the applied potential. If the applied potential is increased, the depletion region extends and the capacity decreases (Table 1 /0.30 / /0.45 V vs. SCE). In Table 1, the n1 values decrease by more negative potentials. This can be explained through the diffusion capacity of the depletion layer; the diffusion character becomes dominant while the capacitive character of the film decreases. The impedance of a constant phase element ZCPE is defined by two values, C and n. With Eq. (6) the impedance ZCPE can be calculated. ZCPE
1 n
C(iv)
pffiffiffiffiffiffi (i 1):
(6)
If n /1, then the equation is identical to that of a capacitor. C is determined as real capacitance. Usually, in a model, a CPE is used instead of a capacitor to describe the non-homogeneities in the system, e.g. a rough or porous surface can cause a double-layer capacity appearing as a constant phase element with a n value smaller than 1 [25]. The CPE1 in the circuit 2a has a capacity of ca. 4.2 mF cm 2 at /0.2 V versus SCE. This value grows up to ca. 4.7 mF cm 2 at /0.3 V versus SCE and then decreases in the negative potential direction gradually due to the decreasing of the conductivity of the film. A CPE with a n value of 0.5 can also be used to produce an infinite length Warburg impedance, ZILW. A Warburg element describes the diffusion of the charge carrier in a material. Lower frequencies in an impedance plot correspond to diffusion deep into the material. Generally, if a material is thick enough and the lowest applied frequency is not fully penetrated the inner phase of the material, the Warburg impedance can be interpreted as infinite. In that case, the ILW impedance can be explained through Eq. (7) [25]. ZILW
Fig. 1. Complex impedance plots of a polyaniline electrode at /0.3 V vs. SCE in MeOH/LiClO4 solution. From 20 kHz to 0.1 Hz: (A) PAn in MeOH/LiClO4; (B) PAn in MeOH/LiClO4/1.4 ml 0.5 M H2SO4; (C) PAn in CO2 saturated MeOH/LiClO4/1.4 ml H /H2O. The solid lines represent the best fitting results according to the equivalent circuits. Inset diagram: all three curves are zoomed.
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RILW tanh(iTILW v)n (iTILW v)n
(7)
Typically, the ILW impedance leads to a 458 slope in the Nyquist diagram (Fig. 1A). The ZILW includes the transport phenomena in the solution in the presence of trace water (the autoprotolysis is mainly performed by
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Fig. 2. Equivalent circuits for the polyaniline film (a), PAn film with H /H2O in the electrolyte (b), and PAn with addition of H /H2O and CO2 saturated electrolyte (c). Re, electrolyte resistance; R1, resistance of the Pt/polymer depletion layer; R2, charge transfer resistance; ZILW, infinite length Warburg impedance; ZFLW, finite length Warburg impedance; CPE1, constant phase element of the depletion layer capacitance (contact capacitance); CPE2, constant phase element of the double layer capacitance.
proton transfer from the trace of water to the organic solvent). The depletion layer resistance increases rapidly at more negative potentials and this growth cannot be influenced by the little concentration of H ions in the solution. However, the values of TILW and RILW in Table 1 are quite independent on the applied potential. With addition of 1.4 ml of H /H2O in the basic electrolyte, the shape of the Nyquist curve (Fig. 1B) changes significantly due to the H2 evolution, and this change is characterized by using the electrical circuit in Fig. 2b. In that case, a second RCPE circuit with the charge transfer resistance R2 and CPE2 was added to the elements in the equivalent circuit in Fig. 2a instead of the ZFLW (the diffusion of H in the solution is neglected), considering the forming of Had atoms. Those results, achieved by our EIS measurements, correspond to the findings in Ref. [16], as well. The Nyquist diagram of polyaniline in presence of 1.4 ml H /H2O in MeOH/LiClO4 saturated with CO2 is presented in Fig. 3. For comparison, the Bode plot is given as well. An additional semi-circle (II) at frequencies between 1174 and 5 Hz is observable only when the electrolyte is saturated with CO2 (Fig. 3). This may correspond to the charge transfer resistance of the CO2 reduction. A twostep reaction process is the reason for the occurrence of
Fig. 3. Nyquist plot of CO2 reduction on a polyaniline electrode at / 0.3 V vs. SCE in the frequency range of 20 kHz /0.1 Hz in CO2 saturated MeOH/LiClO4/H /H2O; inset diagram: Bode plot. Solid lines indicate the simulated data with the fitted parameters in the frequency range of 60 kHz /0.1 Hz.
Table 1 The best-fit values of the equivalent circuit in Fig. 2a for a polyaniline electrode at different potentials without H /H2O and CO2 Potential (V vs. SCE)
/0.20 /0.25 /0.30 /0.35 /0.40 /0.45
R1 (V cm2)
82.71 149.60 285.10 655.00 1014.00 1322.00
(8.33) (2.74) (2.07) (1.46) (1.48) (1.77)
Infinite length Warburg impedance (ZILW, n2 /0.5) a
CPE1 C1 (mF cm 2)
n1
4.24 4.46 4.68 4.14 4.29 3.60
0.84 0.82 0.80 0.79 0.77 0.77
(26.22) (8.21) (6.43) (4.64) (4.55) (6.11)
(2.49) (0.90) (0.74) (0.59) (0.67) (0.84)
TILW (s)
RILW (V cm2)
5.95 13.01 11.68 9.25 8.11 6.86
9276 (8.63) 14 875 (10.14) 13 293 (10.47) 10 691 (8.59) 7677 (12.28) 6085 (14.89)
(20.28) (20.34) (21.41) (18.04) (26.20) (32.18)
Re /5 V cm2; n1, correlation coefficient; TILW, time constant of the infinite length Warburg impedance; RILW, resistance of the infinite length Warburg impedance. a Fixed parameter; ( ) deviation in percent.
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semi-circle (II), i.e. an intermediate state is involved. The first semi-circle (I) at high frequencies may describe the metal/polymer depletion layer. The third semi-circle (III) may represent a finite length Warburg impedance ZFLW, with the time constant TFLW and real part RFLW, describing the transport phenomena of CO2 into the polymer film and the transport of reduction products out of the film. If the material (polymer film) is thin, the low frequencies can also penetrate into the inner bulk phase completely, and create a finite length Warburg element. In that case, the impedance ZFLW can be calculated by using the following equation [25]: ZFLW
RFLW coth(iTFLW v)n : (iTFLW v)n
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reactions. The depletion layer resistance is about two decades lower than the values in Table 2. In other words, a further activity of the polymer film is given through the adsorption of CO2. In this case, the formation of molecular hydrogen is not preferred, and the formation of HCOOad is easier (Eq. (2)). Thus, the reaction obeys an EC mechanism where CO2 reacts chemically with adsorbed hydrogen. In Table 3, the time constant TFLW becomes greater at potentials from / 0.275 to /0.450 V versus SCE, which indicates diffusion of CO2 at these potentials deeper into the film pffiffiffiffiffiffiffiffiffiffiffi ( TFLW L): The current densities increase while the R2 values decrease. However, a limitation of the current densities with values of ca. /0.9 mA cm 2 occurs at more negative potentials to ca. /0.4 V versus SCE. This is also verified with a stationary I /V diagram, recorded during the impedance measurements. As can be seen in Fig. 4, the optimum electrolysis potential is found to be /0.4 V versus SCE. In Nyquist plots, the real part of the Warburg impedances RFLW becomes gradually larger in both directions, to more negative and to more positive potential values than /0.3 V versus SCE. The Nyquist plots of the CO2 reduction obtained at different potentials are presented in Fig. 5. With respect to the CO2 reduction, no EIS measurements were carried out at potentials more positive than /0.2 V versus SCE due to the fact that at such high anodic potentials, the conductance mechanism of the polymer changes, and the reduction of CO2 does not occur. The determination of the kinetic data, such as the exchange current density j0, is only possible by the prior knowledge of the thermodynamic potential for the CO2 electroreduction on polyaniline in methanol. However, according to the formulated mechanism in [5], an electron transfer directly to CO2 molecule cannot occur due to energetic reasons. Only the formation of Had atoms as initial step leads to the reaction products, so that we can assume for our following calculations the equilibrium potential of Had formation.
(8)
Eqs. (7) and (8) can only be used with fixed n values (n /0.5). TFLW and TILW must be equal to L2/D , where L is the effective diffusion thickness, and D is the effective diffusion coefficient. The equivalent circuits (Fig. 2b and c) with the elements Re, R1, R2, CPE1, CPE2 and ZFLW are describing the behaviours of the polymer film at negative potentials in the solution coupled with electrochemical electron transfer steps very well (Tables 2 and 3). With the addition of extra protons into the solution, the conductivity of the polymer film may be increased in the depletion layer and the resistance becomes smaller. This result corresponds to the decrease of the R1 values in Table 2, compared to the values in Table 1. In Table 2, one can find high charge transfer resistance values for R2. The formation of molecular hydrogen (Had/Had 0/H2) seems to be hindered. Consequently, the reaction rate of hydrogen evolution is low. Measurements on blank platinum in the same system show that the reaction rate of the hydrogen formation is ca. two decades larger. As can be seen in Table 3, the values for R1 and R2 are decreased significantly by the introduction of CO2 in the electrolyte which may indicate a coupling between the
Table 2 The fitted data of the EIS measurements on a polyaniline electrode in the presence of 1.4 ml H /H2O in MeOH/LiClO4 at different potential values obtained by using equivalent circuit in Fig. 2b Potential (V vs. SCE)
/0.20 /0.25 /0.30 /0.35 /0.40 /0.45
R1 (V cm2)
78.48 90.53 102.80 115.20 138.50 183.30
(1.66) (1.42) (2.02) (2.56) (3.45) (4.13)
R2 (V cm2)
CPE1 C1 (mF cm 2)
n1
5.66 5.53 5.49 4.32 4.08 4.60
0.76 0.76 0.76 0.78 0.78 0.77
(11.65) (9.29) (12.24) (14.03) (15.54) (13.51)
Re /5 V cm2; n1, n2, correlation coefficients; ( ) deviation in percent.
(1.36) (1.09) (1.44) (1.61) (1.78) (1.56)
61 619 (12.73) 39 580 (8.27) 12 033 (2.10) 11 662 (2.66) 10 952 (2.82) 8767 (2.11)
CPE2 C2 (mF cm 2)
n2
100.50 79.18 64.58 61.07 57.73 52.24
0.76 0.78 0.79 0.76 0.72 0.68
(1.43) (1.40) (1.90) (2.27) (2.76) (2.97)
(0.43) (0.40) (0.52) (0.65) (0.81) (0.90)
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Table 3 The fitted data of the EIS measurements on a polyaniline electrode in the presence of 1.4 ml H /H2O in MeOH/LiClO4 saturated with CO2 at different potential values obtained by using equivalent circuit in Fig. 2c Potential (V vs. SCE) R1 (V cm2)
CPE1
R2 (V cm2)
C1 (mF cm 2) n1 /0.200 /0.225 /0.250 /0.275 /0.300 /0.325 /0.350 /0.375 /0.400 /0.425 /0.450
12.36 9.18 7.00 6.58 6.49 6.98 7.66 7.85 7.74 7.94 7.84
CPE2
Finite length Warburg impedance (ZFLW, n3 /0.5) a
C2 (mF cm 2)
n2
TFLW (s)
(2.15) 512.73 (9.57) 0.50 (1.70) 936.00 (5.40) 314.26 (6.10) 1.07 (1.37) 11.57 (228.87) (3.09) 116.73 (27.85) 0.62 (3.82) 179.60 (4.72) 598.47 (3.72) 0.98 (1.23) 2.48 (12.36) (1.23) 14.26 (21.27) 0.80 (2.34) 48.07 (3.24) 715.84 (3.99) 1.00 (1.19) 1.08 (3.66) (0.99) 9.91 (19.40) 0.83 (2.06) 22.99 (2.44) 830.42 (4.99) 0.97 (1.20) 0.99 (2.31) (1.12) 16.39 (20.02) 0.78 (2.23) 11.98 (2.54) 982.41 (7.48) 0.94 (1.64) 0.99 (1.71) (1.37) 24.77 (21.53) 0.75 (2.50) 7.36 (3.37) 1051.00 (12.12) 0.93 (2.53) 1.11 (1.59) (1.85) 45.36 (23.66) 0.69 (2.92) 4.18 (5.40) 1010.70 (22.27) 0.94 (4.40) 1.28 (1.55) (1.56) 43.68 (2.43) 0.69 a ( /) 2.67 (9.01) 1874.40 (43.03) 0.78 (8.85) 1.48 (1.64) (3.07) 39.27 (3.62) 0.69 a ( /) 2.32 (12.91) 555.65 (61.11) 0.80 (11.28) 1.72 (1.78) (3.14) 36.74 (3.98) 0.69 a ( /) 2.67 (13.03) 176.25 (62.39) 0.89 (9.77) 1.83 (2.14) (3.45) 33.98 (5.90) 0.69 a ( /) 1.78 (14.05) 40.68 (29.64) 1.05 (1.70) 2.33 (3.50)
RFLW (V cm2) 387.30 130.70 91.53 75.28 68.84 77.00 91.59 113.70 146.50 193.90 278.50
(116.56) (6.53) (1.95) (1.14) (0.84) (0.80) (0.80) (0.89) (0.97) (1.02) (1.92)
Re /5 V cm2; n1, n2, correlation coefficients; TFLW, time constant of the finite length Warburg impedance; RFLW, resistance of the finite length Warburg impedance. a Fixed parameter; ( ) deviation in percent.
Fig. 4. Stationary I /V plot of the CO2 reduction on a polyaniline electrode in a MeOH/LiClO4 electrolyte system with 1.4 ml H /H2O. Data have been obtained at the impedance measurements.
The EIS measurements with respect to the CO2 reduction cannot been carried out in the region positive to /0.2 V versus SCE, due to the changing conductance behaviour of the polymer film. However, one can assume that the equilibrium potential of the reaction should be in the range of /0.2 / /0.1 V versus SCE. An equilibrium potential corresponds to the linear segment of a Butler /Volmer curve i.e. that of a maximum in the charge transfer resistance R2. The R2 values increase drastically in the direction of a maximum value positive to /0.25 V versus SCE. With the
Fig. 5. Complex impedance plots on a polyaniline electrode during the reduction of CO2 at potential values varying from /0.2 to /0.45 V vs. SCE in 0.025 V steps in CO2 saturated MeOH/LiClO4 with 1.4 ml H / H2O. Frequency range from 20 kHz to 0.1 Hz. Simulated data points are indicated by solid lines.
assumption of an equilibrium potential, the overpotential of the charge transfer process can be calculated through the R2 values from Table 3. According to Eq. (9), ln R2 versus overpotential h , a Tafel plot can be obtained [26]. Detailed information about Eq. (9) can be found in Ref. [27]. ln R2 ln
RT bzF h: bzFj0 RT
(9)
z is the number of transferred electrons. The h values
F. Ko¨leli et al. / Electrochimica Acta 48 (2003) 1595 /1601
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Acknowledgements F. Ko¨leli thanks the Department of Chemistry of the Universita¨t Oldenburg, for the opportunity provided to carry out the EIS measurements.
References
Fig. 6. Tafel plots to the CO2 reduction on a polyaniline electrode in the presence of 1.4 ml H /H2O in MeOH/LiClO4 saturated with CO2. E0, assumed equilibrium potential; R2, coefficient of determination.
are IR -corrected (about 9 mVmax), including both, the part of potential-dropping on the electrolyte resistance Re and the R1 from Table 3. The Tafel plots of the CO2 reduction on the polyaniline electrode are presented in Fig. 6 by using /0.2 and /0.1 V versus SCE as the equilibrium potentials. From Eq. (9), we could determine for the transfer coefficient b a value of 0.6 by using z / 1, T / 293 K, and exchange current densities j0 between 10 4 and 10 5 A cm 2.
4. Conclusion The CO2 reduction on polyaniline electrode was studied via EIS to obtain kinetic data. The thermodynamic equilibrium potential was assumed to be between /0.2 and /0.1 V versus SCE. The transfer coefficient b , has a value of 0.6 and for the exchange current density j0, a value in the range of 10 4 /10 5 A cm 2 was found in our investigation. As optimum electrolysis potential, a value between /0.35 and /0.40 V versus SCE was determined.
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