Glassy Carbon Electrode Formation and Elec

Journal of The Electrochemical Society, 159 (6) E115-E121 (2012) 0013-4651/2012/159(6)/E115/7/$28.00 © The Electrochemical Society

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Poly(3,5-dithiophene-2-yldithieno[3,2-b;2 ,3 -d]thiophene-coEthylenedioxythiophene)/Glassy Carbon Electrode Formation and Electrochemical Impedance Spectroscopic Study Murat Ates,a,z Ipek Osken,b and Turan Ozturkb,c,z a Department

of Chemistry, Faculty of Arts and Sciences, Namik Kemal University, Degirmenalti Campus, 59030 Tekirdag, Turkey of Chemistry, Faculty of Arts and Sciences, Istanbul Technical University, Maslak, Istanbul, Turkey c TUBITAK UME, Chemistry Group Laboratories, 41470 Gebze-Kocaeli, Turkey b Department

3,5-Dithiophene-2-yldithieno[3,2-b;2 ,3 -d]thiophene (Thy2 DTT) and ethylenedioxythiophene (EDOT) were electro-copolymerized on glassy carbon electrode (GCE) in 0.1 M sodium perchlorate (NaClO4 )/acetonitrile (ACN)/dichloromethane (CH2 Cl2 ) (8:2) solution. Poly(Thy2 DTT-co-EDOT)/GCE thin film was characterized by various techniques, such as Cyclic Voltammetry (CV), Fourier Transform Infrared Spectroscopy – Attenuated Transmittance Reflectance (FTIR-ATR), Scanning Electron Microscopy Energy Dispersive X-ray analysis (SEM-EDX) and Electrochemical Impedance Spectroscopy (EIS). The effects of monomer mole fractions (mole fraction, XThy2DTT = nThy2DTT /nEDOT + nThy2DTT ) (0.5, 0.66 and 0.83) during the preparation of modified electrodes were examined by EIS. Capacitive behaviors of the modified GCE were defined via Nyquist, Bode-magnitude, Bode-phase and Capacitance plots. The circuit model was used to fit the theoretical and experimental data through Kramers-Kronig Transform test. The lowest frequency capacitance (CLF ) value was obtained as CLF = 0.89 mFcm−2 for poly(Thy2 DTT). However, the highest CLF was obtained for the copolymer as CLF = 1.11 mFcm−2 for XThy2DTT = 0.66 and 0.83. Potential application of the copolymer could be energy-storage devices. © 2012 The Electrochemical Society. [DOI: 10.1149/2.041206jes] All rights reserved. Manuscript submitted November 30, 2011; revised manuscript received February 14, 2012. Published March 29, 2012.

Since their discovery in the mid-1970s, interest in conducting polymers (CPs) has been an ever growing research area in polymer chemistry.1 They constitute an interesting class of materials, which combine some of the mechanical features of plastics with electrical properties typical of metals. The ability to tailor the structure of conjugated polymers allow the rational syntheses of conducting polymers with a broad range of electronic properties as the nature of the repeating units and the functional groups alter the electronic character of polymers.2, 3 CPs are distinguished by the presence of alternating single and double bonds between carbon atoms along the polymer chain and due to such bond conjugation, broad valence and conduction bands are generated.4 Research on conducting polymers is largely due to their wide range of possible applications, owing to their facile syntheses and good environmental and long term electrical conductivity stabilities.5 Besides, conducting polymer films or coatings are widely exploited due to their special features, such as low densities, high strength, ease of fabrication, design flexibility, stability and low cost.6 As a versatile class of conjugated polymers, polythiophenes are of current interest, owing to the easy modification of their starting monomers, which provide the control of the properties of the resulting polymers, such as electronic, optical, conductivity, etc..7 They have the potential of being used in a variety of applications, such as light emitting diodes,8 electrochromic devices,9, 10 biosensors,11, 12 field effect transistors,13, 14 solar cells,15, 16 electrochromic displays17 and electrochemical capacitors.18, 19 Synthesis of new polythiophene derivatives with improved electronic and optical properties is an exciting part of conducting polymer research.20 As an important building block of polythiophenes, ethylenedioxythiophene (Fig. 1a) constitutes one of the most attractive monomer of conjugated conducting polymers. Poly(ethylenedioxythiophene) (PEDOT) has interesting properties, such as low half-wave potential, small bandgap, high conductivity, high thermal and electrochemical stabilities and it forms highly stable pale blue transparent films upon doping, while it is a dark blue material in its undoped state.21–24 Such interesting properties have been explained to be due to the strong electron-donating ethylene-

z

E-mail: [email protected]; [email protected]

dioxy bridge situated between carbons 3- and 4- of the thiophene ring.25 Electrochemical polymerization is one of the efficient methods to prepare PEDOT through electro-oxidation of its monomer, which yields a film on the anode surface.26–28 Additionally, copolymerization of EDOT with various monomers is widespread to alter the physical and chemical properties of the final material.29–36 Electrochemical polymerization has the advantage of obtaining conductive polymer film on a suitable surface, which does not only enlarge the scope of candidate polymers, but also omits the procedure of film coating.37, 38 Moreover, it can effectively modify the structures and properties of the final materials, such as electrochemical activity, thermal stability and electrochromic properties.39 Despite a great deal of work focused on fundamental research on conventional CPs, design and syntheses of novel CPs with unique properties are still considered as necessary and significant task for widespread applications.40 Generally, the properties of CPs can be tuned by adjusting the electronic characteristics of the p-orbital along the neutral polymer backbone, including main-chain and pendant-group structural modifications and copolymerization. As a general synthetic design, copolymerization is a promising strategy to obtain materials with better properties than those of the respective homopolymers.41–43 Preparation of copolymers with different monomer concentrations have recently been examined using CFMEs.44 Dithienothiophenes (Fig. 1b) (DTT) and their polymers are among the materials exhibiting promising properties applicable to electronics and optoelectronics such as photovoltaic devices, energy storage, organic semiconductors, organic field effect-transistors and electrochromic devices.45 As they possess three sulfur atoms, they are rich in sulfur which makes them good electron donors. In this study, 3,5-dithiophene-2-yldithieno[3,2-b;2 ,3 -d]thiophene (Thy2 DTT) (Fig. 1c)/ethylenedioxythiophene (EDOT) in different mole fractions were electro-copolymerized on CFME, which correspond to different film thickness and features. Polymer modified CFMEs were characterized via FTIR-ATR and evaluation of capacitor performance was conducted by electrochemical impedance spectroscopic (EIS). Additionally, surface morphological analyzes were achieved via scanning electron microscopy-Energy Dispersive X-ray analysis (SEM-EDX). The circuit model of Rs (Q1 (Rct (Q2 R2 (Q3 (R3 W)))) was used to the simulation of experimental data.

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Journal of The Electrochemical Society, 159 (6) E115-E121 (2012) where diffusion is represented by a Warburg element, with a frequency dispersion obtained from the solution of the one-dimensional Fick’s second law for diffusion in a finite region with a blocking boundary for ions at the GCE/polymer interface.50 The best fitting value of all the parameters for the Rs (Q1 (Rct (Q2 R2 (Q3 (R3 W)))) circuit elements are given in Table II. The mean error of modulus was determined to be less than 15% and χ2 ≤ 10−4 .

Figure 1. Structures of a) ethylenedioxythiophene b) Dithienothiophenes and c) 3,5-dithiophene-2-yldithieno[3,2-b;2 ,3 -d]thiophene were given.

Experimental Materials.— 3,4-Ethylenedioxythiophene (EDOT) was purchased from Sigma-Aldrich. Sodium perchlorate (>98%), dichloromethane (> 98) and acetonitrile were purchased from Merck. In all experiments, 0.1 M NaClO4 in acetonitrile (ACN)/Dichloromethane (CH2 Cl2 ) (8:2) was used as electrolyte. All chemicals were high grade reagents and were used as received. Instrumentation.— A PARSTAT 2273 model of potansiostat (software: PowerSuite and Faraday cage: BAS Cell Stand C3 ) was used for electrochemical studies, which was equipped with a three-electrode electrochemical cell, employing CFME as a working electrode, platinum disk as a counter electrode, encapsulated in epoxy resin (geometric area: 0.020 cm2 ) and a saturated calomel electrode (SCE) as a reference electrode. All potentials are presented on the SCE scale. Scanning electron microscopic (SEM) and Energy Dispersive Xray analysis (EDX) measurements were performed on Carl Zeiss Leo 1430 VP. Average values of the increase in thickness were obtained from SEM images taking into account the diameter of the uncoated CFME. Preparation of the carbon fiber microelectrodes.— High strength (HS) carbon fibers C 320.000 (CA) (Sigri Carbon, Meitingen, Germany) containing 320.000 single filaments in roving and high modulus (HM) carbon fibers were used as working electrodes. All of the electrodes were prepared using a 3 cm long bundle of CFME (with average diameter of 7 μm) attached to a copper wire with Teflon tape. The number of carbon fibers in the bundle was about 20. One centimeter of CFME was dipped into 0.1 M NaClO4 /(ACN/CH2 Cl2 ) (8:2) and monomer solution to keep the electrode area constant (∼0.044 cm2 ) and the rest of the electrode was covered with Teflon tape. The CFMEs were first cleaned with acetone and then dried with an air-dryer prior to the experiments. EIS and modeling.— The electrochemical impedance spectroscopy (EIS) measurements were recorded at room temperature (25◦ C ± 1) using a conventional three electrode cell configuration. The electrochemical parameters of the poly(Thy2 DTTco-EDOT)/GCE was evaluated using ZSimpWin (Version 3.10) software from Princeton Applied Research. EIS measurements were conducted in monomer-free electrolyte solutions with a perturbation amplitude 10 mV (rms) over a frequency range of 10 mHz to 100 kHz on PARSTAT 2273. DC voltage was applied as 0 for all experiments. The results were interpreted on equivalent circuit model of Rs (Q1 (Rct (Q2 R2 (Q3 (R3 W)))). Electrochemical impedance spectroscopy (EIS) was used as a sensitive technique for biosensors46 and (super) capacitors.47 For the fitting the data obtained from EIS, ZSimpWin program was applied, having equivalent circuit48 as a model for describing the electrolyte/GCE interphases among 90 circuit models. The Rs (Q1 (Rct (Q2 R2 (Q3 (R3 W)))) circuit model was built using a series of components, where Rs is the solution resistance of the modified polymer electrode and electrolyte (NaClO4 ) system. Rct is the charge transfer resistance in between polymer film electrode and electrolyte interface, which is the parallel combination of constant phase element (Q1 ). Rct and Cdl values are given as capacitive and resistive features of electrode/electrolyte interphases of an analyte.49 R2 has a parallel connection to constant phase element (Q2 ). Warburg impedance (W),

Synthesis procedure.— Synthesis of 3,5-dithiophene-2yldithieno[3,2-b;2 ,3 -d]thiophene was achieved applying 1,8diketone ring formation reaction,51, 52 details of which will be published elsewhere. 3,5-dithiophene-2-yldithieno[3,2-b;2 ,3 -d]thiophene: 1 H-NMR (600 MHz, CDCl3 ) δ 7.47 (s, 2H), 7.45 (dd, J = 3.9 Hz, J = 1.1 Hz, 2H), 7.32 (dd, J = 4.9 Hz, J = 1.1 Hz, 2H), 7.15 (dd, J = 4.9 Hz, J = 3.9 Hz, 2H); 13 C-NMR (150 MHz, CDCl3 ) δ 142.1, 133.9, 132.0, 130.5, 127.3, 126.8, 123.2, 123.2; EIMS (m/z) M+ +1 161. Results and Discussion Electrocopolymerization process and polymer film formation test.— The cyclic voltammograms (CV), obtained during polymer film growth on GCE, are presented in Figs. 2a–2e. While the CVs of poly(Thy2 DTT)/GCE, (Fig. 2a) and poly(EDOT)/GCE (Fig. 2b), obtained with a scan rate of 100 mVs−1 , showed broad and irreversible redox waves at E = 0.55 V and E = 0.61 V, respectively, poly(Thy2 DTT-co-EDOT)/GCE displayed an irreversible behaviors for XEDOT = 0.5 (E = 1.25 V) (Fig. 2c), XEDOT = 0.66 (E = 1.75 V) (Fig. 2d), and reversible behavior for XEDOT = 0.83 (E = 0.07 V) (Fig. 2e). Roncali et al. reported that poly(EDOT) had an irreversible redox wave at E = 0.80 V in a monomer concentration of 1 M in 0.1 M Bu4 NPF6 /MeCN at a scan rate of 100 mVs−1 .53 While the oxidation peak potentials of Thy2 DTT and EDOT were obtained at −0.13 and 0.24 V, respectively, the copolymers, XEDOT = 0.5, XEDOT = 0.66 and XEDOT = 0.83 displayed oxidation potentials at 0.48, 0.96, and 0.99 V, respectively. The anodic peak potentials of the copolymers inreased from 0.48 to 0.99 V with the increase of EDOT monomer concentration. Same trend was also observed for reduction processes, such as for Thy2 DTT at −0.68 V and for EDOT at −0.37 V. The cathodic peak potential for copolymer increases from −0.77 V for XThy2DTT = 0.5 to 1.06 V for XThy2DTT = 0.83. During electrogrowth process, the total charge was obtained as Q = 3.60 mC for Thy2DTT and Q = 61.72 mC for EDOT. Reflection of this trend was observed with the increase of total charge of the copolymers in parallel to the increase of their EDOT content, such as for XEDOT = 0.5, XEDOT = 0.66 and XEDOT = 0.83, the total charges are Q = 5.48 mC, Q = 8.88 mC and Q = 20.59 mC, respectively. Electro-growth of Thy2 DTT with EDOT in different mole fractions, showed different redox behaviors. Appearance of well defined redox peaks during cyclic voltammetry; having increasing peaks currents from cycle to cycle clearly demonstrated the formation of an electro-active film on the electrode surface. The poly(Thy2 DTT-coEDOT)/GCE was prepared in a mole fraction of XThy2DTT = 0.5 and the thin film was characterized by CV in 0.1M NaClO4 /acetonitrile (ACN)/Dichloromethane (CH2 Cl2 ) (8:2) monomer free solution (Fig. 3). The film was inserted into monomer-free electrolyte solution and its redox behavior was investigated, which had oxidation (∼ −0.089 V) and reduction (∼ −0.098 V) peaks. The scan rate dependence of the electro-active film current density was investigated only on the first reversible system (∼ −0.089 V). The peak current density (ip ) for a reversible voltammogram at 25◦ C is given by the following equation: ip = (2.69×105 )xAxD1/2 xC0 xν1/2 , where ν is the scan rate, A is the electrode area, D is the diffusion coefficient of electroactive species in solution.54 The scan rate dependence of the anodic and cathodic peak current densities shows a linear dependence on scan rates from 25 mV s−1 to 1000 mV s−1 (RAnot = 0.99608, and RCat. = −0.99860) for poly(Thy2 DTT-co-EDOT), XEDOT = 0.5/GCE against ν (Fig. 3b), where thin layer behavior applies. Peak current


Journal of The Electrochemical Society, 159 (6) E115-E121 (2012)

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Figure 2. Cyclic voltammograms for electro-growth of a) Thy2 DTT (Q = 3.60 mC), b) EDOT (Q = 61.72 mC), c) Thy2 DTT with PEDOT, XThy2DTT = 0.5 (Q = 5.48 mC), d) XThy2DTT = 0.66 (Q = 8.88 mC), XThy2DTT = 0.83 (Q = 20.59 mC). Q is obtained as the deposition charge after electropolymerization process. Electropolymerization was performed on GCE by 8 cycles in scan rate of 100 mV s−1 in 0.1 M NaClO4 /(ACN:CH2 Cl2 ) (8:2), [Thy2 DTT]0 = 10 mM and [PEDOT]0 = 10 mM.

density was also determined to be proportional with ν1/2 in the range of square root of scan rates from 5 mV s−1 to 31.6 mV s−1 (Regression fit (RAnot ) = 0.99235, and Rcat. = −0.98676) where diffusion control applies55 (Fig. 3c). This demonstrates that the electrochemical process has both diffusion controlled and thin layer behavior processes.56

SEM-EDX characterization.— The morphological features of the electrocoated carbon fiber micro electrodes were determined via SEM-EDX. The fibers were attached on copper plate, using a double sided carbon tape. The high-resolution images obtained by SEM-EDX analysis of the electro-coated CFMEs. Electropolymerization of different monomers and combination of these two monomers in different

Figure 3. (a) CV of poly(Thy2 DTT-co-PEDOT) in monomer-free formed by 8 cycles in different scan rates from 25 mV s−1 to 1000 mV s−1 on GCE in 0.1 M NaClO4 /(ACN:CH2 Cl2 ) (8:2), [XThy2DTT ] = 0.5. b) Change in anodic and cathodic peak current densities with scan rate, c) Square root of scan rate of Poly(Thy2 DTT-co-EDOT)/GCE film was coated with different scan rates (25, 50, 100, 250, 500 and 1000 mV s−1 ) in monomer free solution. Downloaded on 2014-10-06 to IP 169.232.141.232 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

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Figure 4. SEM pictures of a) uncoated CFME, b) poly(Thy2 DTT), c) poly(EDOT) and copolymer structures with various mole fractions d) XThy2DTT = 0.5, e) XThy2DTT = 0.66, f) XThy2DTT = 0.83. Polymer films were obtained at a scan rates of 100 mVs−1 for 8 cycles in 0.1 M NaClO4 /ACN:CH2 Cl2 (8:2).

mole fractions produced grainy structures particularly for XThy2DTT = 0.66. SEM images of uncoated CFME (Fig. 4a), poly(Thy2 DTT) (Fig. 4b), poly(EDOT) (Fig. 4c) and Poly(Thy2 DTT-coEDOT)/CFME with mole fractions of XThy2DTT = 0.5 (Fig. 4d), XThy2DTT = 0.66 (Fig. 4e) and XThy2DTT = 0.83 (Fig. 4f) are given. Energy-Dispersive X-ray (EDX) images indicated the formation of the copolymer (Figs. 5a–5d, Table I). EDX data from point analysis experiments (Table I) for uncoated CFME (Fig. 5a), poly(Thy2 DTT)

(Fig. 5b), poly(EDOT) (Fig. 5c) and copolymer of XThy2DTT = 0.5, 0.66, 0.83 (Figs. 5d–5f) in 0.1 M NaClO4 /(ACN/CH2 Cl2 ) (8:2) on CFME showed that Thy2 DTT was included in the copolymer structure. This was evidenced by the existence of sulfur in a ratio of 3.25%, compared to its ratio in homopolymers poly(Thy2 DTT) (5.75%) and poly(EDOT) (0.84%). The ratio of sulfur in copolymer structure as a mol fraction (XThy2DTT = 0.5) is higher than poly(EDOT). The presence of the peaks, belonging to oxygen and chlorine indicates the

Figure 5. EDX point analysis results of a) Uncoated CFME b) Poly(EDOT) c) Poly(Thy2 DTT) and copolymer structures with various mole fractions d) XThy2DTT = 0.5 e) XThy2DTT = 0.66 f) XThy2DTT = 0.83. Polymer films were obtained at a scan rate of 100 mVs−1 for 8 cycles in 0.1 M NaClO4 /ACN:CH2 Cl2 (8:2). Downloaded on 2014-10-06 to IP 169.232.141.232 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).


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Table I. EDX point analysis results of Carbon (C), Oxygen (O), Sodium (Na), Sulfur (S) and Chlorine (Cl) elements, calculated from uncoated CFME, poly(Thy2 DTT), poly(EDOT) and poly(Thy2 DTT-co-EDOT) with different mole fractions, XThy2DTT = 0.5, 0.66 and 0.83. Elements /% Polymer

C/%

O/%

Na/%

S/%

Cl/%

Uncoated CFME Poly(Thy2DTT) Poly(EDOT) Copolymer XThy2DTT = 0.5 XThy2DTT = 0.66 XThy2DTT = 0.83

76.33 70.36 64.92

23.67 22.71 33.42

– 0.54 0.37

– 5.75 0.84

– 0.64 0.45

65.69 76.34 50.23

29.28 21.59 46.30

1.41 1.42 3.47

3.25 0.63 –

0.36 0.03 –

inclusion of dopant anion (ClO4 − ) of the supporting electrolyte into the copolymer during electro-growth process.57 Highest percents of sulfur and chlorine were obtained in copolymer of XThy2DTT = 0.5. However, as the content of Thy2 DTT increased in the copolymers, such as XThy2DTT = 0.66 and XThy2DTT = 0.83, the decrease of these two elements was observed. FTIR-ATR analysis.— FTIR-ATR spectra of poly(Thy2 DTT) (Fig. 6a), poly(EDOT) (Fig. 6b) and the copolymer for XThy2DTT = 0.5 (Fig. 6c) were recorded on CFME in NaClO4 /(ACN:CH2 Cl2 )(8:2). The vibrational bands at 1077 cm−1 , 1060 cm−1 and 1049 cm−1 indicated the presence of the dopant anion (ClO4 − ).58 Characteristic peaks of C–H bond at 3580 cm−1 and 2316 cm−1 , C–S bond at 673 cm−1 and 694 cm−1 , C–C bond at 1351 cm−1 , and anti-symmetric and symmetric C=C stretching deformation peaks at 1635, 1499, and 1628 cm−1 were observed. While the presence of C–O was observed with a stretching band at 1296 cm−1 for poly(EDOT), this stretching shifted to 1307 cm−1

Figure 7. Equivalent electrical Randless circuit model of Rs (Q1 (Rct (Q2 R2 (Q3 (R3 W)))). Rs : Solution resistance, Q1 , Q2 and Q3 : Constant phase elements, Rct : Charge transfer resistance, R2 and R3 : Resistance of polymer film, W: Warburg impedance.

with the copolymer, which is an evidence of the formation of the copolymer. Such a bond was not observed for poly(Thy2 DTT) as it does not have C–O bond. Equivalent circuit analysis.— EIS measurements are in general performed by equivalent electric circuit of R(Q(R(QR(Q(RW)))), which is used for fitting the experimental data. That is, an electric circuit can be built by combing some physical elements that has an impedance spectrum identical to that of the electrochemical system under investigation.59 However, it is often difficult to find an unambiguous physical meaning for particular circuit elements. Moreover, the situation is further complicated in the case of conducting polymers by different opinions concerning the mode of transport of charge carriers within the polymer film.60 On the basis of the impedance diagrams obtained, equivalent circuit model of R(Q(R(QR(Q(RW)))) was suggested (Fig. 7). As one of the important advantages of EIS is that an independent check for the validity of impedance data is possible through the use of Kramers-Kronig (K-K) transform tests, the experimental impedance data were evaluated by the test.61 In their most popular form, the transforms given as Z and Z are the real and imaginary

Figure 6. FTIR-ATR of a) poly(Thy2 DTT)/CFME, b) poly(EDOT)/CFME and c) poly(Thy2 DTT-co-EDOT)/CFME, XThy2DTT = 0.5. Downloaded on 2014-10-06 to IP 169.232.141.232 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

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Table II. Circuit modeling, R(Q(R(QR(Q(RW)))), results of electrochemically modified on GCE with ZSimpWin simulating program. Monomers were deposited electrochemically at a scan rate of 100 mVs−1 , using multiple (8 cycle) in 0.1 M NaClO4 /ACN:CH2 Cl2 (8:2). Components

Poly(Thy2 DTT)

Poly(EDOT)

XThy2DTT = 0.5

XThy2DTT = 0.66

XThy2DTT = 0.83

Rs / Q1 /μSxs−n n1 Rct /m Q2 /μSxs−n n2 R2 /μ Q3 /μSxs−n n3 R3 /m W/μSxs−n χ2

325.3 1.71 0.67 4.8 1.25 0.97 2.42 0.042 0.84 0.21 0.31 5.12 × 10−4

137.9 0.21 0.83 4.9 0.17 0.85 0.20 6.81 0.62 4.22 0.15 6.22 × 10−4

180.6 0.89 0.80 3.5 0.17 0.78 0.30 6.44 0.83 107 0.31 5.16 × 10−4

246 1.55 0.75 0.07 1.01 1 0.085 0.19 0.63 257 0.11 3.46 × 10−4

66.5 1.31 0.66 18.5 3.94 0.53 0.35 0.71 0.99 238 0.17 6.76 × 10−4

components of the impedance. Transform equations provide imaginary axis for real axis transformations, and the real axis-to-imaginary axis transforms. Application of the Kramers-Kronig (K-K) transforms in the analysis of electrochemical impedance data is examined with reference to the conditions of stability and linearity. The K-K transforms are clearly sensitive to the intrusion of the impedance and it has been shown elsewhere that they are sensitive to drift in the system (stability).62 R(Q(R(QR(Q(RW)))) circuit model is given in Table II. Low frequency capacitance (CLF ) values can be calculated via CLF = 1/2πfz . Low frequency capacitance values were obtained for poly(EDOT); CLF = 1.05 mFcm−2 , poly(Thy2 DTT); CLF = 0.89 mFcm−2 , and for the copolymers in the mole fractions of XThy2DTT = 0.5; CLF = 1.06 mFcm−2 , XThy2DTT = 0.66; CLF = 1.11 mFcm−2 , and XThy2DTT = 0.83; CLF = 1.11 mFcm−2 (Fig. 8). A value of double layer capacitance, Cdl , can be calculated from a Bode-magnitude plot by extrapolating the linear section to the value of ω = 1 (log w = 0), employing the relationship IZI = 1/Cdl (Fig. 9).63 The highest double layer capacitance of the system (Cdl = 0.49 mFcm−2 ) was obtained for poly(EDOT) and the results observed from Bode-magnitude plot are 0.32 mFcm−2 , 0.25 mFcm−2 , 0.21 mFcm−2 and 0.19 mFcm−2 for poly(Thy2 DTT) and for the copolymers having XThy2DTT = 0.66, XThy2DTT = 0.5 and XThy2DTT = 0.683 mole fractions, respectively.

While the maximum phase angle was obtained for the copolymer in the mole fraction of XThy2DTT = 0.66 as ∼66◦ at the frequency of ∼7.71 Hz, the phase angle and the frequency shifted to the lower values of 60.7◦ for poly(Thy2 DTT) and the copolymer having mole fraction of XThy2DTT = 0.5 at the frequency of 1.78 Hz. Poly(EDOT) showed two different phase peaks. As the highest frequency was obtained as a phase angle of 58o at 5476.5 Hz, the lowest frequency was obtained as 53.8o at 0.33 Hz. (Fig. 10).

In this study, a novel π-conjugated monomer dithieno[3,2-b:2 ,3 d]thiophene and EDOT were electrochemically copolymerized on GCE by CV method. Poly(Thy2 DTT), poly(EDOT) and their electrochemical copolymerization with various mole fractions (XThy2DTT = 0.5, 0.66 and 0.83) were successfully performed on CFME and GCE in NaClO4 /(ACN:CH2 Cl2 ) (8:2). Poly(Thy2 DTT-co-EDOT) was characterized by CV, FTIR-ATR, SEM-EDX and EIS analysis. The equivalent circuit model of R(Q(R(QR(Q(RW)))) was applied together with Kramers-Kronig Transforms to interpret the EIS behavior of the polymer film and electrolyte solution. A modified copolymer electrode provides enhanced capacitance especially with circuit model evaluation, which may results in performance in energy-storage devices.

Figure 8. Nyquist plots for poly(Thy2 DTT), Poly(EDOT) and poly(Thy2 DTT-co-EDOT) for XThy2DTT = 0.5, 0.66 and 0.83 electrocoated on GCE. [EDOT]0 = 10 mM, [Thy2 DTT]0 = 10 mM. EIS measurements were conducted in monomer-free electrolyte solution with perturbation amplitude 10 mV over a frequency range of 10 mHz to 100 kHz.

Figure 9. Bode-magnitude plot of poly(EDOT), poly(Thy2 DTT) and poly(Thy2 DTT-co-EDOT) for XThy2DTT = 0.5, 0.66 and 0.83 electrocoated on GCE. [EDOT]0 = 10 mM, [Thy2 DTT]0 = 10 mM. EIS measurements were conducted in monomer-free electrolyte solution with perturbation amplitude 10 mV over a frequency range of 10 mHz to 100 kHz.

Conclusions


Journal of The Electrochemical Society, 159 (6) E115-E121 (2012) 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

Figure 10. Bode-phase plot of poly(EDOT), poly(Thy2 DTT), and poly(Thy2 DTT-co-EDOT) for XThy2DTT = 0.5, 0.66 and 0.83 electrocoated on GCE. [EDOT]0 = 10 mM, [Thy2 DTT]0 = 10 mM. EIS measurements were conducted in monomer-free electrolyte solution with perturbation amplitude 10 mV over a frequency range of 10 mHz to 100 kHz.

32. 33. 34. 35. 36. 37. 38. 39.

Acknowledgments We thank to Serhat Tıkız (Afyon Kocatepe University, Technology and Research Center (TUAM), Afyon, Turkey) for recording the SEMEDX analyzes. We thank Unsped Global Lojistik for financial support.

40. 41. 42. 43. 44.

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