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Poly(3,4-ethylenedioxythiophene) electrode grown in the presence of ionic liquid and its symmetrical electrochemical supercapacitor application

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O R I G I N A L P A P E R

Poly(3,4-ethylenedioxythiophene) electrode grown

in the presence of ionic liquid and its symmetrical

electrochemical supercapacitor application

Buket Bezgin Carbas1,2 • Burak Tekin2,3

Received: 9 November 2016 / Revised: 12 May 2017 / Accepted: 1 July 2017 / Published online: 5 July 2017

Ó Springer-Verlag GmbH Germany 2017

Abstract Poly(3,4-ethylenedioxythiophene) polymer film (PEDOT-IL) was elec-trosynthesized in the ionic liquid (IL) 1-ethyl-3-methylimidazolium hydrogen sul-phate (EMIMHSO4) medium, which also contains 0.1 M LiClO4 in ACN. For

comparison reasons in terms of structure and electrode capacitance performance, PEDOT film was also synthesized electrochemically without IL. The SEM results show that PEDOT-IL film has more porous surface and fine textures with nanometer-diameter than PEDOT polymer. Different electrochemical methods including galvanostatic charge–discharge (CD) experiments, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) were carried out to investigate the applicability of the system as a redox supercapacitor for both polymers. PEDOT-IL electrode shows higher optimum specific capacitance than PEDOT film. Additionally, the symmetrical supercapacitor was assembled from two PEDOT-IL electrodes in LiClO4/ACN medium and exhibited a maximum specific

capacitance of 107 F g-1, an energy density of 11.5 Wh kg-1 at a power density 13 kW kg-1, and an excellent cycle life of 96% specific capacitance retention after 1000 cycles.

Keywords Conducting polymer Poly(3,4-ethylenedioxythiophene)  Supercapacitor Ionic liquid

& Buket Bezgin Carbas bcarbas@kmu.edu.tr 1

Department of Energy Systems Engineering, Karamanog˘lu Mehmetbey University, 70100 Karaman, Turkey

2

Conductive Polymers and Energy Applications Laboratory, Karamanoglu Mehmetbey University, 70100 Karaman, Turkey

3 Department of Advanced Technologies, Karamanoglu Mehmetbey University, 70100 Karaman, Turkey

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Introduction

Electrical energy consumption has been or forecasted in the future to become the most challenging and main problem in modern society. In this sense, energy storage systems provide a wide array of technological approaches to overcome this trouble. Supercapacitors and batteries are considered to be two of the most promising energy storage technologies for electric vehicles and renewable energy systems, which are proceeding for low cost and eco-friendly storage systems in many applications [1–4]. Supercapacitors, also known as electrochemical capacitors are one of the popular energy storage devices which bridge the gap between conventional batteries (high energy, low power density) and capacitors (high power density, low energy density) [1,5,6]. In comparison to batteries, supercapacitors can be faster switching (charge/discharge) in a longer cycle period [7]. Depending on the charge storage mechanism, supercapacitors are divided into two types. These are electrochemical double layer capacitors (EDLCs) and pseudocapacitors. In EDLCs, the charge is stored electrostatically at the electrode–electrolyte interface in the form of double layers on either electrode in which these double layers act as two serial capacitors on the electrical equivalent circuit, while in pseudocapacitors charge storage occurs via fast redox reactions [8]. Because of the pseudocapacitors charge storage mechanism consists of electrosorpsion, reduction–oxidation reactions, and interca-lation processes, pseudocapacitors achieve higher specific capacitance (10–100 times higher) and energy densities than EDLCs [9,10]. This is why most of the studies have been oriented towards usage of pseudocapacitive materials with higher capacitance values [11,12].

In pseudocapacitors, the mechanisms are related with fast and reversible Faradaic redox reactions at the surface and/or in the bulk, using materials such as inorganic transition metal oxides or electrically conducting polymers (CPs) [1]. Although some transition metals like RuO2show best performance in pseudocapacitors, there

are some disadvantages of metal oxides such as high cost, toxic nature, narrow potential window, storage of charge on the oxide surface (not in the bulk of material), challenging issues during fabrication (limited area manufacturing) [13–15]. In the case of CPs, they have the advantages with their unique properties such as, high specific capacitance, good conductivity, low cost, easy fabrication, good thermal, chemical stability and environmental friendliness [15,16]. CPs store charge, not only in the electrical double layer, but also throughout the body of the polymer by rapid faradaic charge transfer via three-dimensional charge storage mechanism. For that reason, they have higher capacitance values [9]. Another interesting property of CPs is also their applicability for flexible devices, due to their inherent flexible polymeric nature, which is a crucial requirement for the portable flexible electronics [17]. Polymer-based supercapacitor devices have three configurations as per types of conducting polymers doping process. Type I or also known as symmetric, supercapacitor using the same p-dopable polymer for both electrodes. Type II or known as asymmetric, supercapacitor using two different p-dopable polymers have different potential ranges. Type III or also known as asymmetric, supercapacitor using p-dopable polymer for positive electrode and also

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using n-dopable polymer for negative electrode [1, 18]. Among conducting polymers (CPs), polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh) and their derivatives have been studied extensively as electrode materials for electrochemical capacitors because of their low cost and large pseudocapacitance behaviour [19]. Poly(3,4-ethylenedioxythiophene) (PEDOT), which is a parent polymer of PTh family, exhibits not only a high conductivity but also an unusual stability in the oxidized state, being considered as perhaps the most stable conduct-ing polymer currently available [20]. Therefore, many researchers have been using PEDOT and its derivatives as the electrode materials for supercapacitors due to exceptional electrical and electrochemical properties [21–24].

CP electrodes for supercapacitors can be prepared by chemical or electrochem-ical methods. Although chemelectrochem-ical polymerization, which usually produces powdery materials, is the preferred technique for production of large scale, binders and additives are added to polymer mixture to obtain electrode and this causes high internal resistances during device performance [25]. Polymerization of electrodes via electrochemical methods has the advantage of convenient one-step polymer synthesis. This provides control over the film thickness, an important parameter for fabrication of devices. Therefore, polymerization method, type of dopant and electrolyte have an impact on the film morphology and this also affects both electrode and device performances. In general, the performance of supercapacitors is related to physical properties of both the electrolyte and the electrode components. The electrode is one of the most important components which effects the charge capacity and charge storage times; therefore it plays crucial role in determining the energy and power densities of supercapacitor [12]. As a consequence, different tactics have been practiced to upgrade their stability [26–28]. Ionic liquids (ILs) have been enormously used as effective additives or solvents to enhance their chemical/electrochemical properties. Recent studies have shown that CPs can be electropolymerized using ILs as dopant or electrolyte and those polymers have shown improved cycling stability in ILs and gel electrolytes compared to organic electrolytes [29–31]. There are three main advantages of using ionic liquids for electrochemical capacitors. First, the electrochemical activity of the polymer grown in ionic liquids is less affected by the nature of the cycling solvent than the polymer achieved in other solutions. Second, the porous structure of polymer offers a higher specific surface area that is convenient for dopant ions accessing into the polymer matrix and inducing higher charge to keep stable. And the last one, p type or n type polymer prepared from ionic liquids is more stable [32]. Room temperature ionic liquids (RTILs) are generally preferable with some important advantages over conventional solvents such as almost negligible vapor pressure, good thermal stability, high electrochemical stability and wide potential windows. More importantly, they are environmentally benign and less toxic than most conventional organic solvents, making them especially suitable for applications in electrochemical systems [33]. Among all the ILs, fluorinated ILs generally exhibits wide potential window, low viscosity, and hence high ionic conductivity but non-fluorinated electrolytes are more desirable from the safety and environmental standpoints [34]. RTILs are generally mixed with a molecular solvent to optimize the solvent properties of ILs for specific applications. The

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addition of solvents generally decreases the melting point and viscosity and increases the conductivity [35]. In this work, EMIMHSO4, a non-fluorinated IL, was

chosen as ionic liquid electrolyte and acetonitrile as organic solvent was added to decrease the viscosity.

In literature, the number of studies related type I electrochemical pseudo capacitors based on the electrosynthesis of PEDOT electrodes in RTILs for supercapacitor applications were limited. Recently, Pandey et al. constructed an electrochemical capacitor using PEDOT and a gel electrolyte having specific capacitance as 85 F g-1 [33]. In another study, Fu et al. have prepared PEDOT electrodes in combination with four different ionic liquids medium and PEDOT electrode and reported specific capacitance value of 191 F g-1 for 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] ionic liquid [36]. Liu et al. on

the other hand, reported specific capacitance of 130 F g-1 for the electrochemical capacitors based on PEDOT electrodes in [BMIM][BF4] ionic liquids [37].

In the light of above information, we report the synthesis of PEDOT thin films prepared a non-fluorinated ionic liquid, EMIMHSO4in an organic solution of 0.1 M

lithium perchlorate (LiClO4) in acetonitrile (ACN), electrochemically. To

under-stand the morphological, capacitive and electrochemical influence of the RTIL on the PEDOT electrode, bare PEDOT electrode was also synthesized for comparison reasons in the medium of 0.1 M LiClO4/ACN. A symmetrical supercapacitor based

on two PEDOT-IL electrodes were also constructed in the medium of 0.1 M LiClO4/ACN and characterized with electrochemical methods for a symmetrical

supercapacitor application.

Experimental

Materials

3,4-Ethylenedioxythiophene (EDOT), lithium perchlorate (LiClO4), anhydrous

acetonitrile (ACN) and ionic liquid 1-ethyl-3-methylimidazolium hydrogen sulphate (EMIMHSO4) were purchased from Sigma Aldrich. Acetonitrile was refluxed on

CaH2 and then distilled prior to use. For electrochemical synthesis and analysis,

LiClO4 and EMIMHSO4 were used as supporting electrolytes. A platinum disc

(0.02 cm2) as a working electrode, a platinum wire as a counter electrode and a Ag/ AgCl electrode in 3 M NaCl (aq) solution as a reference electrode were used. The polymer films coated on Pt disc had been switched between the neutral and oxidized states several times to break-in the polymer film in a monomer-free electrolyte solution prior to electrochemical analyses to obtain repeatable results. For scanning electron microscopy (SEM) analysis, fluorine-doped tin oxide (FTO) substrate (1.5 cm 9 1.5 cm) was used as working electrode.

Instrumentation

To examine the influence of EMIMHSO4and polymerization charge capacity on

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carried out using Zeiss Evo model SEM. An atomic force microscopy (AFM) from nanomagnetics instruments was used for investigating the thicknesses of polymer films on FTO electrode for morphological characterization. AFM imaging was performed in the tapping mode using PPP-NCLR cantilevers. The polymer deposition carried out using a Ivium compact Stat system. For chronoamperometry, cyclic voltammetry (CV) and galvanostatic charge discharge (CD) tests, a classical three-electrode system were used during electropolymerization and characterization of polymeric films, respectively. The polymer film behaviour was characterized between 0.2 and 1.2 V electrochemically via CV method in the monomer-free electrolytic medium. For electrochemical impedance, spectroscopy (EIS) was also performed and samples were scanned from 100 Hz to 100 kHz at 0.60 V forward bias with 10 mV ac amplitude. All experiments were carried out at room temperature.

Electrosynthesis and electrochemical characterization tests

Bare PEDOT and PEDOT-IL films were grown potentiostatically in ACN-LiClO4

and ACN-LiClO4/EMIMHSO4, respectively, and the thickness of films was

controlled by the total charge passed through the cell. Chronoamperometry method was used and 1.40 V constant potential was selected for polymerization. Depending on charge amount (5, 10, 15, 20, 25 mC) decided, polymerization time was changed. After polymerization, the polymer films were washed repeatedly with anhydrous ACN to remove monomers over the electrode system.

Cyclic voltammetry method was used to characterize electrochemical behaviors of the films in the monomer-free medium. The masses of polymer films (Wp) were

calculated from the total charge passed through the cell during the film growth process, according to Eq. (1) [38]:

Wp ¼

ðgQdepÞðMÞ

FZ ; ð1Þ

where Qdep is the total charge passed through the cell during the polymer growth

process, assuming a 100% current efficiency (g). M is the molecular weight of monomer. F is the Faraday constant (96,485 C mol-1). Z is the number of electrons transferred per monomer attached to the polymer, in which Z = 2 ? c [39,40]. c is the doping level of the polymer and found from Eq. (2):

Qfmax¼ c cþ 2

 

Qd; ð2Þ

where Qd is the charge density for the formation and deposition of the oxidized

PEDOT film. Qfmax is the faradaic charge density, which gives information about

the maximal quantity of oxidized polymer. From Eq. (2) the variation of Qfmaxas a

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Preparation of symmetric supercapacitor

Two symmetrical PEDOT-IL electrodes (negative and positive) was simply constructed. Ag/AgCl electrode was used as reference electrode. Monomer-free medium, 0.1 M LiClO4/ACN was used as electrolyte/solvent medium for

electro-chemical characterization. Capacitance calculations

Specific capacitance values (Csp) of the materials were calculated from cyclic

voltammograms by means of Eq. (3) [23]:

CspðCVÞ ¼ i DV

Dtm

; ð3Þ

where, Cspis the specific capacitance in F g -1

, i is the charge/discharge current in mA, m is the mass of one electrode material in mg. DV=Dt is the scan rate in V s-1. When the charge discharge method is used, Csp (CD) is calculated according to

Eq. (4) [41]:

CspðCDÞ ¼ it

mV; ð4Þ

where, Cspis the specific capacitance in F g-1, I and t are the discharge current and

time, respectively. V is the potential window during charging and discharging. m is the mass loading of polymer on Pt disc electrode. The specific capacitance value for symmetrical capacitor was calculated according to the total mass of active materials on two electrodes.

From impedance measurements, the specific capacitance value Csp(EIS) can also

be calculated by means of Eq. (5) [23]:

CspðEISÞ ¼ 1 2pfmZIm

; ð5Þ

where, Csp(EIS) is the specific capacitance in F g-1, f and ZImare the frequency in

Hz and imaginary part of impedance in ohms, respectively. Calculations of specific power and specific energy

Specific power (Psp) and specific energy (Esp) are used to characterize capacitive

properties of electrochemical capacitors. The calculations related to these properties are shown in the following equations (Eqs.6,7) [23]:

Esp¼ 1 2CspV

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Psp ¼ Esp

Dt; ð7Þ

where Csp, Psp and Esp are the specific capacitance, specific power (W kg-1) and energy density (Wh kg-1) of the electrode, respectively. V is the potential window (V). Dt is the discharge time for the potential.

Coulombic efficiency (g) can be found according to the following equation: g¼ td

tc  

 100; ð8Þ

where td and tc are the expressions of discharge and charge times [42].

Results and discussion

Electrosynthesis and morphology of polymer films

PEDOT and PEDOT-IL were electropolymerized in the medium of 0.1 M LiClO4/

ACN and 0.1 M LiClO4, 0.1 M EMIMHSO4/ACN, respectively. Using ionic liquid

and keeping the monomer concentration (0.1 M) and polymerization potential constant (1.4 V) during polymerization, we could compare response of these polymer film capacitive properties as electrodes.

Figure1 displays surface morphology of PEDOT and PEDOT-IL at 500 mC cm-2 polymerization charge capacity on FTO electrode. The ions are incorporated into polymer matrix as dopants and this makes a growth pattern and makes a narrow size distribution for PEDOT film. In the case of PEDOT-IL film shows a smooth morphology with randomly distributed particles and exhibits a densely packed structure morphology which comprises of uniform globular grains size of 80–150 nm. Moreover, PEDOT-IL exhibited more porous structure morphology and larger active surface area in comparison to PEDOT. As a supercapacitor material, this porous structure will provide some advantages. One of them is that these nano-sized pores benefit for depolarization of the ion concentration and a high interfacial reaction area. The other advantage is the creation of diffusion paths for electrolyte ions, which speed up the intercalation of ions and enhance the utilization rate of electrode materials [43]. The AFM results

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showed that the polymer films for PEDOT and PEDOT-IL were 5.40 and 4.20 lm, respectively.

Doping level is also an important property to describe the electronic properties of conductive polymers. For this purpose, the electrochemical properties of PEDOT (Fig.2a) and PEDOT-IL (Fig.2b) were analyzed with various total charge densities of 5, 10, 15, 20 and 25 mC (area of electrode 0.02 cm2) using CV method (scan rate 50 mV s-1). From Eq. (2) the variation of Qfmax as a function Qd gives a linear curve for both polymers with a slope of c=ðc þ 2Þ by graphical determination and c value for PEDOT (Fig.2c) and -PEDOT-IL (Fig.2d) were found as 0.35 and 0.40, respectively. Doping level value of PEDOT agrees with those determined for PEDOT as reported in the literature (0.33) [18]. In the case of PEDOT-IL, the doping level value is higher than the reference value (0.35). This phenomenon is most probably related to ionic liquid medium. Intercalation of ionic liquid anion to the polymer chain increased the doping level of polymer film in our work. This study also proves that polymerization system containing ionic liquid compared to one of other traditional electrolyte/solvent systems will result higher doping level [44]. The preliminary capacitive behaviour studies of PEDOT electrodes prepared in ionic liquids were also experimented and finally good results were obtained that will not be underestimated at all [45].

Fig. 2 CVs of a PEDOT, b IL-doped PEDOT electrodes in 0.1 M ACN/LiClO4at different charge values (scan rate 50 mV s-1). Variation of Qfmaxfor c PEDOT, d IL-doped PEDOT as a function of the

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The total charge change affects the mass loading during electropolymerization. Because the mass loading at the lowest charge density was too limited, which is an important property to achieve high capacity and high mass loading is a problem for adsorption to the electrode and redox capability of the film during switching, 10 mC (500 mC cm-2) charged PEDOT and PEDOT-IL polymer films were selected to investigate the ionic liquid intercalation into polymer film during polymer growth and electrochemical capacitor performance. The deposition with a charge density of 500 mC cm-2 for both polymers using Eq. (1), a mass loading of around 0.3 mg cm-2(0.006 mg for 0.02 cm2) was found.

Cyclic voltammetry (CV) is one of the electroanalytical methods to characterize the capacitive properties of polymer films. The pseudo capacitance behaviour of polymer film is related with the diffusion of charged counter ions during redox processes. PEDOT (Fig.3a) and PEDOT-IL (Fig.4a) films were scanned at different scan rates (20–300 mV s-1with an increment of 20 mV s-1) in a potential from 0.0 to 1.2 V, where high conductivity and good electrochemical reversibility of PEDOT can be obtained [46]. The shape of CV plots for both polymers resemble rectangle in the low scan rates. While CV shape of PEDOT-IL maintains rectangle behaviour towards the high scanning speed, showing its ideal capacitor behaviour. Csp (CV) change with the increasing scan rates, a curve indicating the function

Fig. 3 aCVs of PEDOT electrode in 0.1 M ACN/LiClO4at different potential scan rates; b specific capacitance of PEDOT electrode as a function of scan rates; c galvanostatic charge/discharge curves of PEDOT electrode in 0.1 M ACN/LiClO4at different current density; d specific capacitance of PEDOT electrode as a function of current density (polymer charge density 500 mC cm-2)

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between specific capacitance and scan rate was plotted for PEDOT (Fig.3b) and PEDOT-IL (Fig.4b) values can be obtained at the scan rates from 20 to 300 mV s-1. It can be observed that Csp (CV) decreased after 60 mV s-1 as the

scan rates increased for both polymers. It suggests that the measured capacitance was mainly related with the redox mechanism. At low scan rates, supporting anions do have enough time to diffuse into the polymer electrode. On the other hand, at high scan rates, most probably supporting anions do not have enough time to intercalate into the electrode. This causes lower degree of accessibility for ions to the electrode and results a decrease for Csp (CV) value [47]. Csp(CV) values for

PEDOT electrode decreases from 118 F g-1 (60 mV s-1) to 88 F g-1 (300 mV s-1) whereas, Csp(CV) values for PEDOT-IL decreases from 140 F g-1

(60 mV s-1) to 120 F g-1(300 mV s-1). The higher Csp(CV) values for

PEDOT-IL than those of bare PEDOT can be attributed to higher charge storage capacity of polymer film with ionic liquid.

Galvanostatic charge and discharge tests were also investigated to evaluate the capacitance performance of PEDOT (Fig.3c) and PEDOT-IL (Fig.4c) polymer films with increasing current densities using a potential window of 0.2–1.2 V vs. Ag/AgCl. Both electrodes showed typical capacitance behaviour in 0.1 M LiClO4/

ACN. The results were consistent with the CV results. Csp (CD) decreased as the

Fig. 4 aCVs of IL-doped PEDOT electrode in 0.1 M ACN/LiClO4at different potential scan rates; bspecific capacitance of IL-doped PEDOT electrode as a function of scan rates; c galvanostatic charge/ discharge curves of IL-doped PEDOT electrode in 0.1 M ACN/LiClO4 at different current density; dspecific capacitance of IL-doped PEDOT electrode as a function of current density (polymer charge density 500 mC cm-2)

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current density increased for PEDOT (Fig.3d) and PEDOT-IL (Fig.4d) polymers. From discharge curves at different current densities, the IR drop is obviously observed and increased as shown in figures. In calculations iR-drop was considered. Csp (CD) values for PEDOT at the current densities of 15, 20, 25, 30, 35 and

40 A g-1were 150, 120, 108, 90, 84 and 79 F g-1, respectively. When the polymer was grown in ionic liquid, Csp(CD) values were increased with respect to values of

PEDOT, as expected from CV results. The values for PEDOT-IL at the current densities of 15, 20, 25, 30, 35 and 40 A g-1 were 195, 156, 150, 137, 140 and 120 F g-1, respectively.

Nyquist plots for oxidized polymer films at 0.6 V in ACN containing 0.1 M LiClO4 is shown in Fig.5. The polymer films exhibit p-doped property and

electrochemical response to the simulation. p-doping process contains transport of both ions and electrons through the film. The resulting plots possess the characteristic shape for both polymeric films acting as a finite transmission line. Each polymer has electronic (RE) and ionic (RI) resistances, which were estimated

from high (Rhigh) and low (Rlow) frequency limiting resistances. RE and RI of

oxidized polymeric films can be obtained using the following relationships (Eq.9) [48]. Using a bare Pt disc electrode, the resistance of electrolyte (RsÞ was found as 612 X. As oxidation potential increases, the movement of counter ions in the polymer film is promoted. This provides an increase in the ionic conductivity. As a result, RE and RIwere calculated as 1320 and 165 X for PEDOT, respectively. The resistance values of PEDOT-IL (RE¼ 184 X and RI = 75 X) decrease in compar-ison that of PEDOT as more ionic species enter polymer film and increase ionic conductivity. In addition, the ion diffusion to the system is faster than that of PEDOT film,

Fig. 5 Impedance spectra of Pt, PEDOT and IL-doped-PEDOT in 0.1 M ACN/LiClO4. Electrode potential: 0.6 V (polymers charge density 500 mC cm-2). Insets show the extended high-frequency region for both polymers

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1 Rhigh RS ¼ 1 RE þ 1 RI

and 3ðRlow RsÞ¼Rlowþ RI: ð9Þ

From the impedance curve (Fig.5) the specific capacitances of PEDOT and PEDOT-IL were calculated using Eq. (5). The calculated Csp(EIS) for PEDOT and

PEDOT-IL at low frequency [15.8 mHz (PEDOT) and 25.1 mHz (PEDOT-IL)] are 120 and 140 F g-1, respectively. As the frequency increase to 1 Hz for both polymer Csp(EIS) remain constant. At the much higher frequency of 1000 Hz, both

polymers suffer from insufficient counter ion diffusion and the values for PEDOT and PEDOT-IL drop to 3 and 12 F g-1, respectively.

A symmetric electrochemical capacitor based on two PEDOT-IL electrodes with 0.1 M LiClO4 electrolyte in ACN solvent was assembled to determine the

capacitance performance of the PEDOT electrode in a full setup. For specific capacitance calculations, CV, galvanostatic CD and EIS electrochemical charac-terization methods were applied to the system as shown in Figs.6and7. First, the CV curves of the capacitor at different scan rates from 20 to 300 mV s-1 were shown in Fig.6a. The shape of curves did not change even at the high scan rates, indicating stable and reversible kinetic character of electrodes. As seen in Fig.6b, Csp (CV) values up to 60 mV s-1 increased and then at the scan rates from 60 to

300 mV s-1 remain almost constant (85 F g-1). Second, galvanostatic charge discharge curves for the capacitor at different current densities were experimented (Fig.6c). The shape of the curves tends towards triangular-shaped curves, as well as charge and discharge curves were nearly linear and symmetric to each other, indicating that the capacitor has rapid response and good electrochemical reversibility [48]. Csp(CD) of capacitor was 107 F g-1at 5 A g-1and 80% initial

specific capacitance retention was obtained when the current density increased to 26 A g-1 (Fig.6d). The power and energy densities of electrochemical capacitor can be evaluated using galvanostatic charge discharge curves. According to Eqs. (6) and (7), Espand Pspvalues were found, respectively, and Ragone plot of capacitor

were drawn as shown in Fig.6e, which shows the highest energy density of 12.5 Wh kg-1 at a power density of 480 W kg-1 was found and reached 11.7 Wh kg-1energy density with the power density of 13 kW kg-1. The capacitor showed a high cycle stability at 26 A g-1for 2500 cycles, as shown in Fig.6f (95 and 80% retention of the initial specific capacitance for 1000 and 2500 cycles, respectively), making it promising as one of the attractive candidates for energy storage devices. The results also reveal that the capacitor show stable coulombic efficiencies more than 96% over 1000 cycles and 85 for 2500 cycles at the current density of 26 A g-1 (Fig.6f).

Figure7shows the impedance plot of symmetric capacitor based on PEDOT-IL electrodes recorded using perturbation amplitude of 10 mV in a frequency range from 105to 0.1 Hz at the applied potential of 0.6 V versus Ag/AgCl. It can be noted that an ideal impedance response for an ideal capacitor is a straight line parallel to the imaginary axis of the complex impedance plots. At high-frequency region, it is not easy to see a well-defined semicircle, indicating the charge transfer resistance is small and the system is kinetically fast and charge transfer resistance is small. For

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that reason, charges can be readily transferred into the highly conductive PEDOT-IL electrode. Low-frequency region gives information about the mass transport. Rhigh

(233 X) and Rlow(242 X) values for capacitor is lower than that of PEDOT-IL data,

indicating a symmetrical capacitor based on IL-grown polymer was obtained with low internal resistance. From the impedance curve, Csp(EIS) of capacitor was found

as 88 F g-1 at low frequency (0.1 Hz). Curve fitting data are also presented by

Fig. 6 aCVs of type I supercapacitor based on IL-doped PEDOT electrode in 0.1 M ACN/LiClO4at different potential scan rates; b specific capacitance of type I supercapacitor based on IL-doped PEDOT electrode as a function of scan rates; c galvanostatic charge/discharge curves of type I supercapacitor based on IL-doped PEDOT electrode in 0.1 M ACN/LiClO4 at different current density; d specific capacitance of type I supercapacitor based on IL-doped PEDOT electrode as a function of current density (polymer charge densities 10 mC); e relationships between the specific energy and specific power of type I supercapacitor based on IL-doped PEDOT electrode at various current densities; f cyclic performance during 2500 cycles at current density of 26 A g-1in 0.1 M ACN/LiClO4

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dotted lines. The Nyquist plots of capacitor can be fitted by an equivalent circuit shown in the inset of Fig.7. As it is obvious, the circuit consists of parallel combination of Warburg impedance (Zw) and charge transfer resistance (Rct) with a

double layer capacitance (Cdl) in series with a Faradaic capacitance, in addition to

the solution resistance (Rs), at high-frequency region. The Warburg impedance is

related to the diffusion process of ions at the electrode/electrolyte interface.

Conclusions

Films of PEDOT and PEDOT-IL were deposited on Pt disc by simple electropoly-merization process, intending to see the effect of ionic liquid on electrochemical, physical properties of polymer electrode. Incorporation of EMIMHSO4 to the

polymer film enables an increase in the porosity and capacitive properties of polymer film. All the electrochemical characterization methods (cyclic voltamme-try, galvanostatic charge discharge, and impedance methods) show that PEDOT-IL electrode has higher specific capacitance values than that of PEDOT. These results suggest that the presence of IL dopant in the polymer electrode system enhances the capacitive properties for potential supercapacitor applications. Furthermore, Type I pseudo capacitor based on two symmetrical PEDOT-IL electrodes were constructed in a typical three-electrode system. From the results of charge and discharge process, this capacitor has good electrochemical performance, using 0.1 M LiClO4

in ACN in the range of 0.2–1.2 V vs. Ag/AgCl. The capacitor exhibited a maximum specific capacitance of 107 F g-1, an energy density of 11.5 Wh kg-1 at a power density 13 kW kg-1, and an excellent cycle life of 96% specific capacitance

Fig. 7 Nyquist plot of symmetric supercapacitors based on IL-doped-PEDOT in 0.1 M ACN/LiClO4. Electrode potential: 0.6 V (polymers charge density 10 mC). Inset show the extended high-frequency region

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retention after 1000 cycles. These results establish PEDOT-IL electrode as competitive material in the field, capable of being integrated into other organic electronic systems as electrodes. Work along this line is currently underway in our laboratories.

AcknowledgementsWe gratefully acknowledge financial support from Karamanoglu Mehmetbey University (KMU-BAP-38-M-16).

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Şekil

Figure 1 displays surface morphology of PEDOT and PEDOT-IL at 500 mC cm -2 polymerization charge capacity on FTO electrode
Fig. 2 CVs of a PEDOT, b IL-doped PEDOT electrodes in 0.1 M ACN/LiClO 4 at different charge values (scan rate 50 mV s -1 )
Fig. 3 a CVs of PEDOT electrode in 0.1 M ACN/LiClO 4 at different potential scan rates; b specific capacitance of PEDOT electrode as a function of scan rates; c galvanostatic charge/discharge curves of PEDOT electrode in 0.1 M ACN/LiClO 4 at different curr
Fig. 4 a CVs of IL-doped PEDOT electrode in 0.1 M ACN/LiClO 4 at different potential scan rates; b specific capacitance of IL-doped PEDOT electrode as a function of scan rates; c galvanostatic charge/ discharge curves of IL-doped PEDOT electrode in 0.1 M A
+4

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