Journal of the Korean Physical Society, Vol. 59, No. 1, July 2011, pp. 145∼149
Supercapacitive Characteristics of Electrodeposited Polyaniline Thin Films Grown on Indium-doped Tin-oxide Substrates A. I. Inamdar, Young Sam Kim, Jae Sang Sohn and Hyunsik Im∗ Department of Semiconductor Science, Dongguk University, Seoul 100-715, Korea
Hyungsang Kim Department of Physics, Dongguk University, Seoul 100-715, Korea
Dae-Young Kim Department of Biological & Environmental Science, Dongguk University, Seoul 100-715, Korea
R. S. Kalubarme and ChanJin Park Department of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, Korea (Received 25 April 2011, in final form 20 May 2011) Polyaniline (PANI) thin films were successfully synthesized using the electrodeposition (ED) technique from a mixed solution of 0.2M aniline and 0.2M H2 SO4 . PANI films with thicknesses of 220 and 250 nm were synthesized. The formation of compact and amorphous PANI films was confirmed with scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements. The electrochemical supercapacitor properties of the PANI films were examined using cyclic voltammetry and galvanostatic charge-discharge measurements. A specific capacitance of ∼473 Fg−1 at 1 mAcm−2 in a 0.5 M H2 SO4 electrolyte was obtained for the PANI electrodes. The electrochemical stability of the PANI electrodes was investigated using cyclic voltammetry and charge-discharge measurements. Both the films were quite stable with 16% capacitance loss after a few initial charge-discharge cycles. PACS numbers: 84.32.Tt, 72.80.Le, 73.61.Ph Keywords: Supercapacitor, Electrodeposition, Cyclic voltametry, Charge-discharge measurements, Polyaniline DOI: 10.3938/jkps.59.145
of metal oxides is limited due to the high cost of the materials. Thus, a key issue is to develop cost-effective polymers for use in the fabrication of electrochemical supercapacitors. Among the wide variety of conducting polymers, polyaniline (PANI) has received considerable attention due to its good redox reversibility and chemical stability [12–17]. Various methods can be used to grow PANI films, and PANI films prepared by different methods show dissimilar supercapacitive properties with respective advantages and disadvantages. Polyaniline nanofibers were synthesized by using interfacial polymerization, and an electrochemical capacitance of 554 Fg−1 was reported. However, in this case, the capacitance values decreased rapidly during electrochemical cycling. In addition, efforts have been made to improve the stability of PANI films by preparing composite PANI/CNT films [18]. Girija and Sangaranarayanan studied the electrochemical polymerization of PANI by means of potential
I. INTRODUCTION Electrochemical supercapacitors play an important role in power source applications such as hybrid electric vehicles, computers, and short-term power sources for mobile electronic devices [1–3]. They can be classified into two types based on their charge storage mechanisms: (i) electrical double-layer capacitors (EDLCs) and (ii) redox supercapacitors [4]. Compared to the EDLC-based capacitors, redox capacitors based on transition-metal oxides or conducting polymers such as RuO2 [5], MnO2 [6], NiO [7], Co3 O4 [8], V2 O5 [9], and polyaniline [10, 11] may provide much higher specific capacitances up to 1000 Fg−1 of the active material. Metal oxides and electrochemical conducting polymers are widely studied materials and are used as pseudocapacitive electrode materials with remarkable performance. However, the use ∗ E-mail:
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Journal of the Korean Physical Society, Vol. 59, No. 1, July 2011
Fig. 1. (Color online) Structural and morphological characteristics of PANI films: (a) Chronoamperometric (CA) curves for PANI film deposition for two time periods. The insets show a photograph of the uniform PANI film and its scanning electron micrograph (SEM) image. (b) X-ray diffraction pattern of the amorphous PANI films.
cycling between -0.2 and 1.2 V at a scan rate of 300 mVs−1 for 50 cycles and reported capacitance values in the range of 346 to 404 Fg−1 at different current densities [19]. In addition, PANI films coated and uncoated with Naflon were studied, and their enhanced cycle life was reported; however, the presented capacitance values were too low [20]. In this study, electrodeposition (ED) is of particular interest due to its low cost, environmental friendliness, and the feasibility of low temperature growth. As an update to previously studied PANI films, the goal of this study was to test the applicability of the ED technique for supercapacitor applications. We propose the synthesis of PANI films using the ED technique, and we examined the supercapacitive properties of the films by using cyclic voltametry and charge-discharge measurements.
II. EXPERIMENTS PANI thin films were synthesized using the ED technique. The films were deposited on indium-doped tinoxide (ITO, 25 - 27 Ωcm−2 )-coated conducting glass
substrates. Prior to the deposition, the ITO substrates were ultrasonically cleaned consecutively in acetone, methanol, and deionized water. The chemicals for the synthesis of the PANI films were used as-received, without further purification. The reaction bath for the PANI film deposition contained 2M aniline and a 2M H2 SO4 (37%) solution. PANI films were electrodeposited using a three-electrode electrochemical system in which the electrodes were the ITO glass substrate as the working electrode, graphite as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. ED was carried out at 0.75 V (versus SCE) for 15 and 20 min by using a chronoamperometry (CA) technique at room temperature. Figure 1(a) shows the measured CA curves for the PANI films. During the deposition, when the electrode was polarized, the current decayed abruptly after the initial surge due to the reduction of cations near the electrodes. This led to the birth of nucleation centers on the electrode’s surface. As fresh cations diffused, the current began to increase. In the CA curves, the current started to increase steadily with time, suggesting progressive nucleation and growth of the PANI film [21]. After the deposition, the films were immersed in deionized water and dried in natural air. The films deposited for 15 and 20 min were denoted PANI-1 and PANI-2, respectively. The thicknesses of the PANI films were measured using an alpha-step profiler and were found to be 220 and 250 nm for PANI-1 and PANI-2, respectively. The left inset of Fig. 1(a) shows a photograph of the PANI-2 film with uniform coverage on the ITO substrate. The structural properties of the PANI films were studied using high-resolution X-ray diffraction (XRD) with Ni-filtered Cu Kα radiation (Kα = 1.54056 ˚ A) (X pert PRO, Philips, Eindhoven, Netherlands). The surface morphology was observed using field-emission scanning electron microscopy (FE-SEM, Model: JSM6701F, JEOL, Japan). The supercapacitive properties of the film were determined using cyclic voltammetry and galvanostatic charge-discharge measurements. Cyclic voltametry and chronoamperometry are used most often as diagnostic tools for elucidating electrode mechanisms. The supercapacitive properties of the materials are mainly based on the redox reactions at the electrode-electrolyte interfaces. CV is an important tool to measure the redox reaction in terms of the current and the voltage by using computer controlled programs that provide information on the supercapacitive behavior of the electrode. The electrochemical measurements (supercapacitive characteristics) of the PANI films were performed on an electrochemical workstation by using a three-electrode electrochemical cell containing 0.5M H2 SO4 as the electrolyte, an SCE as the reference electrode, and graphite as the counterelectrode. A multichannel potentiostat/glavanostat (model WMPG 1000) interfaced with a computer was used in the galvanostatic mode to record the charging and the discharging of the supercapacitor thin films.
Supercapacitive Characteristics of Electrodeposited Polyaniline Thin Films · · · – A. I. Inamdar et al.
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Fig. 2. (Color online) Electrochemical supercapacitive characteristics of PANI films. (a) CV curves of the PANI-1 film at different scan rates. (b) CV curves of the PANI-2 film at different scan rates. (c) Specific capacitance versus scan rate for the PANI-1 and the PANI-2 electrodes (calculated from the CV measurements).
III. RESULTS AND DISCUSSION
according to the following equation [4]: Csp =
The compact nature of the PANI films was observed using scanning electron microscopic (SEM) measurements. The right inset of Fig. 1(a) shows an SEM image of the PANI-2 film. The structural properties of the PANI films were studied using XRD measurements. Figure 1(b) shows the measured X-ray diffraction spectrum of a PANI film grown on an ITO substrate. The peaks marked with “∗ ” exactly match the XRD peaks of the ITO substrate. This indicates that the PANI film is amorphous. The electrochemical properties and supercapacitor properties of the PANI electrodes were determined using cyclic voltametry. The electrochemical measurements, like cyclic voltammetry, were performed at room temperature. Figure 2 shows measured cyclic voltammograms (CVs) recorded in the H2 SO4 electrolyte for the PANI1 and PANI-2 films at four different scan rates (20, 50, 80, and 100 mVs−1 ). The CVs were cycled between 1 V (versus SCE) to -0.4 V (versus SCE). The CVs started at 1 V, reversed at -0.4 V, and terminated at 1 V (versus SCE). CVs provide valuable information on reductionoxidation (charge-discharge) behavior. Here, the capacitance was mainly based on the redox reaction because the shape of the CVs is distinguished from the shape of electric double-layer capacitance, which is normally close to an ideal rectangle [4]. During the CV scans, two reduction and two oxidation peaks (marked with up and down arrows, respectively) were observed, thus presenting an exchange between the leucoemeraldine/emeraldine and emeraldine/pernigraniline states of the PANI film [1]. As the scan rate was increased, the current response, which is a measure of the capacitance, increased. The relatively higher current density for the PANI-2 film indicates good rate capability for supercapacitor applications. The similar shape of the CVs recorded at different scan rates indicates excellent electrochemical reversibility of the PANI films. The specific capacitance of the PANI electrode can be calculated from the CV curves
i , r×m
(1)
where i is the average cathodic current, r is the scan rate, and m is the mass of the active electrode. The specific capacitance values were found to be 435 Fg−1 and 460 Fg−1 for the PANI-1 and the PANI-2 films, respectively, at a scan rate of 20 mVs−1 . The specific capacitance values at a scan rate of 100 mVs−1 decreased to 320 Fg−1 and 239 Fg−1 for the PANI-1 and the PANI-2 films, respectively because the utilization of active material decreased, leading to a decrease in capacitance [22]. Figure 2(c) shows the specific capacitance values calculated from the CV measurements as a function of scan rate. The decrease in the capacitance values is described according by the Randle-Sevcik relationship, in which the current (which is a measure of the supercapacitive properties) of the electrochemical redox reaction is linearly proportional to the concentration of the electroactive species and the square root of the scan rate. Ip = Kn3/2 AD1/2 Hf 1/2 ,
(2)
where the constant K = 2.72 × 105 , n is the number of moles of electrons transferred per mole of electroactive species, A is the area of the electrode in cm2 , D is the diffusion coefficient in cm2 /s, H is the solution concentration in mole/L, and f is the scan rate of the potential in volt/s. Here, the concentration of the electroactive species is the same at different scan rates. If the peak current increases linearly with the scan rate, then it is reasonable to say that it is a diffusion-controlled (reversible) reaction. According to the inset of Fig. 2(c), the reaction is an irreversible redox reaction at the electrodeelectrolyte interface. This irreversible nature of the electrode reduces the charge-transfer processes at the interface. We believe that this is a possible reason for the decrease in the supercapacitive properties with the scan rate. Figure 3 shows charging-discharging curves recorded in the voltage range from -0.6 V to 0.1 V (versus
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Fig. 3. (Color online) Electrochemical supercapacitive characteristics of the PANI films. (a), (b) Charge-discharge curves for the PANI-1 film and the PANI-2 film, respectively, at a constant current of 1 mAcm−2 in a 0.5M H2 SO4 electrolyte within the potential window of -0.6 V to 0.1 V versus the SCE. (c) Specific capacitance versus number of cycles for the PANI-1 and the PANI-2 electrodes (calculated from the charge-discharge measurements). The inset shows the charge-discharge curve of the PANI-2 film for 100 cycles.
Csp =
Fig. 4. (Color online) Electrochemical cycling stability measurement. (a), (b) CV curves of the PANI-1 and the PANI-2 films, respectively, after 100 cycles in a 0.5M H2 SO4 electrolyte at a scan rate of 100 mVs−1 .
SCE) at an applied current density of 1 mAcm−2 . The charge/discharge curves exhibit reversible characteristics without apparent deviation in each cycle. This suggests good electrochemical stability for the PANI electrodes. The specific capacitances were calculated from the charge-discharge curves by using the following equation [1]:
i∆t , ∆V m
(3)
where i is the charge-discharge current, ∆t is the discharge time, ∆V is the electrochemical potential window and m is the mass of the active material. The specific capacitance values were found to be 446 Fg−1 and 473 Fg−1 for the PANI-1 and the PANI-2 films, respectively. The specific capacitance values of the cells determined using this galvanostatic charge-discharge method followed the trend determined by the CV measurements. These specific capacitance values are in agreement with those reported in the literature for pure PANI films [18–20,23]. The specific capacitance of PANI-based electrochemical capacitors depends on many factors, such as the mass of the active electrode, the conductivity of the active electrode, the concentration of the electrolyte, and the morphology of the electrode. Electrochemical stability is one of the important indicators of device quality. To test the stability of the devices, the films were chargeddischarged repeatedly for 100 cycles in a 5M H2 SO4 electrolyte at a constant current density of 1 mAcm−2 . Figure 3(c) shows the measured specific capacitances of the PANI-1 and the PANI-2 films for 100 cycles. The capacitance values were initially found to decrease after tens of cycles and to subsequently become stable. A 16% capacitance loss occurred during the initial cycles. Even after 100 cycles, the charge-discharge curve appears similar, with only a negligible change in capacitance values. These results suggest that PANI films synthesized using the ED technique are suitable for supercapacitor applications. The inset shows the measured charge-discharge curve of the PANI-2 film. The fast charge-discharge and electrochemical stability of the PANI electrodes was also examined using a CV test. Figures 4(a) and (b) show CVs of PANI-1 and PANI-2 electrodes after 100 reduction-oxidation cycles in 0.5M H2 SO4 electrolyte, respectively. As the number
Supercapacitive Characteristics of Electrodeposited Polyaniline Thin Films · · · – A. I. Inamdar et al.
of cycles increased, the capacitance current decreased for both the PANI-1 and the PANI-2 films. This decrease is attributed to the degradation of polyaniline due to mechanical doping/dedoping by the acid [20]. The shape of the CV is one of the important factors used to judge the electrochemical stability of the electrode during charge-discharge cycling. The shape of the measured CVs for the PANI-2 film remained relatively unchanged compared to that of the PANI-1 film. This indicates that the PANI-2 film is electrochemically more stable. The noticeable change in the CV shape of the PANI-1 film is presumably due to a degradation of the electrode’s surface during the doping/de-doping process.
IV. CONCLUSION PANI thin films with different thicknesses were successfully synthesized onto ITO substrates by using an electrodeposition technique for electrochemical supercapacitor applications. SEM and XRD measurements showed that the PANI films were compact and amorphous. The electrochemical supercapacitor measurements were performed in a 0.5M H2 SO4 electrolyte by using cyclic voltametry and charge-discharge techniques in the galvanostatic mode. The highest supercapacitance values were found to be ∼473 Fg−1 for 250nm thick PANI film (PANI-1) and ∼446 Fg−1 for 220nm-thick PANI film (PANI-2). The capacitance values were almost the same as those calculated from CV measurements. According to electrochemical stability measurements, the thicker PANI film (PANI-2) was electrochemically more stable than the thinner film (PANI-1).
ACKNOWLEDGMENTS This project was supported by the National Research Foundation (NRF) of Korea (Grant Nos. K2090100000209E0100-00210 and 2009-0076332).
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