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Activated carbon coated with polyaniline as an electrode material in supercapacitors
NEW CARBON MATERIALS Volume 23, Issue 3, March 2008 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2008, 23(3): 275–280.
RESEARCH PAPER
Activated carbon coated with polyaniline as an electrode material in supercapacitors WANG Qin1, LI Jian-ling1*, GAO Fei1, LI Wen-sheng2, WU Ke-zhong1, WANG Xin-dong1 1
Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China
2
Jinzhou Kaimei Energy Co. Ltd, Jinzhou 121000,China
Abstract: The performance of activated carbons coated with polyaniline (PANI-C) by polymerization of aniline using cyclic voltammetry was investigated as an electrode for supercapacitors by scanning electron microscopy, electrochemical impedance spectroscopy, and constant current charge/discharge measurement. Results showed that a PANI thin film was uniformly deposited on the surface of the activated carbon, forming an interlinked porous network. The PANI-C composite electrodes had better cycling stability than PANI electrodes and the specific capacitance of the composite electrodes was 587 F/g, which was much higher than that of the pristine activated carbon (140 F/g), owing to the faradic reaction of PANI with the electrolyte. The PANI electrode was less stable than the PANI-C composite electrode with a capacitance decay from 513 to 334 F/g for the former and 415 to 385 for the latter, after 50 cycles. Key Words: Supercapacitor; Polyaniline; Activated carbon; Cyclic voltammetry; Specific capacitance
1
Introduction
In recent years, supercapacitors have been attracting great attention because of their high capacitance and potential applications in electronic devices[1]. There has been more interest in two types of supercapacitors. One is the electric double-layer capacitor, and the other is the redox capacitor. In the former case, energy storage arises mainly from the ionic charge separation at the electrode electrolyte interface[2-6]. In the latter case, a faradic process, due to redox reactions, takes place at the electrode materials (such as conducting polymers and metal oxides) at characteristic potentials[7-8]. An activated carbon electrode, an electric double-layer type, has demonstrated a higher cyclic-life, but a lower capacitance value than those of the materials used in the redox capacitor, such as the conducting polymer. The electrically conducting polymers, a redox type, including polyaniline, polypyrrole, and so on, give high capacitance because capacitive and Faradic currents contribute to the charge storage[9]. Polyaniline (PANI) is one of the most important organic conducting polymers. Its unique advantages include easy preparation in aqueous medium, good environmental stability in air, simplicity in doping, improved electronic properties, electrochromic effects, good electrochemistry, and moderately high conductivity in the doped form. All these have made it an exceptionally versatile material with application in such areas as batteries, electronics, nonlinear optics, sensors, and so on[10-11]. However, the conducting polymers, exhibiting large specific capacitance, have disadvantages that include a
lower cyclic-life in the charge–discharge duty than carbon-based electrodes because the redox sites in the polymer backbone are not sufficiently stable for many repeated redox processes. Therefore, it is interesting to develop feasible composite electrodes with a long cycle life and large specific capacitance, which can be made by incorporating PANI into porous carbons, for application in supercapacitors[12-13]. In the present work, PANI was uniformly deposited onto high surface area carbons by means of the cyclic voltammetry (CV) technique, to improve the performance of the electrodes, and the electrochemical properties of the electrodes were investigated by CV, electrochemical impedance spectroscopy (EIS), and the charge–discharge cycling test.
2 2.1
Experimental Materials
Aniline• •ANI• •and H2SO4 employed in the present work were purchased from the Beijing Chemical Reagent Company and were of analytical grade, and aniline was doubly distilled and the resulting colorless liquid was kept under nitrogen, in darkness. The solvent, N-methylpyrrolidinone (NMP) was also of analytical grade and used directly. The activated carbon powder was bought from Ningde Xinsen Activated Carbon Co., Ltd (China). The exposed geometric area of this stainless steel mesh substrate was 1 cm×1 cm. Deionized and doubly distilled water was employed to prepare all solutions. 2.2
Preparation of electrodes
Received date: 10 October 2007; Revised date: 28 August 2008 *Corresponding author. E-mail: [email protected] Copyright©2008, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
WANG Qin et al. / New Carbon Materials, 2008, 23(3): 275–280
Fig.1 SEM photographs of AC and PANI-C electrode: (a) AC electrode ; (b) PANI-C electrode and (c) magnification of (b)
For the preparation of the carbon electrode, porous carbon powder was mixed with 10% mass fraction of carbon black as a conductive filler and 20% mass fraction of poly(vinylideneflouride) (PVdF) dissolved in NMP as a binder to form a slurry. The carbon electrodes were fabricated by coating the slurry on a stainless steel mesh, followed by evaporating the solvent, NMP. This form of electrode is the activated carbon electrode (AC electrode). PANI film was electrochemically loaded into the as-prepared AC electrode and the stainless steel substrate through CV in a potential range between –0.2 and 0.8 V. The polymerization of aniline was carried out for three CV cycles with a scanning rate of 5 mV/s in a solution containing 0.5 mol/L H2SO4 and 0.2 mol/L aniline. After polymerization, the electrodes were rinsed thoroughly with doubly distilled water and then dried in a vacuum oven at 50 °C, overnight. Such PANI-loaded carbon electrodes and the PANI-stainless steel electrodes are named as PANI-C electrodes and PANI electrodes respectively. 2.3
SEM and electrochemical measurements
The surface morphology of the electrodes was observed by JSM-6301F model SEM (LEO-1450, UK). The electrochemical experiments were carried with a VMP2 Multichannel Potentiosta (AMETEK Princeton Applied Research). Electrochemical measurements (CV, EIS, and capacitance measurement) were carried out at ambient temperature using 0.5 mol/L H2SO4 aqueous solution as the electrolyte, to examine the electrochemical performance of the AC electrode, PANI electrode, and PANI-C electrode. All electrochemical experiments were performed by using a double compartment glass cell with a three-electrode configuration, in which an AC, PANI-C or PANI electrode was used as a working electrode. The stainless steel electrode was used as a counter electrode and a saturated calomel electrode (SCE) was used as a reference electrode.
3
Results and discussion
3.1 Morphological properties of the resulting AC and PANI-C electrode
Fig.1 (a) and (b) show the morphological properties of AC and PANI-C electrode, respectively. The carbon was composed of gravel-like particles with different sizes as shown in Fig.1(a). Activated carbon particles with smooth surface morphology are clearly found. The surfaces of the carbon particles are uniformly covered by PANI film and the morphology of the electrode is significantly changed by the deposition of PANI, as shown in Fig.1(b). Fig.1(c) shows the magnification of Fig.1(b) for PANI film deposited on the carbon surface. It is clearly seen that the PANI film displays an interlinked porous network composed of PANI fibers, thus there are many more pathways through the network to make contact between the electrode materials and electrolyte ions more complete, leading to an improvement in the performance of the electrode. 3.2 Cyclic voltammetry and charge/discharge measurement of AC and PANI-C electrodes Potential sweep cyclic voltammetric measurements have been performed in 0.5 mol/L H2SO4 solution in a potential range between –0.2 and 0.7 V to examine the electrochemical characteristics of the resulting electrodes. The voltammograms of the AC electrode and PANI-C electrode are shown in Fig.2.
Fig.2 Cyclic voltammograms of the resulting electrodes at a scanning rate of 5 mV/s in 0.5 mol/L H2SO4. (a) AC electrode and (b) PANI-C electrode
WANG Qin et al. / New Carbon Materials, 2008, 23(3): 275–280
transitions of PANI[20].
Fig.3 Specific capacitances of the AC electrode and PANI-C electrode as a function of discharge current density. (a) AC electrode and (b) PANI-C electrode
Curve (a) shows a rectangular-like shape, which is a typical i-E response of the AC electrode in aqueous media, indicating an electric double-layer charge/discharge characteristic. In addition, currents rapidly reach a steady state value when the sweep direction changes, indicating that the equivalent series resistance (ESR) on this electrode should be low. Obviously, there are three couples of redox peaks that correspond to three different processes of the PANI redox transition on curve (b). A pair of redox peaks founded at approximately -50 and 150 mV on the negative and positive sweeps are attributed to the redox transition of PANI between the leucoemeraldine and polaronicemeralding forms[14-16]. A pair of redox peaks appearing at around 400 mV rise from the redox transition between p-benzoquinone and the hydroquinone forms of PANI. The last two peaks founded at approximately 650 mV are attributed to the redox transition between the polaronicemeraldine and the bipolaronic pernigraniline forms of PANI [17-19]. Much higher voltammetric currents are clearly found on both the positive and negative sweeps, revealing a significant contribution of the redox currents of PANI to the energy storage. In addition, the charges on the positive sweep are approximately equal to those on the corresponding negative sweep, revealing a good electrochemical reversibility of redox
Fig.4
The constant current charge/discharge test was carried out to investigate the electrochemical performance of the AC electrode and the PANI-C electrode. The constant current charge–discharge cycles were obtained between 0 and 0.7 V at current densities of 0.1-1.0 A/g. The average discharge capacitances of the resulting electrodes calculated between 0 and 0.7 V are shown in Fig.3, as a function of the discharge current density. It was clearly seen that the capacitance of the PANI-C electrode was much higher than that of the carbon electrode. The specific capacitance of the AC electrode and PANI-C electrode at current density of 0.1 A/g were 587 and 140 F/g, respectively. When the current density increased to 1 A/g, the specific capacitance of these two kinds of electrodes was 382 and 121 F/g, respectively. The PANI-C electrode, which was formed by loading the faradaic active species, PANI, onto a large specific surface area material, activated carbon, would contribute to energy storage by both the double-layer and the redox type mechanisms. It could also be seen that the decay of the AC electrode was much less than the PANI-C electrode as the current density increased, indicating that activated carbon was more stable than PANI during charge/discharge cycling. 3.3 Cycling stability of resulting PANI and PANI-C electrodes In order to examine the cycling stability of the PANI-C electrode and PANI electrode, the constant current charge/discharge cycling, CV, and EIS of those electrodes, before and after the charge/discharge cycle, were studied in 0.5 mol/L H2SO4. The cyclic voltammograms measurement has been carried out at 0.5 mol/L H2SO4, with a scanning rate of 5 mV/s. Fig.4 shows the cyclic voltammograms of the PANI-C electrode and PANI electrode. It is clearly seen that Fig.4(1) and Fig.4(2) display three couples of current peaks, which correspond to the conversion of different oxidation-state and reduction-state structures of PANI[21]. For the PANI electrode in Fig.4(2), the sharp peak at ~0.2 V corresponding to the oxidation and
CV curves of PANI-C electrode (1) and PANI electrode (2). (a) before charge/discharge, (b) after 5 charge/discharge cycles, and (c) after 50 charge/discharge cycles
WANG Qin et al. / New Carbon Materials, 2008, 23(3): 275–280
Fig.5 Impedance spectroscopy of the PANI-C electrode (1) and PANI electrode (2). (a) before charge/discharge, (b) after 5 charge/discharge cycles and (c) after 50 charge/discharge cycles
cles. The potential amplitude of ac was kept at 10 mV and the frequency range of 10 kHz to 10 mHz was used. Fig.5 shows the Nyquist diagrams of different electrodes. A single semi-circle in the high-frequency region and a straight line in the low-frequency region can be observed for all spectra.
Fig.6 Variation of capacitance with cycle number for PANI-C electrode and PANI electrode charged and discharged at 0.5 A/g in 0.5 mol/L H2SO4 solutions
p-doping reaction with H+ is much higher than the peaks of the other two couples. However, the difference among the three couples of current peaks is not so obvious in Fig.4(1). This is because the mass of carbon is much larger than that of PANI, which makes the current contributed by PANI much less than that contributed by the activated carbon. On the other hand, voltammetric currents corresponding to these peaks (peak currents) gradually decrease with the cycling test for both electrodes. This loss of redox charges for a PANI electrode is attributed to the degradation of PANI, resulting in the formation of the hydrolysis products, p-benzoquinone and hydroquinone. Furthermore, the redox sites in the PANI backbone are insufficiently stable for many repeated redox processes, which is probably due to swelling and deswelling of PANI under aqueous environment, leading to the degradation of PANI during long-term cycling[22]. In case of the PANI-C electrode, PANI forms a very thin film on the surface of the carbon particles, and the volume change caused by swelling and deswelling of the PANI chain is greatly alleviated by surface tension that tends to restrain the change, leading to a more stable capacitance with the cycling test[23]. EIS were analyzed for PANI and PANI-C electrode in 0.5 mol/L H2SO4 solutions before and after charge/discharge cy-
As the number of charge/discharge cycles increase, the diameter of the semi-circle, which represents the overall contact impedance of the electrode[24], increases dramatically for the PANI electrode, but this change was slight in the PANI-C electrode. The resistance increase for the PANI electrode and the PANI-C electrode after 50 cycles was from ~1.3 to ~47 ȍ and from ~0.9 to~1.5 ȍ, respectively. This was in coincidence with the results of CV, and it was probably the swelling and deswelling of the PANI chain that led to the collapse of the PANI backbone during long-term charge/discharge cycles, making the insertion–deinsertion of ions involved in the charge/discharge process more difficult. However, for the PANI-C electrode, the process involved during the charge/discharge was mainly separation of ionic charges at the interface, between the carbon particles and the electrolyte solution, and the redox reaction for PANI occured just on the surface since the PANI film that formed was very thin. Therefore, the cycling stability of the PANI-C electrode was much better than the PANI electrode. Fig.6 shows the relationship of the specific discharge capacitance with the number of cycles. The specific capacitances of the PANI electrode are ~520F/g at the first cycle and ~334 F/g at the fiftieth cycle, respectively, and a 36% capacitance decrease is found after 50 cycles. The specific capacitances of the PANI-C electrode are ~415 F/g at the first cycle and ~383 F/g at the fiftieth cycle and only a 7% loss is found. Furthermore, it is clearly seen that specific capacitance of PANI electrode is higher than that of the PANI-C electrode only during the first 12 cycles. The capacitance decay is probably due to the degradation of PANI during cycling. Swelling and deswelling of this electroactive polymer may also lead to degradation[25]. In conclusion, the PANI-C electrode shows good cycling property compared to the PANI electrode. This is in coincidence with results of CV and EIS, in which degradation
WANG Qin et al. / New Carbon Materials, 2008, 23(3): 275–280
and deformation occur in the PANI chain as the number of cycles increase, with an increase in resistance and a decrease in the capacitance.
4
polymer films onto electrode surfaces[J]. Electrochimica Acta, 1999, 44: 1901-1910.
Conclusions
[12] Frackowiak E, Béguin F. Carbon materials for the electro-
The preparation of AC, PANI, and PANI-C electrodes are discussed in this article. The morphological properties of AC and PANI-C electrodes have been determined by SEM. The PANI-C composite electrode exhibits a much higher specific capacitance than the AC electrode, due to the addition of faradic pseudocapacitance from PANI, and the PANI-C electrode shows a better cycling stability than the PANI electrode. As a result, loading of PANI onto high surface area activated carbon can greatly improve the performance of the electrode.
References
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RESEARCH PAPER
Activated carbon coated with polyaniline as an electrode material in supercapacitors WANG Qin1, LI Jian-ling1*, GAO Fei1, LI Wen-sheng2, WU Ke-zhong1, WANG Xin-dong1 1
Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China
2
Jinzhou Kaimei Energy Co. Ltd, Jinzhou 121000,China
Abstract: The performance of activated carbons coated with polyaniline (PANI-C) by polymerization of aniline using cyclic voltammetry was investigated as an electrode for supercapacitors by scanning electron microscopy, electrochemical impedance spectroscopy, and constant current charge/discharge measurement. Results showed that a PANI thin film was uniformly deposited on the surface of the activated carbon, forming an interlinked porous network. The PANI-C composite electrodes had better cycling stability than PANI electrodes and the specific capacitance of the composite electrodes was 587 F/g, which was much higher than that of the pristine activated carbon (140 F/g), owing to the faradic reaction of PANI with the electrolyte. The PANI electrode was less stable than the PANI-C composite electrode with a capacitance decay from 513 to 334 F/g for the former and 415 to 385 for the latter, after 50 cycles. Key Words: Supercapacitor; Polyaniline; Activated carbon; Cyclic voltammetry; Specific capacitance
1
Introduction
In recent years, supercapacitors have been attracting great attention because of their high capacitance and potential applications in electronic devices[1]. There has been more interest in two types of supercapacitors. One is the electric double-layer capacitor, and the other is the redox capacitor. In the former case, energy storage arises mainly from the ionic charge separation at the electrode electrolyte interface[2-6]. In the latter case, a faradic process, due to redox reactions, takes place at the electrode materials (such as conducting polymers and metal oxides) at characteristic potentials[7-8]. An activated carbon electrode, an electric double-layer type, has demonstrated a higher cyclic-life, but a lower capacitance value than those of the materials used in the redox capacitor, such as the conducting polymer. The electrically conducting polymers, a redox type, including polyaniline, polypyrrole, and so on, give high capacitance because capacitive and Faradic currents contribute to the charge storage[9]. Polyaniline (PANI) is one of the most important organic conducting polymers. Its unique advantages include easy preparation in aqueous medium, good environmental stability in air, simplicity in doping, improved electronic properties, electrochromic effects, good electrochemistry, and moderately high conductivity in the doped form. All these have made it an exceptionally versatile material with application in such areas as batteries, electronics, nonlinear optics, sensors, and so on[10-11]. However, the conducting polymers, exhibiting large specific capacitance, have disadvantages that include a
lower cyclic-life in the charge–discharge duty than carbon-based electrodes because the redox sites in the polymer backbone are not sufficiently stable for many repeated redox processes. Therefore, it is interesting to develop feasible composite electrodes with a long cycle life and large specific capacitance, which can be made by incorporating PANI into porous carbons, for application in supercapacitors[12-13]. In the present work, PANI was uniformly deposited onto high surface area carbons by means of the cyclic voltammetry (CV) technique, to improve the performance of the electrodes, and the electrochemical properties of the electrodes were investigated by CV, electrochemical impedance spectroscopy (EIS), and the charge–discharge cycling test.
2 2.1
Experimental Materials
Aniline• •ANI• •and H2SO4 employed in the present work were purchased from the Beijing Chemical Reagent Company and were of analytical grade, and aniline was doubly distilled and the resulting colorless liquid was kept under nitrogen, in darkness. The solvent, N-methylpyrrolidinone (NMP) was also of analytical grade and used directly. The activated carbon powder was bought from Ningde Xinsen Activated Carbon Co., Ltd (China). The exposed geometric area of this stainless steel mesh substrate was 1 cm×1 cm. Deionized and doubly distilled water was employed to prepare all solutions. 2.2
Preparation of electrodes
Received date: 10 October 2007; Revised date: 28 August 2008 *Corresponding author. E-mail: [email protected] Copyright©2008, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
WANG Qin et al. / New Carbon Materials, 2008, 23(3): 275–280
Fig.1 SEM photographs of AC and PANI-C electrode: (a) AC electrode ; (b) PANI-C electrode and (c) magnification of (b)
For the preparation of the carbon electrode, porous carbon powder was mixed with 10% mass fraction of carbon black as a conductive filler and 20% mass fraction of poly(vinylideneflouride) (PVdF) dissolved in NMP as a binder to form a slurry. The carbon electrodes were fabricated by coating the slurry on a stainless steel mesh, followed by evaporating the solvent, NMP. This form of electrode is the activated carbon electrode (AC electrode). PANI film was electrochemically loaded into the as-prepared AC electrode and the stainless steel substrate through CV in a potential range between –0.2 and 0.8 V. The polymerization of aniline was carried out for three CV cycles with a scanning rate of 5 mV/s in a solution containing 0.5 mol/L H2SO4 and 0.2 mol/L aniline. After polymerization, the electrodes were rinsed thoroughly with doubly distilled water and then dried in a vacuum oven at 50 °C, overnight. Such PANI-loaded carbon electrodes and the PANI-stainless steel electrodes are named as PANI-C electrodes and PANI electrodes respectively. 2.3
SEM and electrochemical measurements
The surface morphology of the electrodes was observed by JSM-6301F model SEM (LEO-1450, UK). The electrochemical experiments were carried with a VMP2 Multichannel Potentiosta (AMETEK Princeton Applied Research). Electrochemical measurements (CV, EIS, and capacitance measurement) were carried out at ambient temperature using 0.5 mol/L H2SO4 aqueous solution as the electrolyte, to examine the electrochemical performance of the AC electrode, PANI electrode, and PANI-C electrode. All electrochemical experiments were performed by using a double compartment glass cell with a three-electrode configuration, in which an AC, PANI-C or PANI electrode was used as a working electrode. The stainless steel electrode was used as a counter electrode and a saturated calomel electrode (SCE) was used as a reference electrode.
3
Results and discussion
3.1 Morphological properties of the resulting AC and PANI-C electrode
Fig.1 (a) and (b) show the morphological properties of AC and PANI-C electrode, respectively. The carbon was composed of gravel-like particles with different sizes as shown in Fig.1(a). Activated carbon particles with smooth surface morphology are clearly found. The surfaces of the carbon particles are uniformly covered by PANI film and the morphology of the electrode is significantly changed by the deposition of PANI, as shown in Fig.1(b). Fig.1(c) shows the magnification of Fig.1(b) for PANI film deposited on the carbon surface. It is clearly seen that the PANI film displays an interlinked porous network composed of PANI fibers, thus there are many more pathways through the network to make contact between the electrode materials and electrolyte ions more complete, leading to an improvement in the performance of the electrode. 3.2 Cyclic voltammetry and charge/discharge measurement of AC and PANI-C electrodes Potential sweep cyclic voltammetric measurements have been performed in 0.5 mol/L H2SO4 solution in a potential range between –0.2 and 0.7 V to examine the electrochemical characteristics of the resulting electrodes. The voltammograms of the AC electrode and PANI-C electrode are shown in Fig.2.
Fig.2 Cyclic voltammograms of the resulting electrodes at a scanning rate of 5 mV/s in 0.5 mol/L H2SO4. (a) AC electrode and (b) PANI-C electrode
WANG Qin et al. / New Carbon Materials, 2008, 23(3): 275–280
transitions of PANI[20].
Fig.3 Specific capacitances of the AC electrode and PANI-C electrode as a function of discharge current density. (a) AC electrode and (b) PANI-C electrode
Curve (a) shows a rectangular-like shape, which is a typical i-E response of the AC electrode in aqueous media, indicating an electric double-layer charge/discharge characteristic. In addition, currents rapidly reach a steady state value when the sweep direction changes, indicating that the equivalent series resistance (ESR) on this electrode should be low. Obviously, there are three couples of redox peaks that correspond to three different processes of the PANI redox transition on curve (b). A pair of redox peaks founded at approximately -50 and 150 mV on the negative and positive sweeps are attributed to the redox transition of PANI between the leucoemeraldine and polaronicemeralding forms[14-16]. A pair of redox peaks appearing at around 400 mV rise from the redox transition between p-benzoquinone and the hydroquinone forms of PANI. The last two peaks founded at approximately 650 mV are attributed to the redox transition between the polaronicemeraldine and the bipolaronic pernigraniline forms of PANI [17-19]. Much higher voltammetric currents are clearly found on both the positive and negative sweeps, revealing a significant contribution of the redox currents of PANI to the energy storage. In addition, the charges on the positive sweep are approximately equal to those on the corresponding negative sweep, revealing a good electrochemical reversibility of redox
Fig.4
The constant current charge/discharge test was carried out to investigate the electrochemical performance of the AC electrode and the PANI-C electrode. The constant current charge–discharge cycles were obtained between 0 and 0.7 V at current densities of 0.1-1.0 A/g. The average discharge capacitances of the resulting electrodes calculated between 0 and 0.7 V are shown in Fig.3, as a function of the discharge current density. It was clearly seen that the capacitance of the PANI-C electrode was much higher than that of the carbon electrode. The specific capacitance of the AC electrode and PANI-C electrode at current density of 0.1 A/g were 587 and 140 F/g, respectively. When the current density increased to 1 A/g, the specific capacitance of these two kinds of electrodes was 382 and 121 F/g, respectively. The PANI-C electrode, which was formed by loading the faradaic active species, PANI, onto a large specific surface area material, activated carbon, would contribute to energy storage by both the double-layer and the redox type mechanisms. It could also be seen that the decay of the AC electrode was much less than the PANI-C electrode as the current density increased, indicating that activated carbon was more stable than PANI during charge/discharge cycling. 3.3 Cycling stability of resulting PANI and PANI-C electrodes In order to examine the cycling stability of the PANI-C electrode and PANI electrode, the constant current charge/discharge cycling, CV, and EIS of those electrodes, before and after the charge/discharge cycle, were studied in 0.5 mol/L H2SO4. The cyclic voltammograms measurement has been carried out at 0.5 mol/L H2SO4, with a scanning rate of 5 mV/s. Fig.4 shows the cyclic voltammograms of the PANI-C electrode and PANI electrode. It is clearly seen that Fig.4(1) and Fig.4(2) display three couples of current peaks, which correspond to the conversion of different oxidation-state and reduction-state structures of PANI[21]. For the PANI electrode in Fig.4(2), the sharp peak at ~0.2 V corresponding to the oxidation and
CV curves of PANI-C electrode (1) and PANI electrode (2). (a) before charge/discharge, (b) after 5 charge/discharge cycles, and (c) after 50 charge/discharge cycles
WANG Qin et al. / New Carbon Materials, 2008, 23(3): 275–280
Fig.5 Impedance spectroscopy of the PANI-C electrode (1) and PANI electrode (2). (a) before charge/discharge, (b) after 5 charge/discharge cycles and (c) after 50 charge/discharge cycles
cles. The potential amplitude of ac was kept at 10 mV and the frequency range of 10 kHz to 10 mHz was used. Fig.5 shows the Nyquist diagrams of different electrodes. A single semi-circle in the high-frequency region and a straight line in the low-frequency region can be observed for all spectra.
Fig.6 Variation of capacitance with cycle number for PANI-C electrode and PANI electrode charged and discharged at 0.5 A/g in 0.5 mol/L H2SO4 solutions
p-doping reaction with H+ is much higher than the peaks of the other two couples. However, the difference among the three couples of current peaks is not so obvious in Fig.4(1). This is because the mass of carbon is much larger than that of PANI, which makes the current contributed by PANI much less than that contributed by the activated carbon. On the other hand, voltammetric currents corresponding to these peaks (peak currents) gradually decrease with the cycling test for both electrodes. This loss of redox charges for a PANI electrode is attributed to the degradation of PANI, resulting in the formation of the hydrolysis products, p-benzoquinone and hydroquinone. Furthermore, the redox sites in the PANI backbone are insufficiently stable for many repeated redox processes, which is probably due to swelling and deswelling of PANI under aqueous environment, leading to the degradation of PANI during long-term cycling[22]. In case of the PANI-C electrode, PANI forms a very thin film on the surface of the carbon particles, and the volume change caused by swelling and deswelling of the PANI chain is greatly alleviated by surface tension that tends to restrain the change, leading to a more stable capacitance with the cycling test[23]. EIS were analyzed for PANI and PANI-C electrode in 0.5 mol/L H2SO4 solutions before and after charge/discharge cy-
As the number of charge/discharge cycles increase, the diameter of the semi-circle, which represents the overall contact impedance of the electrode[24], increases dramatically for the PANI electrode, but this change was slight in the PANI-C electrode. The resistance increase for the PANI electrode and the PANI-C electrode after 50 cycles was from ~1.3 to ~47 ȍ and from ~0.9 to~1.5 ȍ, respectively. This was in coincidence with the results of CV, and it was probably the swelling and deswelling of the PANI chain that led to the collapse of the PANI backbone during long-term charge/discharge cycles, making the insertion–deinsertion of ions involved in the charge/discharge process more difficult. However, for the PANI-C electrode, the process involved during the charge/discharge was mainly separation of ionic charges at the interface, between the carbon particles and the electrolyte solution, and the redox reaction for PANI occured just on the surface since the PANI film that formed was very thin. Therefore, the cycling stability of the PANI-C electrode was much better than the PANI electrode. Fig.6 shows the relationship of the specific discharge capacitance with the number of cycles. The specific capacitances of the PANI electrode are ~520F/g at the first cycle and ~334 F/g at the fiftieth cycle, respectively, and a 36% capacitance decrease is found after 50 cycles. The specific capacitances of the PANI-C electrode are ~415 F/g at the first cycle and ~383 F/g at the fiftieth cycle and only a 7% loss is found. Furthermore, it is clearly seen that specific capacitance of PANI electrode is higher than that of the PANI-C electrode only during the first 12 cycles. The capacitance decay is probably due to the degradation of PANI during cycling. Swelling and deswelling of this electroactive polymer may also lead to degradation[25]. In conclusion, the PANI-C electrode shows good cycling property compared to the PANI electrode. This is in coincidence with results of CV and EIS, in which degradation
WANG Qin et al. / New Carbon Materials, 2008, 23(3): 275–280
and deformation occur in the PANI chain as the number of cycles increase, with an increase in resistance and a decrease in the capacitance.
4
polymer films onto electrode surfaces[J]. Electrochimica Acta, 1999, 44: 1901-1910.
Conclusions
[12] Frackowiak E, Béguin F. Carbon materials for the electro-
The preparation of AC, PANI, and PANI-C electrodes are discussed in this article. The morphological properties of AC and PANI-C electrodes have been determined by SEM. The PANI-C composite electrode exhibits a much higher specific capacitance than the AC electrode, due to the addition of faradic pseudocapacitance from PANI, and the PANI-C electrode shows a better cycling stability than the PANI electrode. As a result, loading of PANI onto high surface area activated carbon can greatly improve the performance of the electrode.
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