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Fuzzy nanofibrous network of polyaniline electrode for supercapacitor application
Synthetic Metals 160 (2010) 519–522
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Fuzzy nanofibrous network of polyaniline electrode for supercapacitor application D.S. Dhawale, D.P. Dubal, V.S. Jamadade, R.R. Salunkhe, C.D. Lokhande ∗ Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416004 (M.S.), India
a r t i c l e
i n f o
Article history: Received 28 August 2009 Received in revised form 22 December 2009 Accepted 18 January 2010 Available online 4 February 2010 Keywords: Fuzzy nanofibrous network Polyaniline Electrodeposition Supercapacitor
a b s t r a c t Fuzzy nanofibrous network of polyaniline electrode is successfully electrosynthesized for supercapacitor application. The nanofibre network of polyaniline electrode is characterized using Fourier transforms infrared spectroscopy (FTIR), scanning electron microscope (SEM) and optical absorption studies. Network of polyaniline is highly porous with interconnected fuzzy nanofibres having diameter typically between 120 and 125 nm. The supercapacitive performance of polyaniline electrode is tested using cyclic voltammetry (C-V) technique in H2 SO4 electrolyte within potential range of −100 to 800 mV. The effect of scan rate on the capacitance of polyaniline electrode is studied. The highest specific capacitance of 839 F g−1 at the voltage scan rate of 10 mV s−1 is achieved. Additionally stability and charging–discharging of polyaniline electrode are studied. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, electrochemical capacitors or supercapacitors have attracted much attention because of their high-power delivery within a very short time and long cycle life, which can store high amounts of energy. Supercapacitors have found an increasingly important role in power-source applications such as hybrid electric vehicles and short-term power sources for mobile electronic devices [1,2]. Generally, supercapacitors can be classified into two types: (1) electric double-layer capacitors based on carbon materials with a high surface area and (2) faradic pseudo-capacitors based on metal oxides and conducting polymers. Conducting polymers offer advantages of low cost comparison with metal oxide and a higher charge density than carbon, these often have good intrinsic auto conductivity. Among the conducting polymers, polyaniline (PANI) is most investigated and studied for its electrochemical characterization [3–9]. Polyaniline possesses high conductivity, good redox reversibility and environmental stability [10]. Due to these properties, polyaniline has been extensively studied for diverse applications involving rechargeable batteries, electrochromic devices, sensors, etc [11,12]. Polyaniline refers to a general class of conducting polymers composed of benzoid and quinoid character, connected by nitrogen. Polyaniline is built up from reduced (–B–NH–B–NH–)n and oxidized (–B–N1/4 –Q–N1/4 )n repeat units, where “B” denotes benzoid and “Q” denotes quinoid
∗ Corresponding author. Tel.: +91 231 2609229; fax: +91 231 2692333. E-mail address: l [email protected] (C.D. Lokhande). 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.01.021
rings. Thus, the changing ratio of amine to imine yields various structures, such as leucoemeraldine a reduced form of emeraldine base. In addition, emeraldine base is regarded as the most useful form of polyaniline due to its high stability at room temperature and the fact that upon doping the emeraldine salt form of polyaniline is electrically conducting. The polyaniline structure consists of up to 1000 or more repeating units and conductivity 10−11 to 10 S/cm. The materials in the nano-size form with high surface area and high porosity give the best performances as the electrode materials for supercapacitors. Therefore the synthesis and supercapacitive characterization of the high surface area nanomaterials of polyaniline such as nanotubes and nanowire [13–15] have been carried out in the past few years. Prasad and Munichandraiah [16] obtained a maximum specific capacitance of 815 F g−1 by potentiodynamical deposited nanofibre polyaniline on stainless steel substrates. Zhao and Li [17] also reported the highest specific capacitance of 1142 F g−1 for nanowire network of polyaniline. Thus, the high value of the specific capacitance was attributed to the high porous morphology obtained at a high sweep rate for the polyaniline by electrodeposition technique. From the structural point of view, growing of fibres of high aspect ratio by low temperature chemical methods such as electrodeposition can find extensive scope. Electrodeposition is a powerful and interesting process that can be applied in numerous fields. The advantages of electrodeposition compared with other techniques include low cost for raw materials and equipments, capability of controlling composition and morphology by varying electrochemical parameters and the ability to deposit films on a complex surface.
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D.S. Dhawale et al. / Synthetic Metals 160 (2010) 519–522
Our main interest is to prepare fuzzy nanofibrous interconnected polyaniline electrode for supercapacitor application using electrodeposition method. In this work, we have performed cyclic voltammetry measurement of the fuzzy nanofibrous polyaniline electrode deposited on the stainless steel substrate for electrochemical supercapacitor application. 2. Experimental details Aniline (99.5%, Merck) as the monomer was purified by double distillation under reduced pressure prior to use. Sulphuric acid (DBSA, 90%, Fluka) used as dopant. The stainless steel substrate (grade 304, 0.2 mm thick) was polished with emery paper to a rough finish, washed free of emery particles and then air-dried. The electrochemical synthesis of polyaniline was performed in a conventional three-electrode electrochemical cell, containing stainless steel substrate as a working electrode, graphite rod of a 2 cm2 surface area as a counter electrode and a saturated calomel electrode (SCE) as a reference electrode. The electrochemical anodization was carried out onto stainless steel substrates from aqueous acidic solution containing 0.5 M H2 SO4 + 0.45 M aniline (C6 H5 NH2 ) at room temperature (300 K). The deposition of well adherent and uniform green-colored polyaniline electrode was observed by applying a constant potential of +0.8 V/SCE for the deposition time 5 min. Thickness was calculated by gravimetric weight difference method by using sensitive microbalance with relation t = m/( × A) where, ‘m’ is the weight of the film deposited on the substrate in grams; ‘A’ the area of the deposited film in cm2 and ‘’ the density of the deposited material (polyaniline = 1.30 g/cm3 ) in bulk form. The maximum thickness obtained for polyaniline thin film was 0.9 m and surface roughness 592 nm. Fourier transformed infrared spectroscopy (FTIR) was recorded between 4000 and 450 cm−1 at a spectral resolution of 2 cm−1 on a PerkinElmer 1710 spectrophotometer using KBr pellets at room temperature. Surface morphological study was carried out using scanning electron microscopy (JEOL-6360). For this, the films were coated with a 10 nm platinum layer using a polaron scanning electron microscopy (SEM) sputter coating unit E-2500 before taking the image. To study the optical characteristics of the film, absorbance spectra were recorded in the range 450–750 nm by means of UV–vis spectrophotometer-119. The electrochemical supercapacitor performance of the polyaniline electrode was tested by cyclic voltammetry (C-V) using the Potentiostat (263 A EG&G Princeton Applied Research) and forming electrochemical cell configuration comprising platinum electrode as a counter electrode, saturated calomel electrode (SCE) as a reference electrode in H2 SO4 electrolyte.
Fig. 1. The FTIR spectrum of polyaniline thin film.
3.2. Surface morphogical studies The surface morphology of electrochemically deposited polyaniline film was investigated by scanning electron microscopy (SEM). Fig. 2(a and b) shows the SEM images of the polyaniline nanofibre network at different magnifications. From Fig. 2, it can be inferred that the nanofibre network of polyaniline is highly porous. The low magnification (5000× (Fig. 2a)) image confirms interconnected fuzzy nanofibrous network of polyaniline electrode. These fibres are relatively smooth with randomly distributed on the substrate having many pores in diameter and length ranging typically from 400 to 500 nm as observed from high magnification (10,000×). Although the polyaniline nanofibres have been studied
3. Results and discussion 3.1. Fourier transforms infrared (FTIR) study Polyaniline formation is also confirmed by Fourier transform infrared spectroscopy (FTIR) technique. The FTIR spectrum of polyaniline is shown in Fig. 1 and has several distinct bands at 3418, 2928, 1592, 1453, 1298 and 1128 cm−1 that are characteristics of polyaniline [18]. The peaks at 3418 and 2928 cm−1 are attributable to NH2 + and N–H stretching modes, The peaks at 1592 and 1453 cm−1 are attributed to C N and C C stretching modes for the quinoid and benzenoid rings. The peaks at about 1298 and 1243 cm−1 are attributed to C–N stretching mode for benzenoid ring and the peak at 1128 cm−1 is assigned to the plane bending vibration of C–H (modes of N Q N, Q N+H–B and B–N+H–B), which is formed during protonation [19] confirming formation of polyaniline.
Fig. 2. . The SEM images of polyaniline nanofibres at the magnifications of (a) 5000× and (b) 10,000×.
D.S. Dhawale et al. / Synthetic Metals 160 (2010) 519–522
Fig. 4. The CV curves of polyaniline electrode at different scanning rates in a 1.0 M H2 SO4 electrolyte.
Fig. 3. Plot of absorbance (˛t) vs. wavelength () of polyaniline thin film.
for various potential applications here, such a highly porous and large surface area network of polyaniline nanofibres is applied for supercapacitor. 3.3. Optical absorption studies For the optical studies, polyaniline films were deposited on FTO coated glass substrate. Fig. 3 shows the plot of absorbance (˛t) vs. wavelength () of polyaniline film and has two peaks nearly at 638 and 332 nm, these two characteristic peaks assigned to polyaniline thin film. This is excellent agreement with the results reported by Sarno et al. [20] for nanostructured polyaniline. The optical absorption is attributed due to transition of charge carriers through a forbidden energy gap. 3.4. Supercapacitive studies The supercapacitive performance of electrodeposited fuzzy fibrous polyaniline electrode was tested using cyclic voltamogramm (C-V) technique. The capacitance (C) was calculated using following relation: C=
Imax dV/dt
(1)
where I is the average current in ampere and dV/dt is the voltage scanning rate. The interfacial capacitance was calculated using the relation: Ci =
C A
521
−200 to 800 mV. Fig. 5 shows the variation of specific capacitance and interfacial capacitance with scan rate. From Fig. 5 it is seen that, specific and interfacial capacitance values are decreased from 839 to 484 F g−1 and 0.365–0.213 F cm−2 , respectively. The maximum specific capacitance of 839 F g−1 was obtained at 10 mV s−1 . The decrease in capacitance has been attributed to the presence of inner active sites that cannot sustain the redox transitions completely at higher scan rates. This is probably due to the diffusion effect of protons within the electrode. The decreasing trend of the capacitance suggests that parts of the surface of the electrode are inaccessible at high charging–discharging rates [24]. The cyclic stability of the nanofibrous polyaniline electrode at the sweep rate of 20 mV s−1 for 1st, 500th and 1000th cycles are shown in Fig. 6. From the figure it is seen that specific capacitance of polyaniline electrode decreased from 829 to 798 F g−1 in the first 500 cycles and thereafter the specific capacitance remained almost constant. This demonstrates that the material is suitable for energy-storage applications. The specific and interfacial capacitance values are decreased by a small amount with the number of cycles due to the loss of active material caused by the dissolution and/or detachment, during the early charging/discharging cycles in the electrolyte. The 90.71% cyclic stability of the fuzzy nanofibrous polyaniline electrode was observed. The charging and discharging behavior of nanofibrous polyaniline electrode was examined by chronoamperometric technique. Fig. 7 shows the charging–discharging behavior of polyaniline
(2)
where ‘A’ is the area of active material dipped in the electrolyte. The specific capacitance Cs (F g−1 ) of polyaniline electrode was calculated using following relation: Cs =
C W
(3)
where W is the weight of polyaniline film dipped in electrolyte. Due to the high-power demand in supercapacitors, one of the basic requirements for an electrode material in this field is the high or pulse-power characteristic [21,22]. This unique property is a strong function of the electrochemical kinetics of redox transitions within the electro-active materials [23]. Thus, the high or pulse-power property of nanofibrous polyaniline electrode is examined using cyclic voltammetry at different scan rates. Fig. 4 shows the CV curves of nanofibrous polyaniline electrode in 1 M H2 SO4 electrolyte for different scan rates within voltage range of
Fig. 5. Variation of specific and interfacial capacitances of polyaniline electrode at different scan rates. Concentration of H2 SO4 electrolyte was 1.0 M.
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change of polyaniline electrode is occurred due to the insertion of charges. 4. Conclusions In this work, we have successfully synthesized nanofibrous polyaniline electrode for electrochemical supercapacitor by electrochemical anodization method. The presence of characteristics bonds of polyaniline were confirmed by FTIR studies. The SEM study revealed that polyaniline films have fuzzy nanofibrous interconnected architecture. Polyaniline electrode showed maximum specific capacitance of 839 F g−1 at 10 mV s−1 . Acknowledgement Authors are grateful to the Department of Science and Technology, New Delhi for financial support through the scheme no. SR/S2/CMP-82/2006. Fig. 6. The CV curves of polyaniline electrode at different number of cycles. The scanning rate and concentration of H2 SO4 were 20 mV s−1 and 1 M, respectively.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Fig. 7. Charging–discharging plots of polyaniline electrode as a function of time in 1 M H2 SO4 electrolyte.
electrode in 1 M H2 SO4 electrolyte. The charge–discharge study revealed that, as the charging time increases, charge accumulated on nanofibrous polyaniline electrode increases and all curves reflects that all charges are discharged. It concludes that no phase
[19] [20] [21] [22] [23] [24]
L. Cao, F. Xu, Y.Y. Liang, H.L. Li, Adv. Mater. 16 (2004) 1853. R.A. Huggins, Solid State Ionics 134 (2000) 179. V. Khomenko, E. Franckowiak, F. Be guin, Electrochim. Acta 50 (2005) 2499. C.C. Hu, E. Chen, J.Y. Lin, Electrochim. Acta 47 (2002) 2741. C.C. Hu, W.Y. Li, J.Y. Lin, J. Power Sources 137 (2004) 152. V. Gupta, N. Miura, Electrochim. Acta 52 (2006) 1721. D. Belanger, X. Ren, J. Davey, F. Uribe, S. Gottesfeld, J. Electrochem. Soc. 147 (2000) 2923. F. Fusalba, P. Gouerec, D. Villers, D. Belanger, J. Electrochem. Soc. 148 (2001) A1. K.S. Ryu, K.M. Kim, N.G. Park, Y.J. Park, S.H. Chang, J. Power Sources 103 (2002) 305. N. Oyama, T. Tatsuma, T. Sato, T. Sotomura, Nature 373 (1995) 598. W.A. Gazzoti, Facz, M.A. Depach, J. Electroanal. Chem. 415 (1996) 107. G. Inzelt, E. Csahok, V. Kertesz, Electrochim. Acta 46 (2001) 3955. V. Gupta, N. Miura, Electrochem. Commun. 7 (2005) 995. V. Gupta, N. Miura, J. Power Sources 157 (2006) 616. K. Jurewicz, S. Delpeux, V. Bertagna, F. Béguin, E. Frakowiak, Chem. Phys. Lett. 347 (2001) 36. K. Rajendra Prasad, N. Munichandraiah, J. Electrochem. Soc. 149 (2002) 1393. G.Y. Zhao, H.L. Li, Micropor. Mesopor. Mater. 110 (2008) 590. N.S. Sariciftci, H. Kuzmany, H. Neugebauer, A. Neckel, J. Chem. Phys. 92 (1990) 4530. Y. He, J. Appl. Surf. Sci. 249 (2005) 1. D.M. Sarno, S.K. Manohar, A.G. MacDiarmid, Synth. Met. 148 (2005) 237. I. Sapurina, A.Y. Osadchev, B.Z. Volchek, M. Trchova, A. Riede, J. Stejskal, Synth. Met. 129 (2002) 29. Z. Chen, L. Xu, W. Li, M. Waje, Y. Yan, Nanotechnology 17 (2006) 5254. C.C. Hu, T.W. Tsou, Electrochem. Commun. 4 (2002) 105. T.P. Gujar, W. Kim, I. Puspitasari, K.D. Jung, O.S. Joo, Int. J. Electrochem. Sci. 22 (2007) 666.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Fuzzy nanofibrous network of polyaniline electrode for supercapacitor application D.S. Dhawale, D.P. Dubal, V.S. Jamadade, R.R. Salunkhe, C.D. Lokhande ∗ Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416004 (M.S.), India
a r t i c l e
i n f o
Article history: Received 28 August 2009 Received in revised form 22 December 2009 Accepted 18 January 2010 Available online 4 February 2010 Keywords: Fuzzy nanofibrous network Polyaniline Electrodeposition Supercapacitor
a b s t r a c t Fuzzy nanofibrous network of polyaniline electrode is successfully electrosynthesized for supercapacitor application. The nanofibre network of polyaniline electrode is characterized using Fourier transforms infrared spectroscopy (FTIR), scanning electron microscope (SEM) and optical absorption studies. Network of polyaniline is highly porous with interconnected fuzzy nanofibres having diameter typically between 120 and 125 nm. The supercapacitive performance of polyaniline electrode is tested using cyclic voltammetry (C-V) technique in H2 SO4 electrolyte within potential range of −100 to 800 mV. The effect of scan rate on the capacitance of polyaniline electrode is studied. The highest specific capacitance of 839 F g−1 at the voltage scan rate of 10 mV s−1 is achieved. Additionally stability and charging–discharging of polyaniline electrode are studied. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, electrochemical capacitors or supercapacitors have attracted much attention because of their high-power delivery within a very short time and long cycle life, which can store high amounts of energy. Supercapacitors have found an increasingly important role in power-source applications such as hybrid electric vehicles and short-term power sources for mobile electronic devices [1,2]. Generally, supercapacitors can be classified into two types: (1) electric double-layer capacitors based on carbon materials with a high surface area and (2) faradic pseudo-capacitors based on metal oxides and conducting polymers. Conducting polymers offer advantages of low cost comparison with metal oxide and a higher charge density than carbon, these often have good intrinsic auto conductivity. Among the conducting polymers, polyaniline (PANI) is most investigated and studied for its electrochemical characterization [3–9]. Polyaniline possesses high conductivity, good redox reversibility and environmental stability [10]. Due to these properties, polyaniline has been extensively studied for diverse applications involving rechargeable batteries, electrochromic devices, sensors, etc [11,12]. Polyaniline refers to a general class of conducting polymers composed of benzoid and quinoid character, connected by nitrogen. Polyaniline is built up from reduced (–B–NH–B–NH–)n and oxidized (–B–N1/4 –Q–N1/4 )n repeat units, where “B” denotes benzoid and “Q” denotes quinoid
∗ Corresponding author. Tel.: +91 231 2609229; fax: +91 231 2692333. E-mail address: l [email protected] (C.D. Lokhande). 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.01.021
rings. Thus, the changing ratio of amine to imine yields various structures, such as leucoemeraldine a reduced form of emeraldine base. In addition, emeraldine base is regarded as the most useful form of polyaniline due to its high stability at room temperature and the fact that upon doping the emeraldine salt form of polyaniline is electrically conducting. The polyaniline structure consists of up to 1000 or more repeating units and conductivity 10−11 to 10 S/cm. The materials in the nano-size form with high surface area and high porosity give the best performances as the electrode materials for supercapacitors. Therefore the synthesis and supercapacitive characterization of the high surface area nanomaterials of polyaniline such as nanotubes and nanowire [13–15] have been carried out in the past few years. Prasad and Munichandraiah [16] obtained a maximum specific capacitance of 815 F g−1 by potentiodynamical deposited nanofibre polyaniline on stainless steel substrates. Zhao and Li [17] also reported the highest specific capacitance of 1142 F g−1 for nanowire network of polyaniline. Thus, the high value of the specific capacitance was attributed to the high porous morphology obtained at a high sweep rate for the polyaniline by electrodeposition technique. From the structural point of view, growing of fibres of high aspect ratio by low temperature chemical methods such as electrodeposition can find extensive scope. Electrodeposition is a powerful and interesting process that can be applied in numerous fields. The advantages of electrodeposition compared with other techniques include low cost for raw materials and equipments, capability of controlling composition and morphology by varying electrochemical parameters and the ability to deposit films on a complex surface.
520
D.S. Dhawale et al. / Synthetic Metals 160 (2010) 519–522
Our main interest is to prepare fuzzy nanofibrous interconnected polyaniline electrode for supercapacitor application using electrodeposition method. In this work, we have performed cyclic voltammetry measurement of the fuzzy nanofibrous polyaniline electrode deposited on the stainless steel substrate for electrochemical supercapacitor application. 2. Experimental details Aniline (99.5%, Merck) as the monomer was purified by double distillation under reduced pressure prior to use. Sulphuric acid (DBSA, 90%, Fluka) used as dopant. The stainless steel substrate (grade 304, 0.2 mm thick) was polished with emery paper to a rough finish, washed free of emery particles and then air-dried. The electrochemical synthesis of polyaniline was performed in a conventional three-electrode electrochemical cell, containing stainless steel substrate as a working electrode, graphite rod of a 2 cm2 surface area as a counter electrode and a saturated calomel electrode (SCE) as a reference electrode. The electrochemical anodization was carried out onto stainless steel substrates from aqueous acidic solution containing 0.5 M H2 SO4 + 0.45 M aniline (C6 H5 NH2 ) at room temperature (300 K). The deposition of well adherent and uniform green-colored polyaniline electrode was observed by applying a constant potential of +0.8 V/SCE for the deposition time 5 min. Thickness was calculated by gravimetric weight difference method by using sensitive microbalance with relation t = m/( × A) where, ‘m’ is the weight of the film deposited on the substrate in grams; ‘A’ the area of the deposited film in cm2 and ‘’ the density of the deposited material (polyaniline = 1.30 g/cm3 ) in bulk form. The maximum thickness obtained for polyaniline thin film was 0.9 m and surface roughness 592 nm. Fourier transformed infrared spectroscopy (FTIR) was recorded between 4000 and 450 cm−1 at a spectral resolution of 2 cm−1 on a PerkinElmer 1710 spectrophotometer using KBr pellets at room temperature. Surface morphological study was carried out using scanning electron microscopy (JEOL-6360). For this, the films were coated with a 10 nm platinum layer using a polaron scanning electron microscopy (SEM) sputter coating unit E-2500 before taking the image. To study the optical characteristics of the film, absorbance spectra were recorded in the range 450–750 nm by means of UV–vis spectrophotometer-119. The electrochemical supercapacitor performance of the polyaniline electrode was tested by cyclic voltammetry (C-V) using the Potentiostat (263 A EG&G Princeton Applied Research) and forming electrochemical cell configuration comprising platinum electrode as a counter electrode, saturated calomel electrode (SCE) as a reference electrode in H2 SO4 electrolyte.
Fig. 1. The FTIR spectrum of polyaniline thin film.
3.2. Surface morphogical studies The surface morphology of electrochemically deposited polyaniline film was investigated by scanning electron microscopy (SEM). Fig. 2(a and b) shows the SEM images of the polyaniline nanofibre network at different magnifications. From Fig. 2, it can be inferred that the nanofibre network of polyaniline is highly porous. The low magnification (5000× (Fig. 2a)) image confirms interconnected fuzzy nanofibrous network of polyaniline electrode. These fibres are relatively smooth with randomly distributed on the substrate having many pores in diameter and length ranging typically from 400 to 500 nm as observed from high magnification (10,000×). Although the polyaniline nanofibres have been studied
3. Results and discussion 3.1. Fourier transforms infrared (FTIR) study Polyaniline formation is also confirmed by Fourier transform infrared spectroscopy (FTIR) technique. The FTIR spectrum of polyaniline is shown in Fig. 1 and has several distinct bands at 3418, 2928, 1592, 1453, 1298 and 1128 cm−1 that are characteristics of polyaniline [18]. The peaks at 3418 and 2928 cm−1 are attributable to NH2 + and N–H stretching modes, The peaks at 1592 and 1453 cm−1 are attributed to C N and C C stretching modes for the quinoid and benzenoid rings. The peaks at about 1298 and 1243 cm−1 are attributed to C–N stretching mode for benzenoid ring and the peak at 1128 cm−1 is assigned to the plane bending vibration of C–H (modes of N Q N, Q N+H–B and B–N+H–B), which is formed during protonation [19] confirming formation of polyaniline.
Fig. 2. . The SEM images of polyaniline nanofibres at the magnifications of (a) 5000× and (b) 10,000×.
D.S. Dhawale et al. / Synthetic Metals 160 (2010) 519–522
Fig. 4. The CV curves of polyaniline electrode at different scanning rates in a 1.0 M H2 SO4 electrolyte.
Fig. 3. Plot of absorbance (˛t) vs. wavelength () of polyaniline thin film.
for various potential applications here, such a highly porous and large surface area network of polyaniline nanofibres is applied for supercapacitor. 3.3. Optical absorption studies For the optical studies, polyaniline films were deposited on FTO coated glass substrate. Fig. 3 shows the plot of absorbance (˛t) vs. wavelength () of polyaniline film and has two peaks nearly at 638 and 332 nm, these two characteristic peaks assigned to polyaniline thin film. This is excellent agreement with the results reported by Sarno et al. [20] for nanostructured polyaniline. The optical absorption is attributed due to transition of charge carriers through a forbidden energy gap. 3.4. Supercapacitive studies The supercapacitive performance of electrodeposited fuzzy fibrous polyaniline electrode was tested using cyclic voltamogramm (C-V) technique. The capacitance (C) was calculated using following relation: C=
Imax dV/dt
(1)
where I is the average current in ampere and dV/dt is the voltage scanning rate. The interfacial capacitance was calculated using the relation: Ci =
C A
521
−200 to 800 mV. Fig. 5 shows the variation of specific capacitance and interfacial capacitance with scan rate. From Fig. 5 it is seen that, specific and interfacial capacitance values are decreased from 839 to 484 F g−1 and 0.365–0.213 F cm−2 , respectively. The maximum specific capacitance of 839 F g−1 was obtained at 10 mV s−1 . The decrease in capacitance has been attributed to the presence of inner active sites that cannot sustain the redox transitions completely at higher scan rates. This is probably due to the diffusion effect of protons within the electrode. The decreasing trend of the capacitance suggests that parts of the surface of the electrode are inaccessible at high charging–discharging rates [24]. The cyclic stability of the nanofibrous polyaniline electrode at the sweep rate of 20 mV s−1 for 1st, 500th and 1000th cycles are shown in Fig. 6. From the figure it is seen that specific capacitance of polyaniline electrode decreased from 829 to 798 F g−1 in the first 500 cycles and thereafter the specific capacitance remained almost constant. This demonstrates that the material is suitable for energy-storage applications. The specific and interfacial capacitance values are decreased by a small amount with the number of cycles due to the loss of active material caused by the dissolution and/or detachment, during the early charging/discharging cycles in the electrolyte. The 90.71% cyclic stability of the fuzzy nanofibrous polyaniline electrode was observed. The charging and discharging behavior of nanofibrous polyaniline electrode was examined by chronoamperometric technique. Fig. 7 shows the charging–discharging behavior of polyaniline
(2)
where ‘A’ is the area of active material dipped in the electrolyte. The specific capacitance Cs (F g−1 ) of polyaniline electrode was calculated using following relation: Cs =
C W
(3)
where W is the weight of polyaniline film dipped in electrolyte. Due to the high-power demand in supercapacitors, one of the basic requirements for an electrode material in this field is the high or pulse-power characteristic [21,22]. This unique property is a strong function of the electrochemical kinetics of redox transitions within the electro-active materials [23]. Thus, the high or pulse-power property of nanofibrous polyaniline electrode is examined using cyclic voltammetry at different scan rates. Fig. 4 shows the CV curves of nanofibrous polyaniline electrode in 1 M H2 SO4 electrolyte for different scan rates within voltage range of
Fig. 5. Variation of specific and interfacial capacitances of polyaniline electrode at different scan rates. Concentration of H2 SO4 electrolyte was 1.0 M.
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change of polyaniline electrode is occurred due to the insertion of charges. 4. Conclusions In this work, we have successfully synthesized nanofibrous polyaniline electrode for electrochemical supercapacitor by electrochemical anodization method. The presence of characteristics bonds of polyaniline were confirmed by FTIR studies. The SEM study revealed that polyaniline films have fuzzy nanofibrous interconnected architecture. Polyaniline electrode showed maximum specific capacitance of 839 F g−1 at 10 mV s−1 . Acknowledgement Authors are grateful to the Department of Science and Technology, New Delhi for financial support through the scheme no. SR/S2/CMP-82/2006. Fig. 6. The CV curves of polyaniline electrode at different number of cycles. The scanning rate and concentration of H2 SO4 were 20 mV s−1 and 1 M, respectively.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Fig. 7. Charging–discharging plots of polyaniline electrode as a function of time in 1 M H2 SO4 electrolyte.
electrode in 1 M H2 SO4 electrolyte. The charge–discharge study revealed that, as the charging time increases, charge accumulated on nanofibrous polyaniline electrode increases and all curves reflects that all charges are discharged. It concludes that no phase
[19] [20] [21] [22] [23] [24]
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