Activated Carbon/Polyaniline Electrode For Electrochemical Supercapacitor

CHAPTER SIX Activated Carbon/Polyaniline Electrode For Electrochemical Supercapacitor Outline Activated carbon/ Polyaniline (AC/PANI) composite prepared by in situ polymerization method. Deposition was carried out by dip coating technique. The formations of AC/PANI composite revealed by Fourier transform infrared, Fourier transform Raman and X-ray photoelectron spectroscopy techniques. Surface morphology of the films is examined by Field Emission Scanning Electron Microscopy which revealed aggregated nanofibers like structure for PANI and interconnected nanofibers with porous structure for AC/PANI films. The supercapacitive behavior of the electrodes is tested in a three electrode system using 1.0 M H 2 SO 4 electrolyte. The highest specific capacitance value of 534 Fg -1 is observed for the AC/PANI composite film owing to the synergic effect of PANI and AC particle Chapter VI Page 141

6.1 Introduction Up to now, the efforts were focused to enhance the electronic conductivity of the Polyaniline (PANI) electrode by incorporating metal ions (Mn and Ag) into the PANI matrix. Surface area of the electrode plays significant role in the specific capacitance. Present chapter deals with the electrochemical study of Activated carbon /PANI (AC/PANI) composite electrode Activated carbon has attracted much attention because of their low cost, abundance, good mechanical strength and high surface area. Electric double layer capacitance is directly proportional to the available surface area hence activated carbon extensively recognized as the electrode material for Electric double layer capacitors (EDLCs) [1-5]. It is interesting that combining the properties of both the materials to form the promising material with long cycle life and high specific capacitance. Recently there are some reports published on the AC/PANI composite. Quin et al. studied the electrochemical properties of AC coated with PANI [6]. Also the chemically deposited PANI onto the surface of AC powder using V 2 O 5 as oxidant and their electrochemical capacitor behavior studied by Zhang Huo et al. [7]. Chang Hu et al. reported the supercapacitor behavior of Activated carbon fabric and PANI composite in neutral NaNO 3 electrolyte [5]. Electropolymerization of PANI onto the high surface area using different methods carried out by Bleda-Martinez et al. for supercapacitor application [8]. Synthesis and characterization of AC/PANI composite films prepared by chemical polymerization and studied their optical properties reported by Zengin et al. [9]. Present work demonstrates the AC/PANI composite material prepared by in situ polymerization method and their electrochemical performance. Presence of AC in PANI modified the surface of the electrode which enhances the electrochemical performance of the composite electrode. Chapter VI Page 142

6.2 Experimental Details 6.2.1 Preparation of Activated carbon The activated carbon was prepared from dry coconut shell. The shell was crushed into small pieces. Then it was washed with double distilled water and dried in oven at 383 K for 24 hr. Dried sample 25 gm was taken in container 50mL conc. H 2 SO 4 was added as impregnating reagent, then kept it as it is for 24 hr. And after it leaches out the acid, the sample washed off with distilled water till neutral ph. For complete formation as carbaneous material, it was kept in muffle furnace, for 1 hr at 573 K. Dried activated carbon powder was then sieved through BSS-25. 6.2.2 Preparation of Activated carbon/ PANI composite AC/PANI composite was prepared by using a chemical bath consisting of ammonium persulphate (APS), hydrochloric acid (HCl), aniline and distilled water. The AC/PANI composite prepared by using an in situ polymerization method. The aniline monomer was first mixed with Activated carbon in 1 M HCl by ultrasonication to form a homogeneous suspension. Then oxidant was added to this mixture to obtain the homogeneous composite of AC/PANI nanofibers. Uniform deposition of the AC/PANI films is obtained on stainless steel substrates by dip coating technique. The solution was kept under constant stirring throughout the film deposition process. For comparison purposes, pure PANI electrode was also prepared via the similar polymerization as described above without addition of activated carbon. Infrared (IR) spectroscopy was used to confirm the formation of PANI in which the powdered material collected from the deposited film was characterized by infrared spectrometer (Perkin-Elmer, model 783, USA). X-ray photoelectron spectra were recorded by using XPS, VG Multilab 2000, Thermo VG Scientific, UK, for phase evaluation. The surface morphology of the films was examined by analyzing the Field emission scanning electron microscope (FE-SEM), JEOL JSM JSM-6500F equipped with an energy dispersive x-ray spectrometer (EDS). Chapter VI Page 143

The electrochemical measurements were performed in an electrolyte of 1.0 M H 2 SO 4 in a conventional three electrode arrangement comprising graphite counter electrode and saturated calomel electrode (SCE) serving as the reference electrode, using scanning potentiostat (model- CHI-400A) CH Instrument, USA. 6.3 Results and Discussion All the samples of the deposition were subjected to the structural, optical, morphological and electrochemical characterization. 6.3.1 Fourier Transform Infra-Red (FT-IR) Fig. 6.1 (a-c) shows the FTIR spectra of the powder collected from AC, PANI and AC/PANI samples over 450-2000 cm -1. The FTIR spectrum for activated carbon is as shown in Fig. 6.1 (a). It gives the four peaks at 1612, 1163, 1120 and 1021 cm -1. The PANI (Fig.6.1 (b)) spectrum consists of four distinct peaks at 1560, 1492, 1304 & 1131 cm -1. The bands are assigned to the N=Q=N stretching, N-B-N stretching (where Q & B denotes the quinoid & Benzenoid), N-H bending and -N= vibration which are similar to those obtained by Jie Li et al. [10]. The sample AC/PANI composite exhibits all peaks of PANI sample. The peaks corresponding to wavenumber 1560, 1492, 1304 and 1131 cm -1 for PANI showed shift towards the higher wavenumber corresponding to 1586, 1498, 1305 and 1140 cm -1 for AC/PANI composite respectively. This shift in wavenumber observed due to the presence of activated carbon in PANI. The peak at 1021 cm -1 for activated carbon also reflects in AC/PANI which confirms the formation of AC/PANI composite. Chapter VI Page 144

(c) Transmittance (A.U.) 1586 1560 1498 1492 1405 1305 1304 1140 1131 1088 1024 (b) (a) 1612 1163 1120 1021 1800 1600 1400 1200 1000 Wavenumber (cm -1 ) Fig.6.1 FT-IR transmittance spectra of the (a) AC, (b) PANI, (c) AC/PANI samples recorded in the wavenumber range of 900 1900 cm 1. 6.3.2 Fourier Transform Raman (FT-Raman) Raman Intensity (A.U.) (b) (a) 1800 1600 1400 1200 1000 800 Wavenumber (cm -1 ) Fig. 6.2 FT-Raman spectra of the (a) PANI, (b) AC/PANI samples recorded in the wavenumber range of 800-1800 cm -1. Chapter VI Page 145

The PANI and AC/PANI films were characterized for their FT-Raman spectra. The Raman spectra were recorded by a laser radiation at an excitation wavelength of 1064 nm. Fig. 6.2 shows the FT-Raman spectra of PANI and AC/PANI. For PANI sample the characteristic bands at 1590, 1505, 1359 and 1175 cm -1 assigned to C-C stretching of quinoid units, C-C stretching of the benzene ring, C-N + stretching and C-N stretching respectively [Fig.6.2 (a)]. The assignment of peak reveals that the synthesized product is PANI. Similar bands were also observed for the AC/PANI sample. Increase in peak intensity is observed for PANIAg 0.9 which indicates Ag particles act as an electrocatalytic role in PANI. 6.3.3 X-ray Photoelectron Spectroscopy (XPS) XPS N 1s spectra of PANI and AC/PANI samples are presented in Fig. 6.3 (a, b). Usually, XPS spectra of PANI can be deconvoluted into three distinct curves, related to the quinoid imine, the benzenoid amine and positively charged nitrogen. Fig. 6.3 (a, b) shows the major two peaks related to quinoid imine and benzenoid amine are observed at 399.34 (±0.01) ev and 400.53 (±0.53) ev for PANI whereas 399.26 (±0.01) ev and 400.27 (±0.07) ev for AC/PANI respectively. It was observed that the binding energies related to the AC/PANI composite shifted towards the lower value as compared to the PANI. Hence, the slight shifts towards lower binding energies observed for our sample indicate the formation of AC/PANI composite electrodes. Chapter VI Page 146

4.5x10 3 Intensity (CPS) 4.0x10 3 3.5x10 3 3.0x10 3 399.34 ev 400.53 ev 2.5x10 3 392 394 396 398 400 402 404 406 408 410 Binding Energy (ev) Fig. 6.3 (a) N1s XPS core level spectra of (a) PANI 5.0x10 3 4.5x10 3 Intensity (CPS) 4.0x10 3 3.5x10 3 399.26 ev 0.01 400.27 ev 0.07 3.0x10 3 2.5x10 3 392 394 396 398 400 402 404 406 408 410 Binding Energy (ev) Fig. 6.3 (b) N1s XPS core level spectra of AC/PANI 6.3.4 Field Emission Scanning Electron Microscopy (FE-SEM) To investigate surface morphology of the films, they were characterized by FESEM. Fig. 6.4 (a, b) shows surface morphologies of PANI and AC/PANI Chapter VI Page 147

samples at X 50,000 magnification. The aggregated nanofibers like structure is observed for PANI sample [Fig. 6.4 (a)]. However, AC/PANI sample revealed the interconnected nanofibers with nonporous structure. The average diameter of the nanofibers is about 30-50 nm observed for PANI sample whereas the 20-40 nm observed for AC/PANI sample. The nanofiber structure provides large surface area to volume ratio leading to a high charge/discharge rate and specific capacitance. Interconnected nanofibers with nonporous offered relatively larger surface area. There was no drastic change observed between PANI and AC/PANI composite. Only due to the interaction of monomer with activated carbon before polymerization process prevents the aggregation of PANI nanofibers which reduces the diameter of nanofibers. This nanofibers and porous structure is favorable for supercapacitor, because it reduces the diffusion resistance of the electrolyte into electrode matrix (a) (b) Fig. 6.4 FE-SEM images of the (a) PANI, (b) AC/PANI samples at X50000 magnifications. 6.3.5 Electrochemical Measurement To identify the oxidation and reduction potentials and the effect of Activated carbon on the electrochemical performance of PANI, cyclic voltammograms (CV) of all samples have been recorded over 0.2 to 0.8 V versus SCE at 5 mvsec -1 in 1.0 M H 2 SO 4 [Fig. 6.5 (a, b)]. Fig. 6.5 (a) shows a CV for PANI. The oxidation peak corresponding to the leucoemeraldine to

emeraldine salt at about 0.27 V and the reduction peaks corresponding to the leucoemeraldine and emeraldine base are found to be at 0.06 V and 0.64 V respectively were observed in PANI sample. The small peaks between 0.3 V to 0.55 V potential are attributed to transformation of PANI charge carriers consisting of polaron (radical cation) and bipolaron (dication) forms delocalized on PANI chains [11]. All peaks are observed in AC/PANI sample. But there is shift observed in oxidation, reduction peaks corresponds to AC/PANI composite as compared to the PANI electrode. The oxidation peaks of AC/PANI shifted towards the lower potential values whereas the reduction peaks shifted towards the higher potential value. This shift observed in peaks due to the least resistance path provided by the well distributed nanofibers and also porous nature of the electrode. It is also known that the redox behavior of PANI depends largely on morphology and structure that affect the specific surface area and the ion diffusivity [12-15]. The specific capacitance of each film was calculated from CV curves by using following equation [16]: idv C s = 2m V S Where, C s is the specific capacitance, idv is the integrated area of the CV curve, m is mass of active material, V is the potential range, S is the scan rate. The specific capacitance value of 285 Fg -1 and 534 Fg -1 was observed for PANI and AC/PANI electrodes. This enhancement into the specific capacitance attributable to both pseudocapacitance and electric double layer capacitance arises from PANI and activated carbon respectively. Chapter VI Page 149

Current density (macm -2 ) 1.2 0.9 0.6 0.3 0.0-0.3 Current density / macm -2 6 4 2 0-2 -4 5 mvsec -1 10 mvsec -1 20 mvsec -1 40 mvsec -1 60 mvsec -1 80 mvsec -1 100 mvsec -1-0.2 0.0 0.2 0.4 0.6 0.8 Voltage Vs SCE / Volts -0.6-0.2 0.0 0.2 0.4 0.6 0.8 Voltage Vs SCE (Volts) Fig. 6.5 (a) CV of the PANI samples within a potential window of -0.2 to 0.8V versus SCE at 5 mvsec -1. Inset Fig. with different scan rate. Current Density (macm -2 ) 4 3 2 1 0 Current Density / macm -2 16 12 8 4 0-4 -8-12 5 mvsec -1 10 mvsec -1 20 mvsec -1 40 mvsec -1 60 mvsec -1 80 mvsec -1 100 mvsec -1-0.2 0.0 0.2 0.4 0.6 0.8 Voltage Vs SCE / Volts -1-0.2 0.0 0.2 0.4 0.6 0.8 Voltage Vs SCE (Volts) Fig.6.5 (b) CV of the AC/ PANI samples within a potential window of -0.2 to 0.8V versus SCE at 5 mvsec -1. Inset Fig. with different scan rate. Chapter VI Page 150

The variation of specific capacitance with respect to scan rates for PANI and AC/PANI samples is as shown in Fig. 6.6. Generally, the specific capacitance decreases with the increase of potential scan rate. It is accepted that at a low scan, the presence of inner active sites which undergo the redox transitions completely, can lead to produce specific capacitance to that at high scan rate because of the diffusion effect of proton within the electrode [17]. CV curves of PANI and AC/PANI electrodes were recorded at a different potential scan rates are as shown in inset of Fig. 6.5 (a, b). As scan rate increases area under the curve increases and the anodic shift in the oxidation peaks and the cathodic shift in the reduction peaks are observed due to the resistance of electrode. This may be compared with the linear variation of peak current with the square root of the scan rate. The cyclic stability of PANI and AC/PANI were recorded up to the 2000 cycles. The specific capacitance of the samples (PANI and AC/PANI) changed along with the number of cycles as shown in Fig. 6.7. It indicates that the 31 % loss observed for PANI sample and only 14 % loss observed for AC/PANI in value of specific capacitance after 2000 cycles, it indicates that the AC/PANI film becomes more stable than the PANI film. Less decrement in the later case observed because of the contribution of mechanical strength of activated carbon improve the electrochemical stability of PANI Chapter VI Page 151

600 500 Specific capacitance (Fg -1 ) 400 300 200 100 (b) (a) 0 0 20 40 60 80 100 Scan rate (mvsec -1 ) Fig. 6.6 Variation of specific capacitance with respect to scan rate (a) PANI, (b) AC/PANI 500 400 Specific capacitance (Fg -1 ) 300 200 100 (b) (a) 0 0 500 1000 1500 2000 Cycle numbers Fig.6.7 Variation of specific capacitance with respect to cycle numbers (a) PANI, (b) AC/ PANI Chapter VI Page 152

6.4 Conclusions Synthesis of AC/PANI composite electrode is better choice for supercapacitor due to its effective cost and ecofriendly nature. Interconnected nanofibers structure provides the large surface to volume ration improve the electrolyte electrode interface leading high charge/discharge rate and electrochemical performance. The highest specific capacitance of 534 Fg -1 is observed at 5 mvsec -1 and AC/PANI composite electrode in 1.0 M H 2 SO 4 electrolyte. Chapter VI Page 153

6.5 References [1] B. E. Conway, Electrochemical Supercapacitors, Kluwer Academic Publishers, New York, (1999). [2] A. Burke, J. Power Sources, 91 (2000) 37. [3] D. Qu, H. Shi, J. Power Sources, 74 (1998) 99. [4] Y. R. Nian, H. Teng, J. Electrochem. Soc., 149 (2002) A1008. [5] C. C. Hu, W. Y. Li, J. Y. Lin, J. of Power Sources, 137 (2004) 152. [6] W. Qin, L. J. ling, G. Fei, L.W. Sheng, W. K. Zhong, W. X. dong, New Carbon Mate., 23 (2008) 3. [7] Z. H. Zhou, N. C. Cai, Y. Zeng, Y. H. Zhou, Chinese J. of Chem., 24 (2006) 13. [8] M. J. Bleda-Martínez, C. Peng, S. Zhang, G. Z. Chen, E. Morallón, D. C. Amorósa, J. of The Electrochem. Soc., 155 (2008) A672. [9] H. Zengin, G. Kalay, Mat. Chem. and Phy., 120 (2010) 46. [10] J. Li, M. Cui, Y. Lai, Z. Zhang, H. Lu, J. Fang and Y. Liu, Synth. Met., 60 (2010) 1228. [11] D. S. Patil, J. S. Shaikh, D. S. Dalavi, M. M. Karanjkar, R. S. Devan, Y. R. Ma, and P. S. Patil, J. of The Electrochem. Soc., 158 (2011) 1. [12] H. H. Zhou, H. Chen, S. L. Luo, G. W. Lu, W. Z. Wei, and Y. F. J. Kuang, J. Solid State Electrochem., 9 (2005) 574. [13] F. Fusalba, P. Gouerec, D. Villers, and D. Belanger, J. Electrochem. Soc., 148 (2001) A1. [14] S. K. Mondal, K. R. Prasad, and N. Munichandraiah, Synthesis, 148 (2005) 275. [15] M. J. Bleda-Martínez, C. Peng, S. Zhang, G. Z. Chen, E.Morallón, and D. Cazorla- Amorós, J. of The Electrochem. Soc., 155 (2008) A672. [16] A. Yu, I Roes, A. Davies, and Z. Chen, Appl. Phys. Lett., 96 (2010) 253105. [17] G. Li, Z. Feng, J. Zhong, Z. Wang, and Y. Tong, Macrom.s, 43 (2010) 2178. Chapter VI Page 154