J Mater Sci: Mater Electron DOI 10.1007/s10854-017-7988-x

Synthesis and characterization of activated carbon/conducting polymer composite electrode for supercapacitor applications K. M. Vighnesha1 · Shruthi1 · Sandhya1 · D. N. Sangeetha1 · M. Selvakumar1 

Received: 16 May 2017 / Accepted: 4 October 2017 © Springer Science+Business Media, LLC 2017

Abstract  Activated carbon is prepared from coconut shell by heating it around 500 °C for 2 h in a muffle furnace. This method is one of the easiest and most economical methods for the synthesis of Activated carbon. The dried coconut shell is carbonized and it is treated with KOH in the ratio of 1:3 carbon-activating agent (KOH) and the resulting slurry is dried in an oven at 60 °C for overnight. The dried mass is further activated at 450 °C then it is washed with an acid solution to remove KOH. The surface area of the synthesized activated carbon can be obtained by using nitrogen absorption–desorption experiment. Conducting polymer such as polyaniline prepared by oxidative polymerization of the respective monomer in tetrafluoroboric acid solution using potassium persulphate as an oxidant. The structure and doping of polyaniline were studied by FTIR, UV and Cyclic voltammetry studies. The conducting polymer is mixed with AC and prepared composites were characterized by UV, FTIR, and CV. The specific capacitance of composites calculated from charge–discharge at 0.5 mA g− 1 was found to be 99.6 Fg− 1 and for cyclic voltammetry, at the scan rate of 2 mV s− 1, it is 77 Fg− 1.

Electronic supplementary material  The online version of this article (doi:10.1007/s10854-017-7988-x) contains supplementary material, which is available to authorized users. * M. Selvakumar [email protected] 1



Department of Chemistry, Manipal Institute of Technology, Manipal University, Manipal 576104, India

1 Introduction With the growing population and increasing energy demand, there is an urgent need for fast energy storage devices. One among that is supercapacitor which fills up the gap between batteries and capacitors with high power density and fast charge/discharge rate [1]. Electrical double-layer capacitors (EDLC), Pseudocapacitors and Hybrid capacitors are the three categories based on the charge-storage mechanism [2]. EDLC store energy by ion adsorption–desorption process in the electrical double layer at the electrode/electrolyte interfaces. The energy density of EDLCs is lower compared to rechargeable batteries [3]. While their power density and cycle life are considerably good compared to rechargeable batteries [4]. This is because of electrostatic interactions between electrodes and ions in solutions. Important physical properties of EDLC electrodes include high surface area and good electric conductivity [5]. Activated carbon (AC) one of the most commonly studied and used materials because of its abundance, cost effectiveness, and environmentally benign nature. AC electrode materials exhibit higher energy density compared to other conventional capacitors [6]. AC is prepared by carbonizing the carbon precursors at elevated temperature under inert atmosphere. Followed by activation, by treating it with an activating agent (KOH) and later annealing at elevated temperature (600–800 °C) [7]. The surface area of AC is dependent on the amount of activating agent used in the activating process. Experimentally it is determined that more the activating agent used, the porosity development is greater and it will help to enhance the surface area of synthesized AC [8]. Polyaniline (PANi), the mostly studied conducting polymer, is a conjugated polymer and is a pseudocapacitors electrode material working with the principle of fast and reversible redox reactions. Due to its high conductivity, thermal

13

Vol.:(0123456789)



and environmental stability, interesting redox behavior yielding to the good specific capacitance (400 Fg− 1) [9]. Different forms of PANi like nanowire, nanofibers, and nanotubes are made use in rechargeable batteries, microelectronics, sensors, electrochromic displays, light emitting and photovoltaic devices [10]. The electrochemically active PANi contributes to the overall pseudocapacitance with the addition of double layer capacitance of carbon materials. As the combination provides overall supercapacitor performance. The composite of carbon with the conducting polymer (PANI in this case), is together responsible for the better double layer capacitance with additional pseudocapacitance. Thus, enhancing the overall performance of the supercapacitor. The present work involves the use of abundantly available coconut shell for the preparation of activated carbon. And enhancing its surface area at lower temperature activation, at 450 °C. A conductive polymer, polyaniline (PANi) was synthesized via oxidative polymerization technique. The synthesis of PANi was achieved using respective monomeraniline, in tetrafluoroboric acid solution using ammonium persulphate as an oxidant. The as prepared PANi is characterized using UV–Vis and FT-IR spectroscopic techniques [11]. When being studied for supercapacitor application, a single electrode material like PANi, alone doesn’t give a very good electrochemical performance. This is due to its slow solid-state diffusion. Whereas AC works on double layer capacitance. And thus, composites are preferred for better electrochemical responses. AC and PANi composites give an enhanced specific capacitance due to the dual mechanism involved. This may be electron diffusion in terms of an electric double layer of AC and fast redox reactions of pseudocapacitor material, PANi. The AC-PANi composites were prepared by a simple polymer blending technique. Several ratios namely, 1:1, 1:2, 1:3, 2:1, 3:1—AC:PANi were prepared. The best ratio composite was selected and further utilized for supercapacitor application. The prepared composites are characterized through SEM and cyclic voltammetry studies. This blend of AC and PANi proved to give a good specific capacitance of 99.6 Fg− 1. The mechanism of the formation and the morphology of these nanocomposites were evaluated.

2 Experimental section Materials: Aniline brought from Lobal Chemie, Mumbai. Tetrafluoro boric acid purchased from Alfa Aesar, Massachusetts, USA, Ammonium persulphate [(NH4)2SO8] brought from Nice Chemicals Pvt. Ltd., Cochin.

13

J Mater Sci: Mater Electron

2.1 Preparation of activated carbon In the typical experimental procedure, as shown in Fig. 2 coconut shell is used as a carbon precursor. The outer part of the coconut shell was washed with water and dried. Then it was taken in a silica crucible and burned in a muffle furnace at 500 °C for 2 h. After completion of burning it was ground well in mortar and pestle. The burned mass was nonactivated carbon and to activate it 1 g of this sample was mixed with a solution that containing 5 mL of water and 1.5 g of an activating agent, which is KOH pellets in the ratio of 3:1. The mixed sample was then dried in an oven at 60 °C for 2 h. The resulting slurry was dried at 110 °C for overnight in the oven. The samples were heat treated from room temperature to 450 °C for the period of 2 h with a rate of elevation in the temperature of 10 °C min−1. Then it was allowed to cool to room temperature. This pyrolyzed sample was washed frequently by a 5 M solution of HCl and then with distilled water till it was free from chloride ion. Once the activating agent was removed, the sample was dried at 110 °C for 12 h. The obtained sample was activated carbon and it was characterized by surface area analyzer [12]. 2.2 Preparation of polyaniline (PANI) To the 20 mL of Aniline in a beaker, added 300 mL of 1 M tetrafluoro boric with constant stirring and kept in an ice bath. When the reaction mixture attains the 0 °C, 200 mL of 0.1 M (­ NH4)2S2O8 was added dropwise. After complete addition of ­(NH4)2S2O8, the reaction mixture was stirred continuously for 2 h. The green colored precipitate separates out which was polyaniline. The precipitate was filtered through Buckner funnel and the obtained polyaniline was dried and characterized by UV and IR spectroscopic techniques [13]. 2.3 Preparation of electrode materials The electrode is prepared by mechanically mixing different ratios (1:1, 1:2, 1:3, 2:1, 3:1) of AC and PANi, using N-methyl pyrrolidone with a polyvinylidene fluoride binder. It is then pasted onto the current collector—stainless steel. Symmetrical supercapacitor cell is prepared using AC-PANI nanocomposites coated electrodes with a polypropylene separator. 2.4 Characterization techniques The surface morphology and microstructure were characterized by scanning electron microscopy (SEM), ZEISS

J Mater Sci: Mater Electron

EVO18 special edition. The surface area of activated carbon was analyzed with surface area analyzer by using nitrogen absorption–desorption technique of Smart sorb 92/93. UV–Visible spectra of polyaniline were recorded using Shimadzu UV-160 1PC. FT-IR spectra of polyaniline were recorded in the range of 4000–400 cm− 1 by KBr pellet technique using Shimadzu FT-IR 8400S. 2.5 Electrochemical characterisation techniques A three-electrode system was employed with Ag/AgCl used as a reference, Pt as a counter electrode and synthesized nano composites coated on stainless steel (1 cm × 1 cm × 0.2 mm) act as a working electrode. The electrochemical properties of AC-PANI nanocomposites electrodes were analyzed in 0.1 N ­HBF4 electrolyte solution. Electrochemical studies including cyclic voltammetry (CV), AC impedance (measurements in the frequency range of 100 mHz–1 MHz) and galvanostatic charge–discharge (GCD) were carried out using BioLogic SP-150.

3 Results and discussions 3.1 Scanning electron microscopy (SEM) Fig.  1 shows the morphology (a) AC (b) PANi and (c) AC-PANI composites. AC shows homogeneous sheet-like structures, consisting random aggregated, thin sheets of AC closely connected with each other and forming a disordered solid. These sheets have pores on their surface which are visible in the SEM images of AC. The pores on the AC is responsible for the enhancement of the surface area of the

AC. The images of PANI clearly indicates the formation of granule morphology of PANI. The granule formation of PANi is mainly influenced by the reaction temperature [14]. Further, the granule structure is also due to heterogeneous nucleation during polymerization of aniline. Careful inspection exhibits the differences between the morphology of AC, PANi and AC/PANI composites which are the combination of disordered AC with aggregated modules of PANi [15]. In the composites of AC and PANi, the larger surface area of the AC due to the pores on AC provides the platform for the distribution of electroactive redox sites of PANi in the composite material. This distribution of PANi on AC will enhance the electroactive surface of PANi, which further enhances the number of active sites on AC, which is now responsible for the enhancement of specific capacitance of the material (Fig. 2). 3.1.1 Surface area The surface area of prepared AC is found to be 312 m2 g− 1 (see Fig. S1, ESI in Supplementary Material). 3.2 KOH‑Activated carbon as an electrode material for supercapacitors KOH mixed with carbon by impregnating in aqueous solution. Later water is evaporated and the solid sample is put in silica crucible, carbonized at 500 °C (see Scheme 1, ESI). During KOH activation, initially, KOH dehydrates to form ­K2O at 400 °C (a), followed by reaction of carbon with steam emitting ­H2 (b). Finally, ­K2CO3 is formed by the reaction of ­K2O and C ­ O2 (d). At 600 °C KOH completely converted to give K ­ 2CO3 [16]. The carbon framework is etched by a

Fig. 1  SEM Images of a activated carbon (AC), b Polyaniline (PANi) and c AC-PANi

13



J Mater Sci: Mater Electron

Fig. 2  Schematic representation of the synthesis of AC from coconut shell

continuous redox reaction of various potassium compounds which are used as activating agent resulting generation of pore network in a carbon frame. The formation of H ­ 2O and ­CO2 contributes to the development of porosity mainly through gasification of carbon. By removal of so formed ­K2CO3 through acid washing (HCl) surface area is enhanced from 30 to 312 m2g− 1.

2KOH → K2 O + H2 O

(1)

C + H2 O → CO + H2

(2)

CO + H2 O → CO2 + H2

(3)

(4) Carbonaceous materials activated through KOH enhances specific surface area depending on various activation parameters and various carbon sources used, which in turn could be used as electrode material for supercapacitors.

CO2 + K2 O → K2 CO3

3.3 Conduction mechanism in polyaniline

13

J Mater Sci: Mater Electron

Due to the delocalisation of electrons, the conjugated double bonds easily permits electron mobility throughout the molecule. In delocalised condition, electrons are not localized between two atoms where the p-bonding electrons are spread over molecules. This is one of the important conditions which allows the easy movement of electrons and also responsible for the electrical conductivity of polyaniline. In addition, polyaniline structure is associated with three major parts that are conjugated double bond structure, the benzenoid amine, and quinoid imine. PANi exists in three different forms of oxidation states: leuco emeralidine, emarldine, and pernigraniline. Among the three states of PANi, emarldine based from which is partially reduced or partially oxidized form of polyaniline one of the stable form because leuco emaraldine when exposed to air it will be easily oxidized and pernigraniline is easily degradable. Polyaniline is less conductive in the undoped state. The conductivity of polyaniline can be increased by oxidation or reduction either by the chemical or electrochemical method. From chemical synthesis, protonated acid acts as doping reagent and it will protonate the polyaniline by forming polaron or bipolaron molecule which will enhance the conductivity by the generation of mobile charge carriers.

Fig. 3  UV–Vis absorption spectra of PANi

3.4 UV–Visible spectroscopy Figure 3. Shows UV–Vis absorption spectrum of PANi. The sharp intense peak at 300 nm corresponds to the π–π* transition of the benzenoid ring while the shoulder peak at around 390 nm corresponds to n–π* transition [17]. With the blending of PANi with AC, the intensity of the PANi is drastically decreased and there is a blue shift observed. This may be due to the interaction of PANi with the AC due to the decrease in the conjugation of the polyaniline when blended with AC. 3.5 Fourier transform‑infrared (FT‑IR) spectroscopy: Figure 4. shows the FT-IR spectrum of PANI. In the spectrum of polyaniline, characteristic bands are observed at 3735, 1593, 1469, 1298, 1163, 929 and 802 cm− 1. A small

Fig. 4  FT-IR spectra of PANi

peak at 3735 cm− 1 is assigned to free N–H vibrations of an amino group. The high-frequency strong peak at 1593 and 1469 cm− 1 indicates the presence of quinoid ring and the benzenoid ring respectively [18]. The intense peak at 1298 cm− 1 in the spectrum corresponds to the C–N stretching vibrations of the 2° aromatic amine [19]. A broadband

13



at 1163 cm− 1 must be Benzene–NH+=Quinoid (B–NH+=Q) vibrations, indicating PANI is conductive [20]. The remaining peaks at 802 and 929 cm− 1 corresponds to the bending vibrations of C–H in-plane and out-plane respectively. The peak at 1573 and 1196 of AC, corresponds to conjugated C=O stretching and C–O stretching respectively. Peaks at similar wavenumbers are observed in the composite materials also, noting that there is no significant difference in the composition by forming composites. However, it is observable that the intensity of the peaks at 1593 and 1469 cm− 1 corresponding to the presence of quinoid and benzenoid ring reduces, mainly in the ratio 1:1—AC:PANi. As the composites are formed, the peaks of AC ad PANi overlap. In the case of equal ratios, the overlap is prominent and thus the PANi peak is not observable. But as the amount of PANi is increased in the composite material the peak at 1593 and 1469 cm−1 is more prominent. 3.6 Electrochemical measurements

J Mater Sci: Mater Electron

Fig. 6  Specific capacitance plot of 1:1 AC-PANi at different scan rates

With the large specific surface area, activated carbon provides a more active area for delivery of ions into the electrode. Further activated carbon gives backbone support and reliable electrical conductivity to polyaniline and functions as an active material to provide required electrical double layer capacitance. Polyaniline, on the other hand, being a pseudocapacitance material reduces diffusion and migration pathway of the electrolyte ions during the charge storage mechanism. 3.7 Cyclic voltammetry (CV) analysis The supercapacitance performance of AC/PANi composite electrode was studied in the aqueous electrolyte in 0.1 N

Fig. 5  Different ratios of AC:PANi AC—impedance

13

Fig. 7  1:1—AC:PANi a CV at different scan rates and b AC Impedance

J Mater Sci: Mater Electron

­HBF4. AC/PANi composite pasted electrode acts as a working electrode, platinum electrode acts as a counter electrode. CV studies were evaluated at different scan rates (see Fig. S2, ESI). CV profile of AC, PANi, AC/PANi composites carried out in the potential range of 0–1 V versus Platinum electrodes. The AC showed a quasi-rectangular shape of EDLC behavior while the composites comprise the characteristics of EDLC with pseudocapacitive behavior due to the influence of conducting polymer PANi. The Nyquist plot of composites was shown in Fig. 5. The semicircle arc at the lower frequency indicates the internal resistance of the electrode material and electrolyte. Among the various composite prepared 1:1 ratio of AC/PANi gives lesser resistance than the rest. The reduced width of semi circle impedance loop of 1:1—AC:PANi in the region of higher frequency confirms the shorter path for electron transport within the electrode material. Thus, further supercapacitor studies were carried out for 1:1—AC/PANi composite. Figure 6 represents the specific capacitance at different scan rates. The high specific capacitance of 77 Fg− 1 was observed for 2 mV scan rate. To evaluate the specific capacitance of the material, CV was performed at different scan rates 2, 5, 10, 20 and 50 mV s− 1shown in Fig. 7a. The area under the curve for 50 mV s− 1 scans was greater than the rest scan rates and showed a quasi-rectangular profile, arising due to electrical double layer capacitance (EDLC) working on the principle of charge accumulation at the electrode–electrolyte interface. Further pseudocapacitance material also adds up to the profile contribution where charge transfer takes place due to continuous fast and reversible redox reactions. The shape of the cyclic voltammogram remains intact with different scan rates from 2 to 50 mV s− 1. The cathodic and the anodic peak shifts to higher and lower potentials with an increase in scan

rates. The constant shape of the CV suggests good electrochemical reversibility of the electrode materials which is for the application for electrochemical energy storage. From the Fig. 7b the reduced width of semi circle impedance loop of 1:1—AC: PANi in the region of higher frequency confirms the shorter path for electron transport within the electrode material i.e., at the electrode–electrolyte interface. The following vertical line represents the capacitive behavior of the electrode material. The Specific capacitance of the electrode was calculated from the CV profile using the following equations (5) [21].

Cs =

2A × Fg−1 ΔV × v × m

(5)

where A is an integral area of CV loop, ∆V is a potential window, v is scan rate and m is a mass of the material at each electrode. The maximum specific capacitance of 77 Fg−1 was obtained for 2 m s−1. The energy density (E) was calculated from the following Eq. (6):

C × ΔV2 1000 × 2 3600

E=

(6)

WhKg−1

where ∆V represents the potential window, C represents the specific capacitance. The maximum of 11 W h kg−1 was obtained. Figure  8. represents, galvanostatic charge/discharge measurement was carried out for 1:1—AC/PANi composite in 0.1 N ­HBF4 solution with the two-electrode system at a current of 0.5 Ag−1 Fig. 8. The profile was nearly linear. From the charge–discharge curve, the Specific Capacitance of electrode was calculated using following Eq. (7) [22]:

Cs =

I × Δt × Fg−1 ΔV × m

(7)

Power density (PD) is given by -

P=

I × Δv × 1000 2×m

(8)

WKg−1

The effective series resistance is calculated using-

ESR =

C × ΔV 2 ×Ω 2

(9)

Table 1  Comparison of SC, E, P and ESR at different scan rates

Fig. 8  The galvanostatic charge–discharge plot of 1:1—AC:PANi

Number Specific of Cycles Capacitance (SC) (Fg−1)

Energy Power Equivalent density (ED) density (PD) Series (W h Kg− 1) (W kg− 1) Resistance (ESR) (Ω)

1 100 1000

12.0 11.4 10.0

86 82 71

36 34.4 30.0

0.88 0.93 0.98

13



where I is the current density, ∆t is discharge time, ∆V is potential window and m represents the active mass of the electrodes. The obtained specific capacitance was found to be 83 Fg−1, power density was found to be 36 W kg− 1 and ESR value was found to be 0.23 Ω. Energy density, specific capacitance, power density and equivalent series resistance are calculated using the equation 6, 7, 8 and 9 respectively. The series resistance was found to be 62.8 Ω, charge transfer resistance (Rct) to be 62.5 Ω and Warburg diffusion element (Zw) was calculated to be 10.15 Ω ­(s−1/2). Table 1 represents the specific capacitance values at different cycles, retaining it’s cycle stability upto 82% after 1000 cycles.

4 Conclusion Activated carbon was prepared from coconut shell and its surface area enhanced by KOH activation method and it was found to be 312 m2 g−1. Conducting polymer (Polyaniline) was prepared by oxidative polymerization using H ­ BF4 initiator and (­ NH4)2S2O8 as an oxidizing agent. Cyclic voltammetry for the resultant composites shows quasi-rectangular shape with the good specific capacitance of 77 Fg−1 for 2 m s−1. The shape of the charge–discharge plot is nearly triangular owing to good reversibility and specific capacitance was found to be 86 Fg−1 at current density 1 mA g−1. Energy density was found to be 11 W h kg−1 and power density 36 W kg−1 was obtained. Acknowledgements  The author hereby thanks, Department of Chemistry, Manipal Institute of Technology, Manipal University for the laboratory and Central Instrumentation Facility.

References 1. J.R. Miller, Simon, Science 321, 651 (2008) 2. K.L. Van, T.T.L. Thi, Prog. Nat. Sci. Mater. Int. 24, 191 (2014) 3. L.L. Zhang, X.S. Zhao, Chem. Soc. Rev. 38, 2520 (2009)

13

J Mater Sci: Mater Electron 4. M. Natalia, Y.N. Sudhakar, M. Selvakumar, Indian J. Chem. Technol. 20, 392 (2013) 5. I. Tanahashi, J. Appl. Electrochem. 35, 1067 (2005) 6. V.B. Kumar, A. Borenstein, B. Markovsky, D. Aurbach, A. Gedanken, M. Talianker, Z. Porat, J. Phys. Chem. C 120, 13406 (2016) 7. F.C. Wua, R.L. Tseng, R.S. Juang, J. Colloid. Interface Sci. 283, 49 (2015) 8. M. Sevilla, R. Mokaya, Energy Environ. Sci. 7, 1250 (2014) 9. M.D.N. Hyder, S.W. Lee, F.C. Cebeci, D.J. Schmidt, Y.S. Horn, P.T. Hammond, ACS Nano 5, 8552 (2005) 10. J.X. Huang, R.B.A. Kaner, J. Am. Chem. Soc. 126, 851 (2004) 11. H. Sindhu, V.G. Sharanabasava, R. Bhat, H.M.P.N. Kumar, D.S. Huh, A. Venkataraman, IJSR. NET 01, 102 (2012) 12. D. Lozano-Castello, M.A. Lillo-Ro´denas, D. Cazorla-Amoro´s, A. Linares-Solano, Carbon 39, 741 (2001) 13 Y.N. Sudhakar, H. Hemanth, S.S. Nitinkumar, P. Poornesh, M. Selvakumar, Ionics 23, 1267 (2016) 14. H. Kuang, Q. Cao, X. Wang, B. Jing, Q. Wang, L. Zhou, J. Appl. Polym. Sci. 130, 39650 (2013) 15 Z. Tian, H. Yu, L. Wang, M. Saleem, F. Ren, P. Ren, Y. Chen, R. Sun, Y. Sun, L. Huang, RSC Adv. 4, 28195 (2014) 16 J. Wang, S. Kaskel, J. Mater. Chem. 22, 23710 (2012) 17 K.T. Vadiraj, S.L. Belagali, IOSR-JAC 8, 53 (2015) 18 M.D. Bedre, D. Raghunandan, S. Basavaraj, A. Venkataraman, J. Mater. Sci. 2, 39 (2012) 19 H. Swaruparani, S. Basavaraja, C. Basavaraja, S.H. Do, A. Venkataraman, J. Appl. Polym. Sci. 117, 1350 (2010) 20 A.L. Shankarananda, S. Kalyani, Int. J. Eng. Sci. 1, 59 (2012) 21. A. Sarkar, A.K. Singh, D. Sarkar, G.G. Khan, K. Mandal, ACS Sustain. Chem. Eng. 3, 2254 (2015) 22 J. Huang, R.B. Kaner, Chem. Commun. 4, 367 (2006)