voltammetry (CV), galvanostatic charge-discharge and electrochemical .... The supercapacitor shows an increasing specific capacitance from 450 to 466 F/g.
The biomass derived activated carbon for supercapacitor S. T. Senthilkumar, R. Kalai Selvan, and J. S. Melo Citation: AIP Conf. Proc. 1538, 124 (2013); doi: 10.1063/1.4810042 View online: http://dx.doi.org/10.1063/1.4810042 View Table of Contents: http://proceedings.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=1538&Issue=1 Published by the AIP Publishing LLC.
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The Biomass Derived Activated Carbon for Supercapacitor S.T.Senthilkumara, R.Kalai Selvana and J.S.Melob a
Solid State Ionics and Energy Devices Laboratory, Bharathiar University, Coimbatore-641 046, India. b Nuclear Agriculture and Biotechnology Division, BARC, Trombay, Mumbai-400 085, India.
Abstract. In this work, the activated carbon was prepared from biowaste of Eichhornia crassipes by chemical activation method using KOH as the activating agent at various carbonization temperatures (600 °C, 700 °C and 800 °C). The disordered nature, morphology and surface functional groups of ACs were examined by XRD, SEM and FT-IR. The electrochemical properties of AC electrodes were studied in 1M H2SO4 in the potential range of -0.2 to 0.8 V using cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) techniques in a three electrode system. Subsequently, the fabricated supercapacitor using AC electrode delivered the higher specific capacitance and energy density of 509 F/g at current density of 1 mA/cm2 and 17 Wh/kg at power density of 0.416 W/g. Keywords: Eichhornia crassipes; Porous activated carbon; Electric double layer capacitor; Capacitance PACS: 87.85.jf; 61.43.Gt; 82.45.Fk; 82.47.Uv
INTRODUCTION Electric double layer capacitors (EDLCs) also known as supercapacitors have attracted special attention due to their potential applications in various fields. It is well known that carbon (activated carbon, graphene and carbon nanotube) [1], metal oxides (MnO2, NiO and Fe3O4) [2] and conducting polymers (poly (3,4ethylenedioxythiophene, Polyaniline) [3,4] are widely examined as electrodes for supercapacitors. Among these, activated carbon (AC) is believed to be suitable due to their low cost, possibility to produce in large scale, easy processability, higher abundance, inert to corrosion and higher endurance at high operating temperatures. In addition it has the good physical/chemical and electrochemical properties for EDLC applications [1]. Also the selection of biomass waste for AC preparation reduces the waste dumping and landfills, environment crisis and cost of waste management. In the recent past a large number of biomasses such as coffee beans [5], coffee ground [6] and banana peel [7] were identified as the low cost electrode raw materials for supercapacitors. On these lines, we have chosen Eichhornia crassipes which is one of the bio-waste and is available in lakes and rivers in large quantities worldwide and is also naturally renewable. The physico-chemical properties of the prepared activated carbons from Eichhornia crassipes have been explored through various techniques like X-ray diffraction (XRD), Raman spectra, scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FT-IR). The electrochemical performance has been ascertained through cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) techniques.
EXPERIMENTAL METHOD AND MATERIALS Using chemical activation method, the activated carbon was prepared from bio-waste of Eichhornia crassipes at various carbonization temperatures (600 °C, 700 °C and 800 °C). The detailed experimental procedure for the preparation of activated carbons (ACs) is given elsewhere [8]. Briefly, the 20 g of pre-heated (at 200 ºC for overnight) pulverized Eichhornia crassipes was activated in 10% of KOH for 24 h. Then, it was carbonized at 600 °C (KA-1), 700 °C (KA-2) and 800 °C (KA-3) for 2 h under Ar atmosphere. The carbonized samples were washed with distilled water and desired amount of HCl until the pH reached neutrality. The washed samples were dried at 100 ºC for overnight. The working electrodes were fabricated as follows; typically, the activated carbon (20 mg), carbon black (2 mg) and poly (vinylidene fluoride) (PVDF, 2 mg) were mixed and dispersed in 0.4 ml of N-methyl-2-pyrrolidone (NMP) to produce homogenous slurry. Then 12 µl of resulting slurry was coated onto the 0.04 mm thick flexible stainless Carbon Materials 2012 (CCM12) AIP Conf. Proc. 1538, 124-127 (2013); doi: 10.1063/1.4810042 © 2013 AIP Publishing LLC 978-0-7354-1162-3/$30.00
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steel sheet (1 cm × 1 cm). The loaded activated carbon was calculated to be 0.6 mg on each electrode. Finally, the fabricated electrodes were dried at 50 ˚C in oven for overnight. The symmetric supercapacitor (SC) was assembled using prepared activated carbon (KA-2) electrode, separated by polypropylene sheet immersed in 1M H 2SO4. The assembly of supercapacitor was successfully completed in air atmosphere at room temperature.
RESULTS AND DISCUSSION The typical XRD pattern of KA-2 is shown in Fig. 1(a). A strong or carbon peak is observed in between 20º and 30º which corresponds to the (002) plane. It indicates that the ACs contain small disordered structure of aromatic sheets or disordered carbon. The other broad peak around 43º attributed to the plane of (001) diffraction indicates the presence of sp2 hybridized carbon of aromatic structure. The Raman spectra (Fig. 1(a) (inset)) also showed the highly disordered carbon characteristics. The peak at about ~1360 cm-1 corresponds to the D band which indicates the breathing mode vibration of A1g, related to disordered carbon. The observed G band at ~1565 cm-1 corresponds to in-plane stretching vibration mode of E2g in sp2 carbons. The FT-IR spectrum of KA-2 is shown in Fig. 2(b). The broad peak at about 3200 cm-1 corresponds to hydroxyl functional groups. The observed peaks at 1064 and 1224 cm1 are ascribed to stretching vibration of –C-OH and C-O. The peak at 1430 cm-1 is related to -CH2. Similarly, the peak at 1562 cm-1 is assigned to quinone group (-C=O). The additional peaks at 871 cm-1 and 583 cm-1, are related to -C-H and siloxane groups [1, 8]. The SEM image of KA-2 indicates (Fig. 1(c)) the presence of porous morphology. Fig. 1(d) shows the EDAX pattern (Fig. 1(d)) of KA-2. It indicates that the samples contains 84.18% of carbon. Fig. 2(a) shows the CV curve of all the AC electrodes at 5 mV/s in 1M H 2SO4 vs. Ag/AgCl in the potential range of -0.2 to 0.8 V. The quasi-rectangular shape was observed for all AC electrodes (Fig. 2(a)) which infers the response is due to the combination effect of redox capacitance and electric double layer capacitance. This redox reaction is correlated to the charge (electron) transfer reaction between the electrolytic ions and presence of functional groups on active material. Moreover, among the AC electrodes, KA-2 shows the large current area that indicates its higher capacitance behavior. The discharge current response of the AC electrodes with respect to scan rate is given Fig. 2(b). A more linear discharge current behavior was observed for KA-2 electrode which indicates the better charge transfer performance. The deviated line shows the slower charge transfer performance. The mean areal capacitance of the AC electrodes are also calculated [9] by taking slope (Ca=ΔI/Δs) of the fitted line. The maximum areal capacitance of 0.31 mF/cm2 was obtained for KA-2 and 0.28 mF/cm2 for KA-1and 0.26 mF/cm2 for KA-3. (a)
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Figure 2(c) shows the Nyquist plot of all the prepared activated carbon electrodes. At low frequency, the large and small tail expresses the slower and quick access of ions into the porous electrode. Additionally, about 45° inclined line and a vertical line close to 90° reveals the non-ideal (Warburg resistance) and ideal capacitive nature of the electrodes. The observed semicircle at high frequency expresses the presence of possible pseudocapacitive interaction in the electrodes. The diameter of the semicircle decreased with increasing carbonization temperature in the order of KA-3
