Biomass-Derived Activated Porous Carbon from Rice Straw for a High

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Biomass-derived Activated Porous Carbon from Rice Straw for High Energy Symmetric Supercapacitor in Aqueous and Non-aqueous Electrolytes Sudhan Nagarajan, Kaipannan Subramani, Manickavasakam Karnan, Nagarajan Ilayaraja, and Marappan Sathish Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01829 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016

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Biomass-Derived Activated Porous Carbon from Rice Straw for High Energy Symmetric Supercapacitor in Aqueous and Non-aqueous Electrolytes N. Sudhana,c, K. Subramania,b, M. Karnana , N. Ilayarajaa and M. Sathisha,b* a

Functional Materials Division, b Academy of Scientific and Innovative Research (AcSIR), c

Centre for Education, CSIR-Central Electrochemical Research Institute, Karaikudi- 630 003, Tamilnadu, India.

Corresponding authors:

[email protected]; [email protected]

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Abstract Biomass-derived activated carbon materials were prepared via a two-step synthesis via carbonization followed by KOH activation of rice straw at 600 °C in an argon atmosphere. The formation of disordered micro and mesopores on carbon by KOH chemical activation and the high specific surface area of ~1007 m2 g-1 was confirmed by N2 adsorption-desorption. Further, the scanning electron microscopic analysis revealed the formation of disordered pores over the carbon surface and the transmission electron microscopic analysis confirmed the formation and aggregation of ultra-fine carbon nanoparticles of ~5 nm in size after the carbonization and the activation process. The three-electrode cell in aqueous electrolyte shows high specific capacitance of 332 F g-1 with high specific capacitance retention of 99% after 5000 cycles. The fabricated symmetric supercapacitor device in aqueous 1 M H2SO4 electrolyte showed a high specific capacitance of 156 F g-1 with a high energy density of 7.8 Wh kg-1. The symmetric device fabricated using 1-ethyl-3-methyl imidazolium tetrafluoroborate [EMIM][BF4] ionic liquids exhibited a cell voltage of 2.5 V and specific capacitance of 80 F g-1 with a high energy density of 17.4 Wh kg-1. The observed electrochemical performance clearly indicates that the activated carbon derived from rice straw could be used as a promising electrode material in supercapacitor for electrochemical energy storage. The cheaper and readily available rice-straw raw materials, simple chemical activation process and high performance promise that the obtained carbon material is viable for commercial applications in supercapacitors.

Keywords: rice-straw; biomass-derived porous carbon; supercapacitors; ionic liquids; high energy density


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1. INTRODUCTION The raising energy consumption, depletion of natural energy resources, drastic climate change, and environment concern dictate to move towards sustainable energy sources with green energy storage. Among the available energy storage devices, supercapacitors have high power density and long cycle life, while the energy density is low which limits their application in large-scale energy storage. However, high power density is essential for an energy storage device for their potential applications in hybrid electric vehicles and heavy electric vehicles. Meanwhile, various attempts are in progress to increase the energy density of the supercapacitors using different electrode materials.1–7 In general, carbon-based materials such as activated carbon,8 graphene,9 carbon nanotube,10 carbon nanofiber,11 templated porous carbon12 and carbidederived carbon13 are widely used as electrode materials in electrochemical double layer capacitors (EDLC). In general, the specific capacitance and energy density could be improved by tuning the surface area and conductivity of the carbon-based materials. Recently, many attempts have been focused on increasing the energy density of the supercapacitor by combining the EDLC and pseudocapacitive materials such as metal oxides, conductive polymers etc.14,15 Additionally, various attempts also have been made to modify the carbon network by doping pseudocapacitive hetero atoms such as nitrogen and sulphur.16–19 Recently, the synthesis of carbon from various organic wastes and biomass-derived carbon materials are under investigation for energy storage applications due to it's abundant and ecofriendly in nature.20 Also, the carbon materials prepared from biomass or organic materials have a significant amount of hetero-atoms as default dopant based on the nature of the biomass that adds additional capacitance due to their pseudocapacitive nature. Similarly, attempts have been made to prepare porous activated carbon from biomass either by chemical activation or physical 3 ACS Paragon Plus Environment

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activation.21,22 Bio sources such as prawn shells,23 catkins,24sugar-cane bagasse,8cotton,25 dead leaves20 and human hair26 were successfully converted into activated carbon and used as an electrode material for supercapacitors. In general, it is observed that the physical or chemical activation is crucial to for the formation of pores and enhance the surface area of the electrode materials. The physical activation is done by carbonization followed by high-temperature treatment at 600 to 1200 oC under inert atmosphere.27 And, the chemical activation is done by heat treatment of carbon precursors with various pore forming reagents (porogen) like KOH, NaOH, ZnCl2, H3PO3.11,28–30 A high specific capacitance of 372 F g-1 at 1 A g-1current density has been achieved by a high specific surface area (3510 m2 g-1) carbon prepared using catkins as bio-source.24 Microporous carbon derived from plant leaves exhibited a high specific capacitance of 302 F g-1 at 1 A g-1 in 1M H2SO4 electrolyte.20 Cherry stones have been successfully converted into the activated carbon with specific surface area of 1273 m2 g-1 by KOH activation and possessed a capacitance of 232 F g-1 in an aqueous electrolyte.31 The activated carbon prepared from rice husk (outermost layer of the rice grain) exhibited a high capacitance of 367 F g-1 in aqueous electrolyte medium.32 A detailed comparison of surface area and specific capacitance for various biomass-derived activated carbon materials is shown in Table S1. From the above Table, it is clearly seen that various biomass could be converted to porous activated carbon for application in supercapacitors. Burning biomass is the prime air pollution in many countries like India and china. Because, If one ton straw materials burnt in open field nearly 400 kg of carbon goes back to the atmosphere in the form of CO and CO2. In order to prevent it, the straw materials could be converted into carbon by burning in a closed inert atmosphere. Thus, the bio-waste materials could be converted to useful carbon materials.


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Among various bio-waste materials, rice straw is one of the most common material from the agriculture sector. This could be effectively utilized for the preparation of porous activated carbon for application in supercapacitor as an electrode material. In our present study, the attempts have been made to prepare carbon from rice straw (Oryza sativa) and the resulted carbon was subjected to chemical activation using KOH as an activation agent. The KOH activation has a significant role in pore formation on the carbon materials that enhances the specific surface area of the material. The pore formation and specific surface area of activated carbon electrode materials depend on the activation temperature.33 Indeed, the high-temperature activation has some disadvantages such as lower yields; pore shrinking effects that will reduce the active surface area of the electrode materials. Consequently, many attempts have been made to optimize the activation temperature between 400–900 oC and it was apparently fixed around 600 ͦC for satisfactory performance.34,35 Here, we demonstrate a simple and scalable synthesis of activated porous carbon from rice straw and fabrication of symmetric supercapacitor device in aqueous and ionic liquids. The fabricated symmetric cell in aqueous electrolyte exhibits a high specific capacitance of 156 F g-1 at 0.5 A g-1 current density and high electrochemical stability in wider potential window. In addition, the fabricated symmetric cell in 1-ethyl-3-methyl imidazolium tetrafluoroborate ionic liquids (EMIMBF4) exhibits a high specific capacitance of 80 F g-1 with high energy density of 17.4 Wh kg-1 and a power density of 174 W kg-1. 2. EXPERIMENTAL SECTION 2. 1. Preparation of porous rice straw carbon (RSC) The activated porous carbon from rice straw was prepared via carbonization followed by chemical activation. In a typical carbonization and activation process, good quality rice straw


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was collected from a field near Karaikudi, Tamil Nadu, India. Further, the dried rice straw was chopped into small pieces followed by washing with water for several times and dried in a hot air oven at 80 °C for 24 h. The dried straw was carbonized in a tubular furnace at 600 °C for 4 h with a heating rate of 5 °C min-1 under argon (Ar) atmosphere. The resulted carbonized carbon was denoted as BA-RSC (Before activation-rice straw carbon). For the activation, the carbonized rice straw carbon was mixed with KOH slowly in a weight ratio of 1:3 followed by heating at 600 °C for 1 h in Ar atmosphere. The resulted sample was collected and washed with DI water and then washed with 1M HCl solution until the pH of the filtrate was neutral and then finally dried at

80 °C for 5 h. The resulted porous carbon was denoted as AA-RSC in the subsequent

discussions. 2. 2. Characterization The crystalline nature and the phase purity of BA-RSC and AA-RSC were examined by powder X-ray diffraction (XRD) measurements using a PAN Analytical X’ Per PRO Model Xray Diffractometer with Cu Kα radiation (α=1.5418 Å) in the range of 10-60°. Fourier Transform Infrared (FT-IR) spectra were recorded in TENSOR 27 spectrometer (Bruker) using KBr pellet technique from 400 to 4000 cm-1. Thermogravimetric analysis (TGA) of the AA-RSC was carried out using TGA/DTA instruments (Model SDT Q 600) from 30 to 800 °C with a heating rate of 10 °C/min in an air atmosphere. CHNS analysis was measured to estimate the amount of carbon content and heteroatom present in the samples using elementarvario EL III. The surface area of CC-AA was calculated using Quantachrome NOVA 3200e surface area and pore size analyzer. The surface morphology of BA-RSC and AA-RSC carbon was characterized using field emission scanning electron microscopy (FE-SEM) using Carl Zeiss AG (Supra 55VP) with an acceleration voltage of 30kV. The particle size and the nature of BA-RSC and AA-RSC were 6 ACS Paragon Plus Environment

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examined using higher resolution transmission electron microscopy (HR-TEM, Tecnai G2 TF20) working at an accelerating voltage of 200 kV. 2.3. Electrochemical characterization Electrochemical properties of BA-RSC and AA-RSC were studied by Cyclic Voltammetry (CV), galvanostatic charge-discharge (CD) and electrochemical impedance spectroscopy

(EIS)

analysis

using

BioLogic

SP-300

Modular

Research

Grade

Potentiostat/Galvanostat/FRA instrument. The working electrode was fabricated by mixing rice straw-derived carbon (BA-RSC or AA-RSC), conductive carbon black (Super-P) and polytetrafluoroethylene (PTFE, as a binder) in a weight ratio of 80:15:5, respectively. After grinding, the resulted electrode paste was pressed on stainless steel (SS) mesh current collector. The electrochemical supercapacitor performances of rice straw-derived activated carbon electrode materials were studied in three electrode configurations using 1M H2SO4 as the electrolyte. For three electrode measurements, rice straw-derived activated carbon electrode materials, Hg/Hg2SO4, and platinum foil served as a working electrode, reference electrode and counter electrode, respectively. The CV and the CD experiments were carried out from the potential range of -0.8 to 0.4 V (vs. Hg/Hg2SO4) at various scan rates ranging from 10 to 50 mV s-1 and different current densities ranging from 0.5 to 10 A g-1, respectively. The quantitative specific capacitance values of BA-RSC and AA-RSC were estimated from discharge curve using the following equation.

=

(1)

Where, (F g-1) is the specific capacitance of the material, I (A) is the current, ∆ (s) is the discharge time, m (g) is the active mass of the electrode and ∆ is the working potential 7 ACS Paragon Plus Environment

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window. In order to study the potential application of AA-RSC electrode materials in supercapacitor device, the electrode materials were studied in two electrode configuration using aqueous and ionic liquid electrolytes. For the above measurements, the equal amount of electrode materials pressed on SS mesh current collectors act as anode and cathode, respectively. In the case of non-aqueous electrolytes ([EMIM][BF4] ionic liquids), conventional Swagelok type

cell was fabricated using Whatman filter paper and Pt foil as separator and current

collector, respectively. The gravimetric capacitance, energy density and power density were calculated using the following equations:

=

(2)

= 2

(3)

=

(4)

=

(5)

Where, Ct (F g-1) is the total specific capacitance of the electrode materials in a symmetrical system, m (g) is the active mass of the electrode, E (Wh kg-1) is the specific energy density; P (W kg-1) is the specific power density of the supercapacitor system.

3. RESULTS AND DISCUSSIONS The activated porous carbon from rice straw was prepared using a two-step process. The first step is the carbonization of dried rice straw and the second step is the activation of surfaces using KOH as a chemical activation agent. The KOH activation is known for the formation of pores on 8 ACS Paragon Plus Environment

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the carbon due to the intercalation of metallic K. After removal of the intercalated metallic K, the expanded carbon lattice cannot return back to its original lattice structure, thus the porous structure has been achieved.28 The XRD patterns of BA-RSC and AA-RSC are shown in Figure 1a. The BA-RSC sample shows nearly featureless profile with a broad hump at 2θ=23° due to the formation of amorphous carbon. Whereas, the AA-RSC shows two broad peaks at 2θ= 23 and 43° correspond to (002) and (100) reflections, which clearly indicates that the hightemperature treatment during activation process enhances the formation of graphitic carbon structure. Though, the complete graphitization will occur very high temperatures (2600–3300 ºC), the KOH activation process induces a certain amount of graphitization that clearly confirms that the bare rice straw material was successfully converted into amorphous carbon and partially graphitic carbon via carbonization process followed by activation process, respectively.36– 38

Additionally, the high-temperature carbonization process is responsible for the formation of

amorphous carbon and also the formation of porous structure.39 Surface nature of the prepared carbon samples is further evaluated using FTIR spectroscopic analysis (Figure 1b). The O-H stretching was observed in both the samples around 3448 cm-1 was mainly attributed to chemisorbed water molecules and hydroxyl groups on the carbon. The FTIR spectrum of BARSC samples shows several peaks at 3448, 1733, 1369 and 1218 cm-1 owing to various oxygencontaining functional groups like –OH, C=O, C-H, and C-O, respectively,40,41 whereas the activated sample AA-RSC shows only a few functional groups with reduced intensities. This clearly indicates that during the activation process many functional groups such as oxygen and sulphur containing functional groups were reduced significantly on the carbon surface. The existence of various elements on the carbon at before and after activation was evaluated by CHNS analysis. The BA-RSC showed 3.07% of sulfur content but the activated sample AA-RSC


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revealed no significant level of sulfur or other heteroatom doping in the carbon network. (Table S2) TGA is a thermal analysis method which measures the change in the mass of the materials as a function of increasing temperature. It is a characteristic tool to understand the physical and chemical nature of the carbon materials and its purity. When biomass is converted to carbon by carbonization there is a possibility for the existence of some metal/metal ion and carbonaceous impurities based on the nature of the biomass. The TGA profile of AA-RSC (Figure 1c) shows initial weight loss below 100 °C due to the physically adsorbed water molecules and adsorbed moisture. When the temperature increased above 100 °C, indicating a stable profile was observed up to 450 °C, and then a drastic weight loss of 100 % was observed between 450 to 600 °C due to the decomposition of carbon into CO2. The steep weight loss profile indicates that the KOH activation results in the formation of structurally similar and stable carbon in the AA-RSC. The above 600 °C, the decomposition profile shows 100% weight loss, it is confirmed that there are no metal/metal ion impurities present in the biomass-derived activated carbon electrode materials. Nitrogen adsorption-desorption isotherms were used to calculate the specific surface area and the porosity of AA-RSC activated carbon material (Figure 1d). A high specific surface area of ~1007 m2 g-1 was obtained for the AA-RSC carbon due to the formation of various types of pores during the chemical activation. The obtained adsorption-desorption isotherm shows hysteresis due to the existence of porosity due to the existence of meso, micro, and macropores with disordered structure. Further, the BJH pore size distribution (inset Figure 1d) indicates the combination of meso and micro pores in the activated carbon which acts as efficient ion transfer


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channels to improve the overall electrochemical behavior. Further, the morphology studies were carried out using microscopic techniques. The surface morphology and the porous nature of BA-RSC and AA-RSC carbon materials were investigated by using FE-SEM images (Figure 2). Formation of thick and large size carbon particles with smooth surface could be seen in the BA-RSC sample (Figure 2 a-b). This clearly confirms that the carbonization process at 600 °C in Ar atmosphere could convert the dried rice straw to valuable carbon product. The chemical activation of BA-RSC using KOH treatment at high-temperature results the formation of pores on the carbon surface that results in the formation of highly rough surfaces with tiny nano-sized particles (Figure 2 c-d). The above results clearly indicate that the KOH activation can enhance the active surface area of carbon by creating additional pores on the surfaces. It is worthy to note here that the formation of pores and modified surface nature of carbon materials by KOH activation could be clearly seen by comparing the FE-SEM images of BA-RSC and AA-RSC (Figure 2). The microstructure and the particle size of the rice straw-derived carbon were examined using HR-TEM as shown in Figure 3. At low magnification, the formation of small size and porous aggregated carbon particles could be seen in Figure 3a whereas the magnification was increased further, it is very clear that the aggregated carbon particles were made up of ultra-small carbon nanoparticles (Figure 3b). The red circle and the inset are shown in Figure 3c which clearly confirms the existence of few nanometer-size carbon particles (less than 10 nm size) in AA-RSC. The lattice fringes were observed in high-resolution TEM image (Figure 3d), showing the d spacing of 0.33 nm, which is in good correspondence with the (002) plane of the graphitic structure. The aggregation of extremely small size carbon nanoparticles in AA-RSC with the


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porous network is highly warranted for easy electrolyte penetration through the electrode materials to access the entire surface for electrochemical energy storage. Based on the above results, it was believed that the activated porous carbon derived from rice straw is a promising candidate for electrochemical supercapacitor applications. To evaluate their electrochemical performance, cyclic voltammetry experiments were performed using a three-electrode system in 1 M H2SO4 electrolyte. The CV profile of BA-RSC and AA-RSC at a potential range of -0.8 to 0.4 V vs. Hg/Hg2SO4 with a sweep rate of 50 mV s-1 is shown in Figure 4a. Further, it can be clearly seen that the area under the curve of AA-RSC is significantly higher than BA-RSC due to high porosity and surface area. Interestingly, the faradaic humps were observed in the CV curves at low scan rates due to reversible pseudocapacitance from the surface oxygen functional groups.42 In general, heteroatoms (typically oxygen or nitrogen) doped carbon materials showed pseudocapacitance due to the redox reaction induced by the lone pair of electrons in the doped hetero atoms.45,53 Furthermore, Figure 4(b) displays the CV profile of AA-RSC at different scan rates, indicating that the peak current is increased thereby the scan rate. On the other hand, at higher current density the specific capacitance was moderately decreased because of inadequate electrolyte diffusion at the electrode surface.43 Further, the galvanostatic discharge profile of BA-RSC and AA-RSC at 1 A g-1 constant current density is shown in Figure 4(c), which clearly confirms that the AA-RSC electrode has high specific capacitance than the BA-RSC. In addition, the nonlinear discharge profile of AA-RSC clearly confirms the contribution of pseudocapacitance due to the redox reactions by the surface functional groups. This is in good agreement with the redox peak observed due to the pseudocapacitance in the CV studies. Figure 4(d) demonstrates the CD curves of AA-RSC at different current densities. The specific capacitance of 332, 304, 277, 253, 12 ACS Paragon Plus Environment

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237, 229 and 190 F g-1 was attained from the CD profile for the current densities of 0.5, 1, 2, 3, 4, 5 and 10 A g-1, respectively. Generally, the diffusion of electrolyte through the micro and mesoporous surfaces will enhance the energy density of the supercapacitor due to the high surface area. At low charging rate, the electrolyte will diffuse through entire micro and mesoporous thereby the specific capacitance will be improved significantly. However, at high charging rates the electrolyte can access only the mesoporous which will reduce the specific capacitance. The BET analysis show the AA-RSC electrode contains a wide range of pore size distribution that covers the existence of micro, meso and macropores. The specific capacitance as a function of current densities is shown in Figure 4(e) and even at a high current density of 10 A g-1 the initial specific capacitance retention of 57 % was achieved which clearly confirms the good rate performance of AA-RSC electrodes for supercapacitor applications. The long-term cyclability was recorded for 5000 cycles at a high current density of 10 A g-1 (Figure 4f) and the first and last 10 cycles of CD profile during the 5000 cycles are shown as inset of Figure 4f. From the CD profile, it is worthy to note that 99 % capacitance retention was achieved even after 5000 cycles of charge-discharge; this confirmed once again the stability of the AA-RSC electrodes. In order to understand the various resistance factors involved in the electrode/electrolyte interface and charge transfer process, EIS (Electrochemical Impedance Spectroscopy) analysis was carried out for the AA-RSC electrodes. The EIS was carried out at the frequency range of 100 kHz to10 mHz with an amplitude of 5 mV. The Nyquist plot of AARSC electrode contains a semicircle in the high-frequency region and a linear shape in the lowfrequency region (Figure S1†). The obtained EIS was fitted with an appropriate equivalent circuit model (Figure S1† inset) with the physical components such as solution resistance (Rs), charge transfer resistance (Rct) and double layer capacitance (Cdl). The numerical values derived


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from the fitted profile reveal the low Rs and Rct values of 0.9 and 9.9 Ω, respectively. This clearly indicates that the activated high surface area carbon derived from the rice-straw acts as a good electrode material for electrochemical applications. The vertical line in the low-frequency region exhibits the domination of the capacitance behavior at the electrode/electrolyte interface.44 The electrochemical performance of the AA-RSC was investigated in a two electrode system to realize their performance in real device. Figure 5a shows the CV profile of AA-RSC in two electrode cell with a cell voltage of 1.2V. The CV curves of symmetrical AA-RSC electrode materials at different scan rates are also shown in Figure 5a. Further, the CD curves of AA-RSC at different current densities are shown in Figure 5b. The non-linear discharge profiles of the symmetric AA-RSC cell again confirm the contribution of pseudocapacitance by the surface functional groups. The specific capacitance values were calculated at different current densities and the symmetrical AA-RSC system exhibited a high specific capacitance value of 156 F g-1 at 0.5 A g-1 current density. Figure S2† shows the specific capacitance of AA-RSC electrode materials as a function of different current densities. The long-term cycling ability was studied at a high current density of 3 A g-1, indicating that 88% of initial capacitance was observed after 10000 charge-discharge cycles (Figure 5c). The Ragone plot (Figure 5d) for the AA-RSC symmetrical aqueous cell exhibits a high energy density of 7.8 Wh kg-1 at a power density of 150.2 W kg-1. These results are comparable to the reported biomass-derived carbon based symmetrical cells in aqueous electrolyte. Liang et al. reported a symmetrical system using activated carbon from pomelo peel showed a high energy density of 9.4 Wh kg-1 at a power density of 96 W kg-1.45 Recently, Gao et al. reported activated carbon derived from prawn shells showed the energy density of 10 and 7.8 Wh kg-1 for acid and alkali medium respectively.23 Recently, porous carbon derived from catkins based 14 ACS Paragon Plus Environment

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symmetrical system showed an energy density value of 9.5 Wh kg-1 in an aqueous medium.24 Thus, the performance of AA-RSC symmetrical cells confirms their possible applications in energy storage to enervate the energy demands. The energy density of the supercapacitors could be enhanced by increasing the potential window of the electrolytes. But, the decomposition of water at 1.23 V limits the use of aqueous based electrolytes at the high potential window. Conventional organic electrolytes have been used in supercapacitors with a reasonable large potential window. However, there are several issues employing such as low safety, limited ionic conductivity, and toxicity.46,47 To overcome this, ionic liquids have been used, their fascinating properties including the wider potential window up to 5V, large liquid phase range, high ionic conductivity and safety make them excellent electrolytes for electrochemical applications.48 The electrochemical behavior of AARSC electrode was investigated in a symmetry HS (Harmonized System) test cell (Hohsen Corporation) with 1-ethyl-3-methyl imidazolium tetrafluoroborate [EMIM][BF4] as the electrolyte. Figure 6a shows the CV profile of AA-RSC at various scan rates from 10 to 50 mV s-1. The CD curves of AA-RSC symmetric cell at various current densities are shown in Figure 6b with a high specific capacitance value of 80 F g-1 at 0.1 A g-1 current density. In addition, the initial capacitance value of 78% was retained after 5000 cycles at 0.5 A g-1 current density (Figure 6c). Furthermore, the fabricated AA-RSC symmetric supercapacitor cell could power a red LED for more than 5 min upon 20 seconds charging as shown in Figure 6c (inset). The Ragone plot of AA-RSC symmetric supercapacitor cell in ionic liquid electrolyte shows a high energy density of 17.4 Wh kg-1 at a power density of 126 W kg-1. Based on the observed above results, it can be clearly exhibited that the fabricated AA-RSC symmetric supercapacitor cell is


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promising for high energy storage applications and responsible for ameliorating the forthcoming energy storage devices in an effective way.

4. CONCLUSIONS Biomass-derived activated porous carbon was prepared from rice straw via carbonization followed by chemical activation with KOH at 600 °C. The electron microscopic imaging confirmed the formation of pores on carbonized carbon surface by KOH chemical activation and also the aggregation of small carbon nanoparticles on the surface of the AA-RSC. The activated rice straw-derived carbon exhibited a combination of meso and microporous in nature that resulted in the formation of activated carbon with a large specific surface area of ~1007 m2 g-1. The existence of various functional group was confirmed using the IR spectroscopy and TGA profile of the activated carbon confirmed the absence of metal/metal ion and carbonaceous impurities in the activated carbon. The electrochemical measurements in aqueous 1 M H2SO4 showed a high specific capacitance of 332 F g-1 with high specific capacitance retention after 5000 cycles. The assembled symmetric cell in aqueous electrolyte exhibited a high specific capacitance of 156 F g-1 at 0.5 A g-1 current density. Further, the fabricated symmetric cell exhibited a high energy density value of 7.8 Wh kg-1 at a power density of 150W kg-1. In addition, the fabricated supercapacitor cell in EMIMBF4 electrolyte showed a high cell voltage of 2.5 V and the specific capacitance of 80 F g-1 with the high energy density of 17.4 Wh kg-1 and power density of 126 W kg-1. In conclusion, the electrochemical supercapacitor performance of AA-RSC symmetric cell in both aqueous and ionic liquid electrolytes can be utilized as potential application towards the electrochemical energy storage prospect effectively.


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ACKNOWLEDGEMENTS

The authors thank the Science & Engineering Research Board, Department of Science and Technology (DST-SERB), India for financial support. Mr. K. Subramani (IF131153) thanks DST for INSPIRE Fellowship. The authors thank Central Instrumentation Facility, CSIR-CECRI, Karaikudi. NOTES AND REFERENCES †Supporting Information (SI) available: Comparison table of biomass-derived activated carbon based electrode materials, CHNS data, EIS data of AA-RSC electrode material in an aqueous electrolyte, rate performance of AA-RSC symmetric cell. See DOI: 10.1039/b000000x. (1)

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Table of Contents: Biomass-derived Activated Porous Carbon from Rice Straw for High Energy Symmetric Supercapacitor in Aqueous and Non-aqueous Electrolytes N. Sudhan, K. Subramani, M. Karnan, N. Ilayaraja and M. Sathish*

Biomass-derived activated carbon from rice straw was demonstrated via a simple carbonization followed by chemical activation route. The fabricated symmetric supercapacitor cell showed high specific capacitance of 332 F g-1 and 80 F g-1 in aqueous and ionic electrolytes with high specific energy density and power density. The symmetric supercapacitor device in non-aqueous electrolyte could power a red LED for more than 5 min upon charging for the 20s.


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Figure 1. (a) XRD pattern, (b) FT-IR spectra of BA-RSC and AA-RSC materials, (c) TGA profile of AA-RSC and (d) Nitrogen adsorption/desorption isotherms and BJH pore size distribution of AA-RSC (inset).


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Figure 2. FE-SEM images of rice straw-derived carbon materials (a-b) before and (c-d) after KOH activation.


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Figure 3. HR-TEM images of AA-RSC at different magnifications, the red circle and the inset image of (c) show the aggregation of few nanometer-size carbon particles.


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Figure 4. CV profile of (a) BA-RSC and AA-RSC electrode at 50 mV s-1 scan rate and (b) CV profile of AA-RSC at different scan rates, (c) CD profile of BA-RSC and AA-RSC electrodes at 1 A g-1current density and (d) CD profile of AA-RSC at different current densities, (e) specific capacitances as a function of current densities, (f) Specific capacitance retention as a function of cycle number for 5000 cycles and first and last ten CD cycles (inset).


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Figure 5. CV curves of AA-RSC at different scan rates and (b) CD curves of AA-RSC at different current densities, (c) Specific capacitance retention as a function of cycle number for 10000 cycles, (d) Ragone plot of AA-RSC symmetric supercapacitor (calculated from 0.5A/g to 4 A/g).


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Figure 6. (a) CV profile of AA-RSC symmetric two electrode cell in EMIM borate, (b) CD profile of AA-RSC symmetric cell at different current densities, (c) Specific capacitance of AA-RSC symmetric cell as a function of cycle number at 0.5 A/g current density and AA-RSC symmetric cell powered LED (inset), (d) Ragone-plot for AA-RSC symmetric cell in ionic liquid electrolyte.


Biomass-Derived Activated Porous Carbon from Rice Straw for a High

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