Electrochemical performance of activated carbons prepared from rice husk in different types of non-aqueous electrolytes

Biomass and Bioenergy 83 (2015) 216e223

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Research paper

Electrochemical performance of activated carbons prepared from rice husk in different types of non-aqueous electrolytes Seiji Kumagai a, *, Daisuke Tashima b a b

Department of Electrical and Electronic Engineering, Akita University, Tegatagakuen-machi 1-1, Akita 010-8502, Japan Department of Electrical Engineering, Fukuoka Institute of Technology, Wajiro-higashi 3-30-1, Higashi-ku, Fukuoka 811-0295, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 July 2015 Received in revised form 26 September 2015 Accepted 30 September 2015 Available online xxx

Activated carbons (ACs) prepared from rice husk (RH), an agricultural byproduct, have mesoporosity that is obtainable from leaching of the mineral component of silica. To verify the suitability of RH-derived ACs for the use of electrode materials of electrical double-layer capacitors, we evaluated the electrochemical performance of three RH-derived ACs (two micro- and mesoporous ACs and one mesoporous AC). Evaluation was done by using the non-aqueous ionic electrolyte solutions 1 mol dm
3 triethylmethyl ammonium tetrafluoroborate/propylene carbonate (PC) solution, 1.5 mol dm
3 spiro-(1,10 )-bipyrrolidinium tetrafluoroborate/PC (SBP$BF4/PC) solution, and the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIm$BF4). Under low voltage scan rate (1 mV s
1) and low current density (<1 mA cm
2), mesoporous AC, which had the highest specific surface area, showed the highest specific capacitance (120 F g
1) in EMIm$BF4. However, its specific capacitance considerably decreased because of the increase in scan rate and current density. Under high scan rate (10 and 100 mV s
1) and high current density (>10 mA cm
2), micro- and mesoporous AC in 1.5 mol dm
3 SBP$BF4/PC showed the highest specific capacitance and highest retention of specific capacitance, even though its specific surface area was not the highest. Mesoporous AC showed voltage-dependent specific capacitance, indicating that ionic transport in the mesoporous structure was sensitive to electric field. It was finally shown that micro- and mesoporosity developed by utilizing natural structure and composition of RH was useful for the electrode materials of advanced electrical double-layer capacitors requiring more viscous nonaqueous electrolytes. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Electrical double-layer capacitor Supercapacitor Activated carbon Rice husk Porosity Non-aqueous electrolyte

1. Introduction Electric double-layer capacitors (EDLCs), alternatively known as supercapacitors, effectively enable numerous chargeedischarge cycles, as well as provide high power density, long life, and high efficiency. In combination with other power sources, EDLCs have potential of meeting the increasingly stringent specifications of next-generation vehicles (electric vehicles and fuel cell vehicles) [1] and of adopting renewable energy such as solar and wind power [2]. The mechanism of power storage by EDLCs is electrical polarization of the interface between the electrode material and the electrolyte, which allows stable and efficient chargeedischarge cycles. Formation of a wider double-layer with more charge at the interface leads to higher capacitance of EDLC devices. Activated

* Corresponding author. E-mail address: [email protected] (S. Kumagai). http://dx.doi.org/10.1016/j.biombioe.2015.09.021 0961-9534/© 2015 Elsevier Ltd. All rights reserved.

carbons (ACs) are one of the suitable electrode materials for EDLCs because of their large specific surface area (approximately, 1000e3000 m2 g
1), low cost, high chemical stability, and high electrical conductivity. The pore structure of ACs is closely related to the capacitive and resistive behaviors of EDLCs. An abundance of micropores can increase the area of the interface between the electrode and electrolyte. However, ionic mobility in electrolyte in the micropores is insufficient [3]. Recent non-aqueous electrolytes which enable high withstanding voltage and thus high energy density further restrict utilization of micropores because of their high viscosity. The type of electrolyte is a critical factor that determines the accessibility of ions to pores. The appropriate cation, anion, and solvent can enhance the EDLC performance [4e6]. Typical nonaqueous electrolyte systems for EDLC applications are quaternary ammonium salts dispersed in solvents of carbonates, for example, tetraethyl ammonium tetrafluoroborate/propylene carbonate (TEA$BF4/PC) and triethylmethyl ammonium tetrafluoroborate/

S. Kumagai, D. Tashima / Biomass and Bioenergy 83 (2015) 216e223

propylene carbonate (TEMA$BF4/PC) mixtures. In such electrolytes, ionic mobility can be influenced by the solvent type, ion concentration, and pore structure of the electrode [7]. Spiro-(1,10 )-bipyrrolidinium tetrafluoroborate/PC (SBP$BF4/PC) solution has been used as alternative electrolyte to conventional quaternary ammonia electrolytes [8e14]. The performance of SBP$BF4/PC was better in comparison with that obtained with conventional nonaqueous electrolytes [10,14]. Ionic liquids have been used as EDLC electrolytes because of their advantages such as nonvolatility and nonflammability [15e17]. Ionic liquids, which are liquids composed only of salt (anion and cation), may be used to improve the affinity of cations and anions to the pore surfaces to enhance the electrochemical performance [18]. Attention has been paid to the role of mesopores produced in ACs in enhancing the utilization of exposed pore surfaces. Highly mesoporous AC (95% mesopore volume fraction) has been shown to exhibit good electrochemical performance in a non-aqueous electrolyte [19]. The presence of mesopores could shorten the path length for ion transport in micropores and could alleviate pore blockage in micropores due to aggregation of ions while decreasing the interfacial area between the electrode and electrolyte. Rice is a staple food in many countries. Annual rice production worldwide is 571 Tg, about one-fifth of which generates the agricultural byproduct rice husk (RH) [20]. The major constituents of RH are cellulose, hemicellulose, lignin, and, mineral components. Silica is a main ingredient of the mineral components. Silica is distributed in RH in a micro to nano scale, and the mass fraction of silica is ca. 20% [21]. This is a peculiar property which is hardly seen in other biomass species. Silica scarcely develops porous structure in RH char, but it can function as a template for mesopores [22]. Because of the great need for utilizing abundant RH, the preparation of RH-based mesoporous ACs has been intensively studied. This was done by a combination of the silica-leaching process and physical or chemical activation [23e27] or through chemical activation using strong alkalis such as NaOH or KOH [28e30]. Biomass-based ACs intended for EDLC applications have been extensively studied: waste coffee ground [31,32], cherry stone [33], waste paper [34], straw [35], sunflower seed shell [36], ginkgo shell [37], argan seed shell [38], potate starch [39], tannin [40], sucrose [41,42], lignin [43]. The electrochemical performance of RH-derived ACs in aqueous electrolytes, namely, 3 mol dm
3 KCl [44], 6 mol dm
3 KOH [45,46], 1 mol dm
3 Na2SO4 [47], and 0.5 mol dm
3 K2SO4 solutions [48], have been found to be comparable with other biomass-based ACs used for EDLC electrodes; their specific capacitances ranged approximately from 80 F g
1 to 280 F g
1. However, the use in non-aqueous electrolytes with higher viscosity and lower conductivity than those of aqueous electrolytes is capable of exerting the full potential of RH-derived ACs with designed mesoporosity. We proposed the fabrication of micro- and mesoporous ACs for EDLC applications using RH as a main ingredient. We used beet sugar to construct an additional microporous structure from which we removed silica to form a template for mesoporous structure [49]. The electrochemical performance of RH-derived ACs in conventional non-aqueous electrolyte of TEA$BF4/PC [50] or TEMA$BF4/PC [49] has been reported, in which they showed excellent performances. However, the electrochemical performance of RH-derived ACs in other non-aqueous electrolytes has not been reported. It would be environmentally and industrially beneficial to convert RH agricultural waste into electrodes of advanced EDLCs using various types of non-aqueous electrolytes. In the present study, we examined the electrochemical performance of RHderived micro- and mesoporous ACs and of a mesoporous AC in

217

three electrolytes, namely, conventional 1 mol dm
3 TEMA$BF4/PC solution, alternative 1.5 mol dm
3 SBP$BF4/PC solution and ionic liquid of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIm$BF4). The roles of microporosity and mesoporosity in their electrochemical performance were also discussed, which elucidated a compatibility of porosity designed in the RH-derived ACs with both conventional and other non-aqueous electrolytes. The results obtained would help to find promising materials for EDLC electrodes in various non-aqueous electrolytes and to understand the interaction between pore structure in ACs and those electrolytes. 2. Experimental 2.1. Sample ACs and their characterization We obtained raw RHs by threshing rice (Oryza sativa L. subsp. japonica) harvested in south region of Akita Prefecture (40e39 N), Japan. The rice cultivar was Akitakomachi. The RHs were naturally dried and then stored indoors. Prior to the preparation of RHderived ACs, they were dried at 378 K for 3 h in a dry oven. We previously reported the preparation processes of RH-derived ACs using the above raw RHs and beet sugar (BS, Hokuren Agriculture Cooperative Association, Japan) which was added to enhance the microporosity of RH-derived ACs [49]. The three ACs, RHBSAC0.5h, RHBSAC1.0h, and RHAC mentioned in [49], were used as sample ACs of EDLC electrodes. RHBSAC0.5h and RHBSAC1.0h are microand mesoporous ACs prepared from RH and BS through the silica leaching process using NaOH solution. The preparation processes of RHBSAC0.5h and RHBSAC1.0h are summarized as follows. The mixture of BS and distilled water was heated at 333 K, providing beet syrup. The RH pre-carbonized at 523 K in N2 gas was mixed with beet syrup, and the mixture was carbonized in N2 gas at 873 K. The carbonized mixture was immersed in NaOH solution at 1 mol dm
3 at 353 K for 10 h for the silica leaching. Then, it was washed using distilled water and dried in air at 378 K. Finally, it was subjected to activation process in CO2 gas at 1123 K. The durations for the activation of RHBSAC0.5h and RHBSAC1.0h were 0.5 and 1.0 h, respectively. On the other hand, RHAC, which had a mesoporous structure, was prepared without using BS. The raw RH was carbonized in enclosed space at 773 K and then subjected to the silica leaching. The leaching process was similar to that of RHBSAC0.5h and RHBSAC1.0h. The resultant RH was washed, dried and subjected to steam activation at 1273 K, finally providing RHAC sample. All the sample ACs were pulverized into particles of <15 mm size. The textural properties of the RH-derived ACs were evaluated from nitrogen adsorptionedesorption isotherms obtained at 77 K and analyzed according to reported methodology and data [49] summarized in Table 1. The pore size distribution was evaluated by using quenched solid density functional theory under the carbon slit-shaped pore model. The width, w, of micropores was <2 nm, and w of mesopores was from 2 nm to 50 nm. Micropores and mesopores were classified as small or large. Micropores were distinguished as either ultramicropores (w  0.7 nm) or supermicropores (0.7 nm < w  2 nm), in accordance with Sing et al. [51]. Mesopores were distinguished as small mesopores (2 nm < w  5 nm) or large mesopores (5 nm < w  50 nm), which is based on the simulation result that mesopores w  5 nm were influential on the specific area capacitance of diffuse layer in use of 1 mol dm
3 TEMA$BF4/PC solution [52]. There are no strict definitions of micro- and mesoporous structure. In the present study, micro- and mesoporous structure was defined to have micropore and mesopore volume fractions between 40% and 60% [49]. Thus, RHBSAC0.5h and RHBSAC1.0h belong to micro- and mesoporous

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Table 1 Textural properties of the RH-derived ACs, summarized from published data [49].

Total pore volume (cm3 g
1) BET specific surface area (m2 g
1) Pore volume calculated by QSDFT (cm3 g
1)

w  0.7 nm 0.7 < w  2 nm 2 < w  5 nm 5 < w  50 nm

RHBSAC0.5h

RHBSAC1.0h

RHAC

0.78

0.99

1.54

1103

1357

1442

0.23 0.17 0.17 0.18

0.18 0.30 0.19 0.25

0.17 0.25 0.54 0.48

w: pore width.

ACs. RHAC of which mesopore volume fraction is >60% is categorized as a mesoporous AC. The surface chemistry of the RH-derived ACs was also evaluated [49] by X-ray photoelectron spectroscopy to determine the surface elements and oxygen-containing functional groups. However, critical differences in their surface chemistry were not detected. Because the present study used only non-aqueous electrolytes, porosity was the main factor that determined their electrochemical performance. 2.2. Electrode preparation and cell assembly A mixture of the active material (sample AC), the conductive agent acetylene black (Denka Black, 10% in mass fraction; Denki Kagaku Kogyo Kabushikikaisya, Japan), and the molding binder polyfluoroethylene (Polyflon D210-C, 10% in mass fraction; Daikin Industries, Ltd., Japan) was ground with a mortar and pestle. During grinding, ethanol was added to the mixture. The mixture was pressed and molded into a thin sheet. The resultant sheet was dried in air at 373 K for >6 h and was then punched out into f 12 mm electrode discs. Six-electrode discs were prepared to evaluate the electrochemical performance of the sample ACs in the different electrolytes. The thickness and the mass of active material (sample AC) in each disc are shown in Table 2. As the thicknesses of the f 12 mm discs were adjusted to around 0.33 mm, RHBSAC0.5h, RHBSAC1.0h, and RHAC had comparable electrode volumes. The mass of the sample disc at the similar volume, namely bulk density, is related to the sample porosity. The masses of discs decreased with increasing their total pore volume (Table 1). The disc was pressed with a current collector made of f 15 mm aluminum mesh. The two discs, which had similar masses (differing by  5%), were attached with aluminum current collectors, degassed at 413 K for >6 h, and then used as positive and negative electrodes. The positive electrode, two pieces of cellulose-type separators (f 23 mm, 55 mm thickness; TF4050, Nippon Kodoshi Corp., Japan) which was degassed at 393 K for >6 h, and the negative electrode were sealed with the electrolyte (3.0 cm3) in the two-electrode cell system. The system had an outer body of aluminum alloy (HS cell, Hohsen Corp., Japan). We used as electrolytes 1 mol dm
3 TEMA$BF4/PC (Ube Industries, Ltd., Japan, hereafter described as TEMA$BF4/PC), and 1.5 mol dm
3 SBP$BF4/PC (KKE-15, Japan Carlit

Table 2 Thicknesses of the employed disc electrodes (f 12 mm) and masses of their active material, RH-derived AC. Sample

Thickness (mm)

Mass of active material (mg)

RHBSAC0.5h RHBSAC1.0h RHAC

0.32 ± 0 0.34 ± 0.01 0.33 ± 0.01

22.0 ± 0.5 18.7 ± 1.0 10.6 ± 0.2

Data are expressed average ± standard deviation.

Co., Ltd., Japan, hereafter described as SBP$BF4/PC), as well as EMIm$BF4 (Kanto Chemicals Co., Inc., Japan), all which had the BF
4 anion. They were used without further purification. The conductivity, viscosity, and cationic structure of the electrolytes are shown in Table 3. All processes for cell assembly were performed in an argon-filled glove box.

2.3. Cyclic voltammetry (CV) CV was performed at 298 K by using an electrochemical measurement system (HZ-5000, Hokuto Denko Corp., Japan). Voltage was repeatedly applied between the positive and negative electrodes of the assembled cell. It was varied from 0 V to 2.5 V and then from 2.5 V to 0 V at a constant scan rate. Simultaneously, the current was measured. The cell was subjected thrice to this voltage sweep to obtain voltammograms of three cycles. CV curves were acquired at scan rates of 100, 10, and 1 mV s
1. Referring to [40,55], the specific capacitance, CCV (F g
1), of the sample AC at varying applied voltage was calculated through Eq. (1):

CCV ¼

4I mAC v

(1)

where I is the current measured at varying applied voltage (A), mAC is the total mass of active material in the positive and negative electrodes of the cell (g), and v is the voltage scan rate (V s
1). The voltammogram at the second cycle was used to evaluate the specific capacitance of the sample ACs.

2.4. Galvanostatic chargeedischarge (GCD) test The cell was evaluated by GCD test in which the cell voltage was increased from 0 V to 2.5 V (charge) and then decreased from 2.5 V to 0 V (discharge) at constant current. A battery chargeedischarge system (HJ1005SD8, Hokuto Denko Corp., Japan) was used for this test. Cyclic chargeedischarge was executed at various current densities and cycle numbers. The current density was taken as the ratio of the current to the area of the circular electrode disc. Thus, sample ACs having identical dimensions were subjected to similar currents. The current density was increased from 0.1 mA cm
2 to 100 mA cm
2. Table 4 shows the conditions of the GCD test. Voltage waveforms of the cell were acquired during the GCD test. We obtained the specific capacitance of the sample AC in the GCD test, CG (F g
1), which depends on the current density, by calculating the charge quantities (time integral of current) during discharge at different current densities by using Eq. (2):

CG ¼

4QGdis mAC E

(2)

where QGdis is the charge quantity in the discharge process (C), mAC is the total mass of active material in the positive and negative electrodes of the cell (g), and E is the voltage range for chargeedischarge (2.5 V). When the current density was high, the previous process could influence the chargeedischarge behavior. Thus, the cycle number was set to increase with increasing current density to stabilize the chargeedischarge behavior or to confirm the reproducibility of the voltage waveform. As shown in Table 4, the electrochemical performance of the sample AC was evaluated at specific cycles. Comparison of the waveform in the previous cycle revealed that voltage waveforms at specific cycles were sufficiently stabilized. All of the tests were carried out at 298 K.

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219

Table 3 Conductivity, viscosity and structure of cations of the electrolytes at 25  C. Electrolyte

Conductivity (mS cm
1)

1 mol dm
3 TEMA$BF4/PC

12.6

3.79

0.65 [53]

1.5 mol dm
3 SBP$BF4/PC

16

5.1

0.42 [14]

EMIm$BF4

13.6

31.8

0.60 [54]

Viscosity (mPa s)

Cation structure

Ionic diameter of cation (nm)

Data on the conductivity and the viscosity were provided by the supplier. This led to different effective digits. The ionic diameter of BF
4 and the diameter of the solvent molecule, PC, were reported to be 0.46 and 0.55 nm, respectively [53].

Table 4 Experimental conditions for the galvanostatic chargeedischarge test. Sequence

Current density (mA/cm2)

Cycle number

Cycle selected for the capacitance evaluation

1 2 3 4 5 6 7 8 9 10

0.1 0.2 0.5 1 2 5 10 20 50 100

2 2 2 2 5 5 5 11 11 11

second second second second third third third sixth sixth sixth

3. Results and discussion 3.1. CV results Cyclic voltammograms of the RH-derived ACs in TEMA$BF4/PC and in SBP$BF4/PC solutions, and in EMIm$BF4 under scan rates of 100, 10, and 1 mV s
1 are shown in Fig. 1. An ideal capacitor lacks a series resistor and capacitance dependence on the applied voltage, and thus produces a symmetric rectangular voltammogram. As none of the samples underwent any redox reactions that produce local humps or dents in the voltammograms, faradaic reactions between the surface functional groups on the samples and the electrolyte, causing pseudo capacitance, hardly occurred. RHBSAC0.5h and RHBSAC1.0h, which were tested at the lowest scan rate (1 mV s
1), produced nearly rectangular voltammograms in all of the electrolytes, and their behavior showed little dependence on the type of electrolyte. With increasing scan rate, cyclic voltammograms of RHBSAC0.5h and RHBSAC1.0h became distorted and acquired a spindle shape. This distortion is general evidence of increased series resistance [33]. Similarly, the largest distortion in the voltammograms and the lowest specific capacitance for RHBSAC0.5h and RHBSAC1.0h were observed when EMIm$BF4 was used. The use of SBP$BF4/PC solution resulted in the least distortion of RHBSAC1.0h voltammogram and the highest specific capacitance of the sample. At similar scan rate and electrolyte, the specific capacitance of RHBSAC1.0h was higher than that of RHBSAC0.5h. The voltammogram of RHAC at 1 mV s
1 is trapezoidal, showing that higher cell voltage resulted in higher specific capacitance. RHAC produced the most extensive distortion due to the increase in scan rate, indicating that the largest series resistance formed with RHAC. In all electrolytes, the specific capacitance increased with the applied voltage and decreased with decreasing applied voltage. This implies that high applied voltage led to high mobility of cations and anions. The difference in specific capacitance during

charging was larger than that during discharging. In addition, RHAC showed the lowest specific capacitance at the highest scan rate (100 mV s
1); in EMIm$BF4, it showed the highest specific capacitance at a scan rate of 1 mV s
1. 3.2. GCD test results We evaluated the specific capacitances of the RH-derived ACs measured during discharge as a function of the current density, which represent their gross charge-storage performance (see Fig. 2). At the lowest current density, RHBSAC0.5h showed a specific capacitance within 92e96 F g
1 regardless of the electrolyte. The use of TEMA$BF4/PC solution was most effective at controlling the lowering of specific capacitance with increasing current density. The specific capacitance of RHBSAC1.0h was higher than that of RHBSAC0.5h in all electrolytes. The use of EMIm$BF4 led to the highest specific capacitance (112 F g
1) at the lowest current density. At high current density, the use of SBP$BF4/PC solution led to strong retention of the specific capacitance of RHBSAC1.0h. We found the specific capacitance of RHAC in EMIm$BF4 at the lowest current density to be 120 F g
1, which was the highest value in all of the experimental conditions. The largest decrease in specific capacitance was also observed with RHAC, regardless of the electrolyte. Voltage waveforms of the two-electrode cells obtained under constant current density (10 mA cm
2) are shown in Fig. 3. The change in voltage of all samples during charge and discharge was approximately linear except during chargeedischarge or dischargecharge switching. Rapid voltage changes during the switching, termed as IR drop, were observed in all samples. RHAC led to larger IR drops than those produced by RHBSAC0.5h and RHBSAC1.0h. The IR drop with RHAC at dischargeecharge switching was noticeably larger than that at chargeedischarge switching. As the voltage range of polarization decreased, the largest IR drop observed with RHAC was clearly related to the large decrease in specific capacitance. We also observed that the degree of IR drop was dependent on the electrolyte. RHBSAC0.5h produced the largest IR drop in SBP$BF4/PC solution, whereas RHBSAC1.0h and RHAC produced the same in TEMA$BF4/PC solution. 3.3. Role of electrolyte type in the electrochemical performance of RH-derived ACs Yu et al. evaluated the electrochemical performance of an AC in 1.5 mol dm
3 TEMA$BF4/PC and 1.5 mol dm
3 SBP$BF4/PC solutions [14]. Under similar salt concentration and PC solvent, SBP$BF4 led to higher conductivity and lower viscosity than those of TEMA$BF4. Although the porous structure of the AC which they used was unknown, 1.5 mol dm
3 SBP$BF4/PC solution led to higher specific

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Fig. 1. Cyclic voltammograms of the RH-derived ACs in different electrolytes at different scan rates. The negative capacitance indicates measurement during the discharge process.

capacitance than those attained with 1.5 mol dm
3 TEMA$BF4/PC solution at all current densities in their study (50e10000 mA g
1). Nono et al. compared the specific capacitance and rate performance of microporous ACs with average pore size of 1.63e1.83 nm in 1 mol dm
3 TEA$BF4/PC and 1 mol dm
3 SBP$BF4 solutions in the solvents PC and dimethyl carbonate (DMC) [10]. All microporous ACs showed higher specific capacitance in the electrolyte 1 mol dm
3 SBP$BF4 solution in PC/DMC mixture (7:3 volume ratio). Although the conductivity and viscosity of the electrolyte which they used were not mentioned, higher performance in terms of rate was attained when SBP$BF4 and AC with smaller average pore size were used. In both works, the reason for the high performance of SBP$BF4 is the smaller size and higher mobility of SBPþ. In the present study, the conductivity and viscosity of the SBP$BF4/ PC solution were about 1.3-fold higher than those of the TEMA$BF4/ PC solution, as shown in Table 3. In addition, the diameter of SBPþ (0.42 nm) is larger than that of TEMAþ (0.65 nm) and is approximately similar to that of BF
4 (0.46 nm). In contrast, that of PC is 0.55 nm.

3.3.1. Specific capacitance and its stability of RH-derived ACs in CV and GCD test Results of CV and GCD test suggest that differences in specific capacitance and in retention of specific capacitance by all of the RHderived ACs in TEMA$BF4/PC and SBP$BF4/PC solutions were small, except in the case of RHBSAC1.0h at high scan rates (10 and 100 mV s
1) and high current density (>10 mA cm
2). As mentioned, the small size of SBPþ in the micropores, resulted in greater retention of the specific capacitance. Textural properties of the sample ACs (Table 1) indicate that RHBSAC1.0h had the largest

specific volume of micropores (0.48 cm3 g
1, w  2 nm). The high mobility of SBPþ even in highly viscous environments could therefore explain the higher rate performance of RHBSAC1.0h in SBP$BF4/PC solution. In all the electrolytes, RHBSAC1.0h could show good electrochemical performance. This was attributable to the well-balanced micro- and mesoporosity in which large mesopores (5 nm < w  50 nm) were not notably developed.

3.3.2. Electrochemical performance of mesoporous AC (RHAC) in non-aqueous electrolytes Low scan rate (1 mV s
1) and low current density (<1 mA cm
2) minimize the effect of the series resistance, increasing the specific capacitance of all of the RH-derived ACs in EMIm$BF4 with their specific surface area. RHAC in EMIm$BF4 showed the maximum specific capacitance. Similarly, Galinski et al. observed that a mesoporous AC (BET specific surface area, SBET: 3261 m2 g
1, total pore volume, VT: 2.03 cm3 g
1, mesopore volume fraction: 70%) had higher specific capacitance in EMIm$BF4 than in 1 mol dm
3 TEMA$BF4 in acetonitrile [56]. On the other hand, the specific capacitance in the PC-based electrolytes did not increase with the specific surface area. Micro- and mesoporous RHBSAC1.0h (SBET: 1357 m2 g
1) showed higher specific capacitance in the PC-based electrolytes compared with that of mesoporous RHAC (SBET: 1442 m2 g
1). The electrochemical performance of ACs is influenced by the behavior of cations and anions on the pore surface. The presence of solvent comprises the difference between the PC-based electrolytes and EMIm$BF4. The viscosity of EMIm$BF4 (31.8 mPa s) was much higher than those of TEMA$BF4/PC (3.79 mPa s) and SBP$BF4/PC (5.1 mPa s) solutions. The high viscosity of ionic liquid is generally

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221

Fig. 2. Specific capacitances of the RH-derived ACs in different electrolytes, as measured during discharge processes in the GCD test.

Fig. 3. Voltage waveforms of cells with RH-derived ACs used with various electrolytes. Waveforms were obtained under constant current density (10 mA cm
2).

attributable to electrostatic interaction (Coulomb force) and hydrogen bonding between cations and anions [57]. The mechanism of charge-driven polarization at the interface between carbon and ionic liquid has been discussed in previous studies [58e60]. Ivanistsev et al. simulated the structural regime of 1-butyl-3methylimidazolium tetrafluoroborate (BMIm$BF4) between two rigid graphene slabs separated by a distance of 10.4 nm under different degrees of surface charge of graphene [61]. BMIm$BF4 has a structure similar to that of EMIm$BF4, and the diameter of BMImþ is slightly larger than that of EMImþ. The 10.4 nm distance is consistent with the width of mesopores. In the study of Ivanistsev et al. [61], a layer of BMImþ formed next to the surface as the surface charge increased from neutral to more negative values. An þ oppositely charged layer of BF
4 and a subsequent layer of BMIm then alternately formed (multilayer formation). When the surface

became more negatively charged, only a single layer of concentrated BMImþ formed closest to the graphene surface (monolayer formation). When the BMImþ concentration at the monolayer reached the maximum, the dense monolayer could not provide the net counter charge. This layer transition proceeds similarly on the positively charged surface. It should be emphasized that the influence of BMIm$BF4 on the electrochemical performances is not identical to that of EMIm$BF4 [62]. However, the structural transition of EMIm$BF4 on the charging surface probably occurs in a similar manner. In TEMA$BF4/PC and SBP$BF4/PC solutions and at the lowest scan rate (1 mV s
1), the specific capacitances of RHAC during discharge reached 120 F g
1 at high applied voltage (>1.8 V) and then decreased to 80 F g
1 at around 0 V. The specific capacitance of RHAC in EMIm$BF4 was observed to behave similarly, as shown by

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the CV profile, but a higher value (152e100 F g
1) was observed during discharge. Under low CV scan rate and low GCD current density, RHAC showed the highest specific capacitance. All samples in EMIm$BF4 showed the largest decrease in specific capacitance when the scan rate and current density increased. Being different from the transport of cations and anions in PC, the mobility of EMImþ and BF
4 are limited because of their higher interaction, resulting in higher viscosity. As stated, surface charging induces structural transition of EMImþ and BF
4 , causing monolayer formation from multilayers on the carbon surface. Facilitated transport of ions in narrow pores is required to maintain the specific capacitance under high scan rate and high current density. Formation of involute layers of EMImþ and BF
4 in concentrated solution can reduce the ionic mobility, increasing the series resistance, and decreasing the number of ions accessible to the carbon surface. Hence, the specific capacitance was not maintained in EMIm$BF4 under high scan rate and high current density. Voltammograms of RHAC show that, regardless of the electrolyte, its specific capacitance was significantly dependent on the applied voltage both during charge and discharge processes. Voltage waveforms of RHAC in EMIm$BF4 indicate that the IR drop produced during dischargeecharge switching was much larger than that during chargeedischarge switching. This difference suggests that larger resistance was produced during discharging with weaker applied electric field. Usually, micropores and mesopores in ACs play different roles during the chargeedischarge processes [63]. Judging from the pore structure of RHAC (VT: 1.54 cm3 g
1, mesopore fraction: 66%), ionic transport in the mesoporous structure was susceptible to the electric field, which was linked to the stability of polarization at the exposed pore surface. The noticeable increase in specific capacitance of RHAC during charging implies that a large driving force provided by a strong electric field was necessary to fill the mesopores or micropores with ions through mesopores and to create ion layers therein. Therefore, limited adsorptive force (van der Waals force) and strong diffusive force within mesopores restricted pore occupancy and layer formation on the exposed pore surface. Therefore, the large dependence of the specific capacitance on voltage and the large IR drop on RHAC were observed during charging. On the contrary, release of cations and anions in mesopores to the exposed pore surface after layer formation at the highest voltage (2.5 V) was more controllable because of the adsorptive force in the pores. Thus, the change in specific capacitance and the IR drop during discharging were alleviated. Table 1 indicated that RHAC had a large volume of mesopores at 5 < w  50 nm (0.48 cm3 g
1). The excessive mesoporosity developed in RHAC was likely to lead to a strong diffusive force. This restricted a quick formation of ion layers on the exposed pore surface, which finally resulted in the largest decrease in the specific capacitance of RHAC under high scan rate and high current density. 3.3.3. Benefits of RH-derived ACs in EDLC applications Electrochemical performance of RH-derived micro- and mesoporous ACs (RHBSAC0.5h and RHBSAC1.0h) and mesoporous ACs (RHAC) in upcoming non-aqueous electrolytes for advanced EDLCs, namely SBP$BF4/PC and EMIm$BF4, was evaluated in comparison with that in conventional TEMA$BF4/PC. The ACs prepared from RH exhibited characteristic electrochemical performances, which depended on their pore structure and their interaction with the used electrolyte. Under the use of viscous electrolytes, micro- and mesoporosity produced in RHBSAC1.0h by utilizing natural structure and composition of RH led to higher rate performance, smaller IR drop, and lower capacitive dependence on the applied voltage. This new finding can contribute to a design and a manufacture of advanced EDLCs using viscous PC-based non-aqueous electrolytes

and neat ionic liquids. 4. Conclusions The specific capacitance and rate performance of RH-derived micro- and mesoporous ACs and of mesoporous AC in 1 mol dm
3 TEMA$BF4/PC and 1.5 mol dm
3 SBP$BF4/PC solutions and in ionic liquid of EMIm$BF4 were evaluated by CV and GCD test. The roles of microporosity and mesoporosity on the capacitive and resistive characteristics of the RH-derived ACs were studied. In EMIm$BF4, mesoporous RHAC, which had the highest specific surface area among the ACs, showed the highest specific capacitance (120 F g
1) under the lowest current density (0.1 mA cm
2). RHAC also displayed considerable decrease in specific capacitance in EMIm$BF4 due to the increase in scan rate and current density. These results may be explained by the intrinsic characteristic of EMIm$BF4, namely, its high viscosity, and by the complex mechanism of layer formation on the exposed pore surface. Under high scan rate (10 and 100 mV s
1) and high current density (>10 mA cm
2), the highest specific capacitance and highest retention of the capacitance were observed on micro- and mesoporous RHBSAC1.0h in 1.5 mol dm
3 SBP$BF4/PC solution rather than in 1 mol dm
3 TEMA$BF4/PC solution. We showed that the high mobility of SBPþ even in highly viscous electrolyte could enhance the rate performance of the RH-derived ACs. The applied voltage greatly influenced the specific capacitance and series resistance of RHAC, which had a large volume of mesopores at 5 < w  50 nm, implying that ionic transport in the mesoporous structure was sensitive to the electric field. Micro- and mesoporosity produced in RHBSAC1.0h was responsible for its higher rate performance, its smaller IR drop, and its lower capacitive dependence on the applied voltage in the viscous electrolytes of 1.5 mol dm
3 SBP$BF4/PC and EMIm$BF4. It was shown that micro- and mesoporosity developed by utilizing natural structure and composition of RH could contribute to a design and a manufacture of advanced EDLCs requiring more viscous non-aqueous electrolytes. Acknowledgment This research was supported in part by JSPS KAKENHI (24686035 and 15K05925). We would like to thank Mr. Yusuke Miura and Mr. Koji Mukaiyachi of Akita University for their help with the experiments. References [1] F. Beguin, E. Raymundo-Piero, E. Frackowiak, Electrical double-layer capacitors and pseudocapacitors, in: F. Beguin, E. Frackowiak (Eds.), Carbons for Electrochemical Energy Storage and Conversion Systems, CRC Press Inc, Boca Raton FL, 2010. Ch. 8. [2] M. Inagaki, H. Konno, O. Tanaike, Carbon materials for electrochemical capacitors, J. Power Sources 195 (24) (2010) 7880e7903. [3] A.B. Fuertes, G. Lota, T.A. Centeno, E. Frackowiak, Templated mesoporous carbons for supercapacitor application, Electrochim Acta 50 (14) (2005) 2799e2805. [4] K. Chiba, T. Ueda, Y. Yamaguchi, Y. Oki, F. Shimodate, K. Naoi, Electrolyte systems for high withstand voltage and durability I. Linear sulfones for electric double-layer capacitors, J. Electrochem. Soc. 158 (8) (2010) A872eA882. [5] K. Chiba, T. Ueda, Y. Yamaguchi, Y. Oki, F. Saiki, K. Naoi, Electrolyte systems for high withstand voltage and durability II. Alkylated cyclic carbonates for electric double layer capacitors, J. Electrochem. Soc. 158 (12) (2011) A1320eA1327. [6] E. Perricone, M. Chamas, J.C. Lepretre, P. Judeinstein, P. Azais, E. RaymundoPinero, F. Beguin, F. Alloin, Safe and performant electrolytes for supercapacitor. Investigation of esters/carbonate mixtures, J. Power Sources 239 (1) (2013) 217e224. [7] A. Orita, K. Kamijima, M. Yoshida, Allyl-functionalized ionic liquids as electrolytes for electric double-layer capacitors, J. Power Sources 195 (21) (2010) 7471e7479.

S. Kumagai, D. Tashima / Biomass and Bioenergy 83 (2015) 216e223 [8] K. Chiba, T. Ueda, H. Yamamoto, Performance of electrolyte composed of spiro-type quaternary ammonium salt and electric double-layer capacitor using it, Electrochemistry 75 (8) (2007) 664e667. [9] K. Chiba, T. Ueda, H. Yamamoto, Highly conductive electrolyte solution for electric double layer capacitor using dimethylcarbonate and spiro-type quaternary ammonium salt, Electrochemistry 75 (8) (2007) 668e671. [10] Y. Nono, M. Kouzu, K. Takei, K. Chiba, Y. Saito, EDLC performance of various activated carbon in spiro-type quaternary ammonium salt electrolyte solutions, Electrochemistry 78 (5) (2010) 336e338. [11] K. Naoi, Nanohybrid capacitor: the next generation electrochemical capacitors, Fuel cells 10 (5) (2010) 825e833. [12] C. Zheng, J. Gao, M. Yoshio, L. Qi, H. Wang, Non-porous activated mesophase carbon microbeads as a negative electrode material for asymmetric electrochemical capacitors, J. Power Sources 231 (1) (2013) 29e33. [13] E. Perriconea, M. Chamasa, L. Cointeauxa, J.C. Lepretrea, P. Judeinstein, P. Azaisc, F. Beguind, F. Alloin, Investigation of methoxypropionitrile as cosolvent for ethylene carbonate based electrolyte in supercapacitors. A safe and wide temperature range electrolyte, Electrochim Acta 93 (1) (2013) 1e7. [14] X. Yu, D. Ruan, C. Wu, J. Wang, Z. Shi, Spiro-(1,10 )-bipyrrolidinium tetrafluoroborate salt as high voltage electrolyte for electric double layer capacitors, J. Power Sources 265 (1) (2014) 309e316. [15] A. Lewandowski, M. Galinski, Carboneionic liquid double-layer capacitors, J. Phys. Chem. Solids 65 (2
3) (2004) 281e286. [16] Q. Zhu, Y. Song, X. Zhu, X. Wang, Ionic liquid-based electrolytes for capacitor applications, J. Electroanal. Chem. 601 (1
2) (2007) 229e236. [17] H. Liu, G. Zhu, The electrochemical capacitance of nanoporous carbons in aqueous and ionic liquids, J. Power Sources 171 (2) (2007) 1054e1061. [18] M. Galinski, A. Lewandowski, I. Stepniak, Ionic liquids as electrolytes, Electrochim Acta 51 (26) (2006) 5567e5580. [19] X. Zhang, J. Wang, Z. Yu, R. Wang, H. Xie, X. Zhang, Highly mesoporous carbon with high capacitance from carbonization of phenoleformaldehyde resin, Mater Lett. 63 (28) (2009) 2523e2525. [20] Y. Chen, Y. Zhu, Z. Wang, Y. Li, L. Wang, L. Ding, X. Gao, Y. Ma, Y. Guo, Applications studies of activated carbon derived from rice husks produced by chemicalethermal processea review, Adv. Colloid Interface Sci. 163 (1) (2011) 39e52. [21] F. Adam, J.N. Appaturi, A. Iqbal, The utilization of rice husk silica as a catalyst: review and recent progress, Catal. Today 190 (1) (2012) 2e14. [22] S. Tabata, H. Iida, T. Horie, S. Yamada, Hierarchical porous carbon from cell assemblies of rice husk for in vivo applications, Med. Chem. Commun. 1 (1) (2010) 136e138. [23] C. Deiana, D. Granados, R. Venturini, A. Amaya, M. Sergio, N. Tacnredi, Activated carbons obtained from rice husk: influence of leaching on textural parameters, Ind. Eng. Chem. Res. 47 (14) (2008) 4754e4757. [24] P.M. Yeketsky, V.A. Yakovlev, M.S. Mel'gunov, V.N. Parmon, Synthesis of mesoporous carbons by leaching out natural silica templates of rice husk, Microporous Mesoporous Mater. 121 (1
3) (2009) 34e40. [25] T.H. Liou, S.J. Wu, Characteristics of microporous/mesoporous carbons prepared from rice husk under base- and acid-treated conditions, J. Hazard Mater. 171 (1
3) (2009) 693e703. [26] Z.R. Ismagilov, N.V. Shikina, I.P. Andrievskaya, N.A. Rudina, Z.A. Mansurov, M.M. Burkitbaev, M.A. Biisenbaev, A.A. Kurmanbekov, Preparation of carbonized rice husk monoliths and modification of the porous structure by SiO2 leaching, Catal. Today 147 (Supplement) (2009) S58eS65. [27] J. Alvarez, G. Lopez, M. Amutio, J. Bilbao, M. Olazar, Upgrading the rice husk char obtained by flash pyrolysis for the production of amorphous silica and high quality activated carbon, Bioresour. Technol. 170 (1) (2014) 132e137. [28] Y. Guo, S. Yang, K. Yu, J. Zhao, Z. Wang, H. Xu, The preparation of mechanism studies of rice husk based porous carbon, Mater. Chem. Phys. 74 (3) (2002) 320e323. [29] Y. Guo, J. Qi, S. Yang, K. Yu, J. Zhao, Z. Wang, H. Xu, Adsorption of Cr(VI) on micro- and mesoporous rice husk-based active carbon, Mater. Chem. Phys. 78 (1) (2002) 132e137. [30] I. Toda, H. Toda, H. Akasaka, S. Ohshio, S. Himeno, H. Saitoh, Effect of potassium agents at activated carbon fabricated from rice husks on pore structure and hydrogen storage property, J. Ceram. Soc. Jpn. 121 (5) (2013) 464e466. [31] T.E. Rufford, D. Hulicova-Jurcakova, Z. Zhu, G.Q. Lu, Nanoporous carbon electrode from waste coffee beans for high performance supercapacitors, Electrochem. Comm. 10 (10) (2008) 1594e1597. [32] T.E. Rufford, D. Hulicova-Jurcakova, E. Fiset, Z. Zhu, G.Q. Lu, Double-layer capacitance of waste coffee ground activated carbons in an organic electrolyte, Electrochem. Comm. 11 (5) (2009) 974e977.
ndez, M.J. La
zaro, C. Ferna
ndez-Gonz
[33] M. Olivares-Marín, J.A. Ferna alez,
mez-Serrano, F. Stoeckli, T.A. Centeno, Cherry stones as A. Macías-García, V. Go precursor of activated carbons for supercapacitors, Mater. Chem. Phys. 114 (1) (1999) 323e327. [34] D. Kalpana, S.H. Cho, S.B. Lee, Y.S. Lee, R. Misra, N.G. Renganathan, Recycled waste paperea new source of raw material for electric double-layer capacitors, J. Power Sources 190 (2) (2009) 587e591. [35] X. Li, C. Han, X. Chen, C. Shi, Preparation and performance of straw based activated carbon for supercapacitor in non-aqueous electrolytes, Microporours Mesoporours Mater. 131 (1e3) (2010) 303e309. [36] X. Li, W. Xing, S. Zhuo, J. Zhou, F. Li, S.Z. Qiao, G.Q. Lu, Preparation of

[37]

[38]

[39]

[40]

[41] [42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

[53]

[54] [55]

[56]

[57]

[58]

[59]

[60]

[61] [62]

[63]

223

capacitor's electrode from sunflower seed shell, Bioresour. Technol. 102 (2) (2011) 1118e1123. L. Jiang, J. Yan, L. Hao, R. Xue, G. Sun, B. Yi, High rate performance activated carbons prepared from ginkgo shells for electrochemical supercapacitors, Carbon 56 (2013) 146e154. A. Elmouwahidi, Z. Zapata-Benabithe, F. Carrasco-Marín, C. Moreno-Castilla, Activated carbons from KOH-activation of argan (Argania spinosa) seed shells as supercapacitor electrodes, Bioresour. Technol. 111 (2012) 185e190. S. Zhao, C.Y. Wang, M.M. Chen, J. Wang, Z.Q. Shi, Potato starch-based activated carbon spheres as electrode material for electrochemical capacitor, J. Phys. Chem. Solids 70 (9) (2009) 1256e1260. G. Amaral-Labat, A. Szczurek, V. Fierro, N. Stein, C. Boulanger, A. Pizzi, A. Celzard, Pore structure and electrochemical performances of tannin-based carbon cryogels, Biomass Bioenerg. 39 (2012) 274e282. L. Wei, G. Yushin, Electrical double layer capacitors with activated sucrosederived carbon electrodes, Carbon 49 (14) (2011) 4830e4838. L. Wei, G. Yushin, Electrical double layer capacitors with sucrose derived carbon electrodes in ionic liquid electrolytes, J. Power Sources 196 (8) (2011) 4072e4079. W. Zhang, H. Lin, Z. Lin, J. Yin, H. Lu, D. Liu, M. Zhao, 3D hierarchical porous carbon for supercapacitors prepared from lignin through a facile templatefree method, ChemSusChem 8 (2015) 2114e2122. Y. Guo, J. Qi, Y. Jiang, S. Yang, Z. Wang, H. Xu, Performance of electrical double layer capacitors with porous carbons derived from rice husk, Mater. Chem. Phys. 80 (3) (2003) 704e709. X. He, P. Ling, M. Yu, X. Wang, X. Zhang, M. Zheng, Rice husk-derived porous carbons with high capacitance by ZnCl2 activation for supercapacitors, Electrochim. Acta 105 (1) (2013) 635e641. D. Liu, W. Zhang, H. Lin, Y. Li, H. Lu, Y. Wang, Hierarchical porous carbon based on the self-templating structure of rice husk for high-performance supercapacitors, RSC Adv. 5 (2015) 19294e19300. A. Ganesan, R. Mukherjee, J. Raj, M.M. Shaijumon, Nanoporous rice husk derived carbon for gas storage and high performance electrochemical energy storage, J. Porous Mater. 21 (5) (2014) 839e847. K.L. Vann, T.T.L. Thi, Activated carbon derived from rice husk by NaOH activation and its application in supercapacitor, Prog. Nat. Sci. Mater. Int. 24 (3) (2014) 191e198. S. Kumagai, M. Sato, D. Tashima, Electrical double-layer capacitance of microand mesoporous activated carbon prepared from rice husk and beet sugar, Electrochim. Acta 114 (1) (2013) 617e626. Y. Gao, L. Li, Y. Jin, Y. Wang, C. Yuan, Y. Wei, G. Chen, J. Ge, H. Lu, Porous carbon made from rice husk as electrode material for electrochemical double layer capacitor, Appl. Energy 153 (1) (2015) 41e47. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984), Pure Appl. Chem. 57 (4) (1985) 603e619. J. Varghese, H. Wang, L. Pilon, Simulating electric double-layer capacitance of mesoporous electrode with cylindrical pores, J. Electrochem Soc. 158 (10) (2011) A1106eA1114. M. Ue, Mobility and ionic association of lithium and quaternary ammonium salts in propylene carbonate and g-butyrolactone, J. Electrochem Soc. 141 (12) (1994) 3336e3342. Y. Matsuda, T. Osaka, Y. Sato, Capacitor Binran, Maruzen Publishing Co., Ltd., Tokyo, 2009, p. 265 (in Japanese). A. J€ anes, H. Kurig, T. Romann, E. Lust, Novel doubly charged cation based electrolytes for non-aqueous supercapacitors, Electrochem. Commun. 12 (4) (2010) 535e539. M. Galinski, K. Babel, K. Jurewicz, Performance of an electrochemical double layer capacitor based on coconut shell active material and ionic liquid as an electrolyte, J. Power Sources 228 (1) (2013) 83e88. K. Fumino, R. Ludwig, Analyzing the interaction energies between cation and anion in ionic liquids: The subtle balance between coulomb forces and hydrogen bonding, J. Mol. Liq. 192 (1) (2014) 94e102. M.V. Fedorov, A.A. Kornyshev, Towards understanding the structure and capacitance of electrical double layer in ionic liquids, Electrochim. Acta 53 (23) (2008) 6835e6840. K. Kirchner, T. Kirchner, V. Ivanistsev, M.V. Fedorov, Electrical double layer in ionic liquids: structural transitions from multilayer to monolayer structure at the interface, Electrochim. Acta 110 (1) (2013) 762e771. M.V. Fedorov, N. Georgi, A.A. Kornyshev, Double layers in ionic liquids: the nature of the camel shape of capacitance, Electrochem. Comm. 12 (2) (2010) 296e299. V. Ivanistsev, S. O'Connor, M.V. Fedorov, Poly(a)morphic portrait of the electrical double layer in ionic liquids, Electrochem. Comm. 48 (1) (2014) 61e64. R. Palm, H. Kurig, K. Tonurist, A. Janes, E. Lust, Is the mixture of 1-ethyl-3methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium tetrafluoroborate applicable as electrolyte in electrical double layer capacitors? Electrochem. Comm. 22 (1) (2012) 203e206. X. He, H. Zhang, H. Zhang, X. Li, N. Xiao, J. Qiu, Direct synthesis of 3D hollow porous graphene balls from coal tar pitch for high performance supercapacitors, J. Mater. Chem. A 2 (1) (2014) 19633e19640.