Hierarchical porous active carbon from fallen leaves by synergy of K2CO3 and their supercapacitor performance

Journal of Power Sources 299 (2015) 519e528

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Hierarchical porous active carbon from fallen leaves by synergy of K2CO3 and their supercapacitor performance Yin-Tao Li a, Yu-Tong Pi a, Li-Ming Lu a, Shun-Hua Xu a, Tie-Zhen Ren a, b, * a b

School of Chemical Engineering & Technology, Hebei University of Technology, Tianjin 300130, China Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Hierarchical porous active carbon is prepared by activation treatment.
The fallen leaves are used as the carbon source.
The use of mixed K2CO3/KOH can create the hierarchical pores on carbon.
The capacitance of ACs is 242 F/g at 0.3 A g1 in two-electrode system.
The capacitance has no decrease after 2000 cycles.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 June 2015 Received in revised form 6 September 2015 Accepted 9 September 2015 Available online xxx

The active carbon materials (ACs) derived from fallen leaves (FLs) are prepared by the activations of mixed KOH and K2CO3. Characterizations reveal that the use of single activator KOH creates the most of micropores in three dimensional and K2CO3 produces the mesopores/macropores mostly in shallow. When combining KOH with K2CO3, the pore size of ACs is enlarged. The surface areas and hierarchical pore structures are related to the mass ratio of two activators. The electrochemical properties are monitored with the cyclic voltametry, charging-discharging and electrical impedance spectra. The prepared sample displays high specific capacitance of up to 242 F g1 (0.3 A g1, 6 M KOH) in a two-electrode system. Their good cycling stability with nearly no capacitance decrease is obtained after 2000 cycles. © 2015 Elsevier B.V. All rights reserved.

Keywords: Synergy Potassium carbonate Active carbon Fallen leaves Supercapacitor

1. Introduction Active carbon materials (ACs) have been widely used as electrode materials in the electric double-layer supercapacitors (EDLCs) and receive excellent capacitance characters [1e4]. Up to now,

* Corresponding author. School of Chemical Engineering & Technology, Hebei University of Technology, Tianjin 300130, China. E-mail address: [email protected] (T.-Z. Ren). http://dx.doi.org/10.1016/j.jpowsour.2015.09.039 0378-7753/© 2015 Elsevier B.V. All rights reserved.

waste biomass as the precursors of ACs has attracted much attention due to their advantage of cheap, abundant, renewable and environmental friendly. Normally, a high specific surface area (SSA) is the basic requirement for ACs as the electrode materials of EDLCs. The ACs derived from green needle coke show the capacitance increased from 260 F g1 to 348 F g1 with the enhancement of surface areas from 2256 m2 g1 to 3347 m2 g1 [5]. On the other hand the ion transportation by the electrostatic attraction between the electrode and electrolyte interface is judged by the pore structures of ACs [6e8]. The tortuosity, connectivity, size

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distribution, and the shape of the pores mainly influence the ion storage capacity and diffusion rate of the supercapacitor [9e11]. The micropores can enhance the surface area and strengthen the electric-double-layer capacitance [12], but hinder the electrolyte ions diffusing into the pores, resulting in a decrease of specific capacitance. In contrast, the mesopores and macropores display the low-resistance by offering broad pathways for transportation of the ions [2,13], but decrease the capacitance due to the low surface area. Hence, efforts to develop the controllable pore size distribution (PSD) of ACs are necessary. In general, the chemical activation has been a conspicuous pathway to produce ACs with high SSA and homogeneous PSD. Some reagents have been commonly used for chemical activation in ages, such as ZnCl2, H3PO4, KOH, NaOH and K2CO3 [14e18]. Among these activators, KOH and NaOH as a strong basic material are reported to be highly efficient [19e22]. Recently, the ACs derived from Enteromorpha prolifera and activated by KOH show high SSA of 2405 m2 g1 and specific capacitance of 296 F g1 at the current density of 0.5 A g1 in 30 ut% KOH solution [22]. However, the ACs activated by KOH represents only micropores and limited mesopores which limits the application in the storage devices due to the high ion-transport resistance and insufficient ionic diffusion [23]. Besides, the excessive KOH causes serious corrosion to the equipment and further increases the cost of industrial production. The non-hazardous K2CO3 with non-deleterious properties has attracted popular attention as an alternative of chemical activation [24e27]. ACs from several kinds of nutshells have been activated by K2CO3 when the temperature is above 627  C. The carbon in the chars can be removed with the release of CO gas, resulting the increase of SSA and the pore volumes. When the temperature reaches to 877  C, the ACs possess the maximum SSA and a large number of mesopores are produced [18]. However, the ACs activated by K2CO3 show weak capacitance characteristics due to the relative low SSA. The ACs from polyaniline display the SSA of 917 m2 g1 and the specific capacitance of 210 F g1 at the low scan rate of 2 mV s1 [28]. Preparing the ACs with hierarchical pore structures thus becomes the effective way to increase the capacitance and decrease the resistance. Herein, we report a simple strategy to fabricate the ACs using the mixed activators of KOH and K2CO3, of which the fallen leaves are chosen as the precursor. According to adjusting the mass ratio of KOH and K2CO3, a series of ACs with different surface areas and pore volume are obtained. We explore the effect of the mass ratio of KOH and K2CO3 on the activation process in details and fully perform electrochemical properties of prepared ACs by CV, EIS and life-cycle. The activated ACs exhibit high specific capacitance value, up to 242 F g1 (0.3 A g1, 6 M KOH), in a twoelectrode system and also a good cycling stability with nearly no capacitance decrease after 2000 cycles. The mixed activators KOH and K2CO3 can provide the alkaline environment and bring original micropores by the process of corrosion on carbon surface by KOH. On the other hand, the decomposition of K2CO3 enlarges the pore size by releasing CO2 at a certain temperature. Hence, the ACs with perfect hierarchical pore structure improve the electrochemical properties. 2. Experimental 2.1. Sample preparation The fallen leaves mainly from fraxinus chinensis used in this experiment were collected from Hebei university of technology in autumn. The fallen leaves were thoroughly washed with deionized water in order to remove the impurity and dried at 70  C for 12 h. The cleaned leaves were cut to debris. The as-prepared leaves were

carbonized in a tube resistance furnace at 500  C for 2 h with a heating rate of 10  C min1 and a nitrogen flow of 40 ml min1 to obtain fallen leaves cokes (FLC). FLC (1 g) were stirred in the solution of KOH and/or K2CO3 at room temperature for 10 h, and dried at 100  C in oven. Then the mixture were heated in a tube resistance furnace at 700  C for 1 h with a heating rate of 10  C min1 in a nitrogen flow of 40 ml min1. After that, the obtained product were neutralized with 1 mol L1 HCl solution, washed with deionized water and dried at 100  C in oven. The ACs were named as OAC-y (x ¼ 1, 2, 3, 4, the mass ratio of KOH/FLC was x), CAC-y(y ¼ 1, 2, 3, 4, the mass ratio of K2CO3/FLC was y), and MAC-z (z ¼ 1, 2, 3, where the z stands for the gram of K2CO3 and KOH is 1 g), respectively. The impurities of the materials have been tested on a Energy Dispersive Spectrometer (EDS) (Fig.S1). The trace elements, including O, Ca, Mg, Al, Si, S, and so on, are contained in the prepared ACs. 2.2. Characterization methods Powder X-ray diffraction (XRD) analyses were performed on a Bruker D8 Advance diffractometer with Cu Ka radiation (l z 0.154 nm) between 3 and 80 under a scan rate of 0.02 min1. Nitrogen adsorptionedesorption isotherm was obtained on a Quantachrome Autosorb-1MP sorption Analyzer at liquid-N2 temperature (196  C). All samples were outgassed at 200  C for 12 h before adsorption experiments. The SSA of samples were calculated via BrunauereEmmetteTeller (BET) method. The total pore volume distribution was calculated at the relative pressure of 0.995. T-plot method was utilized to calculate micropore surface area and volume from Nitrogen adsorption isotherms. The PSD of samples was determined by Density Function Theory (DFT) method. Morphologies of as-obtained products were observed on the scanning electron microscopy (SEM, JSM-6490LV) and the transmission electron microscope (TEM, JEM 1010). Raman characterization was carried out using a Renishaw InVia confocal Raman spectrometer with a 514 nm wavelength laser. 2.3. Preparation of electrodes and their electrochemical characterization To evaluate the electrochemical properties of the prepared ACs, the working electrodes were prepared as follow. ACs were mixed with acetylene black and polytetrafluoroethylene (PTFE), and their mass ratio was 75:20:5. The above slurry was made using ethanol as a solvent and coated onto nickel foam. After dried at 70  C for 12 h, the coated nickel foam was pressed to adhere better to the electrode material under the pressure of 20 MPa. The cyclic voltammetry (CV), galvanostatic chargeedischarge (GCD) and electrochemical impedance spectroscopy (EIS) of ESs were examined by using an IM6 & ZENNIUM electrochemical workstation in a three-electrode system. The working electrode (1 cm  1 cm) was characterized in 6 M KOH solution using a Ag/ AgCl as reference electrode and a platinum electrode as counter electrode. The voltage window of all the test was from 1.0 V to 0 V. EIS measurements were conducted in a frequency ranging from 100 kHz to 1 mHz with alternating current oscillation of 5 mV. The specific capacitance from GCD was determined from the following equation:

C¼

IDt mDv

(1)

where m is the mass of active carbon, Dv is potential difference, I is the electric current density, Dt is the time of discharge. The GCD was measured further using a coin-type (CR2025) two-

Y.-T. Li et al. / Journal of Power Sources 299 (2015) 519e528

electrode system on the Neware 10V3A. The different current density were used with the voltage ranging from 0.0 to 1.0 V. The specific capacitance was obtained from the equation:

C¼

2IDt mDv

(2)

where m is the mass of single electrode. The energy density (E, Wh kg1) of two-electrode system was calculated following equation:

E¼

1 CDV 2 2

(3)

where C is the total cell specific capacitance (F g1). The power density (P, kW kg1) was determined according to the equation:

P¼

E IDV ¼ Dt m

(4)

3. Results and discussion 3.1. Physicochemical properties of the active carbon Fig. 1 shows the XRD patterns of the three species of ACs activted by different activators. All samples possess two broad diffraction peaks indicating a turbostratic structure, which implies that the building blocks of the ACs are composed of graphitic-like microcrystallites that are randomly oriented and distributed throughout the samples [29,30]. The two broad peaks at 21e27 and 41e43 range correspond to the (002) and (100) crystal plane, which is due to the graphene layers spaced in a distance of d(002) ¼ 0.34 nm in plane (002) and d(100) ¼ 0.21 nm in plane (100). This material does not contain fragments of oriented structures and displays the significant content of amorphous phase [31]. In the low-angle scatter all the carbon materials have a considerable intensity, which suggests the presence of much pores [32]. N2 adsorption/desorption isotherms and the corresponding pore size distribution of the samples are illustrated in Fig. 2 and Table 1. According to the IUPAC classification, the isotherms of all the samples are ascribed to type I and type IV, which is due to the coexistence of micropores and mesopores [33e35]. The steep uptake volume below the relative pressure of 0.01 suggests the main

Fig. 1. XRD patterns of samples.

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contribution to surface area from the micropores [33]. After the relative pressure of 0.01, the uptake of the curve indicates the existence of mesopores. The unconspicuous hysteresis loop (H4) occurring at relative pressures in the range of 0.5e0.9, represents the slit pore type [34,35]. In other word, the sample particles exhibit irregular shape with internal voids and broad size distribution [36]. The SSA of OAC-4, CAC-2 and MAC-2 is 2869, 1003 and 1078 m2 g1, respectively. Fig. 2b shows the curves of PSD calculated by DFT model. It is obvious that OAC-4 possesses a peak centering at ca. 0.9 nm which is defined as ultrafine micropores (<1 nm), and several lower peaks centering at ca.1.2 nm, 2.9 nm and 3.8 nm. However, the pore size of MACs concentrates on 1.2e4.3 nm, and CAC-2 displays the pore size under 5 nm. The SSA and pore-structure parameters of prepared ACs are illustrated in Table 1. The SSA of OAC-4 is as high as 2869 m2 g1, but the ratio of micropore area to mesopore area is 2.26, indicating the presence of large number of micropores. As a contrast, CAC-2 and MACs possess low surface area and smaller area ratio, indicating the quantity of mesopores increases obviously. To prove the existence of mesopore and macropore, SEM and TEM images of carbons activated by different activator are shown in Fig. 3. FLC is the random huge agglomerate which possesses less visible pores (Fig. 3a). After activated by KOH, the big particles with irregular shape are maintained as observed in Fig. 3b(inset). But magnifying the particles, the obvious rough surface is from the packing of aggregated nanoparticles in 10 nm scale (Fig. 3b). With activation of K2CO3, the sample possesses a little of shallow pits, which exists and cannot sculpture all the surface (Fig. 3c). The particle shape and size are almost the same as the sample activated by KOH (Fig. 3c(inset)). However, the sample treated with the mixture of KOH and K2CO3 represents the fluffy fragment in different size (Fig. 3d(inset)). The thickness of the samples is clearly decreased. The observable macropores bigger than 50 nm scale can be observed (Fig. 3d). TEM image of OAC-4 clearly shows the unconspicuous pores and some visible pores appear at the edge of the sample (Fig. 3e). The pore structures of MAC-2 (Fig. 3f) display with the varying sizes including amount of macropores and evident mesopores at the relative thin part. The abundant pores are interconnected and enter the core of the carbon, providing a pathway to the inner cores which is broader than that of OAC-4. Combining the results of Nitrogen adsorptionedesorption isotherm, it is clear that MACs possess the prominent hierarchical pore structure with abundant micropores, mesopores and macropores. The general mechanism of chemical activation of KOH has been reported in literature underling the process complexity with the following reactions [37e41]. 6KOH þ C 4 2K þ 3H2 þ 2K2CO3

(5)

K2CO3 þ C 4 K2O þ 2CO

(6)

K2CO3 4 K2O þ CO2

(7)

2K þ CO2 4 K2O þ CO

(8)

The KOH turn into molten liquid above 360  C, which could reform the solideliquid environment around the carbon species to promote the reaction according to Eq. (5). It has been confirmed that carbon oxidation starts at around 400  C with the formation of K2CO3 and the reaction can be proceeded till 600  C. When the reaction temperature achieves to 700  C, the K2CO3 reacts further as the Eqs. (6)e(8) with the release of CO and CO2 gas to create the pores [42]. Though the reaction between K2CO3 and carbon are generally above 1000  C, the presence of graphite facilitates the reaction starting at about 627  C [18]. K2CO3 produced by the

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Fig. 2. N2 adsorption/desorption isotherms (a) and the corresponding DFT pore size distribution curves (b) of prepared ACs.

Table 1 The specific surface area and pore-structure characteristic of the samples. Samples

SBET (m2/g)

Smicro (m2/g)

Smeso (m2/g)

Vtotal (cc/g)

Vmicro (cc/g)

Vmeso (cc/g)

Smicro/Smeso

Vmicro/Vmeso

OAC-4 CAC-2 MAC-1 MAC-2 MAC-3

2869 1003 1058 1078 1409

1990 713 646 636 791

879 290 412 442 618

1.598 0.599 0.716 0.939 0.675

0.832 0.297 0.270 0.274 0.342

0.766 0.262 0.442 0.597 0.405

2.26 2.45 1.56 1.43 1.28

1.09 1.13 0.62 0.57 0.67

reaction of carbon and KOH is so little that cannot release enough gas to enlarge the pores, leading to the main existence of micropores. As for the activation by single K2CO3, a volatile matter is on a removal below the temperature range of 527  C and a little pores are formed in the ACs. When the temperature range above 627  C, K2CO3 reacts and the carbon is removed as CO to increase the SSA

and the pore volumes [18]. However, the melting point of K2CO3 is above 891  C, the reaction mainly occurs on the surface of carbon before the fully solideliquid environment is formed. When the activation process is performed with mixed KOH and K2CO3, the generative process of MAC is deduced as following schematic diagram (Scheme 1). KOH could induce the formation of micropores in

Scheme 1. The activation process on ACs from fallen leaves by KOH and/or K2CO3.

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Fig. 3. SEM images of the active carbon (a) FLC, (b) CAC-2, (c) OAC-4 and (d) MAC-2; the TEM images of the active carbon (e) OAC-4 and (f) MAC-2. The inset in b,c and d are the corresponding low magnification images.

the FLC framework via eqn (5) in the temperature range of 400e600  C [43]. After 600  C, the K2CO3 starts to decompose and enlarge the pore to form the larger pores. 3.2. Electrochemical tests for carbon electrodes The influences of mass ratio for each reagent are revealed in the CV curves at a scan rate of 5 mV s1 (Fig.S2). All the CV curves exhibit relatively rectangular shape and show near mirror-image current response on voltage reversal, indicating that most of the ion charges are stored by the electrode and bring about a reversible reaction and good capacitive behavior at a low scan rate. It is obvious that all the CACs electrodes possess similar rectangular shape, indicating that the addition quantity of K2CO3 has less impacts on the electrochemical of the materials (Fig. S2a). However, the series of OACs electrodes show the big difference as represented in Fig. S2b. The CV window of OAC-1 is the smallest among the OACs electrodes, which is even smaller than those of CACs. OAC-4 shows excellent performance, indicating the activation by KOH must be accumulated to a relatively large quantity, but the corrosivity of KOH is non-ignorable. The electrodes of MACs display the broad CV window and MAC-2 shows the highest current response (Fig. S2c). To verify the results in CV curves, GCD curves of MACs, OACs and CACs are represented in Fig. S3. It is clear that CACs

possess the similar discharge time, OACs show the excellent electrochemistry performance only when KOH reaches to high quantity. The influence of K2CO3 for MACs is conspicuous, maybe due to the slit pores have been generated by the etching of KOH, the quantities of K2CO3 are significant for enlarging the pore size. We further compare the OAC-4, CAC-2 and MAC-2 with the carbon before the activation. Fig. 4a shows that the CV curve of FLC electrode is triangle shape, displaying the poor storage capability of the ion charges. What's more, the area of CV curve from the MAC-2 electrode is the biggest corresponding to the highest specific capacitance. Fig. 4b displays the steady-state GCD curves for all the samples at a constant current density of 1 A g1. The chargeedischarge times of the MAC-2 electrode is longer than others, suggesting that the electrochemical capacitance of the MAC-2 is obviously the highest. Although the OAC-4 possesses the higher surface area, the abundant micropores cannot provide a thoroughfare for the transportation of ions. While MAC-2 possesses the hierarchical porous structure including the domination of the micropores, mesopores and a part of macropores, which can prompt the ions transportation. The CV curves (Fig. 4c) of MAC-2 electrode at different voltage sweep rates remain similar rectangular shape and go through small changes, indicating a quick charge propagation capability and facile ion transport within its pore channels [22]. But they become more and more deformed with the

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Fig. 4. (a) CV curves of different samples at the scan rate of 5 mV/s, (b) GCD curves of different samples at the current density of 1 A/g, (c) CV curves of MAC-2 at different scan rate, (d) GCD curves of MAC-2 at different current density by a three-electrode cell in 6 M KOH.

increasing of scan rate, in which the storage charge has been recognized to be distributed for the double layer formation mechanism [44e46]. Fig. 4d shows the GCD plots of MAC-2 electrode at constant current density range from 0.5 to 5 A g1. All of the GCD curves at the different current density exhibit good triangle-shapes and excellent coulomb efficient, and the specific capacitance of 310 and 240 F g1 at current density of 0.5 and 5 A g1, respectively. The capacitance retention is as high as 80%, indicating the rapid charge and discharge capability for the symmetric EDLCs with MAC-2 electrodes. In addition, the pair of weak redox peaks around 0.3 to 0.5 in the CV curves and a little deviation from the GCD curve at lower potential can be attributed to a certain number of oxygen-containing functional groups on the surface, which results the pseudocapacitance and improve the electrochemical properties [47]. The specific capacitance of all the electrodes at different current density are calculated by Eq. (1), and listed in Table 2. It is obvious that MAC-2 takes the advantages for application and the specific capacitance is higher even the SSA is lower than others. The ACs activated by KOH and using the precursor of human hair and waste tea-leaves obtain the high SSA of 1306 m2 g1 and 2841 m2 g1 with the specific capacitance of 240 and 255 F g1 at the current density of 5 A g1 [3,44]. However,

those raw materials are relatively hard to get and limit the further application in large scale. EIS is an important analytical technique used to get information about the electrochemical frequency behavior and equivalent series resistance of the capacitor system. Fig. 5a displays the EIS data

Table 2 Specific capacitance of the samples at different current density. Samples

Capacitances at different current density（F/g） 0.5 A/g

1 A/g

2 A/g

3 A/g

5 A/g

OAC-1 OAC-2 OAC-3 OAC-4 CAC-1 CAC-2 CAC-3 CAC-4 MAC-1 MAC-2 MAC-3 FLC

134 219 223 287 226 222 209 216 233 310 224 99

124 203 196 252 197 212 198 206 196 268 211 80

118 186 184 216 184 198 188 198 176 252 184 58

111 180 180 216 174 183 183 192 168 240 177 51

105 180 175 215 170 175 175 185 165 240 170 30

Y.-T. Li et al. / Journal of Power Sources 299 (2015) 519e528

measured in three electrode system at the frequency ranges of 100 kHz to 1 mHz with the amplitude of 5 mV, expressed as Nyquist plot for the carbon electrode. All the Nyquist plot which have a similar shape, can be separated into a semicircle at high frequency and a nearly 45 diagonal line at low frequency. The intercept at real part in the high frequency region is the equivalent series resistance (ESR) resulted from the total resistances of the electrode (Rs), the electrolyte resistance (Relectrode) and the resistance at electrolyte/ electrode interface (Rinterface). The width of the semicircle impedance loop represents charge-transfer resistance in the electrode materials. It is found that all the samples show comparable ESR of 0.18e0.25 U. The electrodes of MAC-2 and MAC-3 have lower ESR than the figure of OAC-4, indicating they possess the higher electrical conductivity. In addition, it is obvious that MAC-2 possesses smaller width of the semicircle impedance loop than others, suggesting the electrode and electrolyte have a good contact. As a results, the internal resistance and the charge transfer resistance increase in the sequence of MAC-2 > OAC-4 > MAC-3 > MAC1 > CAC-2, which can be ascribed to the decreased conductivity

525

because of a decreased graphitization degree [48,49]. A straight line near 45 in the medium-frequency region reflects the ion diffusion process from the electrolyte into electrode materials. All the prepared ACs appear to have almost equal length of Warburg line, except OAC-4 and CAC-2, which have too much micropores to impede the ion diffusion from the electrolyte into electrode materials [50]. The vertical lines closed to 90 at lower frequency suggest the pure capacitive behavior and fast transfer character of electrolyte ions in the structure of the carbon electrode [51]. It is shown that MAC-2 and MAC-3 are more close to 90 , indicating more ideal capacitive behavior. This ideal capacitive behavior should also be ascribed to the hierarchical pore texture of carbons that favors fast ionic diffusion and even ultrafine micropore is difficult to lead the ion transport into the inner pore. In order to further investigate the electrochemical capacitive practical performances for the activated carbon electrodes, the symmetric EDLCs was fabricated with those of carbon electrodes and KOH aqueous solution as electrolyte. Fig. 5b shows the GCD curves of MAC-2 at the current density range from 0.3 to 5 A g1

Fig. 5. (a) Nyquist plot of the active carbon for a three electrode system and (inset) the enlarged part in high frequency; (b) GCD curves of MAC-2 electrode at different current density with a two-electrode cell in 6 M KOH; (c) the specific capacitance of all the samples at different current density, and (d) Raman spectra of CAC-2, OAC-4 and MAC-2.

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than three electrode system. In our current results, the capacitance in two-electrode system is better than those reported data [21,54e57] and the mixed activators can efficiently decrease the corrosion to the equipment. Hierarchical mesoporous carbon materials with large microporosity have been prepared by direct triconstituent co-assembly with the use of resols as the carbon precursor, tetraethyl orthosilicate as the inorganic precursor, and triblock copolymer F127 as the soft template, and the specific capacitance is 126 F g1 at the current density of 10 mA g1 [59]. Otherwise, the ordered mesoporous carbons activated by nitric acid oxidation display the improved specific capacitance of 235 F g1 at the current density of 0.1 A g1 owning to the enriched oxygen functional group [60]. In Raman spectroscopy (Fig. 5d), the microstructure of carbon materials and state of sp2 bonded carbon can be clarified with D-band (~1330) and G-band (~1590 cm1) [61,62]. When the G-band varies strongly, suggesting that the carbon is graphitized or oxidized. However, the XRD patterns show broad diffraction peak and there is not obvious graphite peak indicating it is amorphous carbon. Thus we believe that the enhanced G-band is caused by the oxidized carbon. The relative intensity ratios between the D band and G band (ID/IG) is used to measure the disorder degree and the graphitization of the carbon structure. The value of ID/IG for OAC-4, CAC-2 and MAC-2 are 0.98, 0.93 and 0.85, respectively. The decrease of the value can be ascribed to the conversion of disordered to ordered sp2 states, which maybe due to the degree of etching by the mixture activators [22]. The high level of sp2 type carbon predominantly converts the higher electrical conductivity for amorphous active carbon materials [63,64]. MAC-2 with enhanced G-band displays the biggest capacitance. In contrast, most of hierarchical carbons are involved using the hard template and the process of preparation are tedious. The activation by synergy of K2CO3 is a new breakthrough to prepare hierarchical active carbons. The cycle lifetime for the electrode of material is an important indicator of the electrochemical test. The galvanostatic charge/ discharge cycling was monitored at a current density of 1 A g1 in the 6 M KOH aqueous electrolyte within a potential window of 0.0e1.0 V. As shown in Fig. 6a, all the electrodes of materials almost no decay up to 2000 cycles, indicating the excellent cycling stability, which is significant for the practical application [3]. Fig. 6 ashows the Coulomb efficiency as a function of cycle number, which is calculated using the equation of h (%) ¼ (Td/Tc)  100%, where Td and Tc are discharge time and charge time, respectively

and operating voltage range of 0.0e1.0 V in the 6 M KOH aqueous electrolyte. It is clear that the time of charge/discharge decrease with the current density increase. The charge/discharge profiles of carbon electrodes exhibit almost the isosceles triangle curves, indicating excellent coulomb efficiency, and the specific capacitance is obtained via Eq. (2). The specific capacitance of MAC-2 is 241 F g1, and 190 F g1 at the current density of 0.3 A g1, and 5 A g1. The capacitance retention is as high as 79% of the three electrode system, indicating the rapid charge and discharge capability for the symmetric EDLCs with MAC-2 electrodes. Furthermore, the obvious IR drop is not observed suggesting that these materials have a small ESR and can be used as favorable chemical electrodes. Fig. 5c shows the influence of current density on the specific capacitance of different active carbons. The specific capacitance of all carbons decreases gradually with the increase of current densities from 0.3 A g1 to 5 A g1, which is concerned with the limited transportation of the electrolyte ions on the electrode surface during fast charging. Electrolyte ions do not have enough time to reach the surface of micropores at high current density, therefore the specific capacitance at high current density is lower than that at low current density [22]. However, the capacitances of MAC-2 are still the highest. As a contrast, the capacitances of OAC-4 are the lowest, even lower than CAC-2, which could be due to the increase of internal resistance. At the current density of 5 A g1, the capacitance retention ratios are 61%, 68% and 65% for OAC-4, CAC-2, MAC-2, respectively. As a comparison, the specific capacitance of commercial AC is 196 F g1 at the current density of 0.5 A g1 which is obvious lower than MAC-2, and similar to CAC-2. The reported capacitance of ACs displays a big variation. There are many factors could impact the capacitance performance such as the carbon source, active method and the type of electrolyte [21,22,52e58]. The ACs activated by KOH and using the precursor of human hair and waste tea-leaves obtain the high SSA of 1306 m2 g1 and 2841 m2 g1 with the specific capacitance of 240 and 255 F g1 at the current density of 5 A g1 [3,44]. However, those raw materials are relatively hard to get and limit the further application in large scale. As listed in Table 3, ACs could achieve relative higher capacitance with KOH as activator, and the SSA is larger than others. While the surface area and specific capacitance have no inevitable relationship. The surface area of AC prepared in our works is relatively lower than others, but the specific capacitance is higher than many ACs in the literature [22,53]. Furthermore, identical material in two electrode system always is lower

Table 3 AC materials for EDLC electrodes reported recently in the literature. Carbon source

activators

SBET (g cm1)

C (F g1)

Coconut shell

ZnCl2 steam KOH ZnCl2 ZnCl2/CO2 KOH KOH NaOH KOH KOH H3PO4 NaOH KOH KOH&K2CO3

2007 1532 3332 1674 1939 3557 1003 1778 2283 3302 1498 2455 3322 1078

210 193 238 136 139 188 224 235 296 235 119 131 179 310 223

Cationic starch

Beer lees Coal tar pitch Deoiled asphalt Enteromorpha prolifera Rice husk

Fallen leaves a b c d

The The The The

three electrode system. two electrode system. mass concentration of the electrolyte. unit is mA cm2.

(3)a (3) (2)b (2) (2) (2) (2) (2) (3) (2) (3) (3) (3) (3) (2)

E (V)

I A g1

Electrolyte mol L1

Ref

1 0.8 1.15 1.15 1.15 0.9 1 1 1 1 1 1 1 1 1

1 1 0.37 0.37 0.37 1d 0.1 0.05 0.5 0.5 6.25 6.25 6.25 0.5 0.5

KOH(6) KOH(6) KOH(30%)c KOH(30%) KOH(30%) H2SO4(0.1) KOH(6) KOH(7) KOH(30%) KOH(6) KOH(6) KOH(6) KOH(6) KOH(6) KOH(6)

[53] [52] [54]

[21] [55] [56] [22] [57] [58]

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[44]. The value of Coulomb efficiency is basically kept at 1.0 even after 2000 cyclic charge/discharge process, further indicating MAC2 possess great electrochemical stability and coulomb efficiency. EIS measured for two electrode system is shown in Fig. 6b with obvious differences from the three electrode system at high frequency. The width of semicircle for OAC-4 is markedly increased, but it is decreased for CAC-2 and MAC-2. So the internal resistance and the charge transfer resistance increase in the sequence of OAC4(~0.3U) > CAC-2(~0.1U) > MAC-2(~0.05U). it is assumed that the two electrode system has a higher requirement for the pore structure. MACs possess the mesopore and macropore, so the electrolyte could infiltrate electrode completely. CAC-2 possesses the pitlike pore structures and it has no many micropore and curved channel so that the electrolyte would contact the surface of electrode directly. However, OAC samples possess too much micropores and narrow curved channel, it is more difficult for ion diffusion between electrode and electrolyte, and the resistance would increase distinctly. The electrochemical data of three type of electrode certifies that the different pore structures influence the electrochemical character and the hierarchical pore structure can improve the performance efficiently. Through the extra injection of K2CO3, the pore evolves to hierarchical pore and penetrates into the inner-core which would form the pathway for ions to get through easily. Otherwise, it still exists some micropores on the pore walls, which can increase the SSA and provide a short ion-transport channel through the walls. So the ACs actived by the mixture of KOH and K2CO3 possess better electrochemical properties. It is crucial that less KOH alleviates the corrosion for equipment. In another word, the synergy of K2CO3 is of great importance in the process of activation of ACs, by which the pore channel is enlarged [25]. The energy density and power density are calculated according to Eqs. (3) and (4), respectively. Fig. 7 shows a Ragone plot calculated from the GCD curves. We can observe a decreasing trend in power density with the increase of energy density for all cells. MAC-2 possesss very promising energyepower combinations: 33.9 Wh kg1 at the power density of 1.08 kW kg1, and maintains at 26.4 Wh kg1 even the power density reached 18 kW kg1. CAC-2 and OAC-4 display the poor energy performance for a packaged device, which can be ascribed to the higher ion diffusion resistance. The energy density decreases with the increase of power density, which is attributed to the complex resistance and tortuous

527

Fig. 7. Ragone plots referring to energy and power density of the prepared carbons and commercial sample.

diffusion pathway within the porous textures [65]. Normally, the precursor of ACs would impact the energy density, such as Coffee grounds based AC (7.84 Wh kg1 at the power density of 6 kW kg1), petroleum coke based AC (21.2 Wh kg1 at 0.02 kW kg1), and Sugar cane bagasse based AC (5.9 Wh kg1 at 10 kW kg1) and other materials [52,55,66e68]. As a comparison, the commercial ACs show the comparable energy density value at low current density 0.3 A g1 (29.6 Wh kg1 at 1.08 kW kg1), that is higher than that of OAC-2 and CAC-4. But the drop-off rate with the increase of discharge current density is almost the same as the OAC-2 with the rest energy density of 15.3 Wh kg1 at 18 kW kg1, suggesting the similar pore structures exist for both. It is confirmed that MAC-2 possesses better energy density which is mainly due to the perfect hierarchical pores which decrease the charge transfer resistance and improve the kinetics significantly.

4. Conclusion ACs materials derived by fallen leaves have been successfully

Fig. 6. (a) cycle life of the prepared carbon materials at current density of 1 A/g and variation of the specific capacitance and the Coulomb efficiency over the 2000 cycle times and (b) Nyquist plot of the active carbon for a coin-type device in a two-electrode system.

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activated via the mixture of KOH and K2CO3 through hightemperature carbonization and activation. Structural characterizations show the hierarchical pore structures with relative high surface area. The use of K2CO3 can prompt the formation of macropores during the activation procedure. Electrochemical measurements demonstrate the excellent capacitive performance owning to the existence of hierarchical pore channel. Compared with OAC-4 and CAC-2, MAC-2 has the highest specific capacitance of 310 F g1 and a good cycling stability with nearly no capacitance decrease is obtained after 2000 cycles. Acknowledgments This work was supported by the National Natural Science Foundation of China (21076056), Key Project of Chinese Ministry of Education (210010), Ph.D. Programs Foundation of Ministry of Education of China (20091317120005), Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, IRT1059) and Hebei Provincial Key Lab of Green Chemical Technology & High Efficient Energy Saving, School of Chemical Engineering & Technology, Hebei University of Technology. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.09.039. References [1] W. Huang, H. Zhang, Y. Huang, W. Wang, S. Wei, Carbon 49 (2011) 838e843. [2] Y. Lv, L. Gan, M. Liu, W. Xiong, Z. Xu, D. Zhu, D.S. Wright, J. Power Sources 209 (2012) 152e157. [3] W. Qian, F. Sun, Y. Xu, L. Qiu, C. Liu, S. Wang, F. Yan, Energy & Environ. Sci. 7 (2014) 379e386. [4] S. He, L. Chen, C. Xie, H. Hu, S. Chen, M. Hanif, H. Hou, J. Power Sources 243 (2013) 880e886. [5] J. Wang, M. Chen, C. Wang, J. Wang, J. Zheng, J. Power Sources 196 (2011) 550e558. [6] D. Qu, H. Shi, J. Power Sources 74 (1998) 99e107. [7] D.W. Wang, F. Li, H.T. Fang, M. Liu, G.Q. Lu, H.M. Cheng, J. Phys. Chem. B 110 (2006) 8570e8575. [8] H.-K. Song, Y.-H. Jung, K.-H. Lee, L.H. Dao, Electrochim. Acta 44 (1999) 3513e3519. [9] D.R. Rolison, Sci. (New York, N.Y.) 299 (2003) 1698e1701. [10] D.-W. Wang, F. Li, M. Liu, G.Q. Lu, H.-M. Cheng, Angew. Chem. Int. Ed. 47 (2008) 373e376. [11] J.W. Long, B. Dunn, D.R. Rolison, H.S. White, Chem. Rev. 104 (2004) 4463e4492. [12] J. Chmiola, G. Yushin, Y. Gogotsi, C. Portet, P. Simon, P.-L. Taberna, Science 313 (2006) 1760e1763. [13] T. Morishita, Y. Soneda, T. Tsumura, M. Inagaki, Carbon 44 (2006) 2360e2367. [14] M. Olivares-Marin, C. Fernandez-Gonzalez, A. Macias-Garcia, V. GomezSerrano, Appl. Surf. Sci. 252 (2006) 5967e5971. [15] S. Yorgun, N. Vural, H. Demiral, Microporous Mesoporous Mater. 122 (2009) 189e194. [16] Y. Nakagawa, M. Molina-Sabio, F. Rodriguez-Reinoso, Microporous Mesoporous Mater. 103 (2007) 29e34. [17] S.H. Yoon, S. Lim, Y. Song, Y. Ota, W.M. Qiao, A. Tanaka, I. Mochida, Carbon 42 (2004) 1723e1729. [18] J.I. Hayashi, T. Horikawa, I. Takeda, K. Muroyama, F.N. Ani, Carbon 40 (2002) 2381e2386. [19] S. Roldan, I. Villar, V. Ruiz, C. Blanco, M. Granda, R. Menendez, R. Santamaria, Energ Fuel 24 (2010) 3422e3428. [20] B. Xing, C. Zhang, L. Chen, G. Huang, Preparation of activated carbons from lignite for electrochemical capacitors by microwave and electrical furnace heating, in: J.M. Zeng, T.S. Li, S.J. Ma, Z.Y. Jiang, D.G. Yang (Eds.), Advanced Engineering Materials, Pts 1e3, 2011, pp. 2472e2479. [21] S.G. Lee, K.H. Park, W.G. Shim, M.S. Balathanigaimani, H. Moon, J. Ind. Eng. Chem. 17 (2011) 450e454.

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