The production of hydrochar-based hierarchical porous carbons for use as electrochemical supercapacitor electrode materials

Colloids and Surfaces A: Physicochem. Eng. Aspects 423 (2013) 104–111

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

The production of hydrochar-based hierarchical porous carbons for use as electrochemical supercapacitor electrode materials Lili Ding a , Bo Zou a , Yannan Li a , Hequn Liu a , Zichen Wang a , Chun Zhao b , Ying Su c , Yupeng Guo a,∗ a

College of Chemistry, Jilin University, Qianjin Street 2699, Changchun 130012, China Joint State Key Laboratory on Integrated Optoelectronics, Jilin University, Qianjin Street 2699, Changchun 130012, China c The Department of Electronic Science, Changchun Institute of Engineering Technology, Huayuan Road 1, Changchun 130017, China b

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

 Porous carbons were produced from biomass-based hydrochar by three activators activation.  A comparative study of porous carbons was carried out.  Porous carbons were examined for use in supercapacitors.  The preparation method can be extended to other biomass-based hydrochar.

a r t i c l e

i n f o

Article history: Received 5 November 2012 Received in revised form 24 January 2013 Accepted 2 February 2013 Available online 11 February 2013 Keywords: Porous carbon Supercapacitor Rice husk Hydrochar

a b s t r a c t A comparative study of various hydrochar-based porous carbons was carried out. These porous carbons, synthesized from biomass by H3 PO4 , NaOH and KOH activation, were examined for use in supercapacitors. The micropores and mesopores had been introduced into the hydrochar simultaneously. Porous carbons exhibited different electrochemical performance due to the difference of their porosity. The prepared carbons showed a hierarchical porous structure, the corresponding BET surface areas and pore volumes were 1355–3322 m2 /g and 1.45–2.53 cm3 /g. The highest specific capacitance was 179.4 F/g at a current density of 6.25 A/g, and appeared to be a promising electrode material for supercapacitors. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Supercapacitor is one of the important energy storage devices. It excels at providing back-up power supplies to protect against power sags and disruptions. Recently, interests in the development of porous carbons for supercapacitors have been intensified [1–6]. The energy storage mechanism for the porous carbons can be interpreted as a double layer model, which is an reversible ion adsorption onto the carbon surface, the energy is stored in the

∗ Corresponding author. Tel.: +86 431 8515 5358; fax: +86 431 8515 5358. E-mail address: [email protected] (Y. Guo). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.02.003

interface between an electrolyte and a carbon electrode, therefore resulting in the so-called ‘double layer capacitance’. The surface area, pore size distribution (PSD), resistance and surface functional groups of the carbon materials, the size and desolvation of the electrolyte ions and so on, which all affect the capacitance properties of electrode materials [7–12]. At present, for which kind of porous carbon materials are most suitable for use in electrode material has no uniform standard. According to the requirement of practical applications, the porous carbons should possess controllable pore architecture. Porous carbons could be prepared by hard and soft template methods. Lee and Park synthesized ordered porous carbons with the silica template [13]. Almasoudi and Mokaya used the commercially

L. Ding et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 423 (2013) 104–111

zeolitic imidazolate framework as template to nanocast highly microporous carbons (90–95% microporosity) [14]. Well defined macroporous polymers were prepared by the emulsion template [15]. However, complicated and costly preparation procedures of the template method limit its practical applications. In contrast, chemical activation and physical activation with simple and economically viable advantages have attracted great interest. Porous carbons with narrow PSD and high surface area could be prepared by chemical activation. Carbons produced by chemical activation have higher density and surface area, which almost double those by physical activation. KOH, NaOH, H3 PO4 , ZnCl2 are common chemical activators [16–20]. Porous carbons usually exhibit various physicochemical properties depending on the activation methods and the carbon precursors. The natural carbon-rich organic materials can be used as the carbon precursors, for example: cypress [21] coffee beans [22] pistachio shell [23] sunflower seed shell [24] coconut shells [25] and so on. Rice husk contains cellulose, hemicellulose and lignin, so it also can be used as carbonaceous material precursor. These renewable biomass derived materials have achieved great success in the preparation of porous carbons. But the porous carbons prepared from biomass usually contain a certain amount of impurity and high ash content owing to the complicated components of the precursors, and they could not meet the requirements of some high end materials. Hydrothermal carbonization biomass can solve this problem. Hydrothermal carbonization of biomass and carbohydrates is a cheap, easy and green technique. The corresponding products are pure and they have been applied in some advanced fields, such as electrode materials of supercapacitor and lithium ion battery [26]. Recently, Titirici and Antonietti [27], Hu et al. [28] and Hoekman et al. [29] have presented systematic overviews of functional carbonaceous materials prepared from biomass by the hydrothermal carbonaceous process [27–29]. The porous carbons with more stable performance can be obtained from the hydrothermal carbon spheres. For previous investigations, we have successfully prepared hydrochar by the method of acid hydrolysis of rice husk [30]. In this paper, various hierarchical porous carbons synthesized from rice husk-based hydrochar by KOH, NaOH and H3 PO4 activation

105

were examined for use in supercapacitors. Firstly, the silica and other impurities in the rice husk were separated out through the process of hydrolysis of rice husk, which ensured the purity of the precursor of porous carbons. Then the choices of different activators contributed to systematic studies of porous carbons. The main objective of this work is to compare the structure and capacitive properties of various porous carbons, and explore the relationship among the activators, pore structure and the capacitance. 2. Materials and methods 2.1. Materials Rice husk was obtained from a rice mill and washed with distilled water. Rice husk was dried at 105 ◦ C for 12 h. Sulfuric acid (98 wt.%), phosphoric acid (85 wt.%), sodium hydroxide and potassium hydroxide were analytical grade. 2.2. Methods 2.2.1. Preparation of hydrochar and porous carbons Fig. 1 shows the schematic diagram of the preparation processes of hydrochar and porous carbons. The preparation method of the hydrochar was described in our previous study [30], which was a hydrothermal reaction in low temperature. Firstly, the rice husk was hydrolyzed with 72 wt.% sulfuric acid. Then the acid concentration of the hydrolysis solution was diluted to 54 wt.%. The liquid product was separated from solid by filtration and heated at 95 ◦ C for 6 h. The hydrochar was obtained after filtration and repeated washing using distilled water, and the specific surface area was 257 m2 /g. In KOH activation, the mass ratios of KOH to hydrochar were from 3 to 6, their corresponding porous carbons were named K-3, K4, K-5 and K-6, respectively. The reactions were conducted at 400 ◦ C for 0.5 h, then 800 ◦ C for 1 h. The activation temperature and time of NaOH activation were same as KOH, the obtained porous carbon were labeled N-3, N-4, N-5, N-6 according to NaOH/hydrochar ratios of 3, 4, 5 and 6. In the case of H3 PO4 activation, firstly, the hydrochar was impregnated with H3 PO4 (85 wt.%), then carbonized

Fig. 1. Schematic illustration of the preparation processes of porous carbons.

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at 500 ◦ C for 1 h. The prepared porous carbons were named P-4 and P-6 in accordance with the H3 PO4 /hydrochar ratios of 4 and 6. The porous carbons obtained from different activators were washed with distilled water until neutral pH and dried at 110 ◦ C. Finally, the obtained carbons were heat-treated at 800 ◦ C for 1 h under nitrogen atmosphere. The porous carbons were denoted as K-PC, N-PC and P-PC depending on the activators of KOH, NaOH and H3 PO4 .

measurements were conducted on a CHI 6600 electrochemical workstation. The cyclic voltammetry (CV) measurement was carried out at the scan rate of 10 mV/s in a potential range of −1.1 to −0.1 V, the galvanostatic charge/discharge measurement was performed at the current density of 6.25 A/g (−1.1 to −0.1 V). The electrochemical impedance spectroscopy measurements were tested at the open circuit voltage of 5 mV amplitude in the 1–100 Hz frequency range.

2.2.2. Characterization The BET surface area and the pore structure were analyzed by N2 adsorption at a Micromeritics ASAP 2010 Surface Analyzer. The morphology of the porous carbons was observed using a JSM-6700F scanning electron microscope (SEM). The elemental analysis was carried out on PerkinElmer 2400 C, H, N, O, S analyzer. The electrochemical performance was carried out in the following three electrode configuration in 6 mol/L KOH aqueous solution. The working electrode was fabricated by mixing 80 wt.% porous carbon (8 mg), 10 wt.% acetylene black and 10 wt.% poly (tetrafluoroethylene). The mixture was spread onto a piece of foam nickel (area: 1 cm × 1 cm) and then pressed at 10 MPa, the thickness of obtained electrode was about 0.2 mm. A saturated calomel electrode (SCE) and a platinum foil were used as the reference electrode and the counter electrode, respectively. All the electrochemical

3. Results and discussion

d 800

3 -1

400 200

P-4 P-6 0.0

0.2 0.4 0.6 0.8 Relative pressure P/P 0

3 -1

800 600

200 0

0.0

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N-3 N-4 N-5 N-6 1.0

900 K-3 K-4 K-5 K-6

600 300 0.2

0.4

0.6

20 40 Pore size (Å)

60

N-3 N-4 N-5 N-6

0.06 0.04 0.02 0.00 0

10

20 30 40 50 Pore size (Å)

0.8

Relative pressure P/P0

1.0

60

70

K-3 K-4 K-5 K-6

0.10

1200

0.0

0.01

f

Adsorption isotherms Disorption isotherms

1500

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e

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0.03

0

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3

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b

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N2 adsorption/desorption isotherms and the PSD calculated from adsorption isotherms by the Density Functional Theory (DFT) method of the porous carbons are exhibited in Fig. 2. P-PC and K-PC presented the isotherms similar to types I and II isotherms according to the IUPAC classification. The isotherms of N-PC combined types I and IV. The obvious H1 hysteresis loops in N-PC, occurring at a middle pressure P/P0 = 0.4, suggested the presence of capillary condensation. The steep adsorption below relative pressure 0.1 P/P0 of all the samples, suggested large amounts of N2 adsorption, indicated the high microporosity of these materials. The adsorption isotherms rapidly increased in the region of middle and high relative pressure and no platform appeared in them, it reflected the

dV/dlogD (cm3g-1)

Adsorbed volume(cm 3 g-1 )

a

3.1. The pore-structure properties of porous carbons

0.08 0.06 0.04 0.02 0.00 0

10

20

30

40

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Pore size (Å)

Fig. 2. N2 adsorption and desorption isotherms (a, b, c) and the DFT pore size distribution curves (d, e, f) of the porous carbons.

L. Ding et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 423 (2013) 104–111 Table 1 Texture properties and the specific capacity of the porous carbons.

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Table 2 Elemental analysis of porous carbons (wt.%).

Sample

SBET a (m2 /g)

Vt b (cm3 /g)

Dave c (nm)

Vmic /Vt d (%)

Ce (F/g)

Sample

C

H

N

S

O

P-4 P-6 N-3 N-4 N-5 N-6 K-3 K-4 K-5 K-6

1355 1498 2455 2178 2096 1779 2071 2783 2961 3322

1.26 1.27 1.82 1.80 1.85 1.56 1.45 1.97 2.14 2.53

3.71 3.39 3.52 3.31 3.52 3.52 2.80 2.85 2.89 3.05

45.02 48.16 43.82 47.02 36.01 41.87 54.13 53.08 48.13 43.85

94.0 106.6 133.8 83.1 61.1 51.2 126.5 179.4 173.1 157.2

P-4 P-6 N-3 N-4 N-5 N-6 K-3 K-4 K-5 K-6

90.65 96.26 88.07 91.54 88.01 93.47 90.05 92.88 94.15 96.50

1.01 0.88 0.01 0.45 0.00 0.00 0.28 0.61 0.18 0.00

0.18 0.21 0.23 0.42 0.17 0.26 0.34 0.42 0.15 0.18

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

8.16 2.65 11.69 7.59 11.82 6.27 9.33 6.09 5.52 3.32

a

BET surface area. Total pore volume. c Average pore diameter. d Micropore volume/Total pore volume. e Specific capacity calculated by galvanostatic charge–discharge curves at 6.25 A/g. b

presence of mesopores in the obtained carbons. The porous carbons contained micropores and mesopores, which was consistent with the results of PSD (Fig. 2). All prepared materials presented similar hierarchical porous structure, which was caused by the same precursor. Although the PSD of the samples extended over the micropore and mesopore range (pores can be classified into three types according to the IUPAC classification: microporous < 2 nm, 2 nm < mesoporous < 50 nm, and macroporous > 50 nm), the corresponding PSD were narrow, which were from 1 to 4 nm. The porous texture characterizations of the porous carbons are lists in Table 1. The average pore diameter is from 2.8 to 3.7 nm. Along with the increase of mass ratio of activator/hydrochar, the BET surface areas and total pore volumes of P-PC and K-PC increase. It is noteworthy that the surface areas of N-PC decrease with the increase of the NaOH activation ratios. It because as the dosage of NaOH increase, the carbon layer in the pores continue to react with excessive NaOH, the narrow mircopores are widened and grow up to new mesopores. Previous report demonstrated that NaOH was harder to enter into interior of carbon than KOH [31]. Therefore, the primary etching reaction between NaOH and hydrochar occurred on the external surface of hydrochar in the high ratio of NaOH/hydrochar, which enlarged mircopores in the surface of NPC to mesopores. Consequently, the surface area of N-PC decreased. In the case of K-PC, with the dosage of KOH increased, more KOH entered into the interior of the hydrochar and more mircopores formed. Therefore, the BET surface area and the total pore vol˜ ume of the K-PC increased. The report of Raymundo-Pinero et al. described that the difference between KOH and NaOH activation were related with an additional intercalation step of metallic K or Na produced during the redox reactions, metallic K had the ability to be intercalated in all materials in contrast with Na, metallic Na only intercalated in the very disorganized materials [32]. Taking into account the impact of these factors, it is not surprising that the porous carbon obtained by the activation of two monovalent alkali activators show different trend in BET surface area. 3.2. Morphology of porous carbons The SEM figures (Fig. 3) describe the morphologies of the porous carbons. P-PC appeared rough surface, which differed from the N-PC and K-PC, due to the evaporation of H3 PO4 during carbonization and leaving the space. In the activation process, phosphoric acid interacted with hydrochar to form phosphate and polyphosphate bridges, the pore structures created in the dilation processes along with the insertion of phosphate groups [33]. We cannot see any obvious macropore on the samples. This phenomenon is

consisted with the results of the N2 adsorption/desorption measurements. The pore diameter is 1–4 nm. The porous carbons can maintain the general structures of the original hydrochar (Fig. 4) besides N-5, N-6. N-5 and N-6 appeared lamellar structure rather than spherical, which possibly due to the occurrence of fierce gasification of the small particles on the external surface of hydrochar during activation in a high NaOH/carbon ratio. But the K-PC did not exhibit lamellar structure even in a high KOH/carbon ratio. This can be explained by the lower melting point of the NaOH. At the high dosage of NaOH, the fierce etching reaction occurred on the outer surface, NaOH was hard to reach the interior of the hydrochar particles and react with them. Therefore, the Fig. 3e and f present special lamellar structure. 3.3. Elemental analysis Table 2 lists the results of the elemental analysis of the porous carbons. All the prepared porous carbons were rich in carbon and the carbon content were more than 88 wt.%. As the proportion of the activators increased, the carbon content displayed an increasing trend except N-5, respectively. In addition to carbon, the content of the rest elements in porous carbons were low. All the samples do not contain sulfur element. The percentage of nitrogen in all prepared sample were not more than 0.5%. The hydrogen content of the porous carbon was small and no hydrogen was detected in the N-5, N-6 and K-6. The oxygen content decreased as the mass ratio of activator/hydrochar increased except N-5. It suggested that the reactions between hydrochar and alkali might consume more oxygen in the high ratio of alkali. Moreover, the dehydration and decarboxylation reactions in the calcination process also consumed lots of hydrogen and oxygen. 3.4. Electrochemical performances of porous carbons The electrochemical properties of the porous carbons were investigated by CV, galvanostatic charge/discharge and impedance spectroscopy techniques. Fig. 5 shows the CV curves of the carbons at the scan rate of 10 mV/s. All the curves presented near rectangular shape, indicating capacitive behaviors of these materials. A little deviation from the ideal rectangular plots might be owing to the pseudofaradaic contribution of the N- and O- functionalities of the porous carbons. The CV curve enclosed area for P-6 was larger than P-4, indicating a higher capacitance from the P-6. Among N-PC, N-3 exhibited the highest current response and performed best capacitive behavior. The capacitances of N-PC decreased with activation ratio increased. This result was relative to the surface area of N-PC which followed the same trend with capacitance. Compared with N-PC, the capacitances of K-PC reached high value in the ratio of 4 and 5. Relatively large surface areas and pore volumes of K-4 and K5 contributed to the increase in capacitance. But the largest area of K-6 (Table 1) did not show the maximum capacitance. Reasonable

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Fig. 3. SEM micrograph of the porous carbons (a) P-4, (b) P-6, (c) N-3, (d) N-4, (e) N-5, (f) N-6, (g) K-3, (h) K-4, (i) K-5, (j) K-6.

explanation of this discrepancy laid in the different of pore structures. The hysteresis of K-6 indicated K-6 might exit some blocked pores (pores within the carbon only connected to the exterior by necks). These structures might lead the transport velocity of ions slow. In general, the high surface area could result in relative low conductivity [34] that was another explanation for the increasing

specific surface area and decreasing capacitance of K-5 and K-6. In addition, the pore shape, surface functionality and the size of electrolyte ion also influence the capacitance [35,36]. According to the forming mechanism of the electric double layer, the capacitance of porous carbon predominantly depends on the surface area accessible to the electrolyte ions [36]. But the impact of other factors

L. Ding et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 423 (2013) 104–111

including the above mentioned pore size distribution, conductivity, pore shape, surface functionality and the size of electrolyte ion [34–36] should not be ignored. Taking into account of these factors, it was not surprising that the capacitance of K-PC did not linearly increase with increasing specific surface area. The galvanostatic charge/discharge curves (Fig. 5) of the samples were obtained at a current density of 6.25 A/g. The changes in capacitance of the carbons exhibit the same trend as the results of CV curves. All the curves present similar symmetric triangular shape and a little deviation from ideal voltage–time curves. There was no obvious voltage drop at the current switches, suggesting the low resistance of the porous carbon electrode. The accurate specific capacities are listed in Table 1 and the maximum specific capacity of the K-4 is 179.4 F/g. Fig. 6 illustrates the Nyquist impedance plots tested at the open circuit voltage of 5 mV amplitude in the 1–100 Hz frequency range. All the samples exhibit straight line in the low frequency region. The intermediate frequency region of P-PC, N-PC and K-PC are the 45◦ line, which show the typical features of porous electrodes. There are not obvious semicircular regions of the K-PC and P-PC in the high frequency region. The N-PC and hydrochar curves show small semicircular region, but they are not well defined. The pure hydrochar presents larger solution resistance than the porous carbons, it because the porous structure is benefit to the transmission

of ion in solution. The plots indicated the system is kinetically easy and the mass transfer dominates the Nyquist plot. The chargetransfer resistance was inconsequentially small by comparison to the Warburg impedance and the ohmic resistance.

0.0

Potential(V)

Current(A)

0.00

P-4 P-6

-0.02 -1.2

Fig. 4. SEM micrograph of the hydrochar.

d

a 0.02

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Current(A)

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K-3 K-4 K-5 K-6

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-0.9 -0.6 -0.3 Potentive V vs SCE(V)

0.0

40

-0.8

-1.2

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20 40 Time(s)

60

Fig. 5. CV curves (a, b, c) at the scan rate of 10 mV/s and galvanostatic charge-discharge curves (d, e, f) at the current density of 6.25 A/g of the porous carbons.

L. Ding et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 423 (2013) 104–111

5

0.2 0.1 0.0 0.9

0 4

1.0 1.1 Z' (ohm)

8 Z' (ohm)

c

-Z'' (ohm)

-Z'' (ohm)

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0 2 3 Z' (ohm)

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12 N-3 N-4 N-5 N-6

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Hydrochar

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a 15

-Z'' (ohm)

110

1.0

0.2 0.0

0.6 0.8 Z' (ohm)

1.5 2.0 Z' (ohm)

2.5

Fig. 6. Nyquist impedance plots of (a) Hydrochar, (b) P-PC, (c) N-PC, (d) K-PC measured at frequencies of 1–100 Hz, the insets are magnified portion of the plots near the origin.

4. Conclusions The hierarchical porous carbons have been successfully produced from rice husk-based hydrochars by H3 PO4 , NaOH and KOH activation. The impact of activators on structure and electrochemical performance has been explored. Porous carbons presented similar PSD, the corresponding pore sizes and BET surface areas ranged from 1 to 4 nm and 1355–3322 m2 /g. In this paper, K-PC showed the best capacitance, and the maximum specific capacity could achieve 179.4 F/g. They appeared to be promising electrode materials for supercapacitor. The comparative study of porous carbons will contribute to the systemic investigation of the relationship between the pore structure and the capacitance of these materials. This work can be extended to other biomass-based carbon and lay the foundation for the further investigation and selection of porous carbons for supercapacitors. Acknowledgements This research work was supported by Key Project of the National Eleventh Five-Year Research Program of China (2008BAE66B00), Scientific and Technological Planning Project of Jilin Province (20100326 and 20120311) and Interdisciplinary Research Project of Jilin University (201003030). References [1] H.L. Guo, Q.M. Gao, Boron and nitrogen co-doped porous carbon and its enhanced properties as supercapacitor, J. Power Sources 186 (2009) 551–556. [2] K.S. Xia, Q.M. Gao, J. Jiang, J. Hu, Hierarchical porous carbons with controlled micropores and mesopores for supercapacitor electrode materials, Carbon 46 (2008) 1718–1726. [3] Y. Tian, Y. Song, Z.H. Tang, Q.G. Guo, L. Liu, Influence of high temperature treatment of porous carbon on the electrochemical performance in supercapacitor, J. Power Sources 184 (2008) 675–681. [4] W. Xing, C.C. Huang, S.P. Zhuo, X. Yuan, G.Q. Wang, D. Hulicova-Jurcakova, Z.F. Yan, Hierarchical porous carbons with high performance for supercapacitor electrodes, Carbon 47 (2009) 1715–1722. [5] H. Yamada, I. Moriguchi, T. Kudo, Electric double layer capacitance on hierarchical porous carbons in an organic electrolyte, J. Power Sources 175 (2008) 651–656.

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[24]

[25]

[26]

[27]

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