Structure and Capacitive Performance of Porous Carbons Derived

Article pubs.acs.org/IECR

Structure and Capacitive Performance of Porous Carbons Derived from Terephthalic Acid−Zinc Complex via a Template Carbonization Process Zhong Jie Zhang,†,‡ Peng Cui,*,‡ and Xiang Ying Chen*,‡ †

College of Chemistry & Chemical Engineering, Anhui Province Key Laboratory of Environment-friendly Polymer Materials, Anhui University, Hefei 230039, Anhui, P. R. China ‡ School of Chemical Engineering, Anhui Key Laboratory of Controllable Chemistry Reaction & Material Chemical Engineering, Hefei University of Technology, Hefei 230009, Anhui, P. R. China S Supporting Information *

ABSTRACT: High-performance porous carbons as supercapacitor electrode materials have been prepared by a simple but efficient template carbonization process, in which commercially available terephthalic acid−zinc complex is used as a carbon source. It reveals that the carbonization temperature plays a crucial role in determining the structure and capacitive performance of carbons. The carbon-1000 sample has high surface area of 1138 m2 g−1 and large pore volume of 1.44 cm3 g−1 as well as rationally hierarchical pore size distribution. In a three-electrode system, the carbon-1000 sample possesses high specific capacitances of 266.0 F g−1 at 0.5 A g−1 and good cycling stability. In a two-electrode system, the operation temperature (25/50/ 80 °C) can greatly influence the electrochemical performance of the carbon-1000 sample, especially with an extended voltage window (∼ 3 V). The temperature-dependent operation makes it possible for the application of supercapacitors under extreme conditions.

1. INTRODUCTION Carbon materials have been extensively used for the application of supercapacitors because of their low cost, easy availability, nontoxic nature, environmental friendliness, chemical stability in different solutions (from strongly acidic to basic), as well as versatile existing forms.1−3 An efficient charge propagation of the electrical double layer requires materials with a high surface area and pores matching the size of ions, which is the premise for supercapacitor performance.4 Activated carbons produced by physical/chemical activation processes from natural/man-made precursors are the electrode materials most often used. However, harsh reaction conditions (e.g. high temperature and long time period), complicated equipment, high cost, and severe contamination/corrosion are unavoidable in the activation process.5 To circumvent this problem, scientists have thus far developed many strategies to produce porous carbons possessing a high surface area and large pore volume without using the activation process. Of great interest is the template carbonization process. Generally, two types of templates, classified as hard template or soft template, are used as scaffolds to facilitate the formation of porous materials.6,7 Some excellent examples of templates have been reported, typically including Ni(OH)2,8 MgO,9 as well as dual templates.10 On the other hand, the current research on supercapacitors mostly aims at ambient circumstances. In aqueous electrolyte systems (H2SO4, Na2SO4, or KOH), they are easily volatilizable with a narrow voltage window (∼ 1.0 V), which greatly restricts their temperature-dependent application. To fulfill the supercapacitors application under extreme conditions, a mixture of ionic liquids (ILs) and organic solvents, such as acetonitrile (AN) © 2013 American Chemical Society

and propylene carbonate (PC), have commonly been employed as electrolytes because of their high stabilities in a large voltage window (∼3.0 V).11−14 Notably, Simon and Gogotsi groups have obtained a lot of intriguing research results with temperaturedependent supercapacitors.15 For instance, they can obtain a wide temperature range from −50 to 100 °C (voltage window as 3.7 V) and −50 to 80 °C (voltage window as 3.5 V) toward exohedral nanostructured carbon (mixture of nanotubes and onions)16 and graphene activated by KOH,17 respectively, when using a eutectic mixture composed of PIP13−FSI and PYR14−FSI as the electrolyte. In this work, we demonstrate a template carbonization process to produce porous carbon, using a commercially available terephthalic acid−zinc complex as the carbon source. The impact of carbonization temperature upon structure and capacitive performance of carbons was studied in particular. The electrochemical tests were carried out in a three-electrode system using 6 mol L−1 KOH as the electrolyte, and a two-electrode system using [EMIm]BF4/AN as the electrolyte, respectively. What is more, the impact of operation temperature (25/50/80 °C) upon capacitive performances of supercapacitors was also investigated in the two-electrode system using [EMIm]BF4/AN as a mixed electrolyte. Received: Revised: Accepted: Published: 16211

July 31, 2013 September 28, 2013 October 22, 2013 October 22, 2013 dx.doi.org/10.1021/ie402482s | Ind. Eng. Chem. Res. 2013, 52, 16211−16219

Industrial & Engineering Chemistry Research

Article

Figure 1. Carbon-600/800/1000 samples. (a) XRD patterns. (b) TGA curves.

2. EXPERIMENTAL SECTION All analytical chemicals were purchased from Sinopharm Chemical Reagent Company Ltd. (Shanghai) and used as received without further treatment. 2.1. Typical Room Temperature Synthetic Procedure for Terephthalic Acid−zinc Complex Powder. Zn(NO3)2· 6H2O and terephthalic acid and disodium salt with molar ratio of 1:2 were first dissolved in deionized water to form clear solutions, respectively, which were then mixed together under constant magnetic stirring for 2 h at room temperature, forming a white precipitate. Next, it was filtered off, washed with deionized water and absolute ethanol several times, and then dried under vacuum at 120 °C for 6 h to obtain the terephthalic acid−zinc complex powder. 2.2. Typical Template Carbonization Method for Producing Carbon-600/800/1000 samples. In a typical procedure, terephthalic acid−zinc complex powder was placed in a porcelain boat, flushing with Ar flow for 30 min, and further heated in a horizontal tube furnace up to 600/800 °C at a rate of 5 °C min−1 and maintained at this temperature for 2 h under Ar flow. The resultant product was immersed with dilute HCl solution to remove soluble/insoluble substances at room temperature, which was further washed with adequate deionized water. Finally, the sample was dried under vacuum at 120 °C for 12 h to obtain the carbon-600/800 samples. In the case of the carbon-1000 sample, the synthetic procedure is similar to that of the carbon-600 sample mentioned above. However, no HCl solution is required due to the sublimation of zinc metal derived from the reduction of ZnO by carbon materials at 1000 °C along with Ar flow. 2.3. Structure Characterization. X-ray diffraction (XRD) patterns were obtained on a Rigaku D/MAX2500 V with Cu Kα radiation. The thermal gravimetric analysis (TGA) was conducted using a Shimadzu TGA-50H analyzer (pure air stream 50 mL/min; heating rate 10 °C/min). Field-emission scanning electron microscopy (FESEM) images were taken with a Hitachi S-4800 scanning electron microscope. High-resolution transmission electron microscope (HRTEM) images and selected area electron diffraction (SAED) patterns were performed with a JEM-2100F unit. X-ray photoelectron spectra (XPS) were obtained on a VG ESCALAB MK II X-ray photoelectron spectrometer with a source to excite electrons of Mg Kα (1253.6 eV). The specific surface area and pore structure of the carbon samples were determined by N2 adsorption− desorption isotherms at 77 K (Quantachrome Autosorb-iQ) after being vacuum-dried at 150 °C overnight. The specific

surface areas were calculated by a BET (Brunauer−Emmett− Teller) method. Cumulative pore volume and pore size distribution were calculated by using a slit/cylindrical nonlocal density functional theory (NLDFT) model. 2.4. Electrochemical Measurements. Detailed electrochemical measurements employed in the present work are given in the Supporting Information and schematic illustration of the supercapacitor cell is displayed in Figure S1 of the Supporting Information.

3. RESULTS AND DISCUSSION The composition, phase, and crystallinity of samples were detected by the XRD technique. When directly heating terephthalic acid−zinc complex at 600 °C, large quantities of black powder appeared and it is composed of carbon derived from the carbonization of phenyl group and ZnO, as shown in Figure S2a of the Supporting Information. After being washed with HCl solution and deionized water, pure carbon, named as carbon-600, was achieved and its corresponding XRD is given in Figure 1a. The two broad and low-intensity diffraction peaks that locate at 23.8° and 42.6°, respectively, can be indexed as (002) and (10) planes of standard graphite.18 In addition, the present carbons are amorphous with low-graphitization degree thanks to their XRD results. Analogous results also occur in the case of carbon-800, as illustrated in Figure 1a and Figure S2b of the Supporting Information. When increasing the carbonization temperature up to 1000 °C, we can only obtain pure carbon, named as carbon-1000, without the presence of any Zn (metal) or ZnO substance, as displayed in Figure 1a. It is thus apparent to us that the reaction mechanisms involved at 600/800/1000 °C are different. In detail, a mixture of amorphous carbon and ZnO come into forth at 600/800 °C, without any reaction between them. However, at the temperature higher than 800 °C, the newly produced ZnO can react with amorphous carbon to form Zn (metal).19 And subsequently, Zn (metal) vaporizes away along with the Ar flow at the present temperature of 1000 °C, far beyond its boiling point (908 °C) and leaves amorphous carbon alone.20 Besides, TGA measurements of the carbon-600/800/ 1000 samples were carried out under air circumstance, whose profiles are shown in Figure 1b. The residues of three samples are almost tiny, indicating the high purity of carbons. The shape and size of the present carbon samples were investigated by the FESEM technique. Figure 2a displays the representative FESEM image of the carbon-600 sample, indicating that the sample is made up of large numbers of irregular blocks. To further gain an insight into the intrinsic 16212

dx.doi.org/10.1021/ie402482s | Ind. Eng. Chem. Res. 2013, 52, 16211−16219


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relative pressure (P/P0) close to 1.0 represent the existence of macropores.21,22 The specific surface areas were calculated by a BET (Brunauer−Emmett−Teller) method. The carbon-600/ 800/1000 samples have surface areas of 602, 696, and 1138 m2 g−1, respectively, as shown in Table 1, which basically are proportional to their quantities adsorbed. Moreover, the external surface area attributed to mesopores/macropores within each carbon is much larger than the area derived from the micropore. On the other hand, cumulative pore volume and pore size distribution were calculated by using a slit/cylindrical nonlocal density functional theory (NLDFT) model, as shown in Figure 3 (panels b, d, and f). It assumes a slit pore geometry for the micropores and a cylindrical pore geometry for the mesopores.23 The total pore volume of the carbon-600/800/1000 samples are 1.18, 1.19, and 1.44 cm3 g−1, respectively, as given in Table 1. It reveals that a higher carbonization temperature is favored for the improvement of surface areas and pore volumes of carbons. Obviously, the carbon-1000 sample exhibits the largest surface area and total pore volume among these carbons. This to some extent is incurred by the different reaction mechanism, including the reduction of ZnO by amorphous carbon as well as the subsequent vaporization of zinc metal at 1000 °C, which can create larger pores within carbon. The differential pore volumes shown in Figure 3 (panels b, d, and f) also indicate their hierarchically porous features with multimodal distributions. It should be pointed out that the hierarchical pore system is effective to reduce the ion-transfer resistance, but the presence of large pores lowers the electrode density, primarily resulting in a low volumetric energy density. In order to solve this problem, reducing unnecessary large pores and improving connection of narrow pores are significant.24 The elemental composition, chemical state, and electronic state of the elements that exist within the carbon-600/800/1000 samples, especially from the top 1 to 10 nm, were determined by XPS technique. Figure 4a shows their typical survey spectra, with binding energies ranging from 0 to 1400 eV. All present carbon samples are composed of carbon and oxygen elements, without others assignable to zinc or zinc oxide, implying their high purities. The contents of carbon and oxygen elements within samples are listed in Table 2. The carbon-1000 sample has the highest carbon content (96.62%) and lowest oxygen content (3.38%). By all appearances, increasing the carbonization temperature is beneficial to the enhancement of carbon content, while accompanying with the decrease of oxygen content. To evaluate the chemical environment of the C/O atoms, their high-resolution peaks are deconvoluted into several components with the help of XPS peak software. Figure 4b displays the highresolution C1s spectra with a binding energy scope of 282−295 eV and all of which can be fitted into three peaks. The peaks at ca. 284.8−284.9 eV are due to graphitic bonding sp2 CC.25 The ones at ca. 285.2−285.4 eV correspond to sp3 C−C26 and/or incomplete or defective graphitic structures.27 The peaks located at ca. 287.4−287.6 eV can be attributed to CO.26 Besides, the peak at ca. 289.5 eV originates from the contribution of O−CO.28 On the other hand, the high resolution O1s spectra fitted into three peaks is shown in Figure 4c. The peaks ranging from ca. 532.0 to ca. 532.8 eV might be indexed as CO/O−CO, and the peaks at ca. 533.4−533.9 eV as well as ones at ca. 534.2−534.6 eV are due to C−O−C/C− O−OH/C−OH.29 As for the peak at ca. 535.6 eV, it is due to the physically adsorbed oxygen/water.28 In the present work, we first adopted a three-electrode system to test the electrochemical behaviors of the carbon-600/800/

Figure 2. FESEM and HRTEM images of the carbon samples: (a−b) carbon-600, (c−d) carbon-800, and (e−f) carbon-1000.

structure, the HRTEM technique was adopted. Clearly, the carbon-600 sample consists of numerous porous structures, as shown in Figure 2b. The magnified HRTEM result is given in the inset of Figure 2b, and the lattice fringes that are totally disordered reveal its amorphous nature. Additionally, the SAED pattern with vague diffraction rings (the inset of Figure 2b) also evinces the low-graphitization degrees, which well-agree with the XRD results (Figure 1a). We can obtain similar results in the cases of the carbon-800/1000 samples, as shown in Figure 2 (panels c−d and e−f, respectively). With respect to the formation mechanism toward pores within carbons, in the cases of the carbon-600/800 samples, the ZnO particles in situ produced by the decomposition of the terephthalic acid−zinc complex at 600/800 °C serves as hard templates. The ZnO particles can be removed by washing with aqueous HCl solution and thus leave lots of pores. With regard to the carbon-1000 sample, there exists another reaction mechanism, in which ZnO particles, acting as hard templates, are reduced by amorphous carbon, giving rise to Zn (metal). Then, the zinc metal vaporizes away along with the Ar gas at 1000 °C, acting somewhat as a soft template. With consideration of the synergistic effect of hard/soft templates, the carbon-1000 sample is thereby expected to deliver many more pores having different sizes and shapes within carbon. Porous structures of the present carbon-600/800/1000 samples were analyzed by N2 adsorption−desorption isotherms and the pore size distribution curves. All isotherms displayed in Figure 3 (panels a, c, and e) have almost the same shapes, and they can be ascribed to those of type-IV, according to the classification by IUPAC. At low relative pressure (P/P0) close to 0, each isotherm has one stage nearly vertical to abscissa axis and this stage corresponds to micropores. As a consequence, the content of micropores within the carbon-1000 sample is much higher than that within the carbon-600/800 samples in virtue of their quantities adsorbed, as given in the vertical axis. Besides, the loops that locate at moderate relative pressure (P/P0) (0−1.0) correlate with mesopores. And the steep increase stages at high 16213



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Figure 3. N2 adsorption−desorption isotherms and the pore size distribution curves of the carbon samples: (a−b) carbon-600, (c−d) carbon-800, (e− f) carbon-1000.

1000 samples at room temperature, using 6 mol L−1 KOH as the electrolyte. Figure 5a reveals the three cyclic voltammetry (CV) curves measured at a scan rate of 50 mV s−1. No obvious redox peaks can be observed on these CV curves, implying that the predominant specific capacitances derive from the contribution of the electrical double-layer capacitors (EDLC). Interestingly, CV curve of the carbon-1000 sample takes on a nearly rectangular shape in a potential range of −1−0 V, which is much better than those of the carbon-600/800 samples. Furthermore, from the viewpoint of the integral area surrounded by CV curves, we can clearly discern that the CV area of the carbon-1000 sample is much larger than the other two samples. This also indicates that the carbon-1000 sample exhibits the utmost specific capacitance since it is proportional to the CV area. As a result, at the scan rates of 5−800 mV s−1, the order of

the specific capacitances is primarily as follows: carbon-1000 > carbon-800 > carbon-600, as shown in Figure 5b. Figure 5c represents the galvanostatic charge−discharge curves measured at a current density of 0.5 A g−1, all of which exhibit almost triangular shapes in the potential range of −1−0 V, further revealing the overwhelming majority of EDLC features toward the carbon-600/800/1000 samples. Meanwhile, the longer discharging time of the carbon-1000 sample in Figure 5c indicates its larger specific capacitance. The specific capacitances at various current densities are depicted in Figure 5d. The carbon-1000 sample displays the utmost specific capacitances at the current density of 0.5−40 A g−1. The specific capacitances can reach up to 266.0, 177.3 F g−1 at the current density of 0.5, 1.0 A g−1, respectively, which are much larger than those of the carbon-600/800 samples. Of course, the excellent capacitive performance of the carbon-1000 sample lies in its larger surface 16214



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Table 1. Characteristic Surface Areas and Pore Structures of the Carbon Samples BET surface area (m2 g−1) micropore volume (cm3 g−1)

average pore widthc (nm)

sample

total

Smicroa

Sextb

total pore volume (cm3 g−1)

carbon600 carbon800 carbon1000

602

91

511

1.18

0.04

7.9

696

133

563

1.19

0.08

6.8

1138

487

651

1.44

0.22

5.1

a

Smicro represents the micropore area calculated using the t-plot. bSext represents the external surface area calculated using the t-plot. c Average pore width is calculated based on 4 V/A by BET.

area, bigger pore volume, better pore size distribution (especially the hierarchical distribution of micro/meso-/macro-pores) as well as higher carbon content, as deeply illustrated in Figures 3 and 4. Cycling durability is another indispensable factor in determining the practical application of supercapacitors. Figure 5 indicates the cycling durability within 10000 times of the carbon-1000 sample, which was measured at 20 A g−1. To our surprise, the specific capacitance has increased from 120.1 to 130.0 F g−1 after charging−discharging for 10000 times. That is to say, the specific capacitance has improved to be 108.3%, revealing good cycling durability. The incremental retention of the carbon-1000 sample probably lies in the following two aspects: the whole process is activated by the potential cycling and the exposure of the underlying carbon surface is enlarged.30 On the other hand, this improved retention after 10000 cycles of the carbon-1000 sample has also been evinced by the corresponding Nyquist plots, as displayed in Figure 5f. The small semicircles in the high-frequency region reveal the minor existence of Rct (charge-transfer resistance), while the tilted sloping line after 10000 cycles at low frequencies is more vertical compared with the initial one, reflecting the closer capacitive characters of EDLC.31 Besides, Nyquist plots before cycling measured in a three-electrode system toward the carbon-600/ 800/1000 samples are comparatively displayed in Figure S3 of the Supporting Information. Next, we adopted a two-electrode system to test the electrochemical behaviors of the present carbon-1000 sample, using a mixture of 1-ethyl-3-methyl imidazolium tetrafluoroborate ([EMIm]BF4) and acetonitrile (AN) (weight ratio of 1:1) as the electrolyte. A two-electrode test cell was used because it can provide the most accurate measurement of the material performance for the supercapacitor.32,33 More importantly, various operation temperatures, including 25/50/80 °C were designated in this work to investigate their impacts upon electrochemical performance of supercapacitors. Figure 6a shows the CV curves of the carbon-1000 sample at the operation temperatures of 25/50/80 °C while designating the scan rate as 50 mV s−1. All the three CV curves are close to rectangular shapes, implying their EDLC features. The specific capacitances at different operation temperatures can be estimated by integral areas as follows: 80 °C > 50 °C > 25 °C. In other words, we can see that the higher operation temperature, the larger specific capacitance, especially at a low scan rate. This is incurred by the improved conductivity of the [EMIm]BF4/AN electrolyte, the decrease of Rs (the internal resistance) and Rct

Figure 4. XPS spectra of the carbon samples: (a) survey, (b) C1s, and (c) O1s.

Table 2. XPS Peak Analysis of the Carbon Samples sample

C (at. %)

O (at. %)

carbon-600 carbon-800 carbon1000

93.51 94.25 96.62

6.49 5.75 3.38

(the interfacial charge transfer resistance), as well as the better contact at the interface between the electroactive carbon material and electrolyte.34 The present potential window using [EMIm]BF4/AN as the electrolyte ranges from −1.5 to 1.5 V, which is much larger than that (−1−0 V) using 6 mol L−1 KOH as the electrolyte. Besides, the scan rates can also be extended largely up to 1000 mV s−1. This further indicates the advantage of mixed electrolyte containing ionic liquids and organic solvents in the field of supercapacitors.23 However, along with the increase of 16215



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Figure 5. Carbon-600/800/1000 samples: (a) CV curves at 50 mV s−1. (b) Specific capacitances at various scan rates. (c) Galvanostatic charge− discharge curves at 0.5 A g−1. (d) Specific capacitances at various current densities. Carbon-1000 sample: (e) cycling durability. (f) Nyquist plots before/after 10000 cycles. Note that all results shown Figure 5 were measured in a three-electrode system using 6 mol L−1 KOH as the electrolyte.

in Table 3. On the other hand, Figure 6g represents the specific capacitances as a function of cycles (in a range of 10000 cycles) measured at the operation temperatures of 25/50/80 °C, and all of them exhibit excellent cycling durabilities, which are wellconsistent with the results of Nyquist plots, as shown in Figure 6h. In addition, in the case of the carbon-1000 sample, its CV curves at the scan rates of 20−1000 mV s−1 as well as galvanostatic charge−discharge curves of the first and 10000th cycles measured in a two-electrode system at the operation temperature of 25/50/80 °C are given in Figures S4−S6 of the Supporting Information.

scan rates, the CV curves gradually deteriorate in shapes and the discrepancies between different operation temperatures become smaller, as shown in Figure 6 (panels b−c), primarily due to the insufficient interaction between the electrode material and the electrolyte. The overall specific capacitances at various scan rates are given in Figure 6d, and it is seen that the results measured in a two-electrode system are to a large extent smaller than those measured in a three-electrode system. Figure 6e displays the galvanostatic charge−discharge curves of the carbon-1000 sample at the operation temperatures of 25/ 50/80 °C measured at a current density of 10 A g−1. And the whole specific capacitances calculated from the discharge curves are shown in Figure 6f, also indicating that higher operation temperature favors obtaining larger specific capacitance, especially at lower current density. Specific capacitances of the carbon-1000 sample under different conditions are summarized

4. CONCLUSIONS In this work, we represent a straightforward template carbonization process to prepare porous carbons using terephthalic acid−zinc complex as the carbon source. The carbonization 16216



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Figure 6. Carbon-1000 sample: CV curves at (a) 50 mV s−1, (b) 500 mV s−1, (c) 1000 mV s−1, (d) specific capacitances at various scan rates, (e) galvanostatic charge−discharge curves at 10 A g−1, (f) specific capacitances at various current densities, (g) cycling durability measured at 10 A g−1, and (h) Nyquist plots before/after 10000 cycles. Note that all results shown Figure 6 were measured in a two-electrode system using [EMIm]BF4/AN as the electrolyte, at the operation temperatures of 25/50/80 °C.

temperature is found to exert a crucial influence in determining the structure and capacitive performance of carbons. Several scientific advantages exist as follows: (1) the present template

carbonization process involved is simple and efficient, especially without any physical and/or chemical activation processes. (2) The carbon-1000 sample has good structures (such as surface 16217



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Table 3. Specific Capacitances (F g−1) of the Carbon-1000 Sample under Different Conditions scan rate (mV s−1)

current density (A g−1)

electrolyte

operation temperature (°C)

electrode system

50

500

1000

1

5

20

40

KOH [EMIm]BF4/AN [EMIm]BF4/AN [EMIm]BF4/AN

25 25 50 80

three two two two

272.1 99.5 112.5 122.2

− 72.3 75.2 75.8

− 54.2 55.5 55.8

177.3 101.1 124.6 163.3

145.5 87.3 97.3 112.2

120.0 61.3 66.7 72.6

104.4 − − −

(11) Cericola, D.; Kötz, R.; Wokaun, A. Effect of electrode mass ratio on aging of activated carbon based supercapacitors utilizing organic electrolytes. J. Power Sources 2011, 196, 3114. (12) McDonough, J. K.; Frolov, A. I.; Presser, V.; Niu, J.; Miller, C. H.; Ubieto, T.; Fedorov, M. V.; Gogotsi, Y. Influence of the structure of carbon onions on their electrochemical performance in supercapacitor electrodes. Carbon 2012, 50, 3298. (13) Balducci, A.; Dugas, R.; Taberna, P. L.; Simon, P.; Plée, D.; Mastragostino, M.; Passerini, S. High temperature carbon−carbon supercapacitor using ionic liquid as electrolyte. J. Power Sources 2007, 165, 922. (14) Masarapu, C.; Zeng, H. F.; Hung, K. H.; Wei, B. Effect of temperature on the capacitance of carbon nanotube supercapacitors. ACS Nano 2009, 3, 2199. (15) Simon, P.; Gogotsi, Y. Capacitive energy storage in nanostructured carbon−electrolyte systems. Acc. Chem. Res. 2013, 46, 1094. (16) Lin, R.; Taberna, P. L.; Fantini, S.; Presser, V.; Pérez, C. R.; Malbosc, F.; Rupesinghe, N. L.; Teo, K. B. K.; Gogotsi, Y.; Simon, P. Capacitive energy storage from −50 to 100 °C using an ionic liquid electrolyte. J. Phys. Chem. Lett. 2011, 2, 2396. (17) Tsai, W. Y.; Lin, R.; Murali, S.; Zhang, L. L.; McDonough, J. K.; Ruoff, R. S.; Taberna, P. L.; Gogotsi, Y.; Simon, P. Outstanding performance of activated graphene based supercapacitors in ionic liquid electrolyte from −50 to 80 °C. Nano Energy 2013, 2, 403. (18) Aladekomo, J. B.; Bragg, R. H. Structural transformations induced in graphite by grinding: Analysis of 002 X-ray diffraction line profiles. Carbon 1990, 28, 897. (19) Fletcher, E. A. Solarthermal and solar quasi-electrolytic processing and separations: Zinc from zinc oxide as an example. Ind. Eng. Chem. Res. 1999, 38, 2277. (20) Liu, B.; Shioyama, H.; Akita, T.; Xu, Q. Metal-organic framework as a template for porous carbon synthesis. J. Am. Chem. Soc. 2008, 130, 5390. (21) Li, F.; Morris, M.; Chan, K. Y. Electrochemical capacitance and ionic transport in the mesoporous shell of a hierarchical porous core− shell carbon structure. J. Mater. Chem. 2011, 21, 8880. (22) Woo, S. W.; Dokko, K.; Nakano, H.; Kanamura, K. Preparation of three dimensionally ordered macroporous carbon with mesoporous walls for electric double-layer capacitors. J. Mater. Chem. 2008, 18, 1674. (23) Zhu, Y.; Murali, S.; Stoller, M. D.; Ganesh, K. J.; Cai, W.; Ferreira, P. J.; Pirkle, A.; Wallace, R. M.; Cychosz, K. A.; Thommes, M.; Su, D.; Stach, E. A.; Ruoff, R. S. Carbon-based supercapacitors produced by activation of graphene. Science 2011, 332, 1537. (24) Itoi, H.; Nishihara, H.; Kogure, T.; Kyotani, T. Threedimensionally arrayed and mutually connected 1.2-nm nanopores for high-performance electric double layer capacitor. J. Am. Chem. Soc. 2011, 133, 1165. (25) Ruiz-Hitzky, E.; Darder, M.; Fernandes, F. M.; Zatile, E.; Palomares, F. J.; Aranda, P. Supported graphene from natural resources: Easy preparation and applications. Adv. Mater. 2011, 23, 5250. (26) Okpalugo, T. I. T.; Papakonstantinou, P.; Murphy, H.; McLaughlin, J.; Brown, N. M. D. High resolution XPS characterization of chemical functionalised MWCNTs and SWCNTs. Carbon 2005, 43, 153. (27) Datsyuk, V.; Kalyva, M.; Papagelis, K.; Parthenios, J.; Tasis, D.; Siokou, A.; Kallitsis, I.; Galiotis, C. Chemical oxidation of multiwalled carbon nanotubes. Carbon 2008, 46, 833.

area, pore volume, and pore size distribution) and excellent electrochemical behaviors. (3) Temperature-dependent operation in a two-electrode system, using [EMIm]BF4/AN as the electrolyte, makes it possible for the application of supercapacitors under extreme circumstances.

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ASSOCIATED CONTENT

S Supporting Information *

Electrochemical measurements, schematic illustration of a supercapacitor cell, XRD patterns, Nyquist plots, Carbon-1000 sample measured in a two-electrode system using [EMIm]BF4/ AN as the electrolyte at the operation temperatures of 25/50/80 °C. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Tel/Fax: +86-551-2901450. E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant 21101052) and the China Postdoctoral Science Foundation (Grant 20100480045). Z.J.Z. also thanks the financial support from Anhui Province Key Laboratory of Environment-friendly Polymer Materials, Anhui University, Hefei 230039, China (Grant KF2012009).

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REFERENCES

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Structure and Capacitive Performance of Porous Carbons Derived

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