Nitrogen-Enriched Nanocarbons with a 3-D Continuous Mesopore

Apr 6, 2010 - Jianan Zhang , Yang Song , Maciej Kopeć , Jaejun Lee , Zongyu Wang ... N-Doped Carbon Nanospheres for Supercapacitor Application...
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J. Phys. Chem. C 2010, 114, 8581–8586

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Nitrogen-Enriched Nanocarbons with a 3-D Continuous Mesopore Structure from Polyacrylonitrile for Supercapacitor Application Xiaoqing Yang,† Dingcai Wu,*,†,‡ Xiaomei Chen,† and Ruowen Fu*,†,‡ Materials Science Institute, PCFM Laboratory, School of Chemistry and Chemical Engineering, Sun Yat-sen UniVersity, Guangzhou 510275, People’s Republic of China, and Institute of Optoelectronic and Functional Composite Materials, Sun Yat-sen UniVersity, Guangzhou 510275, People’s Republic of China ReceiVed: February 9, 2010; ReVised Manuscript ReceiVed: March 24, 2010

A kind of polyacrylonitrile-based carbon with a 3-D continuous mesopore structure was prepared by using silica gel as a template. Both the mesopore structure and the nitrogen content could be easily tailored by controlling the carbonization temperature. With decreasing the carbonization temperature, the porosity gradually decreased, while the nitrogen content increased. The sample carbonized at 800 °C and showed the highest specific capacitance of 210 F g-1 at the current density of 0.1 A g-1, which could still stay over 90% when the current density increased by 10 times. The good electrochemical properties could be ascribed to the following three factors: (1) the great pseudocapacitance of nitrogen functionalities, which is 28% of the total capacitance, (2) the 3-D continuous pore structure with a high mesoporosity of 92.5%, and (3) the enhanced wettability resulting from the nitrogen on the carbon skeleton. 1. Introduction As a new type of charge storage devices, supercapacitors are attracting more and more attention because of their high power density and long cyclic life compared with batteries and high energy density relative to traditional capacitors.1–5 According to the energy storage mechanism, supercapacitors can be divided into two classes, electric double-layer capacitors (EDLCs), depending on the pore structure of the electrode materials,6,7 and pseudocapacitors based on the active electrode materials,4,8,9 in which the Faradic redox process occurs. To develop supercapacitors with high performance, various materials have been examined as possible electrode materials. Among them, porous carbons, which have high porosity, a large surface area, and good conductivity, have been used most widely. In general, a large surface area is considered necessary for charge accumulation for a high energy density EDLC electrode material. However, electric double-layer capacitance (CEDL) cannot be enhanced illimitably because the excessive increase of surface area causes a huge decrease of the mesoporosity, thus reducing the mass transfer capacity. Therefore, a number of approaches are under development for optimizing the specific capacitance of carbon materials while keeping a proper mesoporosity. One approach is to introduce heteroatoms with pseudocapacitance.10–17 Recently, nitrogen-enriched carbon (NC) has attracted a lot of attention due to its pseudocapacitance effect for supercapacitors. A number of NCs with favorable electrochemical performance have been prepared.12–17 Nitrogen is often introduced onto carbon frameworks by a postprocess,11,15–17 such as treating it with ammonia or melamine, resulting in a tedious process and instability of pseudocapacitance properties given by nitrogen upon cycling the capacitors. Also, a lower nitrogen content (below 10%) is usually reached * To whom correspondence should be addressed. Tel: +86-02084112759. Fax: +86-020-84115112. E-mail: [email protected] (D.W.), [email protected] (R.F.). † School of Chemistry and Chemical Engineering, Sun Yat-sen University. ‡ Institute of Optoelectronic and Functional Composite Materials, Sun Yat-sen University.

by this strategy. Such shortcomings can be overcome by using nitrogen-enriched materials as the precursor.12,13,18–20 With this method, nitrogen can be preserved at a relatively large quantity by choosing an appropriately low carbonization temperature. Meanwhile, the preparation process is simplified, and the nitrogen on the carbon framework can stay stable under the harsh working conditions. However, until now, this method often uses an expensive silica template of SBA-15 for nanocasting routes13,18 or undergoes a direct carbonization of the nitrogenenriched precursor.12,19,20 The former route results in a low performance-to-price ratio of the as-prepared materials, whereas the later gives rise to a low porosity, which hinders the transfer/ diffusion of ions to a certain extent and then decreases the surface and the nitrogen utilization greatly. Hence, a low-cost NC with a more developed pore structure is urgently needed to meet the demands of the supercapacitors. We report herein the preparation of a high mesoporosity NC with a 3-D continuous pore structure for supercapacitors by using an industrial polyacrylonitrile resin (PAN)/silica gel composite with an appropriately low carbonization temperature. Inexpensive silica gel has been prepared and then used as the template because its well-controlled and 3-D continuous structure allows one to tailor the pore size of the resulting carbons over a wide range of mesopores and as well as to design carbon materials with a 3-D continuous pore structure. Nitrogen on the framework can not only provide great pseudocapacitance and thus increase the energy density but also improve the wettability of the as-prepared material in the electrolyte and thus enhance the mass transfer efficiency. As a result, the loss of the power resulting from the relatively poor conductivity can be made up by the positive mass transfer efficiency provided by the enhanced wettability, high mesoporosity, and highly connective 3-D continuous pore structure. For the silica gel templating method, the pore structure of the carbon materials is generally tailored by controlling the catalyst and template amount.21,22 However, we found surprisingly in this study that the carbonization temperature also plays a decisive role in controlling the pore structure of the resulting

10.1021/jp101255d  2010 American Chemical Society Published on Web 04/06/2010

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Figure 1. (a) Nitrogen adsorption/desorption isotherms at 77 K of the as-prepared samples and (b) their corresponding BJH mesopore size distribution curves.

samples. At the same time, the intrinsic pseudocapacitance resulting from pyrrolic/pyridine, pyridinic, and quinone groups has been successfully figured out by separating the discharge time of pseudocapacitance from the total discharge time of the as-prepared samples. 2. Experimental Section 2.1. Preparation of the PAN-Based Nanoporous Materials. N,N-dimethylformamide (DMF), tetraethyl orthosilicate, deionized water, and hydrofluoric acid (HF) were initially added into a polytetrafluoroethylene (PTFE) flask in a volume proportion of 5:2:1:0.2 successively. The mixture was stirred continuously until complete homogenization. The obtained homogeneous mixture was then sealed up and later quickly gelated and aged at 90 °C for 3 days. The resulting colorless, translucent silica gel was put into a conical flask, followed by adding PAN/ DMF solution (at a concentration of 0.5 g of PAN/10 mL of DMF) in the proportion of 1 g of gel/2 mL of solution. Subsequently, the conical flask was oscillated with a rotational speed of 180 r min-1 at 30 °C for 2 days. After pumping filtration of the mixture, the resulting light yellow gel was dried at 90 °C overnight. The samples were stabilized in a muffle furnace at 300 °C for 12 h and then carbonized at 500-900 °C for 3 h under N2 atmosphere. After carbonization, the black carbon/silica composites were washed using 40 wt % HF solution to remove the silica, followed by filtration and drying of the as-prepared carbon materials. The as-prepared PAN-based nanoporous carbon (NPC) materials were denoted as NPCPANx, where x represented the carbonization temperature of the samples. Another noncarbonized sample was prepared by direct template removing of the stabilized sample and denoted as PAN-NC. 2.2. Microstructure Characterization. The microstructure of the carbon samples was investigated by a transmission electron microscope (TEM, JEOL JEM-2010) and an ASAP 2010 surface area and porosity analyzer (Micromeritics Instrument Corporation). The Brunauer-Emmett-Teller method was utilized to calculate the BET surface area (SBET) using adsorption data in a relative pressure range from 0.05 to 0.21. Micropore volume (Vmic)., micropore surface area (Smic), mesopore size distribution, mesopore surface area (Smes), and mesopore volume (Vmes) of the samples were analyzed by t-plot theory, HK (Horvath-Kawazoe) theory, and BJH (Barrett-Joyner-Halendar) theory, respectively.

2.3. X-ray Photoelectron Spectroscopy (XPS) Measurements. XPS measurements were carried out with an ESCALAB 250 instrument. The compositions were determined by considering the integrated peak areas of N 1s and O 1s from the survey spectra. 2.4. Electrochemical Measurements. The capacitive performance of the as-prepared samples was investigated in 1 M H2SO4 using a sandwich-type two-electrode testing cell. The electrodes were prepared by mixing the carbon samples with PTFE and commercial carbon black in the ratio of 8.5:1:0.5 and then rolling the mixture into a film with a roller, cutting the film into a suitable shape and covering a stainless mesh (current collector) with the film, and finally pressing the ensemble together under about 4 MPa. The electrodes were dried at 110 °C for 8 h. The net weight of each electrode was between 4 and 6 mg. A kind of sandwich-type supercapacitor consisting of two similar sample electrodes was assembled. The electrodes and separators were soaked in 1 M H2SO4 electrolyte for over 8 h before each assembling. All electrochemical measurements were performed with the assembled two-electrode supercapacitors at ambient temperature. The galvanostatic charge-discharge tests were executed using a Land battery test equipment (CT2001A) at different current loads between 0.1 A and 1 A g-1 in a potential window of 0-1 V. Cyclic voltammetry (CV) at a scan rate of 1 mV s-1 and electrochemical impedance spectroscopy (EIS) (excitation signal, 5 mV; frequency range, 0.01-10 000 Hz) were also recorded using the same cell. The specific gravimetric capacitance Cg (in F g-1) per single electrode was calculated from the discharge curves. Specific capacitance per surface area Cs (in µF cm-2) was obtained by dividing the Cg by the SBET. 3. Results and Discussion 3.1. Microstructures of the As-Prepared Samples. Figure 1a and Table 1 show the N2 adsorption-desorption isotherms and pore structure parameters of the as-prepared samples, respectively. We can see that all the carbonized samples show an obvious uptake at the relatively high pressure of 0.9-1.0, indicating that they are all typical mesoporous carbons. In the previous studies, catalyst and template amount have been changed to control the pore structure of the resulting carbon materials.21,22 Interestingly, in our study, we found out that the pore structure also changes significantly, depending on the carbonization temperature. It is clearly seen from Figure 1a and Table 1 that the N2 adsorption quantity and the total pore volume (Vtotal) increase with increasing the carbonization temperature.

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TABLE 1: Textual Characteristics of the As-Prepared Samples sample

SBET (m2 g-1)

Smic (m2 g-1)

Smes (m2 g-1)

Vtotal (cm3 g-1)

Vmic (cm3 g-1)

Vmes (cm3 g-1)

Vmes %a

PAN-NC NPC-PAN500 NPC-PAN600 NPC-PAN700 NPC-PAN800 NPC-PAN900

75 268 305 508 635 608

10 92 119 222 268 236

76 132 163 223 393 323

0.22 0.64 0.84 1.03 1.56 1.87

0 0.04 0.05 0.10 0.12 0.11

0.23 0.59 0.81 0.93 1.47 1.79

100 93.7 94.2 90.3 92.5 94.2

a

Vmes % ) Vmes/(Vmes + Vmic) × 100%.

This might be explained as follows: because of emission of many noncarbon elements and carbon-containing compounds during carbonization, the burnoff and shrinkage of the PAN precursor will increase when the carbonization temperature increases, while the silica gel amount remains unchanged, resulting in an increased silica gel/carbon ratio. The higher the silica gel content, the greater is the volume fraction of it and the larger is the porosity of the samples. Therefore, almost all of the pore parameters increase accordingly. For comparison, owing to the high silica gel/precursor ratio, the noncarbonized sample PAN-NC shows a low Vtotal of 0.2 cm3 g-1. On the other hand, the uptake at the relative low pressure of all the samples suggests that, besides the high mesoporosity (Table 1), they all contain a certain amount of micropores. Figure 1b exhibits the BJH mesopore size distribution curves of the NPC-PANx samples. Although the mesopore size of the

Figure 2. TEM image of NPC-PAN800.

samples carbonized at 500-800 °C is all centered at 23 nm, the arrow in Figure 1b suggests that those pores, which are larger than 23 nm, will increase with increasing the carbonization temperature due to the increased silica gel/carbon ratio mentioned above. When the carbonization temperature is up to 900 °C, the most probable distribution of the mesopore size converts to 30 nm. Meanwhile, it can be vividly seen from the TEM image of NPC-PAN800 in Figure 2 that numerous mesopores and some macropores are connected to each other to form a 3-D continuous pore structure on the carbon skeleton originated from the 3-D silica gel network skeleton. This kind of pore structure is very important for the application perspective, including electrodes for supercapacitors, because they can facilitate better diffusion and transport of electrolyte. 3.2. Electrochemical Performance. Figure 3a represents the CV curves of the as-prepared samples at a scan rate of 1 mV s-1. The curves of NPC-PAN800 and NPC-PAN900 samples exhibit shapes of ideal nanoporous carbon electrodes. Their nanoporous structure and skeleton conductivity are able to provide very fast ion transport and electron pathways, respectively, and thus, the electrical double layer can be reorganized quickly at the switching potentials (e.g., -0.5 and 0.5 V in Figure 3a). Moreover, the curve area of NPC-PAN800 is larger than that of NPC-PAN900, suggesting that the specific capacitance of the former is higher than that of the latter. With further decreasing carbonization temperature, the CV curve shape of the samples is getting more and more distorted and the curve area is getting smaller and smaller. The samples carbonized below 600 °C show little capacitance, indicating that they cannot meet the demands of supercapacitors because of the high internal resistance and low porosity. These above results can be further confirmed by the galvanostatic discharge curves of the as-prepared samples in Figure 3b. NPC-PAN800 shows a much longer discharge time than that of the NPC-PAN900, demonstrating that it has a higher Cg (210 F g-1) than that of NPC-PAN900 (161 F g-1). Considering their similar SBET, a much higher Cs (33 µF cm-2) of NPC-PAN800 than that of NPC-PAN900 (26 µF cm-2) can be obtained. This

Figure 3. (a) CV curves of the NPC-PANx samples at the scan rate of 1 mV s-1and (b) their discharge curves at the current load of 0.1 A g-1.

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TABLE 2: Content (wt %) of Various NFs and OFs in the As-Prepared Samples nitrogen content (wt %)

oxygen content (wt %)

sample

N-X

N-Q

N-5

N-6

O-I

O-II

O-III

NPC-PAN700 NPC-PAN800 NPC-PAN900

0.01 0.14 0.25

1.76 2.56 3.22

7.75 5.20 1.82

9.11 7.71 2.49

1.31 1.19 1.04

6.14 5.11 6.89

0 0 0.39

implies that the pseudocapacitance arising from nitrogen plays an important role in the energy storage because the nitrogen content of NPC-PAN800 is much higher than that of NPCPAN900 (Table 2), which will be discussed later. On the other hand, the increased nitrogen on the carbon skeleton can also enhance the wettability of the electrode material14,15,20 and thus increase the surface utilization. Nevertheless, the Cg of NPCPAN700 is only 155 F g-1 because of the relatively low SBET and the low conductivity, which leads to an obvious IR drop at the beginning of its galvanostatic discharge curve and thus reduces the Coulombic efficiency. The different conductivities of the samples can be further revealed by EIS analysis in Figure 4. It is known that the intrinsic resistance of active electrode materials can be estimated from the diameter of the semicircle in the high-frequency region of Nyquist plots, considering the fact that both the electrolyte and the cell-assembling technique are the same.23,24 On the basis of this, we find that the intrinsic resistance of NPC-PAN700 is as high as 6.98 Ω, whereas those of NPC-PAN800 and NPC-PAN900 decrease to 2.22 and 0.58 Ω, respectively. Many studies illustrate that the charge/discharge curves of the electric double-layer supercapacitor are generally shaped like a straight line, whereas those of the pseudocapacitor are shaped as a curve due to the redox reaction during the discharge processes.25–27 In Figure 3b, all the discharge curves show a different extent of inflection at about 0.8 V. The linear part of the time dependence of the potential indicates that CEDL arises from charge separation of electrolyte at the interface between electrode and electrolyte. The other part of the discharge curve exhibits a gradual slope change, from which we can deduce that it is mixed with redox and CEDL types. To calculate the pseudocapacitance of nitrogen and oxygen, extended lines of the linear parts of the discharge curves have been made in Figure 5, which represents the C EDL parts of the discharge curves. Obviously, TD represents the discharge time of the CEDL type, whereas TP represents the discharge time of the pseudocapacitance type The corresponding pseudocapacitance (CP) calculated

Figure 4. Nyquist plots in the range of 10 kHz to 10 mHz for the as-prepared samples. The inset shows the expanded high-frequency region of the plots.

from the TP of the NPC-PAN900, NPC-PAN800, and NPCPAN700 are 37, 79, and 99 F g-1, respectively. For further investigating the pseudocapacitance contribution of different OFs and NFs, XPS measurement has been taken to obtain the relative surface concentrations of different OFs and NFs, and the results are shown in Table 2. The NFs on the carbon matrix include pyridinic (N-6), pyrrolic/pyridone (N-5), quaternary nitrogen (NQ), and pyridine-N-oxide (N-X), and the OFs include CdO quinone-type groups (O-I), C-OH phenol groups and/or C-O-C ether groups (O-II), and chemisorbed oxygen (COOH carboxylic groups) and/or water (O-III). According to many previous studies,11,13,28–31 O-I and nitrogen located at the edges of graphene layers, that is, N-6 and N-5, are considered representing the pseudocapacitance effect. Therefore, after separating the CP from the total Cg, we can set up a three-variable linear equation by choosing the intrinsic pseudocapacitance of N-5 (CN, N5), N-6 (CN, N6), and O-I (CO, OI) as the unknown number to obtain their actual contribution to the pseudocapacitance as follows:

7.75% × CN, N5 + 9.11% × CN, N6 + 1.31% × CO, OI ) 99 F g-1 5.20% × CN, N5 + 7.71% × CN, N6 + 1.19% × CO, OI ) 79 F g-1 1.82% × CN, N5 + 2.49% × CN, N6 + 1.04% × CO, OI ) 37 F g-1 The calculated CN, N5, CN, N6, and CO, OI are 451, 465, and 1655 F g-1, respectively. As a result, the percentage of electric double-layer capacitance (PEDL), pseudocapacitance of NFs (PN), and pseudocapacitance of OFs (PO) are listed in Table 3.

Figure 5. Discharge time of the pseudocapacitance parts of the asprepared samples at the current load of 0.1 A g-1.

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TABLE 3: Contributions from EDL, NFs, and OFs to the Total Capacitance sample

Cg (F g-1)

PEDL %

PN %

PO %

NPC-PAN700 NPC-PAN800 NPC-PAN900

161 210 155

38 62 76

48 28 13

14 10 11

TABLE 4: Specific Capacitance at Different Current Densities of the As-Prepared Samples Cg (F g-1) sample NPC-PAN900 NPC-PAN800 NPC-PAN700

0.1 A g 161 210 155

-1

0.2 A g-1

0.5 A g-1

1.0 A g-1

159 209 121

157 203 54

150 191 9

From Table 3, it can be obviously found that OFs’ contribution to the Cg is much lower than those of NFs, especially in the NPC-PAN700 and NPC-PAN800 samples, although O-I has the highest intrinsic pseudocapacitance of 1655 F g-1. This is because O-I is hard to be preserved on the carbon skeleton. Therefore, the most effective way to increase pseudocapacitance is to introduce N-5 and N-6, which can be easily tailored by controlling the carbonization temperature. From the above results, we know that the Cs increases from 26 µF cm-2 of NPC-PAN900 to 33 µF cm-2 of NPC-PAN800, owing to the pseudocapacitance provided by nitrogen. Meanwhile, a relatively lower conductivity of NPC-PAN800 than that of NPC-PAN900, which has been proved by the EIS test, may lead to a lower power density. Nevertheless, its high mesoporosity of 92.5%, 3-D continuous pore structure, and wettability enhanced by nitrogen can enhance the transfer efficiency of the electrolyte greatly and, therefore, make up the power loss arising from the lower conductivity. This can be proved by the study of capacitance retention, as follows. Table 4 and Figure 6 show the capacitance and capacitance retention of the three samples calculated from the galvanostatic discharge curves at different current densities, respectively. We can obviously see that the capacitance retention of NPC-PAN800 does not decrease sharply. It shows a competitive capacitance retention with that of NPC-PAN900 when the current density increases from 0.1 to 1.0 A g-1, which stays over 90%. This is consistent with our assumption. On the other hand, the capacitance retention of NPC-PAN700 decreases sharply with increasing the current density because its conductivity is too low. It remains 5.8% at the current density of 1.0 A g-1.

Figure 6. Specific capacitance retention at different current densities of the as-prepared samples.

4. Conclusion This work shows that NCs with a 3-D continuous mesopore structure for supercapacitors can be successfully fabricated by using a PAN/silica gel composite with an appropriately low carbonization temperature. Not only the nitrogen content but also the mesopore structure depends on the carbonization temperature. Owing to the great pseudocapacitance mainly from NFs, the 3-D continuous pore structure with a high mesoporosity of 92.5%, and the wettability enhanced by NFs, the NPCPAN800 sample shows the highest Cs of 33 µF cm-2 and then the highest Cg of 210 F g-1 at the current density of 0.1 A g-1, which can stay over 90%, even though the current density increases by 10 times. Acknowledgment. This research was supported by the Project of NNSFC (50472029, 50632040, and 50802116), the Natural Scientific Foundation of Guangdong Province (8451027501001421), and the Scientific Foundation of Guangzhou (2007Z2-D2041). References and Notes (1) Wang, K.; Teng, H. The performance of electric double layer capacitors using particulate porous carbons derived from PAN fiber and phenol-formaldehyde resin. Carbon 2006, 44, 3218–3225. (2) Hu, C.; Su, J.; Wen, T. Modification of multi-walled carbon nanotubes for electric double-layer capacitors: Tube opening and surface functionalization. J. Phys. Chem. Solids 2007, 68, 2353–2362. (3) Gupta, V.; Miura, N. Electrode compositions for carbon power supercapacitors. J. Power Sources 2006, 157, 616–620. (4) Xie, X. F.; Gao, L.; Sun, J.; Liu, Y. Q.; Kajiura, H.; Li, Y. M.; Noda, K. The effect of electro-degradation processing on microstructure of polyaniline/single-wall carbon nanotube composite films. Carbon 2008, 46, 1145–1151. (5) Guo, H.; Gao, Q. Boron and nitrogen co-doped porous carbon and its enhanced properties as supercapacitor. J. Power Sources 2009, 186, 551– 556. (6) Raymundo-Pin˜ero, E.; Leroux, F.; Be´guin, F. A high-performance carbon for supercapacitors obtained by carbonization of a seaweed biopolymer. AdV. Mater. 2006, 18, 1877–1882. (7) Rufford, T.; Hulicova-Jurcakova, D.; Zhu, Z.; Lu, G. Empirical analysis of the contributions of mesopores and micropores to the doublelayer capacitance of carbons. J. Phys. Chem. C 2009, 113, 19335–19343. (8) Kim, C.; Ngoc, B. T. N.; Yang, K. S.; Kojima, M.; Kim, Y. A.; Kim, Y. J.; Endo, M.; Yang, S. C. Self-sustained thin webs consisting of porous carbon nanofibers for supercapacitors via the electrospinning of polyacrylonitrile solutions containing zinc chloride. AdV. Mater. 2007, 19, 2341–2346. (9) Konno, H.; Ito, T.; Ushiro, M.; Fushimi, K.; Azumi, K. High capacitance B/C/N composites for capacitor electrodes synthesized by a simple method. J. Power Sources 2010, 195, 1739–1746. (10) Hulicova-Jurcakova, D.; Puziy, A. M.; Poddubnaya, O. I.; Sua´rezGarcı´a, F.; Tasco´n, J. M. D.; Lu, G. Q. Highly stable performance of supercapacitors from phosphorus-enriched carbons. J. Am. Chem. Soc. 2009, 131, 5026–5027. (11) Hulicova-Jurcakova, D.; Seredych, M.; Lu, G. Q.; Teresa, J. B. Combined effect of nitrogen- and oxygen-containing functional groups of microporous activated carbon on its electrochemical performance in supercapacitors. AdV. Funct. Mater. 2009, 19, 438–447. (12) Be´guin, F.; Szostak, K. A self-supporting electrode for supercapacitors prepared by one-step pyrolysis of carbon nanotubes/polyacrylonitrile blends. AdV. Mater. 2005, 17, 2380–2384. (13) Frackowiak, E.; Lota, G.; Machnikowski, J.; Vix-Guterl, C.; Be´guin, F. Optimisation of supercapacitors using carbons with controlled nanotexture and nitrogen content. Electrochim. Acta 2006, 51, 2209–2214. (14) Li, W. R.; Chen, D. H.; Li, Z.; Shi, Y. F.; Wan, Y.; Huang, J. J.; Yang, J. J.; Zhao, D. Y.; Jiang, Z. Y. Nitrogen enriched mesoporous carbon spheres obtained by a facile method and its application for electrochemical capacitor. Electrochem. Commun. 2007, 9, 569–573. (15) Kang, K. Y.; Hong, S. J.; Lee, B. I.; Lee, J. S. Enhanced electrochemical capacitance of nitrogen-doped carbon gels synthesized by microwave-assisted polymerization of resorcinol and formaldehyde. Electrochem. Commun. 2008, 10, 1105–1108. (16) Chen, X. L.; Li, W. S.; Tan, C. L.; Li, W.; Wu, Y. Z. Improvement in electrochemical capacitance of carbon materials by nitric acid treatment. J. Power Sources 2008, 184, 668–674.

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