Synthesis and High Electrochemical Capacitance Of

Accepted ManuscriptSynthesis and High Electrochemical Capacitance of N-doped Microporous Car bon/Carbon Nanotubes for Supercapacitor Ki-Seok KIM, Soo-Jin Park PII: DOI: Reference: To appear in: Received Date: Revised Date: Accepted Date: S1572-6657(12)00111-7 10.1016/j.jelechem.2012.03.011 JEAC 819 Journal of Electroanalytical Chemistry 29 September 2011 15 March 2012 21 March 2012

Please cite this article as: K-S. KIM, S-J. Park, Synthesis and High Electrochemical Capacitance of N-doped Microporous Carbon/Carbon Nanotubes for Supercapacitor, Journal of Electroanalytical Chemistry (2012), doi: 10.1016/j.jelechem.2012.03.011

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Paper proposed to Journal of Electroanalytical ChemistryEditorial office J.M. Feliu Depto. de Quimica Fisica, Universidad de Alicante, Campus de Sant Vicent del Raspeig, 03080 Alicante, Spain, Email: [email protected]

Synthesis and High Electrochemical Capacitance of N-doped Microporous Carbon/Carbon Nanotubes for Supercapacitor

Ki-Seok KIM and Soo-Jin PARK

Department of Chemistry, Inha University, Incheon 402-751, Korea

To whom all correspondence should be addressed.

Tel.: +82-32-860-8438, Fax: +82-32-860-8438 E-mail: [email protected]

Abstract Nitrogen-enriched carbon layer-coated multi-walled carbon nanotubes (N/CMWNTs) composed of core MWNTs and carbon shells were prepared to obtain a new type of carbon electrode materials, and then chemically activated using KOH. After carbonization and activation, the activated N/C-MWNTs (A-N/C-MWNTs) contained about 18% nitrogen and showed a 1-D structure like MWNTs. In addition, the A-N/CMWNTs exhibited superior electrochemical performance to that of pristine MWNTs and N/C-MWNTs; the highest specific capacitance (262 F/g) of the A-N/C-MWNTs was obtained at a current density of 0.5 A/g, as compared to 66 F/g for MWNTs and 161 F/g for N/C-MWNTs. This superior performance was attributed to the combination effect between improved EDLC features via microporosity developed by chemical activation and increased redox reaction by high nitrogen content of basic A-N/C-MWNTs in acidic electrolyte.

Keyword:

Multi

walled-carbon

nanotubes,

chemical

activation,

core/shell,

pseudocapacitance, electrochemical performance.

2

1. Introduction Supercapacitors, also called electrochemical capacitors (ECs) when used as energy storage systems, have been studied for various applications, including portable electronic devices and hybrid electric vehicles (HEV). Supercapacitors can replace and/or overcome the disadvantages of conventional batteries and capacitors because of their higher energy and power density [1, 2]. The energy storage mechanism in a supercapacitor is mainly related to the electrical double layer capacitor (EDLC) and pseudocapacitor. The former is based on an electrical double layer at the interface between the electrode and electrolyte, while the latter depends on the transfer of electrons between the electrolyte ions and the surface of the electrode, i.e., a faradaic reaction of the electrode materials [3, 4]. Carbon nanotubes (CNTs) have been studied extensively as electrodes in supercapacitors because of their high specific surface area, superior thermal and mechanical properties, and good electrical property [5, 6]. However, CNTs are currently limited for use as electrode materials of supercapacitors by their low specific capacitance (> about 100 F/g). Thus, control of the structure and morphology of CNTs is a key factor to enhance their specific capacitance. Recently, a considerable amount of research has examined introducing heteroatoms (such as nitrogen (N), oxygen (O),3

boron (B), and phosphate (P)) to the surface of CNTs to create metal-, conductive polymer/CNTs composites [7-9]. In an aqueous electrolyte system, nitrogen groups can be incorporated with the CNTs to enhance the specific capacitance through pseudocapacitive reactions between the surface functionalities and the aqueous electrolyte, as well as the formation of electrical double layers [10, 11]. Several methods have been used for the preparation of N-doped CNTs for in situ CVD synthesis, exposure of reactive gases, and carbonization of N-containing polymers precursors, including polyaniline, polypyrrol, and melamine [12-14]. As an N-enriched carbon precursor, melamine can easily control the nitrogen content and maintain the high nitrogen content (>10 wt.%) after carbonization at higher temperatures (over 800) than other polymer precursors. Li et al. [15] reported the electrochemical performance of a nitrogen-enriched carbon sphere prepared by melamine as a nitrogen-containing precursor, and they showed high specific capacitance (159 F/g) at 0.5 A/g with high specific surface area (1,460 m2/g). This result shows that the presence of a nitrogen group can enhance the surface wettability and reduce the resistance of the carbon spheres. Dong et al. [16] examined nitrogen-containing porous carbon materials and discussed the effect of nitrogen groups and micropores on the4

electrochemical performance of a supercapacitor. They used an anionic surfactant and melamine precursor to prepare microporous carbons with a high surface area of 539 m2/g, and the resultant N-enriched carbon materials showed high capacitance over 200 F/g in H2SO4 solution. Many studies have recently been performed on the preparation and electrochemical performance of microporous and N-enriched carbon materials prepared from melamine [17, 18]. However, in regards to obtaining N-enriched porous carbon materials, there has been limited work comparing the various properties of carbon materials prepared from melamine/carbon materials (such as carbon nanotubes, graphite nanofibers, or grapheme) to polyaniline/carbon materials [19, 20]. Furthermore, for pseudocapacitor electrodes, high microporous and N-enriched carbon-coated carbon nanotubes prepared from melamine/carbon nanotubes composite have thus far hardly been reported.

In the present work, melamine and multi-walled carbon nanotubes (MWNTs) were used as N-enriched carbon precursor and substrate, respectively. High microporous and N-enriched carbon-coated MWNTs (A-N/C-MWNTs) were prepared using a three-step process: 1) MWNTs were first coated with a layer of melamine by polymerization and 2) the melamine-coated MWNTs were converted to carbon layers by carbonization at 850, and (3) resultant carbon/MWNTs composites were chemically activated using5

KOH. The electrochemical performance of the prepared N-doped carbon/MWNTs (N/C-MWNTs) was investigated and the effect of chemical activation on the electrochemical performance of N/C-MWNTs is also discussed.

2. Materials and Methods

2.1. Materials

Multi-walled carbon nanotubes (MWNTs) produced by the chemical vapor deposition (CVD) process were obtained from Nanosolution Co. (Korea). The properties of the MWNTs were as follows: purity>95 % (C: 96 %), diameter 10 to 25 nm, and length 25 to 50 m. MWNTs were used after conventional acid treatment using a mixture of sulfuric acid and nitric acid (3:1). Melamine as nitrogen precursor and formaldehyde were supplied from Aldrich. The sulfuric acid, nitric acid, and all other organic solvents used in this study were of analytical grade and used without further purification.

2.2. Preparation of A-N/C-MWNTs To prepare the A-N/C-MWNTs, 0.1 g of MWNTs was dispersed in 50 ml of distilled water with the assistance of 1 h of ultrasonication. MWNT solution was6

charged into the flask, and then 3 g of melamine and 6 ml of formaldehyde solution were added with constant stirring. The solution was heated at 80 until it became clear. After stirring for 30 min, the resulting solution was cooled down to 40 and the pH value of the mixtures was adjusted to 4.5 by using HCl solution. The mixtures were kept at a static condition for 12 h to obtain the melamine/MWNTs composites. Then, the final product was filtered and washed using water and ethanol. The washed powder was dried in a vacuum oven at 60 for 24 h. The N-enriched carbon/MWNT core/shell structure was obtained by carbonizing the melamine/MWNTs in two steps. First, stabilization was carried out at 250 in air for 2 h with a heating rate of 5/min. Second, carbonization was carried out at 800 under N2 gas for 2 h with a heating rate of 5/min and an N2 flow rate of 200 ml/min. The weight ratio of the carbon layer/MWNTs is about 0.8. To obtain the high specific surface area of the sample, the sample was chemically activated using KOH, as previous work [21]. The KOH impregnation process was initiated by mixing 0.2 g of carbon layer/MWNTs with 0.8 g of KOH solution containing 2 g distilled water. The mixture was stirred for 4 h at 60 and then dried for 24 h at 100 . Activation conditions were 800 for 2 h at a heating rate of 10 /min under N2 flow of 200 ml/min. The activated product was washed using 57

wt.% HCl and hot distilled water, filtered, and then dried at 100 . The samples prepared are referred to as N/C-MWNTs and A-N/C-MWNTs, and the scheme for the preparation of A-N/C-MWNTs is presented in Fig. 1.

2.3. Measurements The morphologies of pristine MWNTs, N/C-MWNTs, and A-N/C-MWNTs were observed by scanning electron microscopy (SEM, S-4200, Hitachi). Surface functional groups of pristine MWNTs, N/C-MWNTs, and A-N/C-MWNTs was determined using X-ray photoelectron spectroscopy (XPS, K-Alpha) with a VG Scientific ESCALAB MK-II spectrometer equipped with an MgK (1253.6 eV) X-ray source and a high performance multichannel detector, operated at 200 W. The structure characteristics were confirmed using X-ray diffraction (XRD, Rigaku D/Max 2200V) at 40 kv and 40 mA using Cu K radiation. The XRD patterns were obtained with a scanning rate of 2 /min. The porous characteristics of pristine MWNTs, N/C-MWNTs, and A-N/C-MWNTs were carried out at 77 K using a gas adsorption analyzer (BELSORP, BEL JAPAN). The samples were degassed at 273 K for 12 h in order to obtain a residual pressure that was less than 10-6 mmHg. Specific surface areas and the micropore volume of the samples

8

were determined from the Brunauere-Emmette-Teller (BET) equation and the DubinineRadushkevitch (DeR) equation, respectively. The amounts of N2 adsorbed at relative pressures (P/P0=0.98) were used to investigate the total pore volumes, which corresponded to the sum of the micropore and mesopore volumes. Electrochemical performance of each sample was characterized by cyclic voltammetry (CV) and galvanostatic charge/discharge measurements using a threeelectrode electrochemical cell, which consisted of a counter electrode (Pt), an Ag/AgCl reference electrode, and stainless steel (SUS) mesh coated with samples as the working electrode. To obtain working electrodes, the MWNTs (N/C-MWNTs or A-N/CMWNTs), carbon black, and PVDF (80:10:10, w/w) were mixed in NMP. The mixture was then coated onto SUS mesh and dried at 100 for 12 h. CV measurements were carried out in 1.0 M H2SO4 solution at scan rates of 5 to 100 mV/s in the voltage range 0.2 V to 1.0 V. Galvanostatic charge/discharge behaviors were measured at different current densities from 0.5 A/g to 2 A/g. All samples were measured after initial stabilization processing of 10 cycles. It can be calculated as:

Cspec =

I t V m

(1)

9

where I is the discharge current, m the electrode mass, t the discharge time, and V is the voltage range.

3. Results and discussion

3.1. Textural properties of A-N/C-MWNTs

The N2 adsorption/desorption isotherms and pore size distribution of pristine MWNTs, N/C-MWNTs, and A-N/C-MWNTs are compared in Fig. 2 and Table 1. The surface area of the N/C-MWNTs (202 m2/g) is lower than that of MWNTs (255 m2/g), resulting from the MWNTs pores being blocked by the coating of the N-enriched carbon layers. However, after chemical activation, the surface area (1,270 m2/g) of the A-N/C-MWNTs is remarkably increased with increasing total pore volume (see Table 1), indicating the formation of many microprores [22]. The micropore volume (0.19 cm3/g) of the A-N/C-MWNTs is significantly increased compared to MWNTs (0.03 cm3/g). Likewise, the porous features of each sample can be also confirmed with meso- and micropore distribution. As shown in Fig. 2b and c, the A-N/C-MWNTs show traditional microporous features, as compared to the mesoporous features of MWNTs and N/C-MWNTs.

As is well known, the electrochemical performance of supercapacitors is highly dependent on the porous features of the electrode materials as well as surface chemistry10

because of the size (0.6 to 0.76 nm) of hydrated ions in an aqueous electrolyte. Therefore, a moderate pore size of about 7 nm, however, the electrolyte ions are not adsorbed to the pore surface usually and then loosely bound, so they do not particularly contribute to the capacitance. In this study, the average pore diameter of mesoporous MWNTs (12.4 nm) decreased with carbon-layer coating and chemical activation, and the average pore size of the N/C-MWNTs and A-N/C-MWNTs is 4.7 nm and 2.8 nm, respectively. Especially, the meso- and micropores of A-N/C-MWNTs is centered at 2.0 nm and 0.67 nm, respectively. It indicates that A-N/C-MWNTs have optimized pore structure as electrode for supercapacitor. From these results, it is expected that hybrid system, which consists of N-doped microporous carbon layer and MWNTs can improve electrochemical performance of MWNTs-based electrodes.

3.2. Morphologies of A-N/C-MWNTs

For electrode materials, the core/shell structure can provide surface modifications and new properties by synergistic effects between core and shell materials. In this study, pristine MWNTs are modified by one process including polymer coating, carbon layer11

coating via carbonization, and chemical activation steps.

Fig. 3 shows SEM images of the pristine MWNTs, N/C-MWNTs, and A-N/CMWNTs. The SEM images clearly illustrate that the pristine MWNTs (Fig. 3a) show a smooth surface, with diameters of about 20 to 30 nm and lengths of a few m. Compared to pristine MWNTs, N/C-MWNTs display a rough surface, and they are covered by a compact carbon layer with increasing total diameters of about 40 to 50 nm, indicating that the carbon layer is over 20 nm. These results indicate that the MWNTs can act as good templates for the formation of uniform coreshell structured carbon layer/MWNT composites. Fig. 3c shows the A-N/C-MWNTs chemically activated by KOH. As seen, after chemical activation, the morphology of A-N/C-MWNTs is similar to that of N/C-MWNTs, indicating that the morphology of A-N/C-MWNTs is not affected to chemical activation process.

3.3. Characterization of A-N/C-MWNTs Structural features of MWNTs and N/C-MWNTs are examined using XRD, as shown in Fig. 4. The pristine MWNTs revealed reflections corresponding to the d(002) and d(100) planes of crystalline graphite-like materials at 2=26 and 43, respectively. From the XRD pattern of the N/C-MWNTs, the main peaks are similar to pristine12

MWNTs. Peak analysis at 2=20 to 26 revealed that the N/C-MWNTs have an aromatic layer [25]. Also, compared to MWNTs, the decreased d(002) peak intensity indicated the formation of stacked layered structures with aromatic crystallinity and the coating of the new carbon layers.

XPS analysis is used to determine the surface chemistry of MWNTs, N/C-MWNTs, and A-N/C-MWNTs. As shown in the survey scans of Fig. 5a, MWNTs consist mainly of oxygen (530 eV) and carbon (285 eV) with negligible low nitrogen content. After carbonization and chemical activation, N/C-MWNTs and A-N/C-MWNTs reveal the nitrogen group at 401 eV, as well as carbon and oxygen, indicating the nitrogen doping on the MWNTs by melamine. As shown in Table 2, the content of nitrogen groups on the N/C-MWNTs and A-N/C-MWNTs is 22.9% and 18.1%, while the molar ratio of nitrogen to carbon (N/C) in the samples is 0.32 and 0.24, respectively. It is well known that the decrease of the nitrogen groups after chemical activation is due to the etching effect of the samples. The N1S of the sample is further confirmed by the fact that the N1S peaks consist mainly of two peaks, i.e., pyridinic (398.5 eV) and quaternary (400.4 eV) nitrogen, and the ratio of the quaternary nitrogen is higher than that of pyridinic nitrogen. This suggests that the pyridinic nitrogen in the carbon precursor is chemically transformed into quaternary type in the high temperature carbonization process [26, 27].13

3.4. Electrochemical performances of A-N/C-MWNTs

As mentioned, the introduction of heteroatoms and the porosity of electrode matrerials are crucial for enhancing the electrochemical performance of supercapacitors. To compare the effect of the two factors on the electrochemical performance of MWNTs, N/C-MWNTs, and A-N/C-MWNTs, we changed the scan rate for obtaining the CV curves of each sample. Fig. 6 reveals the CV curves of each sample at 5 mV/s and 30 mV/s in 1.0 M H2SO4 solution as electrolyte. As shown in Fig. 6, N/C-MWNTs and A-N/C-MWNTs exhibit a greater current density than pristine MWNTs, resulting from the nitrogen doping and increased porous features [28]. However, the electrochemical performance of N/C-MWNTs and A-N/C-MWNTs is affected by the scan rate. At a low scan rate, the current density of the N/C-MWNTs is higher than that of A-N/C-MWNTs, whereas at a high scan rate, the results are the opposite. These phenomena can be explained by the electric double layer capacitor (EDLC) and pseudocapacitance theory. At a low scan rate, the energy storage of electrodes is mainly related to the pseudocapacitive reaction, indicating that the slow motion of electrolyte ions leads to increased chemical reactivity between the nitrogen groups of the electrodes and electrolyte ions. On the other hand, at a high scan rate, the energy storage of the electrodes is significantly influenced by the14

pore features, meaning that the chemical reactivity between the electrodes and electrolyte ions is decreased. Thus, A-N/C-MWNTs, which have well developed pore features, can offer easy ion mobility, leading to a higher capacity for storing energy. Fig. 7 shows a good rate capability for the A-N/C-MWNTs, in the wide range of 5 to 50 mV/s. The shape of the CV curves of the A-N/C-MWNTs is stable and the current density significantly increased without deformation of the CV curves up to 50 mV/s, indicating that the current density of the A-N/C-MWNTs is proportional to the scan rate. The galvanostatic charge/discharge curves of the each sample are shown in Fig. 8. Fig. 8a reveals the charge/discharge curves of MWNTs, N/C-MWNT, and A-N/CMWNTs determined at 0.5 A/g. It is obvious that the charge/discharge curve of the MWNTs is sharper and more triangular in shape, indicating ideal capacitance feature with excellent electrical property. The curve shape of the N/C-MWNTs and A-N/CMWNTs is similar to that of the MWNTs, but the charge/discharge time is remarkably increased by the introduction of N-enriched carbon layers and micropores. The presence of the nitrogen groups allows for the pseudocapacitance reaction, thereby contributing to the enhanced energy storage capability of the MWNTs-based electrodes [29]. Additionally, the energy storage capability of the A-N/C-MWNTs is further enhanced by the carbon layer, which have developed microporosity.15

Fig. 8b~d shows the charge/discharge behaviors of the MWNTs, N/C-MWNTs, and A-N/C-MWNTs with different current densities (0.5 A/g to 2.0 A/g). In all conditions, the charge/discharge curves of all samples remain triangular in shape, although the discharge time is longer than the charge time with as the current density decreases. This result can be attributed to the polarization and increased interaction between the carbon electrode and electrolyte ion by nitrogen groups [30, 31]. With low current density (0.5 A/g), the curves of MWNTs and A-N/C-MWNTs are more stable compared to the N/C-MWNTs, indicating more developed pore structures in the MWNTs and A-N/C-MWNTs. From the charge-discharge curve, the specific capacitance (Cspec) for the MWNTs, N/C-MWNTs, and A-N/C-MWNTs-based electrodes is shown in Fig. 9a. As shown, the highest Cspec of MWNTs, N/C-MWNTs, and A-N/C-MWNTs is 66 F/g, 161 F/g, and 262 F/g, respectively, based on the discharge curves at 0.5 A/g current density. As mentioned, the higher Cspec value of the N/C-MWNTs and A-N/C-MWNTs compared to MWNTs is resulted from the combination effect between increased redox reaction by nitrogen groups and micropore features by chemical activation [32, 33]. The nitrogen groups in the carbon electrodes seem to be able to provide the redox reaction proposed in Fig. 9b. In general, the nitrogen atom is a strong electron donor, which can increase16

the adsorption capability of the electrolyte ions in electrical double layer [34].

4. Conclusions

In this work, highly porous N-enriched carbon core/carbon shell structures for electrode materials were prepared in a three-step process: 1) coating of MWNTs with melamine, 2) carbonization of melamine/MWNTs, and 3) chemical activation of Ncarbon/MWNTs using KOH. The effect of the porosity and nitrogen groups on the electrochemical performance of A-N/C-MWNTs was investigated. From the CV results, the A-N/C-MWNTs exhibited superior electrochemical performances compared to pristine MWNTs and N/C-MWNTs. The highest specific capacitance (262 F/g) of the A-N/CMWNTs was obtained at a scan rate of 0.5 A/g, as compared to 66 F/g for MWNTs and 161 F/g for N/C-MWNTs. This superior performance was mainly due to the synergistic effect between high porosity and pseudocapacitance of the A-N/C-MWNTs.

17

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21

Figure captions Fig. 1. Scheme for the preparation of N/C-MWNTs and A-N/C-MWNTs. Fig. 2. N2 adsorption/desorption isotherm of MWNTs, N/C-MWNTs, and A-N/CMWNTs at 77K, (b) mesopore distribution, and (c) micropore distribution. Fig. 3. SEM images of (a) MWNTs, (b) N/C-MWNTs, and (c) A-N/C-MWNTs. Fig. 4. XRD patterns MWNTs and N/C-MWNTs. Fig. 5. (a) XPS wide-scan of MWNTs, N/C-MWNTs, and A-N/C-MWNTs and (b) N1S peak of A-N/C-MWNTs. Fig. 6. Cyclic voltammetry of MWNTs, N/C-MWNTs, and A-N/C-MWNTs (a) 5 mV/s and (b) 50 mV/s scan rate. Fig. 7. Cyclic voltammetry of A-N/C-MWNTs with different scan rates (5 to 50 mV/s). Fig. 8. Charge-discharge behaviors of (a) all samples at same current density (0.5 A/g) and (b) MWNTs, (c) N/C-MWNTs, and (d) A-N/C-MWNTs with different current densities (0.5 to 2.0 A/g). Fig. 9. (a) Specific capacitance of all samples with different current densities (0.5 to 2.0 A/g) and (b) scheme for the redox reaction of the N-doped MWNTs.

22

1.0MWNTs N/C-MWNTs AC-N/C-MWNTs

Potential (V vs. Ag/AgCl)

0.8 0.6 0.4 0.2 0.0 -0.2 0 100 200 300 400 500 600 700 800 900

Time (sec)

This described the increase of specific capacitance in activated N-doped carbon/MWNT electrodes with increasing microporosity.

23

Research HighlightsCarbon layer coated carbon nanotubes were fabricated as new electrode materials. N-doped carbons layers (N/C) were obtained via carbonization of melamine layer. N/C-carbon nanotubes were further activated using potassium hydroxide(KOH). Activated N/C-CNTs showed highest specific capacitance value (262 F/g). It was due to easy ion transfer by micropores and redox reaction of the electrodes.

24

Table 1. Textural properties of MWNTs, N/C-MWNTs, and A-N/C-MWNTs Sample MWNTs N/C-MWNTs A-N/C-MWNTsa b c d e

SBETa (m2/g) 255 202 1270

VTb (cm3/g) 0.79 0.24 0.88

dMc (nm) 12.4 4.7 2.8

VMed (cm3/g) 0.76 0.18 0.69

VMie (cm3/g) 0.03 0.06 0.19

Specific surface area Total pore volume Average pore diameter Mesopore volume Micropore volume

25

Table 2. The element components of MWNTs, N/C-MWNTs, and A-N/C-MWNTs C MWNTs N/C-MWNTs A-N/C-MWNTs 86.36 70.47 74.79 O 10.6 6.1 7.15 N 0.5 22.9 18.1 N/C ratio 0.006 0.324 0.242

26

Melamine Polymerization Carbonization

850 Activation

Fig. 1

(a)600

Volume adsorbed (cm /g)

MWNTs N/C-MWNTs A-N/C-MWNTs

3

400

200

0 0.0 0.2 0.4 0.6 0.8 1.0

(b)Differential pore volume (cm /g/nm2.0 nm3

P/P0MWNTs N/C-MWNTs A-N/C-MWNTs

1.5

1.0

0.5

0.0 1 10 100

(c)Differential pore volume (cm /g/nm1.0 0.8 0.6 0.4 0.2 0.00.67 nm3

Pore size (nm)MWNTs N/C-MWNTs A-N/C-MWNTs

1

2

3

4

Pore size (nm)

Fig. 2

Fig. 3

MWNTs N/C-MWNTs

Intensity (a. u.)0

10

20

30

40

50

60

70

2

Fig. 4


(a)

Counts/s1200

1000

800

600

400

200

0

Binding energy (eV)

-NH2

(b)-C=N

Counts/s392

396

400

404

408

Binding energy (eV)

Fig. 5

10 5 0 -5 -10 -15

(a)

Current (mA/cm )

2


-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V vs Ag/AgCl)

60

(b)

Current (mA/cm )

2

30 0 -30 -60 -90 -0.2 0.0 0.2 0.4 0.6MWNTs N/C-MWNTs A-N/C-MWNTs

0.8

1.0


Fig. 6

80 60

Current (mA/cm )

40 20 0 -20 -40 -60 -80 -0.2 0.0 0.2 0.4 0.6 0.8 1.05 mV/s 10 mV/s 30 mV/s 50 mV/s

2


Fig. 7

(a)1.0

(b)0.8 0.6 0.4 0.2 0.0 -0.2 0 100 200 300 400 500 600 700 800 900



MWNTs N/C-MWNTs AC-N/C-MWNTs

1.0 0.8 0.6 0.4 0.2 0.0 -0.2 0 20 40 60 80

2.0 A/g 1.5 A/g 1.0 A/g 0.5 A/g

100 120 140 160 180

Time (sec)

Time (sec)

(c)1.0

(d)0.8 0.6 0.4 0.2 0.0 -0.2 0 100 200 300 400 500 600 700



2.0 A/g 1.5 A/g 1.0 A/g 0.5 A/g

1.0 0.8 0.6 0.4 0.2 0.0 -0.2 0 200 400 600

2.0 A/g 1.5 A/g 1.0 A/g 0.5 A/g

800

Time (sec)

Time (sec)

Fig. 8

(a)300 250MWNTs N/C-MWNTs A-N/C-MWNTs

Capacitance (F/g)

200 150 100 50 0 0.5 1.0 1.5 2.0

Crrent density (A/g)

(b)(-) + N e+ H+ N H

Fig. 9

Synthesis and High Electrochemical Capacitance Of

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