Derived Three-Dimensional Porous Nitrogen-Doped Carbon

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Facile Synthesis of Poly(p-phenylenediamine)-Derived Three Dimensional Porous Nitrogen-Doped Carbon Networks for High Performance Supercapacitors Hui Peng, Guofu Ma, Kanjun Sun, Jingjing Mu, Zhe Zhang, and Ziqiang Lei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp508684t • Publication Date (Web): 25 Nov 2014 Downloaded from http://pubs.acs.org on December 1, 2014

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The Journal of Physical Chemistry

Facile Synthesis of Poly(p-phenylenediamine)-Derived Three Dimensional Porous Nitrogen-Doped Carbon Networks for High Performance Supercapacitors Hui Penga, Guofu Ma*a, Kanjun Sunb, Jingjing Mua, Zhe Zhanga and Ziqiang Lei*a a

Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, Key

Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China b

College of Chemistry and Environmental Science, Lanzhou City University, Lanzhou 730070,

China

ABSTRACT: We introduce a facile integrated oxidation polymerization and catalytic carbonization method to prepare three-dimensional porous nitrogen-doped carbon networks (3D N-CNWs) with high nitrogen content (about 8.4 wt%) directly from poly(p-phenylenediamine). In synthesis process, the FeCl3 not only serves as an oxidant for oxidative polymerization of pphenylenediamine monomers but also as the carbonization catalyst to promote porous carbon networks formation. The 3D N-CNWs prepared at 700 oC exhibit an interconnected porous framework with high specific surface area and shows remarkable performances as an electrode material for supercapacitors. The maximum specific capacitance of 304 F g-1 at a current density of 0.5 A g-1, and retains the high values of 226 F g-1 even at a high current density of 20 A g-1 is

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obtained for the N-CNWs electrode in 6 M KOH aqueous solution. Moreover, the as-assembled N-CNWs symmetric supercapacitor exhibits a considerably high energy density of 15.8 Wh kg-1 at a power density of 450 W kg-1 operated in the voltage range of 0-1.8 V in 0.5 M Na2SO4 aqueous solution, and exhibits an excellent cycling performance with 97% specific capacitance retention after 5000 cycles.

1. Introduction Supercapacitors, also called electrochemical capacitors, have received substantial interest from the worldwide scientific community along with batteries and fuel cells for alternative energy storage/conversion applications.1 The advantages of supercapacitors are their high power capability, long cycle lifetime, fast charge and discharge rates, light weight, and environmentally friendly.2-3 The charge storage mechanism of supercapacitors is based on the interfacial electrical double-layer capacitors (EDLCs) with high specific surface area, and based on the pseudocapacitance associated with fast surface redox reactions at some metal oxides/hydroxides, conducting polymers, and heteroatom-doped carbon materials.4-6 Since advance supercapacitors require the development of high performance materials that are able to cope with certain challenges, e.g. low-cost and abundantly available materials, and also a long term stability. One class of materials in whose development and understanding researchers have put strong effort is combining with the electric double layer capacitance and pseudocapacitance performance of carbon materials.6 Therefore, intense efforts have been performed to explore alternative porous heteroatom-doped carbon-based materials to realize a high specific capacity and good cycling ability.


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Carbon-based materials, especially nanostructured three-dimensional (3D) porous carbon materials are being increasingly researched as electrode materials for supercapacitors, because of their not only create the desired high surface area and hierarchical porous channels, but also possess higher electrical conductivity and better structural mechanical stability.7-8 Nevertheless, the properties of porous carbon materials depend to a large extent not only on the raw material and their morphological structure and porosity but also on the heteroatoms built into their surface structures.9 By heteroatom doping the properties are altered compared to crude carbon materials, such as increasing electric conductivity, enhancing capacity and surface wettability of electrode materials.6,

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Nitrogen-doped carbon nanomaterials have been discovered as promising

contenders with regard to their wide availability among heteroatom-doped. Nitrogen-containing functional groups can be introduced carbon skeleton main through either reaction with nitrogencontaining reagents (such as NH3, nitric acid and amines) or direct carbonization of nitrogen-rich carbon precursors, such as polyacrylonitrile, melamine, polyaniline (PANI), and so on.9, 11-12 In comparison with the use of multiple steps and multi-precursors, a single precursor which containing both C and N atom is directly carbonized believed to be more convenient and controllable. Recently, polymers based on aromatic diamines have shown more novel multifunctionality than traditional monoamine polymers, e.g. PANI and polypyrrole (PPy), due to one free amino group per repetitive unit on the polymer chains.13-14 Poly(p-phenyldiamine) (PpPDA) is an typically conjugated polymer of the aromatic diamines family. Its structure is closely related to PANI and therefore encourages its study in electronic devices such as supercapacitors and batteries. Unfortunately, the bare PpPD exhibits a very low specific capacitance (16 F g-1) due to poor electrical conductivity.15 But, it can be selected PpPD as nitrogen-rich carbon precursor due to the polymers based on aromatic diamines and a high percentage of


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carbonaceous residue left after thermal treatment for the proof-of-concept studies. This inspires us to study the high nitrogen content carbon nanomaterials from aromatic diamines polymers as pseudocapacitance electrode material for energy storage applications. To the best of our knowledge, a facile integrated oxidation polymerization and catalytic carbonization strategy to prepare polymer-derived carbon networks have rarely been reported. We expect that the PpPDA derived carbon networks structures will likely reveal interesting properties and applications. 2. Experimental 2.1 Materials p-phenylenediamine monomers (pPDA, Sinopharm Chemical Reagent Co., Ltd, China), Ferric chloride (FeCl3, Shanghai Chemical Works, China). All chemical reagents were in analytical grade. All solutions were prepared in deionized water. 2.2. Synthesis of three-dimensional porous nitrogen-doped carbon networks (3D N-CNWs) PpPDA was synthesized by in situ chemical oxidative polymerization method. In a typical process, pPDA monomer (5 mmol) was dissolved in 25 mL of deionized water with magnetic stirring for 30 min in an ice bath. And then, the resulting solution was cooled below 5 oC in an ice bath and an aqueous solution of FeCl3 (15 mmol in 10 mL of deionized water) cooled in advance was added drop-by-drop into the above solution. The polymerization reaction was carried out with magnetic stirring at below 5 oC for 12 h. Finally, the black mixed solution product was transferred to the porcelain boat and the excess water was evaporated in oven (80 o

C) to form a carbon precursor solid powder.

For synthesis of N-CNWs, the as-synthesized carbon precursor mixtures were carbonized under high pure N2 atmosphere at different carbonization temperature (500 ~ 900 °C) with a heating rate of 5 °C min-1, and maintained for 2 h at ultimate temperature. After cooling down to room


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temperature, the product was treated in HCl (2 mol L-1) to remove inorganic salts, followed by washing with deionized water repeatedly until neutral pH and dried at 60 oC in ambient for 24 h. The obtained samples were denoted by N-CNWs-x (where x is the different carbonization temperature: 500 ~ 900 °C). For comparison purpose, the as-synthesized PpPDA was first thoroughly washed with distilled water to remove FeCl3 oxidant and dried at 60 oC in ambient for 24 h. Finally, the pure PpPDA solid was carbonized at 700 °C for 2 h with a heating rate of 5 oC min-1 in a slow N2 flow. The obtained material was denoted as NC-700. 2.3 Materials Characterization The morphologies of the products were examined with field emission scanning electron microscopy (FE-SEM, Carl Zeiss Ultra Plus, Germany) at an accelerating voltage of 5.0 kV. The structure of the samples was characterized by a transmission electron microscopy (TEM, JEM2010 Japan). The X-ray powder diffraction (XRD) was performed on a diffractometer (D/Max2400, Rigaku). X-ray photoelectron spectroscopy (XPS) measurement was performed on an Escalab 210 system (Germany). The Brunauer-Emmett-Teller (BET) surface area (SBET) of the powders was analyzed by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus (U.S.A.). All samples were degassed at 200 °C prior to nitrogen adsorption measurements. The elemental microanalysis (C, H and N) was carried out using the Elemental Analyzer Vario EL. 2.4 Three-electrode fabrication For conventional three-electrode system, the glassy carbon electrode with a diameter of 5 mm was used as the working electrode. The working electrodes were fabricated similar to our previous reported literature.16 Typically, 4 mg of N-CNWs was ultrasonically dispersed in 0.4


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mL of 0.25 wt% Naﬁon (DuPont, USA). The above suspension of 8 µL using a pipet gun was dropped onto the glassy carbon electrode and dried at room temperature. The three-electrode system was test in 6 mol L-1 KOH aqueous solutions, carbon rod serves as the counter electrode and Hg/HgO as the reference electrode, respectively. 2.5 Two-electrode cell fabrication The capacitive performance of N-CNWs was further investigated using a two-electrode testing cell. The working electrode was prepared by mixing the N-CNWs-700 with polyvinylidene fluoride (PVDF) and commercial carbon black (8:1:1) in N-methyl-2-pyrrolidone (NMP) until homogeneous slurry. The slurry was coated on nickel foam with a working area of 1.0 cm2 and the electrodes were dried at 120 oC for 12 h and then weighted and pressed into sheets under 15 MPa. The total mass was between 3 and 5 mg of each electrode and two electrodes with identical or very close weight were selected for the measurements. Two as-prepared N-CNWs-700 electrodes fitted with the separator (thin polypropylene film) and electrolyte solution (0.5 mol L-1 Na2SO4 aqueous solutions) were symmetrically assembled into sandwich-type cells construction (electrode/separator/electrode). 2.6 Electrochemical measurements The electrochemical properties of the samples were investigated by cyclic voltammetry (CV), galvanostatic

charge/discharge

and

electrochemical

impedance

spectroscopy

(EIS)

measurements in three-electrode and two-electrode cell using a CHI 760E electrochemical workstation (Shanghai Chenghua instrument Co., Ltd, China). The cycle-life stability was performed using computer controlled cycling equipment (LAND CT2001A, Wuhan China). The EIS measurements frequency ranging from 10 mHz to 100 kHz and an impedance amplitude of ±5 mV at open circuit potential.


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3. Results and Discussion 3.1. Microstructure characterizations The facile integrated oxidation polymerization and catalytic carbonization method to prepare 3D N-CNWs, as described in Scheme 1. Firstly, the PpPDA was synthesized by in situ chemical oxidative polymerization method. And then, the as-synthesized mixed solution product was evaporated in oven form a carbon precursor solid powder. Finally, the PpPDA carbon precursor and iron compound mixture were carbonized under a N2 atmosphere form 3D N-CNWs. The obtained samples were denoted by N-CNWs-x (where x is the different carbonization temperature: 500 ~ 900 °C). In synthesis process, the FeCl3 not only serves as an oxidant for oxidative polymerization of pPDA monomers but also as the carbonization catalyst to promote porous carbon networks formation. For comparison purpose, pure PpPDA as carbon precursor without iron compound catalyst was also carbonized under a N2 atmosphere at 700 °C for 2 h, and the obtained material was denoted as NC-700.

Scheme 1. Schematic of the preparation process of 3D N-CNWs materials. Figure 1 shows the typical morphology of the NC-700 and N-CNWs-700 products. The FESEM image of NC-700 exhibits large lumps and no obvious porous morphology (Figure 1a). However, as shown in Figure 1b, the N-CNWs-700 is of 3D interconnected net-like structure which composes of many macroporous structure of carbon networks intertwined together. This


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interesting structure was further confirmed by transmission electron microscopy (TEM), as shown in Figure 1c-d. The N-CNWSs-700 sample exhibits a thin net-like morphology with highly interconnected soft wrinkles and folding simultaneously viewed on the surface. In addition to the many macroporous structure, some layer architectures are decorated with disordered meso/micropores also can be observed under higher magnifications (Figure 1d), which structure would play an important role in charge accommodation. The structural characters are seriously affected by the carbonization temperature. Figure 2 shows the microstructure morphology of all the N-CNWs materials prepared at different carbonization temperature. It is found that the lower of the carbonization temperature, the less porous structure in carbon architectures. For instance, the carbon monoliths have unconspicuous pore structure and a very rough surface when the carbonization temperature is 500 oC (N-CNWs-500, see Figure 2a). As the carbonization temperature increase to 600 oC (N-CNWs-600, Figure 2b), the 3D carbon networks structure gradually appear, but the pore space between them is still tightly stacked. The pore size of N-CNWs-600 is about dozens of nanometers. In contrast, the porous carbon networks material was characteristic of 3D developed void space structure and aperture increased when the carbonization temperature increased to 700 oC (N-CNWs-700, Figure 2c-d). In addition, the pore size ranges of N-CNWs-700 from a few dozen nanometers to a micrometer. Similarly, when the carbonization temperature continues to increase, the N-CNWs-800 and NCNWs-900 have interconnected 3D porous network with integrated and sub-micrometer pores, as shown in Figure 2e and Figure 2f, respectively. Those experimental phenomena demonstrate that the shape-controlled synthesis of carbon networks could be achieved by adjusting the carbonization temperature. In addition, the interconnected 3D porous carbon networks would provide a unique open-pore system and a short diffusion path for electrolyte ions, which is


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conducive

to

improve

their

electrochemical

performance.

Figure 1. (a) FE-SEM images of the NC-700; (b) FE-SEM images of the N-CNWs-700; (c-d) TEM images of the N-CNWs-700 under different magnifications.


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Figure 2. FE-SEM images of N-CNWs prepared at different carbonization temperature: (a) 500 o

C; (b) 600 oC; (c-d) 700 oC; (e) 800 oC; (f) 900 oC.


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In order to more clearly understand the catalytic carbonization process, we have carried out the X-ray powder diffraction (XRD), thermogravimetric analysis and differential scanning calorimetry (TGA and DSC) to analyze the changes of materials in different stages. The XRD patterns of the pure PpPDA, NC-700, before and after carbonized the PpPDA with Fe species sample, and acid treatment carbide (N-CNWs-700) are shown in Figure S1 (Supporting Information). The XRD pattern confirmed the mainly formation of rokuhnite and iron oxide in before and after carbonized PpPDA hybrids sample. Remarkably, an obvious diffraction peak could be identified at 44.5° in the after carbonized PpPDA hybrids sample, indicating that formation of Fe (JCPDS no. 06-0696). On the basis of the results, we conclude that the Fe species first formation of cementite (Fe3C) mesophase under the lower carbonized temperature and the following exuded process (from cementite to Fe and C) at the higher temperature, which plays a significant part for the formation of porous network structure.17 The TGA and DSC characterization of the PpPDA with or without Fe species in N2 are shown in Figure S2 (Supporting Information). In the TGA curve of pure PpPDA, the initial 7.4% weight loss within 100 oC shows evaporation of the entrapped water. The weight loss of 35% between 135 oC to 700 oC indicates the gradual thermal decomposition of the polymer skeleton. For the PpPDA with Fe species sample, the weight losses about 15% at temperatures within 160 oC may be attributed to loss of adsorbed water and the loss of water of crystallization on rokuhnite. As the carbonization temperature rise, the thermal decomposition process including the formation of Fe3C at relatively lower temperatures and their subsequent transformation to Fe at the higher temperature.18 Every stage reactions occurring of the PpPDA with Fe species sample also can be detected during each endothermic peak in the DSC curve (Figure S2b, Supporting Information).


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To further investigate their porosity, the nitrogen sorption isotherms were measured to determine the pore structure and the Brunauere-Emmette-Teller (BET) surface areas of all the N-CNWs (Figure 3). The nitrogen adsorption-desorption isotherms of N-CNWs prepared in different carbonization temperature are of type IV with a H3 hysteresis loop in the range of ca. 0.45-1.0 P/P0 as shown in Figure 3a, which suggests the presence of mesoporous structures.19 Furthermore, sharp increases in volume at low relative pressure (P/P0 < 0.4), especially the NCNWs prepared in high carbonization temperature (700 oC and 800 oC) indicate the presence of obvious micropores. A remarkable increase in the amount of adsorbed N2 at relative pressures P/P0 above 0.90 can be attributed to the N2 adsorption in very large cavities, i.e. void spaces and/or macropores above 50 nm in size.20 Obviously, as the carbonization temperature increases, the adsorbed volume of N-CNWs increases clearly. Therefore, it is reasonable to conclude that the BET specific surface area increases with increasing the carbonization temperature. More information related to pore size distribution and pore volume were obtained using data evaluation by the Barrett-Joyner-Halenda (BJH) method. The BJH pore size distribution of of NCNWs prepared in different carbonization temperature are shown in Figure 3b. The low temperature carbonization sample (N-CNWs-500 and N-CNWs-600) showed low porosity having a very low N2 sorption pore volume, while higher temperature carbonization samples (NCNWs-700 and N-CNWs-800) showed noticeable higher pore volumes and a broad pore size distribution in the range of 2-200 nm. A futher analysis of the results for the estimation of the pore-size distribution based on the density functional theory (DFT) is given (Figure 3c). As is shown in the picture, the DFT pore size distribution of N-CNWs-700 exhibits a hierarchical pore structure that mainly made up of micropores (1.1 nm) and mesopores (2.1 nm). Elemental analysis, BET surface area and pore structure characterization parameters of 3D N-CNWs from


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different carbonization temperature are summarized in Table 1. It is found that the structural characters are seriously affected by the carbonization temperature. The nitrogen contents of NCNWs reduced from 12.6 to 2.4 wt% when the carbonization temperature increased from 500 to 900 oC. It’s worth noting that with the carbonization temperature increased from 700 to 800 oC, the nitrogen content is reduced almost half. In spite of this, the nitrogen contents of N-CNWs700 remained as high as 8.4 wt%. The surface composition of the N-CNWs-700 sample is further studied by XPS (Figure 3d). There are three peaks at around 285.8, 401.3 and 532.8 eV that correspond to the C1s peak of sp2 carbon, the N1s peak of the doped nitrogen and the O1s spectrum, respectively. According to the high resolution N1s spectra (Figure S3, Supporting Information), four types of N-containing groups could be revealed on the surface of N-CNWs700, including pyridinic-N (N-6, 398.2 eV), pyrrolic-N (N-5, 399.5 eV), quaternary-N (N-Q, 401.0 eV), and pyridine-N-oxide (N-X, 403.2 eV), respectively.21 As is seen in Table 1, the NCNWs show specific surface area (SBET) are in the range from 559 to 1513 m2 g-1 and total pore volume are in the range from 0.35 to 1.13 cm3 g-1 during the carbonized temperature increased from 500 to 800 oC. The result reveals that the nitrogen contents of N-CNWs decrease and the specific surface areas of N-CNWs increase gradually owing to the volatilization of heteroatoms to form the micropores when increase of carbonization temperature.12 However, the micropore surface area (Smic) of N-CNWs-800 slightly decreases, and this is may be the collapse of some miropores and mesopores and the aggregation of carbon networks at high carbonization temperature. In addition, as the carbonization temperature is increased up to 900 oC, the specific surface area and pore volume of N-CNWs-900 were obviously reduced, which may be caused by the collapse and stack of some carbon network skeleton. The large specific surface area and


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appropriately porous structure are advantageous to quick mass charge transfer and ion diffusion, consequently, leading to higher capacitance storage and rate capability.22

Figure 3. (a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution curves of N-CNWs prepared at different carbonization temperature, (c) DFT pore size distribution of NCNWs-700, (d) X-ray photoelectron spectroscopy (XPS) of N-CNWs-700.

Table 1. Elemental analysis, BET surface area and pore structure characterization parameters of 3D N-CNWs from different carbonization temperature.


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Elemental analysis

SBETa

Smicb

Dc

Vtotald

C%

N%

H%

(m2 g-1)

(m2 g-1)

(nm)

(cm3 g-1)

N-CNWs-500

74.1

12.6

3.4

559

444

2.48

0.35

N-CNWs-600

76.3

10.8

3.3

969

783

2.46

0.60

N-CNWs-700

77.4

8.4

2.8

1442

862

2.55

0.85

N-CNWs-800

78.7

4.3

2.3

1513

746

2.98

1.13

N-CNWs-900

79.3

2.4

1.7

1326

728

2.52

0.83

Samples

a) Specific surface area determined according to BET (Brunauer-Emmett-Teller) method. b) Micropore surface area from t-plot method. c) Adsorption average pore diameter. d) Total pore volume.

3.2. Electrochemical behavior and characterization The electrochemical performance of 3D N-CNWs from different carbonization temperature is first evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge tests in a threeelectrode mode (Figure 4). Figure 4a displays representative CVs of the NC-700 and N-CNWs electrodes prepared at different carbonization temperature at a scan rate of 25 mV s-1 in 6 M KOH solution. The non-catalytic carbonized NC-700 sample exhibits a small irregular rectangular curve corresponding to low capacitances due to its no obvious porous structure. Similarly, the N-CNWs-600 exhibit irregular rectangular and small CV area. In addition, the NCNWs-600 has a obvious hump peaks at lower potential indicate that the capacitive response comes from the combination of EDLC and redox reactions, which related to the high nitrogen content (about 10.8 wt%) of this material. However, low graphitic degree and low specific surface area are lead to low specific capacitance. In contrast, the N-CNWs-700 electrode exhibit near rectangular shape and has the biggest CV area in all of the materials, indicating a largest


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specific capacitance, because of the linear relation between specific capacitance and CV curve area. High specific capacitance of N-CNWs-700 was claimed due to the combined effect of high nitrogen content and large specific surface area. When the carbonization temperature continues to increase, the N-CNWs-800 exhibit ideal rectangular shape but has the smaller CV area than NCNWs-700, which could be attributed to the lowered nitrogen functional groups at the high carbonization temperature.13 It is known that mesopores contribute a lot of capacitance in an carbon-based supercapacitor.23 In addition, recent experimental and theoretical studies have demonstrated that the materials with pore size about 1 nm also have excellent charge storage ability.24-25 The N-CNWs-700 with large capacitance also can be attributed to the large specific surface area and a large number of microporous and mesoporous. Therefore, N-CNWs-700 with 8.4 wt% nitrogen content was characterized and used for supercapacitor applications unless otherwise specified in this work. Figure 4b presents CV curves for the N-CNWs-700 electrode at various scan rates. The CV curve still remains a rectangular-like shape even the scan rate increases to as high as 200 mV s-1, which result indicates that the N-CNWs-700 electrode possesses a balanced specific surface area and nitrogen species, exhibiting excellent electrochemical behavior and good rate capability. The galvanostatic charge/discharge curves of N-CNWs-700 electrodes at various current densities are shown in Figure 4c. All discharge curves are almost symmetrical to the corresponding charge curves, indicating good electrochemical reversibility of this electrode. The correlation between the specific capacitance and the various current densities for different electrodes is presented in Figure 4d. The gravimetric capacitance from galvanostatic charge/discharge was calculated by using the formula of Cs*=I∆t/(m∆V) for the three-electrode system, where I is the constant current (A) and m is the mass (g) of electrode material, ∆t the discharge time and ∆V the voltage


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change during the discharge process. Obviously, the N-CNWs-700 has the highest specific capacitance among all electrodes at the same current densities, which results are consistent with the CV tests. Furthermore, the specific capacitance of N-CNWs-700 as high as 304 F g-1 at a current density of 0.5 A g-1 and 226 F g-1 even at a high current density of 20 A g-1 (about 74% capacitance retention). The results indicate a high capacitance can be maintained under high current density. The specific capacitance of NC-700, N-CNWs-600 and N-CNWs-800, by contrast, just has 202 F g-1, 257 F g-1 and 248 F g-1 at a current density of 0.5 A g-1, respectively.

Figure 4. (a) CV curves of NC-700 and N-CNWs electrodes prepared at different carbonization temperature at a scan rate of 25 mV s-1 in 6 M KOH solution; (b) CV curves of N-CNWs-700 electrode at different scan rates. (c) Galvanostatic charge/discharge curves of N-CNWs-700


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electrode at various current densities; (d) Discharge capacitances of NC-700 and N-CNWs electrodes at various current densities. To further understand the superior performance of N-CNWs-700, the two-electrode symmetric supercapacitor was also fabricated. It is reported that the neutral Na2SO4 aqueous electrolyte possesses a higher operation voltage than that of acid and alkali solutions.19 Since the energy density (E) is usually limited to the device capacitance (C) and the operating voltage (V) according to the equation E = 1/2CV2. Therefore, the N-CNWs-700 symmetric supercapacitor was assembled with 0.5 M Na2SO4 aqueous solution electrolyte. The as-assembled symmetric cell was first measured at different potential windows from 1.0 to 2.0 V at 20 mV s-1, and the resulted CV curves are exhibited in Figure 5a. The CV curves of the supercapacitor are semirectangular shaped even when the high voltage extends to 1.8 V, indicating ideal capacitive behavior and good reversibility. However, when the voltage increases to 2.0 V, the current is dramatically increased since the electrolyte is being decomposed with hydrogen and/or oxygen evolution.26 Therefore, the wide potential window of 1.8 V is chosen to further investigate the overall electrochemical performance of the symmetric cell. The CV curves of the symmetric cell measured at different scan rates of 10~100 mV s-1 at potential window of 1.8 V are displayed in Figure 5b. CV curve area increases with increasing scan rate, but no obvious change in shape even at a high scan rate of 200 mV s-1. This result suggesting the symmetric cell possesses excellent rate capability. The CV curves of the NCNWs-700 symmetric cell greatly deviated from rectangular shape, probably incurred by higher contents of N-containing functional groups on the surfaces of the electrodes and it can be thought of as Faradaic reactions take place during the sweep processes.27 The galvanostatic charge/discharge curves of N-CNWs-700 symmetric cell at various current densities are shown


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in Figure 5c. The non ideal linearity in the charge and discharge curves particularly at lower current density indicates some contribution of the redox reaction from nitrogen doped in carbon skeleton, which is in agreement with the results of the CV curves. The Ragone plot of the device describing the relationship between energy density and power density was obtained and is shown in Figure 5d. The specific energy density (E, Wh kg-1) and power density (P, W kg-1) for a supercapacitor cell were calculated from the discharge curves at different current densities (Figure 5c) using the following equations: E=1/2CV2 and P=E/t, where C is the specific capacitance of supercapacitor cell, V is voltage change during the discharge process after IR drop in V-t curve, and t is the discharge time. It is obvious that the N-CNWs-700 symmetric cell exhibits the highest energy density of 15.8 Wh kg-1 with a power density of 450 W kg-1 and remained 7.8 Wh kg-1 at 9145 W kg-1. Moreover, the obtained maximum energy density is considerably higher than those of recently reported symmetric cells, such as AC//AC (10 Wh Kg1 28

) , MC//MC (7.84 Wh Kg-1)29, MCSF//MCSF (9.6 Wh Kg-1)30 phosphorus-rich carbon

symmetric cell (13 Wh Kg-1)31. The cycling stability test of the N-CNWs-700 symmetric cell was performed at a current density of 3 A g-1 for 5000 galvanostatic charge/discharge cycles, as depicted in Figure 5e. It can be seen that the specific capacitance increases slightly before the initial 500 cycles, which can be interpreted as a result of the improvement of surface wetting between the electrode and the electrolyte during the charge/discharge process.32 Subsequently, the specific capacitance decreases slightly and remains about 97% of the maximum capacitance after 5000 cycles, meaning that the electrode has good electrochemical stability and a high degree of reversibility. The galvanostatic charge/discharge curves of N-CNWs-700 symmetric cell at the first cycle, 500th cycle and 5000th cycle are shown in the inset of Figure 5e. Apparently, the symmetric cell


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can achieve maximum charge and discharge time after 500 cycles, which has the maximum specific capacitance.

Figure 5. (a) CV curves of the N-CNWs-700 symmetric two-electrode cell at different voltage windows in 0.5 M Na2SO4 aqueous electrolytes, (b) CV curves of the N-CNWs-700 symmetric cell at various scan rates, (c) Galvanostatic charge/discharge curves of N-CNWs-700 symmetric


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cell at various current densities, (d) Ragone plot related to energy and power densities of the NCNWs-700 symmetric cell, (e) Cycling stability of N-CNWs-700 symmetric cell, (f) Nyquist plots of two-electrode symmetric cell base on N-CNWs-700 electrode (the inset of modeled equivalent circuit of EIS). Electrochemical impedance spectroscopy (EIS) analysis was used to gain a deep insight into the resistive and capacitive behavior of symmetric cell. Figure 5f shows Nyquist plot of asymmetric cell with the small semicircle in the high-frequency region and the greater than 45o vertical curve in the low-frequency region, which results indicating a low charge-transfer resistance in the electrochemical system and a pronounced capacitive behavior with small diffusion resistance, respectively.33 The Nyquist plots obtained are modeled and interpreted with the help of an appropriate electric equivalent circuit (the inset of Figure 5f), where Rs stands for a combined resistance of ionic resistance of the electrolyte, intrinsic resistance of the substrate and contact resistance at the active material/current collector interface, Rct is the charge transfer resistance caused by the Faradaic reaction. The slope of the 45o portion of the curve is called the Warburg resistance (W) and is a result of the frequency dependence of ion diffusion/transport in the electrolyte to the electrode surface, CL is the limit capacitance. The N-CNWs-700 symmetric cell not only has the low inner resistance (Rs, 1.706 Ω cm2) calculated from the point of intersecting with the x-axis in the high frequency region, but also possesses a small interfacial charge transfer resistance (Rct, 2.154 Ω cm2), counting from the span of the single semi-circle along the x-axis from high to low frequency region.34 After 5000 cycles, the resistance of N-CNWs-700 symmetric cell is slightly larger than initial cycle, but it also has excellent capacitance behavior due to the Nyquist plot similar inclines to the x-axis in the low-frequency region.


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4. Conclusion In summary, three-dimensional porous nitrogen-doped carbon networks (3D N-CNWs) is synthesized by integrated oxidation polymerization and catalytic carbonization method directly from p-phenylenediamine monomer. Moreover, it should be mentioned that the iron compound is play an integrated role of oxidant and carbonization catalyst. The as-synthesized N-CNWs with interconnected porous framework architecture, high specific surface area and high nitrogen content can be providing fast electron transfer throughout the electrode. As a result, the 3D NCNWs delivers a high specific capacitance of 304 F g-1 and excellent rate capability. Moreover, the as-assembled N-CNWs symmetric supercapacitor device with an operation voltage of 1.8 V in 0.5 M Na2SO4 aqueous electrolyte delivers a high energy density of 15.8 Wh kg-1, and excellent cycling performance (only 3% capacitance loss after 5000 cycles). Therefore, it is a promising strategy to synthesize novel 3D N-CNWs electrode materials with low-cost and ecofriendly for high-performance energy storage applications. Supporting Information Available: XRD patterns of the as-synthesized materials in different stages. TGA and DSC curves of the pure PpPDA and PpPDA with Fe species under N2. High resolution N1s spectra of the N-CNWs-700. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: Guofu Ma, [email protected]; Ziqiang Lei, [email protected].


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was financially supported by the National Science Foundation of China (NO.21164009, 21174114), the program for Changjiang Scholars and Innovative Research Team in University (IRT1177), the Colleges and Universities Scientific Research Program of Gansu Province (2013B-069), Key Laboratory of Eco-Environment-Related Polymer Materials (Northwest Normal University) of Ministry of Education, and Key Laboratory of Polymer Materials of Gansu Province.

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TOC graphic


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Derived Three-Dimensional Porous Nitrogen-Doped Carbon

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