Facile Synthesis of Highly Electrocapacitive Nitrogen-Doped Graphitic

Article pubs.acs.org/JPCC

Facile Synthesis of Highly Electrocapacitive Nitrogen-Doped Graphitic Porous Carbons Kyung Taek Cho,† Sang Bok Lee,‡ and Jae W. Lee*,† †

Department of Chemical and Biomolecular Engineering, KAIST, Daejeon 305-701, Korea Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States

‡

S Supporting Information *

ABSTRACT: The present study introduces a facile method to produce nitrogen-doped graphitic porous carbons (NGPCs) with balanced pore size distribution (0.5−5 nm) and high surface area (above 3000 m2 g−1) by utilizing polyacrylonitrile (PAN) as a precursor. The NGPCs are obtained by the stabilization of PAN at 290 °C and subsequent carbonization with KOH activation at 700−800 °C. It is striking that the activation also changes the morphology of PAN from a grape-like structure to a smooth porous structure accompanying partial layer formations of nitrogen-doped graphene. Generation of evenly distributed micro- and mesopores with high specific surface areas, heteroatom doping on the surface of NGPCs, and low resistance of NGPCs contribute to the enhanced capacitance as supercapacitor electrodes.

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multistep procedures,17,19 and specific alignments of equipment.12 In this study, polyacrylonitrile (PAN) has been employed as a carbon precursor due to its low cost and outstanding physical, mechanical, and electrical properties after thermal stabilization at controlled temperatures.20 For example, PAN has been a useful raw material for the preparation of carbon fibers21 and has also been exploited to produce nanofibers by electrospinning techniques,20,22,23 ordered porous carbons prepared by SBA-15,24,25 and porous carbon by copolymer synthesis routes.26,27 However, this study addresses a simple and reliable method for synthesizing N-doped graphitic porous carbons (NGPCs) directly from PAN by proposing a two-step process of thermal stabilization and subsequent KOH activation. Scheme 1 illustrates that PAN is transformed into NGPCs by undergoing the two-step treatment. In the first step, PAN is stabilized and configured to the ladder structure by cyclization and dehydrogenation.21 Then, the stabilized PAN is carbonized to generate an aromatic ladder-like graphitic plane, simultaneously with the KOH activation leading to the increased surface area,28 at the higher temperature than that of the stabilization step. In contrast to the earlier approaches, this method not only is reproducible but also does not need any extra step to incorporate nitrogen into carbon structures. This work first attempted to elucidate how the morphologies and pore size distributions are controlled while creating high specific surface areas and graphene layers in converting PAN to NGPCs by utilizing mass spectrometry and microscopic

INTRODUCTION Carbon materials have widely been used in many fields as industrial materials. Recently, carbons also have attracted public attention as raw materials for fibers, catalysts, and energy devices such as the electrode of batteries, fuel cells, supercapacitors, and solar cells.1 Among various carbon materials, graphene, which retains a two-dimensional sheet structure consisting of carbon atoms monolayer, is considered to have a great potential in that it shows incomparable electrical conductivity and high specific surface area.2,3 For these reasons, much effort has been made to utilize the graphene in various fields.4 Notwithstanding, the enhancement of electrical conductivity for a large size of graphene at low costs is limited in the current commercial applications.5 On the other hand, porous carbons have also been used for industrial applications because of its low cost and wide availability.6 However, the performance of porous carbons has been limited due to their poor controllability of specific surface area and pore size distribution. Doping heteroatoms, such as boron, nitrogen, oxygen, etc., into carbon frames is one of the promising ways to tune their chemical and electrical properties for improving gas adsorption capacity,7 electrochemical activity,8 or electrocatalytic properties.9 Particularly, as doping nitrogen into carbon materials is effective for these applications, the considerable research interest has been generated.1 To date, there are a variety of procedures to prepare nitrogen-doped carbon materials, for example, using polymers containing nitrogen precusors,10,11 nitrogen plasma process,12 chemical vapor deposition,13−15 and the others.16 However, most of these methods require expensive resources like conducting polymers,17 hazardous precursors such as ammonia gas,13,18 © 2014 American Chemical Society

Received: February 18, 2014 Revised: April 9, 2014 Published: April 14, 2014 9357

dx.doi.org/10.1021/jp501742x | J. Phys. Chem. C 2014, 118, 9357−9367

The Journal of Physical Chemistry C

Article

Scheme 1. Process of Preparing NGPCs and Graphitic Structure of the NGPCs

performed using FLASH 2000 series (for C, H, N, and S) and FlashEA 1112 (for O). Mass spectrometric analyses were carried out by a quadrupole mass spectrometer (QMS 200, Pfeiffer Vacuum), connecting with the outlet of the furnace when annealing the stabilized PAN, to detect the evolution of exhausted gas during the carbonization and activation. X-ray photoelectron spectroscopy (XPS) was employed with a Sigma probe (Thermo VG Scientific) to analyze the chemical structures of NGPCs. The XPS peaks were fitted by using Avantage software (Thermo VG package), in which fitting the peaks was completed by using Gaussian (70%) and Lorentzian (30%) curve fitting procedure, and binding energy was corrected with reference to C 1s at 284.5 eV. Device Fabrication of the Supercapacitor. After removing unused KOH and impurities from the activated carbons, the NGPCs were dried in a vacuum oven and were used for electrodes of the supercapacitor. After NGPCs were mixed with PTFE (60 wt % dispersion in water), a binder, (NGPC/PTFE = 9:1 in weight) together with mortar and pestle, a prepared slurry (about 4 mg) was embedded on a nickel foam that is used as a current collector (surface area: 1 cm × 1 cm). The nickel foam pasted with the slurry was pressed and dried in a vacuum oven at 120 °C. Electrochemical Investigation. For three-electrode cell experiments, a calomel electrode was used as a reference electrode and platinum wire as the counter electrode in a 6 M KOH aqueous solution as the electrolyte. Cyclic voltammetry and galvanostatic charge−discharge data were obtained with a CHI instrument (6002D model, USA) at various sweep rates. A gravimetric capacitance was calculated from galvanostatic charge−discharge curves in a three-electrode cell by using eq 1

observations. Among various applications, we will attest that NGPCs can be excellent electrode materials of supercapacitors which are emerging as an alternative energy storage system.29,30 Since N-doping provides extra electrons to graphitic planes in NGPCs, the electrical properties and interfacial adhesion with ions in the electrolyte can be improved cooperatively. In addition to that, possessing the high surface area and porosity will show that the NGPCs retain unprecedentedly high capacitance.

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EXPERIMENTAL SECTION Materials. Polyacrylonitrile (average Mw 150 000), KOH, and PTFE (60 wt % dispersion in water) were purchased from Sigma-Aldrich (Korea). HCl was commercially available from OCI Chemicals (Korea). All chemicals were used as received without further treatments. Preparation of Nitrogen-Doped Graphitic Porous Carbons. Typically, 1−2 g of PAN was stabilized at 290 °C for 1 h in an air atmosphere (100 mL/min) by heating up at a rate of 2 °C/min. After the thermal treatment, the stabilized PAN was crushed and mechanically mixed with KOH at a mass ratio of KOH to PAN equal to 3. Then, the mixture of KOH and PAN was carbonized at the target temperature (700−800 °C) for 1 or 2 h under an argon gas flow (50 mL/min). The resultant porous carbon was transferred to a glass vial, and HCl (10 wt %) was added to remove KOH and other impurities and then rinsed with hot distilled water continuously until reaching neutral pH. Finally, the end product was dried in a vacuum oven at 120 °C. Characterization. The Brunauer−Emmett−Teller (BET) specific surface area and the pore size distribution (PSD) were obtained by nitrogen adsorption at 77 K with three-flex surface characterization (Micromeritics). Prior to the measurement, the samples remained at 300 °C for 3 h under vacuum. The microstructural characterization was monitored by scanning electron microscope (SEM, a Magellan 400 UHR-SEM) at 1−2 kV. The Raman spectroscopy of NGPCs was performed by an ARAMIS (Horiba Jobin Yvon) with Ar ion 514.5 nm laser excitation, and the X-ray diffraction (XRD) was conducted on D/MAX-2500 with Cu Kα (λ = 0.15418 nm), set at 40 kV and 300 mA. Transmission electron microscopy (TEM) was carried out on a Tecnai G2 F30, and elemental analysis (EA) was

Cg =

I Δt mΔV

(1)

where I is the constant current density, Δt is the discharging time, m is the mass of only carbon material used for the working electrode, and ΔV is the discharging voltage change. The discharge voltage window included the IR drop due to the difficulty in identifying the IR drop region. The Nyquist plot was determined by electrochemical impedance spectroscopy (EIS), which was carried out using Biologic SAS VSP, with a sinusoidal signal of 10 mV over the frequency ranging from 100 9358



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Figure 1. Linear plot of nitrogen sorption isotherm and PSD of NGPCs and p-PAN. (a) Nitrogen sorption isotherms of p-PAN (pyrolyzed at 800 °C) and all NGPCs activated at 700, 750, and 800 °C for 1 h and at 700 °C for 2 h. (b) PSD of p-PAN and all the NGPC, which is based on a DFT model from nitrogen adsorption−desorption data.

centered at 0.6 nm. Thus, the KOH activation increases the SSA of carbon materials by generating specific micropores (size of 0.8 and 1.17 nm) and mesopores (size of 2−5 nm), while preserving the original micropores (0.6 nm) in p-PAN. Moreover, all of the KOH activated samples have BET SSA higher than 3000 m2/g with a tendency that the specific surface area enlarges as the activation temperature or time increases (Table 1). While the activation temperature level affects the

kHz to 0.05 Hz. All electrochemical measurements were conducted at room temperature.

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RESULTS AND DISCUSSION Nitrogen adsorption/desorption analyses based on BET31 and density functional theory (DFT) were conducted to obtain specific surface area (SSA) and pore size characterization of the NGPCs prepared at various activation temperatures and periods. The results from the isotherms of NGPCs, as shown in Figure 1a, reveal the influence of the activation temperature and duration on their specific surface area and porosity. To be specific, the NGPC-700 °C/1 h sample activated at 700 °C for 1 h follows a type 1 isotherm with a small portion of hysteresis 4 type, according to the IUPAC classification, which is the characteristic of microporous materials.32 Indeed, the microporosity is confirmed in the pore size distribution (Figure 1b), where micropores centered at smaller than 2 nm are generated. On the other hand, when the activation temperature increases to 750 and 800 °C, the sorption isotherms change to type 2 and hysteresis 2 type (Figure 1a). These changes are correlated with growing pore size, which can be identified in Figure 1b where the amount of mesopores centered at 3 nm increases with the elevated activation temperature, whereas the micropores centered at 0.6, 0.8, and 1.5 nm become diminished. It is obvious that the micropores are widened gradually, merged with each other, and transitioned into mesopores.33,34 To determine whether the transformation is ascribed to the activation temperature or duration, the carbon sample was also prepared at 700 °C for 2 h. The linear plot of nitrogen isotherm at the NGPC-700 °C/2 h also exhibits type 1, which is the comparable pattern of NGPC-700 °C/1 h in that it is saturated at the relative pressure of ∼0.4 and shows hysteresis 4 type. In Figure 1b, the PSD of NGPCs does not make difference between 1 and 2 h, indicating that the PSD of the NGPCs is not much affected by the activation time but by the temperature. Finally, these PSD results of the NGPCs are totally different from the PSD of p-PAN (pyrolyzed at 800 °C) that did not undergo the KOH activation and whose micropores are only

Table 1. Specific Surface Area and Chemical Composition of Carbon Materials chemical composition (wt %) NGPCs 700 °C/1 700 °C/2 750 °C/1 800 °C/1 p-PAN

h h h h

SBET (m2/g)

O

N

C

H

3034.8 3384.3 3124.4 3336.7 430.6

11.91 11.64 3.93 3.53 8.82

2.53 2.78 1.95 0.84 14.51

84.53 84.60 93.68 91.35 74.28

0.81 0.97 0.43 0.23 1.46

PSD, both temperature level and duration at the KOH activation are dominant variables for improving the SSA. Thus, we obtained these NGPCs with high SSAs and even distribution of micro- and mesopores by the simple KOH activation of the solid mixture of the stabilized PAN and KOH. The PSD and SSA of NGPCs are comparable to those of previous porous carbons synthesized using complicated routes, such as electrospinning,23 synthesis of block copolymer,27 mixing graphene with a polymer, and using silica as a template.35−37 The KOH activation has primarily been used to improve the porosity and enlarge the surface area. However, we find that the KOH activation also affects the morphology of PAN. The SEM images (Figure 2) identify the transformation of morphologies. Originally, the pristine PAN used as a precursor of NGPCs, as portrayed in Figure 2a, is a grape structure consisting of small uneven agglomerated particles. However, after the carbonization with the KOH activation, the structural feature of pristine PAN changes from grapes to sponge or cheese, as 9359



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Figure 2. SEM image of (a) pristine PAN before stabilization, (b) NGPC activated at 750 °C for 1 h, and (c) NGPC activated at 750 °C for 1 h at the same magnification as in (a).

Figure 3. (a) Raman spectrum and (b) XRD profiles of NGPCs and p-PAN.

shown in Figure 2b,c, which display the morphology of NGPC750 °C/1 h. All of the activated samples, regardless of activation temperatures, also exhibit analogous shapes characterized as smooth surfaces and many pores in contrast to PANs (for additional pictures, refer to Figure S1 in Supporting Information). The grapelike morphology is no longer found in Figure 2c, even with the same magnification of the pristine PAN (Figure 2a). In addition, the structure of p-PAN is similar to that of the pristine PAN (Figure 2a), but the surface of pPAN is more even (Figure S1e,f in Supporting Information). According to the difference of morphologies between PANs and NGPCs, the KOH activation process reconstructed the structure of PANs completely, which is integral to obtain the uniform N-doped porous carbons. The smooth solid structure of NGPCs implies that initial powder PAN transformed to graphitic structure.38,39 Therefore, nitrogen atoms could be present at the carbon graphitic layer or on the surface (Scheme 1). We used Raman spectroscopy to determine the structural characterization of these carbon samples (Figure 3a). The broad peaks around ∼1350 cm−1 (D band), ∼1590 cm−1 (G band), and 2500−3300 cm−1 (2D, D′, G′, and other bands) are found for all of the NGPCs, indicating that the pristine PAN was converted to amorphous carbons with nanographitic structures.40 The crystallite size of the graphite domains can be determined by the intensity ratio of G to D bands (ID/IG).41,42 The size of NGPCs was calculated as ∼5 nm (refer to Table S1 in Supporting Information). The intensity ratio in NGPCs remained almost constant compared to the p-PAN case, suggesting that the KOH activation did not prevent PAN from changing to the graphitic carbon with the enlarged crystallite size. Moreover, the broad Bragg peaks on XRD patterns imply that these porous carbons possess the graphitic structures in sp2

carbon materials (Figure 3b). The 002 major peak centered at around 24° and the 100 minor peak centered at around 44° were observed for all of the carbon materials. Their average interlayer distance (0.363 nm) is similar to that of typical graphite (0.34 nm),43 indicating that, on the average, a few layers of nitrogen-doped graphene are stacked despite of the introduction of nitrogen atoms into graphitic carbons. The crude lateral size of graphitic domains could be estimated from the full width at the half-height of diffraction peaks by using Scherrer’s equation, based on the 002 peak. Compared with the p-PAN, the NGPCs show weak and broad peaks indicating lower domain heights. Thus, the KOH activation contributed to deterring from packing nitrogen-doped graphene layers over a certain level. These graphene layers were identified through high-resolution transmission electron microscopy (HRTEM) of NGPC-750 °C/1 h, as shown in Figure 4, which revealed graphite ribbons and crystalline impurities of carbon dots on the surface of the NGPC. The results of elemental analysis for the activated carbons are presented in Table 1. As the activation temperature increases, the nitrogen contents of NGPCs decrease from 2.6 wt % for the NGPC activated at 700 °C to 0.84 wt % for that activated at 800 °C. Furthermore, the samples activated with KOH contain less nitrogen than the p-PAN pyrolyzed at 800 °C. It is evident that the nitrogen content is substantially influenced by the presence of KOH during the activation. The tendency of the diminishing nitrogen content with the increased KOH activation temperature has been shown in previous works.23,27,44 When carbon materials are activated with KOH, the generation of pores is generally attributed to the removal of carbon through the release of carbon oxide gases during the reaction of carbon with potassium. After the activation, the 9360



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nitrogen in the NGPCs prepared at 750 and 800 °C for 1 h are lower than the nitrogen content in the NGPC prepared at 700 °C with a longer time of 2 h, indicating that the activation temperature plays a more important role in the diminution of nitrogen than the activation time. In an effort to elucidate the surface characteristic of the NGPCs, we investigated the nature of nitrogen and carbon bonds in XPS. The N 1s and C 1s core level spectra of the NGPCs at temperatures of 700, 750, and 800 °C are displayed in Figure 6, where the peaks of each nitrogen and carbon were deconvoluted by using Gaussian−Lorentzian curve fitting procedure and the full width at half-maximum (fwhm) was confined to 2.0 eV. The fine structure of the nitrogen can be analyzed by differentiating their binding energies in the N 1s peak in XPS which points to the presence of three major bonds (Figure 6a). The bonds of these nitrogen atoms appear to suggest their location along the outer edges of nanographitic domains, which would explain their electrochemical characteristics. When activated at 700 °C, two intense peaks at 398.1 and 400.7 eV and two small peaks at 401.5 and 403−405 eV can be distinguished, which correspond to N-6, N-5, N-Q, and N-PO, respectively (for each N-bonding structure, refer to Figure S2 in Supporting Information). In contrast, the others produced at 750 and 800 °C show three major peaks at 398.1, 400.7, and 401.5 eV. The portions of each N-configuration in four different samples are summarized in Table 2. From these data in Table 2, the following appealing points can be emphasized by considering the different N bonds in the carbon lattice. To begin with, the declination of total nitrogen contents, as increasing activation temperature, obtained from the XPS results is in agreement with the elemental analysis in Table 1. When a closer look at the N-6 portion that is mostly present in the stabilized PAN is taken (Scheme 1), it reduces continuously on the NGPC samples as the activation time or temperature increases, but the N-Q portion escalates while the total amount of nitrogen decreases. This observation suggests that the KOH activation induces the defects at the edge of the stabilized PAN to transform the nitrogen at the edge (N-6) to the nitrogen enclosed by the carbon lattice (N-Q).46 These Nconfiguration changes are expected to improve the electrical conductivity because the increasing N-Q portion has a positive effect on the conductivity of carbon materials.12 The N-PO functional groups are not observed for the samples activated at higher than 700 °C. Finally, as the activation period moves up from 1 to 2 h at 700 °C, N-configurations also show the increase in the N-Q portion, whereas there is little difference in the total nitrogen content. Thus, depending on the activation temperature, the nitrogen-bonding structure and the amount of nitrogen contents vary, whereas depending on the activation time, only the bonding structure changes. As mentioned earlier, one of the applications using NGPCs can be an electrode for supercapacitors. Porous carbons have widely been used in this application due to their high SSA and conductivity, which are important factors in improving the capacitance because, in the electric double-layer capacitor (EDLC) system, these are directly correlated to the amount of storing ions on the surface of the electrode. Besides, in the case of NGPCs, the presence of nitrogen in the graphitic carbon is expected to create a synergistic effect on the surface, such as pseudocapacitance or faradaic reaction, leading to larger capacitance and higher conductivity than pristine carbons. Again, activated porous carbons with a large SSA so far have been employed as electrodes due to the low cost and relatively

Figure 4. HRTEM image of NGPC activated at 750 °C for 1 h: this image shows carbon dots all over the region, while the arrows indicate the partial structure of graphitic layers.

remaining KOH salts and intercalated K are removed by washing with aqueous acid or water, adding more pores.28 However, the N-doped carbons including the NGPCs lose their nitrogen even if the atom does not react with KOH directly. In general, KOH reacts with the carbon atom at high annealing temperatures (6KOH + 2C → 2K + 3H2 + 2K2CO3).45 The drastic diminishment of nitrogen contents in the NGPCs could be supported by the idea that the reaction between carbon and KOH actively takes place at carbons connected with nitrogen atoms. This is because the potassium ion is strongly attracted to various nitrogen functional groups12 and then the KOH reaction with carbons bonded to nitrogen atoms more easily occurs. This can be confirmed with the mass spectrometry technique (Figure 5). Pyrolysis mass spectra of the samples were recorded when the stabilized PANs were heated both with and without the KOH activation. Originally, the PAN expels various volatile byproducts during the carbonization.21 Among those gases, water (H2O), nitrogen gas (N2), and hydrogen cyanide (HCN) are major gases that evolve from the graphitization of PAN (refer to Scheme S1b in Supporting Information). In the 0−64 amu range in QMS signals, the values of m/z = 28, 18, and 17 in the left figures (Figure 5a,c,e,g,i) represent N2, H2O, and −OH (or NH3). These gases are commonly generated for pPAN and all of the NGPCs. However, the release behavior of other gases is totally different between p-PAN (Figure 5b) and NGPCs activated with KOH (Figure 5d,f,h,j). For the p-PAN, it emitted two major gases, the values of m/z = 27 and 44, which are designated to HCN and CO2. In case of NGPCs, HCN and CO2 were not detected and the evolution of the different byproducts, the values of m/z = 16 and 15, were dominant. These atomic masses represent methane-related gases (CH4 and +CH3), which were not formed in the case of p-PAN. In addition, the generation of N2 and water increased. KOH deters HCN from arising by reacting with the carbon of the CN in the stabilized PAN because KOH is more attracted to the nitrogen atoms. As a result, the KOH activation broke the cyclization of the stabilized PAN and led to the enhanced loss of nitrogen atoms through the release of methane and nitrogen gases. During the KOH activation, the amount of water increases because KOH is dehydrated by transforming into K2O (2KOH → K2O + H2O).28 Moreover, the amounts of 9361



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Figure 5. Quadruple mass spectroscopy (QMS) signals during the KOH acitvation of stabilized PAN. Right figures (b,d,f,h,j) are enlarged ones of left figures around m/z = 17. The intensity unit is ionic current. (a,b) p-PAN, (c,d) NGPC-700 °C/1 h, (e,f) NGPC-700 °C/2 h, (g,h) NGPC-75 0°C/1 h, (i,j) NGPC-800 °C/1 h.

controversial issue.34,47−49 A combination of macro-, meso-, and micropores as appropriate portions is desirable due to the functions of each pore: (i) The presence of macropores helps to reserve ions of electrolyte, and the distance between electrode surface and external electrolyte will be shortened.35

easier synthesis of large quantities. However, there are still limitations on producing the ACs with well-ordered porosity and removing impurities, negatively affecting the capacitance and conductivity. In particular, optimizing the pore size, structure, and distribution of activated carbons is becoming a 9362



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Figure 6. XPS of (a) N 1s and (b) C 1s with their deconvolutions for the NGPC-700 °C/1 h, 700 °C/2 h, 750 °C/1 h, and 800 °C/1 h.

Table 2. Distributions of Nitrogen Configurations and Total Nitrogen Amount Measured by XPS 700 700 750 800

°C/1 °C/2 °C/1 °C/1

h h h h

N-6/N(%)

N-5/N(%)

N-Q/N(%)

N-PO/N(%)

total N (atom %)

50.19 44.97 38.46 17.46

45.08 34.21 31.62 53.97

2.75 15.69 29.91 28.57

1.97 5.11 0 0

5.08 5.67 2.34 0.63

(ii) The mesopores can enhance transportation of ions. (iii) The micropores, together with mesopores, can store much higher capacitance by having pore sizes matched with ion size in electrolyte, regarding the charge accommodation.34 As illustrated in Figures 1b and 2b, the NGPCs have even distributions of micro-, meso-, and macropores which can improve the capacitive performance. Specific capacitance of NGPCs was estimated from cyclic voltammetry (CV) at 20 mV/s to determine the influence of

activation temperature on the electrochemical performance (Figure 7a). The CV shows that each capacitance of samples is different with respect to the activation temperature. At NGPC800 °C/1 h, the shape of the CV is nearly rectangular, unlike the samples activated at the lower temperatures, which indicates that the sample has good conductivity and becomes ideal EDLC as activated at the higher temperature. On the other hand, the other samples show deviations from a rectangular shape and a small hump is observed in midvoltage. 9363



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Figure 7. Electrochemical characterization of supercapacitors consisted of NGPCs: (a) CV curves of different samples with a potential window from −1 to 0 V; (b) galvanostatic charge−discharge curves at 1 A/g in 6 mol L−1 KOH; (c) specific capacitance at different current density ranging from 1 to 10 A/g; (d) cyclability at current density of 10 A/g over 10 000 cycles.

as ππ* disappeared when activated at the higher temperature level, whereas the portion of CO almost was doubled. These two groups perform differently in enhancing capacitance. The O−CO group can be deduced as CO2-type oxygen group, and the CO can be evolved as CO-type group through temperature-programmed desorption. The latter group was discovered as having a more positive contribution to the capacitance.51,52 Based on this account, it is identified that the NGPC activated at 700 °C would be affected by O−CO and the CO groups are supposed to assist NGPC-750 °C/1 h and 800 °C/1 h to form electrical double layers. When the current density increases to 10 A/g, the specific capacitance of the samples decreases (Figure 7c), but the performance for the cells with NGPCs also presents a competitive stability. After 10 000 galvanostatic charge− discharge cycles of the NGPC-750 °C/1 h at a very high current density of 10 A/g, the capacitance retention remains 89.5% of the initial capacitance (Figure 7d). To further understand the capacitive and electrochemical behavior of the samples, EIS was used with a sinusoidal signal of 10 mV over the frequency range from 100 kHz to 0.05 Hz. Nyquist plots of all the NGPCs are displayed in Figure 8. The first intersection with the real axis (x) of the Nyquist plot represents the equivalent series resistance (ESR) which is attributed to the resistance of the electrolyte and the internal resistance of the electrode including the activated sample and current collector. The NGPCs show a relatively low ESR at high frequencies,

Deviating from the rectangular shape can be explained as the presence of the pseudocapacitive behavior coming from the exposure of nitrogen atoms on the surface of the electrode to solution.50 To estimate the accurate capacitance, the galvanostatic charge−discharge experiments were carried out. Figure 7b displays the charge−discharge curves of NGPCs, and the specific capacitances are 189, 219, 270, and 247 F/g for NGPC700 °C/1 h, 700 °C/2 h, 750 °C/1 h, and 800 °C/1 h at a current density of 1 A/g, indicating that the NGPC-750 °C/1 h retains the highest capacitance. The higher SSA of the NGPC700 °C/2 h results in having higher capacitance than the NGPC-700 °C/1 h. However, the capacitance of NGPC-800 °C/1 h is lower than that of NGPC-750 °C/1 h, although the SSA of NGPC-800 °C/1 h is higher than that of NGPC-750 °C/1 h. This is due to the fact that the large amount of nitrogen groups in the NGPC-750 °C/1 h has a positive effect on enhancing capacitance.12,17,23 The presence of the nitrogen groups may induce faradaic current and surface polarity intensifying the double-layer formation. On the contrary, the result that the capacitance of the NGPC activated at 700 °C for 2 h is lower than that of NGPC-800 °C/ 1 h, although the SSA of the former is a little higher than that of the latter, cannot be explained by the amount of nitrogen like the previous argument because the former sample has more nitrogen than the latter. Thus, we can clarify this by using the XPS results. In C 1s spectra (Figure 6b), the O−CO as well 9364



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identified by analyzing the gas phases during the carbonization. As the electrode of supercapacitors, the NGPCs exhibit good electrochemical performance, such as high capacity, low resistance, and good stability. The NGPCs can be attractive for other applications in adsorbents, catalysts, and other energy conversion/storage materials because the PAN is less expensive compared to other conductive polymer precursors and the procedure proposed here for synthesizing the NGPCs is very economical and reliable.

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ASSOCIATED CONTENT

S Supporting Information *

A detailed scheme of stabilization and carbonization of PAN, additional SEM images of carbon samples, Raman peak parameters, and equivalent circuit parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 8. Nyquist plots of NGPC-700 °C/1 h, 700 °C/2 h, 750 °C/1 h, and 800 °C/1 h using a sinusoidal signal of 10 mV over the frequency range from 100 kHz to 0.05 Hz.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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indicating that the carbon materials have good conductivities by transforming from a normal polymer, PAN, to graphitic carbons. Since the ESR is reduced for the NGPCs activated at the higher temperature and with the longer activation time, it is clear that the carbonization decreases resistance by generating more ordered graphitic structure. In addition, the plots of all the electrodes do not exhibit the 45° Warburg region, which is originally located between the semicircle and the long tail and is related to the ion diffusion inside the electrodes. The absence of this region indicates that the ion transport into the electrode pores is facilitated. Fitting the impedance data to the equivalent circuit was carried out for the more detailed analysis (refer to Figure S3 and Table S2 in Supporting Information).53 The charge transfer resistance (RCT), which represents the resistance of the charges at interfaces between electrolyte and electrode, is low because of the well-developed porosity after the KOH activation. A more vertical straight line at the low frequency region was evident for the higher activation temperature than for the lower temperature, which means that the NGPCs activated at higher temperatures show the ideal capacitive behavior. Additionally, the pore shape of the carbon electrode can be derived from the Nyquist plot.54 According to this derivation, the impedance curves with a sharp transition peak in the midfrequency between semicircle and a long tail represent that the pores of NGPCs consist of narrow necks and wide bodies.

ACKNOWLEDGMENTS The authors are grateful for the financial support from the Korea CCS R&D Center funded by the Ministry of Science, ICT, and Future Planning (No. NRF-2013M1MA8A1040703).

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REFERENCES

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CONCLUSIONS Nitrogen-doped graphitic porous carbons have been prepared through a facile two-step method of stabilization of polyacrylonitrile at a lower temperature and KOH activation at a higher temperature. Utilizing different activation temperatures and periods, the surface area and pore size distribution of NGPCs can be controlled. Thus, these N-doped carbon materials exhibit considerably large specific surface area higher than 3000 m2/g with an appropriate combination of specific micro-, meso-, and macropores. In addition, after the KOH activation, the polymer precursors are transformed into the nitrogendoped carbons partially stacked with graphene layers. The KOH activation effects on the amounts of nitrogen were also 9365



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fibers containing carbon nanotubes. J. Power Sources 2008, 185, 676− 684.

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Facile Synthesis of Highly Electrocapacitive Nitrogen-Doped Graphitic

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