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Nitrogen and sulfur co-doped nanoporous carbon material derived from p-nitrobenzenamine within several minutes and the supercapacitor application
Accepted Manuscript Nitrogen and sulfur co-doped nanoporous carbon material derived from p-nitrobenzenamine within several minutes and the supercapacitor application Hai Tao Yi, Yan Qi Zhu, Xiang Ying Chen, Zhong Jie Zhang PII: DOI: Reference:
S0925-8388(15)00848-8 http://dx.doi.org/10.1016/j.jallcom.2015.03.135 JALCOM 33746
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
13 December 2014 15 March 2015 17 March 2015
Please cite this article as: H.T. Yi, Y.Q. Zhu, X.Y. Chen, Z.J. Zhang, Nitrogen and sulfur co-doped nanoporous carbon material derived from p-nitrobenzenamine within several minutes and the supercapacitor application, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/10.1016/j.jallcom.2015.03.135
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Nitrogen and sulfur co-doped nanoporous carbon material derived from p-nitrobenzenamine within several minutes and the supercapacitor application Hai Tao Yi1, Yan Qi Zhu1, Xiang Ying Chen1 *, and Zhong Jie Zhang2 ** 1
School of Chemistry and Chemical Engineering, Anhui Key Laboratory of
Controllable Chemistry Reaction & Material Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, P R China. *Corresponding author: Fax: +86 551 62901450. E-mail address: [email protected]. 2
College of Chemistry & Chemical Engineering, Anhui Province Key Laboratory of
Environment-friendly Polymer Materials, Anhui University, Hefei 230039, Anhui, P. R. China. ** Also the corresponding author. E-mail: [email protected]. Abstract Nanoporous carbon materials co-doped with both nitrogen and sulfur have been synthesized via a sulfuric acid-assisted straightforward carbonization method, in which p-nitrobenzenamine serves as both carbon and nitrogen source, whereas sulfuric acid as sulfur source and catalyst. It is revealed that the nanoporous carbon material named as carbon-RT takes on amorphous features with low crystallinity. More importantly, it possesses high nitrogen content (15.95 %) and sulfur content (3.36 %) in carbon matrix. And the synthesis time is so short that the carbon-RT sample could be obtained within just a few minutes. To improve the electrochemical performance of material, the carbon-RT sample has been further heated at 800 °C, resulting in the carbon-800 sample. As a result, it has delivered a high specific capacitance of 73 F g−1 at the current density of 1 A g–1 when measured in a three-electrode system, using 6 mol L–1 KOH as electrolyte. The present synthesis method has been conveniently implemented to produce nanoporous carbon materials co-doped with high content of nitrogen and sulfur, using inexpensive starting materials, thereby it has broad prospects and potentials for producing carbon materials in a simple and scalable manner. 1
Keywords: p-Nitrobenzenamine; Carbonization; N-S codoping; Nanoporous carbon; Supercapacitor.
1. Introduction In recent years, various carbon materials have received widespread attention due to their remarkable properties including high electrical conductivity, chemical inertia, thermostability, environmental friendliness and low manufacturing cost [1, 2]. These properties combined make them attractive for a wide range of potential applications such as adsorption [3], catalysis [4], energy conversion and storage [5], and so on. Among these studies, there has been growing interest in doping heteroatom into carbon which could effective enhance intrinsic or add new features to materials. Generally, there are two major approaches to accomplish the doping of heteroatom. One is by pyrolyzing heteroatom-containing precursors directly [6, 7], and the other is by post-treating carbons with dopants [8, 9]. The doping candidates have been reported including nitrogen, sulfur, boron and phosphorus etc [10]. In comparison to other elements, nitrogen is easier to be doped into carbon materials. The incorporation of nitrogen profoundly impacts the pore structures, surface states, hydrophilicities and electron-transfer rates of materials, and consequently leading to a broader application prospect [11]. For example, Sun et al. synthesized a nitrogen-containing porous graphitic carbon through a simple coordination-pyrolysis combination process, in which glucose, melamine, tetraethyl orthosilicate and nickel nitrate were adopted as carbon source, nitrogen source and graphitic catalyst, respectively. The as derived nitrogen-containing carbon has a high content of nitrogen up to 7.72 wt.% and exhibits a specific capacitances of 293 F g–1 [12]. And Wang et al. prepared mescoporous carbon with high nitrogen content of 11.3 wt.% by pyrolysis of lysine and melamine using ferric chloride as dopant and SiO2 nanoparticles as hard templates. The nitrogen-rich mesoporous carbon presents better electrocatalytic activity, better durability and higher methanol tolerance for oxygen reduction reaction than commercial Pt/C [13]. 2
Compared with that of the nitrogen doping, the researches about carbon materials doped with sulfur are relatively rare. But those researches have confirmed that the doping of sulfur has unique effects on carbon materials including the inducing of bond polarization, lattice distortion and tailoring of charge density distribution, etc [14]. As a consequence, S-doped carbon materials have great advantages over pure or N-doped carbon materials when applying to many fields, like hydrogen storage, CO2 capture, supercapacitors, and so on. For instance, Park et al. synthesized sulfur-doped graphene by a thermal treatment with induced CS2 gas as sulfur source. The resultant sulfur-doped graphene shows higher electrocatalytic activity with larger limiting current density and durability than those of commercial Pt/C catalyst and N-graphene [15]. Seredych et al. obtained a series of sulfur-doped carbons by carbonizing poly (sodium 4-styrene sulfonate) and grapheme oxide. It has also proved that the incorporation of sulfur can enhance the adsorption capacity of carbon materials to CO2 via acid-base interactions and polar interactions [16]. Apart from the carbon materials individually doped with nitrogen or sulphur, some researchers have also focused on the design of N-S-codoped carbons, since the synergistic effect of heteroatoms can further improve materials performance in certain areas. For example, Zhang et al. successfully synthesized mesoporous graphene containing nitrogen (2.12 %) and sulphur (1.70 %) via self-polymerization of polydopamine thin layer on grapheme oxide sheets, followed by reacting with cysteine and thermal treatment. The as synthesized material exhibits significantly enhanced performance as electrocatalyst for ORR than pristine graphene or nitrogen-doped graphene [17]. Sun et al. fabricated graphene-like microspheres doped with 4.6 wt.% of nitrogen and 1.5 wt.% of sulfur using Ni microspheres as the template and poly(vinylpyrrolidone) and (NH4)2S2O8 as the carbon and nitrogen precursor, respectively. The resulting materials exhibit superior capacity with excellent cycling stability as anode materials for lithium ion batteries [18]. Although some excellent nitrogen and sulfur co-doped carbon materials have been prepared in recent year, most of the synthesis methods involve complicated process or expensive starting materials. Thus it is still significant for material scientists to explore more simple and economic 3
methods to produce carbon materials co-doped with high content of nitrogen and sulfur. Herein, we demonstrate a novel and rapid method to produce nanoporous carbon materials co-doped with nitrogen and sulfur. p-Nitroaniline is an organic compound consisting of a benzene ring substituted with a nitro group and an amino group. The phenyl group acts as carbon source when amino group and nitro group as nitrogen source. H2SO4 can trigger the explosive polymerization of p-nitroaniline and induce sulfur into the nanoporous carbon materials. Alcohol blow lamp is selected as a convenient heat source which can generate a flame up to 800 °C higher than the melting point (148.5 °C) and flash point (165.0 °C) of p-nitroaniline.
2. Experimental 2.1 Synthetic method In this work, a sulfuric acid-assisted straightforward carbonization method has been implemented to synthesize nanoporous carbon materials co-doped with both nitrogen and sulfur, in which p-nitroaniline serves as carbon/nitrogen sources, and sulfuric acid as sulfur source and catalyst. The typical synthetic method can be depicted as follows: p-nitroaniline (0.8 g) was first placed in a crucible, followed by the addition of concentrated sulfuric acid (~0.3 mL, 98%), and then the crucible was heated with an alcohol blow lamp (800~900 °C). After ~2 min, black and cylindrical nanoporous carbon materials (the diameter of ~3 cm, and the length of ~15 cm) suddenly happened. The resultant materials were washed by sufficient deionized water and dried at 80 °C for 4 h in an electric oven to obtain the carbon-RT sample.
(Fig. 1)
2.2 Structure characterization X-ray diffraction (XRD) patterns were obtained on a Rigaku D/MAX2500V with 4
Cu Kα radiation. Raman spectra were recorded at ambient temperature on a Spex 1403 Raman spectrometer with an argon-ion laser at an excitation wavelength of 514.5 nm. X-ray photoelectron spectra (XPS) were recorded on a VG ESCALAB MK II X-ray photoelectron spectrometer with an exciting source of Mg Kα (1253.6 eV). High-resolution transmission electron microscope (HRTEM) images and selected area electron diffraction (SAED) patterns were performed with a JEM-2100F unit. Fourier Transform infrared spectroscopy (FTIR) absorption spectrum of selected samples was obtained on a Nicolet 6700 FTIR spectrometer. The specific surface area and pore structure of the carbon samples were determined by N2 adsorption-desorption isotherms at 77 K (Quantachrome Autosorb-iQ) after being vacuum-dried at 150 °C overnight. Cumulative pore volume and pore size distribution were calculated by using a slit/cylindrical nonlocal density functional theory (NLDFT) model.
2.3 Electrochemical characterization The electrochemical performance of samples were evaluated on a CHI660D electrochemical working station ( ChenHua Instruments Co. Ltd., Shanghai) in a three electrode experimental setup at room temperature with 6 mol L−1 KOH aqueous solution as electrolyte. The working electrodes were fabricated by mixing carbon sample (~4.8 mg), acetylene black and polytetrafluoroethylene (PTFE) binder with a mass ratio of 80:15:5 using ethanol as a solvent. The resulting slurry was pressed on foamed Ni grids, followed by evaporating the solvent in a vacuum drying oven at 120 ºC for 24 h. Platinum slice (~6 cm2) and a saturated calomel electrode (SCE) were chosen as counter and reference electrodes, respectively. From the galvanostatic charge–discharge tests, the specific capacitances per mass of active electrode materials were derived according to the equation: C=
I∆t m∆V
Where C (F g−1) is the specific capacitance; I (A) is the discharge current; ∆t (s) is the discharge time; ∆V (V) is the potential window; and m (g) is the mass of active materials loaded in working electrode.
5
3. Results and discussion XRD technique was performed to investigate the component, crystallinity and purity of the carbon sample. As shown in Fig. 2a, the broad diffraction peak at ca. 23.3° corresponds to the (002) plane of hexagonal graphite (JCPDS, Card No. 74-2329) [19]. The broadness and low intensity in shapes reveal that the sample has amorphous or low crystallinity features [20, 21]. Compared with the standard 2 theta value of graphite (26.6°), the present one has largely shifted to lower direction, suggesting the interlayers between (002) planes of the present carbons are largely expanded [19]. Intrinsic features of the carbon-RT sample were also determined by Raman technique as well as the deconvoluted ones by OriginPro Lorentzian method. As shown in Fig. 2b, the characteristic D and G bands of carbon materials are observed at around 1378.4 and 1592.5 cm–1, respectively. The D band thanks to the crystal defects and disordered structures in carbon materials, and the G band is attributed to graphite in-plane vibrations with E2g symmetry [22]. The integral area ratios of D band and G band of the samples, indexed as ID/IG, is also calculated as 3.71, suggesting that the sample are highly disordered, which is consistent with the XRD results. The
parameters of the graphitic in-plane microcrystallite size, La can be estimated as 1.19 nm. A weak band is observed between 600 cm−1 and 800 cm−1 presumably derives from the vibration of sulfur-containing groups [16]. The porosity and pore structure of the carbon-RT sample were investigated by N2
adsorption/desorption
technique.
Fig.
2c
displays
the
typical
N2
adsorption-desorption isotherm of the carbon-RT sample with a relative pressure range of 0 ~ 1.0 (P/P0). The N2 adsorption-desorption profile can be categorized into type-IV according to the classification of international union of pure and applied chemistry. The sharp rise in the low pressure stage (<0.3 P/P0) indicates the presence of microporosity; the hysteresis in the pressure range (0.45-1.0 P/P) reveals the capillary condensation in mesopores and the almost vertical tail at the high pressure (~ 1.0 P/P0) corresponds to macropores [23, 24]. The total BET surface area is 366.8 m2 g−1, and the total pore volume is 0.27 cm3 g−1. The pore size distribution of carbon-RT
6
sample is presented in Fig. 3d which confirms the multiple pore structures, and the average pore width is 6.82 nm.
(Fig. 2)
The morphology and structure of the carbon-RT sample were investigated by HRTEM technique. As shown in Fig. 3a and b, the carbon presents irregular form with a size of hundreds of nanometers. Fig. 3c and d are magnified HRTEM images which reveal the existence of micropores. The lattice spacings are highly disordered in nature, suggesting its amorphous feature. It is supported by the SAED pattern with vague diffraction rings, as shown in the inset of Fig. 4d. The result is also in good accordance with the XRD pattern and Raman spectrum. And the low crystallinity is probably caused by relatively short carbonization time.
(Fig. 3)
The elemental composition, content and chemical state were analyzed by XPS technique. The XPS survey spectrum in Fig. 4a shows a dominant C1s peak (~284.9 eV), an O1s peak (~531.2 eV), a N1s peak (~399.3 eV), and two S peaks (~167.7 eV, ~231.1 eV), which confirm the presence of elemental S and N in the carbon-RT sample. The contents of N and S elements in the carbon-RT sample are calculated to be 15.95% and 3.36%, respectively. Fig. 4b displays the C1s spectrum as well as three convoluted peaks. The peaks at 284.3 and 285.5 eV are assignable to the bonds of sp2 C=C and sp3 C–C, respectively. With respect to the one at 287.9 eV, it owes to the C–O/C–N bond [25, 26]. Fig. 4c shows the high resolution spectrum of N1s for the carbon-RT sample which can be resolved into four parts centered at 398.9 eV, 399. 8 eV , 400.7 eV and 406.0 eV. Those peaks can be assigned to pyridine nitrogen (N-6), pyridone/pyrrolic nitrogen (N-5), quaternary nitrogen (N-Q) and nitro nitrogen atoms, respectively [27-29]. As shown in Fig. 4d, the XPS spectra for the O 1s signals can be fitted into three peaks 7
located at 530.7 eV, 531.8 eV and 533.2 eV, corresponding to –C=O, C=O/S=O and O–C/O–S, respectively [30, 31]. Regarding the high resolution spectrum of S 2p, it primarily can be deconvoluted into three component peaks. The peak at 164.0 eV agrees with the 2p position of the –C–S– covalent bond of the thiophene-S [32]. In addition, the peak at 167.5 eV originates from –C–SO2– [33, 34], while the peak at 168.7 eV from the –C–SO3– [15].
(Fig. 4)
Infrared spectroscopy was used to ascertain the existence forms of heteroatom within the carbon-RT sample. As shown in Fig. 5, the peaks at 3180 and 3330 cm–1 are assigned to the overlap of O–H and N–H [35]. The peak at 1598 cm–1 is attributed to the stretching vibration of N–H groups [35]. The peaks at 1497 and 1303 cm–1 may be due to asymmetric NO2 stretching and symmetric NO2 stretching [36]. The peak at 1190 cm–1 corresponds to the C–O, C–N, and C–S [35]. The peaks at 1190, 1034 and 750 cm–1 can be indexed to S=O asymmetric stretching, S=O symmetric stretching and C–O–S stretching, respectively [36]. Those sulfur-containing bonds also confirm the existence of sulfone and sulfonic acid, which is consistent with the results of XPS. The peak at 839 cm–1 may be due to C–O vibrations [36]. The peak at 630 cm–1 corresponds to the carbon backbone framework [15].
(Fig. 5)
(Table 1)
Based on our characterization results and relevant literatures, the possible mechanism is proposed for the formation of the carbon-RT sample as follows: As temperature rises, 4-nitrobenzenamine starts to melt (~150 °C), decompose (~160 °C) and polymerize (~200 °C) [37, 38]. Both decomposition and polymerization reaction generate gaseous products and heat which might be aggravated along with the 8
temperature increase. And after being heated for ~2 min, a volume of gas is generated in an instant, causing the materials rapidly expanding and porous structures appearing. At the same time, the materials are carbonized with the assistance of concentrated sulfuric acid, primarily serving as catalyst. Although subsequently some of the nitrogen atoms may get out of the system as gas products like N2 etc, high content of nitrogen is still doped into the materials successfully as pyridine nitrogen, pyridone/pyrrolic nitrogen and quaternary nitrogen. Also, sulfur atoms derived from sulfuric acid can be doped into the materials mainly as sulfur-containing functional groups [39, 40]. In order to evaluate the electrochemical performances of the carbon-800 sample, it was measured in a symmetric three-electrode system using 6 mol L−1 KOH aqueous solution as electrolyte. However, the results reveal this the carbon sample prepared by one-step method, as detailedly illustrated above, possesses unsatisfied electrochemical performances. Thus, the carbon sample was further heated in a horizontal tube furnace up to 800 ºC at a rate of 4 ºC min–1 and maintained at this temperature for 2 h under N2 flow, obtaining the carbon-800 sample. Then, its crystallinity, structure, and components are characterized by XPS and Raman and N2 adsorption/desorption techniques. Also, the electrochemical performances are also investigated in the same three-electrode system. Fig. 2a shows the XPS spectra of the carbon-800 sample, which reveal the existence of carbon (~285.1 eV), oxygen (~532.2 eV), nitrogen (~399.3eV) and sulfur (~164.3 eV, ~228.3 eV) atoms. Compared with the carbon-RT sample, the content of carbon and nitrogen increase to 76.03 at.% and 6.26 at.%, while oxygen and sulfur content fall up to 16.63 at.% and 1.08 at.%. As previously mentioned, nitrogen atoms mainly exist as structural nitrogen which has higher thermal stability than sulfur and oxygen containing surface functional groups like –SO3– etc. The losses of oxygen and sulfur atoms increase the relative content of nitrogen without additional incorporation [25]. The Raman spectrum of the carbon-800 sample is illustrated in Fig. 6b. The sample take on two distinct Raman peaks of 1349.2 cm−1 (D band) and 1596.8 cm−1 (G band). The relative ratios of the D band to the G band (ID/IG) of the carbon-800 9
sample are 2.38. As a result, the La value is 1.85 nm which is bigger than that of the carbon-RT sample (1.19 nm). Apparently, the improvement of carbonization can lead to an increase in crystallite size [41]. The N2 adsorption-desorption isotherm of the carbon-800 sample is shown in Fig. 6c which also can be categorized into type-IV with a BET surface area of 464.3 m2 g−1, total pore volume of 0.42 cm3 g−1. The pore size distribution presented in Fig. 6d indicates the carbon-800 sample have well-developed micropores and mesopores with an average pore width of 4.74 nm. In summary, after further thermal treatment at 800 ºC, the sample presents slight increasing in BET surface area and total pore volume and decreasing in average pore diameter.
(Fig. 6)
Fig. 7a shows the cyclic voltammetry (CV) curves scanned at different rates of the carbon-800 sample. It is seen that the carbon electrode retains as quasi rectangular shapes at low scan rates of 10 and 20 mV s–1, while as distorted shapes at high scan rates between 50 to 400 mV s–1. Meanwhile, no evident redox peaks can be observed at all scan rates. It is believed that the primary contribution of the capacitances is probably due to the electric double-layer capacitance (EDLC) which arises from charge separation at the electrode/electrolyte interface [42, 43]. The traditional galvanostatic charge–discharge (GCD) curves between –1 to 0 V at the current density ranging from 1 ~ 5 A g–1 are illustrated in Fig. 7b. All these GCD curves present good triangular shapes, suggesting that the capacitances are mainly from the double-layer energy storage mechanism with no obvious contribution of the pseudocapacitance. As seen in Fig. 7c, the carbon-800 sample achieves a specific capacitance of 73 F g−1 at the current density of 1 A g–1. Then the specific capacitance notably decreases along with the increase of current density, mainly because of the insufficient ion diffusion at high current densities. Comparison of the specific capacitance of the carbon-800 sample with some literature data is presented in Table 2. Due to the high content of nitrogen and sulfur, the carbon-800 sample shows better electrochemical performance 10
than most of other carbon materials whose specific surface areas are similar to it. Fig. 7d shows the cycling stability of the carbon-800 sample based on three-electrode setup at a current density of 1 A g–1. The sample still retains 91.78% of its original capacitance after charging–discharging for 2000 cycles. The material shows favorable cycling stability and reversibility which is meaningful for practical applications. To further identify the exact electrical conductivity of electrodes, the carbon-800 sample was measured by using Nyquist plots before/after 200 cycles, as show in Fig. 6e and Fig. 6f. Both of the two Nyquist plots exhibit two distinct parts, including a semi-circle loop in the high frequency region and a nearly vertical line in the low frequency region [48-50]. Apparently, the semi-circle loops become a little bit bigger after 200 cycles, indicating the ion diffusion and electrolyte permeation into the pore channels become harder.
(Fig. 7)
(Table. 2)
4. Conclusions In this study, we have demonstrated the synthesis of nanoporous carbon materials co-doped with high content of nitrogen and sulfur via a sulfuric acid-assisted straightforward carbonization method. The obtained carbon materials have been characterized by XRD, HRTEM, Raman, XPS, N2 adsorption/desorption and FTIR techniques. Besides, we also investigated carbon materials electrochemical performance in a three-electrode system using 6 mol L−1 KOH as electrolyte. We believe this approach has several scientific advantages for the synthesis of the nitrogen 11
and sulfur co-doped nanoporous carbon: a) The starting materials are commercially available and relatively inexpensive, showing potentials for large scale applications; b) The synthetic process involved is convenient and simple, since the carbonization and doping of nitrogen and sulfur can accomplish in the same time; c) The synthesis time is so short that nitrogen and sulfur co-doped nanoporous carbon materials can be obtained in a few minutes; d) The nanoporous carbon materials display high doping content of nitrogen (15.95 %) and sulfur (3.36 %).
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Figure Captions: Fig. 1. Schematic route showing the production of nanoporous carbon materials co-doped with both nitrogen and sulfur. Fig. 2. The carbon-RT sample: (a) XRD pattern; (b) Raman spectrum; (c) N2 adsorption-desorption isotherm; (d) Cumulative pore volume and pore size distribution curves (calculated by using a slit/cylindrical NLDFT model). Fig. 3. The carbon-RT sample: HRTEM images as well as the magnified SAED pattern (the inset) Fig. 4 XPS spectra of the carbon-RT sample: (a) survey, (b) C1s, (c) N1s, (d) O1s and (e) S2p; (f) summary of carbon/oxygen/nitrogen/sulfur contents. Fig. 5 FTIR spectrum of the carbon-RT sample. Fig. 6. The carbon-800 sample：(a) XPS survey spectrum; (b) Raman spectrum; (c) N2 adsorption-desorption isotherm; (d) Cumulative pore volume and pore size distribution curves (calculated by using a slit/cylindrical NLDFT model). Fig. 7 The carbon-800 sample： (a) CV curves; (b) GCD curves; (c) specific capacitances calculated from GCD curves; (e) Nyquist plots before/after 200 cycles as well as the magnified ones (f).
16
Fig. 1.
17
(b)
o
23.3
Intensity (a.u.)
(a)
40 50 60 70 2 theta (deg.)
80
400
2000
-1
SBET = 366.8 m g VT = 0.27 cm3 g-1
90 60 30
carbon-RT 0.2
0.4
0.6
0.8
0.5
2.5
0.4
2.0
0.3
1.5
carbon-RT 0.2
1.0 0.1
0.5 0.0
0.0 0
Fig. 2.
18
5
10
15 20 25 30 Pore width (nm)
35
-1
Relative Pressure (P/P0)
1.0
(d)
-1
0.0
3.0
3
0
3.5 4.45
2
3.05
(c)
3
-1
800 1200 1600 -1 Wavenumber (cm )
1.75
120
ID/IG = 3.71
90 Cumulative Pore Volume (cm g )
30
150
3
-1
20
G band 1592.5
Differential Pore Volume dV (cm nm g )
Quantity Adsorbed (cm g STP)
10
carbon-RT
Intensity (a.u.)
carbon-RT
D band 1378.4
Fig. 3.
19
survey N1s
S2p S2s
carbon-RT 200 300 400 500 Binding Energy (eV)
Relative Intensity (a.u.)
(c) N1s
1 2
398
(27.4%) (33.5%) (35.7%) (3.4%)
3
400 402 404 406 Binding Energy (eV)
292
O1--530.7 eV (25.4%) O2--531.8 eV (36.7%) O3--533.2 eV (38.0%)
O1s
2
3
1
528 530 532 534 536 538 540 Binding Energy (eV)
90 80 70 60 50 40 30 20 10 0
2 3
1 166 168 170 Binding Energy (eV)
284 286 288 290 Binding Energy (eV)
(d)
408
S2p
164
2
3
282
S1--164.0 eV (3.7%) S2--167.5 eV (47.5%) S3--168.7 eV (48.8%)
(e)
1
600
4 396
Relative Intensity (a.u.)
N1--398.9 eV N2--399.8 eV N3--400.7 eV N4--406.0 eV
C1s
Relative Intensity (a.u.)
100
C1--284.3 eV (37.0%) C2--285.5 eV (54.3%) C3--287.9 eV (8.7%)
(b)
Relative Intensity (a.u.)
Relative Intensity (a.u.)
O1s
C1s
(a)
172 Fig. 4 20
(f) 62.91%
17.78%
15.95% 3.36%
C
O
N
S
80
630
839
60
1598 1497 1303 1190 1034
40 20 0
3330 3180
Transmittance (%)
100
4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm ) Fig. 5
21
C
O
N
S
Content (at.%)
76.03
6.26
16.63
1.08
N1s O1s
S2p S2s
(d)
-1
VT = 0.42 cm g
30 Carbon-800 1.0
0.45
0.4
0.30 0.15
0.2
0 0.0
22
2
3
4
5 0.00
0
5
10 15 20 25 Pore width (nm)
30
35
-1
Fig. 6.
1
-1
0.2 0.4 0.6 0.8 Relative Pressure (P/P0)
0.6
3.98
3
0.60 1.41
SBET = 464.3 m g
Carbon-800
2.78
-1
0.8
3
-1
800 1200 1600 2000 -1 Wavenumber (cm )
1.78
-1 3
400
3
Quantity Adsorbed (cm g STP)
2
0.0
ID/IG = 2.38
Differential Pore Volume dV (cm nm g )
(c)
0
carbon-800
900
120
60
G band 1596.8
carbon-800
300 600 Binding Energy (eV)
90
D band 1349.2
(b) Intensity (a.u.)
survey
Element
Cumulative Pore Volume (cm g )
Relative Intensity (a.u.)
C1s
(a)
(a)
0.0
-1
10 mV s -1 20 mV s -1 50 mV s -1 100 mV s -1 200 mV s -1 400 mVs
-1.0
-0.8
-0.6 -0.4 Potential / V
-0.2
Potential / V
-1
Current density / A g
0.2
12 8 4 0 -4 -8 -12 -16 -20
(b)
-1
1Ag -1 2Ag -1 3Ag -1 4Ag -1 5Ag
-0.2 -0.4 -0.6 -0.8 -1.0
0.0
40
60
80 100 120 140 160 180 200 Time / s
-1
(c)
70
Specific capacitance / F g
Specific capacitance / F g
-1
80 60 50 40 30 20 10 0 0
1
2 3 4 -1 Current density / A g
5
40 20 0
0
500
1000 1500 Cycle number
2000
(f)
(e)
before cycling after 200 cycles
8 -Z"/ ohm
-Z"/ ohm
91.78%
10
40 30 20
before cycling after 200 cycles
6 4 2
10 0
1Ag
60
6
50
-1
(d)
80
0
0
10
20 30 Z'/ ohm
40
50
Fig. 7. 23
0
2
4 6 Z'/ ohm
8
10
Table Captions: Table 1. Summary of main FTIR frequencies of the carbon-RT sample: Table 2. Comparison of the specific capacitance of the electrodes with literature data:
24
Table 1. Wavenumber
Bond assignment
(cm–1) 3180 and 3330
The over lap of O–H and N–H
1598
N–H
1497 and 1303
NO2
1190
C–O, C–N and C–S
1190, 1034 and 750
S=O and C–O–S
839
C–O
630
carbon backbone framework
25
Table 2. Specific
Total
surface
pore
area
volume
item 2
−1
3
−1
Nitrogen Sulfur
Specific
content
content
capacitance Ref.
(at. %)
(at. %)
(F g−1)
(m g )
(cm g )
Activated CNTs
644
0.450
—
—
53.6
44
Activated carbon
1293.2
0.634
—
—
67
45
N-doped MCNTs
175.3208
0.283
2.27
—
44.3
46
S-doped carbon–graphene composites
545
0.432
—
4.3
109
47
N and S co-doped porous carbon
464.3
0.42
16.63
1.08
73
Present work
26
Graphical Abstract
27
Highlights Nitrogen and sulfur co-doped nanoporous carbon materials derived from p-nitrobenzenamine within 2 minutes. A sulfuric
acid-assisted
straightforward carbonization method has been
implemented to produce nanoporous carbon. Nanoporous carbon possesses high nitrogen content (15.95 %) and sulfur content (3.36 %). Nanoporous carbon materials exhibit potential supercapacitor applications.
28
S0925-8388(15)00848-8 http://dx.doi.org/10.1016/j.jallcom.2015.03.135 JALCOM 33746
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
13 December 2014 15 March 2015 17 March 2015
Please cite this article as: H.T. Yi, Y.Q. Zhu, X.Y. Chen, Z.J. Zhang, Nitrogen and sulfur co-doped nanoporous carbon material derived from p-nitrobenzenamine within several minutes and the supercapacitor application, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/10.1016/j.jallcom.2015.03.135
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nitrogen and sulfur co-doped nanoporous carbon material derived from p-nitrobenzenamine within several minutes and the supercapacitor application Hai Tao Yi1, Yan Qi Zhu1, Xiang Ying Chen1 *, and Zhong Jie Zhang2 ** 1
School of Chemistry and Chemical Engineering, Anhui Key Laboratory of
Controllable Chemistry Reaction & Material Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, P R China. *Corresponding author: Fax: +86 551 62901450. E-mail address: [email protected]. 2
College of Chemistry & Chemical Engineering, Anhui Province Key Laboratory of
Environment-friendly Polymer Materials, Anhui University, Hefei 230039, Anhui, P. R. China. ** Also the corresponding author. E-mail: [email protected]. Abstract Nanoporous carbon materials co-doped with both nitrogen and sulfur have been synthesized via a sulfuric acid-assisted straightforward carbonization method, in which p-nitrobenzenamine serves as both carbon and nitrogen source, whereas sulfuric acid as sulfur source and catalyst. It is revealed that the nanoporous carbon material named as carbon-RT takes on amorphous features with low crystallinity. More importantly, it possesses high nitrogen content (15.95 %) and sulfur content (3.36 %) in carbon matrix. And the synthesis time is so short that the carbon-RT sample could be obtained within just a few minutes. To improve the electrochemical performance of material, the carbon-RT sample has been further heated at 800 °C, resulting in the carbon-800 sample. As a result, it has delivered a high specific capacitance of 73 F g−1 at the current density of 1 A g–1 when measured in a three-electrode system, using 6 mol L–1 KOH as electrolyte. The present synthesis method has been conveniently implemented to produce nanoporous carbon materials co-doped with high content of nitrogen and sulfur, using inexpensive starting materials, thereby it has broad prospects and potentials for producing carbon materials in a simple and scalable manner. 1
Keywords: p-Nitrobenzenamine; Carbonization; N-S codoping; Nanoporous carbon; Supercapacitor.
1. Introduction In recent years, various carbon materials have received widespread attention due to their remarkable properties including high electrical conductivity, chemical inertia, thermostability, environmental friendliness and low manufacturing cost [1, 2]. These properties combined make them attractive for a wide range of potential applications such as adsorption [3], catalysis [4], energy conversion and storage [5], and so on. Among these studies, there has been growing interest in doping heteroatom into carbon which could effective enhance intrinsic or add new features to materials. Generally, there are two major approaches to accomplish the doping of heteroatom. One is by pyrolyzing heteroatom-containing precursors directly [6, 7], and the other is by post-treating carbons with dopants [8, 9]. The doping candidates have been reported including nitrogen, sulfur, boron and phosphorus etc [10]. In comparison to other elements, nitrogen is easier to be doped into carbon materials. The incorporation of nitrogen profoundly impacts the pore structures, surface states, hydrophilicities and electron-transfer rates of materials, and consequently leading to a broader application prospect [11]. For example, Sun et al. synthesized a nitrogen-containing porous graphitic carbon through a simple coordination-pyrolysis combination process, in which glucose, melamine, tetraethyl orthosilicate and nickel nitrate were adopted as carbon source, nitrogen source and graphitic catalyst, respectively. The as derived nitrogen-containing carbon has a high content of nitrogen up to 7.72 wt.% and exhibits a specific capacitances of 293 F g–1 [12]. And Wang et al. prepared mescoporous carbon with high nitrogen content of 11.3 wt.% by pyrolysis of lysine and melamine using ferric chloride as dopant and SiO2 nanoparticles as hard templates. The nitrogen-rich mesoporous carbon presents better electrocatalytic activity, better durability and higher methanol tolerance for oxygen reduction reaction than commercial Pt/C [13]. 2
Compared with that of the nitrogen doping, the researches about carbon materials doped with sulfur are relatively rare. But those researches have confirmed that the doping of sulfur has unique effects on carbon materials including the inducing of bond polarization, lattice distortion and tailoring of charge density distribution, etc [14]. As a consequence, S-doped carbon materials have great advantages over pure or N-doped carbon materials when applying to many fields, like hydrogen storage, CO2 capture, supercapacitors, and so on. For instance, Park et al. synthesized sulfur-doped graphene by a thermal treatment with induced CS2 gas as sulfur source. The resultant sulfur-doped graphene shows higher electrocatalytic activity with larger limiting current density and durability than those of commercial Pt/C catalyst and N-graphene [15]. Seredych et al. obtained a series of sulfur-doped carbons by carbonizing poly (sodium 4-styrene sulfonate) and grapheme oxide. It has also proved that the incorporation of sulfur can enhance the adsorption capacity of carbon materials to CO2 via acid-base interactions and polar interactions [16]. Apart from the carbon materials individually doped with nitrogen or sulphur, some researchers have also focused on the design of N-S-codoped carbons, since the synergistic effect of heteroatoms can further improve materials performance in certain areas. For example, Zhang et al. successfully synthesized mesoporous graphene containing nitrogen (2.12 %) and sulphur (1.70 %) via self-polymerization of polydopamine thin layer on grapheme oxide sheets, followed by reacting with cysteine and thermal treatment. The as synthesized material exhibits significantly enhanced performance as electrocatalyst for ORR than pristine graphene or nitrogen-doped graphene [17]. Sun et al. fabricated graphene-like microspheres doped with 4.6 wt.% of nitrogen and 1.5 wt.% of sulfur using Ni microspheres as the template and poly(vinylpyrrolidone) and (NH4)2S2O8 as the carbon and nitrogen precursor, respectively. The resulting materials exhibit superior capacity with excellent cycling stability as anode materials for lithium ion batteries [18]. Although some excellent nitrogen and sulfur co-doped carbon materials have been prepared in recent year, most of the synthesis methods involve complicated process or expensive starting materials. Thus it is still significant for material scientists to explore more simple and economic 3
methods to produce carbon materials co-doped with high content of nitrogen and sulfur. Herein, we demonstrate a novel and rapid method to produce nanoporous carbon materials co-doped with nitrogen and sulfur. p-Nitroaniline is an organic compound consisting of a benzene ring substituted with a nitro group and an amino group. The phenyl group acts as carbon source when amino group and nitro group as nitrogen source. H2SO4 can trigger the explosive polymerization of p-nitroaniline and induce sulfur into the nanoporous carbon materials. Alcohol blow lamp is selected as a convenient heat source which can generate a flame up to 800 °C higher than the melting point (148.5 °C) and flash point (165.0 °C) of p-nitroaniline.
2. Experimental 2.1 Synthetic method In this work, a sulfuric acid-assisted straightforward carbonization method has been implemented to synthesize nanoporous carbon materials co-doped with both nitrogen and sulfur, in which p-nitroaniline serves as carbon/nitrogen sources, and sulfuric acid as sulfur source and catalyst. The typical synthetic method can be depicted as follows: p-nitroaniline (0.8 g) was first placed in a crucible, followed by the addition of concentrated sulfuric acid (~0.3 mL, 98%), and then the crucible was heated with an alcohol blow lamp (800~900 °C). After ~2 min, black and cylindrical nanoporous carbon materials (the diameter of ~3 cm, and the length of ~15 cm) suddenly happened. The resultant materials were washed by sufficient deionized water and dried at 80 °C for 4 h in an electric oven to obtain the carbon-RT sample.
(Fig. 1)
2.2 Structure characterization X-ray diffraction (XRD) patterns were obtained on a Rigaku D/MAX2500V with 4
Cu Kα radiation. Raman spectra were recorded at ambient temperature on a Spex 1403 Raman spectrometer with an argon-ion laser at an excitation wavelength of 514.5 nm. X-ray photoelectron spectra (XPS) were recorded on a VG ESCALAB MK II X-ray photoelectron spectrometer with an exciting source of Mg Kα (1253.6 eV). High-resolution transmission electron microscope (HRTEM) images and selected area electron diffraction (SAED) patterns were performed with a JEM-2100F unit. Fourier Transform infrared spectroscopy (FTIR) absorption spectrum of selected samples was obtained on a Nicolet 6700 FTIR spectrometer. The specific surface area and pore structure of the carbon samples were determined by N2 adsorption-desorption isotherms at 77 K (Quantachrome Autosorb-iQ) after being vacuum-dried at 150 °C overnight. Cumulative pore volume and pore size distribution were calculated by using a slit/cylindrical nonlocal density functional theory (NLDFT) model.
2.3 Electrochemical characterization The electrochemical performance of samples were evaluated on a CHI660D electrochemical working station ( ChenHua Instruments Co. Ltd., Shanghai) in a three electrode experimental setup at room temperature with 6 mol L−1 KOH aqueous solution as electrolyte. The working electrodes were fabricated by mixing carbon sample (~4.8 mg), acetylene black and polytetrafluoroethylene (PTFE) binder with a mass ratio of 80:15:5 using ethanol as a solvent. The resulting slurry was pressed on foamed Ni grids, followed by evaporating the solvent in a vacuum drying oven at 120 ºC for 24 h. Platinum slice (~6 cm2) and a saturated calomel electrode (SCE) were chosen as counter and reference electrodes, respectively. From the galvanostatic charge–discharge tests, the specific capacitances per mass of active electrode materials were derived according to the equation: C=
I∆t m∆V
Where C (F g−1) is the specific capacitance; I (A) is the discharge current; ∆t (s) is the discharge time; ∆V (V) is the potential window; and m (g) is the mass of active materials loaded in working electrode.
5
3. Results and discussion XRD technique was performed to investigate the component, crystallinity and purity of the carbon sample. As shown in Fig. 2a, the broad diffraction peak at ca. 23.3° corresponds to the (002) plane of hexagonal graphite (JCPDS, Card No. 74-2329) [19]. The broadness and low intensity in shapes reveal that the sample has amorphous or low crystallinity features [20, 21]. Compared with the standard 2 theta value of graphite (26.6°), the present one has largely shifted to lower direction, suggesting the interlayers between (002) planes of the present carbons are largely expanded [19]. Intrinsic features of the carbon-RT sample were also determined by Raman technique as well as the deconvoluted ones by OriginPro Lorentzian method. As shown in Fig. 2b, the characteristic D and G bands of carbon materials are observed at around 1378.4 and 1592.5 cm–1, respectively. The D band thanks to the crystal defects and disordered structures in carbon materials, and the G band is attributed to graphite in-plane vibrations with E2g symmetry [22]. The integral area ratios of D band and G band of the samples, indexed as ID/IG, is also calculated as 3.71, suggesting that the sample are highly disordered, which is consistent with the XRD results. The
parameters of the graphitic in-plane microcrystallite size, La can be estimated as 1.19 nm. A weak band is observed between 600 cm−1 and 800 cm−1 presumably derives from the vibration of sulfur-containing groups [16]. The porosity and pore structure of the carbon-RT sample were investigated by N2
adsorption/desorption
technique.
Fig.
2c
displays
the
typical
N2
adsorption-desorption isotherm of the carbon-RT sample with a relative pressure range of 0 ~ 1.0 (P/P0). The N2 adsorption-desorption profile can be categorized into type-IV according to the classification of international union of pure and applied chemistry. The sharp rise in the low pressure stage (<0.3 P/P0) indicates the presence of microporosity; the hysteresis in the pressure range (0.45-1.0 P/P) reveals the capillary condensation in mesopores and the almost vertical tail at the high pressure (~ 1.0 P/P0) corresponds to macropores [23, 24]. The total BET surface area is 366.8 m2 g−1, and the total pore volume is 0.27 cm3 g−1. The pore size distribution of carbon-RT
6
sample is presented in Fig. 3d which confirms the multiple pore structures, and the average pore width is 6.82 nm.
(Fig. 2)
The morphology and structure of the carbon-RT sample were investigated by HRTEM technique. As shown in Fig. 3a and b, the carbon presents irregular form with a size of hundreds of nanometers. Fig. 3c and d are magnified HRTEM images which reveal the existence of micropores. The lattice spacings are highly disordered in nature, suggesting its amorphous feature. It is supported by the SAED pattern with vague diffraction rings, as shown in the inset of Fig. 4d. The result is also in good accordance with the XRD pattern and Raman spectrum. And the low crystallinity is probably caused by relatively short carbonization time.
(Fig. 3)
The elemental composition, content and chemical state were analyzed by XPS technique. The XPS survey spectrum in Fig. 4a shows a dominant C1s peak (~284.9 eV), an O1s peak (~531.2 eV), a N1s peak (~399.3 eV), and two S peaks (~167.7 eV, ~231.1 eV), which confirm the presence of elemental S and N in the carbon-RT sample. The contents of N and S elements in the carbon-RT sample are calculated to be 15.95% and 3.36%, respectively. Fig. 4b displays the C1s spectrum as well as three convoluted peaks. The peaks at 284.3 and 285.5 eV are assignable to the bonds of sp2 C=C and sp3 C–C, respectively. With respect to the one at 287.9 eV, it owes to the C–O/C–N bond [25, 26]. Fig. 4c shows the high resolution spectrum of N1s for the carbon-RT sample which can be resolved into four parts centered at 398.9 eV, 399. 8 eV , 400.7 eV and 406.0 eV. Those peaks can be assigned to pyridine nitrogen (N-6), pyridone/pyrrolic nitrogen (N-5), quaternary nitrogen (N-Q) and nitro nitrogen atoms, respectively [27-29]. As shown in Fig. 4d, the XPS spectra for the O 1s signals can be fitted into three peaks 7
located at 530.7 eV, 531.8 eV and 533.2 eV, corresponding to –C=O, C=O/S=O and O–C/O–S, respectively [30, 31]. Regarding the high resolution spectrum of S 2p, it primarily can be deconvoluted into three component peaks. The peak at 164.0 eV agrees with the 2p position of the –C–S– covalent bond of the thiophene-S [32]. In addition, the peak at 167.5 eV originates from –C–SO2– [33, 34], while the peak at 168.7 eV from the –C–SO3– [15].
(Fig. 4)
Infrared spectroscopy was used to ascertain the existence forms of heteroatom within the carbon-RT sample. As shown in Fig. 5, the peaks at 3180 and 3330 cm–1 are assigned to the overlap of O–H and N–H [35]. The peak at 1598 cm–1 is attributed to the stretching vibration of N–H groups [35]. The peaks at 1497 and 1303 cm–1 may be due to asymmetric NO2 stretching and symmetric NO2 stretching [36]. The peak at 1190 cm–1 corresponds to the C–O, C–N, and C–S [35]. The peaks at 1190, 1034 and 750 cm–1 can be indexed to S=O asymmetric stretching, S=O symmetric stretching and C–O–S stretching, respectively [36]. Those sulfur-containing bonds also confirm the existence of sulfone and sulfonic acid, which is consistent with the results of XPS. The peak at 839 cm–1 may be due to C–O vibrations [36]. The peak at 630 cm–1 corresponds to the carbon backbone framework [15].
(Fig. 5)
(Table 1)
Based on our characterization results and relevant literatures, the possible mechanism is proposed for the formation of the carbon-RT sample as follows: As temperature rises, 4-nitrobenzenamine starts to melt (~150 °C), decompose (~160 °C) and polymerize (~200 °C) [37, 38]. Both decomposition and polymerization reaction generate gaseous products and heat which might be aggravated along with the 8
temperature increase. And after being heated for ~2 min, a volume of gas is generated in an instant, causing the materials rapidly expanding and porous structures appearing. At the same time, the materials are carbonized with the assistance of concentrated sulfuric acid, primarily serving as catalyst. Although subsequently some of the nitrogen atoms may get out of the system as gas products like N2 etc, high content of nitrogen is still doped into the materials successfully as pyridine nitrogen, pyridone/pyrrolic nitrogen and quaternary nitrogen. Also, sulfur atoms derived from sulfuric acid can be doped into the materials mainly as sulfur-containing functional groups [39, 40]. In order to evaluate the electrochemical performances of the carbon-800 sample, it was measured in a symmetric three-electrode system using 6 mol L−1 KOH aqueous solution as electrolyte. However, the results reveal this the carbon sample prepared by one-step method, as detailedly illustrated above, possesses unsatisfied electrochemical performances. Thus, the carbon sample was further heated in a horizontal tube furnace up to 800 ºC at a rate of 4 ºC min–1 and maintained at this temperature for 2 h under N2 flow, obtaining the carbon-800 sample. Then, its crystallinity, structure, and components are characterized by XPS and Raman and N2 adsorption/desorption techniques. Also, the electrochemical performances are also investigated in the same three-electrode system. Fig. 2a shows the XPS spectra of the carbon-800 sample, which reveal the existence of carbon (~285.1 eV), oxygen (~532.2 eV), nitrogen (~399.3eV) and sulfur (~164.3 eV, ~228.3 eV) atoms. Compared with the carbon-RT sample, the content of carbon and nitrogen increase to 76.03 at.% and 6.26 at.%, while oxygen and sulfur content fall up to 16.63 at.% and 1.08 at.%. As previously mentioned, nitrogen atoms mainly exist as structural nitrogen which has higher thermal stability than sulfur and oxygen containing surface functional groups like –SO3– etc. The losses of oxygen and sulfur atoms increase the relative content of nitrogen without additional incorporation [25]. The Raman spectrum of the carbon-800 sample is illustrated in Fig. 6b. The sample take on two distinct Raman peaks of 1349.2 cm−1 (D band) and 1596.8 cm−1 (G band). The relative ratios of the D band to the G band (ID/IG) of the carbon-800 9
sample are 2.38. As a result, the La value is 1.85 nm which is bigger than that of the carbon-RT sample (1.19 nm). Apparently, the improvement of carbonization can lead to an increase in crystallite size [41]. The N2 adsorption-desorption isotherm of the carbon-800 sample is shown in Fig. 6c which also can be categorized into type-IV with a BET surface area of 464.3 m2 g−1, total pore volume of 0.42 cm3 g−1. The pore size distribution presented in Fig. 6d indicates the carbon-800 sample have well-developed micropores and mesopores with an average pore width of 4.74 nm. In summary, after further thermal treatment at 800 ºC, the sample presents slight increasing in BET surface area and total pore volume and decreasing in average pore diameter.
(Fig. 6)
Fig. 7a shows the cyclic voltammetry (CV) curves scanned at different rates of the carbon-800 sample. It is seen that the carbon electrode retains as quasi rectangular shapes at low scan rates of 10 and 20 mV s–1, while as distorted shapes at high scan rates between 50 to 400 mV s–1. Meanwhile, no evident redox peaks can be observed at all scan rates. It is believed that the primary contribution of the capacitances is probably due to the electric double-layer capacitance (EDLC) which arises from charge separation at the electrode/electrolyte interface [42, 43]. The traditional galvanostatic charge–discharge (GCD) curves between –1 to 0 V at the current density ranging from 1 ~ 5 A g–1 are illustrated in Fig. 7b. All these GCD curves present good triangular shapes, suggesting that the capacitances are mainly from the double-layer energy storage mechanism with no obvious contribution of the pseudocapacitance. As seen in Fig. 7c, the carbon-800 sample achieves a specific capacitance of 73 F g−1 at the current density of 1 A g–1. Then the specific capacitance notably decreases along with the increase of current density, mainly because of the insufficient ion diffusion at high current densities. Comparison of the specific capacitance of the carbon-800 sample with some literature data is presented in Table 2. Due to the high content of nitrogen and sulfur, the carbon-800 sample shows better electrochemical performance 10
than most of other carbon materials whose specific surface areas are similar to it. Fig. 7d shows the cycling stability of the carbon-800 sample based on three-electrode setup at a current density of 1 A g–1. The sample still retains 91.78% of its original capacitance after charging–discharging for 2000 cycles. The material shows favorable cycling stability and reversibility which is meaningful for practical applications. To further identify the exact electrical conductivity of electrodes, the carbon-800 sample was measured by using Nyquist plots before/after 200 cycles, as show in Fig. 6e and Fig. 6f. Both of the two Nyquist plots exhibit two distinct parts, including a semi-circle loop in the high frequency region and a nearly vertical line in the low frequency region [48-50]. Apparently, the semi-circle loops become a little bit bigger after 200 cycles, indicating the ion diffusion and electrolyte permeation into the pore channels become harder.
(Fig. 7)
(Table. 2)
4. Conclusions In this study, we have demonstrated the synthesis of nanoporous carbon materials co-doped with high content of nitrogen and sulfur via a sulfuric acid-assisted straightforward carbonization method. The obtained carbon materials have been characterized by XRD, HRTEM, Raman, XPS, N2 adsorption/desorption and FTIR techniques. Besides, we also investigated carbon materials electrochemical performance in a three-electrode system using 6 mol L−1 KOH as electrolyte. We believe this approach has several scientific advantages for the synthesis of the nitrogen 11
and sulfur co-doped nanoporous carbon: a) The starting materials are commercially available and relatively inexpensive, showing potentials for large scale applications; b) The synthetic process involved is convenient and simple, since the carbonization and doping of nitrogen and sulfur can accomplish in the same time; c) The synthesis time is so short that nitrogen and sulfur co-doped nanoporous carbon materials can be obtained in a few minutes; d) The nanoporous carbon materials display high doping content of nitrogen (15.95 %) and sulfur (3.36 %).
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21101052). Dr. Zhong Jie Zhang thanks the financial support from Anhui Provincial Natural Science Foundation (1508085QE104). References [1] H. Jiang, P. S. Lee, C. Z. Li, Energy Environ. Sci. 6 (2013) 41–53. [2] Y. P. Zhai, Y. Q. Dou, D. Y. Zhao, P. F. Fulvio, R. T. Mayes, S. Dai, Adv. Mater. 23 (2011) 4828–4850. [3] M. E. Casco, M. Martinez-Escandell, J. Silvestre-Albero, F Rodriguez-Reinoso, Carbon 67 (2014) 230–235. [4] H. T. Chung, J. H. Won, P. Zelenay, Nat. Commun. 4 (2013) 1922. [5] Q. Wang, J. Yan, Y. B. Wang, T. Wei. M. L. Zhang, X. Y. Jing, Z. J. Fan, Carbon 67 (2014) 119–127. [6] Z. J. Zhang, C. Chen, P. Cui, X. Y. Chen, J. Power Sources 242 (2013) 41–49. [7] B. Xu, D. F. Zheng, M. Q. Jia, G. P. Cao, Y. S. Yang, Electrochim. Acta 98 (2013) 176–182. [8] M. Wu, Y. Ren, N. Guo, S. Li, X. Sun, M. Tan, D. Wang, J. Zheng, N. Tsubaki, Mater. Lett. 82 (2012) 124–126. [9] B. You, L. L. Wang, L. Yao, J. Yang, Chem. Commun. 49 (2013) 5016–5018. [10] J. P. Paraknowitsch, A. Thomas, Energy Environ. Sci. 6 (2013) 2839–2855. [11] Y. H. Lu, Y. Huang, M. J. Zhang, Y. S. Chen, J. Nanosci. Nanotechno. 6 (2013) 12
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15
Figure Captions: Fig. 1. Schematic route showing the production of nanoporous carbon materials co-doped with both nitrogen and sulfur. Fig. 2. The carbon-RT sample: (a) XRD pattern; (b) Raman spectrum; (c) N2 adsorption-desorption isotherm; (d) Cumulative pore volume and pore size distribution curves (calculated by using a slit/cylindrical NLDFT model). Fig. 3. The carbon-RT sample: HRTEM images as well as the magnified SAED pattern (the inset) Fig. 4 XPS spectra of the carbon-RT sample: (a) survey, (b) C1s, (c) N1s, (d) O1s and (e) S2p; (f) summary of carbon/oxygen/nitrogen/sulfur contents. Fig. 5 FTIR spectrum of the carbon-RT sample. Fig. 6. The carbon-800 sample：(a) XPS survey spectrum; (b) Raman spectrum; (c) N2 adsorption-desorption isotherm; (d) Cumulative pore volume and pore size distribution curves (calculated by using a slit/cylindrical NLDFT model). Fig. 7 The carbon-800 sample： (a) CV curves; (b) GCD curves; (c) specific capacitances calculated from GCD curves; (e) Nyquist plots before/after 200 cycles as well as the magnified ones (f).
16
Fig. 1.
17
(b)
o
23.3
Intensity (a.u.)
(a)
40 50 60 70 2 theta (deg.)
80
400
2000
-1
SBET = 366.8 m g VT = 0.27 cm3 g-1
90 60 30
carbon-RT 0.2
0.4
0.6
0.8
0.5
2.5
0.4
2.0
0.3
1.5
carbon-RT 0.2
1.0 0.1
0.5 0.0
0.0 0
Fig. 2.
18
5
10
15 20 25 30 Pore width (nm)
35
-1
Relative Pressure (P/P0)
1.0
(d)
-1
0.0
3.0
3
0
3.5 4.45
2
3.05
(c)
3
-1
800 1200 1600 -1 Wavenumber (cm )
1.75
120
ID/IG = 3.71
90 Cumulative Pore Volume (cm g )
30
150
3
-1
20
G band 1592.5
Differential Pore Volume dV (cm nm g )
Quantity Adsorbed (cm g STP)
10
carbon-RT
Intensity (a.u.)
carbon-RT
D band 1378.4
Fig. 3.
19
survey N1s
S2p S2s
carbon-RT 200 300 400 500 Binding Energy (eV)
Relative Intensity (a.u.)
(c) N1s
1 2
398
(27.4%) (33.5%) (35.7%) (3.4%)
3
400 402 404 406 Binding Energy (eV)
292
O1--530.7 eV (25.4%) O2--531.8 eV (36.7%) O3--533.2 eV (38.0%)
O1s
2
3
1
528 530 532 534 536 538 540 Binding Energy (eV)
90 80 70 60 50 40 30 20 10 0
2 3
1 166 168 170 Binding Energy (eV)
284 286 288 290 Binding Energy (eV)
(d)
408
S2p
164
2
3
282
S1--164.0 eV (3.7%) S2--167.5 eV (47.5%) S3--168.7 eV (48.8%)
(e)
1
600
4 396
Relative Intensity (a.u.)
N1--398.9 eV N2--399.8 eV N3--400.7 eV N4--406.0 eV
C1s
Relative Intensity (a.u.)
100
C1--284.3 eV (37.0%) C2--285.5 eV (54.3%) C3--287.9 eV (8.7%)
(b)
Relative Intensity (a.u.)
Relative Intensity (a.u.)
O1s
C1s
(a)
172 Fig. 4 20
(f) 62.91%
17.78%
15.95% 3.36%
C
O
N
S
80
630
839
60
1598 1497 1303 1190 1034
40 20 0
3330 3180
Transmittance (%)
100
4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm ) Fig. 5
21
C
O
N
S
Content (at.%)
76.03
6.26
16.63
1.08
N1s O1s
S2p S2s
(d)
-1
VT = 0.42 cm g
30 Carbon-800 1.0
0.45
0.4
0.30 0.15
0.2
0 0.0
22
2
3
4
5 0.00
0
5
10 15 20 25 Pore width (nm)
30
35
-1
Fig. 6.
1
-1
0.2 0.4 0.6 0.8 Relative Pressure (P/P0)
0.6
3.98
3
0.60 1.41
SBET = 464.3 m g
Carbon-800
2.78
-1
0.8
3
-1
800 1200 1600 2000 -1 Wavenumber (cm )
1.78
-1 3
400
3
Quantity Adsorbed (cm g STP)
2
0.0
ID/IG = 2.38
Differential Pore Volume dV (cm nm g )
(c)
0
carbon-800
900
120
60
G band 1596.8
carbon-800
300 600 Binding Energy (eV)
90
D band 1349.2
(b) Intensity (a.u.)
survey
Element
Cumulative Pore Volume (cm g )
Relative Intensity (a.u.)
C1s
(a)
(a)
0.0
-1
10 mV s -1 20 mV s -1 50 mV s -1 100 mV s -1 200 mV s -1 400 mVs
-1.0
-0.8
-0.6 -0.4 Potential / V
-0.2
Potential / V
-1
Current density / A g
0.2
12 8 4 0 -4 -8 -12 -16 -20
(b)
-1
1Ag -1 2Ag -1 3Ag -1 4Ag -1 5Ag
-0.2 -0.4 -0.6 -0.8 -1.0
0.0
40
60
80 100 120 140 160 180 200 Time / s
-1
(c)
70
Specific capacitance / F g
Specific capacitance / F g
-1
80 60 50 40 30 20 10 0 0
1
2 3 4 -1 Current density / A g
5
40 20 0
0
500
1000 1500 Cycle number
2000
(f)
(e)
before cycling after 200 cycles
8 -Z"/ ohm
-Z"/ ohm
91.78%
10
40 30 20
before cycling after 200 cycles
6 4 2
10 0
1Ag
60
6
50
-1
(d)
80
0
0
10
20 30 Z'/ ohm
40
50
Fig. 7. 23
0
2
4 6 Z'/ ohm
8
10
Table Captions: Table 1. Summary of main FTIR frequencies of the carbon-RT sample: Table 2. Comparison of the specific capacitance of the electrodes with literature data:
24
Table 1. Wavenumber
Bond assignment
(cm–1) 3180 and 3330
The over lap of O–H and N–H
1598
N–H
1497 and 1303
NO2
1190
C–O, C–N and C–S
1190, 1034 and 750
S=O and C–O–S
839
C–O
630
carbon backbone framework
25
Table 2. Specific
Total
surface
pore
area
volume
item 2
−1
3
−1
Nitrogen Sulfur
Specific
content
content
capacitance Ref.
(at. %)
(at. %)
(F g−1)
(m g )
(cm g )
Activated CNTs
644
0.450
—
—
53.6
44
Activated carbon
1293.2
0.634
—
—
67
45
N-doped MCNTs
175.3208
0.283
2.27
—
44.3
46
S-doped carbon–graphene composites
545
0.432
—
4.3
109
47
N and S co-doped porous carbon
464.3
0.42
16.63
1.08
73
Present work
26
Graphical Abstract
27
Highlights Nitrogen and sulfur co-doped nanoporous carbon materials derived from p-nitrobenzenamine within 2 minutes. A sulfuric
acid-assisted
straightforward carbonization method has been
implemented to produce nanoporous carbon. Nanoporous carbon possesses high nitrogen content (15.95 %) and sulfur content (3.36 %). Nanoporous carbon materials exhibit potential supercapacitor applications.
28