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Double Soft-Template Synthesis of Nitrogen/Sulfur-Codoped Hierarchically Porous Carbon Materials Derived from Protic Ionic Liquid for Supercapacitor Li Sun, Hua Zhou, Li Li, Ying Yao, Haonan Qu, Chengli Zhang, Shanhu Liu, and Yanmei Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07877 • Publication Date (Web): 17 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017

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ACS Applied Materials & Interfaces

Double Soft-Template Synthesis of Nitrogen/SulfurCodoped Hierarchically Porous Carbon Materials Derived from Protic Ionic Liquid for Supercapacitor Li Sun a, Hua Zhou a, Li Li a, Ying Yao b,*, Haonan Qu a, Chengli Zhang c, Shanhu Liu a, Yanmei Zhou a,* a

Institute of Environmental and Analytical Sciences, College of Chemistry and Chemical

Engineering, Henan University, Kaifeng, 475004, China. b

School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081,

China. c

College of Environment and Planning, Henan University, Kaifeng, Henan 475004, China.

KEYWORDS: Protic ionic liquid; Double soft-template; Hierarchically porous carbon materials; Nitrogen/sulfur-codoped; Supercapacitor.

ACS Paragon Plus Environment

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ABSTRACT: Heteroatom-doped hierarchical porous carbon materials derived from the potential precursors and prepared by a facile, effective and low-pollution strategy have recently been particularly concerned in different research fields. In this study, the interconnected nitrogen/sulfur-codoped hierarchically porous carbon materials have been successfully obtained via one-step carbonization of the self-assembly of [Phne][HSO4] (a protic ionic liquid originated from dilute sulfuric acid and phenothiazine by a straight-forward acid-base neutralization) and the double soft-template of OP-10 and F-127. During carbonization process, OP-10 as macroporous template and F-127 as mesoporous template were removed, while [Phne][HSO4] not only could be used as carbon, nitrogen and sulfur source, but also a pore forming agent to create micropores. The acquired carbon materials for supercapacitor not only hold a large specific capacitance of 302 F g-1 even at 1.0 A g-1, but also fine rate property with 169 F g-1 at 10 A g-1 and excellent capacitance retention of nearly 100% over 5000 circulations in 6 M KOH electrolyte. Furthermore, carbon materials also present eximious rate performance with 70% in 1 M Na2SO4 electrolyte.

1. INTRODUCTION Hierarchically porous carbon materials have been greatly concerned in various realms including catalysis, adsorption and energy storage conversion system by their outstanding thermal stability, excellent chemically stability, high surface area (SBET) and reasonable aperture size in recent years

1-8

. Hierarchical porous carbon materials with micropores, mesopores and

macropores have unique advantage as electrode materials for application in supercapacitor 9. The micropores for charge storage mainly correspond to electrochemical capacitance, and mesopores and macropores are related to high-rate capacitive performance by serving as ion transport path


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10-11

. Currently, some techniques have been suggested to prepare hierarchical porous carbons,

including hard template method, soft template method, hard-soft combined double template method, activation method, template-activated method, etc 12-14. The soft template method is that the surfactant and carbon source self-assembled to form aggregate with a certain structure by synergistic effect of hydrogen bond, electrostatic attraction and hydrophobic interaction

15

, and

the surfactant is removed during pyrolysis in order to realize pore-forming without etching and tedious, time-consuming wash 16-17. In contrast, soft-template method is a simpler, more efficient and lower-pollution strategy. However, the specific capacitance of most carbon materials as electrode materials is still comparatively low (below 300 F g-1) even under relatively small current density, although some researchers have adjusted carbon frame structure and the rationality of aperture distribution as much as possible 18-19. The heteroatom incorporated into carbon materials is advantageous to regulate their electron donor performance and thus adjust the electrical and chemical performance of their surface

20

.

Currently, the heteroatom doped including nitrogen (N), sulfur (S), oxygen (O), boron (B) and phosphorus (P) carbon materials was investigated. According to reports, N-doped carbon materials possess significant electrochemical properties, attributed to N atoms in hexagonal carbon rings that raise the basicity, conductivity and oxidation resistance of carbon materials 3. In addition to N, the other heteroatoms including S, B, O and P into carbon frame can also enhance the electrochemical property

21

. According to the literature, the synergistic effect of multiple

heteroatoms doped carbon materials on the electrochemical properties is better than that of single heteroatom 22. In particular S atom, it was studied by Qiao et al. with experiments and theoretical calculations, which illustrated that N/S double doping led to reallocating spin and charging density

23

. Until now, situ-doped technique via heating heteroatom-contained polymer as the


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precursor has been widely used

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, which is simpler and much more feasible to control the

amount of heteroatoms and easily achieves uniform distribution of heteroatoms in the carbon skeleton 25. Currently, the precursor of polymeric compounds comprising doped atom including polyaniline, polypyrrole and polyacrylonitrile is most prevalent, but they are tedious synthesis, poor solubility and difficult to control the amount of heteroatom

26-28

. Zhang et al. showed

various protic ionic liquids (PILs) of [R-NHx][HSO4] (R and x means carbon chain and the number of hydrogen atom, respectively) and N/S co-doping carbon materials with a low S content by a direct pyrolysis of PILs, resulted in carbon materials for application in carbon dioxide capture, electro-catalysis and photo-catalysis, etc stability, negligible volatilities and easy synthesis

30

29

. PILs have excellent thermal

. In addition, PILs display easily regulated

molecular structure and particular chemical composition, and can be simultaneously used as carbon source and heteroatoms source during the process of preparing doped carbon materials 31. Especially, PILs can also play a pore forming agent by themselves, which is attributed to the release of SO2 and NH3 in the process of high temperature carbonization

29

. These results

demonstrate that PILs are an effective and sustainable precursor for doped porous carbon materials. Nevertheless, developing a multiple heteroatoms doped hierarchical porous carbon materials with high heteroatoms content, large specific capacitance, excellent capacitive performance and prepared by a facile and effective method will still be a huge defiance. In this work, one novel approach for synthesizing N/S co-doped hierarchical porous carbon materials (NSHPC) is displayed, based on the fine regulated self-assembly of one PIL named [Phne][HSO4] and the double soft-template of OP-10 and F-127. The resultant NSHPC with interconnected hierarchical network structure corresponds to desired structural feature for highperformance supercapacitor, such as highly opened macropores of hundreds of nanometers to


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several micrometers, mesopores of around 2-20 nm (to serve as fine ion transport channels to the interior surface) and micropores of around 0.6-2 nm (to respond to the high capacitance). Furthermore, the evenly distributed and high level multi-dopant (3.41 at.% of N and 6.65 at.% of S) can also provide additional contribution to the overall electrochemical performance. 2. EXPERIMENTAL SECTION 2.1. Materials Pluronic F-127 was purchased from Sigma Aldrich. Sulfuric acid, Emulsifier OP-10, phenothiazine and potassium hydroxide were acquired by Aladdin. All experimentation employed deionized water. All chemical reagents were employed without further refinement. 2.2. Preparation of precursor and NSHPC The [Phne][HSO4] used as carbon source and heteroatoms source was prepared according to the previous procedures 32. For a typical process, 1.99 g of phenothiazine was dissolved in 15 mL acetone and 6 mL low concentration of sulfuric acid (1.7 mol L-1) was gently joined with ice bath in N2 atmosphere, followed by stirring under ambient condition for 2 hours. Subsequently, F-127 and OP-10 dissolved in deionized water were slowly added in [Phne][HSO4] solution, keeping stirring for 2 hours. The well-distributed mixture was moved to the culture dish, leaving it for 24 hours and then dried at 100 oC in vacuum for 24 hours. The as-made solids were pyrolyzed at 350 oC by temperature raising speed of 2 oC min-1 in N2 atmosphere for 2 h. Subsequently, the temperature was ramped to 700 °C with temperature raising speed of 5 oC min-1 for 2 h. The achieved samples were denoted as NSHPCx:y:z (x, y and z represent the weight ratio of


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[Phne][HSO4], F-127 and OP-10). In addition, the NSHPC15:20 without OP-10 and NSC without F-127 and OP-10 were manufactured by the analogous course to contrast. 2.3. Characterization The morphological feature of all samples was analyzed through JSM-7610F field emission scanning electron microscope (FE-SEM) and JEM-2100 transmission electron microscope (TEM) at 200 kV. X-ray diffraction (XRD) designs were conducted through Bruker D8 Advance. Raman drawings were found via Renishaw inVia. N2 isotherm was collected in -196 °C via Micromeritics ASAP 2020 analyzer. The SBET and total aperture volume (Vtot) of all samples were analyzed through the N2 isotherm. The micropores volume (Vmic) and the distribution of aperture diameter were conducted through nonlocal density functional theory (DFT). X-ray photoelectron spectrums (XPS) were inspected via AXIS ULTRA Scanning XPS Microprobe. 2.4. Electrochemical measurement The performance of all samples was surveyed via emblematical three-electrode mode with the reference electrode by Hg/HgO or Hg/Hg2SO4 and counter electrode by platinum wire under ambient condition (the electrolyte of 6 M KOH and 1 M Na2SO4). Regarding researching electrode, a compound containing NSHPC (80 wt. %), Super C65 (10 wt. %) and polytetrafluoroethylene (10 wt. %) was moved on a film with an area of 1 cm2 and then push with another same nickel foam, followed by drying at 80 °C for 24 h (the active mass of circa 6 mg cm-1). Electrochemical impedance spectrum (EIS) measurement was checked by amplitude of 5 mV between 100 kHz and 10 mHz. The value of capacitance (C, F g-1) was computed by


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galvanostatic charge/discharge (GCD) and cyclic voltammetry (CV) depends on the following formulas: C = (I × t)⁄(m × ∆V)

(1)

C = A ⁄ (2 × S × m × ∆V)

(2)

Here, the current is denoted as I (A), discharge time is represented by t (s), △V (V) means voltage scope, m (g) signifies the weight of NSHPC, A shows integral acreage of cyclic voltammetry profile and S (mV s-1) implies scan speed. 3. RESULTS AND DISCUSSION The synthesis of NSHPC is illustrated in Figure 1. The sources of C, N, S and microporous forming agents are all acted by [Phne][HSO4]. The F-127 and OP-10 functioned as mesoporous template and macroporous template, respectively. In our process, the amino groups of [Phne][HSO4] interact with F-127 and OP-10 principally through intermolecular hydrogen bonds 9, 16

, which ensures that solvent exhalation guides self-assembly and causes microphase

separation to form aggregate with mesoporous and macroporous structure

15

. Thereafter, the

micelles of F-127 and OP-10 can be removed at 350 oC for 2 h to produce highly opened mesopores and macropores 11, while the micropores are derived from the release of SO2 and NH3 during pyrolysis of [Phne][HSO4], F-127 and OP-10 mixture under N2 atmosphere.


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Figure 1. Process diagram of preparing NSHPC. The typical morphology and microstructure of all samples were shown in Figure 2a-e. It can be observed that the NSHPC15:15:1, NSHPC15:20:1, NSHPC15:24:1 and NSHPC15:20 all have interconnected porous network frameworks with abundant large pores, and NSC appears block structure without visible pore. By comparing the FE-SEM images of all samples, the NSHPC15:20:1 possesses best porosity with highly opened macropores of hundreds of nanometers to several microns in Figure 2b. TEM was aimed at further checking the microstructure. As shown in Figure 2f, NSHPC15:20:1 with interconnected, disordered wormlike channel is not only rather translucent due to its highly porous texture, but also stable opposing degradation in the electron beam. The high resolution TEM of NSHPC15:20:1 reveals typical amorphous carbon morphology. The distinctive pore structure can be beneficial to ion-buffering layer for the electrochemical reaction. Thus, the ion carriage kinetics can be improved to promote the ion migration to inner surface and to reduce the pervasion resistance for electrode materials

33-35

.

High proportion capacitance performance can be presented through these phenomena.


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Figure 2. FE-SEM images: (a) NSHPC15:15:1, (b) NSHPC15:20:1, (c) NSHPC15:24:1, (d) NSHPC15:20 and (e) NSC; (f) TEM image of NSHPC15:20:1. Raman and XRD spectrum were employed to further describe the crystallite structure of all samples. Raman spectrum is an effective method for the graphitization degree of carbon materials. In Figure 3a, all samples reveal two characteristic peaks at 1339 and 1596 cm-1, attributed to typical D- and G-band. The level of unordered structure is related to D-band, whereas G-band is ascribed to the internal key-extending motion of sp2 hybridization for carbon atoms

25

. In addition, intensity ratio of IG/ID is related to graphitized level

32

. NSHPC15:20:1 has

the highest ratio (1.01) than NSHPC15:15:1 (0.96), NSHPC15:24:1 (0.97), NSHPC15:20 (0.94) and NSC (0.90), evidencing the highest degree of graphitization, which is propitious to the conductivity hence the capacitance property of electrode in Table 1. With regard to XRD, all samples exhibit two vertexes of 23.6° and 43.5°, related to the graphite (002) and (100) plane in Figure 3b. The exhibited broad peaks demonstrate the amorphous characteristics of all samples 35

, which is consistent with the TEM.

Table 1. The intensity ratio of IG/ID, SBET and pore structure characterization parameters of NSHPC15:15:1, NSHPC15:20:1, NSHPC15:24:1, NSHPC15:20 and NSC. Samples

IG/ID

SBET (m2 g-1)

Vtot (m3 g-1)

Vmic (m3 g-1)

Da (nm)

NSHPC15:15:1

0.96

463

0.38

0.068

3.8

NSHPC15:20:1

1.01

575

0.55

0.093

4.7

NSHPC15:24:1

0.97

497

0.41

0.059

4.1

NSHPC15:20

0.94

411

0.39

0.081

2.9

NSC

0.90

106

0.09

0.082

1.2


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a

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Average pore diameter.

The N2 sorption isotherm of all samples was appeared in Figure 3c. The NSHPC15:15:1, NSHPC15:20:1, NSHPC15:24:1 and NSHPC15:20 reveal a hybrid type I/IV profile by IUPAC classification, indicating a hierarchical pore size distribution

34

. However, NSC is assigned to

type I without hysteresis loop, representing typically microporous

36-37

. The results of pore size

distribution further confirm these phenomena in Figure 3d. These mesopores and macropores respectively originate from the soft template of F-127 and OP-10 during pyrolysis, whereas the micropores arise from the release of SO2 and NH3 in the process of high temperature carbonization. The SBET of all samples is estimated to be 411 m2 g-1, 575 m2 g-1, 497 m2 g-1, 463 m2 g-1 and 106 m2 g-1, respectively. The NSHPC15:20:1 has the largest SBET and optimal aperture volume distribution, consisting of Vmic of 0.093 m3 g-1, the mesoporous and macropores volume of 0.457 m3 g-1 in Table 1. High proportion capacitor performance can be effectively promoted through the combination of large SBET and hierarchical porous structure.


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Figure 3. (a) XRD spectrums, (b) Raman spectrums, (c) N2 isotherms and (d) pore size distributions of NSHPC15:15:1, NSHPC15:20:1, NSHPC15:24:1, NSHPC15:20 and NSC. The scatter of element for N, S and C was shown in Figure 4a and Figure S1-S4. The zero loss images of all samples and homologous mappings of element distinctly revealed uniform dispersion. The result implies that the situ carbonization of PILs offers a straight-forward technique for synthesizing heteroatom uniformly distributed carbon materials. The exterior feature and chemical component of NSHPC15:20:1 was researched by XPS. In Figure 4b, the survey spectrum reveals four peaks corresponded to C 1s, N 1s, O 1s, and S 2p, showing the successful incorporation of N and S atom into the NSHPC15:20:1 with the content of 3.41 at.% and 6.65 at.%, respectively. High-resolution C 1s spectra of NSHPC15:20:1 presents five sub-peaks in Figure 4c. The peaks of 285.4 eV and 284.5 eV are attributed to sp3 hybrid diamond-like carbon (sp3C) and sp2 hybrid graphite-like carbon (sp2C), respectively 38. Several small peaks originate from surface N and O groups with the binding states of the C=O, C-N and C-S-C at 287.8, 287.2 and 283.8 eV, respectively

39

. In Figure 4d, the N 1s spectra displays the existence of four or

three peaks corresponding to N-5 (pyrrolic) at 399.27 eV, N-6 (pyridine-like structures) at 397.97 eV, N-Q (N substituted groups of aromatic graphite structure) at 400.77 eV and pyridineN-oxides at 403.57 eV 40. The N-5 possesses excellent electron donor feature and higher charge transfer, enhancing catalytic activity during electron shift process and availably increases electrochemical property. The N-6 provides one pair of electrons to conjugate with the πconjugated ring. Consequently, it can introduce the property of the electron donor into the carbon materials and has a significant impact on the enhanced capacitance 41. The other remarkable N-Q groups situated at the disabled graphite lattice center will strongly increase specific conductance 9

. The S 2p spectrum can be decomposed into three distinct peaks of about 168.5, 165.0 and


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163.5 eV in Figure 4e 42. The higher pinnacle is traceable by a kind of oxide in anion, however the latter pinnacles are attributed to the S2p 3/2 and S2p 1/2 of thiophene-type sulfur kinds

21, 42

.

The results further prove that a part of carbon atoms in the NSHPC15:20:1 has been replaced by N and S, which is propitious to improve conductivity and wettability as electrode materials for supercapacitor 24, 43.

Figure 4. (a) a typical FE-SEM image and related mapping of N, S and C for NSHPC15:20:1; (b) XPS survey and high resolution pattern of (c) C 1s, (d) N 1s and (e) S 2p of NSHPC15:20:1. For 6 M KOH electrolyte, capacitance property of all samples was discovered by CV and GCD. In Figure 5a, CV profile of all samples display approximate rectangular-shaped at 50 mV s-1, indicating ideal electric capacity performance and the existence of pseudocapacitance processes due to the oxidation-reduction response by perssads of N/S 44. For the NSC electrode, the CV cure exhibits smaller area than NSHPC15:15:1, NSHPC15:20:1, NSHPC15:24:1 and NSHPC15:20, evidencing lower capacitive response due to smaller SBET without mesopores and


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macropores. In contrast, the NSHPC15:20:1 exhibits much higher capacitive response than other electrodes owing to short ion channels 45. GCD profiles of all samples are depicted at 5 A g-1 in Figure 5b, revealing isosceles triangle shape with imperfect symmetry. The result also demonstrates good capacitive behavior for these electrodes with the pseudocapacitive capacitance owing to the oxidation-reduction response (N and S groups), agreed with CV test. As is shown in Figure 5c, the value of these electrode materials was computed by GCD, using the equation (1). The NSHPC15:20:1 presents relatively high rate performance of 58% from 1 A g-1 to 10 A g-1. As current density is less than 3 A g-1, the value of capacitance decreases sharply. The reason might be attributed that leisurely increasing current density results in reduction of the pseudocapacitive reaction in the rapid charge and discharge process

17

. Even so, the

NSHPC15:20:1 still exhibits a large electric capacity of 169 F g-1 even at 10 A g-1 due to the good electrical conductivity, more reasonable distribution of pore size with relatively high SBET as well as high content of N and S that enhances the hydrophilicity of electrode materials

46-47

. The

electrochemical performance of NSHPC15:20:1 was further research by CV and GCD tests at diverse scan speeds between 0.5 mv s-1 and 200 mv s-1 and current densities from 1 A g-1 to 10 A g-1, individually. In Figure S5a, the CV profiles have the similar rectangular CV shape of typical electrochemical double layer capacitor materials at small scan speeds. However, the existence of small hump from the shape of rectangle originates from the redox reaction owing to N and S groups, and the small hump disappears slowly with the scan rate increasing gradually. The results correspond to the GCD test. In Figure S5b, the GCD profile show linearly symmetric triangular form, suggesting an invertible, perfect double-layer capacitor behavior

48

. Moreover,

the small voltage drop denotes the low internal resistance of NSHPC15:20:1 electrode materials. The comparison of specific capacitances of NSHPC15:20:1 with carbon materials using other


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techniques reported recently is further summarized in Table S1. The results show that this work presents a more potential energy storage materials.

Figure 5. Electrochemical measurements: (a) CV at 50 mV s-1, (b) GCD at 5 A g-1, (c) discharge capacitance from 1.0 to 10 A g-1, (d) Nyquist plots and (e) cyclic stability over 5000 cycles of NSHPC15:15:1, NSHPC15:20:1, NSHPC15:24:1, NSHPC15:20 and NSC. In order to further research all electrode materials, EIS was also carried out in Figure 5d. As is shown, all the electrodes display analogical Nyquist plots. The capacitive deed was described in low-lying frequency part and high frequency part displays charge transfer resistance. For comparison, the NSHPC15:20:1 appears a nearly vertical profile in low-lying frequency part, revealing a good capacitive behavior

30, 49

. From the lofty frequency part, the interfacial charge

translatable resistance is assessed by the diameter of semicircle. The NSHPC15:20:1 has the smallest diameter of the semicircle, indicating that charge/ion is efficiently transported due to the developed hierarchical porous framework and large heteroatom-doped surface areas

46, 50

.


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Furthermore, cycle performance of all samples was researched through CV at 100 mV s-1 in Figure 5e. It can be seen that all samples still have above 83%, indicating their excellent electrochemical stability. Especially, the specific capacitance of NSHPC15:20:1 gradually increased with the capacitance retention reach to 110% in the initial 1000 cycles, and then nearly 100% of electric capacity is persisted over 5000 circulations. Na2SO4 as a neutral electrolyte has been widely used in the electrochemical field due to environmentally friendly, economical, easily synthesized than organic electrolyte

51

. Moreover,

Na2SO4 electrolyte holds a larger voltage window than that of alkali electrolyte 52. All electrode materials were researched in 1 M Na2SO4 electrolytes between 0.8 and -0.8 V. The rectangular CV profile and the isosceles triangular GCD profile prove that all samples have an ideal electrochemical capacitor and NSHPC15:20:1 has optimal electrochemical performance in Figure S6a and S6b. In Figure 6a, NSHPC15:20:1 displays the rectangular CV shape without obvious transformation even if at 1000 mV s-1, ascribing that electrolyte ions rapidly diffuse and move to the electrode interface. Figure 6b illustrates symmetrical triangular GCD profile of NSHPC15:20:1 from 0.3 to 10 A g-1, demonstrating ideal electric double-layer capacitor behavior. The electric capacity retention of NSHPC15:20:1 is shown with 70% in Figure 6c. Besides, NSHPC15:20:1 reveals an excellent long life cycle (84% after 5000 circulations), manifesting hunk electrochemical stability at 200 mV s-1 in Figure 6d. These results demonstrate that NSHPC15:20:1 possess excellent electrochemistry capability and highly invertibility as electrode materials for supercapacitor in both KOH and Na2SO4 electrolytes.


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Figure 6. Electrochemical measurements of NSHPC15:20:1: (a) CV profile, (b) GCD profile, (c) discharge capacitance, (d) cycling performance. 4. CONCLUSIONS In brief, we reported a novel, facile and cost-effective technique to introduce N and S to carbon framework to obtain NSHPC and demonstrated its application in supercapacitor. Through using OP-10 as the macroporous templates, F-127 as the mesoporous templates and a PIL named [Phne][HSO4] as carbon source, N/S source and microporous pore forming agent, the NSHPC was received. The hierarchically porous framework promotes charge transport and provides lots of surface sites. Consequently, the NSHPC15:20:1 exhibits fine electrochemical features of 302 F g-1 at 1 A g-1 in 6 M KOH electrolyte. When tested in 1 M Na2SO4 electrolyte at the voltage


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window of 1.6 V, the rate capability and the long cyclic life (after 5000 cycles) of NSHPC15:20:1 retain 70% and 84%, respectively. These results indicated the exciting practicability that PILs can be used to manufacture heteroatom doped hierarchical porous carbon materials for supercapacitor with eminent capability. ASSOCIATED CONTENT Supporting Information The typical FE-SEM image and the related mapping of C, N, and S for NSHPC15:15:1, NSHPC15:24:1, NSHPC15:20, NSC; CV and GCD profile by distinct scan speeds and current densities of NSHPC15:20:1; CV profile by 500 mV s-1 and GCD by 1.0 A g-1 of NSHPC15:15:1, NSHPC15:20:1, NSHPC15:24:1, NSHPC15:20 and NSC in 1 M Na2SO4 electrolyte, the contrast of capacitance value for this work and other recently works. AUTHOR INFORMATION Corresponding Author * Corresponding author: Tel: +86-371-22868833-3422; Fax: +86-371-23881589 E-mail address: [email protected] (Y.M. Zhou); [email protected] (Y. Yao) ACKNOWLEDGMENT The authors are grateful for the National Natural Science Foundation of China (21576071, 51402018). REFERENCES


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Table of Contents Graphic


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Sulfur-Codoped

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