S-Codoped Hierarchically Porous Carbon

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Solvent-Free Synthesis of N/S-Codoped Hierarchically Porous Carbon Materials from Protic Ionic Liquids for Temperature-Resistant, Flexible Supercapacitors Li Sun, Ying Yao, Yanmei Zhou, Li Li, Hua Zhou, Meixia Guo, Shanhu Liu, Caixia Feng, Zhichong Qi, and Bin Gao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03528 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 13, 2018

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ACS Sustainable Chemistry & Engineering

Solvent-Free

Synthesis

of

N/S-Codoped

Hierarchically Porous Carbon Materials from Protic Ionic Liquids for Temperature-Resistant, Flexible Supercapacitors Li Sun,a Ying Yao,b Yanmei Zhou,*,a Li Li,a Hua Zhou,a Meixia Guo,a Shanhu Liu,a Caixia Feng,a Zhichong Qi,a Bin Gao,c a

Henan Joint International Research Laboratory of Environmental Pollution Control Materials,

College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, China. b

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

China. c

Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL

32611, United States. Corresponding author: [email protected] (Yanmei Zhou) KEYWORDS:

Protic

Ionic

Liquids;

N/S-Codoped

Hierarchically

Porous

Carbon;

Supercapacitors; Temperature-Tolerant; Flexibility

ACS Paragon Plus Environment

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ABSTRACT: A versatile precursor of protic ionic liquids ([Megl][HSO4]) was used to prepare N/S-codoped hierarchically porous carbon materials (N/S-HPC) by a double soft-template solvent-free self-assembly method. [Megl][HSO4] as carbon source, heteroatoms source and microporous forming agent, F127 as mesoporous soft-template and sodium dodecyl sulfate as macroporous soft-template have self-assembly in curing process, and then are direct pyrolyzed to acquire N/S-HPC. The optimal sample shows large specific surface area of 1210 m2 g-1, high heteroatom doping (N: 5.31 at. %, S: 3.02 at. %, O: 5.56 at. %) and hierarchically porous structure with micropores (0.8~1.8 nm), mesopores (2.1~7.3 nm) and macropores (54~147 nm). The incorporation of N, S and O atoms into the carbon skeleton structure improves electrical conductivity and surface wettability, and provides additional pseudocapacitance. The results make N/S-HPC not only display high specific capacitance of 347 F g-1 at 0.5 A g-1 and still 174 F g-1 even at 20 A g-1, but excellent cyclic stability of almost 100 % capacitance retention for 5000 cycles in 6 M KOH electrolyte. Furthermore, the N/S-HPC still maintains excellent electrochemical property and stability under the extreme temperatures (-20 oC~100 oC) and bending (0o~180o). Meanwhile, the as-assembled symmetric supercapacitor displays a superior energy density of 15.8 Wh kg-1 at the power density of 212.4 W kg-1, outstanding capacitive performance of 157 F g-1 at 0.5 A g-1 with large electrochemical window of 1.7 V in 1 M Na2SO4 electrolyte.

INTRODUCTION Hierarchically porous carbon materials have already shown outstanding expression in various applications such as catalysis, adsorption, energy storage and conversion, owing to the well combination of hierarchical pore structure, the excellent stability and large surface area1-5. The hierarchical structure, chemical composition, temperature resistance and bending resistance


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in carbon materials have different effects depending on the application such as supercapacitors and wearable devices4. Especially for supercapacitors, the hierarchal porosity has a significant advantage in enhancing the electrochemical property of carbon materials6. The micropores create large specific surface to offer more active sites and the mesopores/macropores provide the reservoirs of ions, supplying an easy way to reach the larger surface area active sites, consumedly reducing the ionic resistance7-10. According to reports, the soft template route for the synthesis of porous carbon materials has significant advantages due to direct realization of pore forming during pyrolysis process without tedious, time-consuming wash, comparing with the general approaches such as hard template method and hard-soft template combined strategy11,12. Traditionally, the soft template route originates from the solvent evaporation induced selfassembly of phenolic resol and block copolymer by the hydrogen bonding interaction in suitable solvents such as alcohols and tetrahydrofuran, with strong acids or bases as the catalysts13,14. The heavy use of highly toxic aldehyde, acid or base and organic solvent causes the traditional soft template route to be restricted from the point of economy and sustainability14, and the carbon materials prepared by traditional soft template route usually refer to the single pore structure, seriously affecting the application and development of porous carbon materials15,16. Therefore, these currently available methods to prepare hierarchically porous carbon materials are still far from meeting the facile, environment-friendly and scalable demand. Chemical composition, such as heteroatom (N, S, O and P) doping, has a momentous effect on the electrical conductivity, wettability and pseudocapacitance of carbon materials for supercapacitors as well1,17,18.

In addition, the electrochemical performance of multiple

heteroatoms doped carbon materials is obviously better than that of single heteroatom owing to the synergistic effect, especially N and S atom19,20. The content and species of heteroatom doping


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in carbon materials depend on the carbon precursor, which has been well proved to a great extent21. Currently, many precursors, such as biomass, polymer, ionic liquid, small molecular organic compounds, fossil materials etc, have been tried22-27. Protic ionic liquids as a novel precursor are synthetized by a simple process of one-step neutralization reaction between organic bases and inorganic acids or organic acids. In addition, the specially appointed chemical composition is beneficial to realize doping at the molecular level and control the content and type of heteroatom in carbon materials6. Furthermore, protic ionic liquids possess versatile function including carbon source, heteroatoms source and particularly a microporous forming agent by themselves during pyrolysis process20,28. These results show that protic ionic liquids are a promising and versatile precursor for preparing heteroatom doped porous carbon materials. Accordingly, it is extremely significant to find a facile, environment-friendly soft-template method and versatile precursor for preparing high content and multiple heteroatoms doped porous carbon materials synchronously possessing the hierarchical porous channels of micropores, mesopores and macropores. Herein, we demonstrate a novel and versatile precursor of protic ionic liquids ([Megl][HSO4]) with a combined method of double soft-template and solvent-free self-assembly to facilely and greenly prepare the nitrogen/sulfur-codoped hierarchically porous carbon materials (N/S-HPC), which simultaneously possess interconnected structure with micropores, mesopores and macropores, high electrical conductivity, large specific surface area (SBET) and abundant N/S content, responding to desired characteristics for high performance supercapacitors. As expected, the N/S-HPC shows stable cycling at a large scan rate and outstanding rate capability at a high current density in both 6 M KOH and 1 M Na2SO4


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electrolytes. Moreover, the excellent temperature resistance and flexibility are shown at extreme temperature (-20 oC~100 oC) and bending (0o~180o). EXPERIMENTAL SECTION Preparation of [Megl][HSO4] and N/S-HPC The [Megl][HSO4] was prepared according to a typical process: dilute sulfuric acid (1.7 M) was gently joined in the aqueous solution of N-methylglucamine with a mole ratio of 1:1 in ice bath under N2 atmosphere, followed by stirring at room temperature for 2 h, and then dried at 80 o

C for 24 h. Subsequently, [Megl][HSO4], F127 and sodium dodecyl sulfate (SDS) with a mass ratio of

1:0.88:0.088 were mixed and ground for 20 min at room temperature. The solid mixture was transferred into an autoclave and cured at 160 oC for 24 h. The resulting products were pyrolyzed at 600 oC for 2 h, and then ramped to 900 oC for 1 h with a heating rate of 5 oC min-1. The achieved samples were denoted as 16-N/S-HPC900 (the 16 and 900 respectively represent the curing at 160 oC and pyrolysis temperature 900 oC). Furthermore, the carbon materials at different curing, different pyrolysis temperature, 16-N/S-C900 pyrolyzing without F127 and SDS, 16-N/S-PC900 pyrolyzing without SDS were prepared to contrast. Characterization The FT-IR and ESI-MS was researched on Bruker VERTEX 70 and QTRAP 4000, respectively. The 1H NMR and

13

C NMR were analyzed on a Bruker DMX-300 spectrometer.

Thermogravimetric Analysis (TG) was conducted via Mettler Toledo DSC851e. The morphological feature, mapping and energy disperse spectroscopy (EDS) of all samples were investigated using JSM-7610F field emission scanning electron microscope (FE-SEM) and JEM2100 high-resolution transmission electron microscope (TEM) at 200 kV. X-ray diffraction


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(XRD) was found through Bruker D8 Advance. Raman spectrum was conducted via Renishaw inVia. N2 isotherm was collected in -196 °C via Micromeritics ASAP 2020 analyzer. The SBET, total whole volume (Vtot) and pore size distribution (PSD) of all samples were analyzed through the N2 isotherm. X-ray photoelectron spectrum (XPS) was inspected via AXIS ULTRA Scanning XPS Microprobe. Electrochemical measurement in a three-electrode system The working electrode was prepared by a mixture of the N/S-HPC, Super C65 and polytetrafluoroethylene with a mass ratio of 80:10:10, which was moved on nickel foam (1 cm2) and dried at 80 °C for 12 h (the active mass of about 6 mg cm-2). The electrochemical performance was studied with the reference electrode of Hg/HgO and counter electrode of platinum wire at room temperature in the electrolyte of 6 M KOH. The specific capacitance (C, F g-1) computed by the following formulas: C = (I × t)⁄(m × ∆V)

(1)

C = A ⁄ (2 × S × m × ∆V)

(2)

where I (A) means the current density, t (s) represents discharge time, working voltage is denoted by △V (V), m (g) is the weight of N/S-HPC, A shows integral area of cyclic voltammetry curve and S (mV s-1) is scan speed. Electrochemical measurement in a two-electrode system In order to research the temperature-resistant, flexible in 6 M KOH, the symmetrical supercapacitor device was prepared with PVA/KOH gel as electrolyte. The working electrode was prepared by a mixture of the N/S-HPC, Super C65 and polytetrafluoroethylene with a mass ratio of 80:10:10. As-prepared mixture was uniformly coated on the 1 cm × 1 cm area among 1 cm × 3 cm nickel foam. After drying at 80 °C for 12 h, the foam was compressed at 20 MPa to


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form the working electrode. Next, the PVA/KOH solution was cast onto the electrodes and dried at room temperature for 12 h. Finally, two symmetrical electrodes were assembled together and sealed with Kapton tape to fabricate the device. The working electrodes and fibrous paper (as the separator) were assembled in capacitive mould as symmetrical supercapacitors to research the capacitive performance in 1 M Na2SO4 neutral electrolyte. All measurements were completed by CHI660e electrochemical workstation. The capacitance (C) can be calculated the following eqn (3): C = (4×I × t)⁄(m × ∆V)

(3)

Where I represents the discharge current (A), t is the discharge time (s), m is the total mass of two electrodes (g), ∆V is the discharge potential window (V) after IR-drop correction. The energy density (E) and power density (P) were calculated in symmetric supercapacitors by the following equations: E = (C×∆V × 1000)⁄(2×4×3600 )

(4)

P = (E×3600)⁄∆t

(5)

RESULTS AND DISCUSSION The mechanism for preparing N/S-HPC is shown in Scheme 1. [Megl][HSO4] is used as carbon, nitrogen/sulfur source, and a microporous forming agent, while the F127 and SDS respectively serve as the mesoporous and macroporous soft template. The self-assembly process may be the following process: during curing process, the surfactants templates, precursors would melt at the high temperatures, which gives a homogeneous reaction system and results in very good mobility and compatibility of various components14. Since a large number of hydroxyl groups and protonated amino groups exist in the molecular structure of [Megl][HSO4], the


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hydrogen-bonding, charge attraction and Van der Waals force, etc. can act as driving force for the polymerization to form micelles29-31. The combined action of [Megl][HSO4], F127 and SDS makes the block structure and microsphere micelles form. During pyrolysis, F127 and SDS were gradually removed to form the combined structure of loose folding, and the microsphere micelles on the surface of the block form a hollow carbon sphere due to the decomposition of F127 located inside the micelle32. Nevertheless, the micropores are derived from the decomposition of [Megl][HSO4] and release of SO2 and NH36,19. To further confirm the hypothesis for preparing N/S-HPC, the 16-N/S-C900 is obtained when both F127 and SDS no present, as well as 16-N/SPC900 is prepared when only F127 exists. As can be observed from SEM in Figure S2, 16-N/SC900 only appears block structure without folding and carbon spheres, but 16-N/S-PC900 shows a large number of carbon spheres without folding structure. Therefore, only the simultaneous action of [Megl][HSO4], F127 and SDS can make the folding structure and microsphere form. Furthermore, the high content of N (5.31 at. %) and S (3.02 at. %) atom are directly incorporated into these carbon materials skeleton at the molecular level since the [Megl][HSO4] is used as carbon precursor.

Scheme 1. The synthesis process of N/S-HPC. ESI-MS, FTIR spectroscopy, 1H NMR,

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C NMR and TG were employed to demonstrate

the successful synthesis of the [Megl][HSO4] in Figure S1. In Figure 1a, the SEM image shows


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that 16-N/S-HPC900 has the combined structure of loose folding and carbon spheres, which is propitious to the rapid ion diffusion and electron transfer33,34. High resolution SEM image further presents that carbon spheres uniformly distribute on the folds in Figure 1b. Moreover, the TEM image of 16-N/S-HPC900 demonstrates that carbon spheres have hollow morphology in Figure 1c, which is favorable for the energy storage for supercapacitors35. The morphology structure was studied by varying pyrolysis and curing temperature, respectively. In Figure S3, the SEM and TEM images of 16-N/S-HPC800 and 16-N/S-HPC1000 were displayed. Obvious morphology differences were observed for different pyrolysis temperature. For 16-N/S-HPC800 (Figure S3a and S3e), the union of layered stacking and carbon spheres were observed. When the pyrolysis temperature was extended, significant distinctions appeared: porous graphite foam (Figure S3b and S3f) with the pyrolysis temperature. To reveal the effect of curing temperature on morphology structure, the samples of 14-N/S-HPC900 and 18-N/S-HPC900 were designed. Diverse morphologies were presented in Figure S3c and S3d. 14-N/S-HPC900 showed the combination of little carbon spheres and tight folds, while only loose porous structure was emerged with the increase of temperature to 180 oC. Therefore, the following conclusions can be obtained: the combined structure of loose folding and hollow carbon spheres cannot be formed when the curing and pyrolysis temperature are low, and the combined structure of loose folding and hollow carbon spheres is destroyed when the curing and pyrolysis temperature are too high. Furthermore, it can be evidently observed that the elements of C, N, and S are uniformly distributed in the carbon skeleton from the corresponding element mapping images of all samples in Figure 1d-f and Figure S4.


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Figure 1. For 16-N/S-HPC900: (a) SEM image, (b) High resolution SEM image, (c) TEM image and corresponding elemental mapping images of (d) C, (e) N and (f) S. The XRD spectra and Raman spectra were used to research the graphitization degree of all samples, which were closely related to the conductivity of carbon materials36. In Figure 2a, 16N/S-HPC800, 16-N/S-HPC900, 16-N/S-HPC1000, 14-N/S-HPC900, and 18-N/S-HPC900 display two broad diffraction at 24o and 43o, corresponding the typical (002) and (100) reflections of graphitic carbon, which indicate the low crystallinity and the amorphous features37. Raman spectra was clarified in Figure 2b, revealing two main peaks. The D band at 1345 cm-1 is ascribed to the degree of disordered/defectiveness in the structure, while G band at 1590 cm-1 corresponds to the graphitic order38. In addition, the intensity ratio of D band and G band (ID/IG) can be used to express the degree of crystallization or defect density of carbon materials39. As the ratio becomes smaller, the degree of graphitization is higher and the defects are less in the carbon structure40. In Table 1, the ID/IG ratios of 16-N/S-HPC800, 16-N/S-HPC900, 16-N/S-HPC1000, 14N/S-HPC900, and 18-N/S-HPC900 were estimated to be 0.97, 0.89, 0.87, 0.90 and 0.90, respectively. Distinctly, the ratios reduce as the pyrolysis temperature promotes, implying a more graphitic structure and thus existing higher conductivity41.


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Figure 2. (a) XRD spectra, (b) Raman spectra, (c) N2 adsorption-desorption isotherms and (d) PSD spectra. Table 1 Intensity ratio of ID/IG, SBET, pore structure characterization parameters and chemical composition of 16-N/S-HPC800, 16-N/S-HPC900, 16-N/S-HPC1000, 14-N/S-HPC900 and 18-N/SHPC900.

Samples

ID/IG

SBET

Vtot

SBET-mic

D (a)

(m2 g-1)

(cm3 g-1)

(m2 g-1)

(nm)

C

N

S

O

Chemical Composition (at. %)

16-N\S-HPC800

0.97

450

0.18

323

6.3

82.44

5.44

3.10

8.43

16-N\S-HPC900

0.89

1210

0.51

827

3.8

85.73

5.31

3.02

5.56

16-N\S-HPC1000

0.87

580

0.25

397

2.5

89.94

4.33

2.77

2.81

14-N\S-HPC900

0.90

910

0.39

625

3.2

84.21

5.06

2.87

5.72

18-N\S-HPC900

0.90

870

0.33

587

3.1

85.11

4.95

2.82

5.48

(a)

Average pore size


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The energy store of carbon-based supercapacitors mainly via electric double layer is seriously affected by the crucial parameters of SBET and PSD42. The pore structure characteristic of all samples was analyzed by N2 adsorption/desorption isotherms in Figure 2c. All samples display a combination of type I and IV isotherms with an obvious type-H4 hysteresis loop, indicating hierarchical porous features38,43,44. The adsorption curve rises rapidly at very low relative pressure and shows almost horizontal at the high relative pressure, which mean that a large number of micropores exist in carbon materials6. Furthermore, the visible hysteresis loop at higher relative pressure reveals the existence of mesopores and macropores34. Meanwhile, the PSD further confirms these phenomena. In Figure 2d, it can be observed that all samples have micropore (0.6-1.8 nm) for large adsorption capacity and surface area, mesopores (2-10 nm) for rapid mass transport and macropores (>50 nm) for acting as reservoirs or host pores45. In Table 1, 16-N/S-HPC900 exhibits the largest SBET and Vtot of 1210 m2 g-1 and 0.51 cm3 g-1, comparing with 16-N/S-HPC800, 16-N/S-HPC1000, 14-N/S-HPC900 and 18-N/S-HPC900. In addition, the microporous SBET (SBET-mic) of 16-N/S-HPC900 can reach 827 m2 g-1, which is beneficial to provide the high double layer capacitance. Surface elemental compositions of 16-N/S-HPC800, 16-N/S-HPC900, 16-N/S-HPC1000, 14N/S-HPC900 and 18-N/S-HPC900 were researched by XPS spectra and EDS in Figure 3a and Table 1. As expected, all samples have four obvious bands at 283.9 eV, 399.2 eV, 162.5 eV and 531.9 eV, corresponding to carbon (C 1s), nitrogen (N 1s) sulfur (S 2p) and oxygen (O 1s), respectively, which represent the existence of C, N, S and O element. The N and S content of 16N/S-HPC900 can be up to 5.31 at. % and 3.02 at. %, respectively. In order to research C, S and N configuration in the samples, high-resolution N 1s and S 2p spectra of 16-N/S-HPC800, 16-N/SHPC900, and 16-N/S-HPC1000 are shown in Figure 3b-c and Figure S5. For the 16-N/S-HPC900,


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the N 1s band is divided into four individual peaks corresponding to pyrrolic-N (399.17 eV), pyridine-N (398 eV), graphite-N (N substituted groups of aromatic structure at 400.5 eV, and Noxides (405.77 eV)46. The pyrrolic-N can availably promote activity in electron transfer process and enhance electrochemical performance due to excellent electron donor feature46. The pyridine-N with one pair of electrons and the π-conjugated ring can provide the electron donor into carbon materials and has remarkable influence on the increased capacitance6. The graphiteN will prominently upgrade conductivity, and N-oxides enhance the wettability of carbon materials to reduce resistance6. Similarly, the S 2p band can be resolved into three peaks about 167.5, 165.1, and 163.2 eV. The peak of 167.5 eV means a kind of oxide in anion, and the latter peaks are due to the S2p 1/2 and S2p 3/2 of thiophene-type S kinds17,47. The synergistic reaction of rich N/S groups in carbon materials can effectively provide more redox active sites, thus providing large pseudocapacitance and enhancing the specific capacitance. Moreover, The O content of 5.56 at. % exists in 16-N/S-HPC900. The O-containing functional groups can enhance the surface wettability and supply the pseudocapacitance to improve the electrochemical performance48. As shown in Table 1, the XPS spectra and EDS results show that the carbon content raises with the temperature change from 800 °C to 1000 °C, revealing that the remove/stabilization of the sp3 saturated oxygen group makes the recovery of the interarea of sp2 conjugated grapheme, and the reduction of N and S content may be due to the decomposition of S- and N-containing functional groups49.


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Figure 3. (a) The XPS spectra of 16-N/S-HPC800, 16-N/S-HPC900, and 16-N/S-HPC1000. The high resolution XPS spectrum of (b) N 1s and (c) S 2p for 16-N/S-HPC900. In view of large SBET, reasonable PSD, especial morphologies structure and high heteroatom doping, N/S-HPC is expected to be as a preeminent candidate electrode material for supercapacitor. The electrochemical performance is researched via cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectrum (EIS) using a three-electrode system in 6 M KOH aqueous solution. In Figure 4a, the CV curves show that all samples are approximate rectangular shape between -1.0 and 0 V, and 16-N/S-HPC900 has the larger CV area than 16-N/S-C900, 16-N/S-PC900, 16-N/S-HPC800, 16-N/S-HPC1000, 14-N/SHPC900 and 18-N/S-HPC900, revealing outstanding capacitive response36. The weak humps in the CV curves exist owing to the pseudocapacitance originated from reversible redox reactions of the surface N-, S- and O-containing groups, which can improve the electron donor-acceptor property and the acid-alkali characteristic of the carbon materials as the active site of the pseudocapacitance, further increasing the specific capacitance50-52. The CV curves of 16-N/SHPC900 are further analyzed at different scan rates in Figure 4b, and the CV curves still remain approximate rectangular shape even at a high scan rate of 100 mV s-1, revealing typical electrochemical double layer capacitor materials with a good rate capability due to large SBET with micropores, mesopores and macropores. In addition, highly linear and symmetrical GCD


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curves of all samples exhibit in Figure S6a, demonstrating excellent electrochemical reversibility37. The longest discharge time of 16-N/S-HPC900 means the highest gravimetric specific capacitance, related to the CV results. The value of discharge capacitance for all samples is calculated in Figure 4c, and the 16- N/S-HPC900 presents the discharge capacitance of 251 F g1

at the current density of 1.0 A g-1, higher than those of 16-N/S-HPC800 (214 F g-1) , 16-N/S-

HPC1000 (180 F g-1), 14-N/S-HPC900 (201 F g-1), and 18-N/S-HPC900 (228 F g-1). As the current density increases to 20 A g-1, the specific capacitance of 16-N/S-HPC900 can still be maintained at 174 F g-1, showing highest rate performance of 73 %, attributed to the reasonable PSD and high content of N, S and O elements44,51. Moreover, the GCD curves of 16-N/S-HPC900 at various current densities in Figure S6b show a small IR drop, which indicate a low internal series resistance. Additionally, the specific capacitance of 16-N/S-HPC900 contrasts to other previously reported carbon materials in Table S1. The 16-N/S-HPC900 appears a comparable or even superior capacitances performance to advanced carbon materials such as nitrogen/phosphorus codoped hollow carbon microspheres20, nitrogen-containing hierarchical porous carbon spheres26, hierarchical porous carbons34, etc.


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Figure 4. Electrochemical measurement in 6 M KOH: (a) CV curves at the scan rate of 50 mV s1

. (b) CV curves of 16-N/S-HPC900 at different scan rates. (c) Specific capacitance at different

current densities. (d) Cycling performance at a scan rate of 100 mV s-1. CV curves of (e) temperature-tolerant from -20 oC to 100 oC and (f) flexibility for 16-N/S-HPC900. The electron/ion transport process of all samples was further studied using EIS in Figure S6c. The process is introduced by two parts of low frequency for ion diffusion resistance and high frequency part for combined series resistance involving the inherent resistance of electrode materials, the electrolyte and contact resistance between the electrode and current collector53. 16N/S-HPC900 displays nearly vertical lines in the low frequency part implying low ion diffusion resistance, and the smallest diameter of the semicircle at high frequency part, evidencing the small charge transfer resistance on electrode/electrolyte interface. These results are ascribed to the high conductivity, developed hierarchical porous structure and large N/S-doped surface areas10. The cyclic stability can be regarded as the key requirement to measure the practical application of electrode materials37. In Figure 4d, the cycle property of all samples was explored


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using CV in KOH electrolyte. 16-N/S-HPC800, 16-N/S-HPC900, 16-N/S-HPC1000, 14-N/S-HPC900, and 18-N/S-HPC900 exhibit excellent cycling performance of above 93 % after 5000 cycles. Especially, 16-N/S-HPC900 still retains nearly 100 % capacitance, and the CV curves without obvious change are shown before and after 5000 cycles in Figure 4d, revealing excellent cycling stability, which can be due to hierarchical porous structure for rapid diffusion of ions and high surface area to supply more active sites for energy storage. The 16-N/S-HPC900 as electrode was also assembled into the symmetrical supercapacitor device to examine the electrochemical property at various temperatures and bending angle. As is shown in Figure 4e, the CV curves are shown under -20, -10, 0, 25, 50, 80 and 100 oC at the scan rate of 50 mV s-1, which appear small curve change after cooling or heating, comparing with the one at room temperature (25 oC) and a nearly rectangular shape, indicating an ideal capacitive behavior. These datum present that the electrode material can tolerate high temperature of 100 oC and low temperature of -20 oC, which can be further proved by GCD at the current density of 10 A g-1 in Figure S6d. Evidently, few changes that the discharge time at 25 or 100 oC is longer than at -20 oC, attributing to the raise of resistance at low temperature9. Similarly, the well symmetric rectangle of CV curves has almost no change at different bending angle of 45o, 90o, 135o and 180o, contrasting to the one at 0o in Figure 4f. In addition, the GCD curves are almost linear with a symmetrical triangular shape and only a slight voltage drop and they nearly coincide with each other in Figure S6e. Furthermore, 16-N/S-HPC900 shows outstanding stability with small change of discharge capacitance at different temperatures and bend after 1000 cycles in Figure S6f. These results reveal that 16-N/S-HPC900 as electrode material possesses the excellent characteristics of temperature-tolerant and flexible.


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The neutral solution of Na2SO4 as a low-cost, environment-friendly and facilely prepared electrolyte than organic electrolyte and larger electrochemical window than alkaline electrolyte has been widely used for supercapacitors50,54. The electrochemical tests of 16-N/S-HPC900 were executed in the symmetrical supercapacitor mould by 1 M Na2SO4 electrolyte with a stable and large electrochemical window of 1.7 V, displaying in Figure 5. The rectangular CV curves appear from 10 to 200 mV s-1, and have no obvious distortion even if at the scan rate of 200 mV s-1 in Figure 5a. The results reveal an excellent electrochemical behavior due to ions rapid diffusion and transportation to the electrode interface in electrolyte6. In Figure 5b, the GCD curves of 16-N/S-HPC900 show symmetrical triangular at the current density of 0.5 to 20 A g-1, proving excellent electric double-layer capacitor behavior. As the current density is 0.5 A g-1 in Figure 5c, the specific capacitance can be up to 157 F g-1 with good rate capability of 75 %, obviously better than the published work such as nitrogen/sulfur-codoped hierarchically porous carbon materials (54 F g-1 at 1.0 A g-1 ) 6, honeycomb-like carbon foam (114 F g-1 at 0.2 mV s-1 )35, hierarchical porous carbon sheets 48 F g-1 at 1.0 A g-1 )55, and hierarchically porous carbon nanosheets (49 F g-1 at 0.2 A g-1 )56. In addition, the electrochemical long life cycle of 16-N/SHPC900 was analyzed in the large voltage window of 1.7 V at 100 mV s-1 for 5000 cycles in Figure 5d, showing an excellent long life cycle of 96 %.


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Figure 5. Electrochemical measurement for 16-N/S-HPC900 in 1 M Na2SO4: (a) CV curves at different scan rates. (b) GVD curves of 16-N/S-HPC900 at different current densities. (c) Specific capacitance at different current densities. (d) Cycling performance at a scan rate of 100 mV s-1. Figure 6 shows the Ragone plots of 16-N/S-HPC900 in different electrolyte. It can be clearly observed that the energy density decreases as the power density increases, which means a higher power output at a lower energy density57,58. Although the specific capacitances in Na2SO4 electrolyte are obviously lower than those in KOH electrolytes, 16-N/S-HPC900 has superior energy density in Na2SO4 electrolyte due to its much higher voltage window (1.7 V). When the power density is 212.4 W kg-1, the energy density of 15.8 Wh kg-1 can be obtained in Na2SO4 electrolyte. To the best of our knowledge, this is a high value obtained in the reported energy density of carbon-based supercapacitors in an aqueous electrolyte, and is comparable to previously reported symmetrical supercapacitors assembled by other method derived carbon


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materials 20,38,42,49,53,54. Even at a very high output power density of 9024 W kg-1, a high energy density of 11.8 Wh kg-1 can still be delivered.

Figure 6. Ragone plot based on the symmetrical supercapacitor. CONCLUSIONS In summary, a novel, facile, environment-friendly method using the versatile precursor of protic ionic liquids to prepare 16-N/S-HPC900 for the temperature-tolerant, flexible supercapacitor has been realized. As interconnected structure, hierarchical porous channels, high electrical conductivity, SBET and N/S content synchronously appear in 16-N/S-HPC900, the excellent electrochemical properties are displayed in both 6 M KOH (the specific capacitance of 347 F g-1 at 0.5 A g-1) and 1 M Na2SO4 (the specific capacitance of 157 F g-1 at 0.5 A g-1 with large electrochemical window of 1.7 V) electrolytes. More importantly, the 16-N/S-HPC900 still maintains excellent electrochemical properties and stability under the condition of extreme temperatures (-20 oC~100 oC) and bending (0o~180o). The combined method of double softtemplate and solvent-free self-assembly provides new insight for facilely and environmentfriendly preparing heteroatom-doped porous carbon materials with micropores, mesopores and macropores for the energy storage applications.


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ASSOCIATED CONTENT Supporting Information Additional experimental results (ESI-MS, 13C NMR, FTIR, TG, 1H NMR, SEM, TEM, surface C, O, N configurations, Mapping and electrochemical measurements) and comparison of various carbonaceous electrode based supercapacitors in an aqueous electrolyte system. AUTHOR INFORMATION Corresponding Author * Corresponding author: Tel: +86-371-22868833-3422; Fax: +86-371-23881589 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank the National Natural Science Foundation of China (21776061, 21576071) and the program for Science & Technology Innovation Team in Universities of Henan Province (19IRTSTHN029). REFERENCES 1.

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SYNOPSIS: N/S-codoped hierarchically porous carbon materials from N-methylglucamine protic ionic liquid were prepared by a double soft-template solvent-free self-assembly method. Table of Contents


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S-Codoped Hierarchically Porous Carbon

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