Asphaltene-based porous carbon nanosheet as electrode for

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Asphaltene-based porous carbon nanosheet as electrode for supercapacitor Fangfang Qin, Xiaodong Tian, Zhongya Guo, and Wenzhong Shen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04227 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 24, 2018

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ACS Sustainable Chemistry & Engineering

Asphaltene-based porous carbon nanosheet as electrode for supercapacitor Fangfang Qina,b, Xiaodong Tianc, Zhongya Guoa,b, Wenzhong Shena∗ a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese

Academy of Sciences, 27 South Taoyuan Road, Taiyuan, Shanxi, 030001, P.R.China b

University of Chinese Academy of Sciences, No. 19 (A) Yuquan Road, Shijingshan

District, Beijing, 100049, P.R.China c

Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of

Sciences, 27 South Taoyuan Road, Taiyuan, Shanxi, 030001, P.R.China E-mail: [email protected] (Fangfang Qin), [email protected] (Xiaodong Tian), [email protected] (Zhongya Guo)

Abstract Asphaltene with high aromaticity derived from coal direct liquefaction residue is a favorable precursor to prepare new carbon materials for its characteristic of easy to polymerize or crosslink. Here, asphaltene was used as carbon precursor for synthesis porous carbon nanosheet via in-situ sheet-structure-directing agent from urea thermal polymerization. The porous carbon nanosheet with controllable thickness and graphitized-like ribbon structure was obtained after KOH

activation.

As

supercapacitors electrode materials, the as prepared porous carbon nanosheets demonstrated a specific capacitance of 282.9 F/g even at 100 A/g in a three-electrode test and 186.7 F/g at 20 A/g in two-electrode test, the electrolyte was KOH aqueous solution in the both tests; the specific capacitance of the device retained 89.6% after 10000

cycles

showing

a

good

lifetime

and

durability.

Its

specific capacitance of the device was 135.4 F/g and 119.1F/g at 1 A/g, respectively in ionic liquid and organic electrolyte; its highest energy density reached 53.5Wh/kg (at 159.9W/kg) and 35.9 Wh/kg (at 134.9 W/kg), respectively. The synergism of high special

∗

surface

to

volume

ratio,

developed

micro-mesoporous

structure,

Corresponding author. Fax: +86-351-4041153, E-mail: [email protected]. 1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

graphitized-like conduction paths resulted in the excellent specific capacitance, outstanding cycle life and rate performance capability of the prepared porous carbon nanosheets as supercapacitor electrodes. Keywords: asphaltene, in-situ sheet-structure-directing, porous carbon nanosheet, graphitized-like ribbon, supercapacitor

Introduction Porous carbons with superior-developed pore structure were frequently used as catalyst supports, adsorbents, energy storage materials.1-4 The surface chemical composition and pore structure of porous carbon depend its application properties, and controlling and modifying its pore structure or surface chemical group have been widely investigated.5-9 Recently, porous carbon with regular morphologies had attracted considerable attention for these morphologies endowing it with unique physical or/and chemical properties. The honeycomb-like porous carbon provided space for NiCo2S4 hybrids and facilitated electrolyte ion diffusion into the integrated electrode for high efficient charge storage;10 flower-like superstructures mesoporous carbon showed outstanding rate capability as supercapacitor electrode material due to the interconnected pore structure and easy access to mesopores on surface of carbon sheet for its effective charge storage;11 interconnected hollow graphitic vesicle as support of Pt displayed higher electrocatalytic activity and durability in the oxygen-reduction reaction due to its open cross-linked hollow channel, well electrical conductivity, high Pt dispersion and strong anticorrosion. 12 Porous carbon nanosheets with ultrahigh surface-to-volume ratio and thicknesses on the nanoscale could be utilized as ion batteries,13 hydrogen-storage materials,14 catalyst supports,15 or supercapacitor electrodes.16 As new energy-storage device, supercapacitor had higher capacity and energy, wider working temperature range, especially longer service life; it combines the advantages of traditional capacitor and battery, and is a promising chemical power source. Porous carbon is a preferred material for commercial supercapacitors electrode owing to its adjustable pore structure, modifiable surface chemical 2


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Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


properties, specific surface area and contained hetero-atom have important influence on its electrochemical properties. The developed pore structure of commercial porous carbon can increase the specific capacitance when it is used as an electrode material, but it also has the potential to cause low rate performance. Compared with conventional porous carbon, porous carbon nanosheet is proved prominent electrochemical property as supercapacitors electrode material due to its higher surface area, faster electronic-transport behaviors and larger open flat layer.17,18 Porous carbon nanosheets could be prepared by templating, self-assembly, thermal decomposition combined with assembly and solvothermal synthesis routes.19 However, the templating synthetic processes are complex and tedious; directional assembly or/and growth between carbon precursor and/or template via van der Waals, π-π, capillary and hydrogen-bond interaction are essential for self-assembly, resulting a time-consuming procedure The precursors with sheet structures are appropriate candidates for thermal decomposition and assembly approach, but the resultant porous carbon nanosheets are prone to aggregate and it is difficult to precisely control nanosheet structure. While various morphologies (nanotrailers, leaf-like, nanosheets ball, etc.) are facile to generated during hydro-/solvothermal carbonization. So, it is necessary to explore facile approach to synthesize porous carbon nanosheets with developed pore structure and controllable sheet thickness. Moreover, carbon nanosheet owing better electrical conductivity is desired for supercapacitor electrode. Asphaltene with high average molecular weight widely exists coal tar, coal direct liquefaction residue, it is easy to polymerize or crosslink due to its high aromaticity and is an remarkable precursor for preparing new functional carbon material.20,21 Moreover, asphaltene is rich in sp2-hybridized carbon species and polycyclic aromatic hydrocarbons, which endows it as a potential precursor to obtain highly graphitized porous carbon.22 However, it was hard to directly obtain porous carbon nanosheet using asphaltene as precursor due to its thermoplasticity. In our previous work, asphaltene was demonstrated as an appropriate carbon precursor for synthesizing carbon nanosheets using graphene oxide as template, and showed favorable rate 3



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performance as supercapacitor electrode.23 But it is necessary to obtain graphene oxide in advance, the process is tedious and limits it large-scale preparation and extensive utilization. Porous carbon nanosheet with developed pore structure and a certain degree of graphitization derived from asphaltene is beneficial to mass diffusion and electron conduction when it is used as supercapacitor electrode. This will be a sustainable principle, and simultaneously achieve the efficient use of coal liquefaction residue. So, it is important to develop facile and efficient route for fabrication of asphaltene-based porous carbon nanosheet. In here, porous carbon nanosheets with developed micropore structure, high electrically conductive and nitrogen content were synthesized using asphaltene and urea. It was creative using asphaltene as precursor to synthesize porous carbon nanosheet through in-situ template generating route. The prepared asphaltene-based porous carbon nanosheet utilized as electrode material for supercapacitor in a 6 mol/L KOH electrolyte exhibited a superior capacitance of 315.7 F/g at 1A/g in three-electrode setup and could miantain 282.9 F/g at 100 A/g. Porous carbon nanosheet displayed a higher rate capability (85.6 %) and better recycle performance of 89.6% due to short diffusive paths and high electrically conductive networks. Furthermore, it demonstrated preferable capacitance of 135.4 F/g and 119.1 F/g at 1A/g, admirable energy density and high power density when it was applied in ion liquid and organic electrolyte were demonstrated. The work provided a conversion way of coal liquefaction residue, meanwhile, it presented a promising carbon materials precursor for energy applications.

Experimental section Raw materials Asphaltene was extracted from coal liquefaction residue (YanChang Petroleum Co. LTD. China) using tetrahydrofuran; analytical reagent of urea, nitric acid, potassium hydroxide,

1-Ethyl-3-methylimidazolium

tetrafluoroborate

(EMIM

BF4),

triethylmethylammonium tetrafluoroborate/propylene carbonate (MeEt3N BF4/PC) were used as ionic liquid and organic electrolytes.

4


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Synthesis of carbon nanosheets Oxidation of asphaltene: 20 g asphaltene is oxidized in 200 mL 10 mol/L nitric acid using reflux at 80 °C for 12 hours. Then, the resulting yellow suspension is filtered by microporous membrane and leached by water until the filtrate reached neutral. The resultant filter residue is dried at 100 °C for 12 h and designed as ASP(O). Preparation process of carbon nanosheets: ASP(O) is dispersed into a urea solution (H2O-ASP(O)-Urea=40 : 20 : 40 wt%) by stirring at 25 °C for 1 hour, and it was dried in oven at 75 oC overnight. Subsequently, it was heated at 540 oC for 3 hours under nitrogen atmosphere, and following cooled to 25 °C, the obtained black powders were labeled as ASP(O)-U. Finally, the ASP(O)-U and KOH were mixed (ASP(O)-U:KOH as 1:1, 1:1.2 or 1:2 w/w) and heated to 800 oC with a heat rate of 5 oC/min and further carbonized at 800 oC for 1 hour in nitrogen atmosphere. The prepared samples were subsequently washed by 1 mol/L HCl and distilled water to eliminate any alkali metal residue. Finally, the resultant samples were named as ACNs-x, x represented the ratios of KOH and ASP(O)-U. The detailed procedure was described in Scheme 1.

Scheme 1. Flowchart of asphaltene-based porous carbon nanosheet formation process.

Structure Characterization Physical instrument (Micromeritics ASAP 2020) is used to examine nitrogen adsorption-desorption isotherms of ACNs-x and ASP-KOH at -196 oC; non-local density functional theory and Brunauer-Emmett-Teller equation are employed to obtain the pore size distribution and surface area of sample. S-4800 field emission scanning electron microscope is used to obtain the SEM images, and a JEM-2000F transmission electron microscopy (JEOL LTD) is used to gained the TEM images. Weight loss curves of materials are detected on thermogravimetric analyzer at argon atmosphere from 30 oC to 800 oC, the rate during the heating process is 10 oC/min. The exhausted gases during carbonization and activation are detected by subsequent 5



Page 6 of 30

mass spectroscopy (Omni Star). Fourier transform infrared spectra, X-ray photoelectron spectra, elemental analysis, Raman spectra are characterized and analyzed by Nicolet FT-IR 380 spectrometer, ESCALAB 250, monochromatic Al Kα source, Vario EL CUBE, and Renishaw InVia Reflex spectrometer, respectively. Electrochemical measurement Cyclic voltammetry, galvanostatic charge-discharge cycling, and electrochemical impedance spectroscopy surveying are carried out on a electrochemical workstation (CH Instruments Inc., Shanghai, China, CHI660C). The cycling stability is tested and deduced by a standard program (LAND-CT2001A battery test system, Land Inc., Wuhan, China). Aqueous electrolyte (6 M KOH), organic electrolyte (1M MeEt3N BF4/PC) and ionic liquid electrolyte (EMIM BF4) are used.

The working electrode is

prepared by blending resultant -ACNs-x, PTFE and carbon black (85:5:10 w/w) in 5 mL C2H5OH, then it is ultrasonically treated for 30 min. The mixture is rolled on a support film and cut into plate with 10 mm diameter, then, it is put on a current collector and dried at 120 °C for 6 hours, the loaded amount of porous carbon nanosheet was about 5 mg. The nickel foam was used as current collector for 6 M KOH, while the current collector for organic electrolyte and ionic liquid electrolyte was aluminum foil. For three electrode system in 6 mol/L KOH, a Pt plate is selected as counter electrode and an Hg/HgO electrode is used as reference electrode. When tested in two electrode cell, two symmetric carbon electrodes are separated by polypropylene membrane. As for electrochemical measurement in organic electrolyte and ionic liquid electrolyte, a coin-cell (CR2032) was used. Typically, the symmetric device was assembled with two carbon electrodes with the same weight and area, spearated by a glassy

fibrous

separator

and

filled

with

corresponding

electrolytes.

All

electrochemical surveying are performed at 25 oC. Specific capacitance (Cm), energy density (E) and power density (P) in three- and two-electrode setup are calculated according to the previous literatures.24,25

6


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Results and discussion Characterization of porous carbon nanosheets The TEM and SEM were used to observe the morphologies of ASP(O)-U, ACNs-x, ASP-KOH, and ASP(O)-KOH. The images of ASP(O)-U and ACNs-x were displayed in Figure 1, The ASP(O)-U was composed of nanosheet with a average thickness of 40-60 nm, ASP(O) contained much oxygen-containing groups (-OH, -NO2,-COOH, etc), which could form strong interaction with urea, the urea will be decomposed and thermal polymerized during heating from room temperature to 540 oC, the exhausted gases mainly from urea will enlarge the space of pyrolysis carbon and inhibit it aggregating. Moreover, the C3N4 nanosheet will be produced from urea at 350-500 o

C,26 it will serve as sheet-structure-directing agent for ASP(O) arrangement. The

sample of ASP(O)-0 was prepared in the absence of urea at same condition as ASP(O)-U, it had a large particle size (ca. 5-10 µm) and coalesced structure in Figure S1(a). The nanosheet thickness and size of ACNs-x were affected by the ratio of KOH to ASP(O)-U, the thicknesses of ACNs-1, ACNs-1.2 and ACNs-2 were around of 36 nm, 34 nm and 30 nm. To compare the structure evolution, Figure S1 displayed high magnification SEM and TEM images of ACNs-x, ASP-KOH and ASP(O)-KOH. The ACNs-1 showed an uninformed nanosheet structure, while there appeared some degree burnt off nanosheet in ACNs-1.2 and ACNs-2 that might be the drastic reaction of KOH with carbon at higher KOH ratio. The high resolution transmission electron microscope (HRTEM) images revealed the graphitized-like ribbons dispersed in ACNs-x (Figure 1 (e, f, g) and inserted images), and the distance between ribbons were 0.43 nm and 0.37 nm for ACNs-1.2 and ACNs-2. While ASP-KOH and ASP(O)-KOH showed irregular porous structure. The absence of urea led to the inability to form the structure directing agent of g-C3N4, so the TEM of ASP(O)-KOH did not have the worm-like structure. The graphitized-like ribbons was seldom generated in porous carbons derived from polymer or biomass, while there were abundant graphitized-like ribbons appeared in ACNs-x due to the high aromaticity and easy graphitization of asphaltene. The graphitized-like structure could improve

7



the conductivity of ACNs-x. It is a facile way to produce graphitized-like nanosheet porous carbon using asphaltene as precursor and urea as structure-guiding agent, and the nanosheet thickness could be adjusted by activation agent ratio.

Figure 1. The SEM images of ASP(O)-U (a), ACNs-1 (b) and ACNs-1.2 (c), ACNs-2 (d); and TEM images of ACNs-1 (e) , ACNs-1.2 (f) and ACNs-2 (g), inserted high magnification area.

The XRD of ASP(O)-U exhibited two characteristic diffraction at 13.1° and 27.5° (Figure S2 (a)), the former was assigned to the (100) peak of in-planar repeat period, and the latter was ascribed to the (002) peak of interplanar stacking among conjugated-aromatic sheets. The both peak intensities of ASP(O)-KOH at 13.1o and 27.5o were very weak, indicating its typically amorphous structure. This result illustrated that graphitic carbon nitride was existed and it served as template to induce the porous carbon nanosheet forming during KOH activation. In addition, Figure S2 8


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(b) appeared two Raman peaks located at 1354 cm-1 and 1600 cm-1

in ACNs-x,

which were attributed to D peak and G peak, respectively. The in-plane sp2 domain size was reduced during activation and a broad and intense D peak caused by phonon mode was appeared. The G peak was derived from E2g phonon scattering of sp2-phase.27,28 The intensity ratio of Raman peaks denoted as ID/IG was often used to estimate a graphitization degree of substances, the ID/IG values of ACNs- 1, ACNs-1.2 and ACNs-2 were estimated as 2.59, 2.31 and 1.11. The broad peak located between 2680 and 2750 cm−1 were regarded as 2D, which was described as sp2 carbon, it was in line with the graphitized ribbons appeared in HRTEM images. It can be attributed to the planar polyaromatic unit in asphaltene matrix. It illustrated that graphitized structure existed in ACNs-x, more amorphous carbons were degraded at high ratio of KOH to ASP(O)-U during activation. It was reported that carbon nitride nanosheets could be formed using urea as precursor.29-31 The decomposition gases of ASP(O)-U were recorded by with temperature and shown in Figure 2. NH3, H2O, NO and HNCO were produced from urea decomposition less than 250 oC. As temperature raised, urea further decomposed to form HCN, CO2 and NO2, and polycondensed to form g-C3N4 nanosheets with temperature.32 At the same time, these nitrogen-containing compounds derived from urea decomposition also reacted with the oxygen-containing groups of ASP(O), and nitrogen-containing functional groups were formed in resultant porous carbon nanosheets. Meanwhile, asphaltene wrapped on the sheet structure of g-C3N4 and formed a black composite (ASP(O)-U) after carbonization for 3 h. The formed g-C3N4 could be served as structure-guiding agent to induce asphaltene generating nanosheet. The weight losses TGA curves and exhausted gases of ASP, ASP(O), ASP(O)-U, ASP(O)-U mixted KOH at different weight ratio from 20 oC to 800 oC were presented in Figure 3. The weight loss of ASP was mainly occurred between 200 oC and 500 oC due to its volatilization and decomposition of light components, and the weight loss was about 70%. The weight loss of ASP(O) was near 60 % because more small molecules in asphaltene were cross-linked during nitric acid oxidation and its thermal stability was improved. Two prominent weight loss of ASP(O)-U appeared at the 9



ranges of 140 oC to 230 oC and 230 oC to 346 oC that could be attributed to urea degradation, and the corresponding weight loss was about 42 % and 27 %, respectively. A gradually weight loss appeared from 346 oC to 800 oC, which was caused by the structure of cross-linked ring formed by the condensation of asphaltene during its decomposition and pyrolysis process. As for ASP(O)-U with KOH at different weight ratio, the light weight loss lower than 100 oC was mainly ascribed to adsorbed water loss; the second stage from 100 oC to 600 oC was ascribed to the decompositions of urea and ASP(O), reaction between urea and KOH; a fast weigh loss occurred from 600 oC to 800 oC was the pyrolysis of polycyclic aromatic compounds and the activation reaction between KOH and carbon skeleton. The final weight losses of the mixtures of ASP(O)-U and KOH (1:1, 1:1.2, 1:2, w/w) were 41.5%, 39 % and 32.5%, respectively.

NH3

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

NO HCN H2O

HNCO

100

200

NO2

CO2 CO

300

400

500

o

Temperature ( C) Figure 2. The exhausted gases of ASP(O)-U during heating.

The relative intensities vs. temperature of mainly pyrolysized gases produced from ASP(O)-U with KOH at different ratios were shown in Figure S3. These exhausted gases were mainly derived from pyrolysis ASP(O)-U and the chemical reaction of ASP(O)-U with KOH. The CO2 appeared from 150 to 800 oC and reached the highest value at 550 oC. CH4 was produced from 300-800 oC and kept a stable value between 550-700 oC. Both CO and HCN were formed in the range of 550-800 oC. NO was

10


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formed at 200-350 oC and 550-800 oC. The case of HNCO and NO2 were similar, both of them were detected at 150-250 oC with a low intensity and at 400-700 oC with a strong intensity. It indicated that ASP(O)-U was pyrolysized and reacted with KOH during carbonization process and more micropores were produced; in addition, the nitrogen of nitrogen radius that derived from ASP(O)-U decomposition during heat treatment might be incorporated into carbon matrix.

100

ASP(O)-U-KOH=1:1.2 ASP(O)-U-KOH=1:2

80

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ASP(O)-U-KOH=1:1

60 ASP(O) ASP

40

ASP(O)-U

20 0

100

200

300

400

500

600

700

800

Temperature ( C) o

Figure 3. Thermogravimetric analysis of ASP, ASP(O), ASP(O)-U and ASP(O)-U mixed with KOH at the weight ratio of 1:1, 1:1.2 and 1:2 under a nitrogen flow. Heating rate: 10 oC/min.

Figure 4 showed the pore size distributions, adsorption and desorption curves of ACNs-x, their primary structure parameters and elemental compositions of ACNs-x were listed in Table 1. ACNs-1.2 and ACNs-2 exhibited type I isotherms which indicated the existence of significant amount of micropores. While ACNs-1 showed relatively broad knee at P/P0 lower 0.2 indicating a certain amount of mesopore existence. The specific surface areas of ACNs-1, ACNs-1.2 and ACNs-2 were 1620, 1980 and 2168 m2/g, and their pore volumes were 0.84, 1.08 and 1.14 cm3/g, respectively. The mesopore (2-4 nm) content of ACNs-1 was significantly higher than those of ACNs-1.2 and ACNs-2. This suggested that the pore structure of the ACNs-x can be regulated by varying the mixing ratio between KOH and ASP(O)-U. In activation process, KOH reacted with carbon and etched the carbon skeleton to form a great deal of micropores. In addition, CO2 generated from the reaction of KOH and carbon could further react with carbon to generate CO above 700 oC, as a result, the 11



pore structure was further developing. Figure S4 and Table S1 also showed the N2 adsorption and desorption curves and property parameters of ASP-KOH, respectively; its specific surface area (SBET) was 142 m2/g, and its pore volume was 0.12 cm3/g, which were much lower than that of ACNs-x. This phenomenon was mainly due to the thermoplasticity of asphaltene. The softening point of asphaltene without oxidization by HNO3 was 75 oC, thus the asphaltene would agglomerate at a very low temperature so that it cannot react with KOH during the activation process, thereby having a lower SBET and less pore volume. For carbon materials as electrodes, the larger SBET and adaptive pore size of porous carbon nanosheet were expected, which could provide more active sites and promote ion/charge fast transport/exchange. In addition, the nitrogen contents of samples were decreased with the weight ratio of KOH to ASP(O)-U; on the contrary, the oxygen content was increased. Moreover, a certain amount of nitrogen- and oxygen-doping will adjust surface tension of electrolyte and surface wettability of electrode will be improved. This illustrated that ACNs-x could be a promising electrode material with good electrochemical properties. (a)

0.10

ACNs-2

3

Pore volume (cm /g)

800

Volume adsorbed (cm /g)

ACNs-1.2

600

ACNs-1

400

200

0

0.0

0.2

0.4

0.6

0.8

(b)

0.08

ACNs-2

3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 30

0.06

ACNs-1

0.02 0.00

1.0

ACNs-1.2

0.04

1

P/P0

10

Pore size (nm)

Figure 4. (a) N2 sorption-desorption isotherms; (b) pore size distributions of ACNs-1, ACNs-1.2 and ACNs-2.

The FT-IR spectra of ASP, ASP(O), ASP(O)-U, ACNs-1, ACNs-1.2 and ACNs-2 were shown in Figure 5, the broad and intense peak located at 3437 cm-1 was the stretching vibration of N-H and O-H group. There were abundant characteristic peaks of oxygen- and nitrogen-containing groups in ASP(O) due to the oxidation of nitric

12


Page 13 of 30

acid, such as, stretching vibrations of C=O, -NO2 and C-N at 1726 cm-1, 1535 cm-1 and 1344 cm-1, respectively. The unique peak appeared at 2214 cm-1 was -C≡N stretching vibration of ASP(O)-U, it was caused by decomposition of urea at low temperature carbonization. As for ACNs-x, the intensities of above mentioned peaks were markedly weakened, such as C–O, C=O, C–H and–O–H, gradually weaken and finally vanished, which implied that KOH activation promoted the surface hydrophobic of porous carbon nanosheet. However, certain nitrogen content was still retained in the sample of ACNs-x. Table 1: Chemical compositions and physical properties of ASP, ASP(O), ASP(O)-U, ACNs-1, ACNs-1.2 and ACNs-2 Sample

Elemental composition (wt %)

Physical parameters

Yield SBET

Smicro

Vtotal

(m2/g)

(cm3/g)

(cm3/g)

ASP

35

0.008

ASP(O)

54

ASP(O)-U

(%)

N

C

H

O

0.06

0.75

92.27

4.88

1.37

0.02

0.10

9.07

60.81

2.40

27.23

82.91

66.3

0.02

0.12

17.05

70.68

2.23

9.50

61.06

ACNs-1

1620

0.77

0.84

2.76

88.98

0.72

7.54

33.86

ACNs-1.2

1980

0.82

1.08

2.43

89.87

0.67

7.03

26.89

ACNs-2

2168

1.03

1.14

1.27

92.04

0.59

6.10

19.87

SBET was obtained by the Brunauer-Emmett-Teller method; Vmicro was calculated according to a t-plot analysis; Vtotal was measured at related pressure of 0.99; the yield of ACNs-x was based on ASP(O).

ACNs-1.2 ACNs-1 ACNs-2 ASP

Transmitance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ASP(O)-U C،‫ش‬N N-H or O-H

-CH2

ASP(O)

C=O

-NO2 and C-N

4000

3500

3000

2500

2000

1500

1000

Wave number (/cm)

Figure 5. The FT-IR spectra of ACNs-1, ACNs-1.2 and ACNs-2.

13



The surface chemical compositions of ACNs-x were further investigated by EA and XPS. Figure 6 and Figure S5 presented the N 1s, C 1s and O 1s characteristic spectra of ACNs-x. It was confirmed that nitrogen had been successfully co-doped into them. The N 1s high resolution spectra were further deconvoluted into 4 peaks (Figure 6(a)), which were ascribed as pyridinic nitrogen (N-6, 398.5 eV), pyrrolic nitrogen (N-5, 400.1 eV), graphitic or quaternary nitrogen (N-Q, 401.5 eV) and oxidized nitrogen (N-Ox, 403 eV), respectively.33 Figure 6 (b) displayed complex C 1s spectra which consisted of two small peaks and a dominant peak of ACNs-x. The two former peaks located at 286 eV and 288.6 eV were belonged to C-N and O-C=O configurations, and the latter peak located at 284.8 eV was corresponded to C-H vibration,34 respectively.35,36 The O 1s spectra of ACNs-1, ACNs-1.2 and ACNs-2 could be fitted to three different peaks (Figure 6 (c)). The peaks at 531.5 eV, 532.8 eV and 534.1 eV were belonged to carbonyl O (O-I, C=O), ether (C-O-C) or phenol O (C-OH, O-II) and N-O-C (O-Ⅲ), respectively.37 This analysis was also consistent with the elemental composition as shown in Table 1.

N-5

N-6

ACNs-2

(b)

N-Ox

N-Ox

N-5

N-6

ACNs-1

Intensity (a.u.)

ACNs-1.2 N-5

396

398

400

402

ACNs-1.2

C-N

C-O-C ACNs-1

C-N/C-H

N-Q C-N 404

Binding energy (eV)

406

284

286

O-¢َ

C-O-C

C-N/C-H

ACNs-1.2 O-¢ٍ

O-¢ٌ

O-¢َ

288

290

292

294

ACNs-1

O-¢ٍ O-¢ٌ

O-¢َ

C-O-C

Binding energy (eV)

ACNs-2

O-¢ٍ O-¢ٌ

C-N

N-6

(c)

ACNs-2

C-N/C-H

Intensity (a.u.)

(a)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 30

528

530

532

534

536

538

Binding energy (eV) Binding Energy (eV)

Figure 6. The XPS spectra of ACNs-1, ACNs-1.2 and ACNs-2. (a) N 1s spectra, (b) C 1s spectra, (c) O 1s spectra.

The positively charged quaternary N, as so called graphitic N (N-Q) could effectively enhance the sample’s capacitance, electronic conductivity and transfer especially at high rates.38 And the negatively charged N-6 and N-5 also could contribute to additional capacitance by providing plentiful charge storage and active sites which benefited from masses of accessible defects.39 Moreover, the oxygen 14


540

Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


atoms of O-I and O-II typed with the nitrogen in carbon skeleton was favorable in rapid ion diffusion/transport for they could maintain sufficient wetting between aqueous electrolyte and electrode surface.40 The structure characterization results illustrated that ACNs-x possessed high ratio of special surface to volume, developed micropore-mesoporous structure, graphitized-like conduction path, nitrogen- and oxygen-doping on surface, which endowed it admirable electrochemical performance, for instance, much ion accessible active sites, low internal resistance and rapid ion migration.41,42 Electrochemical property of ACNs-x in KOH aqueous solution The structure characterization proved that ACNs-x with high SBET had a thickness of 30-36 nm and hierarchical pore structure of micropores (0.6-2 nm) incorporating mesopores (2-4 nm). These properties are what the supercapacitor electrode required. The electrochemical properties of ACNs-x in both three-electrode and two-electrode system are conducted with different electrolytes. The CV curves of ACNs-x at 5 mV/s were illustrated in Figure 7. The closer to the rectangular shape with a large current response indicated that ACNs-x had admirable capacitive characteristics. At the staring scans, the charge-discharge responses were both fast, which illustrated the outstanding electrochemical reversibility of ACNs-x. At the switching potential, the slopes of the current change were close to 90o proving that the carbon nanosheets had very small time-constants that was the character for ideal capacitor due to their suitable pore structure and improved wetting ability.42,43 Moreover, all curves had slight humps in the low voltage region due to pseudocapacitive existence that was caused by electrochemically active functionalities on electrode surface. CV tests of ACNs-x with various scan rates were also displayed in Figure 7 (b, c, d). It can be clearly observed all three CV curves keeping an almost rectangular shape even at 200 mV/s, this suggested that porous carbon nanosheet showed a near ideal capacitance behavior, exhibiting fast charging and discharging process characteristics. The rectangular-like shape in CV curves at high sweep speed implied that the capacitance characteristic was mainly double layer capacitance

15



contribution.44 It was obviously that the current level of ACNs-2 was the highest among the three samples at the same scan rate, which indicated the ACNs-2 had superior electrochemical performance and admirable specific capacitance (Cm) not only its larger Vtotal but also its higher SBET.

(a)

5mV/s

40

Current density ( A/g)


2 1 0

ACNs-1

-1 ACNs-1.2

-2

-1.0

-0.8

-0.6

ACNs-2

-0.4

-0.2

(b)

20 0 -20 -40

-1.0

0.0

ACNs-1

50 mV/s

-0.8

-0.6

Potential (V)

0

-25

-1.0

-0.8

-0.6

10 mV/s 100 mV/s

-0.4

-0.2

5 mV/s 20 mV/s 200 mV/s


25

50 mV/s

-0.4

-0.2

60 (d)

ACNs-1.2

-50

10 mV/s 100 mV/s

5 mV/s 20 mV/s 200 mV/s

0.0

Potential (V)

50 (c)

Current density (A/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 30

40 20 0 -20 -40 -60 50 mV/s

-80 -1.0

0.0

ACNs-2

-0.8

Potential (V)

-0.6

10 mV/s 100 mV/s

-0.4

-0.2

5 mV/s 20 mV/s 200 mV/s

0.0

Potential (V)

Figure 7. The CV curves of ACNs-x electrodes at 5mV/s (a); the CV curves of ACNs-1 (b), ACNs-1.2 (c) and ACNs-2 (d) from 5 to 200mV/s.

Figure 8 displayed the GCD curves of ACNs-x electrodes at various current densities. All three GCD curves presented a slight deviation from symmetry, which indicated that the supercapacitive behavior of ACNs-x was resulted from both electrical double-layer and the pseudocapacitance; moreover, the electrical double layer was dominant, this was consistent with the CV curves results. The ACNs-2 showed the longest charging/discharging time at 0.5 A/g, corresponding to the largest capacitance. The gravimetric Cm obtained by GCD curve vs. current densities of ACNs-x were represented in Figure 9 (a). The Cm calculated at 0.5 A/g of ACNs-1, ACNs-1.2 and ACNs-2 were 198.5 F/g, 243.6 F/g and 330.5 F/g; it calculated at 100 16


Page 17 of 30

A/g which could be kept as 186.1 F/g, 214.1 F/g and 282.9 F/g, respectively, showing a higher rate capability of 93.7%, 80% and 85.6%, respectively. The Cm of ACNs-x was related with its SBET and Vtotal, the higher of SBET and larger of Vtotal were, the higher of specific capacitance was. Moreover, these special oxygen/nitrogen-doping greatly facilitated the sufficient wetting between aqueous electrolyte and electrode surface by elevating more hydrophilic active sites.45,46Thus the three samples all had good rate capability of 97.3%, 80% and 85.6%. Therefore, the ACNs-1 displayed a better rate capability due to its mesopores structure.

-0 .9

0

10

20

30 0.5 A/g 1 A/g 5 A/g

Potential (V)

0.0 -0.3 -0.6 -0.9

0

2 00

40 0

Potential (V)

10 A/g 20 A/g 50 A/g 10 0 A/g

-0 .3 -0 .6 0

15

30

45 0.5 A/g 1 A/g 5 A/g

0 .0 -0 .3

10 A/g 20 A/g 50 A/g 10 0 A/g

-0.3 -0.6 -0.9

0

10

20

30 0.5 A/g 1A/g 5 A/g

0 .0 -0 .3 -0 .6 -0 .9

6 00

T im e (s)

0 .0 (c ) AC N s -2

-0 .9

Potential (V)

-0 .6

0

200

40 0

6 00

T im e (s)

800

ACNs-1 ACNs-1.2 ACNs-2

(d)

0.0

Potential (V)

Potential (V)

-0 .3

0.0 (b ) AC N s-1 .2

Potential (V)

10 A/g 20 A/g 50 A/g 10 0 A/g

0 .0 (a) AC N s-1

Potential (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.3

-0.6

-0 .6 -0 .9

0

40 0

8 00

1 20 0

-0.9

Tim e (s)

0

100

200

300

Time (s)

400

500

600

Figure 8. The GCD curves of ACNs-1 (a), ACNs-1.2 (b), ACNs-2 (c) and ACNs-x at 1A/g (d) in three-electrode cell.

The electrochemical impedance spectroscopy (EIS) of ACNs-x consisted of high frequency region and low frequency region were presented in Figure 9 (b). The high frequency region showed a semicircular shape whose diameter was associated with the internal resistance and power performance of the electrode material. In the low frequency region, the line approximate to vertical illustrated that the samples had good capacitance characteristics. Obviously, the slops of ACNs-x at the low frequency region were all almost close to 90o, which further confirmed their ideal capacitive 17



behavior as electrode material.

5

(a)

300

ACNs-2

85.6%

ACNs-1 ACNs-1.2 ACNs-2

(b)

4 0.60

80%

200 ACNs-1

93.7%

0.45

3

-Z'' (ohm)

ACNs-1.2

-Z'' (ohm)

Specific capacitance ( F/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

2

100

0.30 0.15 0.00 0.50

1

0.75

0

0

20

40

60

80

1.00

1.25

1.50

Z' (ohm)

0.5 A/g-100 A/g

0

100

0

1


2

3

4

5

Z' (ohm)

Figure 9. (a) Specific capacitance of ACNs-x measured at various current densities; (b) Nyquist plots of ACNs-x at a frequency range between 0.01 Hz and 100 kHz.

From the three-electrode system, it could be seen that the ACNs-2 had the best overall electrochemical performance, comprehensive specific capacitance and rate performance, because of its largest SBET, thin and graphitized nanosheet structure and nitrogen-doping. To investigate its electrochemical properties in the real operating condition, the two-electrode test using ACNs-2 as active electrode was evaluated with a potential range from 0 to 1.0 V at KOH aqueous solution. The galvanostatic charge-discharge curves (GCD), specific capacitance, E and P at variuous current density, CV at various scan rates, electrochemical impedance spectroscopy and cycle stability of the supercapacitor were all shown in Figure 10. Its CV curves presented approximate rectangular shape with a large current response due to its large capacitance and well capacitive characteristics. Furthermore, thin and graphitized nanosheet structure of ACNs-2 facilitated the fast transfer of ions and electrons, thus its CV curve still maintained an alomost rectangular shape at 200 mV/s. The GCD curves displayed nearly symmetrical triangular shape at 0.5-10 A/g, but it appeared a distinct hump may due to the electrode polarization near the end point potential at low current densities. Moreover, its specific capacitance derived from GCD curves in three-electrode setup was higher than that of two-electrode setup. This phenomenon caused by electrode polarization was very normal for electrode material. In addition, the Cm of supercapacitor maintained 186.7 F/g at 20 A/g; rate capability reached up to 18


Page 19 of 30

72.9 %, which is superior to that of most porous carbon nanosheets (graphene-based, N and S doping, or B/N co-doped, etc) as listed in Table S2.

0.5

0.0

0

30

60

0.5

0.0

75 0

15 00

T im e (s )

22 50

10

(c)

140

70

0.0

0

2

4

6

Current density

8 (A/g)

1.0

-Z'' (ohm)

3 2

0.5

0.0

1

1.0

1.5

Z' (ohm)

1

2

3

Z' (ohm)

4

2.0

Coulomb efficiency (%)

4

0.9

1.2

1

100

1000

Power density (W/kg)

10000

(f)

100

80

60

40 KOH (89.6%)

20 0

5

0.6

(d)

10

(e)

0.3

Voltage (V)

0.1

0

0

5 mV/s 10 mV/s 20 mV/s 50 mV/s 100 mV/s 200 mV/s

-30

-60

3 00 0

210

5

0

Energy density (Wh/kg)

Specific capacitance (F/g)

280

0

(b)

30

90 0 .1 A/g 0 .5 A/g 1 A/g

1.0

Voltage (V)

60

2 A/g 4 A/g 6 A/g 8 A/g 10 A/g

Current (mA)

Voltage (V)

1.0 (a )

-Z'' (ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2000

4000

6000

8000

10000

Cycle number

Figure 10. The GCD curves (a), CV curves (b), specific capacitance (c), ragone plots (d), electrochemical impedance spectroscopy (e) and cycling performance (f) of ACNs-2 measured in two-electrode setup.

The device delivered an E value of 8.9 Wh/kg and P value of 90.3 W/kg under 0.1 A/g, and the P value reached to 8262 W/kg at 10 A/g. The long cycle performance of ACNs-2 in two-eletrode system with a 10000 complete charge/discharge cycles was studied at 4 A/g. As shown in Figure 10 (f), the Cm of the device retained 89.6% after 10000 cycles showing a good lifetime and durability. Given the above, the as-prepared carbon nanosheet ACNs-2 had excellent 19



electrochemical properties such as large specific capacitance, good recycle performance, favorable rate performance, making it as an excellent electrochemical energy storage material. Electrochemical performance of ACNs-2 in ionic liquid and organic electrolyte To further assess the electrochemical performance of ACNs-2, two-electrode symmetric supercapacitors was fabricated and tested in 1 mol/L MeEt3N BF4/PC ionic liquid electrolyte and EMIM BF4 organic electrolyte. The operating voltage ranges in the two electrolytes were 0-3.2 V and 0-2.7 V, respectively. The galvanostatic charge-discharge curves (GCD), cyclic voltammetry (CV), specific capacitance, ragone plot were all obtained and showed as Figure 11. The both CV curves in two electrolytes showed a nearly rectangular shape at sweep speed between 5 mV/s and 500 mV/s; the rectangular shape at high sweep speed in the organic electrolyte remained well than that in ionic electrolyte, indicating the device had a higher rate performance in organic system, which was also consistent with EISI results. Its specific capacitance of single electrode at 1 A/g was reached to 135.4 F/g in ionic liquid electrolyte and 119.1 F/g in organic electrolyte, respectively. These electrochemical performances were superior to other carbon-based symmetric supercapacitor in ionic electrolyte and organic electrolyte (Table S3). In ragone plot, both energy densities ion liquid and organic electrolytes decreased slightly with the increasing of P value. The highest E value of the supercapacitor reached 53.5 Wh/kg (at 159.9 W/kg) for ionic electrolyte and 35.9 Wh/Kg (at 134.9 W/kg) for organic electrolyte, respectively. Its energy density was still up to 27 Wh/kg (at 27783 W/kg, ionic electrolyte) and 19.7 Wh/kg (at 23620 W/kg, organic electrolyte) even at high-power density. The cyclic performance of the device with a total of 10000 complete charge/discharge cycles was examined at 2 A/g in EMIM BF4 electrolyte and MeEt3N BF4/PC electrolyte. As shown in Figure 11, capacitance retention rate reached 75.7 % and 79.2 % in organic and ion liquid electrolytes after 10000 cycles, respectively. These all proved that the ACNs-2 had excellent electrochemical properties not only in

20


Page 20 of 30

Page 21 of 30

KOH aqueous solution but also in both ionic and organic electrolytes.

2 1 0 0

10

20

0

M eEt 3 N BF 4 /PC

0 0

800

1600

(c)

5 mV/s 10 mV/s 20 mV/s 100 mV/s 500 mV/s

-10

160

50 mV/s 200 mV/s

0.7

1.4

2.1

2.8

3.5

1000

0 10

50 mV/s 500 mV/s

1

Voltage (V)

-Z'' (ohm)

6

4

0

Current density

40 (A/g)

50

MeEt3N BF4/PC

(g) MeEt3N BF4/PC

EMIM BF4

10

1

1

0.1

0.1 100

1000

0

2

4

6

8

10

Z' (ohm) (h)

100

Coulomb efficiency (%)

30

3

EMIM BF4

2

20

2

8

40

10

5 mV/s 10 mV/s 100 mV/s 800 mV/s

(f)

MeEt3N BF4/PC

0

2000

EMIM BF4

20 mV/s 200 mV/s

4.2

80

0

1500

Tim e (s)

-15

EMIM BF4


500

0

Potential (V) (e)

80 0.1A/g 0.2 A/g 0.5 A/g 1 A/g

15

-30

120

10

0 (d)

0

60

EM IM B F 4

0 30

10

0.0

40

1

MeEt3N BF4/PC

-20

20

2

2400

T im e (s)

0

3

1

2 A/g 5 A/g 10 A/g 20 A/g

1

Voltage (V)

2

(b)

2

40 0.1 A/g 0.5 A/g 1 A/g

20


30


Potential (V)

3

Specific capacitance (F/g)

3

5 A/g 10 A/g 20 A/g 50 A/g

Voltage (V)

Potential (V)

3 (a)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

60

40 EMIM BF4 (75.7%) MeEt3N BF4/PC (79.2%)

20 0

10000

Power density (W/kg)

2000

4000

6000

8000

10000

Cycle number

Figure 11. The GCD curves (a, b), CV curves (c, d), specific capacitance (e), electrochemical impedance spectroscopy (f), Ragone plots (g) and cycling performance (h) of ACNs-2 in ion liquid and organic electrolytes.

21



Conclusion In summary, ACNs-x with superior-developed pore structure and graphitized-like ribbons were successfully fabricated using asphaltene as precursor and urea as sheet-structure-directing agent. The ACNs-x equipped excellent electrochemical properties with a high Cm of 315.7 F/g at 1 A/g and 282.9 F/g even at 100 A/g in three-electrode setup. In two-electrode test with KOH aqueous solution, the Cm of the device was 216.7 F/g at 1 A/g and retained 89.6% after 10000 cycles at 4A/g showing a good lifetime and durability. ACNs-x also displayed desirable electrochemical performance in ion liquid and organic electrolytes. This work presented a feasible conversion strategy of coal liquefaction residue and asphaltene as a prospective precursor to produce novel nanosheet carbon structures for energy storage. Furthetmore, the properties of the porous carbon nanosheet will also make it widely used in catalyst supports, gas adsorption, separation, and other fields.

Acknowledgment This work was supported by National Key Research and Development Program of China (2016YFE0203500) and National Science Foundation of China (U1510122).

Supporting Information SEM/TEM images, XRD diffraction patterns, Raman spectra, Exhausted gases compositions, N2 adsorption-desorption isotherms, broad XPS spectra, structure parameters of samples, specific capacitances of compared samples in KOH aqueous solution, ionic liquid and organic electrolytes.

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For Table of Contents Use Only

Porous carbon nanosheets with developed pore structure and graphitized-like ribbon was synthesized using asphaltene and urea as in-situ sheet-structure-directing agent; and it exhibited an excellent specific capacitance, outstanding rate capability and cycling life as supercapacitor electrodes.

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Caption : Porous carbon nanosheets with developed pore structure and graphitized-like ribbon was synthesized using asphaltene and urea as in-situ sheet-structure-directing agent; and it exhibited an excellent specific capacitance, outstanding rate capability and cycling life as supercapacitor electrodes. 84x43mm (300 x 300 DPI)


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Asphaltene-based porous carbon nanosheet as electrode for

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