Hollow Carbon Spheres

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N-doped Mesoporous Carbon Sheets/Hollow Carbon Spheres Composite for Supercapacitors Lili Zhang, Lei Liu, Xiaolin Hu, Yifeng Yu, Haijun Lv, and Aibing Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02970 • Publication Date (Web): 27 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018

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N-doped Mesoporous Carbon Sheets/Hollow Carbon Spheres Composite for Supercapacitors Lili Zhang, Lei Liu, Xiaolin Hu, Yifeng Yu, Haijun Lv and Aibing Chen* College of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, 70 Yuhua Road, Shijiazhuang 050018, China. *Corresponding Author: E-mail: [email protected] Abstract The composite carbon materials with multiple morphologies (such as spheres/sheets and spheres/tubes, and so on) that combine the merits of both structures have a wide range of applications in electrochemistry, catalysis, energy storage and so on. Therefore, the development of an efficient and simple method for preparing carbonaceous composite materials is one of the research hotspots. mul Basing on the inhomogeneity of 3-aminophenol/formaldehyde (3-AF) polymerization spheres, the hollow 3-AF spheres were obtained after the dissolution of internal 3-AF oligomer. The dispersed 3-AF oligomer reassembled with silicate oligomers on the hard template of Mg(OH)2 sheets and hollow 3-AF spheres linked by CTAB through electrostatic force. The obtained N-MCS/HCS possessed both sheet and sphere structure, showing high specific surface area and uniform mesoporous distribution. As electrode material, N-MCS/HCS exhibited a good specific capacity (270 F g-1 at the current density of 1 A g-1) and outstanding cycling life stability (96.3% after 5000 cycles) at the current density of 5 A g-1, and could be used as new electrode material. Keywords: Composite carbon materials, Dissolution-reassembly, Hard-template, 1

ACS Paragon Plus Environment

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Electrode material. Introduction Supercapacitors have attracted increasing attention for next generation energy storage applications owing to their long cycle life, high cycle efficiency, high power density and fast charge/discharge rates.1-5 According to the charge-storage mechanism, supercapacitors can be divided into two categories: one is electrochemical double-layer capacitors (EDLCs), where the energy stores in the double layer by charge accumulation at the electrode/electrolyte interface, such as carbon materials; the other is the pseudocapacitors, where the capacitance derives from reversible faradaic reactions at the electrode/electrolyte surface, such as metal oxides or conducting polymers.6-10 It is generally believed that the performances of EDLCs primarily depended on the electrode materials. Among available candidates, carbonaceous materials are recognized as the most promising electrodes for supercapacitors because of their notable features including relatively light weight, high conductivity, high chemical stability, controllable porosity and plenty of active sites.11-16 In recent years, carbon materials with different morphologies including sphere, tube, sheet and monolith have been widely used as electrodes materials. Among them, carbon sheet, such as graphene and sheet-like carbon materials, has received extensive attention due to its larruping properties such as superior electrical conductivity and high theoretical specific surface area, which guarantee the good performance in terms of electrochemistry. Furthermore, fast charge transfer and mass transport could be 2


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achieved during the charging/discharging processes for carbon sheet.17 Thus, carbon sheet is particularly emerging as one of the most appealing electrode matrices. Numerous studies have demonstrated that carbon sheet as electrode material of supercapacitors exhibits excellent electrochemical properties.18-20 Hollow carbon spheres (HCS) with uniform spherical morphology, large cavity, high surface area, high thermal stability, large packing density and excellent conductivity are another candidate as electrode materials for supercapacitors.20-25 The unique hollow structure can enhance the surface-to-volume ratio and reduce the transport distance for mass diffusion and ion transport. In addition, the large cavity of HCS leaves immense room for further modify of their architectures and functionalities to optimize their electrochemical performance.26 Carbonaceous materials with one morphology (such as sheet, sphere and so on) have made great progress in supercapacitors, meanwhile, multiple dimensional composite carbon materials (such as spheres/sheets or spheres/tubes) combining the respective merits of different morphology show great prospects as electrode material for supercapacitors. GO is the most commonly used carbon sheets owing to the unique properties of flat layers, high surface area and good electric conductivity.27-29 Unfortunately, GO sheets tend to aggregate due to intersheets Van Der Waals attractions, resulting in the loss of the active surface and the drop in the capacitance.30,31 Therefore, the modification of GO with carbon spheres (such as mesoporous carbon spheres or hollow spheres and so on) has received widespread attention.32 The obtained composite material combining the merits of both sheet and 3


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sphere overcome the disadvantage of GO, showing excellent electrochemical properties. Although the composite materials of GO/carbon spheres exhibit good electrochemical properties, the relatively complex and high-cost preparation of graphene seriously limits their large-scale synthesis and wide use for applications because it violates the principles of easy manipulation and low cost. Hence, in order to expand its applications, it is necessary to develop a facile and low cost route for the preparation of the composite materials of carbon sheets/carbon spheres. Herein, we report a strategy of combining dissolution-reassembly with hard-template to prepare N-doped mesoporous carbon sheets/hollow carbon spheres (N-MCS/HCS) composite materials. 3-AF spheres could be transformed into hollow structure by dissolution with of acetone due to the compositional inhomogeneity inside the spheres. The dispersive 3-AF oligomer would assemble with TEOS on the hard-template of Mg(OH)2 sheets and hollow 3-AF spheres linked by CTAB through electrostatic force, forming mesoporous carbon nanosheets decorated with hollow carbon spheres. The obtained N-MCS/HCS has the characteristics of sheet and hollow sphere, which exhibits good performance for supercapacitors. Experimental Chemicals and Materials Formaldehyde solution (37%), ammonium hydroxide solution (NH3·H2O, 28 wt%), cetyltrimethyl-ammonium bromide (CTAB), anhydrous ethanol, magnesium chloride hexahydrate (MgCl2·6H2O), sodium carbonate (Na2CO3), acetone, Tetraethyl orthosilicate (TEOS) and hydrochloric acid (HCl, 36 wt%) were purchased from 4


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Tianjin Yongda Chemical Corp. 3-Aminophenol (3-AP) was purchased from Aladdin Corp. Preparation of N-MCS/HCS MgO rods were prepared according to previous report.31 0.15 g of the as-prepared MgO rods were homogeneously dispersed in 10 mL deionized water and stirred for 3 h. Then, 0.19 g of CTAB was added into the above solution and stirred for 20 min forming A reaction system. 3-AP, (0.1 g, 0.907 mmol), formaldehyde solution (37 wt%, 0.1 ml, 1.331 mmol), and ammonia aqueous solution (25 wt%) as catalyst were added into 30 ml deionized water and reacted at room temperature. After the reaction continued for 30 min, 20 ml acetone was added to selectively remove the interior part of the forming solid inhomogeneous nanospheres and stirring for another 30 min to form B reaction system. The A reaction system was added into B reaction system. After stirring for 10 min, 0.4 ml TEOS is dripped onto the above reaction system gradually. After stirring for 12 h at room temperature, the Mg(OH)2/hollow 3-AF@SiO2/3-AF composite was collected by centrifugation and washed with water and ethanol for several times and then dried at 60 °C. After carbonized at 800 oC for 3h under an N2 atmosphere, the N-MCS/HCS-0.15 was obtained. N-doped hollow carbon spheres (N-HCS), N-MCS/HCS-0.1 and N-MCS/HCS-0.2 were also synthesized using the same method as described above with the adding amount of MgO with 0, 0.1 and 0.2 g respectively. Preparation of N-MCS 0.15 g the as-prepared MgO rods were homogeneously dispersed in 30 mL deionized 5


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water and stirred for 3 h. Then, 0.19 g of CTAB was added into the above solution and stirred for 20 min. 3-AP, (0.1 g, 0.907 mmol) and ammonia aqueous solution (25 wt%) was added into the above solution and stirred for 30 min at room temperature. formaldehyde solution (37 wt%, 0.1 ml, 1.331 mmol) was added into the above solution and stirred for 10 min. Then 0.4 ml TEOS is dripped onto the above reaction system gradually. After stirring for 12 h at room temperature, the Mg(OH)2 @SiO2/3-AF was collected by centrifugation and washed with water and ethanol for several times and then dried at 60 °C. After carbonized at 800 oC for 3 h under an N2 atmosphere, the N-MCS was obtained. Material Characterization Nitrogen adsorption-desorption isotherms measurements was determined by using a Micromeritics TriStar 3020 volumetric adsorption analyzer at 77 K. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface area of each sample. The pore size was calculated from the adsorption branches of the isotherms, according to Barrett-Joyner-Halenda (BJH) method. Total pore volume was estimated from the N2 amount adsorbed at a relative pressure of P/P0 = 0.97. Transmission electron micrographs (TEM) were obtained on a JEOL JEM-2100 electron microscope. X-ray photoelectron spectrometer (XPS) data were collected by using AXIS ULTRA DLD spectrometer with Al Kα radiation as the excitation source and the peak positions were referenced internally to the C1s peak at 284.6 eV. Electrochemical Tests The

electrochemical

measurements

were

performed

6


in

a

three-electrode

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electrochemical cell with an electrochemical analyser (CHI 760E) in 6 M KOH electrolyte with Hg/HgO electrode and Platinum wire as reference electrode and counter electrode, respectively. The electrode was prepared by using the mixture of 80 wt% active carbon material with 10 wt% polytetrafluoroethylene (PTFE) binder and 10 wt% carbon black. The typical mass loading was about 5 mg/cm2. All electrochemical

behaviors

of

cyclic

voltammetry

(CV),

galvanostatic

charge-discharge (GCD), electrical impedance spectroscopy (EIS) and cycling stability were used to carry out electrochemical behaviors. The voltage for CV and GCD was chosen in the range of -1 to 0 V. The scan rates of CV were from 5 to 200 mV s-1, and current densities of GCD were from 0.5 to 10 A g-1. For the two-electrode system, the specific capacitances (C, F g-1), energy density (E, Wh kg-1) and power density (P, Wkg-1) were calculated by the following equations: C=4 I∆t/∆Vm, E=0.5 C(∆V)2 and P=3600 E/∆t, where I (A), ∆t (s), ∆V (V) and m (g) are GCD current, discharge time, voltage window, and mass of active material, respectively. In the three-electrode system, the specific gravimetric capacitance was according to the GCD measurements: C=I∆t/∆Vm. Results and discussion

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Scheme 1. The formation mechanism of N-MCS/HCS. The fabrication procedure of N-MCS/HCS was depicted in scheme 1. In the process of 3-AF resin polymerization at room temperature, a fast precipitation happened, which would result in the compositional inhomogeneity and a radial difference inside the 3-AF resin nanospheres. Treated with acetone, the resin oligomer inside of the nanospheres was dissolved due to the low crosslinking degree. While the hard surface shell could not be dissolved by acetone. Thus, the hollow 3-AF spheres were obtained. The resin oligomer, originating from dissolution of 3-AF oligomer, would disperse in the reaction solution. At the same time, MgO rods reacted with water to form Mg(OH)2 sheets acting as hard-template. The 3-AF resin oligomer and a layer of silicate oligomers composites were assembled on the hard-template of Mg(OH)2 sheets and hollow 3-AF spheres linked by CTAB through electrostatic force, forming Mg(OH)2/hollow 3-AF@SiO2/3-AF structures. Finally, N-MCS/HCS could be readily obtained after annealing and etching the SiO2 and Mg(OH)2 template. 8


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(a)

(b)

(c)

(d)

43 nm

340

nm

Fig. 1. TEM images of N-MCS (a, b) and N-MCS/HCS-0.15 (c, d). To further analyze the morphology and structure of N-MCS/HCS-0.15, TEM tests were conducted. As a comparison, N-MCS was successfully prepared with Mg(OH)2 sheets as hard template, 3-AF as carbon precursor, CTAB as surfactant and TEOS as structure-directing agent. The TEM images of N-MCS were shown in Fig. 1a and b. The large area carbon sheet with wrinkled structure could be seen from Fig. 1a, indicating that the sheet structure of Mg(OH)2 was completely copied into N-MCS.31 The TEM images of N-MCS/HCS-0.15 were shown in Fig. 1c and d. As shown in Fig. 1c, hollow carbon spheres with uniform particle size were observed on carbon sheets, demonstrating that the 3-AF oligomer and TEOS were successfully reassembled on the Mg(OH)2 sheets, and the sheet structure was duplicated from 9


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Mg(OH)2. The sheet edge was clearly distinguished from the background (the red sign). A higher magnification TEM image of N-MCS/HCS-0.15 in Fig. 1d showed that the thin carbon sheets with wrinkled structure possessed disordered mesopores which may be ascribed to the addition of TEOS. The TEM images further revealed a large cavity of 340 nm and a shell thickness of ~43 nm for hollow carbon spheres attached on carbon sheet.

(b) 1.6

N-MCS N-HCS N-MCS/HCS-0.1 N-MCS/HCS-0.15 N-MCS/HCS-0.2

30

20

10 0.0

0.2

0.4

0.6

0.8

Relative Pressure(P/P0)

1.0

dV/dD pore volume (cm3g-1)

(a) 40 Quantity Adsorbed(mmol g-1)

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

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1.2

0.8

0.4

0.0

5

10

15

20

Pore Diameter (nm)

Fig. 2. (a) Nitrogen adsorption-desorption isotherms of all samples; (b) pore size distribution curves of all samples. In order to investigate the difference in textural properties among N-MCS, N-HCS, N-MCS/HCS-0.1, N-MCS/HCS-0.15 and N-MCS/HCS-0.2, the N2 isothermal adsorption-desorption measurements were performed. As depicted in Fig. 2a, all samples exhibited similar type IV adsorption-desorption isotherms and type H3 hysteresis loops. A pronounced hysteresis in the P/P0 range of 0.4-0.9 was observed for all samples, implying the presence of a large number of mesoporous. Notably, compared with the N2 adsorption-desorption isotherms of N-MCS, the isotherms of N-HCS, N-MCS/HCS-0.1, N-MCS/HCS-0.15 and N-MCS/HCS-0.2 displayed a significant increase of the adsorption at high relative pressures (P/P0 > 0.9), indicating 10


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the existence of large cavity. Using the BJH method and the adsorption branch of the nitrogen isotherm (Fig. 2b), the calculated pore-size distribution was obtained. The detailed structural parameters of different samples are listed in Table 1. Obviously, N-MCS/HCS-0.15 has the largest the BET surface area and pore volume, maybe attributed to multiple dimensional composite structure of sheets/spheres. Table 1. The textural parameters of samples SBET (m2 g-1)

Samples

Vt (cm3 g-1)a

Smicro (m2 g-1)b

Vmicro (cm3g-1)c 0.1

Pore size (nm) 2.8

N-MCS 880 0.89 394 N-HCS 911 0.92 383 0.09 2.7 N-MCS/HCS-0.1 798 0.76 326 0.1 2.5 N-MCS/HCS-0.15 1144 1.3 441 0.08 3.5 N-MCS/HCS-0.2 910 0.91 375 0.09 2.8 aTotal pore volume at P/P ~0.97; bMicropore surface area determined by the t-plot; 0 cMicropore volume calculated by the t-plot method.

(a)

(b)

80

40

0

C

7.7

4.3

O

N

O1s N1s

1200

C1s

Intensity (a.u.)

Intensity (a.u.)

Percentage (%)

C1s 87.9

800

400

295

0

Binding Energy (eV)

(c)

290

285

280

Binding Energy (eV) O1s

(d)

N1s

Intensity (a.u.)

Intensity (a.u.)

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540

535

406

530

Binding Energy (eV)

404

402

400

398

Binding Energy (eV) 11


396

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Fig. 3. (a) XPS spectrum; (b) C1s spectrum; (c) O1s spectrum; (d) N1s spectrum of N-MCS/HCS-0.15. The 3-AF not only played the role of carbon precursor, but also acted as an excellent nitrogen precursor, leading to in-situ nitrogen doping for N-MCS/HCS-0.15. XPS was used to provide qualitative information about the surface elemental composition and its chemical environments, as shown in Fig. 3. Three peaks at around 284.6, 532.5 and 400.4 eV could be observed from the survey spectrum (Fig. 3a), corresponding to the C1s peak of sp2 carbon, the O1s, and the N1s of the doped nitrogen, respectively.33 Elemental analysis of XPS revealed that the carbon, oxygen and nitrogen content in the N-MCS/HCS-0.15 were 87.9, 7.7 and 4.3 wt%, respectively, as shown in the inset of Fig. 3a. These results further confirmed that the heteroatom of nitrogen had been successfully doped into the carbon skeleton of N-MCS/HCS. High-resolution XPS spectra for each element were performed and fitted, and the corresponding forms of each element were analyzed. As shown in Fig. 3b, the spectrum of C1s of N-MCS/HCS-0.15 could be deconvoluted into three type peaks, corresponding to C-C at 284.6 eV, C-N at 285.4 eV, C=O at 288.8 eV, which further indicates the existence of elements N, O in the surface of N-MCS/HCS.34-36 The spectrum of O1s (Fig. 3c) could be deconvoluted into four type peaks with binding energies of 530.0, 531.1, 532.9 and 536.4 eV from contribution of oxygen in carboxyl groups, C=O and chemically adsorbed oxygen, respectively.28,37,38 The high resolution XPS spectrum of N1s (Fig. 3d) can be further deconvoluted into three peaks at 398.3, 400.9, and 403.8 eV corresponding to the contribution of pyridinic 12


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nitrogen (at 398.3 eV), quaternary nitrogen (at 400.9 eV) and pyridine N-oxide (at 403.8 eV).28 The pyridinic nitrogen provide the active sites for electrode materials, and the quaternary nitrogen not only is the most stable nitrogen species during the process of pyrolysis but also can improve the electrical conductivity of the carbon materials.39,40 2

(b)0.0


1 0 -1


-2 -3

-0.8

-0.4

Potential(V)

(a) Current (A g-1)

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

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-0.4

-0.8

0

0.0

100

Potential (V)

200

300

400

500

Time (S)

Fig. 4. Electrochemical evaluation of all samples. (a) CV curves at a scan rate of 5 mV s-1; (b) the representative GCD curves at a current density of 1 A g-1. The N-MCS/HCS combines many advantages such as thin sheets, hollow, mesoporous structure, high surface area and suitable nitrogen doping for better electrochemical performance. Hence, in order to further prove that the multi-dimensional composite structure and the ratio of N-MCS/HCS have a great effect on the performance of supercapacitors, the electrochemical performance of N-MCS, N-HCS, N-MCS/HCS-0.1, N-MCS/HCS-0.15 and N-MCS/HCS-0.2 for electrochemical double-layer capacitance (EDLC) was evaluated respectively. As illustrated in Fig. 4a, the CV curves at 5 mV s-1 scan rate over the range of -1-0 V showed that all samples presented a nearly rectangular shape, indicating a favorable EDLC behavior with a good reversibility. As shown in Fig. 4b, GCD curves of all 13


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samples displayed neat isosceles triangle, suggesting good capacitance performance. N-MCS/HCS-0.15 exhibited a capacitance as high as 270 F g-1 at a current density of 1 A g-1, obviously higher than the N-MCS (198 F g-1), N-HCS (194 F g-1), N-MCS/HCS-0.1 (186 F g-1) and N-MCS/HCS-0.2 (216 F g-1) at the same current density. The results proved that the N-MCS/HCS-0.15 with unique multi-dimensional composite structure and appropriate ratio of N-MCS/HCS had good potential for supercapacitors.

14


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(b)0.0

(a) 40

0.5 A g-1 1 A g-1 2 A g-1 3 A g-1 4 A g-1 5 A g-1 10 A g-1

5 mV s 10 mV s-1 20 mV s-1 30 mV s-1 40 mV s-1 50 mV s-1 100 mV s-1 200 mV s-1

20 0 -20 -40 -60

-0.8

-0.4

0.0

Potential (V)

Current (A g-1)

-1

-0.4

-0.8

0

0.4

400

Potential (V)

(d)

250

1200

50

N-MCS/HCS-0.15 N-MCS

40

76%

150

Ref. [45] Ref. [46] Ref. [47] Ref. [48]

N-MCS/HCS Ref. [14] Ref. [17] Ref. [20] Ref. [44]

100 50 0

2

4

6

8

4

30 20

0

2

1

0

10

10

N-MCS/HCS-0.15 N-MCS

3

-Z'' (ohm)

200

0

800

Time (S)

300

-Z'' (ohm)

Specific Capacitance (F g-1)

(c)

0

1

2

3

4

Z' (ohm)

0

10

20

Current Density(A g-1)

30

40

50

Z' (ohm)

(e)

(f) 300 96.3%

80 (f) 0.0

1st cycle

40

C (F/g)

5000th cycle

Potential (V)

Capacitance retention (%)

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

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-0.4

-0.8

0

20

40

60

200

100

80

TIME (S)

0

0

1000

2000

3000

4000

5000

0

0

2

4

6

8

10

Loading density (mg/cm2)

Cycle Number

Fig. 5. (a) CV curves tested at 5-200 mV s-1 of N-MCS/HCS-0.15; (b) GCD curves tested at 0.5-10 A g-1 of N-MCS/HCS-0.15; (c) The correlation of specific capacitance at various current densities for N-MCS/HCS-0.15; (d) Nyquist plot of N-MCS/HCS-0.15 and N-MCS; (e) Cycling performance of the N-MCS/HCS-0.15 supercapacitors for charging and discharging at a current density of 5 A g-1; (f) Capacitance as a function of the areal mass loading of N-MCS/HCS-0.15 at the current density of 1 A g-1. 15


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Since N-MCS/HCS-0.15 showed the highest capacitance value, more detailed electrochemical tests (such as CV, GCD, EIS and cycle stability) were carried out on it. GCD was measured to test the electrochemical performance of N-MCS/HCS-0.15 at different current density, as shown in Fig. 5a, b. Fig. 5a is the CV curves of N-MCS/HCS-0.15 recorded at different scan rates, and all the CV curves maintain quasi-rectangular shape without obvious redox peaks, indicating a dominant EDLCs behavior and outstanding rate ability of N-MCS/HCS-0.15. As shown in the Fig. 5b, the N-MCS/HCS-0.15 has a specific capacitance of 270 F g-1 at the current density of 1 A g-1. Obviously, all of the GCD curves show quasi-triangular shapes with good symmetry, indicating excellent capacitive performance and electrochemical reversibility, even at high current densities 10 A g-1. In order to investigate the influence of the current densities on the specific capacitances, the charge-discharge measurements are recorded at different current densities, as shown in the Fig. 5c. The N-MCS/HCS-0.15 electrode reveals a specific capacitance retention rate of 76%, even at a high current density of 10 A g-1, which is significantly higher than other materials, such as hollow mesoporous carbon spheres (52.2%), mesoporous carbon nanospheres/ graphene sheets (61.0%) and microporous and mesoporous carbon (73.3%) and so on [41-43]. In addition, the specific capacity of N-MCS/HCS-0.15 is higher than that of many other carbon materials, such as carbon sheets, hollow carbon spheres, graphene sheets/mesoporous carbon nanospheres, and activated carbon, attributing

to

multidimensional

composite

structure

spheres.14,17,20,44-48 16


of

sheets/hollow

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The fast ion response rate in N-MCS/HCS-0.15 was examined by EIS in the frequency range from 10-2 to 105 Hz. In order to compare the electron transport behavior of N-MCS/HCS-0.15 and N-MCS, Nyquist plots of N-MCS/HCS-0.15 and N-MCS were shown in Fig. 5d. The Nyquist plots of the N-MCS/HCS-0.15 and N-MCS electrode consisted of capacitive semicircles in the high frequency region and straight lines at different constant inclining angles in the low frequency region. In the low-frequency region, the Nyquist plots of N-MCS/HCS-0.15 and N-MCS presented a straight line indicating an excellent ion diffusion and charge transportation behavior of the electrode. From the magnified region in the high frequency range, the equivalent series resistance (ESR) of N-MCS/HCS-0.15 and N-MCS obtained from the intersection of the Nyquist plots at the x-axis are 0.65 and 0.68 respectively. These results further demonstrate the electron transport behavior of N-MCS/HCS-0.15 is better than that of N-MCS. This suggests that the N-MCS/HCS-0.15 electrode has a very low resistance with a good ion response at high frequency ranges.49 In addition, the long-time cycling stability is an important factor to identify the feasibility for practical application of the device. Fig. 5e shows superior capacitance maintained up to 96.3% after 5000 cycles at the high current density of 5 A g-1. At the same time, the GCD curve of the 5000th cycle maintains similar shape with linearity and symmetry as the first cycle (inset in Fig. 5e), indicating excellent capacitive property and long-term electrochemical stability, which may be ascribed to fluffy porous structure of N-MCS/HCS-0.15. In order to further prove the effect of loading density of the material on electrochemical performance of the electrodes, we have performed the 17


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loading dependent performance test (F/g vs. mg/cm2). As shown in Fig. 5 (f), Capacitance value of N-MCS/HCS-0.15 decreases with the increase of loading density.

(b) 0.8

4 5 mV s-1 10 mV s-1 20 mV s-1 30 mV s-1 40 mV s-1 50 mV s-1 100 mV s-1

0

-4

Potential (V)

Current(A g-1)

(a)

0.5 A g-1 1 A g-1 2 A g-1 3 A g-1 4 A g-1 5 A g-1 10 A g-1

0.4

0.0 0.0

0.4

0.8

1.2

0

100

Time（ S（

Potential (V)

(c) 200

(d)

150

68.5%

100

50

0

0

4

8

Energy density(Wh kg-1)

Specific Capacitance (F g-1)

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

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-1

Current Density (A g )

This work Hollow carbon nanofibers

N-doped graphene

N-porous carbon

1

100

Hollow carbon spheres /carbon nanotubes Hierarchical porous graphitic carbon Activated porous carbon

1000

10000

Power Density (W kg-1)

Fig. 6. (a) CV curves tested at 5-100 mV s-1 of N-MCS/HCS-0.15 in a symmetric two-electrode supercapacitor in 6 M KOH aqueous solution; (b) GCD curves of N-MCS/HCS-0.15 at difference current density in two-electrode system; (c) Specific capacitances at different GCD current densities of N-MCS/HCS-0.15 in two-electrode system; (d) Ragone plots of N-MCS/HCS-0.15. To fully measure its practical applications, the as-prepared N-MCS/HCS-0.15 was tested in symmetric two-electrode supercapacitor in 6 M KOH. Fig. 6a displays the corresponding CV curves at different scan rates. All of the curves show quasi-rectangular shapes, suggesting that the double-layer capacitance is the main 18


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contribution to the total capacitance. The excellent capacitive behavior of N-MCS/HCS-0.15 is also reflected in their long GCD times and symmetrical isosceles triangle-like profiles (Fig. 6b). The specific capacitance of the N-MCS/HCS-0.15 is calculated to be 175 F g-1 at a current density of 0.5 A g-1. Fig. 6c exhibited the specific capacitance of all the N-MCS/HCS-0.15 at various current densities. Moreover, it could be seen that the N-MCS/HCS-0.15 had high capacitance retentions over 68.5% (Fig. 6c) from current density of 0.5 to 10 A g-1. The energy density and power density of N-MCS/HCS-0.15 calculated from GCD discharging curves is shown in Fig. 6d and compared with other carbon materials [50-55]. The Ragone plot reveals that the maximum energy density of N-MCS/HCS-0.15 is 15.1 Wh kg-1 at a specific power density of 0.78 kW kg-1, and still maintains 10.4 Wh kg-1 at a high specific power density of 14.9 kW kg-1. It is obvious that the energy density of N-MCS/HCS-0.15 is higher than other reported carbon materials. The density of N-MCS/HCS-0.15 is about 0.76 g cm-3 by measurement. The density of N-MCS/HCS-0.15 is higher than ordered mesoporous carbon (0.72 g cm-3), nitrogen-doped carbon nanofibers (0.27 g cm-3), nanotube/graphene fibers (0.59 g cm-3) and so on [55-57]. Based on the calculated density, volumetric capacitance, volumetric energy density, and volumetric power density of the sample are about 205 F/cm3, 11.5 Wh/L and 594 W/L respectively. As shown in table 2, comparing these values with the previously reported carbon electrodes shows that N-MCS/HCS-0.15 exhibits high capacitance and good performance for supercapacitors [58-60]. These results prove that the N-MCS/HCS-0.15 with high surface area and multiple 19


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dimensions composite structure have the potential to be used as electrode materials for supercapacitors. Table 2. Comparison of these values of N-MCS/HCS-0.15 with other reported carbon materials. Samples

Cv (F cm-3)a

Ev (Wh L-1)b

Pv (W L-1)c

This work HPCs

205 15.3

11.5 6.0

594 470

CNT/Graphene

8.6

2.0

630

Graphene 84.7 3.6 350 -3 b -1 cm )=* Cs (three-electrode); Ev (Wh L )=*E (two-electrode); cPv(W L-1)=*P (two-electrode).

aCv(F

Conclusions In summary, a facile and effective dissolution-reassembly strategy combining with hard-template has been developed to fabricate mesoporous carbon sheets/hollow carbon spheres composite. 3-AF resin spheres as carbon precursor are used to create the cavity and 3-AF resin oligomer by dissolution with acetone. The 3-AF oligomers, TEOS and CTAB are reassembled on hard template Mg(OH)2 and hollow 3-AF resin spheres, forming the hollow spheres/sheets composite materials of N-MCS/HCS after annealing and etching the SiO2 and Mg(OH)2 template. Thanks to their inherent cavity, mesoporous carbon sheets, high specific surface area, suitable nitrogen doping, N-MCS/HCS-0.15

exhibits

high

capacitance

and

good

performance

for

supercapacitors with good specific capacity (270 F g-1 at the current density of 1 A g-1) and excellent high rate capability. This dissolution-reassembly strategy combining with hard-template making full use of the difference of polymerization degree of RF resin provides a new idea for the preparation of multidimensional composite carbon 20


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materials. AUTHOR INFORMATION Corresponding Author E-mail address: [email protected] (Aibing Chen). Notes The authors declare no competing financial interests. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21676070), Hebei One Hundred-Excellent Innovative Talent Program (III) (SLRC2017034), Hebei Science and Technology Project (17214304D, 16214510D), The Excellent Going Abroad Experts' Training Program in Hebei Province. References [1] Cohn, A. P.; Erwin, W. R.; Share K.; Oakes, L.; Westover, A. S.; Carter, R. E.; Bardhan, R.; Pint, C. L. All Silicon Electrode Photocapacitor for Integrated Energy Storage and Conversion. Nano. Lett. 2015, 15, 2727-2731. [2] Wu, Z.; Zhang, X. B. N, O-codoped Porous Carbon Nanosheets for Capacitors with Ultra-high Capacitance. Sci. China. Mater. 2016, 59, 59547-59557. [3] Wang, Y. X.; Song, Y. F.; Wang, Y.; Chen, X.; Xia, Y. Y.; Shao, Z. Z. Graphene/Silk Fibroin Based Carbon Nanocomposites for High Performance Supercapacitors. J. Mater. Chem. A 2014, 3, 773-781.

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Yushin, G. High-rate Electrochemical Capacitors Based on Ordered Mesoporous Silicon Carbide-derived Carbon. ACS Nano 2010, 4, 1337-1344. [58] Yao, L.; Wu, Q.; Zhang, P. X.; Zhang, J. M.; Wang, D. R.; Li, Y. L.; Ren, X. Z.; Mi, H. W.; Deng, L. B.; Zheng, Z. J. Scalable 2d Hierarchical Porous Carbon Nanosheets for Flexible Supercapacitors with Ultrahigh Energy Density. Adv. Mater. 2018, 30, 1706054. [59] Pan, Z. H.; Liu,

M. N.; Yang, J.; Qiu, Y. C.; Li, W. F.; Xu, Y.; Zhang, X. Y.;

Zhang, Y. G. High Electroactive Material Loading on a Carbon Nanotube@3d Graphene Aerogel for High-performance Flexible All-solid-state Asymmetric Supercapacitors. Adv. Funct. Mater. 2017, 27, 1701122. [60] Wang, S.; Wu, Z. S.; Zheng, S.; Zhou, F.; Sun, C.; Cheng, H. M. Scalable Fabrication

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Graphene-based

Monolithic

Micro-Supercapacitors with Superior Energy and Power Densities. ACS Nano 2017, 11, 4283-4291.

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Hollow Carbon Spheres

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