sheets Composite for

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N-doped Hollow Carbon Spheres/sheets Composite for Electrochemical Capacitor Juan Du, Lei Liu, Yifeng Yu, Zepeng Hu, Beibei Liu, and Aibing Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16921 • Publication Date (Web): 01 Nov 2018 Downloaded from http://pubs.acs.org on November 2, 2018

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ACS Applied Materials & Interfaces

N-doped Hollow Carbon Spheres/sheets Composite for Electrochemical Capacitor Juan Du†, Lei Liu†, Yifeng Yu†, Zepeng Hu†, Beibei Liu†, 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] ABSTARCT Functional carbon materials with combination of 0 dimension and 2 dimension are particularly interesting in electrochemical field owing to the low density, high surface area and strong ion bearing capacity. Especially, hollow mesoporous carbon spheres (0 dimension) and nanosheet (2 dimension) composite (HMCS/S) have received much attention as electrochemical capacitor electrode materials. However, it is challenging for effective preparation of this complex composite structure. In this work, a novel and simple procedure to prepare N-doping HMCS/S (N-HMCS/S) is presented. This approach adopted silica spheres as the core and employed [C18Mim]Br and TEOS as the structural directing agent for formation of the flaky/spherical hybrid structure. The unique structure of nanosheets embedded by hollow carbon spheres and N-doping characteristics endow NHMCS/S with good performance in electrochemical capacitor with high specific capacity (196.5 F g-1) in three-electrode and excellent high rate capability with retention of 61.5 % in two-electrode system. Keywords: hollow carbon spheres, carbon sheets, N-doped, ion liquid, electrochemical 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

capacitor 1. INTRODUCTION Electrochemical capacitors, which are high-rate energy delivery and storage devices, possess wide potential applications, including energy efficient industrial equipment, consumer electronics, hybrid electric vehicles and memory back-up systems1-2. For improving the performance of supercapacitors, electrode material is one of the key factors. Among those materials, carbonaceous materials have obtained increasing attentions due to their superior chemical and physical features, such as diverse morphology, low mass density, promising electronic conductivity and good chemical stability3-5. It has been demonstrated that the microstructures and morphologies of the carbon materials are very important for their electrochemical performance. Used as electrode materials, carbon materials with characteristics of 0 dimension and 2 dimension may exhibit good performance in supercapacitor due to the high specific surface, low density and strong ion bearing capacity. Hollow mesoporous carbon spheres, which is an outstanding member in the family of 0 dimensional structure, have demonstrated promising potentials for energy-conversion/storage applications where high electronic conductivity is often required6. A large number of approaches for preparing hollow mesoporous carbon spheres have been adopted, such as sol-gel strategy, template based method and chemical vapor deposition. In those syntheses, the hard template method has been well accepted as a common strategy, which involves process of the coating and carbonization of carbon precursor on the surface of the solid spherical template, as well as 2 ACS Paragon Plus Environment

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removing the template7-8. The hard template method could offer better control for desired morphology, mesoporous structure and monodispersity9-10. However, the application of hollow mesoporous carbon spheres for supercapacitor is greatly limited due to the long-distance ion transfer. It is well established that carbon nanosheet (2 dimensional structure) presents large surface area, low density, short distance ion transfer and high electronic conductivity, which can make up for the shortcomings of hollow mesoporous carbon spheres in respect of ion transportation11-13. Hence, the combination of hollow porous carbon spheres and nanosheet (HMCS/S) will not only overcome the shortcomings but also keep the advantages of hollow carbon spheres and carbon sheets. Zhang and his co-authors reported a graphene sheet-sphere nanocomposite with good electrochemical performance using graphene oxide and [Ni2(EDTA)] as precursors14. However, the synthesis of this composite structure is difficult because difficult carbon structures normally form by different methods. Surface functionalization is also one of crucial factors to improve the electrochemical performance of carbonaceous materials, such as N-doping, which can modify the crystalline structures and hydrophily of the carbon host3. The doped carbon materials have demonstrated better electrical conductivity through pseudo-capacitance basing on a mechanism of Faradaic reaction. Especially, the quaternary N can facilitate the electrical conductivity for carbon material, and chemical affinity of carbon host can be improved by pyrrolic N and pyridinic N15. The synthesis of N-doped hollow carbon materials from ionic liquids (ILs)16, which are ideal nitrogen precursors, have been reported by many groups. 3 ACS Paragon Plus Environment


Hollow carbon spheres with tunable size and structure can be facilely synthesized by ILs basing on the favorable interactions between the strongly polar ILs and polar inorganic silica templates17. Herein, a simple sol-assembly binding template strategy was presented to synthesize a novel N-doped HMCS/S (N-HMCS/S) composite with combination of 0 dimension (sphere) and 2 dimension (sheet). In this process, SiO2 sphere and its oligomer were used as a hard template and structure supporter respectively. 1-alkyl-3-methylimidazolium bromide ([C18Mim]Br) was used as surfactant to facilitate the assembly of SiO2 sphere and phenolic resin and also introduced nitrogen into the composite. The SiO2 sphere was used to create large cavity for hollow carbon sphere and the sol-assembly of abundant [C18Mim]Br, TEOS and phenolic resin led to generation of carbon sheet. At the same time, strong force attributing to the [C18Mim]Br resulted in special composite structure in which the hollow carbon sphere was embedded in the carbon sheet. The obtained N-HMCS/S, which combined the advantages of 0 and 2 dimensional structure, possessed spheres with large cavity and sheets with rich porous structure due to the existence of SiO2 sphere and TEOS. Those outstanding structural and N-doping characteristics endow the N-HMCS/S with a good specific capacity in supercapacitor. 2. RESULTS AND DISCUSSION

4 ACS Paragon Plus Environment

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(b)

(a)

2.1 nm

(d)

(c)

160 nm

Figure 1. TEM image of N-MCS (a-b) and SEM image of N-HMCS/S (c-d). N-MCS could be successfully synthesized by sol-assembly of [C18Mim]Br, TEOS and phenolic resin. Figure 1a and b showed the transmission electron microscopy (TEM) images of N-MCS. The laminar morphology like silk veil waves could be seen from Figure 1a. The porous structure was confirmed by higher solution TEM (Figure 1b). The irregular pores with size of 2.1 nm with large area could be seen, which might derive from the silica oligomer generated from the hydrolysis of TEOS. This result revealed that the sol-assembly of [C18Mim]Br, TEOS and phenolic resin could lead to mesoporous carbon sheet. When adding SiO2 sphere into the reaction system of N-MCS, N-HMCS/S was obtained. 5 ACS Paragon Plus Environment


The scanning electron microscopy (SEM) images was used to characterize the microstructure details of obtained N-HMCS/S. Figure 1c and d showed the SEM images of N-HMCS/S. Disorderly stacked sheets with a lateral length of more than 10 μm were clearly observed which form roundabout and connecting macropores. It was obvious that the surface of carbon sheet was crude, attributed to the embedded hollow carbon spheres (Figure 1c). The SEM image with higher resolution (Figure 1d) showed hollow carbon spheres adhere to carbon sheet. Carbon spheres with whole (red circle) or broken (yellow circle) morphology and diameter size of ca. 160 nm were uniformly embedded in carbon sheets.


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(a)

(b)

8 nm 160 nm

(c)

(d) 2.1 nm

2.1 nm

(e)

Figure 2. TEM images (a-d) and STEM-EDS elemental mapping images of N-HMCS/S (e). To further investigate the detailed morphology of the N-HMCS/S, TEM test was conducted. As demonstrated in Figure 2a, the hollow carbon spheres (the red arrow) were evenly dispersed in the carbon nanosheets. The translucent image revealed very thin nanosheets. Moreover, some broken hollow carbon spheres could also be seen (the yellow arrow in Figure 2a). The average diameter of N-HMCS/S was about 160 nm, consistent with the 7 ACS Paragon Plus Environment


SEM results (Figure 2b). And the shell thickness of 8 nm, which was thinner than many other hollow carbon spheres derived from resin18-19 and ILs20. The cavity was attributed to silica sphere core. The hollow carbon spheres with thin shell was beneficial for ion transport, posing a positive effect on the performance of electrochemical capacitor. Notably, all the hollow carbon spheres distributed were adhere to rather than isolating from the carbon sheet, indicating a strong force between carbon hollow spheres and sheets in the process of synchronous formation of carbon hollow spheres and sheets. As shown in Figure 2c, a higher resolution TEM image of the carbon sheet indicated the amorphous carbon of the sheets and disorder mesoporous structure. The wormlike uniform mesopores and microspores on the carbon shells also could be seen from the high resolution TEM image (Figure 2d). Energy dispersive spectra (EDS) was tested to observe the element distribution in N-HMCS/S as shown in Figure 2e, which exhibited the carbonaceous network (Figure 2e). The uniformly distribution of C, N and O elements could be found from the corresponding elemental mapping in this region clearly.


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(b) 2.0

N-MCS N-HMCS/S

40

dV/dlog(D) (cm3 g-1)

-1

Quantity Adsorbed (mmol g )

(a) 30

20

10

0.0

0.2

0.4

0.6

0.8

(c)

1.2 0.8 0.4 0

5

10

O1s N1s

1000

800

600

400

20

(d) C=C

C-C/C-N C-O

COOH 1200

15

Pore diameter (nm)

Intensity (a.u.)

C1s

N-MCS N-HMCS/S

1.6

1.0

Relative pressure (P/P0)

Intensity (a.u.)

294

200

292

Binding Energy (eV)

290

288

286

284

282

280

Binding Energy (eV)

(f)

(e)

Intensity (a.u.)

C-O

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


C=O -OH

544

540

536

532

Pyrrolic-N

Pyridinic-N Quaternary-N

408

528

Binding Energy (eV)

404

400

Binding Energy (eV)

396

Figure 3. N2 adsorption-desorption isotherm (a) and pore size distribution (b) of N-MCS and N-HMCS/S, XPS spectra (c), C1s (d), O1s (e) and N1s (f) spectrum of N-HMCS/S. In order to examine the pore characteristics and surface area of N-MCS and N-HMCS/S, the N2 adsorption experiments was performed. Both of N-MCS and N-HMCS/S presented characteristics of type IV isotherm with a hysteresis loop at P/P0 > 0.4 as shown in Figure 3a, indicating the presence of mesoporous structure. The sharp increase at the relative pressure range of 0.9-1 in the adsorption-desorption isotherm of N-HMCS/S indicated the presence of large cavity. The surface area and pore volume of the composite N-HMCS/S 9 ACS Paragon Plus Environment


were 1230 m2·g-1 and 1.40 cm3·g-1 respectively, which were higher than those of N-MCS (645 m2·g-1 and 0.73 cm3·g-1, respectively) due to the additional hollow carbon sphere. The higher surface area provided larger storage space and more active sites for electrochemical supercapacitor compared with some other hollow carbon spheres21-23. The N-HMCS/S exhibited mesoporous size of 2.1 nm derived from the N2 adsorption isotherm (inset of Figure 3b). In order to investigate the nitrogen distribution and content in the obtained N-HMCS/S, XPS was further characterized. Figure 3c presented the XPS survey spectrum of the NHMCS/S sample. XPS elemental analysis revealed that the carbon, oxygen and nitrogen content in the N-HMCS/S were 95.1, 1.7 and 3.2 wt% respectively. The pure three peaks ascribed to C1s, N1s and O1s demonstrated the high purity of carbon without any undesirable components. Four single peaks, corresponding to C=C (284.6 eV), C-N or CC (285.8 eV), C-O (286.5 eV), and C=N/C=O (289.5 eV) functional groups, could be found from the C1s spectrum (Figure 3d). The O1s of oxygen functionalities for N-HMCS/S (Figure 3e) could be deconvoluted into three peaks, which were the carboxyl C-O (61.2 %) centering at 532.5 eV, C=O (29.5 %) at 531.0 eV, and hydroxyl oxygen (9.3 %) in amorphous hydrogenated carbon at 536.5 eV respectively. The N1s spectrum is shown in Figure 3f. Three peaks located at 402.2, 400.7 and 398.2 eV can be identified as the quaternary nitrogen (35.5 %), pyrrolic nitrogen (46.8 %), and pyridinic nitrogen (17.4 %), respectively24.


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Figure 4. Schematic illustrations of the preparation of the N-MCS (Route 1) and NHMCS/S (Route 2). The synthesis mechanism of N-MCS and N-HMCS/S can be described as shown in Figure 4. N-MCS was synthesized by sol-assembly of [C18Mim]Br, TEOS and phenolic resin (Route 1 of Figure 4). For N-HMCS/S (Route 2 of Figure 4), silica nanoparticles with diameter of 144 nm served as core to create cavity of N-HMCS/S. Meanwhile, [C18Mim]Br and TEOS were employed as the structure-directing agent to form the carbon shell and sheet. When the [C18Mim]Br was added into the prepared silica spheres suspension solution, part of the [C18Mim]+ would be adsorbed on negative charged silica spheres surface through electrostatic attraction 25. After the addition of TEOS and resorcinol, both the TEOS and the resorcinol-formaldehyde resin (RF) precursor interacted with [C18Mim]Br through electrostatic interactions to form hybrid aggregates and mesoporous structures24. Then the hybrid aggregates can further condense and crosslink into interpenetrating 3D rigid frameworks, forming RF polymer and silica composites on the 11 ACS Paragon Plus Environment


surface of SiO2 core. Additionally, the redundant [C18Mim]-coated RF aggregates silica would assemble around the RF polymer and silica composites spheres to form polymer/silica composite sheet. Finally, N-HMCS/S could be readily obtained after pyrolysis and etching the SiO2 template. The pyrolysis of [C18Mim]Br resulted in nitrogen doping in the carbon host. (b)

20

-1

Current Density (A g )

(a)

10

-1

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 -1 200 mV s

0 -10 -20 -0.8

(c) 0.0

(d)

-1

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

-0.4 -0.6

-0.4

0.0

0.4

Potential (V)

0.0

IR-0.1V

-1

20 A g -1 30 A g -1 40 A g -1 50 A g -1 100 A g

-0.2

Potential (V)

-0.2

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

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-0.8

-0.4 -0.6 -0.8

-1.0 0

200

400

600

-1.0

800

0

2

4

Times (s)

6

8

Times (s)

10

12

14

Figure 5. The ion transport model of the N-HMCS/S (a), CV (b) and GCD (c and d) curves at different scan rates and current densities of N-HMCS/S in three-electrode system. Hollow mesoporous carbon sphere materials and porous carbon sheets have been keenly pursued as promising candidates for electrode materials in supercapacitor due to their high specific surface area, abundant porous structure, low density and strong ion bearing 12 ACS Paragon Plus Environment

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capacity. The combination of hollow mesoporous carbon spheres (0 dimension) and porous carbon sheets (2 dimension) can further improve the performance of electrochemical capacitor taking advantages of 0 and 2 dimensional materials. The unique characteristics of porous carbon sheets embedded by hollow carbon sphere and N-doping endow NHMCS/S with good performance in ion transportation and hydrophilicity (Figure 5a). The performance of N-HMCS/S as electrode was tested by cyclic voltammetry (CV) measurements (Figure 5b). It could be seen that the regular rectangular shape was retained from 5 to 100 mV s-1 scan rate, demonstrating an ideal electronic double layer capacitor (EDLC). The good performance of N-HMCS/S was also confirmed by galvanostatic charge-discharge (GCD) as shown in Figure 5c and d. The quasi-triangular and symmetrical GCD curves from 0.5 to 100 A g-1 current densities and voltage drop of 0.10 V at 100 A g-1 could be observed, suggesting the superior charge-discharge reversibility, typical EDLC behavior and low internal resistance of the electrodes possessed.



(b) 80

-1

Specific Capacity (F g )

(a) 200 150

Potential (V)

50

N-HMCS/CS

-0.2 -0.4 -0.6 -0.8

N-MCS

-1.00

100

200

300

0

20

40

2

1

0

Time (s)

60

80

100

-1

Current Density (A g )

0

1

20

2

Z' (Ohm)

40

3

4

60

80

Z' (Ohm)

1.0

(d)

-1

0.2 A g -1 0.5 A g -1 1Ag -1 2Ag -1 3Ag -1 4Ag -1 5Ag

0.6

0

0

1.0

-1

10 A g -1 20 A g -1 30 A g -1 40 A g -1 50 A g

0.8

0.4 0.2

0.6 0.4 0.2

0.0 0

100

200

300 Times (s)

400

0.0

500

(e)

0

2

4 Times (s)

6

8

-1

Specific capacity (F g )

10

1

0.1

200 150

61.5 %

100 50 0 0

20

40 -1

Current density (A g ) 10

100

1000

N-HMCS/S Ref. 32 Ref. 33 Ref. 34 Ref. 35 Ref. 36 Ref. 37 Ref. 38

Capacitance Retention(%)

(f)100 80 60

0.8

40

0.6

First Cycle

0.4 0.2

20

0.0

Last Cycle 0

2

4

6

8

10

12

14

16

18

Times (s)

0

10000

1.0

Potential (V)

Potential (V)

3

20

0.8

-1

4

40 -Z'' (Ohm)

0.0

100

Potential (V)

(c)

57.1%

-Z'' (Ohm)

60

0

Energy density (Wh Kg )

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

2000

4000

6000

8000

10000

Cycle Number

-1

Power density (W Kg )

Figure 6. Specific capacitances of N-HMCS/S (a) and GCD curves of N-MCS and NHMCS/S at 1 A g-1 (inset); Nyquist plots (b) and the fitted equivalent circuits (inset of b); GCD curves at different current density (c and d) of N-HMCS/S in the two-electrode system; Ragone plots and specific capacitances of N-HMCS/S at different GCD (inset) (e) and Cycle stability of the electrode at 5.0 A g-1 (f) of N-HMCS/S in two-electrode system.


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Table 1. Comparison of the gravimetric capacitances for sample N-HMCS/S with previous reported carbon spheres. Samples

Morphology

SSA m2 g-1

Electrolyte

Cs F g-1

I/ma A g-1

Ref.

N-HMCS/S

1230

6 M KOH

196.5

0.5

This work

Porous carbon spheres

2900

1 M H2SO4

182

0.5

26

Porous carbon spheres

735.4

6 M KOH

154

2

27

Core-shell carbon spheres

620

6 M KOH

130

0.5

28

Graphene aerogel

220

6 M KOH

125

0.5

29

Carbon sheets

2287

6 M KOH

156

1

30

Graphene film

128

6 M KOH

58

1

31

a: current density.

Figure 6a exhibited the rate performance of N-HMCS/S at current density from 0.5 to 50 A g-1, indicating the capacitance retention 57.1 %. The specific capacity of 196.5 F g-1 at 15 ACS Paragon Plus Environment


0.5 A g-1 was obtained by discharge branches. The GCD curves of N-MCS and N-HMCS/S at 1 A g-1 (inset of Figure 6a) suggested that the N-HMCS/S possessed higher capacitance and better electrochemical performance than N-MCS ascribed to the advantages of 0 and 2 dimensional structure in N-HMCS/S. In addition, the specific capacity of N-HMCS/S was higher than that of many other kinds of carbon materials, such as graphene, carbon sphere and carbon sheets26-31, as shown in Table 1, confirming the superiority of combination of 0 and 2 dimensional structures. The facilitated ion and electron transport behavior of N-HMCS/S was confirmed by the electrical impedance spectroscopy measurement. The Nyquist plots of the N-HMCS/S electrode consisted of capacitive semicircles in the high frequency region and straight lines at different constant inclining angles in the low frequency region (Figure 6b). The solution resistance were determined to be 0.1 Ω from the intercept at real axis and the semicircle intercepts, showing the excellent stability of the N-HMCS/S. For real supercapacitor, a symmetric capacitor was further investigated to evaluate the capacitive performances of N-HMCS/S in a two-electrode system. The GCD profiles of NHMCS/S at different current density (Figure 6c and d) revealed that the capacitor remained a well quasilinear GCD curves even at high current densities, showing its promising application of N-HMCS/S for high-performance supercapacitor. The calculated specific capacitance of the N-HMCS/S was 195 F g-1 at a current density of 0.2 A g-1. The rate capability of N-HMCS/S was also calculated by discharge branches in two-electrode system and the rate performance at the current density range of 0.2-50 A g-1 was shown in 16 ACS Paragon Plus Environment

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the inset of Figure 6e. It can be seen that the N-HMCS/S had a high capacitance retentions 61.5%. As illustrated in the Ragone plots (Figure 6e), calculated by GCD in the symmetric supercapacitor, the N-HMCS/S exhibited good performance. The energy densities of NHMCS/S accordingly decreased from 27.1 to 10.6 Wh kg-1 when power densities increased from 0.99 to 1.58 kW kg-1. Notably, the N-HMCS/S exhibited relative much better performance than many carbon spheres reported previously with higher power output capability at corresponding energy density, further demonstrating that the special concomitant nanostructure of N-HMCS/S led to the superior performance32-38. In addition, basing on the density (6.25 g cm-3) of N-HMCS/S when coated in the current collector, the volume energy density was 4.34 kWh m-3 calculated in the two-electrode system. Long cycling is another crucial parameter for practical applications of supercapacitors. As displayed in Figure 6f, 78.1 % of initial capacity was retained with ca. 100 % coulombic efficiency after 10 000 cycles in two-electrode system. The GCD curves of the last cycles were almost similar with the first cycles (Figure 6f inset), which were linear and symmetrical (the model of symmetrical supercapacitor was shown inset of Figure 6f), indicating excellent capacitive property and long term electrochemical stability. 3. CONCLUSION In summary, the N-HMCS/S composites combining 0 and 2 dimension architecture have been successfully prepared through a facile sol-assembly binding template strategy. RF was used as a carbon precursor, SiO2 spheres and its oligomer as a structure supporter, and 17 ACS Paragon Plus Environment


[C18Mim]Br simultaneously as nitrogen and partial carbon precursors. The resultant NHMCS/S possessed hollow carbon spheres, porous carbon sheets, a certain of nitrogen content, large specific surface and mesoporous structure. The N-HMCS/S showed good electrochemical performance with a specific capacity (196.5 F g-1 at the current density of 0.5 A g-1) in three-electrode system and excellent high rate capability with retention of 61.5 % at the current density range of 0.2-50 A g-1 as well as outstand electrochemical stability with 85 % of initial capacity after 5000 cyclic tests in two-electrode system. This simple sol-assembly binding template strategy provides a new way to prepare multidimensional composite materials. At the same time, this composite material combined 0 and 2 dimensions will have a wide application prospect in catalysis, adsorption and separation, electrochemistry and so on. 4. EXPERIMENTAL SECTION The detail methods was provided in Supporting Information. ASSOCIATED CONTENT Supporting Information Information as mentioned in text. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (A. Chen) Notes 18 ACS Paragon Plus Environment

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