Uniform fibrous-structured hollow mesoporous carbon spheres for high-performance supercapacitor electrodes

Accepted Manuscript Title: Uniform fibrous-structured hollow mesoporous carbon spheres for high-performance supercapacitor electrodes Author: Qiang Zhanga Lei Lia Yanli Wang Yujin Chen Fei Hea Shili Gaia Piaoping Yang PII: DOI: Reference:

S0013-4686(15)30034-7 http://dx.doi.org/doi:10.1016/j.electacta.2015.06.154 EA 25321

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

23-4-2015 15-6-2015 25-6-2015

Please cite this article as: Qiang Zhanga, Lei Lia, Yanli Wang, Yujin Chen, Fei Hea, Shili Gaia, Piaoping Yang, Uniform fibrous-structured hollow mesoporous carbon spheres for high-performance supercapacitor electrodes, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.06.154 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Uniform fibrous-structured hollow mesoporous carbon spheres for high-performance supercapacitor electrodes Qiang Zhanga, Lei Lia, Yanli Wang a, Yujin Chen,*b Fei Hea, Shili Gaia, and Piaoping Yanga,* a

Key Laboratory of Superlight Materials and Surface Technology, Ministry of

Education, College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, PR China b

College of Sciences, Harbin Engineering University, Harbin, 150001, PR China

*

Corresponding author: Fax. +86 451 82533026; Tel. +86 451 82569890.

E-mail address: [email protected]; [email protected]

Abstract Uniform fibrous-structured hollow mesoporous carbon spheres (FHMCSs) have been synthesized by a simple template method using fibrous-structured mesoporous silica microspheres and resol precursor as silica template and carbon sources, respectively. It is found that FHMCSs have an ultra high specific surface area of 1121 m2 g–1 and large pore volume of 1.3 cm3 g–1. The high surface-to-volume ratio is a favorable factor to obtain high specific capacitance and excellent rate performance. As expected, when used as supercapacitor electrodes, the FHMCSs exhibit high specific capacitance of 359.2 F g–1 at current density of 1 A g–1 and excellent cycle stability (92% retention after 5000 cycles). The energy density of the FHMCSs can be estimated to be 49.9 Wh kg–1 at a power density of 500 W kg–1. Moreover, the FHMCSs electrode shows high energy density of 36.4 Wh kg–1 even at the power density of 10000 W kg–1, suggesting a potential application in supercapacitors and other energy storage fields.

Keywords: Supercapacitor · carbon spheres ·fibrous structure ·electrochemical performance

1. Introduction Supercapacitors as novel energy storage devices have been intensively studied because of their high specific power density, good reversibility, long cycle and environmental safety. As a main influence factor of supercapacitor properties, electrode material has already attracted extensive attention in this direction [1–5]. Carbon is found in various micro-textures, with the most dramatically being active carbon, graphene, nanotubes and mesoporous carbon [6–9]. The varying structures of carbon make it an outstanding material which is widely used in electrochemical applications. Due to their accessibility, processability and relatively low cost, carbon materials as potential electrodes store energy via electric double layer (EDL) mechanism have been extensively researched [10–13]. Among the different carbon materials, mesoporous carbon materials are especially attractive as electrodes for supercapacitors because of their high surface area and suitable pore sizes [14–16]. Moreover, mesoporous carbon possesses a developed chemical and mechanical stability as well as good electrical conductivity. Taking into account all the mentioned characteristics, mesoporous carbon as an electrode material for electrochemical applications should be highly promising [17–19]. Recently, mesoporous carbons with different morphologies and structures which show excellent capacitive performance have been applied as supercapacitor electrode materials [20,21]. Hollow mesoporous carbon spheres have received extensively attention due to their mesoporous shell-hollow inner space shape, high chemical and thermal stability, low density, and large surface area. They also possess excellent flow

performance and large voids [22–25]. Besides, mesoporous shell carbon spheres exhibit more advantages in mass diffusion and ion transportation for application as supercapacitors [26–28]. Till now, template-based method has been widely employed to produce hollow mesoporous carbon spheres with controlled structure and inherited porosity [19,29]. Typically, a spherical hard template is usually infiltrated with carbon precursor, and then by means of the following carbonization and etching of template. Therefore, spherical templates and carbon precursors are playing the vital roles in structures and properties of the final hollow carbon materials. In the past few years, ZnO, Ni(OH)2, sepiolite, zeolite and silica have been widely used as hard templates [30–33]. Also, various types of carbon precursors have been developed due to their readily carbonizable properties, such as sucrose, furfuryl alcohol and phenol formaldehyde resin [34,35]. However, hollow carbon spheres with unique structure and textural properties which are potential for achieving good conductivity and high capacitance are still scarce, it is therefore promising to develop facile and cheap routes to prepare this kind of materials. Herein, we proposed the synthesis of fibrous-structured hollow mesoporous carbon spheres (FHMCSs) by a template-based method employing as-prepared fibrousstructure mesoporous silica microspheres as a template and resol precursor as the carbon sources. The FHMCSs have a specific fibrous-structured shell and hollow space with a large surface area, offering inner space and pathway for electrolyte diffuse and ions transport as negative electrodes for supercapacitor applications, which demonstrated a high specific capacitance and excellent rate capability as well as cycle

stability.

2. Experimental section 2.1. Synthesis of resol precursor The resol precursor used in this work was synthesized according to the previously reported process by using phenol and formaldehyde [36]. In a typical synthesis, 10.0 g of phenol was melted at 40 °С, then 2.13 mL of NaOH solution (20 wt.%) was introduced into slowly. After stirred for 10 min, 17.7 g of formalin (37 wt.%) was added drop by drop, and the mixture was stirred at 70 °С for 60 min to increase the extent of polymerization. After cooling down to room temperature, the pH value was adjusted to about 6.0 using 2.0 mol L–1 HCl solution. Water was then removed from the mixture under vacuum at 50 °С. 2.2. Synthesis of the fibrous-structured silica microspheres template The fibrous-structured silica microspheres were synthesized according to our previous work [37]. At room temperature, 0.95 g of cetytrimethylammonium bromide (CTAB) and 0.6 g of urea were added to 30 ml of deionized water to form solution A. 2.7 mL of tetraethylorthosilicate (TEOS) were dissolved in 1.5 mL of pentanol and 30 mL of cyclohexane to get solution B. Then solution A was mixed with solution B with continuous stirring for 30 min. Subsequently, the mixture was transferred to an autoclave and aged at 120 °С for 4 h. The as-made sample were washed with deionized water and acetone several times, and dried in air for 24 h. To remove the CTAB template, the final silica microspheres were treated by calcination at 550 °С for 6 h with a heating rate of 1 °С min–1.

2.3. Synthesis of FHMCSs 0.04 g of as-prepared fibrous-structure mesoporous silica microspheres were dispersed in 60 mL of tetrahydrofuran (THF) under violently stirring. Simultaneously, 0.04 g of resol precursor was dissolved in 20 mL of THF by sonication for 30 min, and then was added dropwise into the silica microspheres dispersion solution. After stirred for 5 h at room temperature, the mixture was heated at 60 °С in oven to obtain the sample. Then, it was calcined at 900 °С for 2 h under argon atmosphere. The obtained spherical mesoporous C/SiO2 composites were further treated with excessive 5% HF aqueous solution for 24 h. Finally, the mixture was centrifugated, washed with water and dried in oven to get the final FHMCSs. 2.4. Characterization X-ray diffraction (XRD) measurements were inspected on a Rigaku D/max-TTRIII diffractometer using Cu Kα radiation (λ = 0.15405 nm). The morphologies of the samples were examined on a scanning electron microscope (SEM, JSM-6480A, Japan Electronics). Transmission electron microscopy (TEM) analysis was characterized by a FEI Tecnai G2 S-Twin transmission electron microscoper operated at an accelerating voltage of 200 kV. Raman spectra from 500 to 2500 cm–1 were obtained using a JobinYvon HR800 Raman spectrometer with 457.9 nm wavelength laser excitation. Nitrogen adsorption/desorption analysis was carried out on a Micromeritics ASAP 2010 at a liquid nitrogen temperature (77 K). The specific surface area of the sample was determined by the Brunauer-Emmett-Teller (BET) method, and the pore size distribution was calculated through the Barrett-Joyner-Halenda (BJH) method. 2.5. Electrochemical measurement

The electrochemical measurements were conducted with a three-electrode mode in a 6 mol L–1 KOH aqueous solution as the electrolyte using a CHI660D electrochemical workstation. The working electrodes were prepared by mixing the active materials (80 wt.%), acetylene black (15 wt.%) and polytetrafluoroethylene (PTFE, 5 wt.%) in absolute ethanol to promote homogeneity. The mixture was then coated onto the nickel foam substrate (1 × 1 cm) and dried overnight at 60 °C in a vacuum oven. The counter electrode and reference electrode were platinum foil and Hg/HgO electrode, respectively. Cyclic voltammetry (CV) at a scan rate from 5 to 50 mV s–1 and galvanostatic charge/discharge at a current density from 1.0 to 20 A g–1 were examined in the same potential range between -1.0 and 0 V. Electrochemical impedance spectroscopy (EIS) was performed at open circuit potential within the frequency range from 0.1 Hz to 100 KHz. The specific gravimetric capacitance was calculated from discharge regions of the CV using the following equation (1): =2

∆

(1)

where

(F/g) is the specific capacitance of single electrode, I is the current response

(mA),

and

electrode (g),

is the vertex potentials of the voltage range, is the potential scan rate (mV/s), and ∆

is the grams of one

is the voltage window (V).

3. Results and discussion 3.1. Phase, structure and morphology The synthetic procedure is illustrated in Scheme 1. FHMCSs were prepared via template method using fibrous-structure mesoporous silica microspheres and resol as

template and carbon precursor, respectively. The graphitic carbon can be deposited in the surface of mesoporous silica microspheres during the carbonization. After the etching of the silica template by HF, uniform FHMCSs were obtained. The morphology and microstructure of as-prepared silica template and FHMCSs were investigated by SEM and TEM (Fig. 1). As shown in Fig. 1a-b, the silica template consists of uniform microspheres with an average particle size of around 700 nm and exhibits intriguing fibrous structure with dendrimeric fibers in three-dimensional space. The fibrous-structure mesoporous silica microspheres after the sol-gel process were calcined in Ar atmosphere at 900 °C for 2 h. After that, HF solution was used to remove the silica template, and then uniform fibrous-structure hollow mesoporous carbon spheres were prepared. Fig. 1c presents the representative SEM image of FHMCSs. It is found that the hollow carbon sphere with an interesting fibrous surface exhibits abundant mesopore channels. In the TEM images (Fig. 1d, e), we can see that the product is composed of uniform spherical particles with an average diameter of about 600 nm. In addition, the carbon spheres exhibit obviously hollow structures. It is noted that the uniform FHMCSs with a rough shell and an accessible mesopores devoting to ions diffusion and adsorption are beneficial to mass transportation [38]. In the XRD pattern for FHMCSs (Fig. 1f), two broad diffraction peaks centered at around 2θ = 25° and 44° indicates the existence of graphite carbon, which corresponds to the respective (002) and (100) crystal planes of graphitic carbon, confirming the highly disordered structure of the carbon spheres. Furthermore, Raman spectroscopy measurement was carried out to investigate the

local structure of FHMCSs (Fig. 2). Raman spectra of FHMCSs exhibited two major peaks located at 1350 and 1580 cm–1, corresponding to D-peak and G-peak respectively. Generally, G-band associates with graphitic carbon and the D-band corresponds to disordered carbon. The degree of crystallization of the graphitic carbon was indicated the intensity ratio of D- and G-peaks (ID/IG ~1.02), as well as suggests the disordered and graphitic carbon structure information of the FHMCSs N2 adsorption-desorption isotherm is used to study the specific textural properties of the activated material. As shown in Fig. 3, the N2 adsorption-desorption isotherm of FHMCS corresponds to the type IV isotherm with an obvious hysteresis in the relative pressure range of 0.4-1.0, which demonstrates the existence of a large number of mesopores in FHMCS. Moreover, the sharp increase at high pressure region (p/p0＞0.9) indicates the presence of mesopores originating from the solvation of SiO2 template. The BET specific surface area and pore volume of the FHMCSs were 1121 m2 g–1 and 1.3 cm3 g–1, respectively. The sample exhibits the typical pore size distribution of mesoporous hollow spheres (inset, Fig. 3). The average pore calculated from the desorption isotherm by the BJH method is 3.1 nm. The large specific surface areas and high mesoporous volume facilitate the diffusion of ions and thus be able to improve the capacitive properties. 3.2. Electrochemical performance Cyclic

voltammetry,

galvanostatic charge/discharge and electrochemical

impedance spectroscopy measurements were conducted to characterize the electrochemical performance of FHMCSs used as supercapacitor electrodes. Fig. 4a

shows the CV curves of FHMCSs electrode at different scan rates from 5 to 50 mV s–1 in a potential range of -1~0 V (vs. Hg/HgO) in 6 mol L–1 KOH aqueous solution. It can be observed that CV curves exhibit a typical rectangular voltammogram shape, indicating that the fibrous and porous structure of the hollow carbon spheres with good capacitive behaviour and reversibility emerge as an outstanding polarizable manner. The maximum specific capacitance is as high as 326 F g–1 at scan rate of 5 mV s–1. With the increase of scan rate, the CV curve become some tilted but still shows a rectangularlike shape, suggesting a fast charge/discharge process with high power capability and low equivalent series resistance. To evaluate the capacitive properties of FHMCSs, galvanostatic charge/discharge evaluation was also performed. Fig. 4b shows the galvanostatic charge/discharge curves of the FHMCSs at constant current densities from 1.0 to 20 A g–1. It is found that the galvanostatic charge/discharge curves exhibit a typical profile near triangular and highly symmetric shape, which indicates the FHMCSs electrode possesses outstanding electrochemical reversibility and high rate capability. Moreover, not obvious voltage drop (IR) is observed during the charge/discharge even at 20 A g–1, showing ideal capacitive behaviour and little internal resistance. Usually, the specific capacitance of the carbon materials can be calculated by equation (2): =

∙∆ (2) ∆ ∙

Where I is the discharged current (A), ∆ is the discharged time (s), ∆ is the voltage window (V) and m is the grams of active material (g). Fig. 4c further illustrates the characteristically relationship between specific capacitances and scan rates that display

the capacitance decay. The specific capacitance of the FHMCSs electrode can reach 359.2 F g–1 at the current density of 1 A g–1, and about 73% of specific capacitance is retained when the charge/discharge rate is increased from 1 to 20 A g–1, demonstrating a good rate capability. The FHMCSs electrode with the excellent capacitance retention is attributed to the accessibility transport paths and high connectively pore channel. Notably, compared with other carbon materials which synthesized by different template in previous literatures, FHMCSs exhibit higher specific capacitance and more excellent rate capability which is definitely relative with its higher special surface area, as shown in Table 1. Long cycling is another crucial parameter in determining the practical application for supercapacitors. To investigate the reusability, the continuous CV process was conducted at 50 mV s–1 a potential range between -1 and 0 V for 5000 cycles, using 6 mol L–1 KOH aqueous solution as electrolyte. As displayed in Fig. 4d, the capacitance retention remains about 92% even after 5000 cycles. Moreover, the specific capacitance of the first and 5000th cycle exhibit a high value (inset of Fig. 4d), which reveals good capacitive property and long term electrochemical stability resulting from its large specific surface area and hollow structure. The impressive electrochemical capacitive performance of the FHMCSs material can be mainly ascribed to the following factors. (a) The advantageous fibrous shell layer with high specific surface areas can not only provide numerous electroactive sites to easily access the electrolyte but also benefit rapid electron transition and high rate of electrolyte infiltration. (b) The large hollow internal space as a storage can facilitate ion

buffering during continuous ion diffusion with a high rate. (c) Numerous accessible mesopores of FHMCSs can offer short diffusion distances for ions electrosorbing in pore channels, and thus increasing the cycle life. In addition, the energy density and power density measurements are conducted to evaluate the electrochemical capacitive performance of supercapacitors in practical application. The Ragone plot reveals the relationship between energy density and power density of the FHMCSs electrode materials, which is depicted in Fig. 5a. The galvanostatic charge/discharge data are used to calculate the Ragone plot with current densities ranging from 1 to 20 A g–1 according to the following equations (3) and (4): =

1 ∆ 2

= Where

∆

(4)

is the energy density (Wh kg–1),

is the cell voltage (V),

(3)

is the specific capacitance (F g–1), ∆

is the power density (W kg–1), and ∆ is the discharge time

(s). The Ragone plot displays an obvious trend that the energy density increases with the power density decreasing. The energy density of FHMCSs can be estimated as high as 49.9 Wh kg–1 at a power density of 500 W kg–1. More significantly, the energy density still retains 36.4 Wh kg–1 even at a power density of 10000 W kg–1. Such excellent results indicate that as-prepared FHMCSs could act as an excellent potential electrode material for high-performance supercapacitors. Electrochemical impedance spectroscopy (EIS) is an effective method to characterize the supercapacitive behavior of the sample. Fig. 5b displays the Nyquist plots measured for FHMCSs in the frequency range from 0.1 to 100 KHz. At low

frequency region, the product exhibits good capacitive behaviour with vertical line, indicating excellent ion diffusion and charge transportation in mesoporous structure. The impedance spectrum in inset of Fig. 5b shows a small ohmic resistance of about 0.41Ω, suggesting an outstanding ion transport efficiency and a highly conductive framework facilitating charge and ion diffusion.

4. Conclusions In this study, fibrous-structured hollow mesoporous carbon spheres have been successfully synthesized through a simple template method by using fibrous-structure mesoporous silica microspheres as a template and resol precursor as a carbon source. The as-prepared FHMCSs have a high specific surface area (1121 m2 g–1), large pore volume (1.3 cm3 g–1), and highly accessible mesoporous structure. Meanwhile, the FHMCSs as supercapacitor electrodes present excellent capacitive performance in 6 M KOH aqueous solution with high specific capacitance (359.2 F g–1), outstanding rate capability (about 262 F g–1 at 20 A g–1), and excellent cycling stability (92% retention after 5000 cycles). Such promising results can be attributed to the unique textural properties of FHMCS, which can offer good charge storage with rapid ion diffusion. Therefore, this product has a promising capability to serve as the high performance supercapacitors.

Acknowledgements This work was supported by funding from financial supports from the National

Natural Science Foundation of China (NSFC 21271053, 21401032, 51472058, 51301050), Research Fund for the Doctoral Program of Higher Education of China (2014M560248), Natural Science Foundation of Heilongjiang Province (B201403), Harbin Sci.-Tech. Innovation Foundation (2014RFQXJ019) and Fundamental Research Funds for the Central Universities of China.

References [1]

T.Y. Wei, C.H. Chen, H.C. Chien, S.Y. Lu, C.C. Hu, A Cost-Effective Supercapacitor Material of Ultrahigh Specific Capacitances: Spinel Nickel Cobaltite Aerogels from an Epoxide-Driven Sol-Gel Process, Adv. Mater., 22 (2010) 347.

[2]

P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater., 7 (2008) 845.

[3]

H. Jiang, J. Ma, C.Z. Li, Mesoporous Carbon Incorporated Metal Oxide Nanomaterials as Supercapacitor Electrodes, Adv. Mater., 24 (2012) 4197.

[4]

D.D. Zhou, H.J. Liu, Y.G. Wang, C.X. Wang, Y.Y. Xia, Ordered mesoporous/microporous carbon sphere arrays derived from chlorination of mesoporous TiC/C composite and their application for supercapacitors, J. Mater. Chem., 22 (2012) 1937.

[5]

G.P. Wang, L. Zhang, J.J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev., 41 (2012) 797.

[6]

Q. Wu, Y.X. Xu, Z.Y. Yao, A.R. Liu, G.Q. Shi, Supercapacitors Based on Flexible Graphene/Polyaniline Nanofiber Composite Films, Acs Nano, 4 (2010) 1963.

[7]

Z. Chen, D.Q. Zhang, X.L. Wang, X.L. Jia, F. Wei, H.X. Li, Y.F. Lu, HighPerformance Energy-Storage Architectures from Carbon Nanotubes and Nanocrystal Building Blocks, Adv. Mater., 24 (2012) 2030.

[8]

Q. Li, R.R. Jiang, Y.Q. Dou, Z.X. Wu, T. Huang, D. Feng, J.P. Yang, A.S. Yu, D.Y. Zhao, Synthesis of mesoporous carbon spheres with a hierarchical pore structure

for the electrochemical double-layer capacitor, Carbon, 49 (2011) 1248. [9]

J.S. Huang, B.G. Sumpter, V. Meunier, A universal model for nanoporous carbon supercapacitors applicable to diverse pore regimes, carbon materials, and electrolytes, Chem.-Eur. J., 14 (2008) 6614.

[10] O. Barbieri, M. Hahn, A. Herzog, R. Kotz, Capacitance limits of high surface area activated carbons for double layer capacitors, Carbon, 43 (2005) 1303. [11] M.B. Bryning, D.E. Milkie, M.F. Islam, L.A. Hough, J.M. Kikkawa, A.G. Yodh, Carbon nanotube aerogels, Adv. Mater., 19 (2007) 661. [12] S. Tanaka, H. Nakao, T. Mukai, Y. Katayama, Y. Miyake, An Experimental Investigation of the Ion Storage/Transfer Behavior in an Electrical Double-Layer Capacitor by Using Monodisperse Carbon Spheres with Microporous Structure, J. Phys. Chem. C, 116 (2012) 26791. [13] C. Largeot, C. Portet, J. Chmiola, P.L. Taberna, Y. Gogotsi, P. Simon, Relation between the ion size and pore size for an electric double-layer capacitor, J. Am. Chem. Soc., 130 (2008) 2730. [14] J. Lee, J. Kim, T. Hyeon, Recent progress in the synthesis of porous carbon materials, Adv. Mater., 18 (2006) 2073. [15] N. Brun, S.R.S. Prabaharan, C. Surcin, M. Morcrette, H. Deleuze, M. Birot, O. Babot, M.F. Achard, R. Backov, Design of Hierarchical Porous Carbonaceous Foams from a Dual-Template Approach and Their Use as Electrochemical Capacitor and Li Ion Battery Negative Electrodes, J. Phys. Chem. C, 116 (2012) 1408.

[16] A. Walcarius, Mesoporous materials and electrochemistry, Chem. Soc. Rev., 42 (2013) 4098. [17] J. Wei, D.D. Zhou, Z.K. Sun, Y.H. Deng, Y.Y. Xia, D.Y. Zhao, A Controllable Synthesis of Rich Nitrogen-Doped Ordered Mesoporous Carbon for CO2 Capture and Supercapacitors, Adv. Funct. Mater., 23 (2013) 2322. [18] W. Xing, S.Z. Qiao, R.G. Ding, F. Li, G.Q. Lu, Z.F. Yan, H.M. Cheng, Superior electric double layer capacitors using ordered mesoporous carbons, Carbon, 44 (2006) 216. [19] S. Alvarez, A.B. Fuertes, Template synthesis of mesoporous carbons with tailorable pore size and porosity, Carbon, 42 (2004) 433. [20] J.W. Lang, X.B. Yan, X.Y. Yuan, J. Yang, Q.J. Xue, Study on the electrochemical properties of cubic ordered mesoporous carbon for supercapacitors, J. Power Sources, 196 (2011) 10472. [21] H.J.

Liu,

J.

Wang,

C.X.

Wang,

Y.Y.

Xia,

Ordered

Hierarchical

Mesoporous/Microporous Carbon Derived from Mesoporous TitaniumCarbide/Carbon

Composites

and

its

Electrochemical

Performance

in

Supercapacitor, Adv. Energy Mater., 1 (2011) 1101. [22] L.M. Guo, L.X. Zhang, J.M. Zhang, J. Zhou, Q.J. He, S.Z. Zeng, X.Z. Cui, J.L. Shi, Hollow mesoporous carbon spheres-an excellent bilirubin adsorbent, Chem. Commun., 40 (2009) 6071. [23] K. Tang, L.J. Fu, R.J. White, L.H. Yu, M.M. Titirici, M. Antonietti, J. Maier, Hollow Carbon Nanospheres with Superior Rate Capability for Sodium-Based

Batteries, Adv. Energy Mater., 2 (2012) 873. [24] K. Tang, R.J. White, X.K. Mu, M.M. Titirici, P.A. van Aken, J. Maier, Hollow Carbon Nanospheres with a High Rate Capability for Lithium-Based Batteries, Chemsuschem, 5 (2012) 400. [25] R.J. White, K. Tauer, M. Antonietti, M.M. Titirici, Functional Hollow Carbon Nanospheres by Latex Templating, J. Am. Chem. Soc., 132 (2010) 17360. [26] X.C. Chen, K. Kierzek, Z.W. Jiang, H.M. Chen, T. Tang, M. Wojtoniszak, R.J. Kalenczuk, P.K. Chu, E. Borowiak-Palen, Synthesis, Growth Mechanism, and Electrochemical Properties of Hollow Mesoporous Carbon Spheres with Controlled Diameter, J. Phys. Chem. C, 115 (2011) 17717. [27] F. Boettger-Hiller, P. Kempe, G. Cox, A. Panchenko, N. Janssen, A. Petzold, T. Thurn-Albrecht, L. Borchardt, M. Rose, S. Kaskel, C. Georgi, H. Lang, S. Spange, Twin Polymerization at Spherical Hard Templates: An Approach to SizeAdjustable Carbon Hollow Spheres with Micro- or Mesoporous Shells, Angew. Chem. Int. Ed., 52 (2013) 6088. [28] Y.D. Xia, R. Mokaya, Ordered mesoporous carbon hollow spheres nanocast using mesoporous silica via chemical vapor deposition, Adv. Mater., 16 (2004) 886. [29] R. Ryoo, S.H. Joo, S. Jun, Synthesis of Highly Ordered Carbon Molecular Sieves via Template-Mediated Structural Transformation, J. Phys. Chem. B, 103 (1999) 7743. [30] P.M. Barata-Rodrigues, T.J. Mays, G.D. Moggridge, Structured carbon adsorbents from clay, zeolite and mesoporous aluminosilicate templates, Carbon,

41 (2003) 2231. [31] X.H. Xia, J.P. Tu, Y.Q. Zhang, X.L. Wang, C.D. Gu, X.B. Zhao, H.J. Fan, HighQuality Metal Oxide Core/Shell Nanowire Arrays on Conductive Substrates for Electrochemical Energy Storage, Acs Nano, 6 (2012) 5531. [32] Z. Tang, C.H. Tang, H. Gong, A High Energy Density Asymmetric Supercapacitor from Nano-architectured Ni(OH)2/Carbon Nanotube Electrodes, Adv. Funct. Mater., 22 (2012) 1272. [33] W. Fan, M.A. Snyder, S. Kumar, P.S. Lee, W.C. Yoo, A.V. McCormick, R.L. Penn, A. Stein, M. Tsapatsis, Hierarchical nanofabrication of microporous crystals with ordered mesoporosity, Nat. Mater., 7 (2008) 984. [34] A.H. Lu, T. Sun, W.C. Li, Q. Sun, F. Han, D.H. Liu, Y. Guo, Synthesis of Discrete and Dispersible Hollow Carbon Nanospheres with High Uniformity by Using Confined Nanospace Pyrolysis, Angew. Chem. Int. Ed., 50 (2011) 11765. [35] J.P. Han, G.Y. Xu, B. Ding, J. Pan, H. Dou, D.R. MacFarlane, Porous nitrogendoped hollow carbon spheres derived from polyaniline for high performance supercapacitors, J. Mater. Chem. A, 2 (2014) 5352. [36] Y. Meng, D. Gu, F.Q. Zhang, Y.F. Shi, H.F. Yang, Z. Li, C.Z. Yu, B. Tu, D.Y. Zhao, Ordered mesoporous polymers and homologous carbon frameworks: Amphiphilic surfactant templating and direct transformation, Angew. Chem. Int. Ed., 44 (2005) 7053. [37] S.L. Gai, P.P. Yang, P.A. Ma, D. Wang, C.X. Li, X.B. Li, N. Niu, J. Lin, Fibrousstructured magnetic and mesoporous Fe3O4/silica microspheres: synthesis and

intracellular doxorubicin delivery, J. Mater. Chem., 21 (2011) 16420. [38] L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chem. Soc. Rev., 38 (2009) 2520. [39] X.J. He, R.C. Li, J.S. Qiu, K. Xie, P.H. Ling, M.X. Yu, X.Y. Zhang, M.D. Zheng, Synthesis of mesoporous carbons for supercapacitors from coal tar pitch by coupling microwave-assisted KOH activation with a MgO template, Carbon, 50 (2012) 4911. [40] B. Xu, L. Peng, G.Q. Wang, G.P. Cao, F. Wu, Easy synthesis of mesoporous carbon using nano-CaCO3 as template, Carbon, 48 (2010) 2377. [41] W.F. Zhang, Z.H. Huang, G.P. Cao, F.Y. Kang, Y.S. Yang, A novel mesoporous carbon with straight tunnel-like pore structure for high rate electrochemical capacitors, J. Power Sources, 204 (2012) 230. [42] Y. Han, X.T. Dong, C. Zhang, S.X. Liu, Hierarchical porous carbon hollowspheres as a high performance electrical double-layer capacitor material, J. Power Sources, 211 (2012) 92.

Captions Scheme 1 Schematic illustration for the formation of FHMCSs. Fig. 1 SEM (a) and TEM (b) images of fibrous-structured mesoporous silica microspheres, SEM (c) and TEM (d) images of FHMCSs, magnified TEM image of FHMCSs (e), and XRD pattern of FHMCSs (f). Fig. 2 Raman spectra of FHMCSs. Fig. 3 Nitrogen adsorption-desorption isotherm of FHMCSs, and inset is the corresponding pore size distribution curve. Fig. 4 CV curves of FHMCSs at different scan rates (a), galvanostatic charge/discharge plots at various current densities (b), the specific capacitance as a function of the current densities (c), and the variation of specific capacitance retention with cycle number at 50 mV s–1 (d). The inset in Fig. 3d shows the specific capacitance at the first cycle and 5000th cycle. Fig. 5 Ragon plots based on the FHMCSs electrode supercapacitor (a), Nyquist plot of the FHMCSs electrode at open circuit potential in the frequency range of 100 KHz to 0.1 Hz (b). Table 1. Different carbon electrode compared by template, special surface area and special capacitance. These electrode materials are measured in 6 M KOH.

Table 1. Different carbon electrode compared by template, special surface area and special capacitance. These electrode materials are measured in 6 M KOH. Carbon material

Template

Special surface 2

–1

Special Capacitance

Ref

–1

area (m g )

(F g )

Mesoporous carbon

MgO

1003

224

[39]

Mesoporous carbon

CaCO3

892

155

[40]

Mesoporous carbon

Mg(OH)2

1014

217

[41]

Carbon hollow-spheres

Colloidal silica

658

269

[42]

Hollow carbon spheres

SPS

213

213

[35]

FHMCSs

Mesoporous silica microspheres

1121

359.2

--

Table 1

Scheme 1

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5