Synthesis of N-Doped Hollow-Structured Mesoporous Carbon

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Synthesis of N-doped Hollow-Structured Mesoporous Carbon Nanospheres for High-Performance Supercapacitors Chao Liu, Jing Wang, Jiansheng Li, Mengli Zeng, Rui Luo, Jinyou Shen, Xiuyun Sun, Weiqing Han, and Lianjun Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02404 • Publication Date (Web): 04 Mar 2016 Downloaded from http://pubs.acs.org on March 4, 2016

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ACS Applied Materials & Interfaces

Synthesis of N-doped Hollow-Structured Mesoporous Carbon Nanospheres for High-Performance Supercapacitors Chao Liu, Jing Wang, Jiansheng Li*, Mengli Zeng, Rui Luo, Jinyou Shen, Xiuyun Sun, Weiqing Han, Lianjun Wang*

[*] C. Liu, J. Wang, Prof. J.S. Li, M.L. Zeng, R. Luo, J.Y. Shen, X.Y. Sun, W.Q. Han, Prof. L.J. Wang Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse School of Environmental and Biological Engineering Nanjing University of Science and Technology Nanjing 210094, P.R .China Tel: +(86) 025-84315351 E-mail: [email protected]; [email protected]

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Abstract: We have demonstrated a facile and controllable synthesis of monodispersed N-doped hollow mesoporous carbon nanospheres (N-HMCSs) and yolk-shell hollow mesoporous carbon nanospheres (N-YSHMCSs) by a modified “silica-assisted” route. The synthesis process can be carried out by using resorcinol-formaldehyde resin as a carbon precursor, melamine as a nitrogen source, hexadecyl trimethylammonium chloride as a template and silicate oligomers as structure-supporter. The morphological (i.e., particle size, shell thickness, cavity size and core diameter) and textural features of the carbon nanospheres are easily controlled by varying the amount of ammonium. The resultant carbon nanospheres possess high surface areas (up to 2464 m2 g-1), large pore volumes (up to 2.36 cm3 g-1) and uniform mesopore size (~2.4 nm for N-HMCSs, ~4.5 nm for N-YSHMCSs). Through combining the hollow mesoporous structure, high porosity, large surface area and N heteroatomic functionality, the as-synthesized N-doped hollow-structured carbon nanospheres (N-HSCSs) manifest excellent supercapacitors performance with high capacitance (up to 240 F/g), favorable capacitance retention (97.0 % capacitive retention after 5000 cycles), and high energy density (up to 11.1 Wh kg-1). Keywords: controllable, N-doped, hollow mesoporous, yolk-shell, carbon nanospheres, supercapacitors

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1. Introduction Climate change and the decreasing availability of fossil fuels require society to move towards efficient, clean, and sustainable sources of energy, as well as new technologies associated with energy conversion and storage. Supercapacitors, also known as electrochemical capacitors or ultracapacitors, have attracted significant attention because of high power density, long lifecycle, pulse power supply, low maintenance cost, simplicity and better safety compared to secondary batteries1-6. On this basic, supercapacitors offer a promising approach to meet the increasing power demands of energy storage systems in the twenty first century. Currently, supercapacitors have been widely used in consumer electronics, memory back-up systems and industrial power and energy management7-11. However, the disadvantages of supercapacitors, such as self-discharging, low energy density and high production cost, have been regarded as major obstacles for the further application of supercapacitors technologies. The electrode material, as an important factor affecting the properties of supercapacitors, has remained a hot spot of the research in this direction. Since the pioneering work on using high surface area carbons to achieve the supercapacitors devices in 199012, porous carbon materials, including activated carbon powders13-16, carbon fabrics17-18, graphene11, 19-22 and carbon nanotubes23-24 were extensively studied as electrode materials of supercapacitors due to its high specific surface area, excellent electrical conductivity, environmental friendliness and low cost. However, the main disadvantage of the presented carbon materials are the low density of active material, which leads to reduced volumetric capacities. Considering the vital influence of carbon materials’ structures (i.e. specific surface area, pore architecture, morphology and surface functionalization) on the supercapacitors performance, it is highly desirable to develop new porous carbon-based materials with distinctive structures as well as a facile synthesis method. 3



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As a novel kind of porous carbon materials, hollow-structured carbon spheres (HSCSs), such as hollow carbon spheres25 (HCSs) and yolk-shell carbon spheres26 (YSCSs), have long been attractive because of their unique structure and functional behavior, such as very high specific area, low specific density, large controllable inner pore volume, and good mechanical strength. The remarkable properties make it greatly potential to be applied in the areas of energy storage, water treatment and biomedicine27-36. As the electrode materials of supercapacitors, HSCSs have more advantages for mass diffusion of electrolyte and transmission of ions and electrons, due to their unique structures37-39. To further improve the energy density and specific power of supercapacitors, introducing heteroatoms into carbon frameworks is an effective strategy, which can impart the acid/base properties to the carbon, and typically contribute the extra pseudo-capacitance from the striking redox process of the surface heteroatom functionalities. Among these heteroatoms, nitrogen doping seems to be the most useful route for enhancing capacity, the surface polarity, electrical conductivity, surface basic sites and electron-donor tendency, while maintaining the superb cycle ability10, 17, 40-42. Therefore, great efforts have been devoted to the preparation of N-doped hollow-structured carbon spheres (N-HSCSs). In most cases, nitrogen containing materials has been employed as precursors for the synthesis of N-HSCSs43-50. By this route, nitrogen can be preserved at a relatively large content by adjusting the carbonization temperature. However, to date, by that means it often rely on hard templating strategy, which is exceedingly difficult to prepare N-HSCSs with uniform morphologies and high dispersity due to the inevitable excess deposition of carbon precursors onto the surface of hard templates during the infiltration process. The morphology and pore architecture of the replicated N-HSCSs is always limited to the parent template, which is monotonous and uncontrollable. It is still difficult to fabricate N-HSCSs with high dispersity, well-defined and controllable morphologies. Previously, a “silica-assisted” strategy was established by Dai and coworkers for diverse carbon spheres32. 4


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Mesoporous carbon nanospheres, hollow mesoporous carbon nanospheres, and yolk-shell mesoporous carbon nanospheres were obtained. This work provides a facile and efficient route for the preparation of HSCSs with controllable morphologies and high dispersity, which may be extended to synthesis of N-HSCSs. To the best of our knowledge, there is no report on the synthesis of N-HSCSs via this “silica-assisted” strategy. Herein, we demonstrate a facile and controllable synthesis of monodispersed N-doped hollow mesoporous carbon spheres (N-HMCSs) and yolk-shell hollow mesoporous carbon spheres (N-YSHMCSs) by a modified “silica-assisted” route. As shown in Fig. 1, the process used resorcinol-formaldehyde resin as a carbon precursor, melamine as a nitrogen source, tetraethoxysilane (TEOS) as a structure-supporter and hexadecyl trimethylammonium chloride (CTAC) as a template. Carbonization was followed by etching of the silica in the carbon/silica composite, resulting in the formation of N-HSCSs. The morphological (i.e., particle size, shell thickness, cavity size and core diameter) and textural features of the carbon nanospheres are easily controlled by varying the amount of ammonium. Additional porosity is produced by removing the silica present in the walls, which create very high BET surface area and porosity. Both two N-HSCS types exhibit promising properties for use in supercapacitors with high capacitance and favorable capacitance retention and show great potential for prospective applications in energy conversion and storage.

Figure 1. The synthesis of N-doped hollow-structured mesoporous carbon spheres 5



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2. Experimental 2.1 Chemicals and Materials Resorcinol, formaldehyde solution (37-40 %), melamine, anhydrous ethanol and tetraethoxysilane (TEOS) were purchased from Sinopharm Chemical Reagent Co., Ltd. Hexadecyl trimethylammonium chloride (CTAC), ammonia aqueous solution (25-28 %) and hydrofluoric acid solution (40 %) was purchased from Nanjing Chemical Reagent Co., Ltd. All chemicals were used as received without any further purification. Millipore water was used in all experiments.

2.2 Synthesis of N-HMCSs and N-YSHMCSs Typically, ammonia aqueous solution (NH4OH, 25 wt%, 0.2, 0.5 and 1 mL, respectively) was mixed with a solution containing absolute ethanol (EtOH, 40 mL), CTAC (2 g), and deionized water (H2O, 100 mL) at 70 oC under vigorous stirring. Subsequently, resorcinol (0.55 g) was added and continually stirred for 30 min. Then TEOS (3 mL) and formaldehyde solution (0.74 mL) were added to the reaction solution and stirred for another 30 min until a complete dissolution occurred. Next, 0.55 mL of formaldehyde and 0.3 g melamine were added to the above mixture and stirred for 24 h continually. For synthesis of N-YSHMCSs, the reaction mixture for hollow nanospheres was further heated for 24 h at 100 oC under a static condition in a Teflon-lined autoclave. The solid product was recovered by centrifugation and air-dried at 70 oC over night. Calcination was carried out in a tubular furnace at 800 oC for 3 h under N2 flow. The heating rate was 1 oC min-1 below 600 oC and 5 oC min-1 above 600 o

C. The obtained carbon-silica composites, including hollow mesoporous carbon-silica

spheres and yolk-shell hollow mesoporous carbon-silica spheres, were denoted as HMCSSs-x and YS-HMCSSs-x, respectively. The silica was etched using 20% HF solution and the obtained N-doped hollow mesoporous carbon spheres and yolk-shell hollow mesoporous carbon spheres were donated as N-HMCSs-x and N-YSHMCSs-x. On the other hand, by 6


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combustion in air to remove carbon from the two types of mesoporous carbon-silica composite spheres, hollow mesoporous silica spheres (denoted as HMSSs-x) can be generated. The “x” denotes the amount of ammonia aqueous solution used (in mL; for instance 1, 2, and 3 refer to 0.2, 0.5, and 1.0 mL of ammonia aqueous solution, respectively). For comparison, hollow mesoporous carbon spheres (HMCSs) and yolk-shell hollow mesoporous carbon spheres (YSHMCSs) without N-doping were prepared as the same way to N-HMCSs-2 and N-YSHMCSs-2 except adding melamine.

2.3 Characterization TEM (transmission electron microscopy) analysis was conducted on a TECNAI G2 20 LaB6 electron microscope operated at 200 kV. SEM (scanning electron microscopy) analysis was conducted on FEI 250 system. N2 adsorption and desorption isotherms were measured using Micromeritics ASAP-2020 at liquid nitrogen temperature (-196 oC). The XPS spectra were obtained by using a PHI QuanteraⅡESCA System with Al Kα radiation at 1486.8V. TGA measurements were carried out on a SDT Q600 analyzer from 25 to 800 oC under N2 with a heating rate of 5 oC/min.

2.4 Electrochemical Measurements All the electrochemical measurements were conducted on a computer-controlled potentiostat (CHI 760C, CH Instrument, Shanghai) with a three-electrode electrochemical cell at 25 °C using a 1 M H2SO4 electrolyte. The standard three-electrode electrochemical cell was fabricated using glassy carbon with deposited sample as the working electrode, platinum wire as the counter electrode, and Ag/AgCl as the reference electrode. The working electrodes were fabricated as follows: first, 5 mg of the N-doped carbon spheres was mixed with 1 mL of DI water/isopropanol (1:1). The obtained suspension (10 µL) was dropped onto a glassy carbon electrode. Through measurement, the loading mass of hollow carbon spheres was 50 7



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µg.” After drying, a Nafion solution (0.5 wt % in isopropanol) was coated on the sample as the binder. The potentials for electrochemical measurements are reported relative to an Ag/AgCl (saturated KCl) reference electrode and the potential window for cycling was confined between 0 and 0.8 V. To explore the electrochemical capacitor in two-electrode system, the nanocomposite used for the electrode was fabricated by mixing 80 wt % N-doped carbon spheres, 10 wt % carbon black, and 10 wt % PVDF binder, then loaded on the carbon fiber paper. The loading mass of N-doped carbon spheres in each electrode was 5 mg cm-2. The two electrodes were separated by a filter paper (Whatman) soaked with electrolyte (1M H2SO4), then wrapped with parafilm, and at last dipped in the electrolyte solution. The cyclic voltammetry and galvanostatic charge-discharge tests of the electrochemical capacitor were further measured.

3. Results and Discussion As shown in Figure 1, the resol-silica spheres were formed by the polymerization of resorcinol/formaldehyde and TEOS in a mixture of ethanol, CTAC, and ammonia. During this process, emulsion droplets were first generated by the hydrogen bonding of water, ethanol, resorcinol, formaldehyde, and silicates32,

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. The negatively charged silicates and resols

electrostatically interacted with the positively charged CTAC, which took place from the outside of droplets by ammonia catalysis. On this basic, the hollow structure of resol-silica spheres was directly achieved. Then, melamine, as a nitrogen source, was added, which resulted in the nitrogen contained resol-silica spheres. After carbonization and silica etching process, N-doped hollow mesoporous carbon spheres (N-HMCSs) were obtained. Scanning electron microscopy (SEM) images (Fig. 2a, d, g) show that the three representative samples of N-HMCSs, which are assigned as N-HMCSs-1, N-HMCSs-2 and N-HMCSs-3, have uniform and spherical morphology in large domains. The particle sizes of N-HMCSs-1, 8


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N-HMCSs-2 and N-HMCSs-3 are evaluated to be 320, 450 and 550 nm, respectively. At a higher magnification image of N-HMCSs-2 (Fig. S2), the rough surface and hollow space are clearly observed. From the transmission electron microscopy (TEM) images of N-HMCSs-1, N-HMCSs-2 and N-HMCSs-3 (Fig. 2b, e, h), the remarkable feature of the spheres is the obvious contrast between the dark edge and the pale center, as is reported for other hollow particles with a central cavity32. The wall thicknesses of N-HMCSs-1, N-HMCSs-2 and N-HMCSs-3 are measured to be 90, 165, 220 nm, respectively. The corresponding cavity diameters of the three samples are 140, 120 and 110 nm, respectively. The average sizes of N-HMCSs decreased when the amount of ammonium was increased, in accordance with the previous reports52. In addition, radially aligned mesopores in carbon shells can be clearly observed at a higher magnification (Fig. 2c, f, i). The N2 adsorption-desorption isotherms of the N-HMCSs samples (Fig. 3A) exhibit typical type-IV hysteresis, indicating the existence of mesopores. It can be seen that the adsorption isotherm of the sample shows an apparent capillary condensation step at a relative pressure (P/P0) of 0.13-0.32; this corresponds to a small mesopore 32. Expect the pore condensation step, isotherms show increases in adsorption at a low relative pressure, implying the existence of a large amount of micropores in the carbon shells after the silica is removed from the carbon−silica composites. These N-HMCSs have high Brunauer Emmett Teller (BET) surface areas in the range of 2079-2464 m2 g−1. The pore sizes are calculated at 2.4-2.7 nm from the adsorption branch by the nonlocal density functional theory (NLDFT) model (Fig. 3B), and the pore volumes are calculated up to 2.20-2.36 cm3 g−1 (Table 1). Based on the above observations, the monodispersed N-HMCSs with tunable particle size as well as the wall thicknesses and central cavity can be obtained by simply adjustment of the amount of ammonium.

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Figure 2. SEM and TEM images of (a-c) N-HMCSs-1, (d-f) N-HMCSs-2, (g-i) N-HMCSs-3

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1600 1200 800 400

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Figure 3. (A) N2 adsorption-desorption curves and (B) pore size distribution of (a) N-HMCSs-1, (b) N-HMCSs-2, (c) N-HMCSs-3. For clarity, the isotherms curves in (A) are offset by 400 cm3 g-1. Table 1. Textural parameters of the samples of N-HMCSs Sample N-HMCSs-1 N-HMCSs-2 N-HMCSs-3

Particle size (nm) 320 450 550

BET surface area (m2 g-1) 2464 2359 2079

Pore volume (cm3 g-1) 2.36 2.23 2.20

Pore size (nm) 2.4 2.7 2.5 10


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Through hydrothermal treatment, yolk-shell structured hollow carbon spheres were obtained. SEM and TEM images of N-YSHMCSs after carbonization and silica etching, which were coded as N-YSHMCSs-1, N-YSHMCSs-2 and N-YSHMCSs-3, are shown in Fig. 4. It is seen that the yolk-shell structure is well retained after the heat and acid treatment. All the three N-YSHMCSs have rough surface and monodispered spherical morphology. The yolk-shell structure can be directly observed from a broken sphere (Fig. S3). As measured from SEM and TEM images, the average diameter of the N-YSHMCSs could be tailed from 385 to 600 nm, and the core sizes and wall thicknesses were in range of 70-100 nm and 110-180 nm, respectively. It is notable that the size increasing occurred after hydrothermal treatment, which may be attributed to the further condensation of polymer and silica species in the reaction solution. At a higher magnification of TEM images, it can be seen that mesopores with size of about 4.5 nm are well distributed in the shell of N-YSHMCSs. The N2 adsorption−desorption isotherms of the N-YSHMCSs samples (Fig. 5A) also exhibit typical type-IV hysteresis, indicating the existence of mesopores. The pore size distribution of the N-YSHMCSs samples were shown in Fig. 4B. The pore sizes are calculated at 4.2-4.7 nm. These spheres have high BET surface areas of 1050-1159 m2 g-1, with total pore volumes of 2.04-2.20 cm3 g-1 (Table 2). In addition, the carbonic cores and shells of these N-YSHMCSs were confirmed from the TGA curves, from which 100% weight loss was achieved (Fig. S1).

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Figure 4. SEM and TEM images of (a-c) N-YSHMCSs-1, (d-f) N-YSHMCSs-2, (g-i) N-YSHMCSs-3

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Figure 5. (A) N2 adsorption-desorption curves and (B) pore size distribution of (a) N-YSHMCSs-1, (b) N-YSHMCSs-2, (c) N-YSHMCSs-3. For clarity, the isotherms curves in (A) are offset by 700 cm3 g-1. Table 2. Textural parameters of the samples of N-HMCSs Sample N-YSHMCSs-1 N-YSHMCSs-2 N-YSHMCSs-3

Particle size (nm) 380 520 600

BET surface area (m2 g-1) 1050 1061 1159

Pore volume (cm3 g-1) 2.20 2.04 2.08

Pore size (nm) 4.7 4.2 4.5

X-ray photoelectron spectroscopy (XPS) is a powerful technique for the characterization of elemental composition and bonding configuration in materials. As shown in Fig. 6a, b, the 12


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as-synthesized N-HMCSs and N-YSHMCSs have the similar nitrogen form in the carbon matrix. The high-resolution XPS spectra of N1s can be curve-fitted into three-type peaks, which are correlated to different electronic states of the nitrogen functional groups: pyridinic (N-6, 398.6 eV) and pyrrolic groups (N-5, 400.9 eV) and pyridine N-oxide (N-X, 403 eV), implying the successful doping of N in these materials. The surface elemental composition data obtained by the XPS analysis are shown in Table S1. The surface N % for N-HMCSs and N-YSHMCSs is about 2%.

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Figure 6. High resolution N 1s spectrum of the as-synthesized (a) N-HMCSs, (b) N-YSHMCSs. In order to understand the structure evolution of N-HMCSs and N-YSHMCSs, TEM was employed to characterize the samples during the different synthetic step. After carbonizing polymer-silica composites, we could obtain carbon-silica composite spheres, including hollow mesoporous carbon-silica spheres (HMCSSs) and yolk-shell hollow mesoporous carbon-silica spheres (YS-HMCSSs). The resultant HMCSSs, correspond to N-HMCSs, displayed hollow and uniform spherical morphology (Fig. 7a-f), indicating that the hollow structure was formed before silica ethcing. In particular, the particle size, shell thickness and cavity size was consistent with N-HMCSs. Except that, the similar phenomenon was also observed in the samples of YS-HMCSSs and corresponding N-YSHMCSs (Fig. 7g-l). By combustion in air to remove carbon from the two types of mesoporous carbon-silica composite spheres, hollow 13



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mesoporous pure silica frameworks spheres (HMSSs) can be generated. The particle sizes of HMSSs from N-HMCSs-1, N-HMCSs-2 and N-HMCSs-3 are measured to be 350, 460 and 560 nm, which are in agreement with the diameters of N-HMCSs (Fig. 8a-f). The similar observations of YS-HMSSs from N-YSHMCSs were also shown in Fig. 8g-l. The particle sizes of YS-HMSSs from N-YSHMCSs-1, N-YSHMCSs-2 and N-YSHMCSs-3 are measured to be 430, 540 and 620 nm, in accordance with the diameters of N-YSHMCSs. In addition, the wall thicknesses of HMSSs are consistent with the carbon spheres. These results indicated that the two constituents of silica and carbon are “homogeneously” dispersed inside the shells. The hollow structure of the carbon-silica and silica framework further implied that the yolk particles of YSHMCSs are solid and carbonic. The evolution of N-HMCSs to N-YSHMCSs can be ascribed the following process: during the hydrothermal treatment of hollow polymer-silica nanospheres, more resorcinol-formaldehyde-melamine polymers could diffuse into the polymer-silica hollow cavity and further polymerize to form smaller polymer spheres32. Thus, N-YSHMCNs are obtained after carbonization and etching of the silica. Therefore, through our synthesis route, N-HMCSs and N-YSHMCSs with controllable morphological and textural features have been successfully fabricated. On the other hand, comparative hollow mesoporous carbon spheres (HMCSs) and yolk-shell hollow mesoporous carbon spheres (YSHMCSs) without N-doping were prepared as the same way to N-HMCSs-2 and N-YSHMCSs-2 except adding melamine. Characterization results (Fig. S4, 5 and Table S2) indicated that N-free HMCSs and YSHMCSs possessed the similar structural parameters with N-HMCSs-2 and N-YSHMCSs-2. The BET surface area of HMCSs and YSHMCSs is 2268 and 1170 m2 g-1, with total pore volumes of 1.98 and 2.20 cm3 g-1, respectively.

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Figure 7. TEM images of (a, d) HMCSSs-1, (b, e) HMCSSs-2, (c, f) HMCSSs-3, (g, h) YS-HMCSSs-1, (i, j) YS-HMCSSs-2, (k, l) YS-HMCSSs-3

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Figure 8. TEM images of (a, d) HMSSs-1, (b, e) HMSSs-2, (c, f) HMSSs-3, (g, h) YS-HMSSs-1, (i, j) YS-HMSSs-2, (k, l) YS-HMSSs-3 The supercapacitor performances of the obtained N-HMCSs were measured by cyclic voltammogram (CV) and galvanostatic charge-discharge. The sample N-HMCSs-2 was used as active materials in the working electrode in a 1 M H2SO4 acidic electrolyte solution. Fig. 9a show the CV curves of N-HMCSs-2 obtained from a three-electrode cell under a potential window of 0 to 0.8 V (vs. Ag/AgCl) at different scan rates. The rectangular-like shape and the appearance of humps in the CV curves indicate that the capacitive response comes from the combination of electric double-layer capacitance and redox reactions, which relate to the heteroatom functionalities of the material. It can be observed that the N-HMCSs-2 has a relatively good rectangular shape at a voltage scan rate up to 400 mV s-1, indicating typical double-layer capacitance behavior and excellent high rate capacitance. Galvanostatic charge/discharge method is assumed to be the most accurate technique for supercapacitor and applied to calculate the specific capacitances of N-HMCSs-2. Fig. 9b shows the galvanostatic charge/discharge curves at various current densities (1-40 A g-1). According to the formula C=It/m△E (herein, I is the charge/discharge current, t the discharge time, m the mass of sample in the electrode and △E the voltage difference), the specific capacitance of the sample 16


Page 17 of 28

was 240 F/g at 1A g-1, which is much higher than HMCSs (Fig. S6). This may be contributed the nitrogen functionalization, which can enhance the electrical conductivity and electrolyte solution wettability of the carbon materials, and thereby enhanced the capacitance. Moreover, the capacitance value of N-HMCSs is also superior to some other N-doped hollow carbon spheres46,

48, 53, 54

. This may be due to that the high surface areas of N-HMCSs-2, as

demonstrated above, which allow a large amount of electrical charge to accumulate on the electrode/electrolyte interface. The small particle size provides a large additional pseudo capacitance because it shortens the ion transport length and makes ion diffusion in the carbon nanospheres easier and thereby enhanced the capacitance. The specific capacitance of N-HMCSs-2 at 2, 3, 4, 5, 10, 20 and 40 A g-1 still preserved a very high capacitance of 214, 196, 185, 177, 173, 170, 165 F/g, respectively, which corresponds to the capacitance retention of 89, 82, 77, 74, 72, 71, 69 % of the value at 1 A g-1, showing a good rate capability (Fig. 9c). The corresponding coulombic efficiency in the galvanostatic charge-discharge at different current densities was measured to be above 90% (Fig. S7). Cycling performance is another important factor in determining the supercapacitor electrodes for many practical applications. The cycling stability of N-HMCSs-2 electrode was examined using galvanostatic charge-discharge cycling at a current density of 5 A g-1 (Fig. 9d). During the 5000 cycles, the specific capacitances are almost constant (the capacitance retention nearly remaining 97 %), which demonstrates good cycling performance.

a

-1

-1

10mvs -1 20mvs -1 30mvs -1 50mvs -1 100mvs -1 200mvs -1 400mvs

100

0

0.8

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200

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


b

1A/g 2A/g 3A/g 4A/g 5A/g 10A/g 20A/g 40A/g

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Capacitance Retention

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Specific Capacitance (F/g)


200

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d

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50

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Cycle number

Figure 9. (a) CV curves, (b) galvanostatic charge−discharge curves, (c) specific capacitance calculated from galvanostatic charge−discharge curves and (d) cycling stability of N-HMCSs-2 Except N-HMCSs, the supercapacitor properties of as-synthesized yolk-shell carbon spheres, N-YSHMCNs-2, were also text as electrode materials. The CV curves and galvanostatic charge/discharge curves were shown in Fig. 10a. Similar to N-HMCSs-2, the CV curves for N-YSHMCNs-2 are quasi-rectangular shaped at the scanning rate of 10 mv S-1 to 400 mv S-1. The gravimetric specific capacitance of this material was estimated using the discharge portion of the galvanostatic charge-discharge curves. The capacitance value obtained for this carbon at the current density of 1 A/g is 169 F g-1, which is higher than YSHMCSs (Fig. 10b and Fig. S8). The specific capacitance of N-YSHMCNs-2 at 2, 3, 4, 5, 10, 20 and 40 A g-1 are 151, 141, 134, 130, 125, 122 and 118 F g-1, respectively, which corresponds to the capacitance retention of 89, 83, 79, 77, 74, 72 and 70 % of the value at 1 A g-1, also showing a good rate capability (Fig. 10c). The corresponding coulombic efficiency in the galvanostatic charge-discharge at different current densities was measured to be above 90% (Fig. S9). The N-YSHMCNs exhibit a very stable capacitance (97.2 % of the original capacitance) after 5000 cycles of charging and discharging, indicating its long-term electrochemical stability (Fig. 10d). The higher specific capacitance of N-HMCSs-2 than N-YSHMCNs-2 may be ascribed to the higher BET surface area, which makes positive 18


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contribution to the supercapacitor performance. Taking their high specific capacitance and very robust electrochemical stability into account, the N-HMCSs and N-YSHMCNs are promise candidates as an electrode candidate for supercapacitors.

a

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30 0

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10mvs -1 20mvs -1 30mvs -1 50mvs -1 100mvs -1 200mvs -1 400mvs

-1


60

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c Capacitance Retention

Specific Capacitance (F/g)

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Figure 10. (a) CV curves, (b) galvanostatic charge−discharge curves, (c) specific capacitance calculated from galvanostatic charge−discharge curves and (d) cycling stability of N-YSHMCSs-2 To completely determine the electrochemical capacitance and simulate the actual device behaviour of these two carbon spheres, further measurements were conducted in a two-electrode system. As shown in Figure 11a, d, N-HMCSs-2 and N-YSHMCSs show a rectangular-like shaped CV curves between 0 and 0.8 V in 1M H2SO4, which is the characteristic of electrochemical double-layer capacitor. The rectangular-like shape was maintained without drastic change at extremely high scan rate of 400 mV s-1, which may be attributed to the hollow hierarchical porous structure that favours the fast ionic migration. The power density and energy density was calculated based on the data in galvanostatic 19



charge-discharge curves (Fig 11b, e). The Ragone plots for the electrochemical N-HMCSs and N-YSHMCSs capacitor in 1 mol L-1 H2SO4 electrolyte are presented in Figure 11 c, f. The result showed that the specific energy densities for the device of N-HMCSs and N-YSHMCSs are about 11.1 and 7.6 Wh kg-1 at a current density of 1 A g-1, respectively. Notably, the specific energy of N-HMCSs still is 7.9 Wh kg-1 with a high specific power of 16.0 kW kg-1 at a current density of 40.0A g-1, which is higher than that of the commercial carbon supercapacitors55. Meanwhile, the specific energy of N-YSHMCSs is 5.5 Wh kg-1 with a high specific power of 16.0 kW kg-1 at a current density of 40.0A g-1, which also possesses an

0

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10000 -1

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0.6

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f

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-1

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Energy Density (Wh Kg )

10mv/s 20mv/s 30mv/s 50mv/s 100mv/s 200mv/s 400mv/s

Energy Density (Wh Kg )

50

a

Potential (V)

-1


excellent energy storage performance.


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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Figure 11. Electrochemical performances of the capacitor device measured in a two-electrode system. Cyclic voltammograms at different scan rates of (a) N-HMCS-2 and (d) N-YSHMCSs-2, Charge-discharge curves at different current densities (b) N-HMCS-2 and (e) N-YSHMCSs-2. (c) Ragone plot of (c) N-HMCS-2 and (f) N-YSHMCSs-2performed in 1.0 mol L-1 of H2SO4 aqueous solution.

4. Conclusion We have demonstrated a facile and controllable synthesis of monodispersed N-doped 20


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hollow mesoporous carbon spheres (N-HMCSs) and yolk-shell hollow mesoporous carbon spheres (N-YSHMCSs) by a modified “silica-assisted” route. The morphological (i.e., particle size, shell thickness, cavity size and core diameter) and textural features of the carbon nanospheres can be easily controlled by varying the amount of ammonium. The resultant carbon nanospheres possess high surface areas (up to 2464 m2 g-1), large pore volumes (up to 2.36 cm3 g-1) and uniform mesopore size (~2.4 nm for N-HMCSs, ~4.5 nm for N-YSHMCSs). The as-synthesized two kinds of N-HSCSs manifest excellent supercapacitors performance with high capacitance, favorable capacitance retention, and high energy density, which may be contributed to the hollow mesoporous structures, high porosity, large surface area and N heteroatomic functionality. It is except that these kinds of N-HSCSs may show promising prospects as advanced energy storage materials and catalyst supports.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 51478224), the priority academic program development of Jiangsu higher education institutions and Research Innovation Grant for Graduate of Jiangsu Common High School (Grant No. KYZZ15_0115). We acknowledge gratefully Professor Junwu Zhu and Miss Yue Zhang for useful discussion and suggestion.

Supporting information TGA curve of N-YSHMCSs-2, element composition and TEM images of N-HMCSs-2 and N-YSHMCSs-2, TEM images, N2 adsorption-desorption curves, pore size distribution and textural parameters of HMCSs and YSHMCSs, CV curves and galvanostatic charge-discharge curves of N-HMCS-2, HMCSs, N-YSHMCS-2 and YSHMCSs, coulombic efficiency of N-HMCS-2 and N-YSHMCS-2. This information is available free of charge via the Internet at http://pubs.acs.org.

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Graphical abstract

N-doped hollow-structured mesoporous carbon spheres with controllable morphological and textural features have been facilely synthesized.


Synthesis of N-Doped Hollow-Structured Mesoporous Carbon

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