N-doping carbon sheet and core–shell mesoporous carbon sphere composite for high-performance supercapacitor

Accepted Manuscript Title: N-doping carbon sheet and core-shell mesoporous carbon sphere composite for high-performance supercapacitor Authors: Juan Du, Lei Liu, Yifeng Yu, Yue Zhang, Aibing Chen PII: DOI: Reference:

S1226-086X(19)30173-X https://doi.org/10.1016/j.jiec.2019.04.012 JIEC 4492

To appear in: Received date: Revised date: Accepted date:

16 January 2019 11 March 2019 7 April 2019

Please cite this article as: Du J, Liu L, Yu Y, Zhang Y, Chen A, N-doping carbon sheet and core-shell mesoporous carbon sphere composite for highperformance supercapacitor, Journal of Industrial and Engineering Chemistry (2019), https://doi.org/10.1016/j.jiec.2019.04.012 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.

N-doping carbon sheet and core-shell mesoporous carbon sphere

Juan Du, Lei Liu, Yifeng Yu, Yue Zhang, Aibing Chen*

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composite for high-performance supercapacitor

College of Chemical and Pharmaceutical Engineering, Hebei University of Science and

Author: E-mail: [email protected]

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*Corresponding

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Technology, 70 Yuhua Road, Shijiazhuang 050018, China.

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Graphical abstract

A novel N-doped core-shell mesoporous carbon sphere and sheet composite with a high specific capacitance and excellent energy density in supercapactor, which employed Ionic liquid as

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template, phenolic resin as carbon precursor, TEOS as assistant agent and mesoporous resin

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spheres as additive for the formation of the flaky/spherical hybrid structure.

Highlights

· A self-assembly method is employed to prepare N-doped core-shell mesoporous sphere and carbon sheet composite.

· The method used ionic liquids as soft-template and nitrogen precursor, resin as carbon

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precursor, silica as assistant agent, mesoporous resin spheres as additive.

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· The obtained carbon material exhibits outstand promising in supercapacitor with a high

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specific capacity and energy density.

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Abstract

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A self-assembly method using ionic liquids as soft-template and nitrogen precursor, resin as carbon

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precursor, silica as assistant agent, mesoporous resin spheres as additive, is employed to prepare N-doped core-shell mesoporous sphere and carbon sheet composite (N-CMCS/CS). The obtained

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N-CMCS/CS possesses thin sheet, core-shell spheres, dual-mesoporous structure, large specific area and a certain of N-doping, which endows it with superior electrochemical performance as

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electrode material in supercapacitor.

Keywords: supercapacitor, high specific capacitance, core-shell carbon spheres, carbon sheets, dual-mesoporous structure

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1. Introduction The energy crisis increasing stimulates continuous research on sustainable and economical energy conversion or storage such as lithium ion batteries, fuel cells and supercapacitors[1]. As one of the

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most promising energy storage devices, supercapacitors showed fast charge/discharge rate, high power density and long cycle life[2]. Generally, electrode materials strongly influence the electrochemical properties of supercapacitors, including their constituent, morphology, structure, and so on[3]. For this reason, developing electrode materials with outstand performance has been hot topic in materials science.[4, 5] Among numerous available candidates, owing to the light

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weight, high chemical stability, well conductivity, rich porosity and plenty of active sites of

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carbonaceous materials, they are recognized as the most promising electrodes for

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supercapacitors[6, 7].

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The electrochemical capacitor application is strongly based on the structural properties of carbon materials[8-10]. As a class of nanoporous carbon materials, order mesoporous carbon spheres

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(MCS) possess many advantages including regular geometry, high surface area and good packing

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density. Moreover, during the charge-discharge process, MCS allow electrolyte ions to transfer

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rapidly even at high current density due to the well-structured mesopores[11]. Meanwhile, MCS with core-shell structure have been used as an excellent advanced electrode in high-performance

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supercapacitors to achieve high energy density while retaining high power density[12, 13]. Carbon sheet with 2D structure have also received extensive attention in recent years due to their larruping

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properties including high theoretical specific surface area and superior electrical conductivity. Thin sheet structure provides transport path for charge and mass transfer during the charging/discharging processes, which guarantee the good electrochemical performance[13]. For instance, mesoporous thin carbon sheet was prepared by using polyimide chemistry in molten salts,

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which have broad pore channel and large accessible surface for electrolyte/ion adsorption and transport in supercapacitor[3, 6, 14]. In recent years, novel carbon composite materials with dual structures (fiber/sphere, fiber/sheet, tube/sheet, and so on) were developed and showed higher

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performance than single feature[15], enriching the carbonaceous electrode materials for supercapacitors. A good example is graphene and carbon sphere composites, which combines the advantage of carbon spheres and carbon sheet, showing advanced performance[15, 16]. However, such complex composite structure still needs more research for practical application. In our recent work, composites of hollow carbon spheres and carbon sheets were prepared to improve

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electrochemical performance.[17] Nevertheless, the improved performance is not significant.

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Except for the porous structures and morphology, the heteroatom functionalities also affected the

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electrochemistry properties of electrodes materials[18]. For example, nitrogen functional groups

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not only increase the specific capacitance by the faradic reaction in the process of electrochemical charge-discharge but also significantly enhance the electrodes surface wettability[16]. Therefore,

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achieving nitrogen doping in carbonaceous materials is also one of the effective ways to effectively

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improve electrochemical performance. Ionic liquids (ILs) are a class of materials that are molten

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salts existing in the liquid state at relative low temperature and do not evaporate under heating. Many efforts have been took to use ILs in solvents, media or soft templates for preparation of

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carbonaceous materials and also as nitrogen precursor to synthesize N-doped carbon materials[16, 19, 20].

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Considering the respective advantages of thin carbon sheet, mesoporous carbon spheres and suitable nitrogen doping, herein, we prepared a novel composite of N-doping core-shell mesoporous carbon spheres (CMCS) and carbon sheet (N-CMCS/CS), which combined the advantages of spheres and nanosheets. In this process, the mesoporous polymer sphere (MPS),

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which was coated by a layer of, was used to form carbon spheres on carbon sheet (CS). The MPS was by a layer of silica shell to incorporate the mesoporous spheres with the thin carbon sheet derived from the RF and form core-shell finally. In addition, ILS not only acted as structural

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directing agent in the formation of sheets, but also led to the in-situ nitrogen doping of composites. As electrode materials, N-CMCS/CS exhibited outstand electrochemical performance in supercapacitor. 2. Results and discussion

The preparation of N-CMCS/CS process was shown in Scheme 1. For the purpose of comparison,

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the CS and N-MCS/CS composite were also prepared. For the purpose of comparison, the CS and

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N-MCS/CS composite were also prepared. Typically, CS was synthesized by sol-assembly of ionic

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liquid ([C18Mim]Br), tetraethoxysilane (TEOS) and resorcinol-formaldehyde (RF) (Route 1 of

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Scheme 1). During the sol-assembly process, RF was used as an organic phase and carbon precursor, [C18Mim]Br as template and TEOS as inorganic assistant to synthesize carbon sheet

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with meso-structure. In order to incorporate mesoporous spheres with the thin carbon sheet, the

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MPS or polymer/silica nanohybrids (named as MPS@SiO2) were added into the reaction system

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respectively. As shown in Route 2 and 3 of Scheme 1, the thin carbon sheet embedded with MCS or CMCS was obtained. In this process, [C18Mim]+ adsorbed on negative charged MPS or

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MPS@SiO2 spheres surface through electrostatic attraction. Then TEOS and the RF precursor interacted with [C18Mim]Br through electrostatic interactions to form mesoporous hybrid

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aggregates. The hybrid aggregates further crosslinked into interpenetrating 3D rigid frameworks, forming RF polymer/silica composites on the surface of MPS or MPS@SiO2 sphere. Additionally, the redundant [C18Mim]-coated RF aggregates would assemble around the RF polymer and silica composites spheres to form polymer/silica composite sheet. Finally, N-MCS/CS and N-CMCS/CS

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could be readily obtained after pyrolysis and etching the SiO2. Meanwhile, it was worth noting that [C18Mim]Br acted as a template and a nitrogen precursor synchronously, leading to in-situ N-

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doping in the carbon skeleton.

CS could be successfully synthesized by col-sol-assembly of [C18Mim]Br, TEOS and RF. The transmission electron microscopy (TEM) images of CS was shown in Fig. 1a and b. The thin carbon sheet with laminar morphology like silk veil waves could be seen from Fig. 1a. Fig. 1b exhibited the higher resolution TEM images, which confirmed the irregular mesoporous structure

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of CS generated from the etching of SiO2.

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When MPS sphere was added in the reaction system of CS, N-MCS/CS was obtained. The

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morphology of the as-synthesized N-MCS/CS composite was also investigated by TEM. A large

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area of the carbon sheet was uniformly decorated with solid carbon nanospheres as shown in Fig. 1c and d. The CS sheet edge can be clearly distinguished from the background (as shown by the

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arrow in Fig. 1c). A TEM image with higher resolution showed solid carbon nanosphere with size

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of ca. 90 nm (Fig. 1d), which was derived from MPS. Generally, ordered mesoporous carbon

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spheres (MCS) could be synthesized by directly pyrolysis of MPS[21]. TEM images revealed uniform spherical morphology of MCS with diameter of 90 nm (Fig. S1a) and order mesoporous

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structure with pore size of 2.7 nm (Fig. S1b). However, in TEM images of N-MCS/CS, carbon spheres showed no obvious mesoporous structure, which might be ascribed to the impregnation

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and coverage of RF oligomer in the mesopore of MPS during the process of co-assembly. The introduction of MPS enriched the porous structure, but the blocking of the pores would hinder rapid ion transportation.

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To well maintain the mesoporous structure of MPS, a compact silica layer was loaded on MPS forming MPS@SiO2, then MPS@SiO2 was added into the reaction system before subsequentially carbonization and etching silica. The spherical and uniform core-shell structures of MPS@SiO2

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sample were also conformed from the TEM images (Fig. S2a). The thickness of rough layer of amorphous silica was 35 nm (Fig. S2b), indicating a successful coating. At the same time, the ordered mesoporous structure of MPS was well maintained with a silica coating, which was a protective layer. When MPS@SiO2 was added to the reaction system of CS, N-CMCS/CS was obtained. Fig. 1e was the TEM image of the nanocomposite of N-CMCS/CS. A great number of

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core-shell nanoparticles with the particle diameter about 150 nm attached on the thin carbon sheet

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could be clearly seen. From higher resolution TEM image of N-CMCS/CS (Fig. 1f and inset), the

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rich mesoporous structure core with diameter of 90 nm, large mesoporous size of 7.0 nm (the

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yellow circle) and amorphous carbon could be clearly observed, which was confirmed by the Raman measurements as shown in Fig. S3. G band at 1576 cm-1, and D band at 1322 cm-1 is

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ascribed to the ordered graphitic carbon ad disordered carbon respectively. Therefore, the ratio of

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peak intensity (ID/IG) is 0.99, which can be ascribed to the amorphous carbon. This results showed

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that the compact silica shell on the surface of RF not only effectively prevented the resin from entering the pore and protected the mesoporous structure of MPS, but also provided a confined

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space leading to increased mesoporous size [22]. The outer thin shell was 10 nm (Fig. 1f inset),

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which might be derived from the coating of col-sol-assembly of [C18Mim]Br, TEOS and RF.

Nitrogen sorption measurement was further used to analyze the porosity of CS, N-MCS/CS and N-CMCS/CS samples as shown in Fig. 2a. A type-IV isotherm and the hysteresis at relative higher pressure (P/P0 > 0.4) could be observed from all these samples, indicating the presence of

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mesoporous characteristics of these samples. Moreover, compared with the CS, the N-CMCS/CS showed more obvious hysteresis loop, indicating richer mesoporous structure. The Barrett-JoynerHalenda (BJH) method was used to calculate pore-size distribution from the desorption branch of

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the nitrogen isotherm and it showed an average pore size of 2.7 nm (Fig. 2b). Meanwhile, NCMCS/CS also had another wide peak at 22.9 nm, ascribing to the core-shell structure. For comparison purposes, MCS was also examined by nitrogen sorption test accordingly as shown in Fig. S4, which exhibited mesoporous size with 2.7 nm. Table S1 showed the detailed textural parameters of the samples. The MCS possessed maximal specific surface area of 922 m2 g-1 slightly

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more than that of N-CMCS/CS (833 m2 g-1), but the pore volume of 0.54 cm3 g-1 was much lower

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than that of N-CMCS/CS (1.36 cm3 g-1).

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The [C18min]Br not only played the role of template agent, but also acted as an excellent nitrogen

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precursor, leading to in-situ nitrogen doping for N-CMCS/CS. The qualitative information and chemical environments about the surface elemental composition could be provided by the X-ray

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photoelectron spectroscopy (XPS) survey spectrum (Fig. 2c), which showed the dominant C1s

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(284.0 eV), N1s (397.9 eV) and O1s (531.2 eV), respectively. In addition, the CS and N-CMCS/CS

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exhibited similar contents of N and O (inset of Fig. 2c). In XPS spectrum of C1s for both CS and N-CMCS/CS, four sub-peaks could be attributed C=C, C-N/C-C, C=O and COOH at 284.6, 285.6,

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286.8, and 287.9 eV, respectively (Fig. 2d). The O1s spectrum (Fig. 2e) could be divided into three peaks which corresponded to C=O, C-O and hydroxyl oxygen located at 532.32, 530.60 and 533.72

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respectively. High-resolution XPS N1s spectra for CS and N-CMCS/CS revealed the assignments of three types of nitrogen defects, corresponding to pyridinic-N (397.98 eV), pyrrolic-N (400.25 eV), quaternary-N (401.16 eV), respectively, as demonstrated by Fig. 2f [18]. The similar chemical

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environments between CS and N-CMCS/CS indicated that nitrogen had been successfully doped into the carbon skeleton which was derived from [C18Mim]Br. The N-CMCS/CS combined many advantages such as dual mesoporous structure, thin sheet, core-

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shell mesoporous nanosphere, nitrogen doping and large pore volume, which were ideal characteristics of good electrodes. The electrochemical performance of N-CMCS/CS for electrochemical double-layer capacitance (EDLC) was evaluated and compared with CS, MCS, N-MCS/CS. The cyclic voltammetry (CV) curves at 5 mV s-1 scan rate exhibited that all of the samples presented a nearly rectangular shape as illustrated in Fig. 3a, indicating a favorable EDLC

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behavior [4, 5, 23, 24]. The N-CMCS/CS acquired the highest specific capacity among all the

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samples according to a preliminary estimate based on the CV integrated area. The galvanostatic

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charge-discharge (GCD) curves with the time extending at low current density of 0.5 A g-1 (Fig.

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3b) were also confirmed. The specific capacities were 139.7, 156.0, 289.2 and 433.3 F g-1 for CS, MCS, N-MCS/CS and N-CMCS/CS, respectively, calculated by discharge branches. It worth

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mentioning that the capacitance of N-CMCS/CS was higher than other samples and many carbon

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materials with different structures as shown Table S2, indicating the great potential for

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electrochemical applications. The capacitance retentions were 78.4, 59.7, 85.8 and 81.9 % for CS, MCS, N-MCS/CS and N-CMCS/CS, respectively, from the rate performance at the current density

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range of 0.5-10 A g-1 (Fig. 3c). The electrical impedance spectroscopy (EIS) measurements were conducted in the frequency range

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of 105-10-2 Hz, providing more powerful evidences for the capacitive behavior of all samples. All the samples showed almost similar impedance spectra with an arc at the high frequency region and a vertical line at the low frequency region as illustrated in Fig. 3d, indicating an ideal capacitive behavior of EDLC. The fitted equivalent circuit is shown in Fig. S5, including Rs (solution

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resistance), Rct (charge transfer resistance), Cdl (double layer capacitance or constant phase element), CPE (leakage capacitance) and ZW (Warburg impedance). At high frequency, it was clear that the N-CMCS/CS showed relative lower solution resistance and charge transfer resistance

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value. The Nyquist plot of N-CMCS/CS was almost straight in the low-frequency region, indicating ideal EDLC supercapacitor.

The detail performance of N-CMCS/CS electrode in a three-electrode cell was shown in Fig. 3e and f. When the scan rate gradually increased from 5 to 100 mV s-1, a regular rectangular shape was retained as shown in Fig. 3e, indicating a good capacitance performance at a high scan rate.

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Fig. 3f showed the rate capability of N-CMCS/CS over the range of current densities from 0.5 to

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10 A g-1 in GCD curves. The quasi-triangular and symmetrical of all GCD curves at various current

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densities indicated the typical EDLC behavior and superior charge-discharge reversibility of

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electrodes.

N-CMCS/CS not only achieved a high power density due to rapid ion transportation, but also had

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a high energy density due to the nitrogen functional groups and optimum pore size as shown in

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Fig. 4a. The capacitive performances of N-CMCS/CS electrode was evaluated for real

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supercapacitor by building a symmetric capacitor using N-CMCS/CS as the electrode. The CV profiles of N-CMCS/CS at different scan rates were presented in Fig. 4b. It was obvious that the

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capacitor remained a good rectangular CV profiles at both low and high scan rate, demonstrating the N-CMCS/CS electrode could serve as promising electrodes for high-performance

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supercapacitor. The electronic double-layer energy storage mechanism was further confirmed by the almost symmetric triangular-like shapes of GCD curves from 1 to 50 A g-1 (Fig. 4c and d). As shown in Fig. 4d, the discharge curves of N-CMCS/CS capacitor only exhibited a rather small voltage drop (IR drop) (0.19 V) even at high current density of 50 A g-1, suggesting a rather low

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internal resistance. Fig. 4e showed the specific capacities calculated from discharge branches and the rate performance at the different current density range of 1-50 A g-1. The specific capacitance of 331 F g-1 at the current density of 1 A g-1 was higher than many other carbon materials, such as

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solid carbon spheres, carbon sheet or hollow carbon spheres and so on, as shown in Fig. 4e[18, 2534].

Ragone plots showed the relationship between energy densities and powder densities of the electrode materials in Fig. 4f. The GCD in the symmetric supercapacitor was used to calculate the energy and power density and obtained the Ragone plots of the N-CMCS/CS, which exhibited

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much better performance than most of carbon materials reported previously with higher power

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output capability at corresponding energy density[8, 11, 34-42]. The energy density of the N-

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CMCS/CS was estimated to be 26.8 Wh kg-1 at a power density of 994 W kg-1. More significantly,

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the energy density was still as high as 14.3 Wh kg-1 even at a very high power density of 66 000 W kg-1. It was clear that the N-CMCS/CS exhibited relative higher power output capability at

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corresponding energy density, further demonstrating the superior performance of N-CMCS/CS

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due to the special nanostructure. Cycling performance is another crucial parameter or practical

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applications of supercapacitors. Therefore, the long cycle stability of the N-CMCS/CS was tested in two-electrode system at the current density of 2 A g-1. As displayed in Fig. S6, 82.6 % of initial

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capacity can be retained after 10000 cyclic tests, indicating excellent capacitive property and longterm electrochemical stability. Such impressive results showed that the N-CMCS/CS are an

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intriguing supercapacitor electrode material for high-rate applications. 3. Conclusion In summary, we have designed a novel composite of N-doped yolk-core mesoporous sphere combined by thin sheet, with [C18Mim]Br as structure-directing agent, TEOS as assistant and

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mesoporous sphere as additive. The N-CMCS/CS combined the advantages of sheet and mesoporous sphere, dual-mesoporous, high surface area, large pore volume and nitrogen doping, endowed it with high specific capacitance of 433.3 F g-1 at 0.5 A g-1 current density. In addition,

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the nitrogen functional groups and an optimum pore size of N-CMCS/CS endowed its high energy density as well as high power density, showing excellent electrochemical properties and good potential application as an electrode material. This assembly strategy might demonstrate a new direction in designing and synthesizing carbonaceous materials with composite structure for high

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performance applications.

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Interests

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The authors declare no competing financial interests. Acknowledgments

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We thank the National Natural Science Foundation of China (21676070), Hebei Training Program

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for Talent Project (A201500117), Beijing National Laboratory for Molecular Sciences, Hebei

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Science and Technology Project (17214304D, 16214510D).

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[31] Y. Fan, P.-F. Liu, Z.-J. Yang, T.-W. Jiang, K.-L. Yao, R. Han, X.-X. Huo, Y.-Y. Xiong, Bifunctional porous carbon spheres derived from pectin as electrode material for supercapacitors and support material for Pt nanowires towards electrocatalytic methanol and ethanol oxidation, Electrochim. Acta 163 (2015) 140-148. [32] X. Ma, L. Gan, M. Liu, P.K. Tripathi, Y. Zhao, Z. Xu, D. Zhu, L. Chen, Mesoporous size controllable carbon microspheres and their electrochemical performances for supercapacitor electrodes, J. Mater. Chem. A 2 (2014) 8407-8415. [33] H. Sun, Y. Zhu, B. Yang, Y. Wang, Y. Wu, J. Du, Template-free fabrication of nitrogendoped hollow carbon spheres for high-performance supercapacitors based on a scalable homopolymer vesicle, J. Mater. Chem. A 4 (2016) 12088-12097. [34] B. Chang, S. Zhang, H. Yin, B. Yang, Convenient and large-scale synthesis of nitrogen-rich hierarchical porous carbon spheres for supercapacitors and CO 2 capture, Appl. Surf. Sci. 412 (2017) 606-615. [35] Q. Zhang, L. Li, Y. Wang, Y. Chen, F. He, S. Gai, P. Yang, Uniform fibrous-structured hollow mesoporous carbon spheres for high-performance supercapacitor electrodes, Electrochim. Acta 176 (2015) 542-547. [36] X. Li, L. Zhang, G. He, Fe3O4 doped double-shelled hollow carbon spheres with hierarchical pore network for durable high-performance supercapacitor, Carbon 99 (2016) 514-522. [37] L. Mao, Y. Zhang, Y. Hu, K.H. Ho, Q. Ke, H. Liu, Z. Hu, D. Zhao, J. Wang, Activation of sucrose-derived carbon spheres for high-performance supercapacitor electrodes, RSC Adv. 5 (2015) 9307-9313. [38] Y.H. Dai, H. Jiang, Y.J. Hu, Y. Fu, C.Z. Li, Controlled synthesis of ultrathin hollow mesoporous carbon nanospheres for supercapacitor applications, Ind. Eng. Chem. Res. 53 (2014) 3125-3130. [39] F. Sun, H. Wu, X. Liu, F. Liu, H. Zhou, J. Gao, Y. Lu, Nitrogen-rich carbon spheres made by a continuous spraying process for high-performance supercapacitors, Nano Res. 9 (2016) 32093221. [40] M. Li, S. Xiang, X. Chang, C. Chang, Resorcinol-formaldehyde carbon spheres/polyaniline composite with excellent electrochemical performance for supercapacitors, J. Solid State Electrochem. 21 (2016) 485-494. [41] Z. Ye, F. Wang, C. Jia, K. Mu, M. Yu, Y. Lv, Z. Shao, Nitrogen and oxygen-codoped carbon nanospheres for excellent specific capacitance and cyclic stability supercapacitor electrodes, Chem. Eng. J. 330 (2017) 1166-1173. [42] X. Xu, Y. Liu, M. Wang, C. Zhu, T. Lu, R. Zhao, L. Pan, Hierarchical hybrids with microporous carbon spheres decorated three-dimensional graphene frameworks for capacitive applications in supercapacitor and deionization, Electrochim. Acta 193 (2016) 88-95.

15

(c)

(d)

SC RI PT

(b)

D

M

A

N

U

(a)

(f)

A

CC

EP

TE

(e)

Fig. 1. TEM images of CS (a, b), N-MCS/CS (c, d) and N-CMCS/CS (e, f).

16

40

(b) 1.4

CS N-YMCS/CS N-MCS/CS

-1

35

d(V)/dlog(D) (cm g )

30

3

25 20 15 10 5

1.2 1.0 0.8 0.6 0.4 0.2

0.4

0.6

0.8

1.0

0

(c)

40

(d)

CS

C=C

CS N-YMCS/CS

94.8 95.1 95

C-C/C-N COOH C-O

0

3.4 3.2

1.8 1.7 C

Intensity (a.u.)

90 5

N

O

Elements

CS

N-YMCS/CS

N-YMCS.CS 1200

1000

800

600

400

200

296

0

292

CS

N-YMCS/CS

M

Intensity (a.u.)

Intensity (a.u.)

536

532

528

408

404

400

396

Binding Energy (eV)

TE

Bingding Energy (eV)

D

540

Pyridinic-N

A

Quaternary-N

-OH

544

280

Pyrrolic-N

C=O

N-YMCS/CS

284

N

(f)

C-O

288

Binding Energy (eV)

Binding Energy (eV) CS

60

Pore Size (nm)

100

Content (%)

20

C=C

SC RI PT

0.2

P/P0

(e)

CS N-YMCS/CS N-MCS/CS

0.0 0.0

Intensity (a.u.)

22.9 nm 2.7 nm

U

Quantity Adsorbed (mmol g-1)

(a)

Fig. 2. N2 adsorption-desorption isotherm (a) and pore size distribution (b) of CS, N-MCS/CS and

EP

N-CMCS/CS, XPS spectra of CS and N-CMCS/CS (c), C1s (d), O1s (e), N1s (f) spectrum of CS

A

CC

and N-CMCS/CS.

17

(b) 0.0 2

N-CMCS/CS N-MCS/CS MCS CS

Potential (V)

-0.2

0

-2

N-CMCS/CS N-MCS/CS MCS CS

-4 -1.0

-0.8

-0.6

-0.4

-0.2

-0.4

-0.6

-0.8

-1.0

0.0

0

400

800

Potential (V)

(d) 160

15

120

10

N-MCS/CS 85.8%

CS 78.4% 100

5

80 0 0

40

MCS 59.7% 4

6

8

0

10

0

-1

(f) 0.0 -0.2

80

120

160

-0.4 -0.6

D

Potential (V)

-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

-1

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

M

40

-80

TE

-1

40

N-MCS/CS CS

Z' (Ohm)

(e) 80

-40

15

N-CMCS/CS MCS

Current Density (A g )

0

10

Z' (Ohm)

N

2

5

A

0

1600

U

200

-Z'' (Ohm)

300

0

Current Density (A g )

1200

Times (s)

N-CMCS/CS 81.9%

400

-Z'' (Ohm)

-1

Specific Capacitance (F g )

(c)

SC RI PT

-1

Current Density (A g )

(a)

0.0

0.2

-1.0 0

400

800 1200 Times (s)

1600

EP

-1.0 -0.8 -0.6 -0.4 -0.2 Potential (V)

-0.8

Fig. 3. Electrochemical evaluation of CS, MCS, N-MCS/CS and N-CMCS/CS: CV curves at the

CC

scan rates of 5 mV s-1 (a), GCD curves at current density of 0.5 A g-1 (b), specific capacitances at different GCD (c), Nyquist plots with fitting curves (d) and their corresponding high frequency

A

ranges (inset), CV curves at different scan rates (e) and GCD curves at different current density (f) of N-CMCS/CS.

18

(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

-20 0.0

0.2

0.4

0.6

SC RI PT

-10

0.8

1.0

Potential (V) -1

1Ag -1 2Ag -1 3Ag -1 4Ag -1 5Ag

0.6

0.4

(d)

0.8

IR

-1

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

0.6

Potential (V)

0.4

0.0

0.0 0

50

100

0

150

2

18

0 20

30

40

-1

1

10

12

N-YHMCS/S Ref. 35 Ref.37 Ref. 40 Ref. 42

Ref. 8 Ref. 11 Ref. 38 Ref. 41

Ref. 34 Ref. 36 Ref. 39

0.1

50

1000

TE

10

D

50

N-doped HCS 26 Core-shell HCS 28 Carbon spheres 30 Carbon spheres 32 HCS 34 Carbon spheres

10

M

200

Energy Density (Wh Kg )

74.0 % 250

100

8

A

(f) 300

-1

Specific Capacitance (F g )

(e)

N-YMCS/S 25 Carbon spheres 27 Carbon spheres 29 Carbon spheres 31 Carbon spheres 33 HCS

6

Times (s)

Times (s)

150

4

U

0.2

0.2

N

Potential (V)

(c) 0.8

10000

100000 -1

-1

Power Density (W Kg )

Current Density (A g )

EP

Fig. 4. Model of charge transfer of N-CMCS/CS (a), CV curves at different scan rate (b); GCD

CC

curves at different current density (c and d); Specific capacitances at different GCD (e); Ragone

A

plots comparison with the reported carbon spheres (f) of N-CMCS/CS.

19

SC RI PT

Scheme 1. Schematic illustrations of the synthesis of CS (Route 1) and N-MCS/CS (Route 2) and

A

CC

EP

TE

D

M

A

N

U

N-CMCS/CS (Route 3).

20