N-doped hollow mesoporous carbon spheres prepared by polybenzoxazines precursor for energy storage

Journal Pre-proof N-doped hollow mesoporous carbon spheres prepared by polybenzoxazines precursor for energy storage Juan Du, Aibing Chen, Lei Liu, Bo Li, Yue Zhang PII:

S0008-6223(20)30018-X

DOI:

https://doi.org/10.1016/j.carbon.2020.01.018

Reference:

CARBON 14953

To appear in:

Carbon

Received Date: 31 October 2019 Revised Date:

23 December 2019

Accepted Date: 5 January 2020

Please cite this article as: J. Du, A. Chen, L. Liu, B. Li, Y. Zhang, N-doped hollow mesoporous carbon spheres prepared by polybenzoxazines precursor for energy storage, Carbon (2020), doi: https:// doi.org/10.1016/j.carbon.2020.01.018. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.

Credit Author Statement: Juan Du: Data curation, Formal analysis, Writing- Original draft preparation. Aibing Chen: Supervision, Conceptualization, Methodology. Lei Liu: Writing-Reviewing and Editing Bo Li: Visualization, Investigation. Yue Zhang: Writing-Reviewing

N-doped hollow mesoporous carbon spheres prepared by polybenzoxazines precursor for energy storage Juan Du1, Aibing Chen1,*, Lei Liu1, Bo Li2,*, Yue Zhang1 1

College of Chemical and Pharmaceutical Engineering, Hebei University of Science

and Technology, 70 Yuhua Road, Shijiazhuang 050018, China. 2

Hebei Research Centre of Analysis and Testing, Hebei University of Science and

Technology, 26 Yuxiang Street, Shijiazhuang 050018, China. *

Corresponding Author: E-mail: [email protected] (A.Chen); [email protected]

(B. Li).

Table of Contents:

Polybenzoxazines derived from relatively cheap phenol/formaldehyde oligomer and ethylenediamine was used as carbon/nitrogen precursor for the preparation of a series of N-doped hollow carbon spheres with tunable morphologies changing from irregular particle aggregates to hollow spheres with increasing in diameter.

N-doped hollow mesoporous carbon spheres prepared by polybenzoxazines precursor for energy storage Juan Du1, Aibing Chen1,*, Lei Liu1, Bo Li2,*, Yue Zhang1 1

College of Chemical and Pharmaceutical Engineering, Hebei University of Science

and Technology, 70 Yuhua Road, Shijiazhuang 050018, China. 2

Hebei Research Centre of Analysis and Testing, Hebei University of Science and

Technology, 26 Yuxiang Street, Shijiazhuang 050018, China. *

Corresponding Author: E-mail: [email protected] (A.Chen); [email protected]

(B. Li). Abstract A co-assembly approach has been developed to controllably fabricate N-doped hollow mesoporous carbon spheres (N-HMCS) with different morphologies. Cetyl-3-methyl ammonium bromide and tetraethyl orthosilicate (TEOS) are applied to co-assemble with polybenzoxazines (phenol/formaldehyde/ethylenediamine (PB) resin) through electrostatic interaction. The utilization of ethylenediamine (EDA) is to introduce nitrogen into N-HMCS and catalyze hydrolysis of the silica precursor and polymerization of PB. The increasing amount of EDA leads to different morphologies of N-HMCS, changing from irregular particle aggregates to hollow spheres with increasing in diameter by controlling the hydrolysis rate of TEOS. In addition, pre-polymerization of phenol/formaldehyde oligomer is another key factor for the formation of N-HMCS, which is proved by the irregular porous particles obtained by direct assembly of phenol, formaldehyde and EDA. The different morphologies in N-HMCS show variable electrochemical performance when using as electrode materials in supercapacitor. Typically, N-HMCS with regular spherical morphology, small diameter and thin shell shows better performance, proving its excellent prospects in energy storage. 1. Introduction Carbonaceous materials have gained considerable attention owing to their unique properties such as high specific surface area and moderate pore size distribution, low 1

density, good chemical stability, allowing their potential applications.[1, 2] Hollow mesoporous carbon spheres (HMCS) have been one of rapid innovations in carbonaceous materials due to their large cavity, regular or tunable morphologies, ensuring promising potentials in catalysis, adsorption, energy storage/conversion derives, etc.[3] It is known that regular small diameter, hollow space, thin shell and high surface area can provide sufficient space and transport channels for charge retention and high energy storage.[4] To improve the electrochemical properties of HMCS, continuous efforts have been devoted to optimize these parameters. The research on HMCS preparation methods has been in progress for years. Hydrothermal process, template or modified Stöber methods, etc. have been investigated by different groups. In typically hydrothermal method, glucose or other substances are used as the carbon source, and surfactant is used to create cavity.[5, 6] However, the produced HMCS often show irregular morphologies and poor porous structure, resulting in low surface area. Hard or soft template strategies are effective to prepare HMCS and control their structures.[7, 8] Nevertheless, template method is either time-consuming in operation or tough to implement, especially for hard template method.[9] The modified Stöber method shows special prospects considering the facile operation, controlled hollow structure and regular morphology of the prepared HMCS.[10] The preparation principle is related to different hydrolysis rate of silica and carbon precursor. Silica is used as additive agent and HMCS can be obtained by etching process. Resins (especially hydroquinone,

resorcinol,

aminophenol) are common carbon precursors, which are mostly expensive and not good choices in practical production.[11-14] Additionally, the investigation on chemical composition is growing. Traditional HMCS is mainly composed of carbon and has no other heterogeneous elements, which makes them unsatisfactory in their applications because of a small number of specific active sites and unfavorable hydrophobic surface. At present, heteroatom doping with N, S, B, P, etc. is an important way to improve the performance, surface polarity and wettability of HMCS.[3, 15] Nitrogen atom is easy to be incorporated into the carbon skeleton due to its electron cloud similar to carbon atom. Generally, 2

there are four types of doped nitrogen in HMCS depending on the bonding environments, including pyrrolic N, pyridinic N, quaternary N/graphitic N and N oxides of pyridinic N, which have been recently demonstrated to improve the capacitance of N-doped HMCS (N-HMCS) via surface faradaic reactions without sacrificing the high rate capability and long cycle life.[16] Polybenzoxazines (PB) is conventional carbon and nitrogen precursors for N-HMCS synthesis, obtained by the polymerization of phenolic, formaldehyde and organic primary amine (1,6-diaminohexane, ethylenediamine (EDA), aniline, etc.).[17] N-HMCS from PB precursor has characteristics of high char yield, flexible resin design, controlled morphology, composition, and the mechanical properties.[15] More importantly, the organic primary amine not only provide nitrogen precursor, but also catalyze the hydrolysis of the silica precursor and polymerization of the resin simultaneously in modified Stöber system.[14] PB precursor derived from phenols is relative low cost and benefit for the large-scale preparation. However, the preparation of N-HMCS from PB, especially PB using phenol as original carbon source, still needs further exploration, because polymerization of phenol and formaldehyde is believed slow comparing with fast hydrolysis of silica precursor.[10] Herein, N-HMCS were prepared by a co-assembly process employing cetyl-3-methyl ammonium bromide (CTAB) as structural inducer, tetraethoxysilane (TEOS) as silica assistance agent, phenol/formaldehyde (PF) oligomer and EDA as carbon/nitrogen precursor in a modified Stöber system. EDA acts as a catalyst for the hydrolysis of TEOS and polymerization of PF oligomer and EDA, simultaneously. It is revealed that the amount of the EDA plays a vital role in controlling and adjusting the morphology

of

N-HMCS.

Additionally,

the

pre-polymerization

of

phenol/formaldehyde oligomer is another key factor for the formation of N-HMCS, proved by a control sample which is irregular porous particle obtained from phenol, formaldehyde and EDA. The addition of TEOS not only leads to the formation of large cavity, but also results in abundant mesopores. As the electrode materials in supercapacitor, the obtained N-HMCS exhibits excellent electrochemical performance attributable to regular spherical morphology, suitable nitrogen content, thin shell and 3

high surface area. 2. Results and discussion

Scheme 1. Schematic illustration of the fabrication processes of N-HMCS samples and MC. As illustrated in Route 1 of Scheme 1, in a typical synthesis, phenol and formaldehyde undergo a process of pre-polymerization firstly to form PF oligomer, which is necessary to form N-HMCS. Then EDA can react with the PF oligomer, forming PB after further polymerization of aldehyde and amino group.[15, 18, 19] The as-prepared PB acts as the carbon/nitrogen precursor in the modified Stöber system (Frame 1 in the Scheme 1). CTAB is used as the bridge agent to realize the co-assembly of PB precursors and TEOS through electrostatic interaction.[20, 21] After pyrolysis and subsequent removal of the silica, N-HMCS is successfully obtained. EDA synchronously serves as a basic catalyst for the hydrolysis of TEOS and polymerization of PB, the resulted N-HMCS can be the adjusted in morphologies by changing EDA amounts (Frame 2 in the Scheme 1). The process of pre-polymerization of PF also plays an important role in the formation of regular spherical morphology and hollow structure. Only MC with irregular porous structure is obtained when phenol, formaldehyde and EDA react directly (Route 2).

4

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(k)

(l)

(j)

Fig. 1. SEM and TEM images of N-HMCS samples with different morphologies. The SEM image of N-HMCS-0.05 with 0.05 mL EDA in the reaction system (Fig. 1a) exhibits accumulation of irregular nanoparticle. Furtherly, the TEM image of N-HMCS-0.05 (Fig. 1b) clearly shows serious aggregation of irregular nanoparticles. The larger pores in N-HMCS-0.05 may be derived from removing of silica particles. The enlarged TEM images of N-HMCS-0.05 (Fig. 1c and inset) furtherly indicates the irregular morphology and rich porous structure. When 0.1 mL EDA is added in the reaction system, the resultant N-HMCS-0.1 appears a well spherical morphology different from N-HMCS-0.05 (Fig. 1d). The TEM image of N-HMCS-0.1 clearly exhibits relative monodisperse and hollow features with diameter of ca. 100 nm (Fig. 1e) and small shell thickness of 10 nm (Fig. 1f). The higher magnification TEM image of inset of Fig. 1c exhibits spread mesoporous on the shell of N-HMCS-0.1. The EDA amount was further increased to 0.2, 0.4, 0.6 mL to investigate its effect on 5

N-HMCS morphology. N-HMCS-0.2 shows small N-HMCS with diameter of ca. 100 nm similar to N-HMCS-0.1 and also displays some large spheres with diameter ca. 900 nm (the arrow in Fig. 1g). TEM of N-HMCS-0.2 displays hollow structure for both small and large spheres. In enlarged image of the large spheres (Fig. 1i), the shell shows loose porous structure, which is composed of small hollow particles with diameter of 100 nm. The amount of EDA was further increased to 0.4 mL to produce N-HMCS-0.4. The SEM image (Fig. 1j) of N-HMCS-0.4 clearly reveals uniform spherical morphology with diameter of ca. 900 nm and the spheres with diameter of 100 nm disappear. Fig. 1k-l present the TEM images of N-HMCS-0.4, which shows the cavity of 500 nm and shell thickness of 200 nm. An enlarged TEM image of N-HMCS-0.4 shows the fluffier and twisted disordered porous shell. The results of N-HMCS-0.1, N-HMCS-0.2, N-HMCS-0.4 demonstrate that EDA amount has strong effect on N-HMCS structures, and more EDA will give larger, regular spheres. However, this trend is destroyed when EDA amount reaches 0.6 mL. As shown in Fig. S1, N-HMCS-0.6 presents disordered morphology. In this process, the pre-polymerization of phenol/formaldehyde oligomer is a key factor for the formation of N-HMCS. This is proved by a controlled trial, in which MC is obtained by directly co-assembly of phenol, formaldehyde and EDA (EDA amount: 0.1 mL). In addition, no spherical morphology can be found in MC and mainly irregular particles with a large number of large pores are observed in Fig. S2.

6

Fig. 2. Schematic mechanism illustration of the fabrication processes of N-HMCS (a), TEM images of [email protected] (b) and [email protected] (c). Therefore, the presented possible explanation for N-HMCS with different morphologies can be illustrated as Fig. 2a. The PF oligomer can further react with EDA to form PB precursor, and TEOS can be hydrolyzed into silica, then PB precursor and silica further assemble with each other under the electrostatic action of CTAB. The amount of EDA affects the hydrolysis rate of TEOS and polymerization degree of PF oligomer, resulting in different morphology of final N-HMCS. When small amount of EDA is used (e.g., 0.05 mL EDA in 28 mL solvent), hydrolysis process goes slow, resulting in many irregular small silica particles. PB is assembled on the irregular silica particles, which leads to formation of irregular carbon particles replicating the morphology of silica. When the EDA amount increases, the irregular silica particles tend to become regular small spheres, acting as a core. The rest silica sources will be co-assembled with PB to form a silica/PB composite, depositing on the silica core ([email protected]). The [email protected] (Fig. 2b) shows spherical morphology and obvious core-shell structures with a silica core and PB resin shell (the sign inset of Fig. 2b). When the amount of EDA increases to 0.2 mL, a few large silica spheres form due to the fast rate of TEOS hydrolysis. Then some small silica@PB composites will further coat on the surface of large silica sphere, resulting in N-HMCS-0.2 as a mixture of large and small hollow spheres. The addition of 7

0.4mL EDA will lead to formation of more regular large silica sphere. The small silica@PB composite can assemble on the surface of the large silica spheres fully and finally form N-HMCS-0.4 with uniform regular spheres and large cavities. As shown in Fig. 2c, the [email protected] is obviously different from [email protected] showing rougher surface and many protrusions. The magnified TEM image of these protrusions (inset of Fig. 2c) shows a core-shell structure, further confirming that the shell is composed of the small silica@PB composite particles. Furthermore, the SEM images of [email protected] (Fig. S3a) and [email protected] (Fig. S3b) also confirm the smoot surface of [email protected] and crude surface of [email protected], agreeing well with the results of TEM images. 50

(b) 0.6 N-HMCS-0.4

-1

0.5

3

40

dV/dlog(D) (cm g )

N-HMCS-0.05 N-HMCS-0.1 N-HMCS-0.2 N-HMCS-0.4

-1

Quantity Adsorbed (mmol g )

(a)

30

20

10

0.0

0.2

0.4

0.6

0.8

0.4

N-HMCS-0.1 0.3

N-HMCS-0.2 0.2

N-HMCS-0.05

1.0

10

20

C

100

O

40

50

(d)

N

N-HMCS-0.4

95 90

Intensity (a.u.)

Content (%)

Intensity (a.u.)

(c)

30

Pore Size (nm)

P/P0

6 4 2 0 N-HMCS-0.05 N-HMCS-0.1 N-HMCS-0.2 N-HMCS-0.4

N-HMCS-0.4 N-HMCS-0.2

N-HMCS-0.2

N-HMCS-0.1

N-HMCS-0.1 N-HMCS-0.05 1200

1000

N-HMCS-0.05 800

600

400

200

0

292

Binding Energy (eV) Binding Energy (eV)

284

280

(f) N-HMCS-0.4

N-HMCS-0.4 Intensity (a.u.)

Intensity (a.u.)

(e)

288

Binding Energy (eV)

N-HMCS-0.2 N-HMCS-0.1

536

N-HMCS-0.1 N-HMCS-0.05

N-HMCS-0.05 540

N-HMCS-0.2

532

528

408

404

400

396

Binding Energy (eV)

Binding Energy (eV)

Fig. 3. Nitrogen adsorption and desorption curves (a), pore distribution (b), XPS spectrum (c), C1s (d), O1s (e) and N1s (f) of N-HMCS samples. 8

N2 adsorption/desorption measurements are conducted to examine the structural parameters of the N-HMCS samples. The isotherms shown in Fig. 3a are identified as type IV, which is characteristic of mesoporous materials.[22, 23] Large hysteresis loops with shapes that are intermediate H2-type isotherms are observed for these samples, which are believed to be related to the capillary condensation associated with mesoporous channels or due to modulation of the channel structure. Additionally, a hysteresis loop can be found for those N-HMCS samples under the relative pressure of P/P0 > 0.9, indicating the cavity of N-HMCS. In addition, N-HMCS-0.4 show obvious upward curve relative to other samples, ascribing to the larger cavity of N-HMCS. The pore-size distribution obtained from the isotherm indicates pore size at 3.3 nm derived from the etching of silica in the N-HMCS. The BET specific surface area of the samples was calculated from N2 isotherms and is found to be about 636, 876, 643 and 460 m2 g-1 of N-HMCS-0.05, N-HMCS-0.1, N-HMCS-0.2 and N-HMCS-0.4, respectively. The single-point total volume of pores at P/P0 = 0.99 is 0.59, 1.60, 0.89 and 0.82 cm-3 g-1 for N-HMCS-0.05, N-HMCS-0.1, N-HMCS-0.2 and N-HMCS-0.4, respectively. It should be pointed out that the N-HMCS-0.1 possesses relatively high specific surface area and large pore volume due to uniform spherical morphology, small diameter, thin shell and rich mesoporous structure. EDA participates in the polymerization of PF, resulting in the formation of PB to achieve in-situ nitrogen doping for the N-HMCS. XPS was further utilized to investigate the distribution and content of nitrogen in the as-synthesized N-HMCS. The survey XPS spectra in Fig. 3c shows peaks corresponding to C1s, N1s, and O1s in all N-HMCS samples. The nitrogen content of N-HMCS samples is shown in inset of Fig. 3c. It is obvious that the nitrogen content increases with the increasing of EDA amount. However, the N-HMCS-0.2 and N-HMCS-0.4 possess similar nitrogen content. In XPS spectrum of C1s, four sub-peaks at 284.7, 285.7, 286.7, and 287.9 eV, attributable to graphitic C, C-N, C-O, and C=O, respectively,[24] are deconvoluted (Fig. 3d), indicating effective carbonization and successfully N-doping. The oxygen functionalities revealed by deconvolution of the O1s peaks (Fig. 3e) are the carboxyl C=O centering at 532.5 eV, C-O at 532.5 eV, and hydroxyl oxygen in amorphous 9

hydrogenated carbon at 536.7 eV, respectively.[25] The XPS profile of N (Fig. 3f) reveals the presence of graphitic N at 401.4 eV, pyrrolic N at 400.1 eV, and pyridinic N at 398.4 eV.[26] These nitrogen functionalities are from nitrogen precursor of PB. Table S1 shows the detailed features of N-HMCS samples, exhibiting the influence of EDA amount on surface area and N/O contents. In addition, the degree of graphitization is also an important property for carbonaceous electrodes. Raman spectra of the N-HMCS samples was also provided to confirm the structures as shown in Fig. S4. The ratio of the intensities of the D band and the G band (ID/IG) of those N-HMCS samples are calculated to be ca. 0.9, indicating their amorphous structure. 3

(a)

0.0

(b)

N-HMCS-0.05 N-HMCS-0.1 N-HMCS-0.2 N-HMCS-0.4

-1

Current Density (A g )

2 -0.2 0 -1 -2

N-HMCS-0.05 N-HMCS-0.1 N-HMCS-0.2 N-HMCS-0.4

-3 -4

Potential (V)

1 -0.4 -0.6 -0.8 -1.0 -1.0

-0.8

-0.6

-0.4

-0.2

0.0

0

200

400

600

Potential (V) (c) 300

(d)

150 N-HMCS-0.05 N-HMCS-0.1 N-HMCS-0.2 N-HMCS-0.4

-Z'' (Ohm)

-Z'' (Ohm)

200

150 100

2 1 0

0

1

2

3

4

Z' (Ohm)

50

0

N-HMCS-0.05 N-HMCS-0.2

N-HMCS-0.1 N-HMCS-0.4

0 0

2

4

6

8

10

0

50

100

-1

Current Density (A g )

150

200

Z' (Ohm) (f)

(e) 40

0.0 -1

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

-0.2

20 -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 -40 -60 -0.8

-0.4

0.0

Potential (V)

-1

Current Density (A g )

1200

3

-1

Capacitance (F g )

250

50

1000

4

200

100

800

Times (s)

-0.4 -0.6 -0.8 -1.0 0

0.4

Potential (V)

200

400

600

800

1000

1200

Times (s)

Fig. 4. Electrochemical evaluation of the N-HMCS samples in three-electrode system: (a) CV curves at 5 mV s-1 scan rate; (b) representative GCD curves at 0.5 A g-1 current 10

density; (c) specific capacitances at different GCD current densities and (d) the Nyquist plots with fitting curves and their corresponding high frequency ranges (inset); CV curves at different scan rates (e) and GCD curves at different current densities (f) of N-HMCS-0.1. The large cavity, thin mesoporous shell, small diameter and nitrogen doping endowed N-HMCS with good performance in respect of more active sites, faster ion transportation and enhanced hydrophility, which benefit for the electrochemical performance.[27] The electrochemical performance of N-HMCS samples is evaluated by CV and GCD measurements using a three-electrode configuration. Fig. 4a shows the CV curves of all samples at the scan rate of 5 mV s-1. These curves exhibit slight deviation from ideal rectangular shape signifying the combined effect of two different energy

storage

mechanisms

of

double

layer

capacitor

(EDLC)

and

peseudocapacitance.[28, 29] N-HMCS-0.1 demonstrates the largest CV area among four samples, suggesting high charge storage capacity of N-HMCS-0.1 due to its unique uniform spherical morphology, thin shell, small diameter and high surface area. The significant improved capacitances are also confirmed by the GCD curves with time extending at low current density of 0.5 A g-1 (Fig. 4b), where small variation from the linear features (slightly distorted triangular shape) is observed in the GCD tests, signifying the existence of EDLC and peseudocapacitance derived from faradaic reactions.[29] The calculated specific capacities are 198, 307, 206 and 192 F g-1 for N-HMCS-0.05, N-HMCS-0.1, N-HMCS-0.2 and N-HMCS-0.4, respectively. Notably, the N-HMCS-0.1 represents the highest specific capacitance. Fig. 4c shows the rate performance of specific capacitance from 0.5 A g-1 to higher current density of 10 A g-1 for N-HMCS samples. As mentioned above the specific capacitance of N-HMCS-0.1 is higher than other samples and 83 % capacitance is retained even at higher discharge rate. EIS was used to gain a deep understanding of the superior rate capability of the N-HMCS electrode. Fig. 4d shows the Nyquist plots of electrodes prepared with N-HMCS samples. These plots show the presence of semi-circle in high frequency region and straight line in low frequency region indicating ideal capacitive 11

behavior.[30] An equivalent circuit was used to fit the impedance spectra shown in inset of Fig. S5, where Rs is the solution resistance, C is the double layer capacitance or constant phase element, Rct is the charge transfer resistance, ZW is the Warburg impedance, Q is the leakage capacitance. The values of the elements constituting the equivalent circuit are arranged in the table S2. EIS of N-HMCS-0.1 shows smaller solution resistance (Rs) of 0.41 Ω than N-HMCS-0.05 (0.42 Ω), N-HMCS-0.2 (0.45 Ω) and N-HMCS-0.4 (0.44 Ω), suggesting a lower internal resistance desirable for the high-rate delivery.[31] The low Rs value of N-HMCS proposes a fast redox reaction at the electrode electrolyte interface and good accumulation of electrolyte ions on the electrode surface which eventually leads to combined EDLC and pseudocapacitance. The CV and GCD curves of N-HMCS-0.1 at different scan rates and current densities are presented in Fig. 4e and f to confirm its high gravimetric capacitance at a high charge-discharge rate. The retained rectangular CV shape of N-HMCS-0.1at the scan rate from 5 to 200 mV s-1 and its GCD curves from 0.5 to 10 A g-1 indicate good capacitance performance at high scan rate, indicating its potential application value in actual energy storage. Furthermore, in order to confirm the positive effect of N-doping for the enhancing of supercapacitor performance, hollow carbon spheres without N-doping was also prepared by the same method using ammonia as catalysis. As shown in the follow Fig. S6, the hollow structure and spherical morphology were proved by the TEM images (a). The supercapacitor performance of this hollow carbon spheres without N-doping was investigated and a capacitance of 260 F g-1 was obtained, which is lower than that of HMCS-0.1 (b). Furthermore, EIS has been used to gain a deep understanding of the superior rate capability of the N-doping electrode. The higher capacitance and lower internal resistance of N-HMCS-0.1 can be found from the smaller semi-circle and transverse intercept, which is desirable for the high-rate delivery and indicating the N-doping is benefit for increasing the electronic conductive.

12

20

(a)

(b)

0.8

-1

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

0.6 -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

5 0 -5 -10 -15 0.0

0.2

0.4

0.6

0.8

Potential (V)

10

-1

Current Density (A g )

15

0.4

0.2

0.0

1.0

0

50

200

-1

Energy Density (Wh Kg )

250

-1

150

(d)

(c) Capacitancance (F g )

100

Times (s)

Potential (V)

200 N-HMCS-0.1 32 33 Graphene Porous carbon 34 35 N-doped carbon sheet Carbon dots 36 37 Porous carbon N-doped porous carbon 38 N-doped porous carbon 39 40 Holllow carbon sphere Carbon spheres 41 Holllow carbon sphere 42 Carbon nanoparticles

150 100 50 0 0

2

4

6

8

-1

10

10 N-HMCS-0.1 43 44 N/S-doped porous carbon Porous carbon 45 46 NiS-graphere Carbon fiber 47 48 N-doped porous carbon Carbon sheets 49 50 Porous carbon Porous carbon 51 52 Carbon sheets Carbon sphere 53 54 Porous carbon Carbon sheets

1

100

1000

10000 -1

Power Density (W Kg )

Current Density (A g )

Fig. 5. CV curves at different scan rates (a), GCD curves at different current densities (b), Specific capacitances at different GCD (c), Ragone plots (d) of N-HMCS-0.1 in two-electrode system. A symmetric supercapacitor was fabricated using the as-prepared N-HMCS-0.1 in 6 M KOH electrolyte with voltages varying from 0 to 1.4 V. The CV curves of the N-HMCS-0.1 electrode at scan rates from 5 to 100 mV s-1 show rectangular shapes (Fig. 5a), indicating ideal capacitive behavior. In addition, the symmetrical galvanostatic charge/discharge curves shown in Fig. 5b are typical of electrical double-layer capacitor, exhibiting an excellent discharge performance. Specific capacitance reaches 265 F g-1 at 0.5 A g-1, with high capacitance retention of 84 % at 10 A g-1 as shown in Fig. 5c.[32-42] In addition, the specific capacity of N-HMCS-0.1 is higher than that of many other carbonaceous materials, such as porous carbon, carbon sphere and carbon sheets (Fig. 5c). A Ragone plot of the supercapacitor was shown in Fig. 5d, revealing a high potential window and high specific capacitance for the supercapacitor. The energy density level of 11.2 Wh kg-1 at a power density of 660.8 W kg-1 and 10 Wh kg-1 energy density at 9000.5 W kg-1 power density is achieved. Such supercapacitor performance is superior to previously reported supercapacitors made from carbonaceous materials including hollow carbon spheres, 13

carbon sheets, heteroatom doping carbon and porous carbon fibers, etc. as shown Fig. 5d.[43-54] Cycling stability is another crucial performance for supercapacitor applications. As displayed in Fig. S7, the excellent stability of N-HMCS-0.1 can be confirmed by the retained 85.1 % of initial capacity after 10000 cyclic tests at current density of 2 A g-1 in two-electrode system. The high specific capacitance, high energy density and long cycle life of the N-HMCS-0.1 electrode can be attributed to its characteristics of small diameter, thin shell and high surface area, which provides enough space to maximize charge retention and realize high energy storage. The appropriate mesoporous structure provides high-speed channels for the rapid entry and exit of electrolyte ions. Nitrogen containing groups within the carbon framework further contribute to the specific capacitance by redox reaction. 3. Conclusion In summary, a co-assembly strategy has been employed to prepare the N-HMCS with uniform spherical morphology using PB as carbon/nitrogen precursor and silica as additive agent. EDA is used in the preparation to provide the nitrogen precursor and catalyze the hydrolysis of the silica precursor and further polymerization of the PB simultaneously. EDA is a key factor to realize regulation of morphologies of N-HMCS from irregular porous particles to relative uniform hollow spheres with diameter increasing. The pre-polymerization of phenol is another key factor which guarantees formation of N-HMCS. PB is obtained by reaction of pre-polymerized phenol/formaldehyde with EDA. N-HMCS with regular spherical shape, smaller diameter, higher surface area, and appropriate nitrogen content exhibits higher electrochemical performance. The features of N-HMCS are also valuable in catalyst, separation, drug sustained and controlled release or other fields. Acknowledgements We thank the National Natural Science Foundation of China (21676070), Hebei Training Program for Talent Project (A201500117), Hebei One Hundred-Excellent Innovative Talent Program (III) (SLRC2017034), Beijing National Laboratory for 14

Molecular

Sciences.

Major

Scientific

and

Technological

Achievements

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Conflict of interest The authors declared that they have no conflicts of interest to this work (N-doped hollow mesoporous carbon spheres prepared by polybenzoxazines precursor for energy storage). They declare that they do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.