Polyacrylonitrile for Nitrogen

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Cite This: ACS Appl. Energy Mater. 2019, 2, 4402−4410

Confined-Space Pyrolysis of Polystyrene/Polyacrylonitrile for Nitrogen-Doped Hollow Mesoporous Carbon Spheres with High Supercapacitor Performance Juan Du,† Yifeng Yu,† Lei Liu,† Haijun Lv,† Aibing Chen,*,† and Senlin Hou*,‡ †

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College of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, 70 Yuhua Road, Shijiazhuang 050018, China ‡ The Second Hospital of Hebei Medical University, 215 Heping Road, Shijiazhuang, 050000, China S Supporting Information *

ABSTRACT: Nitrogen-doped hollow mesoporous carbon spheres have drawn much attention in many applications, including adsorption, catalysis and energy storage, etc. because of their hollow structure, thin carbon shell, and high specific surface area. Herein, a confined-space pyrolysis method is applied for the preparation of nitrogen-doped hollow mesoporous carbon sphere with uniform spherical morphology, relative large cavity, and high specific surface area, using polystyrene/polyacrylonitrile (PS/PAN, or PSPAN) spheres as a carbon precursor. In this process, mesoporous silica shell is coated on the PSPAN spheres to provide a confined space, in which a regular spherical morphology of nitrogen-doped hollow mesoporous carbon sphere can be obtained after the process of pyrolysis. The in situ generation of CO2 and H2O from PSPAN spheres play the role of active agent, creating a rich and uniform mesoporous distribution for nitrogen-doped hollow mesoporous carbon spheres, which is conducive to fast charge transport. Rich nitrogen content in PAN results in in situ nitrogen doping. Adjusting the PS:PAN ratio can realize the adjustment of diameter and cavity size. As an electrode in a supercapacitor, the nitrogen-doped hollow mesoporous carbon sphere exhibits outstanding performance with large specific capacitance, indicating its excellent promise in energy storage. KEYWORDS: Confined-space pyrolysis, polystyrene/polyacrylonitrile, adjustment, N-doped hollow mesoporous carbon sphere, supercapacitor

1. INTRODUCTION Hollow mesoporous carbon sphere (HCS) materials have been a focus of rapid innovations for their interesting properties in many fields,1,2 such as heterogeneous catalysis,3 adsorption,4 and energy storage or platforms for drug delivery,5 because of their regular spherical morphologies, large cavity, high surface areas, and tunable porosity.6,7 Of particular interest, ascribing to the short diffusion length for reactants to access the active sites in the thin shells, large cavity for charge containment, and volume expansion of HCS, it is potential electrode material in supercapacitors. The HCS also can hold various capabilities for doping elements (e.g., S, P, N, etc.),8,9 being beneficial to promote the electrochemical performance.10−12 Recently, much work has been done on their regular morphology, dispersion, surface properties, or porous structure to boost the performance of HCS.13−15 The template method, especially the hard-template approach, is one of the effective methods to prepare HCS with adjustable pore structure, wall thickness, cavity, and other structural parameters. Polystyrene (PS) spheres, which represent a typical polymer, has been always used as a sacrificial hard template for hollow structure, because of its excellent decomposition characteristics with no carbon residue © 2019 American Chemical Society

and uniform particle size. The use of PS spheres as a hard template has been widely examined in preparation of hollow silica or metal oxide spheres.16−18 PS spheres is also an effective hard template for HCS.19,20 Typical uniform HCS materials with high dispersion are prepared by using resin as a shell and PS as a template. However, the HCS materials prepared via this method show an inert surface, which limits their performance, especially electrochemical performance.12 There is little doubt that the preparation of nitrogen-doped HCS (N-HCS) is one of the key topics for the design of HCS materials with high supercapacitor performance. An effective nitrogen doping can improve the hydrophilicity and conductivity of HCS, which is beneficial to reduce the impedance in solution. There have been two primary strategies to prepare N-HCS: post-treatment and in situ synthetic methods. Compared with the former, in situ nitrogen doping is not only simpler, but it is also easier to obtain a high nitrogen content, and the distribution of elemental nitrogen is more uniform on the surface and bulk phase of carbon materials.19 Received: March 18, 2019 Accepted: May 30, 2019 Published: May 30, 2019 4402

DOI: 10.1021/acsaem.9b00578 ACS Appl. Energy Mater. 2019, 2, 4402−4410

Article

ACS Applied Energy Materials

The pyrolyzed species from PAN would form a thin nitrogendoped carbon shell in the confined space (N-HCS@SiO2). In addition, the in-situ-generated gases (e.g., CO2, H2O, etc.) from the pyrolysis of PS and PAN could not travel quickly outside the confined nanospace ascribed to the remaining PAN and the outer silica shell.23 While residing in the confined space, the pyrolyzed gases played the role of an activation agent, which destroyed the carbon shell, resulting in a high surface area and a rich mesoporous structure. In addition, the silica layer isolates PSPAN, thus avoiding mutual adhesion during pyrolysis and improving the dispersibility. Finally, after etching the silica shell, the N-HCS with a highly dispersed and uniform spherical morphology were obtained. The formation of an inorganic outer mesoporous silica shell, which functioned as a confined shell for the high-temperature pyrolysis of the PSPAN spheres and provided a boundary to prevent conglutination of carbon spheres during high-temperature treatment, was crucial for obtaining highly dispersed NHCS. Taking PSPAN-4:6 as an example, if PSPAN was directly pyrolyzed, carbon materials with irregular shape would be obtained (Figure S1 in the Supporting Information), because of the lack of protection of a mesoporous silica shell. The construction of a mesoporous silica outer shell outside PSPAN-4:6 could be proved by transmission electron microscopy (TEM). The TEM images of the PSPAN@SiO2 nanocomposites exhibited a highly uniform core−shell structure (see Figure S2a in the Supporting Information). Each PSPAN sphere, which possessed a diameter size of 410 nm, was encapsulated by a mesoporous silica shell with a thickness of ca. 20 nm (Figure S2b in the Supporting Information). From the high-resolution TEM image (Figure S2c in the Supporting Information), the existence of a mesoporous silica shell was further proved (the yellow sign). The mesoporous features of silica shell on the surface of PSPAN was measured further. Figure S2d in the Supporting Information shows that the isotherm of PSPAN@SiO2 exhibited a type IV profile, indicating the existence of a mesoporous structure. Furthermore, the pore size distribution (Barrett−Joyner−Halenda, BJH) analysis of the nitrogen desorption isotherm of samples was shown with a pore size of ca. 2.7 nm (see the inset of Figure S2d). In addition, the wide-angle X-ray diffraction (XRD) of PSPAN@SiO2 was tested to investigate the crystal structure of the silica shell, as shown in Figure S3e in the Supporting Information. The only wide peak, observed at 23°, demonstrated the amorphous structure of PSPAN@SiO2. Thermogravimetric analysis (TGA) in N2 was used to illustrate pyrolysis behavior of PSPAN and PSPAN@SiO2 composites (see Figure S3 in the Supporting Information). For PSPAN, the mass loss at 400−500 °C was correlated to the thermal characteristics of complete decomposition of PS and partial decomposition of PAN. The carbon yields of PSPAN with ratios of 4:6, 2:8, and 1:9 were 3.4%, 5.1%, and 8.9%, respectively. Obviously, with the increase of PS ratio, the carbon yields decreased. TGA weight-loss curves for PSPAN@ SiO2 composites had a similar trend of losing weight and the main weight loss was at 400−500 °C. Moreover, the higher loss temperature of PSPAN@SiO2 than PSPAN might suggest that PSPAN pyrolyzed at a higher temperature in the confined nanospace. When the PS:PAN ratio was 1:9, the TEM images of NHCS-1 clearly exhibited hollow nanostructures with a diameter of 200 nm (Figure 1a). The TEM images of N-HCS-1 with

Polyacrylonitrile (PAN) is the common carbon and nitrogen precursor for the preparation of carbon materials with nitrogen doping. The abundant nitrogen-containing moieties in PAN make it an appropriate precursor for in situ nitrogen doping. Although the carbon materials have been obtained using PS as a template and PAN as a carbon and nitrogen precursor, the products show irregular morphology and poor porous distribution, which limits its performance.21,22 Recently, we reported the confined pyrolysis method to prepare N-HCS with definite diameter and cavity.23 The preparation of highquality N-HCS materials with a rich mesoporous structure, adjustable diameter, and cavity from the PS template and PAN precursor is interesting. Herein, the polystyrene/polyacrylonitrile composite sphere (PSPAN) with different PS:PAN ratios was chosen as the hard template, and carbon and nitrogen precursor, at the same time, for the synthesis of N-HCS with a tunable diameter and cavity. A mesoporous silica shell outside PSPAN was the key to maintaining the regular spherical morphology, as well as the highly dispersive and mesoporous structure of N-HCS, which provided a confined space. The PS:PAN ratio strongly affected the cavity and diameter size of N-HCS. Because of the thin shell, uniform mesoporous, large cavity, and suitable nitrogen doping, N-HCS exhibited outstanding electrochemical performance in a supercapacitor.

2. RESULTS AND DISCUSSION The fabrication process of N-HCS by confined-space pyrolysis strategy using PSPAN as carbon and nitrogen precursor was illustrated in Scheme 1. PSPAN spheres were first prepared via Scheme 1. Scheme for Preparation of N-HCS Samples

emulsion polymerization. First, repolymerization of styrene was performed for the formation of the PS core of PSPAN. Acrylonitrile then was added dropwise to the resulting polystyrene dispersion. Using this strategy, the PAN will be coated onto the PS core. In addition, the PS:PAN ratio affects the diameter of PSPAN, because of the concentration of styrene, in which a relative low concentration of styrene will lead to a small PS core, resulting to a relative smaller diameter of the PSPAN sphere. The PSPAN spheres then were uniformly coated by a layer of mesoporous silica, using cetyltrimethylammonium bromide (CTAB) as a surfactant agent to obtain PSPAN@SiO2. During the PSPAN pyrolysis in confined space, the PS acted as a self-sacrifice template and would be decomposed completely at the high temperature to create a large cavity for the formation of a hollow structure. 4403


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Figure 1. TEM images of N-HCS samples: (a and b) N-HCS-1, (c and d) N-HCS-2, and (e and f) N-HCS-4.

the better the dispersity of N-HCS and the smoother the surface. The textural properties of the N-HCS samples were analyzed by N2 adsorption−desorption measurements. As shown in Figure 2a, all of the samples presented similar adsorption− desorption of type IV and type H3 hysteresis loops. There is hysteresis in the P/P0 range of 0.4−0.9, which implied the presence of a mesoporous structure.24 In addition, the adsorption−desorption isotherms were not closed at the relative pressure range of 0.9−1 in all the samples, demonstrating the presence of a large cavity.25 The pore size distribution for N-HCS samples from the adsorption branch is shown in Figure 2b. The pore size of N-HCS-1, N-HCS-2, and N-HCS-4 were distributed at 3.1, 3.0, and 4.1 nm, respectively, indicating that the in-situ-generated CO2 and H2O from this confined pyrolysis played an effective role in activation and created a uniform mesoporous distribution for N-HCS. Compared with micropores, the existence of mesopores would contribute to the high-speed transport of charges, reduce the resistance, and further improve the electrochemical performance. Detailed structural parameters are listed in Table

higher resolution showed that the cavity size and thickness was ca. 110 and 45 nm (Figure 1b), and the inset of Figure 1b showed its rich mesoporous structure. When the PS and PAN ratio increased to 2:8, the obtained N-HCS-2 showed a larger cavity with a larger diameter of 250 nm, as shown in Figure 1c. Figure 1d showed that the cavity of N-HCS-2 was ca. 180 nm, which was larger than that observed for N-HCS-1, and the wall thickness decreased to 35 nm. The inset in Figure 1d also showed the rich mesoporous structure of N-HCS-2. When the PS:PAN ratio was 4:6, the diameter of N-HCS-4 was further increased to 410 nm (Figure 1e). Figure 1f shows a highermagnification TEM image of N-HCS-4 with a ca. 350 nm cavity, which was larger than that of N-HCS-1 and N-HCS-2. The shell of N-HCS-4 also became thinner (ca. 30 nm) and possesses an obvious mesoporous structure (see the inset of Figure 1f). With the increase of the ratio of PS to PAN, the diameter of N-HCS and the size of the cavity increased and the thickness of the shell reduced, which proved that PS had played a sacrificial template role inside and created the cavity structure. At the same time, the larger the diameter of spheres, 4404


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Figure 2. (a) N2 adsorption−desorption isotherm, (b) distribution of pore size, (c) XRD, and (d) Raman spectra of N-HCS samples.

Table 1. Textural Properties of N-HCS Samples with Different PS/PAN Ratios sample

SBET (m2 g−1)

Vta (cm3 g−1)

pore sizeb (nm)

diameter (nm)

cavity (nm)

shell thickness (nm)

N-HCS-1 N-HCS-2 N-HCS-4

559 609 955

0.68 0.77 1.07

3.1 3.0 4.1

200 250 410

110 180 350

45 35 30

Total pore volume at P/P0 ≈ 0.99. bPore size distribution was analyzed via the BJH method, using the desorption branch of the isotherm.

a

1, showing the changing trend of detail features of N-HCS. The wide-angle XRD pattern was used to confirm the carbon structure in N-HCS. As exhibited in Figure 2c, the wide-angle XRD patterns of all the N-HCS samples showed two wide peaks at 24° and 43°, ascribing to the amorphous carbon. In addition, the graphene structure of samples was characterized by Raman scattering, distinguishing ordered and disordered crystal structures of carbon. The ID/IG value of the N-HCS was less than 1.0, indicating the presence of their amorphous carbon. The nitrogen-containing nature of PAN also led to nitrogen doping of N-HCS, thus increasing the hydrophilicity of NHCS and reducing the contact resistance in solution, improving electrochemical behavior. Taking N-HCS-1 as an example, the surface element distribution of N-HCS-1 was disclosed by energy-dispersive spectroscopy (EDS), showing the carbon network (Figure 3a). The uniform distribution of elemental C, N, and O was clearly observed from the corresponding elemental mapping. X-ray photoelectron spectroscopy (XPS) was performed further to characterize the chemical state of the elemental composition in the N-HCS-1. As shown in Figure 3b, the survey curve of N-HCS-1 showed three dominant peaks, centered at 284.6 (C 1s), 531.6 (O 1s), and 400.6 eV (N 1s). No other elements could be found obviously, and carbon (91.1%), oxygen (4.1%), and nitrogen (4.8%) were present, as indicated in Figure 3b. The subpeaks at 284.6, 285.6, 286.7, and 289.9 eV were deconvoluted in the XPS spectrum of C 1s (Figure 3c), attributable to C−C, C−N, C−O, and CO, respectively. Deconvolution of the O 1s peaks (Figure 3d) revealed the oxygen functionalities, corresponding to the carboxyl CO and C−O centering

(532.1 eV) and hydroxyl oxygen (536.7 eV) in amorphous hydrogenated carbon, respectively.26 The presence of graphitic nitrogen (403.7 eV), pyrrolic nitrogen (400.6 eV), and pyridinic nitrogen (398.1 eV) could be obtained by the XPS profile of N (Figure 3e).27 The high surface area, rich mesoporous carbon shell, and nitrogen doping endowed the N-HCS samples with good electrochemical performance.28−30 The electrochemical performance of N-HCS samples was observed in a three-electrode system with 6 M KOH as an electrolyte. The cyclic voltammetry (CV) curves for all samples at a scan rate of 5 mV s−1 over the range of −1−0 V showed an almostrectangular shape, as illustrated in Figure 4a, indicating a favorable electronic double layer capacitor (EDLC) behavior, combined with pseudocapacitance with a good reversibility.31 According to the CV integrated area, the N-HCS-1 presented the highest specific capacity, relative to that of N-HCS-2 and N-HCS-4, indicating the advantages of a small diameter. The galvanostatic charge−discharge (GCD) curves at a current density of 0.5 A g−1 is shown in Figure 4b. The specific capacities of N-HCS-1, N-HCS-2, and N-HCS-4 were 451.5, 442.7, and 438.5 F g−1, respectively. Obviously, the N-HCS-1 possessed the highest capacitance, which might be attributed to its thicker carbon shell and rich mesoporous structure, providing more storage space for the ion. At the current density range of 0.5−10 A g−1 (Figure 4c), the rate performance demonstrates that the capacitance retentions were 82%, 81%, and 80% for N-HCS-1, N-HCS-2, and NHCS-4, respectively, indicating that the N-HCS samples with high dispersion, regular spherical shape, thin shell, large cavity, 4405


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Figure 3. (a) EDS elemental mapping images of the N-HCS-1 sample. Also shown are XPS spectra of the N-HCS-1 sample: (b) overall, (c) C 1s, (d) O 1s, and (e) N 1s.

Figure 4. (a) Cyclic voltammetry (CV) curves of 5 mV s−1; (b) galvanostatic charge−discharge (GCD) curves at a current density of 0.5 A g−1; (c) capacitance at different GCD values; and (d) Nyquist plot curves of N-HCS samples.

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Figure 5. (a) CV curves and (b and c) GCD curves for the N-HCS-1 at different scan rates and current densities. Also shown are (d) the specific capacitance of N-HCS-1 at different GCD current densities, (e) Ragone plots comparison of N-HCS-1, and (f) a long circle test of N-HCS-1 at 2 A g−1 in a two-electrode system.

10 A g−1. The relative quasi-triangular and symmetrical shapes of all GCD curves at various current densities demonstrated the typical EDLC and pseudocapacitance behavior. In order to compare with N-HCS, the electrochemical characteristics of nitrogen-doped carbon also was measured. As shown in Figure S5 in the Supporting Information, the poor CV (Figure S5a) and GCD (Figure S5b) curves indicate the poor electrochemical performance of nitrogen-doped carbon derived from directly carbonization of PSPAN, because of its agglomeration and irregular morphology. With regard to a supercapacitor, application in an organic system can expand the application range of electrode materials. Therefore, we performed electrochemical tests in the organic phase with ionic liquid and organic electrolyte, respectively. As shown in Figure S6 in the Supporting Information, both CV and GCD curves showed relatively good shapes, proving the practicability of the material in the organic phase. Through GCD calculation, the capacitance of N-HCS-1 in an ionic liquid and an organic electrolyte were, respectively, 281.1 and 234.3 F g−1 at a current density of 0.5 A g−1, which demonstrates the high electrochemical performance of N-HCS. The electrochemical behavior of the N-HCS-1 in aqueous electrolyte was investigated further in a two-electrode system. The CV curves in a potential range of 0−1 V (Figure 5a)

and mesoporous structure was fit for the transport and diffusion of electrolyte ions. Electrical impedance spectroscopy (EIS) analysis conducted in the frequency range of 105−10−2 Hz provided more powerful evidence for the capacitive behavior of N-HCS. As illustrated in Figure 4d, N-HCS-1 and N-HSC-2 showed a vertical line in the low-frequency region and impedance spectra with an arc in the high-frequency region, indicating the typical capacitive behavior of EDLC. Among them, the N-HCS-4 showed the highest resistance of 0.42 Ω, compared to N-HCS1 (0.31 Ω) and N-HCS-2 (0.32 Ω) (Figure 4d), suggesting that a small diameter would improve the electrode performance. Because of its relatively higher capacitance value, N-HCS-1 was further investigated. The rate performance was another important issue for the practical supercapacitor. As demonstrated in Figure S4a in the Supporting Information, a relative regular rectangular shape could be retained when the scan rate gradually increased from 5 mV s−1 to 100 mV s−1, further indicating the good capacitance performance at a high scan rate and the behavior of EDLC and pseudo-capacitance for NHCS-1. The good performance was also confirmed by GCD and Figure S4b in the Supporting Information showed the GCD curves of N-HCS-1 in the current density range of 0.5− 4407



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showed that a relative regular rectangular shape and slight offset were still retained when the scan rate gradually increased from 5 mV s−1 to 100 mV s−1, demonstrating the behavior of EDLC combined with pseudocapacitance. The GCD curves of a relative quasi-linear nature at different current densities also confirmed the electrochemical behavior of EDLC and pseudocapacitance features, as shown in Figures 5b and 5c. A rather small decrease in voltage (by 0.158 V) could be obtained from the discharge curves at 50 A g−1 (Figure 5c), because of the internal resistance caused by the change in polarity. The capacitance of the N-HCS-1 was 406 F g−1 calculated by the current density of 0.5 A g−1. At the same time, N-HCS-1 possessed a high capacitance retention at current densities of 0.5−50 A g−1, as shown in Figure 5d, indicating the large transport and diffusion channels for electrolyte ions provided by the rich mesoporous structure. Note that the capacitance of N-HCS-1 was higher than that of many other carbon materials with different morphologies at corresponding current density, as shown in Figure 5d.32−41 Figure 5e shows the Ragone plots obtained by GCD in the symmetric supercapacitor; N-HCS-1 showed much better performance than most of carbon materials, including carbon spheres, N/P-doped porous carbon, carbon nanotube, and carbon sheets, as reported previously.36,41−49 As displayed in Figure 5f, 75.5% of the initial capacitance of N-HCS-1 was retained after 10 000 cyclic tests, showing its excellent stability. Generally, N-HCS-1 exhibited excellent electrochemical properties and has potential as an electrode material in supercapacitors.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A. Chen). *E-mail: [email protected] (S. Hou). ORCID

Aibing Chen: 0000-0002-2764-5234 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (No. 21676070), Hebei Training Program for Talent Project (No. A201500117), Hebei One Hundred-Excellent Innovative Talent Program (III) (No. SLRC2017034).

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REFERENCES

(1) Yan, L.; Li, D.; Yan, T.; Chen, G.; Shi, L.; An, Z.; Zhang, D. Confining Redox Electrolytes in Functionalized Porous Carbon with Improved Energy Density for Supercapacitors. ACS Appl. Mater. Interfaces 2018, 10, 42494−42502. (2) Méndez-Morales, T.; Ganfoud, N.; Li, Z.; Haefele, M.; Rotenberg, B.; Salanne, M. Performance of Microporous Carbon Electrodes for Supercapacitors: Comparing Graphene with Disordered Materials. Energy Storage Materials 2019, 17, 88−92. (3) Yang, W.; Xu, X.; Hou, L.; Li, Z.; Deng, B.; Tian, J.; Yang, F.; Li, Y. Nitrogen-enriched Hollow Carbon Spheres Coupled with Efficient Co-Nx-C Species as Cathode Catalysts for Triiodide Reduction in Dye-sensitized Solar Cells. ACS Sustainable Chem. Eng. 2019, 7, 2679−2685. (4) Hou, P.; Xing, G.; Tian, L.; Zhang, G.; Wang, H.; Yu, C.; Li, Y.; Wu, Z. Hollow Carbon Spheres/graphene Hybrid Aerogels as Highperformance Adsorbents for Organic Pollution. Sep. Purif. Technol. 2019, 213, 524−532. (5) Wang, H.; Zhou, H.; Gao, M.; Zhu, Y.-a.; Liu, H.; Gao, L.; Wu, M. Hollow Carbon Spheres with Artificial Surface Openings as Highly Effective Supercapacitor Electrodes. Electrochim. Acta 2019, 298, 552−560. (6) Qiu, D.; Gao, A.; Xie, Z.; Zheng, L.; Kang, C.; Li, Y.; Guo, N.; Li, M.; Wang, F.; Yang, R. Homologous Hierarchical Porous Hollow Carbon Spheres Anode and Bowls Cathode Enabling High-energy Sodium-ion Hybrid Capacitors. ACS Appl. Mater. Interfaces 2018, 10, 44483−44493. (7) Ma, S.; Wang, L.; Wang, Y.; Zuo, P.; He, M.; Zhang, H.; Ma, L.; Wu, T.; Yin, G. Palladium Nanocrystals-imbedded Mesoporous Hollow Carbon Spheres with Enhanced Electrochemical Kinetics for High Performance Lithium Sulfur Batteries. Carbon 2019, 143, 878−889. (8) Yan, L.; Li, D.; Yan, T.; Chen, G.; Shi, L.; An, Z.; Zhang, D. N,P,S-Codoped Hierarchically Porous Carbon Spheres with WellBalanced Gravimetric/Volumetric Capacitance for Supercapacitors. ACS Sustainable Chem. Eng. 2018, 6, 5265−5272. (9) Duan, H.; Yan, T.; Li, Z.; Chen, G.; Zhang, J.; Shi, L.; Zhang, D. Rapid Synthesis of Self-supported Three-dimensional Bubble-like Graphene Frameworks as High-performance Electrodes for Supercapacitors. Sustainable Energy Fuels 2017, 1, 1557−1567. (10) Xu, M.; Yu, Q.; Liu, Z.; Lv, J.; Lian, S.; Hu, B.; Mai, L.; Zhou, L. Tailoring Porous Carbon Spheres for Supercapacitors. Nanoscale 2018, 10, 21604−21616. (11) Du, W.; Wang, X.; Zhan, J.; Sun, X.; Kang, L.; Jiang, F.; Zhang, X.; Shao, Q.; Dong, M.; Liu, H.; Murugadoss, V.; Guo, Z. Biological Cell Template Synthesis of Nitrogen-doped Porous Hollow Carbon Spheres/MnO2 Composites for High-performance Asymmetric Supercapacitors. Electrochim. Acta 2019, 296, 907−915. (12) Du, J.; Liu, L.; Liu, B.; Yu, Y.; Lv, H.; Chen, A. Encapsulation Pyrolysis Synchronous Deposition for Hollow Carbon Sphere with Tunable Textural Properties. Carbon 2019, 143, 467−474.

3. CONCLUSION In summary, N-HCS samples with well-adjusted diameters, cavities, and uniform mesoporous sizes were obtained via the confined-space pyrolysis method, using PSPAN spheres as the core and carbon/nitrogen precursor. The confined space of the mesoporous silica outer shell could provide a confined environment for PS decomposition to create cavity and PAN pyrolysis to form a carbon shell. The confined space also provided a rich mesoporous structure for N-HCS, which was attributed to the activation of pyrolyzed gas from PSPAN. The diameter, cavity, and shell thickness of N-HCS can be welltuned by adjusting the PS:PAN ratio. The N-HCS exhibited a highly dispersive spherical morphology, large cavity, uniform mesoporous size, and suitable nitrogen doping. At the same time, the N-HCS represented an outstanding potential electrode material in supercapacitors with a maximum capacitance of 451.5 F g−1 and excellent stability. This confined-space pyrolysis method provides a new idea for the preparation of carbonaceous materials with high surface area, mesoporous distribution, regular shape, and adjustable structure.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00578. TEM images, N2 adsorption−desorption isotherm, XRD spectra TGA analysis of PSPAN-4:6@SiO2; CV and GCD curves of N-HCS-1 and N-doped carbon derived from directly carbonization of PSPAN; electrochemical performance in ionic liquid and TEA BF4 electrolyte of N-HCS-1 (PDF) 4408


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