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Abstract

Carbon-based materials with multidimensional structures generally exhibit improved properties compared with single-morphology carbon materials for various applications including catalysis, adsorption, and energy storage. Here, an N-doped mesoporous carbon sphere and sheet composite is prepared by a co-assembly strategy using an ionic liquid ([C₁₈Mim]Br) as the structure-directing agent, ethylenediamine as the catalyst, tetraethyl orthosilicate as the pore-forming agent, and resorcinol formaldehyde resin as the carbon precursor. [C₁₈Mim]Br and ethylenediamine not only induce formation of the unique structure but also lead to in situ nitrogen doping on the N-doped mesoporous carbon skeleton. The obtained N-doped mesoporous carbon shows a unique composite structure of thin sheets embedded with carbon spheres, having high a specific surface area and uniform mesopore distribution. When used as an electrode material, the N-doped mesoporous carbon shows a good specific capacity of 273 F g⁻¹ at a current density of 0.5 A g⁻¹ and a good rate capability (82.1% of the capacitance is retained at a high current density of 10 A g⁻¹). Moreover, the N-doped mesoporous carbon exhibited ideal stability behavior (91.6% capacitive retention after 10,000 cycles), indicating a promising role as an electrode material for excellent performance supercapacitors.

Keywords

carbon sheets carbon spheres co-assembly N-doped supercapacitor

Introduction

Recently, energy demands have attracted worldwide attention in modern society.¹ New energy storage devices and technologies have been widely explored, such as lithium-ion batteries and electrochemical capacitors.^2,3 Among them, supercapacitors are attracting increasing interest because of their high energy density and long cycle life compared with rechargeable batteries and have been extensively applied in areas of portable electronics, digital communication, hybrid electronic vehicles, and renewable energy systems.^4–9 Therefore, the development of high-performance supercapacitors is a significant goal and is also the key to widening the applications of supercapacitors.

It is known that the electrode material is the most important component and greatly determines the performance of supercapacitors.^6,9 Electrode materials can be mainly divided into four categories: carbon materials,¹⁰ metal oxides,¹¹ conductive polymers,¹² and composite materials.¹³ Carbon materials receive ever-increasing attention as supercapacitor electrode materials owing to their superior physical and chemical properties, such as low mass density, diverse morphology, good chemical stability, and promising electrical conductivity.^6,14–16

According to their structural characteristics, carbon materials can be divided into zero-dimensional (sphere), one-dimensional (fiber, nanotube), two-dimensional (sheet), and three-dimensional (monolithic aerogels, bulk carbon).^17–19 In previous literature, various two-dimensional sheet-based multidimensional composite materials have been synthesized, such as sheets/spheres, sheets/rods, and sheets/fibers.^18–21 In particular, the sheet and sphere composites combine the respective merits of zero- and two-dimensions, exhibiting enhanced supercapacitor performance, and have received widespread attention.^20,22 Graphene, the most representative two-dimensional carbon material, can be used to prepare carbon-based sphere and sheet composite materials with polymer spheres such as resin spheres,²² polystyrene spheres,²³ or hollow polypyrrole.²⁴ The obtained multiple-dimensional composite carbon materials have proved their high performance in catalysis, adsorption, and as electrodes for supercapacitors.^20,22 However, most of the syntheses suffer from complex preparation processes, which are time-consuming and detrimental to large-scale industrial production.^9,25,26 Researchers first synthesize sheets, then decorate the spheres on the surface of the sheets by self-assembly, or synthesize the spheres followed by assembly to obtain the sheets/spheres composite structures.

In addition to preparing multidimensional composites, doping of heteroatoms (such as N, S, or B) into carbon frameworks is also an effective approach to further enhance the capacitance of composite carbon-based materials.^25,27–29 In particular, N-doping has attracted more attention because it can modify the electron/donor characteristics and crystalline structures of the carbon host through internal or surface doping and thus greatly improve the capacitance performance of multidimensional carbon materials.^27,30–33 In general, there are two kinds of commonly used nitrogen-doping methods, in situ and post-treatment doping.²⁷ Evidently, in situ doping have drawn much attention because of the unique advantages such as milder method and higher nitrogen content. Hence, exploring an efficient way to synthesize sheet and sphere multidimensional composites with nitrogen doping is a meaningful strategy for improving their electrochemical performance and expanding the application of carbon materials.

Herein, an N-doped mesoporous carbon sphere and sheet (N-MCS) composite is synthesized by a co-assembly strategy improving a one-pot process. The as-obtained N-MCS used resorcinol formaldehyde (RF) resin as the carbon precursor, ethylenediamine (EDA) as the catalyst, tetraethyl orthosilicate (TEOS) as the pore-forming agent, and the ionic liquid ([C₁₈Mim]Br) as the structure-directing agent. This method enabled the preparation of the multidimensional composite, which avoided the preparation of spheres or sheets and is promising for large-scale industrial production. The obtained N-MCS exhibited a unique structure combining the zero- and two-dimensions and possesses a uniform mesopore size, a large specific surface area and a suitable nitrogen content.²⁴ When used as an electrode material, the N-MCS showed good electrochemical properties and cycling stability.

Results and discussion

The procedure for fabrication of the N-MCS is illustrated in Figure 1. The N-MCS, using [C₁₈Mim]Br as the structure-directing agent, EDA as the catalyst, TEOS as the pore-forming agent, and the RF resin derived from resorcinol as the carbon resource, was successfully prepared by a simple one-pot co-assembly method.^22,34 First, some of the resorcinol, formaldehyde, and EDA were polymerized to form nitrogen-containing resin spheres (denoted as N-RF), because the formation rate of N-RF is much faster than that of TEOS hydrolysis with the catalysis of EDA.^34,35 Next, the redundant resorcinol and silicate oligomers are adsorbed on N-RF spheres through the electrostatic attraction of [C₁₈Mim]⁺, and sheet-like structures were formed at the same time, resulting in the formation of a sphere and sheet composite. The EDA and [C₁₈Mim]Br not only led to the formation of the unique composite structure but also resulted in the nitrogen doping.³⁴ TEOS acted as a pore-forming agent, creating mesopores and increasing the surface area. Finally, the N-MCS was obtained after pyrolysis and etching the SiO₂. The N-MCS exhibited a composite structure of sheets and spheres.

Figure 1.

Schematic illustration of the synthesis of N-MCS.

The morphology of N-MCS was investigated by transmission electron microscopy (TEM; Figure 2(a) and (b)). It is apparent that the obtained N-MCS exhibits a composite structure of sheets embedded with spheres (Figure 2(a)).²² The surface of the sheets was crude and relatively thin, with irregular edges. Some irregular, football-like spheres were clearly observed on the surface of the sheet, with diameters of 80–130 nm (Figure 2(a)). Magnification (Figure 2(b)) revealed the presence of disordered mesopores on the N-MCS, which might be attributed to the etching of silica.³⁶ It has been demonstrated that the presence of [C₁₈Mim]Br and EDA was the key to the formation of the composite through a one-pot co-assembly method. When using ammonia instead of EDA as the catalyst, the obtained carbon material would consist of only mesoporous carbon sheets.^22,37 Similarly, using hexadecyl trimethylammonium chloride as a structure-directing agent and EDA as the catalyst, only carbon spheres would be obtained.³⁴

Figure 2.

(a and b) TEM images of N-MCS. (c) Nitrogen adsorption–desorption isotherms and (d) pore size distribution of the N-MCS.

The possible reason for the composite structure was that the rate of resin polymerization was much faster than that of TEOS hydrolysis with the catalysis of EDA at the beginning of the reaction, thus resulting in the rapid preformation of N-RF spheres. With the reaction prolonged, the polymerization rate of the resin matched the hydrolysis rate of TEOS. Also, some of the resorcinol and TEOS interacted with [C₁₈Mim]Br through electrostatic forces and assemble around the N-RF spheres to form the RF and silicate oligomers composite sheet.^22,37 At the same time, the N-RF spheres continued to be assembled and the spheres/sheets composite structure was finally obtained.^22,34,38,39

The surface area and pore size of the N-MCS were investigated by nitrogen absorption and desorption measurements (Figure 2(c) and (d)). From Figure 2(c), it can be observed that the N-MCS displayed a combination of type IV isotherms with an obvious type-H3 hysteresis loop at a relative pressure P/P₀ from 0.45 to 0.9, indicating typical mesoporous features.^10,35,40 In addition, the adsorption–desorption isotherm of N-MCS exhibited a hysteresis loop at a relative pressure range of 0.9–1.0, which might be created due to the accumulation of N₂ between the composite material.^10,41 The corresponding pore size distribution curve (Figure 2(d)), calculated from the adsorption branch by the Barrett–Joyner–Halenda (BJH) method demonstrated two types of pores: (1) relatively small mesoporous (about 3.0 nm) and (2) relatively large mesoporous (average diameter 29.0 nm), indicating the presence of mesopores in the N-MCS. The small mesopores (3.0 nm) of N-MCS were created by the etching of silica, while the large mesopores (29.0 nm) can be ascribed to the accumulation of carbon spheres and carbon flakes with rough surfaces. The surface area and the total pore volume of N-MCS were estimated to be 717 m² g⁻¹ and 0.97 cm³ g⁻¹, respectively.

Both EDA and [C₁₈Mim]Br contain nitrogen and can be used as good nitrogen precursors, which would lead to in situ nitrogen doping on the skeleton of N-MCS. Therefore, to investigate the surface elemental composition of the N-MCS, X-ray photoelectron spectroscopy (XPS) analysis was performed (Figure 3). XPS spectra for each element were obtained and fitted, and the corresponding forms of each element were analyzed. There were three obvious peaks at 285, 401, and 532 eV, which, respectively, corresponded to C1s, N1s, and O1s, with no trace of other elements.^42,43 As seen from the inset of Figure 3(a), the content of C1s, O1s, and N1s was 95.7, 2.0, and 2.3 wt%, respectively. The spectrum of C1s of the N-MCS (Figure 3(b)) could be deconvoluted into three single peaks with binding energies of 284.6, 285.5, and 289.2 eV that corresponded to C-C, C–N, and COOH, respectively.^40,42–44 Three distinguishable peaks at 530.1, 531.8, and 534.5 eV in the O1s spectrum (Figure 3(c)) corresponded to oxygen in C=O, C–O–C, and O=C–O bonds, respectively.^43–45 In addition, the N1s spectrum (Figure 3(d)) could also be deconvoluted into three peaks at 397.8, 400.4, and 403.1 eV, which corresponded to pyridinyl-N, pyrrolic-N, and oxidized-N, respectively.^34,40,43,45 N-doping can largely improve the capacitance of carbon-based materials, which may result from the fact that N-containing species can contribute to pseudocapacitance and serve as electrochemically active sites.^27,28

Figure 3.

(a) XPS spectra of N-MCS, (b) C1s, and (c) O1s and (d) N1s spectra of N-MCS.

The N-MCS, combining the respective merits of sheets and spheres, had uniform mesoporous distribution, a high specific surface area, and a suitable nitrogen content.^21,43 Therefore, as a supercapacitor electrode material, the N-MCS showed good performance in terms of electrochemistry. The electrochemical properties of N-MCS (Figure 4) were measured in a typical three-electrode system with 6 M KOH as the electrolyte, and the typical cyclic voltammetry (CV) profiles at different scan rates from 5 to 200 mV s⁻¹ are shown in Figure 4(a).^43,44 All the curves at different scan rates from 5 to 100 mV s⁻¹ exhibited perfect almost rectangular shapes, indicating typical characteristics of an electric double-layer capacitive (EDLC) energy storage mechanism.^18,44 Even at 200 mV s⁻¹, the CV curve showed no dramatic distortion, implying a good capacitance performance at a high scan rate.²⁷ Figure 4(b) shows the galvanostatic charge–discharge (GCD) curves of N-MCS at 0.5–20 A g⁻¹ within the potential window of −1 to 0 V. It can be seen that all the charge and discharge curves are highly symmetric, which further indicated that the electrode possessed an ideal capacitive performance.^22,38 It is also important to note that the specific capacitance of N-MCS reached 273 F g⁻¹ at 0.5 A g⁻¹, which is much higher than that of many other types of nitrogen-doped carbon materials (Table 1) such as N-doped hollow carbon spheres/sheets composites,²² nitrogen-doping hierarchically porous carbon nanosheets,⁴⁴ nitrogen-doped hollow carbon spheres,⁴⁶ heteroatom-doped porous carbon,⁴⁷ nitrogen-doped hollow mesoporous spherical carbon capsules,⁴⁸ porous nitrogen-doped hollow carbon spheres,⁴⁹ nitrogen-doped macro-/mesoporous carbon foams,⁵⁰ nitrogen-doped graphene aerogels,⁵¹ and porous nitrogen-doped carbon nanotubes.⁵² In addition, the specific capacity of N-MCS was higher than that of carbon spheres (hexadecyl trimethylammonium chloride as the structure-directing agent)³⁴ and carbon sheets (ammonia as the catalyst)^22,37 reported in previous work, which is evidence that the excellent electrochemical performance may be ascribed to the unique composite structure and rich mesoporous structure. The corresponding electrode-specific capacitances at different current densities were calculated based on the GCD curves and are shown in Figure 4(c). The N-MCS maintained a good retention rate of 82.1%, even at a current density of 10 A g⁻¹, and is higher than other previous reported N-doped carbon materials (Table 2).^{22,44,46–52} The inset in Figure 4(d) shows the magnified region in the high-frequency range and the equivalent electric circuit of the Electrochemical Impedance Spectroscopy (EIS), which is composed of five components: the internal resistance (R_s), the Warburg impedance (Z_w), the charge-transfer resistance (R_ct), the interfacial capacitance (C_i), and the Faradic capacitance (C).^51,53–56 The R_s value was obtained by the intercept (Z′) on the real axis, which included the intrinsic resistance of the electrode material, the ionic resistance of the electrolyte, and the contact resistance between the electrode and current collector.^51,53,56 R_ct is the contact interface resistance to adsorption–desorption of ions on the electrode material and is obtained from the diameter of the semi-circle line along the x-axis.^54,55 In the low-frequency region, the Nyquist plot of N-MCS showed a nearly vertical line, meaning a nearly ideal capacitive behavior and low diffusion resistance of electrolyte ions in the electrode material.^{27,43–45,57} The magnified region in the high frequency range (Figure 4(d) insert) indicated a low R_s of 0.51 Ω and R_ct of 0.21 Ω, indicating a short ion diffusion path permitting and excellent electronic conductivity. The cycle life is one of the critical parameters in determining practical supercapacitor applications. To investigate the reusability, 10,000 cycles of charge and discharge (Figure 4(e)) were conducted successively on the N-MCS at a current density of 5 A g⁻¹. Very little change in the specific capacitance was observed throughout the whole experiment, and 93% of the original capacitance was preserved after multiple cycles, which illustrated that the N-MCS electrode displayed good cycling and stability behavior compared to other N-doped carbon-based electrode materials (Table 2).^{43,47–49,51,56,58} In addition, the GCD curve of the 10,000th cycle showed no obvious difference from that of the first cycle (Figure 4(f)),⁴⁵ further indicating that the electrode possessed stable performance and good charge propagation.

Figure 4.

(a) CV curves at 5–200 mV s⁻¹; (b) GCD curves at 0.5–20 A g⁻¹ of N-MCS; (c) the correlation of specific capacitance at various current densities for N-MCS; (d) the Nyquist plots of N-MCS, its corresponding high-frequency ranges, and the fitted equivalent circuit (inset); (e) cycle stability of N-MCS at 5.0 A g⁻¹, and (f) the GCD curve of the first and 10,000th cycles.

Table 1.

Comparison of the specific capacitance and rate capability of N-MCS with those of reported carbon materials in 6 M KOH.

Sample	Voltage window (V)	Special capacitance		Rate capability		Ref.
Sample	Voltage window (V)	Current density (A g⁻¹)	Special capacitance (F g⁻¹)	Current density (A g⁻¹)	Retention rate (%)	Ref.
N-HMCS/S	−1 to 0	0.5	196.5	0.5–50	57.1	²²
N-HPCNs-900	−1 to 0	1.0	201	1–10	78.1	⁴⁴
N-HCS	−1 to 0	0.5	173	0.5–5	75.7	⁴⁶
CSC-2	−1 to 0	0.5	266.5	0.5–20	73.6	⁴⁷
N-HMSCCs	−1 to 0	1.0	206	1–10	61.5	⁴⁸
PNHCS	−1 to 0	0.5	213	0.5–10	55.6	⁴⁹
N-MMCF-3	−1 to 0	1.0	198	1–20	80.3	⁵⁰
N-GAs	−1 to 0	0.5	175	0.5-10	55.4	⁵¹
PNCNT	−1 to 0	0.5	210	0.5–10	61.9	⁵²
N-MCS	−1 to 0	0.5	273	0.5–10	82.1	This work

N-MCS: N-doped mesoporous carbon sphere and sheet composite.

Table 2.

Comparison of the stability in charge–discharge cycles of the N-MCS with those of reported N-doped carbon materials in 6 M KOH.

Sample	Current density (A g⁻¹)	Voltage window (V)	Cycle number	Cycle stability (%)	Ref.
NPCHSs	5	−1 to 0	5000	89.1	⁴³
CSC-2	2	−1 to 0	5000	92.7	⁴⁷
N-HMSCCs	5	−1 to 0	3000	92.3	⁴⁸
PNHCS	1	−1 to 0	5000	91.1	⁴⁹
N-GAs	5	−1 to 0	5000	87	⁵¹
NCM-700	1	−0.9 to 0	5000	86	⁵⁶
N-HMCSs-0.8	5	−1 to 0	5000	88	⁵⁸
N-MCS	5	1 to 0	10000	93	This work

N-MCS: N-doped mesoporous carbon sphere and sheet composite.

Two-electrode symmetric supercapacitors were fabricated to further investigate the capacitive performances of N-MCS as a real supercapacitor (Figure 5).⁴⁶ Figure 5(a) shows the CV curves of N-MCS at different scan rates from 5 to 200 mV s⁻¹. The quasi-rectangular CV curves suggested approximately ideal EDLC capacitive behavior at different scan rates.^10,51 Figure 5(b) shows the GCD curves of N-MCS in a potential range of 0–0.8 V from 0.5 to 10 A g⁻¹. All the GCD curves were close to typical isosceles triangles and the charge and discharge curves were symmetric.⁵¹ The calculated specific capacitance of the N-MCS was 187.5 F g⁻¹ at a current density of 0.5 A g⁻¹. These results indicate that the as-prepared symmetric supercapacitor has high electrochemical reversibility, which is consistent with the results in three-electrode tests. Figure 5(c) displays the specific capacitances at different GCD current densities for N-MCS, which indicated that 72% of the original capacitance could be preserved from current density of 0.5 to 10 A g⁻¹, revealing that the N-MCS exhibited good rate capability.^33,52,53,59 Another important issue for supercapacitor applications is the power density and energy density. Figure 5(d) shows the Ragone plots of the N-MCS symmetric supercapacitor in the potential range of 0–1 V. The N-MCS showed a high energy density of 16.7 W h kg⁻¹ at 800 W kg⁻¹, which remained at 11.96 W h kg⁻¹ even when the power density was elevated to 16,000 W kg⁻¹. These data are highly comparable to those previously reported for symmetric carbon-based supercapacitors, including hollow carbon spheres, pure carbon sheets, and membrane carbon materials with hierarchical porous architectures.^{20,43,60–64} Figure 5(e) shows that the specific capacitance retention of N-MCS was 84% after 10,000 cycles in the two-electrode system, further demonstrating the good electrochemical stability and reversibility. As illustrated in Figure 5(f), the EIS before and after the cycle test showed similar Nyquist plots with a semi-circle in the high-frequency region and a straight line in the low-frequency region. Moreover, the R_s values were determined to be 2.5 Ω (before the cycle test) and 1.2 Ω (after the cycle test), indicating an excellent capacitive behavior. Also, the diameter of the semi-circle after 10,000 charge–discharge cycles had significant increase in size compared with that of the pristine sample, primarily revealing the increase of R_ct, which could be attributed to part of the porous structure being damaged during the stability test.^65–68 All these results showed that the N-MCS is a promising electrode material for supercapacitor application.

Figure 5.

(a) CV curves of N-MCS tested at 5–200 mV s⁻¹ in a two-electrode system, (b) GCD curves of N-MCS at different current densities in a two-electrode system in 6 M KOH aqueous solution, (c) specific capacitances at different GCD current densities of N-MCS in a two-electrode system, (d) Ragone plots of N-MCS, (e) cycle stability of N-MCS at 5 A g⁻¹ in the potential window of 0–0.8 V, and (f) Nyquist plots and their corresponding high frequency ranges (inset) before and after the cycle test in two-electrode system.

Conclusion

In summary, we have demonstrated a co-assembly strategy to synthesize an N-MCS composite through a one-pot process. [C₁₈Mim]Br and EDA were the key to the formation of the unique composite structure and also led to suitable nitrogen doping. TEOS used as a pore-forming agent created the uniform mesopores and increased the surface area. When used as an electrode material, the N-MCS exhibited excellent electrochemical capacitance and good retention capability. The one-pot co-assembly strategy provides a new route for the production of multidimensional carbon-based composite materials, which does not require time-consuming procedures such as preparing the spheres or sheets in advance, and improves the application prospects in industry. Meanwhile, the unique structure of the sheet and sphere composite is expected to be promising for various applications in metal-free catalysts or catalyst supports, adsorption separation processes, and so on.

Experimental

Chemicals and materials

TEOS, formaldehyde solution (37 wt%), ethanol, ammonia solution (28 wt%), and hydrofluoric acid (10%) were of analytical grade and were purchased from Tianjin Yongda Chemical Corp. EDA and resorcinol were purchased from Aladdin Corp. [C₁₈Mim]Br was purchased from Shanghai Chengjie Chemical Corp. All experiments were conducted using deionized water.

Preparation of the N-MCS

Typically, EDA (0.4 mL) was dispersed in a solution of deionized water (52 mL) and ethanol (26 mL). Subsequently, [C₁₈Mim]Br (0.33 g) was added and the mixture stirred for 30 min. Next, resorcinol (0.1 g) was added and the solution was stirred for another 30 min until complete dissolution occurred. TEOS (0.5 mL) and formaldehyde solution (0.14 mL) were then added to the reaction solution. The mixture was stirred for 24 h at room temperature. Afterwards, the mixture was poured into an autoclave and kept in an oven at 80 °C. After 24 h, the precipitate was separated by centrifugation, washed with water and ethanol, and dried at 60 °C overnight. For carbonization, the obtained product was heated at 800 °C for 3 h with a heating rate of 3 °C/min under an N₂ atmosphere. The N-MCS was obtained after removal of silica with 10 wt% hydrogen fluoride (HF) solution.

Material characterization

N₂ adsorption and desorption isotherms were measured using a Micromeritics TriStar 3020 at −196 °C, and the surface area was calculated according to the Brunauer–Emmett–Teller (BET) method. The pore size distribution was calculated from the adsorption branch of the isotherms, according to the BJH method. TEM analysis was conducted on a JEOL JEM-2100 electron microscope. The X-ray photoelectron spectrometry (XPS) spectra were obtained by using an AXIS ULTRA DLD spectrometer with Al Kα radiation as the excitation source and the peak positions were referenced internally to the C1s peak at 284.6 eV.

Electrochemical measurements

The electrochemical properties were measured with a three-electrode and symmetrical electrode system with 6 M KOH using a CHI 760E electrochemical working station. The working electrode consisted of the carbon material, a polytetrafluoroethylene (PTFE) binder, and carbon black in a mass ratio of 80:10:10. The resulting slurry was pasted onto nickel foam and the electrodes were dried at 100 °C for 12 h. For the two-electrode system, the specific capacitances (C, F g⁻¹), energy density (E, W h kg⁻¹), and power density (P, W kg⁻¹) were calculated using the following equations: $C = 4 I Δ t / Δ V m,$ $E = 0.5 C {(Δ V)}^{2},$ and $P = E / Δ t,$ where I (A), ∆t (s), ∆V (V), and m (g) are the GCD current, discharge time, voltage window, and mass of the active material, respectively. In the three-electrode system, the specific gravimetric capacitance was calculated from the GCD process according to $C = I Δ t / Δ V m .$

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: We thank the Beijing National Laboratory for Molecular Sciences, the Hebei Province Introduction of Foreign Intelligence Projects (2018), Hebei Science and Technology Project (20544401D, 20314401D), Tianjin Science and Technology Project (19YFSLQY00070) and the National Natural Science Foundation of China (21676070).

ORCID iD

Aibing Chen

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Preparation of an N-doped mesoporous carbon sphere and sheet composite as a high-performance supercapacitor

Abstract

Keywords

Introduction

Results and discussion

Conclusion

Experimental

Chemicals and materials

Preparation of the N-MCS

Material characterization

Electrochemical measurements

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References