Controllable synthesis of mesoporous carbon nanoparticles based on PAN-b-PMMA diblock copolymer micelles generated via RAFT polymerization as electrode materials for supercapacitors

Abstract

Mesoporous carbon nanoparticles (MCNs) were prepared through a series of annealing procedures using well-controlled diblock copolymer micelles as precursors. The micelles were prepared from poly(acrylonitrile)-block-poly(methylmethacrylate) (PAN-b-PMMA), and synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization. The RAFT-controlled synthesis of block copolymer, PAN-b-PMMA was conducted either in solution or emulsion conditions. It was found that micelles with well-defined morphology could be directly formed from the emulsion polymerization. The as-synthesized micelles containing hydrophilic PAN corona as the carbon source and hydrophobic PMMA core as the sacrificing template underwent a microphase-separation process to form a nanostructure at 250 °C, followed by carbonization at 800 °C to afford MCNs. What's more, the MCNs based supercapacitor exhibited a good capacitance value of 220 F g⁻¹ and an excellent cycling performance of 91% capacitance retention after 10 000 cycles. The as-prepared MCNs are envisioned to provide broad practical applications in the fields of energy storage and nanotechnology.

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Introduction

In recent years, supercapacitors have drawn tremendous attention due to their excellent properties such as high power density, fast charge–discharge rate and exceptional cycling stability.^1–8 Based on the mechanism of charge storage and discharge, supercapacitors can be divided into two categories: electrochemical double-layer capacitors (EDLCs) and pseudo-capacitors.^9–11 EDLCs physically store charges via reversible ion adsorption at the electrode–electrolyte interface, while pseudo-capacitors chemically store charges via redox reaction at the vicinity (a few nanometers) of the surface. Typically, electrode materials for EDLCs are made of carbon-based materials (including activated carbon, carbon nanotubes, graphene, etc.), which play an important role in the performance of EDLCs.^6,12–17 Therefore, nanoporous carbon electrode materials have received considerable interest for EDLCs, owing to their unique structure and functional behavior, such as high specific area, low specific density, specific capacitance, good electrical conductivity, large controllable pore volume, excellent chemical stability and well-defined nano-morphology.^18–20 Among these carbon-based materials, activated carbon-based electrode has high specific surface area, but its irregular microporous structure results in poor accessibility when electrolyte ions are close to the porous surface, which limits its electrochemical performance.^21–23 Therefore, in order to achieve high-performance supercapacitors, mesoporous carbon electrodes with high specific surface area, abundant mesoporous structure, and moderate pore size distribution are on great demand.^24–28

The research on synthesis and self-assembly of block copolymers has aroused great attention in the past decade due to their potential applications in nano-medicine, recombinant DNA technology, energy and electronics, separation science and so on.^29–31 In the past three decades, the self-assembly and micellization of amphiphilic block copolymer to prepare block copolymer nano-objects have been widely documented.^32–34 However, the main disadvantage of the self-assembly strategy is that this method is usually achieved under dilute conditions (typically <1% w/w copolymer).³⁵ Recently, the polymerization-induced self-assembly (PISA) methods have been established as a powerful tool to overcome this problem and prepare concentrated block copolymer nano-objects with the polymer concentration up to 40% w/w directly in either aqueous solution, polar solvents, or non-aqueous media.^36–38 Among PISA methods, the reversible addition–fragmentation chain transfer (RAFT) emulsion polymerization was widely employed to prepare block copolymer nano-objects.^39–41 With the proceeding of the RAFT polymerization, the insoluble segment of the synthesized block copolymer reaches to a critical chain length in which the self-assembly of the in situ synthesized block copolymer into micelles takes place.⁴² It has been demonstrated that several parameters listed below are found to dictate the size and morphology of the synthesized block copolymer nano-objects: the degree of polymerization of the solvophobic block, the chain length of the macro-RAFT agent, the solvent character of the polymerization medium and the concentration of the feeding monomer.^{35,36,42–48} For example, the block copolymer nano-objects experienced a stage from nanospheres to worms and in the end to form vesicles when increasing the degree of polymerization of the solvophobic chain segment during the dispersion RAFT polymerization.^49–52 Therefore, the polymerization-induced self-assembly is mostly based on RAFT polymerization.^53–55

Herein, we successfully synthesized PAN-b-PMMA micelles via RAFT polymerization which contained hydrophilic PAN corona and hydrophobic PMMA core. The diblock copolymer synthesized by RAFT polymerization had well-controlled molecular weight and narrow polydispersity. The as-synthesized PAN-b-PMMA micelles underwent a phase separation to form a self-assembled nanostructure at 250 °C, and subsequently converted to MCNs after carbonization at 800 °C. In addition, the well-defined MCN-based supercapacitor exhibited well specific capacitance and possessed an excellent cycling performance, which provided huge potential application as electrode material for supercapacitors.

Experimental

Materials

Methyl methacrylate (MMA), acrylonitrile (AN), dimethyl sulfoxide (DMSO), anhydrous diethyl ether, 2,2′-azobis(isobutyronitrile) (AIBN), N-methyl-2-pyrrolidone (NMP), potassium hydroxide (KOH), poly(vinylidene fluoride) (PVDF) and acetylene black were all purchased from Aladdin. The chain transfer agent, 4-cyano-4-ethyl-trithio-pentanoic acid (CETP), was synthesized according to the method reported in the literatures.^56,57 All chemicals were used as received without further purification. All aqueous solutions were prepared using deionized water with a resistivity of 18.2 MΩ cm⁻¹. All polymerization reactions were carried out in a high pure nitrogen atmosphere.

Methods

Synthesis of PAN via RAFT polymerization. For the synthesis of PAN homopolymer, AN (5.0 g, 0.09 mol), AIBN (5.2 mg, 0.03 mmol), and CETP (24.8 mg, 0.09 mmol) were mixed in a flask with DMSO (15 mL). The flask was sealed with a rubber plug and subsequently the resulting solution was purged with nitrogen for 0.5 h. Finally the flask was immersed into an oil bath preheated at 70 °C for 12 h. The obtained homopolymer was precipitated in anhydrous diethyl ether and this process was repeated for another two times to completely remove the unreacted monomer. The purified PAN homopolymer was characterized by GPC and ¹H NMR.

Synthesis of PAN-b-PMMA micelles using PAN as the macro-RAFT agent. The PAN-b-PMMA micelles were synthesized either in emulsion or solution conditions where the prepared PAN homopolymer was used as macro-RAFT agent. The synthesis of PAN-b-PMMA micelles was first performed in emulsion condition. Specifically, PAN (M_n = 40 000 g mol⁻¹, 0.05 mmol), MMA (1.9 g, 0.02 mol) and AIBN (3.3 mg, 0.02 mmol) were dissolved in 15 mL DMSO and 5 mL deionized water to afford a homogeneous solution. After deoxygenation with nitrogen for 0.5 h, the resulting mixture was placed in a preheated oil bath at 70 °C for 12 h. And subsequently the PAN-b-PMMA micelles were obtained through RAFT polymerization-induced self-assembly. The obtained PAN-b-PMMA micelles were dropwise added in deionized water at ambient temperature to homogeneous distribution. For comparison, the PAN-b-PMMA copolymer was also synthesized through RAFT solution polymerization. In RAFT solution polymerization procedure, DMSO (15 mL) was used as the solvent and other polymerization conditions and procedures were the same as the mentioned RAFT emulsion polymerization. Subsequently, the PAN-b-PMMA diblock copolymer obtained from the RAFT solution polymerization was dropwise added in deionized water with different temperature (ambient temperature 25 °C, 70 °C and 90 °C) and PAN-b-PMMA micelles were obtained through the self-assembly. The obtained PAN-b-PMMA micelles were characterized by the GPC and ¹H NMR.

Preparation of mesoporous carbon nanoparticles (MCNs). The resulting PAN-b-PMMA micelles prepared through the RAFT emulsion polymerization were stabilized at 250 °C for 6 h with a heating rate of 2 °C min⁻¹ in air for pre-oxidation and cross-linking, to afford black mesoporous carbon nanoparticles which were then pyrolyzed at 800 °C for 2 h under nitrogen gas flow. The resulting products had a Langmuir (nitrogen, 77 K) surface area of 307 m² g⁻¹.

Fabrication of the supercapacitor. The as-fabricated mesoporous carbon nanoparticles were exploited as electrode materials for the fabrication of supercapacitor. The working electrode was prepared through the following process: 85 wt% of the obtained mesoporous carbon nanoparticles (61.7 mg) was mixed with 10 wt% of acetylene black and 5 wt% PVDF binder in an agate mortar until homogeneous black powder was obtained. Then 5 mL NMP was added into the homogeneous black powder, followed by agitation. The resulting mixture was then dried in a vacuum oven at 50 °C for 5 h. In order to prepare a working electrode, the resulting products were embedded into nickel foam which was used as a current collector (surface area: 4.52 cm²) and then was pressed at 15 MPa for 10 s.

Electrochemical measurements of MCNs-based supercapacitor. In the three-electrode cell experiments, the calomel electrode and platinum electrode were used as the reference electrode and counter electrode, respectively. In addition, the as-fabricated MCNs were utilized for the preparation of working electrode and 6 M KOH aqueous solution as the electrolyte. All electrochemical measurements were carried out at room temperature. The test procedure of the electrodes was performed through an electrochemical working station (CHI-760D electrochemical workstation). Cyclic voltammetry and galvanostatic charge–discharge data were measured at various sweep rates. The relevant gravimetric capacitance was calculated from galvanostatic charge–discharge curves by the following eqn (1):where C_m (F g⁻¹) is the specific capacitance, I (A) is the constant current density, Δt (s) is the discharging time, ΔV (V) is the discharging voltage change, and m (g) is the mass of only the carbon materials used for the working electrode. The discharge voltage window includes the IR drop due to the difficulty in identifying the IR drop region. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 10⁵ Hz to 10⁻² Hz at the open circuit potential with an AC perturbation of 5 mV.

Characterization. The morphology of the as-synthesized products was observed using a JEM-1200EX transmission electron microscope (TEM) operating at 120 kV. Scanning electron microscope (SEM) images were acquired on a JEOL JSM-6700F scanning electron microscope by using an accelerating voltage of 5 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Kratos Axis Ultra DLD spectrometer employing a monochromated Al Kα X-ray source (h = 1486.7 eV), hybrid (magnetic/electrostatic) optics, and a multi-channel plate, and delay line detector. Spectra were recorded at a takeoff angle of 35 (angle between the plane of the sample surface and the entrance lens of the analyzer) with pass energy of 150 eV. ¹H NMR spectra were recorded in DMSO-d₆ using a JNM-ECP 600 spectrometer (600 MHz). Fourier-transform infrared (FT-IR) spectroscopy was obtained using Tensor 27 produced by Bruker Corporation. Gel permeation chromatography (GPC) was used to determine number average molecular weight (M_n) and M_w/M_n values. Molecular weight (M_w) and polydispersity index (PDI) were recorded on a Shimadzu modular system, comprising a DGU-12A solvent degasser, an LC-10AT pump, a CTO-10A column oven, and a RID-10A refractive index detector. The analyses were operated at 50 °C at a flow rate of 1 mL min⁻¹ using THF as the eluent. THF was used as the solvent to dissolve polymer. Thermogravimetric analysis (TGA) was carried out on a Mettler Toledo model TGA-SDTA851e Thermogravimetric Analyzer in a nitrogen atmosphere from 25 °C to 800 °C at a heating rate of 10 °C min⁻¹. The specific surface area was measured at 77 K using the Brunauer–Emmett–Teller (BET) method based on nitrogen adsorption–desorption isotherms obtained with a Micromeritics TriStar II 3020 sorption analyzer. The crystallographic structures of the carbon materials were determined by a powder XRD system (DX-2700, λ = 0.15406 nm).

Raman spectra were obtained using RENISHAW RM2000 Raman system.

Results and discussion

Synthesis of PAN-b-PMMA micelles

PAN homopolymer was synthesized via RAFT polymerization and purified with anhydrous diethyl ether. The as-prepared PAN homopolymer was analyzed by ¹H NMR and GPC. The M_n of PAN was measured to be 40 000 g mol⁻¹ and the polydispersity index (PDI) of the homopolymer was approximately 1.18. The as-prepared PAN was utilized as macro-RAFT agent for the co-polymerization of MMA with the feed ratio of [MMA] : [PAN m-CTA] : [AIBN] = 400 : 1 : 0.333 (Scheme 1). The PDI of the purified PAN-b-PMMA diblock copolymer was 1.21 and the M_n was measured to be 56 000 g mol⁻¹ by GPC, indicating a well-controlled RAFT polymerization, which was consistent with the known traits of living radical polymerization. In the micellization process, due to the insolubility of the PMMA segment in the PAN-b-PMMA diblock copolymer, the PAN-b-PMMA micelles were obtained through self-assembly with PMMA segments as the core and PAN segments as the corona.

Scheme 1 Schematic representation for the synthesis of PAN-b-PMMA micelles and the formation of mesoporous carbon nanoparticles.

The successful preparation of PAN homopolymer and PAN-b-PMMA micelles were evidenced by ¹H NMR. As shown in Fig. 1a and b, the proton resonance signal at 2.13 ppm and 3.12 ppm can be corresponded to –CH₂– and –CH(CN)– protons of the PAN segment. The signals at 0.72–0.91 ppm, 2.01 ppm and 3.54 ppm should be ascribed to the protons of –CH₃, –CH₂– and –COOCH₃ from the PMMA segment, respectively. In addition, the as-prepared PAN homopolymer and PAN-b-PMMA micelles were further confirmed by FT-IR spectrum as shown in Fig. 1c. The absorption peak at 2245 cm⁻¹ was assigned to the –CN stretching vibration of PAN segment. Peak signals at 1730 cm⁻¹ and 1152 cm⁻¹ should be resulted from the –C=O group and –C–O–C group of PMMA segment, respectively. The foregoing results gave strong evidence of the successful preparation of the PAN-b-PMMA micelles.

Fig. 1 (a) ¹H NMR spectrum of PAN homopolymer in DMSO-d₆ (298 K, 600 MHz). (b) ¹H NMR spectrum of PAN-b-PMMA micelles in DMSO-d₆ (298 K, 600 MHz). (c) FT-IR spectra of the PAN homopolymer and PAN-b-PMMA micelles. (d) TGA curve of the PAN-b-PMMA micelles.

In addition, the PAN-b-PMMA micelles were analyzed using TGA under nitrogen atmosphere from ambient temperature up to 800 °C with a heating rate of 10 °C min⁻¹. As shown in Fig. 1d, it was obviously observed that the TGA curve of the micelles underwent three clearly stages during the heating process. In the first stage at 200–280 °C, PAN-b-PMMA micelles began to decompose with a mass loss of 4.2 wt%, which was attributed to the cross-linking and dehydrogenation of polymer micelles. Without the stabilization step, the competition between chain scission and cyclization during the pyrolysis process would lead to the destruction of the polymeric structure. In addition, the original structure was well-protected in this step. The second stage appeared at 320–430 °C and the total loss was approximately 50 wt%, which should be ascribed to the decomposition of the PMMA segment. As can be seen from Fig. 1d, the last stage was observed at 440–800 °C, during which the total mass loss was 7 wt%, indicating that the carbonization of PAN was accompanied by a further dehydrogenation and partial denitrogenation process.^58–60

In present work, the micelles were formed via self-assembly of the block copolymer PAN-b-PMMA, which were synthesized via either RAFT emulsion polymerization or solution polymerization. The micelles synthesized through the RAFT emulsion polymerization were characterized by TEM (Fig. 2a). As can be seen, the diameter of the homogeneous polymer microspheres was approximately 120 nm and exhibited excellent microspheres structure. For comparison, it was found that the morphologies of the micelles self-assembled from PAN-b-PMMA synthesized via RAFT solution polymerization could be manipulated by the solution temperatures as confirmed by TEM images. When the self-assembly temperature increased, the morphology gradually evolved from vesicles to porous-microspheres as shown in Fig. 2b–d. When the temperature increased from ambient temperature to 90 °C, the micelles eventually formed with uniform size. It was observed that the micelles self-assembled at 90 °C exhibited spherical structures with better homogeneity, indicating that the self-assembly temperature played an important role in controlling the morphology of the micelles. It was also demonstrated that the morphology of micelles self-assembled from PAN-b-PMMA synthesized via RAFT solution polymerization were uncontrollable and poor reproducibility. In addition, the micelles prepared through the RAFT emulsion polymerization exhibited the excellent morphology as confirmed by TEM image (Fig. 2a). Therefore, PAN-b-PMMA micelles prepared through the RAFT emulsion polymerization were chosen as the material for the following characterization and preparation for the supercapacitor.

Fig. 2 (a) The TEM image of PAN-b-PMMA micelles synthesized via RAFT emulsion polymerization. The TEM images of PAN-b-PMMA synthesized via RAFT solution polymerization self-assembled at (b) ambient temperature 25 °C, (c) 70 °C, (d) 90 °C.

The morphology of the PAN-b-PMMA micelles prepared via RAFT emulsion polymerization was further characterized by SEM. As shown in Fig. 3, the as-prepared micelles exhibited an average diameter of approximately 120 nm. At a higher magnification, uniform clew-like microspheres were clearly observed in Fig. 3b. The exterior surfaces of microspheres were rough and clew-like. On the basis of various structural characterizations, the micelles were successfully prepared using PAN-b-PMMA.

Fig. 3 The SEM images of the PAN-b-PMMA micelles prepared via RAFT emulsion polymerization at different magnifications.

Synthesis of MCNs

Based on the well morphology of PAN-b-PMMA micelles prepared via RAFT emulsion polymerization, these micelles can be selected as the ideal precursors for the preparation of MCNs and the following application as supercapacitor electrode material. As shown in Scheme 2, the nanoporous carbon materials can be prepared from the micelles via a three-step mechanism: dehydrogenation and cyclization, thermal stabilization and carbonization.^59–62 The original structure was well-protected due to the thermal stabilization process of PAN at 250 °C in air. Without the stabilization step, the competition between chain scission and cyclization during the pyrolysis step would lead to the destruction of the polymeric structure. Eventually, PAN was carbonized into graphitic framework upon further dehydrogenation and partial denitrogenation at 800 °C.⁶³

Scheme 2 A proposed mechanism for the preparation of mesoporous carbon nanoparticles (MCNs) from PAN-b-PMMA micelles.

The morphology and structure of MCNs were investigated by SEM and TEM. As shown in Fig. 4a and b, the porous carbon nanoparticles exhibited a framework structure and the spherical nanoparticles were randomly distributed in the framework. Particularly, it could be seen that the spherical nanoparticles were porous shown in Fig. 4c. It was also evidenced by TEM shown in Fig. 4d that the micelles after carbonization became porous. The porous structure will be beneficial to the electrolyte penetration and accelerate the kinetic process of the ion transfer within the electrode materials.^27–64,82 Fig. 4e illustrated a high magnification TEM image of carbon nanoparticles. Highly ordered graphite lattices were clearly observed, indicating that the MCNs were partially consisted of crystal carbon structure. The energy dispersive X-ray spectroscopy (EDX) was also performed on the carbon nanoparticles to map the elements, which indicated that the carbon nanoparticles were enriched with C, N and a small quantity of O and S. The EDX analysis in Fig. 4f also revealed that the contents of C, N, O, and S were 77.24%, 19.34%, 3.1%, and 0.33%, respectively.

Fig. 4 The SEM images of MCNs at varied magnifications (a–c). (d) TEM image of MCNs. (e) HR-TEM image and (f) EDX pattern of MCNs.

The structure of the as-fabricated carbon nanoparticles was characterized by Raman spectroscopy and XRD. As shown in Fig. 5a, the broad peak at 1352 cm⁻¹ (D band) was correlative to the disordered sp³ carbon atoms, whereas G band at 1589 cm⁻¹ was interrelated to sp² carbon atoms. D band and G band indicated that the primary PAN segment was transformed into amorphous carbon with nano-graphitic structure.^61,65 In addition, the broad Raman peak at around 2800 cm⁻¹ was related to the G′ band which was a typical characteristic of undisturbed or highly ordered graphitic lattices.⁶⁶ The intensity ratio of D band to G band (I_D/I_G) depends on the type of graphitic materials and reflects the graphitization degree of the synthesized carbon materials.⁶⁷ The intensity ratio of I_D/I_G was calculated to be 1.58, based on the complete peak areas under the D and G bands. In order to further investigate the structure of the obtained material, the MCNs were characterized using the XRD. As shown in Fig. 5b, the 002 peak centered at around 24° and the 100 peak centered at around 44° were observed from the XRD spectrum of the prepared mesoporous carbon nanoparticles. The broad Bragg peak in XRD pattern evidenced the presence of nano-graphitic structure in pyrolytic carbons.⁶⁸

Fig. 5 (a) Raman spectrum and (b) XRD pattern of MCNs prepared with PAN-b-PMMA micelles (M_n: 56 000 g mol⁻¹ by GPC, PDI: 1.21). XPS spectra of PAN-b-PMMA micelles and MCNs: (c) survey spectrum of PAN-b-PMMA micelles; (d) survey spectrum of MCNs; (e) N 1s spectrum of MCNs; (f) C 1s spectrum of MCNs.

XPS has been recognized as a powerful tool in atomic and element analysis. Therefore, the as-prepared PAN-b-PMMA micelles and the MCNs were characterized by the XPS as well. By comparing Fig. 5c with Fig. 5d, it was found that the content of nitrogen decreased significantly, whereas the content of carbon increased after carbonization. In addition, the MCNs were further manifested by N 1s spectrum (Fig. 5e) and C 1s spectrum (Fig. 5f). The N 1s spectrum was deconvoluted into two different peaks centered at 398.2 eV and 400.8 eV, which corresponded to N-6 and N-5 groups, respectively.⁶⁰ The C 1s spectrum could be fit into two strong peaks at 284.5 eV and 285.0 eV and a small peak at 286.2 eV, which were assigned to C–C, C=C and C–N groups, respectively.^59,60 These results further confirmed the formation of graphitic carbon materials with little nitrogen doping.^27,69

Electrochemical performance

In order to study the electrochemical performance of the as-prepared MCNs, the cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques were performed in 6 M KOH aqueous electrolyte on the basis of three-electrode system (Fig. 6). The CV curves and GCD curves in 6 M KOH revealed nearly rectangle shapes at a potential setup parameter from −1 V to 0 V, respectively, indicating that the working electrode possessed well capacitive behavior in ion transfer and charge propagation. In addition, the values of capacitance of the prepared supercapacitor based on MCNs lie in the range of capacitances typically reported for carbon-based supercapacitors (50–200 F g⁻¹).^59,81

Fig. 6 Electrochemical characterization of the supercapacitor prepared using MCNs as working electrode in 6 M KOH aqueous electrolyte: (a) CV curves at different scan rates from 5 mV s⁻¹ to 100 mV s⁻¹ at a potential window from −1 to 0 V; (b) galvanostatic charge–discharge curves at different current densities from 0.05 A g⁻¹ to 0.5 A g⁻¹; (c) EIS curve measured in the frequency range from 10⁻² Hz to 10⁵ Hz at the open circuit voltage with an alternate current amplitude of 5 mV and the inset represents the magnification of local area; (d) cycling performance of the MCNs working electrode in 6 M KOH aqueous electrolyte and the inset represents the galvanostatic charge–discharge curves before and after 10 000 cycles at the same current density.

The CV curves of the MCNs-based supercapacitor were nearly rectangle shape without obvious redox peaks from −0.4 V to 0.6 V (Fig. 6a), suggesting a typical electric double layer capacitance and good charge transport within the MCNs electrode. The distortion of the CV curves at a high scan rate was due to the migration of the electrolyte ions and limited diffusion in the bulk of materials which was a common disadvantage of carbon-based electrode materials.²⁷ Fig. 6b showed the galvanostatic charge–discharge curves of MCNs-based supercapacitor at different current density and all of the curves displayed isosceles triangle shape without plateaus, indicating its ideal electric double-layer capacitive behavior. The specific capacitance (C_g) was found to be 220 F g⁻¹ at 0.05 A g⁻¹ according to eqn (1). As comparison with other carbon materials, the electrode based on the MCNs exhibited well specific capacitance (Table 1). The EIS technique was used to investigate the kinetic process of the MCNs electrode (Fig. 6c). The EIS results elucidated a relatively low contribution from the resistive component to the device impedance, with the equivalent series and parallel resistances equal to 0.3245 Ω and 0.0589 Ω, respectively. These low resistance values indicated the good rate capabilities of the carbon working electrode. Accordingly, the specific capacitance C_g measured at low frequency (10⁻² Hz) was calculated from the EIS data using the equation, C_g = −1/(2πfZ′′) where f was the frequency. The slope in the low frequency locality declared the ion diffusive resistivity at the electrode/electrolyte interface which was nearly vertical, indicating pure capacitor behavior unimpeded by the resistance.

Table 1 Comparison of the specific capacitance based on different carbon materials electrode

Sample	Specific capacitance (F g^−1)	Ref.
MCNs	220 (0.05 A g^−1)	This work
PC14.5-0-12.5	245 (0.05 A g^−1)	70
NCFs	52 (1 mA cm^−2)	71
HCF	63.1 (0.08 mA cm^−2)	72
CTNC	166 (0.1 A g^−1)	59
YS-HCSs	215 (1 A g^−1)	73
HSCMC-5	254 (0.5 A g^−1)	27
CF-6	283 (1 A g^−1)	74
HPGBAr	321 (0.05 A g^−1)	75
MC-1	208 (0.5 A g^−1)	76
m-WCM	234 (5 mA cm^−2)	77
m-CM6-8-120	232 (5 mA cm^−2)	78
FDU-15	130 (0.5 A g^−1)	79
HPC4.2-16.8-6C	194 (0.1 A g^−1)	80
CNFs	140 (10 mV s^−1)	28

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The variation of specific capacitance with cycle number for the MCNs-based supercapacitor was shown in Fig. 6d. The specific capacitance gradually descended with the increasing cycle number, and the specific capacitance still retained ∼91% of the primary figure after 10 000 cycles of testing. The results illustrated that this MCNs-based supercapacitor owned long lifetime and high degree of reversibility in the repetitive charge–discharge cycling. All results demonstrated that the prepared MCNs-based electrode had the well supercapacitive performance, which could envision huge potential application for EDLSs.

Conclusions

To sum up, the RAFT-controlled synthesis of block copolymer, PAN-b-PMMA was successfully conducted either in solution or emulsion conditions. And it was found that the PAN-b-PMMA micelles with well-defined morphology can be directly formed from the emulsion polymerization. The as-synthesized PAN-b-PMMA micelles underwent a microphase-separation process to form a nanostructure at 250 °C, and were eventually converted to MCNs via carbonization at 800 °C. The electrode based on the MCNs displayed well specific capacitance of 220 F g⁻¹ and an excellent cycling performance (91% capacitance retention after 10 000 cycles at a current density of 0.05 A g⁻¹). The obtained MCNs could envision broad practical applications in energy devices and nanotechnology.

Acknowledgements

This work was supported by Qingdao Innovation Leading Talent Program, the NSF of China (51173087) and Qingdao (12-1-4-2-2-jch) and Taishan Scholars Program.

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