MnO2 Nanoflake-Shelled Carbon Nanotube Particles for High

Research Article pubs.acs.org/journal/ascecg

MnO2 Nanoflake-Shelled Carbon Nanotube Particles for HighPerformance Supercapacitors Donghee Gueon and Jun Hyuk Moon* Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742, South Korea

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S Supporting Information *

ABSTRACT: We introduce MnO2 nanoflake/carbon nanotube (CNT) core−shell particles for high-performance supercapacitors. The CNT particles prepared by drying the CNT-dispersed aerosol produce a tightly intertwined CNT assembly by internal capillary force, and the subsequent growth of MnO2 on the CNT surface produces a high surface area MnO2 nanoflake shell. We control the amount of MnO2 decoration on the CNT particles and obtain a specific capacitance of 370 F/g at current density of 0.5 A/g upon their supercapacitor electrode application. This capacitance is 14 times higher than that of bare CNT particles and 3 times higher than that of bare MnO2 particles. An asymmetric capacitor based on the MnO2/ CNT particle is assembled. The capacitor reveals a remarkably high power density of 225 W/kg. This performance is attributed to the contribution of the high pseudocapacitance of a compact MnO2 nanoflake and the high electrical conductivity of CNT particles with compact packing. KEYWORDS: CNT particles, Compact packing, MnO2 nanoflakes, Supercapacitors, Asymmetric capacitors, Pseudocapacitance

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cm).36−38 In particular, the high tendency of entanglement of CNTs due to their high aspect ratio and tubular morphology enables the facile formation of a freestanding electrode film.10,39−41 Various outcomes have been achieved with composites containing CNTs. For example, CuO nanobelts have been mixed with CNTs to produce flexible hybrid electrodes.42 Thin film electrodes consisting of polyaniline nanofibers and CNTs have been created by performing the layer-by-layer coating of these materials; these electrodes have a nanoscale interpenetrating network structure with well-defined nanopores and exhibit excellent electrochemical performances.43 The simple filtering of a mixture of V2O5 nanowires and CNTs was used to prepare composite paper electrodes.44 Electrodes consisting of MnO2-coated CNTs have been prepared with a modified one-pot reaction process and subsequent drying. Noted that these approaches use CNTs to produce flexible electrodes or/and to provide electrical conductivity and that these composites were prepared through solution or slurry casting, which results in film-like morphologies. Further, these studies did not focus on the engineering of the morphology of CNT assemblies. In the method of the present study, MnO2 nanoflake shells are grown on the surfaces of entangled spherical CNT particles. The spherical CNT particles are prepared by drying CNTdispersed aerosol droplets. In contrast to the CNT film supports that have been previously been reported, the density

INTRODUCTION The performance of supercapacitors is determined by the nanoarchitecture of the electrode materials as well as their intrinsic properties.1−6 Recent approaches to the fabrication of high-performance supercapacitors have utilized composites of carbonaceous materials (e.g., activated carbon,7 mesoporous carbon,8 carbon nanotubes (CNTs),9,10 and graphene11−13) and various transition-metal oxides (e.g., RuO2,14 NiO,15−17 Fe2O3,18,19 Co3O4,20,21 V2O5,22−25 and MnO21,26−28) to harness their pseudocapacitance and electrical double layer capacitance. In particular, many previous studies have used carbonaceous materials as the matrixes or supports and coated them with various pseudocapacitive materials.1,4,14,18,20,22,23,29,30 Thus, the carbon material can not only provide a high electrical conductivity for pseudocapacitive materials, which have poor conductivity but can also provide a skeleton on which to grow or disperse nanosized materials.1−4 Although the components of these composite materials are very similar, they have a variety of nanoarchitectures. For example, various carbon/MnO2 nanoarchitectures, such as graphene/MnO230−32 MnO2/graphitic carbon spheres,33 CNT-MnO2 core−shell tubes,34 MnO2 nanocrystals/hierarchically porous carbon,35 and MnO2-coated vertical carbon nanofibers4 have been demonstrated. Note that the specific capacitances of these carbon/MnO2 nanoarchitectures range from 190 to 360 F/g (see Table S1); this large variation establishes the crucial contribution of the carbon matrix. Of the various carbonaceous materials, CNTs are widely used as carbon supports because of their high intrinsic surface-tovolume ratio and high electronic conductivity (∼104 S/ © 2017 American Chemical Society

Received: November 20, 2016 Revised: December 29, 2016 Published: January 30, 2017 2445

DOI: 10.1021/acssuschemeng.6b02803 ACS Sustainable Chem. Eng. 2017, 5, 2445−2453

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ACS Sustainable Chemistry & Engineering

Figure 1. (up) Schematic fabrication of MnO2/CNT core−shell particles. (down) SEM images of (a) CNT particles, (b) low-MnO2/CNT core− shell particles, and (c) high-MnO2/CNT core−shell particles. High-magnitude SEM images of (d) CNT particles, (e) low-MnO2/CNT core−shell particles, and (f) high-MnO2/CNT core−shell particles. (g) SEM images of the internal space of the MnO2/CNT core−shell particles. used without further purification. The CNTs were multiwalled with an outer diameter of 20 nm and an average length of approximately 10 μm. The CNT spherical particles were obtained using a spray-dryer; the aqueous dispersion of CNTs was sprayed at a speed of 5 mL min−1, and the inlet temperatures were set to 125 °C. The CNT particles were heat-treated in a furnace under inert conditions at 500 °C for 2 h to remove the surfactants used for CNT dispersion. Synthesis of MnO2/CNT Core−Shell Particles. The MnO2 coating on CNT particles was obtained by the reduction of KMnO4 in the presence of CNT particles. Briefly, 10 mg of the prepared CNT particles was dispersed in DI water, and 10 mL of KMnO4 (80 mM) was added to the solution. Then, 0.5 μL of H2SO4 was added to the mixture and stirred for 1 h. The reaction was performed at 80 °C, and the reaction time (30 or 60 min) was varied to control the amount of MnO2 coating. After the reaction, the precipitated particles were washed with DI water several times and were dried in a convection oven at 60 °C. For comparison, pure MnO2 was also prepared by the same procedure without the presence of CNT particles. Electrochemical Measurement. A beaker-type, three-electrode system was used to measure the electrochemical properties of the sample. The three-electrode cell was assembled with MnO2/CNT core−shell particles coated on a glassy electrode as the working electrode, a saturated Ag/AgCl electrode (3 M of NaCl) as the reference, and a Pt rod as the counterelectrode. The working electrode with a geometric surface of 1 cm2 contained approximately 1.3 mg of MnO2/CNT composite. We used a 1.0 M of Na2SO4 (Aldrich) solution as an electrolyte solution. Cyclic voltammetry (CV) and galvanostatic charge−discharge curves were measured by VersaSTAT 3 (AMETEK). CV was performed over the voltage range of 0−1 V versus Ag/AgCl (3 M of NaCl) with a range of scan rates from 5 to 100 mV/s. The galvanostatic charge−discharge measurement was performed under a voltage range between 0 and 1 V with a constant current of 0.5−5 A/g The electrochemical impedance analysis of cells was obtained by electrochemical impedance spectroscopy (EIS) using an impedance analyzer (Versastat, AMETEK). The measurement was carried out in the frequency range of 1 MHz to 0.1 Hz with voltage

of CNTs in these particles is high, which results from the internal capillary force that arises during the drying of the droplets.45 This highly intertwined CNT morphology provides a CNT assembly with high electrical conductivity. This morphology also induces the growth of MnO2 nanoflakes rather than large aggregates of MnO2 particles and, thus, results in a high surface area and thereby a high specific-energy density. Moreover, this composite contains well-defined hierarchical pores among its particles as well as CNTs, which is favorable to ion transport through the copposite electrode film. We compared this MnO2/CNT particle film and a conventional MnO2/CNT film and found that the former film exhibits a 84% higher specific capacitance and a 66% higher capacitance retention with a 20-fold increase in the current density, which clearly confirms the advantages of the CNT particle morphology of the film (see Figure S1). In this study, we prepared MnO2/CNT core−shell particles with various thicknesses of MnO 2 coating. We achieved a specific capacitance of 370 F/g at 0.5 A/g with a composite consisting of 85 wt % MnO2/CNT core−shell particles, which is 14 times higher than achieved with bare CNT particles and higher than the restuls of previous studies for CNT/MnO2 composite electrodes.4,34,46,47 We also fabricated an asymmetric capacitor by using MnO2/CNT core−shell particles. The asymmetric capacitor was found to exhibit an energy density of 27 Wh/kg and a power density of 225 W/kg. This power density, in particular, is much higher than those obtained in previous studies of MnO2/C composites.33,34,39,48

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EXPERIMENTAL PROCEDURES

Preparation of CNT Spherical Particles. An aqueous dispersion of carbon nanotubes (CNTs, 5 wt %, purchased from Wonil Co.) was 2446


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Figure 2. TEM image of (a) MnO2 shell and (b) nanoflakes of high-MnO2/CNT particles. (c) High-resolution TEM image of MnO2 shell of highMnO2/CNT particles. amplitude of 10 mV. The asymmetric supercapacitor was assembled by using two pieces of carbon-coated Al foil electrodes and a filter paper separator. MnO2/CNT electrode was prepared by mixing 70 wt % MnO2/CNT core−shell particles, 15 wt % acetylene black and 15 wt % polyvinylidene fluoride(PVDF) in N-methyl-2-pyrrolidone (NMP). The AC electrode was prepared by the same procedure with mixing AC instead of MnO2/CNT core−shell particles. The electrolyte used was 1 M Na2SO4 aqueous solution. CV and galvanostatic-discharge curves of two-electrode system were measured by VersaSTAT 3 (AMETEK). All the operating current densities were calculated based on the mass of active materials. Material Characterization. Scanning electron microscope images were obtained using a field-emission scanning electron microscope (FESEM, Carl Zeiss, SUPRA 55 VP). Transmission electron microscopy (TEM) images were obtained using a transmission electron microscope (JEM-3010, JEOL). Raman spectra were collected using a Horiba Jobin Yvon LabRAM HR equipped with an air-cooled, Ar-ion laser that was operated at 514 nm. X-ray photoelectron spectroscopy (XPS) was performed using a Leybold spectrometer with an Al Kα monochromatic beam (1486.6 eV) that has an input power of 150 W (ESCALAB250 XPS system, Theta Probe XPS system). Xray diffraction (XRD) patterns were obtained using a Davinci D8 Advance diffractometer using Cu−Kα radiation; the samples were scanned between 5 and 80° at a scan rate of 0.04°/s. A thermogravimetric analysis (TGA) was conducted by heating the sample to 800 °C in air at a heating rate of 10 °C/min (TA Instruments TGA Q50).

Figure S2). Deposition by the reduction of KMnO4 is achieved by two-stage growth; at the initial stage, fast nucleation forms amorphous particles, and these particles aggregate favorably in a certain crystal plane and then coalesce to the sheetlike morphology through Ostwald ripening.49−51 The SEM image of the CNT particles shows a size distribution between 0.5 and 5 μm, as observed in Figure 1a. The magnified SEM image shows a highly entangled CNT assembly, as shown in Figure 1d. The SEM images of the MnO2-coated CNT particles exhibit a compact MnO2 nanoflake layer deposited on the particle surface, as shown in Figure 1b and c. We controlled the growth time of MnO2; with increasing time, the MnO2 nanoflake was enlarged and was more connected with neighboring nanoflakes. The MnO2 nanoflake has approximately 270 nm diameter and 30 nm thickness after 30 min of growth (see the SEM image of Figure 1e), and the diameter and thickness were increased to 500 and 70 nm, respectively, after 60 min of growth (see the SEM image of Figure 1f). The samples with MnO2 deposition for 30 and 60 min were designated as low-MnO2/CNT and high-MnO2/ CNT core−shell particles, respectively. The weight fraction of MnO2 for each MnO2/CNT particle was determined by TGA measurement (see Figure S3); based on the weight after decomposition of CNT, the low- and high-MnO2/CNT core− shell particles contained 70 and 85 wt % MnO2, respectively. The cross-sectioned SEM image of the MnO2/CNT core−shell particles is shown in Figure 1g, respectively. The images reveal that the MnO2 nanoflakes dominantly coated the particle surface, and there was a particle-like MnO2 residue. The reaction inside the particle is limited due to the small cavity among the CNTs and the relatively low diffusion of precursors. In Figure 2a and b, the TEM image shows two-dimensional flaky structure of MnO2 grown on the CNT surface. The high resolution TEM image reveals the MnO2 crystals with the interplanar spacing of 0.241 nm, which corresponds to the spacing of the (200) plane of birnessite-type MnO2.51 The XRD patterns of the CNT particles and MnO2/CNT core−shell particles were measured. The XRD patterns of bare CNT particles displayed a strong diffraction peak at 2θ = 26°, which is a representative peak of graphitic carbon, as observed in Figure 3a.34 The MnO2/CNT core−shell particles had peaks at 12°, 24°, 37°, and 66°, which correspond to the (001), (002), (111), and (312) planes of birnessite-type MnO2 (JCPDS 42-1317, δ-MnO2), respectively.34,52 The reduction of MnO4− produces birnessite-type manganese oxide. Birnessite MnO2 is a layered manganese oxide formed by [MnO6] octahedral sharing edges, with alkaline cations and water molecules between layers. The Raman spectra of MnO2/CNT core−shell particles exhibited peaks in the range of 500−700

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RESULTS AND DISCUSSION The fabrication of the MnO2 nanoflake-shelled CNT particles is shown in Figure 1. A CNT particle was prepared by drying CNT-dispersed aerosol droplets; convective drying induced the assembly of CNTs. The droplet was dried quickly within few seconds at 125 °C. The CNT assembly particles had an irregular surface compared to the CNT spherical particles prepared by emulsion drying, which was due to the fast drying process. The spray drying process can meet requiring development of scalable manufacturing; we obtain a high throughput speed of ∼12 g/h or >20 g of sample within a 2 h. The MnO2 coating was achieved by the reduction of KMnO4. Briefly, KMnO4 oxidized the graphene sheet of multiwalled CNTs under acidic conditions and partly exfoliated the surface, whereas KMnO4 was reduced to MnO2 and MnO2 was deposited on the CNT surface. The overall reaction was described by the following: 4KMnO4 + 3C + 2H 2SO4 ↔ 4MnO2 + 3CO2 + 2K 2SO4 + 2H 2O

(1)

Here, we observed that the particulate MnO2 was deposited at the initial stage and subsequently grew into the nanoflake morphology (see the SEM images for the initial growth in 2447


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Figure 4. XPS spectra of (a) Mn 2p and (b) Mn 3s.

particles displayed a rectangular shape, even at high scan rate, indicating good electrochemical capacitor behavior, as observed in Figure 5a. The low-MnO2/CNT core−shell particles also displayed a rectangular shape but exhibited a weak redox peak, as observed in Figure 5b, whereas the high-MnO2/CNT core− shell particles displayed more prominent peaks, as observed in Figure 5c. The apparent redox peak may be attributed to the fact that high-MnO2/CNT core−shell particles possessed higher coverage of the larger MnO2 nanoflakes compared to the low-MnO2/CNT core−shell particles. First, the presence of the redox peak indicates the presence of a redox reaction on the MnO2 surface; the redox reaction involves the manganese ions Mn4+ and Mn3+.52 The reaction includes the insertion-desertion of the alkali cations or protons from the electrolyte, which is described by MnO2 + X+ + e− ↔ MnOOX, where X+ is Na+ or H+.54,58 Second, the shape of the CV curves is maintained with an increasing scan rate, as observed in Figure 5b and 5c, indicating a strong bonding of MnO2 nanoflakes on the CNT surface and low enough interfacial resistance between MnO2 and CNTs. Third, the current of the MnO2/CNT core−shell particles over the scanning potential range was higher than the CNT particles, and the high-MnO2/CNT core−shell particles exhibited a higher current than the low-MnO2/CNT core−shell particles. The high current density of the high-MnO2/CNT core−shell particles indicates much higher capacitance compared to the low-MnO2/CNT core−shell particles and CNT particles, The galvanostatic charge−discharge measurement of bare CNT and low- and high-MnO2/CNT particle electrodes was conducted in the potential range from 0 to 1 V at current densities of 0.5, 1, 2, 5, and 10 A/g. The MnO2 particles prepared without CNT particles were also tested for comparison. The charge/discharge curves at 0.5 A/g are displayed in Figure 6a. Here, the specific capacitance is calculated from the discharge cycle of a typical voltage−time response curve using the equation: C = (iΔt)/(ΔVm), where C is the specific capacitance obtained from the discharge cycle under constant current charge/discharge measurements, i is the

Figure 3. (a) XRD patterns of the CNT particles and the low- and high-MnO2/CNT core−shell particles. (b) Raman spectra of the lowand high-MnO2/CNT core−shell particles and the MnO2 particles.

cm−1 related to MnO2,53 as observed in Figure 3b. The bands at 565 and 648 cm−1 were attributed to the ν3(Mn−O) stretching vibration in the basal plane of MnO6 sheets and the symmetric ν2(Mn−O) stretching vibration of MnO6 groups, respectively. X-ray photoelectron spectroscopy (XPS) spectra provide the oxidation state (or valent states) of the synthesized MnO2 within the MnO2/CNT core−shell particles. The Mn 2p spectra showed peaks at binding energies of 642 and 653 eV, which correspond to Mn 2p3/2 and Mn 2p1/2, respectively (see Figure 4a). The separation of these peaks was approximately 11.8 eV, which indicates the presence of the Mn4+ oxidation state.54,55 The Mn 3s spectra displayed two peaks of the Mn 3s doublet, as shown in Figure 4b. The energy separations of the Mn4+ doublet and Mn3+ doublet were reported to be 4.78 and 5.41 eV, respectively. Here, the separation was approximately 4.91 eV, representing the coexistence of trivalence and tetravalence. Some of the Mn4+ ions at the center of the MnO6 octahedra were replaced by Mn3+ ions, producing a net negative charge. The net negative charges were compensated by the cations, which led to the formation of the birnessite phase.34,55,56 The O 1s spectra was shown in Figure S4. We observed the Mn−O−C bonding, which indicates the chemical bonding of MnO2 and CNT particles.57 The MnO2/CNT core−shell particles were evaluated as an electrode for supercapacitors. The bare CNT particles were also prepared for comparison. The electrochemical performance was examined by CV using an aqueous Na2SO4 (1 M) solution as the electrolyte. The CV curves were recorded at scan rates of 5, 10, 20, 50, and 100 mV/s. The CV curves of the bare CNT 2448


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Figure 5. CV curves of (a) the CNT particles, (b) the low-MnO2/CNT core−shell particles, and (c) the high-MnO2/CNT core−shell particles measured at various scan rates from 5 to 100 mV/s in a 1 M of Na2SO4 electrolyte solution.

Moreover, radial growth of the MnO2 nanoflakes on the spherical CNT particles allowed for a radial cavity among the flakes, which enhanced the ion diffusion into the MnO2 layer. Thus, the synergistic effects of highly conductive CNT particles and the high specific pseudocapacitance of the MnO 2 nanoflakes layer resulted in high capacitor performance. The specific capacitance 370 F/g at 0.5 A/g for the high-MnO2/ CNT particle electrode was the highest level compared to the previous results of the MnO2/carbon composite electrode listed in Table S1. The capacitance retention after increasing the current density by 20-fold for the bare MnO2, low-MnO2/CNT, and highMnO2/CNT particle electrodes was 14, 76, and 68%, respectively, as observed in Figure 6b. The poor rate capability of the MnO2 particle indicates a high resistance of the MnO2, as reported elsewhere. The high capacitance retention of MnO2/ CNT core−shell particles may be attributed to low electrode resistance by the presence of the CNT scaffold. The ohmic voltage drops in the discharge curves for bare MnO2, highMnO2/CNT, and low-MnO2/CNT core−shell particles were 179, 141, and 14 mV, respectively (see Figure 6a). It is noted that the capacitance retention of the high-MnO2/CNT core− shell particles was the highest, compared to the previous results in Table S1. The cycle performance of the bare MnO2, low-MnO2/CNT, high-MnO2/CNT particle electrode was evaluated by measuring the capacitance over charging/discharging cycles up to 4000 cycles, as observed in Figure 7. The MnO2/CNT core−shell particles maintained the initial capacitance over the cycles (i.e., the capacitance retention was approximately 100% up to 4000 cycles), whereas the MnO2 particle electrode revealed a fast decay and exhibited only 7% of the initial capacitance. The fast fading of the capacitance in MnO2-based electrodes is caused

Figure 6. (a) Galvanostatic charge/discharge curves of the CNT particles, low- and high-MnO2/CNT core−shell particles, and MnO2 particles at current densities of 1 A/g. (b) Calculated specific capacitance of the CNT particles, low- and high-MnO2/CNT core− shell particles, and MnO2 particles at various current densities from 0.5 to 10 A/g.

constant current, Δt is the discharge time, ΔV is the potential range, and m is the mass of the sample.59 The capacitances from the measurement are shown in Figure 6b. The specific capacitance of the bare CNT and the low- and high-MnO2/ CNT core−shell particles at 0.5 A/g was calculated as 26, 147, and 370 F/g, respectively. Thus, the pseudocapacitance of the MnO2 nanoflake shell largely enhanced the overall specific capacitance. Compared to the specific capacitance of the bare MnO2, which was 127 F/g at 0.5 A/g, the high-MnO2/CNT core−shell particles exhibited approximately 3 times higher capacitance. These results, i.e., high specific capacitance for the composite electrode with a large amount of MnO2 coating, demonstrated the advantage of the CNT particle morphology and core/shell morphology.34 The closely intertwined CNTs in the CNT particle may provide highly electrically conductive supports. Comparing the Nyquist plot of MnO2/CNT particle film with MnO2 particulate film in EIS measurement, the charge transfer resistance of MnO2/CNT particle electrode are much lower than that of MnO2 particle (see Figure S5).

Figure 7. Cycle performance of the CNT particles, the low- and highMnO2/CNT core−shell particles and MnO2 measured at a constant current density of 2 A/g. 2449


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Figure 8. (a) CV curves of AC and MnO2/CNT electrodes performed in a three electrode cell in 1 M Na2SO4 aqueous solution at a scan rate of 10 mV/s. (b) CV curves of MnO2/CNT core−shell particles: AC asymmetric supercapacitor measured at different potential window at a scan rate of 50 mV/s. (c) CV curves of the asymmetric supercapacitor measured at different scan rates between 0 and 1.8 V. (d) Galvanostatic charge−discharge curves at different current densities. (e) Energy density vs power density of the MnO2/CNT core−shell particles: AC asymmetric supercapacitor in a Ragone plot. (f) Digital image of a red light-emitting diode (LED) lit by the asymmetric supercapacitor.

MnO2/CNT to AC electrodes were about 0.7, based on charge balance theory.33,50,63 The CV curves of the asymmetric cell exhibit the capacitive behavior with nearly rectangular shape without an apparent redox peak even at the increased operating voltage up to 2.0 V, as shown in Figure 8b. We also measured the CV at various scan rates of 5, 10, 20, and 50 mV/s as observed in Figure 8c. The CV curve maintains a rectangular shape even at a high scan rate of 50 mV/s, exhibiting good charge/discharge properties and rate capability. The charge− discharge curves at various current densities are shown in Figure 8d. The gravimetric capacitance (Ct) of the asymmetric supercapacitor was 60 F/g at a current density of 0.25 A/g based on the total mass of active materials in the two electrodes. The enlarged voltage window improves energy and power density. The energy density (E) and power density (P) of asymmetric supercapacitor can be calculated with the following equation. E = 1/2(CV2) and P = E/t, where C is the specific capacitance, V is the working voltage of the supercapacitor, and t is the elapsed times to discharge. A maximum gravimetric energy density of 27 Wh/kg with power density of 225 W/kg was obtained based on the total weight of both electrodes, as shown in the Ragone plot Figure 8e. The values are superior to those of MnO2-based carbon composite supercapacitor.33,34,39,48 This may be attributed to the

by the detachment of MnO2 during the ion-insertion/desertion and/or the dissolution of MnO2 into aqueous electrolyte solution.60,61 Thus, this result indicates that the MnO2/CNT core−shell particles possess sufficient bonding between the MnO2 nanoflakes (see Figure S6) and the CNTs as well as sufficient electrical conductivity to induce the reversible charging/discharging reaction. Finally, we assembled an asymmetric supercapacitor using the MnO2/CNT core−shell particle as the positive electrode and the activated carbon (AC) as the negative electrode. An asymmetric supercapacitor has an advantage of wide operating voltage and high energy density; an asymmetric capacitor achieve a cell voltage extending up to 2.0 V in an aqueous electrolyte.32,33,62 The CV curves of MnO2/CNT core−shell particles and AC electrodes in 1 M Na2SO4 aqueous solution as observed in Figure 8a. The AC electrode was measured within a potential window of −1 to 0 V (vs Ag/AgCl) and MnO2/CNT core−shell particle electrode was measured with in a potential window of 0 to 1 V (vs Ag/AgCl) at a scan rate of 10 mV/s. Both electrodes showed stable electrochemical performance at their potential windows and thus, it can be expected to widen the operation voltage to 2 V in their application to the asymmetric capacitor electrodes. Upon the assembly of asymmetric supercapacitors, we used the mass ratio of 2450


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hierarchical pores of MnO 2 /CNT core−shell particleassembled electrodes that facilitates an ion transport. We connected our prototype cells charged at 1.8 V in series to a red LED and successfully lighted it, as shown Figure 8f.

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CONCLUSIONS We prepared MnO2 nanoflake-shelled spherical CNT particles. The support consisting of CNT particles were prepared by drying CNT-dispersed aerosol droplets, which was found to produce a highly intertwined assembly of CNTs. These CNT particles were found to provide highly electrically conductive supports for the growth of high surface area MnO2 nanoflake shell. A MnO2/CNT core−shell particle composite was tested as a supercapacitor electrode; we obtained a specific capacitance of 370 F/g at a current density of 0.5 A/g. This MnO2/CNT core−shell particle electrode was also found to exhibit a capacitance retention of 76% with a 20-fold increase in the current density, and also to display remarkably stable cycle performance over 4000 cycles. We assembled an asymmetric supercapacitor based on a MnO2/CNT core−shell particle electrode. The device exhibited reversible charging/discharging at 1.8 V, and an energy density of 27 Wh/kg with a power density of 225 W/kg. Our design consisting of a highly dense CNTs support and a core−shell morphology provides a synergistic method for enhancing the electrochemical capacitance of MnO2-based supercapacitor electrodes. We believe that the proposed synthetic methodology can be extended to the fabrication of composites with other transition metal oxides that can be used as high performance supercapacitors.

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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/acssuschemeng.6b02803. Additional data including specific capacitance of particle and film, SEM/EDX images, TGA analysis, XPS O 1s spectra, Nyquist plots, comparison table (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] ORCID

Jun Hyuk Moon: 0000-0002-4776-3115 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the National Research Foundation of Korea (NRF) (grant nos. 20110030253 and 2016M3D3A1A01913254). The Korea Basic Science Institute is also acknowledged for the SEM and XPS measurements.

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MnO2 Nanoflake-Shelled Carbon Nanotube Particles for High

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