Scalable thermal synthesis of a highly crumpled, highly exfoliated and N-doped graphene/Mn-oxide nanoparticle hybrid for high-performance supercapacitors

Abstract

A scalable thermal method to synthesize a highly crumpled, highly exfoliated and N-doped graphene/Mn-oxide nanoparticle hybrid for high performance supercapacitors has been demonstrated. Reduction of graphene-oxide (GO), nanometer scale crumpling, high level of exfoliation, N-doping of graphene and decoration with Mn-oxide nanoparticles, each of which significantly contributes to a high specific capacitance (958 F g⁻¹ at 5 mV s⁻¹) in a synergetic way, are achieved in a single thermal process, thermal annealing of GO–Gly–Mn(NO₃)₂·4H₂O mixture at 500 °C followed by a rapid quenching with liquid nitrogen. N-doping of graphene is predominantly done in the form of pyrrolic-like and pyridine-like nitrogens, and Mn-oxide nanoparticles are formed on the surface of graphene as MnO₂ and Mn₃O₄. The nanometer scale crumpling of graphene sheets, which is achieved by rapid quenching of graphene in the presence of Mn-oxide nanoparticles on its surface, induces an exceptionally high degree of exfoliation of graphenes and prevents restacking of graphene sheets during a repeated charge–discharge process, providing a high specific surface area (1006 m² g⁻¹) and high cycle stability (94.1% retention after 1000 cycles), respectively. The simplicity of the synthesis process and the high performance of supercapacitors make it an easily scalable and industrially applicable method.

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1. Introduction

Supercapacitors, known as electrochemical capacitors, have attracted great attention due to their high power density, fast charge–discharge time and great cycle stability.^1–6 Such excellent properties of supercapacitors make them promising energy storage components in a wide range of applications such as mobile electronic devices,^6,7 hybrid electric vehicles,^1,5,7,8 memory backup systems⁶ and emergency power supplies.^5,9 However, the lower energy density of supercapacitors compared to that of conventional lithium-ion batteries prevents the use of supercapacitors in various high-energy applications.^1,5,10,11 To increase the energy density of supercapacitors, the specific capacitance and operating voltage should be increased since the energy density of the capacitor (E) depends on the specific capacitance (C) and cell operating voltage (V), E = 1/2(CV²).

Carbon based materials such as graphene,^2,3,12–14 activated carbon,^15,16 carbon nanotubes^17,18 and porous carbon materials^19–21 have been widely investigated as electrode material candidates due to their large specific surface area, high electrical conductivity and good chemical stability. Especially, graphene has been considered as the most promising candidate for electrode material because of their high specific surface area up to 2675 m² g⁻¹ and excellent electrical conductivity.^4,5,12 To fabricate graphene based supercapacitors with high specific capacitance, three approaches have been mainly explored. First, to prevent the restacking of graphene sheets (which causes dramatic decrease of specific surface area) and increase the specific surface area, various nanostructured graphene systems have been intensively explored (for example, graphene sheets with 3D crumpled structure are known to inhibit restacking of graphene and provide large surface area^13,22–25). Second, to enhance electrical conductivity and ion binding efficiency of graphene sheets, various methods for doping graphenes with nitrogen have been explored, including arc-discharge,²⁶ plasma²⁷ and thermal treatments of graphenes²⁸ in the presence of nitrogen precursors. Third, graphene surface are decorated with various transition-metal oxide nanoparticles (such as MnO₂, RuO₂ and TiO₂),^5,29,30 which provides additional high specific capacitance through fast redox reactions between the nanoparticles and carrier ions, so called pseudo-capacitance.^6,17 While two of the three approaches are combined for synergetic improvement of specific capacitance and cycle/scan stability, synthesis methods which utilize all the three approaches simultaneously have not been reported yet. In principle, it can be done by simply utilizing the different approaches in series. However, it may require complex and multi-step process, which will prevent scalable synthesis.

In this paper, we report a simple and scalable method to synthesize highly crumpled, highly exfoliated, and N-doped graphene/Mn-oxide nanoparticle hybrid for high performance supercapacitors. In this method, reduction of graphene oxide (GO), nanometer scale crumpling, high level of exfoliation, and N-doping of graphene and formation of Mn-oxide nanoparticles on graphene surface are done by a simple thermal treatment of GO mixed with glycine (Gly) and Mn-nitrate. The hybrid materials prepared in this method provides high specific capacitance (958 F g⁻¹ at 5 mV s⁻¹) with high cycle stability (94.1% retention after 1000 cycles of charge–discharge). The contributions of crumpled structures, N-doping and Mn-oxide nanoparticles to specific capacitance are identified.

2. Experimental

2.1 Materials and synthesis

GO was prepared from natural graphite powder (SP-1, Bay Carbon Inc.) by the modified Hummers' method as described elsewhere.^31,32 (AFM images, size and length distributions of prepared GO are shown in Fig. S1, ESI†). A mixture of GO, glycine (Gly, NH₂CH₂COOH, Sigma-Aldrich) and manganese(II) nitrate tetrahydrate (Mn(NO₃)₂·4H₂O, Sigma-Aldrich) was prepared with 1 : 2 : 4 mass ratio in purified water and sonicated for 2 hours with a bath type sonicator (DH-WUC-D10H, Daihan Science Inc., 40 kHz, 200 W). The sonicated mixture was poured directly onto an alumina crucible. The temperature of the mixture was gradually increased from room temperature to 500 °C at a rate of 2 °C min⁻¹ and maintained 2 hours under argon atmosphere in a tube type furnace (Dae-Heang Science Inc.). After thermal annealing at 500 °C, the mixture was quickly taken out from the furnace and immersed in liquid nitrogen. The quenched sample was collected from the alumina crucible and used for further characterization. A sample which was slowly cooled down at a rate of 2 °C min⁻¹ after the thermal annealing at 500 °C was also prepared for comparison.

2.2 Characterizations

Thermo-gravimetric analysis (TGA) was performed using thermo-gravimetric analyser (TG-209-F3, NETZSCH Inc.) under nitrogen atmosphere with 5 °C min⁻¹ heating rate. X-ray diffraction (XRD) patterns of the samples were measured using high power powder X-ray diffractometer (D/MAX-2500, Rigaku Inc.). X-ray photoelectron spectroscopy (XPS) measurements were performed using K-alpha XPS (Thermo VG Scientific Inc.) with Al Kα X-ray radiation as a source for excitation. Surface morphologies of samples were measured using a field-emission scanning electron microscope (FE-SEM, Nova 230, FEI Inc.) and a field-emission transmission electron microscope (FE-TEM, JEM-2100F, JEOL Inc., operated at 200 kV). The samples for FE-TEM measurements were prepared by dropping aqueous dispersion of prepared powder samples on carbon-coated copper grids followed by drying. Specific surface area and pore structure of the samples were characterized by physical adsorption and desorption measurement of N₂ gas at 77 K with a surface area and porosity analyzer (Tristar II-3020, Micrometrics Inc.). Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) models were used to determine the specific surface area and the pore size/volume distribution, respectively.

2.3 Small-angle neutron scattering

Small-angle neutron scattering (SANS) measurements were performed using the 40 m SANS instrument at HANARO, the Korea Atomic Energy Research Institute (KAERI) in Daejeon, Republic of Korea. Neutrons of wavelength λ = 6 Å with a wavelength spread of Δλ/λ = 0.12 were used. Two configurations of sample-to-detector distances of 1.5 m and 15.89 m were used to cover the q range of 0.003 Å⁻¹ < q < 0.56 Å⁻¹, where q = (4π/λ)sin(θ/2) is the magnitude of the scattering vector and θ is the scattering angle. Background and empty cell scattering were subtracted from the sample scattering. The sensitivity of individual detector pixels was corrected. The corrected data sets were placed on an absolute scale using the direct beam flux method. All the SANS measurements were carried out at 25 °C using quartz cells of 0.5 mm path length.

2.4 Electrochemical measurements

Working electrodes were prepared by mixing electro-active material, carbon black (EC 600JD-2K, Infochems Chemical) and poly(vinylidene difluoride) (PVDF, Sigma-Aldrich) at a mass ratio of 8 : 1 : 1 in 1-methyl-2-pyrrolidine (NMP, Sigma-Aldrich). The resulting black slurries were dropped onto the Al plate (3 × 2 cm size, 99.99% purity, Alfa-Aesar Inc.) and dried at 80 °C for 12 hours. The mass loading of electro-active materials was ca. 1 mg cm⁻² for all electrodes. All electrochemical experiments were carried out using conventional three-electrode cell method in 1 M Na₂SO₄ aqueous electrolyte solution with electrochemical workstation (Solartron 1287A, Solartron Analytical Inc.), including platinum counter electrode and silver chloride reference electrode. From the cyclic voltammetry (CV) curves, the specific capacitance (F g⁻¹) of each sample was calculated using the following equation:
where, I is the current (A), V is the voltage (V), υ is the scan rate (V s⁻¹) and m is the mass (g) of electro-active material.

3. Results and discussions

A simple thermal treatment of GO–Gly–Mn(NO₃)₂·4H₂O mixture (thermal annealing at high temperature followed by a rapid quenching) was used to synthesize highly crumpled, highly exfoliated, and N-doped graphene/Mn-oxide nanoparticle hybrid (Fig. 1). The thermal decompositions of GO, Gly, Mn(NO₃)₂·4H₂O and GO–Gly–Mn(NO₃)₂·4H₂O mixture were measured by TGA (Fig. 2), showing that the decomposition of the oxy-functional groups of GO (thermal reduction of GO), Gly, and Mn(NO₃)₂·4H₂O occur in a similar temperature region. The early weight loss of Mn(NO₃)₂·4H₂O is due to the dehydration³³ and the early weight loss of GO is due to removal of water molecules intercalated between GO sheets.^34,35 The weight loss of GO–Gly–Mn(NO₃)₂·4H₂O mixture with temperature follows the weight loss behavior of its component materials. The simultaneous decomposition of GO and Mn(NO₃)₂·4H₂O leads to the formation of Mn-oxide nanoparticles on graphene surface. The decomposed Mn ions adsorb on the GO surface by electrostatic interactions and Mn-oxide nanoparticles are formed on graphene surface by chemical reaction GO + Mn^a+ → GO/Mn^a+ → G + Mn_xO_y.²² The NH₃ gas generated by the decomposition of Gly act as the source for N-doping of graphene.^36–38 The NOx gas generated by the decomposition of Mn-nitrate may act as the source for N-doping of graphene but this needs further confirmation.²⁸

Fig. 1 Schematic illustration for the synthesis of the highly crumpled, highly exfoliated, and N-doped graphene/Mn-oxide nanoparticle hybrid.

Fig. 2 TGA profiles of GO, Gly, Mn(NO₃)₂·4H₂O, and GO–Gly–Mn(NO₃)₂·4H₂O mixture.

To understand the thermal treatment process, XRD measurements of GO–Gly–Mn(NO₃)₂·4H₂O mixtures annealed at different temperatures (100, 300 and 500 °C) were performed (Fig. 3). For these measurements, the mixtures were heated up to different temperatures at a rate of 2 °C min⁻¹ and kept at the temperatures for 2 hours under argon atmosphere. The XRD patterns of GO and reduced GO (RGO) were also measured as references. While the XRD pattern of as-prepared GO power shows a clear peak at 2θ = 10.6° indicating that the interlayer distance is 0.83 nm, the XRD pattern of RGO shows a broad peak near 2θ = 26° indicating that the interlayer spacing is ca. 0.34 nm. This difference is due to the removal of both the oxy-functional groups on GO surface and the intercalated water molecules between GO sheets.^34,35 The XRD pattern of GO–Gly–Mn(NO₃)₂·4H₂O mixture annealed at 100 °C shows a single broad peak near 2θ = 25°, indicating that interlayer distance between GO sheet was decreased by removal of intercalated water molecules.³³ In the XRD pattern of the mixture annealed at 300 °C, new clear peaks (which correspond to MnO₂ and Mn₃O₄ nanoparticles) start to show up with the broad peak slightly shifted to higher scattering angle. This indicates that MnO₂ and Mn₃O₄ nanoparticles started to be formed and the removal of surface oxy-functional groups was progressed. The XRD pattern of the mixture annealed at 500 °C shows clear peaks corresponding to α-type MnO₂ nanoparticles (indexed in blue color based on reference values in JCPDS 44-141). Additional peaks at 2θ = 32.5°, 44.3° and 64.8° correspond to the (103), (220) and (400) reflections of γ-type Mn₃O₄ (indexed in green color based on reference values in JCPDS 80-0382), indicating that some amount of Mn₃O₄ nanoparticles are formed together with MnO₂ nanoparticles. The rather broad peaks of MnO₂ and Mn₃O₄ (which cannot be explained by the effect of nanoparticle size only) indicate that the crystallinity of nanoparticles is rather low, which is typical for thermally synthesized MnO₂ and Mn₃O₄ nanoparticles.^11,39 It should be noted that in the XRD pattern of the mixture annealed at 500 °C and followed by a rapid quenching with liquid nitrogen (red line in Fig. 3), the broad peak near 2θ = 25° corresponding to the inter-RGO correlations is completely disappeared while the peaks corresponding to MnO₂ and Mn₃O₄ nanoparticles are maintained. This indicates that the graphene sheets are essentially highly exfoliated by rapid quenching. It should be noted that, to the best of our knowledge, this is the first example of RGO sheets for which the broad graphene–graphene correlation peak is completely disappeared.

Fig. 3 XRD patterns of GO, RGO, and GO–Gly–Mn(NO₃)₂·4H₂O mixtures with different thermal treatments.

The XPS spectra of GO–Gly–Mn(NO₃)₂·4H₂O mixtures annealed at different temperatures were measured (Fig. 4). All peaks in the XPS spectra were fitted with Gaussian functions using a fitting software (Thermo Avantage, version 5.932) for accurate analysis. The C1s XPS spectrum of the mixture annealed at 100 °C shows three separated characteristic peaks at 284.5 eV (from C–C bond of graphene⁴⁰), 285.5 eV (from C–O bond of the oxy-functional groups⁴¹) and 288.3 eV (from O–C=O bond of the oxy-functional groups⁴¹) (Fig. 4a). Upon increasing the annealing temperature to 500 °C, the C1s XPS spectra from oxy-functional groups disappear and new peaks at 285.7 eV (C=N bond from pyridine-like nitrogen⁴²) and 287.6 eV (C–N bond from pyrrolic-like nitrogen⁴²) show up in addition to the C–C bond peak. The N1s XPS spectrum of the mixture annealed at 100 °C shows four peaks which correspond to the Gly (399.0 eV and 401.3 eV)^43,44 and the nitrate counter ion from Mn(NO₃)₂ (399.6 eV and 406.6 eV)^45,46 (Fig. 4b). As the annealing temperature is increased to 300 °C, all the four peaks disappear (indicating that Gly and Mn(NO₃)₂ were decomposed) and new peaks corresponding to pyridine-like nitrogen (398.8 eV) and pyrrolic-like nitrogen (400.3 eV) show up.^3,47 The mixture annealed at 500 °C also shows two peaks from pyridine-like and pyrrolic-like nitrogens. Considering that the intensity of pyrrolic-like nitrogen peak is higher than that of pyridine-like nitrogen, the pyrrolic-like nitrogen is more populated than pyridine-like nitrogen in the as-prepared sample. The total atomic concentration of nitrogen in the as-prepared sample, estimated from the XPS spectra, is 4.3 atom% and the atomic concentration ratio between nitrogen and carbon is 1 : 16.3. The Mn2p XPS spectrum of the mixture annealed at 100 °C shows two spin–orbit split peaks at 641.1 eV (Mn2p_3/2) and 653.1 eV (Mn2p_1/2), which corresponded to Mn²⁺ of Mn(NO₃)₂ (Fig. 4c).⁴⁶ The additional peak at 645.0 eV can be attributed to a shake-up satellite of Mn²⁺.^49–51 Both mixtures annealed at 300 °C and 500 °C show two characteristic Mn2p_3/2 peaks at 641.6 eV and 642.3 eV, which corresponded to Mn³⁺ of Mn₃O₄ nanoparticles and Mn⁴⁺ of MnO₂ nanoparticles, respectively.^48,50,52 The higher intensity of Mn⁴⁺ ion peak than Mn³⁺ ion peak indicates that MnO₂ is more populated than Mn₃O₄ in the as-prepared sample. The Mn2p_3/2 peak at 640.8 eV and an additional peak at 646.0 eV are from Mn²⁺ of Mn–O–H bond⁵¹ and shake-up satellite of Mn²⁺,^49,51 respectively. The O1s XPS spectrum of the mixture annealed at 100 °C shows a single peak at 531.8 eV (Fig. 4d) which corresponds to C–O bonds of oxy-functional groups on graphene surface.⁵³ Upon increasing the annealing temperature to 500 °C, the single peak disappeared and new peaks at 529.7 eV and 531.5 eV showed up, which correspond to Mn–O bond and Mn–O–H bond, respectively.^48,54 The intensity of O1s spectrum is decreased continuously with annealing temperature, indicating that the atomic percentage of oxygen is decreased by thermal decomposition of Gly and nitrate, and reduction of GO. The atomic fractions of the mixture annealed at 500 °C, which are estimated from the XPS measurements, are 70.3% (C), 4.3% (N), 8.6% (Mn), and 16.8% (O). The mass fractions of N-doped RGO and Mn-oxide nanoparticles in the as-prepared sample, which are estimated from the atomic fractions, are 55% and 45%, respectively.

Fig. 4 (a) C1s, (b) N1s, (c) Mn2p, (d) O1s XPS spectra of GO–Gly–Mn(NO₃)₂·4H₂O mixtures with various annealing temperatures. The colors of curves represent annealing temperature. (e) Schematic of N-doped graphene with different N species.

The morphologies of the GO–Gly–Mn(NO₃)₂·4H₂O mixture annealed at 500 °C and the mixture rapidly quenched with liquid nitrogen after annealing at 500 °C were characterized by FE-SEM and FE-TEM measurements. The FE-SEM images of the mixture annealed at 500 °C (Fig. 5a and b) show that the flat graphene sheets are uniformly and densely decorated with sphere-like Mn-oxide nanoparticles. On the other hand, the FE-SEM images of the mixture rapidly quenched after thermal annealing (Fig. 5c and d) show that graphene sheets are highly crumpled and uniformly decorated with bigger and rather irregular shaped Mn-oxide nanoparticles. The crumpling of graphene sheets during quenching is caused by large thermal stress due to rapid temperature change from 500 °C to −270 °C.²³ The bigger and irregular shaped nanoparticles formed by quenching may be attributed to the aggregation of nanoparticles due to the folding and crumpling of graphene sheets.⁵⁵ The FE-TEM images of the mixture annealed at 500 °C show that thin graphene sheets are uniformly decorated with nanoparticles (ca. 14.4 nm) (Fig. 5e and f). The FE-TEM images of the mixture quenched after thermal annealing clearly show that crumpled graphene sheets are uniformly decorated with nanoparticles (ca. 25.6 nm) (Fig. 5g and h) (the size distributions of nanoparticles in both cases are shown in Fig. S2, ESI†). The dark regions show crumpled (marked by an arrow 1), and densely crumpled and protruding (marked by an arrows 2 and 3) graphene sheets decorated with Mn-oxide nanoparticles.

Fig. 5 FE-SEM and FE-TEM images of GO–Gly–Mn(NO₃)₂·4H₂O mixture (a), (b), (e), (f) annealed at 500 °C and (c), (d), (g), (h) annealed at 500 °C followed by a rapid quenching.

To understand the crumpling of graphene sheets by quenching, the FE-SEM images of highly crumpled, highly exfoliated, N-doped graphene/Mn-oxide nanoparticle hybrid (CNG-MO) are compared with those of RGO and quenched RGO sheets (Fig. 6). The RGO sheets were prepared by annealing GO powder only (i.e. without Mn(NO₃)₂·4H₂O and Gly) at 500 °C and the quenched RGO sheets were prepared by rapidly quenching the RGO sheets annealed at 500 °C in the same ways as it was done for the CNG-MO. While the RGO sheets show flat graphene sheets with smooth surface, the quenched RGO sheets (without Mn-oxide nanoparticles) show highly wrinkled structures in low magnification images (with 50 μm and 2 μm scale bars) and wrinkled but still rather smooth surface structure in a higher magnification image (with 400 nm scale bar). The CNG-MO sample (the quenched N-doped RGO sheets decorated with Mn-oxide nanoparticles) shows highly crumpled 3D morphologies which are quite different from those of the quenched RGO sheets at all magnifications measured in this study. It should be noted that the highly crumpled 3D structures of graphene sheets decorated Mn-oxide nanoparticles are clearly visible even in the higher magnification image (with 400 nm scale bar). The close inspection of the image indicates that the crumpling of graphene sheets decorated with Mn-oxide nanoparticles occurs in ca. 10–100 nm length scale. This is much finer than the crumpling of the quenched RGO which occurs in a μm length scale. The nanometer scale crumpling of graphene sheets in the CNG-MO sample should be the origin of the high level exfoliation of graphene sheets (as indicated by the XRD pattern shown in Fig. 3) which is the key for high specific surface area. The fine crumpling of graphene sheets also inhibit their restacking, which provides structural stability. The FE-SEM images in Fig. 7 show that the RGO sheets are densely restacked and the quenched RGO sheets are partially exfoliated with restacking structure remaining (as also confirmed by the broad peak near 2θ = 25° in the XRD pattern shown in Fig. S3 ESI†). However, any restacking structure was not observed for the CNG-MO sample. Considering that the heat capacities of MnO₂ (54.1 J mol⁻¹ K⁻¹) and Mn₃O₄ (139.7 J mol⁻¹ K⁻¹) are quite different from that of graphene (8.5 J mol⁻¹ K⁻¹),^56,57 the presence of Mn-oxide nanoparticles on the surface of graphene induces strong thermal stress on the graphene, making graphene sheets highly crumpled and highly exfoliated. To the best of our knowledge, this is the first demonstration that a rapid quenching of reduced graphene oxides decorated with nanoparticles induces nanometer scale crumpling of graphene sheets, resulting in highly exfoliated graphene sheets without any restacking.

Fig. 6 FE-SEM images of (a) RGO, (b) quenched RGO, and (c) CNG-MO samples.

Fig. 7 FE-SEM images of edge sides of (a) RGO and (b) quenched RGO.

The nitrogen gas adsorption–desorption isotherms were measured for the GO–Gly–Mn(NO₃)₂·4H₂O mixture annealed at 500 °C and the mixture quenched after thermal annealing at 500 °C, respectively (Fig. 8). The adsorption–desorption curves of both samples at the relative pressure P/P_o ranging from 0.5 to 1.0 show similar patterns with distinct hysteresis, which is typical for slit-shaped mesopores. However, the adsorption–desorption curves in the low relative pressure region (P/P_o < 0.4) are different for the two samples. In the low relative pressure region, the initial increase of adsorption curve of the quenched sample is more rapid than that of the unquenched sample, and the desorption curve of the quenched sample does not match with the adsorption curve. This indicates that the quenched sample contains small nanopores,^58,59 which is clearly shown in the pore size distributions (Fig. 8c and d). It should be noted that while the unquenched sample contains mesopores which are mostly distributed in the range of 10–100 nm, the quenched sample contains pores which are highly populated in both the small micropore region (ca. 2 nm) and the mesopore region. The high population of small micropores in the quenched sample can be attributed to the complex nanometer scale 3D crumpled structures of graphene sheets. The specific surface area of unquenched and quenched samples, calculated using the BET method, are 559 m² g⁻¹ and 1006 m² g⁻¹, respectively. The large increase of specific surface area in the quenched sample can be attributed to the high level of exfoliation of graphene sheets by crumpling of graphenes.

Fig. 8 Nitrogen adsorption–desorption isotherms and pore size distributions of GO–Gly–Mn(NO₃)₂·4H₂O mixture (a), (b) annealed at 500 °C and (c), (d) annealed at 500 °C followed by a rapid quenching.

The GO–Gly–Mn(NO₃)₂·4H₂O mixture annealed at 500 °C and the mixture quenched after thermal annealing at 500 °C were also characterized using small-angle neutron scattering (SANS) (Fig. 9). The SANS intensities of both samples show a power law decay with an exponent close to −3 which is typical for rough surface⁶⁰ or nearly space-filling network of mesopores.⁶¹ The complexity of structure (graphenes decorated with Mn-oxide nanoparticles form mesopores) makes it difficult to identify the nature of the power law decay. It should be noted that the quenched sample shows an additional scattering in the high-q region (q > 0.2 Å⁻¹), which can be contributed to the randomly distributed micropores formed by nanometer scale crumpling of graphene sheets. Analyzing the SANS intensity of the quenched sample at the high-q region with the Guinier approximation for the micropores in addition to the power law decay model,⁶¹ the dimension of micropores is ca. 1.4 nm which is consistent with the BET measurement. Therefore, the SANS measurements also confirm the nanometer scale crumpling of graphene sheets by rapid quenching in the presence of nanoparticle decoration.

Fig. 9 SANS intensities of GO–Gly–Mn(NO₃)₂·4H₂O mixture (a) annealed at 500 °C and (b) annealed at 500 °C followed by a rapid quenching.

The electrochemical properties of CNG-MO were measured by using a conventional three-electrode cell system as described in experimental section. The potential window for cyclic electrochemical measurements was determined after examining cyclic voltammetry (CV) curves of the sample measured with a few different potential windows (Fig. S4 ESI†). To safely avoid oxygen evolution reaction at higher potential which adds an irreversible redox process to the pseudocapacitance process, the potential window was set between 0 and 0.6 V vs. Ag/AgCl.^62,63 The CV curve of the sample at 5 mV s⁻¹ scan rate (Fig. 10a) shows a nearly rectangular and mirror image, indicating its charge–discharge process is similar to that of the ideal electrochemical capacitor. The specific capacitance of the sample calculated from the CV curve is 958 F g⁻¹ which shows an excellent energy storage performance. The electrochemical performance of CNG-MO was further investigated with galvanostatic charge–discharge experiment in the same potential range with 1, 2 and 5 A g⁻¹ current densities (inset of Fig. 10a). The potential-time line shows a nearly triangular shape, indicating a rapid I–V response and good electrochemical reversibility. The voltage drop at the initiation of the discharge line is relatively small with all current densities, indicating a low equivalent series resistance (ESR) in the electrode material.^6,7 The charge–discharge cycle stability of CNG-MO sample was measured at 5 mV s⁻¹ scan rate (Fig. 10b) and compared with that of N-doped graphene/Mn-oxide nanoparticle hybrid (NG-MO, prepared by thermal annealing of GO–Gly–Mn(NO₃)₂·4H₂O mixture at 500 °C without quenching) (Fig. S5 ESI†). After 1000 cycles of charge–discharge, the specific capacitance of CNG-MO shows an excellent retention of 94.1% maintaining the nearly rectangular shape of CV curve (inset of Fig. 10b), while NG-MO shows only 80% retention. The high cycle stability of CNG-MO can be attributed to its high structural stability due to the nanometer scale 3D crumpling of graphenes which prevent restacking of graphene sheets during repeated charge–discharge process. Thermal reduction, N-doping, Mn-oxide nanoparticle decoration and 3D crumpled structure of graphene are all important factors which can significantly contribute to specific capacitance. To determine the role of each factor quantitatively, the CV curves of a series of samples prepared with different treatments, CNG-MO, NG-MO, G-MO (graphene/Mn-oxide hybrid), RGO and GO, were measured at a scan rate of 5 mV s⁻¹ (Fig. 10c). It is clear that the current densities of samples increase as additional treatments are performed. The specific capacitance of CNG-MO, NG-MO, G-MO, RGO and GO calculated from the CV curves are 958 F g⁻¹, 603 F g⁻¹, 328 F g⁻¹, 147 F g⁻¹ and 89 F g⁻¹, respectively (Fig. 10d), which indicates that all additional treatments contribute to the increase of specific capacitance significantly. It should be noted that the synthesis method presented here synergistically combines all the contributions in a simple thermal process which can be easily scalable.

Fig. 10 (a) CV curve at a scan rate 5 mV s⁻¹ for CNG-MO and galvanostatic charge–discharge curves with of 1, 2 and 5 A g⁻¹ current densities (inset). (b) Cycle performance of CNG-MO at a scan rate of 5 mV s⁻¹. (c) CV curves and (d) specific capacitances of CNG-MO, NG-MO, G-MO, RGO and GO at a scan rate of 5 mV s⁻¹.

The CV curves of CNG-MO with different scan rates (5, 10, 25, 50 and 100 mV s⁻¹) were measured (Fig. 11a). While the CV curve was slightly distorted with the scan rate, the overall rectangular shape of curve was maintained up to the scan rate of 100 mV s⁻¹, indicating that the fast charge–discharge process of the sample was maintained. The specific capacitances of GO, RGO, G-MO, NG-MO and CNG-MO measured at different scan rates are compared (Fig. 11b). The specific capacitance of CNG-MO at 100 mV s⁻¹ is as high as 642 F g⁻¹.

Fig. 11 (a) CV curves of CNG-MO measured at different scan rates of 5, 10, 25, 50, 100 mV s⁻¹. (b) Specific capacitances of CNG-MO, NG-MO, G-MO, RGO and GO at different scan rates.

4. Conclusions

A scalable thermal method to synthesize highly crumpled, highly exfoliated and N-doped graphene/Mn-oxide nanoparticle hybrid for high performance supercapacitor has been demonstrated. Reduction of GO, nanometer scale crumpling, high level of exfoliation, N-doping of graphenes and decoration with Mn-oxide nanoparticles, each of which significantly contributes to high specific capacitance (958 F g⁻¹ at 5 mV s⁻¹) in a synergetic way, are achieved in a single thermal process, thermal annealing of GO–Gly–Mn(NO₃)₂·4H₂O mixture at 500 °C followed by a rapid quenching with liquid nitrogen. N-doping of graphene is predominantly done in the form of pyrrolic-like and pyridine-like nitrogen, and Mn-oxide nanoparticles are formed on the surface of graphenes as MnO₂ and Mn₃O₄. The nanometer scale crumpling of graphene sheets, which is achieved by a rapid quenching of graphenes in the presence of Mn-oxide nanoparticles on its surface, induces exceptionally high degree of exfoliation of graphenes and prevents restacking of graphene sheets during repeated charge–discharge process, providing high specific surface area (1006 m² g⁻¹) and high cycle stability (94.1% retention after 1000 cycles), respectively. The simplicity of synthesis process makes it easily scalable and industrially applicable method.

Acknowledgements

This research was supported by NRF grants funded by the MEST of the Korean government (no. 2011-0031931 and 2014R1A2A1A05007109) and KAERI grant. We acknowledge the HANARO Neutron Research Center for providing access to the beamline used in this study.

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Footnote

† Electronic supplementary information (ESI) available: The information (size and thickness) of as-prepared GO, size distribution of Mn-oxide nanoparticles, XRD data of quenched RGO, CV curves of CNG-MO sample measrured with different upper cut-off voltages and cycle stability of CNG-MO vs. NG-MO. See DOI: 10.1039/c5ra04163e

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