MWNTs Nanoframework-Embedded

May 12, 2017 - On the basis of the above test results, we propose that the good capacitive performance of the Co3O4/3D N-G/MWNTs is ascribed to the fo...
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Research Article pubs.acs.org/journal/ascecg

Nitrogen-Doped 3D Graphene/MWNTs Nanoframework-Embedded Co3O4 for High Electrochemical Performance Supercapacitors Mingmei Zhang,* Ying Wang, Denghui Pan, Yuan Li, Zaoxue Yan, and Jimin Xie School of Chemistry and Chemical Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, China ABSTRACT: Co3O4 nanoparticles embedded on threedimensional (3D) nitrogen-doped graphene incorporated with multiwalled carbon nanotubes (named as 3D N-G/ MWNTs) are synthesized through a simple hydrothermal process and succedent microwave treatment. Small-sized Co3O4 in situ grow on the surface of N-doped multiwalled carbon nanotubes and N-doped graphene nanosheets via a rapid one-pot microwave-assisted method without adding other reagents. Owing to the good electrical conductivity unique structure of 3D N-G/MWNTs, the composite electrodes suggested a huge surface active area and an abundant three-dimensional porous configuration to provide plenty of paths for the rapid electrons/ions transportation and electron-transfer resistances. When the l A/g current density is applied, capacitances of 2039.4 F/g are fulfilled. Even with the charge−discharge current increasing from 1 to 15 A/g, there is still 84% of the capacitance remaining. In particular, the hybrid also exhibits high energy density (59.34 Wh kg−1) and power density(150.73 W kg−1), as well as cycling stability maintaining more than 94% of its primary capacitance after 6000 charge−discharge cycles at a current density of 15 A g−1. The perfect electrochemical performance of the above composites is attributed to both the particular 3D G/MWNTs interconnected structure which could enhance electric conductivity and the nitrogen-doping networks which could give the graphene/carbon nanotube surface large numbers of defects to support high performance cobalt oxide. KEYWORDS: Three-dimensional graphene/MWNTs frameworks, Co3O4 nanoparticles, Nitrogen doped, Supercapacitor

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sites.6 For example, Yang et al. synthesized an MnO2/Ngraphene nanosheet by a one-step hydrothermal method at lower temperatures, and they found nitrogen doping to be beneficial for anchoring MnO2 nanoparticles on graphene.7 In our previous work, we found that nitrogen doping in graphene can make Co3O4 nanoparticles with smaller particle size and a more uniform interface.8 Currently, more focus has been concentrated on the synthesis of stable 3D graphene hybrid composites.9 Graphene surfaces are usually compromised by reuniting graphene sheets with each other using a strong π−π interaction, which does not suit the fast motion of ions in large size. As a result, many efforts have been made to obtain threedimensional structures for outstanding electrochemical properties, which have been proven to be effective in preventing graphene sheets from agglomerating and providing rapid transfer channels of ions.10,11 Huang12 prepared a threedimensional graphene structure using a foam nickel substrate as a support structure, and electrochemical storage energy properties of the three-dimensional structure is 5 times that of the 2D planar structure graphene.

INTRODUCTION When the need to release large amounts of energy in a short time occurs, a battery will not provide the power output to meet the requirements. A supercapacitor, as a new category of energy storage equipment, can completely discharge its stored energy in a range of microseconds to milliseconds.1,2 Supercapacitors fill the performance space between traditional capacitors and batteries. They can be used in electric or gas− electric hybrid vehicles to recover waste energy during braking or provide energy for portable communication tools, and even in some areas they can replace a battery.3,4 Existing electrode materials have a deficiency in a stable structure and large surface area and are always suffering from limited cycle life and low energy density due to phase changes caused by redox reactions when energy is stored or released on the surface or subsurface of an electrode. In order to meet future applications, preparations of high energy density, vast specific capacitance, and long cycle performance for advanced electrode materials will be the critical study for supercapacitors. Recently, growing or embedding a metallic oxide or double metal oxide nanoparticles on graphene-based materials is a suitable method to enhance the electrochemical activity of electrode materials.5 As is well known, to enhance interactions between a metal oxide and a graphene surface, B, P, S, and N are doped into the lattice of graphene to provide more active sites and nucleation © 2017 American Chemical Society

Received: February 14, 2017 Revised: April 26, 2017 Published: May 12, 2017 5099

DOI: 10.1021/acssuschemeng.7b00453 ACS Sustainable Chem. Eng. 2017, 5, 5099−5107

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Schematic diagram of Co3O4/3D N-G/MWNTs hybrid. added into 20 mL of distilled water and ultrasonicated for 6 h to obtain a uniform dispersion, followed by vigorous magnetic stirring for 30 min to achieve stable mixed suspensions. In this process, C6H12N4 and Co2+ were attached on the GO sheets and MWNTs framework. The mixed suspension solution was then transferred into a hydrothermal reaction vessel (50 mL), sealed, and maintained at 180 °C for 24 h. During the hydrothermal process, N atoms were doped into the graphene and MWNTs networks through slow decomposition of C6H12N4.

Recently, reports have also shown that graphene incorporation with multiwalled carbon nanotubes can effectively avoid the overlapping of graphene sheets and provide more superficial area for uniform dispersion of coated metallic oxide nanoparticles.13−15 Furthermore, the 3D graphene/ carbon nanotube hybrid composite was also developed for the preparation of supercapacitors with high charge−discharge capacity, excellent power density, and superior cycling life.16−18 Fan et al. have prepared 3D CNT/graphene nanomaterials using an in situ growth method. This unique carbon structure can integrate pseudocapacitance with double capacitance, which allows the nanostructure network to have high speed ions and proton transfer.19 In this paper, we have developed a simple green way for the synthesis of 3D nitrogen-doped graphene/multiwalled carbon nanotubes (3D N-G/MWNTs) frame structure as carriers to anchor cobaltosic oxide to obtain new special morphologies with potential application in supercapacitors. A hybrid of Co3O4 nanoparticles with super fine particle sizes were in situ embedded in 3D N-G/MWNTs by a microwave treatment process without adding other reagents. The above composites showed an outstanding reversible capacity and excellent rate capability with very good durability for electrode materials in supercapacitors.

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C6H12N4 + 6H 2O = 6HCHO + 4NH3

(1)

NH3 + H 2O = NH4 + + OH−

(2)

Co2 + + 2OH− = Co(OH)2

(3)

2+

OH¯ ions react with Co and result in the precursor of Co(OH)2 covered on the surface of the N-G/MWNTs structure, followed by intermittent microwave heating (900 W) with a 25 s on and 10 s off procedure for 20 cycles in a microwave reactor at 700 °C. The Co(OH)2 is oxidized to Co3O4 by a microwave heating process; meanwhile, N-G/MWNTs form 3D N-G/MWNTs frameworks. 6Co(OH)2 + O2 = 2Co3O4 + 6H 2O

(4)

Final products were collected and dried to actual weight under vacuum drying conditions at 60 °C. The inductively coupled plasma spectroscopy (ICP, IRIS(HR), USA) analysis gave the actual Co3O4 contents as 89.7 wt %. Preparation of Electrode. The Co3O4/3D N-G/MWNTs composite working electrode was obtained by thoroughly stirring the Co3O4/3D N-G/MWNTs active hybrid (1.0 mg), acetylene carbon black, and polyvinylidene (PVD) (mass ratio of 50:15:2) in ethanol for 24 h. The mixture was compacted tightly on a precleaned nickel foam substrate (1 cm × 1 cm) at a pressure of 10 MPa and dried in vacuum for 48 h at 80 °C. For a contrastive study, other electrodes, such as NG, N-G/MWNTs, Co3O4/N-G, and 3D-N-G/MWNTs, were obtained using a similar preparation process. Characterization of Composites. Structural properties and morphologies of the as-prepared composites were examined using TEM (JEOL-JEM-2010, Japan) operating at 120 kV and XRD D8

EXPERIMENTAL SECTION

Preparation of N-Doped Graphene/MWNTs (N-G/MWNTs) and Co3O4/3D N-G/MWNTs. Graphene oxide (GO) was prepared from purified natural flaky graphite according to our previous work (a modified Hummers method).20 In order to produce considerable amounts of −COOH and −OH groups on the surface of MWNTs (diameters of 20−40 nm, purity >95%, purchased from Shenzhen Bill Technology Co.), a typical mixed acid (H2SO4 and HNO3 in the ratio 3:1) treatment was conducted.21 The nitrogen-doped graphene/ MWNTs networks were fabricated through a hydrothermal reaction process. Typically, 45 mM of Co (NO3)26H2O, 50 mg of GO, 50 mg of MWNTs, and 0.15 g of hexamethylenetetramine (C6H12N4) were 5100

DOI: 10.1021/acssuschemeng.7b00453 ACS Sustainable Chem. Eng. 2017, 5, 5099−5107

Research Article

ACS Sustainable Chemistry & Engineering Advance X-ray powder diffraction (Bruker AXS Company, Germany). X-ray photoelectron spectroscopy (XPS) was conducted on a PHI5702. The structures of the obtained samples were carried out by Raman scattering (AU10) performed on a Renishaw (In Via) spectrometer with a 532 nm laser source. The electrochemical determination was carried out on an IM6e potentiastat (ZahnerElectrik, Germany) in a 6 M KOH electrolyte with three-electrode system, comprised with Co3O4/3D N-G/MWNTs as the working electrode, platinum sheet as the counter electrodes, and saturated calomel electrode (SCE) as the reference electrodes. Electrochemical impedance spectroscopy (EIS) was measured at the open-circuit potential over the frequency range of 105 to 0.02 Hz with an ac amplitude of 5 mV. The specific capacitance (C) of the electrodes was calculated from the galvanostatic charge−discharge curves using the following equation:

C = I Δt /(mΔV )

65.3° are the characteristic peaks of Co3O4 (JCPDS 65-3103), which indicated that cubic Co3O4 was embed in the 3D N-G/ MWNTs structure successfully. All the characteristic peaks of Co3O4, graphene, and MWCNTs are observed in Figure 2(d), demonstrating the coexistence of Co3O4, graphene, and MWCNTs in the composite frameworks. Raman spectra further confirms the change in surface structure of Co3O4/3D N-G/MWNTs. The Raman spectra of GO, N-G/MWNTs, and Co3O4/3D N-G/MWNTs are shown in Figure 3. Two characteristic peaks of the D band at 1350

(5) −1

where C is the specific capacitance (F g ) of the electrode, I is charging current (A), Δt is time of discharge (s), m is the mass of the active material (g), and ΔV is the discharging potential range (V).

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RESULTS AND DISCUSSION The schematic diagram of the Co3O4/3D N-G/MWNTs hybrid synthesis was illustrated in Figure 1. Briefly, the acidactivated MWNTs were combined with GO through π−π stacking by ultrasonification to achieve the graphene oxide and acid carbon nanotube composite coexisting in perfect harmony. After adding hexamethylenetetramine and with hydrothermal treatment at 180 °C for 24 h, N atoms were doped into the graphene and MWNTs networks through slow decomposition of C6H12N4. Co3O4 nanoparticles were embedded into the NG/MWNTs structure followed by an intermittent microwave heating procedure under air conditions at 700 °C. The crystallinities of MWNTS, GO, N-G/MWNTs, and Co3O4/3D N-G/MWNTs were analyzed by XRD patterns. Figure 2(a) shows a peak at 2θ = 26.4° corresponding to the

Figure 3. Raman spectra of GO, N-G/MWNTs, and Co3O4/3D N-G/ MWNTs.

cm−1and G band at 1593 cm−1 are observed in GO, N-G/ MWNTs, and Co3O4/3D N-G/MWNTs, which are attributed to the disorder-induced defects in the curved graphene sheet and the first-order scattering of the E2g phonon of sp2 C atoms.23 In addition, five distinct main peaks of F2g1, Eg, F2g2, F2g3, and A1g modes of the crystalline Co3O4 are observed at 187, 458, 507, 603, and 671 cm−1, respectively, further inferring the coexistence of 3D N-G/MWNTs and Co3O4.24 As shown in Figure 3, the ratio of peak intensities of D and G bands (ID/ IG) for GO, N-G/MWNTs, and Co3O4/3D N-G/MWNTs have been estimated to be around 0.87, 1.08, and 1.13, respectively. Obviously, the ID/IG ratio of Co3O4/3D N-G/ MWNTs is the highest compared with the others under the same conditions, which indicates that GO formed a higher level disordered structure in the Co 3 O 4 /3D N-G/MWNTs composite and could afford more active sites for electron storage.25,26 Figure 4(a) shows SEM images at low magnification of the Co3O4/3D N-G/MWNTs. Interconnected loosely packed 3D frameworks structure of MWNTs have constructed random open pores with graphene layers. Such a 3D-structured network could offer them the biggest accessible surface area, which provides not only more exposed active nucleation sites for the embedding of Co3O4 nanoparticles but also more accessibility to green channels for ions to guarantee a fast mass transfer. To further characterize the composite morphology more clearly, the sample was characterized by low and high magnification TEM (Figure 4(b−d)). The MWNTs were not only absorbed on the surface of GO without twining around together but also participated in the connection of graphene sheets (Figure 4(b)). After microwave-assisted heating treatment, Co3O4 nanoparticles are homogeneously embedded throughout the whole 3D graphene/MWNTs framework. The π−3d orbital interaction makes it possible for the graphene/MWNTs to coordinate strong impaction with Co3O4, which enables high-

Figure 2. XRD patterns of MWNTS, GO, N-G/MWNTs, and Co3O4/ 3D N-G/MWNTs.

layered and hexagonal structure of the (002) carbon nanotubes.22 The diffraction peak at 2θ = 10.2° in Figure 2(b) is indexed to the 001 diffraction peak of oxidation treatment of graphite. The strong diffraction peak of MWNTs is shown in the N-G/MWNTs hybrid structure Figure 2(c), while the broadened peak attributed to the amorphous nature of GO sheets is also detected. More specifically, the sharp characteristic peak of GO is absent, and the peak shifts at around 13.2°, after the hydrothermal process, demonstrating the effective reduction of GO. In the patterns in Figure 2(d), the diffraction peaks at 2θ angles of 19.1°, 31.3°, 36.8°, 38.2°, 44.8°, 59.4°, and 5101

DOI: 10.1021/acssuschemeng.7b00453 ACS Sustainable Chem. Eng. 2017, 5, 5099−5107

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ACS Sustainable Chemistry & Engineering

Figure 4. (a) SEM images of Co3O4/3D N-G/MWNTs at low magnification. (b−d) Low and high magnification TEM of Co3O4/3D N-G/ MWNTs. (e) HRTEM image of Co3O4/3D N-G/MWNTs. (f) EDS of Co3O4/3D N-G/MWNTs (inset is particle size distribution charts).

also investigated by the high-resolution analysis. The N 1s spectra can be divided into mainly three peaks: pyridine-like N (398.3 eV), pyrrole-like N (399.4 eV), and graphite-like N (400.8 eV).29 The Co 2p spectra of Co3O4/3D N-G/MWNTs appear in two major peaks located at 780.1 and 795.3 eV in the magnified spectrum, which can be attributed to the Co 2p3/2 and Co 2p1/2 spin orbit peaks of Co3O4. The satellite peaks are two broad and small peaks of Co at the high binding energy side of the two major peak edges. These demonstrate the presence of Co2+ in the materials.30,31 The porous property and specific surface area of Co3O4/3D N-G/MWNTs and Co3O4 was further performed by measuring N2 adsorption/desorption isotherms and pore size distribution curves. The adsorption isotherm of Co3O4/3D N-G/MWNTs and Co3O4 belong to type IV curves according to IUPAC classification (Figure 6(a)) and the hysteresis loops at relative pressure (P/P0) between 0.7 and 1.0, indicating the presence of a mesoporous structure. Without 3D N-G/MWNTs, pure Co3O4 shows a specific surface area of 217.4 m 2 g−1 and the most probable pore size of 5.2 nm, as shown in Figure 6(b). Due to the introduction of 3D N-G/MWNTs, the hybrid shows increased specific surface areas of 648.5 m2 g−1, which can be ascribed to the three-dimensional interconnected high surface area structure, and N-doping makes Co3O4 nano-

speed electron transport between them. The high-resolution (HRTEM) image of Co3O4/3D N-G/MWNTs reveals Co3O4 nanoparticles with clear lattice fringes with a spacing of 0.26 nm, corresponding to the (311) plane of Co3O4 (Figure 4(e)). The elemental analysis by EDS reveals the presence of C, Co, and O in the Co3O4/3D N-G/MWNTs material (Figure 4(f), the Cu signal comes from the sample holder). Further, the particle size distribution charts (Figure 4(f), inset chart) indicate that the particle sizes around 10.4 nm were well dispersed inside the 3D N-G/MWNTs composite networks. As shown in Figure 5, X-ray photoelectron spectroscopy (XPS) analysis of N-G/MWNTs and Co3O4/3D N-G/ MWNTs further give the element states for the Co3O4 and N−C alloying effect. The N 1s peak located at 402 eV is clearly detected in N-G/MWNTs and Co3O4/3D N-G/MWNTs materials, indicating the success of N doping into the G/ MWNTs framework (Figure 5(a)). The intensity of the carbon and oxygen ratio remarkably increase in comparison with that of N-G/MWNTs, indicating that most of the oxygen originates from cobalt oxide. The high resolution C 1s spectra show the intensities of the three main peaks. The strongest peak at 284.7 eV is attributed to the C−C bonds, and the other peaks at 285.3 and 289.1 eV can be associated with the C−N and CO bonds (Figure 5(b)).27,28 The properties of N doping can be 5102

DOI: 10.1021/acssuschemeng.7b00453 ACS Sustainable Chem. Eng. 2017, 5, 5099−5107

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ACS Sustainable Chemistry & Engineering

Figure 5. XPS spectra of Co3O4/3D N-G/MWNTs: (a) Survey spectra of all. (b−d) Spectra correspond to C 1s, N 1s, and Co 2p spectra, respectively.

Figure 6. (a) Nitrogen adsorption (■) and desorption (▲) isotherms of Co3O4/3D N-G/MWNTs and Co3O4 measured at standard temperature and pressure. (b) Corresponding BJH pore size distribution plots.

Figure 7. (a) CV curves of NG, N-G/MWNTs, 3D-N-G/MWNTs, and Co3O4/3D N-G/MWNTs composite electrodes at a scan rate of 10 mV s−1 within the potential of 0−0.6 V. (b) CV curves of Co3O4/3D N-G/MWNTs electrode at different scan rates.

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DOI: 10.1021/acssuschemeng.7b00453 ACS Sustainable Chem. Eng. 2017, 5, 5099−5107

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ACS Sustainable Chemistry & Engineering

Figure 8. (a) CV curves of NG, N-G/MWNTs, 3D-N-G/MWNTs, Co3O4, and Co3O4/3D N-G/MWNTs composite at a current density of 2 A g−1. (b) GCD curves of Co3O4/3D N-G/MWNTs at different current densities.

Figure 9. (a) EIS of NG, Co3O4/ NG, N-G/MWNTs, 3D-N-G/MWNTs, and Co3O4/3D N-G/MWNTs electrodes within the frequency range from 0.01 Hz to 100 kHz in 0.6 M KCl. (b) Specific capacitance at various current densities for Co3O4/ NG, 3D-N-G/MWNTs, and Co3O4/3D NG/MWNTs electrodes.

CoOOH + OH− = CoO2 + H 2O + e

particles much smaller. The hysteresis loop of Co3O4/3D N-G/ MWNTs can be categorized as Type H1, which illustrates that the hybrid contains not only a mesopore structure but also a fraction of micropore distribution.32 The size of the pores based on desorption data exhibits a wide distribution in the range from 5 to 20 nm in the Co3O4/3D N-G/MWNTs structure, indicating the coexistence of a large number of mesopores and micropores.33 This porous structure with a high surface area helps reduce resistance for electrolyte ions into the deep inner interface area during the charge−discharge process. Co3O4/3D N-G/MWNTs with a particular pore structure and tremendous surface area are further determined as electrode materials for supercapacitors in a 6 M KOH electrolyte by cyclic voltammetry (CV) and galvanostatic charge−discharge measurements. Figure 7(a) shows the CV curves of NG, N-G/MWNTs, 3D-N-G/MWNTs, and Co3O4/ 3D N-G/MWNTs composite electrodes at a scan rate of 10 mV s−1 within the potential of 0−0.6 V. The Co3O4/3D N-G/ MWNTs composite has more larger integrated CV curve area at the same scan rate compared with the other CV curves, indicating that the framework Co3O4/3D N-G/MWNTs electrode has good charge storage and high capacitance. A couple of high redox peaks are visible in the Co3O4/3D N-G/ MWNTs CV curve compared with others, indicating that the capacitance contribution is mainly originating from the faradic capacitance of Co3O4. The redox peaks correspond to the different cobalt redox states such as Co4+/Co3+ and Co3+/Co2+. The redox reactions are given below34,35 Co3O4 + OH− + H 2O = 3CoOOH + e

(7)

The specific capacitance value of the CV curves was calculated on the basis of eq 5. The calculated specific capacitance values of NG, N-G/MWNTs, 3D-N-G/MWNTs, Co3O4, Co3O4/NG, and Co3O4/3D N-G/MWNTs are 394, 508, 785, 942, 1297, and 1983 F g−1 at a scan rate of 10 mV s−1, respectively. The specific capacitance of Co3O4/3D N-G/MWNTs is 2.5 times that of 3D N-G/MWNTs, indicating that the capacitance contribution is mainly contributed from the faradic capacitance of Co3O4. ICP analysis gave the actual Co3O4 content as 89.7 wt %, so the percentage of 3D N-G/MWNTs in Co3O4/3D NG/MWNTs is 10.3 wt %. However, the capacitance of pure Co3O4 only is 942 F g−1 because without carrier Co3O4 nanoparticles were easy to accumulate, which led to a large particle size, small specific surface area, and low specific capacitance. Co3O4/3D N-G/MWNTs display the highest specific capacitance also due to the cooperation effect between Co3O4 and 3D N-G/MWNTs. The CV curves of the Co3O4/ 3D N-G/MWNTs electrode at different scan rates are shown in Figure 7(b). A pair of redox peaks is visible at a low scan rate but migrates to negative and positive potentials, respectively, at a high scan rate because of the polarization in the electrode with the rise in peak currents.36 Galvanostatic charge−discharge measurements were recorded in the potential range between 0 and 0.6 V vs SCE. Figure 8(a) shows that Co3O4/3D N-G/MWNTs has highest specific capacitance (1903 F g−1) at a current density of 2 A g−1, which demonstrates that a 3D N-G/MWNTs special structure with a small size of high crystallinity Co3O4 can give the ions access to the inner sites and store much more electric energy.

(6) 5104

DOI: 10.1021/acssuschemeng.7b00453 ACS Sustainable Chem. Eng. 2017, 5, 5099−5107

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ACS Sustainable Chemistry & Engineering The specific capacitance value obtained in our present study is 1.5−8 times higher than previously reported values at the same condition.37−40 The galvanostatic charge−discharge measurements were also carried out at various current densities. When current density is increased from 1 to 15 A g−1, the Co3O4/3D N-G/MWNTs electrode still maintains 84% of capacitance retention (Figure 8(b)). This should be attributed to the good electrical conductivity of the 3D porous structure of G/ MWNTs and nitrogen-doped active sites, which is advantageous to charge transfer and the ions quick permeability from the external electrolyte to the interior surfaces. In order to understand the charge transfer kinetics of the hybrid supercapacitor electrochemical properties, electrochemical impedance spectroscopy (EIS) was measured within the frequency range from 0.01 Hz to 100 kHz in 6 M KOH. Figure 9(a) presents Nyquist plot impedances spectra of NG, Co3O4/ NG, N-G/MWNTs, 3D-N-G/MWNTs, and Co3O4/3D N-G/ MWNTs electrodes. It can be observed that the intersection of the Nyquist plot at a high frequency of the Co3O4/3D N-G/ MWNTs composite is smaller than the other electrodes, which further illustrates that the Co3O4/3D N-G/MWNTs composite have small solution resistance, charge transfer resistance, and equivalent series resistance.41 Besides, the slope of the Nyquist plot at low frequency for the hybrid electrode was the steepest, indicating faster kinetics of the diffusion of the electrolyte ions in the electrode pore channel, which reflects the higher charge− discharge performance.42,43 These can be attributed to the 3D G/MWNTs framework structure of high electrolyte ions transport and the synergistic effect between Co3O4 nanoparticles and N-doped hybrids. Figure 9(b) shows the specific capacitance at various current densities for the prepared materials. It was obvious that the Co3O4/3D N-G/MWNTs hybrid electrode exhibits practical capacitance as high as 2039 F g−1 at low current density of 1 A g−1. The practical capacitance of this hybrid is still as high as 1703 F g−1 even at high current density of 15 A g−1, which is still larger than the initial capacitance of Co3O4/NG (1587 F g−1). Clearly, 84% of capacitance remained when the current density varies from 1 to 15 A g−1, exhibiting good cycling stability. It can also be found that the 3D N-G/MWNTs were kept in a constant low value practical capacitance during the whole current density cycling test, which further shows that the embedding of Co3O4 plays a major role in the contribution of the capacitance. The energy density and power density of 3D N-G/MWNTs, Co3O4/NG, and Co3O4/3D N-G/MWNTs are also shown in Figure 9(b), which could further evaluate the electrochemical performance of the asymmetric supercapacitor. The energy density (ED) and power density (PD) were calculated from the discharge curves using the following equations44,45 ED = 1/2CV 2 (Wh kg −1)

Figure 10. Co3O4/NG and 3D-N-G/MWNTs average specific capacitances versus the cycle number at a current density of 15 Ag1−.

discharge experiments at a current density of 15 A g−1. The cycle of the Co3O4/3D N-G/MWNTs electrode has indicated that the electrode has very well maintained more than 94% of its primary capacitance after 6000 charge−discharge cycles, even at a high current of 15 A g−1. This high cycling stability could be attributed to the synergistic effect of the stable 3D framestructure and abundant nitrogen-doping active sites. All of the causes discussed above show that the new Co3O4/3D N-G/ MWNTs type composite should be a desirable electrode material for next generation supercapacitors. On the basis of the above test results, we propose that the good capacitive performance of the Co3O4/3D N-G/MWNTs is ascribed to the following three aspects: First, MWNTs as a bridge link between graphene sheets form a highly 3D porous structure, which not only prevents restacking of graphene sheets but also reduces the transport path of ions in favor of its rapid effective migration throughout the porous electrode. Second, the nitrogen-doped networks modulate the surface electronic structure of G/MWNTs, thus offering increased active sites to imbedding nanoparticles. Finally, the Co3O4 embedding the highly conductive 3D N-G/MWNTs create a good way for accelerating the charge transfer and hence capacitor storage.46

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CONCLUSIONS In summary, novel Co3O4/3D N-G/MWNTs hybirds have been successfully prepared by a simple hydrothermal process and succedent microwave treatment and were applied as electrode material for supercapacitors. The improved Co3O4/ 3D N-G/MWNTs electrode exhibits a high specific capacitance up to 2039 F g−1 at a current density of 1 A g−1. After 6000 charge−discharge cycles at a current density of 15 A g−1, the excellent cycle capacity retention of the hybrids is about 94.0%. Besides the enhanced surface area and the high conductivity provided by the free-standing 3D graphene/MWCNTs structure and N-doping property, this electrode possesses high specific capacitance and good cycle stability due to the improved synergistic effect with Co3O4 nanoparticles. Convincingly, intermittent microwave synthetic approaches provide an efficient green route for formation of Co3O4 nanoparticles embedding in 3D N-G/MWNTs networks. We believe that the unique composite material should not only be a promising electrode material for high supercapacitive performance but

(8)

PD = E/Δt (W kg −1)

(9) 1−

Here, C is the specific capacitance (Fg ), V refers to potential window (V), and Δt is discharge time (h). 3D N-G/MWNTs, Co3O4 /NG, and Co3O4/3D N-G/MWNTs show the energy density of 19.27, 38.59, and 59.34 Wh kg−1 with the power density of 131.35,142.26, and 150.73 W kg−1, respectively, at a current density of 2A g−1. To further study the stability of the electrode, we carried out the practical application of long-term cycling stability at a high current density. As shown in Figure 10, the life stability cycling tests for 6000 times were examined by galvanostatic charge− 5105

DOI: 10.1021/acssuschemeng.7b00453 ACS Sustainable Chem. Eng. 2017, 5, 5099−5107

Research Article

ACS Sustainable Chemistry & Engineering

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also evoke a new perspective for other applications, including electrocatalysis, biosensors, and other energy storage systems.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 11 88791708. Fax: +86 11 88791800. E-mail: [email protected]. ORCID

Mingmei Zhang: 0000-0002-8251-7913 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the financial supports through the Natural Science Foundations of Jiangsu (BK20140531), Research Foundation for Talented Scholars of Jiangsu University (14JDG187), and National Natural Science Foundations of China (21306067).

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REFERENCES

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DOI: 10.1021/acssuschemeng.7b00453 ACS Sustainable Chem. Eng. 2017, 5, 5099−5107