MWNTs Nanoframework-Embedded

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Nitrogen-doped 3D Graphene/MWNTs nano-frameworks embedded Co3O4 for high electrochemical performance supercapacitors Mingmei Zhang, Ying Wang, Denghui Pan, Yuan Li, Zaoxue Yan, and Jimin Xie ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 12 May 2017 Downloaded from http://pubs.acs.org on May 14, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Sustainable Chemistry & Engineering is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Nitrogen-doped 3D Graphene/MWNTs nano-frameworks embedded Co3O4 for high electrochemical performance supercapacitors

Mingmei Zhang∗, Ying Wang, Denghui Pan, Yuan Li, Zaoxue Yan, Jimin Xie School of Chemistry and Chemical Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, China ∗

Correspondence author: E-mail address: [email protected]. (M. M. Zhang)

Abstract Co3O4 nanoparticles embedded on three-dimensional (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 size of 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 huge surface active area and abundant three-dimensional porous configuration to provide plenty of paths for the rapid electrons/ions transportation and electron-transfer resistances. When the lA/g current density is applied, capacitances of 2039.4 F/g are fulfilled, even the charge-discharge current increases from 1 to 15 A/g, there is still 84% remain of the capacitance. In particular, the hybrid also exhibits high energy density (59.34Whkg-1) and power density(150.73 Wkg-1), as well as cycling stability maintain more than 94% of its primary capacitance after 6000 charge-discharge cycles at current density of 15 A g-1. The perfect electrochemical performance of 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 offer the graphene / carbon nanotube surface with a large numbers of defects to support high performance cobalt oxide.

∗

Correspondence author Tel: +86 11 88791708; fax: +86 11 88791800. E-mail address: [email protected]. (M. M. Zhang) 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Key words: three-dimensional graphene/MWNTs frameworks; Co3O4 nanoparticles; nitrogen-doped; supercapacitor

1. Introduction

When the need to release large amounts of energy in the short time, the battery will not provide power output to meet the requirements; the supercapacitor as a new categories of energy storage equipment can completely discharged the stored energy in the range of microseconds to milliseconds.1, 2 Supercapacitors fill the performance space between traditional capacitors and batteries. It can be used in electric or gas-electric hybrid vehicles in the recovery of waste energy during braking, or provide energy for portable communication tools, and even in some areas to replace the battery.3, 4 While, existing electrode materials deficiency of stable structure and large surface area are always suffering from limited cycle life and low energy density, due to the phase changes caused by redox reactions when energy is stored/released on the surface or subsurface of the electrode. In order to meet future applications, preparations of high energy density, vast specific capacitance and long cycle performance advanced electrode materials will be the critical study of supercapacitors. Recently, growing or embedding the 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 interaction between metal oxide and graphene surface, the B, P, S, and N are doped into the lattice of graphene to provide more active sites and nucleation sites.6 For example, Yang et al. synthesized MnO2/N-graphene nanosheet by one step hydrothermal method at lower temperatures, they found nitrogen doped are beneficial for anchoring MnO2 nanoparticles on graphene.7 In our previous work, it was found that nitrogen doping in graphene can make Co3O4 nanoparticles with smaller particle size and more uniform the interface.8 Currently, more focus has been concentrated on the synthesis of stable 3D graphene hybrid composites.9 Graphene surface are usually 2


Page 2 of 30

Page 3 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


compromised by reunite of graphene sheet each other in view of the strong π-π interaction, which couldn’t suit the fast motion of ion in large size. As a result of the above, many efforts have been made to obtain three dimensional structures for outstanding electrochemical properties, which have been proven to be effective in preventing graphene sheets from agglomerating and providing rapid transfer channel of the ion.10,11 Huang

12

prepared a three-dimensional graphene structure using foam

nickel substrate as a support structure, electrochemical storage energy properties of the three-dimensional structure is 5 times of 2D planar structure graphene. Recently reports have also showed that graphene incorporation with multiwalled carbon nanotubes can effectively avoid the overlapping of graphene sheets and that provides more superficial area for the uniform dispersion 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 nano materials in situ growth method. This unique carbon structure can be integrated pseudo capacitance with the double capacitance, which allows the nano structure network to have high speed ion 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 come into being new special morphologies with potential application in supercapacitors. A hybrid of Co3O4 nanoparticles with super fine particle sizes were situ embedded in 3D N-G/MWNTs by 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 supercapacitor.

2. Experimental

2.1. Preparation of N-Doped Graphene /MWNTs (N-G/MWNTs) and Co3O4/3D N-G/MWNTs 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 of -COOH, -OH groups on the surface of MWNTs (diameters of 20−40 nm, purity > 95%, purchased from Shenzhen Bill Technology Co.), the 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, 45mM of Co (NO3)26H2O, 50 mg of GO, 50 mg of MWNTs and 0.15 g of hexamethylenetetramine (C6H12N4) were added into 20 mL of distilled water and ultrasonicated for 6 h to obtain a uniformly 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℃ for 24 h. During the hydrothermal process, N atoms were doped into the graphene and MWNTs networks through slow decomposition of C6H12N4. C6 H12N 4 + 6H2 O =6 HCHO + 4NH3

(1)

NH3+ H2 O = NH4++OH-

(2)

Co2+ + 2OH- = Co(OH)2

(3)

OHˉions react with Co2+ and result in the precursor of Co(OH)2 covered on the surface N-G/MWNTs structure, followed by intermittent microwave heating (900 W) with a 25s 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 microwave heating process, meanwhile, N-G/MWNTs form 3D N-G/MWNTs framework. 6Co(OH)2 +O2 = 2Co3O4 + 6 H2O

(4)

Final products were collected and dried to actual weight under vacuum drying condition at 60 °C. The inductively coupled plasma spectroscopy (ICP, IRIS(HR), USA) analysis gave the actual Co3O4 contents as 89.7 wt. %. 2.2. Preparation of electrode The Co3O4/3D N-G/MWNTs composite working electrode was carried out by thoroughly stirring Co3O4/3D N-G/MWNTs active hybrid (1.0 mg), acetylene carbon 4


Page 4 of 30

Page 5 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


black and polyvinylidene (PVD) (mass ratio of 50:15:2) in ethanol for 24h. The mixture were compacted tightly on precleaned nickel foam substrate (1cm ×1cm ) at a pressure of 10 MPa and dried in vacuum for 48 h at 80℃. For a contrastive study, other electrode, such as NG, N-G/MWNTs, Co3O4/N-G and 3D-N-G/MWNTs were obtained using a similar preparation process. 2.3. Characterization of the composites Structural properties and morphology of the as-prepared composites were examined using a TEM (JEOL-JEM-2010, Japan) operating at 120 kV and a XRD D8 Advance X-ray powder diffraction (Bruker axs company, Germany). X-ray photoelectron spectroscopy (XPS) conducted on a PHI-5702. The structures of the obtained samples were carried out by Raman scattering was AU10 performed on a Renishaw (In Via) spectrometer with a 532 nm laser source. The electrochemical determination was carried out on an IM6e potentiastat (Zahner-Electrik, Germany) in a 6 M KOH electrolyte with three-electrode system, comprised with Co3O4/3D N-G/MWNTs as working electrode, platinum sheet as the counter electrodes and saturated calomel electrode (SCE) as the reference electrodes. The electrochemical impedance spectroscopy (EIS) was measured at the open-circuit potential over the frequency range of 105 to 0.02 Hz with an a.c. 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 ∆t/( m ∆V)

(5)

where C- specific capacitance (F g-1) of the electrode, I -charging current (A), ∆ ttime of discharge (s), m- mass of the active material (g) and ∆V -discharging potential range (V), respectively.

3. Results and discussion

The schematic diagram of the Co3O4/3D N-G/MWNTs hybrid synthesis was illustrated in Fig. 1. Briefly, the acid activated MWNTs were combined with GO through π−π stacking by ultrasonic to achieve the graphene oxide and acid carbon 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

nanotube

composite

co-exist

in

perfect

harmony.

Page 6 of 30

After

adding

hexamethylenetetramine and hydrothermal treatment at 180℃ for 24 h, N atoms were doped into the graphene and MWNTs networks through slow decomposition of C6H12N4. Co3O4 nanoparticles were embedded into the N-G/MWNTs structure followed by intermittent microwave heating procedure under air conditions at 700°C. The crystallinities of MWNTS, GO, N-G/MWNTs, Co3O4/3D N-G/MWNTs were analyzed by XRD patterns. Fig. 2(a) shows a peak at 2θ = 26.4° corresponding to layered and hexagonal structure of the (002) carbon nanotubes.22 The diffraction peak at 2θ = 10.2° in Fig. 2(b) is indexed to 001 diffraction peak of oxidation treatment of graphite. Strong diffraction peak of MWNTs is shown in the N–G/MWNTs hybrid structure Fig. 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 Fig. 2(d), the diffraction peaks at 2θ angles of 19.1°, 31.3°, 36.8°, 38.2°, 44.8°,59.4°and 65.3° are the characteristic peaks of Co3O4 (JCPDS 65-3103), which indicated that cubic Co3O4 was embed in 3D N-G/MWNTs structure successfully. All the characteristic peaks of Co3O4, graphene and MWCNTs are observed in Fig. 2(d), demonstrating the coexistence of Co3O4, graphene and MWCNTs in the composite frameworks. Raman spectra further confirms the change of 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 Fig. 3. Two characteristic peaks of D band at 1350 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 co-existence of 3D N-G/MWNTs and Co3O4.24 As shown in Fig. 3 the ratio of peak intensities of D and G bands (ID/IG) for GO, N-G/MWNTs, Co3O4/3D N-G/MWNTs have been estimated to 6


Page 7 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


be around 0.87, 1.08 and1.13, respectively. Obviously, ID/IG ratio of Co3O4/3D N-G/MWNTs is the highest comparing with the others under the same conditions, which indicates that GO was formed higher level disordered structure in the Co3O4/3D N-G/MWNTs composite and could afford more active sites for electron storage.25,26 Fig. 4a shows SEM images at low magnification of the Co3O4/3D N-G/MWNTs. Interconnected loosely packed 3D frameworks structure by MWNTs have constructed random open pores with graphene layers. Such 3D-structured network could offer them a biggest accessible surface area, which provides not only more exposed active nucleation sites for the embedding of Co3O4 nanoparticles but also more accessibility green channel for ions to guarantee a fast-speed mass transfer. To further characterize the composite morphology more clearly, the sample was characterized by low and high magnification TEM (Fig.4b, c and d). The MWNTs were not only absorbed on the surface of GO without twine around together but also participate in the connection of graphene sheets (Fig.4b). After microwave assisted heating treatment, Co3O4 nanoparticles

are

homogenously

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 enable to high-speed electron transport between them. The high-resulution (HRTEM) image of Co3O4/3D N-G/MWNTs reveals that the Co3O4 nanoparticles with clear lattice fringes with a spacing of 0.26 nm, corresponding with the (311) plane of Co3O4 (Fig.4e). The elemental analysis by EDS reveals the presence of C, Co and O in the Co3O4/3D N-G/MWNTs material (Fig. 4f, the Cu signal comes from the sample holder). Further, the particle size distribution charts (Fig.4f insert chart) indicate that the particles sizes around 10.4 nm were well dispersed inside the 3D N-G/MWNTs composite networks. As shown in Fig.5, X-ray photoelectron spectroscopy (XPS) analysis of N-G/MWNTs and Co3O4/3D N-G/MWNTs further give the element states about 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 (Fig. 5a). The intensities of ratio of carbon and oxygen 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

remarkably increase in comparison with that of N-G/MWNTs, indicating that most of the oxygen originates from cobalt oxide. The high resolution C1s spectra shows the intensities of 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 289.1eV can be associated with C−N and C=O bonds(Fig.5b).27,28 The properties of N-doped can be also investigated by the high-resolution analysis. The N 1s spectra can be divided into mainly three peaks, the pyridine-like N(398.3 eV), pyrrole-like N (399.4 eV), and graphite-like N(400.8 eV). 29

Co 2p spectra of Co3O4/3D N-G/MWNTs appear 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 peaks edge. These demonstration 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 (Fig. 6a), the hysteresis loops at relative pressure (P/P0) between 0.7 and 1.0, indicating the presence of the mesoporous structure. Without 3D N-G/MWNTs, the pure Co3O4 show a specific surface area of 217.4 m 2 g-1and the most probable pore size of 5.2 nm, as shown in Fig. 6b. Due to the introduction of 3D N-G/MWNTs, the hybrid show increased specific surface areas of 648.5 m2 g-1, which can be ascribed to three dimensional interconnected high surface area structure and N-doping makes Co3O4 nanoparticles much smaller. The hysteresis loop of Co3O4/3D N-G/MWNTs can be categorized as Type H1 which illustrate that the hybrid contains not only a mesopore structure but also a fraction of micropores distribution. 32 The size of the pores based on desorption data exhibits a wide distribution of the range from 5 to 20 nm in Co3O4/3D N-G/MWNTs structure, indicating the coexistence of a large number of mesoporous and micropores. 33 This porous structure with high surface area helps reduce resistance for electrolyte ions into the deep inner interface area during the charge/discharge process. 8


Page 8 of 30

Page 9 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Co3O4/3D N-G/MWNTs with particular pore structure and tremendous surface area is further determined as electrode materials for supercapacitors in 6 M KOH electrolyte by cyclic voltammetry (CV) and galvanostatic charge-discharge measurements. Fig. 7 (a) show the CV curves of NG, N-G/MWNTs ,3D-N-G/MWNTs and Co3O4/3D N-G/MWNTs composite electrodesat 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 lager 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 highly redox peaks is visible in Co3O4/3D N-G/MWNTs CV curve compared with others, indicating that the capacitance contribution is mainly originate 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 below 34,35 Co3O4+ OH-+H2O = 3CoOOH +e

(6)

CoOOH+ OH- = CoO2+H2O+e

(7)

The specific capacitance value of the CV curves was calculated on the basis of the equation (5). The calculated specific capacitance values of NG, N-G/MWNTs, 3D-N-G/MWNTs, Co3O4, Co3O4/N-G and Co3O4/3D N-G/MWNTs are 394 F g-1, 508 F g-1, 785F g-1, 942F g-1, 1297F g-1 and 1983F g-1 at the scan rate of 10 mV s-1, respectively. The specific capacitance of Co3O4/3D N-G/MWNTs are two and a half times than 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 contents as 89.7 wt. %, so the percentage of 3D N-G/MWNTs in Co3O4/3D N-G/MWNTs is 10.3 wt. %. However, the capacitance of pure Co3O4 only have 942F g-1, because without carrier, Co3O4 nanoparticles were easy to accumulate, which led to the large particle size, small specific surface area and low specific capacitance. Co3O4/3D N-G/MWNTs displays the highest specific capacitance also due to 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 showed in Fig. 7b. A pair of redox peaks is visible at low scan rate, but migrates to negative and positive 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

potentials respectively at high scan rate because of the polarization in the electrode with the rise of peak currents. 36 Galvanostatic charge-discharge measurements were recorded in the potential range between 0 and 0.6 V vs SCE. Fig. 8 a shows the Co3O4/3D N-G/MWNTs has highest specific capacitance (1903F g-1) at a current density of 2 A g-1, which demonstrates 3D N-G/MWNTs special structure with a small size of high crystallinity Co3O4 can made the ions access to the inner sites and store much more electric energy. 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 A g-1 to 15 A g-1, the Co3O4/3D N-G/MWNTs electrode still maintain 84% of capacitance retention (Fig. 8b). This should be attributed to the good electrical conductivity of 3D porous structure of G/MWNTs and nitrogen doped active sites, which is advantageous to charge transfer and ion quickly 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 of 0.01 Hz to 100 kHz in 6M KOH. Fig. 9a presented 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 high frequency of Co3O4/3D N-G/MWNTs composite is smallest than the other electrodes, which further illustrate the Co3O4/3D N-G/MWNTs composite have the small solution resistance, charge transfer resistance and equivalent series resistance.41 Besides, the slope of the Nyquist plot at low frequency for the hybrids electrode was the steepest, indicating faster kinetics of the diffusion of 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. Fig. 9 b shows the specific capacitance at various current densities for the prepared materials. It was obvious that 10


Page 10 of 30

Page 11 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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. And 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 is 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 kept in a constant low value practical capacitance during the whole current density cycling test, which further shows that the embedding of Co3O4 play a major role of 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 Fig. 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 equations 44,45 ED =1/2CV2 (Wh kg-1)

(8)

PD =E /∆t (W kg-1)

(9)

Here C is the specific capacitance (Fg-1), V refers to potential window (V) and ∆t is discharge time (h). 3D N-G/MWNTs, Co3O4 /NG and Co3O4/3D N-G/MWNTs show the energy density of 19.27 Wh kg-1, 38.59 Whkg-1 and 59.34Whkg-1 with the power density of 131.35 W kg-1,142.26W kg-1 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 Fig.10, the life stability

cycling

tests

for

6000

times

were

examined

by

galvanostatic

charge-discharge experiment 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 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

next generation supercapacitors. Based on above test results analyse, 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 prevent restacking of graphene sheets, but also reduce the transport path of ions in favor of its rapid effective migration throughout the porous electrode. Second, the nitrogen doped modulate the surface electronic structure of G/MWNTs, thus offer increased active sites to imbedding nanoparticles. Finally, the Co3O4 embedding the highly-conductive 3D N-G/MWNTs create a high net way for accelerating the charge transfer and hencing capacitor storage. 46

Conclusions

In summary, the novel Co3O4/3D N-G/MWNTs hybirds have been successfully prepared by a simple hydrothermal process and succedent microwave treatment, which were applied as an electrode material for supercapacitors. The improved Co3O4/3D N-G/MWNTs electrode exhibits a high specific capacitance up to 2039 F g-1 at current density of 1 A g-1. After 6000 charge-discharge cycles at 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 freestanding 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 also evoke a new perspective for other applications, including electro catalysis, biosensors, and other energy storage systems.

Acknowledgements 12


Page 12 of 30

Page 13 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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

References [1] Nagamuthu, S.; Vijayakumar, S.; Lee, S.H.; Ryu. K.S. Hybrid supercapacitor devices based on MnCo2O4 as the positive electrode and FeMn2O4 as the negative electrode. Appl. Surf. Sci. 2016, 390, 202–208. [2] Konikkara, N.; Kennedy, L.J.; Vijaya. J.J. Preparation and characterization of hierarchical porous carbons derived from solid leather waste for supercapacitor applications. J. Hazard. Mater. 2016, 318,173–185. [3] Cui, L.F.; Huang, L.H.; Ji, M.; Wang, Y.G.; Shi, H.C.; Zuo, Y.H.; Kang, S.F. High-performance MgCo2O4 nanocone arrays grown on three-dimensional nickel foams: Preparation and application as binder-free electrode for pseudo-supercapacitor. J. Power Sources 2016, 333, 118–124. [4] Gobal, F.; Faraji. M. Electrodeposited polyaniline on Pd-loaded TiO2 nanotubes as active material for electrochemical supercapacitor. J. Electroanal. Chem. 2013, 691, 51–56. [5] Jiang, Y.Q.; Chen, L.Y.; Zhang, H.Q.; Zhang, Q.; Chen, W.F.; Zhu, J.K.; Song, D.M. Two-dimensional Co3O4 thin sheets assembled by 3D interconnected nanoflake array framework structures with enhanced supercapacitor performance derived from coordination complexes. Chem. Eng. J. 2016, 292, 1–12. [6] Zhao, Y.H.; Liu, M.X.; Deng, X.X.; Miao, L.; Tripathi, P.K. Ma, X.M.; Zhu, D.Z.; Xu, Z.J.; Hao, Z.X.; Gan, L.H. Nitrogen-functionalized microporous carbon nanoparticles for high performance supercapacitor electrode. Electrochim. Acta 2015,153, 448–455. [7] Yang, S.H.; Song, X.F.; Zhang, P.; Gao, L. Facile Synthesis of nitrogen-doped 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 30

graphene-ultrathin MnO2 sheet composites and their electrochemical performances. ACS Appl. Mater. Inter. 2013, 5, 3317–3322. [8] Li, Y.; Pan, D.H.; Zhang, M.M.; Xie, J.M.; Yan, Z.X. Ultrafine Co3O4 embedded in nitrogen-doped graphene with synergistic effect and high stability for supercapacitors. RSC Adv. 2016, 6, 48357–48364. [9] Hu, Q.Q.; Gu, Z.X.; Zheng, X.T.; Zhang, X.J. Three-dimensional Co3O4@NiO hierarchical nanowire arrays for solid-state symmetric supercapacitor with enhanced electrochemical performances. Chem, Eng. J. 2016, 304, 223–231. [10] Rao, R.H.; Chen, G.G.; Arava, L.M.R.; Kalaga, K.; Ishigami, M.; Heinz,T.F.; Ajayan, P.M.; Harutyunyan, A.R. Graphene as an atomically thin interface for growth of vertically aligned carbon nanotubes. Sci. Rep. 2013, 3, 1–5. [11] Flexer, V.; Chen, J.; Donose, B.C.; Sherrell, P.; Wallace, G.G., Keller, J. The nanostructure of three-dimensional scaffolds enhances the current density of microbial bioelectrochemical systems. Energy Environ. Sci. 2013, 6, 1291−1298. [12] Huang, Z.D.; Zhang, H.Y.; Chen, Y.M., Wang, W.G.; Chen, Y.T.; Zhong, Y.B. Microwave-assisted synthesis of functionalized graphene on Ni foamas electrodes for supercapacitor application. Electrochim. Acta 2013, 108, 421–428. [13] Cheng, Q.; Tang, J.; Ma, J.; Zhang, H.; Shinya, N.; Qin, L.C. Graphene and carbon nanotube composite electrodes for supercapacitors with ultra-high energy density. Phys. Chem. Chem. Phys. 2011, 13, 17615–17624. [14] Fan, Z.J.; Yan, J.; Zhi, L.J.; Zhang, Q.; Wei, T.; Feng, J.; Zhang, M.L.; Qian, W.Z.; Wei, F. A three-dimensional carbon nanotube/graphene sandwich and its application as electrode in supercapacitors. Adv. Mater. 2010, 22, 3723−3728. [15] Jha, N.; Ramesh, P.; Bekyarova, E.; Itkis, M.E.; Haddon, R.C. High energy density supercapacitor based on a hybrid carbon nanotube-reduced graphite oxide architecture.

Adv. Energy Mater. 2012, 2, 438−444.

[16] Sun, M.Q.; Li, H.; Wang, J.; Wang, G.C. Promising graphene/carbon nanotube foam@π-conjugated

polymer

self-supporting

composite

cathodes

for

high-performance rechargeable lithium batteries. Carbon 2015, 94, 864−871. [17] Erbay, C.; Yang, G.; Figueiredo, P.d.; Sadr, R.; Yu, C.H.; Han, A. 14


Page 15 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Three-dimensional porous carbon nanotube sponges for high-performance anodes of microbial fuel cells. J. Power Sources 2015, 298, 177−183. [18] Huang, H.; Chen,T.; Liu,X.Y.; Ma, H.Y. Ultrasensitive and simultaneous detection of heavy metal ions based on three-dimensional graphene-carbon nanotubes hybrid electrode materials. Anal. Chim. Acta 2014, 852, 45−54. [19] Fan, Z.J.; Yan, J.; Zhi, L.J.; Zhang, Q.; Wei, T.; Feng, J.; Zhang, M.L.; Qian, W.Z.; Wei, F. A three-dimensional carbon nanotube/graphene sandwich and its application as electrode in supercapacitors. Adv. Mater. 2010, 22, 3723–3728. [20] Zhang, M.M.; Li, Y.; Yan, Z.X.; Jing, J.J.; Xie, J.M.; Chen, M. Improved catalytic activity of cobalt core-platinum shell nanoparticles supported on surface functionalized graphene for methanol electro-oxidation. Electrochim. Acta 2015,158, 81–88. [21] Zhang, M.M.; Yan, Z.X.; Xie, J.M. Core/shell Ni@Pd nanoparticles supported on MWCNTs at improved electrocatalytic performance for alcohol oxidation in alkaline media. Electrochim. Acta 2012, 77, 237–243. [22] Yin, C.M.; Tao, C.A.; Cai, F.L.; Song, C.C.; Gong, H.; Wang, J.F. Effects of activation temperature on the deoxygenation, specific surface area and supercapacitor performance of graphene. Carbon 2016, 109,558–565. [23] Tan, Y.M.; Xu, C.F.; Chen, G.X.; Liu, Z.H.; Ma, M.; Xie, Q.J.; Zheng, N.F.; Yao, S.Z. Synthesis of ultrathin nitrogen-doped graphitic carbon nanocages as advanced electrode materials for supercapacitor. ACS Appl. Mater. Inter. 2013, 5, 2241−2248. [24] Dong, X.C.; Xu, H.; Wang, X.W.; Huang, Y.X.; Chan-Park, M.B.; Zhang, H.; Wang, L.H.; Huang, W.; Chen, P. 3D graphene cobalt oxide electrode for high-performance supercapacitor and enzymeless glucose detection. ACS Nano 2012, 6, 3206−3213. [25] Li, Y.; Cui, W.Q.; Liu, L.; Zong, R.L.; Yao, W.Q.; Liang, Y.H.; Zhu, Y.F. Removal of Cr(VI) by 3D TiO2-graphene hydrogel via adsorption enriched with photocatalytic reduction. Appl. Catal. B: Environ. 2016, 199, 412–423. [26] Wang, X.B.; Zhang, Y.J.; Zhi, C.Y.; Wang, X.; Tang, D.M.; Xu, Y.B.; Weng, Q.H.; Jiang, X.F.; Mitome, M.; Golberg, D.; Bando, Y. Three-dimensional strutted graphene 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 30

grown by substrate-free sugar blowing for high-power-density supercapacitors. Nat. Commun. 2013, 4, 2905–2913. [27] Zhao, Y.; Hu, C.G.; Hu, Y.; Cheng, H.H.; Shi, G.Q.; Qu, L.T. A versatile, ultralight, nitrogen-doped graphene framework. Angew. Chem. Inter. Edit. 2012, 51, 11371-11375. [28] Yang, J.; Jo, M.R.; Kang, M.; Huh, Y.S.; Jung, H.; Kang, Y.M. Rapid and controllable

synthesis

of

nitrogen

doped

reduced

graphene

oxide

using

microwave-assisted hydrothermal reaction for high power-density supercapacitors. Carbon 2014, 73,106–113. [29] Gong, X.; Liu, S.S.; Ouyang, C.Y.; Strasser, P.; Yang, R.Z. Nitrogen- and phosphorus-doped biocarbon with enhanced electrocatalytic activity for oxygen reduction. ACS Catal. 2015, 5, 920-927. [30] Su, Y.Z.; Zhang, Y.; Zhuang, X.D.; Li, S.; Wu, D.Q.; Zhang, F.; Feng, X.L. Low-temperature synthesis of nitrogen/sulfur co-doped three-dimensional graphene frameworks as efficient metal-free electrocatalyst for oxygen reduction reaction. Carbon 2013,62, 296-301. [31] Ambare, R.C.; Bharadwaj, S.R.; Lokhande, B.J. Non-aqueous route spray pyrolyzed Ru: Co3O4 thin electrodes for supercapacitor application. Appl. Surf. Sci. 2015, 349, 887-896. [32] Ajay, A.; Paravannoor, A.; Joseph, J.; Amruthalakshmi, V.; Anoop, S.S.; Nair, S.V.; Balakrishnan, A. 2D amorphous frameworks of NiMoO4 for supercapacitors: defining the role of surface and bulk controlled diffusion processes. Appl. Surf. Sci. 2015,326, 39–47. [33] Zhu, J.; Yang, D.; Yin, Z.; Yan, Q.; Zhang, H. Graphene and graphene-based materials for energy storage applications, Small 2014, 10, 3480–3498. [34] Rakhi, R.B.; Chen, W.; Hedhili, M.N.; Cha, D.; Alshareef, H.N. Enhanced Rate Performance of Mesoporous Co3O4 Nanosheet Supercapacitor Electrodes by Hydrous RuO2 Nanoparticle Decoration. ACS Appl. Mater. Inter. 2014, 6, 4196 -4206. [35] Rai, A.K.; Gim, J.; Trang-Vu, T.; Ahn, D.; Cho, S.J.; Kim, J. High Rate Capability and Long Cycle Stability of Co3O4/CoFe2O4 Nanocomposite as an Anode 16


Page 17 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Material for High-Performance Secondary Lithium Ion Batteries J. Phys. Chem. C 2014,118,11234-11243 . [36] Wang, Y.Y.; Lei, Y.; Li, J.; Gu, L.; Yuan, H.Y.; Xiao, D. Synthesis of 3D-nanonet hollow structured Co3O4 for high capacity supercapacitor. ACS Appl. Mater. Inter. 2014, 6, 6739−6747. [37] Liu, W.W.; Li, X.; Zhu, M.H.; He, X. High-performance all-solid state asymmetric supercapacitor based on Co3O4 nanowires and carbon aerogel. J.Power Sources 2015, 282, 179-186. [38] Cai, D. P.; Huang, H.; Wang, D.D.; Liu, B.; Wang, L. L.; Liu, Y.; Li, Q. H.; Wang, T.H. High-Performance Supercapacitor Electrode Based on the Unique ZnO@Co3O4 Core/Shell Heterostructures on Nickel Foam. ACS Appl. Mater. Inter. 2014, 6, 15905-15912. [39] Nguyen, V.H.; Shim, J.J. The 3D Co3O4/graphene/nickel foam electrode with enhanced electrochemical performance for supercapacitors. Mater. Lett. 2015, 139, 377 -381 [40] Balasubramanian, S.; Kamaraj, P.K. Fabrication of Natural Polymer Assisted Mesoporous Co3O4/Carbon Composites for Supercapacitors. Electrochim. Acta 2015,168, 50-58. [41] Jin, X.; Lei, B.B.; Wang, J.; Chen, Z.L.; Xie, K., Wu, F.L., Song, Y.; Sun, D.L.; Fang, F. Pomegranate-like Li3VO4/3D graphene networks nanocomposite as lithium ion battery anode with long cycle life and high-rate capability. J. Alloy.Compd. 2016, 686, 227−234. [42] Kim, J.Y.; Lee, J.Y.; Shin, K.Y.; Jeong, H.; Son, H.J.; Lee, C.H.; Park, J.H.; Lee, S.S.; Son, J.G.; Ko. M.J.; Highly crumpled graphene nano-networks as electrocatalytic counter electrode in photovoltaics. Appl. Catal. B: Environ. 2016,192, 342–349. [43] Kim, J.Y.; Kim, J.Y.; Lee, D.K.; Kim, B.; Kim, H.; Ko, M.J. Importance of 4-tert-butylpyridine in electrolyte for dye-sensitized solar cells employing SnO2 electrode. J. Phys. Chem. C 2012, 116, 22759−22766. [44] Wu, C.H.; Shen, Q.; Mi, R.; Deng, S.X.; Shu, Y.Q.; Wang, H.; Liu, J.B.; Yan, H. 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Three-dimensional Co3O4/flocculent graphene hybrid on Ni foam for supercapacitor applications. J. Mater. Chem. A 2014, 2, 15987-15994. [45] Hong, W.; Wang, J.Q.; Li, Z.P.; Yang, S.R. Hierarchical Co3O4@Au-decorated PPy core/shell nanowire arrays: an efficient integration of active materials for energy storage. J. Mater. Chem. A 2015, 3, 2535-2540. [46] Xia, X.H.; Tu, J.P.; Mai, Y.J.; Wang, X.L.; Gu, C.D.; Zhao, X.B. Self-supported hydrothermal synthesized hollow Co3O4 nanowire arrays with high supercapacitor capacitance. J. Mater. Chem.2011,21, 9319–9325.

18


Page 18 of 30

Page 19 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure captions Fig.1 The schematic diagram of the Co3O4/3D N-G/MWNTs hybrid Fig.2 XRD patterns of MWNTS, GO, N-G/MWNTs, Co3O4/3D N-G/MWNTs Fig.3 The Raman spectra of GO, N-G/MWNTs and Co3O4/3D N-G/MWNTs Fig. 4 SEM images of Co3O4/3D N-G/MWNTs at low magnification (Fig.4a), Low and high magnification TEM of Co3O4/3D N-G/MWNTs (Fig.4b, c and d), HRTEM image of Co3O4/3D N-G/MWNTs (Fig.4e), EDS of Co3O4/3D N-G/MWNTs (the inset is the particle size distribution charts) Fig. 5 XPS spectra of Co3O4/3D N-G/MWNTs: (a) Survey spectra of all,(b) ~ (d) correspond to C 1s, N 1s, Co 2p spectra, respectively. Fig. 6 (A) Nitrogen adsorption (■) and desorption (◄) isotherms of Co3O4/3D N-G/MWNTs and Co3O4 measured at standard temperature and pressure, and (B) the corresponding BJH pore size distribution plots. Fig. 7 (a) CV curves of NG, N-G/MWNTs ,3D-N-G/MWNTs and Co3O4/3D N-G/MWNTs composite electrodesat a scan rate of 10 mV s-1 within the potential of 0-0.6 V. (b) CV curves of the Co3O4/3D N-G/MWNTs electrode at different scan rates. Fig. 8 (a) CV curves of NG, N-G/MWNTs, 3D-N-G/MWNTs 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. Fig. 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 of 0.01 Hz to 100 kHz in 0.6M KCl. (b) specific capacitance at various current densities for Co3O4/ NG, 3D-N-G/MWNTs and Co3O4/3D N-G/MWNTs electrodes. Fig. 10 Co3O4/ NG, 3D-N-G/MWNTs average specific capacitance versus the cycle number at a current density of 15 Ag-1.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig.1 The schematic diagram of the Co3O4/3D N-G/MWNTs hybrid

20


Page 20 of 30

Page 21 of 30

Fig.2 XRD patterns of MWNTS, GO, N-G/MWNTs, Co3O4/3D N-G/MWNTs (311) (220) (111)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(222)

(511) (440)

(400)

(d): Co3O4/3D N-G/MWNTs

C(002)

(c): N-G/MWNTs (b): GO

C(002)

10

20

30

(a): MWNTs

40 50 2 theta (deg.)

60

70

21


80


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

Co3O4

G

(b): N-G/MWNTs (a):GO (c):Co3O4/3D N-G/MWNTs

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

500

1000

1500

2000

2500

-1

Raman Shift (cm )

22


3000

Page 22 of 30

Page 23 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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

23



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

a

C=C

Co2p

C 1s

b

b: Co3O4/3D-N-G/MWNTs

N1s

C1s

Intensity (a.u.)

Intensity (a.u.)

O1s C1s

C-N C=O

N1s

O1s a: N-G/MWNTs

200

400

600

800

1000

1200

280

285 290 Binding Energy (eV)

Binding Energy (eV)

N 1s

pyridinic N

c

d

295

Co2p

2p3/2

Intensity (a.u.)

Co3+ pyrrolic N

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

graphitic N

390

395

400 405 Binding Energy (eV)

770

410

2p1/2

2+

Co

3+

Co

775

780

Co2+

785 790 795 800 Binding energy (eV)

24


805

810

815

Page 25 of 30

Fig. 6 (a) Nitrogen adsorption (■) and desorption (◄) isotherms of Co3O4/3D N-G/MWNTs and Co3O4 measured at standard temperature and pressure, and (b) the corresponding BJH pore size distribution plots. 1000

600

Co3O4/3DN-G/MWCNTs

b

0.7

Co3O4/3DN-G/MWCNTs Co3O4

-1 3 -1

800

0.8

a dV/dD (cm g nm )

Adsorbed Volume (cm3g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400 200

Co3O4

0.6 0.5 0.4 0.3 0.2 0.1 0.0

0 0.0

0.2

0.4

0.6

0.8

1.0

0

10

20

30

Pore Diameter (nm)

Relative pressure (p/p0)

25


40

50


Fig. 7 (a) CV curves of NG, N-G/MWNTs ,3D-N-G/MWNTs and Co3O4/3D N-G/MWNTs composite electrodesat a scan rate of 10 mV s-1 within the potential of 0-0.6 V. (b) CV curves of the Co3O4/3D N-G/MWNTs electrode at different scan rates. 25

-1

a

0.3

3D-N-G/MWNTs NG N-G/MWNTs Co3O4/3D-N-G/MWNTs

15 10

Current (A)

20 Current Density ( A g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

5 0

b

0.2

100 mv/s 50 mv/s 20 mv/s 10 mv/s 5 mv/s

0.1 0.0

-5 -0.1

-10 -15

-0.2

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.0

0.1

0.2

0.3

0.4

Potential (V)

Potential (V vs. SEC)

26


0.5

0.6

Page 27 of 30

Fig. 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. -1

Co3O4/3D-N-G/MWNTs

15 A g -1 12 A g -1 10 A g -1 5Ag -1 2Ag -1 1Ag

0.5

a

Co3O4/N-G

0.5

Co3O4

0.4

3D-N-G/MWNTs N-G/MWNTs NG

0.3

0.4 Potential (V)

0.6

Potential (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.3

b

0.2

0.2

0.1

0.1 0.0

0.0 0

200

400

600 800 Time (s)

1000

1200

1400

0

400

800

27


1200 Time (s)

1600

2000

2400


Fig. 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 of 0.01 Hz to 100 kHz in 0.6M KCl. (b specific capacitance at various current densities for Co3O4/ NG, 3D-N-G/MWNTs and Co3O4/3D N-G/MWNTs electrodes. 12

-1

Specific capacitance ( F g )

a

10 -Z'' (Ohm cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

8 6 3D-N-G/MWNTs N-G/MWNTs NG Co3O4/N-G

4 2

Co3O4/3D-N-G/MWNTs

0 0

1

2

3 4 Z' (Ohm cm2)

5

2400 2200 2000 1800 1600 1400 1200 1000 800 600 400 200 0

b Co3O4/3D-N-G/MWNTs

Co3O4/NG 3D-N-G/MWNTs

0

6

5

10

15

20 -1

Current density (A g )

28


25

Page 29 of 30

Fig. 10 Co3O4/ NG, 3D-N-G/MWNTs average specific capacitance versus the cycle number at a current density of 15 Ag-1. 2000

94% retention 1750 -1

15 A g

1500 1250

0.5

1000

0.4

Potential(V)

-1

Specific Capacitance ( Fg )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


750

0.3

0.2

500 0.1

250

0

150

300

450 59800

60000

60200

Times(s)

0 0

1000

2000

3000

4000

5000

Number cycle

29


6000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

For Table of Contents Use Only

Uniformly Co3O4 embedded on 3D-N-doped graphene/MWNTs via a green microwave-assisted method and demonstrating high specific capacitance and cycling stability.

30


Page 30 of 30

MWNTs Nanoframework-Embedded

Recommend Documents