Ultrafine Co₃O₄ embedded in nitrogen-doped graphene with synergistic effect and high stability for supercapacitors

Abstract

Co₃O₄ embedded in nitrogen-doped graphene (Co₃O₄/N-G-750) is prepared by a two-step synthetic method: polyol microwave heating method and vacuum tube calcination method. The diameter from 2 nm to 4 nm of the Co₃O₄ particles can be easily controlled on a nitrogen-doped graphene surface by adjusting the experimental parameters. The electrochemical characteristics of Co₃O₄/N-G-750 composite have been characterized by cyclic voltammetry (CV) and galvanostatic charge–discharge measurements (GCD) in KOH 6 M electrolyte. The specific capacitance of the electrode modified as-prepared composite exhibited a high specific capacitance of 1288.2 F g⁻¹ at a high discharge current of 1.5 A g⁻¹, as well as excellent cycling stability (91.5% capacitance retention after 5000 cycles). This result indicates Co₃O₄/N-GO-750 will be an excellent candidate for supercapacitor applications.

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1. Introduction

With the rapid development of industrialization and the increasingly serious environmental pollution, energy shortages have restricted economic development which has caused unremitting exploration of green renewable energy such as wind and solar energy. Therefore, as an indispensable energy storage device, supercapacitors have been studied for higher cycle efficiency, reasonable power/energy density and better charge/discharge rate than traditional batteries. Supercapacitors can be classified into electrical double-layer capacitors (EDLCs) and redox electrochemical capacitors according to the charge storage mechanism.¹ The electrical charge of electrical double-layer capacitors is stored at the interface between the electrode and the electrolyte while redox electrochemical capacitors depend on reversible faradaic redox reactions which take place at the electrode/electrolyte interface.² The electrode materials of redox capacitors are usually of two types: metal oxides/hydroxides and conducting polymers, of which the former can exhibit better electrochemical stability than the latter. However, high resistance and low energy density limit pseudocapacitance applications.^3,4

To solve this problem, hybrid composite material mechanism combine the advantages and mitigate the shortcomings of both faradic electrode and capacitive electrode, which have been explored to improve the energy density.⁵ Generally, an ideal supercapacitor material requires high electronic conductivity, large specific surface area, high chemical stability and thermal stability and cheap cost of manufacturing.⁶ As its favorable pseudocapacitive characteristic, lower cost and environmental friendly fabrication process, Co₃O₄ has been considered an ideal activated material.⁷ Recently, substantial efforts have been made for the fabrication of nanostructured Co₃O₄, including hydrothermal process template-assisted synthesis and electrochemical deposition.^8–13 Porous Co₃O₄ particles exhibited a specific capacitance of 150 F g⁻¹ at 1 A g⁻¹.¹¹ Ultralayered Co₃O₄ displayed a specific capacitance of 548 F g⁻¹ at 8 A g⁻¹.¹² A specific capacitance of 3D hierarchical Co₃O₄ displayed 781 F g⁻¹ at 0.5 A g⁻¹.¹³ The specific capacitance of Co₃O₄ of above mentioned are far below their theoretical specific capacitance, which is probably ascribable to the fact that relatively poor electron conductivity of Co₃O₄ results in ineffective electrochemical utilization.¹⁴ However, it is a considerable challenge to prepare a templateless, facile and common procedure for the fabrication of the little crystal size of Co₃O₄ nanoparticles with high electronic conductivity, large specific surface area and high thermal stability.¹⁰

To improve the electrical conductivity of Co₃O₄-based electrodes, Co₃O₄ nanoparticles are combined with large specific surface material, Nguyen et al. synthesized Co₃O₄ nanotube-intercalated graphene composite exhibited outstanding specific capacitance of 1010 F g⁻¹ at a high discharge current of 5 A g⁻¹,³⁷ An showed the good prospects of Co₃O₄/carbon nanofiber composite on methanol electrooxidation,¹⁵ Yan synthesized graphene nanosheet (GNS)/Co₃O₄ composite exhibited specific capacitance of 243.2 F g⁻¹ at a scan rate of 10 mV s⁻¹ in 6 M KOH aqueous solution.¹⁶ Graphene and N-doped graphene are attractive to research among the available carbon materials for their outstanding electrical conductivity and high theoretical surface area (up to 2630 m² g⁻¹).^17,18 Theoretical studies have indicated that substitutional doping can alter the Fermi level and introduce a metal to semiconductor transition in graphene.^19,20 Nitrogen is ideal doping candidate for graphene and N atoms can be substitutionally doped into the graphene lattice during hydrothermal process.^21,22 Recently, series efforts have been proposed to synthesize nitrogen-doped graphene (NG), such as chemical vapor deposition (CVD),^23,24 arc-discharge,^25,26 and post treatments²⁷ were used to produce N-doped graphene, with the assistance of ammonia, hydrazine or pyridine as nitrogen source.

In the present work, a three dimensional (3D) wrinkle ternary system of nitrogen-doped graphene composite was developed and Co₃O₄ nanoparticles were embedded by a facile microwave approach in combination with subsequent calcination process. The prepared Co₃O₄/N-G-750 hybrid material was further employed as an electrode material for supercapacitor, which is 1288.2 F g⁻¹ at the current density of 1.5 A g⁻¹ and 91.5% capacitance retention after 5000 cycles.

2. Experimental

2.1 Materials

Natural flake graphite was obtained from Qingdao Guyu graphite Co., Ltd. with a particle size of 150 nm. NaOH and cobalt(II) sulfate heptahydrate (CoSO₄·7H₂O) were purchased from Sinopharm Chemical Reagent Co., Ltd., China, and used as received without any further purification. Polyacrylamide 10 000 000 (PAM) was also purchased from Sinopharm Chemical Reagent Co., Ltd., China. Deionized water was also used throughout the experiment.

2.2 Preparation of N-GO

Graphene oxide (GO) was prepared from purified natural graphite using our previous work (a modified Hummers method).⁴ After removal of residual metal ion and acid, the solid GO was dispersed again in water by sonication for 30 min and separated by sintered discs. To functionalize GO with nitrogen, strong ammonia was added into the graphite oxide dispersion under vigorous stirring for 20 min, then the mixture was sonication for 20 min. It is well known that the GO sheets bear negative charged position on both sides of the sheet, such as –COOH, –OH which has possibility adapt to positive charged ammonium ions. Hence, driven by the electrostatic interaction, the positively charged nitrogen can intercalate between the GO sheets, resulting in an efficient decoration of GO. The mixture was synthesized by hydrothermal reaction constantly for 12 h at 180 °C, followed by sintered discs to remove the excess ammonia. Finally a successful functionalization of GO sheets with nitrogen was obtained.

2.3 Synthesis of Co₃O₄/N-G-750

For the synthesis of Co₃O₄/N-G-750 nanoparticles, typically, 50 mg polyacrylamide (PAM), 1.4058 g cobaltous sulfate (CoSO₄·7H₂O) and 100 mg nitrogen–graphene were dispersed in 50 mL deionized water in beaker. The resulting solution was uniformly dispersed by sonification for 15 min, and then vigorously stirred for 15 min at room temperature, repeat the multi-dispersion procedure twice, then subjected to microwave (MW) heating in a microwave oven temperature 120 °C operated at 850 W. The pH of the entire solution was adjusted to 8.5 by adding NaOH solution (1.0 M). After natural cooling to room temperature, the product was separated by several washes with deionized water to remove the excess sulfate radical and sodium. Co²⁺ ions were first attached to nitrogen–graphene sheets and the Co(OH)₂ nanoparticles on the graphene sheets takes place with the above procedure. The Co(OH)₂ nanoparticles anchored on graphene surface with the aid of N-GO forming Co₃O₄/N-G composite material after calcined at 350, 450, 550, 650 and 750 °C under air condition, which were denoted as Co₃O₄/N-G-350, Co₃O₄/N-G-450, Co₃O₄/N-G-550, Co₃O₄/N-G-650 and Co₃O₄/N-G-750, respectively. The GO-750 and NG-750 were fabricated by the GO and N-GO at the same temperature outline above. The theoretical Co₃O₄ contents in Co₃O₄ was targeted at 85 wt%, the inductively coupled plasma spectroscopy (ICP, Optima 2000DV, USA) analysis gave the actual Co₃O₄ content as 83 wt% for Co₃O₄/N-G-750 composite.

2.4 Preparation of working electrode

The Co₃O₄/N-G-750 composite working electrode was prepared by magnetically stirred Co₃O₄/N-G-750 composite, acetylene black and polytetrafluoroethylene (PTFE) binder (weight ratio of 85 : 10 : 5) for 48 h. A piece of nickel foam (10 mm × 10 mm × 1.1 mm, 110 PPI, 320 g⁻², Changsha Lyrun Material Co., Ltd. China) was degreased with acetone, etched with 3.0 mol L⁻¹ HCl for 5 min, rinsed with water, soaked in 0.1 mmol L⁻¹ NiCl₂ solution for 6 h, and rinsed again with water extensively. Then the resulting mixture was coated onto the nickel foam substrate with a spatula, which was followed clamped at a pressure of 10 MPa and vacuum drying at 60 °C for 24 h. The compare electrode (GO, N-GO, NG-750, Co₃O₄/G) were dealt with the same producer above. The electrocapacitive measurement was performed on an Autolab CHI660D workstation with a 6 M KOH aqueous electrolyte in a three-electrode cell.

2.5 Structural characterization of the supports and the electrocatalysts

The structures of the obtained samples were examined by powder X-ray diffraction (XRD) on a D8 Advance X-ray powder diffraction from Bruker AXS company (Germany) equipped with Cu-Kα radiation (λ = 1.5406 Å), which employing a scanning rate of 7° min⁻¹ in the 2θ range from 5° to 80°. Fourier transforms infrared (FTIR) were measured by Nicolet NEXUS470 FTIR spectroscopy from 400 cm⁻¹ to 4000 cm⁻¹. Raman scattering was AU10 performed on a Renishaw (In Via) spectrometer with a 532 nm laser source. X-ray photoelectron spectroscopy was characterized by Multifunctional imaging electron spectrometer from United States Thermo Fisher Scientific (America). The morphology and structure of the samples were examined by transmission electron microscopy (TEM) at 120 kV using a JEOL-JEM-2010 (Japan). For the electrochemical tests, all measurements were carried out in a three-electrode cell using Autolab (CHI660D) with a working electrode, platinum plate counter electrode and a mercury oxide reference electrode (SCE) at room temperature. The specific capacitance (C) of the electrodes were calculated from the charge–discharge curves using the following equation:

C = It/(mΔV) (1)

where C, I, t, m, and ΔV are the specific capacitance (F g⁻¹) of the electrode, discharging current (A), discharging time (s), mass of the active material (g), and discharging potential range (V), respectively.

3. Results and discussion

3.1 XRD analysis of reaction products at different temperature

Fig. 1 showed the XRD patterns of the formation of Co₃O₄ from Co(OH)₂ by annealing at different temperatures. All the patterns match the characteristics of Co(OH)₂ crystal (PDF # 74-1057) at 350 °C from Fig. 1a. The diffraction peak intensities of 112 (38.0°) and 110 (58.1°) weakened with the increase of calcination temperature. The patterns of Fig. 1e exhibit a perfect match the characteristics of Co₃O₄ crystal (PDF # 42-1467) which indicates that Co₃O₄ can be synthesized at 750 °C.

Fig. 1 XRD patterns of reaction products at different temperature.

3.2 XRD analysis of GO, N-GO, NG-750 and Co₃O₄/N-G-750

GO, N-GO, NG-750 and Co₃O₄/N-G-750 were measured by wide-angel XRD pattern (Fig. 2). For the sample of GO, the strong diffraction peak at 2θ = 10.2° can be indexed as the GO (002) peak. The introduced oxygen-containing functional groups on graphite sheet layer and the interlayer of hydrone result in d = 0.866 nm which wider than graphite d = 0.334 nm. After hydrothermal reaction, the (002) peak of GO was moved to 2θ = 13.4° indicating that graphite lattice structure has been restored to a certain extent. With nitrogen-doped, the (002) peak of N-GO was observed at 2θ = 27.1°.³² Fig. 2c shows the (002) diffraction peak of GO becomes weak while the (002) diffraction peak of graphite becomes stronger after calcination reaction which indicates the lattice structure of graphite was recovered to a certain degree. In addition, the eight well-defined diffraction peaks observed at the 2θ of 19.0°, 31.2°, 36.9°, 38.5°, 44.9°, 55.7°, 59.4° and 65.3° can be assigned to the lattices of (111), (220), (311), (222), (400), (422), (511) and (440) crystalline planes of the Co₃O₄ (PDF # 42-1467). Furthermore, no other diffraction peaks were observed, which indicates the high purity of Co₃O₄ nanoparticles were produced on the N-GO supports without any noticeable impurity phases. As known that abundant functional groups and surface defects by N-doped on GO are facilitated to the formation of little crystalline Co₃O₄ onto the N-GO surface. With the calculation of Scherrer equation:where D presents the average diameter in nm, K presents the Scherrer constant (0.89), λ presents the wavelength of X-ray (=0.154056 nm), β presents the corresponding full width at half maximum (FWHM) of the (220) diffraction peak, and θ presents the Bragg's diffraction angle, the mean size of Co₃O₄ particles is 2.5 nm for Co₃O₄/N-G-750 capacitor composite.

Fig. 2 XRD patterns of (a) GO, (b) N-GO, (c) NG-750 and (d) Co₃O₄/N-G-750.

3.3 FTIR spectra analysis

The Fourier transform infrared spectroscopy was employed to identify the functional groups and investigate the reduction process. As shown in Fig. 3a, graphene oxide shows evidences of hydroxyl groups on the surface according to the peak at 3410 cm⁻¹ which can be attributed to the O–H stretching vibration. In addition, the characteristic C–C skeleton vibration of carbon ring in graphene is observed at 1562 cm⁻¹ for the skeletal vibration of sp² hybridized carbon.²⁸ The peak at 1714 cm⁻¹ can be assigned to a strong C=O stretch²⁹ and C–OH stretch can be found at 1114 cm⁻¹. The FTIR spectra clearly indicates that the oxygen containing groups, such as hydroxyl, epoxy and carboxyl groups were successfully bound to the edges of grapheme nanosheets through over oxidation. After treatment with ammonia (Fig. 3b), the curve is similar to that GO curve. However, there are small differences between them, for instance the peak at 1714 cm⁻¹ for C=O stretch is vanished, the new peaks such as the C–N stretching vibration band at 1406 cm⁻¹ and the typical C=N stretching vibration band at 1315 cm⁻¹ appears, which are correspond to the characteristic bands of nitrogen doping.³⁰ However, after calcined at 750 °C the oxygen-containing functional groups of NG 750 almost disappeared from Fig. 2c. Two weak absorption peaks at 1557 cm⁻¹ and 1194 cm⁻¹ can be assigned to C=N stretch and C–N stretch, respectively. Comparing with Fig. 3a–d have the strong peak at 665 cm⁻¹ and 575 cm⁻¹, which were attributed to the typical absorption peak of Co²⁺ and Co³⁺.³¹

Fig. 3 FTIR spectra of (a) GO, (b) N-GO, (c) NG-750 and (d) Co₃O₄/N-G-750.

3.4 Raman spectroscopy analysis

Raman spectroscopy is considered a very versatile optical method for the characterization of graphitic materials. The Raman spectra of GO, N-GO and Co₃O₄/N-G-750 are shown in Fig. 4. Two characteristic peaks of GO Fig. 4a are observed at approximately 1350 cm⁻¹ and 1592 cm⁻¹, which are attributed to the disorder-induced D band and G band. Generally, the D band is correspond to defects in the curved graphene sheet and staging disorder, while G band is assigned to the first order scattering of the E_2g phonon of sp² C atoms.³³ After functional of the GO to N-GO, the G band shifted to lower value (1588 cm⁻¹), indicating that the N-GO was produced.³⁴ The integrated intensity ratio of the D and G bands (I_D/I_G) increases with the amount of disorder for graphitic materials, vanishing for completely defect-free graphite. As shown in Fig. 4 the ratio of peak intensities of D and G bands (I_D/I_G) for GO and N-GO have been estimated to be around 0.96, 1.03 and 1.09, respectively.³³ For Co₃O₄/N-G-750, as showed in Fig. 4c, five distinct Raman peaks centered at 188, 459, 506, 604 and 670 cm⁻¹ are clearly observed, which can be indexed as F¹_2g, E_g, F²_2g, F³_2g and A_1g modes of the crystalline Co₃O₄, respectively.³⁵ No peaks from activated carbon are observed in the Co₃O₄/N-G-750 sample indicates that the activated carbon was burned off during the calcinations process in air at 750 °C for 3 h.³⁶

Fig. 4 Raman patterns of (a) GO, (b) N-GO and (c) Co₃O₄/N-G-750.

3.5 Morphology analysis

For better understanding of dimensionality of the nanoparticle morphologies, transmission electron microscopy (TEM) is used. Fig. 5 shows high magnification TEM images of Co₃O₄ nanoparticles deposited on graphene sheets. As shown in Fig. 5A, the transparent graphene sheets with crumpled silk veil waves on the top of the micro-gate film were observed and the rumples were intrinsic to grapheme nanosheets. As observed from Fig. 5B, the overlapping of the nitrogen-doped graphene sheets is seriously flexible which indicates the further repair of graphene stable structure. The 3D structure of graphene oxide and the rough surface on nitrogen-doped graphene sheets provide a large accessible surface area, which is benefit to the loading of Co₃O₄ nanoparticles. Since N-doped graphene are full of nitrogen-doped active sites and wrinkles during the hydrothermal treatment, Co₃O₄ nanoparticles are extremely easy to grow on the N-doped graphene with a microwave method, as showed in Fig. 5C, they are a lot of small-sized Co₃O₄ nanoparticles (2–4 nm in size) homogeneous supported on N-doped graphene. Energy dispersive spectroscopy (EDS), as shown in inset of Fig. 5C, shows the corresponding peaks of C, O, Co and Cu (the Cu signal comes from the sample holder). In particular, the HRTEM image describing the crystalline nature (Fig. 5D) exhibits excellent dispersion of Co₃O₄ nanoparticles on the support.

Fig. 5 TEM images of (A) GO, (B) N-GO and (C) Co₃O₄/N-G-750 (the inset of (C) is EDS of Co₃O₄/N-G-750), HRTEM images of (D) Co₃O₄/N-G-750.

3.6 Element analysis

Surface elemental compositions and chemical state of spheral Co₃O₄ were further determined by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 6a, the binding energies obtained in the XPS analyses was corrected for specimen charging by referencing the C 1s peak to 284.83 eV. Compared to the GO, the XPS of Co₃O₄/N-G-750 composite reveal a relatively low O 1s peak at 529.93 eV and two additional N 1s peak at 399 eV and Co 2p peak. After hydrothermal reaction the atomic ratio of carbon and oxygen (C/O) decreased from 28.08 to 21.97 indicates that the destruction of sp² on C surface and more defects have produced. With microwave processing the radio attain 0.9293, which reveals a large number Co₃O₄ loaded on the N-doped graphene surface. Three different peaks centered at 284.6, 285.4 and 289.1 eV are deconvoluted to fit the C 1s spectra of GO (Fig. 6b), which were assigned to the binding energy of C–C, C–O and C=O bond. The weak peak for the C=O bond indicates the reduction of graphene oxide. The N 1s spectrum of N-GO (Fig. 6c) can be assigned to three peaks at 398.3, 399.4 and 400.8 eV, which represent the nitrogen form of pyridinic-N, pyrrolic-N and graphitic-N, respectively. Fig. 6d shows the high-resolution Co 2p spectrum. It can be seen that main peaks of Co 2p^3/2 and Co 2p^1/2 are shown at binding energies of 779.7 and 794.9 eV, with a spin–orbit splitting of about 15.0 eV. The two fitting peaks at binding energies of 781.8 and 797.5 eV are ascribed to Co²⁺, while another two fitting peaks at 780.0 and 795.2 eV are attributed to Co³⁺.^37,38

Fig. 6 XPS spectra of GO, N-GO and Co₃O₄/N-G-750: (a) survey spectra of all, (b) C 1s spectra of GO, (c) N 1s spectra of N-GO, (d) Co 2p spectra of Co₃O₄/N-G-750.

3.7 Electrochemical characterization

Cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) analysis have been performed to investigate the electrochemical performance of the Co₃O₄/N-G-750 nanocomposite. Fig. 7 presents the electrochemical properties of the obtained electrodes. The CV curves of this electrode in the potential window, between 0 and 0.6 V, at the same scan rate of 20 mV s⁻¹. Fig. 7a shows the Co₃O₄/N-G-750 electrode exhibits a much higher current and a more rectangular shape than the bare GO, N-GO, NG 750 and Co₃O₄ electrode, suggesting that the synergy between ultrathin Co₃O₄ nanoparticles and conducting layer facilitates electron transport. CV curves at scan rates of 5, 10, 20 and 50 mV s⁻¹ for the 83% Co₃O₄–17% N-GO nanocomposite are shown in Fig. 7b. It can be clearly found that there are two strong distinct pairs of redox peaks in the CV curves, which are responsible for the redox process of Co₃O₄ as following redox reactions:

Co₃O₄ + OH⁻ + H₂O ⇔ CoOOH + e⁻ (3)

CoOOH + OH⁻ ⇔ CoO₂ + H₂O + e⁻ (4)

Fig. 7 (a) CV curves of bare GO, N-GO, NG-750, Co₃O₄/G and Co₃O₄/N-G-750 at scan rate of 20 V s⁻¹ in 6 M KOH, (b) CV curves of Co₃O₄/N-G-750 at scan rates of 5, 10, 20 and 50 mV s⁻¹, (c) GCD curves of GO, GO-750, N-GO, NG-750, Co₃O₄/G and Co₃O₄/N-G-750 at current density of 3 A g⁻¹ and (d) GCD curves of Co₃O₄/N-G-750 at different current densities.

Obviously, the shape of the CV curves introduce that the capacitive characteristics of the Co₃O₄ phase are different from those of an electric double-layer capacitor, which would produce a CV curve similar to an ideal rectangular shape. It is clearly show that the shape of the CV curves mainly arisen by redox reaction. From the comparison of the CV curves, the area surrounded by the CV curves at the same scan rate for the composite are dramatically larger than those for the pristine NG-750 and Co₃O₄. This result indicates that a large specific capacitance is caused with the nanoparticles and suggest that its capacitance is mainly from the pseudo capacitance of the electrochemically active Co₃O₄ instead of double-layer capacitance from the graphite sheets. Fig. 7b depicts CV of Co₃O₄/N-G-750 electrode at scan rate of 5, 10, 20 and 50 mV s⁻¹. Because of the limited diffusion time, the anodic potential shifts to high value while the cathodic peak potential shift to low value with the increase of scan rate. The result introduces larger scan rate arouse rapid redox reaction occurred among Co₃O₄/N-G electrode. Fig. 7c shows galvanostatic charge–discharge curves of pristine graphene oxide electrode, annealed graphene oxide, N-doped graphene electrode, annealed N-doped graphene, simple Co₃O₄ and Co₃O₄/N-G-750 electrode at a current density of 3 A g⁻¹ in the potential range from 0 to 0.53 V. The specific capacitance of the electrode at different current densities can be calculated by using formula (1). The specific capacitance values of GO, G-750, N-GO, NG-750, Co₃O₄ and Co₃O₄/N-G-750 are 192.8 F g⁻¹, 239 F g⁻¹, 518.4 F g⁻¹, 606.5 F g⁻¹, 982.5 F g⁻¹ and 1245.3 F g⁻¹ at the current density of 3 A g⁻¹, which show a better performance than the reported N-doped graphene and 3D graphene/Co₃O₄ composite.^39,40 Fig. 7d shows the galvanostatic charge and discharge curves of Co₃O₄/N-G-750 at different current densities from 1.5 to 20 A g⁻¹. The electrode exhibited good pseudo capacitances of 1288.2 F g⁻¹ at 1.5 A g⁻¹, 1245.3 F g⁻¹ at 3 A g⁻¹, 1121.4 F g⁻¹ at 4 A g⁻¹, 1032.6 F g⁻¹ at 6 A g⁻¹, 951 F g⁻¹ at 12 A g⁻¹ and 933.9 F g⁻¹ at 20 A g⁻¹. As shown in Fig. 7d, the capacitance decreases with an increase in the discharge current densities, which is ascribed to the increment voltage drop and relatively insufficient active material involved in redox reaction under higher current densities. The electrolyte solution ions can pass to larger metal particle surface and transfer faster between the ultrafine metal participles and graphene layer, which results in higher specific capacitance. In accordance with CV and GCD analysis, Co₃O₄/N-G-750 composite prepared at 750 °C shows superior capacitive behavior.

The composite electrode showed excellent stability performance from Fig. 8, which retained 91.5% of its initial capacity when it was charged and discharged for 5000 cycles at a rate of 9 A g⁻¹. In particular, the composite electrode showed admirable rate performance, approximately 72.5% capacitance remained even at a current rate of 20 A g⁻¹ comparing with 1.5 A g⁻¹. The discharge-specific capacitance wastage could be explained by the repetitive charge/discharge induced degradation of the ultrafine sphere structure with the graphene sheets of the electrode.

Fig. 8 (a) Average specific capacitance versus the cycle number at a current density of 6 A g⁻¹, (b) specific capacitance of Co₃O₄/N-G-750 at various discharge current densities.

4. Conclusions

In summary, Co₃O₄/N-G-750 composite have been successively synthesized via a microwave hydrothermal reaction followed by a high-temperature calcination treatment in air. The obtained Co₃O₄/N-G-750 composite are constructed by many ultrathin nanoparticles with a diameter of 2 to 4 nm interweaving with each other. The Co₃O₄/N-G-750 modified electrode exhibited excellent electrochemical performance with a very high specific capacitance of 1288.2 F g⁻¹ at current density of 1.5 A g⁻¹. And it showed outstanding cycling retention (91.5%) for high-performance electrochemical capacitor after 5000 cycles in 6 M KOH electrolyte. With a similar onset potential to Co₃O₄/GO composite, our composite exhibited striking advantages of higher electric capacity, more stability and a far lower cost of fabrication. Therefore, the Co₃O₄/N-G-750 composite would be an ideal electroactive material for the application of next-generation supercapacitor.

Acknowledgements

This work was supported by the financial supports from Natural Science Foundations of Jiangsu (BK20140531), China Postdoctoral Science Foundation (2015M570410) Research Foundation for Talented Scholars of Jiangsu University (14JDG187) and Zhenjiang Industry Supporting Plan (GY2014040); Dr Zaoxue Yan thanks China Postdoctoral Science Foundation (2014T70481).

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