Exploration of the Active Center Structure of Nitrogen-Doped

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Exploration of the Active Center Structure of Nitrogen-Doped Graphene to Control Over the Growth of Co3O4 for High-Performance Supercapacitor Xiaoning Tian, Xiaolong Sun, Zhongqing Jiang, Zhong-Jie Jiang, Xiaogang Hao, Dadong Shao, and Thandavarayan Maiyalagan ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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ACS Applied Energy Materials

Exploration of the Active Center Structure of Nitrogen-Doped Graphene to Control Over the Growth of Co3O4 for High-Performance Supercapacitor Xiaoning Tian,†,+ Xiaolong Sun,§,†,+ Zhongqing Jiang,*,† Zhong-Jie Jiang,*, ‡ Xiaogang Hao,*,§ Dadong Shao,£ and T. Maiyalagan┴ † School of Materials and Chemical Engineering, Ningbo University of Technology, Ningbo 315211, Zhejiang, China ‡ Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, New Energy Research Institute, College of Environment and Energy, South China University of Technology, Guangzhou 510006, Guangdong, China § Department of Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China £ Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, P. R. China ┴ SRM Research Institute, Department of Chemistry, SRM University, Kattankulathur-603203, India ABSTRACT: Nitrogen-doped graphene sheets with different active center structures, such as amine N, quaternary N, pyridinic N or pyrrolic N atoms, were successfully fabricated using aimed nitrogen precursors and designed annealing process. Then the nitrogen doped graphene with different structure is used as the active center for the growth of Co3O4 nanoparticles. The investigation results reveal that the interaction between loaded Co3O4 particles and amine N atoms doped in graphene sheets is stronger than those of quaternary N, pyridinic N or pyrrolic N atoms, which forms smaller particle size of Co3O4 and high specific surface area of composite electrodes to perform a better electrochemical behavior. The Co3O4/N-RGO 550 oC dominated with amine N atoms exhibit the highest capacitance of 3553 F g-1 and 1967 F g-1 at 1 A g-1 and 15 A g-1, respectively, which are apparently higher than those of the other Co3O4 composite grown on the nitrogen-doped graphene dominated with pyridinic N, pyrrolic N or quaternary N atoms, respectively, and those of previously reported Co3O4 with different morphology or Co3O4 composite materials. Moreover, electrode prepared from Co3O4/N-RGO 550 oC dominated with amine N atoms also has an excellent cycling stability with >90 % capacity retention after 3000 cycles at 5 A g-1. The stronger interaction between Co3O4 and amine N atoms doped in graphene sheets, which facilitate the formation of smaller Co3O4 particle sizes to form higher specific surface and desired pore size distribution to enhance the capacitance and make the Co3O4/N-RGO 550 oC extremely stable for capacitive energy storage, suggesting the potential usage of Co3O4/N-RGO 550 oC composite as high supercapacitor electrode materials. 1

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KEYWORDS: Nitrogen-doped graphene; amine N atoms; Co3O4 particles; active center; Supercapacitor.

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INTRODUCTION Supercapacitors, mainly acting as the pulse power supplier due to the fast charge–discharge capabilities, have become one of the most promising options for energy storage devices because of the high power density, long cycle life, and low maintenance cost. Based on the mechanism of energy storage supercapacitors can be classified into two main types: (1) the electrical double layer capacitors (EDLCs); and (2) pseudocapacitors.1-6 The pure electrostatic charge accumulated at the electrode/electrolyte interface can form the electrical double layer capacitance, which is called EDLC. And the pseudocapacitance comes from the reversible Faradaic reactions occurring at the interface or near the surface of the electrode material.4-6 Over the past decades carbon-based materials, such as porous carbon, is one of the most popular investigated subjects for electrode materials of EDLCs because of the high surface area and tunable surface chemistry.7-8 The power density and cycle life of carbon-based electrodes for EDLCs are excellent. However the high surface area is not all available for energy storage, therefore the energy density of carbon-based electrodes is not as high as expected, which is limited by the pure electrostatic charge accumulation.9-11 In addition, transition metal oxides have been used to build pseudocapcitors. Due to the fast and reversible surface redox processes occurring at the electrode surface pseudocapcitors often provide better electrochemical performances than the EDLCs. Among the detected transition metal oxides Co3O4 is an attractive choice for the application of supercapacitor electrodes because of its high theoretical capacitance (3560 F g-1), lower toxicity, and good capability retention.12-24 In order to take full advantage of Co3O4 based electrode materials a lot of research work were devoted to figure out the critical factors affecting the pseudocapacitance. It can be summarized that the pseudocapacitance of Co3O4 is mainly depends on its surface area, morphology, and specific orientation of crystalline facets.18, 20-21, 23-25 Therefore, the control over the morphology of Co3O4 with desired shape and uniform size is critical to improve the electrochemical performance of Co3O4 based electrode materials. However, despite all the above merits of Co3O4 the low electronic conductivity and volume change during charge/discharge process greatly compromises its pseudocapacitance and cycle stability, both of which restrict its further commercial application as electrode materials. So combining Co3O4 with a highly conductive and chemical stable substrate is an obvious and effective method to overcome these drawbacks.14, 16, 22 Graphene, possessing outstanding feature of high conductivity, good mechanical property and large specific surface area, is a perfect substrate to support the transition metal oxides.26 Moreover the applications of graphene can be largely improved and expanded after doping with heteratoms such as N, which can tailor the electronic properties of graphene.3, 27-31 According to previous reported work it can be found out that the doped N in graphene sheets could not only facilitate the nucleation and growth of the Co3O4 nanoparticles, but also promote the interaction between Co3O4 nanoparticles and doped graphene sheets, both of which are benefit 3



for enhancing the electrochemical performance of prepared Co3O4/graphene electrodes.17, 32-33 Normally the doped N atoms exhibit five main kinds of molecular structures: amine, pyridinic, pyrrolic, quaternary, and oxidized nitrogen.28, 34-37 In this work doped N atoms in graphene sheets with different active center structures were successfully synthesized by using predesigned N-containing precursors and through specified annealing temperature. This is the first time to report the research work related to the investigation of interaction between the active center structures with different nitrogen doping types and supported Co3O4 particles, which can help us to better understand the relationship between corresponded composite electrode material and the electrochemical performance. Furthermore the resultant N doped graphene supported Co3O4 composites exhibit outstanding electrochemical performance indicating a potential option of commercial supercapacitor electrode materials. EXPERIMENTAL SECTION Material Synthesis. Synthesis of N-RGO. Graphene oxide was prepared by a modified Hummers method as described in previously reported work.38-40 The synthesized graphene oxide solution was then freeze-dried to obtain the graphite oxide powder (GOP). 500 mg GOP, 3 g melamine and 40 mL distilled water were mixed with ultrasonication for 60 min to form a well-dispersed suspension. Then 20 mL above suspension was put into a zirconia ball milling tank, which was further set into a planetary mill machine, to operate ball milling for 20 times. After ball milling the samples were recollected, and followed by the freeze-drying. The final prepared solid samples were put into a quartz boat and then calcinated at 550 °C and 850 °C, respectively, for 1 h with a heating rate of 5°C min-1 under N2 protection. The achieved calcinated samples were grinded, washed and dried under 60 °C for 24 h. The final obtained nitrogen doped graphene are denoted as N-RGO 550 °C and N-RGO 850 °C, respectively. Synthesis of NH2-RGO. 100 mg of GOP was mixed with 40 mL glycol under ultrasonication for 1 h to form a brown color solution. Then 3 mL ammonia was added into above GOP solution and left in static at 180°С for 10 h in a closed Teflon bottle. The resulting sample was then filtered, washed rapidly with DI water and dried at 60°С for 24 h, which is signed as NH2-RGO. Synthesis of PPy/RGO. 20 mg of GOP was dispersed in 20 mL DI water by ultrasonication for 1 h. Pyrrole monomer was dissolve in 1.5 mL methanol to form a solution with concentration of 0.1 M and followed by adding into the dispersed GOP slowly. Then the above prepared mixture was transferred into an ice bath and 0.662 mL FeCl3 (1M), the initiator of polymerization, was introduced under stirring. The whole process need to be shielded from light. The product mixture was washed by ethanol, acetone and DI water respectively. After dried at 60°C, the obtained product was put into a quartz boat to calcinated at 850 °C for 1 h with a heating rate of 5 °C min-1 under N2 protection. After washed with ethanol and DI water the resulting product was obtained and labeled as PPy/RGO. 4


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Synthesis of PANi/RGO. GOP suspension was prepared by ultrasonicating 20 mg GOP in 20 mL deionized water for 1 h. Then 1.51 mL aniline (0.1 M) was dropped into the above suspension slowly under stirring and followed by setting in an ice bath. After 10 min, 0.676 mL ammonium peroxydisulfate (0.1 M), 0.676 mL LiClO4 (1M) and 0.676 mL H2SO4 (1M) were put into the above mixture respectively under stirring in 10 min. The polymerization process of aniline was shielded from light for 8 h. The obtained mixture was washed by ethanol, acetone and DI water, which was then put into a 60°C oven over night. The dried product was put into a quartz boat and calcinated at 850 °C for 1 h with a heating rate of 5°C min-1 under N2 protection, then after washed by ethanol and DI water the designed product can be fabricated, which was marked as PANi/RGO. The nitrogen doped graphene with different structure as the active center for the growth of Co3O4 nanoparticles. 100 mg nitrogen doped graphene with different structure (such as N-RGO, NH2-RGO, PPy/RGO or PANi/RGO) was mixed with 100 mL isopropanol/water (1:1, v/v) solution in a three necks round bottom flask under ultrasonication for 0.5 h. Designed amount of Co(NO3)·6H2O was added into the flask followed by slow dropping of NH3·H2O (25 wt%) under over stirring. The above procedure was operated under the protection of Ar and lasted for 4 h. The resultant precursor was filtrated and dried at 70 °C under vacuum. Then the precursor was calcinated at 450 °C for 2 h to achieve the aimed metal oxide/nitrogen doped graphene composite material. According to the doped nitrogen with different structure the obtained metal oxide/nitrogen doped graphene composites were named as Co3O4/N-RGO550oC, Co3O4/N-RGO850oC, Co3O4/NH2-RGO, Co3O4/Ppy/RGO and Co3O4/PANi/RGO, respectively. For comparison, the pure Co3O4 was also synthesized in the absence of nitrogen doped graphene but with the other parameters constant. Characterizations. Morphologies and microstructures features of the samples were characterized by using a field-emission scanning electron microscope (S-4800, Hitachi) at an operation voltage of 20.0 kV, and transmission electron microscopy (TEM) (JEM 2010, JEOL, Japan) operated at 200 kV. Specific surface areas and porosity of resultant samples were tested by using an automatic volumetric sorption analyzer (Quantachrome, Autosorb-IQ-MP). The XRD patterns of samples were recorded using a X-ray diffraction technique (Bruker D8-Advance, Germany) with Cu Kα radiation. X-ray photoelectron spectroscopy was performed by using an Al Kα source (1486 eV) with a VG ESCALAB 250 spectrometer (Thermo Electron, U.K.). Fourier transform infrared spectrscopy (FTIR) of the samples were measured with a Thermo Scientific Nicolet 6700 FTIR spectrometer (Sugar Land, TX, USA). Electrochemical measurements. The electrochemical behaviors of various obtained metal oxide/nitrogen doped graphene composite materials are tested by a three-electrode configuration in a 6 M KOH aqueous electrolyte at room temperature, where Ni foam coated with the metal oxide/nitrogen doped graphene composite materials act as the working electrodes. A paste, prepared by mixing 85 wt. % of 5



aimed metal oxide/nitrogen doped graphene composite material, 5 wt. % of poly(vinylidene fluoride), and 10 wt. % of acetylene black was spread onto the nickel foam (NF) (surface, 1.0 cm × 1.0 cm) and then dried at 80 °C overnight, which acted as the working electrode. A Pt foil and Hg/HgO electrode served as the counter and reference electrodes, respectively. Cyclic voltammograms (CV) were measured by a CHI 760E electrochemical workstation. CV tests were done between 0.1 and 0.6 V (vs. Hg/HgO) at different scan rates of 5, 10, 20 and 30 mVs−1. Galvanostatic charge-discharge curves was measured by a Arbin BT-G electrochemical workstation at various current densities of 1 to 15 A g-1 in the potential range of 0.05–0.5 V (vs. Hg/HgO). The electrochemical performance data shown here are the average values of the results obtained from the three repeated samples. RESULTS AND DISCUSSION

Figure 1. SEM images of GOP (a), N-RGO 550 oC (b), N-RGO 850 oC (c), NH2-RGO (d), PANi/RGO (e), Ppy/RGO (f).

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Figure 2. TEM images of GOP (a), N-RGO 550 oC (b), N-RGO 850 oC (c), NH2-RGO (d), PANi/RGO (e), Ppy/RGO (f).

The morphology of GOP and nitrogen doped graphene with different active center structures are exhibited in Figures 1 and 2. Figures 1a and 2a reveal that the prepared GOP consists of ultrathin, wrinkled, paper-like nanosheets structure. Because in this work the graphite oxide power (GOP) was obtained through freeze-drying process instead of direct thermo treatment, therefore original sheet like structure of graphite oxide in solution state can be fixed and the restacking of graphene sheets due to the strong inter-sheet interaction can be forbidden. In addition, irregular pore structure is formed following the freeze-drying procedure. Compared with GOP, the sheets of calcined samples N-RGO 550 oC and N-RGO 850 oC are more crumpled and disordered as shown in Figure 1b-c and Figure 2b-c. Because melamine and GOP can be mixed homogeneously by ball milling technique, and the subsequent freeze-drying and calcination procedures are responsible for the generation of more obvious pore structures. Following the hydrothermal treatment with ammonia, NH2-RGO still shows the sheet-like structure as GOP, however, the surfaces of NH2-RGO sheets are more rough than that of GOP (Figures 1d and 2d). With the introduction of polymers, more thick sheets are created in samples PANi/RGO and Ppy/RGO (Figure 1e-f and Figure 2e-f) due to the coating of calcined polyaniline and polypyrrole. Interestingly, the sheet-like structures are maintained well in these two samples because the supported calcined polymers efficiently resist the strong π-π interactions and the van der Waals force between the planar basal planes of GOP. It can be concluded that the doping processes, despite the different techniques and diverse nitrogen precursors involved in this work, can prevent the adherence of GOP sheets effectively. 7



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Figure 3. (a) XPS survey scan of N-RGO 550 oC, N-RGO 850 oC, NH2-RGO, Ppy/RGO and PANi/RGO samples. N1s spectra of N-RGO 550 oC (b), N-RGO 850 o C (c), NH2-RGO (d), PANi/RGO (e) and Ppy/RG-O (f). The XPS survey spectra in Figure 3a suggest the presence of C, O, and N in N-RGO 550 oC, N-RGO 850 oC, NH2-RGO, Ppy/RGO and PANi/RGO samples, implying the successful nitrogen doping of graphene by different nitrogen precursors. Surface species concentration for the resultant various N-containing samples summarized by XPS results is shown in Table 1. The content of nitrogen in each sample is approximately similar. Normally the incorporation of nitrogen atoms with graphene sheets are in the following molecular structures: pyridinic nitrogen (398.3 eV) refers to nitrogen atoms at the edge of graphene planes bonded to two carbon atoms and donates one p-electron to the π system; amine nitrogen (399.3 eV) relates to the nitrogen atoms bonded with one carbon atom and two hydrogen atoms; pyrrolic nitrogen (400.2 eV) refers to nitrogen atoms bonded to two carbon atoms and contribute two p-electrons to the π system; quaternary nitrogen (401.0 eV) is derived from the nitrogen atoms incorporated into the graphene layer and replacing carbon atoms within a graphene plane; oxidized type N-functionalities (402.8 eV) indicates the nitrogen atoms bonded to two carbon atoms and one oxygen atom.35-36 The diagrammatic sketch of doped nitrogen atoms in graphene sheets is shown in Scheme 1a.

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Scheme 1. (a) The diagramatic sketch of doped nitrogen atoms in graphene sheets, the unit structures of polyaniline and polypyrrole, (b) The possible formation mechanism of the functionalization of the graphitic structure of graphene with amine, pyridinic or pyrrolic nitrogens. The N 1s spectra of samples N-RGO 550 oC, N-RGO 850 oC and NH2-RGO were decomposed into five peaks assigned to amine, quaternary, pyridinic, pyrrolic nitrogens, and oxidized type N-functionalities respectively (Figure 3b-d), suggesting five kinds of nitrogen chemical structures are built in these three samples. Moreover, the intensity of these five peaks is quite different, implying non-uniform distribution of relative atomic ratios for diverse doping nitrogen species. Significantly, no decomposition peak referring to amine nitrogen was found for samples PANi/RGO and Ppy/RGO, in contrast, pyridinic and pyrrolic nitrogen doping is the dominated chemical structure for PANi/RGO and Ppy/RGO, respectively. Therefore, from the results of XPS investigation, it is clearly to conclude that cooperation with pre-designed nitrogen precursors and specified annealing temperature are efficient ways to produce aimed doping graphene with expected nitrogen chemical structure.

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Table 1. Surface species concentration for the resultant various N-containing samples summarized by XPS results. Samples Species concentration (atomic%) C O N o N-RGO 550 C 84.89 7.66 7.46 o N-RGO 850 C 85.95 5.34 8.71 NH2-RGO 84.74 8.23 7.03 PANi/RGO 87.60 5.42 6.97 Ppy/RGO 86.36 6.70 6.94 Table 2. Detailed breakdown of N1s spectra from N-RGO 550 oC, N-RGO 850 oC NH2-RGO, Ppy/RGO and PANi/RGO samples from XPS analysis, indicating peak position and relative atomic ratios of N species. N species/ Peak position

N-RGO 550 o C (%N)

N-RGO 850 o C (%N)

NH2-RGO (%N)

PANi/RGO (%N)

Ppy/RGO (%N)

Pyridinic/398.3 eV Amine N/399.3 eV Pyrrolic/400.2 eV Quaternary/401.0 eV Oxidized type N-functionalities/402.8 eV

1.62 4.16 0.55 0.72 0.41

3.69 0.40 1.67 1.50 1.51

1.30 3.90 0.51 0.66 0.66

3.76 2.66 0.55

3.32 2.22 1.40

The relative atomic ratios of N species in fabricated samples are summarized in Table 2, which clearly shows that the percentage of doped N with different chemical structures closely related to the nitrogen precursors and preparation conditions. The nitrogen species doped in samples N-RGO 550 oC and N-RGO 850 oC were achieved with the assistance of ammonia by releasing from melamine. However, the dominated doping nitrogen for these two samples is amine and pyridinic nitrogen respectively, therefore it can be assumed that the higher calcination temperature facilitates the chemical structure transformation of doped nitrogen atoms, which is a convenient and effectively way. Doped nitrogen atoms were introduced into samples NH2-RGO with the assistance of ammonia by hydrothermal method. It can be seen from Table 2 that amine nitrogen is the dominated doping nitrogen species in NH2-RGO, and a relatively small amount of pyridinic and pyrrolic nitrogens is also detected. The possible formation mechanism of the functionalization of the graphitic structure of graphene with amine, pyridinic or pyrrolic nitrogens in N-RGO 550 oC, N-RGO 850 o C, NH2-RGO is shown in Scheme 1b.11, 37 In this process the released NH3 reacts with the oxygen functional groups (such as carboxylic acid groups) exist on the graphene oxide sheets to form amide like intermediates (amine, or amide), which can further undergo dehydration or decarbonylation reactions to create pyridinic or 10


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pyrrolic nitrogens, which have relatively stable structures.41-42

Figure 4. SEM images of the as-prepared Co3O4/N-RGO 550 oC with (a) lower and (b-c) higher magnification; Co3O4/NH2-RGO (d); Co3O4/N-RGO 850 oC (e); Co3O4/PANi/RGO (f); Co3O4/Ppy/RGO with (g) lower and (h-i) higher magnification. Among the diverse doped active centers, which one is beneficial for the load of Co3O4 is the critical matter to improve the capacitance of created composite electrode material. Therefore figuring out the interaction relationship between the chemical structure of doped nitrogen and the metal oxide is an important issue. In this work pyridinic or pyrrolic nitrogen doping dominated graphene were specially designed. Based on the unit structures of polyaniline and polypyrrole (as shown in Scheme 1a), after calcination process pyridinic and pyrrolic nitrogen atoms could be doped onto the graphene sheets, respectively. The imagination can be convinced by the XPS analysis results. As the data shown in Table 2, it is clear to see that by using supported polyaniline or polypyrrole as the nitrogen doping precursors pyridinic or pyrrolic nitrogen doping dominated graphene can be well fabricated. It can be summarized that among the prepared nitrogen doped graphene N-RGO 550 oC and NH2-RGO are amine nitrogen dominated; N-RGO 850 oC and PANi/RGO are pyridinic nitrogen dominated; and Ppy/RGO is pyrrolic nitrogen dominated. Until now the nitrogen doped graphene with predesigned active center structures were successfully synthesized.

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Figure 5. TEM images of the as-prepared Co3O4/N-RGO 550 oC (a) and (b), Co3O4/NH2-RGO (c), Co3O4/ N-RGO 850 oC (d), Co3O4/PANi/RGO (e), Co3O4/Ppy/RGO (f) with arrows pointing to the corresponding graphene and Co3O4 particles in the samples. In this work Co(NO3)2 acted as the precursor of Co3O4. According to the result of our previous work it can be concluded that in an alkaline aqueous alcohol solution the reaction of Co(NO3)2 could form the deposition of Co3O4 nanoparticles onto the N doped graphene sheets.17 Figure 4 shows the SEM images of as-prepared Co3O4 loaded on N doped graphene sheets, which possess different active center structure. It can be seen that except Co3O4/Ppy/RGO the supported Co3O4 particles on the other four samples all exhibit irregular morphology. However, it is clearly to see that the particle sizes of samples Co3O4/N-RGO 550 oC and Co3O4/NH2-RGO (Figure 4a-d) are obvious smaller than those of samples Co3O4/N-RGO 850 oC and Co3O4/PANi/RGO (Figure 4e and f). As we discussed in Table 2 the amine nitrogen doping is the dominated active center structure for N-RGO 550 oC and NH2-RGO. On the contrary, the dominated active center structure of doped N atoms for samples Co3O4/N-RGO 850 oC and Co3O4/PANi/RGO is pyridinic nitrogen. Therefore it can be assumed that the doped amine N atoms would facilitate the adsorption of the Co2+ ions, which further assist the nucleation and growth of Co3O4 particles. However, relatively big particle size for samples Co3O4/N-RGO 850 oC and Co3O4/PANi/RGO indicates that doped pyridinic N atoms is not as active as amine N atoms for the attraction of Co2+ ions, that would lead to the fabrication of Co3O4 particles with bigger sizes because the same amount of Co2+ ions were added into the reaction system. Interestingly, in Figure 4g-i the Co3O4 particles grew on the prepared N doped graphene sheets, which is pyrrolic nitrogen atoms dominated, formed a flower like morphology, suggesting the ability of morphology controlling for doped pyrrolic nitrogen atoms. From the result of SEM images it can be concluded that the chemical structure of different doped N atoms could effectively adjust the particle size and morphology of loaded Co3O4 particles, which will further affect the final 12


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electrochemical performance of synthesized composite electrodes.

Figure 6. (a) HRTEM image of the as-prepared Co3O4/N-RGO 550 oC, (b) the corresponding SAED. (c-d) Magnified HRTEM images of the areas indicated by the red rectangle in (a). The TEM images of resultant samples are demonstrated in Figure 5, which provide an insight of individual particle structure. It can be seen that each sample is composed of loaded Co3O4 particles and N doped graphene sheets, which was pointed out with red arrows in Figure 5. It is obviously to discover that the Co3O4 particle size of Co3O4/N-RGO550oC and Co3O4/NH2-RGO is below 50 nm, which further assembles together to form an irregular particle morphology as investigated in SEM images part. However, the particle size of Co3O4/N-RGO850oC and Co3O4/PANi/RGO is much bigger than that of Co3O4/N-RGO550oC and Co3O4/NH2-RGO, which is also consistent with the exploration result of SEM images. Interestingly, the particles formed in Co3O4/Ppy/RGO is also composed of tiny particles below 20 nm as shown in Figure 5f. Unfortunately these small particles gathered together to create a bigger flower like Co3O4 particles during the hydrothermal process as shown in Figure 4h and i. The High resolution TEM (HRTEM) image and selected-area electron diffraction (SAED) pattern of Co3O4/N-RGO 550 oC are depicted in Figure 6a and b. The lattice fringes showing an interplanar spacing of 0.244 nm, 0.285 nm and 0.467 nm in Figure 6c and d, which are enlarged images of Part 1 and Part 2 in Figure 6a, coinciding well with the theoretical interlayer spacing for (311), (220) and (111) planes of Co3O4 crystals, respectively.16, 20 Moreover, the well-defined rings in the SAED pattern reveal a polycrystalline characteristic of the loaded Co3O4.25

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o

Co3O4/N-RGO 550 C

16.2 wt.%

o

Co3O4/NH2-RGO (SBET=50 m g )

80

0

-1

0.015

Weight (%)

-1

280

Desorption dV(d) (cc g )

320

600

(d)

o

Co3O4/N-RGO 550 C

0.020

700

Wavenumber (cm )

2 θ (° ) (c)360

-1

Co3O4/NH2-RGO

Co3O4/PANi/RGO

-1

Co3O4/PANi/RGO

663 cm

Co3O4/Ppy/RGO

o

Co3O4/N-RGO 850 C Co3O4/NH2-RGO

567 cm

(440)

(511)

(400)

(311)

(220)

(111)

Intensity (a.u.)

(a)

Volume (cc 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

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Co3O4/N-RGO 850 C Co3O4/NH2-RGO Co3O4/PANi/RGO Co3O4/Ppy/RGO

100

200

300

400

500

o

600

700

Temperature ( C)

Figure 7. (a) XRD patterns, (b) FTIR spectra, (c) Nitrogen adsorption/desorption isotherm, and (d) TGA analysis of the as-prepared Co3O4/N-RGO 550 oC, Co3O4/N-RGO 850 oC, Co3O4/NH2-RGO, Co3O4/PANi/RGO, and Co3O4/PPy/RGO, the inset is the pore size distribution plot of the as-prepared Co3O4/N-RGO 550 oC calculated by BJH method using desorption branch of the isotherms. Figure 7a exhibits the XRD patterns of prepared composite materials, which are perfectly consistent with the cubic spinel Co3O4 (JCPDS card no. 42-1467), indicating high purity of the loaded Co3O4. The diffraction peaks at 19.2°, 31.5°, 37°, 45.1°, 59.6° and 65.6° can be denoted as (111), (220), (311), (400), (511) and (440) reflections, respectively.13, 19, 43 In addition, Figure 7b demonstrates the FTIR spectra of obtained samples and all the samples exhibit two deep peaks at 663 and 567 cm-1, assigning to the Co-O vibration adsorption of spinel cobalt oxide.16-17 The XRD and FTIR measurements confirmed that the deposition of Co3O4 particles was obtained through the hydrothermal process in a alkaline aqueous alcohol solution.

14


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(a)

(b)

800

600

Co3s Co3p1/2&3/2

C1s

N1s

1000

400

Co2p3/2 3+ Co2p1/2 3+ 2+ Satellites

200

0

810

805

Binding Energy (eV) (c)

800

795

790

785

780

(d)

775

C1s

Intensity (a.u.)

Intensity (a.u.)

Satellites

Binding Energy (eV) O1s

O1

2+

O2s

CoLMM

O1s

Intensity (a.u.)

Co3O4/N-RGO 550 C

Co2p3/2

Co2p1/2

Intensity (a.u.)

OKLL Co2s

o

O2 O3

C in graphite C-OH C-O&C-N C=O&C=N O=C-OH

534

532

530

528

526

292

Binding Energy (eV) (e)

290

288

(f)

N1s

o

Co3O4/N-RGO-550 C o

Intensity (a.u.)

Co3O4/N-RGO-850 C

406

Amine

Quaternary Pyridinic N-oxide

404

402

400

398

396

286

284

282

Binding Energy (eV)

394

Co3O4/NH2-RGO Co3O4/PANi-RGO Co3O4/Ppy-RGO

Intensity (a.u.)

536

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


Binding Energy (eV)

775 780 785 790 795 800 805 810

Binding Energy (eV)

Binding Energy (eV)

Figure 8. (a) High resolution XPS survey spectra of the Co3O4/N-RGO 550 oC; Deconvoluted Co2p (b), O1s (c), C1s (d), and N1s (e) of the Co3O4/N-RGO 550 oC; (f) Co 2p of the as-prepared Co3O4/N-RGO 550 oC, Co3O4/N-RGO 850 oC, Co3O4/NH2-RGO, Co3O4/PANi/RGO, and Co3O4/PPy/RGO. The inset shows a comparison of the normalized Co 2p spectra. In addition, the XPS patterns in Figure 8 further verified the formation of Co3O4. To detect the oxidation state of Co3O4 loaded on N doped graphene sheets X-ray photoelectron spectroscopy (XPS) measurements of the Co3O4/N-RGO 550 oC were carried out in the region of 0~1100 eV and the survey spectrum in Figure 8a presents the peaks attributable to Co, O, N, and C. Two sharp peaks (Figure 8b) located at binding energies of 780.2 and 795.4 eV with a spin-energy separation of 15.2 eV 15



assigned to the Co 2p3/2 and Co 2p1/2 spin-orbit peaks of Co3O4, respectively.25 Moreover, the fitting peaks at 780.9 and 796.2 eV are corresponded to Co(II), however the fitting peaks at 779.4 and 794.6 eV are indexed to Co(III). Other two shakeup satellite peaks at around 788 and 803 eV are also corresponding to characteritics of Co3O4.14, 24 The presence of Co3O4 can be further confirmed by resolved O1s XPS peaks (Figure 8c) labeled as O1 and O2 with binding energies of 529.98 and 531.35 eV, which assign to the lattice oxygen of spinel Co3O4 and the OH species absorbing onto the surface of the fabricated microstructure. The fitting peak at 532.73 eV according to component O3 indicates to multiple adsorption of water near the surface.24, 43 The C 1s XPS spectrum of the Co3O4/N-RGO 550 oC (Figure 8d) clearly indicates a considerable degree of oxidation with five components at binding energy of 284.7, 285.1, 286.3, 287.9 and 289.1 eV, corresponding to graphitic C, C-OH, C-O&C-N, C=O&C=N and O-C=O, respectively. The spectra deconvolution of the N1s XPS peak in Figure 8e shows that there exist four different components of nitrogen-containing groups in the Co3O4/N-RGO 550 oC, which could be attributed to pyridinic (398.5 eV), amine (399.65 eV), graphitic (400.8 eV), and oxidized (402.5 eV) type N-functionalities, respectively. The XPS characterization verified the complete conversion of the precursor to Co3O4. This result is well consistent with that of XRD and the high resolution TEM analysis. The XPS peaks of Co 2p for Co3O4 loaded on different active centers are parallel exhibited in Figure 8f. It can be seen that the Co 2p peaks of the Co3O4/N-RGO 550 o C and the Co3O4/NH2-RGO appear at a relatively positive position compared with those of the Co3O4/N-RGO 850 oC, Co3O4/PANi/RGO and Co3O4/PPy/RGO, which implies a stronger interaction between Co3O4 and N doped graphene sheets in the Co3O4/N-RGO 550 oC and the Co3O4/NH2-RGO than in the Co3O4/N-RGO 850 oC, Co3O4/PANi/RGO and Co3O4/PPy/RGO. As we assumed before the doped amine N atoms are more active than doped pyridinic or pyrrolic N atoms for the adsorption of the Co2+ ions, which further forms the fabrication of Co3O4 particles with smaller size to strengthen the interaction between Co3O4 particles and N doped graphene sheets. Therefore the above XPS results excellently verify our assupmtion. As we known, capacitive performance especially EDLCs is closely related to the specific surface area (SSA) of the electrode materials, moreover, the pore size distribution of the electrode material is critical for the transportation of electrolyte. Therefore, the Brunauer-Emmet-Taller (BET) measurement was applied to access the SSA of synthesized samples. The hysteresis loops of the N2 adsorption–desorption isotherm (Figure 7c) indicate a typical mesoporous structure of Co3O4 particles loaded on N doped graphene sheets with different active centers, and the corresponding pore size distribution of the Co3O4/N-RGO 550 oC in the inset of Figure 7c can also confirm this implication. Among all resultant samples Co3O4/N-RGO 550oC and Co3O4/NH2-RGO possess higher SSA than other samples, attributed to the smaller particle sizes of supported Co3O4, which implies better electrochemical performance acting as electrode materials. In addition, the TGA analysis was conducted to obtain 16


Page 16 of 27

Page 17 of 27

the weight percentages of Co3O4 in the the nitrogen-doped graphene supported Co3O4. Figure 7d shows that the TGA curves of the as-prepared Co3O4/N-RGO 550 oC, Co3O4/N-RGO 850 oC, Co3O4/NH2-RGO, Co3O4/PANi/RGO, and Co3O4/PPy/RGO could be mainly divided into three stages. The weight loss < 150 °C could be ascribed to the evaporation of residual moisture, the weight loss between 150 and 350 °C is attributable to the further reduction of the corresponding supporters, and the weight loss between 350 and 700 °C is assignable to the air oxidation and decomposition of the corresponding supporters. According to the TGA result shown in Figure 7d, the relatively weight percentages of loaded Co3O4 for the as-prepared Co3O4/N-RGO 550 o C, Co3O4/N-RGO 850 oC, Co3O4/NH2-RGO, Co3O4/PANi/RGO, and Co3O4/PPy/RGO estimated are 83.8 wt.%, 83.3 wt.%, 83.9 wt.%, 83.1 wt.%, and 83.8 wt.%, respectively. According to the high loading amount of Co3O4 the SSA of fabricated samples is much smaller than the theoretical value of graphene. And the well dispersed Co3O4 particles with a high loading amount should lead to a higher pseudocapacitance. (b) 320

o

Co3O4/N-RGO 550 C

-1

-1

Current density (A g )

-1

Current density (A g )

Co3O4/NH2-RGO Co3O4/PANi/RGO Co3O4/Ppy/RGO

40

Co3O4 o

N-RGO 550 C

0 -40

240 160

(c)

o

5 mV s -1 10 mV s -1 20 mV s -1 30 mV s

o

Co3O4/N-RGO 850 C

80

Co3O4/N-RGO 550 C

o

Co3O4/N-RGO 550 C

0.5

Potential (V vs. Hg/HgO)

(a)120

80 0 -80 -160

-80

o

Co3O4/N-RGO 850 C Co3O4/NH2-RGO Co3O4/PANi/RGO

0.4

Co3O4/Ppy/RGO Co3O4

0.3

o

N-RGO 550 C

0.2 0.1

-240

0.3 0.2 0.1 500

1000 Time (S)

1500

2000

0.2 0.3 0.4 0.5 Potential (V vs. Hg/HgO)

(e)4000

0.6

0

Co3O4/PANi/RGO

3000

Co3O4/Ppy/RGO Co3O4

2500

o

N-RGO 550 C

2000 1500 1000

0

0

2

4

6 8 10 12 14 -1 Current density (A g )

20

0

800

Co3O4 o

N-RGO 550 C

1

100 -1 Power density (W kg )

-1

40

600

Co3O4/PANi/RGO

Co3O4/Ppy/RGO

(h)4000

60

400

Co3O4/NH2-RGO

Co3O4/N-RGO 850 C

10

16

80

200

o

2000

500

Co3O4/N-RGO 550 C

0

1500

o

-1

Co3O4/NH2-RGO

1000 Time (S) Co3O4/N-RGO 550 C

o

Co3O4/N-RGO 850 C

o

100

500

(f)

o

Co3O4/N-RGO 550 C

3500

Specific capacitance (F g )

Potential (V vs. Hg/HgO)

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

0.4

0

(g)

-1

o

Co3O4/N-RGO 550 C

0.5

0.1

Energy density (Wh kg )

(d)

0.6

-1

0.2 0.3 0.4 0.5 Potential (V vs. Hg/HgO)

Specific capacitance (F g )

0.1

Capacitance Retention

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


1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 Cycle number, n

1000

This work Ref. 18 Ref. 20 Ref. 23 Ref. 24 Ref. 14 Ref. 16 Ref. 17

3500 3000 2500 2000 1500 1000 500 0

0

2

4 6 8 10 12 -1 Current density (A g )

14

16

Figure 9. (a) CV curves of the as-prepared Co3O4/N-RGO 550 oC, Co3O4/N-RGO 850 oC, Co3O4/NH2-RGO, Co3O4/PANi/RGO, Co3O4/PPy/RGO, Co3O4, and N-RGO 550 oC in 6.0 mol L-1 KOH at a 10 mV s-1 scan rate; (b) CVs of the Co3O4/N-RGO 550 oC in 6.0 mol L-1 KOH solution at different scan rates; (c) Galvanostatic discharge curves of the as-prepared Co3O4/N-RGO 550 oC, Co3O4/N-RGO 850 oC, 17



Co3O4/NH2-RGO, Co3O4/PANi/RGO, Co3O4/PPy/RGO, Co3O4, and N-RGO 550 oC at 1 A g−1 in 6.0 mol L-1 KOH aqueous solution; (d) The discharge curves of the Co3O4/N-RGO 550 oC at different current densities; (e) Plots of specific capacitance per gram versus current density for the as-prepared Co3O4/N-RGO 550 oC, Co3O4/N-RGO 850 oC, Co3O4/NH2-RGO, Co3O4/PANi/RGO, Co3O4/PPy/RGO, Co3O4, and N-RGO 550 oC; (f) Ragone plots of symmetric supercapacitors based on the as-prepared Co3O4/N-RGO 550 oC, Co3O4/N-RGO 850 oC, Co3O4/NH2-RGO, Co3O4/PANi/RGO, Co3O4/PPy/RGO, Co3O4, and N-RGO 550 oC electrodes; (g) Cycling stability of the as-prepared Co3O4/N-RGO 550 oC electrode at 5 A g-1. The insert diagram shows the working principle of the pseudocapacitor. (h) Comparison of the rate capability of the as-prepared Co3O4/N-RGO 550 oC electrode with that of other Co3O4 composite electrodes previously reported. To evaluate the electrochemical performance of Co3O4/N-doped graphene composites with different active center structures, cyclic voltammetry (CV) and galvanostatic charge−discharge measurements were conducted in 6.0 M KOH electrolyte using a conventional three-electrode configuration. The test was carried out under a fixed potential window from 0.05 to 0.5 V. Figure 9a shows the CV curves (CVs) of Co3O4/N-RGO 550 oC, Co3O4/N-RGO 850 oC, Co3O4/NH2-RGO, Co3O4/PANi/RGO, Co3O4/PPy/RGO, Co3O4, and N-RGO 550 oC in 6.0 mol L-1 KOH solution at the scan rate of 10 mV s-1. A pair of redox peaks can be found for the pure Co3O4 and nitrogen-doped graphene supported Co3O4, which are attribute to the reversible and continuous faradic redox reactions of loaded Co3O4 particles arising from Co2+/Co3+ reaction and the conversion between Co3+ and Co4+, respectively.16, 18, 22 Within the potential scan range, the reactions can be expressed as follows: Co3O 4 + OH − + H 2 O ⇔ 3CoOOH + e − CoOOH + OH − ⇔ CoO 2 + H 2 O + e −

Furthermore, the shapes of the CVs for all fabricated composite electrodes and the pure Co3O4 electrode are quite different from that of N-RGO 550 oC, indicating a typical characteristic of pseudocapacitance. The pseudocapacitance comes from the reversible Faradaic reactions occurring at the interface or near the surface of the electrode material. The CV curve of N-RGO 550 oC exhibits a rectangular-like shape, which is a typical characteristic of the electrical double layer capacitors (EDLCs). The insert diagram in Figure 9g shows the working principle of the pseudocapacitor. In addition, the area surrounded by CV curve of Co3O4/N-RGO 550 oC is apparently the largest among all resultant samples at the same san rate, suggesting the highest specific capacitance. Figure 9b and Figure S1 present the CV curves of the Co3O4/N-RGO 550 oC, Co3O4/N-RGO 850 oC, Co3O4/NH2-RGO, Co3O4/PANi/RGO, and Co3O4/PPy/RGO electrodes in 6 mol L−1 KOH at the scan rates of 5, 10, 20 and 30 mV s−1, respectively. As shown in Figure 9b, the peak current increases with increasing scan rate from 5 to 30 mV s-1, which indicates the rapid redox reactions of 18


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Page 19 of 27 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


loaded Co3O4 particles.14, 23 It should also be noted that with an increase of scan rates, a positive shift of oxidation peaks and a negative shift of reduction peaks are demonstrated caused by the resistance of the electrode.17 The same conclusion could be made from the synthesized other composite electrodes (Figure S1, in the supporting information). To further investigate the electrochemical behavior of prepared samples the galvanostatic discharge curves are shown in Figure 9c. The specific capacitance could be calculated as follows: C = I ∆t / m∆V (1) where C (F g ) is the specific capacitance, I (A) represents discharge current, and m (g), ∆V (V), ∆t assigned to the mass of the active material, the potential drop during discharge and the total discharge time, respectively. The specific capacitances of the Co3O4/N-RGO 550 oC, Co3O4/N-RGO 850 oC, Co3O4/NH2-RGO, Co3O4/PANi/RGO, Co3O4/PPy/RGO, Co3O4, and N-RGO 550 oC at 1 A g−1 determined from the discharge curves are 3553, 867, 2108, 368, 887, 359, and 134 F g-1, respectively. The capacitances of both the pure Co3O4 and N-RGO 550 oC are much lower than those of the nitrogen-doped graphene supported Co3O4, as shown in Figure 9. These detection data further confirm that the Co3O4/N-RGO 550 oC and Co3O4/NH2-RGO dominated with amine N atoms exhibit higher capacitance than that of Co3O4/N-RGO 850 oC and Co3O4/PANi/RGO dominated with other types of nitrogen atoms, consisting with the CVs results. Because the similar mass ratio of Co3O4 were supported onto N-doped graphene sheets with different active centers, however, the interaction between Co2+ and different active centers is different. Therefore, the stronger interaction would lead to the deposition of smaller particles, which would further facilitate the enhancement of electrochemical performance. So the specific capacitance of the Co3O4/N-RGO 550 o C and Co3O4/NH2-RGO is higher than that of other samples due to the smaller loaded Co3O4 particles as we investigated in the SEM and TEM images. Moreover, the relatively higher specific surface area and proper pore size distribution of the Co3O4/N-RGO 550 oC also augment the EDLCs and helpful for the transportation of electrolyte during charge-discharge process, both of which would further improve the specific capacitance. Figure 9d and Figure S2 show the galvanostatic discharge curves of the Co3O4/N-RGO 550 oC, Co3O4/N-RGO 850 oC, Co3O4/NH2-RGO, Co3O4/PANi/RGO, and Co3O4/PPy/RGO at the current densities of 1 to 15 A g-1 in the potential range of 0.05–0.5V (vs. Hg/HgO). The specific capacitances per gram versus current density are then calculated according to Equation (1) and the corresponding value are plotted in Figure 8e. Because of the existence of the electrode resistance, the circuitous diffusion of OH- ions into the pores of the electrode materials as well as the relatively low utilization of the loaded Co3O4 under higher discharge current density,16, 44 the capacity of all samples decreases with increasing current density from 1 to 15 A g-1. The utilization ratio of active materials at high discharge current density is relatively -1

19



Page 20 of 27

low, mainly because the diffusion of ions can only occur on the outer surface of the electrode materials, not on the inner surface of the electrode materials. At lower discharge current densities, both the inner and outer surfaces of the electrode materials can be used for capacitive storage, accordingly, the corresponding capacity is relatively high. Compared to the specific capacitance (3553 F g-1) of the Co3O4/N-RGO 550 oC at 1 Ag−1, the value can still reach as high as 1967 F g-1 even at 15 A g−1 (Figure 9e), which is higher than these of Co3O4/N-RGO 850 oC (327 F g-1), Co3O4/NH2-RGO (1357 F g-1), Co3O4/PANi/RGO (247 F g-1), Co3O4/PPy/RGO (410 F g-1), Co3O4 (172 F g-1), and N-RGO 550 oC (47 F g-1) electrodes. Thus, the Co3O4/N-RGO 550 oC dominated with amine N atoms exhibit the highest capacitance, which are apparently higher than those of the other Co3O4 composite grown on the nitrogen-doped graphene dominated with pyridinic N, pyrrolic N or quaternary N atoms (Figure 9h). Simultaneously, the specific capacitance of Co3O4/N-RGO 550 oC is much higher than the capacitance of N-RGO 550 oC as well as Co3O4. This further illustrates that the high capacitance of Co3O4/N-RGO 550 oC is mainly due to the strong synergistic effect between N-RGO 550 oC and Co3O4. The stronger the interaction between Co3O4 and nitrogen doped graphene, the better the electrochemical performance of the composite. For the as-prepared samples, the energy and power densities were calculated from galvanostatic discharge curves and plotted on the Ragone diagram shown in Figure 9f according to the equations as following:4 ૚

۳ = ࡯(∆ࢂ)૛ ૡ

ࡱ

(2) (3)

ࡼ=ࢀ

where C is the specific capacitance per gram obtained at different current densities, ∆V and T are the corresponding potential window and discharging time, respectively. Noteworthy, the Co3O4/N-RGO 550 oC shows the maximum energy and highest power densities of 25 Wh kg-1 (at a power density of 56 W kg-1) and 843 W kg-1 (at an energy density of 14 Wh kg-1), which further demonstrates the superiority of Co3O4/N-RGO 550 oC over other synthesized samples. Cycling stability is the crucial parameter for the as-synthesized electrode materials in the practical supercapacitor applications. Because the Co3O4/N-RGO 550 o C shows the best specific capacitance as discussed in CVs and galvanostatic charge-discharge parts, therefore its galvanostatic charge-discharge test at a current density of 5 A g-1 for 3000 cycles is depicted in Figure 9g. It can be seen that the Co3O4/N-RGO 550 oC demonstrated an excellent stability, which remains at >90% of its initial capacitance after 3000 charge/discharge cycles, indicating its potential commercial usage. In order to prove the superiority of Co3O4/N-RGO 550 oC over previously reported Co3O4 with different morphology or Co3O4 composite materials, a 20


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comparison of the specific capacitance with the previous reported results is essential, as can be listed in Figure 9h (Table S1 gives the detailed comparison). Figure 9h and Table S1 clearly show that the specific capacitance of prepared sample Co3O4/N-RGO 550 º C in this work is higher than previously reported Co3O4 with different morphology or Co3O4 composite materials under different current density.14, 16-18, 20, 23-24 The excellent electrochemical performance should be due to the strong interaction between loaded Co3O4 particles and amine N atoms doped in graphene sheets, which facilitates the deposition of smaller particle size of Co3O4 and form high specific surface area of composite electrodes to perform a better electrochemical behavior.

CONCLUSIONS Nitrogen functionalized graphene with different nitrogen doping types synthesized from diverse nitrogen doping precursors are explored to investigate the interaction between the active center structures and supported cobalt oxide (Co3O4) particles. It is found that the growth mode of Co3O4 particles can be controlled by the doping type of different nitrogen atoms duing to the different interaction, which greatly related with the electrochemical properties of the obtained composite electrodes. It shows that the doped amine N atoms are more active for the attraction of the Co2+ ions than doped quaternary N, pyrridinic N or pyrrolic N atoms, which further assist the nucleation and growth of Co3O4 particles with smaller particle size. The Co3O4/N-RGO 550 oC dominated with amine N atoms exhibits the highest capacitive performance of 3553 F g-1 at 1 A g-1. Compared to the specific capacitance of the Co3O4/N-RGO 550 oC at 1 Ag−1, the value can still reach as high as 1967 F g-1 even at 15 A g−1, which is much higher than these of Co3O4/N-RGO 850 oC (327 F g-1), Co3O4/NH2-RGO (1357 F g-1), Co3O4/PANi/RGO (247 F g-1), Co3O4/PPy/RGO (410 F g-1), Co3O4 (172 F g-1), and N-RGO 550 oC (47 F g-1) electrodes. Moreover, it also clearly shows that the specific capacitance of prepared sample Co3O4/N-RGO 550ºC in this work is much higher than previously reported Co3O4 with different morphology or Co3O4 composite materials. This indicates that the high capacitance of Co3O4/N-RGO 550 oC is mainly due to the strong synergistic effect between N-RGO 550 oC and Co3O4. The stronger the interaction between Co3O4 and nitrogen doped graphene, the better the electrochemical performance of the composite. In addition, electrode prepared from Co3O4/N-RGO 550 oC also has an excellent cycling stability with >90 % capacity retention at 5 A g-1 after 3000 cycles. The enhanced electrochemical performances of Co3O4/N-RGO 550 oC are attributed to the stronger interaction between Co3O4 and amine N atoms doped in graphene sheets, which facilitate the formation of smaller Co3O4 particle sizes to form higher specific surface and desired pore size distribution to enhance the capacitance. Furthermore, our work not only proposes a simple fabrication route for high capacitance Co3O4/nitrogen doped graphene composite materials but also opens up a possible approach for obtaining other capacitive materials by strengthening the electron interaction between metal oxides and supports. 21



ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxx. Detailed information regarding CVs of the Co3O4/N-RGO 850 oC, Co3O4/NH2-RGO, Co3O4/PANi/RGO, and Co3O4/PPy/RGO in 6.0 mol L-1 KOH solution at different scan rates; the discharge curves of the Co3O4/NH2-RGO, Co3O4/PPy/RGO, Co3O4/N-RGO 850 oC, Co3O4/PANi/RGO in 6.0 mol L-1 KOH solution at different current densities; Comparison of the rate capability of the as-prepared Co3O4/N-RGO 550 oC electrode with that of other Co3O4 composite electrodes previously reported. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author ∗ E-mail:[email protected].

∗ E-mail: [email protected] or [email protected]. ∗ E-mail: [email protected] or [email protected] Author Contributions + These Authors contribute equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by the Chinese National Natural Science Foundation (Nos. 11474101, U1532139, and 21476156), the Guangdong Innovative and Entepreneurial Research Team Program (No. 2014ZT05N200), the “Outstanding Talent and Team Plans Program” of South China University of Technology, the Ningbo Natural Science Foundation (Nos. 2017A610059, 2017A610296), and the Guangdong Provincial Natural Science Foundation (No. 2017A030313092). REFERENCES (1) Huang, Y.; Liang, J.; Chen, Y. An Overview of the Applications of Graphene-Based Materials in Supercapacitors. Small 2012, 8, 1805-1834. (2) Tan, Y. B.; Lee, J.-M. Graphene for Supercapacitor Applications. J. Mater. Chem. A 2013, 1, 14814-14843. 22


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Exploration of the Active Center Structure of Nitrogen-Doped

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