Graphene-supported ultra-small Co3O4 nanoparticles for high-performance supercapacitors

LIU Zheng, LI Ji, WU Xiaoliang, WEI Tong, FAN Zhuangjun



Graphene-supported ultra-small Co3O4nanoparticles for high-performance supercapacitors

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(Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, College of Material Scence and Chemical Engineering, Harbin Engineering University, Harbin 150001, Heilongjiang, China)

Ultra-small Co3O4nanoparticles/graphene hybrid material had been synthesized by a facile hydrothermal route and consequent calcination process. The as-obtained ultra-small Co3O4nanoparticles with their sizes of 5–8 nm are tightly anchored on the surface of graphene (GNS). Benefiting from the ultra-small size of Co3O4nanoparticles, the high interconnectivity of hybrid material as well as the high conductive networks constructed by GNS, which can provide a fast and efficient transportation of electron and electrolyte ions for the overall electrode, the as-prepared hybrid material exhibits a high specific capacitance of 462 F·g-1at 5 mV·s-1compared with pure Co3O4(193 F·g-1), and retained 88.2% of its initial capacitance after 2000 cycles, indicating a promising electrode material for supercapacitors.

ultra-small nanoparticles; cobalt oxides; graphene; supercapacitors

Supercapacitors, which operate at high charge and discharge rates, have the ability to sustain millions of cycles as energy storage devices for hybrid electric vehicles (HEVs), back-up power and portable electronic equipments [1–3]. However, superca pacitors still suffer from low energy storage density (usually£10 W·h·kg–1) compared with that of lithium-ion batteries (LIBs) [3]. The energy density of supercapacitors is originally related to the specific capacitance of electrode materials based on the equation of=0.52. Hence, the improvement of specific capacitance of the electrodes to gain high energy density for supercapacitors is extremely important [4]. Recently researches show that the rational design of material structure can improve the utilization of electroactive materials, leading to high specific capacitance[5-8].

Graphene (GNS), owing to its ultrahigh surface area (about 2630 m2·g–1), very high intrinsic electrical conductivity, chemical stability and two dimensional (2D) layered-shaped structure, has shown excellent electrochemical properties for energy storage devices [9]. Unfortunately, their strong sheet-to-sheet van der Waals interactions easily cause the aggregation or restacking of graphene, resulting in the decreased electrochemical activity, e. g. electric double layer capacitance [10]. Up to now, GNS are used as fundamental building blocks to construct metal oxide/GNS nanocomposites [11], such as RuO2/GNS [12], MnO2/GNS [13], Mn3O4/GNS [14], SnO2/GNS [15], ZnO/GNS [16], NiO/GNS [17], and Co3O4/GNS [18], in view of the utilization of the specific surface area and conductive properties of GNS.

Among all metal oxides, Co3O4has attracted tremendous attention owing to its ultrahigh theoretical specific capacitance (3560 F·g-1) and long term stability of electrochemical behavior. Recently, Co3O4nanocrystals with different structures including nanowires [19], nanoparticles [20], nanofibers [21], nanorods [22], and nanotubes [23] have been successfully synthesized and showed excellent performances. For example, 3D hierarchical Co3O4twin-spheres with an urchin-like structure exhibited a high specific capacitance and electrochemical stability at high current densities [24]; 1D Co3O4hollow fibers prepared by a controlled thermolysis of Co(CH3COO)2ethanol solution, showed a capacitance of 278 F·g-1at 0.5 A·g-1and 176 F·g-1at 5 A·g-1[25]; XIANG et al. prepared 20 nm Co3O4nanoparticles on the reduced graphene oxide (rGO), the composite exhibited a specific capacitance of 472 F·g-1at 2 mV·s-1, and 95.6% specific capacitance of Co3O4/rGO electrode was retained at 2.0 A·g-1after 1000 cycles [26]. However, the size of their grain is still relatively large. To make full use of characteristic of nanomaterials, it is urgent to control the size and morphology of Co3O4-based materials for their potential energy storage applications [27].

In this work, we reported the facile synthesis and excellent electrochemical performances of porous ultra-small Co3O4nanoparticles/GNS hybrid material. The novel 3D nano-architecture was synthesized by a facile solvothermal method and followed by air calcination Co3O4spheres, which were constructed by ultra-small particles with a size of 5–8 nm tightly anchored on GNS (Scheme 1). The ultra-small Co3O4nanoparticles and the conductive GNS substrate provide a fast and efficient transport of electron and electrolyte ions for overall electrode. As a result, this novel hybrid material exhibited a high specific capacitance of 462 F·g-1at 5 mV·s-1(based on the mass of active materials) compared with pure Co3O4(193 F·g-1), and retained 88% of its initial capacitance after 2000 cycles.

Scheme 1 Schematic illustration of the fabrication process for preparing C-Co3O4/GNS nanocomposite.

1 Experimental

1.1 Synthesis of Co3O4 and Co3O4/GNS nanocomposite

Graphite oxide (GO) was synthesized from natural graphite (300 μm, Qingdao Graphite Company) by a modified Hummer’s method. For the synthesis of Co3O4/GNS, 16.7 mg of GO was collected from the aqueous solution by centrifugation, and dispersed into 144 mL anhydrous ethanol (EtOH) by ultrasonic vibration for 2 h, followed by the addition of 1.445 mmol Co(CH3COO)2·4H2O and 7.2 mL deionized water, then, the suspension was kept at 80 ℃ with stirring for 10 h. Subsequently, the mixture was transferred to a Teflon-lined autoclave of 50 mL capacity, heated to 150 ℃, sealed and maintained at this temperature for 3 h without shaking or stirring. After the reaction, the mixture was cooled to room temperature naturally; the black precipitate was collected by centrifugation, washed with deionized water and absolute ethanol three times, and dried by lyophilization for 12 h. The as-prepared Co3O4/GNS nanocomposite was calcined at 300 ℃ for 2 h with a heating rate of 3 ℃ min in air, and named C-Co3O4/ GNS. For comparison, similar preparation process was used to prepare pure Co3O4without adding any GO and calcination.

1.2 Characterization

The crystallographic structures of the samples were measured by a powder X-ray diffraction system (XRD, TTR-III) equipped with Cu Kαradiation (= 0.154056 nm), scanned from 10° to 80° (2) at a rate of 10°·min-1. Operation voltage was 40 kV with a current of 150 mA. Morphologies of samples were characterized by using a scanning electron microscopy (SEM, HITACHI SU-70) and transition electron microscopy (TEM, FEI TECNAI TF20) with a field emission gun operating at 200 kV. The N2adsorption-desorption isotherms of the samples were measured at 77 K using NOVA 2000 (Quantachrome, USA). Brunauer-Emmett-Teller (BET) and density functional theory (DFT) methods were introduced to calculate specific surface area and pore size distribution, respectively. X-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI 5700 ESCA spectrometer with a monochromated Al Kαradiation (= 1486.6 eV).

1.3 Electrochemical measurements

The working electrodes consisted of 75 %(weight ratio) active materials (C-Co3O4/GNS, Co3O4/GNS or pure Co3O4), 20 % conductive materials (carbon black) and 5 % binder (polytetrafluoroethylene, PTFE), and dispersed in ethanol. Then the slurry was coated onto the nickel foam substrate (1 cm ×1 cm) to form the working electrodes, and dried at 100 ℃ for 12 h in a vacuum oven.

The electrochemical properties of the products were determined under a three-electrode cell at room temperature. Active material electrode served as the working electrode, platinum foil and saturated calomel electrode (SCE) as the counter and reference electrodes, respectively. Cyclic voltammetric (CV) measurements were conducted from 0 to 0.4 V (. SCE) at 2, 5, 10, 20, 30, 40, 50, 75 and 100 mV·s-1, galvanostatic charge/discharge was measured from 0 to 0.35 V (. SCE) in 6 mol·L-1KOH on an electrochemical workstation (CHI 660C) at room temperature.

2 Results and discussion

Fig. 1 shows the typical XRD patterns of Co3O4, Co3O4/GNS and C-Co3O4/GNS. The dominant diffraction peaks of all samples can be indexed to cubic Co3O4(JCPDS No. 42—1467). Compared with Co3O4/GNS before calcination, the graphene diffraction peak of C-Co3O4/GNS at around 25° (corresponding to the 002 lattice plane of carbon) cannot be clearly observed, indicating that the restacking of the graphene sheets has been effectively eliminated by a uniform distribution of Co3O4nanoparticles after the heat-treatment [28].

The detailed morphology and structure properties were well characterized by SEM and TEM as shown in Fig. 2. Pure Co3O4has a uniform sphere-like structure with the size of 50–70 nm [Fig. 2(a)]. Further TEM observation in [Fig. 2(b)] exhibits that the Co3O4particles actually consist of many ultra-small nanoparticles with their sizes of 5–8 nm. Interestingly, Co3O4spheres still tightly anchor on the surface of GNS after the incorporation with GNS[Fig. 2(c)]. Moreover, from TEM image in [Fig. 2(d)], it is clear seen that the Co3O4spheres on the surface of GNS are also constructed by ultra-small nanoparticles with the sizes of 5–8 nm. Additionally, selected area electron diffraction (SAED) patterns (inset of Fig. 2(d) reveal the polycrystalline feature of Co3O4, and the six most-distinct concentric diffraction rings correspond to (111), (220), (311), (400), (511) and (440) planes of cubic Co3O4, which agrees well with the results obtained from XRD analysis. Notably, the aggregated Co3O4spheres easily spread and recrystallize on the surface of GNS to form uniformed Co3O4nanoparticles/GNS nanostructure due to the growth of the nanoparticles during calcination [Fig. 2(e)]. High- resolution TEM (HRTEM) image in Fig. 2(f) shows that Co3O4nanoparticles closely coupled with graphene sheets (marked by arrow) has a cubic spinel structure and the interlayer spacing around 0.485 nm and 0.230 nm are consistent with the standard (111) and (222) lattice plane of Co3O4, respectively. As a result, this unique structure with ultra-small nanoparticles, high interconnectivity and integrity, facilitating fast ion/electron transportation in/between the Co3O4/GNS nanostructure, is very crucial for the application of high performance supercapacitors [29].

To analyze the chemical compositions and the element bonding configuration for our nanomaterials, the X-ray photoelectron spectroscopy (XPS) was further performed. The survey spectrum of C-Co3O4/GNS nanocomposite in Fig. 3(a) directly reveals the presence of C 1s, Co 2p, and O 1s without any other element, reflecting that the successful incorporation of Co3O4into GNS. In addition, the O 1s XPS peak at 530.1 eV, is related with the oxygen species in the Co3O4phase [30-31]. The peak of C 1s is located at 284.6 eV, which is in agreement with the existence of graphene [32]. Notably, the Co 2p XPS spectrum of the nanocomposite [Fig. 3(b)] shows two major peaks with the binding energies at 780.1 and 795.8 eV, corresponding to Co 2p3/2and Co 2p1/2, respectively, with a spin-energy separation of 15.7 eV, which is characteristic of a Co3O4phase [20, 33]. The results of the XPS analysis are consistent well with the XRD analyses.

The specific surface area and porous structure of C-Co3O4/GNS were calculated by Brunauer-Emmett- Teller (BET) N2adsorption/desorption analysis. As shown in Fig. 4, the isotherm of the C-Co3O4/GNS nanocomposite can be ascribed as a type-IV isotherm, according to the International Union of Pure and Applied Chemistry (IUPAC) classification [34]. Notably, the hybrid material displays a BET surface area of 83.12 m2·g-1, which is beneficial for the transportation of ion and electron for overall electrode material, resulting in superior electrochemical performance as an electrode material for superca pacitors. Furthermore, a small pore size around 1.8 nm (inset in Fig. 4) and a pore volume of 0.34 cm3·g-1are observed from the pore-size-distribution (PSD) curve and N2adsorption/desorption isotherm, respectively, meaning that the porous characteristic of C-Co3O4/GNS hybrid material. All results of nitrogen adsorption experiments are well consistent with the pore size observed from TEM and HRTEM characterizations in Fig. 2.

Based on the above characterized results, a formation mechanism is proposed as schematically illustrated in Scheme 1. The formation of this unique architecture mainly represents as follows: ① GO has a large number of oxygen functional groups such as epoxy and hydroxyl, and the carbonyl and carboxyl groups [35], which can act as anchor sites that the metal ions (Co2+) can be easily adsorbed onto the surface of GO sheets through electrostatic attraction; ② Co3O4spheres are firmly attached on the graphene nanosheets after hydrothermal reaction; ③ Co3O4nanoparticles can spread and recrystallize on the surface of GNS to form Co3O4nanoparticles/GNS nanostructure composite due to the broken Co3O4agglomerates induced by thermal stress during calcination.

The electrochemical behaviors of C-Co3O4/GNS nanocomposite are evaluated as the working electrode for supercapacitor applications. Fig. 5(a) shows the CV curves of C-Co3O4/GNS electrode at different scan rates ranging from 5 to 30 mV·s-1within the potential window of 0 to 0.4 V (. SCE) in 6 mol/L KOH aqueous solution. Two pairs of intense peaks can be seen obviously observed, meaning that the specific capacitance primarily originates from the pseudocapacitive capacitance based on a redox mechanism. For each curve, anodic peaks at around 0.12 V and 0.26 V, and the cathodic peaks at 0.20 V and 0.28 V, are mainly attributed to the Faradaic redox reactions related to Co3O4/CoOOH and CoOOH/CoO2, and the corresponding reactions take place as follows:

Furthermore, all CV curves show a similar shape, and the current density increases with the increasing scan rate, which should be associated with the enhanced mass transportation and electron conduction. Notably, with increasing of potential scan rate, anodic peaks shift to higher potential and cathodic peaks shift to lower potential, which is mainly caused by the limitation of the ion diffusion rate to satisfy electronic neutralization during the redox reaction. More importantly, C-Co3O4/GNS shows a significant increase in the internal areas of CV curve as shown in Fig. 5(b), indicating the enhanced electrochemical activity of C-Co3O4/GNS due to the interconnectivity and integrity of Co3O4/GNS nanocomposite [36-37].

The specific capacitance of the electrode can be calculated according to the following equation:

Whereis the response current density, A·cm-1;is the potential, V;is the potential scan rate, mV·s-1; andis the mass of the electroactive materials in the electrode, g.

The specific capacitance of Co3O4, Co3O4/GNS and C-Co3O4/GNS at different scan rates is presented in Fig. 6(a). Clearly, the specific capacitance of C-Co3O4/GNS nanocomposite is calculated to be 462 F·g-1at 5 mV·s-1, much higher than that of pure Co3O4(193 F·g-1) and Co3O4/GNS (281 F·g-1). Fig. 6(b) shows the galvanostatic charge-discharge curves of Co3O4, Co3O4/GNS and C-Co3O4/GNS at different current densities. It can be seen that the emergence of the charge-discharge platform corresponds to the reversible redox process, demonstrating the pseudo capacitive faradaic characteristic of Co3O4, which is consistent with the redox peaks of CV curves.

The high capacitance of C-Co3O4/GNS nanocom posite can be attributed to the highly conductive GNS substrate, which provides an effective path for electron transportation, as well as the porous Co3O4nanoparticles, which are beneficial for fast electron transport and ion diffusion [38]. More importantly, the ultra-small sized Co3O4nanoparticles anchored on GNS provide high electroactive sites and short diffusion length.

Life-cycle stability is very crucial for superca pacitors in practical applications [39]. The cycle stability of Co3O4and C-Co3O4/GNS was conducted between 0 and 0.4 V (. SCE) at a scan rate of 100 mV·s-1for 2000 cycles as shown in Fig. 7. Interestingly, the specific capacitance of the Co3O4/GNS nanocomposite electrode increases by 5% after the first 100 cycles, indicating that the electrode is fully activated and reach the stability state [40-42]. Subsequently, there is a small decrease of the capacitance, resulting from the growth of particle and crystal size of the electrode material, which caused by the irreversible reaction between the electrode materials and electrolyte [24]. After 2000 cycles, the specific capacitance only decreases to 88% of the initial value at this high current density, much higher than that of Co3O4(75% for 2000 cycles), meaning that the C-Co3O4/GNS nanocomposite has good electrochemical stability and long lifetime. It is believed that the increased cycling stability of the C-Co3O4/GNS nanocomposite is attributed to the GNS provide high specific area and high conductivity, buffering the volume change of Co3O4during charge-discharge process, which could effectively reduce the kinetic difficulties for both charge transfer and ion transport for overall electrode material during the cycling process [41].

3 Conclusions

Ultra-small Co3O4nanoparticles/GNS composite had been synthesized by a facile hydrothermal route and consequent calcinations process. Ultra-small Co3O4nanoparticles with the sizes of 5–8 nm are tightly anchored on graphene sheets. Benefiting from the synergistic effect between graphene sheets and Co3O4nanoparticles as well as the ultras-mall porous Co3O4nanoparticles, the as-obtained Co3O4nanoparticles/GNS nanocomposite exhibits high specific capacitance and good cycling stability. The designed electrode materials with high capacitance and excellent cycling stability offer an important guideline for future design of advanced supercapacitors.

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10.12028/j.issn.2095-4239.2016.0049

TM 911 Document code: A Article ID: 2095-4239（2016）06-841-08

2016-07-18.

Fund support：NaturalScience Foundation of Heilonjiang Province (E201416); the Natural Science Foundation of China (51672055), Ph.D.Programs Foundation of Ministry of Education of China (201223 04110020).

The first author：LIU Zheng (1990—)，male，E-mail：liuzhengbeyond @163.com；the corresponding author：FAN Zhuangjun，professor，E-mail：fanzhj666 @163.com.