Graphitic Carbon Coated CuO Hollow Nanospheres with Penetrated

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Graphitic carbon coated CuO hollow nanospheres with penetrated mesochannels for high–performance asymmetric supercapacitors Jing Zhang, Guofeng Zhang, Wenhao Luo, Yan Sun, Cen Jin, and Wenjun Zheng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00755 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 29, 2016

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ACS Sustainable Chemistry & Engineering

Graphitic carbon coated CuO hollow nanospheres with penetrated mesochannels for high–performance asymmetric supercapacitors Jing Zhanga, Guofeng Zhanga, Wenhao Luoa, Yan Suna, Cen Jina, and Wenjun Zheng*ab a

Department of Chemistry, and Key Laboratory of Advanced Energy Materials Chemistry

(MOE), TKL of Metal and Molecule–based Material Chemistry, College of Chemistry, Nankai University, No.94, Weijin Road, Nankai District, Tianjin, 300071, China. E–mail: [email protected] b

Collaborative Innovation Center of Chemical Science and Engineering, Nankai University,

No.94, Weijin Road, Nankai District, Tianjin, 300071, China.

ABSTRACT: We have developed a simple solvothermal–calcination strategy to synthesize continuous graphitic carbon coated hollow CuO (H–CuO@GC) spheres with excellent electrochemical performance. The H–CuO@GC spheres exhibit a high specific surface area (106.6 m2 g–1), penetrated mesochannels (~ 5–15 nm), a large pore volume (0.313 cm3 g–1), a robust hollow structure, and an integral graphitic carbon layer. The H–CuO@GC sphere electrode presents high capacitance, good rate capability and outstanding cycling ability in supercapacitors. In addition, the asymmetric supercapacitor (ASC) assembled by this structure exhibits a good rate capability (retain 75.7% at 10 A g–1) and an excellent cycling stability 1 ACS Paragon Plus Environment


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(90.2% capacitance retention after 10000 cycles), as well as a high energy density (38.6 W h kg–1 at a power density of 1.018 kW kg–1). This work represents a novel design strategy for the improvement of low–conductive nanomaterials applied in many fields, especially in energy applications.

KEYWORDS:

Nanostructures;

Copper

Oxide;

Three–Dimensional;

Mesoporous;

Pseudocapacitors

INTRODUCTION With the explosive worldwide market demand of emerging consumer electronics (e.g. smart– phones, tablets, and e–readers), great effort has been guided to portable energy storage device as power sources. Thus, it is required a lighter and smaller energy device without reducing its power density. As a new energy storage device to fill the gap between traditional capacitor and secondary battery, supercapacitors are able to cater for the needs because of their superior reversibility, rapid charge and discharge,

and high power density.1–3 Particularly,

pseudocapacitors with a Faradaic redox process to store charges have gained great interests, because they can achieve higher energy density than electrochemical double–layer capacitance (EDLC).4 Among the numerous pseudocapacitance materials, CuO is considered as an excellent candidate because of its inexpensive, abundance, safe, and high stability. Although theoretical capacity of CuO is as high as 1800 F g–1,5 the actual specific capacity is not satisfactory, which make it not attract as much attention as other transition metal oxides, such as MnO2, Co3O4, NiO, and V2O5.6–9 Therefore, fabrication of CuO–based pseudocapacitors with high specific capacitances poses a big challenge.

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In order to fulfill fast and reversible redox process, ideal electrode materials are required to exhibit high specific surface area, large porosity, rapid electron and ion transport, as well as high firm structure.10–12 To increase the specific surface area and porosity of CuO electrode, various nanostructures have been designed, including 1D, 2D, and 3D structures.13–15 Porous hollow structure design is an effective route due to its intriguing structural features with low density, surface permeability, and kinetically opportune open structure.16–18 Conventionally, the most effective synthesis methods of hollow structures reported so far are hard template or soft template method. Nevertheless, it is hard to surmount some inherent defects. For example, the removal of hard template is very cumbersome and energy consuming, the morphologies of hollow structures are normally difficult to control because of the deformability of the soft template.19,20 Hence, it is necessary to explore a facile template–free strategy for the fabrication of CuO hollow structure. The electronic conductivity as another factor restricts the electrochemical performance of CuO–based electrode. A general approach to increase electronic conductivity is combining CuO with carbonaceous materials (such as graphene and CNT).21,22 However, in most cases, unavoidable disintegration and aggregation during the charge/discharge process would limit the development of graphene or CNT–based CuO electrode.23 These limitations can be overcome by coating carbon on the CuO nanostructures.24 Thus, the transfer of electrons can be improved without changing its sharp. In addition, carbon coated layer can contribute to maintain the inherent morphology of CuO nanostructures, thus enhancing their structural stability during the cycling test. In terms of the coated carbon, the electronic conductivity and the diffusion of electrolyte ions within graphitic carbon layers are better than amorphous carbon layers.25 As a result, graphitic carbon coated materials would show better electrochemical performance.



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Therefore, coating with conformal graphitic carbon on CuO hollow nanostructures is feasible to meet with the demand of high performance electrode materials. However, to the best of our knowledge, there is no report related to this electrode design to date. In this paper, we developed a template–free wet chemical route to prepare CuO hollow (denoted as H–CuO) spheres, and then successfully realized a simple coating strategy to prepare continuous graphitic carbon coated hollow CuO (denoted as H–CuO@GC) spheres. The as– prepared H–CuO@GC spheres show high specific surface area and pore volume, rapid electron and ion transport, as well as high firm structure. When used as electrode material for supercapacitors, H–CuO@GC spheres show high specific capacitance, good rate performance, as well as long cycle life. Furthermore, H–CuO@GC spheres and activated carbon (AC) were used as positive and negative electrode to fabricate an asymmetric supercapacitor, which exhibits a specific capacitance of 108.7 F g–1 and a maximum energy density of 38.6 W h kg–1 (0.5 A g–1). EXPERIMENTAL DETAIL Synthesis of hollow CuO spheres All the reagents used in the experiment were analytical reagent grade and were used directly without any further purification. In a typical procedure, 6 mL of deionized water and 16 mL of N,N–dimethyl formamide (DMF) were mixed thoroughly, then 0.17g of CuCl2·2H2O (1 mmol) was added and stirring for 20 min. Next, under vigorous stirring, 2 mL of n–butylamine was added slowly. After 30 min of stirring, the mixture was put into a 30 mL Teflon-lined autoclave to be heated at 140 °C for 8 h. The sample was then collected by centrifugation, rinsed with deionized water and ethanol, finally dried at 60 ℃ for 12 h. Employing NaOH rather than n–


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butylamine as alkaline reagent and adjusting the pH value and experimental conditions as the same, the granular CuO sample would be obtained. Synthesis of graphitic carbon coated hollow CuO spheres 50 mg of the as–prepared hollow CuO spheres were dispersed in 20 mL of glucose solution (0.1 M) with stirring overnight. Then the solid was centrifugated and dried at 60 °C. After that, the obtained glucose adsorbed hollow CuO spheres were annealed at 550 °C for 2 h under a slow ramping rate (2 °C/min) in argon atmosphere. The graphitic carbon coated hollow CuO spheres could be obtained after washing and drying. Characterizations Scanning electron microscopy (SEM) images were recorded on a MERLIN Compact (ZEISS) field emission scanning electron microscope (15 kV) coupled with energy dispersive X– ray spectroscopy (EDS). A Tecnai G2 F20 transmission electron microscope was used for transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images. XRD patterns of the products were obtained on a Rigaku Ultima IV diffractometer with Cu Ka radiation. X-ray photoelectron spectroscopy (XPS) was performed to analyse surface elemental compositions using a Kratos Axis Ultra DLD multi–technique XPS System. The characterization of carbon was carried by Raman spectra (Renishaw inVia, Renishaw). Thermogravimetric (TG) analysis was recorded on TG209 (NETZSCH) within a temperature range of 25 to 800 °C and with a ramping rate (5 °C/min) in air atmosphere. Nitrogen adsorption and desorption isotherm measurement were performed in PCT E&E Siverts-type gas sorption analyzer. Electrochemical measurements



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In half–cell tests, the working electrode (1 cm × 1 cm) consists of 80 wt% of active materials, 10 wt% of conductive acetylene black, and 10 wt% of polymer binder (polyvinylidene fluoride, PVDF). A Hg/HgO electrode and a platinum flake were used as the reference electrode and counter electrode, respectively. 3 M KOH solution was used as electrolyte. Each electrode was coated about 1.6 mg of active materials. The electrochemical tests including cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS, 0.1 Hz-100 KHz) were carried out on an electrochemical workstation (Zahner IM6e), galvanostatic charge-discharge (GCD) measurements were conducted with a LAND battery tester. Asymmetric supercapacitor device was assembled to evaluate the practical applications of H–CuO@GC hollow spheres electrode materials. The active carbon and the as-prepared active materials were used as negative and positive materials, respectively. Then the electrodes (1 cm × 3 cm each) were collected together with a separator (cellulose paper) in between and 3M KOH solution as the electrolyte to form a ASC device. Voltammetric charges (Q) is calculated from the following formula: Q = C × ∆V × m, where C is the specific capacitance (F g−1), ∆V is the potential window (V), and m is the mass for single electrode (g). The voltammetric charges in two electrodes should be match (Q+ = Q-), the mass of negative materials can be calculated. Accurately, the m−/m+ is calculated to be 3.1 in our ASC device. The energy density E (W h kg–1) and power density P (W kg–1) were evaluated using the following formulas:

E =

P =

C × ∆V 2

E ∆t

2

×

1000 3600

(1)

(2)

× 3600


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where C designates specific capacitance (F g−1) and ∆V represents the voltage range (V); ∆t represents the discharge time (s). RESULTS AND DISCUSSION H–CuO@GC spheres are synthesized via a two–step approach, as illustrated in Figure 1a. Firstly, H–CuO spheres are prepared directly through a facile template–free solvothermal method. The characterizations in Figure S1 (Supporting Information) demonstrate that the as– prepared H–CuO spheres are monoclinic CuO microspheres with a mesoporous hollow structure assembled by small nanoparticles. In addition, the formation of CuO hollow structure is found to undergo a typical Ostwald ripening process.26 The scanning electron microscopy (SEM) images (Figure 1b–d) of the product obtained in different reaction time suggest that the hollow structure is gradually evolved from solid structure, an illustration of this formation procedure is shown in Figure 1e. With prolonging the reaction time, the increasing particle sizes in the SEM images and the increasing peak intensities in the powder X–ray diffraction (XRD) patterns (Figure S2) also signify the Ostwald ripening process. Interestingly, n–butylamine in the reaction plays a crucial role for the formation of sphere–like structure. In case of NaOH as alkaline reagent in the same condition, the as–obtained CuO sample is irregular granular CuO aggregation (denoted as G–CuO) as shown in Figure S3. It indicates that the solid spheres will not be formed without n– butylamine and the Ostwald ripening will not occur during the subsequent procedures. The influence of n–butylamine may be related to its adsorption on the surface of CuO crystal. It is reported that amine molecules can interact with cupper ions on the CuO surface by coordinate bonds.27,28 Therefore, the amount of adsorbed amine molecules on different crystal planes are different, because the density of exposed Cu2+ ions are different, which result in a unique structure. With the interaction of the adsorbed n–butylamine and the demand of minimizing the 7 ACS Paragon Plus Environment


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surface energy, CuO granules will accumulate together to form sphere-like structure. The second step is carbon coated process through a simple calcination route. In this step, graphitic carbon would cover on the surface of CuO particles in the mesoporous CuO network shells by the carbonization of glucose (details in the Experimental Section). The SEM and transmission electron microscopy (TEM) images (Figure 2a, b) reveal that the final product H–CuO@GC spheres maintain the hollow structure of the H–CuO spheres after carbonization process. The diameter of the obtained H–CuO@GC spheres ranges from 0.8 to 1 µm, which is slight larger than that of H–CuO spheres. The shell thickness is about 200–250 nm, indicating the attenuation of the shell after calcination. The assembly of numerous nanoparticles can provide suitable mesoporous structure as shown in Figure 2c, which will provide penetrated mesochannels for the diffusion of electrolyte ions. It could also be found a uniform and continuous graphitic carbon layer coating on the surface of the CuO particles (as shown by arrows in Figure 2c, d). The high–resolution TEM (HRTEM) image (Figure 2d) reveals an interplane spacing of 0.271 nm, which indicates the presence of the (110) plane of monoclinic CuO. The elemental mapping images shown in Figure 2e-h reveal the even distribution of O, Cu, and C elements throughout the H–CuO@GC spheres. The XRD pattern of the H–CuO@GC spheres shown in Figure 3a demonstrates that all the diffraction peaks are fully accordance with the monoclinic phase of CuO (JCPDS No. 48–1548). The absence of diffraction peak of carbon may be due to the high crystallinity of CuO and the low content of carbon. To gain insight into the chemical components and valence state of the H– CuO@GC spheres, Raman, X–ray photoelectron spectroscopy (XPS), and thermogravimetric (TG) analysis were performed. Two peaks at 1349 cm–1 and 1583 cm–1 shown in Raman spectrum (Figure 3b) are the characteristic D and G bands of carbon. A significant factor for 8 ACS Paragon Plus Environment

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evaluating the degree of crystallinity of carbon materials is the intensity of the peaks between G and D bands (IG/ID).23,29 Here the value of IG/ID is calculated to be 1.32, demonstrating the high degree of carbon in H–CuO@GC material, that is, graphitic carbon. The XPS result (Figure S4) reveals that the sample contains Cu, O, and C elements, and the valence state of Cu only shows Cu2+ state.30,31 The high intensity of C peak may be because of that carbon is coated on the surface of CuO particles. The carbon content of H–CuO@GC spheres was measured by TG curve, as demonstrated in Figure 3c. The evaporation of the moisture is the main reason for the weight loss below 100 oC. The strict weight loss between 350 and 520 oC can be attributed to the oxidation of carbon in the air. The carbon content is measured to be 12.3%. Figure 3d shows the nitrogen adsorption and desorption isotherms of the H–CuO@GC spheres. The H–CuO@GC spheres show a type IV isotherm, suggesting a mesoporous structure. The BET surface area is determined to be 106.6 m2 g–1. The pore size distribution on the basis of BHJ analysis (inset of Figure 3d) is mainly centered at approximately 5–15 nm. The suitable pore size will supply penetrated mesochannels for the migration of electrolyte ion, which will increase the ion diffusion kinetics.32 Furthermore, the total pore volume is calculated to be as high as 0.313 cm3 g–1. The high surface area and pore volume will provide shorter pathway for the ion diffusion and more active sites for rapid redox reaction. More importantly, the graphitic carbon coated on the surface of H–CuO spheres can effectively enhance the electron conductivity and serves as a “cocoon” for manacling the hollow sphere structure without collapse.23 According to above advantages, the H–CuO@GC spheres are expected to show satisfactory electrochemical performance. The electrochemical performance was investigated in a three–electrode cell using 3 M KOH as electrolyte. The cyclic voltammetry (CV) curves of H–CuO@GC spheres within a potential 9 ACS Paragon Plus Environment


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range of 0–0.65 V are demonstrated in Figure 4a. All the CV curves show different shapes from an ideal rectangle, which indicate the pseudocapacitive characteristic. The previous reports5,14,15,21 indicate that the reversible redox peaks are probably related to the conversion between Cu2+ and Cu+ species. Herein, a possible redox mechanism of H–CuO@GC electrode is proposed as follow: CuO +

1

2

H 2O + e− ⇔

1

2

Cu 2O + OH −

(3)

Figure S5a contrasts the CV curves of H–CuO@GC, H–CuO, and G–CuO electrodes at the same scan rate of 20 mV s–1. The enclosed area of H–CuO@GC electrode is much larger than those for H–CuO and G–CuO electrodes, suggesting the higher specific capacitance of H–CuO@GC electrode. Figure 4b shows the galvanostatic charge–discharge profiles of H–CuO@GC electrode at the voltage window of 0–0.6 V at current densities from 1 to 20 A g−1. The nonlinear shape of the GCD profiles suggests the pseudocapacitive behaviour, which agree well with the CV profiles. The GCD curves for H–CuO@GC, H–CuO, and G–CuO electrodes at a current density of 5 A g–1 are shown in Figure S5b. As expected, the H–CuO@GC electrode exhibits higher capacitances than the H–CuO and G–CuO electrodes at the same current density. The CV and GCD curves demonstrate the evidence that the mesoporous hollow structure shows higher specific capacitance than irregular granular aggregation, and graphitic carbon coating would further improve its electrochemical performance. The rate capability and cycling performance tests further highlight the significance of graphitic carbon coated hollow structure for the high performance supercapacitors. The calculated specific capacitances of different CuO electrodes from the GCD profiles are illustrated in Figure 4c. The specific capacitances of H–CuO@GC, H–CuO, and G–CuO electrode are 677,


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526, and 333 F g–1 at 1 A g–1. When the charge rate reaches to 20 A g–1, the specific capacitances are 538, 362, and 153 F g–1, retaining 79.5%, 68.8%, and 45.9% of the original value. Figure 4d compares the structural stability of three CuO electrodes at 5 A g–1. In the first 600 cycles, it can be observed that the capacitances have been increased slightly, which is related to an activation process with slow escape of Cu2+ ions from CuO crystal lattice.15,22,33 After 8000 cycles, the H– CuO@GC sphere electrode possesses 86.7% capacitance retention, which is much higher than those of H–CuO (72.1%) and G–CuO (40.7%). The aforementioned results indicate that synergistic effect of hollow structure and graphitic carbon coating contributes to the high performance supercapacitors. More detailed values about the electrochemical performance of the three CuO electrodes are shown in Supporting Information Table S1. The SEM images of H– CuO@GC electrode after cyclic measurements are demonstrated in Figure S6a, b. Although the shell of the spheres is tuned thicker, the hollow spherical structures can be held, obviously indicating the high structural stability of H–CuO@GC during the charging–discharging process. For the H–CuO without carbon coating (Figure S6c, d), the CuO shells would rupture without any protection and finally destroyed after long–term testing. The XRD pattern (Figure S7a) of H–CuO@GC spheres after 8000 cycles indicates that the phase is not change during long-term cycles, the weak peaks of Ni come from Ni foam during the collection of the sample. The interplane spacing (0.272 nm) corresponding to monoclinic CuO in the HRTEM image (Figure S7b) also indicates the high stability of H–CuO@GC electrode after cycles. Electrochemical impedance spectrum (EIS) analysis is a good approach to study the interfacial process. The Nyquist plots of the three electrodes are shown in Figure S8. At high frequency region, the semicircle is corresponding to charge transfer resistance (Rct) within Faradaic redox reaction.15 Electronic conductivities can be inferred by comparing the Rct of three 11 ACS Paragon Plus Environment


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samples. The actual values can be obtained through fitting the data by Zview software, the result is shown in Table S2. Obviously, the H–CuO@GC electrode shows a lower Rct than H–CuO and G–CuO electrodes due to its negligible semicircle. This indicates that the graphitic carbon coating can improve the interfacial behaviors and facilitate electrical conductivity. At low frequency region, the inclined line is related to the diffusion of electrolyte ions.34,35 Both of the H–CuO@GC and H–CuO electrodes represent more vertical Warburg slopes compared with G– CuO electrode, suggesting better ion diffusion kinetics in the CuO mesoporous hollow spherical structures. According to the above results, the unique characteristics of H–CuO@GC hollow spheres dominate their excellent electrochemical performance. Firstly, H–CuO@GC exhibits high specific surface area, suitable pore size, and large pore volume, which largely increase the contact area between electrode materials and electrolyte, and shorten the electron transfer path. Secondly, the 3D hollow networks built by small subunits can achieve fast electrochemical process at high rate through internal hierarchical and stable diffusion channels based on the ‘‘ion–buffering reservoirs’’ effect.33,36 The sufficient penetrated channels in the mesoporous hollow structure simultaneously promote the diffusion kinetics of electrolyte ions between the internal and external cavities. Thirdly, the graphitic carbon layers on the surface of CuO particles can remarkably improve the electronic conductivity and result in a lower charge transfer resistance. Fourthly, the continuous graphitic carbon networks served as a “cocoon” for the CuO hollow spheres. The so–called cocoon exhibits mechanical stretching effect, which can overcome the expansion force and protect the structural stability and flexibility. Therefore, the comprehensive electrochemical performance of H–CuO@GC hollow spheres electrode in this


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study is superior to many CuO–based electrode materials (Table S3). The result also shortens the distance between CuO and other oxides, e.g. NiO, Co3O4, MnO2, and RuO2. To further investigate the practical application of H–CuO@GC electrode, a simple H– CuO@GC based two–electrode asymmetric supercapacitor (ASC) was built up by AC and H– CuO@GC. The CV profiles of two single electrodes at 20 mV s–1 are shown in Figure 5a, which indicate potential range of 0–0.65 V and –1–0 V for H–CuO@GC and AC electrodes. For the ASC device, voltage range can be enlarged to 1.6 V from a series of CV curves (Figure S9) with various potential range. However, if the operating voltage is up to 1.8 V, water will be decomposed.37 The CV and GCD measurements were performed to further investigate the capability of the device. As shown in Figure 5b and c, the CV and GCD curves keep the similar shape with increasing the scan rate and current density, indicating excellent rapid charge/discharge ability of the device. The nonrectangular sharp of CV curves and the weak nonlinearity of the GCD curves can be attributed to the Faradaic reaction, revealing the pseudocapacitive behavior. Figure 5d shows the specific capacitances evaluated according to the GCD profiles. The capacitances of the H–CuO@GC//AC–ASC device are as high as 108.7 F g–1 at 0.5 A g–1, and maintain 75.7% initial value even increasing to a high rate (10 A g–1), suggesting its outstanding rate capability. The cycling stability tested by the repeated GCD measurement at 5 A g–1 is also be investigated, as demonstrated in Figure 5e. After 10000 cycles, H–CuO@GC//AC–ASC still retains as high as 90.2% of its capacitance, suggesting the superior cycle stability. The slight change of the charge transfer resistances before and after 10000 cycles (Figure S10) further demonstrates the good stability of the two electrode system. In addition, the CV curves of our device (Figure 5f) illustrate scarce change under bending conditions,



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suggesting superior mechanical flexibility. The result indicates the practical application of the device for flexible energy–storage field. The energy and power density can be evaluated from GCD curves, the Ragone plot of H– CuO@GC//AC–ASC device is revealed in Figure 6a. Significantly, The ASC device is able to express a high energy density of 38.6 W h kg–1 at a power density of 1.018 kW kg–1, and hold 29.3 W h kg–1 even at a high power density of 14.064 kW kg–1. The energy density and power density vary with the various current densities is shown in Table S4. The energy density value in this work is not only higher than that of some Cu-based electrodes, but also better than other metal oxides, such as Co, Ni, V, Bi based electrode materials.4, 15, 38-42 Finally, in the practical applications, two H–CuO@GC//AC–ASC devices in series (after charging) can light up a commercial light–emitting diode (LED) (2 V) indicators for 10 min (Figure 6b). CONCLUSIONS In summary, graphitic carbon coated mesoporous hollow CuO spheres have been prepared by a facile solvothermal method with a post calcination process. The as–prepared H–CuO@GC spheres show a high specific surface area (106.6 m2 g–1), penetrated mesochannels (~ 5–15 nm), a large pore volume (0.313 cm3 g–1), a robust hollow structure, and an integral graphitic carbon layer. When used for three–electrode system in supercapacitors, the H–CuO@GC sphere electrode presents outstanding electrochemical performance. In addition, a flexible asymmetric supercapacitor fabricated by H–CuO@GC sphere electrode shows excellent performance with a high capacitance of 108.7 F g–1 and a high energy density of 38.6 W h kg–1. The H– CuO@GC//AC–ASC device delivers high long-term durability with the capacitance retention as high as 90.2% after 10000 cycles. All the superiority of H–CuO@GC spheres provides a novel


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design strategy for the improvement of low–conductive nanomaterials for high–performance energy storage devices. ASSOCIATED CONTENT Supporting Information. SEM image, TEM image, XRD pattern, and EDS pattern of H-CuO spheres; XRD patterns of HCuO spheres after different reaction time; SEM images and XRD patterns of the G-CuO prepared by NaOH; XPS spectra of the H-CuO@GC spheres; CV and GCD profiles of various CuO materials; the specific capacitance values of the three electrodes at different current densities and the capacitance retentions after 8000 cycles; SEM images of H-CuO@GC spheres and H-CuO spheres after 8000 cycles; XRD pattern and HRTEM image of H-CuO@GC spheres after 8000 cycles; impedance Nyquist plots of three electrodes; internal resistance (Rs) and charge transfer resistance (Rct) of different nanostructures; electrochemical performance of HCuO@GC hollow spheres compared with other oxide materials; CV profiles of the HCuO@GC//AC asymmetric supercapacitor within different potential windows; Nyquist plots before and after 10000 charge–discharge cycles for H-CuO@GC//AC-ASC; various performance parameters for the ASC device AUTHOR INFORMATION Corresponding Author: Dr. Wenjun Zheng E-mail: [email protected]. Tel.: +86 22 23507951, Fax: +86 22 23502458 ACKNOWLEDGMENTS



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Figure caption:

Figure 1 (a) Schematic illustration of the formation of mesoporous graphitic carbon coated hollow CuO (H–CuO@GC) spheres. (b–d) SEM images of mesoporous hollow CuO (H–CuO) spheres after different reaction time: (b) 0.5 h; (c) 2 h; (d) 4 h. (e) Corresponding schematic illustration of the fabrication of CuO hollow structure. All scale bars are 200 nm in parts b–d. Figure 2 (a) SEM, (b, c) TEM, and (d) HRTEM images of the mesoporous H–CuO@GC spheres through a convenient solvothermal–calcination coating strategy. (e−h) EDS mapping of O, Cu, and C elements, implying uniformly elemental distribution in the H–CuO@GC sphere. Figure 3 (a) XRD pattern, (b) Raman spectrum, (c) TGA curve, and (d) nitrogen adsorption−desorption isotherm of the mesoporous H–CuO@GC spheres. The inset of (d) is the BHJ pore size distribution plots. Figure 4 (a) CV curves of H–CuO@GC sphere electrode at different scan rates. (b) Galvanostatic charge–discharge curves of H–CuO@GC sphere electrode at various current densities. (c) Rate performances of H–CuO@GC, H–CuO, and G–CuO electrodes in the range of current density from 1 to 20 A g–1. (d) Cycling performances of different CuO nanostructures with 8000 cycling number at a current density of 5 A g–1. Figure 5 Electrochemical performance of H–CuO@GC//AC asymmetric supercapacitor. (a) Comparative CV curves of H–CuO@GC sphere and AC electrodes at a scan rate of 20 mV s–1 in a three–electrode system. (b) CV curves at different scan rates. (c) Charge–discharge curves at various current densities. (d) Rate performance in the range of current density from 0.5 to 10 A g–1. (e) Long–term cycling stability at 5 A g–1 over 10000 cycles. (f) CV curves of the ASC device under flat and bending conditions at 50 mV s−1. Figure 6 (a) Ragone plot of the H–CuO@GC//AC asymmetric supercapacitor compared with other reported data. (b) A simple application to light a commercial light–emitting diode (LED).



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Figure 1 (a) Schematic illustration of the formation of mesoporous graphitic carbon coated hollow CuO (H–CuO@GC) spheres. (b–d) SEM images of mesoporous hollow CuO (H–CuO) spheres after different reaction time: (b) 0.5 h; (c) 2 h; (d) 4 h. (e) Corresponding schematic illustration of the fabrication of CuO hollow structure. All scale bars are 200 nm in parts b–d.


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Figure 2 (a) SEM, (b, c) TEM, and (d) HRTEM images of the mesoporous H–CuO@GC spheres through a convenient solvothermal–calcination coating strategy. (e−h) EDS mapping of O, Cu, and C elements, implying uniformly elemental distribution in the H–CuO@GC sphere.



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Figure 3 (a) XRD pattern, (b) Raman spectrum, (c) TGA curve, and (d) nitrogen adsorption−desorption isotherm of the mesoporous H–CuO@GC spheres. The inset of (d) is the BHJ pore size distribution plots.


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Figure 4 (a) CV curves of H–CuO@GC sphere electrode at different scan rates. (b) Galvanostatic charge–discharge curves of H–CuO@GC sphere electrode at various current densities. (c) Rate performances of H–CuO@GC, H–CuO, and G–CuO electrodes in the range of current density from 1 to 20 A g–1. (d) Cycling performances of different CuO nanostructures with 8000 cycling number at a current density of 5 A g–1.



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Figure 5 Electrochemical performance of H–CuO@GC//AC asymmetric supercapacitor. (a) Comparative CV curves of H–CuO@GC sphere and AC electrodes at a scan rate of 20 mV s–1 in a three–electrode system. (b) CV curves at different scan rates. (c) Charge–discharge curves at various current densities. (d) Rate performance in the range of current density from 0.5 to 10 A g–1. (e) Long–term cycling stability at 5 A g–1 over 10000 cycles. (f) CV curves of the ASC device under flat and bending conditions at 50 mV s−1.


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Figure 6 (a) Ragone plot of the H–CuO@GC//AC asymmetric supercapacitor compared with other reported data. (b) A simple application to light a commercial light–emitting diode (LED).



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For Table of Contents Graphic Only.

Graphitic carbon coated CuO hollow nanospheres with penetrated mesochannels for high–performance asymmetric supercapacitors Jing Zhana, Guofeng Zhanga, Wenhao Luoa, Yan Suna, Cen Jina, and Wenjun Zheng*ab

Synopsis: Low-cost and environmentally friendly graphitic carbon coated CuO has great potential as fiber supercapacitors for greener and more sustainable energy storage.


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Graphitic Carbon Coated CuO Hollow Nanospheres with Penetrated

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