Hierarchical 3D Mesoporous Conch-like Co3O4 Nanostructure Arrays for High-Performance Supercapacitors

Electrochimica Acta 141 (2014) 248–254

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Hierarchical 3D Mesoporous Conch-like Co3 O4 Nanostructure Arrays for High-Performance Supercapacitors Kangwen Qiu a,1 , Hailong Yan a,1 , Deyang Zhang a,1 , Yang Lu a,b , Jinbing Cheng a , Wanqiu Zhao a , Chunlei Wang a , Yihe Zhang c , Xianming Liu d , Chuanwei Cheng e , Yongsong Luo a,∗ a Henan Key Laboratory of Advanced Micro/Nano Functional Materials, School of Physics and Electronic Engineering, Xinyang Normal University, Xinyang 464000, P. R. China b School of Material Science and Engineering, Hebei University of Technology, Tianjin 300130, P. R. China c School of Materials Science and Technology, China University of Geosciences, Beijing 100083, P. R. China d College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471022, P. R. China e Shanghai Key Laboratory of Special Artificial Microstructure Materials And Technology, School of Physics Science and Engineering, Tongji University, Shanghai 200092, P.R. China

a r t i c l e

i n f o

Article history: Received 25 March 2014 Received in revised form 25 June 2014 Accepted 13 July 2014 Available online 28 July 2014 Keywords: Mesoporous Conch-like Co3 O4 Supercapacitor

a b s t r a c t Hierarchical mesoporous conch-like Co3 O4 nanostructure arrays with the thickness of a few nanometers supported on Ni foam substrate have been fabricated by a hydrothermal approach together with a post-annealing treatment. The highly ordered three-dimensional (3D) nanostructure offers numerous advantages duo to the desired properties of macroporosity, including enhanced mass transport for the electrolyte flow, thereby reducing the device resistance and faster the reaction kinetics. Such unique nanoarchitecture exhibits remarkable electrochemical performance with high capacitance and desirable cycle life. When evaluate as an electrode material for supercapacitors, the Co3 O4 nanostructure arrays (NCAs) electrode is able to deliver high areal capacitance of 4.11 F cm−2 at a current density of 5 mA cm−2 in 2 M NaOH aqueous solution. The electrode also exhibits excellent cycling stability by retaining 93% of the maximum capacitance after 5000 charge-discharge cycles. The fabrication strategy presented here is facile, cost-effective, and can offer a new pathway for large-scale energy storage device applications. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction The ever worsening energy and global warming issues call for not only urgent development of clean alternative energies, but also more advanced energy storage and management systems [1,2]. Supercapacitors, also known as electrochemical capacitors (ECs), have attracted great interests due to a number of desirable properties, including fast charging and discharging, long cycle life, and the ability to deliver up more power than secondary batteries and conventional dielectric capacitors [3–5]. The most commonly used materials for electric double layer capacitors EDLCs are carbonaceous materials including active carbon, graphene and carbon nanotubes [6–9], but their low energy density is a major drawback which reaches at most 250 F g−1 . In contrast, supercapacitors based on transition metal oxides (such as RuO2 , MnO2 ,

∗ Corresponding author. E-mail address: [email protected] (Y. Luo). 1 These authors contribute equally to this work. http://dx.doi.org/10.1016/j.electacta.2014.07.074 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

Co3 O4 , etc.), which produce higher specific capacitance than typical carbonaceous materials [10–14]. Among these available supercapacitive materials, hydrous RuO2 has been recognized as the most promising electrode material for its high specific capacitance and excellent reversibility [15,16]. However, because of its high cost, low porosity, and toxic nature, the commercial application of RuO2 is restricted. It is worth noting that Co3 O4 is considered as one of the better alternate material due to its high theoretical capacity, good electrochemical performance and environmental friendliness [17–19]. Therefore, many Co3 O4 nanostructures were synthesized on different substrates [20–22], and their electrochemical performance has been investigated [23–25]. On the other hand, the performance of ECs is mainly determined by the electrochemical activity and kinetics of the electrodes. Electron and ion transport efficiency for charge storage in a supercapacitor mainly depends on electrode properties such as surface area, morphology, and nanocrystalline phases [26,27]. In order to enhance the redox kinetics, great efforts have been devoted to create porous nanostructured materials with large surface area and short diffusion path of ions [28,29]. Herein, hierarchical 3D

K. Qiu et al. / Electrochimica Acta 141 (2014) 248–254

mesoporous conch-like Co3 O4 nanostructure arrays supported on Ni foam was synthesized by a simple hydrothermal method. First, the 3D structure solve a greater challenge, which is the conflict between the mass loading of electrochemical active material per area (5 mg cm−2 ) and its utilization efficiency (as given by the specific capacitance per gram at 822 F g−1 at 1 A g−1 ), therefore, the 3D mesoporous conch-like Co3 O4 nanostructure arrays exhibit a very high area capacitance (4.11 F cm−2 at 5 mV cm−2 ). Second, Ni foam is chosen as the substrate due to its high electrical conductivity and the desirable 3D porous structure [30]. Third, Co3 O4 has been conceived as a promising supercapacitive material owing to its high specific capacitance, environmental compatibility, and low cost. Fourth, all mesoporous Co3 O4 nanostructures consist of the thin surface layer, which is capable of contributing to interfacial electrochemical reactions as providing a large electrode surface area. As a result, electrochemical tests have revealed that the prepared Co3 O4 nanostructure arrays demonstrate remarkable performance as electrode material in supercapacitor application. 2. Experimental Section 2.1. Materials synthesis Co3 O4 nanoconch arrays (NCAs) were fabricated by a simple hydrothermal method. Ni foam (approximately 1.5 × 4 cm) was cleaned with 6 M HCl solution in an ultrasound bath for 30 min in order to remove the NiO layer on the surface, and then rinsed with ultrapure water. In a typical procedure, Co(NO3 )2 .6H2 O (0.58 g, 2 mmol), NH4 F (0.30 g, 12 mmol), and CO(NH2 )2 (0.72 g, 12 mmol) were dissolved in 40 ml of ultrapure water and stirred for 30 minutes. Then, the fully dissolving solution and the cleaned Ni foam were transferred into a 50 ml Teflon-lined stainless steel autoclave. followed by heating at 140 ◦ C for 9 h in an electric oven. After the autoclave was cooled to ambient temperature, the sample was removed, washed with distilled water, and dried at 60 ◦ C. Finally, the samples were annealed at 400 ◦ C in the air for 3 h to obtain Co3 O4 NCAs. Similarly, the Co3 O4 nanowire arrays (NWAs) and nanosheet arrays (NSAs) were also fabricated at 90 ◦ C and 110 ◦ C, respectively.

249

Fig. 1. (a) The optical image of the Ni foam (a-1), Co(OH)2 NCAs (a-2) and Co3 O4 NCAs (a-3). (b-d) Schematic illustration for the formation processes of Co3 O4 NCAs on Ni foam.

respectively, where I is the constant discharge current, t the discharging time, V the voltage drop upon discharging (excluding the IR drop), m the total mass of the electrode material, and S the geometrical area of the electrode. 3. Results and discussion 3.1. Microstructure characterizations The optical images of the Co3 O4 NCAs at different stage are provided in Fig. 1a. Fig. 1b-d schematically illustrates the fabrication process of highly ordered Co3 O4 NCAs. First, the cleaned Ni foam was obtained with 6 M HCl solution in an ultrasound bath for 30 min. Then, Co(OH)2 NCAs are grown directly on Ni foam via a simple hydrothermal process. Finally, the Co(OH)2 precursor is annealed at 400 ◦ C for 4 h to fabricate mesoporous Co3 O4 nanoconch arrays. The generation of OH− initiates the precipitation of Co2+ ions in the solution to produce cobalt hydroxide species. The equations for describing chemical reaction can be written as follows: hydrolysis

2.2. Characterization The chemical composition of the product was characterized by X-ray diffraction (XRD, Bruker, D8-Advance X-ray Diffractometer, ˚ The morphology of the synthesized product Cu K␣ , ␭= 1.5406 A). was examined using field emission scanning electron microscopy (Hitachi, S4800) and transmission electron microscopy (TEM; JEOL, JEM-2010). Raman spectra were recorded on an INVIA Raman microprobe (Renishaw Instruments, England) with a 532 nm laser excitation. X-ray photoelectron spectroscopy (XPS) spectra were measured on a modified PHI 5600 XPS system.

NH2 CONH2 −→ NH4 + + NCO− −

−

(1)

+

NCO + 3H2 O → HCO3 + NH4 + OH 2+

Co

+ 2OH → Co(OH)2

(3)

6Co(OH)2 + O2 → 2Co3 O4 + 6H2 O

(4)

Fig. 2 shows the XRD patterns of the Co(OH)2 precursor and the mesoporous Co3 O4 NCAs. The XRD pattern of the sample can be #

#

Intensity / a.u.

Electrochemical measurements were carried out by electrochemical workstation (CHI 660E) using three-electrode configuration in 2 M NaOH aqueous solution. The mesoporous conch-like Co3 O4 nanostructure arrays supported on Ni foam was directly used as the working electrode. The reference and counter electrodes were Ag/AgCl and platinum foil, respectively. The electrochemical impedance spectroscopy measurements were carried out with a frequency loop from 0.01 Hz to 100 kHz. The mass loading of the Co3 O4 NCAs, NWAs and NSAs on Ni foam were around 30 mg, 15 mg and 9 mg, respectively. Areal and mass capacitances were calculated according to Ca = I × t/(V × S) and Cm = I × t/(V × m),

(2)

−

#

∗

∗

Ni foam

¤ Co(OH)2 ∗ Co3O4

#

Co3O4 NCAs

2.3. Electrochemical evaluation

−

∗

∗

∗

∗

∗

#

Co(OH)2 NCAs ¤

¤

¤

#

¤

10

20

30

#

¤

40

50

¤

60

70

80

2 Theta / degree Fig. 2. XRD patterns of as-prepared Co(OH)2 NCAs and Co3 O4 NCAs.

K. Qiu et al. / Electrochimica Acta 141 (2014) 248–254

(a)

(b)

Intensity / a.u.

779.8 eV (Co 2P3/2)

-2

O

795.2 eV (Co 2P1/2)

770

780

O 1s

790

800

Binding Energy / eV

Intensity / a.u.

250

-

OH

H2O

528

530

532

534

536

Binding Energy (eV)

Fig. 3. Survey XPS spectra of (a) Co 2p and (b) O 1s regions of the Co3 O4 NCAs.

indexed to a polycrystalline phase of Co3 O4 (JCPDS card no. 421467). Furthermore, after being covered by Co3 O4 , the diffraction intensities of the Ni foam diminish (Fig. S1), maybe because the Co3 O4 has grown on the Ni foam and affect the diffraction intensities of Ni foam. The XPS spectra of the Co3 O4 nanoconchs present two major peaks at binding energies of 779.84 and 795.14 eV with a spin-energy separation of 15 eV (Fig. 3). These two peaks correspond to Co 2p3/2 and Co 2p1/2 , which is the characteristic of Co3 O4 phase. Both the XRD and XPS characterizations confirm that a temperature of 400 ◦ C is high enough for the complete conversion of the precursor to Co3 O4 . Thermogravimetric analysis (TGA) of the Co3 O4 NCAs were further investigated, the thermal behavior of the Co(OH)2 precursor shows a sharp weight loss of 22.8% at 400 ◦ C due to the thermal decomposition (Fig. S2). But, there is no additional weight loss observed at higher temperatures which confirms the formation of monophasic compound. Fig. 4a shows the SEM image of the cleaned Ni foam with an average grain size of 100 ␮m. From images, the surface of the Ni foam is relatively smooth before the nanoconch growth. After the nanoconch growth, the surface of the whole Ni foam becomes roughly. Fig. 4b-d) show the SEM images of the hierarchical 3D mesoporous Co3 O4 NCAs from low to high magnification, respectively. It can be clearly seen that, each conch is made up of many mesoporous nanosheets, which are interconnected with each other forming a highly porous surface morphology. From the high magnification SEM image (Fig. 4d), it can be seen that each 3D conch-like Co3 O4 array has many mesoporous, which results from the release of gas during the decomposition of Co3 O4 NCAs precursor. It is thus optimistically anticipated that this structure is highly accessible by the electrolyte when used as an electrode for ECs. To get further information about the microstructure of these Co3 O4 NCAs, the products are further investigated with TEM. Fig. 5a shows a typical TEM image of an individual nanoconch, in which the edge and center of the nanoconch show strong brightness contrast, further confirming their thin nature. The inset of Fig. 5a is the selected area electron diffraction (SAED) pattern. A typical HRTEM image taken from the edge of Co3 O4 NCAs nanoconch (Fig. 5b), indicating that the subunit particles oriented assemble with each other and finally form the Co3 O4 NCAs. Clear lattice fringes were also observed (Fig. 5c), the image reveals the lattice fringes of 0.283 nm, 0.244 nm and 0.232 nm, corresponding to the (220), (311) and (222) plane of polycrystalline Co3 O4 , further demonstrating the superior crystal quality of Co3 O4 NCAs. This is in agreement with the SAED result [31]. The Co3 O4 nanoconchs are scratched down from the Ni foam to estimate their mesoporosity and textural properties. From the N2 adsorption-desorption isotherm shown in Fig. 5d, a distinct hysteresis loop can be observed with typical IV sorption behavior, indicating the existence of a typical mesoporous microstructure. The pore-size-distribution (PSD) data (the inset in Fig. 5d) shows

that the size of the majority of the pores falls in the optimal range of 3-8 nm for the EC application. The mesoporous structure gives rise to a relatively high Brunauer-Emmett-Teller (BET) specific surface area of 82.5 m2 g−1 . The large specific surface area provides numerous electroactive sites for fast and reversible redox reactions between the electrolyte and electroactive species on the electrode surface. On the other hand, the size of pores ∼4.5 nm can serve as a robust reservoir for OH− ions to shortening the diffusion distance to the interior surfaces and ensuring that sufficient redox reacrions take place at high current densities for energy storage. [34] Moreover, the different morphologies of Co3 O4 nanostructures were also shown in Fig. S3. For example, the Co3 O4 NWAs are 2∼3 ␮m in length and about 50 nm in diameter at the middle section (Fig. S3a,b). Fig. S3c,d shows the SEM images of the Co3 O4 NSAs, the Co3 O4 NSAs lie aslant or perpendicular to the Ni foam support, and are interconnected with each other form an integrated network. After careful examination reveals that the thickness of Co3 O4 nanosheets is only 25 nm. To demonstrate that all arrays are converted into Co3 O4 , their XRD patterns (Fig. S4) and Raman spectra (Fig. S5) were measured, which also demonstrate the pure phase of Co3 O4 [32,33]. 3.2. Electrochemical behavior of Co3 O4 NCAs To demonstrate the advantage of its unique structure and further explore the potential applications of supercapacitor devices, the electrochemical tests were carried out in a three-electrode configuration with a Pt plate counter electrode and an Ag/AgCl reference electrode [35]. Fig. 6a shows the cyclic voltammograms (CVs) of Co3 O4 NCAs electrode at scan rates of 5-30 mV s−1 . One pair of redox peaks over the entire range is clearly observed, and the CV curves are nearly rectangular in shape, displaying excellent high-rate capability of the samples. For comparison, the CV curves of the pristine Ni foam, Co3 O4 NWAs, Co3 O4 NSAs and Co3 O4 NCAs electrode at a scan rate of 5 mV s−1 are shown in Fig. 6b. It can be found that the capacitance contribution from Ni foam substrate is negligible. The capacitance of the Co3 O4 NCAs at the same scan rate is higher than other electrode materials. The increment of the CV integrated area suggests that the Co3 O4 NCAs have a much higher capacitance. The excellent electrochemical capability of Co3 O4 NCAs may be attributed to their unique microstructures, which can contribute to the rapid intercalation of cations in the electrode during reduction and deintercalation upon oxidation. The Co3 O4 NCAs were also characterized using galvanostatic charge/discharge measurements, as shown in Fig. 6c. It can be seen that all the curves are highly linear and symmetrical at various current densities from 5 to 40 mA cm−2 . The areal capacitance and mass capacitance can be calculated according to above-mentioned equations from the discharge curves. On the basis of the above

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Fig. 4. (a) SEM image of the cleaned Ni foam; (b) Low-magnification SEM image of the Co3 O4 NCAs; (c) enlarged SEM image of the Co3 O4 NCAs; (d) SEM image of an individual Co3 O4 NCAs.

results, the areal capacitance of the Co3 O4 NCAs at 5, 10, 15, 25 and 40 mA cm−2 is 4.11, 3.72, 3.45, 3.07, and 2.82 F cm−2 , respectively. The mass capacitance of the Co3 O4 NCAs at 1, 2, 3, 5 and 8 A g−1 is 822.3, 744.1, 690.5, 614.2 and 564.1 F g−1 . The areal capacitance

of the Co3 O4 NCAs is very high as a electrode materical for supercapacitors. Wang et al. reported Co3 O4 nanowire arrays with areal capacitance (1.1 F cm−2 at 30 mA cm−2 ), [36] Han et al. reported Co3 O4 nanowire arrays with areal capacitance (0.36 F cm−2 at

Fig. 5. (a-c) Low-magnification and high-magnification TEM images of Co3 O4 NCAs; The insets of (a) is the corresponding SAED patterns of Co3 O4 NCAs; (d) N2 adsorptiondesorption isotherm and PSD (the inset in (d)) of the Co3 O4 nanoconchs scratched down from the Ni foam.

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Fig. 6. (a) Cyclic voltammograms of Co3 O4 NCAs at different scan rates; (b) Cyclic voltammograms of the different electrode materials at 5 mV s−1 ; (c) Charge/discharge curves of Co3 O4 NCAs at various current densities; (d) Current density dependence of the areal capacitance and mass capacitance of the different electrode materials.

1.2 mA cm−2 ), [37] Lu et al. reported Co3 O4 nanosheet arrays with a high capacitance (2.48F cm−2 at 5 mA cm−2 ). [38] The areal capacitance of the Co3 O4 NCAs is much higher than these reports, which is contributed to the much higher mass loading of electrochemically active material per area (as high as 5 mg cm−2 ). Fig. 6d shows the areal capacitance and mass capacitance of the different eletrode materials with the same current density. It can be seen, the mass capacitance of Co3 O4 NCAs (823.1 F g−1 at 1 A g−1 ) is lower than Co3 O4 NSAs (1045 F g−1 at 1 A g−1 ) or Co3 O4 NWAs (953.8 F g−1 at 1 A g−1 ). However, the areal capacitance of Co3 O4 NCAs (4.11 F cm−2 at 5 mA cm−2 ) is much higher than Co3 O4 NSAs (2.19 F cm−2 at 5 mA cm−2 ) and Co3 O4 NSAs (1.13 F cm−2 at 5 mA cm−2 ).This result is caused by the much higher mass loading of electrochemically active material per area of Co3 O4 NCAs (5 mg cm−2 ) than Co3 O4 NSAs (2.5 mg cm−2 ) and Co3 O4 NWAs (1.5 mg cm−2 ). And the Co3 O4 NCAs as electrode, 68.5% of the areal capacitance was retained when the current density increased from 5 to 40 mA cm−2 and 68.6% of the mass capacitance was retained when the current density increased from 1 to 8 A g−1 . This also implies that the electrode has excellent electrochemical reversibility and charge-discharge properties. These results can be attributed to the 3D mesoporous nanostructure and the more active substances of Co3 O4 NCAs under the same condition. The cycle stability of supercapacitors is a crucial parameter for their practical applications. The long-term stability of the electrodes was examined at 25 mA cm−2 as shown in Fig. 7a. It is found that the Co3 O4 NCAs electrode capacitance retention about is more than 93% of initial value after 5000 cycles. The cycling performance of the Co3 O4 NCAs at progressively increased current density was recorded in Fig. 7b. During the first 500 cycles with a charge/discharge density of 5 mA cm−2 , the nanoconch structure shows a capacitance of 4.06 F cm−2 . In the following 1500 cycles, the charge/discharge rate changes successively. This hierarchical mesoporous nanostructure always demonstrates stable

capacitance. After 2000 cycles, with the current rate being again decreased back to 5 mA cm−2 for another 500 cycles, a capacitance of 4.02 F cm−2 can be recovered and without noticeable decrease, which demonstrates the nanostructure has excellent rate performance and cyclability. The good long-term electrochemical stability was further evident from the stable charge/discharge curves for the last 20 cycles (see inset of Fig. 7b). The electrochemical impedance spectroscopy (EIS) analysis has been recognized as one of the principal methods examining the fundamental behavior of electrode materials for supercapacitors. To further understand the advantage of this electrode material, impedance spectra of the Co3 O4 NCAs, Co3 O4 NSAs and Co3 O4 NWAs were measured at open circuit potential with an AC perturbation of 5 mV in the frequency range from 0.01 Hz to 100 KHz. Fig. 7c shows that the resulting Nyquist plots are in the form of an arc at high frequency region and straight 45◦ sloped line at low frequency region. The arc in high frequency region is associated with the interfacial properties of the electrodes and corresponds to the charge-transfer resistance, and the straight line in low frequency region is ascribed to the diffusive resistance related to the diffusion of electrolyte within the pores of the electrode. It is obvious that Co3 O4 NCAs, Co3 O4 NSAs and Co3 O4 NWAs have similar diffusion resistance, all of these electrode materials have a small charge transfer resistance. However, the Co3 O4 NCAs has the smallest diffusion resistance, this result may be attributed to the vast pores in the nanoconch, which can provide a large number of channels to ensure the electrolyte transmission. The measured impedance spectra of Co3 O4 NCAs were analyzed using the complex nonlinear least-squares fitting method on the basis of the equivalent circuit, which is given in the inset of Fig. S6. The small Rct value increase only slightly from 1st and 5000th cycle owing to good contact between the current-collector and conch arrays, indicating that the Co3 O4 NCAs is a promising supercapacitor electrode material. Schematic representation of rechargeable supercapacitive based on

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Fig. 7. (a) Cycling performance of the Co3 O4 NCAs, NWAs and NSAs electrodes at a current density of 25 mA cm−2 ; (b) Cycling stability of the Co3 O4 NCAs at progressively various current densities and inset in (b) is the charge/discharge curves of the last 20 cycles for the Co3 O4 NCAs; (c) Nyquist plots of Co3 O4 NCAs electrode, Co3 O4 NSAs electrode and Co3 O4 NWAs electrode (The inset is the enlarged impedance spectrum of the three electrodes at high frequencies); (d) Schematic representation of rechargeable supercapacitive based on Co3 O4 NCAs on Ni foam.

Co3 O4 NCAs is shown in Fig. 7d. This unique design has the following important merits required for high-performance electrodes. First, the 3D Co3 O4 NCAs have a large number of mesoporous, this porous structure provides a desired specific surface area of 82.5 m2 g−1 . The mesoporous can facilitate the fast penetration of the electrolyte, in other words, these hierarchical porous channels ensure efficient contact between the surface of electroactive Co3 O4 NCAs and the electrolyte. Second, the directly grown array can ensure good mechanical adhesion and electrical connection to the current collector [39], avoiding the use of polymer binders and conducting additives, which generally increase series resistance and deterioration of capacitance during redox reactions. Furthermore, Ni foam could provide fast electronic transfer channels to improve the electrochemical performances. Third, this Co3 O4 NCAs reached a very high mass loading of electrochemical active material per area (5 mg cm−2 ) and a good specific capacitance (822 F g−1 ), which is the most importance reason that the Co3 O4 NCAs exhibited much higher areal capacitance than Co3 O4 NSAs or Co3 O4 NWAs. Fourth, the 3D nanoconch structure provides a perfect stability, contrast with 1D nanowire and 2D nanosheet, the 3D structure is advantageous to maintain the stability of the structure in the process of charging and discharging, therefore, the Co3 O4 NCAs exhibited a good cycling performance.

NCAs endow fast ion and electron transport, large electroactive surface area, and excellent structural stability. The electrochemical measurements reveal that Co3 O4 NCAs manifest promising supercapacitive properties with a very high capacitance and good retention. Therefore, hierarchical mesoporous Co3 O4 NCAs electrodes might hold some potential for the fabrication of highperformance flexible energy-storage devices. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Nos. U1204501, U1304108 and 11272274), the Science and Technology Key Projects of Education Department Henan Province (No. 13A430758), the Innovative Research Team (in Science and Technology) in University of Henan Province (Nos. 13IRTSTHN018). The authors are indebted to Dr D. L. Xu, Y. X. Liu, for their technical assistances and kind help. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2014.07.074.

4. Conclusions In summary, a facile, cost-effective and scalable two-step route has been developed to produce mesoporous Co3 O4 NCAs on Ni foam. The unique Ni foam-supproted 3D mesoporous Co3 O4

References [1] Y.S. Luo, J.S. Luo, J. Jiang, W.W. Zhou, H.P. Yang, X.Y. Qi, H. Zhang, H.J. Fan, Y.W.Y. Denis, C.M. Li, T. Yu, Controlled synthesis of hierarchical graphene wrapped

254

[2]

[3] [4] [5]

[6]

[7]

[8] [9] [10]

[11]

[12]

[13] [14] [15]

[16]

[17]

[18] [19] [20]

[21]

K. Qiu et al. / Electrochimica Acta 141 (2014) 248–254 TiO2 @Co3 O4 coaxial nanobelt arrays for high-performance lithium storage, J. Mater Chem. A 1 (2013) 273. C.Z. Yuan, L. Yang, X.G. Zhang, X.W. Lou, Growth of ultrathin mesoporous Co3 O4 nanosheet arra- ys on Ni foam for high-performance electrochemical capacitors, Energy Environ. Sci. 5 (2012) 7883. P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845. J.R. Miller, R.A. Outlaw, B.C. Holloway, Graphene double-layer capacitor with ac line-filtering pe- rformance, Science 329 (2010) 1637. W. Chen, R.B. Rakhi, L.B. Hu, X. Xie, Y. Cui, H.N. Alshareef, Substrate dependent self-organizati- on of mesoporous cobalt oxide nanowires with remarkable pseudocapacitance, Nano Lett. 12 (2012) 2559. C. Zhou, Y.W. Zhang, Y.Y. Li, J.P. Liu, Construction of High-Capacitance 3D CoO @ Polypyrrole N- anowire Array Electrode for Aqueous Asymmetric Supercapacitor, Nano Lett. 13 (2013) 2078. J. Huang, J. Zhu, K. Cheng, Preparation of Co3 O4 nanowires grown on nickel foam with superior el- ectrochemical capacitance, Electrochim. Acta 75 (2012) 273. C. Liu, Z. Yu, D. Neff, A. Zhamu, B.Z. Jang, Graphene-based supercapacitor with an ultrahigh ener- gy density, Nano Lett. 10 (2010) 4863. M. Kaempgen, C.K. Chan, J. Ma, Y. Cui, G. Gruner, Printable thin film supercapacitors using single walled carbon nanotubes, Nano Lett. 9 (2009) 1872. X.H. Xia, J.P. Tu, J. Zhang, X.L. Wang, W.K. Zhang, H. Huang, Morphology effect on the electroc- hromic and electrochemical performances of NiO thin films, Electrochim. Acta 53 (2008) 5721. J.H. Kim, K.H. Lee, L.J. Overzet, G.S. Lee, Synthesis and electrochemical properties of spin-capa- ble carbon nanotube sheet/MnOx composites for highperformance energy storage devices, Nano Lett. 7 (2011) 2611. J.W. Lang, L.B. Kong, W.J. Wu, Y.C. Luo, L. Kang, Facile approach to prepare loose-packed NiO nano-flakes materials for supercapacitors, Chem. Commun. 35 (2008) 4213. J.B. Wu, Y. Lin, X.H. Xia, Pseudocapacitive properties of electrodeposited porous nanowall Co3 O4 film, Electrochim. Acta 56 (2011) 7163. Y. Qiu, Z.Y. Lu, X. Sun, X. Duan, Hierarchical Co3 O4 nanosheet@ nanowire arrays with enhanced pseudocapacitive performance, RSC Adv. 4 (2012) 1663. B. Luo, B. Wang, M.H. Liang, J. Ning, X.L. Li, L.J. Zhi, Reduced Graphene Oxide mediated grow- th of uniform tin-core/carbon-sheath coaxial nanocables with enhanced lithium ion storage properties, Adv. Mater. 24 (2012) 1405. Y.H. Lin, T.Y. Wei, H.C. Chien, S.Y. Lu, Manganese oxide/carbon aerogel composite an outstanding supercapacitor electrode material, Adv. Energy. Mater. 1 (2011) 901. X.H. Xia, J.P. Tu, X.L. Wang, C.D. Gu, X.B. Zhao, Mesoporous Co3 O4 monolayer hollow-sphere array as electrochemical pseudocapacitor material, Chem. Commun. 47 (2011) 5786. C. Liu, F. Li, L.P. Ma, H.M. Cheng, Advanced materials for energy storage, Adv. Mater. 22 (2010) E28. E.L. Salabas, A. Rumplecker, F. Kleitz, F. Radu, F. Schüth, Exchange anisotropy in nanocasted Co3 O4 nanowires, Nano Lett. 6 (2006) 2977. Y.Y. Liang, Y.G. Li, H.L. Wang, J.G. Zhou, J. Wang, T. Regier, H.J. Dai, Co3 O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction, Nat. Mater. 10 (2011) 780. L. Gao, X.F. Wang, Z. Xie, W.F. Song, L.J. Wang, X. Wu, F.Y. Qu, D. Chen, G.Z. Shen, High-perfo- rmance energy-storage devices based on WO3 nanowire arrays/carbon cloth integrated electrodes, J. Mater. Chem. A 24 (2013) 7167.

[22] Y.S. Luo, D.Z. Kong, J.S. Luo, Y.L. Wang, D.Y. Zhang, K.W. Qiu, C.W. Cheng, C.M. Li, T. Yu, Se- ed assisted Synthesis of Co3O4@(-Fe2O3 Core/Shell Nanoneedle Arrays for Lithium-ion Battery Anode with High Capacity, RSC Adv. (2008), DOI:10.1039/C3RA9F. [23] N. Du, H. Zhang, B.D. Chen, J.B. Wu, X.Y. Ma, Z.H. Liu, Y.Q. Zhang, D.R. Yang, X.H. Huang, J.P. Tu, Porous Co3 O4 nanotubes derived from Co4 (CO)12 clusters on carbon nanotube templates: A highly efficient material for Li-battery applications, Adv. Mater. 19 (2007) 4505. [24] J. Jiang, Y.Y. Li, J.P. Liu, X.T. Huang, C.Z. Yuan, X.W. Lou, Recent Advances in Metal Oxide- based Electrode Architecture Design for Electrochemical Energy Storage, Adv. Mater. 24 (2012) 5166. [25] X.S. Fang, T.Y. Zhai, L. Li, L.M. Wu, Y. Bando, D. Golberg, ZnS nano-structures: From synthesis to applications, Prog. Mater. Sci. 56 (2011) 175. [26] B.R. Duan, Q. Cao, Hierarchically porous Co3 O4 film prepared by hydrothermal synthesis method based on colloidal crystal template for supercapacitor application, Electrochim. Acta 64 (2012) 154. [27] X.H. Xia, J.P. Tu, X.L. Wang, C.D. Gu, X.B. Zhao, Freestanding Co3 O4 nanowire array for high pe- rformance supercapacitors, RSC Adv. 5 (2012) 1835. [28] Z. Fan, J. Chen, K. Cui, Preparation and capacitive properties of cobalt–nickel oxides/carbon nanot- ube composites, Electrochim. Acta 52 (2007) 2959. [29] Y.S. Luo, J.S. Luo, J. Jiang, W.W. Zhou, H.P. Yang, X.Y. Qi, H. Zhang, H.J. Fan, D.Y.W. Yu, C.M. Li, T. Yu, Seed-assisted synthesis of highly ordered TiO2 @␣Fe2 O3 core/shell arrays on carbon textiles for lithium-ion battery applications, Energy Environ. Sci. 5 (2012) 6559. [30] J.P. Liu, J. Jiang, C.W. Cheng, H.X. Li, J.X. Zhang, H. Gong, H.J. Fan, Co3 O4 Nanowire@ MnO2 Ultrathin Nanosheet Core/Shell Arrays: A New Class of HighPerformance Pseudocapacitive Materials, Adv. Mater. 23 (2011) 2076. [31] J. Zhang, X.C. Wang, J. Ma, S. Liu, X. Yi, Preparation of cobalt hydroxide nanosheets on carbon nanotubes/carbon paper conductive substrate for supercapacitor application, Electrochim. Acta 5 (2012) 235. [32] X.L. Yang, K.C. Fan, Y.H. Zhu, J.H. Shen, X. Jiang, P. Zhao, C.Z. Li, Tailored graphene encapsu- lated mesoporous Co3 O4 composite microspheres for highperformance lithium ion batteries, J. Mater. Chem. 22 (2012) 17278. [33] X.S. Fang, L.F. Hu, K.F. Huo, B. Gao, L.J. Zhao, M.Y. Liao, P.K. Chu, Y. Bando, D. Golberg, New Ultraviolet Photodetector Based on Individual Nb2 O5 Nanobelts, Adv. Funct. Mater. 21 (2011) 3907. [34] H.B. Wu, H. Pang, X.W. Lou, Facile synthesis of mesoporous Ni0.3 Co2.7 O4 hierarchical structures for high-performance supercapacitors, Energy Environ. Sci. 6 (2013) 3619. [35] X.Y. Xue, S. Yuan, L.L. Xing, Z.H. Chen, B. He, Y.J. Chen, Porous Co3 O4 nanoneedle array grow- ing directly on copper foils and their ultrafast charging/discharging as lithium-ion battery anodes, Chem. Commun. 47 (2011) 4718. [36] D.P. Cai, D.D. Wang, B. Liu, L.L. Wang, Y. Liu, H. Li, Y.R. Wang, Q.H. Li, T.H. Wang, Three- Dimensional Co3 O4 @NiMoO4 Core/Shell Nanowire Arrays on Ni Foam for Electrochemical Energy Storage, ACS Appl. Mater. Interfaces 6 (2014) 5050. [37] X.H. Xia, J.P. Tu, Y.Q. Zhang, X.L. Wang, C.D. Gu, X.B. Zhao, H.J. Fan, High-Quality Metal Oxide Core/Shell Nanowire Arrays on Conductive Substrates for Electrochemical Energy Storage, ACS Nano 6 (2012) 5531. [38] Z.Y. Lu, Q. Yang, W. Zhu, Z. Chang, J.F. Liu, X.M. Sun, D.G. Evans, X. Duan, Hierarchical Co3 O4 @Ni-Co-O Supercapacitor Electrode with Ultrahigh Specific Capacitance per Area, Nano Res. 5 (2012) 369. [39] T. Yu, Y.W. Zhu, X.J. Xu, P. Chen, C.T. Lim, J.T.L. Thong, C.H. Sow, Controlled growth and field- emission properties of cobalt oxide nanowalls, Adv. Mater. 17 (2005) 1595.