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nickel foam electrode with enhanced electrochemical performance for supercapacitors
Materials Letters 139 (2015) 377–381
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
The 3D Co3O4/graphene/nickel foam electrode with enhanced electrochemical performance for supercapacitors Van Hoa Nguyen a,b, Jae-Jin Shim a,n a b
School of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeongbuk, 712-749, Republic of Korea Department of Chemistry, Nha Trang University, 2 Nguyen Dinh Chieu, Nha Trang, Vietnam
art ic l e i nf o
a b s t r a c t
Article history: Received 24 September 2014 Accepted 23 October 2014 Available online 1 November 2014
Nickel foam was decorated with graphene and cobalt oxide (Co3O4) sheets by chemical vapor deposition and electrochemical deposition, respectively. After coated by a thin graphene layer, the nickel foam was decorated with Co3O4 nanoparticles, ranging in size from 3 to 5 nm. The resulted electrode allowed the rapid transportation of electrons and ions with a large electroactive surface area, and showed excellent structural stability. The specific capacitance of the electrode was as high as 1126 F g 1 at a high current density of 7.5 A g 1 in KOH 3 M electrolyte, highlighting its promising applications as a high performance electrode for supercapacitors. & 2014 Elsevier B.V. All rights reserved.
Keywords: Co3O4 nanoparticles Nickel foam Graphene Electrodeposition Supercapacitors
1. Introduction Supercapacitors have attracted considerable attention as a unique class of energy storage devices with high power capability, long cyclic life, low maintenance, and rapid dynamics of charge propagation [1–2]. Generally, a supercapacitor consists of electrode materials, electrolytes, separators, and current collectors. Among them, the electrode material is one of the most important components, which governs the overall electrochemical performance of the supercapacitors. An ideal supercapacitor electrode material requires many properties, including (i) high specific surface area, (ii) controlled porosity, (iii) high electronic conductivity, (iv) desirable electroactive sites, (v) high thermal stability and chemical stability, and (vi) low cost of raw materials and manufacturing [1]. Considerable effort has been made to develop hybrid nanocomposites to improve the electrical conductivity of supercapacitor electrodes for enhanced electrochemical performance, where a transition metal oxide is combined with a high conductivity material, such as metal nanoparticles, carbon nanotubes, conducting polymers, or graphene [3–7]. On the other hand, there are still challenges for developing electrodes with mesoporous and thin structures, which are considered as promising morphologies because they can improve the use of the active materials by providing a larger surface area and facilitate the mass transport of electrolytes within the electrodes.
n
Corresponding author. Tel.: þ 82 53 810 2587; fax: þ82 53 810 4631. E-mail address: [email protected] (J.-J. Shim).
http://dx.doi.org/10.1016/j.matlet.2014.10.128 0167-577X/& 2014 Elsevier B.V. All rights reserved.
In this study, a three dimensional (3D) ternary system based on a Ni foam/graphene/Co3O4 composite was developed for high performance electrochemical electrodes. The 3D porous foam substrate provided a high surface area for loading the electroactive materials, and facilitated rapid electron transport from the electroactive materials to the current collector.
2. Experimental All chemicals were of analytical grade and used as received. In the first step, nickel foam (NF) (1 cm 3 cm, approximately 0.15 g) was placed into a quartz tube, heated to 1000 1C at a heating rate of 50 1C min 1, and maintained at that temperature for 10 min at atmospheric pressure with a gas flow of H2 (25 sccm) and Ar (50 sccm) to clean the surface. A small amount of CH4 was then introduced into the reaction tube at ambient pressure at a flow rate of 5 sccm. After 10-min reaction in gas mixture, the samples were cooled to room temperature at 100 1C min 1 under Ar and H2. In the second step, the cobalt hydroxide was electrochemical deposited on the NF/G electrode in a 6 mM Co(NO3)2 6 H2O solution at room temperature at –1.0 V (vs. SCE) for 10 min. The electrode was then rinsed several times with deionized water and then with ethanol, and dried in air. Finally, the sample was placed in a quartz tube and calcined at 300 1C for 2 h at a ramping rate of 1 1C min 1 to convert the hydroxide to Co3O4. Generally, 10 mg of graphene and Co3O4 was deposited per 1 cm 1 cm area of Ni foam. The samples were characterized by scanning electron microscopy (SEM, Hitachi, S-4200), transmission electron microscopy (TEM, Philips, CM-200) at an acceleration voltage of 200 kV, and X-ray
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V. Hoa Nguyen, J.-J. Shim / Materials Letters 139 (2015) 377–381
photoelectron spectroscopy (XPS, Thermo Scientific, K-Alpha) using Al Kα monochromatized radiation. For the electrochemical tests, the NF/G/Co3O4 electrode was used as the working electrode. All measurements were carried out in a three-electrode cell using an Autolab PGSTAT302N (Metrohm, Netherlands) with a working electrode, a platinum plate counter electrode and a saturated calomel reference electrode (SCE) at room temperature. The electrolyte was a 3 M KOH solution. The specific capacitance (Cs) of the electrodes was calculated from the charge-discharge curves using the following equation: C¼
It mΔV
ð1Þ
where C, I, t, m, and ΔV are the specific capacitance (F g r) of the electrodes, discharging current (A), discharging time (s), mass of the active material (g), and discharging potential range (V), respectively.
3. Results and discussion Fig. 1 shows SEM images of the supported Ni foam surface. After deposition and the thermal treatment, the NF was coated with an ultrathin mesoporous layer of graphene and Co3O4. A 3D grid structure with hierarchical macro-pores was observed at lowmagnification (Fig. 1a and b) and high-magnification (inset of Fig. 1a and b). These as-formed sheets, several hundred nanometers in size, were interconnected with each other, which created loose porous nanostructures with abundant open spaces and electroactive surface sites [8]. Fig. 1c and d present TEM images of the graphene/Co3O4 nanosheets on the electrode surface. The wrinkled surface and folding at the edges of the graphene sheets were clearly visible. The surface of the graphene sheets was coated with nanoparticles, ca. 5 nm in size. The spacing
between the adjacent fringes was ca. 0.45 nm, which is close to the theoretical interplane spacing of the spinel Co3O4 (1 1 1) planes. Fig. 2 shows the XPS spectra of the NF/G/Co3O4 electrode. The C1 s spectrum shows a dominant peak centered at 284.5 eV (Fig. 2a), which was assigned to the binding energy of sp2 C-C bonds. This confirmed the presence of graphene. The highresolution spectrum for the O 1 s region (Fig. 2b) was deconvoluted into three peaks at binding energies (BEs) of 529.5, 530.8 and 531.5, which are denoted as OI, OII and OIII, respectively. The component, OI, was assigned to typical metal-oxygen bonds [9]. The component, OII, is normally associated with oxygen in the hydroxyl groups on the Co3O4 surface. Moreover, the presence of this contribution in the O 1s spectrum suggests that the surface of the Co3O4 material is hydroxylated because of either a surface oxydroxide or the substitution of oxygen atoms with hydroxyl groups [10]. The OIII component was assigned to a larger number of defect sites with low oxygen coordination normally observed in the materials with small particles [11]. The Co 2p spectrum (Fig. 2c) was fitted to two spin-orbit doublets, which are characteristic of Co2 þ and Co3 þ , as well as one shake-up satellite (denoted as “Sat.”) [12]. XPS confirmed that the surface of the Co3O4 was comprised of Co2 þ and Co3 þ . Fig. 3 introduced the electrochemical properties of the obtained electrodes. At the same scan rate of 50 mV/s (Fig. 3a), the NF/G/Co3O4 electrode exhibited a much higher current and a more rectangular shape than the bare NF and NF/G electrodes, suggesting that the ultrathin mesoporous Co3O4 conducting layer facilitates electron transport. The CV curves of the NF/G/Co3O4 electrode in the potential window, between 0 to 0.5 V, at various scan rates (5 to 100 mV s 1) exhibited an almost rectangular shape at all scan rates (Fig. 3b), suggesting that the electrode has low resistance and is an ideal supercapacitor [13]. Fig. 3c presents the impedance curves for NF/G and NF/G/Co3O4 electrodes before and after 5000 cycles. The more vertical line in the low and high frequency regions for NF/G/NiCo2O4
Fig. 1. (a) SEM images of Ni foam coated with graphene and Co3O4 and (b) TEM images the NF/G/Co3O4 electrode.
V. Hoa Nguyen, J.-J. Shim / Materials Letters 139 (2015) 377–381
Fig. 2. High-resolution XPS spectra of (a) C 1 s (b) O 1 s, and (c) Co 2p of the NF/G/Co3O4 electrode.
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Fig. 3. (a) CV curves of bare NF, NF/G and NF/G/Co3O4 electrode at scan rate of 50 V s 1 in 3 M KOH; (b) CV curves of the NF/G/Co3O4 electrodes at different scan rates. (c) Nyquist plots of NF/G and NF/G/Co3O4 electrodes; (d) galvanostatic discharge curves of the NF/G/Co3O4 electrode at different current densities; (e, f) average specific capacitance versus the cycle number at a current density of 3.5 A g 1.
indicates the more capacitive behavior of the electrode. The improved electrochemical performance of the NF/G/Co3O4 electrode was also confirmed by galvanostatic charge-discharge tests
performed at different current densities (Fig. 3f). In particular, the composite electrode showed excellent rate performance, i.e., approximately 78.5% capacitance remained even at a current rate of 7.5 A g–1
V. Hoa Nguyen, J.-J. Shim / Materials Letters 139 (2015) 377–381
comparing with that of 1.5 A g–1. When the capacitor was charged and discharged for 5000 cycles at a rate of 4 A g 1, it retained 94.1% of its initial capacity. The discharge-specific capacitance loss could be explained by the repetitive charge/discharge-induced degradation of the porous structure of the electrode (inset in Fig. 3d and Fig. 3e). During charge/discharge process, the shape of the NF/G/Co3O4 electrode remained unchanged. However, a slight mesoporous structure change took place and a solid-electrolyte interface layer was formed on the surface of the electrode, as shown in Fig. S1. 4. Conclusion A high performance 3D electrode for aqueous supercapacitors was produced by the deposition of very thin mesoporous graphene and Co3O4 nanosheets on Ni foam. The as-obtained electrode exhibited an ultrahigh specific capacitance of 1434 and 1126 F g–1 at current densities of 1.5 and 7.5 A g 1, respectively, and showed excellent rate capability, and exceptional cycling stability for high-performance electrochemical capacitors. Acknowledgments This study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012009529).
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Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2014.10.128.
References [1] Zhang LL, Zhou R, Zhao XS. J Mater Chem 2010;20:5983–92. [2] Simon P, Gogotsi Y. Nat Mater 2008;7:845–54. [3] Nguyen VH, Shim JJ. J Power Sources 2014;09:031. http://dx.doi.org/10.1016/j. jpowsour. [4] Zhu S, Cen W, Hao L, Ma J, Yu L, Zheng H, Zhang Y. Mater Lett 2014;135:11–4. [5] Zhao YQ, Zhao DD, Tang PY, Wang YM, Xu CL, Li HL. Mater Lett 2012;76:127–30. [6] Xu Y, Xu S, Li M, Zhu Y, Wang L, Chu PK. Mater Lett 2014;126:116–8. [7] Liu T, Shao G, Ji M. Mater Lett 2014;122:273–6. [8] Yuan C, Li J, Hou L, Zhang X, Shen L, Lou XW. Adv Funct Mater 2012;22:4592–7. [9] Marco JF, Gancedo JR, Gracia M. J Solid State Chem 2000;153:74–81. [10] Roginskaya YE, Morozova OV, Lubnin EN, Ulitina YE, Lopukhova GV, Trasatti S. Langmuir 1997;13:4621–7. [11] Jimenez VM, Fernandez A, Espinos JP, Gonzalez-Elipe AR. J Electron Spectrosc Relat Phenom 1995;71:61–71. [12] Cui B, Lin H, Liu YZ, Li JB, Sun P, Zhao XC, et al. J Phys Chem C 2009;113:14083–7. [13] Conway BE. Electrochemical supercapacitor, scientific fundamentals and technological applications. New York: Kluwer Academic/Plenum; 1999.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
The 3D Co3O4/graphene/nickel foam electrode with enhanced electrochemical performance for supercapacitors Van Hoa Nguyen a,b, Jae-Jin Shim a,n a b
School of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeongbuk, 712-749, Republic of Korea Department of Chemistry, Nha Trang University, 2 Nguyen Dinh Chieu, Nha Trang, Vietnam
art ic l e i nf o
a b s t r a c t
Article history: Received 24 September 2014 Accepted 23 October 2014 Available online 1 November 2014
Nickel foam was decorated with graphene and cobalt oxide (Co3O4) sheets by chemical vapor deposition and electrochemical deposition, respectively. After coated by a thin graphene layer, the nickel foam was decorated with Co3O4 nanoparticles, ranging in size from 3 to 5 nm. The resulted electrode allowed the rapid transportation of electrons and ions with a large electroactive surface area, and showed excellent structural stability. The specific capacitance of the electrode was as high as 1126 F g 1 at a high current density of 7.5 A g 1 in KOH 3 M electrolyte, highlighting its promising applications as a high performance electrode for supercapacitors. & 2014 Elsevier B.V. All rights reserved.
Keywords: Co3O4 nanoparticles Nickel foam Graphene Electrodeposition Supercapacitors
1. Introduction Supercapacitors have attracted considerable attention as a unique class of energy storage devices with high power capability, long cyclic life, low maintenance, and rapid dynamics of charge propagation [1–2]. Generally, a supercapacitor consists of electrode materials, electrolytes, separators, and current collectors. Among them, the electrode material is one of the most important components, which governs the overall electrochemical performance of the supercapacitors. An ideal supercapacitor electrode material requires many properties, including (i) high specific surface area, (ii) controlled porosity, (iii) high electronic conductivity, (iv) desirable electroactive sites, (v) high thermal stability and chemical stability, and (vi) low cost of raw materials and manufacturing [1]. Considerable effort has been made to develop hybrid nanocomposites to improve the electrical conductivity of supercapacitor electrodes for enhanced electrochemical performance, where a transition metal oxide is combined with a high conductivity material, such as metal nanoparticles, carbon nanotubes, conducting polymers, or graphene [3–7]. On the other hand, there are still challenges for developing electrodes with mesoporous and thin structures, which are considered as promising morphologies because they can improve the use of the active materials by providing a larger surface area and facilitate the mass transport of electrolytes within the electrodes.
n
Corresponding author. Tel.: þ 82 53 810 2587; fax: þ82 53 810 4631. E-mail address: [email protected] (J.-J. Shim).
http://dx.doi.org/10.1016/j.matlet.2014.10.128 0167-577X/& 2014 Elsevier B.V. All rights reserved.
In this study, a three dimensional (3D) ternary system based on a Ni foam/graphene/Co3O4 composite was developed for high performance electrochemical electrodes. The 3D porous foam substrate provided a high surface area for loading the electroactive materials, and facilitated rapid electron transport from the electroactive materials to the current collector.
2. Experimental All chemicals were of analytical grade and used as received. In the first step, nickel foam (NF) (1 cm 3 cm, approximately 0.15 g) was placed into a quartz tube, heated to 1000 1C at a heating rate of 50 1C min 1, and maintained at that temperature for 10 min at atmospheric pressure with a gas flow of H2 (25 sccm) and Ar (50 sccm) to clean the surface. A small amount of CH4 was then introduced into the reaction tube at ambient pressure at a flow rate of 5 sccm. After 10-min reaction in gas mixture, the samples were cooled to room temperature at 100 1C min 1 under Ar and H2. In the second step, the cobalt hydroxide was electrochemical deposited on the NF/G electrode in a 6 mM Co(NO3)2 6 H2O solution at room temperature at –1.0 V (vs. SCE) for 10 min. The electrode was then rinsed several times with deionized water and then with ethanol, and dried in air. Finally, the sample was placed in a quartz tube and calcined at 300 1C for 2 h at a ramping rate of 1 1C min 1 to convert the hydroxide to Co3O4. Generally, 10 mg of graphene and Co3O4 was deposited per 1 cm 1 cm area of Ni foam. The samples were characterized by scanning electron microscopy (SEM, Hitachi, S-4200), transmission electron microscopy (TEM, Philips, CM-200) at an acceleration voltage of 200 kV, and X-ray
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V. Hoa Nguyen, J.-J. Shim / Materials Letters 139 (2015) 377–381
photoelectron spectroscopy (XPS, Thermo Scientific, K-Alpha) using Al Kα monochromatized radiation. For the electrochemical tests, the NF/G/Co3O4 electrode was used as the working electrode. All measurements were carried out in a three-electrode cell using an Autolab PGSTAT302N (Metrohm, Netherlands) with a working electrode, a platinum plate counter electrode and a saturated calomel reference electrode (SCE) at room temperature. The electrolyte was a 3 M KOH solution. The specific capacitance (Cs) of the electrodes was calculated from the charge-discharge curves using the following equation: C¼
It mΔV
ð1Þ
where C, I, t, m, and ΔV are the specific capacitance (F g r) of the electrodes, discharging current (A), discharging time (s), mass of the active material (g), and discharging potential range (V), respectively.
3. Results and discussion Fig. 1 shows SEM images of the supported Ni foam surface. After deposition and the thermal treatment, the NF was coated with an ultrathin mesoporous layer of graphene and Co3O4. A 3D grid structure with hierarchical macro-pores was observed at lowmagnification (Fig. 1a and b) and high-magnification (inset of Fig. 1a and b). These as-formed sheets, several hundred nanometers in size, were interconnected with each other, which created loose porous nanostructures with abundant open spaces and electroactive surface sites [8]. Fig. 1c and d present TEM images of the graphene/Co3O4 nanosheets on the electrode surface. The wrinkled surface and folding at the edges of the graphene sheets were clearly visible. The surface of the graphene sheets was coated with nanoparticles, ca. 5 nm in size. The spacing
between the adjacent fringes was ca. 0.45 nm, which is close to the theoretical interplane spacing of the spinel Co3O4 (1 1 1) planes. Fig. 2 shows the XPS spectra of the NF/G/Co3O4 electrode. The C1 s spectrum shows a dominant peak centered at 284.5 eV (Fig. 2a), which was assigned to the binding energy of sp2 C-C bonds. This confirmed the presence of graphene. The highresolution spectrum for the O 1 s region (Fig. 2b) was deconvoluted into three peaks at binding energies (BEs) of 529.5, 530.8 and 531.5, which are denoted as OI, OII and OIII, respectively. The component, OI, was assigned to typical metal-oxygen bonds [9]. The component, OII, is normally associated with oxygen in the hydroxyl groups on the Co3O4 surface. Moreover, the presence of this contribution in the O 1s spectrum suggests that the surface of the Co3O4 material is hydroxylated because of either a surface oxydroxide or the substitution of oxygen atoms with hydroxyl groups [10]. The OIII component was assigned to a larger number of defect sites with low oxygen coordination normally observed in the materials with small particles [11]. The Co 2p spectrum (Fig. 2c) was fitted to two spin-orbit doublets, which are characteristic of Co2 þ and Co3 þ , as well as one shake-up satellite (denoted as “Sat.”) [12]. XPS confirmed that the surface of the Co3O4 was comprised of Co2 þ and Co3 þ . Fig. 3 introduced the electrochemical properties of the obtained electrodes. At the same scan rate of 50 mV/s (Fig. 3a), the NF/G/Co3O4 electrode exhibited a much higher current and a more rectangular shape than the bare NF and NF/G electrodes, suggesting that the ultrathin mesoporous Co3O4 conducting layer facilitates electron transport. The CV curves of the NF/G/Co3O4 electrode in the potential window, between 0 to 0.5 V, at various scan rates (5 to 100 mV s 1) exhibited an almost rectangular shape at all scan rates (Fig. 3b), suggesting that the electrode has low resistance and is an ideal supercapacitor [13]. Fig. 3c presents the impedance curves for NF/G and NF/G/Co3O4 electrodes before and after 5000 cycles. The more vertical line in the low and high frequency regions for NF/G/NiCo2O4
Fig. 1. (a) SEM images of Ni foam coated with graphene and Co3O4 and (b) TEM images the NF/G/Co3O4 electrode.
V. Hoa Nguyen, J.-J. Shim / Materials Letters 139 (2015) 377–381
Fig. 2. High-resolution XPS spectra of (a) C 1 s (b) O 1 s, and (c) Co 2p of the NF/G/Co3O4 electrode.
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Fig. 3. (a) CV curves of bare NF, NF/G and NF/G/Co3O4 electrode at scan rate of 50 V s 1 in 3 M KOH; (b) CV curves of the NF/G/Co3O4 electrodes at different scan rates. (c) Nyquist plots of NF/G and NF/G/Co3O4 electrodes; (d) galvanostatic discharge curves of the NF/G/Co3O4 electrode at different current densities; (e, f) average specific capacitance versus the cycle number at a current density of 3.5 A g 1.
indicates the more capacitive behavior of the electrode. The improved electrochemical performance of the NF/G/Co3O4 electrode was also confirmed by galvanostatic charge-discharge tests
performed at different current densities (Fig. 3f). In particular, the composite electrode showed excellent rate performance, i.e., approximately 78.5% capacitance remained even at a current rate of 7.5 A g–1
V. Hoa Nguyen, J.-J. Shim / Materials Letters 139 (2015) 377–381
comparing with that of 1.5 A g–1. When the capacitor was charged and discharged for 5000 cycles at a rate of 4 A g 1, it retained 94.1% of its initial capacity. The discharge-specific capacitance loss could be explained by the repetitive charge/discharge-induced degradation of the porous structure of the electrode (inset in Fig. 3d and Fig. 3e). During charge/discharge process, the shape of the NF/G/Co3O4 electrode remained unchanged. However, a slight mesoporous structure change took place and a solid-electrolyte interface layer was formed on the surface of the electrode, as shown in Fig. S1. 4. Conclusion A high performance 3D electrode for aqueous supercapacitors was produced by the deposition of very thin mesoporous graphene and Co3O4 nanosheets on Ni foam. The as-obtained electrode exhibited an ultrahigh specific capacitance of 1434 and 1126 F g–1 at current densities of 1.5 and 7.5 A g 1, respectively, and showed excellent rate capability, and exceptional cycling stability for high-performance electrochemical capacitors. Acknowledgments This study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012009529).
381
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2014.10.128.
References [1] Zhang LL, Zhou R, Zhao XS. J Mater Chem 2010;20:5983–92. [2] Simon P, Gogotsi Y. Nat Mater 2008;7:845–54. [3] Nguyen VH, Shim JJ. J Power Sources 2014;09:031. http://dx.doi.org/10.1016/j. jpowsour. [4] Zhu S, Cen W, Hao L, Ma J, Yu L, Zheng H, Zhang Y. Mater Lett 2014;135:11–4. [5] Zhao YQ, Zhao DD, Tang PY, Wang YM, Xu CL, Li HL. Mater Lett 2012;76:127–30. [6] Xu Y, Xu S, Li M, Zhu Y, Wang L, Chu PK. Mater Lett 2014;126:116–8. [7] Liu T, Shao G, Ji M. Mater Lett 2014;122:273–6. [8] Yuan C, Li J, Hou L, Zhang X, Shen L, Lou XW. Adv Funct Mater 2012;22:4592–7. [9] Marco JF, Gancedo JR, Gracia M. J Solid State Chem 2000;153:74–81. [10] Roginskaya YE, Morozova OV, Lubnin EN, Ulitina YE, Lopukhova GV, Trasatti S. Langmuir 1997;13:4621–7. [11] Jimenez VM, Fernandez A, Espinos JP, Gonzalez-Elipe AR. J Electron Spectrosc Relat Phenom 1995;71:61–71. [12] Cui B, Lin H, Liu YZ, Li JB, Sun P, Zhao XC, et al. J Phys Chem C 2009;113:14083–7. [13] Conway BE. Electrochemical supercapacitor, scientific fundamentals and technological applications. New York: Kluwer Academic/Plenum; 1999.