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Cite this: Chem. Commun., 2014, 50, 8246
Three-dimensional carbon nanotube networks with a supported nickel oxide nanonet for high-performance supercapacitors†
Received 13th April 2014, Accepted 30th May 2014
Mao-Sung Wu,* Yo-Ru Zheng and Guan-Wei Lin
DOI: 10.1039/c4cc02725f www.rsc.org/chemcomm
A three-dimensional porous carbon nanotube film with a supported NiO nanonet was prepared by simple electrophoretic deposition and hydrothermal synthesis, which could deliver a high specific capacitance of 1511 F g
1
at a high discharge current of 50 A g
1
due to the
significantly improved transport of the electrolyte and electrons.
Supercapacitors have been studied extensively in recent years due to the increasing demand for power sources for portable electronic devices. Supercapacitors can deliver a higher capacitance than the traditional electric double-layer capacitors. A number of materials such as carbonaceous materials, metal oxides/hydroxides, and conducting polymers have been characterized as promising materials for application in supercapacitors.1–3 Among these materials, nickel oxides/hydroxides are available for application in alkaline supercapacitors due to their high corrosive resistance to the alkaline electrolyte. Heat treatment is known to be one of the major factors that determine the capacitive behavior of nickel hydroxide electrodes. Heat treatment in the temperature range of 300–400 1C gives a non-stoichiometric nickel oxide, which is suitable for electrochemical applications.4–6 As the electrode materials in supercapacitors, a large surface area, suitable pore size and pore size distribution, and good electrical conductivity are required to achieve high specific capacitance and rate performance. Micropores may contribute a huge surface area to the material, but narrow micropores are not easily accessed by the liquid electrolyte. The solution to this problem is to use mesoporous nickel oxide materials.7–13 Previous reports have indicated that the nanostructured nickel oxides exhibit a better capacitive performance than the bulk nickel oxide due to their large surface and improved transport of electrolyte ions.14–17 Thus, the shape control of nickel oxide based nanomaterials has become an important factor in determining their capacitive performance. Nickel oxide with different kinds of morphologies such as nanoflakes, nanoflowers, nanotubes, Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan. E-mail: [email protected]; Fax: +886-7-3830674 † Electronic supplementary information (ESI) available: Experimental details, SEM images, TEM images, TGA analysis, BET analysis, XRD patterns, and electrochemical properties. See DOI: 10.1039/c4cc02725f
8246 | Chem. Commun., 2014, 50, 8246--8248
nanowires, and hollow nanospheres is believed to have a positive effect on the capacitive behavior of the electrode.18–29 In general, the electrical conductivity of electrode materials plays an important role in enhancing the utilization of active materials and mitigating the internal resistance of the supercapacitors. The nickel oxide–carbon composite has shown an improvement in capacitive behavior due to the inclusion of carbon materials that improve the dispersibility of nickel oxide nanoparticles and the electrical conductivity.29–34 Nickel oxide supported on high conductive porous Ni foam or Ni nanowires also showed superior capacitive behavior.35–40 In this work, a novel electrode structure of a three-dimensional (3D) porous CNT (carbon nanotube) network-supported nickel oxide nanonet illustrated in Fig. 1 is proposed as a stable and highcapacitance electrode material for supercapacitors. A 3D porous CNT film with interconnected networks was assembled on a stainless steel (SS) sheet via simple electrophoretic deposition (EPD). A nickel oxide nanonet composed of ultrasmall nanowires could be formed between adjacent CNTs during hydrothermal synthesis. The CNT exhibits excellent electrical conductivity and provides a conductive network for the fast transport of electrons. The well-dispersed nickel oxide nanonet provides a large faradaic capacitance, making the nickel oxide–CNT composite electrode suitable for high rate applications.
Fig. 1 Schematic illustrating a novel electrode structure of 3D porous CNT networks with attached nickel oxide nanonet for supercapacitors.
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Fig. 2a shows the SEM (scanning electron microscopy) image of a NiO nanonet grown on an SS sheet after annealing at 300 1C. An SS surface may have a lot of active sites, which allow for the nucleation of nickel hydroxide during hydrothermal synthesis, leading to the formation of a nickel hydroxide nanonet. The nanonet is composed of nanowires with ultrasmall diameters (o10 nm, Fig. S1 in the ESI†). Part of the nanowires aggregate to form vertically aligned sheets, and the rest of the nanowires form a net-like structure. Vertically aligned nanosheets composed of nanowires allow the electrolyte to migrate from the bulk electrolyte into the porous layer and participate in the electrochemical reaction. Fig. 2b shows the SEM image of the porous CNT film coated on the SS sheet prepared using the EPD method. The CNT film exhibits 3D cross-linked porous structure, providing a high porosity and large surface area. The 3D porous CNT film imparts high electrical conductivity to the
ChemComm
Fig. 3 CVs of the NiO/SS, NiO/CNT, and bare CNT electrodes at a scan rate of 10 mV s 1.
electrode and accommodates the large volume of the liquid electrolyte. Fig. 2c shows the SEM image of the CNT film with attached NiO nanonet after annealing at 300 1C. Clearly, the NiO nanowires grow around the CNT surface and outward from each CNT, forming a CNT-supported NiO nanonet. Probably, the acid-treated CNT surface provides a large amount of active sites for nucleation and growth of nanowires, leading to the formation of well-dispersed nanowires in the porous CNT film. Fig. 3 shows the cyclic voltammograms (CVs) of NiO electrodes and the bare CNT electrode. In addition to the oxygen evolution reaction occurring at a more positive potential than 0.45 V, the NiO electrodes exhibit a pair of redox peaks resulting from the faradaic redox reaction between NiO and NiOOH. The redox reaction of NiO electrode in alkaline solution can be expressed as follows:41 NiO + OH 2 NiOOH + e
Fig. 2 SEM images of (a) NiO nanonet, (b) CNT film, and (c) CNT film with attached NiO nanonet.
This journal is © The Royal Society of Chemistry 2014
(1)
Obviously, the current density of the electrode with the CNTsupported NiO nanonet (NiO/CNT) during CV scanning is significantly higher than that of the electrode with the SS-supported NiO nanonet (NiO/SS). The higher the current density, the higher is the specific capacitance of the NiO electrode. This reflects that the 3D CNT networks may facilitate the transport of the electrolyte and electrons through the porous film. The NiO nanonet with ultrasmall nanowires can shorten the ion migration and diffusion paths, leading to an increase in the effective surface area. The current density of the electrode with the bare CNT film is very small compared to that of NiO electrodes during CV scanning, reflecting that the current density comes mainly from the redox reaction of NiO. Supercapacitors are expected to complement or replace batteries in high power applications. Thus, it is essential for supercapacitors to exhibit better performance at high-rate charging and discharging. Fig. 4 shows the specific capacitances of NiO electrodes obtained at various discharge current densities. The specific capacitance of NiO/ CNT electrode could reach as high as 1636 F g 1 at a low discharge current density of 1 A g 1, which is higher than that of the NiO/SS electrode (1496 F g 1). When discharged at a high current density of 50 A g 1, the NiO/CNT electrode still retains a high specific capacitance of 1511 F g 1, while the specific capacitance of the NiO/SS electrode is decreased to 883 F g 1. The capacitance retention
Chem. Commun., 2014, 50, 8246--8248 | 8247
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Notes and references
Fig. 4 Relationship between specific capacitance and discharge current density of the NiO/SS and NiO/CNT electrodes.
is calculated from the ratio of capacitance values discharged at 50 A g 1 and 1 A g 1. The capacitance retention of the NiO/CNT electrode reaches as high as 92.4%, which is much better than that of the NiO/SS electrode (59.0%). It is noted that a large specific surface does not always guarantee a high specific capacitance because not all the surface area is available for electrolyte access. Open porous NiO structures have been developed to fulfill the requirements of a high-rate performance.42,43 In addition to the amount of electrolyte-accessible surface area, electroactive sites available for charge storage and redox reaction should be considered. The 3D conductive scaffold composed of porous CNT networks is essential for electrically connecting the electroactive sites within the electrode to improve the utilization of the active material and mitigate the charge-transfer resistance of faradaic reaction. The enhanced capacitance retention of the NiO/CNT electrode might be due to the following reasons: (1) the well-dispersed NiO nanonet provides larger open channels for easy transport of electrolyte ions and (2) the porous CNT networks act as a current collector for facilitating the transport of electrons and accommodate the large volume of the liquid electrolyte for easy transport of ions. As a result, the proposed strategy can be used to construct a high-rate NiO electrode for application in supercapacitors. In summary, the nickel oxide nanonet with ultrasmall nanowires was prepared using a simple hydrothermal method. A net-like structure of the NiO electrode could offer a large surface area accessible to the liquid electrolyte, leading to a high specific capacitance. The high-rate performance of NiO electrodes turned out to be significantly affected by the conductive scaffold. The specific capacitance of NiO electrode with the CNT-supported nanonet could reach as high as 1511 F g 1 at a current density of 50 A g 1, which is much higher than that of the NiO electrode with the SS-supported nanonet (883 F g 1). The 3D porous CNT film could offer porous channels and conductive networks for the fast transport of the electrolyte and electrons. Thus, the CNTsupported NiO nanonet was capable of delivering high capacitance during high-rate charge–discharge circumstances due to the reduced charge-transfer and diffusion resistances. The authors acknowledge financial support from the National Science Council, Taiwan (Project No.: NSC101-2221-E-151-055-MY2).
8248 | Chem. Commun., 2014, 50, 8246--8248
1 P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845–854. 2 E. Frackowiak, Phys. Chem. Chem. Phys., 2007, 9, 1774–1785. 3 X. Zhao, B. M. Sanchez, P. J. Dobson and P. S. Grant, Nanoscale, 2011, 3, 839–855. 4 M.-S. Wu and H.-H. Hsieh, Electrochim. Acta, 2008, 53, 3427–3435. 5 V. Srinivasan and J. W. Weidner, J. Electrochem. Soc., 2000, 147, 880–885. 6 C. H. Comniellis and I. A. Groza, Thermochim. Acta, 1988, 136, 19–22. 7 T. Alammar, O. Shekhah, J. Wohlgemuth and A.-V. Mudring, J. Mater. Chem., 2012, 22, 18252–18260. 8 H. Chen, J. Xu, X. Xu and Q. Zhang, Eur. J. Inorg. Chem., 2013, 1105–1108. 9 H. Pang, B. Zhang, J. Du, J. Chen, J. Zhang and S. Li, RSC Adv., 2012, 2, 2257–2261. 10 D. Wang, W. Ni, H. Pang, Q. Lu, Z. Huang and J. Zhao, Electrochim. Acta, 2010, 55, 6830–6835. 11 W. Xing, F. Li, Z.-f. Yan and G. Q. Lu, J. Power Sources, 2004, 134, 324–330. 12 C. Yuan, L. Hou, Y. Feng, S. Xiong and X. Zhang, Electrochim. Acta, 2013, 88, 507–512. 13 D.-D. Zhao, M. W. Xu, W.-J. Zhou, J. Zhang and H. L. Li, Electrochim. Acta, 2008, 53, 2699–2705. 14 S.-I. Kim, J.-S. Lee, H.-J. Ahn, H.-K. Song and J.-H. Jang, ACS Appl. Mater. Interfaces, 2013, 5, 1596–1603. 15 S. Xiong, C. Yuan, X. Zhang and Y. Qian, CrystEngComm, 2011, 13, 626–632. 16 S. K. Meher, P. Justin and G. Ranga Rao, Nanoscale, 2011, 3, 683–692. 17 S. Liu, S. Sun and X.-Z. You, Nanoscale, 2014, 6, 2037–2045. 18 J.-W. Lang, L.-B. Kong, W.-J. Wu, Y.-C. Luo and L. Kang, Chem. Commun., 2008, 4213–4215. 19 J. Liu, C. Cheng, W. Zhou, H. Li and H. J. Fan, Chem. Commun., 2011, 47, 3436–3438. 20 H. Jiang, T. Zhao, C. Li and J. Ma, J. Mater. Chem., 2011, 21, 3818–3823. 21 J. W. Lee, T. Ahn, J. H. Kim, J. M. Ko and J.-D. Kim, Electrochim. Acta, 2011, 56, 4849–4857. 22 C.-Y. Cao, W. Guo, Z.-M. Cui, W.-G. Song and W. Cai, J. Mater. Chem., 2011, 21, 3204–3209. 23 X. Tian, C. Cheng, L. Qian, B. Zheng, H. Yuan, S. Xie, D. Xiao and M. M. F. Choi, J. Mater. Chem., 2012, 22, 8029–8035. 24 D. Han, P. Xu, X. Jing, J. Wang, P. Yang, Q. Shen, J. Liu, D. Song, Z. Gao and M. Zhang, J. Power Sources, 2013, 235, 45–53. 25 H. Pang, Q. Lu, Y. Li and F. Gao, Chem. Commun., 2009, 7542–7544. 26 Y.-B. He, G.-R. Li, Z.-L. Wang, C.-Y. Su and Y.-X. Tong, Energy Environ. Sci., 2011, 4, 1288–1292. 27 M. Hasan, M. Jamal and K. M. Razeeb, Electrochim. Acta, 2012, 60, 193–200. 28 B. Ren, M. Fan, Q. Liu, J. Wang, D. Song and X. Bai, Electrochim. Acta, 2013, 92, 197–204. 29 S. Cheng, L. Yang, Y. Liu, W. Lin, L. Huang, D. Chen, C. P. Wong and M. Liu, J. Mater. Chem. A, 2013, 1, 7709–7716. 30 B. Gao, C. Yuan, L. Su, S. Chen and X. Zhang, Electrochim. Acta, 2009, 54, 3561–3567. 31 K.-W. Nam, K.-H. Kim, E.-S. Lee, W.-S. Yoon, X.-Q. Yang and K.-B. Kim, J. Power Sources, 2008, 182, 642–652. 32 K.-W. Nam, E.-S. Lee, J.-H. Kim, Y.-H. Lee and K.-B. Kim, J. Electrochem. Soc., 2005, 152, A2123–A2129. 33 A. Bello, K. Makgopa, M. Fabiane, D. Dodoo-Ahrin, K. I. Ozoemena and N. Manyala, J. Mater. Sci., 2013, 48, 6707–6712. 34 C. Ge, Z. Hou, B. He, F. Zeng, J. Cao, Y. Liu and Y. Kuang, J. Sol-Gel Sci. Technol., 2012, 63, 146–152. 35 X. Xia, J. Tu, Y. Mai, R. Chen, X. Wang, C. Gu and X. Zhao, Chem. – Eur. J., 2011, 17, 10898–10905. 36 H. Wang, H. Yi, X. Chen and X. Wang, Electrochim. Acta, 2013, 105, 353–361. 37 C. Guan, J. Liu, C. Cheng, H. Li, X. Li, W. Zhou, H. Zhang and H. J. Fan, Energy Environ. Sci., 2011, 4, 4496–4499. 38 Y.-L. Wang, Y.-Q. Zhao and C.-L. Xu, J. Solid State Electrochem., 2012, 16, 829–834. 39 W. Ni, B. Wang, J. Cheng, X. Li, Q. Guan, G. Gu and L. Huang, Nanoscale, 2014, 6, 2618–2623. 40 G. W. Yang, C. L. Xu and H. L. Li, Chem. Commun., 2008, 6537–6539. 41 K. C. Liu and M. A. Anderson, J. Electrochem. Soc., 1996, 143, 124–130. 42 Y. Zhang and Z. Guo, Chem. Commun., 2014, 50, 3443–3446. 43 M.-S. Wu and M.-J. Wang, Chem. Commun., 2010, 46, 6968–6970.
This journal is © The Royal Society of Chemistry 2014
Published on 04 June 2014. Downloaded by University of Pittsburgh on 30/10/2014 17:31:56.
COMMUNICATION
View Journal | View Issue
Cite this: Chem. Commun., 2014, 50, 8246
Three-dimensional carbon nanotube networks with a supported nickel oxide nanonet for high-performance supercapacitors†
Received 13th April 2014, Accepted 30th May 2014
Mao-Sung Wu,* Yo-Ru Zheng and Guan-Wei Lin
DOI: 10.1039/c4cc02725f www.rsc.org/chemcomm
A three-dimensional porous carbon nanotube film with a supported NiO nanonet was prepared by simple electrophoretic deposition and hydrothermal synthesis, which could deliver a high specific capacitance of 1511 F g
1
at a high discharge current of 50 A g
1
due to the
significantly improved transport of the electrolyte and electrons.
Supercapacitors have been studied extensively in recent years due to the increasing demand for power sources for portable electronic devices. Supercapacitors can deliver a higher capacitance than the traditional electric double-layer capacitors. A number of materials such as carbonaceous materials, metal oxides/hydroxides, and conducting polymers have been characterized as promising materials for application in supercapacitors.1–3 Among these materials, nickel oxides/hydroxides are available for application in alkaline supercapacitors due to their high corrosive resistance to the alkaline electrolyte. Heat treatment is known to be one of the major factors that determine the capacitive behavior of nickel hydroxide electrodes. Heat treatment in the temperature range of 300–400 1C gives a non-stoichiometric nickel oxide, which is suitable for electrochemical applications.4–6 As the electrode materials in supercapacitors, a large surface area, suitable pore size and pore size distribution, and good electrical conductivity are required to achieve high specific capacitance and rate performance. Micropores may contribute a huge surface area to the material, but narrow micropores are not easily accessed by the liquid electrolyte. The solution to this problem is to use mesoporous nickel oxide materials.7–13 Previous reports have indicated that the nanostructured nickel oxides exhibit a better capacitive performance than the bulk nickel oxide due to their large surface and improved transport of electrolyte ions.14–17 Thus, the shape control of nickel oxide based nanomaterials has become an important factor in determining their capacitive performance. Nickel oxide with different kinds of morphologies such as nanoflakes, nanoflowers, nanotubes, Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan. E-mail: [email protected]; Fax: +886-7-3830674 † Electronic supplementary information (ESI) available: Experimental details, SEM images, TEM images, TGA analysis, BET analysis, XRD patterns, and electrochemical properties. See DOI: 10.1039/c4cc02725f
8246 | Chem. Commun., 2014, 50, 8246--8248
nanowires, and hollow nanospheres is believed to have a positive effect on the capacitive behavior of the electrode.18–29 In general, the electrical conductivity of electrode materials plays an important role in enhancing the utilization of active materials and mitigating the internal resistance of the supercapacitors. The nickel oxide–carbon composite has shown an improvement in capacitive behavior due to the inclusion of carbon materials that improve the dispersibility of nickel oxide nanoparticles and the electrical conductivity.29–34 Nickel oxide supported on high conductive porous Ni foam or Ni nanowires also showed superior capacitive behavior.35–40 In this work, a novel electrode structure of a three-dimensional (3D) porous CNT (carbon nanotube) network-supported nickel oxide nanonet illustrated in Fig. 1 is proposed as a stable and highcapacitance electrode material for supercapacitors. A 3D porous CNT film with interconnected networks was assembled on a stainless steel (SS) sheet via simple electrophoretic deposition (EPD). A nickel oxide nanonet composed of ultrasmall nanowires could be formed between adjacent CNTs during hydrothermal synthesis. The CNT exhibits excellent electrical conductivity and provides a conductive network for the fast transport of electrons. The well-dispersed nickel oxide nanonet provides a large faradaic capacitance, making the nickel oxide–CNT composite electrode suitable for high rate applications.
Fig. 1 Schematic illustrating a novel electrode structure of 3D porous CNT networks with attached nickel oxide nanonet for supercapacitors.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 04 June 2014. Downloaded by University of Pittsburgh on 30/10/2014 17:31:56.
Communication
Fig. 2a shows the SEM (scanning electron microscopy) image of a NiO nanonet grown on an SS sheet after annealing at 300 1C. An SS surface may have a lot of active sites, which allow for the nucleation of nickel hydroxide during hydrothermal synthesis, leading to the formation of a nickel hydroxide nanonet. The nanonet is composed of nanowires with ultrasmall diameters (o10 nm, Fig. S1 in the ESI†). Part of the nanowires aggregate to form vertically aligned sheets, and the rest of the nanowires form a net-like structure. Vertically aligned nanosheets composed of nanowires allow the electrolyte to migrate from the bulk electrolyte into the porous layer and participate in the electrochemical reaction. Fig. 2b shows the SEM image of the porous CNT film coated on the SS sheet prepared using the EPD method. The CNT film exhibits 3D cross-linked porous structure, providing a high porosity and large surface area. The 3D porous CNT film imparts high electrical conductivity to the
ChemComm
Fig. 3 CVs of the NiO/SS, NiO/CNT, and bare CNT electrodes at a scan rate of 10 mV s 1.
electrode and accommodates the large volume of the liquid electrolyte. Fig. 2c shows the SEM image of the CNT film with attached NiO nanonet after annealing at 300 1C. Clearly, the NiO nanowires grow around the CNT surface and outward from each CNT, forming a CNT-supported NiO nanonet. Probably, the acid-treated CNT surface provides a large amount of active sites for nucleation and growth of nanowires, leading to the formation of well-dispersed nanowires in the porous CNT film. Fig. 3 shows the cyclic voltammograms (CVs) of NiO electrodes and the bare CNT electrode. In addition to the oxygen evolution reaction occurring at a more positive potential than 0.45 V, the NiO electrodes exhibit a pair of redox peaks resulting from the faradaic redox reaction between NiO and NiOOH. The redox reaction of NiO electrode in alkaline solution can be expressed as follows:41 NiO + OH 2 NiOOH + e
Fig. 2 SEM images of (a) NiO nanonet, (b) CNT film, and (c) CNT film with attached NiO nanonet.
This journal is © The Royal Society of Chemistry 2014
(1)
Obviously, the current density of the electrode with the CNTsupported NiO nanonet (NiO/CNT) during CV scanning is significantly higher than that of the electrode with the SS-supported NiO nanonet (NiO/SS). The higher the current density, the higher is the specific capacitance of the NiO electrode. This reflects that the 3D CNT networks may facilitate the transport of the electrolyte and electrons through the porous film. The NiO nanonet with ultrasmall nanowires can shorten the ion migration and diffusion paths, leading to an increase in the effective surface area. The current density of the electrode with the bare CNT film is very small compared to that of NiO electrodes during CV scanning, reflecting that the current density comes mainly from the redox reaction of NiO. Supercapacitors are expected to complement or replace batteries in high power applications. Thus, it is essential for supercapacitors to exhibit better performance at high-rate charging and discharging. Fig. 4 shows the specific capacitances of NiO electrodes obtained at various discharge current densities. The specific capacitance of NiO/ CNT electrode could reach as high as 1636 F g 1 at a low discharge current density of 1 A g 1, which is higher than that of the NiO/SS electrode (1496 F g 1). When discharged at a high current density of 50 A g 1, the NiO/CNT electrode still retains a high specific capacitance of 1511 F g 1, while the specific capacitance of the NiO/SS electrode is decreased to 883 F g 1. The capacitance retention
Chem. Commun., 2014, 50, 8246--8248 | 8247
View Article Online
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Published on 04 June 2014. Downloaded by University of Pittsburgh on 30/10/2014 17:31:56.
Notes and references
Fig. 4 Relationship between specific capacitance and discharge current density of the NiO/SS and NiO/CNT electrodes.
is calculated from the ratio of capacitance values discharged at 50 A g 1 and 1 A g 1. The capacitance retention of the NiO/CNT electrode reaches as high as 92.4%, which is much better than that of the NiO/SS electrode (59.0%). It is noted that a large specific surface does not always guarantee a high specific capacitance because not all the surface area is available for electrolyte access. Open porous NiO structures have been developed to fulfill the requirements of a high-rate performance.42,43 In addition to the amount of electrolyte-accessible surface area, electroactive sites available for charge storage and redox reaction should be considered. The 3D conductive scaffold composed of porous CNT networks is essential for electrically connecting the electroactive sites within the electrode to improve the utilization of the active material and mitigate the charge-transfer resistance of faradaic reaction. The enhanced capacitance retention of the NiO/CNT electrode might be due to the following reasons: (1) the well-dispersed NiO nanonet provides larger open channels for easy transport of electrolyte ions and (2) the porous CNT networks act as a current collector for facilitating the transport of electrons and accommodate the large volume of the liquid electrolyte for easy transport of ions. As a result, the proposed strategy can be used to construct a high-rate NiO electrode for application in supercapacitors. In summary, the nickel oxide nanonet with ultrasmall nanowires was prepared using a simple hydrothermal method. A net-like structure of the NiO electrode could offer a large surface area accessible to the liquid electrolyte, leading to a high specific capacitance. The high-rate performance of NiO electrodes turned out to be significantly affected by the conductive scaffold. The specific capacitance of NiO electrode with the CNT-supported nanonet could reach as high as 1511 F g 1 at a current density of 50 A g 1, which is much higher than that of the NiO electrode with the SS-supported nanonet (883 F g 1). The 3D porous CNT film could offer porous channels and conductive networks for the fast transport of the electrolyte and electrons. Thus, the CNTsupported NiO nanonet was capable of delivering high capacitance during high-rate charge–discharge circumstances due to the reduced charge-transfer and diffusion resistances. The authors acknowledge financial support from the National Science Council, Taiwan (Project No.: NSC101-2221-E-151-055-MY2).
8248 | Chem. Commun., 2014, 50, 8246--8248
1 P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845–854. 2 E. Frackowiak, Phys. Chem. Chem. Phys., 2007, 9, 1774–1785. 3 X. Zhao, B. M. Sanchez, P. J. Dobson and P. S. Grant, Nanoscale, 2011, 3, 839–855. 4 M.-S. Wu and H.-H. Hsieh, Electrochim. Acta, 2008, 53, 3427–3435. 5 V. Srinivasan and J. W. Weidner, J. Electrochem. Soc., 2000, 147, 880–885. 6 C. H. Comniellis and I. A. Groza, Thermochim. Acta, 1988, 136, 19–22. 7 T. Alammar, O. Shekhah, J. Wohlgemuth and A.-V. Mudring, J. Mater. Chem., 2012, 22, 18252–18260. 8 H. Chen, J. Xu, X. Xu and Q. Zhang, Eur. J. Inorg. Chem., 2013, 1105–1108. 9 H. Pang, B. Zhang, J. Du, J. Chen, J. Zhang and S. Li, RSC Adv., 2012, 2, 2257–2261. 10 D. Wang, W. Ni, H. Pang, Q. Lu, Z. Huang and J. Zhao, Electrochim. Acta, 2010, 55, 6830–6835. 11 W. Xing, F. Li, Z.-f. Yan and G. Q. Lu, J. Power Sources, 2004, 134, 324–330. 12 C. Yuan, L. Hou, Y. Feng, S. Xiong and X. Zhang, Electrochim. Acta, 2013, 88, 507–512. 13 D.-D. Zhao, M. W. Xu, W.-J. Zhou, J. Zhang and H. L. Li, Electrochim. Acta, 2008, 53, 2699–2705. 14 S.-I. Kim, J.-S. Lee, H.-J. Ahn, H.-K. Song and J.-H. Jang, ACS Appl. Mater. Interfaces, 2013, 5, 1596–1603. 15 S. Xiong, C. Yuan, X. Zhang and Y. Qian, CrystEngComm, 2011, 13, 626–632. 16 S. K. Meher, P. Justin and G. Ranga Rao, Nanoscale, 2011, 3, 683–692. 17 S. Liu, S. Sun and X.-Z. You, Nanoscale, 2014, 6, 2037–2045. 18 J.-W. Lang, L.-B. Kong, W.-J. Wu, Y.-C. Luo and L. Kang, Chem. Commun., 2008, 4213–4215. 19 J. Liu, C. Cheng, W. Zhou, H. Li and H. J. Fan, Chem. Commun., 2011, 47, 3436–3438. 20 H. Jiang, T. Zhao, C. Li and J. Ma, J. Mater. Chem., 2011, 21, 3818–3823. 21 J. W. Lee, T. Ahn, J. H. Kim, J. M. Ko and J.-D. Kim, Electrochim. Acta, 2011, 56, 4849–4857. 22 C.-Y. Cao, W. Guo, Z.-M. Cui, W.-G. Song and W. Cai, J. Mater. Chem., 2011, 21, 3204–3209. 23 X. Tian, C. Cheng, L. Qian, B. Zheng, H. Yuan, S. Xie, D. Xiao and M. M. F. Choi, J. Mater. Chem., 2012, 22, 8029–8035. 24 D. Han, P. Xu, X. Jing, J. Wang, P. Yang, Q. Shen, J. Liu, D. Song, Z. Gao and M. Zhang, J. Power Sources, 2013, 235, 45–53. 25 H. Pang, Q. Lu, Y. Li and F. Gao, Chem. Commun., 2009, 7542–7544. 26 Y.-B. He, G.-R. Li, Z.-L. Wang, C.-Y. Su and Y.-X. Tong, Energy Environ. Sci., 2011, 4, 1288–1292. 27 M. Hasan, M. Jamal and K. M. Razeeb, Electrochim. Acta, 2012, 60, 193–200. 28 B. Ren, M. Fan, Q. Liu, J. Wang, D. Song and X. Bai, Electrochim. Acta, 2013, 92, 197–204. 29 S. Cheng, L. Yang, Y. Liu, W. Lin, L. Huang, D. Chen, C. P. Wong and M. Liu, J. Mater. Chem. A, 2013, 1, 7709–7716. 30 B. Gao, C. Yuan, L. Su, S. Chen and X. Zhang, Electrochim. Acta, 2009, 54, 3561–3567. 31 K.-W. Nam, K.-H. Kim, E.-S. Lee, W.-S. Yoon, X.-Q. Yang and K.-B. Kim, J. Power Sources, 2008, 182, 642–652. 32 K.-W. Nam, E.-S. Lee, J.-H. Kim, Y.-H. Lee and K.-B. Kim, J. Electrochem. Soc., 2005, 152, A2123–A2129. 33 A. Bello, K. Makgopa, M. Fabiane, D. Dodoo-Ahrin, K. I. Ozoemena and N. Manyala, J. Mater. Sci., 2013, 48, 6707–6712. 34 C. Ge, Z. Hou, B. He, F. Zeng, J. Cao, Y. Liu and Y. Kuang, J. Sol-Gel Sci. Technol., 2012, 63, 146–152. 35 X. Xia, J. Tu, Y. Mai, R. Chen, X. Wang, C. Gu and X. Zhao, Chem. – Eur. J., 2011, 17, 10898–10905. 36 H. Wang, H. Yi, X. Chen and X. Wang, Electrochim. Acta, 2013, 105, 353–361. 37 C. Guan, J. Liu, C. Cheng, H. Li, X. Li, W. Zhou, H. Zhang and H. J. Fan, Energy Environ. Sci., 2011, 4, 4496–4499. 38 Y.-L. Wang, Y.-Q. Zhao and C.-L. Xu, J. Solid State Electrochem., 2012, 16, 829–834. 39 W. Ni, B. Wang, J. Cheng, X. Li, Q. Guan, G. Gu and L. Huang, Nanoscale, 2014, 6, 2618–2623. 40 G. W. Yang, C. L. Xu and H. L. Li, Chem. Commun., 2008, 6537–6539. 41 K. C. Liu and M. A. Anderson, J. Electrochem. Soc., 1996, 143, 124–130. 42 Y. Zhang and Z. Guo, Chem. Commun., 2014, 50, 3443–3446. 43 M.-S. Wu and M.-J. Wang, Chem. Commun., 2010, 46, 6968–6970.
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