- Email: [email protected]
Ionic liquid-assisted synthesis and electrochemical properties of ultrathin Co3O4 nanotube-intercalated graphene composites
Author’s Accepted Manuscript Ionic liquid-assisted synthesis and electrochemical properties of Ultrathin Co3O4 nanotube-intercalated graphene composites Van Hoa Nguyen, Jae- Jin Shim www.elsevier.com
PII: DOI: Reference:
S0167-577X(15)30010-0 http://dx.doi.org/10.1016/j.matlet.2015.05.105 MLBLUE18998
To appear in: Materials Letters Received date: 23 March 2015 Revised date: 21 May 2015 Accepted date: 23 May 2015 Cite this article as: Van Hoa Nguyen and Jae- Jin Shim, Ionic liquid-assisted synthesis and electrochemical properties of Ultrathin Co3O4 nanotubeintercalated graphene composites, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.05.105 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ionic liquid-assisted synthesis and electrochemical properties of ultrathin Co3O4 nanotube-intercalated graphene composites Van Hoa Nguyena,b, Jae- Jin Shim*a
a
School of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeongbuk, 712-749, Republic of Korea
b
Department of Chemistry, Nha Trang University, 2 Nguyen Dinh Chieu, Nha Trang, Vietnam.
*
Tel: +82-53-810-2587; E-mail address: [email protected]
Abstract Ultrathin Co3O4 nanotube-intercalated graphene composites were synthesized using a facile one-step hydrothermal method in a mixture of ionic liquid and ethanol. No templates were used to form the Co3O4 nanotubes. The electrode modified as-prepared composites exhibited a high specific capacitance of 1010 F g–1 at a high discharge current of 5 A g–1, as well as excellent cycling stability (89.2 % capacitance retention after 5000 cycles). Keywords: Co3O4 nanotubes; Graphene; Ionic liquid; Hydrothermal method; Supercapacitors
Introduction Cobalt oxide (Co3O4) has attracted considerable attention as an important magnetic and intrinsic p–type semiconductor (direct optical band gaps at 1.48 and 2.19 eV) for many applications, such as the fabrication of sensors, heterogeneous catalysts, electrochromic devices, Li-ion batteries, supercapacitors, and rotatable magnets [1-4]. Therefore, considerable efforts have been made regarding the fabrication of nanostructured Co3O4 with 1
strategies, including template-assisted synthesis [5,6], solvothermal/hydrothermal process [7], physical vapor deposition [8], and electrochemical deposition [9,10]. A range of Co3O4 nanostructures and morphologies, such as spheres, flakes, rods, wires, and tubes, as well as arrays of Co3O4 nanowires, nanorods, nanoneedles, and hollow-spheres have been obtained using these methods. However, most of these approaches require either porous templates or tedious procedures because the post-synthesis removal of the templates may break the nanostructures and introduce unwanted defects or impurities. Therefore, developing a templateless, facile and versatile procedure for the fabrication of Co3O4 nanotubes (NTs) is a considerable challenge. For supercapacitor electrodes, Co3O4 is considered a promising potential activated material because of its environmental friendliness, low cost and favorable pseudocapacitive characteristics. However, the conductivity of Co3O4 is not high. To improve the electrical conductivity of Co3O4-based electrodes, Co3O4 is combined with a high conductivity material, such as metal nanoparticles, carbon nanotubes, conducting polymers, or graphene. In the present work, a facile hydrothermal and templateless method was developed for the one-step fabrication of ultrathin Co3O4 NTs intercalated graphene composites. The as-obtained composites exhibited an overall specific capacitance of 1010 F g–1 at a high current density of 5 A g–1 as well as good long-term cycle stability. Experimental All the chemical reagents were of analytical grade and used as received. Graphene oxide (GO) was synthesized from graphite powder using a modified Hummers method [11]. An appropriate amount of GO was dispersed in 15 ml of a mixture of ethanol and [Bmim][BF4] (14:1), and sonicated for 30 min. A certain amount of Co(NO3)2·6H2O was dissolved separately in the same mixture as above. The two mixtures were mixed together and stirred 2
for 30 min, and the appropriate amount of ammonic solution (30%) was added. Subsequently, the slurry was poured into a Teflon-lined stainless-steel autoclave and heated to 180°C for 10 h. After cooling to room temperature, the products were washed and dried in an oven at 60°C for 6 h. 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 photoelectron spectroscopy (XPS, Thermo Scientific, K-Alpha) using Al Kα monochromatized radiation. All measurements were carried out in a three-electrode cell using an Autolab PGSTAT302N (Metrohm, Netherlands) with a working electrode, platinum plate counter electrode and a saturated calomel reference electrode (SCE) at room temperature. The working electrodes were fabricated by mixing the as-prepared powder (5 mg, 80 wt. %) with 15 wt. % acetylene black and 5 wt. % polytetrafluorene-ethylene (PTFE) binder, and pressed onto nickel foam current-collectors (1.0 cm 1.0 cm). The electrolyte was a 3M KOH solution. The specific capacitance (Cs) of the electrodes was calculated from the chargedischarge curves using the following equation: C
It m V
1
where C, I, t, m, and ΔV are the specific capacitance (F g−1) of the electrodes, discharge current (A), discharge time (s), mass of the active material (g), and discharging potential range (V), respectively. Results and discussion Fig. 1a and 1b shows SEM images of the bare Co3O4 NTs and RGO/Co3O4 composite. The Co3O4 NTs were uniform long NTs with lengths greater than 3m. The surface of the NTs was smooth and clean (Fig. 1a). As shown in Fig. 1b, the RGO/Co3O4 composite had a 3
hierarchical structure of the Co3O4 NTs and graphene sheets. To further characterize the composite morphology more clearly, the sample was characterized by TEM (Fig.1c and 1d). Co3O4 with an ultrathin nanotube morphology, approximately 20 nm in diameter, intercalated uniformly over the surface of the RGO sheets. Furthermore, the Co3O4 nanotubes were anchored strongly to the surface of the RGO sheets even if they had suffered from long time sonication during preparation of the TEM specimens. This strong interaction enables rapid electron transport between Co3O4 and RGO. The selected-area electron diffraction pattern revealed well-defined diffraction rings and dots, suggesting their polycrystalline characteristics (Fig. 1e). Fig. 1f shows the wide-angle XRD pattern of the RGO/Co3O4 composite. The characteristic peak of GO at 2 of 11.85° was absent. With the exception of the typical peak assigned to the RGO at 2 of around 26.5°, six other well defined diffraction peaks were observed: 21.5°, 31.2°, 43.8°, 49.4°, 54.3° and 63.7° 2 with hkl values of (111), (220), (200), (400), (422), and (440), respectively, which are representative of the Co3O4 crystalline structure (JCPSD file no. 65-3103). The HAADF-STEM image revealed uniform interconnected Co3O4 nanotubes and graphene sheets (Fig. 2a), which agrees well with the TEM images (Fig. 1c). EDX-STEM elemental mapping revealed the K-edge signals of O, Co and C (Fig. 3b–d). The even distribution of Co and O confirmed the uniform deposition of Co3O4 nanotubes, suggesting the successful preparation of the nanotubes on the graphene via a hydrothermal method. XPS was performed to provide surface information and characterize the oxidation state of the detected elements. The mass content of Co3O4 in the composite was about 53 wt%. Fig. 3 presents the survey spectrum, the composition of the composite and the core level C 1s, Co 2p and O 1s peaks. The C 1s spectrum revealed three peaks (Fig. 3a), which were assigned to the binding energy of C=C, C-C and C=O bonds. A small peak for the C=O bond confirmed 4
the reduction of graphene oxide. The Co 2p spectrum (Fig. 3c) was fitted to two spin-orbit doublets, which are characteristic of Co2+ and Co3+, along with two shake-up satellites (denoted as “Sat.”) [9,12]. The high resolution spectrum for the O 1s region (Fig. 3d) was deconvoluted into three peaks at binding energies (BEs) of 529.8, 530.9 and 532.4 eV, which are denoted as OI, OII and OIII, respectively. The component, OI, was assigned to typical metal-oxygen bonds [13]. The component, OII, is associated with oxygen in the hydroxyl groups on the Co3O4 surface. The OIII component was assigned to a larger number of defect sites with low oxygen coordination [14]. Fig. 4 presents the electrochemical properties of the as-prepared composite modified electrodes. The CV curves of this electrode in the potential window, between -0.1 to 0.5V, at different scan rates (2 to 50 mV s-1) shows the pseudocapacitance caused by the electrochemical reactions. Two pairs of redox reaction peaks were visible in the CV curves, which are responsible for the redox process of Co3O4 (Fig. 4a) as following redox reactions: CoOOH OH CoO2 H2O e , and Co3O4 OH 3CoOOH e respectively [15,16]. The shape of the CV curves showed that the capacitive characteristics of the Co3O4 phase are quite different from those of an electric double-layer capacitor, which would produce a CV curve close to an ideal rectangular shape. In addition, the peak current also increased with increasing scan rate from 2 to 50 mV s-1. Fig. 4c presents the impedance curves of the electrode before and after 5000 cycles. Both plots feature a semicircle in the high frequencies which relates to the charge transfer resistance, and a sloped line in the low frequencies which corresponds to the diffusive resistance. As can be seen from the plots (Fig. 4c), the charge transfer resistance of the RGO/Co3O4 electrode meets well with the solution resistance after 5000 cycles. The more vertical line in the low and high frequency regions indicates the more capacitive behavior of the electrode. The improved electrochemical performance of the RGO/Co3O4 modified electrode was also confirmed by galvanostatic charge/discharge tests performed at different current densities (Fig. 4c). The electrode exhibited good pseudocapacitances of 1010 F g–1 at 5 A g–1, 913 F g–1 at 7.5 A g–1, 900 F g–1 at 15 A g–1, 833 F g–1 at 20 A g–1, and 792 F g–1 at 25 A g–1. The composite electrode showed excellent rate performance, i.e., approximately 78.2% capacitance retention, when the charge/discharge rate changed from 5 A g–1 to 25 A g–1. The Co3O4 nanotubes yielded enhanced capacitance performance with about 75% increase in specific capacitance respectively compared to that of the bare RGO electrode. The capacitances obtained, in this study, are much higher than those of graphene/Co3O4 (443 F g−1 at 5 A g−1) [16], 3D graphene/Co3O4 (522 F g−1 at 0.5 A g−1) [17], RGO/Co3O4 (311 F g−1 at 5 A g−1) [18], Co3O4 nanowires (746 F g-1 at 5mA cm-2) [19], Co3O4 nanowires (599 F g-1 at 2 A g-1) [20]. In 5
addition, the electrode retained 89.2 % of its initial capacity when it was charged and discharged for 5000 cycles at a rate of 15 A g–1, which is comparable to that of graphene/Co3O4 (97.1% after 1000 cycles), 3D graphene/Co3O4 (75% after 1000 cycles) [17], and RGO/Co3O4 (122% after 5000 cycles) [18]. Conclusions Ultrathin Co3O4 nanotube-intercalated graphene composites were synthesized using a facile one-step hydrothermal method in a mixture of ionic liquid and ethanol. The RGO/Co3O4 modified electrode exhibited outstanding electrochemical performance with a very high specific capacitance of 1010 and 792 F g–1 at current densities of 5 and 25 A g–1, respectively, as well as excellent rate capability and improved cycling stability for highperformance electrochemical capacitors. This paper reports a novel electroactive material for the design of the next-generation supercapacitors. Acknowledgment This study was supported by the Priority Research Centers Program through the National Research
Foundation
of
Korea
(NRF)
funded
by the
Ministry of
Education
(2014R1A6A1031189).
References 1. Li WY, Xu LN, Chen J. Adv Funct Mater 2005;15:851-857. 2. Xia XH, Tu JP, Mai YJ, Wang XL, Gu CD, Zhao XB. J Mater Chem 2011;21:9319-9325. 3. Nguyen VH, Tran VC, Kharismadewi D, Shim JJ. Mater Lett 2015;147:123–127. 4. Nguyen VH, Shim JJ. Mater Lett 2015;139:377–381 5. Du N, Zhang H, Chen B, Wu J, Ma X, Liu Z, Zhang Y, Yang D, Huang X, Tu J. Adv Mater 2007; 19:4505–4509. 6. Liu Y, Zhao WW, Zhang XG. Electrochim Acta 2008;8:3296-3304. 6
7. Dong C, Xiao X, Chen G, Guan H, Wang Y. Mater Lett 2014;123:187–190. 8. Varghese B, Hoong TC, Yanwu Z, Reddy MV, Chowdari BVR, Wee ATS, Vincent TBC, Lim CT, Sow CH. Adv Funct Mater 2007;17:1932–1939. 9. Nguyen VH, Shim JJ. Mater Lett 2015;139:377–381. 10. Mao Y, Li W, Liu P, Chen J, Liang E. Mater Lett 2014;134:276–280 11. Nguyen VH, Nguyen TT, Shim JJ. Synt Met 2015;199:276–279. 12. Nguyen VH, Shim JJ. J Power Sources 2015;273:110-117. 13. Marco JF, Gancedo JR, Gracia M. J Solid State Chem 2000;153:74–81. 14. Jimenez VM, Fernandez A, Espinos JP, Gonzalez-Elipe AR. J Electron Spectrosc Relat Phenom 1995;71:61–71. 15. Sumanta KM, Rao GR. J Phys Chem C 2011;115:15646-15654. 16. Huang S, Jin Y, Jia M. Electrochim Acta 2013;95:139– 145. 17. Liu H, Gou X, Wang Y,Du X, Quan C, Qi T. J Nanomaterials, Article ID 874245, in press. 18. Li Q, Hu X, Yang Q, Yan Z, Kang L, Lei Z, Yang Z, Liu Z. Electrochim. Acta 2014;119:184– 191 19. Gao YY, Chen SL, Cao DX, Wang GL, Yin JL. J Power Sources 2010;195:1757-1760. 20. Xia X, Tu J, Mai Y, Wang X, Gu C, Zhao X. J Mater Chem 2011;21:9319–9325.
Figure captions 7
Fig. 1. SEM images of (a) bare Co3O4 NTs and (b) RGO/Co3O4 composite; (c,d) TEM images the RGO/Co3O4 composite at different magnifications; (e) SAED pattern of the RGO/Co3O4 composite. The image in (e) is taken from the region marked with a rectangle in (c); (f) XRD pattern of the RGO/Co3O4 composite (the upper-right inset is the XRD pattern of GO). Fig. 2. (a) HAADF-STEM image of the RGO/Co3O4 composite and EDX mapping of (b) oxygen, (c) cobalt, and (d) carbon. Fig. 3. (a) XPS survey of the RGO/Co3O4 composite and high-resolution XPS spectra of (b) C 1s (c) Co 2p, and (d) O 1s. Fig. 4. (a) CV curves of the RGO/Co3O4 modified electrode at different scan rates in 6M KOH. (b) Nyquist plots of the RGO/Co3O4-modified electrode before and after 5000 charge/discharge cycles; (c) galvanostatic discharge curves and the specific capacitance of the bare RGO and RGO/Co3O4-modified electrodes at different current densities ; (d) average specific capacitance versus the cycle number at a current density of 15 A g-1.
8
9
10
11
Research highlights • Ultrathin Co3O4 nanotubes with lengths greater than 3 m were intercalated with graphene sheets. • The electrode exhibits a high specific capacitance of 1010 F g−1 at 5 A g−1. • The electrode presents a high rate capacitance and long-term cycling stability.
12
PII: DOI: Reference:
S0167-577X(15)30010-0 http://dx.doi.org/10.1016/j.matlet.2015.05.105 MLBLUE18998
To appear in: Materials Letters Received date: 23 March 2015 Revised date: 21 May 2015 Accepted date: 23 May 2015 Cite this article as: Van Hoa Nguyen and Jae- Jin Shim, Ionic liquid-assisted synthesis and electrochemical properties of Ultrathin Co3O4 nanotubeintercalated graphene composites, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.05.105 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ionic liquid-assisted synthesis and electrochemical properties of ultrathin Co3O4 nanotube-intercalated graphene composites Van Hoa Nguyena,b, Jae- Jin Shim*a
a
School of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeongbuk, 712-749, Republic of Korea
b
Department of Chemistry, Nha Trang University, 2 Nguyen Dinh Chieu, Nha Trang, Vietnam.
*
Tel: +82-53-810-2587; E-mail address: [email protected]
Abstract Ultrathin Co3O4 nanotube-intercalated graphene composites were synthesized using a facile one-step hydrothermal method in a mixture of ionic liquid and ethanol. No templates were used to form the Co3O4 nanotubes. The electrode modified as-prepared composites exhibited a high specific capacitance of 1010 F g–1 at a high discharge current of 5 A g–1, as well as excellent cycling stability (89.2 % capacitance retention after 5000 cycles). Keywords: Co3O4 nanotubes; Graphene; Ionic liquid; Hydrothermal method; Supercapacitors
Introduction Cobalt oxide (Co3O4) has attracted considerable attention as an important magnetic and intrinsic p–type semiconductor (direct optical band gaps at 1.48 and 2.19 eV) for many applications, such as the fabrication of sensors, heterogeneous catalysts, electrochromic devices, Li-ion batteries, supercapacitors, and rotatable magnets [1-4]. Therefore, considerable efforts have been made regarding the fabrication of nanostructured Co3O4 with 1
strategies, including template-assisted synthesis [5,6], solvothermal/hydrothermal process [7], physical vapor deposition [8], and electrochemical deposition [9,10]. A range of Co3O4 nanostructures and morphologies, such as spheres, flakes, rods, wires, and tubes, as well as arrays of Co3O4 nanowires, nanorods, nanoneedles, and hollow-spheres have been obtained using these methods. However, most of these approaches require either porous templates or tedious procedures because the post-synthesis removal of the templates may break the nanostructures and introduce unwanted defects or impurities. Therefore, developing a templateless, facile and versatile procedure for the fabrication of Co3O4 nanotubes (NTs) is a considerable challenge. For supercapacitor electrodes, Co3O4 is considered a promising potential activated material because of its environmental friendliness, low cost and favorable pseudocapacitive characteristics. However, the conductivity of Co3O4 is not high. To improve the electrical conductivity of Co3O4-based electrodes, Co3O4 is combined with a high conductivity material, such as metal nanoparticles, carbon nanotubes, conducting polymers, or graphene. In the present work, a facile hydrothermal and templateless method was developed for the one-step fabrication of ultrathin Co3O4 NTs intercalated graphene composites. The as-obtained composites exhibited an overall specific capacitance of 1010 F g–1 at a high current density of 5 A g–1 as well as good long-term cycle stability. Experimental All the chemical reagents were of analytical grade and used as received. Graphene oxide (GO) was synthesized from graphite powder using a modified Hummers method [11]. An appropriate amount of GO was dispersed in 15 ml of a mixture of ethanol and [Bmim][BF4] (14:1), and sonicated for 30 min. A certain amount of Co(NO3)2·6H2O was dissolved separately in the same mixture as above. The two mixtures were mixed together and stirred 2
for 30 min, and the appropriate amount of ammonic solution (30%) was added. Subsequently, the slurry was poured into a Teflon-lined stainless-steel autoclave and heated to 180°C for 10 h. After cooling to room temperature, the products were washed and dried in an oven at 60°C for 6 h. 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 photoelectron spectroscopy (XPS, Thermo Scientific, K-Alpha) using Al Kα monochromatized radiation. All measurements were carried out in a three-electrode cell using an Autolab PGSTAT302N (Metrohm, Netherlands) with a working electrode, platinum plate counter electrode and a saturated calomel reference electrode (SCE) at room temperature. The working electrodes were fabricated by mixing the as-prepared powder (5 mg, 80 wt. %) with 15 wt. % acetylene black and 5 wt. % polytetrafluorene-ethylene (PTFE) binder, and pressed onto nickel foam current-collectors (1.0 cm 1.0 cm). The electrolyte was a 3M KOH solution. The specific capacitance (Cs) of the electrodes was calculated from the chargedischarge curves using the following equation: C
It m V
1
where C, I, t, m, and ΔV are the specific capacitance (F g−1) of the electrodes, discharge current (A), discharge time (s), mass of the active material (g), and discharging potential range (V), respectively. Results and discussion Fig. 1a and 1b shows SEM images of the bare Co3O4 NTs and RGO/Co3O4 composite. The Co3O4 NTs were uniform long NTs with lengths greater than 3m. The surface of the NTs was smooth and clean (Fig. 1a). As shown in Fig. 1b, the RGO/Co3O4 composite had a 3
hierarchical structure of the Co3O4 NTs and graphene sheets. To further characterize the composite morphology more clearly, the sample was characterized by TEM (Fig.1c and 1d). Co3O4 with an ultrathin nanotube morphology, approximately 20 nm in diameter, intercalated uniformly over the surface of the RGO sheets. Furthermore, the Co3O4 nanotubes were anchored strongly to the surface of the RGO sheets even if they had suffered from long time sonication during preparation of the TEM specimens. This strong interaction enables rapid electron transport between Co3O4 and RGO. The selected-area electron diffraction pattern revealed well-defined diffraction rings and dots, suggesting their polycrystalline characteristics (Fig. 1e). Fig. 1f shows the wide-angle XRD pattern of the RGO/Co3O4 composite. The characteristic peak of GO at 2 of 11.85° was absent. With the exception of the typical peak assigned to the RGO at 2 of around 26.5°, six other well defined diffraction peaks were observed: 21.5°, 31.2°, 43.8°, 49.4°, 54.3° and 63.7° 2 with hkl values of (111), (220), (200), (400), (422), and (440), respectively, which are representative of the Co3O4 crystalline structure (JCPSD file no. 65-3103). The HAADF-STEM image revealed uniform interconnected Co3O4 nanotubes and graphene sheets (Fig. 2a), which agrees well with the TEM images (Fig. 1c). EDX-STEM elemental mapping revealed the K-edge signals of O, Co and C (Fig. 3b–d). The even distribution of Co and O confirmed the uniform deposition of Co3O4 nanotubes, suggesting the successful preparation of the nanotubes on the graphene via a hydrothermal method. XPS was performed to provide surface information and characterize the oxidation state of the detected elements. The mass content of Co3O4 in the composite was about 53 wt%. Fig. 3 presents the survey spectrum, the composition of the composite and the core level C 1s, Co 2p and O 1s peaks. The C 1s spectrum revealed three peaks (Fig. 3a), which were assigned to the binding energy of C=C, C-C and C=O bonds. A small peak for the C=O bond confirmed 4
the reduction of graphene oxide. The Co 2p spectrum (Fig. 3c) was fitted to two spin-orbit doublets, which are characteristic of Co2+ and Co3+, along with two shake-up satellites (denoted as “Sat.”) [9,12]. The high resolution spectrum for the O 1s region (Fig. 3d) was deconvoluted into three peaks at binding energies (BEs) of 529.8, 530.9 and 532.4 eV, which are denoted as OI, OII and OIII, respectively. The component, OI, was assigned to typical metal-oxygen bonds [13]. The component, OII, is associated with oxygen in the hydroxyl groups on the Co3O4 surface. The OIII component was assigned to a larger number of defect sites with low oxygen coordination [14]. Fig. 4 presents the electrochemical properties of the as-prepared composite modified electrodes. The CV curves of this electrode in the potential window, between -0.1 to 0.5V, at different scan rates (2 to 50 mV s-1) shows the pseudocapacitance caused by the electrochemical reactions. Two pairs of redox reaction peaks were visible in the CV curves, which are responsible for the redox process of Co3O4 (Fig. 4a) as following redox reactions: CoOOH OH CoO2 H2O e , and Co3O4 OH 3CoOOH e respectively [15,16]. The shape of the CV curves showed that the capacitive characteristics of the Co3O4 phase are quite different from those of an electric double-layer capacitor, which would produce a CV curve close to an ideal rectangular shape. In addition, the peak current also increased with increasing scan rate from 2 to 50 mV s-1. Fig. 4c presents the impedance curves of the electrode before and after 5000 cycles. Both plots feature a semicircle in the high frequencies which relates to the charge transfer resistance, and a sloped line in the low frequencies which corresponds to the diffusive resistance. As can be seen from the plots (Fig. 4c), the charge transfer resistance of the RGO/Co3O4 electrode meets well with the solution resistance after 5000 cycles. The more vertical line in the low and high frequency regions indicates the more capacitive behavior of the electrode. The improved electrochemical performance of the RGO/Co3O4 modified electrode was also confirmed by galvanostatic charge/discharge tests performed at different current densities (Fig. 4c). The electrode exhibited good pseudocapacitances of 1010 F g–1 at 5 A g–1, 913 F g–1 at 7.5 A g–1, 900 F g–1 at 15 A g–1, 833 F g–1 at 20 A g–1, and 792 F g–1 at 25 A g–1. The composite electrode showed excellent rate performance, i.e., approximately 78.2% capacitance retention, when the charge/discharge rate changed from 5 A g–1 to 25 A g–1. The Co3O4 nanotubes yielded enhanced capacitance performance with about 75% increase in specific capacitance respectively compared to that of the bare RGO electrode. The capacitances obtained, in this study, are much higher than those of graphene/Co3O4 (443 F g−1 at 5 A g−1) [16], 3D graphene/Co3O4 (522 F g−1 at 0.5 A g−1) [17], RGO/Co3O4 (311 F g−1 at 5 A g−1) [18], Co3O4 nanowires (746 F g-1 at 5mA cm-2) [19], Co3O4 nanowires (599 F g-1 at 2 A g-1) [20]. In 5
addition, the electrode retained 89.2 % of its initial capacity when it was charged and discharged for 5000 cycles at a rate of 15 A g–1, which is comparable to that of graphene/Co3O4 (97.1% after 1000 cycles), 3D graphene/Co3O4 (75% after 1000 cycles) [17], and RGO/Co3O4 (122% after 5000 cycles) [18]. Conclusions Ultrathin Co3O4 nanotube-intercalated graphene composites were synthesized using a facile one-step hydrothermal method in a mixture of ionic liquid and ethanol. The RGO/Co3O4 modified electrode exhibited outstanding electrochemical performance with a very high specific capacitance of 1010 and 792 F g–1 at current densities of 5 and 25 A g–1, respectively, as well as excellent rate capability and improved cycling stability for highperformance electrochemical capacitors. This paper reports a novel electroactive material for the design of the next-generation supercapacitors. Acknowledgment This study was supported by the Priority Research Centers Program through the National Research
Foundation
of
Korea
(NRF)
funded
by the
Ministry of
Education
(2014R1A6A1031189).
References 1. Li WY, Xu LN, Chen J. Adv Funct Mater 2005;15:851-857. 2. Xia XH, Tu JP, Mai YJ, Wang XL, Gu CD, Zhao XB. J Mater Chem 2011;21:9319-9325. 3. Nguyen VH, Tran VC, Kharismadewi D, Shim JJ. Mater Lett 2015;147:123–127. 4. Nguyen VH, Shim JJ. Mater Lett 2015;139:377–381 5. Du N, Zhang H, Chen B, Wu J, Ma X, Liu Z, Zhang Y, Yang D, Huang X, Tu J. Adv Mater 2007; 19:4505–4509. 6. Liu Y, Zhao WW, Zhang XG. Electrochim Acta 2008;8:3296-3304. 6
7. Dong C, Xiao X, Chen G, Guan H, Wang Y. Mater Lett 2014;123:187–190. 8. Varghese B, Hoong TC, Yanwu Z, Reddy MV, Chowdari BVR, Wee ATS, Vincent TBC, Lim CT, Sow CH. Adv Funct Mater 2007;17:1932–1939. 9. Nguyen VH, Shim JJ. Mater Lett 2015;139:377–381. 10. Mao Y, Li W, Liu P, Chen J, Liang E. Mater Lett 2014;134:276–280 11. Nguyen VH, Nguyen TT, Shim JJ. Synt Met 2015;199:276–279. 12. Nguyen VH, Shim JJ. J Power Sources 2015;273:110-117. 13. Marco JF, Gancedo JR, Gracia M. J Solid State Chem 2000;153:74–81. 14. Jimenez VM, Fernandez A, Espinos JP, Gonzalez-Elipe AR. J Electron Spectrosc Relat Phenom 1995;71:61–71. 15. Sumanta KM, Rao GR. J Phys Chem C 2011;115:15646-15654. 16. Huang S, Jin Y, Jia M. Electrochim Acta 2013;95:139– 145. 17. Liu H, Gou X, Wang Y,Du X, Quan C, Qi T. J Nanomaterials, Article ID 874245, in press. 18. Li Q, Hu X, Yang Q, Yan Z, Kang L, Lei Z, Yang Z, Liu Z. Electrochim. Acta 2014;119:184– 191 19. Gao YY, Chen SL, Cao DX, Wang GL, Yin JL. J Power Sources 2010;195:1757-1760. 20. Xia X, Tu J, Mai Y, Wang X, Gu C, Zhao X. J Mater Chem 2011;21:9319–9325.
Figure captions 7
Fig. 1. SEM images of (a) bare Co3O4 NTs and (b) RGO/Co3O4 composite; (c,d) TEM images the RGO/Co3O4 composite at different magnifications; (e) SAED pattern of the RGO/Co3O4 composite. The image in (e) is taken from the region marked with a rectangle in (c); (f) XRD pattern of the RGO/Co3O4 composite (the upper-right inset is the XRD pattern of GO). Fig. 2. (a) HAADF-STEM image of the RGO/Co3O4 composite and EDX mapping of (b) oxygen, (c) cobalt, and (d) carbon. Fig. 3. (a) XPS survey of the RGO/Co3O4 composite and high-resolution XPS spectra of (b) C 1s (c) Co 2p, and (d) O 1s. Fig. 4. (a) CV curves of the RGO/Co3O4 modified electrode at different scan rates in 6M KOH. (b) Nyquist plots of the RGO/Co3O4-modified electrode before and after 5000 charge/discharge cycles; (c) galvanostatic discharge curves and the specific capacitance of the bare RGO and RGO/Co3O4-modified electrodes at different current densities ; (d) average specific capacitance versus the cycle number at a current density of 15 A g-1.
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Research highlights • Ultrathin Co3O4 nanotubes with lengths greater than 3 m were intercalated with graphene sheets. • The electrode exhibits a high specific capacitance of 1010 F g−1 at 5 A g−1. • The electrode presents a high rate capacitance and long-term cycling stability.
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