- Email: [email protected]
Cobalt sulfide nanotube arrays grown on FTO and graphene membranes for high-performance supercapacitor application
G Model
ARTICLE IN PRESS
APSUSC-27988; No. of Pages 6
Applied Surface Science xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Cobalt sulfide nanotube arrays grown on FTO and graphene membranes for high-performance supercapacitor application Hou-Zhao Wan a , Jian-Jun Jiang a,∗ , Jing-Wen Yu a , Yun-Jun Ruan a , Lu Peng a , Li Zhang a , Hai-Chao Chen a , Shao-Wei Bie a,b,∗∗ a
School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China b
a r t i c l e
i n f o
Article history: Received 19 February 2014 Received in revised form 23 May 2014 Accepted 24 May 2014 Available online xxx Keywords: Cobalt sulfide Nanotube array Graphene Membrane Supercapacitor
a b s t r a c t We implement to build uniform cobalt sulfide nanotube arrays on graphene membrane by two-step hydrothermal method. The cobalt sulfide nanotube arrays/graphene membrane (CoSTA/GM) shows excellent electrochemical performance due to the combination of double-layer capacitance of conductive grapheme membrane and superior pseudocapacitance of the cobalt sulfide nanotube arrays. It can be clearly observed that CoSTA/GM exhibits the specific capacitance value of 245 F g−1 at 0.5 A g−1 in 6 M KOH electrolyte. The capacitance only decreases by 10.3% of the initial capacitance after 1000 cycles, demonstrating the excellent electrochemical stability of such electrode material. Therefore, the results demonstrate that the novel CoSTA/GM with enhanced capacitance performance and excellent cycling stability is versatile and may be used for supercapacitors. © 2014 Published by Elsevier B.V.
1. Introduction As energy rapid depletion and environmental deterioration, extensive research has focused on energy storage and conversion from alternative energy sources [1–3]. Recently, supercapacitors have attracted wide interests because they can provide higher power density than storage battery and higher energy density than conventional dielectric capacitors, which make them probably the most important next generation energy storage device [4,5]. Supercapacitors can be categorized as double layer capacitors and pseudocapacitors based on the fundamental mechanisms of capacitance [6–8]. The pseudocapacitors depend on faradic reaction and can put up much higher specific capacitance (Cs ) value than double layer capacitors [9,10]. Recently, transition metal sulfides, such as CoS [11], NiS [12], MoS2 [13], have been perceived as one of the most promising electrode materials of supercapacitors considering both their Cs value and cost effectiveness relative to the high cost of RuO2 .
∗ Corresponding author. Tel.: +86 27 87544472; fax: +86 27 87544472. ∗∗ Corresponding author at: School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China. Tel.: +86 27 87544472; fax: +86 27 87544472. E-mail addresses: [email protected] (J.-J. Jiang), [email protected] (S.-W. Bie).
Cobalt sulfides have been paid more and more attentions due to their many outstanding properties for energy storages [14,15]. For instance, cobalt sulfide nanotubes show a Cs value of 285 F g−1 at 2.5 A g−1 [16]. Amorphous CoSx synthesized by a chemical precipitation demonstrated a high capacitance of 369 F g−1 at a high charge–discharge current density of 50 mA cm−2 [17]. The flower-like cobalt sulfide hierarchitectures show 555 F g−1 at 5 mA cm−2 [18]. Hence, cobalt sulfides can be considered as promising active materials for high-performance supercapacitors. Considering that one dimensional (1D) nanoarrays, such as nanowires, nanobelts and nanotubes, can furnish short transmission route for electron transport, short diffusion pathway for electrolyte adsorption [19–21]. 1D CoS nanotube arrays on conductive substrates would be an ideal and promising candidates for supercapacitors. Graphene is particularly important as it possesses a twodimensional structure consisting of sp2 hybridized carbons with only one atomic thickness and as such also an ideal electrode material which require fast charge storage and transport, excellent mechanical strength, high electrical conductivity [22,23]. More recently, chemically reduced graphene membrane (GM) has attracted much attention, revealing a viable path toward potential applications in the field of energy storage [24]. The Cs value of graphene membrane (or graphene paper) is 138 F g−1 with 3.85% and 83.2 F g−1 with 4.35% capacitance fade in aqueous and organic electrolytes after 2000 cycles, respectively, at a scan rate of 10 mV/s
http://dx.doi.org/10.1016/j.apsusc.2014.05.169 0169-4332/© 2014 Published by Elsevier B.V.
Please cite this article in press as: H.-Z. Wan, et al., Cobalt sulfide nanotube arrays grown on FTO and graphene membranes for highperformance supercapacitor application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.169
G Model APSUSC-27988; No. of Pages 6
ARTICLE IN PRESS H.-Z. Wan et al. / Applied Surface Science xxx (2014) xxx–xxx
2
Fig. 1. XRD pattern of the nanorod precursor and cobalt sulfide nanotube.
[25]. However, the onefold graphene membranes are negative in the wide application due to the limitation of the electric double layer energy storage mechanism. In this work, firstly, we confirmed that cobalt sulfide nanotube arrays on fluorine-doped tin oxide (FTO) have excellent faradaic pseudocapacitance. Secondly, based on the excellent electrochemical performance of cobalt sulfide, we present a simple method to fabricate outstanding performance of cobalt sulfide nanotube arrays on graphene membrane (CoSTA/GM). The novel CoSTA/GM exhibited enhanced capacitance performance, rate capability, and excellent cycling stability. 2. Experimental 2.1. Preparation of cobalt sulfide nanotube arrays on FTO The Co-salt precursor nanowire arrays on FTO were prepared by a simple hydrothermal synthesis method. The solution was prepared by dissolving 10 mmol CoCl2 and 10 mmol urea in 60 mL of distilled water. Then the FTO was immersed into the reaction solution, and then this resulting solution was transferred into 100 mL
Fig. 2. The SEM image of CoSTA/FTO. (b) The TEM image of CoSTA/FTO.
Fig. 3. (a) Schematic structure of the cobalt sulfide nanotube array on FTO. (b) CV curves of cobalt sulfide nanotube arrays measured at different rates. (c) Galvanostatic charge/discharge curves of the cobalt sulfide nanotube array. (d) Cycle life of the cobalt sulfide nanotube arrays at a current density of 1 A g−1 .
Please cite this article in press as: H.-Z. Wan, et al., Cobalt sulfide nanotube arrays grown on FTO and graphene membranes for highperformance supercapacitor application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.169
G Model APSUSC-27988; No. of Pages 6
ARTICLE IN PRESS H.-Z. Wan et al. / Applied Surface Science xxx (2014) xxx–xxx
3
Fig. 4. (a) TEM of graphene. (b) Fabrication process of graphene membrane. (c) Fabrication process of cobalt sulfide nanotube arrays/GM. (d, e), (f, g) and (h, i) are cross-section and surface SEM images of the GM, Co-salt/GM precursor, cobalt sulfide nanotube arrays/GM, respectively.
Teflon-lined stainless steel autoclave at 120 ◦ C for 10 h, and then naturally cooled to room temperature. The samples were collected and rinsed with distilled water several times, after vacuum-drying under 60 ◦ C. The second step was obtaining cobalt sulfide nanotube arrays on FTO, the solution was prepared by dissolving 2.5 mmol Na2 S·9H2 O in 60 mL distilled water. The Co-salt precursor nanowire arrays on FTO were immersed into the reaction solution, and then this resulting solution was transferred into 100 ml Teflon-lined stainless steel autoclave at 180 ◦ C for 8 h, and then naturally cooled to room temperature. The samples were collected and rinsed with distilled water several times, after vacuum-drying under 60 ◦ C.
homogeneous dispersion was mixed with 100 mL deionized water, 2 mL hydrazine solution in a 250 mL round-bottom flask, and then refluxed at 80 ◦ C for 3 h. Finally, the graphene dispersion was filtered, and washed several times with distilled water and alcohol, dried at 60 ◦ C for 12 h in a vacuum oven. To prepare the graphene membranes, 200 mg of graphene power was dispersed in 1 L of DI water with ultrasonication for 1 h, forming homogeneous graphene dispersion. 50 ml of the above graphene dispersion was diluted with 150 mL H2 O. Graphene membranes were prepared by filtration of the diluted dispersion through a Millipore filter membrane (PTFE membrane, 0.45 m in pore size), followed by washing, air drying, and peeling off from the filter.
2.2. Preparation of graphene membranes 2.3. Preparation of CoSTA/GM Graphene oxide was synthesized from natural graphite by a modified Hummers method [26,27]. 100 mg graphene oxide powder was dispersed in DI water with ultrasonication to obtain homogeneous graphene oxide dispersion. The resulting 100 mL
Firstly, Co-salt precursor nanowire arrays on graphene membrane were prepared by a simple hydrothermal synthesis method. The solution was prepared by dissolving 10 mmol CoCl2 and
Please cite this article in press as: H.-Z. Wan, et al., Cobalt sulfide nanotube arrays grown on FTO and graphene membranes for highperformance supercapacitor application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.169
G Model APSUSC-27988; No. of Pages 6
ARTICLE IN PRESS H.-Z. Wan et al. / Applied Surface Science xxx (2014) xxx–xxx
4
(Shanghai Chenhua, China) using a three-electrode configuration with as-prepared electrode as working electrode, Pt foil as counter electrode, and Hg/HgO as reference electrode, all the electrodes were dipped in 6 M KOH aqueous electrolyte. The electrochemical properties of cobalt sulfide nanotube arrays on FTO and CoSTA/GM were investigated using CHI660D. The CoSTA/GM electrode with a diameter of 1.2 cm was used as working electrodes. The load masses of Co sulfide nanotubes on FTO and GM are about 200–300 g. 3. Results and discussion 3.1. Characterization of cobalt sulfide nanotube arrays on FTO
Fig. 5. TEM of cobalt sulfide nanotubes. (a) Low magnification of cobalt sulfide nanotubes and graphene membrane, (b) high magnification and cobalt sulfide nanotube and insert shows the corresponding SAED pattern of the cobalt sulfide nanotube.
10 mmol urea in 60 mL distilled water. The graphene membrane was immersed into the reaction solution. Then this resulting solution was transferred into 100 mL Teflon-lined stainless steel maintained at 120 ◦ C for 10 h and then naturally cooled to room temperature. The samples were collected and rinsed with distilled water several times, after vacuum-drying under 60 ◦ C. Secondly, cobalt sulfide nanotube arrays on graphene membranes were obtained. In a typical procedure, the solution was prepared by dissolving 2.5 mmol Na2 S·9H2 O in 60 mL of distilled water. The Co-salt precursor nanowire arrays/graphene membranes were immersed into the solution. And then this resulting solution was transferred into 100 mL Teflon-lined stainless steel autoclave at 180 ◦ C for 8 h and then natural cooled to room temperature. The CoSTA/GM was collected and rinsed with distilled water several times, after vacuum-drying under 60 ◦ C.
The Co-salt precursor nanowire arrays on FTO were prepared by a simple hydrothermal synthesis method at 120 ◦ C for 10 h. The cobalt sulfide nanotube arrays on FTO were obtained at 180 ◦ C for 8 h. Fig. 1 displays a typical XRD pattern of the nanorod precursor and cobalt sulfide nanotube. Fig. 1a shows a typical XRD pattern of the nanorod precursor. All the reflection peaks in this pattern could be readily indexed to crystalline Co(CO3 )0.35 Cl0.20 (OH)0.11 (JCPDS Card file No. 38-547), without any obvious impurity peaks. Fig. 1b shows a typical XRD pattern of the cobalt sulfide nanotube. All of the reflection peaks in this pattern can be readily indexed to Co9 S8 (JCPDS card no. 65-1765). The SEM and TEM images of the CoSTA/FTO have been added. As shown in Fig. 2, the cobalt sulfide nanotubes are vertically aligned and in uniform diameters, lengths, and densities. The inset of Fig. 2a is a high-magnitude image which shows the details of the resulting nanotube morphology. To further understand the tube-nanostructure, the cobalt sulfide nanotubes are investigated by TEM. As shown in Fig. 2b, the relatively high transparency in the center of tubular illustrates its hollow structure. The cobalt sulfide nanotube arrays are considered as excellent electrode materials of supercapacitors due to their high specific area and ordered nanostructure. The electrochemical properties of the cobalt sulfide nanotube arrays on FTO are shown in Fig. 3. Fig. 3a shows the schematic structure of the cobalt sulfide nanotube arrays on FTO (the inset of Fig. 3a is the SEM image of cobalt sulfide nanotube arrays). Fig. 3b is the CV curves of nanotube arrays with a potential ranging from −0.1 V to 0.6 V (vs Hg/HgO) at different scan rates in 6 M KOH. The CV curves exhibit two pairs of distinct redox peaks. According to the literature, two possible reactions are put forward as the following reactions [28]: CoS + OH− ↔ CoSOH + e−
(1)
CoSOH + OH− ↔ CoSO + H2 O + e−
(2)
Chronopotentiometry charge–discharge tests were then carried out to evaluate the capacitance of cobalt sulfide nanotube arrays at different current densities using a potential window of −0.1 to 0.5 V (vs. Hg/HgO). Cobalt sulfide nanotube arrays show typical capacitance characteristics as displayed in the charge–discharge curves of Fig. 3c. The Cs value is calculated according to the following equation [29]: I×t m × V
2.4. Characterization of samples
C=
X-ray diffraction (XRD) (Philips X’Pert PRO; Cu K␣, = 0.1542 nm), FEI Quanta 200 scanning electron microscope (ESEM), field-emission scanning electron microscopy (FESEM; Sirion 200, Philips-FEI Co., Holland) and transmission electron microscopy (TEM, Philips, TecnaiG2 20) were employed to character the phase and morphology of the samples. Cyclic voltammetry (CV), chronopotentiometry studies were performed on a CHI660D electrochemical workstation
where C, I and t are Cs value, charge–discharge current and time, respectively, V is the potential window, and m is the mass of cobalt sulfide nanotube arrays on FTO. The capacitor properties are tested by galvanostatic charge–discharge at different current densities. The cobalt sulfide nanotube arrays exhibit Cs values with 660 F g−1 at 0.5 A g−1 , 545 F g−1 at 1 A g−1 , 450 F g−1 at 2 A g−1 , 375 F g−1 at 4 A g−1 , 315 F g−1 at 8 A g−1 , and 295 F g−1 at 10 A g−1 , respectively. To evaluate the stability of the cobalt sulfide
Please cite this article in press as: H.-Z. Wan, et al., Cobalt sulfide nanotube arrays grown on FTO and graphene membranes for highperformance supercapacitor application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.169
G Model APSUSC-27988; No. of Pages 6
ARTICLE IN PRESS H.-Z. Wan et al. / Applied Surface Science xxx (2014) xxx–xxx
5
Fig. 6. The electrochemical performance of the CoSTA/GM electrode measured in 6 M KOH with voltage window from −0.9 to 0.5 V. (a) CV curves at different scan rates. (b) Galvanostatic charge–discharge curves at different current densities. (c) Plot of Cs value for electrodes under different current densities. (d) Cyclic stability of CoSTA/GM electrode measured at 1 A g−1 .
nanotube arrays, the Cs value with respect to the cycle numbers at 1 A g−1 between −0.1 and 0.5 V are measured and the corresponding results are shown in Fig. 3d. The cobalt sulfide nanotube arrays are found to sustain and deliver 91% of the highest value after 1000 cycles at 1 A g−1 . The excellent capacitive property of the cobalt sulfide nanotube arrays can be attributed to its special nanostructure, which help to expand the effective contact area between electrode and electrolyte. 3.2. Characterization of cobalt sulfide nanotube arrays/graphene membranes Through the above analysis, the cobalt sulfide nanotube array emerges excellent Pseudocapacitance performance which emerges on FTO. Combined with its superior performance, the CoSTA/GM was fabricated. The fabrication process of CoSTA/GM is described in Fig. 4. The fabrication of CoSTA/GM approximately includes three steps as illustrated in Fig. 4b and c: (1) directional flow guided formation of GM by vacuum filtration of graphene (TEM image as shown in Fig. 4a) dispersion using a filter membrane, as illustrated in Fig. 4b. A GM was obtained and the surface of black membrane was smooth as indicated in Fig. 4c. (2) The well-established texture structure of the Co-salt precursor nanowire arrays grew on the GM. The Co-salt precursor also has been investigated by different applications [30]. The final product still keeps the well-ordered arrangement structure on the GM, Fig. 4b shows the pink Co-salt nanowire arrays grew on the black GM. (3) Formation of CoSTA/GM from Co-salt/GM nanowires is based on the Kirkendall effect by using a simple hydrothermal [31], and the pink Co-salt/Gm transformed into black CoSTA/GM (Fig. 4c). The product picture of GM, Co-salt/GM and CoSTA/GM are inserted in Fig. 4c. The GM has a layered structure as shown in cross-section and surface SEM images (Fig. 4d and e), which is probably assembled by the fast dispersive graphene sheets during vacuum filtration. Fig. 4d presents a typical GM with smooth surface and a uniform thickness
of about 9.3 m. The thickness of the graphene membrane can be easily controlled by adjusting the volume or the concentration of the original graphene suspension. The well-hierarchical structure of the as-synthesized nanowire arrays on GM is also confirmed by SEM (Fig. 4f–i). However, it can be obviously seen that plenty of Co-salt nanowires, were uniformly spread around on the surface of the GM (Fig. 4f and g). Nevertheless, the presence of these Co-salt nanowires does not affect the overall well-hierarchical structure of the hybrid membrane on the whole. Moreover, the fracture edges of Co-salt GM also displayed a well-ordered layered structure through the cross section (Fig. 4g). The inset of Fig. 4g is a high-magnitude image which shows the details of the resulting nanowire morphology. The tip shapes of the Co-salt nanowires showed configuration with about 200 nm in diameter, and the stem was about 10 m in length. As we can see in Fig. 4h and i, SEM analyses of the top reveal that the surface of CoSTA/GM is similar to Co-salt/GM precursor with excellent orderliness. To investigate the morphologies and the array densities of the tapered cobalt sulfide nanotubes, the inset of Fig. 4i is a high-magnitude image which shows the details of the resulting nanotube morphology. The cobalt sulfide nanotubes are vertically aligned and in uniform diameters, lengths, and densities. The tip shapes of the cobalt sulfide nanotubes have about 200 nm in diameter, and the stem about 10 m in length. To further understand the tube-nanostructure, the cobalt sulfide nanotubes are investigated by TEM and SAED pattern. As shown in Fig. 5a and b, the relatively high transparency in the center of tubular illustrates its hollow structure. Based on the above analysis, The CoSTA/GM can provide good electronic conductivity, large electrochemical active contact area and fast ion diffusion rate in order to become a promising candidate for supercapacitors. Fig. 6a exhibits the cyclic CV curves of CoSTA/GM between −0.9 and 0.5 V (vs. Hg/HgO) at different scan rates. The insert CV curve of CoSTA/GM shows a typical rectangular shape, implying pure electrical double-layer capacitor behavior (from −0.9 to 0 V) and faradaic pseudocapacitance (from −0.1 to
Please cite this article in press as: H.-Z. Wan, et al., Cobalt sulfide nanotube arrays grown on FTO and graphene membranes for highperformance supercapacitor application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.169
G Model APSUSC-27988; No. of Pages 6
ARTICLE IN PRESS H.-Z. Wan et al. / Applied Surface Science xxx (2014) xxx–xxx
6
sustain and deliver 91% of the highest Cs value after 1000 cycles at 1 A g−1 . In addition, the CoSTA/GM was fabricated by a simple two-step hydrothermal process. It can be clearly observed that CoSTA/GM exhibits the Cs value of 245 F g−1 at 0.5 A g−1 in 6 M KOH aqueous electrolyte. The capacitance only decreases by 10.3% of the initial capacitance after 1000 cycles, demonstrating the excellent electrochemical stability of such electrode material. The novel CoSTA/GM exhibited high capacitance performance, good rate capability and excellent cycling stability. Acknowledgments
Fig. 7. The energy density of the CoSTA/FTO and CoSTA/GM at different current densities.
0.5 V). The galvanostatic charge/discharge curves of CoSTA/GM are also shown in Fig. 6b. It can be clearly observed that CoSTA/GM exhibits the high Cs value of 245 F g−1 at 0.5 A g−1 . When the discharge current density is 1, 2, 5 and 10 A g−1 , the Cs values of the CoSTA/GM can be calculated from the discharge curves to be 195, 154, 140, and 135 F g−1 , respectively, and the results are shown in Fig. 6b and c. The stability of electrode is a one of crucial parameter for practical applications of supercapacitor which is combined with coupled with electrical double-layer capacitance [32], such as the CoSTA/GM. The reaction mechanism of the CoSTA/GM electrode is inserted in Fig. 6d. To investigate the stability of the as-prepared CoSTA/GM electrode, galvanostatic charge–discharge cycling test was between −1.0 and 0.5 V at 1 A g−1 for 1000 cycles, as shown in Fig. 6d. The coulombic efficiency of the CoSTA/GM is slightly increases from 84% to about 98% after the 21st cycle. The Cs value of CoSTA/GM electrode only decreases by 10.3% of the initial value after 1000 cycles, exhibiting the excellent electrochemical stability. The energy density curves of the CoSTA/FTO and CoSTA/GM at different current densities are shown in Fig. 7. Specific energy was calculated, by taking the total masses into account, by the formula Es = (1/2)C(V)2 /m, where Es is the energy density, C is the specific capacitance, V is the potential window of discharge. The energy density of the CoSTA/FTO that decreased from 22.67 W h kg−1 to 9.7 W h kg−1 from 0.5 to 10 A g−1 . The energy density of the CoSTA/GM that decreased from 76.6 W h kg−1 to 40.9 W h kg−1 from 0.5 to 10 A g−1 . The energy density of the CoSTA/GM is obviously higher than that of CoSTA/FTO, the former has a wider potential window. The outstanding capacitor performances of CoSTA/GM may be attributed to its well-ordered structure. Firstly, GM can provide not only electric double layer capacitances, but also shorten the electrolyte ions diffusion distance due to its layer-by-layer stacking structure [33,34]. Secondly, the well interfacial contact between cobalt sulfide nanotubes and graphene is great benefit to highefficiency electrolyte ion diffusion and fast electron transportation throughout the whole kinetics of electrode process [35]. Thirdly, peculiar to cobalt sulfide nanotubes well-orderliness around on the surface of graphene membranes will enhance the pseudocapacitance properties. Therefore, it can be concluded that these quite orderly CoSTA/GM exhibit great potential as electrode material for supercapacitors. 4. Conclusion We prepared cobalt sulfide nanotube arrays on FTO with excellent electrochemical performance. The Cs value of the cobalt sulfide nanotube arrays can achieve 660 F g−1 at 0.5 A g−1 and is found to
This work was supported by the National Natural Science Foundation of China (Grant 51302097) and Wuhan Planning Project of Science and Technology (No. 2013011801010594). All electron microscopy were carried out in Analytical and Testing Center, Huazhong University of Science and Technology. References [1] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845–854. [2] X.H. Xia, J.P. Tu, Y.Q. Zhang, Y.J. Mai, X.L. Wang, C.D. Gu, X.B. Zhao, J. Phys. Chem. C 115 (2011) 22662–22668. [3] A. Wang, H. Wang, S. Zhang, C. Mao, J. Song, H. Niu, B. Jin, Y. Tian, Appl. Surf. Sci. 282 (2013) 704–708. [4] G. Zhang, X.W. David Lou, Sci. Rep. 3 (2013) 1470. [5] L.Q. Mai, F. Yang, Y.L. Zhao, X. Xu, L. Xu, Y.Z. Luo, Nat. Commun. 2 (2011) 381. [6] P. Justin, G. Ranga Rao, Int. J. Hydrogen Energy 35 (2010) 9709–9715. [7] G. Yu, L. Hu, M. Vosgueritchian, H. Wang, X. Xie, J.R. McDonough, X. Cui, Y. Cui, Z. Bao, Nano Lett. 11 (2011) 2905–2911. [8] R. Väli, A. Laheäär, A. Jänes, E. Lust, Electrochim. Acta 121 (2014) 294–300. [9] H. Wan, J. Jiang, J. Yu, K. Xu, L. Miao, L. Zhang, H. Chen, Y. Ruan, CrystEngCommun. 15 (2013) 7649–7651. [10] D. Ghosh, S. Giri, A. Mandal, C.K. Das, Appl. Surf. Sci. 276 (2013) 120–128. [11] J.-Y. Lin, S.-W. Chou, RSC Adv. 3 (2013) 2043. [12] J. Yang, X. Duan, Q. Qin, W. Zheng, J. Mater. Chem. A 1 (2013) 7880. [13] L. Cao, S. Yang, W. Gao, Z. Liu, Y. Gong, L. Ma, G. Shi, S. Lei, Y. Zhang, S. Zhang, R. Vajtai, P.M. Ajayan, Small 9 (2013) 2905–2910. [14] L. Zhang, H.B. Wu, X.W. Lou, Chem. Commun. 48 (2012) 6912–6914. [15] F. Luo, J. Li, H. Yuan, D. Xiao, Electrochim. Acta 123 (2014) 183–189. [16] H. Wan, X. Ji, J. Jiang, J. Yu, L. Miao, L. Zhang, S. Bie, H. Chen, Y. Ruan, J. Power Sources 243 (2013) 396–402. [17] F. Tao, Y.-Q. Zhao, G.-Q. Zhang, H.-L. Li, Electrochem. Commun. 9 (2007) 1282–1287. [18] Q. Wang, L. Jiao, H. Du, J. Yang, Q. Huan, W. Peng, Y. Si, Y. Wang, H. Yuan, CrystEngComm 13 (2011) 6960. [19] C. Shang, S. Dong, S. Wang, D. Xiao, P. Han, X. Wang, L. Gu, G. Cui, ACS Nano 7 (2013) 5430–5436. [20] G.Q. Zhang, H.B. Wu, H.E. Hoster, M.B. Chan-Park, X.W. Lou, Energy Environ. Sci. 5 (2012) 9453. [21] W. Mei, J. Huang, L. Zhu, Z. Ye, Y. Mai, J. Tu, J. Mater. Chem. 22 (2012) 9315. [22] X. Xia, J. Tu, Y. Mai, R. Chen, X. Wang, C. Gu, X. Zhao, Chemistry 17 (2011) 10898–10905. [23] Y. Jin, M. Jia, M. Zhang, Q. Wen, Appl. Surf. Sci. 264 (2013) 787–793. [24] Z. Weng, Y. Su, D.-W. Wang, F. Li, J. Du, H.-M. Cheng, Adv. Energy Mater. 1 (2011) 917–922. [25] G. Wang, X. Sun, F. Lu, H. Sun, M. Yu, W. Jiang, C. Liu, J. Lian, Small 8 (2012) 452–459. [26] M.C. Hsiao, S.H. Liao, M.Y. Yen, P.I. Liu, N.W. Pu, C.A. Wang, C.C. Ma, ACS Appl. Mater. Interfaces 2 (2010) 3092–3099. [27] L. Peng, Y. Feng, P. Lv, D. Lei, Y. Shen, Y. Li, W. Feng, J. Phys. Chem. C 116 (2012) 4970–4978. [28] C.-Y. Chen, Z.-Y. Shih, Z. Yang, H.-T. Chang, J. Power Sources 215 (2012) 43– 47. [29] Z. Huang, H. Zhang, Y. Chen, W. Wang, Y. Chen, Y. Zhong, Electrochim. Acta 108 (2013) 421–428. [30] S. Xiong, J.S. Chen, X.W. Lou, H.C. Zeng, Adv. Funct. Mater. 22 (2012) 861– 871. [31] A. Lahiri, A. Willert, S.Z.E. Abedin, F. Endres, Electrochim. Acta 121 (2014) 154–158. [32] C. Zhao, X. Wang, S. Wang, Y. Wang, Y. Zhao, W. Zheng, Int. J. Hyd. Energy 37 (2012) 11846–11852. [33] D.-W. Wang, F. Li, J. Zhao, W. Ren, Z.-G. Chen, J. Tan, Z.-S. Wu, I. Gentle, G.Q. Lu, H.-M. Cheng, ACS Nano 3 (2009) 1745–1752. [34] L.L. Zhang, X. Zhao, M.D. Stoller, Y. Zhu, H. Ji, S. Murali, Y. Wu, S. Perales, B. Clevenger, R.S. Ruoff, Nano Lett 12 (2012) 1806–1812. [35] J. Yan, Z. Fan, T. Wei, W. Qian, M. Zhang, F. Wei, Carbon 48 (2010) 3825–3833.
Please cite this article in press as: H.-Z. Wan, et al., Cobalt sulfide nanotube arrays grown on FTO and graphene membranes for highperformance supercapacitor application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.169
ARTICLE IN PRESS
APSUSC-27988; No. of Pages 6
Applied Surface Science xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Cobalt sulfide nanotube arrays grown on FTO and graphene membranes for high-performance supercapacitor application Hou-Zhao Wan a , Jian-Jun Jiang a,∗ , Jing-Wen Yu a , Yun-Jun Ruan a , Lu Peng a , Li Zhang a , Hai-Chao Chen a , Shao-Wei Bie a,b,∗∗ a
School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China b
a r t i c l e
i n f o
Article history: Received 19 February 2014 Received in revised form 23 May 2014 Accepted 24 May 2014 Available online xxx Keywords: Cobalt sulfide Nanotube array Graphene Membrane Supercapacitor
a b s t r a c t We implement to build uniform cobalt sulfide nanotube arrays on graphene membrane by two-step hydrothermal method. The cobalt sulfide nanotube arrays/graphene membrane (CoSTA/GM) shows excellent electrochemical performance due to the combination of double-layer capacitance of conductive grapheme membrane and superior pseudocapacitance of the cobalt sulfide nanotube arrays. It can be clearly observed that CoSTA/GM exhibits the specific capacitance value of 245 F g−1 at 0.5 A g−1 in 6 M KOH electrolyte. The capacitance only decreases by 10.3% of the initial capacitance after 1000 cycles, demonstrating the excellent electrochemical stability of such electrode material. Therefore, the results demonstrate that the novel CoSTA/GM with enhanced capacitance performance and excellent cycling stability is versatile and may be used for supercapacitors. © 2014 Published by Elsevier B.V.
1. Introduction As energy rapid depletion and environmental deterioration, extensive research has focused on energy storage and conversion from alternative energy sources [1–3]. Recently, supercapacitors have attracted wide interests because they can provide higher power density than storage battery and higher energy density than conventional dielectric capacitors, which make them probably the most important next generation energy storage device [4,5]. Supercapacitors can be categorized as double layer capacitors and pseudocapacitors based on the fundamental mechanisms of capacitance [6–8]. The pseudocapacitors depend on faradic reaction and can put up much higher specific capacitance (Cs ) value than double layer capacitors [9,10]. Recently, transition metal sulfides, such as CoS [11], NiS [12], MoS2 [13], have been perceived as one of the most promising electrode materials of supercapacitors considering both their Cs value and cost effectiveness relative to the high cost of RuO2 .
∗ Corresponding author. Tel.: +86 27 87544472; fax: +86 27 87544472. ∗∗ Corresponding author at: School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China. Tel.: +86 27 87544472; fax: +86 27 87544472. E-mail addresses: [email protected] (J.-J. Jiang), [email protected] (S.-W. Bie).
Cobalt sulfides have been paid more and more attentions due to their many outstanding properties for energy storages [14,15]. For instance, cobalt sulfide nanotubes show a Cs value of 285 F g−1 at 2.5 A g−1 [16]. Amorphous CoSx synthesized by a chemical precipitation demonstrated a high capacitance of 369 F g−1 at a high charge–discharge current density of 50 mA cm−2 [17]. The flower-like cobalt sulfide hierarchitectures show 555 F g−1 at 5 mA cm−2 [18]. Hence, cobalt sulfides can be considered as promising active materials for high-performance supercapacitors. Considering that one dimensional (1D) nanoarrays, such as nanowires, nanobelts and nanotubes, can furnish short transmission route for electron transport, short diffusion pathway for electrolyte adsorption [19–21]. 1D CoS nanotube arrays on conductive substrates would be an ideal and promising candidates for supercapacitors. Graphene is particularly important as it possesses a twodimensional structure consisting of sp2 hybridized carbons with only one atomic thickness and as such also an ideal electrode material which require fast charge storage and transport, excellent mechanical strength, high electrical conductivity [22,23]. More recently, chemically reduced graphene membrane (GM) has attracted much attention, revealing a viable path toward potential applications in the field of energy storage [24]. The Cs value of graphene membrane (or graphene paper) is 138 F g−1 with 3.85% and 83.2 F g−1 with 4.35% capacitance fade in aqueous and organic electrolytes after 2000 cycles, respectively, at a scan rate of 10 mV/s
http://dx.doi.org/10.1016/j.apsusc.2014.05.169 0169-4332/© 2014 Published by Elsevier B.V.
Please cite this article in press as: H.-Z. Wan, et al., Cobalt sulfide nanotube arrays grown on FTO and graphene membranes for highperformance supercapacitor application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.169
G Model APSUSC-27988; No. of Pages 6
ARTICLE IN PRESS H.-Z. Wan et al. / Applied Surface Science xxx (2014) xxx–xxx
2
Fig. 1. XRD pattern of the nanorod precursor and cobalt sulfide nanotube.
[25]. However, the onefold graphene membranes are negative in the wide application due to the limitation of the electric double layer energy storage mechanism. In this work, firstly, we confirmed that cobalt sulfide nanotube arrays on fluorine-doped tin oxide (FTO) have excellent faradaic pseudocapacitance. Secondly, based on the excellent electrochemical performance of cobalt sulfide, we present a simple method to fabricate outstanding performance of cobalt sulfide nanotube arrays on graphene membrane (CoSTA/GM). The novel CoSTA/GM exhibited enhanced capacitance performance, rate capability, and excellent cycling stability. 2. Experimental 2.1. Preparation of cobalt sulfide nanotube arrays on FTO The Co-salt precursor nanowire arrays on FTO were prepared by a simple hydrothermal synthesis method. The solution was prepared by dissolving 10 mmol CoCl2 and 10 mmol urea in 60 mL of distilled water. Then the FTO was immersed into the reaction solution, and then this resulting solution was transferred into 100 mL
Fig. 2. The SEM image of CoSTA/FTO. (b) The TEM image of CoSTA/FTO.
Fig. 3. (a) Schematic structure of the cobalt sulfide nanotube array on FTO. (b) CV curves of cobalt sulfide nanotube arrays measured at different rates. (c) Galvanostatic charge/discharge curves of the cobalt sulfide nanotube array. (d) Cycle life of the cobalt sulfide nanotube arrays at a current density of 1 A g−1 .
Please cite this article in press as: H.-Z. Wan, et al., Cobalt sulfide nanotube arrays grown on FTO and graphene membranes for highperformance supercapacitor application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.169
G Model APSUSC-27988; No. of Pages 6
ARTICLE IN PRESS H.-Z. Wan et al. / Applied Surface Science xxx (2014) xxx–xxx
3
Fig. 4. (a) TEM of graphene. (b) Fabrication process of graphene membrane. (c) Fabrication process of cobalt sulfide nanotube arrays/GM. (d, e), (f, g) and (h, i) are cross-section and surface SEM images of the GM, Co-salt/GM precursor, cobalt sulfide nanotube arrays/GM, respectively.
Teflon-lined stainless steel autoclave at 120 ◦ C for 10 h, and then naturally cooled to room temperature. The samples were collected and rinsed with distilled water several times, after vacuum-drying under 60 ◦ C. The second step was obtaining cobalt sulfide nanotube arrays on FTO, the solution was prepared by dissolving 2.5 mmol Na2 S·9H2 O in 60 mL distilled water. The Co-salt precursor nanowire arrays on FTO were immersed into the reaction solution, and then this resulting solution was transferred into 100 ml Teflon-lined stainless steel autoclave at 180 ◦ C for 8 h, and then naturally cooled to room temperature. The samples were collected and rinsed with distilled water several times, after vacuum-drying under 60 ◦ C.
homogeneous dispersion was mixed with 100 mL deionized water, 2 mL hydrazine solution in a 250 mL round-bottom flask, and then refluxed at 80 ◦ C for 3 h. Finally, the graphene dispersion was filtered, and washed several times with distilled water and alcohol, dried at 60 ◦ C for 12 h in a vacuum oven. To prepare the graphene membranes, 200 mg of graphene power was dispersed in 1 L of DI water with ultrasonication for 1 h, forming homogeneous graphene dispersion. 50 ml of the above graphene dispersion was diluted with 150 mL H2 O. Graphene membranes were prepared by filtration of the diluted dispersion through a Millipore filter membrane (PTFE membrane, 0.45 m in pore size), followed by washing, air drying, and peeling off from the filter.
2.2. Preparation of graphene membranes 2.3. Preparation of CoSTA/GM Graphene oxide was synthesized from natural graphite by a modified Hummers method [26,27]. 100 mg graphene oxide powder was dispersed in DI water with ultrasonication to obtain homogeneous graphene oxide dispersion. The resulting 100 mL
Firstly, Co-salt precursor nanowire arrays on graphene membrane were prepared by a simple hydrothermal synthesis method. The solution was prepared by dissolving 10 mmol CoCl2 and
Please cite this article in press as: H.-Z. Wan, et al., Cobalt sulfide nanotube arrays grown on FTO and graphene membranes for highperformance supercapacitor application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.169
G Model APSUSC-27988; No. of Pages 6
ARTICLE IN PRESS H.-Z. Wan et al. / Applied Surface Science xxx (2014) xxx–xxx
4
(Shanghai Chenhua, China) using a three-electrode configuration with as-prepared electrode as working electrode, Pt foil as counter electrode, and Hg/HgO as reference electrode, all the electrodes were dipped in 6 M KOH aqueous electrolyte. The electrochemical properties of cobalt sulfide nanotube arrays on FTO and CoSTA/GM were investigated using CHI660D. The CoSTA/GM electrode with a diameter of 1.2 cm was used as working electrodes. The load masses of Co sulfide nanotubes on FTO and GM are about 200–300 g. 3. Results and discussion 3.1. Characterization of cobalt sulfide nanotube arrays on FTO
Fig. 5. TEM of cobalt sulfide nanotubes. (a) Low magnification of cobalt sulfide nanotubes and graphene membrane, (b) high magnification and cobalt sulfide nanotube and insert shows the corresponding SAED pattern of the cobalt sulfide nanotube.
10 mmol urea in 60 mL distilled water. The graphene membrane was immersed into the reaction solution. Then this resulting solution was transferred into 100 mL Teflon-lined stainless steel maintained at 120 ◦ C for 10 h and then naturally cooled to room temperature. The samples were collected and rinsed with distilled water several times, after vacuum-drying under 60 ◦ C. Secondly, cobalt sulfide nanotube arrays on graphene membranes were obtained. In a typical procedure, the solution was prepared by dissolving 2.5 mmol Na2 S·9H2 O in 60 mL of distilled water. The Co-salt precursor nanowire arrays/graphene membranes were immersed into the solution. And then this resulting solution was transferred into 100 mL Teflon-lined stainless steel autoclave at 180 ◦ C for 8 h and then natural cooled to room temperature. The CoSTA/GM was collected and rinsed with distilled water several times, after vacuum-drying under 60 ◦ C.
The Co-salt precursor nanowire arrays on FTO were prepared by a simple hydrothermal synthesis method at 120 ◦ C for 10 h. The cobalt sulfide nanotube arrays on FTO were obtained at 180 ◦ C for 8 h. Fig. 1 displays a typical XRD pattern of the nanorod precursor and cobalt sulfide nanotube. Fig. 1a shows a typical XRD pattern of the nanorod precursor. All the reflection peaks in this pattern could be readily indexed to crystalline Co(CO3 )0.35 Cl0.20 (OH)0.11 (JCPDS Card file No. 38-547), without any obvious impurity peaks. Fig. 1b shows a typical XRD pattern of the cobalt sulfide nanotube. All of the reflection peaks in this pattern can be readily indexed to Co9 S8 (JCPDS card no. 65-1765). The SEM and TEM images of the CoSTA/FTO have been added. As shown in Fig. 2, the cobalt sulfide nanotubes are vertically aligned and in uniform diameters, lengths, and densities. The inset of Fig. 2a is a high-magnitude image which shows the details of the resulting nanotube morphology. To further understand the tube-nanostructure, the cobalt sulfide nanotubes are investigated by TEM. As shown in Fig. 2b, the relatively high transparency in the center of tubular illustrates its hollow structure. The cobalt sulfide nanotube arrays are considered as excellent electrode materials of supercapacitors due to their high specific area and ordered nanostructure. The electrochemical properties of the cobalt sulfide nanotube arrays on FTO are shown in Fig. 3. Fig. 3a shows the schematic structure of the cobalt sulfide nanotube arrays on FTO (the inset of Fig. 3a is the SEM image of cobalt sulfide nanotube arrays). Fig. 3b is the CV curves of nanotube arrays with a potential ranging from −0.1 V to 0.6 V (vs Hg/HgO) at different scan rates in 6 M KOH. The CV curves exhibit two pairs of distinct redox peaks. According to the literature, two possible reactions are put forward as the following reactions [28]: CoS + OH− ↔ CoSOH + e−
(1)
CoSOH + OH− ↔ CoSO + H2 O + e−
(2)
Chronopotentiometry charge–discharge tests were then carried out to evaluate the capacitance of cobalt sulfide nanotube arrays at different current densities using a potential window of −0.1 to 0.5 V (vs. Hg/HgO). Cobalt sulfide nanotube arrays show typical capacitance characteristics as displayed in the charge–discharge curves of Fig. 3c. The Cs value is calculated according to the following equation [29]: I×t m × V
2.4. Characterization of samples
C=
X-ray diffraction (XRD) (Philips X’Pert PRO; Cu K␣, = 0.1542 nm), FEI Quanta 200 scanning electron microscope (ESEM), field-emission scanning electron microscopy (FESEM; Sirion 200, Philips-FEI Co., Holland) and transmission electron microscopy (TEM, Philips, TecnaiG2 20) were employed to character the phase and morphology of the samples. Cyclic voltammetry (CV), chronopotentiometry studies were performed on a CHI660D electrochemical workstation
where C, I and t are Cs value, charge–discharge current and time, respectively, V is the potential window, and m is the mass of cobalt sulfide nanotube arrays on FTO. The capacitor properties are tested by galvanostatic charge–discharge at different current densities. The cobalt sulfide nanotube arrays exhibit Cs values with 660 F g−1 at 0.5 A g−1 , 545 F g−1 at 1 A g−1 , 450 F g−1 at 2 A g−1 , 375 F g−1 at 4 A g−1 , 315 F g−1 at 8 A g−1 , and 295 F g−1 at 10 A g−1 , respectively. To evaluate the stability of the cobalt sulfide
Please cite this article in press as: H.-Z. Wan, et al., Cobalt sulfide nanotube arrays grown on FTO and graphene membranes for highperformance supercapacitor application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.169
G Model APSUSC-27988; No. of Pages 6
ARTICLE IN PRESS H.-Z. Wan et al. / Applied Surface Science xxx (2014) xxx–xxx
5
Fig. 6. The electrochemical performance of the CoSTA/GM electrode measured in 6 M KOH with voltage window from −0.9 to 0.5 V. (a) CV curves at different scan rates. (b) Galvanostatic charge–discharge curves at different current densities. (c) Plot of Cs value for electrodes under different current densities. (d) Cyclic stability of CoSTA/GM electrode measured at 1 A g−1 .
nanotube arrays, the Cs value with respect to the cycle numbers at 1 A g−1 between −0.1 and 0.5 V are measured and the corresponding results are shown in Fig. 3d. The cobalt sulfide nanotube arrays are found to sustain and deliver 91% of the highest value after 1000 cycles at 1 A g−1 . The excellent capacitive property of the cobalt sulfide nanotube arrays can be attributed to its special nanostructure, which help to expand the effective contact area between electrode and electrolyte. 3.2. Characterization of cobalt sulfide nanotube arrays/graphene membranes Through the above analysis, the cobalt sulfide nanotube array emerges excellent Pseudocapacitance performance which emerges on FTO. Combined with its superior performance, the CoSTA/GM was fabricated. The fabrication process of CoSTA/GM is described in Fig. 4. The fabrication of CoSTA/GM approximately includes three steps as illustrated in Fig. 4b and c: (1) directional flow guided formation of GM by vacuum filtration of graphene (TEM image as shown in Fig. 4a) dispersion using a filter membrane, as illustrated in Fig. 4b. A GM was obtained and the surface of black membrane was smooth as indicated in Fig. 4c. (2) The well-established texture structure of the Co-salt precursor nanowire arrays grew on the GM. The Co-salt precursor also has been investigated by different applications [30]. The final product still keeps the well-ordered arrangement structure on the GM, Fig. 4b shows the pink Co-salt nanowire arrays grew on the black GM. (3) Formation of CoSTA/GM from Co-salt/GM nanowires is based on the Kirkendall effect by using a simple hydrothermal [31], and the pink Co-salt/Gm transformed into black CoSTA/GM (Fig. 4c). The product picture of GM, Co-salt/GM and CoSTA/GM are inserted in Fig. 4c. The GM has a layered structure as shown in cross-section and surface SEM images (Fig. 4d and e), which is probably assembled by the fast dispersive graphene sheets during vacuum filtration. Fig. 4d presents a typical GM with smooth surface and a uniform thickness
of about 9.3 m. The thickness of the graphene membrane can be easily controlled by adjusting the volume or the concentration of the original graphene suspension. The well-hierarchical structure of the as-synthesized nanowire arrays on GM is also confirmed by SEM (Fig. 4f–i). However, it can be obviously seen that plenty of Co-salt nanowires, were uniformly spread around on the surface of the GM (Fig. 4f and g). Nevertheless, the presence of these Co-salt nanowires does not affect the overall well-hierarchical structure of the hybrid membrane on the whole. Moreover, the fracture edges of Co-salt GM also displayed a well-ordered layered structure through the cross section (Fig. 4g). The inset of Fig. 4g is a high-magnitude image which shows the details of the resulting nanowire morphology. The tip shapes of the Co-salt nanowires showed configuration with about 200 nm in diameter, and the stem was about 10 m in length. As we can see in Fig. 4h and i, SEM analyses of the top reveal that the surface of CoSTA/GM is similar to Co-salt/GM precursor with excellent orderliness. To investigate the morphologies and the array densities of the tapered cobalt sulfide nanotubes, the inset of Fig. 4i is a high-magnitude image which shows the details of the resulting nanotube morphology. The cobalt sulfide nanotubes are vertically aligned and in uniform diameters, lengths, and densities. The tip shapes of the cobalt sulfide nanotubes have about 200 nm in diameter, and the stem about 10 m in length. To further understand the tube-nanostructure, the cobalt sulfide nanotubes are investigated by TEM and SAED pattern. As shown in Fig. 5a and b, the relatively high transparency in the center of tubular illustrates its hollow structure. Based on the above analysis, The CoSTA/GM can provide good electronic conductivity, large electrochemical active contact area and fast ion diffusion rate in order to become a promising candidate for supercapacitors. Fig. 6a exhibits the cyclic CV curves of CoSTA/GM between −0.9 and 0.5 V (vs. Hg/HgO) at different scan rates. The insert CV curve of CoSTA/GM shows a typical rectangular shape, implying pure electrical double-layer capacitor behavior (from −0.9 to 0 V) and faradaic pseudocapacitance (from −0.1 to
Please cite this article in press as: H.-Z. Wan, et al., Cobalt sulfide nanotube arrays grown on FTO and graphene membranes for highperformance supercapacitor application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.169
G Model APSUSC-27988; No. of Pages 6
ARTICLE IN PRESS H.-Z. Wan et al. / Applied Surface Science xxx (2014) xxx–xxx
6
sustain and deliver 91% of the highest Cs value after 1000 cycles at 1 A g−1 . In addition, the CoSTA/GM was fabricated by a simple two-step hydrothermal process. It can be clearly observed that CoSTA/GM exhibits the Cs value of 245 F g−1 at 0.5 A g−1 in 6 M KOH aqueous electrolyte. The capacitance only decreases by 10.3% of the initial capacitance after 1000 cycles, demonstrating the excellent electrochemical stability of such electrode material. The novel CoSTA/GM exhibited high capacitance performance, good rate capability and excellent cycling stability. Acknowledgments
Fig. 7. The energy density of the CoSTA/FTO and CoSTA/GM at different current densities.
0.5 V). The galvanostatic charge/discharge curves of CoSTA/GM are also shown in Fig. 6b. It can be clearly observed that CoSTA/GM exhibits the high Cs value of 245 F g−1 at 0.5 A g−1 . When the discharge current density is 1, 2, 5 and 10 A g−1 , the Cs values of the CoSTA/GM can be calculated from the discharge curves to be 195, 154, 140, and 135 F g−1 , respectively, and the results are shown in Fig. 6b and c. The stability of electrode is a one of crucial parameter for practical applications of supercapacitor which is combined with coupled with electrical double-layer capacitance [32], such as the CoSTA/GM. The reaction mechanism of the CoSTA/GM electrode is inserted in Fig. 6d. To investigate the stability of the as-prepared CoSTA/GM electrode, galvanostatic charge–discharge cycling test was between −1.0 and 0.5 V at 1 A g−1 for 1000 cycles, as shown in Fig. 6d. The coulombic efficiency of the CoSTA/GM is slightly increases from 84% to about 98% after the 21st cycle. The Cs value of CoSTA/GM electrode only decreases by 10.3% of the initial value after 1000 cycles, exhibiting the excellent electrochemical stability. The energy density curves of the CoSTA/FTO and CoSTA/GM at different current densities are shown in Fig. 7. Specific energy was calculated, by taking the total masses into account, by the formula Es = (1/2)C(V)2 /m, where Es is the energy density, C is the specific capacitance, V is the potential window of discharge. The energy density of the CoSTA/FTO that decreased from 22.67 W h kg−1 to 9.7 W h kg−1 from 0.5 to 10 A g−1 . The energy density of the CoSTA/GM that decreased from 76.6 W h kg−1 to 40.9 W h kg−1 from 0.5 to 10 A g−1 . The energy density of the CoSTA/GM is obviously higher than that of CoSTA/FTO, the former has a wider potential window. The outstanding capacitor performances of CoSTA/GM may be attributed to its well-ordered structure. Firstly, GM can provide not only electric double layer capacitances, but also shorten the electrolyte ions diffusion distance due to its layer-by-layer stacking structure [33,34]. Secondly, the well interfacial contact between cobalt sulfide nanotubes and graphene is great benefit to highefficiency electrolyte ion diffusion and fast electron transportation throughout the whole kinetics of electrode process [35]. Thirdly, peculiar to cobalt sulfide nanotubes well-orderliness around on the surface of graphene membranes will enhance the pseudocapacitance properties. Therefore, it can be concluded that these quite orderly CoSTA/GM exhibit great potential as electrode material for supercapacitors. 4. Conclusion We prepared cobalt sulfide nanotube arrays on FTO with excellent electrochemical performance. The Cs value of the cobalt sulfide nanotube arrays can achieve 660 F g−1 at 0.5 A g−1 and is found to
This work was supported by the National Natural Science Foundation of China (Grant 51302097) and Wuhan Planning Project of Science and Technology (No. 2013011801010594). All electron microscopy were carried out in Analytical and Testing Center, Huazhong University of Science and Technology. References [1] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845–854. [2] X.H. Xia, J.P. Tu, Y.Q. Zhang, Y.J. Mai, X.L. Wang, C.D. Gu, X.B. Zhao, J. Phys. Chem. C 115 (2011) 22662–22668. [3] A. Wang, H. Wang, S. Zhang, C. Mao, J. Song, H. Niu, B. Jin, Y. Tian, Appl. Surf. Sci. 282 (2013) 704–708. [4] G. Zhang, X.W. David Lou, Sci. Rep. 3 (2013) 1470. [5] L.Q. Mai, F. Yang, Y.L. Zhao, X. Xu, L. Xu, Y.Z. Luo, Nat. Commun. 2 (2011) 381. [6] P. Justin, G. Ranga Rao, Int. J. Hydrogen Energy 35 (2010) 9709–9715. [7] G. Yu, L. Hu, M. Vosgueritchian, H. Wang, X. Xie, J.R. McDonough, X. Cui, Y. Cui, Z. Bao, Nano Lett. 11 (2011) 2905–2911. [8] R. Väli, A. Laheäär, A. Jänes, E. Lust, Electrochim. Acta 121 (2014) 294–300. [9] H. Wan, J. Jiang, J. Yu, K. Xu, L. Miao, L. Zhang, H. Chen, Y. Ruan, CrystEngCommun. 15 (2013) 7649–7651. [10] D. Ghosh, S. Giri, A. Mandal, C.K. Das, Appl. Surf. Sci. 276 (2013) 120–128. [11] J.-Y. Lin, S.-W. Chou, RSC Adv. 3 (2013) 2043. [12] J. Yang, X. Duan, Q. Qin, W. Zheng, J. Mater. Chem. A 1 (2013) 7880. [13] L. Cao, S. Yang, W. Gao, Z. Liu, Y. Gong, L. Ma, G. Shi, S. Lei, Y. Zhang, S. Zhang, R. Vajtai, P.M. Ajayan, Small 9 (2013) 2905–2910. [14] L. Zhang, H.B. Wu, X.W. Lou, Chem. Commun. 48 (2012) 6912–6914. [15] F. Luo, J. Li, H. Yuan, D. Xiao, Electrochim. Acta 123 (2014) 183–189. [16] H. Wan, X. Ji, J. Jiang, J. Yu, L. Miao, L. Zhang, S. Bie, H. Chen, Y. Ruan, J. Power Sources 243 (2013) 396–402. [17] F. Tao, Y.-Q. Zhao, G.-Q. Zhang, H.-L. Li, Electrochem. Commun. 9 (2007) 1282–1287. [18] Q. Wang, L. Jiao, H. Du, J. Yang, Q. Huan, W. Peng, Y. Si, Y. Wang, H. Yuan, CrystEngComm 13 (2011) 6960. [19] C. Shang, S. Dong, S. Wang, D. Xiao, P. Han, X. Wang, L. Gu, G. Cui, ACS Nano 7 (2013) 5430–5436. [20] G.Q. Zhang, H.B. Wu, H.E. Hoster, M.B. Chan-Park, X.W. Lou, Energy Environ. Sci. 5 (2012) 9453. [21] W. Mei, J. Huang, L. Zhu, Z. Ye, Y. Mai, J. Tu, J. Mater. Chem. 22 (2012) 9315. [22] X. Xia, J. Tu, Y. Mai, R. Chen, X. Wang, C. Gu, X. Zhao, Chemistry 17 (2011) 10898–10905. [23] Y. Jin, M. Jia, M. Zhang, Q. Wen, Appl. Surf. Sci. 264 (2013) 787–793. [24] Z. Weng, Y. Su, D.-W. Wang, F. Li, J. Du, H.-M. Cheng, Adv. Energy Mater. 1 (2011) 917–922. [25] G. Wang, X. Sun, F. Lu, H. Sun, M. Yu, W. Jiang, C. Liu, J. Lian, Small 8 (2012) 452–459. [26] M.C. Hsiao, S.H. Liao, M.Y. Yen, P.I. Liu, N.W. Pu, C.A. Wang, C.C. Ma, ACS Appl. Mater. Interfaces 2 (2010) 3092–3099. [27] L. Peng, Y. Feng, P. Lv, D. Lei, Y. Shen, Y. Li, W. Feng, J. Phys. Chem. C 116 (2012) 4970–4978. [28] C.-Y. Chen, Z.-Y. Shih, Z. Yang, H.-T. Chang, J. Power Sources 215 (2012) 43– 47. [29] Z. Huang, H. Zhang, Y. Chen, W. Wang, Y. Chen, Y. Zhong, Electrochim. Acta 108 (2013) 421–428. [30] S. Xiong, J.S. Chen, X.W. Lou, H.C. Zeng, Adv. Funct. Mater. 22 (2012) 861– 871. [31] A. Lahiri, A. Willert, S.Z.E. Abedin, F. Endres, Electrochim. Acta 121 (2014) 154–158. [32] C. Zhao, X. Wang, S. Wang, Y. Wang, Y. Zhao, W. Zheng, Int. J. Hyd. Energy 37 (2012) 11846–11852. [33] D.-W. Wang, F. Li, J. Zhao, W. Ren, Z.-G. Chen, J. Tan, Z.-S. Wu, I. Gentle, G.Q. Lu, H.-M. Cheng, ACS Nano 3 (2009) 1745–1752. [34] L.L. Zhang, X. Zhao, M.D. Stoller, Y. Zhu, H. Ji, S. Murali, Y. Wu, S. Perales, B. Clevenger, R.S. Ruoff, Nano Lett 12 (2012) 1806–1812. [35] J. Yan, Z. Fan, T. Wei, W. Qian, M. Zhang, F. Wei, Carbon 48 (2010) 3825–3833.
Please cite this article in press as: H.-Z. Wan, et al., Cobalt sulfide nanotube arrays grown on FTO and graphene membranes for highperformance supercapacitor application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.169