Materials Chemistry and Physics 137 (2012) 576e579
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Co2SnO4/activated carbon composite electrode for supercapacitor Ping He a, *, Zhengwei Xie a, Yatao Chen a, Faqin Dong a, Hongtao Liu b, * a
State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, Sichuan, PR China b College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, PR China
h i g h l i g h t s < Crystalline Co2SnO4 was synthesized by co-precipitation method. < The speciﬁc capacitance of Co2SnO4/AC composite electrode was up to 285.3 F g1. < Co2SnO4/AC composite electrode showed excellent electrochemical reversibility. < Co2SnO4/AC composite electrode showed long cycle life and high stability.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 March 2011 Received in revised form 18 September 2012 Accepted 6 October 2012
A new kind of Co2SnO4-based electrode materials for supercapacitor was synthesized by co-precipitation method. The microstructure and surface morphology of Co2SnO4 were characterized by X-ray diffraction and scanning electron microscopy, respectively. Cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy were employed for the determination of speciﬁc capacitance and the equivalent series resistance of Co2SnO4/activated carbon composite electrode in KCl solution. It was shown that the composite electrode with 25 wt% Co2SnO4 had excellent speciﬁc capacitance up to 285.3 F g1 at the current density of 5 mA cm2. In addition, the composite electrode exhibited excellent long-term stability and, after 1000 cycles, 70.6% of initial capacitance was retained. Regarding the low cost, easy preparation, steady performance and environment friendliness, Co2SnO4/activated carbon composite electrode could have potentially promising application for supercapacitor. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Composite materials Precipitation Electrochemical properties
1. Introduction Supercapacitor, also known as electrochemical capacitor, is a promising energy storage device. Compared with conventional battery and electrolytic capacitor, it has attracted more and more attention in recent years owing to its particular advantages such as low equivalent series resistance, long charge/discharge life and high power density [1,2]. Generally, three kinds of electrode materials are used for the fabrication of supercapacitor: (i) carbon materials with high surface area, such as activated carbon [3e5], carbon aerogel , carbon nanotube [7,8] and graphene , etc.; (ii) metal oxides materials with various oxidation states, such as RuO2 [10e12], IrO2  and MnO2 [14,15], etc.; (iii) conducting polymers such as polypyrrole  and polyaniline , etc. Hydrous ruthenium dioxide is the
* Corresponding authors. Tel.: þ86 816 6089371; fax: þ86 816 2419201. E-mail addresses: [email protected]
(P. He), [email protected]
(H. Liu). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.10.004
most promising candidate electrode materials for electrochemical capacitor due to its high speciﬁc capacitance and prominent electrochemical properties, and it has demonstrated a desirable capacitance up to 954 F g1 in 1.0 M H2SO4 electrolyte . However, ruthenium is not only highly cost but also environmentally hazardous, which retarded its commercial applications. Therefore, cheaper metal oxides with various oxidation states have been explored as electrode materials for supercapacitor. For example, transition metal oxides (Fe3O4 , V2O5 , NiO [21,22], SnO2 , MnO2 [24,25], etc.) have been considered as promising candidate materials because of their advantages of low cost, environmental friendliness and good electrochemical performance in supercapacitor. Recently, some ternary metal compounds, such as MCo2O4 (M ¼ Zn, Ni, etc.) [26,27], Zn2SnO4  and MFe2O4 (M ¼ Ni, Mn, etc.) [29,30], have been investigated as electrode materials for electrochemical capacitor. Compared with single metal oxide, the higher electrochemical capacitance was obtained, demonstrating that multi-electron reaction is dominant in the electrochemical reaction processes of these compounds.
P. He et al. / Materials Chemistry and Physics 137 (2012) 576e579
In this work, nanostructured Co2SnO4 powders have been synthesized by co-precipitation method. The structural and morphological characterizations of Co2SnO4 powders were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The composite of Co2SnO4 and AC with different weight ratios were prepared as supercapacitor electrode materials. The electrochemical properties of Co2SnO4/AC composite electrode were investigated by cyclic voltammetry (CV), chronopotentiometry and electrochemical impedance spectroscopy (EIS). The composite electrode with 25 wt% Co2SnO4 showed excellent electrochemical performance.
2.1. Synthesis of nanostructured Co2SnO4 CoCl2$6H2O, SnCl4$5H2O, NaOH, ethanol, graphite and AC are analytically pure and were purchased from Chengdu Chemical Reagent Factory (Chengdu, China). Double glass-distilled water was used throughout the experiments. Nanostructured Co2SnO4 powers were synthesized by coprecipitation method in alkaline solution using CoCl2$6H2O and SnCl4$5H2O as raw materials. 2.38 g CoCl2$6H2O and 1.75 g SnCl4$5H2O were dissolved respectively into 100 mL distilled water to form two transparent solutions. The two solutions were mixed together, into which 100 mL NaOH solutions (2.0 M) was added dropwise under magnetic stirring for 2 h. The mixed solution was aged for 24 h at 100 C. Finally, the obtained brown precipitation was collected by centrifugation and washed with distilled water and ethanol thoroughly. The gels were dried at 120 C and the dried gels were calcinated in mufﬂe furnace at 400 C for 2 h. 2.2. Characterization Crystallographic structures of as-prepared Co2SnO4 powers were characterized by XRD (X’ Pert PRO, Netherlands) with Cu Ka radiations (l ¼ 0.154060 nm) performed from 10 to 80 at a speed of 2 per minute. SEM (NOVA600i, USA) was used to observe the morphology of as-prepared Co2SnO4. 2.3. Electrochemical measurements
2θ (degree) Fig. 1. XRD pattern of as-prepared Co2SnO4.
(533) planes, respectively. At the same time, it can be observed that the diffraction peaks were wider than those of standard crystal, indicating that the Co2SnO4 materials had the tendency of amorphous. The surface morphology of electrode is very important in the fabrication of electrochemical capacitors. As shown in Fig. 2, the micro-morphological aspect of as-prepared Co2SnO4 was depicted by SEM with three-dimensional growth of the spherical beads, which are interconnected with each other or aggregated. Average dimension of the spherical beads was less than 100 nm. Such kind of morphology provided high surface area which is feasible for supercapacitor application. 3.2. Electrochemical performances of Co2SnO4/AC composite electrode Shown in Fig. 3 were typical CV curves for Co2SnO4/AC composite electrodes in KCl solution. Fig. 3a showed the CV curves of composite electrodes with different weight ratio of Co2SnO4. When 25 wt% Co2SnO4 was utilized, the electrode exhibited higher
The composite materials of Co2SnO4 and AC were mixed with graphite and polytetraﬂuoroethylene (PTFE) at the mass ratio of 80%:15%:5%. Then, a small amount of ethanol was added to the mixture to make it more homogeneous by ultrasonic. Finally, the mixture was pressed onto a pre-polished Ti sheet (1.0 cm 1.0 cm) to fabricate working electrodes. The prepared electrodes were dried at 60 C for 6 h. Saturated calomel electrode (SCE) and graphite rod were used as reference electrode and counter electrode, respectively. CV, chronopotentiometry and EIS of composite electrode were conducted at the PARSTAT 2273 potentiostat controlled by powersuite software (Princeton Applied Research, USA) using a three-electrode cell. 3. Results and discussion 3.1. Structure and morphology of nanostructured Co2SnO4 Shown in Fig. 1 was the XRD pattern of as-prepared Co2SnO4. It can be seen clearly that the peaks of as-prepared Co2SnO4 were well-deﬁned, indicating the crystalline nature of synthesized Co2SnO4 materials. The observed 2q values were consistent with the standard JCPDS values (JCPDS 29-0514). The diffraction peaks were in conformity with (220), (311), (222), (422), (511), (531) and
Fig. 2. SEM of as-prepared Co2SnO4.
P. He et al. / Materials Chemistry and Physics 137 (2012) 576e579
Potential (V vs. SCE)
25 wt% 50 wt%
75 wt% 0 wt%
8 mA/cm 0.8
10 mA/cm 0.2 0.0
Potential (V vs. SCE)
80 100 120 140 160 Time (s)
Fig. 4. Charge/discharge curves of 25 wt% Co2SnO4/AC composite electrode in 1.0 M KCl solution at different current densities with the potential range from 0 to 1.0 V. The mass of the active materials was 3.0 mg.
1.0 M C ¼ 2 I=½ðdV=dtÞm
2.0 M 3.0 M
Potential (V vs. SCE) Fig. 3. Cyclic voltammograms of Co2SnO4/AC composite electrodes with different weight ratio of Co2SnO4 at the potential scan rate of 5 mV s1 in 1.0 M KCl solution (a) and those of 25 wt% Co2SnO4 at the potential scan rate of 5 mV s1 in KCl solutions with different concentrations (b).
speciﬁc capacitance and nearly rectangular CV curve. Compared with CV curve of 25 wt% Co2SnO4, the peak current of 50 wt% Co2SnO4 was higher, but the rectangular CV curve was distorted. In addition, compared with AC electrode (0 wt% Co2SnO4), the capacitance behaviour of Co2SnO4-added composite electrode was improved markedly. Fig. 3b exhibited typical CV curves of 25 wt% Co2SnO4/AC composite electrode in KCl solutions with different concentrations. The best rectangular-like curve was observed in 1.0 M KCl solution. All the CV curves showed no redox peaks, indicating that the electrodes were charged or discharged at a pseudoconstant rate over the complete voltammetric cycle . Shown in Fig. 4 were the charge and discharge curves of 25 wt% Co2SnO4/AC composite electrodes in 1.0 M KCl solution at different current densities with the potential range from 0 to 1.0 V. It could be seen that the charge curves were very symmetric to the corresponding discharge counterparts over the whole potential region. A perfect linear variation of the voltage was observed during the charge and discharge process, indicating that the composite electrode had good electrochemical capacitance performance . Furthermore, the discharge time increased distinctly with decreasing current density. According to charge/discharge curves, the speciﬁc capacitance (C) of the composite electrode was calculated as follows :
where I is the applied constant current, dV/dt the slope of the chronopotentiometric curves when the curves are approximately linear and symmetric, and m the mass of electroactive materials. When the current density was 5 mA cm2, the speciﬁc capacitance of single electrode was calculated to be 285.3 F g1. The large capacitance was due to the uniform mixture of Co2SnO4 with the AC, which increased effectively the actives sites on the oxide particles. As been well known, EIS technique is a powerful tool to characterize the electrochemical processes occurred at the interface between solution and electrode. Illustrated in Fig. 5 was Nyquist plot of 25 wt% Co2SnO4/AC composite electrode. The composite electrode was measured in 1.0 M KCl solution over the frequency range from 105 Hz to 102 Hz and the applied perturbation amplitude was 5 mV (versus open circuit potential). Obviously the Nyquist plot was composed of a semi-circle at higher frequencies containing information of charge transfer reactions, a 45 line in the intermediate frequencies demonstrating that the characteristic
-Z" / ohm
15 10 5 0 0
Z' / ohm Fig. 5. Nyquist plot for 25 wt% Co2SnO4/AC composite electrode in 1.0 M KCl solution with the applied perturbation amplitude of 5 mV (versus open circuit potential). The frequencies were swept from 105 to 102 Hz.
P. He et al. / Materials Chemistry and Physics 137 (2012) 576e579
and EIS. It was found that the composite electrode showed desirable electrochemical properties and the speciﬁc capacitance of 25 wt% Co2SnO4 composite electrode was up to 285.3 F g1 in 1.0 M KCl solution. The stable electrochemical properties, excellent reversibility and the long cycle life of Co2SnO4-based electrode materials demonstrated that it has potential advantages for the fabrication of electrochemical capacitor.
Potential / V
Specific capacitance / (F/g)
0.6 0.4 0.2
65 0.0 9000
Number of cycles Fig. 6. Cycle-life of 25 wt% Co2SnO4/AC composite electrode at current density of 5 mA cm2 in 1.0 M KCl solution with the potential range from 0 to 1.0 V. The insert was charge/discharge curves of this composite electrode. The mass of the active materials was 3.0 mg.
of ion diffusion, and a straight sloping line at lower frequencies exhibiting the pure capacitance of electrode materials . Correspondingly, an equivalent circuit shown in Fig. 5 was designed, by which the ﬁtted plot was in agreement with the measured plot. Rs is the bulk resistance of the electrochemical system, indicating electric conductivity of electrolyte and electrodes; Rct is the faradic charge-transfer resistance, and it was estimated to about 3.2 U from the point of intercept with the real axis; W is the Warburg impedance reﬂecting the diffusion effects of ion at the interface between active materials and electrolyte. The combination of Rct and W is called faradic impedance, reﬂecting the kinetics of electrochemical reaction . It was obvious that Co2SnO4/AC enhanced the conductivity by reducing the internal resistance. Rct for composite electrode at higher frequencies was very low, indicating low electrochemical reaction resistance. And the diffusing lines at lower frequencies showed approximately ideal capacitive behaviour, with a near vertical line parallel to the imaginary axis (Z00 ). As a whole, the composite electrode exhibited a typical capacitance behaviour over the frequency range. The cycle life is an important factor for electrochemical capacitance performance of supercapacitor. The long-term cycling stability of 25 wt% Co2SnO4/AC composite electrode was investigated, and the variation of speciﬁc capacitance over 1000 cycles was depicted in Fig. 6. It was observed that the capacitance of composite electrode was not signiﬁcantly reduced in the 1000 cycles. After 1000 cycles, the capacitance was still 70.6% of the ﬁrst cycle, indicating that the composite electrode possessed excellent cyclic stability and, within the voltage window 0e1.0 V, charge and discharge processes did not induce signiﬁcant structural or microstructure changes of electrode active materials. The long cycle life implies that the Co2SnO4-based could be a good candidate as electrode materials for electrochemical capacitor. 4. Conclusions By co-precipitation method, crystalline Co2SnO4 was synthesized for electrochemical capacitor. The structure and surface morphology of Co2SnO4 were determined by XRD and SEM, respectively. The corresponding electrochemical properties for Co2SnO4/AC composite electrode were evaluated in KCl solution by several electrochemical techniques, including CV, chronopotentiometry
This work was supported by the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (11zxfk26) and the Foundation from the Technology R&D Program of Sichuan Province (2010GZ0300) and the Postgraduate Innovation Fund Sponsored by Southwest University of Science and Technology (10ycjj15). Also we are grateful for the help of Analytical and Testing Center of Southwest University of Science and Technology. References  B.E. Conway, Electrochemical Supercapacitors, Scientiﬁc Fundamentals and Technological Applications, Kluwer Academic/Plenum Publishers, New York, 1999.  A. Malak, K. Fic, G. Lota, C. Vix-Guterl, E. Frackowiak, J. Solid State Electrochem. 14 (2010) 811e816.  D. Kalpana, S.H. Cho, S.B. Lee, Y.S. Lee, R. Misra, N.G. Renganathan, J. Power Sources 190 (2009) 587e591.  L.H. Wang, T. Morishita, M. Toyoda, M. Inagaki, Electrochim. Acta 53 (2007) 882e886.  C.W. Huang, C.H. Hsu, P.L. Kuo, C.T. Hsieh, H. Teng, Carbon 49 (2011) 895e903.  L. Liu, Q.H. Meng, J. Mater. Sci. 40 (2005) 4105e4107.  F. Picó, J.M. Rojo, M.L. Sanjuán, A. Ansón, A.M. Benito, M.A. Callejas, W.K. Maser, M.T. Martínez, J. Electrochem. Soc. 151 (2004) A831eA837.  B.J. Yoon, S.H. Jeong, K.H. Lee, H.S. Kim, C.G. Park, J.H. Han, Chem. Phys. Lett. 388 (2004) 170e174.  H. Gómez, M.K. Ram, F. Alvi, P. Villalba, E. Stefanakos, A. Kumar, J. Power Sources 196 (2011) 4102e4108.  S.K. Mondal, N. Munichandraiah, J. Power Sources 175 (2008) 657e663.  C.C. Hu, K.H. Chang, M.C. Lin, Y.T. Wu, Nano Lett. 6 (2006) 2690e2695.  U.M. Patil, S.B. Kulkarni, V.S. Jamadade, C.D. Lokhande, J. Alloys Compd. 509 (2011) 1677e1682.  A.A.F. Grupioni, T.A.F. Lassali, J. Electrochem. Soc. 148 (2001) A1015eA1022.  Y.U. Jeong, A. Manthiram, J. Electrochem. Soc. 149 (2002) A1419eA1422.  T. Brousse, M. Toupin, R. Dugas, L. Athouël, O. Crosnier, D. Bélanger, J. Electrochem. Soc. 153 (2006) A2171eA2180.  H.F. An, Y. Wang, X.Y. Wang, L.P. Zheng, X.Y. Wang, L.H. Yi, L.H. Yi, L. Bai, X.Y. Zhang, J. Power Sources 195 (2010) 6964e6969.  F. Fusalba, P. Gouérec, D. Villers, D. Bélanger, J. Electrochem. Soc. 148 (2001) A1eA6.  C.K. Min, T.B. Wu, W.T. Yang, C.L. Li, Mater. Chem. Phys. 117 (2009) 70e73.  X. Du, C.Y. Wang, M.M. Chen, Y. Jiao, J. Wang, J. Phys. Chem. C 113 (2009) 2643e2646.  R.N. Reddy, R.G. Reddy, J. Power Sources 156 (2006) 700e704.  V. Ganesh, S. Pitchumani, V. Lakshminarayanan, J. Power Sources 158 (2006) 1523e1532.  A.I. Inamdar, Y.S. Kim, S.M. Pawar, J.H. Kim, H. Im, H. Kim, J. Power Sources 196 (2011) 2393e2397.  A. Jayalakshmi, N. Venugopal, K.P. Raja, M.M. Rao, J. Power Sources 158 (2006) 1538e1543.  V. Subramanian, H.W. Zhu, R. Vajtai, P.M. Ajayan, B.Q. Wei, J. Phys. Chem. B 109 (2005) 20207e20214.  Y.T. Chen, P. He, P. Huang, L. Wang, X.F. Yi, F.Q. Dong, ECS Trans. 28 (2010) 107e115.  K. Karthikeyan, D. Kalpana, N.G. Renganathan, Ionics 15 (2009) 107e110.  V. Gupta, S. Gupta, N. Miura, J. Power Sources 195 (2010) 3757e3760.  W.S. Yuan, Y.W. Tian, G.Q. Liu, J. Alloys Compd. 506 (2010) 683e687.  P. Sen, A. De, Electrochim. Acta 55 (2010) 4677e4684.  S.L. Kuo, N.L. Wu, J. Power Sources 162 (2006) 1437e1443.  J.H. Jiang, A. Kucernak, Electrochim. Acta 47 (2002) 2381e2386.  M.W. Xu, D.D. Zhao, S.J. Bao, H.L. Li, J. Solid State Electrochem. 11 (2007) 1101e1107.  J. Li, X.Y. Wang, Q.H. Huang, S. Gamboa, P.J. Sebastian, J. Power Sources 158 (2006) 784e788.  M. Arulepp, L. Permann, J. Leis, A. Perkson, K. Rumma, A. Jänes, E. Lust, J. Power Sources 133 (2004) 320e328.  S.J. Bao, Y.Y. Liang, W.J. Zhou, B.L. He, H.L. Li, J. Power Sources 154 (2006) 239e245.