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Electrodeposition of MnO2 nanowires on carbon nanotube paper as free-standing, flexible electrode for supercapacitors
Electrochemistry Communications 10 (2008) 1724–1727
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Electrodeposition of MnO2 nanowires on carbon nanotube paper as free-standing, flexible electrode for supercapacitors Shu-Lei Chou a,b,*, Jia-Zhao Wang a,b, Sau-Yen Chew a,b, Hua-Kun Liu a,b, Shi-Xue Dou a a b
Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia ARC Centre of Excellence for Electromaterials Science, University of Wollongong, Wollongong, NSW 2522, Australia
a r t i c l e
i n f o
Article history: Received 5 August 2008 Received in revised form 26 August 2008 Accepted 28 August 2008 Available online 8 September 2008 Keywords: MnO2 Nanowires Carbon nanotube paper Supercapacitor Flexible electrode
a b s t r a c t MnO2 nanowires were electrodeposited onto carbon nanotube (CNT) paper by a cyclic voltammetric technique. The as-prepared MnO2 nanowire/CNT composite paper (MNCCP) can be used as a flexible electrode for electrochemical supercapacitors. Electrochemical measurements showed that the MNCCP electrode displayed specific capacitances as high as 167.5 F g1 at a current density of 77 mA g1. After 3000 cycles, the composite paper can retain more than 88% of initial capacitance, showing good cyclability. The CNT paper in the composite acted as a good conductive and active substrate for flexible electrodes in supercapacitors, and the nanowire structure of the MnO2 could facilitate the contact of the electrolyte with the active materials, and thus increase the capacitance. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Supercapacitors as one of the most important energy devices need to be designed with not only high energy density, but also high levels of important mechanical properties, especially flexibility to meet the various design and power needs of modern gadgets [1,2]. From the materials point of view, three families of materials have been used in supercapacitors so far, that is, carbon, conducting polymer, and metal oxide [3,4]. Each kind of material has its own advantages and disadvantages. Carbon materials have long cycle life and good mechanical properties (especially carbon nanotubes (CNTs)), but low specific capacitance [5]. Conducting polymers are famous for their high flexibility, but have poor cyclability and relatively low capacitance [6]. As for the metal oxides, MnO2 is considered as the most promising material for the next generation of supercapacitors because of its low cost, environmentally friendly nature, and ideal capacitor performance [7–10]. The problem is that the real specific capacitance value of MnO2 is far from the theoretical specific capacitance value (1110 F g1, calculated by transferring 1 mol electrons) [9b]. Moreover, it has been found that the thin film form has the highest specific capacitance, and the specific capacitance dramatically dropped when the MnO2 thickness increased [11]. It is evident that the capacitance strongly
* Corresponding author. Address: Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia. Tel.: +61 2 4221 5993; fax: +61 2 4221 5731. E-mail address: [email protected] (S.-L. Chou). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.08.051
depends on the morphology and surface area. Therefore, considerable research has been done on the synthesis of nanostructured [12–15] and porous [16–19] MnO2 materials to improve the electrochemical performance. On the other hand, coating MnO2 onto conducting agents (e.g. carbon nanotubes) also results in significant improvement of electrochemical performance [20–25]. However, these MnO2 or its composite materials that have been previously reported were mostly powder-based or non-flexible, which hindered the potential application in flexible supercapacitors. Herein we used CNT paper which is prepared by a simple filtration method as the matrix and electrodeposited MnO2 nanowires into/onto the CNT paper to obtain a free-standing, totally flexible with mechanically tough, high-capacitance, and long-life electrode for supercapacitors. 2. Experimental section Carbon nanotube (CNT) paper was prepared via filtration using double wall CNTs (DW0923, Carbon Nanotechnologies Incorporated, USA). The detailed procedure for preparing CNT paper is described elsewhere [26]. The MnO2 nanowires were electrodeposited by a cyclic voltammetric technique in the potential range between +0.60 and +0.30 V with reference to a standard calomel electrode (SCE) at the scan rate of 500 mV s1 onto the carbon nanotube paper [15]. The pure MnO2 nanowires were also prepared by electrochemical deposition as reported previously [15]. After electrochemical deposition, the MnO2 nanowire/CNT
S.-L. Chou et al. / Electrochemistry Communications 10 (2008) 1724–1727
composite paper (MNCCP) and pure MnO2 nanowires (scraped off the Ni substrate) were washed with deionized water three times, and then dried at 120 °C for 24 h in a vacuum oven for further characterizations. The morphology and microstructure of the as-prepared MNCCP was characterized by X-ray diffraction (XRD; Philips PW1730 diffractometer with CuKa radiation and a graphite monochrometer, k = 1.54184 Å), and scanning electron microscopy (SEM; JEOL JEM-3000 equipped with energy dispersive spectroscopy (EDS)). Electrochemical measurements were carried out in a threeelectrode electrochemical cell, including charge-discharge curves (Neware battery test system) and cyclic voltammetry (CV; CHI 660 electrochemistry workstation). The electrochemical cell contained MNCCP as the working electrode, a Pt foil (4 cm2) counter electrode, a saturated calomel electrode (SCE) as reference electrode, and 0.1 M Na2SO4 solution as the electrolyte. The weight percentage of MnO2 obtained from the weight difference before and after electrochemical deposition MnO2 onto/into CNT paper is 19.8%. 3. Results and discussion Fig. 1 shows SEM images and EDS patterns of the as-prepared MnO2 nanowire/CNT composite paper (MNCCP). Fig. 1a shows a cross-sectional view of the as-prepared sample. It can be seen that there are three different layers, labeled as layer 1 to layer 3. Layer 1, about 1 lm thick, which is the top layer of the electrode, is com-
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posed of MnO2 nanowires. The top view of layer 1 shown in Fig. 1b illustrates the cross-linked MnO2 nanowires, which are about 30 nm in diameter, in good agreement with previously reported data [13,15]. The EDS pattern shown in Fig. 1d confirms that the top layer contains only the elements Mn and O. Layer 2, around 2 lm thick, is the transition layer between the MnO2 nanowire layer and the CNT layer (layer 3). The EDS taken from layer 2 (Fig. 2c) shows that the elements present include Mn, O, and C. The formation of layer 2 occurred because the Mn2+ ions could diffuse into the CNT paper and be deposited inside the CNT paper matrix during the electrochemical deposition of MnO2 nanowires onto the CNT paper. Due to the high density of the CNT paper, there is still a pure CNT layer at the bottom of the CNT paper, which could act as a flexible substrate and maintain the good mechanical properties of the pristine CNT paper. The inset picture in Fig. 1a proved that the electrode of MNCCP is free-standing, highly flexible and tough enough to be folded. XRD patterns of pristine CNT paper, pure MnO2 nanowires and MNCCP are shown in Fig. 2. It can be observed that the CNT paper only shows one broad peak at around 25.3°, which is marked with a triangle in Fig. 2. Despite the low intensity and weakly crystalline nature of electrolytic MnO2 (EMD), the three major diffraction peaks of pure MnO2 nanowires at 2h = 22°, 37.2° and 66.6°, can be assigned as the crystal planes of (1 1 0), (0 2 1), and (0 6 1) in c-MnO2 [27]. The other two peaks at 2h = 42.3° and 57.2° can also observed from the pure MnO2 nanowires but with relative low intensity, confirming that the as-prepared products
Fig. 1. SEM images (a, b) and corresponding EDS patterns (c, d) of MNCCP in a cross-sectional view (a, c) and a top view (b, d). The inset of (a) is a typical photograph of the MNCCP electrode held between two fingers.
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S.-L. Chou et al. / Electrochemistry Communications 10 (2008) 1724–1727
Fig. 2. XRD patterns of pristine CNT paper, pure MnO2 nanowires and MNCCP.
are c-MnO2. However, the MNCCP shows only two additional peaks at 37.2° and 66.6°, excluding the broad peak from the CNT paper. Both of the two diffraction peaks could be matched with cMnO2 confirming the presence of MnO2. It is very hard to observe other three peaks from XRD pattern of MNCCP because of three facts: the poor crystallization, influence of CNT background and the low loading rate of MnO2.
The results of electrochemical tests, including cyclic voltammograms and charge-discharge curves, high-rate capability and cyclability are shown in Fig. 3. Fig. 3a shows cyclic voltammograms of the pristine CNT paper (the inner curve) and MNCCP (the three outside curves). It can be seen that both the pristine CNT paper and the MNCCP show the typical square shaped CV curves that indicate ideal supercapacitor behavior [4]. The higher current densities in the CV curves indicate the higher specific capacitances. The MNCCP shows much higher capacitance than that of the pristine CNT paper. The specific capacitance of pristine CNT paper calculated from the CV curve is 32 F g1 at the scan rate of 5 mV s1, which is comparable to the values reported for chemical vapor deposition (CVD)-grown aligned CNTs [28,29]. The specific capacitances of MNCCP are 129, 117, and 107 F g1 for scan rates of 5, 10, and 20 mV s1, respectively. As the scan rates are increased from 5 mVs1 to 20 mVs1, the shapes of the CV curves of the MNCCP still keep nearly square, indicating the relatively good high-rate capability. Fig. 3b shows the charge-discharge curves at different current densities. The sharp and almost symmetrical charge-discharge curves show high coulombic efficiency and ideal capacitor performance. The total specific capacitance of the MNCCP can be calculated from Eq. (1), and according to the weight percentage of MnO2 (19.8%) in the total composite electrode, the specific capacitance of MnO2 nanowires can be calculated by Eq. (2).
Fig. 3. (a) Cyclic voltammograms of the pure CNT paper (inner curve) and the MNCCP (three outside curves) at the scan rates of 5, 10, and 20 mV s1; (b) Charge and discharge curves of MNCCP electrode at different current densities; (c) Specific capacitances of MNCCP and calculated MnO2 nanowires at different current densities; (d) Cycle life of the pristine CNT paper and MNCCP electrode between 0.1 V and 0.8 V at a current density of 770 mA g1. The electrolyte is 0.1 M Na2SO4 solution.
S.-L. Chou et al. / Electrochemistry Communications 10 (2008) 1724–1727
C¼
It mV
C MnO2 ¼
m C 80:2% m C CNT 19:8% m
ð1Þ
ð2Þ
where C, CCNT, and C MnO2 (F g1) are the specific capacitances of the MNCCP, the pristine CNT paper, and the calculated capacitance of the MnO2 nanowires, respectively, I (mA cm2) is the current density, t (s) is the discharging time, and m (g) is the total weight of the electrode (MNCCP). The specific capacitance of the MNCCP and the calculated specific capacitance of the MnO2 nanowires are shown in Fig. 3c. The specific capacitance of MNCCP is 167.5, 147.5, 121.8, and 107.9 F g1 at a current density of 77, 154, 385, and 770 mA g1, respectively. It should be noted that there is still around 60% initial capacitance retention even when the current density increases as much as 10 times, indicating the relatively good high-rate capability. The specific capacitance of free-standing MNCCP electrode is comparable to the value for powder-based MnO2/CNT composite [23,25]. The advantage of using free-standing MNCCP for flexible electrodes is that the electrode fabrication process could be simplified by avoiding the various steps involved in the conventional fabrication of binder-enriched electrodes [29,30]. Moreover, the highly flexible electrode could offer an opportunity for future totally flexible devices. The calculated specific capacitance of MnO2 nanowires in MNCCP is as high as 516.2 F g1 at the current density of 77 mA g1, which is higher than the value for nanostructured and porous MnO2 that was reported previously [12–19]. The CNT paper could act as a good conductive matrix for contact and dispersion of MnO2 (layer 1 and layer 2 in Fig. 1a), and the nanowire structure of MnO2 could facilitate contact of the electrolyte with the active materials [13–15]. Both of these factors could account for the capacitance enhancement of MnO2. Fig. 3d illustrates the typical cycle life curves of pristine CNT paper and MNCCP. It can be seen that the capacitance of pristine CNT paper remains almost the same after 1000 cycles, confirming its good cyclability. The MNCCP electrode only lost less than 4% of its initial capacitance after 1000 cycles and less than 12% of initial capacitance after 3000 cycles, showing the good cyclability. The coulombic efficiency of MNCCP is around 99% indicating the potential for commercial application. 4. Conclusions Free-standing, totally flexible MnO2 nanowire/CNT composite paper (MNCCP) was successfully prepared by electrochemical deposition of MnO2 nanowires into/onto CNT paper through a CV technique. The MNCCP electrode displayed high specific capacitance with good cyclability. Based on our studies, the improved electrochemical performance of the composite electrode could be
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(1) the CNT paper act as a highly conductive, flexible, and active substrate for supercapacitor electrode, and (2) the nanowire structure of MnO2 facilitate the contact of the electrolyte with the active materials, and thus improve the electrode capacitance. The present MNCCP with high-capacitance and long cycle life may have a great potential for commercial application in totally flexible supercapacitors. Acknowledgments Financial support provided by the Australian Research Council (ARC) through ARC Centre of Excellence funding (CE0561616) is gratefully acknowledged. The authors thank Dr. T. Silver at the University of Wollongong for critical reading of the manuscript. References [1] R.F. Service, Science 313 (2006) 902. [2] V.L. Pushparaj, M.M. Shaijumon, A. Kumar, S. Murugesan, L. Ci, R. Vajtai, R.J. Linhartd, O. Nalamasu, P.M. Ajayan, Proc. Natl. Acad. Sci. USA 104 (2007) 13574. [3] B.E. Conway, J. Electrochem. Soc. 138 (1991) 1539. [4] B.E. Conway, Electrochemical Supercapacitors Scientific Fundamentals and Technological Applications, Plenum Press, New York, 1999. [5] E. Frackowiak, F. Beguin, Carbon 39 (2001) 937. [6] S. Ghosh, O. Inganas, Adv. Mater. 11 (1999) 1214. [7] H.Y. Lee, J.B. Goodenough, J. Solid State Chem. 144 (1999) 220. [8] C.C. Hu, T.W. Tsou, Electrochem. Commun. 4 (2002) 105. [9] (a) M. Toupin, T. Brousse, D. Belanger, Chem. Mater. 14 (2002) 3946; (b) M. Toupin, T. Brousse, D. Belanger, Chem. Mater. 16 (2004) 3184. [10] R.N. Reddy, R.G. Reddy, J. Power Sources 124 (2003) 330. [11] S.C. Pang, M.A. Anderson, T.W. Chapman, J. Electrochem. Soc. 147 (2000) 444. [12] V. Subramanian, H.W. Zhu, R. Vajtai, P.M. Ajayan, B.Q. Wei, J. Phys. Chem. B 109 (2005) 20207. [13] M.S. Wu, Appl. Phys. Lett. 87 (2005) 153102. [14] N. Nagarajan, H. Humadi, I. Zhitomirsky, Electrochim. Acta. 51 (2006) 3039. [15] S.L. Chou, F.Y. Cheng, J. Chen, J. Power Sources 162 (2006) 727. [16] M.W. Xu, D.D. Zhao, S.J. Bao, H.L. Li, J. Solid State Chem. 11 (2007) 1101. [17] T. Xue, C.L. Xu, D.D. Zhao, X.H. Li, H.L. Li, J. Power Sources 164 (2007) 953. [18] H.R. Chen, X.P. Dong, J.L. Shi, J.J. Zhao, Z.L. Hua, J.H. Gao, M.L. Ruan, D.S. Yan, J. Mater. Chem. 17 (2007) 855. [19] M. Nakayama, T. Kanaya, R. Inoue, Electrochem. Commun. 9 (2007) 1154. [20] Y.T. Wu, C.C. Hu, J. Electrochem. Soc. 151 (2004) A2060. [21] E. Raymundo-Pinero, V. Khomenko, E. Frackowiak, F. Beguin, J. Electrochem. Soc. 152 (2005) A229. [22] V. Subramanian, H.W. Zhu, B.Q. Wei, Electrochem. Commun. 8 (2006) 827. [23] X.F. Xie, L. Gao, Carbon 45 (2007) 2365. [24] A.E. Fischer, K.A. Pettigrew, D.R. Rolison, R.M. Stroud, J.W. Long, Nano Lett. 7 (2007) 281. [25] S.B. Ma, K.W. Nam, W.S. Yoon, X.Q. Yang, K.Y. Ahn, K.H. Oh, K.B. Kim, J. Power Sources 178 (2008) 483. [26] S.H. Ng, J.Z. Wang, Z.P. Guo, G.X. Wang, H.K. Liu, Electrochim. Acta 51 (2005) 23. [27] (a) Y. Chabre, J. Pannetier, Prog. Solid State Chem. 23 (1995) 1; (b) M.M. Thackeray, Prog. Solid State Chem. 25 (1997) 1. [28] C. Du, J. Yeh, N. Pan, Nanotechnology 16 (2005) 350. [29] L.J. Ci, S.M. Manikoth, X.S. Li, R. Vajtai, P.M. Ajayan, Adv. Mater. 19 (2007) 3300. [30] K.H. An, W.S. Kim, Y.S. Park, J.M. Moon, D.J. Bae, S.C. Lim, Y.S. Lee, Y.H. Lee, Adv. Funct. Mater. 11 (2001) 387.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Electrodeposition of MnO2 nanowires on carbon nanotube paper as free-standing, flexible electrode for supercapacitors Shu-Lei Chou a,b,*, Jia-Zhao Wang a,b, Sau-Yen Chew a,b, Hua-Kun Liu a,b, Shi-Xue Dou a a b
Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia ARC Centre of Excellence for Electromaterials Science, University of Wollongong, Wollongong, NSW 2522, Australia
a r t i c l e
i n f o
Article history: Received 5 August 2008 Received in revised form 26 August 2008 Accepted 28 August 2008 Available online 8 September 2008 Keywords: MnO2 Nanowires Carbon nanotube paper Supercapacitor Flexible electrode
a b s t r a c t MnO2 nanowires were electrodeposited onto carbon nanotube (CNT) paper by a cyclic voltammetric technique. The as-prepared MnO2 nanowire/CNT composite paper (MNCCP) can be used as a flexible electrode for electrochemical supercapacitors. Electrochemical measurements showed that the MNCCP electrode displayed specific capacitances as high as 167.5 F g1 at a current density of 77 mA g1. After 3000 cycles, the composite paper can retain more than 88% of initial capacitance, showing good cyclability. The CNT paper in the composite acted as a good conductive and active substrate for flexible electrodes in supercapacitors, and the nanowire structure of the MnO2 could facilitate the contact of the electrolyte with the active materials, and thus increase the capacitance. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Supercapacitors as one of the most important energy devices need to be designed with not only high energy density, but also high levels of important mechanical properties, especially flexibility to meet the various design and power needs of modern gadgets [1,2]. From the materials point of view, three families of materials have been used in supercapacitors so far, that is, carbon, conducting polymer, and metal oxide [3,4]. Each kind of material has its own advantages and disadvantages. Carbon materials have long cycle life and good mechanical properties (especially carbon nanotubes (CNTs)), but low specific capacitance [5]. Conducting polymers are famous for their high flexibility, but have poor cyclability and relatively low capacitance [6]. As for the metal oxides, MnO2 is considered as the most promising material for the next generation of supercapacitors because of its low cost, environmentally friendly nature, and ideal capacitor performance [7–10]. The problem is that the real specific capacitance value of MnO2 is far from the theoretical specific capacitance value (1110 F g1, calculated by transferring 1 mol electrons) [9b]. Moreover, it has been found that the thin film form has the highest specific capacitance, and the specific capacitance dramatically dropped when the MnO2 thickness increased [11]. It is evident that the capacitance strongly
* Corresponding author. Address: Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia. Tel.: +61 2 4221 5993; fax: +61 2 4221 5731. E-mail address: [email protected] (S.-L. Chou). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.08.051
depends on the morphology and surface area. Therefore, considerable research has been done on the synthesis of nanostructured [12–15] and porous [16–19] MnO2 materials to improve the electrochemical performance. On the other hand, coating MnO2 onto conducting agents (e.g. carbon nanotubes) also results in significant improvement of electrochemical performance [20–25]. However, these MnO2 or its composite materials that have been previously reported were mostly powder-based or non-flexible, which hindered the potential application in flexible supercapacitors. Herein we used CNT paper which is prepared by a simple filtration method as the matrix and electrodeposited MnO2 nanowires into/onto the CNT paper to obtain a free-standing, totally flexible with mechanically tough, high-capacitance, and long-life electrode for supercapacitors. 2. Experimental section Carbon nanotube (CNT) paper was prepared via filtration using double wall CNTs (DW0923, Carbon Nanotechnologies Incorporated, USA). The detailed procedure for preparing CNT paper is described elsewhere [26]. The MnO2 nanowires were electrodeposited by a cyclic voltammetric technique in the potential range between +0.60 and +0.30 V with reference to a standard calomel electrode (SCE) at the scan rate of 500 mV s1 onto the carbon nanotube paper [15]. The pure MnO2 nanowires were also prepared by electrochemical deposition as reported previously [15]. After electrochemical deposition, the MnO2 nanowire/CNT
S.-L. Chou et al. / Electrochemistry Communications 10 (2008) 1724–1727
composite paper (MNCCP) and pure MnO2 nanowires (scraped off the Ni substrate) were washed with deionized water three times, and then dried at 120 °C for 24 h in a vacuum oven for further characterizations. The morphology and microstructure of the as-prepared MNCCP was characterized by X-ray diffraction (XRD; Philips PW1730 diffractometer with CuKa radiation and a graphite monochrometer, k = 1.54184 Å), and scanning electron microscopy (SEM; JEOL JEM-3000 equipped with energy dispersive spectroscopy (EDS)). Electrochemical measurements were carried out in a threeelectrode electrochemical cell, including charge-discharge curves (Neware battery test system) and cyclic voltammetry (CV; CHI 660 electrochemistry workstation). The electrochemical cell contained MNCCP as the working electrode, a Pt foil (4 cm2) counter electrode, a saturated calomel electrode (SCE) as reference electrode, and 0.1 M Na2SO4 solution as the electrolyte. The weight percentage of MnO2 obtained from the weight difference before and after electrochemical deposition MnO2 onto/into CNT paper is 19.8%. 3. Results and discussion Fig. 1 shows SEM images and EDS patterns of the as-prepared MnO2 nanowire/CNT composite paper (MNCCP). Fig. 1a shows a cross-sectional view of the as-prepared sample. It can be seen that there are three different layers, labeled as layer 1 to layer 3. Layer 1, about 1 lm thick, which is the top layer of the electrode, is com-
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posed of MnO2 nanowires. The top view of layer 1 shown in Fig. 1b illustrates the cross-linked MnO2 nanowires, which are about 30 nm in diameter, in good agreement with previously reported data [13,15]. The EDS pattern shown in Fig. 1d confirms that the top layer contains only the elements Mn and O. Layer 2, around 2 lm thick, is the transition layer between the MnO2 nanowire layer and the CNT layer (layer 3). The EDS taken from layer 2 (Fig. 2c) shows that the elements present include Mn, O, and C. The formation of layer 2 occurred because the Mn2+ ions could diffuse into the CNT paper and be deposited inside the CNT paper matrix during the electrochemical deposition of MnO2 nanowires onto the CNT paper. Due to the high density of the CNT paper, there is still a pure CNT layer at the bottom of the CNT paper, which could act as a flexible substrate and maintain the good mechanical properties of the pristine CNT paper. The inset picture in Fig. 1a proved that the electrode of MNCCP is free-standing, highly flexible and tough enough to be folded. XRD patterns of pristine CNT paper, pure MnO2 nanowires and MNCCP are shown in Fig. 2. It can be observed that the CNT paper only shows one broad peak at around 25.3°, which is marked with a triangle in Fig. 2. Despite the low intensity and weakly crystalline nature of electrolytic MnO2 (EMD), the three major diffraction peaks of pure MnO2 nanowires at 2h = 22°, 37.2° and 66.6°, can be assigned as the crystal planes of (1 1 0), (0 2 1), and (0 6 1) in c-MnO2 [27]. The other two peaks at 2h = 42.3° and 57.2° can also observed from the pure MnO2 nanowires but with relative low intensity, confirming that the as-prepared products
Fig. 1. SEM images (a, b) and corresponding EDS patterns (c, d) of MNCCP in a cross-sectional view (a, c) and a top view (b, d). The inset of (a) is a typical photograph of the MNCCP electrode held between two fingers.
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S.-L. Chou et al. / Electrochemistry Communications 10 (2008) 1724–1727
Fig. 2. XRD patterns of pristine CNT paper, pure MnO2 nanowires and MNCCP.
are c-MnO2. However, the MNCCP shows only two additional peaks at 37.2° and 66.6°, excluding the broad peak from the CNT paper. Both of the two diffraction peaks could be matched with cMnO2 confirming the presence of MnO2. It is very hard to observe other three peaks from XRD pattern of MNCCP because of three facts: the poor crystallization, influence of CNT background and the low loading rate of MnO2.
The results of electrochemical tests, including cyclic voltammograms and charge-discharge curves, high-rate capability and cyclability are shown in Fig. 3. Fig. 3a shows cyclic voltammograms of the pristine CNT paper (the inner curve) and MNCCP (the three outside curves). It can be seen that both the pristine CNT paper and the MNCCP show the typical square shaped CV curves that indicate ideal supercapacitor behavior [4]. The higher current densities in the CV curves indicate the higher specific capacitances. The MNCCP shows much higher capacitance than that of the pristine CNT paper. The specific capacitance of pristine CNT paper calculated from the CV curve is 32 F g1 at the scan rate of 5 mV s1, which is comparable to the values reported for chemical vapor deposition (CVD)-grown aligned CNTs [28,29]. The specific capacitances of MNCCP are 129, 117, and 107 F g1 for scan rates of 5, 10, and 20 mV s1, respectively. As the scan rates are increased from 5 mVs1 to 20 mVs1, the shapes of the CV curves of the MNCCP still keep nearly square, indicating the relatively good high-rate capability. Fig. 3b shows the charge-discharge curves at different current densities. The sharp and almost symmetrical charge-discharge curves show high coulombic efficiency and ideal capacitor performance. The total specific capacitance of the MNCCP can be calculated from Eq. (1), and according to the weight percentage of MnO2 (19.8%) in the total composite electrode, the specific capacitance of MnO2 nanowires can be calculated by Eq. (2).
Fig. 3. (a) Cyclic voltammograms of the pure CNT paper (inner curve) and the MNCCP (three outside curves) at the scan rates of 5, 10, and 20 mV s1; (b) Charge and discharge curves of MNCCP electrode at different current densities; (c) Specific capacitances of MNCCP and calculated MnO2 nanowires at different current densities; (d) Cycle life of the pristine CNT paper and MNCCP electrode between 0.1 V and 0.8 V at a current density of 770 mA g1. The electrolyte is 0.1 M Na2SO4 solution.
S.-L. Chou et al. / Electrochemistry Communications 10 (2008) 1724–1727
C¼
It mV
C MnO2 ¼
m C 80:2% m C CNT 19:8% m
ð1Þ
ð2Þ
where C, CCNT, and C MnO2 (F g1) are the specific capacitances of the MNCCP, the pristine CNT paper, and the calculated capacitance of the MnO2 nanowires, respectively, I (mA cm2) is the current density, t (s) is the discharging time, and m (g) is the total weight of the electrode (MNCCP). The specific capacitance of the MNCCP and the calculated specific capacitance of the MnO2 nanowires are shown in Fig. 3c. The specific capacitance of MNCCP is 167.5, 147.5, 121.8, and 107.9 F g1 at a current density of 77, 154, 385, and 770 mA g1, respectively. It should be noted that there is still around 60% initial capacitance retention even when the current density increases as much as 10 times, indicating the relatively good high-rate capability. The specific capacitance of free-standing MNCCP electrode is comparable to the value for powder-based MnO2/CNT composite [23,25]. The advantage of using free-standing MNCCP for flexible electrodes is that the electrode fabrication process could be simplified by avoiding the various steps involved in the conventional fabrication of binder-enriched electrodes [29,30]. Moreover, the highly flexible electrode could offer an opportunity for future totally flexible devices. The calculated specific capacitance of MnO2 nanowires in MNCCP is as high as 516.2 F g1 at the current density of 77 mA g1, which is higher than the value for nanostructured and porous MnO2 that was reported previously [12–19]. The CNT paper could act as a good conductive matrix for contact and dispersion of MnO2 (layer 1 and layer 2 in Fig. 1a), and the nanowire structure of MnO2 could facilitate contact of the electrolyte with the active materials [13–15]. Both of these factors could account for the capacitance enhancement of MnO2. Fig. 3d illustrates the typical cycle life curves of pristine CNT paper and MNCCP. It can be seen that the capacitance of pristine CNT paper remains almost the same after 1000 cycles, confirming its good cyclability. The MNCCP electrode only lost less than 4% of its initial capacitance after 1000 cycles and less than 12% of initial capacitance after 3000 cycles, showing the good cyclability. The coulombic efficiency of MNCCP is around 99% indicating the potential for commercial application. 4. Conclusions Free-standing, totally flexible MnO2 nanowire/CNT composite paper (MNCCP) was successfully prepared by electrochemical deposition of MnO2 nanowires into/onto CNT paper through a CV technique. The MNCCP electrode displayed high specific capacitance with good cyclability. Based on our studies, the improved electrochemical performance of the composite electrode could be
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(1) the CNT paper act as a highly conductive, flexible, and active substrate for supercapacitor electrode, and (2) the nanowire structure of MnO2 facilitate the contact of the electrolyte with the active materials, and thus improve the electrode capacitance. The present MNCCP with high-capacitance and long cycle life may have a great potential for commercial application in totally flexible supercapacitors. Acknowledgments Financial support provided by the Australian Research Council (ARC) through ARC Centre of Excellence funding (CE0561616) is gratefully acknowledged. The authors thank Dr. T. Silver at the University of Wollongong for critical reading of the manuscript. References [1] R.F. Service, Science 313 (2006) 902. [2] V.L. Pushparaj, M.M. Shaijumon, A. Kumar, S. Murugesan, L. Ci, R. Vajtai, R.J. Linhartd, O. Nalamasu, P.M. Ajayan, Proc. Natl. Acad. Sci. USA 104 (2007) 13574. [3] B.E. Conway, J. Electrochem. Soc. 138 (1991) 1539. [4] B.E. Conway, Electrochemical Supercapacitors Scientific Fundamentals and Technological Applications, Plenum Press, New York, 1999. [5] E. Frackowiak, F. Beguin, Carbon 39 (2001) 937. [6] S. Ghosh, O. Inganas, Adv. Mater. 11 (1999) 1214. [7] H.Y. Lee, J.B. Goodenough, J. Solid State Chem. 144 (1999) 220. [8] C.C. Hu, T.W. Tsou, Electrochem. Commun. 4 (2002) 105. [9] (a) M. Toupin, T. Brousse, D. Belanger, Chem. Mater. 14 (2002) 3946; (b) M. Toupin, T. Brousse, D. Belanger, Chem. Mater. 16 (2004) 3184. [10] R.N. Reddy, R.G. Reddy, J. Power Sources 124 (2003) 330. [11] S.C. Pang, M.A. Anderson, T.W. Chapman, J. Electrochem. Soc. 147 (2000) 444. [12] V. Subramanian, H.W. Zhu, R. Vajtai, P.M. Ajayan, B.Q. Wei, J. Phys. Chem. B 109 (2005) 20207. [13] M.S. Wu, Appl. Phys. Lett. 87 (2005) 153102. [14] N. Nagarajan, H. Humadi, I. Zhitomirsky, Electrochim. Acta. 51 (2006) 3039. [15] S.L. Chou, F.Y. Cheng, J. Chen, J. Power Sources 162 (2006) 727. [16] M.W. Xu, D.D. Zhao, S.J. Bao, H.L. Li, J. Solid State Chem. 11 (2007) 1101. [17] T. Xue, C.L. Xu, D.D. Zhao, X.H. Li, H.L. Li, J. Power Sources 164 (2007) 953. [18] H.R. Chen, X.P. Dong, J.L. Shi, J.J. Zhao, Z.L. Hua, J.H. Gao, M.L. Ruan, D.S. Yan, J. Mater. Chem. 17 (2007) 855. [19] M. Nakayama, T. Kanaya, R. Inoue, Electrochem. Commun. 9 (2007) 1154. [20] Y.T. Wu, C.C. Hu, J. Electrochem. Soc. 151 (2004) A2060. [21] E. Raymundo-Pinero, V. Khomenko, E. Frackowiak, F. Beguin, J. Electrochem. Soc. 152 (2005) A229. [22] V. Subramanian, H.W. Zhu, B.Q. Wei, Electrochem. Commun. 8 (2006) 827. [23] X.F. Xie, L. Gao, Carbon 45 (2007) 2365. [24] A.E. Fischer, K.A. Pettigrew, D.R. Rolison, R.M. Stroud, J.W. Long, Nano Lett. 7 (2007) 281. [25] S.B. Ma, K.W. Nam, W.S. Yoon, X.Q. Yang, K.Y. Ahn, K.H. Oh, K.B. Kim, J. Power Sources 178 (2008) 483. [26] S.H. Ng, J.Z. Wang, Z.P. Guo, G.X. Wang, H.K. Liu, Electrochim. Acta 51 (2005) 23. [27] (a) Y. Chabre, J. Pannetier, Prog. Solid State Chem. 23 (1995) 1; (b) M.M. Thackeray, Prog. Solid State Chem. 25 (1997) 1. [28] C. Du, J. Yeh, N. Pan, Nanotechnology 16 (2005) 350. [29] L.J. Ci, S.M. Manikoth, X.S. Li, R. Vajtai, P.M. Ajayan, Adv. Mater. 19 (2007) 3300. [30] K.H. An, W.S. Kim, Y.S. Park, J.M. Moon, D.J. Bae, S.C. Lim, Y.S. Lee, Y.H. Lee, Adv. Funct. Mater. 11 (2001) 387.