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Development of new nanocomposite based on nanosized-manganese oxide and carbon nanotubes for high performance electrochemical capacitors
Electrochimica Acta 55 (2010) 3428–3433
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Development of new nanocomposite based on nanosized-manganese oxide and carbon nanotubes for high performance electrochemical capacitors Tarik Bordjiba, Daniel Bélanger ∗,1 Département de Chimie, Université du Québec à Montréal, Case Postale 8888, succursale Centre-Ville, Montréal, Québec, H3C 3P8 Canada
a r t i c l e
i n f o
Article history: Received 3 July 2009 Received in revised form 24 December 2009 Accepted 27 December 2009 Available online 11 January 2010 Keywords: Electrochemical capacitors Carbon nanotubes Manganese oxide Nanocomposite Energy storage
a b s t r a c t We report the synthesis of a new composite electrode based on nanosized-manganese oxide and carbon nanotubes (CNTs) by electrophoretic deposition of CNTs on a stainless steel (SS) substrate followed by direct spontaneous reduction of MnO4 − ions to MnO2 to form the multi-scaled SS–CNT–MnO2 electrode. The resulting material was characterized by scanning electron microscopy, energy dispersive X-ray analysis, cyclic voltammetry and galvanostatic charge–discharge in a 0.65 M K2 SO4 aqueous solution. The binderless SS–CNT–MnO2 nanocomposite electrode shows a very high specific capacitance of 869 F/g of CNT–MnO2 and good stability during long galvanostatic charge–discharge cycling. To the best of our knowledge, this is one of the highest capacitance for manganese oxide electrode ever reported. In addition to its applicability in electrochemical capacitors, this methodology could be extended to develop other high performance nanocomposite material electrodes based on carbon nanotubes and metal oxide for the future generation of electrochemical power sources. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction The development of advanced composite materials based on metal oxide-carbon nanotubes is a new route for achieving highly efficient electrode for electrochemical power sources such as fuel cells [1], lithium batteries [2] and electrochemical capacitors [3–5]. For electrochemical capacitors, active electrode materials include carbon, conducting polymers and transition metal oxides [6]. Manganese oxide is a promising electrode material for electrochemical capacitors due to its low cost, natural abundance, environmental safety and its high theoretical capacitance. If one Mn atom in MnO2 is assumed to store one electron, then the specific capacitance of MnO2 should be around 1370 F/g [7,8]. But, practically, this oxide shows a specific capacitance of only one-fifth or one-sixth of the above value [9,10]. Such low practical specific capacitance is due to the intrinsically poor electronic conductivity and dense morphology of the oxide [9,11]. On the other hand, the challenge with the MnO2 lies in maximizing its electrochemical utilization. Currently, there are mainly two efficient ways to reach high specific capacitance with manganese oxide. The first one is by developing nanostructured manganese oxide, which allows reaching a specific capacitance in the range of 700 F/g [12–15]. The second one is by the incorporation of carbon nanotubes in the MnO2 matrix, which
∗ Corresponding author. Tel.: +1 514 987 3000x3909; fax: +1 514 987 4054. E-mail address: [email protected] (D. Bélanger). 1 ISE member. 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.12.070
allows to reach specific capacitance ranging from 325 to 580 F/g [16–19]. In this report, we describe a low cost method for the development of a new class of binderless nanocomposites materials based on nanostructured metal oxide (manganese oxide) and carbon nanotubes. This nanocomposite is prepared following the procedure shown in Scheme 1 which consists of the electrophoretic deposition (EPD) of CNTs on a stainless steel (SS) substrate followed by direct spontaneous reduction of MnO4 − ions on the multi-scaled SS–CNT substrate to form manganese oxide (MnO2 ). An important feature of this nanocomposite is that is does not rely on the use of a binder. In fact, a common approach to fabricate electrodes is to add to the active material, a conductivity enhancer and a binder to promote the adhesion of the particles. Importantly, the use of binder and/or other adhesives may occlude a fraction of the surface of the active materials or electrically isolate some particles. Our CNT–MnO2 nanocomposite is fabricated directly on the SS substrate without any binder or conductive additive, which makes the CNT–MnO2 nanocomposite even more interesting as binderless electrode for electrochemical capacitors. 2. Experimental Purified (>90%) CNTs (Multiwall, O.D. × I.D. × length 10–15 nm × 2–6 nm × 0.1–10 m) prepared by chemical vapor deposition (CVD) method were purchased from Aldrich. All other chemicals used in this study were also purchased from Aldrich and used as received. Distilled water of 18 M obtained through
T. Bordjiba, D. Bélanger / Electrochimica Acta 55 (2010) 3428–3433
Scheme 1. Procedure for the fabrication of the SS, SS–MnO2 -1, SS–MnO2 -30, SS–CNT–MnO2 -1, and SS–CNT–MnO2 -30 electrodes.
Nanopure II (Branstead) was used to prepare the solutions. Stainless steel gauze type 304 (SS) was obtained from Alfa Aesar. Scheme 1 presents the concept for the fabrication of the electrodes used in this work. To obtain CNTs with acidic sites on the surface, typically, (0.8 g) of carbon nanotubes were refluxed in (60 mL) of mixed (1:3 volume ratio) concentrated nitric and sulfuric acids at 130 ◦ C for 30 min [20]. The oxidized tubes were washed with distilled water. For simplicity, thereafter the oxidized CNTs are denoted as CNTs. An aqueous suspension (concentration 0.5 mg/mL) was used for the electrophoretic deposition of CNTs on the SS substrate [21]. The 2 cm × 1 cm stainless steel substrate was used following a cleaning step performed by ultrasonic treatment in acetone for 10 min and drying under vacuum at room temperature for 24 h. Electrophoretic deposition was conducted at constant voltage 40 V, for a deposition time of 4 min, and an electrode separation of 2 cm. After electrophoretic deposition, the samples were dried under vacuum at room temperature for 24 h. This sample was denoted as SS–CNT. The solution for redox deposition (0.25 M KMnO4 + 0.5 M H2 SO4 ) was prepared as mentioned in the literature [22]. The SS and SS–CNT sheets were dipped in a beaker containing the freshly prepared KMnO4 + H2 SO4 solution for 1 and 30 min. The redox deposition was carried out at 50 ◦ C. The following deposition samples were cleaned with distilled water and dried in vacuum at room temperature for 24 h. At the end the resulting samples denoted hereafter as SS, SS–MnO2 -1, SS–MnO2 -30, SS–CNT–MnO2 -1 and SS–CNT–MnO2 -30. The morphologies and chemical composition of the samples were observed by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis using a HITACHI S-4300SE/N (VP-SEM) apparatus, respectively. The sample electrochemical characteristics were determined by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD). A Solartron 1470 battery test operated under Corrware II software (Scribner associates) was used for CV and GCD measurements. Electrochemical measurements were carried out using a three-electrode cell with the reference electrode and counter electrode being an Hg/HgSO4 (saturated K2 SO4 ) and a platinum foil, respectively. The electrochemical impedance measurements were conducted by sweeping the frequency from 100 kHz to 0.01 Hz range by using an ac amplitude of 10 mV at potential of 0 V vs Hg/HgSO4 using a Solartron 1255 B frequency response analyzer. The mass loading (m) per unit of area of one electrode was reported in Table 1. The mass load-
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ing corresponds to the mass of CNT for SS–CNT, the mass of MnO2 for SS–MnO2 -1 and SS–MnO2 -30 and the mass of CNT + MnO2 for SS–CNT–MnO2 -1 and SS–CNT–MnO2 -30 respectively. These values were used to compute the specific capacitance of these electrodes. The loading mass was determined by weighing the electrode before and after the deposition of MnO2 . It is important to note that we have used a very accurate balance for these measurements (m = 0.01 mg). In our experiment, samples were of 2 cm2 area and only 1 cm2 area was immersed in the electrolyte. The electrical connection to the potentiostat was made by a crocodile clip to the non-immersed area. Care was taken to avoid a contact of the clips with the electrolytic solution. All the experiments were conducted at 25 ◦ C. The electrolyte was 0.65 M K2 SO4 for all experiments. It was degassed with N2 prior to electrochemical measurements and a blanket of the gas was maintained throughout the course of the experiments. The values of specific capacitance C (F/g) were estimated by cyclic voltammetry by integrating the cyclic voltammogram curve to obtain the voltammetric charge (Q), and subsequently dividing this charge by the mass of the active material (m) and the width of the potential windows (E), C=
Q Em
(1)
The capacitance from the constant current charge–discharge curve was calculated using: C=
Itd ECC m
(2)
where I is the total current, td the discharge time, ECC the CC potential drop during constant current discharge. 3. Results and discussion Scanning electronic microscopy (SEM) of the various electrodes is shown in Fig. 1. The stainless steel substrate presents a smooth surface (Fig. 1A and B), whereas a rougher morphology is obtained following the electrophoretic deposition of CNTs on its surface (Fig. 1C and D). The CNTs film is highly porous with open space between the entangled nanotubes although it appears to completely coat the SS substrate. Both the SS and SS–CNT electrodes were subsequently used as substrate (and reducing agent) for the spontaneous formation of MnO2 from MnO4 − ions. The SEM images of SS–MnO2 -1 electrode shown in Fig. 1E and F illustrate the deposition of MnO2 nanowires on the SS substrate. A longer time of deposition (30 min) allows to increase the amount of deposited MnO2 (see also Table 1). The SEM images of the SS–CNT–MnO2 -1 sample displayed in Fig. 1G–I show that the carbon nanotubes are covered by manganese oxide and the thickness of the MnO2 deposited layer is estimated to few nanometers. Presumably, the deposition of manganese oxide on carbon nanotubes in this thin porous layer form allows penetration of the electrolyte and maximization of its electrochemical utilization, which is very important for electrochemical capacitor application. Fig. 1J–L presents SEM images of the SS–CNT–MnO2 -30 electrode. This material is characterized by a nanostructured morphology which consists of manganese oxide nanowires (diameter: around 5 nm, length: more than 100 nm). In this sample, nanosized-manganese oxide covers totally the carbon nanotubes. Based on SEM images, we can notice that a 30 min dipping time allows the formation of
Table 1 The mass loading (m) per unit of area of each electrode. Sample
SS–CNT
SS–MnO2 -1
SS–MnO2 -30
SS–CNT–MnO2 -1
SS–CNT–MnO2 -30
Active mass (mg)
0.01
0.09
0.216
0.03
0.08
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Fig. 1. SEM images of: (A and B) SS, (C and D) SS–CNT, (E and F) SS–MnO2 -1, (G–I) SS–CNT–MnO2 -1, (J–L) SS–CNT–MnO2 -30.
nanosized-manganese oxide which provides a better covering of the substrate (SS or SS–CNT) by nanosized-manganese oxide in comparison to a 1 min dipping time. From the images of Fig. 1I and L, the thickness of the active material (CNT–MnO2 ) in the SS–CNT–MnO2 -1 and SS–CNT–MnO2 -30 electrodes is estimated to be 300 nm.
The EDX analysis (results not shown) revealed that the atomic ratio of oxygen and manganese is 2. Therefore, the as-formed coating is presumably MnO2 . Also, MnO2 is known to be the most thermodynamically stable form of manganese species in an aqueous solution containing MnO4 − ions. Therefore, MnO4 − ions tend to oxidize water with the concurrent evolution of oxygen and spon-
T. Bordjiba, D. Bélanger / Electrochimica Acta 55 (2010) 3428–3433
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Fig. 2. Cyclic voltamograms of SS–CNT, SS–MnO2 -1, SS–MnO2 -30, SS–CNT–MnO2 -1 and SS–CNT–MnO2 -30 electrodes in 0.65 M K2 SO4 aqueous solution at a scan rate of 1 mV/s. Inset: cyclic voltamograms of SS and SS–CNT electrodes in 0.65 M K2 SO4 aqueous solution at a scan rate of 1 mV/s.
taneous precipitation of MnO2 [23]. Buriak and coworkers have observed that metal nanoparticle form spontaneously on semiconductor and metal surfaces when soaked in a solution containing certain redox-active species [24]. The spontaneous reduction of metal ions to metallic nanoparticles on the CNTs in aqueous solutions of noble metal ions has been reported [25]. Our results are in agreement with previous reports on the spontaneous formation of MnO2 from MnO4 − ions on CNTs [19,22,26]. The driving force for this reaction is due to the difference in the reduction potential between the CNT and MnO4 − ions [19]. The deposition of MnO2 on SS can be also due to difference in reduction potential between the metal and permanganate ions. The work function of carbon and Fe are 5 and 4.81 eV, respectively [27]. This means that Fe can also reduce MnO4 − ions as well as carbon. In the present work, the SS gauze was soaked in the KMnO4 + H2 SO4 solution at 50 ◦ C. The autocatalytic nature of permanganate decomposition in acidic solutions can also explain the deposition of MnO2 on the SS sheet [28]. Moreover, it has been shown that in acidic solution, permanganate is first reduced to Mn2+ , which subsequently reacts with KMnO4 to generate MnO2 [29]. Fig. 2 (inset) shows the cyclic voltammogram of a SS–CNT electrode in 0.65 M K2 SO4 which exhibits non-ideal capacitive behaviour since their shape is far from rectangular. The increase of the current at the positive and negative (for the SS–CNT electrode) potential limits is due to unidentified redox processes. The CV response of the SS–CNT electrode is 5 times larger than that of the bare SS electrode of an equivalent geometric area. Nyquist plot of the SS–CNT electrode (results not shown) shows a resistance of 2.98 (three-electrode cell and not optimized connections), which is an indication of the good electrical contact between CNT and SS. However, in a practical device a much lower contact resistance would be mandatory. This will be required more work on surface treatment of the current collector to decrease this contact resistance. Fig. 2 shows the CV for SS–CNT, SS–MnO2 -1, SS–MnO2 -30, SS–CNT–MnO2 -1 and SS–CNT–MnO2 -30 electrodes in the same experimental conditions. The five CVs display an almost rectangular shape over 800 mV even trough strong polarization can be noticed. The SS–CNT electrode shows a specific capacitance of about 145 F/g, which is consistent with values reported for CNT in the literature [30,31]. The specific capacitances of the SS–MnO2 electrodes are much lower (260–360 F/g) than that of the SS–CNT–MnO2 electrode (480–813 F/g).
Fig. 3. (A) Cyclic voltammograms of SS–CNT–MnO2 -30 electrodes in a 0.65 M K2 SO4 aqueous solution at different scan rates. Inset: variation of the specific capacitance of SS–CNT–MnO2 -30 electrode as function of scan rate. (B) Charge–discharge curve (cell voltage vs time) of SS–CNT–MnO2 -30 electrode at a current of 2.5 A/g. Inset: variation of the specific capacitance of SS–CNT–MnO2 -30 electrode as a function of applied current.
Fig. 3A shows the effect of the scan rate on the CV response of a SS–CNT–MnO2 -30 electrode. An increase of the current with an increase of the scan rate is observed for the electrode with no significant modification of shape of the cyclic voltammograms at high scan rate. The specific capacitance of SS–CNT–MnO2 electrodes decreased upon increasing of the scan rate (inset Fig. 3A). The specific capacitance decreased to 448 F/g at scan rate of 50 mV/s, which is only 55% of that measured at 1 mV/s. This decrease is attributed to the low conductivity of the MnO2 layer. The specific capacitance of the SS–CNT–MnO2 -30 electrode decreases to 195 F/g at a scan rate 200 mV/s (results not shown), which is 24% of the value measured at 1 mV/s (813 F/g). Therefore, this nanocomposite electrode shows a rate capability similar to that based on manganese oxide and carbon nanotubes developed in our former study [16]. Typical constant current charge–discharge curves presented in Fig. 3B shows an initial potential drop that is followed by a linear variation of potential. Specific capacitances determined as a function of the applied current are reported as inset in Fig. 3B. The specific capacitance calculated at a constant current of 2.5 A/g is estimated to be 869 F/g (equivalent to 70 mF/cm2 ), which is close to that one calculated from the CV at 1 mV/s. As expected, it can be seen that the capacitance slightly decreased as the charge/discharge current increased. The capacitance of this nanocomposite which is higher than the previously reported value (325 F/g) for a nanocomposite electrode based on carbon nanotubes and manganese oxide deposited on carbon paper, CP–CNT–MnO2 [16] is due to: (i) the morphology and mass loading of nanosized-manganese oxide, (ii) the use of carbon nanotubes with manganese oxide, (iii) the binderless character of the electrode and the preparation method of the nanocomposite CNT–MnO2 electrode. Firstly, unlike a dense
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film, nanosized-manganese oxide nanowires provide high specific capacitance due to its porous structure that allows the electrolyte to penetrate deeply into the active electrode material [12–15,32]. This maximizes the electrochemical utilization of manganese oxide and overcome the principal disadvantage of manganese oxide, which is its high density. In addition the mass loading of the SS–CNT–MnO2 -30 and SS–CNT–MnO2 -1 electrode is lower than that of the CP–CNT–MnO2 electrode also maximizes the relative electrochemical utilization of manganese oxide. Several studies have reported the beneficial effect of the nanostructured manganese oxide for which specific capacitance ranging between 480 and 700 F/g have been reported [12–15,32]. Thus, our nanocomposite electrode shows a 24% improvement of its capacitance in comparison with these studies. Secondly, carbon nanotubes have been also recognized as promising material for electrochemical capacitors due to their interesting properties such as high electric conductivity, good chemical stability and nanosized effect [30,31]. The use of carbon nanotubes with manganese oxide leads to high specific capacitance because the carbon nanotubes act as a highly conductive current collector, the through-connected porosity serves as a continuous pathway for electrolyte transport, and the nanostructured MnO2 minimizes the transport distance for ions into the oxide [28]. Several studies have reported the beneficial effect of the combination of manganese oxide with carbon nanotubes (in the present study we have combined carbon nanotubes with manganese oxide nanowires), for example, the specific capacitance of the MnO2 in a MnO2 –CNT nanocomposite can be as high as 580 F/g [19]. Thirdly, the binderless character of the electrode and the preparation method of the CNT–MnO2 nanocomposite allows to increase the specific capacitance. Conventional methods of preparation of these nanocomposites comprise three steps: 1—dispersion of CNTs at the manganese oxide surface, 2—formation of composite film with an appropriate binder, and 3—assembly of the composite film on a current collector. It is well known that the dispersion is rather difficult and the adhesion of nanotubes to the MnO2 matrix material still present considerable challenges. Indeed the effective utilization of CNTs when used in a composite depends strongly on the ability to disperse the CNTs individually and uniformly throughout the host matrix without destroying their integrity or reducing their aspect ratio. In the present work, the nanosized-manganese oxide was directly deposited on carbon nanotubes which were previously deposited on the current collector to allow a better distribution of the nanosized-manganese oxide on the surface of carbon nanotubes and also avoid the use of binder. The combination of the three previous factors leads to a 24% increase of the specific capacitance of manganese oxide but which is still far from the theoretical specific capacitance of MnO2 which should be around 1370 F/g [7,8]. The cycling stability of the SS–CNT–MnO2 -30 nanocomposite electrode was also investigated for 1000 galvanostatic charge–discharge cycles in 0.65 M K2 SO4 electrolyte (Fig. 4). A slight decrease of the specific capacitance is observed. Thus, this nanocomposite electrode shows a relatively good cycling stability. The synthesis methods used in our work (electrophoretic deposition of CNTs on the conductor substrate and direct redox deposition of metal oxide from corresponding metallic ions) present the advantages to be simple, low cost and easily transferable to an industrial process. Another binderless manganese oxide/CNT composite has been recently reported but its fabrication requires a more complex experimental procedure involving chemical vapour deposition of carbon nanotubes arrays [4]. Furthermore, the directed growth of CNTs on a conducting substrate may provide CNTs with low purity, and further purification process (chemical or thermal) of this material led to the alteration and/or oxidation of the conductor substrate [18]. On the other hand, electrophoretic deposition has been shown to be versatile and effective to deposit
Fig. 4. Variation of the specific capacitance of a SS–CNT–MnO2 -30 nanocomposite electrode as function of cycle number. Electrolyte: 0.65 M K2 SO4 aqueous solution, charge and discharge at a current of 2.5 A/g.
purified, and/or functionalized CNTs on conductor substrate [21]. The direct redox deposition of metal oxide on a conductor substrate has attracted great interest due to simplicity of operation, cost effectiveness, high throughput, and lack of elaborate equipment [24]. The combination of the carbon nanotubes and nanosized metal oxide synthesized by using simple methods can open new routes for developing high performance electrode for future generation of electrochemical power sources. In this study, we have presented the development of a new composite based on nanosized-manganese oxide and carbon nanotubes. The specific capacitance of this nanocomposite is as high as 869 F/g. To the best of our knowledge, this is one of the highest capacitance for manganese oxide electrode ever reported in the open literature. 4. Conclusion To summarize, new low cost nanocomposite materials based on nanosized-manganese oxide and carbon nanotubes were synthesized by using the electrophoretic deposition (EPD) of CNTs on a stainless steel (SS) substrate followed by direct spontaneous reduction of MnO4 − ions on the multi-scaled SS–CNT substrate to form MnO2 . The specific capacitance of the nanocomposite SS–CNT–MnO2 electrode is as high as 869 F/g and stable over long cycling. In addition to applicability in electrochemical capacitors, this methodology could be extended to develop a new high performance nanocomposite material electrode based on carbon nanotubes and metal oxide for future generation of electrochemical power sources. This strategy can find application in not only electrochemical power sources devices but also for catalysis, sensors and microelectronics. Acknowledgements The authors would like to thank Université du Québec à Montréal (UQAM), NanoQAM and Defence Research and Development Canada for their support for this work. References ` [1] N. Mackiewicz, G. Surendran, H. Remita, B. Keita, G. Zhang, L. Nadjo, A. Hagege, E. Doris, C. Mioskowski, J. Am. Chem. Soc. 130 (2008) 8110. [2] J. Li, S. Tang, L. Lu, H.C. Zeng, J. Am. Chem. Soc. 9 (2007) 9401. [3] S.-L. Chou, J.-Z. Wang, S.-Y. Chew, H.-K. Liu, S.-X. Dou, Electrochem. Commun. 10 (2008) 1724. [4] H. Zhang, G. Cao, Z. Wang, Y. Yang, Z. Shi, Z. Gu, Nano Lett. 8 (2008) 2664. [5] W.C. Fang, O. Chyan, C.L. Sun, C.T. Wu, C.P. Chen, K.H. Chen, Electrochem. Commun. 9 (2007) 239. [6] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845. [7] D. Bélanger, T. Brousse, J.W. Long, Interface 17 (2008) 49. [8] M. Toupin, T. Brousse, D. Bélanger, Chem. Mater. 16 (2004) 3184.
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[21] B.J.C. Thomas, A.R. Boccaccini, M.S.P. Shaffer, J. Am. Ceram. Soc. 88 (2005) 980. [22] M. Wu, L. Zhang, J. Gao, Y. Zhou, S. Zhang, A. Chen, J. Electrochem. Soc. 155 (2008) A355. [23] S.B. Ma, K.Y. Ahn, E.-S. Lee, K.H. Oh, K.B. Kim, Carbon 45 (2007) 375. [24] L.A. Porter, H.C. Choi, J.M. Schmeltzer, A.E. Ribbe, L.C.C. Elliott, J.M. Buriak, Nano Lett. 2 (2002) 1369. [25] C. Choi, M. Shim, S. Bangsaruntip, H. Dai, J. Am. Chem. Soc. 124 (2002) 9058. [26] X. Xie, L. Gao, Carbon 45 (2007) 2365. [27] CRC Handbook on Chemistry and Physics Version, 2008, pp. 12–114. [28] A.E. Fischer, K.A. Pettigrew, D.R. Rolison, R.M. Stroud, J.W. Long, Nano Lett. 7 (2007) 281. [29] X. Han, L. Zhou, H. Liu, Y. Hu, Polym. Degrad. Stab. 92 (2007) 75. [30] A.G. Pandolfo, A.F. Hollenkamp, J. Power Sources 157 (2006) 11. [31] E. Frackowiak, F. Béguin, Carbon 39 (2001) 937. [32] K. Prasad, N. Miura, J. Power Sources 135 (2004) 354.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Development of new nanocomposite based on nanosized-manganese oxide and carbon nanotubes for high performance electrochemical capacitors Tarik Bordjiba, Daniel Bélanger ∗,1 Département de Chimie, Université du Québec à Montréal, Case Postale 8888, succursale Centre-Ville, Montréal, Québec, H3C 3P8 Canada
a r t i c l e
i n f o
Article history: Received 3 July 2009 Received in revised form 24 December 2009 Accepted 27 December 2009 Available online 11 January 2010 Keywords: Electrochemical capacitors Carbon nanotubes Manganese oxide Nanocomposite Energy storage
a b s t r a c t We report the synthesis of a new composite electrode based on nanosized-manganese oxide and carbon nanotubes (CNTs) by electrophoretic deposition of CNTs on a stainless steel (SS) substrate followed by direct spontaneous reduction of MnO4 − ions to MnO2 to form the multi-scaled SS–CNT–MnO2 electrode. The resulting material was characterized by scanning electron microscopy, energy dispersive X-ray analysis, cyclic voltammetry and galvanostatic charge–discharge in a 0.65 M K2 SO4 aqueous solution. The binderless SS–CNT–MnO2 nanocomposite electrode shows a very high specific capacitance of 869 F/g of CNT–MnO2 and good stability during long galvanostatic charge–discharge cycling. To the best of our knowledge, this is one of the highest capacitance for manganese oxide electrode ever reported. In addition to its applicability in electrochemical capacitors, this methodology could be extended to develop other high performance nanocomposite material electrodes based on carbon nanotubes and metal oxide for the future generation of electrochemical power sources. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction The development of advanced composite materials based on metal oxide-carbon nanotubes is a new route for achieving highly efficient electrode for electrochemical power sources such as fuel cells [1], lithium batteries [2] and electrochemical capacitors [3–5]. For electrochemical capacitors, active electrode materials include carbon, conducting polymers and transition metal oxides [6]. Manganese oxide is a promising electrode material for electrochemical capacitors due to its low cost, natural abundance, environmental safety and its high theoretical capacitance. If one Mn atom in MnO2 is assumed to store one electron, then the specific capacitance of MnO2 should be around 1370 F/g [7,8]. But, practically, this oxide shows a specific capacitance of only one-fifth or one-sixth of the above value [9,10]. Such low practical specific capacitance is due to the intrinsically poor electronic conductivity and dense morphology of the oxide [9,11]. On the other hand, the challenge with the MnO2 lies in maximizing its electrochemical utilization. Currently, there are mainly two efficient ways to reach high specific capacitance with manganese oxide. The first one is by developing nanostructured manganese oxide, which allows reaching a specific capacitance in the range of 700 F/g [12–15]. The second one is by the incorporation of carbon nanotubes in the MnO2 matrix, which
∗ Corresponding author. Tel.: +1 514 987 3000x3909; fax: +1 514 987 4054. E-mail address: [email protected] (D. Bélanger). 1 ISE member. 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.12.070
allows to reach specific capacitance ranging from 325 to 580 F/g [16–19]. In this report, we describe a low cost method for the development of a new class of binderless nanocomposites materials based on nanostructured metal oxide (manganese oxide) and carbon nanotubes. This nanocomposite is prepared following the procedure shown in Scheme 1 which consists of the electrophoretic deposition (EPD) of CNTs on a stainless steel (SS) substrate followed by direct spontaneous reduction of MnO4 − ions on the multi-scaled SS–CNT substrate to form manganese oxide (MnO2 ). An important feature of this nanocomposite is that is does not rely on the use of a binder. In fact, a common approach to fabricate electrodes is to add to the active material, a conductivity enhancer and a binder to promote the adhesion of the particles. Importantly, the use of binder and/or other adhesives may occlude a fraction of the surface of the active materials or electrically isolate some particles. Our CNT–MnO2 nanocomposite is fabricated directly on the SS substrate without any binder or conductive additive, which makes the CNT–MnO2 nanocomposite even more interesting as binderless electrode for electrochemical capacitors. 2. Experimental Purified (>90%) CNTs (Multiwall, O.D. × I.D. × length 10–15 nm × 2–6 nm × 0.1–10 m) prepared by chemical vapor deposition (CVD) method were purchased from Aldrich. All other chemicals used in this study were also purchased from Aldrich and used as received. Distilled water of 18 M obtained through
T. Bordjiba, D. Bélanger / Electrochimica Acta 55 (2010) 3428–3433
Scheme 1. Procedure for the fabrication of the SS, SS–MnO2 -1, SS–MnO2 -30, SS–CNT–MnO2 -1, and SS–CNT–MnO2 -30 electrodes.
Nanopure II (Branstead) was used to prepare the solutions. Stainless steel gauze type 304 (SS) was obtained from Alfa Aesar. Scheme 1 presents the concept for the fabrication of the electrodes used in this work. To obtain CNTs with acidic sites on the surface, typically, (0.8 g) of carbon nanotubes were refluxed in (60 mL) of mixed (1:3 volume ratio) concentrated nitric and sulfuric acids at 130 ◦ C for 30 min [20]. The oxidized tubes were washed with distilled water. For simplicity, thereafter the oxidized CNTs are denoted as CNTs. An aqueous suspension (concentration 0.5 mg/mL) was used for the electrophoretic deposition of CNTs on the SS substrate [21]. The 2 cm × 1 cm stainless steel substrate was used following a cleaning step performed by ultrasonic treatment in acetone for 10 min and drying under vacuum at room temperature for 24 h. Electrophoretic deposition was conducted at constant voltage 40 V, for a deposition time of 4 min, and an electrode separation of 2 cm. After electrophoretic deposition, the samples were dried under vacuum at room temperature for 24 h. This sample was denoted as SS–CNT. The solution for redox deposition (0.25 M KMnO4 + 0.5 M H2 SO4 ) was prepared as mentioned in the literature [22]. The SS and SS–CNT sheets were dipped in a beaker containing the freshly prepared KMnO4 + H2 SO4 solution for 1 and 30 min. The redox deposition was carried out at 50 ◦ C. The following deposition samples were cleaned with distilled water and dried in vacuum at room temperature for 24 h. At the end the resulting samples denoted hereafter as SS, SS–MnO2 -1, SS–MnO2 -30, SS–CNT–MnO2 -1 and SS–CNT–MnO2 -30. The morphologies and chemical composition of the samples were observed by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis using a HITACHI S-4300SE/N (VP-SEM) apparatus, respectively. The sample electrochemical characteristics were determined by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD). A Solartron 1470 battery test operated under Corrware II software (Scribner associates) was used for CV and GCD measurements. Electrochemical measurements were carried out using a three-electrode cell with the reference electrode and counter electrode being an Hg/HgSO4 (saturated K2 SO4 ) and a platinum foil, respectively. The electrochemical impedance measurements were conducted by sweeping the frequency from 100 kHz to 0.01 Hz range by using an ac amplitude of 10 mV at potential of 0 V vs Hg/HgSO4 using a Solartron 1255 B frequency response analyzer. The mass loading (m) per unit of area of one electrode was reported in Table 1. The mass load-
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ing corresponds to the mass of CNT for SS–CNT, the mass of MnO2 for SS–MnO2 -1 and SS–MnO2 -30 and the mass of CNT + MnO2 for SS–CNT–MnO2 -1 and SS–CNT–MnO2 -30 respectively. These values were used to compute the specific capacitance of these electrodes. The loading mass was determined by weighing the electrode before and after the deposition of MnO2 . It is important to note that we have used a very accurate balance for these measurements (m = 0.01 mg). In our experiment, samples were of 2 cm2 area and only 1 cm2 area was immersed in the electrolyte. The electrical connection to the potentiostat was made by a crocodile clip to the non-immersed area. Care was taken to avoid a contact of the clips with the electrolytic solution. All the experiments were conducted at 25 ◦ C. The electrolyte was 0.65 M K2 SO4 for all experiments. It was degassed with N2 prior to electrochemical measurements and a blanket of the gas was maintained throughout the course of the experiments. The values of specific capacitance C (F/g) were estimated by cyclic voltammetry by integrating the cyclic voltammogram curve to obtain the voltammetric charge (Q), and subsequently dividing this charge by the mass of the active material (m) and the width of the potential windows (E), C=
Q Em
(1)
The capacitance from the constant current charge–discharge curve was calculated using: C=
Itd ECC m
(2)
where I is the total current, td the discharge time, ECC the CC potential drop during constant current discharge. 3. Results and discussion Scanning electronic microscopy (SEM) of the various electrodes is shown in Fig. 1. The stainless steel substrate presents a smooth surface (Fig. 1A and B), whereas a rougher morphology is obtained following the electrophoretic deposition of CNTs on its surface (Fig. 1C and D). The CNTs film is highly porous with open space between the entangled nanotubes although it appears to completely coat the SS substrate. Both the SS and SS–CNT electrodes were subsequently used as substrate (and reducing agent) for the spontaneous formation of MnO2 from MnO4 − ions. The SEM images of SS–MnO2 -1 electrode shown in Fig. 1E and F illustrate the deposition of MnO2 nanowires on the SS substrate. A longer time of deposition (30 min) allows to increase the amount of deposited MnO2 (see also Table 1). The SEM images of the SS–CNT–MnO2 -1 sample displayed in Fig. 1G–I show that the carbon nanotubes are covered by manganese oxide and the thickness of the MnO2 deposited layer is estimated to few nanometers. Presumably, the deposition of manganese oxide on carbon nanotubes in this thin porous layer form allows penetration of the electrolyte and maximization of its electrochemical utilization, which is very important for electrochemical capacitor application. Fig. 1J–L presents SEM images of the SS–CNT–MnO2 -30 electrode. This material is characterized by a nanostructured morphology which consists of manganese oxide nanowires (diameter: around 5 nm, length: more than 100 nm). In this sample, nanosized-manganese oxide covers totally the carbon nanotubes. Based on SEM images, we can notice that a 30 min dipping time allows the formation of
Table 1 The mass loading (m) per unit of area of each electrode. Sample
SS–CNT
SS–MnO2 -1
SS–MnO2 -30
SS–CNT–MnO2 -1
SS–CNT–MnO2 -30
Active mass (mg)
0.01
0.09
0.216
0.03
0.08
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Fig. 1. SEM images of: (A and B) SS, (C and D) SS–CNT, (E and F) SS–MnO2 -1, (G–I) SS–CNT–MnO2 -1, (J–L) SS–CNT–MnO2 -30.
nanosized-manganese oxide which provides a better covering of the substrate (SS or SS–CNT) by nanosized-manganese oxide in comparison to a 1 min dipping time. From the images of Fig. 1I and L, the thickness of the active material (CNT–MnO2 ) in the SS–CNT–MnO2 -1 and SS–CNT–MnO2 -30 electrodes is estimated to be 300 nm.
The EDX analysis (results not shown) revealed that the atomic ratio of oxygen and manganese is 2. Therefore, the as-formed coating is presumably MnO2 . Also, MnO2 is known to be the most thermodynamically stable form of manganese species in an aqueous solution containing MnO4 − ions. Therefore, MnO4 − ions tend to oxidize water with the concurrent evolution of oxygen and spon-
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Fig. 2. Cyclic voltamograms of SS–CNT, SS–MnO2 -1, SS–MnO2 -30, SS–CNT–MnO2 -1 and SS–CNT–MnO2 -30 electrodes in 0.65 M K2 SO4 aqueous solution at a scan rate of 1 mV/s. Inset: cyclic voltamograms of SS and SS–CNT electrodes in 0.65 M K2 SO4 aqueous solution at a scan rate of 1 mV/s.
taneous precipitation of MnO2 [23]. Buriak and coworkers have observed that metal nanoparticle form spontaneously on semiconductor and metal surfaces when soaked in a solution containing certain redox-active species [24]. The spontaneous reduction of metal ions to metallic nanoparticles on the CNTs in aqueous solutions of noble metal ions has been reported [25]. Our results are in agreement with previous reports on the spontaneous formation of MnO2 from MnO4 − ions on CNTs [19,22,26]. The driving force for this reaction is due to the difference in the reduction potential between the CNT and MnO4 − ions [19]. The deposition of MnO2 on SS can be also due to difference in reduction potential between the metal and permanganate ions. The work function of carbon and Fe are 5 and 4.81 eV, respectively [27]. This means that Fe can also reduce MnO4 − ions as well as carbon. In the present work, the SS gauze was soaked in the KMnO4 + H2 SO4 solution at 50 ◦ C. The autocatalytic nature of permanganate decomposition in acidic solutions can also explain the deposition of MnO2 on the SS sheet [28]. Moreover, it has been shown that in acidic solution, permanganate is first reduced to Mn2+ , which subsequently reacts with KMnO4 to generate MnO2 [29]. Fig. 2 (inset) shows the cyclic voltammogram of a SS–CNT electrode in 0.65 M K2 SO4 which exhibits non-ideal capacitive behaviour since their shape is far from rectangular. The increase of the current at the positive and negative (for the SS–CNT electrode) potential limits is due to unidentified redox processes. The CV response of the SS–CNT electrode is 5 times larger than that of the bare SS electrode of an equivalent geometric area. Nyquist plot of the SS–CNT electrode (results not shown) shows a resistance of 2.98 (three-electrode cell and not optimized connections), which is an indication of the good electrical contact between CNT and SS. However, in a practical device a much lower contact resistance would be mandatory. This will be required more work on surface treatment of the current collector to decrease this contact resistance. Fig. 2 shows the CV for SS–CNT, SS–MnO2 -1, SS–MnO2 -30, SS–CNT–MnO2 -1 and SS–CNT–MnO2 -30 electrodes in the same experimental conditions. The five CVs display an almost rectangular shape over 800 mV even trough strong polarization can be noticed. The SS–CNT electrode shows a specific capacitance of about 145 F/g, which is consistent with values reported for CNT in the literature [30,31]. The specific capacitances of the SS–MnO2 electrodes are much lower (260–360 F/g) than that of the SS–CNT–MnO2 electrode (480–813 F/g).
Fig. 3. (A) Cyclic voltammograms of SS–CNT–MnO2 -30 electrodes in a 0.65 M K2 SO4 aqueous solution at different scan rates. Inset: variation of the specific capacitance of SS–CNT–MnO2 -30 electrode as function of scan rate. (B) Charge–discharge curve (cell voltage vs time) of SS–CNT–MnO2 -30 electrode at a current of 2.5 A/g. Inset: variation of the specific capacitance of SS–CNT–MnO2 -30 electrode as a function of applied current.
Fig. 3A shows the effect of the scan rate on the CV response of a SS–CNT–MnO2 -30 electrode. An increase of the current with an increase of the scan rate is observed for the electrode with no significant modification of shape of the cyclic voltammograms at high scan rate. The specific capacitance of SS–CNT–MnO2 electrodes decreased upon increasing of the scan rate (inset Fig. 3A). The specific capacitance decreased to 448 F/g at scan rate of 50 mV/s, which is only 55% of that measured at 1 mV/s. This decrease is attributed to the low conductivity of the MnO2 layer. The specific capacitance of the SS–CNT–MnO2 -30 electrode decreases to 195 F/g at a scan rate 200 mV/s (results not shown), which is 24% of the value measured at 1 mV/s (813 F/g). Therefore, this nanocomposite electrode shows a rate capability similar to that based on manganese oxide and carbon nanotubes developed in our former study [16]. Typical constant current charge–discharge curves presented in Fig. 3B shows an initial potential drop that is followed by a linear variation of potential. Specific capacitances determined as a function of the applied current are reported as inset in Fig. 3B. The specific capacitance calculated at a constant current of 2.5 A/g is estimated to be 869 F/g (equivalent to 70 mF/cm2 ), which is close to that one calculated from the CV at 1 mV/s. As expected, it can be seen that the capacitance slightly decreased as the charge/discharge current increased. The capacitance of this nanocomposite which is higher than the previously reported value (325 F/g) for a nanocomposite electrode based on carbon nanotubes and manganese oxide deposited on carbon paper, CP–CNT–MnO2 [16] is due to: (i) the morphology and mass loading of nanosized-manganese oxide, (ii) the use of carbon nanotubes with manganese oxide, (iii) the binderless character of the electrode and the preparation method of the nanocomposite CNT–MnO2 electrode. Firstly, unlike a dense
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film, nanosized-manganese oxide nanowires provide high specific capacitance due to its porous structure that allows the electrolyte to penetrate deeply into the active electrode material [12–15,32]. This maximizes the electrochemical utilization of manganese oxide and overcome the principal disadvantage of manganese oxide, which is its high density. In addition the mass loading of the SS–CNT–MnO2 -30 and SS–CNT–MnO2 -1 electrode is lower than that of the CP–CNT–MnO2 electrode also maximizes the relative electrochemical utilization of manganese oxide. Several studies have reported the beneficial effect of the nanostructured manganese oxide for which specific capacitance ranging between 480 and 700 F/g have been reported [12–15,32]. Thus, our nanocomposite electrode shows a 24% improvement of its capacitance in comparison with these studies. Secondly, carbon nanotubes have been also recognized as promising material for electrochemical capacitors due to their interesting properties such as high electric conductivity, good chemical stability and nanosized effect [30,31]. The use of carbon nanotubes with manganese oxide leads to high specific capacitance because the carbon nanotubes act as a highly conductive current collector, the through-connected porosity serves as a continuous pathway for electrolyte transport, and the nanostructured MnO2 minimizes the transport distance for ions into the oxide [28]. Several studies have reported the beneficial effect of the combination of manganese oxide with carbon nanotubes (in the present study we have combined carbon nanotubes with manganese oxide nanowires), for example, the specific capacitance of the MnO2 in a MnO2 –CNT nanocomposite can be as high as 580 F/g [19]. Thirdly, the binderless character of the electrode and the preparation method of the CNT–MnO2 nanocomposite allows to increase the specific capacitance. Conventional methods of preparation of these nanocomposites comprise three steps: 1—dispersion of CNTs at the manganese oxide surface, 2—formation of composite film with an appropriate binder, and 3—assembly of the composite film on a current collector. It is well known that the dispersion is rather difficult and the adhesion of nanotubes to the MnO2 matrix material still present considerable challenges. Indeed the effective utilization of CNTs when used in a composite depends strongly on the ability to disperse the CNTs individually and uniformly throughout the host matrix without destroying their integrity or reducing their aspect ratio. In the present work, the nanosized-manganese oxide was directly deposited on carbon nanotubes which were previously deposited on the current collector to allow a better distribution of the nanosized-manganese oxide on the surface of carbon nanotubes and also avoid the use of binder. The combination of the three previous factors leads to a 24% increase of the specific capacitance of manganese oxide but which is still far from the theoretical specific capacitance of MnO2 which should be around 1370 F/g [7,8]. The cycling stability of the SS–CNT–MnO2 -30 nanocomposite electrode was also investigated for 1000 galvanostatic charge–discharge cycles in 0.65 M K2 SO4 electrolyte (Fig. 4). A slight decrease of the specific capacitance is observed. Thus, this nanocomposite electrode shows a relatively good cycling stability. The synthesis methods used in our work (electrophoretic deposition of CNTs on the conductor substrate and direct redox deposition of metal oxide from corresponding metallic ions) present the advantages to be simple, low cost and easily transferable to an industrial process. Another binderless manganese oxide/CNT composite has been recently reported but its fabrication requires a more complex experimental procedure involving chemical vapour deposition of carbon nanotubes arrays [4]. Furthermore, the directed growth of CNTs on a conducting substrate may provide CNTs with low purity, and further purification process (chemical or thermal) of this material led to the alteration and/or oxidation of the conductor substrate [18]. On the other hand, electrophoretic deposition has been shown to be versatile and effective to deposit
Fig. 4. Variation of the specific capacitance of a SS–CNT–MnO2 -30 nanocomposite electrode as function of cycle number. Electrolyte: 0.65 M K2 SO4 aqueous solution, charge and discharge at a current of 2.5 A/g.
purified, and/or functionalized CNTs on conductor substrate [21]. The direct redox deposition of metal oxide on a conductor substrate has attracted great interest due to simplicity of operation, cost effectiveness, high throughput, and lack of elaborate equipment [24]. The combination of the carbon nanotubes and nanosized metal oxide synthesized by using simple methods can open new routes for developing high performance electrode for future generation of electrochemical power sources. In this study, we have presented the development of a new composite based on nanosized-manganese oxide and carbon nanotubes. The specific capacitance of this nanocomposite is as high as 869 F/g. To the best of our knowledge, this is one of the highest capacitance for manganese oxide electrode ever reported in the open literature. 4. Conclusion To summarize, new low cost nanocomposite materials based on nanosized-manganese oxide and carbon nanotubes were synthesized by using the electrophoretic deposition (EPD) of CNTs on a stainless steel (SS) substrate followed by direct spontaneous reduction of MnO4 − ions on the multi-scaled SS–CNT substrate to form MnO2 . The specific capacitance of the nanocomposite SS–CNT–MnO2 electrode is as high as 869 F/g and stable over long cycling. In addition to applicability in electrochemical capacitors, this methodology could be extended to develop a new high performance nanocomposite material electrode based on carbon nanotubes and metal oxide for future generation of electrochemical power sources. This strategy can find application in not only electrochemical power sources devices but also for catalysis, sensors and microelectronics. Acknowledgements The authors would like to thank Université du Québec à Montréal (UQAM), NanoQAM and Defence Research and Development Canada for their support for this work. References ` [1] N. Mackiewicz, G. Surendran, H. Remita, B. Keita, G. Zhang, L. Nadjo, A. Hagege, E. Doris, C. Mioskowski, J. Am. Chem. Soc. 130 (2008) 8110. [2] J. Li, S. Tang, L. Lu, H.C. Zeng, J. Am. Chem. Soc. 9 (2007) 9401. [3] S.-L. Chou, J.-Z. Wang, S.-Y. Chew, H.-K. Liu, S.-X. Dou, Electrochem. Commun. 10 (2008) 1724. [4] H. Zhang, G. Cao, Z. Wang, Y. Yang, Z. Shi, Z. Gu, Nano Lett. 8 (2008) 2664. [5] W.C. Fang, O. Chyan, C.L. Sun, C.T. Wu, C.P. Chen, K.H. Chen, Electrochem. Commun. 9 (2007) 239. [6] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845. [7] D. Bélanger, T. Brousse, J.W. Long, Interface 17 (2008) 49. [8] M. Toupin, T. Brousse, D. Bélanger, Chem. Mater. 16 (2004) 3184.
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