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Preparation and electrochemical characterizations of MnO2-dispersed carbon aerogel as supercapacitor electrode material
Journal of Non-Crystalline Solids 355 (2009) 2461–2465
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Preparation and electrochemical characterizations of MnO2-dispersed carbon aerogel as supercapacitor electrode material Guifen Lv a,b, Dingcai Wu a, Ruowen Fu a,c,* a
Materials Science Institute, PCFM Laboratory, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, PR China Soil and Fertilizer & Resources and Environment Institute, Jiangxi Academy of Agricultural Sciences, Nanchang 330200, PR China c Institute of Optoelectronic and Functional Composite Materials, Sun Yat-Sen University, Guangzhou 510275, PR China b
a r t i c l e
i n f o
Article history: Received 28 May 2008 Received in revised form 27 July 2009 Available online 24 September 2009 PACS: 66.70.Df 81.05.Uw 82.47.Uv
a b s t r a c t The nano-sized amorphous a-MnO2-dispersed carbon aerogels (CAs) were prepared by liquid phase coprecipitation technique. Different MnO2 contents and heating temperatures were taken to prepare the composites. The composites prepared were used to make electrodes of electrochemical supercapacitors. Electrochemical properties of the electrodes were studied by cyclic voltammetry and galvanostatic charge–discharge. The specific capacitances of the MnO2-dispersed CAs and the nano-MnO2 ingredient in the composite are up to 219 F g 1 and 401 F g 1 at 5 mA.cm 2, 1.6 times and 5.0 times greater than that of neat CA and MnO2, respectively. Ó 2009 Published by Elsevier B.V.
Keywords: Electrochemical properties Porosity
1. Introduction Electrochemical supercapacitors are unique energy storage devices [1,2]. There are two different types electrochemical capacitors based on their energy storage mechanisms [1,3]: electrochemical double-layer capacitors (EDLC) based on carbon electrodes and pseudocapacitors with certain metal oxides and others as electrode materials. Carbon aerogels (CA) are promising materials as electrodes for EDLCs due to their attractive properties such as high electrical conductivity, high porosity, controllable pore structure and high surface area [4,5]. CA-based supercapacitor offers fast response and long cycle life, but usually has lower specific capacitance (30–150 F g 1) [6–8]. Hence, there have been considerable efforts to improve the capacitance of CA-based supercapacitor electrodes by various techniques. Transition metal oxides attached to CAs have been studied recently and showed improved capacitance behavior due to their enhanced stability and high conductivity [9,10]. Among the transition metal oxides, manganese oxide has been extensively used as an electrode material in pseudocapacitor because of abundance, low-cost, electrochemical reactivity and environmental friendliness [11–13]. * Corresponding author. Address: Materials Science Institute, PCFM Laboratory, School of Chemistry and chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, PR China. Tel./fax: +86 20 84115112. E-mail address: [email protected] (R. Fu). 0022-3093/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2009.08.035
Recently, Wang and co-workers prepared a type of MnO2xH2O/ CA composite electrode materials. In this approach, the used carbon aerogel had a lot of macroporous resulting from the stack of. ca. 100–200 nm carbon particles (determined by the reported SEM image of CA [14]), and thus nanosized MnO2xH2O particles (50–80 nm) were deposited onto the surface of porous carbon aerogels and filled the inner big pores of the carbon aerogel. More recently, in our experiment, we found that the pore structure of CA plays an important part in the formation of the microstructure of CA/MnO2 composite. When a typical mesoporous (15 nm) CA material was used to prepare CA/MnO2 composite, a new nanostructural-filamentous nano-MnO2 can be formed between the carbon nano-frameworks of CA. In this paper, the technique parameters for the preparation of this type of novel filamentous nano-MnO2-dispersed CAs with optimal electrochemical performances were explored by changing heating temperature and loading mass of MnO2. The structure of CA/MnO2 composites thus obtained was investigated by XRD and SEM; and their electrochemical properties were revealed by cyclic voltammetry and galvanostatic charge–discharge.
2. Experimental CA was fabricated by the microemulsion-templated sol–gel polymerization method developed by our group [15]. CA/MnO2
G. Lv et al. / Journal of Non-Crystalline Solids 355 (2009) 2461–2465
were carried out. Specific capacitances were calculated from galvanostatic charge curves at current density of 5 mA cm 2 according to the equation: C = I*t/m, where I, t, and m are current, charge time and the mass of the active material. 3. Results Fig. 1 shows the X-ray diffraction patterns of CA, MnO2 and their composites. CA shows the typical graphite-like micro-crystalline diffraction peaks, which center at around 23° and 44°, corresponding to 0 0 2 and 1 0 1 [14]. MnO2 shows the weak and broad peaks appeared at around 36° and 65°, indicating the amorphous nature and the small size of the crystalline of a-MnO2 [16]. CA/MnO2T100 shows both the characteristic peaks of amorphous MnO2 and CA. The peak intensity of MnO2 increases with increasing temperature from 50 to 150 °C, indicating that the crystallization degree of MnO2 increases with heating temperature. SEM observation (Fig. 2(a)) shows that the MnO2 nano-grains aggregate to form porous submicron balls with network structure. Fig. 2 (b) shows that the nano-carbon particles (ca. 20–30 nm in diameter) of the used CA are interconnected into a three-dimensional network, in which many mesopores among the carbon nano-particles are observed. These mesopores have a narrow pore size distribution with a maximum at 15 nm (see Fig. 3). These results show the CA used here is a typical mesoporous material with
0.10 2 -1
SBET: 620m g
-1
composites were prepared by a liquid phase co-precipitation technique. According to the predetermined formulations, CA was added into 0.1 M KMnO4 solution and then stirred for 12 h with a magnetic stirrer. After complete homogenization of the mixture, 0.15 M Mn(CH3COO)24H2O was added drop-wise under constant stirring. The resulting MnO2-dispersed CA was filtered and dried at predetermined heating temperatures for 12 h. In this study, the as-prepared CA/MnO2 composites with various heating temperatures and weight ratios were referred to as CA/MnO2-Txx and CA/MnO2-Mnyy, respectively. For blank test, neat MnO2 was prepared using the same procedure but without adding CA. The structures of the samples prepared here were studied by studied by scanning electron microscopy (SEM, JSM-6330F, Japan), X-ray diffraction (XRD, D-MAX 2200 VPC, Japan), and ASAP 2010 surface area analyzer (Micromeritics, USA). MnO2-dispersed CA was mixed with 10 wt% of conductive carbon black by ultrasonic vibration in ethanol for 30 min, and the resulting mixture was thoroughly homogenized with 5 wt% PTFE binder to make slurry. After that, the half-dried slurry was rolled to get appropriate thickness sheets, cut into 1 cm2 slices and then coated with nickel current collector, followed by drying the electrodes at 110 °C under vacuum for 12 h. Electrodes from neat MnO2 or CA were fabricated using the same procedure. A kind of sandwich-type supercapacitor consisting of two similar electrodes was assembled. The assembled two-electrode supercapacitors were measured in 1 M Na2SO4 aqueous solutions on IME6X electrochemical working station at ambient temperature. Cyclic voltammetry (CV) and galvanostatic charge–discharge tests
Differential Pore Volume (cm g nm )
2462
350 300
CA 250
Intensity
0.08
3 -1
Vmes: 1.00cm g
3 -1
400
MnO2
200 150
CA/MnO2-T50
100
CA/MnO2-T100
50
CA/MnO2-T150
3 -1
Vmic: 0.16cm g 0.06
0.04
0.02
0.00
20
40
60
80
Pore Width (nm)
0 20
30
40
50
60
70
2 Fig. 1. X-ray diffraction patterns of various electrode materials.
Fig. 3. BJH pore size distribution of carbon aerogels. Note: SBET: BET specific surface area; Vmes: BJH desorption cumulative specific pore volume; Vmic: Specific micropore volume. The errors of the pore structure parameters were ±1% of the reported values.
Fig. 2. SEM images of (a) MnO2; (b) CA and (c) CA/MnO2-T100.
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high surface area (620 m2 g 1) and great mesopore volume (1.00 cm3 g 1). Fig. 2 (c) shows that in the CA/MnO2-T100 composite, many filamentous MnO2 nano-materials are observed to be well dispersed in CA’s three-dimensional nanoporous networks. The CV curves recorded for MnO2, CA and CA/MnO2-T100 composite are shown in Fig. 4. CV curves of CA and CA/MnO2-T100 show typical capacitive behavior with a constant charge and discharge over the complete cycle. The CV curves are much closer to an ideal rectangle, which show a mirror image with respect to the zero current line and a rapid current response on voltage reversal at each end potential. CV curve of MnO2 is a little distorted. Pseudocapacitive behavior for MnO2 has been well studied [17], it is attributed to reversible redox reactions in an aqueous system involving protons and cations exchange with the electrolytes, the insertion of protons in the MnO2 electrode surface with the associated redox reactions of high and low oxidation states is responsible for the electron exchange reaction. Typical charge–discharge curves in Fig. 5 show that the MnO2, CA and CA/MnO2-T100 composite have stable electrochemical properties. The capacitor voltage varies linearly with time during
0.009
0.006
(c) (b)
Current (A)
0.003
(a)
0.000
charging and discharging. As well known, the IR drop is associated with the equivalent series resistance of the supercapacitor cell. The IR drop values for CA and CA/MnO2-T100 are negligible. However, such a drop is obvious for MnO2, indicating the large inner resistance of MnO2. Without doubt, this obvious IR drop difference is evidence that the CA can improve the electronic conductivity of CA/MnO2-T100. The specific capacitances are listed in Table 1. For MnO2 and CA, their specific capacitances are only 80 F g 1 and 133 F g 1, while that for CA/MnO2-T100 composite is up to 219 F g 1. The effects of heating temperature and MnO2 content on the electrochemical properties were also studied. Galvanostatic charge–discharge curves of composites with different heating temperatures are shown in Fig. 6. It can be seen that the IR drop decreases with increasing the temperature. Form Fig. 7, it can be seen that the CV curves become distorted with increasing content of MnO2. Table 2 displays the dependence of the specific capacitance on the mass fraction of MnO2 of the composite. It is obvious that MnO2 takes an important part in the specific capacitance. The specific capacitance come to the maximum value (i.e., 219 F g 1) when the MnO2 content is 37.2%. Table 2 also shows the specific capacitance of MnO2 composition in MnO2-dispersed CA with various MnO2 contents. For CA/MnO2–Mn40, since CA takes 62.8 wt% of the electrode mass and its specific capacitance is 133 F g 1, its capacitance contribution is 84 F. Based upon this, the capacitance contribution of MnO2 ingredient is calculated to be 135 F. Since MnO2 takes 37.2 wt%, the specific capacitance of MnO2 is 364 F g 1. Likewise, the specific capacitance of MnO2 composition in CA/MnO2–Mn30 and CA/MnO2–Mn50 can be calculated to be 401 F g 1 and 264 F g 1, respectively.
-0.003 Table 1 Specific capacitance of CA, MnO2 and CA/MnO2 composites with various heating temperatures.
-0.006
-0.009 0.0
0.2
0.4
0.6
0.8
1.0
Potential (V)
Sample
Heating temperature (°C)
Capacitance (F g
CA MnO2 CA/MnO2–T50 CA/MnO2–T100 CA/MnO2–T150
/ / 50 100 150
133 ± 1 80 ± 2 160 ± 2 219 ± 2 78 ± 1
1
)
Fig. 4. Cyclic voltammograms of (a) CA; (b) MnO2; (c) CA/MnO2-T100 at scan rate of 5 mV s 1.
1.2
(c)
(a) (b)
1.0
(c)
(b)
(a)
1.0
0.8
Voltage (v)
Potential (v)
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0 0
0.0 0
200
400
600
800
100
200
300
400
500
600
700
Time (seconds)
Time (seconds) Fig. 5. Charge–discharge curves for (a) CA; (b) MnO2; (c) CA/MnO2-T100 composite.
Fig. 6. Charge–discharge curves for MnO2 deposited at various heating temperatures: (a) CA/MnO2-T50; (b) CA/MnO2-T100; (c) CA/MnO2-T150.
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(b)
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Current (A)
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Current (A)
(c)
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0.02 0.00 -0.02 5mV/s 10mV/s 50mV/s 100mV/s
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(a)
0.02 0.00 -0.02 5mV/s 10mV/s 50mV/s 100mV/s
-0.04 -0.06 0.0
1.0
0.2
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1.0
Potential (V)
0.02 0.00 -0.02 5mV/S 10mV/S 50mV/S 100mV/S
-0.04 0.0
0.2
0.4
0.6
0.8
1.0
Potential (V)
Fig. 7. Cyclic voltammogram of MnO2 deposited at various content: (a) CA/MnO2–Mn30; (b) CA/MnO2–Mn40 and (c) CA/MnO2–Mn50.
Table 2 Specific capacitance of CA/MnO2 composites with various MnO2 contents. Sample
MnO2 content (wt%)
Capacitance (F g
CA/MnO2–Mn30 CA/MnO2–Mn40 CA/MnO2–Mn50
28.7 37.2 53.1
210 ± 2 219 ± 2 203 ± 2
1
)
CMn (F g
1
)
401 ± 4 364 ± 3 264 ± 3
Note: The specific capacitance of CA used in the MnO2-dispersed CAs is 133 F g 1. CMn denotes the specific capacitance of MnO2 composition of MnO2-dispersed CAs.
4. Discussion Though the mesopore dimension of CA used is larger than the diameter of MnO2 wire produced, the nano-carbon particles could bridge the filamentous MnO2 and connected them together (Fig. 2 (c)). At the same time, the three-dimensional carbon network of CA could provide a conductive pathway for MnO2. Therefore, the conductivity of the composites can be improved and active sites can be enhanced for electrochemical process due to good interconnection of the porous composite. Electrochemical improvement is attributed to the two reasons: Firstly, the CA with well developed three-dimensional nano-network offers abundant connected sites for MnO2, thus the filamentous nano-MnO2 can be formed and well dispersed. Undoubtedly, the effective specific area of MnO2 is improved sharply and then the number of active sites of MnO2 is greatly increased for electrochemical process. Secondly, the filamentous MnO2 nano-materials are interconnected through the high-conductivity carbon nanoframewrok of carbon aerogels, therefore the conductivity of MnO2 in the composite can be improved greatly. Such an improvement resulting from the addition of CA was also found in other metal oxide/CA composites, such as RuO2xH2O/CA compostites [9] and ZnO/CA composites [10]. To get a further understanding of the CA/MnO2 composite formation process, a schematic diagram is shown in Fig. 8. CA possesses an open porous network. With the limitation of the CA pore structure, nano-MnO2 filaments form in the pores along the connected network. Therefore, CA/MnO2
Fig. 8. Schematic diagram of the microstructure of MnO2/CA composite.
composite consists of MnO2 redox species and CA with high electrical conductivity, and can be considered to be promising electrode material for supercapacitor. With the temperature increases, the IR drop decreases (In Fig. 6). There may be two reasons for this: one is that when the temperature increases, the composite will lose adsorption water and chemical hydration water, the resistance decreases. Thus lower temperature with more water would result in larger inner resistance. The other is that with a higher heating temperature, the crystallization degree of MnO2 increases (see Fig. 1), and the array of the crystals is in a better order, which helps to reduce the inner resistance. The specific capacitances are listed in Table 1. It is suggested that higher heating temperature can result in lower capacitance. The largest specific capacitance 219 F g 1 appears at a heating temperature of 100 °C. However, the specific capacitance of the composite heating at 150 °C is only 78 F g 1. The similar law is found in the RuO2 capacitor behavior [18]. The reason is that when the samples are treated at lower temperature, the amorphous MnO2 with more adsorption water and chemical hydration water has a looser surface structure, which can improve the protons/ions diffusion and adsorption/desorption during the redox transitions, so that more protons will insert electrode material surface or inside to participate in the redox reactions. Though lower heating temperature is good for electrode material activity, the inner resistance is another consideration for the comprehensive electrochemical properties. In the experiments, 100 °C is the suitable temperature for the activation of electrode material. When the CA content increases, there is a corresponding increase in the specific capacitance of MnO2 ingredient. This supports our argument that suitable content MnO2 is necessary, the content is too small, the specific capacitance of the composite decreases, while the content is too large, and the resistance of the system increases. In our experiments, 20–40 wt% of MnO2 content is recommended. 5. Conclusions A homogenous nano-sized amorphous a-MnO2-dispersed CA for supercapacitor electrode material has been synthesized by liquid phase co-precipitation technique. Filamentous nano-MnO2 has been formed and homogeneously dispersed among the threedimensional mesoporous network of CA. The as-prepared MnO2dispersed CAs exhibit good supercapacitor properties, the highest specific capacitance of MnO2-dispersed CAs is 219 F g 1, and that for MnO2 ingredient in the composite is as high as 401 F g 1. The reason for the improved performance for the composites is the formation of nano-filamentous MnO2 between the carbon nanoframework of CA, which helps to improve the dispersion and conductivity of MnO2. The effects of heating temperature and MnO2
G. Lv et al. / Journal of Non-Crystalline Solids 355 (2009) 2461–2465
content on the electrochemical properties of the composites are discussed, 100 °C of heating temperature and 20–40 wt% of MnO2 content are recommended. Acknowledgements This research was supported by the Project of NNSFC (50472029, 50632040), and the Scientific Foundation of Guangzhou (2007Z2-D2041). References [1] B.E. Conway, J. Electrochem. Soc. 138 (6) (1991) 539. [2] R. Kötz, M. Carlen, Electrochim. Acta 45 (2000) 2483. [3] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Publishers, New York, 1999. p. 18.
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[4] S.T. Mayer, R.W. Pekala, J.L. Kaschimitter, J. Electrochem. Soc. 140 (1993) 446. [5] X. Lu, R. Caps, J. Fricke, C.T. Alviso, R.W. Pekala, J. Non-Cryst. Solids 8 (1995) 226. [6] R. Saliger, U. Fischer, C. Herta, J. Fricke, J. Non-Cryst. Solids 225 (1998) 81. [7] S.J. Kim, S.W. Hwang, S.H. Hyun, J. Mater. Sci. 40 (2005) 725. [8] B. Fang, Y.Z. Wei, K. Maruyama, M. Kumagai, J. Appl. Electrochem. 35 (2005) 229. [9] J. Li, X. Wang, Q. Huang, J. Appl. Electrochem. 37 (2007) 1129. [10] D. Kalpana, K.S. Omkumar, S. Suresh Kumar, N.G. Renganathan, Electrochim. Acta 52 (2006) 1309. [11] N. Nagarajan, M. Cheong, I. Zhitomirsky, Mater. Chem. Phys. 103 (2007) 47. [12] R.N. Reddy, R.G. Reddy, J. Power Sources 124 (2003) 330. [13] M. Wu, G.A. Snook, G.Z. Chen, D.J. Fray, Electrochem. Commun. 6 (2004) 499. [14] J. Li, X. Wang, Q. Huang, S. Gamboa, P.J. Sebastian, J. Power Sources 160 (2006) 1501. [15] D. Wu, R. Fu, M.S. Dresselhaus, G. Dresselhaus, Carbon 44 (2006) 675. [16] Z. Zhou, N. Cai, Y. Zhou, Mater. Chem. Phys. 94 (2005) 371. [17] E. Raymundo-Piñero, V. Khomenko, E. Frackowiak, J. Electrochem. Soc. 152 (2005) 229. [18] C. Lin, J.A. Ritter, B.N. Popov, J. Elctrochem. Soc. 146 (1999) 3155.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Preparation and electrochemical characterizations of MnO2-dispersed carbon aerogel as supercapacitor electrode material Guifen Lv a,b, Dingcai Wu a, Ruowen Fu a,c,* a
Materials Science Institute, PCFM Laboratory, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, PR China Soil and Fertilizer & Resources and Environment Institute, Jiangxi Academy of Agricultural Sciences, Nanchang 330200, PR China c Institute of Optoelectronic and Functional Composite Materials, Sun Yat-Sen University, Guangzhou 510275, PR China b
a r t i c l e
i n f o
Article history: Received 28 May 2008 Received in revised form 27 July 2009 Available online 24 September 2009 PACS: 66.70.Df 81.05.Uw 82.47.Uv
a b s t r a c t The nano-sized amorphous a-MnO2-dispersed carbon aerogels (CAs) were prepared by liquid phase coprecipitation technique. Different MnO2 contents and heating temperatures were taken to prepare the composites. The composites prepared were used to make electrodes of electrochemical supercapacitors. Electrochemical properties of the electrodes were studied by cyclic voltammetry and galvanostatic charge–discharge. The specific capacitances of the MnO2-dispersed CAs and the nano-MnO2 ingredient in the composite are up to 219 F g 1 and 401 F g 1 at 5 mA.cm 2, 1.6 times and 5.0 times greater than that of neat CA and MnO2, respectively. Ó 2009 Published by Elsevier B.V.
Keywords: Electrochemical properties Porosity
1. Introduction Electrochemical supercapacitors are unique energy storage devices [1,2]. There are two different types electrochemical capacitors based on their energy storage mechanisms [1,3]: electrochemical double-layer capacitors (EDLC) based on carbon electrodes and pseudocapacitors with certain metal oxides and others as electrode materials. Carbon aerogels (CA) are promising materials as electrodes for EDLCs due to their attractive properties such as high electrical conductivity, high porosity, controllable pore structure and high surface area [4,5]. CA-based supercapacitor offers fast response and long cycle life, but usually has lower specific capacitance (30–150 F g 1) [6–8]. Hence, there have been considerable efforts to improve the capacitance of CA-based supercapacitor electrodes by various techniques. Transition metal oxides attached to CAs have been studied recently and showed improved capacitance behavior due to their enhanced stability and high conductivity [9,10]. Among the transition metal oxides, manganese oxide has been extensively used as an electrode material in pseudocapacitor because of abundance, low-cost, electrochemical reactivity and environmental friendliness [11–13]. * Corresponding author. Address: Materials Science Institute, PCFM Laboratory, School of Chemistry and chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, PR China. Tel./fax: +86 20 84115112. E-mail address: [email protected] (R. Fu). 0022-3093/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2009.08.035
Recently, Wang and co-workers prepared a type of MnO2xH2O/ CA composite electrode materials. In this approach, the used carbon aerogel had a lot of macroporous resulting from the stack of. ca. 100–200 nm carbon particles (determined by the reported SEM image of CA [14]), and thus nanosized MnO2xH2O particles (50–80 nm) were deposited onto the surface of porous carbon aerogels and filled the inner big pores of the carbon aerogel. More recently, in our experiment, we found that the pore structure of CA plays an important part in the formation of the microstructure of CA/MnO2 composite. When a typical mesoporous (15 nm) CA material was used to prepare CA/MnO2 composite, a new nanostructural-filamentous nano-MnO2 can be formed between the carbon nano-frameworks of CA. In this paper, the technique parameters for the preparation of this type of novel filamentous nano-MnO2-dispersed CAs with optimal electrochemical performances were explored by changing heating temperature and loading mass of MnO2. The structure of CA/MnO2 composites thus obtained was investigated by XRD and SEM; and their electrochemical properties were revealed by cyclic voltammetry and galvanostatic charge–discharge.
2. Experimental CA was fabricated by the microemulsion-templated sol–gel polymerization method developed by our group [15]. CA/MnO2
G. Lv et al. / Journal of Non-Crystalline Solids 355 (2009) 2461–2465
were carried out. Specific capacitances were calculated from galvanostatic charge curves at current density of 5 mA cm 2 according to the equation: C = I*t/m, where I, t, and m are current, charge time and the mass of the active material. 3. Results Fig. 1 shows the X-ray diffraction patterns of CA, MnO2 and their composites. CA shows the typical graphite-like micro-crystalline diffraction peaks, which center at around 23° and 44°, corresponding to 0 0 2 and 1 0 1 [14]. MnO2 shows the weak and broad peaks appeared at around 36° and 65°, indicating the amorphous nature and the small size of the crystalline of a-MnO2 [16]. CA/MnO2T100 shows both the characteristic peaks of amorphous MnO2 and CA. The peak intensity of MnO2 increases with increasing temperature from 50 to 150 °C, indicating that the crystallization degree of MnO2 increases with heating temperature. SEM observation (Fig. 2(a)) shows that the MnO2 nano-grains aggregate to form porous submicron balls with network structure. Fig. 2 (b) shows that the nano-carbon particles (ca. 20–30 nm in diameter) of the used CA are interconnected into a three-dimensional network, in which many mesopores among the carbon nano-particles are observed. These mesopores have a narrow pore size distribution with a maximum at 15 nm (see Fig. 3). These results show the CA used here is a typical mesoporous material with
0.10 2 -1
SBET: 620m g
-1
composites were prepared by a liquid phase co-precipitation technique. According to the predetermined formulations, CA was added into 0.1 M KMnO4 solution and then stirred for 12 h with a magnetic stirrer. After complete homogenization of the mixture, 0.15 M Mn(CH3COO)24H2O was added drop-wise under constant stirring. The resulting MnO2-dispersed CA was filtered and dried at predetermined heating temperatures for 12 h. In this study, the as-prepared CA/MnO2 composites with various heating temperatures and weight ratios were referred to as CA/MnO2-Txx and CA/MnO2-Mnyy, respectively. For blank test, neat MnO2 was prepared using the same procedure but without adding CA. The structures of the samples prepared here were studied by studied by scanning electron microscopy (SEM, JSM-6330F, Japan), X-ray diffraction (XRD, D-MAX 2200 VPC, Japan), and ASAP 2010 surface area analyzer (Micromeritics, USA). MnO2-dispersed CA was mixed with 10 wt% of conductive carbon black by ultrasonic vibration in ethanol for 30 min, and the resulting mixture was thoroughly homogenized with 5 wt% PTFE binder to make slurry. After that, the half-dried slurry was rolled to get appropriate thickness sheets, cut into 1 cm2 slices and then coated with nickel current collector, followed by drying the electrodes at 110 °C under vacuum for 12 h. Electrodes from neat MnO2 or CA were fabricated using the same procedure. A kind of sandwich-type supercapacitor consisting of two similar electrodes was assembled. The assembled two-electrode supercapacitors were measured in 1 M Na2SO4 aqueous solutions on IME6X electrochemical working station at ambient temperature. Cyclic voltammetry (CV) and galvanostatic charge–discharge tests
Differential Pore Volume (cm g nm )
2462
350 300
CA 250
Intensity
0.08
3 -1
Vmes: 1.00cm g
3 -1
400
MnO2
200 150
CA/MnO2-T50
100
CA/MnO2-T100
50
CA/MnO2-T150
3 -1
Vmic: 0.16cm g 0.06
0.04
0.02
0.00
20
40
60
80
Pore Width (nm)
0 20
30
40
50
60
70
2 Fig. 1. X-ray diffraction patterns of various electrode materials.
Fig. 3. BJH pore size distribution of carbon aerogels. Note: SBET: BET specific surface area; Vmes: BJH desorption cumulative specific pore volume; Vmic: Specific micropore volume. The errors of the pore structure parameters were ±1% of the reported values.
Fig. 2. SEM images of (a) MnO2; (b) CA and (c) CA/MnO2-T100.
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high surface area (620 m2 g 1) and great mesopore volume (1.00 cm3 g 1). Fig. 2 (c) shows that in the CA/MnO2-T100 composite, many filamentous MnO2 nano-materials are observed to be well dispersed in CA’s three-dimensional nanoporous networks. The CV curves recorded for MnO2, CA and CA/MnO2-T100 composite are shown in Fig. 4. CV curves of CA and CA/MnO2-T100 show typical capacitive behavior with a constant charge and discharge over the complete cycle. The CV curves are much closer to an ideal rectangle, which show a mirror image with respect to the zero current line and a rapid current response on voltage reversal at each end potential. CV curve of MnO2 is a little distorted. Pseudocapacitive behavior for MnO2 has been well studied [17], it is attributed to reversible redox reactions in an aqueous system involving protons and cations exchange with the electrolytes, the insertion of protons in the MnO2 electrode surface with the associated redox reactions of high and low oxidation states is responsible for the electron exchange reaction. Typical charge–discharge curves in Fig. 5 show that the MnO2, CA and CA/MnO2-T100 composite have stable electrochemical properties. The capacitor voltage varies linearly with time during
0.009
0.006
(c) (b)
Current (A)
0.003
(a)
0.000
charging and discharging. As well known, the IR drop is associated with the equivalent series resistance of the supercapacitor cell. The IR drop values for CA and CA/MnO2-T100 are negligible. However, such a drop is obvious for MnO2, indicating the large inner resistance of MnO2. Without doubt, this obvious IR drop difference is evidence that the CA can improve the electronic conductivity of CA/MnO2-T100. The specific capacitances are listed in Table 1. For MnO2 and CA, their specific capacitances are only 80 F g 1 and 133 F g 1, while that for CA/MnO2-T100 composite is up to 219 F g 1. The effects of heating temperature and MnO2 content on the electrochemical properties were also studied. Galvanostatic charge–discharge curves of composites with different heating temperatures are shown in Fig. 6. It can be seen that the IR drop decreases with increasing the temperature. Form Fig. 7, it can be seen that the CV curves become distorted with increasing content of MnO2. Table 2 displays the dependence of the specific capacitance on the mass fraction of MnO2 of the composite. It is obvious that MnO2 takes an important part in the specific capacitance. The specific capacitance come to the maximum value (i.e., 219 F g 1) when the MnO2 content is 37.2%. Table 2 also shows the specific capacitance of MnO2 composition in MnO2-dispersed CA with various MnO2 contents. For CA/MnO2–Mn40, since CA takes 62.8 wt% of the electrode mass and its specific capacitance is 133 F g 1, its capacitance contribution is 84 F. Based upon this, the capacitance contribution of MnO2 ingredient is calculated to be 135 F. Since MnO2 takes 37.2 wt%, the specific capacitance of MnO2 is 364 F g 1. Likewise, the specific capacitance of MnO2 composition in CA/MnO2–Mn30 and CA/MnO2–Mn50 can be calculated to be 401 F g 1 and 264 F g 1, respectively.
-0.003 Table 1 Specific capacitance of CA, MnO2 and CA/MnO2 composites with various heating temperatures.
-0.006
-0.009 0.0
0.2
0.4
0.6
0.8
1.0
Potential (V)
Sample
Heating temperature (°C)
Capacitance (F g
CA MnO2 CA/MnO2–T50 CA/MnO2–T100 CA/MnO2–T150
/ / 50 100 150
133 ± 1 80 ± 2 160 ± 2 219 ± 2 78 ± 1
1
)
Fig. 4. Cyclic voltammograms of (a) CA; (b) MnO2; (c) CA/MnO2-T100 at scan rate of 5 mV s 1.
1.2
(c)
(a) (b)
1.0
(c)
(b)
(a)
1.0
0.8
Voltage (v)
Potential (v)
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0 0
0.0 0
200
400
600
800
100
200
300
400
500
600
700
Time (seconds)
Time (seconds) Fig. 5. Charge–discharge curves for (a) CA; (b) MnO2; (c) CA/MnO2-T100 composite.
Fig. 6. Charge–discharge curves for MnO2 deposited at various heating temperatures: (a) CA/MnO2-T50; (b) CA/MnO2-T100; (c) CA/MnO2-T150.
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(b)
0.06
0.04
0.06
Current (A)
0.04
Current (A)
(c)
0.08
0.02 0.00 -0.02 5mV/s 10mV/s 50mV/s 100mV/s
-0.04 -0.06 0.0
0.2
0.4
0
0.8
0.04
Current (A)
(a)
0.02 0.00 -0.02 5mV/s 10mV/s 50mV/s 100mV/s
-0.04 -0.06 0.0
1.0
0.2
Potential (V)
0.4
0.6
0.8
1.0
Potential (V)
0.02 0.00 -0.02 5mV/S 10mV/S 50mV/S 100mV/S
-0.04 0.0
0.2
0.4
0.6
0.8
1.0
Potential (V)
Fig. 7. Cyclic voltammogram of MnO2 deposited at various content: (a) CA/MnO2–Mn30; (b) CA/MnO2–Mn40 and (c) CA/MnO2–Mn50.
Table 2 Specific capacitance of CA/MnO2 composites with various MnO2 contents. Sample
MnO2 content (wt%)
Capacitance (F g
CA/MnO2–Mn30 CA/MnO2–Mn40 CA/MnO2–Mn50
28.7 37.2 53.1
210 ± 2 219 ± 2 203 ± 2
1
)
CMn (F g
1
)
401 ± 4 364 ± 3 264 ± 3
Note: The specific capacitance of CA used in the MnO2-dispersed CAs is 133 F g 1. CMn denotes the specific capacitance of MnO2 composition of MnO2-dispersed CAs.
4. Discussion Though the mesopore dimension of CA used is larger than the diameter of MnO2 wire produced, the nano-carbon particles could bridge the filamentous MnO2 and connected them together (Fig. 2 (c)). At the same time, the three-dimensional carbon network of CA could provide a conductive pathway for MnO2. Therefore, the conductivity of the composites can be improved and active sites can be enhanced for electrochemical process due to good interconnection of the porous composite. Electrochemical improvement is attributed to the two reasons: Firstly, the CA with well developed three-dimensional nano-network offers abundant connected sites for MnO2, thus the filamentous nano-MnO2 can be formed and well dispersed. Undoubtedly, the effective specific area of MnO2 is improved sharply and then the number of active sites of MnO2 is greatly increased for electrochemical process. Secondly, the filamentous MnO2 nano-materials are interconnected through the high-conductivity carbon nanoframewrok of carbon aerogels, therefore the conductivity of MnO2 in the composite can be improved greatly. Such an improvement resulting from the addition of CA was also found in other metal oxide/CA composites, such as RuO2xH2O/CA compostites [9] and ZnO/CA composites [10]. To get a further understanding of the CA/MnO2 composite formation process, a schematic diagram is shown in Fig. 8. CA possesses an open porous network. With the limitation of the CA pore structure, nano-MnO2 filaments form in the pores along the connected network. Therefore, CA/MnO2
Fig. 8. Schematic diagram of the microstructure of MnO2/CA composite.
composite consists of MnO2 redox species and CA with high electrical conductivity, and can be considered to be promising electrode material for supercapacitor. With the temperature increases, the IR drop decreases (In Fig. 6). There may be two reasons for this: one is that when the temperature increases, the composite will lose adsorption water and chemical hydration water, the resistance decreases. Thus lower temperature with more water would result in larger inner resistance. The other is that with a higher heating temperature, the crystallization degree of MnO2 increases (see Fig. 1), and the array of the crystals is in a better order, which helps to reduce the inner resistance. The specific capacitances are listed in Table 1. It is suggested that higher heating temperature can result in lower capacitance. The largest specific capacitance 219 F g 1 appears at a heating temperature of 100 °C. However, the specific capacitance of the composite heating at 150 °C is only 78 F g 1. The similar law is found in the RuO2 capacitor behavior [18]. The reason is that when the samples are treated at lower temperature, the amorphous MnO2 with more adsorption water and chemical hydration water has a looser surface structure, which can improve the protons/ions diffusion and adsorption/desorption during the redox transitions, so that more protons will insert electrode material surface or inside to participate in the redox reactions. Though lower heating temperature is good for electrode material activity, the inner resistance is another consideration for the comprehensive electrochemical properties. In the experiments, 100 °C is the suitable temperature for the activation of electrode material. When the CA content increases, there is a corresponding increase in the specific capacitance of MnO2 ingredient. This supports our argument that suitable content MnO2 is necessary, the content is too small, the specific capacitance of the composite decreases, while the content is too large, and the resistance of the system increases. In our experiments, 20–40 wt% of MnO2 content is recommended. 5. Conclusions A homogenous nano-sized amorphous a-MnO2-dispersed CA for supercapacitor electrode material has been synthesized by liquid phase co-precipitation technique. Filamentous nano-MnO2 has been formed and homogeneously dispersed among the threedimensional mesoporous network of CA. The as-prepared MnO2dispersed CAs exhibit good supercapacitor properties, the highest specific capacitance of MnO2-dispersed CAs is 219 F g 1, and that for MnO2 ingredient in the composite is as high as 401 F g 1. The reason for the improved performance for the composites is the formation of nano-filamentous MnO2 between the carbon nanoframework of CA, which helps to improve the dispersion and conductivity of MnO2. The effects of heating temperature and MnO2
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content on the electrochemical properties of the composites are discussed, 100 °C of heating temperature and 20–40 wt% of MnO2 content are recommended. Acknowledgements This research was supported by the Project of NNSFC (50472029, 50632040), and the Scientific Foundation of Guangzhou (2007Z2-D2041). References [1] B.E. Conway, J. Electrochem. Soc. 138 (6) (1991) 539. [2] R. Kötz, M. Carlen, Electrochim. Acta 45 (2000) 2483. [3] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Publishers, New York, 1999. p. 18.
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