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Sparse MnO2 nanowires clusters for high-performance supercapacitors
Materials Letters 73 (2012) 194–197
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Sparse MnO2 nanowires clusters for high-performance supercapacitors Y.F. Yuan ⁎, Y.B. Pei, S.Y. Guo ⁎, J. Fang, J.L. Yang College of Machinery and Automation, Zhejiang Sci-Tech University, Hangzhou 310018, China
a r t i c l e
i n f o
Article history: Received 26 November 2011 Accepted 9 January 2012 Available online 15 January 2012 Keywords: MnO2 Supercapacitor Nanocrystalline materials Energy storage and conversion
a b s t r a c t MnO2 nanowires clusters are electrodeposited onto Ni foam by a cyclic voltammetric technique. MnO2 nanowires, about 20 nm in diameter and up to 200 nm in length, present a sparse grass clusters structure. As cathode material for supercapacitors, MnO2 nanowires clusters exhibit superior pseudocapacitance performances with high specific capacitances (1080 F g− 1 at 4 A g− 1 and 415 F g− 1 at 30 A g− 1) as well as excellent largecurrent cycling stability, making it suitable for high-performance supercapacitor application. The improved pseudocapacitance performances are attributed to small size, large surface area and high dispersion degree of MnO2 NWs, as well as 3-Dimension structure of Ni foam, which provide faster ion and electron transfer, larger reaction surface area, higher electrochemical activity, leading to faster reaction kinetics and higher material utilization ratio. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Supercapacitors represent a unique class of energy storage devices that exhibit high power capability, excellent reversibility and long cycle life, including the two mechanisms: electrical double layers capacitance and pseudocapacitance. Compared with double layer carbonaceous materials, pseudocapacitive transition metal oxides such as RuO2 [1], MnO2 [2], NiO [3], and Co3O4 [4], have much higher capacitances (several times) and have been widely studied as electrode materials for electrochemical supercapacitors. Among them, MnO2 is an attractive pseudocapacitive material owing to its low cost, environmentally friendly nature, and ideal capacitor performances. However, the poor electrical conductivity (10 − 5–10 − 6 S cm − 1) and low electrochemical kinetic properties lead to compromised capacitance of MnO2. Considerable research has been done on nanostructured and porous MnO2 [5], or coating MnO2 onto conducting agents (e.g. carbon nanotubes [6], graphene [7]). Nevertheless, these MnO2 materials still need mix with conducting agents and binders to prepare the supercapacitor electrodes, which weaken the nanometer effect and the composite effect, and increase the complexity and the cost of the preparation process. One of the ways to address these issues is to directly grow MnO2 nanomaterials on conductive substrates, which can facilitate the diffusion of active species and transport of electrons, save the tedious process of mixing active material with ancillary materials such as carbon black and polymer. Xu et al. prepared mesoporous MnO2 NWs on the conductive AAO/Ti/Si substrates and obtained the specific capacitance as high as 493 F g − 1 at current density of 4 A g − 1 [8]. Chen et al.
⁎ Corresponding authors. Tel.: + 86 571 86843343. E-mail addresses: [email protected] (Y.F. Yuan), [email protected] (S.Y. Guo). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.01.046
electrodeposited nanoneedle film, nanorod film, nanoflake film on Ni sheets and the highest capacitance was 324 F g − 1 [9]. Lots of similar researches have been reported in recent years [10]. Nevertheless, their capacitances are not superior in comparison with the performance of MnO2/conducting material composites, e.g. 1250 F g − 1 of MnOx/spin-capable carbon nanotube sheet composite [6]. The conventional plain conductive substrates, high compactness and slightly large size of MnO2 nanomaterials are possible causes. By using Ni foam as the substrate and adjusting the electrodeposited growth of MnO2 NWs, MnO2 NWs cluster grew on Ni foam, putting NWs to better use. MnO2 NWs clusters exhibit superior supercapacitor performances with higher specific capacitance and better cycling capability. 2. Experimental MnO2 NWs clusters were electrodeposited onto Ni foam (2 × 4 cm 2) by a cyclic voltammetry technique in the potential range between 0.1 and 0.6 V with reference to an Ag/AgCl electrode at the scan rate of 500 mV s − 1 at room temperature. The electrolyte was composed of 0.1 M manganous acetate and 0.1 M sodium sulfate. The counter electrode was a Pt foil with 2 × 3 cm 2. The working electrode was Ni foam (area ~8 cm 2) pretreated by hydrochloric acid. After the electrodeposition, the samples were rinsed several times with deionized water and dried at 110 °C for 5 h in air. The weight of the electroactive material was about 0.04 mg, measured by Mettler, AG285. The microstructure of the as-prepared MnO2 NWs clusters were characterized by scanning electron microscopy (SEM, S4800) and transmission electron microscope (TEM, JEM-2100) equipped with energy dispersive spectroscopy (EDS). Electrochemical measurements were carried out in a threeelectrode cell, with the integrative electrode of MnO2 NWs/Ni foam as the working electrode, a Pt foil (2 × 3 cm 2) as the counter
Y.F. Yuan et al. / Materials Letters 73 (2012) 194–197
electrode, and an Ag/AgCl electrode as the reference electrode. 0.1 M Na2SO4 was used as the electrolyte. The cyclic voltammetry was done on an electrochemical station (Gaoss Union, EC550) at room temperature, and the galvanostatic charge–discharge on LAND battery test system.
195
(JCPDS No.42-1316) (Fig. 1c). A group of crystal plain parallel to the axial line of the NW can be observed at the NW bottom (Fig. 1d). The interplanar spacing is about 0.34 nm, close to the interplanar spacing 0.33 nm of crystal plain (201) of γ-MnO2. The EDS analysis on this NW discloses that atomic ratio of Mn and O is close to 1:2 and O atom is slightly excessive. Ni, Cu, Cr and C elements come from Ni foam, Cu grid, Cr specimen holder and C film on Cu grid, respectively. The cyclic scanning potential range has a large effect on the morphology of MnO2. The potential less than 0.7 V grow MnO2 NWs. The lower the potential, the slower the growth rates of NWs, the sparser the NWs. The potential over 0.7 V will form a cross-like flake structure, and the growth rate remarkably increases. Fig. 2a is cyclic voltammograms (CV) of MnO2 NWs clusters at the different scanning rate. At 5 mV s − 1, CV curve is almost symmetrical. The approximate rectangular voltammogram suggests an ideal capacitive behavior. With the scanning rate increasing, the symmetry gradually decreases and the anodic peak is more and more remarkable, which differ from symmetrical CV curves of other MnO2
3. Results and discussion After the electrodeposition growth, Ni foam was covered by a khaki film. This khaki film is made up of NWs with diameters of ~ 20 nm and lengths up to 200 nm (Fig. 1a). NWs grow like grass. Several NWs grow from a common root, forming a grass cluster. The NWs on Ni surface is uniform and uncompact, which ensure the sufficient contact between each NW with the electrolyte. The microstructures of NWs are further studied by TEM (Fig. 1b). The diameter (~20 nm) can be clearly measured. The surface is very rough. Two selected area are observed by HRTEM and shown in Fig. 1 c and d. The lattice characterization on the top NW well coincides with γ-MnO2
a
b
c d
e
400
Atomic% O : 38.78 Mn : 16.13
Cu
300
Counts
O,Cr,Mn
Mn,Cr
200
100
C
Ni Cu
Mn Ni
Cr 0 0
1
2
3
4
5
6
7
8
9
10
11
KeV Fig. 1. (a) SEM image of MnO2 NWs clusters on Ni surface; (b) TEM image of single MnO2 NW; (c, d) HRTEM images of the selected area of MnO2 NW; and (e) EDS pattern of MnO2 NW.
196
Y.F. Yuan et al. / Materials Letters 73 (2012) 194–197
a
b
1.5 1.0
Potential / V vs. Ag/AgCl
0.6
Current / mA
0.5 0.0 -0.5 -1 5 mV s -1 10 mV s -1 20 mV s -1 50 mV s -1 100 mV s
-1.0 -1.5 -2.0 -2.5 -3.0
0.7
0.0
0.1
0.5 0.4 0.3 0.2 0.1 0.0
0.2
0.3
0.4
0.5
0.6
30 20
-0.1
0.7
0
10 50
8 100
Potential / V vs. Ag/AgCl
d
1200 0.75 V 1100 1000 0.70 V 900 800 0.65 V 700 600
0
2
4
6
8
10
150
200
250
300
Time / s
Specific Capacitance / F g-1
Specific Capacitance / F g-1
c
4 A g-1
6
12
14
16
800 700 600 500 400 300 200
20 A g-1
10 A g-1
30 A g-1
100 0
0
500
Cycling Number
1000
1500
2000
2500
3000
Cycling Number −1
Fig. 2. (a) Cyclic voltammograms of MnO2 NWs clusters at the scanning rate of 5, 10, 20, 50 and 100 mv s ; (b) Charge–discharge curves of MnO2 NWs at different current density; (c) Effect of the cut-off voltage on the specific capacitance; and (d) Cycling performance of MnO2 NWs clusters at high current density. The electrolyte is 0.1 M Na2SO4.
nanomaterials reported [9], indicating that the MnO2 NWs clusters have higher electrochemical oxidation activity. Fig. 2b shows the charge–discharge curves of MnO2 NWs clusters at different current density. The charge–discharge curves are not symmetrical. Both the increasing rate of charge potential and the decreasing rate of discharge potential at the later stage are relatively slow, presenting high coulombic efficiency and ideal pseudocapacitance performance. Their specific capacitances are 830 F g − 1 at 4 A g − 1, 692 F g − 1 at 6 A g − 1, 554 F g − 1 at 8 A g − 1, 554 F g − 1 at 10 A g − 1, 415 F g − 1 at 20 A g − 1, 415 F g − 1 at 30 A g − 1. Moreover, the cut-off voltage has a large effect on the capacitance of MnO2 NWs clusters (Fig. 2c). With the increasing cut-off voltage, the capacitance gradually rises. At 0.75 V, the capacitance is 1080 F g − 1, showing a high material utilization rate. We further study cycling performance of MnO2 NWs clusters at current density of 10, 20 and 30 A g − 1, and the cut-off voltage is 0.65 V (Fig. 2d). At 10 A g − 1, the pseudocapacitance of NWs clusters increases to 692 F g − 1 after initial 100 activated cycles; maintains 692 F g − 1 until 500 cycles; and then gets back to 554 F g − 1. When the current density increases to 20 and 30 A g − 1, the pseudocapacitance maintains 415 F g − 1; reaches 60% of the maximum value at 10 A g − 1 and the coulombic efficiency is always more than 80%; exhibiting an excellent pseudocapacitance retention capability (Fig. 2d). The capacitances values are slightly lower than that of MnO2 nanoparticles/carbon nanotubes composites [6], but higher than MnO2 NWs/carbon nanotube composite paper (516.2 F g − 1) [11], manganese oxide/carbon aerogel composite (503 F g − 1) [12], γ-MnO2 nanoflake films (324 F g − 1) [9], birnesite-type MnO2 NWs
(191 F g − 1) [2], birnessite MnO2 nanotubes (350 F g − 1) [13], manganese oxide nanorod arrays (583.6 F g − 1) [10], and α-MnO2 nanorod (166.2 F g − 1) [14]. The high pseudocapacitance and excellent cycling stability of MnO2 are due to the unique 3-Dimension NWs clusters electrode. Each NW has its own contact with the substrate at the bottom, reducing internal resistance and ensuring every NW participates in the electrochemical reaction. The large specific surface area of NWs increases the electrochemical reaction rate at the interfaces, enhancing large-current charge–discharge capability. Ni foam is 3-Dimension; and the space between neighboring NWs is large enough, favoring easier diffusion of the electrolyte into the entire surface of all NWs, decreasing the polarization, enhancing large-current reaction capability. The length and the diameter of NWs are relatively small, shortening diffusion paths of electrons and ions within NWs, reinforcing electrochemical kinetics performance of MnO2 and favoring the large-current performance.
4. Conclusions We get MnO2 NWs clusters on Ni foam by a cyclic voltammetric technique. The capacitance achieved can be as high as up to 1080 F g − 1, and the large-current cycling performance is excellent, too. The small size and large dispersion degree of NWs clusters, as well as 3-Dimension structure of Ni foam favors faster ion/electron transfer; provides larger reaction surface area; and decreases the polarization, leading to faster kinetics and enhanced pseudocapacitive
Y.F. Yuan et al. / Materials Letters 73 (2012) 194–197
properties. It is believed that this study puts forward a significant and feasible strategy for high-performance MnO2 supercapacitors. Acknowledgement This work is supported by Research Projects of Education Department of Zhejiang Province, China under Grant No. Y201122147. References [1] Wang YG, Wang ZD, Xia YY. Electrochim Acta 2005;50:5641. [2] Vargas OA, Caballero A, Hernan L, Morales J. J Power Sources 2011;196:3350. [3] Yuan YF, Xia XH, Wu JB, Yang JL, Chen YB, Guo SY. Electrochim Acta 2011;56:2627.
197
[4] Yuan YF, Xia XH, Wu JB, Huang XH, Pei YB, Yang JL, et al. Electrochem Commun 2011;13:1123. [5] Wang YT, Lu AH, Zhang HL, Li WC. J Phys Chem C 2011;115:5413. [6] Kim JH, Kyung HL, Lawrence JO, Gil SL. Nano Lett 2011;11:2611. [7] Wu ZS, Ren WC, Wang DW, Li F, Liu BL, Cheng HM. ACS Nano 2010;4:5835. [8] Xu CL, Zhao YQ, Yang GW, Li FS, Li HL. Chem Commun 2009:7575. [9] Chou SL, Cheng FY, Chen J. J Power Sources 2006;162:727. [10] Lu XH, Zheng DZ, Zhai T, Liu ZQ, Huang YY, Xie SL, et al. Energy Environ Sci 2011;4:2915. [11] Chou SL, Wang JZ, Chew SY, Liu HK, Dou SX. Electrochem Commun 2008;10:724. [12] Lin YH, Wei TY, Chien HC, Lu SY. Adv Energy Mater 2011;1:901. [13] Roberts AJ, Slade RCT. Energy Environ Sci 2011;4:2813. [14] Li Y, Xie HQ, Wang JF, Chen LF. Mater Lett 2011;65:403.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Sparse MnO2 nanowires clusters for high-performance supercapacitors Y.F. Yuan ⁎, Y.B. Pei, S.Y. Guo ⁎, J. Fang, J.L. Yang College of Machinery and Automation, Zhejiang Sci-Tech University, Hangzhou 310018, China
a r t i c l e
i n f o
Article history: Received 26 November 2011 Accepted 9 January 2012 Available online 15 January 2012 Keywords: MnO2 Supercapacitor Nanocrystalline materials Energy storage and conversion
a b s t r a c t MnO2 nanowires clusters are electrodeposited onto Ni foam by a cyclic voltammetric technique. MnO2 nanowires, about 20 nm in diameter and up to 200 nm in length, present a sparse grass clusters structure. As cathode material for supercapacitors, MnO2 nanowires clusters exhibit superior pseudocapacitance performances with high specific capacitances (1080 F g− 1 at 4 A g− 1 and 415 F g− 1 at 30 A g− 1) as well as excellent largecurrent cycling stability, making it suitable for high-performance supercapacitor application. The improved pseudocapacitance performances are attributed to small size, large surface area and high dispersion degree of MnO2 NWs, as well as 3-Dimension structure of Ni foam, which provide faster ion and electron transfer, larger reaction surface area, higher electrochemical activity, leading to faster reaction kinetics and higher material utilization ratio. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Supercapacitors represent a unique class of energy storage devices that exhibit high power capability, excellent reversibility and long cycle life, including the two mechanisms: electrical double layers capacitance and pseudocapacitance. Compared with double layer carbonaceous materials, pseudocapacitive transition metal oxides such as RuO2 [1], MnO2 [2], NiO [3], and Co3O4 [4], have much higher capacitances (several times) and have been widely studied as electrode materials for electrochemical supercapacitors. Among them, MnO2 is an attractive pseudocapacitive material owing to its low cost, environmentally friendly nature, and ideal capacitor performances. However, the poor electrical conductivity (10 − 5–10 − 6 S cm − 1) and low electrochemical kinetic properties lead to compromised capacitance of MnO2. Considerable research has been done on nanostructured and porous MnO2 [5], or coating MnO2 onto conducting agents (e.g. carbon nanotubes [6], graphene [7]). Nevertheless, these MnO2 materials still need mix with conducting agents and binders to prepare the supercapacitor electrodes, which weaken the nanometer effect and the composite effect, and increase the complexity and the cost of the preparation process. One of the ways to address these issues is to directly grow MnO2 nanomaterials on conductive substrates, which can facilitate the diffusion of active species and transport of electrons, save the tedious process of mixing active material with ancillary materials such as carbon black and polymer. Xu et al. prepared mesoporous MnO2 NWs on the conductive AAO/Ti/Si substrates and obtained the specific capacitance as high as 493 F g − 1 at current density of 4 A g − 1 [8]. Chen et al.
⁎ Corresponding authors. Tel.: + 86 571 86843343. E-mail addresses: [email protected] (Y.F. Yuan), [email protected] (S.Y. Guo). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.01.046
electrodeposited nanoneedle film, nanorod film, nanoflake film on Ni sheets and the highest capacitance was 324 F g − 1 [9]. Lots of similar researches have been reported in recent years [10]. Nevertheless, their capacitances are not superior in comparison with the performance of MnO2/conducting material composites, e.g. 1250 F g − 1 of MnOx/spin-capable carbon nanotube sheet composite [6]. The conventional plain conductive substrates, high compactness and slightly large size of MnO2 nanomaterials are possible causes. By using Ni foam as the substrate and adjusting the electrodeposited growth of MnO2 NWs, MnO2 NWs cluster grew on Ni foam, putting NWs to better use. MnO2 NWs clusters exhibit superior supercapacitor performances with higher specific capacitance and better cycling capability. 2. Experimental MnO2 NWs clusters were electrodeposited onto Ni foam (2 × 4 cm 2) by a cyclic voltammetry technique in the potential range between 0.1 and 0.6 V with reference to an Ag/AgCl electrode at the scan rate of 500 mV s − 1 at room temperature. The electrolyte was composed of 0.1 M manganous acetate and 0.1 M sodium sulfate. The counter electrode was a Pt foil with 2 × 3 cm 2. The working electrode was Ni foam (area ~8 cm 2) pretreated by hydrochloric acid. After the electrodeposition, the samples were rinsed several times with deionized water and dried at 110 °C for 5 h in air. The weight of the electroactive material was about 0.04 mg, measured by Mettler, AG285. The microstructure of the as-prepared MnO2 NWs clusters were characterized by scanning electron microscopy (SEM, S4800) and transmission electron microscope (TEM, JEM-2100) equipped with energy dispersive spectroscopy (EDS). Electrochemical measurements were carried out in a threeelectrode cell, with the integrative electrode of MnO2 NWs/Ni foam as the working electrode, a Pt foil (2 × 3 cm 2) as the counter
Y.F. Yuan et al. / Materials Letters 73 (2012) 194–197
electrode, and an Ag/AgCl electrode as the reference electrode. 0.1 M Na2SO4 was used as the electrolyte. The cyclic voltammetry was done on an electrochemical station (Gaoss Union, EC550) at room temperature, and the galvanostatic charge–discharge on LAND battery test system.
195
(JCPDS No.42-1316) (Fig. 1c). A group of crystal plain parallel to the axial line of the NW can be observed at the NW bottom (Fig. 1d). The interplanar spacing is about 0.34 nm, close to the interplanar spacing 0.33 nm of crystal plain (201) of γ-MnO2. The EDS analysis on this NW discloses that atomic ratio of Mn and O is close to 1:2 and O atom is slightly excessive. Ni, Cu, Cr and C elements come from Ni foam, Cu grid, Cr specimen holder and C film on Cu grid, respectively. The cyclic scanning potential range has a large effect on the morphology of MnO2. The potential less than 0.7 V grow MnO2 NWs. The lower the potential, the slower the growth rates of NWs, the sparser the NWs. The potential over 0.7 V will form a cross-like flake structure, and the growth rate remarkably increases. Fig. 2a is cyclic voltammograms (CV) of MnO2 NWs clusters at the different scanning rate. At 5 mV s − 1, CV curve is almost symmetrical. The approximate rectangular voltammogram suggests an ideal capacitive behavior. With the scanning rate increasing, the symmetry gradually decreases and the anodic peak is more and more remarkable, which differ from symmetrical CV curves of other MnO2
3. Results and discussion After the electrodeposition growth, Ni foam was covered by a khaki film. This khaki film is made up of NWs with diameters of ~ 20 nm and lengths up to 200 nm (Fig. 1a). NWs grow like grass. Several NWs grow from a common root, forming a grass cluster. The NWs on Ni surface is uniform and uncompact, which ensure the sufficient contact between each NW with the electrolyte. The microstructures of NWs are further studied by TEM (Fig. 1b). The diameter (~20 nm) can be clearly measured. The surface is very rough. Two selected area are observed by HRTEM and shown in Fig. 1 c and d. The lattice characterization on the top NW well coincides with γ-MnO2
a
b
c d
e
400
Atomic% O : 38.78 Mn : 16.13
Cu
300
Counts
O,Cr,Mn
Mn,Cr
200
100
C
Ni Cu
Mn Ni
Cr 0 0
1
2
3
4
5
6
7
8
9
10
11
KeV Fig. 1. (a) SEM image of MnO2 NWs clusters on Ni surface; (b) TEM image of single MnO2 NW; (c, d) HRTEM images of the selected area of MnO2 NW; and (e) EDS pattern of MnO2 NW.
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Y.F. Yuan et al. / Materials Letters 73 (2012) 194–197
a
b
1.5 1.0
Potential / V vs. Ag/AgCl
0.6
Current / mA
0.5 0.0 -0.5 -1 5 mV s -1 10 mV s -1 20 mV s -1 50 mV s -1 100 mV s
-1.0 -1.5 -2.0 -2.5 -3.0
0.7
0.0
0.1
0.5 0.4 0.3 0.2 0.1 0.0
0.2
0.3
0.4
0.5
0.6
30 20
-0.1
0.7
0
10 50
8 100
Potential / V vs. Ag/AgCl
d
1200 0.75 V 1100 1000 0.70 V 900 800 0.65 V 700 600
0
2
4
6
8
10
150
200
250
300
Time / s
Specific Capacitance / F g-1
Specific Capacitance / F g-1
c
4 A g-1
6
12
14
16
800 700 600 500 400 300 200
20 A g-1
10 A g-1
30 A g-1
100 0
0
500
Cycling Number
1000
1500
2000
2500
3000
Cycling Number −1
Fig. 2. (a) Cyclic voltammograms of MnO2 NWs clusters at the scanning rate of 5, 10, 20, 50 and 100 mv s ; (b) Charge–discharge curves of MnO2 NWs at different current density; (c) Effect of the cut-off voltage on the specific capacitance; and (d) Cycling performance of MnO2 NWs clusters at high current density. The electrolyte is 0.1 M Na2SO4.
nanomaterials reported [9], indicating that the MnO2 NWs clusters have higher electrochemical oxidation activity. Fig. 2b shows the charge–discharge curves of MnO2 NWs clusters at different current density. The charge–discharge curves are not symmetrical. Both the increasing rate of charge potential and the decreasing rate of discharge potential at the later stage are relatively slow, presenting high coulombic efficiency and ideal pseudocapacitance performance. Their specific capacitances are 830 F g − 1 at 4 A g − 1, 692 F g − 1 at 6 A g − 1, 554 F g − 1 at 8 A g − 1, 554 F g − 1 at 10 A g − 1, 415 F g − 1 at 20 A g − 1, 415 F g − 1 at 30 A g − 1. Moreover, the cut-off voltage has a large effect on the capacitance of MnO2 NWs clusters (Fig. 2c). With the increasing cut-off voltage, the capacitance gradually rises. At 0.75 V, the capacitance is 1080 F g − 1, showing a high material utilization rate. We further study cycling performance of MnO2 NWs clusters at current density of 10, 20 and 30 A g − 1, and the cut-off voltage is 0.65 V (Fig. 2d). At 10 A g − 1, the pseudocapacitance of NWs clusters increases to 692 F g − 1 after initial 100 activated cycles; maintains 692 F g − 1 until 500 cycles; and then gets back to 554 F g − 1. When the current density increases to 20 and 30 A g − 1, the pseudocapacitance maintains 415 F g − 1; reaches 60% of the maximum value at 10 A g − 1 and the coulombic efficiency is always more than 80%; exhibiting an excellent pseudocapacitance retention capability (Fig. 2d). The capacitances values are slightly lower than that of MnO2 nanoparticles/carbon nanotubes composites [6], but higher than MnO2 NWs/carbon nanotube composite paper (516.2 F g − 1) [11], manganese oxide/carbon aerogel composite (503 F g − 1) [12], γ-MnO2 nanoflake films (324 F g − 1) [9], birnesite-type MnO2 NWs
(191 F g − 1) [2], birnessite MnO2 nanotubes (350 F g − 1) [13], manganese oxide nanorod arrays (583.6 F g − 1) [10], and α-MnO2 nanorod (166.2 F g − 1) [14]. The high pseudocapacitance and excellent cycling stability of MnO2 are due to the unique 3-Dimension NWs clusters electrode. Each NW has its own contact with the substrate at the bottom, reducing internal resistance and ensuring every NW participates in the electrochemical reaction. The large specific surface area of NWs increases the electrochemical reaction rate at the interfaces, enhancing large-current charge–discharge capability. Ni foam is 3-Dimension; and the space between neighboring NWs is large enough, favoring easier diffusion of the electrolyte into the entire surface of all NWs, decreasing the polarization, enhancing large-current reaction capability. The length and the diameter of NWs are relatively small, shortening diffusion paths of electrons and ions within NWs, reinforcing electrochemical kinetics performance of MnO2 and favoring the large-current performance.
4. Conclusions We get MnO2 NWs clusters on Ni foam by a cyclic voltammetric technique. The capacitance achieved can be as high as up to 1080 F g − 1, and the large-current cycling performance is excellent, too. The small size and large dispersion degree of NWs clusters, as well as 3-Dimension structure of Ni foam favors faster ion/electron transfer; provides larger reaction surface area; and decreases the polarization, leading to faster kinetics and enhanced pseudocapacitive
Y.F. Yuan et al. / Materials Letters 73 (2012) 194–197
properties. It is believed that this study puts forward a significant and feasible strategy for high-performance MnO2 supercapacitors. Acknowledgement This work is supported by Research Projects of Education Department of Zhejiang Province, China under Grant No. Y201122147. References [1] Wang YG, Wang ZD, Xia YY. Electrochim Acta 2005;50:5641. [2] Vargas OA, Caballero A, Hernan L, Morales J. J Power Sources 2011;196:3350. [3] Yuan YF, Xia XH, Wu JB, Yang JL, Chen YB, Guo SY. Electrochim Acta 2011;56:2627.
197
[4] Yuan YF, Xia XH, Wu JB, Huang XH, Pei YB, Yang JL, et al. Electrochem Commun 2011;13:1123. [5] Wang YT, Lu AH, Zhang HL, Li WC. J Phys Chem C 2011;115:5413. [6] Kim JH, Kyung HL, Lawrence JO, Gil SL. Nano Lett 2011;11:2611. [7] Wu ZS, Ren WC, Wang DW, Li F, Liu BL, Cheng HM. ACS Nano 2010;4:5835. [8] Xu CL, Zhao YQ, Yang GW, Li FS, Li HL. Chem Commun 2009:7575. [9] Chou SL, Cheng FY, Chen J. J Power Sources 2006;162:727. [10] Lu XH, Zheng DZ, Zhai T, Liu ZQ, Huang YY, Xie SL, et al. Energy Environ Sci 2011;4:2915. [11] Chou SL, Wang JZ, Chew SY, Liu HK, Dou SX. Electrochem Commun 2008;10:724. [12] Lin YH, Wei TY, Chien HC, Lu SY. Adv Energy Mater 2011;1:901. [13] Roberts AJ, Slade RCT. Energy Environ Sci 2011;4:2813. [14] Li Y, Xie HQ, Wang JF, Chen LF. Mater Lett 2011;65:403.