Manganese Oxide Nanowires Grown on Ordered Macroporous

ARTICLE pubs.acs.org/JPCC

Manganese Oxide Nanowires Grown on Ordered Macroporous Conductive Nickel Scaffold for High-Performance Supercapacitors Chia-Ling Ho and Mao-Sung Wu* Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan ABSTRACT: Macroporous α-MnO2 electrodes composed of nanowires were prepared by electrophoresis and electrodeposition with the help of monodispersed polystyrene spheres. The effects of macropore and conductive scaffold on the pseudocapacitive behavior of the MnO2 nanowire electrodes were investigated by cyclic voltammogram (CV) and galvanostatic charge/discharge tests in 1 M Na2SO4 electrolyte. Due to a better ion transport in macropores, the specific capacitance of macroporous MnO2 electrode (282 F g
1) was higher than that of bare MnO2 electrode (181 F g
1) at a scan rate of 10 mV s
1. Macroporous conductive nickel scaffold could further increase the active surface area and the utilization of MnO2 nanowire electrode due to the fast ion and electron transport in the macroporous MnO2 electrode. Hence, the specific capacitance of MnO2 electrode with nanowires grown on the macroporous conductive scaffold could reach as high as 472 F g
1 at a scan rate of 10 mV s
1.

1. INTRODUCTION Recently, supercapacitor also called electrochemical capacitor has become one of the most promising energy storage devices due to the increasing power demand for electric vehicles and portable electronics. Several metal oxides including ruthenium oxides, nickel oxides, cobalt oxides, and manganese oxides exhibit high specific capacitance and high rate capability which render them as attractive electrode materials for supercapacitor applications.1
3 Among these metal oxides, the manganese oxide-based supercapacitor has attracted much research attention because of its low cost, natural abundance, and friendly environmental compatibility compared with noble metal oxides.2 It is generally believed that precursors, manufacturing processes, and posttreatments greatly influence the physical properties, chemical properties, and electrochemical characteristics of the manganese oxide electrodes. Various synthetic methods such as chemical coprecipitation,4,5 thermal decomposition,6 hydrothermal method,7
11 sol
gel process,12
14 electrochemical deposition,14
18 and microemulsion19,20 have been used to prepare the nanostructured manganese oxides for supercapacitor materials. Electrochemical deposition method has turned out to have some advantages over the other methods: morphology, weight, and film thickness of manganese oxide can be varied by tuning the deposition parameters including current/potential, bath composition, and bath temperature. According to the faradaic reaction between MnO2 and MnOOH, the theoretical specific capacitance of MnO2 electrode can be calculated to be approximately 1110 F g
1.14 However, the practical capacitance values of manganese oxides reported in the literature are much less than the theoretical value. Actually, there are many different factors that affect the specific capacitance of the manganese oxide electrodes. The most important factors are as follows: microstructure (porous structure) of electrode, electrical r 2011 American Chemical Society

conductivity of the electrode, and proton transport through the manganese oxide lattice.14 Ultrathin manganese oxide film prepared by sol
gel method has been reported to have a very high specific capacitance of approximately 700 F g
1.14 Such high capacitance can be attributed to a very short distance for transportation of electron and proton through the ultrathin film. However, such ultrathin film limits its practical application in terms of energy density. Nanostructured manganese oxides with high specific surface area and short proton-transport path such as nanorods, nanowires, nanofibers, and nanoflakes have been demonstrated to be promising candidates for supercapacitor applications.21
26 The porous film structure is beneficial to the electrochemical performance of manganese oxide electrode because it can provide more channels for electrolyte (proton) transport through porous film compared with a compact film. A compact thick film makes electrolyte more difficult to penetrate into the inner layer of the film, only the outer layer of the film is exposed to the electrolyte. The thick film is also harmful to the electron conduction from the current collector to the outer layer of the manganese oxide film due to the inherently poor electrical conductivity of manganese oxide. To improve the electrical conductivity of manganese oxide film, conductive materials, such as carbon powder, carbon nanotubes, graphite, and graphene oxide, have been combined into the MnO2 films.27
33 In this work, we propose a novel architecture, manganese oxide nanowires grown on ordered macroporous conductive nickel scaffold, to enhance the capacitive behavior of manganese oxide electrode. This manganese oxide architecture expedites the electron conduction and electrolyte transport Received: August 23, 2011 Revised: September 30, 2011 Published: October 03, 2011 22068

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Figure 1. Schematic illustration of the proposed procedure for fabricating (a) macroporous MnO2 electrode and (b) macroporous conductive MnO2 electrode.

through film electrode and consequently improves the pseudocapacitive behavior of the manganese oxide nanowires.

2. EXPERIMENTAL SECTION The monodispersed PS (polystyrene) spheres (about 500 nm in diameter) with negatively charged surfaces were suspended in water (2.5 wt %, Alfa Aesar). The monolayer PS-sphere template was assembled onto the stainless steel (SS) substrate by electrophoretic deposition (EPD). EPD of the ordered PS monolayer was carried out by applying a potential difference of 60 V (Keithley, 2400 source meter) between the working (positive electrode, SS, 2 cm  2 cm) and counter (negative electrode, platinum, 2 cm  2 cm) electrodes for 60 s at room temperature. The EPD bath was prepared by suspending PS spheres in an isopropyl alcohol solution (IPA, 50 mL). The ordered PS/Ni composite with core
shell structure was prepared by EPD in a bath containing IPA (50 mL), PS suspension (0.24 g), and nickel nitrate (15 mg). Prior to EPD, the SS foil was cut into pieces of 2 cm  2 cm, which were then soaked in acetone and ultrasonically vibrated for 10 min to remove any contaminants from their surfaces. Deionized (DI) water was then used to rinse the tailored SS foils in ultrasonic vibration for another 15 min. The distance between the working and counter electrodes was maintained at approximately 1 cm. After EPD, the template electrodes were dried at room temperature for 24 h in air. Manganese oxide nanowires were anodically deposited onto the bare SS, PS-coated SS, and PS/Ni-coated SS in a plating solution of 0.1 M manganous acetate and 0.1 M sodium sulfate at a current density of 0.125 mA cm
2. The plating solution was stirred by a Teflon stir bar on a magnetic hot plate during deposition. After electrochemical deposition, the as-deposited

electrodes with PS-sphere templates were immersed in a toluene solution for 30 min to remove PS-sphere template. The manganese oxide electrodes were rinsed several times in DI water and then annealed at 300 °C for 1 h in air. The weight of the manganese oxide deposits was measured by the microbalance (Mettler Toledo XS105) with an accuracy of 0.01 mg. The surface morphology of the manganese oxide electrodes was examined with a field-emission scanning electron microscope (FESEM, JEOL JSM-6700F). The nanowire structure of the manganese oxide was characterized by a transmission electron microscopy (TEM, JEOL JEM-2010). TEM specimen was prepared by the following procedure: the manganese oxide nanowires were stripped off and suspended in ethanol with ultrasonic vibrations for 5 min; a drop of the manganese oxide supernatant was then transferred onto a standard holey carbon-covered-copper TEM grid. The electrochemical characteristic of the manganese oxide electrodes was determined by cyclic voltammetry in a homemade three-electrode cell with 1 M Na2SO4 aqueous electrolyte. The electrochemical test cell was equipped with a working electrode (2 cm  2 cm), a platinum counter electrode (2 cm  2 cm), and a reference electrode (saturated calomel electrode, SCE). The potential was swept linearly in time using a potentiostat/galvanostat (CH Instruments, CHI 608) in a potential range of 0
1.1 V at a scan rate of 10 mV s
1. Galvanostatic charge/discharge and cycle-life stability were performed at various current densities by a source meter (Keithley, 2400 source meter) in a potential range of 0
1.1 V versus SCE.

3. RESULTS AND DISCUSSION Figure 1 illustrates the proposed procedures for fabricating macroporous manganese oxide electrodes. EPD of the PS-sphere 22069

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template is carried out via transport of PS spheres with negatively charged surfaces toward a positive electrode and via deposition of PS spheres under an applied electric field (Figure 1a). The formation of monolayer PS-sphere template can be achieved by adjusting the strength of applied electric field and/or deposition time. Manganese oxide nanowires are deposited in the interstitial spaces between PS spheres during anodic deposition. The electrochemical growth of manganese oxide nanowires from an aqueous solution containing Mn2+ ions can be expressed as34 Mn2þ þ 2H2 O f MnO2 þ 4Hþ þ 2e


ð1Þ

The negative charges are scattered around outside the PS-sphere surfaces on which Mn2+ ions can be absorbed due to the electrostatic force. However, it is difficult to directly deposit manganese oxide nanowires on the PS surface because PS is an electrical insulator, which cannot conduct electrons for the electrochemical reaction to occur. When the Mn2+ ions in the bulk electrolyte arrive at the exposed SS surface (anode) due to diffusive transport, the electrochemical reaction occurs simultaneously, forming a manganese oxide layer on the exposed SS surface. The deposited manganese oxide film allows electrons to be conducted from the SS substrate to the outer layer of deposits. Thus, the manganese oxide continues to be deposited. The manganese oxide grows perpendicular to the SS substrate and finally covers the PS spheres with a further increase in the deposition time. After deposition, the PS-sphere template is dissolved by toluene, leading to the formation of macroporous film. In this work, the formation of manganese oxide film with open macropores can be achieved in a short deposition time. Macroporous conductive nickel scaffold is carried out by means of EPD of PS spheres in the presence of nickel nitrate. When adding nickel nitrate to the PS-sphere suspension, nickel ions dissolved from the nickel nitrate are adsorbed by the PS spheres of negative charged surfaces to form more positively charged PS spheres. When the PS sphere arrives at the SS substrate (negative electrode), the electrostatic driving force between nickel ion and the SS surface is much greater than that between nickel ion and the PS surface. Therefore, the nickel ions absorbed on the PS sphere surface may move to the bottom of the PS sphere along the PS surface and may react electrochemically with the electrons from SS substrate. In this case, a nickel layer grows around the PS sphere, forming a partially spherical nickel shell. Manganese oxide nanowires are deposited onto the core
shell structure of PS/Ni composite by anodic electrodeposition. After deposition, the PS template is dissolved by toluene, leading to the formation of manganese oxide nanowires grown on the macroporous conductive nickel scaffold shown in Figure 1b. Figure 2 shows the XRD patterns of SS, MnO2 grown on SS, and MnO2 grown on conductive nickel scaffold after annealing at 300 °C for 1 h in air. In addition to the diffraction peaks of SS, the XRD pattern of MnO2 grown on SS could be assigned to α-MnO2 (JCPDS 44-0141).35 As can be seen from Figure 2, the crystal structure of MnO2 grown on macroporous conductive nickel scaffold is similar to that of MnO2 grown on SS substrate. The diffraction peak at about 2θ = 37.5° (211) is broad, which indicates a poor crystallinity and a small grain size of the electrodeposited MnO2 after 300 °C annealing. A smaller grain size can provide a larger specific surface area for electrolyte access. To achieve high current output from the capacitor, the surface area of the electrodes in contact with the electrolyte must be as large as possible.

Figure 2. XRD patterns of SS substrate, MnO2 grown on SS, and MnO2 grown on conductive nickel scaffold.

Figure 3 shows the SEM and TEM micrographs of MnO2 nanowires grown on the bare SS, macroporous template, and macroporous conductive template after removal of PS spheres. The MnO2 grown on SS substrate, denoted as bare MnO2 electrode, shows uniformly distributed nanowires with 10
15 nm in diameter (Figure 3a). TEM observation (Figure 3b) reveals that the deposited MnO2 is composed of hollow nanowires. The MnO2 deposited on the PS-coated SS, denoted as macroporous MnO2 electrode, has monodispersed open macropores (about 500 nm in diameter) and consists of MnO2 nanowires with a diameter of approximately 10
15 nm (Figure 3c). The SEM micrograph of MnO2 grown on macroporous conductive nickel scaffold, denoted as macroporous conductive MnO2 electrode, is similar to that of macroporous MnO2 electrode, except that the electrode contains the partially spherical nickel shells as the conductive scaffolds (Figure 3d). It was previously reported that the nanowire and nanorod architectures could improve the electrochemical performance of MnO2 thin films because it has a high specific surface area for redox reactions and a shortened ion diffusion path in the solid phase.21
26 The pore size of MnO2 films also plays an important role in determining the electrolyte penetration, ion diffusion, and ion migration resistances.36
39 It is reasonable that a larger pore is more easily accessible to the electrolyte, whereas a smaller pore is unfavorable to electrolyte penetration and ion transport. As can be seen in Figure 3, MnO2 film electrodes prepared with the help of PS template have monodispersed open macropores (about 500 nm in diameter). Hence, we can expect that the MnO2 electrodes with nanowires and open macropores may facilitate their pseudocapacitance behavior. The pseudocapacitance properties of MnO2 electrodes were evaluated by cyclic voltammetry. Figure 4 shows the cyclic voltammograms (CVs) of Ni scaffold and MnO2 electrodes at a scan rate of 10 mV s
1. These MnO2 electrodes are almost similar in CV shape. CV curves of the MnO2 electrodes are roughly rectangular mirror images with respect to the zero current line, which is the characteristic of an ideal pseudocapacitive behavior. Generally, the specific capacitance of electrode is directly proportional to its CV area. The CV area of MnO2 grown on macroporous conductive nickel scaffold is much higher than that of macroporous MnO2 electrode and bare MnO2 electrode. 22070

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Figure 3. (a) SEM micrograph of bare MnO2 electrode. (b) TEM micrograph of MnO2 nanowires. (c) SEM micrograph of macroporous MnO2 electrode. (d) SEM micrograph of macroporous conductive MnO2 electrode.

Figure 4. Cyclic voltammograms of the Ni scaffold and MnO2 electrodes carried out at a scan rate of 10 mV s
1.

Specific capacitance (C) of the electrodes can be calculated from CV curves according to the following equation: C¼

i ν

ð2Þ

where C is the specific capacitance of manganese oxide electrode (F g
1), ν is the scan rate (V s
1), and i is the average cathodic

current density (A g
1). For the macroporous conductive MnO2 electrode, the specific capacitance is calculated on the basis of the weight of MnO2 which was measured by subtracting the weight of Ni-coated SS (after removal of PS) from the weight of macroporous conductive MnO2 electrode after 300 °C annealing. The specific capacitance of macroporous conductive MnO2 electrode at a scan rate of 10 mV s
1 was measured to be approximately 472 F g
1, much higher than that of macroporous MnO2 electrode (282 F g
1) and bare MnO2 electrode (181 F g
1). The specific capacitance of macroporous MnO2 electrode is higher than that of bare MnO2 electrode due to the macroporous structure which allows fast electrolyte ion transport through electrode. However, macroporous structure seems to have less effect on the specific capacitance of MnO2 electrode than the conductive effect of nickel scaffold described above. A significant increase in specific capacitance can be achieved by employing the macroporous conductive nickel scaffold. This finding suggests that a combination of conductive scaffold and porous structure plays a key role in enhancing the specific capacitance of MnO2 electrode. In addition, the CV curve of the Ni scaffold (after heating at 300 °C for 1 h in air) in 1 M Na2SO4 aqueous solution, which is also shown in Figure 4, was scanned for comparison; capacitance value was very low (less than 30 F g
1) compared with the main MnO2 material. Conclusively, the capacitance came mainly from the MnO2 rather than the Ni scaffold. Generally, supercapacitor uses fast and reversible surface or near-surface reactions for charge storage. Two mechanisms have 22071

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Figure 5. Schematic illustration of the MnO2 nanowires grown on macroporous conductive nickel scaffold for facilitating both the electron and ion transport through electrode.

been proposed to explain the charge storage in MnO2 materials. The first one is based on the surface adsorption/desorption of alkali metal cations (C+) from the neutral electrolyte40
42 ðMnO2 Þsurface þ Cþ þ e
T ðMnO2
Cþ Þsurface +

+

ð3Þ

where C represents the alkali metal cation such as K , Na , or Li+. Most of the adsorption and desorption processes take place only on the outermost surface layer of the manganese oxide. In this case, the amount of charge storage in MnO2 electrode may be less dependent on the charge/discharge current density due to the fast surface reaction. Previous study demonstrated that only the Mn surface atoms are involved in the pseudocapacitive redox processes in 0.1 M Na2SO4 aqueous electrolyte.40 Thus, the high capacitance seems to be related to the high surface area of the MnO2 powder rather than the intercalation of Na+ ions and/or protons in the bulk of α-MnO2.40 On the basis of the first mechanism, the pseudocapacitance of MnO2 depends on the effective surface area which is significantly affected by the particle size and pore size distribution. It was previously reported that the pore structure and structural morphology of MnO2 electrode can be varied by using surfactant template in an attempt to obtain the high effective surface area for enhanced specific capacitance of electrode.5,43 In this work, with the help of macropores, the specific capacitance of macroporous MnO2 electrode is somewhat higher than that of bare MnO2 electrode. Although the open macroporous structure can improve the electrolyte ion transport through MnO2 electrode, there is still a need for improved pseudocapacitive performance. We believe that in addition to porous structure, the electrical conductivity may play an important role in enhancing the specific capacitance of MnO2 nanowire electrode. Poor electrical conductivity makes electron more difficult to conduct from the SS substrate to the top of the nanowires; only the region near the bottom of the nanowires is electrochemically active. The second mechanism involves the intercalation/deintercalation of protons or alkali metal cations in the manganese oxide bulk as follows:14,41 MnO2 þ H þ þ e
T MnOOH MnO2 þ C

þ




þ e T MnOOC

+

ð4Þ ð5Þ

According to previous reports, this mechanism is expected to be predominant in crystalline manganese oxide.44 The MnO2 has different crystal forms including α-MnO2, β-MnO2, δ-MnO2,

and γ-MnO2, etc. The pseudocapacitive behavior of MnO2 electrodes significantly depends on their crystalline structure, especially the size of the tunnels able to provide intercalation and deintercalation of cations.45 Among these MnO2 materials, α-MnO2 exhibits high specific capacitance because it contains larger (2  2) tunnels within its crystal structure for facilitating intercalation and deintercalation of cations.46 β-MnO2 structure is inherently more stable than α-MnO2 because its structure consists of narrow (1  1) tunnels. However, this stable structure with narrow tunnels impedes the intercalation and deintercalation of cations, therefore resulting in a very low capacitance value.47 Due to the limited electrical conductivity of bulk MnO2, the adsorption/desorption and intercalation/deintercalation of cations may only occur at the nanowire/SS/electrolyte interface. The α-MnO2 nanowires grown on macroporous conductive nickel scaffold showed a better pseudocapacitive behavior than the bare and macroporous MnO2 electrodes. This was attributed to the fact that the conductive scaffold featuring ordered macropores shown in Figure 5 provides a large number of fast electron and ion transport routes for charge storage through eqs 3
5. Figure 6 shows the effect of scan rate on the specific capacitance of MnO2 electrodes during CV tests. It is generally believed that the pseudocapacitive behavior of electrodes is affected by charge-transfer, mass-transfer, and ohmic resistances. Nanoscaled MnO2 materials can provide large surface area for reducing the charge-transfer resistance of redox reactions (eqs 4 and 5). In addition, the diffusion resistance of protons within the bulk of MnO2 nanowires can be mitigated due to the short diffusion paths in nanowires. The diffusion resistance of protons in electrolyte may be improved by use of porous electrode materials. The ohmic resistance results from the electron transport in solid electrode, which can be reduced by incorporating the conductive scaffold into the electrode structure. An electrode with macropores is in favor of the electrolyte penetration and ion migration (diffusion). In this work, the MnO2 nanowires were electrochemically grown on the macroporous conductive nickel scaffold. This unique architecture facilitates both the ion transportation and electron conduction, thus substantially increasing the utilization of MnO2 electrode. Clearly, the specific capacitance of macroporous conductive MnO2 electrode at each scan rate was much higher than that of bare MnO2 electrode and macroporous MnO2 electrode. The capacitance value of macroporous conductive MnO2 electrode 22072

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Figure 6. Specific capacitance variations of the MnO2 electrodes associated with the CV scan rate.

reached as high as 472 F g
1 at a low rate of 10 mV s
1, and reduced to 380 F g
1 at a very high rate of 200 mV s
1. On the other hand, the bare MnO2 electrode only reached 181 F g
1 at a low rate of 10 mV s
1 and reduced to 126 F g
1 at a high rate of 200 mV s
1. As can be seen from Figure 6, the scan rate has little effect on the capacitance value, indicating that the electrochemical reaction on the α-MnO2 surface is fast enough. A small decrease in capacitance value with increasing scan rate can be explained by the slow diffusion of protons into the 2  2 tunnels of α-MnO2 and/or mesoporous electrode. At lower scan rates, most of the active sites can be utilized for charge storage. However, at high scan rates, diffusion limits the movement of protons due to time limit, and only the outer active surface can be utilized for the charge storage. An electrode with high electrical conductivity can provide much more active sites for charge storage, leading to a large specific capacitance. Therefore, the MnO2 nanowires grown on macroporous conductive nickel scaffold have two significant advantages: (i) the nanowire configuration ensures that each nanowire participates in the charge storage reactions because each nanowire is in electric contact with the macroporous nickel scaffold and also interfaced with the electrolyte solution, and (ii) the open macropores and spaced nanowires make the diffusion and migration of the electrolyte easier. Therefore, this electrode architecture is particularly promising for high-power application when the pseudocapacitor is used at high-rate charge and discharge conditions. Figure 7 displays the galvanostatic charge and discharge curves of MnO2 electrodes at a current density of 5 A g
1. All electrodes showed a sloping potential profile during charging and discharging, reflecting an ideal pseudocapacitive behavior of MnO2 nanowire electrodes. A closer look at the charge and discharge curves and the charging time and discharging time for all electrodes are almost identical, implying a high reversibility of the faradaic reaction taking place on the α-MnO2 surface. The charging time and discharging time of bare MnO2 and macroporous MnO2 electrodes in each cycle were much shorter than those of macroporous conductive MnO2 electrode. The specific capacitance of MnO2 electrodes during the galvanostatic test could be calculated according to the following equation: C¼

iΔt ΔV

ð6Þ

Figure 7. Galvanostatic charge/discharge curves of the MnO2 electrodes at a current density of 5 A g
1: (a) bare MnO2 electrode, (b) macroporous MnO2 electrode, and (c) macroporous conductive MnO2 electrode.

Figure 8. Capacitance retention of the MnO2 electrodes during galvanostatic charge/discharge cycling at a current density of 20 Ag1
.

where ΔV (V) is the potential window and i is the discharge current density (A g
1) applied for time Δt (s). The specific capacitances of bare MnO2, macroporous MnO2, and macroporous conductive MnO2 electrodes were calculated to be approximately 163, 261, and 443 F g
1, respectively. These capacitance values were comparable to those of obtained from the CV curves at a scan rate of 25 mV s
1 shown in Figure 6. The specific capacitance of the macroporous conductive MnO2 electrode is lower than that of RuO2 electrode and is much higher than that of SnO2 electrode and V2O5 electrode.1,2 The cycle-life stability of MnO2 electrodes was carried out by galvanostatic charging/discharging at a high current density of 20 A g
1. Figure 8 shows the capacitance retention of MnO2 electrodes during charge and discharge cycling tests. Clearly, the macroporous structure has little impact on the cycle-life stability. The MnO2 electrodes maintain over 90% of their initial capacitances after 3000 cycles. The high capacitance retention reflected a high durability of α-MnO2 nanowires grown on macroporous conductive substrate for electrochemical capacitor application in neutral electrolyte. 22073

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4. CONCLUSIONS The macroporous α-MnO2 nanowire electrodes were prepared by electrophoretic deposition of PS-sphere template, followed by anodic electrodeposition of MnO2 nanowires. The formation of PS-sphere monolayer can be achieved via transport of PS spheres with negatively charged surfaces toward SS substrate and via deposition of PS spheres under an applied electric field. The conductive scaffold with partially spherical nickel shells can be formed by EPD of PS spheres in the presence of nickel nitrate additive. The results indicated that the open macroporous structure has less effect on the pseudocapacitive behavior of MnO2 electrode than the macroporous conductive scaffold. The specific capacitance of macroporous conductive MnO2 electrode was much higher than that of macroporous MnO2 electrode and bare MnO2 electrode in all CV scan rates. The improved pseudocapacitive behavior resulted from the unique architecture featuring macroporous conductive nickel scaffold which facilitates both the electrolyte ion transport and electron conduction through MnO2 electrode. Macroporous conductive MnO2 electrode exhibited high capacitance and stable capacitance retention during charge/discharge cycling and therefore is a promising candidate for long-term applications in high-performance supercapacitors. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: 886-9-45614423.

’ ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the National Science Council, Taiwan, Republic of China (Project No. NSC 100-2221-E-151-059). ’ REFERENCES (1) Zhang, Y.; Feng, H.; Wu, X.; Wang, L.; Zhang, A.; Xia, T.; Dong, H.; Li, X.; Zhang, L. Int. J. Hydrogen Energy 2009, 34, 4889. (2) Wei, W.; Cui, X.; Chen, W.; Ivey, D. G. Chem. Soc. Rev. 2011, 40, 1697. (3) Zhao, X.; S
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