ARTICLE pubs.acs.org/JPCC
Investigation of Pseudocapacitive Charge-Storage Behavior in Highly Conductive Ordered Mesoporous Tungsten Oxide Electrodes Changshin Jo,† Ilkyu Hwang,‡ Jinwoo Lee,*,† Chul Wee Lee,‡ and Songhun Yoon*,‡ † ‡
Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Kyungbuk 790-784, Korea Green Chemical Technology Division, Korea Research Institute of Chemical Technology (KRICT), Daejeon 305-600, Korea
bS Supporting Information ABSTRACT: Herein, a pseudocapacitive charging behavior of highly conductive ordered mesoporous tungsten oxide (m-WO3-X) is investigated. For this purpose, various electrochemical analysis methods such as cyclic voltammetry (CV), galvanostatic charge discharge experiment and electrochemical impedance spectroscopy (EIS) were employed. From CV experiment, a relationship analysis between voltammetric charge and scan rate resulted in total (67 C g1), outer (61 C g1) and inner charge (6 C g1), which was related with the well-developed crystalline structure of m-WO3-X. In galvanostatic chargedischarge profiles with change of applied current, a more severe decrease of cathodic capacity than the anodic one was observed, which was attributed to larger cathodic resistance. This resistance dependency on potential was clarified with EIS fitting analysis. Here, the charge transfer resistance and Warburg diffusion resistance became larger with increasing potential, which was relevant to the change of oxidation state during redox reaction. Using these electrochemical analysis results, a schematic illustration of the pseudocapacitive charging mechanism was proposed.
’ INTRODUCTION Great attention has been given to nanostructured materials for their energy storage application.15 One promising high power energy source, supercapacitors (SCs), has been intensively investigated as charge storage devices for digital communications, hybrid electric vehicles (HEVs), and electric vehicles (EVs).68 In pseudocapacitors, the charges are accumulated within an interphase between electrode and electrolyte from the Faradaic reaction, which resulted in a higher capacitance than electric double-layer capacitors (EDLCs). As electrode materials, various transition metal (Ni, Ru, Mn, Zn, and W) oxides and conducting polymer (polypyrrole and polyaniline) materials have been investigated.916 Because the specific capacitance (Csp) of Ru and Mn oxides are as high as 768 and 243 F g1, respectively, they have been considered as the most suitable pseudocapacitor electrodes.1214 Furthermore, a high rate capability of SCs has been considered as another important requirement. To improve rate capability, resistance of SCs should be minimized.6,7 In general, it is well-known that the resistance of pseudocapacitors is composed of several components: a bulk electrolyte resistance, an electrical resistance of the electrode, a charge transfer resistance associated with the faradaic reaction at the interphase, and diffusion resistance of proton conduction during charging process.1720 In order to increase the electrical conductivity and shorten the diffusion path of the proton for the purpose of resistance reduction, highly conductive and nanostructured metal oxides are highly recommended.7,20 Along this line, we developed a new ordered mesoporous WO3-X (hereafter m-WO3-X) with a high electrical conductivity and applied it to r 2011 American Chemical Society
pseudocapacitor electrodes.21,22 From the performance investigation of the m-WO3-X electrode, it exhibited a high Csp and a good rate capability due to electrical conductivity and ordered mesoporous structure.22 In addition, the m-WO3-X electrode showed a much higher volumetric capacitance (Cvol) due to its intrinsically high density. Here we report a detailed study of the characterization of the m-WO3-X pseudocapacitor electrode. In order to clarify the pseudocapacitive charge storage mechanism of the m-WO3-X electrode, various electrochemical analysis methods are employed. From cyclic voltammetry (CV) with a change of scan rate, the voltammetric charges are obtained and utilized for the estimation of proton charging capacity at the inner and outer surfaces. Using galvanostatic chargedischarge dependency on applied current, resistance is obtained in a cathodic direction and an anodic direction. Electrochemical impedance spectroscopy (EIS) is conducted with a change of applied potential, and the resistance components are obtained separately. Using the change of obtained resistances, a charging storage mechanism of the proton in the m-WO3-X electrode is presented.
’ EXPERIMENTAL METHODS Synthesis and Characterization of m-WO3-X and m-WO3. Hard template mesoporous silica KIT-6 was prepared following Received: April 20, 2011 Revised: May 11, 2011 Published: May 20, 2011 11880
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The Journal of Physical Chemistry C the reported procedure.2123 As-synthesized KIT-6 was synthesized using P123 ((EO)20(PO)70(EO)20) as a structure agent and butanol as a structure modifier. Calcined KIT-6 was impregnated with phosphotungstic acid in two steps by using the solvent evaporation infiltration method.21,22 In the first step, 1.2 g of phosphotungstic acid was dissolved in 20 mL of ethanol followed by addition of 0.45 g of KIT-6. After mixing, the mixture was dried at 60 C and calcined at 350 C under air. In this process, phosphotungstic acid infiltrates into the KIT-6 pores and the precursor is decomposed into WO3. In the second impregnation step, the prepared composite was infiltrated with 0.6 g of phosphotungstic acid in the same way for the first step impregnation method and calcined at 550 C again under air to obtain WO3/KIT-6. Hydrofluoric acid (HF) etching of WO3/KIT-6 generated m-WO3. m-WO3-X was prepared by heat-treatment of WO3/KIT-6 composite at 600 C under Ar/H2 (4 wt %) atmospehre for 4 h, followed by HF etching for the removal of silica. Pore size distribution (PSD) was analyzed by N2 adsorption measurement (Micromeritics ASAP 2010). External morphology of carbon was examined using a scanning electron microscope (SEM, JEOL JSM-840A), whereas the pore image was scanned by a transmission electron microscopy (TEM, JEOL JEM-2010). The X-ray diffraction (XRD) patterns were obtained with a Rigaku D/Max-3C diffractometer equipped with rotating anode and Cu KR radiation (λ = 0.15418 nm). Electrochemical Characterization. For the preparation of working electrodes, a mixture of prepared tungsten oxide powders, polytetrafluoroethylene (PTFE) binder and carbon additive Ketjenblack ECP-600JD (KB) for conductivity enhancement (10:1:1 in weight ratio) was dispersed in isopropyl alcohol and coated on 1 cm 1 cm stainless steel Exmet as the current collector. The electrode resistance was negligible when 10 wt % conducting materials was added.24 The resulting electrode plate was pressed and dried under vacuum at 120 C for 12 h. Electrochemical performance of the m-WO3-x electrodes was analyzed with three-electrode configuration in aqueous 2 M H2SO4 electrolyte. A Pt flag and Ag/AgCl electrode was used as the counter and reference electrode, respectively. The interelectrode gap between the working electrode and the reference one was fixed as 0.4 cm. EIS and CV were conducted by Iviumstat of Ivuim Technology. Ac-impedance spectra were recorded from 0 to 0.6 V vs Ag/AgCl at 10 mV magnitude from 0.05 to 105 Hz frequency. CV was carried out in the potential range of 0.1 to 0.8 V (vs Ag/AgCl) with a scan rate of 0.250 mV s1. For the nonlinear least-squares (NLLS) fitting of ac-impedance spectra, the program EQUIVALENT CUIRCUIT, version 3.95 by EG&G PARC was used. The electrode fabrication method was identical to the literature procedure except for the electrode loading of 10 mg cm2.21
’ RESULTS AND DISCUSSION Material Characterization. Figure 1 shows small-angle X-ray scattering (SAXS) and wide-angle XRD (WAXRD) patterns for m-WO3-X material. In Figure 1a, in the SAXS pattern of m-WO3-X, three typical peaks, (110), (211), and (200) peaks, are clearly observed, which is typical for mesostructured materials templated from KIT-6 mesoporous silica with cubic Ia3d structure. This I4132 or I4332 mesostructure was generated by a unique replication process of KIT-6 template in which one of the enantiomeric pairs in the channels of KIT-6 was exclusively
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Figure 1. (a) SAXS pattern of m-WO3-X. (b) WAXRD pattern (JCPDS: 46-1096) of m-WO3-X.
occupied with a WO3-X solid network. The SAXS pattern of m-WO3 is the same as that of m-WO3-X, meaning that both materials have the same ordered mesoporous structure (Figure S1, Supporting Information). The XRD peaks of m-WO3-X (Figure 1b) can be indexed to the cubic WO3-X phase (JCDPS: 46-1096). This electrically conducting cubic WO3-X phase was obtained through the reduction of the WO3 phase with tetragonal structure (Figure S2, JCDPS: 89-1287). This result indicates that W5þ ions generated by the partial reduction of WO3 phase are located all over the WO3-X skeletons. Figure 2 displays the SEM and TEM images of m-WO3-X. As is shown in the SEM image, ordered mesopores can be observed all over the m-WO3-X particles, which was also confirmed by the TEM image. Figure 3 displays nitrogen adsorptiondesorption isotherms of m-WO3-X. Here, PSDs of m-WO3-X estimated from the adsorption branch using the BarettJoynerHalenda (BJH) method was inset within the figure. The pore structure of m-WO3-X remained the same after reduction of m-WO3. An N2 adsorption shows two distinct jumps at ∼0.5 P/P0 and ∼0.9 P/P0, corresponding to uniform 3.5 nm pores and ∼20 nm pores observed in PSDs, respectively. Approximately 20 nm sized pores might be produced by filling phosphotungstic acid in either one of two chiral channels and removal of KIT-6 template.21 The measured pore volume (Vpore) and BrunauerEmmettTeller (BET) surface area (ABET) were 0.18 cm3 g1 and 54.3 m2 g1, respectively. Analysis of Pseudocapacitive Proton Charging. Figure 4 shows the cyclic voltammograms of m-WO3-X, which were measured from 0.1 to 0.8 V vs Ag/AgCl with a change of scan rate of 0.550 mV s1. Characteristic redox peaks of SCs were 11881
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Figure 4. Cyclic voltammograms of m-WO3-X in 2 M H2SO4 at room temperature with a scan rate of 0.550 mV s1.
Figure 2. Representative SEM (a) and TEM (b) image of m-WO3-X.
Figure 5. Capacitance versus voltage profiles obtained from cyclic voltammograms of m-WO3-X.
Figure 3. Nitrogen adsorptiondesorption of m-WO3-X. (Inset) Corresponding PSD calculated from adsorption isotherm of m-WO3-x using the BJH method.
observed. As can be seen, the current became larger, and peak potentials moved to higher voltage according to the increase in scan rate, which was probably due to a finite time constant in the SC electrodes.7,24 In order to investigate the change of redox peak patterns, capacitance versus voltage profiles were plotted in Figure 5, which were derived by dividing the measured current in Figure 4 by the scan rate.2528 Clearly, large anodic and cathodic
peak capacitances were observed: 220 and 250 F g1, respectively. As reported in our previous paper, capacitance per unit area (Carea) and volumetric capacitance (Cvol) of the m-WO3-X electrode were as high as 200 F cm2 and 639 F cm3, respectively, which were calculated by Vpore (0.18 cm3 g1) and the calculated density (dcal = 3.21 g cm3).22 Here, dcal was obtained from true density (dtrue) and pore volume (Vpore): dcal = 1/(Vpore þ dtrue1). From dcal, Cvol was estimated: Cvol = Csp dcal 3 dtrue of WO3, and WO3-X was 7.16 g cm3. When compared with the Cvol of the Mn oxide electrode, which is smaller than 500 F cm3, our m-WO3-X electrode exhibited a high Cvol.1116 It was reported that the redox peaks in Figure 5 located between 0.05 and 0.1 V vs Ag/AgCl are associated with the change of transition state between W6þ and W5þ.29 According to an increase in the scan rate, the anodic peak potential moved to higher values, and a 11882
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Figure 7. Galvanostatic chargedischarge profiles of m-WO3-X.
of voltammetric charge (q*) on scan rate (v) was examined.19,20 The voltammetric charge was calculated by the integration of the internal area in the cyclic voltammogram. ð1Þ q t ¼ q i þ q o
Figure 6. Dependence of (a) peak current on scan rate (v), (b) 1/q* on v1/2, and (c) q* on v1/2 for the m-WO3-X electrode.
smaller total internal area that is proportional to the available voltammetric charge (q*) in Figure 5 was observed. In Figure 6, the scan rate dependency of the peak current and q* were analyzed. In Figure 6a, the peak current increased linearly below 10 mV s1. This linearity is characteristic for an SC electrode. Above 10 mV s1, however, a deviation from the linearity was observed, and the redox peak became collapsed gradually in Figure 5. This deviation from SC behavior was attributed to the finite time constant (τ) of the m-WO3-X electrode with a relatively high loading of 10 mg cm2, which resulted in a high value because it is proportional to the resistance (R) and capacitance (C): τ = RC.6,7,25,28 In order to measure the electrochemically active sites coupled with the proton exchange between the oxide and the aqueous electrolyte, the dependency
Here, q*t, q*i, and q*o are total charge, inner charge, and outer charge, respectively.19 From the literature, q*o corresponds to the region directly accessible to protons.20 On the contrary, q*i is relevant to the proton partially accessible region, such as micropores, grain-boundaries, crevices, cracks, and so forth.19 q*o can be obtained by the extrapolation of q* to v = ¥ from the plot of q* versus v1/2.20 Similarly, q*t can be calculated from the extrapolation of q* to v = 0 from the 1/q* versus v1/2 plot. As shown Figure 6b,c, the estimation procedure of q*t, q*o, and q*i values is displayed, and they were 67, 61, and 6 C g1, respectively. These results demonstrated that a very high ratio of outer voltammetric charge was observed in the m-WO3-X material, which was probably due to a well-developed crystalline structure induced by the high calcination temperature and negligible proton partial accessible regions. Figure 7 shows the galvanostatic chargedischarge profiles of m-WO3-X electrode, which were measured with a change of the applied current from 1 to 50 mA cm2. Evidently, a characteristic pseudocapacitive behavior below 0.4 V and a steep potential increase were observed above 0.4 V, which was relevant to the current decrease in Figure 5.22 With a change of the applied current, the capacity decreased gradually and interestingly, and the decrease of cathodic capacity (downward direction) was more severe than that of the anodic one (upward direction), indicative of lower rate capability in the cathodic capacity.25 Typically, it is well known that a voltage change during current switching was proportional to the resistance of SCs.6,7,24 At first glance, the voltage rise at 0.1 V was smaller than its drop at 0.8 V, which reflected that the former resistance was smaller than the latter case. The resistance is calculated by dividing the voltage change during switching by the applied current and discussed later. In addition, note that the voltage rise at 0.1 V under 50 mA cm2 was below 0.2 V, which was similar to that of the microporous carbon electrode in EDLC.25 In principle, the charge mechanism in EDLC is based on the double-layer charging of electrolyte, which is basically much faster than the pseudocapacitor electrode that utilizes a redox reaction on the 11883
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Figure 9. (ad) Nyquist plots (empty circles) with voltage changes (00.6 V) and the best-fitted results (solid lines) using the equivalent circuit in panel e. Figure 8. (a) Capacity profiles at different applied currents. (b) Voltage drop versus applied current profiles. Rectangle and circle are anodic and cathodic directions, respectively.
interphase of electrode surface. Hence, the voltage rise of the m-WO3-X electrode, similar to the EDLC one, exhibited a high rate capability, which was probably due to its fast proton diffusion within the metal oxide wall and the short path of electrolyte transport through the well-ordered mesopores.2527 In Figure 8a, the capacity change was plotted against the applied current. Here, an anodic capacity remained less influenced, but a cathodic capacity became much smaller with an increasing current. In order to identify this different current dependency of capacity, Figure 8b displays the voltage drops at 0.1 (rectangle) and 0.8 V (circle) vs Ag/AgCl against the applied current. Here, the best-fitted results using the leastsquares method are shown as a solid or dot line. From the slopes of these lines, the resistance (R) can be acquired; the estimated resistance in cathodic and anodic scans were 11.0 and 4.4 Ω, respectively. This indicated that the cathodic resistance was 2.5 times larger than the anodic one. Because of this larger resistance, cathodic capacity decreased largely under high current as shown in Figure 7. In order to investigate the potential dependency of resistance, the EIS was carried out with a change of potential. In Figure 9, the EIS data expressed as a Nyquist plot are shown from 0 to 0.6 V vs Ag/AgCl. As can be seen, characteristic impedance spectra for a
pseudocapacitor were observed. At a very high frequency, the intercept Zreal corresponds to the bulk electrolyte resistance (Rb) between the reference electrode and the working one, which remained unchanged with potential change.7,24,25 In high frequency region, a semicircle was commonly observed and became larger with increase of potential. This semicircle resistance was relevant to the electrical conduction of the electrode and mostly to the resistance associated with proton charge-transfer reaction at the electrode/electrolyte interphase (Rct). Also, this semicricle was related with the double layer charging capacity (Qdl) within the electrode surface.17,18 With decreasing frequency, the slope change from 45 to vertical was observed, which is related with Warburg diffusion (W) of the proton within the interphase.17 The following pseudocapacitive charging (Qps) in the m-WO3-X electrode was observed for vertical rise. These electrochemical processes in the Nyquist plot can be represented as a combination of the above linear circuit elements in an equivalent circuit. In Figure 9e, an equivalent circuit is shown. Here, Q is a constant phase element (CPE) expressed as an admittance form by Boukamp;25 Q ; Y ðωÞ ¼ Y o ðjωÞn
ð2Þ
Here, ω = 2πf. For n = 1 and 0.5, Q represents an ideal capacitance and Warburg diffusion, respectively. Hence, it is expected that the n values of Qdl and Qps should be near 1. For 11884
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Table 1. Best Fitting Results of Nyquist Spectra According to Change of Measuring Potential Qdl Vmea /V Rbulk /Ω
Y0/F
Qw n
Rct /Ω Rw/ Ω
Scheme 1. Schematic Illustration of the Pseudocapacitive Charge Storage Mechanism in the m-WO3-X Electrodea
Qps n
Y0/F
n
0
0.42
1.98 104 0.85
1.62
0.81
0.50 2.83
1.0
0.2
0.43
1.27 104 0.89
2.08
1.33
0.45 0.92
1.0
0.4 0.6
0.43 0.45
0.89 104 0.91 0.62 104 0.94
2.19 2.24
2.18 3.00
0.43 0.28 1.0 0.35 0.10 0.91
QW, n should be close to 0.5.28 Furthermore, the Warburg resistance can be obtained: RW = 1/Yo in the QW fitting result.17 Using the proposed equivalent circuit, NLLS fittings of the EIS data in Figure 9 were conducted. The best-fitted results are displayed as a solid line in Figure 9 and listed in Table 1 (χ2 < 103). In this table, it was observed that the fitted value of Rb (∼0.43 Ω) remained invariant, irrespective of potential change. For capacitance comparison, Qdl was much smaller than Qps, indicating that pseudocapacitance was the dominant process for charge storage in the m-WO3-X electrode. As expected, n values of Qdl and Qps were above 0.9, reflecting that the employed equivalent circuit model was suitable for explanation of electrochemical processes. With increase of voltage, a decreasing tendency of Qdl and Qps was observed, but the latter decreased very abruptly, which indicates that pseudocapacitive charging decreased greatly at higher voltage. This coincided with cyclic voltammogram and galvanostatic chargedischarge profiles. On the contrary, Rct and RW became larger according to voltage increase as their contribution in total resistance varied. At 0 V, Rct had a major contribution, but RW became dominant according to voltage increase, which reveals that proton diffusion became much more sluggish at higher voltage. EIS fitting results thereby demonstrated that the larger voltage drop at 0.8 V was attributed to the large Rct and RW. Also, note that the calculated resistance by potential jump in Figure 8 was different from the summation of Rct and RW because an additional resistance can be raised by the pseudocapcitive charging process.18 This resistance dependency on potential can be explained by a change of oxidation state between W6þ and W5þ during the redox reaction. In our previous report, the electrical conductivity increase with crystal structure transformation was observed during reduction from m-WO3 to m-WO3-X (1.76 S cm1).21,22 In m-WO3, the oxidation state of W metal is þ6 (W6þ). However, W6þ and W5þ coexisted in m-WO3-X material, which was proven by our preliminary experiment using electron paramagnetic resonance (EPR). In this experiment, it was observed that paramagnetically active W5þ species existed in m-WO3-X. In addition, open circuit voltages of m-WO3 and m-WO3-X electrodes in fresh cells were 0.37 and 0.25 V, respectively, which was attributed to the difference in oxidation state.7 Scheme 1 elucidates the pseudocapacitive charge storage mechanism in the m-WO3-X electrode. For convenience, three phases within the WO3-X wall are schematically separated. In the electrode phase, tungsten metal has two oxidation states of W5þ and W6þ, and the proton is provided from the electrolyte phase.7,18 An interphase is assumed to exist between electrode and electrolyte, where the pseudocapacitive faradaic reaction mostly happens. From the literature, 47% of Ru atoms are involved in solidphase redox reaction in RuO2 film electrode.7,20 From a simple calculation using the estimated total charge (q*t = 67 C g1) in
a
Upper part is for the schematic transport of electron and proton within m-WO3-X. Bottom part indicates a schematic change of the oxidation state during pseudocapacitive chargingdischaring. Note that transports of electron (e) and proton (Hþ) within the interphase at 0 V became sluggish at 0.6 V vs Ag/AgCl.
CV experiment, charge utilization for the pseudocapacitive reaction was 15 533 C mol1, (67 C g1 231.84 g mol1), which indicated that 16% of W atoms in m-WO3-X was utilized for charge storage. Hence, it is assumed that the interphase occupied about 16% fraction of m-WO3-X electrode. When the electrode potential increases to 0.6 V, most W5þ species within the interphase are converted into W6þ state. This oxidation state change can result in the conductivity decrease and so Rct becomes larger. When lowered to 0 V, the state of the interphase is reduced to W5þ, and an increase of electrical conductivity (lower resistance) is expected. Along with the conductivity change, it was observed that RW became larger with voltage increase, indicative of a retarded proton transport within electrode interphase.18,20 The slow proton transport was probably relevant to a decrease of charge storage capability in W6þ state of m-WO3 material, which was expressed as the capacitance per area (Carea); m-WO3-X (366 F cm2) was much larger than that of m-WO3 (236 F cm2) in our previous report.22 In Scheme 1, furthermore, the merits of the ordered mesoporous structure in m-WO3-X electrode for pseudocapacitor is presented. As shown in Figure 7, the m-WO3-X electrode displayed a potential jump similar to that of the EDLC electrode, which was possibly due to a fast electrolyte transport through the mesopores and a well-developed electrical percolation within theWO3-X wall. Also, a rapid proton diffusion can be expected within the electrode interphase in the W5þ state when compared with the m-WO3 electrode.
’ CONCLUSIONS In summary, the pseudocapacitive chargingdischarging mechanism of the m-WO3-X electrode was proposed. To this 11885
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The Journal of Physical Chemistry C purpose, various electrochemical analysis methods were employed. From a relationship analysis between voltammetric charge and scan rate, total, inner, and outer voltammetric charge were estimated separately. Using a change of galvanostatic capacity with an increasing current, a large cathodic resistance resulted in a more severe decrease of cathodic capacity than anodic. The resistance increases associated with charge transfer resistance and Warburg diffusion were observed with increasing potential, which can be explained by the change of oxidation state during redox reaction. Conclusively, the electrochemical charge storage mechanism based on pseudocapacitive redox reaction was presented.
’ ASSOCIATED CONTENT
bS
Supporting Information. XRD pattern and SAXS pattern of m-WO3 are presented. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (J.L.);
[email protected] (S.Y.).
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’ ACKNOWLEDGMENT This work was financially supported by Korea Research Institute of Chemical Technology (KRICT). This research is further supported by a National Research Foundation of Korea grant funded by the Korean government (2009-0064640, 20090084771) and by the second stage of the BK 21 program of Korea. We appreciate Dr. Jong H. Jang for kind comments. ’ REFERENCES (1) Zhu, T.; Chen, J. S.; Lou, X. W. J. Mater. Chem. 2010, 20, 7015–7020. (2) Shi, Y.; Guo, B.; Corr, S. A.; Shi, Q.; Hu, Y. S.; Heier, K. R.; Chen, L.; Seshadri, R.; Stucky, G. D. Nano Lett. 2009, 9, 4215–4220. (3) Chen, J. S.; Cheah, Y. L.; Madhavi, S.; Lou, X. W. J. Phys. Chem. C 2010, 114, 8675–8678. (4) Nishihara, H.; Itoi, H.; Kogure, T.; Hou, P.-X.; Touhara, H.; Okino, F.; Kyotani, T. Chem.Eur. J. 2009, 15, 5355–5363. (5) Kwon, T.; Nishihara, H.; Itoi, H.; Yang, Q. H.; Kyotani, T. Langmuir 2009, 25, 11961–11968. (6) K€otz, R.; Carlen, M. Electrochim. Acta 2000, 45, 2483–2498. (7) Conway, B. E. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications; Springer: New York, 1999. (8) Hwang, S. R.; Teng, H. J. Electrochem. Soc. 2002, 149, A591–A596. (9) Ding, S.; Zhu, T.; Chen, J. S.; Wang, Z.; Yuan, C.; Lou, X. W. J. Mater. Chem. 2011, 21, 6602–6606. (10) Wang, H.; Casalongue, H. S.; Liang, Y.; Dai, H. J. Am. Chem. Soc. 2010, 132, 7472–7477. (11) Kalpana, D.; Omkumar, K.; Kumar, S. S.; Renganathan, N. Electrochim. Acta 2006, 52, 1309–1315. (12) Wu, M. S.; Chiang, P. C. J. Electrochem. Solid-State Lett. 2004, 7, A123–A126. (13) Yang, X.; Wang, Y.; Xiong, H.; Xia, Y. Electrochim. Acta 2007, 53, 752–757. (14) Jeong, Y.; Manthiram, A. J. Electrochem. Soc. 2002, 149, A1419– A1422. (15) Kim, H.; Popov, B. N. J. Electrochem. Soc. 2003, 150, D56–D62. (16) Zolfaghari, A.; Ataherian, F.; Ghaemi, M.; Gholami, A. Electrochim. Acta 2007, 52, 2806–2814. 11886
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