Remarkable Capacity Retention of Nanostructured Manganese Oxide

Mar 24, 2009 - (1-8) Especially, there has been intense research on energy storage to satisfy the present day demand for high and continuous power sup...
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J. Phys. Chem. C 2009, 113, 6303–6309

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Remarkable Capacity Retention of Nanostructured Manganese Oxide upon Cycling as an Electrode Material for Supercapacitor P. Ragupathy,† Dae Hoon Park,*,‡ Guy Campet,‡ H. N. Vasan,*,† Seong-Ju Hwang,§ Jin-Ho Choy,§ and N. Munichandraiah| Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore- 560012, India, Institut de Chimie de la Matiere Condensee de Bordeaux du CNRS, AVenue du Dr. Albert Schweitzer 87, 33608, Pessac France, Center of Intelligent Nano-Bio Materials (CINBM), DiVision of Nano Sciences and Department of Chemistry, Ewha Women’s UniVersity, Seoul 120-750, Korea, and Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore- 560012, India ReceiVed: December 25, 2008; ReVised Manuscript ReceiVed: February 18, 2009

Electrochemical capacity retention of nearly X-ray amorphous nanostructured manganese oxide (nanoMnO2) synthesized by mixing directly KMnO4 with ethylene glycol under ambient conditions for supercapacitor studies is enhanced significantly. Although X-ray diffraction (XRD) pattern of nanoMnO2 shows poor crystallinity, it is found that by Mn K-edge X-ray absorption near edge structure (XANES) measurement that the nanoMnO2 obtained is locally arranged in a δ-MnO2-type layered structure composed of edge-shared network of MnO6 octahedra. Field emission scanning electron microscopy and XANES measurements show that nanoMnO2 contains nearly spherical shaped morphology with δ-MnO2 structure, and 1D nanorods of R-MnO2 type structure (powder XRD) in the annealed (600 °C) sample. Volumetric nitrogen adsorption-desorption isotherms, inductively coupled plasma analysis, and thermal analysis are carried out to obtain physicochemical properties such as surface area (230 m2 g-1), porosity of nanoMnO2 (secondary mesopores of diameter 14.5 nm), water content, composition, etc., which lead to the promising electrochemical properties as an electrode for supercapacitor. The nanoMnO2 shows a very high stability even after 1200 cycles with capacity retention of about 250 F g-1. Introduction Recently nanostructured materials have attracted great interest because of their unique physical and chemical properties and wide applicability in various fields such as energy storage (lithium secondary battery, solar cell, supercapacitor, and fuel cell), catalysts, sensors, and so forth.1-8 Especially, there has been intense research on energy storage to satisfy the present day demand for high and continuous power supply to drive the fast-changing development in wireless communication equipments and electric transportation.9-11 Among the various kinds of energy storages, electrochemical supercapacitors have gained enormous interest.12-30 In the Ragone plot (specific power vs specific energy), these devices occupy the area between batteries and conventional capacitors. Active electrode materials for supercapacitor are broadly classified into three categories: (1) activated high surface area carbon, (2) conducting polymers, and (3) transition metal oxides.12 Among the various transition metal oxides investigated for supercapacitor electrode materials over the years, ruthenium oxide is considered as the most promising material offering high specific capacitance (760 F g-1) with excellent cyclability to replace carbon which is commercialized.13-15 However, ruthenium oxide still suffers on its commercialization due to high cost. Toward this end, scientists of different disciplines have * To whom correspondence should be addressed. E-mail: vasan@ sscu.iisc.ernet.in. Phone: +91 80 2293 3310. Fax: +91 80 2360 1310. † Solid State and Structural Chemistry Unit, Indian Institute of Science. ‡ Institut de Chimie de la Matiere Condensee de Bordeaux du CNRS. § Ewha Women’s University. | Department of Inorganic and Physical Chemistry, Indian Institute of Science.

made great efforts in developing less expensive candidates such as MnO2, NiO, V2O5, MoO3, PEDOT, etc. as a substitute for ruthenium oxide.16-33 Among these, manganese oxide has received special attention, due to its environmental compatibility and low cost. MnO2 exists in different crystallographic forms; in particular δ- and R-MnO2 forms (Figure 1) are of interest in view of their good electrochemical capacitance behavior.26,27 The performance as capacitor or Li intercalation/deintercalation of nanostructured MnO2 is found to be better than the bulk MnO2, and many reports are found in the literature for synthesizing nanoMnO2 from KMnO4 by using various inorganic reducing agents such as KBH4, Na2S2O4, NaH2PO2, HCl, H2SO4, and Mn(II) salts.16-18,24 Also, remarkable efforts are made to synthesis nanostructured manganese oxide by using organic reducing agents such as fumaric acid,21 aniline,23 glucose,34 BMIM-BF4,35etc., because of the advantage of most of the organic intermediates obtained have low decomposition temperature which prevent further growth and agglomeration of particles.21,23 Abundant previous research work enabled nanoMnO2 to provide distinct enhancement in specific capacitance from 100 to 250 F g-1.16-18,20-23,25-27,36-43 To the best of our knowledge most of the hitherto studied nanoMnO2 in the powder form, which is useful for practical application for high power density devices showed poor capacitance retention upon repeated cycling.16,17,20,22,23,25-27,42 Thus, it remains a challenge to overcome the capacitance fade upon repeated cycling. In the present work, we report ethylene glycol mediated one pot process of synthesizing nanoMnO2. The obtained product is nearly X-ray amorphous with high surface area of about 230 m2 g-1, suitable for supercapacitor electrode. Even after 2000

10.1021/jp811407q CCC: $40.75  2009 American Chemical Society Published on Web 03/24/2009

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Figure 1. Crystal structure of δ-MnO2 (a) and R-MnO2 (b).

cycles, the nanoMnO2 exhibits a remarkable capacitance retention of 250 F g-1, with little fading (8%) after 1200 cycles. Experimental Section Synthesis. The nanostructured MnO2 (nanoMnO2) sample was prepared by mixing KMnO4 with ethylene glycol in a beaker under ambient conditions. Because permanganate is a strong oxidizing agent and ethylene glycol is a strong reducing agent, the exothermic redox reaction between them leads to the formation of nanoMnO2 and oxidized products of glycol, presumably some aldehydes. In a typical preparation, about 0.022 mol (3.47 g) of commercial KMnO4 was dissolved in 200 mL of distilled water and then slowly mixed with 0.090 mol (5 cc) of commercial ethylene glycol. The mixture was aged for 20 min on stirring at ambient conditions. The resulting powder was washed with distilled water several times followed by ethanol and dried in air at 60 °C for overnight. To see the evolution of the structure, the sample was heated at intervals of 100 °C for 3 h starting from 200 up to 600 °C. The nanoMnO2 can be scaled up with the employed method as it is very simple and economical. Characterization. To obtain the information on the crystal structure and its morphology for the resulting nanoMnO2, powder X-ray diffraction (XRD) was recorded in Philips XRD X’PERT PRO diffractometer with Cu KR (λ ) 1.5418 Å) as a source. Field emission scanning electron microscopic/energydispersive spectroscopic (FE-SEM/EDS) studies were done in Jeol microscope model JSM-6700F. TEM and selected area electron diffraction (SAED) studies along with high resolution transmission electron microscopy (HRTEM) were done in model TECNAI F30. The local crystalline structure of the nearly X-ray amorphous of nanoMnO2 was obtained by XANES studies operated at 2.5 GeV and 180 mA. XANES data was collected at room temperature in a transmission mode using a Si(111) single crystal monochromator and gas ionization detectors. The calibration was done by measuring the reference spectrum of Mn metal simultaneously. Data analysis of the experimental spectra was carried out using the standard procedure.44 Chemical composition and thermal behavior of nanoMnO2 were probed by inductively coupled plasma (ICP) analysis (Varian inductively coupled plasma atomic emission spectrometer model Vista-PRO) and thermogravimetric analysis (TGA) (NETZSCH TG 209 F1). Surface area and porosity of the sample were obtained from N2 adsorption-desorption isotherm and BarretJoyner-Halenda (BJH) plots (Micromeritics porosimeter ASAP 2010). For electrochemical characterization, electrodes were

Figure 2. Powder XRD patterns of (a) nanoMnO2 and samples calcined for 3 h at (b) 200, (c) 300, (d) 400, (e) 500, and (f) 600 °C, respectively.

fabricated on a high purity stainless steel (SS) (WEBER) as a current collector. The SS was polished with successive grades of emery paper and washed thoroughly with detergent and distilled water. 75 wt % of nanoMnO2, 20 wt % of ketjen black EC-600 JD (KB), and 5 wt % of polyvinylidene fluoride (PVDF) were ground in a mortar. Several drops of n-methyl pyrrolidinone (NMP) were added to make a syrup. This was then coated on to the pretreated SS foil and dried at 100 °C under reduced pressure for 12 h. The electrochemical studies were performed in a three electrode-configuration-cell consisting of nanoMnO2 as the working electrode, platinum foil as a counter electrode and Hg/HgO, OH- (6 M) as the reference electrode by using a galvanostat/potentiostat (TACUSSEL electronique, model PGS201T). Results and Discussion Powder XRD, SEM/EDX, and TEM Analysis. Structural analysis was carried out by powder XRD. Figure 2 shows the powder XRD patterns of nanoMnO2 and annealed samples. The patterns are nearly identical up to 300 °C (parts a-c of Figure 2) indicating that nanoMnO2 sustains its amorphous framework up to 300 °C, whereas R-MnO2 with tunnel structure matching with the JCPDS 44-0141 appears for heat treated sample higher

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Figure 3. SEM images of as nanoMnO2 (a), corresponding EDAX (b), and annealed sample at 600 °C low (c) and high (d) magnifications.

Figure 4. TEM image of nanoMnO2 (a) and an annealed sample at 600 °C (b). The corresponding SAED patterns are shown as insets. HRTEM image of nanorod MnO2, as inset in Figure b (lattice spacing is 0.49 nm corresponds to d(200)).

than 400 °C/3 h (parts d-f of Figure 2) along with concomitant change in morphology from nearly spherical to rod type (see SEM). In fact, considering the inside temperature of operating electrode of energy storages from the performance efficiency and safety, it is important for active materials to maintain the original structure even at high temperatures. Thus, observing the XRD pattern for the heated sample up to 300 °C (Figure 2c), the obtained nanoMnO2 can be regarded as a good candidate with considerable thermal stability as an electrode material for supercapacitors. From the SEM images (Figure 3a) of nanoMnO2, we observe a nearly spherical nanopatricles with diameter of around 5-13 nm and corresponding EDAX (Figure 3b) showing the presence of potassium. We also see that primary particles of nanoMnO2 with secondary pores (interparticle space) are agglomerated in part forming secondary particles of size ∼30 nm and growing 1D nanorods of length of about 200-300 nm and diameter of around 40 nm upon heat treatment (600 °C) (parts c and d of Figure 3). We presume that the stabilized nanorod shaped MnO2 with tunnel structure of R-MnO2 may be very useful as electrode

material for lithium ion secondary batteries due to its nano characteristics and waterfree lattice. The TEM picture for the nanoMnO2 (Figure 4a) shows not well-defined particle shape and the corresponding SAED shows a diffused pattern characteristic of an amorphous phase. Whereas for the annealed sample one can see rod shape of high crystal quality as seen in welldefined circles in the SAED pattern and also the lattice fringes seen in HR-TEM image (Figure 4b as inset) of single nanorod with the inter spacing distance of 0.49 nm corresponding to d(200) of R-MnO2. TGA, ICP, Brunauer-Emmett-Teller (BET), and IR Analysis. According to the TGA (Figure 5), the nanoMnO2 shows around 14% weight loss upon heating from room temperature to 200 °C, corresponding to the loss of water molecules which exist both on the surface and in the lattice of the nanostructure. The corresponding DTA curve shows an endothermic (75 °C) and exothermic peak (200 °C) and a small exothermic peak around 480 °C, with slight increase in weight possibly due to oxygen absorption. A considerable amount of water content is essential for an electrode material for ionic

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Figure 5. TGA/DTA curves of nanoMnO2. Figure 7. IR spectra of nanoMnO2 (a) and an annealed sample at 600 °C (b).

Figure 6. BET curves of nanoMnO2 (a) and annealed sample at 600 °C (b). Pore size distribution of nanoMnO2 as inset.

transportation through electrolyte to enhance the electrochemical performance as a supercapacitor.14,39 Therefore, the water loss upon the heat treatment can be considered as detrimental for electrochemical properties of MnO2. Atomic absorption spectroscopy (AAS) ICP analyses were carried out to determine the composition of the obtained and found that molar ratio between potassium and manganese as K/Mn ) 0.234. The BET isotherm shows a hysteresis loop around P/P0 between 0.8 and 1 (curve a of Figure 6) for the nanoMnO2 indicating the existence of secondary mesopores formed between the particles,27 which is also seen in the SEM image. The BET surface area is found to be 230 m2 g-1 highlighting the overall formation of nanostructure. The corresponding pore size distribution (BJH) curve indicates a narrow pore size distribution, with the average pore size of around 14.5 nm, demonstrating that it is associated with agglomerates or compacts of uniform spheres. We presume that the existence of secondary pores plays an important role in part to enhance the electrochemically activity of the nanostructures, leading to enhanced electrochemical performance. However, in the case of annealed sample it shows the absence of hysteresis (curve b of Figure 6). The surface area of the samples heated at various temperatures are given in Table 1, showing that the surface area decreases with increase in annealing temperature.

The IR spectra for the nanostructures were recorded in order to check the presence of water molecules and some organic remnant in the material. As shown in Figure 7, only IR bands for the nanoMnO2 corresponding to the bending and stretching modes of water molecules are detected at around 1630 and 3450 cm-1, respectively, and no other IR bands indicate the absence of any organic entities, which is further confirmed in the elemental analysis of C and H, which showed very little traces of carbon and hydrogen. The absence of IR bands corresponding to water molecules in the heat treated sample at 600 °C indicates that the sample is anhydrous, which is considered as one of the detrimental factors for good electrochemical properties as an electrode material for supercapacitor. The rather strong IR bands between 400 and 600 cm-1 correspond to metal-oxygen bond of MnO2.45 XPS and Mn K-Edge XAS Analysis. The surface effect of supercapacitor is believed to be dominant in amorphous MnO2 over intercalation process in the proposed two operating mechanisms on MnO2 as an electrode material.23 Thus, it is important to know the oxidation state and the chemical composition of manganese on the surface of the nanostructures. Such a study can be made by X-ray photoelectron spectroscopy (XPS), a powerful tool for the surface characterization of materials. However, earlier studies have shown that, due to the localized positive and negative charge distribution on irradiation of X-rays on nonconductive samples, such as MnO2 prepared in the present study, leads to the broadening or tailing of peaks at lower binding energy causing difficulty in accurately determining the valence state of Mn only from the Mn 2P2/3 peak.46,47 They have overcome this difficulty by analyzing O1s and Mn 3s spectra and by determining the splitting width of Mn 3s peaks and comparing with the standard. Such an XPS spectra for nanoMnO2 and annealed sample at 600 °C are shown in Figure 8, and on analysis (Table 2) it is found that the oxidation state of Mn is +4 as reported earlier.37,40,46,47 In Figure 8b, the deconvoulated XPS spectra of O1s are shown, resulting in one sharp peak (1) and two broad peaks (2,3) with three binding energies corresponding to the following three different types of oxygen bonds: 529.3-530.0 eV (Mn-O-Mn), 530.5-531.5 eV (Mn-O-H), and 531.8-532.8 eV (H-O-H).46,48 Also, slightly higher binding energy (533.4 eV) has been reported for such a molecule.49 Comparing these values and from the areas under each deconvulated curve (Table 2), it is seen further that most of the Mn at the surface is attached to oxygen in the

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Figure 8. XPS spectra of nanoMnO2 and annealed sample at 600 °C.

TABLE 1: Structure Type, Surface Area (SBET), and Specific Capacitance (F g-1) of the Nanostructures at Different Temperatures sample

structure type

surface area (m2 g-1)

specific capacitance (F g-1)a

as prepared 200 °C 300 °C 400 °C 500 °C 600 °C

δ-MnO2 (local) δ-MnO2 (local) δ-MnO2 (local) R-MnO2 R-MnO2 R-MnO2

230 212 194 36 34 22

250 225 207 85 73 61

a All the measurements are carried out in the voltage window between 0 and 1 V vs Hg/HgO, OH- (6 M) at constant current density 0.5 mA cm-2.

TABLE 2: XPS Data Analysis of Manganese Oxidea Mn 2p3/2

a

Mn 3s

O 1s

BE

BE (1)

BE (2)

∆E(1-2)

BE

% area

valencestate(n)

1

642.69

88.9

84.3

4.6

642.7

89.0

84.3

4.7

45 27 28 61 23 16

4

2

530.0 531.9 533.6 530.2 531.7 533.5

γ-Mn2O3b β-MnO2b

641.7 642.2

88.8 89.4

83.6 84.7

5.2 4.7

sample

4 3 4

Sample 1, nanoMnO2; sample 2, annealed sample at 600 °C; BE values are in eV. b From ref 46.

form of oxide (Mn-O-Mn), in both nanoMnO2 and annealed sample at 600 °C. However in the latter there is an increase in concentration of the oxide form. Still the presence of H-O-H bond in the annealed sample (600 °C) is due to the surface adsorbed water, which is not seen in IR spectra for the same sample, which is thoroughly dried and recorded (Figure 7b). To study the local arrangement and electronic structure of manganese ions, we have performed Mn K-edge XANES analysis on the obtained nanoMnO2, which is nearly X-ray amorphous and compared the data with that of Mn2O3, MnO2, R-MnO2, and δ-MnO2 compounds. The sample shows weak preedge peaks P and/or P′ corresponding to dipole-forbidden 1s f 3d transitions, indicating the stabilization of Mn in the octahedral site. It is well-known that the increase of Mn oxidation state gives rise to the enhancement of the peak P′. While the only feature of P and P′ is observable for the reference Mn2O3 and MnO2, respectively, the obtained sample shows two features P and P′, indicative of the mixed oxidation state of Mn3+/Mn4+. In the main-edge region, the phase of the obtained nanoparticles display an intense resonance peak B corresponding to the dipole-allowed 1s f 4p transitions at around 6563 eV as

seen in Figure 9. It is also well-known that its sharpness and intensity is proportional to the relative concentration of edgesharing over corner-sharing of MnO6 octahedra.50 In the samples investigated, the observed intense and sharp peak for the δ-MnO2 phase is compatible well with its layered structure consisting of edge-shared MnO6 octahedra. The result is also clearly supported by the overall feature of the curve (Figure 9) obtained for the sample which is almost same with that of the reference δ-MnO2 structure. In comparison, the broader feature appears for the R-MnO2 type structure, confirming its network of corner and edge-sharing of MnO6 octahedra. The overall features of the obtained sample are also apparently close to that of δ-MnO2 type structure. On the basis of the above findings, it clearly demonstrates that the obtained sample has δ-MnO2-type structure locally even though it is nearly X-ray amorphous. Electrochemical Studies. Electrochemical studies of MnO2 were reported in several electrolytes, and it was found that an aqueous solution of Na2SO4 is the most suitable in the view of the maximum specific capacitance measured.25,26,37 Accordingly, 0.1 M Na2SO4 solution was used for electrochemical charac-

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Figure 9. Mn K-edge XANES spectra of nanoMnO2 (solid line) and annealed sample at 600 °C (dash-dot line), in comparison with those for the references Mn2O3 (dashed line), δ-MnO2 (solid line), and R-MnO2 (dash-dot-dot-dash line).

terization in the present studies also. Cyclic voltammograms (Figure 10a) of the nanoMnO2 for the first, 1200th, and 2000th cycles in 0.1 M Na2SO4 have rectangular shape, indicating characteristic ideal capacitive behavior9,12 of the samples. These three voltammograms almost merge with each other, indicating good stability of the nanoMnO2 electrode even after it was cycled 2000 times. Galvanostatic charge-discharge cycling data of nanoMnO2 electrode for the first few cycles at a current density of 0.5 mA cm-2 are shown in Figure 10b. The variation of potential with time is linear during both charging and discharging processes. This type of linear variation of potential is another important criterion for capacitance behavior of an electrode material. The data similar to shown in Figure 10b were recorded for all 2000 cycles tested in the present study. Specific capacitance (SC) of the electrode was calculated using the formula given below

SC ) It/(Em)

(1)

where I is the current, t the discharge time of a cycle, E the potential window (1.0 V), and m the mass of MnO2. The value of discharge capacitance obtained from the data presented in Figure 10b is 250 F g-1. This value is reasonably high in comparison with the values reported in the literature16,27,42 for similarly synthesized MnO2 samples. For instance, specific capacitance of 200 F g-1 was reported by Lee and Goodenough16 for MnO2 prepared from reduction of KMnO4 by MnSO4. Reddy and Reddy42 reported a value of 130 F g-1 for MnO2 prepared by reduction of KMnO4 with sodium fumarate. About 167 F g-1 for the samples prepared by hydrothermal synthesis was reported by Xu et al.27 Thus, specific capacitance of about 250 F g-1 measured in the present study for nanoMnO2 is quite attractive. The variation of specific capacitance during 2000 cycles tested is shown in Figure 10c. It is seen that nanoMnO2 possesses a high electrochemical stability with less than 8% fall in capacitance after 2000 charge-discharge cycles. Specific capacitance values for MnO2 samples heated at different temperatures are listed in Tale 1. It is found that there is a gradual decrease in specific capacitance for the samples heated at 200 and 300 °C due to loss of water molecules and also

Figure 10. Cyclic voltammograms in 0.1 M Na2SO4 at 10 mV S-1 of first (solid line), 1200th (dashed line), and 2000th (solid line) cycle (a), charge-discharge curves of first few cycles at a constant current density 0.5 mA cm-2 (b), and specific capacitance as a function of cycle number (c) for nanoMnO2.

decrease in surface area. However, for the samples heated at 400, 500, and 600 °C, there is a drastic decrease in specific capacitance. The capacitance retention upon electrochemical cycling is strongly attributed to the nature of nanoMnO2 possessing high surface area, secondary pores between particles, and appropriate water content. As illustrated in Table 1, it was found that both the surface area and specific capacitance decreased slightly on heating up to 300 °C. However, we could observe a dramatic decrease of surface area and in specific capacitance between the samples annealed at 300 and 400 °C, together with a structural change. By assumption that high specific capacitance is closely related to surface area, secondary pores, and water content, which support the proposed surface effect in operating

Remarkable Capacity Retention of NanoMnO2 mechanism as an electrode material for supercapacitor, these properties decrease by increasing temperature. As a consequence, among the present samples, we could obtain the highest specific capacitance and surface area from the nanoMnO2 sample. Unlike the previous reports wherein poor specific capacitance retention is reported, 16,17,20,22,23,25-27,42 it is worthy to note that present nanoMnO2 possess remarkable stability of specific capacitance of about 250 F g-1 up to 1200 cycles and a very slight decrease (8%) further up to 2000 cycles (Figure 10c). Conclusion In this study, we have successfully enhanced electrochemical properties of nanoMnO2 synthesized through one-pot reaction under ambient conditions from KMnO4 and ethylene glycol. We were able to stabilize the capacitance retention of nanoMnO2 upon redox cycling up to 2000 cycles with very little capacitance loss of 8%. We believe that the obtained good electrochemical performance is strongly related to the physicochemical properties of nanoMnO2 with high surface area mostly owing to the existence of secondary mesopores and proper water content. Moreover, we were able to control the morphology of nanoMnO2 into 1D nanorod shape via heat treatment at 600 °C, which is considered as a suitable electrode material for lithium secondary batteries. On the basis of the optimized physicochemical properties of the obtained nanoMnO2, it is effective to use ethylene glycol as a reducing agent in synthesizing nanoMnO2 as an electrode material for supercapacitors. Acknowledgment. H.N.V. thanks the Department of Science and Technology, India, for financial assistance, and P.R. acknowledges LAFICS/IFLaSc program for providing financial grant to carry out experiments in ICMCB, Bordeaux. The XANES experiments were carried out at the Pohang Accelerator Laboratory in South Korea. References and Notes (1) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J.-M. Nature 2000, 407, 496. (2) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (3) Cheng, F.; Zhao, J.; Song, W.; Li, C.; Ma, H.; Chen, J.; Shen, P. Inorg. Chem. 2006, 45, 2038. (4) Arico, A. S.; Bruce, P. G.; Scrosati, B.; Tarascon, J.-M.; Schalkwijk, W. V. Nat. Mater. 2005, 4, 366. (5) Qian, H.-S.; Antonietti, M.; Yu, S.-H. AdV. Funct. Mater. 2007, 17, 637. (6) Shanmugam, S.; Gedanken, A. Small 2007, 3, 1189. (7) Barone, P. W.; Strano, M. S. Angew. Chem., Int. Ed. 2006, 45, 8138. (8) Zhang, H.; Zhu, Q.; Zhang, Y.; Wang, Y.; Zhao, L.; Yu, B. AdV. Funct. Mater. 2007, 17, 2766. (9) Conway, B. E. Electrochemical Supercapacitors, Scientific Fundamentals and Technological Applications; Kluwer Academic/Plenum: New York, 1999.

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