Fabrication of Porous β-Co(OH)2 Architecture at Room Temperature

Jun 27, 2013 - *E-mail: [email protected] (T.P.). ... The specific capacitance obtained from charge–discharge measurements made at a dischar...
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Fabrication of Porous β‑Co(OH)2 Architecture at Room Temperature: A High Performance Supercapacitor Chanchal Mondal,† Mainak Ganguly,† P. K. Manna,‡ S. M. Yusuf,‡ and Tarasankar Pal*,† †

Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India

‡

S Supporting Information *

ABSTRACT: A facile, cost-effective, surfactant-free chemical route has been demonstrated for the fabrication of porous βCo(OH)2 hierarchical nanostructure in gram level simply by adopting cobalt acetate as a precursor salt and ethanolamine as a hydrolyzing agent at room temperature. A couple of different morphologies of β-Co(OH)2 have been distinctly identified by varying the mole ratio of the precursor and hydrolyzing agent. The cyclic voltammetry measurements on β-Co(OH)2 displayed significantly high capacitance. The specific capacitance obtained from charge−discharge measurements made at a discharge current of 1 A/g is 416 F/g for the Co(OH)2 sample obtained at room temperature. The charge−discharge stability measurements indicate retention of specific capacitance about 93% after 500 continuous charge−discharge cycles at a current density of 1 A g−1. The capacitive behavior of the other synthesized morphology was also accounted. The nanoflower-shaped porous β-Co(OH)2 with a characteristic three-dimensional architecture accompanied highest pore volume which made it promising electrode material for supercapacitor application. The porous nanostructures accompanied by high surface area facilitates the contact and transport of electrolyte, providing longer electron pathways and therefore giving rise to highest capacitance in nanoflower morphology. From a broad view, this study reveals a low-temperature synthetic route of β-Co(OH)2 of various morphologies, qualifying it as supercapacitor electrode material. other materials are invoked.16 The research, based on pseudocapacitive behavior of metal oxides, such as NiO,17 V2O5,18 MnO2,19 Ni(OH)2,20 TiO2,21 and Co3O4,22 has been carried out in the past decades due to their high power density, energy density, mass density, and cycle stability. In this regard, being an inexpensive, nontoxic, electrochemically active redox material, Co(OH)2 nanomaterial, with layered structure having large interlayer spacing, has found its innumerable utilizations as a supercapacitor. For example, Kong et al. showed a pseudocapacitive behavior of the Co(OH)2 nanoflakes.23 Hu et al. showed that the specific capacitance of sheet-like Co(OH)2 is about 416.7 F/g.24 Gupta et al. have shown a high specific capacity of cobalt hydroxide nanomaterial deposited electrochemically on a stainless steel electrode.25 Zhou et al. reported a very high specific capacitance (1084 F g−1) of mesoporous Co(OH)2 film deposited electrochemically on a titanium substrate.26 Although a high specific capacitance was achieved using electrochemical methods, a poor yield of the electroactive material is in the main drawback of these methods. Therefore, to prepare Co(OH)2 in gram scale having high specific capacitance is a burning problem for researchers. The surface property of the electrode materials is very crucial in determining its pseudocapacitance. High specific capacitance needs a large number of electroactive sites in the electrodes for the enhanced transport rate of electrolyte ions and electrons.27

1. INTRODUCTION Cobalt hydroxide is a promising transition metal hydroxide, which has received a significant interest in view of its application in industry as a solar selective absorber1 and catalyst for oxygen evolution and oxygen reduction reactions,2−4 gas sensor,5 photovoltaic cell,6 batteries,7 lubricating material,8 and supercapacitor.9 The hydroxides of cobalt are well-known to crystallize in α- and β- polymorphic forms.10 The brucite-like Co(OH)2 has a hexagonal layered structure composed of a Co layer sandwiched between two O layers. It has reversible electrochromic11 and antiferromagnetic property.12 The field of supercapacitors has received a noteworthy attention for its importance in energy efficient, eco-friendly, high-power, and high-energy devices. This makes supercapacitors an advantageous alternative to fossil fuels as one of the most promising candidates for next-generation power devices.13 The electrochemical capacitors are divided mainly into two categories: (i) electrical double layer capacitor, which arises from the charge separation at the electrode/electrolyte interface, and (ii) pseudocapacitor, which arises from fast, reversible redox processes taking place at or near a solid electrode surface.14 The research on supercapacitor, based on pseudocapacitance, has been oriented around metal oxide and metal hydroxide nanomaterials as they possess a large capacitance and a fast redox kinetics.15 In this regard, RuO2 is the most studied system having a very high specific capacitance value. However, due to its toxicity and high cost, © 2013 American Chemical Society

Received: January 10, 2013 Published: June 27, 2013 9179

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Table 1. Reaction Conditions for the Growth of a Couple of β-Co(OH)2 Diverse Nanoarchitectures entry

vol of cobalt acetate (0.1 M) taken (mL)

final conc of cobalt acetate (mM)

vol of ethanolamine (mL)

final conc of ethanolamine (mM)

Co2+/ ethanolamine

product’s morphology

1 2 3

5 5 5

98 96.15 95.24

0.1 0.2 0.25

324.87 637.25 788.97

0.302 0.151 0.121

nanoflower stacked plate ill-defined plate

For this reason from the past decade several groups have explored various methods for the synthesis of cobalt hydroxide nanomaterial to generate various morphologies, e.g., nanowire,28 nanosheet,29 nanoneedles,30 nanorod,31 hexagonal nanoplate,32 and nanocolumn.33 There are few reports on the synthesis of Co(OH)2 nanoflower. Qiao et al. have synthesized β-Co(OH)2 nanoflower by mixing cobalt nitrate, poly(ethylene glycol), and ammonium hydroxide in the presence of a template polyvinylpyridine under a hydrothermal condition.34 Zhu et al. have reported a facile route for the synthesis of cobalt hydroxide, having flower-like morphology, by hydrothermal heating of cobalt acetate and glycerol.35 Wang et al. discussed the preparation of self-assembled cobalt hydroxide nanoflowers using a microemulsion as a nanoreactor.36 However, fabrication of three-dimensional nanoflower of Co(OH)2 at room temperature is still a big challenge. The aim of this article is the synthesis of Co(OH)2 nanoflower at low temperature. Herein, we first report a straightforward methodology for the synthesis of 3D Co(OH)2 nanoflower at room temperature. The present methodology is indeed a promising one, as it does not require any sophisticated instrument, giving high yield from a mild and trouble-free route. We obtained distinctly different morphology simply by varying the mole ratio of precursor salt, cobalt acetate, and hydrolyzing agent, ethanolamine. The asprepared nanomaterial exhibits a higher specific capacitance with excellent cycling stability. The corresponding variation in the morphology and electrochemical properties, as supercapacitor electrodes, has been investigated and discussed elaborately.

Figure 1. Observed and Rietveld-refined X-ray diffraction patterns of β-Co(OH)2. The prominent Bragg peaks are indexed.

Co(OH)2 nanomaterial which exhibits a predominant wellcrystalline brucite-like phase which is consistent with the peak positions in the literature (JCPDS file no. 30-443) in which the 2θ scan has peaks at 19.0°, 32.4°, and 37.9°, corresponding to the (001), (100), and (101) diffractions. The strong low-angle reflection peak at 19° can be found with d-spacing of 4.64 Å, which is in good agreement with the reported interlayer spacing value of β-Co(OH)2.37 Figure 1 also shows the Rietveld-refined X-ray diffraction patterns of the β-Co(OH)2 compound which is described in the Supporting Information. Figure S1 displays the FTIR spectrum of as prepared brucitelike cobalt hydroxide nanoflower. The absorption around 489 cm−1 can be ascribed to vibrations due to Co−O bond and bending vibrations of Co−O−H in the brucite-like β-Co(OH)2 nanoflowers. The broad peak at around 3445 cm−1 indicates the presence of surface adsorbed water molecule, while the weak signature at 1656 cm−1 ascribes to hydrogen bound hydroxyl groups. The relatively weaker peak at 1560 cm−1 is due to trace amount of acetate ligand. The sharp peak at 3631 cm−1 authenticates the presence of free hydroxyl groups in brucitelike structure. Further evidence for the chemical state of the as-synthesized Co(OH)2 nanoflower is obtained by the X-ray ptotoelectron spectroscopy (XPS) measurements, as displayed in Figure S2. It demonstrates two main peaks at 780.87 and 795.93 eV, which correspond to Co 2p3/2 and Co 2p1/2. This feature endorses the presence of the Co(II) state in the as-prepared sample. The morphology of the β-Co(OH)2 nanoflower was examined by the field emission scanning electron microscopy (FESEM) analysis. Figure 2a,b displays the FESEM images of β-Co(OH)2 nanoflower at different magnifications. The flowers look like carnation having size range 6−6.2 μm. The flowery architecture is comprised of numerous curvy nanosheets. The average width of the nanosheet was found to be 40.74 nm (Figure S3a). It was found that the curved nanosheets stacked one by one and grows in a three-dimensional fashion to generate hierarchical flower-like morphology. More interestingly, these petals intercrossed each other, and a closer examination of the porous structures discloses the self-assembly directed oriented attachment of the nanosheets. The flowers are standing toward the radial directions starting from the center of the microsphere, and they apparently look quite flexible, reflecting their ultrathin feature. The generation of such

2. EXPERIMENTAL SECTION 2.1. Materials and Instruments. The relevant information has been provided as S1 in the Supporting Information. 2.2. Synthesis of Co(OH)2 Nanomaterial. In a typical synthesis of cobalt hydroxide nanoflower, 0.1 mL of ethanolamine was added to a 5 mL of 0.1 M Co(CH3COO)2·4H2O solution at room temperature in a glass vial. The resultant mixture was kept at room temperature for 12 h. After completion of the reaction, a pink precipitate was generated at the bottom of the glass vial. The precipitate was collected and washed several times with water and ethanol. The pink product was dried at 60 °C for characterization. The same experimental procedure was performed with different volumes of starting materials to obtain different morphology (Table 1). 2.3. Magnetization Measurement. The information is depicted in the Supporting Information as S2. 2.4. Capacitance Measurement. We used a three-electrode system for the electrochemical characterizations, where Ag−AgCl electrode, Pt electrode, and Co(OH)2 fabricated on glassy carbon electrode using 1% nafion binder acted as reference electrode, counter electrode, and working electrode, respectively, in 1 M KOH electrolyte.

3. RESULTS AND DISCUSSION 3.1. Characterization of Cobalt Hydroxide Nanoflower. The structural information and phase purity of the products have been obtained by powder X-ray diffraction (XRD) measurement. Figure 1 shows the XRD pattern of 9180

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Figure 2. FESEM images of Co(OH)2 nanoflower at medium magnification (a) and a single nanoflower at high magnification (b), Co(OH)2 stacked nanoplate at medium (c) and high magnification (d), and ill-defined plate-like morphology at medium (e) and high magnification (f).

anisotropic aesthetically beautiful flower could be rationalized by a diffusion-limited aggregation model, where the irreversible aggregation of randomly moving particles causes self-clustering growth inheriting fractal dimensions. When the volume ratio of cobalt acetate/ethanolamine was decreased, stacked plate-like morphology (Figure 2c,d) appeared. The plates stack together and generate a stacked plate-like morphology. This is because at this concentration of ethanolamine a prolonged reaction time does not produce flower-like morphology. An increasing concentration of the base in the reaction mixture also does not generate flower-like morphology; rather, an ill-defined plate-like morphology appears as shown in Figure 2e,f. It can be suggested that a particular concentration of cobalt acetate and

ethanolamine favors the formation of carnation flower-like morphology at room temperature. Various morphologies of Co(OH)2 are obtained due to the different rate of dissociation of the hydrolyzing agent, i.e., the base in its different concentrations. It may be assumed that at a particular concentration fast anisotropic growth of crystals takes place due to a suitable amount of nuclei in the reaction mixture. The concentration of the base has a marked effect on the kinetics of the reaction between dissociated OH− and the counterions of the cobalt precursor. The evolution of Co(OH)2 starts from the replacement of counterions of Co2+ with OH−. This creates small nuclei, followed by aggregation/self-assembly into bigger particles bearing a particular but habitual crystal 9181

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isotherm (Figure 4a) and the BJH pore-size distribution (Figure 4b), provides the information about the porosity of the flower-like nanoarchitectures of β-Co(OH)2. The obtained BET specific surface area of the nanoflower is 190.19 m2 g−1. The plot of BJH pore-size distribution the pore-size distribution centered at ∼50 Å addresses the mesoporous nature of the studied nanomaterial. 3.2. Magnetic Moment Measurement. A detailed description of magnetic moment measurement is provided in S3 of the Supporting Information. 3.3. Capacitance Measurement. The structure and morphology of the electrode material have a great influence on its electrochemical performance. Nanoparticles due to their larger surface to volume ratio facilitate better intercalation of charges into the material. Since Co(OH)2 is an electroactive material, the as-prepared nanoflower, stacked plate, and ill-defined platelet β-Co(OH)2 were fabricated into supercapacitor electrodes, and their capacitive behavior is estimated by employing the cyclic voltammetry (CV), galvanostatic charge−discharge (GCD), and electrical impedance spectroscopy (EIS) measurements. Figure 5a shows the CV curves of the β-Co(OH) 2 nanoflower. The measurements were performed in a potential range of −0.2 to +0.8 V (versus SCE) using 1 M KOH electrolyte at different scan rates of 10, 20, 30, 40, and 50 mV s−1. The plot indicates redox peaks due to the following faradaic reactions of Co(OH)2

orientation and morphology. The whole phenomenon happens with a view to minimize the surface energy.38 A nanoflower of Co(OH)2, achieved at lower OH− concentration, formed simply by stacking of Co(OH)2 nanosheets. Low concentration of OH− controls the kinetics of Co(OH)2 formation and creates a relatively small number of nuclei. These nuclei get sufficient time and room for particle growth.39 This ultimately leads to the formation of larger size flower-shaped Co(OH)2 with an energetically favored state that can be clarified by the Ostwald ripening process.38 On the other hand, at higher OH− concentration, a large amount of OH− in the reaction medium initiates the generation of a large number of Co(OH)2 nuclei at the initial stage. Thus, there remains a relatively limited number of Co2+ in the solution. As a result, further growth of the nuclei into larger particles to obtain nanoflower morphology cannot be achieved because of the nonavailability of Co2+. The energy dispersive spectroscopy (EDS) (Figure S4) analysis confirms the presence of Co and O in the cobalt hydroxide nanoflowers. However, the presence of hydrogen cannot be detected by such analysis because this tool only permits an identification of the elements after boron in the periodic table. Further insight into the morphology of the nanoflowers is revealed from the transmission electron microscopy (TEM) analysis. Figures 3a and 3b display the TEM images of the

Co(OH)2 + OH− = CoOOH + H 2O + e−

However, the electrochemical reaction occurs at higher potential as follows: CoOOH + OH− = CoO2 + H 2O + e−

The peaks present in the voltammogram authenticates that a redox mechanism involves during the capacitance measurement, and it was not governed by pure electric double layer capacitance. One quasi-reversible electron transfer process takes place which is displayed in every curve, signifying that the measured capacitance is mainly based on redox mechanism. Furthermore, the shape of the curves (Figure 5a) indicates that the capacitance characteristic was fully dissimilar from that of the electric double layer capacitor, which would generate a CV curve resemble to an ideal rectangular shape. The presence of the peaks indicates a pseudocapacitive mechanism is operative due to reversible electrochemical reactions. The oxidation and reduction peaks in the voltammogram of the electrode material corresponding to anodic and cathodic scan are not symmetric. This arises due to kinetic irreversibility of the redox process.40 The phenomenon can also be due to ohmic resistance and polarization because of the diffusion of electrolyte ions in the porous electrode during the redox reactions. The equation which was used for calculating the specific capacitance (SC) is as follows:

Figure 3. TEM image of Co(OH)2 nanoflower (a) and tip of the nanoflower (b), SAED pattern of the nanoflower (c), and fringe spacing of the nanoflower (d).

synthesized nanoflowers and tips, respectively. It is in well agreement with the FESEM images. It is observed clearly that the plates are stacked over each other to attain a stacked plate arrangement. Figures S3b and S3c display the obtained TEM image of ill defined platelets and stacked plates, respectively. The selected area electron diffraction (SAED) pattern of the nanoflower is shown in Figure 3c. It indicates that the sample is polycrystalline in nature. The calculated interlayer spacing, as displayed in Figure 3d, was found to be about 0.27 ± 0.2 nm, which coincides well with the interlayer spacing of the (100) crystal plane in β-Co(OH)2. The Brunauer−Emmett−Teller (BET) gas sorptometry measurement, in view of the nitrogen adsorption/desorption

V2

specific capacitance C sp =

∫V 1 iV dv mv(V2 − V1)

where the denominator indicates total charge, m is the electrode mass, ν is the scan rate, and (V2 −V1) is the potential window. The measurement was performed at different scan rates, and their corresponding specific capacitance was calculated. At 10, 9182

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Figure 4. Nitrogen adsorption−desorption isotherm of Co(OH)2 nanoflower (a) and the corresponding BJH pore size distribution plot (b).

Figure 5. (a) Cyclic voltammograms of Co(OH)2 nanostructure at different scan rate recorded using 1 M KOH solution: (a) nanoflower, (b) stacked plate, and (c) and ill-defined plate. (d) Comparison of specific capacitance of Co(OH)2 nanostructure with three different morphologies.

20, 30, 40, and 50 mV s−1 scan rate the specific capacitance was found to be 306, 272, 243, 230, and 210 F g−1, respectively. With increasing scan rate the corresponding current increases. Figure 5b indicates the CV curves of Co(OH)2 stacked plate at different scan rates. The calculated values of specific capacitance

for stacked plate were 284, 254, 231, 213, and 196 F/g at 10, 20, 30, 40, and 50 mV/s scan rates, while the ill-defined platelike morphology showed lowest specific capacitance of 271, 242, 222, 201, and 183 F/g at different scan rates of 10, 20, 30, 40, and 50 mV/s, respectively (Figure 5c). The specific 9183

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curves corroborates the pseudocapacitance property of the Co(OH)2 nanomaterial, which coincides well with the CV results.42,43 A slight curvature of the charge−discharge curves confirms the pseudocapacitance nature due to redox reaction at the electrode/electrolyte interface along with the double layer capacitance behavior due to the charge separation between the electrode/electrolyte interfaces. The specific capacitance from the discharge plot was calculated by using the equation

capacitance values are plotted as a function of scan rates for the three different morphologies of the as-synthesized materials in Figure 5d. It was found that at a low scan rate the specific capacitance is maximum which might be due to the slow faradaic reaction. Solution and electrode resistance have the capability to distort current response at the switching potential, and this distortion is dependent upon the scan rate. Because of this reason, the shape of the CV has altered with the increase of scan rate. These results reveal that the redox mechanism is mainly responsible for the measured capacitance. At high current, the diffusion of ions occurs mainly into the outer regions of the pores, while at lower scan rate, both inner and outer surface are accessed by the electrolyte ions, and this leads to high capacitance at low scan rate.41 The nanoflower morphology due to its porous nature helps better diffusion of ions compare to other morphology and results its higher capacitance than others. To investigate the specific capacitance and to realize the sustainability of this as-synthesized Co(OH)2 nanomaterials as a supercapacitor, galvanostatic charge−discharge experiments of all the samples were carried out at a constant current density of 1 A/g and a comparatively high current density of 5 A/g (Figure 6a). All the nonlinear nature of the charge−discharge

specific capacitance C = it /mΔV

where I is the constant current, m is the electrode mass, t is the discharge time, and ΔV is the potential. For the charge− discharge study the potential range was fixed as 0 V to (+)0.8 V. The working potential of the electrode material was chosen in such a way within which the electrode material showed maximum Coulombic efficiency, and beyond this potential range a flat charging plot was obtained leading to very low Coulombic efficiency. The material exhibits a specific capacitance of 416 F/g at 1 A/g constant current and also retained 320 F/g specific capacitance even at the higher current density of 5A/g (Figure 6a). To compare the supercapacitor performance, other morphologies of Co(OH)2 are also fabricated into electrode, and their capacitive behavior is measured using galvanostatic charge−discharge experiments. Figure 6b shows the measured capacitances adopting charge discharge technique at 1 and 5 A/g are 384 and 298 F/g, respectively, for stacked plate, while for ill-defined plate morphology as depicted in Figure 6c, the capacitance value at 1 and 5 A/g was found to be 371 and 279 F/g, respectively. The nanoflower-shaped structure is beneficial for supercapacitance property over other shapes as it has distinct threedimensional networks with high porosity. The high BET surface area along with the presence of pore channels (as shown in FESEM images) are favorable for the accessibility of electrolyte OH− to the electrode active surface and a fast diffusion rate of electrons within the redox phase, which increases the specific capacitances. Moreover, the nanoflower has the greatest portion of pore volume in which water molecules are absorbed. A literature report suggests that the nanopores in metal oxide can behave as a buffering reservoir to accommodate OH− ions for the redox reaction, facilitating maximized contact and fast diffusion, which in turn increases the kinetics of the reversible redox process for charge storage.44 Compared to stacked nanoplate and ill-defined nanoplate

Figure 6. Charge/discharge curves for Co(OH)2 nanostructure measured at 5 and 1 A/g current density: (a) nanoflower, (b) stacked plate, and (c) ill-defined plate.

Figure 7. (a) Cycling behavior of Co(OH)2 nanoflower measured at 1 A/g current density. (b) Nyquist plot of Co(OH)2 electrodes; the inset shows the modified Randles circuit model. 9184

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4. CONCLUSIONS In summary, a trouble-free, low-cost, surfactant or template free wet chemical methodology has been accepted for the formation of hierarchical β-Co(OH)2 3D nanoflowers at room temperature. This is achieved simply by ethanolamine assisted hydrolysis of aqueous solution of Co(CH3COO)2. The Co(OH)2 nanoflowers are found to crystallize in the β phase, bearing a flower-like morphology. The BET gas sorption measurement ascertains the mesoporous nature of the nanoflowers, having a high specific surface area. A couple of diverse structures have been achieved by manipulating the concentration of the precursor and hydrolyzing agent. The assynthesized nanoflower exhibits better electrochemical performance in KOH electrolyte solution compared to other morphologies and hence found its application as a supercapacitor. The high surface area along with porous structure makes β-Co(OH)2 nanoflower a superior electrode material to act as a supercapacitor. Our impressive shape-selective synthetic methodology, with the advantages of inexpensive experimental setup and a greener synthetic route, thus promises to be transformed into a proliferation of innovative structures and materials that may be scaled up readily for industrial production. Furthermore, the high capacitance, high energy density, and long cyclic stability of the studied nanomaterials are useful for applications in batteries.

Co(OH)2 with lower surface area, the nanoflower-shaped Co(OH)2 nanostructure has large electroactive surface sites for better electrolytic contact; consequently, nanoflower-shaped Co(OH)2 bears the highest capacitance among the fabricated morphologies. Inspired by the highest capacitance obtained from nanoflower morphology of Co(OH)2, its cyclic charge/discharge measurement was carried out. This measurement is an important tool for understanding the stability as well as service life of the capacitor for its practical use. The charge−discharge test was continued 500 times at 1 A/g current density to evaluate their stability as supercapacitor electrode material, which responded meaningfully by retaining 93% specific capacitance, as shown in Figure 7a. The imposed mechanical stress to the electrode active materials due to insertion or deinsertion of electrolyte ions possibly the reason for capacity fading.45 Figure 7b displays the electronic impedance spectroscopy, in the form of a Nyquist plot, of the as-synthesized Co(OH)2 nanoflowers, measured within the frequency range of 1 mHz to 1 MHz, to obtain the resistive and capacitive elements associated with the electrode. The impedance spectra comprised of one semicircle at high-frequency region, while the straight line arises at low-frequency region. The Nyquist plot designates basically the plot of the imaginary component (Z″) of the impedance against the real component (Z′). These kinds of plots can be fitted with an equivalent circuit as shown in inset of Figure 7b, where the R1 stands for bulk resistance of the electrochemical system, R2 denotes Faradic charge transfer resistance, W is the Warburg impedance, Q1 is the constant phase element (CPE). The Co(OH)2 nanoflower exhibited a very low solution resistance of 0.57 Ohm, and a charge transfer resistance of 5.95 Ohm. The deep rising of the nyquist plot in the lower frequency region is a clue of good capacitive behavior. The CPE constant (n) determines the capacitive behavior of the electrode material. For our as prepared material n is 0.89, which is very close to one signifies its advanced electrode performance. The energy density and the power density from the CV measurement for Co(OH)2 nanoflower was calculated from the following equations: energy density (E) =

■

S Supporting Information *

FTIR spectrum, XPS spectrum, EDS spectrum, FESEM and TEM image, a polyhedral representation of the crystal structure, data obtained from Reitveld analysis, and magnetic moment measurement. This material is available free of charge via the Internet at http://pubs.acs.org.

■

*E-mail: [email protected] (T.P.). Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The authors are thankful to IIT Kharagpur, BRNS, CSIR, New Delhi, for financial assistance.

1 C(ΔV 2) 2

Table 2. Different Values of Energy Density and Power Density at Different Scan Rates 20 75.6

30 67.8

40 63.9

50 58.33

3060

5443.2

7322.4

9201

10499.4

REFERENCES

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where C is the specific capacitance at particular scan rate, ΔV is the potential window, and T is the discharge time. The various values of energy density and power density at different scan rates are given in Table 2. The moderate energy density and high power density indicate that the as-synthesized material can behave as a supercapacitor.

10 85

AUTHOR INFORMATION

Corresponding Author

power density (P) = E /T

scan rate (mV/s) energy density (W·h/kg) power density (W/kg)

ASSOCIATED CONTENT

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(45) Meher, S. K.; Rao, G. R. Enhanced activity of microwave synthesized hierarchical MnO2 for high performance supercapacitor applications. J. Power Sources 2012, 215, 317−328.

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dx.doi.org/10.1021/la401752n | Langmuir 2013, 29, 9179−9187