Performance evaluation of symmetric supercapacitor based on cobalt hydroxide [Co(OH)2] thin film electrodes

Electrochimica Acta 98 (2013) 32–38

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Performance evaluation of symmetric supercapacitor based on cobalt hydroxide [Co(OH)2 ] thin film electrodes A.D. Jagadale, V.S. Kumbhar, D.S. Dhawale, C.D. Lokhande ∗ Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, MS, India

a r t i c l e

i n f o

Article history: Received 25 October 2012 Received in revised form 11 February 2013 Accepted 19 February 2013 Available online 1 March 2013 Keywords: Electrodeposition Symmetric supercapacitor Cyclic voltammetry Charge–discharge Electrochemical impedance spectroscopy

a b s t r a c t In the present investigation, we have successfully assembled symmetric supercapacitor device based on cobalt hydroxide [Co(OH)2 ] thin film electrodes using 1 M KOH as an electrolyte. Initially, potentiodynamic electrodeposition method is employed for the preparation of Co(OH)2 thin films onto stainless steel substrate. These films are characterized for structural and morphological elucidations using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The XRD reveals formation of ␤-Co(OH)2 material with hexagonal crystal structure. The SEM images show formation of nanoflakes like microstructure with average flake width 100 nm. Electrochemical characterizations of Co(OH)2 based symmetric supercapacitor cell are carried out using cyclic voltammetry, charge–discharge and electrochemical impedance spectroscopy (EIS) techniques. In the performance evaluation the maximum values of specific capacitance, specific energy and specific power are encountered as 44 F g−1 , 3.96 Wh kg−1 and 42 kW kg−1 . The value of equivalent series resistance (ESR) is estimated as 2.3  using EIS. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Existing investigations and growth of electrochemical power sources are mainly paying attention on fuel cells, batteries and electrochemical capacitors (ECs) and are intended for achieving high specific energy, high specific power, long cycle life, etc., with relatively low cost [1]. ECs are also known as supercapacitors which store energy using either ion adsorption (electrochemical double-layer capacitors, EDLCs) or fast surface redox reactions (pseudocapacitors) [2]. Supercapacitors have higher energy density than conventional capacitors and higher power density than batteries. Due to these inimitable properties the field of supercapacitors has been attracted by the research community [3]. In most of the cases, carbonaceous materials are used in the fabrication of supercapacitors due to its excellent cyclic stability and high specific capacitance. But, they are suffered from poor specific energy density and limited cell voltage. These difficulties could be minimized with the employment of metal oxides and conducting polymers [4,5]. Metal oxides like ruthenium oxide (RuO2 ) [6], Indium oxide (In2 O3 ) [7], cobalt oxide (Co3 O4 ) [8], nickel oxide (NiO) [9] and manganese oxide (MnO2 ) [10] have been reported as electrode materials in supercapacitor. Among these metal oxides, cobalt oxide/hydroxides are most prominent electrode materials for supercapacitors owing to their attractive properties like high

∗ Corresponding author. Tel.: +91 231 2609225; fax: +91 231 26092333. E-mail address: l [email protected] (C.D. Lokhande). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.02.094

conductivity and good cyclic stability [11,12]. The supercapacitive performance of the material strongly depends on the morphological creation and thereby on the specific surface area. Xue et al. have reported the dual-template synthesis of mesoporous Co(OH)2 nanowires with value of specific capacitance 993 F g−1 [13]. Also, Pan and co-workers have improved the value of specific capacitance by preparing Co(OH)2 /Ni composite nanoflakes array with the exploitation of simple chemical techniques and showed the maximum specific capacitance of 1310 F g−1 [14]. The cobalt hydroxide/carbon nanotubes composite has been studied for supercapacitive properties which showed the value of specific capacitance 322 F g−1 [15]. Recently, we have reported an effect of deposition scan rates on the film microstructure of ␤-Co(OH)2 nanoflakes and corresponding supercapacitive performance of the film electrode, found maximum value of specific capacitance of 890 F g−1 [16]. Above different morphological evidences of cobalt hydroxide showed flaky nanostructure can be effectively utilized for the fabrication of supercapacitors. According to the cell configuration, there are two kinds of supercapacitors, symmetric and asymmetric supercapacitors. Symmetric supercapacitors are formed with two similar electrode materials as a cathode and anode, while two dissimilar electrode materials form the asymmetric supercapacitors. Different reports have been made by researchers on the performance evaluation of symmetric and asymmetric supercapacitors. Xia et al. have reported the specific capacitance of 52.66 F g−1 for symmetric RuO2 /RuO2 supercapacitor cell with operating voltage 1.6 V [17]. Symmetric supercapacitors based on multilayers of conducting polymers

A.D. Jagadale et al. / Electrochimica Acta 98 (2013) 32–38

PEDOT and poly (N-methylpyrrole) PNMPy showed the specific capacitance up to 90 F g−1 [18]. The asymmetric type of supercapacitor formed with Co(OH)2 and graphene has been proposed to be a power-oriented supercapacitive device by Hu et al. with lower value of specific energy 1.7 Wh kg−1 [19]. However, above reports suggest that asymmetric supercapacitors are endured from low value of working voltage and specific energy. This difficulty can be inhibited through the fabrication of symmetric supercapacitors. So far, there is still no report on the symmetric supercapacitor based on Co(OH)2 to our best knowledge. In the present work, the electrochemical performance of symmetric Co(OH)2 –Co(OH)2 supercapacitor using 1 M KOH electrolyte solution is investigated for the first time. The Co(OH)2 (nanoflakes) thin film electrodes are prepared by potentiodynamic technique, which is simple, convenient and favorable for obtaining porous and nanostructured thin films. The electrochemical performance of the symmetric supercapacitor is evaluated using cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) techniques. 2. Experimental 2.1. Chemicals All chemicals were used without further purification, the cobalt (II) nitrate hexahydrate [Co(NO3 )2 ·6H2 O] (purity: 90.0%) and potassium hydroxide [KOH] (purity: 85.0%) was obtained from SD Fine Chem. Ltd. (Mumbai, India). 2.2. Co(OH)2 thin film electrode preparation and characterization The Co(OH)2 thin film material was deposited on to the stainless steel SS (grade 304, 0.1 mm thick) substrate by CV in an 0.1 M Co(NO3 )2 ·6H2 O solution at room temperature (300 K). Preparative parameters were used from the article published elsewhere [16]. Thin film electrodes were formed with stainless steel substrates of dimensions 3 cm × 3 cm. The 8-channel automatic battery cycler (WBCS3000) was employed for the potentiodynamic deposition. The deposition was performed in the potential window 0 to −1.2 V/SCE at the scan rate 200 mV s−1 . After the deposition, the

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electrodes were dried in air. These thin film electrodes were further used for the characterization and fabrication of symmetric supercapacitor test cell. The weight of deposited material loaded onto the stainless steel substrate was measured using weight difference method by employing microbalance. The total weight measured on both of the electrode was 6.21 mg. The X-ray diffraction pattern (Cu ˚ of the film was obtained using Bruker axs K␣ radiation  = 1.5406 A) D8 Advance model within a diffraction angle (2) from 10 to 90◦ . The surface morphology of Co(OH)2 was studied by scanning electron microscopy (SEM) (JEOL JSM model 6360). The value of specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method (ASAP 2410 model). The pore size was obtained from the adsorption and desorption branch of the nitrogen isotherms by the Barrett–Joyner–Halenda (BJH) method. 2.3. Symmetric Co(OH)2 |KOH| Co(OH)2 test cell fabrication The symmetric supercapacitor test cell was fabricated with twoelectrode configuration. The electrodes were made of Co(OH)2 nanoflakes; Fig. 1a shows the actual schematic of supercapacitive test cell which consists of two Co(OH)2 electrodes separated by a thin polypropylene separator in 1 M KOH aqueous electrolyte solution. Fig. 1b shows the photographs of ‘L’ shaped stainless steel substrate coated with Co(OH)2 thin film having dimensions 3 cm × 3 cm. Finally, a photograph of symmetric supercapacitor test cell with two leads is shown in Fig. 1c. 2.4. Electrochemical measurements The electrochemical properties of symmetric Co(OH)2 –Co(OH)2 supercapacitor were studied in a two-electrode system using CV and charge–discharge. In order to evaluate the capacitive and resistive properties of the cell, an electrochemical impedance spectroscopy (EIS) was employed using CH instruments (Model CH16112D). The CV response of the cell was measured at different scan rates varying from 5 mV s−1 to 100 mV s−1 . Galvanostatic charge–discharge was carried out with potential window of 0 to +1.2 V. Impedance spectroscopy measurement was carried out at a dc bias of −0.2 V with sinusoidal signal of 5 mV over the frequency range from 1 MHz to 0.01 Hz.

Fig. 1. Symmetric Co(OH)2 –Co(OH)2 supercapacitor test cell. (a) Schematic diagram of Co(OH)2 based supercapacitor test cell. (b) Photographs of Co(OH)2 thin film electrodes with dimensions 3 cm × 3 cm. (c) Photograph of symmetric Co(OH)2 –Co(OH)2 supercapacitor test cell used in this study.

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Fig. 2. X-ray diffraction (XRD) patterns of (a) As-deposited ␤-Co(OH)2 , (b) ␤-Co(OH)2 after 5000 charge–discharge cycles, (c) Co(OH)2 thin film after immersion, kept in the 1 M KOH solution at room temperature for 15 h, deposited on stainless steel substrate (‘*’ indicates peaks originated due to stainless steel).

3. Results and discussion 3.1. Structural and morphological study of Co(OH)2 XRD study was employed to understand the crystal structure, the structural stability after electrochemical cycling and effect of long term immersion of the film in KOH solution. XRD pattern of as deposited film showed a formation of ␤-Co(OH)2 material with hexagonal crystal structure (Fig. 2a) [JCPDS card no. 30-0443]. After 5000 charging–discharging cycles, the XRD pattern (Fig. 2b) of film revealed that, peak positions are slightly shifted toward left side and intensities increased with decrease in broadening which may be attributed to poor crystallinity of cycled ␤-Co(OH)2 [20].

Besides, an effect of long term immersion on structural property of as deposited film was studied in 1 M KOH electrolyte for 15 h at room temperature. The XRD pattern (Fig. 2c) shows the formation of CoOOH phase since Co3+ state is more stable in KOH solution [21]. In addition, peaks marked with an asterisk (*) are assigned to the characteristic peaks of the stainless steel substrate. Liu et al. have reported similar kind of crystal structure for single-crystal platelets of ␤-Co(OH)2 [22]. Fig. 3(a) and (b) shows SEM images of Co(OH)2 thin film at magnifications 5000× and 100,000×. From Fig. 3a, it can be clearly seen that the prepared Co(OH)2 consists of uniformly spread nanoflakes over the stainless steel substrate. Fig. 3b shows higher magnification (100,000×), which demonstrates the formation of nanoflakes

Fig. 3. Scanning electron micrograph (SEM) images of Co(OH)2 thin film deposited at 100 mV s−1 at (a) 5000× and (b) 100,000× magnifications.

-2

Current density (Acm )

250

3

-1

Amount adsorbed (cm g )

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200 150 100

0.0008

35

-1

a: 5 mVs

e

Oxidation

-1

b: 10 mVs

0.0006

-1

c: 20 mVs

d -1

0.0004

d: 50 mVs

-1

c

e: 100 mVs

b

0.0002

a

0.0000

-0.0002

50

Reduction

-0.0004 -0.2

0 0.0

0.2

0.4

0.6

0.8

0.0

0.2

1.0

Relative pressure (P/P0) Fig. 4. Adsorption–desorption isotherms (inset shows the pore size distribution obtained by BJH method) of Co(OH)2 .

with an average thickness of 100 nm. Previously, Chen et al. have reported similar kind of microstructure for cobalt hydroxide which has been deposited by electrodeposition method [23].

0.4

0.6

0.8

1.0

1.2

Voltage (V) Fig. 5. Cyclic voltammograms of symmetric Co(OH)2 –Co(OH)2 cell at scan rates 5, 10, 20, 50, 100 mV s−1 .

The specific capacitance (SC) values are calculated by integrating area under the IV curves and then dividing by the scan rate (mV s−1 ) and weight deposited W (0.34 mg cm−2 ). The capacitance was calculated using the following relation:



C=

I dt

(2)

dV/dt



3.2. BET study Textural parameters of Co(OH)2 were obtained by nitrogen adsorption–desorption measurement. Fig. 4 shows the N2 adsorption–desorption isotherm of the Co(OH)2 sample. From the graph, it is clear that the isotherm is of type IV according to IUPAC classification and exhibit H1 hysteresis with a featured capillary condensation in the mesopores, indicating the presence of mesopores in the prepared Co(OH)2 thin film. The BJH desorption pore size distribution of the sample is presented in the inset of Fig. 4. As can be seen in an inset, the sample shows a narrow pore size distribution with huge pore volume. The specific surface area, pore size and pore volume of the Co(OH)2 thin film are 113.7459 m2 g−1 , 3.46 nm and 0.356402 cm3 g−1 , respectively. The physical parameters together with a sharp capillary condensation step clearly reflect the presence of the mesoporous nature of the prepared Co(OH)2 thin film. 3.3. Electrochemical characterization of test cell

where, I dt is the area under curve and dV/dt is voltage scanning rate. SC of the symmetric test cell is calculated by dividing ‘W’ on stainless steel substrate. Specific capacitance =

C 2W

(3)

Fig. 5 shows that the scan rate increases with increase in area under the curve with reduction and oxidation potentials shifting toward higher and lower potential sides, respectively. The peak current varies linearly with the scan rate confirming the pseudocapacitive behavior. This is due to the scan rate dependent diffusion of OH− ions into the film matrix. At lower scan rate, both the outer and inner surfaces of flakes are effectively utilized, while at higher scan rates mainly outer surface is accessed by the OH− ions [25]. Also, it indicates the kinetics of interfacial Faradaic redox reactions and the rates of electronic and ionic transport are rapid enough at high scan rate of 100 mV s−1 [26]. Symmetric Co(OH)2 –Co(OH)2 supercapacitor cell exhibited specific capacitances of 44, 32, 25, 19 and 15 F g−1 at the scan rates of 5, 10, 20, 50 and 100 mV s−1 ,

3.3.1. Cyclic voltammetry Fig. 5 shows the cyclic voltammograms (CVs) of symmetric Co(OH)2 –Co(OH)2 supercapacitor test cell, measured in the voltage window of 0 to +1.2 V with, different scan rates of 5, 10, 20, 50, and 100 mV s−1 . Initially, at the scan rate of 5 mV s−1 two distinct redox peaks (at +1.02 and +0.65 V) are found, which appear during the extended cycles and are also reproducible. These redox peaks can be considered as the reversible electron transfer reaction between Co(OH)2 and OH− ions from the electrolyte [24]. The electrochemical reaction corresponding to the redox peaks can be expressed as follows: Co(OH)2 + OH−

oxidation/reduction

←→

CoOOH + H2 O + e−

(+1.02 V) (1)

During charging of Co(OH)2 –Co(OH)2 symmetric test cell, positively charged electrode gets oxidized to form CoOOH at the outer layer of Co(OH)2 . While at negative electrode K+ ions from the solution gets physically adsorbed on the surfaces of nanoflakes without Faradaic reactions. The process of charging and discharging of the cell is represented in Fig. 6.

Fig. 6. Schematics of charging–discharging Co(OH)2 –Co(OH)2 supercapacitor.

mechanism

of

symmetric

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Fig. 7. Galvanostatic charge–discharge curves for symmetric Co(OH)2 –Co(OH)2 test cell at current densities of (a) 0.0007 A cm−2 , (b) 0.001 A cm−2 , (c) 0.003 A cm−2 and (d) 0.005 A cm−2 .

Specific power :

S.E. = S.P. =

Coulombic eficiency :

V × Id × Td W

(4)

V × Id W

(5)

 (%) =

Td × 100 Tc

(6)

40

3.8

35

3.6

30

3.4

25

3.2

20

3.0

15

2.8

10

2.6

-1

2.4 0.000

5 0.001

0.002

0.003

0.004

-1

Specific energy :

45 4.0

Specific power (kWkg )

3.3.2. Charge–discharge study Fig. 7(a)–(d) shows the charge discharge profiles of symmetric cell at charging current densities of 0.0007. 0.001, 0.003 and 0.005 A cm−2 (corresponding to a charge–discharge specific current densities of 0.1, 0.2, 0.4 and 0.8 A g−1 ), respectively. These charge–discharge studies were carried out within potential window 0.0 to +1.2 V. At lower current densities (0.1 and 0.2 A g−1 ), during charging a non-linear increasing behavior of the supercapacitor cell is observed whereas at current densities 0.4 and 0.8 A g−1 behavior was linear. The symmetry of the charging and discharging characteristics at higher current densities indicates good charge release behavior. As seen in Fig. 7, the discharge curves are linear in the total range of potential with constant slopes, showing nearly perfect capacitive behavior [29]. The specific energy, power and Coulombic efficiency have been estimated from the following equations,

where, Id is discharge current, Td is discharge time and W is the total mass of Co(OH)2 film material coated onto both of the stainless steel substrates. The symmetric supercapacitor exhibited excellent supercapacitive performance with maximum values of specific energy and power as 3.96 Wh kg−1 and 42 kW kg−1 at current densities 0.1 and 0.2 A g−1 , respectively. The value of specific energy is quite higher than that of reported by Hu et al. for asymmetric supercapacitor based on Co(OH)2 and graphene electrodes [19]. Additionally, Fig. 8 reflects that the values of specific energy and power decrease and increase, respectively as per the increment of discharge current density. This may be due to the increment of insufficient active material involved in the redox reaction under higher current densities. These results suggest the symmetric test cell based on Co(OH)2 has a high-quality rate

Specific energy (Whkg )

respectively. The maximum value of 44 F g−1 is comparable with the value reported in literature. Ganesh et al. have reported the value of specific capacitance 22 F g−1 for porous Ni |KOH| porous Ni symmetric supercapacitor cell [27]. Previously, Kong et al. have reported the value of specific capacitance of 72.4 F g−1 for asymmetric supercapacitor formed with loose packed cobalt hydroxide nanoflakes and activated carbon. However, the surplus value of specific capacitance may be ascribed to the huge surface area of activated carbon electrode [28].

0.005

-2

Current density (Acm ) Fig. 8. Variations of specific energy and power as the function of current densities 0.0007, 0.001, 0.003 and 0.005 A cm−2 .

37

40 36 32

1.4 1.2

28 Voltage (V)

-1

Specific capacitance (Fg )

A.D. Jagadale et al. / Electrochimica Acta 98 (2013) 32–38

24 20

1.0 0.8 0.6 0.4 0.2

16

0.0 0

20

40

60

80

Time (s)

12 0

1000

2000

3000

4000

5000

Cycle numbers Fig. 9. Ragone plot of symmetric Co(OH)2 –Co(OH)2 test cell.

Fig. 11. Variation of specific capacitance as a function of cycle numbers.

capability at a large current density, which is very important for the supercapacitor to provide high power density. Fig. 9 shows the Ragone plot of Co(OH)2 –Co(OH)2 symmetric test cell. In comparison with the previously reported symmetric configurations, [17,27] our Co(OH)2 –Co(OH)2 symmetric cell exhibit comparable energy density and higher power density, which can be attributed to the nanoflakes like morphology with high surface area of Co(OH)2 thin film electrode, which thus improves the surface area at electrode/electrolyte interface and power density of symmetric test cell. Coulombic efficiency of symmetric test cell at current densities of 0.0007, 0.001, 0.003, and 0.005 mA cm−2 are found to be 43.35, 63.95, 77.05 and 78.15%, respectively (Fig. 10). This indicates more feasibility of the redox process even at higher current density condition. Charge–discharge cyclic stability of symmetric test cell was carried out under constant current density of 0.0007 A cm−2 by performing 5000 number of charge–discharge cycles. Fig. 11 shows the variation of specific capacitance and number of charge–discharge cycles. It was observed that Co(OH)2 –Co(OH)2 symmetric test cell showed 81% of cyclic stability with no significant change in specific capacitance.

signal of 5 mV over the frequency range of 10 MHz and 0.01 Hz. A sharp increase of the imaginary part of EIS at lower frequency is due to capacitive behavior of the cell, where a semicircular loop at higher frequencies is due to charge-transfer resistance. For an ideal double-layer capacitor, the impedance plot should be a vertical line, parallel to the imaginary axis, which is generally observed in the case of CNT electrodes [30]. An inclined behavior of Co(OH)2 –Co(OH)2 symmetric cell is due to pore size distribution. In the intermediate frequency region, the 45◦ line to Z -axis is the indication of ion diffusion into electrode materials. A bottom inset of Fig. 12, the semicircle, which represented the charge transfer resistance at the electrode/electrolyte interface, was easily detected in the high frequency range of the plot. The cell showed a small semicircle in the high frequency range, indicating a charge transfer resistance caused by oxidation of Co(OH)2 surfaces under the highly alkaline condition. A simplified equivalent circuit consists of CPE1 , R1 , R2 , and W1 elements are shown in top inset of Fig. 12. The CPE1 is the constant phase element, R1 is the solution resistance, R2 is the charge transfer resistance and W1 is the Warburg diffusion resistance. The value of ESR was estimated as 2.3 . Ganesh and co-workers have reported the values of ESR for symmetric porous Ni |KOH| porous Ni supercapacitor ranging from 10 to 20 . The lower ESR in the present case is ascribed to the lower intrinsic resistance of cobalt hydroxide film which improves the redox activity during charging–discharging of supercapacitor [27].

Coulombic efficiency η (%)

3.3.3. Electrochemical impedance spectroscopy Fig. 12 presents complex plane electrochemical impedance spectrum (EIS) of symmetric Co(OH)2 –Co(OH)2 test cell. EIS measurement was carried out at a dc bias of 0.073 V with a sinusoidal

80 75 70 65 60 55 50 45 40 0.000

0.001

0.002

0.003

0.004

0.005

-2

Current density (Acm ) Fig. 10. Variation of Coulombic efficiency of symmetric Co(OH)2 –Co(OH)2 test cell as a function of current densities 0.0007, 0.001, 0.003 and 0.005 A cm−2 .

Fig. 12. The Nyquist plot of symmetric Co(OH)2 –Co(OH)2 test cell (top inset: equivalent circuit derived from the Nyquist plot, bottom inset: zoomed region of higher frequency).

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4. Conclusions In summary, potentiodynamic electrodeposition method has been employed for the deposition of Co(OH)2 thin film onto stainless steel substrate. These Co(OH)2 thin film electrodes have been assembled as a symmetric Co(OH)2 –Co(OH)2 supercapacitor test cell. This symmetric Co(OH)2 –Co(OH)2 supercapacitor showed maximum specific capacitance of 44 F g−1 at the scan rate of 5 mV s−1 along with increased values of specific power and specific energy as 42 kW kg−1 and 3.96 Wh kg−1 , respectively. Acknowledgement Authors are grateful to the Council for Scientific and Industrial Research (CSIR), New Delhi (India) for financial support through the scheme. No. 03(1165)/10/EMR-II. References [1] M. Winter, R.J. Brodd, What are batteries, fuel cells, and supercapacitors? Chemical Reviews 104 (2004) 4245. [2] B.E. Conway, Electrochemical Capacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum, London, 1999. [3] C. Arbizzani, M. Mastragostino, F. Soavi, New trends in electrochemical supercapacitors, Journal of Power Sources 100 (2001) 164. [4] C.D. Lokhande, D.P. Dubal, Oh-Shim Joo, Metal oxide thin film based supercapacitors, Current Applied Physics 11 (2011) 255. [5] Mastragostino, C. Arbizzani, F. Soavi, Polymer-based supercapacitors, Journal of Power Sources 97 (2001) 812. [6] V.D. Patake, C.D. Lokhande, O.S. Joo, Electrodeposited ruthenium oxide thin films for supercapacitor: effect of surface treatments, Applied Surface Science 255 (2009) 4192. [7] K.R. Prasad, K. Koga, N. Miura, Electrochemical deposition of nanostructured indium oxide: high-performance electrode material for redox supercapacitors, Chemistry of Materials 16 (2004) 1845. [8] S.G. Kandalkar, C.D. Lokhande, R.S. Mane, S.H. Han, A non-thermal chemical synthesis of hydrophilic and amorphous cobalt oxide films for supercapacitor application, Applied Surface Science 253 (2007) 3952. [9] U.M. Patil, R.R. Salunkhe, K.V. Gurav, C.D. Lokhande, Chemically deposited nanocrystalline NiO thin films for supercapacitor application, Applied Surface Science 255 (2008) 2603. [10] D.P. Dubal, D.S. Dhawale, R.R. Salunkhe, C.D. Lokhande, Conversion of chemically prepared interlocked cubelike Mn3 O4 to birnessite MnO2 using electrochemical cycling, Journal of the Electrochemical Society 157 (2010) A812. [11] S.K. Meher, G.R. Rao, Ultralayered Co3 O4 for high-performance supercapacitor applications, Journal of Physical Chemistry C 115 (2011) 15646. [12] V. Gupta, T. Kusahara, H. Toyama, S. Gupta, N. Miura, Potentiostatically deposited nanostructured ␣-Co(OH)2 : a high performance electrode material for redox-capacitors, Electrochemistry Communications 9 (2007) 2315.

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