mont K10 composite for high electrochemical capacitive energy storage

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 1

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MnO2/mont K10 composite for high electrochemical capacitive energy storage Vijayakumar Badathala*, Justin Ponniah 1 Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India

article info

abstract

Article history:

MnO2/mont K10 composite was prepared by reacting KMnO4 and ethylene glycol in pres-

Received 28 December 2015

ence of montmorillonite K10 (mont K10) at room temperature. The composite was char-

Received in revised form

acterized by X-ray diffraction, X-ray photoelectron spectroscopy, BET specific surface area

26 April 2016

(136 m2/g), and pore-size analysis (mesopores of diameter 3.9 nm and macropores of

Accepted 18 May 2016

diameter 5.7 nm), infrared spectroscopy, and thermogravimetric analysis for its physico-

Available online xxx

chemical properties. Morphology was studied by scanning electron microscopy. The electrochemical studies of MnO2/mont K10 were carried out using cyclic voltammetry in 0.5 M

Keywords:

Na2SO4 aqueous electrolyte. The MnO2/mont K10 composite showed remarkable specific

MnO2/Mont K10

capacitance of 460 Fg
1 at a scan rate of 5 mV
1.

Montmorillonite K10

© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Composite Electrode Supercapacitor

Introduction The nanostructured materials find application in catalysts, sensors, and electrochemical energy systems (lithium ion battery, solar cell, supercapacitor, and fuel cell) because of their unique physical and chemical properties [1e4]. Particularly, 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 equipment and electric transportation [5]. Among the energy storage devices, electrochemical capacitors have gained special interest because of their ability in harvesting the kinetic energy that is wasted whenever vehicles or large machines are slowed or stopped

[6]. The electrochemical capacitors store the charge electrostatically using reversible adsorption of ions of an electrolyte onto the active materials as explained by GouyeChapmaneSterneGrahame model (electrical double-layer capacitors, EDLCs) or by utilizing the surface redox reactions of an active material (pseudocapacitors) [5]. The electrochemical capacitors can operate at high charge and discharge rates over an almost unlimited number of cycles than batteries, and enable energy recovery in heavier duty vehicles like trucks and buses. Activated high surface area carbons, conducting polymers and various transition metal oxides have been investigated as supercapacitors [7]. RuO2 possesses pseudocapacitance property and exhibits much higher specific capacitance (1000 F g
1in aqueous acid electrolyte) than that of conventional carbon materials, but is

* Corresponding author. Present address: Department of Chemistry, Vel Tech High Tech Dr. Rangarajan Dr.Sakunthala Engineering College, Avadi, Chennai 600062, India. Tel.: þ91 44 2684 0181. E-mail address: [email protected] (V. Badathala). 1 Present address: Rajiv Gandhi University of Knowledge Technologies RGUK IIIT, RK Valley, Vempalli, 516329, Kadapa District, Andhra Pradesh, India. http://dx.doi.org/10.1016/j.ijhydene.2016.05.173 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Badathala V, Ponniah J, MnO2/mont K10 composite for high electrochemical capacitive energy storage, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.173

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prohibitive in price [8]. Efforts have been made to develop more practical low cost pseudocapacitance materials resulting MnO2 [6,9e11], NiO [12e15], Co3O4 [16e18], Fe3O4 [19], V2O5 [20], SnO2 [21], TiO2 [22], CuO [23], and Bi2O3 [24] as pseudocapacitance electrode materials. In addition to these oxides, hydroxides [25e27], sulfides [28,29], electronically conducting polymers [30e33], mixed transition metal oxides and their composites [34] have also been tested as pseudocapacitor electrode materials. Among the transition metal oxides, MnO2 [6,9e11] has been extensively studied as a pseudocapacitor electrode material due to its large abundance, high surface area, environmentally friendly behavior, and low cost with a good possibility of enhancing its performance by improving the preparative methods. In addition, MnO2 possesses an internal tunnel structure which is more ideal host materials for the insertion and desertion of ions in electrode materials for energy storage devices like battery and electrochemical capacitor. Normally the manganese oxides exhibit specific capacitances in the range of 100e200 Fg
1 in alkali salt solutions, which are much lower than those for RuO2 based electrodes. Extensive efforts have been dedicated to improve the limited specific capacitance (low energy density), long-term stability problems and low rate-capacity associated with MnO2 electrodes. Mesoporous MnO2 with uniform nanorod morphology and mesoporous b-MnO2 using SBA-15 and KIT-6 as templates [35], graphene [36], graphene/polyaniline composite [37,38], graphene oxide-MnO2 [39], and MnO2-coated carbon nanotubes between graphene sheets [40] have been reported as electrochemical capacitor electrodes. Recently, graphene/MnO2 [41], MnO2/rGO/Ni composite [42], graphene-polyaniline-MnO2 hybrids [43], honeycomb MnO2 nanospheres/carbon nanoparticles/graphene composites [44], hierarchical porous activated carbon @MnO2core/shell nanocomposite [45], polyaniline@MnO2/graphene ternary composites [46], MnO2-deposited TiO2 nanotube arrays [47], molybdenum disulfide/carbon composite [48], cobalt nickel sulphide dendrite/quasispherical nanocomposite [49], graphdiyne nanostructures [50] and nanoarchitectured MoS2 [51] have also been employed as electrochemical capacitor electrode materials. The excellent electronic conductivity, high stability, and mechanical flexibility of additive materials enable the improved electrochemical and mechanical properties of MnO2 composite electrodes for electrochemical capacitor. Clays are naturally available layered materials with wide applications in various fields [52,53]. To the best of our knowledge, few reports on modified clays as electrodes are available in the literature. In continuation of our work on clay composites [54] as electrode materials, we report here the preparation, characterization and use of MnO2/mont K10 composite as electrode material for supercapacitors. The electrochemical properties of MnO2/mont K10 composite as obtained were investigated, together with MnO2 for comparison.

stirring. About 7.9 g (0.5 M) of KMnO4 was added to the 1.5% clay solution and stirred the mixture for 30 min and then slowly 31.0 g (5 M) of ethylene glycol was added. KMnO4 oxidizes ethylene glycol and MnO2 is formed. The mixture was aged for 4 h on stirring at ambient conditions. The resulting powder was washed with distilled water several times followed by ethanol and dried in air at 100  C for 14 h. For comparative studies, MnO2 was also prepared using the same conditions without adding montmorillonite K10.

Physical characterization To obtain the information on the crystal structure and its morphology for the resulting MnO2/mont K10 composite, the X-ray powder diffraction (XRD) data were collected on a Bruker AXS D8 Discover diffractometer with Cu Ka radiation (l ¼ 0.15418 nm). X-ray photoelectron spectroscopy (XPS) signals were collected on Omicron Nanotechnology instrument using Mg Ka X-rays at 1253.6 eV operated at 300 W. All the elemental binding energies were referenced to the C (1 s) line situated at 284.6 eV. The specific surface areas of the samples were determined by BET method using N2 adsorption-desorption at 77 K on a Micromeritics ASAP 2020 surface area analyzer. The IR spectra were recorded in the range of 450e4000 cm
1 on a PerkineElmer FTIR spectrometer using KBr pellet. The thermogravimetry analysis (TGA) was conducted on a PerkineElmer TGA-7 analyzer in N2 with a heating rate of 20  C min
1 from room temperature to 800  C. Field emission scanning electron microscopic/energy dispersive spectroscopic (FE-SEM/EDS) studies were done in Jeol microscope model JSM-6700F.

Electrochemical characterization For electrochemical characterization, electrodes were fabricated on a high purity nickel as a current collector. The nickel was polished with successive grades of emery paper, sonicated and washed thoroughly with detergent and distilled water. 75 wt % of MnO2/mont K10, 20 wt % of acetylene black, and 5 wt % of polyvinylidene fluoride (PVDF) were ground in a mortar. Few drops of n-methyl pyrrolidinone (NMP) were added to make syrup. This was then coated on to the pretreated nickel foil and dried at 60  C under reduced pressure for 12 h. The electrochemical studies were performed on a CHI 7081C electrochemical workstation using a three electrodeconfiguration-cell consisting of MnO2/mont K10 as the working electrode, platinum foil (1  2 cm2) as a counter electrode and SCE (Saturated Calomel Electrode) as reference electrode, all dipped in 0.5 M Na2SO4 aqueous electrolyte.

Results and discussion XRD

Experimental Materials and methods Preparation of MnO2 and MnO2/mont K10 composite In a typical preparation, 1.5 g of montmorillonite K10 (mont K10) was added to 100 ml of distilled water with constant

Fig. 1 shows powder XRD patterns of montmorillonite K10, assynthesized MnO2, and MnO2/mont K10 composite. The samples were also heat treated at 300 and 400  C. The XRD analysis reveals that MnO2 and MnO2/mont K10 composite are in amorphous phase and on thermal treatment at 400  C, they exhibit a-MnO2 phase (JCPDS 44e0141) [55]. It is also evident that the MnO2/mont K10 composite possesses layered

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montmorillonite since the peaks of both mont K10 and MnO2 are present. A considerable amount of water content is essential for an electrode material for ionic transportation through electrolyte to enhance the electrochemical performance as a supercapacitor [6]. Therefore, water molecules present in the interlayers of mont K10 serve this purpose for MnO2/mont K10 composite dried at 100  C to be a good electrode material for supercapacitors.

XPS The surface compositional information of MnO2 and MnO2/ mont K10 composite was collected by X-ray photoelectron spectroscopy (XPS). The survey spectrum of MnO2 (not shown) showed the distinctive XPS peaks of manganese, oxygen, carbon, potassium and their auger peaks. On the other hand, the survey spectrum of MnO2/mont K10 composite (Fig. 2a) showed XPS peaks of manganese, oxygen, carbon, silicon, potassium and their auger peaks. Fig. 2b shows the region where the XPS signals of Mn 2p3/2 and Mn 2p1/2 are expected in MnO2/mont K10 composite. The Mn 2p3/2 peak is centered at 642.1 eV and the Mn 2p1/2 peak at 653.7 eV, with a spin-

energy separation of 11.6 eV. These results of Mn 2p3/2 and Mn 2p1/2 in MnO2 agree with the data reported in the literature [56].

FTIR Fig. 3 shows the infrared spectrum of the samples. Bands around 3435 and 1631 cm
1 were observed in all the samples, due to the stretching and bending vibrations of water molecules or hydroxyl groups in the interlayer or surface water. In Mont K10, a strong band near 1100 cm
1 is assigned to the SieO bending vibration, while the SieOeSi stretching vibration has appeared near 1040 cm
1 as a strong band. The shoulder band at 920 cm
1can be attributed to AleOH group and the band near 800 cm
1 is due to the skeletal vibrations of quartz. MnO2 sample showed a strong band at about 562 cm
1 corresponding to metaleoxygen bond of MnO2 [57]. MnO2/ mont K10 composite showed the bands of both MnO2 and mont K10 which is in accordance with XRD results.

TGA Fig. 4 shows the TG profiles of mont K10, MnO2 and MnO2/ mont K10 composite. The TGA curve of mont K10 shows an initial sharp decrease in weight ~6% within the temperature range of 80e150  C is attributed to the loss of adsorbed water and interlayer water. A steady weight loss of about 6% in the temperature range of 150e800  C is attributed to the loss of

250000

a

Intensity (CPS)

200000 Mn 2p3/2 Mn 2p1/2 O 1s

150000 100000 50000

C K

Si

0 0

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Mn 2p3/2

40000

Mn 2p1/2

35000 30000 642.1

25000

653.7

20000 640

645

650

655

660

Binding Energy (eV) Fig. 1 e a XRD profiles of mont K10, MnO2, and MnO2/mont K10 composite and b Magnified XRD profiles of MnO2, and MnO2/mont K10 composite.

Fig. 2 e a Survey XPS spectrum of MnO2/mont K10 composite, and b XPS spectrum of MnO2/mont K10 composite at Mn 2p3/2 and Mn 2p1/2.

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Mn-O

Mont K10

Al-OH

H-O

Si-O-Si

H-O

MnO2/mont K10

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm-1)

Fig. 3 e Infrared spectra of mont K10, MnO2, and MnO2/ mont K10 composite.

dehydroxylation caused by breaking of structural hydroxyl groups of the montmorillonite. MnO2 shows about 10% weight loss upon heating from room temperature to 200  C, corresponding to the loss of adsorbed water molecules. The weight loss of about 2.5% in the temperature range from 200 to 450  C is due to the removal of crystalline water. A slight increase in weight after 470  C may possibly be due to oxygen absorption. MnO2/mont K10 composite shows about 14% weight loss upon heating from room temperature to 200  C, corresponding to the loss of adsorbed water and interlayer water. The gradual weight loss of about 7% in the temperature range from 200 to 800  C is due to the removal of crystalline water and loss of dehydroxylation caused by breaking of structural hydroxyl groups of the montmorillonite. Further, a weight loss at 400e580  C may also be due to the reduction of manganese from tetravalent to trivalent form [58].

Surface characteristics

of mont K10, MnO2, and MnO2/mont K10 composite. The sorption isotherms of the samples can be attributed to type IV which is characteristic of capillary condensation in mesopores. Mont K10 exhibits a hysteresis loop in the relative pressure (P/Pο) range of 0.42e0.99, MnO2 exhibits a hysteresis loop in the relative pressure (P/Pο) range of 0.40e0.99, whereas MnO2/mont K10 exhibits relatively a big hysteresis loop in the relative pressure (P/Pο) range of 0.45e~0.99. The hysteresis loop for MnO2 in the relative pressure (P/Pο) range of 0.40e0.99 suggests that it has mesopores of 3.6 nm as well as considerable amounts of larger mesopores or inter-particular porosity of around 12.6 nm with broad (secondary) pore-size distributions. Mont K10, and MnO2/mont K10 composite have mesopores with narrow pore-size distributions of 3.8e5.7 nm. The BET specific surface areas of the samples follow the sequence (Table 1): Mont K10 (208 m2/g) > MnO2 (180 m2/g) > MnO2/mont K10 (136 m2/g), whereas the total pore volumes of the samples follow the sequence: MnO2 (0.3441 cm3/g) > Mont K10 (0.2815 cm3/g) > MnO2/mont K10 (0.1800 cm3/g). It is evident from the results that use of mont K10 helped in obtaining MnO2/mont K10 composite with unique narrow pore-size distribution of 3.9e5.7 nm.

SEM The surface morphologies of mont K10, MnO2, and MnO2/ mont K10 composite are shown in Fig. 6. Fig. 6a shows SEM

Volume adsorbed (cm3g-1, STP)

Transmittance (a.u.)

MnO2

200 150

Mont K10

100 50

MnO2/mont K10

0 0.0

1.4

Mont K10

90

MnO2

85 MnO2/mont K10

80

Pore volume (cm3/g)

95

1.2

150

300

450

600

750

Temperature (°C) Fig. 4 e TG profiles of mont K10, MnO2, and MnO2/mont K10 composite.

0.4

0.6

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1.0

Mont K10

b

MnO2/mont K10

MnO2

1.0 0.8 0.6 0.4 0.2 0.0

0

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Relative pressure (p/po)

100

Weight (%)

MnO2

250

Fig. 5 shows the nitrogen adsorption-desorption isotherms and Barret-Joyner-Halenda (BJH) pore-size distribution curves

75

a

300

100

Pore diameter (Å) Fig. 5 e a Nitrogen adsorption-desorption isotherms, and b Barret-Joyner-Halenda (BJH) pore-size distribution curves of mont K10, MnO2, and MnO2/mont K10 composite.

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image of mont K10 which exhibited platelet morphology. MnO2 shows particles of different shapes and sizes (Fig. 6b), where as MnO2/mont K10 composite shows particles of smaller sizes compared to MnO2 (Fig. 6c) that helps the composite in obtaining enhanced specific capacitance.

Electrochemical studies Cyclic voltammetry Effect of scan rate Fig. 7 displays the cyclic voltammograms (CV) of MnO2 and MnO2/mont K10 sample measured in the potential range of
0.2e1.0 V employing the scan rates of 5, 10, 20, and 50 mV. The corresponding specific capacitances, obtained from the CV curves are also presented in Fig. 7. Generally, the charge storage occurs in an electrical double-layer capacitor by electrostatic reversible adsorption of ions of the electrolyte. This type of capacitance behavior gives CV curves close to ideal rectangular shape. But these samples show CV curves distinctly different from rectangular shape. This pseudocapacitance behavior can be attributed to the charge storage mechanism essentially by redox reactions related to Mno2 þ Naþ þ e
#MnOONa þ ðMnO2 ÞSurface þ Naþ þ e
# MnO
2 Na


Surface

ðMnO2 =mont K10ÞSurface þ Naþ 
þ þ e
# MnO
mont K10 Surface 2 Na There is an inverse relation observed between the scan rate and the total amount of charge stored. In other words, there are kinetic limitations associated with the diffusion of ions through the MnO2/mont K10 electrode matrix which limits the full storage of charge at higher scan rates. For example, at 5 mV
1 scan rate, MnO2/mont K10 electrode reaches maximum specific capacitance of 460 F g
1while at 50 mV
1 it reaches only 313 F g
1. At 5 mV
1 scan rate, MnO2 electrode reaches maximum specific capacitance of 327 Fg
1 while at 50 mV
1 it reaches 65 Fg
1. Fig. 6 e SEM images of a mont K10, b MnO2, and c MnO2/ mont K10 composite.

Chronopotentiometry The ability of the synthesized MnO2/mont K10 and MnO2 as electrode materials for electrochemical capacitors was evaluated by carrying out galvanostatic charge/discharge measurements in 0.5 M Na2SO4. The durations of charging and discharging are almost equal for each electrode, implying high

coulombic efficiency of charge/discharge cycling. However, the durations of charge and discharge cycles are different for MnO2/mont K10 and MnO2, suggesting that the specific capacitance values are also different, similar to the

Table 1 e Some physicochemical properties of montmorillonite K10, MnO2, and MnO2/mont K10. Entry 1 2 3

Sample

Specific surface area (m2/g)

Total pore volume (cm3/g)

Avg. pore diameter (nm)

Montmorillonite K10 MnO2 MnO2/mont K10

208 180 136

0.2815 0.3441 0.1800

3.8 3.6, 12.6 3.9, 5.7

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capacitance is only marginal, by about of available capacity from 231 to 223 Fg
1 when the current density is increased from 200 to 500 mAg
1. It is observed that the MnO2/mont K10 composite shows better specific capacitance than MnO2 even at higher current density. The dependence of specific capacitance on current density and scan rates of MnO2/mont K10 electrode is summarized in Fig. 8b.

Cyclic test

Fig. 7 e Cyclic voltammograms (CV) measured at different scan rates of a MnO2, and b MnO2/mont K10.

observations made from cyclic voltammograms (Fig. 9). The specific capacitances (Cs, Fg
1) of MnO2/mont K10 and MnO2 electrodes were estimated by dividing the current (i, A) with the active mass (1.35 mg) of the electrode (m, g) and the discharge slope of chronopotentiometric discharge curve [(dV/ dt), Vs
1] according to the following equation, Cs ¼ i/[m (dV/ dt)] [23]. The specific capacitance values at a current density of 200 mAg
1 were found to be 231 F g
1 and 148 Fg
1 for MnO2/ mont K10 and MnO2 respectively. Because pseudocapacitance is an interfacial phenomenon, the materials with high specific surface area with suitable pore-size distributions should have high charge storage capabilities. The MnO2/mont K10 composite has a specific surface area of 136 m2g
1. This value is smaller than that of MnO2 (180 m2g
1). However, the narrow distribution of the pore diameter is significant in the case of MnO2/mont K10 (Fig. 3b). The galvanostatic charge discharge curves for the two electrodes from
0.05e0.9 V at different current densities in 0.5 M Na2SO4 aqueous solutions are shown in Fig. 8. It is clear that there is a decrease in specific capacitance as the discharge current density increases. This is generally caused by an increment of voltage drop and insufficient use of the active material involved in the redox reaction under applied high current densities. At higher current densities, the ions reach the outer surface of the electrode swiftly, but only a small fraction of ions get intercalated into the pore interiors of MnO2/mont K10 matrix. This may result in drastic decrease of specific capacitance. However, the reduction of the specific

To evaluate the electrochemical stability of MnO2/mont K10 as an electrode material for electrochemical capacitors, galvanostatic charge/discharge measurements were carried out for 200 cycles at a fairly high current of 500 mAg
1 between
0.05 and 0.9 V in 0.5 M Na2SO4 electrolyte. Fig. 9 shows cycle life of MnO2/mont K10 and MnO2. MnO2/mont K10 showed 8% reduction in specific capacitance from 223 to 206 Fg
1 while MnO2 showed 70% reduction in its specific capacitance from 124 to 39 Fg
1. The cycle performance and the Coulombic efficiency are shown in Fig. 9. The typical charge/discharge curves of the first 25 cycles are shown as an inset. A gradual decrease in specific capacitance to a value of 206 Fg
1 over 200 cycles was observed in MnO2/mont K10. This demonstrates that, within the voltage window
0.05 to 0.9 V, the charge and discharge processes do not seem to induce significant structural or microstructural changes of the MnO2/mont K10 electrode material, as expected for pseudocapacitance reactions. The slight decrease of specific capacitance observed may be attributed to the detachment of some of the MnO2/mont K10 material from the smooth Ni foil current collector. Any structural changes in electrodes affect the stability of the material and decrease the measured specific capacitance value. However, during the cycling process, the Coulombic efficiency remained above 95%, which demonstrates excellent reversibility of the MnO2/mont K10 electrode. Therefore, the MnO2/ mont K10 composite with long-term stability and reversibility is a suitable material for pseudocapacitor electrodes.

Electrochemical impedance spectroscopy The MnO2/mont K10 composite and MnO2 are subjected to electrochemical impedance study between 0.01 Hz and 100 000 Hz to evaluate the electrochemical resistance. Fig. 10 shows Nyquist plots (real part, Z0 versus imaginary part, Z00 ) of electrochemical impedance measured for MnO2/mont K10 and MnO2 at different bias potentials. The impedance behavior shows three major processes occurring in the high, medium, and low-frequency regions. The two partial semicircles at high and medium frequency region which are often attributed to different resistances due to different processes occurring in the system as a result of faradic reactions. The expanded view of the semicircles at different bias potentials is shown in the Fig. 10. The EIS spectra have been evaluated through a simple circuit model, proposed by Hu et al. [59] and is shown in the insets of Fig. 10. The partial semicircle at high frequency region (shown as an inset in Fig. 10) is characteristic of the processes occurring at the oxide/clayeelectrolyte interface. This can be modeled as a double-layer capacitor Cd in parallel with an ionic charge-transfer resistor, Rict. The Rict is a resistance due to different conductivity in the solid oxide/

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Potential (V vs SCE)

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1000 mA g-1 (114 F g-1)

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2000 mA g-1 (2 F g-1)

600

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1.0

50

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0.0 0

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200 mA g-1 (148 F g-1)

5

b

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Fig. 8 e Chronopotentiometry of a MnO2, and b MnO2/mont K10 electrodes measured at different current densities in 0.5 M Na2SO4 electrolyte.

clay (electronic conductivity) and the aqueous electrolyte phase (ionic conductivity). So, the resistance is due to some discontinuity in the charge-transfer process at the solid oxide/ clay/liquid electrolyte interface. Here, the resistance is independent of applied biased potential due to steady electronic

and ionic conductivity of the solid oxide/clay and liquid electrolyte, respectively. The partial kinetic semicircles at medium frequency regions correspond to the charge-transfer resistance due to faradic redox processes in the system involving the exchange

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80 40 0 -40 -80 -120

0 20

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40

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-160

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8000

80

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T im e ( s )

-200

Coulombic efficiency (%)

500 mA a P o te n tia l ( V v s S C E )

Specific capacitance (Fg-1)

120

100 20 40 60 80 100 120 140 160 180 200

0

Cycle number

150 120 90

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0.6 0.4

60

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-80 MnO2/mont K10

0.4V

b

-60 0.6V

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-40 Z"

20 0.8

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Specific capacitance (Fg-1)

1.0

180

0

0

Coulombic efficiency (%)

500 mA b

210

-20

20 40 60 80 100 120 140 160 180 200

Cycle number

Fig. 9 e The specific capacitance and coulombic efficiency as a function of cycle number at a current density of 500 mAg¡1 of a MnO2, and b MnO2/mont K10. The insets show 25 cyclic chargeedischarge curves of MnO2, and MnO2/mont K10 respectively at a current density of 500 mAg¡1.

of Naþ. This is associated with the surface phenomena of the MnO2/mont K10 or MnO2 electrode. This impedance characteristic can be modeled as a film impedance due to the faradic redox processes involving electron hopping in the MnO2/mont K10 particle and Naþ ion diffusion. This is assumed to be consisting of a film capacitor, Cf in parallel with an electron-transfer resistor, Rect and is dependent upon the applied bias potentials. With increase in biased potential, there is a decrease in impedance due to increase in Naþ ion diffusion, thereby increasing the feasibility of electron-transfer. The linear part at lower frequencies corresponds to the Warburg impedance, W, which is described as a diffusive resistance of the Naþ ion within the MnO2/mont K10 electrode pores. At a biased potential of 0.6 V, the Nyquist plot of MnO2/montK10 displays a nearly vertical line along the imaginary axis at lower frequencies. When the dc potential is reduced to 0.6 V and 0.2 V, a considerable deviation from near 90 slopes is observed at the low-frequency region. This deviation along the imaginary axis is ascribed to the pseudocapacitance, Cs, due to facile and reversible faradic redox reactions at the electrodeeelectrolyte interface and an easy access of the Naþ ions with the electroactive MnO2/mont K10 electrode under such low frequencies. The zero slope at higher voltage is attributed to the effect of high operation voltages on the supercapacitive behavior and more pronounced surface redox reactions at electrode/

0 0

5

10

15

20

25

Fig. 10 e Nyquist plots of a MnO2, and b MnO2/mont K10 electrodes at different bias potentials showing two semicircles at high frequency region. Insets show the equivalent circuit. electrolyte interface. For all biased potentials at higher frequencies, the intercept at real part (Z0 ) is 0.1U. This constant value is due to the identical combination of (i) ionic and electronic charge-transfer resistances, (ii) intrinsic chargetransfer resistance of the active MnO2/mont K10 material, and (iii) diffusive as well as contact resistance at the active material/current collector interface. The experimental impedance data are also converted to specific capacitance (Cs) using the following equation: Cs ¼
1=2pf Z

00

and plotted against frequency, f in Fig. 11. The specific capacitance inevitably decreases with biased potential which is consistent with the scan rate dependent current density observed in the cyclic voltammetry. Further the specific capacitance calculated from impedance measurements at different bias potentials (Fig. 11) is also in agreement with the results obtained from the cyclic voltammetry measurements.

Conclusion A simple method has been developed to synthesize MnO2/ mont K10 composite with improved electrochemical capacitance behavior. This method allows us to tune the

Please cite this article in press as: Badathala V, Ponniah J, MnO2/mont K10 composite for high electrochemical capacitive energy storage, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.173

Specific capacitance (F g-1)

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a

120

references

100 80

0.2 V, Cs= 98 Fg-1

60

0.4 V, Cs=105 Fg-1

40

0.6 V, Cs=111 Fg-1

20 0 0.01

0.1

1

10

100 1000 10000100000

Frequency (Hz) b

140 Specific capacitance (F g-1)

9

120 100

0.2 V, Cs=138 Fg-1

80

0.4 V, Cs=142 Fg-1

60

0.6 V, Cs=144 Fg-1

40 20 0 0.01

0.1

1

10

100 1000 10000100000

Frequency (Hz) Fig. 11 e The specific capacitances of a MnO2, and b MnO2/ mont K10 electrodes calculated from impedance spectra as a function of frequency at different bias potentials.

microtextural properties, which include the specific surface area, pore volume, and pore dimension and distribution. The MnO2/mont K10 possesses platelet morphology with a pore volume of 0.18 cm3g
1 and a specific surface area of 136 m2g
1. These properties enable the MnO2/mont K10 composite to have better electrochemical capacitive properties by reducing its charge-transfer resistance. In a life cycle test, MnO2/mont K10 exhibits an excellent cycle profile with a Coulombic efficiency of over 95% and shows a specific capacitance of 206 Fg
1 over 200 cycles. The method employed in this study shows that MnO2/mont K10 with improved properties can be useful for high-performance pseudocapacitor applications. Further, these results encourage research on preparation of various mixed transition metal oxide/montmorillonite composites and their application as electrode materials for supercapacitors.

Acknowledgments The authors thank the Department of Science and Technology, Government of India, New Delhi (Project No.SR/FTP/CS11/2006) for financial assistance. The authors are grateful to Prof. G. Ranga Rao, IIT Madras for his encouragement and support.

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