Journal of Solid State Electrochemistry

, Volume 12, Issue 2, pp 207–211

Surfactant stabilized nanopetals morphology of α-MnO2 prepared by microemulsion method

Authors

  • S. Devaraj
    • Department of Inorganic and Physical ChemistryIndian Institute of Science
    • Department of Inorganic and Physical ChemistryIndian Institute of Science
Short Communication

DOI: 10.1007/s10008-007-0364-7

Cite this article as:
Devaraj, S. & Munichandraiah, N. J Solid State Electrochem (2008) 12: 207. doi:10.1007/s10008-007-0364-7

Abstract

α-Manganese dioxide is synthesized in a microemulsion medium by a redox reaction between KMnO4 and MnSO4 in presence of sodium dodecyl sulphate as a surface active agent. The morphology of MnO2 resembles nanopetals, which are spread parallel to the field. The material is further characterized by powder X-ray diffraction, energy dispersive analysis of X-ray, and Brunauer–Emmett–Teller surface area. Supercapacitance property of α-MnO2 nanopetals is studied by cyclic voltammetry and galvanostatic charge–discharge cycling. High values of specific capacitance are obtained.

Keywords

Manganese dioxideMicroemulsionNanopetalsSupercapacitorCyclic voltammetry

Introduction

Synthesis of nanomaterials is one of the recent scientific interests in almost all areas of research. Materials of such interest include powders of metals, metal oxides, ceramics, polymers, semiconductors, etc. The shape of nanomaterials is very wide, which include spherical particles, wires, tubes, rods, etc. The experimental conditions of preparation influence the shape, morphology, and properties of nanoscale materials [1]. Two-dimensional nanostructures of materials are rarely reported. Nanowall materials, which are two-dimensional structures, include carbon nanowalls [2, 3], ZnO nanowalls [4], and GaS and GaSe [5]. Besides nanowalls, which generally stand vertically and are interconnected, nanoflower structures are also reported for ZnO [6].

An electrochemical supercapacitor or ultracapacitor is a charge-storage device, which can be used as an energy source for high-power, short pulse applications [7]. Active materials, which are useful for supercapacitors, include high surface area carbon, hydrated ruthenium dioxide, electronically conducting polymers, etc [8]. Although RuO2·xH2O is extensively investigated owing to its high specific capacitance (about 760 F g−1) [9], its high cost is a discouraging factor for practical supercapacitors. Manganese-based compounds are inexpensive, abundant in nature, and environmentally friendly, and therefore, MnO2 is investigated for this purpose [10]. However, specific capacitance of MnO2 is lower (about 165 F g−1) than that of RuO2·xH2O [11]. MnO2 can be prepared by oxidation of Mn2+ either by an electrochemical route [12] or a chemical route [13]. Decreasing the particle size is considered to be a significant approach for increasing specific capacitance of MnO2 [14]. Studies reported on synthesis of nanoscale MnO2 and its characterization for capacitor properties are scarce [15]. In the present work, MnO2 is prepared with nanopetals morphology by a redox reaction between KMnO4 and MnSO4 in a microemulsion medium, which consists of sodium dodecyl sulphate as a surface active agent.

Materials and methods

Analytical grade or high purity chemicals, namely, KMnO4, MnSO4·H2O, cyclohexane, sodium dodecyl sulphate (SDS, all from Merck), n-butanol (SD Fine Chemicals), and Na2SO4 (BDH) were used for the experiments. Solutions were prepared in doubly distilled water. MnO2 was synthesized from a quaternary microemulsion consisting of water-cyclohexane-SDS-n-butanol. A solution was prepared by mixing 51.2 ml of cyclohexane (oil), 6.2 ml of n-butanol (cosurfactant), and 0.45 g of SDS (surfactant) and stirred well until it became optically transparent. The solution was divided into two equal parts. To one part of the nonaqueous medium, 10 ml of aqueous solution of 0.1 M KMnO4 was added, and to the other part, 10 ml of aqueous solution of 0.15 M MnSO4·H2O was added. Each portion of the emulsion was copiously mixed using a magnetic bar before they were mixed together. The final emulsion was stirred for 12 h to get a dark-brown precipitate. The product was separated and dried at 70 °C for 12 h in air.

For electrochemical characterization, electrodes were prepared on high-purity battery grade Ni foil (0.18 mm thick) as the current collector. A Ni foil of 10 mm width and 80 mm length was sectioned out of a sheet; 2 cm2 area at one end was used to prepare the MnO2-coated electrode, and the rest of its length was used as a tag for electrical connection. The Ni foil was polished with successive grades of emery, cleaned with detergent, washed copiously with doubly distilled water, rinsed with acetone, dried in air, and weighed. MnO2 (70%), acetylene black (20%), and polyvinylidene difluoride (10%) were ground in a mortar; a few drops of 1-methy-2-pyrrolidone was added to form a syrup. It was coated on to the pretreated Ni foil (2 cm2) and dried at 110 °C under vacuum. Coating and drying steps were repeated to get a required loading level of active material (0.5 mg cm−2). Finally, the electrodes were dried at 110 °C under vacuum for 12 h. Average thickness of MnO2 coating was about 40 μm, which was measured using a Digitrix Mark II digital micrometer 901-151 EDl-25. A glass cell, which had provision for introducing MnO2-working electrode, Pt auxiliary electrode and a reference electrode were employed for electrochemical studies. A saturated calomel electrode (SCE) was used as the reference electrode, and potential values are reported against SCE. All electrochemical experiments were done at 20 ± 2 °C.

Powder X-ray diffraction (XRD) patterns were recorded using Philips XRD X’PERT PRO diffractometer using CuKα as a source. Morphology and energy dispersive analysis of X-ray (EDAX) were recorded using FEI Sirion scanning electron microscope (SEM), which was coupled with Oxford Instrument super ultra thin window EDAX. Brunauer–Emmett–Teller (BET) surface area measurements in N2 atmosphere were carried out using SMARTSORB-92/93 surface area analyzer. Sartorious balance of model CP22D-OCE with 10-μg sensitivity was used for weighing electrodes and materials. Electrochemical studies were carried out using a potentiostat/galvanostat EG&G PARC model Versastat II or Solartron model 1286. Thermogravimetric analysis (TGA) was performed for powder MnO2 in the temperature range ambient to 900 °C in air at a heating rate of 10 °C per minute using NETZSCH TG 209 F1 thermogravimetric analyzer.

Results and discussion

In the present synthetic procedure, manganese dioxide is obtained from both the reactants, namely, MnSO4 and KMnO4. Oxidation of Mn2+ by the reduction in Mn7+ results in the formation of Mn4+ from both the reactant species:
$$ 3{\text{ Mn}}^{{{\text{2 + }}}} + 2{\text{ Mn}}^{{7 + }} \to {\text{ }}5{\text{ Mn}}^{{4 + }} $$
(1)
Mn4+ precipitates as MnO2 in an aqueous medium:
$$ {\text{Mn}}^{{4 + }} + 2{\text{ H}}_{{\text{2}}} {\text{O }} \to {\text{MnO}}_{2} {\text{ }} + 4{\text{ H}}^{ + } $$
(2)

In the reaction medium, which is a microemulsion, the manganese salts are present in the aqueous phase, and the surfactant molecules orient with their polar head group towards the aqueous phase and their nonpolar hydrocarbon chain towards the organic phase [16]. Owing to shielding of surfactant molecules, the reaction zone is limited to the microemulsion. The product, namely MnO2, formed possesses a nanostructure.

SEM micrograph (Fig. 1a) shows that the sample consists of petal-like structures distributed uniformly along with some amorphous-like nanoparticles. High magnification SEM image (Fig. 1b) presents a view of nanopetal surface parallel to the field. Unlike the general belief that nanowalls stand vertically and are interconnected, Fig. 1a shows that nanopetals are distributed parallel to the field with a wide separation. The petal surface is smooth, which spreads 1 to 2 μm in length. A rough estimation of petal thickness provides values of approximately in the range of 100–200 nm. The observation of nanopetals of MnO2 obtained in the present study (Fig. 1a,b) is similar to nanoflowers reported for ZnO [6]. Surface area measurements using N2 adsorption on nanopetals of α-MnO2 powder by BET isotherm provided a value of 119 m2 g−1.
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Fig. 1

SEM micrographs (a and b) in different magnifications and EDAX spectrum (c) of MnO2

The samples were subjected to EDAX and XRD studies. The EDAX spectrum (Fig. 1c) indicates the presence of Mn and O as the major elements. Peaks corresponding to C, Na, K, and S are also present. Whereas K is derived from KMnO4, the origin of C, Na, and S is SDS used for synthesis. It is known that MnO2 exhibits polymorphism with α, β, γ, and δ forms, which depend on the conditions of synthesis. The XRD pattern (Fig. 2) was indexed on a tetragonal phase with lattice constants of a = b = 9.802 Å, c = 2.863 Å, and unit cell volume 275.075 Å3 in the space group I4/m (JCPDS no: 44-0141). The lattice constants are in agreement with the reported values [15]. The broad XRD peaks (Fig. 2) reflect nanocrystalline nature of the compound. As the surfactant and potassium salt are present at low concentration, they are not detectable in the XRD studies.
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Fig. 2

XRD pattern of nanopetal MnO2

On subjecting the nanopetal α-MnO2 to a thorough washing in methanol and doubly distilled water followed by drying at 70 °C for 12 h, it was found that the nanopetal morphology collapsed, and nanoparticles of spherical morphology resulted as seen in SEM micrograph (Fig. 3). It is thus concluded that the nanopetal morphology of MnO2 is due to the action of surface active molecules. It is likely that the MnO2 particles are aggregated by adsorption of SDS molecules resulting in the morphology of nanopetals.
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Fig. 3

SEM micrograph of nanoparticles of α-MnO2 obtained after washing

TGA thermogram of nanopetal MnO2 (Fig. 4) shows 29% weight loss below 260 °C, which corresponds to a loss of water, organic reactants, and trace amount of oxygen. Weight loss of about 4% at around 575 °C corresponds to the loss of oxygen from MnO2 lattice resulting in the phase transformation to Mn2O3, and another 3% weight loss at 790 °C corresponds to further loss of oxygen resulting in phase transformation from Mn2O3 to Mn3O4. The nature of TGA thermogram and different phase transitions present in Fig. 4 are in agreement with the literature reported for MnO2 [17].
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Fig. 4

TGA of nanopetal α-MnO2 recorded in air at a heating rate of 10 °C min−1

Nanopetal α-MnO2 electrodes were subjected to electrochemical studies in 0.1 M Na2SO4 aqueous electrolyte [18]. Cyclic voltammograms recorded between 0 and 1.0 V at several sweep rates (Fig. 5a) exhibit rectangular shape. There is a linear increase in voltammetric current density with an increase in sweep rate (Fig. 5b), suggesting capacitive behavior of the electrodes. In addition to the existence of double-layer capacitance, MnO2 possesses pseudocapacitance because of the reversible redox processes between Mn4+ and Mn3+. This process is accompanied by reversible insertion/deinsertion of alkali cation (Na+) and/or proton (H+) present in the electrolyte [19].
$$ {\text{MnO}}_{2} {\text{ }} + {\text{Na}}^{ + } + {\text{e}}^{ - } {\text{ }} \rightleftharpoons {\text{ MnOONa}} $$
(3)
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Fig. 5

a Cyclic voltammograms of MnO2 recorded at 5 (1), 10 (2), 20 (3), 50 (4), and 100 mV s−1 (5) in 0.1 M Na2SO4. b Linear plot of voltammetric current density as a function of sweep rate

Specific capacitance of the electrode was measured by conducting constant current charge–discharge cycling of the electrodes. Typical cycling curves showing variation of electrode potential with time of cycling at a current density (c.d.) of 0.5 mA cm−2 are shown in Fig. 6a. There is a linear variation of potential during both charging and discharging processes. Specific capacitance (SC) is calculated using Eq. 4:
$$ {\text{SC }} = {I{\text{ }}t} \mathord{\left/ {\vphantom {{I{\text{ }}t} {{\left( {\Delta E{\text{ }}m} \right)}}}} \right. \kern-\nulldelimiterspace} {{\left( {\Delta E{\text{ }}m} \right)}} $$
(4)
where I is the discharge (or charge) current, t is the time of discharge (or charge), ΔE (=1.0 V) is the potential window of cycling, and m is the mass of MnO2. Discharge SC obtained from the second cycle (Fig. 6a) is 228.3 F g−1. As the SC of charging process is 240.2 F g−1, the coulombic efficiency of charge–discharge cycling is 0.95 at a c.d. of 0.5 mA cm−2. Cycling of the α-MnO2 electrodes were performed at several c.d.’s in the range from 0.5 to 10 mA cm−2. The variation of SC with c.d. is shown in Fig. 6b. There is a decrease in SC with increase in c.d., which is due to a decrease in efficiency of utilization of the active material. Nevertheless, coulombic efficiency remained at 0.95–0.96 at all c.d.’s used for the experiments. The SC obtained is fairly constant during an extending charge–discharge cycling as shown in Fig. 6c.
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Fig. 6

a Galvanostatic charge–discharge cycles recorded at 0.5 mA cm−2 current density in 0.1 M Na2SO4, b dependence of specific capacitance on current density of galvanostatic charge–discharge cycling, and c cycling data at 0.5 mA cm−2 for 100 cycles

Conclusions

MnO2 synthesized from a redox reaction between KMnO4 and MnSO4 in a microemulsion medium consisting of sodium dodecyl sulphate crystallizes in nanopetals morphology with α crystallographic structure. The electrochemical properties of α-MnO2 in 0.1 M Na2SO4 medium support its use for supercapacitor application. High values of specific capacitance are obtained.

Acknowledgments

Authors thank Dr. H. N. Vasan for discussions, Mr. P. Ragupathy for XRD, and Mr. K. C. Suresh for surface area measurements. One of the authors (SD) acknowledges the senior research fellowship from the Council of Scientific and Industrial Research, New Delhi, India.

Copyright information

© Springer-Verlag 2007