Ultrasonics Sonochemistry 21 (2014) 1933–1938

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Sonochemically synthesized MnO2 nanoparticles as electrode material for supercapacitors Balasubramaniam Gnana Sundara Raj a, Abdullah M. Asiri b, Abdullah H. Qusti b, Jerry J. Wu c, Sambandam Anandan a,⇑ a b c

Nanomaterials and Solar Energy Conversion Lab, Department of Chemistry, National Institute of Technology, Trichy 620 015, India Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21413, P.O. Box 80203, Saudi Arabia Department of Environmental Engineering and Science, Feng Chia University, Taichung 407, Taiwan

a r t i c l e

i n f o

Article history: Received 21 October 2013 Received in revised form 26 November 2013 Accepted 28 November 2013 Available online 8 December 2013 Keywords: Sonochemical synthesis Amorphous materials X-ray diffraction Electrochemical properties Energy storage Supercapacitor

a b s t r a c t In this study, manganese oxide (MnO2) nanoparticles were synthesized by sonochemical reduction of KMnO4 using polyethylene glycol (PEG) as a reducing agent as well as structure directing agent under room temperature in short duration of time and characterized by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscope (SEM), Transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET) analysis. A supercapacitor device constructed using the ultrasonically-synthesized MnO2 nanoparticles showed maximum specific capacitance (SC) of 282 Fg1 in the presence of 1 M Ca(NO3)2 as an electrolyte at a current density of 0.5 mA cm2 in the potential range from 0.0 to 1.0 V and about 78% of specific capacitance was retained even after 1000 cycles indicating its high electrochemical stability. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Electrochemical capacitors (ECs) or supercapacitors have attracted increasing attention in recent years due to their higher power density and longer cycle life than batteries and higher energy density than conventional capacitors [1]. Accordingly, they have been utilized in a wide range of applications such as consumer electronics, memory back-up systems and hybrid electric vehicles [2,3]. Electrochemical capacitors bridge the gap between batteries and conventional capacitors. Depending on the charge storage mechanism, electrochemical capacitors are basically classified into two types as electrical double layer capacitor (EDLC) and pseudo capacitors. In the EDLC, capacitance arises as a result of charge separation at the electrode–electrolyte interface, whereas the charge transfer from reversible faradaic reactions takes place at the electrode surface of pseudo capacitance. Many researchers have been focusing on the development of electrode active materials with improved electrochemical properties [4–6]. Generally, high surface area carbons [7], conducting polymers [8] and transition metal oxides are used as electrode active materials for supercapacitors [9–12]. Various transition-metal oxides have been shown to be excellent electrode active materials

⇑ Corresponding author. Tel.: +91 431 2503639; fax: +91 431 2500133. E-mail addresses: [email protected], [email protected] (S. Anandan). 1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.11.018

due to their chemical stability, variable valence etc. [13]. Manganese oxide (MnO2) acts as an attractive electrode active material for supercapacitors because of its structure flexibility, long cycle life, environmental compatibility and low cost [14–16]. The performance of MnO2 depends on different synthetic methods owing to its crystal structure, particle size and morphology. MnO2 usually has low intrinsically electronic conductivity and clustered morphology [17]. MnO2 nanoparticles were synthesized by different methods including co-precipation [18,19], micro emulsion [20], sol–gel [21], sonochemical [22], hydrothermal [23] and electrochemical methods [14,24]. Sonochemical method is a useful technique for synthesising of nanostructured metal oxide materials at room temperature, ambient pressure and short reaction times. The benefits of sonochemistry, in creating nanostructures materials arise principally from acoustic cavitation; the formation, growth, and implosive collapse of bubbles in a liquid. Bubble collapse stimulated by cavitation produces intense local heating and high pressures [25]. The storage capacity of MnO2 electrolytes with univalent cations has been accounted by one electron transfer processes. However, studies with electrolytes containing polyvalent cations are scarce. The specific capacitance of MnO2 in electrolytes containing bivalent cations is greater than that of univalent cation [26,27]. Recently, a high specific capacitance of 283 Fg1 was reported for MnO2 in bivalent cation (Ca2+) containing electrolytes, whereas 188 Fg1 was reported in monovalent cation (Na+) containing electrolytes [28].

B. Gnana Sundara Raj et al. / Ultrasonics Sonochemistry 21 (2014) 1933–1938

In this investigation, we followed a simple route to prepare MnO2 nanoparticles using a sonochemical method by reduction of KMnO4 using PEG as a reducing agent as well as structure directing agent under room temperature in short reaction time. The supercapacitive behavior of the product was evaluated in aqueous electrolytes containing bivalent cations, which showed that the ultrasonically prepared MnO2 nanoparticles exhibited remarkable capacitive behavior in bivalent cation containing electrolytes. 2. Experimental section 2.1. Materials Reagent-grade KMnO4, Ca(NO3)2 (MERCK) and Poly Ethylene Glycol (PEG; mw: 55,000) (Aldrich), Vulcan XC-72, poly-vinylidene fluoride (PVdF) and N-methyl-2-pyrrolidone (NMP) were used. All solutions for the experiment were prepared with doubly distilled (DD) water. 2.2. Material preparation MnO2 was prepared by reduction of KMnO4 with PEG aided by sonication. A horn type 20 kHz Sonics sonifier (100 W/cm2) with a tip diameter of 13 mm was used. Typically, 0.5 g of KMnO4 was dissolved in 60 ml of double distilled (DD) water and then 5.5 g of PEG was added with continuous stirring for 5 min in a 100 ml sonochemical glass vessel and then ultrasonicated for 20 min. After 20 min, brown color precipitate was formed. Then the precipitate was filtered, washed in DD water repeatedly for several times followed by ethanol washing three times and dried in an air oven at 70 °C for 8 h. 2.3. Characterization In order to confirm the crystal structure and phase purity of the product, powder X-ray diffraction patterns were recorded on a Philips XPertPro X-ray diffractometer with Cu Ka (k = 0.15418 nm) radiation. FT-IR spectra were obtained using a BRUKER Optik GmbH MODEL TENSOR 27 FT-IR spectrometer with a detector RT DLaTGS using a KBr pellet. The surface property measurements Nitrogen adsorption–desorption was carried out by using micromeritics surface area analyzer model ASAP 2020. The morphology and structural properties of the as-prepared manganese oxides were studied using a JEOL 7401F and JEOL JEM 2010 model.

electrode in grams and DE is the operating potential window in volts of charge or discharge. 3. Results and discussion As mentioned in the experimental details, sonochemical preparation processes is followed for the synthesis of MnO2 nanoparticles based only on the redox reactions between potassium permanganate and polyethylene glycol (PEG) without any other additives such as templates or surfactants, under mild conditions (room temperature) in short duration of time (20 min) [29,30]. PEG is used as a structure directing agent used for the preparation of porous nanomaterials [31]. Using PEG, the nanostructured materials such as nanoparticles, nanowires, nanorods, etc. are formed due to the presence of both hydrophilic and hydrophobic groups which can form micelles in aqueous solutions [32]. During the ultrasonic irradiation, the organic radicals (glycols or aldehydes) that are generated during pyrolysis of PEG due to the extreme temperature conditions generated within the cavitation bubbles lead to the reduction of KMnO4 to MnO2 [33,34]. Fig. 1. shows the XRD pattern of the as prepared MnO2 show broad peaks positions at 2h values 12.6°, 37.2° and 66° matches well with the d-MnO2

(001)

Intensity / a.u.

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(111)

(005)

10

20

30

40

50

60

2 θ / degree

70

80

Fig. 1. XRD spectrum of sonochemically prepared MnO2 nanoparticles.

2.4. Electrode fabrication and electrochemical characterization

SC ¼ It=m DE

ð1Þ

where I is the charge–discharge current in amps, t is the discharge time in seconds, m is the mass of the active material present on the

υ

(O-H) bend

% Transmittance

For electrochemical characterization methods, electrodes were prepared on high-purity stainless steel plate as a current collector. The plate was polished with successive grades of emery paper, cleaned with soap solution, washed with DD water, rinsed with acetone, dried and weighed. As prepared MnO2 (75 wt.%) as an active material, Vulcan XC-72 carbon (20 wt.%) as a conductive agent, PVdF (5 wt.%) as a binder were ground in a mortar, and a few drops of NMP was added to form slurry. It was coated onto the pretreated SS plate (coating electrode area is 1.0 cm2) and dried at 100 °C under vacuum for 12 h. Electrochemical studies were carried out using a potentiostat/galvanostat (AUTOLAB 302 N module) in a three-electrode system with the MnO2 coated plate as the working electrode, Pt foil as the counter electrode and Ag/AgCl as the reference electrode. The discharge specific capacitance (SC) of MnO2 was calculated using the formula

4000

υ

(O-H) str

υ

(Mn-O)

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 2. FT-IR spectrum of sonochemically prepared MnO2 nanoparticles.

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Fig. 3. SEM (a), TEM (b), HRTEM (c), SAED (d) and EDX (e) images of sonochemically prepared MnO2 nanoparticles.

(a)

(b)

0.012 0.010

300

0.008

2

Pore area m /g.nm

3

Quantity Adsorbed (cm /g STP)

400

200

100

0.006 0.004 0.002 0.000

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (p/p°)

0

10

20

30

40

50

60

70

80

90

100

Pore diameter / nm

Fig. 4. (a) N2 adsorption–desorption isotherms and (b) BJH pore-size distribution curve of sonochemically prepared MnO2 nanoparticles.

JCPDS value (JCPDS No. 80-1098). All the above reflections can be indexed to the corresponding crystal planes ((h k l) (0 0 1), (1 1 1)

and (0 0 5)) [35,36]. The peaks broadening suggest that the sample is poorly crystalline in nature.

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0.016 0.016

f

-2

Current density / A cm

0.008

Current density / A cm

-2

0.012

0.012

0.008 0.004

e

0.000 -0.004 -0.008 0.0

0.2

0.4

0.6

0.8

1.0

d

Potential / V vs. Ag/AgCl

0.004

c b a

0.000 -0.004 -0.008 0.0

0.2

0.4

0.6

0.8

1.0

Potential / V vs. Ag/AgCl Fig. 5. CV for sonochemically prepared MnO2 nanoparticles at different scan rates 5 mV s1,10 mV s1, 20 mV s1, 40 mV s1, 80 mV s1, 160 mV s1 in the potential range of 0.0–1.0 V vs. Ag/AgCl in aqueous solution of 1 M Ca(NO3)2 electrolyte (a–f). Inset shows the fitted rectangular shape in the cyclic voltammetric behavior.

The FT-IR spectrum for the as-prepared MnO2 nanoparticles is shown in Fig. 2. The absorption band at 525 cm1 can be assigned to Mn–O bending vibrations, arise from MnO6 octahedron vibra-

(a)

(b)

1.0 Potential / V vs. Ag/AgCl

1.0

Potential / V vs. Ag/AgCl

tion mode. The observed broad band around 3400 cm1 is attributed due to hydroxyl stretching vibrations and the weak band around 1630 cm1 corresponds to the bending vibrations of the OH group, which are related to adsorbed crystalline water molecules [37]. The morphology of the as prepared MnO2 nanoparticles is spherical as suggested by SEM and TEM micrographs (Fig. 3a and b). The size range of the particles is in the range from 10 to 20 nm. From high resolution transmission electron microscopy (HRTEM) image (Fig. 3c), lattice fringes are clearly seen and match well with the MnO2 planes. The amorphous pattern is viewed in the selected area electron diffraction (SAED) (Fig. 3d). The chemical composition of the as prepared MnO2 was determined by Energydispersive X-ray spectroscopy (EDX) analysis (Fig. 3e) indicates the presence of pure manganese and oxygen as elements in the nanoparticles. The nitrogen adsorption/desorption isotherm and pore-size distribution were measured for the prepared MnO2 nanoparticles and it is shown in Fig. 4. The observed Brunauer–Emmett–Teller (BET) surface area of the sample is 60 m2 g1 and it shows type IV hysteresis loops for a typical mesoporous structure as defined by the IUPAC [38]. The Barrett–Joyner–Halenda (BJH) analysis shows a narrow pore-size distribution of about 10–14 nm. The BET surface area is not unique influence parameter for the capacitance, but an

0.8

0.6

0.4

0.2

st 1 cycle th 500 cycle th 1000 cycle

0.8 0.6 0.4 0.2 0.0

0.0 0

1000

2000

3000

4000

5000

0

6000

100

Time / s

200

300

400

500

600

Time/s

Specific Capacitance / Fg

-1

300

(c)

250

200

150

100 0

200

400

600

800

1000

Cycle numbers Fig. 6. (a) Charge–discharge cycles of sonochemically prepared MnO2 nanoparticles in aqueous solution of 1 M Ca(NO3)2 at a current density of 0.5 mA cm2 between 0.0 and 1.0 V vs. Ag/AgCl. Area of the electrode: 1.0 cm2, (b) charge–discharge curves of sonochemically prepared MnO2 nanoparticles recorded on 1st, 500th and 1000th cycles, and (c) cycling behavior of sonochemically prepared MnO2 nanoparticles.

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increase tendency of the specific capacitance is connected with the relative large BET surface area [39,40]. For electrochemical measurements, cyclic voltammetry and charge–discharge cycling experiments were performed in aqueous electrolytes. There are two mechanisms proposed for charge storage in MnO2 nanoparticles [41,42]. The first step is based on the intercalation/deintercalation of protons (H+) or alkali metal cations (M+) such as Li+ in the bulk of the material upon reduction/oxidation of Mnn+ into MnO2. þ

MnO2 þ Mþ þ e ¢ MnOOMðMþ ¼ Li ; Naþ ; Kþ Þ

ð2Þ

The second step is based on the surface process, which involves the adsorption/desorption of electrolyte cations (M+) on MnO2.

ðMnO2 Þsurface þ Mþ þ e ¢ ðMnO2 Mþ Þsurface

ð3Þ

Electrochemical studies of MnO2 nanoparticles were reported using several electrolytes and as it is known that MnO2 nanoparticles exhibit higher SC in Ca (NO3)2 electrolyte than in the conventional Na2SO4 electrolyte because it contains bivalent cations which can reduce two Mn4+ to Mn3+ ions [26,27,43], so the electrochemical capacitance properties of MnO2 was studied in 1 M Ca (NO3)2 electrolyte. Fig. 5 shows cyclic voltammetric data of as prepared MnO2 nanoparticles electrode at various scan rates from 5 to 160 mV s1 in the potential range between 0.0 and 1.0 V vs. Ag/AgCl in aqueous 1 M Ca(NO3)2 solution. The rectangular shape of voltammogram (see inset Fig. 5.) without any redox current peaks indicates the ideal capacitive behavior. The size of the voltammogram increases with an increase in the sweep rate which indicates that the voltammetric currents are directly proportional to the scan rate [44]. At the low-scan rate, the ions from the electrolyte can occupy all the available sites in the active electrode material, because the ions have enough time to diffuse into all the sites which leads to the higher capacitance. On the other hand, at high-scan rate, the ions from electrolyte confront the difficulty to access all the available sites in the active electrode material due to their partial rate of movement in the electrolyte [45]. As prepared MnO2 nanoparticles were subjected to galvanostatic charge–discharge cycling studies. Charge–discharge curves recorded at a current density (c.d) of 0.5 mA cm2 in the potential range from 0.0 to 1.0 V in 1 M Ca(NO3)2 as electrolyte solution are shown in Fig 6a. There is a linear variation of potential with time during charging and discharging processes, which is another criterion for capacitive behavior of as prepared MnO2 electrode material. The initial value of discharge capacitance in Ca (NO3)2 was 282 Fg1. Ragupathy et al. report that the capacitive behavior of amorphous manganese oxides with short range crystallinity is predominantly due to the surface adsorption– desorption of cations rather than to insertion/deinsertion [46], which yield lower SC value for MnO2 nanoparticle while compared to its theoretical value. However, the higher SC in the bivalent cation system may possibly be assigned to the bivalent electrolyte cation reducing two Mn4+ to Mn3+, which means a doubling of the number of electrons involved compared to that with a univalent cation-containing electrolyte system [28,34]. The observed specific capacitance values are comparatively high when compared with the literature values especially, 169 Fg1 at a current density of 250 mA g1 in 1 M Na2SO4 reported by Yu et al. [47]. Jiang et al. reported that the specific capacitance of MnO2 electrode is increased from 77 to 176 Fg1 in the presence of added surfactant P123, since the particle size of MnO2 was decreased [37]. Fig. 6b presents charge–discharge curves of as prepared MnO2 recorded on 1st, 500th and 1000th cycles. The stability of the as prepared MnO2 was studied by charge–discharge cycling at a current density of 0.5 mA cm2 (Fig. 6c). The discharge capacitance falls from an initial value of

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282–262 Fg1 after 100 cycles. The SC of MnO2 after 500 cycles was 247 Fg1, which is 87% of the initial capacitance. At the end of 1000 cycles, 78% of the initial specific capacitance (220 Fg1) is retained indicates its high electrochemical stability. 4. Conclusions In this work, a simple sonochemical route is followed for the synthesis of MnO2 nanoparticles using PEG as a reducing agent and studied its impact on structure, morphology and electrochemical performance. XRD results indicate that the prepared MnO2 nanoparticle is poorly crystalline in nature. The morphology study shows that the particles are spherical in shape and the size ranges from 10 to 20 nm. The electrochemical performance of MnO2 delivers a maximum SC of 282 Fg1 at a current density of 0.5 mA cm2 in 1 M Ca(NO3)2 electrolyte. The higher SC of MnO2 was assigned to the narrow pore size distribution with small pores and using an electrolyte containing bivalent cations. The prepared MnO2 nanoparticle exhibits high electrochemical stability suggesting that it is a suitable electrode material for electrochemical supercapacitors. Acknowledgment The research work was financially supported by Council of Scientific and Industrial Research (CSIR), New Delhi (CSIR Reference No. 02 (0021)/11/EMR-II). References [1] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Acedamic/Plenum Publishers, New York, 1999. [2] J.R. Miller, A.F. Burke, Electrochemical capacitors: challenges and opportunities for real-world applications, Electrochem. Soc. Interface 17 (2008) 53–57. [3] L.T. Lam, R. Louey, Development of ultra-battery for hybrid-electric vehicle applications, J. Power Sources 158 (2006) 1140–1148. [4] S. Sarangapani, B.V. Tilak, C.P. Chen, Materials for electrochemical capacitors. Theoretical and experimental constraints, J. Electrochem. Soc. 143 (1996) 3791–3799. [5] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845–854. [6] G. Yu, X. Xie, L. Pan, Z. Bao, Y. Cui, Hybrid nanostructured materials for highperformance electrochemical capacitors, Nano Energy 2 (2013) 213–234. [7] L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chem. Soc. Rev. 38 (2009) 2520–2531. [8] G.A. Snook, P. Kao, A.S. Best, Conducting-polymer-based supercapacitor devices and electrodes, J. Power Sources 196 (2011) 1–12. [9] M.T. Brumbach, T.M. Alam, P.G. Kotula, B.B. McKenzie, B.C. Bunker, Nanostructured ruthenium oxide electrodes via high-temperature molecular templating for use in electrochemical capacitors, ACS Appl. Mater. Interfaces 2 (2010) 778–787. [10] E. Beaudrouet, A.L.G.L. Salle, D. Guyomard, Nanostructured manganese dioxides: synthesis and properties as supercapacitor electrode materials, Electrochim. Acta 54 (2009) 1240–1248. [11] S. Anandan, B. Gnana Sundara Raj, G.J. Lee, J.J. Wu, Sonochemical synthesis of manganese (II) hydroxide for supercapacitor applications, Mater. Res. Bull. 48 (2013) 3357–3361. [12] K. Liang, X. Tang, W. Hu, High-performance three-dimensional nanoporous NiO film as a supercapacitor electrode, J. Mater. Chem. 22 (2012) 11062– 11067. [13] J.P. Zheng, P.J. Cygan, T.R. Jow, Hydrous ruthenium oxide as an electrode material for electrochemical capacitors, J. Electrochem. Soc. 142 (1995) 2699– 2703. [14] S.C. Pang, M.A. Anderson, T.W. Chapman, Novel electrode materials for thinfilm ultra capacitors: comparison of electrochemical properties of sol–gelderived and electrodeposited manganese dioxide, J. Electrochem. Soc. 147 (2000) 444–450. [15] Z.Q. Li, Y. Ding, Y.J. Xiong, Y. Xie, Rational growth of various a-MnO2 hierarchical structures and b-MnO2 nanorods via a homogeneous catalytic route, Cryst. Growth Des. 5 (2005) 1953–1958. [16] D. Zheng, S. Sun, W. Fan, H. Yu, C. Fan, G. Cao, Z. Yin, X. Song, One-step preparation of single-crystalline b-MnO2 nanotubes, J. Phys. Chem. B 109 (2005) 16439–16443. [17] S. Devaraj, N. Munichandraiah, High capacitance of electrodeposited MnO2 by the effect of a surface-active agent, Electrochem. Solid-State Lett. 8 (2005) A373–A377.

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