Sonochemically synthesized MnO2 nanoparticles as electrode material for supercapacitors
Graphical abstract
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], [5], [6]. Generally, high surface area carbons [7], conducting polymers [8] and transition metal oxides are used as electrode active materials for supercapacitors [9], [10], [11], [12]. Various transition-metal oxides have been shown to be excellent electrode active materials 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], [15], [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 Fg−1 was reported for MnO2 in bivalent cation (Ca2+) containing electrolytes, whereas 188 Fg−1 was reported in monovalent cation (Na+) containing electrolytes [28].
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.
Section snippets
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.
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
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
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 Fg−1 at a current density of
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).
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