Sonochemical synthesis, characterization, and electrochemical properties of MnMoO4 nanorods for supercapacitor applications
Graphical abstract
Introduction
Supercapacitors, also called as electrochemical capacitors (ECs) that have attracted much attention due to their high power density, low maintenance cost, long cycle life and its applications in renewable energy sources, unbroken power supplies and hybrid electrical vehicle systems [1], [2]. In general, ECs can be classified into two types such as electrical double layer capacitance (EDLCs) and pseudocapacitance based on the mechanism of charge storage [3]. In the former case, charge builds up at the electrode and electrolyte interfaces with reversible electrolyte ion movement and adsorption–desorption resulting the capacitance. In the later case, charge storage arises from the reversible redox reactions at the electrode surface [4]. Carbon materials such as activated carbon, mesoporous carbon, carbon nanotubes and graphene nanosheets have been used for EDLCs [5], [6]. Several transition metal oxide/hydroxides nanostructures have been investigated for pseudocapacitance applications [6]. In recent years, researches on pseudocapacitive materials such as Ni(OH)2, Co(OH)2, RuO2, MnO2, Co3O4, NiO, CuO have been investigated [7], [8], [9], [10], [11], [12], [13]. Eventhough, these materials possess higher specific capacitance, there are several demerits such as high cost and toxicity of RuO2, and poor electrical conductivity of MnO2 which limits their applications in commercialization [14], [15].
In recent years, the researchers focused on developing novel pseudocapacitive materials for improving the performance of the supercapacitors. The studies on nanostructured metal molybdates showed superior electrochemical performances [16], [17], [18], [19], [20]. Manganese molybdate (MnMoO4) is one of the metal molybdate nanostructures with excellent electrochemical properties. Considering the recent works performed on the supercapacitive behavior of MnMoO4, only limited reports are available. Mai et al., reported the hydrothermal synthesis of MnMoO4 nanowires with specific capacitance 9.7 F g−1 and improved their specific capacitance into 187 F g−1 at 1 A g−1 of current density by forming hierarchical structures of MnMoO4/CoMoO4 nanowires which is due to the high surface/body ratio [17]. Senthilkumar et al. reported synthesis of MnMoO4 by solution combustion method with specific capacitance of 126 F g−1 at 5 mA cm−2 of current density [19]. However, the development of molybdate materials with simple and reliable synthetic method still remains massive challenge [21]. To best of our knowledge, there are no reports available on the synthesis of MnMoO4 nanorods using facile sonochemical method towards electrochemical capacitor applications. The aim of this study is to examine the electrochemical properties of sonochemically synthesized α-MnMoO4 nanorods for supercapacitor applications.
Section snippets
Materials and methods
Manganese acetate tetrahydrate (Mn(CH3COO)2.4H2O), absolute alcohol (C2H5OH) were purchased from Daejung chemicals, South Korea. Sodium molybdate (Na2MoO4) was purchased from Sigma Aldrich Ltd. All chemicals used in this research were research grade and doubly distilled water was used further purification. The ultrasonic sound was carried out on a SONIX throughout this experiment without VCX 750 model (20 kHz, 750 W) using a direct immersion Titanium horn.
Synthesis of MnMoO4 nanorods
The α-MnMoO4 nanorods are prepared via
Results and discussion
In this study, α-MnMoO4 nanorods was prepared by a facile sonochemical method using manganese acetate and sodium molybdate as starting precursors. Under sonochemical conditions, the molybdate and manganese ions reacts with each other which results in the formation of MnMoO4.xH2O. It is known that achieving pure metal molybdates via wet chemistry approach is difficult due to the formation of hydrates. In order to achieve the pure MnMoO4, the MnMoO4.xH2O is calcined at 500 °C which further leads
Conclusion
In conclusion, a facile sonochemical approach for the preparation of α-MnMoO4 nanorods has been demonstrated. The XRD analysis, Raman spectrum, and HR-TEM studies revealed the high crystallinity of the prepared α-MnMoO4 nanorods. The electrochemical studies using CV and EIS measurements suggested the pseudocapacitive nature of the α-MnMoO4 nanorods. A maximum specific capacitance value of 168.32 F g−1 was obtained from the galvanostatic charge discharge analysis at the current density of
Acknowledgments
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013R1A1A2064471) and the 2014 Jeju Sea Grant College Program funded by the Ministry of Land, Transport and Maritime Affairs (MLTM), Korea. A part of S. -J. Kim's research was supported by the SBS Cultural Foundation.
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