Synthesis, characterization, and electrochemical properties of CoMoO4 nanostructures
Graphical abstract
Introduction
Electrochemical capacitors, also called supercapacitors or ultracapacitors, are considered to be emerging energy-storage devices due to their high power density and long cycling life [1]. In general, there are two mechanisms for charge storage in electrochemical supercapacitors: (i) electrical double-layer capacitance (EDLC) and (ii) pseudocapacitance [2]. EDLC is due to reversible electrolyte ion adsorption at the electrode/electrolyte interface; pseudocapacitance is due to redox reactions at the electrode surface [3]. Carbonaceous materials, such as activated carbon, mesoporous carbon, carbon nanotubes (CNTs), and graphene nanosheets, have all been widely investigated for EDLC devices [4], [5]. In contrast, several metal oxide/hydroxides with various nanostructured morphologies have been investigated for pseudocapacitance applications [5]. Extensive research has focused on materials for pseudocapacitors due to their high energy density compared to carbon-based EDLC devices [6]. A variety of transition metal oxides, such as RuO2, MnO2, CuO, NiO, MoO3, MoO2, Co3O4, and their corresponding hydroxides, having various nanostructures, such as nanoparticles, nanosheets, nanowires, nanorods, and hierarchical structures, have been investigated for their pseudocapacitance behavior over the last decade [7], [8], [9]. Although these materials possess higher specific capacitances, they have several disadvantages, such as the high cost and toxicity of RuO2 and poor electrical conductivity of MnO2, which limit their use in commercial applications [10], [11]. The increasing demand for energy-storage devices has motivated researchers to develop novel materials for this application.
Recently, research has focused on the development of novel nanostructures that are environmentally benign and possess enhanced electrochemical properties [12]. With respect to these criteria, metal molybdate nanostructures are well suited for energy-storage devices because they are environmentally safe and exhibit enhanced performance compared to their corresponding oxides. Nanostructured NiMoO4, CoMoO4, MnMoO4, and their hierarchical structures showed superior electrochemical performance [13], [14], [15]. Electrochemical studies based on metal molybdates motivated us to examine the detailed supercapacitive behavior of CoMoO4 nanostructures. CoMoO4 is advantageous because it is low cost, non-toxic, and exhibits enhanced electrochemical properties [16], [17]. There have been few reports on the supercapacitive behavior of CoMoO4. Mai et al. reported the hydrothermal synthesis of CoMoO4 nanowires with a specific capacitance 62.8 F g−1 and improved the specific capacitance to 187 F g−1 by forming hierarchical structures of MnMoO4/CoMoO4 nanowires; the improvement was due to the high surface-to-volume ratio [14]. Xu et al. demonstrated that CoMoO4/CNTs composites possess a specific capacitance of 170 F g−1 [18]. Recently, Xia et al. reported hydrothermally synthesized CoMoO4 nanoparticles with a specific capacitance of 72 F g−1 [19]. Recent studies by other groups also demonstrated that CoMoO4·H2O has a higher specific capacitance than pure CoMoO4 [17], [20].
Various synthetic strategies, such as hydrothermal synthesis, microwave-assisted synthesis, and wet chemical routes, have been used to achieve CoMoO4 nanostructures [13], [14], [18]. It is known that synthesis method, starting materials, and reaction parameters play a vital role in the physico-chemical properties of nanomaterials, which can strongly influence their optical, electrical, and electrochemical properties [1]. Moreover, the size, shape, and surface effects of nanostructures can also significantly alter their electrochemical properties [21]. Recently, sonochemical synthesis has become a popular approach for the fabrication of nanostructured materials, such as metals, metal oxides, metal chalcogenides, and graphene [22], [23]. In this study, we used a facile sonochemical approach for the synthesis of CoMoO4 nanostructures. The advantage of sonochemical synthesis, compared to conventional methods, is the acoustic cavitation phenomenon (i.e., the formation, growth, and collapse of bubbles in liquid medium) [24]. The reaction conditions used in the sonochemical approach, including high temperature (5000 K), pressure (20 MPa), and cooling rate (1010 K s−1), provide a large number of reactive sites, which are typically not available during conventional reactions. This results in the unique properties of the synthesized nanomaterials [25], [26]. To our knowledge, this is the first report on the sonochemical synthesis of CoMoO4 nanostructures.
In this study, we used a facile sonochemical approach for the synthesis of CoMoO4 nanostructures and investigated their electrochemical properties for supercapacitor applications. Techniques such as cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge cycles were used to study the electrochemical properties of the prepared CoMoO4.
Section snippets
Materials and methods
Sodium molybdate (Na2MoO4) was purchased from Sigma–Aldrich Ltd., South Korea. Cobalt chloride hexahydrate (CoCl2·6H2O) and methanol were purchased from Daejung chemicals Ltd., South Korea. All chemicals were of research grade, and double-distilled water was used throughout the experiments. Ultrasound irradiation (US) was carried out on a SONIX VCX 750 (20 kHz, 750 W) using a direct-immersion titanium horn.
Synthesis of CoMoO4 nanostructures
The CoMoO4 nanostructures were synthesized by a facile sonochemical approach using sodium
Results and discussion
In this study, a facile sonochemical approach was used for the synthesis of CoMoO4 nanostructures. Solutions of sodium molybdate and cobalt chloride were mixed under US, resulting in the formation of CoMoO4·H2O. It is difficult to achieve pure metal molybdate using a wet chemistry approach due to the formation of hydrates by the intercalation of water molecules between the interlayers. To achieve pure CoMoO4, the CoMoO4·H2O was calcined at 500 °C, forming CoMoO4 as follows:CoCl2·6H2O + Na2MoO4
Conclusions
In conclusion, CoMoO4 nanostructures were successfully synthesized by a facile sonochemical approach, and the structure, morphology, and bonding nature were investigated. The electrochemical studies, including CV and EIS analysis, demonstrated the pseudocapacitance nature of the prepared CoMoO4 nanostructures. A maximum specific capacitance of ∼133 F g−1 was obtained during discharge cycles at a constant discharge current density (1 mA cm−2). Nearly 62% of the specific capacitance was retained
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).
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