Synthesis, characterization, and electrochemical properties of CoMoO4 nanostructures

https://doi.org/10.1016/j.ijhydene.2014.01.069Get rights and content

Highlights

  • Nano CoMoO4 was synthesized by sonochemical method.

  • FE-SEM studies show the plate like morphology of CoMoO4.

  • XRD studies show the presence of monoclinic phase of CoMoO4.

  • Specific capacitance of 133 F g−1 was achieved using charge–discharge analysis.

  • CoMoO4 electrode shows capacitance retention of about 84% after 1000 cycles.

Abstract

We report a facile sonochemical approach for the synthesis of cobalt molybdate (CoMoO4) nanostructures and their application as electrodes for supercapacitors. X-ray diffraction analysis showed the formation of monoclinic CoMoO4. The surface morphology was investigated using a field-emission scanning electron microscope, which showed the formation of plate-like CoMoO4 nanostructures. The growth mechanism and formation of the CoMoO4 nanostructures is discussed. Further, the electrochemical performance of the CoMoO4 nanostructures was examined using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge analysis. The CV curves showed the presence of redox pairs and, along with the EIS data (using Nyquist and Bode plots), demonstrated the pseudocapacitance nature of the synthesized CoMoO4. The galvanostatic studies showed non-symmetric discharge curves, and a maximum specific capacitance of ∼133 F g−1 was obtained at a constant discharge current density (1 mA cm−2). The cyclic stability tests demonstrated capacitance retention of about 84% after 1000 cycles, suggesting the potential application of CoMoO4 in energy-storage devices.

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).

References (47)

  • C. Deng et al.

    One-pot sonochemical fabrication of hierarchical hollow CuO submicrospheres

    Ultrason Sonochem

    (2011)
  • V. Safarifard et al.

    Sonochemical syntheses of a nano-sized copper(ІІІ) supramolecule as a precursor for the synthesis of copper(ІІІ) oxide nanoparticles

    Ultrason Sonochem

    (2012)
  • G. Kianpour et al.

    Precipitation synthesis and characterization of cobalt molybdate nanostructures

    Superlattice Microst

    (2013)
  • A. Paravannoor et al.

    Chemical and structural stability of porous thin film NiO nanowire based electrodes for supercapacitors

    Chem Eng J

    (2013)
  • K. Liu et al.

    Electropolymerization of high stable poly(3,4-ethylenedioxythiophene) in ionic liquids and its potential applications in electrochemical capacitor

    J Power Sources

    (2008)
  • K.K. Purushothaman et al.

    Supercapacitor behavior of α-MnMoO4 nanorods on different electrolytes

    Mater Res Bull

    (2012)
  • K. Krishnamoorthy et al.

    Supercapacitive properties of hydrothermally synthesized sphere like MoS2 nanostructures

    Mater Res Bull

    (2014)
  • J. Yan et al.

    High performance supercapacitor electrodes based on highly corrugated graphene sheets

    Carbon

    (2012)
  • I. Shakir et al.

    Structural and electrochemical characterization of α-MoO3 nanorod-based electrochemical energy storage devices

    Electrochim Acta

    (2010)
  • L. Wang et al.

    Preparation and electrochemical properties of mesoporous Co3O4 crater-like microspheres as supercapacitor electrode materials

    Curr Appl Phys

    (2010)
  • V.D. Patake et al.

    Electrodeposited porous and amorphous copper oxide film for application in supercapacitor

    Mater Chem Phys

    (2009)
  • J. Xu et al.

    Preparation and electrochemical capacitance of cobalt oxide (Co3O4) nanotubes as supercapacitor material

    Electrochim Acta

    (2010)
  • J. Yan et al.

    Rapid microwave assisted synthesis of graphene nanosheet/Co3O4 composite for supercapacitor

    Electrochim Acta

    (2010)
  • Cited by (118)

    View all citing articles on Scopus
    View full text