Elsevier

Ceramics International

Volume 46, Issue 8, Part A, 1 June 2020, Pages 10893-10902
Ceramics International

Construction of polypyrrole-wrapped hierarchical CoMoO4 nanotubes as a high-performance electrode for supercapacitors

https://doi.org/10.1016/j.ceramint.2020.01.103Get rights and content

Abstract

For the creation of advanced electrochemical energy storage devices, a large challenge still remains in the designing and engineering of active electrodes with tailored nanoarchitectures and components that provide optimized electrochemical performances. In this study, CoMoO4@polypyrrole nano-heterostructures (NHs) are constructed by wrapping a polypyrrole (PPy) shell around the surface of CoMoO4 nanotubes (NTs) using a self-templated reaction and a subsequent in situ gas-phase polymerization reaction. CoMoO4 NTs possess a large amount of electroactive sites, short ion diffusion pathways, and provide sufficient buffering space. The PPy shell, on the other hand, is conductive, thereby allowing for efficient electron transport and fast charge transfer kinetics. By using their respective advantageous qualities for energy storage, along with the synergistic effect between the CoMoO4 NTs and PPy shell, the CoMoO4@PPy NHs electrode demonstrated improved specific capacitances of 1203 F g−1 at 2 A g−1 and 974 F g−1 at 20 A g−1, as well as 96% capacitance retention after 5000 cycles at 10 A g−1. Furthermore, asymmetric supercapacitor (ASC) fabricated using the CoMoO4@PPy//N-doped carbon NTs (N-CNTs) provided an energy density of 40.3 Wh kg−1 at a power density of 749 W kg−1. These results suggest the considerable potential of CoMoO4@PPy NHs for use in high-performance energy-storage devices.

Introduction

Driven by the fast-growing demands for portable electronic devices and hybrid vehicles, the development of safe, high-efficiency energy storage devices at a low cost is currently highly sought after. Supercapacitors (SCs) have attracted considerable attention owing to their high power density, long cyclic lifetime, low maintenance cost, and good safety [[1], [2], [3], [4]]. It is well known that the electrode materials play a key role in determining the capacitive performances of SCs. In general, the charge-storage mechanisms for the electrode materials can be classified into electrical double-layer capacitors (EDLCs) and pseudocapacitors [5,6]. In comparison with carbon-based materials that possess the typical characteristic of EDLCs, transition metal oxides have been documented to possess higher specific capacitances due to their fast and reversible Faradaic redox reactions [[7], [8], [9]]. Therefore, research concerning the design and fabrication of various transition oxides for use as high-performance electrodes in SCs has become increasingly popular.

Recently, metal molybdate compounds such as NiMoO4 [10], MnMoO4 [11], and CuMoO4 [12], which are binary metal oxides, have been shown to serve as promising electrode materials for SCs because they demonstrate higher electrochemical performances than the two corresponding types of single oxides (metal oxides and molybdenum oxide) [13,14]. Among these compounds, CoMoO4 has gained more attention because of its low cost and natural abundance, along with its particularly distinctive reversible and electrochemical activities that combine the high theoretical specific capacitance of cobalt oxide with the reversible small-ion storage capabilities and rich polymorphisms of molybdenum oxide [15,16]. However, the further application of CoMoO4 as an electrode in high-performance SCs remains impeded by the limited ion-accessible surface area and low electrical conductivity of this material, which result in its unsatisfactory practical specific capacitance and rate capability [14]. Therefore, the current key challenges in fabricating CoMoO4 electrode materials involve designing the hierarchical micro/nanostructures with high specific surface areas and increasing the conductivity to permit further applications of these electrodes in the field of SCs.

To date, various CoMoO4 micro/nanostructures including nanorods [17], nanowires [18], nanoneedles [19], nanosheets [20], nano/microspheres [21,22], and hollow microplates/spheres [23,24] among others have been reported upon for use as electrodes in SCs. For example, Lee et al. reported the synthesis of low-crystalline CoMoO4 nanorods with a maximum specific capacitance of 420 F g−1 at 1 A g−1 [17]. Teng et al. synthesized ultra-thin CoMoO4 nanosheets that exhibited a specific capacitance of 153 F g−1 at 1 mA cm−2 [20]. Wang et al. constructed CoMoO4 microspheres from the assembly of nanoflakes, which demonstrated a capacitance value of 384 F g−1 at 1 A g−1 [22]. Han et al. fabricated hollow CoMoO4 microplates arrays derived from a metal-organic framework, which could deliver an areal capacitance of 12.2 F cm−2 at 2 mA cm−2 [23]. In spite of the impressive progress that has been made concerning the design and synthesis of CoMoO4-based nano/microstructures for use in SCs, it is still highly desirable to engineer the distinctive micro/nanostructures to optimize the electrochemical performances of CoMoO4-based materials. In this way, the requirements of high-performance SCs may be achieved.

PPy as a typical conjugated polymer, has been reported to serve as a competitive electrode material for SCs owing to its high conductivity and notable electrochemical oxidation-reduction reversibility [25,26]. In particular, recent reports have shown that combining PPy with metal compounds could greatly improve the electrochemical performance of the resulting composites. This is because PPy can not only help to decrease the internal resistances of the active materials and thereby promote the electron transport, but also provide additional pseudocapacitance [[27], [28], [29]]. Therefore, this motivates us to integrate the advantageous features of CoMoO4 and PPy for the purpose of energy storage to construct high-performance composite electrodes consisting of these two components.

On the basis of the above considerations, we herein report the design and optimization of PPy-wrapped hierarchical CoMoO4 NTs as electrodes for high-performance SCs. In this design, one-dimensional CoMoO4 NTs are constructed. These structures differ from previously reported CoMoO4 nanostructures, because NTs are expected to possess many advantages for energy storage, including their high surface area, direct current channel, short diffusion length, sufficient space, and good mechanical stability. A PPy shell is wrapped around the surface of the CoMoO4 NTs to form CoMoO4@PPy NHs. This shell not only provides extra electroactive sites, but also increases the electric conductivity of the material and promotes the charge transfer process. Therefore, the CoMoO4@PPy NHs possess a high specific capacitance of 1203 F g−1 at 2 A g−1 and 974 F g−1 at 20 A g−1 along with a capacitance retention of 96% over 5000 cycles at 10 A g−1. Moreover, the fabricated ASC device based on CoMoO4@PPy//N-CNTs yields an energy density of 40.3 Wh kg−1 at a power density of 749 W kg−1, suggesting that the engineered CoMoO4@PPy NHs are a competitive candidate for use as advanced SCs electrodes.

Section snippets

Synthesis of CoMoO4 NTs

First, MoO3 nanorods as a self-template were synthesized according to the literature [30]. In this process, 1 mmol of ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O) was dissolved into 35 ml of an HNO3 solution (composed of 5 ml of concentrated HNO3 and 30 ml of H2O), and was transferred to a 50-ml Teflon-lined stainless-steel autoclave. After heating at 180 °C for 8 h, the MoO3 nanorods were harvested by centrifugation, washed several times with deionized water and absolute ethanol,

Results and discussion

The synthetic preparation of the CoMoO4@PPy NHs is schematically depicted in Fig. 1. First, the MoO3 nanorods were prepared via a hydrothermal reaction between MoO42− and H+ ions. Second, MoO3 nanorods served as a self-template to generate CoMoO4 NTs via a solvothermal reaction in the presence of Co(CH3COO)2, followed by calcination. Finally, CoMoO4@PPy NHs were achieved by wrapping the PPy shell on the surface of the CoMoO4 NTs using a gas phase polymerization process. The possible reactions

Conclusions

In summary, the CoMoO4@PPy NHs were constructed by wrapping a PPy shell onto a CoMoO4-nanotubes backbone using a self-templated reaction method and a subsequent in situ vapour polymerization reaction. With contributions from the CoMoO4 and PPy electroactive components, along with the charge storage advantages provided by the unique NHs, the CoMoO4@PPy NHs electrodes demonstrated enhanced specific capacitances, rate capabilities, and cyclic stability compared to the pristine CoMoO4 NTs

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledge

This work was supported by the National Natural Science Foundation of China (Grant no. 51774128), the Natural Science Foundation of Hunan Province of China (Grant no. 2018JJ4009), the Scientific Research Fund of Hunan Provincial Education Department (Grant no. 17A055), Zhuzhou Key Science & Technology Program of Hunan Province and Green Packaging and Security Special Research Fund of China Packaging Federation (Grant no. 2017ZBLY14).

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