Construction of polypyrrole-wrapped hierarchical CoMoO4 nanotubes as a high-performance electrode for supercapacitors
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).
References (43)
- et al.
Nano Energy
(2018) - et al.
A review on recent advances in hybrid supercapacitors: design, fabrication and applications
Renew. Sustain. Energy Rev.
(2019) - et al.
Advanced materials and technologies for hybrid supercapacitors for energy storage-A review
J. Energy Storage
(2019) - et al.
Design of oxygen-deficient NiMoO4 nanoflake and nanorod arrays with enhanced supercapacitive performance
Chem. Eng. J.
(2018) - et al.
Facile synthesis of rod-like manganese molybdate crystallines with two-dimensional nanoflakes for supercapacitor application
Electrochim. Acta
(2017) - et al.
Structural, morphological and magneto-optical properties of CuMoO4 electrochemical nanocatalyst as supercapacitor electrode
Ceram. Int.
(2018) - et al.
Hierarchical CoMoO4 nanoneedle electrodes for advanced supercapacitors and electrocatalytic oxygen evolution
Electrochim. Acta
(2018) - et al.
Ultrathin nanosheet-assembled hollow microplate CoMoO4 array derived from metal-organic framework for supercapacitor with ultrahigh areal capacitance
J. Power Sources
(2019) - et al.
Nanostructured polypyrrole as a flexible electrode material of supercapacitor
Nano Energy
(2016) - et al.
Decoration NiCo2S4 nanoflakes onto Ppy nanotubes as core-shell heterostructure material for high-performance asymmetric supercapacitor
Chem. Eng. J.
(2018)
Electrochemical and magneto-optical properties of cobalt molybdate nano-catalyst as high-performance supercapacitor
Ceram. Int.
Facilely synthesized NiMoO4/CoMoO4 nanorods as electrode material for high performance supercapacitor
J. Alloy. Comp.
Facile synthesis of MnO2-Ni(OH)2 3D ridge-like porous electrode materials by seed-induce method for high-performance asymmetric supercapacitor
Electrochim. Acta
Efficient storage mechanisms for building better supercapacitors
Nat. Energy
Asymmetric supercapacitor electrodes and devices
Adv. Mater.
Towards establishing standard performance metrics for batteries, supercapacitors and beyond
Chem. Soc. Rev.
Recent advances in chemical methods for activating carbon and metal oxide based electrodes for supercapacitors
J. Mater. Chem.
Hollow structural transition metal oxide for advanced supercapacitors
Adv. Mater.
Transition metal oxides with one-dimensional/one-dimensional-analogue nanostructures for advanced supercapacitors
J. Mater. Chem.
Nanostructured Mo-based electrode materials for electrochemical energy storage
Chem. Soc. Rev.
Fabrication of metal molybdate micro/nanomaterials for electrochemical energy storage
Small
Cited by (21)
In-situ grown sweet alyssum flowers-like CoMoO<inf>4</inf> for high performance hybrid supercapacitors
2023, Journal of Alloys and CompoundsPolyphosphazene-derived P/S/N-doping and carbon-coating of yolk-shelled CoMoO<inf>4</inf> nanospheres towards enhanced pseudocapacitive lithium storage
2023, Journal of Colloid and Interface ScienceInsights on the capacitance degradation of polypyrrole nanowires during prolonged cycling
2022, Polymer Degradation and StabilityCitation Excerpt :In addition, the much lower S/NTotal values may be related to the fluffy inner structure of PPy/pTS NWs, making the doping pTS− easily escape from the PPy chains during the degradation process. The PPy nanotubes [3,43,44] and PPy shells [32,37] also show similar features. For the PPy/pTS NWs prepared under different Ipoly, their Nδ+/NTotal, =N−/NTotal, and outer surface S/NTotal values did not show apparent differences (Table 3).