Hierarchical porous CoFe2O4/CoFe2Se4 nanosheet as supercapacitor electrodes: Effect of selenium content and pore structure on electrochemical performance
Graphical Abstract
By controlling the pore structure of CoFe2O4 nanosheet, the CoFe2O4/CoFe2Se4 composites with different pore structure were obtained after selenization.
Introduction
Mechanical energy generated by human activities, if it can be collected efficiently and utilized, may become a third type of sufficient green energy [1]. In addition, although mechanical energy can be used at any time, devices that collect and store mechanical energy need to be flexible and convenient, and have large power density and energy density. Therefore, supercapacitors stand out from many stockpile equipment and become one of the candidates of mechanical energy storage equipment. Because of its fast charging speed, high current tolerance, and power density, energy density and high capacity are also excellent [2]. According to energy storage mechanism, supercapacitors can be divided into two types: double-layer capacitor and pseudocapacitor [3]. The former is the charge stored on surface of electrode, while the latter is a reversible Faraday redox reaction on electrode [4]. It is observed that specific surface area and conductivity of electrode play a key role in the property of supercapacitors. Herein, improving conductivity and specific surface area of the electrode material can certainly improve electrochemical performance of the supercapacitor [5].
In the preliminary work of our research group, the conductivity and specific surface area properties of CoFe2Se4 are excellent [2]. It has been proved in other work that metal selenides have better conductivity and electrochemical properties than metal oxides. For example, Chandu et al. demonstrated CoFe2Se4 nanorods electrode for asymmetric supercapacitors (183.4 mAh g−1) [6]. Mohammadi et al. prepared GW-FeS2-FeSe2-CSS with specific capacity of 352.3 F g−1 [7]. Song et al. reported CoFe2O4/CoFe2Se4 nanobox electrode with 463.27 F g−1 [2]. Li et al. illustrated that NiCo2Se4 nanotubes have good specific capacity 461 F g−1 [8]. And Yu et al. reported NiSe nanosheets electrode with 443 mA h g−1 [9]. According to the above, the synergies between selenides and other materials can improve the performance of supercapacitors. Moreover, binary metal oxides are often used as the positive pole of supercapacitors. Because of its multistage oxidation state, the property of the binary metal oxides are better than that of the single metal oxides, for instance NiFe2O4 [10], MnFe2O4 [11], and CoFe2O4 [12]. Among of them, spinel CoFe2O4 electrodes have outstanding electrochemical performances and low cost. The direct selenization of CoFe2O4 into a composite of selenide and oxide by sodium selenite can not only improve the conductivity of CoFe2O4, but also increase the specific surface area [13].
In general, we prepared various CoFe2O4 nanosheet on NF by a simple method for controlling solvent ratio and structural inducer. The nanosheet of CoFe2O4 can be adjusted easily by changing ratio of several solvents and combination of structural inducer. In addition, CoFe2O4 is directly formed a CoFe2O4/CoFe2Se4 core-shell structure composite by selenation. (i) CoFe2O4/CoFe2Se4 not only improves conductivity of electrode, but also increases the specific surface area of composite electrode;(ii) the controllable preparation of CoFe2O4 nanosheet offers the possibility of multiple composite structures for the composite electrode. Benefiting from these advantages, the CoFe2O4/CoFe2Se4 electrode has a specific capacity of 933.6 C g−1 at 1 A g−1 in 3 M KOH electrolyte. The CoFe2Se4 and CoFe2O4 composites with different selenium content and pore structure can be prepared in a controllable manner, providing guidance for the preparation of porous selenide in the future.
Section snippets
Results and discussion
Fig. 1 exhibits synthesis process of CoFe2O4 porous nanosheet and CoFe2O4/CoFe2Se4 porous nanosheet by simple hydrothermal method, and the morphology and structure of CoFe2O4 are controlled by structural inducer and solvent ratio. First, the suitable solvent is selected and the ratio of solvent to water is adjusted to prepare CoFe2O4 porous nanosheet with different morphology. Secondly, choose the appropriate structural inducer, and use the combination of solvent and structural inducer to
Conclusion
In conclusion, we controllably design and synthesize CoFe2O4 porous nanosheets by solvent ratio and structural inducer. And the CoFe2O4/CoFe2Se4 composite material with different selenium content is obtained by selenization. The as-fabricated CoFe2O4/CoFe2Se4 electrode as the positive supercapacitor electrode displays high specific capacity of 933.6 C g−1 at 1 A g−1. The CoFe2O4/CoFe2Se4//CNTs achieved excellent specific capacity of 505 C g−1 at 1 A g−1 and the energy density is 112.2 Wh kg−1
Experimental Section
The experimental materials and the preparation process of the materials, detailed characterization and electrochemical test requirements and formulas are included in the Supporting information.
CRediT authorship contribution statement
Kun Song: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Rui Yang: Validation, Formal analysis, Visualization. Xiaoshuang Chen: Resources, Writing - review & editing, Supervision, Data curation. Yongjie Zheng: Resources, Writing - review & editing, Supervision, Data curation. Guoli Chen: Resources, Writing - review & editing, Supervision, Data curation. Nan Zhao: Writing, Formal analysis, Visualization.
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.
Acknowledgements
This work is financially supported by the Natural Science Foundation of Heilongjiang Education department (YSTSXK201810), the National Natural Science Foundation of China (NSFC, no. 21905146).
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