Hydrothermally synthesized FeCo2O4 nanostructures: Structural manipulation for high-performance all solid-state supercapacitors
Introduction
With gradually increased demand of sustainable and environmental-friendly energy storage devices, supercapacitors with higher efficient energy harvest and energy delivery have been attracted widespread attentions [1], [2], [3], [4], [5], [6]. In general, according to the differences of the energy storage mechanism, there are two types of supercapacitors, which are electric double layer capacitor (EDLCs) and capacitor with fast faradaic oxidation-reduction reaction [7], [8], [9]. Previous studies are mainly focused on EDLCs, such as graphene and porous carbons, although carbon occupies the dominant position of the electrode material in commercial supercapacitors, their energy density remains to be improved [10], [11]. Most recently, due to the higher energy density and electrochemical stability, studies associated with metal-oxides-based materials have drawn great interests [12], [13], [14], [15], [16]. Thus, numerous studies have been devoted to discover and develop metal-oxides-based materials. In this respect, a series of electrode materials such as NiO [17], Co3O4 [18], MnO2 [19], and Fe3O4 [20] have been developed for supercapacitors. Whereas, the conductivity of metal-oxides-based materials is too low and greatly limited the improvement of its electrochemical performance, so that cannot be satisfied at the need of practical applications [21]. Consequently, further exploration of novel alternative materials with high energy density and preferable conductivity are the need of the hour to promote practical applications in supercapacitors.
Recently, mixed binary transition metal oxides MCo2O4 (M = Ni [22], Cu [23], Mn [24] and Zn [25]) depict excellent performance electrochemical and structural stability. MCo2O4, benefitted from the synergy effects between M and cobalt ions demonstrates improved redox reactions as well as endorsed structural variations [26], so that spinel MCo2O4 multiple oxidation states are predicted to possess preferable electrical conductivity as well as improved electrochemical performance when compared to that of mono-metal oxides [27]. Presently, several reports have been devoted to the supercapacitors performance of mixed binary transition metal oxides MCo2O4 [28]. Guan et al. reported that the ZnCo2O4 nanowire cluster as asymmetric supercapacitors exhibited high energy densities (41 and 16.63 Wh kg−1) at power densities (384 and 2561 W kg−1) [29]. Yuan group has reported the ultrathin mesoporous NiCo2O4 nanosheets, which deliver ultrahigh specific capacitance of 2010 and 1450 F g−1 at current densities of 2 and 20 A g−1, respectively [30]. Amongst the various MCo2O4, a typical Fe-Co oxide (FeCo2O4) where iron compounds denote excellent electrical conductivity, is generally regarded to gain higher capacitive property [31]. Zhu et al. reported a novel FeCo2O4 submicron-tube for supercapacitors, which demonstrated a good capacitance of 1254 F g−1 at the current density of 2 A g−1 [32]. Besides that, constructing well-designed architectures, such as creating a hybrid FeCo2O4-based hetero-structure, are important and prospective methods to realize enhanced electrochemical property as well as high energy densities in supercapacitors [33]. Liu et al. fabricated a nanoflake@nanowire hetero-structure of NiCo2O4, and contributed the superior electrochemical performance of nanoflake@nanowire NiCo2O4 to the strong synergistic effect between separated ingredients [34]. In this way, the optimized electronic interaction in hybrid hetero-structure can provide superior electron collection efficiency, large specific surface area, and eventually lead to an improved performance [33]. Thus, it is a feasible way to implement the above rational designation and investigate the energy storage performance of FeCo2O4 hetero-structure.
Herein, three different FeCo2O4 nanostructures (FeCo2O4 nanowires, FeCo2O4 nanoflake@nanowire hetero-structure and FeCo2O4 hierarchical nanospheres) have been achieved by rational adjustments on the duration of hydrothermal reaction. Subsequently, we characterize the electrochemical properties of as-prepared samples, and find that FeCo2O4 nanoflake@nanowire hetero-structure that was prepared by 18 h hydrothermal reaction depicts the superior supercapacitors performance. We utilize FeCo2O4 nanoflake@nanowire hetero-structure as both the anode and cathode electrodes to fabricate symmetric all solid-state supercapacitor. The as-prepared FeCo2O4 nanoflake@nanowire hetero-structure exhibits a high specific capacitance of about 969 F g−1 at 2 A g−1, as well as more excellent electrochemical performance than that of nanowires and nanospheres. It must also be mentioned that the fabricated device exhibits a high specific capacitance of 146 F g−1 at the current density of 0.5 A g−1. The results demonstrate that the rational designed and synthesized FeCo2O4 nanoflake@nanowire hetero-structure preserve a great prospect for optimal electrochemical performance.
Section snippets
Materials preparation
In the experiment, all chemicals reagents were received without any purification process. To remove the surface oxide layer, the Ni foam substrates (1 cm × 2 cm in a rectangular shape) were corroded and ultrasonicated in 6 M HCl for 15 min, and then the clean Ni foam can be obtained after wash for three times by using acetone, ethanol and ultrapure water, and dried at room temperature for subsequent experiments.
Synthesis of Fe-Co binary Metal Oxide: In a typical procedure, 262 mg Co(No3)2·6H2O and
Results and discussion
In this work, FeCo2O4 nanowires, FeCo2O4 nanoflake@nanowire hetero-structure and FeCo2O4 hierarchical nanospheres were acquired by a simple hydrothermal process followed with annealing treatment. X-ray diffraction (XRD) pattern of FeCo2O4 is presented on Fig. 1. The value of the diffraction peak at 19.0°, 31.3°, 44.8°, 55.8°, 59.5° and 65.4° can be assigned to the (111) (220), (311), (422), (511) and (440) planes of the FeCo2O4 spinel phase (JCPDS 04-0850), respectively.29 And the rest peaks
Conclusion
In this paper, the FeCo2O4 nanowires, nanoflake@nanowire hetero-structure and hierarchical nanospheres are designed and fabricated by employing a controlled hydrothermal strategy. The dependence of the microstructures on electrochemical properties of the FeCo2O4 samples that are prepared by different hydrothermal reaction time has been investigated. As a result, the sample prepared by 18 h delivers the maximum specific capacitance of 969 F g−1 at current density of 2 A g−1. In addition, the
Acknowledgments
This work was supported by the Grants from National Natural Science Foundation of China (No. 11504312), Project supported by Provincial Natural Science Foundation of Hunan (No. 2016JJ2132), Open Fund based on innovation platform of Hunan Colleges and Universities (No. 15K128), Scientific Research Fund of Hunan Provincial Education Department (No. 15C1322), as well as the Program for Changjiang Scholars and Innovative Research Team in University (IRT_17R91).
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