Hierarchical NiCo2S4@PANI core/shell nanowires grown on carbon fiber with enhanced electrochemical performance for hybrid supercapacitors
Graphical abstract
Introduction
With the growing developments of backup power sources, portable electronics devices, renewable energy power plants, and electric vehicles, further improvement of energy storage and conversion technology is imperative [1], [2], [3], [4]. Supercapacitors have attracted considerable attention as high-power energy storage devices for various applications, owing to the favorable properties such as fast charge/discharge rate, high power density, low maintenance cost, and long service life [5], [6], [7], [8], [9]. According to their energy storage mechanism, supercapacitors can be divided into two categories: electrical double-layer capacitors (EDLCs) and pesudocapacitors [10]. EDLCs store energy through charge separation at the electrode/electrolyte interface, while pesudocapacitors store energy through redox reactions that occur at or near the interface of electrode (with approximately 5 nm) [11], [12]. By contrast, pesudocapacitors can deliver almost 10 times the capacitance and energy density of EDLCs. Therefore, it is extremely significant to develop new electrode materials with pesudocapacitance for constructing asymmetric supercapacitors with high energy density.
Pseudocapacitive materials mainly consist of metal oxides/hydroxides/suldes and conductive polymers [7], [13], [14], [15]. The ternary transition metal sulfides offer higher electrochemical activity than metal oxides/hydroxides and conductive polymers and thus have been investigated extensively for use in supercapacitor electrodes [16], [17], [18]. NiCo2S4 is a current research focus on account of its remarkable theoretical capacitance, cost efficiency, and simple preparation process [18], [19], [20]. However, NiCo2S4 suffers from low electrical conductivity and large volume change in its charge/discharge processes, leading to poor rate capability and cycling stability of its electrodes. As a result, the practical application of pure NiCo2S4 in supercapacitors has been restricted [18]. In recent years, considerable research has been performed to overcome the limitations of NiCo2S4 without sacrificing its advantages. One competitive strategy to enhance the performance of NiCo2S4 is to build free-standing electrodes by growing NiCo2S4 on conductive substrate materials [21], [22], such as nickel foam [23] and carbon cloth [24]. Construction of heterostructures by introducing other components is another effective approach to improve supercapacitive performance [25], [26], [27]. For example, hybrid NiCo2S4@MnO2 heterostructures were investigated for high-performance supercapacitor, hybrid NiCo2S4@MnO2 heterostructured electrodes possessed a remarkable specific capacitance of 1337.8 F/g at a current density of 2.0 A/g and excellent cycling stability (82% retention after 2000 cycles) because of the synergistic effects of NiCo2S4 and MnO2 [28]. Moreover, Cobalt sulfide (CoSx) nanosheets were coated on NiCo2S4 nanotube arrays as electrode materials for high-performance supercapacitors [29]. However, the conductivity and structural stability of the shells reported to date are not sufficient to meet the requirements for high-performance supercapacitors.
Polyaniline (PANI) is promising to combine with other materials because of its high theoretical pseudocapacitance, excellent chemical stability, high conductivity, low cost and easy preparation [14], [30], [31]. By combining PANI with NiCo2S4, many complementary characteristics such as abundant accessible electroactive sites and high electron diffusion efficiency may be realized. In addition, construction of a core/shell structure is efficient to improve cycling stability [32], [33]. However, covering NiCo2S4 with PANI layers to fabricate supercapacitor electrodes has not been reported.
Herein, we use a PANI layer as a shell coating on core NiCo2S4 nanowires grown on carbon fiber cloth (denoted as NiCo2S4@PANI/CF) as shown in Fig. 1. Benefiting from the core/shell heterostructure, the fabricated NiCo2S4@PANI/CF shows excellent electrochemical performance, especially in terms of superior cycling life. Furthermore, the NiCo2S4@PANI/CF electrode as positive electrode was assembled with graphene/CF as negative electrode to fabricate an asymmetric supercapacitor. The device delivered a high energy density of 64.92 Wh/kg and a power density of 276.23 W/kg, while displaying considerable flexibility at wider bending angles.
Section snippets
Materials
NiCl2·6H2O, CoCl2·6H2O, CO(NH2)2, Na2S·9H2O, sodium p-toluenesulfonate (p-TSS, C7H7SO3Na) and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd., of analytical grade and were used as received without any further purification. Carbon fiber cloth (W0S1002) was supplied by Taiwan CeTech Co., Ltd. Deionized (DI) water was used throughout.
Synthesis of NiCo2S4/CF
NiCo2S4 nanowires grown on carbon fiber cloth were prepared via two-step hydrothermal method. Before hydrothermal growth, carbon fiber cloth (4 cm × 2 cm)
Materials characterizations
The morphologies and microstructures of obtained samples were investigated by field-emission transmission electron microscope (FE-TEM, FEI Tecnai G2 F20, USA) and field-emission scanning electron microscopy (FE-SEM, ZEISS SUPRA 55, German) equipped with an energy dispersive X-ray spectrometer (EDS). The structural and componential analyses were detected by X-ray diffractometer (XRD, Rigaku Ultima III, Japan), Fourier Transform Infrared Spectrometer (FTIR, Thermo Nicolet 5700 system, USA) and
Electrochemical measurements
The electrochemical measurements were operated on the electrochemical workstation CHI 660D in a standard three-electrode system with 6 M KOH solution as electrolyte. The counter electrode and reference electrode was platinum foil and Hg/HgO electrode, respectively. The core/shell structural NiCo2S4@PANI grown on carbon fiber was used directly as self-standing working electrode. Cyclic voltammogrammic (CV) and galvanostatic charge-discharge behaviors were tested within a potential range of 0–0.6
Results and discussion
The growth of NiCo2S4@PANI core/shell structure on CF was traced by FE-SEM. The morphology of each sample is shown in Fig. 2. The bare CF is rough and the diameter is nearly 10 μm (Fig. 2(a)), which is conducive to the growth of NiCo2S4 nanowires. Through the hydrothermal reaction, a great many of NiCo2S4 nanowires were grown on the CF (Fig. 2(b)). The high-magnification image in Fig. 2(c) reveals that NiCo2S4 grew vertically and divergently. After the further electrochemical deposition, the
Conclusions
In summary, a core/shell heterostructural NiCo2S4@PANI/CF composite was fabricated by a hydrothermal method and potentiostatic deposition, and then employed as supercapacitor electrode. Benefiting from the unique heterostructure of PANI shell cladding on core NiCo2S4 nanowires, the NiCo2S4@PANI/CF electrode shows more electrochemical activity sites and faster ionic diffusion, then exhibited enhanced electrochemical performance compared with NiCo2S4/CF: an areal capacitance of 4.74 F/cm2 (1823
Acknowledgement
The authors greatly acknowledge the financial supports by the Department of Science & Technology of Jiangxi Province (Grant No. 20153BCB23011).
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