Controllable synthesis of hierarchical NiCo₂S₄@Ni₃S₂ core–shell nanotube arrays with excellent electrochemical performance for aqueous asymmetric supercapacitors

Abstract

In this study, unique NiCo₂S₄@Ni₃S₂ core–shell nanotube arrays (NTAs), a promising positive electrodes for supercapacitors, have been successfully synthesized on Ni foam via a novel method. Electrochemical tests show the highest area specific capacity of 4.25 C cm⁻² at 4 mA cm⁻², maintained at 3.12 C cm⁻² at 40 mA cm⁻². In addition, a 3D reduced graphene oxide (rGO) aerogel has been fabricated as a negative electrode for supercapacitors, and this displays an excellent capacitance performance of 286.9 C g⁻¹ at 1 A g⁻¹. An asymmetric supercapacitor denoted as NiCo₂S₄@Ni₃S₂//rGO has been assembled based on NiCo₂S₄@Ni₃S₂ core–shell NTAs and rGO aerogel. The NiCo₂S₄@Ni₃S₂//rGO device achieves an outstanding performance with a specific capacity of 163.15 C g⁻¹, an energy density of 32.75 W h kg⁻¹ at a power density of 0.36 kW kg⁻¹. Moreover, it displays a remarkable cycling performance (77.5% capacity retention after 5000 cycles). These results indicate potential applications of NiCo₂S₄@Ni₃S₂//rGO in asymmetric supercapacitors (ASCs).

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1. Introduction

In response to environmental problems and the energy crisis, the development of environmentally-friendly and renewable sources of energy has become an urgent worldwide necessity. Supercapacitors, also called electrochemical capacitors, have attracted intense interest due to their high power density, long cycle life, fast charge–discharge capability, and excellent reversibility.^1–3 Early research mainly focused on electrical double-layer capacitors (EDLCs), such as porous carbons and graphenes, whose capacity stems from the interface between electrolyte and electrode.^4,5 Nevertheless, their low energy density and low specific capacity greatly limit their application in energy storage devices.^6,7 Currently, redox-based supercapacitors have shown potential applications for energy storage devices owing to their high specific capacity and reversible redox reaction.^8,9 Universally, faradaic materials are mainly transition metal oxides,^10–12 transition metal sulfides,^13,14 transition metal nitrides,^15,16 transition metal phosphides,^17,18 hydroxides,^19,20 and conductive polymers.^21,22 Although these supercapacitive materials can display a remarkably high faradaic capacity compared to carbon materials resulting from a reversible redox reaction, there is an urgent problem to be addressed by researchers: relatively low energy density hinders the promotion and application of supercapacitors. Lately, numerous researchers have begun to develop ASCs due to their high energy density and high power density on account of their high operating voltage and high specific capacity.^23,24 Hence, we usually fabricate ASCs by the combination of faradaic and EDLC materials to accomplish high power density, high energy density and a stable cycling life.²⁵ Considering the above, it is critical to seek a prominent faradaic material and an outstanding EDLC material in order to boost the performance of ASCs.

Recently, metal sulfides have emerged as one of the most outstanding candidates for supercapacitors, such as binary nickel sulfides,^26,27 cobalt sulfides²⁸ and manganese sulfides.²⁹ Compared to the low capacity of carbon materials, the poor electrical conductivity of transition metal oxides, and the poor cycle performance of conducting polymers, transition metal sulfides have displayed numerous superior electrochemical properties.³⁰ Among metal sulfides, NiCo₂S₄ possesses richer redox reactions than the corresponding binary nickel sulphide (NiS) and cobalt sulphide (Co₉S₈), and exhibits a major superiority over NiCo₂O₄ in terms of higher conductivity and higher specific capacity.^31,32 In fact, there have been some significant achievements in the application to supercapacitors of NiCo₂S₄ electrode materials.^33,34 For instance, Wu et al. assembled an ASC using NiCo₂S₄ mesoporous nanosheets as the positive electrode and active carbon (AC) as the negative electrode. This ASC exhibited an energy density of 25.5 W h kg⁻¹ at a power density of 334 W kg⁻¹.³⁵ However, single NiCo₂S₄ electrode materials cannot meet the requirements of supercapacitors for high power density and high energy density. The facile and recommended strategy to conquer this defect is to construct a core–shell structure with different supercapacitive materials, where the differences are due to their rich accessible redox reaction sites, easy diffusion of electrolyte, enlarged contact surface area between electrode and electrolyte, as well as the fantastic synergy effect between core and shell materials. There have been some reports on constructing core–shell structures that have achieved supercapacitors with superior electrochemical performances. Fu et al. successfully fabricated hierarchical NiCo₂S₄@CoS_x hybrid electrodes by a two-step method, which show an excellent specific capacity of 2.37 C cm⁻² at 5 mA cm⁻².³⁶ Wan et al. constructed NiCo₂S₄ nanotube@NiMn-LDH arrays, which show a high specific capacity of 0.87 C cm⁻² at 1 mA cm⁻².³⁷ Evidently, NiCo₂S₄-based hybrid core–shell structures have proved to be promising positive electrodes in applications to make high performance ASCs.

As for the negative electrode materials in ASCs, 3D porous rGO aerogels have lately been considered as ideal EDLC materials for high-performance supercapacitors, because of their superior properties like large surface areas, excellent cycle stability, high electrical conductivity and well-controlled structures.^38–40 Moreover, 3D rGO aerogels can guarantee multi-dimensional electron transport pathways and ease of access to the electrolyte, which are of great importance for achieving high-rate energy storage.⁴¹ Therefore, 3D porous rGO aerogels can be considered as candidate negative electrodes for ASCs.

In this paper, we have controlled the synthesis of unique hierarchical NiCo₂S₄@Ni₃S₂ core–shell NTAs in situ grown on Ni foam and used as positive electrodes. Moreover, a 3D rGO aerogel with an excellent capacity performance was fabricated as a negative electrode through a facile hydrothermal reaction. First, a core material of NiCo₂S₄ NTAs was constructed via a facile and effective anion-exchange reaction. This offered a stable scaffold for the shell and a short diffusion path for electrolyte ions and abundant active sites for the faradaic process. Then ultrathin Ni₃S₂ nanosheets are coated on the surface of NiCo₂S₄ NTAs by a simple electrodeposition process. To the best of our knowledge, the enlarged surface of the Ni₃S₂ nanosheets is favorable for the rapid diffusion of electrolyte ions and electrolyte efficient infiltration.^42,43 Remarkably, the NiCo₂S₄@Ni₃S₂ core–shell NTAs as-fabricated through the combination of NiCo₂S₄ nanotubes and Ni₃S₂ nanosheets exhibited a superior electrochemical performance as a supercapacitor, which could be ascribed to the synergistic effect of its unique core–shell nanostructures. Furthermore, the as-assembled NiCo₂S₄@Ni₃S₂//rGO device exhibits relatively high energy density, high power density and excellent cycling stability, indicating its great potential in ASCs.

2. Experimental sections

2.1. Synthesis of NiCo₂S₄ NTAs on Ni foam

Unique NiCo₂S₄ NTAs were synthesized according to previously published work.⁴⁴ Prior to the synthesis, the Ni foam (1 cm × 1 cm × 1.6 mm) was pretreated with 5% HCl, ethanol and deionized (DI) water for 15 min each to remove impurities and oxides from the surface and dried in an oven. NiCo₂S₄ NTAs were obtained through a two-step hydrothermal reaction. Firstly, 2 mmol (0.4754 g) NiCl₂·6H₂O, 4 mmol (0.9517 g) CoCl₂·6H₂O and 12 mmol (0.7207 g) urea were dissolved in 35 mL DI water with continuous stirring. Then the solution was transferred into a 50 mL Teflon-lined stainless steel autoclave. Subsequently, we strung six nickel foams by using a bent iron wire and put them in the center of the Teflon. The autoclave was kept at 120 °C for 6 h in the oven and cooled down to room temperature naturally. The samples were collected by washing with DI water and ethanol, then dried in the oven at 60 °C for 12 h to make precursors. Secondly, the precursors were placed in a 50 mL Teflon-lined stainless steel autoclave with 35 mL sodium sulfide solution (0.2 M) and kept at 120 °C for 14 h. Finally, the products were collected by washing with DI water and ethanol, and then dried at 60 °C for 12 h to obtain NiCo₂S₄ NTAs. The mass loading of NiCo₂S₄ on Ni foam was approximately 8.0 mg.

2.2. Synthesis of NiCo₂S₄@Ni₃S₂ core–shell NTAs

Ni₃S₂ nanosheets were synthesized via an electrodeposition process carried out on a CHI630D electrochemical analyzer.⁴⁵ Including a Ni foam sheet supported NiCo₂S₄ NTAs as the working electrode, a saturated Ag/AgCl as the reference electrode and a Pt sheet (1 cm × 1 cm) as the counter electrode. The typical deposition was formed in a mixed solution of 50 mM NiCl₂·6H₂O and 1 M thiourea by cyclic voltammetry (CV) with the potential ranging from −1.2 V to 0.2 V (vs. Ag/AgCl) at 5 mV s⁻¹ for 6 cycles. The NiCo₂S₄@Ni₃S₂ core–shell NTAs were collected by washing with DI water and ethanol, then dried at 60 °C for 12 h. As a comparison, bare Ni₃S₂ was obtained through the same synthesis method, except that the working electrode was the clean Ni foam. The mass loading of Ni₃S₂ was about 2.0 mg and the whole weight of NiCo₂S₄@Ni₃S₂ core–shell NTAs on Ni foam was 10.0 mg.

2.3. Preparation of the 3D rGO aerogel

A graphene oxide (GO) solution was initially prepared from natural graphite by a modified Hummers' method.⁴⁶ The obtained GO solution was diluted to 2 mg mL⁻¹. Then 70 mL of the diluted GO solution was added to a 100 mL enclosed Teflon-lined autoclave and maintained at 180 °C for 12 h. The autoclave was cooled to room temperature naturally. A self-assembled columnar graphene was formed and vacuum-freeze-dried for 24 h to obtain the 3D rGO aerogel.

2.4. Characterization

Crystallite structures of the as-prepared samples were characterized by X-ray diffraction (XRD, Rigaku Smart Lab) using Cu Kα (λ = 1.5418 Å) radiation. The morphology and microstructure of the synthesized products were examined by field emission scanning electron microscopy (FESEM, SU-8000, Japan) and high resolution transmission electron microscopy (HRTEM, JEM-2100). Raman spectra were obtained using a micro-Raman system (LabRAM HR800) with an excitation energy of 2.41 eV (514 nm).

2.5. Electrochemical measurements

The electrochemical performances of the obtained samples were investigated in a three-electrode cell in a 2 M KOH aqueous solution. The obtained samples were used as the working electrode, a platinum plate (3 cm × 3 cm) was used as the counter electrode and Hg/HgO was used as the reference electrode. Cyclic voltammograms (CV), galvanostatic charge–discharge (GCD) measurements were carried out on an electrochemical workstation (CHI660E) to evaluate the electrochemical behaviors. And the long cycling life of the ASC devices was inspected through a LAND battery test system (CT2001A). The average specific capacity determined from galvanostatic charge–discharge (GCD) tests can be calculated from eqn (1) as follows:here C_a is the area specific capacity (C cm⁻²), C_m is the mass specific capacity (C g⁻¹), i is the discharge current (A), t is the discharge time (s), S is the surface area (cm⁻²) and m is the mass (g) of a single electrode.

2.6. Fabrication of ASC devices

The ASC devices were fabricated in a two-electrode configuration by taking NiCo₂S₄@Ni₃S₂ NTAs as the positive electrode and rGO aerogel as the negative electrode with a separator made by polyvinylidene fluoride (PVDF) sandwiched between the two electrodes. Prior to the assembly of the ASC devices, the rGO electrode was obtained by mixing rGO, polytetrafluorene-ethylene (PTFE) and acetylene black in ethanol with a mass ratio (%) of 80 : 10 : 10. The prepared mixture was ultrasonicated for 1 h and coated on Ni foam according to the matched weight. Afterwards, the Ni foam with the rGO coating was pressed and dried at 60 °C for at least 12 h.

3. Results and discussion

3.1. Structural characterization of NiCo₂S₄@Ni₃S₂ core–shell NTAs

Hydrothermal synthesis combined with an electrodeposition method were used for the hierarchical NiCo₂S₄@Ni₃S₂ core–shell NTAs on Ni foam, which is schematically illustrated in Fig. 1. Initially, aligned NiCo₂S₄ NTAs were grown on Ni foam by a two-step hydrothermal method. Subsequently, the NiCo₂S₄ NTAs served as the scaffold for the growth of interconnected Ni₃S₂ ultrathin nanosheets via an electrodeposition process and formed unique hierarchical core–shell hybrid structures. The hierarchical NiCo₂S₄ NTAs on Ni foam are shown in Fig. 2 at different magnifications. In Fig. 2(a), the Ni foam is covered with a large number of NiCo₂S₄ NTAs. Obviously, the hierarchical structures adhere to the Ni foam substrate tightly and uniformly. The magnified image (Fig. 2(b)) reveals that the NiCo₂S₄ nanotubes have diameters ranging from 50 to 150 nm and lengths of 2–4 μm from their tips to their roots in contact with the Ni foam. The unique morphology and structure of NiCo₂S₄ nanotubes can be attributed to the process of the anion-exchange reaction during the conversion from precursors to NiCo₂S₄.^47,48

Fig. 1 Schematic illustration of a facile method of NiCo₂S₄@Ni₃S₂ core–shell NTAs on Ni foam.

Fig. 2 FESEM images of (a) and (b) NiCo₂S₄ NTAs on Ni foam.

For the sake of a closer exploration of the morphologies of the NiCo₂S₄@Ni₃S₂ core–shell NTAs, a series of parallel experiments were carried out. The NiCo₂S₄ nanotubes covered with slim Ni₃S₂ nanosheets with different CV cycles of electrodeposition are displayed in Fig. 3. As shown in Fig. 3(a), Ni₃S₂ nanosheets can be observed; the core–shell structures are not very evident. This results from the small amount of Ni₃S₂ with just three deposition cycles. In Fig. 3(b), after nine cycles, the NiCo₂S₄ nanotubes are not only covered by a large number of Ni₃S₂ nanosheets, but there also appear to be vast agglomerations of particles, leading to less space between the core–shell nanotubes. In terms of experimental parameters, the optimal morphologies are obtained after six cycles, as shown in Fig. 3(c) and (d). The Ni₃S₂ nanosheets are interconnected with each other, thus forming a highly porous surface morphology. The morphology of Ni₃S₂ was also characterized by FESEM (Fig. S1†). In addition, the spaces between each core–shell nanotube constitute ordered channels on a large scale. In such a unique core–shell structure, the opened channels served as diffusion pathways, making the active materials more accessible to electrolyte ions.⁴¹ Moreover, those ultrathin and curled Ni₃S₂ nanosheets greatly increase the redox sites during electrochemical behaviours.⁴⁹

Fig. 3 FESEM images of (a) NiCo₂S₄ NTAs covered by three cycles Ni₃S₂ nanosheets; (b) NiCo₂S₄ NTAs covered by nine cycles Ni₃S₂ nanosheets; (c) and (d) NiCo₂S₄ NTAs covered by six cycles Ni₃S₂ nanosheets.

The sample structures were further confirmed by XRD analysis. Fig. 4 shows the wide-angle XRD patterns of the NiCo₂S₄ NTAs and NiCo₂S₄@Ni₃S₂ core–shell NTAs supported on the Ni foam. The three typical peaks at 44.7°, 52.1° and 76.5° originate from the Ni substrate concerned with the (111), (200) and (220) planes (JCPDS no. 87-0712).⁵⁰ The five major peaks at 31.6°, 38.3°, 47.4°, 50.5°, and 55.3° were indexed to the (311), (400), (422), (511) and (440) planes of the NiCo₂S₄ phase (JCPDS card no. 20-0782). The EDX spectrum (Fig. S2(a)†) indicates that the NiCo₂S₄ nanotubes consist mainly of Ni, Co and S elements. Moreover, the weak peak at 21.8° coincides with the (101) plane in the standard Ni₃S₂ spectrum (JCPDS card no. 44-1418), indicating poor crystallinity or the low mass loading of Ni₃S₂ nanosheets obtained by electrodeposition. The chemical compositions of the Ni₃S₂ nanosheets were confirmed by EDX (Fig. S2(b)†). The results clearly reveal that the Ni₃S₂ nanosheets are composed mainly of Ni and S. The Ni/S atomic ratio is about 1.45, which indicates that the chemical composition of the Ni₃S₂ nanosheets is pure Ni₃S₂.

Fig. 4 Powder XRD patterns of the NiCo₂S₄ NTAs and NiCo₂S₄@Ni₃S₂ core–shell NTAs on Ni foam.

TEM analysis provides further insight into the morphology and detailed structure of the as-obtained NiCo₂S₄@Ni₃S₂ core–shell NTAs. The whole of the core–shell structures can be clearly observed, as shown in Fig. 5(a)–(c), while the core NiCo₂S₄ nanotubes are tightly wrapped by Ni₃S₂ nanosheets. Also the hollow part of the NiCo₂S₄ nanotube is filled up with the shell. As shown in Fig. 5(d) and (e), there are straight and orderly lattice fringes with interplanar gaps of 0.28 nm, corresponding well to the (311) plane of NiCo₂S₄. The other image with bending and relatively coarse lattice fringes represents an interplanar spacing of 0.41 nm which can be indexed to the (101) plane of Ni₃S₂, suggesting poor crystallinity compared with the NiCo₂S₄ nanotubes as well as in the XRD analysis.

Fig. 5 (a–c) HRTEM images of NiCo₂S₄@Ni₃S₂ core–shell NTAs; (d) HRTEM images of Ni₃S₂ nanosheets; (e) HRTEM images of NiCo₂S₄ NTAs.

3.2. Electrochemical performance of the NiCo₂S₄@Ni₃S₂ core–shell NTAs

To explore the electrochemical performances of the samples, cyclic voltammograms (CV) measurements were performed on the three-electrode cell in 2 M KOH aqueous solution. Fig. 6(a) shows the CV curves of pure NiCo₂S₄ and NiCo₂S₄@Ni₃S₂ core–shell NTAs electrodes in the voltage range of 0–0.6 V (vs. Hg/HgO) at a scan rate of 5 mV s⁻¹. The obvious redox peaks indicated the faradaic capacitive characteristics of the samples. For the NiCo₂S₄ NTAs electrode, a pair of redox peaks at 0.23 V and 0.48 V (Ni²⁺/Ni³⁺ and Co³⁺/Co⁴⁺) can be clearly observed. However, compared with the CV curve of bare NiCo₂S₄, the NiCo₂S₄@Ni₃S₂ core–shell NTAs electrode shows a pair of significantly enhanced redox peaks at voltages of 0.22 V and 0.51 V, respectively, which may be derived from the abundant faradaic redox reactions and the short ion diffusion path offered by Ni₃S₂, which can be attributed to the reversible reactions of Ni²⁺/Ni³⁺ caused by the addition of Ni₃S₂ nanosheets (the supercapacitor performance of Ni₃S₂ is supplied in Fig. S3†). Moreover, the area surrounded by the CV curve is dramatically enlarged by the introduction of Ni₃S₂ nanosheets, suggesting the larger capacity of NiCo₂S₄@Ni₃S₂ core–shell NTAs. Fig. 6(b) shows CV curves of NiCo₂S₄@Ni₃S₂ electrodes at 5–50 mV s⁻¹. With an increase in the scan rate, the CV curves have a tendency to further augment the shape and shifting of the redox peaks, suggesting a diffusion-controlled electrochemical process. The charge storage mechanism of NiCo₂S₄ and NiCo₂S₄@Ni₃S₂ core–shell NTAs can be described by the following equations in alkaline electrolyte.^40,51

NiS + OH⁻ ↔ NiSOH + e⁻ (2)

CoS + OH⁻ ↔ CoSOH + e⁻ (3)

CoSOH + OH⁻ ↔ CoSO + H₂O + e⁻ (4)

Ni₃S₂ + 3OH⁻ ↔ Ni₃S₂(OH)₃ + 3e⁻ (5)

Fig. 6 (a) CV curves of the NiCo₂S₄ NTAs and NiCo₂S₄@Ni₃S₂ core–shell NTAs at 5 mV s⁻¹; (b) CV curves of NiCo₂S₄@Ni₃S₂ core–shell NTAs at various scan rates; (c) galvanostatic charge–discharge curves of NiCo₂S₄ NTAs and NiCo₂S₄@Ni₃S₂ core–shell NTAs at 4 mA cm⁻²; (d) galvanostatic charge–discharge curves of NiCo₂S₄@Ni₃S₂ core–shell NTAs at different current densities; (e) and (f) C_a and C_m of NiCo₂S₄ NTAs and NiCo₂S₄@Ni₃S₂ core–shell NTAs at different current densities.

Fig. 6(c) depicts a comparison of charge–discharge curves for the NiCo₂S₄ NTAs and NiCo₂S₄@Ni₃S₂ core–shell NTAs between 0 and 0.5 V at a current density of 4 mA cm⁻². As expected, the NiCo₂S₄@Ni₃S₂ core–shell NTAs electrode achieves a much longer charge–discharge time than the bare NiCo₂S₄, demonstrating its smaller polarization and higher electrochemical reactivity. According to eqn (1), the area specific capacity of the NiCo₂S₄@Ni₃S₂ electrode is calculated to be 4.25 C cm⁻² (425 C g⁻¹), which is evidently higher than the bare NiCo₂S₄ with 2.62 C cm⁻² (328 C g⁻¹) at the lowest current density of 4 mA cm⁻². The charge–discharge curves of the NiCo₂S₄@Ni₃S₂ core–shell NTAs at various current densities are recorded in Fig. 6(d). Fig. 6(e) further illustrates the area specific capacity (C_a) of the NiCo₂S₄ NTAs and NiCo₂S₄@Ni₃S₂ core–shell NTAs at various current densities. Meanwhile, the mass specific capacity (C_m) based on the mass loading of all active materials versus current density plots are shown in Fig. 6(f). The specific capacities decrease gradually with increasing current density, because the ions don't have enough time to penetrate into the inner portion of the active materials at a high current density. Nevertheless, the areal specific capacity of NiCo₂S₄@Ni₃S₂ is still sustained at 3.12 C cm⁻² at 40 mA cm⁻² (capacity retention of 73.4% at 40 mA cm⁻²). Furthermore, the specific capacity and capacity retention of NiCo₂S₄@Ni₃S₂ electrode in this study are superior to numerous NiCo₂S₄-based nanomaterials reported before, such as NiCo₂S₄@CoS_x (2.37 C cm⁻² at 5 mA cm⁻², capacity retention of 47.7% at 50 mA cm⁻²),³⁶ NiCo₂S₄@Ni–Mn LDH arrays/GS (0.87 C cm⁻² at 1 mA cm⁻², capacity retention of 72.8% at 10 mA cm⁻²),⁴¹ NiCo₂S₄@MnO₂ (1.30 C cm⁻² at 3 mA cm⁻², capacity retention of 46.2% at 20 mA cm⁻²).⁵²

3.3. Structural characterization and electrochemical performance of 3D rGO aerogel

Obviously, the high-resolution SEM image in Fig. 7(a) indicates that rGO aerogel has an interconnected 3D porous network with pore sizes ranging from submicrometers to micrometers. This can be clearly observed in the TEM (Fig. 7(b)). The phase structures of rGO and GO were checked by XRD (Fig. S4(a)†). The curve of GO displays a peak at 2θ = 11.5°, which corresponds to an interlayer distance of 0.76 nm originating from some functional groups between the layers, such as carboxyl, hydroxyl, and epoxy. The curve of rGO shows a broad peak at 2θ = 24.4°, indicating its poor ordering characteristic. The changes in the structure were researched by Raman spectroscopy (Fig. S4(b)†). As shown in the figure, the peak for rGO (1589 cm⁻¹) at the G band is down-shifted compared with that of GO (1593 cm⁻¹), indicating that the hexagonal network of carbon atoms in rGO recovers with defects from the structure with rich isolated double bonds in GO.^53,54 The I_D/I_G ratio of GO is 0.98 and increases to 1.10 in rGO, which demonstrates that some defects introduced in rGO result from hydrothermal reduction, which is related to the disordered carbon structure. Examples are the vacancies, boundaries, and amorphous structure, which can provide more active sites for charge storage and boost electrochemical performance. To discuss the pore structure of 3D rGO aerogel, a N₂ adsorption–desorption measurement was carried out at 77 K. Fig. S5† gives the N₂ adsorption–desorption isotherms of the 3D rGO aerogel, based on which the BET specific surface area is calculated to be 168.6 m² g⁻¹ (Table S1†). Pore size distribution analysis using the Barrett–Joyner–Halenda (BJH) method shows that the 3D rGO aerogel has a wide pore-size distribution, from micropores to macropores (inset of Fig. S5†), which is consistent with the FESEM observations.

Fig. 7 (a) The high-resolution FESEM images of rGO aerogel; (b) the high-resolution TEM images of rGO aerogel; (c) CV curves of rGO aerogel at various scan rates; (d) galvanostatic charge–discharge curves of rGO aerogel at different current densities; (e) C_m of rGO aerogel and AC at different current densities; (f) long-term cycle life graph of rGO aerogel and AC.

The CV curves of the rGO aerogel show a similar rectangular shape at different scan rates, with a potential window of −1.0 to 0.0 V (vs. Hg/HgO) in a 2 M KOH electrolyte solution, as can be seen in Fig. 7(c), which reveals the typical double-layer capacity. The GCD curves of the rGO aerogel are relatively symmetrical and linear, which also indicates the double-layer capacitive behavior (Fig. 7(d)) of the rGO aerogel. Fig. 7(e) displays the calculations of the specific capacity for the rGO aerogel according to the galvanostatic discharge curves. The specific capacity for the rGO aerogel can achieve a value of 286.9 C g⁻¹ at a current density of 1 A g⁻¹, which is superior to AC (240.1 C g⁻¹ at 1 A g⁻¹). When the current density increases to 10 A g⁻¹, the specific capacity of the rGO aerogel can also achieve 210.1 C g⁻¹, manifesting an excellent property of rGO aerogel at a high discharge rate. The cycling performance of the rGO aerogel at a current density of 10 A g⁻¹ is also shown in Fig. 7(f). The capacity retention can remain as high as 93.2% of the maximum value after 5000 cycles, showing the superior cycling stability of the 3D rGO aerogel. In contrast, the specific capacity retention of AC is only 83.6% under the same conditions. In sum, the rGO aerogels, which possess an interconnected 3D porous network exhibiting a symmetrical linear characteristic, demonstrate admirable EDLC properties, including outstanding cycle stability and excellent rate capability, due to their high specific surface areas, large pore volumes and superior electrical conductivity.

3.4. Electrochemical performances of the ASC device

To further investigate the electrochemical performances of the composite material, an ASC was fabricated by using NiCo₂S₄@Ni₃S₂ core–shell NTAs as the positive electrode and 3D rGO aerogel as the negative electrode, and NiCo₂S₄//rGO was assembled for comparison. For the two-electrode system, it is crucial to keep the charges balanced with the relationship q⁺ = q⁻.⁵⁵ According to eqn (6) and (7):

q⁺ = m⁺ × C_m⁺ (6)

q⁻ = m⁻ × C_sp⁻ (7)

According to the charge balance (q⁺ = q⁻), the mass of rGO was decided by following eqn (8):

here, q⁺, m⁺ and C_m⁺ represent the charge (C), mass (g) and specific capacity (C g⁻¹) of the positive electrode; q⁻, m⁻, and C_sp⁻ represent the charge (C), mass (g), and specific capacity (C g⁻¹) of the negative electrode in the three-electrode cell. After this calculation, the mass loadings of rGO in NiCo₂S₄//rGO and NiCo₂S₄@Ni₃S₂//rGO were 9.25 and 15.20 mg, respectively. Fig. 8(a) exhibits the CV curves of NiCo₂S₄//rGO and NiCo₂S₄@Ni₃S₂//rGO at a scan rate of 5 mV s⁻¹ under 1.6 V in 2 KOH electrolyte. It is apparent that the area enclosed by the CV curves of the NiCo₂S₄@Ni₃S₂//rGO is considerably larger than that of the NiCo₂S₄//rGO device, which benefits from the construction of NiCo₂S₄@Ni₃S₂ core–shell structure. Galvanostatic charge–discharge tests were further conducted in the voltage range from 0 to 1.6 V to estimate the capacity of NiCo₂S₄@Ni₃S₂//rGO and NiCo₂S₄//rGO. As shown in Fig. 8(b) and (c), the NiCo₂S₄@Ni₃S₂//rGO still shows a longer discharge time than bare NiCo₂S₄//rGO, indicating that the core–shell structure is endowed with stable and exceptional electrochemical performance both in an asymmetric supercapacitor and in the three-electrode system. Furthermore, the C_m values of the NiCo₂S₄//rGO and NiCo₂S₄@Ni₃S₂//rGO devices depended on the discharging curves with different current densities, as plotted in Fig. 8(d). Evidently, the mass specific capacity of NiCo₂S₄@Ni₃S₂//rGO achieves 163.15 C g⁻¹ at a current density of 0.5 A g⁻¹ and 122.80 C g⁻¹ at a high current density of 4 A g⁻¹.

Fig. 8 (a) CV curves of the NiCo₂S₄//rGO and NiCo₂S₄@Ni₃S₂//rGO at 5 mV s⁻¹; (b) galvanostatic charge–discharge curves of the NiCo₂S₄//rGO and NiCo₂S₄@Ni₃S₂//rGO at 0.5 A g⁻¹; (c) galvanostatic charge–discharge curves of NiCo₂S₄@Ni₃S₂//rGO at different current densities; (d) C_m of NiCo₂S₄//rGO and NiCo₂S₄@Ni₃S₂//rGO at different current densities.

The cycling performances of NiCo₂S₄//rGO and NiCo₂S₄@Ni₃S₂//rGO were also investigated at a current density of 2 A g⁻¹. As shown in Fig. 9(a), an obviously increasing capacity is captured owing to the activation process of the electrode materials for the first 500 cycles in the test.^56,57 In particular, the mass loading of the ASC devices was so high that it took some time to activate the active materials. After reaching the maximum, both devices show a fluctuating decline, ending up with 77.6% and 69.4% of the initial capacity after 5000 cycles, respectively. The excellent cycling performance of NiCo₂S₄@Ni₃S₂//rGO with little deformation expresses a favourable stability due to the structure of NiCo₂S₄@Ni₃S₂ and the strong adherence to the substrate Ni foam. We have also provided the XRD pattern and FESEM image of NiCo₂S₄@Ni₃S₂ on the surface of the Ni foam after the 5000 cycling tests in Fig. S6 and S7.† The test results showed that the core–shell nanotube structure of NiCo₂S₄@Ni₃S₂ remains mainly stable, which also proved the long cycle life of the ACS device. Fig. S8† shows the impedance Nyquist plots of the NiCo₂S₄, Ni₃S₂ and NiCo₂S₄@Ni₃S₂ composite electrodes. Remarkably, the internal resistances (R_e) at the high-frequency intercept of the real axis are measured to be 0.75, 0.46 and 0.32 Ω for NiCo₂S₄, NiCo₂S₄@Ni₃S₂ and Ni₃S₂, respectively. The NiCo₂S₄@Ni₃S₂ exhibits a slightly smaller semicircle than the Ni₃S₂ and NiCo₂S₄ electrode at the medium-frequency, which indicates the lower charge transfer resistance (R_ct).⁵⁸ NiCo₂S₄@Ni₃S₂ also reveals a higher slope at low frequency compared to the Ni₃S₂ and NiCo₂S₄ electrodes, demonstrating its lower diffusive resistance (W).⁵⁹ The above are responsible for the excellent electrochemical performance of the NiCo₂S₄@Ni₃S₂ core–shell NTAs.

Fig. 9 (a) long-term cycle life graph of NiCo₂S₄//rGO and NiCo₂S₄@Ni₃S₂//rGO; (b) Ragone plots of energy density and power density of NiCo₂S₄@Ni₃S₂//rGO.

To highlight the superior electrochemical performance of the ASC device, an advanced Ragone plot of NiCo₂S₄@Ni₃S₂//rGO (energy density vs. power density) is displayed in Fig. 9(b). The equations given below were used to calculate the energy density and power density of the ASC devices:

P = E/Δt (10)

where E is the energy density (W h kg⁻¹), P is the power density (W kg⁻¹), V is the cell voltage (V) and t is the discharge time (s) of the ASC devices. It is worth noting that the NiCo₂S₄@Ni₃S₂//rGO device exhibits an outstanding energy density of 32.75 W h kg⁻¹ at a power density of 0.36 kW kg⁻¹ and still retains a considerable energy density of 25.50 W h kg⁻¹ at a large power density of 2.98 kW kg⁻¹, implying its feasible and promising applications for energy storage. Additionally, the obtained high energy and power densities of our supercapacitors are superior to the values reported so far among the ASC devices based on NiCo₂S₄, such as hollow NiCo₂S₄ (microspheres)//AC (24.7 W h kg⁻¹ at 0.428 kW kg⁻¹),⁶⁰ and mesoporous NiCo₂S₄ (nanoparticles)//AC (28.3 W h kg⁻¹ at 0.245 kW kg⁻¹).⁶¹ Remarkably, a series of ASC devices based on core–shell structures are also displayed for comparison. The value of the NiCo₂S₄@Ni₃S₂//rGO device also possesses a competitive superiority compared with CNT@NiO//PCPs (25.4 W h kg⁻¹ at 0.4 kW kg⁻¹),⁶² NiCo₂O₄@NiMoO₄//AC (21.7 W h kg⁻¹ at 0.157 kW kg⁻¹)⁶³ and so on. In order to investigate the practical application of NiCo₂S₄@Ni₃S₂//rGO, two ASC devices were assembled by series connection. After charging, the NiCo₂S₄@Ni₃S₂//rGO device is able to light up a red commercial light-emitting diode (LED) with a diameter of 5 mm (2.2 V, 20 mA) as shown in Fig. 9(b).

Based on the above results, NiCo₂S₄@Ni₃S₂ core–shell NTAs electrodes possess an ultrahigh specific capacity, a superior rate capability and a fast electron transport and ion diffusion rate, because: (I) NiCo₂S₄ with intrinsic properties of good conductivity and high specific capacity is made into a hollow tubular structure, resulting in a sufficient connection between the electrolyte and the electrode which can significantly promote full utilization of the electrode;^47,48,64 (II) the ultrathin interconnected Ni₃S₂ nanosheets are tightly wrapped on the surface of NiCo₂S₄ NTAs, contributing to enhanced mechanical stability and effective electrolyte accessibility via short ion-transport pathways; (III) Ni₃S₂ and NiCo₂S₄ are favorable faradaic electrode materials, whose combination can provide numerous electroactive sites for the faradaic redox reactions, showing a superior capacity. In short, these favorable features will eventually lead to enormous enhancement of electrochemical performance in energy storage.

4. Conclusions

In summary, we have successfully synthesized a unique NiCo₂S₄@Ni₃S₂ core–shell NTAs supported on Ni foam through a facile and preferable method with boosted electrochemical performance for supercapacitors. In contrast to bare NiCo₂S₄ electrodes and other electrode materials, the obtained hierarchical NiCo₂S₄@Ni₃S₂ core–shell NTAs electrode exhibits a higher specific capacity and outstanding rate capacity due to its homogeneous core–shell nanotube structure and the faradaic characteristics of NiCo₂S₄ and Ni₃S₂. Moreover, the ASC device assembled by taking NiCo₂S₄@Ni₃S₂ NTAs as the positive electrode and rGO aerogel as the negative electrode can achieve a mass specific capacity of 163.15 C g⁻¹ at 0.5 A g⁻¹ and a maximum energy density of 32.75 W h kg⁻¹ at a power density of 361 W kg⁻¹, as well as high cycle stability (77.5% retention at 2 A g⁻¹ after 5000 cycles). Even at the highest power density of 2.9 kW kg⁻¹, the NiCo₂S₄@Ni₃S₂//rGO device still remains at 25.50 W h kg⁻¹. Therefore, these results not only light up the importance of judiciously designed core–shell nanostructures for achieving enhanced performance, but also demonstrate that NiCo₂S₄@Ni₃S₂//rGO holds promising application in ASCs.

Acknowledgements

The authors are grateful for the testing support from the Analysis and Testing Center (Xiamen University).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21284k

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