Dual carbon-modified nickel sulfide composites toward high-performance electrodes for supercapacitors

Abstract

Composites with reduced graphene oxide (rGO) modification or carbon-coated structures are usually constructed to enhance the electrochemical performance of electrode materials. Herein, we develop a sequential freeze-drying, calcination and sulfidation strategy to prepare dual carbon-modified nickel sulfide composites (Ni₃S₂@C/rGO). Owing to the ultrasmall particle size, stable structure and high electrical conductivity, the as-prepared composites exhibit enhanced performance with high specific capacitance (1023.44 F g⁻¹ at a current density of 5 A g⁻¹), good rate capability (848 F g⁻¹ at a current density of 20 A g⁻¹) and long-term cycling stability (70.1% retention over 5000 cycles) as electrode materials for supercapacitors. Moreover, the asymmetric supercapacitor displays a good cycle life and a superior energy density of 52.5 W h kg⁻¹ at a power density of 750 W kg⁻¹. The facile fabrication and excellent electrochemical performances of Ni₃S₂@C/rGO demonstrate that constructing dual carbon-modified composites is a promising strategy for high-performance electrode materials.

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Introduction

With the rapid consumption of fossil fuels and serious deterioration of the environment, various clean and renewable energy sources have drawn much attention. As one of the most promising energy storage devices, supercapacitors (SCs) have been widely investigated owing to the advantages of high power density, low cost, fast charging capability, excellent cycling stability and good environment benignity.^1–3 Among various electrode materials, metal oxides/sulfides (NiO, Ni(OH)₂, MnO₂, NiCo₂S₄etc.)^4–7 are reported to have potential application. Nickel sulfides, such as NiS, NiS₂, Ni₃S₂, and Ni₃S₄ have attracted increasing attention owing to their high theoretical capacitances, rich redox chemistry, low cost and nature abundance.^8–12 Specially, Ni₃S₂ is reported to have excellent performance in supercapacitors.¹³ Unfortunately, the intrinsically poor conductivity and large volume change occurring during a charge/discharge process lead to rapid capacitance fading and poor rate capability.

With an aim to address the above problems, numerous approaches have been rationally developed to improve the electrochemical performance, such as tailoring the size of the nanoparticles,¹⁴ designing micro-nano structures¹⁵ and preparing composites.^16–20 It is worth noting that designing and preparing carbon-modified composites is an effective approach to improve the electrochemical properties of SCs.^21–24 As we all know, with the advantages of light weight, high conductivity and high surface area, graphene is preferred to produce high conductivity and abundant active sites for redox reactions.^25–27 Moreover, carbon coating has also been employed to improve the conductivity in electrical devices.^28–30 The capsule structure not only shortens the electron diffusion path, but also avoids the reunion of nanoparticles and prevents the polysulfide from dissolving, resulting in excellent electrochemical performance. Therefore, double carbon-modified structures are expected to improve the electrochemical performances. For example, Chen et al.¹⁴ prepared (Co₉S₈ QD@HCP)@rGO sponge-like composites by simultaneous thermal reduction, carbonization, and sulfidation processes. Wang et al.³¹ synthesized graphene encapsulated hollow FeP@carbon nanocomposites (H-FeP@C@GR). Han et al.³² reported a type of sandwich-type CoS-based coaxial nanocable with a conductive CNT backbone core, well-confined CoS nanoparticle interlayer and a conformal carbon coating shell (denoted as CNT@CoS@C). When the above dual carbon-modified composites were used as anodes for sodium/lithium ion batteries, they all exhibited outstanding electrochemical performance, which was a benefit from the conductive network, structural integrity, and the synergistic effect of each component.

Herein, we report a simple three-step method to fabricate dual carbon-modified Ni₃S₂@C/rGO composites. The introduction of graphene networks endows the ultrasmall Ni₃S₂ nanoparticles with favourable dispersion and enhanced conductivity, thus accelerating electron transport and ion diffusion reactions. Furthermore, the carbon coating layer plays an important role in alleviating the volume expansion of Ni₃S₂, resulting in long-term cycling with high capacity retention rates. In addition, the rGO and carbon layers play a synergistic effect for avoiding aggregation of Ni₃S₂ nanoparticles during the charge/discharge process. While used as an electrode for supercapacitors, Ni₃S₂@C/rGO delivers excellent cycling stability and good rate properties. It is demonstrated that the dual carbon-modified method is an effective strategy to improve the electrochemical performance of electrode materials.

Experimental

Synthesis of NiO@C/rGO precursors

A dual carbon-modified nickel oxide composite was synthesized by a freeze-drying method, followed by an annealing process. The typical process was as follows: 0.02 g of graphene oxide (GO) was dispersed in 50 mL of deionized water under stirring, and then 0.3 g of Ni(NO₃)₂·6H₂O and 0.3 g of citric acid were added into the above mixture. After stirring for 30 min, the blended solution was freeze-dried under vacuum for 48 hours. After that, the above sample was calcined under argon for 2 h at 450 °C to obtain NiO@C/rGO precursors. For comparison, Ni/rGO was prepared under the same conditions without the addition of citric acid. The NiO@C composite was also prepared without the addition of GO.

Synthesis of Ni₃S₂@C/rGO

Ni₃S₂@C/rGO was obtained via a sulfidation of NiO@C/rGO. Typically, 50 mg of the as-obtained NiO@C/rGO precursor was dispersed in 35 mL of deionized water by sonication, followed by the addition of 1.0 g Na₂S·9H₂O under vigorous stirring. Then the suspension was transferred into 50 mL Teflon lined stainless steel autoclaves for hydrothermal reaction at 120 °C for 10 h. Finally, the precipitates were washed with deionized water and ethanol several times and dried at 60 °C overnight, and the Ni₃S₂@C/rGO composite was obtained. For comparison, Ni₃S₂/rGO and Ni₃S₂@C composites were also prepared under the same conditions with Ni/rGO and NiO@C as precursors, respectively.

Material characterization

The crystallographic characterization of the samples was done by powder X-ray diffraction (XRD, Bruker, D8 ADVANCE powder diffractometer, Cu Kα radiation, λ = 0.15418 nm). The morphologies were characterized by field emission scanning electron microscopy (SEM, Hitachi, SU-8010) linked with X-ray energy-dispersive spectroscopy (EDS, EDAX PW9900), and transmission electron microscopy (TEM, FEI, Tecnai G20). X-ray photoelectron spectroscopy (XPS) was conducted on a VG Multilab 2000 (VG Inc.) photoelectron spectrometer using monochromatic Al Kα radiation under vacuum at 2 × 10⁻⁶ Pa. The thermal behavior of the as-prepared samples has been investigated by the thermogravimetric analyzer-derivative thermogravimetry technique (TGA-Q500). The specific surface areas and porous features of the samples were further investigated by nitrogen adsorption/desorption measurements (Autosorb-IQ2-VP).

Electrochemical measurements

The working electrodes were fabricated by mixing active material (Ni₃S₂@C/rGO, Ni₃S₂/rGO or Ni₃S₂@C), acetylene black and a binder (PVDF) in a weight ratio of 80 : 10 : 10. The formed paste was pressed on a piece of nickel foam (1.0 cm × 1.0 cm) at 2 MPa, and dried under vacuum at 80 °C for 10 h. Electrochemical measurements were conducted in a three-electrode arrangement in 2 M KOH electrolyte. A platinum sheet and HgO/Hg electrode were used as the counter electrode and reference electrode, respectively. Cyclic voltammetry (CV) was conducted with a Zahner IM6 electrochemical workstation with voltage scan rates of 1, 2, 5 and 10 mV s⁻¹. The galvanostatic charge–discharge tests were conducted on a LAND battery system at the current densities of 1, 2, 5, 10, and 20 A g⁻¹. The specific capacitance is calculated according to the following equation:where C (F g⁻¹) is the specific capacitance, I (A) represents the discharge current, and m (g), ΔV (V) and Δt (s) denote the mass of active materials, potential drop during discharge and the total discharge time, respectively. The energy density E (W h kg⁻¹) and power density P (W kg⁻¹) are calculated from the following equations:where C_cell (F g⁻¹) is the specific capacitance of the supercapacitor device, ΔV (V) is the working potential window, and Δt (s) is the time for full discharge.

Results and discussion

The schematic illustration for the fabrication of dual carbon-modified Ni₃S₂@C/rGO is shown in Fig. 1 and the specific details are described in the Experimental section. The whole formation process of the dual carbon-modified Ni₃S₂@C/rGO is mainly based on three stages: freeze-drying, calcination and sulfidation. At the initial stage of freeze-drying, a 3D GO network structure was formed by self-assembly, meanwhile, nickel citrate was formed and anchored on GO sheets by electrostatic interaction. Subsequently, nickel citrate and excessive citric acid were decomposed during the calcination process, leading to the formation of NiO with a carbon coating structure and the reduction of GO. As a result, uniform NiO@C nanoparticles anchored on rGO (see ESI Fig. S1†) were prepared and employed as precursors. As shown in Fig. S1a,† the diffraction peaks are mainly indexed to Ni (JCPDS no. 70-989) and partially indexed to NiO (JCPDS no. 73-1519). From the SEM and TEM images (Fig. S1b–e†), the nanoparticles display a diameter of 20–30 nm and uniformly embed in the rGO matrix. Fig. S1f† shows that the NiO nanoparticles are encapsulated by amorphous carbon. Therefore, the NiO@C/rGO precursor already has a dual carbon-modified structure. Subsequently, ion exchange reaction occurs during the following sulfidation process, converting NiO@C/rGO into Ni₃S₂@C/rGO.

Fig. 1 Schematic illustration of the synthesis of dual carbon-modified Ni₃S₂@C/rGO.

The crystal structure of Ni₃S₂@C/rGO was examined by XRD. As shown in Fig. 2a, the diffraction peaks at 21.8°, 31.2°, 37.8°, 44.4°, 49.8°, 50.3° and 55.3° are assigned to the (010), (−110), (111), (020), (120), (−120) and (−211) crystal planes of rhombohedral Ni₃S₂ (JCPDS no. 71-1682), respectively. Meanwhile, impurity diffraction peaks of NiS (JCPDS no. 3-760) with low intensities are also observed. The morphology of Ni₃S₂@C/rGO is shown in Fig. 2b and c. It can be seen that Ni₃S₂ nanoparticles with a small size of 20–30 nm uniformly disperse and tightly anchor on the conductive rGO networks. The low magnification TEM image shown in Fig. 2d further confirms the aforementioned observation. HRTEM images shown in Fig. 2e and f present a clear lattice spacing of 0.28 nm, 0.41 nm and 0.24 nm corresponding to the (−110), (010) and (111) planes of Ni₃S₂, respectively, indicating excellent crystallinity. From Fig. 2f, it is clearly seen that Ni₃S₂ nanoparticles are enwrapped by an amorphous carbon shell with a thickness of ∼5 nm, which is formed by the decomposition of citric acid. In addition, EDS mapping has been carried out to demonstrate the distribution of elements. Obviously, the atoms of Ni, S and C distribute uniformly in the composite (Fig. 2g). This dual carbon-modified structure will greatly enhance the conductivity of the Ni₃S₂ electrode materials, and will further be beneficial for the improvement of the electrochemical performances.

Fig. 2 (a) XRD pattern, (b, c) SEM images and (d–f) TEM images and (g) EDS mapping of Ni₃S₂@C/rGO.

XPS was conducted to determine the surface electronic states and chemical composition of Ni₃S₂@C/rGO. The survey spectrum shown in Fig. 3a shows that four elements of Ni 2p, S 2p, C 1s and O 1s exist in the as-prepared Ni₃S₂@C/rGO composite. The element O may be derived from rGO or decomposition of oxygen-containing groups in citric acid during the pyrolysis process.²⁸ As shown in Fig. 3b, the two peaks at approximately 855.8 eV and 873.2 eV in the high-resolution Ni 2p spectrum can be assigned to Ni 2p3/2 and Ni 2p1/2, respectively. The Ni 2p spectrum can be fitted with two spin–orbit dual characteristics of Ni²⁺ and Ni³⁺, corresponding to the oxidation state of Ni in Ni₃S₂ and NiS.^9,33 The peaks located at 861.7 eV and 879.6 eV correspond to the satellite peaks.^34,35 The S 2p spectrum displays characteristic peaks of S 2p3/2 and S 2p1/2 at about 159.9 eV and 162.4 eV, respectively (Fig. 3c). In the C 1s spectrum (Fig. 3d), the peaks appearing at 284.6 eV and 285.4 correspond to the C=C and C–C bonds, which exist in rGO and the pyrolytic carbon. A peak at 286.2 eV is obviously detected, which can be assigned to the C–O configuration, confirming the existence of oxygenated functional groups in the dual carbon decoration.³⁶

Fig. 3 XPS spectra of the Ni₃S₂@C/rGO composite: (a) full spectrum, (b) Ni 2p, (c) S 2p and (d) C 1s.

In order to verify the effect of the dual carbon-modified structure on the electrochemical properties, Ni₃S₂/rGO and Ni₃S₂@C composites were obtained for comparison using Ni/rGO and NiO@C as precursors. In Fig. S2a,† the as-prepared Ni/rGO sample exhibits high purity, which is consistent with JCPDS no. 70-1869. Fig. S2b–d† show that metal Ni nanoparticles with small sizes uniformly distribute on rGO sheets. For the NiO@C precursor, both peaks of Ni (JCPDS no. 70-1869) and NiO (JCPDS no. 71-1179) diffusion peaks are observed (Fig. S3a†). The NiO particles coated by carbon display a good distribution, which is confirmed by Fig. S3b–d.† Through the sulfidation process, Ni₃S₂/rGO and Ni₃S₂@C composites were obtained. As shown in Fig. 4a, the sharp diffraction peaks indicate well the degree of crystallinity of Ni₃S₂/rGO, and the crystal lattice shown in Fig. 4d also demonstrates it. In the Ni₃S₂/rGO composite, Ni₃S₂ nanoparticles with an ultrasmall size of ∼20 nm uniformly distribute and tightly anchor on the rGO conductive substrate (Fig. 4b and c). As for Ni₃S₂@C, it exhibits a weak degree of crystallinity with a low diffraction peak intensity as shown in Fig. 4e. Impurity diffraction peaks of NiS (JCPDS no. 3-760) are also found for both of Ni₃S₂/rGO and Ni₃S₂@C, similar to Ni₃S₂@C/rGO. Fig. 4f–h shows that Ni₃S₂@C is composed of amorphous carbon coated Ni₃S₂ nanoparticles with a diameter of ∼10 nm, which is smaller than Ni₃S₂/rGO and Ni₃S₂@C/rGO.

Fig. 4 (a) XRD pattern, (b) SEM image and (c, d) TEM images of Ni₃S₂/rGO, and (e) XRD pattern, (f) SEM image and (g, h) TEM images of the Ni₃S₂@C composite.

TGA of Ni₃S₂@C/rGO, Ni₃S₂/rGO and Ni₃S₂@C composites in air was conducted from room temperature to 800 °C. As shown in Fig. S4,† slight weight loss in the initial stage results from the removal of water and residual organics. Approximately 57.8%, 86.3% and 66.8% of the original sample weight remained after TGA in air and the resultant solid combustion product is NiO.²⁷ The overall reaction is given in eqn (4).

2Ni₃S₂ + 7O₂ = 6NiO + 4SO₂ (4)

Therefore, the Ni₃S₂ content in Ni₃S₂@C/rGO, Ni₃S₂/rGO and Ni₃S₂@C was estimated to be 61.9%, 92.3% and 71.6%, respectively. Besides, the specific surface area and porous characteristics of the samples are characterized through N₂ adsorption–desorption measurements, which are shown in Fig. S5.† It can be found that the specific surface area of Ni₃S₂@C (170.2 m² g⁻¹) is higher than that of Ni₃S₂@C/rGO (144.2 m² g⁻¹) and Ni₃S₂/rGO (58.5 m² g⁻¹). The results demonstrate that the introduction of amorphous carbon can greatly improve the specific surface area of Ni₃S₂@C/rGO.

The typical cyclic voltammetry (CV) curves of Ni₃S₂@C/rGO at sweep rates ranging from 1 to 10 mV s⁻¹ are presented in Fig. 5a. The observed pair of redox peaks indicates the existence of faradaic redox reactions between Ni²⁺/Ni³⁺ and OH⁻ during the electrochemical process, which is shown as follows:^37,38

Ni₃S₂ + 3OH⁻ = Ni₃S₂(OH)₃ + 3e⁻ (5)

Fig. 5 (a) CV curves of the Ni₃S₂@C/rGO electrode at different scan rates, (b) charge–discharge curves of the Ni₃S₂@C/rGO electrode at different current densities, (c) cycle life and coulombic efficiency of the Ni₃S₂@C/rGO, Ni₃S₂/rGO and Ni₃S₂@C electrodes at 5 A g⁻¹, (d) rate capability of the Ni₃S₂@C/rGO, Ni₃S₂/rGO and Ni₃S₂@C electrodes, (e) the EIS curves of the freshly prepared Ni₃S₂@C/rGO, Ni₃S₂/rGO and Ni₃S₂@C electrodes, and (f) the EIS curves of the Ni₃S₂@C/rGO electrode after 1000 cycles at 5 A g⁻¹.

Moreover, the CV curves measured with variable scan rates display similarly shapes without any obvious distortion, confirming its pseudocapacitance characteristic and good electrochemical reversibility. In addition, the galvanostatic charge–discharge curves of the Ni₃S₂@C/rGO electrode collected at different current densities show the platform nearly at 0.35/0.45 V (Fig. 5b), which is well consistent with the CV observations. In comparison, Ni₃S₂/rGO (Fig. S6a†) and Ni₃S₂@C (Fig. S6b†) electrodes show similar charge/discharge curves, but much lower capacitances. Furthermore, a comparison of the cyclic voltammetry (CV) curves at different scan rates is shown in Fig. S7a–c.† The integral areas of the CV curves illustrate that the Ni₃S₂@C/rGO electrode displays significantly enhanced electrochemical performance compared with Ni₃S₂/rGO and Ni₃S₂@C. Based on the CV curves, the redox peak current follows well a power-law relationship with the scan rate (I_p = aν^b), where a b-value of 0.5 indicates that the redox reaction of Ni₃S₂ electrodes can be observed as a diffusion-controlled battery-type faradaic process.⁷

Long cycling stability is also a significant factor for supercapacitors. Repeated charging/discharging measurement at a current density of 5 A g⁻¹ is conducted to further evaluate the electrochemical properties. As shown in Fig. 5c, it is noted that the Ni₃S₂@C/rGO electrode presents good cycling stability after 5000 cycles compared with Ni₃S₂/rGO and Ni₃S₂@C. The initial discharge capacitance of Ni₃S₂@C/rGO is about 1023.44 F g⁻¹, and the specific capacitance remains at 724.32 F g⁻¹ and after that, repeated charging/discharging proceeds 5000 times, with a capacity retention ratio of 70.1%. While for Ni₃S₂/rGO and Ni₃S₂@C, there is a short activation process at the initial charge/discharge stage, followed by sharp capacitance degradation with retention ratios of 51.9% and 45.0%, respectively. The rate capabilities of the Ni₃S₂@C/rGO, Ni₃S₂/rGO and Ni₃S₂@C electrodes measured at 1, 2, 5, 10 and 20 A g⁻¹ are shown in Fig. 5d and Table S1.† Obviously, it can be seen that the Ni₃S₂@C/rGO electrode delivers the highest specific capacitance of 1171 F g⁻¹ at 1 A g⁻¹, whereas only 980 F g⁻¹ and 910 F g⁻¹ are obtained for Ni₃S₂/rGO and Ni₃S₂@C, respectively. Upon increasing the high current density to 20 A g⁻¹, the Ni₃S₂@C/rGO electrode can still retain a large specific capacitance of 848 F g⁻¹, demonstrating good rate capability. Electrochemical impedance spectroscopy (EIS) was further conducted to investigate the redox reaction kinetics of the Ni₃S₂@C/rGO, Ni₃S₂/rGO and Ni₃S₂@C electrodes. As shown in Fig. 5e, a semicircle in the high-frequency region and a straight line in the low-frequency region are obviously observed, which are related to the charge transfer resistance (R_ct) and the capacitive behavior of the electrode, respectively.^39,40 The R_ct of Ni₃S₂@C/rGO, Ni₃S₂/rGO and Ni₃S₂@C electrodes are 0.21, 0.24 and 0.25 Ω, respectively, demonstrating that the introduction of rGO and the carbon layer accelerates the electron transfer reaction kinetics. Furthermore, the line in the low frequency of Ni₃S₂@C/rGO is more vertical than Ni₃S₂/rGO and Ni₃S₂@C that confirms the ideal capacitive behavior of the Ni₃S₂@C/rGO electrode.⁴¹ It is observed that even after 1000 cycles at 5 A g⁻¹, the Ni₃S₂@C/rGO electrode displays a similar EIS curve to the freshly prepared electrode (Fig. 5f), illustrating the excellent stability of the electrode.

To evaluate the practical application of the Ni₃S₂@C/rGO composite, an asymmetric supercapacitor (ASC) has been fabricated employing commercial activated carbon (AC) as the negative electrode with a working potential window of 1.5 V in 2 M KOH. The CV curves of the AC electrode display nearly rectangular shape, showing typical electric double-layer capacitance properties (Fig. S8a†). As shown in Fig. S8b,† the AC electrode delivers an average specific capacitance of 167 F g⁻¹ at 5 A g⁻¹ with a high coulombic efficiency of ∼99%. To balance the charge between the positive and negative electrodes, the optimum loading mass ratio of Ni₃S₂@C/rGO : AC is about 1 : 3. Fig. 6a shows the CV profiles of Ni₃S₂@C/rGO//AC ASC at various scan rates of 1–10 mV s⁻¹. The redox peaks of each CV curve indicate the faradaic pseudocapacitive nature of Ni₃S₂@C/rGO//AC.^16,17 When the scan rate increases, the redox peaks shift slightly and remain stable, demonstrating fast charging and discharging characteristics. According to the galvanostatic charge–discharge curves, the specific capacitance of Ni₃S₂@C/rGO//AC is calculated to be 168, 138, 113 and 100 F g⁻¹ at 1, 2, 5 and 10 A g⁻¹, respectively, showing a good rate performance. Fig. 6c shows the cycling stability of the Ni₃S₂@C/rGO//AC ASC. The specific capacitance still remains at 166.8 F g⁻¹ after 1000 cycles. The power performance of Ni₃S₂@C/rGO//AC is also evaluated in the Ragone plot. As shown in Fig. 6d, Ni₃S₂@C/rGO//AC delivers an energy density of 52.5 W h kg⁻¹ at a power density of 750 W kg⁻¹ and a power density of 7500 W kg⁻¹ at an energy density of 31.25 W h kg⁻¹, which are equal or higher than those previously reported nickel sulfide electrode materials.^17,42–44

Fig. 6 (a) CV curves of the Ni₃S₂@C/rGO//AC ASC at different scan rates. (b) Galvanostatic charge–discharge curves of the Ni₃S₂@C/rGO//AC ASC at various current densities and (c) cycling performance of the Ni₃S₂@C/rGO//AC ASC at 1 A g⁻¹.

The excellent electrochemical performance of the dual carbon-modified Ni₃S₂@C/rGO electrode can be attributed to the following effects: (i) the small size of Ni₃S₂ can facilitate the fast reaction kinetics and boost the effective surface area of the active material, resulting in high specific capacity; (ii) TEM images of the electrodes after 1000 cycles at 5 A g⁻¹ confirm that the quite stable nanostructure does favor the good cycling performance of the Ni₃S₂@C/rGO composite (Fig. S9†). On one hand, the uniform and tight distribution of Ni₃S₂ anchored on rGO can not only benefit electron transfer, but also protect Ni₃S₂ nanoparticles from aggregation during cycling (Fig. S9a†). On the other hand, the capsule structure of Ni₃S₂ formed by carbon coating further prevents the aggregation and dissolution of Ni₃S₂ during the charging/discharging process (Fig. S9b†); (iii) the dual carbon-modified structure significantly improves the conductivity of the active materials, thus helping to enhance the rate performance of the electrodes. Therefore, the realization of the dual carbon-modified nanostructure can greatly generate remarkable electrochemical performance of the Ni₃S₂@C/rGO electrode.

Conclusions

In summary, dual carbon-modified Ni₃S₂@C/rGO electrode materials have been prepared by sequential freeze-drying, calcination and sulfidation processes. The unique dual carbon-modified structure effectively protects the aggregation of Ni₃S₂ nanoparticles, constructs integral conductive networks and buffers the volume change of active materials during a charge/discharge process. Benefiting from the ultrasmall particle size, good conductivity and stable nanostructure, the as-prepared Ni₃S₂@C/rGO electrode exhibits superior electrochemical performance for supercapacitors with an outstanding cycle life and rate performance (with the specific capacitance of 1023.44 F g⁻¹ at 5 A g⁻¹ retaining 70.1% after 5000 cycles). The Ni₃S₂@C/rGO//AC ASC also delivers satisfactory energy density and power density, demonstrating its potential in practical application. In addition, this work opens a new avenue for improving the electrochemical performance of electrode materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the Natural Science Foundation of Jiangsu Province (BK20160213, BK20170239), the National Natural Science Foundation of China (51702138), the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18-2106) and the Scientific Research Foundation of Qufu Normal University (xkj201601).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8qi01024b

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