English

• 0.   Introduction

  The sharpening of energy and environmental problems and the intermittent nature of alternative renewable energy (wind, solar, tidal, etc.) have pushed efficient energy storage systems into the research boom. Until now, the available energy storage techniques contained rechargeable batteries, fuel cells, and supercapacitors. What stands out is that supercapacitors possess faster charging rates, a high power density, a wide operating temperature range, and a long cycle life compared with other energy storage devices^[1-3]. It is worth mentioning that the electrode is the most key component, compared with the other two components (electrolyte and separator)^[4-5]. It is the performance of electrode materials, for instance, their thermal and structural stability, electronic conductivity, large specific surface area, high porosity, and redox activity, that determines the performance of supercapacitors^[6-8].

  Transition metal sulfides (TMSs) have been testified to be promising electrode materials for supercapacitors^[9-11], which can be ascribed to their rich redox active sites, low cost, environmental friendliness, and high theoretical capacitance. Particularly, TMSs demonstrate better specific capacity than transition metal oxides, which are mainly derived from the sulfur element and exhibit a lower electronegativity than oxygen, making them have a flexible phase structure. This feature promotes electron transfer and further improves electrochemical performance^[12-13]. According to the research that has been reported, TMSs can be divided into Mo-based, Co-based, Ni-based, Cu-based, and W-based sulfides^[14]. Although TMSs have been employed as electrode materials in the research of supercapacitor performance, it is proven that they still suffer from intrinsic low conductivity, large volume expansion, low surface area, poor cyclability, and nanoparticle aggregation issues^[15-17]. As a consequence, it is urgent to develop novel electrode materials that can overcome the above problems.

  In recent years, researchers have testified several approaches such as structure engineering (hollow structure), interface engineering (heterostructure), interface expansion, phase transformation, heteroatom doping, and combination with carbon materials (graphene) or conductive polymers (polypyrrole, polyaniline)^[18-19]. For example, Wu et al. adopted a simple nitrilotriacetic acid-assisted hydrothermal technique to prepare a series of hierarchical NiS/carbon hexahedrons that consist of self-assembling nanosheets or nanorods^[20]. Owing to the synergetic effect of void space and hierarchical assembly mode, the specific capacitance of optimized NiS/carbon electrode can reach up to 1 530.4 F·g^-1 at 1 A·g^-1 and it showed excellent cycle stability with 85.6% after 5 000 cycles. Yan et al. reported double-layered yolk-shell microspheres of NiCo₂S₄-Ni₉S₈-C by using bimetallic MOFs as self-template^[21]. The prepared sample showed a high specific capacity of 293.6 mAh·g^-1 at 1 A·g^-1 and excellent rate capacity (81.1% from 1 to 20 A·g^-1). Benefitting from the hetero-interfaces of NiCo₂S₄-Ni₉S₈ can optimize the electronic distribution, coupled with the excellent electroconductivity of carbon within microspheres, which boosts the electrochemical performance. Except for Co or Ni-based sulfides, several works have confirmed that Co-Ni-based sulfides also exhibit remarkable performance^[22-23], which is caused by they can produce a generous redox reaction derived from the coexistence of Co and Ni ions instead of single metal sulfides^[24-25]. In summary, it is still challenging to prepare high-performance electrode materials that can simultaneously achieve both composition optimization and structure design. Metal-organic frameworks (MOFs), which possess the advantages of unique morphology, high specific surface area, rich metal sites, and adjustable pore size^[26-28], can solve this issue. Except for direct use, MOFs are acted as excellent precursors or templates to form unique hollow architecture.

  In this work, the novel electrode materials CoNi_xS_y/NC (NC: nitrogen-rich doped carbon hollow nanocage) were fabricated by core-shell MOFs precursor ZIF-8@Ni-ZIF-67 through the pyrolysis-sulfurization two-step method. CoNi_xS_y/NC well inherits the polyhedron morphology of ZIFs, and multi-component metal sulfides uniformly disperse in the MOFs-derived hollow carbon nanocage, which can alleviate the agglomeration tendency of active nanoparticles, restrain the volume expansion, and accelerate the electron and ion transfer, resulting in high electrochemical performance.

  1.   Experimental

  1.1   Materials

  Co(NO₃)₂·6H₂O and 2-methylimidazole were obtained from Aladdin Chemical Co., Ltd. Meanwhile, Zn(NO₃)₂·6H₂O, S powder and potassium hydroxide (KOH, 85%) were purchased from China Pharmaceutical Chemicals Co., Ltd., respectively. Finally, dimethylglyoxime nickel (DNi) and Nafion were achieved from Alfa-Aesar. In particular, all reagents were analytically pure and without further purification.

  1.2   Synthesis of core-shell precursor ZIF-8@Ni/ZIF-67

  ZIF-8@Ni/ZIF-67 precursor can be prepared through simple methods of epitaxial growth and ion exchange. In the first place, the crystal seed (ZIF-8) was synthesized by the reported technique^[29]. Specifically, Zn(NO₃)₂·6H₂O (8.926 g) and 2-methylimidazole (9.248 g) were dissolved in 225 mL methanol, respectively. After ultrasonic treatment for 5 min, the ligand solution was poured into another solution. Then, the mixed solution was constantly stirred for 24 h at room temperature. ZIF-8 can be obtained by centrifugation, methanol washing, and drying at 70 ℃. As-prepared ZIF-8 (0.747 g) was added to 150 mL methanol by ultrasound (5 min). Simultaneously, DNi (0.913 g) was dissolved in 200 mL methanol containing 8.76 g Co(NO₃)₂·6H₂O and the mixture was subjected to ultrasonic treatment for 25 min. 2-Methylimidazole (9.24 g) was also dissolved in 50 mL methanol and the mixture was subjected to ultrasonic treatment for 5 min. Subsequently, all solutions were uniformly mixed and stirred magnetically for 24 h. After standing for 1 h, the precipitate was collected by centrifuging with methanol, finally, dried in a 70 ℃ vacuum to obtain precursor ZIF-8@Ni/ZIF-67.

  1.3   Synthesis of CoNi/NC and CoNi_xS_y/NC

  CoNi/NC was obtained by pyrolysis precursor. Briefly, ZIF-8@Ni-ZIF-67 was placed in a tube furnace and heated to 850 ℃ with a ramp rate of 2 ℃·min^-1 and kept for 2 h in flowing N₂. To obtain N-doped carbon-supported CoNi_xS_y composite materials (CoNi_xS_y/NC), CoNi/NC was further mixed with S powder in a mass ratio of 2∶1 and the mixture was ground for 10 min. Finally, the mixture was heated to 600 ℃ with a ramp rate of 3 ℃ min^-1 and kept for 2 h in a N₂ atmosphere.

  1.4   Synthesis of ZnS/CoNi_xS_y

  ZnS/CoNi_xS_y was prepared by using the same procedure to directly sulfurize the precursor ZIF-8@Ni/ZIF-67.

  1.5   Characterization

  The crystal phase of products was determined by X-ray diffraction (XRD, Bruker AXS D8 Advance, Cu Kα radiation, λ=0.154 18 nm), with a step size of 0.2 (°)·s^-1 in the 2θ ranging from 5° to 80°. In addition, the test voltage and current were 40 kV and 40 mA. Simultaneously, microstructure and morphology were analyzed by scanning electron microscope (SEM, FEI Quanta FEG 250) at an accelerating voltage of 30 kV.

  1.6   Electrochemical measurements

  The electrochemical performance was also investigated by a typically three-electrode system. The working electrode was prepared via the reported method by our group^[30]. Briefly, a mass fraction of 75% of the active material was mixed with 20% acetylene black and 5% Nafion to form a uniform slurry, then coated on a piece of nickel foam (1 cm²). The active mass loading on the electrode was 1.5 mg·cm^-2. Simultaneously, Hg/HgO electrode and platinum sheet were used as reference electrodes and counter electrodes respectively, and 6 mol·L^-1 KOH was employed as the electrolyte. The cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) test, and electrochemical impedance spectroscopy (EIS) test of electrode materials were performed on the CHI760E electrochemical workstation. The specific capacitance (C, F·g^-1) of electrode material was calculated by the equation^[31]: C=IΔt/(ΔVm), where I is the discharge current (A), Δt is the discharge time (s), ΔV is the charge-discharge voltage window (V), and m is the mass of active material (g). In addition, the specific capacity (Q, mAh·g^-1) of the battery type of the individual electrode was calculated according to the equation^[32]: Q=IΔt/m.

  An asymmetric two-electrode supercapacitor (CoNi_xS_y/NC//CoNi_xS_y/NC) was assembled by taking the active material and 6 mol·L^-1 KOH as cathode/anode and electrolyte, respectively.

  2.   Results and discussion

  2.1   SEM, XRD, and N₂ adsorption-desorption analysis

  As depicted in Scheme 1, the structural design philosophy of CoNi_xS_y/NC was delivered. First, the core-shell precursor ZIF-8@Ni-ZIF-67 was synthesized by an epitaxial growth strategy. In particular, the nanorod-shaped DNi was introduced as a Ni source.

  Scheme 1

  

  Scheme 1.  Schematic illustration of the production of CoNi_xS_y/NC

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  During the fabrication of the precursor, the Co nodes in ZIF-67 were partially substituted with Ni by ion exchange reaction, and the Ni-ZIF-67 shell with metal Co and Ni alternating was constructed. This regular alternation avoids the formation of large-size Co nanoparticles and agglomeration between particles during high-temperature carbonation, which is conducive to exposing more active sites. From Fig. 1, it can be seen that the distributions of Zn, Ni, Co, N, and C atoms were uniform in the core-shell precursor ZIF-8@Ni-ZIF-67. The N and C atoms were distributed all around the precursor, while the Zn atom was located in the core, and the Co or Ni atoms in the outer shell, indicating the successful preparation of the core-shell structure and the introduction of Ni atom. The comparison material ZnS/CoNi_xS_y can be obtained by directly sulfurizing the precursor at 600 ℃. To achieve a porous hollow carbon nanocage structure, the precursor was pyrolyzed at 850 ℃ under a N₂ atmosphere, the hollow structure is favorable to the rapid diffusion of electrolytes and ions, enhancing the rate performance of material^[33-34]. Ultimately, the intermediate product CoNi/NC was evenly mixed with sulfur powder and sulfurized by the same process.

  Figure 1

  

  Figure 1.  Element mapping images of ZIF-8@Ni-ZIF-67

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  The micromorphology and element composition of prepared samples were investigated by SEM images and corresponding energy dispersive X-ray spectra (EDS). It can be seen from Fig. 2a that the directly sulfurized product ZnS/CoNi_xS_y still maintained the morphology of the precursor, which means the sulfurization process will not make the polyhedron collapse. In addition, the amplified SEM image (Fig. 2b) revealed that the metal sulfide particles were uniformly loaded on the surface of the whole polyhedron. Fig. 2c verified the existence of Zn, Co, Ni, C, and O elements. It can be speculated that uniformly dispersed metal sulfide particles may be ZnS, cobalt, or nickel sulfides. The micromorphology and element composition of CoNi/NC were also analyzed, as shown in Fig. 2d-2f. The polyhedron morphology of the precursor was still inherited and the corresponding diameter was about 1.5 μm. After calcination at high temperatures, the surface of the polyhedron happened slight shrinkage due to the carbon restructure, and rich carbon nanotubes grown on the surface under in-situ catalysis of metal Co and Ni, which is consistent with the previously reported results^[35-36]. The in-situ grown carbon nanotubes fabricated a 3D conductive network which can enhance the conductivity of composites and promote electronic and ions rapid shuttle. Especially, when the calcination temperature reached 850 ℃, the core ZIF-8 derived N-doped carbon (Zn will evaporate) employs as a nanostructured hollow framework to accelerate the diffusion kinetics^[37-39]. Simultaneously, the produced gas is conducive to metal ions moving outside, promoting the formation of the hollow structure. The hollow structure has a large specific surface and the hollow interior space can also be used as an electrolyte ion reservoir^[40], which can make it possible that fast Faraday redox reactions during charge/discharge. Meanwhile, this unique hollow structure can alleviate volume expansion during the electrochemical process and improve the cycle stability of electrode materials. From Fig. 2f, it is observed that Co, Ni, and C were uniformly distributed in CoNi/NC material. Moreover, the Zn peak was not detected in the EDS, indicating that it has completely evaporated at high temperatures.

  Figure 2

  

  Figure 2.  (a, b, d, e) SEM images and (e, f) EDS of (a-c) ZnS/CoNi_xS_y and (d-f) CoNi/NC

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  To further improve the electrochemical performance of materials, the low-activity CoNi nanoparticles were transformed into multi-component metal sulfides with higher activity by the sulfurization process. The achieved CoNi_xS_y/NC still maintained the morphology of polyhedron and the produced metal sulfides were uniformly dispersed on the surface of hollow carbon nanocage, as shown in Fig. 3a and 3b. From Fig. 3c and 3d, the hollow nanorod structure with carbon nano tube (CNTs) and nanoparticles on the surface was also seen, which came from the unreacted DNi. The metal Ni nanoparticles can effectively catalyze the formation of CNTs at 850 ℃. Then, after sulfurization, the evenly dispersed Ni nanoparticles were transformed into NiS and Ni₉S₈. Many studies have confirmed that NiS and Ni₉S₈ can be used as active components to improve the supercapacitor′s performance of electrode materials^[41-44]. Therefore, the unreacted DNi can increase the electrochemical performance of the prepared material.

  Figure 3

  

  Figure 3.  SEM images of CoNi_xS_y/NC

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  The corresponding elemental content of CoNi_xS_y/NC can be obtained through EDS, as shown in Fig. 4a. The contents of Co, Ni, and S were 4.5%, 1.3%, and 4.3%, respectively. The XRD patterns for ZnS/CoNi_xS_y, CoNi/NC, and CoNi_xS_y/NC are presented in Fig. 4b. For ZnS/CoNi_xS_y, the diffraction peaks of 28.93°, 43.87°, 47.83°, and 56.64° correspond to the (0120), (1064), (1073) and (0219) planes, respectively, suggesting the formation of ZnS phase (PDF No.83-1700). The peaks at 2θ of 39.6° and 52.2° can be ascribed to the (331) and (440) planes of Co₉S₈ (PDF No.86-2273). In addition, some weak diffraction peaks can be owned to the CoNi₂S₄ (PDF No.86-2247), NiS (PDF No.75-0612), and Ni₉S₈ (PDF No.78-1886) phases. The results confirm the coexistence of multi-component metal sulfides after direct vulcanization of precursor, including ZnS, Co₉S₈, CoNi₂S₄, Ni₉S₈, and NiS, which is in good agreement with the EDS result. From the XRD pattern of CoNi/NC, it can be seen three major diffraction peaks at 26.22°, 44.38°, and 51.68°, which can be ascribed to the graphitic carbon, metal Co or Ni, and CoNi alloy, respectively^[29]. Compared with CoNi/NC, the peak of graphitic carbon in ZnS/CoNi_xS_y was not detected, indicating that the high-temperature pyrolysis process is conducive to the formation of graphitic carbon with outstanding conductivity. The XRD pattern of CoNi_xS_y/NC still possessed the diffraction peaks of Co₉S₈, CoNi₂S₄, Ni₉S₈, and NiS. The weak peaks of ZnS are due to lots of Zn having been evaporated and removed during high-temperature carbonization. It is noteworthy that the peak intensities of CoNi₂S₄, Ni₉S₈, and NiS increased significantly, demonstrating that there were a large amount of nickel sulfide and Co-Ni-based sulfides in the material. The peak at 26.58° can be well assigned to the graphitic carbon. These results are also consistent with EDS. The N₂ adsorption-desorption isotherm of CoNi_xS_y/NC was further analyzed (Fig. 4c) which displayed the reversible type Ⅳ isotherm, indicating its mesoporous structures. CoNi_xS_y/NC possessed a specific surface area of 313 m²·g^-1. Simultaneously, the pore size distribution curve (Fig. 4d) presented that the average pore diameter of CoNi_xS_y/NC was about 6.3 nm. A highly abundant porous structure and large specific surface area can provide more active sites and rich channels for rapid electronic transport, and then the electrocatalytic performance of CoNi_xS_y/NC can be enhanced.

  Figure 4

  

  Figure 4.  (a) EDS of CoNi_xS_y/NC; (b) XRD patterns of ZnS/CoNi_xS_y, CoNi/NC, and CoNi_xS_y/NC; (c) N₂ adsorption-desorption isotherm and (d) pore size distribution of CoNi_xS_y/NC

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  2.2   Electrochemical performance

  The electrochemical performance of ZnS/CoNi_xS_y, CoNi/NC, and CoNi_xS_y/NC was investigated in a three-electrode system with 6 mol·L^-1 KOH. As shown in Fig. 5a, it can be seen from different CV curves of three materials that ZnS/CoNi_xS_y and CoNi_xS_y/NC all exhibited significant redox peaks compared with CoNi/NC, which is due to the Faraday redox reaction between multi-component metal sulfides and OH^-. Particularly, the CV curve of CoNi_xS_y/NC manifested a larger integral area in the whole potential range than others, suggesting the importance of the combination of multi-component metal sulfides with a unique hollow carbon nanocage for improved capacitance. Remarkably, CoNi_xS_y/NC exhibited a larger closed CV curve area than ZnS/CoNi_xS_y, indicating the electrochemical activity of material that contains high-content of CoNi₂S₄, Ni₉S₈, and NiS is higher than ZnS-based material. Some researchers have testified that ternary Co-Ni-based sulfides can be considered as the hopeful electrode materials for high-performance supercapacitors, owing to Co ions and Ni ions can realize rich redox reactions^[45]. For the CV curve of CoNi/NC, there was the smallest CV closed area due to the lack of an active component of metal sulfides in this material, which will not undergo a typical redox reaction. The corresponding conclusion could also be verified from the GCD curves, as shown in Fig. 5b. The CoNi_xS_y/NC possessed a maximum discharge time of 314.6 s at 1 A·g^-1 compared with ZnS/CoNi_xS_y (165.3 s) and CoNi/NC (21.8 s). Corresponding to the redox peak of CV curves, the GCD curves of all samples presented non-linear with a stable discharge-charge platform.

  Figure 5

  

  Figure 5.  Electrochemical performance of ZnS/CoNi_xS_y, CoNi/NC, and CoNi_xS_y/NC in 6 mol·L^-1 KOH electrolyte: (a) CV curves at the scan rate of 10 mV·s^-1; (b) GCD curves at the current density of 1 A·g^-1

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  Subsequently, the CV curves at different scan rates and GCD curves at different current densities of CoNi_xS_y/NC, ZnS/CoNi_xS_y, and CoNi/NC were evaluated. From Fig. 6a, 6c, and 6e, it can be seen that all electrodes remained electrochemical stable at the potential window of 0 to 0.6 V when the scan rates are different. For ZnS/CoNi_xS_y and CoNi_xS_y/NC, with the increase in scan rate, the redox peak can be observed in each curve, revealing that the charge storage mainly depended on Faraday redox transformation. Simultaneously, the curve can be well retained and showed a symmetrical redox peak, proving the remarkable discharge-charge reversibility of electrodes. Fig. 6b, 6d, and 6f show the GCD curves of CoNi_xS_y/NC, ZnS/CoNi_xS_y, and CoNi/NC at various current densities. The obvious voltage plateau can be well consistent with the Faraday property in CV curves. There is no doubt that the discharge times of CoNi_xS_y/NC at different current densities were highest than ZnS/CoNi_xS_y and CoNi/NC.

  Figure 6

  

  Figure 6.  Electrochemical performance in 6 mol·L^-1 KOH electrolyte: CV curves of (a) CoNi_xS_y/NC, (c) ZnS/CoNi_xS_y, and (e) CoNi/NC at scan rates from 5 to 100 mV·s^-1, respectively; GCD curves of (b) CoNi_xS_y/NC, (d) ZnS/CoNi_xS_y, and (f) CoNi/NC at different densities

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  Specifically, the specific capacitances of all prepared electrode materials at different current densities can be observed in Fig. 7a. When the current density was 1, 2, 3, 5, 8, and 10 A·g^-1, the specific capacitance of CoNi_xS_y/NC reached 629.2, 618.4, 610.2, 600.0, 585.6, and 578.0 F·g^-1, respectively. Which is superior over than some reported Co or Ni-based electrode materials (Table 1). Simultaneously, the specific capacitance of ZnS/CoNi_xS_y was 330.2, 331.8, 331.6, 323.8, 310.2, and 303.8 F·g^-1. The specific capacitance of CoNi/NC was respectively 41.5, 43.6, 43.2, 40.0, 36.8, and 34.0 F·g^-1. In addition, the calculated specific capacity of CoNi_xS_y/NC was 87.4, 85.9, 84.8, 83.3, 81.3, and 80.3 mAh·g^-1 under different current densities. To further evaluate the ion/electron transfer behavior of three electrodes, the Nyquist plots were measured, as shown in Fig. 7b. The high-frequency semicircle area is considered as transfer resistance (R_ct), which mainly comes from the electrode reaction. Notably, CoNi_xS_y/NC had a smaller R_ct than others, indicating CoNi_xS_y/NC electrode material is endowed with high conductivity, which benefits from the formation of graphitic carbon and fabrication of 3D hollow conductive network can accelerate the ion and electron transfer and shorten diffusion path.

  Figure 7

  

  Figure 7.  (a) Specific capacitance comparison at 1-10 A·g^-1 and (b) Nyquist plots of the samples; (c) Cycle life of CoNi_xS_y/NC at 10 A·g^-1

  Inset: GCD curves at the first 20 cycles and SEM images of CoNi_xS_y/NC after cycles.

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  Table 1

  

  Table 1.  Electrochemical performance of some reported similar electrode materials

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  Composite	Rate capability	Cycle performance	Electrolyte	Ref.
  CoNixSy/NC	629.2 F·g-1 (1 A·g-1)	93.4% (1 000 cycles, 10 A·g-1)	6 mol·L-1 KOH	This work
  Flower-like CoS	357 F·g-1 (0.5 A·g-1)	87.3% (2 000 cycles, 0.5 A·g-1)	6 mol·L-1 KOH	[46]
  PPy/CoS/BC	614 F·g-1 (0.7 A·g-1)	62.4% (3 00 cycles, 0.7 A·g-1)	2 mol·L-1 NaCl	[47]
  CoS/N-doped CDs-3	697 F·g-1 (1 A·g-1)	ca. 100% (10 000 cycles, 10 A·g-1)	3 mol·L-1 KOH	[48]
  CC/Co9S8/PPy	429 F·g-1 (1 mA·cm-2)	83.5% (2 500 cycles, 4 mA·cm-2)	2 mol·L-1 KOH	[49]
  Co9S8@S-rGO	348.45 F·g-1 (1 A·g-1)	92.6% (5 000 cycles, 1 A·g-1)	6 mol·L-1 KOH	[50]
  CN-Co/Co9S8	334 F·g-1 (1 A·g-1)	—	6 mol·L-1 KOH	[51]
  Co9S8 NPs	502 F·g-1 (1 A·g-1)	98.2% (7 000 cycles, 8 A·g-1)	1 mol·L-1 KOH	[43]
  Ni9S8 NSs	281 F·g-1 (1 A·g-1)	82% (5 000 cycles, 10 A·g-1)	1 mol·L-1 KOH	[52]
  NiS/CNFs-2	133.3 mAh·g-1 (1 A·g-1)	82.52% (1 000 cycles, 2 A·g-1)	6 mol·L-1 KOH	[53]
  w-NiS-3D-Nf	770 F·g-1 (1 A·g-1)	92.67% (2 200 cycles, 1A·g-1)	2 mol·L-1 KOH	[54]

  As expected, the electrode material CoNi_xS_y/NC with its unique structure and optimization component displayed excellent rate performance, which is closely related to the N-doped carbon hollow network and various metal sulfide compositions in the electrode. Given the high specific capacitance and outstanding rate performance, the long cycle life of the CoNi_xS_y/NC electrode was also evaluated, as shown in Fig. 7c. After for 1 000 charge-discharge cycles at 10 A·g^-1, CoNi_xS_y/NC still maintained 93.4% of initial capacitance value. It can be seen from the inset of Fig. 7c that the discharge time can be well-kept without significant reduction at the first 20 cycles. The higher capacitance retention manifests the electrode material is endowed with superior cycle stability and electrochemical stability. The morphology of CoNi_xS_y/NC after a long-term cycle test was also characterized. It can be seen from the SEM image that the polyhedron and rod-like structure were still clearly visible and without apparent collapse, implying the excellent structural stability of the material.

  To verify the potential practical application of CoNi_xS_y/NC, the powder CoNi_xS_y/NC was coated on the Ni foam (NF). Then, two pieces of CoNi_xS_y/NC/NF were used as cathode and anode to assemble a symmetrical supercapacitor device. The schematic diagram of this device can be observed from the inset of Fig. 8d. In two-electrode system tests, CV curves of different scan rates can be obtained at the voltage window range from 0 to 1.6 V (Fig. 8a). A rectangular shape and a pair of obvious redox peaks appeared in all CV curves, indicating the capacitance contribution is determined by electric double layer capacitance and redox reaction^[55]. The GCD curves at different current densities can be achieved, as shown in Fig. 8b. The discharge time decreased with the increase in current density. Fig. 8c displays the specific capacitances at different current densities, the specific capacitances of the symmetrical supercapacitor device were 207.2, 132.8, 75.2, 53.3, and 34.6 F·g^-1 at 1, 2, 3, 5 and 10 A·g^-1, respectively. The result reveals poor performance at high current density than low. Finally, the cycle stability was also evaluated at 5 A·g^-1 for 1 000 cycles. As shown in Fig. 8d, the capacitance retention can maintain 85.36% and undergoes 1 000 cycles, indicating the assembled device has excellent long cycle life. Therefore, the better electrochemical performance of the device exhibits the potential for the practical application of CoNi_xS_y/NC.

  Figure 8

  

  Figure 8.  Electrochemical performances of CoNi_xS_y/NC//CoNi_xS_y/NC device in 6 mol·L^-1 KOH: (a) CV curves at different scan rates from 10-100 mV·s^-1; (b) GCD curves at different current densities at 1-10 A·g^-1; (c) specific capacitance comparison at different current densities; (d) cycle life at 5 A·g^-1

  Inset: schematic diagram of the CoNi_xS_y/NC//CoNi_xS_y/NC device and GCD curves at the first 10 cycles.

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  3.   Conclusions

  In summary, the multi-component Co-Ni-based sulfides that decorated the N-doped carbon hollow nanocage structure were synthesized via pyrolysis-sulfurization in two steps. Benefitting from the unique hollow structure and optimized component, the prepared CoNi_xS_y/NC composite delivered a high electrochemical performance, including an excellent specific capacitance reaching 629.2 F·g^-1 at 1 A·g^-1 and superior long cycle stability of 93.4% retention after 1 000 cycles. In addition, the assembled symmetrical supercapacitor device also showed high specific capacitance (207.2 F·g^-1 at 1 A·g^-1) and long cycle life (85.36% retention after 1 000 cycles). The enhanced performance can be ascribed: (ⅰ) optimized components can fabricate multi-component metal sulfides, and the interaction between them can provide an adequate number of active sites and improve the conductivity of materials. (ⅱ) Rich N-doped carbon enhances the wettability between the electrode and electrolyte and changes the electronic structure of carbon to improve conductivity. (ⅲ) Fabricated 3D hollow carbon network not only makes metal sulfides uniformly dispersed and exposes more active sites, resulting in remarkable improvements in ion/electron conductivity but also can effectively restrain the volume expansion of electrode material during the charge-discharge process. Consequently, the high performance of multi-component Co-Ni-based sulfides can be further modified to become a promising electrode material for supercapacitors.

  
  
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  Scheme 1  Schematic illustration of the production of CoNi_xS_y/NC

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  Figure 1  Element mapping images of ZIF-8@Ni-ZIF-67

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  Figure 2  (a, b, d, e) SEM images and (e, f) EDS of (a-c) ZnS/CoNi_xS_y and (d-f) CoNi/NC

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  Figure 3  SEM images of CoNi_xS_y/NC

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  Figure 4  (a) EDS of CoNi_xS_y/NC; (b) XRD patterns of ZnS/CoNi_xS_y, CoNi/NC, and CoNi_xS_y/NC; (c) N₂ adsorption-desorption isotherm and (d) pore size distribution of CoNi_xS_y/NC

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  Figure 5  Electrochemical performance of ZnS/CoNi_xS_y, CoNi/NC, and CoNi_xS_y/NC in 6 mol·L^-1 KOH electrolyte: (a) CV curves at the scan rate of 10 mV·s^-1; (b) GCD curves at the current density of 1 A·g^-1

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  Figure 6  Electrochemical performance in 6 mol·L^-1 KOH electrolyte: CV curves of (a) CoNi_xS_y/NC, (c) ZnS/CoNi_xS_y, and (e) CoNi/NC at scan rates from 5 to 100 mV·s^-1, respectively; GCD curves of (b) CoNi_xS_y/NC, (d) ZnS/CoNi_xS_y, and (f) CoNi/NC at different densities

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  Figure 7  (a) Specific capacitance comparison at 1-10 A·g^-1 and (b) Nyquist plots of the samples; (c) Cycle life of CoNi_xS_y/NC at 10 A·g^-1

  Inset: GCD curves at the first 20 cycles and SEM images of CoNi_xS_y/NC after cycles.

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  Figure 8  Electrochemical performances of CoNi_xS_y/NC//CoNi_xS_y/NC device in 6 mol·L^-1 KOH: (a) CV curves at different scan rates from 10-100 mV·s^-1; (b) GCD curves at different current densities at 1-10 A·g^-1; (c) specific capacitance comparison at different current densities; (d) cycle life at 5 A·g^-1

  Inset: schematic diagram of the CoNi_xS_y/NC//CoNi_xS_y/NC device and GCD curves at the first 10 cycles.

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  Table 1.  Electrochemical performance of some reported similar electrode materials

  Composite	Rate capability	Cycle performance	Electrolyte	Ref.
  CoNixSy/NC	629.2 F·g-1 (1 A·g-1)	93.4% (1 000 cycles, 10 A·g-1)	6 mol·L-1 KOH	This work
  Flower-like CoS	357 F·g-1 (0.5 A·g-1)	87.3% (2 000 cycles, 0.5 A·g-1)	6 mol·L-1 KOH	[46]
  PPy/CoS/BC	614 F·g-1 (0.7 A·g-1)	62.4% (3 00 cycles, 0.7 A·g-1)	2 mol·L-1 NaCl	[47]
  CoS/N-doped CDs-3	697 F·g-1 (1 A·g-1)	ca. 100% (10 000 cycles, 10 A·g-1)	3 mol·L-1 KOH	[48]
  CC/Co9S8/PPy	429 F·g-1 (1 mA·cm-2)	83.5% (2 500 cycles, 4 mA·cm-2)	2 mol·L-1 KOH	[49]
  Co9S8@S-rGO	348.45 F·g-1 (1 A·g-1)	92.6% (5 000 cycles, 1 A·g-1)	6 mol·L-1 KOH	[50]
  CN-Co/Co9S8	334 F·g-1 (1 A·g-1)	—	6 mol·L-1 KOH	[51]
  Co9S8 NPs	502 F·g-1 (1 A·g-1)	98.2% (7 000 cycles, 8 A·g-1)	1 mol·L-1 KOH	[43]
  Ni9S8 NSs	281 F·g-1 (1 A·g-1)	82% (5 000 cycles, 10 A·g-1)	1 mol·L-1 KOH	[52]
  NiS/CNFs-2	133.3 mAh·g-1 (1 A·g-1)	82.52% (1 000 cycles, 2 A·g-1)	6 mol·L-1 KOH	[53]
  w-NiS-3D-Nf	770 F·g-1 (1 A·g-1)	92.67% (2 200 cycles, 1A·g-1)	2 mol·L-1 KOH	[54]

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