Sponge-like NiCo₂S₄ nanosheets supported on nickel foam as high-performance electrode materials for asymmetric supercapacitors

Abstract

Herein, binder-free sponge-like NiCo₂S₄ nanosheets supported on Ni foam with ultra-high mass loading were synthesized via a facile one-step hydrothermal route. The unique 3D sponge-like network structure of the obtained NiCo₂S₄ nanosheets is feasible for the fast transport of ions and electrons. The NiCo₂S₄ nanosheet-based electrode could achieve a high area capacity of 11.97 F cm⁻² at 15 mA cm⁻², demonstrating its superior electrochemical performance. Besides, the corresponding asymmetric supercapacitor possesses a maximum area energy density of 0.47 mW h cm⁻² at a power density of 6.59 mW cm⁻² and a great cycle life with 80.1% capacitance retention even after 5000 cycles. The outstanding performances confirm that the obtained sponge-like NiCo₂S₄ nanosheets grown on Ni foam can be a prospective candidate for electrochemical energy storage.

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Introduction

With the rapid consumption of fossil fuels in the world, it is urgent to develop new energy storage devices such as supercapacitors (SC).^1,2 Since pseudocapacitive materials exhibit higher specific capacitance and energy density compared with carbonaceous materials, they have attracted great interest among researchers for supercapacitors.^3,4 Until now, a large number of pseudocapacitive materials, such as transition metal oxides⁵/hydroxides⁶/sulfides,⁷ conductive polymers⁸ and metal–organic frameworks,⁹ have been investigated and developed for enhanced pseudocapacitance. Among the various materials above, transition metal sulfides, especially ternary transition metal sulfides, have become a research hotspot in the field of SC due to their high theoretical capacitance and excellent electrical conductivity.¹⁰

As a typical ternary transition metal sulfide, NiCo₂S₄ has gained great attention because of its low cost and rich redox reactions.¹¹ Nowadays, NiCo₂S₄ with various nanostructures and morphologies, such as nanosheets,¹² nanoneedles,¹³ nanorods,¹⁴ nano-petals¹⁵ and core–shell structures,⁷ has been widely reported. It is worth noting that NiCo₂S₄ nanosheets exhibit excellent electrochemical performance. The large specific surface and ultra-thin thickness of NiCo₂S₄ nanosheets could provide abundant active sites as well as a short transport channel for ions and electrons.¹⁶ For example, Lin et al. prepared NiCo₂S₄ nanosheet arrays with self-decorated nanoneedles on nickel foam by a facile and efficient two-step hydrothermal approach.¹⁷ It was demonstrated that the morphology of NiCo₂S₄ can be transformed from pony-sized and curly nanosheet arrays to large but still thin nanosheet arrays with the increase of reaction time. After that, Liu and co-workers synthesized NiCo₂S₄ nanosheets arrays on carbon cloth via an in situ growth and subsequent hydrothermal sulfidation, which delivered a high capacity of 1653 F g⁻¹ at a current density of 1 A g⁻¹, superior rate capability and extraordinary cycling stability.¹⁸ However, the mass loading of active materials on carbon cloth was only 0.6 mg cm⁻², far less than the requirement for commercial equipment (8–10 mg cm⁻²).⁵ Although the NiCo₂S₄ nanosheets mentioned above demonstrate outstanding electrochemical characteristics, the complex fabrication process and low mass loading limit their further application.¹⁹

It is well known that the rational design of electrode materials with a well-defined morphology and microstructure is an effective strategy to further enhance their electrochemical properties.²⁰ Recently, electrode materials with a hierarchical structure have been reported widely due to their exceptional properties and remarkable potential in the field of SC.²¹ Taking 3D sponge-like electrode materials as an example, the presence of a rich open framework and large contact-specific surface area were proved to achieve extraordinary electrochemical performance for SC significantly, especially high-rate performance and long-term cycling stability.²² For instance, Feng reported that 3D sponge-like porous Mn₂O₃ tiny nanosheets grown on a Ni(OH)₂/Mn₂O₃ nanosheet array-based electrode delivered an outstanding specific capacitance of 2274.4 F g⁻¹ at a current density of 1 A g⁻¹.²³ Meanwhile, the corresponding specific capacitance and rate performance are obviously superior to those of pristine Ni(OH)₂/Mn₂O₃ nanosheet arrays. In addition, Li demonstrated a series of 3D sponge-like mesoporous Mn₃O₄ electrodes, which exhibited superior rate and cycling performance to that of non-sponge-like porous Mn₃O₄ microspheres.²⁴ It was found that the synthesis of electrode materials with a sponge-like structure is an effective strategy to achieve excellent electrochemical properties. Nevertheless, to the best of our knowledge, there are no reports focused on the synthesis of sponge-like NiCo₂S₄-based electrodes and their applications for supercapacitors.

Herein, novel 3D sponge-like NiCo₂S₄ nanosheets with an ultra-high mass loading (18.7 mg cm⁻²) were obtained via a one-step hydrothermal process. The synthesized sample demonstrates an impressive electrochemical performance with an outstanding area capacitance of 11.97 F cm⁻² at a current density of 15 mA cm⁻² and superior rate performance. Besides, the corresponding asymmetric supercapacitor presents a high area energy density of 0.47 mW h cm⁻², a superior power density of 65.88 mW cm⁻² at an energy density of 0.18 mW h cm⁻² as well as a great cycle life with 80.1% capacitance retention even after 5000 cycles.

Experimental section

Materials

Ni(NO₃)₂·6H₂O (AR), Co(NO₃)₂·6H₂O (AR), CH₃COONa (AR), polyethylene glycol 200 (GD) and (NH₂)₂CS (AR) were supplied by Sinopharm Chemical Reagent Co., Ltd. Active carbon (AC) was provided by Nanjing XFNANO Materials Tech Co., Ltd. All of the reagents were used without further purification. Ni foam (NF) was cut to 3 × 1 cm², immersed in 3 M HCl solution, ultrasonicated for 30 min and carefully washed with deionized water and absolute ethanol three times separately.

Growth of NiCo₂S₄ on NF

The growth of NiCo₂S₄ on nickel foam was realized through a facile one-step hydrothermal method. In a typical procedure, 0.291 g of Ni(NO₃)₂·6H₂O, 0.582 g of Co(NO₃)₂·6H₂O, 0.492 g of CH₃COONa and 0.36 g of (NH₂)₂CS were added in a mixed solution of 36 mL of deionized water and 4 mL of polyethylene glycol 200 with magnetic stirring for 30 min to form a clear and transparent pink solution. Then, the above solution and two pieces of NFs were transferred to a 100 mL Teflon-lined stainless-steel autoclave and heated at 180 °C for 12 h. After the autoclave was cooled to room temperature naturally, the NFs were taken out and rinsed with deionized water and absolute ethanol three times separately and dried for 12 h at 80 °C. The obtained materials were labeled as NiCo₂S₄/NF.

Characterization

The crystal structure of the as-synthesized products was characterized using X-ray diffraction (XRD, Rigaku Ultima IV) with Cu-Kα radiation at 35 kV. The morphologies and microstructures of the products were studied by scanning electron microscopy (SEM, Thermo VEG 3 TESCAN) with energy-dispersive X-ray (EDS) and elemental mapping equipment. Transmission electron microscopy (TEM) was performed by using a FEI tecnai G2 F20 S-Twin system operated at 200 kV. The mass loading of NiCo₂S₄ on NF was studied with an atomic absorption spectrometer (AAS, PERSEE TAS-990).

Electrochemical measurements

Electrochemical measurements were conducted in a three-electrode cell with 2 M KOH as the electrolyte. The as-prepared NiCo₂S₄/NF was used as the working electrode directly, while a platinum wire was used as the counter electrode and an Ag/AgCl electrode acted as the reference electrode. The electrochemical properties of the as-synthesized sample including cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) were measured on a CHI 660E (Chenhua) electrochemical workstation. The nominal area of the sample immersed in the KOH electrolyte was around 1 cm². The areal and specific capacitance of the electrode was calculated from GCD and based on the following equations:

where C_a (F cm⁻²) is the areal capacitance, C_s (F g⁻¹) is the specific capacitance, I (A) is the discharge current, t (s) is the discharge time, S (cm²) is the geometrical area of the electrode, m (g) is the mass of the active material on the NF, and ΔV is the potential window. For assembling the asymmetric supercapacitor (ASC), the NiCo₂S₄/NF electrode and active carbon (AC) were utilized as the positive electrode and negative electrode respectively. 2 M KOH solution was used as the electrolyte. The mass ratio between the positive and negative electrodes is determined by balancing the charge stored in each electrode according to the following equation:
where m⁺ and m⁻ are the mass of NiCo₂S₄ and AC on the NF separately, C_s⁻ and C_s⁺ are the specific capacitance of the positive and negative electrodes respectively, and ΔV₋ and ΔV₊ are the potential window of the positive and negative electrodes, respectively. The corresponding energy density (E, mW h cm⁻²) and power density (P, mW cm⁻²) of the ASC were calculated according to the following equations:

where C (F cm⁻²) is the specific capacitance of the ASC calculated from the GCD curve, V (V) is the potential window of the ASC, and Δt (S) is the discharge time of the ASC obtained from the GCD curve.

Results and discussion

The synthesis procedure of sponge-like 3D interconnected NiCo₂S₄ nanosheets on Ni foam is depicted in Scheme 1. The NiCo₂S₄/NF material is directly obtained via a simple hydrothermal process.

Scheme 1 Preparation process of 3D interconnected NiCo₂S₄ nanosheets with ultra-high mass loading supported on Ni foam.

The structure and morphology of NiCo₂S₄/NF were studied using a scanning electron microscope as shown in Fig. 1a and b. As depicted in Fig. 1a, a sponge-like 3D interconnected network structure of NiCo₂S₄ nanosheets on Ni foam can be observed. The presence of high-density nanosheets with a hierarchical and porous structure could provide abundant contact surface area and active sites for redox reactions. More importantly, as shown in Fig. 1b, the 3D sponge-like configuration was made up of interconnected nanosheets with a thickness of around 30 nm, which was beneficial for the materials to be fully utilized. Based on the test using an atomic absorption spectrometer, the mass loading of NiCo₂S₄ on NF was about 18.7 mg cm⁻². As revealed in Fig. 1c, an ultra-thin and porous NiCo₂S₄ nanosheet-based 3D hierarchical structure directly growing on the Ni foam substrate could not only improve the conductivity of the as-obtained materials, but also supply fast ion diffusion channels and efficient transport pathways, which were feasible for the electrochemical reactions.²⁵

Fig. 1 (a) and (b) SEM images of sponge-like 3D interconnected NiCo₂S₄ nanosheets supported on Ni foam at different magnifications, (c) possible transportation paths of ions and electrons in NiCo₂S₄/NF, (d) XRD pattern and (e) SEM image and the corresponding elemental mappings of NiCo₂S₄/NF.

In addition, the crystallographic structure of the NiCo₂S₄ nanosheets was examined by XRD as shown in Fig. 1d. There were five characteristic diffraction peaks at 31.6°, 38.3°, 50.5°, 55.3° and 65.1°, which could be well indexed to the (222), (400), (511), (440), and (533) planes of the spinel NiCo₂S₄ crystalline structure (JCPDS card no. 20-0782), separately.²⁶ Excep for three characteristic diffraction peaks derived from Ni foam, there were no additional diffraction peaks, indicating the successful synthesis of NiCo₂S₄ on NF. Furthermore, elemental mapping of the sample was carried out as shown in Fig. 1e. Ni, Co and S were uniformly distributed on the surface of the materials and no other elements were detected. The above results further proved that NiCo₂S₄/NF with an interconnected network structure was successfully synthesized by a facile one-step hydrothermal method.

What's more, the structure of NiCo₂S₄ nanosheets was clarified by TEM and HRTEM as shown in Fig. 2a and b. As depicted in Fig. 2a, the interconnected network structure was consistent with the SEM images of NiCo₂S₄/NF. In addition, as shown in Fig. 2b, the HRTEM image showed clear lattice fringes with the lattice spacing of 0.166 nm and 0.235 nm, corresponding to the (440) and (400) planes of the spinel NiCo₂S₄ crystalline structure, which was also in accordance with the XRD pattern.²⁷ Meanwhile, the corresponding selected area electron diffraction (SAED) pattern, as shown in the inset of Fig. 2b, exhibits obvious diffraction rings.

Fig. 2 (a) TEM image of the as-obtained NiCo₂S₄/NF, (b) HRTEM image with the inset showing the corresponding SAED pattern.

In order to study the electrochemical performance of NiCo₂S₄/NF, the CV curves, galvanostatic charge–discharge curves (GCD) and EIS Nyquist plot were obtained in a three-electrode system with 2 M KOH as the electrolyte. As depicted in Fig. 3a, the CV curves were surveyed in a potential window between −0.3 V and 0.65 V at different scanning rates ranging from 5 mV s⁻¹ to 25 mV s⁻¹. In particular, a part of redox peaks can be observed, indicating that NiCo₂S₄ is a typical psudocapacitance material with reversible redox based on the following reaction equations:⁶

NiCo₂S₄ + 2OH⁻ + H₂O ↔ NiSOH + 2CoSOH + 2e⁻

NiSOH + OH⁻ ↔ NiSO + H₂O + e⁻

CoSOH + OH⁻ ↔ CoSO + H₂O + e⁻

Fig. 3 Three-electrode test of NiCo₂S₄/NF: (a) CV curves at different sweep rates from 5 mV s⁻¹ to 25 mV s⁻¹, (b) galvanostatic charge–discharge (GCD) curves at different current densities from 15 mA cm⁻² to 40 mA cm⁻², (c) area capacitance of NiCo₂S₄/NF at various current densities calculated by GCD, and (d) the corresponding EIS Nyquist plot; the inset shows the magnified image at a high frequency.

It is noted that the shape of CV curves does not twist significantly with the increase in sweep rates, demonstrating the excellent stability and high rate performance of the electrode.²⁸ Moreover, the area capacitance of the NiCo₂S₄/NF electrode was estimated by the GCD test from 0 V to 0.45 V. As shown in Fig. 3b, there are a part of obvious potential voltage plateaus between 0.15 V and 0.35 V, which are in agreement with the CV curves and previous reports.²⁹ As revealed in Fig. 3c, the NiCo₂S₄/NF electrode shows a series of ultra-high area capacitances of 11.97, 11.54, 11.14, 10.75 and 10.06 F cm⁻² at current densities of 15, 20, 25, 30 and 40 mA cm⁻², respectively. Impressively, even when the current density was increased to 40 mA cm⁻², the area capacitance still retained 84% of the initial value, demonstrating outstanding rate performance and superior psudocapacitance. In addition, the Nyquist plot in Fig. 3d shows the low charge-transfer resistance of the sample. The high slope of the Warburg straight line reflects the low diffusion resistance of electrolyte ions in sponge-like NiCo₂S₄/NF, which suggests that the synthesized NiCo₂S₄/NF electrode has good electrical conductivity.³⁰

To further explore the practical use of the NiCo₂S₄/NF electrode, an asymmetric supercapacitor was packaged. The CV curves of the device were measured at different potential windows as shown in Fig. 4a, and the optimum operation window can be up to 1.65 V. When the potential window was further extended to 1.75 V, the oxygen evolution reaction occurred on the surface of the NiCo₂S₄/NF electrode.³¹Fig. 4b shows a series of CV curves of the ASC with different scan rates in the voltage range of 0 V–1.65 V. The change of the redox peak is almost negligible, which illustrates the excellent charge/discharge performance of the ASC.³² Furthermore, a series of symmetrical GCD curves of the NiCo₂S₄/NF//AC device were measured with the potential range of 0 V–1.65 V as shown in Fig. 4c. The observed voltage plateaus from 1.0 V to 1.65 V are consistent with the CV curves of the device, exhibiting the apparent pseudocapacitance characteristic of the electrode material.³³ Furthermore, the area capacitances of the NiCo₂S₄/NF//AC device were calculated from the GCD curves. As shown in Fig. 4d, the highest area capacitance of the device could reach up to 1.23 F cm⁻² at 8 mA cm⁻². Even when the current density increased to 80 mA cm⁻², the device delivered a capacitance retention of 40% and still maintained the area capacitance of 0.49 F cm⁻², indicating the superior rate performance of the assembled ASC.

Fig. 4 Electrochemical performance of the NiCo₂S₄/NF//AC asymmetric supercapacitor: (a) CV curves of ASC at different potential windows with the scanning rate of 20 mV s⁻¹, (b) CV curves of the ASC at different scanning rates with the potential window from 0 V to 1.65 V, (c) GCD curves of the ASC at different current densities, (d) area capacitances of the ASC at various current densities.

Meanwhile, Fig. 5a shows the cycling stability and coulombic efficiency of the device. After cycling at 40 mA cm⁻² for 5000 cycles, the capacitance retention and coulombic efficiency of the NiCo₂S₄/NF//AC device were 80.1 and 91.5%, respectively. Furthermore, the capacitance retention of the ASC surpassed 100% after 1500 cycles, which can be attributed to the activation of the electrode material.²⁷ What's more, the energy density and power density of the ASC were calculated according to the GCD curves. As shown in Fig. 5b, the highest area energy density was up to 0.47 mW h cm⁻² at an area power density of 6.59 mW cm⁻², and an energy density up to 0.18 mW h cm⁻² at a maximum power density of 65.88 mW cm⁻², demonstrating the excellent application potential. Obviously, the assembled NiCo₂S₄/NF//AC ASC device in this work is superior to the results reported for similar supercapacitors as depicted in Table 1.^34–38

Fig. 5 (a) Cycling performance and coulombic efficiency of the ASC at 40 mA cm⁻² after 5000 cycles, with the inset presenting the two ASC configurations connected in series to illuminate a red LED, (b) Ragone plots of the NiCo₂S₄/NF//AC ASC device in comparison with other reported SC devices.

Table 1 Comparison of the electrochemical performance of the asymmetric supercapacitor based on NiCo₂S₄/NF in this work with other materials as the substrate

Asymmetric supercapacitor	Energy density (mW h cm^−2)	Power density (mW cm^−2)	Ref.
Porous carbonized cotton/ZnS/CuS//AC	0.39	4.32	34
Cu–W–S/Ni//graphene	0.10	0.68	35
NiS nanotrees//PAC	0.47	3.92	36
NiCo2S4@NiCo2S4//AC	0.28	3.05	37
NiCo2S4@CNT//Ti3C2Tx@CCT	0.18	2	38
NiCo2S4/NF//AC	0.47	6.59	Our work

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The excellent electrochemical performance of the NiCo₂S₄/NF-based electrode can be explained as follows: firstly, the Ni foam underneath can act as a current collector for the NiCo₂S₄/NF electrode, which is feasible for the rapid transport of electrons and ions as well as decreasing the transfer resistance of the NiCo₂S₄/NF electrode material significantly. Secondly, ultra-high loading of NiCo₂S₄ nanosheets on Ni foam could provide plenty of active supporters for energy storage in a unit area. Thirdly, the 3D sponge-like structure consisting of ultra-thin porous nanosheets could offer a larger effective specific surface area, more active sites, and a shorter transport channel for ions and electrons, which plays a vital role in the redox reaction to realize exceptional function. As a result, NiCo₂S₄/NF with ultra-high mass loading exhibits excellent electrochemical performance.

Conclusion

In summary, sponge-like NiCo₂S₄ nanosheets supported on NF with ultra-high mass loading were successfully obtained through a facile method. Since the unique 3D porous sponge-like network structure could provide more accessible surface areas and active sites, the NiCo₂S₄/NF-based electrode exhibited superior electrochemical performance. Noticeably, NiCo₂S₄/NF delivered a high area capacitance of 11.97 F cm⁻² at a current density of 15 mA cm⁻². Meanwhile, the corresponding ASC was assembled and achieved the maximum area energy density of 0.47 mW h cm⁻² with a power density of 6.59 mW cm⁻² and 80.1% capacitance retention even after 5000 cycles. The above results demonstrated that the 3D sponge-like NiCo₂S₄ nanosheets supported by NF based electrodes with high mass loading could have great potential for energy storage applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21405105), the Shanghai Natural Science Foundation (14ZR1429300), and the State Key Laboratory of Green Catalysis of Sichuan Institutes of Higher Education (LZJ1703).

Notes and references

1. S. Chen, C. Lu, L. Liu, M. Xu, J. Wang, Q. Deng, Z. Zeng and S. Deng, A hierarchical glucose-intercalated NiMn-G-LDH@NiCo₂S₄ core-shell structure as a binder-free electrode for flexible all-solid-state asymmetric supercapacitors, Nanoscale, 2020, 12, 1852–1863.
2. B. Li, P. Gu, Y. Feng, G. Zhang, K. Huang, H. Xue and H. Pang, Ultrathin nickel-cobalt phosphate 2D nanosheets for electrochemical energy storage under aqueous/solid-state electrolyte, Adv. Funct. Mater., 2017, 27, 1605784.
3. W. Chen, X. Zhang, L. E. Mo, Y. Zhang, S. Chen, X. Zhang and L. Hu, NiCo₂S₄ quantum dots with high redox reactivity for hybrid supercapacitors, Chem. Eng. J., 2020, 388, 124109.
4. Y. Yan, B. Li, W. Guo, H. Pang and H. Xue, Vanadium based materials as electrode materials for high performance supercapacitors, J. Power Sources, 2016, 329, 148–169.
5. Z. X. Tong, Y. J. Ji, Q. Z. Tian and W. M. Ouyang, High mass loading and high-density flower-like NiCo₂O₄ nanosheets on Ni foam for superior capacitance, Chem. Commun., 2019, 55, 9128–9131.
6. J. Zhao, C. Ge, Z. Zhao, Q. Wu, M. Liu, M. Yan, L. Yang, X. Wang and Z. Hu, Sub-nanometer-scale fine regulation of interlayer distance in Ni-Co layered double hydroxides leading to high-rate supercapacitors, Nano Energy, 2020, 76, 105026.
7. A. Singh, S. K. Ojha, M. Singh and A. K. Ojha, Controlled synthesis of NiCo₂S₄@NiCo₂O₄ core@Shell nanostructured arrays decorated over the rGO sheets for high-performance asymmetric supercapacitor, Electrochim. Acta, 2020, 349, 136349.
8. X. Huang and L. Gou, High performance asymmetric supercapacitor based on hierarchical flower-like NiCo₂S₄@polyaniline, Appl. Surf. Sci., 2019, 487, 68–76.
9. C. Ye, Q. Qin, J. Liu, W. Mao, J. Yan, Y. Wang, J. Cui, Q. Zhang, L. Yang and Y. Wu, Coordination derived stable Ni-Co MOFs for foldable all-solid-state supercapacitors with high specific energy, J. Mater. Chem. A, 2019, 7, 4998–5008.
10. Y. Liu, Y. Wen, Y. Zhang, X. Wu, H. Li, H. Chen, J. Huang, G. Liu and S. Peng, Reduced CoNi₂S₄ nanosheets decorated by sulfur vacancies with enhanced electrochemical performance for asymmetric supercapacitors, Sci. China Mater., 2020, 63, 1216–1226.
11. M. Liang, M. Zhao, H. Wang, J. Shen and X. Song, Enhanced cycling stability of hierarchical NiCo₂S₄@Ni(OH)₂@PPy core-shell nanotube arrays for aqueous asymmetric supercapacitors, J. Mater. Chem. A, 2018, 6, 2482–2493.
12. Y. Liu, Z. Li, L. Yao, S. Chen, P. Zhang and L. Deng, Confined growth of NiCo₂S₄ nanosheets on carbon flakes derived from eggplant with enhanced performance for asymmetric supercapacitors, Chem. Eng. J., 2019, 366, 550–559.
13. M. Dong, Z. Wang, G. Yan, J. Wang, H. Guo and X. Li, Confine growth of NiCo₂S4 nanoneedles in graphene framework toward high-performance asymmetric capacitor, J. Alloys Compd., 2020, 822, 153645.
14. M. Zhang, H. Liu, Z. Song, T. Ma and J. Xie, Self-assembling NiCo₂S₄ nanorods arrays and T-Nb₂O₅ nanosheets/three-dimensional nitrogen-doped garphene hybrid nanoarchitectures for advanced asymmetric supercapacitor, Chem. Eng. J., 2020, 392, 123669.
15. Y. Wen, S. Peng, Z. Wang, J. Hao, T. Qin, S. Lu, J. Zhang, D. He, X. Fan and G. Cao, Facile synthesis of ultrathin NiCo₂S₄ nano-petals inspired by blooming buds for high-performance supercapacitors, J. Mater. Chem. A, 2017, 5, 7144–7152.
16. T. Chen, S. Wei and Z. Wang, NiCo₂S₄-Based Composite Materials for Supercapacitors, ChemPlusChem, 2019, 85, 43–56.
17. L. Lin, J. Liu, T. Liu, J. Hao, K. Ji, R. Sun, W. Zeng and Z. Wang, Growth-controlled NiCo₂S₄ nanosheet arrays with self-decorated nanoneedles for high-performance pseudocapacitors, J. Mater. Chem. A, 2015, 3, 17652–17658.
18. T. Liu, J. Liu, L. Zhang, B. Cheng and J. Yu, Construction of nickel cobalt sulfide nanosheet arrays on carbon cloth for performance-enhanced supercapacitor, J. Mater. Sci. Technol., 2020, 47, 113–121.
19. W. Kong, C. Lu, W. Zhang, J. Pu and Z. Wang, Homogeneous core-shell NiCo₂S₄ nanostructures supported on nickel foam for supercapacitors, J. Mater. Chem. A, 2015, 3, 12452–12460.
20. G. Qu, C. Li, P. Hou, G. Zhao, X. Wang, X. Zhang and X. Xu, Hierarchically hollow structured NiCo₂S₄@NiS for high-performance flexible hybrid supercapacitors, Nanoscale, 2020, 12, 4686–4694.
21. Y. K. Wang, M. C. Liu, J. Cao, H. J. Zhang, L. B. Kong, D. P. Trudgeon, X. Li and F. C. Walsh, 3D hierarchically structured CoS nanosheets: Li⁺ storage mechanism and application of the high-performance lithium-ion capacitors, ACS Appl. Mater. Interfaces, 2020, 12, 3709–3718.
22. N. Kumar, D. Mishra, S. Y. Kim, T. Na and S. H. Jin, Two dimensional, sponge-like In₂S₃ nanoflakes aligned on nickel foam via one-pot solvothermal growth and their application toward high performance supercapacitors, Mater. Lett., 2020, 279, 128467.
23. L. Feng, J. Sun, Y. Liu, X. Li, L. Ye and L. Zhao, 3D sponge-like porous structure of Mn₂O₃ tiny nanosheets coated on Ni(OH)₂/Mn₂O₃ nanosheet arrays for quasi-solid-state asymmetric supercapacitors with high performance, Chem. Eng. J., 2018, 339, 61–70.
24. S. Li, L. L. Yu, R. B. Li, J. Fan and J. T. Zhao, Template-free and room-temperature synthesis of 3D sponge-like mesoporous Mn₃O₄ with high capacitive performance, Energy Storage Mater., 2018, 11, 176–183.
25. X. Xu, L. Liang, Q. Liu, X. Zhang, Y. Zhao and S. Qiao, In situ induced sponge-like NiMoS₄ nanosheets on self-supported nickel foam skeleton for electrochemical capacitor electrode, Colloids Surf., A, 2020, 602, 125099.
26. Z. Sun, C. Zhao, X. Cao, K. Zeng, Z. Ma, Y. Hu, J. H. Tia and R. Yang, Insights into the phase transformation of NiCo₂S₄@rGO for sodium-ion battery electrode, Electrochim. Acta, 2020, 338, 135900.
27. H. Wang, M. Liang, C. Ma, W. Shi, D. Duan, G. He and Z. Sun, Novel dealloying-fabricated NiCo₂S₄ nanoparticles with excellent cycling performance for supercapacitors, Nanotechnology, 2019, 30, 235402.
28. F. Yu, Z. Chang, X. Yuan, F. Wang, Y. Zhu, L. Fu, Y. Chen, H. Wang, Y. Wu and W. Li, Ultrathin NiCo₂S₄@graphene with a core-shell structure as a high performance positive electrode for hybrid supercapacitors, J. Mater. Chem. A, 2018, 6, 5856–5861.
29. Y. Gao, B. Wu, J. Hei, D. Gao, X. Xu, Z. Wei and H. Wu, Self-assembled synthesis of waxberry-like open hollow NiCo₂S₄ with enhanced capacitance for high-performance hybrid asymmetric supercapacitors, Electrochim. Acta, 2020, 347, 136314.
30. Y. J. Ji, Y. L. Deng, F. Chen, Z. Q. Wang, Y. Z. Lin and Z. H. Guan, Ultrathin Co₃O₄ nanosheets anchored on multi-heteroatom doped porous carbon derived from biowaste for high performance solid-state supercapacitors, Carbon, 2020, 156, 359–369.
31. T. Xing, Y. Ouyang, Y. Chen, L. Zheng, H. Shu, C. Wu, B. Chang and X. Wang, Preparation and performances of 3D hierarchical core-shell structural NiCo₂S₄@NiMoO₄·xH₂O nanoneedles for electrochemical energy storage, Electrochim. Acta, 2020, 351, 136447.
32. B. S. Soram, I. S. Thangjam, J. Y. Dai, T. Kshetri, N. H. Kim and J. H. Lee, Flexible transparent supercapacitor with core-shell Cu@Ni@NiCoS nanofibers network electrode, Chem. Eng. J., 2020, 395, 125019.
33. W. Zhao, G. Yan, Y. Zheng, B. Liu, D. Jia, T. Liu, L. Cui, R. Zheng, D. Wei and J. Liu, Bimetal-organic framework derived Cu(NiCo)₂S₄/Ni₃S₄ electrode material with hierarchical hollow heterostructure for high performance energy storage, J. Colloid Interface Sci., 2020, 565, 295–304.
34. S. Zhai, Z. Fan, K. Jin, M. Zhou, H. Zhao, Y. Zhao, F. Ge, X. Li and Z. Cai, Synthesis of zinc sulfide/copper sulfide/porous carbonized cotton nanocomposites for flexible supercapacitor and recyclable photocatalysis with high performance, J. Colloid Interface Sci., 2020, 575, 306–316.
35. P. Pazhamalai, K. Krishnamoorthy, S. Sahoo, V. K. Mariappan and S. J. Kim, Copper tungsten sulfide anchored on Ni-foam as a high-performance binder free negative electrode for asymmetric supercapacitor, Chem. Eng. J., 2019, 359, 409–418.
36. G. S. R. Raju, E. Pavitra, G. Nagaraju, S. C. Sekhar, S. M. Ghoreishian, C. H. Kwak, J. S. Yu, Y. S. Huh and Y. K. Han, Rational design of forest-like nickel sulfide hierarchical architectures with ultrahigh areal capacity as a binder-free cathode material for hybrid supercapacitors, J. Mater. Chem. A, 2018, 6, 13178–13190.
37. J. Xie, Y. Yang, G. Li, H. Xia, P. Wang, P. Sun, X. Li, H. Cai and J. Xiong, One-step sulfuration synthesis of hierarchical NiCo₂S₄@NiCo₂S₄ nanotube/nanosheet arrays on carbon cloth as advanced electrodes for high-performance flexible solid-state hybrid supercapacitors, RSC Adv., 2019, 9, 3041–3049.
38. J. Yu, J. Zhou, P. Yao, J. Huang, W. Sun, C. Zhu and J. Xu, A stretchable high performance all-in-one fiber supercapacitor, J. Power Sources, 2019, 440, 227150.

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