In situ self-assembly of Ni₃S₂/MnS/CuS/reduced graphene composite on nickel foam for high power supercapacitors

Abstract

Transition metal sulfides (TMS), as promising electroactive materials for asymmetric supercapacitors, have been limited due to their relatively poor conductivity and cycle stability. Here ternary Ni₃S₂/MnS/CuS composites were assembled in situ on nickel foam (NF) using a hydrothermal method via electrostatic adsorption of Ni⁺, Mn²⁺ and Cu²⁺ ions on a reduced graphene (rGO) nanosheet template. The chemical structure was characterized by various analytic methods. Ni₃S₂/MnS/CuS has spherical morphology assembled from closely packed nanosheets, while Ni₃S₂/MnS/CuS@rGO has a three-dimensional porous spherical structure with much lower diameter because rGO nanosheets can play the role of a template to induce the growth of Ni₃S₂/MnS/CuS. At a current density of 1 A g⁻¹, the specific capacitance was obtained to be 1028 F g⁻¹ for Ni₃S₂/MnS/CuS, 628.6 F g⁻¹ for Ni₃S₂/MnS@rGO, and 2042 F g⁻¹ for Ni₃S₂/MnS/CuS@rGO, respectively. Charge transfer resistance (R_ct) of Ni₃S₂/MnS/CuS@rGO (0.001 Ω) was much lower than that of Ni₃S₂/MnS@rGO by 0.02 Ω, and lower than that of Ni₃S₂/MnS/CuS by 0.017 Ω. After 5000 cycles, the Ni₃S₂–MnS–CuS@RGO electrode maintains 78.3% of the initial capacity at 20 A g⁻¹. An asymmetric supercapacitor was subsequently assembled using Ni₃S₂/MnS/CuS@rGO as the positive electrode and rGO as the negative electrode. The specific capacitance of asymmetric batteries was maintained at 90.8% of the initial state after 5000 GCD.

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Introduction

The demand for high-performance energy storage devices has attracted intensive attention around the world.^1,2 Among different energy storage devices, supercapacitors are considered to have superior performance due to their superior performance characteristics, including high power density, ultra-fast charge–discharge rate, low maintenance, long service life, excellent stability and safety.^3–5 Based on the interface storage mechanism between the electrode and the electrolyte, the supercapacitors are classified into electric double layer capacitors and pseudocapacitors.⁶ The capacity of double layer capacitors and pseudocapacitors mainly depends on the characteristics of the electrolyte, the surface morphology, surface area and structure of the electrodes. Among them, double layer capacitors rely on charge storage for ion adsorption and desorption, while pseudocapacitors rely on charge storage and involve fast Faraday redox reactions at the electrode/electrolyte interface. Compared with double layer capacitors, pseudocapacitors usually display higher energy density and specific capacitance, transition metal oxides (TMO) and transition metal sulfides (TMS) are generally utilized in pseudocapacitors.⁷ It has been reported that the composites of graphene with Ni-, Mn- and Co- rich TMO or TMS show excellent electrochemical performance.^8,9 More importantly, due to multiple oxidation, the ternary transition metal sulfide exhibits greater redox activity and higher specific capacitance than the single transition metal sulfide counterpart.^10–13 However, the application of TMO or TMS is limited due to their relatively poor conductivity and cycle stability.^14,15 The transfer impedance (R_ct) of Ni₃S₂, Ni₃S₂/MWCNT-NC,¹⁶ Ni₃S₂/rGO,¹⁷ pure MnS, γ-MnS/rGO-60, a-MnS and a-MnS/N-rGO,¹⁸ Ni₃S₂/MnS,¹⁹ CuS and CuS-rGO²⁰ have been reported to be 8.99 Ω, 0.7 Ω, 1.4 Ω, 0.3 Ω, 0.2 Ω, 8.68 Ω, 3.74 Ω, 0.52 Ω, 12.2 Ω and 4.2 Ω, respectively. The solution impedance (R_s) of pure MnS, γ-MnS/rGO-60, Ni₃S₂/MnS and CuS-rGO are 8.8 Ω, 1.3 Ω, 1.1 Ω, 0.25 Ω and 0.5 Ω, respectively.

In this study, ternary TMS/rGO composites were in situ assembled on nickel foam (NF) using hydrothermal method. Ternary TMS/rGO composites were fabricated via electrostatic adsorption of Ni⁺, Mn²⁺ and Cu²⁺ ions on rGO nanosheet, followed by the sulfurization which utilizes thiourea as sulfur source.²¹ The objective of this research is to reduce R_ct through the incorporation of Cu²⁺ into the binary TMS system and utilization of rGO nanosheets as template. On the one hand, CuS and rGO nanosheets can functionalize as template to induce the growth of Ni₃S₂/MnS,²² forming a three-dimensional porous spherical structure self-assembled by nanosheets, generating a more efficient electron transport structure. On the other hand, CuS electrode exhibits higher conversion efficiency and faster charge transfer. The presence of Cu vacancies in CuS is able to improve the ionic and electronic conduction, as well as the free holes in the composites, resulting in the increase of conductivity and the decrease of R_ct.²³ The as-prepared composite in this study was found to have an ultra-low R_ct (0.001 Ω) without the addition of a binder and a conductive agent.

Moreover, the Ni₃S₂/MnS/CuS/@rGO-NF and rGO-NF were selected as electrodes to build asymmetric supercapacitor. Generally, the asymmetric supercapacitor consists of a battery-type electrode (tantalum capacitor) and a capacitor-type electrode (electrochemical double layer), exhibiting better reversibility, high energy density, and power density.^24,25 Depending on the electrolyte, asymmetric supercapacitors are classified into two types, non-aqueous and aqueous. Non-aqueous electrolytes have slow insertion kinetics, resulting in moderate decoupling power delivery performance and greatly reduced energy density due to the low viscosity and high pH value of the aqueous electrolyte. However, ion mobility is an essential prerequisite for rapid reaction kinetics. Therefore, aqueous electrolytes got more and more attention. Currently, CuS‖AC-ASC,²⁶ ZNACOs‖AC,²⁷ ZnFe₂O₄‖Ni(OH)₂ (ref. 28) and CoFe₂O₄‖rGO under aqueous electrolytes have been reported,²⁹ their specific capacitance reached 48.2 F g⁻¹, 232 F g⁻¹, 118 F g⁻¹ and 38 F g⁻¹. In contrast, the Ni₃S₂/MnS/CuS/@rGO-NF‖rGO-NF asymmetric supercapacitor exhibited a high specific capacitance of 522 F g⁻¹ at 2 A g⁻¹.

Experimental part

Materials and reagents. Graphite powder (purity 99%) was provided by Tian City Kermel Chemical Reagent (China), H₂SO₄ (98.08%), NaNO₃ and HCl (36%) were purchased from Tianjin city Kermel Chemical Reagent (China). MnCl₂·4H₂O and H₂O₂ (30%, 500 ml) were purchased from Tianjin Kaitong Chemical Reagent Co., Ltd. NiCl₂·6H₂O (AR) were purchased from Tianjin Dongli District Tianda Chemical Reagent Factory. CuCl₂·2H₂O (AR) was ordered from Tianjin Komiou Chemical Reagent Co., Ltd. CH₄N₂S (Tu) (AR) was ordered from Henan Dongyang Chemical Products Co., Ltd. KMnO₄ was ordered from National Group Chemical Reagent Co., Ltd. Nickel foam (NF) was ordered from Kunshan Yujiaxin Electronic Technology Co., Ltd. DMF (AR) was ordered from Tianjin Tianli Chemical Reagent Co., Ltd. Ethylene glycol (AR) was purchased from Tianjin Hongyan Reagent Factory. Ultra-purified (UP) water with resistivity of 18.2 MΩ cm was used throughout the experiment.

Preparation of GO. Graphite oxide (GO) was prepared by the modified Hummers' method.¹⁵ Typically, 5 g graphite power and 2.5 g sodium nitrate (NaNO₃) were added to 115 ml of concentrated sulfuric, the reaction was kept in the ice bath for 30 min at 5 °C. Then 15 g KMnO₄ was introduced, followed by a reaction at 35 °C for 30 min. Afterwards, 230 ml of water was added dropwise into the suspension, and the obtained suspension was heated up to 98 °C. After 15 min, the suspension was further diluted to 420 ml and treated by 50 ml 30 wt% hydrogen peroxide. The graphite oxide (GO) was thereby obtained after repeated centrifugation, washing, and drying.

Nickel foam pretreatment. The nickel foam (NF) was placed into a rectangular plate of dimension 100 mm × 100 mm × 1 mm, followed by ultrasonic treatment with HCl. Then NF was washed with acetone and deionized water for four times at a frequency of 100 Hz to remove the oxides on the NF surface. Afterwards, NF was dried in the vacuum oven under 70 °C for 12 h, and cut into a size of 10 mm × 10 mm × 1 mm for utilization.

Synthesis of Ni₃S₂/MnS/CuS@rGO-NF nanocomposites. NiCl₂·6H₂O (1 mmol), MnCl₂·4H₂O (1 mmol), CuCl₂·2H₂O (1 mmol), Tu (8 mmol) were dissolved in 40 ml of anhydrous N,N-dimethylformamide (DMF) by sonication. Then, 0.5 mg ml⁻¹ GO dispersion was prepared by dispersing 20 mg GO in 40 ml of ethylene glycol. Afterwards, the pretreated NF was blended with the above-mentioned solutions and transferred into a 100 ml Teflon-lined autoclave vessel and the reaction was remained at 180 °C for 24 h. The Ni₃S₂/MnS/CuS@rGO-NF composites were thereby obtained. Ni₃S₂/MnS/CuS@rGO was also prepared without the addition of NF based on the above experimental procedure. The as-prepared Ni₃S₂/MnS/CuS@rGO-NF and Ni₃S₂/MnS/CuS@rGO were washed with deionized water and ethanol for three times respectively and then dried in a vacuum oven at 70 °C for 12 h. The preparation scheme of Ni₃S₂/MnS/CuS@rGO is shown in Fig. 1.

Fig. 1 Schematical fabrication of Ni₃S₂/MnS/CuS@rGO-NF nanocomposites.

Assembly of the asymmetric supercapacitor. In order to evaluate the Ni₃S₂/MnS/CuS@rGO composite electrode for practical application, asymmetric supercapacitor (ASC) was assembled with Ni₃S₂/MnS/CuS@rGO-NF as positive electrode (PE), graphene-loaded Ni foam as negative electrode (NE), 6 M KOH solution as electrolyte, and cellulose paper as separator.³⁰ During the preparation of PE and NE, 2 ml PVDF (0.025 g ml⁻¹) was utilized as binder and 0.05 g of acetylene black was adopted as conductive agent.

Characterization. The phase information was obtained by an X-ray diffraction spectrometer (XRD, TTR-III, Rigaku, Japan) equipped with Cu target radiation. Elemental analysis was performed on an X-ray photoelectron spectrometer (XPS, KRATOS XSAM 800, United Kingdom) equipped with Mg Kα X-ray radiation. The morphology and structure were elucidated by scanning electron microscope (SEM, JSM-6360, JEOL, Japan), transmission electron microscope (TEM, TECHNI-G2, FEI, US), energy dispersive X-ray spectroscopy (EDS, JEOL, JEM-2100F, Japan) and the high-resolution TEM (HRTEM, JEM-2100F, JEOL, Japan).

Electrochemical measurements. Electrochemical measurements for electrodes were taken using a half-cell three-electrode system on electrochemical work station (CHI660E, Shanghai Chenhua Corp. China) at room temperatures (25 °C), using saturated calomel electrode (SCE) as reference electrode, 1 cm × 1 cm platinum plate as counter electrode, 6 M KOH aqueous electrolyte as electrolyte and Ni₃S₂/MnS/CuS@rGO-NF as working electrode. Cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) experimental analysis were performed under various scan rates and current densities with a potential window from −0.3 to 0.4 V. The electrochemical impedance spectroscopy (EIS) plots were recorded in frequency width of 0.01 Hz to 100 kHz with an ac voltage of 5.0 mV. The mass specific capacitance (C_m) is calculated based on eqn (1):where I is the discharge current, Δt is the discharge time (s), m is the mass (g) of the active material in the electrode, and ΔV is the voltage window (V).

The energy density (E, W h kg⁻¹) and power density (P, W kg⁻¹) of electrode were calculated according to eqn (2) and (3):^31,32

where ΔV is the potential window (V), C is the specific capacity (F g⁻¹), and Δt is the discharge time (s).

Electrochemical measurements such as CV, GCD and EIS for asymmetric supercapacitor were also carried out on electrochemical work station (CHI660E, Shanghai Chenhua Corp. China) at room temperatures (25 °C) using four-electrode system. CV and GCD analysis were performed under various scan rates and current densities with a potential window from 0 to 1.5 V. The EIS plots were recorded in frequency width of 0.01 Hz to 100 kHz with an ac voltage of 5.0 mV.

The energy density (E, W h kg⁻¹) and power density (P, W kg⁻¹) of the battery are calculated from the discharge curve according to the following equation:

E = C_m(ΔV)²/8 (4)

P = E/Δt (5)

where C_m, ΔV, and Δt are the mass-specific capacitance, cell voltage (1.5 V), and discharge time, respectively.

Results and discussion

Phase and elemental analysis

XRD patterns of Ni₃S₂/MnS@rGO and Ni₃S₂/MnS/CuS@rGO are shown in Fig. 2a and b, respectively. By comparison of Fig. 2a and b, the introduction of Cu²⁺ ions did not change the structural phase of Ni⁺ and Mn²⁺. The characteristic diffraction peaks of r-MnS are detected at 29.62°, 34.62°, 49.34°, 58.60°, 61.48°, 72.35°, 80.06°, 82.59°, which correspond to the (111), (200), (220), (311), (222), (400), (331), (420) of r-MnS (JCPDS, PDF no: 64-3413). The diffraction peaks at 2θ = 21.76°, 31.10°, 37.80°, 44.35°, 49.73°, 50.11°, 54.61° and 55.16° correspond to (010), (−110), (111), (020), (120), (−120), (121) and (−121) crystal planes in Ni₃S₂ (JCPDS-no. 44-1418).³³ The diffraction peaks at 2θ values of 27.37°, 29.54°, 32.07°, 33.13° in Fig. 2b are attributed to the (100), (102), (103) and (006) crystal planes of CuS phase (JCPDS, PDF no. 06-0464).¹⁰

Fig. 2 XRD patterns of (a) Ni₃S₂/MnS@rGO and (b) Ni₃S₂/MnS/CuS@rGO.

For determining the chemical state and electronic states of the elements on the surface Ni₃S₂/MnS/CuS@rGO composites, XPS analysis is conducted, as shown in Fig. 3. The XPS survey spectrum confirms the coexistence of Ni, Cu, Mn, O, C and S (Fig. 3a). The contents of Ni, Cu, Mn, O, C and S elements are calculated to be 7.8 at%, 6.73 at%, 0.33 at%, 40.26 at%, 28.49 at% and 16.4 at%, respectively. The characteristic peak at 569.08 eV for Cu LMM is the typical binding energy value for CuS (Fig. 3a), corroborating that Cu element is existent in the form of bivalent state.

Fig. 3 X-ray photoelectron spectra of N₃S₂/MnS/CuS@rGO: (a) survey spectrum, (b) Ni 2p spectrum, (c) Mn 2p spectrum, (d) Cu 2p spectrum, (e) S 2p spectrum and (f) high-resolution C 1s spectrum respectively.

The Ni 2p spectrum is deconvoluted into two spin–orbit bimodal peaks of Ni²⁺ and Ni³⁺ and two shake-up satellite peaks (Fig. 3b).²⁰ The characteristic peaks at 857.5 eV and 874.9 eV are assigned to Ni 2p_3/2 and Ni 2p_1/2, respectively. The binding energy difference of Ni 2p_3/2 and Ni 2p_1/2 is 17.4 eV, indicating the co-existence of di/tri-valent state.

The Mn peaks in Fig. 3c are observed in the range of 635.0–655 eV. Mn 2p_3/2 and Mn 2p_1/2 peaks at 642.3 eV and 654.1 eV with the spin energy separation of ∼11.8 eV, indicate the presence of Mn²⁺ oxidation (Fig. 3d).³⁴ Two peaks at 932.3 and 952.2 eV are due to Cu 2p_3/2 and Cu 2p_1/2.³⁵ The peaks at 161.9 eV and 162.8 eV are associated with S 2p_3/2 and S 2p_1/2 in the metal sulfide. The peak at 168.6 eV could be ascribed to sulfur in oxidation state (Fig. 3f).³⁶ C 1s peak can be fitted into three Gaussian peaks centered at 284.6 eV, 286.0 eV and 288.6 eV, which are attributed to C=C, alkyl C–C, and O=C–O, respectively.^36–39

Morphology analysis

The SEM morphology of Ni₃S₂/MnS/CuS@rGO and Ni₃S₂/MnS/CuS are provided, as shown in Fig. 4. Without the addition of rGO, Ni₃S₂/MnS/CuS displays spherical morphology assembled by closely packed nanosheets (Fig. 4a and b). With the incorporation of rGO template, Ni₃S₂/MnS/CuS@rGO also shows spherical morphology with much lower diameter. rGO nanosheets can functionalize as template to induce the growth of Ni₃S₂/MnS/CuS, forming a three-dimensional porous spherical structure self-assembled by nanosheets (Fig. 4c and d), generating a more efficient electron transport structure. EDS spectra of Ni₃S₂/MnS/CuS@rGO composite certify the presence of C, Ni, Mn, Cu and S elements (Fig. 5), confirming the successful fabrication of the Ni₃S₂/MnS/CuS@rGO composite.

Fig. 4 Morphology of Ni₃S₂/MnS/CuS@rGO at low magnification (a) and high magnification (b); morphology of Ni₃S₂/MnS/CuS at low magnification (c) and high magnification (d).

Fig. 5 The EDS spectra of C, Cu, Mn, Ni and S on spherical Ni₃S₂/MnS/CuS@rGO nanostructures.

TEM and HRTEM morphology of Ni₃S₂/MnS/CuS@rGO is further offered, as shown in Fig. 6. Spherical morphology is also observed for Ni₃S₂/MnS/CuS@rGO. HRTEM images (Fig. 6b) show clear fringes with a measured interplanar spacing of 0.326 nm, 0.24 nm and 0.12 nm that correspond to CuS (100), Ni₃S₂ (111) and MnS (331), respectively, which is in good agreement with the XRD results. The SAED pattern (Fig. 6c) shows a concentric diffraction ring, demonstrating the existence of polycrystalline structure.

Fig. 6 (a) TEM of Ni₃S₂/MnS/CuS@rGO, and (b) the HRTEM of Ni₃S₂/MnS/CuS@rGO. (c) Selected-area electron diffraction (SAED) pattern of Ni₃S₂/MnS/CuS@rGO.

Electrochemical analysis of electrodes

CV curves of Ni₃S₂/MnS/CuS@rGO, Ni₃S₂/MnS@rGO and Ni₃S₂/MnS/CuS composite electrodes are compared at 4 mV s⁻¹ with a potential window from −0.3 to 0.4 V, as shown in Fig. 7a. All CV curves show a pair of strong redox peaks, demonstrating that the Ni₃S₂/MnS/CuS@rGO electrode presents pseudocapacitance behavior, as shown in Fig. 7a. Ni₃S₂/MnS/CuS@rGO displays more intense current density than Ni₃S₂/MnS/CuS@rGO and Ni₃S₂/MnS@rGO, suggesting enhanced activity and specific capacitance.³⁷

Fig. 7 (a) CV plots of Ni₃S₂/MnS/CuS@rGO, Ni₃S₂/MnS/CuS at a 4 mV s⁻¹. (b) CV plots of Ni₃S₂/MnS/CuS@rGO composites at various voltage sweeps (1, 2, 4, 6, 8, 10, 20, 30, 40 mV s⁻¹) in 6.0 M KOH. (c) GCD analyses of Ni₃S₂/MnS/CuS@rGO composites at various current densities (1, 2, 4, 6, 8 and 10 A g⁻¹). (d) Specific capacitance of Ni₃S₂/MnS/CuS@rGO, Ni₃S₂/MnS/CuS composites at different current densities (1, 2, 4, 6, 8 and 10 A g⁻¹) in 6.0 M KOH.

As shown in Fig. 7b, the redox peak in the CV curve may be correlated with the redox reaction between M²⁺ and M³⁺ (M represents Ni, Cu and Mn elements), as expressed by eqn (6)–(9).^40–42 The peaks are located at 0.12 V and 0.32 V when the scan rate is 2 mV s⁻¹. With the 20-fold increase in the scan rate from 2 to 40 mV s⁻¹, the position of the cathodic peak shifts from 0.12 V to −0.02 V, suggesting the lower resistance of the electrode.³ Moreover, almost no deformation takes place in the CV peak shape as the scan rate increases, certifying rapid electron transport and charge separation.

Ni₃S₂ + 3OH⁻ − 3e⁻ ⇔ Ni₃S₂(OH)₃ (6)

MnS + OH⁻ − e⁻ ⇔ MnSOH (7)

MnSOH + OH⁻ − e⁻ ⇔ MnSO + H₂O (8)

CuS + OH⁻ − e⁻ ⇔ CuSOH (9)

Fig. 7c shows the galvanostatic charge–discharge curves for the Ni₃S₂/MnS/CuS@rGO electrode at various current densities in the range of −0.3 V to 0.4 V. A plateau is distinctly observed in the discharge curves due to the electrochemical and concentration polarizations.²⁰ At a current density of 1 A g⁻¹, the specific capacitance is 1028 F g⁻¹ for Ni₃S₂/MnS/CuS, 628.6 F g⁻¹ for Ni₃S₂/MnS@rGO, and 2042 F g⁻¹ for Ni₃S₂/MnS/CuS@rGO, respectively. Ni₃S₂/MnS/CuS@rGO shows better stability and higher specific capacitance than Ni₃S₂/MnS/CuS and Ni₃S₂/MnS@rGO at various current densities (Fig. 7d). These data show that the electrochemical performances of Ni₃S₂/MnS/CuS@rGO have been enhanced due to the intervention of Cu²⁺ and graphene.

The Nyquist plots of the Ni₃S₂/MnS/CuS/@rGO, Ni₃S₂/MnS/CuS and Ni₃S₂/MnS@rGO electrodes are obtained by electrode impedance spectroscopy (EIS), as shown in Fig. 8a. Ni₃S₂/MnS/CuS@rGO exhibits smaller charge transfer resistance (R_ct). The R_ct of Ni₃S₂/MnS/CuS@rGO (0.001 Ω) is much lower than that of Ni₃S₂/MnS@rGO by 0.02 Ω, and lower than that of Ni₃S₂/MnS/CuS by 0.017 Ω, which can be ascribed to the high electroconductivity of active materials and the structure of active materials.⁴³ After 5000 cycles, Ni₃S₂–MnS–CuS@RGO electrode maintains 78.3% of the initial capacity. This indicates that the (Ni₃S₂/MnS/CuS@rGO)/NF asymmetric electrode exhibits good capacitance retention at 20 A g⁻¹ (Fig. 8b). Fig. 8c shows the Nyquist plots of Ni₃S₂/MnS/CuS@rGO electrode at 0.3 V bias potential before and after 5000 GCDs. R_ct only increases by 0.024 Ω after the 5000 cycles, proving that Ni₃S₂/MnS/CuS@rGO has high durability and electrochemical stability even after long-term cycling.

Fig. 8 (a) Nyquist plots of the Ni₃S₂/MnS/CuS@rGO, Ni₃S₂/MnS/CuS composites electrodes in 6.0 M KOH in the frequency width from 0.01 to 100 kHz. (b) Cyclic performance of the Ni₃S₂/MnS/CuS@rGO electrodes at 20 A g⁻¹ in 6.0 M KOH (aq). (c) Nyquist plots of Ni₃S₂–MnS–CuS@rGO before and after 5000 cycles.

The electrochemical properties of Ni₃S₂/MnS/CuS@rGO electrode are also compared with those electrodes reported in the literatures, as listed in Table 1. It demonstrates that the Ni₃S₂/MnS/CuS@rGO electrode is endowed with excellent specific capacitance and capacity, as well as low R_ct.

Table 1 Comparison analysis of electrochemical properties of Ni₃S₂/MnS/CuS@rGO in three-electrode system

Electrode materials	Specific capacitance		Capacity retention	R s	R ct	Reference
	Low current density	High current density
Ni3S2	—	—	—	0.72	0.26	37
Ni3S2@rGO	—	—	—	0.37	0.29	37
a-MnS/N-rGO	933.6 F g^−1 at 1 A g^−1	469.1 F g^−1 at 20 A g^−1	70% after 2000 cycles at 10 A g^−1	0.78	3.7	18
The γ-MnS/rGO-60 composite	547.6 F g^−1 at 1 A g^−1	351.4 F g^−1 at 20 A g^−1	70% after 3000 cycles at 5 A g^−1	1.1	0.20	14
MnS/NixSy	1073.81 F g^−1 at 1 A g^−1	428.57 F g^−1 at 20 A g^−1	89% after 10 000 cycles at 6 A g^−1	0.25	0.52	48
CoNi2S4/CNLDHs	1027 F g^−1 at 0.5 A g^−1	600 F g^−1 at 10 A g^−1	Decreased ∼18% after 2000 cycles at 10 A g^−1	—	0.35	46
CoNi2S4/RGO	1706 F g^−1 at 0.5 A g^−1	1268 F g^−1 at 10 A g^−1	Decreased ∼8% after 2000 cycles at 10 A g^−1	—	0.20	46
Ni0.67Cu0.33Co2S4	1341 F g^−1 at 1 A g^−1	1078 F g^−1 at 10 A g^−1	71.7% after 10 000 cycles at 4 A g^−1	0.33	0.08	47
CZTS/RGO	591 F g^−1 at 0.25 A g^−1	—	80.6% after 1000 cycles at 1 A g^−1	0.51	0.04	48
Ni3S2/MnS/CuS@rGO	2042 F g^−1 at 1 A g^−1	1655 F g^−1 at 10 A g^−1	78.3% after 5000 cycles at 20A g^−1	0.46	0.001	This work

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Electrochemical analysis of asymmetric supercapacitor

The 3D schematic model and energy storage mechanism of asymmetric supercapacitor is illustrated in Fig. 9, which use Ni₃S₂/MnS/CuS@rGO as the positive electrode, rGO as the negative electrode and 6 M KOH as electrolyte. In battery dynamics, the charge storage process of two ions is faster than that of single ion shuttle reaction. Therefore, hybrid battery power equipment such as asymmetric supercapacitor is able to achieve higher energy and higher power performance. As shown in the 3D schematic model, K⁺ and OH⁻ ions move towards the negative and positive electrodes during the process of charge–discharge process. The cell works by redox reactions with OH⁻ on the Ni₃S₂/MnS/CuS@rGO positive electrode and simultaneous adsorption/desorption of K⁺ cations on the rGO negative electrode ^44.

Fig. 9 3D model diagram of the Ni₃S₂/MnS/CuS@rGO‖rGO asymmetric supercapacitor.

The Ni₃S₂/MnS/CuS@rGO asymmetric supercapacitor devices exhibit some excellent electrochemical performances on the basis of CV curves at various voltage sweeps and GCD current at various densities (Fig. 10a and b). Fig. 10a shows the CV cures of the ASC plotted in the potential window of 0 V to 1.5 V at varying scan rates (5–30 mV s⁻¹). All the CV plots are similar in shape, maintaining a pair of cathodic and anodic peaks. The redox peaks in the CV plots are found to shift away from each other with increasing scan rates. The redox peaks are indicative of the pseudocapacitive nature of ASC, which is clear from the large area of the corresponding CV plots.²⁵ The specific capacity is calculated by using the net mass of Ni₃S₂/MnS/CuS@rGO electroactive material. Hybrid power supply devices provide a specific capacitance of 522.2 F g⁻¹ at 2A g⁻¹.

Fig. 10 (a) CV curves of Ni₃S₂/MnS/CuS@rGO supercapacitor for battery mixers. (b) GCD curves at different current densities. (c) Specific capacitance and coulombic efficiency (η) of the ASC equipment.

After 5000 GCD cycles, the specific capacity of asymmetric batteries is maintained at 90.8% of the initial state (Fig. 10c), exhibiting excellent cycle stability. A simple application to power a red commercial light-emitting diode (LED) is conducted (Fig. 11). The asymmetric supercapacitor device is able to power a red LED indicator for 240 min.

Fig. 11 The button batteries supplied power for a red LED indicator for 240 min.

Conclusions

Ternary Ni₃S₂/MnS/CuS composites were successfully fabricated on nickel foam via electrostatic adsorption of Ni⁺, Mn²⁺ and Cu²⁺ ions on rGO nanosheet template based on hydrothermal method. The chemical structure was certified by XRD, XPS, SEM and HRTEM. Ni₃S₂/MnS/CuS displayed spherical morphology assembled by closely packed nanosheets, while Ni₃S₂/MnS/CuS@rGO showed 3D porous spherical morphology with much lower diameter, generating a more efficient electron transport structure. Ni₃S₂/MnS/CuS@rGO was demonstrated to have better stability and higher specific capacity than Ni₃S₂/MnS/CuS, and Ni₃S₂/MnS@rGO at various current densities. The R_ct of Ni₃S₂/MnS/CuS@rGO was also much lower owing to the high electroconductivity of active materials and the unique structure of active materials. The electrochemical performances of Ni₃S₂/MnS/CuS@rGO electrode have been significantly improved due to the intervention of Cu²⁺ and graphene. The asymmetric supercapacitor assembled with Ni₃S₂/MnS/CuS@rGO as positive electrode and rGO as negative electrode also presented excellent cycle stability. Ni₃S₂/MnS/CuS@rGO will have promising applications in the field of energy storage devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful to the National Natural Science Foundation of China (No. 21501104, 21978164), the Natural Science Foundation of Heilongjiang Province (B2015014), Industrialization Project of Shaanxi Education Department (No. 19JC010) and the National High-end Foreign Expert Project (No. GDW20186100428) for their support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra05435a

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