Controllable Synthesis of Flower-like Hierarchical CuCo₂S₄ Nanostructure Arrays for High-Performance Hybrid Supercapacitors

Abstract

Transition metal sulfides (TMSs) are considered as attractive materials in the areas of energy storage because of their unique redox properties, excellent electronic conductivity, as well as environmental friendliness. However, poor cyclic stability and limited electrochemical active sites hinder their further application. To address this issue, a flower-like hierarchical CuCo₂S₄ structure is constructed by a two-step hydrothermal method. In this nanostructure, CuCo₂S₄ grows outward to form a tightly bound hierarchical structure on the nickel foams (NFs). This oriented structure can provide more laminar gaps for electrolyte ion diffusion, exposing more reaction sites to increase the ion transport efficiency between the layers, reducing the ion transport resistance and improving the reaction kinetics. Thus, the CuCo₂S₄ electrode exhibits excellent energy storage performance, exhibiting a high specific capacity of 1415.6 F g⁻¹ at 1 A g⁻¹. After 10,000 cycles of 10 A g⁻¹, it still has 91.9% of the initial performance. In addition, an asymmetrical supercapacitor (ASC) was constructed by choosing CuCo₂S₄ as the anode and RGO as the cathode, which has the maximum energy density (61.8 Wh Kg⁻¹) at 812.1 W Kg⁻¹ and significant cycling endurance (92.05% retention) at 10,000 turns. Briefly, the researchers successfully constructed an array of CuCo₂S₄ flower-like hierarchical nanostructures and confirmed their potential application in supercapacitors.

1. Introduction

The escalating issues of environmental pollution and energy crisis have garnered global attention, prompting researchers to urgently explore efficient, renewable, and clean sources of sustainable energy [1,2,3,4,5]. Supercapacitors (SCs) are expected to be the promising alternatives for next-generation high-performance power supplies due to their fast charging and discharging processes, excellent power density, high cycle life, and high safety [6,7,8,9,10,11]. Based on the charge storage mechanism, SCs can be classified into two categories [12]: pseudo-capacitors, which utilize Faraday reversible reactions, and electric double-layer capacitors (EDLCs), which involve the accumulation of the electrostatic charges at the electrode/electrolyte interface. However, the relatively low energy density is still the main problem that impedes the large-scale application of SCs. In most cases, the energy density of SCs can be affected by the structure of the electrode materials, such as hollow nanostructures, core-shell structures, and lamellar structures [13]. Several recent studies have reported that optimizing and designing the morphology structure of electrode materials is one of the defining means of raising the energy density of SCs [14,15,16]. However, the development of high-performance electrode materials to bolster their energy density remains an exceptionally challenging undertaking [17,18].

In the last decades, carbon materials (e.g., porous carbon, graphene, etc.), transition metal oxides (e.g., Co₃O₄, Mn₃O₄), and conductive polymers (e.g., polypyrrole, polyaniline, etc.) have often been used to fabricate pseudocapacitors [19,20]. However, the low performance of activated carbon and the poor thermal stability and cycling performance of conductive polymers have limited the practical application of SCs to some extent [21]. In terms of transition metal oxides, they have higher structural flexibility and excellent specific capacitance, but their slower electron conduction may hinder their practical application in the commercial storage [22]. Transition metal sulfides have more preparation pathways and multiple redox states compared to transition metal oxides and are capable of obtaining more nanostructures, which can effectively promote redox reactions in capacitors. They also have many advantages, such as abundant resources, low cost, and better conductivity, which make them more widely used in SCs. Transition metal sulfides have superior thermal stability and electrical conductivity and higher electrochemical activity than the corresponding transition metal oxides [20,22]. The M-S bonding energy of metal sulfides is lower than the M-O bonding energy of metal oxides, which is more favorable for the electrochemical storage [23]. In addition, compared with binary metal sulfides, polymetallic sulfides have higher electrical conductivity and specific capacitance due to the participation and synergistic effect of multiple cations in redox reactions and the excellent electrochemical activity [24,25]. Further, due to its low electronegativity, sulfur possesses a more flexible structure, which is conducive to improving the electron transport potency. For these reasons, TMSs have attracted extensive attention from researchers as a promising electrode material for SCs [26,27].

Metal sulfides, such as NiCo₂S₄ [28], Co₃S₄ [29], and CuCo₂S₄ [30], due to their excellent pseudocapacitance properties, have been recognized as a popular research direction for SCs electrode materials in recent years. Among them, CuCo₂S₄, in which Cu occupies a tetrahedral position and Co occupies an octahedral position, has attracted special attention owing to its abundant redox state and environmental friendliness [31,32,33]. However, CuCo₂S₄ cannot meet the current demand of SCs due to its limited electrochemical active sites [24,25,34]. In order to solve the problems above, an effective approach is to improve the electrochemical behavior of transition metal sulfides by optimizing morphology and structural engineering strategies [26,35]. The morphology of the electrode material can determine its effective contact area with the electrolyte, thus affecting the electrochemical reaction active sites and charge diffusion paths. For instance, Zhang [23] et al. synthesized three-dimensional hollow nanotube CuCo₂S₄ arrays by the solvothermal method, with a specific capacitance of 458.8 C g⁻¹ at 1.0 A g⁻¹ and a capacity retention of 96% after 1000 consecutive charge/discharge cycles. Fan [36] team prepared a CuCo₂S₄ nanosheet array under hydrothermal conditions, possessing a high specific capacitance of 3132.7 F g⁻¹ at 1 A g⁻¹ and a maximum energy density (46 Wh kg⁻¹) at a power density of 992 W kg⁻¹. Wang [37] et al. synthesized oriented nanograss arrays consisting of CuCo₂S₄ nanocrystals by using a hydrothermal method with a retention rate higher than 99% after 5000 cycles and obtained a high energy density of 32 Wh kg⁻¹. Jia et al. [33] prepared hollow sea urchin-like CuCo₂S₄ microspheres by using the hydrothermal and vulcanization methods. A high specific capacitance of 1069 F g⁻¹ was obtained at 1 A g⁻¹, with 88% capacity retention after 5000 charge/discharge cycles. Although metal sulfides have high electrochemical activity and excellent specific capacity, the electrode materials are subject to phase transition reactions during electrochemical reactions, which cause volume changes in the electrodes, thereby reducing the reaction kinetics, as well as generating harmful side reactions. Therefore, the controlled preparation of CuCo₂S₄ with the desired microstructure remains a great challenge.

In this work, flower-like hierarchical CuCo₂S₄ nanostructures were synthesized on NFs by a simple two-step solvent heat method with a well-designed structural design strategy. Because of the unique flower-like hierarchical structure, the CuCo₂S₄ electrode material significantly enhances the kinetics of ionic reactions. Moreover, the binder-free CuCo₂S₄ grown directly on the NFs exhibits good electrical conductivity. With the advantages of short electron transport distance, fully exposed active sites, and synergistic effects of copper and cobalt, CuCo₂S₄ demonstrates excellent cycling stability and multiplicative properties. These findings suggest that the strategy proposed in this work provides a practical and effective method for improving the performance and nanostructure design of SCs.

2. Experimental Section

2.1. Materials

The NFs were purchased from Tianjin Damao Chemical Reagent Co., Ltd. (Tianjin, China) Cobalt nitrate hexahydrate (Co(No₃)₂·6H₂O), Thioacetamide (TAA), Urea (CH₄N₂O), Copper nitrate trihydrate (Cu(No₃)₂·3H₂O), and Potassium Hydroxide (KOH) were purchased from the official website of Aladdin Reagent (Shanghai, China), and all of the chemicals were analytically pure and not further purified. The deionized water used came from local sources.

2.2. Synthesis of (Cu⁺², Co)₂(CO₃)(OH)₂ Nanosheets

The NFs (1 × 2 cm⁻¹) were washed with 2 M HCl under ultrasonic conditions for 20 min and followed by three repeated washes with ethanol and deionized water (DI), then dried for 6 h. Meanwhile, 0.045 g of Cu(NO₃)₂·3H₂O, 0.525 g of Co(NO₃)₂·6H₂O, and 0.565 g of urea were dissolved in 40 mL of DI. The resulting solution was transferred to a 50 mL autoclave, and the NFs were suspended in the center of the solution and reacted at 90 °C for 12 h. The obtained NFs were then washed with DI and ethanol and finally dried under vacuum at 60 °C; this product is marked as Cu-CCO.

2.3. Synthesis of CuCo₂S₄

The Cu-CCO obtained previously was put into an autoclave containing a solution of 0.06 mol L⁻¹ C₂H₅NS (TAA) and sulfurized at 120 °C for 6 h. The obtained NFs were rinsed with ethanol and DI 3–4 times and then dried under vacuum at 60 °C for 10 h. The final product was labeled as CuCo₂S₄-6M. As a control test, the electrode materials with TAA concentrations of 0.05 mol L⁻¹ and 0.07 mol L⁻¹ were synthesized and labeled as CuCo₂S₄-5M and CuCo₂S₄-7M, respectively. Furthermore, while keeping the sulfidation concentration constant at 6 M, different reaction times of 5 h, 6 h, and 7 h were used to evaluate the implication of sulfidation time on the properties of CuCo₂S₄. These samples were labeled as CuCo₂S₄-5h, CuCo₂S₄-6h, and CuCo₂S₄-7h, respectively.

2.4. Characterization

The morphology and microstructure were characterized by scanning electron microscopy (SEM, SU8010) and transmission electron microscopy (TEM, Tecnai G2 F20 (FEI Company, Hillsboro, OR, USA)). X-ray diffraction (XRD, a Rigaku D/max-2550 PC) tests were carried out to analyze the p crystal structure with Cu Kα radiation. The chemical state of the samples was analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi (ThermoFisher Scientific, Waltham, MA, USA)). Fourier-transform infrared spectroscopy (FT-IR) was performed on a Perkin-Elmer Spectrum One B instrument using the KBr pellet technique.

2.5. Electrochemical Measurements

The CHI 660E electrochemical workstation (CH Instruments, Inc., Bee Cave, TX, USA) was used for cyclic voltammetry (CV), constant current charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) in the range of 0.01 to 100 kHz. The cyclic stability of the electrodes was tested by a Land cell measurement system (Wuhan Land Electronics Co., Ltd., Wuhan, China) at a current density of 10 A g⁻¹. In the three-electrode system, 2 M KOH was used as the aqueous electrolyte, and CuCo₂S₄ was employed as the working electrode, where both the platinum plate and the saturated calomel electrode (SCE) were used as the counter and reference electrodes. The calculation of specific capacity (C_g (F g⁻¹)) is based on GCD curves by the following equation:

$${\mathrm{C}}_{\mathrm{g}}=\frac{\mathrm{It}}{\mathrm{m}}$$

(1)

where I (A), t (s), and m (g) represent discharge current density, discharge time, and loading mass, respectively.

2.6. Fabrication of Asymmetrical Supercapacitor (ASC)

ASC devices were assembled in 2 M KOH by using reduced graphene oxide and CuCo₂S₄ as negative and positive electrodes of ASC, respectively. The negative electrode material was prepared as follows: reduced graphene oxide (RGO), polyvinylidene fluoride (PVDF), and acetylene black were dispersed in N-methylpyrrolidone (NMP) at a mass ratio of 8:1:1 and milled for thirty minutes to form a homogeneous slurry. The slurry was then uniformly applied to the NFs, dried under vacuum at 60 °C for 24 h, and pressed at 10 MPa for 2 min. The mass ratio of the active material between the two electrodes is determined by means of the principle of charge balance (Equations (2) and (3)).

$${\mathrm{Q}}^{+}={\mathrm{Q}}^{-}$$

(2)

$$\frac{{\mathrm{m}}^{+}}{{\mathrm{m}}^{-}}=\frac{{\mathrm{I}}_{\mathrm{m}}^{-}\times \Delta \mathrm{t}}{{\mathrm{I}}_{\mathrm{m}}^{+}\times \Delta \mathrm{t}}$$

(3)

where ${\mathrm{m}}^{+}$ and ${\mathrm{m}}^{-}$ refer to their loadings of active substances (g); ${\mathrm{I}}_{\mathrm{m}}^{-}$ and ${\mathrm{I}}_{\mathrm{m}}^{+}$ are discharge currents of negative and positive electrodes; and both ${\mathrm{Q}}^{-}$ and ${\mathrm{Q}}^{+}$ are stored charges in negative and positive electrodes (C), respectively.

3. Results and Discussion

The synthesis process of flower-like hierarchical CuCo₂S₄ nanostructure arrays is shown in Figure 1. First, Cu-CCO precursor nanosheets were grown in NFs substrates by use of simple solvent methods (Figure S2). Then, the prepared Cu-CCO precursor was immersed in TAA solution via a secondary hydrothermal method and stored at 120 °C for 6 h. CuCo₂S₄ arrays with flower-like hierarchical nanostructure were obtained through ion etching and exchange.

XRD tests were performed on CuCo₂S₄, as shown in Figure 2a,b. Among them, the diffraction peaks at positions 44.4°, 51.7°, and 76.3°are from the nickel foam substrate [38]. As shown in Figure 2a, all the peaks of CuCo₂S₄ obtained after the vulcanization of Cu-CCO in the presence of TAA match exactly with the standard card (JCPDS no.42-1450) [39]. The diffraction peaks at 2θ of 54.83°, 50.01°, 46.15°, 38.13°, and 31.27° are attributed to the (044), (115), (224), (004), and (113) crystal planes of the CuCo₂S₄ phase, respectively. A weak diffraction peak can be seen at 22.1° when the TAA concentration is 0.07 M. This is due to the reaction of S²⁻ with the NFs substrate, forming Ni₃S₂ when the sulfide concentration is too high [40]. In Figure 2b, all the peaks of CuCo₂S₄ correspond well to the peaks of the standard card. In addition, no other impurity peaks in the XRD spectra indicates that there is no growth of other impurity elements on the surface of NFs.

The chemical state and chemical structure of CuCo₂S₄ were tested using XPS. The full XPS spectrum of Figure 2c contains Cu, Co, and S elements, which is consistent with the XRD results, indicating the successful synthesis of CuCo₂S₄. The Cu 2p spectrum in Figure 2d exhibits the 2p_2/3 and 2p_1/2 orbitals of Cu, with two peaks observed at 933.7 and 955.2 eV. In parallel, the Cu 2p spectra reveal two peaks for Cu⁺ at 932.1 and 952.0 eV, as well as two peaks for Cu²⁺ at 955.6 and 935 eV, suggesting the coexistence of Cu⁺ and Cu²⁺ [41,42]. As depicted in Figure 2e, in the high-resolution spectrum of Co 2p, the two peaks at 797.2 and 781.6 eV can be observed, corresponding to the 2p_1/2 and 2p_2/3 orbitals of Co. Additionally, the peaks at 780.6 eV (Co 2p_3/2) and 796.4 eV (Co 2p_1/2) are attributed to Co²⁺, while the peaks at 778.5 eV (Co 2p_3/2) and 793.5 eV (Co 2p_1/2) indicate the presence of Co³⁺, suggesting the existence of Co³⁺ and Co²⁺ [43]. Figure 2f displays the spectrum of S 2p, which exhibits two peaks around 161.7 and 162.8 eV, belonging to S 2p_3/2 and S 2p_1/2, respectively [44].

Figure 3a,b illustrate the flower-like hierarchical CuCo₂S₄-5M nanostructures uniformly dispersed on the NFs. Figure 3c,d exhibit SEM images of CuCo₂S₄-6M at varying magnifications. The SEM image in Figure 3d reveals the intricate flower-like hierarchical nanostructures. At lower magnification in Figure 3c, it is evident that the nanoflower is a stack of nanosheets with a thickness of approximately 50–60 nm. These nanosheets intersect with each other, forming a multilayer structure that provides ample space and facilitates the diffusion of electrolyte ions. Figure 3e,f show the microscopic morphology of CuCo₂S₄-7M. It can be observed that an excessively high concentration of TAA etching destroys the morphology of flower-like hierarchical CuCo₂S₄ nanostructures, resulting in their accumulations and agglomerations on the NFs.

In order to investigate the optimal time for vulcanization, this study also explores the effect of hydrothermal time on the morphology of the material after vulcanization. It is worth noting that even after further sulfuration treatment, the Cu-CCO precursor was successfully converted to CuCo₂S₄, while maintaining its flower-like hierarchical nanostructures (Figure 4a,b). As shown in the SEM images captured after a hydrothermal time of 6 h in Figure 4c,d, the flower-like hierarchical CuCo₂S₄ nanostructure arrays grown on the NFs are more upright and ordered; the electrodes have more contact with the electrolyte, thus providing more active reaction sites. On the other hand, as shown in Figure 4e,f, the vertically grown flower-like hierarchical nanostructures on NFs appear less ordered and structurally intact compared with CuCo₂S₄-6h.

To further observe the microstructure and crystallinity of CuCo₂S₄, it was analyzed using TEM. Figure 5a shows a high-magnification scanning transmission electron micrograph of CuCo₂S₄, and the facet spacing of 0.33 nm and 0.28 nm obtained from the TEM maps corresponds to the (133) and (022) facets of the CuCo₂S₄ spinel phase [45]. Figure 5b displays the selected electron diffraction map of CuCo₂S₄, where diffraction rings correspond to the (022), (224), and (004) crystal planes of CuCo₂S₄. These findings align with the results obtained from the XRD. The elemental mapping of CuCo₂S₄-6M (Figure 5c) shows that Cu, Co, and S elements are uniformly distributed in the sample.

The electrochemical performance of CuCo₂S₄ was evaluated using a three-electrode system, including CV, GCD, and EIS tests. To further understand the relationship between the sulfide concentration and the electrochemical performance of CuCo₂S₄, CV tests were conducted. From Figure 6a, it can be seen that the curve shapes are roughly the same, and all three samples have a pair of distinct reduction peaks and oxidation peaks; their oxidation and reduction peaks are approximate, manifesting that the CuCo₂S₄ electrode has good pseudocapacitance characteristics and electrochemical reversibility. The sample CuCo₂S₄-6M has the maximum area of the closed interval, indicating a higher specific capacitance. Figure 6b shows the GCD comparison of CuCo₂S₄ at 1 A g⁻¹. Unlike the triangles in the electrodes of the double-layer capacitor, all the samples have a clear plateau region in the 0.2–0.4 potential interval, which further proves that the CuCo₂S₄ electrode is a battery-type material. And at the same current density, CuCo₂S₄-6M has a longer discharge time, indicating that it has a larger electric capacity. Figure 6c displays Nyquist plots of CuCo₂S₄. The equivalent circuit composed of equivalent series resistance (R_S), double-layer capacitance (C_dl), Warburg impedor (W), Faraday capacitance (C_F), and charge transfer resistance (R_ct) is shown in Figure S3. It is clear that CuCo₂S₄-6M shows an almost negligible semicircle and a smallest real axis intercept, demonstrating R_S and R_ct. From the tests in Figure 6a–c, it is clear that the CuCo₂S₄-6M electrode has superior electrochemical performance. Therefore, the impact of vulcanization time on the electrode material properties was further explored by GCD, CV, and EIS in Figure 6d–f, while maintaining the optimal vulcanization concentration. Figure 6d shows the CV curves obtained at 50 mV s⁻¹, illustrating that CuCo₂S₄-6h owns a larger area of the closed interval. Figure 6e displays the GCD curve at the same current density, revealing specific capacitances of 998.2, 1415.6, and 1172.3 F g⁻¹ for CuCo₂S₄-4h, CuCo₂S₄-6h, and CuCo₂S₄-8h, respectively. Notably, CuCo₂S₄-6h owns the largest specific capacity, which may be attributed to the optimal maintenance of its flower-like hierarchical nanostructure achieved by an appropriate sulfidation time. The electrochemical kinetics were further characterized by EIS (Figure 6f), demonstrating that CuCo₂S₄-6h exhibits the smallest R_ct and R_s indicative of faster charge transfer and better electrochemical performance. To further highlight the excellent electrochemical performance of CuCo₂S₄-6h, CV tests were performed at different scan rates (Figure 6g). Remarkably, the CV curves remain relatively unchanged even at high scan rates, attributable to the low polarization resistance and good reversibility of CuCo₂S₄-6h. The redox reaction can be described by the following equations [46]:

$${\mathrm{CuCo}}_2{\mathrm{S}}_4{+\mathrm{OH}}^{-}{+\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{CuSOH}+2\mathrm{CuSOH}+\mathrm{e}}^{-}$$

(4)

$${\mathrm{CoSOH}+\mathrm{OH}}^{-}\leftrightarrow {\mathrm{CoSO}+\mathrm{H}}_2{\mathrm{O}+\mathrm{e}}^{-}$$

(5)

$${\mathrm{CuSOH}+\mathrm{OH}}^{-}\leftrightarrow {\mathrm{CuSO}+\mathrm{H}}_2{\mathrm{O}+\mathrm{e}}^{-}$$

(6)

In Figure 6h, the specific capacitances of the CuCo₂S₄ were calculated to be 1415.6, 1398.5, 1362.8, 1315.1, 1257.3, and 1198.2 F g⁻¹ for 1–10 A g⁻¹, and have a charging and discharging plateau, which is consistent with CV results in Figure 6g. The specific capacitance retention at 10 A g⁻¹ compared to 1 A g⁻¹ is 84.6%, proving its excellent multiplicity performance. Figure 6i displays the cyclic charge/discharge test of CuCo₂S₄-6h. After 10,000 cycle tests, the capacity of CuCo₂S₄-6h was 91.9% of the initial capacity, which further proves that the CuCo₂S₄ nanoflower electrode synthesized under optimized reaction conditions possesses excellent cycling stability.

Figure 6. CuCo₂S₄-5M, CuCo₂S₄-6M, and CuCo₂S₄-7M (a) CV curves at 50 mV s⁻¹; (b) GCD curves at 1 A g⁻¹; (c) EIS curves; CuCo₂S₄-4h, CuCo₂S₄-6h, and CuCo₂S₄-8h (d) CV curves at 50 mV s⁻¹; (e) GCD curves at 1 A g⁻¹; (f) EIS curves; (g) CV curve and (h) GCD curve of CuCo₂S₄-6h; (i) Cycling properties of CuCo₂S₄-6h. The blue line represents capacity retention and corresponds to the blue vertical axis. The green line represents Coulombic efficiency, corresponding to the green vertical axis.

Figure 6. CuCo₂S₄-5M, CuCo₂S₄-6M, and CuCo₂S₄-7M (a) CV curves at 50 mV s⁻¹; (b) GCD curves at 1 A g⁻¹; (c) EIS curves; CuCo₂S₄-4h, CuCo₂S₄-6h, and CuCo₂S₄-8h (d) CV curves at 50 mV s⁻¹; (e) GCD curves at 1 A g⁻¹; (f) EIS curves; (g) CV curve and (h) GCD curve of CuCo₂S₄-6h; (i) Cycling properties of CuCo₂S₄-6h. The blue line represents capacity retention and corresponds to the blue vertical axis. The green line represents Coulombic efficiency, corresponding to the green vertical axis.

For an analysis of the charge storage behavior of CuCo₂S₄ and to determine whether it is surface-controlled battery behavior or capacitance-controlled during the charge and discharge process, a CV test on CuCo₂S₄ was conducted at small scan rates (Figure 7a); the peak data were then fitted to obtain the b value. The relevant calculation formulas are as follows [47,48,49]:

$${\mathrm{i}=\mathrm{a} \times \mathrm{v}}^{\mathrm{b}}$$

(7)

$${\mathrm{i}\left(\mathrm{V}\right)=\mathrm{k}}_1{ \times \mathrm{v}+\mathrm{k}}_2{ \times \mathrm{v}}^{1/2}$$

(8)

where IP is the peaks current, a and b are the constants, k₁v represents the surface contribution, $K_2{V}^{\frac{1}{2}}$ represents the diffusion contribution, and b is the slope of the straight line in Figure 7b, which is closely related to different electrochemical behaviors. When b = 1, surface control presents capacitive behavior, whereas when b = 0.5, diffusion control presents battery behavior. For the CuCo₂S₄ electrode, b values of 0.78 and 0.81 can be obtained from the calculations, indicating that the surface capacitance and diffusion control exist simultaneously, with the b value being close to 1. Thus, its contribution was fitted and analyzed by Equations (7) and (8). From Figure 7c, it can be observed that the surface control accounts for 82.56%, 84.36%, 85.63%, 87.46%, and 89.27% of the total capacitance as the rate increases. Additionally, the surface-controlled capacitance contribution increases with the scan rate, indicating that the electrode is dominated by capacitive behavior during the electrochemical reactions.

To further evaluate the practicality of the ASC device, we assembled a two-electrode supercapacitor with CuCo₂S₄ as the positive electrode and RGO as the negative electrode in 2 mol L⁻¹ KOH electrolyte. Figure 8a shows the CV curves of the RGO at 10–100 mV s⁻¹, where the quasi-rectangular shape of the bilayer capacitance behavior can be clearly seen. The GCD curves of RGO at 1–10 A g⁻¹ (Figure 8b) show a clear, distinct symmetric triangle, indicating high reversibility. Meanwhile, the CV curves for the two electrodes RGO (−1–0 V) and CuCo₂S₄ (0–0.6 V) were tested in a three-electrode system at 50 mV s⁻¹ (Figure 8c).

Figure 9a shows the schematic diagram of ASC assembled by CuCo₂S₄//RGO. In order to determine the most suitable operating voltage range, we performed CV tests at 50 mV s⁻¹ (Figure 9b). It is seen that there is no significant change in the curve until the voltage window reaches 0–1.7 V, where a significant polarization occurs. This indicates that the ASC has a stable voltage window at 1.6 V [50]. The CV curve of ASC is shown in Figure 9c. As the sweep speed increases, the area around the CV curve expands, showing good capacitive behavior and multiplier performance. The specific capacitances of CuCo₂S₄//RGO were 228.1, 198.3, 183.6, 163.6, 146.2, and 141.5 F g⁻¹ at 1–10 A g⁻¹ (Figure 9d). Figure 9e shows the Ragone diagram of CuCo₂S₄//RGO, which demonstrates that the CuCo₂S₄//RGO device has a high power density (812.1 W kg⁻¹) and energy density (61.8 Wh kg⁻¹), which outperforms many copper–cobalt-based supercapacitors (Table S1) [35,51,52,53,54]. This indicates that the energy density of the ASC device is well maintained. The CuCo₂S₄//RGO device exhibits excellent stability, maintaining a specific capacitance of 82.8% after 10,000 cycle measurements at a high current density of 10 A g⁻¹ (Figure 9f).

4. Conclusions

In summary, a flower-like layered CuCo₂S₄ nanostructure was obtained by a two-step solvent–thermal method. Remarkably, the unique hierarchical structure of the CuCo₂S₄ electrodes exhibit superior cycle stability (high capacitance retention of 91.90% after 10,000 cycles at high currents) and excellent energy storage performance (1425.6 F g⁻¹ at 1 A g⁻¹). In addition, ASC devices have a voltage range of 0–1.6 V and offer power density (812.1 W kg⁻¹) and energy density (61.8 Wh kg⁻¹). The device offers excellent stability, with 92.05% capacity maintenance after 10,000 cycles. Its good cycling stability is attributed to the good electrochemical properties and special nanostructure of CuCo₂S₄. Therefore, this paper offers an effective and practical strategy for elaborate nanostructure design and enhancement of supercapacitors’ performance.