High-Performance Asymmetric Supercapacitor Based on a Bilayer Cu_0.7Zn_0.3CoNiS_yO_4−y/Ni₃S₂ Electrode

Abstract

Supercapacitors have begun to successfully compete with Li-ion batteries in various portable energy storage applications, owing to their ability to enable fast charging, deliver high power and energy, and offer an exceptionally long cycle life. This paper presents the results of a study on the performance of a positive electrode composed of a Cu_xZn_1−xCoNiS_yO_4−y whisker layer and an underlying porous Ni₃S₂ layer, synthesized in a single step via the hydrothermal method. The coating with the nominal composition Cu_0.7Zn_0.3CoNiS₃O/Ni₃S₂ exhibited a high specific capacitance of 4.10 C cm⁻² at a current density of 2 mA cm⁻² or 9535 F g⁻¹ at a current density of 1 A g⁻¹, attributed to the synergistic contribution of both layers and the optimized ratio of the four transition metals in the sulfoxide matrix. The assembled asymmetric supercapacitor (ASC), employing the obtained composite as the positive electrode and activated carbon as the negative electrode, exhibited a specific capacitance of 115 F g⁻¹ (200 C g⁻¹). It achieved a high energy density of 48.3 Wh kg⁻¹ at a power density of 870 W kg⁻¹. After 20,000 charge–discharge cycles at a current density of 10 A g⁻¹, the ASC retained 74% of its initial capacitance, highlighting the potential of the Cu_xZn_1−xCoNiS_yO_4−y electrode for high-performance energy storage applications.

1. Introduction

Asymmetric supercapacitors (ASCs) are devices in which one electrode accumulates charge via a battery-type Faradaic process, while the other relies on a capacitive mechanism. Transition metal (TM) sulfides are among the most promising materials for the positive electrode in ASCs. Currently, research is focused on developing combinations of two or three TMs within AB₂S₄ or ABCS₄-type structures [1], resulting in sulfides with variable compositions. In these frameworks, the A, B, and C sites can be occupied by metals such as Ni, Co, Cu, and Zn, and less commonly by Mn [2], Sn [3], Mo [4], or W [5]. These materials are also being studied in composite form with oxides (e.g., [6]), hydroxides [7,8] (including core–shell nanostructures [9]), conducting polymers [10,11], reduced graphene oxide (rGO) [12], and other components.

It is widely recognized that TM sulfides exhibit superior electrical conductivity and electrochemical performance compared to the oxides and hydroxides of the same metals [1]. Therefore, incorporating an additional element into the structural matrix or partially substituting one of the metal components is considered an effective strategy for enhancing supercapacitor performance. For this reason, the fabrication of battery-type positive electrodes is often a multi-step process [1,6,7,8,9,10,11,12,13], typically involving hydrothermal synthesis. In many cases, precursor materials are first synthesized and subsequently sulfided [2,12,14,15]. However, this can result in only partial replacement of oxygen atoms by sulfur, or lead to the formation of an additional surface layer composed of a different compound [7,8,9,10,11].

Hydrothermal synthesis is an effective method for producing composites based on oxide–sulfide systems of transition metals. However, it is important to note that the hydrothermal process is a quasi-equilibrium method; so, the synthesis of compounds that are outside the equilibrium stability region can be challenging. Sulfur-containing materials exhibit excellent electrochemical performance and long-term cycling stability; however, achieving stoichiometric composition and single-phase samples is not always feasible [16,17,18,19]. Consequently, there is significant potential for designing composites in which the synergistic effects of individual elements contribute to the compound’s distinctive redox behavior.

ASCs based on CuCo₂S₄ [13,20,21,22] and NiCo₂S₄ [10,11,18,23] and Zn-Co-S compounds [7,9,15,19,24,25,26,27] are actively being developed. Gao et al. [1] synthesized a series of Cu(Co,Ni)₂S₄ materials and demonstrated that cobalt and nickel can be readily substituted within the spinel lattice. However, a high nickel content resulted in the formation of a multiphase composite comprising CuCo₂S₄, Cu₉S₈, and Ni₇S₆. The authors identified CuCo_1.25Ni_0.75S₄ as the optimal composition, which exhibited a high specific capacitance of 647 F g⁻¹ at 1 A g⁻¹ and excellent cycling stability, retaining 98% of its capacitance after 10,000 charge–discharge cycles. In the study by V. Vignesh and R. Navamathavan [17], spherical Ni–Co–Zn–S particles were synthesized via a one-step hydrothermal method. However, the resulting product was not single-phase, as indicated by the diffraction patterns presented. To characterize the material, the diffraction peaks were compared with standard patterns for Co₃S4, Ni₃S₄, and ZnS, suggesting that the microspheres likely consist of a mixture of these three sulfides. Y. Liu et al. [28] successfully synthesized a single-phase CoZnNiS compound and assembled a hybrid supercapacitor using a CoZnNiS/CNTs/rGO film as the positive electrode and carbon spheres/rGO as the negative electrode, which exhibited high-performance characteristics. H. Zhang et al. [29] reported that Ni/Co/S composites demonstrated optimal electrochemical performance when the Ni/Co molar ratio was 1:1. Additionally, a study on the incorporation of copper into ZnCo₂O₄ [30] was reviewed. M. Sharma et al. identified Zn_0.7Cu_0.3Co₂O₄ as the most effective composition, achieving a high specific capacitance of 1425 F g⁻¹—1.55 times greater than that of the original ZnCo₂O₄ (917 F g⁻¹). Another recently reported oxide, NiCuCoO [31], exhibited a specific capacity of 596 C g⁻¹ at a current density of 1 A g⁻¹. The assembled NiCuCoO/AC asymmetric supercapacitor delivered a capacity of 168 C g⁻¹ at 1 A g⁻¹, with an energy density of 96 Wh kg⁻¹, a power density of 841 W kg⁻¹, and a capacity retention of 95% after 5000 cycles at 10 A g⁻¹.

When selecting the chemical composition of the synthesized compound, it is reasonable to consider that crystallographically coherent bimetallic sulfides—those with similar lattice parameters and identical space group symmetries—combine most effectively within spinel sublattices. For example, the carrollite CuCo₂S₄ sublattice (FCC, a = 9.474 Å, JCPDS card No. 42-1450) can be effectively combined with the siegenite NiCo₂S₄ sublattice (FCC, a = 9.417 Å, JCPDS card No. 43-1477) or the CoNi₂S₄ sublattice (FCC, a = 9.427 Å, JCPDS card No. 24-0334). In contrast, zinc–cobalt sulfide Zn_0.76Co_0.24S has a significantly smaller lattice parameter (a = 5.394 Å) and belongs to the F4$\overline{3}$m space group. Therefore, the incorporation of zinc into mixed-type Fd$\overline{3}$m spinel structures should be approached with caution, as it may induce lattice reconstruction and promote the formation of multiphase composites. Nonetheless, there remains considerable potential for investigating the mutual substitution of copper and zinc, as well as nickel and cobalt, without altering the space group of the Fd$\overline{3}$m phase.

We previously synthesized a thio/oxy spinel, CuCoNiS_xO_4−x, capable of accommodating various sulfur–oxygen substitution ratios. Among the studied compositions, CuCoNiS₂O₂ exhibited the highest specific capacitance of 3612 F g⁻¹ at a current density of 1 A g⁻¹. The ASC assembled using this material demonstrated promising performance, with a specific capacitance of 133.5 F g⁻¹ at 1 A g⁻¹ and a wide potential window of 1.7 V. It also achieved an energy density of 53.6 Wh kg⁻¹ at a power density of 805 W kg⁻¹, along with good cycling stability [32]. In that study, we also conducted an extensive comparative analysis of previously reported sulfides containing three transition metals and summarized electrode and ASC performance characteristics in a comprehensive table.

Thus, device architectures and composite electrodes based on transition metal sulfides—when optimized for high specific capacity and long-term stability—exhibit distinct characteristics in each specific case. Composite materials that effectively integrate the unique properties of their individual components hold great potential for achieving enhanced redox performance. However, achieving such a synergistic effect—resulting in high performance of supercapacitor electrodes and ASC devices overall—is a challenging task due to the complexity of integrating electrical, electrochemical, and mechanical properties within composites that contain multiple crystalline, amorphous, or polymeric phases. In this study, Zn-doped CuCo₂S₄ filamentous crystals were synthesized on nickel foam (NF) as a current-collecting substrate using a simple one-step hydrothermal method. It was observed that the formation of the active layer on NF begins with the synthesis of a Ni₃S₂ film, which serves as a foundation for the subsequent growth of the CuCo₂S₄ whisker layer. This electrode architecture significantly contributes to the high performance of the resulting supercapacitor device. Based on the synthesized composite, an ASC was fabricated, employing the composite as the positive electrode and activated carbon as the negative electrode.

2. Materials and Methods

Copper nitrate trihydrate (Cu(NO₃)₂·3H₂O, 98%), zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, 99%), cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O, 98%), nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O, 98.5%), thiourea (N₂H₄CS, 99%), sodium hydroxide (NaOH, 98%), and N-methyl-2-pyrrolidone (NMP) were purchased from Merck KGaA (Darmstadt, Germany). PVDF Kynar (PolyK, 2124 Old Gatesburg Rd, State College, PA, USA), conductive acetylene black from MTI KJ Group (Richmond, CA, USA), and activated carbon (AC) (YEC-8A, Fuzhou Yihuan Carbon Co., Fuzhou, China) were used to synthesize the negative electrodes. NF was obtained from Kunshan Guangjiayuan New Materials Co., Ltd. (Kunshan, China).

The phase analysis of samples was performed by the X-ray diffraction (XRD) with D8 ADVANCE (Bruker, Bilerica, MA, USA) using Cu Kα radiation (40 kV, 40 mA). Morphology and energy dispersive analysis (EDA) were observed on a TM4000Plus microscope (Hitachi, Tokyo, Japan) with an accelerating voltage of 15 keV. Raman spectra were studied on an NTEGRA Spectrometer (NT-MDT, Zelenograd, Russia) at an excitation wavelength of 532 nm, collected for 200 s from different 2 μm patches in the spectral range of 0–1000 cm⁻¹ with a resolution of ±4 cm⁻¹. The electrochemical tests of both electrodes in a three-electrode system and ASC were determined with a CorrTest CS2350 bipotentiostat (Wuhan Corrtest Instruments Corp., Ltd., Wuhan city, China) and a workstation P-40X-FRA-24M (Elins, Chernogolovka, Russia).

3. Synthesis of the Cu_xZn_1−xCoNiS_yO_4−y

A simple one-step hydrothermal method, similar to that described previously [32], was employed to synthesize samples with the nominal composition Cu_xZn_1−xCoNiS_yO_4−y on a pretreated NF substrate. In each case, the corresponding molar proportions of the precursor components were used. We prepared three samples, designated Cu0.7-Zn0.3, Cu0.5-Zn0.5, and Cu0.3-Zn0.7, with different levels of copper and zinc. For example, to obtain a Cu0.7-Zn0.3 sample, 0.7 mmol of copper nitrate, 0.3 mmol of zinc nitrate, and 1 mmol each of cobalt and nickel nitrates were used, along with 4 mmol of thiourea and 14 mmol of NaOH. The precursors were dissolved in two separate aqueous solutions: one containing all the metal nitrates, and the other containing thiourea and NaOH. The solutions were then combined and placed into a Teflon-lined autoclave along with a nickel foam substrate. The autoclave was heated at a rate of 10 °C per minute to a temperature of 180 °C and maintained at this temperature for 3 h. After the hydrothermal reaction, the autoclave was removed from the furnace and allowed to cool naturally to room temperature. The resulting coatings were black in color. The samples were subsequently washed several times with deionized water and ethanol, and then dried at 80 °C for 12 h.

4. Results

4.1. Structural and Morphology Characterization

The phase composition of the samples was determined by X-ray diffraction (XRD). In all diffraction patterns where the loading mass exceeded 1 mg cm⁻², a distinct peak appeared at 21.75°, and peaks beyond 35° exhibited complex doublet or triplet structures. XRD analysis revealed that these peaks correspond to the Ni₃S₂ phase. As will be shown later in the SEM analysis, a porous layer forms beneath the whisker-like structures. This occurs due to the etching of the nickel foam by the decomposition products of thiourea, leading to the formation of the Ni₃S₂ phase. As shown in Figure 1a, the black curve of the diffraction pattern reveals overlapping peaks corresponding to both the spinel phase and the Ni₃S₂ phase, the latter being crystallographically rhombohedral with a rhombohedral-centered lattice (space group R32). The Ni₃S₂ phase exhibits all characteristic peaks according to JCPDS card No. 71-1682, indicating a polycrystalline structure of the porous layer.

In contrast, the whisker array—composed of single crystals—exhibits only a few distinct peaks, specifically at 31.27°, 37.97°, 49.99°, and 54.79°, corresponding to the (113), (004), (115), and (044) planes of a carrollite-like spinel structure, as described in JCPDS card No. 42-1450. A detailed examination of the regions near the peaks at 37.97° and 54.79° (Figure 1b,c) confirms that the spinel and Ni₃S₂ peaks can be clearly distinguished.

The blue curve in Figure 1a shows the XRD pattern of the powder sample obtained using the same synthesis method. The Ni₃S₂ phase is absent in the powder, and all the intense characteristic peaks correspond to the spinel-like phase. In this case, the calculated lattice parameter is 9.392 Å, compared to 9.474 Å for pure carrollite. The deviation of the lattice parameter a from that of pure carrollite may indicate the influence of a high zinc concentration within the CuCo₂S₄ powder crystals.

This elevated zinc content in the hydrothermally synthesized powders is further supported by elemental analysis data (Figure 2a). Since the EDA results for nickel, cobalt, copper, and zinc in the samples synthesized on the NF substrate were influenced by the substrate itself, elemental analysis for Co and Zn was conducted on a powder sample synthesized under the same conditions as the Cu0.7-Zn0.3 sample (Figure 2a). The data show that the atomic ratio of copper to zinc in the powder closely matches the molar ratio of Cu (0.7) to Zn (0.3) in the precursor solution. Additionally, the molar concentrations of cobalt and nickel are approximately in a 1:1 ratio. Therefore, the composition of the Cu0.7-Zn0.3 sample can be nominally expressed as Cu_0.7Zn_0.3CoNiS_yO_4−y. However, the actual composition of the deposited layer may slightly deviate from this nominal formula. Nevertheless, the presence of zinc and nickel impurities significantly influences the properties of the CuCo₂S₄ layer.

Additionally, microprobe EDA analysis was performed on the Ni foam in an area where the whisker layer had detached (Figure 3b). The results revealed that the atomic ratio of nickel to sulfur in this region is approximately 3.2:2.0 (see Figure 2b). Thus, the Ni₃S₂ layer—whose XRD reflections are shown in Figure 1—is formed through the sulfidation of the nickel foam and serves as a seed layer for the growth of the whisker layer. The EDA peak at 8.25 keV corresponds to the Kβ1 line of nickel (Figure 2b), and the EDA peak at 1.49 keV corresponds to aluminum and originates from the aluminum sample holder; it was excluded from the quantitative analysis.

Table 1. Elemental composition of samples (mass %).

Sample	Cu	Zn	Co	Ni	S	O
Cu0.3-Zn0.7	5.96	14.37	21.40	19.27	30.68	8.62
Cu0.5-Zn0.5	10.87	9.69	21.45	18.01	26.76	13.21
Cu0.7-Zn0.3	14.74	5.55	20.76	17.92	30.58	10.45

shows the elemental composition of the samples.

The morphology and dimensions of the structural elements in the Cu0.7-Zn0.3 sample were examined using scanning electron microscopy (SEM). As shown in Figure 3a, the nickel foam is uniformly coated with a layer of filamentous crystals approximately 3 µm in length and several tens of nanometers in diameter. Following galvanostatic charge–discharge (GCD) cycling (after 2000 cycles), SEM analysis revealed areas where the whisker layer had detached. In these regions, the surface appears porous with shallow depressions (Figure 3b), and the elemental composition corresponds to the Ni₃S₂ phase. This porous layer clearly formed as a result of nickel foam sulfidation during the initial stage of synthesis.

The Raman scattering data for both phases are presented in Figure 4. The Raman spectrum of the whisker layer (Figure 4a) exhibits five characteristic peaks at 150, 250, 304, 354, and 374 cm⁻¹, which can be attributed to the vibrational modes of the spinel structure: F2g(1), E2g, F2g(2), F2g(3), and A1g [32]. For samples with varying copper and zinc contents, the spectra revealed splitting of one of the latter four modes.

Figure 4b shows the Raman spectrum of a region where the whisker layer was absent due to exfoliation after electrochemical cycling. The spectrum closely matches the known Raman signature of Ni₃S₂, which is typically characterized by six combination modes [33,34]: 187.6 cm⁻¹ (A1(2)), 202.1 (E(4)), 223.6 (E(3)), 303.6 (E(2)), 324.6 (A1(1)), and 350.3 cm⁻¹ (E(1)).

In our case, four out of six phonon modes characteristic of the Ni₃S₂ phase were reliably observed. Previously, other researchers have reported Raman spectra of synthesized Ni₃S₂ containing only two bands [35]. Chen et al. [36] noted that various research groups have published highly inconsistent Raman data for Ni₃S₂—ranging from a complete absence of peaks to the presence of a single band, and in some cases, up to ten Raman bands. These discrepancies are attributed to the challenges in synthesizing high-purity, stoichiometric Ni₃S₂ with a well-defined crystal structure.

4.2. Electrochemical Evaluation of Manufactured Electrodes

Electrochemical tests were conducted in a three-electrode system comprising a working electrode, a reference electrode (Ag/AgCl), and a platinum counter electrode, with 3.5 M aqueous KOH solution as the electrolyte. The galvanostatic charge–discharge (GCD) curves for the three synthesized samples are presented in Figure 5a. The Cu0.7–Zn0.3 sample (mass loading 1 mg cm⁻²) exhibited the highest specific capacitance. At a current density of 2 mA cm⁻² (equivalent to 2 A g⁻¹) and within an operating potential window of 0.43 V, the Cu0.7–Zn0.3 sample delivered a capacity of 4.1 C cm⁻², corresponding to 9535 F g⁻¹ or 9.53 F cm⁻², with an internal resistance (IR drop) of 1.75 Ω. The other two samples exhibited a potential window of up to 0.405 V and were tested at a current density of 1 A g⁻¹. The Cu0.5–Zn0.5 sample, with a mass loading of 1.8 mg cm⁻², achieved a specific capacity of 1.47 C cm⁻², corresponding to 2012 F g⁻¹ or 3.62 F cm⁻². For the Cu0.3–Zn0.7 sample (mass loading of 2.5 mg cm⁻²), the values were 2.0 C cm⁻², 1980 F g⁻¹, or 4.95 F cm⁻². Since the Cu0.7–Zn0.3 sample exhibited significantly higher capacitance compared to the Cu0.5–Zn0.5 and Cu0.3–Zn0.7 samples, all subsequent tests were conducted using samples synthesized with a copper-to-zinc precursor molar ratio of 7:3.

Figure 5b presents the cyclic voltammetry (CV) curves of the Cu0.7–Zn0.3 sample recorded at various scan rates ranging from 5 to 100 mV s⁻¹ within a potential window of –0.4 to 0.8 V. The shape of the CV curves is characteristic of ternary metal sulfides [7,10,11,13,14,16,17,25,26,31] and is typical of battery-type materials. At a low scan rate of 5 mV s⁻¹, the operating potential range narrows to approximately 0.5 V (Figure 5b), with the discharge process occurring around 0.2 V. This corresponds well to the discharge voltage range observed in the GCD curves (Figure 5c,d). Figure 5c,d show the GCD curves of the electrode at different current densities from 2 to 100 A g⁻¹ in the potential range from 0 to 0.43 V. At a current density of 100 mA cm⁻² (100 A g⁻¹), the specific capacitance was 48% of the capacity at low currents, as shown in Figure 5e, while the IR drop remained at 0.78 Ohm, demonstrating the high conductivity and excellent rate characteristics of the electrode.

Electrochemical impedance spectroscopy (EIS) was performed on the electrode in the frequency range of 0.01 Hz to 100 kHz, with an AC amplitude of 5 mV. The resulting Nyquist plot is shown in Figure 5f. The data were fitted using the equivalent circuit shown in the inset, which includes two constant phase elements (CPEs), as described by the Equation:

$$CPE={\displaystyle \frac{1}{{T\left(j\omega \right)}^p}}.$$

(1)

In this equation, ω is the frequency (s⁻¹), T is the amplitude (Ω⁻¹cm²s⁻ⁿ), and the fitted parameters are p₁ = 0.67 and p₂ = 0.79, indicative of pseudocapacitive behavior. The solution resistance (Rₛ = R₁) was found to be 0.56 Ω cm², and the charge-transfer resistance (R_ct = R₂) was 0.31 Ω cm², suggesting efficient charge transfer at the electrode–electrolyte interface, which contributes to enhanced electrochemical performance.

A coin cell was assembled to study the electrochemical performance of an asymmetric supercapacitor, consisting of a capacitor-type negative electrode and a battery-type positive electrode. The contact area of the positive electrode was 1 cm², and filter paper was used as the separator. To prepare the negative electrode, a slurry containing 90 wt.% activated carbon, 5 wt.% polyvinylidene fluoride (PVDF), and 5 wt.% acetylene black was dispersed in N-methyl-2-pyrrolidone (NMP) as the solvent and applied onto nickel foam. The electrode was then dried under vacuum at 80 °C for 12 h and pressed at a pressure of 10 MPa. The mass of both electrodes was adjusted to ensure charge balance between the positive and negative electrodes at their respective optimal operating voltages. The electrochemical properties of the electrode materials and the performance of the asymmetric hybrid supercapacitor (ASC) were evaluated using galvanostatic charge–discharge (GCD) measurements. The specific capacitance of the device C_device, measured in C cm⁻² or F g⁻¹ units, was calculated using Equation (2):

$${C_{device}={\displaystyle \frac{I∆t}{S}}\left(\mathrm{C}{\mathrm{cm}}^{-2}\right);C}_{device}={\displaystyle \frac{I∆t}{m_{total}∆V}}\left(\mathrm{F}{\mathrm{g}}^{-1}\right),$$

(2)

where I is the discharge current (A), Δt is the discharge time (s), S is the contact area (cm²), m_total is the mass of active material of electrodes (g), and ΔV is the potential window (V). Energy density E_D (Wh kg⁻¹) and power density P_D (W kg⁻¹) values were calculated by the following formulas:

$$E_D={\displaystyle \frac{C_{device}∆V^2}{7.2}},$$

(3)

$$P_D={\displaystyle \frac{3600E_D}{∆t}}.$$

(4)

Figure 6a shows the CV curves of the positive and negative electrodes used to manufacture the ASC. The CV curves were measured at a scan rate of 20 mV s⁻¹. It can be seen that the operating voltage range for the positive electrode is no more than 0.6 V, and for the negative electrode, an increase in current is not observed up to −1.0 V. The CV curves of the ASC device (Figure 6b) in the range of 0–1.74 V reflect the combined contribution of Faradaic pseudocapacitance and electric double-layer capacitance (EDLC). According to the CV curves shown in Figure 6c, recorded at various voltage windows, the operating voltage can be extended up to 1.6 V at a scan rate of 10 mV s⁻¹. However, further increasing the voltage to 1.8 V leads to the appearance of leakage current.

Figure 6d displays the GCD (galvanostatic charge–discharge) curves of the ASC. The specific capacitance is calculated to be 115 F g⁻¹ (200 C g⁻¹) at a current density of 1 A g⁻¹. The maximum energy density achieved is 48.3 Wh kg⁻¹ at a power density of 870 W kg⁻¹. Figure 7 shows the capacitance retention and Coulombic efficiency over 20,000 cycles at a current of 70 mA cm⁻², equivalent to a current density of 10 A g⁻¹. The Coulombic efficiency remains nearly constant at just below 100%, while the capacitance retention gradually decreases from 100% to 74% after 20,000 cycles.

5. Discussion

The results presented in this study are consistent with literature data indicating that composites based on sulfides and oxides of transition metals such as nickel (Ni), cobalt (Co), copper (Cu), and others [37,38,39,40,41] are highly promising materials for energy storage applications. Their excellent electrochemical performance arises from a combination of unique properties that produce a synergistic effect when various metal sulfides are combined or integrated with other materials. This synergy is manifested through enhanced electrical conductivity of the sulfides, which facilitates efficient charge transfer; an increased active surface area that improves access to electrochemically active sites; rapid and reversible redox kinetics; and structural stability during cycling, attributed to the direct growth of nickel-based sulfides on the surface of the nickel foam (NF). Furthermore, the inclusion of multiple redox-active metals such as Ni and Co introduces a range of redox transitions, thereby increasing the overall capacitance and energy storage efficiency of the composite. Metal sulfides possess high theoretical specific capacities, which are rarely achieved in single-phase materials; however, these limitations can often be overcome through the design of composite structures. In particular, composites based on CuCo₂S₄ and related compounds, such as NiCo₂S₄, have shown promising results. For example, a hierarchical matrix of heterostructured NiCo₂S₄@polypyrrole (PPy) core–shell nanotubes on nickel foam (NF) demonstrated an exceptionally high specific capacitance of 9.78 F cm⁻² [10]. In that study, the binder-free NiCo₂S₄@PPy/NF electrode retained 80.64% of its initial capacitance after 2500 cycles at a current density of 50 mA cm⁻². A CuCo₂S₄/graphene composite [12] exhibited excellent cycling stability, retaining 97% of its initial capacity after 5000 cycles at 2 A g⁻¹, with a specific capacitance of 665 F g⁻¹. Similarly, in [13], a CuCo₂S₄/NF electrode achieved a specific capacitance of 3132.7 F g⁻¹. An ASC device based on this electrode delivered an energy density of 46.1 Wh kg⁻¹ at a power density of 991.6 W kg⁻¹ and retained 70.8% of its capacitance after 4000 cycles at 2 A g⁻¹. W. Chen and co-authors [42] synthesized a series of Ni_xCu_1−xCo₂S₄ compounds with varying nickel and copper contents and demonstrated that the composition Ni_0.67Cu_0.33Co₂S₄ exhibited the best electrochemical performance. The corresponding asymmetric supercapacitor (ASC) achieved an energy density of 40 Wh kg⁻¹ at a power density of 412.5 W kg⁻¹, with 71.7% of its initial capacity retained after 10,000 cycles at a current density of 4 A g⁻¹. Cobalt–zinc sulfides also show strong potential for supercapacitor applications. C. Cheng et al. [24] developed a battery-type electrode based on ZnCo₂S₄ hollow core–shell nanospheres and fabricated a hybrid device using a carbon-based negative electrode. The resulting ASC demonstrated an energy density of 51.7 Wh kg⁻¹, a power density of 1700 W kg⁻¹, and outstanding cycling stability, retaining more than 98.7% of its capacity after 2000 cycles at 6 A g⁻¹.

The improved electrochemical performance of the asymmetric supercapacitors in this study was achieved through the development of a composite based on a zinc-doped CuCo₂S₄ filamentous structure, grown on a Ni₃S₂ buffer layer. Zinc alloying enhances the electronic conductivity and structural stability of the spinel matrix, while also introducing additional redox-active sites. These effects contribute to higher specific capacity and improved rate performance of the electrode material. The filamentous fibers exhibit a high specific surface area, which facilitates greater ion accessibility and enhances charge storage capacity. Additionally, the fibers demonstrate excellent electrochemical activity, enabling efficient charge transfer and improved cyclic stability. The filaments are grown on a nickel (Ni) buffer layer, which serves as a robust foundation for the active material. The Ni₃S₂ layer, formed through the sulfidation of a foamed nickel substrate, provides exceptional electrical conductivity, ensuring efficient electron transport. Furthermore, this sulfidation process enhances the mechanical adhesion of the active material to the NF substrate, contributing to the structural integrity of the electrode. The synergistic combination of these properties results in superior ASC performance compared to conventional electrode materials. This approach underscores the potential of zinc-doped CuCo filamentous structures for advancing high-performance energy storage devices.

6. Conclusions

In summary, binder-free composite electrodes based on Ni, Co, Cu, and Zn sulfides were successfully synthesized on nickel foam substrates using a simple one-step hydrothermal method. The structural foundation of the resulting Cu_xZn_1−xCoNiS_yO_4−y composite consists of two layers. A thin Ni₃S₂ layer, formed by sulfidation of the nickel foam surface, provides excellent electrical contact and a robust mechanical structure capable of withstanding repeated charge–discharge cycles. This porous Ni₃S₂ layer also acts as a transition layer for the growth of a whisker-like spinel structure of the zinc-doped CuCo₂S₄ type on its surface. The optimal level of zinc doping was found to occur when the molar ratio of copper to zinc precursors in the growth solution is approximately 7:3. The electrode, owing to the combination of a whisker array and a porous Ni₃S₂ sublayer in its structure, delivers a high specific capacitance of 4.10 C cm⁻² or 9535 F g⁻¹ at a current density of 2 mA cm⁻² or 1 A g⁻¹, respectively, in a 3.5 M KOH aqueous electrolyte. The composite electrode material, in addition to its high capacity, demonstrates excellent cycling stability, retaining 74% of its initial capacity after 20,000 charge–discharge cycles at a current density of 10 A g⁻¹. This performance underscores the superior energy storage capabilities of the newly developed electrode material.