2.1. Synthesis of NiS/CoS/NF

NiS/CoS were deposited on NF substrates using CBD technique as outlined in Figure 1. The substrates were cleaned using an ultrasonic cleaner in the presence of 1 M hydrochloric acid (HCl), methanol, acetone, and deionized (DI) water. CBD process was carried out in a growth beaker, which consists of 50 mL of DI water, 0.10 M urea as the complexing agent, 0.05 M of cobalt (II) nitrate, and 0.05 M of thioacetamide as the cobalt and sulfur source, respectively.

The mixture was stirred for 10 min leading to formation of a homogeneous solution. The precleaned NF were clipped vertically and submerged into the growth solution which was then placed into a stirring water bath setup. The deposition was carried out at 90°C for 1, 2, 2.5, and 3 h, in which the samples were named accordingly as NiS/CoS-1, NiS/CoS-2, NiS/CoS-2.5, and NiS/CoS-3. Upon complete deposition, the samples were rinsed in DI water and subsequent oven drying for 6 h prior to further characterization.

2.2. Material Characterization

The crystalline properties of the bimetallic sulfides were studied using an X-ray diffractometer (Bruker AXS D8 ADVANCE) at a scanning rate of 2° per minute. The Merlin, Gemini II field emission scanning electron microscope (FESEM) coupled with Oxford Instruments energy dispersive X-ray spectrometry (EDX) was used to analyze the morphological characteristics and composition of elements. The electronic composition of the samples were analyzed using X-ray photoelectron spectroscopy (XPS) with a Shimadzu system and a kα X-ray source.

2.3. Electrochemical Characterization

The measurements of the supercapattery cell were conducted using a two-electrode setup, with NiS/CoS/NF as the cathode and activated carbon (AC) smeared on NF serving as the anode. The negative electrode was fabricated using a slurry comprising AC, acetylene black, and PVdF with the ratio of 8:1:1 in the presence of N-methyl 2-pyrrolidone (NMP). The prepared slurry is applied to NF with a dimension of 1 cm × 2 cm and heated at 100°C for 3 h before it is ready for use.

3.1. Structural Properties of NiS/CoS/NF

The effect of CBD time on crystallinity of the active material was analyzed based on the X-ray diffractograms shown in Figure 2.

The samples exhibited significant XRD peaks at 44.52°, 51.90°, and 76.32°, which correspond to the NF (JCPDS No. 04-0850). Formation of NiS/CoS was evident due to the presence of new diffraction peaks at 45.33°, 47.76°, 52.82°, and 77.34°. These peaks coincide with the standard peaks of hexagonal NiS (JCPDS No. 01-077-1624) [10] and hexagonal CoS (JCPDS No. 65-3418) [11]. Upon hydrolysis, thioacetamide releases hydrogen sulfide, which reacts with the cobalt ion (Co²⁺) originating from the salt precursor and forms cobalt sulfide [12]. Concurrently, nickel ions (Ni²⁺) released from the NF substrate also react with the sulfur source to produce nickel sulfide. As predicted, the crystallinity of the bimetallic sulfide improved at higher deposition time. At the beginning of the deposition process, rapid nucleation takes place, leading to the formation of small and isolated structures with poor crystallinity. As the reaction progresses, these individual nuclei merge to form well-defined nanostructures with more well-ordered lattice structures, thus, enhancing the crystallinity.

Figure 3 depicts the XPS analysis of NiS/CoS-2.5, which reveals the chemical valence states in the active material. The survey scan spectrum confirms the presence of Ni, Co, S, O, and C (Figure 3a). The high resolution XPS spectrums of Ni 2p, Co 2p, and S 2p were deconvoluted using the Gaussian fitting technique. The Co 2p high resolution XPS spectrum resolves into two spin–orbit doublets and two shakeup satellites corresponding to Co²⁺ and Co³⁺ states (Figure 3b). The Co 2p^3/2 spectrum in the lower energy region deconvolutes into peaks at 781.4 and 784.9 eV which corresponds to Co³⁺ and Co²⁺, respectively [13]. The Co 2p^1/2 spectrum shows a doublet ascribed to Co³⁺ and Co²⁺ at 796.6 and 799.1 eV. Similarly, deconvolution of the Ni 2p spectrum reveals two pairs of peaks assigned to Ni³⁺ and Ni²⁺ species. As shown in Figure 3c, the Ni 2p^3/2 and Ni 2p^1/2 signals at Ni³⁺ were detected at 856.5 and 874.2 eV, respectively. Meanwhile, the Ni 2p^3/2 and Ni 2p^3/2 signals of Ni²⁺ are located at 860.1 and 878.5 eV [14]. Figure 3d exhibits the high resolution XPS spectrum of S 2p, which resolved into two peaks at 163.7 and 168.6 eV attributed to S 2p^1/2 and 2p^3/2. Previous literature indicates that the peak at 163.7 eV is associated with the metal–sulfur bonds [15]. The peak at 172.3 eV, which is typically attributed to oxidized sulfur species, was also present in the spectrum [16].

3.2. Morphological Properties of NiS/CoS/NF

The evolution of the morphology of NiS/CoS nanostructures with increasing deposition time is obvious from the observation of FESEM micrographs of the samples. In NiS/CoS-1, the surface of NF still remains smooth with minimal coverage of the active material throughout the substrate (Figure 4a). With 2 h of deposition, NiS/CoS-2 showed a higher loading of active material on the surface of NF. Higher magnification images verified the formation of nanoclusters with more uniform and well-defined shapes (Figure 4b). At 2.5 h of deposition time, the sample showed the most interesting morphology. The nanostructures coalesced to form a spherical configuration with the spherical threaded surface, resembling a pollen grain-like structure (Figure 4c). The improvement in the morphology can be attributed to a higher extent of Ostwald ripening and surface energy minimization, which are facilitated by a longer deposition time. Finally, when the CBD process was extended up to 3 h, NiS/CoS-3 was arranged into more regularly-sized spheres (Figure 4d). With prolonged deposition time, the hollow surface structure found in NiS/CoS-2.5 was completely absent. Figure 4e depicts the EDX mapping of the FESEM micrographs to depict the uniform distribution of nickel, cobalt, and sulfur throughout the NF surface.

To assess the performance of NiS/CoS as an electrode component in supercapattery, electrochemical tests such as CV, GCD, EIS, and cycle stability were carried out. Figure 5a shows the CV plots of all fabricated electrodes. The CV measurement was studied in a three-electrode setup, where the Hg/HgO served as the reference electrode, platinum sheets as the counter electrode, and the prepared sample as the working electrode. The measurement was carried out within a potential window of −0.3 to 0.8 V in 1.0 M KOH electrolyte at scan rates between 5 and 30 mV s⁻¹. From the CV curves, a pair of redox peaks can be seen, demonstrating pseudocapacitive properties.

3.3. Electrochemical Performance of NiS/CoS/NF

The NiS/CoS-2.5 sample underwent a series of CV measurements at scan rates between 5 and 30 mV s⁻¹. The tests were carried out to gather more comprehensive data on the electrochemical performance of the electrodes. The CV curves are displayed in Figure 5b.

As shown in Figure 5e, the diffusion contribution of NiS/CoS-1, NiS/CoS-2.5, NiS/CoS-2.5, and NiS/CoS-3 were 39.8%, 59.4%, 60.7%, and 60.6%. The higher diffusion contribution in samples with deposition time longer than 2 h is attributed to the higher loading of active materials with abundant redox-active sites which promote faradaic reactions. At low scan rates, ions have sufficient time to diffuse into the deeper layers of the electrode, leading to a higher contribution from diffusion-controlled processes. Thus, for all the samples, as scan rate increased the diffusion contribution decreased.

The energy storage capacity of the samples was assessed through GCD analysis. Figure 6a displays the GCD curves recorded at a current density of 2 mA cm⁻² across the potential range of 0–0.47 V. The peaks observed in the CV profile correspond to the nonlinear charge and discharge curve, confirming the pseudo capacitance contribution from the electrode. The specific capacity is determined using Equation (1) and displayed in Figure 6a insets. The C_s emphasizes the impact of varying deposition times during the CBD process for sample fabrication on the charge storage performance of the samples. The highest C_s is observed when the sample is heated for 2.5 h. This is likely because the uniform NiS/CoS growth provides ample active sites for more effective OH⁻ ion absorption. The increase in C_s can also be attributed to the hollow shape of the active material, providing more efficient pathways for ion diffusion. Extending the heating duration to 3 h leads to a decrease in C_s, associated to the increase in particle size as observed in the FESEM analysis. The increase in particle size leads to NiS/CoS agglomeration, which reduces the availability of sites for OH⁻ ion absorption and desorption. Hence, the C_s for sample NiS/CoS-3 is 0.80 C cm⁻², which is lesser than the C_s value of 2.60 C cm⁻² found for sample NiS/CoS-2.5 measured at 2.0 mA cm⁻² in 1.0 M KOH. The high specific capacity observed in sample NiS/CoS-2.5 is attributed to the rapid ion exchange facilitated by the well-organized NiS/CoS nanostructures on the substrate [21]. Comparison of GCD curves across different current densities for NiS/CoS-2.5 at current densities of 2 and 20 mA cm⁻² are 2.60 and 0.74 C cm⁻², respectively (Figure 6b). With the current density interval from 2 to 5 mA cm⁻², the C_s retention is 79%. A high-capacity retention suggests strong charge storage capability, potentially due to rapid charge transfer associated with increased current densities [22]. The specific capacity value decreased to 1.82 C cm⁻² as the current density increased to 6 mA cm⁻². The value was further reduced to 1.21, 0.83, and 0.74 C cm⁻² at current densities of 8, 10, and 20 mA cm⁻². This shows that the sample could no longer sustain being charged at higher current densities than 5 mA cm⁻². In this study, NF serves as both the substrate and a reactive source of nickel for the in situ formation of NiS/CoS. This process consumes a significant portion of the nickel at the surface of the substrate, potentially limiting the formation of Ni(OH)₂ through corrosion in the alkaline KOH electrolyte. This observation is supported by the CV curves and GCD scan of pristine NF (Figure S2), which shows significantly lower capacitance compared to the NiS/CoS electrode, confirming the dominant role of the active material.

The EIS study of the electrodes is measured in a three-electrodes configuration as well with 1.0 M KOH electrolyte. The Nyquist plots of all samples are shown in Figure 6c. Similar electrochemical impedance characteristics were observed across all plots. The equivalent circuit corresponding to the Nyquist plot is included in Figure 6d. The EIS entails a high-frequency section forming a semicircle and a low-frequency region displaying an oblique line. Each region corresponds to its specific frequency range. The y-intercept of the semicircle on the x-axis in the high-frequency region is known as the equivalent series resistance (R_s). This value represents the total ionic resistance of the electrolyte, contact resistance, and the internal resistance of the material [23]. The R_S values of sample NiS/CoS-1, NiS/CoS-2, NiS/CoS-2.5, and NiS/CoS-3 are 1.42, 1.71, 1.40, and 1.67 Ω. The charge transfer resistance (R_ct) is calculated based on the redox reaction resistance, as shown by the high-frequency semicircle [24]. In the low-frequency range, an oblique line with a slope exceeding 45° can be seen, linked to diffusion and kinetic processes, referred to as Warburg impedance (W). The R_ct values of sample NiS/CoS-1, NiS/CoS-2, NiS/CoS-2.5, and NiS/CoS-3 are 1.41, 0.54, 0.44, and 0.73 Ω. Compared to all the samples, NiS/CoS-2.5 electrode displayed the smallest R_ct value. This low resistance supports more efficient electron diffusion and migration, contributing to its superior charge transfer mechanism. The homogeneous growth of NiS/CoS onto the NF greatly enhances ionic access to the active material surface. This observation is consistent with the FESEM findings. Table 1 summarizes the comparative charge storage performances from other contemporary works on TMSs. Many prior works on composite sulfides achieving high capacitance utilize the hydrothermal method, which often involves high temperature, long reaction times, and complex setups [33, 34]. In contrast, this work employs a CBD method—a simple, scalable, and cost-effective approach. This makes it highly suitable for industrial applications and overcomes some limitations of hydrothermal synthesis. In addition, unlike the common approach of synthesizing NiS/CoS separately and then coating them onto a substrate, this work employs an in situ growth process using NF as both the substrate and a source of nickel ions. This results in a tightly adhered active material layer on the NF surface, improving electron transport and mechanical stability during electrochemical cycling. From the table, sample NiS/CoS-2.5 has prospectively high specific capacity compared to other materials.

An asymmetric cell with the configuration NiS/CoS-2.5//PVA + KOH//AC was constructed to demonstrate the practical application of NiS/CoS as the positive electrode and to evaluate the cycling stability. Figure 7a shows the CV profile of AC coated NF which exhibits double-layer capacitive behavior, as evident from its near-rectangular curve shape, which reflects its ability to store charge through the formation of an electrochemical double layer at the electrode–electrolyte interface. No significant redox peaks were observed, indicating pure EDLC behavior with negligible pseudocapacitive contribution. The CV curve emphasizes that AC operates stably within the voltage range of 0 to −1.0 V in 1 M KOH. GCD measurements were conducted across a range of current densities between −0.65 and 0 V. As illustrated in Figure 7b, the GCD curves exhibit similar pattern, characterized by a typical nearly symmetrical triangular shape which is characteristic for EDLC material. The Nyquist plot of the AC electrode shows a R_S and R_CT of 7.44 and 3.16 Ω, respectively (Figure 7c). The stability of AC in the negative potential range complements the pseudocapacitive behavior of NiS/CoS, enabling a wide operating voltage range and enhancing the overall energy storage capacity of the asymmetric supercapattery.

The CV curves of the cell at different scan rates are shown in Figure 7d. The CV curves show considerable asymmetry and consistent shape at all scan rates. It is evident that the broadening of the device’s redox peaks indicates a combination of EDLC and pseudocapacitance processes [37]. The redox peak shifts from lower to higher scan rates, indicating charge polarization within the device [38]. This confirms the superiority of the battery-type charge process, which leads to a decrease in charge as the scan rate increases. This suggests that ion diffusion is restricted to the outer active surface area of the electrodes [39]. Figure 7e presents the charge–discharge curves of NiS/CoS-2.5//PVA + KOH//AC at different current densities. The symmetric and nonlinear characteristics of the GCD profiles reflect the hybrid and reversible charge storage mechanism of the supercapattery device. The ASCs exhibited specific capacities of 0.207, 0.195, 0.156, 0.139, and 0.101 C cm⁻² at current densities of 1, 2, 4, 6, and 8 mA cm⁻², respectively. The percentage of retention rate of specific capacities is about 49.8% when the current density increases from 1 to 8 mA cm⁻². The Nyquist plot of the ASC shows a R_S and R_CT of 9.78 and 0.75 Ω, respectively (Figure 7f). The overall charge storage performance of the asymmetric cell is shown in the Ragone plot (Figure 7g). At a specific capacity of 0.207 C cm⁻² and an operating voltage of 1.0 V, the device exhibited a maximum energy density of 103.5 mWh cm⁻² with a power density of 0.95 kW cm⁻². The maximum power density was obtained at 5.86 kW cm⁻² with an energy density of 50.5 mWh cm⁻².

This cell was tested continuously for 2000 cycles at a current density of 4 mA cm⁻² during charging and discharging. The NiS/CoS-2.5 electrode used in the cell had a total mass load of approximately 0.01120 g. The cell maintains excellent stability during continuous cycling. Figure 8a displayed the trend in specific capacity throughout the charge–discharge cycles. The specific capacity for the first 200 cycles was increased to 111% of the beginning value. Such a phenomenon is associated with the active material’s activation process, which led to inadequate utilization of NiS/CoS throughout the initial cycles [40]. Over the subsequent 200 cycles, the specific capacity decreased to 103%. Following 2000 cycles, the cell maintains approximately 90% of its original capacitance and can store charge. The minor attenuation in capacitance retention at high current densities indicates that the NiS/CoS-2.5 electrode maintains excellent electrochemical performance. This can also be confirmed by the GCD graph in Figure 8b having only slight changes between the 1^st and 2000^th cycle with specific capacity value of 0.19 and 0.17 C cm⁻², respectively. The energy density and power density of the fabricated device is 115.0 mW h cm⁻² and 2.62 kW cm⁻², respectively before stability test and 102.8 mW h cm⁻² and 2.74 kW cm⁻² after 2000 cycles. The low-capacity value is mostly attributed to the usage of PVA in the electrolyte which is used to prevent leakage and enhance mechanical stability. However, this can reduce ionic conductivity compared to liquid electrolytes due to the restricted mobility of ions within the polymer matrix. In future studies, the performance of the device can be improved by replacing the negative electrode with RGO or graphite instead of AC. These materials can provide wider potential window, therefore, increase the specific capacity of the device. Figure 8c presents the EIS measurements taken before and after the cycling tests. After 2000 cycles, the Nyquist plot was shown to shift near higher resistance and the diameter of the semicircle extended. The R_ct values were recorded as 0.33 Ω before cycling and 1.60 Ω after cycling. The higher resistance observed is probably due to the repeated charge–discharge processes. This low resistance is likely due to the strong synergistic interaction between NF and NiS/CoS [41]. Additionally, the porous nature of NF is advantageous for smooth electrolyte diffusion into and out of the electrode material, creating greater channels for swift ionic transitions [42].

To better grasp the deviations in chemical composition states and morphology of NiS/CoS after the stability test, the samples were characterized to obtain XPS spectrum and FESEM micrographs. As seen in Figure 9a–c, upon continuous charge–discharge cycles, NiS/CoS-2.5 showed a similar spherical shape as before. However, the thread-like surface texture has completely diminished, forming a dense and opaque nanostructure. This transformation in morphology is identified as the primary rationale behind the decline in performance. The comparison of EDX spectrum before and after the stability test demonstrates a significant increase in the percentage of oxygen present in the sample in Figure 9d,e. After stability studies, the deconvoluted XPS spectrum of the sample showed similar electronic configurations in the valence states of nickel and copper, as shown in Figure 9f. However, in agreement with the EDX spectrum, the peak attributed to oxidized sulfur species displayed a substantial increase in area. This can originate from the electrochemical reaction upon consecutive charge–discharge cycles and surface oxidation following interaction between the electrode surface and the alkaline electrolyte.