Facile synthesis of self-supported amorphous cobalt-nickel tungstate thin films for high performance asymmetric supercapacitor: role of Co and Ni variation

1 Introduction

With the rapidly growing industrialization and urbanization, the energy demand around the globe has notably increased. To counter the sudden surge in the energy demand, not only conventional sources of energy like coal, hydroelectric, thermoelectric and nuclear energy, but also sources of renewable energy, including solar energy, wind energy and tidal energy, etc., are being used widely (Wang et al., 2024; Algarni et al., 2023; Wang et al., 2019). However, their discontinuity in energy-delivering capacity has led scientists to find an alternative for energy storage systems with high power and energy density. Among the available Systems for storing energy, batteries and supercapacitors have recently gained significant research attention owing to their high cycle life, long-term stability and moderate energy and power density (Hao et al., 2024; Mandal et al., 2025). Considering their superior characteristics, such as high power density, quick charging and discharging, and excellent stability, supercapacitors have been considered a promising candidate for future energy storage devices. Supercapacitors store energy primarily through two mechanisms: electrical double-layer capacitance (EDLC) and pseudocapacitance. Their very low energy density remains a significant drawback, prompting extensive research efforts aimed at enhancing their energy storage capabilities through advanced material design and device engineering (Shuja et al., 2024; Malavekar et al., 2024a; Hegazy et al., 2024).

Numerous materials have been explored to achieve higher specific capacitance. Various oxide, sulfide, hydroxide, and nitride-based materials are promising candidates for supercapacitor applications (Jayakumar et al., 2024; Zhu et al., 2025; Malavekar et al., 2024b; Adalati et al., 2022). Among them, tungstate-based materials are attracting significant attention due to the multiple oxidation states of tungsten and high conductivity. Recently, binary metal oxides such as NiCo₂O₄ (Kannangara et al., 2024), FeWO₄ (Gnana Sundara Raj et al., 2023), NiMoO₄ (Luo et al., 2022), CoWO₄ (Bagwade et al., 2022), ZnWO₄ (Guan et al., 2014) and NiWO₄ (Araújo et al., 2024) have been synthesized and displayed outstanding pseudocapacitive performance. However, limited surface area, compact morphology and non-porous nature hinder their use in practical applications. To overcome these limitations, various experimental strategies were employed. For instance, Gopi et al. (V Muralee Gopi et al., 2024) synthesised a NiCo₂O₄/CuS composite electrode that enhanced the specific capacitance of 141.13 mAh g⁻¹ at 1 A g⁻¹ due to the increased surface area from 52.16 m² g⁻¹ for bare NiCo₂O₄ to 91.38 m² g⁻¹ for NiCo₂O₄/CuS. Joe et al. (Babu et al., 2019) prepared β-Co(OH)₂/CMC nanoflakes with a mesoporous structure, resulting in an enhanced supercapacitive performance of 306 C g⁻¹ at a scan rate of 2 mV s⁻¹. Other strategies, including doping, hybridization, and morphology tuning, proved to be beneficial for obtaining enhanced electrochemical performance (Jenitha et al., 2025; Malavekar et al., 2023). In this context, the variation in cationic ratio also has a significant impact, modulating the structural and electronic properties of the resultant material. In many cases, the variation in ratio affected both morphology and electrical conductivity (Marje et al., 2022; Dong et al., 2025). Therefore, optimization of the cationic ratio could enable modified physicochemical properties, thereby increasing the overall electrochemical performance.

Electrochemical performance is primarily determined by the synthesis method and the selection of the prepared material. Although many physical deposition methods are available, chemical deposition methods are advantageous due to their low power requirements, cost-effectiveness, freedom of choice in substrate selection, and excellent control over all parameters (V Magdum et al., 2024). The successive ionic layer adsorption and reaction (SILAR) method is widely used for the deposition of many inorganic materials due to its operational flexibility and control over the thickness of the thin film (Malavekar et al., 2021; V Magdum et al., 2025). Also, the growth rate can be controlled using the SILAR method. Thin films are preferred over bulk to ensure a binder-free approach, which provides the additional benefit of avoiding the resistance of the binder. Additionally, direct growth can help maintain the morphology intact during the electrochemical processes (Liu et al., 2014).

Although NiWO₄, CoWO₄, and various mixed-metal tungstates have been extensively explored as electrode materials for supercapacitor applications, systematic investigations into the influence of Ni/Co compositional variation in directly grown Ni–Co tungstate thin films remain scarce (Bagwade et al., 2022; Bagwade et al., 2023; Patil et al., 2022). In particular, the effect of cationic composition on the structural, morphological, and electrochemical properties of amorphous Ni–Co tungstates have not been comprehensively elucidated. In this study, composition engineering is employed by precisely tuning the Ni/Co ratio to optimize the electrochemical performance of Ni_xCo_1-xWO₄ thin film electrodes. Unlike conventional crystalline powder electrodes that require polymeric binders and conductive additives (V Sadavar et al., 2022), the present work utilizes amorphous, binder free Ni_xCo_1-xWO₄ thin films directly deposited on stainless steel (SS) substrates via the successive ionic layer adsorption and reaction (SILAR) technique. The amorphous structure provides a high density of unsaturated electrochemically active sites, abundant structural defects, and shortened ion diffusion pathways, thereby facilitating rapid redox kinetics. Furthermore, the direct growth of the active material on the conductive substrate ensures efficient electron transport, strong interfacial adhesion, and reduced charge-transfer resistance (Bachankar et al., 2024; Wang et al., 2021; Zhao et al., 2022). The synergistic integration of compositional optimization, amorphous phase engineering, and direct binder-free SILAR deposition enhances the charge-storage performance while providing fundamental insights into the composition structure electrochemical property relationship of Ni–Co tungstate thin film electrodes.

Herein, we report a binder-free approach for depositing Ni_xCo_1-xWO₄ (NCW) thin films on SS substrates using the cost-effective SILAR method with varying Co: Ni ratios. The effect of Co:Ni ratio variation on the physicochemical and electrochemical properties is examined by various characterization techniques. Eventually, the practical application of the best-performing thin film was evaluated by fabricating an asymmetric supercapacitor device using NCW and reduced graphene oxide (rGO) electrodes and their electrochemical performance was assessed.

2 Experimental section

2.1 Chemicals

All the chemicals used in the experiments were of analytical reagent (AR) grade, ensuring their high purity. No additional purification procedures were performed for all chemicals used in the study. Cobalt chloride hexahydrate (CoCl₂‧6H₂O, purity ≥98%), nickel chloride hexahydrate (NiCl₂‧6H₂O, purity ≥98%), potassium hydroxide (KOH, purity ≥85%), sodium nitrate (NaNO₃, purity ≥99%), sodium tungstate (Na₂WO₄‧2H₂O, purity ≥99%), Graphite flakes (99% carbon basis, 325 mesh particle size ≥99%), hydrochloric acid (HCl, 37% assay), sulfuric acid (H₂SO₄, 97% assay), hydrogen peroxide solution (35% by wt. In H₂O), hydrazine hydrate solution (80% by wt. In H₂O), and potassium permanganate (KMnO₄, purity ≥99%) were acquired from Sigma Aldrich.

2.2 Synthesis of NCW thin films

The binder-free NCW electrocatalysts were deposited on the SS substrate by a simple SILAR method. The deposition was carried out by dipping the SS substrate separately into cationic and anionic precursor solutions, as shown in Figure 1. Before deposition, the SS substrates (304-grade) were polished using fine sandpaper and then washed with acetone, followed by ultrasonic cleaning in double-distilled water for 20 min. For deposition of NCW series materials, in the first beaker, different concentrations of cobalt chloride (0–0.1 M) and nickel chloride (0–0.1 M) were made as cationic precursors. To accomplish synthesis, 50 mL of DDW was used to fully dissolve stoichiometric quantities of cobalt chloride (CoCl₂‧6H₂O) and nickel chloride hexahydrate (NiCl₂‧6H₂O). These solutions acted as the sources of Ni²⁺ and Co²⁺ cations, respectively, which are essential for the subsequent deposition process. Rinsing was performed using the DDW contained in the second and fourth beakers to ensure the removal of loosely bound ions. The anionic source in the third beaker was 50 mL of 0.1 M sodium tungstate dihydrate (Na₂WO₄‧2H₂O) aqueous solution, which supplied the required WO₄ ²⁻ ions as a precursor essential for the development of the mixed metal tungstate phase. The careful preparation of this solution ensured the uniform availability of tungstate ions for reaction with the metal cations. The precise ratio variations of the Ni, Co with WO₄ ²⁻ precursors for the deposition of Ni_xCo_1-xWO₄ material are listed in Supplementary Table S1.

Because of the attraction force between the substrate and the Ni and Co ions, varying amounts of these ions were adsorbed onto the SS substrates during the synthesis process. The substrate was then immersed in DDW to remove any remaining traces of contaminants and loosely bound cations. The substrate was then dipped in the tungstate precursor, where the final product was formed after reaction between the adsorbed cations and WO₄ ^2- anions. In the fourth beaker, DDW was used to remove unreacted species, thereby maintaining the purity of the (Ni_xCo_1-xWO₄ material. For the deposition process. Metal ions adsorption, reaction and rinsing time were optimized to be 10, 10, and 20 s, respectively. A uniform, adherent film was deposited after 100 cycles, and a variation in film colour was observed, ranging from light apple green to purple. The varying mole ratios of Ni and Co are represented as NCW1 (100:0), NCW2 (75:25), NCW3 (50:50), NCW4 (25:75), and NCW5 (0:100) (given in Supplementary Table S1). The mass loading of all NCW series films deposited for 100 SILAR cycles is presented in Supplementary Figure S1. A continuous increase in mass loading was observed with increasing Co content, rising from 0.271 mg cm⁻² for the NCW1 film to 0.610 mg cm⁻² for the NCW5 film.

The immersion times used in the SILAR process, namely, 10 s in the cationic precursor solution, 10 s in the anionic precursor solution, and 20 s for rinsing with deionized water, were selected based on preliminary optimization studies. Shorter immersion times resulted in incomplete adsorption of precursor ions, leading to insufficient nucleation and poor surface coverage. In contrast, longer immersion times promoted excessive ion adsorption, resulting in the formation of loosely adhered deposits that compromised the film uniformity and adhesion to the SS substrate. Similarly, the number of SILAR deposition cycles was systematically optimized, and 100 cycles were found to provide the optimum balance between active material loading and electrochemical performance. Films deposited with fewer cycles contained an insufficient amount of electroactive material, whereas those prepared with a larger number of cycles produced relatively thicker films, which impeded electrolyte penetration, increased ion diffusion resistance, and consequently deteriorated the electrochemical performance.

2.3 Synthesis of GO and rGO

The details of the synthesis and characterizations of GO are given in Supplementary Material S1; Supplementary Figure S2, respectively, in the ESI file. The details of the synthesis and characterizations of reduced graphene oxide (rGO) are given in Supplementary Material S2; Supplementary Figure S3, respectively, in the ESI file.

2.4 Material characterizations

Details pertaining to the characterization are documented in Supplementary Material S3 of the ESI file.

2.5 Electrochemical study

Supplementary Material S4 in the ESI file includes the specifics of the electrochemical study.

3 Results and discussion

3.1 Physicochemical studies

The X-ray diffraction (XRD) technique was employed to analyze the crystal structure of the NCW series samples. As shown in Figure 2a, the XRD patterns indicate that the films are amorphous. The NCW series thin films exhibit no distinct diffraction peaks, confirming their amorphous structure. The peaks marked with ♦ correspond to the SS substrate. The reduced intensity of the substrate peaks after NCW deposition suggests effective coverage of the substrate surface by the deposited films. Furthermore, no additional peaks were observed, confirming the successful and uniform deposition of the NCW series on the substrate. Raman spectroscopy (Figure 2b) was used to assess the chemical structure and molecular bonding characteristics of NCW films. The distinct crystal structures and chemical interactions within the NCW series result in slight shifts in their Raman spectral peaks. Typically, the Raman spectra of NiWO₄ and CoWO₄ show peaks related to the symmetric and antisymmetric stretching of the W-O bond within the WO₆ octahedra. The Raman spectra exhibited a strong peak at 945 cm⁻¹ for NiWO₄. The red shift of the Raman band from 945 to 882 cm⁻¹ after Co incorporation indicates a modification of the local WO₆ octahedral environment. Such a shift may originate from changes in the local W-O bonding environment, octahedral distortion, and increased structural disorder resulting from the substitution of Ni by Co ions (Malavekar et al., 2022; Bagwade et al., 2023). In general, the bond lengths for Ni-O and Co-O are nearly identical; however, for W-O, they differ, resulting in asymmetric bonding and structural defects. WO₆ octahedra are hence responsible for the main peak. Furthermore, the vibrational modes of Ni-O and Co-O bonds are attributed to the mild Raman peaks at 286, 345, 411, 531, and 689 cm⁻¹. Consequently, the synthesis of NiWO₄, CoWO₄, and Ni_xCo_1-xWO₄ is significantly confirmed by Raman spectra.

Transmission electron microscopy (TEM) images (Figures 2c,d) confirm the formation of interconnected spherical particles. The high-resolution TEM (HR-TEM) image shows no line pattern, indicating that the material is amorphous. This observation is further supported by the selected area electron diffraction (SAED) pattern (Figure 2e). The SAED pattern exhibits a broad bright spot at the centre surrounded by the diffuse rings, indicating the absence of long range crystallinity and confirming the predominantly amorphous nature of the NCW films. The diffuse scattering observed in the SAED pattern is consistent with the XRD features, further supporting the formation of an amorphous structure. Energy-dispersive X-ray spectroscopy (EDS) analysis was conducted to examine the elemental distribution, confirming the presence of Ni, Co, W, and O (Figures 2f–j). The EDS spectra for NCW4 are presented in Supplementary Figure S4, confirming the formation of material with the optimal atomic composition.

The surface morphologies of the NCW series were studied using FE-SEM and EDS. Figure 3 presents different magnifications (20kx and 5kx) for a compressive analysis. The morphological evolution was observed from sample NCW1 to NCW5 due to variations in Ni and Co composition. The SEM images of pristine NiWO₄ (NCW1) exhibited in Figures 3a1,a2 reveal that the sample consists of an agglomerated nano-pebbles-like structure having an average particle size of 340 nm. Moreover, with the increase in the concentration of cobalt in the NCW2 sample (Figures 3b1,b2), the spherical pebble-like morphology was slightly distorted with an average particle size of 160 nm. Additionally, the sample (NCW3) with the same Ni:Co (50:50) concentration, as shown in Figures 3c1,c2, exhibits an interconnected, overlapped nano-pebble-like structure with an average particle size of 105 nm and a highly porous structure. Furthermore, increasing the concentration of Co, followed by Ni (NCW4), results in a nanopebble-like morphology with an average particle size of 160 nm, as shown in Figures 3d1,d2. The pristine CoWO₄ sample (NCW5), displayed in Figures 3e1,e2, reveals a compact particle with a large average particle size of 320 nm than other samples. At lower magnification, images from Figures 3a2–e2, seem to have a compact, uniform, well covered over the substrate. Thus, it is concluded that the developed structure exhibits high porosity, enabling effective electrolyte penetration into the electrode matrix.

Additionally, the EDS technique was used to evaluate the elemental compositions of the NCW series samples, as shown in the Supplementary Material (Supplementary Figures S5a–e). The different Co and Ni concentrations are reflected in the unique peaks observed in each EDS spectrum for Ni, Co, W, and O. These results indicate that the experimental and observed composition ratios are closely aligned, thereby confirming the successful formation of the NCW series samples.

3.2 Electrochemical study

3.2.1 Electrochemical performance of NCW thin film electrodes

Using a three electrode configuration in 1 M KOH electrolyte, the electrochemical performance of the NCW series electrodes for supercapacitor applications was examined. Cyclic voltammetry (CV) curves were acquired over a potential range of 0–0.6 V vs. Hg/HgO at a fixed scan rate of 10 mV s⁻¹, as illustrated in Figure 4a. The CV profiles of the electrodes at different scan rates between 2 and 10 mV s⁻¹ are shown in Figure 4b; Supplementary Figures S6a–d. Depending on the Co:Ni ratio, each electrode exhibits a unique current response. A battery-type charge storage mechanism is indicated by the distinct redox peaks in the NCW1 CV curve (Supplementary Figure S6a). With increasing Co content, the shape of the CV curves changes, showing broader redox features associated with the Co²⁺/Co³⁺, Co³⁺/Co⁴⁺, and Ni²⁺/Ni³⁺ redox couples. The NCW4 electrode exhibits the largest enclosed CV area and the highest current response among the series, indicating a higher charge storage capacity (Figure 4b). Furthermore, Figure 4c shows that the oxidation peak current increases proportionally to the square root of the scan rate, suggesting a dominant charge-storage process mediated by diffusion. The anodic and cathodic peaks shift toward more positive and negative potentials, respectively, as the scan rate increases, indicating that it combines diffusion-controlled Faradaic processes with surface-controlled capacitive processes (Afzal et al., 2024). To gain further insight into the charge storage mechanism, the CV data were analyzed using the Trasatti method. According to this approach, the overall stored charge comprises two parts. One arising from surface-related processes, including both non-Faradaic (electric double-layer) and near-surface Faradaic reactions; and the other from bulk diffusion-controlled Faradaic reactions occurring within the electrode material (Reddy Pallavolu et al., 2024; Schoetz et al., 2022; Hao et al., 2019). These charge storage behaviours are governed by a power law relationship (Equation 1), reflecting the contribution of both mechanisms to the overall capacitance,where “a” and “b” are variable parameters, where the value of “b” provides insight into the charge storage mechanism. A diffusion-controlled process, where the current response is proportional to the square root of the scan rate, is generally indicated by a ‘b’ value of 0.5. Conversely, a “b” value of 1 suggests a surface-controlled process, where the current is linearly proportional to the scan rate, characteristic of capacitive behaviour occurring at the electrode/electrolyte interface (Pholauyphon et al., 2024). The “b” value can be determined from the slope of the linear fit to the plot of log (i) versus log (ʋ). The calculated ‘b’ values for NCW1, NCW2, NCW3, NCW4, and NCW5 are 0.50, 0.53, 0.71, 0.82, and 0.95, respectively (Figure 4d). The calculated b-values increase from approximately 0.50–0.95 with increasing Co content, indicating a gradual evolution in the charge storage mechanism. The lower b-values observed for the Ni-rich compositions suggest that the electrochemical process is predominantly diffusion controlled, whereas the higher b-values of the Co-rich samples indicate a greater contribution from surface controlled capacitive processes. Particularly, NCW4 exhibits a b-value close to unity, demonstrating that the charge storage mechanism is largely governed by surface redox reactions. This transition from diffusion controlled to surface controlled behaviour with increasing Co incorporation contributes to the enhanced electrochemical performance of NCW4. The Equation 2 that follows can be used to assess how much each of these processes contributed to the overall current response:

i = a ʋ b (1)

i ʋ = k 1 ʋ + k 2 ʋ 1 / 2 = i s u r f a c e + i b u l k (2)

The values of k ₁ (slope) and k ₂ (intercept) were determined from the linear plot of i/ʋ ^1/2 verses ʋ ^1/2 (Supplementary Figure S7), using the peak current values corresponding to the oxidation peak. The respective current contributions for each electrode at the scan rate of 10 mV s⁻¹ are illustrated in Figure 4e. Figure 4f displays the current contributions for the NCW4 electrode at various scan rates and that of for NCW1, NCW2, NCW3, and NCW5 are provided in Supplementary Figures S8a–d. It was observed that variations in the Ni:Co ratio significantly influence the charge storage mechanism, with a higher contribution from surface-controlled processes as the Co content increases. For instance, the NCW1 electrode stores approximately 99% of its charge via diffusion-controlled mechanisms, whereas the NCW5 electrode shows a diffusion-controlled contribution of about 12%. The surface-controlled contribution increased from approximately 50% at 2 mV s⁻¹ to 59%, 63%, 66%, and 69% at 4, 6, 8, and 10 mV s⁻¹, respectively, while the diffusion-controlled contribution decreased correspondingly from 50% to 31% in NCW4. The increasing contribution of surface-controlled processes with increasing scan rate indicates that charge storage is progressively dominated by rapid surface redox reactions. Consequently, the NCW4 electrode maintains efficient charge storage even at higher scan rates, resulting in its excellent rate capability.

Galvanostatic charge-discharge (GCD) measurements for the NCW series electrodes were performed at current densities ranging from 1.1 to 4.1 A g⁻¹. The comparative GCD curves at fixed current density of 1.1 A g⁻¹ for all NCW series electrodes are presented in Figure 5a. The GCD profile of the NCW1 electrode exhibits a distinct potential plateau during charging, characteristic of a battery-type charge-storage mechanism dominated by reversible Faradaic redox reactions. With increasing Co content, the charging curves exhibit a sloped profile, and by NCW5, a quasi-triangular GCD shape is observed, indicating a change in charge storage behaviour. Among all samples, the NCW4 electrode exhibits the longest charge-discharge duration at each current density, reflecting its superior charge storage capacity. GCD profiles recorded at different current densities for the NCW4 electrode are shown in Figure 5b, and those of NCW1, NCW2, NCW3, and NCW5 are displayed in Supplementary Figures S9a–d. The specific capacitance and specific capacities at 1.1 A g⁻¹ for NCW1, NCW2, NCW3, NCW4, and NCW5 are 494.47 ± 20, 1380.33 ± 23, 1447.43 ± 28, 1571.43 ± 25, and 373.78 ± 12 F g^-1, and 247.24 ± 10, 690.17 ± 11, 723.50 ± 14, 785.72 ± 12, and 186.88 ± 6 C g⁻¹, respectively. Figure 5c shows the variation of specific capacity with current densities, while Supplementary Figure S10 shows the variation of specific capacitance with current densities. The improved electrochemical behavior of NCW4 may be associated with the cooperative contribution of Ni and Co ions toward the redox processes and charge transfer characteristics. Furthermore, the electrochemical performance of NCW4 was benchmarked against previously reported Ni and Co-based tungstate materials (Supplementary Table S2), highlighting its competitive or superior potential for high performance supercapacitor applications.

The effect of Ni and Co compositional variation on the resistive characteristics of the NCW series electrodes was assessed using electrochemical impedance spectroscopy (EIS). Figure 5d displays the Nyquist plots, and the inset displays the matching equivalent circuit model that was utilized to derive values for the charge transfer resistance (R_ct) and series resistance (R_s). The R _s values for NCW1, NCW2, NCW3, NCW4, and NCW5 are 0.448 ± 0.01, 0.411 ± 0.01, 0.450 ± 0.01, 0.277 ± 0.01, and 0.432 ± 0.01 Ω cm⁻², respectively. All electrodes exhibited comparable R _s values, indicating similar intrinsic and contact resistances. However, significant differences were observed in the R _ct values. Notably, the NCW4 nanohybrid electrode exhibited the lowest R _ct value (25.72 ± 0.6 Ω cm⁻²), compared to 73.70 ± 0.8, 29.01 ± 0.3, 43.33 ± 0.4, and 29.31 ± 0.6 Ω cm⁻² for NCW1, NCW2, NCW3, and NCW5, respectively, indicating improved charge transfer kinetics for NCW4. Warburg impedance, which reflects the restrictions on ion diffusion within the electrode, is further indicated by the presence of a linear tail in the low frequency region of the Nyquist plots. Supplementary Table S3 lists all the fitted parameters along with their values.

The superior electrochemical performance of NCW4 can be attributed to several complementary factors. The optimized Ni/Co composition promotes enhanced electrochemical activity, while the nanospherical morphology facilitates efficient electrolyte penetration and ion transport. Moreover, doble layer capacitance (C _dl) measurements performed in non-faradaic potential window 0–0.1 V vs. Hg/HgO (Supplementary Figure S11) demonstrates that NCW4 possesses the highest C _dl (4.36 mF A⁻¹, Supplementary Figure S11f) among all the investigated electrodes, indicating the presence of a larger number of accessible electroactive sites for faradaic reactions. EIS further reveals that NCW4 exhibits the lowest R _ct (25.72 ± 0.6 Ω), suggesting faster electron transport and improved interfacial charge transfer kinetics. In addition, the higher b-value indicates a greater contribution of surface-controlled charge transfer processes, enabling rapid redox reactions and improved rate capability. Therefore, the enhanced electrochemical performance of NCW4 arises from the combined effects of increased C _dl, optimized charge transfer kinetics, favorable morphology, and optimized Ni/Co composition.

The NCW4 electrode’s cycling stability is assessed at a current density of 4.1 A g⁻¹ over 5,000 charge-discharge cycles, as shown in Figure 5e. The NCW4 electrode exhibited outstanding long-term performance, retaining 77% of its initial capacity and 95% Columbic efficiency. The Nyquist plots of the NCW4 electrode after stability measurements are shown in the inset of Figure 5e, indicating that after 5,000 continuous GCD cycles, a slight increase in R _s and R _ct is observed. These values are tabulated in Supplementary Table S3. The superior cycling stability of NCW4 is attributed to the optimized Ni:Co ratio and the synergistic interaction between the two metal components, which collectively enhance structural stability and electrochemical reversibility. Columbic efficiency remains close to 95% throughout the cycling test, indicating that the faradaic charge storage reactions are highly reversible. Nevertheless, the electrode remains approximately 77% of its initial capacity after 5,000 cycles. This suggests that, although the electrochemical reactions remain largely reversible, gradual degradation of the electrode occurs during long-term cycling. Such capacity fading may arise from structural relaxation of the amorphous NCW framework, partial loss or inactivation of electrochemically active sites, slight dissolution of transition metal species, and increased interfacial resistance induced by repeated ion insertion/extraction. Consequently, the number of accessible active sites decreases over extended cycling while the charge storage reactions remain predominantly reversible, resulting in high Columbic efficiency despite moderate capacity decay.

The structural stability of the NCW4 electrode after prolonged electrochemical cycling was investigated by post cycling XRD, FE-SEM, and Raman analyses (Supplementary Figure S12). The post cycling XRD pattern (Supplementary Figure S12a) reveals the emergence of two diffraction peaks, which are likely associated with the formation of oxyhydroxide phases generated during repeated charge–discharge cycling. However, these reflections do not exactly match the standard diffraction patterns of reported oxyhydroxide phases, possibly due to K⁺ intercalation into the electrode lattice during electrochemical cycling, resulting in structural distortion and phase evolution (Malavekar D. B. et al., 2024; Malavekar et al., 2026). FE-SEM images (Supplementary Figure S12b) demonstrate the formation of surface cracks after long term cycling, indicative of repeated volume expansion and contraction during the Faradaic reactions. Nevertheless, the characteristic spherical morphology of the active material is largely preserved, although partial particle agglomeration is observed. The post cycling Raman spectrum (Supplementary Figure S12c) exhibits characteristic vibrational bands at 284, 343, 412, 530, 692, 765, and 881 cm⁻¹, corresponding to Ni–O, Co–O, and O–W–O bonding (Patil et al., 2022; Patil et al., 2026), confirming that the fundamental crystal framework of the NCW4 electrode remains intact despite the structural reconstruction induced during electrochemical cycling. These results demonstrate the excellent structural integrity of the NCW4 electrode, which contributes to its outstanding long term cycling stability.

3.2.2 Electrochemical performance of rGO electrode

Using a conventional three-electrode setup, with a Pt plate serving as the counter electrode, Hg/HgO as the reference electrode, and the rGO-coated substrate as the working electrode in a 1 M KOH electrolyte, the electrochemical performance of the rGO electrode for supercapacitor applications was assessed. CV analysis, conducted at scan rates ranging from 5 to 100 mV s⁻¹ (Supplementary Figure S13a), revealed a quasi-rectangular voltammetric profile at lower scan rates, indicative of dominant electric double-layer capacitive (EDLC) behaviour (Karade and Sankapal, 2018). This profile progressively evolved into a more elongated shape at elevated scan rates due to increased ionic transport resistance. The consistent increase in the integrated area of CV curve with increasing scan rate signifies effective charge storage capability of material. The GCD measurements at a current density of 0.3 A g⁻¹ yielded a quasi-triangular curve (Supplementary Figure S13b), from which a specific capacitance of 39.24 F g⁻¹ was calculated. Supplementary Figure S13c shows that this capacitance decreased with increasing current density. This is a common occurrence explained by the shorter time required for complete ion diffusion into the porous electrode structure at higher rates. The EIS presented in the Nyquist plot (Supplementary Figure S13d) was modelled using an equivalent circuit, yielding a low R _s of 0.65 Ω cm⁻² and a R _ct of 24.25 Ω cm⁻²; full parameter details are provided in Supplementary Table S4. Based on its performance, the rGO electrode was subsequently employed as the anode in a two-electrode asymmetric supercapacitor assembly.

3.2.3 Aqueous hybrid supercapacitor (HSC) device

Recent studies have demonstrated significant interest in hybrid supercapacitor devices due to their ability to deliver high specific energy while maintaining superior specific power for practical applications. To evaluate the practical level feasibility of the synthesized electrodes, a two electrode aqueous hybrid supercapacitor (HSC) device was assembled, as illustrated in the schematic diagram (Figure 6a). For the fabrication of the HSC, the optimised NCW4 electrode was used as the positive electrode, and rGO as the negative electrode. rGO was selected as the negative electrode material because of its high electrical conductivity, large specific surface area, excellent rate capability, and predominantly EDLC behavior. The 2D conductive network of rGO facilitates rapid electron transport and efficient ion diffusion. Making it highly suitable for asymmetric supercapacitor applications. To maximise the energy storage capacity of the HSC, the mass ratio among the two electrodes was carefully optimised using the charge-balancing equation (Supplementary Equation S3. The comparative CV profiles of the NCW4 and rGO electrodes, recorded at a scan rate of 50 mV s⁻¹, are presented in Figure 6b. Furthermore, the operational potential window of the HSC was optimized by evaluating CV curves at 50 mV s⁻¹ (Figure 6c) and GCD profiles at a current density of 4.5 A g⁻¹ across varying upper cut-off voltages from 1.3 to 1.7 V (Supplementary Figure S14). The maximum stable cell voltage was extended to 1.6 V, beyond which a sharp current surge, ascribed to the onset of the oxygen evolution reaction, was observed. In the HSC, charge storage is governed by two distinct mechanisms: (i) intercalation/de-intercalation of OH⁻ ions at the NCW4 electrode-electrolyte interface, and (ii) adsorption/desorption of K⁺ ions at the rGO electrode-electrolyte interface during charge/discharge processes.

CV was used to assess the electrochemical behavior of the HSC at scan rates ranging from 5 to 100 mV s⁻¹ within a potential window of 0–1.6 V (Figure 6d). Excellent rate capacity is demonstrated by the preservation of quasi-rectangular CV profiles at high scan rates. Notably, the current response exhibited a systematic increase with scan rate, indicating robust electrochemical kinetics. Complementary GCD analysis (Figure 6e) performed at current densities of 2.0–5.6 A g⁻¹ revealed slightly non-linear profiles with consistent charge-discharge characteristics, further confirming the hybrid nature of the charge storage mechanism in the HSC. The HSC exhibits a specific capacitance of 141.6 F g⁻¹ at 2.0 A g⁻¹, maintaining a capacitance of 104 F g⁻¹ at an elevated current density of 5.6 A g⁻¹ (Figure 6e). The specific capacitance values, derived from GCD measurements, exhibit a characteristic decrease with increasing current density, consistent with typical electrochemical energy storage systems (Figure 6f). Furthermore, energy and power characteristics were quantified through systematic evaluation of specific energy and specific power using Supplementary Equations S6, S7, respectively. The HSC device delivers a maximum specific energy of 50.41 Wh kg⁻¹ at a specific power of 1.74 kW kg⁻¹, while retaining 37.03 Wh kg⁻¹ at a specific power of 4.30 kW kg⁻¹ (Figure 6g). Furthermore, the specific energy and specific power values acquired in the current study were compared with previously reported cobalt tungstate and nickel tungstate electrode-based supercapacitor devices, with detailed comparisons summarized in Supplementary Table S5.

The EIS study provides valuable insights into the charge transfer kinetics of HSC. Specifically, Figure 6h presents a Nyquist plot of the NCW4//rGO HSC, indicating favourable electrical and ionic conductivity, along with an efficient electrode-electrolyte interface, as reflected by the low series resistance (R _s = 1.483 Ω) and charge transfer resistance (R _ct = 12.73 Ω). The HSC was subjected to extended cycling tests at 5.6 A g⁻¹ to evaluate its long term electrochemical stability. Notably, the device demonstrated a capacitive retention of 95.1% and a columbic efficiency of 90% following 5,000 cycles, as illustrated in Figure 6i. The electrochemical activation of the electrode material and the progressive opening of its mesoporous structure, which facilitates effective ion intercalation, are responsible for the HSC device’s initial rise in capacitive retention with progressive charge–discharge cycling. However, the subsequent slight decline in capacitance suggests possible surface distortion or structural degradation of the active material (Pujari et al., 2022). Furthermore, EIS measurements recorded after 5,000 charge-discharge cycles (also shown in Figure 6h) and the corresponding fitted parameters in Supplementary Table S6 indicate a minor increase in R _s and R _ct, likely due to slow or structural breakdown of the active material over extended cycling (Kachura et al., 2025; Sánchez-romate et al., 2021). Supplementary Figure S15 depicts the electrical equivalent circuit. According to these findings, Ni_0.25Co_0.75WO₄//rGO exhibits outstanding electrochemical properties, suggesting that it could be utilised as an electrode material in energy storage applications.

4 Conclusion

In this study, self-supported nanospherical nickel cobalt tungstate electrodes were successfully synthesized via the SILAR method. Owing to the strong synergistic interaction between Ni²⁺ and Co²⁺, the optimized NCW4 electrode delivers a high specific capacity of 785.72 C g⁻¹ at 1.1 A g⁻¹, which is nearly three times higher than that of pristine nickel and cobalt tungstate electrodes. The electrode also exhibits excellent cycling stability, retaining 77% of its capacity after 5,000 cycles. A hybrid supercapacitor (NCW4//rGO) assembled with rGO as the negative electrode demonstrates a high specific energy of 50.41 Wh kg⁻¹ at a specific power of 1.74 kW kg⁻¹, along with outstanding cycling stability (95.1% retention after 5,000 cycles). These results demonstrate how important composition engineering is for improving charge storage behavior and offer important new information for the development of NCW-based electrode topologies in the future for high-performance supercapacitor applications.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fbael.2026.1903174/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: Jamile Mohammadi Moradian, University of Science and Technology of China, China

Reviewed by: V Balasubramanian, Sona College of Technology, India

Nishat Fatima, Government Sadiq College Women University, Pakistan