Interface-engineered Co-Ni-S composite electrode for ultrahigh capacitance and water oxidation

Abstract

Transition metal sulfides (TMSs) are promising candidates for electrochemical energy storage and water splitting, but their practical application is hindered by limited conductivity and sluggish ion transport. Herein, we present an interface-engineered Co-Ni-S composite electrode prepared through a two-step process involving electrodeposition followed by hydrothermal sulfurization, which effectively preserves cobalt active sites while optimizing interfacial characteristics. This architecture enhances redox kinetics, electron mobility, and ion diffusion, resulting in a highly porous nanosheet structure with exceptional electrochemical performance. The Co-Ni-S composite electrode delivers an ultrahigh specific capacitance of 3586 F g⁻¹ at 1 A g⁻¹ and maintains 97% capacity retention over 5000 cycles. Simultaneously, it exhibits outstanding oxygen evolution reaction (OER) activity, requiring a low overpotential of 210 mV at 10 mA cm⁻² and showing long-term stability over 50 hours. Density functional theory (DFT) calculations confirm the presence of stable Co-Ni-S bonding and synergistic charge transfer. When assembled in an asymmetric supercapacitor device, the electrode achieves a remarkable energy density of 172 Wh kg⁻¹ and high-rate capability. These findings highlight the potential of interface-engineered bimetallic sulfides as multifunctional materials for next-generation energy storage and water-splitting technologies.

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1. Introduction

The global pursuit of clean and reliable energy systems has intensified efforts to develop multifunctional materials for both energy storage and conversion. Among various electrochemical technologies, supercapacitors (SCs) have attracted considerable attention due to their ability to deliver high power density, rapid charge–discharge capability, and long operational life.^1–3 However, their relatively low energy density remains a significant limitation, especially when compared to conventional batteries. Addressing this challenge requires innovative approaches to electrode design that can simultaneously improve capacitance, conductivity, and stability.^4–9 One promising class of materials for next-generation supercapacitor electrodes is transition metal sulfides (TMSs). These materials offer abundant redox-active sites and better electrical conductivity than their oxide or hydroxide counterparts.^10,11

Nevertheless, TMSs are often limited by poor ion transport, low active site accessibility, and structural instability during cycling. Overcoming these challenges requires strategies that precisely control material composition, morphology, and surface chemistry at the nanoscale.^12,13 Bimetallic sulfides, particularly those based on cobalt and nickel, have recently demonstrated significant potential. The combination of two transition metals introduces synergistic effects that can enhance electronic conductivity, increase active site density, and improve overall electrochemical performance. These materials often exhibit a hybrid energy storage mechanism, combining battery-type faradaic reactions with capacitive processes. Additionally, precise control over the interface structure achieved through techniques like controlled sulfurization can further optimize ion diffusion and charge transport.^14–16

In addition, interface-engineered Co-Ni-S synthesized via Co-retained sulfurization has emerged as an effective strategy for enhancing electrochemical performance in energy storage and conversion applications.¹⁷ This controlled sulfurization process preserves cobalt sites while modulating interfacial properties, resulting in improved conductivity, accelerated charge transfer, and enhanced active site exposure, which is crucial for high-performance SCs and the oxygen evolution reaction (OER).^18,19 Beyond energy storage, cobalt–nickel sulfides have also demonstrated strong electrocatalytic activity, particularly in the OER, a key process in water splitting for hydrogen production. The ability to engineer a single material system that supports both supercapacitor and OER functions provides a valuable platform for multifunctional energy devices.

This study reports a cobalt-retained Co-Ni-S electrode fabricated via a two-step process involving cathodic electrodeposition followed by controlled hydrothermal sulfurization. This method preserves active cobalt sites while refining the material's interfacial chemistry, resulting in vacancy-rich Co-Ni-S nanosheet arrays on Ni-foam. The nanosheet architecture enhances rapid charge transport and structural robustness, delivering exceptional performance in both supercapacitors and the OER. When integrated into an asymmetric device with activated carbon (AC), the optimized electrode achieves an energy density of 172 Wh kg⁻¹ at a power density of 7.4 kW kg⁻¹, retains 97% capacitance after prolonged cycling, and operates with 100% coulombic efficiency. As an OER catalyst, it requires only 210 mV to reach 10 mA cm⁻² and exhibits a low Tafel slope of 38 mV dec⁻¹, sustaining activity for over 50 hours. The successful LED demonstration further underscores its practical viability. Collectively, these findings highlight how interface-engineering and vacancy-rich surfaces in bimetallic sulfides harmonize battery-type and pseudocapacitive behavior, projecting a promising path toward multifunctional energy-conversion and storage technologies.

2. Experimental

The materials used in this study are described in the SI, and the detailed procedures for electrochemical analysis, fabrication of the solid-state asymmetric supercapacitor (ASC), OER measurements, and material characterization are also provided.

2.1. Electrodeposition of CoNi alloys on NF

In a typical synthesis procedure, multiple nickel foam (NF) pieces (2 × 1 cm) were pre-treated before use. The NF substrates underwent sequential ultrasonication in acetone, deionized water, and 1.0 M HCl for 10 minutes each, followed by drying in a vacuum oven. Electrodeposition was carried out using a three-electrode setup, consisting of a saturated Ag/AgCl reference electrode, a platinum wire counter electrode, and the NF substrate as the working electrode. For the CoNi alloy deposition, the electrolyte was prepared by dissolving 0.3 M Ni(NO₃)₂, 0.3 M Co(NO₃)₂, 2.0 M NH₄Cl, and 0.1 M HCl in 50 mL of deionized water, resulting in a uniform solution with a pH of 3.5. The electrodeposition is performed on the NF substrate by applying a constant voltage from 0 to −1 V at a scan rate of 10 mV s⁻¹. For the synthesis of CoNi alloy-10 Seg (segments), CoNi alloy-20 Seg, and CoNi alloy-30 Seg samples, the dual metal salts were added in varying molar ratios while keeping the other chemical conditions unchanged.

2.2. Synthesis of Co-Ni-S/NF composite

Following the previous step, Na₂S was dissolved and stirred in a solution for 10 minutes. This prepared solution was then transferred into a 100 mL Teflon-lined stainless-steel autoclave containing the CoNi-NF alloy-coated substrates and subjected to hydrothermal treatment at 170 °C for 12 hours (Scheme 1). After the reaction, the products were carefully rinsed with ethanol and water and then dried in a vacuum oven at 70 °C, yielding the black Co-Ni-S/NF-170 °C sample. Similarly, Co-Ni-S/NF-150 °C and Co-Ni-S/NF-160 °C samples were obtained by varying the hydrothermal temperature of 150 °C and 160 °C, respectively, while maintaining all other experimental conditions constant.

Scheme 1 Schematic fabrication of thin Co-Ni-S nanosheet composite arrays on the NF via simple electrodeposition and hydrothermal approach.

To ensure consistent active material loading, all Co-Ni precursor films were prepared under identical electrodeposition conditions, using different segments and electrolyte concentrations. The deposited films were then subjected to hydrothermal sulfurization at 150, 160, and 170 °C for 12 hours. These parameters were chosen to investigate the influence of temperature on phase evolution and electrochemical behavior while keeping the initial metal loading nearly constant. The minor variations in mass loading observed after sulfurization mainly arise from differences in surface porosity and crystallinity, rather than discrepancies in precursor deposition.

3. Results and discussion

3.1. Structural characterization

The schematic of the dual-stage electrodeposition and hydrothermal process for fabricating a surface-modified CoNi alloy/sulfur heterostructure is shown in Scheme 1. In the first stage, a hierarchical CoNi alloy nanostructure is synthesized via cathodic electrodeposition, where Co²⁺ and Ni²⁺ ions undergo electrochemical reduction to form a solid CoNi alloy, facilitated by water decomposition (Co²⁺ + 2e⁻ → Co, Ni²⁺ + 2e⁻ → Ni) (Fig. S1), yielding well-defined CoNi alloy layers. The electrolyte solution for this deposition is prepared by dissolving 0.3 M nickel salt, 0.3 M cobalt salt, 2.0 M NH₄Cl, and 0.1 M HCl in 50 mL of deionized water, resulting in a uniform mixture with a pH of 3.5. Electrodeposition is performed on an NF substrate by applying a constant voltage between 0 and −1 V, with a scan rate of 10 mV s⁻¹. Subsequently, an additional electrodeposition step is conducted to optimize the alloy composition and structural properties using a Co-Ni electrolyte under the same voltage range (Fig. S2). In the second step, hydrothermal methods are applied with varying temperatures in a Na₂S solution to convert the CoNi alloys into Co-Ni-S/NF. During hydrothermal sulfurization, it may be possible to form multiple sulfide phases such as Ni₃S₂, CoS, Co₉S₈, and ternary Co-Ni sulfides. The oxidation peak in the CV profile indicates the formation of formamidine disulfide from Na₂S, whereas the reduction peak corresponds to the generation of M–S.^20,21 A gradual increase in peak current density was observed with continued cycling, suggesting steady nucleation and growth of the active layer. Visually, the electrode surface transitioned from dark grey to black, indicating progressive material deposition. The applied current promotes ion migration toward the substrate, where ions are reduced and accumulate, forming a sheet-like morphology. These reactions, driven by electrochemical conditions, involve various ions that contribute to the formation of sulfide nanosheets through controlled surface interactions.²² These findings indicate that the nanosheet structure remains stable across various ion types, while optimized ion incorporation enhances electrolyte access, active site availability, and ion transport. This results in lower internal resistance and improved reaction kinetics. Careful ion selection during synthesis enhances performance without compromising structural integrity.²³ Future work should explore the specific roles of different ions to better understand their influence on electrochemical efficiency.

The sulfurization process conducted at different temperatures (150, 160, and 170 °C) enables precise interface engineering that markedly improves the morphological and electrochemical characteristics of the Co-Ni-S heterostructure. This transformation produces a sulfur-enriched surface, which improves conductivity, increases the number of active sites, and reinforces the material's stability. Such tailored interfaces contribute significantly to the improved effectiveness of the material in energy storage and conversion applications. The XRD patterns reveal the structural evolution of CoNi alloys grown on Ni-foam under varying conditions, including segmental growth (10, 20, and 30 segments) and different Co : Ni ratios (1 : 0.5, 1 : 1, and 1 : 1.5) (Fig. S3a and b). The diffraction peaks confirm successful formation of the CoNi alloy, with changes in peak intensity and sharpness reflecting variations in crystallinity, grain size, and phase composition. Specifically, increasing the number of segments to 30 and raising the Co content to a 1 : 1.5 ratio leads to enhanced peak intensity, indicating improved crystallinity and phase purity. The observed peak shifts across different samples suggest the presence of lattice strain and compositional tuning effects, which play a crucial role in influencing the electrochemical performance of the materials. The combined optimization of segmental growth and Co : Ni ratio thus enhances the structural integrity, electrical conductivity, and catalytic activity of the alloys, making them highly promising as active materials. Furthermore, Fig. S3c shows the structural characterization of the Co-Ni-S/NF obtained by sulfurizing the CoNi alloys via a hydrothermal method. The XRD patterns align well with reference data for (JCPDS #73-0698, Heazlewoodite)²⁴ and Co₉S₈ (JCPDS #65-6801),²⁵ confirming that the synthesized material is not a single phase but rather a composite, indicating the successful formation of the Co-Ni-S composite. Based on our detailed XRD analysis, the material is predominantly composed of Ni₃S₂, with a minor amount of Co₉S₈. The diffraction peaks of Ni₃S₂ dominate the pattern, forming the primary crystalline framework, while the small Co₉S₈ fraction reflects the limited cobalt incorporation from the electrodeposition precursor. Since Co atoms closely resemble Ni in size, their incorporation slightly modifies the lattice without changing the overall Ni₃S₂ crystal structure, justifying the classification of the composite as Co-Ni-S. The well-defined peaks indicate high crystallinity, which is essential for efficient electron transport, and subtle peak shifts and intensity variations suggest structural modulation due to cobalt incorporation.²⁶ These structural changes enhance catalytic performance by increasing active sites and improving electronic conductivity, while the hydrothermal process effectively transforms Co-Ni precursors into the Co-Ni-S composite, making it a highly promising material for energy storage and conversion (Fig. S3d).

The reactions for the sulfurization process can be expressed as:

2Ni³⁺ + 3S²⁻ → Ni₃S₂ (1)

Co²⁺ + S²⁻ → CoS (2)

9Co + 8S → Co₉S₈ (3)

Co²⁺ + 3Ni²⁺ + 4S²⁻ → CoNi₃S₂ (4)

These reactions suggest that a variety of sulfide peaks may occur at the sulfurization, with Ni₃S₂ predominating and minimal Co₉S₈ and potential ternary sulfides.

The FESEM images display the morphological evolution of CoNi alloys under different CV segments. Fig. S4a and b shows a porous nanosheet network, which enhances surface area and active sites. Fig. S4c and d exhibit crack formation and densification, likely due to structural stress or material degradation. Fig. S4e and f reveals a shift to a compact, roughened surface, suggesting surface reconstruction or phase transformation. In addition, the FESEM images depict the morphological evolution of synthesized CoNi alloys with varying Co/Ni salt ratios. Fig. S5a–c reveals a nanostructured, spherical morphology with increasing magnification from 10 μm to 100 nm, indicating the uniform formation of aggregated nanoparticles. Fig. S5d–f displays a distinct layered, sheet-like structure, suggesting a transition to a more hierarchical architecture. Fig. S5g–i exhibits an intricate, interconnected nanosheet network, with higher magnifications emphasizing the porous nature of the material. Further, FESEM images of sulfurized samples (Co-Ni-S) at different hydrothermal temperatures (150 °C, 160 °C, and 170 °C) demonstrate morphological evolution of the synthesized material, highlighting the impact of temperature on structural development. At 150 °C (Fig. S6a–c), the material exhibits a rough and poorly defined nanostructure with irregular, loosely packed nanosheets, indicating incomplete crystallization. As the temperature increases to 160 °C (Fig. S6d–f), the nanosheets become more interconnected and uniform, with enhanced surface roughness and porosity, suggesting improved structural organization. Further increasing the temperature to 170 °C (Fig. S6g–i) results in a highly porous, interconnected, and well-developed nanosheet network, reflecting enhanced nucleation and growth at elevated temperatures. The sample synthesized at 170 °C achieves an optimal balance, containing crystalline Ni₃S₂, low-crystallinity Co₉S₈, and amorphous regions. This mixed-phase structure is advantageous because the crystalline regions provide efficient electron pathways and stability, amorphous regions create abundant defects and active sites for ion adsorption and redox reactions, and the porous nanosheet morphology ensures rapid electrolyte penetration and ion diffusion.²⁷ Together, these features synergistically enhance redox kinetics and charge storage, resulting in the highest observed specific capacitance. This mechanism highlights the critical role of temperature in controlling phase composition and morphology to maximize electrochemical performance.

The TEM analysis of Co-Ni-S-170 composite (Fig. 1a) reveals both crystalline and amorphous phases, which play crucial roles in energy storage and conversion applications. In Fig. 1b, the TEM and HR-TEM images show lattice fringes along with defects and vacancies, indicating a crystalline structure with structural imperfections. These defects can enhance ion and electron transport, improving electrochemical performance. In contrast, Fig. 1c displays a featureless, uniform texture, characteristic of an amorphous structure lacking long-range atomic order. The combination of crystalline and amorphous phases offers synergistic advantages: the crystalline domains ensure high electrical conductivity for efficient charge transfer, while the amorphous regions provide abundant active sites and structural flexibility, enhancing ion distribution and electrochemical reaction kinetics. HR-TEM analysis reveals lattice spacings of 0.157 nm and 0.151 nm, corresponding to the (620) and (533) planes of Co₉S₈ crystals, respectively (Fig. 1d).²⁵ The SAED pattern (Fig. 1e) confirms a clear polycrystalline structure. Additionally, defects and vacancies increase the active surface area and accelerate ion diffusion. The elemental mapping shows a uniform distribution of Ni, Co, O, N, and S, indicating the effective synthesis of nanosheets and confirming the successful formation of the metal sulfide Co-Ni-S composite (Fig. 1f and S7).

Fig. 1 (a) TEM images of Co-Ni-S composite, (b) HR-TEM microstructure, (c and d) amorphous and crystalline phases of lattice spacing, (e) SAED pattern, and (f) EDX mapping analysis of Co-Ni-S composite.

X-ray photoelectron spectroscopy (XPS) was used to examine changes in the electronic structure of Co and Ni after sulfur incorporation and to identify the oxidation states in the Co-Ni-S composite. The survey spectrum of Co-Ni-S-170 composite (Fig. 2a) shows clear peaks for Co, Ni, S, O, and C, confirming the composite's successful formation and elemental composition. Additionally, the atomic ratio analysis indicates the successful incorporation of sulfur, which plays a crucial role in altering the oxidation states of Co and Ni (Fig. 2b). The XPS spectra provide a detailed analysis of the material's surface chemistry through peak deconvolution. In the Co 2p spectrum, peaks at 779.6 eV and 794.7 eV correspond to Co²⁺, while peaks at 781.2 eV and 796.8 eV correspond to Co³⁺. Shake-up satellite peaks near 785.0 eV and 802.0 eV suggest strong electronic interactions (Fig. 2c). The Ni 2p spectrum in Fig. 2d exhibits Ni²⁺ peaks at 855.3 eV and 873.0 eV, Ni³⁺ peaks at 857.8 eV and 875.5 eV with characteristic satellite peaks near 861.0 eV and 879.0 eV, indicating mixed valence states that enhance redox activity. The dominant intense peaks of Ni³⁺, Co³⁺, and their satellite peaks indicate the formation of Co-S and Ni-S phases Ni₃S₂ and the presence of Co-Ni-S.²⁸ The S 2p spectrum in Fig. 2e reveals S²⁻ peaks at 162.1 eV (S 2p_3/2) and 163.3 eV (S 2p_1/2), along with a sulfate (SO₄²⁻) peak at 168.5 eV, confirming successful sulfurization and formation of the Co-Ni-S composite.²⁹ In the O 1s band (Fig. 2f), metal–oxygen bonding (M–O) appears at 529.8 eV, hydroxyl groups (O–H) at 531.2 eV, carbonyl (C=O) at 532.5 eV, and ether or hydroxyl oxygen (C–O) at 533.8 eV, contributing to enhanced surface activity. The N 1s spectrum consists of pyridinic-N at 398.5 eV, pyrrolic-N at 400.3 eV, and graphitic-N at 401.8 eV, which contribute to enhance conductivity and structural stability (Fig. 2g). The C 1s spectrum shows graphitic C–C/C=C at 284.6 eV, C–O at 286.2 eV, and carbonyl/carboxyl groups (C=O) at 288.5 eV (Fig. 2h). The XPS analysis reveals that the material contains mixed-valence metal centers, as evidenced by the coexistence of Co²⁺/Co³⁺ and Ni²⁺/Ni³⁺ species in the Co 2p and Ni 2p spectra. This mixed-valence state indicates the metal vacancies and structural defects, while the observed satellite peaks further confirm the presence of unpaired electrons at defect-rich metal sites. The S 2p spectrum shows both metal-bonded sulfide (S²⁻) and surface-oxidized sulfur species (SO_x), suggesting sulfur vacancies that provide additional active sites and enhance electrolyte ion adsorption.²⁸ Together with oxygen- and nitrogen-related defect sites, these features create a highly defect-rich surface that promotes improved charge transport, faster ion diffusion, and enhanced electrochemical activity.

Fig. 2 (a) Survey spectra of Co-Ni-S composite, (b) atomic percentages, high-resolution XPS spectra of (c) Co 2p, (d) Ni 2p, (e) S 2p, (f) O 1s, (g) N 1s, and (h) C 1s.

3.2. Electrochemical analysis

Electrochemical properties of CoNi alloy electrodes with different (cyclic voltammetry) CV segments and Co/Ni ratios were evaluated using CV, (Galvanostatic charge–discharge) GCD, specific capacitance, and (electrochemical impedance spectroscopy) EIS. The CV curves (Fig. S8–S11) show quasi-rectangular shapes with clear redox peaks, indicating both (electric double-layer capacitance) EDLC and pseudocapacitive behavior arising from reversible faradaic reactions. The optimized Co/Ni ratio resulted in the highest charge storage capacity. Symmetrical GCD profiles demonstrated good electrochemical reversibility, with the optimized ratio showing an extended discharge time and a prominent plateau, signifying enhanced capacitance. Electrochemical reversibility is confirmed by stable CV profiles over 30 cycles, while GCD analysis revealed high-rate performance. The retention of specific capacitance at elevated current densities affirms long-term stability. EIS spectra (Fig. S8–S11) revealed low charge-transfer resistance and efficient ion diffusion, attributed to the enhanced porosity and conductivity of the optimized nanostructure, ensuring superior electrochemical performance. Following the sulfurization, the Co-Ni-S composite exhibited noticeably enhanced electrochemical properties. The CV curves of Co-Ni-S-170 composite in 1 M KOH show prominent redox peaks, reflecting a pseudocapacitive charge storage process governed by reversible faradaic reactions (Fig. 3a). The redox process primarily involves the oxidation and reduction of Ni and Co species, following the reactions: Ni²⁺ + OH⁻ ⇌ Ni³⁺ + e⁻, Ni³⁺ + OH⁻ ⇌ Ni⁴⁺ + e⁻, and Co²⁺ + OH⁻ ⇌ Co³⁺ + e⁻, Co³⁺ + OH⁻ ⇌ Co⁴⁺ + e⁻.

Ni₃S₂ + OH⁻ ↔ Ni₃S₂OH + e⁻ (5)

CoS + OH⁻ ↔ CoSOH + e⁻ (6)

Fig. 3 Electrochemical performance analysis. The (a) CV curves at 5 mV s⁻¹ and (b) GCD curves at 1 A g⁻¹. (c) The CV curves and (d) the GCD curves of Co-Ni-S-170 composite. (e) Specific capacitance, and (f) EIS spectra. (g) Stability of Co-Ni-S-170 at 10 A g⁻¹, and (h) schematic diagram for energy storage materials properties.

These surface reactions occur under the cycle process of CoNi₃S₂ in KOH solution, contributing to the pseudocapacitive or battery-type capacity.

CoNi₃S₂ + OH⁻ ↔ CoNi₃S₂OH + e⁻ (7)

Among the tested samples, the Co-Ni-S composite synthesized at 170 °C showed the highest current density, indicating enriched SC performance due to improved crystallinity, conductivity, and an optimized nanostructure that assists ion distribution and charge transfer (Fig. 3a). The 160 °C sample exhibited moderate redox behavior, while the 150 °C sample showed the lowest current response, likely due to incomplete phase formation or poor conductivity.

Fig. 3b presents the GCD curves, illustrating the electrochemical performance of Co-Ni-S composite samples prepared at 150 °C, 160 °C, and 170 °C. The variations in potential profiles suggest temperature-dependent morphological and structural modifications influencing the electrochemical performance. The Co-Ni-S synthesized at 170 °C exhibits a higher discharge time, indicating enhanced specific capacity and improved charge storage capability, possibly due to optimized amorphous/crystallinity and conductivity. The distinct plateaus in the curves denote redox reactions, confirming a faradaic charge storage mechanism. The CV curves in Fig. 3c show clear redox peaks, indicating strong reversibility of the faradaic reaction in the Co-Ni-S composite. This reflects efficient electron and ion transport, supporting its excellent electrochemical performance. Additionally, the CV curves of Co-Ni-S-150 and Co-Ni-S-160 at various scan rates are presented in Fig. S12a and c, further illustrate the material's stable redox behavior and rate-dependent electrochemical characteristics, reinforcing its potential for supercapacitor applications. Further, GCD analyses of the 3-electrodes were performed in the 3-electrode system at different current densities to validate our findings of the charge-storage mechanism. The GCD profiles exhibited a near symmetric shape and were well consistent with the CV curves, demonstrating excellent rate capability with both pseudo and battery-type features. Fig. 3d shows the GCD results of 3 electrodes at different current densities of 1–20 A g⁻¹. The 1 A g⁻¹ GCD curve had a significant amount of non-linearity in contrast to 20 A g⁻¹, which is due to the phenomenon that can be ascribed to the high surface pseudocapacitance of the electrode at lower current density. Increasing the current density promotes deeper ion penetration, causing the GCD profiles to appear triangular. The specific capacitance/capacity values were estimated from the GCD curves, as shown in Fig. 3e. The estimated specific capacitances (capacity) of 3 electrodes are Co-Ni-S-170 (3586 F g⁻¹ (1793 C g⁻¹)), Co-Ni-S-160 (3501 F g⁻¹ (1751 C g⁻¹)), and Co-Ni-S-150 (3238 F g⁻¹ (1619 C g⁻¹)) at 1 A g⁻¹ (inset Fig. 3e). Co-Ni-S-170 showed high specific capacitances of 3586, 3432, 3408, 3392, 3373, and 3368 F g⁻¹ at current densities of 1, 2, 4, 8, 10, and 20 A g⁻¹, respectively surpassing Co-Ni-S-150 and Co-Ni-S-160. The slight decrease in capacitance/capacity at higher current densities is likely due to increased electrode resistance and limited participation of faradaic redox reactions.

To investigate the energy storage behavior and reaction kinetics of Co-Ni-S-170, CV curves were recorded at scan rates from 5 to 50 mV s⁻¹. As shown in Fig. 3c, all curves display distinct redox peaks, confirming highly reversible faradaic reactions. To gain deeper insight into kinetics, the relationship between peak current (i) and scan rate (v) was further analyzed.

i = av^b (8)

log i = log a + b log θ (9)

where a and b are constants, the slope of the log(v) vs. log(i) plot helps determine the b-value, which is key to understanding charge storage behavior. Typically, b ≈ 0.5 suggests a diffusion-controlled (battery-type) process, while b ≈ 1.0 indicates a surface-controlled (capacitive) mechanism.³⁰ For Co-Ni-S-170, the calculated b-values are 0.59 and 0.62 (Fig. S13a), revealing that the charge storage is predominantly diffusion-controlled with a partial capacitive contribution. The root mean square (R²) values of the cathodic and anodic peaks are 0.997 and 0.999, respectively (Fig. S13b), further supporting the battery-type behavior of the material. To quantify the capacitive and diffusive contributions, the following equation was used:

i(v) = k₁v + k₂v^1/2 (10)

where i(v) is current, v is the scan rate, k₁ and k₂ are constants. To evaluate these parameters, a graph was generated by plotting v^1/2 against i/v^1/2. From this plot, the slope of the fitted line corresponds to the value of k₁, while the y-intercept represents k₂. Fig. S13c shows the capacitive and diffusion-controlled contributions of the optimized Co-Ni-S electrode at a scan rate of 5 mV s⁻¹. Fig. S13d illustrates how the contributions vary with different scan rates, highlighting the increasing capacitive dominance at higher scan rates. The capacitive-controlled mechanism accounts for 13.02% of the total capacitance. However, for the Co-Ni-S-170 composite electrode, this fraction increases from 13.02% to 32.13% as the scan rate rises from 5 to 50 mV s⁻¹. Additionally, due to limited ion diffusion into the inner layers at higher scans, the influence of diffusion-controlled processes is significantly reduced. Notably, the capacitance behavior of Co-Ni-S-170 is associated with constrained ion movement and efficient electron transport, suggesting that the surface redox reaction plays a predominant role in charge storage at elevated scan rates.

To further assess the electrochemical performance, EIS was conducted to evaluate electrochemical performance, specifically analyzing charge transfer and electrolyte diffusion processes using an equivalent circuit insertion method (Fig. 3f). The charge-transfer resistances of Co-Ni-S-150, Co-Ni-S-160, and Co-Ni-S-170 were determined to be 0.85, 0.81, and 0.75 Ω, respectively. The EIS plot data values of all the samples are supplied in Table S1. The lower resistance of Co-Ni-S-170 is attributed to its well-developed multilevel structure and the modulation of sulfur vacancies, which increase the effective contact area and provide numerous electrolyte-active sites. Additionally, the impedance and morphology of the Co-Ni-S-170 composite electrode remain unchanged even after extended charge/discharge cycles (Fig. S14). The Co-Ni-S-170 composite electrode exhibited remarkable stability, retaining 99% of its capacity after 5000 cycles at a high current density of 10 A g⁻¹ (Fig. 3g). Its excellent cycling stability and high specific capacity retention can be attributed to the robust 2D porous nanostructure and the strategic introduction of sulfur vacancies. The Co-Ni-S-170 composite electrode significantly enhances the energy storage capability of SCs through several key mechanisms (Fig. 3h). Its optimized nanostructure, featuring a 2D porous architecture, provides a larger electroactive surface area, facilitating enhanced ion diffusion and charge storage. The presence of sulfur vacancies further improves electrical conductivity and charge transport, reducing charge-transfer resistance and enabling efficient faradaic redox reactions.^31,32 The strong pseudocapacitive behavior, combined with battery-type capacitance, ensures high energy density while maintaining rapid charge–discharge characteristics. Additionally, the Co-Ni-S-170 composite electrode demonstrates remarkable cycling stability (99% retention after 5000 cycles), attributed to its structural integrity and the ability to accommodate volume expansion during repeated redox processes. These features enhance coulombic efficiency, boost ion transport, and ensure long-term stability, making Co-Ni-S-170 composite a strong contender for SCs. Many transition metal sulfides (TMSs), such as Ni₃S₂/CoNi₂S₄ (ref. 33) or NiS₂, often suffer from poor cycling stability due to structural degradation, volume changes, or dissolution during repeated redox reactions. Our Co-Ni-S electrode shows enhanced performance due to its interface-engineered composite structure. The coexistence of crystalline Ni₃S₂, low-crystallinity Co₉S₈, and amorphous regions provides both structural robustness and abundant redox-active sites, while the porous nanosheet morphology ensures rapid ion diffusion and electrolyte penetration, reducing mechanical stress during cycling. Additionally, strong interfacial interactions between Co and Ni sulfides improve electron transport and stabilize reaction intermediates, mitigating degradation over long-term cycling.

Fig. 4a presents a schematic illustration of the asymmetric supercapacitor (ASC) device. To construct the ASC, the Co-Ni-S-170 composite electrode was chosen as the cathode due to its superior electrochemical properties. AC was used as the anode, and a PVA/KOH electrolyte with a 1 : 1 mass ratio was employed. The details of the AC are supplemented in Fig. S15. The mass loading of the device was 3.2 mg, which was estimated by the mass balance equation (eqn (S2)). Fig. 4b shows a well-matched asymmetric device with the Co-Ni-S-170 active material operating from 0 to 0.6 V and AC from −1.0 to 0 V, without overlapping voltages. Fig. 4c shows the CV curves of the Co-Ni-S-170//AC ASC device across potential windows from 0–1.6 V, with the curves maintaining their shape without distortion. Fig. 4d further demonstrates the device's fast and reversible charge/discharge behavior, as the CV curves remain stable even at a high scan rate of 5 mV s⁻¹ within 0–1.6 V. The broad redox peaks retain a consistent quasi-rectangular shape at various scan rates, reflecting the combined battery-type behavior of the Co-Ni-S-170 positive electrode and the EDLC of the AC negative electrode, indicating excellent rate performance and reversibility. No significant irreversible current was detected in the CV plots over the operating voltage range of 1.7 V, indicating that the device can reliably operate up to 1.7 V. The GCD curves of the Co-Ni-S-170//AC device exhibit a stable discharge profile across current densities from 1.5 to 30 mA cm⁻² within a voltage range of 0–1.6 V (Fig. 4e), consistent with the CV curves. Simultaneously, a small iR drop was observed in Fig. S15(a) upon increasing current densities, confirming improved conductivity and supporting the faradaic redox nature of the GCD curves. Further validating this discussion, the log relationship of redox peak current vs. scan rates, root mean square plot of total currents vs. scan rates, and current contribution upon increasing scan rates were analyzed, as shown in Fig. S16(b–d). The b value of 0.68 and R² value of >9 indicate a dominant diffusion mechanism with surface-controlled behaviour, which was further validated by the current contributions in Fig. S16d and was also crucial to attaining higher charge storage values. The GCD curves exhibit non-linear, quasi-rectangular shaping with longer charge–discharge durations, indicating excellent reversibility and very high charge storage capacity. The specific capacitance values were calculated from GCD, as shown in Fig. 4f. The specific capacitance values of the device were 484, 393, 448, 612, 547, 467, and 456 F g⁻¹ at 1.5–30 mA cm⁻² current densities. With an increase in current density, capacitance values decreased due to the increase in internal resistance and the insufficient time to complete redox reactions at higher current density.³⁴Fig. 4g presents the outstanding cycling stability of 97% over 5000 long cycles with 100% coulombic efficiency. The high retention of the device indicates the superior ability of charge storage with excellent stability and durability. The Co-Ni-S-170 composite electrode's fast ion intercalation and strong redox activity, combined with AC's porous structure and high conductivity, deliver excellent performance in the Co-Ni-S-170//AC SC device. Fig. 4h shows Nyquist plots before and after 5000 stability cycles. The lower internal resistance values of 0.75 and 0.82 Ω with lower charge transfer resistances of 1.5 and 1.63 Ω before and after stability, respectively, indicate superior energy storage capabilities with extraordinary cycling stability and high structural integrity.^35,36 The slight rise in R_ct shows the active material's durability and efficient charge transfer, boosting the SC device's performance. The mid to low-frequency linear section at 60–70° indicates a mix of diffusion control (Warburg impedance) and capacitive behavior (double-layer capacitance), confirming the supercapattery mechanism seen in CV and GCD results. Energy and power densities are calculated using eqn (S3) and (S4), and the results are shown in the Ragone plot (Fig. 4i). The device delivered the highest and ultra-high energy density of 172 Wh kg⁻¹ at a power density of 400 Wh kg⁻¹. Further, these results are compared with other devices ZnCo₂O₄@Ni-Co-S (53.1 Wh kg⁻¹@1125 W kg⁻¹),³⁷ Ni₃S₂/CoNi₂S₄/NF (50.7 Wh kg⁻¹@1594.1 W kg⁻¹),³⁸ NiCoS@NCNi@CF (64.77 Wh kg⁻¹@420.13 W kg⁻¹),³⁹ (CoMoS) (37.14 Wh kg⁻¹@700 W kg⁻¹),⁴⁰ Se-CoNi₂S₄ (44.6 Wh kg⁻¹@827 W kg⁻¹),⁴¹ and Ov-NCM (71 Wh kg⁻¹@801 W kg⁻¹).⁴² Even at an elevated power output of 7400 W kg⁻¹, the device maintained an energy density of 92.5 Wh kg⁻¹, signifying a ∼18.5% enhancement in power with a merely ∼47% decrease in energy. A key highlight of this study is the device's excellent cyclic stability, maintaining over 97% capacity retention in both three-electrode (Table S2) and two-electrode setups. Such high stability is crucial for real-world use since SCs often undergo frequent charge–discharge cycles in applications like hybrid energy storage, electric vehicles, and renewable energy systems. Without durable cyclic performance, efficiency would decline over time, leading to shorter device lifespan and higher maintenance costs. Therefore, long-term reliability and consistent energy delivery are essential for practical viability.

Fig. 4 (a) Schematic diagram of the assembled Co-Ni-S-170//AC ASC. (b) The Co-Ni-S-170 and AC CV curves at a scan rate of 5 mV s⁻¹, (c) CV curves, (d) CV curves (5 to 50 mV s⁻¹), (e) GCD curves (1 to 20 A g⁻¹). (f) Specific capacitance values, (g) stability at a current density of 10 A g⁻¹. (h) The EIS spectrum of the ASC device before/after cycling, (i) Ragone plot, (j) photo of two ASC devices in series, lighting different colored LEDs (3.2 V), and (k) after 10 minutes.

The energy and power densities of this device are compared in Fig. 4i with various energy storage technologies, including Li-ion, Ni–MH, Ni–Cd, Pb–acid batteries, EDLCs, conventional capacitors, and hybrid-ion capacitors. The device achieves energy densities comparable to the lower end of Li-ion batteries and matches or exceeds commercial batteries like Ni–MH, Ni–Cd, and Pb–acid. It outperforms EDLCs and traditional capacitors in energy-power capabilities. Its power density reveals that hybrid-ion capacitors, such as Li-ion, Na-ion, and K-ion types, surpass most conventional batteries. This combination of high-energy capacity from metal-ion batteries and rapid power delivery from SCs positions it as a strong candidate for next-generation energy storage technologies. Finally, the practical utility of the solid-state SC devices was proved by connecting them in series, achieving a working voltage of 3.2 V (Fig. 4j). The SC devices effectively powered different colored LEDs (green, blue, and red) for a long period, displaying their strong energy storage capabilities and stability. This remarkable performance may be ascribed to the optimized electrode materials, which offer high surface-active sites, efficient ion transport, and improved redox activity, resulting in superior charge storage and quick charge/discharge cycles. Additionally, the low internal resistance and stable electrochemical interface ensure minimal energy loss and long-term durability. The device's high energy density and power output are the result of these variables working together, which makes it a strong contender for applications involving next-generation energy storage.

The ultra-high energy density achieved in our work is attributed to some key factors, such as (i) the Co-retained sulfurizing preserves cobalt sites, improving electrical conductivity, and facilitating improved charge transportation, (ii) the hierarchical porous structure optimizes active site exposure, leading to improved ion diffusion and charge storage capacity, (iii) the synergistic interaction between pseudocapacitive and battery type behaviour optimizes the overall charge storage capacity, (iv) the optimized interface achieves lower internal resistances to deliver longer discharge time, superior power density and cycling stability, (v) the electrode material doesn't degrade at 97% capacitance even after 5000 cycles, ensuring proper operation for a long time. All the arguments together result in an ultra-high energy density of 172 Wh kg⁻¹ for Co-Ni-S and make it an extremely promising future energy storage material.

To explore the structural and electronic properties of the Co-Ni-S composite electrode, in this investigation, we performed density functional theory (DFT) calculations using the Vienna Ab initio Simulation Package (VASP).^43,44 To balance accuracy and computational efficiency, a plane-wave basis set was used with a 500 eV cutoff energy to expand the electronic wavefunctions. The Perdew–Burke–Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) was chosen to describe the exchange–correlation interactions of electrodes.^44,45 Since spin effects in Co, Ni, and S are significant, a spin-polarized approach was applied for precise electronic structure calculations of Co-Ni-S. The self-consistent field (SCF) calculations were converged to 10⁻⁶ eV, and forces were relaxed to below −0.02 eV Å⁻¹ to ensure reliable equilibrium geometries of the electrode. The Brillouin zone was sampled using the Monkhorst–Pack scheme.^46,47 For the structural optimization of Co-Ni-S, a 5 × 5 × 1 k-point mesh was employed, providing adequate sampling for the accurate determination of equilibrium geometries. The corresponding optimized structure is illustrated in Fig. 5(a) and (b). The bond lengths are 2.21 Å for Co-S, 2.41 Å for Co-Ni, and range from 2.21 to 2.24 Å for Ni-S. These bond lengths indicate that the metal–sulfur interaction remains stable in the Co-Ni-S structure. Further, the stable interaction of Co with Ni₃S₂ is confirmed by the Charge Density Difference (CDD) map, which provides valuable insights into charge transfer processes between them. As shown in Fig. 5(c), the redistribution of charge density is depicted by the green region where electrons are accumulated, and blue regions show electron depletion zones, which indicate potential charge transfer between the Co, Ni, and S atoms of Co-Ni-S, which suggests strong electronic interactions. The presence of cobalt and nickel suggests the involvement of d-orbital electrons, which are crucial for redox reactions in the Co-Ni-S composite. The high electronegativity of sulfur atoms could facilitate electron transfer and enhance the electronic conductivity of Co-Ni-S. This combination of charge redistribution and strong metal–sulfur interactions could make this system a promising candidate for supercapacitor electrodes, offering high energy density. Additionally, in the Co-Ni-S structure, the Co atom has a Bader charge of 8.634e, indicating that 0.366e has been transferred from Co to Ni₃S₂. Within Ni₃S₂, sulfur atoms gain more charge from Co. This result also suggests that the Co interacts strongly with a sulfur atom of Ni₃S₂.

Fig. 5 (a) Top view and (b) side view of optimized Co-Ni-S system, and (c) charge density difference map (CDD), (d) band structure and density of states of Co-Ni-S composite.

Fig. 5d shows the density of states (DOS) and band structure of Co-Ni-S, where a denser 12 × 12 × 1 k-point mesh is utilized to achieve high-resolution electronic structure details of the Co-Ni-S electrode. The electronic band structure and DOS calculations for Co-Ni-S indicate its metallic nature, as multiple bands cross the Fermi level at 0 eV, confirming the absence of an electronic bandgap. The band structure along high-symmetry points Γ–M–K–Γ in the Brillouin zone reveals a complex dispersion with a high density of states near the Fermi level, suggesting strong electronic interactions. The spin-resolved DOS further highlights an asymmetry between spin-up (black) and spin-down (red) states, indicating spin polarization and potential magnetic behaviour. The DOS at the Fermi level suggests high electrical conductivity, which is influenced by hybridization between Co, Ni, and S atomic orbitals. These properties make Co-Ni-S composite a promising multifunctional material for energy storage and conversion applications, particularly in SCs and electrocatalytic systems, where high conductivity and charge transfer efficiency are critical factors.

3.3. Electrocatalytic water oxidation performance

In addition to energy storage performance, the versatile characteristics of the Co-Ni-S composite were tested for multidisciplinary energy applications such as OER performance. The electrocatalytic performance of the nanostructured samples for the OER was assessed by LSV at a scan rate of 5 mV s⁻¹ in 1 M KOH. The overpotentials needed to achieve a current density of 10 mA cm⁻² differed among the samples: Co-Ni-S composite synthesized at 150 °C, 160 °C, and 170 °C required 280 mV, 240 mV, and 210 mV, respectively (Fig. 6a and b). Among them, Co-Ni-S-170 °C showed the lowest overpotential of 210 mV at 10 mA cm⁻² (Table S3), highlighting its enhanced electrocatalytic activity and faster OER kinetics.⁴⁸ For comparison, NiFe-LDH shows an overpotential of 270 mV at 50 mA cm⁻² with a Tafel slope of 68.1 mV dec⁻¹ and ECSA of 120 cm²,⁴⁹ while NiFeIII(1:1)-LDH exhibits 382 mV at 50 mA cm⁻² with a Tafel slope of 31.1 mV dec⁻¹.⁵⁰ These results confirm that our Co-Ni-S electrode achieves excellent intrinsic OER activity, with low overpotential, favorable Tafel slope, and high ECSA, demonstrating that the interface-engineered structure effectively enhances active site utilization and electron transfer. The improved stability and efficiency of the Co-Ni-S-170 °C catalyst can be attributed to the full coverage of the CoNi alloy on sulfur, facilitating efficient electron transfer and promoting catalytic activity. Although full coverage improves initial activity, it also speeds up degradation during the OER, potentially reducing the catalyst's long-term stability. This enhanced performance suggests that simply utilizing the CoNi alloy as a catalyst is insufficient to provide an optimal amount of co-catalyst required for high efficiency. The reaction kinetics of the electrocatalysts were assessed using Tafel plots derived from their LSV polarization curves (Fig. 6c). The Tafel slopes measured for Co-Ni-S synthesized at 150 °C, 160 °C, and 170 °C were 54, 43, and 38 mV dec⁻¹, respectively. Notably, the Co-Ni-S-170 °C catalyst showed the lowest Tafel slope of 38 mV dec⁻¹, indicating superior reaction kinetics compared to the others. This reduced Tafel value is attributed to the efficient synthesis of Co-Ni-S-170 °C, which significantly accelerated the electron transfer process, thereby enhancing its OER electroactivity. EIS analysis, fitted with the circuit in Fig. 6d, was used to assess the charge transfer resistance of the electrocatalysts. Among the tested samples, the Co-Ni-S-170 °C electrocatalyst exhibited the lowest charge transfer resistance (R_ct) of 15 Ω, compared to 16.8 Ω for Co-Ni-S-150 °C and 20.6 Ω for Co-Ni-S-160 °C (Fig. 6d). The EIS plot data values of all the samples are supplied in Table S1. This lower R_ct value indicates superior charge transfer capability, contributing to enhanced OER kinetics and overall efficiency. The C_dl was evaluated via CV at different scan rates in the non-faradaic region, as shown in Fig. S17. The estimated C_dl values were 56.82 mF cm⁻² for Co-Ni-S-170 °C, 36.58 mF cm⁻² for Co-Ni-S-160 °C, and 20.08 mF cm⁻² for Co-Ni-S-150 °C. Among these, Co-Ni-S-170 °C exhibited the highest C_dl value (Fig. 6d), indicating a significantly larger electrochemically active surface area (ECSA). Additionally, the ECSA values calculated for the electrocatalysts further support this trend, with Co-Ni-S-170 °C showing the highest ECSA at 1420.5 cm², compared to 914.5 cm² of Co-Ni-S-160 °C and 502 cm² of Co-Ni-S-150 °C (Fig. 6e). The larger ECSA of Co-Ni-S-170 °C enhances its electrocatalytic activity and OER efficiency. Long-term durability was assessed via chronopotentiometry at 100 and 300 mA cm⁻² for 50 hours (Fig. 6g), confirming stable performance. The Co-Ni-S-170 °C electrocatalyst exhibited excellent durability, maintaining stable OER performance throughout the entire test. However, a slight decline in performance over time was observed (insert Fig. 6g), likely to be due to surface oxidation or nominal degradation of the catalyst. Even with this slight decrease, the Co-Ni-S-170 °C electrocatalyst outperformed the other catalysts in long-term stability, reinforcing their efficiency and reliability for OER applications. For further confirmation, post-stability characterizations were conducted by XRD, SEM, and XPS measurements to find possible sulfur degradation and phase transformations. The XRD pattern of Co-Ni-S recorded after the OER stability test shows slight but not noticeable changes compared with the pristine sample, reflecting a minor phase transformation during prolonged electrochemical operation. The characteristic reflections of Ni₃S₂ (PDF#73-0698) and Co₉S₈ (PDF#65-6801) remain visible, yet the relative intensity and position of some peaks exhibit small shifts, indicative of structural rearrangement and local lattice distortion (Fig. S18). Such changes suggest that partial reconstruction occurs at the catalyst surface, driven by the harsh oxidative environment, while the bulk phase largely retains its crystallinity. The slight phase shift implies a redistribution of sulfur coordination and subtle modifications in the Co-Ni-S-S lattice, which may be associated with sulfur depletion and progressive replacement of S²⁻ by O²⁻/OH⁻ species during OER. The SEM images recorded after the OER stability test demonstrate that the overall nanosheet-like morphology of the Co-Ni-S catalyst is largely preserved, indicating good structural robustness under prolonged electrochemical operation (Fig. S19).

Fig. 6 Electrocatalytic OER performance. (a) LSV curves (95% iR correction) of Co-Ni-S-150 °C, Co-Ni-S-160 °C, and Co-Ni-S-170 °C in 1.0 M KOH. (b) Overpotential, (c) Tafel plots derived from the LSV curves in (a). (d) EIS measurements of Co-Ni-S-150 °C, Co-Ni-S-160 °C, and Co-Ni-S-170 °C at 1.44 V 1.0 M KOH. (e) C_dl values, (f) ECSA, and (g) Long-term stability analysis of Co-Ni-S-170 °C in 1 M KOH (inset: before and after stability LSV curves).

In addition, the XPS spectra of Co-Ni-S before and after the OER stability test clearly demonstrate a uniform positive binding-energy shift across Co 2p, Ni 2p, S 2p, and O 1s regions, reflecting a decrease in electron density around the metal centers and stronger interaction with electronegative oxygen (Fig. S20). After stability, the Co 2p spectrum shows a relative enrichment of Co³⁺/Co⁴⁺ components at the expense of Co²⁺, while the Ni 2p spectrum displays a parallel increase in Ni³⁺ contribution, both accompanied by attenuated satellite peaks, signifying surface oxidation into CoOOH and NiOOH phases. In the S 2p region, a decrease in the sulfide signal alongside the emergence of oxidized-S species indicates sulfur depletion and conversion to SO_x, further exposing metal centers to oxygen coordination. The O 1s spectrum broadens considerably after stability, with dominant –OH and M–O components consistent with the formation of hydrated oxy (hydroxide) layers on the surface. Importantly, the observed positive shifts suggest that the metals become more electron-deficient, thereby lowering the Fermi level and increasing M–O covalency, which in turn enhances the adsorption and activation of OER intermediates (*OH, *O, and *OOH). This electronic restructuring not only improves charge transfer but also stabilizes the higher-valence redox couples (Co³⁺/Co⁴⁺ and Ni²⁺/Ni³⁺) that are directly involved in the OER cycle. Thus, the positive binding-energy shift serves as clear evidence of an in situ reconstruction process, where the Co-Ni-S precursor evolves into an oxy (hydroxide)-rich surface that optimally balances electronic conductivity, active-site availability, and catalytic durability. Overall, the Co-Ni-S-170 °C electrocatalyst is a promising candidate for effective and stable OER activity, which is confirmed by the low overpotential, suitable Tafel slope, lower R_ct values, large electrochemically active site area, and superb long-term stability.

Finally, the sulfurization conditions were found to play a pivotal role in dictating the surface structure and electrochemical behavior of the electrode. Controlled sulfurization not only ensured the formation of strong metal–sulfur bonds but also introduced sulfur vacancies that acted as additional redox-active sites. Moreover, the process triggered surface reconstruction, producing a defect-rich and porous morphology that exposed a higher density of electrochemically accessible sites. This optimized surface structure facilitated rapid ion transport and accelerated charge transfer, thereby contributing to the enhanced capacitance and catalytic activity of the electrode.

4. Conclusion

In summary, this study presents a robust strategy for designing multifunctional electrode materials by engineering the interface of cobalt–nickel sulfides through a two-step route involving electrodeposition and a controlled sulfurization process. The resulting Co-Ni-S composite nanosheet architecture effectively combines high electrical conductivity, abundant electroactive sites, and structural stability. These features contribute to a remarkable specific capacitance of 3586 F g⁻¹ and excellent cycling durability, with 97% capacity retention after 5000 charge–discharge cycles. Beyond its performance as a supercapacitor electrode, the Co-Ni-S composite material also exhibits strong catalytic activity for the OER, achieving a low overpotential of 210 mV at 10 mA cm⁻² and maintaining stable performance over extended operation. DFT calculations further support the experimental findings, confirming stable metal–sulfur bonding and efficient charge transfer within the heterostructure. The combination of pseudocapacitive and battery-type behavior, facilitated by tailored interfacial properties and structural defects, underlines the material's versatility. These insights offer a promising pathway for developing next-generation energy storage and conversion devices that demand both high energy density and long-term reliability.

Author contributions

Samikannu Prabu: conceptualization, methodology, data analysis, writing draft, and review – editing. Subramani Mohanapriya: conceptualization, methodology, data analysis, and writing draft. M. R. Pallavolu: data validation; visualization; software, review – editing. Bor Kae Chang: conceptualization, revising, and supervision. Kung-Yuh Chiang: conceptualization, revising, and supervision.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

Additional raw data or computational files (if applicable) can be made available from the corresponding author upon reasonable request.

The data that support the findings of this study are available within the article and its supplementary information (SI). Supplementary information: contains materials, electrochemical analysis, fabrication of solid state asymmetric supercapacitor, electrochemcial measurements of OER, characterization techniques, CV of electrodeposited CoNi alloy and varying with Co/Ni ratio with XRD patterns, FESEM studies, TEM-EDS of CONIS-170, electrochemial studies of CoNi alloy, CoNiS-170, and AC, OER studies of CoNiS and their XRD, FESEM and XPS after stability test, and tables of comparative energy storage performance and OER results. See DOI: https://doi.org/10.1039/d5ta06065f.

Acknowledgements

The authors acknowledge the grant NSTC-110-2211-E-008-042-MY3 and NSTC-113-2218-E-008-010 of the National Science and Technology Council (NSTC) of Taiwan to support this study financially.

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