Optimization of CNT-induced Ni₉S₈ nanoplates for balanced charge-transfer kinetics and conductivity in high-performance battery-type supercapacitors

Abstract

The growing need for sustainable and eco-friendly energy solutions has intensified the efforts to develop advanced electrode materials with superior electrochemical traits. In this study, we synthesized Ni₉S₈ (NS), and its carbon nanotube (CNT) based composites containing 3, 6, and 9% CNTs (NS-3, NS-6, and NS-9) for supercapacitor applications. NS exhibited an initial plate-like morphology, which transformed into a CNT-entangled structure in composites. NS-6 showed an increased surface area of 61.72 m² g⁻¹, reflecting the coexistence of both micro and mesopores. Cyclic voltammetry revealed well-defined redox peaks across multiple scans, signifying a hybrid charge storage behavior, as validated through Dunn's model analysis. Among all composites, NS-6 showed the best electrochemical performance, achieving a specific capacity of 1382.47 C g⁻¹ at 2.0 A g⁻¹ and excellent cyclic stability with 95.5% retention after 10 000 cycles. The composite also presented enhanced ionic conductivity (4.4 × 10⁻² S cm⁻¹), reduced charge transfer resistance (2.51 Ω), and a diffusion coefficient of 7.48 × 10⁻¹¹ cm² s⁻¹. Based on these optimized results for NS-6, an asymmetric device was assembled, delivering 301.11 C g⁻¹, an energy density of 50.18 Wh kg⁻¹, and a power density of 705.88 W kg⁻¹. These results highlight NS-6 as a leading contender for efficient electrode materials for future energy solutions.

Hierarchical Ni₉S₈-based nanoarchitecture with enhanced redox activity, charge storage and cyclic stability.

1. Introduction

The escalating global population and rapid technological advancements have significantly intensified energy demands. Dependence on conventional fossil fuels has, in turn, resulted in substantial greenhouse gas emissions, primarily carbon dioxide and methane, driving climate change and disturbing the planet's ecological equilibrium.^1,2 The finite nature of fossil fuel reserves and their severe environmental consequences necessitate a transition to sustainable and environmentally friendly energy sources such as wind, solar, and hydropower to achieve carbon neutrality.^3,4 Nevertheless, the intermittent nature of these renewable sources poses a significant challenge to their large-scale integration into the power grid. To maintain a consistent supply and stable output, energy harvested from such variable sources must be efficiently stored and delivered on demand. This need has prompted extensive research into advanced energy storage devices (ESDs) with enhanced energy and power capabilities, improved durability, and higher overall efficiency, reflecting their increasing commercial relevance.^5–7 Thus, the development of innovative materials and technologies capable of effectively storing and utilizing renewable energy is pivotal in the global pursuit of a carbon-free economy.^8,9

Extensive research has been devoted to energy storage technologies, including batteries, capacitors, and supercapacitors (SCs). Among these, SCs have emerged as a focal point due to their ability to bridge the performance gap between conventional capacitors and batteries.^9,10 Their high-power density (P_d), excellent reversibility, and broad operational temperature range make them a better choice for applications such as backup power systems and microelectronic devices.¹¹ Based on the electrode material and process of charge storage, SCs are broadly categorized into two different classes. Electric double-layer capacitors (EDLCs) employ carbon-based materials to store charge through electrostatic interactions, enabling ultrafast charge/discharge processes but relatively low energy density (E_d).¹² In contrast, pseudo-capacitors (PCs) store charge via rapid surface or near-surface redox reactions, typically using transition metal oxides (TMOs), transition metal sulfides (TMSs), nitrides, hydroxides, or conducting polymers. Although PCs offer significantly high energy storage capacity, these are often limited by slower charge/discharge rates and shorter cycling stability.^13,14 To overcome these limitations, hybrid capacitors have been developed by integrating EDLC and PC materials into composite electrodes. This hybrid configuration exploits a synergistic effect, achieving a balance between E_d and rapid charge/discharge performance.^15,16 The efficiency of SCs is reliant on the nature and the morphology of the electrode materials. Two-dimensional (2D) materials offer extensive surface area and abundant active sites, facilitating enhanced redox reactions. There are different electrode materials used in SCs, such as metal–organic frameworks (MOFs), TMOs, TMSs, carbon nanotubes (CNTs), and activated carbon (AC). Among these, MOFs are often limited by poor conductivity, while conductive polymers typically suffer from mechanical instability. Similarly, TMOs face challenges due to subpar mechanical durability and limited electron transport traits.¹⁷ Conversely, TMSs offer outstanding theoretical capacity values and rich redox chemistry enabled by variable oxidation states. Due to the inherently low electronegativity of sulfur, TMSs adopt adaptable structural configurations that facilitate both electron mobility and ion transport, thereby boosting overall electrochemical (EC) attributes. Notably, mono-metallic sulfides such as Co₃S₄, MoS₂, Ni₃S₄, and Ni₉S₈ are readily synthesized, facilitated by the low electronegativity of sulfur. Among these, Ni₉S₈ (NS) has emerged as a particularly attractive prospect for use in SC applications, due to its rich chemical complexity, excellent electrical conductivity, high specific capacitance (C_p), and environmental benignity.^18,19

Despite their promising EC properties, TMSs still encounter several challenges, including limited cycling stability, particle aggregation, and sluggish charge transfer kinetics. To address these limitations, the incorporation of porous electrode materials has proven effective in enhancing EC performance by increasing electrolyte-accessible surface area and maximizing the utilization of active sites. In this context, carbon-based materials like AC, graphene, and CNTs have received extensive recognition due to their specific surface area (SSA) and excellent electrical conductivity. Furthermore, CNTs stand out for their tubular structure, exceptional electrical conductivity, robust mechanical properties, and enhanced structural stability.^20–22

In this context, Fang et al. prepared MnCo₂O₄/Ni₃S₄ nanocomposites by a hydrothermal process. At the current density (I_d) of 1 A g⁻¹, MnCo₂O₄/Ni₃S₄ demonstrated a specific capacity (Q_SC) of 904.7 C g⁻¹ with a higher E_d of 50.7 Wh kg⁻¹ by providing an outstanding P_d of 405.8 W kg⁻¹.²³ Likewise, Mohammadi et al. prepared a Co₃S₄/CNTs hybrid material by the hydrothermal technique, in which the composite provides an exceptional C_p of 966 F g⁻¹ at 1 A g⁻¹. Furthermore, 43.5 Wh kg⁻¹ of E_d was provided by the material at a P_d of 391 Wh kg⁻¹. Additionally, the electrode exhibited 90% of its initial C_p after 3000 charge–discharge intervals.²⁴ Subsequently, Wang et al. reported Ni₃S₄ by utilizing a hydrothermal procedure, which delivered a C_p of 1797 F g⁻¹ at 0.5 A g⁻¹.²⁵ Hierarchical Ni₃S₄/MoS₂ nanostructures were synthesized by Zhang et al. by the hydrothermal method. Ni₃S₄/MoS₂ nanospheres exhibit excellent C_p of 1440.9 F g⁻¹ and retain 90.7% capacitance retention at 2 A g⁻¹.²⁶ Recently, Huang et al. reported a nanoflower of Ni₃S₄/MoS₂, which exhibited an excellent C_p of 1296 F g⁻¹ with good cyclic stability.²⁷

In this study, NS and its composites incorporated with varying percentages of CNTs (3, 6, and 9%) were synthesized via hydrothermal and solvothermal processes and labelled as NS-3, NS-6, and NS-9, respectively. This study establishes a systematically optimized morphology and CNT concentration in the NS matrix, revealing the critical balance between conductive network formation and active surface utilization for enhanced electrochemical performance.²⁸ Unlike prior reports, the work provides a quantitative correlation between CNTs concentration and charge transport behavior identifying optimal composition governing electrochemical efficiency. Furthermore, a distinctive aspect of this work lies in the rigorous extraction and analysis of ion diffusion coefficients using the galvanostatic intermittent titration technique (GITT), which, to the best of our knowledge, remains largely unexplored for NS–CNT composites. This enables a deeper insight into the charge storage mechanism along with the ion diffusion dynamics of electrode materials. These findings provide valuable insights and guidance for researchers in shaping future directions.

2. Experimental

2.1. Preparation of NS

The NS material was prepared through the hydrothermal method to ensure a controlled and uniform reaction environment. In a typical procedure, stoichiometric amounts of nickel (Ni) nitrate and thiourea were dispersed in deionized (DI) water, followed by the addition of ethylene glycol (a few drops). Cetyltrimethylammonium bromide (CTAB) served as both a stabilizer and a structure-directing reagent. The resulting solution was stirred for 30 min to achieve complete homogeneity. The uniform solution was subsequently placed in a Teflon-lined stainless-steel autoclave and exposed to hydrothermal processing at 150 °C for 16 h. Upon completion, the autoclave was allowed to cool naturally to normal temperature. The precipitates obtained were collected and washed several times with ethanol and DI water to eliminate residual impurities and unreacted precursors. After purification, the final product was dried on a hot plate at 70 °C overnight to ensure the elimination of residual moisture and solvents, thereby yielding the final NS material.

2.2. Synthesis of NS/CNTs

A solvothermal chemical method was employed to synthesize the NS/CNTs composites with varying concentrations of CNTs. Initially, CNTs were dispersed in 40 mL of ethanol and continuously stirred for 30 min to achieve uniform dispersion. Subsequently, pre-synthesized NS material was gradually added to the CNTs suspension under constant stirring to ensure a homogeneous mixture. To further enhance the interaction between NS and CNTs, the resulting mixture was subjected to ultrasonic treatment in a sonication bath for 1 h. After the sonication, the uniform suspension was introduced into a Teflon-lined autoclave and heated in a muffle furnace at 150 °C for 3 h. Upon completion of the reaction, the autoclave was allowed to cool naturally to normal temperature. The resulting product was collected and washed several times with ethanol and DI water using centrifugation. The final product was then dried on a hot plate at 70 °C until complete solvent evaporation, yielding the NS/CNTs composite powder.²⁹ The entire synthesis process is presented in Fig. 1.

Fig. 1. Schematic illustration of the synthesis process, (a) Ni₉S₈ by the hydrothermal method, and (b) Ni₉S₈/CNT composites by the solvothermal method.

2.3. Electrode fabrication and slurry preparation

The electrode fabrication process began by cutting Ni foam (NF) into pieces measuring 1 × 1.25 cm². NF was selected as the substrate due to its low cost, high surface area, and porous structure, which are beneficial for efficient charge transport and strong adhesion of active materials. To remove surface oxides and improve conductivity, the NF pieces were etched using an acid solution composed of 37 mL DI water and 13 mL hydrochloric acid (HCl). The NF was immersed in this solution and sonicated for 1 h to ensure effective surface cleaning and activation. Next, a binder solution was developed by diluting 0.3 g of polyvinylidene fluoride (PVDF) in 15 mL of dimethylformamide under magnetic stirring at 250 rpm. The electrode slurry was then formulated by mixing the active material (80%), AC (10%), and PVDF binder solution (10%). This mixture was stirred continuously for 8 h at 250 rpm to guarantee uniform dispersion of components. The prepared slurry was uniformly coated onto the pretreated NF substrates and dried in an oven at 60 °C to ensure complete solvent evaporation and firm adhesion of the active layer.^28,29 The loaded mass (active material) of the electrodes was carefully measured before and after the deposition, yielding a net mass of around 3 ± 0.1% mg cm⁻².

3. Characterization techniques

Structural attributes of the prepared samples were investigated using a Bruker D8 Advance X-ray diffractometer (XRD; λ = 0.154 nm) and Raman spectroscopy (Renishaw; λ = 514 nm). The morphological and microstructural features were examined using the Nova-Nano SEM-450 model of scanning electron microscopy (SEM), and the JEOL (JEM-2100) instruments were used to perform transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analysis to investigate detailed morphology and structural properties. Additionally, the surface area and pore volume were analyzed by the adsorption–desorption of nitrogen (N₂) gas using the Tristar-II 3020 analyzer. An Oxford Instruments energy-dispersive X-ray spectroscopy (EDX) was employed to investigate the elemental composition. For the evaluation of EC characterizations of prepared samples, an EC workstation CS 350M (Corrtest Electrochemical & Corrosion Studio, 6.4) was utilized.³⁰ A 1 M KOH solution was used as an electrolyte in this setup. Additionally, asymmetric assembly was used to perform EC investigations and diffusion tests to elucidate the performance of all the electrodes.

4. Results and discussion

4.1. Structural analysis

To investigate the crystal structure of NS and its composites (NS-3, NS-6, and NS-9), XRD analysis was employed, as shown in Fig. 2(a). These analysis were performed using the prepared electrodes, with Ni foam serving as a substrate. Initially, the sharp and intense peaks of Ni-related diffraction were observed in all samples, which are consistent with the ICSD card no. 00-004-0850. To see the crystallographic attributes of the NS sample, a zoomed-out diffraction region is shown along with the original diffraction pattern. For the NS sample, prominent diffraction peaks were observed approximately at 2θ values of 31.9°, 37.4°, 40.2°, 47.5°, 51.3°, 57.7° and 73.6°, corresponding to the (132), (240), (114), (243), (261), (154), and (227) planes, respectively. These peaks are consistent with the ICSD card no. 00-022-1193, confirming the orthorhombic crystal structure. The low diffraction from these planes is attributed to the lower mass fraction of the active material compared to the Ni foam, as mentioned in the synthesis section. In addition, the data were used to design the crystal structure of pristine NS using VESTA software, as is presented in Fig. 2(b). The XRD patterns of the NS-3, NS-6 and NS-9 exhibited the same pattern as for the pristine NS without noticeable change in the peak position and intensity upon integration of CNTs. This shows that the addition of CNTs does not alter or change the crystal structure of the NS matrix.³¹ Moreover, no additional peaks appeared in the XRD spectra of all the composites, which is again due to the lower concentrations of CNTs in the active material on the Ni substrate. A similar patterns observed for all compositions, indicating that the incorporation of CNTs doesn't significantly change the structure of the NS matrix.

Fig. 2. (a) XRD patterns of NS, NS-3, NS-6, and NS-9, (b) crystal structure of NS, (c) RAMAN analysis of NS-6, and (d) EDX spectra of all the prepared samples.

4.2. Raman spectroscopy

Raman spectroscopy is a powerful, non-destructive technique widely employed to understand the lattice vibrations, low-frequency modes, phase purity, and chemical bonding in materials. It is specifically effective for differentiating structural defects, metal sulfides, and carbonaceous materials, making it markedly suitable for the examination of composite materials. Herein, a Raman study was performed to affirm the phase development of the individual constituents and to validate structural interactions inside the composites. The Raman spectrum of the NS-6 material shows three prominent peaks characteristic of NS positioned at approximately 275, 373 and 456 cm⁻¹, as shown in Fig. 2(c). The lower frequency band at 275 cm⁻¹ is assigned to low-frequency acoustic modes involving NS lattice vibrations, whereas peaks at 373 cm⁻¹ correspond to symmetric NS stretching vibrations. The intense and sharp band observed at 456 cm⁻¹ corresponds to antisymmetric NS stretching modes arising from out-of-phase vibrations within the sulfide framework.³² These characteristics of the Raman features confirm the successful preparation of the NS phase. In addition, two extra peaks appeared at 1347 cm⁻¹ and 1553 cm⁻¹, which correspond to the integration of CNTs within the composite. The peaks appeared at 1347 cm⁻¹ and 1553 cm⁻¹, corresponding to the D and G bands of CNTs, respectively.^33,34 The D band arises from defect-induced vibrations and lattice imperfections, while the G band appears due to the stretching vibration of sp²-hybridization carbon atoms.³⁵ The presence of these bands indicates the successful addition of CNTs into the composite without introducing additional impurity phases.

4.3. Compositional analysis

EDX is a well-known technique used for identifying and quantifying the elemental composition of a sample, offering reliable detection of all the elements with high sensitivity. Fig. 2(d) presents the EDX spectra of NS, NS-3, NS-6, and NS-9. The EDX results confirmed the presence of Ni, and S, with elemental ratios in close agreement with the expected stoichiometric values. Furthermore, C and Au related peaks were also detected in the EDX spectrum that are not associated with the synthesized material, but these peaks are due to the carbon tape and gold coating utilized for sample mounting and surface coating prior to this analysis. For the NS sample, the presence of Ni and S in the spectrum verifies the successful formation of the intended composition. The EDX spectrum of NS reveals no additional peaks, confirming the absence of contamination in the prepared material. In the NS/CNTs composites, a slight increase in the intensity of carbon peaks is observed with the higher CNTs content, validating the successful incorporation of CNTs into the pristine NS framework.³⁶

4.4. Morphological analysis

SEM analysis was employed to investigate the morphological characteristics of the synthesized samples, which play a crucial role in determining their EC performance. Fig. 3(a and b) presents the SEM images of NS demonstrating plate-like morphology with interconnected plates, moderate porosity, and surface roughness, offering reasonable ion transport and surface area suitable for dual redox reactions.³⁷ The presence of CTAB directed anisotropic growth, while urea provided a controlled alkaline environment for uniform nucleation, resulting in well-defined nanosheets. In contrast, the SEM images of the NS-3 composite, as presented in Fig. 3(c and d), reveal a highly porous, fibrous structure, suggesting improved ion diffusion and possibly enhanced redox activity due to its better electrolyte interaction. This morphological evolution is driven by thermal energy, promoting reorganization, crystallite growth, and the integration of CNTs, which prevents dense stacking, enhances conductivity, and creates a more open framework. The resulting structure provides greater electrolyte accessibility, more active sites, and improved ion transport pathways. In the same way, SEM images of NS-6 in Fig. 3(e and f) displayed a highly porous, well-integrated CNTs network that can offer improved conductivity and electrolyte accessibility.^38,39 Further increasing the CNTs content to 9% resulted in noticeable clustering between the tubular CNTs and NS nanoparticles, as observed in Fig. 3(g and h). This clustered morphology likely hinders efficient ion diffusion to the electrode surface, thereby leading to reduced EC performance. The agglomeration of CNTs can be attributed to the excessive CNTs loading itself, which promotes self-entanglement and particle bridging. Such aggregation not only reduces the effective surface area but also disrupts the formation of continuous conductive pathways.⁴⁰

Fig. 3. (a–h) SEM images of NS, NS-3, NS-6, and NS-9 across various magnifications, (i–k) TEM and HRTEM analysis of NS-6, (l–o) elemental mapping of NS-9, and (p and q) BET analysis of NS and NS-6.

The microstructure and crystallinity of the NS-6 composite were further examined by TEM. The TEM images illustrated in Fig. 3(i and j) clearly exhibit the porous architecture of thin, plate-like nanostructures, consistent with the SEM analysis. These nanoplates are uniformly distributed and interconnected, developing a porous architecture. Moreover, the CNTs can be observed as elongated fibrous structures entangled with the NS nanoplates, providing channels for optimal ion transport. This unique architecture facilitates efficient electrolyte penetration and provides shortened ion diffusion pathways, which are beneficial for enhanced electrochemical performance. The HRTEM image of the NS-6 composite, as presented in Fig. 3(k), reveals well-defined lattice fringes, confirming the crystalline nature of the material. The measured interplanar spacing of 0.26 nm corresponds to the (312) crystal plane of the NS material, which agrees well with standard crystallographic data.⁴¹ The clear lattice fringes further authenticate the high crystallinity of the nanoplates. In addition, the intimate interfacial contact between CNTs and NS nanoplates at the nanoscale is expected to enhance charge transfer kinetics and ionic conductivity, thereby improving the overall electrochemical response of the composite. Furthermore, an elemental mapping test for NS-6 was carried out to examine the spatial distribution of its constituent elements, with the findings depicted in Fig. 3(l–o). The mapping indicates the uniform dispersion of expected elements such as Ni, S, and carbon throughout the fragmented composite.

4.5. Brunauer–Emmett–Teller analysis

The SSA of the electrode materials plays a crucial role in determining the EC behavior of SCs. To examine the porous structure of these materials, nitrogen (N₂) adsorption–desorption analysis is commonly used. The resulting isotherms offer valuable insight into textural parameters, including SSA and pore size distribution.⁴²Fig. 3(p and q) displays the N₂ adsorption–desorption curves for NS and NS-6, along with their corresponding pore size profiles. Based on Brunauer–Emmett–Teller (BET) calculations, the SSAs of NS and NS-6 are determined as 56.34 and 61.72 m² g⁻¹, respectively. The larger SSA of NS-6 indicates a greater number of accessible active sites for ion adsorption, potentially improving SC performance. At low relative pressures (P/P₀), a nearly linear rise in nitrogen uptake is noticed, reflecting monolayer formation on the internal pore surfaces. As P/P₀ approaches 0.8, the curves level off, suggesting the coexistence of micro and mesopore structures. The sharp increase in gas adsorption within the P/P₀ range of 0.0–0.01 reveals considerable interlayer porosity, which is advantageous for both gas diffusion and ionic transport. The pore size distributions, derived from the Barrett–Joyner–Halenda (BJH) method using the adsorption branches, indicate cumulative pore volumes of 0.089 and 0.067 cm³ g⁻¹ for NS and NS-6 within the 13.6–14.8 nm range. Such pore volumes support efficient ion mobility during EC cycling.

4.6. Cyclic voltammetry analysis

To investigate the charge storage mechanism of the synthesized samples, cyclic voltammetry (CV) was employed. The CV profiles of NS, NS-3, NS-6, and NS-9 were recorded within a potential range of 0 to 0.5 V. The possible reaction pathway during the redox reaction is illustrated in Fig. 4(a). To evaluate the rate-dependent EC behavior, CV measurements were carried out at sweep rates of 1, 2, 4, 6, 8, and 10 mV s⁻¹, presented in Fig. 4(b–e). Additionally, CV loops retain their distinct shapes regardless of the sweep rate escalation, pointing to a strong affinity between the electrolyte species and the electrode surface. This stability supports consistent performance across varied conditions.⁴³ Moreover, the presence of well-defined redox peaks within the CV loops at all scan rates confirmed the co-existence of faradaic and non-faradaic processes, supporting the presence of hybrid charge storage mechanisms. When NS is combined with OH⁻ ions, it shows the following redox reaction, with a change in oxidation state from Ni²⁺ to Ni³⁺, and the reaction mechanism is given below.^44,45Ni₉S₈ + OH⁻ ⇌ Ni₉S₈OH + e⁻Ni₉S₈OH + OH⁻ ⇌ Ni₉S₈O + H₂O + e⁻

Fig. 4. (a) Schematic illustration of the reaction pathway, cyclic voltammograms of (b) NS, (c) NS-3, (d) NS-6, (e) NS-9 at different scans, (f) comparison of CV loops of all the samples at a scan rate of 1 mV s⁻¹, and (g) variation in specific capacity for all samples with scan rate.

In the first step, NS is combined with OH⁻ and loses a single electron, with Ni₉S₈OH as a byproduct. In the second step, the byproduct is again combined with OH⁻ and releases a single electron, and Ni₉S₈O and H₂O are byproducts. During reduction, the whole above process inverts and results in Ni₉S₈. The anodic peaks for NS, NS-3, NS-6, and NS-9 appeared within the potential intervals of 0.42–0.49, 0.42–0.50, 0.41–0.48, and 0.40–0.49 V, respectively, and the corresponding cathodic peaks were detected between 0.17–0.24, 0.15–0.23, 0.15–0.22, and 0.13–0.22 V, over all sweep rates. Notably, the redox peaks persist even at 10 mV s⁻¹, highlighting the electrode's superior tolerance to fast kinetics. Analysis of the voltammetric profiles shows consistent peak symmetry across all scan rates, which reflects highly reversible redox activity and a reliable charge storage mechanism. Minor shifts of cathodic peaks, slightly moving to lower voltages and anodic peaks towards higher potential, suggest a rapid yet reversible charge carrier transport dynamic. This implies that the active component within the electrode promotes enhanced ion diffusion and streamlined electron mobility within the EC setup. A collective CV representation of all materials at 1 mV s⁻¹ is displayed in Fig. 4(f). Moreover, Fig. 4(g) demonstrates that as the scan rate increases, the Q_SC of all synthesized materials decreases. A gradual reduction in Q_SC with an increase in scan rates suggests that at the lower sweep rates, the system allows adequate time for metal ions to diffuse thoroughly into the electrode framework, activating nearly all available redox sites, which results in enhanced capacity. However, when the scan rate is increased, the intercalation window narrows, restricting the EC activity mainly to surface layers. This limits access to internal active regions, leading to a decrease in the Q_SC values.

4.7. Dunn's model investigations

To check the charge storage mechanism theoretically, the Dunn model was applied by plotting the peak current (I_p) as a function of scan rate (v) for NS, NS-3, NS-6, and NS-9. This analysis provides critical insights into reaction kinetics and charge storage mechanisms, with the correlation quantitatively described by eqn (1) and (2).⁴⁶

I_p = av^b 1

I(v) = k₁v + k₂v^1/2 2

In this equation, a and b are adjustable parameters, where I_p represents the peak anodic or cathodic peak current and v represents the sweep rate. The b value plays a key role in the determination of charge storage behavior. The b value, which is close to 0.5, suggests diffusion-dominant processes, whereas a value of b close to 1 indicates capacitive behavior. If the value of b ranges between 0.5 and 1 or below the diffusive threshold 0.5, it highlights mixed or hybrid (battery type) behavior.⁴⁷ The log(I_p) vs. log(v) shown in Fig. 5(a–d) yields the value of b, which is 0.50, 0.52, 0.53, and 0.56, for NS, NS-3, NS-6, and NS-9, respectively, confirming the coexistence of diffusive as well as capacitive contributions. Eqn (2) was used to further separate the capacitive and diffusive-controlled contributions of each process. Herein, I(ν) denotes the peak current at a specific potential, whereas constants k₁ν and k₁ν^1/2 are the current from surface-controlled as well as diffusion-controlled processes, respectively.^48,49 Capacitive and diffusive contribution graphs of all prepared samples at the lowest scan rate are presented in Fig. 5(e–h). Moreover, the proportion of both diffusive and capacitive-controlled contributions at different scan rates is presented in Fig. 5(i–l). Fig. 5(i) shows that for pristine NS, the diffusive-controlled contribution dominates over the capacitive contributions, having 94% and 6%, respectively, at a lower scan rate of 1 mV s⁻¹. This is because the metal sulfides predominantly exhibit a diffusive-dominant nature, where charge storage is significantly contributed to by ion diffusion. At the sweep rate of 1 mV s⁻¹, the diffusive contributions are 94, 88, 85, and 81% for NS, NS-3, NS-6 and NS-9, respectively. This indicates that overall, the capacitive contribution increases as the CNTs content increases throughout the sample compositions. Fig. 5(i–l) shows that, as the scan rate increases, the diffusive-controlled behavior decreases among all synthesized samples, a trend attributed to restricted ion transport at higher sweep rates, which in turn accentuates the dominance of capacitive charge storage. Collectively, these analysis confirm that the charge storage arises from both faradaic and non-faradaic processes, highlighting the hybrid nature of these electrode materials.

Fig. 5. (a–d) Log of peak current versus log of scan rate plots of NS, NS-3, NS-6, and NS-9, respectively, (e–h) diffusive and capacitive participations at 1 mV s⁻¹ of all prepared samples, and (i–l) contribution ratios of capacitive and diffusive processes of all samples at different scan rates.

4.8. Charge–discharge investigations

To gain deeper insights into the EC performance and charge–discharge characteristics of the fabricated NS and NS/CNTs electrodes, galvanostatic charge–discharge (GCD) analysis was carried out over a stable voltage window of 0 to 0.5 V. The resulting GCD curves were revealed to be non-linear and consistent with CV findings, confirming the hybrid nature of the electrode materials.⁵⁰ The GCD profiles of NS, NS-3, NS-6, and NS-9 were explored under the varied I_d of 2.0, 3.1, 4.1, 6.2, 8.3, and 10.4, as depicted in Fig. 6(a–d). Notably, enlargement in the area beneath the charge–discharge curves indicates improved charge storage capability, pointing toward higher Q_SC values with increasing I_d. However, at higher I_d, a decreasing trend was observed. This reduction is attributed to shorter time intervals that hinder effective ion migration, limiting full access to internal active sites. Additionally, the drop in Q_SC at higher I_d stems from partial utilization of the electrode material, as the rapid process restricts ion transportation through the electrolyte medium.⁵¹Fig. 6(e) represents the comparison of all the prepared electrode materials at the lowest I_d of 2.0 A g⁻¹, demonstrating that NS-6 has the maximum discharge time among all the prepared samples. The consistent enhancement in charge storage capability was seen with the increased CNTs content up to 6%, attributed to exceptional electrical conductivity as well as the tubular structure of CNTs, which provides a path for fast ion kinetics. However, at 9% CNTs loading, a decrement in the storage capability was observed. This can be attributed to the excessive CNTs content, which sometimes acts as a blocking channel, restricting ion diffusion, ultimately leading to the inefficient EC response. Furthermore, the agglomeration of CNTs in NS-9 can form barriers that hinder ion transport into the porous electrode channels, leading to a reduction in discharge performance. The Q_SC values along with E_d and P_d were computed from the GCD data using eqn (3–5), respectively.

3

4

5

Here I is the applied current, Δt is the discharge time, and m is the active mass. The calculated Q_SC values at 2.0 A g⁻¹ are 794.18, 1029.68, 1382.47, and 813.97 C g⁻¹, for NS, NS-3, NS-6, and NS-9, respectively, as depicted in Fig. 6(f). The progressive enhancement in GCD performance with the increasing CNTs content up to 6% can be directly attributed to the intrinsic traits of CNTs. Their exceptional electrical conductivity establishes rapid electron pathways, while the high surface area ensures uniform dispersion of NS and exposure of electroactive sites. At 3% CNTs integration, the benefits began to appear but remained limited due to insufficient coverage. Increasing the CNTs content to 6% established an optimal conductive and porous framework, enhanced ion transport, and resulted in remarkable performance. However, further increasing the CNTs content to 9% led to dilution of NS, blocked active pores, and CNTs bundling, thereby reducing electrolyte accessibility and leading to a decline in capacity. The E_d and P_d for all the synthesized samples were determined as 55.15, 71.50, 96.01, and 56.52 Wh kg⁻¹ and 520.83 W kg⁻¹ at 2.0 A g⁻¹, respectively. Such high capacities and E_d have been reported in the literature, particularly for Li–S cathodes, which is attributed to the low band gap of sulfides and the lower electronegativity of sulfur compared to oxygen in oxides.⁵² The calculated values of E_d and P_d of all prepared samples at different I_d are listed in Table 1. Moreover, the variations of E_d with P_d are depicted in Fig. 6(g), indicating that E_d decreases with increases in P_d due to the limited time available for the complete utilization of the stored ionic charge. In addition, the three-electrode system highlights the intrinsic capabilities of the material, a two-electrode assembly is required to access the realistic device performance, and this is elaborated in the following section.

Fig. 6. GCD profiles of (a) NS, (b) NS-3, (c) NS-6, (d) NS-9 at various current densities, (e) GCD profiles of all samples at 11.7 A g⁻¹, (f) variation of specific capacity from sample to sample, (g) interplay between energy density and power density, (h) cyclic stability of NS-9 electrode at minimum current density, and (i–l) GIIT profiles of NS, NS-3, NS-6, and NS-9 to probe the ionic diffusion behavior under current pulses.

Table 1. Specific capacity, energy density, and power density of NS, NS-3, NS-6, and NS-9 from GCD analysis.

Sample	Current density (A g−1)	Discharge time (s)	Specific capacity (C g−1)	Energy density (Wh kg−1)	Power density (W kg−1)
NS	2.0	111.28	794.18	55.15	520.83
	3.1	58.96	649.46	45.10	781.25
	4.1	36.23	483.0	33.54	1041.66
	6.2	62.66	391.62	27.19	1562.50
	8.3	40.4	336.66	23.37	2083.33
	10.4	28.47	296.56	20.59	2604.16
NS-3	2.0	494.25	1029.68	71.50	520.83
	3.1	215.64	673.87	46.79	781.25
	4.1	137.96	574.83	39.91	1041.66
	6.2	73.48	459.25	31.89	1562.50
	8.3	47.0	391.66	27.19	2083.33
	10.4	32.67	340.31	23.63	2604.16
NS-6	2.0	663.59	1382.47	96.01	520.83
	3.1	395.33	1235.40	85.79	781.25
	4.1	252.81	1053.37	73.15	1041.66
	6.2	166.17	1038.56	72.12	1562.50
	8.3	103.47	862.25	59.87	2083.33
	10.4	67.95	707.81	49.15	2604.16
NS-9	2.0	390.71	813.97	56.52	520.83
	3.1	183.9	574.68	39.91	781.25
	4.1	114.62	477.58	33.16	1041.66
	6.2	59.89	374.31	25.99	1562.50
	8.3	37.54	312.85	21.72	2083.33
	10.4	25.76	268.33	18.63	2604.16

Furthermore, to assess the long-term viability of the material for SC applications, an extensive cyclic stability test was conducted. NS-6 underwent 10 000 consecutive charge–discharge cycles at a lower I_d 0.19 A g⁻¹. Remarkably, it retained 95.5% of its initial capacity along with excellent coulombic efficiency of 98.5% throughout the prolonged EC operation, as shown in Fig. 6(h). This stability highlights the material's robust structural integrity and durability. With such promising cyclic performance, NS-6 emerges as a strong candidate for next-generation SC technologies, particularly in portable electronic devices and advanced energy storage systems where both high E_d and P_d are critical. Furthermore, the literature-based comparison of various composites is given in Table 2 to benchmark our findings.^53–57

Table 2. A comparative analysis of synthesis methodologies and electrochemical performance metrics reported for sulfide-based electrode materials in half-cell assemblies and the last row in bold highlighting the key findings of this work.

Composition	Method	Specific capacity	Diffusion coefficient	Ionic conductivity	Cyclic stability (%)	References
NiMoO4/NiCo2S4	Two-step hydrothermal	1283 C g−1 at 2 A g−1	N/A	N/A	83.4% (5k) cycles	53
NiCo2O4/NiCo2S4	Three-steps method	1341 C g−1 at 1 A g−1	N/A	N/A	82.88% (5k) cycles	54
Ni9S8	Hydrothermal	952 C g−1 at 11.8 A g−1	5.71 × 10−15 cm2 s−1	N/A	98.6% (3k) cycles	18
Ni9S8/Co3S4	Two-pot hydrothermal	1080.3 C g−1 at 1 A g−1	N/A	N/A	83.3% (12k) cycles	55
Co-Ni9S8/MXene	Solvothermal	1904.75 C g−1 at 5.88 A g−1	6.65 × 10−9 cm2 s−1	4.6 × 10−1 S cm−1	99% (5k) cycles	56
NiCo2S4/Ni/Ni9S8	Hydrothermal	696 C g−1 at 1 A g−1	N/A	N/A	95% (3k) cycles	57
Ni 9 S 8 /CNTs	Hydrothermal	1382.47 C g− 1 at 2 A g− 1	7.48 × 10− 11 cm2 s−1	4.4 × 10− 2 S cm −1	96.6% (5k) cycles	This work

4.9. Diffusion dynamics study

GITT analysis was employed to explore the diffusion dynamics, which describe how charge carriers like ions migrate from the region of high to low concentration, and are typically characterized by the diffusion coefficient. A widely adopted method for accurately probing these dynamics in electrode materials is the GITT. This technique involves applying a constant current to the system while periodically recording the voltage response during controlled relaxation periods. After each current perturbation, the relaxation period was carefully selected to enable the electrode potential to reach a quasi-equilibrium condition. The initiation of a current pulse causes an instantaneous voltage elevation, depicted in the GITT profiles (Fig. 6(i–l)). This is explained by the ohmic IR drop that results from the inherent resistance of the electrolyte, working electrode, and interfacial contacts. This resistive response was measured from the preliminary potential difference (E₀ − E₁) at the pulse's beginning and was deliberately extracted from subsequent diffusion-related analysis. The transient potential response (ΔE_t) was defined as ΔE_t = E₂ − E₁, where E₁ is the electrode potential immediately following the IR drop and E₂ is the potential at the end of the applied current pulse. It was obtained from the part of the curve related to concentration gradient development. Additionally, the steady-state potential variation (ΔE_s) was derived from the relaxation phase, where ionic redistribution reduces concentration gradients, and was written as ΔE_s = E_n − E_o. Where E_n is the equilibrium potential following relaxation and E_o is the electrode's initial open-circuit potential. A detailed mechanism regarding these calculations has also been reported in the literature, and the diffusion behavior quantified using eqn (6).^58,59

6

In this context, D_K⁺ represents the diffusion coefficient of the potassium ions (K⁺) in the KOH electrolyte. The relaxation time (τ), fixed at 10 min in our study, reflects the periods allowed for ionic species to return towards equilibrium. Additional parameters, such as the m_b active mass, M_B is the molar mass, V_m is the molar volume, and S is the effective electrode surface area. ΔE_s is the steady-state voltage change during relaxation, and ΔE_t is the total voltage change during the current pulse, excluding the IR drop.⁶⁰ The calculated diffusion coefficients for NS, NS-3, NS-6, and NS-9 are 6.64 × 10⁻¹¹, 7.39 × 10⁻¹¹, 7.48 × 10⁻¹¹, and 6.83 × 10⁻¹¹ cm² s⁻¹. The improvement in D_K⁺ is attributed to the strong interfacial bonding between CNTs and the NS surface, which promotes reversible redox kinetics and facilitates ion transport during charge/discharge cycles. Moreover, the integration of CNTs substantially reduces interfacial resistance, thereby enhancing electronic conductivity and overall electrode performance. However, the reduction in D_K⁺ at 9% CNTs arises from CNTs agglomeration, as discussed in section 4.3, Fig. 3(h), which blocks ion-accessible pores and increases interfacial resistance. Moreover, excess CNTs attenuate the active NS phase, limiting redox participation and slowing ion kinetics. These findings demonstrated that optimized CNTs incorporation can markedly improve the rate capability and cycling stability of NS-based hybrid SC electrodes.

4.10. Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) was employed to assess the resistive and capacitive behavior of the prepared samples. In EIS, a sinusoidal voltage was applied across a frequency range from 10⁻² to 10⁵ Hz, and the current was measured. The ratio of the voltage to current gives impedance, which is a complex quantity whose real part gives the resistive components, while the imaginary part gives the reactance. When these two quantities are plotted against each other, the resulting Nyquist plot (NQP) offers valuable insights into ion transport both at the electrode–electrolyte interface and within electrode channels.

Fig. 7(a) represents the NQPs for all prepared electrode materials, while Fig. 7(b–e) showcases individual NQPs alongside their equivalent circuit models, obtained through curve fitting using EC Biological Lab V11.60 software based on their Randles circuit. Fig. 7(a) displays distinct semicircles followed by line segments for all the synthesized samples, highlighting the faradaic and non-faradaic reactions at the electrode surface. The diameter of the semicircle reflects the charge transfer resistance (R_ct); the greater the R_ct, the lower the electrical conductivity and vice versa. Moreover, the line segment in the region of low frequency indicates Warburg impedance (W_R), which indicates the ion diffusion; the steeper the line, the better the ion diffusion performance, and vice versa. The initial intercept of the semicircle on the x-axis gives us the electrode resistance (R_s), which indicates the ionic conductivity of the electrode material. NQPs revealed that the incorporation of CNTs significantly reduced the R_ct, W_R (M_a), and R_s, owing to their outstanding electrical conductivity and tubular structure, which supports the formation of interconnected pathways that facilitate rapid electron transport. At 3% CNT loading, this improvement is present but limited by insufficient network development. Furthermore, the NS-6 composite exhibited the most pronounced reduction in resistance, as the uniformly dispersed CNTs framework ensures efficient ion migration and strong interfacial interactions with NS. In contrast, NS-9 leads to aggregation and partial pore blockage, as observed in the SEM of NS-9 in Fig. 3(h), which hinders ion diffusion and offsets the impedance benefits, thereby correlating with the decrement in overall performance of NS-9.⁵⁹ The quantified parameters obtained from the EC Lab are listed in Table 3. Overall, the EIS analysis revealed that NS-6 exhibited superior reaction kinetics compared to all other prepared samples, indicating its potential as a more effective electrode material for SC applications.

Fig. 7. (a) Combined EIS spectra of NS, NS-3, NS-6, NS-9, (b–e) fitted curves of all samples (the inset shows Randle circuits), (f) Bode plot for all samples, (g) variation of ionic conductivity and relaxation time for all prepared electrodes, and (h) radar graph illustrating the all-around performance of optimized sample across different evaluated parameters.

Table 3. Values of elements for the corresponding fitted circuit, R₁ = R_s, R₂ = R_ct, and M_a = restricted diffusion element.

Sample	R 1 (Ω)	R 2 (Ω)	M a (Ω)
NS	1.31	5.14	1.76
NS-3	1.18	4.71	1.62
NS-6	1.05	2.51	0.49
NS-9	1.17	2.67	0.63

4.10.1. Ionic conductivity

Ionic conductivity is an essential parameter in SC studies, which determines the EC efficiency of materials. It has been calculated for all the samples by using eqn (7).

7

Herein, R_i is the electrolyte resistance, σ is the ionic conductivity, and A and L are the surface area and thickness of the electrode. The values of ionic conductivity are listed in Table 4 for all samples. It was seen that NS-6 has a maximum conductivity of 4.4 × 10⁻² S cm⁻¹ among all these samples, indicating a better choice for the electrode material.^52,61 CNTs incorporation markedly boosts ionic conductivity at optimal loading by forming efficient conductive networks that accelerate charge carrier transportation. However, NS-9, due to higher CNT content, tends to agglomerate and block ionic channels, while simultaneously masking electroactive sites, thereby impeding ion migration and lowering the conductivity.

Table 4. Calculated values of relaxation time, conductivity, transference number, and diffusion coefficient for NS, NS-3, NS-6, and NS-9.

Sample	Frequency (Hz)	Relaxation time (s)	Conductivity (S cm−1)	Transference number	Diffusion coefficient (cm2 s−1)
NS	1.14	0.14	3.9 × 10−2	0.41	6.64 × 10−11
NS-3	1.16	0.13	4.1 × 10−2	0.43	7.39 × 10−11
NS-6	1.65	0.09	4.4 × 10−2	0.67	7.48 × 10−11
NS-9	1.44	0.11	4.2 × 10−2	0.64	6.83 × 10−11

4.10.2. Transference number

The cation transference number (t₊) represents the proportion of the total ionic current that was exclusively transported by cations in an EC system. Since ionic conduction is not uniformly distributed among all charge carriers, this parameter is critical for discerning the specific role of cations in the overall transport process. The disparity in ion contributions arises from several intrinsic and extrinsic factors, including ion size, mobility, temperature-dependent dynamics, structural defects, and polarization effects. By quantifying t₊, one can effectively differentiate the individual contributions of cations and anions to the total ionic conductivity. In the context of this study, t₊ was calculated to assess the dominant charge carriers and their impact on ion transport efficiency. The value of t₊ was derived using eqn (8), providing insight into the selective mobility of ion species within the system.

8

Here, Z_d (0) is the Warburg impedance, and R_b is the electrolyte resistance; t₊ indicates the transference number for cations.⁵⁹ In this experiment, the calculated t₊ for NS, NS-3, NS-6, and NS-9 are tabulated in Table 4. Based on the data presented in Table 4, t₊ increases as the integration of CNTs increases up to 6%, which suggests EC neutrality at the electrode and electrolyte interface by alleviating the negative concentration gradient, thereby markedly promoting ion intercalation processes.⁶²

4.10.3. Relaxation time

The Bode plot serves as a valuable analytical tool for determining the relaxation time (τ), a critical parameter that indicates the rate at which ions within the electrode matrix return to equilibrium following energy uptake. This relaxation time, defined by (τ = 1/2πf_c), was derived from the frequency-dependent phase angle data to evaluate the charge–discharge kinetics of the electrode materials. Fig. 7(f) illustrates the dynamic correlation between frequency and phase angle, showcasing the material's phase behavior across a spectrum of frequencies. The calculated τ values for all synthesized composites are summarized in Table 4. A reduced τ corresponds to diminished internal resistance, which is beneficial for SCs' performance by facilitating more rapid and efficient power delivery. Among the evaluated materials, the NS-6 composite demonstrated the shortest τ of 0.09 s, indicating superior EC responsiveness and lower resistive losses.⁴²Fig. 7(g) represents the variation of the relaxation time and ionic conductivity with composition. Furthermore, Fig. 7(h) shows a radar chart illustrating the EC performance metrics of all fabricated electrode materials. Among them, NS-6 demonstrates outstanding performance across all assessed parameters, primarily due to its unique architecture and highly interconnected porous framework. The NS-6 electrode delivered EC performance, maintaining low resistance values and a large diffusion coefficient value, underscoring its strong potential for next-generation SC applications, particularly in portable electronics, electric vehicles, and large-scale energy storage systems.

4.11. Evaluation of asymmetric supercapacitor device

Asymmetric SCs constitute an important category of EC energy storage that incorporates compositionally and functionally diverse electrode materials as the anode and cathode. This arrangement broadens the effective EC potential range, resulting in considerable improvements in both E_d and P_d. In such a configuration, the positive electrode exhibits PC behavior through a faradaic redox mechanism, while the negative electrode accumulates charge through the EDLC (carbon-based materials) mechanism.

To verify practical applicability, an asymmetric cell was employed using a two-electrode system, where NS-6 acted as the cathode, and AC served as the anode, as illustrated in Fig. 8(a). The charge balancing equation q⁺ = q⁻(q = CmΔV) governs the EC equilibrium of the developed device, indicating that the charges are collected equally on negative and positive electrodes. The given equation shows the mathematical representation of all factors that govern the potential for charge storage of fabricated electrodes.⁶²

9

Here, C⁺ represents CS, m⁺ corresponds to active mass, and Δv⁺ displays the potential window for the positive electrode, while exact parameters are represented by C⁻, m⁻, and Δv⁻ for the negative electrode.⁵⁶

Fig. 8. (a) Schematic representation of the prepared device, (b) CV profiles of NS-6 at various scans, at 0.5 V, (c) CV profiles at different potential windows, (d) CV profiles at different scan rates within a 1.2 V, (e) GCD profiles at 1.2 V potential window, (f) variation of specific capacity with current densities, (g) Ragone plot of E_d and P_d, and (h) Nyquist plot of the device.

The EC response of the device was systematically assessed using CV, GCD, and EIS in a 1 M KOH electrolyte solution. The CV traits of the NS-6//AC device demonstrated the combined effect of EDLC and PC mechanisms, functioning efficiently over a potential range of 0.0–0.5 V. Importantly, the CV curves retained almost identical shapes even at higher scan rates, as shown in Fig. 8(b), indicating excellent reversibility and EC stability. Moreover, Fig. 8(c) shows that the CV loops remain consistent when the operating voltage window is extended, thereby widening the working window from 0.5 to 1.2 V. This extended operating voltage is primarily attributed to the AC electrode, which provides high EC stability and accommodates charge storage through the EDLC mechanism, thereby ensuring smooth and reversible reactions. Additionally, Fig. 8(d) demonstrates the CV profiles recorded at various sweep rates within the 1.2 V range, further validating stable capacitive performance. This expanded applied voltage window contributes directly to improved E_d and P_d.⁵⁸

Complementary GCD studies carried out across the 0.0–1.2 V region further confirmed the reliable charge/discharge capability of the device depicted in Fig. 8(e). The calculated Q_SC values were 301.11, 215.85, 150.88, 130.82, and 116.45 C g⁻¹ at 1.17, 1.56, 1.96, 2.35, and 3.13 A g⁻¹, respectively. A maximum E_d of 50.18 Wh kg⁻¹ was achieved along with a P_d of 705.88 W kg⁻¹, highlighting the material's promise for device applications. This difference in I_d is due to the different mechanisms involved in full and half-cell assembly. In a full-cell configuration, both electrodes work complementarily, and their individual potentials are combined to determine the overall cell potential, often leading to different optimal operating conditions compared to the half-cell measurements. The variations of these values (Q_SC, E_d, and P_d) are given in Fig. 8(f and g). Additionally, a literature-based comparative analysis has been included in Table 5, indicating a strong agreement between the reported data and the findings of this work.⁶⁵Fig. 8(h) shows the NQP of the NS-6//AC device, which reveals a nearly vertical line in the low-frequency region, parallel to the imaginary axis, signifying efficient ion transport within the electrodes and minimal diffusion resistance.^63,64 These results highlight the excellent potential of this hybrid electrode for practical applications, particularly in systems requiring both high E_d and P_d, including electric vehicles, portable electronics and grid-scale energy storage systems.

Table 5. A comparison of synthesis methodologies and electrochemical performance metrics for nickel-based sulfide electrode materials in full-cell assemblies is presented, with the last row in bold highlighting the key findings of this work.

Composition	Method	Specific capacity (loading mass)	Energy density	Power density	References
NiCo2S4/Ni/Ni9S8	Two-step hydrothermal	216 C g−1 at 1 A g−1 (∼2–4 mg cm−2)	53 Wh kg−1	887 W kg−1	57
NiCo2S4/Ni9S8/C	Sulfidation	143.5 C g−1 at 2 A g−1 (∼2 mg cm−2)	51 Wh kg−1	1399.40 W kg−1	65
Ni9S8	Hydrothermal	104.2 C g−1 at 0.7 A g−1 (N/A)	17.3 Wh kg−1	423.5 W kg−1	18
Ni9S8/Co3S4	Two-pot hydrothermal	128.1 C g−1 at 1 A g−1 (N/A)	45.5 Wh kg−1	800 W kg−1	55
Co-Ni9S8/MXene	Solvothermal	228.70 C g−1 at 3.52 A g−1 (∼1 mg cm−2)	31.76 Wh kg−1	1769.70 W kg−1	56
Ni 9 S 8 /CNTs	Hydrothermal	301.1 C g− 1 at 1.17 A g− 1 (∼3 mg cm− 2 )	50.18 Wh kg− 1	705.88 W kg− 1	This work

5. Conclusion

In conclusion, NS and its derivatives with CNTs, NS-3, NS-6, and NS-9, were synthesized through a combination of hydrothermal and solvothermal techniques. The XRD analysis confirmed that all samples preserved their orthorhombic crystal structure irrespective of CNTs loading. Raman analysis exhibited successful formation of NS frameworks, evidenced by characteristic vibrational modes corresponding to NS lattice bending and stretching, indicating good structural stability and phase development. Morphological studies revealed the formation of well-defined nano-sheet architectures, while EDX and elemental mapping confirmed the homogeneous dispersion of CNTs within the NS matrix and the absence of unwanted impurity phases. The optimized sample NS-6 showed an impressive surface area of 61.72 m² g⁻¹, reflecting the coexistence of micro and mesoporous nature of the electrode material. EC investigations indicated that the energy storage process is primarily dominated by a hybrid redox mechanism, as substantiated by Dunn's model. Among all samples, NS-6 exhibited the highest Q_SC of 1382.47 C g⁻¹ at 2.0 A g⁻¹, along with an impressive E_d and P_d. Additionally, NS-6 retained 95.5% of its initial capacity after 10 000 cycles with an excellent coulombic efficiency of 98.5%. EIS analysis further demonstrated its optimized ionic conductivity of 4.4 × 10⁻² S cm⁻¹ and cation transference number of 0.67. Furthermore, GITT analysis showed that NS-6 has D_K⁺ of 7.48 × 10⁻¹¹ cm² s⁻¹, as compared to other prepared electrode materials. The asymmetric device achieved a Q_SC of 301.11 C g⁻¹, coupled with E_d and P_d values of 50.18 Wh kg⁻¹ and 705.88 W kg⁻¹, respectively. Collectively, these findings underscore NS-6 as a strong candidate for advanced hybrid SC applications.

Conflicts of interest

We declare that there are no competing/financial or any other type of interests to declare.

Data availability

Data will be available on request.