Nickel Selenide Electrodes with Tuned Deposition Cycles for High-Efficiency Asymmetric Supercapacitors

Abstract

This study aims to develop high-performance nickel selenide (NiSe) electrodes via a controlled electrodeposition approach, optimizing the number of deposition cycles to enhance electrochemical energy storage capabilities. Nickel selenide electrodes were synthesized at varying electrodeposition cycles (2CY–5CY) and systematically evaluated in both three-electrode and asymmetric supercapacitor (ASC) configurations to determine the optimal cycle for superior performance. Among all, the NiSe-3CY electrode demonstrated the best electrochemical characteristics, delivering a high specific capacitance of 507.42 F/g in a three-electrode setup. It also achieved an energy density of 22.89 Wh/kg and a power density of 584.61 W/kg, outperforming its 2CY, 4CY, and 5CY counterparts. Notably, the 3CY electrode exhibited the lowest series resistance (1.59 Ω), indicative of enhanced charge transport and minimal internal resistance. When integrated into an ASC device (NiSe-3CY//activated carbon), it maintained a specific capacitance of 18.78 F/g, with an energy density of 8.45 Wh/kg and power density of 385.03 W/kg. Furthermore, the device exhibited impressive areal and volumetric capacitances of 351 mF/cm² and 1.09 F/cm³, respectively, with a corresponding volumetric energy density of 0.49 mWh/cm³. Long-term cycling tests revealed excellent durability, retaining 91% of its initial capacity after 10k cycles with a high Coulombic efficiency of 99%. These results confirm that the 3CY electrode is a highly promising candidate for next-generation energy storage systems, offering a balanced combination of high capacitance, energy density, and cycling stability.

1. Introduction

The excessive reliance on fossil fuels across industrial, transportation, and domestic sectors has led to severe environmental pollution and an escalating global energy crisis. This alarming situation necessitates urgent action and the adoption of sustainable energy solutions that can mitigate ecological degradation while meeting the rising energy demands. Renewable energy sources such as solar, wind, and hydropower have emerged as promising alternatives to fossil fuels. However, their intermittent and variable nature necessitates the development of efficient energy storage systems to ensure a stable and reliable power supply. In this context, energy storage devices play a critical role in enhancing the utility of renewables by storing excess energy during peak generation and delivering it during demand surges, thereby reducing dependency on conventional energy sources [1,2]. Among various energy storage technologies, batteries and supercapacitors have gained considerable attention. Supercapacitors, in particular, are distinguished by their rapid charge–discharge capability, superior power density, environmental friendliness, and excellent cyclic stability [3]. Nevertheless, their widespread deployment is constrained by a key limitation: relatively low energy density compared to batteries. The energy density (E) of a supercapacitor is governed by the equation E = 0.5 CV², where “C” denotes the specific capacitance of the electrode material and “V” represents the operational voltage window [3,4]. Consequently, strategies to improve energy density revolve around enhancing the specific capacitance or expanding the potential window. Among these, optimizing capacitance via material innovation and structural engineering has proven highly effective. The design and synthesis of electrode materials with tailored morphology, crystallinity, and surface area are essential to achieving high-performance supercapacitors. Electrode materials are broadly classified into carbon-based materials for electric double-layer capacitors (EDLCs) and redox-active compounds for pseudocapacitors [5,6]. While EDLCs rely on physical adsorption and are often composed of activated carbon, graphene, or carbon nanotubes, pseudocapacitors store charge via reversible faradaic redox reactions, offering higher specific capacitance. Transition metal oxides, sulfides, and, more recently, chalcogenides have emerged as front-runners for pseudocapacitive applications due to their tunable redox activity and structural versatility [7,8]. Nickel-based materials such as nickel oxide (NiO) [9,10,11,12], nickel sulfide (NiS) [13,14,15,16,17], and nickel hydroxide (Ni(OH)₂) [18,19,20,21,22] have been extensively explored due to their rich electrochemical characteristics and multiple valence states. NiO, while offering high theoretical capacitance and redox reversibility, suffers from poor intrinsic electrical conductivity, which hinders its rate capability and overall electrochemical performance. To address this, strategies such as hybridization with conductive matrices (e.g., graphene, CNTs) and metal-ion doping have been implemented. NiS presents improved conductivity and electrochemical kinetics compared to NiO, but it is often plagued by structural degradation and limited long-term stability. Nickel sulfide (NiS), on the other hand, exhibits superior capacitance and conductivity compared to NiO. The improved electrical conductivity of NiS is attributed to its unique electronic structure, which facilitates efficient charge transport. Additionally, NiS demonstrates excellent chemical stability and high electrochemical activity, making it a suitable alternative to NiO for energy storage applications. Transition metal chalcogenides, which include sulfides, selenides, and tellurides, have emerged as promising materials for pseudocapacitive energy storage [23,24,25,26]. In recent years, bimetallic transition metal selenides (BTMSs) have emerged as promising candidates for high-performance supercapacitor electrodes due to their superior electrical conductivity, redox activity, and structural stability compared to their monometallic counterparts. For instance, Ni₂MnSe₄ nanostructures with pinecone-like arrays synthesized via electrodeposition exhibit excellent areal capacitance (2.1 F cm⁻²) and long-term stability (92% retention over 10k) [27]. Similarly, the fabrication of NiSe/MnSe@CoS heterostructures on carbon paper has demonstrated enhanced ion transport and electrical conductivity, yielding a high specific capacity of 884 C/g and excellent durability over 20k cycles [28]. Additionally, NiSe₂@CNT composites prepared by microwave synthesis deliver high capacitance (980.5 F/g) and energy density (25.61 Wh/kg), underscoring the benefits of carbon-based hybridization [29]. The NiSe₂@MoSe₂/MWCNT composite further improves ion accessibility and reduces resistance due to hierarchical porosity and conductive pathways [30]. Furthermore, NiSe-SnSe composites demonstrate notable energy–power metrics (55.4 Wh/kg and 8400 W/kg) and robust cycling stability [31]. These studies collectively emphasize the importance of nanostructure engineering, heterointerface modulation, and composite strategies in overcoming the intrinsic limitations of selenides, paving the way for advanced hybrid and asymmetric supercapacitor systems.

In this regard, nickel selenide (NiSe) has emerged as a next-generation pseudocapacitive material due to its superior metallic conductivity, enhanced redox activity, and structural robustness. The incorporation of selenium—a heavier and less electronegative chalcogen than sulfur—into nickel-based compounds imparts higher electrical conductivity owing to its larger atomic radius and stronger metallic bonding characteristics [32,33]. These features facilitate efficient electron transport, rapid redox kinetics, and improved utilization of active material, resulting in higher specific capacitance, better rate performance, and prolonged cycle life. Given these advantages, NiSe and its derivatives represent a promising class of materials for next-generation supercapacitors. The rational design of NiSe-based electrodes through advanced synthesis techniques, heterostructuring, and elemental doping offers a viable route to overcome the energy density limitations of conventional supercapacitors. This work aims to explore and develop novel NiSe-based electrode architectures with enhanced electrochemical performance, contributing to the ongoing quest for high-performance, scalable, and sustainable energy storage systems. The primary purpose of this study is to enhance the energy storage performance of supercapacitors by synthesizing nickel selenide (NiSe) electrodes through a controlled electrodeposition process. This work systematically investigates the influence of electrodeposition cycles (ranging from two to five) on the structural and electrochemical properties of the resulting NiSe electrodes. The objective is to identify the optimal deposition cycle that yields the best balance of specific capacitance, energy density, and cycling stability. The study employs cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) to evaluate the electrochemical performance of the electrodes. Based on these results, the best-performing electrode is further integrated into an asymmetric supercapacitor (ASC) device. The ultimate goal is to demonstrate a scalable and efficient approach for fabricating high-performance NiSe-based electrodes suitable for next-generation energy storage systems.

2. Methods and Materials

2.1. Material

Nickel foam (1.6 mm thick) was procured from MTI Corporation, Seoul, South Korea. Nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O) and selenium dioxide (SeO₂) powder, hydrochloric acid, acetone, potassium hydroxide, and ethanol were supplied by were obtained from Daejung Chemicals, Seoul, South Korea.

2.2. Synthesis of Nickel Selenide

Nickel selenide thin films were prepared using a simple electrodeposition method. For the preparation of the nickel selenide thin film, 10 mM nickel nitrate and 3 mM selenium oxide were dissolved in 40 mL of deionized (DI) water and stirred for 30 min using a magnetic stirrer to obtain a homogeneous solution. The electrodeposition was carried out using a three-electrode configuration, where platinum served as the counter electrode, an Ag/AgCl electrode as the reference electrode, and nickel foam was used as the working electrode for the deposition of the nickel selenide film. Before deposition, the nickel foam electrodes were cleaned by sonicating in 1 M HCl for 10 min, followed by thorough washing with ethanol and DI water multiple times, then dried in an oven. The electrodeposition of nickel selenide was performed using cyclic voltammetry (CV) within the potential window of −1.2 V to 0.2 V with a 5 mV/s scan rate. Electrodeposition was carried out for 2, 3, 4, and 5 cycles, and the resulting electrodes were labeled as 2CY, 3CY, 4CY, and 5CY for further analysis and discussion. Selenium is first reduced to selenium ions, which then combine with nickel ions to form nickel selenide (NiSe). The formation of nickel selenide by electrodeposition is given by Equations (1) and (2) [34,35]. The schematic of the preparation of nickel selenide by electrodeposition is shown in Figure 1. After deposition, the electrodes were rinsed with DI water and ethanol several times and dried for 12 h in an oven at 60 °C before use.

$$\mathrm{S}\mathrm{e}+{2\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\to {\mathrm{H}}_2\mathrm{S}\mathrm{e}$$

(1)

$${\mathrm{H}}_2\mathrm{S}\mathrm{e}+{\mathrm{N}\mathrm{i}}^{2+}\to \mathrm{N}\mathrm{i}\mathrm{S}\mathrm{e}+{2\mathrm{H}}^{+}$$

(2)

2.3. Material Characterization

Material characterization was essential for understanding the physicochemical properties of the synthesized samples. The crystalline structure and phase formation were examined using X-ray diffraction (XRD) patterns, recorded on a PANalytical diffractometer equipped with Cu Kα radiation (λ = 1.5406 Å). The surface chemical composition and oxidation states of the elements were analyzed using X-ray Photoelectron Spectroscopy (XPS) performed with Thermo Fisher Scientific’s K-alpha system (Waltham, MA, USA). Morphological features were investigated through field emission scanning electron microscopy (FE-SEM, model S-4800, HITACHI Ltd., Tokyo, Japan). Electrochemical behavior was evaluated on a ZIVE SP5 electrochemical workstation from WonAtech (Seoul, South Korea).

2.4. Electrochemical Studies

Electrochemical studies of all electrodes were conducted using the same three-electrode configuration with 2 M KOH as the electrolyte. The techniques used include cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS). An asymmetric supercapacitor (ASC) device was fabricated using the optimized nickel selenide electrode as the positive electrode and activated carbon (AC) as the negative electrode. The AC electrode was prepared by forming a slurry of activated carbon, carbon black, and polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) solvent, which was then drop-cast onto Ni foam and dried for 12 h at 60 °C. The optimized 3CY electrode and the AC electrode were assembled using paraffin paper, with filter paper serving as the separator and 2 M KOH as the electrolyte. The electrochemical performance of the ASC device was studied to evaluate its practical applicability.

3. Results and Discussion

3.1. The Characterizations of Morphology, Composition, and Structure

Figure 2a presents the XRD patterns of nickel selenide electrodes prepared at different electrodeposition cycles: two, three, four, and five cycles. The diffraction peaks observed at 28.13°, 32.84°, 33.60°, 44.37°, 49.78°, 58.16°, 61.41°, 68.87°, 70.63°, and 80.02° correspond to the (100), (101), (001), (102), (110), (200), (112), (202), (004), and (210) crystal planes of nickel selenide, confirming its hexagonal structure. The common peaks for the Ni foam substrate, denoted as *. These diffraction peaks are consistent with the standard JCPDS card No. 01-075-0610, further verifying the successful formation of nickel selenide. Additionally, the XRD spectra reveal extra peaks at 45°, 52.40°, and 76.79°, which are attributed to the underlying Ni foam substrate. These additional peaks indicate that the deposited nickel selenide layers are not completely covering the Ni substrate, especially at lower deposition cycles. As the deposition cycles increase, the intensity of the nickel selenide peaks becomes more pronounced, suggesting an enhancement in crystallinity and material growth. The crystallinity of the nickel selenide electrodes is directly influenced by the number of electrodeposition cycles. At two deposition cycles, the nickel selenide formation is relatively incomplete, with lower-intensity diffraction peaks, indicating smaller crystallites or amorphous phases present within the structure. As the deposition cycles increase to three and four cycles, the XRD peaks become sharper and more intense, signifying improved crystallinity and increased material deposition. However, at five deposition cycles, slight peak broadening is observed, which may be attributed to increased strain in the material, defects introduced during prolonged deposition, or possible agglomeration of nanoparticles. XPS is an essential tool for determining the oxidation states of elements and confirming the presence of specific chemical bonds in the material. Figure 2b displays the XPS survey scan spectra, revealing the presence of nickel (Ni) and selenium (Se) as the primary elements in the synthesized material. The absence of any impurity-related peaks suggests high-purity nickel selenide formation. The obtained XPS results are in good agreement with the energy-dispersive X-ray spectroscopy (EDS) analysis performed for elemental mapping, further confirming the stoichiometric composition of the material. The high-resolution XPS spectrum of the Ni 2p region is shown in Figure 2c. The spectrum is deconvoluted into distinct peaks corresponding to different oxidation states of Ni. The two main peaks are observed at 855.41 eV and 872.96 eV, which are assigned to Ni 2p_3/2 and Ni 2p_1/2, respectively. Furthermore, the peaks appearing at 855.06 eV and 872.69 eV correspond to Ni²⁺, indicating the presence of divalent nickel species. Additionally, peaks observed at 856.24 eV and 874.50 eV suggest the existence of Ni³⁺ oxidation states, which could arise due to partial oxidation at the surface [36,37,38]. The presence of Ni³⁺ is important as it can influence the electrochemical performance of the material, particularly in charge storage applications. In addition to the primary peaks, two satellite peaks appear at 861.10 eV and 879.60 eV, which originate from electron excitation processes [38]. These satellite features further confirm the Ni²⁺ state within the 3CY sample. The ratio of Ni²⁺ to Ni³⁺ suggests that the material predominantly exists in the Ni²⁺ oxidation state, which aligns well with previously reported nickel selenide structures. The high-resolution XPS spectrum of Se 3d is presented in Figure 2d. The spectrum is deconvoluted into two major peaks located at 55.41 eV and 55.34 eV, which correspond to Se 3d_5/2 and Se 3d_3/2, respectively [39,40,41,42,43]. These peaks are characteristic of Se^2-, confirming the presence of selenide anions in the prepared sample. The observation of Se^2- further supports the formation of nickel selenide, as it indicates the correct stoichiometry between nickel and selenium. A minor peak shift towards higher binding energy may be observed due to possible surface oxidation, leading to the presence of selenium oxide species. However, this effect is minimal and does not significantly impact the bulk material’s composition.

Figure 3(a1–a3) shows the FESEM micrographs of nickel selenide electrodes prepared using two deposition cycles. The micrographs reveal a non-uniform microball surface morphology, where individual microballs are completely overlapped with each other. The overlapping nature of these structures leads to a compact arrangement with limited surface exposure. This kind of morphology could impact the electrochemical properties by restricting the availability of active sites for redox reactions. The compact and overlapped microballs reduce the effective surface area, which may limit ion transport during electrochemical cycling. However, the interconnected nature of the microballs may contribute to enhanced electrical conductivity by providing continuous electron pathways. Figure 3(b1–b3) illustrates the FESEM images of nickel selenide electrodes subjected to three deposition cycles. As observed, the microball structures exhibit a more defined and slightly modified surface morphology compared to the two-cycle deposition sample. The roughness of the microballs increases, and smaller microballs appear to grow over the Ni electrode surface. This increase in surface roughness can be beneficial for electrochemical applications as it enhances the electrode–electrolyte interaction, thereby improving charge storage capability.

The formation of smaller microballs over the primary structures suggests that an increase in deposition cycles promotes secondary nucleation and growth. The enhancement in surface morphology and roughness at three deposition cycles contributes to an increased effective surface area, which is advantageous for electrochemical reactions. As a result, the electrochemical performance, such as charge storage capability and reaction kinetics, is expected to improve for these electrodes compared to those prepared with two deposition cycles.

Figure 3(c1–c3) depicts the FESEM micrographs of nickel selenide electrodes prepared with four deposition cycles. A notable change in morphology is observed at this stage. The microballs begin to aggregate and exhibit overlapping structures, leading to reduced porosity and accessibility of the active sites. The clumping effect is attributed to the continuous growth of nickel selenide microstructures, which eventually results in the loss of individual microball definition. As the microballs start to overlap significantly, the electrode–electrolyte interaction is negatively impacted. The reduction in accessible surface area and porosity limits the diffusion of electrolyte ions into the electrode, leading to decreased electrochemical performance. This observation suggests that an optimal deposition cycle exists beyond which the morphological changes become detrimental to the material’s electrochemical behavior. The FESEM micrographs presented in Figure 3(d1–d3) correspond to nickel selenide electrodes deposited over five cycles. At this deposition stage, the microball surface morphology continues to evolve, with an increasing presence of additional microstructures forming over the existing microballs. However, rather than contributing positively to the electrochemical performance, this excessive growth leads to a reduction in surface roughness. The newly formed particles cover the existing microball structures, causing a more compact and less porous morphology. The increase in deposition cycles leads to the saturation of microball growth, reducing the overall electrochemically active surface area. As a result, the energy storage capacity of the nickel selenide electrode decreases. The excessive material deposition may also introduce stress and defects, which could further impact the electrochemical stability and cycling performance of the electrode. To complement the morphological analysis, Energy Dispersive X-ray Spectroscopy (EDS) was performed on all nickel selenide electrodes prepared at different deposition cycles. Figure 4 presents the EDS spectra of the nickel selenide electrodes, where distinct peaks corresponding to nickel (Ni) and selenium (Se) are observed. The presence of these elements confirms the successful deposition of nickel selenide.

3.2. Electrochemical Study

Cyclic voltammetry is a vital technique for understanding the redox behavior and charge storage mechanism of electrode materials. Figure 5a illustrates the comparative CV profiles of nickel selenide electrodes prepared through varying electrodeposition cycles, specifically 2, 3, 4, and 5 cycles. These cycles represent the number of times the potential was applied during electrodeposition, affecting the material’s growth, morphology, and ultimately its electrochemical performance. Each CV curve features two well-defined redox peaks, which clearly indicate a pseudocapacitive charge storage mechanism rather than electric double-layer capacitance. This pseudocapacitive behavior is a result of faradaic redox reactions occurring at the electrode-electrolyte interface. These redox peaks correspond to the reversible oxidation and reduction of nickel selenide species within the electrode structure, affirming the material’s suitability for high-performance energy storage applications. Among all samples, the electrode subjected to 3 cycles of electrodeposition (3CY) demonstrated the largest enclosed area under the CV curve, which is directly proportional to its charge storage capacity. This observation highlights the enhanced electrochemical activity of the 3CY electrode. The superior performance is primarily attributed to its distinct surface morphology, which was characterized by a microball-like structure. These spherical microstructures, uniformly distributed over the nickel foam substrate, exhibit rough surfaces that offer a higher surface area. The enhanced surface area increases the number of electrochemically active sites and facilitates better electrolyte penetration, thereby supporting more efficient redox reactions during the charge–discharge process. The variation in electrochemical response across the differently cycled electrodes suggests a correlation between electrodeposition cycles and surface morphology. At 3 cycles, the morphology seems to reach an optimized configuration, where the microball formation is complete, uniform, and appropriately compact. In contrast, at higher deposition cycles (4 and 5 cycles), the particles appear more aggregated or irregularly grown, potentially leading to reduced electrochemical activity due to blocked active sites or decreased electrolyte diffusion pathways. This critical role of surface microstructure underscores the importance of precise control over synthesis parameters in tuning material performance. To further evaluate the energy storage capability, galvanostatic charge–discharge (GCD) measurements were performed at a constant current density. Figure 5b presents the GCD curves of the nickel selenide electrodes at various electrodeposition cycles. The 3CY electrode displayed the longest charge–discharge duration, reflecting a higher specific capacitance compared to electrodes deposited with 2, 4, and 5 cycles. This prolonged charge–discharge time can be directly linked to the high-capacity retention of the electrode, facilitated again by the microball architecture. The rough texture of the microballs not only promotes rapid electron transport but also improves the ion diffusion from the electrolyte into the electrode, making the redox reactions more efficient and sustaining higher capacitance over repeated cycles. Capacitance (C_s,a,v,) was calculated using Equations (3)–(5) from the GCD curves [44,45,46]. The values obtained for 2, 3, 4, and 5 electrodeposition cycles were 397 F/g (1.62 F/cm²), 507 F/g (1.97 F/cm²), 485 F/g (1.79 F/cm²), and 407 F/g (1.67 F/cm²), respectively. The 3CY electrode again stood out with the highest specific capacitance value of 507 F/g. Comparison of different selenide material electrode material performance is listed in

Table 1. Comparison of different selenide material electrode material performance.

Sr. No.	Method	Electrode Material	Capacitance	Ref.
1.	Electrodeposition	NiSe	507 F/g, 8 A/g	Present work
2.	Hydrothermal	NiSe2@rGO	467 mAh/g, 1 A/g	[47]
3.	Hydrothermal	NiSe2-CoSe	374 F/g, 8 A/g	[48]
4.	Hydrothermal	NiSe-SnSe	217 F/g, 5 A/g	[31]
5.	In-suit solid-phase synthesis (ISPS)	NiSe2/CNTs	172.70 mAh/g, 1 A/g	[49]
6.	Hydrothermal	NiSe2-MoSe2	121 F/g, 7 A/g	[30]

. This remarkable performance is credited to its optimized morphology that balances active surface area, porosity, and electron/ion conductivity. Figure 5c,e provide a comparative graphical representation of the specific capacitance and areal capacitance, respectively, further supporting the conclusion that 3-cycle deposition is optimal for achieving maximum electrochemical performance. The energy density (ED) and power density (PD) of the electrodes were calculated using Equations (6) and (7), and the values were plotted on a Ragone plot as shown in Figure 5d,f.

$$\mathrm{S}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{c} \mathrm{c}\mathrm{a}\mathrm{p}\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e} \left({\mathrm{C}}_{\mathrm{s}}\right)={\displaystyle \frac{\mathrm{I}·{\mathrm{T}}_{\mathrm{d}}}{\mathrm{m}·∆\mathrm{V}}}$$

(3)

$$\mathrm{A}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{l} \mathrm{c}\mathrm{a}\mathrm{p}\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e} \left({\mathrm{C}}_{\mathrm{a}}\right)={\displaystyle \frac{\mathrm{I}·{\mathrm{T}}_{\mathrm{d}}}{\mathrm{A}·∆\mathrm{V}}}$$

(4)

$$\mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{c} \mathrm{c}\mathrm{a}\mathrm{p}\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e} \left({\mathrm{C}}_{\mathrm{v}}\right)={\displaystyle \frac{\mathrm{I}·{\mathrm{T}}_{\mathrm{d}}}{\mathrm{V}·∆\mathrm{V}}}$$

(5)

$$\mathrm{E}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y} \mathrm{D}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y} \left({\mathrm{E}\mathrm{D}}_{(\mathrm{s},\mathrm{a},\mathrm{v})}\right)={\displaystyle \frac{1}{2}}{\mathrm{C}}_{(\mathrm{s},\mathrm{a},\mathrm{v})}{\mathrm{V}}^2$$

(6)

$$\mathrm{P}\mathrm{o}\mathrm{w}\mathrm{e}\mathrm{r} \mathrm{D}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y} \left({\mathrm{P}\mathrm{D}}_{(\mathrm{s},\mathrm{a},\mathrm{v})}\right)={\displaystyle \frac{{{\mathrm{T}}_{\mathrm{d}}·\mathrm{E}\mathrm{D}}_{(\mathrm{s},\mathrm{a},\mathrm{v})}}{3600}}$$

(7)

These plots provide a clear understanding of how energy and power outputs of the electrodes compare across deposition cycles. The 3CY electrode exhibited a maximum energy density of 22.89 Wh/kg (0.089 mWh/cm²) and a power density of 584 W/kg (2.27 mW/cm²). These values indicate the electrode’s potential for applications requiring both high energy and power outputs, such as hybrid supercapacitors and portable energy storage devices. To understand the charge transfer and diffusion processes at the electrode-electrolyte interface, EIS measurements were conducted in the frequency range from 0.01 MHz to 0.1 Hz with an AC amplitude of 10 mV. Figure 5g illustrates the Nyquist plots of the electrodes, where the intercept at the high-frequency region corresponds to the solution or series resistance (R_s), and the semicircle in the mid-frequency region represents the charge transfer resistance (R_ct). The R_s values for the 2CY, 3CY, 4CY, and 5CY electrodes were 1.86 Ω, 1.59 Ω, 1.60 Ω, and 1.64 Ω, respectively. The 3CY electrode demonstrated the lowest series resistance, further validating its superior electrical conductivity and ion transport kinetics. The reduced R_s indicates minimal internal resistance and better electrical contact between the electrode material and the current collector, facilitating faster and more efficient charge transfer during electrochemical processes. The observed electrochemical behavior-longer GCD durations, higher specific capacitance, favorable energy/power densities, and lower resistance-highlights that the unique microball-like surface morphology formed at 3 electrodeposition cycles plays a critical role in improving overall performance. This suggests that 3CY deposition not only optimizes the physical structure but also enhances the electrochemical characteristics of nickel selenide electrodes. Figure 6a–d illustrates the CV profiles of the electrodes recorded at a wide range of scan rates from 2 mV/s to 50 mV/s. These profiles offer an initial understanding of the electrochemical behavior of the electrodes under different electrochemical environments. Meanwhile, Figure 6e–h presents the corresponding GCD curves recorded at various current densities ranging from 8 mA/cm² to 20 mA/cm². The shapes of these curves and the times for charge and discharge are critical for evaluating the rate capability, reversibility, and practical energy storage performance of the electrodes. The electrochemical behavior of the nickel selenide electrodes was further analyzed using the CV profiles to understand the fundamental reaction kinetics and energy storage mechanisms. The Trasatti method, a commonly used analytical tool in electrochemical studies, was employed to dissect the total current response into its capacitive and diffusion process. This method enables a more nuanced understanding of how the charge storage occurs-whether it is dominated by surface reactions or controlled by ion diffusion within the electrode matrix.

To quantify the nature of the current contribution, the b-value was determined. The b-value, calculated from the slope of the log(i_p) versus log(v) plots (where i_p is the peak current and v is the scan rate), serves as a key indicator of the charge storage mechanism. When the b-value approaches 1.0, it indicates a capacitive-controlled process dominated by surface or near-surface redox reactions. Conversely, a b-value around 0.5 suggests a diffusion-controlled process, in which the electrochemical reactions are influenced by the movement of ions through the bulk of the active material. Equation (8) was used to derive the b-values from the log i_p vs. log v relationship between peak current and scan rate [50,51,52,53,54,55]. Figure 7a,b displays the linear fittings used to extract these values. The calculated b-values for the electrodes with 2, 3, 4, and 5 deposition cycles were found to be 0.48, 0.49, 0.416, and 0.46, respectively. These values indicate that all electrodes exhibit mixed behavior with a strong diffusion-controlled component. Among them, the 3CY electrode, with a b-value of 0.49, most closely approximated a diffusion-dominated process. The nearly 0.5 b-value observed for the 3CY electrode is a clear indication that the redox reactions occurring during the charge–discharge process are primarily governed by ion diffusion. This can be attributed to the unique microball-like morphology observed on the surface of the 3CY electrode, which is developed as a result of the three-cycle electrodeposition. The interconnected voids and rough surface texture of these microballs facilitate better electrolyte penetration and ion transport, supporting diffusion-controlled electrochemical reactions. To further validate the diffusion-dominated charge storage mechanism, the relationship between i_p (peak current) and v^0.5 (square root of the scan rate) was also analyzed. This method provides a secondary confirmation of the dominant current contribution process and helps to differentiate between the capacitive and diffusion-based behaviors at specific potentials. The linearity of the i_p vs. v^0.5 plots further reinforced the conclusion that the current process in the 3CY electrode is primarily governed by ion diffusion. In addition to the b-value analysis, a quantitative breakdown of the capacitive and diffusion-controlled current contributions was conducted using Equation (9) [50,51,52,53,54,55]. This analysis, visualized in Figure 7c,d, was performed at a fixed scan rate of 40 mV/s and across a range of scan rates. For the 3CY electrode, the analysis revealed that 65.33% of the total current contribution originated from diffusion-controlled processes, while the remaining 34.66% was attributed to capacitive behavior. This predominance of the diffusion-controlled component aligns well with the b-value and i_p-v^0.5 analyses, providing a consistent and comprehensive understanding of the charge storage mechanism.

$$i_p=a{v}^b$$

(8)

$$i_p=k_{c}v+k_d{v}^{1/2}$$

(9)

$${\displaystyle \frac{i_p}{\surd v}}=2.69\times A\times C\times \surd D\times \surd n$$

(10)

$$i_p=0.227ACFn{k}^{0}exp\left[{\displaystyle \frac{\propto nF}{RT}}\left(E_p-E^0\right)\right]$$

(11)

The enhanced diffusion-dominated behavior observed in the 3CY electrode is attributed to its favorable structural features. The rough microball surface morphology not only increases the effective surface area but also introduces void spaces that facilitate ion migration and intercalation. Such features are essential for achieving high-rate capability and long-term cycling stability in supercapacitor electrodes. Further insight into the ion transport properties was obtained by calculating the diffusion coefficient (D) of each electrode. This parameter, derived from the CV data and the linear fitting of i_p vs. v^0.5 using Equation (10), quantifies the ease with which ions move within the electrode material. Figure 7e,f depicts the calculated diffusion coefficients for all electrode samples [50,51,52,53,54,55].

The 3CY electrode demonstrated the highest diffusion coefficient, measured at 1.70 × 10⁻⁶ cm²/s, which was significantly higher than the values observed for other samples. This superior diffusivity again underscores the influence of the optimized surface structure created during the three-cycle electrodeposition process. The presence of interparticle voids, high surface roughness, and a well-connected porous network allows for rapid ion transport, which is essential for efficient redox reactions and high-power performance. To deepen our understanding of the electrochemical reaction kinetics, two additional parameters were examined: the charge transfer coefficient (α) and the standard rate constant (k⁰). These were determined using the CV profiles and Equation (11), with the results presented in Figure 7g,h. The charge transfer coefficient provides information about the symmetry of the energy barrier associated with electron transfer during the redox process. All electrodes exhibited α values in the range of 0.16 to 0.18, indicating that the redox reactions proceed via a quasi-reversible mechanism rather than a fully reversible or irreversible pathway. The standard rate constant, k⁰ is a measure of the intrinsic speed of electron transfer reactions. The values for all electrodes fell within the range of 10⁻¹ < k⁰ < 10⁻⁵, a range typically associated with quasi-reversible electrochemical processes. Notably, the 3CY electrode exhibited a standard rate constant of 3.5 × 10⁻⁴ s⁻¹, further supporting the representing that the redox reactions occurring at this electrode are quasi-reversible in nature. This is a favorable characteristic for supercapacitor electrodes, as quasi-reversible reactions strike a balance between stability and reactivity, allowing for efficient energy storage and release without significant degradation over time. Collectively, the electrochemical parameters obtained by b-value, diffusion coefficient, charge transfer coefficient, and standard rate constant reveal a comprehensive picture of the charge storage behavior of the 3CY electrode. The dominance of diffusion-controlled processes, along with high diffusivity and a quasi-reversible redox mechanism, highlights the suitability of the 3CY electrode for high-performance supercapacitor applications. The comparative b-value, transfer coefficient (α), the standard rate constant, and diffusion coefficient of all NiSe electrodes were listed in

Table 2. The b-value, transfer coefficient (α), the standard rate constant, and diffusion coefficient all NiSe electrode.

Parameter and Electrode code	2CY	3CY	5CY	5CY
b-value	0.49	0.5	0.47	0.49
Transfer coefficient (α)	0.17	0.19	0.17	0.18
Standard rate constant (k0) (×10−4) cm/S	3.3	3.7	3.5	3.7
Diffusion coefficient (D) (×10−6) cm2/S	1.01	1.70	1.56	1.56
Rs(Ω)	1.86	1.59	1.59	1.64

. The key structural advantage lies in its microball-like morphology, which combines roughness, porosity, and interconnected pathways that enhance electrochemical accessibility and charge transport efficiency.

The detailed electrochemical study demonstrates that the three-cycle electrodeposition condition yields a nickel selenide electrode with superior kinetic properties and an ideal morphology for efficient energy storage. The 3CY electrode’s enhanced performance is attributed to its unique structural features, which enable a predominantly diffusion-controlled charge storage process supported by a quasi-reversible redox mechanism. These findings pave the way for the design and development of advanced electrode materials for next-generation supercapacitor devices.

4. Asymmetric Supercapacitor (ASC) Device

Based on the findings from the electrochemical analyses, the 3CY electrode was selected for the fabrication of an asymmetric supercapacitor (ASC) device. The device was assembled using the 3CY electrode as the positive electrode and activated carbon as the negative electrode. This configuration was chosen to evaluate the real-world applicability of the optimized nickel selenide electrode in energy storage systems. The ASC device is expected to benefit from the synergistic combination of the high capacitance and excellent diffusion behavior of the 3CY electrode and the well-established capacitive behavior of activated carbon. To evaluate the practical applicability of the optimized electrode material, an asymmetric supercapacitor (ASC) device was fabricated and thoroughly analyzed. The ASC was designed to integrate the strengths of the synthesized nickel selenide electrode and activated carbon (AC), utilizing their complementary charge storage mechanisms. In this configuration, the nickel selenide electrode, synthesized via electrodeposition at three cycles (referred to as 3CY), served as the positive electrode, while the AC functioned as the negative electrode due to its outstanding electrical conductivity and electric double-layer capacitive behavior. The device assembly was carried out by sandwiching the 3CY nickel selenide electrode and AC using filter paper as a separator. The complete structure was then tightly wrapped with parafilm to minimize electrolyte evaporation and ensure device integrity during measurements. Aqueous 2 M KOH was used as the electrolyte, chosen for its high ionic conductivity, wide electrochemical stability window, and environmental friendliness. This configuration was selected based on the electrochemical compatibility and charge balance between the pseudocapacitive and electric double-layer materials to yield a high-performance hybrid energy storage device. Figure 8a illustrates the CV curves of the ASC recorded at varying potential windows ranging from 1.0 V to 1.6 V at a fixed scan rate. The CV profiles maintained a consistent, nearly rectangular shape even as the potential window increased. This observation suggests excellent electrochemical reversibility and structural stability of the ASC across a wide voltage range. More importantly, there was no significant distortion or loss in the CV shape, even at the highest tested potential of 1.6 V, indicating the absence of decomposition reactions or side reactions within the operating window. This result confirms the suitability of the ASC for stable operation up to 1.6 V, which is relatively high for aqueous-based systems and contributes to enhanced energy density. Figure 8b displays the CV curves obtained at various scan rates from 10 to 100 mV/s within the selected 1.6 V window.

All CV curves showed well-defined capacitive behavior with quasi-rectangular shapes, demonstrating a combination of double-layer and Faradaic charge storage. Notably, as the scan rate increased, the area enclosed by the CV curves also increased proportionally. This trend is a clear indicator of good rate capability, as it signifies the ability of the electrode to maintain capacitance at higher charge/discharge rates. The unaltered shape of the CV profiles, even at high scan rates, reveals the excellent kinetics of ion transport within the electrode, implying a low diffusion resistance and efficient interaction between the electrolyte and the active material. The GCD profiles for the ASC are presented in Figure 8c for different potential windows ranging from 1.0 V to 1.6 V at a constant current density. All discharge curves displayed symmetric triangular shapes, which is an ideal capacitive behavior. Moreover, the charging and discharging durations increased with increasing voltage window, further confirming the ASC’s ability to store higher amounts of energy at extended operating potentials. The lack of IR drops and nonlinearity in the GCD curves also signifies low internal resistance and good electrode–electrolyte compatibility. Figure 8d shows the GCD curves recorded at different current densities to assess the rate capability and calculate key performance parameters such as specific capacitance (C_s), areal capacitance (C_a), and volumetric capacitance (C_v). These curves were analyzed using standard electrochemical equations, including Equations (3)–(5), to extract the capacitance values at various current densities.

The calculated capacitances are illustrated in Figure 8e (specific capacitance), Figure 8g (areal capacitance), and Figure 8i (volumetric capacitance). The ASC delivered a specific capacitance of 18.77 F/g, an areal capacitance of 350 mF/cm², and a volumetric capacitance of 1.09 F/cm³. These values are significant, especially when considering the hybrid nature of the device, and point to the efficient utilization of the active materials within the electrode system. However, a noticeable trend was observed, where the capacitance values declined as the current density increased. This behavior is commonly observed in supercapacitor devices and can be attributed to the kinetics of ion diffusion and charge transfer. At low current densities, the charge/discharge process occurs more gradually, allowing electrolyte ions to deeply penetrate the porous network of the electrode and access more active sites. Conversely, at high current densities, the rapid charge/discharge cycles restrict ion diffusion to only the outer surface of the electrode, limiting the active area involved in the electrochemical process and thus reducing the effective capacitance. This kinetic limitation is particularly pronounced in electrodes with a thick or densely packed morphology, where inner active sites become electrochemically inaccessible under fast cycling conditions. To further assess the energy storage capability of the ASC, both energy density (ED) and power density (PD) were calculated using Equations (6) and (7), based on the specific, areal, and volumetric capacitance values. These performance metrics were plotted as Ragone plots, which are presented in Figure 8f (specific), Figure 8h (areal), and Figure 8j (volumetric). The Ragone plots demonstrate the trade-off relationship between energy and power density in the ASC. The optimized 3CY ASC exhibited competitive energy and power densities, placing it within the performance range of commercial supercapacitor technologies. The combination of high energy density and excellent power delivery capability highlights the ASC’s potential for real-world applications where both fast charging and high energy output are required, such as in electric vehicles, portable electronics, and renewable energy storage systems. An essential requirement for practical application is the long-term cycling stability of the energy storage device. To this end, the ASC was subjected to an extended charge–discharge cycling test for 10,000 cycles at a constant current density. The cyclic performance, shown in Figure 8k, revealed excellent durability, with 91% capacitance retention after 10,000 cycles. Moreover, the Coulombic efficiency—a measure of the charge recovery in each cycle—remained consistently high at 99%, indicating minimal energy loss and excellent reversibility of the electrochemical processes. Such high retention values demonstrate the mechanical and structural stability of the 3CY nickel selenide electrode and confirm that the active material does not undergo significant degradation, phase transformation, or detachment from the current collector during long-term operation. This durability is vital for the commercial deployment of ASC devices, where reliability and lifespan are key criteria for adoption. Electrochemical impedance spectroscopy was performed to understand the charge transport and resistance characteristics of the ASC, both before and after cycling. The EIS Nyquist plots of ASC before and after stability shown in Figure 8l were analyzed to extract the series resistance (R_s) values. Initially, the R_s was measured at 4.9 Ω, indicating good electrical conductivity and efficient ion transport pathways within the device. After 10,000 charge–discharge cycles, the R_s increased slightly to 6.5 Ω, which may be attributed to minor structural changes or degradation of the electrode material and electrolyte interface over time. Despite this increase, the R_s remained within an acceptable range, further confirming the robust nature of the electrode material and device configuration. The impact of cyclic stability was studied using FESEM analysis. Figure 9a–c shows the FESEM micrographs of the 3CY electrode after the long-term cycling test. The surface modifications observed in the FESEM images, despite the high capacitance retention (91% over 10k cycles), typically result from repetitive charge–discharge processes. During cycling, the NiSe electrode undergoes continuous redox reactions (Ni²⁺/Ni³⁺ and Se⁰/Se²⁻), which gradually induce surface restructuring. The formation of surface hydroxide or oxide layers (e.g., Ni(OH)₂ and SeO_x species) can alter the surface morphology, even though the underlying bulk structure remains electrochemically active. The comprehensive electrochemical characterization of the ASC device assembled using the 3CY nickel selenide electrode and activated carbon reveals outstanding performance metrics, including high specific/areal/volumetric capacitance, excellent rate capability, good energy and power densities, and superior cycling stability. The electrode structure effectively combines Faradaic redox reactions and electric double-layer charge storage to deliver hybrid capacitive behavior, making it a promising candidate for advanced energy storage systems.

5. Conclusions

This study demonstrated the impact of electrodeposition cycles on the electrochemical behavior of a nickel selenide electrode, highlighting the nickel selenide 3CY electrode as the optimal configuration. The nickel selenide 3CY electrode delivered the highest specific capacitance (507.42 F/g). It also exhibited superior energy and power densities (22.89 Wh/kg and 584.61 W/kg, respectively), supported by the lowest R_s value (1.59 Ω), indicating efficient charge conduction. In the asymmetric supercapacitor configuration with activated carbon, the 3CY//AC device showed remarkable specific capacitance (18.78 F/g), areal capacitance (351 mF/cm²), and volumetric capacitance (1.09 F/cm³). The device also provided an energy density of 8.45 Wh/kg and sustained a power output of 385.03 W/kg, proving its applicability in high-performance energy storage. Post-stability analysis revealed minor resistance increase (R_s from 4.9 to 6.5 Ω) and excellent cycling retention (91% over 10,000 cycles) with 99% Columbic efficiency, underscoring its long-term durability. Overall, the results validate that nickel selenide-3CY synthesized via optimized electrodeposition is a viable and scalable electrode material for high-performance asymmetric super capacitors.