Rare-Earth-Induced Structural Modulation of NiFe₂O₄ for High-Energy Asymmetric Supercapacitor Devices

Abstract

The rational design of electrode materials with tailored composition and architecture is crucial for advancing high-capability electrochemical energy storage systems. This study reports that gadolinium-modified NiFe₂O₄ nanosheet electrodes were effectively synthesized on nickel foam via a hydrothermal approach followed by thermal treatment. A series of compositions (NiFe, NiFe–Gd1, NiFe–Gd2, and NiFe–Gd3) were prepared to systematically examine the effect of Gd incorporation on structural features and electrochemical properties. X-ray diffraction (XRD) analysis confirmed the formation of the cubic spinel NiFe₂O₄ phase without detectable secondary phases, indicating that the crystal structure remains intact after Gd introduction. X-ray photoelectron spectroscopy (XPS) further verified the presence of Ni²⁺, Fe³⁺, and Gd³⁺ species within the lattice environment. Morphological analysis using field-emission scanning electron microscopy (FESEM) revealed a nanosheet-based architecture, where the optimized NiFe–Gd2 electrode exhibited a porous and interconnected nanosheet framework with abundant exposed edges. This structural configuration improves electrolyte penetration and facilitates efficient ion transport during charge storage processes. Electrochemical measurements demonstrated that the NiFe–Gd2 electrode delivers an areal capacitance of 5235 mF cm⁻² at 10 mA cm⁻², along with improved reaction kinetics and low internal resistance. An asymmetric supercapacitor assembled using NiFe–Gd2 as the positive electrode and activated carbon as the negative electrode operated stably within a 0–1.5 V potential window, achieving an energy density of 0.136 mWh cm⁻² and a power density of 3.14 mW cm⁻², while retaining 86.55% of its initial capacitance after 7000 cycles. These results highlight the potential of rare-earth engineering as a viable strategy for designing advanced spinel ferrite electrodes and pave the way for the development of high-performance, durable, and scalable supercapacitor systems for practical energy storage applications.

1. Introduction

The decarbonization of modern energy infrastructures requires electrochemical storage systems that can efficiently manage intermittent power generation while sustaining high-rate energy delivery [1,2]. The rising integration of intermittent renewable sources into power grids, together with the expanding market for portable and miniaturized electronics, requires energy storage devices that can operate efficiently under high power conditions without compromising durability [3]. While lithium-ion batteries dominate high-energy applications, their inherently slow charge–discharge kinetics, coupled with safety concerns at high-rate operation, restrict their suitability for fast energy buffering [4]. In this context, supercapacitors have emerged as complementary systems due to their high-power density and fast electrochemical response, and extended cycle life. However, their comparatively lower energy density remains a fundamental limitation, necessitating advanced material design strategies to overcome the intrinsic performance disparity between batteries and conventional capacitors [5]. The performance of supercapacitors is intrinsically governed by their charge storage mechanism. Electrochemical double-layer capacitors (EDLCs), typically based on porous carbon materials, store charge via reversible electrostatic ion adsorption/desorption at the electrode–electrolyte interface, ensuring excellent cycling stability but limited capacitance [6]. Conversely, pseudocapacitors rely on fast, reversible Faradaic charge-transfer reactions of redox-active materials, enabling significantly higher charge storage capacity [7]. Transition metal oxides such as Ruthenium dioxide (RuO₂), Manganese dioxide (MnO₂), Nickel oxide (NiO), and Cobalt oxide (Co₃O₄) have been widely investigated because of their rich redox chemistry and high theoretical capacitance [8]. While RuO₂ exhibits excellent electrochemical reversibility, its high-cost limits practical applications. The other oxides, although economically viable, often suffer from insufficient low electrical conductivity coupled with structural instability during prolonged redox cycling. These boundaries have prompted exploration of multicomponent oxide systems with enhanced electronic and structural stability [9]. Spinel-structured mixed transition metal oxides (AB₂O₄) have attracted particular attention due to the synergistic interaction between two metal cations within a single crystalline lattice [10]. The coexistence of multiple redox-active centers promotes enhanced electrochemical activity, while the spinel framework provides structural robustness during ion insertion and extraction [11]. Among these materials, Nickel ferrite (NiFe₂O₄) stands out as a promising pseudocapacitive electrode owing to its low cost, environmental compatibility, and chemical stability [12]. The presence of Ni²⁺/Ni³⁺ and Fe²⁺/Fe³⁺ redox couples enable multiple electron-transfer pathways, contributing to improved charge storage performance compared with single-component oxides [13].

Emerging research highlights the significant promise of NiFe₂O₄-derived systems in energy storage technologies. Vidyadevi et al. reported that NiFe₂O₄ thin films synthesized by spray pyrolysis exhibited a cubic spinel structure and delivered a high specific capacitance of 707 F g⁻¹ at 2 mV s⁻¹ with an energy density of 83.88 Wh kg⁻¹ in 1 M KOH [14]. Hsu et al. investigated Bi-substituted NiFe₂O₄ (NiFe_2−x Bi_xO₄, 20% Bi), achieving 339.16 F g⁻¹ with 90.76% capacitance retention after 5000 cycles [13]. Zhu et al. enhanced electrochemical performance through composite engineering by integrating NiFe₂O₄ with N-doped porous carbon, increasing capacitance from 136.4 to 201.4 F g⁻¹ at 1 A g⁻¹ with 85.7% retention after 10,000 cycles [15]. Furthermore, Yan et al. reported Mn-doped NiFe₂O₄ nanosheet arrays delivering an areal capacitance of 8.68 F cm⁻² and demonstrating excellent cycling stability in hybrid supercapacitor devices [16]. Although these investigations confirm that morphology control, composite formation, and transition metal substitution can improve electrochemical performance, the intrinsic electronic conductivity and lattice-level defect chemistry of NiFe₂O₄ remain only partially optimized. Most reported modifications focus on surface engineering or secondary phase integration, while controlled structural modulation within the spinel lattice has received comparatively less systematic attention [17].

Cation substitution at the lattice level offers a direct strategy to regulate crystal structure, electronic environment, and defect formation. Transition metal dopants such as Co, Mn, and Zn can influence cation distribution and moderately enhance conductivity [18]. However, rare-earth elements introduce stronger lattice perturbations because of their larger ionic radii and distinct electronic configurations. Their incorporation can induce lattice strain, alter metal–oxygen bond characteristics, and generate oxygen vacancies, thereby facilitating improved charge transport and electrochemical kinetics. Among rare-earth ions, Gadolinium (Gd) is particularly attractive for structural engineering of spinel ferrites. Partial substitution of Gd into the Fe sublattice can lead to controlled lattice expansion, modified cation redistribution, and enhanced defect density [19]. Such rare-earth-induced structural modulation is expected to improve electronic conductivity, increase electroactive site accessibility, and accelerate ion diffusion during redox cycling [20]. Despite these promising attributes, a systematic correlation between Gd-induced lattice modification and device-level electrochemical performance in NiFe₂O₄-based asymmetric supercapacitors has not been comprehensively established [21,22].

The present study focuses on evaluating the impact of rare-earth doping on both the structural features and electrochemical performance of NiFe₂O₄ through the controlled incorporation of Gd into its spinel structure. A series of Gd-doped NiFe₂O₄ samples with different dopant concentrations was synthesized to determine the optimum Gd content and to better understand how doping influences the crystal structure, morphology, and charge storage behavior. Detailed structural and electrochemical studies were performed to establish a clear relationship between Gd-induced structural changes and supercapacitor performance. In addition, the optimized material was assembled into an asymmetric supercapacitor device to evaluate its energy density, power density, and cycling stability under practical operating conditions.

2. Experimental Section

Preparation of NiFe₂O₄ and Gd-Doped NiFe₂O₄ Electrodes

NiFe₂O₄ and Gd-doped NiFe₂O₄ electrodes were fabricated in situ on nickel foam using a hydrothermal process followed by annealing. For the preparation of 200 mL precursor solution, 0.10 mol of nickel nitrate hexahydrate (Ni (NO₃)₂·6H₂O) and 0.20 mol of ferric nitrate nonahydrate (Fe(NO₃)₃·9H₂O) were dissolved in deionized water under continuous stirring, maintaining a Ni²⁺:Fe³⁺ molar ratio of 1:2. The mixture was continuously stirred until a clear and uniform solution was obtained. For Gd doping, calculated amounts of gadolinium nitrate (Gd(NO₃)₃) were added to the precursor solution to obtain 1 mol %, 2 mol %, and 3 mol % substitution levels. The mixture was stirred thoroughly to ensure uniform distribution of Gd ions. The solution pH was subsequently adjusted to 9–10 through the gradual addition of ammonia solution under constant stirring to promote controlled precipitation of the mixed hydroxides. Nickel foam (NF) substrates (2 × 3 cm²) were cleaned by ultrasonication in 3 M hydrochloric acid (HCl), ethanol, and deionized water for 10 min each to remove surface impurities. The cleaned substrates were immersed in the prepared precursor solution, which was then transferred into a Teflon-lined stainless-steel autoclave(4science.net, Seongnam-si, Gyeonggi-do, Republic of Korea) and heated at 180 °C for 12 h. Upon cooling naturally to ambient temperature, the samples were removed, rinsed thoroughly with deionized water and ethanol, and dried at 80 °C. Finally, the electrodes were annealed in air at 500 °C for 3 h to obtain crystalline NiFe₂O₄. The prepared samples were labeled as NiFe (undoped), NiFe–Gd1 (1 mol % Gd), NiFe–Gd2 (2 mol % Gd), and NiFe–Gd3 (3 mol % Gd). The as-prepared electrodes were directly used as binder-free working electrodes for electrochemical measurements.

Table 1. Precursor composition and nominal stoichiometry of synthesized samples.

Sample	Ni (mol)	Fe (mol)	Gd (mol%)	Nominal Composition
NiFe	0.10	0.20	0	NiFe2O4
NiFe–Gd1	0.10	0.20	1 mol%	NiFe2O4:Gd (1 mol%)
NiFe–Gd2	0.10	0.20	2 mol%	NiFe2O4:Gd (2 mol%)
NiFe–Gd3	0.10	0.20	3 mol%	NiFe2O4:Gd (3 mol%)

Table 1. Precursor composition and nominal stoichiometry of synthesized samples.

Sample	Ni (mol)	Fe (mol)	Gd (mol%)	Nominal Composition
NiFe	0.10	0.20	0	NiFe2O4
NiFe–Gd1	0.10	0.20	1 mol%	NiFe2O4:Gd (1 mol%)
NiFe–Gd2	0.10	0.20	2 mol%	NiFe2O4:Gd (2 mol%)
NiFe–Gd3	0.10	0.20	3 mol%	NiFe2O4:Gd (3 mol%)

3. Sample Characterization and Electrochemical Measurements

The structural, surface, and electrochemical properties of the synthesized NiFe and Gd-doped NiFe₂O₄ electrodes were examined using standard characterization techniques. Phase identification was carried out by XRD using a PANalytical X’Pert PRO (XRD: PANalytical, Almelo, The Netherlands) diffractometer with Cu-Kα radiation (λ = 1.5406 Å), operated at 40 kV and 40 mA. The chemical states and surface composition were further investigated by XPS using a Thermo Scientific Kα⁺ system (XPS: Cheshire, UK analysis system) equipped with a monochromatic Al Kα source (hν = 1486.6 eV). The surface morphology of the samples was analyzed using field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800, Chiyoda, Tokyo, Japan) at an accelerating voltage of 5 kV. Prior to imaging, a thin platinum coating was applied to avoid surface charging. Elemental composition and distribution of Ni, Fe, Gd, and O were determined by energy-dispersive X-ray spectroscopy (EDS) attached to the FE-SEM system. Electrochemical measurements were performed using a Biologic WBCS3000 (BioLogic Science Instruments, Seyssinet-Pariset, France) workstation in a conventional three-electrode configuration. The as-prepared NiFe and NiFe–Gd electrodes grown on nickel foam served directly as the working electrodes, while a platinum wire and Ag/AgCl electrode were used as the counter and reference electrodes, respectively. A 2 M KOH aqueous solution was used as the electrolyte. Cyclic voltammetry (CV) measurements were recorded within an appropriate potential window at various scan rates to evaluate redox behavior. Galvanostatic charge–discharge (GCD) tests were conducted at different current densities to determine specific capacitance and rate performance. Electrochemical impedance spectroscopy (EIS) was carried out over a frequency range from 100 kHz to 0.01 Hz with a small AC amplitude to analyze charge transfer resistance and internal resistance characteristics.

4. Results and Discussions

4.1. XRD Elucidation

The crystal structure of the prepared NiFe and Gd-modified NiFe₂O₄ samples was examined by XRD, and the corresponding patterns are depicted in Figure 1. All the observed diffraction peaks can be well indexed to the cubic spinel structure of NiFe₂O₄ and are consistent with the standard JCPDS card No. 00-044-1485 [23]. The characteristic reflections appearing at approximately 2θ ≈ 18.3°, 30.2°, 35.5°, 37.13°, 43.3°, 47.7°, 53.6°, 57.2°, and 62.8° correspond to the (111), (220), (311), (222), (400), (331), (511), (440) and (533) crystallographic planes, respectively. Among these reflections, the (311) peak exhibits the highest intensity, which is a typical feature of crystalline spinel NiFe₂O₄. After the incorporation of Gd, the overall diffraction profiles remain almost unchanged, indicating that the fundamental spinel framework is preserved. No additional diffraction peaks related to secondary phases or Gd-based oxides are detected within the resolution limit of the instrument, suggesting that Gd is effectively incorporated into the NiFe₂O₄ lattice rather than forming a separate crystalline phase. A slight variation in peak position and intensity can be noticed for the Gd-modified samples compared with the bare NiFe. A closer examination of the primary diffraction peak reveals a slight shift toward lower 2θ values with increasing Gd content. Specifically, the peak position shifts from 35.60° for pristine NiFe to 35.32° for the NiFe-Gd3 sample, indicating lattice expansion. This behavior can be attributed to the substitution of Fe³⁺ ions by larger Gd³⁺ ions, resulting in lattice distortion within the spinel framework. The lattice parameter (a) was calculated using Bragg’s law and the cubic spinel relation based on the (311) reflection. The calculated values show a gradual increase from 8.36 Å for pristine NiFe, 8.38 Å for NiFe-Gd1, 8.40 Å for NiFe-Gd2 to 8.42 Å for the NiFe-Gd3 sample, further confirming the successful incorporation of Gd into the host lattice. Furthermore, the average crystallite size was estimated using the Debye–Scherrer Equation (1) [24]:

$$D={\displaystyle \frac{0.9\lambda }{\beta cos\theta }}$$

(1)

where λ is the X-ray wavelength (1.5406 Å), β is the full width at half maximum (FWHM), and θ is the Bragg angle. The calculated crystallite size decreases from approximately 34 nm for pristine NiFe, 28 nm for NiFe-Gd1, 24 nm for NiFe-Gd2, to 19 nm for NiFe-Gd3, indicating that Gd incorporation influences nucleation and growth kinetics, leading to refined nanocrystalline domains. From above all discussion, the XRD results confirm that the introduction of Gd does not alter the basic cubic spinel structure of NiFe₂O₄, while slightly modifying the lattice environment. It is important to note that Gd ions were introduced in small molar percentages (1–3 mol%) relative to Fe³⁺ without altering the initial Ni:Fe ratio. Despite this, XRD analysis shows no secondary phases and a systematic shift in primary peaks toward lower 2θ values along with lattice expansion, confirming effective incorporation of Gd into the NiFe₂O₄ lattice. This incorporation is likely achieved through partial substitution and defect-site occupation, leading to lattice distortion and enhanced electrochemical activity. Such structural modulation is expected to influence the electrochemical behavior of the material [25].

Figure 1. XRD patterns of pristine NiFe and Gd-incorporated NiFe₂O₄ samples.

4.2. XPS Analysis

The surface chemical composition and oxidation states of the optimized NiFe–Gd2 sample were analyzed using XPS, and the corresponding high-resolution spectra are presented in Figure 2. The Ni 2p spectrum (Figure 2a) exhibits multiple components associated with the spin–orbit splitting of nickel. Two main peaks appear at binding energies of approximately 855.7 eV and 873.5 eV, corresponding to the Ni 2p_3/2 and Ni 2p_1/2 states. Additional components located near 856.6 eV and 874.6 eV are also observed, which are generally attributed to the multiplet splitting and satellite features characteristic of Ni²⁺ species in spinel ferrite structures. The presence of these satellite-related features indicates the oxidized state of nickel and confirms its participation in the NiFe₂O₄ lattice. The Fe 2p spectrum (Figure 2b) shows characteristic peaks located at approximately 710 eV and 724 eV, corresponding to Fe 2p_3/2 and Fe 2p_1/2, respectively. A component appearing near 712 eV is also evident, which is commonly associated with the multiplet splitting of iron and is indicative of the Fe³⁺ oxidation state in ferrite materials [26]. The observed binding energy positions are consistent with previously reported values for spinel NiFe₂O₄, confirming that the ferrite framework remains stable after the introduction of Gd. The coexistence of Ni²⁺ and Fe³⁺ species provides multiple redox-active sites, facilitating efficient charge storage through reversible electron transfer processes. Further evidence for the incorporation of gadolinium is provided by the Gd 3d spectrum (Figure 2c), where two distinct peaks are observed at approximately 1190.5 eV and 1224.4 eV, corresponding to the Gd 3d_5/2 and Gd 3d_3/2 states. These peak positions are characteristic of Gd³⁺, indicating that gadolinium is present in its trivalent oxidation state within the material. The incorporation of Gd ions into the ferrite structure can modify the local electronic environment and influence the interaction between the metal cations [25]. Unlike transition metals, Gd³⁺ does not directly participate in redox reactions; however, its presence significantly alters the local electronic structure. Owing to its larger ionic radius and strong electrostatic interaction, Gd³⁺ substitution induces lattice distortion and modifies the cation distribution within the spinel structure. The O 1s spectrum (Figure 2d) can be deconvoluted into two main components. The dominant peak centered at around 530.5 eV is attributed to lattice oxygen bonded to metal cations, representing the metal–oxygen framework of the spinel structure. A second component located at higher binding energy in the range of 531.8 eV corresponds to surface-adsorbed oxygen species or hydroxyl groups. These oxygen species are often associated with surface defects or oxygen vacancies, which can contribute to enhanced surface activity and improved charge transfer characteristics during electrochemical processes [27]. Notably, the relative intensity of this defect-related oxygen component is quit high in the Gd-modified sample, suggesting an increased concentration of oxygen vacancies, consistent with previous reports on doped spinel ferrites. The formation of these vacancies is primarily driven by lattice distortion and charge imbalance introduced by Gd³⁺ incorporation. Oxygen vacancies in oxide and spinel systems are known to exist in different charge states, including neutral, singly ionized, and doubly ionized vacancies, as reported in previous studies [28,29,30]. Such defect states are closely associated with lattice distortion and charge compensation mechanisms in complex oxides. In this context, the possible presence of oxygen vacancies in the present system is interpreted based on indirect experimental indications (XPS and XRD) in conjunction with established literature reports. These oxygen vacancies play a pivotal role in improving electrochemical performance by acting as active sites for OH⁻ adsorption, facilitating ion diffusion, and enhancing surface reactivity.

Figure 2. High-resolution XPS spectra of the optimized NiFe–Gd2 electrode showing (a) Ni 2p, (b) Fe 2p, (c) Gd 3d, and (d) O 1s regions.

4.3. Morphological and Elemental Composition

The surface morphology of the prepared samples was examined using FESEM, and the corresponding images are shown in Figure 3. The bare NiFe sample (Figure 3(a1–a3)) exhibits a structure composed mainly of stacked nanosheets with relatively smooth surfaces. These nanosheets are closely packed and partially overlapped, forming a compact layered arrangement with limited intersheet spacing. Such dense stacking may reduce the accessibility of electrolyte ions to the active surface. After the introduction of Gd, a noticeable change in the nanosheet morphology can be observed. The NiFe–Gd1 sample (Figure 3(b1–b3)) shows comparatively thinner nanosheets with a slightly more open arrangement. The sheets appear loosely interconnected and form a layered network with increased spacing between adjacent nanosheets. A more pronounced structural evolution is observed for the NiFe–Gd2 sample (Figure 3(c1–c3)). The morphology consists of well-developed nanosheets with sharp edges and thin protruding features distributed over the surface. The nanosheets appear more uniformly arranged, creating a porous framework with sufficient void spaces between the layers. Such nanosheet-based architecture can provide a larger exposed surface area and facilitate electrolyte diffusion through the structure. In contrast, the NiFe–Gd3 sample (Figure 3(d1–d3)) shows a relatively compact morphology. The nanosheets appear thicker and partially aggregated, and several cracks are visible across the surface. The formation of these compact regions may decrease the effective surface area and limit ion transport pathways. These observations suggest that moderate incorporation of Gd influences the growth of the nanosheet structure. Among the studied samples, NiFe–Gd2 exhibits a more favorable nanosheet arrangement with a relatively open and accessible structure, which may contribute to improved electrochemical behavior [31].

Figure 3. FESEM images of (a1–a3) NiFe, (b1–b3) NiFe–Gd1, (c1–c3) NiFe–Gd2, and (d1–d3) NiFe–Gd3 samples.

A semi-quantitative morphological analysis was carried out using FESEM images through scale-bar calibration. The nanosheet thickness was estimated by measuring multiple regions across different micrographs to ensure statistical reliability. The pristine NiFe electrode shows relatively thick nanosheets (~45 nm), while Gd incorporation progressively reduces thickness, with NiFe–Gd1 have thickness around 40.18 nm. Further NiFe–Gd2 exhibiting ultrathin nanosheets with ~27 nm thickness and enhanced porosity. In contrast, NiFe–Gd3 shows increased thickness and partial aggregation. These structural variations strongly influence electrolyte accessibility and ion diffusion kinetics, thereby governing electrochemical performance.

The elemental composition of the prepared samples was examined by energy dispersive EDS, and the corresponding spectra of NiFe, NiFe–Gd1, NiFe–Gd2, and NiFe–Gd3 are shown in Figure 4(a1–d1). The spectrum of the NiFe sample displays characteristic peaks of Ni, Fe, and O, confirming the successful formation of NiFe₂O₄. The quantitative results listed in the inset table indicate the relative weight percentages of the detected elements. With the introduction of Gd, additional peaks corresponding to Gd appear in the spectra of NiFe–Gd1, NiFe–Gd2, and NiFe–Gd3, confirming the successful incorporation of Gd into the NiFe₂O₄ framework. Furthermore, the Gd content gradually increases from NiFe–Gd1 to NiFe–Gd3, consistent with the designed variation in dopant concentration. Elemental mapping analysis was carried out to visualize the spatial distribution of the elements. The mapping images of NiFe (Figure 4(a2–a4)) reveal a uniform distribution of Ni, Fe, and O across the observed region, indicating good compositional homogeneity. Similar distribution patterns are observed for the Gd-containing samples. For NiFe–Gd1 (Figure 4(b2–b5)), Ni, Fe, O, and Gd are evenly dispersed, suggesting effective integration of the dopant within the host structure. The NiFe–Gd2 and NiFe–Gd3 samples (Figure 4(c2–c5,d2–d5) also exhibit homogeneous elemental distribution without noticeable aggregation. The EDS spectra and mapping results confirm the presence of all expected elements and verify the successful incorporation of Gd into the NiFe₂O₄ structure [14].

Figure 4. EDS spectra and elemental mapping images of (a1–a4) NiFe, (b1–b5) NiFe–Gd1, (c1–c5) NiFe–Gd2, and (d1–d5) NiFe–Gd3 showing the uniform distribution of Ni, Fe, O, and Gd elements.

4.4. Electrochemical Analysis

The electrochemical behavior of the pristine and Gd-substituted NiFe electrodes (1, 2, and 3 mol % Gd) was comprehensively evaluated in a three-electrode configuration using 2 M KOH electrolyte. CV, GCD, and EIS were employed to systematically assess the influence of rare-earth incorporation on charge storage characteristics. Figure 5a presents the CV curves recorded between 0 and 0.5 V at a scan rate of 1 mV s⁻¹. All electrodes exhibit well-defined anodic and cathodic peaks, confirming that the capacitance originates predominantly from reversible faradaic reactions rather than purely electrostatic double-layer charging. These redox features are associated with the reversible oxidation and reduction in transition metal species within the spinel framework. The distinct peak formation verifies the battery-type behavior typical of ferrite-based pseudocapacitive materials [32]. To further examine the kinetic response, CV measurements were conducted at scan rates ranging from 1 to 100 mV s⁻¹ for NiFe, NiFe-Gd1, NiFe-Gd2, and NiFe-Gd3 (Figure 5b–e). A progressive increase in current response with increasing scan rate was observed for all samples, while the overall curve shape remained preserved. This behavior reflects good structural integrity and rapid charge transfer capability under accelerated polarization conditions. The quasi-symmetrical profiles suggest efficient OH⁻ diffusion and fast interfacial redox kinetics. Among the investigated compositions, the NiFe-Gd2 electrode demonstrates the most intense redox peaks and the largest integrated CV area, signifying superior charge storage capability. The enhanced electrochemical performance can be attributed to the optimized Gd incorporation level, which effectively modifies the electronic structure and defect chemistry without compromising crystallinity. Moderate Gd substitution likely introduces lattice distortion and oxygen vacancies, thereby increasing electrical conductivity and creating additional electroactive sites [14,33]. The superior behavior of the NiFe-Gd2 electrode is further supported by its favorable nanosheet morphology, characterized by uniformly distributed and interconnected structures. This architecture promotes extensive electrode–electrolyte contact, short ion diffusion pathways, and efficient electron transport. In contrast, the NiFe-Gd1 sample exhibits comparatively lower current response, which can be linked to insufficient structural evolution and limited exposure of active sites. Excessive doping in NiFe-Gd3 leads to partial structural densification and aggregation, which may hinder electrolyte accessibility and restrict ion migration. The undoped NiFe electrode displays the lowest electrochemical activity, reflecting its relatively less optimized nanostructural features and reduced defect density. The charge storage mechanism in NiFe_2−xGd_xO₄ under alkaline conditions is governed by reversible redox transitions of Ni and Fe species. The overall electrochemical reaction can be expressed as Equation (2) [25]:

$$Ni{Fe}_{2-x}{{Gd}_{x}O}_4+H_{2}O+{OH}^{-}\rightleftharpoons NiOOH+FeOOH+e^{-}$$

(2)

Figure 5. CV analysis of NiFe and Gd-incorporated electrodes: (a) CV curves at 1 mV s⁻¹, (b–e) CV curves at different scan rates for all electrodes, (f) Peak current vs. square root of scan rate, (g) Diffusion coefficient comparison, and (h) b-value determination.

The dominant contribution arises from the Ni²⁺/Ni³⁺ transformation, accompanied by Fe²⁺/Fe³⁺ redox transitions. Importantly, Gd³⁺ ions remain electrochemically stable within the spinel lattice and do not directly participate in faradaic reactions. Instead, their role is structural and electronic in nature. The incorporation of larger Gd³⁺ ions induce local lattice distortion and facilitates the formation of oxygen vacancies, which enhance charge carrier mobility and improve OH⁻ adsorption and diffusion at the electrode surface. Consequently, optimized Gd substitution effectively amplifies pseudocapacitive behavior through defect-mediated conductivity enhancement rather than direct redox activity [25,34].

To further clarify the kinetic characteristics of the electrodes, the CV responses recorded at scan rates ranging from 1 to 100 mV s⁻¹ were systematically examined. The dependence of peak current (i_p) on the square root of scan rate (v^1/2) is illustrated in Figure 5f. A strong linear relationship is observed for all compositions, indicating that the electrochemical process is predominantly governed by diffusion-controlled faradaic reactions. This behavior confirms that ion transport within the electrode–electrolyte interface plays a decisive role in determining overall charge storage performance. To quantitatively evaluate ion transport properties, the apparent diffusion coefficients (D) of the electroactive species were estimated using the Randles–Sevcik formalism applicable to reversible redox systems (3) [35]:

$$D={\displaystyle \frac{i_p}{2.69 \times {10}^5 \times n^{3/2} \times A \times C \times v^{1/2}}}$$

(3)

where i_p represents the peak current, n is the number of electrons transferred, A denotes the effective electroactive surface area (cm²), C is the concentration of the electrolyte species, and v is the scan rate. The resulting values are summarized in Table 2 and comparatively visualized in Figure 5g. Among the investigated samples, the NiFe-Gd2 electrode exhibits the highest diffusion coefficients for both anodic and cathodic processes, signifying substantially improved ion mobility and faster redox kinetics. The enhancement in diffusion behavior can be directly correlated with the optimized rare-earth incorporation level. Moderate Gd³⁺ substitution induces controlled lattice distortion, which collectively improve ionic accessibility and reduce transport resistance. In contrast, the undoped NiFe electrode demonstrates comparatively sluggish diffusion characteristics, attributable to its relatively less favorable nanostructure. Similarly, the NiFe-Gd1 sample shows only moderate improvement, suggesting that insufficient doping does not fully activate defect-assisted ion pathways. Excessive substitution in NiFe-Gd3, however, appears to compromise structural homogeneity, leading to partial densification and restricted electrolyte infiltration, thereby lowering the effective diffusion coefficient. The superior diffusion kinetics exhibited by NiFe-Gd2 highlight the critical role of controlled rare-earth doping in tailoring ion transport dynamics within spinel ferrite electrodes. By optimizing concentration and nanoscale architecture simultaneously, the electrode achieves a favorable balance between structural integrity and rapid electrochemical reactivity, rendering it highly promising for advanced high-rate energy storage systems.

Table 2. Performance metrics of diffusion coefficients (D), b-values, and ESR of studied electrodes.

Sample Code	Diffusion Coefficient (cm2/s) × 10−7		b-Value	Rs (Ω)
	Oxidation	Reduction
NiFe	8.628	9.107	0.40	0.7306
NiFe-Gd1	14.626	17.874	0.31	0.6943
NiFe-Gd2	27.38	31.805	0.44	0.6798
NiFe-Gd3	7.804	7.846	0.37	0.8321

Further, the intrinsic kinetics governing charge storage in the NiFe, NiFe-Gd1, NiFe-Gd2, and NiFe-Gd3 electrodes was performed (Figure 5h). The relationship between peak current and scan rate was quantitatively examined using the widely accepted power-law model Equations (4) and (5) [36,37]:

$$i ={av}^b$$

(4)

$$\mathit{log}\left(i\right)=\mathit{log}\left(a\right)+b log\left(v\right)$$

(5)

where i denotes the redox peak current at a scan rate (v), a is a constant related to the electroactive surface characteristics, and b is the kinetic exponent that reflects the dominant charge storage pathway. The magnitude of the b-value serves as a reliable indicator of electrochemical behavior: a value near 0.5 implies a semi-infinite diffusion-controlled process, while a value approaching 1.0 corresponds to a surface-controlled capacitive mechanism [38]. Linear regression of the log(i) versus log(v) plots yielded b-values in the range of 0.3–0.45 for all investigated electrodes (Table 2). These intermediate values clearly demonstrate that the redox process is purely dominated by diffusion-limited with minor influence of surface-driven; it arises from a synergistic contribution of diffusion-assisted faradaic reactions and interfacial pseudocapacitive effects. Such hybrid kinetics are characteristic of transition metal oxide systems, where charge storage proceeds through reversible redox transitions accompanied by ion insertion/extraction within the near-surface lattice region [39]. Notably, the optimized NiFe-Gd2 electrode exhibited comparatively b-value near to 0.5 range, suggesting an enhanced diffusion-controlled contribution. This behavior can be attributed to the structural modulation induced by the appropriate Gd³⁺ incorporation, which generates lattice distortion that promote electrolyte accessibility and accelerate charge-transfer dynamics.

To quantitatively distinguish between interfacial capacitive effects and bulk diffusion-controlled processes, the measured current response was deconvoluted according to the relationship Equation (6) [40]:

$$i\left(V\right)=k_{1}v{+k_{2}v}^{1/2}$$

(6)

In this expression, the term k₁v corresponds to the surface-controlled contribution originating from rapid redox reactions and electrostatic charge accumulation at or near the electrode–electrolyte interface. Conversely, k₂v^1/2 represents the diffusion-governed component associated with ion transport into the interior of the active material and the accompanying faradaic transformations. The constants k₁ and k₂ were extracted by plotting i(V)/v^1/2 as a function of v^1/2, enabling precise separation of the two charge storage pathways. Accordingly, the total stored charge (Q_t) can be expressed as the sum of surface-derived charge (Q_s) and diffusion-mediated charge (Q_d) Equation (7) [40]:

$$Q_t=Q_s+Q_d$$

(7)

At a scan rate of 1 mV s⁻¹, the relative capacitive/diffusion contributions for NiFe, NiFe-Gd1, NiFe-Gd2, and NiFe-Gd3 were determined to be 8.0/92.0%, 6.0/94.0%, 4.0/96.0%, and 14.0/86.0%, respectively (Figure 6a). These results clearly indicate that Gd incorporation substantially amplifies the diffusion-dominated fraction of charge storage, with the NiFe-Gd2 electrode exhibiting the highest diffusion-controlled contribution (~96%). Such a dominant bulk-redox participation signifies accelerated ion migration and highly efficient faradaic kinetics within the optimized structure. The evolution of charge storage behavior with increasing scan rate (1–20 mV s⁻¹) was also systematically examined (Figure 6b–e). The b-values in the range of 0.31–0.44 indicate that the charge storage process is predominantly governed by diffusion-controlled faradaic reactions. However, further quantitative analysis using Dunn’s method reveals a finite capacitive contribution at all scan rates, confirming that surface-controlled processes also participate to a limited extent. Thus, the overall mechanism is best described as diffusion-dominated with a secondary capacitive contribution. For all compositions, the fraction of surface-controlled contribution progressively increased at higher sweep rates. This trend reflects the kinetic limitation imposed by rapid potential variation, where electrolyte ions cannot effectively penetrate into deeper regions of the electrode and thus predominantly participate in near-surface reactions [41]. Despite this expected shift toward capacitive dominance at elevated scan rates, NiFe-Gd2 consistently preserved a comparatively larger diffusion-driven component than the other samples. This retention of bulk-redox contribution even under fast polarization conditions underscores its superior ion accessibility, structural robustness, and highly reversible electrochemical behavior.

Figure 6. (a) Capacitive and diffusion-controlled contribution at 1 mV s⁻¹, (b–e) Charge storage contribution analysis showing capacitive and diffusion-controlled contributions for NiFe and Gd-modified electrodes at different scan rates.

The GCD characteristics were systematically examined to further elucidate the charge storage efficiency and rate capability of the synthesized electrodes. A comparative set of GCD profiles obtained at 10 mA cm⁻² within the potential window of 0 to 0.45 V is presented in Figure 7a, clearly highlighting the compositional dependence of electrochemical performance. To comprehensively assess rate behavior, the measurements were extended over a broad current density range (10–50 mA cm⁻²) for pristine NiFe and the Gd-doped variants (NiFe-Gd1, NiFe-Gd2, and NiFe-Gd3), as illustrated in Figure 7b–e. All electrodes exhibited distinctly nonlinear discharge curves accompanied by well-defined voltage plateaus, a characteristic signature of battery-type pseudocapacitive materials dominated by diffusion-assisted faradaic redox reactions [42]. Amongst the investigated samples, NiFe-Gd2 demonstrated the most pronounced characteristics with a smooth and gradual voltage decay, reflecting highly reversible ion insertion/extraction processes coupled with surface-confined redox activity. More importantly, its discharge duration was significantly prolonged relative to the other compositions, directly signifying enhanced charge storage capacity [43]. The nearly comparable symmetry between charge and discharge branches across all electrodes indicates high coulombic efficiency and excellent electrochemical reversibility. Especially, NiFe-Gd2 exhibited the smallest instantaneous voltage drop (IR-drop) at the onset of discharge, along with an almost ideal symmetry in its GCD profile. These features confirm reduced internal resistance, accelerated charge-transfer kinetics, and improved electrical conductivity. The IR-drop behavior as a function of current density (Figure 8a) further substantiates this observation: lower current densities produced diminished ohmic drops for all electrodes, consistent with reduced polarization and improved interfacial contact [44]. However, NiFe-Gd2 consistently maintained the lowest IR-drop across the entire current range, underscoring its optimized electronic pathways and superior ion-transport dynamics. To rigorously quantify performance metrics, the areal capacitance (C_A), energy density (ED), and power density (PD) were calculated using integral expressions appropriate for nonlinear discharge curves Equations (8)–(10) [2,45]:

$$C_A={\displaystyle \frac{I \times 2 \times \int V\left(t\right)dt}{A \times {\left(dV\right)}^2}}$$

(8)

$$ED={\displaystyle \frac{1}{2\times 3600}} C_A\times {dV}^2$$

(9)

$$PD={\displaystyle \frac{ED\times 3600}{T_d}}$$

(10)

Figure 7. GCD profiles of NiFe and Gd-incorporated electrodes (a) at 10 mA cm⁻² and (b–e) at different current densities.

Figure 8. Electrochemical performance comparison including (a) IR drop, (b) Areal capacitance, (c) Nyquist plots, and (d) Cycling stability of NiFe–Gd2.

Here, I represents the applied discharge current, ∫V(t)dt accounts for the time-dependent voltage integral during discharge (ensuring accurate treatment of non-ideal pseudocapacitive behavior), A is the effective electrode area, dV denotes the potential window, and T_d is the discharge time. The use of integrated voltage profiles is essential for faradaic systems, where deviation from linear discharge behavior arises from redox kinetics rather than ideal double-layer capacitance. At 10 mA cm⁻², the calculated areal capacitances were 2864 mF cm⁻² (NiFe), 5136 mF cm⁻² (NiFe-Gd1), 2074 mF cm⁻² (NiFe-Gd3), and a substantially higher 5235 mF cm⁻² for NiFe-Gd2 (Table 3; Figure 8b). The outstanding performance of NiFe-Gd2 clearly validates the beneficial role of optimized Gd³⁺ incorporation. Its superior capacitance can be attributed to multiple synergistic effects: (i) expansion of the electrochemically active surface area exposing abundant redox-active sites, (ii) improved electronic conductivity through well-connected nanosheet frameworks that minimize charge-transfer resistance, and (iii) enhanced electrolyte accessibility enabled by interconnected porous channels that shorten ion diffusion pathways. The optimized rare-earth content plays a pivotal role in preserving structural integrity. Controlled Gd substitution regulates nucleation and growth, suppresses particle aggregation, and prevents excessive nanosheet restacking, thereby maintaining a porous and accessible architecture essential for rapid redox kinetics. In contrast, insufficient or excessive dopant concentrations compromise structural uniformity, leading to reduced electrochemical utilization. As anticipated, increasing current density resulted in a gradual decline in both capacitance and energy density for all samples due to limited ion diffusion depth and incomplete activation of inner electroactive regions at high rates (Table 3) [12]. Nevertheless, NiFe-Gd2 exhibited remarkable rate performance, retaining approximately 48.12% of its initial capacitance even at 50 mA cm⁻². This robust retention highlights its structural stability, efficient electron transport, and sustained ionic mobility under high-rate conditions.

Table 3. Evaluation of electrochemical performance parameters: areal capacitance (CA), areal capacity (C), energy density (ED), and power density (PD) at variable current densities.

Sample Code	I (mA cm−2)	Areal Capacitance CA (mF cm−2)	Capacity (mAh cm−2)	Energy Density ED (mWh cm−2)	Power Density PD (mW cm−2)
NiFe	10	2864	0.179	0.081	2.27
	20	2825	0.177	0.079	4.54
	30	2519	0.157	0.071	6.71
	40	1857	0.116	0.052	8.55
	50	1383	0.086	0.039	10.77
NiFe-Gd1	10	5136	0.321	0.144	2.49
	20	3299	0.206	0.093	4.45
	30	2400	0.150	0.068	6.57
	40	2212	0.138	0.062	8.62
	50	1481	0.093	0.042	10.49
NiFe-Gd2	10	5235	0.327	0.147	2.09
	20	5136	0.321	0.144	4.41
	30	3556	0.222	0.100	6.43
	40	3319	0.207	0.093	9.60
	50	2519	0.157	0.071	11.09
NiFe-Gd3	10	2074	0.130	0.058	2.50
	20	1995	0.125	0.056	4.46
	30	1956	0.122	0.055	6.92
	40	1738	0.109	0.049	9.12
	50	790	0.049	0.022	9.64

EIS was conducted to evaluate the charge transfer resistance and ion diffusion behavior of NiFe, NiFe-Gd1, NiFe-Gd2, and NiFe-Gd3 electrodes in 2 M KOH (Figure 8c). The Nyquist plots exhibit a high-frequency intercept (solution resistance, Rs), a semicircle in the intermediate region (charge transfer resistance, Rct), and a low-frequency linear tail. Among all samples, NiFe-Gd2 shows the smallest semicircle diameter, indicating the lowest Rct and the most efficient electron transfer at the electrode–electrolyte interface. It also presents a steeper low-frequency line, approaching vertical behavior, which confirms faster OH⁻ ion diffusion and improved capacitive characteristics. To gain deeper insight into the charge transport characteristics, EIS data were further analyzed using an equivalent circuit model (R1+Q1/(R2+Q2/R3)), as shown in Figure 8c. The extracted fitting parameters reveal a clear dependence of resistance on Gd incorporation (Table 2). The pristine NiFe electrode exhibits an Rs of 0.7306 Ω and a relatively high Rct of 7.117 Ω, indicating comparatively slower charge-transfer kinetics. Upon Gd incorporation, both resistive components decrease significantly, with the NiFe-Gd2 electrode showing the lowest Rs (0.6798 Ω) and Rct (4.973 Ω). Notably, the optimized NiFe-Gd2 electrode demonstrates superior electrochemical kinetics, which can be associated with its enhanced electronic conductivity induced by Gd incorporation. In contrast, the NiFe-Gd1 electrode exhibits a reduced Rs of 0.6943 Ω and Rct 5.179 Ω compared to pristine NiFe, indicating improved electrochemical kinetics; however, its performance remains inferior to the optimally doped NiFe–Gd2, suggesting that lower Gd incorporation is insufficient to fully activate the defect-enhanced charge transport pathways. Further increasing Gd content (NiFe-Gd3) results in an increase in both Rs (0.8321 Ω) and Rct (7.424 Ω), indicating that excessive doping leads to structural distortion and possible agglomeration, thereby hindering charge transport. These findings clearly demonstrate that an optimal Gd concentration is crucial for achieving improved electrochemical performance.

To assess the long-term electrochemical durability of the optimized NiFe-Gd2 electrode, extended GCD cycling was carried out for 12,000 continuous cycles at a high current density 80 mA cm⁻², as shown in Figure 8d. Remarkably, the electrode retained approximately 85.81% of its initial capacitance after prolonged cycling, accompanied by a remarkable coulombic efficiency of 94.38%, confirming highly reversible redox kinetics and minimal performance degradation. Such high capacitance retention clearly reflects the excellent structural integrity, electrochemical reversibility, and mechanical robustness of the Gd-modified NiFe framework. The superior cycling stability of NiFe-Gd2 can be primarily attributed to the optimized Gd³⁺ incorporation within the NiFe₂O₄ lattice. The controlled substitution induces lattice distortion and defect sites, which not only enhance electrical conductivity but also buffer the volumetric variations associated with repetitive OH⁻ ion insertion and extraction. This structural adaptability effectively mitigates stress accumulation and suppresses pulverization or structural collapse during long-term cycling. Furthermore, the interconnected nanosheet network and porous architecture of NiFe-Gd2 facilitate continuous electrolyte penetration and uniform ion diffusion throughout the electrode matrix. Such an open framework minimizes localized current concentration and polarization effects, thereby preserving interfacial stability and sustaining efficient faradaic reactions over extended operation. The slight capacitance decay observed after extensive cycling may originate from gradual electrolyte ion trapping within microporous domains, minor surface restoration, or partial blockage of electroactive sites due to repeated redox transformations [46,47]. However, the limited magnitude of degradation confirms that the structural framework remains largely intact and electrochemically accessible. Overall, the outstanding cyclic retention of the NiFe-Gd2 electrode highlights the crucial role of optimized rare-earth doping in stabilizing spinel NiFe₂O₄ systems.

To evaluate the practical significance of the developed electrode, a comparative analysis with recently reported NiFe₂O₄-based and hybrid supercapacitor systems is presented in Table 4 [47,48,49,50,51,52]. It is evident that the electrochemical performance of the optimized NiFe-Gd2 electrode is highly competitive with, and in some cases superior to, previously reported materials. Specifically, the NiFe-Gd2 electrode delivers an areal capacitance of 5.235 F/cm², which surpasses many conventional NiFe₂O₄ and composite-based systems. In addition to high capacitance, it demonstrates a favorable energy–power balance. These values are notably higher than those of several reported NiFe₂O₄-based electrodes, indicating the effectiveness of the present material design strategy. Furthermore, the electrode maintains excellent cycling stability, retaining 85.81% of its initial capacitance after 12,000 cycles, which confirms its structural robustness and long-term electrochemical reliability. The superior performance of the NiFe-Gd2 electrode can be attributed to the synergistic interplay between compositional tuning and structural engineering. The incorporation of Gd³⁺ ions induce lattice distortion, which enhances electrical conductivity and promotes rapid charge transfer. A deeper insight into the enhanced electrochemical performance of the NiFe-Gd2 electrode can be attributed to the multifaceted role of Gd incorporation in modulating the structural and electronic properties of NiFe₂O₄. Owing to its larger ionic radius compared to Fe³⁺, the insertion of Gd into the spinel lattice induces local lattice distortion, which in turn promotes the formation of oxygen vacancies and defect sites. These defects serve as additional electroactive centers and significantly enhance the adsorption and diffusion of OH⁻ ions during redox reactions. Moreover, the introduction of Gd alters the electronic structure by creating localized states that facilitate improved electron transport and reduce charge-transfer resistance, as evidenced by the lower impedance observed in EIS measurements. The presence of such defect-induced states enhances electrical conductivity and accelerates faradaic reaction kinetics. In addition to electronic modulation, Gd incorporation also plays a crucial role in tailoring the morphology of the electrode material. It influences the nucleation and growth dynamics during hydrothermal synthesis, leading to the formation of a porous and interconnected nanosheet architecture. This structural configuration provides a high surface area and open diffusion pathways, thereby enabling efficient electrolyte penetration and rapid ion transport. Collectively, these structural, defect, and electronic modifications induced by Gd doping synergistically contribute to the superior electrochemical performance of the NiFe-Gd2 electrode, highlighting the effectiveness of rare-earth engineering in optimizing spinel ferrite-based supercapacitor materials.

Table 4. Comparative analysis of electrochemical performances metrics of NiFe-Gd2 with recently reported NiFe₂O₄-based systems.

Sr. No.	Material	Specific Capacitance	Current Density	Energy Density	Power Density	Cycle Stability	Ref.
1.	NiFe2O4	0.668 F/cm2	1 mA/cm2	0.0257 mWh/cm2	0.26 mW/cm2	92.5% (7000 cycles)	[48]
2.	Mn-doped NiFe2O4	8.68 F/cm2	1 mA/cm2	0.39 mWh/cm2	0.544 mW/cm2	95.2% (5000 cycles)	[47]
3.	PEDOT–NiFe2O4	251 F/g	5 mA/cm2	-	-	84% (500 cycles)	[49]
4.	NiFe2O4@ AAC	1130 F/g	1 A/g	23.1 Wh/kg	5329 W/kg	88% (10,000 cycles)	[50]
5.	NiFe2O4	0.7227 F/cm2	3 mA/cm2	30.45 mWh/cm2	420 mW/cm2	130% (2000 cycles)	[51]
6.	NiFe2O4/MoS2	506 F/g	1 A/g	-	-	90.7% (3000 cycles)	[52]
7.	NiFe-Gd2	5.235 F/cm2	10 mA/cm2	0.147 mWh/cm2	2.09 mW/cm2	85.81% (12,000 cycles)	This work

5. Electrochemical Performance of Asymmetric Supercapacitor Device

To bridge the gap between half-cell evaluation and practical applicability, an asymmetric supercapacitor device (ASD) was assembled using the optimized NiFe-Gd2 electrode as the positive electrode and activated carbon (AC) as the negative electrode. The selection of AC was strategic, owing to its stable electric double-layer capacitive (EDLC) behavior, high surface area, and excellent rate capability, which effectively complement the faradaic pseudocapacitive nature of the NiFe-Gd2 electrode. This rational pairing enables synergistic integration of diffusion-controlled redox reactions and surface-controlled charge storage, thereby maximizing the overall device performance. Both electrodes were prepared on nickel foam substrates to ensure low contact resistance and mechanical robustness. The negative electrode was fabricated by preparing a homogeneous slurry consisting of activated carbon, acetylene black, and polyvinylidene fluoride (PVDF) binder in weight ratio of 8:1:1, dispersed in N-methyl-2-pyrrolidone (NMP). The well-mixed slurry was uniformly coated onto nickel foam and dried at 60 °C for 12 h to ensure complete solvent evaporation and strong adhesion to the current collector. The assembled device employed 2 M KOH aqueous electrolyte with a porous separator to facilitate efficient ionic conduction while preventing electrical shorting. The electrochemical behavior of the NiFe-Gd2//AC device was systematically examined using CV, GCD, and EIS. CV of the NiFe-Gd2//AC device was recorded at progressively extended voltage windows from 0 to 1.0 V to 0–1.5 V to determine the maximum stable operating range (Figure 9a). The CV curves maintained their shape and structural integrity up to 1.5 V without any distortion or sharp current rise, indicating suppressed electrolyte decomposition. The gradual increase in enclosed CV area with increasing voltage confirms enhanced charge storage capability. These results validate the stable and widened operating potential window of the assembled asymmetric device. The CV profiles recorded at scan rates ranging from 10 to 100 mV s⁻¹ (Figure 9b) exhibited quasi-rectangular shapes superimposed with distinct redox humps, confirming the coexistence of EDLC contribution from AC and battery-type pseudocapacitive behavior from the NiFe-Gd2 electrode. Importantly, the device operated stably within an extended potential window of 1.5 V, which is comparatively wide for aqueous electrolyte systems. This expanded voltage range is attributed to the complementary potential domains of the positive and negative electrodes, enabling enhanced energy density without inducing significant polarization. The corresponding GCD curves (Figure 9c) displayed nonlinear charge–discharge characteristics with well-defined voltage plateaus, further validating the dominant faradaic contribution of the NiFe-Gd2 electrode. At a current density of 10 mA cm⁻², the device delivered an areal capacitance of 436 mF cm⁻², achieving an energy density of 0.136 mWh cm⁻² and a power density of 3.14 mW cm⁻² (Table 5). These values demonstrate a well-balanced energy–power relationship. Even at elevated current densities, the device retained a substantial fraction of its capacitance, confirming excellent rate capability and rapid charge propagation across the electrode–electrolyte interfaces.

Figure 9. Electrochemical performance of the NiFe–Gd2//AC asymmetric supercapacitor showing (a) CV curves at variable voltage window, (b) CVs at different scan rates, (c) GCD profiles, (d) Nyquist plot, and (e) Cycling stability.

Table 5. Energy storage performance of the NiFe-Gd2 device.

Sample Code	I (mA)	Areal Capacitance CA (mF cm−2)	Capacity (mAh cm−2)	Energy Density ED (mWh cm−2)	Power Density PD (mW cm−2)
NiFe-Gd2 device	10	436	0.091	0.136	3.14
	20	324	0.068	0.101	1.66
	30	136	0.028	0.043	3.12
	40	50	0.010	0.016	2.98
	50	20	0.004	0.006	2.81

EIS analysis further substantiated the superior electrochemical kinetics of the assembled device (Figure 9d). The equivalent series resistance (Rs) was determined to be approximately 1.88 Ω, reflecting minimal internal resistance and efficient ionic/electronic transport pathways. Long-term cycling durability was evaluated over 7000 continuous charge–discharge cycles at a high current density of 60 mA cm⁻², shown in Figure 9e, accompanied with 1st, 4000th, and 7000th GCD cycles. Impressively, the device preserved approximately 86.55% of its initial capacitance while maintaining a high coulombic efficiency of 94.07%, demonstrating excellent electrochemical reversibility and structural resilience. The outstanding stability originates from the robust spinel framework of NiFe-Gd2, where controlled Gd³⁺ substitution induces lattice distortion and defect sites that buffer volumetric strain during repetitive ion insertion and extraction. Simultaneously, the porous nanosheet architecture accommodates mechanical stress and preserves continuous electron/ion transport channels, thereby minimizing structural degradation over prolonged operation. Overall, the NiFe-Gd2//AC asymmetric supercapacitor exhibits an exceptional combination of extended operating voltage, high areal capacitance, favorable energy–power output, low internal resistance, and durable cycling performance. These results clearly demonstrate that optimized rare-earth doping in spinel NiFe₂O₄ not only enhances half-cell electrochemical properties but also translates effectively into superior device-level performance. The present study highlights the critical importance of compositional engineering and structural optimization in advancing high-performance, practical supercapacitor systems suitable for next-generation portable and hybrid energy storage applications.

6. Conclusions

In this study, Gd-incorporated NiFe₂O₄ nanosheet electrodes were successfully synthesized on nickel foam to explore the influence of rare-earth modification on structural features and electrochemical performance. Structural analysis confirmed the formation of a stable cubic spinel NiFe₂O₄ phase, while XPS results verified the presence of Ni²⁺, Fe³⁺, and Gd³⁺ species within the lattice. The incorporation of Gd significantly influenced the growth behavior of the material, leading to noticeable changes in the nanosheet morphology. Among the investigated samples, the NiFe–Gd2 electrode exhibited a well-developed and interconnected nanosheet architecture, which provides improved electrolyte accessibility and a higher density of electroactive sites. As a result, the optimized electrode delivered a high areal capacitance of 5235 mF cm⁻² at a current density of 10 mA cm⁻², along with improved charge transfer characteristics. Furthermore, an asymmetric supercapacitor assembled using NiFe–Gd2 as the positive electrode and activated carbon as the negative electrode operated within a 0–1.5 V potential window, achieving an areal capacitance of 436 F cm⁻², an energy density of 0.136 mWh cm⁻², and a power density of 3.14 mW cm⁻². The device also demonstrated good cycling stability, retaining 86.55% of its initial capacitance after 7000 cycles. These results suggest that controlled Gd incorporation effectively improves the electrochemical performance of NiFe₂O₄ nanosheet electrodes and highlights their potential for practical asymmetric supercapacitor applications.