Natural stabilizer-assisted sol–gel synthesis of medium-entropy spinel ferrites: structural and electrochemical insights for supercapacitor applications

Abstract

Multi-cation spinel ferrites have gained significant interest due to their tuneable properties, enhanced stability, conductivity, and ion mobility, making them promising for supercapacitor applications. In the present study, (MnCoNiZn)Fe₂O₄ spinel ferrite was synthesized by a sol–gel route using albumen (egg white) as a natural stabilizer as well as a chelating agent for the preparation of ferrite. The formation of a single-phase cubic spinel structure with a space group of Fd3̄m has been established by XRD studies. The morphology analysis using FESEM and HRTEM revealed well-defined, uniformly distributed pyramidal-shaped particles, confirming the controlled growth achieved by the synthesis route. The optical band gap, determined from UV–Vis spectroscopy, is found to be 2.86 eV. The electrochemical evaluation was carried out in a three electrode configuration using aqueous 2 M KOH electrolyte. The prepared ferrite exhibited a specific capacitance as high as 408.18 F g⁻¹ at a significantly high current density of 4 A g⁻¹ during charge–discharge, highlighting its superior charge storage capacity. The coulombic efficiency was found to be 91.6% up to 1500 continuous charge–discharge cycles, which reveals excellent reversal behaviour. Additionally, a maximum energy density of 36.31 Wh kg⁻¹ is achieved at a power density of 1601.28 W kg⁻¹. Overall, this study establishes albumen-assisted sol–gel synthesis as a non-toxic stabilizer and effective strategy for producing medium-entropy spinel ferrites, emphasizing their potential as sustainable and high-performance electrode materials for superior performance supercapacitor applications.

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1. Introduction

The rapidly advancing technologies of wearable electronics, electric transportation, and smart energy infrastructure have created demands for new sustainable, high efficiency energy storage systems that will accommodate the rising global demands for energy.^1–4 Among various energy storage technologies, supercapacitors (electrochemical capacitors) are perhaps the most important, filling the gap between traditional dielectric capacitors and rechargeable batteries.^5–7 The unique advantages of these devices, including ultrafast charge–discharge characteristics, superior power density, excellent rate performance, and outstanding cycling stability, make them indispensable for modern applications ranging from microelectronic sensors to hybrid electric vehicles.^8–10 However, despite these advantages, the energy density of conventional supercapacitors is still low (less than 10 Wh kg⁻¹) and is thus a limiting factor in systems with high energy requirements. Accordingly, present work is directed toward new electrode materials that can be expected to impart high power and good energy density to the capacitive devices.^11–13

Transition metal oxides (TMOs) have emerged as promising pseudocapacitive materials due to their redox active sites, chemical versatility, and low cost.^9,12,13 Among them, ferrite-based oxides (MFe₂O₄) are particularly attractive owing to their structural stability, rich redox chemistry, and flexible spinel framework. The spinel structure, consisting of tetrahedral (A) and octahedral (B) sites, enables tunable cation distribution and multiple valence states, facilitating reversible faradaic reactions and adjustable electronic properties.^14–17 All of these properties render ferrites very suitable for pseudocapacitor electrodes where rapid and reversible faradaic reactions occur at the interface between the electrode and electrolyte. Nevertheless, traditional binary ferrites often exhibit poor intrinsic conductivity and limited electrochemical reversibility, resulting in suboptimal rate capability and rapid capacity fading during extended cycling.^1,18

In order to address these problems, a more complex composition in the ferrite structure has been developed as a compelling design tool. Inspired by the principles of medium entropy and high entropy oxides (MEOs/HEOs), this has been reached via the substitution of different metal cations into the ferrite spinel structure. The thermodynamic stabilization of such systems is governed by configurational entropy, which can be expressed as:

where R is the gas constant and x_i represents the mole fraction of each component.^18,19 For the present system, the cationic sublattice consists of four divalent metal ions, namely Mn²⁺, Co²⁺, Ni²⁺, and Zn²⁺. With an equimolar distribution of four cations (Mn²⁺, Co²⁺, Ni²⁺, and Zn²⁺), the entropy is calculated as ΔS_config = R ln 4 (∼1.386R). According to established classification, systems with ΔS_config between 1R and 1.5R are categorized as medium-entropy materials, while values ≥1.5R correspond to high-entropy systems. Therefore, the present (MnCoNiZn)Fe₂O₄ composition can be classified as a medium-entropy spinel ferrite. These materials gain thermodynamic stabilisation by high configurational entropy, resulting in the minimisation of phase segregation and formation of a single phase even at moderate temperatures. The simultaneously existent different transition metals also result in several redox active species and synergistic cationic interactions that enhance charge transfer, ionic diffusion and electronic conduction.^20,21

The synergistic effects of the cations are obviously important. The Mn²⁺ ions enhance both ionic and electronic transport and promote reversible redox surface changes between the Mn²⁺/Mn³⁺ states. Meanwhile, cationic centres that are readily attainable such as Co²⁺/Co³⁺ and Ni²⁺/Ni³⁺ redox systems are attained via the cations Co²⁺ and Ni²⁺. The Zn²⁺ ions show a stabilising structural role by their ability to stabilise the spinel structure and also mitigate the cation drift during cycling, whereas the Fe³⁺ ions show some pseudocapacitive phenomena shown in the Fe²⁺/Fe³⁺ transitions.^22–24 The existence of these species in the one matrix should therefore give a favourable arrangement, where each cation has its own separate contribution to redox activity, conductivity or spinel integrity, allowing an increase of activity in terms of electrochemical performance of the whole array.¹⁵

Although entropy-stabilized ferrites show great potential, their synthesis via conventional solid-state techniques is challenging, due to high temperature calcination (≥1000 °C) for a prolonged period of time resulting in grain coarsening, agglomeration and non-uniformity in cation mixing, and the lack of accessibility to electrochemical surface area, and redox reaction.²⁵ In contrast, wet chemical techniques, particularly sol–gel methods, offer many advantages including: high homogeneity at the molecular level, stoichiometry control, and convenience of accessing nano scale particle systems at reduced temperatures of calcination. It also offers the advantage of incorporating organic or biogenic agents to facilitate control of morphology and porosity, important traits for efficient ion transport.^2,14,26

Many recent works have validated the advantages of ferrite systems with multiple cations in electrochemical energy storage devices. For instance, Ganesh et al. (2024) developed Cu–Zn–Mg ferrite electrode systems with a specific capacitance of 508.25 F g⁻¹ at 1.75 A g⁻¹.²⁷ Agale et al. (2023) synthesized Cu_1−xNi_xMnFeO₄ ferrite and achieved a capacitance of 975 F g⁻¹ at a high energy density of 20.8 Wh kg⁻¹.²⁸ Yi Yin et al. (2023) reported entropy stabilized oxide (FeCoCrMnNi)₃O₄ with a capacitance of 332.2 F g⁻¹ at 0.3 A g⁻¹ in alkaline medium.²⁹ In continued research, Aparna et al. (2018)⁵⁷ have made a systematic study of MFe₂O₄ (M = Fe, Co, Ni, Mn, Cu, Zn) observing a high range of capacitance (from >= 101 to 444.78 F g⁻¹), showing a profile of cation-type effectiveness on electrochemical behaviour.³⁰ The most recent examples are by Hsu et al. (2025) reporting Bi-doped NiFe₂O₄ exhibiting a high specific capacitance of 339.16 F g⁻¹ at 5 mV s⁻¹, showing the advantages of modifications in composition.³⁰ Collectively, these studies highlight the importance of cation engineering in enhancing electrochemical performance.

On the other hand, the concept of green synthesis approaches has emerged, in which biogenic compounds are used as natural stabilizers, complexing agents or structure-directing templates in the synthesis of materials. These biologically active products have tendencies that are inherently beneficial, such as the ability to complex metal ions, gel formation with defined properties, and residues of carbonised material after processing at elevated temperatures.^31–33 Albumen (egg white) is an environment friendly agent, and plays a multifunctional role during the synthesis of the ferrite system. Firstly, the proteinaceous functional groups present in albumen (–NH₂, –COOH, –OH) facilitate chelating interactions with metal cations (Mg²⁺, Co²⁺, Ni²⁺, Zn²⁺, Fe³⁺), ensuring homogeneous mixing at the molecular level.³⁴ Secondly, albumen acts as a soft template, guiding the nucleation and growth of nanoparticles and preventing agglomeration, thereby contributing to controlled morphology and porosity. Finally, upon thermal treatment, albumen undergoes decomposition to form in situ carbonaceous species, which can enhance the electrical conductivity and structural stability of the final material.^35,36

This is ultimately beneficial in enhancing the electroactive interface called up in the terminology. The synthesis employing albumen as the catalyst agent has been shown to be useful for simple binary oxides such as Fe₂O₃ and ZnO. The procedure has previously been little exploited for the production of compositionally complex, entropy-stabilised spinel ferrites.^37–39

In this work, we report the use of albumen-assisted sol–gel synthesis to yield [Mn_0.25Co_0.25Ni_0.25Zn_0.25]Fe₂O₄, which is a medium-entropy, spinel ferrite developed to have high electroactive and structurally sound properties for supercapacitor uses. When albumen is selected as the biogenic precursor, it not only leads to uniform cation distributions and particle miniaturisation, but also produces a conductive carbonaceous support on calcining the products further improving charge transport. The combined effect of multiple redox-active cationic participants (Mn, Co, Ni, and Fe) and the stabilising effect of Zn is expected to lead to beneficial pseudo-capacitance, improved electrical conductivity and significantly good cycling stability.

The structural and morphological properties of the synthesized ferrite [MnCoNiZn]Fe₂O₄ were investigated through extensive analyses such as X-ray diffraction (XRD), Rietveld refinement, X-ray photoelectron spectroscopy (XPS), and electron microscopy. The electrochemical performance was evaluated to establish the relationship between entropy-driven compositional design and charge storage behaviour. The study demonstrates how a biogenic synthesis technique combined with an entropy-driven compositional design can produce a sustainable high-performance electrode material for next-generation supercapacitors.

2. Experimental section

2.1. Materials

The following reagents were used in the synthesis of [MnCoNiZn]Fe₂O₄: ferric nitrate nonahydrate (Fe(NO₃)₃·9H₂O), manganese nitrate hexahydrate (Mn(NO₃)₂·6H₂O), cobalt nitrate hexahydrate (Co(NO₃)₃·6H₃O), nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O), and zinc nitrate hexahydrate (Zn(NO₃)₃·6H₂O). All were of analytical grade with 98–99% purity and were provided by Sisco Research Laboratories (SRL) Pvt. Ltd, India. Pellets of potassium hydroxide (KOH) were also acquired from SRL. Graphite foil (GF) was obtained from Falconi Industries, Hyderabad, while carbon black and polyvinylidene fluoride (PVDF) were acquired from Alfa Aesar, India. In the experiment, distilled water was used as the solvent. The albumen was extracted from fresh chicken eggs that were gathered from a local farm, which acts as a natural stabilizing and templating agent owing to its inherent capping properties. All the reagents were utilized directly as received, without any further purification.

2.2. Preparation of [MnCoNiZn]Fe₂O₄

An amount of 40 mL of egg albumen, freshly extracted from chicken eggs, was mixed with 60 mL of distilled water and stirred vigorously to make it a homogeneous mixture. The stoichiometrically calculated amounts of corresponding raw metal nitrate salts of Fe, Mn, Co, Ni, and Zn were subsequently added into the albumen-water solution, where Mn, Co, Ni, and Zn were maintained in equimolar proportions (1 : 1 : 1 : 1), while Fe was added to satisfy the spinel stoichiometry of [Mn_0.25Co_0.25Ni_0.25Zn_0.25]Fe₂O₄. The mixture was maintained at a temperature of 70 °C on a hot plate magnetic stirrer with bead rotation at 400 rpm until it transformed into a viscous gel, signifying the completion of gelation. The gel was then dried in an oven at a temperature of 100 °C for a duration of 6 hours to remove the residual moisture content, resulting in a solid like mass. This dried mass was then finely ground with an agate mortar and subjected to calcination at various temperatures – 400 °C, 600 °C, 800 °C, 1000 °C and 1100 °C for 4 hours duration with 3 °C min⁻¹ of rise and fall temperature in a muffle furnace. To maintain its phase uniformity and regulate particle growth, the material was intermittently ground during each calcination process. The final calcined material appeared as a blackish-brown powder, confirming the successful synthesis of [Mn_0.25Co_0.25Ni_0.25Zn_0.25]Fe₂O₄. The albumen-assisted final product was designated as MCNZF, representing the composition of [Mn_0.25Co_0.25Ni_0.25Zn_0.25]Fe₂O₄ obtained by employing this bio-mediated sol–gel route. A schematic representation of the synthesis strategy is provided in Fig. 1.

Fig. 1 Schematic flow of MCNZF synthesis.

2.3. Material characterization

The synthesized MCNZF spinel ferrite was characterized to analyse its structural, chemical, compositional, morphological, optical, and electrochemical properties. Crystallographic analysis was conducted using X-ray diffraction (XRD) (PANalytical Empyrean diffractometer, Cu Kα radiation (λ = 1.5404 Å), scanning 2θ range of 10° to 80°, step size of 0.020°) to confirm phase purity and determine lattice parameters. Rietveld refinement was performed using the FullProf software tool with the ICSD database, to obtain precise structural information. Raman spectroscopy was carried out (Horiba Jobin Yvon T64000 triple monochromator, 514 nm Ar⁺ laser, Backscattering mode) to identify the vibrational modes of spinel ferrites. Surface elemental composition and oxidation states were examined via X-ray photoelectron spectroscopy (XPS) (Kratos AXIS Supra). The morphology and microstructure of the samples were investigated using Field Emission Scanning Electron Microscopy (FESEM, JEOL JSM-7610F) and High-Resolution Transmission Electron Microscopy (HRTEM, FEI Talos F200X) to resolve lattice fringes and confirm the crystal structure at the desired scale. The specific surface area of the sample was evaluated using the Brunauer Emmett Teller (BET) theory method based on N₂ adsorption–desorption isotherms recorded at 77 K. The analysis was performed using an Autosorb iQ Station 1 instrument. The pore size distribution was further calculated employing the Barrett Joyner Halenda (BJH) method. Optical properties were measured (using Shimadzu UV-2401 UV-Visible spectrophotometer (UV)) to identify the absorption spectra and bandgap of the material, while Fourier-transform infrared (FTIR) spectroscopy in the 400–4000 cm⁻¹ range (PerkinElmer Spectrum 100) was employed to identify metal-oxygen bonding and molecular vibrations. Electrochemical performance was evaluated through cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) experiments on a ZIVE MP5 multichannel workstation (WonATech Co., Ltd, Korea), and electrochemical impedance spectroscopy (EIS) measurements were performed (Metrohm AutoLAB 302N equipped with NOVA 2.1 software) to assess charge transfer and capacitive behaviour.

2.4. Preparation of the electrode

The working electrode was fabricated by blending the synthesized MCNZF powder as the primary material, carbon black (CB) to enhance the conductivity and polyvinylidene fluoride (PVDF) as a binder, added in a weight ratio of 80 : 10 : 10. To this combination, a consistent amount of N-methyl-2-pyrrolidone (NMP) is added to make a homogeneous slurry. The resulting mixture was then uniformly coated on a graphite foil (GF) substrate (3 cm × 1 cm) using a doctor blade, with electrode material spreading on an effective area of 1 cm². The GF used as the current collector has a thickness of ∼0.5 mm, while the deposited active material forms a thin coating layer in the range of ∼5–20 µm. The loading of the active material was maintained at roughly 1 mg per electrode. After coating, the electrodes were dried in an oven at a temperature of 70 °C for 12 hours duration. This MCNZF-coated GF electrode (MCNZF/GF) was subsequently used as the working electrode in a three-electrode supercapacitor configuration for electrochemical evaluations.

2.5. Three electrode system (3E)

The MCNZF/GF electrode was integrated into a three-electrode supercapacitor configuration operated at ambient conditions. In this setup, the prepared electrode was used as a working electrode, while a platinum wire acted as a counter electrode and an Ag/AgCl (saturated KCl) electrode was used as a reference. The electrolyte used for the configuration was 2 M KOH. An illustration of the complete experimental arrangement is displayed in Fig. 2.

Fig. 2 Schematic diagram of the three-electrode system (3E).

3. Results and discussion

3.1. Structural characterization

The structural characterization of the synthesized MCNZF sample was done using XRD analysis, and the corresponding diffraction patterns are shown in Fig. 3. The sample was calcined at various temperatures from 400 °C to 1100 °C to determine the optimal condition for achieving well-defined crystalline phases with minimal impurities. The respective XRD patterns are presented in the supplementary information (SI) (Fig. S1). At all calcination temperatures, the dominant (major) diffraction peak appeared at 2θ = 35.60°, corresponding to the principal reflection of the Fe₃O₄ structure, confirming the formation of a spinel phase (JCPDS No. 65-3107).^14,40,41 However, at 800 °C, the XRD pattern revealed a minor impurity peak at 2θ = 33.37°, attributed to Fe₂O₃, indicating partial structural instability. This instability is due to the partial oxidation of Fe²⁺ to Fe³⁺ ions and the redistribution of cations within the spinel lattice.^42,43 In the case of the multicomponent MCNZF sample, which contains multiple transition metal cations (Mn²⁺/Mn³⁺, Co²⁺/Co³⁺, Ni²⁺, Zn²⁺, and Fe²⁺/Fe³⁺), this instability is further influenced by cation migration between tetrahedral (A) and octahedral (B) sites. At intermediate temperatures, the unequal diffusion rates and ionic radii differences among these cations cause local lattice distortion and strain.^44–46 However, with further calcination at higher temperatures, enhanced atomic mobility promotes configurational entropy stabilization, allowing a homogeneous distribution of cations over both sublattices. Upon increasing the calcination temperature to 1000 °C, the impurity peak diminishes significantly, suggesting improved structural ordering. Further heating to 1100 °C results in complete elimination of the impurity, confirming the formation of a single-phase cubic spinel structure with no detectable secondary phases. The diffraction pattern and corresponding Rietveld refinement of the MCNZF sample calcined at 1100 °C, as shown in Fig. 3(a), reveal well-defined reflections characteristic of a cubic spinel ferrite structure (space group: Fd3̄m), closely resembling that of Fe₃O₄. Prominent diffraction peaks observed at 2θ ≈ 18.48°, 30.26°, 35.60°, 37.25°, 43.24°, 53.61°, 57.12°, and 62.70° correspond to the (111), (022), (113), (222), (004), (224), (115), and (044) planes, respectively, consistent with the ICSD collection code 158745.^47–49 The Rietveld refinement yielded R_p = 37.0, R_wp = 21.2, and R_e = 19.1, with a goodness-of-fit value chi-square (χ²) ≈ 1.23, indicating a satisfactory agreement between the experimental and calculated diffraction patterns and confirming the formation of a single-phase cubic spinel structure with excellent structural homogeneity and phase purity. The respective Rietveld refinement parameters are shown in Table S1 in the SI.

Fig. 3 (a) XRD and Rietveld fit and (b) Raman spectra of MCNZF.

The Raman spectrum of the synthesized MCNZF is shown in Fig. 3(b). The MCNZF sample exhibits characteristic peaks corresponding to the vibrations of the spinel structure. These vibrational modes are related to the stretching and bending of bonds in the spinel lattice, specifically involving the oxygen ions and the metal cations in the tetrahedral (A) and octahedral (B) sites.⁵⁰ The Raman-active modes of a spinel structure with Fd3̄m symmetry are derived from group theory and typically include A_1g (Symmetric stretching of the A-site metal–oxygen bonds), E_g (Symmetric stretching vibrations of oxygen atoms), and T_2g (Various bending and stretching vibrations). For ferrites, these modes generally appear in the following regions for A_1g: ∼600–700 cm⁻¹ (strong peak, associated with the tetrahedral A-site metal-oxygen bonds), and T_2g: ∼200–500 cm⁻¹ (weaker peaks, associated with vibrations of the B-site metal-oxygen bonds). In the synthesized MCNZF, the strong A_1g mode splits into two distinct modes (A_1g(1), A_1g(2)) due to cation inversion in the spinel structure, around at 683 cm⁻¹ and 613 cm⁻¹ corresponding to the Fe–O bond vibrations. The multiple T_2g modes (T_2g(1), T_2g(2), T_2g(3)) are at 475 cm⁻¹, 373 cm⁻¹ and 182 cm⁻¹, reflecting the interactions, due to Mn, Co, Ni, and Zn ions. The shift of T_2g(3) below 200 cm⁻¹ is due to the influence of Co and Zn. The E_g mode appeared at 321 cm⁻¹.^2,40 The factors influencing the Raman peaks are the cation distribution of Mn, Co, Ni, Zn, and Fe between the tetrahedral and octahedral sites, affecting the peak positions and intensities. The broadening of the peaks is due to the increased disorder by entropy effects from multiple cations. The particle size and synthesis approach may also affect the shifting or broadening of peaks due to strain, defects, or quantum confinement.^6,41 The presence of residual carbon is identified in the Raman spectrum and displayed in Fig. S2(a).

XPS was employed to identify the elemental composition and oxidation states of the synthesized MCNZF sample. The corresponding spectra are displayed in Fig. 4, including both the wide survey scan spectra and high-resolution core-level spectra for elements Mn, Co, Ni, Zn, Fe, C, and O. The survey spectrum (Fig. 4a) confirms the presence of all the elements in the synthesized MCNZF material, while the specific deep scans provide detailed insights into their chemical states. In Fig. 4b, the Mn 2p spectrum shows peaks at 640.2 eV (Mn 2p_3/2) and 651.7 eV (Mn 2p_1/2), which deconvolute into components corresponding to Mn³⁺ and Mn²⁺ oxidation states. The Co 2p spectrum (in Fig. 4c) displays two main peaks at 780.4 eV and 794.3 eV, along with two satellite peaks at 785.8 eV and 802.4 eV respectively, confirming the coexistence of Co³⁺ and Co²⁺ oxidation states. In Fig. 4d, the Ni 2p peaks are observed at 854.1 eV (2p_3/2) and 872.2 eV (2p_1/2), accompanied by a satellite peak at 860.1 eV, indicating the exclusive presence of Ni²⁺ in the sample. The Zn 2p spectrum (Fig. 4e) shows peaks at 1019.6 eV and 1042.2 eV, characteristic of Zn²⁺. The Fe 2p spectrum (Fig. 4f) exhibits peaks at 709.9 eV and 723.3 eV, along with satellite peak features at 717.6 eV and 731.8 eV, confirming the coexistence of Fe³⁺ and Fe²⁺ oxidation states.^21,27,51,52 The O 1s spectrum (Fig. 4g) presents a dominant peak at 529.6 eV, corresponding to lattice oxygen (metal-oxygen bonds), with additional components attributed to C=O, Fe–O–C, and Fe–O–Fe linkages. The C 1s spectrum (Fig. 4h) shows a primary peak at 284.6 eV, assigned to C–C, C–O, and O–C=O functional groups, indicating the presence of residual surface carbon species.^1,53 Overall, the XPS results confirm the successful incorporation of Mn, Co, Ni, Zn, and Fe into the spinel lattice, with mixed valence states that are crucial for the conductivity and electrochemical behaviour of the MCNZF spinel ferrite.

Fig. 4 (a) Wide survey spectrum, (b) Mn 2p, (c) Co 2p, (d) Ni 2p, (e) Zn 2p, (f) Fe 2p, (g) O 1s and (h) C 1s.

3.2. Morphology characterization

The surface morphology of the synthesized MCNZF sample was investigated using FESEM, as displayed in Fig. 5(a). The micrograph of the MCNZF calcined at 1100 °C reveals well-defined crystalline particles with particles ranging from 200 nm to 2 µm. Additional FESEM images captured at various magnifications are provided in Fig. S2 of the SI, which further confirm the uniform surface texture and distribution of particles. The increase in calcination temperature facilitates grain growth and densification, resulting in larger particle dimensions due to enhanced diffusion and coalescence processes.^54,55 The morphology displays a predominantly polygonal structure with sharp-edged grains, and in some regions, pyramidal-shaped crystallites are observed, suggesting the presence of anisotropic growth facets during thermal treatment. Notably, no significant evidence of agglomeration is detected, indicating effective stabilization of the particles during synthesis, likely due to the albumen-assisted route that prevented uncontrolled particle clustering. The Energy-Dispersive X-ray (EDX) spectrum, presented in Fig. 5(b), confirms the presence of all constituent elements – Mn, Co, Ni, Zn, Fe, and O, corresponding to the stoichiometric formula of the MCNZF composition. The elemental distribution, as depicted in the EDX elemental mapping (Fig. 5(c)), demonstrates a homogeneous dispersion of metal cations throughout the sample without any phase segregation. This uniformity in elemental distribution suggests successful incorporation of all transition metals into the spinel lattice, reinforcing the compositional stability and purity of the synthesized medium-entropy spinel ferrite. The albumen-derived carbon residue is shown in the magnified EDX image in Figure S2(b) of the SI.

Fig. 5 (a) FESEM images, (b) EDX and (c) elemental mapping.

The HRTEM analysis was carried out on the synthesized MCNZF sample calcined at 1100 °C to gain deeper insights into its microstructural properties, as shown in Fig. 6(a) and (b). The micrographs display irregular, non-spherical particles with sizes ranging from a few hundred nanometres to several micrometres, suggesting the formation of well-crystallized grains at elevated temperatures. The SAED pattern shown in Fig. 6(c) exhibits distinct concentric diffraction rings, which can be indexed to the (022), (113), (004), (115), and (044) planes. These reflections are in good agreement with the XRD results, confirming the polycrystalline nature of the spinel MCNZF material. The lattice-resolved HRTEM image in Fig. 6(d) reveals clear and well-defined lattice fringes with interplanar spacings of 0.239 nm and 0.253 nm, corresponding to the (222) and (113) planes, respectively. These values are consistent with the XRD analysis, further validating the formation of the spinel structure and crystalline phase formation. The HAADF-STEM images and elemental mapping (Fig. 6(e)) reveal a uniform spatial distribution of Mn, Co, Ni, Zn, Fe, and O, indicating successful incorporation of the constituent cations within the spinel lattice and the absence of phase segregation, characteristic of a homogeneous medium-entropy ferrite structure.

Fig. 6 (a) HR-TEM image at 200 nm, (b) HR-TEM image at 500 nm, (c) SAED pattern, (d) lattice spacing images and (e) HAADF-STEM image.

3.3. Textural characterization

To investigate the textural characteristics of the material, nitrogen (N₂) adsorption–desorption measurements were performed to estimate the specific surface area, pore size distribution, and pore volume. The obtained adsorption–desorption isotherm (relative pressure versus adsorbed quantity), along with the BJH pore size distribution curve (pore diameter versus pore volume) and the corresponding BET plot (inset), are provided in Fig. 7(a) and (b). The isotherm exhibits a typical Type IV profile with an H3 hysteresis loop, indicating the presence of mesoporous structures.⁵⁶ The absence of a limiting adsorption at high relative pressure suggests slit-shaped pores formed by the aggregation of particles rather than well-defined cylindrical pores.³⁴ Such a pore structure arises from interparticle voids and results in a non-uniform pore network. The calculated BET surface area of the sample is 4.161 m² g⁻¹, accompanied by a total pore volume of 0.0086 cm³ g⁻¹. The average pore radius is determined to be 4.12 nm, further confirming the mesoporous nature of the material. Despite the relatively low surface area, the presence of mesopores suggests a moderately porous framework with accessible channels that can support efficient ion diffusion. This textural feature supports diffusion-dominated charge storage behaviour, consistent with the low surface area and high diffusion-controlled contribution observed in electrochemical analysis.

Fig. 7 (a) N₂ adsorption–desorption isotherm and (b) BJH plot and BET plot (Inset).

3.4. Optical characterization

FT-IR spectroscopy was performed to identify the functional groups present in the MCNZF composition in the wavenumber range of 400–4000 cm⁻¹. The recorded spectrum, displayed in Fig. 8(a), shows the distinct absorption bands observed at 578, 1103, 1631, and 3436 cm⁻¹. The strong band observed at 578 cm⁻¹ is due to the metal–oxygen (M–O) stretching vibrations, corresponding to the cations occupying tetrahedral and octahedral sites in the spinel lattice. The band observed at 1103 cm⁻¹ is due to the C–OH stretching vibration, while the peak around 1631 cm⁻¹ is related to C–O bending vibrations. The broad band near 3436 cm⁻¹ corresponds to O–H stretching vibrations, arising from adsorbed or coordinated water molecules.^14,57,58 These spectral features collectively confirm the formation of a pure spinel phase, with no evidence of amides related to albumen remains.

Fig. 8 (a) FTIR and (b) Tauc plot.

The optical behaviour of the synthesized MCNZF sample was analyzed using UV-Visible spectroscopy over the wavelength range from 200–800 nm (displayed in Fig. S3 of SI) to gain insights into its electronic structure and optical transition mechanisms. The obtained absorption spectrum exhibited a distinct absorption edge in the visible region, indicating the semiconducting nature of the material. The optical band gap energy was evaluated using Tauc's relation, which correlates with the absorption coefficient (α) and the incident photon energy (hν).^59–61 For the MCNZF sample, the plot of (αhν)^1/2 vs hν was constructed, as shown in Fig. 8(b), suggesting an indirect allowed transition mechanism. The linear portion of the plot was extrapolated to intersect the energy axis at (αhν)^1/2 = 0. The calculated optical band gap energy using eqn (S1) (in SI) is of 2.86 eV. The calculated band gap value reflects a moderate electronic separation between the conduction band and valence band, confirming the semiconducting nature of the MCNZF sample. This intermediate band gap lies within the visible region, which allows effective absorption energy conversion applications. Furthermore, the observed optical response suggests possible charge transfer transitions between the metal cations, contributing to the absorption in the visible range. Such transitions are beneficial for electronic conductivity and surface reactivity, both crucial for electrochemical properties.^62,63

3.5. Electrochemical performance

The electrochemical performance of the synthesized MCNZF sample was evaluated using a three-electrode configuration in an electrolyte of 2M KOH. Cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements were performed to investigate the capacitive behaviour and charge transfer characteristics of the electrode material.

3.6. Cyclic voltammetry study

CV was performed on the fabricated MCNZF/GF electrode in a potential window range of −1.4 V to −0.4 V at a different scan rate varying from 5 mV s⁻¹ to 200 mV s⁻¹, as illustrated in Fig. 9(a). The CV curves exhibited a progressive increase in the area of the enclosed loop and also in the corresponding current response with increasing scan rate, indicating the enhanced charge storage kinetics and improved ion diffusion at a higher sweep rate. This trend reflects the good rate of capability and electrochemical reversibility of the MCNZF/GF electrode. A comparison of the MCNZF/GF electrode with the bare GF electrode is presented in Fig. S5(a) of the SI, confirming the enhanced electrochemical activity upon MCNZF coating.

Fig. 9 (a) Cyclic voltammetry at various scan rates. (b) Linear fit for selected scan rates (c) capacitance contribution %. (d) Capacitive and diffusion-controlled regions at 5 mV s⁻¹.

The CV profile at a scan rate of 5 mV s⁻¹ (Fig. S5(b) of the SI) exhibits well-defined redox peaks, with an anodic (oxidation) peak around −0.84 V and a cathodic (reduction) peak around −1.12 V. These distinct peaks indicate the pseudocapacitive nature of the material, originating from faradaic redox reactions involving multiple valence states of the transition metal ions (Fe³⁺/Fe²⁺, Mn³⁺/Mn²⁺, Co³⁺/Co²⁺, and Ni²⁺, Zn²⁺) which are identified from XPS. The coexistence of divalent and trivalent cations enables rapid and reversible electron transfer, contributing to high capacitive performance through surface processes or near-surface redox processes.⁶⁴

To quantitatively distinguish between the capacitive and diffusion-controlled charge storage mechanisms, the scan rate-dependent CV data were analyzed using Dunn's method.⁶⁵ In this approach, the total current at a selected potential (V) and scan rate (ν) is expressed as a linear combination of capacitive and diffusion-controlled contributions:

i(V,ν) = k₁(V)ν + k₂(V)ν^1/2 (2)

Here, k₁(V)ν represents the surface-controlled (capacitive or electrochemical double-layer capacitance (EDL)-type) current, while k₂(V)ν^1/2 corresponds to the diffusion-controlled (pseudocapacitive) current. Rearranging the equation yields:

i(V/ν)/ν^1/2 = k₁(V)ν^1/2 + k₂(V) (3)

By plotting i(V/ν)/ν^1/2 versus ν^1/2 at selected potentials across different scan rates (5, 10, 20, and 50 mV s⁻¹), as shown in Fig. 9(b), the parameters k₁ and k₂ were extracted to evaluate the quantitative contributions of the capacitive and diffusion-controlled processes. The Dunn analysis was conducted in the potential window of −1.4 to −0.4 V. The analysis was restricted to moderate scan rates to avoid deviations arising from polarization and diffusion limitations at higher scan rates, ensuring reliable interpretation of charge storage contributions.

At 5 mV s⁻¹, the pseudocapacitive contribution accounted for 84.85% of the total current, while the capacitive (EDL) contribution was 15.16%. The respective capacitive and diffusion-controlled regions derived from the CV curve at 5 mV s⁻¹ are illustrated in Fig. 9(d). The relatively smaller capacitive region suggests that charge storage in MCNZF/GF is predominantly governed by diffusion-controlled faradaic processes rather than purely EDL behaviour.^20,41,66 As the scan rate increases, the capacitive fraction gradually rises due to faster surface-limited processes, as shown in the bar graph in Fig. 9(c). The variation of pseudocapacitive and EDL contributions with scan rate is depicted in Fig. S5(c) of the SI, where the pseudocapacitive and EDL fractions were 63.10% and 36.10%, respectively, at 50 mV s⁻¹. This confirms the synergistic coexistence of EDL capacitance and faradaic reactions governing the overall charge storage behaviour of the MCNZF/GF electrode.

3.7. Galvanostatic charge–discharge test (GCD) study

The charge storage behaviour of the MCNZF/GF electrode was further examined by GCD analysis within the potential range of −1.2 V to −0.4 V at varying current densities of 4, 8, 12, and 16 A g⁻¹, as illustrated in Fig. 9(a). The high current density range (up to 16 A g⁻¹) was used to probe the ability of the electrode to sustain rapid charge–discharge processes under stringent conditions. The GCD curves display nearly symmetrical triangular shapes with minor deviations resembling plateaus, particularly at lower current densities. Such features are indicative of a hybrid charge storage mechanism involving both EDL and pseudocapacitive contributions.^15,51,66,67 The small plateau-like regions appearing within the potential range of −1.2 to −0.8 V in Fig. 10(a), mostly at a current density of 4 A g⁻¹, can be attributed to faradaic redox reactions involving transition metal ions, which operate in conjunction with the non-faradaic double-layer charging process. This observation confirms that the MCNZF/GF electrode exhibits a combined EDL and pseudocapacitive behaviour, consistent with the CV and Dunn method analyses. Based on these results, a current density of 4 A g⁻¹ was selected for long-term GCD cycling, and 1500 cycles were performed with selected cycle numbers presented in Fig. S5(d) of the SI. The electrode achieved a maximum specific capacitance of 408.18 F g⁻¹ at 4 A g⁻¹ calculated using eqn (2) shown in the SI, which gradually decreased with increasing current density due to reduced ion diffusion efficiency and limited active material utilization at higher charge–discharge rates. The dependence of specific capacitance (C_sp) on current density is plotted in Fig. 10(b), and the corresponding numerical values are summarized in Table 1.

Fig. 10 (a) GCD at various current densities. (b) Variation of C_sp w.r.t current density.

Table 1 Variation of C_sp with current density and cycle number

Current density (A g^−1)	Csp (F g^−1)	Cycle number	Csp (F g^−1)
4	408.18	1	408.18
8	235.74	500	293.305
12	186.75	1000	288.085
16	97	1500	285.695

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3.8. Cycling stability

The long-term cycling stability of the MCNZF/GF electrode was performed through GCD measurements at a selected current density of 4 A g⁻¹ for 1500 continuous cycles. The variation of specific capacitance (C_sp) with the cycle number is depicted in Fig. 11(a), and the corresponding data are summarized in Table 1. As shown in Fig. 11(b), the coulombic efficiency remained relatively stable with minimal degradation throughout the cycling process, but capacitance retention is moderately stable. After 1500 cycles, the electrode preserved 66.79% of its initial specific capacitance, indicating good structural robustness and electrochemical reversibility during repeated charge–discharge operations. Moreover, a high coulombic efficiency of 91.55% was maintained, suggesting efficient charge–discharge kinetics and excellent energy recovery during prolonged cycling.^53,68 The moderate capacitance retention observed can be primarily attributed to the bulk nature and also due to the changes induced during repeated ion insertion/extraction processes, rather than solely to the bulk nature of the MCNZF material. In bulk electrodes, ion transport and electrolyte penetration are often restricted to the surface or near-surface regions, resulting in limited utilization of the inner active sites during cycling. Over prolonged charge–discharge operation, the repetitive volume expansion and contraction associated with faradaic redox reactions may induce mechanical stress, partial cracking, and also loss of electrical contact between active particles with the current collector. These effects collectively contribute to gradual performance fading.^3,69,70 Nevertheless, the high coulombic efficiency confirms that the charge storage process remains largely reversible, demonstrating stable electrochemical kinetics despite structural limitations inherent to bulk materials. It is important to note that the reported electrochemical values are derived from a 3E configuration and reflect the intrinsic behaviour of the material. Due to the narrow potential window of ferrite-based electrodes in symmetric systems, these materials are more effectively utilized in asymmetric configurations to achieve enhanced operating voltage and energy density.

Fig. 11 (a) Variation of C_sp w.r.t cycle number. (b) Capacitance retention and coulombic efficiency.

3.9. Electrochemical impedance spectroscopy (EIS)

EIS was performed to examine the intrinsic resistance and charge-transfer behaviour of the MCNZF/GF electrode.^14,27,70 The measurements were carried out in 2 M KOH electrolyte over a frequency range of 0.1 Hz–100 kHz with an AC amplitude of 5 mV at open-circuit potential. The Nyquist plot presented in Fig. 12(a), together with the equivalent circuit shown in the inset, was analyzed using a modified Randles-type model. The Bode plots (phase angle vs frequency) before and after GCD are presented in Fig. S6(a) and (b) of the SI. In the high-frequency region, a small semicircular arc is observed, corresponding to the charge-transfer resistance (R_ct) at the electrode–electrolyte interface. For the new electrode, the low semicircle diameter indicates a small R_ct of approx. 24.91 Ω, implying efficient redox kinetics and facile charge transport through the bulk structure. After performing GCD cycling, the semicircle expands, yielding an increased R_ct value of about 38.75 Ω. The other parameters are included in Table S2 in the SI. This rise can be attributed to the formation of resistive surface films, microstructural rearrangements, and partial blockage of electroactive sites within the bulk matrix, leading to diminished ion-diffusion and decreased electron conduction pathways. Consequently, the overall conductivity and specific capacitance decrease upon prolonged cycling. The favourable impedance response of MCNZF/GF mainly arises from the synergistic role of multiple transition metals such as Co, Ni, and Fe, which provide abundant redox-active centres and enhance the electronic conductivity.^3,24,40 Meanwhile, Mn and Zn contribute primarily to structural stability and lattice uniformity, minimizing the mechanical degradation during cycling.

Fig. 12 (a) Nyquist plot of MCNZF/GF before and after GCD. (b) Ragone plot.

3.10. Ragone plot

The Ragone plot of the MCNZF/GF was constructed to evaluate and compare the energy and power density characteristics, as shown in Fig. 12(b). The energy density (E) and power density (P) were calculated using eqn (S4) and (S5) of the SI, respectively. The MCNZF/GF exhibited a maximum energy density of 36.31 Wh kg⁻¹ at a corresponding power density of 1601.28 W kg⁻¹ at a current density of 4 A g⁻¹. The energy and power densities are calculated based on a 3E configuration and represent electrode-level performance. These results are comparable to or exceed previously reported values for other high-performance transition metal-based spinel and entropy-stabilized supercapacitor electrodes.^6,11,25 Upon increasing the current density up to 16 A g⁻¹, the MCNZF/GF electrode still maintained an energy density of 8.63 Wh kg⁻¹ at a high-power density of 6405.12 W kg⁻¹, demonstrating significantly good rate capability and fast charge–discharge kinetics. The superior energy power performance of the MCNZF/GF electrode can be attributed to its highly crystalline spinel structure, rich redox-active sites due to multiple transition metals, and the synergistic effect of entropy stabilization, which promotes enhanced charge storage behaviour. These electrochemical characteristics alongside the high specific capacitance, long-term cycling durability and robust structural integrity highlight the potential of MCNZF as a promising electrode material for advanced energy storage applications.

Collectively, the EIS, CV, and GCD analyses reveal that the MCNZF/GF electrode exhibits low interfacial resistance, acceptable ion transport, and moderately stable electrochemical behaviour, demonstrating its potential for hybrid or pseudocapacitive energy storage applications.

To examine the morphological changes before and after cycling, SEM analysis was carried out. The SEM images of MCNZF/GF before GCD and after GCD with their surface roughness plots are displayed in Fig. 13. The surface roughness analysis derived from profile data reveals a noticeable increase after electrochemical cycling, with the average roughness (R_a) rising from 34.61 to 41.18 and the root mean square roughness (R_q) increasing from 43.08 to 48.24. This enhancement in roughness indicates the development of a more uneven and textured surface, which can be attributed to repeated ion insertion/extraction and associated structural rearrangements during cycling.⁷¹ Such changes are consistent with the SEM observations, where the electrode surface becomes comparatively rough without the formation of visible cracks, suggesting mechanical stability of the material. The increased roughness may initially provide additional active sites; however, excessive surface irregularities can also hinder effective electron transport and ion diffusion pathways, contributing to the observed decrease in capacitance upon prolonged cycling.⁷²

Fig. 13 (a) SEM image of MCNZF/GF before GCD. (b) SEM image of MCNZF/GF after GCD. (c) Surface roughness plot before GCD. (d) Surface roughness plot after GCD.

The comparison of various synthesis techniques for binary, ternary, and quaternary (entropy) spinel ferrite compounds, along with the different electrolytes, potential windows, current densities and corresponding specific capacitances reported earlier for supercapacitor applications, is presented in Table 2.

Table 2 Comparative summary of recent reports

Material	Method	Electrode mass load (mg)	Electrolyte	I (A g^−1)	PW (V)	Csp (F g^−1)	Stability @cycles	Ref.
[NiCo]Fe2O4	Sol–gel	—	1 M KOH	1	−1 to 1	50.0	20%@200	73
[NiCu]Fe2O4	Sol–gel	—	1 M KOH	1	−1 to 1	44.0	5%@200	73
[CuCo]Fe2O4	Sol–gel	—	1 M KOH	1	−1 to 1	76.9	80%@200	73
[CuCo]Fe2O4	Sol–gel	∼1	1 M KOH	1		397.0	50%@300	74
[CoMg]Fe2O4	Sol–gel combustion	∼1	2 M KOH	0.5	0 to 0.4	579.3	96.49%@1000	75
[AlxCuyCoz]Fe2O4 (x + y + z = 1)	Sol–gel	—	1 M KOH	2	−1 to 1	256-540	88% @300	74
NiFe2O4/CF	Hydrothermal	—	3 M KOH	1	0 to 0.6	490.0	91.3%@5000	76
[ZnMg]Fe2O4	Sol–gel citrate	∼1	1 M Na2SO4	1	0 to 0.4	484.6	—	77
[ZnMgCu]Fe2O4	Solvothermal reflux	∼1	1 M KOH	1.75	0 to 0.6	508.25	—	27
[CaZnMg]Fe2O4	Solvothermal reflux	—	2 M KOH	0.5	0 to 0.6	66.8	—	78
[MnCoNiZn]Fe2O4	Sol–gel	∼1	2 M KOH	4	−1.2 to −0.4	408.18	66.79%@1500	This work

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4. Conclusion

A medium-entropy spinel ferrite [MnCoNiZn]Fe₂O₄ was synthesized via a sol–gel route, forming a cubic spinel single phase with pyramidal shaped particles (200 nm–2 µm). The optical properties of the material showed a semiconductive nature with an optical band gap of 2.86 eV. Electrochemical analyses revealed a hybrid charge-storage mechanism combining both pseudocapacitive and electric double-layer behaviour, confirmed by Dunn's method. The electrode delivered a significantly high specific capacitance of ∼408 F g⁻¹ at 4 A g⁻¹ current density, retaining 66.79% of its initial capacitance after 1500 cycles with 91.55% coulombic efficiency. The EIS results showed low equivalent series and charge transfer resistance, indicating efficient ion transport and good electrical conductivity. The performance enhancement arises from the synergistic interaction among Fe, Co, and Ni redox-active centres and the structural stability provided by Mn and Zn. Being a bulk material, the limited surface area and longer ion diffusion pathways contribute to moderate cycling stability. Incorporation of graphene oxide or reduced graphene oxide could improve conductivity, surface reactivity, and durability, making [MnCoNiZn]Fe₂O₄ a promising candidate for hybrid supercapacitor applications.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data [e.g., raw data, characterization data, or analytical methods] supporting the findings of this study are available from the corresponding author. Data can be obtained from the corresponding author upon reasonable request.

Supplementary information is available. See DOI: https://doi.org/10.1039/d5ya00340g.

Acknowledgements

The authors gratefully acknowledge the financial support from the Science and Engineering Research Board (SERB), Government of India, by grant No: CRG/2019/001574. The authors would like to acknowledge the support offered by CSIR-Indian Institute of Chemical Technology. Additionally, the authors gratefully acknowledge the University Grants Commission (UGC), India, for financial assistance through the National Fellowship for Other Backward Classes (NFOBC) during the course of this research.

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