In Situ Synthesized Manganese Ferrite/Carbon Composite Nano-Material: A Novel Electrode Material for High-Performance Supercapacitors

Abstract

This study presents an in situ synthesis of a novel manganese ferrite/carbon (MF/C) composite material via a citrate sol–gel route followed by calcination in an inert argon (Ar) atmosphere. The structural and morphological and porosity properties were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDX), and N₂ gas physisorption analysis. Electrochemical evaluation of the MF/C in a 3 M KOH electrolyte in a three-electrode configuration showed a high specific capacity of 39.26 mAh g⁻¹ at 1 Ag⁻¹ and a rate capability of 69% at 5 Ag⁻¹ and an equivalent series resistance (ESR) of 0.798 Ω. Subsequently, an asymmetric hybrid supercapacitor device (MF/C//AC) was fabricated using MF/C as the positive electrode and human-derived activated carbon (AC) as the negative electrode. The assembled device exhibited remarkable performance, with a wide operating voltage window of 1.4 V, a high sweeping potential of 1 V s⁻¹, a specific capacity, energy, power and maximum power of 42.4 mAhg⁻¹, 16.35 Wh kg⁻¹, 1944 W kg⁻¹ and 236 kW kg⁻¹, respectively, and excellent capacitance retention of 92% after 15,000 charge–discharge cycles. The in situ preparation approach significantly reduced synthesis time and cost compared to conventional multi-step methods, as less equipment was required, while still achieving comparable or superior electrochemical performance to other supercapacitors in the literature.

1. Introduction

To meet the world’s growing energy demands, scientists have recently focused a great deal of attention on the development of environmentally friendly and sustainable energy storage and conversion technologies [1,2]. The need for effective sustainable energy transmission and storage around the world has led to a growing interest in supercapacitors among electrochemical energy storage devices [3]. In addition to being a desirable medium for storing renewable energy, supercapacitors can serve as an effective power source for portable electronic devices due to their high power density, high charge–discharge rate, and exceptional stability. Their effective applications include regenerative braking, emergency doors, system backup, and in some automobiles as the primary power source [3,4].

Most widely utilized supercapacitors are pseudo/Faradic capacitors, owing to their superior electrochemical performance compared to electric double-layer capacitors (EDLCs). Their enhanced performance is primarily due to their fast and reversible redox reactions, which contribute to higher specific capacity and energy [5]. Furthermore, aqueous supercapacitors offer higher power density, higher cycling stability, and more safety with lower cost, making them a more sustainable option than commercial lithium-ion batteries and sodium/potassium intercalation batteries, which typically use more expensive, flammable, and toxic materials. The choice of pseudocapacitors is to provide an intermediate state, where batteries and EDLCs typically fall short—by combining the high energy density of batteries with the quick discharge rates of EDLCs [6].

As a result of the limitations associated with current energy storage materials, several approaches have been employed to enhance the performance of pseudocapacitors. These include the synthesis of iron oxides with distinct structures, the production of multicomponent iron oxides, and the blending of carbon with iron oxides. The electrochemical characteristics of iron oxides can also be modified by introducing different metallic cations. Among these, manganese has attracted attention due to its multiple valence states, which facilitate redox reactions and improve conductivity and mechanical stability during discharge. Moreover, the possible synergistic interaction between manganese and iron may enhance the electrochemical activity of manganese ferrite (MnFe₂O₄) [1,7,8,9]. Our group has previously reported similar results for materials such as CoFe₂O₄ [10] and NiFe₂O₄ [11] in supercapacitor applications. A CoFe₂O₄-based device showed an enhanced pseudocapacitive charge storage performance with a maximum specific capacitance and capacity values of 756.5 Fg⁻¹ and 57.16 mAhg⁻¹, while a NiFe₂O₄-based device demonstrated optimum electrochemical charge storage performance with specific capacitance and capacity values of 1128 Fg⁻¹ and 58 mAhg⁻¹, respectively.

The inherent characteristics of MnFe₂O₄ make it a promising electrode material for energy storage applications [6,12]. Its distinct spinel structure promotes enhanced electrochemical activity by enabling numerous redox-active sites. Prior research has demonstrated that Mnfe₂O₄ can deliver strong electrochemical performance, including cyclic stability and good capacitance. However, challenges such as inadequate electronic conductivity and particle aggregation continue to limit its full potential [6,12,13]. To advance its application in supercapacitors, more research into the structural and electrochemical behaviour of MnFe₂O₄ as well as a deeper understanding of its charge storage mechanisms are required.

Despite considerable progress in material design, the most commonly employed synthesis approaches remain conventional multi-step methods. Typically, these involve first preparing the metal oxide using techniques such as sol–gel, hydrothermal, solvothermal, or auto-combustion techniques, then annealing in air [14,15,16,17]. Simultaneously, carbon-based materials are often synthesized separately through processes such as carbonization/activation [18,19,20], chemical vapor deposition (CVD) [21,22], Hummers’ method [23,24], or arc discharge [25]. The two components are then combined into a composite using methods like hydrothermal treatment [26] or ball milling [27]. These multi-step procedures often are time-consuming, require numerous chemicals, and demand a wide range of specialized equipment, increasing production cost and complexity.

In contrast, this work adopts a simplified in situ sol–gel and calcination method, where the dried gel from a sol–gel precursor is directly calcined in an inert argon (Ar) atmosphere. This approach not only promotes effective dispersion of the metal oxide within the carbon matrix but also significantly reduces the number of synthesis steps, the material and equipment requirements, and the overall cost. Additionally, the method offers improved simplicity and scalability, making it suitable for practical and large-scale applications. Moreover, the MF/C electroactive material prepared using this novel method yielded electrochemical results comparable to those reported in the literature using more complex and resource-intensive techniques [17,28,29], further validating the efficiency and potential of this in situ strategy.

2. Experimental Details

2.1. Materials Used

Manganese chloride tetrahydrate (MnCl₂·4H₂O, 98%), iron (III) chloride hexahydrate (FeCl₃·6H₂O, 99.99%), and citric acid C₈H₈O₇ (99.5%) were purchased from Sigma-Aldrich (St. Louis, MO, USA), and were used as starting materials for the synthesis of MnFe₂O₄/carbon composite nano-powder and calcinated in an argon (Ar) environment. The synthesized material was utilized to prepare the electrodes together with polyvinylidene fluoride (PVDF), 1-methyl-2-pyrrolidone (NMP), and conductive acetylene black (CAB). A 0.2 mm thick commercial nickel foam (NF) was sectioned into rectangular 1 × 3 cm² and circular 16 mm diameter pieces for three- and two-electrode configuration measurements, respectively, then subjected to a chemical dip-washing process using 3 M hydrochloric acid (HCl), followed by acetone, and ethanol, with each solution dipped for 15 min before being rinsed with deionized water (DI Water) and dried in an electric oven at 60 °C for 4 h [30].

2.2. Material Synthesis

MF/C nano-powder was synthesized through the citrate sol–gel [31] and a calcination/carbonization technique [18,19,20]. In summary, a solution was formulated using a 1:2 (Mn:Fe) ratio of metal ions and 1:1 ratio of metal ions to citric acid (CA), accomplished by dissolving 1.98 g of MnCl₂·4H₂O, 5.40 g of FeCl₃·6H₂O, and 5.76 g of CA in 30 mL of DI Water. The CA served as a chelating agent ensuring metal ion separation and a source of carbon during the calcination/carbonization process. The solution was continuously stirred in a magnetic stirrer at a constant temperature of 80 °C for 2 h until the prepared solution turned into a gel, which was further dried in an oven at 100 °C for 3 h until a xerogel was obtained. The prepared xerogel was calcinated at 500 °C in a quartz tube furnace for 3 h in an Ar environment with a flow rate of 300 sccm (standard cubic centimetres per minute). The obtained powder was then crushed using a mortar and pestle. From a total precursor mass of 13.14 g, approximately 2.5 g of MF/C composite was obtained, corresponding to a yield of ~19%, with the mass loss mainly attributed to precursor decomposition, volatilization of organics, and chloride removal during carbonization. The preparation (synthesis) schematic is displayed in Figure 1.

Figure 1. Schematic diagram illustrating the citrate sol–gel synthesis process of MF/C.

2.3. Characterization

A Brucker 2D PAN analytical BV diffractometer (Amsterdam, The Netherlands), equipped with CuKα (λ = 0.154 nm) over a 2θ range of 5 to 90°, was utilized to acquire X-ray diffraction patterns for assessing the crystallinity of the prepared material. The FTIR spectra of the materials were recorded in absorption mode using a Nicolet 750 FTIR spectrometer (Bruker, Billerica, MA, USA). The nitrogen adsorption–desorption isotherms of MF/C were measured using a NOVA Touch NT 2LX-1 surface area and porosity analyser (Quantachrome Instruments, Boynton Beach, FL, USA), operated at 220 V and controlled by TouchWin software version 1.22. The sample surfaces were degassed at 180 °C for 12 h under vacuum to eliminate impurities and moisture. To study the specific surface areas of the adsorption branch, the Brunauer–Emmett–Teller (BET) method was used, operating in a relative pressure range (P/P0) of 0.01 to 0.2. In addition, the density functional theory (DFT) method was used to calculate the pore-size distributions from the desorption branch of the isotherms. The surface morphologies and elemental compositions of the material were studied using an energy-dispersive spectrometer (EDS) coupled with a Zeiss Ultra Plus 55 Field Emission Scanning Electron Microscope (FE-SEM) (Carl Zeiss Microscopy GmbH, Oberkochen, Germany) running at accelerating voltages of 20 kV and 2.0 kV, respectively.

2.4. Electrochemical Characterization

All electrochemical analyses were conducted at room temperature using a Biologic VMP-300 potentiostat (Bio-Logic Science Instruments, Seyssinet-Pariset, France), controlled by EC-Lab software version V11.50. In the three-electrode system, a glassy carbon plate was used as the counter electrode, while a Ag/AgCl (with a 3 M KCl electrolyte inside) electrode functioned as the reference electrode. The working electrodes were fabricated by uniformly mixing the electroactive material (MF/C, 80 wt%) (wt% = weight percentage), conducting agent (CAB, 10 wt%), and binder (PVDF, 10 wt%), using NMP as a solvent, onto a piece of rectangular and circular NF. After coating, the electrode was dried overnight at 60 °C in an electric oven. The resulting mass-loading was determined and recorded in milligrams, ranging between 2.1 and 3.2 mg. Cyclic voltammetry (CV) was conducted at scan rates of 5–1000 mVs⁻¹ within an applied potential window of 0.0–0.4 V vs. Ag/AgCl. The galvanostatic charge/discharge (GCD) measurements were conducted at different specific currents, which ranged from 1 to 5 Ag⁻¹. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range of 100 kHz to 10 mHz with an amplitude of 10 mV as the open-circuit potential. Furthermore, the active material was electrochemically analysed using a two-electrode asymmetric device. In this device (MF/C//AC), MF/C served as the positive electrode, while AC derived from human hair served as the negative electrode. All electrochemical measurements for the three- and two-electrode configurations were carried out in a 3 M KOH aqueous electrolyte solution. Note that the negative electrode AC material was previously reported by our group [32,33], where it was analysed structurally and morphologically, and also tested for its supercapacitor capabilities.

3. Results and Discussion

3.1. Structural and Morphological Analysis

The powder XRD analysis was utilized to examine the crystal structure and phase changes of the synthesized material. The powder XRD pattern of the MF/C is displayed in Figure 2a. The powder XRD pattern of the MF/C composite is displayed in Figure 2a, together with standard reference peaks of MnFe₂O₄ spinel (JCPDS no: 10-0319 [34]). The diffraction peaks were indexed to the face-centred cubic spinel structure of MnFe₂O₄ with space group $Fd\overline{3}m$, where Mn²⁺ and Fe³⁺ ions occupy the tetrahedral and octahedral sites, respectively. The indexed diffraction peaks correspond to the (220), (311), (400), (422), (511), and (440) crystallographic planes, confirming the formation of the MnFe₂O₄ spinel phase. No additional impurity peaks were observed, indicating high phase purity of MnFe₂O₄ in the MF/C composite [35]. The average crystallite size of MnFe₂O₄ was calculated using the Scherrer equation [36] applied to the most intense (311) diffraction peak, yielding a crystallite size of 12.5 nm, which is consistent with previous reports for MnFe₂O₄ [37,38].

Figure 2. (a) X-ray diffraction, (b) FTIR spectrum, (c) unit cell, (d) N₂ adsorption/desorption isotherm, (e) pore-size distribution, and (f) SEM micrographs of MF/C.

The FTIR spectrum of the MF/C composite shown in Figure 2b exhibits several characteristic absorption bands, confirming the successful integration of carbon material with MnFe₂O₄ particles. The broad band centred at ~3448 cm⁻¹ is attributed to O–H stretching vibrations, originating from surface hydroxyl groups. The weak band observed around 2450 cm⁻¹ is associated with atmospheric CO₂ adsorption or overtone vibrations commonly observed in carbon-based materials. The absorption peak at ~1636 cm⁻¹ corresponds to C=O stretching vibrations or H–O–H bending modes, indicating the presence of oxygen-containing functional groups on the carbon matrix. The band located at ~1356 cm⁻¹ is assigned to C–O stretching vibrations, further confirming the presence of carbon–oxygen functional groups, while the strong absorption band at ~556 cm⁻¹ is characteristic of metal–oxygen (M–O–M) stretching vibrations within the MnFe₂O₄ spinel lattice, confirming the formation of the ferrite structure. These FTIR features are consistent with previous reports for MnFe₂O₄, other spinel carbon-based composites and heteroatom-doped carbon materials, indicating successful bonding and interaction between MnFe₂O₄ nanoparticles and the carbon framework [38,39,40].

The unit cell structure of MnFe₂O₄ has been visualized using VESTA software (Ver. 3.5.2, 64-bit Edition), showing the positions and bonding of Mn²⁺, Fe³⁺ and O²⁻ within the MnFe₂O₄ structural matrix, as shown in Figure 2c. The prepared material was taken for surface textural analysis, and the obtained N₂ adsorption–desorption isotherms are displayed in Figure 2d. The figure reveals that the N₂ adsorption–desorption isotherm displays type-IV isotherms, which represent low-pressure monolayer adsorption and high-pressure multilayer adsorption [41]. Furthermore, the BET analysis estimated the average specific surface area of MF/C to be 29.62 m²/g. Moreover, the material has a micropore surface area of 1.709 m²/g and pore diameter of 2.65 nm. The pore-size distribution in Figure 2e shows a small dense area of mesopores and micropores. The SEM micrographs presented in Figure 2f reveal agglomerated MgAl₂O₄ particles dispersed on the AC surface, exhibiting relatively large pore structures. The EDS spectrum and mapping are displayed in Figure S1 (Supplementary Materials) and Figure 3, respectively, showing the presence of manganese (Mn), iron (Fe), oxygen (O), nitrogen (N), chlorine (Cl) and carbon (C) emanating from the precursor, while the carbon peak/concentration might have increased due to carbon tape during the SEM-EDS sample preparation. As shown in Figure 3, the elemental mapping reveals a relatively uniform distribution of all detected elements across the sample surface.

Figure 3. EDS mapping of MF/C.

3.2. Electrochemical Performance

Three-electrode configurations were employed to assess the electrochemical performance of the novel in situ-prepared MF/C. The electrolyte was optimized by testing different KOH concentrations as shown in Figure S2, with 3 M KOH showing the best performance and subsequently being used throughout this entire study. CV was performed at different scan rates ranging from 5 to 100 mVs⁻¹ within a potential window of 0–0.4 vs. Ag/AgCl, as shown in Figure 4a. The CV curves of MF/C exhibit prominent redox peaks, which signify quick redox reactions attributed to its pseudocapacitive characteristics. The consistent observation of these redox peaks at all scan rates indicates a viable and reversible redox reaction [13], shown below in Equations (1) and (2):

$$\mathrm{M}\mathrm{n}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_4+{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}\mathrm{H}}^{-}\leftrightarrow \mathrm{M}\mathrm{n}\mathrm{O}\mathrm{O}\mathrm{H}+2\mathrm{F}\mathrm{e}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{e}}^{-}$$

(1)

$$\mathrm{F}\mathrm{e}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{O}\mathrm{H}}^{-}\leftrightarrow \mathrm{F}\mathrm{e}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}$$

(2)

Figure 4. (a) CV curves at scan rates of 5–100 mVs⁻¹, (b) GCD curves at specific currents ranging from 1 to 5 Ag⁻¹, (c) specific capacity versus specific current, (d) Nyquist plot, and (e) phase angle analysis of MF/C.

With increasing scan rates, the oxidation peaks moved to higher potentials while the reduction peaks shifted to lower potentials. This happens because the electrolyte ions have limited time to access the electrode surface. This indicates that the oxidation and reduction reactions taking place during the process are quasi-reversible in nature [42]. The cyclic process undergoes extensive redox reactions as a result of polarization effects and ohmic resistance [7]. To better understand the charge storage mechanism, the relationship between peak current (i) and scan rate (ν) was analysed using the power-law expression, $i = a{\nu }^{\mathrm{b}}$, where the $b$ value indicates the dominant charge storage process. The $b$ values calculated from the slope of log(i) versus log(ν) plots displayed in Figure S3 for the cathodic peaks were found to be 0.72, suggesting that the charge storage involves both surface-controlled capacitive processes and diffusion-controlled Faradaic reactions [43,44].

Figure 4b illustrates the GCD curve of the MF/C electrode at different specific currents within the potential range of 0–0.4 vs. Ag/AgCl. All GCD curves exhibit a nonlinear characteristic with discharge plateaus, indicating the presence of Faradaic redox reactions as seen on the CV curves displayed in Figure 4a. The specific capacities ($Q_s$) in mAhg⁻¹ of the electrodes were calculated from the discharge curve of the GCD in Figure 4b using Equation (3) [45]:

$$Q_s={\displaystyle \frac{I_s\times \mathit{\Delta }t}{3.6m}}$$

(3)

where $I_s$ represents the constant current (in amperes), Δt is the discharge time (in seconds), and m is the mass (in milligrams) of the MF/C material. The GCD curves at a specific current of 1 Ag⁻¹ were used to determine the high specific capacity of 39.26 mAhg⁻¹, suggesting the higher-performance nature of the MF/C electrode. Figure 4c shows the rate capability of the MF/C active material, showing a specific capacity retention of 69% at 5 Ag⁻¹ of the initial 39.26 mAhg⁻¹. The reduction in specific capacity at high current density may be caused by the restricted access of OH⁻ ions to the internal active sites of the MF/C electrode, leading to incomplete redox reactions or time restrictions for redox reactions to occur. Moreover, the slower diffusion rate of electrolyte ions into the electrode material also contributed to the decrease in capacity [13]. The EIS result in Figure 4d was measured to further analyse the characteristic resistances and impedances of MF/C electrodes. The x-axis intercept corresponds to the equivalent series resistance (ESR), which is the total electrolyte resistance, inherent resistance of the electrode, and contact resistance between the current collector, electrode material, and electrolyte. In the high-frequency range, the diameter of the semicircle represents the charge transfer resistance (R_ct) of the electrode, which is associated with the charge transfer processes at the electrode–electrolyte interface. The prepared sample showed an ESR and R_ct of 0.798$\Omega$ and 2.5 $\Omega$, respectively. In the low-frequency region, the 45° slanted line represents the Warburg impedance (W_s) of electrolyte diffusion, which is caused by the diffusion of ions from the electrolyte to the electrode surface [46]. The pseudocapacitive behaviour seen on the CV (Figure 4a) and GCD (Figure 4b) curves and Nyquist plot (Figure 4d) is confirmed by the phase angle of $-61°$ displayed in Figure 4e, which falls within the range of $-45°$ to $-80°$ symbolizing Faradaic behaviour.

The Faradaic behaviour observed in Figure 4a,b, together with the material characterization results shown in Figure 2 and Figure 3, suggests that MnFe₂O₄ nanoparticles are uniformly embedded and well-dispersed within the conductive carbon matrix, as illustrated in Figure 5. The carbonized carbon derived from citric acid provides abundant electric double-layer capacitance (EDLC) adsorption sites and forms a continuous conductive network that facilitates rapid electron transport throughout the electrode. In contrast, the MnFe₂O₄ nanoparticles act as Faradaic-active sites, contributing pseudocapacitance through reversible redox reactions. The intimate interfacial contact between MnFe₂O₄ and the carbon matrix shortens ion diffusion pathways and enables efficient electrolyte penetration into electroactive sites. Additionally, nitrogen and oxygen introduced via metal nitrate stabilizers result in heteroatom-doped carbon, which increases defect density and active sites while further improving electronic conductivity [47,48]. The presence of oxygen-containing functional groups enhances the hydrophilicity of the electrode surface, promoting improved wettability and faster ion transport at the electrode–electrolyte interface and enhancing the electrochemical performance of the MF/C composite [47].

Figure 5. Cross-sectional schematic of MF/C positive electroactive material.

To thoroughly evaluate the electrochemical performance of MF/C nano-powder, a two-electrode asymmetric device was constructed, with MF/C as the positive and AC derived from human hair as the negative material [32,33]. The CV curves of the individual electrode materials, recorded within their respective potential windows, are presented in Figure 6a. As evident from the CV profiles, the AC electrode exhibits typical EDLC behaviour, characterized by a nearly rectangular shape, while the MF/C electrode demonstrates clear Faradaic features, indicating pseudocapacitive charge storage. The prepared device was then tested in a 3 M KOH electrolyte as they both perform optimally within that electrolyte. The charge disparity within the specific capacities/capacitance of the two electrodes was rectified by charge balance as described in Equations (S1)–(S3) [49]. The MF/C//AC device exhibited a CV, presented in Figure 6b, measured at scan rates ranging from 5 mVs⁻¹ to 1 Vs⁻¹, with a maximum voltage of 1.4 V, which exceeds the combined potential window of 0.0–0.4 vs. Ag/AgCl for MF/C and −0.8–0.0 vs. Ag/AgCl for AC due to the synergistic effect between the two electrodes. The CV curves at different scan rates clearly show the combined contribution of EDLC behaviour from AC and Faradaic behaviour from MnFe₂O₄, which is typical of a hybrid asymmetric supercapacitor. The GCD curves at different specific currents in the range of 1–5 Ag⁻¹ (Figure 6c) show potential steps, confirming the Faradaic behaviour of the device as indicated by the CV curves (Figure 6b). Since the device predominantly exhibits Faradaic behaviour, its specific capacity ($Q_s$) was determined from the GCD curves using Equation (3) and was found to be 42.4 mAhg⁻¹ at a specific current of 1 Ag⁻¹. The rate capability curve displayed in Figure 6d shows a quick decay due to the above-mentioned reasons. Additionally, the specific energy (E_d in Wh kg⁻¹) and power density (P_d in W kg⁻¹) were determined from the GCD curves using the following Equations (4) and (5):

$$E_d={\displaystyle \frac{I_s}{3.6}}{\int }_{Vi}^{Vf}V dt$$

(4)

$$P_d={\displaystyle \frac{E_d}{\Delta t}}3600$$

(5)

where $I_s$, $\Delta t$, and ${\int }_{Vi}^{Vf}V$ dt represent the specific current (Ag⁻¹), discharge time (s), and integral area under the curve of the GCD plot (Vs), respectively. The MF/C//AC hybrid device provided a specific energy and specific power, as shown in Figure 6e, where 1 Ag⁻¹ shows a higher specific energy of 16.35 Wh kg⁻¹ while the specific current of 5 Ag⁻¹ shows a higher specific power of 1944.45 W kg⁻¹. The MF/C//AC device was assessed for material stability and degradation under constant CD for 15,000 cycles, as displayed in Figure 6f. The material capacity retention (CR) is 92% of its initial capacity and the coulombic efficiency ($n_c$) is 99% [50].

$$\mathrm{C}\mathrm{a}\mathrm{p}\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y} \mathrm{R}\mathrm{e}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} (\%)=\left({\displaystyle \frac{\mathrm{C}\mathrm{a}\mathrm{p}\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y} \mathrm{a}\mathrm{t} \mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}\mathrm{e} \mathrm{n}}{\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{C}\mathrm{a}\mathrm{p}\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y}}}\right)\times 100$$

(6)

$${\mathrm{n}}_{\mathrm{c}} (\%)=\left({\displaystyle \frac{{\mathrm{Q}}_{\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r}\mathrm{g}\mathrm{e}}}{{\mathrm{Q}}_{\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r}\mathrm{g}\mathrm{e}}}}\right)\times 100$$

(7)

where $Q$ is the charge. To further investigate the electrochemical behaviour of the device, including its conductivity and charge transport properties at the electrode/electrolyte interface, EIS was performed on the MF/C//AC asymmetric device; see Figure 6g. The Nyquist plots display an ESR of 0.74 Ω and the absence of a semicircle in the high-frequency range, while the low-frequency region shows a curve diverging away from the imaginary axis (y-axis), indicating voltage leakage. The maximum power P_max of the device was estimated from Equation (8) [49] below to be 236 kW kg⁻¹:

$$P_{max}={\displaystyle \frac{\Delta V^2}{4m\left(ESR\right)}}$$

(8)

where ΔV, m and ESR are the cell potential (V) of 1.4 V, mass of 2.8 × 10⁻⁶ kg and ESR of 0.74 Ω determined from the Nyquist plot in Figure 6g, respectively. The EIS fitting results presented as an inset in Figure 6g were obtained using the ZFIT module in EC-Lab software (version 11.50), employing the Randomise + Simplex fitting algorithm. The fitting quality was evaluated using optimized minimization parameters, including the chi-squared value ($X^2$), its normalized form ($X/\sqrt{N}$), and the normalized chi-squared statistic ($X^2/{\left|Z\right|}^2$). As observed from the Nyquist plot in Figure 6g, there is no significant discrepancy between the experimental EIS data and the fitted equivalent circuit model. The equivalent circuit consists of a solution resistance (R_s) connected in series with a constant phase element (Q), which is arranged in parallel with the charge transfer resistance (R_CT) and the Warburg diffusion element (W). The Warburg element, which governs ion diffusion from high- to low-frequency regions, is connected in series with R_CT. In an ideal scenario, a purely capacitive and perfectly polarizable electrode would exhibit a vertical line parallel to the imaginary axis at low frequencies, corresponding to the mass capacitance (C). However, the Nyquist plot in Figure 6g shows a deviation from this ideal capacitive behaviour, which is attributed to the presence of a resistive component associated with the capacitance. This resistance is identified as the leakage resistance (R_L), connected in parallel with C. Figure 6h presents the Bode plot showing the impedance phase angle as a function of frequency for the MF/C//AC asymmetric device. The phase angle in the low-frequency region is approximately −50°, indicating Faradaic behaviour. Table 1 compares the electrochemical results obtained in this work with those found in previous studies that used Mn/Fe/carbon-based materials prepared via different synthesis techniques. Our device yields comparable results.

Figure 6. (a) CV curves of MF/C and AC electrodes recorded in the positive and negative potential windows, respectively; (b) CV curves of the MF/C//AC asymmetric device recorded at scan rates ranging from 5 to 1000 mV s⁻¹; (c) galvanostatic charge–discharge (GCD) curves of the MF/C//AC device measured at specific currents from 1 to 10 A g⁻¹; (d) specific capacity as a function of applied current density; (e) specific energy and specific power as functions of applied current density; (f) capacity retention and coulombic efficiency of the MF/C//AC device during cycling; (g) Nyquist plot of the MF/C//AC device (inset: enlarged high-frequency region) along with the fitted equivalent circuit; and (h) phase angle analysis of the MF/C//AC device.

Table 1. The electrochemical properties of supercapacitor devices prepared using different synthesis methods with Mn/Fe/spinel/carbon-based composite materials. SPani—semi-polycrystalline polyaniline; PANI—polyaniline; AC—activated carbon; and rGO—reduced graphene oxide.

Device	Method	Electrolyte	Cell Voltage (Volt)	Cell Capacitance (F g−1)	Specific Energy (Whkg−1)	Specific Power (Wkg−1)	Capacitance Retention (%)/Cycle Number	Ref.
SPani-AC//SPani-AC	Chemical polymerization	1 M Na2SO4	1.5	45 @ 5 Ag−1	14	-	96/4500	[19]
Graphene/PANI	Electrochemical co-deposition polymerization	1 M H2SO4	0.8	128 @ 0.5 Ag−1	11.38	199.8	85/10,000	[51]
Mo-doped NiFe2O4/NiF	Sol–gel auto-combustion	1 M KOH	0.5	268 @ 1 Ag−1	20.55	567	-	[52]
Mn-NiFe2O4//AC	Simple chemical etching and annealing treatment	1 M KOH	1.5	1.726 @ 1 mA cm−2	390 µWh cm−2	544 µW cm−2	95.2/5000	[53]
MnFe2O4@rGO//AC	Hydrothermal synthesis	1 M KOH	0.55	325 @ 5 Ag−1	11.8	540	75/10,000	[13]
MF/C//AC	In situ citrate sol–gel and calcination method	3 M KOH	1.4	42.4 mAhg−1 @ 1 Ag−1	16.35	1944.45	92/15,000	This work

4. Conclusions

In this research, MF/C was successfully synthesized using the in situ citrate sol–gel and calcination method. The structural and morphological analysis of the MF/C electrode revealed XRD peaks corresponding to MnFe₂O₄. The FTIR analysis of the MF/C electrode exhibited characteristic absorption bands. The SEM images of MnFe₂O₄ reveal some fractured nanospheres, indicating the presence of hollow structures. The electrochemical performance of the MF/C working electrode was evaluated in a three-electrode cell setup with a 3 M KOH electrolyte. Following this, an asymmetric device was successfully constructed using MnFe₂O₄ as the positive electrode and AC as the negative electrode. The MnFe₂O₄//AC asymmetric device that was fabricated demonstrated high performance at an applied potential difference of 1.4 V in 3 M KOH. The device delivered a specific capacity of 42.4 mAh g⁻¹, a specific energy of 16.35 Wh kg⁻¹, and a maximum specific power of 236 kW kg⁻¹. Furthermore, it retained 92% of its capacitance after 15,000 charge–discharge cycles, confirming excellent long-term stability. Moreover, this work demonstrates an effective and low-cost synthesis route for MF/C composites, offering high electrochemical performance in both three- and two-electrode configurations, making it a promising electrode material for next-generation high-performance supercapacitor applications.