Effective and High-Performance MgFe₂O₄/Mg-MOF Composite for Direct Methanol Fuel Cells

Abstract

The development of efficient and sustainable electrocatalysts for optimizing methanol oxidation reactions (MORs) in direct methanol fuel cells (DMFCs) is crucial for the innovation of clean electrode energy technologies. This study highlights the synthesis and characterization of magnesium ferrite (MgFe₂O₄) and magnesium-based metal–organic framework (Mg-MOF) composites, utilizing cost-effective and scalable methods such as co-precipitation and ultrasound-assisted synthesis. The composite material, prepared in a 1:1 ratio, demonstrated enhanced catalytic performance due to the synergistic integration of MgFe₂O₄ and Mg-MOF. Comprehensive structural and morphological analyses, including X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), the Brunauer–Emmett–Teller (BET) technique, and X-ray photoelectron spectroscopy (XPS), confirmed the successful formation of the composite. Also, the modification of magnetic properties, particularly the values of coercive force (Hc), led to a significant enhancement in electrical and catalytic performance. The material exhibited mesoporous characteristics and an improved surface area. Electrochemical evaluations revealed superior MOR activity for the composite electrode, achieving a current density of 31.5 mA∙cm⁻² at 1 M methanol with an onset potential of 0.34 V versus Ag/AgCl, measured at a scan rate of 100 mV/s. Remarkably, the composite electrode showed a 75% improvement in current density compared to its components. Additionally, the composite exhibited a low overpotential of 350 mV and favorable Tafel slopes of 22.54 and 4.27 mV∙dec⁻¹ at high and low potentials, respectively, confirming rapid methanol oxidation kinetics on this electrode. It also demonstrated excellent stability, retaining 97.4% of its current density after 1 h. Electrochemical impedance spectroscopy (EIS) further revealed a reduced charge transfer resistance of 9.26 Ω, indicating enhanced conductivity and catalytic efficiency. These findings underscore the potential of MgFe₂O₄/Mg-MOF composites as cost-effective and high-performance anode materials for DMFCs, paving the way for sustainable energy solutions.

1. Introduction

The risks of climate change are escalating daily due to pollutants and harmful emissions, particularly greenhouse gases that contribute to global warming. These emissions result from increasing energy demands associated with population growth and unsustainable fossil fuel consumption. Recent global climate conferences have emphasized the urgent need to adopt renewable energy solutions and advanced energy conversion technologies, including solar power, wind energy, hydroelectric power, and fuel cells, as sustainable alternatives to fossil fuels [1,2,3]. Among these technologies, liquid fuel cells stand out due to their high energy efficiency, minimal emissions, environmental friendliness, ease of operation, and portability [4,5]. Their low operating temperatures have made them a focus for researchers seeking to enhance overall efficiency. Unlike polymer electrolyte membrane fuel cells (PEMFCs), which require hydrogen as a fuel source, liquid fuel cells directly utilize methanol, eliminating the need for hydrogen conversion and making them particularly suitable for portable electronic devices [6,7].

Methanol is the most commonly used fuel in liquid fuel cells owing to its low molecular weight, cost efficiency, and widespread availability compared to alternatives such as ethanol, ethylene glycol, acids, or hydrazine [8]. The primary reaction in these fuel cells involves the electrochemical oxidation of alcohols. However, the slow kinetics of this reaction, coupled with the generation of intermediates and byproducts, significantly hampers overall efficiency, necessitating the use of catalysts to enhance reaction rates, especially at the anode [9,10]. A major challenge lies in the high cost and substantial loading requirements of traditional catalysts, particularly platinum and palladium, which are widely used as anodic materials [11,12]. Their expense, limited availability, and susceptibility to surface poisoning contribute to sluggish reaction kinetics. Furthermore, the high platinum loading required to break the strong carbon–oxygen bond in methanol further escalates costs. Additional issues, including fuel crossover, cathodic flooding, undesirable byproduct formation, fuel safety concerns, and long-term stability, are largely influenced by catalyst efficiency, driving the search for more abundant and sustainable catalytic alternatives [13,14].

Bimetallic catalysts often exhibit superior catalytic performance compared to monometallic counterparts due to synergistic and ligand–electronic interactions [15]. Ferrites, with the general formula MFe₂O₄, represent a unique class of mixed-metal oxides characterized by the incorporation of various cations within their structure. These materials are notable for their low cost, reduced toxicity, biocompatibility, high electrical conductivity, chemical stability, and remarkable electrochemical and magnetic properties [16,17]. Magnesium ferrite, a soft magnetic material with a cubic inverse spinel structure, displays n-type semiconducting behavior, making it particularly effective for electrochemical catalysis [18,19]. Beyond its magnetic properties, magnesium ferrite is widely used as a heterogeneous catalyst due to its favorable material characteristics. It can be synthesized through various methods, including sol–gel, co-precipitation, hydrothermal, solvothermal, microemulsion, and dip-coating techniques, with co-precipitation offering distinct advantages in simplicity and versatility, allowing for precise control over material properties [20,21,22,23,24,25,26,27,28].

Despite its success in applications such as capacitors, microwave devices, catalytic systems, and photovoltaic cells, magnesium ferrite’s conductivity remains a limiting factor, particularly in electronic applications, necessitating modifications to enhance efficiency [29,30,31]. Strategies to improve conductivity and overall performance include tailoring intrinsic properties and promoting electron hopping through Fe²⁺/Fe³⁺ redox interactions [30,31]. Researchers have explored doping and hybrid material designs, incorporating supports such as carbon-based materials (e.g., graphene, carbon nanotubes, and carbon fibers), metal oxides, nitrides, metal–organic frameworks (MOFs), and conductive polymers. These supports enhance conductivity while increasing surface area, improving dispersion, ensuring structural stability, and reducing costs. Magnesium-based MOFs further enhance electrical conductivity and mechanical/thermal properties. This study synthesized a magnesium ferrite/Mg-MOF composite to modify magnetic properties and improve methanol oxidation performance. Electrochemical techniques (chronoamperometry (CA), electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), and cyclic voltammetry (CV)) assessed the composite’s activity in alkaline conditions, while structural, microstructural, magnetic, and thermal analyses confirmed enhanced conductivity, surface area, adsorption sites, and stability. The composite demonstrated superior current density compared to individual components, highlighting its potential as a cost-effective, high-performance anode material for direct methanol fuel cells.

2. Results and Discussions

2.1. X-Ray Diffraction Analysis

The X-ray diffraction (XRD) patterns for MgFe₂O₄, Mg-MOF, and their composite are presented in Figure 1. Firstly, the peaks of MgFe₂O₄ (JCPDS card No. # 01-075-4395) were observed in Figure 1(S1), confirming the presence of a well-crystallized cubic structure belonging to the Fd-3m space group. The peaks appeared at 2θ = 30.2°, 35.5°, 43.2°, 53.6°, 57.2°, 62.8°, 71.2°, and 75.3° corresponding to the planes (220), (311), (400), (422), (511), (440), (620), and (622), respectively, exhibiting sharpness and intensity, indicating high crystallinity. Sharp, intense MgFe₂O₄ peaks (e.g., 35.5° for (311) confirm a crystalline cubic spinel structure (Fd-3m space group), which facilitates electron hopping between Fe sites (Figure 1(S1)). The absence of secondary peaks or intermediate phases within the indexing X-ray indicates the high purity of the composition. Secondly, the XRD pattern for Mg-MOF was confirmed by the appearance of peaks in Figure 1(S2) at angles of 6.8°, 11.8°, 18.5°, and 38.2°. The two major sharp peaks at approximately 6.76° and 11.76° were assigned to the (110), (300), and (510) diffraction peaks, which indicated that Mg-MOF-74 was successfully synthesized. It has shown good agreement with the previous report and confirms its defective, porous structure [32,33]. In addition, a higher angle with the broadening of the XRD patterns suggests that the Mg-MOF-74 exhibits a less crystalline lattice with defects and/or disordered structure [34]. The secondary peak related to Mg (OH)₂ indexed to 2θ = 58.7° (JCPDS card No. 00-001-1169) can provide additional information regarding the effect of the synthesis condition on the structure of the MOF. Moreover, a broad hump at 2θ ~ 26.5° and 54.6° indicates the presence of carbon that is predominantly amorphous, aiding electron transfer from MgFe₂O₄ to intermediates [35,36]. Also, (Figure 1(S2)) broadened peaks (e.g., 38.2°) indicate disorder, enhancing adsorption capacity.

Finally, in Figure 1(S3), the XRD pattern of the composite revealed distinctive peaks characteristic of both (MgFe₂O₄) and (Mg-MOF). Broadened and less intense peaks were shown due to a synergetic effect. Meanwhile, modification in the magnetic behavior of the composite led to the crystals not being random and a preference for the specific orientation. This validates the successful creation of the composite.

2.2. Fourier Transform Infrared Spectroscopy Analysis

The FTIR curves for MgFe₂O₄, Mg-MOF, and the composite are depicted in Figure 2. The spectra were measured at room temperature in the frequency range of 400–4000 cm⁻¹, as shown in Figure 2(S1). Firstly, for MgFe₂O₄, small peaks were observed at a higher frequency (ν1) at 565 cm⁻¹ and a lower frequency (ν2) at 430 cm⁻¹. These peaks were attributed to the stretching vibrations of the metal–oxygen bonds in the tetrahedral and octahedral interstitial sites within the crystal lattice of the ferrite [37]. The peaks at both low and high frequencies indicate the stretching vibrations occurring in the interstitial positions, highlighting the bond interactions between oxygen and metals. The highest value in tetrahedral sites may be attributed to the short bond length. The absorption bands ν1 and ν2 are found in the expected range, and the difference in band position is due to the difference in the Fe³⁺-O²⁻ for the octahedral and tetrahedral complexes [38]. Secondly, for Mg-MOF in Figure 2(S2), the peak corresponding to M-O was observed, similar to ν (M-O) and especially at the absorption value of 430 cm⁻¹, similar to those in MgFe₂O₄, indicating the presence of M-O bonds in both compounds. However, the M-O peak in the MOF appeared weaker compared to Mg ferrite, likely due to the different structural environments and coordination states of Mg in MO. Additionally, the band with small peaks at 1034 to 1146 cm⁻¹ of ν (C-O) and the peak at (1403) cm⁻¹ are associated with the carboxylate-based linker ν (-COO-), whereas the crests at 3124 and 3414 are correlated with ν (C-H) and ν (OH), respectively. This was attributed to the water that formed and became trapped within the framework of the MOF due to its exposure to dry air for a few minutes [39]. The O–H and C–H stretching bands gradually disappear through the creation of MOFs. These peaks are indicative of the functional groups present in the MOF structure, reflecting its complex organic framework. The absence of peaks in the range 2700–3000 cm⁻¹, associated with the presence of DMF, serves as evidence that the samples were thoroughly washed of any traces of the mother liquor from the reaction. Furthermore, Figure 2(S3) characterized the composites of MgFe₂O₄ and Mg-MOF with a ratio of 1-1. The appearance of weak peaks between 430 cm⁻¹ and 765 cm⁻¹ related to the M-O bond can be due to synergistic interaction. As the peak strength decreased, the Mg-MOF reached its highest level, and some peaks overlapped, indicating that MgFe₂O₄ and Mg-MOF functioned effectively together, as shown in Figure 2(S2,S3).

2.3. Magnetic Properties

The magnetization curves illustrated in Figure 3 for the three compounds MgFe₂O₄, Mg-MOF, and their composite highlight the differences in their magnetic properties, influenced by factors such as spin canting, anisotropic magnetism, interstitial sites, and coordination bonds within the MOF structure and ferrite [40,41].

Table 1. Saturation magnetization (Ms), coercivity (Hc), retentivity (Mr) and width of (S1) MgFe₂O₄, (S2) Mg MOF, and (S3) composite with ratio of 1:1.

Materials	Ms (emu/g)	Mr (emu/g)	Hc (G)	(Mr/Ms)
Mg Ferrite	24.7	1.81	23.15	0.07
Mg Mof	0.45	0.189	1695.3	0.42
Composite (1:1)	15.07	2.72	114.22	0.17

summarizes the values related to the magnetic properties of saturation magnetization (Ms), magnetic remanence (Mr), coercivity force Hc, the and Ms/Mr ratio of the synthesized materials MgFe₂O₄, Mg-MOF, and their composite ratios (1:1).

Magnesium ferrite exhibits the highest saturation magnetization (Ms) at 24.7 emu/g, indicating a strong magnetic response under an external field. This pronounced magnetization can be attributed to the ordered arrangement of spins and the absence of significant spin canting, which facilitates the effective alignment of magnetic moments [42]. In contrast, Mg-MOF shows a significantly lower Ms of 0.45 emu/g, reflecting its weaker magnetic properties. This reduction in magnetization can be explained by its anisotropic magnetic behavior and the presence of coordination bonds that may disrupt the alignment of spins [43]. The composite, with a Ms of 15.07 emu/g, demonstrates an intermediate magnetic response, suggesting a synergistic interaction between the two components [44]. Here, the magnetic properties of magnesium ferrite enhance those of the MOF, resulting in improved overall magnetization.

The remanence (Mr) values further elucidate these differences, with the composite achieving the highest Mr at 2.72 emu/g, compared to 1.81 emu/g for MgFe₂O₄ and 0.18 emu/g for Mg-MOF. The higher remanence observed in the composite indicates a superior retention of magnetization, which is advantageous for enhancing electrocatalytic performance. This can be linked to the interplay between the magnetic phases and the structural characteristics of the MOF and Mg ferrite, including the presence of interstitial sites that facilitate magnetic interactions and contribute to the composite’s overall magnetic stability [45].

In terms of coercivity (Hc), Mg-MOF has the highest value at 1695.3 G, indicating a strong resistance to demagnetization. However, this elevated coercivity may impede the dynamic magnetic behavior essential for catalytic processes, as it restricts the material’s responsiveness to changing magnetic fields. The composite, with a Hc of 114.2 G, strikes a balance between stability and responsiveness, making it particularly suitable for effective electrocatalysis. Conversely, magnesium ferrite, with the lowest Hc at 23.15 G, is more easily demagnetized, which could limit its catalytic efficiency.

Finally, the remanence ratio (Mr/Ms) is highest for magnesium MOF at 0.42, followed by the composite at 0.17 and magnesium ferrite at 0.07. A higher remanence ratio indicates a greater ability to retain magnetization after the removal of an external magnetic field, which is beneficial for maintaining catalytic activity [46]. This characteristic can be attributed to the unique structural features of the MOF, including coordination bonds and the arrangement of interstitial sites, which enhance its magnetic properties and potential for improved electrocatalytic performance. Overall, the interplay of these factors across the three materials underscores the complex relationship between structural characteristics and magnetic behavior, highlighting the composite’s promising attributes for future applications.

2.4. Morphological Characterization

Figure 4 depicts the FESEM images of the synthesized materials. Figure 4a presents the FESEM images of MgFe₂O₄ at two distinct magnification levels. The morphological composition of MgFe₂O₄ consists of grooves and overlapping rocky granules with the particles hunching and sticking together. The SEM images for Mg-MOF in Figure 4b illustrate the adhesion sheets and rods of the MOF arranged in a multilayered stacked configuration. Figure 4c demonstrates the successful integration of Mg-MOF with MgFe₂O₄, showcasing the two-dimensional stacking morphology of MgFe₂O₄ deposited onto the Mg-MOF substrate. The SEM micrographs reveal elongated, twisted platelet-like structures, which suggest a high surface area and a morphology conducive to efficient electrocatalytic processes. The physical merging of Mg ferrite does not change the framework of the Mg-MOF crystals. The crystal size of Mg-MOF is much bigger than that of MgFe₂O₄. Additionally, after one hour in 1 MeOH and 0.5 M NaOH at 100 mV/s, the morphological configuration of the 1:1 ratio composite remains unchanged from when it was used to oxidize methanol, confirming the material’s durability in alkaline conditions (Figure 4d).

2.5. BET Surface Area Measurements

The BET surface area and textural properties of the synthesized materials investigated were analyzed and tabulated using N₂ adsorption–desorption isotherms as shown in Figure 5 and

Table 2. Specific surface area, total pore volume, and pore width of MgFe₂O₄, Mg-MOF, and their composite (1:1).

Materials	BET m2∙g−1	Total Pore Volume cc.g−1	Pore Width nm
Mg Ferrite	8.684	10.05	2.3
Mg-Mof	89.318	10.44	4.69
Composite (1:1)	18.887	10.11	3.98

. The magnesium ferrite (S1) and composite (S3) exhibited typical Type I/II adsorption–desorption isotherms in low pressure, respectively, as defined by IUPAC [47]. Also, the magnesium MOF (S2) and composite at high pressure showed type IV isotherms with a high N₂ adsorption at p/po values and slightly sharp adsorption at higher p/po values (0.8–1.0), indicating the coexistence of micro-, meso-, and/or macropores in the Mg-MOF samples [48]. This multi-scale porosity enhances methanol and OH⁻ adsorption. The specific surface area, total pore volumes, and average pore diameter of the synthesized materials were computed using the Brunauer–Emmett–Teller (BET) model (Micromeritics, Norcross, GA, USA). The values of the surface area of the MgFe₂O₄, Mg-MOF, and their composite were related to be 8.684, 89.318, and 18.887 m²∙g⁻¹, respectively. The MgFe₂O₄ layers were merged with the Mg MOF. An optimized surface area for the composite with a 1:1 ratio can likely result in a superior electrode–electrolyte interface, enhancing charge or ion gathering. Based on the Barrett–Joyner–Halenda (BJH) model, the pore volume of the MgFe₂O₄, Mg-MOF, and composite was measured to be 1.30, 4.69, and 2.98, respectively. The materials exhibit average pore sizes of 2.30, 4.69, and 3.98 nm, respectively, characteristic of mesoporous materials [49]. Higher porosity and volume increase the surface area (S_BET), improving active site accessibility and reducing diffusion resistance.

All the data mentioned above reveal the structural modification, including the hierarchically porous framework, wide pore size distribution, and enlarged surface area. Consequently, they provide efficient routes for electron transfer and ion transport, making them strong candidates as electrocatalytic electrode materials for methanol oxidation in DMFCs [50].

2.6. XPS Spectra Investigation

The X-ray Photoelectron Spectroscopy (XPS) analysis of the composite material (1:1 ratio of MgFe₂O₄ and Mg-MOF) before and after methanol oxidation, as shown in Figure 6 and Figure 7, reveals significant insights into the chemical states of the elements involved. The survey spectra indicate that the peaks for Fe, Mg, C, and O were indexed and visible before and after methanol use. The XPS analysis of Fe 2p (Figure 6a,b) reveals two distinct peaks at 713.6 eV (Fe³⁺) and 727.12 eV (Fe²⁺) before methanol oxidation, with satellite features confirming the coexistence of both oxidation states. Post-oxidation, minimal shifts to 713.5 eV (Fe³⁺) and 727.3 eV (Fe²⁺) indicate retention of the Fe³⁺/Fe²⁺ redox pairs, demonstrating their stability during catalysis. These redox-active sites play a critical role in methanol oxidation:

Fe³⁺ acts as the primary oxidizing agent, converting methanol to intermediates (e.g., CH₃O⁻).

Fe²⁺ regenerates Fe³⁺ via electron donation, sustaining the catalytic cycle through reversible redox transitions: (Fe³⁺ + e⁻ ↔ Fe²⁺).

The stability of Fe³⁺ in the form of FeOOH (evidenced by binding energies) further supports its role in facilitating electron transfer and intermediate formation [51].

The Mg 1s XPS spectra (Figure 6c,f) show a pre-oxidation peak at 1307.6 eV (MgO) with satellite features (1308.1 eV, 1306.9 eV), confirming Mg²⁺ in MgFe₂O₄. Post-oxidation, a shift to 1306.2 eV suggests the formation of Mg-OH or Mg-O-CH₃ species, driven by interactions between Mg-MOF and intermediates (e.g., CH₃O⁻) in the alkaline medium (0.5 M NaOH). This shift reflects hydroxylation (MgO → Mg(OH)₂ [52]) and coordination of intermediates with oxygenated groups on Mg-MOF’s porous framework, which stabilizes reactive species and creates new active sites via electron redistribution between Mg²⁺ and Fe³⁺. In contrast, iron’s minimal interaction with methanol due to stable alkoxy intermediates on its surface highlights the distinct roles of Mg (intermediate stabilization) and Fe (redox cycling) in the composite [53,54]. These synergistic interactions enhance methanol adsorption, oxidation efficiency, and structural stability, underpinning the composite’s superior catalytic performance. The oxygen (O 1s) and carbon (C 1s) peaks, as shown in Figure 7a,c, and Figure 7b,d, respectively, remained consistent before and after oxidation. The O 1s peaks at 533.6 eV, 534.7 eV, and 531.7 eV suggest the presence of different oxygen species such as hydroxyl groups and oxides, while the C 1s peaks at 287 eV indicate the presence of carbonyl or carboxyl groups, reflecting the composite’s structure [55]. Cross-checking with Fe 2p or O 1s shifts could confirm the synergistic interaction between ferrite and MOF. These results confirm the composite’s stability and effectiveness as an anodic catalyst in methanol fuel cells, as evidenced by the unchanged peak positions and the specific binding energies corresponding to the chemical states of the elements involved.

2.7. The Electrochemical Reactions of Prepared Electrodes

In this study, three anodic electrodes, MgFe₂O₄, Mg-MOF, and a 1:1 composite of both, were evaluated for their electrochemical performance in a 0.5 M NaOH electrolyte at a potential window of (0–0.6) V and various scan rates. Figure 8 illustrates that the current density for all electrodes was influenced by the scan rate, with an increase in scan rate leading to a corresponding increase in current density, indicating that the process is kinetically controlled. Specifically, Figure 8a shows that the current density for magnesium ferrite reached 13 mA.cm⁻² at a scan rate of 100 mV.s⁻¹. For the Mg-MOF, as depicted in Figure 8b, the current density was 2.5 mA.cm⁻². Finally, Figure 8c demonstrates that the composite electrode achieved a current density of 15.5 mA.cm⁻², reflecting its superior electrochemical activity. These results highlight the enhanced catalytic properties of the composite electrode, suggesting its suitability as a strong candidate for methanol oxidation in fuel cells.

2.8. Electrochemical Oxidation Reactions of Methanol (MOR)

Figure 9 demonstrates the electrochemical performance of the synthesized materials (MgFe₂O₄, Mg-MOF, and the composite with a 1:1 ratio) in the methanol oxidation reaction. The electrochemical oxidation of methanol on the surface of mg ferrite was conducted at different methanol concentrations as shown in Figure 9a. The increase in the concentration of methanol from 0.5 M to 1 M was followed by a rise in the current density up to 18 mA cm⁻². This highest value is related to the completely covered active site by methanol molecules. After increasing the methanol concentration above 1 M, the sensitivity of the electrode toward oxidation is lowered due to blocking the active side by methanol and oxidation products [56]. Meanwhile, Figure 9b depicts the variation of current density with concentrations of methanol from 0.5 M to 3 M. The elevated current density was monitored at 6.9 mA cm⁻² at 1 M methanol on the Mg-MOF electrode and slightly decreased from 1 M to 3 M. That is attributed to the shortage of active sites and the full coverage which hindered the diffusion of methanol molecules [57,58]. Lastly, Figure 9c demonstrates that using the composite as the electrode resulted in consistent behavior, even with the addition of methanol at various concentrations to the 0.5 M NaOH electrolyte. Increasing the efficiency of the oxidation process resulted in a current density of 31.5 mA cm⁻² at 1 M methanol. This inversed the available active site for methanol adsorption [59].

As shown in Figure 9d, the CV profiles of MgFe₂O₄, Mg-MOF, and the 1:1 composite were tested in 0.5 M NaOH with 1 M methanol. The onset potentials for methanol oxidation were found to be 0.4 V, 0.36 V, and 0.34 V vs. Ag/AgCl, respectively. The composite (1:1) displayed the lowest onset potential, meaning it needs minimal energy to initiate the redox reaction. This highlights the 1:1 ratio as an efficient electrocatalyst, particularly when compared to the onset potentials reported in prior research [60,61] (see Table 3).

Methanol oxidation in an alkaline medium (0.5 M NaOH) typically involves proton abstraction and electron transfer. Based on XPS and FTIR, the following mechanism is proposed in three steps:

Initial adsorption of methanol

$${\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}+{\mathrm{M}\mathrm{g}\left(\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}\right)}^{+2}+\mathrm{O}{\mathrm{H}}^{-}\to {\mathrm{M}\mathrm{g}}^{+2}-\mathrm{O}-{\mathrm{C}\mathrm{H}}_3+{\mathrm{H}}_2\mathrm{O}$$

(1)

(Methanol adsorbs onto the catalyst surface (likely Mg or Fe sites in MgFe₂O₄), with Mg-MOF contributing high porosity to enhance adsorption.)

Dehydrogenation and electron transfer

$${\mathrm{M}\mathrm{g}}^{+2}-\mathrm{O}-{\mathrm{C}\mathrm{H}}_3\stackrel{{\mathrm{F}\mathrm{e}}^{+3}/{\mathrm{F}\mathrm{e}}^{+2}}{\to }{\mathrm{M}\mathrm{g}}^{+2}-\mathrm{O}\mathrm{H}+\mathrm{H}\mathrm{C}\mathrm{H}\mathrm{O}+2{\mathrm{e}}^{-}$$

(2)

(MgFe₂O₄ acts as an active site for electron transfer, leveraging its magnetic and electronic properties, while Mg-MOF improves dispersion and prevents aggregation.)

Final oxidation:

$$\mathrm{H}\mathrm{C}\mathrm{H}\mathrm{O}+\mathrm{O}{\mathrm{H}}^{-}\stackrel{\mathrm{M}\mathrm{g}\text{-}\mathrm{M}\mathrm{O}\mathrm{F} \mathrm{s}\mathrm{i}\mathrm{t}\mathrm{e}\mathrm{s}}{\to }{\mathrm{H}\mathrm{C}\mathrm{O}\mathrm{O}}^{-}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}$$

(3)

The final product (formate) depends on the applied potential (0 to 0.6 V) and the concentrations of methanol (1 M) and NaOH (0.5 M), and MOF’s high porosity stabilizes intermediates.

2.9. The Electrochemical Reaction and Stability

Figure 10 shows the results of utilizing techniques like LSV, Tafel, EIS, and CA. The synthesized electrodes were evaluated for their electrochemical performance in a methanol oxidation reaction (MOR). The composite (1:1) exhibited the highest efficiency, making it the most promising electrode material. Figure 10a depicts the LSV curves of the tested electrodes in (0.5 M) NaOH, and 1 M methanol, whereas the overpotentials (V-V_o) were calculated. The overpotentials for Mg Fe₂O₄, Mg-MOF, and the composite at 5 mA cm⁻² were 450, 400, and 350 mV, respectively. The composite exhibited a lower overpotential than the other electrocatalyst electrodes for oxidation reactions, suggesting the suitability and high efficiency for MOR.

The Tafel slope is one of the most important factors used to evaluate the kinetics of the electrocatalyst in the electrooxidation reactions. The lower value of the Tafel slope indicates a higher catalytic activity. Figure 10b shows the Tafel slopes of the electrocatalyst for the synthesized electrodes in (0.5 M) NaOH, 1 M methanol, involving two regions at high and low potentials. The slopes at a high potential and low potential reflect the changes in the reaction mechanism or the rate-determining step, whereas the slope of the straight line at a high potential refers to oxidative removal of CO-like species [62,63], and at a low potential it is related to the step of methanol adsorption then a dehydrogenation reaction. At low potentials, the Tafel slopes for Mg Fe₂O₄, Mg-MOF, and the composite were 4.83, 6.11, and 4.27 mV dec⁻¹, respectively. That means the step of dehydrogenation of methanol on the composite electrode is faster than ferrite and Mg-MOF. Similarly, the slopes at a high potential were calculated to be 27.5, 33.65, and 22.54 mV dec⁻¹ for Mg Fe₂O₄, Mg-MOF, and the composite, respectively. Thus, the composite has a higher performance for the removal of poisonous species than ferrite and Mg MOF.

Table 3. Comparison of catalyst preparation techniques, current densities, onset potentials, and references to prior research.

Materials	Synthesis Method	Scan Rate (mV/s)	Current Density (mA∙cm−2)	Onset Potential (V)	Reference
Ni-MOF-74/CPE	Hydrothermal	25	13.46	0.70	[64]
MnCo2O4/NiCo2O4/rGO	Hummers Method	20	24.76	0.58	[65]
MOF-74(Ni)/NiOOH	Self-Sacrificing Template	50	27.62	-	[66]
Co-MOF-71@GO	Hydrothermal	50	29.1	0.60	[67]
Cu@AC(1:1)	Carbonized at 700 °C	50	2.11	0.9	[68]
MWCNTs@Ni3S2/CuxS	Solvothermal	50	22.3	-	[69]
Mg-MOF	Ultrasound-Assisted	100	6.9	0.360	This Work
Mg Fe2O4	Co-Precipitation	100	18	0.41	This Work
Composite (1:1)	Physical Mixing	100	31.5	0.340	This Work

Table 3. Comparison of catalyst preparation techniques, current densities, onset potentials, and references to prior research.

Materials	Synthesis Method	Scan Rate (mV/s)	Current Density (mA∙cm−2)	Onset Potential (V)	Reference
Ni-MOF-74/CPE	Hydrothermal	25	13.46	0.70	[64]
MnCo2O4/NiCo2O4/rGO	Hummers Method	20	24.76	0.58	[65]
MOF-74(Ni)/NiOOH	Self-Sacrificing Template	50	27.62	-	[66]
Co-MOF-71@GO	Hydrothermal	50	29.1	0.60	[67]
Cu@AC(1:1)	Carbonized at 700 °C	50	2.11	0.9	[68]
MWCNTs@Ni3S2/CuxS	Solvothermal	50	22.3	-	[69]
Mg-MOF	Ultrasound-Assisted	100	6.9	0.360	This Work
Mg Fe2O4	Co-Precipitation	100	18	0.41	This Work
Composite (1:1)	Physical Mixing	100	31.5	0.340	This Work

The stability of MgFe₂O₄, Mg-MOF, and composite electrodes towards MOR were estimated by chronoamperometric techniques. As shown in Figure 10c, the composite has excellent stability toward MOR, achieving current density retention rates reaching up to 97.4% after 1 h which confirms Mg-MOF’s role in preventing intermediate poisoning (e.g., CO accumulation).

The EIS technique was applied within a frequency range of 0.01 to 100,000 Hz, using a 10 mV AC signal, a 1 s stabilization time, a potential of 0.6 V vs. Ag/AgCl, and a solution of 0.5 M NaOH with 1 M methanol. This setup was used to analyze the electrochemical behavior of MgFe₂O₄, Mg-MOF, and composite electrodes. In Figure 10d, the Nyquist plots have exhibited variation in quasi-semicircles at a high-frequency region which reflects the difference in resistance of each electrode. A small value of the charge transfer resistance (Rct) at the electrode–electrolyte interface is associated with the high conductivity of the electrode. As shown in Figure 10d, the semicircle appears with a value of Rct 39.4 Ω, 16.4 Ω, and 9.26 Ω related to Mg Fe₂O₄, Mg-MOF, and composite electrodes, respectively. The obtained data emphasized that the composite electrode has a lower impedance than MgFe₂O₄ and Mg MoF. Consequently, the composite electrode (1:1 ratio) achieves higher efficiency in the methanol oxidation reaction (MOR) compared to MgFe₂O₄ and Mg-MOF individually. The synergistic effect of incorporating carbon, metal, and metal oxides plays a key role in enhancing the electrode’s performance for methanol oxidation.

3. Experimental Section

3.1. Material

Magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O), iron nitrate hexahydrate (Fe (NO₃)₂·6H₂O), citric acid (HOC (CO₂H) (CH₂CO₂H)₂), 1,4-benzene dicarboxylic acid (C₈H₆O₄) (BDC), isopropanol, and sodium hydroxide (NaOH) were acquired from LABAL Chemie (Mumbai, India). Also, in this study, the following sources were used: all chemicals were delivered from Merck company (Boston, MA, USA) including N,N-dimethyl formamide (DMF), 5 wt% Nafion solution, graphite sheet (thickness: 1 mm, area: 1 × 1 cm²), methanol (Me-OH), ethanol, and hydrogen peroxide (H₂O₂, 30%). Also, all chemicals and solvents were utilized with no requirement for additional purification of any of the preparation steps.

3.2. Synthesis of Magnesium Ferrite (MgFe₂O₄)

Generally, the synthesis of magnesium ferrite (MgFe₂O₄) was prepared through the co-precipitation method. Briefly, in a round-bottom flask solution of Mg (NO₃)₂·6H₂O (1 g, 4.1 mmol), Fe (NO₃)₃·6H₂O (2 g, 5.6 mmol) in bi-distilled water (50 mL) was dissolved under vigorous stirring for 30 min to ensure complete dissolution. Also, citric acid (1.84 g, 9.6 mmol) was added to the solution due to its dual role as a capping agent to form complexes with metal ions and as a consumption agent. The pH value was modified by adding NaOH (1 M) drop by drop under constant stirring to reach 12, and the reaction temperature was maintained on a hot plate at 160 °C for 2 h. This step facilitates the formation of a homogeneous precipitate. The mixture was then supplemented with a few drops of 30% H₂O₂ to promote an oxidation reaction until an intermediate change into dark brownish precipitation. Hydrogen peroxide acts as an oxidizing agent, ensuring the complete oxidation of Fe⁺² to Fe⁺³. This step is crucial for achieving the desired stoichiometry and phase purity of the ferrite. The dark brownish precipitate was washed with bi-distilled water multiple times to remove any impurities and unreacted precursors. The filtrate was put into a dryer overnight at 100 °C. This step removes any residual moisture from the sample. The dried powder was placed in a crucible and annealed in a furnace at 850 °C for 4 h. Annealing enhances the crystallinity and phase purity of the magnesium ferrite.

3.3. Synthesis of Mg-MOF

The synthesis of magnesium MOF was conducted using the ultrasound-assisted technique. As follows, 0.35 g (2.11 mmol) of 1,4-benzene dicarboxylic acid (BDC) (C₈H₆O₄) was dissolved in 15 mL DMF using a magnetic stirrer until the ligand had completely dissolved. Metal salt solutions were prepared by complete dissolution of 1.12 g (4.37 mmol) Mg (NO₃)₂∙6H₂O of magnesium nitrate hexahydrate in a 15 mL mixture of DMF, water, and ethanol solution (15:1:1, v/v). The 1,4-benzene dicarboxylic acid (BDC) solution was then added dropwise to the magnesium nitrate solution, under stirring, to avoid rapid precipitation. Following thorough mixing, the liquid was put into an ultrasonic cleaner for 15 min at (40 KHz, 120 W) at room temperature. A few drops of NaOH 1 M were added to adjust the pH to 9. The product solution was split into 5 filter tubes (20 mL each), capped, and transferred to the centrifuge under the spine at (5000 rpm) for 90 min with intervals of 15 min each. After centrifugation, the supernatant was decanted and then centrifugation was used to gather the crystals, and they were then regularly cleaned with a total of 6–8 washes with ethanol and distilled water. Finally, the product was dried in a watch glass at 60 °C for 24 h followed by sintering the dried powder at 80 °C for 12 h in a vacuum oven. The yield of the reaction was based on the starting mass of the organic linker, the 1,4-benzene dicarboxylic acid (BDC).

3.4. Synthesis of Magnesium Ferrite (MgFe₂O₄) and Mg-MOF Composite

A gram of magnesium ferrite was physically mixed with 1 g of magnesium MOF to form a composite in a 1:1 weight ratio. The mixed powders were then dissolved in a solvent mixture of 5 mL DMF and 2.5 mL ethanol. The solution was stirred until the powders were well dispersed. The mixture was placed in an ultrasonic cleaner and sonicated for 15 min at 40 kHz and 120 W at room temperature to ensure thorough mixing. After sonication, the mixture was dried at 60 °C for 24 h to remove any residual solvents and moisture. Finally, the dried composite powder was sintered at 300 °C for 12 h to enhance its crystallinity and phase purity.

3.5. Characterization Techniques

X-ray diffraction measurement was performed using an X-ray diffractometer (PANalytical, Empyrean, Cambridge, MA, USA) equipped with Cu Kα radiation of wavelength 1.54045 Å, operating at a voltage of 40 kV and a current of 30 mA. To confirm the existence of the intended functional groups, Fourier transform infrared (FTIR) spectra of the samples were captured using a Vertex 70 (Bruker, Bremen, Germany) device in the 400–4000 cm⁻¹ wave number range. The materials’ surface characteristics were depicted utilizing scanning electron microscopy (SEM, ZEISS (Oberkochen, Germany) and Sigma 500 VD, Aizu, Japan). The textural characteristics of the samples, including surface areas and pore size distributions, were evaluated by exploiting a Tri-Star 3020 analyzer (Micromeritics, Norcross, GA, USA) through nitrogen (N₂) adsorption–desorption isotherms. The magnetic behavior of materials under various conditions was measured using a vibrating sample magnetometer (VSM, LakeShore VSM 7400, Carson, CA, USA).

3.6. Fabrication of the Working Electrode

A homogeneous suspension was prepared by mixing 2 mg of the materials under study, each separately, with 380 μL of an isopropanol–water solution in a ratio of 1:2. Subsequently, 10 μL of a 5 wt% Nafion solution was added as a binding agent. The mixture was then sonicated for 30 min. Finally, approximately 15 μL of the slurry was drop-coated onto the graphite sheet (thickness: 1 mm; area: 1 × 1 cm²) and allowed to dry at 60 °C.

3.7. Electrochemical Measurements

The electrochemical tests proceeded utilizing a potentiate/galvanostatic (AUTOLAB PGSTAT 302N, Metrohm, Herisau, Switzerland) and NOVA 1.11 software, employing various techniques, including (CV) cyclic voltammetry, (LSV) linear sweep voltammetry, (CA) chronoamperometry, and electrochemical impedance spectroscopy (EIS). The experimental structure included a three-electrode cell consisting of a reference electrode Ag/AgCl/KCl (saturated) (HANA Company, Milano, Italy) and an auxiliary electrode based on a platinum wire of area 1 cm². The working electrode under study is made up of a graphite sheet (G) loaded with three different electrocatalysts: G/MgFe₂O₄, G/Mg-MOF, and G/composite with an active surface area of 1 cm². The efficacy of the fabricated electrodes and their electrocatalytic behavior were evaluated both in the presence and absence of methanol at varying concentrations, using an alkaline electrolyte solution (0.5 M NaOH).

Cyclic voltammetry experiments of the modified electrodes were conducted over a potential range of 0.0 to 0.6 V, with scan rates ranging from 10 to 100 mV/s. The linear sweep voltammetry (LSV) test was conducted on the catalyst at low scan rates to investigate the electron transfer kinetics in the MOR process. Chronoamperometry (CA) was performed at a constant voltage of 0.6 V for 3600 s. Electrochemical impedance spectroscopy (EIS) was conducted to monitor changes in the resistance and conductivity of the electrodes, covering a frequency range from 100 kHz to 0.01 Hz. Measurements were taken at 0.6 V with an amplitude of 10 mV.

4. Conclusions

In this study, low-temperature methods were employed to synthesize magnesium ferrite (MgFe₂O₄) via the co-precipitation method and magnesium MOF using ultrasound-assisted method techniques. A 1:1 physical mixture of these materials was created to form a composite aimed at enhancing the properties of magnesium ferrite for use as an electrocatalytic anode in methanol fuel cells. The magnetic properties, particularly the coercivity (Hc), were also modified due to the synergistic effects within the composite, leading to improved activity of the surface. High coercivity (Hc) reflects increased resistance to magnetic reorientation, indicating higher nanoparticle stability under electrochemical stress. The intermediate saturation magnetization (Ms) balances the strong magnetism of ferrite (due to Fe³⁺) and weak magnetism of MOF, enhancing particle dispersion and reducing aggregation. This synergistic interaction between MgFe₂O₄ and Mg-MOF enhances current density, improves surface properties, and optimizes electrocatalytic efficiency. MgFe₂O₄ facilitates efficient electron transfer via its Fe³⁺/Fe²⁺ redox-active sites, while Mg-MOF stabilizes reactive intermediates during the catalytic process.

Surface area analysis indicated that the pores were in the mesoporous scale (2.30–4.69) nm, which was beneficial for enhancing catalytic efficiency by providing more active sites and facilitating better mass transport. The lowest values of the overpotential 350 mV and Tafel slopes (22.45, 4.27) mV dec⁻¹ for high and low overpotentials confirmed the efficiency of the composite in the methanol oxidation process. Additionally, electrochemical impedance spectroscopy (EIS) results highlighted significant improvements in the electrical properties of the composite compared to pure magnesium ferrite and Mg-MOF, evidenced by reduced charge transfer resistance to 9.26 Ω. The chronoamperometric technique demonstrated stability up to 97.4% for the composite, and the current density optimized at about 75%. These findings collectively underscore the potential of the MgFe₂O₄/MOF composite as a highly efficient anode catalyst for methanol fuel cells, offering enhanced catalytic and electrical properties.