Synthesis of Co₃Fe₇/CoFe₂O₄ incorporated porous carbon catalysts via molten salt method: applications in the oxygen reduction reaction and 4-nitrophenol reduction

Abstract

Developing high-performance, multifunctional non-precious metal catalysts is essential for enhancing the efficiency of future energy utilization. In this study, four types of magnetic, recyclable Co₃Fe₇/CoFe₂O₄ incorporated porous carbon composite catalysts were synthesized using citric acid as the carbon source and ammonium chloride (NH₄Cl) as the salt medium. Iron and cobalt salts, in four different proportions, were uniformly incorporated using freeze-drying technology and subsequently processed through in situ calcination. Among the synthesized catalysts, Co₃Fe₇/CoFe₂O₄@NC-1, demonstrated outstanding catalytic reduction performance, with a reaction rate constant (k) of 0.031 min⁻¹, along with excellent cycle stability for 4-NP. The resulting Co₃Fe₇/CoFe₂O₄@NC-3 catalyst exhibited good ORR activity in an alkaline medium (E_onset = 0.99 V, E_1/2 = 0.83 V, J_L = −5.2 mA cm⁻²), along with long-term durability and resistance to methanol poisoning. These hybrid materials hold promise as non-precious metal electrocatalysts for fuel cell ORRs and introduce a new class of catalytic candidates for 4-NP reduction.

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1. Introduction

Designing eco-friendly catalyst materials is a key strategy for achieving green energy conversion and plays a crucial role in addressing global environmental and energy challenges.^1–3 The partially filled d-electron orbitals of precious metals make their surfaces highly receptive to adsorbing reactants, facilitating the formation of reaction intermediates. Noble metal catalysts exhibit outstanding catalytic activity and possess unique properties, including high-temperature tolerance, oxidation resistance, and corrosion resistance.⁴ However, their widespread application is hindered by high costs, rapid deactivation, limited reusability, resource scarcity, and other drawbacks.^5,6 Transition metals are abundant and cost-effective, making them ideal candidates for developing efficient catalysts. Over the past few decades, metal-based nanomaterials, including metal oxides and alloys, have gained significant attention due to their crucial roles in physical and chemical interactions.^7–9 Transition metal catalysts demonstrate excellent thermal stability, strong resistance to toxins, extended lifespan, and enhanced selectivity in oxidation reactions. Similarly, the presence of oxygen vacancies increases the number of catalytic activation sites, further improving their performance.

Recently, there has been increasing interest in developing alternative catalysts that do not rely on precious metals. These include nanosized transition metals,¹⁰ transition metal oxides,¹¹ nitrides,¹² chalcogenides,¹³ and carbides.¹⁴ Magnetic metal-based compounds, such as Co, Fe, and Ni,^15,16 have garnered significant attention due to their potential to overcome challenges related to catalyst recovery and reuse. As a result, various transition metal catalysts and their oxides, including Ni@NiO, CuO, NiO/NiS, CoO, Co₃O₄/CoFe₂O₄, and CuFe₂O₄/Ag, have been developed and utilized for the catalytic reduction of 4-nitrophenol (4-NP).^17–19 However, excessive clustering of magnetic metal nanoparticles (NPs) during synthesis adversely affects their performance in practical applications. To prevent catalyst aggregation, these NPs are typically coupled with a carrier. Carbon-based carriers are particularly effective due to their ability to facilitate rapid electron transport, enhancing electron transfer between individual metal particles.^20–24 Common materials used for carbon carriers include polymers, graphene oxide (GO), and graphene. For example, Kong et al.²⁰ developed a thermal reduction synthesis technique to fabricate CoFe nanoparticles encapsulated within reduced graphene oxide (RGO) sheets via hydrogenation heat treatment. The resulting CoFe/RGO nanocomposite exhibited excellent catalytic efficiency for the reduction of 4-NP to 4-aminophenol (4-AP). Sun et al.²³ employed an electrospinning-assisted technique to produce one-dimensional carbon fibers containing MIL-53(Fe) as a precursor for fabricating carbon nanofiber (CNF)-based catalysts with enclosed Fe₃C or CoFe₂O₄ NPs. The aggregation and corrosion of Fe₃C/CoFe₂O₄ are minimized when encapsulated in carbon fibers during the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Bai et al.²⁵ synthesized CoFe NPs by pyrolyzing metal and nitrogen/carbon-containing precursors, followed by their confinement within nitrogen-rich C₃N₄ nanosheets. ZIF-8 crystals were grown in situ on the surface of C₃N₄. This structure significantly enhances the graphitization of the carbon carrier during high-temperature treatment at 900 °C. Moreover, these NPs are specifically designed to prevent the agglomeration of alloy particles effectively. These studies highlight the diverse interactions between magnetic metal NPs and carbon-based auxiliary materials. The carbon component, particularly RGO, plays a crucial role in preventing particle aggregation and providing nucleation sites during synthesis.²⁶ However, the high cost of many carbon sources poses a significant barrier to their widespread industrial application. Therefore, identifying an affordable carbon source and developing a simple, efficient synthesis method for carbon-based nanocatalysts is essential.

In this study, four types of magnetic, recyclable Co₃Fe₇/CoFe₂O₄ incorporated porous carbon composite catalysts were synthesized using citric acid as a carbon source to inhibit the hydrolysis of metal salts and ammonium chloride as a salt medium to generate gas during high-temperature pyrolysis. Iron and cobalt salts, in four different proportions, were uniformly incorporated via freeze-drying and subsequently processed through in situ calcination. Among the synthesized catalysts, Co₃Fe₇/CoFe₂O₄@NC-1 demonstrated exceptional catalytic performance and strong cycling stability for the reduction of 4-NP. Furthermore, the Co₃Fe₇/CoFe₂O₄@NC-3 catalyst exhibited improved performance in the ORR under alkaline conditions, demonstrating excellent long-term stability and strong resistance to methanol poisoning. These hybrid materials hold potential for diverse applications and present a novel strategy for developing advanced catalysts.

2. Experimental section

2.1 Chemical and reagents

Ferrous chloride (FeCl₂, anhydrous), cobalt chloride (CoCl₂, anhydrous), citric acid (C₆H₈O₇), and NH₄Cl were purchased from Aladdin (Shanghai), while Nafion (5 wt%) and commercial platinum/carbon (20 wt% Pt/C) were obtained from Sigma-Aldrich Co. Ltd.

2.2 Synthesis of catalyst materials

First, a mixture of 6.0 g of NH₄Cl, 1.0 g of C₆H₈O₇, 0.2 g of FeCl₂, and 0.04 g of CoCl₂ was dissolved in 70 mL of deionized water and stirred for 2 hours until a homogeneous solution was achieved. Citric acid acted as both a carbon source and a hydrolysis inhibitor for the metal salts. The homogeneous solution was frozen at −50 °C and subsequently freeze-dried at −50 °C and 1.0 Pa. During this process, the ice in the precursor sublimated, yielding a uniform precursor powder. The obtained powder was then placed in a high-temperature tube furnace and calcined under an N₂ gas flow (50 mL min⁻¹) at 900 °C for 3 hours, with a heating rate of 3 °C min⁻¹. During calcination, the NH₄Cl template decomposed, releasing gas, while citric acid carbonized to form a porous carbon layer. The metal ions (Fe²⁺, Co²⁺) fused to form metal NPs, which were then dispersed on the porous carbon layers. Finally, the tube furnace was cooled to room temperature under an N₂ atmosphere, yielding the final sample, Co₃Fe₇/CoFe₂O₄@NC-3 (mole ratio 4 : 1). The Fe²⁺/Co²⁺ molar ratios of 1 : 1, 2 : 1, 5 : 1, 0 : 3, and 3 : 0 were adjusted by varying the feed ratios of FeCl₂ and CoCl₂ while maintaining a total molar amount of 3 mmol. Co₃Fe₇/CoFe₂O₄@NC-1 (mole ratio 1 : 1), Co₃Fe₇/CoFe₂O₄@NC-2 (mole ratio 2 : 1), Co₃Fe₇/CoFe₂O₄@NC-4 (mole ratio 5 : 1), Co@NC (without FeCl₂), and Fe/Fe₂O₃@NC (without CoCl₂) were synthesized under identical conditions. To determine the optimal carbonization temperature, precursors with a mole ratio of 4 : 1 were also subjected to heat treatment at 800 °C and 1000 °C. Details regarding the characterization techniques and electrochemical measurements are provided in the ESI.†

2.3 Material characterization

Laboratory powder X-ray diffraction (XRD) patterns were recorded using a Rigaku Ultima IV diffractometer with a Cu Kα source (40 kV, 40 mA). The morphology and structure of the samples were examined using a field-emission scanning electron microscope (FE-SEM, Quant 250FEG) and a high-resolution transmission electron microscope (HR-TEM, JEM-2100F) operated at 200 kV. A Micromeritics Belsorp-max analyzer was employed to measure the Brunauer–Emmett–Teller (BET) surface area and pore size distribution (PSD). X-ray photoelectron spectroscopy (XPS) measurements were performed using a Kratos Axis Ultra instrument with monochromatic Al Kα radiation. Ultraviolet-visible (UV-vis) spectroscopy was conducted using a Hitachi U-4100 UV-vis spectrophotometer. Magnetic measurements were performed at room temperature with a vibrating sample magnetometer (VSM, LakeShore, USA, Model: 7404).

2.4 Electrochemical measurements

The electrochemical studies were conducted using a Gamry RDE710 electrochemical workstation with a three-electrode configuration. An Ag/AgCl (KCl-saturated) electrode served as the reference electrode, while a carbon rod was used as the counter electrode. To ensure consistency, the working electrodes for all four catalysts were prepared using the same procedure. First, 5 mg of catalyst powder was dissolved in 0.8 mL of ethanol. Next, 40 μL of a 5 wt% Nafion solution (Sigma-Aldrich) was added, and the mixture was sonicated to achieve a uniform suspension. Finally, 10 μL of the resulting catalyst ink, with a concentration of 0.30 mg cm⁻², was applied to coat a glassy carbon electrode.

ORR studies were conducted using 0.1 M KOH electrolytes, saturated with nitrogen (N₂) or oxygen (O₂), at a scan rate of 50 mV s⁻¹ applied to the electrodes. The ORR activity was measured using a rotating disk electrode (RDE) in an O₂-saturated 0.1 M KOH solution with linear sweep voltammetry (LSV) from 0.2 V to −0.8 V. Moreover, the ORR stability was assessed through chronoamperometric i–t tests at 1600 rpm in an O₂-saturated 0.1 M KOH solution over time. The performance of the developed catalysts was compared to that of a commercial Pt/C (20 wt%) electrocatalyst (HiSPEC3000, Alfa Aesar).

2.5 Catalytic reduction of 4-NP

To evaluate the catalytic properties, the reduction of 4-NP to 4-AP was investigated in the presence of sodium borohydride (NaBH₄). In the experiment, 2.7 mL of a 4-NP solution was mixed with 0.3 mL of a 0.1 M aqueous NaBH₄ solution, resulting in a yellowish hue in the final mixture. Subsequently, 5 mg of the hybrid catalyst was added to the solution. As no immediate color change was observed, it was inferred that the reaction proceeded at a very slow rate. The entire process was quantitatively monitored using a UV-visible spectrophotometer (Metash UV-6100). At regular intervals, 2.5 mL of the reaction solution was withdrawn and promptly analyzed using the UV-vis spectrophotometer. Measurements were conducted at room temperature within a scanning range of 250–500 nm.

3. Results and discussion

The preparation process for the Co₃Fe₇/CoFe₂O₄@NC-3 (mole ratio 4 : 1) catalyst is illustrated in Fig. 1. Initially, a homogeneous solution was obtained by dissolving NH₄Cl (6.0 g), citric acid (C₆H₈O₇, 1.0 g), FeCl₂ (1.23 mmol), and CoCl₂ (0.308 mmol) in 70 mL of deionized water, followed by continuous stirring for two hours. In this process, citric acid serves as both a carbon source and an acidifier, preventing the hydrolysis of the metal salts. The resulting homogeneous solution was then frozen at −50 °C and subsequently freeze-dried under vacuum conditions to obtain the precursor powder. During this process, NH₄Cl acted as a rigid template, effectively preventing the aggregation of citric acid and metal salts. Further, the carboxyl functional groups in citric acid strongly interacted with the metal ions, forming a uniform FeCl₂–CoCl₂–C₆H₈O₇ complex that coated the surface of the NH₄Cl template.²⁷ The precursor powder was then subjected to high-temperature calcination, during which NH₄Cl decomposed into gas and escaped, while citric acid carbonized to form a carbon layer. The metal ions fused to generate metal NPs, as illustrated in Fig. 1. Co₃Fe₇/CoFe₂O₄@NC-1 (mole ratio 1 : 1), Co₃Fe₇/CoFe₂O₄@NC-2 (mole ratio 2 : 1), and Co₃Fe₇/CoFe₂O₄@NC-4 (mole ratio 5 : 1), Co@NC (without FeCl₂), and Fe/Fe₂O₃@NC (without CoCl₂) were synthesized using same conditions for comparison.

Fig. 1 Schematic illustration of the Co₃Fe₇/CoFe₂O₄@NC-1 and Co₃Fe₇/CoFe₂O₄@NC-3 nanocomposite formation.

3.1 Morphological and structural characterization

The electrocatalysts, Co₃Fe₇/CoFe₂O₄@NC-1 and Co₃Fe₇/CoFe₂O₄@NC-3 were initially examined using FE-SEM to investigate their morphological and structural characteristics. Co₃Fe₇/CoFe₂O₄@NC-1 displayed a complex three-dimensional structure with thick, curved plates (Fig. 2A). However, Co₃Fe₇/CoFe₂O₄@NC-3 exhibited a flat groove structure, with rice-like nanostructures faintly visible along the grooves (Fig. 2E). For a more detailed comparison, SEM images of Co@NC, Fe/Fe₂O₃@NC, Co₃Fe₇/CoFe₂O₄@NC-2, and Co₃Fe₇/CoFe₂O₄@NC-4 are provided in the ESI (Fig. S1–S3†). Co₃Fe₇/CoFe₂O₄@NC-1 and Co₃Fe₇/CoFe₂O₄@NC-3 morphology was further analyzed using TEM and HR-TEM, as shown in Fig. 2B–D, F and G. The TEM images of Co₃Fe₇/CoFe₂O₄@NC-1 and Co₃Fe₇/CoFe₂O₄@NC-3 clearly show fringe distances of 0.20 nm and 0.30 nm, corresponding to the (110) plane of the Co₃Fe₇ alloy²⁸ and the (220) plane of the CoFe₂O₄ spinel structure.^29,30 Furthermore, Co₃Fe₇/CoFe₂O₄@NC-1 and Co₃Fe₇/CoFe₂O₄@NC-3, exhibit a consistent horizontal band, identified as defective carbon with an interplanar distance of 0.34 nm, suggesting the presence of graphitic carbon.³¹

Fig. 2 Representative SEM (A and E), TEM (B and F), and lattice fringe HR-TEM (C, D and G) images of Co₃Fe₇/CoFe₂O₄@NC-1, and Co₃Fe₇/CoFe₂O₄@NC-3.

The XRD technique was used to evaluate the composition and phases of the synthesized Co₃Fe₇/CoFe₂O₄@NC-1, Co₃Fe₇/CoFe₂O₄@NC-2, Co₃Fe₇/CoFe₂O₄@NC-3, and Co₃Fe₇/CoFe₂O₄@NC-4 (Fig. 3A and S4†). The four composite catalysts exhibited similar peak patterns, corresponding to the Co₃Fe₇ phase at approximately 44.75° and 65.11° (JCPDS no. 48-1816),³² and the CoFe₂O₄ phase at around 35.45°, 57.16°, and 62.72° (JCPDS no. 22-1086).²⁸ Moreover, slight shifts in the diffraction peaks (around 44.75° and 65.11°) suggest that increasing the iron content decreases the peak angles for the Co₃Fe₇ phase. This phenomenon can be attributed to the higher oxidation tendency of Fe²⁺ ions relative to Co²⁺ ions during the heating process, as shown in the ESI (Fig. S4†). Furthermore, the diffraction peak at approximately 26°, corresponding to the graphitized structure, shifted from a broad bulging shape to a sharp diffraction peak.³³

Fig. 3 (A) PXRD patterns; (B) Raman spectra; (C) N₂ sorption isotherms and BJH pore size distributions (inset); (D) the magnetic hysteresis loop at room temperature (T = 300 K) of Co₃Fe₇/CoFe₂O₄@NC-1 and Co₃Fe₇/CoFe₂O₄@NC-3.

Raman spectra of Co₃Fe₇/CoFe₂O₄@NC-1, Co₃Fe₇/CoFe₂O₄@NC-2, Co₃Fe₇/CoFe₂O₄@NC-3, and Co₃Fe₇/CoFe₂O₄@NC-4 showed two peaks at around 1330 cm⁻¹ and 1590 cm⁻¹ (Fig. 3B and S5†).^34,35 The order of I_D/I_G values is as follows: I_D/I_G (Co₃Fe₇/CoFe₂O₄@NC-1) = 1.12 > I_D/I_G (Co₃Fe₇/CoFe₂O₄@NC-2) = 1.09 > I_D/I_G (Co₃Fe₇/CoFe₂O₄@NC-3) = 1.07 > I_D/I_G (Co₃Fe₇/CoFe₂O₄@NC-4) = 1.03. Similarly, several weak signal peaks were detected in the high-wavelength region around 3000 cm⁻¹. The presence of these peaks, along with the high degree of graphitization, suggests a reduction in the charge transfer barrier.^36,37 Raman spectroscopy is widely regarded as an effective technique for identifying metal oxides. As shown in Fig. 3B, the Co₃Fe₇/CoFe₂O₄@NC-3 material exhibited weak peaks at approximately 181 (T_2g(1)), 304 (E_g), 463 (T_2g(2)), and 663 (A_1g(1)) cm⁻¹, which correspond to the characteristic peaks of the CoFe₂O₄ spinel.^28,38

To precisely determine the surface areas of Co₃Fe₇/CoFe₂O₄@NC-1, Co₃Fe₇/CoFe₂O₄@NC-2, Co₃Fe₇/CoFe₂O₄@NC-3, and Co₃Fe₇/CoFe₂O₄@NC-4, nitrogen adsorption–desorption isotherms were measured (Fig. 3C, S6 and Table S1†). Co₃Fe₇/CoFe₂O₄@NC-3 exhibited a larger accessible surface area (517.58 m² g⁻¹) compared to Co₃Fe₇/CoFe₂O₄@NC-1 (439.99 m² g⁻¹), Co₃Fe₇/CoFe₂O₄@NC-2 (393.64 m² g⁻¹), and Co₃Fe₇/CoFe₂O₄@NC-4 (447.37 m² g⁻¹). The larger surface area of Co₃Fe₇/CoFe₂O₄@NC-3 enhances electrolyte infiltration and facilitates oxygen adsorption, thus improving its electrocatalytic performance.^39,40 Furthermore, all samples exhibited type IV isotherms with a hysteresis loop in the relative pressure (P/P⁰) range of 0.45 to 0.99, indicating the presence and distribution of mesopores, as shown by the pore size distribution curves.^41,42

The FeCo alloy is well known for its magnetic properties.^43,44 The magnetic behavior of the synthesized catalysts was evaluated using a vibrating sample magnetometer (Fig. 3D and S7†).⁴⁵ The magnetic behavior of micro-/nanosystems is influenced by factors such as particle size, shape, surface oxidation, and crystalline structure, with slight compositional changes significantly affecting saturation magnetization (M_s). The magnetic hysteresis loops of the catalysts, exhibiting typical ferromagnetic characteristics at 300 K, are shown in Fig. 3D and S7.† The Co₃Fe₇/CoFe₂O₄@NC-1 exhibited a slightly higher M_s (43.5 emu g⁻¹) compared to Co₃Fe₇/CoFe₂O₄@NC-2 (35.3 emu g⁻¹), Co₃Fe₇/CoFe₂O₄@NC-3 (35.7 emu g⁻¹), and Co₃Fe₇/CoFe₂O₄@NC-4 (35.4 emu g⁻¹).

The surface compositions and chemical states of Co₃Fe₇/CoFe₂O₄@NC-1 and Co₃Fe₇/CoFe₂O₄@NC-3 were analyzed using XPS (Fig. 4 and Table S2†). The survey spectra in Fig. 4A show the presence of Co, Fe, C, N, and O elements. Fig. S8† demonstrates that the C 1s spectra display three main peaks corresponding to C–C, C–N, and O–C=O bonds.⁴⁶ Fig. 4B presents the high-resolution Co 2p spectra of Co₃Fe₇/CoFe₂O₄@NC-1 and Co₃Fe₇/CoFe₂O₄@NC-3. In the Co₃Fe₇/CoFe₂O₄@NC-1 sample, distinct peaks appear at 779.2 and 792.6 eV, 780.6 and 795.6 eV, 783.4 and 798.3 eV, and 786.9 and 803.6 eV, corresponding to zero-valence cobalt in the CoFe alloy, the Co–N_x bond, the ionic state of cobalt, and satellite peaks, respectively.^47,48 Fig. 4C presents the Fe 2p spectra of Co₃Fe₇/CoFe₂O₄@NC-1 and Co₃Fe₇/CoFe₂O₄@NC-3. For Co₃Fe₇/CoFe₂O₄@NC-1, the deconvoluted peaks appear at 709.1 and 720.0 eV (zero-valence Fe), 710.7 and 722.9 eV (Fe–N_x), 713.9 and 726.7 eV (Fe ions), and 717.2 and 732.1 eV (satellite peaks).⁴⁹ In Co₃Fe₇/CoFe₂O₄@NC-3, the zero-valence Co peak shifted to higher binding energies, appearing at 784.5 and 790.2 eV. Meanwhile, the Co peak exhibited a shift toward lower binding energies, indicating a transition to an ionic state, with the most pronounced shift at 782.1 and 797.9 eV. The zero-valence Fe binding energy decreased to 707.9 and 720.0 eV. This shift, along with the strong Co–Fe interaction, suggests electron transfer from Co to Fe. The core of the CoFe alloy undergoes charge redistribution, leading to enhanced surface energy. This modification facilitates oxygen species adsorption and desorption on the alloy NP surface, therefore improving the interfacial catalytic rate.⁵⁰ Fig. 4D presents the N 1s spectra of both samples, deconvoluted into pyridinic-N (398.3 eV), Co(Fe)–N_x (400.2 eV), pyrrolic-N (401.0 eV), and graphitic-N (402.3 eV).^51,52 The presence of M–N_x and other nitrogen-doping configurations indicates strong metal–nitrogen bonding and effective nitrogen incorporation into the carbon framework. Co₃Fe₇/CoFe₂O₄@NC-3 exhibited a higher pyridinic-N/graphitic-N ratio (2.78) than Co₃Fe₇/CoFe₂O₄@NC-1 (2.05). Pyridinic-N, with its lone-pair electrons, acts as an efficient catalytic site for the ORR.^53,54 Comparison of the XPS O 1s spectra revealed a single peak at 531.9 eV for Co₃Fe₇/CoFe₂O₄@NC-1, whereas Co₃Fe₇/CoFe₂O₄@NC-3 exhibited two peaks at 532.3 eV and 530.4 eV. The O1 peak, typically associated with low oxygen coordination at defect sites, appears in smaller particles. As shown in Fig. S9,† the presence of O₂ suggests metal–oxygen bonds, indicating Fe₂O₃ in the material.²⁵ The XPS data provide insights into the surface chemistry of Co₃Fe₇/CoFe₂O₄@NC-1 and Co₃Fe₇/CoFe₂O₄@NC-3, revealing the presence of pyridinic-N/graphitic-N, CoFe alloy, and various oxidation states of iron and cobalt. These surface characteristics are anticipated to play a crucial role in the material's electrochemical properties and catalytic performance.

Fig. 4 XPS spectra of Co₃Fe₇/CoFe₂O₄@NC-1, and Co₃Fe₇/CoFe₂O₄@NC-3. (A) Survey spectra; (B) Co 2p; (C) Fe 2p; and (D) N 1s core levels.

3.2 Electrochemical properties for ORR

Cyclic voltammetry (CV) was conducted to assess the electrocatalytic ORR performance of the synthesized nanocomposites in 0.1 M KOH under O₂- and N₂-saturated conditions at a scan rate of 50 mV s⁻¹ (Fig. 5A and Table S3†). Among the four catalysts, Co₃Fe₇/CoFe₂O₄@NC-3 demonstrated the highest ORR activity, highlighting the significance of its active components in enhancing ORR catalysis.⁵⁵

Fig. 5 (A) CV curves of Co₃Fe₇/CoFe₂O₄@NC-1, Co₃Fe₇/CoFe₂O₄@NC-2, Co₃Fe₇/CoFe₂O₄@NC-3, and Co₃Fe₇/CoFe₂O₄@NC-4 in a 0.1 M KOH solution (the solid lines represent O₂-saturated conditions, while the dotted lines represent N₂-saturated conditions). (B) LSV curves were obtained at 1600 rpm for the four produced nanocomposite samples and a commercial 20 wt% Pt/C catalyst in an oxygen-saturated 0.1 M KOH electrolyte. (C) Tafel graphs of the four synthesized nanocomposite samples and 20% Pt/C were generated using LSV data. (D) The limiting current density (J_L) and the electron transfer number (n) comparisons of Co₃Fe₇/CoFe₂O₄@NC-1, Co₃Fe₇/CoFe₂O₄@NC-2, Co₃Fe₇/CoFe₂O₄@NC-3, and Co₃Fe₇/CoFe₂O₄@NC-4.

LSV polarization curves were used to assess ORR activity (Fig. 5B). Among all samples, Co₃Fe₇/CoFe₂O₄@NC-3 exhibited the highest performance (Fig. 5B, S10–S13 and Table S3†), with E_onset = 0.99 V, E_1/2 = 0.83 V, and J_L = −5.20 mA cm⁻². The activity followed the order: Co₃Fe₇/CoFe₂O₄@NC-2 (E_onset = 1.07 V, E_1/2 = 0.82 V, J_L = −5.80 mA cm⁻²) > Co₃Fe₇/CoFe₂O₄@NC-4 (E_onset = 0.94 V, E_1/2 = 0.83 V, J_L = −5.61 mA cm⁻²) > Co₃Fe₇/CoFe₂O₄@NC-1 (E_onset = 0.91 V, E_1/2 = 0.83 V, J_L = −3.29 mA cm⁻²). These results highlight the significant impact of metal ratio adjustments on active catalyst composition and ORR performance. For comparison, CV and LSV profiles with the RDE configuration were used to evaluate the catalytic activities of Co₃Fe₇/CoFe₂O₄@NC-3 synthesized via pyrolysis at 800 °C and 1000 °C, denoted as Co₃Fe₇/CoFe₂O₄@NC-3-800 and Co₃Fe₇/CoFe₂O₄@NC-3-1000, respectively. The ORR activity of these samples was lower than that of Co₃Fe₇/CoFe₂O₄@NC-3, particularly at pyrolysis temperatures below or above 900 °C (Fig. 5B, S14, S15 and Table S3†). Co₃Fe₇/CoFe₂O₄@NC-3, pyrolyzed at 900 °C, exhibited high onset (0.99 V), half-wave (0.83 V), and limiting current densities (−5.20 mA cm⁻²), comparable to the performance of commercial 20 wt% Pt/C.

The Tafel slope is expressed as follows:⁵⁶

Overpotential g = a + b log(j)

where j represents the current density, b denotes the Tafel slope, and a is constant. A lower b value indicates a smaller increase in overpotential with rising current density, suggesting faster reaction kinetics for the rate-determining step. In this study, the selected log(j) range considers a current density of 10 mA cm⁻², which serves as a benchmark for catalyst comparison.⁵⁷ The Tafel slope of Co₃Fe₇/CoFe₂O₄@NC-3 was 93 mV dec⁻¹, closely aligning with Pt/C (97 mV dec⁻¹) and Co₃Fe₇/CoFe₂O₄@NC-4 (109 mV dec⁻¹). However, Co₃Fe₇/CoFe₂O₄@NC-1 exhibited a lower value of 61 mV dec⁻¹, indicating distinct catalytic behavior, while Co₃Fe₇/CoFe₂O₄@NC-2 showed a significantly higher slope of 167 mV dec⁻¹, suggesting reduced ORR efficiency (Fig. 5C). Meanwhile, the LSV curves, recorded at various rotational speeds (400, 625, 900, 1225, 1600, 2025, and 2500 rpm) were used to investigate the ORR mechanism of the four nanocomposites (Fig. 5D). Based on the RDE test, the electron transfer number (n) was computed using the Koutechy–Levich (K–L) equations:⁵⁸

1/J = 1/J_L + 1/J_K = 1/Bω^1/2 + 1/J_K (1)

B = 0.62nFC₀(D₀)^2/3ν^−1\6 (2)

J_K = nFkC₀ (3)

In these equations, J represents the current density in the LSV curve, while J_L and J_K represent diffusion-limiting and kinetic-limiting current densities, respectively. B represents the reciprocal of the K–L curve slope, and ω represents the electrode rotation rate. C₀ (1.2 × 10⁻⁶ mol cm⁻³) and D₀ (1.9 × 10⁻⁵ cm² s⁻¹) represent the bulk concentration and diffusion coefficient of oxygen in the 0.1 M KOH solution, respectively. F denotes the Faraday constant (96 485 C mol⁻¹), ν represents the dynamic viscosity and k represents the rate constant of electron transfer. The K–L plots exhibited excellent linearity from 0.4 V to 0.6 V vs. RHE, confirming first-order reaction kinetics for the ORR. The electron transfer number (n) for Co₃Fe₇/CoFe₂O₄@NC-3, determined from K–L analysis, was 3.31 (Fig. S12†), closely aligning with Pt/C (4.0). This suggests that Co₃Fe₇/CoFe₂O₄@NC-3 follows a four-electron ORR pathway for oxygen reduction.⁵⁹

Long-term durability is crucial for assessing ORR catalyst performance. Stability tests for Co₃Fe₇/CoFe₂O₄@NC-3 and Pt/C in an oxygen-saturated 0.1 M KOH solution were conducted using time-amperometric measurements at 0.8 V and 1600 rpm. After 10 000 seconds, Co₃Fe₇/CoFe₂O₄@NC-3 retained 88% of its initial current density, whereas Pt/C retained only 60% (Fig. S16A†). The superior stability of Co₃Fe₇/CoFe₂O₄@NC-3 is attributed to its higher pyridinic-N content, which enhances Fe and Co anchoring, improving the dispersibility of active components on the carbon carrier.^60–62 The methanol crossover experiment confirmed the robust stability of Co₃Fe₇/CoFe₂O₄@NC-3, as its current density remained unchanged after methanol introduction. However, Pt/C exhibited a sharp decline due to methanol oxidation (Fig. S16B†). Cycling-accelerated durability testing was conducted to evaluate catalyst stability under alkaline conditions (Fig. S17†). After 10 000 CV cycles, the E_1/2 potential loss was 14 mV for Co₃Fe₇/CoFe₂O₄@NC-3 and 33 mV for Pt/C, confirming the improved stability of Co₃Fe₇/CoFe₂O₄@NC-3 compared to commercial Pt/C. The slight decrease in catalytic activity may be attributed to the partial growth of FeCo clusters in the spent catalyst, as evidenced by SEM analysis (Fig. S18†). The XRD patterns of Co₃Fe₇/CoFe₂O₄@NC-3 exhibited diffraction peaks similar to the original ones, confirming that its chemical composition remained unchanged after the durability test, demonstrating excellent electrochemical stability (Fig. S19†). These findings underscore the superior stability and methanol tolerance of Co₃Fe₇/CoFe₂O₄@NC-3 compared to Pt/C. Further, its catalytic efficiency surpassed that of previously reported iron oxide ORR catalysts (Table S4†).

3.3 Catalytic properties for 4-nitrophenol reduction

The reduction of aromatic nitro compounds to amines plays a vital role in organic synthesis and industrial production. Among these, 4-NP is a priority pollutant due to its high toxicity, carcinogenicity, and teratogenicity. Its conversion to 4-AP using sodium borohydride serves as a standard reaction for assessing the catalytic efficiency of metal or alloy catalysts.^63–65 Various catalysts have been explored for the degradation of 4-NP, including gold nanoparticles (AuNPs),⁶⁶ silver nanoparticles (AgNPs),⁶⁷ titanium dioxide (TiO₂), and⁶⁸ metal–organic frameworks (MOFs).⁶⁹ In this study, the catalytic performance of the synthesized nanocomposites was assessed via the reduction of 4-NP to 4-AP (Fig. 6). The reaction kinetics were examined using time-resolved spectra. As shown in Fig. 6A and B, the absorption peak of 4-NP at 400 nm decreased, while that of 4-AP at 300 nm increased, indicating successful conversion. The corresponding color change from yellow to colorless further confirmed the reaction.^19,20 For better comparison, the degradation of the other three composite catalysts (Co₃Fe₇/CoFe₂O₄@NC-2 and Co₃Fe₇/CoFe₂O₄@NC-4) was also performed (Fig. S20 and S21†).

Fig. 6 The UV-vis absorption spectra of the reduction process were measured in the presence of (A) Co₃Fe₇/CoFe₂O₄@NC-1 and (B) Co₃Fe₇/CoFe₂O₄@NC-3. (C) The ln(C_t/C₀) plots compare the response times for the reduction of 4-NP catalyzed by Co₃Fe₇/CoFe₂O₄@NC-1, Co₃Fe₇/CoFe₂O₄@NC-2, Co₃Fe₇/CoFe₂O₄@NC-3, and Co₃Fe₇/CoFe₂O₄@NC-4. (D) Cycling performances of the Co₃Fe₇/CoFe₂O₄@NC-1.

This confirms the superior catalytic performance of Co₃Fe₇/CoFe₂O₄@NC-1 in 4-NP reduction. Pseudo-first-order kinetics was applied to determine kinetic parameters. The concentration ratio C_t/C₀ (where C_t and C₀ represent the concentrations of 4-NP at time t and 0, respectively) was calculated based on light absorption at 400 nm. The linear relationship between ln(C_t/C₀) and time (t) is shown in Fig. 6C, comparing the degradation efficiency of Co₃Fe₇/CoFe₂O₄@NC-1, Co₃Fe₇/CoFe₂O₄@NC-2, Co₃Fe₇/CoFe₂O₄@NC-3, and Co₃Fe₇/CoFe₂O₄@NC-4 catalysts. After 50 minutes, Co₃Fe₇/CoFe₂O₄@NC-1 achieved 79% degradation of 4-NP, followed by Co₃Fe₇/CoFe₂O₄@NC-4 (55%), Co₃Fe₇/CoFe₂O₄@NC-2 (42%), and Co₃Fe₇/CoFe₂O₄@NC-3 (28%). These results confirm Co₃Fe₇/CoFe₂O₄@NC-1 as the most efficient catalyst for 4-NP reduction.⁷⁰ The apparent rate constant k for the catalysts synthesized under different conditions was determined as follows: k(Co₃Fe₇/CoFe₂O₄@NC-1) = 0.031 min⁻¹ > k(Co₃Fe₇/CoFe₂O₄@NC-4) = 0.017 min⁻¹ > k(Co₃Fe₇/CoFe₂O₄@NC-2) = 0.011 min⁻¹ > k(Co₃Fe₇/CoFe₂O₄@NC-3) = 0.006 min⁻¹. The highest k value of 0.031 min⁻¹ for Co₃Fe₇/CoFe₂O₄@NC-1 confirms its superior catalytic efficiency in the reduction of 4-NP.

To evaluate the reusability and stability of the synthesized catalysts, degradation experiments of 4-NP were conducted using Co₃Fe₇/CoFe₂O₄@NC-1 over five cycles. As shown in Fig. 6D, the k values and corresponding linear correlations were plotted. After five cycles, Co₃Fe₇/CoFe₂O₄@NC-1 exhibited only a 6% performance decrease, retaining over 70% of its initial catalytic activity and demonstrating excellent stability. For comparison, the k values for Co₃Fe₇/CoFe₂O₄@NC-1, tested for reusability, are presented in Fig. S22,† showing a decline in stability and reusability. These findings underscore the superior catalytic efficiency and reusability of Co₃Fe₇/CoFe₂O₄@NC-1.

4. Conclusions

In summary, four composite catalyst materials (Co₃Fe₇/CoFe₂O₄@NC-1, Co₃Fe₇/CoFe₂O₄@NC-2, Co₃Fe₇/CoFe₂O₄@NC-3, and Co₃Fe₇/CoFe₂O₄@NC-4) were synthesized using the molten salt method. Among these, Co₃Fe₇/CoFe₂O₄@NC-1 exhibited superior catalytic reduction efficiency and maintained favorable durability over multiple cycles in 4-NP reduction. Meanwhile, Co₃Fe₇/CoFe₂O₄@NC-3 demonstrated excellent ORR performance in an alkaline environment, with improved stability and resistance to methanol-induced deactivation. The surface chemistry characteristics, including the proportion of pyridinic-N/graphitic-N, the oxidation states of cobalt and iron, and the presence of oxidized or/and unoxidized CoFe alloy, significantly influence the catalytic effectiveness and electrochemical properties of Co₃Fe₇/CoFe₂O₄@NC-1 and Co₃Fe₇/CoFe₂O₄@NC-3. Furthermore, all hybrid materials exhibited excellent ferromagnetic properties at ambient temperature, allowing for efficient manipulation by an external magnetic field. Their outstanding catalytic performance, ease of recyclability, and cost-effectiveness make these newly developed composite catalysts highly promising for applications in catalysis.

Data availability

The authors declare that the data supporting the findings of this study are available within the paper and its ESI.†

Author contributions

Writing—original draft and writing—review & editing, Y.-l. Wu; investigation, H. He; data curation, X. Tang and Q.-y. Luo; supervision, H.-j. Zhang; funding acquisition, Q.-g. Hou; resources, W.-k. Fu. All authors have read and agreed to the published version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by the Natural Science Foundation of Shandong Province (ZR2023QB023 and ZR2021MB027), and the Transportation Technology Innovation Plan of Shandong Province (2024B112-02). The authors would like to thank MJEditor (https://www.mjeditor.com) for its linguistic assistance during the preparation of this manuscript.

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Footnote

† Electronic supplementary information (ESI) available: SEM images of the Co@NC, Fe/Fe₂O₃@NC, Co₃Fe₇/CoFe₂O₄@NC-2 and Co₃Fe₇/CoFe₂O₄@NC-4; PXRD patterns, Raman spectra, N₂ sorption isotherms, and the magnetic hysteresis loop at room temperature (T = 300 K) of the Co₃Fe₇/CoFe₂O₄@NC-2, Co₃Fe₇/CoFe₂O₄@NC-3, and Co₃Fe₇/CoFe₂O₄@NC-4; the high-resolution XPS spectra of C 1s, and O 1s core levels of Co₃Fe₇/CoFe₂O₄@NC-1 and Co₃Fe₇/CoFe₂O₄@NC-3 composite; LSV curves of Co₃Fe₇/CoFe₂O₄@NC-1, Co₃Fe₇/CoFe₂O₄@NC-2, Co₃Fe₇/CoFe₂O₄@NC-3, Co₃Fe₇/CoFe₂O₄@NC-4, Co₃Fe₇/CoFe₂O₄@NC-3-800, and Co₃Fe₇/CoFe₂O₄@NC-3-1000; amperometric i–t curves of Co₃Fe₇/CoFe₂O₄@NC-3 and 20 wt% Pt/C; SEM images of Co₃Fe₇/CoFe₂O₄@NC-3 catalyst after stability tests; PXRD patterns of Co₃Fe₇/CoFe₂O₄@NC-3 catalyst before and after stability tests; UV-vis absorption spectra of the reduction reaction systems in the presence of Co₃Fe₇/CoFe₂O₄@NC-2 and Co₃Fe₇/CoFe₂O₄@NC-4; cycling performances of the Co₃Fe₇/CoFe₂O₄@NC-3 catalyst. See DOI: https://doi.org/10.1039/d5ra00893j

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