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. 2025 Apr 8;15(14):10884–10895. doi: 10.1039/d5ra00893j

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

Yanling Wu a,b, Xi Tang a, Hui He a, Qingyuan Luo a, Wenkai Fu a, Qinggao Hou a,✉, Haijun Zhang c,✉
PMCID: PMC11977103  PMID: 40201206

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 (Eonset = 0.99 V, E1/2 = 0.83 V, JL = −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.

Four Fe/Co ratio composite catalysts were synthesized via freeze-drying and in situ calcination. Among them, Co3Fe7/CoFe2O4@NC-1 demonstrated superior 4-NP reduction activity, while Co3Fe7/CoFe2O4@NC-3 exceled in alkaline ORR performance.graphic file with name d5ra00893j-ga.jpg

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.

Fig. 1

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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.

Fig. 2

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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.

Fig. 3

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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 ID/IG values is as follows: ID/IG (Co3Fe7/CoFe2O4@NC-1) = 1.12 > ID/IG (Co3Fe7/CoFe2O4@NC-2) = 1.09 > ID/IG (Co3Fe7/CoFe2O4@NC-3) = 1.07 > ID/IG (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 (T2g(1)), 304 (Eg), 463 (T2g(2)), and 663 (A1g(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 (Ms). 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 Ms (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 Created by potrace 1.16, written by Peter Selinger 2001-2019 O bonds.46Fig. 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–Nx bond, the ionic state of cobalt, and satellite peaks, respectively.47,48Fig. 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–Nx), 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.50Fig. 4D presents the N 1s spectra of both samples, deconvoluted into pyridinic-N (398.3 eV), Co(Fe)–Nx (400.2 eV), pyrrolic-N (401.0 eV), and graphitic-N (402.3 eV).51,52 The presence of M–Nx 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.

Fig. 4

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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 (JL) 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.

Fig. 5

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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 Eonset = 0.99 V, E1/2 = 0.83 V, and JL = −5.20 mA cm−2. The activity followed the order: Co3Fe7/CoFe2O4@NC-2 (Eonset = 1.07 V, E1/2 = 0.82 V, JL = −5.80 mA cm−2) > Co3Fe7/CoFe2O4@NC-4 (Eonset = 0.94 V, E1/2 = 0.83 V, JL = −5.61 mA cm−2) > Co3Fe7/CoFe2O4@NC-1 (Eonset = 0.91 V, E1/2 = 0.83 V, JL = −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:56Overpotential 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/JL + 1/JK = 1/Bω1/2 + 1/JK 1
B = 0.62nFC0(D0)2/3ν−1\6 2
JK = nFkC0 3

In these equations, J represents the current density in the LSV curve, while JL and JK 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 E1/2 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(Ct/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.

Fig. 6

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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 Ct/C0 (where Ct 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(Ct/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.

Supplementary Material

RA-015-D5RA00893J-s001

Acknowledgments

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.

†

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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Associated Data

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Supplementary Materials

RA-015-D5RA00893J-s001
RA-015-D5RA00893J-s001.pdf (2.8MB, pdf)

Data Availability Statement

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

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