Scalable synthesis of N-doped graphene-oxide-supported FeCo(OH)_x nanosheets for efficient Co-doped Fe₃O₄ nanoparticle-based oxygen reduction reaction electrocatalysis

Abstract

Developing efficient and cost-effective materials is crucial for advancing the electrochemical oxygen reduction reaction (ORR). By introducing Co²⁺ ions into formamide, our method prevents rapid Fe²⁺ oxidation to Fe³⁺, promoting the formation of well-defined Fe₃O₄ nanoparticles rather than Fe₂O₃. This study presents a synthesis route for high-performance spinel Fe and Co oxide nanoparticles on N-doped reduced graphene oxide (NRGO). This solvothermal synthesis in formamide yields well-dispersed, ultrafine FeCo(OH)_x nanoparticles (∼5 nm) anchored on NRGO. These nanoparticles can be employed for the formation of spinel Fe_xCo_3−xO₄ oxide nanoparticles, potentially due to their high surface area and strong interaction with the NRGO support. This, in turn, facilitates the successful decoration of highly dispersed spinel Fe_xCo_3−xO₄ oxide nanoparticles (∼30 nm) onto the NRGO support, even after calcination at 900 °C, which represents the critical temperature for conventional graphitization. This unique approach results in significantly reduced particle aggregation compared with that of conventional methods. The (Co)Fe₃O₄-NRGO nanocomposite exhibits remarkable ORR activity, achieving an electron number of ∼3.7 and a current density of 5.01 mA cm⁻² at E = 0.75 V_RHE, comparable to those of commercial Pt/C catalysts. Furthermore, the catalyst exhibits remarkable stability, maintaining a reducing current density that is 42% lower after 40 000 s of uninterrupted operation at 0.75 V_RHE compared with a 75% reduction observed with Pt/C. This exceptional performance is attributed to the strong interaction between the (Co)Fe₃O₄ nanoparticles and NRGO, facilitated by the Co ion precursor during annealing.

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Introduction

The increasing demand for clean-energy technology, particularly fuel cells, has created a growing need for low-cost, high-performance transition metal catalysts for the oxygen reduction reaction (ORR).^1–5 The advancement of the electrochemical ORR relies on the development of materials that are both efficient and low-cost, as they can address crucial environmental problems.^5–9 ORR electrocatalysis is important in fuel cell cathodes, spurring significant research efforts to address its limitations and enhance fuel cell efficiency and stability.^10,11 ORR catalysis based on platinum-group metals is widely employed; however, the high cost and poor durability of the metals hinder their practical applications.^9,11–14

Transition metal oxides, particularly spinel Co₃O₄ and its inverse spinel counterpart Fe₃O₄, have emerged as attractive alternatives to Pt/C catalysts for the ORR because of their cost-effectiveness, inherent activity, and superior resistance to particle aggregation, compared with Pt/C.^15,16 Notably, Fe₃O₄ offers advantages over Co₃O₄ because of its earth abundance.^17,18 However, achieving high ORR performance with Fe₃O₄ necessitates careful control of its structure and morphology.^19,20 Crystalline Fe₃O₄ nanoparticles with a single phase are crucial for maximizing active sites, whereas small particle sizes enhance surface area and mass transport.^19–22

To address these limitations and unlock the full potential of Fe₃O₄ for the ORR, we propose a synthetic approach based on intermediate control. This method involves the transformation of layered double hydroxide (LDH)-like Fe(OH)_x intermediates into well-dispersed, nanosized Fe(OH)_x species anchored on N-doped reduced graphene oxide (NRGO). This approach leverages our previously demonstrated control over intermediate hydroxide species.¹ In our prior work, Co(OH)₂ single layers were employed to successfully decorate RGO, affording well-dispersed Co₃O₄ nanoparticles, approximately 5 nm in size.¹ This established method offers a promising strategy for synthesizing spinel nanoparticles, such as Fe₃O₄, while preventing particle aggregation even at high calcination temperatures.

In conventional Fe₃O₄ nanoparticle synthesis, undesirable byproducts may form when carried out in organic or aqueous media, such as Prussian blue in the presence of CN⁻ ions and α-Fe₂O₃ in environments with excess O₂.^23–25 To overcome these limitations and create precisely defined nanoparticles, this study uses the intermediate control method. Through this process, ultra-small FeCo(OH)_x intermediate nanoparticles are carefully grown on NRGO. This intermediate phase not only supports the uniform distribution of nanoparticles but also minimizes particle agglomeration, ensuring a stable and well-dispersed catalyst structure. This approach offers several advantages: (1) the prevention of particle aggregation during high-temperature calcination, (2) the enhancement of ORR electrocatalytic performance, (3) the improvement of crystallinity and single-phase magnetite formation, and (4) the production of well-defined, small particles on the conducting layer even at elevated temperatures (up to 900 °C). Additionally, the incorporation of Co precursors simplifies the process by eliminating the need for a separate hydroxide synthesis step and effectively prevents particle aggregation.

We introduce a solvothermal treatment to prepare LDH-like FeCo(OH)_x nanosheets on NRGO. Subsequent calcination at 900 °C, the critical temperature for conventional graphitization, results in (Co)Fe₃O₄ spinel nanocrystals on highly graphitized NRGO supports. This optimized temperature enables the production of uniformly distributed spinel (Co)Fe₃O₄ nanoparticles (∼30 nm) on the NRGO support.

Scheme 1 illustrates the synthesis process, including the formation of FeCo(OH)_x nanosheets via the solvothermal reaction, their anchoring on NRGO, and their subsequent calcination to form (Co)Fe₃O₄ nanoparticles uniformly deposited on the NRGO. The Fe-only path introduces a mixture of Prussian blue and Fe(OH)₃, affording an irregular mixture of α-Fe₂O₃ and Fe₃O₄. In this work, the incorporation of Co²⁺ in the Fe²⁺ precursor solution leads to the formation of an ultrathin LDH-like FeCo(OH)_x phase on the NRGO surface and the production of well-decorated spinel (Co)Fe₃O₄ on the NRGO sheet for ORR electrocatalysis.

Scheme 1 Schematic illustration of FeCo(OH)_x nanosheet precursor preparation and transformation into spinel (Co)Fe₃O₄ on a NRGO support. This approach offers a controlled synthesis route compared to conventional methods that often lead to a mixture of α-Fe₂O₃ and Fe₃O₄ during calcination of commercial magnetite.

Results and discussion

Role of Co ions in the formation of (Co)Fe₃O₄-NRGO

The X-ray diffraction (XRD) pattern of (Co)Fe₃O₄-NRGO (Fig. 1A) showed the successful formation of spinel (Co)Fe₃O₄ and the absence of peaks corresponding to α-Fe₂O₃. This suggested the successful inhibition of α-Fe₂O₃ formation during the calcination of FeCo(OH)_x at 900 °C. The formation of intermediate hydroxides and their calcined products was characterized by XRD analysis. For comparison, a control group was prepared without the addition of Co ions, affording Fe(OH)₃ during the solvothermal synthesis. The characteristic peaks of Fe(OH)₃ were observed in the XRD pattern of Fe(OH)_x–NRGO, which corresponded to 17.4° and 34.8° (Fig. 1A). An additional broad peak at approximately 24.5° was attributed to the glass holder. The presence of NRGO was confirmed by the XRD peak at 43.2° (Fig. 1A, labeled NRGO). This peak suggested the presence of partially oxidized graphene sheets, likely due to the solvothermal or cleaning processes.

Fig. 1 XRD patterns of (A) FeCo(OH)_x–NRGO, Fe(OH)_x–NRGO, (Co)Fe₃O₄–NRGO (calcined at 900 °C), and FeO_x–NRGO (900). (B) (Co)Fe₃O₄−NRGO calcined at various temperatures (450–1200 °C). The inset specifies that G and GO represent graphite and graphene oxide, respectively.

Following calcination, a new peak appeared, as shown in Fig. 1A (labeled G), corresponding to the characteristic diffraction peak (26.5°) of graphite. The XRD analysis of the Fe(OH)_x–NRGO composite after calcination at 900 °C revealed a transformation into a mixture of α-Fe₂O₃ and Fe₃O₄ phases. Additionally, the diffraction peaks corresponding to metallic iron were observed in both samples. While Fe(OH)₃ can be a precursor to α-Fe₂O₃ and Fe₃O₄ under specific conditions, it may decompose during the high-temperature calcination process (900 °C), resulting in a mixture of α-Fe₂O₃ (33.2°, 35.7°, 49.4°, 53.9°, and 62.5°) and Fe₃O₄ (30.4°, 35.7°, 43.2°, 57.1°, and 63.1°). The XRD patterns of α-Fe₂O₃ and Fe₃O₄, including reference data from the PDF file database, are provided in Fig. S1† for comparison. The FeCo(OH)_x–NRGO sample showed no diffraction peaks, except for the NRGO support before calcination. However, dominant peaks for the calcined forms of the (Co)Fe₃O₄ phases, 29.9°, 35.4°, 43.1°, 57.1°, and 62.4°, appeared along with weak diffraction signals for metallic iron (44.8° and 65.0°). The result indicated that Co influenced the formation of the FeCo(OH)_x phase, potentially promoting the phase transition to the spinel (Co)Fe₃O₄ phase during calcination.

The synthesis of the spinel (Co)Fe₃O₄ nanoparticles was further supported by observations from the solvothermal reaction conducted without the NRGO support (Fig. S1†). Interestingly, the solvothermal reaction without the NRGO support did not yield any precipitate after 6 h; however, precipitation was observed after 12 h (Fig. S1A†). The transmission electron microscopy (TEM) image of the 6 h sample might suggest the presence of nanosheets within the colloidal solution, composed of Co–Fe hydroxide (Fig. S1B†). However, without Co, the solvothermal reaction afforded a predominant precipitate of α-Fe₂O₃ (Fig. S1C†). This observation suggested that the presence of Co caused the FeCo(OH)_x nanosheet to suppress the formation of α-Fe₂O₃. The 12 h FeCo(OH)_x sample, exhibiting initial powder formation, contained both Fe(OH)₃ and Prussian blue phases according to XRD (Fig. S1C†). After calcination at 900 °C, FeCo(OH)_x transformed primarily into Fe₃O₄, as evidenced by the XRD result (Fig. 1). However, the presence of metallic iron likely stems from the partial reduction of iron oxides, a common side reaction during high-temperature calcination. Interestingly, commercially available Prussian blue also showed metallic iron phases after calcination at 900 °C (Fig. S2†), similar to our (Co)Fe₃O₄–NRGO and (Co)Fe₃O₄ samples (Fig. 1, S1, and S2†).

The XRD patterns of the FeCo(OH)_x-NRGO samples calcined at elevated temperatures revealed a shift in the (002) peak of the NRGO component (Fig. 1B, labeled G(002)). This peak shifted from 24.1° to 26.5° as the calcination temperature increased from 450 °C to 900 °C, indicating improved graphitization of the NRGO sheets. No further peak shift was observed above 900 °C, suggesting the completion of graphitization at 900 °C. Additionally, severe particle aggregation on the NRGO was observed at temperatures exceeding 1050 °C (Fig. S4†). Achieving highly crystalline nanoparticles often requires high crystallinity in the supporting graphene.^7,26 However, high-temperature treatments typically used for graphene synthesis can damage its crystalline structure.²⁷ These high temperatures can promote unwanted graphene-sheet breakdown, further affecting the crystallinity of the nanoparticles grown on them.^27,28 Importantly, the XRD patterns of all samples exhibited diffraction peaks corresponding solely to the (Co)Fe₃O₄ phase, and no peaks for α-Fe₂O₃ were detected. Therefore, this method offers a significant advantage in decorating crystalline spinel Fe₃O₄ nanocatalysis onto the highly graphitized NRGO support, affording high ORR activity due to the synergistic effect between the (Co)Fe₃O₄ nanoparticles and the conductive NRGO sheet.

Scanning electron microscopy (SEM), high-resolution TEM (HR-TEM), and scanning TEM were employed to investigate the morphology, size distribution, and elemental distribution of the as-prepared hydroxides and their corresponding calcined products (Fig. 2). Both Fe(OH)_x–NRGO and its calcined product at 900 °C showed large aggregates on the NRGO supports (Fig. S5†), probably due to fast aggregation of the Fe(OH)_x phase during the solvothermal reaction. In contrast, FeCo(OH)_x–NRGO showed fine particles smaller than 10 nm on the NRGO surface. Notably, the calcined (Co)Fe₃O₄–NRGO exhibited well-dispersed nanoparticles. The SEM results of the FeCo(OH)_x–NRGO composite depicted the uniform dispersion of the FeCo(OH)_x nanoparticles, averaging approximately 5 nm in size, anchored on the NRGO sheets (Fig. 2A–C). Upon calcination at 900 °C, these particles transformed into (Co)Fe₃O₄ nanoparticles, exhibiting an average size of 20.1 ± 5.8 nm, while maintaining excellent dispersion on the entire NRGO surface (Fig. 2D and E). Energy dispersive X-ray spectroscopy (EDS) validated the co-localization of Fe and Co within the (Co)Fe₃O₄ particles (Fig. 2G), indicating a well-defined mixed metal oxide.

Fig. 2 (A–C) SEM and TEM images of FeCo(OH)_x—NRGO. (D–F) SEM and TEM images of Co(Fe₃O₄)–NRGO, and inset of (F) shows HR-TEM lattice images. (G) Scanning transmission electron microscopy (STEM) image of Co(Fe₃O₄)–NRGO with the corresponding elemental mapping images for Fe, Co, C and N (scale bar: 100 nm).

X-ray photoelectron spectroscopy (XPS) was employed to analyze the atomic chemical composition of FeCo(OH)_x–NRGO and (Co)Fe₃O₄–NRGO, specifically focusing on investigating the Fe and Co species through the photoelectric effect (Fig. S6†). A comparison of the C 1s spectra between FeCo(OH)_x–NRGO and (Co)Fe₃O₄–NRGO (900 °C) revealed a pronounced disparity in the relative intensities of C=C and C–C peaks. Notably, (Co)Fe₃O₄–NRGO (900 °C) exhibited approximately 1.5 times higher intensity for these peaks, indicating a significant enhancement in NRGO crystallization. This observation was consistent with the calcination process, wherein the removal of oxygen and hydroxyl groups facilitated C–C bond formation.

Conversely, the intensities of the C–O and C=O peaks were approximately 4 times lower in (Co)Fe₃O₄–NRGO (900 °C) than in FeCo(OH)_x–NRGO, indicative of reduced oxygen functionalities (Fig. S6A†). The analysis of the Fe 2p_3/2 spectra provided insights into the oxidation states of iron in both FeCo(OH)_x–NRGO and (Co)Fe₃O₄–NRGO (900 °C). The presence of a peak at 709.9 eV in both samples confirmed the existence of Fe²⁺, indicative of magnetite (Fe₃O₄) formation in the final products. Furthermore, the detection of Fe metal peaks at 707.5 and 708.4 eV in both samples suggested the coexistence of metallic iron, a finding corroborated by XRD patterns, which identified the presence of an iron metal phase (Fig. S6B†). O 1s analysis revealed a peak at 530.5 eV in both FeCo(OH)_x–NRGO and (Co)Fe₃O₄–NRGO (900 °C), confirming the presence of metal–oxygen bonding (Fig. S6C†). Co 2p analysis indicated weakly detected Co peaks at 795.0 eV in the Co 2p spectra of both FeCo(OH)_x–NRGO and (Co)Fe₃O₄–NRGO (900 °C), suggesting a low concentration of Co ions in the samples (Fig. S6D†).

Characterization of FeCo(OH)_x nanosheets

Our unique synthesis method is presented for the fabrication of FeCo(OH)_x nanoparticles (approximately 5 nm) grown directly onto NRGO. These FeCo(OH)_x precursors successfully transformed into well-dispersed (Co)Fe₃O₄ nanoparticles with an average size of 20.1 nm during calcination. To gain a deeper understanding of the FeCo(OH)_x precursor structure and its transformation mechanism, further characterization techniques are warranted based on our previous studies.

Fig. 3A presents the thermogravimetric analysis (TGA) curves of the FeCo(OH)_x–NRGO and Fe(OH)_x–NRGO composites at temperatures between 50 °C and 900 °C. The Fe(OH)_x–NRGO sample underwent a rapid weight loss at temperatures between 220 °C and 290 °C, likely because of the decomposition of layered hydroxide structures.¹ This aligns with our previous observations of a rapid weight loss at approximately 200 °C associated with the collapse of hydroxide layers in thick-β-Co(OH)₂, releasing water molecules.¹ In contrast, single-layered Co(OH)₂ on RGO had a relatively high decomposition temperature because there was no layer collapse. Compared with Fe(OH)_x, which likely formed thick particles in Fe(OH)_x–NRGO, FeCo(OH)_x showed a nanosheet behavior in FeCo(OH)_x–NRGO, as shown in the TGA curve. The TEM inspection strongly evidenced the thin-plate structure of FeCo(OH)_x (Fig. 3B and C) prepared under specific solvothermal conditions without the NRGO support. This indicates the presence of few hydroxide layers in FeCo(OH)_x, leading to smaller (Co)Fe₃O₄ nanoparticles compared with the large α-Fe₂O₃ particles obtained from the Fe(OH)_x particles on NRGO.

Fig. 3 (A) TGA curves of FeCo(OH)_x–NRGO and Fe(OH)_x–NRGO under a N₂ atmosphere, along with (B) TEM images of the products obtained after a 6 hour solvothermal reaction at 120 °C using only metal precursors (FeCl₂ and CoCl₂ in a 2 : 1 ratio) and formamide without NRGO (inset: photographs of FeCo(OH)_x solutions after different solvothermal reaction times).

Compared with FeCl₂ alone, which produced Fe(OH)_x and large α-Fe₂O₃ aggregates, the addition of CoCl₂ in FeCl₂ produced a transparent and red colloid without a significant aggregation (Fig. 3B). Further examination using HR-TEM revealed diffraction patterns reminiscent of the (012) and (018) peaks, akin to the LDH structure composed of Co²⁺–Fe³⁺ (Fig. S7A and B†).^28,29 We suggest that the presence of Co²⁺ might induce the formation of a Co²⁺–Fe³⁺ LDH nanosheet, leading to tight binding on a two-dimensional NRGO layer. The presence of weakly layered regions within the Co²⁺–Fe³⁺ LDH nanosheet accounted for the diffraction peaks observed at (012) and (018), indicating preferential diffraction from these crystallographic planes.²⁸ Furthermore, the EDS mapping demonstrates the overlapping regions of Fe and Co (Fig. S7C†).

Contribution of Co²⁺ in FeCo(OH)_x nanosheets

To elucidate the relationship between the Co²⁺ and Fe²⁺ precursors in this reaction, we investigated the actual atomic ratio by inductively coupled plasma atomic emission spectroscopy and compared it with the expected ratio based on the solvothermal reaction. Interestingly, abnormal behavior was observed when the experimental Co/Fe ratio (Co_exp/Fe_exp) ranged from 0.5 to 1. This deviated from the anticipated trend. When the Co_exp/Fe_exp ratio reached 0.5, the findings supported prior research indicating the substantial influence of the Co²⁺ ions on the formation of the spinel (Co)Fe₃O₄ form. Additionally, a high Fe content encouraged the development of bulk Fe(OH)₃ particles, resulting in the aggregation of the α-Fe₂O₃ phase (Fig. 1A and S8†). The optimal experimental ratio of Co to Fe precursors (Co_exp/Fe_exp) was 0.5, resulting in an actual Fe : Co ratio of 2.75 : 0.25. At this ratio, XRD analysis confirmed the successful incorporation of Co into Fe₃O₄, as expected (Fig. S8A,† shown as a 2 : 1 ratio). However, when the Co_exp/Fe_exp ratio was reduced to 0.2 (corresponding to an actual Fe : Co ratio of 2.56 : 0.44), α-Fe₂O₃ was also observed in the XRD pattern (Fig. S8A,† shown as 2.5 : 0.5). This suggested that a low Co content enables the fast conversion of Fe(OH)_x into α-Fe₂O₃, whereas a high Co content favors the formation of spinel (Co)Fe₃O₄, albeit at a relatively low rate. In contrast, when the Co ratio exceeded 1, it fostered the autonomous formation of Co₃O₄, as evidenced by the XRD pattern at a ratio of 1.5 : 1.5 (Fig. S8†). Without the Fe ions (using only Co ions), small quantities of Co₃O₄ were formed, as evidenced by a weak and slightly diffracted peak at 36.7° (311) in the XRD pattern (Fig. S8†), which was not prominent on the NRGO in the HR-TEM images (Fig S9†).

This is likely due to the limited oxidation of Co²⁺ ions in our solvothermal system. For the (Co)Fe₃O₄–NRGO samples, the slight shift of the (311) diffraction peaks toward lower angles supported the substitution of large Co²⁺ ions (radius = 0.69 Å) compared with Fe³⁺ with high spin (radius = 0.60 Å) in the spinel structure. Upon examining the reaction time dependence, it was observed that the Co content peaked at 6 h in the solvothermal reaction. This implied that the contribution of Co gradually increased during synthesis, reaching its peak at 6 h. Beyond this point, it was believed that the contribution of the Fe ions became increasingly prominent as Prussian blue formed, resulting from the decomposition of the solvent formamide and its reaction with the Fe ions.

Additionally, XPS analysis (Fig. S6†) reveals a 0.3 eV positive shift in the Fe³⁺ peak, from 711.2 eV in FeO_x–NRGO(900) to 711.5 eV in (Co)Fe₃O₄–NRGO(900). This suggests modifications in Fe³⁺ within the Fe oxide particles of (Co)Fe₃O₄–NRGO(900). This positive shift of Fe³⁺ upon doping with other transition metals aligns with previous studies, as seen in the Cr-doped Fe₃O₄ procedure published by Chen and co-workers.³⁰ This claim is further supported by the XRD pattern in Fig. S8B,† which suggests the substitution of some Fe³⁺ ions with Co²⁺ ions due to their similar ionic radii.^4,9 Therefore, the Co incorporation gave better ORR performance than FeO_x–NRGO(900).

ORR activity of (Co)Fe₃O₄–NRGO(900)

The ORR activity of (Co)Fe₃O₄–NRGO(900) was investigated using cyclic voltammetry (CV), linear sweep voltammetry (LSV), and chronoamperometry with a rotating disk electrode (Fig. 5, S10 and 11†). Identical quantities of the samples were deposited onto glassy carbon electrodes for the electrochemical experiments. The CV measurements in O₂-saturated and N₂-saturated 0.1 M KOH electrolytes revealed the electrocatalytic activity for the ORR (Fig. 5A and S10A†). In contrast, (Co)Fe₃O₄–NRGO(900) exhibited a more positive and well-defined ORR peak at 0.87 V vs. RHE, indicating a higher half-cell potential and potentially superior ORR activity compared with those of FeO_x–NRGO(900) and its counterparts incorporating (Mn, Ni, and Zn)FeO_x–NRGO(900) (Fig. 5A and S10A†). The relatively broad and less intense ORR peak observed for FeO_x–NRGO(900) at 0.80 V vs. RHE can be attributed to the low conductivity of the α-Fe₂O₃ phase (Fig. 1A and B). Fig. 5B and S10B† display the ORR polarization curves obtained by subtracting the N₂-saturated background from the corresponding CV curves. The onset potential and half-wave potential are crucial indicators of the ORR catalytic activity (Table S1†). As expected, (Co)Fe₃O₄–NRGO(900) exhibited a more positive onset potential (0.95 V vs. RHE) and half-wave potential (0.86 V vs. RHE) than FeO_x–NRGO(900), other metal-incorporated FeO_x–NRGO (Mn, Ni, and Zn), and bare NRGO(900). For (Co)Fe₃O₄–NRGO(900), a region characterized by mixed kinetic and diffusion control was observed between 1.0 and 0.7 V versus RHE, followed by a region mainly limited by oxygen transport, as evidenced by the increase in current with higher electrode rotation rates (Fig. S11†). Conversely, FeO_x–NRGO(900) displayed significantly low ORR activity due to the presence of ORR inactive phases, likely including α-Fe₂O₃ (Fig. 5A–D and S2D–F†). In comparison, commercial Pt/C exhibited a mixed kinetic and diffusion-controlled region at higher electrode rotation rates (Fig. S12†).

Fig. 5D presents bar charts comparing the ORR activity, as determined from the LSV curves (Fig. 5B and S10B†), of (Co)Fe₃O₄–NRGO(900) on a glassy carbon electrode to those of bare NRGO(900), FeO_x–NRGO(900), other metal-incorporated FeO_x–NRGO catalysts (Mn, Ni, and Zn), and commercial Pt/C (20 wt% Pt). The specific metrics evaluated included current density at 0.75 V vs. RHE and potential at a current density of 2.5 mA cm⁻² (Co)Fe₃O₄–NRGO(900) exhibited a potential of 0.87 V vs. RHE at 2.5 mA cm⁻² and a current density of 5.01 mA cm⁻² at 0.75 V vs. RHE. These values were comparable to the 0.88 V potential and 5.16 mA cm⁻² current density observed for commercial Pt/C, demonstrating comparable ORR performance (Fig. 5D).

Beyond Co, this work investigated the influence of other dopant elements (Mn, Ni, and Zn) on ORR activity in our intermediate control approach. While the SEM images shown in Fig. S13B† confirm the formation of well-crystallized particles on NRGO for (Mn)FeO_x–NRGO(900), its ORR performance deviates from that of (Co)Fe₃O₄–NRGO(900). (Mn)FeO_x–NRGO(900) exhibited a potential of 0.84 V vs. RHE at 2.5 mA cm⁻², comparable to that of (Co)Fe₃O₄–NRGO(900) (0.87 V vs. RHE). However, Mn doping offered a less pronounced enhancement in ORR activity compared with Co (Table S1†). Conversely, (Ni)FeO_x–NRGO(900) displayed inferior ORR activity compared with bare FeO_x–NRGO (Fig. 5A and Table S1†). This suggested minimal to no positive effect of Ni incorporation within the Fe(OH)_x intermediate system. Finally, (Zn)FeO_x–NRGO(900), as shown in Fig. S13F,† exhibits bulk particle formation, rendering it inactive for ORR catalysis. These observations collectively highlight the crucial role of Co incorporation within our intermediate control system for optimizing ORR performance.

FeCo(OH)_x–NRGO exhibited a lower ORR potential (0.68 V vs. RHE) than (Co)Fe₃O₄–NRGO(900) (0.87 V vs. RHE) at 2.5 mA cm⁻², likely because of the inherently low electrical conductivity of the hydroxide phase relative to the (Co)Fe₃O₄ nanocrystals dispersed on the NRGO support (Table S1†). The absence of the Fe₃O₄ phase in FeCo(OH)_x–NRGO, as confirmed by TEM images (Fig. 2A–C) and TGA analysis (Fig. 4A), directly translates to poor ORR activity, as evidenced by its high onset potential of 0.84 V vs. RHE (Fig. S14 and Table S1†). This highlights the critical role of Fe₃O₄ in facilitating efficient oxygen reduction. As shown in Table S1,† these differences are reflected in the significantly higher onset and half-wave potentials observed for FeCo(OH)_x–NRGO compared to those of (Co)Fe₃O₄–NRGO(900). To further understand the reaction kinetics, Tafel slope analysis was employed for the ORR process. The Tafel slope is measured in the kinetic-controlled region in the range between 1.0 V and 0.8 V. A low Tafel slope signifies fast electron transfer kinetics and high intrinsic activity. As expected, (Co)Fe₃O₄–NRGO(900) exhibited the lowest Tafel slope (−59 mV dec⁻¹) compared with Pt/C (−81 mV dec⁻¹), FeO_x–NRGO(900) (−103 mV dec⁻¹), (Mn)FeO_x–NRGO(900) (−83 mV dec⁻¹), (Ni)FeO_x–NRGO(900) (−82 mV dec⁻¹), (Zn)FeO_x–NRGO(900) (−160 mV dec⁻¹) and NRGO(900) (−114 mV dec⁻¹). This indicated that (Co)Fe₃O₄–NRGO(900) enabled the fastest ORR. This Tafel slope analysis, coupled with the positive onset potential of (Co)Fe₃O₄–NRGO(900), suggested its exceptional potential as a high-performance ORR electrocatalyst (Fig. 5C and S10C†).

Fig. 4 (A) Comparison of the expected atomic ratios of Fe and Co, derived from the reactant precursors, with the actual atomic ratios (shown in square brackets) determined by ICP-AES. (B) Actual atomic ratios obtained at different solvothermal reaction times for a Co_exp/Fe_exp feed ratio of 0.5.

Fig. 5 ORR polarization curves and kinetics of the electrochemical catalysis. (A) CV, (B) LSV curves, and (C) Tafel plots of GC electrodes modified with NRGO(900), FeO_x–NRGO(900) and (Co)Fe₃O₄–NRGO (Mn)FeO_x–NRGO(900), (Ni)FeO_x–NRGO(900), (Zn)FeO_x–NRGO(900), and commercial Pt/C. The ORR kinetics of samples with NRGO(900), FeO_x–NRGO(900), (Co)Fe₃O₄–NRGO and Pt/C are shown in (A)–(C). The CV curves were recorded in 60 mL of 0.1 M KOH electrolyte solution under O₂-saturated conditions (solid line) and N₂-saturated conditions (dashed line) at a scan rate of 25 mV s⁻¹. The LSV curves were recorded in the same electrolyte solution at a scan rate of 5 mV s⁻¹ with a rotation rate of 1600 rpm. (D) Bar chart depicting the ORR kinetic current density at 0.75 V vs. RHE and the required potentials for J = 2.5 mA cm⁻² for the samples.

Furthermore, our high-temperature calcination (above 900 °C) successfully produced electrochemically active (Co)Fe₃O₄ nanocrystals on the NRGO layer without significant aggregation or damage to the graphene layers (Fig. 1A, B and 2A–F). This method offers a valuable strategy for synthesizing Fe₃O₄ at high temperatures while suppressing the formation of less active α-Fe₂O₃ (Fig. S4†). Koutecky–Levich (K–L) plots were employed to evaluate the electron transfer number (n) for the ORR process on (Co)Fe₃O₄–NRGO(900) (Fig. S11†).^1,4,9 The electron transfer number of 3.70 for (Co)Fe₃O₄–NRGO(900) supports its exceptional ORR performance, suggesting a near-complete reduction of oxygen to water. Chronoamperometric measurements at 0.75 V vs. RHE (Fig. S10D†) revealed the outstanding stability of (Co)Fe₃O₄–NRGO(900). Compared with Pt/C (75% decrease), the (Co)Fe₃O₄–NRGO(900) hybrid exhibited a significantly lower current density decay (42%) over a prolonged 40 000 s test, demonstrating superior long-term stability. Fuel cells can utilize methanol as a fuel through both oxidation and reduction reactions. More importantly, methanol crossover, a major challenge in direct methanol fuel cells (DMFCs), can poison catalysts and hinder performance. Therefore, developing catalysts with high methanol tolerance is critical in DMFCs. We evaluated the methanol tolerance of (Co)Fe₃O₄–NRGO(900) and Pt/C by introducing 1 mL of methanol into an O₂-saturated electrolyte at 200 s (Fig. 6). Notably, the ORR current density of (Co)Fe₃O₄–NRGO(900) remained stable, indicating a negligible impact from methanol. Conversely, Pt/C exhibited a sharp decrease in current, highlighting its susceptibility. This superior methanol tolerance suggests that (Co)Fe₃O₄–NRGO(900) holds promise as a DMFC electrocatalyst. Additionally, to further evaluate the surface poisoning of the catalyst, ORR performance was measured after the addition of 10 mM KSCN to 0.1 M KOH (Fig. S15†). For (Co)Fe₃O₄–NRGO(900), the current density decreased by 13.4% at 0.7 V, while for FeO_x–NRGO(900), it decreased by 73.4% at the same potential. The half-wave potential (E_1/2) dropped by only 0.04 V for (Co)Fe₃O₄–NRGO(900) (from 0.86 to 0.82 V) but by a larger 0.21 V for FeO_x–NRGO(900) (from 0.91 to 0.70 V). This suggests that the Co ions are uniformly integrated into the Fe₃O₄ lattice, reducing SCN⁻ adsorption on exposed iron oxide nanoparticles.^31–33

Fig. 6 Chronoamperometric responses for the ORR at 0.75 V vs. RHE with a rotation rate of 200 rpm following the addition of 1 mL of methanol (99.9%) for Pt/C and (Co)Fe₃O₄–NRGO(900).

Our results demonstrate that (Co)Fe₃O₄–NRGO(900) exhibits competitive ORR performance and superior stability compared with the commercial Pt/C catalyst. This highlights the effectiveness of our approach, which utilizes the initial formation of nanosized FeCo(OH)_x–NRGO and its high-temperature transformation into crystalline, electrochemically active magnetite nanoparticles on NRGO. The high double-layer capacitance of (Co)Fe₃O₄–NRGO(900) likely stems from its large surface area, contributing to its excellent performance. Additionally, it requires low activation energy for charge transportation, further enhancing its practicality. These figures-of-merit compare favourably with those exhibited by Ni catalysts at various calcination temperatures, as well as Ni supported on reduced graphene oxide.³⁴ At 450 °C, the E_1/2 value reaches 0.800 V, which is comparable to that of our sample. Notably, unlike many other nanocomposite (N)rGO samples, our sample retains high ORR efficiency even at the critical temperature of 900 °C.

Conclusions

This approach involved the high-temperature calcination of nanosized FeCo(OH)₂ nanosheets deposited on NRGO. Both the (Co)Fe₃O₄ nanoparticles and the NRGO support retained excellent quality after high-temperature calcination (900 °C), exhibiting no signs of severe aggregation or breakdown, which is critical for optimal ORR electrocatalysis. Interestingly, TGA analysis revealed the presence of CoFe LDH nanosheets as the initial species deposited on NRGO. This transformation facilitated their efficient decomposition into well-dispersed Co(Fe₃O₄) nanoparticles at high temperatures. The presence of Co²⁺ in this system played a crucial role in the formation of the LDH nanosheet phase. It prevented the formation of less-active α-Fe₂O₃ by promoting the rapid oxidation of iron hydroxide precursors. Our approach offers a simple and effective method for decorating NRGO with electrochemically active magnetite nanoparticles. The intimate interaction between the Co-doped Fe₃O₄ nanocrystals and the porous NRGO support, likely due to covalent coupling, significantly enhanced the catalyst's ORR performance. This is evident from the low onset potential of 0.95 V vs. RHE, the high electron transfer number of 3.7, and superior stability compared with those of commercial Pt/C. This method, applicable to various layered metal hydroxide precursors, holds promise for the development of improved ORR catalysts supported on graphene-based materials.

Experimental

Detailed experimental procedures can be found in Section S1 of the ESI.†

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (RS-2024-00341027 and RS-2024-00343195) and the Research Fund-2023 of the Catholic University of Korea.

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Footnote

† Electronic supplementary information (ESI) available: XRD, photos, SEM, TEM, elemental maps, XPS data, ORR data, and K–L plots of (Co)Fe₃O₄–NRGO. See DOI: https://doi.org/10.1039/d4ta06684g

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