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Article

Anodic Catalytic Oxidation of Sulfamethoxazole: Efficiency and Mechanism on Co3O4 Nanowire Self-Assembled CoFe2O4 Nanosheet Heterojunction

by
Han Cui
1,
Qiwei Zhang
2,* and
Shan Qiu
3,*
1
Engineering Research Center of Low-Carbon Treatment and Green Development of Polluted Water in Northeast China, Ministry of Education, Northeast Normal University, Changchun 130117, China
2
WSGRI Engineering & Surveying Incorporation Limited, Wuhan 430080, China
3
State Key Laboratory of Urban Water Resource & Environment, Harbin Institute of Technology, Harbin 150001, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(9), 854; https://doi.org/10.3390/catal15090854
Submission received: 21 July 2025 / Revised: 26 August 2025 / Accepted: 1 September 2025 / Published: 4 September 2025
(This article belongs to the Section Electrocatalysis)
Browse Figures
Versions Notes

Abstract

By modulating the mass ratio of hydrothermal agents to cobalt/iron precursors, Co3O4 nanowires were successfully integrated into spinel-type Co/Fe@NF, forming a heterojunction anode for alkaline water electrolysis (AWE) hydrogen production. This Co3O4 nanowire-assembled CoFe2O4 nanosheet anode (Co/Fe(5:1)@NF) exhibits exceptional electrochemical oxygen evolution reaction (OER) performance, requiring only 221 mV overpotential to achieve 10 mA cm−2. Sulfamethoxazole (SMX) was employed as a model pollutant to investigate the anode sacrificial material; it achieved approximately 95% SMX degradation efficiency, reducing the OER potential of 50 mV/10 mA cm−2. SMX oxidation coupled with Co/Fe heterojunction structure partially substitutes the OER. Co/Fe heterojunction generates an internal magnetic field, which induces the formation of novel active species within the system. ·O2− is the newly formed active oxygen species, which enhanced the proportion of indirect SMX oxidation. Quantitative analysis reveals that superoxide radical-mediated indirect oxidation of SMX accounts for approximately 38.5%, Fe(VI) for 9.4%, other active species for 6.1%, and direct oxidation for 46.0%. The nanowire–nanosheet assembly stabilizes a high-spin configuration on the catalyst surface, redirecting oxygen intermediate pathways toward triplet oxygen (3O2) generation. Subsequent electron transfer from nanowire tips facilitates rapid 3O2 reduction, forming superoxide radicals (·O2−). This study effectively driven by indirect oxidation, with cathodic hydrogen production, providing a novel strategy for utilizing renewable electricity and reducing OER while offering insights into the design of Co/Fe-based catalyst.

Graphical Abstract

1. Introduction

Hydrogen energy is regarded as a key carrier in the global transition to sustainable energy, and alkaline water electrolysis (AWE) has emerged as one of the most promising technologies for hydrogen production due to its mature application [1,2]. However, the AWE system is still hindered by the anodic oxygen evolution reaction (OER). The effective strategies to enhance the AWE system involve coupling the electrochemical oxidation of anodic sacrificial agent and modifying the electrode surface [3,4,5], which requires lower theoretical potentials than OER. Commonly used small molecules as reactants include urea, hydrazine, ethanol [6], and renewable alcohols [7,8], certain organic pollutants can be utilized to enhance the AWE system. However, macromolecular pollutants were rarely utilized in this field. The Co3O4 nanowire-on-nickel-foam (NF) anode (Co3O4-NW@NF) exploits its high-aspect-ratio nanotips to generate intense local electric fields and interfacial Joule heating within the diffusion layer; these nanotip-enhanced fields increase the local concentration of OH− ions and elevate the interfacial temperature [9]. Complementarily, the Fe2O3/NiFe2O4@NF anode couples anodic oxidation of SMX with in situ electro-generation of Fe(VI), which acts as a powerful oxidant for indirect oxidation of SMX. The Co/Fe anodes are designed to replace the sluggish oxygen evolution reaction with more energy-efficient anodic processes—either accelerated OER or pollutant oxidation [10].
Sulfamethoxazole (SMX), a typical sulfonamide antibiotic (SAs), is widely used in the treatment of animal and plant diseases [11], leading to its global detection in natural environments. Specifically, SMX could be easily oxidized and identified as an optimal anodic sacrificial agent to enhance the AWE system. However, SMX removal primarily occurs through direct oxidation, which may lead to some corrosion of the electrode surface in the AWE system and decrease the lifespan of the electrode [12]. The appropriate method to modify the electrode should be considered. Current promising strategies include tuning the electronic structures [13], enhancing the gas diffusion behavior [14] and fabricating nanostructured catalysts [15]. It has been demonstrated that the incorporation of transition metal elements into the catalyst can effectively modulate the electronic properties of the electrode surface [16]. Furthermore, ultrathin nanosheets contribute to the formation of highly efficient ion transport pathways, whereas nanowire-based structures are capable of locally amplifying the electric and thermal fields through the lightning rod effect [10]. Collectively, these three strategies substantially improve both mass and charge transfer processes occurring at the electrode surface and across the electrode-electrolyte interface. It is worth noticing that altering the anode structure can modify the oxidation pathway of the sacrificial agent [11]. Non-Faradaic reactions taking place on the electrode surface typically generate hydroxyl radicals (·OH). Moreover, the Fe present in the electrode can also be transformed into hexavalent iron (Fe(VI)) under alkaline condition [17,18,19]. These two substances will engage in an oxidation-reduction reaction on the electrode surface to degrade pollutants. In this research, the nanowire arrays onto the nanosheets were anchored via hydrothermal reaction by adjusting the proportion of ammonium fluoride and harness the directional crystallization of fluoride ions. Electrons accumulate at nanowire tips and may transfer to downward-generated oxygen through tip discharge, forming superoxide radicals (·O2−) that degrade pollutants. As a result, the conventional OER is partially replaced by the oxidation reaction of SMX macromolecules and the electrode is also protected, which finally helps reduce the overall system overpotential. Furthermore, the incorporation of Fe can modulate the spin state of the electrode, alter the reaction pathway of oxygen intermediates, enhance the likelihood of generating ·O2−, facilitate the indirect oxidation of SMX to become the dominant pathway, and prolong the service life of the system [18].
Therefore, this study aims on (1) modify the Co/Fe ratio to synthesize Co3O4 nanowire-modified CoFe2O4 nanosheets based on NF; (2) investigating the feasibility of employing organic pollutants as anodic sacrificial agents via SMX decomposition; and (3) evaluate the possible route of generation of Reactive Oxygen Species (ROS).

2. Results and Discussion

2.1. Physical Characterization

SEM was employed to examine the morphologies of the samples. As shown in Figure 1a–f, a noticeable morphological difference was observed among the electrodes with varying Co contents. As depicted in Figure S1, only Co3O4 nanosheets were uniformly grown on the NF without Co/Fe ratio doping. Figure 1a,b shows that when the Co/Fe ratio was lower than 1:1, the surface of the sample still consisted of nanosheets, with only slight variations in their structure. Upon reaching a Co/Fe ratio of 1:1 (Figure 1c), a low density of nanowires emerged. The density of nanowires increased with the rising Co content, and at a Co-to-Fe ratio of 5:1, the density of nanowires was observed (Figure 1e,f). Therefore, the SEM results confirmed that adjusting the Co-to-Fe ratio effectively led to the formation of structures that anchored nanowires onto nanosheets. It is speculated that Co doping contributed to the generation of nanowires. EDS results (Figure 1g) demonstrated an even distribution of Co, Fe, and O elements.
X-ray diffraction (XRD) was then conducted to further identify the phases of the electrocatalyst. The XRD patterns of CoFe-based@NF with varying Fe to Co ratios exhibited similar diffraction peaks. The prominent peaks at approximately 18.24°, 30.06°, 35.45°, 37.28°, 43.47°, 53.89°, and 57.17° correspond to the (111), (220), (311), (222), (400), (511), and (440) planes, respectively, and match well with the typical diffraction pattern of CoxFe3−xO4 [20,21]. Notably, all the diffraction peaks shifted slightly towards higher angles, likely due to the incorporation of Co atoms, which have a smaller radius, into the original CoxFe3−xO4 lattice [21]. This positive shift indicates that the electrocatalyst’s microstructure underwent lattice contraction [22,23]. Lattice contraction reduces the reaction energy barriers of the rate-determining step while increasing the energy barrier for lattice oxygen oxidation, thereby enhancing both the activity and stability of the catalyst during the OER process [24]. Furthermore, it was observed that the (111) plane appeared in all samples with different Co/Fe ratios, and it was the most obvious when the Co/Fe ratio reached 5:1 compared with other samples. This could be attributed to the replacement of a limited amount of Fe with Co, while any excess Co accumulated in the form of Co3O4 around the grain boundaries.
TEM was employed to examine the exposed facets for further confirmation of the hypothesis. As shown in Figure 1i,j, the primary lattice fringe observed at 0.253 nm corresponds to the (311) facet of CoFe2O4, while the lattice fringe on the nanowires at 0.486 nm is attributed to the (111) facet of Co3O4. Additionally, the junction between these two structures also reveals the (111) facet. These observations indicate that the doping of NWs induces the growth of the (111) facet, which is consistent with the XRD results. The adjacent bright spots near the center correspond to the (1-11) and (220) crystal planes (JCPDS 00-003-0864; a = b = c = 8.377 Å, α = β = γ = 90°, space group Fd-3m) (Figure S2a). The (311) crystal plane is also identified, with angles of 58.5° and 31.5° relative to (1-11) and (220), respectively, matching theoretical calculations. The zone axis of the sample is determined to be [1-1-2]. Figure S2b confirms similar results using JCPDS 00-042-1467 with the same lattice parameters and space group.
XPS was further conducted to investigate the changes in valence states and active species. In the high-resolution Co 2p XPS spectrum, peaks at approximately 782 and 794 eV are attributed to Co3+ species, while peaks around 784 and 797 eV correspond to Co2+ (Figure 2a). In the Fe 2p XPS spectra, the peaks at 712 and 724 eV correspond to the 2p3/2 and 2p1/2 of Fe2+, respectively, while the peaks at 714 and 727 eV are attributed to the 2p3/2 and 2p1/2 of Fe3+, respectively. The positive shift of approximately 5 eV and the increased content of Fe3+ confirm the partial oxidation of Fe2+ to Fe3+ (Figure 2b) [25]. Previous research revealed that standard peak of 2p3/2 of Co was 785.9 eV and 2p3/2 of Fe was 710.7 eV in CoFe2O4, respectively [26,27].
The incorporation of Co 2p leads to a positive shift in the Fe 2p spectra, as Co can modulate the intrinsic properties of the catalysts, thereby altering the chemical environment of the Fe sites. Meanwhile, the main peak of Co 2p also exhibits a positive shift, indicating an enhanced interaction between Fe and Co. Additionally, the valence states of both Fe and Co are elevated to some extent, resulting in the formation of more oxyhydroxides. The high-valence Fe and Co species are the active species during electrocatalysis [23,28].
The O 1s spectra in Figure 2c were deconvoluted into three peaks. The peaks located at approximately 529.9, 531.3, and 533.4 eV correspond to lattice oxygen, oxygen vacancies, and chemisorbed oxygen, relatively. As the proportion of Co increases, the density of oxygen vacancies also increases [29]. This is consistent with our previous conclusion that the introduction of nanowires enhances the ratio of oxygen vacancies on the electrocatalyst surface, thereby improving its electrocatalytic performance [9]. Similarly, the main peak of O 1s shows a slight positive shift, which is attributed to the metal-oxygen interaction, both of which contribute to the formation of metal oxides/hydroxides. In summary, with the increasing proportion of Co, the share of high-valence Fe and Co species, as well as the proportion of oxygen vacancies, increases, all of which positively affect the catalytic performance of the electrocatalysts.

2.2. Electrochemical Characterization

To evaluate the OER activity of electrodes with varying Fe/Co ratios, a series of electrochemical tests were performed. As shown in Figure 3a, the CV curve of Co/Fe(5:1)@NF exhibits the largest area, which suggests it may demonstrate superior catalytic activity. The unique structure of Co3O4 modified Co/Fe nanosheet formed bear-spinel structure which provides a suitable situation for electron transfer and anodic oxidation. The high electrochemical activity then performed redox peaks in CV curves. The other kinds of anodes without spinel structure could not perform high electrochemical activity and as a result the CV curve showed little differences.
Figure 3b further demonstrates that Co/Fe(5:1)@NF exhibits the lowest overpotential (275.5 mV) to reach a current density of 10 mA cm−2, consistent with the CV results. Additionally, it shows a smaller Tafel slope (91.3 mV dec−1) compared to Co/Fe(1:5)@NF (133.6 mV dec−1), Co/Fe(1:2)@NF (128.4 mV dec−1), Co/Fe(1:1)@NF (141.0 mV dec−1), and Co/Fe(2:1)@NF (164.5 mV dec−1), signifying faster reaction kinetics on the CoFe2O4-Co3O4/NF surface (Figure 3c). Electrochemical impedance spectroscopy (EIS) was performed to further investigate the charge transfer and electrode kinetics. The unique architecture of nanowires anchored on nanosheets likely contributes to the enlarged active surface area, resulting in faster charge transfer and mass transport (Figure S3). Furthermore, Co/Fe(5:1)@NF exhibited 0.05 V reduction in the OER potential in 1 M KOH in the presence and absence of SMX (Figure 3d).
The superior catalytic activity can be attributed to the following factors. The increased Co content altered the morphology, which gradually transitioned from nanosheets to a nanowire-assembled nanosheet structure, thereby enhancing the specific surface area of the electrode. Additionally, the nanowires facilitated mass and charge transfer on the electrode surface by enhancing the local electric and temperature fields [8]. Previous research indicated that the unique structure of electrode could elicit electric and temperature fields [10].
Furthermore, the elevated Co content suppressed the formation of Fe2O3, which further enhanced the catalytic activity of the electrocatalyst [29,30].

2.3. SMX Degradation via Anodic Oxidation

Assembling Co3O4-NSWs on CoFe2O4 (5:1@NF) enhances the inherent catalytic activity of CoFe2O4. The next question we addressed was whether this strategy could also influence the pollutant removal efficiency. To investigate this, a series of pollutant removal experiments were conducted on anodes with different Co/Fe ratios. As shown in Figure 4a, Co/Fe(5:1)@NF exhibited the highest removal efficiency of 95%, outperforming the other samples. With the increasing percentage of Fe, the removal efficiency decreased from 93.3% to 78.2% (Figure 4b). The respective scavengers test was proposed to clarify the contribution of different radicals to SMX removal (Figure 4c). When Butyl alcohol (TBA) was added to the system, the SMX removal efficiency reached 88.1%, indicating that the amount of ·OH was negligible. The result is consistent with our previous report [10].Additionally, the doping of Furfuryl alcohol (FFA) did not decrease SMX removal, further supporting the minimal role of ·OH in this system. Based on our prior conclusion, Fe(VI) exists in the electrolyte [31,32], and (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) ABTS was used to capture Fe(VI) [17,33]. The addition of ABTS reduced SMX removal to 75.8%. Moreover, when pBQ (1,4-Benzoquinone) (the specific scavenger for ·O2−) was added, SMX removal dropped to 58.4%. To further confirm the generation of ·O2− and the relationship between morphologies and ·O2−, EPR was performed. As indicated by the EPR results (Figure 4d), the production of superoxide radicals in heterojunction is the highest, likely due to two factors. First, the generation pathway of superoxide radicals has been altered. In addition to the direct electron acquisition by oxygen molecules, triplet oxygen can also be converted into superoxide radicals. Second, the radical oxidation reaction that transforms superoxide radicals into singlet oxygen can occur in the bulk solution phase. This process not only prevents catalyst corrosion but also significantly enhances the structural stability of the catalyst [16]. It is worth noting that the ·O2− spin concentration was found 3.518 × 1011 spins/mm3 in Co/Fe(5:1)@NF compared with Co/Fe(2:1)@NF (5.361 × 1011 spins/mm3) and Co/Fe(1:5)@NF (6.068 × 1012 spins/mm3) (Table 1). In conclusion, these results indicate that Fe(VI) and ·O2−, particularly ·O2−, are the main active species involved in the indirect oxidation of SMX. The correlation between EPR and Ov in XPS revealed negative. As increasing percentage of Co, the ·O2− spin concentration increased combined with decreasing peak height of Ov in XPS.

2.4. Mechanism and Kinetics of Generation of ·O2−

The dynamic kinetic simulations revealed that SMX degradation followed a first-order reaction, with Co/Fe(5:1)@NF showing a larger reaction rate constant (k) of 0.0122 min−1 (Figure 5a), indicating its superior ability to remove SMX. The other electrodes with different morphologies exhibited reaction rate constant (k) between 0.0062 and 0.0107. This finding strongly suggests that the Co/Fe(5:1)@NF electrode not only possesses excellent catalytic activity but also demonstrates remarkable capabilities in oxidatively degrading pollutants. The underlying cause of the excellence performance of Co/Fe(5:1)@NF in SMX removal was due to both direct oxidation on the electrode surface and indirect oxidation by active species contribute to SMX degradation [10,34]. Therefore, the relationship between active species and morphologies was explored. The active species masking experiments were carried out in all electrodes serving as the anode. The oxidation pathway of SMX was predominantly direct oxidation and as the ratio of Co increased in anode, the unique structure of nanowire on nanosheet decreased the percentage of direct oxidation (Table 2). This could be attributed to the fact that the addition of Co suppressed the formation of Fe2O3. Furthermore, Co/Fe(5:1)@NF serving as anode performed 38.53% with ·O2− oxidation and 9.37% with Fe(VI) oxidation identified as indirect oxidation path comparing with the direct oxidation path decreased to 45.99% (Table 2). The XTT (2,3-Bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide) sodium salt assay results demonstrated a progressive increase in ultraviolet spectrophotometric values with prolonged reaction time (Figure 5b), thereby providing further evidence for the generation and accumulation of superoxide anion radicals (·O2−) within the system. It is worth noticing that the active species within the system also demonstrate a morphology-dependent characteristic.
·O2− serves as one of the primary free radicals in the indirect oxidation process, demonstrating a stronger oxidation capability than Fe(VI). Unlike direct oxidation at the electrode surface, indirect oxidation plays a predominant role in the degradation of SMX. These two modifications were explored to enhance the overall efficiency of this system. Firstly, the redox potential of ·O2−/O2, (0.56 V vs. SCE) is significantly lower than that of Fe(VI)/Fe(IV) (0.76 V vs. SCE), indicating that Fe(VI) has a greater ability to oxidize pollutants than ·O2− [35,36]. When Co/Fe(5:1)@NF was used as the anode, the yield of ·O2− was maximized, suggesting a correlation between ·O2− generation and the electrode morphology.
Furthermore, as indirect oxidation predominates, less SMX is directly degraded on the electrode surface, resulting in a reduction in transfer resistance at the electrolyte-electrode interface. Meanwhile, the lower production of Fe(VI) mitigates electrode surface erosion. The measured ratios of direct to indirect oxidation show that the introduction of nanowires significantly suppresses anodic corrosion and iron element precipitation (Figure 4d). Additionally, the superoxide radicals generated further compensate for the indirect efficiency of SMX oxidation and degradation (Figure 5c).

3. Materials and Methods

3.1. Reagents and Chemicals

Ferric nitrate (Fe(NO3)3·9H2O, 98.5%), urea (CO(NH2)2, 99%), cobalt nitrate (Co(NO3)2·6H2O, 98.5%), ammonium fluoride (NH4F, 96.0%) and potassium hydroxide (KOH, 99.0%) were all sourced from Aladdin Reagent (Shanghai) Co., Ltd. (Shanghai, China) Nickel foam, with a thickness of 5 mm, was supplied by Kunshan Longsheng Electronic Materials Co., Ltd. (Suzhou, China), while platinum sheet electrodes (Pt, 1 × 2 cm2, 99%) were obtained from Shanghai Fanyue Electronic Technology Co., Ltd., Shanghai, China. The radical quenchers including tert-Butyl alcohol (TBA), Furfuryl alcohol (FFA), 1,4-Benzoquinone (pBQ), XTT (2,3-Bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide) sodium salt, DMPO (5,5-Dimethyl-1-pyrroline-N-oxide) and Ethanol were all purchased from Aladdin Reagent (Shanghai) Co., Ltd. (Shanghai, China).

3.2. Synthesis of CoFe2O4@NF Electrodes in Different Ratios of Co/Fe

The CoFe2O4-NSW (Assembly of nanosheets from nanowires), CoFe2O4-NS (Nanosheets) and CoFe2O4-NW (Nanowires) loaded on NF (Nickel form) electrode were prepared Via a typical hydrothermal-annealing method. Initially, the NF (3 × 4 cm2) was pretreated with 6 M HCl for 15 min, followed by several ultrasonic washes with ethanol and ultrapure water. Next, 0.5 mmol of Co(NO3)2·6H2O, 2.5 mmol of Fe(NO3)3·9H2O, 10 mmol of urea (CO(NH2)2), and 8 mmol of NH4F were dissolved in 70 mL of deionized (DI) water with continuous stirring for 30 min. The solution was then transferred to a Teflon-lined stainless-steel autoclave, along with the cleaned NF substrate, and heated in an electric oven at 120 °C for 6 h. After cooling to room temperature, the precursor was removed from the autoclave, washed several times with DI water and ethanol, and dried at 60 °C for 6 h. Subsequently, the NF substrate was annealed in air at 400 °C for 2 h. The preparation of different Co/Fe ratios which loaded on NF follows the same procedure as CoFe2O4@NF, with the exception of the varying molar ratios during the hydrothermal process. The resulting samples were labeled as Co3O4-CoFe2O4@NF (Co/Fe(5:1)@NF), CoFe2O4@NF (Co/Fe(2:1)@NF), Co1.5Fe1.5O4@NF (Co/Fe(1:1)@NF), Co2FeO4@NF (Co/Fe(1:2)@NF), and Co0.5Fe2.5O4@NF (Co/Fe(1:5)@NF).

3.3. Characterizations

X-ray diffraction (XRD) a Cu Kα radiation source (λ = 0.1541 nm, 40 kV) on a Rigaku Ultima IV (Tokyo, Japan). Scanning electron microscopy (SEM) was performed on a ZEISS Sigma500, equipped with an energy dispersive X-ray spectroscopy (EDS) system. Transmission electron microscopy (TEM) was conducted on a FEI Tecnai G2 F30 with an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed using an Al Kα radiation source on a Thermo Scientific K-Alpha. Metal dissolution from the electrode surface in solution was analyzed by inductively coupled plasma-optical emission spectrometer (ICP-OES) on an Agilent 5110. The magnetic properties were determined using a vibrating sample magnetometer (VSM, Lakeshore, 7404, Westerville, OH, USA). The concentration variation of ·O2− during the reaction was determined by analyzing the changing trend of the spectrophotometer readings at a wavelength of λ = 470 nm by using the XTT probe [37]. An electron paramagnetic resonance (EPR) spectrometer was utilized to measure the ·O2− only at a specific moment. The instrumental settings were as follows: microwave frequency, 9.72 GHz; microwave power, 19.57 mW; central magnetic field, 3470 G; sweep width, 100 G; sweep time, 20 s; modulation frequency, 100 kHz. A 30 μL sample was mixed with 30 μL of DMPO (0.1 mol L−1 in methanol). After thorough mixing, a small volume was loaded into a capillary tube, which was then inserted into a quartz tube and placed in the EPR spectrometer cavity for superoxide radical measurement. To investigate the involvement of ROS in the degradation of pollutants through the quenching approach, six widely utilized ROS scavengers (TBA, FFA, ABTS and pBQ) were introduced into the anodic oxidation treatment [38].

3.4. Electrochemical Measurements

Electrochemical measurements were conducted using a CHI 760e electrochemical workstation (Chenhua Co., Shanghai, China) with a typical three-electrode setup in a 1.0 M KOH electrolyte. A saturated calomel electrode (SCE) and Pt sheet were used as the reference and counter electrodes, respectively, while either bare NF or the prepared electrodes with dimensions of 1 × 2 cm2 served as the working electrode (Figure 6). When the applied voltage equals the standard electrode potential of the oxygen evolution reaction (OER, 1.23 V), the water electrolysis reaction will commence. Nevertheless, in an actual electrochemical reaction system, kinetic barriers exist. As a result, the actual voltage required is higher than the theoretical voltage. At this time, the potential difference between the applied voltage and the standard voltage is defined as the overpotential (η). The overpotentials of the OER can be derived from the linear sweep voltammetry (LSV) curve. Typically, the overpotential at a current density of 10 mA cm−2 is chosen to evaluate the performance of catalysts.
The parameters for cyclic voltammetry (CV), LSV, Tafel analysis, electrochemical impedance spectroscopy (EIS), and electrochemical active surface area (ECSA) are provided in Supplementary Information. The reaction rate constant (k) of the pollutant was calculated from its degradation curve based on Equation (1).
ln C / C 0 = − k t

4. Conclusions

In summary, by modulating the mass ratio of cobalt-to-iron precursors and utilizing the One-pot hydrothermal method, Co3O4 nanowires were successfully assembled onto CoFe2O4 nanosheets. The heterojunction structure demonstrates excellent electrochemical performance, with an OER overpotential of 221 mV at a current density of 10 mA/cm2. The newly constructed heterojunction anode generates superoxide radicals under an applied electric field. Singlet oxygen captures electrons and is converted into triplet oxygen atoms. Ultimately, superoxide radicals are formed. Along with the hydroxyl radicals formed at the anode originally, singlet oxygen, and hexavalent iron generated under alkaline conditions, they jointly contribute to the indirect oxidation efficiency of SMX compared with a non-heterojunction electrode. These radicals efficiently oxidize SMX in water, achieving a removal rate of 95%, while simultaneously reducing the OER potential by approximately 50 mV/10 mA cm−2. This study shows that stable, large organic molecules like SMX can act as sacrificial agents to lower the OER potential. Indirect oxidation of SMX reduces electrode corrosion and improves anode stability by reducing formation of Fe(VI). Furthermore, this approach provides new insights into using wastewater instead of fresh water in electrochemical catalysis applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15090854/s1, Figure S1: Co3O4 loaded on NF as anode; Figure S2: FFT images in TEM analysis with 2:1@NF (a) and 5:1@NF (b); Figure S3: EIS with different Co/Fe ratio anode serving in the system.

Author Contributions

Conceptualization, H.C. and Q.Z.; methodology, Q.Z.; data curation, H.C. and Q.Z.; writing—original draft preparation, H.C.; Writing—review and editing, S.Q.; funding acquisition, S.Q. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (QA202442), National Natural Science Foundation of China (No. 52300152), China Postdoctoral Science Foundation (No. 2024M750395), Project supported by the Science Foundation for Young Scholars of China Minmetals Corporation (Grant No. 2024QNJJB13), Hubei Provincial Special Program for Cultivating Young Sci-Tech Talents (2025DJA113) and supported by Engineering Research Center of Low-Carbon Treatment and Green Development of polluted Water in Northeast China, Ministry of Education.

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

Author Qiwei Zhang was employed by the company WSGRI Engineering & Surveying Incorporation Limited. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Catalysts 15 00854 g001
Figure 1. SEM of (a,b) Co/Fe(1:5)@NF and Co/Fe(1:2)@NF, (c,d) Co/Fe(1:1)@NF and Co/Fe(2:1)@NF, and (e,f) Co/Fe(5:1)@NF; (g) EDS mapping of Co/Fe(5:1)@NF; (h) XRD of different electrodes; (i,j) TEM of Co/Fe = 2:1@NF, and Co/Fe = 5:1@NF.
Figure 1. SEM of (a,b) Co/Fe(1:5)@NF and Co/Fe(1:2)@NF, (c,d) Co/Fe(1:1)@NF and Co/Fe(2:1)@NF, and (e,f) Co/Fe(5:1)@NF; (g) EDS mapping of Co/Fe(5:1)@NF; (h) XRD of different electrodes; (i,j) TEM of Co/Fe = 2:1@NF, and Co/Fe = 5:1@NF.
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Catalysts 15 00854 g002
Figure 2. XPS spectrum of (a) Co 2p, (b) Fe 2p and (c) O 1s of Co/Fe(5:1)@NF, Co/Fe(2:1)@NF, Co/Fe(1:1)@NF, Co/Fe(1:2)@NF and Co/Fe(1:5)@NF.
Figure 2. XPS spectrum of (a) Co 2p, (b) Fe 2p and (c) O 1s of Co/Fe(5:1)@NF, Co/Fe(2:1)@NF, Co/Fe(1:1)@NF, Co/Fe(1:2)@NF and Co/Fe(1:5)@NF.
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Catalysts 15 00854 g003
Figure 3. (a) CV, (b) LSV, and (c) Tafel of Co/Fe(5:1)@NF, Co/Fe(2:1)@NF, Co/Fe(1:1)@NF, Co/Fe(1:2)@NF and Co/Fe(1:5)@NF; (d) LSV on Co/Fe(5:1)@NF electrode in 1 M KOH in the presence and absence of SMX.
Figure 3. (a) CV, (b) LSV, and (c) Tafel of Co/Fe(5:1)@NF, Co/Fe(2:1)@NF, Co/Fe(1:1)@NF, Co/Fe(1:2)@NF and Co/Fe(1:5)@NF; (d) LSV on Co/Fe(5:1)@NF electrode in 1 M KOH in the presence and absence of SMX.
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Catalysts 15 00854 g004
Figure 4. (a) SMX degradation on different anodes, (b) Pseduo-first-order kinetic of SMX degradation on different anodes, (c) Effect of scavengers on SMX removal, (d) EPR spectrum of DMPO-·O2−.
Figure 4. (a) SMX degradation on different anodes, (b) Pseduo-first-order kinetic of SMX degradation on different anodes, (c) Effect of scavengers on SMX removal, (d) EPR spectrum of DMPO-·O2−.
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Catalysts 15 00854 g005
Figure 5. Pseduo-first-order kinetic (a) and XTT spectrophotometric values of SMX degradation on different anodes (b) and ·O2− Possible generation of path diagrams (c), symbol “*” related to Reactive oxygen terminal.
Figure 5. Pseduo-first-order kinetic (a) and XTT spectrophotometric values of SMX degradation on different anodes (b) and ·O2− Possible generation of path diagrams (c), symbol “*” related to Reactive oxygen terminal.
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Catalysts 15 00854 g006
Figure 6. Experimental setup diagram. The black side on the power supply is positive electrode and the red side is negative electrode.
Figure 6. Experimental setup diagram. The black side on the power supply is positive electrode and the red side is negative electrode.
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Table 1. Results of quantitative analysis of ·O2− by EPR.
Table 1. Results of quantitative analysis of ·O2− by EPR.
Electrode5:1@NF2:1@NF1:5@NF
·O2− spin concentration
(spins/mm3)		3.518 × 10115.361 × 10116.068 × 1012
Table 2. Direct versus indirect oxidation normalized treatments in SMX degradation rates at different morphology electrodes.
Table 2. Direct versus indirect oxidation normalized treatments in SMX degradation rates at different morphology electrodes.
ElectrodeDirect-Oxidize Percentage (%)Fe(VI) Percentage (%)·O2−
Percentage (%)		Other Radicals Percentage (%)
5:1@NF45.999.3738.536.11
2:1@NF66.2711.79Undetected21.94
1:1@NF76.2711.29Undetected12.44
1:5@NF79.8511.46Undetected8.69
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Cui, H.; Zhang, Q.; Qiu, S. Anodic Catalytic Oxidation of Sulfamethoxazole: Efficiency and Mechanism on Co3O4 Nanowire Self-Assembled CoFe2O4 Nanosheet Heterojunction. Catalysts 2025, 15, 854. https://doi.org/10.3390/catal15090854

AMA Style

Cui H, Zhang Q, Qiu S. Anodic Catalytic Oxidation of Sulfamethoxazole: Efficiency and Mechanism on Co3O4 Nanowire Self-Assembled CoFe2O4 Nanosheet Heterojunction. Catalysts. 2025; 15(9):854. https://doi.org/10.3390/catal15090854

Chicago/Turabian Style

Cui, Han, Qiwei Zhang, and Shan Qiu. 2025. "Anodic Catalytic Oxidation of Sulfamethoxazole: Efficiency and Mechanism on Co3O4 Nanowire Self-Assembled CoFe2O4 Nanosheet Heterojunction" Catalysts 15, no. 9: 854. https://doi.org/10.3390/catal15090854

APA Style

Cui, H., Zhang, Q., & Qiu, S. (2025). Anodic Catalytic Oxidation of Sulfamethoxazole: Efficiency and Mechanism on Co3O4 Nanowire Self-Assembled CoFe2O4 Nanosheet Heterojunction. Catalysts, 15(9), 854. https://doi.org/10.3390/catal15090854

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