Electrochemical Synergies of Heterostructured Fe2O3-MnO Catalyst for Oxygen Evolution Reaction in Alkaline Water Splitting

For efficient electrode development in an electrolysis system, Fe2O3, MnO, and heterojunction Fe2O3-MnO materials were synthesized via a simple sol–gel method. These particles were coated on a Ni-foam (NF) electrode, and the resulting material was used as an electrode to be used during an oxygen evolution reaction (OER). A 1000-cycle OER test in a KOH alkaline electrolyte indicated that the heterojunction Fe2O3-MnO/NF electrode exhibited the most stable and highest OER activity: it exhibited a low overvoltage (n) of 370 mV and a small Tafel slope of 66 mV/dec. X-ray photoelectron spectroscopy indicated that the excellent redox performance contributed to the synergy of Mn and Fe, which enhanced the OER performance of the Fe2O3-MnO/NF electrode. Furthermore, the effective redox reaction of Mn and Fe indicated that the structure maintained stability even under 1000 repeated OER cycles.


Introduction
Hydrogen has the highest energy density per unit mass and is known as a clean energy source that does not emit any exhaust gas except for water when used as a fuel. Although significant amounts of H 2 have been produced through the modification of fossil fuels [1], global warming and depletion of fossil fuels have recently led to attempts to obtain H 2 via new conversion methods in various countries around the world. In particular, the method of electrolyzing water to produce H 2 is an innovative and ecofriendly technology because it uses the cleanest and abundant resource on earth-water-as a raw material for H 2 [2]. The water electrolysis reaction, which is described by 2H 2 O ↔ H 2 + O 2 , consists of two half reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) [3]. HERs are one-electron reactions, and catalysts that exhibit excellent activity have been reported because of the relatively simple mechanism of the reaction: Pt electrodes with a high reduction potential are widely used [4], and in recent years, C-doped electrodes with excellent conductivity, such as graphene, have been introduced [5]. In contrast, the OERs are four-electron reactions, and the reaction mechanism is complex; many overvoltages are required in the O 2 -generating reaction [6]. Over the years, many researchers performed the experiments related to electrochemical oxygen evolution reactions (OER). Typically, noble metal oxides such as RuO 2 [7] and IrO 2 [8] have shown excellent catalytic activity and stability in both acidic and alkaline electrolytes. However, because of the scarcity and high cost of metals, there are limitations in usage for industrial

Synthesis of MnO, Fe 2 O 3 , and Fe 2 O 3 -MnO Materials
MnO and Fe 2 O 3 materials were prepared via the typical sol-gel method, as follows. For the synthesis of MnO, 4.0 g of manganese dichloride tetrahydrate (MnCl 2 ·4H 2 O, ≥97%, Sigma-Aldrich., MO, USA) was dissolved in 100 mL of distilled (DI) water with stirring for approximately 2 h. The resulting homogeneous mixture was heated at 70 • C for 4 h to evaporate the DI water solvent and obtain a brown precipitate. The resulting precipitate was washed several times with water and ethanol alternately and dried overnight at 70 • C. MnO crystalline catalysts were finally synthesized via sintering at 500 • C for 4 h in an Ar gas atmosphere to prevent the Mn from being oxidized into trivalent or tetravalent ions. Fe 2 O 3 was prepared via a similar procedure. First, 6.0 g of ferric chloride hexahydrate (FeCl 3 ·6H 2 O, ≥99%, Sigma-Aldrich., MO, USA) was dissolved in 100 mL of DI water. After approximately 2 h of stirring, ammonia water (NH 4 OH, ≥25% NH 3 in H 2 O, Sigma-Aldrich., MO, USA) was added dropwise until the pH of the solution was 8.0. The orange precipitate was washed several times with DI water and ethanol and dried overnight at 70 • C. The resulting powder was sintered at 500 • C for 4 h to produce an Fe 2 O 3 crystalline catalyst.
Finally, the heterojunction Fe 2 O 3 -MnO material was prepared via impregnation, as follows. The prepared crystalline Fe 2 O 3 and MnO were mixed with absolute ethanol (CH 3 CH 2 OH, ≥99.8%, Sigma-Aldrich., MO, USA) at a weight ratio of 1:1, followed by stirring for 2 h to obtain a homogeneous solution. Then, a red-brown-colored powder was obtained through evaporation at 70 • C. The obtained powder was heat-treated at 400 • C for 3 h in an Ar atmosphere to produce the heterojunction This study focused on identifying the synergistic effects of Fe and Mn. Although the manuscript did not include data, the OER reactions were already performed by varying the mass ratio of Fe 2 O 3 to MnO (Fe 2 O 3 :MnO = 1:2, 1:1 and 2:1). As a result, the catalyst with 1:1 mass ratio exhibited the higher catalytic activity. This is probably due to the difference in particle size or shape of Fe 2 O 3 and MnO, which may have disrupted the synergistic effect with other particles as more molecules aggregated among themselves. Therefore, when Fe 2 O 3 and MnO were mixed in a 1:1 ratio, it was determined that the synergistic effect was increased with the most uniform and stable heterojunction. In other words, in order to clearly compare the performance of bimetallic and monometallic materials, only one Fe 2 O 3 -MnO material mixed in a 1:1 ratio was presented in this study and the synergy effect was clarified.

Fabrication of MnO/NF, Fe 2 O 3 /NF, and Fe 2 O 3 -MnO/NF Electrodes
The electrodes for OER were prepared as follows in Scheme 1a: The surface was treated with 5.0% HCl for approximately 30 min to remove surface impurities on the substrate, i.e., NF (1.0 cm × 1.0 cm). The surface of the NF was then washed with DI water and absolute ethanol and dried at room temperature. The coating solution was prepared by mixing 30 mg of the synthesized catalyst powder (MnO, Fe 2 O 3 , and Fe 2 O 3 -MnO) with 600 µL of ultrapure water, 600 µL of absolute ethanol, and 60 µL of a 5.0 wt.% Nafion solution (Sigma-Aldrich., MO, USA), followed by dispersion for 3 h. The coating solution was evenly coated onto the surface-treated NF using a dip-coating method and then dried at 70 • C. Subsequently, MnO/NF, Fe 2 O 3 /NF, and Fe 2 O 3 -MnO/NF electrodes were prepared via heat treatment at 200 • C for 3 h.

Characterizations
The crystal structures of the MnO, Fe2O3, and Fe2O3-MnO electrodes were examined via X-ray diffraction (XRD) analysis (Miniflex, Rigaku, Tokyo, Japan) using Ni-kV, 15.0 mA) in the 2θ range of 20°-90°. The surface morphology of the particles was observed using a Hitachi S-4100 field-emission scanning electron microscope. Energy-dispersive X-ray spectroscopy (EDS) and EDS elemental mapping of the particles were performed using EDAX (EX-250, Horiba). Transmission electron microscopy (TEM) was performed using an H-7600 (Hitachi, Tokyo, Japan) instrument. X-ray photoelectron spectroscopy (XPS) was performed using an AXIS-NOVA (Kratos Inc., San Diego, USA) equipped with AlK (1486.6 eV) radiation.

Electrochemical Measurements
All electrochemical measurements were performed at room temperature (25 °C) using an electrochemical cell test system and IVIUMnSTAT (Ivium Technologies, The Netherlands). The electrochemical OER activity was tested in an alkaline electrolyte (1 M KOH, pH = 14) using a standard electrochemical device (Scheme 1b): MnO/NF, Fe2O3/NF, and Fe2O3-MnO/NF were used as the working electrode, and Hg/Hg2Cl2 (saturated calomel electrode, SCE) and Pt wire were used as the reference and counter electrodes, respectively. The measured potential was converted into the potential of a reversible hydrogen electrode (VRHE) using the following calibration equation [25].
For the initial stabilization of the electrode, 100 cycles of cyclic voltammetry (CV) scans (50.0 mV/s) were performed at approximately 1.0-1.7 V (vs. RHE). Linear sweep voltammetry (LSV) was performed at a scan rate of 5.0 mV/s in 1 M KOH. All electrochemical data was obtained with iR compensation by automatic current interrupt. Figure 1A,B shows the XRD pattern and TEM image of the powder particles before the electrode assembly. The XRD pattern of MnO is consistent with the cubic structure with the Fm-3 space group, and the peaks corresponding to the (111), (200), and (220) diffraction planes are observed at 2θ = 34°, 40°, and 58°, respectively (JCPDS card no. 01-072-1533) [26]. The XRD peaks of Fe2O3 correspond to Scheme 1. Electrode fabrication sequence (a) and electrochemical device image for oxygen evolution reaction (OER) test (b).

Characterizations
The crystal structures of the MnO, Fe 2 O 3 , and Fe 2 O 3 -MnO electrodes were examined via X-ray diffraction (XRD) analysis (Miniflex, Rigaku, Tokyo, Japan) using Ni-filtered CuKα radiation (30.0 kV, 15.0 mA) in the 2θ range of 20 • -90 • . The surface morphology of the particles was observed using a Hitachi S-4100 field-emission scanning electron microscope. Energy-dispersive X-ray spectroscopy (EDS) and EDS elemental mapping of the particles were performed using EDAX (EX-250, Horiba). Transmission electron microscopy (TEM) was performed using an H-7600 (Hitachi, Tokyo, Japan) instrument. X-ray photoelectron spectroscopy (XPS) was performed using an AXIS-NOVA (Kratos Inc., San Diego, USA) equipped with AlKα (1486.6 eV) radiation.

Electrochemical Measurements
All electrochemical measurements were performed at room temperature (25 • C) using an electrochemical cell test system and IVIUMnSTAT (Ivium Technologies, The Netherlands).
The electrochemical OER activity was tested in an alkaline electrolyte (1 M KOH, pH = 14) using a standard electrochemical device (Scheme 1b): MnO/NF, Fe 2 O 3 /NF, and Fe 2 O 3 -MnO/NF were used as the working electrode, and Hg/Hg 2 Cl 2 (saturated calomel electrode, SCE) and Pt wire were used as the reference and counter electrodes, respectively. The measured potential was converted into the potential of a reversible hydrogen electrode (V RHE ) using the following calibration equation [25].
For the initial stabilization of the electrode, 100 cycles of cyclic voltammetry (CV) scans (50.0 mV/s) were performed at approximately 1.0-1.7 V (vs. RHE). Linear sweep voltammetry (LSV) was performed at a scan rate of 5.0 mV/s in 1 M KOH. All electrochemical data was obtained with iR compensation by automatic current interrupt. Figure 1A,B shows the XRD pattern and TEM image of the powder particles before the electrode assembly. The XRD pattern of MnO is consistent with the cubic structure with the Fm-3 space group, and the peaks corresponding to the (111), (200), and (220) diffraction planes are observed at 2θ = 34 • , 40 • , and 58 • , respectively (JCPDS card no. 01-072-1533) [26]. The XRD peaks of Fe 2 O 3 correspond to the diffraction planes attributed to the rhombohedral structure of the R-3c spatial group and were observed at 2θ = 24 • (012), 33 • (104), 35 • (110), and 40 • (JCPDS card no. 01-089-0598) [27]. The heterojunction Fe 2 O 3 -MnO particles exhibited the crystal-structure patterns of both MnO and Fe 2 O 3 . Unexpectedly, the intensity of the XRD peaks was increased overall for the heterojunction Fe 2 O 3 -MnO particles. This may be due to the further growth of crystals during the sintering process for grafting the two types of pure crystals. The peak corresponding to Fe 2 O 3 was more distinct, suggesting that the Fe 2 O 3 crystals entirely covered the MnO crystals. This is confirmed by the TEM image in Figure 1B; the MnO sample exhibited a broad and thin sheet shape (approximately 400-500 nm thick), and in the Fe 2 O 3 sample, a thin cloth piece of approximately 100 nm was rounded to a 15 × 100 nm rod shape. Eventually, it appeared similar to a petal. In the heterojunction Fe 2 O 3 -MnO sample, a piece of Fe 2 O 3 covered the MnO sheet, which is consistent with the XRD results. the diffraction planes attributed to the rhombohedral structure of the R-3c spatial group and were observed at 2θ = 24° (012), 33° (104), 35° (110), and 40° (JCPDS card no. 01-089-0598) [27]. The heterojunction Fe2O3-MnO particles exhibited the crystal-structure patterns of both MnO and Fe2O3. Unexpectedly, the intensity of the XRD peaks was increased overall for the heterojunction Fe2O3-MnO particles. This may be due to the further growth of crystals during the sintering process for grafting the two types of pure crystals. The peak corresponding to Fe2O3 was more distinct, suggesting that the Fe2O3 crystals entirely covered the MnO crystals. This is confirmed by the TEM image in Figure 1B; the MnO sample exhibited a broad and thin sheet shape (approximately 400-500 nm thick), and in the Fe2O3 sample, a thin cloth piece of approximately 100 nm was rounded to a 15 × 100 nm rod shape. Eventually, it appeared similar to a petal. In the heterojunction Fe2O3-MnO sample, a piece of Fe2O3 covered the MnO sheet, which is consistent with the XRD results.  Figure 2A shows the results of EDS analysis of selected surfaces of the film-type electrodes assembled using the manufactured powders. According to the EDS analysis results, the atomic ratio  Figure 2A shows the results of EDS analysis of selected surfaces of the film-type electrodes assembled using the manufactured powders. According to the EDS analysis results, the atomic ratio of Mn:O on the surface of MnO was approximately 1.0:1.2, and the atomic ratio of Fe:O on the Fe 2 O 3 surface was approximately 2.0:3.3. This indicates that the metals were stoichiometrically contained in the particles and that the metal oxides were manufactured reliably. In the case of Fe 2 O 3 -MnO, the ratio of Mn:Fe:O was 1.2:1.0:5.5, although it was physically grafted. Figure 2B shows a scanning electron microscopy (SEM) image of the surface of the electrode coated with catalyst particles on the NF substrate. Regardless of the type of catalyst, the catalyst uniformly covered the NF. Compared with the powder catalyst form in Figure 1, the catalyst was evenly coated on the NF in its original shape, without deformation. Figure 2C shows the element mapping of the electrode surface, confirming that Mn, O f , and Fe were uniformly distributed. The intensity of the color is proportional to the concentration of the element, and the color intensity changes stoichiometrically. In particular, the color of Fe in the Fe 2 O 3 -MnO heterojunction is similar to that of Fe 2 O 3 , indicating that the Fe 2 O 3 crystal covered the surface of MnO, which agrees with the results of XRD and TEM.  Figure 2B shows a scanning electron microscopy (SEM) image of the surface of the electrode coated with catalyst particles on the NF substrate. Regardless of the type of catalyst, the catalyst uniformly covered the NF. Compared with the powder catalyst form in Figure 1, the catalyst was evenly coated on the NF in its original shape, without deformation. Figure 2C shows the element mapping of the electrode surface, confirming that Mn, Of, and Fe were uniformly distributed. The intensity of the color is proportional to the concentration of the element, and the color intensity changes stoichiometrically. In particular, the color of Fe in the Fe2O3-MnO heterojunction is similar to that of Fe2O3, indicating that the Fe2O3 crystal covered the surface of MnO, which agrees with the results of XRD and TEM.     [28]. Repeated CV tests for 100 cycles were performed to evaluate the stability of the electrodes for oxidation-reduction. In addition, CV cycle repetition is necessary for the full activity of the electrode. As the number of CV scans increased, the oxidation and reduction peaks of the NF were shifted to a positive voltage, and after about 100 cycles the CV curve was constantly stabilized [29]. However, no potential change was observed in the MnO/NF, Fe 2 O 3 /NF, and Fe 2 O 3 -MnO/NF electrodes assembled here, despite cycling. This indicates that the MnO, Fe 2 O 3 , and Fe 2 O 3 -MnO catalysts formed a stable structure at the beginning, suggesting that they contributed to the activity of the catalyst. Notably, for the heterojunction Fe 2 O 3 -MnO catalyst, no peak change was observed during the 100 cycles, and the current density was more stable than those of the other two samples. This indicates that the MnO and the Fe 2 O 3 particles were very stable and strongly bonded to each other; thus, there was little loss of catalytic active sites. The current density and peak area in the CV curve are closely related to the reversible redox reaction and the electrical active surface area [30]. In all NF-based catalysts, the peak area is expanded by increasing the volume of Ni(OH) 2 /NiOOH with cyclic repetition according to the following equations [31].

Result and Discussion
peaks of the NF were shifted to a positive voltage, and after about 100 cycles the CV curve was constantly stabilized [29]. However, no potential change was observed in the MnO/NF, Fe2O3/NF, and Fe2O3-MnO/NF electrodes assembled here, despite cycling. This indicates that the MnO, Fe2O3, and Fe2O3-MnO catalysts formed a stable structure at the beginning, suggesting that they contributed to the activity of the catalyst. Notably, for the heterojunction Fe2O3-MnO catalyst, no peak change was observed during the 100 cycles, and the current density was more stable than those of the other two samples. This indicates that the MnO and the Fe2O3 particles were very stable and strongly bonded to each other; thus, there was little loss of catalytic active sites. The current density and peak area in the CV curve are closely related to the reversible redox reaction and the electrical active surface area [30]. In all NF-based catalysts, the peak area is expanded by increasing the volume of Ni(OH)2/NiOOH with cyclic repetition according to the following equations [31].
Ni 0 + 2OH→Ni(OH)2 + 2e − Ni(OH)2 ↔ NiOOH + e − + H + Furthermore, the overall current-density range for the NF electrode coated with the catalyst was significantly increased compared with that of pure NF, which may be related to the catalytic activity. A uniformly dispersed metal or metal oxide has been reported to provide a large number of active sites in the electrocatalytic reaction, resulting in a significant increase in the current density [32]. Both the current density and the peak area of the heterojunction Fe2O3-MnO/NF electrodes were significantly increased. This is presumably due to the excellent redox performance of Mn and Fe, and it is expected that the junction of the heterojunction structure generated more active sites and had a high electron density to promote electron transfer during the catalytic reaction [33].  Furthermore, the overall current-density range for the NF electrode coated with the catalyst was significantly increased compared with that of pure NF, which may be related to the catalytic activity. A uniformly dispersed metal or metal oxide has been reported to provide a large number of active sites in the electrocatalytic reaction, resulting in a significant increase in the current density [32]. Both the current density and the peak area of the heterojunction Fe 2 O 3 -MnO/NF electrodes were significantly increased. This is presumably due to the excellent redox performance of Mn and Fe, and it is expected that the junction of the heterojunction structure generated more active sites and had a high electron density to promote electron transfer during the catalytic reaction [33].
The LSV curves were measured at the 100-cycle OER test. The results are shown in Figure 4A. In general, the activity of electrocatalysts is compared according to the overvoltage at a current density of 10 mA/cm 2 in a water electrolysis reaction [34]. For a pure NF electrode, an overvoltage (η) of 430 mV was needed to reach a current density of 10 mA/cm 2 . In contrast, the MnO/NF and Fe 2 O 3 /NF electrodes exhibited relatively low over-voltages of 410 and 400 mV, respectively, indicating that they had better activity than the pure NF. The heterojunction Fe 2 O 3 -MnO/NF exhibited an overvoltage of 370 mV, confirming that it had the best OER activity among the electrodes examined in this study. Figure 4B shows the Tafel slopes derived from the LSV polarization curves. The Tafel slope indicates the inherent characteristics of electrocatalysts, and a smaller Tafel slope corresponds to better water oxidation reaction kinetics [35]. As shown in Figure 4B, the Tafel slope for the Fe 2 O 3 -MnO/NF electrode was 66 mV/dec, which was significantly smaller than those for the NF (100 mV/dec), MnO/NF (74 mV/dec), and Fe 2 O 3 /NF (78 mV/dec) electrodes. The stability of the electrocatalyst is very important for the long life of the water electrolysis system. In order to assess the durability of the MnO/NF, Fe 2 O 3 /NF, and Fe 2 O 3 -MnO/NF electrodes, a chronopotentiometry test was performed for 20 h, and the applied current density was set to 10 mA cm −2 . Figure 4C shows the chronopotentiometric curves of water electrolysis for different catalyst electrodes with a constant current density of 10 mA cm −2 . The potentials required to achieve a current density of 10 mA cm −2 at the MnO/NF, Fe 2 O 3 /NF, and Fe 2 O 3 -MnO/NF electrodes were~1.64, 1.63, and 1.60 V, respectively; the potential was maintained the same throughout the 20 h electrolysis reaction. Looking closely at the results of the first two hours of the reaction of all the electrodes, the catalyst was stably activated as the potential gradually decreased. This showed that the NF used as the substrate electrode was partially exposed to improve the electrical conductivity of the catalyst and can easily transfer the current density. Jaramillo et al. [36] reported that OER stability losses could be due to many factors such as corrosion, material degradation, surface passivation, and so on. As demonstrated in the SEM image of the electrode surface after OER, the catalyst developed in this study was stably bound to the NF surface without any leaching or collapsing during the reaction. Thus, these results show that Fe 2 O 3 -MnO/NF electrodes can exhibit good electrochemical durability and excellent OER activity in practical alkaline aqueous solutions. Figure 5A,B shows the LSV curves at 1000 cycle for the OER and SEM images of the electrode surface before and after the 1000 cycled OER when the heterojunction Fe 2 O 3 -MnO/NF was used as the working electrode. Surprisingly, when the Fe 2 O 3 -MnO/NF electrodes were used, the LSV curve retained its performance as early as in the 1000-cycle OER test. The Tafel plots obtained after 1000 cycles exhibited negligible slope differences, indicating that the Fe 2 O 3 -MnO/NF electrode had very good durability. Figure 5B shows an SEM image of the Fe 2 O 3 -MnO/NF electrode surface before and after 1000 cycles in the OER. On the surface of the NF electrode after the OER, the Fe 2 O 3 -MnO heterojunction particles were well attached equally before the reaction, and they surprisingly had a similar size and shape to the fresh Fe 2 O 3 -MnO particles. This is presumably because the heterojunction structure was well maintained owing to the self-redox reaction of Mn and Fe, which facilitated the electron transfer, and the voltage application did not damage the NF electrode as a substrate.   Figure 5A,B shows the LSV curves at 1000 cycle for the OER and SEM images of the electrode surface before and after the 1000 cycled OER when the heterojunction Fe2O3-MnO/NF was used as the working electrode. Surprisingly, when the Fe2O3-MnO/NF electrodes were used, the LSV curve retained its performance as early as in the 1000-cycle OER test. The Tafel plots obtained after 1000 cycles exhibited negligible slope differences, indicating that the Fe2O3-MnO/NF electrode had very good durability. Figure 5B shows an SEM image of the Fe2O3-MnO/NF electrode surface before and after 1000 cycles in the OER. On the surface of the NF electrode after the OER, the Fe2O3-MnO heterojunction particles were well attached equally before the reaction, and they surprisingly had a similar size and shape to the fresh Fe2O3-MnO particles. This is presumably because the heterojunction structure was well maintained owing to the self-redox reaction of Mn and Fe, which facilitated the electron transfer, and the voltage application did not damage the NF electrode as a substrate.  Figure 6 shows the XPS results of Fe2p, Mn2p, Ni2p, and O1s for the Fe2O3-MnO/NF electrode after the OER. An Ni2p3/2 peak corresponding to metal Ni appeared at 852.6 eV, and an Ni2p3/2 peak corresponding to Ni 2+ appeared at 855-856 eV [37]. Unfortunately, no complete metal-Ni component was observed in the NF electrode before the OER. This is evidence that the NF was partially oxidized during the process of coating the particles on the NF. Perhaps the OER performance could have been improved if the NF used as the substrate electrode had maintained a perfect metal state. However, 2+ Figure 5. LSV curves at 1000 cycle for the OER (A) and SEM images of the electrode surface before and after the 1000 cycled OER (B). Figure 6 shows the XPS results of Fe2p, Mn2p, Ni2p, and O1s for the Fe 2 O 3 -MnO/NF electrode after the OER. An Ni2p 3/2 peak corresponding to metal Ni appeared at 852.6 eV, and an Ni2p 3/2 peak corresponding to Ni 2+ appeared at 855-856 eV [37]. Unfortunately, no complete metal-Ni component was observed in the NF electrode before the OER. This is evidence that the NF was partially oxidized during the process of coating the particles on the NF. Perhaps the OER performance could have been improved if the NF used as the substrate electrode had maintained a perfect metal state. However, only the Ni2p 3/2 highest occupied molecular orbital peak attributed to Ni 2+ was broadly observed at 856.1 eV for NF before the reaction (fortunately, NF was not completely oxidized to NiO, as lowest unoccupied molecular orbital (LUMO) moiety was not observed). After the OER, the binding energy shifted slightly to a higher value (856.4 eV), and the peak shape changed sharply. Furthermore, the Ni2p 1/2 peak corresponding to the LUMO was observed at 874.1 eV after the reaction. This indicates that some of the Ni in the NF was oxidized to Ni 2+ during the OER. The Fe2p 3/2 peak for fresh Fe 2 O 3 -MnO/NF was separated into peaks in various oxidation states, and peaks attributed to Fe 0 , Fe 2+ , and Fe 3+ were observed at binding energies of 706.4, 708.5, and 711.2 eV, respectively [38]. Additionally, the Fe2p 1/2 peak corresponding to Fe 3+ was observed at 724.9 eV. The observation of Fe 0 and Fe 2+ in the XPS analysis indicates that the Fe 3+ accepted electrons from the NF, becoming reduced and oxidizing the NF, during the loading of the Fe 2 O 3 -MnO particles onto the NF. However, after the OER, the isolated peaks converged to one peak at 711.2 eV. This suggests that the oxidation state of Fe during the OER was stable at 3+ [39]. In the high-resolution Mn2p XPS spectrum obtained before the OER, two main spin-orbitals corresponding to Mn2p 3/2 and Mn2p 1/2 (indicating the presence of Mn 2+ ) were observed at 641.6 and 653.6 eV, respectively [40]. Additionally, the Mn2p 3/2 peak attributed to Mn 3+ exhibited a low binding energy (644.7 eV). After the OER, the Mn2p 3/2 peak corresponding to Mn 2+ converged at 641.6 eV as a single peak, the intensity of the peak decreased, and the peak width increased, similar to the results for Fe2p. This indicates that the oxidation state of Mn was stabilized to 2+ during the OER. According to the XPS results for Fe2p and Mn2p, the active metals that constituted the electrode during the OER were reduced to Fe 3+ → Fe 2+ (Mn 2+ → Mn 3+ ) when charged, and the electrons moved in the opposite direction during discharging. At this time, redox couples were formed between Fe 3+ and Mn 2+ , and they were repeatedly oxidized-reduced and then restored. It was confirmed that their redox reaction was fairly stable and that the OER performance continued to be maintained after 1000 cycles, indicating a synergy caused by the coexistence of Mn and Fe. Before the OER, the XPS spectra of O1s were separated by various binding energies at 529.9, 531.9, and 533.8 eV, which can be attributed to M-O, H 2 Oads, and M-OH, respectively [41]. After the OER, the peaks were slightly shifted to higher binding energies. The H 2 Oads peak because of adsorbed water was stronger than the M-O peak corresponding to O 2 in the lattice. This is because during the OER, when the active metals underwent repeated oxidation-reduction reactions, a part of the M-O bond of the lattice was broken. The ratio of H 2 O ads peak and lattice M-O peak after OER changed significantly. It can be expected as a result derived from the partial participation in the reaction in which the surface lattice oxygen atoms of the metal oxide are exchanged with the oxygen atoms of the electrolyte solution. OHspecies adsorb to MO via one electron transfer, and then O-O bonds of MO(OH) are formed. Here, additionally, OH − species were adsorbed to generate MOO(OH). At this time, a portion of the surface lattice oxygen may be released as the higher valent oxide is transferred to the lower valent oxide. It is believed that the OER and the redox transition of the transition metal oxide in the alkaline medium are correlated. Moreover, recent research papers [42] have demonstrated that oxidation of lattice oxygen in metal oxides can enhance the OER activity and induce non-concerted proton-electron transfer.
MO(OH) are formed. Here, additionally, OH − species were adsorbed to generate MOO(OH). At this time, a portion of the surface lattice oxygen may be released as the higher valent oxide is transferred to the lower valent oxide. It is believed that the OER and the redox transition of the transition metal oxide in the alkaline medium are correlated. Moreover, recent research papers [42] have demonstrated that oxidation of lattice oxygen in metal oxides can enhance the OER activity and induce non-concerted proton-electron transfer. However, this did not appear to be a major factor affecting the oxidation state of Mn or Fe; rather, it may have been related to the partially damaged NF electrode.

Conclusions
Fe2O3 and MnO were synthesized via a sol-gel method, and an Fe2O3-MnO heterojunction mixture was prepared. These particles were coated onto NF electrodes, and the electrochemical However, this did not appear to be a major factor affecting the oxidation state of Mn or Fe; rather, it may have been related to the partially damaged NF electrode.

Conclusions
Fe 2 O 3 and MnO were synthesized via a sol-gel method, and an Fe 2 O 3 -MnO heterojunction mixture was prepared. These particles were coated onto NF electrodes, and the electrochemical properties of the resulting materials were evaluated. XRD patterns and TEM images of the powder particles were analyzed to confirm their crystallinity and shape. Through SEM and elemental mapping of the NF surface after a 1000-cycle OER test, the adhesion degree of the particles on the NF was examined. The change in and stability of the oxidation state of the active metals were determined based on the XPS spectra before and after the OER. As a result of the OER, a lower overvoltage (1.60 V/10 mAcm −2 ) was obtained for the Fe 2 O 3 -MnO/NF (containing the heterojunction of the two oxides) than for the pure Fe 2 O 3 /NF and MnO/NF, and the Tafel slope for the heterojunction electrode was as small as 66 mV/dec. Even after 1000 cycles, the OER performance of Fe 2 O 3 -MnO/NF remained stable, without deterioration. These results indicate that during Mn and Fe self-redox reactions in Fe 2 O 3 -MnO/NF hetero-electrodes, the Mn oxidizes O 2 in water and the Fe reduces H 2 , facilitating the migration of electrons. Eventually, it appears that this is a synergistic effect in the OER.