Effect of α-FeOOH in KOH Electrolytes on the Activity of NiO Electrodes in Alkaline Water Electrolysis for the Oxygen Evolution Reaction

Abstract

Iron cation impurities reportedly enhance the oxygen evolution reaction (OER) activity of Ni-based catalysts, and the enhancement of OER activity by Fe cations has been extensively studied. Meanwhile, Fe salts, such as iron hydroxide and iron oxyhydroxide, in the electrolyte improve the OER performance, but the distinct roles of Fe cations and Fe salts have not been fully clarified or differentiated. In this study, NiO electrodes were synthesized, and their OER performance was evaluated in KOH electrolytes containing goethite (α-FeOOH). Unlike Fe cations, which enhance the performance via incorporation into the NiO structure, α-FeOOH boosts OER activity by adsorbing onto the electrode surface. Surface analysis revealed trace amounts of α-FeOOH on the NiO surface, indicating that physical contact alone enables α-FeOOH to adsorb onto NiO. Moreover, interactions between α-FeOOH and NiO were observed, suggesting their potential role in OER activity enhancement. These findings suggest that Fe salts in the electrolyte influence OER performance and should be considered in the development of OER electrodes.

1. Introduction

Alkaline water electrolysis (AWE), which involves a hydrogen evolution reaction (HER) at the cathode and an oxygen evolution reaction (OER) at the anode, is a key process for generating green hydrogen to achieve carbon neutrality [1,2,3]. However, OER is slower than HER owing to the complex four-electron transfer process, which significantly affects the overall AWE efficiency [4,5]. Iridium- and ruthenium-based noble metals are suitable for overcoming sluggish OER; however, due to their high cost and low stability, Ni- and Fe-based electrochemical catalysts are currently the most studied and industrially used [5,6,7]. The most common catalysts in AWE are pure Ni, Ni oxide, and Ni hydroxide, and various studies have been conducted to increase their efficiency [6,8,9].

Recent findings have shown that extrinsic cations in an alkali electrolyte affect Ni-based electrochemical catalysts and are being investigated to increase OER activity [10,11]. Corrigan reported that the OER performance of NiO film electrodes significantly increases by incorporating Fe impurities (free ions) in the electrolyte, suggesting that such trace amounts have a significant effect on Ni-based electrodes [12]. In a similar study, Boettcher et al. confirmed that the OER activity of NiOOH in a Fe-impurity-free KOH electrolyte was lower than that in KOH with Fe impurity and reported that the incorporation of Fe increased the conductivity of NiOOH by more than 30 times and affected the electronic structure of NiOOH [13]. Boettcher et al. subsequently reported that an extrinsic Fe(III) impurity in an alkaline electrolyte can be deposited on Ni-based electrodes during the OER process as a mixed metal oxyhydroxide, which improves the OER performance [14]. Moreover, Bell et al. reported that Ni(OH)₂/NiOOH aged in KOH containing Fe impurities for an extended period leads to the formation of NiFe-layered double (oxy)hydroxide (LDH) as Fe incorporates into NiOOH, and Fe-sites within this mixed Ni-Fe LDH phase play a critical role in enhancing the OER activity [15]. These studies indicate that Fe has a positive effect on Ni-based electrochemical catalysts; however, the precise role of Fe is not fully understood thus far. Moreover, Fe(III) cations in the electrolyte have been recently reported to coordinately interact with chemisorbed OH⁻ and assist in OER [10]. Thus, Fe impurities in the KOH electrolyte affect OER efficiencies.

Practical AWE systems inevitably contain Fe in the electrolyte as they constitute components such as steel pipes, steel vessels, and KOH impurities. However, the solubility of Fe in concentrated KOH electrolyte is extremely low, leading to the presence of Fe in the form of salts [9,16]. Reportedly, adding large amounts of Fe salts to alkaline solutions with OH⁻ produces Fe(OH)₂ or Fe(OH)₃, and the addition of Fe salts to concentrated alkaline solutions produces α-FeOOH (goethite) [16,17,18]. These salts (Fe(OH)_x or α-FeOOH) are active in OER in their own form, and studies have demonstrated that α-FeOOH on Ni-based surfaces increases the OER activity [2,7,19]. Moreover, the effect of Fe salts generated in the electrolyte on the OER activity of Ni-based electrodes has been studied [10,16,19]. Zhang et al. significantly increased the OER activity of NiFeOH via the adsorption of negatively charged Fe(OH)₃ colloidal particles and attributed it to the charge transfer from Ni²⁺ to Fe³⁺, which enhanced the adsorption of oxygen intermediates and the OER electron transfer kinetics [16]. Kriek et al. confirmed the presence of α-FeOOH in Ni and NiO electrochemical catalysts via in-situ Raman spectroscopy by increasing the amount of Fe in the electrolyte and explained that Ni and NiO electrochemical catalysts with an enhanced OER activity by Fe are related to the Fe on the catalyst surface, not to the Fe incorporated in the bulk of the catalyst [19]. Therefore, both Fe cations and Fe salts present in the electrolyte can influence the enhancement of OER activity of Ni-based electrodes, and it is important to study their varying influences.

In our previous study, we confirmed the effect of Fe impurities in KOH electrolyte on the increased OER activity at NiO electrodes [20]. We found that simply aging NiO electrodes in KOH with Fe impurities did not increase the OER activity owing to the inability to incorporate Fe; however, prolonged exposure (~4 h) under OER conditions (1.5 A cm⁻²) increased the OER activity. Furthermore, this occurred because it is difficult to incorporate Fe impurities in NiO with a rock-salt structure; however, the NiO structure under OER conditions was changed to make it easier for Fe incorporation [20]. In an alkaline electrolyte, NiO transforms to NiOOH during OER [21,22,23]. In addition, at a higher potential, the incorporation of Fe³⁺ into NiOOH is more favorable [24]. Therefore, in this OER improvement study, we used these observations to distinguish between the effects of Fe salts in the electrolyte and that of incorporating Fe impurities (cations) into the electrode. We calcined Ni(OH)₂ to synthesize the previously developed stacked NiO nanosheet electrode [20]. The synthesized electrode was then tested for OER activity in electrolytes in which α-FeOOH was intentionally generated by adding Ni(NO₃)₂∙6H₂O to KOH containing the Fe impurity. Unlike previous studies, we demonstrated here that the OER activity of NiO was significantly enhanced even before exposure to OER at 1.5 A cm⁻², which further increased after exposure. The immediately increased OER activity could be attributed to the α-FeOOH present on the NiO surface, which was confirmed by analyzing the α-FeOOH adsorbed on the NiO when it was simply exposed to an electrolyte containing α-FeOOH. Therefore, the presence of both Fe cation impurities and Fe salts (α-FeOOH) in a KOH electrolyte differently affects the OER activity of NiO electrodes.

2. Results

2.1. Preparation and Characterization of the NiO Electrode

To determine the effect of the Fe cation impurity and Fe salts in the electrolyte on the NiO electrochemical catalysts, we synthesized the NiO electrodes used in our previous study [20], as schematically shown in Figure 1a. Ni(OH)₂ was synthesized utilizing stacked nanosheets via a hydrothermal method, as demonstrated by the SEM image shown in Figure 1b. The nanosheets were stacked to form a single unit with a thickness of approximately 1 μm. The as-synthesized Ni(OH)₂ was then heat-treated at 400 °C for 2 h in air, and the NiO retained the morphology of the original Ni(OH)₂ (Figure 1c). Furthermore, low-magnification SEM images showed that all units had a consistent structure throughout the process (Figure S1). Thermogravimetric analysis confirmed that 400 °C is a sufficient temperature for Ni(OH)₂ to transform into NiO (Figure S2). Moreover, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) demonstrated that the ratio of Ni to O was approximately 1:1 in the synthesized NiO (Figure S3). The stacked structure of NiO was further analyzed via transmission electron microscopy (TEM), clearly identifying the nanosheet structure shown in Figure 1d,e. TEM-EDS mapping revealed uniform Ni and O distributions in the synthesized NiO (Figure S4). X-ray diffraction (XRD) analysis was conducted because SEM and TEM analyses are limited to localized parts. The XRD patterns are consistent with those of Ni(OH)₂ (Figure S5) [25]. Figure 1f demonstrates the XRD pattern of NiO synthesized via Ni(OH)₂. The diffraction peaks at 37.26, 43.30, and 62.90° were attributed to the (111), (200), and (220) planes, demonstrating the formation of NiO. The XRD peaks corresponding to Ni are from the Ni substrate. X-ray photoelectron spectroscopy (XPS) was also performed to further analyze the electronic structure. The binding energies of the XPS results presented in this study were calibrated using the C 1s peak at 284.8 eV. For Ni(OH)₂, the peaks corresponding to Ni(OH)₂ in the Ni 2p and O 1s spectra were precisely defined (Figure S6). The XPS analysis of NiO also enabled the identification of the characteristic peaks of NiO (Figure 1g,h). The Ni 2p_3/2 peak of NiO could be separated into two prominent peaks at 853.7 and 855.4 eV, attributed to Ni²⁺ and Ni³⁺, respectively [26]. Moreover, the Ni²⁺ peak is related to the face-centered cubic structure of the as-prepared NiO, and the Ni³⁺ peak ascribes to the nickel vacancies. As shown in Figure 1h, the O1s spectrum of NiO was split into two major peaks, which are typically found in NiO [26,27]. Various analyses confirmed that NiO with a nanosheet-stacked structure was sufficiently fabricated and utilized as the electrode in this study.

2.2. Fabrication of α-FeOOH (Fe Salt) in KOH Electrolyte

Ni-based electrodes are affected by trace amounts of ionic Fe impurities in KOH [12,13,14,15]. Our previous study confirmed the effect of Fe impurities in KOH by utilizing NiO electrodes and demonstrated the difficulty in incorporating Fe into NiO in a rock-salt structure when placing it in KOH with Fe impurities; however, when exposed to an OER environment of 1.5 A cm⁻² for 4 h, Fe was incorporated and the OER activity increased [20]. Furthermore, the performance did not increase when NiO was exposed to the same OER environment in Fe impurity-free KOH, indicating that the performance increase in OER was owing to the Fe impurity [20]. Here, we developed an approach to determine the effect of Fe impurity as ions and as salts in the electrolyte. Several studies have recently reported that Fe salts in the electrolyte can affect the OER performance of Ni-based electrodes, suggesting its significance [10,16,19]. We added iron nitrate (Fe(NO₃)₃∙9H₂O) to KOH to intentionally create a salt in the electrolyte and it immediately precipitated in the electrolyte, as shown in Figure 2a. As Fe has an extremely low solubility in concentrated KOH, the added iron nitrate is mostly present as Fe(OH)₂ or FeOOH salts, and the concentration of the ionic Fe impurity in the electrolyte is not expected to significantly change [9,16]. The salt was then characterized by SEM, as shown in Figure 2b. The generated salt had an acicular structure and was less than 1 μm in length. The EDS analysis showed that the atomic ratio of Fe and O was 1:2.28, which is approximately 1:2 if water adsorption is considered (Figure S7). Figure 2c presents the XRD pattern of the produced salt, confirming that the structure of the precipitated Fe salt matches that of α-FeOOH (JCPDS 29-0713) [28]. The following XPS analysis was performed to analyze the electronic structure of α-FeOOH. Figure 2d,e present the XPS spectra of Fe 2p and O 1s of α-FeOOH, respectively. In the Fe 2p spectrum, α-FeOOH can be characterized by the Fe³⁺ peaks at 711.0 eV (Fe 2p_3/2) and 724.4 eV (Fe 2p_1/2) [29,30,31]. In addition, the O 1s spectrum was separated into three peaks, which were identified as 529.1 eV (Fe–O–Fe), 530.6 eV (Fe–O–H), and 532.0 eV (H₂O), respectively [32,33], demonstrating typical features of α-FeOOH. The intentionally prepared Fe salt (α-FeOOH) was also identified. Therefore, as α-FeOOH can be generated in the electrolyte by Fe precursors and is active in OER, it is likely to be present in the electrolyte affecting the OER activity, which must be further investigated.

2.3. Influence of the Ionic Fe Impurity and α-FeOOH in KOH on the Electrochemical Performance

We prepared three experiments to determine the effects of the Fe impurity and Fe salts on NiO electrodes. The electrodes were electrochemically tested by preparing three different electrolytes: (1) commercial KOH without any treatment; (2) KOH with α-FeOOH generated and then filtered to remove the α-FeOOH; and (3) KOH with the generated α-FeOOH. The experimental setup is schematically shown in Figure 3a. The actual photos of setups (2) and (3) are shown in Figure S8. In condition (2), α-FeOOH was clearly removed by the filter, whereas in condition (3), α-FeOOH precipitated in the electrolyte. LSVs were performed utilizing the synthesized NiO electrode at different electrolyte conditions, and the electrochemical performance was evaluated. We also aimed to compare the initial performance with that after exposure to an OER environment (1.5 A cm⁻² for 4 h), where the Fe impurity can be incorporated. Figure 3b–d show the linear sweep voltammetry (LSV) curves for each experiment, demonstrating that the initial performance of the untreated KOH electrolyte had an overpotential of 407 mV at 10 mA cm⁻². After exposure to the OER environment, the overpotential decreased by approximately 41 mV to 366 mV, which is consistent with the trend in our previous study [20]. Subsequently, the same experiment was performed on the electrolyte after α-FeOOH was first generated and then removed by a filter, for which the results were similar to those of the experiment with the untreated KOH (Figure 3c). The initial overpotential was 407 mV in KOH with α-FeOOH removed, which decreased to 362 mV after exposure to the OER condition. This indicated that the filter could not remove the ionic Fe impurity present in the electrolyte. Furthermore, the amount of ionic Fe impurities in the electrolyte did not increase owing to the iron nitrate added to the electrolyte to generate α-FeOOH. In fact, increasing the amount of ionic Fe impurities reportedly leads to a larger performance enhancement as the amount of incorporated Fe increases [9]. This can be attributed to the low solubility of Fe, as previously discussed, which results in the conversion of nearly all the added iron nitrate into α-FeOOH salts; similar results have been reported in other studies [9,16]. However, a noteworthy result was obtained for the KOH electrolyte containing α-FeOOH, in which the initial performance of the NiO electrode increased (384 mV@10 mA cm⁻²) (Figure 3d), demonstrating that the α-FeOOH present in the electrolyte directly affects the NiO electrode, which differs from the increased performance of NiO caused by the Fe impurity. In addition, after exposure to the OER environment, the overpotential decreased to 324 mV, which was consistent with the previous trend, and can be attributed to the increased performance owing to the incorporation of Fe. In addition, the overpotential reduction was 60 mV, higher than the previous reduction (approximately 40 mV), which can be attributed to the influence of α-FeOOH on the incorporation of Fe into NiO; however, further research is needed. In summary, the OER improvement effects of the ionic Fe impurity and α-FeOOH were independent. Moreover, α-FeOOH is presumed to improve the initial performance by affecting the OER activity at the NiO electrode surface, whereas the ionic Fe impurity was found to increase the effective OER activity when incorporated into NiO.

Whether α-FeOOH actually affects the electrode surface of NiO or is simply present in the electrolyte was experimentally confirmed. The NiO electrode was simply immersed in KOH containing α-FeOOH for 30 min, washed with distilled water, and immersed in untreated KOH without α-FeOOH to perform the same electrochemical performance tests as in the previous experiment. Figure 4a presents the results of the LSV curves obtained in this manner. The results exhibited nearly the same initial performance (379 mV@10 mA cm⁻²) as that for the KOH electrolyte with α-FeOOH, indicating that: α-FeOOH is more likely to be present on the electrode surface than in the electrolyte, which affects the OER performance; and simply immersing the NiO electrode in the electrolyte with α-FeOOH may deposit a certain amount of α-FeOOH on the electrode surface. Additionally, we verified that NO³⁻, the anion of iron nitrate added to generate α-FeOOH, did not affect the performance of the electrode, confirming that the performance improvement of the NiO electrode was owing to α-FeOOH. We added 0.375 g of KNO₃ to the KOH electrolyte to exclude the effect of Fe while maintaining the same NO³⁻ molar concentration as when 0.5 g of iron nitrate was added. As a result, both the initial performance (412 mV@10 mA cm⁻²) and that after exposure to the OER environment (368 mV@10 mA cm⁻²) were nearly equal to the result of the untreated KOH (Figure 4b). This indicated that the anion (NO³⁻) did not have any effect in the previous electrolyte conditions where iron nitrate was added.

Here, we focused more on α-FeOOH than on the well-known effects of the ionic Fe impurities on the NiO electrodes. Based on previous experiments, we speculated that the initial performance of the NiO electrode improved in the presence of α-FeOOH in the electrolyte owing to the presence of α-FeOOH on the electrode surface, which is the active site of OER. Therefore, further experiments were conducted under conditions in which the amount of α-FeOOH on the electrode surface could be increased to confirm the change in the OER activity. In the first experiment, as the amount of α-FeOOH precipitation in the electrolyte was already sufficient (Figure S8b), the electrochemical performance was evaluated via LSV after stirring the NiO electrode in the KOH containing α-FeOOH for 30 min and for 1 h; Figure 5a presents the LSV results with and without stirring. When exposed to the electrolyte with α-FeOOH for the same amount of time, there was a significant difference between the results with and without stirring. Under 30 min of resting condition (original condition), the overpotential was 384 mV (@10 mA cm⁻²), whereas under 30 min of stirring, the overpotential was significantly lower at 328 mV (@10 mA cm⁻²). As intended, the probability of α-FeOOH contacting the NiO electrode increased, resulting in more α-FeOOH on the electrode surface, which is likely responsible for the significant increase in the initial performance. Under the condition of stirring for 1 h, the overpotential (324 mV@10 mA cm⁻²) no longer significantly decreased, suggesting that there may be a limit to the amount of α-FeOOH that can be present on the electrode surface. Therefore, when using an electrolyte with Fe salts, the effect of increasing the OER performance owing to Fe salts can be further maximized under conditions where the electrolyte is cycled, such as in a real water electrolysis system. Subsequently, we aimed to achieve a similar effect by increasing the amount of α-FeOOH in the electrolyte by increasing the amount of iron nitrate by six times (3.0 g) and 10 times (5.0 g) the original amount (0.5 g); the NiO electrode was rested in the prepared electrolyte for 30 min, and the OER performance was confirmed via an electrochemical evaluation. Figure 5b presents a bar diagram summarizing the overpotential at 10 mA cm⁻², which was obtained from the electrochemical test in KOH according to the amount of iron nitrate added. The overpotential apparently decreased linearly with increasing amounts of iron nitrate, which was consistent with the stirring experiment, demonstrating that increasing the amount of α-FeOOH on the NiO electrode increased the OER performance as intended. However, compared to the stirring experiment, the overpotential was 360 mV despite a ten-fold increase in the amount of iron nitrate added, which was higher than that obtained by stirring for 30 min. This indicated that the NiO electrode is more efficient in making good contact with α-FeOOH than the absolute amount of α-FeOOH in the electrolyte. That is, in a real water electrolysis system with electrolyte cycling, Fe salts, even in small amounts, can have a greater effect on the electrode.

2.4. Confirmation of α-FeOOH on the NiO Electrode

By adjusting the electrolyte conditions, α-FeOOH in the electrolyte had a significant effect on the OER performance of NiO; experimental results demonstrated the effectiveness of α-FeOOH on the NiO electrode surface. Therefore, we aimed to directly identify and characterize α-FeOOH on the NiO electrodes. The NiO electrode was immersed in the KOH electrolyte containing α-FeOOH for 30 min while stirring, which maximally influenced the performance change; this electrode was referred to as NiO + α-FeOOH. We first attempted to confirm the presence of NiO and α-FeOOH simultaneously via TEM. As expected, α-FeOOH was present in significantly small amounts, complicating the detection of the presumed α-FeOOH at a low magnification (Figure S9). EDS mapping was performed for further confirmation, enabling the identification of the acicular α-FeOOH composed of Fe and O separated from NiO (Figure 6a). The atomic ratios of the EDS mapping area showed the following compositions: Ni: 44.0%; Fe: 4.2%; and O: 51.8%, which is a close approximation considering that it is composed of NiO and α-FeOOH (Figure S10). Kriek et al. also recently reported that the presence of α-FeOOH can be confirmed via an in-situ Raman analysis after 30 cyclic voltammetry (CV) cycles using NiO electrodes in a KOH electrolyte mixed with a certain amount of iron nitrate nonahydrate, suggesting that α-FeOOH can significantly improve the OER performance of NiO electrodes [19]. However, the improved OER performance was not clearly explained. Furthermore, although Kriek et al. described the adsorption of Fe onto the NiO surface during the CV cycle, our study demonstrated that it can be adsorbed by simple physical contact. An XPS analysis was performed to characterize α-FeOOH on the NiO surface and to confirm the changes in the electronic structure of NiO and α-FeOOH in NiO + α-FeOOH. Figure 6b presents the Ni 2p XPS spectra of NiO and NiO + α-FeOOH. The results revealed that NiO + α-FeOOH also has a Ni 2p_3/2 peak consisting of Ni²⁺ (853.7 eV) and Ni³⁺ (855.4 eV) at almost the same position as NiO. The ratio of the Ni²⁺ peak area to that of Ni³⁺ for each electrode was 3.22 for NiO and 3.59 for NiO + α-FeOOH. This indicates that the NiO in NiO + α-FeOOH is composed of the same Ni²⁺ and Ni³⁺ as in the case without α-FeOOH; however, the proportion of Ni³⁺ slightly increased, owing to the influence of α-FeOOH. The NiO + α-FeOOH sample was analyzed for Fe 2p via XPS and compared to α-FeOOH (Figure 6c), indicating that NiO + α-FeOOH was composed of the same Fe³⁺ as α-FeOOH; however, the binding energy was reduced by approximately 0.6 eV. Thus, electrons can be transferred from the Ni²⁺ in NiO to the Fe³⁺ in α-FeOOH, which is likely related to the increase in the previously indicated proportion of Ni³⁺. These results demonstrated that α-FeOOH adsorbs onto the NiO surface and interacts with one another. Similar to our experimental results, Zhang et al. immersed a NiFeOH electrode in an electrolyte containing Fe(OH)₃ colloids for 5 min to adsorb the Fe(OH)₃ colloids and reported the electron transfer from Ni²⁺ to Fe³⁺ via XPS analysis [16]. They also found that the electron deficiency in Ni simplifies the transformation of Ni into a structure favorable for OER, and the high-valence Fe³⁺ sites on the electrode surface lower the energy barrier for OH* oxidation to O*, a potential limiting step in OER. Consequently, the synergistic interactions between Fe³⁺ and Ni²⁺ significantly enhance the OER activity. Our experimental results are similar and suggest that α-FeOOH is responsible for the significant increase in the OER performance in the previous electrochemical experiments; however, further studies are needed to investigate this possibility. Changes in the electronic structure potentially induced by the chemical state and interaction between NiO and α-FeOOH are anticipated to be more precisely characterized using atomic resolution electron energy loss spectroscopy (EELS) and X-ray absorption spectroscopy (XAS) [34]. These analyses are expected to confirm the synergistic interaction between NiO and α-FeOOH. We directly confirmed the adsorption of α-FeOOH on a NiO electrode during exposure to an electrolyte in the presence of α-FeOOH, confirming the interaction between the adsorbed α-FeOOH and NiO, which can be inferred as a possible cause for the improved OER performance.

3. Materials and Methods

3.1. Fabrication of NiO Electrode

Prior to the hydrothermal deposition, nickel plate substrates were sandblasted (0.1 mm alumina, 6–8 bar), cleaned by 60 min of sonication in deionized water, and dried. Ni(OH)₂ films were hydrothermally deposited onto the nickel plate substrate and annealed. A nickel bath was prepared, which included 0.150 g of Ni(NO₃)₂∙6H₂O (97%, Daejung Chemicals & Metals Co., Siheung-si, Republic of Korea), 20 mL of deionized water, and 80 mL of NH₃ (28.0–30.0 wt.%, Junsei Chemical Co., Tokyo, Japan). The bath was transferred into a glass cylinder with nickel plates. After closing the lid, the cylinder was heated to 90 °C in an oven and aged for 6 h to obtain a Ni(OH)₂ film on the nickel plate. The nickel substrate with the Ni(OH)₂-coated surface was removed from the bath, washed, and dried. The coated samples were heated in an electric furnace (working temperature of 1000–1200 °C; box-type) in an air atmosphere at a rate of 2 °C min⁻¹ to 400 °C for 2 h [20].

3.2. Electrolyte Modification

Three types of electrolytes were analyzed in this study: commercial KOH without any treatment; KOH with Fe salt added and then filtered to remove the salt; and KOH with Fe salt added. For the KOH with Fe salt added and then removed, the salt was removed using filter paper (5C filter paper, 100 circles, 185 mm, 0.28 mg/circle, ADVANTEC, Tokyo, Japan). For the KOH with Fe salt, 0.5–5.0 g of Fe(NO₃)₃·9H₂O (98%, Daejung Chemicals & Metals Co., Siheung-si, Republic of Korea) was added into 500 mL of commercial KOH to obtain an iron precipitate. In this study, the commercial KOH used as the electrolyte was modified from a 1 M KOH solution (Samchun Pure Chemical Co., Ltd., Anyang, Republic of Korea).

3.3. Incorporating Ionic Fe Impurities into NiO

Ionic Fe impurities were incorporated into the NiO electrode by applying a constant current (CC) of 1.5 A cm⁻² for 4 h in the three types of modified electrolytes.

3.4. Electrochemical Measurement

Electrochemical measurements were conducted on an electrochemical workstation (ZIVE SP2, WonATech, Seoul, Republic of Korea) with a three-electrode system at 25 °C. The three-electrode system consisted of the prepared NiO electrode as the working electrode, Hg/HgO (0.115 V vs. SHE, XR 400, Radiometer Analytical, Lyon, France) as the reference electrode, and a nickel plate as the counter electrode.

The OER activity was measured using LSV with a scan rate of 0.2 mV s⁻¹ at a potential range of 0.2–0.8 V (vs. Hg/HgO). The measured results compensated for IR drop by utilizing the solution resistance obtained via electrochemical impedance spectroscopy (EIS). EIS measurements were performed in the range of 300 kHz to 30 mHz at an overpotential of 560 mV.

3.5. Characterization

SEM (Regulus 8220, Hitachi, Tokyo, Japan) was employed to analyze the surface morphology and structure of the electrode, whereas EDS (X-MAX 50, HORIBA, Tokyo, Japan) was used to determine the electrodeposited components on the electrode surface. The inner structure and morphology of the electrodes were obtained using TEM (JEM-F200, JEOL, Tokyo, Japan) operated at 200 kV. The content and distribution of elements in the microscopic regions of the electrodes were characterized via EDS, which was connected to the TEM. The samples for TEM analysis were prepared by dispersing the coated particles in ethanol, and a small aliquot was dripped onto a carbon-coated copper grid using a fine mesh with a 3-mm diameter. The sample was then air-dried to form a thin film. The crystal structures of the electrodes and filtered Fe were characterized via XRD (Smart Lab High Temp/Smart Lab High Resolution, Rigaku, Tokyo) using Cu Kα (λ = 1.5406 Å) radiation with a work potential of 45 kV and tube current of 200 mA. XPS (NEXSA G2, Thermo Fisher Scientific, Waltham, MA, USA) was employed to determine the phase of the electrode surface, which was difficult to obtain via XRD, and the binding energy was adjusted based on the C1s peak at 284.8 eV.

4. Conclusions

We determined how the Fe impurity and salt present in KOH individually affect the OER performance of NiO electrodes. We intentionally added iron nitrate to untreated KOH to generate Fe salts, which were identified to be α-FeOOH. Electrochemical evaluations via LSV using NiO in three different electrolyte conditions (untreated KOH, α-FeOOH removed KOH, and KOH + α-FeOOH) showed that α-FeOOH significantly affected the OER performance of NiO and that the ionic Fe impurity in KOH led to performance increase as it was incorporated into NiO after exposure to the OER environment; thus, the different roles of the ionic Fe impurity and α-FeOOH were clarified. We focused on α-FeOOH rather than the more well-known Fe ionic impurities. Electrochemical measurements revealed that the effect of α-FeOOH is absent in the electrolyte, but present on the NiO surface. Furthermore, stirring the electrolyte with α-FeOOH significantly improved the performance compared to simple immersion, as well as increasing the amount of α-FeOOH in the electrolyte. Thus, OER performance improvement is more valid in environments where α-FeOOH is in contact with the electrode surface of NiO. Furthermore, we directly identified α-FeOOH in the NiO electrode, even in trace amounts, and confirmed the interaction of α-FeOOH with NiO via TEM and XPS analyses. In conclusion, this study demonstrated that trace amounts of ionic Fe impurities in KOH electrolytes and generated Fe salts in the electrolyte can significantly affect the OER performance. Fe salts derived from steel pipes, and steel vessels, among other components that are in contact with the electrolyte in actual water electrolysis systems, can affect the electrode and should be considered in the study of water electrolysis electrodes.