One-Step Synthesis of High-Efficiency Oxygen Evolution Reaction Catalyst FeS_x(Y/MB) with High Temperature Resistance and Strong Alkali

Abstract

Given the energy crisis and escalating environmental pollution, the imperative for developing clean new energy is evident. Hydrogen has garnered significant attention owing to its clean properties, high energy density, and ease of storage and transportation. This study synthesized four types of catalysts—FeS(DI/MB), FeS(ET/MB), Fe(DI/MB), and Fe(ET/MB)—using two distinct solution systems: DI/MB and ET/MB. The FeS(DI/MB) catalyst, synthesized using the layered solution system (DI/MB), demonstrates a uniformly distributed and dense nanosheet structure, exhibiting excellent resistance to strong bases and superior catalytic properties. The FeS(DI/MB) electrode showed OER overpotentials of 460 mV and 318 mV in 1 M and 6 M, respectively, at current densities of up to 500 mA cm⁻². Under industrial electrolysis test conditions, the FeS(DI/MB) electrode required only 262 mV to achieve a current density of 500 mA cm⁻², operating in a high-temperature, strong alkaline environment of 6 M at 60 °C. Furthermore, the FeS(DI/MB) electrode exhibited excellent OER catalytic activity and stability, as evidenced by a 60 h stability test These findings provide valuable insights into the preparation of iron nickel sulfide-based catalysts, and further in-depth and comprehensive exploration is anticipated to yield the excellent catalytic performance of these catalysts in the realm of electrolytic water hydrogen production.

1. Introduction

Hydrogen, as an ideal clean energy source, is in increasing demand. Currently, hydrogen production from electrolyzed water is the most important method [1]. Due to the reaction mechanism, the oxygen evolution reaction (OER) controls the process of electrolyzed water [2,3]. Thus, finding efficient and stable OER catalysts is necessary to enhance the efficiency of hydrogen production [4,5]. For ease of comparison, the academic community typically selects the oxygen evolution overpotential (η) corresponding to a current density (j) of 10 mA cm⁻² to assess the catalyst’s catalytic performance [6]. A current density (j) of 10 mA cm⁻² equates to the minimum current density required for water decomposition achievable by solar water electrolysis equipment operating at 10% efficiency under standard sunlight intensity, significantly lower than that needed for industrial hydrogen production. The increase in current density affects the catalytic performance of the material, making it of great significance for industrial production that the catalyst material maintains high-efficiency catalytic activity even under high-current conditions [7]. In testing the electrochemical performance of hydrogen evolution and oxygen evolution catalysts, laboratories commonly use 1 M KOH as the electrolyte [8,9]. Nonetheless, testing hydrogen and oxygen evolution reactions in a 1 M KOH electrolyte leads to higher overpotential, hindering the water electrolysis process, thus reducing hydrogen production efficiency and resulting in relatively high electrolytic energy consumption, insufficient for industrial production requirements [10,11,12]. To enhance hydrogen production efficiency, industrial electrolytes typically consist of 6 M KOH, with electrolysis conducted at 60 °C [13,14,15]. Currently, among numerous reported non-metallic materials, nickel–iron-based compounds exhibit notable OER electrocatalytic activity [16,17,18]. These materials predominantly manifest as metal (hydrogen) oxides (e.g., Ni-Fe-LDH), nitrides (e.g., Ni3FeN), phosphides (e.g., NiP), selenides (e.g., NiSe), etc. [19,20,21,22,23]. The majority exhibit superior OER catalytic activity compared to noble metal catalysts [24,25]. Nonetheless, synthesis methods reported in the literature mainly involve two or three-step processes, which are cumbersome and inconvenient, resulting in products rarely suitable for high-temperature and strong alkali environments [22,26]. Thus, we developed a one-step synthesis method for a high-temperature and strong alkali-resistant OER catalyst (Fe/Ni)Sx/NF. To distinguish the solution systems, the solution of deionized water and methylbenzene is denoted as DI/MB, the mixture of anhydrous ethanol and methylbenzene as ET/MB, and the catalyst as FeSx(Y/MB), where X = 0, 1; Y = DI, ET.

This paper’s design concept involves dissolving sulfur powder in methylbenzene as the sulfur source, ferric nitrate in ethanol or deionized water as the iron source, and utilizing nickel foam as the nickel source, all through a one-step hydrothermal method for direct material synthesis. The reaction equation can be simply expressed as:

Fe³⁺ + Ni(NF)→ Fe²⁺ + Ni²⁺

(1)

Fe²⁺ + Ni²⁺ + Ni + S → (Fe/Ni)S_x/NF (FeS_x(Y/MB))

(2)

Fe³⁺ exhibits strong oxidizing properties and tends to form Ni/Fe structures on the surfaces of nickel foam. Dissolving Fe(NO₃)₃ in deionized water and adding sulfur-containing toluene resulted in intriguing solution layering phenomena.

Anhydrous ethanol was used instead of deionized water to explore the impact of layered and non-layered solutions on the morphology, structure, and performance of the catalyst. This paper prepared four catalysts, FeS(DI/MB), FeS(ET/MB), Fe(DI/MB), and Fe(ET/MB), using two different solution systems, namely DI/MB and ET/MB. Structural characterization revealed that the FeS(DI/MB) and Fe(DI/MB) catalysts exhibited a dense nanosheet structure, whereas the FeS(ET/MB) and Fe(ET/MB) catalysts lacked such a structure. The lower electronegativity of sulfur facilitates its loading on the edges of Ni/Fe nanosheets during the hydrothermal process, thereby enhancing the material’s resistance to strong bases and its catalytic activity. With the KOH concentration increasing from 1 M to 6 M, the OER overpotentials of FeS(DI/MB) were 460 mV and 318 mV at a current density of 500 mA cm⁻², respectively. Additionally, at 6 M 60 °C, FeS(DI/MB) requires only 262 mV to achieve a current density of 500 mA cm⁻². Tested under simulated industrial electrolysis conditions, it was found to exhibit stable performance in high-temperature and strong alkaline environments, indicating its potential for industrial applications. These findings offer novel approaches for NiFeS catalyst synthesis, and further in-depth exploration is anticipated to reveal the excellent catalytic performance of NiFeS catalysts in hydrogen production from electrolyzed water.

2. Results and Discussion

Figure 1 illustrates the preparation of the FeS(DI/MB) electrode via a one-step hydrothermal method. The catalyst was directly synthesized through a one-step hydrothermal method by dissolving sulfur powder in toluene as the sulfur source, iron nitrate in deionized water or anhydrous ethanol as the iron source, and utilizing nickel foam as the nickel source.

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to characterize the morphology and structure of the catalyst. Figure 2a,b reveals that the surface of the FeS(DI/MB) electrode exhibits a uniformly distributed ultrathin nanosheet array structure, facilitating the provision of abundant active sites and enhancing catalytic performance. Elemental mapping in Figure 2c–g demonstrates the uniform distribution of Ni (d), O (e), Fe (f), and S (g) on the surface of the FeS(DI/MB) electrode, indicating successful S doping. Figure S1 illustrates that the microscopic morphology of the FeS(ET/MB) electrodes prepared from the unstratified solution exhibits a distinct granularity, contrasting significantly with the nanosheet structure of FeS(DI/MB). To further explore the impacts of these two solution systems on catalyst performance, we fabricated Fe(DI/MB) and Fe(ET/MB) electrodes using the same method. Figures S2 and S3 reveal that Fe(DI/MB) also possesses a dense nanosheet structure, while the microstructure of Fe(ET/MB) is visibly granular, mirroring the SEM images of FeS(DI/MB) and FeS(ET/MB). SEM images reveal that the FeS(DI/MB) and Fe(DI/MB) electrodes prepared using the DI/MB system exhibit a uniformly distributed ultrathin nanosheet structure, providing more active sites and improved gas permeability.

The structural composition and chemical state of the catalysts were further analyzed by XRD and XPS tests. As shown in Figure 3a, the XRD characteristic peaks of FeS(DI/MB) correspond to the standard PDF cards of nickel foam and Fe_3.6Fe_0.9(O, OH, SO₄)₉ [27]. The blue vertical lines correspond to the (111), (200), and (220) crystal planes of the standard PDF card (PDF#04-0850) for nickel foam, and the red vertical lines correspond to the (003), (102), and (009) crystal planes of the standard PDF card (PDF #13-0090) for Fe_3.6Fe_0.9(O, OH, SO₄)₉ [27]. The XPS spectra in Figure 3b further confirm the presence of Ni, Fe, S, and O elements in the FeS(DI/MB) electrode, providing evidence of successful S doping. Figure 3c shows the spectrum of Ni 2p with the characteristic peaks at 875.2 and 857.6 eV corresponding to the two peaks Ni 2p1/2 and Ni 2p3/2, respectively, and Sat corresponding to the satellite peaks. These peaks confirm the presence of elemental nickel and indicate that it exists mainly in the Ni²⁺ chemical state [28]. Figure 3d shows the spectrum of Fe 2p and two characteristic peaks can be observed at 725.2 and 713.1 eV, corresponding to Fe 2p1/2 and Fe 2p3/2, respectively, indicating that the Fe element on the catalyst surface exists mainly in the form of Fe³⁺ [29]. Figure 3e shows the spectra of O 1s and it can be seen that the O element is present in the form of bonding with metal and hydroxyl groups [30]. This suggests that the FeS(DI/MB) surface may be rich in highly reactive intermediate states that significantly enhance the OER activity of the catalyst. Figure 3f shows the spectrum of S 2p, again confirming the presence of the S element [31]. In summary, the FeS(DI/MB) electrode exhibits uniform distribution of Fe, Ni, S, and O elements, consistent with the EDX plots of Figure 2c–g.

A three-electrode electrochemical test system was assembled in a 1 M KOH solution to evaluate the OER catalytic activity of FeS(DI/MB), FeS(ET/MB), Fe(DI/MB), Fe(ET/MB) electrodes, and NF. The results are depicted in Figure 4. According to Figure 4a, the FeS(DI/MB) electrode requires an overpotential of 250 mV to achieve a current density of 100 mA cm⁻². The current densities achievable by FeS(ET/MB), Fe(DI/MB), Fe(ET/MB), and NF at a 250 mV potential are 46.32, 19.91, 7.736, and 4.28 mA cm⁻², respectively. At this overpotential, the current densities of FeS(DI/MB) electrodes surpass those of FeS(ET/MB), Fe(DI/MB), Fe(ET/MB), and NF by factors of 2.16, 5.02, 12.93, and 23.36, respectively. This suggests that S doping enhances the catalytic activity of self-supporting electrodes, facilitating the formation of a uniformly distributed nanosheet structure with increased active sites and improved oxygen evolution performance. Consequently, the oxygen evolution performance of the nanosheet FeS(DI/MB) electrode surpasses that of the FeS(ET/MB), Fe(DI/MB), Fe(ET/MB) electrodes, and NF. Figure 4b reveals that the Tafel slopes for the FeS(DI/MB), FeS(ET/MB), Fe(DI/MB), and Fe(ET/MB) electrodes are 20.62 mV dec⁻¹, 38.97 mV dec⁻¹, 93.91 mV dec⁻¹, and 51.04 mV dec⁻¹, respectively. A smaller Tafel slope corresponds to a higher reaction rate, indicating a faster reaction. The Tafel slope of the FeS(DI/MB) electrode is significantly smaller than that of the FeS(ET/MB), Fe(DI/MB), and Fe(ET/MB) electrodes. Furthermore, the nanosheet structure formed by S-doped FeNi significantly enhanced the reaction kinetics of the electrode. The stability test of the FeS(DI/MB) electrode is depicted in Figure 4d, showing that the LSV curve remains essentially unchanged after 5000 cycles of CV, demonstrating the excellent stability of the FeS(DI/MB) electrode. The zoomed out graph in Figure 4d indicates that the OER performance of the FeS(DI/MB) electrode decreases almost unchanged after 60 h of continuous i-t testing, further affirming its outstanding stability.

In order to assess the relative electrochemically active surface area of the electrodes, the electrochemical double-layer capacitance (C_dl) was estimated by cyclic voltammetry (CV) [32]. Figure S5 summarizes the converted Cdl values of each electrode. It is evident that the Cdl value of the FeS(DI/MB) electrode (10.24 mF cm⁻²) is significantly larger than that of the FeS(ET/MB) electrode (9.16 mF cm⁻²), the Fe(DI/MB) electrode (5.15 mF cm⁻²), and the Fe(ET/MB) electrode (9 mF cm⁻²). This suggests that the FeS(DI/MB) electrode has more active sites. The electrochemically active surface area (ECSA) is closely related to C_dl. The larger the C_dl value, the greater the ECSA value, indicating that the electrochemically effective active surface area of the catalyst is larger, and bubbles are more conducive to separation from the electrode surface, resulting in better OER catalytic activity [33].

The electrolyte alkalinity was increased from 1 M KOH to 6 M KOH, while maintaining a temperature of 25 °C. A three-electrode system was assembled to evaluate the OER catalytic activity of the FeS(DI/MB), FeS(ET/MB), Fe(DI/MB), Fe(ET/MB) electrodes, and the NF. The results are depicted in Figure 5. As can be seen in Figure 5a, from the perspective of whether the solution is layered or not, the OER catalysis of the FeS(DI/MB) electrode is superior to that of FeS(ET/MB), and the OER performance of Fe(DI/MB) is superior to that of Fe(ET/MB). From the point of view of whether vulcanization takes place or not, the OER catalytic performance of the FeS(DI/MB) electrode is superior to that of Fe(DI/MB), and the OER catalytic performance of the FeS(ET/MB) electrode is superior to that of Fe(ET/MB). The Tafel slope also confirms the above relationship, as shown in Figure 5b. The Tafel slope of FeS(DI/MB) is 18.33 mV dec⁻¹, which is significantly smaller than the 34.92, 28.01, and 50.14 mV dec⁻¹ of FeS(ET/MB), Fe(DI/MB), and Fe(ET/MB). This indicates that FeS(DI/MB) still exhibits excellent reaction kinetics in a strong alkaline environment of 6 M. As can be seen from Figure 5c, the FeS(DI/MB) catalytic electrode requires an overpotential of 318 mV to release 500 mA cm⁻² under this test condition, and the current densities that can be achieved by FeS(ET/MB), Fe(DI/MB), Fe(ET/MB), and nickel (Ni foam) in empty foam at this potential are 242.56, 250.84, 140.68, and 5.91 mA cm⁻², respectively. At the same overpotential, the current density of FeS(DI/MB) is 2.06, 1.99, 3.55, and 84.60 times higher than that of FeS(ET/MB), Fe(DI/MB), Fe(ET/MB), and NF. Figure 5d shows the stability test of the FeS(DI/MB) electrodes in 6 M KOH. After 5000 cycles of CV, the LSV curve remains basically unchanged, indicating that FeS(DI/MB) batteries have excellent stability, which proves that the FeS(DI/MB) electrode still has excellent stability in a strong alkaline environment of 6 M KOH.

The electrochemical double-layer capacitance (C_dl) was estimated using cyclic voltammetry (CV) to assess the relative electrochemically active surface area of the electrodes in a high-concentration electrolyte. Figure S7 illustrates that the C_dl value for the FeS(DI/MB) electrode (15.18 mF cm⁻²) significantly exceeds those of the FeS(ET/MB) electrode (14.16 mF cm⁻²), the Fe(DI/MB) electrode (9.74 mF cm⁻²), and the Fe(ET/MB) electrode (7.92 mF cm⁻²), suggesting a higher number of active sites on the FeS(DI/MB) electrode.

Further, the simulated industrial electrolysis conditions were tested, the alkalinity of the electrolyte was kept constant on the basis of the former experiment (6 M KOH), the ambient temperature was increased from a room temperature of 25 °C to 60 °C: i.e., the tests were carried out at high temperatures and in strong alkaline conditions, and a three-electrode system was constructed for evaluating the catalytic activity of FeS(DI/MB), FeS(ET/MB), Fe(DI/MB), Fe(ET/MB) electrodes, and NF OER, and the results of the tests are shown in Figure 6. As can be seen in Figure 6a, from the perspective of whether the solution is layered or not, the OER catalysis of the FeS(DI/MB) electrode is superior to that of FeS(ET/MB), and the OER performance of the Fe(DI/MB) is superior to that of the Fe(ET/MB). The OER catalytic performance of the FeS(DI/MB) electrode was superior to that of the Fe(DI/MB) electrode in terms of whether it was sulfated or not. And the OER performance of the FeS(ET/MB) electrode is superior to the Fe(ET/MB) electrode. Their Tafel slopes (b) similarly corroborate the above relationship. Figure 6b shows that the Tafel slope of FeS(DI/MB) is 15.31 mV dec⁻¹, which is much smaller than the 28.01, 29.14, and 77.57 mV dec⁻¹ of FeS(ET/MB), Fe(DI/MB), and Fe(ET/MB). This indicates that the FeS(DI/MB) electrode still exhibits excellent reaction kinetics under 6 M KOH and 60 °C conditions. As can be seen from Figure 6c, the FeS(DI/MB) catalytic electrode requires an overpotential of 262 mV to release a current density of 500 mA cm⁻² under simulated industrial electrolysis conditions. At this potential, the current densities of FeS(ET/MB), Fe(DI/MB), Fe(ET/MB), and NF were 250.84, 242.56, 140.68, and 26.95 mA cm⁻², respectively. At the same overpotential, the current density of FeS(DI/MB) is 1.99, 2.06, 3.55, and 18.55 times higher than that of FeS(ET/MB), Fe(DI/MB), Fe(ET/MB), and NF. As shown in Figure 6d, the LSV curves before and after 5000 CV cycles of charge discharge tests are basically consistent, indicating that the FeS(DI/MB) electrode containing FeNi sulfide catalyst has very high electrochemical stability under simulated industrial electrolysis conditions (6 M KOH, 60 °C). In summary, FeS(DI/MB) electrodes exhibit excellent OER catalytic activity and stability under simulated industrial electrolysis conditions, and their preparation process is simple and low-cost, with good industrial application prospects.

3. Materials and Methods

3.1. Chemicals and Materials

The ferric nitrate nonahydrate (Fe(NO₃)₃·9H₂O, 98%), hydrochloric acid (HCl, 37 wt%), sublimated sulfur (S), methylbenzene (C₆H₆), and potassium hydroxide (KOH, ≥98%) were prepared from Aladdin Reagent Co., Ltd., Shanghai, China. The Ni foam was obtained from Kunshan Jiayisheng Electronics Co., Ltd., Kunshan, China. The anhydrous ethanol was purchased from Tianjin Kaitong Chemical Reagent Co., Ltd., Tianjin, China.

3.2. Synthesis of FeS(DI/MB)

The nickel foam (NF), measuring 0.5 cm × 0.5 cm × 0.01 cm, underwent successive ultrasonic cleaning with 3 M HCl, anhydrous ethanol, and deionized water for 30 min to eliminate the inert oxide film from the surface. After cleaning, dry the nickel foam base in an oven at 60 °C to obtain the prefabricated nickel foam base.

Employ an electronic balance to precisely measure 0.30 g of ferric nitrate nonahydrate dissolved in 20 mL of deionized water, followed by 0.05 g of sublimated sulfur dissolved in 30 mL of methylbenzene solution. Stir each solution individually. Subsequently, combine the two solutions to form a layered solution, with the organic and aqueous layers remaining immiscible. Submerge the prepared nickel foam sheet vertically into the lower layer of the aforementioned layered solution, then subject it to calcination in a muffle furnace at 150 °C for 5 h. Following cooling, wash the catalyst with distilled water and dry it to obtain the nickel iron sulfide catalyst prepared from the layered solution system, designated as FeS(DI/MB).

3.3. Synthesis of FeS(ET/MB)

Utilize an electronic balance to precisely measure 0.30 g of ferric nitrate nonahydrate dissolved in 20 mL of anhydrous ethanol, followed by 0.05 g of sublimated sulfur dissolved in 30 mL of methylbenzene solution. Stir each solution individually. Subsequently, combine the two solutions to obtain a homogeneous mixture. Submerge the pretreated nickel foam sheet vertically into the aforementioned mixed solution, then subject it to calcination in a muffle furnace at 150 °C for 5 h. After cooling, wash it with distilled water and dry it to obtain nickel–iron sulfide catalysts prepared from the layered solution system, designated as FeS(ET/MB).

3.4. Synthesis of Fe(DI/MB)

Accurately weigh 0.30 g of ferric nitrate hydrate using an electronic balance, then dissolve it in 20 mL of distilled water. Next, weigh 30 mL of methylbenzene solution and combine it with the previously prepared solution to form a layered solution where the organic and aqueous layers remain immiscible. Vertically place the pretreated nickel foam sheet in the lower layer of the layered solution mentioned above, then calcine it in a muffle furnace at 150 °C for 5 h. After cooling, wash the catalyst with distilled water and dry it to obtain the nickel–iron sulfide catalyst prepared from the layered solution system, designated as Fe(DI/MB).

3.5. Synthesis of Fe(ET/MB)

Accurately weigh 0.30 g of ferric nitrate hydrate using an electronic balance, then dissolve it in 20 mL of distilled water. Next, weigh 30 mL of methylbenzene solution and combine it with the previously prepared solution to form a mixed solution. Vertically place the pretreated nickel foam sheet in the lower layer of the mixed solution mentioned above, and then calcine it in a muffle furnace at 150 °C for 5 h. After cooling, wash the catalyst with distilled water and dry it to obtain the nickel–iron–sulfide catalyst prepared from the mixed solution, designated as Fe(DI/MB).

3.6. Electrochemical Measurements

All electrochemical measurements are carried out at the CHI440 electrochemical workstation under ambient conditions. A standard three-electrode system was adopted for OER. The electrodes (FeS(DI/MB), FeS(ET/MB), Fe(DI/MB), and Fe(ET/MB)) prepared in the above steps were used as working electrodes (0.25 cm⁻²), and carbon rod and Hg/HgO were used as counter and reference electrodes, respectively. The electrochemical test was performed at room temperature, 1 M and 6 M KOH. The potential obtained by calibration with a reversible hydrogen electrode (RHE) and the electrochemical test of the overall water decomposition were performed in a two-electrode system. The reversible hydrogen (RHE) electrode was used as a reference scale through the potential conversion equation:

E_RHE = E_Hg/HgO + 0.098 + 0.059 × pH (pH = 14.778)

(3)

Linear scanning voltammetry (LSV) was measured at 1 mV s⁻¹. Electrochemical impedance spectra (EIS) were tested in the frequency range of 100 kHz to 10 mHz. The electrochemical double-layer capacitance (C_dl) values were obtained by testing cyclic voltammetry (CV) at scan rates from 20 to 200 mV s⁻¹. Cyclic stability tests were performed for 5000 cycles at a scan rate of 50 mV s⁻¹. Chronopotentiometric stability tests were performed at a constant current density of 100 mA cm⁻².

3.7. Characterization Tests

Using an X-ray diffractometer (30–80°), the crystal characteristics of the electrocatalysts as-prepared were examined. Transmission electron microscopy (TEM) and field emission scanning electron microscopy (SEM) were used to observe morphology and microstructure at a 20 kV accelerating voltage. The VG-ESCA Scientific Theta Probe spectrometer and Al-K α Perform X-ray photoelectron spectroscopy (XPS) were used with a beam (1486.6 eV) to analyze the types of elements on the surface of the sample and to quantitatively and qualitatively analyze the chemical states of the elements contained within.

4. Conclusions

To sum up, through the step-by-step change in the test environment from 25 °C, 1 M KOH to the chamber and finally to the simulated industrial electrolysis conditions, a high temperature of 60 °C and a strong alkali of 6 M KOH, it is concluded that increasing the alkaline electrolyte and temperature are conducive to water decomposition. The OER performance of the FeS(DI/MB) catalytic electrode prepared with stratified solution system (DI/MB) is better than that of FeS(ET/MB) prepared with unstratified solution system (ET/MB), and the OER performance of Fe(DI/MB) is better than that of Fe(ET/MB). The results show that the catalyst prepared by the layered system has better reaction kinetics and is more suitable for water electrolysis. The OER performance of FeS(DI/MB) is better than that of Fe(DI/MB), and the OER performance of FeS(ET/MB) is better than that of Fe(ET/MB), indicating that the performance becomes better after vulcanization. The test also shows excellent electrochemical stability under simulated industrial electrolysis conditions, which is suitable for use as an industrial electrolytic water catalyst. This provides a new research idea for the research of electrolytic water, that is, to increase the electrochemical performance of the catalyst by changing the testing environment of the electrolyte.