Facile Immersing Synthesis of Pt Single Atoms Supported on Sulfide for Bifunctional toward Seawater Electrolysis

Abstract

Seawater electrolysis for hydrogen production represents a substantial opportunity to curtail production expenditures and exhibits considerable potential for various industrial applications. Platinum-based precious metals exhibit excellent activity for water electrolysis. However, their limited reserves and high costs impede their widespread use on a large scale. Single-atom catalysts, characterized by low loading and high utilization efficiency, represent a viable alternative, and the development of simple synthesis methods can facilitate their practical application. In this work, we report the facile synthesis of a single-atom Pt-loaded NiCoFeS_x (Pt@NiCoFeS_x) bifunctional catalytic electrode using a simple impregnation method on a nickel foam substrate. The resulting electrode exhibits low overpotentials for both HER (60 mV@10 mA cm⁻²) and OER (201 mV@10 mA cm⁻²) in alkaline seawater electrolytes. When incorporated into a seawater electrolyzer, this electrode achieves a direct current energy consumption of only 4.18 kWh/Nm³H₂ over a 100 h test period with negligible decay. These findings demonstrate the potential of our approach for industrial-scale seawater electrolysis.

1. Introduction

The integration of renewable energy with hydrogen production stands as a vital element within the burgeoning new energy industry [1,2,3,4,5]. In the industrial domain, the water electrolysis for hydrogen production necessitates the utilization of highly efficient catalysts to mitigate the overpotential. The oxygen evolution reaction (OER) [6,7,8,9,10,11,12,13], which encompasses a complex four-electron-proton coupled transfer process, is characterized by a significant energy barrier and is identified as the rate-determining step in the electrocatalytic water splitting for hydrogen production. At present, the hydrogen evolution reaction (HER) electrocatalyst most frequently employed is Pt/C [14,15,16], while the OER is catalyzed by IrO₂ [8,17,18,19]. Despite their commendable performance in terms of low overpotentials during electrocatalytic water splitting, the extensive application of these catalysts in the field of electrocatalysis is constrained by the prohibitive costs and limited availability of the constituent precious metals.

In recent years, numerous researchers have focused on investigating single-atom catalysts [20,21,22,23] to minimize the loading of precious metals and maximize their utilization efficiency. Nowadays, many single-atom catalysts, such as Pt and Ir, have been studied and applied in the field of water electrolysis. However, during the synthesis of single-atom catalysts, metal single atoms are prone to migration or agglomeration, forming clusters or nanoparticles in the preparation and reaction stages, leading to poor catalyst stability during long-term catalysis. The stability of metal single atoms is associated with the preparation method and local coordination environment, and it can be enhanced by modulating the interaction between the metal single atoms and the support to reduce surface energy. Optimizing the water-splitting performance of single-atom catalysts from the perspectives of rational structure design and durability is both significant and challenging.

Utilizing seawater as a feedstock for hydrogen production not only significantly reduces water cost but also mitigates the risk of a freshwater crisis that may arise from large-scale water electrolysis once hydrogen energy becomes widespread. However, the presence of abundant Cl⁻ ions in seawater can cause severe corrosion to non-precious metal-based electrodes [24,25,26,27,28]. Consequently, researchers have dedicated efforts to developing Ni-based phosphides, sulfides, nitrides, selenides, and other materials. For instance, Kuang et al. [29] developed an in situ synthesis method where a nickel sulfide passivation layer was first grown on the surface of nickel foam, followed by the in situ electrodeposition of NiFe layered double hydroxides (LDH) on the nickel sulfide surface to enhance its activity, successfully synthesizing a NiFe/NiS_x-Ni anode. This seawater electrolysis anode demonstrated stable operation for over 1000 h at industrial current densities ranging from 400 to 1000 mA cm⁻². In the research on bifunctional electrodes for seawater electrolysis, Shrestha et al. [30] synthesized the CoFeOF/NF bifunctional electrode, whose high electronegativity could electrostatically suppress chloride ions, achieving high stability at a current density of 400 mA cm⁻² for 145 h.

In this work, we successfully loaded Pt single atoms onto the surface of NiCoFeS_x nanoarray electrodes using a simple impregnation method. The prepared catalytic electrodes exhibited exceptional HER and OER activities in both 1 M KOH and 1 M KOH + 0.5 M NaCl (alkaline simulated seawater) electrolytes. Notably, in alkaline simulated seawater, the overpotentials for HER and OER at a current density of 10 mA cm⁻² were remarkably low, reaching only 71 mV and 201 mV, respectively. Furthermore, both electrodes maintained stability for over 200 h at 200 mA cm⁻² with negligible performance degradation. Finally, we assembled an electrolysis device for seawater, achieving an average cell voltage of only 1.75 V and a hydrogen production energy consumption as low as 4.18 kWh/Nm³H₂.

2. Results

The synthesis of Pt@NiCoFeS_x was achieved through a simple three-step reaction. Initially, a ternary NiCoFe-LDHs nanosheet array structure was grown in situ on the surface of Ni foam substrate via a hydrothermal method. Subsequently, NiCoFeS_x was synthesized through in situ topological reduction. Finally, Pt single atoms were loaded onto the surface of the NiCoFeS_x electrode by immersing the electrode in a solution containing H₂PtCl₆ at 60 °C for 12 h. Scanning electron microscopy (SEM) images of the samples revealed that the Ni foam substrate exhibited a three-dimensional cross-linked pore structure (Figure 1a). After the in situ growth of NiCoFe-LDHs via the hydrothermal method, a nanosheet array was formed on the surface of the substrate (Figure 1b). The NiCoFeS_x material prepared through topological transformation maintained the original nanosheet array structure (Figure 1c). This nanosheet array structure increased the specific surface area of the electrode, favoring the loading and dispersion of Pt single atoms on the electrode surface. The surface of the Pt@NiCoFeS_x catalyst prepared by the impregnation method appeared smooth (Figure 1d), indicating that the loading of Pt single atoms did not alter the morphology of the NiCoFeS_x material.

Transmission electron microscopy (TEM) images of the Pt@NiCoFeS_x electrode showed a clear and smooth nanosheet structure (Figure 1e) with an average size of approximately 30 nm. Bright spots in the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image demonstrated the highly uniform dispersion of Pt atoms on the material surface (Figure 1f), confirming the successful loading of Pt single atoms. The lattice spacing is 0.234 nm, which corresponds to the (021) plane of Ni3S2 at the diffraction peak of 38.01° in XRD. Compared to the PDF standard card’s lattice spacing of 0.236 nm, there is a certain degree of contraction, which is caused by the doping of Co and Fe. To investigate the distribution of various elements in the material, energy dispersive spectrometer (EDS) mapping was employed. The results showed that S, Ni, Co, Fe, and Pt were all uniformly distributed on the surface (Figure 1g).

The surface electronic structure of the Pt@NiCoFeS_x catalyst was characterized through X-ray photoelectron spectroscopy (XPS) to analyze the valence states and bonding behavior of Ni, Co, Fe, S, and Pt elements. In the Ni 2p spectrum (Figure 2a), peaks were identified at binding energies of 855.28 eV and 873.08 eV, which correspond to the Ni 2p3/2 and Ni 2p1/2 states, respectively, indicating the presence of Ni²⁺. Additional satellite peaks were observed at 861.28 eV and 879.13 eV, further corroborating the assignments for Ni 2p3/2 and Ni 2p1/2. A peak at 852.43 eV was attributed to 2p3/2 of Ni⁰, suggesting the coexistence of a nickel sulfide phase. Turning to the Co 2p spectrum (Figure 2b), peaks at binding energy of 778.11 eV and 793.15 eV were attributed to Co-S bonds. Signals at 780.75 eV and 796.25 eV were identified as Co 2p3/2 and Co 2p1/2, respectively, confirming the presence of Co²⁺. Corresponding satellite peaks were also observed at 785.05 eV and 802.65 eV. In the Fe 2p spectrum (Figure 2c), a signal at 709.78 eV was ascribed to Fe-S bonds. Meanwhile, peaks at 712.88 eV and 725.58 eV were assigned to Fe 2p3/2 and Fe 2p1/2, respectively, indicating the existence of Fe³⁺ ions. The S 2p spectrum (Figure 2d) revealed a peak at 168.63 eV, characteristic of S-O bonds. This peak likely arises from the oxidation of the catalyst upon exposure to air. Peaks at 161.38 eV and 162.43 eV were attributed to S 2p3/2 and S 2p1/2 of M-S bonds, respectively, confirming the presence of sulfides within the sample. In the Pt 4f spectrum (Figure 2e), peaks were fitted at binding energies of 72.93 eV and 76.23 eV, which were assigned to Pt²⁺ in the Pt 4f7/2 and Pt 4f5/2 states, respectively. The absence of peaks corresponding to metallic Pt in the spectrum provides further evidence that Pt is anchored on the support surface in the form of isolated single atoms.

Furthermore, a comparative X-ray diffraction (XRD) analysis was conducted between the NiCoFeS_x and Pt@NiCoFeS_x catalysts (Figure 2f). Notably, in the XRD pattern of Pt@NiCoFeS, no metallic diffraction peaks corresponding to Pt were observed, which aligns with the findings from HRTEM characterization. This absence of Pt peaks suggests that Pt is highly dispersed or in an amorphous state within the catalyst. Distinct peaks were identified at 2θ values of 44.62°, 52.05°, and 76.58°, which correspond to the crystalline structure of the nickel foam substrate. Peaks located at 2θ of 38.01° and 50.19° are attributed to the presence of Ni₃S₂. Similarly, peaks at 2θ of 31.31° and 55.31° represent the Co₃S₄ phase, while the peak at 2θ of 22.08° indicates the presence of Fe₉S₁₀. These observations provide further evidence for the successful synthesis of the Pt@NiCoFeS_x catalyst, confirming the incorporation of Pt into the NiCoFeS_x matrix without forming large Pt crystallites.

Linear sweep voltammetry (LSV) tests were conducted in a 1 M KOH solution to compare the HER performance of Pt@NiCoFeS_x single-atom catalysts synthesized under different conditions. At room temperature, when the reaction immersion times were 6, 9, and 12 h, the overpotentials at a current density of 10 mA cm⁻² were found to be 112 mV, 104 mV, and 102 mV, respectively (Figure S1a). These results demonstrate that increasing the immersion time can enhance the Pt loading, leading to improve HER activity. Upon maintaining a stirring time of 12 h and elevating the reaction temperature from room temperature to 60 °C, a remarkable decrease in HER overpotential was observed, dropping from 102 mV to 57 mV (Figure S1b). This significant improvement is potentially due to the intensified etching effect of H₂PtCl₆ on the substrate at higher temperatures, resulting in a higher number of metal vacancies. During the immersing process, the acidity of chloroplatinic acid may cause etching of the substrate, creating metal vacancies, thereby enabling the successful loading of Pt. As a result, the loading of Pt single atoms increases further, exhibiting superior HER catalytic activity. The HER performance of the Pt@NiCoFeS_x single-atom catalyst can be optimized by adjusting the synthesis conditions, including the immersion time and reaction temperature. These findings provide valuable insights into the design and synthesis of efficient HER catalysts based on Pt single atoms.

To investigate the versatility and universality of this method for synthesizing single-atom catalysts and further explore the electrocatalytic water-splitting performance of NiCoFeS_x electrodes loaded with different noble metal single atoms, Pt@NiCoFeS_x, Ir@NiCoFeS_x, and Ru@NiCoFeS_x electrodes were prepared under identical synthesis conditions. Using these electrodes as working electrodes in a 1 M KOH electrolyte, the catalytic activities of these three single-atom catalysts for HER and OER were analyzed and compared. The LSV polarization curves for electrocatalytic HER are shown in Figure 3a. At a current density of 10 mA cm⁻², the overpotentials of Pt@NiCoFeS_x, Ir@NiCoFeS_x, and Ru@NiCoFeS_x electrodes are 57 mV, 130 mV, and 133 mV, respectively. The HER catalytic activity of the Pt@NiCoFeS_x electrode was significantly better than that of Ir@NiCoFeS_x and Ru@NiCoFeS_x. More importantly, the HER electrocatalytic activity advantage of the Pt@NiCoFeS_x electrode became even more pronounced at high current densities, with an overpotential of only 169 mV at a current density of 200 mA cm⁻², demonstrating the crucial role of Pt noble metal single atoms in the electrocatalytic HER reaction, and showcasing the enormous potential of Pt@NiCoFeS_x electrodes for application in alkaline water electrolysis for hydrogen production. As shown in Figure 3b, all three electrodes, Pt@NiCoFeS_x, Ir@NiCoFeS_x, and Ru@NiCoFeS_x, exhibited great activity for OER. There is no significant difference in the catalytic OER performance among the three electrodes. At a current density of 10 mA cm⁻², the overpotentials of Pt@NiCoFeS_x, Ir@NiCoFeS_x, and Ru@NiCoFeS_x electrodes were 200 mV, 211 mV, and 214 mV, respectively. Based on the above characterization of the alkaline water-splitting performance of these three noble metal single-atom catalysts, it can be concluded that Pt@NiCoFeS_x exhibits superior activity in catalyzing both HER and OER reactions compared to the other two electrodes.

To further analyze and compare the activity before and after Pt single-atom loading, as well as with commonly used commercial catalysts such as IrO₂ and Pt/C, the electrocatalytic HER and OER performances of Ni foam, NiCoFe-LDH, NiCoFeS_x, and Pt@NiCoFeS_x electrodes, and commercial catalysts IrO₂ and Pt/C under alkaline conditions were evaluated using a standard three-electrode system.

In the LSV curves that assessed the HER performance of the samples (Figure 4a), the Pt@NiCoFeS_x electrode achieved an overpotential of only 57 mV at a current density of 10 mA cm⁻², which was comparable to the commercial Pt/C catalyst and 95 mV lower than that of the NiCoFeS_x electrode. Since alkaline water electrolysis for hydrogen production usually necessitates high current density, we further examined the HER catalytic activity of the catalysts at a current density of 200 mA cm⁻². The Pt@NiCoFeS_x electrode attained an overpotential of only 169 mV at a current density of 200 mA cm⁻², significantly surpassing the NiCoFeS_x electrode (327 mV) and the NiCoFe-LDH electrode (406 mV). These findings indicated that the Pt@NiCoFeS_x electrode retained excellent activity even under high current densities, suggesting its potential for practical applications. Moreover, the Tafel slope of Pt@NiCoFeS_x (Figure 4b) was solely 51.3 mV dec⁻¹, less than that of NiCoFeS_x (88.4 mV dec⁻¹) and NiCoFe-LDH (118.8 mV dec⁻¹). This exemplified the kinetic superiority of Pt@NiCoFeS_x in HER. From the LSV polarization curves for electrocatalytic OER (Figure 4c), it can be observed that the catalytic performance of the NiCoFeS_x electrode obtained through topological transformation is improved compared to the NiCoFe-LDH electrode. The catalytic activity of the Pt@NiCoFeS_x electrode, loaded with Pt noble metal single atoms, was further enhanced compared to the NiCoFeS_x electrode. At a current density of 10 mA cm⁻², the voltage was 1.43 V, corresponding to an overpotential of 200 mV, which was 20 mV lower than that of the NiCoFeS_x electrode, indicating that the uniform dispersion of Pt single atoms on the surface of the NiCoFeS_x catalyst is beneficial for enhancing the OER activity of the catalyst. More importantly, compared to the currently available commercial catalyst IrO₂, the Pt@NiCoFeS_x electrode exhibits superior performance, with η10 (overpotential at 10 mA cm⁻²) that is 37 mV lower. The Pt@NiCoFeS_x electrode not only improved the electrocatalytic OER activity but also reduced the usage of precious metals and lowered the cost, demonstrating its potential for large-scale applications. An analysis of the Tafel slopes of the samples (Figure 4b) revealed that the Tafel slope of the Pt@NiCoFeS_x electrode was 22 mV dec⁻¹, which was lower than that of NiCoFeS_x (25.4 mV dec⁻¹) and NiCoFe-LDH electrodes (33.4 mV dec⁻¹). This indicated that Pt@NiCoFeS_x exhibits a faster reaction kinetic rate. Based on the Tafel slope kinetic model, the rate-determining step for the fabricated catalytic electrode is the Tafel step [31]. Examination of the electrochemical impedance spectroscopy (EIS) plots for the NiCoFe-LDHs, NiCoFeS_x, and Pt@NiCoFeS_x electrodes (Figure S2) reveals that the Pt@NiCoFeS_x electrode exhibits the lowest charge transfer resistance. This reduced resistance indicates a faster charge transport rate, which contributes to the superior OER and HER catalytic activity observed in the Pt@NiCoFeS_x electrode. To further validate the stability of the electrode, a chronoamperometric stability test was conducted using a two-electrode system (Figure 4e). The electrode demonstrated remarkable stability exceeding 200 h with minimal performance decay. Furthermore, a comparison of the HER and LSV curves before and after the stability test (Figure S3) reveals that there is minimal change in the overpotential of the electrode.

To evaluate the performance of the bifunctional electrode in seawater, HER (Figure 5a) and OER (Figure 5b) tests were conducted in a 1 M KOH + 0.5 M NaCl electrolyte, which served as an alkaline simulated seawater solution. The results were then compared to those of other samples. At a current density of 10 mA cm⁻², the Pt@NiCoFeS_x electrode exhibited HER and OER overpotentials of 60 mV and 201 mV, respectively. These values were comparable to its performance in a 1 M KOH electrolyte alone and were superior to those of the other samples tested. As alkaline water or seawater electrolysis has already entered the industrialization stage, we sought to verify the applicability of our prepared electrode in seawater electrolysis devices. To this end, we scaled up the Pt@NiCoFeS_x electrode to a diameter of 50 mm, leveraging the simplicity of our synthetic approach, which involved merely enlarging the synthesis container. Subsequently, two of these 50 mm-diameter Pt@NiCoFeS_x electrodes were installed as the cathode and anode, respectively, in an electrolytic cell, which primarily consisted of a bipolar plate, sealing gasket, and diaphragm (Figure 5c). In a 6 M KOH + 0.5 M NaCl electrolyte, operating at 80 °C and a current density of 200 mA cm⁻², the cell demonstrated an impressive performance over a 100 h test period (Figure 5d). The average cell voltage remained low at just 1.75 V, translating to a direct current energy consumption of only 4.18 kWh/Nm³ H₂ for hydrogen production. This level of performance positions our system at the forefront of alkaline electrolyzers. Furthermore, by using seawater as the feedstock, our approach offers additional cost savings in water usage. After the stability test, we re-evaluated the HER and OER LSV curves of the electrodes (Figure S4), observing negligible changes in overpotential compared to their pre-test states. SEM characterization of the post-test electrodes (Figure S5) confirmed that the nanosheet array structure on the electrode surface remained intact. In addition, we also tested the Faradaic efficiency of H₂ and O₂ at different current densities within 50 s using the water displacement method. In the electrolytic cell test, the theoretical gas production was relatively large (producing 197.25 mL of H₂ at an electrolysis of 150 mA cm⁻² for 50 s), which allowed for a more accurate assessment of the Faradaic efficiency. The data results are shown in Table S1, where the Faradaic efficiencies of H₂ and O₂ are both 99.6%. This finding underscores the electrode’s remarkable stability in a seawater environment, highlighting its potential for industrial applications.

3. Material and Methods

3.1. Chemicals

All reagents employed in this study were of analytical grade and utilized as-is, without any need for further purification processes. Ni(NO₃)₂·6H₂O, Co(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O, H₂PtCl₆, Na₂S, CO(NH₂)₂, and NH₄F were purchased from Aladdin Industrial Corporation (Shanghai, China). KOH, NaCl, and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the solutions in this work were prepared by deionized water purchased from Hangzhou Wahaha Co., Ltd. (Hangzhou, China).

3.2. Synthesis of NiCoFe-LDHs

Ni foam with a thickness of 1.7 mm, purchased from Kunshan Teng Er Hui Electronic Technology Co., Ltd. (Kunshan, China), was cut into 3 × 4 cm pieces. These Ni foam pieces were individually soaked in hydrochloric acid, acetone, anhydrous ethanol, and deionized water for 10–15 min of ultrasonic cleaning to remove any oil and oxide contaminants from their surfaces. The cleaned Ni foam was then wrapped in plastic wrap and stored for further use. The NiCoFe-LDHs electrode was grown in situ on the Ni foam substrate using a hydrothermal method. Specifically, 0.096 g of Ni(NO₃)₂·6H₂O, 0.096 g of Co(NO₃)₂·6H₂O, 0.13 g of Fe(NO₃)₂·9H₂O, 0.60 g of CO(NH₂)₂, and 0.30 g of NH₄F were dissolved in 36 mL of deionized water to form a homogeneous solution. This solution was then transferred to a high-pressure reactor, along with the cleaned 3 × 4 cm Ni foam pieces. The reactor was placed in an oven set to 120 °C for a reaction duration of 12 h. After the reaction, the electrodes were repeatedly washed with deionized water and anhydrous ethanol and then dried overnight at 60 °C to obtain NiCoFe-LDHs.

3.3. Synthesis of NiCoFeS_x

The NiCoFeS_x electrode was prepared from the NiCoFe-LDHs material on the surface of the Ni foam using a topological transformation method. Specifically, 1.07 g of Na₂S·9H₂O was dissolved in 36 mL of deionized water to form a solution, which was then transferred into a high-pressure reactor. Subsequently, the 3 × 4 cm NiCoFe-LDHs-covered Ni foam was placed into the high-pressure reactor, and the reactor was sealed and heated in an oven at 120 °C for 4 h. After the reaction, the product was repeatedly washed with deionized water and anhydrous ethanol to remove any unreacted precursors or impurities. Finally, the NiCoFeS_x-covered Ni foam was dried overnight at 60 °C to obtain the NiCoFeS_x electrode.

3.4. Synthesis of Pt@NiCoFeS_x

The NiCoFeS_x electrode was cut into 1 × 2 cm pieces, which were subsequently immersed in separate aqueous solutions of 20 μmol/L hexahydro chloroplatinic acid, hexahydro chloroiridic acid, and ruthenium chloride trihydrate. Under continuous stirring at 60 °C, the NiCoFeS_x electrodes were allowed to react for 12 in each respective solution. This process resulted in the formation of Pt@NiCoFeS_x, Ir@NiCoFeS_x, and Ru@NiCoFeS_x electrodes, respectively.

3.5. Electrochemical Measurements

The electrochemical measurements were conducted under ambient conditions using a three-electrode configuration housed in a glass cell, connected to an electrochemical workstation (model CHI 660e, manufactured by CH Instruments, Shanghai, China). Electrocatalysts, measuring 1 cm × 1 cm, were directly used as the working electrode, while a carbon rod electrode served as the counter electrode and a saturated calomel electrode (SCE) provided the reference potential. A freshly prepared 1 M KOH solution was utilized as the electrolyte for catalyst activation assessment via cyclic voltammetry (CV), performed at a scan rate of 100 mV s⁻¹ for 20 consecutive cycles within a potential range of 0 to 1 V versus SCE. The electrochemical tests were then carried out in a 1 M KOH + 0.5 M NaCl solution, with all measured potentials referenced to SCE and converted to the reversible hydrogen electrode (RHE) scale using the Nernst equation (E_RHE = E + E_SCE + 0.059 pH). Polarization curves were obtained using CV at a scan rate of 5 mV s⁻¹ and corrected for Ohmic drops determined through impedance spectroscopy. Additionally, electrochemical impedance spectroscopy (EIS) measurements were conducted in the 1 M KOH solution by applying an AC voltage with a 10 mV amplitude at the open-circuit potential, over a frequency range from 100 kHz to 0.1 Hz. This comprehensive experimental approach ensures accurate and reproducible results for electrochemical analysis. The IR correction is applied using the following formula: E_RHE = E_SCE + 0.059pH −iR₀.

3.6. Materials Characterization

The specimens’ dimensions and microscopic structure were meticulously examined by employing a field-emission scanning electron microscope (FE-SEM, model JEOL JSM6335, Akishima, Japan) operating at a 20 kV accelerating voltage. Before HADDF-STEM imaging, the prepared catalyst was added to anhydrous ethanol using ultrasonic waves to form a very dilute colloidal suspension. Then, 20 microliters of this suspension were dropped onto a 230-mesh Cu grid coated with an ultrathin carbon film. Imaging was performed using a Thermo Fisher Spectrum 300 microscope (Waltham, MA, USA) equipped with an aberration corrector for the probe-forming lens. Under the conditions of a beam current less than 40 pA, a probe convergence semi-angle of approximately 25 mrad, and a probe size of about 0.6 Å, HADDF-STEM images were acquired. The sample could be moved in both the α (±35°) and β (±30°) directions using a double-tilt holder, and it was tilted at −30° (α) to obtain the images. To simulate the HADDF-STEM images, Dr. Probe software (1.10.8) was used with parameters set to match the experimental conditions. The precision of the simulation results was 0.008 nanometers per pixel. To delve deeper into their structural properties, X-ray diffraction (XRD) patterns were acquired using a Rigaku D/max 2500 X-ray diffractometer (Akishima, Japan) equipped with Cu Kα radiation, operating under conditions of 40 kV, 30 mA, and a wavelength of 1.5418 Å. These measurements were precisely executed over a 2θ angular range spanning from 5° to 80°, with a scanning rate of 5° per minute. Furthermore, comprehensive X-ray photoelectron spectroscopy (XPS) analyses were performed using the PHI Quantera II XPS Scanning Microprobe system (Physical Electronics, Chanhassen, MN, USA), ensuring a thorough investigation of the specimens’ properties.

3.7. Performance Evaluation of Seawater Electrolysis Devices

The testing system for the seawater electrolysis device, provided by Zhangjiakou Ruiqing Technology Co., Ltd. (Zhangjiakou, China), consists of several essential elements, including an electrolyzer, gas–liquid separator, DC power supply, electrolyte circulation pump, and temperature control system. It is worth mentioning that the electrolyzer stands out because of its dual-cell configuration and the incorporation of the AGFA ZIRFON UTP500 composite membrane (Mozer, Belgium), ensuring optimal performance. Inside the electrolytic cell, both the cathode and the anode are made of electrodes prepared in this work. The testing conditions were meticulously defined, maintaining a temperature of 80 °C and an electrolyte flow rate of 3 L/min. According to the Chinese National Standard GB3211-2015 [32], the unit DC energy consumption value W_d (kWh/Nm³) of the electrolyzer is calculated as W_d = 2390 × E₀/1000. After simplification, the formula is then E₀ × 2.39.

4. Conclusions

In summary, we present a straightforward method for synthesizing a bifunctional catalytic electrode comprised of single-atom Pt-loaded NiCoFeS_x (Pt@NiCoFeSx) using a basic impregnation technique on a nickel foam base. The resulting electrode showcases exceptional performance with low overpotentials for both HER and OER in alkaline seawater electrolytes, achieving 60 mV and 201 mV at 10 mA cm⁻², respectively. When integrated into a seawater electrolyzer, this electrode maintains robust performance over a 100 h test period, consuming only 4.18 kWh/Nm³H₂ of direct current energy with minimal degradation. These results underscore the promise of our method for large-scale seawater electrolysis applications.