Direct 2400 h Seawater Electrolysis Catalyzed by Pt-Loaded Nanoarray Sheets

Abstract

Seawater electrolysis offers a sustainable route for large-scale, carbon-neutral hydrogen production, but its industrial application is limited by the poor efficiency and durability of current electrocatalysts under high current densities. Herein, we synthesized ultrasmall Pt nanoclusters uniformly anchored on FeCoNi phosphide nanosheet arrays, forming a composite catalyst with outstanding hydrogen evolution reaction (HER) performance in alkaline seawater. The catalyst achieves an ultralow overpotential of 17 mV at −10 mA cm⁻², far surpassing commercial Pt/C, and stably delivers industrial-level current densities up to 2000 A m⁻² for over 2400 h with minimal voltage degradation and low energy consumption (4.16 kWh/Nm³ H₂). X-ray photoelectron spectroscopy revealed strong interfacial electronic interactions between Pt and Fe/Co species, involving electron transfer from Pt that modulates its electronic structure, weakens hydrogen adsorption, and enhances both HER kinetics and Pt dispersion. This work presents a scalable and robust catalyst platform, bridging the gap between laboratory research and industrial seawater electrolysis for green hydrogen production.

1. Introduction

Hydrogen energy has emerged as a promising solution to address global energy and environmental challenges due to its high energy density and clean combustion characteristics [1,2,3]. With the rapid development of offshore renewable energy sources, particularly offshore wind power, the integration of electricity generation with water electrolysis for hydrogen production offers a sustainable energy strategy [4,5]. However, the scarcity of freshwater resources significantly restricts the large-scale application of traditional electrolysis technologies in offshore environments [6]. As seawater accounts for over 96% of the Earth’s water resources, directly utilizing seawater for electrochemical hydrogen production presents a cost-effective and resource-abundant alternative, especially for remote coastal or island-based renewable energy systems [5,7,8]. Realizing efficient and durable direct seawater electrolysis holds transformative potential for carbon-neutral energy infrastructure.

Despite its promise, direct seawater electrolysis still faces considerable technical challenges. Natural seawater contains a wide range of dissolved salts and organic impurities, among which chloride ions alone account for up to 55.0% of the total solute content [9,10]. These aggressive anions can severely reduce the catalytic activity of electrode materials, accelerate corrosion, and compromise the stability and lifespan of electrolytic systems—hindering their scalability and practical deployment in marine environments [11,12]. Due to the inherently low conductivity of natural seawater, the kinetics of the hydrogen evolution reaction (HER) are significantly hindered, necessitating the development of highly active HER catalysts. To address this challenge, researchers have explored various strategies such as heteroatom doping, defect engineering, and surface modification. These approaches aim to enhance catalytic activity by constructing heterostructures, increasing surface area, and introducing synergistic effects. Yang et al. demonstrated that molybdenum (Mo) doping can accelerate electron transfer within the catalyst and shift the d-band center of Ni in NF@Ni₀.₈₅Se downward, thereby reducing the hydrogen adsorption energy and facilitating hydrogen desorption [13]. Similarly, Liu et al. fabricated a multiphase CoNiP/Co_xP heterostructure on nickel foam via electrodeposition followed by phosphidation. The strong electronic interactions at the heterointerfaces were found to significantly promote water dissociation and hydrogen desorption processes [14]. Platinum (Pt) is widely recognized as the most effective catalyst for the HER due to its optimal hydrogen adsorption energy [15]. However, commercial Pt/C catalysts suffer from high material costs, low atomic utilization, and poor long-term stability under harsh alkaline and chloride-rich conditions [16]. To address these issues, synthesizing ultrasmall Pt nanoparticles with high dispersion is critical to enhance atomic utilization and reduce cost without sacrificing activity [17,18]. These small Pt particles form strong metal–support interactions with transition metals [19,20], optimizing the electronic environment of the Pt active sites. Furthermore, the vertically aligned substrate structure enables rapid gas bubble release during electrolysis. Furthermore, constructing an efficient support structure that can electronically interact with Pt—such as incorporating iron (Fe) and cobalt (Co) species—can not only modulate the electronic environment of Pt to enhance its intrinsic activity but also helps anchor the Pt nanoparticles more securely, thereby improving long-term stability and resistance to current fluctuations in variable offshore energy scenarios [21,22,23].

In this work, we developed a highly efficient Pt@FeCoNi-P catalyst by anchoring ultrasmall Pt nanoparticles onto FeCoNi trimetallic phosphide nanosheets. This design not only maximizes Pt atomic utilization but also leverages strong interactions with Fe, Co, and Ni to boost catalytic activity and stability. The catalyst exhibits a low overpotential of just 17 mV at −10 mA cm⁻² and maintains excellent performance under industrial current densities. In a membrane electrode assembly, it enabled stable seawater electrolysis over 2400 h at 2000 A m⁻² with low energy consumption, showing strong potential for practical large-scale hydrogen production.

2. Results

The synthesis route of the catalyst is illustrated in Figure 1a. Initially, a vertically aligned nanoarray of FeCoNi–layered double hydroxides (FeCoNi-LDH) was grown in situ on a nickel foam substrate via a hydrothermal method. As confirmed by scanning electron microscopy (SEM) in Figure S1, the as-synthesized FeCoNi-LDH exhibited a well-defined nanosheet architecture uniformly distributed on the Ni foam. This nanostructured configuration provided a high density of exposed surface area and structural channels conducive to subsequent modification and mass transfer. Afterward, the FeCoNi-LDH underwent a phosphorization treatment in a tube furnace under a phosphorus-containing atmosphere, converting the hydroxide phase into transition metal phosphides, denoted as FeCoNi-P. SEM images (Figure S2) indicate that the nanosheet array morphology was well-preserved after the phosphorization step. X-ray diffraction (XRD) analysis was performed to determine the phase composition and crystalline structure of the catalyst. As shown in Figure S9 (Supporting Information), the XRD pattern of the Pt@NiFeCo-P catalyst displays distinct diffraction peaks corresponding to Ni₂P (JCPDS No. 74-1385), FeP (JCPDS No. 39-0809), and CoP (JCPDS No. 29-0497), confirming the successful formation of a multiphase phosphide system. Subsequently, platinum nanoparticles (Pt NPs) were deposited on the surface of FeCoNi-P via a second hydrothermal process. This yielded the final catalyst, referred to as Pt@FeCoNi-P. The SEM image in Figure 1b shows that the overall nanoarray structure remained intact after Pt loading, with no observable agglomeration or collapse. The vertically oriented nanosheet arrays were beneficial for facilitating charge transport and electrolyte diffusion while providing abundant anchoring sites for the Pt NPs. High-resolution transmission electron microscopy (HRTEM) was employed to investigate the structural details of the catalyst. As presented in Figure 1c and Figure S3, the Pt@FeCoNi-P retained its two-dimensional nanosheet morphology, confirming the successful phase transformation without significant degradation of the nanostructure. Upon further magnification, multiple lattice fringes corresponding to different phosphide phases were observed (Figure 1d). The lattice spacing measured in region d₁ was approximately 0.28 nm, which matched the (011) plane of CoP, while the fringes in region d₂ showed a spacing of around 0.24 nm, corresponding to the (111) plane of FeP. These findings indicate that the substrate of Pt@FeCoNi-P is composed of a multiphase polycrystalline mixture of transition metal phosphides. Further observation revealed that Pt NPs were uniformly distributed across the FeCoNi-P substrate (Figure 1e). In the enlarged region e₁, lattice fringes with a spacing of 0.23 nm can be clearly discerned, which could be attributed to the (111) plane of face-centered cubic Pt. The nearby region e₂ displays lattice spacings of approximately 0.27 nm, consistent with the (011) plane of FeP, demonstrating the close integration of Pt with the phosphide substrate. The Pt nanoparticles exhibit a narrow size distribution, with diameters ranging from 1 to 3 nm, indicating their ultrasmall size and high dispersion. This morphology was expected to contribute significantly to catalytic performance by exposing more active Pt atoms and facilitating strong metal–support interactions.

To elucidate the interfacial electronic interactions between platinum nanoparticles and the multimetallic phosphide support, X-ray photoelectron spectroscopy (XPS) was employed for both Pt@FeCoNi-P and FeCoNi-P samples. As shown in Figure 2a, the Pt 4f region of Pt@FeCoNi-P exhibits two sets of doublets corresponding to metallic Pt⁰ and oxidized Pt²⁺ states [24]. The appearance of higher-valence Pt is indicative of a strong interaction between Pt and the surrounding phosphide species. This phenomenon is often attributed to electron redistribution at the metal–support interface, wherein Pt donates partial electron density to the adjacent electronegative elements, such as Fe and Co. XPS analysis was conducted on both Pt/C and the Pt@FeCoNi-P catalyst. As shown in Figure S10 (Supporting Information), the Pt 4f spectra exhibit a pronounced binding energy shift toward lower values for Pt@FeCoNi-P compared to Pt/C, indicating a lower oxidation state of Pt⁰ in the composite catalyst. This shift is attributed to strong electronic interactions between the Pt nanoparticles and the phosphorus-rich transition metal phosphide substrate–interactions that are absent in conventional Pt/C systems. These interfacial electronic modifications led to a noticeable downshift in the d-band center of Pt, which in turn mitigated the adsorption strength of hydrogen intermediates (*H) on the Pt surface. This weakening of hydrogen binding not only optimized the thermodynamics of the hydrogen evolution pathway but also reduced the energy barrier for the Volmer–Heyrovsky steps, thereby significantly promoting the HER kinetics in alkaline environments. The Fe 2p spectrum (Figure 2b) provides further evidence of this electronic interaction. Upon Pt loading, the Fe 2p_3/2 peak in Pt@FeCoNi-P shifted positively by 0.4 eV compared to the pristine FeCoNi-P, indicating a reduction in the Fe oxidation state [25]. This binding energy shift supports the presence of electron transfer from Pt to Fe, suggesting the formation of an interfacial electronic gradient. Such Pt–Fe charge coupling was previously shown to stabilize Pt against oxidation or agglomeration under electrolytic conditions while simultaneously modifying Fe sites to participate in dual-functional catalytic roles such as hydrogen adsorption or OH^– dissociation [26]. Moreover, this interfacial engineering likely induced an “electron bridge” effect, facilitating long-range conductivity across the nanoarray. A similar trend can be observed in the Co 2p spectrum (Figure 2c), where the Co 2p_3/2 peak also shifted to lower binding energy by approximately 0.2 eV upon Pt deposition. This shift implies a lower Co oxidation state and confirms the electron delocalization between Pt and Co [27]. Such modulation of Co valency was correlated with enhanced electronic conductivity and increased surface charge density, both of which are beneficial for HER performance in alkaline environments. Additionally, CoP-based species promoted the initial water dissociation step by lowering the energy barrier for OH^– cleavage, which synergistically worked with Pt’s high activity in proton reduction. This interfacial cooperation between Pt and CoP establishes a bifunctional HER mechanism, resulting in improved reaction kinetics and enhanced catalytic stability during long-term alkaline electrolysis [28]. In contrast, the Ni 2p spectrum (Figure 2d) remained largely unchanged after Pt introduction, suggesting that Ni played a more electrochemically inert or supportive role in this system. Its stability, however, may still contribute to maintaining the structural integrity of the array during long-term operation. Taken together, the XPS data reveal clear evidence of strong interfacial charge interactions between Pt and both Fe and Co species within the multimetallic phosphide framework. These interactions not only tuned the local electronic environment of active sites but also enhanced the anchoring and dispersion of Pt NPs, preventing migration or sintering under high-current-density seawater electrolysis. The synergistic effect between Pt and the Fe/CoP support likely played a vital role in boosting HER activity and long-term operational durability.

To evaluate the HER performance in alkaline simulated seawater, linear sweep voltammetry (LSV) measurements were conducted in the 1.0 M NaOH + 0.5 M NaCl electrolyte using four different electrodes: Pt@FeCoNi-P, commercial Pt/C, FeCoNi-P, and bare Ni foam. As shown in Figure 3a, Pt@FeCoNi-P exhibits the most favorable catalytic activity among all tested samples. The overpotentials required to reach specific current densities are summarized in Figure 3b. At a current density of −10 mA cm⁻², the overpotential of Pt@FeCoNi-P was only 17 mV, which was 24 mV lower than that of Pt/C (41 mV), 69 mV lower than FeCoNi-P (86 mV), and 240 mV lower than Ni foam (257 mV), respectively. The performance gap became even more pronounced under industrial-level current densities. At −100 mA cm⁻², Pt@FeCoNi-P required an overpotential of only 91 mV, outperforming Pt/C (142 mV), FeCoNi-P (266 mV), and Ni foam (443 mV). At a much higher current density of −400 mA cm⁻², Pt@FeCoNi-P still maintained remarkable activity, with an overpotential of just 168 mV, significantly lower than that of Pt/C (231 mV), FeCoNi-P (447 mV), and Ni foam (762 mV). These values corresponded to overpotentials that were 1.4, 2.7, and 4.5 times higher than that of Pt@FeCoNi-P, respectively. To further probe the intrinsic reaction kinetics, Tafel slope analysis was performed (Figure 3c). Pt@FeCoNi-P showed a low Tafel slope of 53.5 mV dec⁻¹, indicating fast HER kinetics in alkaline media. In contrast, Pt/C exhibits a slightly higher slope of 61.0 mV dec⁻¹. The FeCoNi-P and Ni foam electrodes display much larger Tafel slopes of 161.0 and 150.1 mV dec⁻¹, respectively, suggesting sluggish HER kinetics due to the absence of efficient active sites or synergistic interfaces [29]. Electrochemical impedance spectroscopy (EIS) was also employed to assess the charge transfer behavior (Figure 3d). Among all the electrodes, Pt@FeCoNi-P exhibits the smallest semicircle in the Nyquist plot, indicating the lowest charge transfer resistance. Equivalent circuit fitting results for the three catalysts (Figures S6–S8 and Table S3) further confirm that Pt@FeCoNi-P exhibits the lowest charge-transfer resistance (R_ct), indicating more effective exposure of active sites and faster interfacial electron transport. These characteristics collectively account for its superior electrocatalytic performance compared to the other tested materials. This confirmed that the introduction of ultrasmall Pt NPs and the conductive FeCoNi phosphide support facilitated rapid electron transport and interfacial charge transfer, both of which are essential for sustained high-rate hydrogen evolution.

The remarkable catalytic activity of Pt@FeCoNi-P in three-electrode configurations motivated further investigation of its viability in practical seawater electrolysis devices. To bridge the performance gap between lab-scale testing and industrial-scale operation under high current densities, a large-area circular Pt@FeCoNi-P electrode with a diameter of 11.8 cm was fabricated and integrated into a membrane electrode assembly (MEA)-based electrolyzer system. The prepared electrode served as the cathode, while FeCoNi-P was employed as the anode. A two-cell electrolyzer stack was assembled using these components. A schematic diagram of the electrolyzer configuration is shown in Figure S4. Each individual cell within the electrolyzer stack comprised a cathode, anode, bipolar plate, sealing gasket, and Zirfon UTP 500 membrane (AGFA, Mortsel, Belgium). The complete system was further coupled with a gas–liquid separator, power supply system, electrolyte circulation system, and temperature control system to construct a fully functional alkaline electrolysis device suitable for long-term operation and high-rate hydrogen production. As illustrated in Figure 4a, Pt@FeCoNi-P exhibits superior performance compared to commercial Pt/C electrodes, delivering lower cell voltages at identical current densities. This clearly indicates its excellent efficiency in converting electrical energy into chemical fuel. In addition to catalytic activity, long-term operational stability is critical for industrial applications. To assess this, the durability of the Pt@FeCoNi-P electrode was evaluated in a two-electrode configuration at an industrial current density of 2000 A m⁻². The chronopotentiometry results (Figure 4b) demonstrate that Pt@FeCoNi-P could continuously sustain hydrogen production for over 2400 h in 20 wt.% NaOH with saturated NaCl electrolyte, without any noticeable increase in cell voltage. Throughout the operation, the average cell voltage was maintained at approximately 1.74 V, corresponding to a hydrogen production energy consumption of 4.16 kWh/Nm³ H₂—placing it among the best-performing commercial-scale alkaline water electrolyzers reported to date. Post-reaction SEM analysis was carried out to assess the structural integrity of the Pt@FeCoNi-P catalyst after HER test. The result showed that the nanosheet array morphology was well-retained (Figure S5). As shown in Figure S11, the Pt 4f binding energy exhibited a pronounced negative shift after the long-term stability test, while residual Pt²⁺ species were still detectable. This result confirms the sustained electronic interaction between Pt and the phosphorus-rich transition metal phosphide substrate. The metallic Pt⁰ species played a dominant role in facilitating *H adsorption and H₂ desorption during the HER. Meanwhile, the sub-stoichiometric Pt²⁺ component contributed to interfacial stabilization by inhibiting nanoparticle sintering and modulating the energy barriers associated with water dissociation. Together, these dual valence states synergistically maintained both catalytic activity and structural integrity under prolonged electrochemical operation. The HER performance of the Pt@FeCoNi-P catalyst after long-term stability testing was assessed via post-reaction LSV. As shown in Figure S12, the overpotential at a current density of −10 mA cm⁻² exhibits a negligible shift, with only a 3 mV decrease observed. This minimal change in performance underscores the catalyst’s outstanding electrochemical durability and structural stability under prolonged operating conditions. Faradaic efficiency, another key metric in electrolysis, was evaluated using a gas drainage method over 300 s at various current densities. Given the large gas output under industrial current conditions, this approach provided a realistic and precise assessment. As shown in Figure 4c and Figure S6 and summarized in Table S1, the Faradaic efficiencies for H₂ and O₂ reached 99.49% and 99.08%, respectively, confirming nearly quantitative conversion of charge to gas products with negligible secondary reactions. When benchmarked against recently reported HER catalysts in alkaline environments (Figure 4d and Table S2), Pt@FeCoNi-P outperforms many existing materials in terms of long-term durability. A photo overview of the electrochemical device is depicted in Figure 4e. These results confirm its excellent adaptability and robustness, underscoring its practical potential for industrial-scale alkaline seawater electrolysis under real-world energy conditions.

3. Material and Methods

3.1. Chemicals

All reagents used in this study were of analytical grade and utilized without further purification. Cobalt(II) nitrate hexahydrate (Co(NO₃)₂·6H₂O, 99.99%), iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O, 99.99%), ammonium fluoride (NH₄F, 99.9%), and urea (CO(NH₂)₂, 99.9%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Hexachloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O, 99.99%) was purchased from Suzhou Platinum-Hydrogen New Energy S&T Co., Ltd. (Suzhou, China). Potassium hydroxide (NaOH, ≥96%), sodium hypophosphite (NaH₂PO₂, 99.99%), sodium chloride (NaCl, ≥99.5%), and ethanol (CH₃CH₂OH, ≥99.7%) were purchased from Fuchen Chemical Reagent Co., Ltd. (Tianjin, China). High-purity deionized water (resistivity >18.2 MΩ·cm) was supplied by Hangzhou Wahaha Co., Ltd. (Hangzhou, Zhejiang, China) and used in all solution preparations and washing procedures.

3.2. Synthesis of Pt@FeCoNi-P

The synthesis of Pt@FeCoNi-P involved a three-step process: hydrothermal growth of NiCoFe-LDH on nickel foam, phosphidation to form NiCoFe-P, and subsequent platinum loading.

• Step 1: Synthesis of NiCoFe-LDH precursor

Commercial nickel foam (3 × 3 cm²) was first pretreated by sequential ultrasonication in ethanol and deionized water for 15 min to remove surface impurities and oxides. Subsequently, 0.873 g of Co(NO₃)₂·6H₂O, 0.404 g of Fe(NO₃)₃·9H₂O, 0.3 g of urea, and 0.778 g of NH₄F were accurately weighed and dissolved in 30 mL of deionized water under magnetic stirring to form a homogeneous solution. The cleaned nickel foam was immersed into the precursor solution, which was then transferred to a 50 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 120 °C for 12 h to promote hydrothermal crystallization. After cooling to room temperature, the resulting substrate was removed, washed thoroughly with deionized water and ethanol several times to eliminate residual ions, and dried at 60 °C overnight to obtain NiCoFe-LDH.

• Step 2: Phosphidation to form NiCoFe-P

The obtained NiCoFe-LDH was placed in a porcelain boat along with 0.5 g of sodium hypophosphite (NaH₂PO₂·H₂O), positioned upstream in a quartz tube furnace under nitrogen flow. The temperature was ramped to 300 °C at a rate of 5 °C/min and maintained for 2 h to complete the phosphidation process. After naturally cooling to room temperature under nitrogen, the product was rinsed with deionized water and ethanol and dried to yield the NiCoFe-P sample.

• Step 3: Platinum deposition

For platinum deposition, the NiCoFe-P sample was immersed in 30 mL of a 0.001 mol·L⁻¹ H₂PtCl₆ aqueous solution and maintained at 80 °C for 30 min to allow for spontaneous redox-driven Pt crystallization onto the surface. After deposition, the catalyst was thoroughly rinsed with deionized water and ethanol and dried under vacuum, yielding the final Pt@FeCoNi-P catalyst.

3.3. Materials Characterization

The morphology and microstructure of the samples were investigated using a field-emission scanning electron microscope (FE-SEM, Zeiss Supra 55, Oberkohen, Germany) operated at an accelerating voltage of 20 kV. High-resolution transmission electron microscopy (HRTEM) was carried out using a JEOL JEM-2100 instrument (Tokyo, Japan) operated at 200 kV to observe lattice fringes and nanoparticle dispersion. The surface elemental composition and chemical valence states were analyzed using X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA) equipped with a monochromatized Al Kα radiation source (hν = 1486.6 eV). All XPS spectra were calibrated using the C 1s peak at 284.8 eV as the reference.

3.4. Electrochemical Measurements

All electrochemical tests were performed using a CHI760E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) in a standard three-electrode setup. The working electrodes were the synthesized catalyst materials supported on nickel foam with a geometric surface area of 1 cm². A graphite rod and a saturated calomel electrode (SCE; Tianjin Aida Hengsheng Technology Development Co., Ltd., Tianjin, China) were employed as counter and reference electrodes, respectively. The electrolyte was composed of freshly prepared 1 M NaOH and 0.5 M NaCl to simulate an alkaline seawater environment. During electrochemical testing, the cell was placed in a constant-temperature oil bath to maintain thermal stability. The measured potentials versus SCE were converted to reversible hydrogen electrode (RHE) potentials using the following Nernst equation:

E(RHE) = E(SCE) + 0.059 × pH + 0.244 V

Linear sweep voltammetry (LSV) was conducted in the potential range of 0–1.0 V vs. SCE at a scan rate of 5 mV·s⁻¹. Prior to measurements, the catalyst surface was activated by 20 cycles of cyclic voltammetry (CV) at a scan rate of 100 mV·s⁻¹. Electrochemical impedance spectroscopy (EIS) was performed at an overpotential of 10 mV with an AC perturbation of 5 mV in the frequency range from 1000 kHz to 0.1 Hz. All polarization curves were corrected for iR drop based on the Ohmic resistance derived from EIS measurements.

3.5. Performance Evaluation of Water Electrolysis Devices

To evaluate the practical applicability and long-term durability of catalysts under industrially relevant conditions, many research groups have developed and implemented specialized testing platforms [30]. In this study, we utilized a commercial water electrolysis system for performance assessment, enabling a more realistic evaluation of the catalyst’s operational stability and scalability for hydrogen production. The system was supplied by Zhangjiakou Ruiqing Technology Co., Ltd. (Zhangjiakou, China) and included essential components such as an electrolyzer, gas–liquid separator, power supply, temperature controller, and electrolyte circulation pump. The electrolyzer consisted of two chambers connected in series, with each chamber equipped with two current collectors, one cathode, one anode, a sealing gasket, and a ZIRFON UTP500 composite membrane (AGFA, Agfa-Gevaert N.V., Mortsel, Belgium). The electrodes used were circular with a diameter of 118 mm. The electrolyte used was a mixture of 20 wt.% NaOH and saturated NaCl, mimicking the high ionic strength and alkalinity of industrial electrolytes. Electrolysis experiments were conducted at a constant temperature of 80 °C, representative of typical large-scale alkaline water electrolysis systems. The electrolyte circulation rate was maintained at 3 L·min⁻¹ to ensure efficient mass transport and effective thermal management. This setup enabled stable operation at high current densities and provided a robust platform for evaluating electrode performance under realistic electrolysis conditions. In terms of data acquisition and analysis, the reported cell voltage refers to the average voltage per cell, obtained by dividing the total measured stack voltage by two, given that the electrolyzer operates with two cells connected in series. To evaluate the energy efficiency of hydrogen production, the direct current energy consumption was calculated by multiplying the average cell voltage by a coefficient of 2.39.

4. Conclusions

This work developed a highly efficient and durable Pt@FeCoNi-P catalyst for alkaline seawater electrolysis. It achieved ultra-low overpotentials (17 mV at −10 mA cm⁻²) and superior HER kinetics, significantly outperforming commercial Pt/C. When scaled to an industrial two-cell electrolyzer, it sustained 2000 A m⁻² for over 2400 h with minimal voltage increase and low energy consumption (4.16 kWh/Nm³ H₂), demonstrating exceptional operational stability and practical viability. Based on these findings, we propose the following strategies to further enhance catalyst performance for direct seawater electrolysis: (1) enhance active site exposure via nanostructure design; (2) employ porous support materials to improve active site density; (3) introduce desirable defects (e.g., oxygen vacancies) via defect engineering; (4) utilize heteroatom doping to fine-tune the electronic structure; (5) apply phase engineering to optimize material properties; and (6) construct heterogeneous interfaces to promote charge transport and catalytic synergy. This technology offers in situ conversion of offshore renewable electricity, addressing long-distance power transmission challenges and fostering innovation in marine energy and global energy decarbonization. Beyond marine applications, seawater electrolysis can be extended to hydrogen production from high-salinity water sources such as salt lakes, oil/gas field produced water, and industrial effluents, breaking freshwater dependence and supporting the development of a diverse, sustainable hydrogen energy system.