Supersolidophobic Pt catalyst for long-term natural seawater electrolysis with hydrogen production and magnesium extraction

Domain: physics
Published: 2025-12-12
Editorial outcome: major_revision

Abstract
The electrochemical synthesis of solid compounds is a critical emerging area in electrocatalysis; however, a major challenge is still remaining on severe catalyst scaling driven by strong solid-solid interactions, giving rise to rapid electrode deactivation. In this study, for the example of simultaneous synthesis of Mg(OH)2 and H2 from natural seawater electrolysis, we demonstrate that Pt catalyst with surface-coordinated halogens (F, Cl, Br, I) can alleviate this scaling effect (i.e., solidophobicity) by like-charge repulsion and thus regulating the local environment. Specifically, Pt-I coordination results in supersolidophobicity, achieving a successful extraction of Mg(OH)2 (>99% purity) while stably producing H2 (under 100 mA cm-2 for over 5000 hours). A combination of experimental and theoretical studies reveals that, due to the like charge repulsion between I- and insitu generated OH-, the Pt-I catalyst regulates the surface pH gradient which increases the distance from the electrode surface to the Mg(OH)2 nucleus (>4 μm) and facilitates homogeneous nucleation. Additionally, the scalability of the Pt-I catalyst, along with the techno-economic analysis and life cycle assessment of the natural seawater electrolysis technology are systematically demonstrated.

Introduction
The production of green hydrogen fuel through water electrolysis is crucial for the advancement of renewable energy systems1–4. According to the World Energy Council, global hydrogen demand is projected to reach at least 148 million tons per year by 20505,6. However, compared to gray hydrogen, which is produced via methane reforming or coal gasification, the cost of green hydrogen production remains significantly higher (~US$2 and ~US$5 per kg H2 for gray and green hydrogen, respectively)7. In addition to the cost reduction of renewables, coupling water electrolysis with other value-increment reactions is considered an effective strategy to offset energy input costs8–10. The general approach involves replacing the oxygen evolution reaction (OER) with alternative anodic reactions that exhibit lower thermodynamic equilibrium potential and/or reduced kinetic overpotential11–13, such as electrooxidation of biomass molecules14, plastic wastes15, and organic compounds16. These alternatives have been extensively explored, achieving significant progress17–19. Another strategy is to capitalize on the surface alkaline environment generated in situ during the hydrogen evolution reaction (HER) at the cathode, which facilitates the extraction of valuable metal resources from the electrolyte by forming easily separable hydroxide precipitates. For instance, the electrolysis of natural seawater enriched with Mg2+ (>0.79 g/L) enables the simultaneous production of H2 and high-value Mg(OH)2 at the cathode20.

Although the aforementioned process elevates the benefits of seawater electrolysis, the severe adhesion of precipitates to the electrode surface, which leads to rapid electrode deactivation, limits its sustainability21–23. According to classical nucleation theory, heterogeneous nucleation on the electrode surface is thermodynamically more favorable than homogeneous nucleation due to the significantly lower nucleation energy barrier24,25. Therefore, from a kinetic perspective, it is essential to regulate the local reaction environment at the cathode during the hydrogen evolution reaction (HER) to favor homogeneous nucleation of Mg(OH)2 in the electrolyte. However, the lack of in-depth studies on the local reaction environment significantly complicates solidophobic catalyst design26. In principle, the catalyst should simultaneously achieve high HER activity under near-neutral environment and exhibit solidophobic surface properties.

In this study, we demonstrate that the coordinated halogens (F, Cl, Br, I) on Pt catalysts are critical for regulating the local reaction environment while maintaining high HER activity. In experiments, the Pt-I catalyst loaded on a Ti substrate (Pt-I/Ti) exhibits the best solidophobic behavior (i.e., supersolidophobicity) and maintains stable operation at 100 mA cm−2 in natural seawater electrolyte for over 5000 h. In contrast, the unmodified Pt/Ti electrode is rapidly covered by Mg(OH)2, leading to a drastic degradation in current density (100 mA cm−2, <1 h, Fig. 1a). The stability of Pt-I/Ti is attributed to its ability to regulate the local surface reaction environment. This regulation arises from the like-charge repulsion between the surface halogen ligands and OH− generated during the HER process, which drives OH− away from the electrode and allows it to react with Mg2+ in the bulk seawater (Fig. 1b). In situ experiments and theoretical calculations indicate that surface-coordinated halogens remain stable throughout the HER process, with the peak OH− concentration occurring ~8 μm away from the surface, much farther than on an unmodified electrode (~3 μm). The practical applicability of this supersolidophobic electrode is demonstrated by assembling it in a scaled electrolyzer (electrode area: 100 cm2), which exhibits stable performance (10 A, >1000 h) for the simultaneous production of green H2 and Mg(OH)2 (5.54 g h−1, >99% purity). The double-product strategy contributes to reducing the H2 production cost and environmental impact in seawater electrolysis.

Results
Preparation and characterization of the Pt-X/Ti (X: F, Cl, Br, I) electrocatalyst
The synthesis of Pt-X/Ti involves first electrodeposition of H2PtCl6 on Ti substrate to form Pt-Cl/Ti electrode, which is subsequently utilized to fabricate Pt-F/Ti, Pt-Br/Ti and Pt-I/Ti electrodes by exchanging the coordinated halogens, as schemed in Fig. 1a and Fig. S1 (details can be seen in Methods). The contrastive sample of Pt/Ti without coordinated halogens was also prepared, characterized and evaluated. The scanning electron microscopy (SEM), selected area electron diffraction (SAED) and X-ray diffraction (XRD) results of five samples exhibited negligible morphological differences of Pt and the same predominant exposed crystallographic plane (1, 1, 1) (Fig. S2-6), consistent with the samples obtained via electrodeposition methods reported previously27.

The energy dispersive spectroscopy (EDS) results of Pt-X/Ti from transmission electron microscopy (TEM) are shown in Fig. 2b–e, where the distributions of halogen elements align well with those of Pt. The results from ion chromatography and Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) indicate that the four samples exhibit similar halogen/platinum content ratios (Fig. S7). Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) was further employed to characterize the interaction between halogens and Pt nanoparticles28,29, as shown in Fig. 2f-i and Fig. S8. The 3D TOF-SIMS results demonstrated that PtX2- (X: F, Cl, Br, I) were primarily existed on the surface of four samples, while the concentrations gradually decreased with the increase of depth, directly proving that halogens coordinated with Pt were predominantly located at the surface. In contrast, negligible halogens can be detected on the surface of Pt nanoparticles for the Pt/Ti electrode without halogen-modification (Fig. S9).

Electrocatalytic performance in natural seawater
The HER activities of Pt/Ti, Pt-X/Ti (X: F, Cl, Br, I) and Pt foil were firstly evaluated in a standard three-electrode system with natural seawater (from Ningbo Bay after sediment removal, Fig. S10, 11, and Supplementary Table 1) as the electrolyte. As shown in Fig. 3a, the Pt-X/Ti required small overpotentials (445, 428, 450 and 465 mV for Pt-F/Ti, Pt-Cl/Ti, Pt-Br/Ti and Pt-I/Ti, respectively) for achieving industrial current density (~100 mA cm−2), better than those of Pt/Ti electrode and Pt foil (472 and 550 mV, respectively). The polarization curves without iR correction are presented in Fig. S13a. Moreover, the Tafel slope of Pt-X/Ti (163–196 mV dec−1) and the charge transfer resistance (3.4–9.5 Ω) further confirmed the advanced HER kinetics (Fig. S12b and 13, Supplementary Table 2)30–32. The enhanced HER activity can be explained by the enhanced first-electron-affinity due to surface coordinated halogens33. Density functional theory (DFT) calculations of the HER process in a neutral environment, as shown in Fig. S14, 15, reveal that the coordinated halogens weaken the adsorption energy of H* at adjacent Pt sites, thereby accelerating the Heyrovsky step and promoting the HER process34. The chlorine weaken the adsorption energy the most while iodine shows the weakest effect, consistent with the difference in HER activities.

Compared with HER activity, stability issue requires much more attention due to the severe scaling effect on electrode surface, resulting in fast electrode deactivation21,22,35,36. Thus, in order to screen the stabilities of different cathodes, the commercial boron-doped diamond (BDD) and 0.5 M Na2SO4 were adopted as anode and anolyte to ensure the stability of the anodic side, and the experiments were conducted in a three-electrode system equipped with a cation exchange membrane (Fig. S16, 17). Under the current density of 100 mA cm−2, the applied bias of Pt/Ti electrode without halogen-modification quickly increased ~100 mV within 30 min (Fig. 3b), which was majorly due to the decrease of working area caused by the coverage of white flocculent scale (Mg(OH)2, Fig. S18-20). In sharp contrast, all four Pt-X/Ti electrodes exhibited much enhanced stabilities under the same conditions. Specifically, Pt-F/Ti, Pt-Cl/Ti, and Pt-Br/Ti electrodes displayed potential increases of ~33 mV, ~29 mV, and ~17 mV, respectively, while Pt-I/Ti electrode maintained stable performance with negligible potential increase, demonstrating its supersolidophobic property. It is worth noting that, through comprehensive evaluation, the Pt-I/Ti electrode ranks among the first class for HER in natural seawater environment (Fig. S21, 22)37–40.

Meanwhile, optical images and XRD patterns of the four electrodes before and after the reaction further supported the difference in stabilities (Fig. S23, 24).

Benefiting from the simplicity of synthesis process, the Pt-I/Ti electrode can be easily scaled up to 100 cm2 in the laboratory, as shown in the inset of Fig. 3c and Fig. S25. The large-area consistency was demonstrated by selecting and evaluating, five different regions of the 100 cm2 electrode in terms of the HER activity and stability in natural seawater (Fig. 3c and Fig. S26). Moreover, as shown in Fig. 3e and Fig. S27-30, Micro X-ray Fluorescence (Micro-XRF) and SEM results revealed that most regions exhibited relatively identical elements distribution and morphology, further proving the scaling-up feasibility of this Pt-I/Ti electrode. More importantly, for practical application, the lifetime of the electrode is required at least thousands of hours (>1000 h)41,42. Therefore, in this case, we have extended the HER testing of Pt-I/Ti electrode in natural seawater over 5000 h and observed stable operation with a negligible increase in applied bias (<10 mV) (Fig. 3d and Fig. S31), which demonstrates competitive stability among the natural seawater cathodes reported previously (Supplementary Table 3)40,43–47.

The stability of Pt-I/Ti electrode heavily lies on the firmness of Pt-I coordination, which has been demonstrated in several ex situ and in situ characterizations. The Micro-XRF pattern of Pt-I/Ti electrode after 1000 h test demonstrated Pt and I elements maintain a uniform distribution. (Fig. 3e and Fig. S32, 33). As shown in Fig. 3f, the high-resolution X-ray photoelectron spectroscopy (XPS) spectra of I 3d for fresh and used Pt-I/Ti exhibited nearly identical peaks48,49, confirming the stable presence of coordinated iodine on Pt surface. The presence of coordinated halogens (F, Cl, Br) were also confirmed by the observation of ν(Pt-X, X: F, Cl, Br) peaks in Far-Infrared and XPS results (Fig. S34-36). In addition, in situ Raman spectroscopy revealed that, with the increase of applied bias from OCP to −1.0 V vs. RHE, the ν(Pt-I) remained detectable and unchanged at ~147 cm−150,51, while the δ(Pt-OH) at 1063 cm−1 did not appear at all33,52, as shown in Fig. 3g (experimental set-up is shown in the Fig. S37). This phenomenon demonstrates that the coordinated iodine is stable and cannot be replaced by the generated OH- during the HER process, and other halogens (F, Cl, Br) also show the same behavior (Fig. S37). The observable ν(S-O) peak at 980 cm−1 is attributed to the background signal from the electrolyte (Experimental details and Fig. S38).

Mechanistic investigation
Indeed, the synthesis of hydroxides on the electrode surface (i.e., Mg(OH)2 in this case) is closely related to the local reaction environment (i.e., pH) near the cathode during the HER process26,53. To further investigate the mechanism on how halogen coordination regulates the local reaction environment, a two-dimensional (2D) COMSOL Multiphysics model was firstly constructed to map out the pH distribution near the cathode surface (Fig. S39). It is found that the Pt/Ti electrode without halogen-modification exhibited a maximum pH value (~10.2) at ~3 μm from the electrode surface while the pH peak (~10.3) for the Pt-I/Ti electrode shifted to ~8 μm under a current density of 100 mA cm-2, as shown in Fig. 4a and Fig. S40. To verify this simulation, we developed a micro-region pH measurement system capable of in situ surface pH monitoring, and incorporated a stepper motor to measure the pH variation with distance (experimental set-up is shown in the Fig. S41). As shown in Fig. 4b and Fig. S42, the local pH of Pt/Ti electrode surface rapidly increased, achieving the minimum pH required for Mg(OH)2 precipitation (pH ≈ 9.6) at ~2 μm and complete precipitation (pH ≈ 11.1) at ~4 μm. In sharp contrast, for the Pt-I/Ti electrode, the pH reached 9.6 at ~4 μm and exceeded 11.1 at ~8 μm. These theoretical and experimental results both revealed that the surface diffusion of OH- was accelerated on the Pt-I/Ti electrodes, which were confirmed by other current densities (Fig. S43-45). It is worth noting that, for both electrodes, the maximum local pH during the HER process was lower than the minimum pH (~13.0) for starting the precipitation of Ca2+, giving rise to the potential in Ca-Mg separation from seawater. Further, open circuit potential (OCP) decay transients was used to verify interfacial pH swings during electrocatalysis54, and the results (Fig. 4c and Fig. S46, 47) demonstrated that the Pt-I/Ti electrode exhibited stronger resistance to pH increases which also indicated the accelerated OH- diffusion on electrode surface. It should be noted that the different OH- diffusion properties are consistent with *OH binding strength on Pt-X/Ti, as observed in the CO stripping experiments55,56, where Pt-I/Ti electrode exhibited the strongest repulsive capacity (Fig. S48).

Molecular dynamics (MD) simulations were used to validate the ion (e.g., Cl- or OH-) exclusion ability of the electrode surface57,58. The Pt and Pt-I slabs covered with solvent layers were constructed as model systems to explore the distribution of OH− close to the electrode surface (Fig. 4d, e, and Fig. S49). It is observed that the pure Pt surface would be surrounded by OH−, while the OH− ions on the Pt-I surface were repelled away from the surface. The number of OH− within 30 Å of the surface was halved (Fig. 4f), and the diffusion coefficients of OH- increased from 8.64 × 10−5 to 9.49 × 10−5 cm2 s−1 (Fig. S50).

Combined with the experimental and theoretical results, the HER process on electrode surfaces with or without halogen-modification in natural seawater can be illustrated in Figs. 4g, h. For the pure Pt surface, the diffusion of OH- generated by hydrogen evolution is dependent on the electrode repulsion and concentration gradient. Nevertheless, for the Pt-X surface, besides the above forces, like-charge repulsion from the halogen layer led to the faster OH- diffusion and expanded low-pH region. Thus, the combination of Mg2+ and OH- occurred closer to the pure Pt surface, which facilitated the heterogeneous nucleation process (i.e., Mg(OH)2 scaling). In contrast, for the Pt-X surface, as the position for Mg2+ precipitation was much farther from the electrode, homogeneous nucleation of Mg(OH)2 in the electrolyte was preferred (i.e., supersolidophobicity).

Performance and techno-economic analysis of scale electrolyzer
Compared with individual Mg2+ extraction and hydrogen production from seawater, this study achieves simultaneous production of H2 and high-purity Mg(OH)2 from seawater in virtue of the stable and supersolidophobic cathode (Fig. 5a). Next, the practical applicability of the supersolidophobic electrode was demonstrated in a large-scale direct seawater electrolyzer (electrode area: 100 cm2), assembled by a cation exchange membrane, an IrO2-Ti anode (Fig. S51-53), and a scaled Pt-I/Ti cathode, as schemed in Fig. 5b and Fig. S54-55. Natural seawater and 0.5 M Na2SO4 were used as the catholyte and anolyte, respectively. Fig. 5c demonstrated that the lab-made electrolyzer could be stably operated with a cell potential of ~2.75 V under 10 A for over 1000 h. During this process, the white Mg(OH)2 product was continuously produced at a rate of 5.54 g h−1, with a purity exceeding 99% and a particle size of ~35 μm (Fig. S56-61 and Supplementary Table 4). The catholyte pH was maintained at ~9.5 by flowing the natural seawater, resulting in a OH− availability of ~53.1% (Fig. S62). In contrast, for the traditional chemical precipitation method, the OH− availability was only ~16.7% when the solution pH was adjusted to ~9.6. With the pH increased further to ~11.0 by adding NaOH dropwise, the OH- availability rose to ~36.1%, although the purity of Mg(OH)2 ~91.4% (Fig. 5d, Fig. S63, 64, and Supplementary Table 5 and details can be seen in the Supplementary Discussion 1). The effective utilization of local pH indicates much enhanced OH− availability, which also represents a notable advantage of this value-increment natural seawater electrolysis strategy.

The TEA results were shown in Fig. 5e and Fig. S65, which summarized the preliminary and rough estimation of expenses associated with the direct conversion of natural seawater into valuable H2 and Mg(OH)2 (details can be seen in the Supplementary Discussion 2). The co-produced Mg(OH)2 could balance the hydrogen production cost, bringing the hydrogen production cost down to US$0.61 per kg of H2, which was below the DOE 2025 and 2030 targets (US$1.40 and US$1.00 per kg of H2, respectively)59–61 (Fig. S65). As shown in Fig. S66, 67, variations in each economic parameter from −10% to +10% of their original values produce linear trends with specific slopes. Compared to the original value, electricity cost shows a relatively smaller effect on hydrogen production costs. However, the slopes of the curves related to the prices of magnesium hydroxide and sodium hydroxide are steeper, indicating that any change in these parameters could have a significant impact on hydrogen production costs. We also conducted a Life Cycle Assessment (LCA) to quantify its environmental impact compared to traditional hydrogen production processes (Fig. 5f, and Supplementary Table 6 and 7). In this assessment, the system expansion approach was adopted as the allocation method, assuming that the co-produced byproducts could equivalently replace the same quantities of conventionally produced materials in the market, thereby generating positive environmental benefits62. As shown in Fig. 5f, the process produces 8 t of O2 and 1.45 t of Mg(OH)2 along with 1 t of H2. Compared with average H2 production, the seawater electrolysis-based magnesium extraction and hydrogen production process reduces abiotic depletion potential (ADP) and global warming potential (GWP), while exhibiting higher values only in the ADP-element and ozone layer depletion potential (ODP) categories. The results indicate that this double-product strategy can reduce the overall environmental impact.

In summary, this study developed a series of halogen-modified Pt-X (X: F, Cl, Br, I) catalysts aiming at meeting the strict requirement (both high HER activity and solidophobic surface property) of cathode in natural seawater electrolysis. The supersolidophobic Pt-I/Ti cathode demonstrated stable performance for over 5000 h under 100 mA cm−2 in natural seawater electrolysis, while also enabling the co-electrosynthesis of high-purity Mg(OH)2 through the utilization of localized alkaline environment. Detailed characterizations, alongside COMSOL and classical MD simulations, revealed that the local reaction environment regulation and supersolidophobicity arise from the iodine modification on the electrode surface, inducing like-charge repulsion between I− and in situ generated OH- during the HER process and promoting homogeneous nucleation of solid product. Considering the high economic value of double-product advantage, the proposed seawater electrolysis strategy would reduce the sell price and environmental impact of green hydrogen. Furthermore, we can foresee that this strategy in constructing supersolidophobic electrode will be of significant important for future electro-synthesis of solid compounds.

Methods
Chemicals
Chloroplatinic(IV) acid (H2PtCl6, Aladdin Industrial, ≥99.95%), potassium nitrate (KNO3, China National Pharmaceutical Group Corporation, ≥99.0%), sodium fluoride (NaF, Macklin reagent, 99.99%), sodium bromide (NaBr, Macklin reagent, 99.9%), sodium iodide (NaI, Macklin reagent, 99.9%), sodium sulfate (Na2SO4, Macklin reagent, ≥99.0%), sulfuric acid (H2SO4, China National Pharmaceutical Group Corporation, ≥96.0%), sodium hydroxide (NaOH, Aladdin Industrial, 97%), sodium phosphate dibasic (Na2HPO4, Aladdin Industrial, 99.9%), sodium dihydrogen phosphate (NaH2PO4, Aladdin Industrial, 99.9%), 5,5-Dimethyl-1-pyrroline N-oxide (DMPO, Aladdin Industrial, 97%). Ti mesh (99.5%, 100 mesh) and IrO2-Ti felt (thickness is 0.25 mm, loading amount: 2 mg cm−2) were purchased from Suzhou sinero technology Co., LTD. Carbon fiber paper (TGP-H-060, 0.19 mm) was purchased from TORAY Industries. Boron doped diamond (BDD) was purchased from Suzhou shure technology Co., LTD. Cation exchange membrane (Nafion 117, thickness is 183 μm) was purchased from Suzhou sinero technology Co., LTD. All deionized water was prepared with OmniaPure ultra-pure water system (resistivity of 18.2 MΩ). All chemical reagents were used as received without further purification.

Preparation of Ti mesh matrix electrode
First, the Ti mesh was cut into pieces with the size of 1 cm × 2 cm. Second, these pieces were sequentially degreased through sonication in acetone, ethanol, and deionized water for 10 min each. Subsequently, they were cleaned with 2 M H2SO4 in an ultrasound bath for 10 min to remove the surface impurity layer. Finally, the Ti mesh was washed with deionized water until attaining a neutral pH and dried at 60 °C for 3 h.

Preparation of Pt-Cl/Ti electrode
The Pt-Cl/Ti electrode was synthesized through electrochemical deposition strategy in the three-electrode system, wherein the Ti mesh, graphite paper and saturated calomel electrode (SCE) electrode served as the working electrode, counter electrode, and reference electrode, respectively. The electrodeposition solution contained 0.0015 M H2PtCl6 and 0.1 M KNO3. The Pt-Cl/Ti was prepared using the i-t method at a potential of −0.3 V for 15 min, followed by washed with deionized water and dried at 60 °C for 3 h. Pt loading amount: Pt-Cl/Ti (0.254 mg cm-2).

Preparation of Pt-X/Ti (X: F, Br, I) electrodes by coordination exchange
The Cl ligand can be removed under the irradiation of visible light in water. CEL-HXUV300E from China Education Au-light with a spectral region from 320 nm to 780 nm was used as the light source with an output power density of 3.75 mW cm−2. After 30 min of irradiation, the electrode was immersed in 2 M NaX (X: F, Br, I) at 40 °C for 4 h under magnetic stirring. Finally, the Pt-X/Ti (X: F, Br, I) electrode was washed with deionized water and dried at 60 °C for 3 h. Pt loading amount: Pt-F/Ti (0.253 mg cm−2), Pt-Br/Ti (0.248 mg cm−2) and Pt-I/Ti (0.247 mg cm−2).

Preparation of Pt/Ti electrode
The Pt/Ti electrode was synthesized by calcining the Pt-Cl/Ti electrode in a tube furnace with a 5% H2/Ar flow under 200 °C for 4 h. Pt loading amount: Pt/Ti (0.248 mg cm−2).

Preparation of Pt-X/CFP (X: F, Cl, Br, I) and Pt/CFP electrodes
The synthesis method for the Pt-X/CFP (X: F, Cl, Br, I) and Pt/CFP electrodes is similar to that for the Pt-X/Ti (X: F, Cl, Br, I) and Pt/Ti electrodes, with the only difference being the substitution of the substrate with carbon fiber paper.

Preparation of scaled Pt-I/Ti electrode
The synthesis procedure of scaled Pt-I/Ti electrode is similar to that of the 1 cm × 2 cm Pt-I/Ti electrode, with the only difference being the substitution of the Ti matrix (10 cm × 10 cm). The electrode was then electrodeposited using a laboratory-made electrodepositing cell for 20 min, followed by washing with deionized water and drying at 60 °C for 3 h.

Seawater Pretreatment
Seawater pretreatment only sediment removal by filtration system (filter paper diameter = 150 mm).

Material characterizations
Scanning electron microscopy (SEM) images were taken on Hitachi S4800 working at 20 keV. The aberration-corrected HAADF-STEM measurements were taken on a JEM-ARM200F instrument at 200 keV. The crystal structures of the samples were examined by X-ray diffraction (XRD, Bruker D8) with Cu Kα radiation (λ = 0.154178 nm). The surface structures of Pt-X/Ti (X: F, Cl, Br, I) electrodes were analyzed by PHI nanoTOF II TOF-SIMS (ULVAC-PHI, Japan). X-ray photoelectron spectroscopy (XPS) measurements were performed by using a Thermo ESCALAB 250 high-performance electron spectrometer using monochromatized Al Ka (hν = 1486.6 eV) as the excitation source. Raman measurements were conducted by using a RENISHAW inVia Raman spectrometer. Far-Infrared spectra were obtained on a Bruker Vertex 70V. The content of cations in seawater was measured by Agilent ICP 720 inductively coupled plasma optical emission spectrometer (ICP-OES). Micro X-ray Fluorescence (Micro-XRF) images were taken on a Bruker M4 TORNADO. The product particle size was measured by Malvern Zetasizer Nano ZS90. Faradaic efficiencies were tested by using gas chromatography (GC, Agilent 7890B) method30. The electron paramagnetic resonance (EPR) spectra were collected on a Bruker Elexsys 500 spectrometer at 25 °C. Micro-region pH measurements were performed by using a commercial glass-type pH microelectrode with a tip diameter of 500 μm (Unisense, pH-500c), together with an internal reference.

Electrochemical measurements
Except for the electrolyzer stabilization process, all of the electrochemical measurements were measured on electrochemical workstation (ChenHua CH instrument 760E) with a three-electrode configuration at 25 °C. Except in stability tests, the fabricated samples, graphite paper and saturated calomel electrode (SCE, saturated KCl) served as the working electrode, counter electrode, and reference electrode, respectively. In a stability test, the fabricated samples, commercial boron-doped diamond (BDD, 3 cm × 4 cm) and saturated calomel electrode (SCE, saturated KCl) acted as the reference electrode, counter electrode, and reference electrode, respectively. The as-fabricated electrodes were clamped by pure Ti holder (100%) to make electrical contact. The electrolyte (100 mL natural seawater, pH = 7.71 ± 0.02 and 100 mL 0.5 M Na2SO4, pH = 7.0 ± 0.02) was purged with pure N2 (for HER) or O2 (for OER) for at least 30 min. The polarization curves of HER were documented at a rate of 5 mV s−1. LSV is measured independently three times.

The reference electrodes were calibrated in an electrolyte saturated with high-purity hydrogen, using two platinum foils (1 × 1 cm2) as the working and counter electrodes, respectively. Cyclic voltammetry (CV) was performed at a scan rate of 5 mV s−1. The thermodynamic potential of the hydrogen electrode reaction was determined as the average of the two potentials at which the current crossed zero during the CV scans, and this value was used to convert the measured potentials to the reversible hydrogen electrode (RHE). All potentials of the measured curves were corrected with iR (100%)63 compensation and then converted to RHE according to the following two equations:

E(RHE) = E(Ag/AgCl) + 0.0592 × pH + 0.1989 (1)

E(RHE) = E(SCE) + 0.0592 × pH + 0.2440 (2)

The service life of the electrodes in the electrolyzer was tested using a direct current (DC) power supply (MAISHENG, MP1560D) with a constant current density in a two-electrode mode, and the cell potential was recorded every hour, commercial IrO2-Ti (10 cm × 10 cm) acted as the counter electrode. Electrochemical impedance spectroscopy (EIS) curves were obtained by a frequency range from 1000 kHz to 0.1 Hz under different potentials.

All the solutions used as electrolytes are prepared freshly and used immediately, and are stored in a dark place.

MEA measurement
The MEA setup with the geometric area of 100 cm2 was designed and manufactured by the lab and equipped with a designed flow channel. The as prepared electrodes (Pt-I/Ti, 10 cm × 10 cm) and commercial IrO2-Ti felt (10 cm × 10 cm) electrode were selected as HER and OER electrodes, respectively. The cation exchange membrane was Nafion 117 (Dupont, USA). The membranes were pretreated by boiling sequentially in 3% H2O2 solution for 1 h and 0.5 M H2SO4 for 3 h, followed by rinsing in deionized water. The stability was carried out on DC power supply. The anolyte (0.5 M Na2SO4, 20 L) and catholyte (natural seawater, 20 L) were replaced every 6 h to maintain a constant pH, with the cathode effluent pH consistently stabilized at ~9.5.

Electrochemical in situ Raman measurements
Electrochemical in situ Raman spectroscopy was carried out using a RENISHAW inVia Raman spectrometer equipped with a Leica TCS SP8 CARS microscope and a Spectra-Physics 532 nm Ar laser. All in situ Raman measurements were performed with a custom-made spectro-electrochemical cell. The electrochemical activities were measured in a three-electrode configuration at 25 °C. The as prepared electrodes, Pt wire and Ag/AgCl (saturated KCl) electrode served as the working electrode, counter electrode, and reference electrode, respectively. The electrolytes were 0.5 M Na2SO4. Noted that, to prevent side reactions, a sulfate solution is used. The laser beams were focused on the sample through the quartz window to collect the Raman spectrum. The chronoamperometry method was employed to applied different potentials to the electrode with 100 mV intervals in the range of 0 V to −1.0 V vs. RHE. The acquisition time for each spectrum was 10 s with 3 sweeps from 100.92 to 1811.54 cm−1. The optical images of custom-built electrochemical cells for in situ Raman measurements is provided in Supplementary Fig. 37.

CO stripping measurements
The CO stripping experiment was performed in a three-electrode H-cell configuration. The system employed a graphite rod as the counter electrode, a saturated calomel electrode (SCE, saturated KCl) as the reference electrode, and the as-prepared electrode as the working electrode. Initially, 10% CO/Ar gas was introduced into a CO-saturated 0.5 M H2SO4 electrolyte, allowing CO adsorption onto the working electrode at a constant potential of 0.1 V (vs. RHE) for 30 min. Subsequently, the electrode was transferred to an Ar-saturated 0.5 M H2SO4 electrolyte for CV analysis. Prior to measurement, the solution was purged with Ar gas, and CO stripping CV curves were recorded at a scan rate of 20 mV s−1.

Micro-region pH measurements
To test the pH on the electrode surface, a laboratory-made micro-region pH system equipped with a microscope (ZOOM-650, Shenzhen MicroVision Technology Co., Ltd.) and a stepper motor was used to measure pH variation with distance (Marzhauser Wetzlar MM33 with tilting device, step size: 1 μm). The electrolyte was 0.5 M Na2SO4, and the as prepared electrodes, Pt foil, and saturated calomel electrode (SCE, saturated KCl) served as the working electrode, counter electrode, and reference electrode, respectively. Initially, the glass-type pH microelectrode tip (Unisense, pH-500c) was positioned ~1 μm from the electrode surface using the stepper motor. A constant current was then applied, and the pH electrode was controlled to move away from the electrode surface in 1 μm increments. At each test point, the pH electrode was held for 1 s before moving to the next point. Each set of measurements was repeated three times to minimize error. The optical image of custom-built system for micro-region pH measurements is provided in Supplementary Fig. 41.

Open circuit potential (OCP) decay transients measurements
The open circuit potential (OCP) decay transients can be used to verify interfacial pH swings during electrocatalysis54. The electrolyte was 100 mL 0.5 M NaPi (Phosphate buffer, pH = 6.8 ± 0.02), and the as prepared electrodes, Pt foil, and saturated calomel electrode (SCE, saturated KCl) served as the working electrode, counter electrode, and reference electrode, respectively. Multi-step chronopotentiometry (CP) was employed to galvanostatically polarize the working electrode at current densities of 10, 25, 50 and 100 mA cm−2 for 2 min, with a data collection interval of 0.25 s, allowing a steady-state potential to be reached. Subsequently, a second CP step at the same current density, but with a higher data collection interval of 0.0002 s, was performed for 1 second. A third CP step at 0 mA cm−2 was used to collect the open circuit potential decay transient with a data collection interval of 0.0002 s for 1 s. Finally, a fourth CP step at 0 mA cm−2, with a lower data collection interval of 0.25 s, was conducted to capture the remainder of the open circuit potential decay. The data collection intervals were carefully selected to ensure high timer resolution during the termination of polarization, while avoiding the data buffering capacities of the potentiostat.

Theoretical computation details
All density functional theory (DFT) calculations were carried out using the Vienna Ab initio Simulation Package (VASP) with the projector augmented-wave (PAW) method64. The exchange-correlation interactions were described using the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional. A plane-wave energy cutoff of 450 eV was employed, and partial occupancies of the Kohn-Sham orbitals were treated using the Gaussian smearing method with a width of 0.2 eV. The self-consistent calculations apply a convergence energy threshold of 10−5 eV, and the force convergency was set to 0.05 eV Å−1. The p(3 × 3) Pt(111) surface was built with 4 layers, where two of the layers at the bottom was fixed in all the calculations. The coordinated halogen atoms (F, Cl, Br, I) was adsorbed on the top site of Pt, and the relevant affected site for HER was considered closed to the adsorbed halogen atoms. The k-point of 3 × 3 × 1 was used. The reaction free energy of H+ involved steps was calculated following the computational hydrogen electrode (CHE) model65. Free energy corrections were considered at the temperature of 298 K, following:

ΔG = ΔE + ΔGZPE + ΔGU - TΔS (3)

Here, ΔE, ΔGZPE, ΔGU, and ΔS represent the DFT-calculated energy change, the zero-point energy correction, the internal energy correction, and the entropy correction, respectively66.

All Molecular dynamic (MD) simulations were applied to calculation the number distribution and diffusion coefficients of OH− in seawater at pristine Pt (111) and I− adsorbed Pt (111) surfaces under an external electric field of 0.2 V Å−1 along z direction. The solid–liquid interface calculations were conducted with COMPASS III force field using the Forcite module in Materials Studio (MS) 2020. The dimensional lengths of the Pt substrate are 24.89 Å × 23.95 Å in xoy plane, which is composed of four layer Pt atoms. The solution contains 24 OH−, 36 Na+, 2 Mg2+, 16 Cl− and 1200 H2O molecules in a rectangular box with length scales of 24.89 Å × 23.95 Å × 61.86 Å. After geometry optimization, the aqueous solution was placed on pristine Pt (111) and I− adsorbed Pt (111) surfaces. All MD calculations were performed under the NVT ensemble (T = 298.0 K) with a timestep of 1 fs and a total simulation time of 400 ps, during which simulation trajectories were recorded every 4000 steps. The running time was long enough for system energy and temperature reaching stable. The temperature was controlling by a Nose–Hoover thermostat. The Ewald scheme and atom-based cutoff method (i.e., a radius of 12.5 Å) were applied to treat electrostatic and van der Waals (vdW) interactions, respectively. The number profiles of OH− along perpendicular direction were analyzed. The solid–liquid interfaces snapshots were displayed. The diffusion coefficients of OH− can be estimated from the slope of the mean square displacement (MSD) curves by the Einstein relationship.

The atomic coordinates of the optimized crystal structures can be found in Supplementary Data 1.

LCA analysis
An LCA was conducted using the SimaPro software following the international standard ISO 14040. The CML 2001 method was selected to evaluate the environmental impacts of the studied system, and background data were obtained from the Ecoinvent 3.11 database. The production of 1 t of hydrogen was defined as the functional unit, and the system boundary was set on a cradle-to-gate basis, covering the production processes of hydrogen and magnesium hydroxide in the seawater electrolysis-based magnesium extraction and hydrogen production process. A life-cycle inventory was established to clearly distinguish the system inputs and outputs. The main inputs include seawater, sodium hydroxide, catalyst materials, and electricity, while the outputs consist of hydrogen, oxygen, and magnesium hydroxide. A system expansion approach was applied for the attributional LCA, assuming that the co-produced byproducts could substitute conventionally produced materials in the market and thereby generate environmental benefits. Based on the CML 2001 method, the characterization analysis quantified the specific contributions of each environmental impact category (AP, GWP, FAETP, MAETP, TETP, ADP, EP, HTP, ADP element, ODP, and POCP) to represent the overall life-cycle environmental impacts.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript addresses the modification of a Pt catalyst with halogens to suppress Mg(OH)₂ deposition on the electrode surface via OH⁻ repulsion. This work builds upon the authors' previous publications in the field (e.g., https://pubs.acs.org/doi/10.1021/acs.nanolett.4c01484). Here, the authors demonstrate excellent long-term stability and scalability of the catalyst. I would say the manuscript can be recommended for publication. However, I have several suggestions for improvement:\n1. The term supersolidophobic is not clearly defined in the existing literature. It remains unclear why the system is referred to as supersolidophobic compared to the authors’ prior “solidophobic” reports. A clearer definition and distinction are needed.\n2. The procedure for preparing Pt-halogen electrodes via light irradiation and treatment with NaX solutions is insufficiently explained. If Pt electrodes can be modified in seawater containing halides, what prevents similar modifications from occurring naturally? Furthermore, the pure Pt/Ti electrode is calcined at 200 °C, but this is not mentioned for halogen-modified electrodes. For consistency, the preparation protocol should be the same or explicitly justified. If light treatment significantly alters the surface, the appropriate reference should be the light-treated Pt/Ti electrode.\n3. What happens when the electrodes are tested in deionized or distilled water? Is the enhanced efficiency of halogen-modified electrodes still observed, or is it specific to seawater?\n4. The proposed repulsion between I⁻ and OH⁻ is plausible, but in real seawater, halogens may undergo redox reactions or ion exchange. To support the stability of halogen coordination, I recommend complementing micro-XRF with ICP analysis of the electrolyte before and after electrolysis to track halogen leaching and quantify surface species.\n5. The focus is largely on halogen content, but little is said about the oxidation states of Pt. Halogen coordination should affect Pt’s electronic environment, which can be detected via XPS. The manuscript should include quantification of halogen-to-metal ratios and discuss electronic effects on Pt: how much is in reduced state and oxidized...\n6. The reported shift in the pH gradient from 3 μm to 8 μm is interesting, but the spatial and temporal resolution of the measurement is not described. Clarifying the method’s resolution and limitations is essential for evaluating the significance of this observation.\n7. The HER activity of Pt-I is slightly lower than Pt-Cl or Pt-F, yet Pt-I is the chosen system. The rationale behind this choice —possibly superior solidophobicity or long-term performance—should be better justified.\n8. While Faradaic efficiency for H₂ is reported to be near 100%, there is no mention of Mg(OH)₂ or O₂ balances. Additionally, what happens to the residual chloride ions after Mg precipitation?\n9. Could the accumulation or partial passage of Mg(OH)₂ impact membrane performance over time? This possibility should be addressed or ruled out experimentally.\n10. How does this system compare with commercial seawater electrolysis technologies in terms of cost, durability, and performance metrics? Including such a benchmark would enhance the manuscript's relevance to applied research.\n11. The statement that this system “outperforms” previously reported seawater cathodes may be overstated. Some of the cited systems were tested for shorter durations, which does not necessarily imply inferior performance.", "reviewer_2": "In the manuscript entitled “Supersolidophobic Pt catalyst for long-term natural seawater electrolysis with high-purity magnesium extraction”, Lu and co-workers present a Pt catalyst with surface-coordinated halogens (e.g., F, Cl, Br, I) which can promote like-charge repulsion mechanism to achieve a successful extraction of high-purity Mg(OH)2 with the stable green H2 production from seawater. The manuscript presents innovative research methods and promising preliminary results. However, it requires minor revisions to address several points before it can be considered for publication. Below are detailed comments and suggestions:\n1. The authors should ensure numerical consistency throughout the manuscript. For instance, the purity of Mg(OH)2 is stated as 99.0% in the main text but appears as 99% in Figure 5. Which value is correct? Similarly, the Mg(OH)2 production rate is given as 5.54 g h-1 (two significant figures) in the main text, but the corresponding balance data in Supplementary Figure 54 appears to show only one significant figure. Please verify and correct these discrepancies.\n2. The provided 3D TOF-SIMS images indicate that Pt-Br/Ti exhibits the most abundant PtBr2⁻ signal, surpassing that of the Pt-Cl/Ti precursor. This finding is somewhat confusing, given that Pt-X/Ti (F, Br, I) samples are synthesized via halogen exchange from Pt-Cl/Ti; one might expect similar PtX2⁻ abundances across these samples. The EDX data shows a similar trend. However, Supplementary Figure 34 (XPS) reveals a much weaker Cl signal compared to F and Br. The authors should clarify this apparent contradiction between the TOF-SIMS/EDX and XPS results regarding halogen abundance/distribution.\n3. To robustly support the claim of Pt-X coordination (beyond physical adsorption), additional characterizations such as comparative XPS analysis should be provided. Specifically, comparing the binding energies of Pt and relevant halogens in the catalyst samples with those of bare Pt and PtX2 reference compounds would be highly informative to demonstrate chemical shifts indicative of coordination.\n4. There appears to be an error in Supplementary Figure 12: the y-axis should be labeled \"-Z’’\" instead of \"Z’’\". Additionally, the axes scales should be consistent across the relevant panels. Please correct this figure.\n5. The authors selected Ti felt as the catalyst substrate. Could the rationale for not using potentially cheaper alternatives, such as carbon fiber paper, be briefly discussed? Are there specific stability or performance considerations favoring Ti felt in this seawater electrolysis system?\n6. Additional characterization of the produced Mg(OH)2, such as TEM imaging to examine its morphology and crystallinity, would strengthen the analysis of the extraction product.", "reviewer_3": "This study presents a novel platinum (Pt) catalyst modified with surface-coordinated halogens (F, Cl, Br, I) for long-term natural seawater electrolysis, enabling simultaneous high-purity Mg(OH)₂ extraction and green hydrogen production. The Pt–I/Ti cathode exhibits supersolidophobicity, attributed to like-charge repulsion between iodide (I⁻) ions and in situ-generated hydroxide (OH⁻) ions. This repulsion shifts Mg(OH)₂ nucleation away from the electrode surface, promoting homogeneous rather than heterogeneous nucleation. As a result, the system achieves outstanding operational stability (>5,000 hours at 100 mA cm⁻²), yields high-purity Mg(OH)₂, and reduces hydrogen production costs to $0.52/kg—outperforming U.S. DOE targets. Overall, this work is highly innovative and suitable for publication in Nature Communications.\nRegarding the molecular dynamics (MD) simulations, I have a few comments to provide more simulation details:\n1. Figure 4d–f demonstrates that OH⁻ ions are repelled from the Pt–I surface but accumulate near pristine Pt. Which specific intermolecular interactions or force field parameters in the COMPASS III model (as detailed in the Supplementary Methods) are responsible for the electrostatic repulsion between surface-bound iodide (I⁻) and OH⁻ ions?\n2. The MD system (Supplementary Figure 47) contains 24 OH⁻, 36 Na⁺, 2 Mg²⁺, 16 Cl⁻, and 1,200 H₂O molecules. What was the rationale behind selecting these specific ion concentrations?\n3. Figure 4f shows that the number of OH⁻ ions within 30 Å of the Pt–I surface is approximately half that near pristine Pt. How was this spatial distribution of OH⁻ ions quantified—e.g., using radial distribution functions (RDFs)?\n4. The MD simulations applied an external electric field of 0.2 V/Å along the z-direction (as described in the Supplementary Information). How was this field strength chosen, and does it reflect the experimental electrolysis conditions?\n5. Were MD simulations also conducted for other halogen-modified surfaces (Pt–F, Pt–Cl, Pt–Br)? If so, how did their OH⁻ exclusion behaviors compare with Pt–I? If not, could the trend in halogen electronegativity (F > Cl > Br > I) provide mechanistic insight into the unique supersolidophobicity observed for Pt–I?", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_5": "Dear authors,\nFirst of all, I would like to congratulate you on your insightful work concerning the ‘Supersolidophobic Pt catalyst for long-term natural seawater electrolysis with high-purity magnesium extraction’. The paper addresses the challenge of catalyst deactivation innovatively, while simultaneously pursuing the ‘double-product strategy’ of producing both green and cheap hydrogen and high-purity Mg(OH)2. The manuscript is high of quality and very well written. Besides, the techno-economic assessment and its details in the supplementary information are very well explained and followed an understandable order.\nI have several points of suggestion to be considered to improve the quality of the manuscript and increase the robustness of your outcomes. These suggested points will improve the quality of your assessments and will benefit the visibility of your work:\nMAIN TEXT:\n•Line 1-2: Title\no The main focus of the paper is the double-product strategy to produce green/sustainable hydrogen and high-purity magnesium. However, in the title, hydrogen is not mentioned. Consider adding hydrogen as well. By doing so, you will increase your visibility.\n•Line 122: Fig. 3:\no Make sure that the figures are readable in the article. Or consider restructuring to improve readability and visibility.\n•Line 252: Fig. 5:\no Fig. 5a: Positive effects are colored in red, which, in my opinion, are mostly associated with a negative outcome. I would prefer to see the colour green for the following sentence: “Co-electrosynthesis H2 and Mg(OH)2, no preprocessing, low cost, high Mg(OH)2 purity (>99%)”.\no Fig. 5e: Make the benefits of Mg(OH)2 better visible.\no Fig. 5f: For ‘This work’ (which represents your work), I would also use the colour green to represent positive results (see comment above). As for now, it looks like a negative result. Consider changing.\nSeveral DISCUSSION POINTS need to be considered:\n• Sensitivity analysis:\no Discussion point based on the sensitivity assessment in the Supplementary Information (SI) (see SI part below).\n• Electrolyser\no What is your assumption about how the technology would perform on an industrial scale? As the assumptions are based on 100 cm2.\no Scaling up the electrolyser to industrial scale will most likely benefit the economic and environmental outcome due to learning effects and economies of scale.\no Doi: https://doi.org/10.1016/j.spc.2023.11.002.\n•Linked to the Electrolyser: Catalyst: Discuss the cost and environmental impacts of precious metals\no Environmental impacts: For GHG missions and other environmental impacts:\no Anode: IrO2-Ti anode => Iridium\no Cathode: Pt-I/Ti cathode => platinum\no Potential article: doi:10.1371/journal.pone.0101298.\no Consider discussing the future impact of scalability and future improvement in technologies, electricity mixes, and other aspects. The environmental impacts can be significantly reduced by …\no Scaling up : Doi: https://doi.org/10.1016/j.spc.2023.11.002\no Consider the future improvement in technologies and electricity mixes: Doi: https://doi.org/10.1016/j.crsus.2024.100286.\no Is there any indication of recycling of these materials? I would assume this would significantly increase the economic viability and decrease environmental impacts.\n• Sodium hydroxide\no Based on your explanation to keep the pH of the anolyte stabilized, you need to add NaOH, which tells me that OH is depleting (due to OH- usage) and Na is increasing in the seawater that is potentially released into the sea again? What are the environmental implications of the increasing Na content in the seawater, considering that in the future it will be on an industrial scale? Does it need to be extracted/treated, and if yes, discuss the potential cost for additional equipment and operational costs.\n• Environmental aspect of the novel technology\no It would be nice to include a critical discussion point about the environmental implications of the technology (specifically the catalyst and its precious metals) in comparison to the conventional benchmarks. As your article states the sustainable nature of the products, environmental implications are required. I understand that the core of the assessment is the techno-economic feasibility; however, a discussion point about the environmental implications is highly desired and beneficial for the paper. It is not necessary to conduct a full-scale life cycle assessment, but indicating how environmentally sound the technology might be, based on literature, is highly appreciated.\nSUPPLEMENTARY INFORMATION\n•Sensitivity analysis:\no A TEA usually includes a sensitivity analysis showing how certain parameters might affect the economic outcome, specifically important for technologies under low TRL with high uncertainty. Particularly for some of your assumptions, it would be beneficial to add the following sensitivity analyses (see below). This will make your results more robust and accountable for price uncertainties, associated with different conditions (e.g. different production/purchase locations, etc.). Besides, your assumptions are usually based on Chinese conditions. The journals of ‘Nature’ aim for a more general view that can relate to other parts of the world (e.g. US and the EU). Hence, some of the parameters are asked to be evaluated based on different regions (see below).\nIMPORTANT!!: Discuss the sensitivity analysis briefly in the main text. Calculations can be kept in the supplementary information.\n• Mg(OH)2:\no How will the result (H2 production cost per ton or kg) change if the price fluctuates ±10%? (can be a different value)\n• Electrolyser (almost 22% contribution)\no You chose a price of 600 $/kW. (SI: line 229). This is based on Chinese conditions.\no How will the result (H2 production cost per ton or kg) change if you use the European and the US conditions with prices around 2000-3000 $/kW? (SI line 227-228).\n• NaOH: (most influential parameter when it comes to cost: 58%)\no Base assumption is USD 0.30 /kg.\no How will the result (H2 production cost per ton or kg) change if the price fluctuates ±10% (can be a different value)?\n• Electricity (21%):\no Chosen = USD 0.03/kWh\no The European and the US electricity prices substantially deviate from the Chinese ones.\no How will the result (H2 production cost per ton or kg) change if the electricity price is chosen based on….\no Average EU price: 0.1899 EUR /kWh\no Fluctuates between 0.067 EUR/kWh (Finland) and 0.25 EUR/kWh (Cyprus). Keep in mind it is in €, not $!!!\no Website: https://ec.europa.eu/eurostat/statistics-explained/index.php?title=Electricity_price_statistics#Electricity_prices_for_non-household_consumers\no Average US price: 01318 USD/kWh.\no Website: https://www.eia.gov/electricity/monthly/epm_table_grapher.php?t=epmt_5_3\n• Capacity\no Line 236 SM: Why do you focus on the capacity of 100 kg H2 production per day? How did you calculate the 100 kg?\nPlease elaborate?\no Line 237 SM: What is 1/0.99 representing?\n• Material cost: Line 259: Natural seawater: Line 238 – 240 vs Line 259 – 261:\no I have the feeling they are not calculated on the same basis:\no Please elaborate why. In case it is wrong, please correct it.\no In Line 238: 1/2 e- vs line 259 1/4e-. Why is there a difference? I would assume it would be the same. Please elaborate. In case it is wrong, correct it.\n• SM Line 124, you are stating that anolyte (0.5 M Na2SO4, 20 L) and catholyte (natural seawater, 124 20 L) are assumed to be replaced every 6 h to maintain a constant pH. However, I do not see the cost for Na2SO4. Is there any reason why the cost of Na2SO4 is not included?\n•LINE 623-722 = something is off, a part is missing. Correct it."}, "author_response": "Response to Reviewer 1\nGeneral Comment: The manuscript addresses the modification of a Pt catalyst with\nhalogens to suppress Mg(OH) deposition on the electrode surface via OH- repulsion.\nhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c01484). Here, the authors\ndemonstrate excellent long-term stability and scalability of the catalyst. I would say the\nmanuscript can be recommended for publication. However, I have several suggestions\nfor improvement:\nResponse: We do appreciate the reviewer’s attention to our previous work and the\npositive comments provided; the point-by-point response is shown below.\nComment 1: The term supersolidophobic is not clearly defined in the existing literature.\nIt remains unclear why the system is referred to as supersolidophobic compared to the\nauthors’ prior “solidophobic” reports. A clearer definition and distinction are needed.\nResponse: The term “supersolidophobic” is derived from superhydrophobicity and\nsuperaerophobic1,2, referring to the suppression of solid detachment and adsorption on\nthe surface of other material surface. In our previous work, we introduced the concept\nof solidophobic properties; however, the stability of the electrode remained to be\nimproved3. In this manuscript, we have demonstrated the enhanced stability of the\nelectrode, and therefore, we refer to it as “supersolidophobic”, indicating a material that\nsimultaneously exhibits solidophobic properties along with unprecedented long-term\nstability and high durability.\nComment 2: The procedure for preparing Pt-halogen electrodes via light irradiation\nand treatment with NaX solutions is insufficiently explained. If Pt electrodes can be\nmodified in seawater containing halides, what prevents similar modifications from\noccurring naturally? Furthermore, the pure Pt/Ti electrode is calcined at 200 °C, but\nthis is not mentioned for halogen-modified electrodes. For consistency, the preparation\nprotocol should be the same or explicitly justified. If light treatment significantly alters\nthe surface, the appropriate reference should be the light-treated Pt/Ti electrode.\nResponse: During the halogen coordination exchange process, the first step involves\nthe removal of Cl- from the Pt-Cl/Ti. In this process, light exposure is essential4-6; this\nreaction does not occur in the absence of light. Subsequently, halogen exchange (F, Br,\nI) takes place. The reaction cannot occur under the reaction conditions without the\nnecessary light exposure. The calcination process at 200°C and reduction atmosphere\naims to obtain a non-coordinated Pt/Ti electrode, which serves as a comparison sample\nto demonstrate the role of halogen coordination. To confirm that calcination does not\ninduce other effects on the sample, we performed thermal treatment on Pt-I/Ti in a N\n2\natmosphere. As shown in Figure R1a, the calcined Pt-I/Ti sample requires an\noverpotential of 450 mV to reach 100 mA cm-2, which is almost identical to the value\nbefore calcination (465 mV). Under a current density of 100 mA cm-2, the applied bias\nof the calcined Pt-I/Ti sample remains consistent with that before calcination (Figure\nR1b). Additionally, we conducted light exposure treatment on the Pt/Ti sample, and the\ntreated sample exhibited almost the same behavior as the initial sample (Figure R2).\nFigure R1. (a) Polarization curves with iR correction of Pt-I/Ti and Pt-I/Ti after\ncalcination in natural seawater (pH=7.71). (b) Short-term durability test of Pt-I/Ti and\nPt-I/Ti after calcination in natural seawater electrolysis under 100 mA cm-2.\nFigure R2. (a) Polarization curves with iR correction of Pt/Ti and Pt/Ti after light\ntreatment in natural seawater (pH=7.71). (b) Short-term durability test of Pt/Ti and Pt/Ti\nafter light treatment in natural seawater electrolysis under 100 mA cm-2.\nComment 3: What happens when the electrodes are tested in deionized or distilled\nwater? Is the enhanced efficiency of halogen-modified electrodes still observed, or is it\nspecific to seawater?\nResponse: The enhancement of platinum site activity by halogen modification is\nattributed to the reduction of the adsorption energies of H* on the platinum active sites7,\nmaking it applicable in various solution environments6. As shown in Figure R3, LSV\ntesting was conducted in 0.5 M Na SO , and the Pt-X/Ti electrodes still required small\n2 4\noverpotentials (458, 428, 465, and 474 mV for Pt-F/Ti, Pt-Cl/Ti, Pt-Br/Ti, and Pt-I/Ti,\nrespectively) to achieve the industrial current density (~100 mA cm-2), outperforming\nthe Pt/Ti electrode (488 mV).\nFigure R3. Polarization curves with iR correction of Pt/Ti and Pt-X/Ti in 0.5 M Na SO\n2 4\n(pH=7).\nComment 4: The proposed repulsion between I⁻ and OH⁻ is plausible, but in real\nseawater, halogens may undergo redox reactions or ion exchange. To support the\nstability of halogen coordination, I recommend complementing micro-XRF with ICP\nanalysis of the electrolyte before and after electrolysis to track halogen leaching and\nquantify surface species.\nResponse: To verify the stability of halogen coordination before and after the reaction,\nwe conducted a 100 h electrolysis test (Figure R4) with samples taken at different time\npoints for analysis. As shown in Figure R5, the iodine content in the solution remained\nconsistently below the detection limit of ion chromatography, indicating that the Pt-I\ncoordination remained stable throughout the reaction process, with negligible I-\nleaching.\nFigure R4. Stability tests of Pt-I/Ti electrode under 100 mA cm-2.\nFigure R5. The raw data of Pt-I/Ti from ion chromatography during the stability tests.\n(a) 0 h, (b) 5 h, (c) 20 h, (d) 50 h, (e) 75 h and (f) 100 h.\nNote: The choice of using Na SO as the electrolyte was made because, when\n2 4\ncirculating natural seawater is used as the electrolyte, the renewal of the electrolyte may\nresult in a low concentration of leached ions, making detection difficult. Therefore,\nNa SO was used as the electrolyte, and circulation was eliminated to investigate\n2 4\npossible ion leaching during the reaction. Trace amounts of Cl- originated from the\nNa SO (Macklin reagent, 99.95%) used.\n2 4\nComment 5: The focus is largely on halogen content, but little is said about the\noxidation states of Pt. Halogen coordination should affect Pt’s electronic environment,\nwhich can be detected via XPS. The manuscript should include quantification of\nhalogen-to-metal ratios and discuss electronic effects on Pt: how much is in reduced\nstate and oxidized.\nResponse: Halogen coordination with platinum induces electron transfer from\nplatinum to the halogen8. The 4f peaks of Pt in the Pt-F/Ti, Pt-Cl/Ti, Pt-Br/Ti, and Pt-\n7/2\nI/Ti samples are located between Pt2+ (72.7 eV) and Pt4+ (74.9 eV) (Figure R6),\nindicating that the average valence states of these platinum species range between +2\nand +49. To quantitatively analyze the halogen-to-platinum ratio, we dissolved the\nelectrode samples with different halogen coordination and performed ion\nchromatography and ICP-OES tests. As shown in Figure R7, the ratios of halogen to\nplatinum are close and Cl/Pt ratio is the highest , indicating that some loss occurs during\nthe halogen exchange process.\nFigure R6. High-resolution XPS spectra of Pt 4f for (a) Pt/Ti, (b) Pt-F/Ti, (c) Pt-Cl/Ti,\n(d) Pt-Br/Ti and (e) Pt-I/Ti. (f) Binding energy of Pt 4f .\n7/2\nNote: The effect of fluorine on platinum is smaller than that of chlorine because the\nelectron affinity of fluorine is lower than that of chlorine, making it more difficult for\nplatinum to transfer electrons to fluorine.\nFigure R7. The ratio of platinum to halogen in different samples.\nRevision in the revised Supplementary Information (Figure 7 and 36):\nSupplementary Figure 7. The ratio of platinum to halogen in different samples.\nNote: The ratios of halogen to platinum are similar and all less than the Cl/Pt ratio,\nindicating that some loss occurs during the halogen exchange process.\nSupplementary Figure 36. High-resolution XPS spectra of Pt 4f for (a) Pt/Ti, (b) Pt-\nF/Ti, (c) Pt-Cl/Ti, (d) Pt-Br/Ti and (e) Pt-I/Ti. (f) Binding energy of Pt 4f .\n7/2\nNote: The effect of fluorine on platinum is smaller than that of chlorine because the\nelectron affinity of fluorine is lower than that of chlorine, making it more difficult for\nplatinum to transfer electrons to fluorine.\nComment 6: The reported shift in the pH gradient from 3 μm to 8 μm is interesting, but\nthe spatial and temporal resolution of the measurement is not described. Clarifying the\nmethod’s resolution and limitations is essential for evaluating the significance of this\nobservation.\nResponse: Due to the limitations of existing technologies, such as stepper motors and\nmicroscopes, the minimum spatial resolution for micro-region pH testing is 1 μm, with\na minimum pH accuracy of 0.01 and a single-point response time of 1 s. In the future,\nwe plan to further develop a micro-region pH 3D testing system to enable more precise\nanalysis of pH variations on the material surface during catalytic reactions. We are sorry\nthat the test is not such precise, thanks for your understanding!\nComment 7: The HER activity of Pt-I is slightly lower than Pt-Cl or Pt-F, yet Pt-I is\nthe chosen system. The rationale behind this choice—possibly superior solidophobicity\nor long-term performance—should be better justified.\nResponse: Stability is a crucial issue for seawater electrolysis for hydrogen\nproduction10-12. To meet the demands of future industrial applications, electrodes must\nexhibit a very high stablity13,14, especially in seawater electrolysis with hydrogen\nproduction and magnesium extraction. The degradation of electrode performance in this\nsystem primarily results from the adhesion of Mg(OH) , which negatively impacts the\n2\noverall system. Therefore, we selected the Pt-I/Ti electrode, which demonstrates the\nhighest stability. Notably, as shown in Figure R8, after 0.5 hour of reaction, the\noverpotential of Pt-I/Ti at 100 mA cm-2 (436 mV) is superior to that of other halogen-\ncoordinated electrodes.\nFigure R8. Short-term durability test of Pt/Ti and Pt-X/Ti in natural seawater\nelectrolysis under 100 mA cm-2.\nComment 8: While Faradaic efficiency for H₂ is reported to be near 100%, there is no\nmention of Mg(OH)₂ or O₂ balances. Additionally, what happens to the residual\nchloride ions after Mg precipitation?\nResponse: Faradaic efficiency for gas generation during the reaction was measured\nusing gas chromatography (GC). As shown in Figure R9, the oxygen generation\nFaradaic efficiency (FE) of Pt-I/Ti in a three-electrode system in 0.5 M Na SO is close\n2 4\nto 100%. The conversion efficiency of magnesium hydroxide reflects the utilization of\nOH-, which in the natural seawater electrolysis system is 53.1% (Figure R10). During\nelectrolysis, as magnesium ions precipitate, Na+ in the anode region is transported to\nthe cathode region through the ion-exchange membrane under the influence of diffusion\nand electric field forces. This results in an increase in the NaCl concentration in the\ncathode region. This conclusion is supported by the increase in Na+ content in the\ncathode solution before and after the reaction, as shown in Figure R11.\nFigure R9. Oxygen generation faradaic efficiency of Pt-I/Ti in three-electrode system\nin 0.5 M Na SO under 100 mA cm-2.\n2 4\nFigure R10. OH- availability of seawater electrolysis to synthesis Mg(OH) .\nComment 9: Could the accumulation or partial passage of Mg(OH)₂ impact membrane\nperformance over time? This possibility should be addressed or ruled out\nexperimentally.\nResponse: The membrane used in the MEA tests was Nafion 117 (Dupont, USA),\nwhich has a chemical structure formed by copolymerizing perfluorovinyl ether\nmonomers containing sulfonic acid groups with tetrafluoroethylene, giving the surface\nan acidic character15. This property makes it difficult for magnesium hydroxide to\nadhere to the surface. As shown in Figures R12 and R13, after 100 h of stability testing,\nmagnesium was almost undetectable on the membrane surface, indicating the\nmembrane's resistance to degradation in seawater environments.\nFigure R12. Optical image of the Nafion 117.\nFigure R13. SEM-EDX mappings of Nafion 117 after 100 h of stability testing.\nComment 10: How does this system compare with commercial seawater electrolysis\ntechnologies in terms of cost, durability, and performance metrics? Including such a\nbenchmark would enhance the manuscript's relevance to applied research.\nResponse: A representative commercialized seawater electrolysis case is the offshore\ntesting conducted by Xie et al. in collaboration with Dongfang Electric Corporation in\nFujian Province, China12 (https://news.cgtn.com/news/2023-06-02/China-produces-\nhydrogen-by-direct-seawater-electrolysis-1kjxYMpo2I0/index.html). As shown in\nFigure R14, a comparison is made between the seawater electrolysis with hydrogen\nproduction and magnesium extraction system and the system proposed by Xie et al. in\nterms of cost, stability, and other factors. It can be observed that while the seawater\nelectrolysis with hydrogen production and magnesium extraction system has higher\nenergy consumption, it offers a cost advantage in hydrogen production and stability.\nFigure R14. Comparison of stability, energy consumption, H production cost and\n2\ncurrent density for different systems.\nComment 11: The statement that this system “outperforms” previously reported\nseawater cathodes may be overstated. Some of the cited systems were tested for shorter\ndurations, which does not necessarily imply inferior performance.\nResponse: Thanks for the reviewer’s comments, which will help improve the rigor of\nthe manuscript. We have revised the term “outperforms” and replaced it with\n“demonstrates higher stability”.\nRevision in the revised manuscript (page 8): Therefore, in this case, we have\nextended the HER testing of Pt-I/Ti electrode in natural seawater over 5,000 hours and\nobserved a very impressive stability with negligible increase in applied bias (<10 mV)\n(Fig. 3d and Fig. S31), which demonstrates superior stability among the natural\nseawater cathodes reported previously (Supplementary Table 3).\nReferences\n1860 (2002).\nMoS Nanostructured Electrodes. Adv. Mater. 26, 2683-2687 (2014).\n2\n2\nScience 352, 797-800 (2016).\nprocesses. Water Res. 102, 421-427 (2016).\nefficient alkaline hydrogen evolution reaction. Nat. Commun. 13, 6875 (2022).\nunderstandings, recent advances, and future prospects. Energy Environ. Sci. 13, 3185-3206\n(2020).\n3\nreduction reaction. Science 348, 1230-1234 (2015).\nphotoelectron spectroscopy. Appl. Surf. Sci. Adv. 5, 100112 (2021).\nNiFeBa-LDH. Adv. Mater. 36, 2411302 (2024).\nAdv. 9, eadi7755 (2023).\n673-678 (2022).\nHydrogen. Adv. Mater. 36, 2311322 (2024).\n30, 644-687 (2005).\nResponse to Reviewer 2\nGeneral Comment: In the manuscript entitled “Supersolidophobic Pt catalyst for\nlong-term natural seawater electrolysis with high-purity magnesium extraction”, Lu\nand co-workers present a Pt catalyst with surface-coordinated halogens (e.g., F, Cl, Br,\nI) which can promote like-charge repulsion mechanism to achieve a successful\nextraction of high-purity Mg(OH) with the stable green H production from seawater.\n2 2\nThe manuscript presents innovative research methods and promising preliminary\nresults. However, it requires minor revisions to address several points before it can be\nconsidered for publication. Below are detailed comments and suggestions:\nResponse: We sincerely appreciate the insightful comments and constructive\nsuggestions from the reviewer, which help us significantly improve the quality of our\nmanuscript. We have revised our manuscript, and the details are further discussed in\nthe following point-by-point response.\nComment 1: The authors should ensure numerical consistency throughout the\nmanuscript. For instance, the purity of Mg(OH) is stated as 99.0% in the main text but\n2\nappears as 99% in Figure 5. Which value is correct? Similarly, the Mg(OH)\n2\nproduction rate is given as 5.54 g h-1 (two significant figures) in the main text, but the\ncorresponding balance data in Supplementary Figure 54 appears to show only one\nsignificant figure. Please verify and correct these discrepancies.\nResponse: Thanks for the reviewer’s suggestions regarding the numerical values in the\nmanuscript. We have conducted a thorough review of all the numerical values\nthroughout the manuscript and made the necessary revisions. It should be noted that the\ndiscrepancy between Supplementary Figure 54 and the figures in the main text arises\nfrom the use of an electronic scale with a division value of 0.1 g for weighing the 500\nhours product.\nRevision in the revised manuscript (page 2, 4, 12): Specifically, Pt-I coordination\nresults in supersolidophobicity, achieving a successful extraction of high-purity\nMg(OH) (>99% purity) while efficiently and stably producing green H (under 100\n2 2\nmA cm-2 for over 5,000 hours of continuous operation).\nThe practical applicability of this supersolidophobic electrode is demonstrated by\nassembling it in a scaled electrolyzer (electrode area: 100 cm2), which exhibits stable\nperformance (10 A, >1,000 hours) for the simultaneous production of green H and\n2\nrate of 5.54 g h-1, with a purity exceeding 99% and a particle size of ~35 μm (Fig. S56-\n61 and Supplementary Table 4).\nComment 2: The provided 3D TOF-SIMS images indicate that Pt-Br/Ti exhibits the\nmost abundant PtBr ⁻ signal, surpassing that of the Pt-Cl/Ti precursor. This finding is\n2\nsomewhat confusing, given that Pt-X/Ti (F, Br, I) samples are synthesized via halogen\nexchange from Pt-Cl/Ti; one might expect similar PtX ⁻ abundances across these\n2\nsamples. The EDX data shows a similar trend. However, Supplementary Figure 34\n(XPS) reveals a much weaker Cl signal compared to F and Br. The authors should\nclarify this apparent contradiction between the TOF-SIMS/EDX and XPS results\nregarding halogen abundance/distribution.\nResponse: In the TOF-SIMS tests, the yield of ions differs for the same substrate with\ndifferent ions. A higher yield corresponds to a stronger peak intensity, but this does not\nnecessarily indicate a higher content. Only ions of the same type in different samples\nof the same material can be directly compared1,2. Therefore, as shown in Fig. 2f-i, the\n3D TOF-SIMS results only indicate that halogen ions are located on the catalyst surface.\nAs shown in Figure R15, the molar content of different halogen coordination is similar,\nslightly lower than that of the Cl coordination, due to the fact that Pt-X/Ti (F, Br, I)\nsamples are synthesized via halogen exchange from Pt-Cl/Ti. We thank the reviewer\nfor pointing out the inconsistency in the XPS results presented in Supplementary Figure\n34. We have re-conducted the XPS test on the Pt-Cl/Ti sample and replaced the data in\nSupplementary Figure 34 with the updated results.\nFigure R15. The content of halogen in different samples.\nRevision in the revised Supplementary Information (Figure 34):\nSupplementary Figure 34. High-resolution XPS spectra of (a) F 1s for Pt-F/Ti, (b)\nCl 2p for Pt-Cl/Ti and (c) Br 3d for Pt-Br/Ti.\nComment 3: To robustly support the claim of Pt-X coordination (beyond physical\nadsorption), additional characterizations such as comparative XPS analysis should be\nprovided. Specifically, comparing the binding energies of Pt and relevant halogens in\nthe catalyst samples with those of bare Pt and PtX reference compounds would be\n2\nhighly informative to demonstrate chemical shifts indicative of coordination.\nResponse: Based on the reviewer’s suggestion, we performed XPS tests on Pt foil,\nPtCl , PtBr and PtI . As shown in Figure R16, halogen coordination with platinum\n2 2 2\ninduces electron transfer from platinum to the halogen. This result is consistent with\nthe changes in the oxidation states of platinum observed in the Pt-X/Ti (X: F, Cl, Br, I)\nsamples (Figure R17). However, due to the difficulty in synthesizing or obtaining of\nplatinum tetrafluoride, we did not measure it. The results obtained from the standard\nsample, consistent with those from Pt-X/Ti (X: F, Cl, Br, I), provide strong evidence\nthat the interaction between platinum and halogens is not merely a physical adsorption\nbut rather the formation of Pt-X coordination.\nFigure R16. High-resolution XPS spectra of Pt 4f for (a) Pt foil, (b) PtCl , (c) PtBr ,\n2 2\nand (d) PtI . (e) Binding energy of Pt 4f .\n2 7/2\nFigure R17. High-resolution XPS spectra of Pt 4f for (a) Pt/Ti, (b) Pt-F/Ti, (c) Pt-Cl/Ti,\n(d) Pt-Br/Ti and (e) Pt-I/Ti. (f) Binding energy of Pt 4f .\n7/2\nNote: The effect of fluorine on platinum is smaller than that of chlorine because the\nelectron affinity of fluorine is lower than that of chlorine, making it more difficult for\nplatinum to transfer electrons to fluorine.\nComment 4: There appears to be an error in Supplementary Figure 12: the y-axis\nshould be labeled \"-Z’’\" instead of \"Z’’\". Additionally, the axes scales should be\nconsistent across the relevant panels. Please correct this figure.\nResponse: Thanks for the reviewer’s suggestions regarding the standardization of\nnotations. We have revised the manuscript accordingly.\nRevision in the revised Supplementary Information (Figure 12):\nSupplementary Figure 12. EIS Nyquist plots of Pt/Ti, Pt-F/Ti, Pt-Cl/Ti, Pt-Br/Ti and\nPt-I/Ti at an applied potential of -0.1 V.\nComment 5: The authors selected Ti felt as the catalyst substrate. Could the rationale\nfor not using potentially cheaper alternatives, such as carbon fiber paper, be briefly\ndiscussed? Are there specific stability or performance considerations favoring Ti felt\nin this seawater electrolysis system?\nResponse: As the reviewer correctly pointed out, carbon fiber paper is less expensive\nthan titanium felt. However, its mechanical strength is insufficient to meet the\nrequirements of electrolyzer assembly in industrial applications. It tends to break during\nassembly or under the impact of electrolyte flow and bubble flushing, which\ncompromises the stability of the electrolysis system. Since stability is of paramount\nimportance in seawater electrolysis systems3,4, we chose titanium felt as the catalyst\nsupport for the seawater electrolysis with hydrogen production and magnesium\nextraction system.\nComment 6: Additional characterization of the produced Mg(OH) , such as TEM\n2\nimaging to examine its morphology and crystallinity, would strengthen the analysis of\nthe extraction product.\nResponse: As shown in Figure R17, the Mg(OH) produced during seawater\n2\nelectrolysis exhibits a hexagonal plate-like morphology with high crystallinity,\nconsistent with the XRD results in Figure R18. In addition, elemental mapping confirms\nthat the magnesium hydroxide product possesses high purity (Figure R19).\nFigure R17. TEM and SAED images of the produced Mg(OH) .\nSupplementary Figure 59. (a) TEM and (b) SAED images of the produced Mg(OH) .\nchemical and imaging surface analysis. Analyst 129, 483-487 (2004).\n271 (2000).\nNiFeBa-LDH. Adv. Mater. 36, 2411302 (2024).\nAdv. 9, eadi7755 (2023).\nResponse to Reviewer 3\nGeneral Comment: This study presents a novel platinum (Pt) catalyst modified with\nsurface-coordinated halogens (F, Cl, Br, I) for long-term natural seawater electrolysis,\nenabling simultaneous high-purity Mg(OH) extraction and green hydrogen production.\nbetween iodide (I-) ions and in situ-generated hydroxide (OH-) ions. This repulsion\nshifts Mg(OH) nucleation away from the electrode surface, promoting homogeneous\n2\nrather than heterogeneous nucleation. As a result, the system achieves outstanding\noperational stability (>5,000 hours at 100 mA cm-2), yields high-purity Mg(OH) ,\n2\nand reduces hydrogen production costs to $0.52/kg—outperforming U.S. DOE targets.\nOverall, this work is highly innovative and suitable for publication in Nature\nCommunications.\nRegarding the molecular dynamics (MD) simulations, I have a few comments to provide\nmore simulation details:\nResponse: We sincerely appreciate the insightful comments and constructive\nsuggestions provided by the reviewer, and the responses for questions are showing\nbelow.\nComment 1: Figure 4d–f demonstrates that OH⁻ ions are repelled from the Pt–I\nsurface but accumulate near pristine Pt. Which specific intermolecular interactions or\nforce field parameters in the COMPASS III model (as detailed in the Supplementary\nMethods) are responsible for the electrostatic repulsion between surface-bound iodide\n(I⁻) and OH⁻ ions?\nResponse: The COMPASS III force field (Supplementary Methods) was used to\nanalyze the interactions between I- and OH-, which explicitly include: (i) Coulombic\ninteractions (electrostatic repulsion) between charged species; and (ii) van der Waals\n(vdW) interactions (Lennard–Jones potential). Since both are anions, electrostatic\nrepulsion dominates, resulting in fewer OH- on the Pt-I surface.\nCharge distribution: (i) Iodine (I-) adsorbed on Pt was modeled as a point charge\n(q=-1eq=-1e) based on its anionic state; (ii) OH- was similarly assigned q=-1eq=-1e.\nKey parameter: The repulsive force between I- and OH- follows Coulomb’s\nlaw:𝐹 = 𝑘 𝑞1𝑞2, where q , q are negative charges of I- and OH-, respectively, and 𝑟 is\ne 𝑟2 1 2\ntheir separation distance. The specific force field parameters were automatically\nassigned to I- and OH- by the Forcite module in the COMPASS III software.\nComment 2: The MD system (Supplementary Figure 47) contains 24 OH⁻, 36 Na⁺, 2\nMg²⁺, 16 Cl⁻, and 1,200 H₂O molecules. What was the rationale behind selecting these\nspecific ion concentrations?\nResponse: In the MD system, the number of ions was determined to reflect the real\nconditions during the reaction. The OH- concentration was elevated to model the locally\nalkaline environment during hydrogen evolution reaction (pH≈13.0). The amounts of\nNa⁺, Mg2⁺, and Cl- were set close to their levels in seawater to maintain electroneutrality.\nConsequently, the ion ratios approximate those of seawater (Supplementary Table 1),\nwhile OH- was amplified to account for in situ generation during HER.\nComment 3: Figure 4f shows that the number of OH⁻ ions within 30 Å of the Pt–I\nsurface is approximately half that near pristine Pt. How was this spatial distribution of\nOH⁻ ions quantified—e.g., using radial distribution functions (RDFs)?\nResponse: The number of OH- was quantified not by using the radial distribution\nfunction, but by a 1D planar-averaged method. Specifically, the solution was divided\nalong the z-direction into slices at 3 Å intervals, and the number of OH- within each\nslice was counted to generate a histogram of OH- distribution as a function of distance\nalong the z-axis. From this analysis, it was determined that the OH- count within 30 Å\nof Pt-I was approximately half that observed near pristine Pt.\nComment 4: The MD simulations applied an external electric field of 0.2 V/Å along\nthe z-direction (as described in the Supplementary Information). How was this field\nstrength chosen, and does it reflect the experimental electrolysis conditions?\nResponse: The field strength was chosen based on the interfacial electric field during\nHER, corresponding to 100 mA cm-2 (overpotential ~465 mV). It was applied along the\nz-direction (normal to the electrode surface) to simulate cathodic polarization. This\nenhanced repulsion from the negatively charged surface (synergistic with the I- charge)\nand accelerated OH- diffusion into the bulk electrolyte.\nComment 5: Were MD simulations also conducted for other halogen-modified surfaces\n(Pt–F, Pt–Cl, Pt–Br)? If so, how did their OH⁻ exclusion behaviors compare with Pt–\nI? If not, could the trend in halogen electronegativity (F > Cl > Br > I) provide\nmechanistic insight into the unique supersolidophobicity observed for Pt–I?\nResponse: MD simulations were performed only for Pt–I (Fig. 4d-f, Supplementary\nFigure 47). The electronegativity trend is F (4.000) > Cl (3.475) > Br (3.219) > I (2.778),\nwhere lower electronegativity indicates a stronger anionic character1. Accordingly, I-\nretains more negative charge, resulting in stronger repulsion of OH-.\nThis\nelectronegativity trend is consistent with the experimentally observed order of stability\nand solidophobicity: Pt-I > Pt-Br > Pt-Cl > Pt-F (Fig. 3b and 4b, Supplementary Figure\n42). Therefore, while all halogen-coordinated samples exhibit solidophobic\ncharacteristics, Pt–I demonstrates a supersolidophobic behavior.\nFig. 3. b, Short-term durability test of Pt/Ti and Pt-X/Ti in natural seawater electrolysis\nunder 100 mA cm-2.\nFig. 4. b, Micro-region pH measurements as a function of distance during the HER\nprocess of Pt/Ti and Pt-I/Ti electrodes at 100 mA cm-2.\nSupplementary Figure 42. Optical images of the pH microelectrode and the test\nelectrode at the start of the pH measurements and micro-region pH measurements as a\nfunction of distance during the HER process at 100 mA cm-2. (a,d) Pt-F/Ti, (b,e) Pt-\nCl/Ti and (c,f) Pt-Br/Ti.\nReferences\nResponse to Reviewer 4\nGeneral Comment: I co-reviewed this manuscript with one of the reviewers who\nprovided the listed reports. This is part of the Nature Communications initiative to\nfacilitate training in peer review and to provide appropriate recognition for Early\nCareer Researchers who co-review manuscripts.\nResponse: We appreciate Nature Communications for promoting the co-review model\nto facilitate training in peer review and to provide appropriate recognition for Early\nCareer Researchers.\nResponse to Reviewer 5\nGeneral Comment: First of all, I would like to congratulate you on your insightful\nwork concerning the ‘Supersolidophobic Pt catalyst for long-term natural seawater\nelectrolysis with high-purity magnesium extraction’. The paper addresses the challenge\nof catalyst deactivation innovatively, while simultaneously pursuing the ‘double-\nproduct strategy’ of producing both green and cheap hydrogen and high-purity\nMg(OH) . The manuscript is high of quality and very well written. Besides, the techno-\n2\neconomic assessment and its details in the supplementary information are very well\nexplained and followed an understandable order.\nI have several points of suggestion to be considered to improve the quality of the\nmanuscript and increase the robustness of your outcomes. These suggested points will\nimprove the quality of your assessments and will benefit the visibility of your work:\nResponse: We do thank the reviewer for their thorough review of our manuscript and\nappreciate the valuable comments and suggestions provided.\nMAIN TEXT:\n•Line 1-2: Title\no The main focus of the paper is the double-product strategy to produce\ngreen/sustainable hydrogen and high-purity magnesium. However, in the title,\nhydrogen is not mentioned. Consider adding hydrogen as well. By doing so, you will\nincrease your visibility.\nResponse: We thank the reviewer for the comments regarding the article title. We agree\nthat hydrogen is a key product of this study, and revising the title will help enhance the\nvisibility of the manuscript. The revised title is: “Supersolidophobic Pt catalyst for long-\nterm natural seawater electrolysis with hydrogen production and high-purity\nmagnesium extraction”.\nRevision in the revised manuscript (page 1):\nSupersolidophobic Pt catalyst for long-term natural seawater\nelectrolysis with hydrogen production and high-purity magnesium\nextraction\n•Line 122: Fig. 3:\no Make sure that the figures are readable in the article. Or consider restructuring\nto improve readability and visibility.\nResponse: We have revised the content of Fig. 3 to ensure clarity and to help readers\nbetter understand the data and conclusions.\nRevision in the revised manuscript (page 6):\nFig. 3. HER performance and characterizations of Pt/Ti and Pt-X/Ti (X: F, Cl, Br,\nI) electrodes in natural seawater electrolysis. a, Polarization curves with iR\ncorrection of Pt/Ti, Pt-X/Ti and Pt foil in natural seawater (pH=7.71). b, Short-term\ndurability test of Pt/Ti and Pt-X/Ti in natural seawater electrolysis under 100 mA cm-2.\nc, Five representative polarization curves with iR correction of Pt-I/Ti in different areas\nof a scaled electrode (electrode size: 100 cm2). d, Long-term durability tests of Pt-I/Ti\nelectrode under 100 mA cm-2. e, Micro-XRF mapping of scaled Pt-I/Ti electrode after\nstability tests. f, High-resolution XPS spectra of I 3d for fresh and used Pt-I/Ti. g, In\nsitu electrochemical Raman spectroscopy of Pt-I/Ti from OCP to -1.0 V vs. RHE.\n•Line 252: Fig. 5:\no Fig. 5a: Positive effects are colored in red, which, in my opinion, are mostly\nassociated with a negative outcome. I would prefer to see the colour green for the\nfollowing sentence: “Co-electrosynthesis H and Mg(OH) , no preprocessing, low cost,\n2 2\nhigh Mg(OH) purity (>99%)”.\n2\no Fig. 5e: Make the benefits of Mg(OH) better visible.\n2\no Fig. 5f: For ‘This work’ (which represents your work), I would also use the\ncolour green to represent positive results (see comment above). As for now, it looks like\na negative result. Consider changing.\nResponse: As the reviewer noted, green is commonly used to indicate positive results.\nAccordingly, we revised Fig. 5a and 5f to display positive results in green and negative\nresults in red. In addition, we modified the content of Fig. 5e to make the benefits of\nMg(OH) more clearly visible. Additionally, we have re-positioned Fig. 5f\n2\n(Comparison of hydrogen production cost of DOE 2025 target, DOE 2030 target, and\nthis work.) within the manuscript. The revised figure is now located in Supplementary\nFigure 65.\nRevision in the revised manuscript (page 11):\nFig. 5. a, Schematic diagram of three strategies including extraction of Mg(OH) from\n2\nseawater, H production via seawater electrolysis and extraction of Mg(OH) coupled\n2 2\nwith H production from seawater.\nRevision in the revised Supplementary Information (Figure 65):\nSupplementary Figure 65. Comparison of hydrogen production cost of DOE 2025\ntarget, DOE 2030 target and this work.\nNote: Mg(OH) reasonably as USD 0.68 per kg and NaOH reasonably as USD 0.30 per\n2\nkg.\nElectrolyzer system cost of USD ~600 kW-1.\nElectrovalence=USD 0.03 kW·h-1.\nSeveral DISCUSSION POINTS need to be considered:\n• Sensitivity analysis:\no Discussion point based on the sensitivity assessment in the Supplementary\nInformation (SI) (see SI part below).\nResponse: We thank the reviewer for highlighting the important issue of sensitivity\nanalysis. We will add sensitivity analyses for different regions (e.g., the United States\nand Europe) in the Supplementary Information. Specifically, we will discuss how\nvariations in electricity prices, NaOH prices, and electrolyzer costs across regions affect\nthe overall hydrogen production cost.\n• Electrolyser\no What is your assumption about how the technology would perform on an\nindustrial scale? As the assumptions are based on 100 cm2.\no Scaling up the electrolyser to industrial scale will most likely benefit the\neconomic and environmental outcome due to learning effects and economies of scale.\no Doi: https://doi.org/10.1016/j.spc.2023.11.002.\nResponse: Alkaline electrolyzers for hydrogen production and chlor-alkali\nelectrolyzers for NaOH production are already well developed1-4. Integrating these\nmature processes with seawater-based magnesium extraction and hydrogen production\ncan definitely accelerate the industrial-scale application of this technology. Large-scale\ndeployment of this process can significantly reduce the cost of green H production\n2\nwhile providing high-purity Mg(OH) . Moreover, scaling up the electrolyzer to\n2\nindustrial scale is expected to further improve both economic and environmental\noutcomes due to learning effects and economies of scale5. Based on the design and\nmanufacturing process of a 100 cm2 electrolyzer, we have developed an electrolyzer\nwith an effective area of 1800 cm2 (Figure R20). In the future, we will further advance\nthe preparation of square-meter-scale electrodes, along with the design and fabrication\nof corresponding electrolyzers, which will help reduce energy consumption and\nenhance production efficiency.\nFigure R20. Optical image of 1800 cm2 natural seawater electrolysis system.\n•Linked to the Electrolyser: Catalyst: Discuss the cost and environmental impacts\nof precious metals\no Environmental impacts: For GHG missions and other environmental impacts:\no Anode: IrO -Ti anode => Iridium\n2\no Cathode: Pt-I/Ti cathode => platinum\no Potential article: doi:10.1371/journal.pone.0101298.\no Consider discussing the future impact of scalability and future improvement in\ntechnologies, electricity mixes, and other aspects. The environmental impacts can be\nsignificantly reduced by …\no Scaling up : Doi: https://doi.org/10.1016/j.spc.2023.11.002\no Consider the future improvement in technologies and electricity mixes: Doi:\nhttps://doi.org/10.1016/j.crsus.2024.100286.\no Is there any indication of recycling of these materials? I would assume this\nwould significantly increase the economic viability and decrease environmental\nimpacts.\nResponse: To achieve a greener and more sustainable future, reducing reliance on\nprecious metals is essential6. With the development of various catalysts, many\ninexpensive metals have shown strong catalytic performance and can, to some extent,\nserve as alternatives to precious metal catalysts. However, considering only earth-\nabundant metals (inexpensive metals) as substitutes for precious metals is insufficient\nto make catalysis greener and more sustainable7. Although inexpensive metals\ngenerally have lower environmental impacts (e.g., global warming potential,\ncumulative energy demand, and terrestrial acidification) than precious metals such as\nPt and Ir, precious metal catalysts offer advantages in terms of longer lifetimes, lower\nmaintenance costs, and easier scalability, which can effectively reduce overall\nenvironmental impact8-12. Furthermore, while deactivation of precious metals during\nelectrocatalytic reactions can often be mitigated through regeneration13, the\ndeactivation of inexpensive metals is usually accompanied by dissolution or similar\nprocesses, making regeneration more difficult14,15. Therefore, although precious metals\nare used in the reaction, advanced recycling and regeneration technologies can further\nmitigate their environmental impact16. In addition, with learning effects, economies of\nscale, and the widespread adoption of renewable energy in the future17,18, the\nenvironmental footprint of seawater electrolysis with hydrogen production and\nmagnesium extraction system will be further reduced, while its economic benefits will\ncontinue to improve.\n• Sodium hydroxide\no Based on your explanation to keep the pH of the anolyte stabilized, you need to\nadd NaOH, which tells me that OH is depleting (due to OH- usage) and Na is increasing\nin the seawater that is potentially released into the sea again? What are the\nenvironmental implications of the increasing Na content in the seawater, considering\nthat in the future it will be on an industrial scale? Does it need to be extracted/treated,\nand if yes, discuss the potential cost for additional equipment and operational costs.\nResponse: As the reviewer rightly pointed out, the discharge of excess Na+ without\ntreatment could negatively affect the marine ecosystem. However, with technological\nadvances, state-of-the-art anodes with high oxygen evolution selectivity, high activity,\nand long lifetimes will be applied in electrolysis systems19-22, enabling the direct use of\nseawater as both the catholyte and anolyte. Under such conditions, seawater electrolysis\nfor hydrogen production and magnesium extraction would only result in a minor loss\nof Mg2+ and a decrease in pH. The slight magnesium depletion is unlikely to\nsignificantly impact the marine ecosystem, and the lowered pH can be adjusted by\ncalcining shell-derived, abundant coastal resources to obtain quicklime (CaO)23.\nTherefore, this technology, when applied on an industrial scale, will not cause\nadditional adverse effects on marine ecosystems.\n• Environmental aspect of the novel technology\no It would be nice to include a critical discussion point about the environmental\nimplications of the technology (specifically the catalyst and its precious metals) in\ncomparison to the conventional benchmarks. As your article states the sustainable\nnature of the products, environmental implications are required. I understand that the\ncore of the assessment is the techno-economic feasibility; however, a discussion point\nabout the environmental implications is highly desired and beneficial for the paper.\nIt is not necessary to conduct a full-scale life cycle assessment, but indicating how\nenvironmentally sound the technology might be, based on literature, is highly\nappreciated.\nResponse: In response to the reviewer’s comments, we have conducted a Life Cycle\nAssessment (LCA) of the environmental impact of the seawater electrolysis-based\nmagnesium extraction and hydrogen production process using the Simapro. Further\ndetails are provided in the Supplementary Information. Due to the advantages of noble\nmetal catalysts, such as longer lifespan, lower maintenance costs, and easier\nscalability8-12, they can effectively reduce the overall environmental impact.\nFurthermore, during electrocatalytic reactions, the deactivation of noble metals can\ntypically be mitigated through regeneration13, extending their lifespan and\ncompensating for the environmental impact associated with their extraction and\nprocessing. Therefore, in the LCA analysis, the environmental impact of the catalyst\ncomponent was not considered for any hydrogen production system. As shown in\nFigure R21 and Table R1, we compared the environmental impacts of hydrogen\nproduced by seawater electrolysis–based magnesium extraction under green electricity\nand conventional electricity with those of commercial hydrogen. Even when powered\nby conventional electricity, the key indicators GWP and ADP remain lower than those\nof commercial hydrogen. Additionally, in response to the reviewer’s comments, we\nhave incorporated the LCA analysis into the main text, as shown in Fig. 5f. We\ntherefore conclude that leveraging the locally generated alkaline environment at the\ncathode during seawater electrolysis to precipitate Mg2+ offers an effective alternative\nto conventional, high-impact Mg(OH) production methods while simultaneously\n2\ncoupling green hydrogen production. This “double-product” strategy advantage can\nsubstantially reduce the overall environmental impact.\nFigure R21. Life Cycle Assessment (LCA) of the seawater electrolysis-based\nmagnesium extraction and hydrogen production process (use of green electricity).\nTable R1. A comprehensive comparison between average hydrogen production (Ⅰ),\nelectrolyzing seawater for hydrogen production using green electricity (ⅠⅠ) and\nelectrolyzing seawater for hydrogen production using conventional electricity (ⅠⅠⅠ).\nMethod TETP\nFAPTP MAETP ADP- ODP POCP\nof H2 AP (SO2- GWP (1,4- ADP EP (PO4 HTP (1,4-\n(1,4-DCB (1,4-DCB element (CFC-11 ethylene\nproducti Eq) (CO2 Eq) DCB (MJ) Eq) DCB Eq)\nEq) Eq) (Sb Eq) Eq) Eq)\non Eq)\nⅠ 26.8 18740.1 5845.8 12496000 48.6 173996.4 21.4 12433.0 0.02 0.00004 1.7\nⅠⅠ -142.9 -50477.2 -4691.9 -24391536.7 -139.3 -426318.9 -36.08 -35208.8 0.11 0.0015 -7.9\nⅠⅠⅠ 147.4 7611.4 30347.3 86404925.3 16.2 137892.7 67.0 36449.3 0.16 0.0011 6.6\nNote: Acidification potential (AP), global warming potential (GWP), freshwater\naquatic ecotoxicity potential (FAETP), marine aquatic ecotoxicity potential (MAETP),\nterrestrial ecotoxicity potential (TETP), abiotic depletion potential (ADP),\neutrophication potential (EP), human toxicity potential (HTP), abiotic depletion\npotential of elements (ADP element), ozone layer depletion potential (ODP) and\nphotochemical ozone creation potential (POCP).\nRevision in the revised manuscript (page 11):\nFig. 5. f, Life Cycle Assessment (LCA) of the seawater electrolysis-based magnesium\nextraction and hydrogen production process.\nRevision in the revised manuscript (page 13):\nWe also conducted a Life Cycle Assessment (LCA) to quantify its environmental\nimpact compared to traditional hydrogen production processes (Fig. 5f and\nSupplementary Table 6). Our evaluation focused on eleven environmental impact\ncategories: acidification potential (AP), global warming potential (GWP), freshwater\naquatic ecotoxicity potential (FAETP), marine aquatic ecotoxicity potential (MAETP),\nterrestrial ecotoxicity potential (TETP), abiotic depletion potential (ADP),\neutrophication potential (EP), human toxicity potential (HTP), abiotic depletion\npotential of elements (ADP element), ozone layer depletion potential (ODP) and\nphotochemical ozone creation potential (POCP). The results indicate that this “double-\nproduct” strategy can substantially reduce the overall environmental impact.\nRevision in the revised Supplementary Information (Table 6):\nSupplementary Table 6. A comprehensive comparison between average hydrogen\nproduction (Ⅰ), electrolyzing seawater for hydrogen production using green electricity\n(ⅠⅠ) and electrolyzing seawater for hydrogen production using conventional electricity\n(ⅠⅠⅠ).\nMethod TETP\nFAPTP MAETP ADP- ODP POCP\nof H2 AP (SO2- GWP (1,4- ADP EP (PO4 HTP (1,4-\n(1,4-DCB (1,4-DCB element (CFC-11 ethylene\nproducti Eq) (CO2 Eq) DCB (MJ) Eq) DCB Eq)\nEq) Eq) (Sb Eq) Eq) Eq)\non Eq)\nⅠ 26.8 18740.1 5845.8 12496000 48.6 173996.4 21.4 12433.0 0.02 0.00004 1.7\nⅠⅠ -142.9 -50477.2 -4691.9 -24391536.7 -139.3 -426318.9 -36.08 -35208.8 0.11 0.0015 -7.9\nⅠⅠⅠ 147.4 7611.4 30347.3 86404925.3 16.2 137892.7 67.0 36449.3 0.16 0.0011 6.6\nNote: Acidification potential (AP), global warming potential (GWP), freshwater\naquatic ecotoxicity potential (FAETP), marine aquatic ecotoxicity potential (MAETP),\nterrestrial ecotoxicity potential (TETP), abiotic depletion potential (ADP),\neutrophication potential (EP), human toxicity potential (HTP), abiotic depletion\npotential of elements (ADP element), ozone layer depletion potential (ODP) and\nphotochemical ozone creation potential (POCP).\nRevision in the revised Supplementary Information (Characterizations and\nMeasurements):\nLCA analysis. An LCA was conducted using Simapro software (Database: ecoinvent\n3.11), focusing on categories such as acidification potential (AP), global warming\npotential (GWP), freshwater aquatic ecotoxicity potential (FAETP), marine aquatic\necotoxicity potential (MAETP), terrestrial ecotoxicity potential (TETP), abiotic\ndepletion potential (ADP), eutrophication potential (EP), human toxicity potential\n(HTP), abiotic depletion potential of elements (ADP element), ozone layer depletion\npotential (ODP) and photochemical ozone creation potential (POCP). The functional\nunit for this assessment was defined as the production of one ton of magnesium\nhydroxide.\nSUPPLEMENTARY INFORMATION\n•Sensitivity analysis:\no A TEA usually includes a sensitivity analysis showing how certain parameters\nmight affect the economic outcome, specifically important for technologies under low\nTRL with high uncertainty. Particularly for some of your assumptions, it would be\nbeneficial to add the following sensitivity analyses (see below). This will make your\nresults more robust and accountable for price uncertainties, associated with different\nconditions (e.g. different production/purchase locations, etc.). Besides, your\nassumptions are usually based on Chinese conditions. The journals of ‘Nature’ aim for\na more general view that can relate to other parts of the world (e.g. US and the EU).\nHence, some of the parameters are asked to be evaluated based on different regions\n(see below).\nResponse: We thank the reviewer for the constructive comments on the TEA analysis,\nparticularly the suggestion to include a sensitivity analysis. This will help provide more\ncomprehensive results applicable to other regions of the world. Following the\nreviewer’s recommendation, we have added the sensitivity analysis. Through\nsensitivity analysis, we compare the impact of key parameters (such as magnesium\nhydroxide price, hydrogen price, and electricity price) on hydrogen production costs.\nAs shown in Figure R22, variations in each economic parameter from -10% to +10%\nof their original values produce linear trends with specific slopes. The steeper the slope,\nthe greater the impact of that economic parameter on hydrogen production costs.\nCompared to the original value, electricity cost has a relatively smaller effect on\nhydrogen production costs. However, the slopes of the curves related to the prices of\nmagnesium hydroxide and sodium hydroxide are steeper, indicating that any change in\nthese parameters could have a significant impact on hydrogen production costs.\nFigure R22. Analysis of hydrogen production cost sensitivity to variation in specific\neconomic parameters.\nIMPORTANT!!: Discuss the sensitivity analysis briefly in the main text.\nCalculations can be kept in the supplementary information.\nResponse: Following the reviewer’s suggestion, we have added a discussion on the\nsensitivity analysis in the main text and included the detailed calculation results in the\nSupplementary Discussions.\nRevision in the revised manuscript (page 13):\nAs shown in Fig. S64-65, variations in each economic parameter from -10% to\n+10% of their original values produce linear trends with specific slopes. Compared to\nthe original value, electricity cost has a relatively smaller effect on hydrogen production\ncosts. However, the slopes of the curves related to the prices of magnesium hydroxide\nand sodium hydroxide are steeper, indicating that any change in these parameters could\nhave a significant impact on hydrogen production costs.\n• Mg(OH) :\n2\no How will the result (H production cost per ton or kg) change if the price\n2\nfluctuates ±10%? (can be a different value)\nResponse: When the market price of magnesium hydroxide fluctuates by ±10%, the\nhydrogen production cost varies between USD -0.38 and USD 1.60 per kg H (Figure\nFigure R23. Changes in hydrogen production costs due to fluctuations in the price of\nMg(OH) .\n2\n• Electrolyser (almost 22% contribution)\no You chose a price of 600 $/kW. (SI: line 229). This is based on Chinese\nconditions.\no How will the result (H production cost per ton or kg) change if you use the\n228).\nResponse: During the TEA analysis, when using electrolyzer manufacturing costs for\nthe European and U.S. regions, we considered the application of the seawater\nelectrolysis-based magnesium extraction and hydrogen production technology in these\nregions. Therefore, we adjusted the costs of sodium hydroxide (NaOH) and magnesium\nhydroxide (Mg(OH) ), as well as the electricity prices for the respective regions. When\n2\nthe electrolyzer price is USD 2500/kW, the hydrogen production cost is USD 1.61 in\nthe European and USD 0.80 in the US (Figure R24).\nFigure R24. Changes in hydrogen production costs due to fluctuations in the price of\nelectrolyzer.\n• NaOH: (most influential parameter when it comes to cost: 58%)\no Base assumption is USD 0.30 /kg.\no How will the result (H production cost per ton or kg) change if the price\n2\nfluctuates ±10% (can be a different value)?\nResponse: When the market price of sodium hydroxide fluctuates by ±10%, the\nhydrogen production cost varies between USD -0.08 and USD 1.12 (Figure R25).\nFigure R25. Changes in hydrogen production costs due to fluctuations in the price of\nNaOH.\n• Electricity (21%):\no Chosen = USD 0.03/kWh\no The European and the US electricity prices substantially deviate from the\nChinese ones.\no How will the result (H production cost per ton or kg) change if the electricity\n2\nprice is chosen based on….\no Average EU price: 0.1899 EUR /kWh\no Fluctuates between 0.067 EUR/kWh (Finland) and 0.25 EUR/kWh (Cyprus).\nKeep in mind it is in €, not $!!!\no Website: https://ec.europa.eu/eurostat/statistics-\nexplained/index.php?title=Electricity_price_statistics#Electricity_prices_for_non-\nhousehold_consumers\no Average US price: 01318 USD/kWh.\no Website:\nhttps://www.eia.gov/electricity/monthly/epm_table_grapher.php?t=epmt_5_3\nResponse: Based on the electricity prices provided by the reviewer for different regions,\nthe hydrogen production cost will fluctuate between USD -3.27 and USD 12.90 (Figure\nR26).\nFigure R26. Changes in hydrogen production costs due to fluctuations in the price of\nelectricity.\nRevision in the revised Supplementary Information (Figure 66)：\nSupplementary Figure 66. Changes in hydrogen production costs due to price\nfluctuations caused by various factors. (a) Mg(OH) price, (b) NaOH price, (c)\n2\nelectricity price and (d) electrolyzer price.\nNote: The price of Mg(OH) in the US: USD 1700 per ton\n2\n(https://chinese.alibaba.com/product-detail/Factory-price-magnesium-carbonate-with-\n99-1600691031574.html?spm=a2700.7724857.0.0.14025b82TWtkCl).\nThe price of Mg(OH) in the EU: USD 2050 per ton\n2\n(https://www.wholesalesuppliesplus.com/).\nThe price of NaOH in the US: USD 330 per ton (https://businessanalytiq.com/).\nThe price of NaOH in the EU: USD 290 per ton (https://businessanalytiq.com/).\nThe price of electrolyzer in the US and EU: USD 2500/kW.\n• Capacity\no Line 236 SM: Why do you focus on the capacity of 100 kg H production per\n2\nday? How did you calculate the 100 kg?\nPlease elaborate?\nResponse: Producing 100 kg of hydrogen per day corresponds to the hydrogen output\nof an electrolyzer with a 1 Nm3 hydrogen production capacity, operating for 12 hours\nper day. An electrolyzer with a 1 Nm3 hydrogen production capacity is typically used\nas a benchmark for evaluating industrial applicability24,25. Therefore, we chose the\nproduction of 100 kg of hydrogen per day as the basis for our calculations.\no Line 237 SM: What is 1/0.99 representing?\nResponse: In the calculation of the total required current, *1/0.99 accounts for the\npresence of other competing reactions in the electrolysis system. The total current is\nadjusted to compensate for this consumption, ensuring that the current available for\nhydrogen production meets the expected value26,27. Specifically, 0.99 represents the\nhydrogen generation Faradaic efficiency (FE).\n• Material cost: Line 259: Natural seawater: Line 238 – 240 vs Line 259 – 261:\no I have the feeling they are not calculated on the same basis:\no Please elaborate why. In case it is wrong, please correct it.\no In Line 238: 1/2 e- vs line 259 1/4e-. Why is there a difference? I would assume\nit would be the same. Please elaborate. In case it is wrong, correct it.\nResponse: We thank the reviewer for pointing out the errors in our calculations. We\nhave made the necessary corrections.\nRevision in the revised Supplementary Information (Discussion 1):\nNatural seawater: 112800 A × (1/2e-) × (1/96485 C/mol) × (0.018 kg/mol) ×\n(0.2642 t/kg) × (86400 s/day) × (USD 0.00378/gal) × (1 day/0.1t)=USD 90.0 per ton\nH .\n263.2+USD 105.3+USD 90+USD 6000=USD 10469.7 per ton H .\n2\n• SM Line 124, you are stating that anolyte (0.5 M Na SO , 20 L) and catholyte\n2 4\n(natural seawater, 124 20 L) are assumed to be replaced every 6 h to maintain a\nconstant pH. However, I do not see the cost for Na SO . Is there any reason why the\n2 4\ncost of Na SO is not included?\n2 4\nResponse: Controlling the pH of the solution stably in the laboratory is challenging;\nhowever, in industrial settings, real-time pH monitoring systems and automatic dosing\ndevices can achieve this28,29. Therefore, in the laboratory stability tests, we maintained\na constant pH by replacing the sodium sulfate solution. In the TEA analysis, which\ncalculates the economic benefits of large-scale electrolyzers, we accounted for the cost\nof adding alkali to maintain the solution’s pH. As such, we only considered the cost of\nNaOH and did not account for the consumption of Na SO in this cost calculation.\n2 4\n•LINE 623-722 = something is off, a part is missing. Correct it.\nResponse: Due to unforeseen errors during the upload process, some content was\nmissing. We have re-uploaded the missing content in the revised version.\nReferences\nand applications. Prog. Energy Combust. Sci. 36, 307-326 (2010).\nproduction: Performance improvement and applications. Int. J. Hydrogen Energy 49, 458-488\n(2024).\noxygen depolarized cathodes: history, present status and future prospects. J. Appl. Electrochem.\n38, 1177-1194 (2008).\nEnviron. Policy. 16, 225-234 (2014).\n5 de Souza, N. R. D., Matt, L., Sedrik, R., Vares, L. & Cherubini, F. Integrating ex-ante and\nprospective life-cycle assessment for advancing the environmental impact analysis of emerging\nbio-based technologies. Sustain. Prod. Consum. 43, 319-332 (2023).\n33 (2020).\n2608 (2023).\ncatalyst recycling. J. Clean. Prod. 142, 2618-2628 (2017).\nRecovery Techniques for a Circular Economy. Catalysts 11 (2021).\nbased catalyst recycling process. Green Chem. 22, 1919-1933 (2020).\nSustain. Metall. 9, 909-926 (2023).\npresence of organic molecules and chloride ions. Rsc. Adv. 13, 7980-7987 (2023).\nElectrocatalysts. ACS Catal. 5, 233-240 (2015).\nSolutions. J. Electrochem. Soc. 111, 897 (1964).\nof halide on Ni-based anode. Nat. Commun. 14, 4822 (2023).\nONE 9, e101298 (2014).\nand their impact on climate change. Int. J. Hydrogen Energy 47, 33112-33134 (2022).\nthe transition to greener chemical processes. Cell Rep. Sustain. 2, 100286 (2025).\ncatalyst. Nat. Energy 8, 264-272 (2023).\nExchange Membrane Electrolyzer. Adv. Funct. Mater. 35, 2415143 (2025).\nan intrinsically-selective chlorine-phobic nickel polysulphide nanostructured electrocatalyst for\npure oxygen evolution. Nano Energy 111, 108439 (2023).\nelectrolysis. Nano Energy 136, 110714 (2025).\ndeoxygenation of triglyceride via natural waste shell derived Ca(OH) nanocatalyst. J. Anal.\nNiFeBa-LDH. Adv. Mater. 36, 2411302 (2024).\nmodulating 3d metal oxidation cycle energetics. Nat. Catal. 3, 985-992 (2020).\nselectivity. Nat. Sustain. 6, 827-837 (2023).\nand on-site co-synthesis of H and Mg hydroxide nanoflakes. Chem 10, 3067-3087 (2024).\n2\nfor Real-Time Monitoring and Controlling of Wastewater. IEEE Access 10, 6528-6540 (2022).\nof urban wastewater systems—where do we stand today? J. Hydrol. 299, 335-348 (2004)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed most of my comments. However, I find it strange that in Supplementary Figure 7 the Pt-to-halogen ratio is about 0.1, which would imply that each Pt atom stabilizes around 10 halogen atoms. Is this correct or could there be a mistake ?", "reviewer_2": "The authors have addressed all my concerns in a proper way. I do think this nice piece of work now is ready for publication.", "reviewer_3": "The authors have adequately addressed all of my questions and concerns. I believe the manuscript is ready for publication in its current form.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_5": "Dear authors,\nI am pleased to see the improvements made to the manuscript and the inclusion of LCA work, which has the potential to significantly enhance the quality of the paper by demonstrating the sustainability of the technology in terms of its technical capabilities, economic feasibility, and environmental performance. Most of the previous comments have been addressed appropriately. However, two major aspects still require improvement: (1) the sensitivity analysis and (2), in particular, the LCA. Further details are provided in the comments below.\nIMPORTANT: It is essential that the sensitivity analysis and the LCA are presented with the highest level of transparency. At present, some of the reported calculations appear questionable, either due to insufficient description of the methodology or potential errors in the calculations themselves. For the next revision, please ensure that any errors are corrected and/or that the calculation methodology is described in sufficient detail to enable reproducibility and proper verification.\nMANUSCRIPT\nAbstract\nLine 38-40: The authors performed a life cycle assessment. Hence, I would also mention it in the abstract.\nMain Body:\nLine 257: Fig. 5:\no For Fig. 5e: If possible, highlight the margin between costs and benefits as potential “profits”. By doing this, it will be clear to the reader that it is economically profitable just by looking at Figure 5e.\no Fig. 5f: While it shows the relative comparison between two alternatives, the magnitude is not evident here. The scale goes from 15 to -15. What exactly does it mean? For the mid-point impact categories abbreviation, I would write down the full meaning of it in the caption, or refer to the supplementary material where it is fully written down.\nLine 300-309: Instead of listing the different categories, I would rather discuss the outcome in comparison to the conventional H2 production. I would move the categories in the supplementary material. Listing up the impact factor categories without any context in the main text seems to be unprofessional.\nMANUSCRIPT/SUPPLEMENTAL MATERIAL:\nThe most crucial aspect is the performed LCA. While the reviewer greatly appreciates the effort put into this analysis and fully acknowledges its importance, there are several critical points that must be (i) addressed, (ii) revised, and (iii) checked before the work can be considered for publication.\n(1) The absence of the catalyst from the assessment may lead to a substantial underestimation of the environmental impacts. In particular, assumptions regarding catalyst lifespan, regeneration potential, and low maintenance costs remain insufficiently substantiated, as they are primarily based on laboratory-scale results. The suggested study (doi:10.1371/journal.pone.0101298) demonstrates that the GHG emissions of iridium and platinum are immense. While it is understood that modelling the catalyst’s contribution may be challenging, this aspect should be addressed at a minimum through a dedicated discussion in the main text or supplementary material. At present, it is mentioned solely in the rebuttal and not incorporated into the manuscript or supplementary files. The omission of such a discussion risks undermining the credibility of the LCA, especially among LCA experts.\n(2) Unfortunately, the manuscript does not yet include essential elements that constitute the minimum requirements for a robust and publishable LCA:\no What is the functional unit?\no What are the system boundaries?\no Does your LCA align with the ISO 14044?\no What approach has been used to define your system boundaries (e.g. cradle-to-gate or grave-to-grave, etc.)\no What LCA approach has been used? (consequential or attributional)?\no What allocation method has been used as you have a multi-product process? (e.g. Economic, mass allocation, or system expansion, see ISO 14040).\no What LCIA (life cycle impact assessment) method has been used? It is indicated in SimaPro.\n(3) Line 761-771, Supplementary material: While the improvement from average H2 production (I) to electrolysing seawater for hydrogen production using conventional electricity (III) seems to be reasonable and consistent with the described technology, the reported improvement from (III) to (II = seawater for hydrogen production using green electricity) remains highly questionable. The negative values, representing beneficial contributions (e.g., carbon sinks), appear to result solely from the assumed use of green electricity. However, it is difficult to reconcile this with the implication that the technology becomes a net carbon sink from a GWP/greenhouse gas emissions perspective. The same applies to all other indicators reported as negative. For instance, even when renewable energy is used, greenhouse gas emissions (kg CO₂-eq.) are still generated, which also holds true across all other impact categories:\nReCiPe2016 from SimaPro:\n- Green electricity in Germany (kg CO2-eq/kWh)\n• Photovoltaic: 0.0981\n• Wind: 0.0073\n• Nuclear: 0.00653 (debatable if green or not)\n- Green electricity in China (kg CO2-eq/kWh)\n• Photovoltaic: 0.0785\n• Wind: 0.049\n• Nuclear: 0.000783\n- Average conventional electricity mixes (including different national-based electricity sources) (kg CO2-eq/kWh) in…,\n• China: 1.26.\n• World: 0.861.\n• USA: 0.577.\n=> Hence, it is highly unlikely that the use of green electricity alone would render the technology a net (carbon) sink. This outcome suggests either a significant error in the LCA calculation/modelling or the presence of another factor that has not been sufficiently explained. It is therefore essential to correct and/or provide a detailed elaboration of this result, as the current information is insufficient to verify the LCI and LCA. Transparency and comprehensive explanation are critical in LCA studies, particularly when certain information must remain confidential.\nSUPPLEMENTAL MATERIAL (referring to figures and text that have been included)\n(1)Sensitivity analysis: Fig. 66, line 725:\nFig. 66a: correct.\nFig. 66b: almost the same results. Probably due to different digits after the comma.\nAccording to the reviewer’s calculation: Sensitivity analysis: NaOH:\n• 10% higher = 1.209 USD/kg H2\n•-10% -= still positive = 0.09 USD/kg H2\n=> For both 66a and b: Discuss and explain what implications the negative outcomes have for your technology and economic feasibility.\nFig. 66c: correct\nFig. 66d: seems to be incorrect, or the calculation approach is not sufficiently described to be followed. Please make sure that the calculation way and method is transparent enough to reproduce the numbers/outcome.\nREBUTTAL part:\n(1)Sensitivity analysis: Electrolyser (page 46-487):\no The sensitivity analysis for the electrolyser seems to be off, given the calculation method that has been provided by lines 256-257, lines 288-289, and line 292 in the supplementary materials.\no Taking a price of USD 2500/kW (as indicated in the rebuttal) and US electricity price of 0.1331 USD/kWh, the hydrogen cost rises to approx. 11.76 USD/ kg H2. Even if the electricity price is kept constant (based on the benchmark of 0.03 USD/kWh), the outcome would still be higher than calculated, which is 8.42 USD/ kg H2.\no Correct this mistake and provide a transparent and detailed calculation.\n(2) Sensitivity: Electricity price (Page 48):\no The calculation seems to have an error. The benchmark electricity price is USD 0.03/kWh, and the Finnish electricity price is 0.067 EUR/kWh (= 0.07906 USD/kWh => more than double). Hence, it is not possible that the Finnish electricity price will lead to a negative hydrogen production cost (or decrease => it must increase). According to line 262 in the supplementary material: Electricity: 315.85 kW x 24h/day USD 0.079/kWh x (day/0.1 ton) = USD 5992 per ton H2. => 4.33 USD/kg H2. The same issues apply to the other electricity prices. Hence, there must be a calculating error. Please correct it."}, "author_response": "Response to Reviewer 1\nGeneral Comment: The authors have addressed most of my comments. However, I\nfind it strange that in Supplementary Figure 7 the Pt-to-halogen ratio is about 0.1,\nwhich would imply that each Pt atom stabilizes around 10 halogen atoms. Is this correct\nor could there be a mistake?\nResponse: We express our gratitude to the reviewers for pointing out the error, and the\ncorresponding content has been corrected in the revised manuscript (Figure R1).\nFigure R1. The ratio of platinum to halogen in different samples.\nRevision in the revised Supplementary Information (Figure 7):\nSupplementary Figure 7. The ratio of platinum to halogen in different samples.\nResponse to Reviewer 2\nGeneral Comment: The authors have addressed all my concerns in a proper way. I do\nthink this nice piece of work now is ready for publication.\nResponse: We sincerely thank the reviewer for the kind support and recognition of our\nwork. We greatly appreciate the time and effort dedicated to reviewing our manuscript.\nYour interest and detailed comments are highly encouraging and provide valuable\nguidance for our current and future research.\nResponse to Reviewer 3\nGeneral Comment: The authors have adequately addressed all of my questions and\nconcerns. I believe the manuscript is ready for publication in its current form.\nResponse: We sincerely thank the reviewer for the kind support of our work. We\ngreatly appreciate the time and effort dedicated to reviewing our manuscript. All of the\nreviewer’s comments are highly constructive and have been very helpful in improving\nour research.\nResponse to Reviewer 4\nGeneral Comment: I co-reviewed this manuscript with one of the reviewers who\nprovided the listed reports. This is part of the Nature Communications initiative to\nfacilitate training in peer review and to provide appropriate recognition for Early\nCareer Researchers who co-review manuscripts.\nResponse: We appreciate Nature Communications for promoting the co-review model\nto facilitate training in peer review and to provide appropriate recognition for Early\nCareer Researchers.\nResponse to Reviewer 5\nGeneral Comment: I am pleased to see the improvements made to the manuscript and\nthe inclusion of LCA work, which has the potential to significantly enhance the quality\nof the paper by demonstrating the sustainability of the technology in terms of its\ntechnical capabilities, economic feasibility, and environmental performance. Most of\nthe previous comments have been addressed appropriately. However, two major\naspects still require improvement: (1) the sensitivity analysis and (2), in particular, the\nLCA. Further details are provided in the comments below.\nIMPORTANT: It is essential that the sensitivity analysis and the LCA are presented\nwith the highest level of transparency. At present, some of the reported calculations\nappear questionable, either due to insufficient description of the methodology or\npotential errors in the calculations themselves. For the next revision, please ensure that\nany errors are corrected and/or that the calculation methodology is described in\nsufficient detail to enable reproducibility and proper verification.\nResponse: We sincerely appreciate the reviewers’ valuable suggestions and\nconstructive comments that have helped improve the quality of our manuscript. The\nmanuscript has been revised accordingly, and our detailed responses to the reviewers’\ncomments are provided in the following point-by-point reply.\nMANUSCRIPT\nAbstract\nLine 38-40: The authors performed a life cycle assessment. Hence, I would also\nmention it in the abstract.\nResponse: We thank the reviewer for the comments regarding the article abstract, we\nagree that the life cycle assessment should be mentioned in the abstract, and the section\nhas been revised accordingly.\nRevision in the revised manuscript (page 2):\nAdditionally, the scalability of the Pt-I catalyst, along with the techno-economic\nanalysis and life cycle assessment of the natural seawater electrolysis technology are\nsystematically demonstrated.\nMain Body:\nLine 257: Fig. 5:\no For Fig. 5e: If possible, highlight the margin between costs and benefits as\npotential “profits”. By doing this, it will be clear to the reader that it is economically\nprofitable just by looking at Figure 5e.\nResponse: As the reviewer correctly pointed out, adding the difference between cost\nand revenue in Fig. 5e allows readers to more intuitively understand how the double-\nproduct strategy effectively reduces the cost of hydrogen production. Therefore, Fig.\n5e has been revised accordingly.\nRevision in the revised manuscript (page 12):\nFig. 5. e, Brief division of natural seawater electrolysis total costs and benefits.\no Fig. 5f: While it shows the relative comparison between two alternatives, the\nmagnitude is not evident here. The scale goes from 15 to -15. What exactly does it mean?\nFor the mid-point impact categories abbreviation, I would write down the full meaning\nof it in the caption, or refer to the supplementary material where it is fully written down.\nResponse: The numerical differences among various environmental impact factors are\nsubstantial. To facilitate a more intuitive comparison of different hydrogen production\ntechnologies, the data for all impact factors were normalized to a range between -15\nand 15, as shown in Table R1. In addition, the specific meanings of the impact category\nabbreviations have been provided in the Supplementary Information, which will help\nreaders better understand the implications of each environmental impact factor.\nTable R1. A comprehensive comparison between average hydrogen production (Ⅰ),\nelectrolyzing seawater for hydrogen production using green electricity (ⅠⅠ) and\nelectrolyzing seawater for hydrogen production using conventional electricity (ⅠⅠⅠ).\nTETP\nMethod FAPTP ADP-\nAP (10-1 GWP (10- MAETP (10-1 kg EP (10-1 HTP (10-4 ODP (103 POCP (kg\nof H2 (10-3 kg ADP (10-5 element\nkg SO2- 4 kg CO2 (10-7 kg 1,4- 1,4- kg PO4 kg 1,4- kg CFC- ethylene\nproducti 1,4-DCB MJ) (102 kg\nEq) Eq) DCB Eq) DCB Eq) DCB Eq) 11 Eq) Eq)\non Eq) Sb Eq)\nEq)\nⅠ 2.68 1.87 5.85 1.25 4.86 1.74 2.14 1.24 2.28 0.05 1.66\nⅠⅠ -10.05 -5.03 -4.06 -2.35 -13.60 -4.22 -3.49 -3.47 13.11 1.55 -7.12\nⅠⅠⅠ 18.98 0.78 30.98 8.73 1.94 1.43 6.82 3.70 17.24 1.11 7.34\nLine 300-309: Instead of listing the different categories, I would rather discuss the\noutcome in comparison to the conventional H production. I would move the categories\n2\nin the supplementary material. Listing up the impact factor categories without any\ncontext in the main text seems to be unprofessional.\nResponse: In response to the reviewer’s comments, the main text of the manuscript has\nbeen revised to include a discussion of the environmental impacts of the seawater\nelectrolysis-based magnesium extraction and hydrogen production process compared\nwith conventional hydrogen production technologies. In addition, a detailed description\nof the impact category definitions has been added to the Supplementary Material, which\nwill help readers from different disciplines better understand these environmental\nfactors.\nRevision in the revised manuscript (page 13 and 14):\nIn this assessment, the system expansion approach was adopted as the allocation\nmethod, assuming that the co-produced byproducts could equivalently replace the same\nquantities of conventionally produced materials in the market, thereby generating\npositive environmental benefits. As shown in Fig. 5f, the process produces 8 t of O\n2\nand 1.45 t of Mg(OH) along with 1 t of H . Compared with average H production, the\n2 2 2\nseawater electrolysis-based magnesium extraction and hydrogen production process\nsignificantly reduces abiotic depletion potential (ADP) and global warming potential\n(GWP), while exhibiting higher values only in the ADP-element and ozone layer\ndepletion potential (ODP) categories.\nRevision in the revised Supplementary Information (Table 6):\nSupplementary Table 6. A comprehensive comparison between average hydrogen\nproduction (Ⅰ), electrolyzing seawater for hydrogen production using green electricity\n(ⅠⅠ) and electrolyzing seawater for hydrogen production using conventional electricity\n(ⅠⅠⅠ).\nTETP\nMethod FAPTP ADP-\nAP (10-1 GWP (10- MAETP (10-1 kg EP (10-1 HTP (10-4 ODP (103 POCP (kg\nof H2 (10-3 kg ADP (10-5 element\nkg SO2- 4 kg CO2 (10-7 kg 1,4- 1,4- kg PO4 kg 1,4- kg CFC- ethylene\nproducti 1,4-DCB MJ) (102 kg\nEq) Eq) DCB Eq) DCB Eq) DCB Eq) 11 Eq) Eq)\non Eq) Sb Eq)\nEq)\nⅠ 2.68 1.87 5.85 1.25 4.86 1.74 2.14 1.24 2.28 0.05 1.66\nⅠⅠ -10.05 -5.03 -4.06 -2.35 -13.60 -4.22 -3.49 -3.47 13.11 1.55 -7.12\nⅠⅠⅠ 18.98 0.78 30.98 8.73 1.94 1.43 6.82 3.70 17.24 1.11 7.34\nNote: Acidification potential (AP), global warming potential (GWP), freshwater\naquatic ecotoxicity potential (FAETP), marine aquatic ecotoxicity potential (MAETP),\nterrestrial ecotoxicity potential (TETP), abiotic depletion potential (ADP),\neutrophication potential (EP), human toxicity potential (HTP), abiotic depletion\npotential of elements (ADP element), ozone layer depletion potential (ODP) and\nphotochemical ozone creation potential (POCP).\nMANUSCRIPT/SUPPLEMENTAL MATERIAL:\nThe most crucial aspect is the performed LCA. While the reviewer greatly\nappreciates the effort put into this analysis and fully acknowledges its importance, there\nare several critical points that must be (i) addressed, (ii) revised, and (iii) checked\nbefore the work can be considered for publication.\n(1) The absence of the catalyst from the assessment may lead to a substantial\nunderestimation of the environmental impacts. In particular, assumptions regarding\ncatalyst lifespan, regeneration potential, and low maintenance costs remain\ninsufficiently substantiated, as they are primarily based on laboratory-scale results.\nThe suggested study (doi:10.1371/journal.pone.0101298) demonstrates that the GHG\nemissions of iridium and platinum are immense. While it is understood that modelling\nthe catalyst’s contribution may be challenging, this aspect should be addressed at a\nminimum through a dedicated discussion in the main text or supplementary material.\nAt present, it is mentioned solely in the rebuttal and not incorporated into the\nmanuscript or supplementary files. The omission of such a discussion risks undermining\nthe credibility of the LCA, especially among LCA experts.\nResponse: We fully agree with the reviewer that the environmental impacts associated\nwith the use of catalysts should not be overlooked. Accordingly, based on the catalyst\nlifetime and loading obtained from our laboratory stability tests, as well as the\nenvironmental impact data for platinum and iridium reported in the ecoinvent 3.11\ndatabase and previous studies (DOI: 10.1371/journal.pone.0101298), we reanalyzed the\nenvironmental impacts of the seawater electrolysis-based magnesium extraction and\nhydrogen production process (Table R1 and R2). According to the assumptions for\nindustrial-scale production described in the Supplementary Information, under an\nelectrode area of 112 m2 and a current density of 100 mA cm-2, the production of 1 t of\nhydrogen requires 240 h of operation, corresponding to 4.8% of the catalyst lifetime\n(5000 h). The required catalyst loading is 276.64 g of Pt (for the cathode, 112 m2) and\n1920 g of Ir (for the anode, 112 m2). Therefore, for every 1 t of hydrogen produced,\n13.28 g of Pt and 92.16 g of Ir are consumed, and these values were used for the\nenvironmental impact assessment.\nTable R1. A comprehensive comparison between average hydrogen production (Ⅰ),\nelectrolyzing seawater for hydrogen production using green electricity (ⅠⅠ) and\nelectrolyzing seawater for hydrogen production using conventional electricity (ⅠⅠⅠ).\nTETP\nMethod FAPTP ADP-\nAP (10-1 GWP (10- MAETP (10-1 kg EP (10-1 HTP (10-4 ODP (103 POCP (kg\nof H2 (10-3 kg ADP (10-5 element\nkg SO2- 4 kg CO2 (10-7 kg 1,4- 1,4- kg PO4 kg 1,4- kg CFC- ethylene\nproducti 1,4-DCB MJ) (102 kg\nEq) Eq) DCB Eq) DCB Eq) DCB Eq) 11 Eq) Eq)\non Eq) Sb Eq)\nEq)\nⅠ 2.68 1.87 5.85 1.25 4.86 1.74 2.14 1.24 2.28 0.05 1.66\nⅠⅠ -142.9 -50477.2 -4691.9 -24391536.7 -139.3 -426318.9 -36.08 -35208.8 0.11 0.0015 -7.9\nⅠⅠⅠ 147.4 7611.4 30347.3 86404925.3 16.2 137892.7 67.0 36449.3 0.16 0.0011 6.6\nTable R2. A comprehensive comparison between different system inputs and outputs.\nAP TETP\nGWP FAPTP ADP-\n(10-1 MAETP (10-1 kg EP (10-1 HTP (10-4 ODP (103 POCP (kg\nType of (10-4 kg (10-3 kg ADP (10-5 element\nkg (10-7 kg 1,4- 1,4- kg PO4 kg 1,4- kg CFC- ethylene\nsubstance CO2 1,4-DCB MJ) (102 kg\nSO2- DCB Eq) DCB Eq) DCB Eq) 11 Eq) Eq)\nEq) Eq) Sb Eq)\nEq) Eq)\nO2 3.46 0.83 4.36 1.38 2.32 0.92 1.94 0.66 0.82 0.04 1.84\nMg(OH)2 27.20 7.66 35.60 7.99 43.70 7.21 11.02 9.04 39.61 1.97 15.47\nConventional\n32.44 6.45 48.29 13.00 22.59 6.36 12.66 9.10 30.85 0.18 16.68\nelectricity\nGreen\n3.41 0.64 13.25 1.92 7.05 0.72 2.35 1.94 26.72 0.06 2.21\nelectricity\nNaOH 12.96 2.80 22.02 5.01 25.04 3.15 7.00 4.25 25.11 2.94 7.20\nPt 0.39 0.06 0.25 0.03 0.03 0.008 0.05 0.02 0.29 0.0003 0.16\nIr 3.85 0.08 0.38 0.05 0.30 0.04 0.07 0.03 1.41 0.0012 0.61\nRevision in the revised Supplementary Information (Table 7):\nSupplementary Table 7. A comprehensive comparison between different system\ninputs and outputs.\nAP TETP\nGWP FAPTP ADP-\n(10-1 MAETP (10-1 kg EP (10-1 HTP (10-4 ODP (103 POCP (kg\nType of (10-4 kg (10-3 kg ADP (10-5 element\nkg (10-7 kg 1,4- 1,4- kg PO4 kg 1,4- kg CFC- ethylene\nsubstance CO2 1,4-DCB MJ) (102 kg\nSO2- DCB Eq) DCB Eq) DCB Eq) 11 Eq) Eq)\nEq) Eq) Sb Eq)\nEq) Eq)\nO2 3.46 0.83 4.36 1.38 2.32 0.92 1.94 0.66 0.82 0.04 1.84\nMg(OH)2 27.20 7.66 35.60 7.99 43.70 7.21 11.02 9.04 39.61 1.97 15.47\nConventional\n32.44 6.45 48.29 13.00 22.59 6.36 12.66 9.10 30.85 0.18 16.68\nelectricity\nGreen\n3.41 0.64 13.25 1.92 7.05 0.72 2.35 1.94 26.72 0.06 2.21\nelectricity\nNaOH 12.96 2.80 22.02 5.01 25.04 3.15 7.00 4.25 25.11 2.94 7.20\nPt 0.39 0.06 0.25 0.03 0.03 0.008 0.05 0.02 0.29 0.0003 0.16\nIr 3.85 0.08 0.38 0.05 0.30 0.04 0.07 0.03 1.41 0.0012 0.61\nNote: According to the assumptions for industrial-scale production, under an electrode\narea of 112 m2 and a current density of 100 mA cm-2, the production of 1 t of hydrogen\nrequires 240 h of operation, corresponding to 4.8% of the catalyst lifetime (5000 h).\nThe required catalyst loading is 276.64 g of Pt (for the cathode, 112 m2) and 1920 g of\nIr (for the anode, 112 m2). Therefore, for every 1 t of hydrogen produced, 13.28 g of Pt\nand 92.16 g of Ir are consumed, and these values were used for the environmental\nimpact assessment. For every 1 t of H produced, 8 t of O and 1.45 t of Mg(OH) are\n2 2 2\ngenerated, accompanied by the consumption of 2 t of NaOH and 75803 kWh of\nelectricity.\nRevision in the revised manuscript (page 12):\nFig 5. f, Life Cycle Assessment (LCA) of the seawater electrolysis-based magnesium\nextraction and hydrogen production process.\n(2) Unfortunately, the manuscript does not yet include essential elements that\nconstitute the minimum requirements for a robust and publishable LCA:\no What is the functional unit?\no What are the system boundaries?\no Does your LCA align with the ISO 14044?\no What approach has been used to define your system boundaries (e.g. cradle-to-\ngate or grave-to-grave, etc.)\no What LCA approach has been used? (consequential or attributional)?\no What allocation method has been used as you have a multi-product process?\n(e.g. Economic, mass allocation, or system expansion, see ISO 14040).\no What LCIA (life cycle impact assessment) method has been used? It is indicated\nin SimaPro.\nResponse: In our manuscript, the production of 1 t of hydrogen was defined as the\nfunctional unit, and the system boundary was set on a cradle-to-gate basis. The\nboundary mainly includes the production processes of hydrogen and magnesium\nhydroxide in the seawater electrolysis-based magnesium extraction and hydrogen\nproduction process. The environmental impact assessment follows the international\nstandard ISO 14044. Furthermore, a system expansion approach was adopted for the\nattributional LCA, assuming that the co-produced byproduct could substitute\nconventionally produced materials in the market and thus generate environmental\nbenefits. All analyses were conducted using the CML 2001 method, in which the\ncharacterization results quantify the specific contributions of various environmental\nimpact categories (AP, GWP, FAETP, MAETP, TETP, ADP, EP, HTP, ADP element,\nODP, and POCP) to represent the overall life-cycle environmental impacts. The\nSupplementary Information has been revised to provide a more comprehensive\nexplanation of the environmental impact assessment methodology.\nRevision in the revised Supplementary Information:\nLCA analysis. An LCA was conducted using the SimaPro software following the\ninternational standard ISO 14040. The CML 2001 method was selected to evaluate the\nenvironmental impacts of the studied system, and background data were obtained from\nthe Ecoinvent 3.11 database. The production of 1 t of hydrogen was defined as the\nfunctional unit, and the system boundary was set on a cradle-to-gate basis, covering the\nproduction processes of hydrogen and magnesium hydroxide in the seawater\nelectrolysis-based magnesium extraction and hydrogen production process. A life-cycle\ninventory was established to clearly distinguish the system inputs and outputs. The main\ninputs include seawater, sodium hydroxide, catalyst materials, and electricity, while the\noutputs consist of hydrogen, oxygen, and magnesium hydroxide. A system expansion\napproach was applied for the attributional LCA, assuming that the co-produced\nbyproducts could substitute conventionally produced materials in the market and\nthereby generate environmental benefits. Based on the CML 2001 method, the\ncharacterization analysis quantified the specific contributions of each environmental\nimpact category (AP, GWP, FAETP, MAETP, TETP, ADP, EP, HTP, ADP element,\nODP, and POCP) to represent the overall life-cycle environmental impacts.\n(3) Line 761-771, Supplementary material: While the improvement from average\nH production (I) to electrolysing seawater for hydrogen production using conventional\n2\nelectricity (III) seems to be reasonable and consistent with the described technology,\nthe reported improvement from (III) to (II = seawater for hydrogen production using\ngreen electricity) remains highly questionable. The negative values, representing\nbeneficial contributions (e.g., carbon sinks), appear to result solely from the assumed\nuse of green electricity. However, it is difficult to reconcile this with the implication\nthat the technology becomes a net carbon sink from a GWP/greenhouse gas emissions\nperspective. The same applies to all other indicators reported as negative. For instance,\neven when renewable energy is used, greenhouse gas emissions (kg CO₂-eq.) are still\ngenerated, which also holds true across all other impact categories:\nReCiPe2016 from SimaPro:\n- Green electricity in Germany (kg CO -eq/kWh)\n2\n• Photovoltaic: 0.0981\n• Wind: 0.0073\n• Nuclear: 0.00653 (debatable if green or not)\n- Green electricity in China (kg CO -eq/kWh)\n2\n• Photovoltaic: 0.0785\n• Wind: 0.049\n• Nuclear: 0.000783\n- Average conventional electricity mixes (including different national-based\nelectricity sources) (kg CO -eq/kWh) in…,\n2\n• China: 1.26.\n• World: 0.861.\n• USA: 0.577.\n=> Hence, it is highly unlikely that the use of green electricity alone would render\nthe technology a net (carbon) sink. This outcome suggests either a significant error in\nthe LCA calculation/modelling or the presence of another factor that has not been\nsufficiently explained. It is therefore essential to correct and/or provide a detailed\nelaboration of this result, as the current information is insufficient to verify the LCI and\nLCA. Transparency and comprehensive explanation are critical in LCA studies,\nparticularly when certain information must remain confidential.\nResponse: As the reviewer correctly pointed out, the use of green electricity also results\nin CO emissions, leading to questions regarding how the seawater electrolysis-based\n2\nmagnesium extraction and hydrogen production process can achieve carbon sinks. It\nshould be noted that, in this study, a system expansion approach was adopted, assuming\nthat the co-produced byproducts could substitute conventionally produced materials in\nthe market and thereby generate environmental benefits. Consequently, when the\nenvironmental impacts of the products generated by the seawater electrolysis-based\nmagnesium extraction and hydrogen production system are lower than those of\nequivalent products manufactured through conventional processes, a net positive\nenvironmental benefit is achieved. Moreover, the environmental impacts of seawater\nelectrolysis for hydrogen production are highly dependent on the source of electricity.\nWhen green electricity is used and the substitution effects of the byproducts are\nconsidered, the seawater electrolysis-based magnesium extraction and hydrogen\nproduction system can indeed function as a carbon sink.\nSUPPLEMENTAL MATERIAL (referring to figures and text that have been\nincluded)\n(1)Sensitivity analysis: Fig. 66, line 725:\nFig. 66a: correct.\nFig. 66b: almost the same results. Probably due to different digits after the comma.\nAccording to the reviewer’s calculation: Sensitivity analysis: NaOH:\n• 10% higher = 1.209 USD/kg H\n2\n•-10% -= still positive = 0.09 USD/kg H\n2\n=> For both 66a and b: Discuss and explain what implications the negative\noutcomes have for your technology and economic feasibility.\nResponse: The cost of hydrogen produced via the seawater electrolysis-based\nmagnesium extraction and hydrogen production strategy is influenced by fluctuations\nin the prices of magnesium hydroxide and sodium hydroxide. A decrease in the price\nof magnesium hydroxide or an increase in the price of sodium hydroxide would lead to\na higher hydrogen production cost, posing additional challenges for cost control and\ntechnological optimization. Future cost reductions could be achieved by employing\nmore affordable alkaline sources (e.g., calcium oxide derived from calcined seashells)\nor by comprehensively optimizing the system to reduce energy consumption. It should\nbe noted, however, that the primary focus of this study remains on reducing the\nhydrogen production cost to promote the large-scale application of water electrolysis.\nEven under potential price fluctuations, the cost of hydrogen produced using the current\ntechnology remains close to the DOE 2025 target, which clearly demonstrates the\napplication potential of the seawater electrolysis-based magnesium extraction and\nhydrogen production strategy.\nRevision in the revised Supplementary Information (Figure 67):\nNote: The variations in each economic parameter from -10% to +10% of their original\nvalues produce linear trends with specific slopes. The steeper the slope, the greater the\nimpact of that economic parameter on hydrogen production costs. Compared to the\noriginal value, electricity cost has a relatively smaller effect on hydrogen production\ncosts. However, the slopes of the curves related to the prices of magnesium hydroxide\nand sodium hydroxide are steeper, indicating that any change in these parameters could\nhave a significant impact on hydrogen production costs.\nA decrease in the price of magnesium hydroxide or an increase in the price of\nsodium hydroxide would lead to a higher hydrogen production cost, posing additional\nchallenges for cost control and technological optimization. Future cost reductions could\nbe achieved by employing more affordable alkaline sources (e.g., calcium oxide derived\nfrom calcined seashells) or by comprehensively optimizing the system to reduce energy\nconsumption.\nFig. 66c: correct\nFig. 66d: seems to be incorrect, or the calculation approach is not sufficiently\ndescribed to be followed. Please make sure that the calculation way and method is\ntransparent enough to reproduce the numbers/outcome.\nResponse: Considering the regional variations in electrolyzer cost, electricity price,\nand the prices of magnesium hydroxide and sodium hydroxide, using identical price data\nfor all regions would clearly be inappropriate. Therefore, region-specific prices were\nadopted in our calculations. The price of Mg(OH) in the US: USD 1700 per ton\n2\n(https://chinese.alibaba.com/product-detail/Factory-price-magnesium-carbonate-with-\n99-1600691031574.html?spm=a2700.7724857.0.0.14025b82TWtkCl); The price of\nMg(OH) in the EU: USD 2050 per ton (https://www.wholesalesuppliesplus.com/). The\n2\nprice of NaOH in the US: USD 330 per ton (https://businessanalytiq.com/); The price\nof NaOH in the EU: USD 290 per ton (https://businessanalytiq.com/). The price of\nelectrolyzer in the US and EU: USD 2500 kW-1. The price of electricity in the US: USD\n0.13 kWh-1; The price of electricity in the average EU: USD 0.22 kWh-1. The price of\nelectricity in the Finland: USD 0.078 kWh-1. The price of electricity in the Cyprus: USD\n0.29 kWh-1. The price of Mg(OH) in the China: USD 680 per ton. The price of NaOH\n2\nin the China: USD 300 per ton. The price of electrolyzer in the China: USD 600 kW-1.\nThe price of electricity in the China: USD 0.03 kWh-1.\nThe price in the US:\nCapital cost:\n1) Electrolyzer: Electrolyzer cost per ton H : 315.8 kW × USD 2500 kW-1 ×\n2\n(1/5year) × (year/360 day) × (day/0.1 ton)=USD 4386.11 per ton H .\n2\n2) BOP: BOP cost per ton H : USD 4386.11 per ton H × 0.6=USD 2631.67\n2 2\nper ton H .\n2\n3) Separation equipment: The separation equipment cost as 5% of the electroly\nzer cost. USD 4386.11 per ton H × 0.05=USD 219.31 per ton H .\n2 2\nOperating cost:\n1) Electricity: 315.84 kW × 24h/day × USD 0.13 kW·h-1 × (day/0.1 ton)=USD\n9990.65 per ton H .\n2\n2) Maintenance: 315.8 kW × USD 2500 kW-1 × 0.05 × (year/360 day) × (day\n/0.1 ton)=USD 1096.67 per ton H .\n2\n3) Separation: Since H is the sole gaseous product of seawater reduction and\n2\nthe gas, solid, and liquid phases are automatically separated, the separation\ncost is expected to be minimal. The separation equipment cost as 10% of t\\he electrolyzer cost. USD 4386.11 per ton H × 0.1=USD 438.61 per ton\n1) Natural seawater: 112800 A × (1/2e-) × (1/96485 C/mol) × (0.018 kg/mol)\n× (0.2642 t/kg) × (86400 s/day) × (USD 0.00378/gal) × (1 day/0.1t)=USD\n90 per ton H .\n2\n2) Sodium hydroxide: The pH of the anolyte was stabilized by adding NaOH\n(For every mole of Mg(OH) need 2 moles of NaOH). USD 9860 per ton\n2 2\nThe total cost: USD 4386.11+USD 2631.67+USD 219.31+USD 9990.65+U\nSD 1096.67+USD 438.61+USD 90+USD 6600=USD 25453.80 per ton H .\n2\n5=USD 24650 per ton H .\nCapital cost:\n1) Electrolyzer: Electrolyzer cost per ton H : 315.8 kW × USD 2500 kW-1 ×\n2\n(1/5year) × (year/360 day) × (day/0.1 ton)=USD 4386.11 per ton H .\n2\n2) BOP: BOP cost per ton H : USD 4386.11 per ton H × 0.6=USD 2631.67\n2 2\nper ton H .\n2\n3) Separation equipment: The separation equipment cost as 5% of the electrol\nyzer cost. USD 4386.11 per ton H × 0.05=USD 219.31 per ton H .\n2 2\nOperating cost:\n1) Electricity: 315.84 kW × 24h/day × USD 0.22 kW·h-1 × (day/0.1 ton)=USD\n16676.35 per ton H .\n2\n2) Maintenance: 315.8 kW × USD 2500 kW-1 × 0.05 × (year/360 day) × (day\n/0.1 ton)=USD 1096.67 per ton H .\n2\n3) Separation: Since H is the sole gaseous product of seawater reduction and\n2\nthe gas, solid, and liquid phases are automatically separated, the separation\ncost is expected to be minimal. The separation equipment cost as 10% of t\\he electrolyzer cost. USD 4386.11 per ton H × 0.1=USD 438.61 per ton\n1) Natural seawater: 112800 A × (1/2e-) × (1/96485 C/mol) × (0.018 kg/mol)\n× (0.2642 t/kg) × (86400 s/day) × (USD 0.00378/gal) × (1 day/0.1t)=USD\n90 per ton H .\n2\n2) Sodium hydroxide: The pH of the anolyte was stabilized by adding NaOH\n(For every mole of Mg(OH) need 2 moles of NaOH). USD 9860 per ton\n2 2\nThe total cost: USD 4386.11+USD 2631.67+USD 219.31+USD 16676.35+\nUSD 1096.67+USD 438.61+USD 90+USD 5800=USD 31339.50 per ton H .\n2\n5=USD 29725 per ton H .\nThe price of electricity in the US: USD 0.13 kWh-1 (https://www.eia.gov/electri\ncity/monthly/epm_table_grapher.php?t=epmt_5_3).\nThe price of electricity in the average EU: USD 0.22 kWh-1 (https://ec.europa.e\nu/eurostat/statistics-explained/index.php?title=Electricity_price_statistics#Electricity_\nprices_for_non-household_consumers).\nThe price in the US:\nCapital cost:\n1) Electrolyzer: Electrolyzer cost per ton H : 315.8 kW × USD 2500 kW-1 ×\n2\n(1/5year) × (year/360 day) × (day/0.1 ton)=USD 4386.11 per ton H .\n2\n2) BOP: BOP cost per ton H : USD 4386.11 per ton H × 0.6=USD 2631.67\n2 2\nper ton H .\n2\n3) Separation equipment: The separation equipment cost as 5% of the electroly\nzer cost. USD 4386.11 per ton H × 0.05=USD 219.31 per ton H .\n2 2\nOperating cost:\n1) Electricity: 315.84 kW × 24h/day × USD 0.13 kW·h-1 × (day/0.1 ton)=USD\n9990.65 per ton H .\n2\n2) Maintenance: 315.8 kW × USD 2500 kW-1 × 0.05 × (year/360 day) × (day\n/0.1 ton)=USD 1096.67 per ton H .\n2\n3) Separation: Since H is the sole gaseous product of seawater reduction and\n2\nthe gas, solid, and liquid phases are automatically separated, the separation\ncost is expected to be minimal. The separation equipment cost as 10% of t\\he electrolyzer cost. USD 4386.11 per ton H × 0.1=USD 438.61 per ton\n1) Natural seawater: 112800 A × (1/2e-) × (1/96485 C/mol) × (0.018 kg/mol)\n× (0.2642 t/kg) × (86400 s/day) × (USD 0.00378/gal) × (1 day/0.1t)=USD\n90 per ton H .\n2\n2) Sodium hydroxide: The pH of the anolyte was stabilized by adding NaOH\n(For every mole of Mg(OH) need 2 moles of NaOH). USD 9860 per ton\n2 2\nThe total cost: USD 4386.11+USD 2631.67+USD 219.31+USD 9990.65+U\nSD 1096.67+USD 438.61+USD 90+USD 6600=USD 25453.80 per ton H .\n2\n5=USD 24650 per ton H .\nCapital cost:\n1) Electrolyzer: Electrolyzer cost per ton H : 315.8 kW × USD 2500 kW-1 ×\n2\n(1/5year) × (year/360 day) × (day/0.1 ton)=USD 4386.11 per ton H .\n2\n2) BOP: BOP cost per ton H : USD 4386.11 per ton H × 0.6=USD 2631.67\n2 2\nper ton H .\n2\n3) Separation equipment: The separation equipment cost as 5% of the electrol\nyzer cost. USD 4386.11 per ton H × 0.05=USD 219.31 per ton H .\n2 2\nOperating cost:\n1) Electricity: 315.84 kW × 24h/day × USD 0.22 kW·h-1 × (day/0.1 ton)=USD\n16676.35 per ton H .\n2\n2) Maintenance: 315.8 kW × USD 2500 kW-1 × 0.05 × (year/360 day) × (day\n/0.1 ton)=USD 1096.67 per ton H .\n2\n3) Separation: Since H is the sole gaseous product of seawater reduction and\n2\nthe gas, solid, and liquid phases are automatically separated, the separation\ncost is expected to be minimal. The separation equipment cost as 10% of t\\he electrolyzer cost. USD 4386.11 per ton H × 0.1=USD 438.61 per ton\n1) Natural seawater: 112800 A × (1/2e-) × (1/96485 C/mol) × (0.018 kg/mol)\n× (0.2642 t/kg) × (86400 s/day) × (USD 0.00378/gal) × (1 day/0.1t)=USD\n90 per ton H .\n2\n2) Sodium hydroxide: The pH of the anolyte was stabilized by adding NaOH\n(For every mole of Mg(OH) need 2 moles of NaOH). USD 9860 per ton\n2 2\nThe total cost: USD 4386.11+USD 2631.67+USD 219.31+USD 16676.35+\nUSD 1096.67+USD 438.61+USD 90+USD 5800=USD 31339.50 per ton H .\n2\n5=USD 29725 per ton H .\n(1)Sensitivity analysis: Electrolyser (page 46-487):\no The sensitivity analysis for the electrolyser seems to be off, given the calculation\nmethod that has been provided by lines 256-257, lines 288-289, and line 292 in the\nsupplementary materials.\no Taking a price of USD 2500/kW (as indicated in the rebuttal) and US electricity\nprice of 0.1331 USD/kWh, the hydrogen cost rises to approx. 11.76 USD/ kg H .\nUSD/kWh), the outcome would still be higher than calculated, which is 8.42 USD/ kg\nH .\n2\no Correct this mistake and provide a transparent and detailed calculation.\nResponse: This manuscript primarily focuses on materials research. In the prev\nious version, the sensitivity analysis was presented in a relatively concise mann\ner, which may have led to misunderstandings among reviewers and readers. Th\nerefore, this section has been supplemented in the revised manuscript (see Sup\nplementary Discussions), providing a clearer understanding of the sensitivity an\nalysis. Considering the regional variations in electrolyzer cost, electricity price,\nand the prices of magnesium hydroxide and sodium hydroxide, using identical\nprice data for all regions would clearly be inappropriate. Therefore, region-spec\nific prices were adopted in our calculations. In the previous revision, following\nthe reviewer’s suggestion, we included an analysis of the regional price variati\nons associated with the deployment of the seawater electrolysis-based magnesiu\nm extraction and hydrogen production technology. When the electrolyzer cost\nwas assumed to be USD 2500 kW⁻¹ (for the US), the corresponding price of\nmagnesium hydroxide was USD 1700 per ton，the corresponding price of sodiu\nm hydroxide was USD 330 per ton，the corresponding price of electricity was\nUSD 0.13 kWh-1.\nCapital cost:\n1) Electrolyzer: Electrolyzer cost per ton H : 315.8 kW × USD 2500 kW-1 ×\n2\n(1/5year) × (year/360 day) × (day/0.1 ton)=USD 4386.11 per ton H .\n2\n2) BOP: BOP cost per ton H : USD 4386.11 per ton H × 0.6=USD 2631.67\n2 2\nper ton H .\n2\n3) Separation equipment: The separation equipment cost as 5% of the electroly\nzer cost. USD 4386.11 per ton H × 0.05=USD 219.31 per ton H .\n2 2\nOperating cost:\n1) Electricity: 315.84 kW × 24h/day × USD 0.13 kW·h-1 × (day/0.1 ton)=USD\n9990.65 per ton H .\n2\n2) Maintenance: 315.8 kW × USD 2500 kW-1 × 0.05 × (year/360 day) × (day\n/0.1 ton)=USD 1096.67 per ton H .\n2\n3) Separation: Since H is the sole gaseous product of seawater reduction and\n2\nthe gas, solid, and liquid phases are automatically separated, the separation\ncost is expected to be minimal. The separation equipment cost as 10% of t\\he electrolyzer cost. USD 4386.11 per ton H × 0.1=USD 438.61 per ton\n1) Natural seawater: 112800 A × (1/2e-) × (1/96485 C/mol) × (0.018 kg/mol)\n× (0.2642 t/kg) × (86400 s/day) × (USD 0.00378/gal) × (1 day/0.1t)=USD\n90 per ton H .\n2\n2) Sodium hydroxide: The pH of the anolyte was stabilized by adding NaOH\n(For every mole of Mg(OH) need 2 moles of NaOH). USD 9860 per ton\n2 2\nThe total cost: USD 4386.11+USD 2631.67+USD 219.31+USD 9990.65+U\nSD 1096.67+USD 438.61+USD 90+USD 6600=USD 25453.80 per ton H .\n2\n5=USD 24650 per ton H .\n2\n(2) Sensitivity: Electricity price (Page 48):\no The calculation seems to have an error. The benchmark electricity price is USD\n0.03/kWh, and the Finnish electricity price is 0.067 EUR/kWh (= 0.07906 USD/kWh =>\nmore than double). Hence, it is not possible that the Finnish electricity price will lead\nto a negative hydrogen production cost (or decrease => it must increase). According\nto line 262 in the supplementary material: Electricity: 315.85 kW x 24h/day USD\n0.079/kWh x (day/0.1 ton) = USD 5992 per ton H . => 4.33 USD/kg H . The same\n2 2\nissues apply to the other electricity prices. Hence, there must be a calculating error.\nPlease correct it.\nResponse: Considering the regional variations in electrolyzer cost, electricity\nprice, and the prices of magnesium hydroxide and sodium hydroxide, using identical\nprice data for all regions would clearly be inappropriate. Therefore, region-specific\nprices were adopted in our calculations. The baseline hydrogen production cost was\nderived from the prices of the electrolyzer, magnesium hydroxide, sodium hydroxide,\nand electricity in China. In Europe, these prices vary to different extents, resulting in\nregional differences in the economic performance of the seawater electrolysis-based\nmagnesium extraction and hydrogen production technology. The price of Mg(OH) in\n2\nthe EU: USD 2050 per ton. The price of NaOH in the EU: USD 290 per ton. The price\nof electrolyzer in the EU: USD 2500 kW-1. The price of electricity in the average EU:\nUSD 0.22 kWh-1. The price of electricity in the Finland: USD 0.078 kWh-1. The price\nof electricity in the Cyprus: USD 0.29 kWh-1. The price of Mg(OH) in the China: USD\n2\n680 per ton. The price of NaOH in the China: USD 300 per ton. The price of electrolyzer\nin the China: USD 600 kW-1. The price of electricity in the China: USD 0.03 kWh-1.\nThe price in the average EU:\nCapital cost:\n1) Electrolyzer: Electrolyzer cost per ton H : 315.8 kW × USD 2500 kW-1 ×\n2\n(1/5year) × (year/360 day) × (day/0.1 ton)=USD 4386.11 per ton H .\n2\n2) BOP: BOP cost per ton H : USD 4386.11 per ton H × 0.6=USD 2631.67\n2 2\nper ton H .\n2\n3) Separation equipment: The separation equipment cost as 5% of the electrol\nyzer cost. USD 4386.11 per ton H × 0.05=USD 219.31 per ton H .\n2 2\nOperating cost:\n1) Electricity: 315.84 kW × 24h/day × USD 0.22 kW·h-1 × (day/0.1 ton)=USD\n16676.35 per ton H .\n2\n2) Maintenance: 315.8 kW × USD 2500 kW-1 × 0.05 × (year/360 day) × (day\n/0.1 ton)=USD 1096.67 per ton H .\n2\n3) Separation: Since H is the sole gaseous product of seawater reduction and\n2\nthe gas, solid, and liquid phases are automatically separated, the separation\ncost is expected to be minimal. The separation equipment cost as 10% of t\\he electrolyzer cost. USD 4386.11 per ton H × 0.1=USD 438.61 per ton\n1) Natural seawater: 112800 A × (1/2e-) × (1/96485 C/mol) × (0.018 kg/mol)\n× (0.2642 t/kg) × (86400 s/day) × (USD 0.00378/gal) × (1 day/0.1t)=USD\n90 per ton H .\n2\n2) Sodium hydroxide: The pH of the anolyte was stabilized by adding NaOH\n(For every mole of Mg(OH) need 2 moles of NaOH). USD 9860 per ton\n2 2\nThe total cost: USD 4386.11+USD 2631.67+USD 219.31+USD 16676.35+\nUSD 1096.67+USD 438.61+USD 90+USD 5800=USD 31339.50 per ton H .\n2\n5=USD 29725 per ton H .\nCapital cost:\n1) Electrolyzer: Electrolyzer cost per ton H : 315.8 kW × USD 2500 kW-1 ×\n2\n(1/5year) × (year/360 day) × (day/0.1 ton)=USD 4386.11 per ton H .\n2\n2) BOP: BOP cost per ton H : USD 4386.11 per ton H × 0.6=USD 2631.67\n2 2\nper ton H .\n2\n3) Separation equipment: The separation equipment cost as 5% of the electrol\nyzer cost. USD 4386.11 per ton H × 0.05=USD 219.31 per ton H .\n2 2\nOperating cost:\n1) Electricity: 315.84 kW × 24h/day × USD 0.078 kW·h-1 × (day/0.1 ton)=US\nD 5920.10 per ton H .\n2\n2) Maintenance: 315.8 kW × USD 2500 kW-1 × 0.05 × (year/360 day) × (day\n/0.1 ton)=USD 1096.67 per ton H .\n2\n3) Separation: Since H is the sole gaseous product of seawater reduction and\n2\nthe gas, solid, and liquid phases are automatically separated, the separation\ncost is expected to be minimal. The separation equipment cost as 10% of t\\he electrolyzer cost. USD 4386.11 per ton H × 0.1=USD 438.61 per ton\n1) Natural seawater: 112800 A × (1/2e-) × (1/96485 C/mol) × (0.018 kg/mol)\n× (0.2642 t/kg) × (86400 s/day) × (USD 0.00378/gal) × (1 day/0.1t)=USD\n90 per ton H .\n2\n2) Sodium hydroxide: The pH of the anolyte was stabilized by adding NaOH\n(For every mole of Mg(OH) need 2 moles of NaOH). USD 9860 per ton\n2 2\nThe total cost: USD 4386.11+USD 2631.67+USD 219.31+USD 5920.10+U\nSD 1096.67+USD 438.61+USD 90+USD 5800=USD 20583.25 per ton H .\n2\n5=USD 29725 per ton H .\nCapital cost:\n1) Electrolyzer: Electrolyzer cost per ton H : 315.8 kW × USD 2500 kW-1 ×\n2\n(1/5year) × (year/360 day) × (day/0.1 ton)=USD 4386.11 per ton H .\n2\n2) BOP: BOP cost per ton H : USD 4386.11 per ton H × 0.6=USD 2631.67\n2 2\nper ton H .\n2\n3) Separation equipment: The separation equipment cost as 5% of the electrol\nyzer cost. USD 4386.11 per ton H × 0.05=USD 219.31 per ton H .\n2 2\nOperating cost:\n1) Electricity: 315.84 kW × 24h/day × USD 0.29 kW·h-1 × (day/0.1 ton)=USD\n22088.59 per ton H .\n2\n2) Maintenance: 315.8 kW × USD 2500 kW-1 × 0.05 × (year/360 day) × (day\n/0.1 ton)=USD 1096.67 per ton H .\n2\n3) Separation: Since H is the sole gaseous product of seawater reduction and\n2\nthe gas, solid, and liquid phases are automatically separated, the separation\ncost is expected to be minimal. The separation equipment cost as 10% of t\\he electrolyzer cost. USD 4386.11 per ton H × 0.1=USD 438.61 per ton\n1) Natural seawater: 112800 A × (1/2e-) × (1/96485 C/mol) × (0.018 kg/mol)\n× (0.2642 t/kg) × (86400 s/day) × (USD 0.00378/gal) × (1 day/0.1t)=USD\n90 per ton H .\n2\n2) Sodium hydroxide: The pH of the anolyte was stabilized by adding NaOH\n(For every mole of Mg(OH) need 2 moles of NaOH). USD 9860 per ton\n2 2\nThe total cost: USD 4386.11+USD 2631.67+USD 219.31+USD 22088.59+\nUSD 1096.67+USD 438.61+USD 90+USD 5800=USD 36751.74 per ton H .\n2\n5=USD 29725 per ton H .\n2"}, {"round": 3, "reviewer_comments": {"reviewer_5": "The authors have adequately addressed all of my remaining questions and concerns. In my opinion, the manuscript is ready for publication in its current form. Congratulations on the remarkable work."}, "author_response": ""}]

Decision letter