In-situ direct seawater electrolysis using floating platform in ocean with uncontrollable wave motion

Domain: earth_science
Published: 2024-06-21
Editorial outcome: minor_revision

Abstract
Direct hydrogen production from inexhaustible seawater using abundant offshore wind power offers a promising pathway for achieving a sustainable energy industry and fuel economy. Various direct seawater electrolysis methods have been demonstrated to be effective at the laboratory scale. However, larger-scale in situ demonstrations that are completely free of corrosion and side reactions in fluctuating oceans are lacking. Here, fluctuating conditions of the ocean were considered for the first time, and seawater electrolysis in wave motion environment was achieved. We present the successful scaling of a floating seawater electrolysis system that employed wind power in Xinghua Bay and the integration of a 1.2 Nm3 h−1-scale pilot system. Stable electrolysis operation was achieved for over 240 h with an electrolytic energy consumption of 5 kWh Nm−3 H2 and a high purity (>99.9%) of hydrogen under fluctuating ocean conditions (0~0.9 m wave height, 0~15 ms−1 wind speed), which is comparable to that during onshore water electrolysis. The concentration of impurity ions in the electrolyte was low and stable over a long period of time under complex and changing scenarios. We identified the technological challenges and performances of the key system components and examined the future outlook for this emerging technology.

Introduction
Seawater electrolysis using offshore renewable energy as input offers a promising alternative to the unsustainable utilization of fossil fuels1–7. The ocean, which accounts for more than 96% of global water storage, is like a giant hydrogen mine. Moreover, vast offshore wind power resources are difficult to develop and utilize in an economically manner, so seawater electrolysis is becoming increasingly attractive for converting wind power into hydrogen energy.

At present, indirect seawater electrolysis (seawater electrolysis after pre-desalination) has been used in several demonstration projects. In 2017, Japan’s Toyota Motor Corporation developed the world’s first vessel powered from hydrogen fuel, Energy Observer, in which hydrogen was generated from seawater electrolysis8. In 2019, Tractebel Overdick proposed a hydrogen generation platform using offshore wind as the energy input9. In 2022, Lhyfe was revealed to generate green hydrogen in the sea10. The system was equipped with a 1-MW PEM electrolyser capable of producing 400 kg of hydrogen per day. These demonstration projects suggest that the coupling of pre-desalination and conventional electrolysers may be a feasible solution for generating hydrogen in the oceans. However, the use of purification systems to eliminate impurity ions may increase energy consumption and engineering costs, making in situ hydrogen production in the oceans difficult2,11.

Direct seawater electrolysis eliminates the need for additional water purification processes provides an attractive pathway for hydrogen production in the oceans, but many challenges still exist. The composition of seawater is complex, containing at least 92 elements, many of which can exert a significant negative impact on electrolysis12–14, e.g. the chlorine oxidation reaction not only competes with the oxygen evolution reaction to reduce hydrogen production efficiency but also releases toxic Cl2, which could cause severe corrosion of electrolysis systems; the deposition and adhesion of insoluble substances such as Mg(OH)2 and Ca(OH)2 generated during electrolysis may weaken the transfer functions of membranes and lead to the rapid deactivation of catalysts. Moreover, the neutral environment and lower ionic conductivity of seawater limit electrolysis15,16. More importantly, variations in seawater composition due to various factors (such as geographical location and seasonal changes) pose significant challenges in designing seawater electrolysers17,18.

Since the concept of direct seawater electrolysis was proposed in the 1970s19, scientists have developed many strategies involving catalyst engineering20–29, asymmetric electrolytes30–38, and hydrophilic membrane-based seawater electrolysis4,6,39,40 to achieve highly efficient hydrogen generation. Reference 41 developed an anodic catalyst consisting of three-dimensional standing arrays of heterolateral Ni3S2/Co3S4 (NiCoS) nanosheets uniformly grown on Ni foam, and a double electrode electrolyser was used to produce hydrogen from alkaline seawater for 100 h at 2.4 A and 2.08 V. Reference 42 designed an asymmetric feed electrolyser, which operated at 5 A and 2.0 V for 100 h in alkaline seawater with a dry cathode. Reference 43 developed a seawater electrolyser combined with Fe, P-NiSe NFs catalyst that could operate for 200 h at 7.2 A and 1.8 V in natural seawater. Although these strategies have demonstrated promising outcomes in laboratory settings in specific cases, there are currently no reports of direct seawater electrolysis conducted in situ. Moreover, the design and implementation of direct seawater electrolysis within marine environment using renewable energy remain largely unexplored.

In our previous work1, seawater electrolysis driven by water phase-transition migration was proposed. This strategy is realised by applying a hydrophobic porous membrane as a gas-path interface and employing a concentrated KOH solution as a self-dampening electrolyte (SDE). During operation, the water vapour pressure difference between the seawater and the SDE induces spontaneous seawater gasification on the seawater side and the diffusion of water vapour through the porous membrane to the SDE side, where it is reliquified by absorption by the SDE. When the water migration rate equals the electrolysis rate, continuous and stable water migration via a ‘liquid–gas–liquid’ mechanism is realised to provide freshwater for electrolysis (Fig. 1a). A demo-type seawater electrolyser at the 386 l h−1 H2 generation scale was fabricated, and it showed long-term stable operation for over 3200 h at 250 mA cm−2 in Shenzhen Bay seawater with a low energy consumption of 5.0 kWh Nm−3 H2. This strategy realises efficient size-flexible and scalable direct seawater electrolysis in a way similar to fresh water splitting without a notable increase in operation cost and has high potential for practical application in the ocean. To further validate the viability of large-scale applications integrating renewable energy in the oceans, comprehensive tests on direct seawater electrolysis must be conducted under fluctuating wave conditions.

Here, for the first time, we present a scaled floating platform for in situ direct seawater electrolysis using offshore wind energy and operating at a 1.2 Nm3 h−1 H2 generation rate under uncontrolled and fluctuating ocean conditions. First, the influence of the fluctuating environment on the water migration process and on seawater electrolysis was investigated in a laboratory. Then, expanding upon the design of the laboratory-scale demonstration model, three seawater electrolysers were integrated into a system with coupled modules for energy storage, current conversion, H2 detection and transportation modules. This configuration is advantageous because it enables the stable operation of the platform in a fluctuating ocean environment, and wind power is used to supply energy to this type of platform. In the future, there is potential for increasing the economic viability of in situ direct seawater electrolysis based on renewable energy inputs.

Results
Process variable correlations during seawater electrolysis
The concentration of seawater in the oceans is a crucial factor influencing electrolysis. Specifically, the composition of seawater varies in different ocean regions (Fig. 1b), leading to distinct vapour pressure differences between seawater and the SDE, thereby influencing the water migration rate and electrolysis balance. To investigate the relationship between seawater and the SDE, a detailed theoretical model was proposed, as outlined in 'Methods'. When the Xinghua Bay seawater concentration approached 2.98 wt% and the KOH solution concentration was 30 wt%, the interfacial water vapour pressure difference was ~1.28 kPa, which was lower than the water vapour pressure difference between the Shenzhen Bay seawater (~1.51 wt%) and the SDE (Fig. 1c). In addition, fluctuations in ocean waves mainly affect the system through their influence on the mass transfer area. The migration rate depends on the instantaneous mass transfer area, which indicates that the water migration mass is an integral function that varies over time. To investigate and compare the difference in water mass transfer in fluctuating environments, we conducted a water migration behaviour test in various simulated seawater modes (calm, constant, pulsed, tidal and turbulence modes) (Fig. 1d and Supplementary Fig. 1). Compared to those in the calm mode, the water migration masses in the fluctuating environment exhibited slightly greater values. On the one hand, the larger contact area between the moving seawater and the membrane increased the water migration mass per unit time; on the other hand, the movement of seawater could counterbalance the momentary increase in the saline water concentration on the surface of the membrane. The average migration rate of water at the different wave heights and lengths significantly increased (Supplementary Fig. 1b). The water migration rates were ~0.0136, 0.0177, and 0.0194 g g–1 SDE min−1 at wave heights of 2.9, 3.2 and 3.4 cm, as well as the corresponding wavelengths of 54.5, 59.4 and 62.3 cm, respectively (Fig. 1e). Fortunately, dynamic balancing of our strategy could overcome such fluctuations in the oceans. As previously described, the water migration behaviour could be self-regulated by adjusting the water vapour pressure difference with the concentration. For example, if the initial electrolysis rate exceeds the water migration rate, the SDE concentration increases, leading to an increase in the water vapour pressure difference; consequently, the water migration rate also increases to match the electrolysis rate to reach a dynamic balance.

In addition, a 500-h durability test of seawater electrolysis in a simulated fluctuating environment was carried out in the laboratory. The SEM images of the clear catalyst and membrane pores provide evidence of the system’s adaptability and robustness in such dynamic settings (Fig. 1f, g and Supplementary Fig. 2).

Floating platform system overview
The kilowatt-scale floating system of in situ direct seawater electrolysis is driven by renewable energy in the oceans (Fig. 2a), and the design is described further in the 'Methods'. An uninterruptible power supply (UPS) containing an energy storage system plays a significant role in regulating stable power to ensure continuous green energy input into the device under intermittent winds (Fig. 2b). A technical illustration of the integrated current conversion, seawater electrolysis, H2 detection and transportation module is shown in Fig. 2c and Supplementary Fig. 3, in which the seawater electrolysis module at the 1.2 Nm3 h−1 H2 scale is based on the phase-transition migration strategy previously proposed and comprises three demo-type electrolysers in parallel. Moreover, a condensation device incorporated within the H2 detection module was used to purify the hydrogen gas to obtain the high-purity H2 for subsequent transportation and utilization. Each condensation device receives approximately 700~900 ml of condensed solution per day, which can be further replenished into the electrolysers.

The seawater electrolysis module is a significant part of the floating seawater electrolysis platform, and each electrolyser is immersed in seawater with a maximum water migration area of 13,168 cm2. The water migration interface consists of a hydrophobic polytetrafluoroethylene (PTFE) membrane with gas paths. To allow the electrolyser to adapt to different ocean environments, it is crucial to calculate the relationship between system parameters (such as membrane pores, membrane thickness, SDE concentration, etc.) and the seawater mass transfer rate before operation based on seawater parameters (temperature, concentration, etc.). In addition, a COMSOL simulation was performed to study the distributions of the pressure, concentration, and diffusion rates during operation (Supplementary Fig. 4). As expected, due to the channel design, the electrolyte inside and outside the stack was constantly exchanged, essentially maintaining a 30 wt% concentration. The solution concentration on the electrode surface increased due to the rapid consumption of a large amount of water in the electrolysis process. However, the gas flow also drove the electrolyte exchange process around the stacks, especially oxygen promoting the electrolyte solution flow rate, which is beneficial for the renewal of the electrolyte. In addition, the gas flow of the generated hydrogen gradually increased from bottom to top, which may have occurred to the resistance of the electrolytes, and the hydrogen gas flow drove the renewal and mass transfer of the KOH electrolyte in the cathode cavity. Moreover, due to the accumulation of hydrogen gas and the resistance of the electrolyte solution, the pressure increased from the top (~500 Pa) to the bottom (~3000 Pa). Due to compromises made during implementation at the pilot scale, some of the designs were not optimised, and the non-flowing electrolyser may have resulted in gas pressurization to a certain extent. Nonetheless, the pressure would not damage the diaphragm because the diaphragm can resist higher pressures. The hydrogen generated by 11 electrolytic units flowed smoothly into the confluence pipeline at a rate of 0.2~0.6 ms−1. However, there were some design parameters that were worth optimising, e.g. the generated hydrogen that accumulated at the corners of the cavity.

Given the originality and scale of the in situ direct seawater electrolysis platform, notable design and operational challenges had to be overcome, as detailed in 'Methods'. To alleviate fluctuations related to waves, the inclusion of a seawater cabin connected to ocean was proposed so that the waves did not directly impact the electrolysers; instead, the wave fluctuations were partially offset through the floating platform itself (Fig. 2d). This method ensures the application of real-world seawater. Additionally, shaking can be reduced by adding a certain amount of counterweight and anchor (Supplementary Fig. 5). Furthermore, due to salt mist and a high humidity (usually >RH 80%) over the oceans, it was necessary to use a dehumidification system and air circulation system to prevent system corrosion and reduce impurities and a high-humidity atmosphere inside the system (Supplementary Fig. 6).

Performance under uncontrolled fluctuating ocean conditions
An offshore wind turbine with 10 MW of power located in Xinghua Bay, Fuzhou City, Fujian Province, was one part of a larger wind farm network (Fig. 3a and Supplementary Fig. 7). In this energy network, the operation of the wind turbines is based on optimal energy capture and power output considerations. Each wind turbine captures and converts wind energy at wind speeds ranging from 3 to 25 ms−1 through adjustments to the blade angle and rotation speed of the turbine. Power generation is related to the instantaneous wind speed, as shown in Fig. 3b. Data over a period of 900 min from May 17 to 18 were selected for analysis. During this period, the maximum and minimum power generation levels of the wind turbine were 10.16 MW and 0.73 MW, respectively. Fluctuations in the wind velocity cause instability in the power output of wind turbines, which is a challenge for the AWE electrolysers due to their lower response rate. Accordingly, a UPS with an energy storage device was used to stabilize the output voltage of the turbine. The UPS could stably output 19.8 kW of power, providing continuous energy input for the floating seawater electrolysis platform (Supplementary Fig. 8), while excess electricity was stored in the energy storage device. When the offshore wind was insufficient to drive the wind turbine, the energy storage device served as the main power source, providing a stable voltage for the floating platform. This design could ensure the stability of the hydrogen production process.

To ensure the stability and safety of the floating platform in the ocean, the finite element simulation was used to analyse the motion, pressure, and stress distribution of the floating platform under a force 8 wind (~17.2 ms−1) and a 1-m significant wave height44,45. The floating platform, stabilized by mooring ropes, could swing between 5.5 and 7.9 m along the X direction (sway motion), while it could surge and heave along the Y direction (−0.55~0.56 m) and Z direction (0.54~0.9 m), respectively. The floating platform rotated from −2.5 to 2.0° around the X-axis (row), from −8.5 to 7.2° around the Y-axis (pitch), and from −12.4 to 10.2° around the Z-axis (yaw), which was within a controllable range (Supplementary Fig. 9). In addition, as shown in Fig. 3c, maximum stress occurs at the anchor frame (~286 MPa), and the maximum fluid pressure on the wet surface of the floating platform body was ~13,041 Pa. These values were lower than the yield stress of the ship structural steel employed, ensuring adequate strength for the floating platform.

The floating platform was located approximately southeast of the wind turbine (Fig. 3d), and was connected by 200-m submarine cables (Supplementary Fig. 10). In situ direct seawater electrolysis was performed over a period of 10 days from 17 May to 26 May 2023 at a 1.2 Nm3 h−1 H2 generation scale (Fig. 3f). This was the most suitable period for experiments in this ocean area; in other periods, the Maritime Bureau requested evacuation due to typhoon weather conditions. During the 10 days of hydrogen production from seawater, the average temperature of the ambient atmosphere and shallow seawater ranged from ~20 to 25 °C. However, the wind speed and wave height varied considerably over time, as shown by the following data: for example, 12:00 AM, May 17, 2023-0.4 m wave height and 7 ms−1 wind speed; 0:00 AM, May 18, 2023-0.7 m wave height and 9 ms−1 wind speed; 20:00 PM, May 22, 2023-0.9 m wave height and 15 ms−1 wind speed. Over the course of one day, the environment conditions also greatly varied, e.g. at 4:00 AM, May 21, 2023-0 m wave height and 3 ms−1 wind speed; at 20:00 PM, May 21, 2023-0.6 m wave height and 8 ms−1 wind speed. During this period, the winds were primarily from the northeast and southwest (Fig. 3e).

At a constant current of 957 A for electrolysis of each electrolyser in Xinghua Bay seawater and an uncontrolled fluctuating ocean environment with a 0.3~0.9 m wave height and 0~15 ms−1 wind speed, the platform system exhibited good and stable performance for over 240 h with an energy consumption of ~5.0 kWh Nm−3 H2 (Fig. 4a-d and Supplementary Fig. 11). The voltage of each electrolyser was ~2.14, 2.12 and 2.14 V at 250 mA cm−2, which was ~0.1 V higher than the voltage tested in the laboratory (Supplementary Fig. 12). This may have occurred due to a slight loosening of the electrode plates and catalysts during transportation on the road or in the ocean, which led to an increase in the electronic transmission resistance, but the values were within an acceptable range. The energy consumption was also similar to that of onshore industrial alkaline electrolysis using pure water, while the extra investment of pre-desalination was eliminated. The hydrogen purity was over 99.9% (Supplementary Fig. 13), satisfying requirements for commercial use. Moreover, no obvious increase in impurities, such as Cl-, Mg2+, and SO42-, was detected in the SDE after long-term electrolysis; the electrolyser exhibited over 99.99% ion-blocking efficiency (Fig. 4e and Supplementary Fig. 14). The SDE remained clear without precipitate formation, supporting this point. The above research further illustrated that the system could withstand certain wave impacts. Scanning electron microscopy (SEM) analysis revealed that the catalyst layer maintained its original morphology even after long-term operation (Fig. 4f). In addition, the OH- concentration of the KOH SDE was stable and ranged from 25 to 28%, indicating that the electrolysis and water migration rates of the system tended to remain consistent at an average seawater temperature of ~23 °C (Supplementary Fig. 15).

The PTFE membrane was examined after 10 days of operation. SEM analysis revealed distinct pores in the membrane, suggesting no notable biofouling or pore blocking (Fig. 4g). The contact angles of the hydrophobic PTFE membrane were ~147.05° (with KOH droplets) and 150.18° (with Xinghua Bay seawater droplets) after long-term operation (Fig. 4h), verifying the suitable environmental tolerance of the multi-fluorine structure. To confirm that the mass transfer performance of the PTFE membrane was not affected by seawater waves or impurities, a water migration behaviour test was further conducted, and the results were compared to the membrane’s performance before the test. As shown in Fig. 4i, the average water migration in the PTFE membrane after 10 days of use was ~0.097 g g–1 SDE min−1, which was similar to that with the new membrane (0.096 g g–1 SDE min−1).

Methods
Water migration test in the laboratory
Static water migration behaviour was observed according to the weight changes of the SDE electrolytes at 25 °C. Here, KOH and 0.5 M NaCl were considered as the SDE electrolyte and simulated seawater, respectively. A certain quantity of KOH solution and 0.5 M NaCl were placed on both sides of the porous PTFE membrane. The weight changes in the KOH solution were recorded at regular intervals to prepare a curve showing the mass of water migrating over time.

Additionally, an open 60×40×20 cm3 container was used as a seawater cabin, in which a 0.5 M NaCl solution was used as the simulated seawater, and a wave maker was used to create different kinds of simulated waves (calm, constant, pulsed, tidal and turbulence modes). A 50 wt% KOH solution as the SDE was placed in a square container and a PTFE membrane with 0.1 μm pore size was lined on the five walls of the container with mass transfer area of 75 cm2. Water migration tests were carried out under simulated wave conditions with different wave types and heights. The wave heights of 3.4, 3.2, and 2.9 cm corresponded to wavelengths of 62.32, 59.40, and 54.52 cm, respectively, and the periods were all 1.34 s.

Seawater electrolysis in a simulated fluctuating environment in the laboratory
To test the durability of the seawater electrolyser in a fluctuating seawater environment, an open 1.4×1.2×0.6 m3 container containing seawater from Xinghua Bay was used as the seawater cabin, and irregular waves were created to simulate the fluctuating environment of the ocean. The seawater electrolyser was immersed in seawater with a specific water migration area according to the concentration of seawater and SDE. Linear sweep voltammetry (LSV) was conducted using a multimeter. The corresponding voltages between the anode and cathode plates of the electrolyser were obtained at current densities of 0 mA cm−2, 10 mA cm−2, 50 mA cm−2, 100 mA cm−2, 150 mA cm−2, 200 mA cm−2, and 259 mA cm−2. The electrolysers were operated for more than 500 h at a current of 957 A. The voltage of each electrolyser was recorded as the result of the electrolysis process. After electrolysis, the microstructure of the gas pathways in the PTFE membrane and the morphology of the commercial catalyst were observed via SEM analysis (Thermo Scientific Apreo 2C). The water contact angle of the PTFE membrane was measured at room temperature using a contact angle goniometer (DSA25, Krüss) with seawater droplets.

Floating platform design
UPS module. A UPS, which contains a lithium-ion battery as an energy storage device, is mainly used to provide an uninterrupted electricity for floating platform. When the wind in the oceans is sufficient to drive a wind turbine, the electricity generated by the wind turbine is stably transported to the platform through the UPS, while the excess electricity is stored in an energy storage device. When the wind is insufficient, that is, the electricity generated by the wind turbine is insufficient for the platform operation, the UPS provides 220 V power to the floating platform from the energy storage device. The energy storage device mainly consists of a system cabinet and a battery pack. The system cabinet (2400 mm×800 mm×1800 mm) contains rectifiers, inverters and bypasses. The battery pack (4153.9 mm×600 mm×1800 mm) is composed of a lithium iron phosphate battery, and its rated energy, rated voltage, and maximum discharge current are 188.16 kWh, 224 V, and 400 A, respectively. Power conversion from the wind turbine to the battery supply could be completed within 10 ms to ensure the continuous and normal operation of the floating platform.

Current conversion module. The intelligent current conversion module converts the AC power generated from the wind turbine into high-current DC power with low voltage. The maximum output voltage is 8 V, and the maximum current is 1200 A. This approach provides a reliable and stable current for seawater electrolysis.

Seawater electrolysis module
1.2 Nm3 H2 h−1 seawater electrolysis module, which consists of three seawater electrolysers, is the core component of the floating platform. The seawater cabin is connected to the ocean environment, and electrolysers are directly immersed in the seawater cabin for hydrogen generation. The electrolyser mainly consists of an outer frame, a “gas phase” isolation interface, and a core stack. The lower part of the outer frame (70.5 cm×82 cm×62 cm) is equipped with a porous grid. The inner wall of the outer frame is lined with a PTFE membrane, which forms a “gas phase” isolation interface in seawater and contains the electrolyte. The core electrolysis stack consists of 11 independent electrolysis units connected in parallel. Each unit includes a positioning frame, an anode plate, an anode catalyst layer, a hydrophilic diaphragm, and a cathode catalyst layer, and a cathode plate. A stainless-steel beam is welded to the outer frame of the electrolyser and fixed to the roof of the seawater chamber to resist swinging environment. A transparent PC board between the fixing beam of the electrolyser and steel beam in the seawater tank is used to seal the seawater cabin and support the electrolyser. Fluororubber gaskets are used at the connections for shock absorption, and they are connected to each other with bolts. Before testing, COMSOL was used to validate the design of the electrolyser.

H2 detection module. The hydrogen generated from the electrolyser is transferred to the hydrogen detection module. The temperature of electronic condenser (a kind of gas-liquid separator) is set to 1.5 °C, and the setpoint accuracy is ±0.1 °C. After the hydrogen gas enters the electronic condenser, the alkaline steam from the hydrogen gas flow is condensed into a liquid solution, achieving gas-liquid separation as the temperature decreases. The condensed alkaline liquid is discharged through a drainage device, which works cyclically for 30 s and stops for 10 s after every 20 s. Approximately 700~900 ml of alkaline liquid was removed from a single electrolyser every 24 h. After cooling, the dehydrated hydrogen enters the detector for analysis.

Strategies for overcoming crucial implementation challenges
Here, we provide an overview of the crucial challenges that were faced during the design, installation, and investigation of the kilowatt-scale floating seawater electrolysis platform and the mitigation strategies that were used to avoid or resolve each challenge. (a) The compatibility of the alkaline seawater electrolyser with fluctuating wind power. To obtain stable power from the wind turbine, a UPS with an energy storage system was installed. When the wind was sufficient, the UPS ensured a stable voltage output, while excess electricity was saved by the energy storage device, and when wind was insufficient, the energy storage device can provided stable power for the floating platform. (b) The connection and installation of the cable. To prevent the cables from sinking into the seabed and causing the floating platform to be dragged, a 30-m-long cable was secured to a series of floating piles. (c) Fixing the position of the floating platform. To prevent the floating platform from being dragged by cables or affected by strong winds and waves, a counterweight module and four anchors were installed on the floating platform body. (d) Corrosion protection. To prevent corrosion on the surface of the floating platform, a composite anti-corrosion coating was required on the floating system surface. In addition, it was necessary to install zinc blocks for cathodic protection on the platform. Moreover, considering salt mist and the relative humid (usually >80%) environment over the ocean, a dehumidification module had to be installed in the floating system to extend service life. (e) Hydrogen safety. Several vents and monitoring devices were installed inside the platform. In addition, a flow meter was installed to monitor whether H2 crossed over or leaked to stop electrolysis before dangerous conditions. (f) Alleviating the impact of waves. A seawater cabin directly linked to the external ocean via a control valve was constructed inside the floating platform. This design not only ensured direct interaction between the electrolyser and seawater but also enabled the floating structure (~48.5 t) to withstand the impact of waves. (g) Convenience and reliability. For ease of operation in an uncontrolled and fluctuating ocean environment, three lifting devices were installed in the floating system. This approach facilitated electrolyser installation, debugging, and emergency response procedures.

Platform simulation and offshore testing in Xinghua Bay
To ensure the safety of the platform in the ocean, it was necessary to conduct a finite element simulation to analyse the stress in the platform in a fluctuating ocean environment. A model was established for the primary load-bearing components of the floating platform (the main body of the platform and four mooring racks). The simulation was carried out under the following conditions: a water depth of 10 m, a water density of 1025 kg m−3, force 8 wind (~17.2 ms−1), an irregular wave with a peak period of 4 s, and an effective wave height of 1 m. The motion response and mooring force, as well as the stress distribution and fluid pressure of the platform under the action of wind and waves, were calculated to determine the kinematic stability and structural strength of the platform.

LSV was carried out to compare the electrolytic performance in the uncontrolled ocean environment with that in the laboratory. During the stability test, the voltage monitoring module collected the real-time voltage of the electrolyser with a high-conductivity and anticorrosive copper probe. The real-time voltage value was recorded every minute with an accuracy of ±0.01 V. A current sensing ring with an accuracy of ±1 A was used to monitor the real-time current of each electrolyser, and the current was recorded every minute. The hydrogen flow rate was detected via a gas monitoring device to determine whether the electrolyser was operating properly. All monitoring information was transmitted to the monitoring system cloud through 4G so that all the information could be obtained in real-time. In addition, SDE samples were collected daily from the electrolysers and tested for ion concentration to ensure that the water phase-transition migration process was not affected by wave fluctuations. The temperature, relative humidity, wave height, wind speed, and direction were recorded every 4 h.

Gas analysis
H2 produced by the electrolyser was dried and dealkalized. Ar gas was used as the carrier gas to push the collected H2 into a gas chromatograph for the H2 purity test. The contents of N2, O2, and CO impurities were determined simultaneously, after which the purity of the hydrogen was calculated via the area normalisation method.

Ion concentration test
The concentrations of SO42–, Cl–, F–, and Br– ions were measured using an ion chromatograph (Thermo Scientific ICS 100). Mg2+, Na+, K+, Ca2+, and Sr2+ were analysed using an inductively coupled plasma optical emission spectrometer (PE Avio 200 ICP‒OES). The hypochlorite (ClO−) concentration in the SDE was measured by the o-tolidine method31.

Electrolytic energy consumption associated with H2 production
The energy consumption for producing hydrogen per Nm3 of H2 of the three electrolysers, Q, was calculated as follows:

W = ∫ I × U dt

where I is the current and U is the voltage. W of the three electrolysers corresponds to W1, W2 and W3. Next, the volume of H2 produced, V, was calculated as follows:

V = 22.4 × I × t / (Z × F)

where the number of electrons transferred for the hydrogen evolution reaction, Z, is 2 and the Faraday constant, F, is 96,485 C mol−1. V of the three electrolysers corresponds to V1, V2 and V3. Thus, the electrolytic energy consumption is Q = W/V (kWh Nm−3), and the average electrolytic energy consumption is Q = (W1 + W2 + W3) / (V1 + V2 + V3).

Model
The theoretical model was based on the following assumptions: (1) In the instantaneous process, temperature changes, thermal convection, and concentration polarization on both sides of the membrane are not considered. (2) The amount of water migration per unit time is equal to that absorbed by the KOH solution (SDE) for the same area and time.

According to Darcy’s law, the water migration mass flux is assumed to be proportional to the vapour pressure difference across the PTFE membrane and is given by (1):

J(x) = Bm ΔP = Bm (Ps − P(x))

where J(x) is the mass flux (g m−2 s−1), Bm is the membrane permeability (g m−2 s−1 Pa−1), and Ps and P(x) are the water vapour pressures at the seawater and the SDE side surfaces (Pa), respectively.

The water migration rate and water migration mass can be calculated with (2) and (3):

dM/dt = J(x) S = Bm (Ps − P(x)) S

∫0t Mmigration = ∫0t J(x) S = ∫0t Bm (Ps − P(x)) S

Seawater can be considered as a NaCl solution. The water vapour pressure on the seawater side can be calculated with (4)46:

Ps = (1 − xs) × (1 − 0.5xs − 10xs2) × exp(23.1964 − 3816.44 / (T − 46.13))

where xs is the molar fraction of NaCl.

The water vapour pressure on the SDE side can be calculated with (5)47:

lg(P(x)/105) = 55.56x / (1 − x) − 0.01508 × (55.56x / (1 − x))2 − 0.0016788 × (55.56x / (1 − x))3 + 2.25887 × 10−5 × (55.56x / (1 − x))4 + (1 − 0.0012062 × (55.56x / (1 − x)) + 5.6024 × 10−4 × (55.56x / (1 − x))2 − 7.8228 × 10−6 × (55.56x / (1 − x))3) × (35.4462 − 3343.93 / T − 10.9 lg T + 0.0041645 T)

where, x is the molar fraction of KOH solution.

The water consumption during electrolysis can be calculated with Eqs. (6) and (7):

It = nZF

M = 18It / zF

where I is the current (A), z is the number of electrons transferred for the HER, and F is the Faraday constant (96,485 C mol−1).

When the water consumption for water migration and electrolysis are equal, the system becomes balanced.

ΔM = Mmigration − Melectrolysis = ∫0t Bm (Ps − P(x)) S − 18It / zF

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "1. What are the noteworthy results?\nIn this study, expanding upon the design of the lab-scale demonstration model, a large-scale and\nefficient floating system of in-situ direct seawater electrolysis, based on the water phase-transition\nmigration mechanism under interfacial vapour pressure difference, using renewable offshore wind\nenergy, was operated in the ocean with uncontrollable fluctuating wind and wave conditions for the\nfirst time.\n2. Will the work be of significance to the field and related fields? How does it compare to the\nestablished literature?\nYes, this work will be of significance to the field due to the transition from lab-scale demonstration\nmodels to large-scale functional ones in real conditions of waves and wind.\nHydrogen production directly from natural resources like seawater using renewable energies offers\na promising pathway to achieve a sustainable energy industry and fuel economy as evidenced by\nthe large number of articles in this area of research. Various direct seawater electrolysis methods have\ndemonstrated effective at the lab-scale.\n3. Does the work support the conclusions and claims, or is additional evidence needed?\nThere is no additional evidence needed\n4. Are there any flaws in the data analysis, interpretation and conclusions? Do these prohibit\npublication or require revision?\nAfter the Abstract from paragraphs 21-38, it would be indicated to appear the \"Introduction\"\nsection for paragraphs 42-104.\nIn my opinion, the conclusions should be outlined in a separate \"Conclusions\" section.\n5. Is the methodology sound? Does the work meet the expected standards in your field?\nThroughout the manuscript there are interpretations or discussions repeated several times, which\nmakes reading difficult.\nAlso, the methods and analyses used in the laboratory-scale model are not presented separately\nfrom those applied to the real model.\n6. Is there enough detail provided in the methods for the work to be reproduced?\nIn my opinion, it is a problem that many details and information are provided in the\nSupplementary file, so that there are always references in the text to this additional material.\nPerhaps it would be advisable for the additional material to contain only figures, and the\nexplanations to be given in the article, i.e. the complete model and pages 11-12.", "reviewer_2": "In-situ direct seawater electrolysis in uncontrollable fluctuating ocean\nThis article studies the electrolysis of seawater without purification in a floating platform subject to\nthe motion caused by the waves, the wind and currents. The topic is interesting for the audience of\nNature Communications. However, it requires substantial improvement prior to being accepted for\npublication.\nRegarding the title, it should be a composition o keywords relevant to the article. The ocean is\nuncontrollable and fluctuating, so the two descriptive adjectives that appear in the title add no\ninformation. I would suggest including the words “floating platform” and “wave motion” in the title.\nAbout the arrangement of the article. Right now, the article presents the following structure:\n• Abstract\n• Results\no Process variable correlations and model\no System overview\no Performances in uncontrollable fluctuating ocean\no Potential and outlook\n• Discussion\n• Methods\no Pilot plant design and operation\n♣ UPS module\n♣ Current conversion module\n♣ Seawater electrolysis module\n♣ H2 detection module\no Integrated testing\no Electrochemical test\no Ion concentration test\no Material characterization\no Gas analysis\no Model\n• Reference\n• Acknowledgements\n• Author contributions\n• Competing interests\nThe current structure must be improved. It is not possible for the reader to jump directly into the\nresults. The article must have first a comprehensive literature review in the Introduction to state\nthe problem this article is going to solve and why no other previous research has been able to\ntackle the specific problem. The Introduction shall also include the scope of the article. The\nintroduction must include as well a summary of the structure of the article at the end.\nFurthermore, the introduction should include what the reader should expect to find in the main\nbody of the article and what could find in the supplementary information document. Normally after\nthe introduction comes the Methods, then the results, the discussion and the conclusions.\nCurrently there is no conclusions section.\nThe supplementary information document arrangement must include an index in addition to the\nindex of figures. The supplementary information document must include an introduction to explain\nthe reader the structure of the document. The article shall be able to stand alone without the need\nto refer to the supplementary information document. Right now it is not possible to read without\nconsulting the supplementary information document. The supplementary information document\nmust be able to stand alone as well. The arrangement of the main body of the article and the\nsupplementary information document must be revised.\nRegarding the physics modelling of the floating platform, wave characteristics are not just defined\nby amplitude. In addition, the system is dynamic, so it is important to consider other parameters\nlike the moments of inertia, the masses, the added masses and the hydrostatics of the platform. I\nsuggest the authors to carefully read the following references:\n• Edition, F., Journée, J. M. J., & Massie, W. W. (2001). Offshore hydromechanics. Delft University\nof Technology.\n• Chakrabarti, S. (2005). Handbook of Offshore Engineering (2-volume set). Elsevier.\nI suggest providing the orientation of the floating platform and the directional rose of waves and\nwinds to complete all the information.\nAbout the references to previous works. The article presents only 23 references, out of them none\ninclude DOI. Two of them correspond to press releases, in which the URL of the article is provided\nbut no the accession date. The literature review must be much more extensive to state the\nproblem and the rationale to this work.\nConcerning the reproducibility of the research. In the current status the research described in the\narticle is impossible to reproduce, as the current information is not properly organized and it is\nincomplete. The authors must improve the article to ensure that the research could be reproduced\nby any other researcher. In addition, some of the problem that this article covers are not properly\ndescribed. It is not possible for the reader to understand problems like the water migration\nproblem in Figure 1. The electrolysis cell is not properly described, thus, it is not possible to\nunderstand their architecture, and how seawater is separated from the SDE. In Figure 2, what is\nthe gas condenser? Is it actually a moisture remover?.\nAbout the units in the results. You can provide results in any units you wish but SI units are\nmandatory. Use MJ for kWh and use kg for Nm3.\nAbout the English language: Proof-reading is highly recommended.\nHappy New Year!", "reviewer_3": "Manuscript number: NCOMMS-23-55237\nThe manuscript entitled \" In-situ direct seawater electrolysis in uncontrollable fluctuating ocean\"\naddressed an important and interesting research study. It is significant to explore to potential\nopportunities to employ direct seawater electrolysis in the offshore environment. Fluctuating\nconditions of the ocean are considered in the model development to guide the in-situ direct\nelectrolysis for hydrogen generation in the fluctuating ocean. The methodology is sound and\nresults are noteworthy and support the conclusions drawn from the manuscript, however, further\nclarification, justification and additional results would help consolidate the idea being presented.\nSome critical points as follows need to be addressed:\nAdditional details need to be added on sizing of the proposed model. On what basis 1.2 Nm3h-1\nscale was selected?\nA 240-h stable electrolysis operation with an energy consumption of 5 kWh Nm-3 H2. How is this\nenergy consumption is determined and how the components such as seawater electrolysis and\nenergy storage behave in real-time on the fluctuation in wind power?\nAuthors assumed the wind speed range of 0-15 m/s. How will the model behave if wind speed\nincreases from the assumed limit?\nSome insights and atleast two cases would be required how the modeled system is producing 1.2\nNm3h-1 of hydrogen when the input power source is intermittent in nature, how imperative part is\nbeing played by the energy storage system and some details on the storage system sizing would\nadd value.\nSEM images are shows for the catalysts and PTFE membrane after 10 days seawater electrolysis,\nhowever, it would be imperative to see the long term effects on the electrodes, catalysts and\nmembrane to predict the lifetime."}, "author_response": "Point-by-point responses to the reviewers’ comments\nReferee #1\nComments：\n1. What are the noteworthy results?\nIn this study, expanding upon the design of the lab-scale demonstration model, a large-scale and\nefficient floating system of in-situ direct seawater electrolysis, based on the water phase-transition\nmigration mechanism under interfacial vapour pressure difference, using renewable offshore wind\nenergy, was operated in the ocean with uncontrollable fluctuating wind and wave conditions for the\nfirst time.\nResponse:\nWe would like to thank the reviewer very much for his/her positive comments and precise summary\nof the noteworthy results of our work.\nComments：\n2. Will the work be of significance to the field and related fields? How does it compare to the\nestablished literature?\nYes, this work will be of significance to the field due to the transition from lab-scale demonstration\nmodels to large-scale functional ones in real conditions of waves and wind.\nHydrogen production directly from natural resources like seawater using renewable energies offers a\npromising pathway to achieve a sustainable energy industry and fuel economy as evidenced by the\nlarge number of articles in this area of research. Various direct seawater electrolysis methods have\ndemonstrated effective at the lab-scale.\nResponse:\nWe are very grateful to the reviewer for his/her positive comments on our work of direct seawater\nelectrolysis in fluctuating ocean, which is greatly encouraging us. This work is a new exploration on\nthe flied of seawater electrolysis from the laboratory to real ocean environments, which could provide\na solution for developing a more mature mode of hydrogen production directly from seawater using\nrenewable energy. This approach is expected to promote the efficient application of offshore renewable\nenergy and reduce the cost of large-scale hydrogen fuel production in the future. In addition, this work\nmay guide researchers to pay more attention to the impact of the ocean environment on seawater\nelectrolysis, thereby promoting the development of this field towards to industrial application.\nComments：\n3. Does the work support the conclusions and claims, or is additional evidence needed?\nThere is no additional evidence needed.\nResponse:\nWe would like to thank the reviewer for his/her recognition of the adequacy and comprehensiveness\nof our work.\nComments：\n4. Are there any flaws in the data analysis, interpretation and conclusions? Do these prohibit\npublication or require revision?\nAfter the Abstract from paragraphs 21-38, it would be indicated to appear the \"Introduction\" section\nfor paragraphs 42-104.\nIn my opinion, the conclusions should be outlined in a separate \"Conclusions\" section.\nResponse:\nWe apologize that the title “Introduction” was not included in the original manuscript. In the revised\nmanuscript, the heading “Introduction” was added to differentiate for paragraphs (in line 42-106) after\nthe “Abstract” section. Additionally, the “Conclusion” of our manuscript is included in the\n\"Discussion\" section (in line 325-353) by following the style of the most recent literature appearing in\nNature Communications and “Formatting Instructions” given by Nature Communications (Liu, Z. et\nal. Synergistic dual-phase air electrode enables high and durable performance of reversible proton\nceramic electrochemical cells. Nat Commun 15, 472 (2024); Sun, M. et al. Gas diffusion enhanced\nelectrode with ultrathin superhydrophobic macropore structure for acidic CO electroreduction. Nat\nClass Lithium-based batteries. Nat Commun 15, 536 (2024); Meng, J. et al. Rapid-charging aluminium-\nsulfur batteries operated at 85 °C with a quaternary molten salt electrolyte. Nat Commun 15, 596\n(2024)).\nComments：\n5. Is the methodology sound? Does the work meet the expected standards in your field?\nThroughout the manuscript there are interpretations or discussions repeated several times, which\nmakes reading difficult.\nAlso, the methods and analyses used in the laboratory-scale model are not presented separately from\nthose applied to the real model.\nResponse:\nWe apologize that the repeated interpretations or discussions in our original manuscript made reading\nmore difficult. In the revised manuscript, we have revised the content carefully to make the manuscript\nmore concise and easier to read. In addition, we have added “Seawater electrolysis in a simulated\nfluctuating environment in the laboratory” to the “Methods” section to separate the laboratory-scale\nelectrolysis from the real model in ocean and added more details to make our work more solid and\ncomprehensive (in line 371-384).\n‘To test the durability of the seawater electrolyser in a fluctuating seawater environment, an open 1.4\n× 1.2 × 0.6 m3 container containing seawater from Xinghua Bay was used as the seawater cabin, and\nirregular waves were created to simulate the fluctuating environment of the ocean. The seawater\nelectrolyser was immersed in seawater with a specific water migration area according to the\nconcentration of seawater and SDE. Linear sweep voltammetry (LSV) was conducted using a\nmultimeter. The corresponding voltages between the anode and cathode plates of the electrolyser were\nobtained at current densities of 0 mA cm−2, 10 mA cm−2, 50 mA cm−2, 100 mA cm−2, 150 mA cm−2, 200\nmA cm−2, and 259 mA cm−2. The electrolysers were operated for more than 500 h at a current of 957\nA. The voltage of each electrolyser was recorded as the result of the electrolysis process. After\nelectrolysis, the microstructure of the gas pathways in the PTFE membrane and the morphology of the\ncommercial catalyst were observed via SEM analysis (Thermo Scientific Apreo 2C). The water contact\nangle of the PTFE membrane was measured at room temperature using a contact angle goniometer\n(DSA 25, Krüss) seawater droplets.’\nComments：\n6. Is there enough detail provided in the methods for the work to be reproduced?\nIn my opinion, it is a problem that many details and information are provided in the\nSupplementary file, so that there are always references in the text to this additional material. Perhaps it would be\nadvisable for the additional material to contain only figures, and the explanations to be given in the\narticle, i.e. the complete model and pages 11-12.\nResponse:\nWe would like to thank the reviewer for his/her comment on our supplementary materials. In response\nto the reviewer’s suggestion, some important explanations, such as the complete model and “Strategies\nfor overcoming crucial implementation challenges”, have been added to the “Methods” section of the\nrevised manuscript rather than in the supplementary information. We have condensed and revised the\nrelevant content for improved readability. In addition, some figures originally in supplementary\ninformation also have been placed in the revised manuscript (Fig. R1_1 and Fig. R1_2), so that the\nmanuscript contains more useful information.\nFig. R1_1 | Overview of the platform. a, Illustration of the overall system consisting of the UPS\nmodule, current conversion module, seawater electrolysis module, H detection module and\n2\ntransportation module. b, Process diagram of the device (I=current value, U=voltage value, Q=flow\nvalue). c, Photo of the interior space of the floating platform. d, Diagram of seawater cabin connected\nwith ocean.\nFig. R1_2 | Durability test in uncontrollable fluctuating ocean. a, Variation in the temperature and\nhumidity in Xinghua Bay. The data were recorded every 4 hours. The wind speed (b) and wave height\n(c) during the period of May 17, 2023 to May 26, 2023. The data were recorded every 4 hours. d,\nStability of the seawater electrolysis system in uncontrollable fluctuating ocean. e, The ions\nconcentration of SDE and Xinghua Bay seawater. SEM images of catalyst (f) and PTFE membrane (g)\nafter electrolysis in fluctuating ocean. h, Contact angle of PTFE membrane after electrolysis. i, Water\nmigration behaviour using the PTFE membrane before and after 10 days used. All error bars indicate\nthe standard deviation at three measurements.\nReferee #2\nComments：\n1. In-situ direct seawater electrolysis in uncontrollable fluctuating ocean\nThis article studies the electrolysis of seawater without purification in a floating platform subject to\nthe motion caused by the waves, the wind and currents. The topic is interesting for the audience of\nNature Communications. However, it requires substantial improvement prior to being accepted for\npublication.\nResponse:\nWe are very grateful to the reviewer for his/her positive evaluation of our work. We have extensively\nrevised the structure and content of our manuscript based on his/her comments to enhance the research\ncontent and viewpoints presented in our articles.\nComments：\n2. Regarding the title, it should be a composition o keywords relevant to the article. The ocean is\nuncontrollable and fluctuating, so the two descriptive adjectives that appear in the title add no\ninformation. I would suggest including the words “floating platform” and “wave motion” in the title:\nResponse:\nWe would like to thank the reviewer for his/her advice about the title. We have seriously considered\nthe reviewer's suggestion and highlighted key words in the title. The new title is as follows:\n‘In-situ direct seawater electrolysis using floating platform in ocean with uncontrollable wave motion’\nComments：\n3. About the arrangement of the article. Right now, the article presents the following structure:\n• Abstract\n• Results\no Process variable correlations and model\no System overview\no Performances in uncontrollable fluctuating ocean\no Potential and outlook\n• Discussion\n• Methods\no Pilot plant design and operation\n(cid:0) UPS module\n(cid:0) Current conversion module\n(cid:0) Seawater electrolysis module\n(cid:0) H detection module\n2\no Integrated testing\no Electrochemical test\no Ion concentration test\no Material characterization\no Gas analysis\no Model\n• Reference\n• Acknowledgements\n• Author contributions\n• Competing interests\nThe current structure must be improved. It is not possible for the reader to jump directly into the results.\nThe article must have first a comprehensive literature review in the Introduction to state the problem\nthis article is going to solve and why no other previous research has been able to tackle the specific\nproblem. The Introduction shall also include the scope of the article. The introduction must include as\nwell a summary of the structure of the article at the end. Furthermore, the introduction should include\nwhat the reader should expect to find in the main body of the article and what could find in the\nsupplementary information document. Normally after the introduction comes the Methods, then the\nresults, the discussion and the conclusions. Currently there is no conclusions section.\nResponse:\nWe would like to thank the reviewer for his/her helpful comment, which is important in improving the\nreadability of our work.\nIn response to reviewer’s suggestion, the article structure has been revised and the “Conclusion”\nsection of our manuscript is included in the \"Discussion\" section by following the style of the most\nrecent literature appearing in Nature Communications and “Formatting Instructions” given by Nature\nCommunications (Liu, Z. et al. Synergistic dual-phase air electrode enables high and durable\nperformance of reversible proton ceramic electrochemical cells. Nat Commun 15, 472 (2024); Sun, M.\net al. Gas diffusion enhanced electrode with ultrathin superhydrophobic macropore structure for acidic\nCO electroreduction. Nat Commun 15, 491 (2024); Zhang, X. et al. A dicarbonate solvent electrolyte\n2\nfor high performance 5 V-Class Lithium-based batteries. Nat Commun 15, 536 (2024); Meng, J. et al.\nRapid-charging aluminium-sulfur batteries operated at 85 °C with a quaternary molten salt electrolyte.\nNat Commun 15, 596 (2024)).\nThe new structure of our manuscript is as follows:\n‘• Abstract\n• Introduction\n• Results\no Process variable correlations during seawater electrolysis\no Floating platform system overview\no Performance under uncontrolled fluctuating ocean conditions\no Potential and outlook\n• Discussion\n• Methods\no Water migration test in the laboratory\no Seawater electrolysis in a simulated fluctuating environment in the laboratory\no Floating platform design\no Strategies for overcoming crucial implementation challenges\no Platform simulation and offshore testing in Xinghua Bay\no Gas analysis\no Ion concentration test\no Electrolytic energy consumption associated with H production\n2\no Model\n• Reference\n• Acknowledgements\n• Author contributions\n• Competing interests’\nIn addition, we apologize that the title of the “Introduction” was not previously included in the original\nmanuscript, which led to a misunderstanding by the reviewers. We have revised and improved the\ncontent of the introduction according to reviewer’s advice, including the significance of our work,\noverview and issues in this field, innovation of the article, and summary of the article (in line 42-106).\nComments：\n4. The supplementary information document arrangement must include an index in addition to the\nindex of figures. The supplementary information document must include an introduction to explain the\nreader the structure of the document. The article shall be able to stand alone without the need to refer\nto the supplementary information document. Right now it is not possible to read without consulting\nthe supplementary information document. The supplementary information document must be able to\nstand alone as well. The arrangement of the main body of the article and the supplementary information\ndocument must be revised.\nResponse:\nWe would like to thank the reviewer for his/her suggestions on our supplementary information section.\nWe have added the structure explanation to the first page of supplementary information file (in line\n24-29), including the title, author information, table of contents, supplementary figures and references\nto illustrate the content of each section. This information was based on the most recent paper appearing\nin Nature Communications (Zhong, T. et al. A wireless battery-free eye modulation patch for high\nmyopia therapy. Nat Commun 15, 1766 (2024); Peng, Q. et al. Extreme Li-Mg selectivity via precise\nion size differentiation of polyamide membrane. Nat Commun 15, 2505 (2024)). Furthermore, a\ncatalogue was created to clearly list the figures and their page numbers.\nIn addition, we apologize that the original supplementary information contains much information. To\nmake our manuscript clearer, some necessary descriptions and data that were originally included in\nthe supplementary information, including the complete model, the strategies for overcoming\nchallenges, and important data, have been added to the revised manuscript, and relevant explanations\nhave been provided. In addition, related data in the supplementary information have been combined to\nmake the file more readable (as shown in Fig. R2_1 and Fig. R2_2).\nFig. R2_1 | Overview of the platform. a, Illustration of the overall system consisting of the UPS\nmodule, current conversion module, seawater electrolysis module, H detection module and\n2\ntransportation module. b, Process diagram of the device (I=current value, U=voltage value, Q=flow\nvalue). c, Photograph of the interior space of the floating platform. d, Diagram of the seawater cabin\nconnected to the ocean.\nFig. R2_2 | Durability test in an uncontrollable fluctuating ocean environment. a, Variation in the\ntemperature and relatively humidity in Xinghua Bay. The data were recorded every 4 hours. The wind\nspeed (b) and wave height (c) during the period from May 17, 2023 to May 26, 2023. The data were\nrecorded every 4 hours. d, Stability of the seawater electrolysis system in an uncontrolled fluctuating\nocean environment. e, The ion concentrations of SDE and Xinghua Bay seawater. SEM images of the\ncatalyst (f) and PTFE membrane (g) after electrolysis in a fluctuating ocean. h, Contact angle of the\nPTFE membrane after electrolysis. i, Water migration behaviour of the PTFE membrane before and\nafter 10 days of use. All error bars indicate the standard deviation at three measurements.\nComments：\n5. Regarding the physics modelling of the floating platform, wave characteristics are not just defined\nby amplitude. In addition, the system is dynamic, so it is important to consider other parameters like\nthe moments of inertia, the masses, the added masses and the hydrostatics of the platform. I suggest\nthe authors to carefully read the following references:\n• Edition, F., Journée, J. M. J., & Massie, W. W. (2001). Offshore hydromechanics. Delft University\nof Technology.\n• Chakrabarti, S. (2005). Handbook of Offshore Engineering (2-volume set). Elsevier.\nResponse:\nWe are very grateful to the reviewer for his/her suggestions on the physical modelling of our floating\nplatform, which is important for improving our work.\nFor the laboratory-scale model, the characteristic parameters of the simulated waves, including the\nwave period and wavelength, were considered in the water migration behaviour test based on the\nreviewer’s suggestion. According to the experiment, the period of different simulated waves is\napproximately 1.34 s, while the wave heights of 3.4, 3.2 and 2.9 cm correspond to the wavelengths of\n62.32, 59.40 and 54.52 cm, respectively. We have added a relevant description of the parameters of\nthe simulated wave in the manuscript (in line 129-132):\n‘The average migration rate of water at the different wave heights and lengths significantly increased.\nThe water migration rates were approximately 0.0136, 0.0177 and 0.0194 g g-1 min-1 at wave\nH2O SDE\nheights of 2.9, 3.2 and 3.4 cm, as well as the corresponding wave lengths of 54.5, 59.4 and 62.3 cm,\nrespectively’\nFor physical modelling of the floating platform, the motion and surface pressure distribution of the\nfloating platform under the comprehensive action of wind and waves were simulated by finite element\nanalysis. The geometric model of the platform was composed of a platform body (7 m × 9 m) and four\nmooring racks (Fig. R2_3a), in which the depth was 2.5 m and the draft depth was 1 m. The floating\nplatform was symmetrical about the xz and yz planes, and its moment of inertia Ixx, Iyy and Izz about\nthe coordinate axes are 76798, 411172 and 592647 kg m2, respectively. The mooring arrangement of\nthe platform was mainly composed of four moored cables at four corners (Fig. R2_3b). The simulation\nwas carried out under the following conditions: a water depth of 10 m; a water density of 1025 kg m-\n3; a force 8 wind (~17.2 m s-1); and an irregular wave with a peak period of 4 s and an effective\nsignificant wave height of 1 m.\nRecording the evolution of the six degrees of freedom of the platform enabled determination of the\nplatform's motion transformation over time (Fig. R2_4). Under the action of wind and waves, the\nfloating platform remained stable between 5.5 and 7.9 m in the X direction (sway motion) under the\naction of the mooring cable. The displacement of the platform in the Y direction (surge motion) was\nwithin the range of -0.55 to 0.56 m, and the displacement in the Z direction (heave motion) was within\nthe range of -0.54 to 0.9 m. The floating platform deviated from -2.5 to 2.0° around the X-axis (row),\nfrom -8.5 to 7.2° around the Y-axis (pitch), and from -12.4 to 10.2° around the Z-axis (yaw). To analyse\nthe structural stability of the platform, we calculated the stress distribution and fluid pressure of the\nplatform body when the floating platform was subjected to a maximum mooring force of 73.2 kN. The\nmaximum stress of the platform was located at mooring rack with 286 MPa, and the maximum fluid\npressure on the wet surface was 13041 Pa (Fig. R2_5). The forces are less than the yield stress of the\nfloating structure steel, which could ensure the stability of platform.\nWe have added the corresponding description of the platform analysis in the manuscript as follows (in\nline 236-246):\n‘To ensure the stability and safety of the floating platform in the ocean, the finite element simulation\nwas used to analyse the motion, pressure and stress distribution of the floating platform under a force\n8 wind (~ 17.2 m s-1) and a 1-m significant wave height. The floating platform, stabilized by mooring\nropes, could swing between 5.5 and 7.9 m along the X direction (sway motion), while it could surge\nand heave along the Y direction (-0.55~0.56 m) and Z direction (0.54~0.9 m), respectively. The floating\nplatform rotated from -2.5 to 2.0° around the X-axis (row), from -8.5 to 7.2° around the Y-axis (pitch),\nand from -12.4 to 10.2° around the Z-axis (yaw), which was within a controllable range\n(Supplementary Fig. 9). In addition, as shown in Fig. 3c, maximum stress occurs at the anchor frame\n(~286 MPa), and the maximum fluid pressure on the wet surface of the floating platform body was\napproximately 13041 Pa. These values were lower than the yield stress of the ship structural steel\nemployed, ensuring adequate strength for the floating platform.’\nFig. R2_3. a, Floating platform model. b, Simulation conditions.\nFig. R2_4. Motion distance in the X direction (sway) (a), Y direction (surge) (b) and Z direction (heave)\n(c). Deviation angle in the X direction (row) (d), Y direction (pitch) (e) and Z direction (yaw) (f).\nFig. R2_5. a, The stress distribution of the floating platform. b, Hydrodynamic pressure distribution\non the wet surface of the floating platform.\nComments：\n6. I suggest providing the orientation of the floating platform and the directional rose of waves and\nwinds to complete all the information.\nResponse:\nWe are very grateful to the reviewer for his/her suggestions on the details of our study.\nThe orientation figure of the floating platform is shown in Fig. R2_6a (also shown in Fig. 3d). The\nfloating platform is about southeast of the wind turbine, and it is connected by cables. We have added\nthe relevant description in manuscript as follows (in line 247):\n‘The floating platform is about southeast of the wind turbine’\nThe rose diagram of wind directional based on wind direction data recorded every 4 h is shown in Fig.\nR2_6b (also shown in Fig. 3e). And we have added the relevant description in manuscript as follows\n(in line 259-260):\n‘During this period, winds are primarily from the northeast and southwest’\nFig. R2_6. a, Orientation diagram of floating platform in ocean. b, Directional rose chart of wind\nduring offshore test.\nComments：\n7.About the references to previous works. The article presents only 23 references, out of them none\ninclude DOI. Two of them correspond to press releases, in which the URL of the article is provided\nbut no the accession date. The literature review must be much more extensive to state the problem and\nthe rationale to this work.\nResponse:\nWe apologize that the references were not clear or accurate. According to the “Formatting Instructions”\ngiven by Nature and the reviewer’s suggestion, we have added more previous work about direct\nseawater electrolysis, and revised the format of all references (in line 537-674). In addition, more\nreferences about seawater electrolysis have been added to the manuscript:\n‘20. Fan R, Liu C, Li Z, et al. Ultrastable electrocatalytic seawater splitting at ampere-level current\ndensity. Nature Sustainability 7, 158-167 (2024). https://doi.org/10.1038/s41893-023-01263-w\n21. Liu X, Yu Q, Qu X, et al. Manipulating electron redistribution in Ni P for enhanced alkaline\n2\nseawater electrolysis. Advanced Materials 36, 2307395 (2024).\nhttps://doi.org/10.1002/adma.202307395\n22. Li P, Zhao S, Huang Y, et al. Corrosion resistant multilayered electrode comprising Ni N nanoarray\n3\novercoated with NiFe‐Phytate complex for boosted oxygen evolution in seawater electrolysis.\nAdvanced Energy Materials 14, 2303360 (2024). https://doi.org/10.1002/aenm.202303360\n23. Xu W, Wang Z, Liu P, et al. Ag nanoparticle‐induced surface chloride immobilization strategy\nenables stable seawater electrolysis. Advanced Materials 36, 2306062 (2024).\nhttps://doi.org/10.1002/adma.202306062\n24. Zhou L, Guo D, Wu L, et al. A restricted dynamic surface self-reconstruction toward high-\nperformance of direct seawater oxidation. Nature Communications 15, 2481 (2024).\nhttps://doi.org/10.1038/s41467-024-46708-8\n25. Duan X, Sha Q, Li P, et al. Dynamic chloride ion adsorption on single iridium atom boosts seawater\noxidation catalysis. Nature Communications 15, 1973 (2024). https://doi.org/10.1038/s41467-024-\n46140-y\n26. You H, Wu D, Si D, et al. Monolayer NiIr-layered double hydroxide as a long-lived efficient oxygen\nevolution catalyst for seawater splitting. Journal of the American Chemical Society 144, 9254-9263\n(2022). https://doi.org/10.1021/jacs.2c00242\n27. Tan L, Yu J, Wang C, et al. Partial sulfidation strategy to NiFe‐LDH@ FeNi S heterostructure\n2 4\nenable high‐performance water/seawater oxidation. Advanced Functional Materials 32, 2200951\n(2022). https://doi.org/10.1002/adfm.202200951\n28. Yu L, Zhu Q, Song S, et al. Non-noble metal-nitride based electrocatalysts for high-performance\nalkaline seawater electrolysis. Nature Communications 10, 5106 (2019).\nhttps://doi.org/10.1038/s41467-019-13092-7\n29. Keane T P, Veroneau S S, Hartnett A C, et al. Generation of pure oxygen from briny water by binary\ncatalysis. Journal of the American Chemical Society 145, 4989-4993 (2023).\nhttps://doi.org/10.1021/jacs.3c00176\n30. Dresp S, Thanh T N, Klingenhof M, et al. Efficient direct seawater electrolysers using selective\nalkaline NiFe-LDH as OER catalyst in asymmetric electrolyte feeds. Energy & Environmental Science\n13, 1725-1729 (2020). https://doi.org/10.1039/d0ee01125h\n31. Sun F, Qin J, Wang Z, et al. Energy-saving hydrogen production by chlorine-free hybrid seawater\nsplitting coupling hydrazine degradation. Nature Communications 12, 4182 (2021).\nhttps://doi.org/10.1038/s41467-021-24529-3\n32. Zhang L, Wang Z, Qiu J. Energy‐saving hydrogen production by seawater electrolysis coupling\nsulfion degradation. Advanced Materials 34, 2109321 (2022).\nhttps://doi.org/10.1002/adma.202109321\n33. Xin Y, Shen K, Guo T, et al. Coupling hydrazine oxidation with seawater electrolysis for energy‐\nsaving hydrogen production over bifunctional CoNC nanoarray electrocatalysts. Small 19, 2300019\n(2023).\n34. Guo L, Chi J, Zhu J, et al. Dual-doping NiMoO with multi-channel structure enable urea-assisted\n4\nenergy-saving H production at large current density in alkaline seawater. Applied Catalysis B:\n35. Zhu D, Zhang H, Miao J, et al. Strategies for designing more efficient electrocatalysts towards the\nurea oxidation reaction. Journal of Materials Chemistry A 10, 3296-3313 (2022).\nhttps://doi.org/10.1039/D1TA09989B\n36. Jiang X, Dong Z, Zhang Q, et al. Decoupled hydrogen evolution from water/seawater splitting by\nintegrating ethylene glycol oxidation on PtRh @Rh nanowires with Rh atom modification.\n0.02\nJournal of Materials Chemistry A 10, 20571-20579 (2022). https://doi.org/10.1039/D2TA05469H\n37. Zhu W, Wei Z, Ma Y, et al. Energy‐efficient electrosynthesis of high value‐added active chlorine\ncoupled with H generation from direct seawater electrolysis through decoupling electrolytes.\n38. Liu K, Gao X, Liu C X, et al. Energy‐saving hydrogen production by seawater splitting coupled\nwith PET plastic upcycling. Advanced Energy Materials, 2304065 (2024).\nhttps://doi.org/10.1002/aenm.202304065\n39. Shi L, Rossi R, Son M, et al. Using reverse osmosis membranes to control ion transport during\nwater electrolysis. Energy & Environmental Science 13, 3138-3148 (2020).\nhttps://doi.org/10.1039/D0EE02173C\n40. Veroneau S S, Nocera D G. Continuous electrochemical water splitting from natural water sources\nvia forward osmosis. Proceedings of the National Academy of Sciences 118, e2024855118 (2021).\nhttps://doi.org/10.1073/pnas.2024855118\n41. Veroneau S S, Hartnett A C, Thorarinsdottir A E, et al. Direct seawater splitting by forward osmosis\ncoupled to water electrolysis. ACS Applied Energy Materials 5, 1403-1408 (2022).\nhttps://doi.org/10.1021/acsaem.1c03998\n42. Marin D H, Perryman J T, Hubert M K A, et al. Hydrogen production with seawater-resilient\nbipolar membrane electrolyzers. Joule 7, 765-781 (2023). https://doi.org/10.1016/j.joule.2023.03.005\n46. Journe J M J, Massie W W. Offshore hydromechanics. Delft University of Technology, (2001).\n47. Chakrabarti S. Handbook of offshore engineering (2-volume set). Elsevier, (2005).\n48. Lawson K W, Lloyd D R. Membrane distillation. Journal of Membrane Science 124, 1-25 (1997).\nhttps://doi.org/10.1016/S0376-7388(96)00236-0\n49. Balej J. Water vapour partial pressures and water activities in potassium and sodium hydroxide\nsolutions over wide concentration and temperature ranges. International Journal of Hydrogen Energy\n10, 233-243 (1985). https://doi.org/10.1016/0360-3199(85)90093-X’\nComments：\n8. Concerning the reproducibility of the research. In the current status the research described in the\narticle is impossible to reproduce, as the current information is not properly organized and it is\nincomplete. The authors must improve the article to ensure that the research could be reproduced by\nany other researcher. In addition, some of the problem that this article covers are not properly\ndescribed. It is not possible for the reader to understand problems like the water migration problem in\nFigure 1. The electrolysis cell is not properly described, thus, it is not possible to understand their\narchitecture, and how seawater is separated from the SDE. In Figure 2, what is the gas condenser? Is\nit actually a moisture remover?\nResponse:\nIn response to the reviewer’s comments, we have provided more details about our seawater electrolysis\nstrategy, the experiment and the specific functions of the floating system in the revised manuscript.\nFor the research description, we separated the laboratory-scale and pilot-scale experiments, and\nprovided a detailed description of the experimental operation process in the “Methods” section of the\nmanuscript.\nTo explain water migration, we have added a principle description of our seawater electrolysis\nstrategy based on the water phase transition migration mechanism (Fig. R2_7, also shown as Fig. 1a),\nand the following sentences and figure have been added to the “Introduction” section (in line 83-89)\nand Fig. 1a respectively.\n‘This strategy is realized by applying a hydrophobic porous membrane as a gas-path interface and\nemploying a concentrated KOH solution as a self-dampening electrolyte (SDE). During operation, the\nwater vapour pressure difference between the seawater and the SDE induces spontaneous seawater\ngasification on the seawater side and the diffusion of water vapour through the porous membrane to\nthe SDE side, where it is reliquified by absorption by the SDE. When the water migration rate equals\nthe electrolysis rate, continuous and stable water migration via a ‘liquid–gas–liquid’ mechanism is\nrealized to provide fresh water for electrolysis’\n‘\nFig. R2_7. Seawater electrolysis drive by water phase transition migration mechanism.’\nFor the gas condenser, it is a kind of gas-liquid separator (Fig. R2_8). The principle of semiconductor\nrefrigeration is adopted to achieve refrigeration and further separate the gas-liquid mixture. In our\nsystem, the hydrogen gas (containing alkali steam), generated from an electrolyser, enters the gas\ncondenser through the gas inlet for cooling (1.5 ℃), and the alkaline steam is liquified. The cooled\nand dried hydrogen leaves the condenser through the gas outlet. The description and figure of\ncondenser has been added to the “Methods” section (in line 420) and supplementary information:\n‘The temperature of electronic condenser (a kind of gas-liquid separator) is set to 1.5 ℃, and the\nsetpoint accuracy is ± 0.1 ℃.’\nFig. R2_8. Photograph of the condenser.\nComments：\n9. About the units in the results. You can provide results in any units you wish but SI units are\nmandatory. Use MJ for kWh and use kg for Nm3.\nResponse:\nWe would like to thank the reviewer very much for the suggestions on physical units. We wrote the\nunits (kWh and Nm3) by following the style of recent literature published by Nature Communications\n(Sun, F. et al. Energy-saving hydrogen production by chlorine-free hybrid seawater splitting coupling\nhydrazine degradation. Nat Commun 12, 4182 (2021); Shi, H. et al. A sodium-ion-conducted\nasymmetric electrolyzer to lower the operation voltage for direct seawater electrolysis. Nat\nCommun 14, 3934 (2023)).\nComments：\n10. About the English language: Proof-reading is highly recommended.\nResponse:\nIn response to the reviewer’s comment, the manuscript and supplementary information have been\ncarefully proofread. In addition, the language of the entire manuscript has been revised by a\nprofessional editor from Springer Nature.\nReferee #3\nComments：\n1. The manuscript entitled \" In-situ direct seawater electrolysis in uncontrollable fluctuating ocean\"\naddressed an important and interesting research study. It is significant to explore to potential\nopportunities to employ direct seawater electrolysis in the offshore environment. Fluctuating\nconditions of the ocean are considered in the model development to guide the in-situ direct electrolysis\nfor hydrogen generation in the fluctuating ocean. The methodology is sound and results are noteworthy\nand support the conclusions drawn from the manuscript, however, further clarification, justification\nand additional results would help consolidate the idea being presented. Some critical points as follows\nneed to be addressed:\nResponse:\nWe would like to thank the reviewer for his/her comments and for summarizing our work. According\nto the opinions and suggestions of the reviewer, we have revised the manuscript to make the content\nmore solid. Our point-by-point responses to Reviewer 3's comments are as follows.\nComments：\n2. Additional details need to be added on sizing of the proposed model. On what basis 1.2 Nm3h-1\nscale was selected?\nResponse：\nIn our study, the size of the floating platform is 7 m (width) × 9 m (length) × 2.5 m (height) (the actual\nspace available for layout was 4.4 m (width) × 6.4 m (length) × 2.2 m (height), as shown in Fig. R3_1a),\nand the total capacity of seawater electrolysis module was 1.2 Nm-3 H h-1. The seawater electrolysis\n2\nmodule was composed of three parallel electrolysers, and the scale of each electrolyser was 0.4 Nm-3\nH h-1. The size design of the floating platform was mainly based on the following two considerations:\ncm × 82 cm × 62 cm) and compared their operating effects in parallel. This could provide a basis for\nthe development of large-scale seawater electrolysis floating platforms with multiple parallel devices\nin the future.\nSecond, to verify the impact of a fluctuating environment on seawater electrolysis, the floating\nplatform did not include a propulsion system to avoid interference from other factors; thus, freight\nequipment had to transport it to the test area, which had many limitations. According to some\nregulations and restrictions on maritime transportation in China and corresponding safety\nconsiderations, only long-distance land transportation from the shipyard to the wharf of the test area,\nand then a short trip by ship towing to the test area, was allowed. During transportation, limited by\nroad width and height, some road infrastructure such as traffic lights had to be removed (Fig. R3_1b).\nIn addition, according to the Chinese Highway Engineering Technical Standards (4), for a secondary\nhighway, the roadbed width for a design speed of 80 km h-1 was 15.0 m; thus, it was best for the width\nof the devices to be less than 7.5 m.\nOverall, in order to verify the parallel mode of three devices in a limited space, we had to design the\nfloating body to have dimensions of 7 m × 9 m × 2.5 m, with layout dimensions of 4.4 m × 6.4 m ×\n2.2 m. If the total scale of the electrolyser was further expanded, the floating platform would also be\nenlarged, which would make it more difficult to transport it on land, so we chose a hydrogen production\nequipment of 1.2 Nm3 h-1H after comprehensive consideration.\nthe floating platform land transport site.\nComments：\n3. A 240-h stable electrolysis operation with an energy consumption of 5 kWh Nm-3 H . How is this\n2\nenergy consumption is determined and how the components such as seawater electrolysis and energy\nstorage behave in real-time on the fluctuation in wind power?\nResponse:\nWe would like to thank the reviewer for his/her comment. The calculation method of electrolytic\nenergy consumption is as follows, which has been added in the ‘Energy consumption associated with\nH production’ of the Methods section (in line 492-502).\n2\n‘The energy consumption for producing hydrogen per Nm3 of H of the three electrolyzers, Q, was\n2\ncalculated as follows:\nwhere I is the current and U is the voltage. W of the three electrolysers corresponds to W , W and W .\n1 2 3\nNext, the volume of H produced, V, was calculated as follows:\n2\nwhere the number of electrons transferred for the hydrogen evolution reaction, Z, is 2 and the Faraday\nconstant, F, is 96,485 C mol-1. V of the three electrolysers correspond to V , V and V .\n1 2 3\nThus, the electrolytic energy consumption is Q = W/V (kWh Nm-3), and the average electrolytic energy\nconsumption is Q = (W +W +W )/(V +V +V ).’\n1 2 3 1 2 3\nIn response to reviewer’s question of how the components behave, the mechanism of the real-time\nresponse of energy storage and our seawater electrolyser to fluctuation in wind power is as follows\n(Fig. R3_2): When the offshore wind was sufficient, the offshore wind turbine could be used as the\nmain power supply, and a 220 V±1% stable constant frequency AC is output through the\nuninterruptible power supply system. Then, the output current was converted into stable DC to supply\nthe seawater electrolysis equipment via a DC-regulated power supply. Due to the fluctuation in\noffshore wind, the excess electricity was stored in the energy storge device when the wind power was\nsufficient, while the electricity stored in energy storage device was used to supply for floating platform\nduring a lack of wind power. Since the power of our floating platform was only 19.8 kW, the electricity\ngenerated from the 10 MW wind turbine during the offshore test could basically meet the requirements\nof the platform. Nevertheless, the energy storage device ensured a stable supply of electricity and was\nable to provide additional power to the platform within 10 ms when the offshore wind became\ninsufficient (Fig.R3_3). In light of this, electrolysers exhibited excellent stability for 10 days under\nthis fluctuating environment, as shown in Fig. 4 in the manuscript.\nFig. R3_2. Flowchart of a system using fluctuating offshore wind power as an energy source for\ndirect electrolysis of seawater to produce hydrogen.\nFig. R3_3. Stability of the energy storage device.\nComments：\n4. Authors assumed the wind speed range of 0-15 m/s. How will the model behave if wind speed\nincreases from the assumed limit?\nResponse:\nWe would like to thank the reviewer for his/her comments, which helps further improving our\nmanuscript. In the manuscript, the wind speed range of 0~15 m s-1 was the real date during the offshore\ntest in Xinghua Bay. In this condition, a 10 days stable electrolysis operation with an electrolytic\nenergy consumption of 5 kWh Nm-3 H and high-purity of over 99.9% was achieved. This mainly\n2\nresulted from the fact that the floating platform body resisted most of the wind and wave impacts. In\nfact, we only had 10 days to carry out the offshore experiment according to the China Meteorological\nAdministration and the Maritime Safety Administration, and we had to evacuate the testing area and\nreturn to the dock when the wind level exceeded level 8.\nIt is necessary to study the influence of stronger wind and waves on our floating platform. Thus,\naccording to the reviewer’s suggestion, a finite element simulation has been carried out to analyse the\nmotion characteristics and stress distribution of the floating platform under force 8 wind (about 17.2\nm s-1) and the significant wave height of 1 m (it is worth noting that it is dangerous for vessels and\nplatforms to work at sea under force 8 wind). As shown in Fig. R3_4, the floating platform is kept\nstable between 5.5 and 7.9 m in the X direction (sway motion) under the action of the mooring cable.\nThe displacement of the platform in the Y direction (surge motion) is within the range of -0.55 to 0.56\nm, and the displacement in the Z direction (heave motion) is within the range of -0.54 to 0.9 m. The\nfloating platform deviates from -2.5 to 2.0° around the X-axis (row), from -8.5 to 7.2° around the Y-\naxis (pitch), and from -12.4 to 10.2° around the Z-axis (yaw). As shown in Fig. R3_5, when the\nmaximum mooring force on the floating platform is 73.2 kN, the maximum stress occurs at the anchor\nframe (~286 MPa), and the maximum fluid pressure on the wet surface of the floating platform body\nis approximately 13041 Pa, which are lower than the yield stress of ship structural steel (355 MPa) we\nused. Therefore, it is sufficiently safe to carry out our offshore work under 0~0.9 m wave height, 0~15\nm s-1 wind speed. Anyway, it is necessary to further optimize the design of a floating platform for\nindustrial applications in marine environments in the future.\nThe corresponding description has been added in the manuscript (in line 236-246 and line 349-351):\n‘To ensure the stability and safety of the floating platform in the ocean, the finite element simulation\nwas used to analyse the motion, pressure and stress distribution of the floating platform under a force\n8 wind (~ 17.2 m s-1) and a 1-m significant wave height46-47. The floating platform, stabilized by\nmooring ropes, could swing between 5.5 and 7.9 m along the X direction (sway motion), while it could\nsurge and heave along the Y direction (-0.55~0.56 m) and Z direction (0.54~0.9 m), respectively. The\nfloating platform rotated from -2.5 to 2.0° around the X-axis (row), from -8.5 to 7.2° around the Y-\naxis (pitch), and from -12.4 to 10.2° around the Z-axis (yaw), which was within a controllable range\n(Supplementary Fig. 9). In addition, as shown in Fig. 3c, maximum stress occurs at the anchor frame\n(~286 MPa), and the maximum fluid pressure on the wet surface of the floating platform body was\napproximately 13041 Pa. These values were lower than the yield stress of the ship structural steel\nemployed, ensuring adequate strength for the floating platform.’\n‘For use in the deep seas, the resistance of the floating platform to stronger winds and waves should\nbe considered; moreover, the stability and strength of the hydrophobic porous membrane and\nelectrolysis system still need to be optimized for long-term use under controlled ocean conditions.’\nFig. R3_4. Motion distance in the X direction (sway) (a), Y direction (surge) (b) and Z direction (heave)\n(c). Deviation angle in X direction (row) (d), Y direction (pitch) (e) and Z direction (yaw) (f).\nFig. R3_5. a, The stress distribution of the floating platform. b, Hydrodynamic pressure distribution\non the wet surface of the floating platform.\nComments：\n5. Some insights and at least two cases would be required how the modeled system is producing 1.2\nNm3h-1 of hydrogen when the input power source is intermittent in nature, how imperative part is\nbeing played by the energy storage system and some details on the storage system sizing would add value.\nResponse:\nWe would like to thank the reviewer very much for this comment.\nWhen the offshore wind is sufficient, the wind turbine can generate 220 V±1% AC power to the\nfloating plate through an uninterrupted power supply (UPS), and then the DC-regulated power can\nsupply stable DC power to the seawater electrolysers. Moreover, the excess electricity output by the\nwind turbine is stored in the energy storage device. When the offshore wind is insufficient to drive the\nwind turbine’s operation, the energy storage serves as the main power source to provide a stable\nvoltage for the floating platform. This design could ensure the stability of the hydrogen production\nprocess. The corresponding description has been added to the revised manuscript (in line 217-224):\n‘Fluctuations in the wind velocity cause instability in the power output of wind turbines, which is a\nchallenge for the AWE electrolysers due to their lower response rate. Accordingly, a UPS with an\nenergy storage device was used to stabilize the output voltage of the turbine. The UPS could stably\noutput 19.8 kW of power, providing continuous energy input for the floating seawater electrolysis\nplatform, while excess electricity was stored in the energy storage device. When the offshore wind was\ninsufficient to drive the wind turbine, the energy storage device served as the main power source,\nproviding a stable voltage for the floating platform. This design could ensure the stability of the\nhydrogen production process.’\nIn addition, the energy storage device mainly consists of a system cabinet (2400 mm × 800 × 1800\nmm) and a battery pack (4153.9 mm × 600 mm × 1800 mm; rated energy of 188.16 kWh, rated voltage\nof 224 V, and maximum discharge current of 400 A). The power conversion from the wind turbine to\nthe battery supply could be completed within 10 ms to ensure the continuous normal operation of the\nfloating platform.\nA corresponding detailed description of storage energy device has been added to the “Methods”\nsection of the manuscript (in line 393-399)\n‘The energy storage device mainly consists of a system cabinet and a battery pack. The system cabinet\n(2400 mm × 800 mm × 1800 mm) contains rectifiers, inverters and bypasses. The battery pack (4153.9\nmm × 600 mm × 1800 mm) is composed of a lithium iron phosphate battery, and its rated energy, rated\nvoltage and maximum discharge current are 188.16 kWh, 224 V, and 400 A, respectively. Power\nconversion from the wind turbine to the battery supply could be completed within 10 ms to ensure the\ncontinuous and normal operation of the floating platform.’\nComments：\n6. SEM images are shows for the catalysts and PTFE membrane after 10 days seawater electrolysis,\nhowever, it would be imperative to see the long term effects on the electrodes, catalysts and membrane\nto predict the lifetime.\nResponse:\nWe sincerely appreciate the reviewer’s useful suggestion, which is very important for improving the\nquality of our paper.\nIn fact, the seawater electrolysis strategy was based on our previous paper (Nature 612, 673-678\n(2022)). In that paper, the system was stably operated for over 3200 h with no obvious electrocatalyst\ncorrosion or membrane wetting was observed, which lays the foundation for long-term use.\nHere, a seawater cabin (1.4 m × 1.2 m × 1.0 m) with a wave simulation function (wave height: ~3 cm;\nwavelength: ~30 cm) was built (Fig. R3_6a). After operating for over 500 h in a fluctuating\nenvironment, the anode/cathode catalysts and PTFE membrane were observed via SEM (Fig. R3_6b),\nand contact angle was further investigated (Fig. R3_6c). The pores of the PTFE membrane were clear,\nand there was no obvious pore plugging or morphological changes. The catalysts exhibited no obvious\ncorrosion phenomenon, and the catalytic site was clear and uniform. The PTFE membrane still exhibited\nhydrophobic performance. The experimental results showed that the seawater electrolyser had a durability\nfor over 500 h in a fluctuating environment and had the potential to contribute to long-duration and\nstable seawater electrolysis in the ocean.\nWe have added the following description to the manuscript (in line 138-141):\n‘In addition, a 500-h durability test of seawater electrolysis in a simulated fluctuating environment\nwas carried out in the laboratory. The SEM images of the clear catalyst and membrane pores provide\nevidence of the system's adaptability and robustness in such dynamic settings’\nFig. R3_6. a, Photograph of the seawater electrolyser testing in the seawater cabin. b, SEM images of\nthe anode catalyst and PTFE membrane before and after the 500-h durability test. c, Contact angle of\nPTFE membrane before and after test. d, Stability curve of electrolysis."}, {"round": 2, "reviewer_comments": {"reviewer_1": "From my point of view, the authors fully answered all my questions.\nBeing satisfied with the answers and the additions made to the manuscript, my opinion is that the\nwork can be published in its current form.", "reviewer_2": "The Reviewer is satisfied with the changes. Good work.", "reviewer_3": "The authors of the manuscript entitled “In-situ Direct Seawater Electrolysis Using a Floating\nPlatform in the Ocean with Uncontrollable Wave Motion” have made significant and substantial\nimprovements to the manuscript and addressed all the comments and suggestions accordingly.\nThe revisions made have significantly improved the clarity, coherence, and scientific rigor of the\nmanuscript. Based on the revised content, I am confident in recommending the possible\npublication of this manuscript."}, "author_response": "Point-by-point responses to the reviewers’ comments\nReferee #1\nComments：\nFrom my point of view, the authors fully answered all my questions.\nBeing satisfied with the answers and the additions made to the manuscript, my opinion is that the work can\nbe published in its current form.\nResponse:\nWe would like to thank the reviewer very much for his/her positive comments and this would\nsignificantly encourage us.\nReferee #2\nComments：\nThe Reviewer is satisfied with the changes. Good work.\nResponse:\nWe are very grateful to the reviewer for his/her positive evaluation of our work.\nReferee #3\nComments：\nThe authors of the manuscript entitled “In-situ Direct Seawater Electrolysis Using a Floating Platform in\nthe Ocean with Uncontrollable Wave Motion” have made significant and substantial improvements to the\nmanuscript and addressed all the comments and suggestions accordingly. The revisions made have\nsignificantly improved the clarity, coherence, and scientific rigor of the manuscript. Based on the revised\ncontent, I am confident in recommending the possible publication of this manuscript.\nResponse:\nWe would like to thank the reviewer for his/her comments and for summarizing our revised manuscript\npreviously. We also greatly appreciate the specific comments provided by the reviewers in the early\nstage, which have enabled our work to meet the requirements of Natural Communications."}]

Decision letter