Hydrogen production via seawater electrolysis involves two half-reactions that are identical to those in pure water electrolysis. At the cathode, the primary reaction is hydrogen evolution (HER), which is essentially a two-electron transfer process resulting in the formation of hydrogen molecules through the coupling of electrons and protons^[16]. Generally, the HER reaction mechanism is referred to as the Volmer-Heyrovsky mechanism or the Volmer-Tafel mechanism, involving three basic reactions as shown in equations (1) to (3)^[17].

the first step of HER is known as the Volmer reaction, in which a water molecule gains an electron to form a hydrogen atom adsorbed on the electrode surface, while simultaneously generating a hydroxide ion. The hydrogen atoms on the electrode surface desorb via two pathways: one is the Heyrovsky reaction involving electron participation, where water molecules around the adsorbed hydrogen atoms are reduced and combine with them to form hydrogen gas molecules; the other is the Tafel reaction without electron involvement, where two hydrogen atoms adsorbed on the catalyst directly combine to form hydrogen gas molecules^[18-19]. Finally, as shown by the overall HER reaction equation under alkaline and neutral conditions in Equation (4), after H₂is produced and detached from the electrode, a large number of OH^-remain, leading to an increase in pH near the electrode, which is the direct cause of alkaline scale deposition.

In the direct electrolysis of seawater, the oxygen evolution reaction (OER) predominantly occurs at the anode. As shown in Figure 1^[20], M represents the catalytic surface active site, with the blue line indicating the reaction pathway under acidic conditions and the red line representing the pathway under alkaline conditions. Under alkaline conditions, OH^- forms M-OH and M-O intermediates through two elementary reactions, which then proceed via two distinct reaction pathways to generate O₂. The first pathway involves the absorption of hydroxyl radicals by the surface-bound M-O to form a superoxide intermediate M-OOH, followed by electron transfer and coupling with hydroxyl radicals to produce O₂ molecules and release the active site. The second pathway involves the direct coupling of two M-O intermediates to form O₂^[21-23]. Regardless of the reaction pathway, OER involves a four-electron transfer process^[24]. Under standard conditions, the thermodynamic equilibrium potential for OER is 1.23 V; however, in practical reactions, an additional potential is required to drive the reaction forward, known as overpotential. Additionally, the coupling of multiple electrons and protons results in a larger activation energy barrier for OER, leading to significant energy consumption and a sluggish OER kinetics^[25]. Clarifying the energy relationships among these reactions is crucial for achieving a low overpotential.

As mentioned above, during the electrolysis of seawater, a large amount of OH^- is generated on the cathode surface and cannot diffuse into the solution system in time, leading to a significant increase in pH at the electrode surface, thereby inducing the combination of Mg²⁺ and Ca²⁺. Figure 4 illustrates the mechanism of alkaline scale formation during seawater electrolysis. Studies have shown that OH^- primarily combines with Mg²⁺ to form crystals, which continue to grow on the electrode surface, providing numerous deposition sites for subsequent Mg²⁺ and Ca²⁺ deposition^[34-36]. In actual electrolysis processes, the collected alkaline scale crystals are predominantly co-precipitates of magnesium hydroxide and calcium carbonate, due to the promotion of OH^- in the conversion of HCO₃^- to CO₃^2- in seawater, as shown in the following equation^[37].

Changes in external factors such as temperature, pressure, pH, and electrochemical parameters can all affect the deposition of alkaline scale, leading to differences in the morphology of alkaline scale crystals. Studies have shown that magnesium hydroxide crystals exist exclusively in the brucite form, whereas calcium carbonate can exist in multiple forms, including calcite, aragonite, and amorphous calcium carbonate^[38].

Limited by the mass transfer rate, the OH^- generated by HER cannot diffuse into the solution system in time, leading to its significant accumulation on the cathode surface, which is a major cause of alkaline scale deposition on the cathode surface^[14]. Developing HER catalysts with porous structures, high specific surface areas, high electrical conductivity, and high mass transfer efficiency can alleviate, to some extent, the negative effects caused by alkaline scale deposition^[14]. Due to the low conductivity of natural seawater, most of the high-performance HER catalysts reported so far can only exhibit excellent performance under acidic or alkaline seawater conditions; catalysts that function effectively in neutral seawater still require further research^[39-41]. Considering that adding auxiliary reagents during seawater electrolysis not only increases electrolysis costs but may also affect electrode lifespan, developing HER catalysts that perform efficiently under neutral conditions is of great significance for direct seawater electrolysis^[42].

Constructing special catalyst surface structures through electrode morphology control is an effective approach to reduce alkaline scale deposition. Kang et al.^[35]fabricated highly dispersed conical nanobeam arrays on copper meshes using anodic oxidation, and further immersed the copper mesh to form a hydrophobic interface. The nanobeam arrays with a hydrophobic interface exhibited unique anti-scaling performance. As shown in Figure 5a,due to the tip effect, active sites are mainly concentrated at the tips of the nanobeams, where water molecules decompose, and the generated H₂forms a bubble layer on the hydrophobic surface, preventing Ca²⁺and Mg²⁺from contacting the cathode surface. Alkaline scale crystals form only at the nanoscale tips and accumulate layer by layer. When the crystal particles grow to micrometer size and continue to stack, the adhesion between the crystals and anchor points weakens. Meanwhile, continuously generated H₂escapes from the cathode surface and ruptures at the liquid-air interface. The vibration caused by the rupture disrupts the mechanical equilibrium between the crystals and the tip anchor points, enabling the alkaline scale crystals to automatically detach from the nanotips. By repeatedly carrying out this process, the cathode achieves scale-free functionality. Similarly, Yu et al.^[43]modified nickel phosphide (Ni_xP) microparticle arrays with nickel-cobalt nitride (NiCoN) nanoparticles, obtaining a hierarchical nanostructured catalyst NiCoN|Ni _xP|NiCoN. Thanks to its hierarchical 3D nanostructure, the catalyst allows for rapid diffusion of generated bubbles, thereby inhibiting the deposition of cathodic scale. It demonstrates excellent HER catalytic stability in neutral seawater, as shown in Table 1.The above studies also indicate that constructing nanostructures on the catalyst surface can effectively promote mass transfer processes, thus suppressing scale deposition on the catalyst surface. Cai et al.^[44]developed a honeycomb-like three-dimensional electrode NiCoP/PC. Acting as a microbubble/precipitate transport system, it continuously and uniformly releases small-sized H₂bubbles to achieve in-situ removal of scale. This electrode exhibits outstanding electrocatalytic performance and stability in both alkaline and neutral seawater environments, operating stably for over 1000 hours at a current density of 1 A·cm^-2.

The essence of cathodic alkaline scale formation lies in the drastic pH change near the cathode surface, where large amounts of Ca²⁺ and Mg²⁺ combine with OH^- at the cathode surface, leading to scale deposition. In addition to the above-mentioned approach of regulating catalyst surface morphology and improving catalysts to optimize their surface microenvironment, this is also an effective method to prevent cathodic alkaline scale deposition. The electrostatic repulsion effect is frequently applied in studies on chlorine corrosion inhibition on the anode side^[45-46]. By adding anionic groups such as phosphate, sulfate, and borate to the electrolyte system, Cl^- can be effectively prevented from reaching the anode surface and competing with OER^[47]. Similarly, utilizing this phenomenon to repel Ca²⁺ and Mg²⁺ can partially inhibit the deposition of alkaline scale. Liang et al.^[44] fully mixed the organic superbase DMAN with the cathode catalyst, leveraging the electrostatic repulsion effect. DMAN acts as a proton sponge, absorbing and storing large amounts of H⁺, thereby generating a significant repulsive effect on Ca²⁺ and Mg²⁺ in seawater, which partially inhibits the formation of alkaline scale (Figure 5b). However, the anti-scaling effect of this strategy is limited, and alkaline scale still deposits on the cathode surface during long-term seawater electrolysis tests.

Currently, research on seawater electrolysis hydrogen production catalysts still focuses primarily on suppressing chlorine corrosion on the anode side. Most catalysts used in neutral seawater environments can only fully demonstrate their effectiveness at low current densities^[48-50]. However, to meet future hydrogen energy demands, developing catalysts that perform well at high current densities is of great significance for future seawater electrolysis hydrogen production. Kasani et al.^[51]summarized the overpotentials of several seawater electrolysis catalysts at high current densities of 1000 and 500 mA·cm^-2, as shown in Table 2and Figure 6. It is evident that current research on seawater electrolysis catalysts continues to focus on alkaline seawater environments while also progressing toward higher current densities. At high current densities, apart from Pt-Co-Mo, various Ni-based catalysts exhibit excellent performance, suggesting that developing high-performance catalysts based on Ni could be a promising research direction in the field of seawater electrolysis catalysts.

As mentioned above, the alkaline scale deposited on the cathode is essentially a precipitate formed by the combination of Ca²⁺, Mg²⁺, and OH^- from seawater at the electrode surface. In addition to electrode morphology engineering, constructing a protective layer on the cathode surface to prevent the binding of alkaline scale-forming ions can also effectively eliminate the deposition of alkaline scale.

Yi et al^[60] constructed a special "hydrophobic-solid" layer on the surface of NiCu alloy. The formation of this layer benefits from the high surface energy of the alloy, which promotes water adsorption. A large number of water molecules adsorb onto the alloy surface, forming a complete hydrogen-bond network and subsequently creating a dense hydrated layer, known as the "hydrophobic-solid" layer, as shown in Figure 7a. The presence of this dense hydrated layer significantly increases the energy barrier for Mg²⁺ penetration, forcing it to nucleate uniformly in the electrolyte and preventing it from reaching the electrode surface, thereby effectively reducing alkali scale deposition. Accelerated durability test results indicate that the base material Ni experiences voltage rise due to alkali scale deposition on the cathode surface after only 10 minutes of electrolysis in the simulated solution. In contrast, the presence of the "hydrophobic-solid" layer ensures stable catalyst performance over prolonged electrolysis periods, achieving stable operation for thousands of hours at a current density of 100 mA·cm^-2, as shown in Table 3. Similarly, to prevent Mg²⁺ from reaching the cathode surface, Liu et al^[61] first verified through quantitative experiments and XRD analysis that the primary ion causing alkali scale deposition is Mg²⁺ (Figure 7b). To address the deposition issue, they then in situ coated a layer of Ni(OH)₂ ion-separating layer on nickel foam (NF). The working mechanism of this separating layer is based on electrostatic repulsion: since the surface of the Ni(OH)₂ film carries a positive charge, it effectively blocks the transport of Mg²⁺ to the cathode surface. Meanwhile, the nanoscale cracks on its surface accelerate the mass transfer of OH^- and H₂O required for HER, resulting in higher catalytic performance compared to the Ni electrode. The NF electrode modified with this film can reduce precipitation by approximately 98.3% during electrolysis in natural seawater and exhibits operational stability exceeding 100 hours at a current density of 10 mA·cm^-2.

As mentioned above, the construction of the electrode protective layer has significantly inhibited alkali scale deposition; however, the protective layer may have some negative impact on the electrode's catalytic performance. For instance, an excessively thick protective layer could increase interfacial mass transfer resistance and reduce the kinetics of the hydrogen evolution reaction. Additionally, an overly dense protective layer might partially cover the active sites on the electrode surface, leading to an increase in overpotential. In practical design, material innovation can be employed to minimize these negative effects of the protective layer, thereby achieving a balance between catalytic performance and anti-deposition capability.

From the perspective of managing OH^-, the pH in the micro-regions on the electrode surface can be regulated to consume the excess OH^- produced on the surface, thereby inhibiting the deposition of alkaline scale. Unlike physical anti-scaling methods such as adjusting the morphology of the catalyst surface or constructing an anti-scaling layer on its surface, regulating the pH in the micro-regions on the cathode surface to neutralize OH^- is a chemical process that fundamentally disrupts the necessary conditions for alkaline scale formation. Compared to these other methods, this approach offers more stable operation and significantly superior descaling performance.

The regulation of local pH on the cathode surface is achieved by creating an H⁺-rich or OH^--rich environment near the cathode. Guo et al.^[62] captured OH^- by introducing a Lewis acid layer on the surface of transition metal oxide catalysts. Due to the strong binding interaction between Lewis acids and OH^-, Mg²⁺ and Ca²⁺ in seawater would find it difficult to combine with them on the electrode surface to form precipitates (Figure 8a). Meanwhile, thanks to their strong binding with OH^-, a strongly alkaline microenvironment is established within the electrical double layer, and even the small amount of dissociated OH^- will be neutralized by buffering ions such as HCO₃^- in seawater, thereby preventing the rapid increase of pH to some extent and inhibiting the deposition of alkaline scale. This inhibition strategy achieved long-term stability for more than 100 hours in untreated natural seawater and demonstrated an industrial-level current density of 1.0 A·cm^-2 in a flowing electrolytic cell, confirming its effectiveness in suppressing alkaline scale deposition. Different from the above-mentioned research, Han^[15] explored the effect of the porous cathode/bipolar membrane (BPM) interface on slowing down the formation of inorganic precipitates in direct seawater electrolysis systems, proposing an optimized design of the interface between the porous electrode and BPM in zero-gap seawater electrolyzers to mitigate alkaline scale deposition (Figure 8b). Han introduced open areas on the BPM that do not come into contact with the porous cathode, facilitating the diffusion of unreacted free protons and protons generated from water dissociation at the BPM to the cathode surface, thus creating a localized acidic environment at the cathode. This strategy helps further acidify the seawater, slows down the rise of pH on the cathode surface, and inhibits the formation of inorganic precipitates.

In the field of hydrogen production via water electrolysis, pulsed electrolysis is often employed to enhance mass transfer at the electrode interface during the electrolysis process. However, due to the presence of numerous impurity ions such as Cl^-, Mg²⁺, and Ca²⁺ in seawater, pulsed power supply is rarely used for seawater electrolysis. Here, we summarize a novel electrolysis method that achieves pH balance in the electrolytic system by periodically reversing the electrode polarity. This method has been extensively studied in wastewater treatment, and its working principle involves using the H⁺ generated after polarity reversal to neutralize the OH^- produced by the electrode before reversal, thereby achieving in-situ pH regulation, as shown in Figure 9. Zhou et al.^[63] applied the concept of polarity reversal electrolysis to water softening, demonstrating through continuous experiments over 20 cycles that periodically reversing electrode polarity can effectively remove hardness ions such as Mg²⁺ and Ca²⁺, extending electrode lifespan while reducing energy losses and safety hazards caused by scale deposition on the electrode surface. Similarly, Jin et al.^[64] also utilized polarity reversal to remove scale deposits from the electrode surface in situ. The difference is that Jin et al. quantitatively monitored changes in scale deposition on the electrode surface within a single polarity reversal cycle under ultra-high current densities ranging from 100 to 250 A·m^-2. The experimental results indicate that this electrolysis method can effectively remove scale deposits, reduce softening energy consumption, and improve operational stability, thus proving the applicability of polarity reversal under high current density conditions.

The aforementioned literature demonstrates the effectiveness of polarity reversal in removing scale, but does not explicitly address its inhibitory effect on alkali deposit formation during seawater electrolysis for hydrogen production. It is undeniable that there are certain similarities between the "alkali deposits" formed on the cathode surface during seawater electrolysis and the "scale" deposited on electrode surfaces during hard water softening. First, in terms of compositional makeup, both primarily consist of Mg(OH)₂and Ca(OH)₂; second, from a generation mechanism perspective, both "scale" and "alkali deposits" result from the substantial accumulation of OH^-, which causes a sharp increase in pH at the electrode surface, thereby inducing the precipitation of Mg²⁺and Ca²⁺through combination with OH^-. As shown in Figure 10, Yan et al.^[65]were the first to apply the concept of polarity reversal to the field of hydrogen production via water electrolysis. They designed a membrane-free electrolytic cell with two independent chambers connected by a Ni²⁺/Ni³⁺redox couple, which facilitates chamber connection and gas separation. To ensure the continuous and stable operation of the redox couple, it is necessary to periodically reverse the electrode polarity to achieve the cyclic conversion of Ni²⁺/Ni³⁺. Since the electrode material used is Ni-based and exhibits poor resistance to chlorine corrosion, this exploration was conducted entirely under alkaline electrolyte conditions and has not yet been applied to seawater electrolysis. However, this work indirectly highlights the potential of using polarity-reversal electrolysis for hydrogen production via water electrolysis, suggesting that applying this electrolysis method to descaling the cathode in seawater electrolysis represents a promising future research direction. Of course, frequent polarity reversals can disrupt the microstructure of the catalytic layer, leading to the detachment of active sites on the electrode surface and reducing electrode lifespan^[6,66]. Therefore, how to mitigate the "damage" caused by such reversals still requires further discussion and research.

Traditional electrolyzers such as alkaline water electrolyzers (AWE), PEM water electrolyzers (PEMWE), AEM water electrolyzers (AEMWE), and high-temperature solid oxide electrolyzers (SOEC) have already achieved quite high performance requirements when electrolyzing conventional pure water-based electrolytes^[67-69]. However, they remain largely unsuitable for electrolytes containing numerous contaminants and impurities, such as seawater. As mentioned earlier, most current seawater hydrogen production systems operate in an alkaline environment, and thus predominantly utilize traditional AEM electrolyzers. Jin et al.^[70] summarized the issues associated with directly applying traditional electrolyzers to seawater electrolysis (Figure 11). In general, traditional electrolyzers or electrolysis systems were not designed with consideration of the negative impacts caused by impurity ions in seawater, leading to problems such as electrode corrosion, electrode material blockage, and membrane fouling during electrolysis. Regarding the issue of cathode or membrane blockage due to alkali scale deposition, this section summarizes three strategies reported so far.

First, traditional seawater electrolyzers commonly employ a symmetric feed approach to achieve electrolysis efficiency comparable to state-of-the-art electrolyzers. However, this conventional feeding method struggles to cope with the significant corrosion and deposition of Cl^-, Ca²⁺, Mg²⁺, and other substances in seawater on electrode materials and membranes, and still faces challenges in terms of flexibility in regulating electrolyte composition. In contrast, adopting an asymmetric feed supply can effectively address the issue of alkali scale deposition on electrode or membrane surfaces. Shi et al.^[71] designed a pH-asymmetric electrolyzer using a sodium ion exchange membrane as a separator, which prevents the transport of Cl^- toward the anode and OH^- toward the cathode, thereby effectively alleviating Cl^- corrosion and the deposition of Ca²⁺ and Mg²⁺ (Figure 12a). Additionally, this electrolyzer circulates NaCl solution or natural seawater in the cathode compartment and NaOH solution in the anode compartment. This pH-asymmetric design reduces the overpotential of the two half-reactions in seawater electrolysis, lowering energy consumption. The circulating flow also accelerates the renewal rate of the double-layer components, mitigating the deposition of Ca²⁺ and Mg²⁺. The asymmetric feed approach can also be implemented by circulating electrolytes between the anode and cathode compartments. During electrolysis, large amounts of OH^- and H⁺ inevitably accumulate in the anode and cathode compartments, respectively, resulting in a significant pH gradient between the electrodes, reducing electrolysis efficiency and promoting alkali scale deposition. Kato et al.^[72] designed a special seawater electrolysis circulation system based on a traditional PEM electrolyzer, where seawater flowing out of the anode compartment is sent to the cathode compartment for electrolysis. The OH^- generated during the HER process at the cathode neutralizes the H⁺ in the liquid flowing from the anode, thus reducing the pH gradient. Similarly, reducing the pH gradient significantly lowers energy consumption and disrupts the external conditions conducive to alkali scale deposition (pH > 9.5), partially alleviating alkali scale accumulation (Figure 12b). Likewise, Rossi et al.^[73] designed a novel PEM electrolyzer that uses vapor feed on the anode side and simulated seawater feed on the cathode side. This unique feeding method reduces the competitive transport of Na⁺ against H⁺ across the membrane, enhancing the mass transfer efficiency of H⁺ within the system and significantly lowering the pH gradient compared to the traditional symmetric feed approach. Badreldin et al.^[74] designed a membrane-free electrolyzer that also employs an H⁺-rich anolyte to alleviate the localized pH increase on the cathode surface (Figure 12c), further demonstrating the feasibility of the asymmetric feed approach in seawater electrolysis.

Another strategy to address cathodic alkaline scale deposition is to pre-treat natural seawater to completely isolate Ca²⁺ and Mg²⁺ from the electrolysis system. Seawater purification is also a key preliminary step in hydrogen production from seawater, with reverse osmosis (RO) being one of the commonly used membrane processes for seawater purification. However, to overcome the effect of osmotic pressure, this process requires applying high pressure on one side of the membrane, resulting in relatively high energy consumption. In contrast, forward osmosis (FO), a technology with higher energy efficiency, has emerged as a cost-effective alternative for seawater purification prior to electrolysis. Veroneau et al.^[75] proposed an electrolyzer design utilizing FO for water electrolysis to produce hydrogen, which can directly use brine without any pretreatment or purification, thereby avoiding the negative impacts and energy waste caused by impurity ions, as shown in Figure 12d. Veroneau's water splitting system consists of an electrochemical compartment separated from the external brine by a semi-permeable membrane. The compartment is filled with a higher-concentration buffer solution at pH=7, which is separated from the external NaCl solution by an acetate cellulose semi-permeable membrane. Under the driving force of the concentration gradient, water molecules continuously transfer into the compartment, thus achieving in-situ purification of the brine. Furthermore, Veroneau et al.^[76] replaced the external NaCl solution with untreated natural seawater and demonstrated that when the water splitting rate and forward osmosis rate reach equilibrium, continuous water splitting effectively maintains the concentration gradient, ensuring the sustained in-situ purification of seawater. The FO-based direct seawater electrolysis system for hydrogen production exhibits high flexibility and scalability, making it suitable for large-scale H₂ collection from seawater.

Building upon FO-driven seawater purification, Xie et al.^[77]introduced another innovation. Their designed electrolysis system uses concentrated KOH as a self-regulating electrolyte (SDE) and hydrophobic porous polytetrafluoroethylene (PTFE) as a semi-permeable membrane. Unlike previous systems, the two sides of the semi-permeable membrane in this setup are no longer a traditional concentrated solution and a dilute solution, but rather a concentrated solution and water vapor (Figure 12e). This design enables the preferential diffusion of water vapor driven by the vapor pressure difference between seawater and the SDE, while completely preventing the penetration of liquid seawater and impurity ions. Specifically, seawater on the seawater side spontaneously vaporizes, and the water vapor diffuses through short gas channels within the membrane to the SDE side, where it is re-liquefied via absorption by the SDE—a "liquid-gas-liquid" phase transition process. This allows for the in-situ generation of pure water from seawater for electrolysis, while the water consumed during electrolysis maintains the interfacial pressure difference. Experimental results demonstrate that this system can stably produce hydrogen from natural seawater and operate continuously for 3200 hours at a current density of 250 mA·cm^-2.

In addition to the seawater electrolyzers operating in low-temperature environments mentioned above, high-temperature solid oxide electrolysis cells (SOECs) driven by thermal energy can also be used to alleviate the issue of alkaline scale deposition. In high-temperature SOECs, water evaporates into steam, which is transported to the cathode to produce H₂. The generated O^2- ions pass through the solid oxide membrane to reach the anode, where O₂ is produced^[78]. Lim et al.^[79] studied the effects of pure water vapor and seawater vapor on hydrogen production by simulating seawater electrolysis with steam. Their research found that the initial performance and degradation rates of pure water vapor and seawater vapor electrolysis were nearly identical. Seawater electrolysis using solid oxide electrolysis cells can stably operate at a current density of 0.8 A·cm^-2 for 70 hours. Furthermore, in high-temperature environments, part of the energy required for water splitting to produce hydrogen can be supplied by thermal energy, thus reducing electricity consumption to some extent and providing certain advantages over low-temperature electrolyzers. However, there are still some challenges associated with seawater electrolysis using solid oxide electrolysis cells, such as electrode lifespan and the damaging effects of trace impurities in seawater vapor on the electrolyzer. Liu et al.^[80] used SOECs to electrolyze natural seawater, demonstrating that seawater vapor can effectively prevent the deposition of alkaline scale; however, during long-term operation, trace amounts of salt in the vapor can still cause blockage of the intake pipes or the electrolyzer.