Metal–Organic Frameworks for Seawater Electrolysis and Hydrogen Production: A Review

Abstract

Electrolysis utilizing renewable electricity is an environmentally friendly, non-polluting, and sustainable method of hydrogen production. Seawater is the most desirable and inexpensive electrolyte for this process to achieve commercial acceptance compared to competing hydrogen production technologies. We reviewed metal–organic frameworks as possible electrocatalysts for hydrogen production by seawater electrolysis. Metal–organic frameworks are interesting for seawater electrolysis due to their large surface area, tunable permeability, and ease of functional processing, which makes them extremely suitable for obtaining modifiable electrode structures. Here we discussed the development of metal–organic framework-based electrocatalysts as multifunctional materials with applications for alkaline, PEM, and direct seawater electrolysis for hydrogen production. Their advantages and disadvantages were examined in search of a pathway to a successful and sustainable technology for developing electrode materials to produce hydrogen from seawater.

1. Introduction

Climate change issues are forcing countries to take measures without affecting economic development and prosperity. Carbon-neutral targets by 2050 on energy, transportation, and industry require substantial investment in renewable energy, energy efficiency, energy storage, and alternative fuels. Future climate scenarios have promoted hydrogen as one of the solutions for energy, transportation and industrial fuel (118 MJ/kg energy content) along with renewable energy. Since it is not available naturally, the cleanest way to produce it to utilize renewable energy along with electrolyzer technologies, which is key for decarbonization of several sectors that drive economic development and sustainable growth. There are different electrolysis technologies dictated by the pH of electrolyte. Both acid and base electrolytes operate at 1.23 V total voltage. However, high pH with alkaline electrolyte brings down electrode voltages lower, and therefore the use of non-precious metals becomes possible as electrodes. Existing Proton Exchange Membrane (PEM) electrolyzers are challenged by the high cost of Nafion membrane and electrocatalyst. Alternatively, the Anion Exchange Membrane (AEM) water electrolyzer is one of the most promising electrolysis technologies. However, the present membrane and electrocatalyst are performing below the requirement, and further development is desired to enhance the efficiency of the electrolyzer and bring its material from idea to technology demonstration. Two areas that are commercially competitive are chlor-alkali electrolysis and aluminum production. Now that carbon taxes have been implemented, hydrogen from electrolysis may become competitive with fossil fuel technologies by 2030. Using seawater to produce hydrogen is a promising approach that is low-cost and sustainable [1,2]. Clean hydrogen has attracted a lot of interest and support, with increasing global projects and scientific development strategies. There are several inherent technological challenges with the main types of water electrolysis systems, including size, corrosion and high energy consumption, which are affecting the efficiency and the durability of electrolyzer cells. Use of a noble metal catalyst in the electrodes is another challenge which affects the electrolyzer system integration and production costs [3].

The main challenge of seawater electrolysis comes from the complexity of seawater components sediment, microbes, and complicated ion species. Furthermore, the physical and chemical characteristics of natural seawater are subject to seasonal variations and geographic location, rather than a set composition. The purity of seawater electrolyzers is mostly impacted by sediment and microbes, which can be eliminated through pretreatment. Many ions, such as Cl⁻, SO₄²⁻, Br⁻, F⁻, Na⁺, Mg²⁺, Ca²⁺, K⁺, Sr²⁺, Cu²⁺, Cd²⁺, HCO₃⁻, and CO₃²⁻, found in seawater, however, are difficult to filter (Figure 1). The impact of these ions needs to be taken into account in seawater electrolysis [4].

The water feedstocks used in commercially available water electrolysis devices are usually freshwater resources. However, the growing human population, increasing water demand, pressure from industrial expansion and anthropogenic climate change poses a growing threat to global freshwater systems [5]. Seawater, which is an infinite resource, can be applied in hydrogen production technology. Compared with pure water electrolysis, saltwater electrolysis is still in its infancy due to the complexity of the electrochemical processes and the limitations imposed by the properties of seawater. Therefore, multiple seawater purification techniques are required, including desalination, deionization, and purification [6].

During the electrocatalytic seawater splitting process, both the hydrogen evolution reaction and the oxygen evolution reaction are significant half-reactions. Noble metal oxides have been used as potential catalysts, such as platinum for hydrogen evolution reaction (HER) and RuO₂ for oxygen evolution reaction (OER). However, they have two disadvantages: (i) the high cost and limited supply of materials prevent large-scale implementation; and (ii) the loading of catalysts onto glassy carbon requires the use of binders and additives, which improves electrocatalytic performance and increases contact resistance [7].

In comparison to traditional metallic transition metal oxides, metal–organic frameworks (MOFs) are thought to be a useful material for constructing porous structures with increased electrochemically active surface area and an enhanced activity–stability factor. However, the unique metal coordination and constrained mass transfer kinetics in basic MOFs prevent water molecules from diffusing correctly. MOF coordination has been enhanced by pyrolysis, creating more porous reactive sites that enhance gas desorption and ion diffusion. The activity and antitoxicity of the active sites in Cl⁻ containing electrolytes are improved by altering the catalyst surface with active oxyhydroxide. Researchers have discovered that the electrocatalyst’s specific surface area and mass transfer capacity are determined by its porous structure. Fast mass transfer kinetics are ensured by the 3D-ordered porous structures with distinctive surface properties, which also increase exposed surface areas, permit interfacial interaction with the active core sites, and encourage electrolyte penetration to lessen ion diffusion and electron transfer paths [8].

This review article discusses the difficulties in producing pure hydrogen by seawater electrolysis as well as the use of MOFs as appropriate electrocatalysts in direct, alkaline, and PEM seawater electrolysis. We also discussed challenges and future perspectives for designing effective MOF-based electrocatalysts for seawater electrolysis to produce hydrogen.

2. Seawater for Green Hydrogen Production

Seawater electrolysis is a technology that has potential for obtaining clean energy as green hydrogen (Figure 1). Developing efficient electrocatalysts is very important for hydrogen production through direct seawater electrolysis [9]. Green hydrogen production from water is one of the best options because it does not make pollution or cause climate change when used. Since there is an abundance of seawater, using seawater electrolysis to produce hydrogen is also the cheapest and easiest way. Designing effective, bifunctional electrocatalysts requires careful consideration of precursor and additive selection, as materials development for sustainable energy conversion technologies emphasizes [10].

Seawater electrolysis is a technology full of potential for generating hydrogen from an abundant source of water (Figure 2). The most challenging issue utilizing seawater as a feed in the electrolyzer is the presence of several cations and anions that will take part in the electrochemical reactions either to corrode or precipitate on the electrode [11,12,13]. The lifetime of the electrodes and electrolyzers will be shorten significantly, eliminating their potential for commercial deployment.

The ultra-low conductivity and the presence of bacteria, microbes, and stray ions in seawater generate delayed kinetics that impede efficient seawater splitting [14]. The extremely corrosive nature of complex marine environments, such as wave erosion, seawater spray, and seawater corrosion, as well as the attachment of numerous ocean microorganisms and large marine organisms like mussels and mollusks, pose significant challenges for electrodes used in seawater electrolysis [15,16]. MOFs have become a novel class of materials with wide-ranging applications in a variety of domains, such as chemical sensing, corrosion protection, and catalysis, because of their easily configurable pore configurations and unique structural features. Through the combination of active coating protection and passive protection, MOFs serve as a container for the storage and release of corrosion inhibitors, enhancing the material’s resistance to corrosion [17,18,19,20,21]. Among many alternative structures of materials, MOFs are a current research interest as electrocatalysts for seawater electrolysis.

The Content of Salts in Seawater

Seawater possesses high electrical conductivity (33.9 ms cm⁻¹ at 25 °C) and neutral conditions (pH 6.44), taking up 75% of the earth [22]. The content of seawater is complicated and varied in different areas. The main solute in seawater consists of various salts. The composition of the surface seawater of the Atlantic Ocean has been studied and it has been found that potassium (K⁺), sodium (Na⁺), chloride (Cl⁻) magnesium (Mg²⁺), sulfate (SO₄²⁻), and calcium (Ca²⁺) ions took up more than 99% of the total amount of salts in seawater [23]. The mentioned ions, while enhancing the conductivity of seawater and encouraging the splitting of seawater, also hindered the reaction. To smooth the tests, simulated seawater has been extensively applied. Numerous researchers have adopted 3.5% NaCl solution as simulated seawater samples by complying with the global average salinity [24,25,26,27]. Other researchers have reported that they added Na₂S, Na₂SO₃ or glycerol to the NaCl solution as sacrificial agents [20,21,22,23,24] or introduced more magnesium, calcium, potassium, bromine, and sulfate salts to simulate natural seawater more practically [28,29,30,31,32,33,34,35].

3. Metal–Organic Framework Electrocatalysts for Seawater Electrolysis

MOFs are special materials made by coordinating metal centers with organic ligands. These coordination forms are big, repeating structures that create multidimensional networks. MOFs have unique structural properties such as tunable pore sizes, high surface areas, and open metal sites that allow potential applications as catalysts, gas storage, sensors, magnetism, and luminescence [36,37,38,39,40,41]. MOFs are coordination structures of repeating metal complexes that can be electrochemically active depending on the type of metal ligands and ions. This can be used to design electrochemically functional frameworks with good electrocatalytic activity and electrochemical properties (Figure 3) [42,43].

To improve catalytic efficiency, highly reactive molecules can be incorporated and distributed into the pores in MOFs. Intrinsic activity and active sites are crucial for electrocatalytic water splitting. The large number of active centers increases the activity, which is necessary to give the electrocatalysts strong electrocatalytic activity [44]. Additionally, the guest and host exhibit synergistic actions that assist the electrocatalytic activity. Even if the addition of highly active molecules can reduce the individual surface area of the MOF, the combined effects will be sufficiently effective [45]. Particularly when the catalytic molecules are enclosed within MOFs, the porous structure facilitates the electrolytes’ penetration into the interior active sites, increasing the percentage of available active sites and speeding up the catalytic performance of MOFs. An increase in external activity can result in a rise in electrocatalytic activity, which can help alleviate the transport issues brought on by excessive catalyst loading [46]. The difference in pore size between mesoporous materials and microporous zeolites has been believed to be bridged by MOFs, which have been extremely diverse structures compared to conventional catalysts such as zeolite, activated carbon, etc., and have extremely controllable pore sizes (typically 0–3 nm–9.8 nm). The constant pores and shape have made reaction catalysts of specific shapes and sizes accessible, allowing MOFs to selectively catalyze reactions. Their extremely high surface area (BET, typically > 1000 m² g⁻¹, up to 7000 m² g⁻¹) has further increased the adsorption and aggregation of molecules at the active sites, further enhancing the resulting activation and catalytic conversion. Lattice strain engineering can enhance the regional coordination environment of atoms to produce synergism of active sites and quick tuning of electrocatalysts, which had benefits for superior bifunctional water splitting. Specifically, MOFs’ extremely crystalline form gives them an ordered structure, which is essential for understanding basic processes and catalytic efficiency. Molecular-level oxidation site engineering is a crucial component of post-electrochemical structure change and has been successfully applied to achieve high electrochemical performance [47]. The development of electrocatalysts with significant water oxidation efficiency requires the dynamic construction of adaptive heterojunctions while incorporating lattice strain to enhance the redox activity of transition metal catalysts [48]. Furthermore, several methods of designing porous structures, including pore space partitioning, metal atom dropping, and hydrophilic sheets, are crucial for the catalytic activities of MOFs [49]. Following pore separation, MOFs’ tiny pores may result in improved guest-guest interactions and enhanced catalytic activity. Interestingly, new heterometallic species can be created in MOFs using the pore partitioning technique, which can also increase the catalytic activity of MOFs [50].

At least two techniques can be used to expand the variety of catalytic reactions in which MOFs can take part: post-synthetic modification, functionalized modification utilizing a multivariate method, or encapsulation or confinement of pores [51]. The pore space of MOFs can host a number of additional catalytically active molecules as guests and work as a nanoreactor, for example, metal complexes, inorganic nanoparticles, organic molecules, etc. [52]. Nowadays, another popular strategy to enhance the conductivity and electrocatalytic properties of MOFs is to load them onto conductive frameworks like nickel foam, carbon cloth, and tin-doped indium oxide-coated glass substrates using solvothermal, electrochemical, or direct deposition techniques [53]. MOFs can be coated on rigid, porous, and highly conductive templates to create a huge mass transport, many active catalysis sites, and highly conductive pathways. The created structures will have a number of advantages over conventional materials. For instance, the calcination of the MOF will produce highly active moieties of the template surface, and the template will assist in maintaining the porosity for mass transport and permeable roads. Consequently, there is a chance to achieve a mix of rapid mass dissemination, structural stability, and high activity. High activity, high porosity, structural stability, and high conductivity are important factors to consider while designing MOF catalysts [54].

MOF structures are integrated with suitable diffusion media and current collectors based on the type of electrolyte used. Ni-plated stainless steel mesh and diffusion media are used for alkaline and AEM electrolyzers. PEM electrolyzers utilize Pt-coated titanium structures for corrosion resistance and mechanical stability under pressure. Catalysts are either uploaded onto the porous transport layer or on the membrane. Nafion or anion exchange ionomers are used as binders for electrode fabrication (Figure 4).

• MOFs possess high surface area and porosity, which provides additional active sites for catalytic reactions, making them very promising for advancing sustainable energy solutions.
• The tunable chemical structure of MOFs by modifying its surface with various metal nodes and organic linkers allows tailoring the structure depending on the further application.
• MOFs are very versatile, as they can be easily functionalized with various metal nanoparticles, improving catalytic sites and performance in hydrogen production.
• MOFs can be easily synthesized by green synthesis methods, facilitating scalable and environmentally friendly production [55].

The first step in designing MOFs is choosing the two primary interacting elements: metal and bridging organic ligands. Coordination polymers must be synthesized in a single step, mainly through self-assembly, since once they are generated, they are insoluble (Figure 5). Commercially accessible starting chemicals are used nearly exclusively in MOF research since they are quick and affordable to prepare. Some research teams have used conventional organic synthesis and unconventional in situ synthesis to design and create their own organic compounds. In situ modifications have been made to ligand precursors, which then attach to the metal centers. The process could involve hydrolysis, alkylation, hydroxylation, decarboxylation, etc. [56]. In conventional synthesis, the organic ligands are first prepared, and self-assembly of the MOF follows at a later stage. Post-modification of existing bridges in a ready-made frame has also been used [57].

Despite being extremely stable in pure water electrolysis, conventional electrocatalysts are too costly for broad commercial use. Conventional oxide catalysts are contaminated by side reactions or salt precipitation because seawater conditions have a lot of side reactions. Rather than using pure Pt, Vidales and Semali suggest Ptx/Ni_0.6Mo_0.4-oxide electrodes for HER in acidic environments. Improved performance is anticipated when such catalysts are made using MOF-based structures. MOF architectures with composite structures are hence preferable [59].

MOFs are a new class of electrocatalysts for electrochemical energy conversion reactions such as OER, oxygen reduction reaction (ORR), and HER and have attracted the interest of many researchers. Many MOFs have been investigated for their electrocatalytic properties, such as Fe, Co, and Ni-based [54], Cu-based [42], bimetallic [60,61], inkjet, and other MOFs [62] (see

Table 1. MOF-based Electrocatalysts for Seawater Electrolysis.

MOF-Based Electrocatalysts	Results Obtained	Ref.
NiRu-PTA/NF	Have been used directly for alkaline seawater electrolysis as a bifunctional HER/OER electrocatalyst. At a very low overpotential of 16 mV and 298 mV, respectively, the NiRu-PTA/NF, a self-supporting electrode, has efficiently catalyzed both the HER and OER to produce 10 mA cm−2 for the HER and 20 mA cm−2 for the OER. Furthermore, following a 200-h chronopotentiometry test, the NiRu-PTA/NF//NiRu-PTA/NF integrated water electrolysis cell with NiRu-PTA/NF as both cathode and anode in a minimal cell configuration demonstrated excellent long-term stability with an unattenuated cell voltage and a very low cell voltage of 1.54 V to drive the current density of 10 mA cm−2.	[63]
NiCo/MXene	The hybrid seawater electrolyzer has completely eliminated the chlorine risks to cell performance in neutral or alkaline seawater by enabling hydrogen production at ultralow cell voltages of 0.7–1.0 V. Meanwhile, stable seawater electrolysis for 140 h at 500 mA cm–2 with high Faradaic efficiency can produce hydrogen at an intense rate of 9.2 mol h–1 gcat–1. Compared to seawater electrolyzers, the electricity cost has been significantly decreased by 30–52% at a high current density of 500 mA cm–2.	[64]
NiCo-LDH nanostructures	The electrocatalyst has demonstrated low overpotentials of 356 mV for HER and 433 mV for OER at 400 mA·cm−2 in alkaline seawater, which is significantly better than the majority of documented non-noble metal catalysts. With remarkably low cell voltages of 1.56 and 1.89 V at current densities of 10 and 400 mA·cm−2, respectively, the derived CF electrode loading of CCH@NiCo LDH has demonstrated exceptional performance as anodes and cathodes for total alkaline seawater electrolysis. Furthermore, in alkaline seawater, the strong stability of 100 h has also been shown at levels exceeding 200 mA·cm−2.	[65]
NiCo/MOF	Ni0.15Co0.85-MOF material has revealed exceptional electrocatalytic performance, as evidenced by low values of overpotential (1.33 V vs. RHE at 10 mA cm–2), TOF (0.47 s–1), and Tafel slope (125 mV dec–1). At a 40 mA cm–2 current density, Ni0.15Co0.85-MOF also has shown excellent stability during the 72 h tests.	[66]
CC@CoNC	Using the optimized CC@CoNC as both cathode and anode, the hydrazine-assisted water electrolysis system has been successfully completed. To reach 200 mA cm−2, it requires only an ultra-low cell voltage of 0.557 V and an electricity usage of 1.22 kW h per cubic meter of H2. Additionally, in the hydrazine-assisted seawater electrolysis system for H2 production, the optimized CC@CoNC has demonstrated much enhanced stability, operating steadily for more than 40 h at ≈10 mA cm−2.	[67]
CdFe-BDC	The electrocatalyst has possessed outstanding overpotentials of 290 mV at 100 mA cm–2 and 148 mV at 10 mA cm–2, respectively. HER and OER have demonstrated the lowest performance. In real seawater media, the entire water splitting performance needs 1.68 V to reach a current density of 10 mA cm–2, and at ambient alkaline conditions, it has produced H2 and O2 at competitive rates of 6.4 and 3.1 μL s–1, respectively.	[68]
NiFe-LDH/MOF	Seawater oxidation has required just 235 and 307 mV OER overpotentials to reach current densities of 20 and 100 mA·cm−2, respectively, with nearly no attenuation for a 100-h stability test at 20 mA·cm−2.	[69]
Ir@NiFe-MOF/NF	With overpotentials of 445 and 233 mV and a current density of 1000 mA cm−2, the material has shown remarkable alkaline seawater OER and HER characteristics. It also outperforms other previously reported bifunctional electrocatalysts, operating steadily for 400 h in alkaline seawater and requiring just 2.11 V to drive 250 mA cm−2 in a membrane electrode device.	[70]
MIL-(IrNiFe)@ NF	The electrocatalyst has worked really well and lasts a long time when helping to split water into hydrogen and oxygen in seawater. It only has needed a little bit of power (1.9 volts) to produce a lot of hydrogen.	[71]
αNPANI/MIL53(Al)	Scientists have found that adding both naphthylamine and a MOF to PANI helped it produce a lot of hydrogen better. They also have tested how well this new material has worked for at least 48 h without breaking down.	[72]
FeNi-LDH/MIL-88A	The electrocatalyst has worked really well, producing a good amount of energy. The electrocatalyst has also been tested for a long time—more than 200 h—and has been found to have stayed strong and worked just as well the whole time, even when using seawater.	[73]
Pt2/Ni(OH)2/NF	The catalyst has shown an excellent HER performance, requiring only 283 mV at 1000 mA cm−2 and high stability for 200 h.	[74]
3D B-NiCoP hollow nanorod	In alkaline natural seawater electrolytes, 3D B-NiCoP hollow nanorod arrays have been reported to be capable of driving a cathodic current density of 10 mA cm–2 at an overpotential of 98 mV. In seawater electrolytes, the electrocatalyst has exhibited outstanding long-term stability for over 85 h at a high current density of 113 mA cm–2. Furthermore, in natural seawater, the 3D B-NiCoP hollow nanorod array electrocatalyst has demonstrated exceptional HER activity and stability.	[75]

).

4. Electrochemical Green Hydrogen Production by Seawater Splitting

Water electrolysis, as an important way for hydrogen generation, is widely used in hydrogen energy production due to its environmental friendliness and sustainability [76]. One possible technique for creating a cleaner, greener future is splitting water and producing hydrogen using electricity derived from renewable energy sources. Hydrogen is a clean and sustainable energy carrier since it has the highest gravimetric energy density. A global goal is to develop seawater electrolysis technology as a sustainable way to produce and transform hydrogen into electrical energy in the future (Figure 6).

One of the extensively researched carbon-free methods for producing green hydrogen is water splitting via an electrochemical process called electrolysis. A single cell with two electrodes (cathode and anode) that are either electrically linked or immersed by an electrolyte or membrane is called an electrolyzer [77]. Hydrogen and oxygen are produced by passing electricity through the electrolyte for electrolysis [78]. An OER and/or chlorine occur at the anode, and an HER occur at the cathode, respectively. The O₂ and chlorine are released back into the atmosphere, while the H₂ is then used as a fuel [79]. The basic reaction of water electrolysis is as follows in Equation (1) [80]:

H₂O + Heat (48.6 kJ mol⁻¹) + Electricity (237.2 kJ mol⁻¹) → H₂ + 1/2 O₂

(1)

At room temperature, the thermodynamic cell’s theoretical voltage is 1.23 V, but the practical voltage needed for efficient separation falls between 1.3 and 1.48 V. The extra energy is given to overcome the material’s ohmic resistance and reaction kinetic fluctuations. By storing electrons as H₂, which may either be utilized as fuel or transformed back into electricity, electrolyzers turn electrical energy into chemical energy [81,82]. Water splitting is still not cost-effective when compared to traditional sources, though, because of the high energy input costs and the efficiency of hydrogen production. The electrolysis of water is undoubtedly an important reaction, as seen by the positive Gibbs free energy value; typically, the process overcomes a sizable kinetic barrier [83,84,85]. Electrode design is crucial for reducing the kinetic barrier and increasing the overall efficiency of water splitting. One of the major tasks facing scientists is the creation of highly active electro-catalysts that are resistant to charge transfer, inexpensive, and use less energy [86].

MOFs represent a promising class of electrocatalysts for seawater electrolysis, because of their remarkable catalytic qualities, customizable structure, and high porosity. They are perfect for HER and OER in seawater separation because of their special structure, which consists of metal nodes synchronized with organic linkers. This structure offers an extremely high surface area, a large number of active sites, and superior ion transport channels. The abundance of active sites, especially metal clusters (e.g., Co, Ni, Fe) and nitrogen-rich linkers, which improve HER and OER performance; structural tunability, which allows them to be transformed into metal oxides, carbides, phosphides, or single-atom catalysts, further improving their stability and activity in seawater electrolysis; and high surface area and porosity, which improve the mass transport of reactants and intermediates, thereby improving the overall reaction kinetics. In spite of these benefits, pure MOFs frequently have low electrical conductivity and limited stability in challenging electrochemical circumstances. In order to get around these restrictions, scientists have created MOF derivatives and composites that have far better catalytic performance. MOF-based electrocatalysts have been synthesized using a variety of methods. In hydrothermal and solvothermal synthesis, metal salts react with organic linkers under regulated pressure and temperature to create extremely crystalline structures with adjustable pore widths. In order to improve conductivity and stability, MOFs can be directly pyrolyzed into metal/nitrogen-doped carbon (M-N-C), metal oxides, or phosphide catalysts. Charge transfer and mechanical stability are improved by in situ growing on conductive supports such as indigo, graphene, or nickel foam. Bifunctional electrocatalytic activity for both HER and OER is improved by metal doping and hybridization techniques that involve extra metal atoms, such as Ni, Fe, or Mo. NiFe-MOFs, for instance, have shown remarkable OER performance in alkaline seawater [87].

The efficiency of MOF-derived catalysts in tackling the main issues of seawater electrolysis, such as long-term stability, high current density operation, and resistance to chloride corrosion, has been demonstrated in recent investigations. Because of their strong Co-P bond, which improves HER kinetics, MOF-derived cobalt phosphide (CoP)-based catalysts have excellent inherent activity and endurance in alkaline seawater. NiFe-MOFs are also very effective anode materials for direct seawater splitting, with reports showing an OER overpotential of only 296 mV at 1000 mA cm⁻². Moreover, under actual seawater circumstances, single-atom catalysts (SACs) made from MOFs, namely Fe-SAC and Co-SAC supported on nitrogen-doped graphene, have shown remarkable stability and activity, attaining high turnover frequencies and long-term durability. These successes demonstrate how MOF-based electrocatalysts have the potential to transform seawater electrolysis for environmentally friendly hydrogen production [88].

The corrosive marine environment, which is characterized by high salinity, chloride ion attack, and electrochemical deterioration, poses serious stability problems for nanostructured electrocatalysts used in seawater electrolysis [89]. Aggressive chloride ions (Cl⁻) hasten the structural degradation of electrocatalysts, which eventually results in catalyst deactivation and reduced operational efficiency [90]. Metal leaching is a major problem that impairs catalyst longevity, particularly in catalysts based on transition metals like Ni, Fe, and Co. The catalytic efficacy of these metals is diminished by their propensity to dissolve during extended electrolysis [91].

Nanoparticle aggregation presents another difficulty since smaller catalytic particles have a propensity to group together and create larger agglomerates. By decreasing the active surface area, this aggregation lowers the catalyst’s overall efficiency. Furthermore, despite having a high intrinsic catalytic activity, single-atom catalysts (SACs) are prone to clumping and migration, which causes a quick loss of active sites [89]. The long-term use of these materials in seawater electrolysis is eventually limited by the performance degradation that is further aggravated by the dissolution of metal nanoparticles into the electrolyte [90]. Often, electrode passivation—the formation of a protective but non-conductive layer by insoluble metal hydroxides and oxides—causes electrocatalyst deterioration. Efficiency is decreased and overpotential is increased by this layer [92].

Moreover, seawater electrolysis produces hydroxyl and hypochlorite radicals, which speed up the breakdown of catalytic materials by contributing to their structural deterioration. Frequently employed to spatially confine metal atoms, MOFs and covalent organic frameworks degrade in alkaline seawater conditions, resulting in hydrolysis and framework collapse. The catalyst’s lifespan is further constrained by this [88]. Electrocatalysts that are exposed to seawater conditions on a regular basis have significant electrochemical and chemical deterioration, which affects the efficiency of the HER and OER [93].

Research on Ni-Fe-based catalysts shows that surface oxidation in settings with a lot of chloride causes phase transitions that reduce the catalysts’ conductivity and reactivity [90]. Efficiency is eventually decreased by metal hydroxides that are formed as a result of prolonged high current density operation. These hydroxides block active sites and decrease mass transport [91]. When exposed to the severe conditions of alkaline saltwater, high-performance HER catalysts, such MoS₂ and Ni-Mo alloys, experience surface rearrangement and a decrease in catalytic activity [94].

One of the most promising and difficult methods for achieving extremely effective electrochemical separation of seawater is the sensible modification of MOFs’ chemical components. In seawater electrolysis, MOFs with multimetallic components can have three-dimensional hierarchical nanostructures that offer a large number of active sites, high intrinsic catalytic activity, and quick electron transport [95]. For natural seawater electrolysis, have been synthesized a hierarchical flower-like structure of RhCoNi-MOF as a highly stable and active electrocatalyst. With a voltage requirement of 1.52 V at 10 mA cm⁻², their RhCoNi-MOF can be utilized as a bifunctional catalyst for thorough seawater separation and have been exhibited stability for over 80 h [96].

The OER should oxidize H₂O to O₂, but in real seawater conditions, the presence of about 0.5 M Cl⁻ provides a competing pathway: two-electron chlorine evolution reaction (CER) (2Cl⁻ → Cl₂ + 2^e−) or subsequent hypochlorite formation under alkaline conditions (Figure 7) [97,98]. Kinetically and thermodynamically, this poses a serious problem. The standard potential for CER (E° ≈ 1.36 V vs. SHE at pH 0) is close to that of the OER (E° ≈ 1.23 V vs. SHE), and interfacial conditions and overpotentials often shift these values into a range where CER becomes significantly easier to initiate. The CER is a more straightforward two-electron transfer than the 4-electron OER, which includes several intricate proton-coupled electron transfer steps, including the creation of O-O bonds. Because of this complexity gap, halogen oxidation is kinetically more accessible, enabling Cl₂ evolution to fiercely contend and, in certain cases, dominate [99]. Based on the mechanism and working conditions, seawater electrolysis can be divided into three main technologies: alkaline electrolysis initiated by OH⁻ ions; seawater electrolysis of a proton exchange membrane initiated by H⁺ ions; seawater electrolysis by direct split water 2H₂O → 2H₂ + O₂.

4.1. MOF-Based Approaches to Prevent Chloride-Induced Corrosion and Undesired Side Reactions During Seawater Electrolysis

Seawater electrolysis, as an alternative to freshwater electrolysis, has become a viable and sustainable technique for producing hydrogen on a big scale. However, obstacles like corrosion caused by chloride, catalyst degradation, and competition between the OER and CER prevent it from being widely used [86].

MOFs have garnered attention in recent decades as possible electrocatalysts for seawater electrolysis. However, because to intrinsic difficulties such low chemical stability and restricted electrical conductivity, current studies are still far from reaching commercial needs, despite its potential. The conductive MOFs, which combine high porosity with effective charge transport to provide qualities for creating extremely stable and active electrocatalysts. Regretfully, the majority of conductive MOF electrocatalysts have trouble achieving long-term stability and industrial current density [100].

4.1.1. Enhanced Electrostatic Repulsion Force

The seawater electrolysis is significantly hindered by the presence of Cl⁻ions poses challenges in terms of stability and selectivity for OER. Seawater containing chloride ions (Cl⁻) can cause metal chloride precipitates and CER or hypochlorite formation on the anode. Despite being thermodynamically unfavorable, CER and hypochlorite formation are kinetically beneficial. They so actively compete with the targeted OER, resulting in higher overpotentials and, ultimately, higher energy consumption. The produced chlorine and hypochlorite are dangerous and poison the catalysts, which lowers the electrolysis process’s efficiency considerably [101,102,103,104].

The corrosion resistance of NF as an anode electrode has been enhanced by the addition of sulfate, extending operating stability. According to the research, sulfate anions have formed a negative charge layer on the anode surface through preferential adsorption, which electrostatically repels the chloride ions from the anode. In comparison to the conventional alkaline saltwater electrolyte, it was discovered that the adsorbed sulfate might result in stability that was roughly five times higher while maintaining a comparable OER activity [105,106]. OER selectivity is improved by phosphate anions’ preferential adsorption on the anode surface, which creates a negatively charged layer and further repels Cl-ions. This reduces corrosion of the anode electrocatalyst because of the electrostatic repulsion effect [107].

4.1.2. Design of MOF Electrocatalysts

The design of electrocatalysts with high activity and stability is essential for enhancing the electrolysis cell’s hydrogen generation performance, which is constrained by the harsh oxidation and corrosion environment of the OER in seawater electrolysis [108].

The OER and the CER compete with one another in seawater electrolysis. Because OER involves a four-electron transfer and CER involves a two-electron transfer, the CER side reaction is kinetically more advantageous than OER. Thermodynamically, in the lower pH range, hypochlorite production is accompanied by a progressive increase in the thermodynamic barrier as pH rises. With a potential difference of 480 mV, the thermodynamic barrier reaches its maximum and the CER takes place when the pH rises over 7.5. Chloride ions also have a tendency to adsorb on the anode during direct seawater electrolysis, resulting in metal-chloride coordination that corrodes the anode substrate and causes electrode deactivation see Equations (2) to (4) [109,110,111].

M + Cl⁻ → MCl + e⁻

(2)

MCl + Cl⁻ → MCl⁻_x

(3)

MCl⁻_x + OH⁻ → M(OH)_x + Cl⁻

(4)

Designing OER-selective catalysts with greater activation energies for Cl⁻ oxidation than for water splitting is one of the best ways to reduce CER. For instance, by altering the local electronic structure, doping with heteroatoms like Mo or S might enhance the OER kinetics while also raising the barrier to CER [112]. Moreover, Cl⁻ adsorption and corrosion susceptibility are lessened by self-passivating oxyhydroxide layers that are created during electrolysis (for example, on NiFe or CoFe LDH) [113,114]. Hierarchical and porous nanostructured catalysts further inhibit CER by enhancing water availability and impeding the transfer of Cl⁻. By separating the active sites from corrosive anions, these physical architectures—particularly hollow and core–shell systems—help to promote effective and selective water oxidation [115,116,117,118].

4.1.3. Functionalizing MOFs with Lewis Acids

In comparison to alkaline seawater, neutral seawater has lower activity for both OER and HER catalysts. Therefore, in order to achieve the promising current density in practice, greater applied voltages are needed. In this instance, detrimental chloride oxidation/corrosion occurs, further diminishing the total electrolysis capacity and negatively eroding the catalyst. Moreover, the pH of the seawater close to the cathode rises noticeably during the electrolysis process, which causes a lot of insoluble precipitates to develop because of the presence of Mg²⁺, which physically inhibits the cathode.

Accordingly, a Lewis acid layer can be added to a transition metal oxide catalyst to enable the dynamic separation of water molecules and the trapping of hydroxyl anions. Both electrodes’ reaction kinetics are favored by the in situ production of localized alkalinity. Additionally, OH⁻ is preferentially enriched to withstand the arrival of Cl⁻ on the catalyst surface, preventing the chemical interaction with Cl⁻ on the catalyst surface because of the existence of localized alkalinity. Additionally, because of its high affinity for the Lewis acid layer, OH⁻ is much less likely to be captured by Mg²⁺ and Ca²⁺ cations in the seawater electrolyte, which lessens the issue of scale development [119].

4.2. MOF-Based Electrocatalysts for Alkaline Seawater Electrolysis

Alkaline seawater electrolysis (ASWE) is used to produce pure hydrogen fuel from salt water. Despite the applications that this method of producing hydrogen it has a number of disadvantages such as sophisticated equipment that can be damaged by salt water and uses a lot of energy. These issues make the process less efficient and cause the parts that help the reaction to wear out faster. Also, they often need special, expensive metals to help the process work, which can make the whole system cost more [3]. As shown in Figure 8, these reactions occur at the cathode and anode surfaces, respectively, and are preferably separated by an ion exchange membrane. Among other indicators and parameters such as pressure, temperature, electrode type, electrolyte type and concentration, and cell configuration, both OER and HER are very sensitive to the pH of the electrolyte. Under alkaline conditions, OH^- generated from water at the cathode surface act as electrochemical charge carriers. The oxidation of OH^- at the anode surface leads to the formation of O₂ and electrons, which are further used to generate H₂ at the cathode surface. According to Faraday’s law of electrolysis, electrochemical reactions occur at the interface between the electrode surface and the electrolyte, and the rate at which gas evolves is exactly proportional to the current passing through the electrical circuit [120].

ASWE is a new and exciting way to make green hydrogen, which is a clean and green energy sources for the environment. But there are some problems that need solving before it can be used widely. These problems include making the technology cheaper, work better, and be easier to expand. One big problem is that it costs a lot of money to build the system, especially the part called the electrolyzer, which is like a machine that splits water into hydrogen and oxygen. The electrolyzer can be very expensive because it uses special materials like nickel (Ni) and iridium (Ir), which are costly. Another problem is that the process does not always work as efficiently as we would like. It only uses about 60 to 70% of the energy it gets, which means a lot of energy is wasted during the process [121]. This inefficiency is caused by a number of variables, such as the alkaline solution’s poor conductivity, the high overvoltage needed for electrolysis, and the electrolyzer’s increased power consumption. Furthermore, the energy capacity that can be supplied to the electrolysis cell limits the electrolyzer’s size. As a result, ASWE is currently employed for small-scale hydrogen generation applications, like those found in labs and research facilities. Furthermore, the electrolysis cell’s narrow operating temperature range limits ASWE’s potential for producing green hydrogen. Usually run between 60 and 80 degrees Celsius, the electrolysis process becomes more efficient as the temperature rises [122]. Consequently, ASWE should not be used in cold locations or in situations where the temperature drops below the electrolysis cell’s operational range. The electrolysis process operates at pressures ranging from 1 to 3 bar, and its efficiency decreases as the pressure drops. As a result, ASWE cannot be used in high-pressure settings like those found in fuel cells or the chemical industry. The progressive attrition of the electrodes and membrane over time is the cause of the electrolysis cell’s reduced lifespan. Another barrier to ASWE for green hydrogen production is the scarcity of alkaline solutions. Large quantities of alkaline solutions, such as NaOH or KOH, are necessary for the electrolysis process but can be costly and time-consuming to obtain [123]. MOF-containing materials studied for alkaline seawater electrolysis are presented in

Table 2. MOF-based Electrocatalysts for alkaline seawater electrolysis.

MOF-Based Electrocatalysts	Results Obtained	Ref.
(Ni@ZIF67)-(Ni–Co–CoO@C)	In comparison to the ZIF67-derived nanocomposites, the material has demonstrated good HER activity and significantly increased OER activity. In alkaline seawater, the NZ700 catalyst has demonstrated the best OER activity (η10 = 281 mV versus RHE) among the catalysts, whereas the NZ500 catalyst has shown the best HER activity (η50 = 196 mV vs. RHE).	[124]
NiCoP-MOF	A NiCoP-MOF catalyst based on carbon paper has been used to electrolyze natural seawater for the acid-free in situ generation of hydrogen or alkaline additives due to its strong electrocatalytic activity (overpotential = 166 mV and Tafel slope = 181.2 mV dec–1).	[125]
Ru@CoNi-MOF	At a current density of 10 mA/cm2, the catalyst has needed an overpotential below 47 and 279 mV in order to reach OER and HER, respectively. Notably, Ru@CoNi-MOF’s mass activity for OER and HER was 25.9 and 10.6 mA mg−1, respectively, almost 15.2 and 8.8 times greater than Ni-MOF’s.	[126]
NiFe-based MOF	The material has demonstrated remarkable OER performance in alkaline seawater settings, with an η40 of 285 mV, in addition to a surprisingly low overpotential (η200) of just 286 mV at a current density of 200 mA cm−2 in 1 M KOH solution. Furthermore, after responding for 100 h at a high current density of 200 mA cm−2 in alkaline seawater, NFN-MOF/NF only showed 2.3% and 4.8% chronopotentiometric degradation.	[127]
Ni–Co-MOF	Because of their high voltammetric charge density and enhanced electrochemically accessible active surface, the electrodes have shown remarkable performance and endurance in ASWE. By focusing on kinetic parameters, electrochemical impedance spectroscopy analysis has examined the kinetics of the water oxidation reaction in the presence of Cl– ions (at concentrations ranging from 0.5 M to 3.5 M). The results have indicated that the chemical process following the initial electron transfer was the step that determines the rate.	[128]
Pt2/Ni(OH)2/NF	In seawater splitting, the catalyst has demonstrated exceptional catalytic activity. The overpotential was higher than the commercial 20% Pt/C, measuring 19 mV at 10 mA cm−2. A seawater electrolyzer using a Pt2/Ni(OH)2/NF cathode catalyst and an AEM has a cell voltage of just 1.46 V at 10 mA cm−2. At a current density of 200 mA cm−2, the energy consumption for generating 1 m3 H2 was 3.8 kW h, which was less than that of NF = |NF (4.3 kW h).	[74]
Ni-BDC/NH2-MIL-88B(Fe)	A long stability of 200 h and low overpotentials of 232 and 299 mV at 100 mA cm−2 in seawater solutions have been demonstrated by this effective OER electrocatalyst for extremely efficient seawater electrolysis.	[129]
Pt/T-NiFe-BDC	In alkaline seawater, the material has demonstrated competitive HER activity, achieving ultralow overpotentials of 158 and 266 mV at 500 and 1000 mA cm–2 with exceptional stability and quick kinetics. In a 500-h continuous test at 500 mA cm–2, an asymmetric water electrolyzer using Pt/T-NiFe-BDC as the cathode demonstrated no attenuation and only needed a voltage of 1.89 V to generate an industrial density of 1000 mA cm–2.	[130]
NH2–NiCoFe-MIL-101	In electrolytes based on natural seawater, the bifunctional electrode has demonstrated exceptional stability and catalytic activity. Remarkably, the NH2-NiCoFe-MIL-101 two-electrode urea-assisted alkaline natural seawater electrolysis cell required only 1.56 mV to produce 100 mA cm–2, which is significantly less than the 1.78 V required for alkaline natural seawater electrolysis cells. It also has shown excellent long-term stability at a current density of 80 mA cm–2 for 80 h.	[131]
Ru-CoMo-LDH	Ru-CoMo-LDH∥Pt/C has demonstrated excellent electrochemical performance (i.e., overpotentials of 1.5545 and 1.731 V to generate current densities of 10 and 200 mA cm–2, respectively) and great stability in a comprehensive water splitting test conducted in natural seawater.	[132]
NiFe-VOx/NF	According to electrochemical tests, in alkaline natural seawater, the improved NiFe4-VOx/NF have needed only 285 mV overvoltage to reach a current density of 100 mA cm−2. Additionally, in alkaline natural seawater, the catalyst has functioned steadily at high current density for at least 100 h.	[133]
NC-CoNi2S4@ReS2/CC	In comparison to the control samples, thhe material has shown lesser overpotentials of 87 and 253 mV for OER and HER at 10 mA cm−2, as well as a lower Tafel slope and Rct. A 56-h CA test and 1000 cycles of cyclic voltammetry have been used to verify the higher catalytic stability. Furthermore, because of the sulfur particles’ resistance to corrosion at the interface, NC-CoNi2S4@ReS2/CC has demonstrated remarkable electrocatalytic activity in both alkaline and saltwater electrolytes.	[134]
4-GInMx@NF	The material has exhibited outstanding performance, achieving low HER overpotentials (η) of 110 and 104 mV with Tafel slopes of 76 and 83 mV/dec at current density (J) of 10 mA/cm−2 in natural seawater.	[135]
P-Ni4Mo/CF	In alkaline seawater, the catalyst has demonstrated significant HER performance and stability, with an overpotential as low as 260 mV at a current density of 100 mA cm−2. At an overpotential of 551 mV, the P-Ni4Mo/CF achieved 1.0 A cm−2, which was marginally less than that of the Pt/C catalyst (453 mV). Furthermore, following more than 200 h of durability testing, P-Ni4Mo/CF has shown strong durability with virtually no activity loss.	[136]

.

4.3. MOF-Based Electrocatalysts for Proton Exchange Membrane Seawater Electrolysis

Proton exchange membrane (PEM) seawater electrolysis is key for transforming and depositing excess renewable energy (Figure 9). PEM electrolysis provides advantages over alkaline water electrolyzers, including a high responsiveness, high current densities, large dynamic range, and pressures. High operating pressures are important because they contribute to reducing the costs and energy use related to downstream mechanical shrink [137]. Despite that, noble metal electrode catalysts and other components in PEM seawater electrolyzers prevent the popularization of this technique due to reserve issues and cost [138]. To increase electrolytic efficiency and lower the cost of PEM electrolyzers, significant efforts are now being made to create improved electrocatalysts [139].

The seawater electrolysis technique has been used to build PEM electrolyzers, which can manage acceptable current densities at high applied input voltages. 99.995% pure hydrogen can be produced using the PEM electrolyzer. PEMs, or Nafion-based membranes, are used in electrolyzers to isolate the anode from the cathode. The mechanical strength, conductivity, thermal and chemical stability, and stability under wildly fluctuating pressure and temperature are all highly intriguing characteristics of these membranes. The primary drawbacks of the PEM electrolyzer with regard to overall endurance are the PEM conductive membrane and the expense of the electrocatalyst [86,140,141,142,143].

High-purity hydrogen gas, which may be utilized in fuel cells and other industrial sectors, can be produced in PEM seawater electrolysis systems utilizing green power provided from renewable energy sources such as wind or solar. Significant progress has been made recently in increasing the productivity of PEM seawater electrolysis through the design of stack components; however, problems with their long-term use and large-scale implementation still exist because of their high cost and durability in acidic environments [144]. According to recent studies, there is increasing interest in introducing MOFs into PEMs to increase their proton conductivity. Nevertheless, the rules for choosing appropriate MOFs for creating high-performing composite membranes have remained tricky due to the vast quantity of MOFs [145]. MOF-containing materials studied for PEM seawater electrolysis are presented in

Table 3. MOF-based Electrocatalysts for polymer membrane seawater electrolysis.

MOF-Based Electrocatalysts	Results Obtained	Ref.
BPM Fe- MOF	Over the MEA containing 1.0 mg∙cm−2 of Fe-MOF, the greatest hydrogen production rate of 1.45 L∙h−1 has been recorded at 500 mA∙cm−2, which has been observed over the different current density range of 0–500 mA∙cm−2.	[146]
NiFe-LHD	The material has demonstrated exceptional long-term stability over 9000 h under 1.0 A cm−2 in alkaline natural seawater, as well as an industrial-level current density of 1.0 A cm−2 at overpotentials of 200 and 220 mV in alkaline simulated (1 M KOH + 0.5 M NaCl) and natural (1 M KOH + seawater) seawater, respectively.	[147]
NF/Ni3N@NiFe-PA	The NF/Ni3N@NiFe-PA has exhibited notable OER activity in seawater thanks to its regulated electronic state caused by the synergism between Ni and Fe species and enhanced proton-coupled electron transfer through accelerated proton movement with the help of phytic acid, which has been incorporated as a proton transfer relay, and has enhanced mass transfer provided by special superhydrophilic and superaerophobic properties.	[148]
Ni0.1Co0.9Se2/ NCHP	At a current density of 10 mA cm–2 in 0.5 M H2SO4, the material has demonstrated a low overpotential of 89.8 mV and a tiny Tafel slope of just 48.3 mV dec–1. Additionally, it might have shown remarkable qualities in both 0.1 M PBS and 1.0 M KOH environments at the same time. With Ni0.1Co0.9Se2/NCHP as the cathode, the PEM electrolyzer has demonstrated exceptional stability (500 mA cm–2@100 h) and a notably high critical current density (1.83 V@500 mA cm–2).	[149]
PVDF-modified PEM	A PEM with a high sulfonic acid density that has been modified by PVDF has been created. In liquid water electrolysis, vapor-phase water electrolysis, and direct seawater, the membrane containing 25 weight percent Nafion™/sulfonated Quino-PVDF (also known as QuinoCEM-0.25) has demonstrated good performance. It has achieved maximum current densities of 130, 480, and 240 mA cm−2 over a cell voltage of 1.8 V at 80 °C, respectively.	[150]

.

4.4. MOF-Based Electrocatalysts for Direct Seawater Electrolysis

Seawater would not only be a sustainable supply but also help avoid the bigger space needs and auxiliary procedures involved with system engineering if it could be split directly into renewable electricity without the need for reverse osmosis purification. because the component primarily impacted by the pollutant in seawater is the membrane. The technology may eliminate the high expenses related to sensitive PEM or AEM because it is membrane-less. Additionally, membrane-less electrolyzers will simplify the research strategy by allowing researchers to concentrate just on OER selective electrode design [151].

Because of the reaction’s slow kinetics and high activation barrier, extra electrical energy is typically needed to propel the process. The minimal voltage needed for seawater electrolysis is 1.23 V [29]. However, for practical use, the cell must have a voltage greater than 1.23 V. The pH of the electrolyte affects the chemical process [152]. Furthermore, seawater splitting is hampered by the presence of dangerous impurity ions, microbes, and tiny particles, which prevent hydrogen from being produced. The generation of hydrogen at the cathode, for example, generates a lot of OH⁻, which makes the Mg²⁺ and Ca²⁺ ions more likely to form Mg(OH)₂ and Ca(OH)₂ precipitates on the cathode. As a result, the active site of the catalyst becomes blocked, which lowers the catalytic activity [37,38]. The hydrogen plant’s lifespan is also affected by the hazardous chloride oxidation process (ClOR), which takes place at the anode [13,153,154].

Direct seawater electrolysis (DSWE) creates an opportunity toward low-cost H₂ and O₂ using plentiful seawater feedstocks (Figure 10). However, high concentrations of ions available in seawater complicate electrolyzer functionalization. Cl⁻ ions, for example, can oxidize to corrosive and toxic byproducts. This poses a challenge for the safe and long-term operation of seawater electrolysis apparatus [155].

DSWE is a sustainable technology for green hydrogen production. Nevertheless, the use of this technology remains a major challenge due to limited lifetime and poor catalytic activity, side reactions, and metal deposits resulting from corrosion associated with CERs [156]. DSWE allows a viable solution to the excessive consumption of seawater resources in existing industrial seawater electrolysis. In spite of electrolysis efficiency and material stability are affected by electrode material corrosion caused by CERs on the anode surface [157]. MOF-containing materials studied for direct seawater electrolysis are presented in

Table 4. MOF-based Electrocatalysts for Direct seawater electrolysis.

MOF-Based Electrocatalysts	Results Obtained	Ref.
S-Ni-MOF/Fe-MOF	The material has achieved low overpotentials of 281 and 279 mV at 100 mA cm−2 current density and has remained stable for at least 100 h.	[157]
(Cr2O3–CoOx)	At 500 mA cm−2, the direct electrolysis of actual seawater that has not been acidified nor alkalized produced results that have been stable for 100 h. Lewis acid-modified electrodes (Cr2O3–CoOx) in a flow electrolyzer for natural seawater have demonstrated the industrially necessary current density of 1.0 A cm−2 at 1.87 V and 60 °C.	[158]
Fe2P–NiCoP	In seawater media, the material has demonstrated outstanding OER catalytic activity and stability. With a Faraday efficiency of about 100%, the electrolyzer has attained a current density of 10 mA cm−2, Pt/C || Fe2P–NiCoP only needs the ultra-low voltage in saltwater media (1.525 V).	[159]
MOF/Fe2O3	At an overpotential of 410 mV, the material has achieved a current density of 1 A cm−2, which was approximately 200 percent greater than that of IrO2 that is employed in commercial settings. Because of its selective anodic reaction and anti-corrosive operating mode, the heterostructured catalyst has demonstrated long-lasting performance against chlorine corrosion for over 350 h at a higher current density of ∼1.5 A cm−2.	[160]
2D Ni Nanoplates	Compared to RuO2, Ni nanoplates have demonstrated significantly lower cell voltages of 267 and 393 mV at current densities of 500 and 1000 mA cm–2. Interestingly, a durability test at 100 mA cm–2 revealed that the cell voltage changed very little over the course of 90 h.	[161]
RuFe-MOF-PA60	This material has outlasted commercial Pt/C||IrO2 systems in terms of durability and has shown stable operation for 70 h at an industrial current density of 50 mA cm−2. The catalyst outperformed the majority of MOF-based bifunctional systems, achieving ultralow overpotentials of 255 mV (OER) and 70 mV (HER) at 10 mA cm−2.	[162]
Pt@FeCoNi phosphide nanosheet arrays	The material has outperformed conventional Pt/C by accomplish an ultralow overpotential of 17 mV at −10 mA cm−2. It has also steadily delivered industrial-level current densities up to 2000 A m−2 for more than 2400 h with low energy consumption (4.16 kWh/Nm3 H2) and no voltage deterioration.	[163]
Ce–Co(OH)2@FeOOH	The system has shown exceptional stability, sustaining electrolysis for 400 h in high-salinity conditions (2 M NaCl) and for 2500 h at 2 A cm–2.	[164]

.

5. Challenges and Future Perspectives

Developing efficient, cost-effective electrocatalysts is crucial for efficient hydrogen production using seawater electrolysis. MOFs pose significant advantages owing to their extensive area, tunable permeability, and ease of functional processing to develop novel, high-performance, robust, and sustainable electrocatalysts for seawater electrolysis. While MOFs exhibit unique properties, including pore size modifications, high surface areas open metal sites, and high durability in harsh environments, some challenges should be addressed regarding the MOF-based electrolysis systems: (i) poor conductivity of pristine MOFs, (ii) stability of MOF-based catalysts in real seawater conditions, and (iii) the use of scalable synthesis routes for practical applications. Understanding the reaction mechanism is crucial to alleviating the mentioned challenges. Binary and ternary hybridization of MOFs with conductive materials or transformation of MOF-structures into different conductive metal oxides, phosphides, sulfides, or carbides with high electrocatalytic activity are some of the main approaches to alleviate the low conductivity issue. The future focus on MOF-based seawater electrocatalysts should be directed to synthesizing intrinsically conductive MOF structures with high electrocatalytic activity.

While seawater offers significant advantages as a green hydrogen source due to its abundance, the electrolysis process encounters a crucial problem due to the presence of chloride ions (Cl⁻) as an interfering ion in seawater. At the anode of the electrolyzer, Cl⁻ ions are involved in the chlorine CER, resulting in the evolution of chlorine gas, which has detrimental effects on both the membrane and the catalyst. The formation of chlorine gas decreases the electrolyzer efficiency and degrades the membrane and the catalysts. In addition, the produced chlorine gas has a toxic nature. Different approaches have been implemented to address this issue, including chemical modification of the electrolyte and catalysts [165]. MOFs can be utilized to mitigate or alleviate the CER issue in this regard. Some reports have shown that MOF-based electrocatalysts with tailored active sites and electronic structures exhibit higher selectivity for OER than CER. Fathima et al. for instance, designed an electrocatalyst derived from ZIF67 to be used in seawater electrolysis [124]. In the given work, the authors prepared a zeolite imidazole framework (Ni@ZIF67) to obtain nickel–cobalt–cobalt oxide nanoparticles embedded in amorphous carbon nanocomposite and evaluated the electrochemical performance as well as the chlorine resistance. The composite electrocatalysts exhibited high OER and HER performance and retained their stability after operating in alkaline seawater for 170 h.

Additionally, it is worth noting that the use of alkaline seawater also suppressed the CER. Another MOF-derived nanocatalyst of Mo4N-Ni3N was also used for seawater electrolysis, and the durability of the catalyst layer was assessed against CER [166]. The electrocatalysts exhibited potentials of 330 and 319 mV at 500 mA/cm² in alkaline freshwater for OER and HER, respectively, which are lower than the critical potential value for CER, making it possible to achieve chlorine-free seawater electrolysis. It should be considered that the theoretical potential of the CER is 480 mV in 1.0 M KOH. Therefore, the enhanced electrocatalytic activity of MOFs and MOF-derived nanocomposites, with their abundant active sites and tunable physical properties, is of great importance in alleviating or preventing chlorine formation in seawater electrolysis. Another approach for CER suppression is the use of MOF-based electrocatalysts to exclude Cl⁻ ions, preventing their transport to the anode. MOFs with a smaller pore size than the adequate size of Cl⁻ ions can act as a sieving material by preventing the Cl⁻ transport while allowing the transport of smaller OH⁻ ions. In addition, the surface charge of MOFs can be exploited to expel the Cl⁻ ions [167,168].

The d-band theory is used to explain and predict the catalytic activity of transition metal catalysts. Based on the d-band theory, the electrocatalytic activity and the selectivity of the catalysts are highly dependent upon the d-band energy position relative to the Fermi level [169,170]. For the case of the CER, if the d-band energy level is closer to the Fermi level, the adsorption of the reaction intermediates on the active sites is strong, resulting in promoted CER activity since Cl⁻ ions can adsorb strongly on the active sites of the catalyst. In the reversed condition, when the d-band level is considerably lower than the Fermi level, the reaction intermediates adsorb the surface weakly, yielding a sluggish OER performance [171]. Therefore, the d-band level of the catalysts should be tailored to an optimum energy level, allowing high OER activity while limiting the chlorine gas formation. One of the first approaches to having a moderate d-band energy level to have enhanced OER and limited CER is choosing the metal nodes used in the MOF structure. For example, some researchers displayed that the NiFe-MOF shows higher OER activity due to the tuned electronic structure [172,173].

Wan et al. demonstrated the change in the d-band center of NiFe-MOF catalysts when Cr atoms are doped as the modifier. The Cr-NiFe-MOF catalysts exhibited high OER activity and durability. The authors explained the enhanced electrocatalytic activity in terms of the change in the d-band level. The introduction of Cr atoms into the NiFe system resulted in a replacement of Ni atoms with the Cr dopant in the vicinity of Fe sites, resulting in a lower d-band center and a concomitant optimum adsorption behavior of the reaction intermediates. Besides tuning the composition of the metal nodes in MOFs, the band of the MOF-based catalysts can also be tuned by ligand functionalization. In this approach, ligands with electron donor or withdrawing properties result in a change in the d-band level. For instance, while –NH₂ and –OH can increase the d-band level, –F, –NO₂, and -SO₃H can lower the d-band level [174,175,176]. Therefore, by engineering the surface, structure, and electrochemical activity of MOF-based electrocatalysts, the CER issue in seawater electrolysis can be alleviated.

Density functional theory (DFT), artificial intelligence (AI), and machine learning (ML) approaches play pivotal roles not only in alleviating the challenges of MOFs in seawater electrolysis applications but also in boosting the performance of the catalysts by finely tuning the catalyst structure and composition. For example, as discussed, adsorption energies of the reaction intermediates are of great interest to improve the reaction rate and efficiency and to design novel catalysts with high Cl⁻ tolerance. The d-band level of the catalysts and the adsorption energies of the reaction intermediates to the active sites can be predicted as a function of the applied modification strategy. Therefore, not only the electrochemical performance but also the selectivity of the catalysts can be predicted before their synthesis. Based on computational results, the intended composition may be modified to achieve higher catalyst performance. In addition, the effect of the defects or dopants on the catalyst performance can be evaluated theoretically using DFT approaches. The properties of MOF-based catalysts and the corresponding electrochemical behavior of performance can be assessed using machine learning and artificial intelligence approaches. In addition, by employing a reverse direction approach, new MOF-based catalysts with the desired properties can be designed for seawater electrolysis.

Rinawati et al. found that the MOF-derived NiFe-LDH has been shown to have excellent activity, owing to a low overpotential of 299 mV for reaching a current density of 10 mA cm⁻², a low Tafel slope of 48.7 mV dec⁻¹, and good stability over 24 h compared with pristine NiFe-LDH (319 mV), RuO₂ (333 mV), and Ni-25 (395 mV). According to the obtained result, it can be expected that this strategy can provide a new perspective for the design of MOF electrocatalysts, increase the range of MOF-derived structures, and explore higher-performance electrocatalysts [177].

For the electrocatalysts we have chosen in this study, the aim is not to compare them with each other but to emphasize their relevance and significance in future scientific and practical applications for seawater electrolysis. There is data in the literature on stability and overpotential. It has been suggested that an energy-band engineering approach be used to fully comprehend the connections between asymmetric electronic structure and activity in MOFs. A new approach to the design of effective catalysts for the OER has been provided by the local symmetry breaking and adsorption/desorption capabilities of the reaction species, which suggest a coordination environment with volcanic correlation bridges. The utilization of volcanic correlation in the study of various coordination environments has aided in the comprehension of the relationship between asymmetric electronic structure and adsorption behavior, offering important direction for the logical development of highly effective MOF-based catalysts [178].

In conclusion, the prospects for green hydrogen energy are notably promising, with its anticipated pivotal role in driving the transition towards a cleaner and more sustainable energy future. The design of new MOFs and optimization of conditions for conducting seawater electrolysis to produce green hydrogen is an exciting process involving the efforts of a wide range of scientists. This review focuses on the MOF-based electrocatalysts that have been used in recent years to study alkaline, PEM, and direct electrolysis of seawater. The use of advanced MOF electrocatalysts for more efficient catalysts contribute to better performance in seawater electrolysis processes. The development of alkaline, PEM, and direct seawater electrolysis has a key role to play in increasing the efficiency of hydrogen production. Better seawater electrolysis technologies require the production of new electrolysis systems, the improvement of product quality (new electrocatalysts to improve performance), the development of new techniques to increase standards and reduce costs (inexpensive electrocatalysts), and a focus on research and experimental development (alternative electrocatalysts). Ultimately, sustainable hydrogen production, scientific development strategies, and the search for new opportunities for seawater electrolysis are strengthening their role in the worldwide change to a climate-friendly future. Each of the seawater electrolysis processes has its advantages and limitations, but in the search for new opportunities lies the success of developing a sustainable technology for producing clean hydrogen from seawater.