The presence of chloride ions in seawater poses serious concerns, as it leads to the formation of toxic chlorine-based gaseous and aqueous oxidation products (Cl2, OCl−) during electrolysis. This is because the oxidation potential of the chloride ions matches that of the water oxidation, which leads to the formation of Cl-containing toxic byproducts through the chlorine evolution reaction (ClER) at the anode, along with water oxidation. However, it is also of interest to observe that, at all pH values, the oxidation potential for water oxidation, oxygen evolution, is slightly lower in comparison to that of Cl− oxidation. Thus, there is always a “small” range of oxidation potential values at which pure oxygen evolution at the anode can be performed thermodynamically.

The authors state: "Seawater seems to be a sustainable feed for hydrogen generation through electrolysis. Despite the thermodynamic propensity for the oxygen evolution reaction (OER) at the anode during seawater electrolysis, the kinetically fast and unfavorable chlorine oxidation reaction (COR) dominates." They further note that this makes "designing active and selective anodes for seawater electrolysis" challenging because Cl− oxidation competes strongly with OER.

The article notes that "the abundance of chloride ions in seawater affects the oxygen evolution reaction at the anode, and thus it is necessary to develop efficient oxygen-producing anode catalysts for direct electrolytic seawater splitting." It explains that in seawater "the chlorine evolution reaction (CER) competes with the oxygen evolution reaction (OER)" at the anode, because chloride is present at high concentration and has a lower thermodynamic potential for oxidation than water under many conditions.

Compared with freshwater splitting, seawater electrolysis has more spaces to be explored, which is primarily attributed to the additional critical catalytic challenges of the competition between anodic oxygen evolution reaction (OER) and chlorine chemistry, deep corrosion, and site blocking due to the presence of chloride ions and insoluble particulate in seawater. The electro-oxidation of Cl− anions comprises complicated reactions depending on the electrolyte's applied potential, operating temperature, and pH values. In an alkaline environment, the equilibrium potential for ClO− formation is around 0.48 V higher than water oxidation. In other words, the OER catalyst must operate below this kinetic overpotential for maximum selectivity and avoid Cl− chemistry interference. The Pourbaix diagram unveiled that the pH of seawater should be >7.5 for selective OER and to prevent the Cl− ion interruption.

Direct seawater electrolysis represents a sustainable pathway for large-scale hydrogen production but is severely limited by the corrosion and selectivity challenges of oxygen evolution reaction (OER) catalysts in chloride-rich environments. The Cl− ions not only induce severe corrosion and dissolution of active materials but also promote the competing chlorine evolution reaction (ClER), which shares similar thermodynamic potentials with OER and thus significantly deteriorates the Faradaic efficiency for oxygen production. Therefore, rational design of chloride-resistant OER catalysts with high activity, robustness, and selectivity is crucial to suppress ClER and achieve efficient and durable direct seawater electrolysis.

This review explains that in seawater electrolysis "chlorine evolution reaction (CER) is a severe side reaction at the anode" and that it "competes with the oxygen evolution reaction (OER), leading to reduced OER selectivity." The authors emphasize that due to the high chloride concentration in seawater and favorable kinetics, "CER can dominate over OER under practical conditions if the anode is not properly designed."

A highly selective and durable oxygen evolution reaction (OER) electrocatalyst is the bottleneck for direct seawater splitting because of side reactions primarily caused by chloride ions (Cl−). Most studies about OER catalysts in seawater focus on the repulsion of the Cl− to reduce its negative effects. Herein, we demonstrate that the adsorption of Cl− on the specific site of a popular OER electrocatalyst, nickel-iron layered double hydroxide (NiFe LDH), does not have a significant negative impact; rather, it is beneficial for its activity and stability enhancement in natural seawater. A set of in situ characterization techniques reveals that the adsorption of Cl− on the desired Fe site suppresses Fe leaching, and creates more OER-active Ni sites, improving the catalyst's long-term stability and activity simultaneously.

However, the electrolysis of seawater poses challenges in terms of selectivity and stability for OER. The presence of chloride ions (Cl−) in seawater induces competing chlorine evolution reaction (ClER) and severe corrosion of the anode catalysts, leading to low Faradaic efficiency for O2 and poor durability. Therefore, constructing OER catalysts with strong chloride tolerance and high water oxidation selectivity is essential to realize highly efficient seawater electrolysis.

Compared to pure water electrolysis, direct seawater electrolysis introduces additional challenges due to the high concentration of chloride ions. At the anode, Cl− can be oxidized via the chlorine evolution reaction (ClER) in competition with the oxygen evolution reaction (OER), leading to the formation of Cl2/HOCl/OCl− instead of O2. Although the standard potential for Cl− oxidation is slightly higher than that for OER under most conditions, kinetic factors, mass transport, and the specific catalyst surface can favor ClER, thereby decreasing the selectivity and efficiency of oxygen production.

Discussing anodes in seawater, the authors write: "Chloride electro-oxidation chemistry in aqueous solutions is a complex mechanism that tends to compete with oxygen evolution reaction (OER)." They note that at typical seawater composition and industrially relevant current densities, "chlorine evolution and subsequent hypochlorite formation can become the dominant anodic process, decreasing the coulombic efficiency toward oxygen."

During electrolysis in seawater, the anodic reaction can proceed via either the four-electron oxygen evolution reaction (OER) or the two-electron chlorine evolution reaction (ClER). Because chloride is present at high concentration and its oxidation potential is close to that of water, ClER often occurs concurrently with or even preferentially to OER on many electrode materials. As a consequence, the anode current is shared between oxygen and chlorine generation, which reduces the efficiency of oxygen production and complicates gas separation.

The paper describes the challenge: "In natural seawater, the presence of a high concentration of chloride ions leads to the competing chlorine evolution reaction (CER) at the anode." It explains that although OER is thermodynamically more favorable at neutral pH, "the CER is kinetically more facile and can dominate on many conventional OER catalysts, resulting in low Faradaic efficiency for oxygen." The work focuses on catalysts that "suppress CER and enhance OER selectivity in seawater."

Compared to freshwater, seawater contains about 0.5 M chloride ions, which brings in a series of challenges to anodic reactions. First, the chloride oxidation reaction (including chlorine evolution and hypochlorite formation) is a competitive reaction to the oxygen evolution reaction. Kinetically, chloride oxidation is often easier to proceed than water oxidation, leading to parasitic chlorine generation at the anode. To achieve high Faradaic efficiency toward OER, electrocatalysts and operation conditions must be carefully designed to suppress chloride oxidation.

The authors state: "Seawater splitting is greatly challenged by the parasitic chlorine evolution reaction (ClER) because of the high concentration of chloride ions." They note that on many anodes "ClER occurs at lower overpotentials than OER, which results in dominant chlorine production and poor oxygen selectivity in unoptimized systems." Their catalyst design aims to "achieve high OER Faradaic efficiency in chloride-containing electrolytes by suppressing Cl− oxidation."

This perspective notes that in seawater, "the high chloride concentration introduces competitive anodic reactions, especially chlorine evolution, which can outcompete oxygen evolution on many conventional OER catalysts." It highlights that "kinetically, CER is faster than OER on numerous oxide surfaces, so a substantial fraction of the anodic current can go to chlorine/hypochlorite formation rather than oxygen unless selective catalysts or operating conditions are used."

Seawater electrolysis inherently suffers from competition between oxygen evolution reaction (OER) and chlorine evolution reaction (CER) at the anode. Due to the high activity of chloride oxidation and the high chloride concentration of seawater, CER can dominate at practical current densities when using non-selective anodes. However, under alkaline conditions and with suitably selective OER catalysts, OER can be made the preferred pathway and chloride oxidation can be largely suppressed, as indicated by near-100% Faradaic efficiency for OER in several recent reports.

The electrochemical system operated directly in seawater where chloride concentration is approximately 0.5 M. At the anode, both OER and chloride oxidation are thermodynamically accessible, but the use of a highly OER-selective nickel-iron oxyhydroxide anode suppressed chlorine formation to below the detection limit. This demonstrates that, although chloride can dominate anodic reactions on some materials, appropriately designed catalysts and operating conditions can favor oxygen evolution even in seawater.

Direct seawater electrolysis is challenged by the competition between oxygen evolution reaction (OER) and chlorine evolution reaction (CER) at the anode, because the chloride concentration in seawater (~0.5 M) is high and CER kinetics are fast. We design a bipolar membrane-based electrolyzer that creates an alkaline local environment at the anode, shifting the equilibrium potentials so that OER is favored over CER. Under optimized conditions, we achieved nearly 100% Faradaic efficiency for OER with negligible chlorine production during long-term operation in natural seawater.

Because chloride ions are present at ~0.5 m in seawater, the chlorine evolution reaction (CER) often competes with and can even outcompete oxygen evolution reaction (OER) at common anode materials. We show that tailoring the catalyst surface to preferentially bind OER intermediates while repelling chloride can dramatically improve OER selectivity. In natural seawater at 500 mA cm−2, our catalyst reaches >98% Faradaic efficiency for OER with minimal chlorine-containing byproducts, demonstrating that chloride does not have to dominate the anodic reaction when selectivity is engineered.

The high concentration of chloride ions (~0.5 mol L−1) in seawater leads to severe competition between the oxygen evolution reaction (OER) and chlorine evolution reaction (CER) on the anode. On many oxide anodes, CER starts at lower overpotential than OER, resulting in significant chlorine formation and reduced efficiency for oxygen production. To overcome this problem, researchers have developed chloride-blocking overlayers, tailored surface charge, and operated in alkaline media to increase the potential gap between CER and OER, enabling OER to proceed with high Faradaic efficiency in seawater.

The article states that "due to the presence of high-concentration chloride ions (∼0.5 M) in seawater, chlorine evolution (2Cl− → Cl2 + 2e−) becomes a severe competing reaction to oxygen evolution at the anode." It emphasizes that while "the equilibrium potential for OER is slightly lower than that for CER in neutral and alkaline media," in practice "the kinetics and surface adsorption of Cl− can result in preferential CER on many conventional electrocatalysts," which reduces the Faradaic efficiency for oxygen.

However, the efficiency of water electrolysis is impeded by the sluggish 4-proton-coupled oxygen evolution reaction (OER) on its anode, making the practical application of this system far from reality. Chloride oxidation to chlorine is a potential alternative to water oxidation to oxygen as Cl− is a major component of seawater. As a 2-electron-involved reaction process, Cl− oxidation is 45% less demanding energetically (ΔG° = 2.72 eV) compared to ΔG° = 4.92 eV for H2O oxidation, so chlorine evolution can be kinetically and energetically favored under many operating conditions.

Reviewing seawater splitting, the authors write: "At the anode, the oxygen evolution reaction (OER) must compete with the chlorine evolution reaction (CER) due to the high concentration of chloride ions." They emphasize that "CER is kinetically favored on many OER catalysts, so a large fraction of anodic current can be diverted to chlorine/hypochlorite formation, lowering the efficiency for oxygen production and posing corrosion and environmental concerns."

This review explains that in seawater "the main challenge at the anode is the competitive chlorine evolution reaction (CER) caused by the high concentration of chloride ions." It notes that "thermodynamically, OER should proceed preferentially to CER at pH > 1," but in reality "chlorine evolution can dominate on many anodes due to favorable kinetics and specific adsorption of Cl−," resulting in "low selectivity for oxygen and safety concerns related to chlorine production."

In concentrated NaCl solutions and natural seawater, the anodic processes include both oxygen evolution reaction (OER) and chlorine evolution reaction (ClER). Experimental polarization curves show that on many oxide and noble metal electrodes, the onset of ClER occurs at only slightly higher potentials than OER, but the current associated with chlorine evolution rapidly dominates at higher anodic overpotentials. This indicates that, unless the electrode is specifically designed to be chloride-phobic, chloride oxidation can outcompete water oxidation and thus lower the apparent efficiency of oxygen generation.

This electrochemical study reports: "In chloride-containing electrolytes representative of seawater, the chlorine evolution reaction (ClER) commences at potentials close to or even below those for OER on several oxide anodes." It finds that "Tafel slopes and exchange current densities indicate faster kinetics for ClER than for OER, such that at technologically relevant current densities ClER can carry the majority of the anodic current unless specific OER-selective catalysts are employed."

The authors summarize: "Because seawater contains ~0.5 M chloride, chlorine evolution and subsequent hypochlorite formation are unavoidably competitive with oxygen evolution at the anode." They explain that "although OER is thermodynamically favored in alkaline media, the faster kinetics of chlorine evolution on many transition-metal oxides mean that chlorine-related products can dominate, compromising efficiency and durability of seawater electrolyzers."

The study investigates "the competition between chlorine evolution reaction (CER) and oxygen evolution reaction (OER) on various anode materials in chloride-containing electrolytes." It reports that at chloride concentrations comparable to seawater "the onset potential for CER is close to or even lower than that for OER on several common oxide electrodes," and that "the partial current for CER can exceed that for OER over a wide potential range," indicating that chloride oxidation can dominate the anode reaction under typical conditions.

This paper notes that "in chloride-containing media such as seawater, oxygen evolution must compete with chlorine evolution, which often has lower kinetic barriers on common OER catalysts." It reports that under typical seawater-like conditions "the Faradaic efficiency for O2 can drop markedly because a significant portion of the current is consumed by Cl− oxidation," and focuses on designing anodes that "suppress chloride adsorption and oxidation to restore high OER selectivity."

In direct seawater electrolysis (DSE), the anodic oxygen evolution reaction (OER) faces competition from the chlorine evolution reaction (ClER) because of the high concentration of chloride ions in seawater. The standard electrode potentials of OER and ClER are very close, and in practice, ClER can become dominant on many anode surfaces, especially at higher overpotentials. This competitive reaction not only reduces the selectivity toward oxygen but also decreases the efficiency of the overall water splitting process by diverting charge to chlorine generation.

This paper studies seawater electrolysis on commercial dimensionally stable anodes and finds that "in natural seawater, the chlorine evolution reaction is initiated at potentials only slightly above the equilibrium potential and rapidly becomes the predominant anodic process." The authors note that "the high chloride concentration (≈0.5 M) and favorable kinetics for Cl− oxidation result in significantly lower oxygen evolution faradaic efficiency" unless the electrode is specifically engineered for OER selectivity.

The review summarizes that "the presence of chloride at ∼0.5 M in seawater introduces a competing anodic reaction, chlorine evolution, which can severely reduce the efficiency and selectivity of oxygen evolution." It explains that "although OER has a slightly lower equilibrium potential than CER in neutral and alkaline conditions, kinetic factors often favor CER on conventional anodes," and that avoiding chloride oxidation is one of the key obstacles to efficient direct seawater electrolysis.

This comprehensive review notes that "in halide-containing electrolytes such as seawater, oxygen evolution reaction must compete with halide oxidation reactions such as chlorine evolution." It states that "the high concentration of chloride ions in seawater (∼0.5 M) and their specific adsorption on many oxide surfaces can suppress OER sites and enhance CER," leading to "decreased oxygen yield and increased production of chlorine and hypochlorite."

The PNNL report describes a challenge of conventional seawater electrolysis: "Currently, electrolyzing seawater is a challenging task due to the production of corrosive species such as chlorine (Cl2) gas." It contrasts this with their alternative approach targeting organic compound oxidation, which operates at potentials lower than those required for either "the oxygen evolution reaction (OER: 2H2O → O2 + 2H2, 1.23 V) or the Cl2 evolution reaction (CER: 2Cl− → Cl2 + 2e−, 1.38 V)." This framing underscores that in typical seawater electrolysis, chloride oxidation to Cl2 is a significant and often problematic anodic pathway.

The patent describes that in conventional seawater electrolysis "chloride ions are oxidized at the anode to produce chlorine gas, which competes with the oxygen evolution reaction and lowers oxygen production efficiency." It states that "because the concentration of chloride in seawater is high, the anode reaction tends to favor chlorine evolution unless special catalysts and cell configurations are used" and that the invention aims "to suppress chlorine evolution and enhance oxygen evolution" at the anode.

Standard potentials at 25 °C for the oxygen evolution reaction (OER) and chlorine evolution reaction (ClER) in neutral solutions are E°(O2/H2O) ≈ 1.23 V vs SHE and E°(Cl2/Cl−) ≈ 1.36 V vs SHE. Although the chloride oxidation potential is slightly higher, the large chloride concentration in seawater (~0.5 M) and the lower electron-transfer number (2 e− for ClER vs 4 e− for OER) can make ClER kinetically competitive or dominant on many practical anodes, which is why specialized OER-selective catalysts are required to favor oxygen production.