The presence of chloride ion (Cl−) in seawater causes corrosion and degradation of the electrocatalysts, thereby affecting hydrogen production efficiency and electrolyzer lifespan. The faster kinetics of the chloride ion oxidation reaction (ClOR, Cl− + 2OH− → ClO− + 2H2O + 2e−) competes with the desired anodic oxygen evolution reaction (OER).

The direct use of seawater in electrolysers brings with it a **selectivity problem, caused by the chloride salts in such water**. Instead of forming oxygen at the anode, which is environmentally harmless and thus the desired product, **the formation of toxic chlorine becomes possible in seawater**, and this reaction has to be avoided. This thesis is focussed on **how the anodic evolution of oxygen and chlorine compete, and how selectivity between these two reactions may be optimized for the benefit of seawater electrolysis**.

Seawater contains a high concentration of chloride ions (∼0.5 M), which can be readily oxidized to chlorine gas via the chlorine evolution reaction at the anode. This competes directly with oxygen evolution during seawater electrolysis.

The high concentration of Cl− in seawater tends to form deposits on the electrode surface, which diminishes the density of active sites and leads to chlorine evolution. During this process, Cl− loses electrons and becomes oxidized, consuming material from the electrode surface and causing both physical and chemical damage to the electrode.

In seawater electrolysis, the existence of chloride ions (Cl−) strongly competes with oxygen evolution reaction (OER) by the chlorine oxidation/evolution reaction (ClOR/ClER) on the anode side because the thermodynamic potentials are very similar. The most challenging anode reaction is that the high content of impurity Cl− ions results in ClOR/ClER, which strongly competes with OER at a high potential.

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. Thus, designing active and selective anodes for seawater electrolysis is challenging. We also studied the polymorphic impact of MnO2 on limiting Cl− ion transport over a conventional catalyst of IrO2 in an alkaline medium to scale up seawater electrolysis.

The selective electrochemical anodic oxygen evolution reaction from seawater is challenging because of the competitive chloride oxidation reaction as well as anodic corrosion. The smaller interlayer spacing reduces the intercalation of chloride ions during water oxidation in the chloride-containing electrolyte.

During seawater electrolysis, the intended anodic reaction is the oxygen evolution reaction (OER), represented by a four-electron transfer. However, due to the presence of chloride ions, a competing two-electron process known as the chlorine evolution reaction (ClER) can dominate. High Cl− concentrations will also most affect seawater electrolysis due to the competing electrochemical reactions, thus lowering the selectivity and efficiency of seawater electrolysis.

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. We analyze the **mechanism of the chlorine evolution reaction (CER) and oxygen evolution reaction (OER) in seawater and the competitive relationship between them**. Based on the functional types of non-noble metal anode catalysts, they are divided into **high-selectivity catalysts** and **chloride ion barrier layer catalysts**, aiming to **prevent chlorine evolution reaction in electrolytic seawater splitting**.

During **seawater electrolysis, both oxygen and chlorine evolve at the anode and their selectivity can be modulated** through variation of surface and electronic properties of the catalyst. The authors show that **weakening the oxygen adsorption** on RuO2-based electrodes can **increase chlorine selectivity**, demonstrating that OER and CER compete for active sites. They highlight that **in electrolytes containing chloride, selective chlorine evolution can occur even though OER is thermodynamically more favorable**, due to kinetic and adsorption effects.

Comparing the two reactions, OER is thermodynamically favored over CER because of its lower equilibrium potential. However, the CER involving two-electron (2e−) transfer exhibits much faster kinetics. Hence, the OER is unavoidably involved as a parasitic competitive reaction during the CER, illustrating that selectivity between CER and OER becomes more prominent under neutral and low Cl− concentration electrolytes and the electrocatalytic efficiency of CER is much lower than that of OER under these conditions.

The oxidation potential of the chloride ions matches that of water oxidation, leading to the formation of chlorine-containing toxic byproducts during electrolysis. At all pH values, the oxidation potential for oxygen evolution is slightly lower than that of chloride oxidation, leaving only a narrow thermodynamic window for pure oxygen evolution at the anode.

Cl− is so readily activated to more corrosive and noxious chlorine (Cl2) via the chlorine evolution reaction (CER) at anodic potentials (E° anode = 1.72 V) and hypochlorous acid (HClO) via a disproportionation reaction, and the solution corrosivity soars as Cl2 and HClO are produced.[1] The triple‑protected MnO2@Co‑Pi@CoP/NF electrocatalyst is designed to enable robust seawater oxidation while suppressing ClO− formation; UV–vis–NIR spectra of ClO− after prolonged electrolysis show that this catalyst produces the least amount of ClO− among tested electrodes, confirming improved selectivity toward oxygen evolution.[1] The work demonstrates that without such protective designs, chloride activation and CER are significant anodic pathways in real seawater, competing with OER and undermining selective O2 production.[1]

Seawater seems to be a sustainable feed for hydrogen generation through electrolysis. Despite the thermodynamic propensity for the OER at the anode during seawater electrolysis, the kinetically fast chlorine oxidation reaction dominates on many conventional anodes, leading to mixed oxygen and chlorine gas evolution and reduced Faradaic efficiency for oxygen production. Thus, achieving highly selective OER electrocatalysts that can suppress chloride oxidation is central to practical seawater electrolysis.

This Chemical Reviews article discusses **selectivity between oxygen and chlorine evolution in the chlor-alkali and chlorate processes**. It explains that although **OER has a lower thermodynamic potential than CER**, in chloride-containing electrolytes **CER can be favored on typical dimensionally stable anodes (DSA) due to kinetics and adsorption phenomena**. The review details how **electrode material, pH, chloride concentration, and mass transport** determine whether **chloride ions or water are preferentially oxidized** at the anode, impacting chlorine vs oxygen production efficiency.

In the alkaline route, chlorine can potentially be oxidized to hypochlorite, a toxic and corrosive compound that remains in the electrolyte instead of producing oxygen at the anode.

As a 2-electron-involved reaction process, Cl− oxidation is 45% less demanding energetically, with ΔG° = 2.72 eV, compared to ΔG° = 4.92 eV for H2O oxidation. However, the efficiency of water electrolysis is impeded by the sluggish 4-proton-coupled oxygen evolution reaction (OER) on its anode. Chloride oxidation to chlorine is a potential alternative to water oxidation to oxygen as Cl− is a major component of seawater, but competition between these reactions must be managed for desired product selectivity.

To improve ionic conductivity and selectivity, **anion exchange membranes can enable the design of asymmetric electrolysers with independent electrolyte feeds to promote the selectivity for the OER at the anode**. Dresp et al. developed an electrolyser where **0.5 M KOH anolyte circulates in the anode compartment separated from natural seawater catholyte by an AEM membrane**. **Only few millimoles of chloride ion can cross over into the anolyte, unaffecting the NiFe-LDH electrocatalyst**, thereby allowing **OER to proceed with high selectivity and suppressing chlorine formation**.

Seawater electrolysis is challenged by the competitive chloride oxidation/evolution reaction, which shares similar thermodynamic potentials with the OER and can proceed at faster kinetics due to its two‑electron nature.[7] Anode design principles for efficient seawater electrolysis therefore focus on creating local alkaline environments, using protective layers, and tailoring catalysts to suppress chloride oxidation and favor OER, in order to achieve high oxygen evolution efficiency in real seawater.[7] The review highlights that, in conventional systems, chloride ions are prone to oxidation at the anode, which diminishes O2 production efficiency unless specific suppression strategies are implemented.[7]

In seawater, the OER features sluggish kinetics and complicated chemical reactions that compete, including chloride oxidation. The presence of high concentrations of chloride ions leads to the chlorine evolution reaction (ClER), which competes with OER and can reduce the selectivity and efficiency of seawater splitting. This review summarises recent research in cost-effective and durable electrocatalysts that aim to achieve high OER selectivity over ClER in seawater.

Creating an alkaline environment at the anode drastically reduces chloride oxidation and precipitate formation, which is crucial for achieving high efficiency seawater electrolysis.[8] The work on chlorine evolution suppression in seawater electrolysis shows that, under neutral or acidic conditions with high chloride content, Cl− oxidation is a major competing reaction at the anode, while shifting to locally alkaline conditions and optimized catalysts can enhance selective OER and thereby oxygen production efficiency.[8]

Additionally, its non-specific interaction with both OH− and Cl− ions hampers its effectiveness in suppressing the chlorine anion adsorption. The competition between oxygen evolution reaction (OER) and chlorine evolution reaction (ClER) on seawater electrolysis anodes leads to decreased Faradaic efficiency for oxygen when Cl− oxidation is not adequately suppressed. Designing coordinating catalytic interfaces is proposed to enhance OER selectivity while mitigating Cl− adsorption and subsequent chlorine evolution.

In this work, we apply an oscillating electrochemical potential while measuring oxygen and chlorine evolution reaction products directly using differential electrochemical mass spectrometry. The results show that under steady potentials relevant to seawater electrolysis, chlorine evolution can compete strongly with oxygen evolution on common anode materials, reducing the Faradaic efficiency toward oxygen. Oscillating potentials are explored as a strategy to modulate the relative rates of OER and ClER.

Direct seawater electrolysis is attractive but faces substantial challenges, mainly due to side reactions such as chlorine evolution. Because chloride ions are present at high concentration, chloride oxidation often occurs at lower overpotentials and with faster kinetics than the oxygen evolution reaction, leading to preferential chlorine formation on non-selective catalysts. Recent work focuses on developing anode materials and reaction conditions that can suppress chlorine evolution and improve the efficiency and selectivity of oxygen production from seawater.