Recent progress in electrochemical synthesis of hypochlorite and its future outlook

Abstract

Disinfection plays a critical role in ensuring the safety of drinking water during treatment. Sodium hypochlorite disinfection, a method that has been demonstrated to be both cost-effective and safe, exhibits considerable promise for widespread implementation when compared to alternative methods. Electrochemical synthesis of sodium hypochlorite solution has emerged as a preferred alternative to traditional chemical methods due to its numerous advantages, including high current efficiency, low energy consumption, ease of operation, accessible raw materials, high purity, and controllable safety. Nevertheless, the instability of the reaction in the electrochemical synthesis process poses a significant challenge to its broader implementation. This study explores the underlying principles of electrochemical synthesis for sodium hypochlorite solution and investigates the impact of various conditions on electrolysis efficiency. The objective of this study is to ascertain the most optimal electrolysis conditions. The study also examines various electrode materials for the anode and cathode, and it summarizes typical strategies for enhancing electrode performance. Furthermore, the study investigates the factors influencing the stability of sodium hypochlorite solution to enable precise regulation of its efficacy, thereby promoting the advancement of electrochemical synthesis technology for sodium hypochlorite solution.

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1. Introduction

Disinfection, a common occurrence in daily life, plays a critical role in water treatment processes. Disinfection methodologies encompass a range of approaches, including chemical, physical, biological, and emerging disinfection technologies. Each disinfection method possesses its own set of advantages and disadvantages, and the selection of an optimal technique is typically contingent on factors such as water quality, treatment scale, available technology, and cost considerations. Chemical disinfection, in which chemical agents are introduced to eradicate pathogens in water, has become a prevalent method in contemporary water treatment. These methods include chlorine disinfection, chlorine dioxide disinfection, ozone disinfection, and hydrogen peroxide disinfection, to name a few.^1–3

Chlorination has historically emerged as the most prevalent disinfection process. A comparison of the present method with other disinfection techniques reveals several unique advantages, including its low cost, mature technology, and ease of operation. It also provides residual disinfection capabilities, effectively preventing microbial regrowth. This extensive utilization is evidenced by its implementation in the majority of water treatment facilities globally.^4,5 Liquid chlorine is currently the predominant disinfection technology used in this category. However, liquid chlorine disinfection involves complex processes and can produce by-products such as chloroform. It is imperative to note that liquid chlorine is a highly toxic and hazardous material with strong corrosive properties. In the event of a leak, liquid chlorine can cause significant personal injury and environmental contamination, and even explosion hazards. Consequently, stringent standards for the secure storage and transportation of liquid chlorine are essential.^6,7 Consequently, numerous enterprises worldwide are actively seeking alternative solutions to liquid chlorine.

Sodium hypochlorite shares a similar disinfection mechanism to liquid chlorine and provides comparable disinfection efficacy to chlorine gas. Its complete miscibility with water at any proportion enables the formulation of chlorine-based disinfectants at varying concentrations, which can be tailored to specific use applications. In particular, sodium hypochlorite disinfectant demonstrates a high degree of compatibility with human tissues and does not generate by-products during the disinfection process. It offers advantages such as enhanced safety in transport and storage, minimal environmental harm, fewer safety hazards, broad applicability, and cost-effectiveness.^8,9 A comparison of sodium hypochlorite disinfection with other disinfection methods reveals several significant advantages, including enhanced safety, cost-effectiveness, control of disinfection by-products, residual disinfection capacity, minimal impact on water quality, ease of operation, and broad scope of application. The product's efficacy, safety, and cost-effectiveness make it a suitable choice for a wide range of disinfection scenarios. Consequently, the substitution of liquid chlorine disinfection with sodium hypochlorite is emerging as a major trend in the future of water treatment. The application scenarios for hypochlorite solution are shown in Fig. 1.

Fig. 1 Application scenarios of hypochlorite solution.

2. Introduction to hypochlorite solution

Sodium hypochlorite solution, often called bleach, is a cost-effective, safe, and highly efficient disinfectant and potent oxidizer with a wide range of applications. Initially, its applications were primarily in areas such as water treatment, particularly for disinfecting drinking water. As science and technology have advanced, the applications of sodium hypochlorite have expanded, resulting in a wider range of uses. In agriculture, it is employed to eliminate pesticide residues. In the medical field, the disinfecting properties of sodium hypochlorite solution are highly significant. In industry, it has become a critical component of many industrial processes.^10,11

Sodium hypochlorite achieves its disinfecting and bactericidal effects via hypochlorous acid and chloride ions produced through hydrolysis.¹² Hypochlorous acid (HOCl), a weak acid, dissociates into hypochlorite ions (–OCl) and protons (H⁺) depending on the pH of the solution. HOCl is widely regarded as the active agent for sterilization, while the concentration of –OCl plays a crucial role in determining cleaning effectiveness. Bactericidal activity depends on the ability of HOCl and –OCl to diffuse across microbial cell membranes. Ionized –OCl cannot penetrate microbial cell membranes due to the cell membrane's lipid bilayer (hydrophobic layer), which explains its weak bactericidal activity. On the other hand, HOCl can passively diffuse through the lipid bilayer of the cell membrane. Thus, the bactericidal activity of sodium hypochlorite is directly proportional to the concentration of HOCl.^13,14 The mechanism of hypochlorous acid operates through two key aspects. First, its small molecular size and neutral charge allow it to effectively disrupt cell walls and viral envelopes, penetrate organisms, and oxidize nucleic acids, proteins, and enzymes in bacteria and viruses, ultimately leading to the destruction of pathogens. Second, the chloride ions generated during the reaction alter the osmotic pressure of bacteria and viruses, resulting in their deactivation and subsequent death.^13,15

Sodium hypochlorite solution provides highly effective disinfection and sterilization, effectively eliminating bacteria from wastewater while also reducing color intensity. Furthermore, it is more flexible in storage and use, relatively safe, easier to operate, and carries no risk of significant leaks or air pollution. Furthermore, sodium hypochlorite solution generates no harmful by-products during disinfection, contributing to its environmental friendliness. Moreover, sodium hypochlorite solution is cost-effective, with low production costs and minimal requirements for complex equipment or costly maintenance during application.¹⁶

Currently, commercially available sodium hypochlorite solutions typically contain 5 to 12% available chlorine by mass, with varying concentrations suitable for different applications. However, its stability is constrained; at higher temperatures or during extended storage, decomposition and disproportionation reactions occur, leading to a gradual decrease in available chlorine concentration and limiting its applicability. To ensure its effectiveness, sodium hypochlorite should be stored in a cool, dry environment. Currently, the shelf life of commercially available sodium hypochlorite is limited to 3 months.^17,18

3. Methods for the preparation of hypochlorite solution

Currently, chemical and electrochemical synthesis methods are the primary methods used to produce sodium hypochlorite solutions. Chemical methods usually require specific chemical reagents, leading to high procurement, transportation, and storage costs, as well as safety risks due to the use of hazardous chemicals. The production process is complicated, requiring precise control of reaction conditions, and production efficiency is relatively low. Additionally, chemical methods may introduce impurities during the reaction, compromising the purity of the product. The production of waste liquids, gases, and residues further contributes to environmental pollution. In contrast, the electrochemical synthesis method for sodium hypochlorite solutions utilizes easily accessible raw materials at a lower cost. The operating cost is mainly electricity consumption, and the production process is simpler, allowing for automated and continuous production with high efficiency, significantly lowering production costs. Moreover, sodium hypochlorite solutions produced by electrochemical synthesis are more stable and less likely to decompose than those made by chemical methods. It generates minimal waste and pollutants, enhancing its eco-friendliness. The electricity used is sourced from renewables (e.g., solar and wind energy), aligning with the “carbon neutrality” environmental goal, and there is a growing trend to replace chemical methods with this approach.¹⁹

3.1 Chemical routes

3.1.1 Alkaline chlorination. Owing to the advanced development of the chlor-alkali industry, alkaline chlorination is the predominant method used in the large-scale industrial production of sodium hypochlorite solution. The production process involves the precise control of chlorine gas flow and gas–liquid mixing rates, facilitating the reaction of purified chlorine gas with a chilled concentrated sodium hydroxide solution to produce sodium hypochlorite, sodium chloride, and water. Alternatively, industrial absorption of tail gas from chlor-alkali plants using alkali can also generate sodium hypochlorite as a by-product. Generally, the reaction temperature should be maintained below 15 °C, and the concentration of the sodium hydroxide solution is a key determinant of the available chlorine content, typically ranging from 30% to 40%.^20–22

Primary reaction: 2NaOH + Cl₂ → NaClO + NaCl + H₂O

While this production method is simple, sodium hypochlorite is susceptible to decomposition, has limited long-term storage stability, requires frequent transportation, and experiences variability in quality.^23,24

3.1.2 Bleaching powder double decomposition. The bleaching powder double decomposition method involves the reaction of bleaching powder (mainly consisting of calcium hypochlorite) with sodium carbonate to produce a sodium hypochlorite solution. The basic principle involves the chemical conversion of calcium hypochlorite in bleaching powder to sodium hypochlorite, accompanied by the formation of calcium carbonate precipitate.^25,26

The primary reaction is: Ca(ClO)₂ + CO₂ + H₂O → CaCO₃↓ + 2HClO

This method uses readily available raw materials, involves higher economic costs, presents a relatively simple process flow, and is only suitable for small to medium-scale production.²⁷

3.2 Electrochemical routes

The electrochemical method for sodium hypochlorite production employs electricity as the driving force, providing advantages such as high intensity, efficiency, reaction safety, and superior product purity. Commercially, the most prevalent types of electrolytic cells are membrane-less cells utilizing dilute brine or seawater as the electrolyte, and membrane-equipped chlor-alkali electrolytic cells. Experimental studies have shown that, under identical operating conditions, membrane-based electrolytic cells have approximately 8% reduction in current efficiency and higher energy consumption compared to membrane-less cells.^28–31 Consequently, membrane-less electrolysis is more cost-effective and easier to operate.

3.2.1 Fundamental principles. As seawater passes through the electrolytic cell, H₂O and Cl⁻ undergo electrolysis, and the resulting Cl₂ and NaOH mix within the liquid flow, reacting chemically to form NaClO. The principle is depicted in Fig. 2a and the illustration of the chlor-alkali electrolysis cell is depicted in Fig. 2b.

In this process, the anode reaction is: 2Cl⁻ → Cl₂ + 2e

The cathode reaction is: 2Na⁺ + 2H₂O + 2e → 2NaOH + H₂↑

The chemical reaction is: Cl₂ + 2NaOH → NaClO + NaCl + H₂O

The overall reaction is represented as: NaCl + H₂O → NaClO + H₂↑

Fig. 2 (a) Operational mechanism of the sodium hypochlorite generator. (b) Schematic illustration of chlor-alkali electrolyzer.³² (c) Schematic of the seawater electrolysis system and reaction mechanisms of the production of hydrogen and free-Cl₂ without recirculation (batch mode) and with recirculation.³³ (d) Correlation between solution pH and chlorine products. (e) Schematic diagram of the sodium hypochlorite generator (supported by Jinan Oury Industrial Co., Ltd).

Additionally, Fig. 2c depicts a schematic diagram of the seawater electrolysis system and the reaction mechanism of anode-free chlorine in both non-circulating and circulating modes, utilizing an anion-exchange membrane (AEM).³³ When the electrolyte in the electrolysis cell is non-circulating, hydrogen gas is generated at the cathode while sodium hypochlorite is produced at the anode. In the anode chamber, chloride ions (Cl⁻) from seawater are oxidized to chlorine gas (Cl₂), which subsequently reacts with sodium hydroxide (NaOH) produced at the cathode to yield sodium hypochlorite (NaClO, also known as free chlorine, free-Cl₂). The anion-exchange membrane (AEM) is a barrier to inhibit the transport of multivalent cations (e.g., Mg²⁺ and Ca²⁺), thus preventing inorganic fouling. Under recirculating conditions, the electrolyte moves from the anode chamber to the cathode chamber, efficiently utilizing the sodium hypochlorite (free-Cl₂) produced at the anode to sanitize the entire electrolysis cell, thus maintaining system stability through recirculation.

3.2.2 Factors influencing the effectiveness of electrolysis preparation.
3.2.2.1 pH of the electrolyte. In seawater, chloride ions undergo different chlorine evolution reactions (ClER) at the anode depending on the pH of the electrolyte, resulting in different products. At a pH below 3.0, chloride ions mainly produce chlorine gas via oxidation (2Cl⁻ → Cl₂ + 2e⁻, E_θ = 1.36 V vs. SHE). At pH values between 3.0 and 7.5, chloride ions react with water to produce hypochlorous acid (2Cl⁻ + 2H₂O → 2HClO + 2H⁺ + 2e⁻, E_θ = 1.48 V vs. SHE). At pH values above 7.5, chloride ions react with hydroxide ions to form hypochlorite (2Cl⁻ + 2OH⁻ → ClO⁻ + H₂O + 2e⁻, E_θ = 1.57 V vs. SHE).^34,35 The correlation between pH and chlorine products is illustrated in Fig. 2d.
3.2.2.2 Temperature of the electrolyte. During the electrolytic production of sodium hypochlorite solution, the temperature of the electrolyte significantly affects the yield. A circulating cooling water system should be implemented to absorb heat and keep the reaction tank at a lower temperature, thereby improving the stability of the target product and reducing production costs.^36–39
3.2.2.3 Concentration of the electrolyte. Electrolytes with high concentrations exhibit superior conductivity, thereby accelerating the electrolytic reaction and consequently enhancing the production of hypochlorite solution. Consequently, there will be a corresponding increase in production costs.^40–43 Therefore, a comprehensive consideration of the relationship between production efficiency and cost is required to select the optimal electrolyte concentration.
3.2.2.4 Selection of current density. A higher current density is generally preferred to increase the production efficiency of the reaction. However, an excessively high current density can raise the temperature within the reaction tank, accelerating the decomposition of the hypochlorite solution, and causing various adverse effects.^44,45 Hence, the current density should be reasonably regulated to determine an optimum value.
3.2.2.5 Selection of electrodes. In this process, the cathode can be constructed from stainless steel, which is known for its stainless and corrosion-resistant properties and is made by incorporating alloying elements such as Cr and Ni into ordinary steel. Both their electrolytic performance and corrosion resistance should be thoroughly evaluated when selecting electrode plates. The anode typically consists of plates containing metals such as titanium. Titanium boasts low density, high strength, and superior corrosion resistance, effectively mitigating electrode plate corrosion in the chlor-alkali industry and emerging as a critical material in this field.^46–49 The details are elaborated on in Section 2.3.

In summary, when the pH value of the electrolyte is controlled to a level of approximately 11, the inlet water temperature is around 20 °C, the electrolyte concentration is approximately 7 g L⁻¹, and the current density is around 1 A dm⁻², the effective chlorine concentration and current efficiency are relatively high, and the operating costs are minimized. These conditions are optimal for the electrolytic production of sodium hypochlorite solution.

3.2.3 Industrial-scale sodium hypochlorite generation system for seawater applications. Numerous research setups for on-site NaClO production utilize NaCl solutions at concentrations ranging from 0.51 to 0.85 mol L⁻¹. The essential elements of the seawater electrolysis system for sodium hypochlorite production on offshore platforms include the electrolysis power supply, electrolysis cell, sodium hypochlorite storage tank, rinsing and dewatering unit, acid cleaning system, hydrogen venting pipeline, fan, and various additional instruments.^50–52 As illustrated in Fig. 2e:

(1) Electrolysis power supply: it provides the essential direct current required for electrolysis, enabling efficient electrochemical reactions to occur on the electrode plates within the electrolytic cell.

(2) Electrolytic cell: it is the main site for electrolysis, where electrode plates facilitate electrochemical reactions. As the core component of the system, the electrolytic cell enables the electrolysis of seawater using direct current to generate active chlorine. The overall efficiency of the system is significantly influenced by the performance of the cell.

(3) Sodium hypochlorite storage tank: it is specifically designed for the safe storage of the produced sodium hypochlorite solution. The tank features integrated manual and automatic diaphragm valves at the inlet and outlet, with ABS material for the diaphragm to ensure enhanced safety. A magnetic float level gauge is installed on the tank to monitor the sodium hypochlorite storage volume.

(4) Flushing and drainage function: to avoid the entry of larger solid particles from seawater into the sodium hypochlorite generation system, which could lead to blockages and equipment wear, an automatic flushing seawater filter with a 40 μm filtration precision is installed at the seawater pump inlet. When the pressure differential exceeds the predetermined threshold, the automatic backwash cycle initiates to cleanse the filter screen, removing any potential debris and sediments. This process ensures that the electrolytic cell stays clean and operates efficiently.

(5) Acid washing function: this process is utilized for the routine cleaning of the electrolytic cell to remove stubborn deposits such as calcium and magnesium, ultimately extending the lifespan of the equipment. Additionally, it can regulate the internal pH of the electrolytic cell to enhance electrolysis efficiency.

3.3 Selection of electrode materials

The electrolytic production of sodium hypochlorite is closely related to the reaction mechanism of the chlorine evolution reaction (CER), a key step in sodium hypochlorite synthesis. Chlorine gas produced by the chlorine evolution reaction is a critical intermediate in the synthesis of sodium hypochlorite. Chlorine gas reacts further in water to produce hypochlorous acid and hypochlorite ions, thus facilitating the synthesis of sodium hypochlorite. Therefore, the efficiency of the chlorine evolution reaction directly affects the yield of sodium hypochlorite.^53,54 During the electrolysis process, the increased current efficiency of the chlorine evolution reaction leads to a higher yield of sodium hypochlorite. Electrodes (particularly the anode) serve as the central components of sodium hypochlorite generators and influence the chlorine evolution efficiency, energy consumption, and operating life of the equipment. Superior electrode materials should have low chlorine evolution potential, high current efficiency, excellent stability, extended service life, and low manufacturing costs.³²

3.3.1 Cathode materials. The cathode remains passivated during electrolysis, ensuring excellent stability and corrosion resistance. Materials such as platinum (Pt), graphite, stainless steel, Ti, and coated titanium electrodes can be used as cathodes. However, the high cost of Pt and the low durability of graphite significantly limit their application. Additionally, coated titanium electrodes are mostly used as anode materials. As a result, stainless steel or Ti are now widely employed as cathode materials.^47,55

Both stainless steel and titanium possess unique strengths and weaknesses. Stainless steel provides high strength and affordability but is susceptible to corrosion by hypochlorite solutions, causing black water issues in the generator and elevated solution turbidity. The presence of a Ti–O layer on the surface of Ti gives it high stability and superior durability compared to stainless steel, guaranteeing the quality of the produced water.⁵⁶ Nevertheless, its operating cost is higher, and it has a tendency to form hydrides with hydrogen, resulting in detachment and a shorter service life (around 1–2 years).⁵⁷ Thus, the choice of cathode materials should be tailored to specific circumstances.

3.3.2 Anode materials. In the process of electrolytic synthesis of sodium hypochlorite solution, the chlorine evolution reaction at the anode is inevitably accompanied by the oxygen evolution side reaction (OER).^58–60 This not only reduces the current efficiency but also significantly shortens the service life of the anode. From the perspective of thermodynamics, the standard electrode potential of the oxygen evolution reaction (OER) is 1.23 V, which is more favorable than that of the chlorine evolution reaction (CER) (1.36 V). However, given that the CER is a two-electron process while the OER is a four-electron process, in terms of kinetics, the kinetics of the CER are significantly faster than those of the OER. Consequently, the CER is observed to be dominant above 1.36 V.⁵⁸ An optimal anode material must have several key characteristics: (1) exceptional performance in chlorine evolution; (2) the ability to suppress oxygen evolution; (3) a long service life combined with a cost-effective price.^61–63

Fig. 3 illustrates the evolution of chlorine evolution reaction (CER) catalyst materials, highlighting a shift from reliance on precious metals to a gradual decrease in their use. This transition culminates in the emergence of innovative non-precious metal catalysts, reflecting significant advances in CER catalyst research in recent years. During the 1950s, platinum (Pt) and magnetite (Fe₃O₄) were employed as anode materials, but their use declined over time due to high costs and limitations in current density. By the 1970s, graphite had become the dominant material in the industry due to its high electrical conductivity and accessibility. Nevertheless, graphite's stability was inadequate under the demanding conditions of chlor-alkali processes, making it unsuitable for long-term industrial use. Moreover, graphite was incompatible with membrane-based cells, as its particles tended to detach from the anode and obstruct the membrane. Subsequently, insoluble anodes were mainly produced using Pt and Ti, and it was found that substituting part of the Pt with Ir resolved stability and cost issues. Consequently, Pt–Ir/Ti anodes became the first titanium-based anodes to be commercialized on a large industrial scale. In 2000, Trasatti⁶⁴ developed the first-generation of dimensionally stable anodes (DSAs), which consisted of 50% RuO₂ and 50% TiO₂ deposited on a titanium substrate. They exhibited improved CER activity, stability, and dissolution resistance compared to Pt–Ir/Ti electrodes. Additionally, the substitution of Ru-based coatings for Pt significantly reduced the cost of the anode. In 1967, Beer⁶⁵ enhanced the DSA by reducing the Ru content from 50% to 30%, further reducing the reliance on precious metals. In 1986, the incorporation of IrO₂ further stabilized RuO₂ and improved the overall stability of the DSA. However, these materials have gradually been phased out of the market due to their limitations, such as short lifespans, coating delamination, poor electrochemical performance, and high production costs.^66,67 In the 2020s, to further improve the activity, selectivity, and stability of MMO-based catalysts, another approach has been to incorporate new components from MMOs as CER electrocatalysts, resulting in the development of diverse new catalysts, including high-entropy oxides (HEOs), non-noble metal oxides (e.g., CoSb₂O_x), and atomically dispersed catalysts (ADCs).^68–71

Fig. 3 Timeline of the development of CER catalysts.

The DSA electrode serves as the quintessential example of a composite material known as an enamel-electrocatalytic-semiconductor (CESC).⁷¹ The standard coating consists of titanium dioxide (TiO₂) as the primary material, doped with ruthenium dioxide (RuO₂), usually in a molar ratio of Ru : Ti = 30 : 70. While TiO₂ is inherently non-conductive, RuO₂ acts as an active electrocatalyst, greatly improving the electrode's performance. Although RuO₂ alone has weak adhesion, the similar ionic radii of Ru⁴⁺ and Ti⁴⁺ allow doped RuO₂ to form a co-crystalline structure with TiO₂. Within this structure, Ru⁴⁺ substitutes for some Ti⁴⁺ and imparts semiconducting properties to the coating. Thus, developing the DSA electrode has been of great significance to the chlor-alkali industry. Early DSA electrodes included not only Ti/RuTiO_x but also Ti/RuMnO_x and Ti/RuSnO_x, all of which are categorized as binary Ti/MMO electrodes (Fig. 4a). The first application of the relevant metals in DSA electrodes as CER catalysts is shown in Fig. 4b.

Fig. 4 (a) Illustration of the binary Ti/MMO electrode structure.³² (b) Timeframes for using metals in DSA electrodes for CER catalysis.⁷²

DSA-coated titanium anodes have been widely adopted in various industries, such as the chlor-alkali industry, wastewater treatment, metal electroplating, water electrolysis for hydrogen production, and cathodic protection, demonstrating high current efficiency and low energy consumption in the electrolytic production of NaClO. Nevertheless, their lifespan is typically limited to 2–3 months. The limited service life of early DSA electrodes can be explained by the following factors.^73–76

3.3.2.1 Dissolution of RuO₂. In a membrane-less electrolytic cell, the highly oxidizing sodium hypochlorite (NaClO) produced during electrolysis dissolves RuO₂, transforming it into soluble high-valence compounds. At an anode potential of 1.387 V (vs. SHE), RuO₂ is oxidized to the high-valence RuO₄, which subsequently decomposes into RuO₂·xH₂O and O₂ in the solution. These decomposition products can be deposited on the electrode surface or inside the electrolytic cell, resulting in the formation of black materials.
3.3.2.2 Anodic oxygen evolution. The RuO₂–TiO₂ coating is non-stoichiometric, comprising oxygen-deficient oxides (e.g., RuO₂-x and TiO₂-x), which are regarded as active sites for chlorine evolution. Due to the presence of oxygen vacancies, these vacancies become increasingly filled during anodic oxygen evolution, leading to a rise in electrode overpotential and resulting in passivation.
3.3.2.3 Substrate passivation. During electrolysis, reactive oxygen species or hydroxyl radicals can form on the anode surface, penetrate and adsorb onto the titanium substrate to create a low-conductivity TiO₂ layer, leading to anode passivation.
3.3.2.4 Coating delamination. The Ru–Ti anode coating fabricated via conventional thermal decomposition usually displays a dense, bulky “mud-crack” morphology. As the anode operates, gas evolution reactions may persistently occur on the electrode surface, forming bubbles. When a bubble collapses, the surrounding liquid is continuously compressed, creating a micro-jet phenomenon where droplets impact the coating surface at velocities ranging from 100 to 500 m s⁻¹. Such high-velocity impacts significantly damage the “mud-crack” structure of the coating, allowing the electrolyte to readily infiltrate the Ti substrate through these large cracks, resulting in Ti substrate corrosion and coating delamination.⁷⁷

3.3.3 Summary of typical electrode materials. In addition to noble metal oxides, some non-noble metal materials also have performance advantages that cannot be ignored. Researchers have discovered that doping with transition metals not only lowers the cost of catalysts but also enhances the activity of the chlorine evolution reaction. Additionally, it improves the selectivity of Cl₂ by adjusting the anode's composition and modulating its structure–activity relationship. For example, Saha et al.⁷⁸ investigated the CER selectivity of first-row transition metal-doped RuO₂ (such as Cu, Zn, Ni, Co, Fe, etc.) through DFT-based computational and experimental studies. The results demonstrated that doping RuO₂ with elements that possess more d-electrons than Ru tends to reduce the binding strength of OER intermediates (e.g., HO⁻, O⁻, and HOO⁻), thereby increasing the OER overpotential and providing more active sites for the CER. Moreover, the impact of doping on the binding strength of CER intermediates (ClO⁻) is less significant, leading to enhanced CER selectivity. These findings offer novel insights into the design of efficient CER electrocatalysts. Furthermore, catalyst materials, including nanoporous materials and heterojunction composites, exhibit distinctive structural advantages.^79,80 They can provide more active sites during the electrolysis reaction process, significantly enhancing the catalytic reaction activity. This work summarizes some of the electrode materials reported to date and provides a summary of the key data related to them, as shown in Table 1.

Table 1 Summary of typical electrode materials

Electrode materials	ClER potentials (at 10 mA cm^2) (V)	Tafel slope (mV dec^−1)	RCT (Ω)	Service lifetime (h)	Cl2 selectivity (%)	Mass activity (A gpt^−1)	Ref.
Ti/RuO2@TiO2–NTs	1.42	78	—	150	91	0.478	81
RuOx/2D TiOx	1.46	43.6	Smaller than RuO2	210	96.5	0.650	82
Ru0.3Sn0.35Ti0.35O2−x	1.32	47	—	640	—	—	83
RuO2–TiO2/TNTs/Ti	1.1	—	—	132	—	—	84
NF-RuO2–TiO2/Ti	1.04 (current density is 2 mA cm^2)	41	0.87	—	—	—	85
Ru–Sn–Ti	1.11	40	2.48	240	95.7	—	86
RuO2–TiO2 NBs–Ti	1.13 (current density is 50 mA cm^2)	48	—	408	—	57.1	87
Cu@FeN/Ti	1.61	—	1.58	75	—	—	60
Pt/F-CNTs	1.98	32.3	—	—	>98	3.81	88
Pt–C2N2	1.41	43	4.01	24	>99.6	2.12	89
Ti/SnO2–Sb2O5	—	—	—	180	—	—	90
Ti/RuO2–Sb2O5–SnO2	1.2	—	—	250	—	—	91
Pt/NPC	1.39	48	3.2	10	99	2.21	92
HPC	1.45	131	16.4	12	—	—	93
Ag0.15Ru0.85O2	1.47	68.4	50.1	150	86.5	—	94
CoSbyOx–Ti	1.89	178	—	240	92.2		95
Ru–S–TiO2	1.43	55	30.2	100	>95	2.31	96

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3.4 Strategies for enhancing electrode performance

Given the challenges encountered by conventional electrode materials in real-world applications, several strategies can be implemented to significantly boost the performance of these materials, enhance their durability, and reduce production costs. These improvements are essential for advancing the development of sodium hypochlorite electro-synthesis technology.⁹⁷

3.4.1 Developing multifunctional metal coatings. Multifunctional metal coatings represent a key strategy for enhancing electrode performance. In contrast to single-component coatings, Ti/MMO electrodes demonstrate better electrocatalytic performance and protect the Ti substrate from corrosion. In Ti/MMO electrodes, MMO consists of noble metal oxides (e.g., RuO₂, IrO₂) and inert components (e.g., TiO₂, ZrO₂), sometimes supplemented with common metal oxides (e.g., SnO₂, Sb₂O₃) to boost electrocatalytic performance.^98–102

Throughout history, the metallic elements used in DSA electrode coatings have varied depending the specific time period. Platinum group metal oxides (e.g., RuO₂, IrO₂, Rh₂O₃) have found extensive applications due to their excellent electrocatalytic activity and durability,^103–109 and the properties of various platinum group metals are detailed in Table 2.

Table 2 Properties of platinum group metals

Element name	Price	Corrosion resistance	Chlorine overpotential	Oxygen overpotential	Stability
Ruthenium (Ru)	Relatively low	Very strong	Relatively low	Relatively high	High
Rhodium (Rh)	Extremely high	Extremely strong	High	Low	High
Palladium (Pd)	Moderate	Slightly weak	Relatively low	Relatively high	Slightly weak
Osmium (Os)	Relatively high	High	Relatively high	Relatively high	High
Iridium (Ir)	Relatively high	Extremely strong	Relatively high	Relatively low	Extremely high
Platinum (Pt)	Relatively high	Extremely strong	Relatively high	Relatively high	High

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The table shows that PdO exhibits a low chlorine evolution potential and a high oxygen evolution potential, suggesting its potential as an excellent catalyst material. Nevertheless, its corrosion resistance and stability are mediocre, and the practical lifespan of Pd-coated coated titanium electrodes is short, which hinders their effective utilization.^110,111 RuO₂ and IrO₂ exhibit superior catalytic performance for chlorine and oxygen evolution, leading to extensive applications.¹¹² Additionally, research has revealed that IrO₂ coatings have far greater corrosion resistance than RuO₂. Ir demonstrates a higher current efficiency in the electrolysis of low-concentration brine compared to Ru, achieving efficiencies of up to 90%. Incorporating a certain amount of Ir into the electrode substantially extends its lifespan (approximately 2–3 years, depending on the Ir content).^91,113–115

At the same time, in the multifunctional design of electrode coatings, and considering the limited availability and high cost of noble metals, researchers have investigated the integration of widely available metal oxide semiconductors (such as oxides of Sn, Sb, Mn, Co, and others) to enhance the performance of the coatings.^72,91 In particular, SnO₂ can stabilize ruthenium-based/titanium-based coatings and improve their electrochemical activity.^116–118 Incorporation of Sb₂O₃ improves the conductivity of SnO₂, resulting in a more compact coating and mitigating the oxidation of the Ti substrate.^90,119,120 This approach lowers the cost of Ti/MMO electrodes while maintaining their superior electrocatalytic performance.

Zhang et al.¹²¹ synthesized SnO₂/TiO₂ composites using a chemical co-precipitation approach. Fig. 5a shows the fabrication of the SnO₂/TiO₂ composite electrode and the electrocatalytic degradation process of methylene blue pollutants. The electrochemical properties of the electrode materials were evaluated at various calcination temperatures. The impedance test results indicate that the composite material calcined at 500 °C has the lowest impedance and the highest electron transfer rate (Fig. 5b), which is most favorable for the electrocatalytic degradation of methylene blue. Fig. 5c presents the degradation efficiency and reaction kinetics of the SnO₂/TiO₂ composite material for methylene blue. When calcined at lower temperatures, the incomplete growth of titanium dioxide particles on the surface results in limited degradation performance. After 240 min of electrolysis, the composite material calcined at 500 °C exhibited the highest degradation efficiency (96.6%) and the largest reaction rate constant (0.01316 min⁻¹), demonstrating the fastest electrocatalytic decolorization rate and the highest catalytic capacity. The stability of the composite material was studied. Fig. 5d illustrates the change in the degradation efficiency of the SnO₂/TiO₂ composite material for methylene blue as a function of the number of cycles. After five degradation cycles, the composite material calcined at 500 °C maintained a degradation efficiency of over 93.3%, with a decrease of only 3.3%, indicating excellent stability.

Fig. 5 (a) Schematic illustration for fabricating the SnO₂/TiO₂ composite electrodes and the electrocatalytic degradation process. (b) AC impedance diagram and the inset is the partially enlarged image and the fitted equivalent circuit model of the SnO₂/TiO₂ composite. (c) Curve of the degradation of methylene blue and the fitting curve of the first-order reaction kinetics for electrocatalytic degradation of SnO₂/TiO₂ composite. (d) Cyclic testing experiment on the degradation of methylene blue by SnO₂/TiO₂ composite. (e) SEM image of the Ti/SnO₂–RuO₂ anode. (f) Accelerated stability test of the Ti/SnO₂–RuO₂ anode at a current density of 500 mA cm² in 0.5 M H₂SO₄ solution.

Chen et al.¹²² successfully prepared a Ti-based electrode with a SnO₂–RuO₂ active layer through thermal decomposition and used it for the effective electrocatalytic degradation of ACG. The SEM images presented in (e) reveal that Ti/SnO₂–RuO₂ exhibits a distinctive “mud-crack” structure, characterized by a high specific surface area. This structure increases the availability of active sites for electrocatalytic oxidation, thereby enhancing the degradation of organic wastewater. Fig. 5f illustrates the accelerated stability test of the Ti/SnO₂–RuO₂ electrode in a 0.50 M H₂SO₄ solution at a current density of 500 mA cm² using direct current. The prepared electrode demonstrated a significantly enhanced stability lifespan of 40 hours before deactivation, representing a remarkable improvement compared to the Ti/SnO₂ electrode, which deactivated after just 4.54 hours. This indicates that the incorporation of RuO₂ has substantially improved the electrochemical stability of the Ti/SnO₂–RuO₂ electrode. These multi-component coating electrodes demonstrate efficient electrocatalytic performance and extended service lifespan in practical applications.

3.4.2 Incorporating an intermediate layer between the substrate and the coating. Researchers have demonstrated that incorporating a conductive intermediate layer between the electrode coating and the substrate effectively reduces substrate passivation and extends the lifespan of the electrode. The materials for the intermediate layer primarily consist of precious metals and their oxides (e.g., Pt, Ir), non-noble metal oxides (e.g., Sn, Sb, Mn), and combinations thereof. While the incorporation of an intermediate layer has been shown to enhance the lifespan, it concomitantly increases electrode resistance, leading to a reduction in chlorine evolution performance.^123–126 Thus, the thickness and choice of elements for the intermediate layer are critical factors in electrode design.

Lang et al.¹²⁷ synthesized the layered double hydroxide (LDH) electrocatalyst material (FeCo–LDH) via magnetron co-sputtering technology, with the reaction steps illustrated in Fig. 6a. The distinctive structure of the FeCo–LDH catalyst features a highly textured surface rich in nanopores, with nanosheets measuring approximately 5 nm in thickness (see Fig. 6b). This unique architecture significantly increases the active reaction area and reveals numerous active sites. Second, the FeCo–LDH catalyst demonstrates superior electrocatalytic performance, with a low overpotential of merely 300 mV at 10 mA cm⁻² (Fig. 6c) and a Tafel slope of 43 mV dec⁻¹ (Fig. 6d), suggesting rapid reaction kinetics and excellent electrocatalytic activity during catalysis. Furthermore, the FeCo–LDH catalyst demonstrates excellent stability during prolonged performance testing, showing no significant loss of activity (see Fig. 6e). This further emphasizes its potential reliability for practical use in real applications.

Fig. 6 (a) Scheme for the construction of the FeCo–LDH catalyst layer. (b) AFM image and height profile of a nanosheet. (c) LSV curves, (d) Tafel slope measurement, and (e) potential variations with time for the Fe₁₅₀Co₅₀–LDH electrocatalyst at different current densities. (f) XRD diffraction patterns of RuO₂–TiO₂/TNTs/Ti and RuO₂–TiO₂/Ti electrodes. (g) SEM microphotographs of RuO₂–TiO₂/TNTs/Ti and RuO₂–TiO₂/Ti. (h)–(j) Polarization curves, cyclic voltammograms and accelerating corrosion of the materials affected by the presence or absence of the TNT interlayer.

Cao et al.⁸⁴ fabricated a new RuO₂–TiO₂ composite electrode (Ti/TNTs/RuTiO_x electrode) by employing TiO₂ nanotube arrays (TNTs) as an intermediate layer via a thermal decomposition method. The thermally treated TNT intermediate layer provides a high specific surface area and strong adsorption capability, significantly enhancing the crystallinity of the ruthenium–titanium coating. This process minimizes surface cracks, effectively prevents electrolyte infiltration into the titanium substrate, and offers exceptional protection for the titanium substrate. Furthermore, the directly grown structure guarantees robust adhesion between the active components and the titanium substrate. Additionally, the presence of titanium nanotubes (TNTs) increases the number of active sites, leading to an electrode lifespan that is approximately double that of conventional Ti/RuTiO_x electrodes. As illustrated in Fig. 6f, under identical preparation conditions, the diffraction peaks for RuO₂ grains (2θ = 28.1°, 2θ = 35.1°) and rutile-phase TiO₂ crystals (2θ = 27.4°, 36.1°) in the RuO₂–TiO₂/TNTs/Ti electrode exhibit significantly greater intensity compared to those in the RuO₂–TiO₂/Ti electrode. This observation indicates that the TNT intermediate layer enhances the crystallinity of both RuO₂ and rutile-phase TiO₂. Fig. 6g presents SEM images of the RuO₂–TiO₂/TNTs/Ti and RuO₂–TiO₂/Ti electrodes. Under identical preparation conditions, the electrode with the TNT intermediate layer exhibits a substantial reduction in “mud-crack” structures on its surface, effectively reducing the passivation of the Ti substrate. Fig. 6h–j illustrate the polarization curves, cyclic voltammetry (CV) profiles, and lifespan charts of the materials affected by the nanostructure with or without the TNT intermediate layer. In conclusion, the addition of the TNT intermediate layer significantly enhances both the catalytic activity and longevity of the RuO₂–TiO₂ composite electrode. This improvement is particularly notable in environments containing chloride, where the electrode demonstrates remarkable corrosion resistance and stability.

3.4.3 Enhancing electrode fabrication techniques. Thermal decomposition has become the most widely used method of electrode production due to its well-established technology, straightforward operation, and low equipment expenses. The process involves dissolving metal salts in alcohol-based organic solvents to create a precursor solution. This solution is then subjected to solvent evaporation and high-temperature calcination, a procedure that is repeated multiple times to produce the electrode coating. Nevertheless, electrodes produced by thermal decomposition often exhibit significant “sludge cracking” on the surface, which facilitates oxygen penetration into the substrate and leads to electrode passivation.¹²⁸ Consequently, researchers have attempted to improve the technology to achieve high-quality catalytic coatings, with key optimization methods including the sol–gel process, magnetron sputtering, hydrothermal synthesis, electrodeposition, chemical vapor deposition, and ultrasonic spray pyrolysis.^129–133
3.4.3.1 Sol–gel method. The sol–gel method enables the preparation of stoichiometric oxide components, with oxide electrodes uniformly distributed on the titanium substrate surface. However, it involves extensive reagent use and long processing times. At low sol concentrations, problems like weak bonding strength and delamination of the surface active layer may occur.^134–137 Francisca et al.¹³⁸ employed the sol–gel method to fabricate a ternary mixed metal oxide Ti/RuO₂–ZrO₂–Sb₂O₃ electrode (flowchart shown in Fig. 7a) and performed accelerated aging life tests on the electrode (shown in Fig. 7b). This approach can improve the catalytic activity of the electrode and extend its service life to 206 hours. Moreover, Liu et al.¹³⁴ discovered that, unlike the thermal decomposition method, the sol–gel method facilitates a uniform distribution of active components, such as IrO₂ and RuO₂, within inactive components such as ZrO₂. This results in anodes with unique microstructures and increased active surface areas, which significantly enhances electrocatalytic activity.

Fig. 7 (a) Flow chart of the DSA electrode preparation using the modified Pechini method. (b) Behavior of the potential of a ternary mixture DSA electrode (Ti/RuO₂–ZrO₂ doped with Sb₂O₅) during an accelerated life test. (c) Cyclic voltammetry for anodes annealed at 600 °C, 100 mV s⁻¹, and 1 mol L⁻¹ H₂SO₄. (d) Model of the layer-by-layer degradation mechanism for the anode, (e) cyclic voltametric behavior of Ti/Ni–Sb–SnO₂ anodes with different Ni doping concentrations in the precursor solution. (f) The calculation models of SnO₂ doped with 12.5 at% Sb atoms, 12.5 at% Sb and 6.25 at% Ni atoms and 12.5 at% Sb and 12.5 at% Ni atoms. (g) Fermi energy level and work function of Ti/Ni–Sb–SnO₂.

3.4.3.2 Magnetron sputtering method. Magnetron sputtering enables the fabrication of dense and uniform coatings, although the equipment is intricate and expensive.^127,139,140 Yan et al.¹⁴¹ employed magnetron sputtering to finely tune the microstructure of the coating, fabricating Sb-doped Ti/SnO₂ electrodes. The electrodes fabricated using this method feature a microrod structure, significantly differing from the porous or mud-cracked surfaces produced by other techniques. The microrod structure increases the specific surface area of the anode and the number of active sites, thereby reducing the actual current density and improving the density and adhesion of the coating. Additionally, magnetron sputtering can create coatings with uniform and well-crystallized structures, enhancing the anode's conductivity and stability. This unique structure significantly enhances the performance of the anode, resulting in an oxidation overpotential of approximately 2.2 V for the microrod-structured SnO₂ anode in a 1.0 mol L⁻¹ sulfuric acid solution (as illustrated in Fig. 7c). This performance is comparable to, or even superior to, that of Sb-doped SnO₂ anodes prepared by conventional methods. Moreover, a layer-by-layer degradation mechanism model for the anode has been introduced in the literature (depicted in Fig. 7d), which effectively explains the periodic oscillations observed during accelerated life testing of the microrod-structured anode and provides important theoretical foundations for understanding the anode failure mechanism.
3.4.3.3 Ultrasonic spray pyrolysis method. The ultrasonic spray pyrolysis technique is advantageous due to its low equipment cost and the ease with which it allows for the control of decomposition conditions. Moreover, the generation of ultrafine droplets generated through ultrasonic atomization has been demonstrated to refine the microstructure of the film surface, thereby enhancing the performance of the film.^142–144 Chen et al.¹⁴⁵ employed spray pyrolysis to fabricate Ti/Ni–Sb–SnO₂ anodes, demonstrating excellent density. In contrast to conventional thermal decomposition or spin-coating methods, the surface displayed no noticeable cracks, enhancing the anode's durability and service life. Fig. 7e illustrates the cyclic voltammetry behavior of Ti/Ni–Sb–SnO₂ electrodes with different nickel doping concentrations in 0.5 M sulfuric acid solution. As shown, the oxygen evolution onset potential for the undoped Ti/Sb–SnO₂ electrode is around 2.0–2.4 V (vs. NHE), while nickel doping raises the onset potential to above 2.4 V, with a slight increase as the nickel concentration increases (reaching up to 2.5 V). This finding indicates that nickel doping enhances the anode's oxidation capacity, thereby facilitating the enhanced efficiency of electrochemical treatment for organic pollutants. To further understand the impact of nickel doping on the oxygen evolution onset potential, the team calculated the work function of Sb–SnO₂ with nickel atomic ratios ranging from 0 to 12.5 at%. The computational model is illustrated in Fig. 7f and calculations were performed using density functional theory (DFT, Fig. 7g). The results indicate that an increase in the nickel doping concentration results in a higher work function of SnO₂. This, in turn, aids in elevating the anode's oxidation potential and strengthening its oxidation capability. This finding offers further theoretical backing for the experimental results.

Furthermore, a variety of synthesis methods exist, each exhibiting distinct strengths and weaknesses. These methods are constantly being optimized and explored. For example, hydrothermal synthesis can produce electrodes with distinctive structures by controlling the concentration of hydrochloric acid and has demonstrated superior performance.⁸⁵ Electrodeposition has been demonstrated to yield electrodes with strong adhesion, uniform thickness, and crack-free surfaces. This methodology also enables precise tuning of the coating's structure and performance. However, it requires significant amounts of metal precursor salts, especially for the deposition of precious metals, resulting in increased costs and environmental concerns such as water contamination.¹⁴⁶ Chemical vapor deposition (CVD) is a widely used technique for the fabrication of high-purity, high-density films. This methodology allows for the creation of gradient concentrations or variable compositions by modifying the gas phase composition. Nonetheless, the high cost of gas phase precursors limits its widespread use in DSA electrode synthesis.¹⁴⁷

4. Efficacy of hypochlorite solution

The production capacity of an electrolytic sodium hypochlorite solution system is typically evaluated based on the chlorine content available per unit of time. However, seawater contains various salts in addition to NaCl, including Ca²⁺ and Mg²⁺. Calcium and magnesium ions form calcium carbonate and magnesium carbonate on the surface of the cathode, where they adhere firmly. These deposits increase the cell voltage and decrease the yield of sodium hypochlorite. This directly impacts the energy consumption of the entire chlorine generation system.^148–151 At the same time, different electrode materials and electrolytic cell designs can affect current efficiency, resulting in the actual yield not meeting the theoretical value. Additionally, the conditions under which reactions occur also play a critical role in determining the stability of the solution.

Fig. 8 illustrates the electrochemical reactions occurring on the surface of the cathode electrocatalyst within the reverse electrodialysis (RED) system, as well as the mechanisms behind the formation of inorganic scaling on different electrocatalysts. Diagram a illustrates the nucleation and growth process of scaling (Mg(OH)₂ deposition) on bulk Pt cathode electrocatalysts. On the cathode surface, water undergoes electrolysis to produce hydroxide ions (OH⁻). These OH⁻ ions react with multivalent cations in seawater (e.g., Mg²⁺ and Ca²⁺) to form hydroxides (such as Mg(OH)₂), which progressively deposited on the electrocatalyst surface. The kinetics of crystal growth in precipitates have a significant effect on the properties of cathode electrocatalysts. These properties include the scaling factor (whether at the nanoscale or bulk level), porosity, crystallinity, and surface chemistry. Consequently, these factors can lead to a reduction in electrocatalytic activity and charge transfer efficiency. Diagram b displays the morphological variations in inorganic scaling formed by different cathode electrocatalysts (e.g., bulk Pt, nano Pt/C, heat-treated carbon, untreated carbon, and acid-air plasma-treated carbon) within the RED system, highlighting the influence of electrocatalyst design on scaling behavior. Hence, therefore, the selection of suitable electrode materials can effectively mitigate the effects of precipitates on electrolysis efficiency.

Fig. 8 (a) Schematic of the electrochemical reactions at the cathodic electrocatalyst during real-time RED evaluation. (b) Schematic of the scale formation on bulk Pt, nano Pt/CB/CC, HT CB/CC, bare CB/CC, and AA CB/CC.¹⁵²

4.1 Measurement of available chlorine content

Numerous methods exist for measuring the available chlorine content in solutions, including the test strip method, iodometric titration, colorimetry, and gas chromatography. Nonetheless, these methods are either labor-intensive and time-consuming or require high reagent consumption and costly equipment.¹⁵³ In contrast, UV-visible spectrophotometry provides notable advantages in sensitivity and accuracy, allowing chlorine content to be measured quickly and accurately using a simple procedure. It offers swift analysis, user-friendly operation, excellent reproducibility, and high analytical precision, contributing to its widespread application in water quality assessment across both laboratory and industrial settings. A schematic diagram of the available chlorine method is shown in Fig. 9.

Fig. 9 Schematic diagram of the method for measuring the available chlorine content.

4.1.1 Test strip method. The test strip method involves dipping a chemically treated test strip into a water sample and assessing the available chlorine concentration through color changes. This method is very simple to use, ideal for quick on-site screening, and highly cost-effective, but it has low precision and can only give a rough concentration range. Moreover, ambient humidity and temperature greatly affect the test strip method.

4.1.2 Iodometric titration method. The principle of iodometric titration involves the reaction of available chlorine with potassium iodide in an acidic solution to produce iodine, which is subsequently titrated with a sodium thiosulfate standard solution to calculate the available chlorine content. This method is mature, relatively simple, applicable, and appropriate for routine laboratory analysis. However, it requires specialized chemical reagents and titration apparatus, requires subjective factors that may influence the operator's advanced technical skills, and requires determination of the titration endpoint.

4.1.3 Iodometric photometric method. The iodometric photometric method entails the reaction of available chlorine with potassium iodide in an acidic solution to release iodine, which is then measured colorimetrically using photometry to determine the available chlorine content. This method is applicable for determining high and low concentrations of available chlorine, integrating the high sensitivity and selectivity of photometry to yield more accurate measurements. However, it requires equipment such as a colorimeter, which increases the cost, and requires more extensive sample preparation, which can be affected by sample color or turbidity.¹⁵⁴

4.1.4 Gas chromatography method. Gas chromatography effectively isolates chloride components from the sample using a chromatographic column. It measures the concentration of each component with a detector, such as an electron capture detector (ECD) or a thermal conductivity detector (TCD). Finally, it determines the available chlorine content by referencing a standard curve or applying a correction factor. The thermal conductivity detector operates by measuring the difference in thermal conductivity between the separated components and the carrier gas. Variations in gas components and concentrations lead to changes in the temperature and resistance of the thermal sensor, with the detection signal magnitude proportional to the component concentration. This method offers exceptional sensitivity and selectivity, along with simple operation and reliable results. It generates more representative analytical data, but requires specialized equipment and trained personnel.¹⁵⁵

4.1.5 UV spectrophotometry method. The principle of UV spectrophotometry involves the reaction of chloride ions with specific reagents to generate colored products that display characteristic absorption peaks in the UV-visible spectrum. The absorbance of these colored products is proportional to the concentration of chloride ions, allowing the chloride ion content to be accurately determined from the absorbance and a standard curve. This detection method offers several advantages, including high sensitivity for identifying low concentrations of chloride ions, simple experimental procedures that are ideal for rapid laboratory testing, and the ability to produce highly accurate results that allow accurate quantitative analysis based on a standard curve.^156–158

4.2 Enhancing the stability of the solution

4.2.1 pH effects. Under alkaline conditions (pH > 7), the monovalent chlorine present in sodium hypochlorite solution primarily exists as hypochlorite ions (ClO⁻), which is the most stable form and demonstrates a slow decomposition rate. Additionally, a higher OH⁻ concentration in the solution promotes the reversible hydrolysis reaction of sodium hypochlorite to shift to the left, minimizing the degradation of available chlorine. Nevertheless, an excessively high pH can reduce the bactericidal effectiveness of sodium hypochlorite solution.^159–162 To ensure optimal stability and bactericidal efficacy, the pH of the disinfectant is typically maintained between 7 and 8 by adjusting the free alkali content.

4.2.2 Light effects. Due to its strong oxidizing properties, sodium hypochlorite is subject to natural decomposition at room temperature, a process that is significantly accelerated by exposure to light. The reaction is represented as . According to experimental data, cumulative sunlight exposure exceeding 20 hours under natural conditions reduces the available chlorine content to below 10%. With increasing temperature and light intensity, the motion of organic chlorine molecules accelerates, and the activation energy for decomposition decreases, further hastening the decomposition rate. This process results in precipitation during storage or use, ultimately affecting the available chlorine content.¹⁶³ Hence, light protection is essential during the production, transportation, and storage of sodium hypochlorite solution. For optimal storage, it is advisable to use opaque or dark packaging materials, including rigid PVC, polyethylene, steel containers, or brown bottles.

4.2.3 Solution concentration effects. The concentration of hypochlorite solution is inversely correlated with the tendency of the decomposition reaction. Higher chlorine content in sodium hypochlorite solution increases the likelihood of disproportionation reactions, resulting in accelerated decomposition and reduced stability.^164,165 Low concentrations can effectively slow the decomposition rate and improve stability. In practice, to balance performance and cost effectiveness, the initial concentration of sodium hypochlorite solution should be reduced, or the solution should be diluted to a lower concentration for storage. This method effectively slows the rate of decomposition of the sodium hypochlorite solution, minimizing the loss of active ingredients and increasing its stability during extended storage. Moreover, lowering the concentration reduces safety risks during storage by preventing potential chemical reactions or leaks due to high concentrations.

4.2.4 Stabilizer effects. To enhance the stability of sodium hypochlorite, it is common practice to incorporate an appropriate amount of stabilizer into its aqueous solution. The choice of stabilizers must ensure that they do not diminish the disinfecting efficacy of sodium hypochlorite. Typical stabilizers include inorganic compounds (e.g., sodium silicate, boron nitride, sodium bromide, disodium EDTA) and organic compounds (e.g. β-cyclodextrin, cellulose, sulfamic acid, phenanthroline). Research shows that, under the same experimental conditions, sodium hypochlorite solutions containing stabilizers effectively maintain a stable level of available chlorine compared to a control group without stabilizers. This significantly extends both the shelf life and half-life of the solution, thereby enhancing its practicality and economic value.

In addition, factors such as temperature, storage conditions, metal ions and contaminants, and storage duration can all affect the available chlorine content in sodium hypochlorite solutions. It is essential to maintain optimal conditions to preserve the available chlorine content as much as possible and improve the stability of sodium hypochlorite solution. A schematic diagram of methods to increase the stability of sodium hypochlorite solution is shown in Fig. 10.

Fig. 10 Schematic diagram of methods to enhance the stability of sodium hypochlorite solution.

5. Conclusions and prospects

Electrolytic synthesis of sodium hypochlorite solution is an efficient, safe, and environmentally friendly production method with extensive applications in water treatment, healthcare, and other sectors. As global attention toward environmental protection and public health continues to grow, the electrolytic synthesis of sodium hypochlorite solution technology, with its distinct advantages, can effectively mitigate pollutant emissions and safety risks associated with traditional chemical synthesis. This technology is progressively replacing other synthesis methods, emerging as a new industry trend, delivering substantial economic and environmental benefits, and is poised to play a significant role in more emerging areas.

Significant advancements have been made in the electrolytic synthesis of sodium hypochlorite solution technology, particularly in the areas of equipment design, process optimization, and application development. The development of electrode materials has advanced significantly from unitary to diversified types, with ongoing research expected to produce new electrode materials featuring high current efficiencies, extended lifespans, and low costs. At the same time, ensuring the stability of the sodium hypochlorite solution is imperative and cannot be ignored. Strategies such as adjusting storage conditions and incorporating external additives have proven to be effective in ensuring the stability of the solution.

The electrolytic synthesis of sodium hypochlorite solution technology is poised for continued evolution. First, advances in technology and continued innovation will lead to more efficient and energy-saving production processes while improving product purity and stability. Second, the diversification of market demand indicates that electrolytic synthesis of sodium hypochlorite solution technology has tremendous market potential and will play an increasingly important role in areas such as food processing, textiles and renewable energy, providing new momentum for industry growth. Lastly, the national focus on environmental safety and the push for environmental policies will steer the industry toward greener and more sustainable development. Achieving a mutually beneficial outcome for both economic and environmental interests is possible through technological innovations in multiple domains, including energy conservation and emission reduction, resource recycling, green chemistry, and intelligent automation. Electrolytic synthesis technology fulfills the requirements for sustainable development and is poised to become the dominant process for sodium hypochlorite solution production.

In conclusion, the technology for electrolytic synthesis of sodium hypochlorite solution strongly supports current market needs, highlights significant environmental and economic benefits, and provides a solid foundation for future sustainable development. This technology is poised to drive industry advancements and expand the application of sodium hypochlorite solution into broader markets.

Data availability

Data will be made available on request.

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

This work was financially supported by the Guangxi Natural Science Fund for Distinguished Young Scholars (2024GXNSFFA010008) and the National Natural Science Foundation of China (22469002).

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