The non-diaphragm method explains the production of sodium hypochlorite from dilute salt water at room temperature, which overcomes the difficulties and dangers in transportation, storage and use of chlorine preparations such as liquid chlorine. The fields of disinfection of water and restaurant canteen tableware are more and more widely used. However, the electrolytic production of sodium hypochlorite, due to the use of low-concentration brine and low reaction temperature, greatly intensifies the oxygen evolution side reaction of the anode during the electrolysis process, which reduces the anode current efficiency on the one hand, and increases the anode corrosion efficiency on the other hand. increase, which is an order of magnitude higher than that of the chlor-alkali industry. These harsh conditions put forward high requirements for the selection of anode materials for sodium hypochlorite generators, so that the anode must have good reaction selectivity and excellent corrosion resistance. The anode materials commonly used in the electrochemical industry such as carbon and graphite in the early days have been used in the production of sodium hypochlorite, but they were eliminated because they could not meet the harsh conditions. Titanium-plating grade metal anodes made by electroplating technology have been widely promoted in the production of sodium hypochlorite for the first time due to their good corrosion resistance and convenient production. The price makes it more and more unsatisfactory for users.

Over the past decade, many modifications have been made to titanium platinum-coated anodes and ruthenium-titanium anodes, especially by adding iridium.

Equal corrosion-resistant active components and the use of multi-layer coating structure have made a breakthrough in the life and electrochemical performance indicators of the anode, and promoted the popularization and application of sodium hypochlorite generators. Base metal oxide anodes such as lead dioxide for the electrolysis of sulfuric acid solutions still have a place in the production of sodium hypochlorite due to their low cost. Recently, an amorphous alloy anode with excellent electrochemical performance has come out. In theory, this anode is an ideal anode material for brine electrolysis.

In 1957, Bill invented titanium, tantalum, niobium, zirconium and other valve metal anodes plated with platinum group metals. Titanium platinum-plated metal anodes were first used in the production of sodium hypochlorite on a large scale. Because the platinum coating is particularly hard, wear-resistant, dimensionally stable and simple to manufacture, and its lifespan can reach 15,000 hours in the production of sodium hypochlorite, its corrosion resistance is unmatched by any other electrode at that time. However, the fatal defect of titanium platinum-plated anode is that the chlorine discharge potential is too high, and its performance depends on the bonding strength of platinum microstructure and titanium matrix, and the actual surface area of the coating is related to the grain size and the exposed crystal plane. Compared with the geometric area, the area does not increase much, which affects the improvement of the chlorine release activity of the anode.

Moreover, the porosity of the platinum coating is as high as 15%-30%, and the electrolyte can often directly contact the substrate, resulting in the passivation of the titanium base and the failure of the platinum coating. To this end, it is proposed to add a corrosion-resistant platinum group metal oxide intermediate layer under the platinum coating to block the penetration of the electrolyte and improve the electrochemical performance, which is a major breakthrough for noble metal anodes. Among platinum group metal oxides, iridium dioxide interlayers are the most effective

, Because IrO2 has the characteristics of low chlorine evolution potential and no passivation, it can significantly improve the electrochemical performance of the anode and overcome the characteristics of platinum plating that is easy to fall off. The MODE anode with IrO2 intermediate layer has doubled the life span compared to the platinum-plated titanium anode, reaching 6 years, and the current efficiency has also increased by 10%. It can be seen that although the cost of the MODE anode is too high, the indicators are indeed outstanding.

Another major breakthrough of precious metal anodes in recent years is to use thermal decomposition instead of electrodeposition to prepare electrodes.

The real surface area of the solution coating can reach 100-1000 times the original geometric area, which greatly exceeds the real surface area of the coating. greatly exceeds the true surface area of the coating. The actual current density can be significantly reduced when used. Therefore, the titanium electrode prepared by thermal decomposition method has greatly improved lifespan and electrochemical performance compared with the platinum-plated electrode. The addition of a corrosion-resistant active component iridium to the thermally decomposed platinum coating has created today's Pt-Ir anode, in which platinum, as a corrosion-resistant conductive phase, plays a role in the other electrode catalytic phase, iridium dioxide. The role of the adhesive. The electrode has strong corrosion resistance and superior chemical properties, and in the electrolysis test, the current efficiency can reach about 80%. The corrosion rate is only 0.3 micrograms/ampere/hour. For titanium platinized anode and 12-1/4, the conversion life can reach 26000 hours. At present, Pt-Ir anode is known as the most active electrode in foreign countries. It has replaced platinum-plated electrodes and has become the most important anode material for sodium hypochlorite production.

Noble metal oxide anodes (titanium anodes), which are very stable in most electrochemical processes, are also known as dimensionally stable anodes. Because of its simple production and lower cost than platinum-plated anodes, and its electrochemical performance due to platinum-plated anodes, it has become an important anode material in the electrochemical industry including chlor-alkali industry, chlorate industry, cathodic protection, and electrolytic extraction. However, the ruthenium-titanium electrode, which has a lifespan of about 10 years in the chlor-alkali industry, has a lifespan of only 2-3 months in the production of sodium hypochlorite. In order to increase the lifespan, people try to thicken the coating, but its effect is very limited, because the coating is easy to fall off after reaching a certain thickness, and this cannot solve the disadvantage of poor reaction performance of the titanium electrode.

In order to enhance the life and performance of titanium anodes, people have studied adding platinum group elements to the coating, and found many anodes containing iridium, rhodium, palladium, platinum and other elements. They have their own characteristics in catalytic activity and service life. Although palladium has the highest chlorine evolution activity, it has an adverse effect on the life of the anode, and the titanium anode containing iridium and rhodium is the main direction of development in the future, especially the anode containing iridium. It has good performance, will not be passivated, and has strong corrosion resistance. To develop a high-life titanium anode, it must be a high-performance titanium anode. At present, the titanium anodes that have been successfully applied at home and abroad are basically iridium-containing anodes.

The biggest progress of noble metal oxide anodes in recent years is the invention of the anode with a multilayer structure, that is, one or more intermediate layers are added under the active surface layer. The addition of the intermediate layer can not only block the infiltration of oxygen during the electrolysis process, slow down the passivation of the matrix, but also improve the reaction selectivity of the anode and reduce the contact potential between the active surface layer and the zinc matrix. It can be increased several times to ten times. At present, there are two commonly used intermediate layers: one is the intermediate layer of platinum, iridium, palladium, germanium and other noble metal oxides that are not easily oxidized, and some non-metallic elements can also be added, and the other is the intermediate layer of base metal oxides , the most common are Sn-Sb and Sn-Nb oxides dominated by tin, which not only considers the rutile phase structure SnO2 is not easy to be oxidized, but also adds Sb, Nb can improve the conductivity of Sno2, which can significantly Slow down the oxidation of titanium base and improve the electrochemical performance of anode.