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Introduction to the main structure of the electrolytic cell

2022/01/14 16:55
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[Abstract]:
The functions of anode and cathode are different, and the material requirements are also different.

1. Anode
​​Electrolyzer
The functions of anode and cathode are different, and the material requirements are also different.
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Divided into soluble and insoluble. In an electrolytic cell for copper refining, the anode material is soluble blister copper to be refined. It dissolves into the solution during electrolysis to replenish the copper that comes out of solution at the cathode. In electrolyzers for electrolysis of aqueous solutions (such as brine solutions), the anodes are insoluble, and they do not change substantially during the electrolysis process, but they often catalyze the anodic reactions that take place on the electrode surface. In the chemical industry, insoluble anodes are mostly used.
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In addition to meeting the basic requirements of general electrode materials (such as electrical conductivity, catalytic activity strength, processing, source, and price), anode materials also need to be insoluble and passive in strong anodic polarization and high temperature anolyte , with high stability. * Since then, graphite is the most widely used anode material. However, graphite is porous, has poor mechanical strength, and is easily oxidized into carbon dioxide. The chlorine evolution overpotential on the graphite electrode is also higher when it is used for electrolysis of saline solution.
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In the molten salt electrolysis cell, because the electrolysis temperature is much higher than that in the aqueous solution electrolysis cell, the requirements for the anode material are stricter. For the electrolysis of molten sodium hydroxide, steel, nickel and its alloys are generally available. For electrolytic molten chloride, only graphite can be used.
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2. Cathode
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When a metal or alloy is used as the cathode, because it works at a relatively negative potential, it can often play a cathodic protection role, and the corrosiveness is small, so the cathode material is easier to choose. In an aqueous electrolyzer, the cathode generally produces a hydrogen evolution reaction with a high overpotential. Therefore, the main improvement direction of cathode materials is to reduce the hydrogen evolution overpotential. In addition to the use of lead or graphite as the cathode when sulfuric acid is used as the electrolyte, low carbon steel is a commonly used cathode material. In order to reduce power consumption, various methods are currently used to prepare cathodes with high specific surface area and catalytic activity, such as porous nickel-plated cathodes.
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In order to improve product quality, special cathode materials can also be used. For example, in the mercury cathode for the production of caustic soda by electrolysis of brine solution by mercury method, the high overpotential of mercury hydrogen evolution is used to discharge sodium ions to generate sodium amalgam, which is then used in In the equipment, high-purity and high-concentration lye is prepared by decomposing sodium amalgam with water. In addition, in order to save electricity, an oxygen-consuming cathode can also be used to reduce oxygen at the cathode to replace the hydrogen evolution reaction. According to the theoretical calculation, the cell voltage can be reduced by 1.23V.
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3. Diaphragm
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In order to prevent the mixture of cathode and anode products and avoid possible harmful reactions, in the electrolytic cell, the cathode and anode chambers are basically separated by a diaphragm. The diaphragm needs to have a certain porosity, so that ions can pass through, but not molecules or bubbles. When there is current flowing, the ohmic voltage drop of the diaphragm should be lower. These performance requirements are basically unchanged during use, and require good chemical stability and mechanical strength under the action of the cathode and anode chamber electrolytes. When electrolyzing water, the electrolyte in the cathode and anode chambers is the same, and the diaphragm of the electrolytic cell only needs to separate the cathode and anode chambers to ensure the purity of hydrogen and oxygen, and to prevent the explosion of hydrogen and oxygen mixture. The more common and more complicated situation is that the electrolyte composition of the cathode and anode chambers in the electrolytic cell is different. At this time, the diaphragm also needs to prevent the mutual diffusion and interaction of electrolysis products in the electrolytes of the cathode and anode chambers. For example, the diaphragm in the diaphragm electrolytic cell in the production of chlor-alkali can increase the resistance of the diffusion and migration of hydroxide ions in the cathode chamber to the anode chamber.
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Diaphragms are made of inert materials, such as asbestos diaphragms, which are widely used in the chlor-alkali industry. However, the performance of the asbestos diaphragm is unstable. When the brine contains calcium and magnesium impurities, it is easy to form hydroxide precipitation in the diaphragm, reducing the permeability. take off. To this end, resin can be added to asbestos as a reinforcing material, or a microporous diaphragm can be made of resin as the main body, which has great improvement in stability and mechanical strength. The cation exchange membrane developed in the chlor-alkali production in recent years is a new type of membrane material. It has the selectivity of ion permeation, which can make the chloride ions basically not enter the cathode chamber, so that the alkali solution with extremely low sodium chloride content can be obtained.