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The electrolytic cell consists of a cell body, an anode and a cathode, and most of them are separated from the anode chamber and the cathode chamber by a diaphragm. According to the different electrolytes, it is divided into three types: aqueous electrolytic cells, molten salt electrolytic cells and non-aqueous electrolytic cells. When direct current passes through the electrolytic cell, oxidation reaction occurs at the interface between the anode and the solution, and reduction reaction occurs at the interface between the cathode and the solution to produce the desired product. Optimizing the structure of the electrolytic cell and selecting the materials of the electrodes and diaphragms are the keys to improving the current efficiency, reducing the cell voltage and saving energy.
Anode
The functions of the anode and the cathode are different, and the material requirements are also different.
Divided into soluble and insoluble categories. 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 electrolytic cells for electrolyzing aqueous solutions (such as brine solutions), the anodes are insoluble, and they do not change substantially during the electrolysis process, but often have a catalytic effect on the anodic reaction that occurs on the electrode surface. In the chemical industry, insoluble anodes are mostly used.
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. Graphite has long been 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.
The metal oxide electrode formed by coating ruthenium oxide and titanium oxide on the titanium base proposed by H. Bill in the 1960s is a major innovation in anode materials. Ruthenium dioxide has good catalytic activity for certain anode reactions such as chlorine evolution and oxygen evolution, and can work at high current density with relatively low cell voltage. The most prominent feature is that it has good chemical stability, and its working life is much longer than that of graphite anodes. For example, in the diaphragm electrolytic cell for chlor-alkali production, its life span can reach more than 10 years. Because it is not easy to corrode and is dimensionally stable, it is called a dimensionally stable anode. In order to adapt to different requirements and uses, other components can be added to the coating. For example, adding tin and iridium can increase the overpotential of oxygen and improve the selectivity of the anode. For example, adding platinum can improve the stability of the electrode. At present, precious metal-coated metal anodes have been widely promoted in the chemical industry.
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.
Cathode
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.
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 the mercury electrolysis of brine solution, the high overpotential of mercury hydrogen evolution is used to discharge sodium ions to generate sodium amalgam, which is then used in special In the equipment of high-purity and high-concentration lye, the sodium amalgam is decomposed 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.
Diaphragm
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.
Diaphragms are made of inert materials, such as asbestos diaphragms long 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; at relatively high temperatures and under the action of the electrolyte, swelling and loose 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.
