Main structure of electrolytic cell

electrode anode
The anode and cathode have different functions and have different material requirements.
Divided into two categories: soluble and insoluble. In electrolytic cells for refining copper, 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 the solution at the cathode. In electrolytic cells used to electrolyze aqueous solutions (such as salt water solutions), the anodes are insoluble and they basically do not change during the electrolysis process, but they often have a catalytic effect on the anode reactions carried out 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 conductivity, catalytic activity strength, processing, source, price), anode materials must also be insoluble and non-passivated in strong anodic polarization and higher-temperature anolytes. , 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. It is constantly corroded and peeled off during the electrolysis process, causing the electrode distance to gradually increase and the cell voltage to increase. When used for electrolysis of salt water solution, the overpotential of chlorine evolution on the graphite electrode is also high.
The metal oxide electrode formed by coating ruthenium oxide and titanium oxide on a titanium base proposed by H. Beer in the 1960s was 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 outstanding feature is that it has good chemical stability and its working life is much longer than that of graphite anodes. For example, in diaphragm electrolyzers used in chlor-alkali production, their lifespan 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. 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 molten salt electrolyzers, because the electrolysis temperature is much higher than that in aqueous solution electrolyzers, the requirements for anode materials are stricter. For electrolysis of molten sodium hydroxide, steel, nickel and their alloys are generally used. For electrolysis of molten chloride, only graphite can be used.
cathode
When metal or alloy is used as the cathode, since it works at a relatively negative potential, it can often play a role in cathodic protection and is less corrosive, so the cathode material is easier to select. In an aqueous electrolytic cell, the cathode generally produces a hydrogen evolution reaction and has a high overpotential. Therefore, the main improvement direction of cathode materials is to reduce the hydrogen evolution overpotential. Except when using sulfuric acid as the electrolyte, lead or graphite must be used as the cathode, 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 used to electrolyze salt water solution to produce caustic soda using the mercury method, the high overpotential of hydrogen evolution from mercury is used to discharge sodium ions to generate sodium amalgam, which is then used in a special In the equipment, sodium amalgam is decomposed with water to prepare high-purity, high-concentration alkali solution. In addition, in order to save electric energy, an oxygen-consuming cathode can also be used to reduce oxygen at the cathode to replace the hydrogen evolution reaction. According to theoretical calculations, the cell voltage can be reduced by 1.23V.
diaphragm
In order to prevent the mixing of cathode and anode products and avoid possible harmful reactions, in electrolytic cells, diaphragms are basically used to separate the cathode and anode chambers. The diaphragm needs to have a certain porosity to allow ions to pass through without allowing molecules or bubbles to pass through. When current flows through the diaphragm, the ohmic voltage drop of the diaphragm must be low. These performance requirements remain basically unchanged during use, and they require good chemical stability and mechanical strength under the action of the electrolytes in the cathode and anode chambers. When electrolyzing water, the electrolytes in the cathode and anode chambers are the same. The diaphragm of the electrolytic cell only needs to separate the cathode and anode chambers to ensure the purity of hydrogen and oxygen and prevent explosions caused by mixing hydrogen and oxygen. A more common and complicated situation is that the electrolyte compositions in the cathode and anode chambers of the electrolytic cell are different. At this time, the diaphragm also needs to prevent the mutual diffusion and interaction of electrolytic products in the electrolytes of the cathode and anode chambers. For example, the diaphragm in the diaphragm electrolytic cell in chlor-alkali production can increase the resistance of the hydroxide ions from the cathode chamber to the anode chamber.
Diaphragms are made of inert materials, such as the asbestos diaphragms long used in the chlor-alkali industry. However, the performance of asbestos separators is unstable. When the brine contains calcium and magnesium impurities, hydroxide precipitation is easily generated in the separator, reducing the permeability. At relatively high temperatures and under the action of electrolyte, swelling and loosening may occur. Take off. For this purpose, resin can be added to asbestos as a reinforcing material, or a microporous membrane can be made with resin as the main body, which can greatly improve the stability and mechanical strength. The cation exchange membrane developed in chlor-alkali production in recent years is a new type of membrane material. It has selectivity for ion permeation, which can basically prevent chloride ions from entering the cathode chamber, so that an alkali solution with extremely low sodium chloride content can be produced.