electrolysis electrolys
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DC electric through the electrolyte solution or melt to cause an electrochemical reaction at the electrode interface. Electrolysis is carried out in an electrolytic cell (cell). The electrolytic reaction is the reverse reaction of the primary cell reaction. During the electrolysis process, electrical energy is converted into chemical energy and has been widely used in the fields of metal smelting , chlor-alkali industry, inorganic salt production, organic chemical industry, electroplating and electropolishing.
1. Brief History
1807 H. David Electrolytic melting caustic to produce potassium and sodium, opening up new fields to obtain high-purity substances.
1818 M. Faraday proposed the law of electrolysis in the 1930s.
1886 C.M. Hall made aluminum electrolytic successfully.
1890 The first electrolytic potassium chloride to produce chlorine gas factory was put into production. In 1893, diaphragm electrolysis was used to electrolyze saline to make caustic soda. In 1897, mercury electrolytic method achieved industrialization. At this point, electrolysis method has become an important production method in the chemical industry and metallurgical industry.
2. Principle
ions in electrolyte solution or melt are usually in a state of irregular motion. When direct current is turned on, the ions move in a direction under the action of an electric field, which is called the electromigration of the ions. At the same time, an electrochemical reaction occurs at the interface of the two electrodes, which is called electrode reaction . cation migrates toward the cathode direction, and electrons obtained on the cathode are reduced; anion migrates toward the anode direction, and electrons lost on the anode are oxidized.
For example, when electrolyzing copper chloride aqueous solution (see figure), copper ions migrates toward the cathode direction, and the cathode reaction is:
(1)
chloride ions migrates toward the anode direction, and the anode reaction is:
(2) The sum of the cathode reaction and the anode reaction is the electrolytic reaction:
Therefore, when electrolyzing the copper chloride solution, metal copper is deposited on the cathode, and chlorine gas is generated on the anode. During the electrolysis process, the amount of substance generated on any electrode follows the Faraday's electrolysis law.
Even if the same electrolyte is electrolyzed, if the electrode materials are different, or electrolyzed at different temperatures and electrode potential , the rate of electrode reactions may be different. When
is electrolyzed, a battery is formed between the electrode, the precipitated substance and the electrolyte. The battery electromotive force is numerically equal to the theoretical electrolytic voltage of the electrolyte, which can be calculated by the Nesch formula. Since the electrode polarization and the electrolytic cell have an internal resistance ratio of the actual electrolytic voltage E (also known as the voltage) than the theoretical electrolytic voltage, that is, the
neutralization is the superpotential of the cathode and anode respectively, which is the voltage drop consumed by the internal resistance of the electrolytic cell. The overpotential and resistance of the entire electrolytic cell can be reduced by selecting electrode materials and surface treatment and improving the structure of the electrolytic cell to minimize the electrolytic voltage. Any ions in the
electrolytic cell and its discharge product form an electrode, and the potential of the electrode is called the precipitation potential of this ion. In the cathode, ions with higher precipitation potential are preferred for electrode reactions; in the anode, ions with lower precipitation potential are preferred for electrode reactions.
electrolysis is a powerful redox method. Many redox reactions , which are difficult to occur in general, can be achieved through electrolysis. For example, only by electrolysis can the molten fluoride be oxidized to elemental fluorine on the anode. When the lithium salt is electrolyzed, the lithium ions that are difficult to reduce can be reduced to metal lithium on the cathode.
3. Type
According to the different states of the electrolyte, electrolysis can usually be divided into aqueous solution electrolysis and molten salt electrolysis. According to the process that occurs on the electrode, it can be divided into simple electron transfer, gas release, metal corrosion, metal precipitation, oxide generation and organic electrolysis.
3.1. Aqueous solution electrolysis
The chlor-alkali industry based on electrolytic saline, electrolytic refining, metal plating and electropolishing in the metallurgical industry, all belong to this type of electrolysis.
electrolytic refining uses crude metal as the anode of the electrolytic cell, and after oxidation, it turns into ions and enters the solution. Then, a higher-grade metal is deposited on the cathode to achieve the purpose of refining. During the electroplating process, in order to improve the surface state of the metal precipitated on the cathode, one or several additives (such as animal glue, thiourea and cheese) must be added to the electrolyte .
electropolishing is a method of finishing the metal surface. It uses metal products suspended in the electrolytic cell as an anode, and electrolyzes under specific conditions. The dissolution of the anode metal is eliminated to eliminate subtle unevenness on the surface of the product and make it have a mirror-like glossy appearance.
In order to solve environmental pollution, electrolysis is also commonly used to remove certain harmful metal ions in sewage.
3.2. molten salt electrolytic
is mainly used in the production of light metal and rare metals such as aluminum and magnesium. The molten salt electrolyte used in electrolysis should have low melting point, appropriate viscosity, density, surface tension, , a sufficiently high conductivity, and quite low volatility and insoluble metal melts that are electrolyzed.
In most cases, the current efficiency of molten salt electrolysis is lower than that of aqueous solution electrolysis. Factors affecting current efficiency include temperature, current density, interpole distance and the properties of electrolytes. Among them, the dissolution of metal by electrolytes is the main factor in reducing current efficiency.
3.3. Organic electrolysis
electrolysis has important applications in organic synthesis of . Compared with general organic synthesis, the main feature of electro-organic synthesis is that there is no need to introduce additional oxidants and reducing agents into the system, such as reducing glucose to sorbitol and mannitol on the cathode. This not only reduces the consumption of precious chemicals, but also eliminates certain environmental pollution. Electrosynthesis can cause some reactions that are poor in stability and are not suitable for high temperature and high pressure, so as to be carried out at normal temperature and pressure.
In addition, in electrical synthesis, the reaction rate of side reactions can be greatly reduced by selecting electrode materials and controlling temperature and voltage, and the operation of separation and purification in production can be simplified.
Excerpted from: "China Encyclopedia (2nd Edition)" Volume 5, China Encyclopedia Press , 2009
