Aqueous Solution Electrolytic Cells
Aqueous solution electrolytic cells can be divided into two types: diaphragm cells and non-diaphragm cells. Diaphragm cells can be further divided into isotropic membranes (asbestos wool), ion membranes, and solid electrolyte membranes (such as β-Al2O3); non-diaphragm cells can be further divided into mercury cells and oxidation cells.
The cell structure varies depending on the electrolyte used.
Aqueous solution electrolytic cells are divided into two types: diaphragm cells and non-diaphragm cells. Diaphragm cells are generally used. Non-diaphragm cells are used in chlorate production and in the mercury process for the production of chlorine and caustic soda. Maximizing the electrode surface area per unit volume can improve the cell's production efficiency. Therefore, the electrodes in modern diaphragm electrolytic cells are mostly upright. Electrolytic cells exhibit different performance and characteristics depending on the material, structure, and installation of their internal components.
Molten Salt Electrolyzers
Mostly used to produce low-melting-point metals, they operate at high temperatures and minimize moisture ingress to prevent hydrogen ion reduction at the cathode. For example, when producing metallic sodium, the cathode reduction potential of sodium ions is very negative, making reduction difficult. Therefore, anhydrous molten salts or molten hydroxides that do not contain hydrogen ions must be used to prevent hydrogen deposition at the cathode. Therefore, the electrolysis process must be conducted at high temperatures, such as 310°C for the electrolysis of molten sodium hydroxide. For mixed electrolytes containing sodium chloride, the electrolysis temperature is around 650°C.
Higher electrolytic cell temperatures can be achieved by varying the spacing between the electrodes, converting the electrical energy consumed by the ohmic voltage drop into heat. When electrolyzing molten sodium hydroxide, the cell can be made of iron or nickel. When electrolyzing molten electrolytes containing chlorides, the inevitable introduction of small amounts of water from the raw materials can generate moist chlorine gas at the anode, which is highly corrosive to the cell. Therefore, cells for the electrolysis of molten chlorides are generally made of ceramic or phosphate materials, while areas protected from chlorine gas can be made of iron. The cathodic and anodic products in molten salt electrolyzers must also be properly separated and removed from the cell as quickly as possible to prevent the cathode product, sodium metal, from floating on the electrolyte surface for extended periods, where it could react with the anode product or oxygen in the air.
Non-aqueous Electrolyzers
Because the complex chemical reactions often involved in producing organic products or electrolyzing organic matter in non-aqueous electrolyzers limit their application and are rarely used industrially. Commonly used organic electrolytes have low conductivity and slow reaction rates. Therefore, a lower current density and minimal interelectrode spacing are essential. Fixed-bed or fluidized-bed electrode structures offer a larger electrode surface area, increasing electrolyzer capacity.
