an apparatus in which electrolysis is carried out, consisting of one or many electrolytic cells. An electrolyzer is a vessel (or system of vessels) filled with an electrolyte, in which electrodes—a cathode and anode—have been placed; the cathode is connected to the negative pole of the direct-current source and the anode is connected to the positive pole.

Various types of electrolyzers are used in industry and the laboratory, for example, open and hermetically sealed electrolyzers, electrolyzers for batch and continuous operation, electrolyzers with fixed or removable electrodes, and electrolyzers with various systems for separating the electrolysis products. Depending on their intended use, electrolyzers are designed for operation at different temperatures, from below 0°C (for example, the electrochemical synthesis of unstable oxygen compounds) to high temperatures (for example, in the electrolysis of molten electrolytes in the production of aluminum, calcium, and other metals), and they are accordingly equipped with systems for heating or cooling the electrolyte or electrodes. Some electrolyzers have a diaphragm—a porous barrier or membrane that separates the cathodic space from the anodic space and that is permeable to ions but retards mechanical mixing and diffusion. Asbestos, polymeric materials, and ceramics are used to make these diaphragms. Electrolyzers with ion-exchange membranes are also used. A distinction is made between unipolar and bipolar electrolyzers on the basis of how the electrolyzers are connected to the electric circuit. Unipolar electrolyzers consist of one electrolytic cell with electrodes of one polarity, each of which may consist of several elements connected in parallel to the circuit. Bipolar electrolyzers consist of many cells, as many as 100–160, which are connected in series to the circuit, and each electrode, with the exception of the two terminal electrodes, functions as a cathode at one end and an anode at the other.

The anodes are made of graphite, carbon-graphite materials, platinum, the oxides of several metals, lead, or lead alloys. Heavy-duty titanium anodes are also used, with an active coating made of a mixture of ruthenium oxide and titanium oxide or of platinum and its alloys. The cathode in most electrolyzers is made of steel. Liquid-electrode electrolyzers are also used (for example, mercury is used as the cathode in one method for the production of chlorine and sodium hydroxide). Some electrolyzers operate under pressure, for example, water is dissociated at pressures up to 4 meganewtons/m² (40 kilograms-force/cm²). The development of electrolyzers operating at higher pressures is presently under way. When selecting the materials for the construction of electrolyzers, the corrosiveness of the electrolyte and electrolysis products, temperature, and other factors are taken into account. Steel, including steel with various protective coatings, plastics, glass, fiberglass, and ceramics are used. Large modern electrolyzers operate under large loads: unipolar electrolyzers, up to 400–500 kiloamperes, and bipolar electrolyzers, up to 1,600 kiloamperes.