Core Structure Analysis of Chlor-Alkali Electrolyzers: Anode, Cathode, and Membrane Assembly

The essence of a chlor-alkali electrolyzer is a precisely controlled electrochemical reactor. Direct current flows in through the anode and out through the cathode. Within an extremely narrow gap, brine is decomposed into chlorine gas, hydrogen gas, and caustic soda. Driving all of this are three precision-engineered core components working in concert: the anode, the cathode, and the ion-exchange membrane.

1. Anode Structure and Catalytic Properties

The anode is where chloride ions surrender their electrons. The anode of a modern chlor-alkali electrolyzer is a titanium metal expanded mesh, with a surface coating of mixed metal oxides only a few micrometers thick. Titanium is chosen as the substrate material because, under the anodic polarization conditions of electrolysis, a dense titanium dioxide (TiO₂) passivation film spontaneously forms on the titanium surface. This film is extremely stable in wet chlorine gas and concentrated brine. However, while the passivation film protects titanium from corrosion, it also strips the titanium surface of its electrical conductivity. Therefore, a micrometer-scale catalytic coating is sprayed onto the titanium surface. The core component of the coating is ruthenium dioxide (RuO₂), an oxide with metallic-level conductivity. The work function of RuO₂ falls precisely within the bandgap of TiO₂, forming a “conductive bridge” within the TiO₂ passivation film that allows electrons to pass from the electrolyte through the passivation film to the titanium substrate. Meanwhile, RuO₂ exhibits extremely high catalytic activity for the chlorine evolution reaction (2Cl⁻ → Cl₂ + 2e⁻). At the same voltage, the chlorine generation rate is far greater than the oxygen generation rate, improving current efficiency. However, a pure RuO₂ coating slowly dissolves during long-term operation in acidic anolyte (pH 2–4). Therefore, iridium dioxide (IrO₂) is added to the coating of commercial DSAs (Dimensionally Stable Anodes). Iridium dioxide is far more stable than binary ruthenium oxide in acidic environments, extending coating life. Some formulations also contain SnO₂ as a stabilizer and dispersant. Diaphragm cell anode coatings typically contain more SnO₂ because the diaphragm cell anolyte has a higher pH, creating a greater need to suppress the oxygen evolution side reaction.

2. Membrane Layer Structure and Ion Selectivity

The modern chlor-alkali membrane is a perfluorinated polymer composite material comprising four functional layers: the thickest layer is the sulfonic acid layer, facing the anode. Perfluorosulfonic acid (-SO₃H) is a superacid. This layer has high water content, and sodium ions pass through it unimpeded; it is the main conductive layer of the membrane, providing a low-resistance pathway. Embedded behind the sulfonic acid layer is a PTFE fabric that provides the mechanical skeleton. The chemical inertness of the perfluorocarbon chain endows the membrane with durability in the dual extreme environment of strong oxidizing and strong alkaline conditions. The PTFE skeleton also ensures that the membrane does not undergo thermal creep during high-temperature operation. Localized shrinkage or expansion of the membrane would cause changes in the gap with the electrodes, leading to uncontrolled local current density. Facing the cathode is the carboxylic acid layer; perfluorocarboxylic acid (-COOH) is a weak acid. On the acidic, water-rich anode side, -COOH partially remains as neutral molecules, with ion channels half-open; on the alkaline, water-poor cathode side, -COOH fully dissociates into -COO⁻, and the dense negative charge forms a Donnan exclusion barrier that prevents OH⁻ back-migration. The outermost layer is an inorganic coating a few micrometers thick (typically porous oxides such as ZrO₂). By forming a porous, rough surface structure, it reduces the membrane surface's adhesion to bubbles, allowing the chlorine and hydrogen gases produced by electrolysis to rapidly detach from the membrane surface. A single sulfonic acid membrane does not have sufficient negative charge density to effectively block OH⁻ back-migration. The OH⁻ concentration in the catholyte is far higher than that in the anolyte, and the excessive concentration gradient drives OH⁻ to migrate backward through the membrane to the anode, greatly reducing current efficiency. This was later addressed by laminating a carboxylic acid layer onto the sulfonic acid layer. The negative charge density of -COO⁻ in the carboxylic acid layer is far higher than that of -SO₃⁻, enhancing the repulsion of OH⁻. Although the total membrane resistance increases slightly, current efficiency leaps from 80% to 96–97%.

3. Core R&D Directions of Key Components

These three core components of the chlor-alkali electrolyzer represent three dimensions of research and development: the durability of electrocatalytic coated anodes in strongly oxidizing environments; the stability of high specific surface area catalytic cathodes in strong alkali; and the efficient ion selectivity of perfluorinated membranes under extreme pH gradients.

FAQ：

1. Why does the titanium anode need a mixed metal oxide coating?

A natural TiO₂ passivation film on titanium prevents corrosion but loses conductivity. The RuO₂-IrO₂ composite coating builds a conductive bridge, provides high chlorine evolution catalytic activity, and improves acidic environment stability.

2. What is the function of IrO₂and SnO₂in anode coatings?

IrO₂ improves the long-term acid resistance of the coating to avoid dissolution. SnO₂ acts as a stabilizer and dispersant, which helps suppress oxygen evolution side reactions for diaphragm cells.

3. Why does the chlor-alkali membrane adopt a dual-layer sulfonic acid + carboxylic acid structure?

The sulfonic acid layer ensures high ion conductivity and low resistance. The carboxylic acid layer forms a strong Donnan exclusion barrier to block OH⁻ back-migration, greatly improving current efficiency from 80% to 96%–97%.