In the field of electrochemistry, a performance of anode materials directly determines an efficiency, energy consumption or service life of the electrolysis process. Metal-Metal Oxide Coated Titanium Anode (MMO Titanium Anode) is a new type of electrochemical anode material. Since its introduction in the 1960s, it has quickly become an ideal substitute for traditional lead-based and graphite-based anodes due to its excellent electrochemical performance, corrosion resistance and long life. MMO titanium anodes are widely used in the chlor-alkali industry, water treatment, metal electrodeposition, cathodic protection and other fields by coating one or more layers of metal oxide coating on the surface of the titanium substrate, combining the high strength and light weight of the titanium substrate with the high catalytic activity of the metal oxide. With the growing global demand for green, environmentally friendly and energy-efficient technologies, MMO titanium anodes, with their unique technical advantages, play an increasingly important role in promoting the sustainable development of the electrochemical industry.
1. Types of MMO titanium anodes
1.1 Classification by coating metal oxide composition
DSA type (dimensionally stable anode): This is the most common type of MMO titanium anode, mainly based on titanium-based ruthenium dioxide (Ti/RuOâ‚‚), modified by adding other metal oxides (such as TiOâ‚‚, IrOâ‚‚, Taâ‚‚Oâ‚…, etc.). DSA type anodes perform well in chlorine evolution reactions, and their coatings have high chlorine evolution catalytic activity and good electrochemical stability. For example, in the chlor-alkali industry, Ti/RuOâ‚‚-TiOâ‚‚ coated anodes can efficiently produce chlorine at high current density while maintaining low overpotential and reducing energy consumption. For example, adding IrOâ‚‚ can improve the corrosion resistance and oxygen evolution catalytic activity of the anode in an acidic environment.
Oxygen evolution anode: This type of anode is mainly composed of titanium-based iridium dioxide (Ti/IrOâ‚‚) or titanium-based iridium tantalum oxide (Ti/IrOâ‚‚-Taâ‚‚Oâ‚…), and is mainly used in oxygen evolution reaction processes, such as electrolysis of water to produce oxygen, electroplating wastewater treatment, etc. IrOâ‚‚ has excellent oxygen evolution catalytic activity and chemical stability, and can maintain good performance in both acidic and alkaline environments. Ti/IrOâ‚‚-Taâ‚‚Oâ‚… coated anode further enhances the corrosion resistance and mechanical strength of the coating by compounding Taâ‚‚Oâ‚… with IrOâ‚‚, thereby extending the service life of the anode. This type of anode can effectively reduce the oxygen evolution overpotential and improve the electrolysis efficiency under high voltage and strong oxidation environments.
Multifunctional anode: In order to meet the needs of complex working conditions, MMO titanium anodes with multiple functions have been developed. For example, titanium-based platinum oxide coating anodes (such as Ti/Pt-IrOâ‚‚) combine the high catalytic activity of platinum and the corrosion resistance of IrOâ‚‚. They are suitable for chlorine evolution, oxygen evolution and organic oxidation reactions, and are often used to treat industrial wastewater containing multiple pollutants. In addition, there are MMO titanium anodes doped with rare earth elements (such as cerium Ce and lanthanum La). The addition of rare earth elements can improve the conductivity and catalytic activity of the coating and broaden the application range of the anode.
1.2 Classification by preparation process
Preparation of anodes by thermal decomposition: This is the most traditional and widely used preparation process. First, a metal salt (such as ruthenium salt, iridium salt) is prepared into a solution and coated on the surface of the titanium substrate by brushing, dipping or spraying, and then thermally decomposed at high temperature (400-600℃) to form a metal oxide coating. This method is simple and low in cost, but the uniformity and density of the coating are difficult to control, and multiple coating-thermal decomposition cycles may cause thermal stress between the titanium substrate and the coating, affecting the bonding strength.
Preparation of anode by sol-gel method: The hydrolysis and polycondensation reaction of metal alkoxides (such as ruthenium tetraisopropoxide and iridium tetrabutoxide) is used to prepare a uniform and stable sol, which is then coated on a titanium substrate by dipping or spin coating, and then dried and heat treated to form an oxide coating. The coating prepared by the sol-gel method has a nanoscale structure, a large specific surface area, high catalytic activity, and a tight bond between the coating and the substrate. However, this method has a long preparation cycle, strict requirements on process conditions, and relatively high costs.
Preparation of anode by electrochemical deposition method: Metal ions are deposited on the surface of a titanium substrate by electrochemical means, and then oxidized to form a metal oxide coating. This method can accurately control the thickness and composition of the coating, and can prepare a uniform and dense coating with strong bonding between the coating and the substrate. However, the electrochemical deposition method requires special electrolysis equipment, and the process parameter control is complex. It is currently mainly used in laboratory research and high-end customized anode preparation.
2. Advantages of MMO titanium anode
2.1 Excellent electrochemical performance
Low overpotential: The metal oxide coating of MMO titanium anode has high catalytic activity and can significantly reduce the overpotential of reactions such as chlorine evolution and oxygen evolution. For example, in the chlor-alkali industry, compared with traditional graphite anodes, the chlorine evolution overpotential of MMO titanium anode can be reduced by 200-300mV, which means that at the same current density, MMO titanium anode can reduce power consumption and production costs. The low overpotential characteristic also enables MMO titanium anode to maintain efficient and stable working state at high current density, which is suitable for large-scale industrial production.
High current efficiency: Due to the high catalytic activity of the coating, MMO titanium anode can promote the target reaction and inhibit the occurrence of side reactions, thereby improving the current efficiency. In the electroplating industry, when using MMO titanium anode for metal electrodeposition, the current efficiency can be increased to more than 90%, reducing the ineffective consumption of metal ions and improving the quality of the coating and production efficiency.
2.2 Excellent corrosion resistance
Chemical stability: The metal oxide coating on the surface of the titanium substrate has good chemical stability and can resist corrosion from a variety of corrosive media such as acids, alkalis, and salts. In the electrolytic cell of the chlor-alkali industry, the anode is in a highly corrosive environment of high-concentration sodium chloride solution and chlorine gas. The oxide coating of the MMO titanium anode can effectively protect the titanium substrate and prevent it from being corroded and dissolved. Studies have shown that the service life of the MMO titanium anode in the chlor-alkali electrolysis environment can reach 5-10 years, which is much longer than the 1-2 years of the traditional lead-based anode.
Abrasion resistance: By optimizing the coating formula and preparation process, the coating of the MMO titanium anode has high hardness and wear resistance. During the continuous electrolysis process, the coating is not easy to fall off due to mechanical friction or bubble scouring, and can maintain stable electrochemical properties and extend the overall service life of the anode.
2.3 Long life and low cost
Long service life: The comprehensive performance of the MMO titanium anode enables it to maintain stable operation under various complex working conditions, greatly reducing the frequency of anode replacement. In the water treatment industry, the MMO titanium anode used for electrolytic disinfection can run continuously for 3-5 years without replacement, reducing equipment maintenance costs and downtime, and improving system reliability and stability.
Reduce overall costs: Although the initial purchase cost of the MMO titanium anode is higher than that of traditional anodes, its advantages such as long life, low energy consumption and high current efficiency significantly reduce its overall cost over the entire life cycle. Taking the chlor-alkali industry as an example, although the initial investment of using MMO titanium anodes increases, the cost can be recovered within 2-3 years by saving electricity and reducing maintenance and replacement costs, and long-term economic benefits can be achieved.
2.4 Environmental friendliness
No heavy metal pollution: Traditional lead-based anodes will gradually corrode and dissolve during use, releasing lead ions, causing serious harm to the environment and human health. The titanium matrix and metal oxide coating of the MMO titanium anode are non-toxic and harmless, and no heavy metal pollution will be generated during use, which meets modern environmental protection requirements.
Energy saving and consumption reduction: The low overpotential and high current efficiency characteristics of the MMO titanium anode can effectively reduce the energy consumption of the electrolysis process. For example, in the chlor-alkali industry, the use of MMO titanium anodes can reduce electricity consumption per ton of alkali by 10%-15%, reduce energy consumption and carbon emissions, and help achieve the “double carbon” goal.
3. Application of MMO titanium anodes
3.1 Chlor-alkali industry
Chlorine production: The chlor-alkali industry is one of the main application areas of MMO titanium anodes. In the process of electrolyzing brine to produce chlorine, hydrogen and sodium hydroxide, the MMO titanium anode significantly improves the production efficiency and quality of chlorine with its high chlorine catalytic activity and corrosion resistance. Modern large-scale chlor-alkali electrolyzers generally use DSA-type MMO titanium anodes, which can operate stably at a high current density of 3-4kA/m², and the purity of chlorine can reach more than 99%, while reducing the maintenance cost and energy consumption of the electrolyzer.
Sodium hydroxide preparation: While the MMO titanium anode efficiently produces chlorine in the anode chamber, the ion exchange membrane in the cathode chamber separates the anode and the cathode, allowing sodium ions to migrate to the cathode chamber to react with water to generate sodium hydroxide. Due to the stable performance of MMO titanium anode, the purity and concentration of sodium hydroxide solution can be guaranteed, meeting the demand for high-quality caustic soda in industrial production.
3.2 Water treatment field
Electrolytic disinfection: The strong oxidizing substances such as hypochlorous acid and ozone produced by electrolyzing water with MMO titanium anode can effectively kill bacteria, viruses and algae in water. In drinking water treatment, MMO titanium anode electrolytic disinfection technology has the characteristics of rapid response and no secondary pollution. It can replace traditional chlorine disinfection and ultraviolet disinfection methods to ensure the safety of drinking water. In swimming pool water purification, the electrolytic disinfection system uses MMO titanium anode, which can produce disinfectants in real time, avoiding the safety hazards of chemical storage and addition.
Wastewater treatment: MMO titanium anode can be used to treat various types of industrial wastewater and domestic sewage, degrading organic matter and removing pollutants such as heavy metal ions and ammonia nitrogen through electrolytic oxidation and electrocoagulation. For example, in the treatment of printing and dyeing wastewater, the oxygen-evolving MMO titanium anode can produce hydroxyl radicals (・OH), oxidizing and decomposing the refractory organic dye molecules into carbon dioxide and water; in heavy metal-containing wastewater, heavy metal ions are reduced to single substances through electrodeposition, achieving harmless treatment of wastewater.
3.3 Metal electrodeposition
Electroplating industry: MMO titanium anodes are widely used in copper plating, nickel plating, chromium plating and other processes in the field of electroplating. Compared with insoluble lead anodes, MMO titanium anodes have higher current efficiency and stability, which can ensure the uniformity and density of the coating and improve the quality of electroplated products. At the same time, since MMO titanium anodes do not participate in electrochemical reactions, they will not produce metal impurities like soluble anodes, reducing the pollution of the plating solution and maintenance costs.
Electroforming and electrolytic refining: In the electroforming process, MMO titanium anodes, as inert anodes, can provide stable current output, ensure the accuracy and quality of metal deposition, and are used to manufacture precision molds, electronic components, etc. In the electrolytic refining industry, such as copper electrolytic refining, MMO titanium anodes can replace traditional lead anodes, reduce energy consumption, and improve copper purity and production efficiency.
3.4 Cathodic protection
Marine engineering: In the marine environment, metal structures such as ships, offshore platforms, and submarine pipelines face serious corrosion problems. Through the impressed current cathodic protection (ICCP) system, using MMO titanium anodes as auxiliary anodes, cathodic current can be applied to the protected metal structure to form cathodic polarization on its surface, thereby inhibiting corrosion. MMO titanium anodes have good corrosion resistance and conductivity in seawater, and can provide current to the cathodic protection system for a long time.
Underground pipeline protection: For buried oil and natural gas pipelines, the cathodic protection system using MMO titanium anodes as auxiliary anodes can effectively extend the service life of the pipeline. MMO titanium anodes can adapt to the complexity of the soil environment, maintain stable electrochemical properties in soils with different pH values ​​and salinity, and provide reliable cathodic protection for pipelines.
3.5 Other applications
New energy field: In the process of hydrogen production by water electrolysis, oxygen-evolving MMO titanium anodes (such as Ti/IrOâ‚‚-Taâ‚‚Oâ‚…) are ideal for alkaline electrolyzers and proton exchange membrane electrolyzers due to their high oxygen evolution catalytic activity and corrosion resistance. In addition, in the research of anode catalysts for fuel cells, the coating design concept of MMO titanium anodes also provides ideas for the development of new and efficient catalysts.
Organic synthesis: MMO titanium anodes can be used for organic electrosynthesis reactions to synthesize pharmaceutical intermediates, fine chemicals, etc. through electrochemical oxidation or reduction processes. Its high catalytic activity and selectivity can promote the target reaction, reduce side reactions, and improve product yield and purity, providing a new technical approach for green organic synthesis.
IV. Summary
MMO titanium anodes play an irreplaceable role in many fields such as chlor-alkali industry, water treatment, metal electrodeposition, and cathodic protection due to their excellent electrochemical properties, excellent corrosion resistance, long life, and environmental friendliness. With the global emphasis on sustainable development and energy conservation and emission reduction, the application demand for MMO titanium anodes will continue to grow. In the future, the development of MMO titanium anodes will mainly focus on the following directions: First, by optimizing the coating formula and preparation process, develop new anode materials with higher catalytic activity, lower overpotential and longer life; second, combine nanotechnology, composite technology, etc. to prepare coatings with special structures and functions to adapt to complex working conditions and diversified application needs; third, expand the application of MMO titanium anodes in emerging fields such as new energy and organic synthesis, and promote technological progress in related industries.
At the same time, in order to further reduce the production cost of MMO titanium anodes and improve their market competitiveness, it is necessary to strengthen industry-university-research cooperation and accelerate the industrialization transformation of scientific research results. In addition, the establishment of a complete anode performance evaluation system and life prediction model will help optimize the design and use of anodes and provide users with more reliable technical support. With the continuous innovation and breakthroughs in technology, MMO titanium anodes will play a more important role in promoting the green and intelligent development of the electrochemical industry and contribute to global sustainable development.
