In the field of electrochemistry, the performance of anode materials plays a decisive role in the efficiency, energy consumption and service life of the electrolysis process. Titanium anodes, especially metal oxide coated titanium anodes (MMO titanium anodes), have quickly become an ideal substitute for traditional anode materials (such as graphite, lead alloys, etc.) since their introduction due to their unique performance advantages. Titanium anodes are based on titanium metal and coated with a specific metal oxide coating on the surface. They have both the high strength and corrosion resistance of the titanium substrate and the high catalytic activity of the coating material. They are widely used in chlor-alkali industry, water treatment, metal electrodeposition, cathodic protection and other fields. With the growing demand for green, environmentally friendly and energy-efficient technologies in the global industry, the manufacturing technology and application value of titanium anodes have received more and more attention. A deep understanding of the manufacturing process and application of titanium anodes is of great significance to promoting the sustainable development of the electrochemical industry and improving industrial production efficiency.
1. Advantages of titanium anodes
1.1 Excellent corrosion resistance
Titanium can quickly form a dense titanium dioxide (TiO₂) passivation film in the air. The thickness of this oxide film is about 1-3nm. It has extremely high chemical stability and can effectively resist the erosion of various corrosive media such as acids, alkalis, and salts. In the electrolytic cell environment of the chlor-alkali industry, the anode needs to withstand the dual corrosion of high-concentration sodium chloride solution and chlorine gas for a long time. The traditional lead-based anode will gradually wear out due to corrosion, while the passivation film of the titanium anode can significantly delay the corrosion process. Its service life can reach 5-10 years, far exceeding the 1-2 years of the lead-based anode. In addition, in highly corrosive environments such as oceans and chemical wastewater, titanium anodes also show excellent corrosion resistance, reducing equipment maintenance and replacement costs.
1.2 Efficient electrochemical performance
The metal oxide coating (such as RuO₂, IrO₂, etc.) on the surface of the titanium anode has high catalytic activity and can significantly reduce the overpotential of electrochemical reactions such as chlorine evolution and oxygen evolution. Taking the chlor-alkali industry as an example, compared with the graphite anode, the chlorine evolution overpotential of the titanium anode can be reduced by 200-300mV, which means that at the same current density, the titanium anode can significantly reduce the energy consumption. At the same time, its high catalytic activity can also improve the current efficiency and reduce the occurrence of side reactions. In the electroplating industry, when using titanium anodes for metal electrodeposition, the current efficiency can be increased to more than 90%, effectively improving the quality of the coating and production efficiency.
1.3 Long life and stability
The corrosion resistance and electrochemical stability of the titanium anode give it a long service life. In the electrolytic disinfection equipment of the water treatment industry, the titanium anode can operate continuously and stably for 3-5 years without frequent replacement, reducing equipment downtime and maintenance costs. In addition, the performance of the titanium anode decays slowly during long-term use, and it can maintain stable current output and reaction efficiency, providing reliable protection for industrial production.
1.4 Environmental friendliness
Traditional lead-based anodes will continue to corrode and dissolve during use, releasing toxic lead ions, causing serious harm to the environment and human health. The titanium substrate and metal oxide coating of the titanium anode are non-toxic and harmless, and will not produce heavy metal pollution. At the same time, the low energy consumption characteristics of the titanium anode help reduce carbon emissions, which is in line with the global trend of energy conservation and emission reduction. It is a green and environmentally friendly anode material.
2. Types of titanium anodes
2.1 Classification by coating material
DSA type (dimensionally stable anode): This type is based on titanium-based ruthenium dioxide (Ti/RuO₂) and is modified by adding metal oxides such as TiO₂, SnO₂, and Ta₂O₅. DSA type anodes perform well in chlorine evolution reactions and are often used in chlorine production in the chlor-alkali industry. For example, Ti/RuO₂-TiO₂ coated anodes can still maintain efficient chlorine evolution at high current densities (3 – 4kA/m²), with low overpotential and significantly reduced energy consumption. By adjusting the ratio of each oxide in the coating, the performance of the anode in different media can be optimized. For example, adding Ta₂O₅ can enhance the corrosion resistance of the coating.
Oxygen evolution anode: mainly titanium-based iridium dioxide (Ti/IrO₂) or titanium-based iridium tantalum oxide (Ti/IrO₂-Ta₂O₅) as coating materials, suitable for oxygen evolution reactions, such as electrolysis of water to produce oxygen, electroplating wastewater treatment and other fields. IrO₂ has excellent oxygen evolution catalytic activity and chemical stability, and can work stably in both acidic and alkaline environments. The Ti/IrO₂-Ta₂O₅ coated anode further improves the corrosion resistance and mechanical strength of the coating by compounding Ta₂O₅, so that it can still maintain efficient oxygen evolution performance under high voltage and strong oxidizing environment.
Precious metal coated anode: precious metal or alloy coatings such as platinum (Pt) and ruthenium iridium alloy (Ru-Ir) are coated on the surface of the titanium substrate. This type of anode has extremely high catalytic activity and chemical stability, and is suitable for scenarios with extremely high requirements for anode performance, such as high-end electroplating, organic electrosynthesis, etc. However, due to the high cost of precious metals, their large-scale application is limited.
2.2 Classification by application field
Anodes for chlor-alkali industry: specially designed for chlor-alkali electrolysis, requiring anodes to have high chlorine evolution catalytic activity and chlorine corrosion resistance. In addition to DSA anodes, it also includes improved multi-metal oxide coated anodes to meet the needs of higher current density and longer service life.
Anodes for water treatment: according to different water treatment processes, they can be divided into electrolytic disinfection anodes, electrocoagulation anodes and electrocatalytic oxidation anodes. Oxygen evolution anodes are often used for electrolytic disinfection and electrocatalytic oxidation, degrading organic matter by producing strong oxidizing substances such as hydroxyl radicals (・OH); while electrocoagulation anodes need to have good conductivity and moderate corrosion resistance to release metal ions to form flocculants.
Anodes for metal electrodeposition: used in processes such as electroplating and electroforming, requiring anodes to have stable electrochemical properties to ensure uniform and dense coatings. Common types include insoluble titanium anodes (such as Ti/RuO₂) and soluble titanium-based composite anodes (such as titanium-based lead alloy coated anodes).
Anodes for cathodic protection: Mainly used in impressed current cathodic protection systems for metal structures such as marine engineering and underground pipelines. Such anodes need to maintain stable current output in different environmental media. For example, titanium-based mixed metal oxide anodes used in seawater need to have good conductivity and seawater corrosion resistance.
3. Manufacturing process of titanium anodes
3.1 Pretreatment of titanium substrate
Machining: According to design requirements, titanium sheets or pipes are processed into the required anode shape through cutting, stamping, welding and other processes. For anodes with complex shapes (such as mesh and porous structures), precision machining technology is required to ensure dimensional accuracy.
Surface grinding and polishing: Use sandpaper, polishing machine and other tools to grind the surface of the titanium substrate to remove impurities such as oxide scale and oil stains, so that the surface roughness reaches Ra 0.8 – 1.6μm. A smooth surface helps to improve the bonding strength between the coating and the substrate.
Degreasing treatment: Immerse the titanium substrate in an alkaline degreasing agent (such as a mixed solution of sodium hydroxide and sodium carbonate) at 60-80℃ for 10-20 minutes to remove residual grease on the surface. Then rinse with clean water to ensure that there is no chemical residue.
Pickling activation: Immerse the titanium substrate in a mixed solution of hydrofluoric acid and nitric acid (such as HF:HNO₃ = 1:3) at room temperature for 5-10 minutes to remove the surface oxide film and activate the substrate surface. Rinse with deionized water immediately after pickling to prevent secondary oxidation.
3.2 Coating preparation
Thermal decomposition method
Coating liquid preparation: Dissolve metal salts (such as ruthenium trichloride and iridium tetrachloride) in an organic solvent (such as ethanol and isopropanol) to prepare a uniform coating liquid. According to the coating formula, other metal salts or additives can be added to adjust the coating composition.
Coating: The coating liquid is evenly coated on the surface of the titanium substrate by brushing, dipping or spraying. After each coating, it is necessary to dry at 100-120℃ for 10-15 minutes to remove the solvent.
Thermal decomposition: The coated titanium substrate is placed in a high-temperature furnace and thermally decomposed at 400-600℃ for 10-30 minutes to convert the metal salt into a metal oxide. Repeat the coating-drying-thermal decomposition steps 3-10 times until the desired coating thickness (usually 5-20μm) is reached.
Sol-gel method
Sol preparation: Metal alkoxides (such as ruthenium tetraisopropoxide and iridium tetrabutoxide) are mixed with water and ethanol, and hydrolyzed and polycondensed under the action of acidic or alkaline catalysts to form a stable sol. The viscosity and particle size of the sol are controlled by adjusting the reaction conditions (such as temperature and pH value).
Coating and gelation: The sol is coated on the surface of the titanium substrate by dipping or spin coating, and left at room temperature for several hours to gel the sol to form a wet gel film.
Drying and calcination: Dry the wet gel film at 100-150℃ for several hours to remove water and organic solvents to form a dry gel. Then calcine at 400-600℃ for 1-2 hours to convert the dry gel into a nanoscale metal oxide coating.
Electrochemical deposition method
Electrolyte preparation: Dissolve metal salts (such as chlororuthenic acid, chloroiridic acid) in a suitable electrolyte, and add buffers, complexing agents and other ingredients to adjust the pH value and conductivity of the electrolyte.
Electrodeposition: With the titanium substrate as the cathode and the platinum electrode or other inert electrode as the anode, electrodeposition is carried out under constant current or constant potential conditions. The thickness and composition of the coating can be precisely controlled by controlling the current density, deposition time and temperature.
Post-treatment: The titanium anode after electrodeposition needs to be cleaned, dried, and heat treated at high temperature (300-500℃) to improve the crystal structure and electrochemical properties of the coating.
3.3 Post-treatment and quality inspection
Heat treatment optimization: The prepared titanium anode is annealed at 400-600℃ to eliminate the stress inside the coating, improve the bonding between the coating and the substrate, optimize the crystal structure of the oxide, and improve the electrochemical performance of the anode.
Quality inspection
Appearance inspection: Check whether the anode surface is uniform, crack-free, and peeling-free by visual or microscopic inspection.
Coating thickness detection: X-ray fluorescence spectroscopy (XRF), scanning electron microscopy (SEM) and other technologies are used to measure the coating thickness to ensure that it meets the design requirements.
Electrochemical performance test: The chlorine and oxygen evolution overpotentials of the anode are tested by cyclic voltammetry (CV), linear polarization (LP) and other means to evaluate its catalytic activity; the service life and stability of the anode are tested by constant current electrolysis test.
Binding strength test: A thermal shock test (such as repeated heating-cooling cycles) or a bending test is used to detect the bonding strength between the coating and the substrate to prevent the coating from falling off.
IV. Application of titanium anodes
4.1 Chlor-alkali industry
Chlorine production: Chlor-alkali industry is the largest application field of titanium anodes, which produces chlorine, hydrogen and sodium hydroxide by electrolysis of saturated salt water. DSA-type titanium anodes have become the mainstream anode material for chlor-alkali electrolyzers due to their high chlorine catalytic activity and corrosion resistance. After modern large-scale chlor-alkali electrolyzers use titanium anodes, the current density can be increased to 3-4kA/m², the purity of chlorine reaches more than 99%, and the power consumption per ton of alkali is reduced by 10%-15%.
Sodium hydroxide preparation: During the chlor-alkali electrolysis process, the titanium anode efficiently produces chlorine in the anode chamber, while the cathode chamber is isolated from the anode chamber by an ion exchange membrane, and the sodium ions migrate to the cathode chamber to react with water to produce sodium hydroxide. The stable performance of the titanium anode ensures the high purity and high concentration of the sodium hydroxide solution, which meets the needs of industrial production.
4.2 Water treatment field
Electrolytic disinfection: The strong oxidizing substances such as hypochlorous acid and ozone produced by the electrolysis of water using titanium anodes can kill bacteria, viruses and algae in the water. In drinking water treatment, titanium anode electrolytic disinfection technology has the advantages of rapid response and no secondary pollution, and can replace traditional chlorine disinfection and ultraviolet disinfection. In swimming pool water purification, the titanium anode electrolysis system can produce disinfectants in real time, avoiding the safety hazards of chemical storage and addition.
Wastewater treatment
Electrocatalytic oxidation: Oxygen-evolving titanium anodes (such as Ti/IrO₂-Ta₂O₅) produce hydroxyl radicals (・OH) in wastewater treatment to oxidize and decompose difficult-to-degrade organic matter (such as pesticides and dyes) into carbon dioxide and water. This technology is suitable for the treatment of high-concentration organic wastewater such as printing and dyeing wastewater and pharmaceutical wastewater.
Electrocoagulation: Titanium-based iron and aluminum coated anodes release metal ions through the electrolysis process to form flocculants to remove suspended matter, colloids and heavy metal ions in wastewater. Electrocoagulation technology has the characteristics of high treatment efficiency and small footprint, and is widely used in industrial wastewater pretreatment and deep treatment.
4.3 Metal Electrodeposition
Electroplating Industry: Titanium anodes are widely used in electroplating processes such as copper plating, nickel plating, and chromium plating. Compared with insoluble lead anodes, titanium anodes have higher current efficiency and stability, which can ensure uniform and dense coatings and improve the quality of electroplated products. At the same time, since titanium anodes do not participate in electrochemical reactions, they will not produce metal impurities like soluble anodes, reducing the pollution of plating solutions and maintenance costs.
Electroforming and electrolytic refining: In the electroforming process, 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), titanium anodes can replace traditional lead anodes, reduce energy consumption, and improve metal purity and production efficiency.
4.4 Cathodic Protection
Marine Engineering: In the impressed current cathodic protection (ICCP) system of marine metal structures such as ships, offshore platforms, and submarine pipelines, titanium-based mixed metal oxide anodes are used as auxiliary anodes to apply cathodic current to the protected metal structure to inhibit corrosion. Titanium anodes have good conductivity and corrosion resistance in seawater, and can provide long-term and stable protection current.
Underground pipeline protection: For buried oil and natural gas pipelines, the cathodic protection system using titanium anodes as auxiliary anodes can effectively extend the service life of the pipeline. Titanium anodes can adapt to different soil environments (such as changes in pH and salt content) and provide reliable cathodic protection for pipelines.
4.5 Other applications
New energy field: In the process of hydrogen production by electrolysis of water, oxygen-evolving titanium anodes (such as Ti/IrO₂-Ta₂O₅) 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 titanium anodes also provides a reference for the development of new and efficient catalysts.
Organic synthesis: 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, improve product yield and purity, and provide a new way for green organic synthesis.
V. Summary
Titanium anodes have become an indispensable key material in the modern electrochemical industry due to their excellent corrosion resistance, efficient electrochemical performance, long life and environmental friendliness. From the chlor-alkali industry to water treatment, from metal electrodeposition to cathodic protection, the application scope of titanium anodes has been continuously expanded, promoting the technological progress and green development of related industries. Its manufacturing process covers multiple links such as titanium substrate pretreatment, coating preparation and post-treatment. The precise control of each step of the process directly affects the final performance of the anode.