Popular Science Knowledge (1) An Overview of the Development of Titanium Anodes

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Electrodes are important components in electrolysis processes; their performance directly affects the efficiency of electrolysis as well as the cost and quality of the products obtained through electrolysis. Since the material used for electrodes determines their performance, the development of new electrode materials with superior properties has always been a focus of researchers and engineering professionals around the world throughout the progress of the electrochemical industry. The development of electrode materials has gone through several stages, including graphite electrodes, iron oxide electrodes, lead-based alloy electrodes, precious metal electrodes, and titanium-coated electrodes. In 1896, E.G. Acheson successfully produced artificial graphite using the electrothermal crystallization method, and it was applied in saltwater electrolysis production; since then, the electrolysis industry entered the era of graphite electrodes. Saltwater electrolysis requires electrode materials to possess good electrocatalytic performance for chlorine evolution, excellent durability, and the ability to suppress oxygen evolution. At high salt water concentrations, graphite electrodes can fully meet the aforementioned requirements. However, during long-term operation, the graphite anodes were found to have the following drawbacks: high resistance and high power consumption ； As the electrochemical reaction proceeds, electrode wear increases and the electrode spacing changes, leading to instability in electrolytic production and making it difficult to maintain a stable active surface for the chlorination reaction. To overcome the aforementioned disadvantages of graphite electrodes, there is an urgent need to replace the non-metallic graphite electrode material with metallic electrode materials. Under such circumstances, lead-based alloy electrodes were invented to replace graphite electrodes. Lead-based alloy electrodes have advantages such as low cost, easy shaping, the ability to self-repair even when the surface oxide is damaged, and stable operation in electrolyte. However, long-term production experience has revealed the following fatal drawbacks: (1) The electrodes are heavy and have low strength, making them prone to deformation during use, which leads to short circuits and reduces current efficiency. (2) The electrical conductivity of the electrodes is not good enough, resulting in high power consumption. Therefore, there is an urgent need to find a new electrode to replace lead-based alloy electrodes. In the 1960s, the Dutchman Henri Bernard Beer, through years of effort, developed a new type of anode – an insoluble ruthenium oxide-based anode coated on a titanium substrate (referred to as DSA) – that featured a long service life, high electrochemical catalytic performance, and no secondary pollution. Industrial production of this anode was established in 1968. The advent of coated titanium anodes overcame the shortcomings of traditional graphite and lead-based alloy electrodes, solved many problems encountered in daily life and industrial practice, greatly improved the production landscape in the electrolysis industry, and is regarded as a major technological advancement in the chlor-alkali industry. From then on, the development of electrodes entered the era of titanium electrodes. Since the industrialization of metal anodes, considering factors such as price and machinability, titanium metal is currently widely used in industry as the substrate for anodes. In the 1970s, China also began the industrial use of titanium anodes. The first metal anode to achieve industrialization was the chlorination-type metal anode. A ruthenium-titanium anode (Ti/RuO2-TiO2) with titanium as the substrate and RuO2-TiO2 as the active coating ； Ruthenium-tin anode (Ti/RuO2-SnO2) with titanium as the substrate and RuO2-SnO2 as the active coating. In the aforementioned anode coating, the active oxide RuO2 plays a role in electrocatalysis, while TiO2 and SnO2 act as auxiliary oxides to stabilize RuO2. The ruthenium-titanium oxide-coated anodes, which are widely used in the chlor-alkali industry, are durable when used in the electrolysis of chloride solutions. However, since the overpotentials for chlorine evolution and oxygen evolution are not very different, these ruthenium-titanium coated anodes release a small amount of oxygen alongside chlorine. Under normal production conditions, the oxygen content in chlorine is approximately 2-4%. This gives rise to two problems: first, the purity of chlorine products is low ； Second, it shortens the lifespan of the anode; the chlorine gas released not only destroys the solid solution structure of the coating, which contains chlorine and oxygen defects, but also diffuses from the surface of the coating to the interface between the coating and the titanium substrate, gradually forming a passivation film of titanium oxide. This results in a reverse resistance at the P-N junction, causing the electrode coating to lose its activity. To overcome these two drawbacks, ternary or quaternary coating formulations are generally used in order to raise the oxygen evolution potential without increasing the chlorine evolution potential. This allows for an extension of the electrode’s operational life, while also enabling an increase in current density. For example, adding semi-active elements such as Co, Sn, and Sb to a Ru-Ti base helps to reduce the activity of the coating and decrease the amount of precious metals required ； Similarly, the addition of inactive elements such as Zr, Ta, Pb, W, V, etc., can help improve certain properties of the electrode. Generally speaking, compared with traditional graphite electrodes and lead-based alloy electrodes, DSA anodes have the following advantages: (1) The size of the anode remains stable, and the distance between the electrodes does not change during electrolysis, which ensures that the electrolytic process takes place under stable voltage conditions ； (2) The chlorine release overpotential is low, resulting in a low operating voltage and thus low power consumption ； (3) It can overcome the dissolution problems of graphite and lead electrodes, preventing contamination of the electrolyte and cathode products, thereby improving the purity of metal products ； (4) Both the coating and the substrate exhibit excellent corrosion resistance, enabling stable operation during production and reducing maintenance costs. It can operate at high current densities, thereby increasing the production capacity per unit area ； (5) The base metal titanium can be reused multiple times; titanium anodes are easy to manufacture with high precision, and their light weight helps reduce the workload. Due to the above advantages of DSA anodes, they are widely used in the chlor-alkali industry, water electrolysis industry, and hydrometallurgy industry. However, during long-term use, the following disadvantages of DSA anodes have been observed: (1) The use of a highly active coating formulation increases its activity but simultaneously reduces the service life of the coating ； (2) The increase in the chlorine evolution activity of the coating often leads to an increase in oxygen evolution activity as well, reducing the reaction selectivity of the electrode and resulting in a decline in product quality ； (3) The active components of the coating are primarily precious metal elements, which are expensive and require large quantities, resulting in high electrode costs ； (4) The backbone of the coating is mainly Ti and Ta, whose prices are also high. DSA-coated anodes have demonstrated advantages in the chlor-alkali industry, such as energy savings, high production efficiency, reduced maintenance costs, lower labor intensity, and significant environmental benefits. To date, it has been applied in various fields such as water treatment, metal foil manufacturing, chlorate production, metal surface treatment, cathodic protection, electroplating, and seawater purification. At the same time, DSA-coated anodes also have many of the aforementioned disadvantages. In order to further improve the performance of anodes and reduce costs, the development of coated titanium anodes in recent years has focused on the following directions: (1) diversification of coatings ； (2) Prepare the intermediate layer ； (3) Nanoscale oxide coating ； (4) Search for new active components and inert components. Since the invention of the DSA anode, electrochemists have achieved significant progress in improving coating formulations, with the main approach being the addition of other precious or non-precious metal elements to create multi-component coatings. A research team from Fuzhou University conducted a relatively systematic study on the effects of adding various elements, including Sn, Co, Mn, Ce, Si, etc. The studies showed that the addition of Si can control the grain size of the coating, refine the particle size of the resulting material, and also help to prevent the formation of metal ruthenium. The addition of Ce and Co can effectively enhance the electrocatalytic activity of the coating; however, an increase in their concentration has a significant negative impact on the coating’s lifespan ； Japanese scholar Fumio Noguchi has also studied the effect of cobalt on the properties of titanium anode coatings. His research shows that, compared to pure iridium coatings, the addition of cobalt can effectively enhance the electrocatalytic activity and stability of the coatings ； As a non-precious metal oxide, MnO2 is considered to be an oxygen evolution anode material with high activity aside from RuO2 and IrO2 electrodes; however, it has poor stability. MnO2 receives special attention due to its good catalytic activity and low cost. In recent years, nanomaterials have attracted the attention of many materials scientists due to their excellent properties. Due to the surface effect, volume effect, size effect, and macroscopic tunneling effect of nanoparticles, they are widely used in fields such as electronics, optics, chemical ceramics, biology, and medicine. As an important category of nanomaterials, nanooxides have significant applications in areas such as precision ceramics, photovoltaic cells, magnetic recording and sensors, catalysts, and luminescent materials. However, there is little research on their use as electrode materials. Based on the excellent properties of nanomaterials, some researchers have proposed combining the DSA anode with nanotechnology in order to develop electrodes with even better performance. Methods such as Sol-gel were also employed to explore the preparation and characterization of binary and ternary nanoscale oxide coatings including RuO2-TiO2, RuO2-SiO2, and RuO2-TiO2-Co3O4. It has achieved certain results.