Metal Oxide Titanium Electrodes Classification And Preparation

1. Classification of metal oxide electrodes

The metal oxide electrode (also called DSA electrode) uses titanium as a substrate, and a metal oxide coating of a certain thickness is prepared on its surface. The coating is mainly composed of platinum group metal oxides, and then other inert metal oxides, such as TiO₂, Ta₂0₅, etc. are added. There are many classification methods.

According to the number of components, it can be divided into unit coating (such as PbO/Ti, MnO2/Ti, etc.), binary coating (such as Ti0₂Ru0₂/Ti, lr0₂Ta₂0₅/Ti, etc. ternary coating (such as RuIrTi/ Ti, RuCoTi/Ti, RuSnTi/Ti, RuSnlr/Ti, etc.), quaternary coatings (such as RuIrSnTi/Ti) and five-component coatings (such as RuIrSnCoTi/Ti), etc.

According to the main active components of the electrode coating, it can be divided into manganese-based anodes, lead-based anodes, ruthenium-based anodes, and iridium-based anodes. As shown in Table 1.1.

Tab．1．1 Classification and application of DSA

According to the main reaction that occurs on the electrode surface, it is mainly divided into electrodes for chlorine evolution (mainly coated with ruthenium such as Ti02Ru02/Ti) and electrodes for oxygen evolution (mainly coated with iridium such as Ir02Ta205/Ti) The preparation of metal oxide electrodes mostly uses thermal oxidation to obtain a certain thickness of metal oxide on a titanium substrate.

1.1 Pretreatment of titanium substrate

Before painting the metal oxide, the surface treatment of the titanium substrate must be carried out. Its purpose is to remove oil stains and oxide film on the surface of the substrate, so that the substrate is in an active state, so as to improve the bonding force between the coating and the titanium substrate, improve the conductivity of the electrode, and prolong the service life of the electrode.

The pretreatment of the titanium substrate has the following steps: sandblasting, degreasing, acid etching, cleaning and drying.

1.2 Sandblasting on the surface of the titanium substrate is powered by compressed air, and small particles of sand (or metal pellets) are sprayed on the surface of the titanium substrate with a high-speed airflow and a certain inclination angle. Coating) falls off from the titanium surface to obtain a uniform pitted surface.

1.3 After sandblasting, the surface of the titanium substrate has oil stains. Solvent degreasing (or electrolytic degreasing) is required until the surface of the substrate is free of oily water droplets. Otherwise, the presence of oil will greatly reduce the bonding force between the coating and the substrate. Acid etching is to immerse the degreasing titanium substrate in a 0.1kg/L oxalic acid solution (or HF solution) and etch it for 1 to 3 hours in a boiling state. According to the x-ray diffraction analysis, titanium hydride and oxides coexisted in the phase structure of the titanium matrix after acid washing (as shown in Figure 1.2). The composition of the titanium hydride formed on the surface is close to TiH1.79, and its free energy of formation is 82.9-85.9 kj/mol, and the composition is quite stable. Add 2h at 200℃, its basic composition can still remain unchanged, which is very beneficial for long-term storage. In order to improve the bonding force of the coating and improve the conductivity, the pickling treatment is very important, and it is an important step to realize the surface activation of the titanium substrate.

In general, the binding force of noble metals and their oxides with titanium oxide is greater than their binding force with pure titanium. Therefore, in addition to etching the titanium substrate before coating, the surface of the titanium substrate must be activated to make it porous. Titanium oxide layer, so the process of matrix treatment is actually a process of activating the titanium matrix metal. After the titanium substrate is sandblasted, degreasing and acid treated, there are pits of different depths on the surface. The existence of these pits is beneficial to improve the bonding strength of the coating and the substrate. Before coating, the titanium substrate needs to be cleaned with an ultrasonic instrument to remove the deposited powder and dirt in the pit and on the surface of the substrate. Because in the oxalic acid etching, titanium oxalate is produced and attached to the surface of the titanium substrate. If the titanium substrate is taken out of the acid tank, it is impossible to remove the deposits by simply rinsing, otherwise the bonding strength of the coating and the titanium substrate will be affected. The cleaned titanium substrate should be placed in distilled water for later use to prevent oxidation of the titanium substrate. Before opening. The moisture on the surface of the titanium substrate and the micropores must be dried. Otherwise, during coating, the unbaked water interacts with the titanium salt (or tin salt) in the coating solution to produce precipitates, which will cause the coating to fall off and affect the service life of the electrode.

2. Electrode preparation

Process parameters such as the composition of the coating solution, the concentration of the coating solution, the drying temperature and time, and the thermal oxidation temperature and time directly affect the performance of the electrode. The number of brushing times and the concentration of the coating solution are related to the amount of brushing; the number of thermal oxidation times, time and temperature have an impact on the electrochemical performance and corrosion resistance of the electrode. Fewer thermal oxidation times, low temperature, and short time, resulting in incomplete oxidation of the coating and uneven oxide crystallization, which will reduce the catalytic performance and service life of the electrode; while the number of thermal oxidation increases, the temperature rises, and the time is prolonged, It will cause oxidation of the titanium matrix and increase of oxide particles, which will also reduce the catalytic performance of the electrode and reduce the service life of the electrode. Therefore, under the premise of not affecting the coating performance, a thermal oxidation coating process should be used after a few brushings to appropriately reduce the number of thermal oxidations. In addition, the following points should be paid attention to during the preparation process:

2.1 Each time you paint, the coating should be thin and even. The coating solution is generally applied in about 15 to 18 times to avoid a large amount of accumulation or agglomeration of the coating solution on the surface of the substrate.

2.2 Under the infrared lamp, the solvent evaporates slowly, and the temperature is determined according to the boiling point of the solvent; the time is appropriate for the solvent to evaporate completely to avoid carbonization of the solvent at high temperature and affect the performance of the coating.

2.3 The completely dried electrode is sent to the muffle furnace, and the oxidation temperature and time are determined according to the coating composition, generally 5-15 minutes.

2.4 After the electrode is thermally oxidized, it must be cooled to room temperature before the next brushing can be performed to prevent the oxide coating from being damaged by the cold and heat.

2.5 After finishing brushing and drying, thermally oxidize in the muffle furnace for 1 hour to fully oxidize the electrode coating.

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