Lead Dioxide Anode Introduction
1. Lead dioxide anode introduction
With the continuous development of industry and science and technology, traditional anode materials are increasingly showing their limitations. For example, the cost of platinum is too high; the corrosion resistance of graphite in the chlor-alkali industry and the oxygen evolution system is not ideal, and the strength is low: lead alloy anodes have poor corrosion resistance, low electrocatalytic performance, and large power consumption. From the requirements of so-called "green materials" such as energy saving, consumption reduction, and pollution-free, people hope to find new anodes with long life, high electrochemical performance, and no secondary pollution. Under the environment of oxygen evolution, people have developed the lead dioxide electrode (PbO2): a non-stoichiometric compound that is deficient in oxygen and contains excessive lead. It has multiple crystal forms, using the anode electrodeposition to produce the β-PbO2, which has Oxidation, corrosion resistance (high stability in strong acid H2S04 or HN03), high oxygen overpotential, good electrical conductivity, strong binding force, strong oxidation ability when electrolyzed in aqueous solution, can bear large current, etc. At present, it has been widely used in the fields of electroplating, smelting, wastewater treatment, etc., and cannot be replaced by many other electrode materials (such as DSA, lead, titanium coating with platinum).
1.1 Lead dioxide anode characteristic
It is widely used in the electrolytic preparation of various organic substances and in the process of sewage treatment and high-purity water preparation, application range is wide. Pb02 has the advantages of excellent electrical conductivity, good charge and discharge reversibility, and low price. It is widely used as a positive electrode for lead-acid batteries. At present, the utilization rate of lead dioxide, the positive active material of lead-acid batteries, is not high, and generally does not exceed 50%. The oxygen evolution potential is high, generally 1.75V (relative to the calomel electrode), and has a strong reducing force of the degradation for organic material (COD).
1.2 Bottom layer of Lead dioxide anode
The materials currently used as the bottom layer are: platinum group metals and their oxides, tin antimony oxide, iridium tantalum composite oxide bottom layers, etc., their properties are as follows: (1) platinum group metals and their oxides: the bottom layer has good electrical conductivity, which can greatly improve the bonding performance of the coating and the substrate. (2) Tin antimony oxide: The tin antimony oxide layer obtained by thermal decomposition method is dense and uniform. With this underlayer, it is difficult for the electrolyte to penetrate the titanium surface, oxygen atoms or 02-. The diffusion of ions into the titanium matrix is also blocked, thereby avoiding the formation of Ti02. In addition, Ti02 is a wide bandgap N-type semiconductor. After doping with Sb, the extra electron in the Sn02 lattice replaced the pentavalent Sn atom in the Sn02 lattice with an extra electron entering the conduction band, which greatly increased the electron concentration in the conduction band. However, when Sb is too much, the disorder degree of the sn02 lattice will be increased, and the electrical conductivity of the sn02 will be reduced. Therefore, the content of the Sb is related to the superiority and inferiority of the underlying performance. This bottom layer also has the effect of reducing the internal stress of the coating. (3) Titanium-tantalum composite oxide bottom layer: This bottom layer has the characteristics of good conductivity, good corrosion resistance, and low electrochemical activity. Even if the bottom layer is exposed during the electrolysis process, no electrolytic reaction occurs, so there is no problem that the plating layer peels off due to this.
1.3 Surface active layer of lead dioxide anode
The PbO2 surface active layer is generally prepared by an electrodeposition method. It has two crystal forms, α and β, and β-PbO2 has good corrosion resistance and electrical conductivity, and is usually used as the surface active layer of an electrode. However, α-PbO2 has a strong binding force, and its O—O atomic distance is between “bottom layer” and β-PbO2, which can act as a buffer fusion, reduce electrodeposition distortion and increase the affinity between the surface and the bottom layer. Therefore, in the electroplating process, α-type PbO2 can be deposited under strong alkaline conditions first, and β-type PbO2 can be deposited under acidic conditions to improve the service life of the electrode.
2. Application fields of lead dioxide titanium-based electrode
Under the environment of oxygen evolution, lead dioxide electrodes is developed. PbO2 is a non-stoichiometric compound that is deficient in oxygen and contains excessive lead. It has a variety of crystal forms. Corrosion (higher stability in strong acid H2S04 or HN03), high oxygen overpotential, good electrical conductivity, strong binding force, strong oxidation ability when electrolyzed in aqueous solution, can bear large current, etc., it is very promising. At present, it has been widely used in the fields of electroplating, smelting, waste water treatment, cathode anti-corrosion, etc., which cannot be replaced by many other electrode materials (such as DSA, lead, titanium platinum plating).
Lead dioxide electrodes have low resistivity, stable chemical properties, good corrosion resistance, good electrical conductivity, and can be used for large currents. They are widely used in the electrolytic preparation of various organic and inorganic substances, sewage treatment and high-purity water preparation processes. The application field is very wide.
2.1 Inorganic Chemical Industry
2.1.1 Chlorate, PbO2 electrode has been used in chlorate industry for a long time. The production of bromate and iodate using PbO2 electrodes is relatively mature, especially iodate. Due to the surface structure of PbO2 electrodes, in addition to electrochemical reactions, it also plays a catalytic role.
2.1.2 Electrolyzed H2O2
H2O2 produced by electrolysis generally uses Pt as an electrode. Some people have studied the use of MnO2, Fe3O4, graphite, etc. as anode materials, but they have not been successful, and PbO2 as an anode has achieved good economic benefits. Because the overpotential of PbO2 electrode to oxygen is slightly lower than that of Pt, people have conducted research on replacing Pt electrode with PbO2 electrode. During World War II, Japan lacked platinum and H2O2 was a military necessity, so in 1944-1945, it realized the industrialization of substrate-free PbO2 electrodes instead of Pt-based H2O2.
2.2 Organic Chemical Industry
The application of PbO2 electrodes in organic synthesis is not as mature as in inorganic synthesis applications, and many are still being explored.
2.2.1 Chloroform.
In the preparation of chloroform, the PbO2 electrode is used instead of the expensive Pt electrode. The effect is ideal. The most suitable conditions for the electrosynthesis of chloroform: NaCl 300g / L, EtOH 25ml / L, PH 8 ~ 10, temperature 60 ~ 70 ℃; The anode current density is 0.3 to 0.5A / m2, the current efficiency is 80% to 90%, the cell voltage is 5V, the conversion rate is 98% to 99%, and the purity is 99.5% to 99.9%. In the preparation of bromoform, the current efficiency is 92.5%, platinum is 87%, and graphite is 86%. PbO2 is the most effective anode material in iodoform electrosynthesis. The current efficiency is 90%, and the anode loss is negligible.
2.2.2 Isobutyric acid
Industrially, isobutyric acid is made from KMnO4 of isobutanol in an alkaline medium and oxidized and rectified to produce 1t of isobutyric acid. In addition to the main raw material isobutanol, it still needs about 3.2tKMnO4, 1.6tH2SO4, Auxiliary materials such as 0.3tNa2CO3 have high cost and produce nearly 2tMnO2 waste residue, which pollutes the environment. The use of lead-based lead dioxide electrodes to indirectly oxidize isobutanol to isobutyric acid reduces environmental pollution.
2.2.3 Sewage treatment
Titanium-based PbO2 electrodes are used to treat difficult-to-bio-degradable organic pollutants, bio-toxic pollutants, and high-temperature organic wastewater. Degradation of a 10 mg / L methyl orange solution with a titanium-based PbO2 electrode showed that the removal rate of methyl orange was nearly 100% when treated at a current density of 36 mA / cm for 12 min, and has higher electrocatalytic activity. . Using a new PbO2 electrode to treat nitrobenzene wastewater, it was found that the PbO2 electrode had a higher COD removal rate than the ordinary graphite electrode. After 5 hours of electrolysis, the COD removal rate was up to 65%. The high electrolysis efficiency is mainly due to the high oxygen evolution potential of the PbO2 electrode. Under anodic polarization, the surface of the PbO2 electrode is prone to generate · OH, which will react with nitrobenzene which migrates to the electrode surface. Characteristics of Ti / PbO2 anode electrocatalytic oxidation of organic pollutants. The experimental results show that the electrode shows a good electrocatalytic activity for the degradation of phenol, and has good environmental protection application prospects. The PbO2 electrode showed good catalytic performance for the degradation of aniline. Within 3 hours, the aniline could obtain a higher removal rate. At the same time, the PbO2 electrode also showed good stability and service life. The results of the research on the treatment of hydroxystyrene wastewater with PbO2 electrode prove that it generally takes only 3 ~ 6h to completely degrade it into inorganic or CO2.
Metal has the incomparable mechanical properties compared to other materials, which makes it the most attractive choice for the substrate of lead dioxide electrode . However, not all metals are suitable for the substrate of lead dioxide electrode . It must be valve-shaped metal with unidirectional current-carrying properties, such as Ti, Ta, Nb, Zr and so on. Among the above metals, Ta has the best corrosion resistance and low resistivity, and is the best material for use as a substrate in terms of performance. However, because Ta has a high affinity for oxygen, it generally needs to be in an anoxic environment, and Ta metal is expensive, so it is not commonly used in actual production. Ti is cheap, has low density, high strength, and has a thermal expansion rate close to that of lead dioxide. Therefore, Ti is generally selected as the substrate of the lead dioxide electrode. The titanium substrate generally adopts a mesh structure. This is because the Ti mesh is tough and firmly bonded to the electrodeposited layer. The lead dioxide electrode based on the Ti mesh can reduce the resistance to electrolyte flow and improve the current efficiency, especially at high current density Effectively prevent the electrode from overheating.
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