Why DSA Titanium Anodes are Used in Chlorine Evolution Reactions?

Why DSA Titanium Anodes are Used in Chlorine Evolution Reactions?

The chlorine evolution reaction is one of the most important practical applications of electrochemical reactions carried out on an industrial scale. In this reaction, sodium chloride is electrolyzed producing chlorine gas at the anode as well as sodium hydroxide and hydrogen at the cathode.

Industrial chlorine evolution is performed on “dimensionally stable anodes″, on which corrosion and other degradation processes progress only slowly. Dimensionally stable anodes consist of materials for which electrode degradation is limited to the electro-active surface layer. Commonly employed electrode materials are ruthenium coating titanium anodes or ruthenium iridium coating titanium anodes.

Typical oxide electrodes for the chlorine evolution reaction are composed of mixed oxides of ruthenium−titanium or ruthenium−iridium−titanium. Especially Ru−Ti−Ox mixed oxides show a high catalytic performance toward chlorine evolution. Unfortunately, these electrodes degrade under harsh working conditions (high current densities and temperatures, low pH). To increase the lifetime and stability of such catalysts, iridium oxide is often added to the catalyst composition.

The most common method for the synthesis of Ru−Ti−Ox or Ru−Ir−Ti-Ox catalysts is the dissolution of ruthenium chloride, iridium chloride, titanium chloride, or di-isopropoxide in solvents followed by brushing of the solution onto an electrode substrate, drying, and finally thermal treatments to convert the deposit into ruthenium oxide. The coated electrodes usually possess a compact morphology featuring mud-cracks within the coating. Depending on the synthesis procedure the oxide coatings can also contain textural porosity. The morphology of the electrode coating can be influenced by varying the coating technique like brushing, dipping, or spinning.

At high current densities, the gas evolution takes place not only at the coatings outer surface but also inside of the pores that are provided by the textural porosity of the studied coatings. Furthermore, the pores are necessary to enable a sufficiently fast chlorine removal at high current densities from the electrode due to the formation of gas channels inside the pores.