Why Ruthenium Coated Titanium Anodes are Best for Cl2 Evolution?

Why Ruthenium Coated Titanium Anodes are Best for Cl2 Evolution?

In Cl2 production, an aqueous solution of about 30% brine (NaCl), having a pH of 2-4 and a temperature of 75-90oC is usually circulated through the electrochemical cells. A direct electric current is passed through the brine to discharge Cl2 at the coated titanium anode. O2 evolution, as in many anodic processes, is an unavoidable side reaction occurring at the anode with Cl2.  

RuO2-TiO2 coated titanium electrodes are among the best anodes for Cl2 evolution for the following reasons:
1. They do not undergo passivation easily;
2. They have little dissolution problems since the reaction occurs at low overpotential;
3. They are not easily wetted by Hg in Hg cells and so Cl2 gas is readily released to avoid gas entrapment and coating wear.

In dilute chloride solutions at higher pH, the efficiency of Cl2 evolution decreases as a result of occurrences of the side reaction.Cl2 evolution efficiency depends on electrolyte concentration and composition, current density, nature of electrode, and temperature.
In chlorate electrolysis, chlorine is produced at the coated titanium anode from chloride ions. The chlorine molecules then hydrolyze and subsequently form the chlorate product by chemical oxidation.

The O2 evolution reaction can occur favorably in both acidic and alkaline media on RuO2-TiO2 anodes. RuO2-TiO2 based anodes are however unstable for O2 evolution as they exhibit much shorter lifetimes in such applications due to coating dissolution.

Alkaline media would have been the preferred option for practical applications; however, the media tend to attack the RuO2-TiO2 anodes. Thus, the acidic solution option is preferred for O2 evolution on RuO2 based anodes. O2 evolution is to decrease as chloride concentration increases. Furthermore, at low current density, O2 evolution decreases as RuO2 in the active layer decreases. The shorter lifetimes of RuO2 based anodes for O2 evolution are attributed to the oxidation of ruthenium to soluble RuO3 or to the highly volatile oxidation states of RuO4  respectively.