Mechanism of the chlorine evolution on a ruthenium oxide/titanium oxide electrode and on a ruthenium electrode

The mechanism of the chlorine evolution on titanium electrodes coated with a layer of ruthenium oxide and titanium oxide under different experimental conditions, and on a ruthenium electrode, both in acidic chloride solution, has been investigated. Potentiodynamic current density—potential curves were recorded as a function of the time anodic pre-polarisation, the composition of the solution and the temperature. Moreover, potential decay curves were determined. Theoretical potential decay curves were deduced for both the Tafel reaction (2 Clad→Cl2) and the Heyrovsky reaction (Cl− + Clad → Cl2 + e−) as a rate determining step in the formation of molecular chlorine. They were compared with those found experimentally. The influence of possible diffusion of atomic chlorine out of the electrode was also taken into consideration. It was found that for all the electrodes investigated, molecular chlorine is formed both at anodic polarisation and on open circuit according to the Volmer—Heyrovsky mechanism, where the Heyrovsky reaction is the rate-determining step. The transfer coefficient is 0.5 for the chlorine evolution at an “ideal” ruthenium oxide titanium oxide electrode and at a ruthenium electrode.