On the electrolysis of dilute chloride solutions: Influence of the electrode material on Faradaic efficiency for active chlorine, chlorate and perchlorate

In the present work, the electrolytic process of diluted aqueous chloride solutions was investigated at Ti/RuO2·2SnO2 and Ti/Pt electrodes, at different values of current density, temperature and electrolysis time. The time evolution of chlorine-related species (i.e., active chlorine (dissolved Cl2, HClO, OCl−), chlorite, chlorine dioxide, chlorate and perchlorate) was investigated in order to establish whether their formation and consumption was related to either chemical or electrochemical path of reactions. The estimated Faradic efficiencies demonstrated the better catalytic activity of the Ti/RuO2·2SnO2 electrode towards the chlorine evolution reaction with respect to the Ti/Pt anode, and the key-role played by the temperature, which reflects the different activation energies of the two competing electrochemical reactions, i.e., chloride and water oxidations. The concentration trends of chlorate and perchlorate indicated that the electrochemical route was responsible for their presence in the bulk solution, instead of a chemical path. The low concentration levels assessed for chlorites throughout the tests did not suggest the preponderance of the chemical over the electrochemical depletion process; however, further potentiodynamic tests suggested their high reactivity towards both anodic and cathodic surfaces, thus suggesting that the electrochemical path of depletion could prevail over the chemical. Conversely, due to a solution pH unfavourable to the stability of chlorine dioxide, its low concentration level was associated with a chemical depletion route.