Effects of electrolyte on the electrocatalytic activities of RuO₂/Ti and Sb-SnO₂/Ti anodes for water treatment

The study of high efficiency electrochemical water treatment systems is of great importance in contributing to a sustainable water supply. In this study, we prepared RuO₂/Ti and Sb-SnO₂/Ti electrodes and investigated their electrocatalytic activities for the oxidation of water and three model substrates (methylene blue, acid orange 7, and 4-chlorophenol) in two supporting electrolytes: NaCl vs. Na₂SO₄. Irrespective of the electrolyte, the particulate RuO₂/Ti anode was found to oxidize water at ca. 0.8 V lower potential ranges with significantly higher currents than the cracked-mud type of Sb-SnO₂/Ti, indicating that the latter is more suitable for substrate oxidation. In the system of Sb-SnO₂/Ti anode-stainless steel cathode couple, the degradation rates of all the substrates were highly enhanced in NaCl, whereas their complete oxidation (i.e., CO₂ evolution) occurred more markedly in Na₂SO₄. Additional detailed experimental results indicated that the relative superiority of Sb-SnO₂/Ti over RuO₂/Ti for treating the model substrates depended greatly on the employed supporting electrolytes, and that the superiority particularly vanished when NaCl was used as the electrolyte. Using this electrolyte, active chlorine species-mediated indirect reactions seemed to occur at both anodes, while in the Na₂SO₄ electrolyte, the surface-specific reaction occurred. Finally, surface analysis and diverse electrochemical experiments were performed to compare both anodes in a more quantitative way and to investigate the effect of the electrolytes on the electrocatalytic activities of the anodes.