Controlling the Selectivity of Chlorine Evolution Reaction by IrTaOₓ/TiO₂ Heterojunction Anodes: Mechanism and Real Wastewater Treatment

This study investigated the effects of varied loadings of TiO₂ overlayers in heterojunction with conventional Ir_0.7Ta_0.3O_x (IrTaO_x) anodes on chlorine evolution reaction (ClER) and real (waste)water treatment at circum-neutral pH. With an optimized design of IrTaO_x/TiO₂, elevated ClER selectivity was attained by more facile chemisorption of chloride ions to a thin TiO₂ layer on IrTaO_x. The current efficiency (CE) of ClER in galvanostatic electrolysis of 50 mM NaCl solutions (at 30 mA cm^–2) was maximized to ∼80% by a heterojunction architecture with ∼605 μg cm^–2 of IrTaO_x and ∼265 μg cm^–2 of TiO₂ after specific rounds of drop casting. Further increases in loading resulted in escalated film-pore resistance or deterioration of ClER selectivity. The observed CE values were correlated with experimental descriptors, such as potential of zero charge and flat band potential, demonstrating that the weaker metal–oxygen bond strength on TiO₂ could enhance the ClER selectivity compared to bare IrTaO_x. We concluded that ClER primarily occurs on TiO₂ near the junction owing to the nanoporous nature of the TiO₂ layer, while IrTaO_x serves as ohmic contact. The optimized IrTaO_x/TiO₂ anodes effectively improved the treatment of reverse osmosis concentrate, but phosphate ions in livestock wastewater caused adverse effects due to complexation on TiO₂. The heterojunction architecture effectively tunes the surface charge density for selective generation of oxidants, which can facilitate electrochemical water treatment with reduced use of the precious metals.