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

Abstract

This study investigated the effects of varied loadings of TiO2 overlayers in heterojunction with conventional Ir0.7Ta0.3O 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.