Controlling the Selectivity of Chlorine Evolution Reaction by IrTaO<sub><i>x</i></sub>/TiO<sub>2</sub> Heterojunction Anodes: Mechanism and Real Wastewater Treatment
Evandi Rahman, Jieun Shin, Sukhwa Hong, Sukhwa Hong, Sunmi Im, Jiseon Kim, Chong Min Chung, Seok Won Hong, Seok Won Hong, Michael R. Hoffmann, Kangwoo Cho
Abstract
This study investigated the effects of varied loadings of TiO 2 overlayers in heterojunction with conventional Ir 0.7 Ta 0.3 O 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 /TiO 2, elevated ClER selectivity was attained by more facile chemisorption of chloride ions to a thin TiO 2 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 TiO 2 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 TiO 2 could enhance the ClER selectivity compared to bare IrTaO x . We concluded that ClER primarily occurs on TiO 2 near the junction owing to the nanoporous nature of the TiO 2 layer, while IrTaO x serves as ohmic contact. The optimized IrTaO x /TiO 2 anodes effectively improved the treatment of reverse osmosis concentrate, but phosphate ions in livestock wastewater caused adverse effects due to complexation on TiO 2 . 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.