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

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journal contribution
posted on 2024-05-15, 17:09 authored by Evandi Rahman, Jieun Shin, Sukhwa Hong, Sunmi Im, Jiseon Kim, Chong Min Chung, Seok Won Hong, Michael R. Hoffmann, Kangwoo Cho
This study investigated the effects of varied loadings of TiO2 overlayers in heterojunction with conventional Ir0.7Ta0.3Ox (IrTaOx) anodes on chlorine evolution reaction (ClER) and real (waste)­water treatment at circum-neutral pH. With an optimized design of IrTaOx/TiO2, elevated ClER selectivity was attained by more facile chemisorption of chloride ions to a thin TiO2 layer on IrTaOx. 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 IrTaOx 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 IrTaOx. We concluded that ClER primarily occurs on TiO2 near the junction owing to the nanoporous nature of the TiO2 layer, while IrTaOx serves as ohmic contact. The optimized IrTaOx/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.