Revealing the roles of oxygen vacancies in NiO-CeO₂ redox catalysts for electrocatalytic ozonation: Mechanistic study via in situ Raman spectroscopy

Improving the O₃ activation via electrocatalysis is a promising approach to achieve higher hydroxyl radical (^[rad]OH) production, however, the mechanisms and performance of electrocatalytic ozonation (ECO) remains unclear. Herein, we investigated the redox enhancement effect via preparing the asymmetric oxygen vacancies (O_V) contained NiO-CeO₂ and doped on carbon felt electrode to enhance anodic O₃ activation for atrazine (ATZ) demineralization. XPS and ESR liquid analysis results revealed that O_V content in NiO-CeO₂ electrode are crucial sites for O₃ decomposition and facilitating the ^[rad]OH generation. In situ Raman spectroscopy finding evidenced the intermetallic redox behaviour between NiO and CeO₂ in ECO. As a result, ECO using NiO-CeO₂ exhibited 3.4-times higher ATZ demineralization rate and 84.4% lower energy consumption (149 kWh/mg) than sole ozonation (960 kWh/mg). In addition, NiO-CeO₂ electrode in ECO also showed excellent demineralization performance toward SMX and BPA. Meanwhile, the leaching of Ce(III) and Ni(II) from NiO-CeO₂ were not detected in ECO. The toxicities of ATZ degradation by-products were reduced, as proved by QSAR predictions and seed germination. Notably, NiO-CeO₂ electrode exhibited excellent reusability performance in consecutive seven cycles ECO tests and outstanding stability in continuous-stirred-tank-reactor experiments, with 84% ATZ demineralization for up to 10 hr. This study provides mechanistic insights into the roles of O_V in redox enhancement to improve ^[rad]OH generation for water treatment.