Direct Electron Transfer Coordinated by Oxygen Vacancies Boosts Selective Nitrate Reduction to N₂ on a Co–CuOx Electroactive Filter

Atomic hydrogen (H*) is used as an important mediator for electrochemical nitrate reduction; however, the Faradaic efficiency (FE) and selective reduction to N₂ are likely compromised due to the side reactions (e.g., ammonia generation and hydrogen evolution reactions). This work reports a Co-CuOx electrochemical filter with CoOx nanoclusters rooted on vertically aligned CuOx nanowalls for selective nitrate reduction to N₂, utilizing the direct electron transfer between oxygen vacancies and nitrate to suppress the contribution by H*. At a cathodic potential of -1.1 V (vs Ag/AgCl), the Co-CuOx filter showed 95.2% nitrate removal and 96.0% N₂ selectivity at an influent nitrate concentration of 20 N-mg L⁻¹. Meanwhile, the energy consumption and FE were 0.60 kW h m⁻³ and 53.5%, respectively, at a permeate flux of 60 L m⁻² h⁻¹. The presence of abundant oxygen vacancies on Co-CuOx was due to the change in the electron density of the Cu atom and a decrease of the coordination numbers of Cu-O via cobalt doping. Theoretical calculations and electrochemical tests showed that the oxygen vacancies coordinated nitrate adsorption and subsequent reduction reactions, thus suppressing the contribution of H* to nitrate reduction and leading to a thermodynamically favorable process to N₂ via direct electron transfer.