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 N 2 are likely compromised due to the side reactions (e.g., ammonia generation and hydrogen evolution reactions). This work reports a Co–CuO x electrochemical filter with CoO x nanoclusters rooted on vertically aligned CuO x nanowalls for selective nitrate reduction to N 2, 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–CuO x filter showed 95.2% nitrate removal and 96.0% N 2 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–CuO x 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 N 2 via direct electron transfer.