Direct Electron Transfer Coordinated by Oxygen Vacancies Boosts Selective Nitrate Reduction to N 2 on a Co-CuO x 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.