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.