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-CuOx electrochemical filter with CoOx nanoclusters rooted on vertically aligned CuOx 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-CuOx 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-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 N-2 via direct electron transfer.

Automated summary

This study reports a new Co-CuOx electrochemical filter for selective nitrate reduction to N-2. By growing CoOx nanoclusters on vertically aligned CuOx nanowalls, the contribution of H* is suppressed by utilizing the direct electron transfer between oxygen vacancies and nitrate. The Co-CuOx filter exhibits high efficiency in nitrate removal and N-2 selectivity.