Highly selective electrochemical nitrate reduction using copper phosphide self-supported copper foam electrode: Performance, mechanism, and application

•A binder-free composite electrode (Cu3P/CF) was prepared by Cu3P directly growing on Cu foam.•The Cu3P/CF showed high activity and selectivity for electrochemical denitrification.•Electrochemical NO3− reduction was achieved via the redox reaction of Cu0/NO3−.•Cu3P was a bifunctional electrocatalyst for electron transfer and NO2− conversion.•A 2-stage process (per-oxidation and post-reduction) was designed for real wastewater treatment. A highly active and selective electrode is essential in electrochemical denitrification. Although the emerging Cu-based electrode has attracted intensive attentions in electrochemical NO3− reduction, the issues such as restricted activity and selectivity are still unresolved. In our work, a binder-free composite electrode (Cu3P/CF) was first prepared by direct growth of copper phosphide on copper foam and then applied to electrochemical NO3− reduction. The resulting Cu3P/CF electrode showed enhanced electrochemical performance for NO3− reduction (84.3%) with high N2 selectivity (98.01%) under the initial conditions of 1500 mg L−1 Cl− and 50 mg N L−1 NO3−. The cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) demonstrated that electrochemical NO3− reduction was achieved through electron transfer between NO3− and Cu0 originated from CF. The in-situ grown Cu3P served as the bifunctional catalyst, the electron mediator or bridge to facilitate the electron-transfer for NO3− reduction and the stable catalyst to produce atomic H* toward NO2− conversion. Meanwhile, the Cu3P/CF remained its electrocatalytic activity even after eight cyclic experiments. Finally, a 2-stage treatment strategy, pre-oxidation by Ir-Ru/Ti anode and post-reduction by Cu3P/CF cathode, was designed for electrochemical chemical oxygen demand (COD) and total nitrogen (TN) removal from real wastewater. [Display omitted]