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

Abstract

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.

Keywords: Binder-free electrode; Copper foam; Copper phosphide; Electrochemical denitrification; Reaction mechanism.