Tuning the Oxidation State of Cu Electrodes for Selective Electrosynthesis of Ammonia from Nitrate

Electrochemical reduction of nitrate (NO₃^-) to ammonia (NH₃) provides a promising route for recycling nitrate from wastewater to balance the nitrogen cycle and sustainable production of ammonia. Among various catalytic materials for NO₃^- electroreduction, Cu shows a favorable selectivity to NH₃. However, Cu can be easily oxidized, while the effect of the Cu oxidation state on NO₃^- reduction remains to be elucidated. Here, we report that oxidic Cu formed on a Cu electrode can enhance its activity and selectivity for NO₃^- reduction to NH₃. We first used a polished Cu foil as a model catalyst for NO₃^- reduction and found that a brief exposure of the Cu electrode to air could increase its yield rate and Faradaic efficiency for NH₃ production. The improved catalytic performance was attributed to the formed Cu⁺ sites that can reduce the energy barrier for NO₃^- reduction to NH₃ and suppress the competing HER reaction. Based on this finding, an oxide-derived Cu (OD-Cu) electrode was prepared by annealing a Cu foil in O₂ gas followed by electroreduction, which exhibited superior performance for NO₃^- reduction to NH₃, with a Faradaic efficiency of 92% and a yield rate of 1.1 mmol h^-1 cm^-2 for NH₃ production at -0.15 V versus reversible hydrogen electrode. Moreover, an OD-Cu foam electrode was similarly developed to demonstrate NO₃^- recycling from a low-concentration NO₃^- solution, which showed a nearly 100% conversion of NO₃^- to NH₃ using a circulating flow cell.