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.