Sulfur-Dopant-Promoted Electrocatalytic Reduction of Nitrate by a Self-Supported Iron Cathode: Selectivity, Stability, and Underlying Mechanism

The electrocatalytic reduction of nitrate (NO3−) to ammonia (NH4+) and further oxidation to N2 via active chlorine is an effective method for denitrification. High activity, NH4+–N selectivity and long-term stability of a cathode are essential for the application. Herein, we reported a sulfur-modified Fe foam (FeSx @FF) cathode, which achieved high NO3−–N conversion (94.96%) and NH4+–N selectivity (82.29%) due to its enhanced electron transfer, atomic hydrogen (H*) generation, and adsorption of NO3−. Combined in-situ differential electrochemical mass spectrometry and theoretical calculations reveal that the ultrafast *NO3 reduction to *NO2 and low formation energy of *NO are crucial in reduction of *NO3 to *NH3. The FeS0.2 @FF cathode for realistic industrial wastewater (∼350 mg L−1 NO3−–N) treatment shows high TN removal (90.75%) and N2 selectivity (98.88%) with an ultralow electrical cost (∼0.22 kWh g−1N). This designed system has potential for efficient NO3− removal in the saline wastewater.