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 (FeS_x @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 FeS_0.2 @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.

将硝酸盐（NO ₃ ^-）电催化还原为氨（NH ₄ ⁺）并通过活性氯进一步氧化为N ₂是一种有效的反硝化方法。正极的高活性、NH ₄ ⁺ -N 选择性和长期稳定性对于该应用至关重要。在此，我们报道了一种硫改性的泡沫铁 (FeS _{x @FF )}阴极，^由于其增强的电子转移、原子氢气 (H*) 的产生和 NO _3的吸附^-. 结合原位差分电化学质谱和理论计算表明，*NO ₃超快还原为*NO ₂和*NO的低形成能是*NO ₃还原为*NH ₃的关键。^{用于实际工业废水（~350 mg L -1} NO ₃ ^- -N）处理的 FeS _0.2 @FF 阴极显示出高 TN 去除率（90.75%）和 N ₂选择性（98.88%）以及超低的电力成本（~0.22 kWh g ^-1 N)。这种设计的系统具有在含盐废水中有效去除NO ₃ ^-的潜力。