• Ultrathin 2D FeS nanosheets synthesized via a facile, scalable method exhibit high conductivity and efficient electrocatalytic N2 reduction.
• The FeS catalyst achieves an NH3 yield of 9.0 μg·h−1·mg−1 and a Faraday efficiency of 12.4%, outperforming most reported NRR catalysts.
• The 2D mackinawite structure is identified as the key factor for enhanced NRR performance, offering insights for low-cost Fe-based electrocatalysts.
• This work advances sustainable NH3 synthesis, contributing to carbon neutrality and practical NRR applications.