Electrocatalytic N₂ reduction provides an attractive alternative to Haber-Bosch process for artificial NH₃ synthesis. The difficulty of suppressing competing proton reduction, however, largely impedes its practical use. Herein, we design a hydrophobic octadecanethiol- modified Fe₃P nanoarrays supported on carbon paper (C18@Fe₃P/CP) to effectively repel water, concentrate N₂, and enhance N₂-to-NH₃ conversion. Such catalyst achieves an NH₃ yield of 1.80 × 10^–10 mol·s^–1·cm^–2 and a high Faradaic efficiency of 11.22% in 0.1 M Na₂SO₄, outperforming the non-modified Fe₃P/CP (2.16 × 10^-11 mol·s^–1·cm^–2, 0.9%) counterpart. Significantly, C18@Fe₃P/CP renders steady N₂-fixing activity/selectivity in cycling test and exhibits durability for at least 25 h. First-principles calculations suggest that the surface electronic structure and chemical activity of Fe₃P can be well tuned by the thiol modification, which facilitates N₂ electroreduction activity and catalytic formation of NH₃.