The electrochemical N 2 reduction reaction (NRR) offers a direct pathway to produce NH 3 from renewable energy. However, aqueous NRR suffers from both low Faradaic efficiency (FE) and low yield rate. The main reason is the more favored H + reduction to H 2 in aqueous electrolytes. Here we demonstrate a highly selective Ru/MoS 2 NRR catalyst on which the MoS 2 polymorphs can be controlled to suppress H + reduction. A NRR FE as high as 17.6% and NH 3 yield rate of 1.14 × 10 -10 mol cm -2 s -1 are demonstrated at 50 °C. Theoretical evidence supports a hypothesis that the high NRR activity originates from the synergistic interplay between the Ru clusters as N 2 binding sites and nearby isolated S-vacancies on the 2H-MoS 2 as centers for hydrogenation; this supports formation of NH 3 at the Ru/2H-MoS 2 interface.