The nitrate reduction reaction is emerging as having tremendous potential to mitigate nitrate pollution and simultaneously produce valuable ammonia. Here, we propose Co3O4 nanoparticles embedded in porous carbon nanofibers (Co3O4@CNF) as a high-efficiency catalyst to convert nitrate to ammonia, and it achieves a high faradaic efficiency of 92.7% and an extremely large NH3 yield of 23.4 mg h-1 mg-1cat, and also presents excellent electrochemical stability. Theoretical calculations reveal that the potential determining step (PDS) reaches as low as 0.28 eV. This work is expected to open a new avenue to rationally design robust noble-metal-free catalysts for the electrochemical synthesis of ammonia.