FeCo nanoparticles exsolved from Co-doped Sm0.9FeO3 nanofibers with abundant oxygen vacancies (Vos) are proposed as an efficient electrocatalyst to promote nitrate reduction reaction (NITRR). Such catalyst achieves a maximum Faradaic efficiency (FE) of 90.3 % and a large NH3 yield of 17.2 mg h−1 mg−1cat. at a negatively shifted potential of −0.9 V in 0.1 M PBS with 0.1 M NaNO3, and the alloy nanoparticles socketed into nanofibers remain extremely stable during long-term electrolysis. The reaction pathway favoring the formation of NH2OH is uncovered by in situ electrochemical tests and theoretical calculations reveal the exsolution of FeCo alloy combined with the generation of Vos enhances nitrate adsorption and lowers energy increase of the potential determining step. Finite-element simulations unveil the applied current and charges are localized on the alloys along the nanofiber, which confirms the exsolved FeCo nanoparticles are the main active sites for NITRR.