Electrocatalytic　nitrate　reduction　reaction　(NO3-RR)　to　ammonia　under　ambient　conditions　is　expected　to　be　a　green　process　for　ammonia　synthesis　and　alleviate　water　pollution　issues.　We　report　a　CuO　nanoparticles　incorporated　on　nitrogen-doped　porous　carbon　(CuO@NC)　catalyst　for　NO3-RR.　Part　of　Cu(II)　is　reduced　to　Cu(I)　during　the　NO3-RR　process　to　construct　Cu(I)-Cu(II)　pairs,　confirmed　by　in　situ　X-ray　photoelectron　spectroscopy　(XPS)　and　Raman　spectroscopy.　Density　functional　theory　(DFT)　calculations　indicated　that　the　formation　of　Cu(I)　could　provide　a　reaction　path　with　smaller　energy　barrier　for　NO3-RR,　while　Cu(II)　effectively　suppressed　the　competition　of　hydrogen　evolution　reaction　(HER).　As　a　result,　CuO@NC　catalyst　achieved　a　Faradaic　efficiency　of　84.2%　at　-0.49　V　versus　reversible　hydrogen　electrode　(RHE),　and　a　NH3　yield　rate　of　17.2　mg　h-1　mgcat.-1　at　-0.79　V　vs.　RHE,　higher　than　the　HaberBosch　process　(＜3.4　g　h-1　gcat.-1　).　This　work　may　open　a　new　avenue　for　effective　NO3-RR　by　modulating　oxidation　states.　(c)　2024　Science　Press　and　Dalian　Institute　of　Chemical　Physics,　Chinese　Academy　of　Sciences.　Published　by　ELSEVIER　B.V.　and　Science　Press.　All　rights　are　reserved,　including　those　for　text　and　data　mining,　AI　training,　and　similar　technologies.