Moderate B-doped Cu^δ+ sites have outstanding performance for electrocatalytic NO₃⁻-to-NH₃. The faradaic efficiency can reach 96.8% with a current density of 26.3 mA cm⁻² and an NH₃ yield rate of 7.2 mg h⁻¹ mg_cat.⁻¹. The active and stable Cu^δ+ sites contribute to enhanced electron transport and higher reactivity. In situ experiments and theoretical calculations show that the Cu^δ+ sites with moderate B-doping can decrease the reaction barrier and facilitate the formation of *NOH, leading to high catalytic activity for the NO₃⁻-to-NH₃ reaction.