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Electrochemical nitrate reduction reaction (NO3−RR) is an ideal route to produce ammonia (NH3) under ambient conditions. Although a markedly improved NH3 production rate has been achieved on the NO3−RR compared with the nitrogen reduction reaction (NRR), the NH3 production rate of NO3−RR is still well below the industrial Haber–Bosch route due to the lack of robust electrocatalysts for yielding high current densities with concurrently good suppression of hydrogen evolution reaction (HER). Herein, we describe an in situ electrochemical strategy for the synthesis of hollow carbon-coated Cu nanoparticles (NPs) (HSCu@C) with abundant grain boundaries (HSCu-AGB@C) for highly efficient NO3−RR in both alkaline and neutral media. Impressively, in alkaline media, the HSCu-AGB@C can achieve a maximum NH3 Faradaic efficiency of 94.2% with an ultrahigh NH3 rate of 487.8 mmol g−1cat h−1 at −0.2 V versus a reversible hydrogen electrode, more than 2.4-fold of the rate obtained in the Haber–Bosch. Both theoretic computations and experimental results uncover that the grain boundaries play the key to improve the NO3−RR performance. Herein, the industrial-scale NH3 production rate may open exciting opportunities for the practical electrosynthesis NH3 under ambient conditions. © 2022 CCS Chemistry. All rights reserved.