The electrochemical nitrate (NO₃⁻) reduction reaction (NO₃⁻RR), with much faster kinetics than the nitrogen (N₂) reduction, provides new opportunities to harvest ammonia (NH₃) under ambient conditions. However, the NH₃ production rate of NO₃⁻RR is still much inferior to that of the industrial Haber–Bosch route due to the lack of robust electrocatalysts for suppressing the hydrogen evolution reaction (HER) at large current densities. Herein, we demonstrate an electrocatalyst synthesis strategy based on the in situ electrochemical reduction of ultrathin copper-oxide nanobelts under NO₃⁻RR conditions, which favorably exposes Cu(100) facets and abundant surface defects, thereby markedly facilitating the NO₃⁻RR yet hindering the HER. We discover that the intermediates of NO₃⁻RR (i.e., N*) can serve as capping agents for controlling the exposed facets during the reduction. Impressively, in alkaline media, the NO₃⁻RR catalyzed by defective Cu(100) facets gives a NH₃ yield rate which is 2.3-fold higher than that of the Haber–Bosch process. The synergy of Cu(100) facets and defects, which upshifts the d band center of Cu, is the key to excellent performance. The reaction intermediate-mediated strategy demonstrated in this study offers a fresh concept and robust methodology for directional electrocatalyst synthesis to achieve markedly enhanced performance.