Electrochemical methods have been proven to effectively eliminate nitrates in sewage and convert them into high value-added ammonia products. Here, after annealing treatment of metal boron cluster organic polymers formed by the combination of 1,10-phenanthroline, Cu2+ and closo-[B12H12](2)(-), a Cu single-atom doped BCN (B-doped CN) with a diamond-shaped nanosheet structure was obtained. In the electrochem-ical reduction reaction of nitrate, BCN-Cu exhibits excellent catalytic activity, specifically: 1) the ammonia yield rate reached as high as 498.85 mu g h(-1) cm(-2), 1047.14 mu g h(-1) cm(-2), 1900.07 mu g h(-1) cm(-2) and 3358.74 mu g h(-1) cm(-2) at -0.3 V, -0.4 V, -0.5 V and -0.6 V vs reversible hydrogen electrode, respectively, and Faradaic efficiency is 95.90%, 97.28%, 98.23% and 97.37%; 2) after repeated use of BCN-Cu 10 times or continuous operation for 16 h, the activity against electrochemical reduction reaction of nitrate anions is almost unchanged. The (NO3-)-N-15 isotopic labeling experiment proved that the detected NH3 comes from the reduction of NO3- on BCN-Cu. Control experiments show that the presence of Cu determines whether BCN-Cu has the possibility of catalyzing electrochemical reduction reactions of nitrate, and the presence of the B element enhances the catalytic activity of BCN-Cu. Density functional calculations indicate that in the water phase the process of reducing NO3- to NH3 on Cu-0 is an exothermic reaction, and that the adsorption process of NO3- on Cu-0 is the rate-determining step. (C) 2021 Elsevier Ltd. All rights reserved.