Constructing Cu-CuO heterostructured skin on Cu cubes to promote electrocatalytic ammonia production from nitrate wastewater.

Electroreducting nitrate (NO) into ammonia (NH), i.e. NORA, can not only relieve NO pollution but also produce high value-added NH. Cu-based species have been taken as a promising catalyst for NORA because of their relatively high Faradaic efficiency (FE), benefiting from the weak side hydrogen evolution reaction (HER). However, their NH yield rates are still unsatisfactory due to the multiple electron paths of NORA. Herein, we report a Cu cube with Cu-CuO heterostructured skin, prepared by electrochemically induced reconstruction from a CuO cube. This novel Cu-based catalyst presents a mol-level NH yield rate of 3.17 mol h g ranking at the top level among non-noble metal catalysts and an ultrahigh FE of 98.7 %. These excellent performances attribute to the Cu-CuO heterostructured skin of Cu cubes, which has favorable energy for the hydrogenation of *NO to *NOH during the NORA process and an unfavorable one for HER. For the NO removal of real river water, this novel Cu-based catalyst presents a high NO removal rate of 95.5 % after the NORA test for 12 h, resulting in a lower NO concentration than the maximum residual amount of NO in drinking water limited by WHO and China. This study provides a feasible strategy by the electrochemical reconstruction method to prepare superior Cu-based electrocatalysts with mol-level NORA performances for the purification of nitrate wastewater.