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

Electroreducting nitrate (NO3-) into ammonia (NH3), i.e. NO3-RA, can not only relieve NO3- pollution but also produce high value-added NH3. Cu-based species have been taken as a promising catalyst for NO3-RA because of their relatively high Faradaic efficiency (FE), benefiting from the weak side hydrogen evolution reaction (HER). However, their NH3 yield rates are still unsatisfactory due to the multiple electron paths of NO3-RA. Herein, we report a Cu cube with Cu-CuO heterostructured skin, prepared by electrochemically induced reconstruction from a Cu2O cube. This novel Cu-based catalyst presents a mol-level NH3 yield rate of 3.17 mol h-1 g-1 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 NO3-RA process and an unfavorable one for HER. For the NO3- removal of real river water, this novel Cu-based catalyst presents a high NO3- removal rate of 95.5 % after the NO3-RA test for 12 h, resulting in a lower NO3- concentration than the maximum residual amount of NO3- 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 NO3-RA performances for the purification of nitrate wastewater.