Abstract: With the advancement of industry and agriculture, substantial nitrate-containing pollutants have been discharged. This leads to severe environmental pollution and imbalance in nitrogen cycle, threatening ecology environment and human health. Electrocatalytic nitrate reduction to ammonia can convert pollutants into important chemical feedstocks and energy-storage molecules, providing a feasible route for ammonia production using renewable energy. However, this reaction is complicated and involves multiple steps and intermediates, suffering from high overpotential and limited selectivity and activity. To solve this issue, herein we designed a mixed-valence Cu/CuO_x catalyst which is primarily composed of Cu and Cu₂O and shows three-dimensional nanowire structure, achieving efficient nitrate reduction reaction for ammonia synthesis. The current density for ammonia production reaches 1.89 A·cm^-2 at -0.22 V vs. RHE (reversible hydrogen electrode), with a Faradaic efficiency of 80.9%. Cu/CuO_x also maintains excellent stability during prolonged electrolysis. The apparent and normalized performances of Cu/CuO_x are much higher than those of the control systems, such as Cu and Cu₂O. Experimental and theoretical results prove that the Cu⁰ and Cu⁺ species in Cu/CuO_x play synergistic roles, promoting the conversion of ^*NO intermediate to ^*NOH and accelerating the desorption of NH₃. The nanowire structure provides abundant active sites, thereby enhancing the performance of nitrate reduction to ammonia. This work not only provides an efficient catalyst for nitrate reduction to ammonia but also serves as a guideline for the rational design of such catalysts.

Key words: Electrocatalytic nitrate reduction, Ammonia synthesis, Cu/CuO_x, Mixed-valence Nanowire, Synergistic effect