Long-Term Efficient Ammonia Synthesis by Cu-Co Tandem Active Sites at High Current Density

Abstract

Electrocatalytic nitrate reduction reaction (NO3RR) holds significant promise for both sustainable ammonia (NH3) synthesis and industrial wastewater treatment. However, its efficiency in neutral electrolytes is severely limited by the insufficient supply of reactive hydrogen species (*H), leading to the accumulation of nitrite (NO2-), which not only deactivates catalysts but also suppresses subsequent hydrogenation steps. In addition, under high-current-density conditions, the competing hydrogen evolution reaction (HER) intensifies, leading to a decrease in the Faradaic efficiency (FE) for NH3. In this work, we present a Cu-Co3O4 tandem catalyst, where trace Cu doping into octahedral sites of Co3O4 promotes nitrate adsorption, while the altered coordination environment of Co facilitates water dissociation to generate *H. The Cu-Co active sites facilitate the efficient transfer of *H to adsorbed nitrate species, consequently enhancing both the ammonia production rate and catalytic stability under high-current-density and neutral conditions. The optimized catalyst can achieve a maximum FE of 93.2% and a high ammonia yield rate of 11.3 mg h-1 cm-2. Furthermore, we coupled nitrate reduction (NO3RR) with 5-hydroxymethylfurfural oxidation (HMFOR) in a membrane electrode assembly (MEA) electrolyzer, and a comprehensive techno-economic analysis demonstrates the profitability and commercial viability of this coupled system.