Abstract:

The practical application of electrocatalytic nitrate reduction to ammonia (NO₃RR) is limited by insufficient selectivity and stability under industrial current densities, primarily due to competitive side reactions and catalyst structural degradation. A critical challenge lies in the rational design of stable and active sub-nanometer metal clusters, where achieving strong yet precise anchoring of metal atoms to specific nitrogen sites within a carbon matrix is essential to prevent aggregation and tune the electronic structure for optimal performance. In this work, by precisely engineering the local coordination environment of nitrogen dopants, we created tailored anchoring sites that effectively stabilize cobalt atoms, thereby fostering the formation of well-defined cobalt sub-nanometer clusters (Co SNC). Mechanistic investigations reveal that the optimized Co SNC sites facilitate the selective hydrogenation of the key *NH₂OH intermediate to NH₃ via adsorbed hydrogen (*H_ads). Consequently, the engineered catalyst achieves an exceptional ammonia production rate of 6.9 mol h^-1 g_cat^-1 with a Faradaic efficiency of 91.4% at -0.6 V vs. RHE. Moreover, it demonstrates outstanding durability, operating steadily for over 100 h at an industrial-level current density of 500 mA cm^-2@2.09 V in a membrane electrode assembly. This work elucidates the critical role of enhancing the dispersion of metal active sites, providing a viable strategy for designing efficient and stable electrocatalysts toward nitrate-to-ammonia conversion and related processes.

Key words: Sub-nanometer clusters, Nitrate reduction, Electrocatalyst, Industrial-current density, Selective hydrogenation