Deciphering Cation-Stabilized *NO₂ at the Molecular Level in Electrocatalytic Nitrate Reduction

Electrochemical nitrate reduction (NO₃RR) offers a sustainable pathway for ammonia (NH₃) production, yet its advancement is limited by a lack of molecular-level understanding of interfacial reaction dynamics. Here, we unravel a synergistic interfacial mechanism for rational catalyst design based on cooperative interfacial modulation. By integrating in situ Raman spectroscopy with multiscale simulations, we uncover a cation-mediated stabilization mechanism of the key *NO₂ intermediate on atomically defined Au single-crystal surfaces. We demonstrate that electrolyte cations play a critical role in modulating the local electric field and stabilizing *NO₂ via interfacial coordination, thereby controlling its interfacial reactivity and subsequent transformation. Armed with this insight, we employ Sn heteroatom modification to achieve dual-function optimization. The electronic interaction between Sn and Au weakens *NO₂ adsorption, lowering the hydrogenation barrier, while the altered interfacial water structure facilitates proton transfer through a reinforced hydrogen-bond network. This synergistic modulation leads to enhanced NH₃ selectivity and suppressed hydrogen evolution. Our findings highlight a paradigm shift from an active-site-centric catalyst design to an integrated approach that considers the entire electrochemical interface, where electronic, ionic, and solvent effects are concurrently tuned. This work provides both molecular-level mechanistic insights and a generalizable strategy for designing advanced electrocatalysts for NO₃RR and broader proton-coupled electron transfer reactions.