Engineering Cu/Ru Heterointerface-Shelled Nanocavities by the Kirkendall Effect for Highly Efficient Nitrate Electroreduction to Ammonia

Electrochemical nitrate (NO₃^-) reduction to ammonia (NH₃) offers a sustainable approach for NH₃ synthesis while concurrently addressing NO₃^- pollution. However, achieving efficient NO₃^--to-NH₃ conversion remains challenging due to sluggish multistep proton-coupled electron transfer processes and poor intermediate converison. Here, we present a nanocatalyst featuring a hollow nanocavity encased within a shell rich in Cu/Ru heterointerfaces, which synergistically leverages both interfacial and structural advantages to effectively lower energy barriers and accelerate intermediate conversion kinetics, thereby enhancing the overall catalytic performance for NH₃ production. Density functional theory (DFT) computations, supported by operando and control experiments, reveal that CuRu heterointerfaces with their optimized electronic structure act as the primary active sites, establishing a favorable NO₃^--to-NH₃ reaction pathway. Simultaneously, the catalytic synergy between Cu and CuRu sites enables tandem catalysis, which is further amplified by nanocavity-induced spatial confinement of the key intermediate NO₂^-. This nanocatalyst is realized via a Kirkendall effect-driven strategy, with its structural features systematically optimized. The resulting catalyst demonstrates outstanding NH₃ production performance in a 0.1 M KNO₃ + 0.1 M KOH electrolyte, delivering a Faradaic efficiency of 97.4%, a yield of 152.6 mg h^-1 mg_metal^-1, and an energy efficiency of 40% at a low potential of -0.1 V_RHE─positioning it as a top contender among state-of-the-art NO₃^--to-NH₃ electrocatalysts. By elucidating mechanistic insights into interfacial effects, tandem catalysis, and nanoconfinement, this work highlights the synergistic impact of compositional and structural engineering and offers a generalizable design strategy for advancing NO₃^--to-NH₃ electroconversion and broader sustainable catalytic transformations.