Synergistic Active Sites in a Defect-Engineered FeMoO ₄ /CeS ₂ Diphase for Highly Efficient Ambient Electrocatalytic Ammonia Synthesis

The development of efficient electrocatalysts for the ambient nitrogen reduction reaction (NRR) remains challenging due to sluggish N2 activation and the competing hydrogen evolution reaction (HER). Here, we report a defect-engineered FeMoO4/CeS2 diphase grown directly on nickel foam (NF) for efficient electrochemical ammonia synthesis under ambient conditions. Systematic tuning of the Fe/Mo/Ce molar ratio establishes a clear composition-activity relationship, with the optimized FeMoCe (2:4:1) (2-FeMoCe) catalyst delivering an NH3 yield rate of 5124 μg cm-2 h-1 (1139 μg mgcat-1 h-1) and a Faradaic efficiency (FE) of 46% at -0.70 V vs RHE in neutral 0.5 M Na2SO4. In a membrane-separated single stack-cell configuration, the catalyst maintains strong performance, achieving 2451 μg cm-2 h-1 (613 μg mgcat-1 h-1) with an FE of 35.01%, highlighting its practical relevance. Comprehensive structural and electronic analyses reveal a defect-rich oxide-sulfide diphase featuring abundant oxygen vacancies (VO2+), mixed-valence Fe2+/Fe3+ and Ce3+/Ce4+ species, and enhanced interfacial electronic coupling. Spectroscopic and electrochemical results confirm defect enrichment upon Fe incorporation, accompanied by increased electrochemically active surface area and lower charge-transfer resistance (Rct). In situ vibrational analysis identifies surface-bound -NHx intermediates, consistent with the sequential hydrogenation of adsorbed nitrogen species toward NH3 formation. The superior NRR performance is attributed to synergistic interfacial active sites that promote defect-mediated N2 activation, facilitate charge redistribution, and suppress HER. This work demonstrates that rational defects and diphase engineering are an effective strategy for advancing sustainable ammonia electrosynthesis.