Sulfur-Bridge Engineering Enables Reverse Hydrogen Spillover to Atomic Cu for Nitrate-to-Ammonia Electrocatalysis

Copper-based catalysts are widely regarded as promising candidates for electrocatalytic nitrate reduction (NO₃RR), an environmentally benign route to ammonia synthesis, yet their efficiency is often constrained by nitrite accumulation and insufficient active hydrogen (*H) supply at high current densities. Here, isolated Cu atoms were anchored into a hollow Co₃S₄ polyhedral framework (Cu-Co₃S₄), generating a sulfur bridged asymmetric active center. Electrochemical and computational studies reveal that Co₃S₄ functions as an efficient *H donor, transferring hydrogen species to Cu sites through a sulfur bridge mediated reverse hydrogen spillover process, thereby accelerating the hydrogenation of nitrogen intermediates. By precisely tuning the Cu site density to balance nitrogen intermediate adsorption with *H supply, the optimized Cu_1.01wt%-Co₃S₄ catalyst delivers an exceptional NH₃ yield rate of 94.52 mg h^-1 mg_cat. ^-1 (18.90 mg h^-1 cm^-2) and a Faradaic efficiency (FE) of 95.18% at -0.8 V vs reversible hydrogen electrode. The catalyst also exhibits remarkable durability over 300 h at -200 mA cm^-2 and performs effectively in zinc-nitrate batteries. These findings highlight the importance of coupling intermediate activation with hydrogenation kinetics and provide guiding principles for the rational design of high efficiency NO₃RR electrocatalysts.